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Medium-voltage DC power distribution technology
6
Medium-voltage DC power distribution technology Biao Zhao 1 , Rong Zeng 1,2 , Qiang Song 1 , Zhanqing Yu 1 , Lu Qu 1,2 1 Department of Electrical Engineering, Tsinghua University, Beijing, China; 2 Energy Internet Research Institute, Tsinghua University, Beijing, China
Chapter Outline 6.1 Development background 124 6.2 Application advantages and scenarios 6.3 System architecture technology 126
125
6.3.1 Topology 126 6.3.2 Bus structure 127 6.3.3 Grounding form 128 6.3.3.1 Grounding location 128 6.3.3.2 Grounding type 129 6.3.4 Organization forms of distributed sources 131 6.3.5 Connection forms between different buses 131
6.4 Key equipment technology
132
6.4.1 Voltage source converter 132 6.4.2 DC transformer 133 6.4.3 DC breaker 135
6.5 Control technology
137
6.5.1 Converter control 137 6.5.2 Multisource coordination control 137 6.5.2.1 Bus voltage control 137 6.5.2.2 Power quality management 139 6.5.3 Multibus network-level control 139
6.6 Protection technology 139 6.7 Practical medium-voltage DC Energy Internet systems in China 6.7.1 Medium-voltage DC Energy Internet system in Shenzhen 140 6.7.1.1 Technical demands from Baolong Industrial Park 140 6.7.1.2 Two-terminal “Hand in Hand” architecture 141 6.7.1.3 Key equipment scheme 141 6.7.1.4 Multifunctional operation ways 143 6.7.1.5 Protection scheme 147 6.7.2 Medium-voltage DC Energy Internet system in Zhuhai 149 6.7.2.1 Technical demands from Tangjiawan Science Park 149 6.7.2.2 Three-terminal architecture 150 6.7.2.3 Key equipment scheme 150 6.7.2.4 Control scheme 151
6.8 Summary
151
The Energy Internet. https://doi.org/10.1016/B978-0-08-102207-8.00006-0 Copyright © 2019 Elsevier Ltd. All rights reserved.
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6.1
The Energy Internet
Development background
In the beginning of the power grid, there was a debate between the use of AC and DC grids. However, AC grids won and became prevalent because the voltage and power levels of DC grids were very low and voltage conversion was very difficult at that time. In recent years, as the development of power semiconductor technology and power electronics technology has progressed, the technical limitations of DC grids are gradually being overcome, making the DC technology appear to be popular again. In DC grids, the DC transmission technology is a current research hotspot. This technology is divided into high-voltage DC (HVDC) based on a line-commutated converter (LCC-HVDC) and HVDC based on a voltage source converter (VSC-HVDC). Compared with the LCC-HVDC, commutation of the VSC-HVDC is independent of an AC grid, thus there is no commutation failure issue and it can supply power for a passive system. Because of the employment of pulse width modulation in the VSC-HVDC, the harmonic contents of the voltage and current are very low, meaning the connection transformer and filter can be omitted. In addition, the VSC-HVDC can control active power and reactive power flexibly. Based on the advantages above, the VSC-HVDC has great prospect in the connection of renewable energy and is developing rapidly. Compared with traditional power loads, the loads in modern distribution systems are changing a lot, as more and more distributed sources and DC loads (almost all electronic products are DC loads) are connected to distribution systems. To handle such a change, a low-voltage DC (LVDC) microgrid is a good solution. Compared with a low-voltage AC system, there are no issues in terms of phase and frequency synchronization; consequently, the connection between different systems becomes simple and the reliability is increased. In addition, the conversion stages can be reduced, such that efficiency, volume, and cost are improved. Based on these advantages, the LVDC systems are also developing rapidly. As mentioned above, the HVDC and LVDC systems have been developing rapidly in recent years. However, the HVDC systems need to distribute their power to load and the LVDC systems need to connect with the power grid. If we still employ a medium-voltage AC (MVAC) power distribution system as an intermediate link, a line-frequency power transformation and ACeDC converter are also needed, which cause high loss, high cost, and high volume. In this case, a medium-voltage DC (MVDC) power distribution system will be a good choice as an intermediate link for HVDC and LVDC systems. In addition, just like with the application of HVDC and LVDC systems, the MVDC also has its own advantages and great prospects to integrate different renewable energies and power loads with a medium-voltage level. Therefore, considering that the MVDC power distribution system has great potential to integrate different DC systems, energy sources and power loads for Energy Internet, this chapter will give an introduction to this kind of technology.
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Application advantages and scenarios
As discussed in Section 6.1, the application scenarios of MVDC distribution systems can be summarized based on the development of HV power transmission and LV power distribution. However, the distribution features should be emphasized compared with HV power transmission, and the MV features should be emphasized compared with LV power distribution. As shown in Fig. 6.1, according to the shift from HV transmission to MV distribution, the application advantages and scenarios of MVDC distribution systems can be summarized as:
•
•
•
•
Providing distribution interface for HVDC transmission systems An MVDC distribution system can connect with an HVDC transmission system directly based on a DC transformer. Therefore, the transition link of the AC grid is not needed, meaning the conversion stages can be reduced and the reliability can be improved. Improving power quality of MVAC distribution systems An MVDC distribution system can isolate voltage drop, suppress harmonics, and compensate reactive power for an MVAC distribution system by controlling the operation of an acedc converter, such that the MVDC distribution system can improve the power quality of the MVAC distribution system. Improving power supply reliability and utilization An MVDC distribution system is an easy-to-form multiterminal power supply structure, in which the multiple power sources can compensate for and support each other to improve power supply reliability. In addition, compared with 1:1 cold standby for substations in a traditional AC grid, an MVDC distribution system can be used to achieve hot standby, so the power equipment utilization can be increased. Increasing power supply capacity With the same insulation level, wire cross section and current density, a DC distribution has a higher power supply capacity than an AC distribution, which also can increase the power supply radius. As shown in Fig. 6.2, according to the shift from LV distribution to MV distribution, the application advantages and scenarios of MVDC distribution systems can be summarized as: Providing a customizable power supply solution with high power quality
HVDC transmission
MVAC distribution
AC/DC VSC
MVDC distribution C
C
VS
MVDC distribution
VS
MVDC distribution
C VS
Fault DC transformer
VS C
•
Figure 6.1 Application scenarios of MVDC distribution for grid-level connection.
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MVDC distribution
DC/AC
High-power highsensitive ac loads
DC/DC
DC/DC
High-power high- High-power dcsensitive dc loads distributed sources
DC/DC or DC/AC
ac or dc microgrid
Figure 6.2 Application scenarios of MVDC distribution for load-level connection.
•
•
In a power grid, there are a lot of high-power AC loads, such as semiconductor factories, banks, data centers, etc., which are sensitive to power supply and have strict requirements with regard to reliability and power quality. Because of their high reliability and high power quality requirements, an MVDC distribution system is a good solution to provide customizable power supply for these sensitive loads. Providing a grid-connected interface for high-power DC sources In the future power grid, there will be a lot of high-power DC sources, such as electric vehicle charging stations, battery energy storage stations, and photovoltaic power stations. An MVDC distribution system can provide a grid-connected interface for them to reduce conversion stages, cost, and power loss. Providing a grid-connected interface for various LV microgrid systems In the future power grid, there will be a lot of LV AC or DC microgrid systems. Because there are no issues regarding phase and frequency synchronization for MVDC distribution systems, an MVDC distribution system can serve as a grid-connected interface to connect different LV microgrid systems. Then the reliability will be high and it can also satisfy the requirements of harmonic control, reactive power compensation, and energy feedback for microgrid systems.
6.3
System architecture technology
System architecture is related to safety, quality, economy, etc., for the power supply of an MVDC distribution system, and has significance in developing a standard of MVDC distribution systems. The system architecture of MVDC distribution systems contains the following key issues: topology, bus structures, grounding form, organization forms of loads and distributed sources, and connection forms between different buses.
6.3.1
Topology
As shown in Fig. 6.3, the main topology for an MVDC distribution system can be a radial topology, ring topology, or two-terminal topology. In general, the radial topology has the lowest reliability; however, the fault identification and control protection
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(b) (a) DC/AC
DC/DC
DC/AC
DC/DC
VSC VSC
DC/AC
VSC
DC/DC DC/AC
DC/DC
DC/AC
DC/DC
(c) VSC
VSC
DC/AC
DC/DC
DC/AC
DC/DC
Figure 6.3 Main topologies for MVDC distribution systems. (a) Radial topology. (b) Ring topology. (c) Two-terminal topology.
technologies of the others are difficult. The ring topology has the highest reliability but also the most difficult implementation for control and protection. In practice, the topology of an MVDC distribution system is the first issue that should be selected according to the system-level design requirements of the specific system.
6.3.2
Bus structure
Similar to the HVDC transmission, the typical MVDC bus structures are a unipolar asymmetry structure, unipolar symmetry structure, and bipolar structure, as shown in Fig. 6.4. Compared with unipolar asymmetry, unipolar symmetry has some advantages, such as a rated voltage that is half that of the unipolar asymmetry structure, and
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(a)
(b)
(c)
Vdc
+ Vdc/2
+ Vdc/2
0
– Vdc/2
– Vdc/2
0
Figure 6.4 Typical MVDC bus structure. (a) Unipolar asymmetry structure. (b) Unipolar symmetry structure. (c) Bipolar structure.
the DC bias of the transformer can be eliminated with the symmetry operation. The unipolar symmetry structure is more popular in MVDC distribution systems. Compared with the unipolar structure, the bipolar structure has three lines and can keep working even during a polar break, giving it a higher reliability. However, more converters are required to achieve a bipolar structure in an MVDC distribution system, meaning the cost will be high, so this kind of structure is usually used for specific systems that require high reliability or high voltage and high power. As for the LVDC bus, the typical structures are similar to the MVDC bus structures. Moreover, a multilayer structure can also be used, such as not only a 380-V bus, but also 48 and 5 V LVDC buses, to connect electronic products.
6.3.3
Grounding form
The ground terminal will provide a reference voltage for the whole MVDC distribution system. It is also the basis for designing insulation, the lighting arrester, etc. As for the grounding form in an MVDC distribution system, there are two issues: grounding location and grounding type.
6.3.3.1
Grounding location
For the unipolar asymmetry and bipolar structures in Section 6.3.2, the grounding location is simple and is usually the line with zero voltage. For the unipolar symmetry structure, there is no apparent grounding location, so one needs to be created artificially. There are two ways to this: grounding in the AC side or in the DC side. For grounding in the AC side, a neutral point can be grounded by a resistor directly if there is a neutral point for the transformer in the converter side; this way is simple and has the fewest additional components, as shown in Fig. 6.5(a). Otherwise, if there is no neutral point for the transformer in the converter side, the grounding location can be created by a Y-type inductor, as shown in Fig. 6.5(b). In this way, the Y-type inductor will consume a lot of reactive power. The consumed reactive power is great if the inductance is designed with a small value and the production is difficult if the inductance is designed with a large value. As for grounding in the DC side, because there are capacitors in the DC side for the traditional VSCs, such as two-level and three-level systems, the grounding location can be the neutral point of the DC capacitor, as shown in Fig. 6.5(c). However, for new VSC systems, such as modular multilevel converters (MMCs), there is no capacitor in the DC side, thus the clamping resistor should be used to create a neutral point, as shown in Fig. 6.5(d). In this way, the grounding performance is related to the value
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(a)
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(b) VSC
VSC
(c)
(d)
VSC
VSC
Figure 6.5 Grounding forms. (a) AC side grounding 1. (b) AC side grounding 2. (c) DC side grounding 1. (d) DC side grounding 2.
of the clamping resistor: the power loss is high if the resistance is designed with a small value and the grounding connection is weak if the resistance is designed with a large value. Considering fault recovery time and steady-state power loss, the grounding in the AC side is recommended for MVDC distribution systems based on MMC.
