IEEE Transactions on Power Delivery, Vol. 13, No. 1,January 1998
Development of a Control System for a High-Performance Self-Commutated AC/D C Converter
Koji Sakamoto Masashi Yajima
Power Electronics Department, Power Engineering R&D Center, Tokyo Electric Power Co., Inc., 4-1 Egasaki, Tsurumi, Yokohama 230, Japan
/*东京电力株式会社电力工程研发中心电力电子部,4-1Egasaki,Tsurumi,横滨 230,日本＊ ／
Tadao Ishikawa, Member, IEEE
Power Electronics Group ，Komae Research Laboratory，Central Research Institute of Electrical Power Industry (CIUEPI), Japan
/*Tadao Ishikawa, IEEE 会员,日本，电源工业研究中心，狛江实验室，电力电子组*/
Electric Power Research & Development Center Chubu Electric Power Co., Inc., Japan
Substation Section，Power System Engineering & Operation，Kansai Electric Power Co., Inc., Japan
Substation & HVDC Engineering Section，Electrical Engineering Department Electric Power Development Co., Ltd., Japan
Abstract : A self-commutated ac/dc converter composed of controlled turn-off devices to be applied to future system interconnection is described. The advantages of a control system for this type of converter are:
(1) Commutation does not fail when the system voltage is decreased or distorted by a power system fault.
It needs no equipment for reactive power supply, such as static capacitors or synchronous rotating condensers, when used in a low short-circuit capacity power system.
(3) It can independently control active power through dc lines
and reactive power from each terminal.
/* 它可能独立地控制通过直流线路的有功功率和每个终端的无功功率。 */
The proposed back-to-back (BTB) control system using the voltage margin method was verified with a power system simulator, and the results demonstrated the excellent features of the high-performance self-commutated converter.
Keywords : Self-commutated AC/DC converter, Pulse width modulation, Power system simulator
PE-790-PWRD-0-04-1997 A paper recommended and approved by the IEEE Transmission and Distribution Committee of the IEEE Power Engineering Society for publication in the IEEE Transactions on Power Delivery. Manuscript submitted December 18, 1996; made available for printing April 11, 1997.
Due to the growth in power demand, power systems are being expanded and power stations are being located further away from load centers. The application of power electronics to power networks, such as HVDC and FACTS equipment, is expected to enhance power transmission capacity and improve power system stability in the future.
Conventional line-commutated converters using thyristors have been in operation for many years in various countries around the world, however numerous studies have been conducted to cope with problems such as commutation failures during ac system faults and instability arising when the converter is connected to a power system with a small short-circuit capacity ratio.
Therefore a new type of ac/dc converter having the following high-performance is expected to be realized
(1)The converter will not fail to carry out commutation even when a voltage dip or waveform distortion has occurred in the ac system.
(2) The converter will be able to send power from a healthy ac system even when another ac side has completely failed.
(3) The converter will be able to easily control reactive power.
The recent development of high-power semiconductor switching devices with controlled turn-off functions has made it possible to apply large-capacity converters to power systems. In Japan, two sets of 50 MVA static var compensators using gate turn-off thyristor (GTO) converters with a nominal dc voltage of 16 kV and another of 80 MVA with dc 4 kV, which are referred to as "static synchronous compensators (STATCOMs)" in the FACTS projects, have already been installed and are in operation . Since these converters are of the voltage-source type, a scheme for high-speed control of dc voltage and reactive power was developed, and satisfactory operating experience in actual grid systems has been obtained.
/* 受控大功率的半导体开关器件的新发展使为电力系统提供大容量变换器成为可能。 在日本，两套50MVA的 静态无功补偿器使用GTO和普通16KV直流，另一套80MVA的则使用4KV，它们被称为柔性交流输电工程的“静态同步补偿器”，已经安装了并且运转中。 因为这些变换器是压源型，直流电压和无功功率高速控制的一份方案被开发出来，并且在实际网格系统获得的令人满意的运营经验*/
This paper describes a cooperative control scheme for a dc two-terminal system consisting of voltage-source self-commutated converters . Using this scheme, each terminal can be operated independently, with little data communication. Prototype controllers have also been developed and tested with an analogue power system simulator. The test results have verified that active and reactive power can be controlled without interference, and it has been confirmed that stable operation is continued without commutation failure even in the event of an ac system fault
/* 本文描述了一种由压源自整流变换器构成的双端直流系统的合作控制方案。 使用这份方案，每端可以独立地管理，只用很少的数据通信。 原型控制器也开发出来，并在一套电力系统模拟器测试。 测试结果证实有功功率和无功功率可以是受控的，不用干涉，并且被证实甚而在交流系统失败的情形下，稳定的操作继续，不会换向失败*/
11. CONFIGURATION OF CONTROL SYSTEM
The control system we are proposing for the high-performance converter consists of terminal controllers and a master controller, as shown in Fig. 1.
The main functions for controlling the active power, reactive power and dc voltage are incorporated in the terminal controllers. The master controller is provided with the minimum set of functions necessary for coordinated operation of the terminals in the dc circuit, such as start and stop sequences, power flow reversal.
Fig. 1.Hierarchy of control system.
