- Email: [email protected]
Application of power electronics technologies to the 21st century's bulk power transmission in Japan
r~UTTERWORTH E I N E M A N N
I~I
Electrical Power & Energy Systems, Vol. 17, No. 3, pp. 181-193, 1995 Elsevier Science Ltd Printed in Great Britain 0142-0615(95)00009-7 0142-0615/95 $10.00 + 0.00
Application of power electronics technologies to the 21st century's bulk power transmission in Japan Y Sekine Science University of Tokyo, Japan
K Takahashi and 1' Hayashi Central Research Institute of Electric Power Industry, Japan
part of the country. Meanwhile, more generating plants are being installed in remote areas, as siting places near a load centre become more difficult to obtain. Moreover, the size of generating stations is getting larger. For these reasons, every indication shows that serious regional imbalance between supply and demand in each service area is becoming noticeable year by year. At the same time, it is becoming harder to predict various uncertain factors for the future, such as unexpected delay in new generating-plant construction, unanticipated change in annual peak demand growth, and so forth. This leads to a situation in which power utilities have to secure more flexibility against the uncertainties. To cope with this growing regional imbalance and increasing uncertainties, emphasis is being placed on the long-distance bulk transmission as well as the interchanges among neighbouring power utilities. Today, it is becoming necessary not only to maintain wide-area power system operation, but also to share the reserve margin among interconnected power utilities even at the stage of system expansion planning. From 1990 to 1992, a study committee on the 21st century vision of the interconnected power systems of Japan was conducted by both power utilities and universities under the Agency of Natural Resources and Energy (ANRE), national government organization. It was pointed out by the study committee that, concerning the interconnection between the 50 Hz and 60 Hz systems around the year 2010, inter-capacity of 2.0 to 2.5 GW will be required to meet increasing uncertain factors. It was also suggested that two power flows of 3.5 to 6.0 GW will be observed from both ends of the main island to the central part by the year 2010. By referring to this long-range prospect, each power utility has studied its own planning measures such as reinforcement of existing 500 kV transmission networks, construction of UHV AC trunk lines or HVDC transmission cables, and installation of new back-to-back
The electric power industry in Japan & now carrying out a nation-wide research and development programme for the effective application of power electronics technologies to the bulk transmission network and interconnected power systems for the 21st century. The programme will continue for eight years funded by ten power utilities and subsidy from the national government. The research and development programme specifically consists of the following three study projects." A C high-voltage/large-current transmission (feasibility study on enhancement measures for AC transmission capability, and verification test of their effectiveness using a power system simulator); multiterminal HVDC transmission (feasibility study on multiterminal HVDC transmission, and development and verification test of a prototype control/protection device); high performance AC/DC converter (development of a partial model for a 300 M W self-commutated converter, and field test of the developed partial model). Keywords: power system, power electronics, transmission, A C/DC converter
HVDC
I. I n t r o d u c t i o n Today, nine electric service areas in Japan are connected to each other through EHV transmission lines and two frequency converter stations. The total peak load amounted to approximately 150 GW in 1992. The interconnected network is divided into two parts of different system frequencies, almost at the centre of the main island. The western part is composed of six service areas of 60 Hz, and the eastern part is composed of three areas of 50 Hz. Figure I illustrates briefly the interconnected power systems in Japan. In recent years, power consumption has been concentrated in the metropolitan areas located in the central
181
182
Appfication of power electronic technologies to the 21st century. Y. Sekine
et al
~
annual ~ak load : 147 GW kwh consumption : 682 ~ (in 1992)
4 GW
~
DC Link
!i!~!!~!~!!if!!~!~!!!!i!~i!!i~!!!!!!!i!!~!!![55Z]
600~BTB
5 ~
k ~ 1 1 GW
('96)
13 GW
1
\
10 GW
)
,
28 GW
23
GW l u
\
51 GW
BTB 1400MW
DC Link (2001) 4 GW
300HW BTB ('97)
50Hz SystemI [45%3
Figure 1. Interconnected power systems in Japan
DC converter stations. It has already been determined that by the beginning of the 21st century, the present 500 kV transmission networks will be expanded further by forming double routes of 500 kV lines in the 60 Hz system, while operation of UHV (nominal: 1000 kV AC) transmission will be newly commenced in the 50 Hz system. The interconnection capacity between the 60 Hz and 50 Hz systems will be expanded from 0.9 GW at the present time to 1.2 GW in 1997 when the third frequency converter station will be newly installed. Another new back-to-back HVDC converter station of 0.3 GW will be installed in 1996 solely for the purpose of tie-line load flow control between two power utilities in the 60 Hz system. Concerning HVDC transmission, upgrading transfer capability from 0.3 GW to 0.6 GW at HokkaidoTohoku cable interconnection, submerged about 40 km long, was completed in 1993. Another possibility of 1.4GW HVDC cable, also submerged approximately 40 km long between Shikoku and Kansai, is now under study. Various research and development required for the long-term power system vision are now going on. Of course, in order to have more fully utilized existing AC transmission facilities, improvement of the conventional control schemes is being carried out. Enhancement of controlling performance of PSS (power system stabilizer) and SVC (static var compensator) is also under way, along with employment of new technologies such as SVC with self-commutation. Concerning long-distance bulk power transmission in the 50Hz system, new UHV technology is now extensively being developed to meet the reliability and environmental requirements. Some
UHV designed lines have already been put in use in 500 kV operation, and a UHV substation test facilities is also under construction. Now, as for HVDC transmission technologies, the above-mentioned study committee strongly pointed out that one of the most promising and indispensable measures will be the effective utilization of power electronics technologies in the design of trunk power systems in the 21st century. It was commonly recognized among the ten power utilities that in order to enhance the performance of power facilities efficiently in the future bulk transmission networks, including inter-regional interconnected systems, more attention should be paid to application of power electronics technologies, adoption of HVDC transmission systems, and development of new AC/DC converter equipment. In meeting the requirements, ten power utilities in Japan established a long-range research and development programme called 'Reinforced Power System Interconnection', in March 1992. This programme will continue for eight years, up to 1999, financially supported partly by subsidy from the national government and partly by funds from the power industry. Based upon this long-range programme, the 'Technical Committee on Interconnection Reinforcement Technology' was organized, sponsored by the ten power utilities and the Central Research Institute of Electric Power Industry (CRIEPI). In the fiscal years 1992 to 1999, the Committee will obtain a budget of about 9 billion yen in total from the ten power utilities and the ANRE. Presently, three cooperative research groups under the committee are being engaged in the following
Application of power electronic technologies to the 21st century: Y. Sekine et am three technical study projects:
Project 2: Multi-terminal HVDC transmission Applicability of multi-terminal HVDC to the future long-distance and bulk-power transmission will be focused upon. When a large scale HVDC transmission is adopted in power system planning, how to secure more flexibility is one of the essential problems. Multi-terminal HVDC is a key means of coping with this problem. For this reason, current technological progress and the actual operation in the multi-terminal HVDC transmission system in Japan and abroad will be surveyed. Advantages and disadvantages of the HVDC application will be evaluated through simulation studies on stable operation of a system fault using model power systems. In particular, operation performance of a converter at commutation failure will be analysed in order to assess the introduction to a practical power system and to identify possible technological problems for the development. In addition, to establish efficient control/protection schemes for multi-terminal HVDC transmission, a prototype control/protection device will be designed and manufactured, and then will be subjected to verification tests on a power system simulator.
(l) Project 1: AChigh-voltage/large-current transmission; (2) Project 2: Multi-terminal HVDC transmission; (3) Project 3: High performance AC/DC converter.
II. General o u t l i n e of t h e p r o g r a m m e I1.1 Research activities in three projects Project 1. AC high- w,ltage/large-current transmission In this project, emphasis will be placed on upgrading AC power transmission capability in a large power system, in particular lay employing power electronics technologies. With the aim of stability improvement in interconnected power systems and efficient utilization of power transmission facilities, the current progress in various measures utilizing power electronics in Japan and abroad will be reviewed. Effects in applying the enhancement measures will be analysed on the models of regionally interconnected systems in the future. Costs of the application will be evaluated for more specific identification of the research and development subjects. In addition, a miniature model of the control/protection device incorporating the most promising measure, identified by the evaluation studies, will be designed and manufactured, and the performance and effectiveness will be verified by using a power system simulator.
I
Project 3: High performance AC/DC converter A high performance AC/DC converter is expected to be incorporated effectively into power facilities such as power system stabilizing controllers and HVDC interconnection installations, as well as power generation plants of small dispersed type in the future. Technological developments for higher voltage and larger capacity rating, higher reliability and low loss performance
TECHNICAL COMMITTEEON REINFORCEDINTERCONNECTIONTECHNOLOGY advice "I funds
~
I T,-,.0w,. UTILITIES I-- i oii ni:Zt
EEEEEE~~E6~EII EE]E]EEEEE~EE E I~I}IIEIII~!~S6EEEEEEEEEE
i reporting ~
(coordination)
subsidy
..............
............... reporting
EEEEEE~EEEEEE E EE~IIIHHHIE]~EEEEEEE?EE)IEE E~T~E!E6EE]EEEEE"
NATIONALI
GOVERNMENT
):EEEEEEEEEE~E~I~IE~E E EEEEEE E EEEI}[~I]I~I]]]~EEEE]~:
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ======================================================================== :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
RI'SEARCHGROUP#1
RESEARCHGROUP#2
RESEARCHGROUP#3
Chief coordinator: KEPCO
Chief coordinator: EPDC
Chief coordinator: TEPCO
Deputy coordinator: TEPCO, CEPCO, CRIEPI
Deputy coordinator: TEPCO, KEPCO, CRIEPI
Deputy coordinator: CEPCO, KEPCO, EPDC CRIEPI
Member: Hokkaido-, Tohoku-, Hokuriku-, Chugoku-, Shikoku-, KyusyuPower Utilities and EPDC
Member: Hokkaido-, Tohoku-, Chubu-, Hokuriku-, Chugoku-, Shikoku-, and KyusyuPower Utilities
Figure 2. Organizations for research implementation
183
Member: Hokkaido-, Tohoku-, Hokuriku-, Chugoku-, Shikoku- and KyusyuPower Utilities
Appfication of power electronic technologies to the 21st century: Y. Sekine et al
184
will be implemented in this project. Studies on selfcommutated converters, which are strong against power system disturbances, will be implemented. More specifically, converter component technologies for series connection of elements, gate drive by high voltage pulse, and lost energy recovering at turn-on/off will be established. Verification tests will be also conducted for these technologies. In addition, concerning new elements of the converter, technological subjects for the development of self-commutated elements will be clarified by simulation analysis, prototype design and manufacture. Finally, field verification tests of the developed partial model will be performed on an actual power system. 11.2 Research organization The research activities for the three projects are to be implemented by three research groups under the structure presented in Figure 2. Four major parties in the electric power industry, Central Research Institute of Electric Power Industry (CRIEPI), Kansai Electric Power Company (KEPCO), Electric Power Development Company (EPDC), and Tokyo Electric Power Company (TEPCO), coordinate the whole research programme and preside over each research group. A committee designated the 'Technical Committee on Interconnect Reinforcement Technology', composed of experts and utility representatives, supervises all the research activities and gives advice to each research group.