6.3.3.2
Grounding type
There are mainly two types of grounding in MVDC distribution systems: highresistance grounding and low-resistance grounding. In high-resistance grounding, the current flowing to the earth is low when the system operates in an unbalanced state; however, the overvoltage is high, as shown in Fig. 6.6. When there is a monopolar grounding fault, because the grounding resistance is high (equivalent to the open circuit in Fig. 6.6), the fault current is limited. However, because the VSC continue to control voltage in the DC side, the voltage of the nonfault polar line will increase to double the normal voltage, and the voltage of the neutral line will also increase to the rated voltage of the polar line. Considering line insulation, an MVDC distribution system usually does not allow operation in an unbalanced state for a long time, so the system will stop working. In fact, if we want to ensure the system can operate under a monopolar grounding fault state for a long time, the insulation level of the distribution lines should be increased, which will increase the cost. In low-resistance grounding, there is no overvoltage when the system operates in an unbalanced state; however, the fault current is high, as shown in Fig. 6.7. When there is a monopolar grounding fault, because the grounding resistance is low (equivalent to short circuit in Fig. 6.7), the DC line is short-circuited directly and the fault current is very high. This kind of grounding type does not cause overvoltage, so the insulation monitoring device cannot be installed, meaning the DC breaker should be used to cut off the fault current.
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DC
+ –
Vdc E DC
+ – Grid overvoltage 2 x DC
DC
+ –
Vdc E DC
DC
+ –
Vdc E DC
+ –
Figure 6.6 High-resistance grounding.
DC
+ –
E DC
+ –
Large earth fault current, affecting one of the poles I
DC
+ –
E DC
DC
+ –
E DC
Figure 6.7 Low-resistance grounding.
+ –
+ –
+ –
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In practice, the grounding form of an MVDC distribution system should be designed according to the system-level conditions and requirements.
6.3.4
Organization forms of distributed sources
The distributed sources used in practice include renewable energy, such as photovoltaic and wind energy, and non-renewable energy, such as fuel cells and gas turbines. The performances, in terms of voltage, power, controllable degree, etc., are different for different distributed sources, such that the converter topology for the connection of each source type is also different. Therefore, the organization form should be selected and optimized based on the practical situation in different application scenarios.
6.3.5
Connection forms between different buses
In an MVDC distribution system, because of the existence of distributed sources, the power exchange between the MVDC bus and the LV bus can be unidirectional or bidirectional according to different operation requirements of the system. Then the converter topology for the connection between different buses can also be different with different operation requirements. In addition, with the development of power electronics technology, the connection form between different buses can be linefrequency or high-frequency conversion, as shown in Fig. 6.8. The line-frequency transformer is used to achieve voltage matching and isolation, whereas the dceac converter is used to acquire line-frequency AC sources. Alternatively, the high-frequency transformer is integrated into the DCeDC converter directly to achieve voltage
(a)
MVDC bus
(b)
DC
MVDC bus
AC DC
LVDC bus
High-frequency integration
Line-frequency transformer
AC
DC
DC DC
DC DC
DC
LVDC bus
Figure 6.8 Different connection forms between different buses. (a) Line-frequency isolation. (b) High-frequency isolation.
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matching and isolation. In fact, because of the advantages of its high density, low cost, and low noise, power conversion technology based on high-frequency isolation is a research hotspot in DC distribution systems.
6.4
Key equipment technology
In an MVDC distribution system, the DC system needs to exchange power with the AC grid, different DC buses need to connect with each other, and various distributed sources, storage systems, and loads need to access the DC distribution system. Therefore, power electronics equipment will be the key to achieve voltage conversion and power management of different links. At present, the LV and low power converters that connect distributed sources, storage systems, and loads have been widely studied, showing that the MV and high-power converters that connect different DC and AC buses will be the key to MVDC distribution. In addition, the DC breaker is also a key equipment to ensure safe and reliable operation of an MVDC distribution system.
6.4.1
Voltage source converter
A VSC is the key equipment used to connect an MVDC distribution system with an AC distribution system. In VSCs, the two-level converter is the most common topology. However, because of the limitation of voltage and current capacity, it is difficult for the normal two-level converter to achieve high-voltage and high-power application directly; hence the switch-series technology should be used. That said, the dynamic balance issue is a difficulty for the switch series; moreover, the switch series only increases voltage leveldthe harmonic and power loss performances are not improved. After the two-level converter, a three-level neutral point clamped converter was proposed to achieve better harmonic and power loss performances; however, the switch-series technology is also needed to increase voltage and power for MVDC distribution. In recent years, a MMC has been proposed, which has been widely applied in HVDC transmission systems. The basic topologies of two-level, three-level, and modular multilevel converters are shown in Fig. 6.9. Table 6.1 gives a comparison of these topologies. Compared with
(a)
(b)
(c) SM2
SM2
SM2
SMn
SMn
SMn
SM2
SM2
SM2
SMn
SMn
SMn
Figure 6.9 Basic topologies for VSCs in MVDC distribution systems. (a) Two-level converter. (b) Three-level converter. (c) MMC.
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Table 6.1 Comparison of VSC topology Performance
Two-level and three-level VSCs
MMC
Technological difficulty
Switch-series increases technological difficulty
Module-series decreases technological difficulty
Harmonic
High
Low
Filter
Required
Not required
Switching frequency
High
Low
Power loss
High
Low
EMI
High
Low
Economy
Low
High
Flexibility
Low
High
Manufacturer
ABB
ABB, Siemens, Nanrui, Rongxin, etc.
two-level and three-level VSCs, an MMC employs module-series technology, which reduces technological difficulty and increases reliability. The output voltage of an MMC has good harmonic performance and the filter can be omitted. At the same time, the switching frequency of MMC is reduced greatly and the power loss is low. In fact, not only are there advantages in terms of performance, but the module-type IGBT for an MMC application is also more mature and the number of manufacturers is more than that of press-pack IGBT for two-level and three-level VSCs; therefore, an MMC will be a good choice for a VSC in an MVDC distribution application. Of course, the practical choice should be decided according to the requirements of the specific system.
6.4.2
DC transformer
In MVDC distribution, the DC transformer is a key equipment to achieve connection between the MVDC bus and LVDC buses. In fact, it is difficult for the DC transformer to realize power conversion through a simple magnetic transformer, which is widely used in AC distribution; instead, it should be based on power electronics technology. The solutions for a DC transformer in an MVDC distribution system can be divided into three kinds: switch-series type, modular-multilevel type, and multiple-module type, as shown in Fig. 6.10. Table 6.2 gives a basic comparison of these DC transformer topologies. The switchseries DC transformer employs series structure to increase the voltage level and a high-frequency scheme to provide voltage match and electrical isolation. When this topology has access to the LVDC bus, the LV converter employs the common full-bridge, that is m ¼ 1, so the power capacity is limited for the individual switch used in the
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(a)
(b)
(c) SM
SM
SM
SM
SM
SM
SM
SM
SM
SM
SM
SM
T
T
DC/DC
DC/DC
DC/DC
DC/DC
DC/DC
DC/DC
ISOS
ISOP
DC/DC
DC/DC
SM
SM
SM
SM
SM
SM
SM
SM
DC/DC
DC/DC
SM
SM
SM
SM
DC/DC
DC/DC
IPOS
IPOP
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Figure 6.10 Basic topologies for a DC transformer in an MVDC distribution system. (a) Switch-series type. (b) Modular-multilevel type. (c) Multiplemodule type.
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Table 6.2 Comparison of DC transformer Topology
Characteristics
Application
Difficulty
Seriesswitch type
Series-switch to increase voltage
MV/HV in both sides
Voltage sharing of series switches; high capacity of HF transformer
Modular multilevel type
Series-MMCsubunit to increase voltage
MV/HV in both sides
Voltage sharing of subunit; high capacity of HF transformer
Multiplemodule type
Series-H-bridge to increase voltage; parallel-Hbridge to increase power
MV/HV/LV
Voltage and power sharing of H-bridge
LVDC side. Furthermore, the voltage sharing technology of the series switches in the HV side is very difficult to implement. Compared to the series-switch type, the HV side of a multilevel-modular DC transformer employs the modular multilevel structure to increase the voltage level. The topology is appropriate for the application of MV/HV in both sides; however, the power capacity is also limited, for the power capacity of the high-frequency isolation transformer is limited. Also, similar to the series-switch type, when this topology has access to the LVDC bus, the individual cell is used; thus, the power capacity is further limited. Multiple module is a typical solution for a DCeDC converter to increase the voltage/power capacity in the DCeDC field, such as inputserieseoutput-series, input-serieseoutput-parallel, input-paralleleoutput-series, and input-paralleleoutput-parallel. For the multiple-module DC transformer, the adjustment of the power/voltage level is flexible and the modularity is high; moreover, to change the series/parallel scheme, the DC transformer can be applied in various voltage levels, making it a good choice for an MVDC distribution system.
6.4.3
DC breaker
In MVDC distribution, the DC breaker is a key protection equipment to prevent fault-spread and ensure safe operation. Different from in AC system, there is no natural zero-crossing point in a DC system, so the DC breaker technology is more difficult than AC breaker technology. At present, the solutions for a DC breaker can be divided into three kinds: mechanical type, power electronics type, and hybrid type, as shown in Fig. 6.11. Table 6.3 gives a basic comparison of the three types of DC breakers. The mechanical DC breaker has the lowest conduction loss; however, because resonant elements are required to cause the zero-crossing point, the circuit and control are complicated, the volume is bulky, the reliability is low, and the break time is long. The power electronics DC breaker has the simplest structure, in which the break time is very short, and the reliability is high; however, because the voltage drop of the power electronics
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(a)
(b)
G
L
DB1
C
CB
DB2
S DB1
MOV
DB2
MOV
(c) FD
DB2
DB1
MS MOV
Figure 6.11 Basic solutions for a DC breaker in an MVDC distribution system. (a) Mechanical type. (b) Power electronics type. (c) Hybrid type.
Table 6.3 Comparison of DC breaker Performance
Mechanical DC breaker
Power electronics DC breaker
Hybrid DC breaker
Voltage level
MV
LV
HV/MV
Commutation speed
Slow
Fast
Fast
Break ability
High
Low
Medium
Control
Difficult
Easy
Medium
Cost
Low
High
Medium
Reliability
Low
High
High
Conduction Loss (10 kV/1 kA)
<100 W
>10 kW
>2 kW (power electronics branch) <100 W (mechanical branch)
switch is high, the operation loss is also high. The hybrid DC breaker combines the advantages of the mechanical and power electronics DC breakers, so the break time is short, the reliability is high, and the operation loss is high, making it a good choice for MVDC distribution systems.
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Control technology
According to the analysis above, there are various buses, distributed sources and loads in an MVDC distribution system. Moreover, various power electronics converters are used to achieve voltage conversion and power management for these components, and the operation states will be different with different system-level requirements. Therefore, control technology plays a significant role to ensure effective operation of an MVDC distribution system. In this section, the control of an MVDC distribution system will be divided into three levels: unit level, microgrid level, and distribution grid level. The corresponding controls consist of power electronics converter control, multisource coordination control, and multibus network-level control.
6.5.1
Converter control
In an MVDC distribution system, the power electronics converters have different forms. According to the different operation states of distributed sources, loads, and distribution systems, the power electronics converters should control their voltage, current, and power. In fact, the control of interface converters between the LVDC bus and distributed sources and loads is simple, whereas the control of a VSC and a DC transformer is complicated. However, many control methods have been proposed and successfully used for VSCs and DC transformers, thus the converter control for an MVDC distribution system will not be a technical bottleneck.
6.5.2
Multisource coordination control
Compared with unit-level control of power electronics converters, the multisource coordination control is mainly related to microgrid control, which can be divided into two types: bus voltage control and power quality management.
6.5.2.1
Bus voltage control
In an MVDC distribution system, there are a lot of power electronics converters that are connected to the DC bus. Because the output impedance is inconsistent and control voltage differences exist, there will be circulating current between parallel sources. Therefore, the current sharing control should be employed to ensure the voltage stability of the DC bus. Fig. 6.12 gives typical current sharing control methods. In centralized control, a separate centralized controller is added to the whole parallel system, such that each parallel unit operates according to control signals from a centralized controller to ensure that the output has good consistency. The main problem with centralized control is low reliability, as the whole system stops working when a failure is detected in the centralized controller. There is no separate centralized control for master-slave control, which selects one unit as the master controller, meaning the others are slave controllers. Compared with centralized control, the reliability of the master-slave control is increased to a certain degree, though it is still low.
