UHV Converter Stations and the Main Electrical Equipment

Chapter 16

UHV Converter Stations and the Main Electrical Equipment 16.1 UHVDC CONVERTER STATIONS A converter station mainly comprises converters and the DC and AC switchyards. The converter mainly consists of the converter transformer and converter valve. The DC switchyard accommodates smoothing reactors, DC filters, DC measuring devices, arresters, surge capacitors, coupling capacitors, switchgears, etc. The AC switchyard accommodates AC switchgears, AC filters, reactive power compensation devices, arresters, etc. Converter stations of 6500-kV and 6600-V (Itaipu, Brazil) HVDC links that have been commissioned and 6660 kV EHV DC links under construction in China and abroad all employ the configuration of one 12-pulse converter per pole. Converter valves of one pole are arranged in one valve hall, making two valve halls for two poles in each converter station. UHVDC links refer to those at a voltage level of 6800 kV and above. At present, China is the only country with UHVDC links that are either operating (6800 kV Xiangjiaba
Shanghai and Yunnan
Guangdong), under construction (6800 kV Jinping-unan), or being planned (6800 kV Xiluodu
Zhexi and Hami
Zhenzhou, as well as 61100 kV Zhundong
Chongqing). Due to the restrictions in equipment manufacturing and transportation, UHVDC converter stations adopt such a configuration that each pole has multiple series-connected 12-pulse converters arranged in multiple valve halls (HV-end and LV-end valve halls). Their design is different from that of conventional HVDC converter stations.

16.1.1

HVDC Converter Stations With a Single 12-Pulse Converter Per Pole

For a single-circuit bipolar DC link, there is one valve hall for each pole at each converter station under the scheme of one 12-pulse converter per pole. Thus, there are in total two valve halls at each end. Both valve halls are arranged in line. In particular, the converter transformer is installed close to the AC side of the valve hall with its valve-side bushing directly protruding into the valve hall. The DC and AC switchyards are located on the two sides of the valve hall.

16.1.2

UHVDC Converter Station With Two Series-Connected 12-Pulse Converters Per Pole

16.1.2.1 Electrical Structure The double 12-pulse series converter structure refers to the monopole converter with two 12-pulse converters composed of a converter series. The two series-connected 12-pulse converters may be at the same voltage level. For example, an 6800-kV DC transmission project may consist of two 6400-kV 12-pulse converters in series, represented as 400 kV 1400 kV; a 61000-kV DC project may consist of two 6500-kV 12-pulse converters in series, represented as 500 kV 1 500 kV. Alternatively, the two 12-pulse converters may be at different voltage levels. For example, at the design stage of the 6800 kV Xiangjiaba
Shanghai UHVDC Transmission Project, two 500 kV 1 300 kV or 600 kV 1 200 kV 12-pulse converters were considered.

16.1.2.2 Configuration of Valve Halls Irrespective of them having the same or different voltage levels, the two 12-pulse converters may be placed in one valve hall, i.e., two valve halls at each converter station; or placed in two separate valve halls, i.e., four valve halls at each converter station, one for each 12-pulse converter. As indicated by analysis and demonstration on economic indexes of the engineering design of UHVDC transmission projects, it is more economical to have the two 12-pulse converters installed in two separate end valve halls (HV-end and LV-end valve hall). This is justified by the fact that the HV-end valve hall is designed with greater height and volume to UHV Transmission Technology. DOI: http://dx.doi.org/10.1016/B978-0-12-805193-1.00016-1 Copyright © 2018 China Electric Power Press. Published by Elsevier Inc. All rights reserved.

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provide sufficient air clearance for the required high operating impulse insulation level, while the LV-end valve hall may be designed with smaller height and volume as the required low operating impulse insulation level can be achieved with a small air clearance. The installation of high- and low-end 12-pulse converter valves in one large and one small valve hall, respectively, will reduce the construction difficulties and overall cost of valve halls, when compared to arranging the two 12-pulse converters in the same valve hall.

16.1.2.3 Layout of Converter Stations The general layout of a UHVDC converter station features a streamline arrangement of “DC switchyard
valve hall, converter transformer
AC switchgear.” The design takes into account the topography, land requisition, direction of AC and DC outgoing lines, switchgears and other conditions so as to make the construction, operation and maintenance as reasonable and economical as possible. A UHVDC converter station mainly consists of a converter transformer, valve hall, DC switchyard, and AC switchyard, among which the layout of the converter transformer and valve hall is of great importance and will directly affect the overall layout of the converter station.

16.1.2.4 Structure and Arrangement of Converter Valves Valve towers are the main equipment in valve halls, and dictate the design of valve halls. Valve towers may be either supported or suspended. In general, supported valve towers are not suitable for converter stations located in active seismic areas or those with high aseismic requirements, as they require numerous post insulators which will make the valve towers complex and heavy. Suspended valve towers have a chained mechanical structure. Their suspended parts are flexible to enable them freely swing in a horizontal direction, thus achieving good aseismic performance. For this reason, suspended valve towers now play a dominant role in China’s HVDC and UHVDC projects. Converter valves are available in a double-valve structure and a quadruple-valve structure. 1. Characteristics of double-valve units. The 12-pulse converter valve group consists of two six-pulse valve groups connected in series and arranged independently. Each six-pulse valve group comprises three phases in parallel, each of which has two series-connected valve arms placed on one valve tower to form a double-valve unit. So one 12-pulse converter valve group has six double-valve units in total, i.e., six valve towers are suspended in each valve hall. The two six-pulse valve groups are electrically independent from each other such that in the case of failure in one valve arm, the other sixpulse valve group will maintain normal operation through switchover. In terms of structure, double-valve units are of low height, which is conducive to reducing the height of the valve halls. Also, the connections between converter transformers and valve towers are clear and simple. However, a disadvantage is that the valve hall occupies a large area and has a large volume. The 6 800-kV Xiangjiaba
Shanghai and Yunnan
Guangdong UHVDC projects are examples adopting double-valve units. 2. Characteristics of quadruple-valve units. The 12-pulse converter valve group is composed of four series-connected converter valve arms in each of the three phases, and the four valve arms of each phase are closely connected on one valve tower to form a quadruple-valve unit. So one 12-pulse converter valve group has three quadruple-valve units, i.e., three valve towers are suspended in each valve hall. Compared with double-valve units, quadruple-valve units are taller, but the valve hall has a smaller floor area and volume. However, the connections between converter transformers and valve towers are more complex.

16.1.2.5 Layout of Converter Transformers Given the manufacturing capability and transportation requirements, UHV converter transformers are all of the singlephase two-winding type. Each 12-pulse converter valve group needs six converter transformers. Converter transformers shall be properly arranged to allow convenient incoming and outgoing lines, simple connections, and clear layout. Typically, converter transformers are placed close to valve halls, with valve-side bushings protruding through the walls of the valve halls. The layout of converter transformers relative to thevalve hall is available in three main schemes: Scheme 1 is a double-valve tower, with converter transformers installed close to one side of the valve hall and arranged in a line, as shown in Fig. 16.1. Scheme 2 is a quadruple-valve tower, with converter transformers installed closely to one side of the valve hall and arranged in a line, as shown in Fig. 16.2. Scheme 3 is a quadruple-valve tower, with converter transformers installed close to the valve hall and arranged on two sides of the valve hall, as shown in Fig. 16.3.

UHV Converter Stations and the Main Electrical Equipment Chapter | 16

YY–A

YY–B

YY–C

YΔ–A

YΔ–B

YΔ–C

Converter transformer

Converter transformer

Converter transformer

Converter transformer

Converter transformer

Converter transformer

Valve tower

Valve tower

Valve tower

Valve tower

Valve tower

Valve tower

UH

VH

WH

UL

VL

WL

Valve hall

593

FIGURE 16.1 Layout scheme 1 of converter transformers relative to the valve hall.

YY–A

YY–B

YY–C

YΔ–A

YΔ–B

YΔ–C

Converter transformer

Converter transformer

Converter transformer

Converter transformer

Converter transformer

Converter transformer

Valve hall

Valve tower

Valve tower

Valve tower

U

V

W

FIGURE 16.2 Layout scheme 2 of converter transformers relative to the valve hall.

In comparison, connections in valve halls are relatively simple in schemes 1 and 2, and complex in scheme 3. In terms of bus wiring of converter transformers, schemes 1 and 2 are relatively simple, while scheme 3 is relatively complex as it requires a dedicated bus area. In the 6800-kV Xiangjiaba
Shanghai UHVDC Transmission Project, converter transformers are arranged in scheme 1.

16.1.2.6 Layout of Valve Halls In a UHVDC converter station with double 12-pulse converters per pole, there are a total of four valve halls which may be arranged appropriately based on project-specific conditions. Where double-valve units are used, there are two basic layout schemes.

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YY–A

YY–B

YY–C

Converter transformer

Converter transformer

Converter transformer

Valve tower

Valve tower

Valve tower

U

V

W

Converter transformer

YΔ–A

Converter transformer

YΔ–B

FIGURE 16.3 Layout scheme 3 of converter transformers relative to the valve hall.

Converter transformer

YΔ–C Earth electrode line

Pole 2 DC filter of pole 2

Neutral busbar equipment

DC filter of pole 1

Smoothing reactor

Smoothing reactor Smoothing reactor

HV-end valve hall of pole 2

HV-end converter transformers of pole 2

Pole 1

LV-end valve hall of pole 2

LV-end converter transformers of pole 2

Smoothing reactor LV-end valve hall of pole 1

HV-end valve hall of pole 1

LV-end converter transformers of pole 1

HV-end converter transformers of pole 1

Control building

AC busbar AC switchyard

FIGURE 16.4 Layout scheme 1 of valve halls for system configuration of double 12-pulse converters per pole.

1. Scheme 1: Similar to the arrangement in 6500-kV HVDC projects, two HV-end valve halls, two LV-end valve halls, 24 converter transformers, and a control building arranged inline, as shown in Fig. 16.4. 2. Scheme 2: Two LV-end valve halls are arranged back-to-back, and two HV-end valve halls are located on the two sides. The six converter transformers for each valve hall are arranged inline, as shown in Fig. 16.5. Comparative analysis suggests that the two layout schemes have their own merits and demerits. In particular, scheme 2 is more advantageous in reducing noise, simplifying connections, and saving land as well as clear function divisions. In the 6800-kV Xiangjiaba
Shanghai UHVDC Transmission Project, the valve halls are arranged in scheme 2.

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Earth electrode line Pole 1

Pole 2

LV-end LV-end valve hall valve hall of pole 2 of pole 1

LV-end converter transformers of pole 1

Smoothing reactor

Smoothing reactor LV-end converter transformers of pole 2

HV-end valve hall of pole 2

HV-end converter transformers of pole 2

Smoothing reactor

DC filter of pole 1

Neutral busbar equipment

Smoothing reactor

HV-end converter transformers of pole 1

DC filter of pole 2

HV-end valve hall of pole 1

AC busbar

AC switchyard

FIGURE 16.5 Layout scheme 2 of valve halls for system configuration of double 12-pulse converters per pole.

16.1.2.7 Layout of DC Switchyards DC switchyards are generally located on one side of valve halls, and shall be arranged on a pole basis whenever practical while facilitating patrol inspection, operation, handling, maintenance, and testing of equipment. DC switchyards may be of indoor or outdoor types depending on the pollution level of the converter stations and the manufacturing capability of the DC equipment. 1. Outdoor DC switchyard. The outdoor DC switchyard is basically of a symmetrical configuration by poles with the DC neutral busbar equipment at the center of the yard. DC filters are arranged between the DC line pole and the neutral busbar equipment. 2. Indoor DC switchyard. The indoor DC switchyard is basically arranged in a symmetrical manner in polar. To reduce the floor areas of indoor DC switchyard and cut the construction cost, often only equipment on the DC line pole is placed indoors, such as disconnectors, smoothing reactors, arrestors, HV capacitor towers of DC filters, etc.

16.1.2.8 Layout of AC Switchyards The layout of AC switchyards shall, in addition to conventional practices, take into account the arrangement of AC filters, reactive power compensation devices, and converter transformers. Usually, AC switchyards are located on the line side of converter transformers, and their layout depends mainly upon the terrain of converter station sites and the land requisition. Fig. 16.6 shows an aerial view of Fulong Converter Station of the 6800-kV Xiangjiaba
Shanghai UHVDC Transmission Project.

16.1.3

UHVDC Converter Stations With Three 12-Pulse Converters Per Pole

16.1.3.1 Electrical Structure Converter stations configured with three series-connected 12-pulse converters per pole refer to those with three 12-pulse converters connected in series per pole. Similarly, the three 12-pulse converters may be at the same voltage level. For example, a 61000-kV DC link may have three 6333-kV 12-pulse converters in series, represented as “333 kV 1 333 kV 1333 kV”. Alternatively, the three converters may be at different voltage levels. For example, a 61000-kV DC link may use series combinations of “200 kV 1 400 kV 1 400 kV” or “250 kV 1 350 kV 1 400 kV.”

