Study On Overvoltage Analysis And Insulation Fault Detection Of Critical Elecric Equipment In Converter Station

With the continuous development of the economic strength and people’s living standard in China, the dependent degree of the society on energy, especially the power energy, is higher and higher. The converter station is the main conversion place of the AC and DC transmission. The key electrical equipments of converter station are the transformers and power capacitors. Any equipment fault may cause the large area blackout. According to the requirements of "The Twelfth Five-Year Plan" in China, the power department should sum up the experiences and lessons of power equipment fault. At the same time, the electric power standard system and fortification standard should be improved reasonably to guarantee the power supply safe and reliable. The key electrical equipment in the converter station was the study object in this paper. The overvoltage mechanism and fault feature of transformer and power capacitor would be revealed. The insulation fault detection and lifetime prediction method would be improved. The research results were of important academic significance and engineering value.The simulation model of converter station which contains transformers, capacitor voltage transformers and breakers was established. The ferroresonance overvoltage of transformer in the converter station was studied. The effect of excitation characteristics inflection point and capacity of transformer on ferroresonance was evaluated. The main measures to restrain the resonance include changing short-term investment in damping load, grading capacitance of breaker and capacitance value of capacitor voltage transformer(CVT). And the restrain effects of these measures were compared in the paper. Feasible scheme to restrain the resonance was put forward. When inter-turn short-circuit fault of transformer occur, the equivalent circuit and sequence diagrams could be given according to symmetrical component method. Then the distribution of voltage and current of internal fault was got. The change of voltage distribution in the winding can be found. The results show that the duration time of the resonant voltage shock and the overvoltage spikes change obviously when the breaker is interrupted at different times. The series resonance circuit is formed by the grading capacitor and the excitation impedance of transformer. It leads that the ferromagnetic resonance process presents synchronized characteristics with power frequency. Increasing the inflection point of excitation characteristics, increasing the transformer capacity, reducing the grading capacitor of the breaker and short-term investment in damping load could help to restrain the ferroresonance overvoltage. However, series resistance in the neutral point of transformer has no effect on the inhibition of ferroresonance. Transformer internal fault is mainly due to the focused field resulted from insulating weak link.According to statistical analysis on the data of field operation for a long-term, the overvoltage distribution characteristic caused by the inner components fault of the power capacitor was studied. The number limit of inner fault components which could avoid insulation of capacitor destroyed by overvoltage was discussed. The breakdown simulation model of capacitor’s inner components fault was established. The degree that equivalent circuit parameters of breakdown components and the voltage of capacitor’s two ends influence on discharge current at fault time was analyzed. The protective set value of unbalanced current of the reactive power compensation capacitor bank was calculated. According to manufacturing technology and operation maintenance of capacitor, the suggestion was presented. The results show that dangerous over-voltage will not develop in the capacitors of the other two phases regardless of the number of series inner components which occur penetrating-breakdown when single-phase fault occurs in the capacitor bank. The value of inductance and resistance in the equivalent circuit varies approximately in inverse proportion to discharge current. The high bus voltage will directly affect the life of the capacitors. The techniques against fault about power capacitors need to be improved in terms of both the quality and structure. The adjusted set value of imbalance current could ensure the effective action of the imbalance current protection.Aiming at the special experiment of high altitude and low air pressure, the partial discharge testing techniques of converter transformer was improved. Anti-interference measures were put forward. For the practical transformers, the pressurized method of electro-acoustic detection was improved. Accuracy of the method was verified. According to the oil temperature rise model at the top and bottom of transformer, an expert analysis system was established. The system could predict and monitor the oil temperature, hot-spot temperature and life loss of transformer.