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(a)
(b)
Center controller
Converter 3
1 e
Converter 2
3
Lin
Line 1 Master Slave converter 1 converter 2
e
Line 2
Lin
Converter 1
Line 2
+
Slave converter 3
+
–
– Low voltage DC bus
Low voltage DC bus
(c)
(d)
+
Converter 3
Converter 2
Converter 1
Line 3 Converter 3
Line 2 Converter 2
Converter 1
Line 1
Communication bus
+
–
– Low voltage DC bus
Low voltage DC bus
Figure 6.12 Typical bus voltage control method. (a) Centralized control. (b) Master-slave control. (c) Nonemaster-slave control with communication. (d) Nonemaster-slave control without communication.
In nonemaster-slave control, every unit has the same control model and controls its own operation state by itself. The nonemaster-slave control can be divided into two types: control with communication and control without communication. In nonemaster-slave control with communication, there is a common line to transfer the information (such as current, active power, reactive power, etc.) of each unit. Then, the parallel control can be simplified; however, the communication line is easy to be disturbed, which decreases reliability. The nonemaster-slave control without communication changes output impendence by detecting its own current and voltage to maintain consistent output characteristics. Thus current sharing can be achieved. There is no communication between modules in this method, so the reliability is high, but its dynamic effectiveness is not that good. From the current research, the nonemaster-slave control is the development trend, especially droop
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control without communication, which conforms to the distribution characteristics of distributed systems, making it a research hotspot.
6.5.2.2
Power quality management
In an MVDC distribution system, the power of a microgrid is not high and the disturbance resisting ability is weak. There will be a lot of dynamic situations, such as sudden change of output power for distributed sources and loads, switching between grid-connection operation and isolation operation, etc., which may lead to voltage fluctuation and flicker. Furthermore, the control system may operate by mistake, causing the whole microgrid to potentially break down. At present, to prevent these kinds of situations, various energy storage systems (such as supercapacitors, flywheels, batteries, etc.) are used to manage power quality of the system. In addition, to assure the process of energy transition and energy current balance, it is necessary to manage and configure distributed sources, storage systems, and loads.
6.5.3
Multibus network-level control
When a lot of microgrid systems are connected to an MVDC distribution grid, their interaction will be complicated. To ensure stable and reliable operation of the MVDC distribution grid, the network-level control based on multiterminal multibus distribution system should be emphasized, such as the operational impact to an MVDC distribution grid with a highly permeable microgrid, and the optimization dispatch of AC and DC distribution systems. Nowadays, the system-level control mainly focuses on an LVDC microgrid; the network-level control of an MVDC distribution system needs to be further developed.
6.6
Protection technology
Compared with an AC distribution system, the system architecture and operation modes of DC distribution are different, so traditional protection solutions are not suitable for DC distribution systems. In addition, compared with HVDC transmission systems, there are more components in MVDC distribution systems, and the failure of any part can affect the reliable operation of the system. Therefore, the protection strategy will be more complicated in MVDC distribution systems. In general, the protection technology of MVDC distribution systems can be categorized in two ways: protection equipment and fault diagnosis and treatment. The most important protection equipment for an MVDC distribution system is a DC breaker, which has been introduced in Section 6.4. In addition, if we still use traditional AC plug and wiring board in an LVDC system, there will be a great electric arc when one pulls out or inserts plugs. Therefore, a new DC plug and wiring board are also key protection equipment to promote the development of DC distribution systems. The faults of MVDC distribution systems can be divided into bipole faults and ground faults, according to the fault type. These can be further divided into bus faults
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and branch faults, according to the fault position. Different faults have different protection priority, making their treatment methods different. Because the protection method in AC distribution cannot be used in DC distribution directly, new fault diagnosis and protection methods should be developed to ensure safe operation. An MVDC distribution system can keep working even after several components fail. In addition, based on reliable detection and fault clearing, the LVDC microgrid can still keep working despite disconnecting from the MVDC distribution bus when the distribution bus fails. Currently, although fault diagnosis and treatment methods have been widely discussed in literature, reliable and simple solutions are still emphasized to reduce difficulty. In particular, it is very difficult to diagnose failures when the neutral point is not connected with the ground or the DC line is very short. Overall, the protection technology of MVDC distribution systems still lacks standards and practical experiences, which should be researched in detail.
6.7
Practical medium-voltage DC Energy Internet systems in China
6.7.1
Medium-voltage DC Energy Internet system in Shenzhen
This section will take Baolong Industrial Park in China as an example to introduce an application scheme of the Energy Internet based on an MVDC distribution system.
6.7.1.1
Technical demands from Baolong Industrial Park
The application scenario of Baolong Industrial Park is shown in Fig. 6.13. The technical demands of the Energy Internet from Baolong Industrial Park mainly
Load
473 m
Load
829 m
PV station
1.9 km
Conductor plant
Conductor plant
1.2 km
1.5 km
Battery storage station
2.5 km
EV station
Biling power substation
Figure 6.13 Application scenario for Baolong Industrial Park.
Danhe power substation
Medium-voltage DC power distribution technology
141
focus on: (1) increasing power supply reliability; (2) improving power quality; and (3) integrating new energy, battery storage systems, and sensitive loads. The specific connection demands are as follows: • • • •
AC-sensitive loads, of which the voltage is 10 kV and the power is 8 MW. An AC microgrid system with high-power PV and AC loads, a voltage of 380 V and a power of 3.5 w þ5 MW. A DC microgrid system with PV and EV charging equipment and DC loads, a voltage of 400 V and a power of 2.5 MW; in this system, power is consumed in the DC mircogrid system and is not transferred to the grid. A DC microgrid system with a high power battery storage station, a voltage of 400 V, and a power of 4 w 4 MW.
6.7.1.2
Two-terminal “Hand in Hand” architecture
There is no DC transmission line in Baolong Industrial Park. Instead the system relies on 110 kV Biling AC Station and 110 kV Danhe AC Station as its main sources. To increase the reliability of the power supply, the topology of the Energy Internet based on MVDC distribution is designed as a two-terminal “hand in hand” structure, as shown in Fig. 6.14. In Fig. 6.13, the two-terminal AC sources connect with the MVDC distribution bus through VSCs to improve the power quality of the AC distribution grid. VSC1 and VSC2 are designed with an MMC structure, and the MVDC bus is designed using a unipolar symmetry structure to decrease the rated voltage of the distribution line and eliminate DC bias of the AC transformer. However, there is no natural neutral point in the DC side for the MMC, so the AC transformer is designed with a “O/Y” structure to create a neutral point in the AC side of the MMC. Then the neutral point can be grounded with the earth. In this system, we use high-resistance grounding to ensure that the system can keep working under a monopolar grounding fault.
6.7.1.3
Key equipment scheme
According to the technical demands, the converter equipment in this Energy Internet can be divided into four types, as shown in Table 6.4. Type 1: Interface for a 10 kV MVAC distribution bus, of which the power is 20 MVA, and interface for a 380 V AC microgrid bus, of which the power is 5 MVA. This type of converter has bidirectional power transfer ability and can achieve DC voltage control, AC voltage control, and power control functions according to the system-level operation command. Type 2: Interface for 10 kV AC sensitive loads, of which the power is 8 MVA. This type of converter does not need bidirectional power transfer ability, as the MVDC distribution bus will provide power. It just needs to achieve an AC voltage control function. Type 3: Interface for a 400 V DC microgrid without a power backflow requirement, of which the power is 2.5 MVA. This type of converter does not need bidirectional power transfer ability, as the MVDC distribution bus will provide power. It just needs to achieve an LVDC voltage control function.
142
10 kV ac
10 kV ac
±10 kV dc
±10 kV dc 110/10 kV
110/10 kV 1
Power station 1
4
3
2
VSC1
VSC2 #1
Transformer DC breaker
VSC
UVSC
1
Line
DC/DC converter
Bus
DC/AC converter
DC AC
UDCSST
#4 DC DC
#6
#7
DC load
AC sensitive loads
#5
VSC3 400 Vdc
10 kV ac
DC transformer
#1
#3
#2
EV charger
DC microgrid without power backflow requirement
Power station 2
DCSST 380 V ac
400 V ac #9
#8
PV
#10
AC load
PV
AC microgrid
DC microgrid with power backflow requirement
Figure 6.14 System architecture of Energy Internet based on an MVDC distribution system in Baolong Industrial Park. The Energy Internet
Medium-voltage DC power distribution technology
143
Table 6.4 Key converters for the Energy Internet in Baolong Industrial Park Type
Equipment
Interface object
Control function
1
VSC1 w VSC3
10 kV MVAC distribution bus, 380 V AC microgrid bus
DC voltage control, AC voltage control, power control
2
UVSC
10 kV AC sensitive loads
AC voltage control
4
UDCSST
400 V DC microgrid without power backflow requirement
LVDC voltage control
5
DCSST
400 V DC microgrid with power backflow requirement
MVDC voltage control, LVDC voltage control, power control
Type 4: Interface for a 400 V DC microgrid with a power backflow requirement, of which the power is 4 MVA. This type of converter has bidirectional power transfer ability and can achieve MVDC voltage control, LVDC voltage control, and power control functions according to the system-level operation command. In this system, all the VSCs are designed with an MMC structure to improve AC harmonics and efficiency, as shown in Fig. 6.15(a). To connect with a 20 kV DC bus, a 1200 V IGBT is used, and the DC link voltage of the submodule is 800 V. Then the number of submodules in the MMCs is 25. All the DC transformers are designed with multiple-module types based on a dual-active-bridge DCeDC converter to achieve high reliability and high dynamic response speed, as shown in Fig. 6.15(b). In addition, to ensure safe operation of the system and increase fault break speed and operation efficiency, the MVDC breaker in this system is designed to be a hybrid-type, as shown in Fig. 6.15(c). The break time is 5 ms.
6.7.1.4
Multifunctional operation ways
The Energy Internet not only can be powered by both VSC1 and VSC2 but also can be powered by only one of them when the other VSC fails. In addition, the operation way of the system may be changed when the line is broken or other equipment fail. Therefore, the control modes of the controlled equipment should change with the shift of the operation way of the system to ensure high reliability of the power supply. According to the system architecture in Fig. 6.13, the operation of the Energy Internet based on an MVDC distribution system can be designed in seven ways: two-terminal power supply operation, single-terminal power supply operation, twoterminal isolation operation, power support operation, STATCOM operation, backto-back operation, and island operation, as shown in Table 6.5. Operation ways are defined based on the control mode change of converters and topology change of the Energy Internet. Power flow direction does not affect operation ways of the system;
144
(b) SM1
SM1
SM2
SM2
SM2
SM25
SM25
SM25
SM1
SM1
SM1
SM2
SM2
SM2
(c) Mechanical switch
Dual active bridge DC/DC cell 1 S11
S13
Q11
Q13 PE switch
S12
S14
Q12
Q14
Dual active bridge DC/DC cell 2
LVDC microgrid bus
SM1
MVDC distribution bus
MVDC distribution bus
(a)
Balance Snubber
MOV
MOV
MOV
SM25
SM25
SM25
Dual active bridge DC/DC cell 25 The Energy Internet
Figure 6.15 Schemes of key equipment for the Energy Internet based on MVDC distribution systems. (a) VSC. (b) DC transformer. (c) DC breaker.