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FIGURE 16.6 Aerial view of Fulong Converter Station of the 6800-kV Xiangjiaba
Shanghai UHVDC Transmission Project.

16.1.3.2 Configuration of Valve Halls Converter stations configured with three series-connected 12-pulse converters per pole follow the same design philosophy as those with double series-connected 12-pulse converters per pole. From the above analyses of the configuration of UHVDC converter valve halls with double 12-pulse converters per pole, for the sake of reducing manufacturing difficulty and construction cost of valve halls, it is suggested that the HV-, MV-, and LV-end 12-pulse converter valves be installed in three valve halls of large, medium, and small size, respectively. Hence, there are six valve halls in total at one converter station. Converter stations in such a configuration adopt the same arrangement of valve towers, converter transformers, and DC and AC switchyards as those with double 12-pulse converters per pole. The biggest difference is that the number of valve halls at each station increases from four to six, and correspondingly the number of converter transformers increases from 24 to 36, which makes the valve hall design more challenging. This section will only discuss the layout of converter transformers and valve halls for system configuration of three series-connected 12-pulse converters per pole. The following design schemes are available for consideration: 1. Scheme 1: Similar to the arrangement in 6500-kV HVDC, two LV-end valve halls are located at the center, two MVend valve halls on two sides of the LV-end valve halls, and two HV-end valve halls on two sides of the MV-end valve halls; 36 converter transformers are installed close to one side of the valve halls and arranged inline, as shown in Fig. 16.7. 2. Scheme 2: Two LV-end valve halls are arranged back-to-back, and two MV-end and two HV-end valve halls are successively located on the two sides. The six converter transformers for each valve hall are arranged inline (see Fig. 16.8). 3. Scheme 3: The six valve halls at one station are arranged in delta fashion. The six converter transformers for each valve hall are arranged close to the valve hall inline. These converter transformers may be placed inside or outside the delta shape. Arranging the 36 converter transformers inside the delta shape will facilitate noise reduction but create difficulty in arrangement of the incoming lines of MV- and HV-end converter transformers, while arranging them outside the delta shape is unfavorable to controlling noise emitted from the station but beneficial in arrangement of the incoming and outgoing lines of converters. Fig. 16.9 shows a schematic design with converter transformers placed outside the delta shape, DC switchyards at the center, and AC switchyards on the outside.

16.2 UHVDC CONVERTER VALVES The UHVDC converter valve is the core equipment of a UHVDC transmission system and mainly serves to convert alternating current to direct current and vice versa. It is, in essence, a high-voltage controlled semiconductor switch of large capacity based on six-inch thyristors. The valve conducts only in one direction, namely in the forward direction from

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Earth electrode line

Pole 2 DC filter of pole 2

Smoothing reactor

Smoothing reactor

Smoothing reactor

HV-end valve hall of MV-end valve hall of LV-end valve hall of pole 2 pole 2 pole 2

HV-end converter transformers of pole 2

MV-end converter transformers of pole 2

Control building

LV-end converter transformers of pole 2

Pole 1

DC filter of pole 1

Neutral busbar equipment

Smoothing reactor

LV-end valve hall of MV-end valve hall of HV-end valve hall of pole 1 pole 1 pole 1

LV-end converter transformers of pole 1

MV-end converter transformers of pole 1

HV-end converter transformers of pole 1

AC busbar AC switchyard

FIGURE 16.7 Layout scheme 1 of valve halls with three 12-pulse converters (inline).

Earth electrode line

Pole 2

MV-end valve hall of pole 2

Smoothing reactor

LV-end LV-end valve hall valve hall of pole 2 of pole 1

Pole 1

DC filter of pole 1

LV-end converter transformers of pole 1 MV-end converter transformers of pole 1

Smoothing reactor MV-end converter transformers of pole 2 LV-end converter transformers of pole 2

HV-end valve hall of pole 2

HV-end converter transformers of pole 2

Smoothing reactor

Neutral busbar equipment

Smoothing reactor

MV-end valve hall of pole 1

HV-end converter transformers of pole 1

DC filter of pole 2

HV-end valve hall of pole 1

AC busbar AC switchyard

FIGURE 16.8 Layout scheme 2 of valve halls with three 12-pulse converters (back-to-back).

positive pole to negative pole; in the reverse direction, the valve is in high-resistance off state. As the core equipment in the converter station and the component where current conversion happens, converter valves normally take one-fourth of the total investment of all equipment in a converter station. Converter valves must be capable of safely and reliably operating under predetermined external ambient and system conditions in accordance with relevant provisions and of satisfying the requirements in terms of loss, installation, maintenance, and investment. DC converter valves are typically of modular design, and their fundamental functional assemblies are thyristor modules. In various applications, a DC converter valve may consist of a different number of thyristor modules connected in series to provide sufficient voltage withstand capability as required under different voltage levels. The modular design of a DC converter valve determines that 6800-kV converter valves are not much different in internal insulation from conventional DC converter valves and it is only necessary to ensure that sufficient through-current capability is provided and that there is no significant increase in the number of thyristor levels. These demands can be fulfilled with the application of new large-power six-inch thyristors. The six-inch thyristor has a peak blocking voltage of 8500 kV, and a single thyristor will be able to accommodate 4500 A direct current.

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LV-end converter transformers of pole 2

LV-end converter transformers of pole 1 AC busbar

Smoothing reactor

AC busbar

Smoothing reactor

MV-end

MV-end

valve hall

valve hall Neutral busbar equipment

of pole 2 DC filter of pole 2

of pole 1 DC filter of pole 1

HV-end

HV-end

valve hall

valve hall

of pole 2

of pole 1

Smoothing reactor

Pole 2

Smoothing reactor

Earth electrode line

Pole 1

MV-end converter transformers of pole 1

MV-end converter transformers of pole 2 HV-end converter transformers of pole 2

LV-end valve hall of pole 1

HV-end converter transformers of pole 1

LV-end valve hall of pole 2

AC busbar

FIGURE 16.9 Layout scheme 3 of valve halls with three 12-pulse converters (delta).

FIGURE 16.10 Typical UHVDC double valve towers.

16.2.1

Valve Structure

Characterized by high voltage and large power, the UHV converter valves are configured in a 400-kV 1 400-kV scheme in which two 12-pulse converters are connected in series per pole. Each 12-pulse bridge has a rated DC voltage of 400 kV and a rated power of 1600 MW. Under normal operating conditions, the LV-end 12-pulse converter connects zero and 400 kV DC potentials, while the HV-end 12-pulse converter connects 400 kV and 800 kV DC potentials. A DC bypass circuit breaker is connected across the 12-pulse converter, through which the 12-pulse converter can be switched in or out of service. Each 12-pulse bridge is installed in a separate valve hall. Valve halls at 0
400 kV are referred to as LV valve halls and those at 400
800 kV as HV valve halls. Each 12-pulse bridge comprises two series-connected six-pulse bridges and six double-valve units. Fig. 16.10 shows typical double-valve towers in UHVDC transmission systems, and Fig. 16.11 gives an example of the electrical layout of converter valves of a complete monopole in UHVDC converter stations.

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FIGURE 16.11 Structural arrangement of four double-valve units in one phase of a complete monopole.

Double valve unit 3

Double valve unit 4 To 800 kV line pole

Double valve unit 1 0 kV(DC)

Double valve unit 2

400 kV(DC)

600 kV(DC)

200 kV(DC)

Gate unit

Gate unit

Gate unit

Gate unit

Gate unit

Gate unit

Gate unit

Gate unit

Gate unit di/dt reactor

Grading capacitor of thyristor module

FIGURE 16.12 Schematic diagram of a typical thyristor module used in UHVDC links.

Each double-valve tower consists of two single-valve units, and is suspended from the top of a valve hall through suspension insulators so as to provide the air clearance and creepage distance required by the high potential of the valve tower. The overvoltage protection for converter valves is provided by ZnO arresters connected in parallel to single-valve units. Shields are installed at the top and bottom of valve towers to effectively reduce the risk of electrostatic discharge. Edges and corners of the shields are arc-shaped to ensure no sparkover will occur between them and the ground under high voltage and to protect the converter valves from external electromagnetic interference so as to allow uniform distribution of internal fields. As the voltage is as high as 800 kV, special attention shall be paid to the insulation design of converter valves. Located in clean valve halls, UHV converter valves are designed with the same creepage distance as conventional 500-kV converter valves, i.e., specific creepage distance larger than 14.0 mm/kV. The air clearance of converter valves is dependent on the operating impulse voltage level which is in a nonlinear relationship with flashover distance. In view of this, the air clearance required by 800-kV systems is much larger than that designed based on the linear voltage distribution in 500-kV systems.

16.2.2

Valve Design

16.2.2.1 Electrical Design To improve the reliability, availability, and flexibility in installation and maintenance, converter valves are typically of modular design. The converter valves can, based on topology complexity and inclusion relationship, be divided into thyristor level, thyristor module, valve module, single-valve unit, and double-valve unit (six-pulse bridge). The thyristor level is comprised of thyristor elements and the associated firing, protection, and monitoring electronic circuits and snubber circuit. A thyristor level is the most fundamental functional unit of a converter valve. A thyristor module is the most fundamental functional assembly of a converter valve, and is a structural assembly composed of multiple thyristor levels and the auxiliary equipment and anode reactor (also known as a di/dt reactor, or saturable reactor) in their immediate vicinity. Fig. 16.12 shows the schematic diagram of a typical thyristor module used in UHVDC links. A valve module is an independent converter valve unit, and can be electrically used as a complete single-valve unit. It is generally composed of two thyristor modules connected in series. If six-inch thyristors with a nonrepetitive peak offstate voltage of 7.2 kV are used and the redundancy of thyristor levels is provided, the number of thyristor levels in a

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single-valve unit of an 6800-kV UHVDC system is 70 (including three in redundancy).Therefore, if one valve module has 18 thyristor levels, a single-valve unit may be comprised of four series-connected valve modules, and a double-valve tower will have four such modules in series. Figs. 16.13 and 16.14, respectively, give the three-dimensional diagram of a typical valve module and an HV valve tower used in UHVDC applications. Table 16.1 lists the typical parameters of an 6800-kV single-valve unit.

FIGURE 16.13 Three-dimensional diagram of a typical valve module used in UHVDC link.

FIGURE 16.14 Three-dimensional diagram of typical HV valve towers used in UHVDC link.

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TABLE 16.1 Typical Parameters of an 6800-kV Single-Valve Unit Component and Design Parameter

Value

Component and Design Parameter

Value

Parameters of thyristor

7.2 kV/5000 A

Number of reactors in each valve module

4

Total number of thyristor

70 μF

Damping grading capacitor (μF)

1.6

Number of redundant thyristor

3

Damping resistor (Ω)

40

Number of thyristor levels in a valve module

18

Structure of valve tower

Double-valve unit

Number of saturable reactors

16

16.2.2.2 Mechanical Design The mechanical design of a converter valve is the carrier of its electrical design and the physical foundation for its electrical functions. Also, the mechanical design shall take into account aseismic design and fire protection design, and effectively integrate various components of converter valves, so as to achieve the basic electrical functions of converter valves. The structural design of converter valves is not the simple addition of aesthetic and electrical requirements, but the result of comprehensive consideration of the interaction between various complicated factors including creepage distance, electrical distance, internal interference, stray inductance and capacitance, water cooling requirement, gravity distribution, convenience in installation, maintenance, and test operation, etc. In addition, proper selection of equipment materials is also of great importance to building a system with higher reliability, extended service life, and minimal fire risk. Generally, steel frame structures are made of high-strength aluminum alloy, and insulation materials are eco-friendly, fireproof halogen-free materials meeting UL94V-0 flame-retardant level specified by Underwriters Laboratory.

16.2.2.3 Aseismic Design The UHVDC converter valve tower is of flexible suspension structure. It has large physical size and experiences frequent turn-on and -off, thus necessitating a strong structure capable of withstanding an M7 earthquake. The DC converter valve shall have sufficient seismic resistance to guarantee its structural integrity when subjected to external seismic load in addition to its self-weight, and to ensure that in the event of an earthquake, little or no obvious structural damage is caused and most equipment can continue normal operation. To meet the aseismic design requirements, the valve tower and arrester suspension system has to be designed as a flexible system. Seismic analysis shall be made on DC converter valves to understand the vibration characteristics of the valve structure, i.e., modality and natural frequency. After that, seismic analysis shall be made on DC converter valves based on the construction and installation needs and in accordance with IEEE 693-2005 IEEE Recommended Practice for Seismic Design of Substations or GB 50260-1996 Code for Seismic Design of Electrical Installations. For UHVDC converter valves with a seismic fortification intensity of magnitude 8, the design basic horizontal acceleration shall be 0.2 g, the vertical acceleration shall be 65% of horizontal acceleration, and the structural damping shall be 2%. On this basis, the structural strength calculations shall be made for both static and dynamic states to verify the conformity of the mechanical design with aseismic design requirements.

