Studies on the Effects of Thyristor Dynamic Extinction angle on HVDC Commutation Failure

Transcription

1 IEEE PEDS 2017, Honolulu, USA December 2017 Studies on the Effects of Thyristor Dynamic Extinction angle on HVDC Commutation Failure Weihua Xu, Xiaoguang Wei, Kunpeng Zha, Zhiyuan He, Jie Yang Abstract This paper focuses on the key technology of ultra high voltage direct current (UHVDC) converter valve equipment which is resistant to commutation failures. The simulation model is established, which can reflect the whole turn-off process of thyristor. The paper also offers the simulation models of converter valve model and of the double-fed high voltage direct current (HVDC) system. The transient fault and the failure mechanism of commutation failure are studied. The internal and external factors that affect commutation failure are analyzed in depth and the impact of operating conditions on commutation failure is quantified. The calculation method of the thyristor dynamic turn-off time and the related key technical indexes are proposed to optimize the commutation characteristics of the high power thyristor. This paper presents the method of intelligent trigger detection, such as forward voltage monitoring method, turn-off angle direct measurement method and minimum turn-off angle estimation method, together with their application approach in multi-infeed HVDC system (MIDC). The intelligent trigger monitoring device and the converter valve module prototype which is resistant to commutation failure are developed, and then verified by functional test. Index Terms Commutation Failure, Converter Valve Equipment, Control Strategy C I. INTRODUCTION OMMUTATION failure is one of the common faults in the inverter side in the operation of HVDC system. Commutation failure leads to the reduction of the inverter direct current (DC) voltage over a period of time, to the increasing of DC current, to the reduction of DC transmission power, as well as the intensify of converter transformer DC magnetic bias. The This work was supported by State Grid Corporation of China, with the project of Research on Topology of UHVDC bypass impedance device. Weihua Xu. Author is with Global Energy Interconnection Research Institute, Changping District, Beijing, , China. ( xuweihua@geiri.sgcc.com.cn). Xiaoguang Wei. Author is with Global Energy Interconnection Research Institute, Changping District, Beijing, , China. ( weixiaoguang@geiri.sgcc.com.cn). Kunpeng Zha. Author is with C-EPRI Electric Power Engineering Co. Ltd, Changping District, Beijing, , China. ( zhakp@chinaepri.sgcc.com.cn). Zhiyuan He. Author is with Global Energy Interconnection Research Institute, Changping District, Beijing, , China. ( hezhiyuan@geiri.sgcc.com.cn). Jie Yang. Author is with Global Energy Interconnection Research Institute, Changping District, Beijing, , China. ( yangjie@geiri.sgcc.com.cm). life of the converter valve is shortened by it and the voltage instability is increased in the inverter side of weak alternating current (AC) system and other adverse consequences [1]. If the system is not properly controlled after commutation failure, it will lead to a continuous commutation failure and eventually shut down the DC system. In addition, the input current of AC system will also change during the period of commutation failure, which changes the direction of the system trend and may lead to the misoperation of AC protection system [2]. Therefore, the study of commutation failure is critical to the safe operation of the system and the grid. Up to now, there has been a great deal of research outcomings on commutation failure at home and abroad, but there is little research on converter valve device itself, and rarer research has considered the actual turn-off time of the converter valve. Literatures [3]-[4] analyzed the causes of commutation failure. Literatures [5]-[6] analyzed the influencing factors of commutation failure. Literatures [7]-[8] studied the criteria for commutation failure. Literatures [9]-[10] studied the method of detecting the commutation failure, such as method of wavelet transform, etc. Literature [11] proposed some recovery measures from commutation failure. Literature [12] used different simulation softwares to simulate commutation failure. Literature [13] studied the influence from AC system failure on commutation failure. In literature [14], the simulation of the detection of commutation failure, of fault recovery and of related protection are carried out according to the specific HVDC project. Literature [15] studied the commutation failure of multi-infeed HVDC system (MIDC). Literature [4] analyzed the fault current of the inverter commutation failure ideally. In the above research results, the critical extinction angle of the converter valve is defined as a fixed value (about 8 ), which is independent of the operating conditions and operating environment. However, in actual operations, the turn-off time of the converter valve is dynamically co-related to the operating conditions and operating environment, whether directly determines the commutation failure of converter valve occurs, and has an indirect effect on the control margin of the extinguish angle of the control system. Therefore, it is of practical significance to study the commutation failure on the basis of the dynamic critical extinction angle of converter valve equipment and to study the method to resist commutation failure from the perspective of the converter valve equipment. In this paper, based on the analysis of the thyristor turn-off process, a simulation model which could reflect the whole turn-off process of thyristor is established. The internal and external factors that affect commutation failure are analyzed in depth and the impact of operating conditions on commutation /17/$ IEEE 795