Medium-voltage DC power distribution technology
145
Table 6.5 Operation ways and control states of the Energy Internet system Operation ways
VSC1
VSC2
VSC3
DCSST
UVSC
UDCSST
Two-terminal power supply
Vdc&Q
P&Q
Vac/P
Vlvdc/P
Vac/P
Vlvdc/P
P&Q
Vdc&Q
Vdc&Q
e
Vac/P
Vlvdc/P
Vac/P
Vlvdc/P
e
Vdc&Q
Two-terminal isolation
Vdc&Q
Vdc&Q
Vac/P
Vlvdc/P
Vac/P
Vlvdc/P
Power support
Vdc&Q
Vac/P
Vac/P
Vlvdc/P
Vac/P
Vlvdc/P
Vac/P
Vdc&Q
Vac/P
Vac/P
Vdc&Q
Vlvdc/P
Vac/P
Vlvdc/P
Vac/P
Vhvdc
Single-terminal power supply
STATCOM
Vdc&Q
Vdc&Q
e
e
e
e
Back-to-back
Vdc&Q
P&Q
e
e
e
e
P&Q
Vdc&Q
e
e
Vdc&Q
Vlvdc/P
Vac/P
Vlvdc/P
Vac/P
Vhvdc
Island
the system should self-adapt bidirectional power flow. In Table 6.5, Vdc and Vac represent voltages in the DC and AC sides for the AC/DC converters, respectively; Vlvdc and Vmvdc represent voltages in the LVDC and MVDC sides for the DC/DC converters, respectively; P and Q represent active power and reactive power transfering from the MVDC bus to a converter, and vice versa., respectively; represents power transfer from the MVDC bus to a converter, and vice versa.
Two-terminal power supply operation When all the equipment operates normally, two-terminal AC sources supply power to the DC distribution bus through VSC1 and VSC2. In this operation, one VSC controls the DC bus voltage and another controls power. Specific control priority can be defined in advance. For the AC microgrid with power backflow ability, the interface converter VSC3 can be operated in AC voltage control or active power control mode. For the DC microgrid with power backflow ability, the interface converter DCSST can be operated in LVDC voltage control or active power control mode. For the AC loads, the interface converter UVSC can be operated in AC voltage control or active power control mode. As for the DC microgrid without power backflow ability, the interface converter UDCSST can be operated in LVDC voltage control or active power control mode; however, the power can simply flow from the DC distribution to the DC microgrid.
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The Energy Internet
Single-terminal power supply operation When one of VSC1 and VSC2 fails, the DC distribution bus can be powered by another normal VSC. Other equipment keeps the same operation modes with a twoterminal power supply operation way.
Two-terminal isolation operation When the DC distribution bus is opened, the two-terminal “hand in hand” distribution system can change into two single-terminal radial distribution systems. Both VSC1 and VSC2 operate in DC voltage control mode to power these two separate distribution systems.
Power support operation When AC stations lose power, to ensure an uninterruptible power supply for important loads that connect to them, the DC distribution system can provide short-term support to the AC stations. In this case, if just one AC station needs to be powered, the corresponding VSC operates in AC voltage control mode and another VSC operates in DC voltage control mode; if both AC stations need to be powered, then both VSC1 and VSC2 operate in AC voltage control mode and the MVDC distribution bus is controlled by VSC3 or DCSST.
STATCOM operation In STATCOM operation, there is no active power transmissiondVSC1 and VSC2 operate in STATCOM state to provide reactive power to the AC stations. It should be noticed that although VSC1 and VSC2 can also operate in reactive power mode, the main control object is still active power, so the reactive power compensation is limited.
Back-to-back operation In back-to-back operation, all the LV buses are disconnected from the MVDC distribution bus, and the two AC stations exchange power through VSC1, VSC2, and the MVDC buses. One VSC operates in DC voltage control mode and the other operates in power control mode. Compared with the reactive power compensation in STATCOM operation, there is mainly active power exchange between the two AC stations.
Island operation When both VSC1 and VSC2 fail, the MVDC distribution system can disconnect from the AC grid and operate in island operation, whereas VSC3 and DCSST will control the MVDC bus voltage. Island operation is similar to power support operation; however, the DC distribution does not provide power to the AC grid. In island operation, because the source power is not enough, loads with low priority should be cut off according to the predefined priority. Fig. 6.16 presents the changing principle of operation ways for the Energy Internet based on an MVDC distribution system. In practice, there is just unibidirectional changing between some operation ways. When two-terminal power supply operation
Medium-voltage DC power distribution technology
147
Figure 6.16 Changing principle of operation ways for the Energy Internet based on MVDC distribution.
changes to STATCOM operation, there will be a transition way, which is singleterminal power supply operation or two-terminal isolation operation. When the system changes between island operation and other grid-connected operations, rapid detection and configuration of the island state can be achieved based on passive detection of voltage and current, or active detection by injecting disturbance signals. Because the position of the broken lines may be different, the distributed sources, energy storage system, and loads may be different in the island, so the control system should configure the topology and changing control modes of the converters rapidly to achieve multi-island networking and ensure normal power supply of loads with high priority.
6.7.1.5
Protection scheme
According to the system architecture in Fig. 6.14, the possible fault types of the Energy Internet are as follows: 1. AC system fault, including AC transformer fault, AC line fault, etc. 2. VSC fault, including switch short-circuit fault, switch open-circuit fault, inductor shortcircuit fault, etc. 3. DC transformer fault, including switch short-circuit fault, switch open-circuit fault, highfrequency transformer fault, etc. 4. DC line fault, including monopolar grounding fault, dipolar short-circuit fault, DC line opencircuit fault, switch equipment fault, etc. 5. Control system fault.
Based on the fault types above, to simplify configuration and increase reproducibility, the protection system is divided into 13 districts, as shown in Fig. 6.17.
148
1
District 2
T1
DCM1
District 5
2
L1 DCM2 DCM3
L2
District 7
3 DCM5
DCM6
District 9
4 DCM9 DCM10
L3
L4
5
District 11
DCM12 DCM13 L5
DCB1
District 4
AC1
DCM16
6 District 12
DCF2
DCF3
DCF4
DCF5
DCF6
DCF7
T2 ACM3
DCM7
DCM11
DCM14
DCF9
DCF10
DCF11
DCF12
UDCSST
DCF13 T3
VSC3
T4
DCM8
ACM4
VSC2 DCF8
DCM4
UVSC
District 13
L
DCB2 DCF1
District 10
VSC1
ACB1
District 8
ACM1
L ACM2
District 3
District 6
District 1
ACB2 AC2
DCSST
DCF14 DCF15
ACB4 ACB3 ACM6
ACM5
DC load
AC sensitive loads
PV EV charger
DC microgrid without power backflow requirement
AC load
PV
DC microgrid with power backflow requirement
AC microgrid
The Energy Internet
Figure 6.17 Protection configuration for the Energy Internet based on MVDC distribution.
Medium-voltage DC power distribution technology
149
The AC system is a protection district, including an AC transformer and AC line. Every VSC is a protection district, including a converter and its accessory components. Every DC transformer is a protection district, including a converter and its accessory components. Every MVDC line is a protection district, including a line and DC breakers. Based on the protection design, the system can cover all faults under different operation ways.
6.7.2
Medium-voltage DC Energy Internet system in Zhuhai
6.7.2.1
Technical demands from Tangjiawan Science Park
In addition to the MVDC system in Shenzhen, another Energy Internet system based on MVDC distribution is being built in Tangjiawan Science Park in Zhuhai. It is the first demonstration project for a hybrid AC/DC distribution system with a multilevel grid (10 kV, 375 V, 400 V) in the world at present. The application scenario of Tangjiawan Science Park is shown in Fig. 6.18. The technical demands mainly focus on: (1) increasing power utilization of substations; (2) increasing power supply flexibility and reliability; (3) improving power quality; and (4) integrating PVs, battery storage systems, DC loads, and sensitive loads. The specific connection demands are as follows: AC sensitive loads with a power of 0.5 MW. DC loads with a power of 0.5 MW. Tangjia power substation PV station
Energy storage station
km 0.5
EV station
0.5 km
Threeterminal breaker
Figure 6.18 Application scenario for Tangjiawan Science Park.
1 km Jishan power substation II
1 km
AC loads
DC loads
1 km
N
Jishan power substation I
• •
150
• • •
The Energy Internet
PV station with a power of 1 MW. Energy station with a power of 0.5 MW. EV charge station with a power of 1 MW.
6.7.2.2
Three-terminal architecture
Different from the system in Baolong Industrial Park, for the Tangjia AC station, Jishan AC station I and II will be used as its main sources. The MVDC system in Tangjiawan Science Park has three terminals, as shown in Fig. 6.19. There are more new energy sources that can be used to support the main grid, so the power supply flexibility and reliability in Tangjiawan Science Park will be higher. In addition, the MVDC bus will still employ a unipolar symmetry structure, in which the voltage is 10 kV; however, the LVDC bus will employ a bipolar structure, in which the voltage is about 375 V. The result is that more LV loads can be easily integrated into the Energy Internet system.
6.7.2.3
Key equipment scheme
Just like in Baolong Industrial Park, the key equipment in Tangjiawan Science Park is a VSC, DC transformer, and DC breaker, such that the same scheme also can be used, as shown in Fig. 6.15. However, to further decrease the equipment volume and cost, a three-terminal DC breaker and a VSC with the integration of the DC breaker are employed, as shown in Fig. 6.20. In addition, an IGCT switch is used to build the VSC in the Tangjia power station to increase efficiency and overload capacity.
Jishan station I
10 MW
10 MW
±10 kV
Tangjia station
DC loads
±375 V
AC loads
1.5 MW
±375 V
1 MW 500 kW
Interface for future
10 MW
P3 EV
P2 ES
P1 PV
1 MW 0.5 MW 1 MW
Jishan station II
Figure 6.19 Three-terminal architecture based on the MVDC distribution system in Tangjiawan Science Park.
Medium-voltage DC power distribution technology
151
SM 1
SM 2
SM 2
SM N
C S2 (IGCT)
D2
SM N
Lc
Lc
Lc
Lc
Lc
+
ics1 TS
udc
SM 1
SM 1
SM 1
SM 2
SM 2
SM 2
SM N
SM N
SM N
SM
+
Mechanical switch
SM N
Lc
SC
CS1 +
ucs1
–
–
usc
–
PW switch Energy absorb branch in line side
D1
SM 1
SM 2
SM
Energy absorb branch in converter side
S1 (IGCT)
SM 1
imain
+10 kV
iline ics2 DS
–
ucs2 +
CS2
MVDC distribution bus
immc
–10 kV
Figure 6.20 VSC with the integration of a DC breaker based on IGCT.
6.7.2.4
Control scheme
To ensure the operation of the Energy Internet system in Fig. 6.14, the control system is designed as shown in Fig. 6.21. In the control system, the key equipment, including VSCs, DC transformers, and DC breakers, is controlled to establish stable MVDC and LVDC bus voltage, whereas other equipment and loads are self-controlled to achieve “plug and play.” In Fig. 6.21, the control system is divided into three layers. The top layer is the energy optimization layer. Under the given constraint conditions (such as system stability, voltage fluctuation limitation, equipment tolerance, etc.), the top layer uses optimal and economical operation as a target to dispatch energy between different sources and provide optimal commands to the middle layer. The middle layer is the central control layer. According to the optimal commands from the top layer and the real-time operation parameters, the middle layer provides mode, voltage, and power references to key equipment through logic and mathematical analysis. The bottom layer is the self-control layer. Key equipment controls its own state to achieve the control targets from the middle layer, whereas other controlled equipment controls its own state to connect the Energy Internet.
6.8
Summary
This chapter introduces the MVDC power distribution technology. The development background, application advantages and scenarios, system architecture and topology, key equipment, and control and protection technology are discussed. In particular, two practical MVDC Energy Internet systems in China are introduced. According to the introduction in this chapter, the MVDC power distribution system has advantages and great prospects to integrate different renewable energies and power loads with a medium-voltage level for the Energy Internet and is also a good choice as an intermediate link for HVDC and LVDC systems.
152
1. MVDC optimal dispatch 2. AC system optimal dispatch
Energy optimization
Optimal commands
1. System start and stop 2. Voltage coordination control 3. Operation ways management 4. Fault ride-through control
AC system real-time states
Energy management
MVDC real-time states
Operation feedback
Central control center
Central control
1000M ethernet
100M ethernet
Figure 6.21 Control structure for the Energy Internet based on MVDC distribution.