11.3 Schedule of research programme The schedule of research activities described below is summarized in Table 1.
Project 1:A C high- voltage/large- current transmission In fiscal years 1992 and 1993, the present status in Japan and abroad will be surveyed. The technical merits and problems will be analysed and evaluated on model power systems. Development issues for enhancing transmission performance will also be identified. In the three years from fiscal years 1994 to 1996, a miniature sample model of the control/protection device will be developed and tested by using a power system simulator. Project 2: Multi-terminal HVDC transmission In fiscal year 1992 and fiscal year 1993, the current progress in Japan and abroad on multi-terminal HVDC transmission technologies will be surveyed. The feasibility of the introduction will be evaluated on model power systems. In fiscal years 1994 to 1996, a prototype of control/protection device will be developed and simulator tests will be implemented to assess the performance and effects in the practical application. Project 3: High performance A C/DC converter In fiscal year 1992, a component model (module) will be developed and tested. In fiscal years 1993 to 1995, the development and tests will be continued for a partial
Table 1. Schedule of research programme
Research item (1) AC (i) (ii) (iii) (iv) (v) (vi)
high-voltage/large-current transmission Survey on present progress Analysis and evaluation using model systems Cost evaluation Identification of research and development issues Development and verification of miniature sample model* Full-scale verification test
(2) Multi-terminal HVDC transmission (i) Survey on present progress (ii) Analysis and evaluation using model systems (iii) Identification of research and development issues (iv) Development and verification of control/protection device (v) Full-scale verification test (3) High performance AC/DC converter (i) Survey of features of converters and elements (ii) Development of converter • Development/tests of component model (module) • Development/tests of partial model (arm) • Study of control/protection device • 1/8 scale verification test (iii) Development of new element • Prototype designing and performance estimation • Prototype manufacturing and function tests • Development of elements
'92
'93
- -
'94
'95
'96
'97
'98
'99
Design, manufact., test
- - - D e s i g n , manufact., test
Analysis, design, manufact., test
*To be selected from self-commutated SVC, thyrister controller series capacitor, thyrister controlled phase shifter, and variable speed fly-wheel machine. • Intermediate check and review will be performed on the research achievements up to fiscal year 1994. Fundamental policy on fiscal year 1996 and after will be determined, including abortion of each project.
Application of power electronic technologies to the 21st century: Y. Sekine et al model (arm). Also, a st ady on control/protection devices will be pursued until[ fiscal year 1995. In addition, verification tests on an actual power system of a 1/8 scale model of 300 MW HVDC facility will be implemented by fiscal year 1999. On the other hand, for the new elements, prototype design and manufacturing will be conducted, and functional tests will be implemented by fiscal year 1995.
III. Project 1: AC high voltage/large current transmission In the future interconrLected power systems, application of power electronics technology is widely expected to play an important role as a new means for enhancing transmission performance. The objective of Project 1 is to identify effectiveness and related problems of the new technology. Suitable methods of application are also to be clarified in reference to the specific characteristics of each power system. For this objective, more specifically, the following targets are defined, and the associated technical studies and research activities are being implemented: (1) to assess the applicability of various measures for enhancing AC transmission performance; (2) to develop technologies needed for designing a control/protection device for the optimal control of a power system. In fiscal years 1992 to 1993, the state of the art on the measures for transmission performance enhancement and the current progress of their development work in Japan and abroad were surveyed. Based on the survey, the measures to be studied in Project 1 have been screened. Also, model power systems were decided upon and the applicability of the various measures have been evaluated on the model power systems.
185
II1.1 Survey of progress in Japan and abroad II1.1.1 Comparison of measures
The current status of the following conventional and new measures were reviewed partly through enquiry among the research group members: (1) upgrading to higher transmission voltage; (2) expansion to multiple transmission route; (3) stabilizing control by generators with PSS (power system stabilizer); (4) reactive power control including SVC (static var compensator); (5) real power control by TCSC (thyrister controlled series capacitor); (6) adoption of HVDC transmission, etc. The principle, features, effectiveness, operating modes, problems, etc. were reviewed for each measure, and comparison among these has been done with special respect to future possibilities. II 1.1.2 Survey of progress in overseas nations The current status of verification programmes of power electronics technology applied to power system control in the US (FACTS programme) was surveyed by direct interviews. This time, the present situation on a thyrister controlled series capacitor was investigated at Slatt Substation, BPA, at Kayenta Substation, WAPA, and at Kanawha River Substation, AEP. 111.1.3 Identification of study measures Based on the above two surveys, various measures to improve transmission performance were evaluated, taking into consideration factors such as features of the power system to which the measures would be applied, feasibility of the technologies, and applicability of the measures (cost, environment, etc.). Some conventional means such as voltage upgrading were also selected for comparative study. Table 2 presents the measures to be studied in this research project.
Table 2. Measures to be studied and their priority
Measures to be studied
Priority
Conventional means
Upgraded voltage Multiple route
B B
Generation control
Ultra-quick response exciter with PSS (power system stabilizer)
B
Reactive power control
Conventional SVC (static var compensator) Self-commutated SVC
C A
Real power control
TCSR (thyrister controlled series capacitor) TCPS (thyrister controlled phase shifter) TCDR (thyrister controlled damping resistor) SMES (superconducting magnetic energy storage) Variable speed fly-wheel machine
A A C C A
Others (HVDC)
HVDC including BTB (back-to-back) Self-commutated HVDC (including BTB)
B C
A: for major study, B: for comparative study, C: for supplementary study
1 86
Application of power electronic technologies to the 21st century: Y. Sekine et al
II1.1.4 Selection of analytical tool In order to make it possible to compare the effectiveness of various measures, mathematical models of the measures should be formulated and incorporated in the analytical tool. A survey was conducted in Japan and abroad on analytical tools to be used in the research works of Project 1. As a result, the 'Power System Dynamics Analysis Program' developed by CRIEPI was selected in terms of the following points: (l) to be capable of simulating all the measures to be studied; (2) to be available easily anytime and anywhere in Japan. The effectiveness of the CRIEPI's software has been tested and verified on different examples of the measures to be compared and evaluated.
II 1.2 Analysis and evaluation on model systems II 1.2.1 Definition of model systems Two simple power system models illustrated in Figure 3 have been defined in view of the image of inter-regional interconnected power systems of Japan in the future (around 2010). This was done with emphasis on the features of the model power systems so that the effects on stability enhancement by various thyrister controlled equipment can be clearly identified. Realistic conditions of the power systems under planning at each utility company were taken into consideration in the model systems. The radial power system model is based on the image of the 60 Hz interconnected power system, in which six double circuit 500kV transmission lines are interconnected in tandem, and the total generation is 115 GW;
i!!!iiiii i : i i;Eii iiiiiii i i i: iii iiiiiiil ( "Sm0w' / TOTAL GENERATION = 115 GW TOTAL LOAD = 115 GW
\ L = 6 GW / ~._ J
5ookv I 2cct ~200kin
,oo v
I
YS, M oo ,_j TOTALGENERATION = 86.5 GW TOTAL LOAD = 86.5 GW
500kV I 2cct J 120kin
lO00kV ~
500kV I I
/
lO00kV
500kV I I
\
5OOkV
Figure 3. Interconnected power system models
/
500kV II
\
5ookv
/
\
Appfication of power electronic technologies to the 21st century: Y. Sekine et al whereas the looped power system has been defined on the image of the 50 Hz :system, in which seven sub-systems are interconnected by 1000 kV and 500 kV transmission lines of double circuits, with total generation of 68.5 GW. 111.2.2 Simulation analysis
Stability enhancement effects of various measures applied to the two interconnected power system models were comparatively evaluated by using CRIEPI's analysis tool. Broadly speaking, power system stability is classified into transient stability, dynamic stability and steady state stability. In this analysis, only transient stability was considered, and the behaviour of a power system under serious disturbance due to a system fault was analysed. Specifically, the purpose of the simulation was to check whether or not the target amount of power exchange between two subsystems in each model can be stably achieved by each measure against all the given disturbances. The results of the simulation analysis are summarized in Table 13.The minimum required capacity of each measure is presented in this table. Radial power system model Power flow of 3 GW from System 1 to System 6 was set as the target value of power exchange. The power supply in System 1 was made to increase. It should be noted that, as indicated in t]ae middle column of Table 3, the minimum capacity required for series equipment (TCSC, TVPS) is smaller than that of parallel equipment (SVC, TCDR), with the exception of DC interconnection. Looped power systero model Power generation in System IV was assumed to be shifted to System I, and the incremental power flow from System I to System VI was se,t at the target value of 3 GW. It was shown that the equipment capacity required for the measures is minimum with TCPS, as indicated in the right-hand column of Table 3.
187
111.2.3 Evaluation The effectiveness of various measures was studied in terms of the transfer capability enhancement and generating capacity reallocations by analysing the two model power systems typically representing the future interconnected power systems of Japan. Although the analysis results indicated an encouraging effectiveness of all the methods, the following four promising measures were identified as the major subjects with high priority, taking into consideration other factors such as equipment cost (Table 3): (1) (2) (3) (4)
self-commutated SVC (static var compensator); TCSR (thyrister controlled series capacitor); TCPS (thyrister controlled phase shifter); variable speed fly-wheel machine.
It was also noticed that evaluation of their effectiveness should be implemented from viewpoints other than transient stability, such as power flow control and voltage stability, considering the further development requirements for their commercialization.
IV. Project 2: Multi-terminal HVDC transmission In adopting an HVDC transmission system in longdistance/bulk-power networks, assurance of power system reliability is one of the main issues. A highly reliable control/protection scheme is also indispensable. In addition, it is commonly thought that a multi-terminal HVDC system will be needed for the future power systems of Japan because it provides more flexibility to power system operation and configuration. For this reason, the capability of the multi-terminal HVDC transmission system to operate stably against disturbances such as AC and DC system faults is the main subject of Project 2.
