Commutation Failure Study Of Yunnan-Guangdong UHVDC Transmission System

Commutation failure is a kind of phenomenon the converter valves fail to complete of commutation in the commutation time. To judge and prevent commutation failure need to make its happen mechanism, leading factor, the harm to the system, etc. When commutation failure occurs in UHVDC transmission system, it will make DC voltage and DC current suddenly changed and seriously affect the stability of DC transmission system. DC control system occupies the core position of UHVDC transmission system, which means that the output of UHVDC transmission system changes according to the control modes of it. The effective diagnosis of commutation failure in UHVDC is of great significance to take appropriate control measures after the failure and to avoid the malfunction of AC system protection equipment. This paper refers to:(1) Study the basic theory of commutation failure. Analyze the inverter commutation process. Understand of commutation failure mechanism. Ratio of transformer, transformer leakage resistance value, and the converter bus voltage value, the DC current value, the leading triggering angle value, and the AC system fault are the key factors of leading to commutation failure. The commutation failure fundamental judge method is to determine the relationship between the arc angle and minimum arc angle. Another method commonly used in engineering is the minimum voltage drop method, and there are some other methods. Because of some defects in these methods in use, it is necessary to study some of new method. Commutation failure in the inverter side will bring great harm to AC and DC system:①DC voltage drops, and DC current rises.②Converter valve will overheat, and its service life is shorten.③Converter transformer will bias, accompanied by low frequency noise.④Reactive power will not balance.⑤The resonance overvoltage because of the alternating current into dc system.⑥Continuous commutation failure force HVDC system blocking, and influence the stability of the system.(2) Study the effect of the rectifier side control mode on commutation failure. DC control system occupies the core position of UHVDC transmission system, which means that the output of UHVDC transmission system changes according to the control modes of it. In the paper, the cause of commutation failure is analyzed; The rectifier side control modes are introduced; The simulation model of Yunnan-Guangdong UHVDC transmission system is built; The effect of the constant current control and the constant power control in Yunnan-Guangdong UHVDC transmission system rectifier side on commutation failure is studied. The simulation results show that when rectifier side under current control mode, the critical voltage ratio of inverter transformer that commutation failure just doesn’t happen is greater than that under constant power control mode; When rectifier side under current control mode, the critical resistance value that continuous commutation failures just doesn’t happen is smaller than that under constant power control mode when three-phase grounding fault, two-phase short circuit fault or single-phase ground fault happens at inverter side AC bus. The control ability for commutation failure of constant current control in the rectifier side is better than constant power control.(3) Using digital technology multi-scale morphology research to judge commutation failure occurred. Based on Yunnan-Guangdong±800k VUHVDC transmission system simulation model, simulate DC line short-circuit fault and commutation failures in different fault conditions. The aerial mode component of DC bus current is analyzed which is decomposed into six scales based on the modified morphology filter, and each of scales of spectral form is extracted. Coupled with energy spectrum entropy and singular spectrum entropy to extract fault features, define two fault diagnosis index as a criterion of the judging system running state, and then respectively setting up four threshold to diagnose DC line short circuit fault, normal operation and commutation failure in allusion to the two measures. DC line short circuit fault and commutation failures in different fault conditions are simulated many times. The simulation results indicate the method can differentiate between DC line short circuit fault and commutation failure accurately.(4) Give a method based on Hilbert-huang transform technology to diagnose commutation failure. For the normal operation, the commutation failure, and the DC line short-circuit fault of UHVDC transmission system, using Hilbert-Huang transform to distinguish of the three kinds of operation. First, reduce the noise of DC current signal using the SVD filter. Then, extract the signal of high frequency component form using the morphological filter, process it with Hilbert-Huang transform. Analysis the instantaneous frequency and the amplitude of IMF1, and put forward the method of threshold value to judge the system running state and fault type. Set the maximum instantaneous frequency f0 and average amplitude A0 of IMF1 as the two threshold values. If f≥f0, the system is in fault state, or the system is in normal state. If the system was in fault state, judge A≥A0. If A≥A0 is true, the DC line short-circuit fault happened, if not, it is the commutation failure. And, the results of validation indicate the method can differentiate the operation state of the system, DC line short-circuit fault and commutation failure accurately.