AC and DC loads
operation states
DG (uncontrolled)
The Energy Internet
Controlled equipments
Self-control
Energy stroage
Self-control
DG
Self-control
Key equipments
Self-control
Monitoring system
MVDC real-time states
Control modes, voltage, current
1. Current control 2. Voltage control 3. Power control 4. Modes control
AC system dispatch
Medium-voltage DC power distribution technology Biao Zhao 1 , Rong Zeng 1,2 , Qiang Song 1 , Zhanqing Yu 1 , Lu Qu 1,2 1 Department of Electrical Engineering, Tsinghua University, Beijing, China; 2 Energy Internet Research Institute, Tsinghua University, Beijing, China
Chapter Outline 6.1 Development background 124 6.2 Application advantages and scenarios 6.3 System architecture technology 126
125
6.3.1 Topology 126 6.3.2 Bus structure 127 6.3.3 Grounding form 128 6.3.3.1 Grounding location 128 6.3.3.2 Grounding type 129 6.3.4 Organization forms of distributed sources 131 6.3.5 Connection forms between different buses 131
6.4 Key equipment technology
132
6.4.1 Voltage source converter 132 6.4.2 DC transformer 133 6.4.3 DC breaker 135
6.5 Control technology
137
6.5.1 Converter control 137 6.5.2 Multisource coordination control 137 6.5.2.1 Bus voltage control 137 6.5.2.2 Power quality management 139 6.5.3 Multibus network-level control 139
6.6 Protection technology 139 6.7 Practical medium-voltage DC Energy Internet systems in China 6.7.1 Medium-voltage DC Energy Internet system in Shenzhen 140 6.7.1.1 Technical demands from Baolong Industrial Park 140 6.7.1.2 Two-terminal “Hand in Hand” architecture 141 6.7.1.3 Key equipment scheme 141 6.7.1.4 Multifunctional operation ways 143 6.7.1.5 Protection scheme 147 6.7.2 Medium-voltage DC Energy Internet system in Zhuhai 149 6.7.2.1 Technical demands from Tangjiawan Science Park 149 6.7.2.2 Three-terminal architecture 150 6.7.2.3 Key equipment scheme 150 6.7.2.4 Control scheme 151
6.8 Summary
151
The Energy Internet. https://doi.org/10.1016/B978-0-08-102207-8.00006-0 Copyright © 2019 Elsevier Ltd. All rights reserved.
140
124
6.1
The Energy Internet
Development background
In the beginning of the power grid, there was a debate between the use of AC and DC grids. However, AC grids won and became prevalent because the voltage and power levels of DC grids were very low and voltage conversion was very difficult at that time. In recent years, as the development of power semiconductor technology and power electronics technology has progressed, the technical limitations of DC grids are gradually being overcome, making the DC technology appear to be popular again. In DC grids, the DC transmission technology is a current research hotspot. This technology is divided into high-voltage DC (HVDC) based on a line-commutated converter (LCC-HVDC) and HVDC based on a voltage source converter (VSC-HVDC). Compared with the LCC-HVDC, commutation of the VSC-HVDC is independent of an AC grid, thus there is no commutation failure issue and it can supply power for a passive system. Because of the employment of pulse width modulation in the VSC-HVDC, the harmonic contents of the voltage and current are very low, meaning the connection transformer and filter can be omitted. In addition, the VSC-HVDC can control active power and reactive power flexibly. Based on the advantages above, the VSC-HVDC has great prospect in the connection of renewable energy and is developing rapidly. Compared with traditional power loads, the loads in modern distribution systems are changing a lot, as more and more distributed sources and DC loads (almost all electronic products are DC loads) are connected to distribution systems. To handle such a change, a low-voltage DC (LVDC) microgrid is a good solution. Compared with a low-voltage AC system, there are no issues in terms of phase and frequency synchronization; consequently, the connection between different systems becomes simple and the reliability is increased. In addition, the conversion stages can be reduced, such that efficiency, volume, and cost are improved. Based on these advantages, the LVDC systems are also developing rapidly. As mentioned above, the HVDC and LVDC systems have been developing rapidly in recent years. However, the HVDC systems need to distribute their power to load and the LVDC systems need to connect with the power grid. If we still employ a medium-voltage AC (MVAC) power distribution system as an intermediate link, a line-frequency power transformation and ACeDC converter are also needed, which cause high loss, high cost, and high volume. In this case, a medium-voltage DC (MVDC) power distribution system will be a good choice as an intermediate link for HVDC and LVDC systems. In addition, just like with the application of HVDC and LVDC systems, the MVDC also has its own advantages and great prospects to integrate different renewable energies and power loads with a medium-voltage level. Therefore, considering that the MVDC power distribution system has great potential to integrate different DC systems, energy sources and power loads for Energy Internet, this chapter will give an introduction to this kind of technology.
Medium-voltage DC power distribution technology
6.2
125
Application advantages and scenarios
As discussed in Section 6.1, the application scenarios of MVDC distribution systems can be summarized based on the development of HV power transmission and LV power distribution. However, the distribution features should be emphasized compared with HV power transmission, and the MV features should be emphasized compared with LV power distribution. As shown in Fig. 6.1, according to the shift from HV transmission to MV distribution, the application advantages and scenarios of MVDC distribution systems can be summarized as:
•
•
•
•
Providing distribution interface for HVDC transmission systems An MVDC distribution system can connect with an HVDC transmission system directly based on a DC transformer. Therefore, the transition link of the AC grid is not needed, meaning the conversion stages can be reduced and the reliability can be improved. Improving power quality of MVAC distribution systems An MVDC distribution system can isolate voltage drop, suppress harmonics, and compensate reactive power for an MVAC distribution system by controlling the operation of an acedc converter, such that the MVDC distribution system can improve the power quality of the MVAC distribution system. Improving power supply reliability and utilization An MVDC distribution system is an easy-to-form multiterminal power supply structure, in which the multiple power sources can compensate for and support each other to improve power supply reliability. In addition, compared with 1:1 cold standby for substations in a traditional AC grid, an MVDC distribution system can be used to achieve hot standby, so the power equipment utilization can be increased. Increasing power supply capacity With the same insulation level, wire cross section and current density, a DC distribution has a higher power supply capacity than an AC distribution, which also can increase the power supply radius. As shown in Fig. 6.2, according to the shift from LV distribution to MV distribution, the application advantages and scenarios of MVDC distribution systems can be summarized as: Providing a customizable power supply solution with high power quality
HVDC transmission
MVAC distribution
AC/DC VSC
MVDC distribution C
C
VS
MVDC distribution
VS
MVDC distribution
C VS
Fault DC transformer
VS C
•
Figure 6.1 Application scenarios of MVDC distribution for grid-level connection.
126
The Energy Internet
MVDC distribution
DC/AC
High-power highsensitive ac loads
DC/DC
DC/DC
High-power high- High-power dcsensitive dc loads distributed sources
DC/DC or DC/AC
ac or dc microgrid
Figure 6.2 Application scenarios of MVDC distribution for load-level connection.
•
•
In a power grid, there are a lot of high-power AC loads, such as semiconductor factories, banks, data centers, etc., which are sensitive to power supply and have strict requirements with regard to reliability and power quality. Because of their high reliability and high power quality requirements, an MVDC distribution system is a good solution to provide customizable power supply for these sensitive loads. Providing a grid-connected interface for high-power DC sources In the future power grid, there will be a lot of high-power DC sources, such as electric vehicle charging stations, battery energy storage stations, and photovoltaic power stations. An MVDC distribution system can provide a grid-connected interface for them to reduce conversion stages, cost, and power loss. Providing a grid-connected interface for various LV microgrid systems In the future power grid, there will be a lot of LV AC or DC microgrid systems. Because there are no issues regarding phase and frequency synchronization for MVDC distribution systems, an MVDC distribution system can serve as a grid-connected interface to connect different LV microgrid systems. Then the reliability will be high and it can also satisfy the requirements of harmonic control, reactive power compensation, and energy feedback for microgrid systems.
6.3
System architecture technology
System architecture is related to safety, quality, economy, etc., for the power supply of an MVDC distribution system, and has significance in developing a standard of MVDC distribution systems. The system architecture of MVDC distribution systems contains the following key issues: topology, bus structures, grounding form, organization forms of loads and distributed sources, and connection forms between different buses.
6.3.1
Topology
As shown in Fig. 6.3, the main topology for an MVDC distribution system can be a radial topology, ring topology, or two-terminal topology. In general, the radial topology has the lowest reliability; however, the fault identification and control protection
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(b) (a) DC/AC
DC/DC
DC/AC
DC/DC
VSC VSC
DC/AC
VSC
DC/DC DC/AC
DC/DC
DC/AC
DC/DC
(c) VSC
VSC
DC/AC
DC/DC
DC/AC
DC/DC
Figure 6.3 Main topologies for MVDC distribution systems. (a) Radial topology. (b) Ring topology. (c) Two-terminal topology.
technologies of the others are difficult. The ring topology has the highest reliability but also the most difficult implementation for control and protection. In practice, the topology of an MVDC distribution system is the first issue that should be selected according to the system-level design requirements of the specific system.
6.3.2
Bus structure
Similar to the HVDC transmission, the typical MVDC bus structures are a unipolar asymmetry structure, unipolar symmetry structure, and bipolar structure, as shown in Fig. 6.4. Compared with unipolar asymmetry, unipolar symmetry has some advantages, such as a rated voltage that is half that of the unipolar asymmetry structure, and
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(a)
(b)
(c)
Vdc
+ Vdc/2
+ Vdc/2
0
– Vdc/2
– Vdc/2
0
Figure 6.4 Typical MVDC bus structure. (a) Unipolar asymmetry structure. (b) Unipolar symmetry structure. (c) Bipolar structure.
the DC bias of the transformer can be eliminated with the symmetry operation. The unipolar symmetry structure is more popular in MVDC distribution systems. Compared with the unipolar structure, the bipolar structure has three lines and can keep working even during a polar break, giving it a higher reliability. However, more converters are required to achieve a bipolar structure in an MVDC distribution system, meaning the cost will be high, so this kind of structure is usually used for specific systems that require high reliability or high voltage and high power. As for the LVDC bus, the typical structures are similar to the MVDC bus structures. Moreover, a multilayer structure can also be used, such as not only a 380-V bus, but also 48 and 5 V LVDC buses, to connect electronic products.
6.3.3
Grounding form
The ground terminal will provide a reference voltage for the whole MVDC distribution system. It is also the basis for designing insulation, the lighting arrester, etc. As for the grounding form in an MVDC distribution system, there are two issues: grounding location and grounding type.
6.3.3.1
Grounding location
For the unipolar asymmetry and bipolar structures in Section 6.3.2, the grounding location is simple and is usually the line with zero voltage. For the unipolar symmetry structure, there is no apparent grounding location, so one needs to be created artificially. There are two ways to this: grounding in the AC side or in the DC side. For grounding in the AC side, a neutral point can be grounded by a resistor directly if there is a neutral point for the transformer in the converter side; this way is simple and has the fewest additional components, as shown in Fig. 6.5(a). Otherwise, if there is no neutral point for the transformer in the converter side, the grounding location can be created by a Y-type inductor, as shown in Fig. 6.5(b). In this way, the Y-type inductor will consume a lot of reactive power. The consumed reactive power is great if the inductance is designed with a small value and the production is difficult if the inductance is designed with a large value. As for grounding in the DC side, because there are capacitors in the DC side for the traditional VSCs, such as two-level and three-level systems, the grounding location can be the neutral point of the DC capacitor, as shown in Fig. 6.5(c). However, for new VSC systems, such as modular multilevel converters (MMCs), there is no capacitor in the DC side, thus the clamping resistor should be used to create a neutral point, as shown in Fig. 6.5(d). In this way, the grounding performance is related to the value
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(a)
129
(b) VSC
VSC
(c)
(d)
VSC
VSC
Figure 6.5 Grounding forms. (a) AC side grounding 1. (b) AC side grounding 2. (c) DC side grounding 1. (d) DC side grounding 2.
of the clamping resistor: the power loss is high if the resistance is designed with a small value and the grounding connection is weak if the resistance is designed with a large value. Considering fault recovery time and steady-state power loss, the grounding in the AC side is recommended for MVDC distribution systems based on MMC.