There is no need for an exchange of signals between the two terminals; the system achieves a high degree of independence with minimal reliance on communication systems. The automatic frequency control (AFC), emergency power dispatch and other functions related to ac system control that are employed in conventional line-commutated converters can be also added above the master controller if necessary, according to the ac system conditions.
/*两个终端之间不需要交换信号； 系统达到高度独立以对通信系统的最小的信赖。如果交流系统需要，传统线性变化器的自动频率控制，应急电源，和其他与交流系统有关的功能 也可以被加进主控器*/
The high-performance ac/dc converter is able to control active and reactive power independently by employing the amplitude and the phase of the ac output voltage as two state variables. In order to realize this function, the instantaneous values of the three-phase voltage and current are converted to the d-q coordinates in the ACR block of the terminal controller so as to suppress interaction between the active and reactive components, as shown in Fig. 2.
INTERCHANGE POWER CONTROL USING VOLTAGE MARGIN
In a voltage-source converter, if there is any imbalance between the input power of one terminal and the output of the other, the energy stored in the dc capacitor may change and therefore the dc voltage may fluctuate from its nominal level. In order to prevent dcovervoltages and undervoltages and to continue operation of the converter, the interchange power between the rectifier (REC) and the inverter (NV)should be coordinated.
The voltage margin method for interchange power control was applied to the dc voltage control block (DC-AVR) and the active power control block (APR) in the terminal controller. When the active power is injected from the dc circuit to the ac system at one terminal, the power is supplied from the ac side to the dc side at the other terminal so as to compensate for dc voltage fluctuation. Consequently, the two terminals can be coordinated without relying on a communication system to maintain constant dc voltage.
The voltage margin is defined as the difference between the dc reference voltages of the two terminals, for the purpose of active power interchange between terminals having the characteristics shown in Fig. 3(a) and (b). When power is to be transferred from terminal A to terminal B, the voltage margin is subtracted from the dc reference voltage for terminal B.
The intersection of the characteristics of the two terminals in Fig. 3(c) is the operating point; the dc voltage is determined by the dc reference voltage of
terminal A (REC), and the active power is assigned by the lower limit of terminal B 。In the block diagram of Fig. 2, the voltage margin is the input value to the DC-AVR block. The lower limit of the DC-AVR block is a variable limit, and is controlled by the output value of the APR block. When the active power reference value is modified, the lower limit changes and the interchanged
active power can be controlled.
The power flow direction of the two terminals can be easily reversed with the voltage margin method. As indicated in Fig. 3(d), by resetting the dc reference voltage to its nominal value for terminal B and subtracting the voltage margin for terminal A instead, the operating point is moved and the power flow can be reversed. Thus, in the voltage margin method, the dc voltage and the active power are separately controlled by the rectifier and the inverter, respectively.
In a two-terminal configuration when a terminal is tripped out, the active power is no longer interchanged. Hence the operating point, which has been at the intersection of the characteristic lines for the two terminals, shifts to the zero active power point along the characteristic of the healthy terminal in order to maintain constant dc voltage. This operation is automatically completed, even if no communication signals are exchanged. Even in the event of a serious fault, as long as a healthy terminal remains, the dc voltage control system continues its function. Thus, dc voltage fluctuations are maintained within the small range of the voltage margin.
IV.PROTOTYPE CONTROL EQUIPMENT
In order to confirm the operating performance of the control method, prototype control equipment was manufactured and tested with an analogue simulator. An external view of the equipment is shown in Fig. 4.
Since the high-performance converter is of the voltage- source type, in the event of a power system fault the control system should respond instantaneously so that overcurrents do not occur. The prototype equipment uses high-speed digital signal processors and microprocessors.and enables a sampling period of 200 ps (an electrical degree of 3.6 at 50 Hz) for the ACR and PWM blocks,where rapid response is required. In the prototype equipment, all of the control functions including 9-pulse PWM are carried out by digital processing.
V. SIMULATOR TEST RESULTS
A. Configuration of Test Circuit
In order to verify the performance of the prototype equipment for active and reactive power control, and to confirm continuous operating performance for voltage decreases, waveform distortions and other disturbances in the ac power system, we conducted various tests combining converter models employing actual switching devices with an analogue power system simulator. The basic specifications for the power system simulator are shown in TABLE I.
As shown in Fig. 5, the test circuit consists of a 50 Hz and a 60 Hz power network connected to a back-to-back (BTB) system. AC/DC converters are linked to the tertiary winding of a main transformer (66 kv> at a 275 kV / 500 kV substation on the 50 Hz system side, and to a 275 kV bus on the 60 Hz side. The nominal active power is 37.5 MW,and each terminal comprises four converter bridges. /*如图5所示，测试电路包括连接了BTB系统的50Hz和60Hz的网络。交直流变换器连接在275KV/500KV变电所的66KV主变压器的三相绕组和60Hz边的275KV总线上。正常有功功率是37.5MW，每一端包含4个变换器桥。*/
B. Simulator Test Results
The following tests were conducted.
