RESEARCH ON THE ADAPTIVE CONTROL AND MONITORING SYSTEM OF 10 kv CLASS THYRISTOR SWITCHED CAPACITOR EQUIPMENT

Transcription

1 RESEARCH ON THE ADAPTIVE CONTROL AND MONITORING SYSTEM OF 10 kv CLASS THYRISTOR SWITCHED CAPACITOR EQUIPMENT Guangfu TANG, Jiao ZHANG, Yuanliang LAN, Menggan REN China Electric Power Research Institute ABSTRACT Thyristor Switched Capacitor (TSC) is very useful to control the reactive power and system voltage in power system. China Electric Power Research Institute (CEPRI) has been developed a set of 10kV TSC equipment. The equipment has been commissioned in NanYang, Henan Province. An overall digital control and monitoring system has been used in the equipment. The control system adopts adaptive voltage-reactive power control strategy, which distinguishes the system condition automatically. Based on the system condition, the control system can decide which control strategy is adopted, and achieves the optimal control. Field bus technology is used in the system. It is easy for all control units to realize data sharing. At the same time, a redundant configuration of the control system and capability of distinguishing fault unit are obtained. The synchronization with system voltage is very important for power electronics equipment suppliers. In the control system, a special synchronization and detecting technology is used. The technology can strictly ensure the control synchronization with system voltage. At last, a distributed layer structure is used in the controller. The control and monitoring system can detect the fault unit and record the fault type easily by the structure. Thus, the function and reliability of the whole control system is improved greatly. 1 INTRODUCTION In distribution systems capacitor banks are often used for power factor compensation. When being used to switch capacitor banks mechanical breakers generate large current transients. These transients may cause serious system disturbances, premature wear of the breaker contacts, and possibly shorten the life of capacitors. These problems may be acceptable if the capacitors are not switched often. However, if switching occurs frequently every day the disturbances caused could be intolerable. Unlike mechanical breaker, the thyristor switched capacitor (TSC) realizes control of instant of switching, and eliminates almost completely the voltage and current transients. The TSC provides instantaneous reactive power by switching the appropriate capacitors. In this paper, a 10kV class TSC technology was developed at China Electric Power Research Institute, with the Sponsorship of State Power Corporation, China. TSC is one of the perfect equipment, which meets the demand of the reactive power in distribution system. The typical TSC consists of breaker, reactor, capacitor, thyristor valve and controller. The control and monitor system of TSC is the intelligentized equipment that exclude the use of regulating, monitoring and protecting functions. It can realize rapid adaptive regulation, and has friendly human machine interface as well. The powerful calculating ability of the controller is necessary to utilize the rapid response characteristic of the thyristor valve. Meanwhile, it can meet the need of different operation mode in distribution system. As intelligentized equipment, TSC s controller should have the function of self- monitoring and protection. Furthermore, it must be highly reliable and easy maintenance. In order to realize these functions mentioned above, the distributed layer structure is applied in TSC s controller. In this paper, the design conception of an adaptive control and monitoring system of the TSC is introduced, including adaptive control, protection system, monitoring system, valve s triggering, and multi-synchronization technology. 2 GENERAL DESCRIPTION OF 10kV TSC The equipment has been put into operation in Dongjiao substation in NanYang, Henan Province. Figure 1 shows the system connection single line diagram. The substation is a 10kV switch substation. The figure 2 shows basic structure diagram of the distribution system before installing of TSC. 110kV Transformer (110kV/10kV) 7. 2Mvar capaci t or St ep- s down u vol t age North Fig.1 Single-line Diagram of Power Supply 10kV FC km 10kV Fig.2 Single-line Diagram of Substation The structure of 10kV bus in Dongjiao substation is double buses that can be connected by a circuit breaker. In normal mode, the two buses operate respectively, and the breaker connected the two buses is open. In the case of system fault, the bus connection breaker is closed. So double bus structure is used in single bus operation. Of course, there is only one power supply available. As shown in figure 2, a set of fixed TSC ( ) ( ) Switch substation FC2 10kV South Load1 Load2 Load3 CEP_GFTang_A1 Session 1 Paper No