6.3.3.2
Grounding type
There are mainly two types of grounding in MVDC distribution systems: highresistance grounding and low-resistance grounding. In high-resistance grounding, the current flowing to the earth is low when the system operates in an unbalanced state; however, the overvoltage is high, as shown in Fig. 6.6. When there is a monopolar grounding fault, because the grounding resistance is high (equivalent to the open circuit in Fig. 6.6), the fault current is limited. However, because the VSC continue to control voltage in the DC side, the voltage of the nonfault polar line will increase to double the normal voltage, and the voltage of the neutral line will also increase to the rated voltage of the polar line. Considering line insulation, an MVDC distribution system usually does not allow operation in an unbalanced state for a long time, so the system will stop working. In fact, if we want to ensure the system can operate under a monopolar grounding fault state for a long time, the insulation level of the distribution lines should be increased, which will increase the cost. In low-resistance grounding, there is no overvoltage when the system operates in an unbalanced state; however, the fault current is high, as shown in Fig. 6.7. When there is a monopolar grounding fault, because the grounding resistance is low (equivalent to short circuit in Fig. 6.7), the DC line is short-circuited directly and the fault current is very high. This kind of grounding type does not cause overvoltage, so the insulation monitoring device cannot be installed, meaning the DC breaker should be used to cut off the fault current.
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DC
+ –
Vdc E DC
+ – Grid overvoltage 2 x DC
DC
+ –
Vdc E DC
DC
+ –
Vdc E DC
+ –
Figure 6.6 High-resistance grounding.
DC
+ –
E DC
+ –
Large earth fault current, affecting one of the poles I
DC
+ –
E DC
DC
+ –
E DC
Figure 6.7 Low-resistance grounding.
+ –
+ –
+ –
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In practice, the grounding form of an MVDC distribution system should be designed according to the system-level conditions and requirements.
6.3.4
Organization forms of distributed sources
The distributed sources used in practice include renewable energy, such as photovoltaic and wind energy, and non-renewable energy, such as fuel cells and gas turbines. The performances, in terms of voltage, power, controllable degree, etc., are different for different distributed sources, such that the converter topology for the connection of each source type is also different. Therefore, the organization form should be selected and optimized based on the practical situation in different application scenarios.
6.3.5
Connection forms between different buses
In an MVDC distribution system, because of the existence of distributed sources, the power exchange between the MVDC bus and the LV bus can be unidirectional or bidirectional according to different operation requirements of the system. Then the converter topology for the connection between different buses can also be different with different operation requirements. In addition, with the development of power electronics technology, the connection form between different buses can be linefrequency or high-frequency conversion, as shown in Fig. 6.8. The line-frequency transformer is used to achieve voltage matching and isolation, whereas the dceac converter is used to acquire line-frequency AC sources. Alternatively, the high-frequency transformer is integrated into the DCeDC converter directly to achieve voltage
(a)
MVDC bus
(b)
DC
MVDC bus
AC DC
LVDC bus
High-frequency integration
Line-frequency transformer
AC
DC
DC DC
DC DC
DC
LVDC bus
Figure 6.8 Different connection forms between different buses. (a) Line-frequency isolation. (b) High-frequency isolation.
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matching and isolation. In fact, because of the advantages of its high density, low cost, and low noise, power conversion technology based on high-frequency isolation is a research hotspot in DC distribution systems.
6.4
Key equipment technology
In an MVDC distribution system, the DC system needs to exchange power with the AC grid, different DC buses need to connect with each other, and various distributed sources, storage systems, and loads need to access the DC distribution system. Therefore, power electronics equipment will be the key to achieve voltage conversion and power management of different links. At present, the LV and low power converters that connect distributed sources, storage systems, and loads have been widely studied, showing that the MV and high-power converters that connect different DC and AC buses will be the key to MVDC distribution. In addition, the DC breaker is also a key equipment to ensure safe and reliable operation of an MVDC distribution system.
6.4.1
Voltage source converter
A VSC is the key equipment used to connect an MVDC distribution system with an AC distribution system. In VSCs, the two-level converter is the most common topology. However, because of the limitation of voltage and current capacity, it is difficult for the normal two-level converter to achieve high-voltage and high-power application directly; hence the switch-series technology should be used. That said, the dynamic balance issue is a difficulty for the switch series; moreover, the switch series only increases voltage leveldthe harmonic and power loss performances are not improved. After the two-level converter, a three-level neutral point clamped converter was proposed to achieve better harmonic and power loss performances; however, the switch-series technology is also needed to increase voltage and power for MVDC distribution. In recent years, a MMC has been proposed, which has been widely applied in HVDC transmission systems. The basic topologies of two-level, three-level, and modular multilevel converters are shown in Fig. 6.9. Table 6.1 gives a comparison of these topologies. Compared with
(a)
(b)
(c) SM2
SM2
SM2
SMn
SMn
SMn
SM2
SM2
SM2
SMn
SMn
SMn
Figure 6.9 Basic topologies for VSCs in MVDC distribution systems. (a) Two-level converter. (b) Three-level converter. (c) MMC.
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Table 6.1 Comparison of VSC topology Performance
Two-level and three-level VSCs
MMC
Technological difficulty
Switch-series increases technological difficulty
Module-series decreases technological difficulty
Harmonic
High
Low
Filter
Required
Not required
Switching frequency
High
Low
Power loss
High
Low
EMI
High
Low
Economy
Low
High
Flexibility
Low
High
Manufacturer
ABB
ABB, Siemens, Nanrui, Rongxin, etc.
two-level and three-level VSCs, an MMC employs module-series technology, which reduces technological difficulty and increases reliability. The output voltage of an MMC has good harmonic performance and the filter can be omitted. At the same time, the switching frequency of MMC is reduced greatly and the power loss is low. In fact, not only are there advantages in terms of performance, but the module-type IGBT for an MMC application is also more mature and the number of manufacturers is more than that of press-pack IGBT for two-level and three-level VSCs; therefore, an MMC will be a good choice for a VSC in an MVDC distribution application. Of course, the practical choice should be decided according to the requirements of the specific system.
6.4.2
DC transformer
In MVDC distribution, the DC transformer is a key equipment to achieve connection between the MVDC bus and LVDC buses. In fact, it is difficult for the DC transformer to realize power conversion through a simple magnetic transformer, which is widely used in AC distribution; instead, it should be based on power electronics technology. The solutions for a DC transformer in an MVDC distribution system can be divided into three kinds: switch-series type, modular-multilevel type, and multiple-module type, as shown in Fig. 6.10. Table 6.2 gives a basic comparison of these DC transformer topologies. The switchseries DC transformer employs series structure to increase the voltage level and a high-frequency scheme to provide voltage match and electrical isolation. When this topology has access to the LVDC bus, the LV converter employs the common full-bridge, that is m ¼ 1, so the power capacity is limited for the individual switch used in the
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(a)
(b)
(c) SM
SM
SM
SM
SM
SM
SM
SM
SM
SM
SM
SM
T
T
DC/DC
DC/DC
DC/DC
DC/DC
DC/DC
DC/DC
ISOS
ISOP
DC/DC
DC/DC
SM
SM
SM
SM
SM
SM
SM
SM
DC/DC
DC/DC
SM
SM
SM
SM
DC/DC
DC/DC
IPOS
IPOP
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Figure 6.10 Basic topologies for a DC transformer in an MVDC distribution system. (a) Switch-series type. (b) Modular-multilevel type. (c) Multiplemodule type.
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Table 6.2 Comparison of DC transformer Topology
Characteristics
Application
Difficulty
Seriesswitch type
Series-switch to increase voltage
MV/HV in both sides
Voltage sharing of series switches; high capacity of HF transformer
Modular multilevel type
Series-MMCsubunit to increase voltage
MV/HV in both sides
Voltage sharing of subunit; high capacity of HF transformer
Multiplemodule type
Series-H-bridge to increase voltage; parallel-Hbridge to increase power
MV/HV/LV
Voltage and power sharing of H-bridge
LVDC side. Furthermore, the voltage sharing technology of the series switches in the HV side is very difficult to implement. Compared to the series-switch type, the HV side of a multilevel-modular DC transformer employs the modular multilevel structure to increase the voltage level. The topology is appropriate for the application of MV/HV in both sides; however, the power capacity is also limited, for the power capacity of the high-frequency isolation transformer is limited. Also, similar to the series-switch type, when this topology has access to the LVDC bus, the individual cell is used; thus, the power capacity is further limited. Multiple module is a typical solution for a DCeDC converter to increase the voltage/power capacity in the DCeDC field, such as inputserieseoutput-series, input-serieseoutput-parallel, input-paralleleoutput-series, and input-paralleleoutput-parallel. For the multiple-module DC transformer, the adjustment of the power/voltage level is flexible and the modularity is high; moreover, to change the series/parallel scheme, the DC transformer can be applied in various voltage levels, making it a good choice for an MVDC distribution system.
6.4.3
DC breaker
In MVDC distribution, the DC breaker is a key protection equipment to prevent fault-spread and ensure safe operation. Different from in AC system, there is no natural zero-crossing point in a DC system, so the DC breaker technology is more difficult than AC breaker technology. At present, the solutions for a DC breaker can be divided into three kinds: mechanical type, power electronics type, and hybrid type, as shown in Fig. 6.11. Table 6.3 gives a basic comparison of the three types of DC breakers. The mechanical DC breaker has the lowest conduction loss; however, because resonant elements are required to cause the zero-crossing point, the circuit and control are complicated, the volume is bulky, the reliability is low, and the break time is long. The power electronics DC breaker has the simplest structure, in which the break time is very short, and the reliability is high; however, because the voltage drop of the power electronics
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(a)
(b)
G
L
DB1
C
CB
DB2
S DB1
MOV
DB2
MOV
(c) FD
DB2
DB1
MS MOV
Figure 6.11 Basic solutions for a DC breaker in an MVDC distribution system. (a) Mechanical type. (b) Power electronics type. (c) Hybrid type.
Table 6.3 Comparison of DC breaker Performance
Mechanical DC breaker
Power electronics DC breaker
Hybrid DC breaker
Voltage level
MV
LV
HV/MV
Commutation speed
Slow
Fast
Fast
Break ability
High
Low
Medium
Control
Difficult
Easy
Medium
Cost
Low
High
Medium
Reliability
Low
High
High
Conduction Loss (10 kV/1 kA)
<100 W
>10 kW
>2 kW (power electronics branch) <100 W (mechanical branch)
switch is high, the operation loss is also high. The hybrid DC breaker combines the advantages of the mechanical and power electronics DC breakers, so the break time is short, the reliability is high, and the operation loss is high, making it a good choice for MVDC distribution systems.
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137
Control technology
According to the analysis above, there are various buses, distributed sources and loads in an MVDC distribution system. Moreover, various power electronics converters are used to achieve voltage conversion and power management for these components, and the operation states will be different with different system-level requirements. Therefore, control technology plays a significant role to ensure effective operation of an MVDC distribution system. In this section, the control of an MVDC distribution system will be divided into three levels: unit level, microgrid level, and distribution grid level. The corresponding controls consist of power electronics converter control, multisource coordination control, and multibus network-level control.
6.5.1
Converter control
In an MVDC distribution system, the power electronics converters have different forms. According to the different operation states of distributed sources, loads, and distribution systems, the power electronics converters should control their voltage, current, and power. In fact, the control of interface converters between the LVDC bus and distributed sources and loads is simple, whereas the control of a VSC and a DC transformer is complicated. However, many control methods have been proposed and successfully used for VSCs and DC transformers, thus the converter control for an MVDC distribution system will not be a technical bottleneck.
6.5.2
Multisource coordination control
Compared with unit-level control of power electronics converters, the multisource coordination control is mainly related to microgrid control, which can be divided into two types: bus voltage control and power quality management.
6.5.2.1
Bus voltage control
In an MVDC distribution system, there are a lot of power electronics converters that are connected to the DC bus. Because the output impedance is inconsistent and control voltage differences exist, there will be circulating current between parallel sources. Therefore, the current sharing control should be employed to ensure the voltage stability of the DC bus. Fig. 6.12 gives typical current sharing control methods. In centralized control, a separate centralized controller is added to the whole parallel system, such that each parallel unit operates according to control signals from a centralized controller to ensure that the output has good consistency. The main problem with centralized control is low reliability, as the whole system stops working when a failure is detected in the centralized controller. There is no separate centralized control for master-slave control, which selects one unit as the master controller, meaning the others are slave controllers. Compared with centralized control, the reliability of the master-slave control is increased to a certain degree, though it is still low.