-Tests to confirm basic operating performance /*验证基本工作性能的测试*/
* Start and stop/*启动停止*/
* Modification of references (active power, reactive power, ac voltage)
* Power flow reversal /*电流反向*/
-Tests to confirm continuous operating performance
AC voltage disturbances (voltage fluctuations, voltage imbalance, frequency fluctuations)
Adjacent ac equipment operation (opening/closing of capacitor banks, transformers, transmission lines)
AC system disturbances (ground faults in ac transmission lines)
DC terminal tripping-out
1) Tests to Confirm Basic Operating Performance
Fig. 6 shows the results of the test for power flow reversal under the condition that the active power is nominal. The voltage margin switches in the terminal controllers of both converters are opened and closed so as to reverse the power flow in accordance with requests from the master controller. The active power flow is reversed quickly, within 90 ms, with very little effect on the reactive power output. A slight fluctuation in the dc voltage of less than 10% is observed during power flow reversal, however after reversal, the dc voltage is stabilized at unity.
2) Tests to Confirm Continuous Operating Performance
Fig. 7 shows the response of the converter when a one-line grounding fault (1LG) occurs at the ac transmission line on the 60 Hz side (terminal B: INV). Even during such a malfunction, the converter continues to operate without causing overcurrents in the output current.
The active power at terminal B contains the second harmonic ripple owing to the negative-phase-sequence component of the system voltage; however, the average power is still interchanged at the level of about 2/3 before the fault. At the same time the output power of terminal A (REC) is limited automatically, so that the dc voltage is maintained at 1 pu. During this period, there is no exchange of signals between the terminals. Immediately after the fault is cleared, the system quickly returns to its power flow condition prior to the fault.
Fig. 8 shows the response in the event of tripping-out due to gate block at the 50 Hz terminal (terminal A: REC). When terminal A is tripped out, active power can no longer be exchanged; however, the healthy 60 Hz terminal (terminal B: INV) can control reactive power continuously. After the rectifier, which has been regulating dc voltage, undergoes tripping-out, the healthy inverter terminal should maintain the dc voltage in order to continue operation.
In the voltage margin method, the operating point can be shifted at P=O in the characteristics of either terminal. When the inverter remains in operation, by switching its mode from active power control to dc voltage control, the decrease in dc voltage can be held within the range of the voltage margin (10%). The switching of control mode mentioned above is completed automatically, by detecting the decrease in dc voltage on its own side even when there is no exchange of signals.
It was also confirmed through various tests that the active power and reactive power can be controlled without interference and that they can follow their reference values accurately and quickly. Further, it was verified that the converters can continue stable operation during ac system voltage fluctuations, adjacent ac equipment operation and ac system disturbances. These results demonstrated that the control scheme for the high-performance ac/dc converter has been successfully established.
VI. CONCLUSIONS /*结论*/
A control system for the high-performance ac/dc converter has been established. This system employs the voltage margin method to realize interchange power control, interference-free control of active and reactive power, and suppression of overcurrents with high-speed sampling processing.
The operating performance of the control system confirmed through tests using a power system simulator is summarized as follows:
(1)Active and reactive power can be controlled with no mutual interference.
(2) Continuous operation is possible even in the event of disturbances in the ac power system.
(3)In the event of terminal tripping-out as well, the healthy terminal can continue operation without depending on any means of communication between terminals.
This R&D program is also engaged in the development of high-performance ac/dc converter main circuits and converter transformers. We are planning to combine
control/protection equipment with 37.5 MW GTO converters and converter transformers at the ShinShinano Substation of Tokyo Electric Power Co., Inc. A
three-terminal BTB system is to be connected to actual power systems and field testing will be conducted.
VII. ACKNOWLEDGEMENTS /*致谢*/
The authors would like to thank the Agency of Natural Resources and Energy for supporting this project and the electric power companies of Hokkaido, Tohoku, Hokuriku, Chugoku, Shikoku and Kyushu for their cooperation as members of the committee for the project. The authors also wish to acknowledge Toshiba Corp., Hitachi Ltd., and Mitsubishi Electric Corp. for their help in manufacturing the prototype controllers.
VIII. REFERENCES /*参考文献*/
[l] Y. Sekine, T. Hayashi, et al., "Application of Power Electronics Technologies to Future Interconnected Power System in Japan," Proc. of the 1995 CIGRE Tokyo Symposium, N0.210-03, May 1995.
 H. Suzuki, T. Nakajima, et al., "Development and Testing of Prototype Models for a High-Performance 300 Mw Self-Commutated ACDC Converter," IEEE Power Engineering Society Summer
Meetiing,NO.96 SM 448-1,1996.
 S. Mori, K. Matsuno, T. Hasegawa, et al., "Development of a Large Static Var Generator Using Self-Commutated Inverters for Improving Power System Stability," LEEE Power Engineering Society Winter M~tiw,N0.92 WM 165-1, 1992.
 F. Ichikawa, K. Suzuki, T. Nakajima, et al., "Development of Self－Commutated SVC for Power System," IEEE Conference Record of the Power Conversion Conference, Yokohama, April 1993.
 S. Horiuchi, et al., "Control System for High Performance Self-commutated Power Converter," CZGRE Group 14 Session, No.14-304, Paris, August 1996.