2 2 Weihua Xu et al.: Studies on Effects of Dynamic Extinction angle of the Thyristor on HVDC Commutation Failure failure is quantified. The calculation method of the thyristor dynamic turn-off time is put forward to optimize the commutation characteristics of the high power thyristor. This paper presents the method of intelligent trigger detection and puts forward their application approach in MIDC. The intelligent trigger monitoring device and the converter valve module prototype which is resistant to commutation failure are developed, and verified by functional test. characteristic into the classic macro model of thyristor. The model could reflect the thyristor turn-off process completely, could simulate the various operating states and characteristics of thyristor accurately. It also contributes to the study of the commutation failure and expands the scope of macro model. II. COMMUTATION PROCESS ANALYSIS OF CONVERTER VALVE The Silicon Controlled Rectifier (SCR) valve used in HVDC system is the semi-controlled power device with switching characteristic. It must withstand the forward voltage and the gate trigger pulse to turn from the off state into the conduction state. Only when the current flowing through the valve is zero, i.e. the excess carrier in the valve disappears, it will turn from the conduction into the off state. When current switches from one valve to another, the effect of the inductance in commutation circuit makes it impossible to change the current through the valve suddenly (hence the current through the inductor is continuous and can t change suddenly). It takes a while to complete the current conversion process between the two valves, and such process is called the commutation process. In the commutation process, the two valves which participate in commutation in the same half-bridge are in conduction state, which makes the converter transformer valve side winding into short-circuit state. In the newly conducted valve, the current direction is the same as the short circuit current direction, the current rises from zero to the steady value of DC current. In the closed valve, the current direction is opposite to the short circuit current, the current falls from the steady value of DC current to zero and then turns off. During a period of reverse voltage effect, it is not possible to restore the blocking capability when the valve just exits the conduction state or the commutation process has not been completed. In both cases, when the valve voltage is turned into positive, the valve which is predetermined to conduct is inverted from the valve which is predetermined to exit the conduction state, it is called commutation failure. The valve needs a certain time to restore the positive blocking capability, since that the carriers deionization recovery time is about 400~900 microseconds. The recovery time corresponds to an electrical angle, which is defined as the critical extinction angle [3][4], and the minimum value to turn off the valve is denoted as γ min. Triggering angle error, AC voltage changes, DC current changes and so on will directly lead to the decrease of turn-off angle γ. Commutation failure occurs when γ <γ min. III. ACCURATE THYRISTOR SIMULATION MODEL The conventional macro model does not clearly illustrate the forward blocking recovery characteristics of the thyristor, while the forward blocking recovery characteristic directly affects the success or failure of the thyristor turn-off process. Therefore, in order to study the commutation failure process accurately, as well as the operating characteristics of the converter valve, we introduce a description of the forward blocking recovery Fig. 1. Thyristor macro model structure The basic structure of the thyristor macro model is as shown in Fig. 1, and the whole model consists of four parts: main block, control block, reverse recovery block and forward blocking recovery block. The model uses different branch circuits to describe the various operating states and characteristics of thyristor, including turn-on and turn-off control, turn-on delay, latching current, forward voltage rise rate, forward and reverse leakage current, forward and reverse breakdown voltage, forward bias, reverse bias, reverse recovery process and forward blocking recovery process. The voltage and current waveforms of the thyristor turn-on process and turn-off process are as shown in Fig.2 and Fig.3. Fig. 2. Voltage and current waveforms of the thyristor turn-on process Fig. 3. Voltage and current waveforms of the thyristor turn-off process 796