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(a)
(b)
Center controller
Converter 3
1 e
Converter 2
3
Lin
Line 1 Master Slave converter 1 converter 2
e
Line 2
Lin
Converter 1
Line 2
+
Slave converter 3
+
–
– Low voltage DC bus
Low voltage DC bus
(c)
(d)
+
Converter 3
Converter 2
Converter 1
Line 3 Converter 3
Line 2 Converter 2
Converter 1
Line 1
Communication bus
+
–
– Low voltage DC bus
Low voltage DC bus
Figure 6.12 Typical bus voltage control method. (a) Centralized control. (b) Master-slave control. (c) Nonemaster-slave control with communication. (d) Nonemaster-slave control without communication.
In nonemaster-slave control, every unit has the same control model and controls its own operation state by itself. The nonemaster-slave control can be divided into two types: control with communication and control without communication. In nonemaster-slave control with communication, there is a common line to transfer the information (such as current, active power, reactive power, etc.) of each unit. Then, the parallel control can be simplified; however, the communication line is easy to be disturbed, which decreases reliability. The nonemaster-slave control without communication changes output impendence by detecting its own current and voltage to maintain consistent output characteristics. Thus current sharing can be achieved. There is no communication between modules in this method, so the reliability is high, but its dynamic effectiveness is not that good. From the current research, the nonemaster-slave control is the development trend, especially droop
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control without communication, which conforms to the distribution characteristics of distributed systems, making it a research hotspot.
6.5.2.2
Power quality management
In an MVDC distribution system, the power of a microgrid is not high and the disturbance resisting ability is weak. There will be a lot of dynamic situations, such as sudden change of output power for distributed sources and loads, switching between grid-connection operation and isolation operation, etc., which may lead to voltage fluctuation and flicker. Furthermore, the control system may operate by mistake, causing the whole microgrid to potentially break down. At present, to prevent these kinds of situations, various energy storage systems (such as supercapacitors, flywheels, batteries, etc.) are used to manage power quality of the system. In addition, to assure the process of energy transition and energy current balance, it is necessary to manage and configure distributed sources, storage systems, and loads.
6.5.3
Multibus network-level control
When a lot of microgrid systems are connected to an MVDC distribution grid, their interaction will be complicated. To ensure stable and reliable operation of the MVDC distribution grid, the network-level control based on multiterminal multibus distribution system should be emphasized, such as the operational impact to an MVDC distribution grid with a highly permeable microgrid, and the optimization dispatch of AC and DC distribution systems. Nowadays, the system-level control mainly focuses on an LVDC microgrid; the network-level control of an MVDC distribution system needs to be further developed.
6.6
Protection technology
Compared with an AC distribution system, the system architecture and operation modes of DC distribution are different, so traditional protection solutions are not suitable for DC distribution systems. In addition, compared with HVDC transmission systems, there are more components in MVDC distribution systems, and the failure of any part can affect the reliable operation of the system. Therefore, the protection strategy will be more complicated in MVDC distribution systems. In general, the protection technology of MVDC distribution systems can be categorized in two ways: protection equipment and fault diagnosis and treatment. The most important protection equipment for an MVDC distribution system is a DC breaker, which has been introduced in Section 6.4. In addition, if we still use traditional AC plug and wiring board in an LVDC system, there will be a great electric arc when one pulls out or inserts plugs. Therefore, a new DC plug and wiring board are also key protection equipment to promote the development of DC distribution systems. The faults of MVDC distribution systems can be divided into bipole faults and ground faults, according to the fault type. These can be further divided into bus faults
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and branch faults, according to the fault position. Different faults have different protection priority, making their treatment methods different. Because the protection method in AC distribution cannot be used in DC distribution directly, new fault diagnosis and protection methods should be developed to ensure safe operation. An MVDC distribution system can keep working even after several components fail. In addition, based on reliable detection and fault clearing, the LVDC microgrid can still keep working despite disconnecting from the MVDC distribution bus when the distribution bus fails. Currently, although fault diagnosis and treatment methods have been widely discussed in literature, reliable and simple solutions are still emphasized to reduce difficulty. In particular, it is very difficult to diagnose failures when the neutral point is not connected with the ground or the DC line is very short. Overall, the protection technology of MVDC distribution systems still lacks standards and practical experiences, which should be researched in detail.
6.7
Practical medium-voltage DC Energy Internet systems in China
6.7.1
Medium-voltage DC Energy Internet system in Shenzhen
This section will take Baolong Industrial Park in China as an example to introduce an application scheme of the Energy Internet based on an MVDC distribution system.
6.7.1.1
Technical demands from Baolong Industrial Park
The application scenario of Baolong Industrial Park is shown in Fig. 6.13. The technical demands of the Energy Internet from Baolong Industrial Park mainly
Load
473 m
Load
829 m
PV station
1.9 km
Conductor plant
Conductor plant
1.2 km
1.5 km
Battery storage station
2.5 km
EV station
Biling power substation
Figure 6.13 Application scenario for Baolong Industrial Park.
Danhe power substation
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focus on: (1) increasing power supply reliability; (2) improving power quality; and (3) integrating new energy, battery storage systems, and sensitive loads. The specific connection demands are as follows: • • • •
AC-sensitive loads, of which the voltage is 10 kV and the power is 8 MW. An AC microgrid system with high-power PV and AC loads, a voltage of 380 V and a power of 3.5 w þ5 MW. A DC microgrid system with PV and EV charging equipment and DC loads, a voltage of 400 V and a power of 2.5 MW; in this system, power is consumed in the DC mircogrid system and is not transferred to the grid. A DC microgrid system with a high power battery storage station, a voltage of 400 V, and a power of 4 w 4 MW.
6.7.1.2
Two-terminal “Hand in Hand” architecture
There is no DC transmission line in Baolong Industrial Park. Instead the system relies on 110 kV Biling AC Station and 110 kV Danhe AC Station as its main sources. To increase the reliability of the power supply, the topology of the Energy Internet based on MVDC distribution is designed as a two-terminal “hand in hand” structure, as shown in Fig. 6.14. In Fig. 6.13, the two-terminal AC sources connect with the MVDC distribution bus through VSCs to improve the power quality of the AC distribution grid. VSC1 and VSC2 are designed with an MMC structure, and the MVDC bus is designed using a unipolar symmetry structure to decrease the rated voltage of the distribution line and eliminate DC bias of the AC transformer. However, there is no natural neutral point in the DC side for the MMC, so the AC transformer is designed with a “O/Y” structure to create a neutral point in the AC side of the MMC. Then the neutral point can be grounded with the earth. In this system, we use high-resistance grounding to ensure that the system can keep working under a monopolar grounding fault.
6.7.1.3
Key equipment scheme
According to the technical demands, the converter equipment in this Energy Internet can be divided into four types, as shown in Table 6.4. Type 1: Interface for a 10 kV MVAC distribution bus, of which the power is 20 MVA, and interface for a 380 V AC microgrid bus, of which the power is 5 MVA. This type of converter has bidirectional power transfer ability and can achieve DC voltage control, AC voltage control, and power control functions according to the system-level operation command. Type 2: Interface for 10 kV AC sensitive loads, of which the power is 8 MVA. This type of converter does not need bidirectional power transfer ability, as the MVDC distribution bus will provide power. It just needs to achieve an AC voltage control function. Type 3: Interface for a 400 V DC microgrid without a power backflow requirement, of which the power is 2.5 MVA. This type of converter does not need bidirectional power transfer ability, as the MVDC distribution bus will provide power. It just needs to achieve an LVDC voltage control function.
142
10 kV ac
10 kV ac
±10 kV dc
±10 kV dc 110/10 kV
110/10 kV 1
Power station 1
4
3
2
VSC1
VSC2 #1
Transformer DC breaker
VSC
UVSC
1
Line
DC/DC converter
Bus
DC/AC converter
DC AC
UDCSST
#4 DC DC
#6
#7
DC load
AC sensitive loads
#5
VSC3 400 Vdc
10 kV ac
DC transformer
#1
#3
#2
EV charger
DC microgrid without power backflow requirement
Power station 2
DCSST 380 V ac
400 V ac #9
#8
PV
#10
AC load
PV
AC microgrid
DC microgrid with power backflow requirement
Figure 6.14 System architecture of Energy Internet based on an MVDC distribution system in Baolong Industrial Park. The Energy Internet
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Table 6.4 Key converters for the Energy Internet in Baolong Industrial Park Type
Equipment
Interface object
Control function
1
VSC1 w VSC3
10 kV MVAC distribution bus, 380 V AC microgrid bus
DC voltage control, AC voltage control, power control
2
UVSC
10 kV AC sensitive loads
AC voltage control
4
UDCSST
400 V DC microgrid without power backflow requirement
LVDC voltage control
5
DCSST
400 V DC microgrid with power backflow requirement
MVDC voltage control, LVDC voltage control, power control
Type 4: Interface for a 400 V DC microgrid with a power backflow requirement, of which the power is 4 MVA. This type of converter has bidirectional power transfer ability and can achieve MVDC voltage control, LVDC voltage control, and power control functions according to the system-level operation command. In this system, all the VSCs are designed with an MMC structure to improve AC harmonics and efficiency, as shown in Fig. 6.15(a). To connect with a 20 kV DC bus, a 1200 V IGBT is used, and the DC link voltage of the submodule is 800 V. Then the number of submodules in the MMCs is 25. All the DC transformers are designed with multiple-module types based on a dual-active-bridge DCeDC converter to achieve high reliability and high dynamic response speed, as shown in Fig. 6.15(b). In addition, to ensure safe operation of the system and increase fault break speed and operation efficiency, the MVDC breaker in this system is designed to be a hybrid-type, as shown in Fig. 6.15(c). The break time is 5 ms.
6.7.1.4
Multifunctional operation ways
The Energy Internet not only can be powered by both VSC1 and VSC2 but also can be powered by only one of them when the other VSC fails. In addition, the operation way of the system may be changed when the line is broken or other equipment fail. Therefore, the control modes of the controlled equipment should change with the shift of the operation way of the system to ensure high reliability of the power supply. According to the system architecture in Fig. 6.13, the operation of the Energy Internet based on an MVDC distribution system can be designed in seven ways: two-terminal power supply operation, single-terminal power supply operation, twoterminal isolation operation, power support operation, STATCOM operation, backto-back operation, and island operation, as shown in Table 6.5. Operation ways are defined based on the control mode change of converters and topology change of the Energy Internet. Power flow direction does not affect operation ways of the system;
144
(b) SM1
SM1
SM2
SM2
SM2
SM25
SM25
SM25
SM1
SM1
SM1
SM2
SM2
SM2
(c) Mechanical switch
Dual active bridge DC/DC cell 1 S11
S13
Q11
Q13 PE switch
S12
S14
Q12
Q14
Dual active bridge DC/DC cell 2
LVDC microgrid bus
SM1
MVDC distribution bus
MVDC distribution bus
(a)
Balance Snubber
MOV
MOV
MOV
SM25
SM25
SM25
Dual active bridge DC/DC cell 25 The Energy Internet
Figure 6.15 Schemes of key equipment for the Energy Internet based on MVDC distribution systems. (a) VSC. (b) DC transformer. (c) DC breaker.