Table 3. Summary of analysis results on two system models
System model
Radial system model
Looped system model
Purpose
To increase transfer capability from Sys. I to Sys. VI by 3 GW
To move generation source in Sys. IV to Sys. I by 3 GW
___2100 MVA + 2100 MVA 600 MVA (17f~) 500 MVA (+ 10°) 3000 MW 900 MW
-I-1500 -I- 1500 1600 150 2700 900
Measure
SVC (line-commutated) SVC (self-commutated) TCSC TCPS TCDR Fly-wheel* Upgraded voltage Multiple route Exciter with PSSt DC (line-commutated) DC (self-commutated)
500 500 180 3000 3000
Note: The minimum required capacity (or length) of each measure is presented. *Variable speed fly-wheel machine. t Ultra-quick response exciter with PSS.
km km MVA MW MW
200 30 2700 900
MVA MVA MVA (6.3f~) MVA (+0.9 °) MW MW -km MVA MW MW
188
Application of power electronic technologies to the 21st century: Y. Sekine et al
The studies are conducted with emphasis on the power system models representing the future bulk power transmission networks of Japan. With this objective, the following targets were set up for the research study: (1) to select the control/protection schemes which enable the keeping of stable operation against disturbances; (2) to develop a prototype control/protection device and perform the verification tests. So far, examples of a control/protection device employed in a multi-terminal system in overseas countries have been surveyed. Analysis and evaluation by means of model system simulation have been performed so as to identify the challenging subjects for technological development. IV.1 Survey of progress in Japan and abroad IV.1.1 Multi-terminal HVDC transmission and control
technologies Multi-terminal control system Six typical control schemes have been proposed. They are originally based on a scheme of constant-current vs. constant-voltage, or constant-power vs. constantmarginal-angle. These schemes are generally adopted in a two-terminal system and have been extended to a multi-terminal system. Also, another control system is proposed incorporating VDCOL (voltage dependent current order limiter) or alpha-limiter characteristics on the inverter side. This control system can maintain stable operation without a signal transmission channel even in the case when the system voltage drops or one terminal is lost.
Power system control Both in a two-terminal system and a multi-terminal system, new DC power controls are envisaged. These aim at the optimal condition of interconnected AC systems, such as frequency control and system stability control. The examples are frequency control at one terminal at Quebec-New England and at Sacoi, generator instability prevention control and AC system stabilization control, both under study at Mead-Phoenix. Signal transmission channel High reliability in the signal transmission channel, adequately coordinated with control/protection schemes, is essential in a multi-terminal HVDC system. It is a fact that the unavailability factor (outage rate) of a signal transmission channel, which has a double route configuration consisting of microwave and optical fibre, is seemed to be almost equivalent to that of a redundant control system with duplicated control computers. Against momentary disruption of one train in a signal transmission channel, either means of train lock or output lock would be effective; whereas, against the anomaly of two trains, withholding previous data to continue operation would be available. DC circuit breaker A verification test on an actual power system was conducted on DC 400 kV, 14(K)-4000 A circuit breakers at EPRI, WH and BBC. Whereas, in Japan a prototype circuit breaker rated at DC 250kV, 1200-8000A has
been developed, and completed with a shop verification test. Effects of DC circuit breakers were analysed by computer simulation on a model power system. As a result, there is a prospect of obtaining a DC circuit breaker that could substantially reduce the fault duration time, in comparison with the combination of a high-speed line switch and a control system.
IV.1.2 Multi-terminal HVDC facilities in overseas nations Quebec-New England In order to export the abundant hydro-electric power of Quebec to the US, a five terminal DC system ( _ 500 kV, 2250 MW) was constructed and commissioned in 1992. It was found out later that a commutation failure would not be recovered at two terminals connected to weak AC power systems. This multi-terminal system will be operated with two or three terminals for the time being. Power control and connection/disconnection operation can be performed at each converter station even when a signal transmission channel gets into outage.
Sardinia-Corsica-Italy The third terminal of 5 0 M W was installed and commissioned in 1987 in Corsica between the existing Sardinia-Italy two-terminal HVDC transmission ( _ 200 kV, 300 MW). The converter station in Corsica is being operated with a large marginal angle (40 °) so as to prevent a commutation failure. The emergency control system is so designed that when a commutation failure occurs due to reduced system voltage or other causes, the whole system will be shut down at once, and Corsica will be restarted after the AC voltage has been established at the other terminals. Others Other examples of the multi-terminal HVDC system include the new and old converter stations in parallel at Pacific Inter-Tie, and the emergency parallel operation of converters at Nelson River and Itaipu.
IV.2 Analysis and evaluation on model systems IV.2.1 Model systems and analysis conditions
Application of a three-terminal HVDC system to bulk power transmission from remote generating sources was assumed, and three simple model systems illustrated in Figure 4 were selected. Model A is an AC/DC combined system having one generating source, in which a power of 13 G W is transmitted through hybrid routes of a 1000kV AC transmission line and a _+500kV DC transmission line, and the receiving-end systems are interconnected by two links. On the other hand, Model B is a system having a single route of DC transmission line from one generating source. Model B is the case of the elimination of the AC transmission line from Model A. Model C is also a system having a DC route, and power transmission from two generating sources is assumed in this model. The fault analyses were conducted under the conditions of AC system fault, DC line fault and AC/DC converter terminal fault. In order to clarify the difference in the control schemes, the assumed faults are irrelevant to
Appfication of power electronic technologies to the 21st century." Y. Sekine et al
MODEL 300km
__+500kV bipole 2 c c t
II
UHV 200km
I i
a
5GW~ ~ INV~
UHV 200km
~
,I, ---
~
~
I
I
300km
___500kV bipole 2cct
REC [ ] IOGW
2
I
I
MODEL
II UHV 200km 5GWE~
INV~
AI
UHV 600km
I,I
J I i
5GW~ INVl ~
900km
189
900km
BI
UHV 200km
_~L II i,i ~ 5GW~]
I
REC
I
[] IOGW
INVl _~~
S IOGW@
I
I
I
48GW
MODEL 5GW I
cl
REC2
SGW
300km
±500kV bipole
I
UHV 200km IOGW~
i
I
2cct
1
UHV 200km
1, I
900km
I,
REC1
~
~ 5GW
INV.___~%
I
I
I
IOGW
Figure 4. System models for multi-terminal HVDC transmission
the signal transmission channels. Namely, analytical conditions in this study are as follows: (1) AC system fault: three phase line-to-ground near converter terminal and lines left not reclosed; (2) DC system fault: one line-to-ground and converter is restarted; (3) converter terminal fault: one-pole outage and converter is not restarted.
IV.2.2 Multi-terminal control scheme
In assuring stable operating performance of a DC multi-terminal transmission system, the control system plays an important role. It should be less affected by changes in operating condition of the interconnected power systems. Of various multi-terminal control schemes being researched in Japan and abroad, two control schemes, a voltage margin scheme and a
190
Application of power electronic technologies to the 21st century: Y. Sekine et al
Table 4. Performance of two typical control schemes Scheme
Basic configuration and operation
Voltage margin scheme
(1) A constant-current control circuit (ACR) and a constant-voltage control circuit (AVR) are both provided at each terminal. (2) The current setting value at each terminal is determined taking into account the current margin (A/d). (3) The voltage setting values of all terminals are the same except at one terminal which defines the DC system voltage, and at which the value is lower by the voltage margin (AVd). VOr
Vdi I
Vd i2
Idr
2-stage ACR scheme
Idil
Idi2
(1) Two ACR and one AVR circuits are provided at each terminal. (2) The voltage setting values at each terminal have the following relation, so that they do not interfere with one another. Minimum Vdr for rectifiers > Maximum Vdi for inverters (3) Either converter having minimum Vdr or maximum Vdi defines the DC system voltage. (4) The No. 1 ACR circuit has the function similar to a voltage margin scheme. The rectifier has VDCOL characteristics. (5) The No. 2 ACR circuit works when the normal current balance is not maintained due to signal transmission outage. 70r ACR2
bld
V0ii
• :
I .
.
.
.
.
.
.
.
.
.
.
7012 I
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
I I c=2 Idr
.
.
.
.
.
.
.
.
ACRI .
1'c"21 Idil
Idi2
Note: Central computer transmits the current and voltage setting values satisfying the above relations to the controllers at each terminal.
two-stage ACR (automatic current regulation) with VDCOL, were selected as the typical subjects for analysis and evaluation. With both schemes, stable operation in an emergency is expected without depending on a signal transmission channel, because the voltage-defining terminal is automatically determined when the AC voltage is reduced. Table 4 presents the performance characteristics of the two control schemes. IV.2.3 Analytical results The two control schemes of Table 4 were analysed and comparatively evaluated for the three system models of Figure 4. The power system dynamics analysis program of CRIEPI was used in the analysis. The results are presented in Table 5. As indicated in the table, there is a possibility that a commutation failure occurs in the process of voltage recovery from a three phase line-to-ground fault near an inverter terminal in an AC system. In particular, with the two-stage ACR scheme in the event of an AC system fault near the rectifier in Model A, the build up of the DC system after voltage recovery is somewhat slow. This is due to the reduction and fluctuation of DC current even with a slight drop in DC system voltage. It should also be noticed that the voltage margin scheme cannot assure stable operation in the
event of rectifier terminal outage, whereas the two-stage ACR scheme can maintain stable operation even under the outage of a converter terminal. IV.2.4 Evaluation
The two-stage ACR scheme seems to be useful for maintaining stable operation even when a power system fault occurs during the abnormal condition of a signal transmission channel. Prevention of a commutation failure in the process of recovery from an AC system fault is also expected with the two-stage ACR scheme as long as it is provided with the VDCOL function. The VDCOL is helpful for voltage recovery after an AC system fault. However, the two-stage ACR scheme is required to make its setting values with due consideration on the system voltage variation, if it is provided with the VDCOL function. Another problem to be studied further with the two-stage ACR scheme is the slow build up of the DC system in the recovery process at a fault. Based on these considerations, an advanced control scheme is deemed appropriate to assure stable operation in a multi-terminal HVDC system, such as a new two-stage ACR scheme incorporating the automatic selection function of the voltage-defining terminal, which is a feature of the voltage margin system.