2 capacitor bank has been installed in either bus. It can be switched on/off according to the system reactive power requirement. The capacity of the fixed capacitor bank installed in north-bus is 2.7Mvar (FC1) and that in south-bus is 2.2Mvar (FC1). After analyzing the whole system, it is concluded that the capacity of the fixed capacitor bank is unreasonable. Therefore, it cannot meet the system reactive power requirement. Figure 3 shows the reactive power change with the fixed capacitor bank switched on/off. It is easy to understand that the maximum over-compensation is 700kvar, and the maximum under-compensation is 350kvar per phase. The normal dynamic reactive power requirement is approximative 1MVar. Now, a new compensation scheme is adopted. TSC and fixed capacitor (FC) take the place of the original fixed capacitor bank FC2. The compensation capacity of FC and TSC is 1.2MVar, 1Mvar respectively. The TSC is divided into two step banks. The capacity ratio of the two step banks is 1:2. Therefore it can realize three level compensation modes. Human machine interface Valve monitoring unit Triggering unit As mentioned above, the control and monitoring system is a distributed layer structure. Among these units, redundant protection unit, valve monitoring unit and human machine interfaces are intelligentized devices. The system share data by CAN field bus that will be discussed below. This technology improves the reliability and the flexibility of the whole control system. Figure 5 shows the structure of the control and monitoring system. Human machi ne interface Chief monitor RS485 CAN BUS Redundant pr ot ect i on Regul at i on Val ve moni t or Tr i gger i ng dr i ver Fig.5 Block Diagram of the Control and Monitoring System Fig.3 Reactive Power Curves with Automatic Switching on/off Capacitor Bank 3 DESCRIPTION CONTROL AND MONITORING SYSTEM 3.1 General introduction The main functions of the control and monitoring system are protection, measurement and regulation. The main function blocks of the TSC are shown in figure 4. CT PT Sensing circuit 10kV Power System Control Triggering Snubber circuit circuit circuit Switching circuit Fig.4 Function Block Diagram of the TSC The control and monitoring system of TSC contains the following units: monitor unit Redundant protection unit Regulation unit C L CT C L Protection circuit Field Bus becomes popular in 1990s. It is a kind of advanced industrial measure and control technology. It introduces communication network technology into industrial measure and control filed. In fact, field bus is a digital communication protocol, and a communication network that connects field devices with control systems. It is also a digital bi-directional transmission system. Its structure is distributed layer and multi-branch. It covers the measure and control technology, industrial instrument technology, and computer network technology. Above all, it represents the development of industry measure and control technology in the future. By now, the following field buses technology are wildly used, such as CAN, LONWORKS, FF, PROFIBUS, DUPLINE, HART, LAN and so on. The main reasons choosing CAN bus in this system is described as following. The protocol is simple The CAN bus has been a well-proved technology System structure is reliable The cost is cheaper 3.2 Monitoring and protection Because of the limitation of the traditional relay protection, it can t be applied in the TSC s equipment as the major protection mode. It just can be applied as a backup of the protection system. The main limitations are presented below. (1) The response time of TSC is very fast, about 30 ms. Thus it is evident that the traditional relay protection cannot meet the requirement of TSC. (2) The protection types in traditional relay are very restricted. That is to say, it has a big dead-area. For these reasons mentioned above, an advanced digital protection method has been adopted for the TSC. The CEP_GFTang_A1 Session 1 Paper No

3 protection is embedded in the control system. Monitoring system consists of major monitoring unit and redundant protection unit. The main task of major monitoring unit is to monitor the status of thyristor valve and controller. Meanwhile it can locate fault position in the equipment, and acts exactly according to preestablished program. The redundant protection unit carries out the tasks of serious fault protection and monitors the real-time state of the major monitoring unit. The major functions of protection and monitoring system are given as following: bus overvoltage, phase-phase fault, phase-ground fault, capacitor protection, valve overcurrent, valve monitoring, temperature rise of valve hall, power supply fault of controller, hardware fault of controller, and incorrect trigger signal. All signals obtained by monitoring unit are transformed into standard I/O signals or analog signals, and then sent to a processor by sampling. In order to ensure the correct response, a logic flow diagram is adopted in software, which is shown in Figure 6. No Faul t Cont r ol l er star t up P1 TSC Breaker Tri pped Act i ng Upon ItemTAB. 