16.2.2.4 Key Components 1. Thyristor A thyristor, as a key part of a converter valve, is a semiconductor device of PNPN structure. Currently, largepower phase controlled thyristors (PCT) are used in HVDC applications, including electrically triggered thyristors (ETTs) and light-triggered thyristors (LTTs). Except for direct light triggering and integrated overvoltage protection functions, LTTs have the same microcosmic on/off mechanism as ETTs. The silicon wafer of thyristors used in UHVDC applications has a diameter up to six inches (150 mm), a nonrepetitive reverse blocking voltage up to 8.5 kV, and an average on-state current up to 5000 A, which can meet the demands of UHVDC transmission with higher voltage, larger capacity, and longer distance. 2. Saturable reactor The valve reactor is an important part of the DC converter valve. As it is usually connected in series to the anode of the thyristor, it is also referred to as an anode reactor. It makes up a thyristor module by connecting multiple thyristors in series, and two thyristor modules constitute a valve module. Depending on different voltage levels, multiple

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valve modules form a single valve unit or a complete converter valve. The valve reactor is a nonlinear saturable reactor and a special component unique to the HVDC converter valve. It mainly serves the following functions: a. Suppresses on-state di/dt peak current; b. Limits off-state du/dt; c. Facilitates voltage grading in sudden transient conditions. With the main function of suppressing on-state and off-state di/dt, it has the following characteristics: a. High inductance at low currents; b. Appropriate inductance level and gradual saturation in fully saturated state; c. Satisfactory controlled damping characteristics; d. Low conduction and magnetic hysteresis loss; e. Small end-to-end capacitance; f. Small current step at conduction. 3. Damping circuit A thyristor damping circuit is comprised of a damping capacitor, damping resistor, and DC voltage grading resistor. Each thyristor level is connected with a RC circuit in parallel for voltage grading under normal operating conditions. At the instant of thyristor turn-on and turn-off, the RC damping circuit can control transient voltage and current stresses while powering the gate circuit of each thyristor level. The DC voltage grading resistor is able to balance voltage when the converter valve is under pure DC voltage, and also measure the voltage of the thyristor in the gate circuit. In some applications, a thyristor module capacitor is also connected to improve voltage distribution in the case of steep surge voltages. 4. Thyristor firing and monitoring unit The operation of converter valves also needs peripheral equipment, namely firing and on-line monitoring devices. Fig. 16.15 shows the relationship between the converter valve and the firing and on-line monitoring devices. The firing system consists of a set of equipment over the whole process from valve base electronics (VBEs) receiving the firing signal from the valve control to thyristor electronics delivering the pulse encoded by VBEs to the thyristor gate after logic analysis. The entire system include VBEs, thyristor electronics (TE board), valve control (VC), and station control (SC) equipment as well as some connecting optical cables. The valve control is the main control system of the converter station, and it controls the firing pulse phase of the converter. It mainly comprises the phase control circuit, firing pulse generator, and some regulators. It can adjust the phase of firing pulse as required by the system and deliver 120 broad pulse to VBEs. In addition, it receives the current zero-crossing signals from VBEs for extinction angle calculation. The valve control is also connected to the thyristor on-line monitoring device via optical cables, through which control signals are sent to the thyristor on-line monitoring device to enable the latter to make correct judgment based

Thyristor as part of valve

Thyristor control unit (TE) Optical receiver

Optical transmitter

Optical signal transmission Firing signal returned after check

Firing signal Optical transmitter Remote monitoring equipment

Optical receiver

Valve base electronics (VBEs)

Valve control (VC)

FIGURE 16.15 Schematic diagram of converter valve and its firing and on-line monitoring device.

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on operating conditions. The station control is an operation control system for the entire station, and is responsible for the switching operation of circuit breakers at the entire station. It receives fault signals detected by the thyristor on-line monitoring device and initiates alarm or tripping according to the severity of the fault. a. Thyristor electronics, including thyristor control and monitoring equipment. The thyristor control unit serves to fire and monitor thyristors, and is equipped with an emergency (protective) forward firing device. The emergency forward firing device trips thyristors in emergency relying on voltage tapped from the RC circuit connected in parallel with the thyristors, so as to protect the thyristors from excessive forward overvoltage. The setting of emergency firing voltage shall be determined first as a standby firing circuit and then as a backup for HV lightning arresters. In addition, the thyristor electronics also provide protection and voltage measurement for thyristors during recovery, and deliver thyristor operation monitoring signals to VBEs via optic fibers. b. VBEs. VBEs are units which collect various signals in converter valves and thyritor operation data, and exchange information with such equipment as the thyristor control unit, thyristor monitoring equipment, valve unit control and protection system, value arrester monitoring, and leak detection of valve cooling system. VBEs convert the pulse issued by the valve control into the pulse sequence required by the gate control, and make statistics of operation condition signals, du/dt overvoltage protection, and negative voltage detection data from each thyristor and then send the same to the thyristor monitoring equipment. Generally, one set of VBEs is provided for one quadruple (or double) valve unit. c. Thyristor monitoring equipment, connected to the valve control via optic fibers and connected to the station control via cables. Its main functions include: i. Detect and locate faulty thyristors and thyristor control units; ii. Send alarm or tripping signals to the station control when the number of faulty thyristors and operated times of break over diode (BOD) protection exceeds the set point; iii. Detect the margin of optical transmitter and receiver in the range from the valve control to the thyristor control. d. Converter valve operation control is one of the core pieces of equipment in a DC transmission system, and is classified into system control, bipole control, valve unit control, and firing control. e. Converter valve protection may be divided into two parts: internal protection and external protection. The internal protection mainly includes arrester protection and BOD protection. The BOD protection protects thyristors from forward overvoltage in the absence of normal control pulse. The directional overvoltage protection is provided by valve arresters.The cooling system control and protection must also be included. The external protection is mainly composed of overcurrent protection, valve bridge differential protection, pole differential protection, and overvoltage protection within the valve unit and pole protection area. 5. Valve cooling system The cooling system is an integral part of converter valves, as it dissipates the heat produced by various elements of converter valves to outside the valve hall so as to ensure that the thyristor operates within a normal junction temperature range. The UHVDC converter valve is generally air-insulated and water-cooled. As a direct contributor to the safe and reliable operation of converter valves, the cooling system is required to have sufficient cooling capacity and high reliability. Generally, the cooling system is divided into internal and external water cooling systems.

16.2.3

Valve Test

To verify whether a UHVDC converter valve meets the design requirements and complies with the relevant provisions in IEC 60700
1 Thyristor Valves for Use in HVDC Transmission System, Part 1, Electrical Test, the valve shall be tested and accompanied with complete test reports. The valve test may be classified into type test, production test, and special test by test purposes and items. 1. Type tests refer to those tests which are carried out to verify that the valve design will meet the requirements specified. In IEC 60700-1, type tests are classified under two major categories: dielectric and operational tests. Type tests may be conducted on a complete valve or valve sections. In the case of the latter, the number of tested valve sections will be not less than the total number of valve sections in a single-valve unit. All type tests must be performed on the same valve or valve sections. The operational tests must be performed after dielectric tests. Correction factors for project site, relative air density, temperature rise in the valve hall, and atmospheric pressure variations must be included in the determination of the test voltages for air insulation. In all dielectric tests other than periodic firing tests, the redundant thyristor levels shall be shorted. While in operational tests, the redundant thyristor levels need not be shortened, but the test voltage shall be increased according to the specified proportional coefficient.

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2. Production tests refer to those tests which are carried out to verify proper manufacture, so that the properties of a valve correspond to those specified. Production tests are classified under two major categories: routine tests and sample tests. 3. Special tests refer to those additional tests to be carried out as agreed between the user and the manufacturer.

16.3 CONVERTER TRANSFORMERS 16.3.1

Functions and Features

The winding of a converter transformer connected to AC system is called the line side winding, and the other winding connected to converter valves is called the valve side winding. The converter transformer serves the function of transmitting energy from the sending-end AC system to the rectifier or transmitting energy from the inverter to the receiving-end AC system. It enables power transmission through electromagnetic coupling of windings on both sides, while providing electrical insulation and galvanic isolation between AC and DC systems. The line-side busbar and the DC side of the converter bridge can reach their respective rated voltage and meet the allowable voltage deviation. In operation, the valve side winding of the converter transformer is subjected to the combined load stress of DC and AC voltage, added to which is the harmonic current in windings on both sides. Therefore, its design, manufacture, and operation are different from conventional power transformers. 1. Short-circuit impedance The converter transformer should have sufficient short-circuit impedance to limit the current following the valve arm and DC busbar short-circuit faults to a level that will not damage thyristors of the converter valve. However, excessive short-circuit impedance will increase the reactive power loss and hence the required compensation equipment, and will result in excessive commutation voltage drops. The short-circuit impedance of a large-capacity converter transformer is usually 12%
18%. Three-phase asymmetry is one of the main causes for noncharacteristic harmonics. To reduce noncharacteristic harmonics, the allowable difference between the measured value and nominal value of three-phase short-circuit impedance should be as small as possible. The allowable deviation in short-circuit impedance is 67.5%
10% for conventional power transformers, which is higher than that for converter transformers. The allowable deviation for a converter transformer at principal tap is 65%. For a converter transformer with a tapping range of no more than 30%, the impedance within the normally used tapping range shall not go beyond the value corresponding to principal tap 65% and that within other tapping ranges shall not go beyond the value corresponding to principal tap 610%. More stringent requirements are placed on UHV converter transformers. The allowable deviation in impedance at the principal tap shall be 63.75%. For those UHV converter transformers designed for the same purpose or interchangeable converter transformers, the impedance at principal tap and the impedance changes over the entire tapping range of each transformer neither shall exceed the average measured value 62%. 2. Insulation design The most noticeable difference in insulation design between a converter transformer and conventional power transformer lies in that the latter only considers the AC voltage withstand strength. However, the valve side winding of a converter transformer needs to withstand DC biasing that is superimposed on the AC voltage. It operates under the combination of AC and DC voltages and is at extremely high potential to ground. In a 12-pulse converter composed of two series-connected six-pulse converters, the DC component in valve-side winding-to-ground voltage is 0.5 Ud (Ud is the DC voltage of a six-pulse converter) for the first six-pulse converter counted from the grounding side and is 1.5 Ud for the second six-pulse converter. Obviously, the valve side windings of converter transformers withstand stress produced by both AC and DC voltages. The dashed lines in Fig. 16.16B show the potential to ground of valve side windings at all levels. Additionally, DC full voltage starting and polarity reversal make the insulation structure of a converter transformer far more complex than conventional AC transformers. As the composite insulation characteristics of oil and paper are different under AC and DC voltages, it is not feasible to copy the insulation requirements for conventional transformers. Instead, due attention must be paid to the influence of the DC electric field to oil and paper in the transformer and the field strength distribution in oil, paper, and porcelain of the bushing bottom end. Also, it should consider the excessive voltage stress on the oil gap due to changes of electric charges distribution in composite insulation following rapid change of DC voltage polarity. 3. Influence of high-order harmonic current Operating converters will produce harmonic voltage and current on both AC and DC sides. The harmonic component of leakage flux may give rise to local overheating of some metallic components and the oil tank as well as increased stray loss. Also, magnetostriction due to harmonic current may cause vibration and noise sensitive to the human auditory system. Hence, the design, manufacture, and onsite installation of a converter transformer should take

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FIGURE 16.16 Potential to ground of valve side winding of a converter transformer for a 12-pulse converter in a bipolar link: (A) Connection diagram; (B) potential to ground.

1 u2

605

2

3 u2

4 1 2

5 3

u2 6

4

5 ωt

7

6 7

u2 8

9

AC system (A)

8 9 (B)

these factors into account. Thermal design of windings, iron core, and steel construction and testing of transformers must provide a margin for impact of harmonic currents and employ suitable cooling equipment. Fasteners used at positions with strong harmonics bias should be made of nonmagnetic materials, and magnetic shielding measures shall be provided between windings and housing. If necessary, sound-absorbing wall may be built on site or the converter transformers may be installed in sound-proof rooms. 4. DC bias DC bias refers to the asymmetry of the iron core magnetization curve as a result of the DC flux produced by direct current flowing through a converter transformer. For a converter transformer, DC bias is mainly attributed to: (1) different phases fired during communication; (2) power frequency current flows through DC lines; (3) positive-sequence second-harmonic voltage occurring on an AC busbar; and (4) ground potential at a converter station rising owing to the injection of current into earth electrode under a monopolar earth return operation mode. DC flux causes serious saturation of iron core, serious distortion of exciting current, and production of a large amount of harmonics, which increase the reactive power loss, reduce the voltage of the transmission system, and even lead to misoperation of system protection. The severe flux saturation in iron core of a converter transformer will lead to serious leakage flux and hence local overheating of internal metallic parts, damage to insulation, and even shortened lifespan of the transformer. In abnormal operation conditions, long-term conduction of particular valves may also cause DC bias. 5. On-load tapping range In order to maintain the optimal operation of the DC transmission system, converter transformers are usually equipped with many load regulation taps to allow a wide tapping range of 15%
30%. The regulation of each tap position is typically small, within 1%
2%, to avoid any dead zone and frequent back-and-forth motions during collaboration between the tap changer and converter firing angle control. 6. Tests Apart from type tests and routine tests as conducted for conventional AC transformers, the converter transformer needs to receive additional DC tests, such as a DC voltage test, DC voltage partial discharge test, and DC voltage polarity reversal test.