3 Weihua Xu et al.: Studies on Effects of Dynamic Extinction angle of the Thyristor on HVDC Commutation Failure 5 IV. DYNAMIC TURN-OFF TIME OF THE THYRISTOR Through the front accurate simulation of the thyristor whole turn-off process, we get the factors that affect the turn-off of thyristor. First, when the forward voltage is applied to the thyristor again, the residual carrier density in the N-base region is determined by the forward on-state current I F, the current decrease rate di/dt, the turn-off reverse voltage V rr, the carrier life τ, thyristor junction temperature T j and forward voltage application time. The magnitude and duration of the forward recovery current are determined by the residual carrier density in the N-base region, the applied dv/dt amplitude and the loop inductance. And the trigger sensitivity of the thyristor determines whether the amplitude and duration of the forward recovery current can arrive the recovery current pulse amplitude that causes the opening of the thyristor gate. Residuals Residual Case Order Plot Case Number Fig. 5. Residual plot Fig. 4. Relationship between turn-off time and di/dt at different temperatures Multiple linear regression models could be established and given as follows, while 0 through 4 are coefficients in the model: ( di di tq f, Tj, Vrr, IT ) Tj + 3Vrr + 4I (1) T dt dt With multiple value pairs of the raw data from Fig. 4, the multiple linear regression analysis is performed using MATLAB modeling tool. Regression analysis generates the fitted regression equation, i.e., the thyristor turn-off time curve is as follows: tq di Tj +0 Vrr I (2) F dt Residual analysis of regression model is performed, and the residual plot is obtained as shown in Fig. 5. From Fig. 5, it could be intuitively seen that the absolute values of the residuals are relatively small and the points depicted are randomly distributed up and down around a straight line of 0, i.e. the horizontal axis. The regression line is well fitted to the observed values. There is a significant linear correlation between dependent variables and independent variables. The difference between the observed value (or actual value) and the estimated value (or theoretical value) of the fitted regression equation is within ± 35μs, which is basically consistent with the original data. According to the eq.(2), specific index requirements of thyristor craft are proposed, according to the actual situation of project and to reduce the inherent turn-off time of the thyristor. V. MONITORING METHOD OF COMMUTATION FAILURE The monitoring/predicting and defensive measures for commutation failure are relatively mature from the perspective of DC system, although the measures are blank from the perspective of the converter valve equipment. While suppressing the minimum turn-off time of thyristor, it also performs real-time monitoring of commutation failure through the valve control device in the converter valve. Real-time monitoring could be performed using the following methods. A. A method of intercepting voltage from the sampling window By observing the valve voltage at the moment after the commutation of the valve to determine whether a commutation failure exists, we can compare the valve voltage waveform intercepted by sampling window at that time with the valve voltage waveform of the normal commutation. The observed valve s opening time is selected as the time reference point, as is shown in Fig μ γ t= α Fig. 6. Sampling window and reference time Fig. 7. Sampling after commutation failure (loss of data) For a certain arm, t=0 is selected as the reference, commutation is completed at t=300-α, whether the commutation is failed depends on whether the arm restores forward blocking at that moment, that is, whether the valve voltage is greater than the on-state voltage drop. The normal commutation sampling window is as shown in Fig. 6, and when the sampling window can t sample the 797

4 2 Weihua Xu et al.: Studies on Effects of Dynamic Extinction angle of the Thyristor on HVDC Commutation Failure forward voltage, that is commutation failure, and is as shown in Fig. 7. The commutation failure monitoring method relies on the following: 1) normal operation cycle data of a valve provide the timing reference point, 2) the trigger angle given by real time measuring system determine the sampling start time, 3) real-time measurement of valve voltage. VI. DEVELOPMENT AND TESTING OF CONVERTER VALVE EQUIPMENT A. Development of converter valve equipment The converter valve module prototype which is successfully resistant to commutation failure is developed with the application of this research results. The following improvements are made: the turn-off angle is fixed by synthetic test device control system at 12. Compare the blocking recovery capability of two types of thyristor under the same operating conditions. (2) Function verification of intelligent trigger monitoring device: 1 to adjust the water temperature and current to increase the thyristor γ0 gradually to the commutation failure period, monitor TTM and VBE, calculate the system turn-off angle γ and thyristor γ0, and then output the monitoring and calculation Result, 2 once the turn-off angle margin is less than the limit, the trigger leading angle β will be adjusted by the control & protection system. 