Application of power electronic technologies to the 21st century: Y. Sekine et
al
191
Table 5. Analysis resullts of DC multi-terminal transmission
Model system
Fault condition
Voltage margin scheme
2-stage ACR scheme
Model A
AC system fault DC line fault DC terminal outage
Unsafe* Safe Safe
Unsafet Safe Safe
Model B
AC system fault DC line fault
Unsafe++ Safe
Unsafe$ Safe
Model C
AC system fault DC line fault DC terminal outage
Unsafe + Safe Serious§
Unsafe+ Safe Safe
* Restart is impossible witl" fault near inverter when AC system is weak. t DC current fluctuates with fault near rectifier. **Commutation failure occurs at inverter terminal but system gets restored. §One pole gets shut down when rectifier drops off.
Table 6. Fundamental specification of 300 MW self-commutated converter
Item
Specification
System
Rating Voltage harmonic
300 MW-100 MVAr (316 MVA), AC 275 kV Distortion rate: total < 1%; component <0.5%
Converter
Type/Element Insulation/Cooling Control Configuration
Voltage-type, GTO (6 kV-3 kA) Air-insulated, Water-cooling Multi-phase PWM (possibility of 9-pulse) 8-stage (possibility of 4-stage)
Transformer
Configuration
Double step-up (possibility of direct step-up)
V. Project converter
3: Higl~ p e r f o r m a n c e
AC/DC
Operation of the conventional converter of the linecommutated type depends upon the system voltage. Generally, a high performance converter should never fail commutation even when the AC voltage is reduced or its waveform is distorted. It should have the ability to continue operation ew;n in the event of power supply interruption due to failures in the AC system, and should not generate any higher harmonics. The technology development for a 30,9MW AC/DC converter of the self-commutated type, which meets the requirements for high performance, is being implemented in Project 3. It is essential that not only the converter itself should have high quality, but also that the performance of the elements should be improved as the core component of a self-commutated converter. To clarify the technological development issues of the self-commutated elements, analysis and design by computer simulation are also in progress. For the analytical study by computers, the simulation technology is not yet sufficiently mature. Currently, a large number of iteration calculations are required to solve the equation representing the voltagecurrent relation during switching phenomena of the elements. The improvement of computation methods is also one of the fundamental research subjects.
Development of high performance converter Development target In Japan, SVCs of 30 to 80 MVA with DC voltage of several tens of kV, employing a self-commutated converter, have been developed, and verification tests were performed at Inuyama Switchyard, KEPCO, and Shin-Shinano Frequency Converter Station, TEPCO. In order to employ SVCs in practical HVDC power transmission in the future, roughly ten times more capacity rating is required. Also, further improvements in efficiency and reliability are still needed. For this reason, a basic specification on the development target has been defined. As presented in Table 6, the target is to develop a 300 MW-38.5 kV self-commutated converter for HVDC transmission. It is assumed that in making up this converter system, eight converter units will be connected in parallel on the DC side. Higher harmonics will be suppressed by the multiplex connection of transformers on the AC side. V.1
V.1.1
Converter component technologies for large capacity Table 7 summarizes the recent major achievements on the component technologies for a high performance converter of large capacity, which has been developed by model test on a single arm (4 modules: 16 GTOs of 6 kV, V.1.2
192
Appfication of power electronic technologies to the 21st century." Y. Sekine et al
Table 7. Major achievements on converter component technologies Technology
Series connection
Gate drive supply
Low loss circuit
Target
To make a large number of elements be connected in series with a lumped scheme by adjusting the gate timing of each element with great accuracy,
To achieve high reliability by simplifying the circuit so as to supply the gate drive power from the high voltage circuit.
To reduce total loss by recovering the energy lost in the snubber and reactor.
Test item
Voltage share of each element when the gate timing is deviated.
Turn-on/off performance of the gate drive supply circuit.
Operating performance of the energy recovery circuit, and the amount of energy recovered.
Major achievements
(1) The voltage share varies within 10% or less if the deviation of turn-off timing is within 1/~s. (2) The number of series elements, currently ten or so, could be increased by three to five times.
(1) Characteristics of voltage and current share at turn-on/off has been cleared. (2) Supply of gate drive power from anodecathode in the main circuit is possible.
(1) Approximately 3/4 of the energy for turn-on/off was recovered. (2) The energy could be utilized for gate drive power source.
3 kA each in series) of the converter unit. Three results of technological developments, series connection of elements, gate drive power supply with high voltage, and energy recovering from snubber circuits, are presented in the table. Series connection of elements In order to realize the self-commutated HVDC transmission in the future, the unit, say, 300 MW B-T-B facility, will be required to have a capacity at least five times larger than the largest self-commutated SVC now existing in Japan. For this reason, a large number of elements must be connected in series. Today, there is a prospect that three to five times as many elements as currently used (ten elements or so) can be successfully connected in series. This has been achieved by adjusting the timing of the gate signal to each element with high precision, and equalizing the voltage shared by each element of a single arm. The deviation of divided voltage turn-on and turn-offofeach element was made to be adjustable within the range of _+10% by controlling the gate signal timing. Gate drive power In order to reduce the number of parts in a module and realize high reliability of a converter, high voltage supply of gate pulse is a promising technology. At present, the gate drive power is supplied from a low voltage source via an insulation transformer. With this scheme, however, the size of the transformer becomes too large and the number of parts increases too much to realize a large capacity converter. To cope with this problem, a scheme of directly supplying the gate from the anode-to-cathode in the main circuit of high voltage has been studied. High reliability is also expected to assure the gate drive power supply by adopting this scheme. The power supply circuit, illustrated in Figure 5, was installed in each GTO element to confirm the operation. Tests were performed on one arm referred to above, verifying that a stable operation was made possible.
power supply unit
GTO 7
Figure 5. Power supply circuit for gate drive
anode reactor
insulating transformer
~1 ectifier
condenser auxiliary GTO Figure 6. Operating principle of low loss circuit Low loss snubber circuit As far as the existing scheme is concerned, power loss in the snubber circuit and anode reactor at the time of turn-on and turn-off becomes too large to expand the rated capacity. Therefore, the energy generated in the circuit must be recovered positively. Aiming at the reduction of converter loss, a method to recover the energies in the snubber condenser and anode reactor at the time of switching was developed. In addition, the
Application of power electronic technologies to the 21st century: Y. Sekine et al possibility of utilizing a part of the recovered energy for the gate drive power was experimentally studied to improve the total efficiency, and promising results have been obtained. In this experiment, it has been made clear that 70 to 80% of the loss could be recovered. This was achieved with the circuit, the simplified diagram and operating principle of which are illustrated in Figure 6. The principle is roughly that the energy lost in the snubber condenser and anode reactor is stored in the circuit, and the stored energy is used again through the insulation transformer to the DC circuit at the time of turn-on and turn-off. V.2 Development of high performance elements V.2.1 Characteristics of self-commutated elements IGBT The [GBT (insulated gate bipolar transistor) is a bi-polar element with the structure of a MOS (metal oxide semiconductor). This e]Lement looks most promising as a future high performance element, because its drive power is so small as it can be driven by voltage, also it has a high switching speed and strong rupturing resistivity, and its structure makes it easy to manufacture. The development of high voltage and large capacity elements while maintaining their high response will be the future issue to be challenged. GTO
The GTO (gate turn-oil" thyrister) is a bi-polar element having a four-layer structure of p-n-p-n. The development of high voltage and large capacity units is now in progress with the improvement in the technology for miniaturizing gate pattern. Since the gate is turned off by draining the carriers accumulated inside, the switching frequency is currently less than 300 Hz or so. The future task is to upgrade the frequency performance while maintaining high voltage and large capacity. S l thyrister
The SI (static induction) thyrister is a bi-polar element of a static induction type, and the gate is structured in channels. The element can be turned on and turned off by controlling the gate voltage. For this reason, it has a high switching speed and low turn-on voltage. The future task is to realize high voltage and large current ratings. V.2.2 Definition of development targets Based on the survey on self-commutated characteristics, and considering the element parameters needed for a high performance converter, the target rating of the required element has been set at 4.5kV - 3kA class with a switching frequency of 5 kHz. It has also been decided to implement the characteristics analysis and design by computer simulation. Three elements, IGBT, SI thyrister and GTO, have been selected for the study. It has been determined to explore the possibility of higher withstand voltage and larger current for the IGBT and SI thyrister, and higher frequency for the GTO.
VI. Conclusion The background and outline of the long-range research and development programme for 'Reinforced Power
193
System Interconnection' currently being carried out by the power industry in Japan have been briefly introduced. The purpose, organization, schedule and main results up to now have also been presented. The major achievements and future plan of the three study projects in the research programme are summarized as follows.
A C high-voltage/large-current transmission The effectiveness in the application of various measures for enhancing AC transmission capability to interconnected systems has been analysed and evaluated on model power systems. The results indicated that series equipment such as TCSC and TCPS will be realized as small size facilities and may have impact on the future transmission networks. In the next stage, with the aim of enhancing operation and control performance and realizing high reliability, the verification tests using a power system simulator will be implemented on the measures which have been identified as the important subjects to be further studied. Multi-terminal HVDC transmission
The feasibility study on the control schemes to be applied to a multi-terminal HVDC transmission system has been implemented. A guideline for the development of a practical control/protection device has been clarified. In the next step, a prototype of the control/protection device will be designed and manufactured. The verification tests will then be carried out by a power system simulator, as it has been confirmed that assurance of reliability is especially important in introducing the multi-terminal HVDC system to trunk power networks.
High performance AC/DC converter From the viewpoint of enhancing the performance of a converter which is the core of the above two study projects, a technology to enable a series connection of 16 elements has been developed for the partial model of a 300 MW self-commutated converter. A device to supply the gate pulse at high voltage and a circuit to recover the energy consumed at the time of switching have been also developed. In future, the converter operation performance in the bridge configuration will be clarified and a prototype of the control/protection device will be manufactured and tested. These will be done for the interim step to the verification field test on the developed partial model in an actual power system. VII. A c k n o w l e d g e m e n t s
The authors would like to express their thanks to Dr Masahiro Takasaki, Senior Research Engineer, Department of Power System, CRIEPI, and Dr Bahman Kermanshahi, Visiting Scientist, CRIEPI, for their helpful support for arranging this paper.