16.3.2

Type and Parameter Selection

16.3.2.1 Type Selection The converter transformer is available in four designs, namely three-phase three-winding, three-phase two-winding, single-phase three-winding, and single-phase two-winding. The specific choice should be made depending on the system voltage on the AC and DC sides, the capacity of the transformer, transport conditions, and layout of the converter station.

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16.3.2.2 Selection of Parameters 1. AC rated voltage UVN on the valve side Udi0N π Udi0N UVN 5 pffiffiffiffiffiffiffiffiffiffiffiffi 3 5 3 1:35α 2cosα

(16.1)

where, Udi0N is the DC voltage of one six-pulse converter; α is the rated firing angle at the rectifier side, and the rated extinction angle at the inverter side. At the rectifier side: Udi0NR 5

ðUdNR =nÞ 1 UT cosαN 2 ðdXNR 1 drNR Þ

(16.2)

Udi0NI 5

ðUdNR =RdN IdN Þ=n 2 UT cosγ N 2 ðdXNI 1 drNI Þ

(16.3)

At the inverter side:

where, n, number of six-pulse converters per pole (in a UHVDC link configured with two 12-pulse converters per pole, n 5 4); UdNR, rated DC voltage on the rectifier side; UT, forward conduction voltage drop of the converter valve; αN, rated firing angle on the rectifier side; γ N, rated extinction angle on the inverter side; dXNR, dXNI, inductive p.u. voltage drop on the rectifier side and inverter side, respectively, for the principal tap of the converter transformer; drNR, drNI, resistive p.u. voltage drop on the rectifier side and inverter side, respectively, for the principal tap of converter transformer; RdN, resistance of the DC transmission line; IdN, rated DC current. 2. AC rated voltage IVN on the valve side For six-pulse converters, the RMS rated AC voltage on the valve side can be expressed by: pffiffiffi 2 IVN 5 pffiffiffi IdN 5 0:816IdN (16.4) 3 where, IVN refers to the RMS rated AC voltage on the valve side; and IdN refers to the rated direct current. 3. Rated capacity SN For six-pulse converters, the rated capacity of a three-phase converter transformer can be expressed by: pffiffiffi π SN 5 3UVN IVN 5 Udi0N IdN 3

(16.5)

For 12-pulse converters, the rated capacity of a single-phase three-winding converter transformer can be expressed by: SN 5

pffiffiffi 2 2π UdioN IdN 3UVN IVN 3 5 3 9

(16.6)

For 12-pulse converters, the rated capacity of a single-phase two-winding converter transformer can be expressed by: SN 5

pffiffiffi 1 π 3UVN IVN 3 5 UdioN IdN 3 9

(16.7)

4. Short-circuit impedance Optimal selection of short-circuit impedance is a vital step in the design of an HVDC link. A too large ( .22%) or too small (,12%) short-circuit impedance will lead to increased manufacturing cost of the converter transformer. The selection of short-circuit impedance shall consider the following factors: (1) short-circuit impedance determines leakage inductance of the converter transformer and short-circuit surge current allowed by the thyristor; (2) the larger the short-circuit impedance is, the larger the voltage drop becomes. Hence, for given rated transmission power of an HVDC link, the converter transformers and converter valves are required to have as large nominal capacity as possible; (3) short-circuit impedance dictates the overlap angle and hence affects the value of the advance angle or extinction angle of the inverter station; (4) short-circuit impedance affects the demands for reactive power and thus the capacity of compensation equipment; (5) short-circuit impedance affects the amplitude of the harmonic current. Generally, increasing short-circuit impedance will reduce the harmonic current. The above is simply some main considerations for the selection of short-circuit impedance. How the short-circuit impedance affects the total cost of the converter station shall be determined according to the specific conditions. For

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HVDC links transmitting bulk power over a long distance, the large transmission capacity requires thyristors with high current capacity and good capability to withstand large short-circuit current. Therefore, the reducing the short-circuit impedance may not lead to an excessive rise in the short-circuit current beyond the capacity of thyristors. In a back-toback converter station, to cut the cost of the converter station, the DC voltage is relatively low and the DC current is usually large and approximate to the allowable short circuit of converter valve. Under this circumstance, reducing the short-circuit impedance will lead to a higher DC voltage and possible rise in the cost of the converter station.

16.3.3

R&D of UHV Converter Transformers

1. Calculation of AC and DC electric fields As compared with conventional power transformers, the valve side windings of a converter transformer need to withstand not only the combined stress of full-wave and chopped-wave lightning impulse voltage, operating impulse voltage, externally applied power frequency voltage, and induced test voltage, but also the long-term DC voltage and DC polarity reversal voltage. Therefore, insulation of a converter transformer is much more complex. The insulation of a converter transformer comprises composite insulation materials consisting of oil, paper, and paperboard. When the converter transformer is subjected to sinusoidal AC voltage, its electric field distribution in different insulation materials is capacitive, i.e., depending on the permittivity (ε) of different materials. Electric field strength is high in transformer oil with low permittivity; while AC electric field strength is low in paperboard with high permittivity. When the converter transformer is subjected to steady-state DC voltage, its DC electric field distribution depends upon the resistivity of different materials in composite insulation. Steady-state DC field is concentrated in insulation paper and paperboard with high resistivity. But the transformer oil with a minimum resistivity is substantially short-circuited, i.e., quite low voltage in the oil. When the converter transformer is subjected to electric field polarity reversal, the electric field distribution varies with the time of voltage application. During polarity reversal, a strong electric field is found in the low-resistivity transformer oil. This reveals that the distribution of electric field strength takes on a capacitive feature, i.e., permittivity of insulation materials determines the electric field distribution during polarity reversal. Transformer oil withstands a weaker electric field than paper or paperboard, but is subjected to a higher electric field strength during polarity reversal, giving it a weak position as an insulation for a converter transformer. As time elapses, the shape of equipotential lines twists, winds, and even loops at some positions. This indicates the presence of isolated space charges at joints of insulating media during polarity reversal and their distribution is quite complex. A large amount of space charges cause the local potential to rise or drop, forming a “potential peak” or “potential valley.” Moreover, these space charges produce an additional electric field that will be superimposed on the original electric field, causing distortion of the electric field manifesting as twisting and winding of the equipotential line. It can be seen that the space charges have a substantial effect on the transient electric field distribution following polarity reversal. 2. Main insulation and longitudinal insulation of windings The main insulation determines the overall construction of a converter transformer, and is closely related to impedance, loss, quality, outline dimensions, and transportation conditions of the transformer. Longitudinal insulation determines the structure and impulse voltage withstand capability of the valve side winding. a. Structure of main insulation. The insulation of the active part of a converter transformer adopts the configuration of two limbs in parallel and the windings are arranged as follows: iron core—regulating winding—line-side winding—valve-side winding—oil tank. In line with the higher insulation level of windings, the main insulation distance between windings and terminal insulation should be increased accordingly. Measures like increasing the number of angle rings, paper cylinders, and paper rings, and properly devised arrangement may be taken to ensure desirable distribution of the electric field of insulation under AC, DC, and polarity reversal voltages and to effectively improve the insulation strength of oil
paper insulation. b. Structure of longitudinal insulation. The head end of line-side winding needs to withstand a full-wave lightning impulse voltage of 1550 kV. Line-side windings may be of the interleaved type and sections at the winding head are interleaved sections. Electrostatic plates should be installed at upper and lower terminals of windings to improve the electric field distribution at winding terminals and insulation strength. Both the head and tail ends of valve side windings withstand a full-wave lightning impulse voltage of 1800 kV, and therefore the valve-side windings employ uniform insulation. Featuring a with large current, low AC voltage, and small number of turns, the valve-side windings are usually of the continuous internal shield type or helical type. The continuous internal shield type is usually coiled by semihard, self-adhesive transposed conductors (conductors on several coils at the head and tail ends are provided with shielding wire). This kind of winding uses longitudinal zoned capacitive compensation with desirable distribution of impulse voltage and well-controlled field strength distribution to ensure that no partial discharge occurs in windings. As the valve-side winding is subjected to high AC and DC

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voltages, electrostatic plates with proper radius of curvature shall be installed at its upper and lower terminals to effectively improve the electric field distribution there. Regulating windings are bobbin windings coiled by semihard, selfadhesive transposed conductors and there is no oil path between turns and segments. The taps are led out after welding of cables and tap leads, and ZnO nonlinear resistors are provided between taps to limit lightning impulse overvoltage on regulating windings. 3. Arrangement of valve side leads The transport dimensions of a converter transformer shall be determined by its technical parameters and structure. In particular, the valve side lead has a significant impact on the transport dimensions. The valve side lead may be either placed inside or outside the oil tank, and comprises large-diameter grading tubes, covered insulation, and multilayer insulating cylinders. Properly designed number of insulating cylinders and installation positions of leads can minimize the insulation distance between lead grading tubes and grounding positions like the oil tank and iron core. Placing the valve-side lead outside the oil tank can make full use of the external space and downsize the converter transformer proper. However, the complex interface between valve-side lead and winding poses strict requirements on manufacturing errors and great difficulties for assembly in the workshop and on site, thus requiring sufficient process safeguards/measures and special tooling. Placing a valve side lead in the oil tank can simplify onsite installation and reduce risks. However, it is quite challenging to arrange the valve side lead in the limited-space oil tank, which requires tremendous calculations and analyses to optimize design, allow reasonable arrangement of inner lead structure, and effectively control electric field strength. Besides, strict control is required over the manufacturing errors in the active part, leads, and oil tank during manufacture. 4. Valve side bushing and outlet device Bushing manufacturers in China are currently making vigorous efforts in the development of valve side bushings. However, 6500-kV and above bushings still rely on foreign manufacturers.

16.4 UHVDC SMOOTHING REACTORS 16.4.1

Functions

The smoothing reactor, also known as a DC reactor, is generally connected in series between converters and DC lines. It serves the following primary functions: 1. In the case of a disturbance or incident in the DC system, it suppresses the rate of rise of DC current to avoid fault propagation; 2. Where the inverter fails, it can avoid commutation failure; 3. Reduce the probability of commutation failure of the inverter in the event of an AC voltage drop; 4. In the case of a short circuit of the DC line, it coordinates with the regulator to limit the short-circuit current peak; 5. Works together with the DC filter to suppress and reduce the harmonic voltage and current produced in the course of conversion, thereby significantly reducing the interference to the communication facilities along the DC line; 6. Under low DC load, prevents overvoltage on inductive elements such as the converter transformer due to interruption of the direct current; 7. Limits the discharge current flowing to the converter valve from lines and capacitive equipment installed at the line ends.

16.4.2

Structural Characteristics

The smoothing reactor falls into two categories based on insulation and magnetic structure: a dry-type smoothing reactor with air core and an oil-immersed type smoothing reactor with iron core, both having proven track records in HVDC projects. They have the following advantages and disadvantages: 1. Insulation. a. For a dry-type smoothing reactor with air core, the insulation against ground is simple, the 6800-kV DC voltage is solely carried by support insulators, interturn insulation strength is low, and no critical electric field strength occurs at power flow reversal. Meanwhile for an oil-immersed type smoothing reactor with iron core, the insulation is comprised of oil
paper insulation, and the distribution of the DC electric field is difficult to calculate precisely, thus making the DC electric field difficult to control and entailing a large amount of tests and studies as well as extensive experiences.