2) Test results The optimization test results of thyristor are as shown in Fig ) High-power thyristor with optimized turn-off characteristics Reduce the commutation failure probability caused by the core elements. 2) Intelligent trigger monitoring system Accurately monitor the inherent turn-off angle of the thyristor and the turn-off angle from the control system, obtain the control margin in real time, and quickly monitor the failure of commutation, as is shown in Fig.8. Fig. 10. Comparison diagram of thyristor turn-off time before and after optimization Fig. 8. Acquisition and calculation of the turn-off angle in intelligent trigger monitoring board The converter valve module prototype is as shown in Fig. 9. It could be seen from the Fig.10, that the turn-off time of the optimized thyristor in the same operating conditions reduces significantly from the original 724μs to 460μs. The functional verification results of the intelligent trigger monitoring device are as shown in Fig. 11 (a) and (b) respectively. As can be seen from Fig. 11, that the intelligent trigger monitoring device could test the operating state of the converter valve voltage effectively and send a detection signal within 1.7ms after the commutation failure occurs. Fig. 9. Intelligent trigger monitoring system valve module prototype B. Functional test verification The test is divided into two parts: performance verification of improving the commutation characteristics of thyristor and function verification of intelligent trigger monitoring device. 1) Experimental ideas (1) Performance verification of improving the commutation characteristics of thyristor: one thyristor without improving the commutation characteristics (γ0 = 13.6 ) and 8 thyristors which are improved the commutation characteristics (γ0 = 10.8 ) are pressed in the same TCA, VII. CONCLUSION In this paper, the mechanism of commutation failure is studied, the research tool is designed and the improvement technology to active defense commutation failure of the converter valve is put forward. Conclusions are as follows: (1) The precise thyristor macro model proposed in this paper can reflect the complete turn-off process of the thyristor compared with the traditional macro model. (2) The power upgrade of the high power thyristor leads to the increase of the inherent turn-off time of the thyristor, the effect of the real-time operating condition is more obvious and the method of fixing the turn-off time will no longer be desirable. It will increase the risk of commutation failure if the current ± 800kV continues to use the past turn-off angle control margin. The existing inverter turn-off angle control margin will be increased by about 3-4 by the dynamic turn-off time adjustment strategy. (3) The monitoring for commutation failure from the 798

5 Weihua Xu et al.: Studies on Effects of Dynamic Extinction angle of the Thyristor on HVDC Commutation Failure 5 perspective of converter valve equipment is achieved for the first time, the commutation failure monitoring cycle is 1.67ms. The converter valve module prototype applied to MIDC system is developed successfully and verified by functional test. The test results show that the turn-off time of the converter valve module prototype is shorter than before. (a) control studies[j]. Eletra., 1991, 135(4): [10] Chen Shuyong, Li Xinnian, Yu Jun, et a1. A method based on the sin cos components detection mitigates commutation failure in HVDC[J]. Proceedings of the CSEE, 2005, 25(14): 1-6(in Chinese). [11] Ou Kaijian.Ren Zhen.Jing Yong. Reaearch on commutation failure in HVDC transmission system.part 2:measures against commutation failures [J].Electric Power Automation Equipment, 2003, 23(6): 6-9(in Chinese). [12] Liu Hongchao, Li Xingyuan, Wang Lu, et al. Coordination and optimization of HVDC modulations in multi-infeed HVDC transmission system[j]. Power System Technology, 2004,28(1):5-9(in Chinese). [13] Xu Zheng. The characteristics of HVDC systems to weak AC systems part II: control modes and voltage stability[j]. Power System Technology, 1997, 21(3):1-4(in Chinese). [14] Li Feng,Guan Lin,Zhong Jiefeng, et al. Study on stability of Guang dong AC/DC hybrid power systerm [J]. Power System Technology, 2005, 29(11): 1-5(in Chinese). [15] Mao Xiaoming, Guan Lin, Zhang Yao, et al Researchs on HVDC modeling for AC/DC Hybrid Grid with Multi-Infeed HVDC [J]. Proceedings of the CSEE,2004,9(24): 68-73(in Chinese). Fig. 11. device (b) Functional verification of the intelligent trigger monitoring Weihua Xu received her B.Eng., M.Eng. and Ph.D. degrees in electrical engineering from North China Electric Power University, China, in 2004, 2006 and 2010, respectively. Since 2010, she has been a postdoctoral Fellow with the China Electric