VIII. References 1 The Agency of Natural Resources and Energy Study on the long-range prospects for interconnection of power systems in Japan of the 21st century (June 1992) (in Japanese) 2 Central Research Institute of Electric Power Industry General report on technological developments for reinforced power system interconnection (March 1994) (in Japanese)
I~I
Electrical Power & Energy Systems, Vol. 17, No. 3, pp. 181-193, 1995 Elsevier Science Ltd Printed in Great Britain 0142-0615(95)00009-7 0142-0615/95 $10.00 + 0.00
Application of power electronics technologies to the 21st century's bulk power transmission in Japan Y Sekine Science University of Tokyo, Japan
K Takahashi and 1' Hayashi Central Research Institute of Electric Power Industry, Japan
part of the country. Meanwhile, more generating plants are being installed in remote areas, as siting places near a load centre become more difficult to obtain. Moreover, the size of generating stations is getting larger. For these reasons, every indication shows that serious regional imbalance between supply and demand in each service area is becoming noticeable year by year. At the same time, it is becoming harder to predict various uncertain factors for the future, such as unexpected delay in new generating-plant construction, unanticipated change in annual peak demand growth, and so forth. This leads to a situation in which power utilities have to secure more flexibility against the uncertainties. To cope with this growing regional imbalance and increasing uncertainties, emphasis is being placed on the long-distance bulk transmission as well as the interchanges among neighbouring power utilities. Today, it is becoming necessary not only to maintain wide-area power system operation, but also to share the reserve margin among interconnected power utilities even at the stage of system expansion planning. From 1990 to 1992, a study committee on the 21st century vision of the interconnected power systems of Japan was conducted by both power utilities and universities under the Agency of Natural Resources and Energy (ANRE), national government organization. It was pointed out by the study committee that, concerning the interconnection between the 50 Hz and 60 Hz systems around the year 2010, inter-capacity of 2.0 to 2.5 GW will be required to meet increasing uncertain factors. It was also suggested that two power flows of 3.5 to 6.0 GW will be observed from both ends of the main island to the central part by the year 2010. By referring to this long-range prospect, each power utility has studied its own planning measures such as reinforcement of existing 500 kV transmission networks, construction of UHV AC trunk lines or HVDC transmission cables, and installation of new back-to-back
The electric power industry in Japan & now carrying out a nation-wide research and development programme for the effective application of power electronics technologies to the bulk transmission network and interconnected power systems for the 21st century. The programme will continue for eight years funded by ten power utilities and subsidy from the national government. The research and development programme specifically consists of the following three study projects." A C high-voltage/large-current transmission (feasibility study on enhancement measures for AC transmission capability, and verification test of their effectiveness using a power system simulator); multiterminal HVDC transmission (feasibility study on multiterminal HVDC transmission, and development and verification test of a prototype control/protection device); high performance AC/DC converter (development of a partial model for a 300 M W self-commutated converter, and field test of the developed partial model). Keywords: power system, power electronics, transmission, A C/DC converter
HVDC
I. I n t r o d u c t i o n Today, nine electric service areas in Japan are connected to each other through EHV transmission lines and two frequency converter stations. The total peak load amounted to approximately 150 GW in 1992. The interconnected network is divided into two parts of different system frequencies, almost at the centre of the main island. The western part is composed of six service areas of 60 Hz, and the eastern part is composed of three areas of 50 Hz. Figure I illustrates briefly the interconnected power systems in Japan. In recent years, power consumption has been concentrated in the metropolitan areas located in the central
181
182
Appfication of power electronic technologies to the 21st century. Y. Sekine
et al
~
annual ~ak load : 147 GW kwh consumption : 682 ~ (in 1992)
4 GW
~
DC Link
!i!~!!~!~!!if!!~!~!!!!i!~i!!i~!!!!!!!i!!~!!![55Z]
600~BTB
5 ~
k ~ 1 1 GW
('96)
13 GW
1
\
10 GW
)
,
28 GW
23
GW l u
\
51 GW
BTB 1400MW
DC Link (2001) 4 GW
300HW BTB ('97)
50Hz SystemI [45%3
Figure 1. Interconnected power systems in Japan
DC converter stations. It has already been determined that by the beginning of the 21st century, the present 500 kV transmission networks will be expanded further by forming double routes of 500 kV lines in the 60 Hz system, while operation of UHV (nominal: 1000 kV AC) transmission will be newly commenced in the 50 Hz system. The interconnection capacity between the 60 Hz and 50 Hz systems will be expanded from 0.9 GW at the present time to 1.2 GW in 1997 when the third frequency converter station will be newly installed. Another new back-to-back HVDC converter station of 0.3 GW will be installed in 1996 solely for the purpose of tie-line load flow control between two power utilities in the 60 Hz system. Concerning HVDC transmission, upgrading transfer capability from 0.3 GW to 0.6 GW at HokkaidoTohoku cable interconnection, submerged about 40 km long, was completed in 1993. Another possibility of 1.4GW HVDC cable, also submerged approximately 40 km long between Shikoku and Kansai, is now under study. Various research and development required for the long-term power system vision are now going on. Of course, in order to have more fully utilized existing AC transmission facilities, improvement of the conventional control schemes is being carried out. Enhancement of controlling performance of PSS (power system stabilizer) and SVC (static var compensator) is also under way, along with employment of new technologies such as SVC with self-commutation. Concerning long-distance bulk power transmission in the 50Hz system, new UHV technology is now extensively being developed to meet the reliability and environmental requirements. Some
UHV designed lines have already been put in use in 500 kV operation, and a UHV substation test facilities is also under construction. Now, as for HVDC transmission technologies, the above-mentioned study committee strongly pointed out that one of the most promising and indispensable measures will be the effective utilization of power electronics technologies in the design of trunk power systems in the 21st century. It was commonly recognized among the ten power utilities that in order to enhance the performance of power facilities efficiently in the future bulk transmission networks, including inter-regional interconnected systems, more attention should be paid to application of power electronics technologies, adoption of HVDC transmission systems, and development of new AC/DC converter equipment. In meeting the requirements, ten power utilities in Japan established a long-range research and development programme called 'Reinforced Power System Interconnection', in March 1992. This programme will continue for eight years, up to 1999, financially supported partly by subsidy from the national government and partly by funds from the power industry. Based upon this long-range programme, the 'Technical Committee on Interconnection Reinforcement Technology' was organized, sponsored by the ten power utilities and the Central Research Institute of Electric Power Industry (CRIEPI). In the fiscal years 1992 to 1999, the Committee will obtain a budget of about 9 billion yen in total from the ten power utilities and the ANRE. Presently, three cooperative research groups under the committee are being engaged in the following
Application of power electronic technologies to the 21st century: Y. Sekine et am three technical study projects:
Project 2: Multi-terminal HVDC transmission Applicability of multi-terminal HVDC to the future long-distance and bulk-power transmission will be focused upon. When a large scale HVDC transmission is adopted in power system planning, how to secure more flexibility is one of the essential problems. Multi-terminal HVDC is a key means of coping with this problem. For this reason, current technological progress and the actual operation in the multi-terminal HVDC transmission system in Japan and abroad will be surveyed. Advantages and disadvantages of the HVDC application will be evaluated through simulation studies on stable operation of a system fault using model power systems. In particular, operation performance of a converter at commutation failure will be analysed in order to assess the introduction to a practical power system and to identify possible technological problems for the development. In addition, to establish efficient control/protection schemes for multi-terminal HVDC transmission, a prototype control/protection device will be designed and manufactured, and then will be subjected to verification tests on a power system simulator.
(l) Project 1: AChigh-voltage/large-current transmission; (2) Project 2: Multi-terminal HVDC transmission; (3) Project 3: High performance AC/DC converter.
II. General o u t l i n e of t h e p r o g r a m m e I1.1 Research activities in three projects Project 1. AC high- w,ltage/large-current transmission In this project, emphasis will be placed on upgrading AC power transmission capability in a large power system, in particular lay employing power electronics technologies. With the aim of stability improvement in interconnected power systems and efficient utilization of power transmission facilities, the current progress in various measures utilizing power electronics in Japan and abroad will be reviewed. Effects in applying the enhancement measures will be analysed on the models of regionally interconnected systems in the future. Costs of the application will be evaluated for more specific identification of the research and development subjects. In addition, a miniature model of the control/protection device incorporating the most promising measure, identified by the evaluation studies, will be designed and manufactured, and the performance and effectiveness will be verified by using a power system simulator.
I
Project 3: High performance AC/DC converter A high performance AC/DC converter is expected to be incorporated effectively into power facilities such as power system stabilizing controllers and HVDC interconnection installations, as well as power generation plants of small dispersed type in the future. Technological developments for higher voltage and larger capacity rating, higher reliability and low loss performance
TECHNICAL COMMITTEEON REINFORCEDINTERCONNECTIONTECHNOLOGY advice "I funds
~
I T,-,.0w,. UTILITIES I-- i oii ni:Zt
EEEEEE~~E6~EII EE]E]EEEEE~EE E I~I}IIEIII~!~S6EEEEEEEEEE
i reporting ~
(coordination)
subsidy
..............
............... reporting
EEEEEE~EEEEEE E EE~IIIHHHIE]~EEEEEEE?EE)IEE E~T~E!E6EE]EEEEE"
NATIONALI
GOVERNMENT
):EEEEEEEEEE~E~I~IE~E E EEEEEE E EEEI}[~I]I~I]]]~EEEE]~:
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ======================================================================== :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
RI'SEARCHGROUP#1
RESEARCHGROUP#2
RESEARCHGROUP#3
Chief coordinator: KEPCO
Chief coordinator: EPDC
Chief coordinator: TEPCO
Deputy coordinator: TEPCO, CEPCO, CRIEPI
Deputy coordinator: TEPCO, KEPCO, CRIEPI
Deputy coordinator: CEPCO, KEPCO, EPDC CRIEPI
Member: Hokkaido-, Tohoku-, Hokuriku-, Chugoku-, Shikoku-, KyusyuPower Utilities and EPDC
Member: Hokkaido-, Tohoku-, Chubu-, Hokuriku-, Chugoku-, Shikoku-, and KyusyuPower Utilities
Figure 2. Organizations for research implementation
183
Member: Hokkaido-, Tohoku-, Hokuriku-, Chugoku-, Shikoku- and KyusyuPower Utilities
Appfication of power electronic technologies to the 21st century: Y. Sekine et al
184
will be implemented in this project. Studies on selfcommutated converters, which are strong against power system disturbances, will be implemented. More specifically, converter component technologies for series connection of elements, gate drive by high voltage pulse, and lost energy recovering at turn-on/off will be established. Verification tests will be also conducted for these technologies. In addition, concerning new elements of the converter, technological subjects for the development of self-commutated elements will be clarified by simulation analysis, prototype design and manufacture. Finally, field verification tests of the developed partial model will be performed on an actual power system. 11.2 Research organization The research activities for the three projects are to be implemented by three research groups under the structure presented in Figure 2. Four major parties in the electric power industry, Central Research Institute of Electric Power Industry (CRIEPI), Kansai Electric Power Company (KEPCO), Electric Power Development Company (EPDC), and Tokyo Electric Power Company (TEPCO), coordinate the whole research programme and preside over each research group. A committee designated the 'Technical Committee on Interconnect Reinforcement Technology', composed of experts and utility representatives, supervises all the research activities and gives advice to each research group.