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b. The dry-type air core smoothing reactor has smaller ground capacitance as compared to the oil-immersed iron core type and requires a lower impulse insulation level. The ground insulation voltage is withstood by insulators, and the HV-end and LV-end coils are interchangeable, for which only one spare coil is reserved at each station. For the oil-immersed type smoothing reactor however, the withstand voltage of the main insulation at the HV end and LV end varies and hence also the design, therefore needing separate spares. 2. Incremental inductance. a. Without iron core and composed of metallic structures made of aluminum alloy or nonmagnetic stainless steel with a permeability of 1, the dry-type smoothing reactor with air core is a linear element with a constant incremental inductance. Because of the absence of iron core, each smoothing reactor has low incremental inductance, generally not higher than 100 mH. b. Where larger incremental inductance is required, two or more air core smoothing reactors should be connected in series. For an oil-immersed type smoothing reactor, it is easy to increase the incremental inductance of an individual reactor as it has an iron core. The incremental inductance in previous 6500-kV DC projects may be as large as 300 mH at a current of 3000 A. The oil-immersed type smoothing reactor is a nonlinear element, and so the incremental inductance increases when a small DC current flows through the reactor. c. Noise level. The two smoothing reactors have comparable noise levels. As the dry-type smoothing reactor with air core is installed at elevated places, the noises produced have a greater impact on the surrounding environment, necessitating additional measures. d. Electromagnetic interference. The magnetic field lines of a dry-type smoothing reactor with air core are distributed in the space, having a greater impact on the surrounding environment. In contrast, the oil-immersed type smoothing reactor with iron core causes less electromagnetic interference to the surrounding environment because it is enclosed in an oil tank. e. Equipment protection. The dry-type smoothing reactor with air core is not susceptible to main insulation faults even following interturn faults, and hence does not need online monitoring to detect internal faults, simplifying the secondary control and protection equipment. However, the oil-immersed type smoothing reactor with iron core needs sound protection to facilitate online monitoring and prevention of faults. f. Transportation. As compared with the oil-immersed type smoothing reactor, the dry-type smoothing reactor contains no iron core or oil, and is therefore lightweight for ease of transport. The oil-immersed type smoothing reactor is large and heavy, imposing great constraints on its transportation and making the cost rise. g. O&M. The dry-type smoothing reactor with air core is oil-free, needs no auxiliary operation system, and is virtually maintenance-free, which contributes to low O&M cost. However, in the case of faults, it has to be replaced instead of being repaired. In contrast, an oil-immersed type smoothing reactor needs such auxiliary equipment as an oil filter, leading to large maintenance workload and high operational costs. h. Aseismic performance. A single 800-kV dry-type smoothing reactor with air core may have a weight of 30
40 t and an installation height of above 10 m, which makes it top-heavy with poor aseismic performance. In contrast, the oil-immersed type smoothing reactor is installed on the ground with low center of gravity, contributing to good aseismic performance. i. Arrangement. The arrangement of a dry-type smoothing reactor is quite flexible. While for the oil-immersed type smoothing reactor, associated oil sump, firewall, and other civil works are needed, resulting in less arrangement flexibility.

16.4.3

Tests

The following text discusses the items and procedures of routine tests, type tests, and field tests. Given that a dry-type smoothing reactor with air core and an oil-immersed type smoothing reactor with iron core differ greatly in terms of test items and procedures, they are introduced separately herein.

16.4.3.1 Routine Test of Dry-Type UHV Smoothing Reactor With Air Core 1. Winding DC resistance measurement. The DC resistance can be measured using a bridge method or volt-ampere method. The measurement shall be conducted on the entire winding with DC power source after the DC current becomes stable. 2. Impedance measurement. Use the volt
ampere method to measure the loss and impedance under power frequency sinusoidal current, and then convert them into rated transient fault current. 3. Inductance measurement. The inductance shall be measured at a frequency of 50
2500 Hz with either the bridge method or volt-ampere method.

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4. Full-wave lightning impulse test (terminal-to-terminal). During the test, the smoothing reactor proper shall be assembled in the same way as on-site installation (post insulators are not required to be fully assembled) and full-wave lightning impulse shall be applied on each terminal of the smoothing reactor in turns while the other terminal is directly grounded. The test voltage is the voltage across terminals as specified in particular technical specifications. 5. Operating impulse test (terminal-to-terminal). During the test, the smoothing reactor proper shall be assembled in the same way as on-site installation (post insulators are not required to be fully assembled) and operating impulse shall be applied on each terminal of the smoothing reactor in turns while the other terminal is directly grounded. The test voltage is the voltage across terminals as specified in particular technical specifications. 6. Loss measurement. The measured loss shall be corrected against the reference temperature of 80 C. 7. Load current test. To verify the current-carrying capacity of smoothing reactors, the rated load current shall be applied for at least 12 h. Also, possible overheating and abnormal temperature shall be detected through infrared imaging and the infrared imaging mapping shall be supplied to users.

16.4.3.2 Type Test of Dry-Type UHV Smoothing Reactor With Air Core 1. Full-wave lightning impulse test (terminal-to-ground). During the test, the smoothing reactor proper shall be assembled in the same way as on-site installation and the two terminals shall be short-circuited. The test voltage is the voltage between the terminal and the ground as specified in particular technical specifications. 2. Chopped-wave lightning impulse test. The test procedures for chopped-wave and full-wave lightning impulse tests are basically the same. In the former test, the test equipment shall be placed as close to the test piece as possible to reduce the impedance of terminals and test lines and maintain the waveform of chopped wave. a. Terminal-to-terminal. The full-wave lightning impulse shall be applied on each terminal of the smoothing reactor in turns, while the other terminal is directly grounded. The test voltage is the voltage across terminals as specified in particular technical specifications. b. Terminal-to-ground. During the test, the smoothing reactor proper shall be assembled in the same way as on-site installation and the two terminals shall be short-circuited. The test voltage is the voltage between the terminal and the ground as specified in particular technical specifications. 3. Operating impulse test (terminal-to-ground). During the test, the smoothing reactor proper shall be assembled in the same way as on-site installation and the two terminals shall be short-circuited. The test voltage is the voltage between the terminal and the ground as specified in particular technical specifications. In addition, the wet operating impulse test shall be taken into account, during which the smoothing reactor proper and at least one post insulator shall be in wet conditions. 4. Applied wet DC voltage withstand test. When installed outdoors, the dry-type smoothing reactor with air core shall operate normally. The wet voltage withstand test on the post insulator shall also be included in the test. The test voltage is the manufacturer guarantee voltage applied to the post insulator and should be of positive polarity. 5. Temperature rise test. During this test, the hotspot temperature rise should be measured with an embedded thermocouple. The measured value shall be corrected against the maximum ambient temperature, and the corrected value cannot go beyond the limit. Also, infrared imaging shall be performed during the test and the map indicating the possible overheating and abnormal temperature points should be retained. Upon completion of the test, measure the thermal time constant of the reactor and calculate the short-time overload characteristic based on it.

16.4.3.3 Field Test of a Dry-Type UHV Smoothing Reactor With Air Core Primary test items: measurements of winding resistance, inductance, and noise. The deviation of the measured winding DC resistance and inductance from the values obtained in the factory test shall be within 62%.

16.4.3.4 Routine Test of an Oil-Immersed Type UHV Smoothing Reactor With Iron Core 1. Winding DC resistance measurement. The winding DC resistance can be measured using the bridge method or volt
ampere method, during which a DC power source shall be used. 2. Measurement of insulation resistance, absorption ratio, or polarization index of winding including bushing. Use a 5000-V insulation resistance meter to measure the winding to ground insulation resistance after thorough discharging of the winding under test. 3. Measurement of insulation dielectric loss factor tanδ and capacitance of winding including bushing. The dielectric loss factor of windings shall be measured using the bridge method when the oil surface temperature is in the range of 10 C
40 C. 4. Measurement of insulation resistance of core and clamps. The insulation resistance of core and clamps shall be measured using a 2500-V or 5000-V insulation resistance meter for 1 min, during which no flashover or breakdown shall occur.

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5. Measurement of mechanical strength of the oil tank. This includes the vacuum residual pressure test and pressure test of the oil tank. 6. Insulation test of the auxiliary circuit. The insulation resistance of the cooler oil pump and fan motor shall be measured using a 2500-V insulation resistance meter, during which no flashover or breakdown shall occur. 7. Insulating oil test. Test items: measurement of insulation strength, moisture content, dissolved gas in oil (chromatographic test), gas content, and suspended particles, etc. 8. Measurement of incremental inductance. The rated incremental inductance shall be measured by superimposing typical harmonic current on the rated DC current. Measure the incremental inductance under several intermediate DC current values between zero and the rated DC current and check the linearity of the smoothing reactor. The incremental inductance can be obtained by measuring the voltage and current or other methods, which are subject to agreement between the user and manufacturer. 9. Temperature rise test. For the oil-immersed type smoothing reactor, the influence of harmonic current shall be taken into consideration when determining (through calculation and test) the temperature of the oil, winding, and other metallic structures in operation. After obtaining the top oil temperature rise, continue the test with DC current equivalent to the winding loss under rated operation condition. The said condition shall last for 1 h in the winding during which the temperature of the oil and cooling medium shall be measured. Upon completion of the test, the winding temperature rise shall be determined. 10. Loss measurement. The measured loss shall be corrected against the reference temperature of 80 C. 11. Full-wave lightning impulse test. The test procedure is detailed below: a. Apply impulses on each terminal of the winding in turns while the other terminal is directly grounded. The test voltage shall be the voltage across the two terminals of the winding as specified. b. If the winding to ground insulation level and the insulation level across two terminals of winding are different as specified, the lightning impulse test procedure shall be agreed by the user and manufacturer. Consideration may be given to grounding the untested terminal of winding through an appropriate resistor. This resistor shall be selected to ensure that the desired test voltage can be achieved across two terminals of the winding when the voltage between the terminal subjected to impulse and the ground is as specified. This test shall be performed on each terminal of the winding. When the terminal-to-ground and terminal-to-terminal lightning impulse levels are identical, only the first test is necessitated. 12. Operating impulse test. Two terminals of the winding shall be connected when the operating impulse test voltage is applied between the winding and the ground. It is not necessary to perform an operating impulse test between two terminals of the winding. 13. Applied DC withstand voltage test with partial discharge measurement. During the applied DC withstand voltage test, the oil temperature shall be 10 C
30 C. During the test, two terminals of the winding shall be connected, and the test voltage shall be of positive polarity and applied between the connected terminals and the ground. 14. Polarity reversal test with partial discharge measurement. During the polarity reversal test, the test voltage shall be applied between the connected terminals and the ground. Also, the partial discharge level shall be measured throughout the test. Upon completion of the test, conduct thorough discharge; otherwise, there may be enormous residual charges on the insulation structure which may influence the partial discharge measurement thereafter. 15. Applied AC withstand voltage test with partial discharge measurement. During the test, the test voltage shall be applied between the connected terminals and the ground. Also, partial discharge measurement shall be made throughout the test. The permissible maximum partial discharge shall not exceed 300 pC.

16.4.3.5 Type Test of Oil-Immersed Type UHV Smoothing Reactor With Iron Core Primary test items include chopped-wave lightning impulse test and sound level measurement. The procedures for chopped-wave and full-wave lightning impulse tests are identical.

16.4.3.6 Field Test of Oil-Immersed Type UHV Smoothing Reactor With Iron Core 1. Measurement of winding resistance. The difference between the measured winding resistance under DC condition and the value obtained in the factory test shall be within 62%. 2. Measurement of insulation resistance, absorption ratio, or polarization index of winding including bushing. The insulation resistance shall not be lower than 70% of the value obtained in the factory test. The measured absorption ratio should have no obvious deviation from the value obtained in the factory test. 3. Measurement of dielectric loss factor tanδ and capacitance of winding including bushing. The tanδ of windings including bushing shall not be larger than 130% of the value obtained in the factory test.

612

4. 5. 6. 7. 8.

9.

10. 11.

Part | II DC

Measurement of insulation resistance of core and clamps. Insulation test of auxiliary circuit. Test of insulating oil. Measurement of sound level. During handover test on site, measure the sound level at sensitive points and 2 m from the smoothing reactor under rated operating condition with all cooling equipment in service. Measurement of DC leakage current. The DC leakage current shall be measured after the insulation resistance, dielectric loss factor, and capacitance of winding including bushing are measured as qualified. The test shall be conducted at the terminal with application of 60-kV DC voltage. Read the leakage current at 1 min which is generally up to 30 μA. Measurement of vibration of the oil tank. The measured vibration of the tank wall (peak-to-peak displacement) is expressed by the height of the main crest of the vibration wave, the maximum horizontal amplitude of which shall not exceed 200 μm under rated operation conditions. Measurement of temperature distribution over the oil tank surface. The temperature of the oil tank surface shall be measured with an infrared thermometer and other equipment to check for hot spots and their distribution. Checking of measuring, protection, and detection system. Some special test items of a UHV smoothing reactor shall be as agreed between the user and manufacturer before conclusion of the contract, such as measurement of highfrequency impedance and testing of transient fault current.

16.5 UHVDC ARRESTERS The UHVDC arrester is critical to overvoltage protection and is decisive for insulation level of equipment and has a direct bearing on the volume, cost, and operational safety of the equipment. As such, despite the small proportion of the cost of the arrester in the total project cost, the performance of the UHVDC arrester has a great effect on the whole project.