Medium-voltage DC power distribution technology
145
Table 6.5 Operation ways and control states of the Energy Internet system Operation ways
VSC1
VSC2
VSC3
DCSST
UVSC
UDCSST
Two-terminal power supply
Vdc&Q
P&Q
Vac/P
Vlvdc/P
Vac/P
Vlvdc/P
P&Q
Vdc&Q
Vdc&Q
e
Vac/P
Vlvdc/P
Vac/P
Vlvdc/P
e
Vdc&Q
Two-terminal isolation
Vdc&Q
Vdc&Q
Vac/P
Vlvdc/P
Vac/P
Vlvdc/P
Power support
Vdc&Q
Vac/P
Vac/P
Vlvdc/P
Vac/P
Vlvdc/P
Vac/P
Vdc&Q
Vac/P
Vac/P
Vdc&Q
Vlvdc/P
Vac/P
Vlvdc/P
Vac/P
Vhvdc
Single-terminal power supply
STATCOM
Vdc&Q
Vdc&Q
e
e
e
e
Back-to-back
Vdc&Q
P&Q
e
e
e
e
P&Q
Vdc&Q
e
e
Vdc&Q
Vlvdc/P
Vac/P
Vlvdc/P
Vac/P
Vhvdc
Island
the system should self-adapt bidirectional power flow. In Table 6.5, Vdc and Vac represent voltages in the DC and AC sides for the AC/DC converters, respectively; Vlvdc and Vmvdc represent voltages in the LVDC and MVDC sides for the DC/DC converters, respectively; P and Q represent active power and reactive power transfering from the MVDC bus to a converter, and vice versa., respectively; represents power transfer from the MVDC bus to a converter, and vice versa.
Two-terminal power supply operation When all the equipment operates normally, two-terminal AC sources supply power to the DC distribution bus through VSC1 and VSC2. In this operation, one VSC controls the DC bus voltage and another controls power. Specific control priority can be defined in advance. For the AC microgrid with power backflow ability, the interface converter VSC3 can be operated in AC voltage control or active power control mode. For the DC microgrid with power backflow ability, the interface converter DCSST can be operated in LVDC voltage control or active power control mode. For the AC loads, the interface converter UVSC can be operated in AC voltage control or active power control mode. As for the DC microgrid without power backflow ability, the interface converter UDCSST can be operated in LVDC voltage control or active power control mode; however, the power can simply flow from the DC distribution to the DC microgrid.
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The Energy Internet
Single-terminal power supply operation When one of VSC1 and VSC2 fails, the DC distribution bus can be powered by another normal VSC. Other equipment keeps the same operation modes with a twoterminal power supply operation way.
Two-terminal isolation operation When the DC distribution bus is opened, the two-terminal “hand in hand” distribution system can change into two single-terminal radial distribution systems. Both VSC1 and VSC2 operate in DC voltage control mode to power these two separate distribution systems.
Power support operation When AC stations lose power, to ensure an uninterruptible power supply for important loads that connect to them, the DC distribution system can provide short-term support to the AC stations. In this case, if just one AC station needs to be powered, the corresponding VSC operates in AC voltage control mode and another VSC operates in DC voltage control mode; if both AC stations need to be powered, then both VSC1 and VSC2 operate in AC voltage control mode and the MVDC distribution bus is controlled by VSC3 or DCSST.
STATCOM operation In STATCOM operation, there is no active power transmissiondVSC1 and VSC2 operate in STATCOM state to provide reactive power to the AC stations. It should be noticed that although VSC1 and VSC2 can also operate in reactive power mode, the main control object is still active power, so the reactive power compensation is limited.
Back-to-back operation In back-to-back operation, all the LV buses are disconnected from the MVDC distribution bus, and the two AC stations exchange power through VSC1, VSC2, and the MVDC buses. One VSC operates in DC voltage control mode and the other operates in power control mode. Compared with the reactive power compensation in STATCOM operation, there is mainly active power exchange between the two AC stations.
Island operation When both VSC1 and VSC2 fail, the MVDC distribution system can disconnect from the AC grid and operate in island operation, whereas VSC3 and DCSST will control the MVDC bus voltage. Island operation is similar to power support operation; however, the DC distribution does not provide power to the AC grid. In island operation, because the source power is not enough, loads with low priority should be cut off according to the predefined priority. Fig. 6.16 presents the changing principle of operation ways for the Energy Internet based on an MVDC distribution system. In practice, there is just unibidirectional changing between some operation ways. When two-terminal power supply operation
Medium-voltage DC power distribution technology
147
Figure 6.16 Changing principle of operation ways for the Energy Internet based on MVDC distribution.
changes to STATCOM operation, there will be a transition way, which is singleterminal power supply operation or two-terminal isolation operation. When the system changes between island operation and other grid-connected operations, rapid detection and configuration of the island state can be achieved based on passive detection of voltage and current, or active detection by injecting disturbance signals. Because the position of the broken lines may be different, the distributed sources, energy storage system, and loads may be different in the island, so the control system should configure the topology and changing control modes of the converters rapidly to achieve multi-island networking and ensure normal power supply of loads with high priority.
6.7.1.5
Protection scheme
According to the system architecture in Fig. 6.14, the possible fault types of the Energy Internet are as follows: 1. AC system fault, including AC transformer fault, AC line fault, etc. 2. VSC fault, including switch short-circuit fault, switch open-circuit fault, inductor shortcircuit fault, etc. 3. DC transformer fault, including switch short-circuit fault, switch open-circuit fault, highfrequency transformer fault, etc. 4. DC line fault, including monopolar grounding fault, dipolar short-circuit fault, DC line opencircuit fault, switch equipment fault, etc. 5. Control system fault.
Based on the fault types above, to simplify configuration and increase reproducibility, the protection system is divided into 13 districts, as shown in Fig. 6.17.
148
1
District 2
T1
DCM1
District 5
2
L1 DCM2 DCM3
L2
District 7
3 DCM5
DCM6
District 9
4 DCM9 DCM10
L3
L4
5
District 11
DCM12 DCM13 L5
DCB1
District 4
AC1
DCM16
6 District 12
DCF2
DCF3
DCF4
DCF5
DCF6
DCF7
T2 ACM3
DCM7
DCM11
DCM14
DCF9
DCF10
DCF11
DCF12
UDCSST
DCF13 T3
VSC3
T4
DCM8
ACM4
VSC2 DCF8
DCM4
UVSC
District 13
L
DCB2 DCF1
District 10
VSC1
ACB1
District 8
ACM1
L ACM2
District 3
District 6
District 1
ACB2 AC2
DCSST
DCF14 DCF15
ACB4 ACB3 ACM6
ACM5
DC load
AC sensitive loads
PV EV charger
DC microgrid without power backflow requirement
AC load
PV
DC microgrid with power backflow requirement
AC microgrid
The Energy Internet
Figure 6.17 Protection configuration for the Energy Internet based on MVDC distribution.
Medium-voltage DC power distribution technology
149
The AC system is a protection district, including an AC transformer and AC line. Every VSC is a protection district, including a converter and its accessory components. Every DC transformer is a protection district, including a converter and its accessory components. Every MVDC line is a protection district, including a line and DC breakers. Based on the protection design, the system can cover all faults under different operation ways.
6.7.2
Medium-voltage DC Energy Internet system in Zhuhai
6.7.2.1
Technical demands from Tangjiawan Science Park
In addition to the MVDC system in Shenzhen, another Energy Internet system based on MVDC distribution is being built in Tangjiawan Science Park in Zhuhai. It is the first demonstration project for a hybrid AC/DC distribution system with a multilevel grid (10 kV, 375 V, 400 V) in the world at present. The application scenario of Tangjiawan Science Park is shown in Fig. 6.18. The technical demands mainly focus on: (1) increasing power utilization of substations; (2) increasing power supply flexibility and reliability; (3) improving power quality; and (4) integrating PVs, battery storage systems, DC loads, and sensitive loads. The specific connection demands are as follows: AC sensitive loads with a power of 0.5 MW. DC loads with a power of 0.5 MW. Tangjia power substation PV station
Energy storage station
km 0.5
EV station
0.5 km
Threeterminal breaker
Figure 6.18 Application scenario for Tangjiawan Science Park.
1 km Jishan power substation II
1 km
AC loads
DC loads
1 km
N
Jishan power substation I
• •
150
• • •
The Energy Internet
PV station with a power of 1 MW. Energy station with a power of 0.5 MW. EV charge station with a power of 1 MW.
6.7.2.2
Three-terminal architecture
Different from the system in Baolong Industrial Park, for the Tangjia AC station, Jishan AC station I and II will be used as its main sources. The MVDC system in Tangjiawan Science Park has three terminals, as shown in Fig. 6.19. There are more new energy sources that can be used to support the main grid, so the power supply flexibility and reliability in Tangjiawan Science Park will be higher. In addition, the MVDC bus will still employ a unipolar symmetry structure, in which the voltage is 10 kV; however, the LVDC bus will employ a bipolar structure, in which the voltage is about 375 V. The result is that more LV loads can be easily integrated into the Energy Internet system.
6.7.2.3
Key equipment scheme
Just like in Baolong Industrial Park, the key equipment in Tangjiawan Science Park is a VSC, DC transformer, and DC breaker, such that the same scheme also can be used, as shown in Fig. 6.15. However, to further decrease the equipment volume and cost, a three-terminal DC breaker and a VSC with the integration of the DC breaker are employed, as shown in Fig. 6.20. In addition, an IGCT switch is used to build the VSC in the Tangjia power station to increase efficiency and overload capacity.
Jishan station I
10 MW
10 MW
±10 kV
Tangjia station
DC loads
±375 V
AC loads
1.5 MW
±375 V
1 MW 500 kW
Interface for future
10 MW
P3 EV
P2 ES
P1 PV
1 MW 0.5 MW 1 MW
Jishan station II
Figure 6.19 Three-terminal architecture based on the MVDC distribution system in Tangjiawan Science Park.
Medium-voltage DC power distribution technology
151
SM 1
SM 2
SM 2
SM N
C S2 (IGCT)
D2
SM N
Lc
Lc
Lc
Lc
Lc
+
ics1 TS
udc
SM 1
SM 1
SM 1
SM 2
SM 2
SM 2
SM N
SM N
SM N
SM
+
Mechanical switch
SM N
Lc
SC
CS1 +
ucs1
–
–
usc
–
PW switch Energy absorb branch in line side
D1
SM 1
SM 2
SM
Energy absorb branch in converter side
S1 (IGCT)
SM 1
imain
+10 kV
iline ics2 DS
–
ucs2 +
CS2
MVDC distribution bus
immc
–10 kV
Figure 6.20 VSC with the integration of a DC breaker based on IGCT.
6.7.2.4
Control scheme
To ensure the operation of the Energy Internet system in Fig. 6.14, the control system is designed as shown in Fig. 6.21. In the control system, the key equipment, including VSCs, DC transformers, and DC breakers, is controlled to establish stable MVDC and LVDC bus voltage, whereas other equipment and loads are self-controlled to achieve “plug and play.” In Fig. 6.21, the control system is divided into three layers. The top layer is the energy optimization layer. Under the given constraint conditions (such as system stability, voltage fluctuation limitation, equipment tolerance, etc.), the top layer uses optimal and economical operation as a target to dispatch energy between different sources and provide optimal commands to the middle layer. The middle layer is the central control layer. According to the optimal commands from the top layer and the real-time operation parameters, the middle layer provides mode, voltage, and power references to key equipment through logic and mathematical analysis. The bottom layer is the self-control layer. Key equipment controls its own state to achieve the control targets from the middle layer, whereas other controlled equipment controls its own state to connect the Energy Internet.
6.8
Summary
This chapter introduces the MVDC power distribution technology. The development background, application advantages and scenarios, system architecture and topology, key equipment, and control and protection technology are discussed. In particular, two practical MVDC Energy Internet systems in China are introduced. According to the introduction in this chapter, the MVDC power distribution system has advantages and great prospects to integrate different renewable energies and power loads with a medium-voltage level for the Energy Internet and is also a good choice as an intermediate link for HVDC and LVDC systems.
152
1. MVDC optimal dispatch 2. AC system optimal dispatch
Energy optimization
Optimal commands
1. System start and stop 2. Voltage coordination control 3. Operation ways management 4. Fault ride-through control
AC system real-time states
Energy management
MVDC real-time states
Operation feedback
Central control center
Central control
1000M ethernet
100M ethernet
Figure 6.21 Control structure for the Energy Internet based on MVDC distribution.
AC and DC loads
operation states
DG (uncontrolled)
The Energy Internet
Controlled equipments
Self-control
Energy stroage
Self-control
DG
Self-control
Key equipments
Self-control
Monitoring system
MVDC real-time states
Control modes, voltage, current
1. Current control 2. Voltage control 3. Power control 4. Modes control
AC system dispatch