11.3 Schedule of research programme The schedule of research activities described below is summarized in Table 1.
Project 1:A C high- voltage/large- current transmission In fiscal years 1992 and 1993, the present status in Japan and abroad will be surveyed. The technical merits and problems will be analysed and evaluated on model power systems. Development issues for enhancing transmission performance will also be identified. In the three years from fiscal years 1994 to 1996, a miniature sample model of the control/protection device will be developed and tested by using a power system simulator. Project 2: Multi-terminal HVDC transmission In fiscal year 1992 and fiscal year 1993, the current progress in Japan and abroad on multi-terminal HVDC transmission technologies will be surveyed. The feasibility of the introduction will be evaluated on model power systems. In fiscal years 1994 to 1996, a prototype of control/protection device will be developed and simulator tests will be implemented to assess the performance and effects in the practical application. Project 3: High performance A C/DC converter In fiscal year 1992, a component model (module) will be developed and tested. In fiscal years 1993 to 1995, the development and tests will be continued for a partial
Table 1. Schedule of research programme
Research item (1) AC (i) (ii) (iii) (iv) (v) (vi)
high-voltage/large-current transmission Survey on present progress Analysis and evaluation using model systems Cost evaluation Identification of research and development issues Development and verification of miniature sample model* Full-scale verification test
(2) Multi-terminal HVDC transmission (i) Survey on present progress (ii) Analysis and evaluation using model systems (iii) Identification of research and development issues (iv) Development and verification of control/protection device (v) Full-scale verification test (3) High performance AC/DC converter (i) Survey of features of converters and elements (ii) Development of converter • Development/tests of component model (module) • Development/tests of partial model (arm) • Study of control/protection device • 1/8 scale verification test (iii) Development of new element • Prototype designing and performance estimation • Prototype manufacturing and function tests • Development of elements
'92
'93
- -
'94
'95
'96
'97
'98
'99
Design, manufact., test
- - - D e s i g n , manufact., test
Analysis, design, manufact., test
*To be selected from self-commutated SVC, thyrister controller series capacitor, thyrister controlled phase shifter, and variable speed fly-wheel machine. • Intermediate check and review will be performed on the research achievements up to fiscal year 1994. Fundamental policy on fiscal year 1996 and after will be determined, including abortion of each project.
Application of power electronic technologies to the 21st century: Y. Sekine et al model (arm). Also, a st ady on control/protection devices will be pursued until[ fiscal year 1995. In addition, verification tests on an actual power system of a 1/8 scale model of 300 MW HVDC facility will be implemented by fiscal year 1999. On the other hand, for the new elements, prototype design and manufacturing will be conducted, and functional tests will be implemented by fiscal year 1995.
III. Project 1: AC high voltage/large current transmission In the future interconrLected power systems, application of power electronics technology is widely expected to play an important role as a new means for enhancing transmission performance. The objective of Project 1 is to identify effectiveness and related problems of the new technology. Suitable methods of application are also to be clarified in reference to the specific characteristics of each power system. For this objective, more specifically, the following targets are defined, and the associated technical studies and research activities are being implemented: (1) to assess the applicability of various measures for enhancing AC transmission performance; (2) to develop technologies needed for designing a control/protection device for the optimal control of a power system. In fiscal years 1992 to 1993, the state of the art on the measures for transmission performance enhancement and the current progress of their development work in Japan and abroad were surveyed. Based on the survey, the measures to be studied in Project 1 have been screened. Also, model power systems were decided upon and the applicability of the various measures have been evaluated on the model power systems.
185
II1.1 Survey of progress in Japan and abroad II1.1.1 Comparison of measures
The current status of the following conventional and new measures were reviewed partly through enquiry among the research group members: (1) upgrading to higher transmission voltage; (2) expansion to multiple transmission route; (3) stabilizing control by generators with PSS (power system stabilizer); (4) reactive power control including SVC (static var compensator); (5) real power control by TCSC (thyrister controlled series capacitor); (6) adoption of HVDC transmission, etc. The principle, features, effectiveness, operating modes, problems, etc. were reviewed for each measure, and comparison among these has been done with special respect to future possibilities. II 1.1.2 Survey of progress in overseas nations The current status of verification programmes of power electronics technology applied to power system control in the US (FACTS programme) was surveyed by direct interviews. This time, the present situation on a thyrister controlled series capacitor was investigated at Slatt Substation, BPA, at Kayenta Substation, WAPA, and at Kanawha River Substation, AEP. 111.1.3 Identification of study measures Based on the above two surveys, various measures to improve transmission performance were evaluated, taking into consideration factors such as features of the power system to which the measures would be applied, feasibility of the technologies, and applicability of the measures (cost, environment, etc.). Some conventional means such as voltage upgrading were also selected for comparative study. Table 2 presents the measures to be studied in this research project.
Table 2. Measures to be studied and their priority
Measures to be studied
Priority
Conventional means
Upgraded voltage Multiple route
B B
Generation control
Ultra-quick response exciter with PSS (power system stabilizer)
B
Reactive power control
Conventional SVC (static var compensator) Self-commutated SVC
C A
Real power control
TCSR (thyrister controlled series capacitor) TCPS (thyrister controlled phase shifter) TCDR (thyrister controlled damping resistor) SMES (superconducting magnetic energy storage) Variable speed fly-wheel machine
A A C C A
Others (HVDC)
HVDC including BTB (back-to-back) Self-commutated HVDC (including BTB)
B C
A: for major study, B: for comparative study, C: for supplementary study
1 86
Application of power electronic technologies to the 21st century: Y. Sekine et al
II1.1.4 Selection of analytical tool In order to make it possible to compare the effectiveness of various measures, mathematical models of the measures should be formulated and incorporated in the analytical tool. A survey was conducted in Japan and abroad on analytical tools to be used in the research works of Project 1. As a result, the 'Power System Dynamics Analysis Program' developed by CRIEPI was selected in terms of the following points: (l) to be capable of simulating all the measures to be studied; (2) to be available easily anytime and anywhere in Japan. The effectiveness of the CRIEPI's software has been tested and verified on different examples of the measures to be compared and evaluated.
II 1.2 Analysis and evaluation on model systems II 1.2.1 Definition of model systems Two simple power system models illustrated in Figure 3 have been defined in view of the image of inter-regional interconnected power systems of Japan in the future (around 2010). This was done with emphasis on the features of the model power systems so that the effects on stability enhancement by various thyrister controlled equipment can be clearly identified. Realistic conditions of the power systems under planning at each utility company were taken into consideration in the model systems. The radial power system model is based on the image of the 60 Hz interconnected power system, in which six double circuit 500kV transmission lines are interconnected in tandem, and the total generation is 115 GW;
i!!!iiiii i : i i;Eii iiiiiii i i i: iii iiiiiiil ( "Sm0w' / TOTAL GENERATION = 115 GW TOTAL LOAD = 115 GW
\ L = 6 GW / ~._ J
5ookv I 2cct ~200kin
,oo v
I
YS, M oo ,_j TOTALGENERATION = 86.5 GW TOTAL LOAD = 86.5 GW
500kV I 2cct J 120kin
lO00kV ~
500kV I I
/
lO00kV
500kV I I
\
5OOkV
Figure 3. Interconnected power system models
/
500kV II
\
5ookv
/
\
Appfication of power electronic technologies to the 21st century: Y. Sekine et al whereas the looped power system has been defined on the image of the 50 Hz :system, in which seven sub-systems are interconnected by 1000 kV and 500 kV transmission lines of double circuits, with total generation of 68.5 GW. 111.2.2 Simulation analysis
Stability enhancement effects of various measures applied to the two interconnected power system models were comparatively evaluated by using CRIEPI's analysis tool. Broadly speaking, power system stability is classified into transient stability, dynamic stability and steady state stability. In this analysis, only transient stability was considered, and the behaviour of a power system under serious disturbance due to a system fault was analysed. Specifically, the purpose of the simulation was to check whether or not the target amount of power exchange between two subsystems in each model can be stably achieved by each measure against all the given disturbances. The results of the simulation analysis are summarized in Table 13.The minimum required capacity of each measure is presented in this table. Radial power system model Power flow of 3 GW from System 1 to System 6 was set as the target value of power exchange. The power supply in System 1 was made to increase. It should be noted that, as indicated in t]ae middle column of Table 3, the minimum capacity required for series equipment (TCSC, TVPS) is smaller than that of parallel equipment (SVC, TCDR), with the exception of DC interconnection. Looped power systero model Power generation in System IV was assumed to be shifted to System I, and the incremental power flow from System I to System VI was se,t at the target value of 3 GW. It was shown that the equipment capacity required for the measures is minimum with TCPS, as indicated in the right-hand column of Table 3.
187
111.2.3 Evaluation The effectiveness of various measures was studied in terms of the transfer capability enhancement and generating capacity reallocations by analysing the two model power systems typically representing the future interconnected power systems of Japan. Although the analysis results indicated an encouraging effectiveness of all the methods, the following four promising measures were identified as the major subjects with high priority, taking into consideration other factors such as equipment cost (Table 3): (1) (2) (3) (4)
self-commutated SVC (static var compensator); TCSR (thyrister controlled series capacitor); TCPS (thyrister controlled phase shifter); variable speed fly-wheel machine.
It was also noticed that evaluation of their effectiveness should be implemented from viewpoints other than transient stability, such as power flow control and voltage stability, considering the further development requirements for their commercialization.
IV. Project 2: Multi-terminal HVDC transmission In adopting an HVDC transmission system in longdistance/bulk-power networks, assurance of power system reliability is one of the main issues. A highly reliable control/protection scheme is also indispensable. In addition, it is commonly thought that a multi-terminal HVDC system will be needed for the future power systems of Japan because it provides more flexibility to power system operation and configuration. For this reason, the capability of the multi-terminal HVDC transmission system to operate stably against disturbances such as AC and DC system faults is the main subject of Project 2.