1 S1 TSC Br eaker Closed Command Cl ose TSC Br eak er P2 Success Unbl ock del ay Cont rol l er fai l ure Reset Fai l ur e TSC Breaker Cl osed Fig.6 State flow chart of monitor logic The software was written in C language. The technology of object-oriented programming is used. So each event is regarded as an object whose parameter, protection configuration, transformation vector and disposal measure are combined in a single object package. A unique event is triggered when a monitoring item reaches its set value. The event-triggering principle is used in the software. It makes the complex logic process become a simple event. Furthermore, it prevents potential logic errors occurring. At the same time, it makes software compact and easy to maintain by the modularization programming technology. 3.3 Adaptive control strategies The control strategy of the controller is very critical important to the performance of whole equipment. There are two important factors for the controller performance. One is the hardware that must have a strong calculation capability. Another is how to select the suitable control strategy in order Trigger Bl ocked Ac t i ng Upon ItemTAB. 2 S2 Ac t i ng Upon ItemTAB. 2 S3 Unbl ocked TSCFai l ur e Tri gger Not Bl ocked Ac t i ng Upon ItemTAB. 3 S4 to reach an optimal control. Here, an improved PID control method is adopted. The control objects of controller are system reactive power and 10kV-bus voltage. The controller can realize optimal control by selecting the appropriate ratio coefficient of the controller object. Figure 7 shows the control block diagram. u i Ku Ki K1/(1+K2S) K3/(1+K4S) Fig. 7 Control Block Diagram During the development of the equipment, it becomes more and more evident that a single control strategy cannot meet the need of system multi-operation mode. It is obvious that there are two operation modes by the main circuit in figure 2. One is single bus operation mode. The other is double bus operation in parallel connection. At the same time, another complicated operation mode needs to be considered. If each bus is installed a set of TSC equipment, the control strategy must be improved again so that the two TSCs can operate concordantly in this situation. The system operation mode can be automatically distinguished and identified by collecting the breaker working status. Thus, it is necessary to establish codes for every breaker working status, code 1 denotes the ON status of the breaker, and code 0 denotes OFF status of the breaker. A six-bit vector is used to distinguish the system operation status. It indicates respectively south-bus fixed capacitor breaker, north-bus fixed capacitor breaker, TSC breaker, bus connection breaker, south-bus breaker, and north-bus breaker from the minimum bit to the maximum bit. The vector is defined as a pointer, which points to a 26 vector table. In this vector table, some parameters determine the control strategy. These parameters include the constant of proportion, the time constant of voltage integral, the time constant of voltage differential, the time constant of reactive power differential, the constant of reactive power integral, the weight coefficient of reactive power and voltage. In actual equipment, a priority coefficient is also defined to coordinate the probable operation of the two sets of TSC equipment. For example, table 1 gives the priority coefficient. The parameter that the operation vector points to shows whether there are two sets of TSC equipment operation or not. Of course, If only a set of TSC equipment is put into operation for the distribution system, the parameter will be set equal to zero. Table 1. Priority coefficient for two sets of TSC equipment Level TSC1 TSC2 Kd Ke Fu(S) ² Wz(S) Wc(S) Z Fi(S) CEP_GFTang_A1 Session 1 Paper No

4 Operation Nonoperation Operation Nonoperation On the other hand, the TSC controller must abandon the ability of regulating reactive power in order to keep the system voltage stability in some special operation mode. For example, when the system voltage is much higher or lower from a standard value, the TSC controller will adjust the weight coefficient of reactive power and voltage, and increase weight coefficient of system voltage till voltage regulation mode according to system voltage level. Therefore, the controller can realize the optimal control by coordinating system voltage level and the parameters that come from the vector table in all kinds of situation. 