16.5.1

Characteristics

16.5.1.1 Diversity in Type, Performance, and Parameters The internal overvoltage of DC transmission systems is more complex than that of an AC system in many aspects such as the causes, mechanism of development, amplitude, and waveform. The waveform and burden vary greatly for arresters installed at different positions of the DC system. In addition, as compared with HVDC projects, the UHVDC project has a higher voltage level and more complex structure, and arresters are subjected to a larger burden. As such, a UHVDC project needs a good variety of DC arresters with better performance parameters. Besides, these arresters vary greatly with types. Arresters in a UHVDC converter station are arranged in basically the same way as those in an HVDC converter station, and a typical arrangement is shown in Fig. 16.17. In this arrangement, each arrester serves the following functions: the DC valve arrester protects the valves from being exposed to overvoltage; the bridge arrester protects the six-pulse converters bridged over from being exposed to overvoltage; the DC line arrester protects the equipment in the DC switchyard that is connected to DC line pole from being exposed to overvoltage; the DC busbar arrester protects the equipment connected to the DC line pole on the converter side of the smoothing reactor from being exposed to overvoltage; the converter arrester limits the magnitude of lightning overvoltage occurring in valve hall; the neutral busbar arrester protects the neutral busbar and the equipment connected to it from being exposed to overvoltage. In particular, under balanced bipolar operation, the operating voltage of the neutral busbar is close to zero, while under monopolar metallic return mode the operating voltage shall be taken into account. In the case of a ground fault, the arrester will be subjected to strong energy impulses and, therefore, more arresters are often required. The smoothing reactor arrester protects the smoothing reactor from being exposed to overvoltage; the DC filter arrester protects the reactor, resistor, and LV capacitor of the DC filter from being exposed to overvoltage; the MRTB arrester protects the DC circuit breaker; the mid-point DC busbar arrester protects the lower six-pulse converter of the 12-pulse converter from being exposed to overvoltage, and is connected in series with the valve arrester to protect the valve side windings of the converter transformer corresponding to the upper six-pulse converter.

16.5.1.2 Higher Protection Level For the purpose of protecting the equipment and reducing the insulation level of the equipment, thereby lowering the project cost, the DC arresters are required to provide good protection, i.e., low residual voltage level at certain coordination current (coordination current refers to the current with particular waveform used for determining the protection level of arresters). In addition, the energy absorbed by DC arresters should be checked; too low residual voltage may cause the

UHV Converter Stations and the Main Electrical Equipment Chapter | 16

Line1

2

4

5

613

3 DB2 6a 7

V11 6b V1*

DB1

CBH

8

V12 V2*

MH V2

9a 9b

V2

10 12

22 EM

CB 1.2

V3

13a

DC Filters

11

13b

14

V2

15 ML

V2

16a 16b

V2

17

CBN1

18

19

CBN2

21

20

EL

E

Line2 FIGURE 16.17 Typical configuration of arresters in a UHVDC converter station.

arrester to absorb excessive energy. For UHVDC arresters, the operating impulse coordination current is generally taken as 1
3 kA and the lightning impulse coordination current is taken as 5
30 kA. From the perspective of operational safety and the stability of arresters, the reference voltage of a DC arrester shall be as high as possible so as to reduce the voltage ratio (ratio of CCOV to reference voltage) of arresters under normal operation conditions. The above two requirements interact with each other and thorough studies should be conducted to determine a proper scheme. An important indicator to reflect the protection level of arresters is voltage ratio, i.e., the ratio of residual voltage at the coordination current to the reference voltage at the reference current. The higher the voltage ratio under the same coordination current and reference current, the better the protection performance of te arresters. Two methods may be used to improve the protection level of arresters: (1) decrease the voltage ratio of the varistors, in which case the residual voltage of arresters can be reduced for a certain reference voltage; and (2) adopt a structure of multiple parallel arresters, thus reducing the coordination current corresponding to each varistor column and hence the residual voltage of the arrester.

16.5.1.3 High Energy Absorption Capacity Following the occurrence of overvoltage, UHVDC arresters absorb much more energy than AC arresters. It can be learned from the simulation results of DC systems that the absorbed energy of UHVDC arresters generally ranges from several MJ to tens of MJ. To allow for such a huge amount of energy, it is necessary to increase the energy absorption capacity of DC varistors and connect multicolumn varistors in parallel. By improving the energy absorption capacity of varistors, the number of varistors connected in parallel is decreased, thereby reducing the volume, space occupation, and cost of

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α+μ

T/2

α Commutation overshoots Commutation overshoots

(A)

Commutation overshoots

T/2

T/2

(B)

(C)

FIGURE 16.18 Continuous operating voltage of a valve arrester, bridge arrester, and DC busbar arrester (rectifier operation: α 1 μ 5 34 degrees, sixpulse waveform): (A) Valve arrester; (B) bridge midpoint arrester; (C) DC busbar arrester.

arresters. Currently, the per unit volume energy absorption capacity of advanced varistors at home and abroad can reach 300 J/cm3.

16.5.1.4 Complex Continuous Operating Voltage Conditions In contrast to AC arresters, the voltage waveform of DC arresters which consists of a DC component, AC component, and high-frequency component varies with their installation position. In particular, DC line arresters are subjected to DC voltage; DC valve arresters are subjected to sinusoidal voltage superimposed with transient voltage when the thyristor is turned off, and zero voltage when the thyristor is turned on; the bridge arrester (bridge midpoint M) and DC busbar arrester are subjected to DC component superimposed with pulse voltage; filter arresters are subjected to a high-order harmonic voltage. Fig. 16.18 shows typical continuous operating voltage of the valve arrester, bridge arrester, and DC busbar arrester. Given the complex voltage waveforms that the DC arresters are subjected to, three continuous operating voltages are specified for arresters installed on the valve side, namely peak value of continuous operating voltage (PCOV), which refers to the highest continuously occurring crest value of the voltage including commutation overshoots; crest value of continuous operating voltage (CCOV), which refers to the highest continuously occurring crest value of voltage excluding commutation overshoots; and equivalent continuous operating voltage (ECOV), which refers to the voltage value that generates the same power losses as the actual operating voltage. The power loss characteristics of a metal oxide varistor under direct current, high frequency, and AC voltage are different. Relevant test results show that the conditions of an accelerated aging test under direct current are most severe, followed by six-pulse bridge and 12-pulse bridge voltage and then the valve voltage. At low CCOV voltage ratio ( ,85%), losses are small under various voltage waveforms. However, with an increase in voltage ratio, the power loss of the pole busbar arrester under pure DC current increases rapidly, while that of bridge-side and valve-side arresters under highorder harmonic voltage takes turns to increase at a fast rate when the CCOV voltage ratio exceeds 100%. Under DC voltage, the conditions of an accelerated aging test of varistors are most severe and the power loss increases rapidly with the increasing temperature and voltage ratio. Therefore, the voltage ratio of DC line pole arresters cannot be too high, and is generally about 85%. For other arresters, the voltage ratio can be increased accordingly.

16.5.1.5 Complex Structure As required by the protection level and energy withstand capability, most DC arresters adopt a configuration of multiple parallel columns, including multiple varistor columns connected in parallel in the arrester and multiple arresters connected in parallel. The number of parallel columns ranges from two to a dozen columns or even tens of columns as required. For example, a common valve arrester has four or five columns in parallel, a DC line pole arrester has two, four, or five columns in parallel, and MRTB usually has several parallel-connected arresters, each of which consists of multiple varistor columns in parallel. The housing structure of arresters falls into three categories, i.e., open type, porcelain housing, and composite housing. Greatly affected by the external environment, open type arresters are generally used in the valve hall of converter stations,

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and valve arresters adopt a similar design. Arresters with porcelain and composite housing can be used either indoors or outdoors.

16.5.1.6 External Insulation and Pollution The external insulation of UHVDC arresters is generally designed by taking into account the actual pollution level and altitude of the project, so as to ensure that no flashover occurs in the external insulation under normal continuous operating voltage and overvoltage. Also, the effects of pollution on internal varistors shall be considered, which mainly manifests as an internal partial temperature rise due to surface discharge and temperature rise due to pollution layer current of a multisection arrester flowing into the arrester core through the flange. The latter case is especially prominent under nonuniform pollution. Currently, for a DC line pole arrester which has great size due to the demand for a large creepage distance of porcelain housing, generally composite housing or a combination of porcelain housing and composite housing may be adopted. With relatively low continuous operating voltage, other types of arresters installed outdoors can use porcelain or composite housing.

16.5.2

Key Performance and Design Features

16.5.2.1 R&D of DC Varistors In consideration of the characteristics of UHVDC arresters, higher requirements are placed on varistors, including low voltage ratio, more energy absorption, and high actual operating voltage (combination of direct current, power frequency, and high frequency) withstand capability (i.e., good aging resistance). Many factors may affect these performance indicators, and important ones include additives, sintering temperature curve, additive process/procedure, composition and process of DC high resistance layer, calcination temperature and time of additives, DC coating process, etc. The uniformity of varistors is a major factor determining the absorption capacity of varistors, and a uniform varistor can give full play to the energy withstand capability of various parts of the varistor. Therefore, the energy absorption capacity of varistors can be effectively improved by ensuring uniformity across the varistor from processing of raw materials (granularity of powder and full, uniform mixing), stamping (make the density of each part of the varistor as uniform as possible), and sintering (make the temperature of each part of the varistor as uniform as possible). In addition, researches show that the energy absorption capacity of varistors can be improved by reducing the clearance between the aluminizing layer at the end surface and the side glazing.

16.5.2.2 Current Sharing by Paralleling Multiple Varistor Columns To allow high through-current capability and reduce the insulation level of the equipment under protection, a great many UHVDC arresters have to be designed with multiple parallel-connected varistor columns. In high through-current applications, the absorbed energy must be uniformly distributed among the multiple parallel varistor columns. As all varistor columns are exposed to the same voltage, the energy distribution is actually the current distribution. However, due to manufacturing tolerance, current flowing through different varistor columns varies, and thus the permissible energy absorption of the arrester depends on the varistor column whose residual voltage is the minimum under the decisive current. If an arrester is composed of several parallel-connected varistor columns in one or several porcelain housings, the nonuniformity factor β (β 5 n 3 Imax/Iarr) of the current sharing under critical energy stress shall be ensured. In the bracketed formula, Iarr is the peak value of total current of the arrester under the decisive energy stress, which is obtained through system study; Imax is the maximum guaranteed peak value of current flowing through any column; and n is the number of parallel-connected varistor columns. During the grouping of multiple parallel varistor columns, the current sharing between columns is generally achieved by ensuring uniformity between the reference voltage and particular residual voltage under switching impulse current.

16.5.2.3 Temporary Overvoltage Withstand Capability The UHVDC arresters may possibly be exposed to temporary overvoltage for a certain period of time, and therefore should have corresponding withstand capability. However, the temporary overvoltage conditions are quite complex with irregular waveforms and vary with projects and operating conditions, making it difficult to place requirements on the temporary overvoltage withstand time characteristic curve as was done for arresters in the AC system. Generally, sufficient margin shall be reserved in design. If necessary, the power frequency voltage withstand time characteristic curve should be provided in the same way as the AC arresters.

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16.5.3

Part | II DC

Testing

16.5.3.1 Protection Level (Residual Voltage Test) For the purposes of the residual voltage test, the residual voltage at the steep-front impulse current (1/10 μs), lightning impulse current (8/20 μs), and operating impulse current (30/60 μs) should be given for a particular magnitude of the coordination current. Under special operating conditions, such as overvoltage lasting for milliseconds that may possibly occur in valve arresters, the residual voltage at slow-front (1/2 ms) impulse current shall also be provided.

16.5.3.2 Accelerated Aging Test For the purposes of the accelerated aging test, the actual voltage waveform at the position where the arresters are installed shall be used. Other voltage waveforms may also be applied in the test provided that these waveforms can be proved to be equivalent to or more accurate than the actual waveforms. To conduct the equivalent test, for arresters subjected to commutation overshoots (e.g., valve and bridge arresters), the PCOV/CCOV ratio used to simulate the actual voltage waveform shall be as close to that of the actual waveform as possible. If the former is smaller than the latter, the voltage ratio under the actual PCOV should be used; otherwise, the voltage ratio under CCOV should be used.

16.5.3.3 Energy Withstand Test The UHVDC arrester may withstand overvoltage for a long period of time and, therefore, its energy withstand capability is often verified by impulse current waveform (rectangular wave) having a long duration (4
10 ms). Alternatively, to a certain extent, the energy of 2 ms square wave can be equivalent to that of 4
10 ms square wave. Technical brochure CIGRE 33/14.05 Guidelines for the Application of Metal Oxide Arresters Without Gaps for HVDC Converter Stations gives the formula to convert energy withstand capability for different waves. W 5 W0 ðT=T0 Þγ

(16.8)

where W is the energy capability for a rectangular current wave of duration T; W0 is the energy capability for a rectangular current wave of duration T0; and γ is set to 0.25. It should be noted that γ cannot be taken as 0.25 all the time. Test results show that varistors that can withstand the energy of 2 ms rectangular wave do not necessarily withstand the energy of 10 ms rectangular wave as converted according to Eq. (16.8). As such, the energy withstand test shall be conducted based on the actual wave duration as required.

16.5.3.4 Operating Duty Test The DC arresters and AC arresters are similar in terms of the procedure of operating duty test, and vary in preliminary test and number of energy injections. Also, the voltage applied 100 ms following the last energy injection is different for DC and AC arresters, which in particular is power frequency voltage for AC arresters and CCOV or ECOV for DC arresters. Simulating actual CCOV is difficult for tests with high power in laboratory and the transmission wire will reduce the PCOV/CCOV ratio, causing great difficulty in the operating duty test with CCOV. Therefore, how to achieve CCOV and ECOV is a key link in the operating duty test of DC arresters. AC, DC, or combined waves may be used as ECOV for equivalent verification during which the power loss generated by ECOV must equal that generated by CCOV at the temperature of the varistor after energy injection since different waveforms change with temperature in quite different ways. In addition, the requirements for PCOV/CCOV shall be same as those in the accelerated aging test. Table 16.2 shows the procedure for the operating duty test of DC arresters.