Table 3. Summary of analysis results on two system models
System model
Radial system model
Looped system model
Purpose
To increase transfer capability from Sys. I to Sys. VI by 3 GW
To move generation source in Sys. IV to Sys. I by 3 GW
___2100 MVA + 2100 MVA 600 MVA (17f~) 500 MVA (+ 10°) 3000 MW 900 MW
-I-1500 -I- 1500 1600 150 2700 900
Measure
SVC (line-commutated) SVC (self-commutated) TCSC TCPS TCDR Fly-wheel* Upgraded voltage Multiple route Exciter with PSSt DC (line-commutated) DC (self-commutated)
500 500 180 3000 3000
Note: The minimum required capacity (or length) of each measure is presented. *Variable speed fly-wheel machine. t Ultra-quick response exciter with PSS.
km km MVA MW MW
200 30 2700 900
MVA MVA MVA (6.3f~) MVA (+0.9 °) MW MW -km MVA MW MW
188
Application of power electronic technologies to the 21st century: Y. Sekine et al
The studies are conducted with emphasis on the power system models representing the future bulk power transmission networks of Japan. With this objective, the following targets were set up for the research study: (1) to select the control/protection schemes which enable the keeping of stable operation against disturbances; (2) to develop a prototype control/protection device and perform the verification tests. So far, examples of a control/protection device employed in a multi-terminal system in overseas countries have been surveyed. Analysis and evaluation by means of model system simulation have been performed so as to identify the challenging subjects for technological development. IV.1 Survey of progress in Japan and abroad IV.1.1 Multi-terminal HVDC transmission and control
technologies Multi-terminal control system Six typical control schemes have been proposed. They are originally based on a scheme of constant-current vs. constant-voltage, or constant-power vs. constantmarginal-angle. These schemes are generally adopted in a two-terminal system and have been extended to a multi-terminal system. Also, another control system is proposed incorporating VDCOL (voltage dependent current order limiter) or alpha-limiter characteristics on the inverter side. This control system can maintain stable operation without a signal transmission channel even in the case when the system voltage drops or one terminal is lost.
Power system control Both in a two-terminal system and a multi-terminal system, new DC power controls are envisaged. These aim at the optimal condition of interconnected AC systems, such as frequency control and system stability control. The examples are frequency control at one terminal at Quebec-New England and at Sacoi, generator instability prevention control and AC system stabilization control, both under study at Mead-Phoenix. Signal transmission channel High reliability in the signal transmission channel, adequately coordinated with control/protection schemes, is essential in a multi-terminal HVDC system. It is a fact that the unavailability factor (outage rate) of a signal transmission channel, which has a double route configuration consisting of microwave and optical fibre, is seemed to be almost equivalent to that of a redundant control system with duplicated control computers. Against momentary disruption of one train in a signal transmission channel, either means of train lock or output lock would be effective; whereas, against the anomaly of two trains, withholding previous data to continue operation would be available. DC circuit breaker A verification test on an actual power system was conducted on DC 400 kV, 14(K)-4000 A circuit breakers at EPRI, WH and BBC. Whereas, in Japan a prototype circuit breaker rated at DC 250kV, 1200-8000A has
been developed, and completed with a shop verification test. Effects of DC circuit breakers were analysed by computer simulation on a model power system. As a result, there is a prospect of obtaining a DC circuit breaker that could substantially reduce the fault duration time, in comparison with the combination of a high-speed line switch and a control system.
IV.1.2 Multi-terminal HVDC facilities in overseas nations Quebec-New England In order to export the abundant hydro-electric power of Quebec to the US, a five terminal DC system ( _ 500 kV, 2250 MW) was constructed and commissioned in 1992. It was found out later that a commutation failure would not be recovered at two terminals connected to weak AC power systems. This multi-terminal system will be operated with two or three terminals for the time being. Power control and connection/disconnection operation can be performed at each converter station even when a signal transmission channel gets into outage.
Sardinia-Corsica-Italy The third terminal of 5 0 M W was installed and commissioned in 1987 in Corsica between the existing Sardinia-Italy two-terminal HVDC transmission ( _ 200 kV, 300 MW). The converter station in Corsica is being operated with a large marginal angle (40 °) so as to prevent a commutation failure. The emergency control system is so designed that when a commutation failure occurs due to reduced system voltage or other causes, the whole system will be shut down at once, and Corsica will be restarted after the AC voltage has been established at the other terminals. Others Other examples of the multi-terminal HVDC system include the new and old converter stations in parallel at Pacific Inter-Tie, and the emergency parallel operation of converters at Nelson River and Itaipu.
IV.2 Analysis and evaluation on model systems IV.2.1 Model systems and analysis conditions
Application of a three-terminal HVDC system to bulk power transmission from remote generating sources was assumed, and three simple model systems illustrated in Figure 4 were selected. Model A is an AC/DC combined system having one generating source, in which a power of 13 G W is transmitted through hybrid routes of a 1000kV AC transmission line and a _+500kV DC transmission line, and the receiving-end systems are interconnected by two links. On the other hand, Model B is a system having a single route of DC transmission line from one generating source. Model B is the case of the elimination of the AC transmission line from Model A. Model C is also a system having a DC route, and power transmission from two generating sources is assumed in this model. The fault analyses were conducted under the conditions of AC system fault, DC line fault and AC/DC converter terminal fault. In order to clarify the difference in the control schemes, the assumed faults are irrelevant to
Appfication of power electronic technologies to the 21st century." Y. Sekine et al
MODEL 300km
__+500kV bipole 2 c c t
II
UHV 200km
I i
a
5GW~ ~ INV~
UHV 200km
~
,I, ---
~
~
I
I
300km
___500kV bipole 2cct
REC [ ] IOGW
2
I
I
MODEL
II UHV 200km 5GWE~
INV~
AI
UHV 600km
I,I
J I i
5GW~ INVl ~
900km
189
900km
BI
UHV 200km
_~L II i,i ~ 5GW~]
I
REC
I
[] IOGW
INVl _~~
S IOGW@
I
I
I
48GW
MODEL 5GW I
cl
REC2
SGW
300km
±500kV bipole
I
UHV 200km IOGW~
i
I
2cct
1
UHV 200km
1, I
900km
I,
REC1
~
~ 5GW
INV.___~%
I
I
I
IOGW
Figure 4. System models for multi-terminal HVDC transmission
the signal transmission channels. Namely, analytical conditions in this study are as follows: (1) AC system fault: three phase line-to-ground near converter terminal and lines left not reclosed; (2) DC system fault: one line-to-ground and converter is restarted; (3) converter terminal fault: one-pole outage and converter is not restarted.
IV.2.2 Multi-terminal control scheme
In assuring stable operating performance of a DC multi-terminal transmission system, the control system plays an important role. It should be less affected by changes in operating condition of the interconnected power systems. Of various multi-terminal control schemes being researched in Japan and abroad, two control schemes, a voltage margin scheme and a
190
Application of power electronic technologies to the 21st century: Y. Sekine et al
Table 4. Performance of two typical control schemes Scheme
Basic configuration and operation
Voltage margin scheme
(1) A constant-current control circuit (ACR) and a constant-voltage control circuit (AVR) are both provided at each terminal. (2) The current setting value at each terminal is determined taking into account the current margin (A/d). (3) The voltage setting values of all terminals are the same except at one terminal which defines the DC system voltage, and at which the value is lower by the voltage margin (AVd). VOr
Vdi I
Vd i2
Idr
2-stage ACR scheme
Idil
Idi2
(1) Two ACR and one AVR circuits are provided at each terminal. (2) The voltage setting values at each terminal have the following relation, so that they do not interfere with one another. Minimum Vdr for rectifiers > Maximum Vdi for inverters (3) Either converter having minimum Vdr or maximum Vdi defines the DC system voltage. (4) The No. 1 ACR circuit has the function similar to a voltage margin scheme. The rectifier has VDCOL characteristics. (5) The No. 2 ACR circuit works when the normal current balance is not maintained due to signal transmission outage. 70r ACR2
bld
V0ii
• :
I .
.
.
.
.
.
.
.
.
.
.
7012 I
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
I I c=2 Idr
.
.
.
.
.
.
.
.
ACRI .
1'c"21 Idil
Idi2
Note: Central computer transmits the current and voltage setting values satisfying the above relations to the controllers at each terminal.
two-stage ACR (automatic current regulation) with VDCOL, were selected as the typical subjects for analysis and evaluation. With both schemes, stable operation in an emergency is expected without depending on a signal transmission channel, because the voltage-defining terminal is automatically determined when the AC voltage is reduced. Table 4 presents the performance characteristics of the two control schemes. IV.2.3 Analytical results The two control schemes of Table 4 were analysed and comparatively evaluated for the three system models of Figure 4. The power system dynamics analysis program of CRIEPI was used in the analysis. The results are presented in Table 5. As indicated in the table, there is a possibility that a commutation failure occurs in the process of voltage recovery from a three phase line-to-ground fault near an inverter terminal in an AC system. In particular, with the two-stage ACR scheme in the event of an AC system fault near the rectifier in Model A, the build up of the DC system after voltage recovery is somewhat slow. This is due to the reduction and fluctuation of DC current even with a slight drop in DC system voltage. It should also be noticed that the voltage margin scheme cannot assure stable operation in the
event of rectifier terminal outage, whereas the two-stage ACR scheme can maintain stable operation even under the outage of a converter terminal. IV.2.4 Evaluation
The two-stage ACR scheme seems to be useful for maintaining stable operation even when a power system fault occurs during the abnormal condition of a signal transmission channel. Prevention of a commutation failure in the process of recovery from an AC system fault is also expected with the two-stage ACR scheme as long as it is provided with the VDCOL function. The VDCOL is helpful for voltage recovery after an AC system fault. However, the two-stage ACR scheme is required to make its setting values with due consideration on the system voltage variation, if it is provided with the VDCOL function. Another problem to be studied further with the two-stage ACR scheme is the slow build up of the DC system in the recovery process at a fault. Based on these considerations, an advanced control scheme is deemed appropriate to assure stable operation in a multi-terminal HVDC system, such as a new two-stage ACR scheme incorporating the automatic selection function of the voltage-defining terminal, which is a feature of the voltage margin system.