Power Research Institute, Beijing, China. In 2012, she joined the State Grid Smart Grid Research Institute, Beijing. Her research interests are in UHVDC converter valve electrical design, dynamic reactive power compensation of and protection of the converter in UHVDC transmission system. REFERENCES [1] Lin Lingxue, Zhang Yao, Zhong qing, et al. A survey on commutation failure in multi-infeed HVDC transmission systerms[j]. Power System Technology, 2006, 30(17): 40-46(in Chinese). [2] Wei jianwei, Lin Li, Cheng Tao,et al.commutation failure factors analysis in HVDC transmission[j]. Proceedings of the Chongqing University, 2006, 29(5): 16-18(in Chinese). [3] C. V. Thio, J. B. Davies, K. L. Kent.Commutation failures in HVDC transmission systems[j]. IEEE Transmission on Power Delivery, 1996, 11(2): [4] Zhejiang University DC transmission research group. DC transmission [M]. Beijing: Electric Power Industry Press, 1982: (in Chinese). [5] Ou Kaijian, Ren Zhen, Jing Yong. Reaearch on commutation failure in HVDC transmission system. Part 1: commutation failure factors analysis[j].electric Power Automation Equipment, 2003, 23(5): 5-8(in Chinese). [6] Zhou Changehun, Xu Zheng. Simulation and analysis of recovery characteristics of HVDC connected to AC system with weak strength[j]. Power System Technology, 2003, 27(11): 18-21(in Chinese). [7] Ren Zhen, Chen Yongjin, Liang Zhensheng, et al. Probability analysis of commutation failure in HVDC transmission systems [J]. Automation of Electric Power Systems, 2004, 28(24): 19-22(in Chinese). [8] He Chaorong, Li Xingyuan, Jin Xiaoming, et al. Simulation analysis on commutation failure criteria for HVDC transmission [J]. Power System Technology, 2006, 30(22): 19-23(in Chinese). [9] Szechtman M, Wess T, Thio C V. First benchmark model for HVDC Xiaoguang Wei received his B.Eng. and M.Eng. degree in electrical engineering from North China Electric Power University, Baoding, China, in 1999 and 2003, respectively, and the Ph.D. degree in electrical engineering from China Electric Power Research Institute (CEPRI), Beijing, China, in In 2007, he joined CEPRI, where he led the research and development team of UHVDC converter valve, and was the manager for CEPRI in the areas of HVDC technology. In 2012, he joined the State Grid Smart Grid Research Institute, Beijing. Since 2014, he has been vice president of engineering of DC Transmission Technology Institute, mainly engaged in DC transmission technology, the development of DC circuit breaker, high-power electronic technology research. Kunpeng Zha was born in Kaifeng, HenanChina, on May 13, He received the B.Sc. degree in electrical engineering from the Harbin University of Science and Technology, Harbin, China, and the M.Sc. and Ph.D. degrees in electrical engineering fromchina Electric Power Research Institute (CEPRI), Beijing, in 2002 and 2005, respectively. In 2005, he joined CEPRI as an Electrical Engineer. In 2012, he joined the State Grid Smart Grid Research Institute, Beijing, as a Senior Engineer. His special fields of interest include high-voltage test technology, ultra-high-voltage dc transmission systems, thyristor valve test methods, test facility, high-power electronics device test technology, measurement, and automation. Currently, he is working on the synthetic circuit for the high-power thyristors and test method research of HVDC valves. 799

6 2 Weihua Xu et al.: Studies on Effects of Dynamic Extinction angle of the Thyristor on HVDC Commutation Failure Zhiyuan He received the B.Eng. degree in electrical engineering from Sichuan University, Chengdu, China, in 2000 and the M.Eng. and Ph.D. degrees in electrical engineering from the China Electric Power Research Institute (CEPRI), Beijing, China, in 2003 and 2006, respectively. In 2006, he joined CEPRI, where he led the Voltage-Source-Converter-Based High-Voltage DC (VSC-HVDC) Transmission Systems Group from 2008 to 2009 and was the Manager in the areas of HVDC technology. In the past years, he has accomplished a theoretical study on high-power electronics technology for reliable operation of large interconnected power grids, relocatable dc deice system, and VSC-HVDC transmission, including the first VSC-HVDC project commissioning in 2011 in Asia. He was a Member of CIGRE B4 Working Group 48 and researched on Components Testing of VSC System for HVDC Applications from 2006 to During , he was a member of IEC SC22F Working Group 19 and researched on High-Voltage Direct Current (HVDC) Power Transmission Using Voltage Sourced Converters (VSC). He has published more than 50 papers and is the holder of 36 patents in his research field. Moreover, he was a recipient of two Provincial Scientific and Technological Progress Awards. Jie Yang received the B.Eng. and Ph.D. degrees in electrical engineering, from Tsinghua University, Beijing, China, in 2005 and 2011, respectively. In 2011, he joined China Electric Power Research Institute (CEPRI), Beijing, China. In 2012, he joined the State Grid Smart Grid Research Institute, Beijing. His research direction is the system design and analysis of VSC-HVDC systems. 800