16.5.3.5 Current-Sharing Characteristic Test For multiple columns of arresters in one or more porcelain housings, the current sharing characteristic test shall be conducted on all arrester units or varistors. The manufacturer shall specify a proper switching impulse current within the range of 100
1000 A and the current flowing through each column shall be measured simultaneously. For testing the resistance, the apparent wavefront time of the impulse current shall not be shorter than 7 μs,while the half peak time is not specified. For testing the parallel resistance group, the apparent wavefront time of the impulse current shall not be shorter than 30 μs, while the half peak time is not specified.

16.5.3.6 Pollution Test As the pollution test method of DC arresters is still under discussion, the solid layer method, solid layer cool fog method, and nonuniform pollution coating method for AC arresters provide references. Theoretically the conditions of each section

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TABLE 16.2 Procedure for Operating Duty Test of DC Arresters Test Procedure

Arresters with Significant Continuous Operating Voltage (e.g., valve arrester and bridge arrester)

DC Busbar Arrester and Line Arrester

Arresters without Significant Continuous Operating Voltage (e.g., neutral busbar arrester)

1

Current distribution measurement (if necessary), reference voltage measurement, 10 kA lightning impulse current test

Current distribution measurement (if necessary), reference voltage measurement, 10 kA lightning impulse current test

Current distribution measurement (if necessary), 10 kA lightning impulse current test

2

Six high energy discharges with cooling down to 25 C 6 10 C between two discharges

Six groups of high energy discharges (three for each ground) with cooling down to 25 C 6 10 C between two groups

Seven high energy discharges with cooling down to 25 C 6 10 C between two discharges

3

Cooling down to 25 C 6 10 C, and three sections

Cooling down to 25 C 6 10 C, and three sections

Cooling down to 25 C 6 10 C, and three sections

4

One 4/1 0 and 100 kA high current impulse lasting for 50
60 s, and one 4/10 and 100 kA high current impulse

One 4/10 and 100 kA high current impulse lasting for 50
60 s, and one 4/10 and 100 kA high current impulse

One 4/10 and 100 kA high current impulse lasting for 50
60 s, and one 4/10 and 100 kA high current impulse

5

Preheated to 60 C 6 3 C

Preheated to 60 C 6 3 C




6

As short as possible, not longer than 5 min

As short as possible, not longer than 5 min




7

One high energy discharge

Two high energy discharges at an interval of 50
60 s




8

Not longer than 100 ms

Not longer than 100 ms




9

CCOV or ECOV, 30 min

CCOV or ECOV, 30 min




of a multisection DC arrester are basically the same under uniform pollution conditions as they are not affected by potential distribution, but differ greatly under nonuniform pollution conditions.

16.5.3.7 Type Test Items All types of DC arresters shall be type tested. Test items of a full type test include: visual inspection, specific creepage distance inspection, resistive current test, energy withstand test, residual voltage test, power frequency reference voltage test, DC reference voltage test, leakage current test under 0.75 p.u. DC reference voltage, operating duty test, sealing test, external insulation withstand test, pressure relief test, radio interference voltage and partial discharge test, current distribution test, thermomechanical and boiling water immersion test (only for arresters with composite housing), mechanical performance test, antielectric corrosion and tracking test (only for arresters with composite housing), and artificial pollution test.

16.6 UHVDC BUSHINGS UHVDC bushings must withstand high DC voltage containing tiny AC voltage waveform, and allow HV conductors to pass through objects with different potential, serving the functions of insulation and mechanical support. Integrating electric, thermal, mechanical, and chemical properties, they are one of the core pieces of equipment in the development of UHVDC transmission. Generally, bushings used for leading electrical current into or out of the metal cases of electrical equipment like transformers, reactors, and breakers are called electrical equipment bushing; while bushings designed to lead electrical current through walls of buildings (e.g., converter valve hall) are called wall bushing. Compared with HVAC bushings, HVDC bushings have the following characteristics: 1. Electric field distribution of HVDC bushings depends on the electrical conductivity (or resistivity) of insulating material and they are susceptible to field strength, temperature, and humidity. Therefore, it shows strong nonlinearity and

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needs electric-thermal coupled-field calculations while electric field distribution of HVAC bushings depends on the permittivity of insulating materials and the field strength varies slightly with external factors like temperature. 2. During polarity reversal, the DC bushings show dynamic electric field distribution, and are subjected to combined stresses of DC and AC components as well as space charges; the electric field is not uniformly distributed at the bushing end. Besides, electric field distribution at the bushing end and changes of field strength at critical positions are of a linear nature at different moments. 3. Internal insulation of DC bushings has no obvious regulation effects on the electric field distribution of external insulation. Therefore, the electric field distribution on their outer surface is less favorable than that of AC bushings. Due to the electrostatic adhesion under HVDC, a great deal of contaminants deposit on the external insulation surface, which is detrimental to the potential control and electric field distribution of bushings, and moreover, uneven exposure to rain often leads to electric field distortion and flashover of external insulation. The electrical insulation structure of a UHVDC bushing is quite typical, and it withstands high electrical, thermal, and mechanical stresses. It is a device with both internal and external insulation. Its electric field distribution is complex because of the typical insertion structure with one electrode inserting into another electrode. In particular, the electric field has a large component perpendicular to the dielectric surface and the distribution is extremely uniform on the surface and depends on factors such as dielectric constant, resistivity, and temperature. Besides, the electric field distribution is greatly affected by the internal and external insulation structures due to the complex bushing structure. Additionally, heating of the conductor, dielectric loss, thermal breakdown, and sealing shall also be taken into account. Hence, there are great difficulties in the material, process, structural design, and manufacturing technology of UHVDC bushings. At present, the highest voltage level of DC bushings is 6 800 kV worldwide, and bushings at this voltage level are mainly provided by ABB and Siemens. China is now working actively on the R&D of HVDC dry-type bushings.

16.6.1

Structural Style of UHVDC Bushings

Currently, UHVDC bushings are mainly available in three structural styles, namely epoxy resin-impregnated dry-type, oilimpregnated paper, and pure SF6 insulated structure. 1. Epoxy resin impregnated dry-type bushing The core of dry-type bushing is an epoxy resin-impregnated condenser core which is wound from insulating paper and aluminum foil electrode, subsequently impregnated in epoxy resin under vacuum conditions and then heated for curing. The resin-impregnated condenser core is designed following a similar principle to oil
paper bushings. Based on capacitance or resistance grading principle, the design mainly involves the selection of the maximum radial operating field strength and the design of the grading electrode. The external insulation of bushings uses a composite insulating envelope and the space between the core and insulating envelope is filled with SF6 at a certain pressure. The advantages of this kind of bushing include good electrical performance, reasonable internal and external distribution of electric field strength, high partial discharge withstand capability, high mechanical strength, good pollution resistance, fine hydrophobicity, small size, light weight, nonfragile, easy to pack and transport, high reliability, and easy to maintain. However, with a long length, the winding, vacuum resin impregnation and curing process of the condenser core are quite challenging, and place high requirements on workshop conditions, manufacturing equipment, and workmanship. Fig. 16.19 shows an 6800-kV dry-type converter transformer bushing produced by Siemens. 2. Oil-impregnated paper bushing The oil-impregnated paper bushing is typically provided with a porcelain envelope for external insulation, the oilimpregnated paper core is for internal insulation, and the space between the core and the insulating envelope is filled with transformer oil. Its advantages are mature process and high pass percentage. Hence it is widely used in AC applications at various voltage classes. In China, the Nanjing Electric (Group) Co., Ltd. has provided UHVDC and AC oilimpregnated paper wall bushings for the environmental control chamber of the China Electric Power Research Institute and the State Grid Electric Power Research Institute. The internal insulation of oil-impregnated paper bushing is characterized by good electrical performance, reasonable field strength distribution, low dielectric loss, high partial discharge inception voltage, strict control of material, and mature manufacture equipment and process; the external insulation is achieved via high-voltage electric porcelain and has good insulation performance and chemical stability. However, oil-impregnated paper bushing is internally filled with oil and provided with an external porcelain envelope, leading to a heavy weight and, making it difficult to transport and install. Moreover, when the voltage level is high, its length becomes long, in which case the mechanical strength and balance cannot be guaranteed. Also, the bushing has potential hazards such as oil leakage, out-of-limit of oil chromatography, and explosion. Additionally, DC electric field distribution of external insulation of a DC wall bushing provided with a porcelain envelope is sensitive to changes in

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619

FIGURE 16.19 An 6800-kV dry-type converter transformer bushing produced by Siemens.

surface conductivity caused by pollution and humidity, which may result in distortion of the electric field and flashover of external insulation due to uneven exposure to rainwater. 3. Pure SF6 insulated bushing The SF6 insulated bushing has the simplest structure, and mainly consists of composite hollow insulator, conductor component, internal shield, and external grading ring. The composite hollow insulator is for external insulation and SF6 is for internal insulation. It has several metal shield cylinders to regulate the internal electric field and external grounding electric field, but such regulation is less effective because there are great interactions between the internal and external field distribution and the bushing diameter has to be very large. The advantages of this kind of bushings are high mechanical strength, good pollution resistance, light weight, and ease of transportation. However, the shield electrode with a large diameter and thin wall places stringent requirements on structural design, manufacturing processes, and installation and fixing techniques.

16.6.2

Design of UHVDC Bushings

The design of UHVDC bushings is a worldwide challenge that involves various complex factors such as various complex fields, structural style, selection of materials, process control, mechanical strength, grading shield, and design of internal and external insulation. 1. Fields Electric field distribution of the bushings is determined by resistivity, which changes with temperature in an exponential manner. The electric field is concentrated at the edge of the intermediate flange, which is likely to cause surface gliding spark discharge and flashover. Also, an electric field between flanges and conduct tubes is strong and so the insulating medium is susceptible to breakdown. Bushings are associated with other electrical equipment, and are required to be compact and small in size. The bushing is of composite structure made of organic, inorganic, gas, liquid, and solid materials. Under a strong electric field, various dialectic characteristics change in a complicated way, many factors will affect the bushing performance, and there are prominent problems like partial discharge. Therefore, improper structural design may lead to partial discharge or surface flashover, harming the reliability of the insulation. In conclusion, analysis of the HVDC electric field distribution is critical to the insulation design. In the event of power reversal in a DC link, the electric field distribution at the bushing end is not uniform, and such distribution and field strength at various key positions vary with time in a linear manner. Therefore, the design of a UHVDC bushing should consider steady-state distribution of the electric field under positive and negative polarity reversal test voltages and transient electric field and surface space charges during polarity reversal to avoid damage to bushings during this process. The condenser core shall be designed such that no partial discharge occurs under the largest electric field produced by the test voltage. The condenser core is like a cylindrical resistor bank consisting of multiple series stages under DC voltage, and it is designed to control the electric field by adjusting the resistance of each resistor. During polarity reversal, the electric field distribution depends on the permittivity and dielectric constant, and so dielectric properties of various materials in a composite structure should be considered to ensure as uniform distribution of the axial and radial fields as possible and to increase the inception voltage of harmful partial discharge.