3.4 Multi-synchronization technology The reliability of synchronizing circuit is very important for power electronics-based equipment. Generally, the controller obtains synchronous signal from the system voltage by its voltage transformer (PT), gain adjustor, low-pass filter, phase shift, and zero across detector. In fact, some abnormality situations could be met as follows: PT broken. The element fault of the signal adjustment channel. The system voltage disturbance. The temperature excursion. All situations mentioned above can causes synchronous failure and this is very dangerous for the thyristor valve. Now, a novel type synchronizing circuit is designed in order to obtain correct and reliable synchronous signal. False triggering must be avoided in the TSC equipment that are caused by the PT broken or the severe distortion of system voltage, which could be detected by monitoring-controlling system with some time delay because of acquiring and acting. Therefore, a new fast and sensitive method is adopted to reduce this risk. The new type synchronizing circuit is showed in Figure 8. system voltage signal conditioner A signal conditioner B crystal oscillator zero voltage comparator A zero voltage comparator B digital shifter A digital shifter B Long per i od synchr oni zat i on counter/timer zero crossing point 1 zero crossing point 2 zero crossing point 3 zero crossing pulse zero crossing point detector Fig.8 Block Diagram of the Zero Crossing Channel zero crossing fault signal A digital phase shifter, witch has a shorter integrate time constant and better transient performance, replaced an analog shifter. At the same time, two signal adjusting circuits work together that have different turndown frequencies and integrate time constant. There different transient response time between the two circuits when a large disturbance happens in distribution system. Additionally, a high steady crystal oscillator is used to implement the third synchronizing circuit, which is also used for zero cross detecting. The high frequency and steady pulse generated by the high steady crystal oscillator is transferred to calculator which is used for cycle timer and once synchronized according to system voltage every 5 to 10 cycles. When the point crossing system voltage zero suddenly varies with some outside disturbance, the position crossing zero produced by the third synchronizing circuit will keep the same as before during the interval of synchronization. Then, an excursion value from the actual zero crossing is generated by a special zero crossing detectors. The synchronization monitoring system will stop sending out the triggering signal when the excursion value is beyond a set value. When any phase synchronization monitoring system produces malfunction for three-phase system, all triggering signal will immediately be stopped. 5 THE TECHNOLOGY OF VALVE S TRIGGERING AND MONITORING The design of triggering circuit is an important aspect for power electronics equipment because it is directly related to the thyristor valve s switching on/off capability. The reliability of valve which consists of semiconductor devices with limited voltage/current stresses will determine the reliability of the equipment in comparatively extent. Of course, monitoring and protection circuits are also necessary for thyristor valve especially in series or parallel operation except for triggering circuit. Semiconductor devices such as thyristors in series operation are widely applied in transmission and distribution system, for instance, HVDC, SVC, TCSC, and so on. Triggering and monitoring methods are different between devices in series manner and a single device. At present, three kinds of triggering methods are used in power electronics equipment as following: optic-electric triggering electric-magnetic triggering light triggering directly Although the monitoring method is not same according to different application, insulation method can only be divided into two types: fiber-optic insulation and electromagnetic insulation in theory. Electric-magnetic triggering and fiber-optic monitoring are used in the TSC equipment with small capacity in order to reduce the cost. The advantage of the electric-magnetic triggering is that it does not need complicated circuit of fetching energy in high potential. If the fetching energy method at high potential is selected for the TSC application, fetching energy by voltage and current CEP_GFTang_A1 Session 1 Paper No

5 manner need to be synchronously considered during the on-state and off-state of a thyristor valve level. The fetching energy electronic circuit is relatively non-reliable for whole equipment. On the other hand, the electric-magnetic triggering can simplify design, improve reliability, and reduce cost. The electric-magnetic triggering circuit of high frequency for the TSC equipment is shown in figure 9. Here, the primary current waveform of the pulse transformer PT is shown in figure 10, and the triggering current waveform of thyristor is shown in figure 11. main controller Fig.9 Principal Diagram of the Triggering Circuit 1 > triggering latching detecti ng opticelectri c couplers detecti ng triggering latching logical switching amplifier detecti ng ci rcuit on/off 1) Ref A: 1 Volt 50 us Fig.10 Waveform of PT Primary Current (1A/div) R C switching power pulse transformer power transi stor thyri stor at high potential, where the thyristor voltage can be transformed into a high frequency light pulse sequence. Then, the high frequency light pulse sequence is transferred to controller by fiber-optic, and is transformed into electric pulse sequence and decoded again. The energy of these monitoring circuits is sent through pulse transformer from low potential. Of course, the voltage of the thyristor valve level can be expected when the valve is not triggered. In the non-triggering condition, zero voltage indicates that the thyristor valve level is burnt-out or fuses of capacitors are broken. In the triggering condition, the controller doesn t response the voltage. 