Application of power electronic technologies to the 21st century: Y. Sekine et
al
191
Table 5. Analysis resullts of DC multi-terminal transmission
Model system
Fault condition
Voltage margin scheme
2-stage ACR scheme
Model A
AC system fault DC line fault DC terminal outage
Unsafe* Safe Safe
Unsafet Safe Safe
Model B
AC system fault DC line fault
Unsafe++ Safe
Unsafe$ Safe
Model C
AC system fault DC line fault DC terminal outage
Unsafe + Safe Serious§
Unsafe+ Safe Safe
* Restart is impossible witl" fault near inverter when AC system is weak. t DC current fluctuates with fault near rectifier. **Commutation failure occurs at inverter terminal but system gets restored. §One pole gets shut down when rectifier drops off.
Table 6. Fundamental specification of 300 MW self-commutated converter
Item
Specification
System
Rating Voltage harmonic
300 MW-100 MVAr (316 MVA), AC 275 kV Distortion rate: total < 1%; component <0.5%
Converter
Type/Element Insulation/Cooling Control Configuration
Voltage-type, GTO (6 kV-3 kA) Air-insulated, Water-cooling Multi-phase PWM (possibility of 9-pulse) 8-stage (possibility of 4-stage)
Transformer
Configuration
Double step-up (possibility of direct step-up)
V. Project converter
3: Higl~ p e r f o r m a n c e
AC/DC
Operation of the conventional converter of the linecommutated type depends upon the system voltage. Generally, a high performance converter should never fail commutation even when the AC voltage is reduced or its waveform is distorted. It should have the ability to continue operation ew;n in the event of power supply interruption due to failures in the AC system, and should not generate any higher harmonics. The technology development for a 30,9MW AC/DC converter of the self-commutated type, which meets the requirements for high performance, is being implemented in Project 3. It is essential that not only the converter itself should have high quality, but also that the performance of the elements should be improved as the core component of a self-commutated converter. To clarify the technological development issues of the self-commutated elements, analysis and design by computer simulation are also in progress. For the analytical study by computers, the simulation technology is not yet sufficiently mature. Currently, a large number of iteration calculations are required to solve the equation representing the voltagecurrent relation during switching phenomena of the elements. The improvement of computation methods is also one of the fundamental research subjects.
Development of high performance converter Development target In Japan, SVCs of 30 to 80 MVA with DC voltage of several tens of kV, employing a self-commutated converter, have been developed, and verification tests were performed at Inuyama Switchyard, KEPCO, and Shin-Shinano Frequency Converter Station, TEPCO. In order to employ SVCs in practical HVDC power transmission in the future, roughly ten times more capacity rating is required. Also, further improvements in efficiency and reliability are still needed. For this reason, a basic specification on the development target has been defined. As presented in Table 6, the target is to develop a 300 MW-38.5 kV self-commutated converter for HVDC transmission. It is assumed that in making up this converter system, eight converter units will be connected in parallel on the DC side. Higher harmonics will be suppressed by the multiplex connection of transformers on the AC side. V.1
V.1.1
Converter component technologies for large capacity Table 7 summarizes the recent major achievements on the component technologies for a high performance converter of large capacity, which has been developed by model test on a single arm (4 modules: 16 GTOs of 6 kV, V.1.2
192
Appfication of power electronic technologies to the 21st century." Y. Sekine et al
Table 7. Major achievements on converter component technologies Technology
Series connection
Gate drive supply
Low loss circuit
Target
To make a large number of elements be connected in series with a lumped scheme by adjusting the gate timing of each element with great accuracy,
To achieve high reliability by simplifying the circuit so as to supply the gate drive power from the high voltage circuit.
To reduce total loss by recovering the energy lost in the snubber and reactor.
Test item
Voltage share of each element when the gate timing is deviated.
Turn-on/off performance of the gate drive supply circuit.
Operating performance of the energy recovery circuit, and the amount of energy recovered.
Major achievements
(1) The voltage share varies within 10% or less if the deviation of turn-off timing is within 1/~s. (2) The number of series elements, currently ten or so, could be increased by three to five times.
(1) Characteristics of voltage and current share at turn-on/off has been cleared. (2) Supply of gate drive power from anodecathode in the main circuit is possible.
(1) Approximately 3/4 of the energy for turn-on/off was recovered. (2) The energy could be utilized for gate drive power source.
3 kA each in series) of the converter unit. Three results of technological developments, series connection of elements, gate drive power supply with high voltage, and energy recovering from snubber circuits, are presented in the table. Series connection of elements In order to realize the self-commutated HVDC transmission in the future, the unit, say, 300 MW B-T-B facility, will be required to have a capacity at least five times larger than the largest self-commutated SVC now existing in Japan. For this reason, a large number of elements must be connected in series. Today, there is a prospect that three to five times as many elements as currently used (ten elements or so) can be successfully connected in series. This has been achieved by adjusting the timing of the gate signal to each element with high precision, and equalizing the voltage shared by each element of a single arm. The deviation of divided voltage turn-on and turn-offofeach element was made to be adjustable within the range of _+10% by controlling the gate signal timing. Gate drive power In order to reduce the number of parts in a module and realize high reliability of a converter, high voltage supply of gate pulse is a promising technology. At present, the gate drive power is supplied from a low voltage source via an insulation transformer. With this scheme, however, the size of the transformer becomes too large and the number of parts increases too much to realize a large capacity converter. To cope with this problem, a scheme of directly supplying the gate from the anode-to-cathode in the main circuit of high voltage has been studied. High reliability is also expected to assure the gate drive power supply by adopting this scheme. The power supply circuit, illustrated in Figure 5, was installed in each GTO element to confirm the operation. Tests were performed on one arm referred to above, verifying that a stable operation was made possible.
power supply unit
GTO 7
Figure 5. Power supply circuit for gate drive
anode reactor
insulating transformer
~1 ectifier
condenser auxiliary GTO Figure 6. Operating principle of low loss circuit Low loss snubber circuit As far as the existing scheme is concerned, power loss in the snubber circuit and anode reactor at the time of turn-on and turn-off becomes too large to expand the rated capacity. Therefore, the energy generated in the circuit must be recovered positively. Aiming at the reduction of converter loss, a method to recover the energies in the snubber condenser and anode reactor at the time of switching was developed. In addition, the
Application of power electronic technologies to the 21st century: Y. Sekine et al possibility of utilizing a part of the recovered energy for the gate drive power was experimentally studied to improve the total efficiency, and promising results have been obtained. In this experiment, it has been made clear that 70 to 80% of the loss could be recovered. This was achieved with the circuit, the simplified diagram and operating principle of which are illustrated in Figure 6. The principle is roughly that the energy lost in the snubber condenser and anode reactor is stored in the circuit, and the stored energy is used again through the insulation transformer to the DC circuit at the time of turn-on and turn-off. V.2 Development of high performance elements V.2.1 Characteristics of self-commutated elements IGBT The [GBT (insulated gate bipolar transistor) is a bi-polar element with the structure of a MOS (metal oxide semiconductor). This e]Lement looks most promising as a future high performance element, because its drive power is so small as it can be driven by voltage, also it has a high switching speed and strong rupturing resistivity, and its structure makes it easy to manufacture. The development of high voltage and large capacity elements while maintaining their high response will be the future issue to be challenged. GTO
The GTO (gate turn-oil" thyrister) is a bi-polar element having a four-layer structure of p-n-p-n. The development of high voltage and large capacity units is now in progress with the improvement in the technology for miniaturizing gate pattern. Since the gate is turned off by draining the carriers accumulated inside, the switching frequency is currently less than 300 Hz or so. The future task is to upgrade the frequency performance while maintaining high voltage and large capacity. S l thyrister
The SI (static induction) thyrister is a bi-polar element of a static induction type, and the gate is structured in channels. The element can be turned on and turned off by controlling the gate voltage. For this reason, it has a high switching speed and low turn-on voltage. The future task is to realize high voltage and large current ratings. V.2.2 Definition of development targets Based on the survey on self-commutated characteristics, and considering the element parameters needed for a high performance converter, the target rating of the required element has been set at 4.5kV - 3kA class with a switching frequency of 5 kHz. It has also been decided to implement the characteristics analysis and design by computer simulation. Three elements, IGBT, SI thyrister and GTO, have been selected for the study. It has been determined to explore the possibility of higher withstand voltage and larger current for the IGBT and SI thyrister, and higher frequency for the GTO.
VI. Conclusion The background and outline of the long-range research and development programme for 'Reinforced Power
193
System Interconnection' currently being carried out by the power industry in Japan have been briefly introduced. The purpose, organization, schedule and main results up to now have also been presented. The major achievements and future plan of the three study projects in the research programme are summarized as follows.
A C high-voltage/large-current transmission The effectiveness in the application of various measures for enhancing AC transmission capability to interconnected systems has been analysed and evaluated on model power systems. The results indicated that series equipment such as TCSC and TCPS will be realized as small size facilities and may have impact on the future transmission networks. In the next stage, with the aim of enhancing operation and control performance and realizing high reliability, the verification tests using a power system simulator will be implemented on the measures which have been identified as the important subjects to be further studied. Multi-terminal HVDC transmission
The feasibility study on the control schemes to be applied to a multi-terminal HVDC transmission system has been implemented. A guideline for the development of a practical control/protection device has been clarified. In the next step, a prototype of the control/protection device will be designed and manufactured. The verification tests will then be carried out by a power system simulator, as it has been confirmed that assurance of reliability is especially important in introducing the multi-terminal HVDC system to trunk power networks.
High performance AC/DC converter From the viewpoint of enhancing the performance of a converter which is the core of the above two study projects, a technology to enable a series connection of 16 elements has been developed for the partial model of a 300 MW self-commutated converter. A device to supply the gate pulse at high voltage and a circuit to recover the energy consumed at the time of switching have been also developed. In future, the converter operation performance in the bridge configuration will be clarified and a prototype of the control/protection device will be manufactured and tested. These will be done for the interim step to the verification field test on the developed partial model in an actual power system. VII. A c k n o w l e d g e m e n t s
The authors would like to express their thanks to Dr Masahiro Takasaki, Senior Research Engineer, Department of Power System, CRIEPI, and Dr Bahman Kermanshahi, Visiting Scientist, CRIEPI, for their helpful support for arranging this paper.
VIII. References 1 The Agency of Natural Resources and Energy Study on the long-range prospects for interconnection of power systems in Japan of the 21st century (June 1992) (in Japanese) 2 Central Research Institute of Electric Power Industry General report on technological developments for reinforced power system interconnection (March 1994) (in Japanese)