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During operation, dielectric conductance, heat, and loss, heat transfer of transformer oil and ambient temperature changes may all lead to changes in bushing temperature. Also, the resistivity varies with temperature, which will lead to electric field changes. Hence, electric-thermal coupled-field calculations are needed for DC bushings. In recent years, thanks to rapid development of computer technology, finite element and boundary element numerical simulation have been widely used in complex project issues. Mature finite element software can implement modeling and loading based on the actual structure. Its advantages include simple treatment of boundary conditions, high accuracy of calculation results, and display of calculation results using figures, curves, or charts, thus facilitating the analysis with expertise. Relying on software and workstations with powerful functions like solid modeling, resolving, data analysis, and post-processing, calculation and analysis can be performed for three-dimensional distribution of AC and DC fields, transient fields, and the electric-thermal coupled field of UHVDC bushings. In addition, by optimizing insulation, uniform distribution of axial, radial, and end electric fields can be realized to improve the operational reliability of UHVDC bushings. 2. Selection of material Intensive studies have been made on characteristics of insulating oil, cable paper, and relevant auxiliary materials of oil-impregnated paper bushings. For epoxy resin-impregnated bushings, the dielectric properties, curing properties, bonding, intermiscibility, impregnation, and electric-thermal aging properties of epoxy resin, crepe paper, electrode material, epoxy resin cylinder, and silicon rubber have a great impact on the partial discharge, thermal resistance, and mechanical properties of the entire bushing. Studies have suggested that the fiber structure and distribution of crepe paper significantly affect its mechanical properties and the impregnation in epoxy resin. Crepe paper with fine fibers and uniform distribution of fibers is conducive to improving the impregnation and elongation of the epoxy resin, thus improving the quality of the condenser core. 3. Check of mechanical strength UHVDC bushings have long length, large diameter, and heavy weight and so in addition to ensure compliance with the electrical performance requirements, their mechanical performance and aseismic capability should also be analyzed and calculated to ensure the reliability of bushings. Bushing quality, internal pressure, end load, icing load, operating load, wind load, and seismic load may all exert bending stress on the bushing. Hence the mechanical strength of bushings shall be checked at the design stage for conventional expected load and unconventional extreme load schemes as per the relevant standards. With the development of computer technology, using powerful ANSYS software can accurately solve bushing displacement and stress caused by external loads. 4. External insulation design Internal insulation of DC bushing cannot effectively adjust the electric field distribution of the external insulation, resulting in less uniformity of electric field distribution than AC bushings. Under DC voltage, the electric field on the bushing surface features large distortion and high field strength, for which reason the voltage for inception of flashover and breakdown are much lower than AC bushings. Due to the electrostatic adhesion under HVDC, a great deal of contaminants deposit on the external insulation surface, which is detrimental to the potential control and electric field distribution of bushings. As the pollution becomes more and more severe, the insulation level reduces, and consequently flashover may occur under certain weather conditions. In consideration of these, enough creepage distance should be provided and the shed profile of the external insulating envelope shall be optimized in the insulation design so as to increase the pollution flashover voltage of external insulation. The increasing voltage level poses stricter requirements for creepage distance and adds to the difficulty of manufacturing satisfactory bare porcelain products. Inevitably, simply inheriting the old external insulation design practice will not work. Therefore, bushings in the DC yard are usually provided with composite insulating envelopes or RTV-coated porcelain envelopes to reduce the height of external insulation and mitigate uneven moisture of bushing surface so as to ensure uniform electric field distribution on the surface.

16.7 UHVDC FILTERS 1. Configuration principle The configuration of UHVDC filters shall give due consideration to the amplitude of harmonics of individual orders and the proportion of them in the equivalent disturbing current, which means coupling factor and weighting factor of harmonic current in the calculation of disturbing current. Theoretically, a 12-pulse converter only produces harmonic voltage of 12 k (k 5 1, 2, 3. . .) orders on the DC side. Actually, however, noncharacteristic harmonics are also present due to the existence of a variety of asymmetric factors, including ground stray capacitance of converter transformers. Noncharacteristic harmonics of low orders caused by stray capacitance has large amplitude, the filtering of which requires large filters. The main path of these harmonics is converter transformer—converter valve—ground, and only a small portion of them enter DC lines.

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Given below is a typical filter configuration scheme adopted by DC transmission projects worldwide: a. Connect a bank of surge capacitors between the neutral busbar on the LV end of the 12-pulse converter and ground to eliminate low-order noncharacteristic harmonics. Filters for low-order harmonics are generally not installed for cost-saving purposes. b. Connect two banks of double-tuned or one bank of triple-tuned passive DC filters between the DC busbar of each pole and the neutral busbar in parallel. The center tuning frequency shall focus on characteristic harmonics of high amplitude as well as high-order harmonics having great influence upon equivalent disturbing current in order to achieve better filtering effects. 2. Types of DC filter circuit DC filter circuits are usually arranged as parallel filters between the DC pole busbars and the station neutral busbar or ground. Similar to AC filters, DC filter circuits come in various types, and common types include single-tuned with or without high-pass, double-tuned, and triple-tuned filters. The structure of a DC filter circuit is determined based on the equivalent disturbing current produced by DC lines. Considering the dominant amplitude of characteristic harmonic currents, the DC filter circuit shall match those harmonics (of 12, 24, 36, etc., orders). Generally, the cost of a DC filter does not take up a large proportion of the whole converter station cost. The most cost-intensive component in the DC filter is the HV capacitor as it must be designed to withstand high DC voltage. One of the major means to reduce cost is to use a double-tuned or multituned filter circuit sharing a common HV capacitor. Generally, filtering capacitors are installed between the neutral point of the converter station and the ground, to offer a low-impedance path for current on the DC side with 3k-order harmonics as the main components. As the ground stray capacitance of the converter transformer winding provides a path for DC harmonics, especially the lower-order DC harmonic current, parameters of the neutral capacitor shall be dependent upon these harmonics. Generally, the capacitance of the capacitor shall be selected from dozens of microfarad to several millifarad and it avoids shunt resonance created with inductance of earth electrode lines at a critical frequency. The DC filter circuit usually uses a band-pass double-tuned filter circuit. In a 12-impulse converter, the combination of orders is usually 12/24 and 12/36 when a double-tuned filter is used. Fig. 16.20 shows DC filters for one 12-pulse converter of one pole. 3. Selection of the HV capacitor of a DC filter The DC filter comprises HV and LV capacitors and reactors, etc., among which the HV capacitor is the core component. a. Average operating field strength of HV capacitor. HV capacitors mainly serve to withstand DC voltage and the AC voltage component is extremely small. The DC voltage is distributed among the series-connected capacitor units depending on the magnitude of the leakage resistance of the porcelain insulators at the end of each capacitor unit. Currently, the average operating field strength ranges from 60 kV/mm to 70 kV/mm for HVAC capacitors and ranges from 90 kV/mm to 110 kV/mm for HVDC capacitors. The HV capacitors are mainly subjected to DC operating voltage whose distribution takes into account the unevenness of leakage resistance distribution among the porcelain bushings at the ends of the capacitor unit as well as the influence of harmonic voltage.

Lde Pole busbar 12/24

C2

12/36 C1

C1

L1

L1

L2

C2

L2

Neutral busbar CN

FIGURE 16.20 Schematic diagram of DC filters for a 12-pulse converter of one pole.

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b. Selection of fuse protection for HV capacitors. The HV capacitors may be protected by an external fuse, an internal fuse, or no fuse. At present, the internal fuse is the most widely used type. As DC filters are simply used for filtering without reactive power compensation function, the total current of all filtering branches is often smaller than that of an AC filter. Where it is technically and economically justifiable, reliable no-fuse HV capacitors can be used.

16.8 UHVDC MEASURING INSTRUMENTS Measuring instruments in the DC switchyard are classified into DC voltage measuring instruments and DC current measuring instruments. LV measuring equipment in an 6800-kV UHVDC link is the same as that in conventional HVDC links. The principles of HVDC measuring instruments are the same as those of conventional HVDC projects, that is, increasing the external insulation voltage. The DC voltage measuring instrument should have desirable transient response and frequency response to ensure that at the maximum tolerances, the measured value is within the accuracy requirements required for HVDC system control and protection purposes. The output signal of a DC voltage measuring system must be of sufficient quality to ensure that the signal is usable at all levels of DC voltage (of both polarities) from 0.1 p.u. to 1.5 p.u. and the specified accuracy shall be met in this range. HV connection on the primary side of measuring instruments should be isolated from the output signal. If this fails, protection equipment must be installed to limit possible output signal within 2 kV when faults occur on measuring instruments. The measuring instrument output signal shall be of sufficient amplitude to ensure that the signal is usable at all levels of primary current from 1% to 300% of the rated current, with an instantaneous value of output up to 600% transiently. 1. DC current measuring instruments The DC current measuring instruments, also called the DC current transformer, are usually installed at the HVDC line end, the neutral busbar, and earth electrode line in the converter station. The output signals are used for control and protection of the DC system. In terms of their main technical performance, the DC current measuring instruments are required to ensure sufficient dielectric strength between the output circuit and the measured main circuit, and have high antielectromagnetic interference capability, high measuring accuracy, and quick response. Generally, the HVDC current measuring instruments are classified into two types: inductive and photoelectric types. a. Inductive DC current measuring instrument. Inductive DC current measuring instruments are categorized into serial and shunt types. Fig. 16.21 shows the schematic diagram of an inductive DC current transformer, which is similar to a magnetic amplifier. Its main components include saturable reactor, auxiliary AC power source, rectifier circuit, and burden resistor. As reactor core material has high rectangular factor and small coercive force, a secondary DC signal that is proportional to the primary current will be produced on the burden resistor when the DC current in the main circuit changes. Its main performance ratings are as follows: measuring accuracy is from class 0.5 to class 1.5, response time is from 50 μs to 100 μs, and incorrect response is 0.5%
3% when the primary current is less than 10% of the rated value. b. Fig. 16.22 shows the structural block diagram of a photoelectric DC current transformer. Components of a photoelectric current transformer include: i. High-accuracy shunt. This can be shunt resistor or a Rogovski coil. ii. Electro-optical module (remote module shown in Fig. 16.22). This is also located at the HV part of the instrument to allow analogue-to-digital conversion of signals and data sending. Electronic devices of the remote module are powered by a separate optical fiber from the light source in the control room. FIGURE 16.21 Schematic diagram of an inductive DC current transformer: (A) Serial type; (B) shunt type.

I

∼

e

L1

Auxiliary AC power source

Id

L1

Id RL

L2

(A)

∼ RL

Burden

L2

(B)

UHV Converter Stations and the Main Electrical Equipment Chapter | 16

Remote module

Local module

Power source converter

Laser diode

Digital-to -analog converter

Digital-toanalog converter

Data receiver

Data transfer

Current divider

Power source

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FIGURE 16.22 Structural block diagram of a photoelectric DC current transformer. FIGURE 16.23 Schematic diagram of a DC voltage transformer: (A) Resistive voltage divider; (B) resistive-capacitive voltage divider.

R1 R1

Ud C1 DC amplifier

R2

(A)

U2

R2

C2

Amplifier

(B)

iii. Signal transmission optical fiber. iv. Optical interface (local module shown in Fig. 16.22). This is located in the control room, and is used to receive digital signals transmitted via optic fiber and send the same to the relevant control and protection devices after checking by the processor chip in the module. The DC current measured by photoelectric current transformer is sent to the receiver in the control room via an up to 300 m long optic fiber in the form of a digital optical signal. Its measuring accuracy can reach 0.5% and the measuring frequency can be up to 7 kHz. As compared with an inductive current transformer, the greatest advantages of a photoelectric current transformer are small diameter of the post insulator and simple electronic circuit, which are significantly favorable for reducing arcing faults, electromagnetic interference, and manufacturing costs. Its disadvantage is slow response speed. Optic fibers of a photoelectric current transformer may be attached onto insulators including porcelain insulators, post insulators, and composite insulators commonly used in converter stations. The length of the insulator may be simply designed according to usual practice. Therefore, a photoelectric current transformer is the norm for DC current measurement of a UHV pole line. The only difference between 800-kV and 500-kV photoelectric DC current transformers is the length of optic fiber for transmission. 2. DC voltage measuring instrument The DC voltage measuring instrument usually uses a DC voltage divider consisting of a resistive voltage divider plus a DC amplifier, and its working principle is shown in Fig. 16.23A. If faster response speed is required, a resistive-capacitive voltage divider as shown in Fig. 16.23B can be used. As the HV resistor R1 has a high resistance and withstands high voltage, it is usually of an oil-filled type or gas-filled type. In an 6800-kV UHVDC transmission system, DC voltage dividers are of the resistive-capacitive type with effective voltage grading measures. The core component of a voltage sensor is the DC resistive voltage divider (including the HV arm resistor and LV arm resistor which are connected in series). As the current through the HV arm resistor is equal to that through the LV arm resistor, it is possible to calculate the voltage of the HV side by measuring the voltage across the LV arm resistor. In order to ensure the measuring accuracy under different ambient temperatures and voltages, the resistors of the HV and LV arms must have a small temperature coefficient and inductance and remain

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stable under voltage effect. Since the magnitude of a HV resistor is as high as hundreds of megohms, the resistive current flowing through the voltage divider is usually of milli-amperes. For a purely resistive voltage divider, although the voltage is uniformly distributed when a direct current flows through it, the voltage distribution under lightning impulses will be extremely nonuniform because of the different ground stray capacitance of resistor elements at varying heights. Specifically, a resistor element at the HV side is subjected to a much higher voltage than that applied on the middle and bottom resistor elements, and is therefore susceptible to breakdowns and unable to meet the requirements of test and operation. Two compensation capacitors are connected in parallel across the resistor to improve the distribution of electric fields. Their compensating capacity should match with the resistor’s resistance. Moreover, a purely resistive voltage divider usually has unsatisfactory frequency response characteristics, and the compensation capacitors can improve such characteristics, ensuring compliance with the relevant requirements of monitoring, control, and protection requirements of HVDC transmission lines. The major difference between a UHVDC voltage divider and a 6500-kV one lies in the internal and external insulation. The UHVDC voltage divider operates at a higher voltage level and hence requires an increased insulation level. Therefore, it is necessary to re-design the terminal insulation and compensation devices to avoid coronas and improve the distribution of impulse voltages. During the design and manufacturing, due consideration should be given to the temperature rise control of resistor elements to ensure a desirable measuring accuracy. Also, the effects of wet pollution on the distribution of internal and external voltages should be taken into account. In order to ensure system safety and reduce interference, an electrostatic shield should be provided between the primary circuit and the secondary output circuit of the UHVDC voltage divider.