6 ELECTROMAGNETIC COMPATIBILITY (EMC) During development of the control and regulating system, EMC is one of important topics. A strong EMC for a system could guarantee the equipment operating reliably in a strong noise environment. The EMC of industry equipment includes not only the knowledge of electrotechnics theory but also practical experience. After extensive research and experiment, it is concluded that the fast transient disturbance has the most serious negative effect to control system. And this kind of signal disturbs the control system via its power circuit, signal circuit and communication circuit. So some methods have been done to deal with minimize this disturbance in the control system. These methods are as following: To provide absolute power supply with a different grounding system for all units. To isolate all input/output signals by optical coupler. The circuit is shown in fig.12. To separate digital ground from protection ground. To suppress the fast transient signal by a lot of special components. As a conclusion, the control system has passed all kinds of the type test according to IEC IEC > V1 VC Signal output V2 1) Ref A: 50 mvolt 20 us Signal ground 1 Signal input Fig.11 Triggering Current of Thyristor (125mA/div) Electric-magnetic triggering and monitoring/detecting insulated by fiber-optic are used in the TSC with small capacity in order to reduce the cost. Namely fiber-optic is used in the monitor/detecting circuit as channel of status signal transfer and insulation medium between high potential and low potential. The voltage of every thyristor valve level is monitored by a monitoring electronics circuit 7 CONCLUSION Communication ground Unit A Outside Connection Unit B Fig.12 Schematic Circuit of Isolated I/O Signal ground 2 TSC equipment provides a very effective method to control the reactive power and system voltage. The TSC equipment, CEP_GFTang_A1 Session 1 Paper No

6 which is developed by CEPRI, has been commissioned in NanYang, Henan province. This equipment is the first TSC equipment that is developed in China. Because of fast switching on/off of thyristor valves, the equipment has a very fast response with the change of reactive power and system voltage according to control strategy. Now in China, this equipment has a very good prospect in distribution system, which faces the problems of insufficiency of reactive power and power quality. REFERENCE [1]EMC and The Printed Circuit Board: design, Theory, and layout Made Simple Mark I. Montrose, IEEE PRESS, 1999, ISBN X [2]K.Engberg, H.Frank, B.Klerfors: Thyristor Switched Capacitor, TSC, in Theory and Practice. Paper presented at IEE Fourth International Conference on AC and DC Power Transmission, London UK, September 1985 [3]M.A.El-Sharkawi, S.S.Venkata, S.V.Vadari, M.L.Chen, Development and Field Testing of an Adaptive Power Factor Controller, IEEE Transations on Energy Conversion, Vol. EC-2, No. 4, December 1987 [4] M.A.El-Sharkawi, S.S.Venkata, T.J. Williams and N.G.Butler, An adaptive Power Factor Controller for Three-Phase Induction Generators, IEEE Transaction on Power Apparatus and Systems, Dec 1985, pp [5] Guy Olivier, et al, Minimal Transient Switching of Capacitors IEEE Trans. On Power Delivery, Vol.8, No 4 October [6] M.F.McGranaghan, et al, Impact of Utility Switched Capacitors on Customer Systems-Magnification at Low Voltage Capacitors, IEEE Trans. on Power Delivery, Vol.7, No.2, April He received his B.Eng. in electrical engineering in 1990 from Xi an Jiaotong University, China. His M.Sc. and Ph.D. in Electrical Engineering were received from the Institute of Plasma Physics, Academia Sinica in 1993 and 1996 respectively. Then he joined EPRI China, where he is currently a senior engineer. Zhang Jiao was born in 1972, China. He received his B.Eng. in electrical engineering in 1993 from Southeast University, China. In 1993, he joined the EPRI, where he is currently an engineer. Lan Yuanliang was born in 1970, China. He received his B.Sc. in application of computer in power system and M.Sc. in Electric Engineering from Northeast Institute of Electric Power, China in 1994 and 1997 respectively. In 1997, he joined the China EPRI, where he is currently an engineer. Ren Menggan was born in 1974, China. He graduated from the Dalian University of technology in 1996 and received his bachelor degree. In 2002, he receives his master degree from China Electric Power Research Institute (CEPRI). Now he joined Power Electronics Corporation, CEPRI. His special fields of interest included power electronics, automation and computer. Now he is developing static var compensators equipment.. Tang Guangfu was born in 1966, China. CEP_GFTang_A1 Session 1 Paper No