Protection Methods For Transmission Line Of VSC-HVDC Based On The Generalized S-transform

VSC-HVDC presents lots of advantages such as:rapid responding,easy to control,flexible operation mode,supplying power to passive network,fewer commutation failure,ready to construct multi-terminal DC system and so on.After the fault of transmission line,the fault current rises rapidly,and generally reaches the steady state within 10ms,which causes great damage to the whole VSC-HVDC system.Therefore,it is required to clear the fault current quickly.In order to overcome the problem that the protection method is not accurate and fast,a protection method for transmission line of VSC-HVDC based on the generalized S-transform is proposed in this paper.The main contents are as follows:According to the higher time-frequency resolution of generalized S-transform,a line pilot protection method based on generalized S-transform and phase mode transformation is proposed.Firstly,the electromagnetic transient simulation diagram is established according to the mathematical models and control mode of rectifier,inverter and cables in VSC-HVDC.Various faults of transmission line are simulated.Secondly,the phase mode transformation method is employed to decouple the current in the bipolar cable and the suitable modulus current is selected which is the basis of the protection method.The generalized S-transform is used to analyze the time-frequency of the modulus current.Then,extract the amplitude curve of the specific high frequency band and use the convex points of the amplitude curve to identify the fault and its time.Then,the zero frequency curve of the module current after generalized S transform is applied to distinguish the noise interference or the cables’ fault.Finally,the positive and negative value of the current increment after the fault is utilized to decide whether the cable has an internal fault or an external fault.The simulation experimental results show that the proposed method in this paper can not only identify faults rapidly and accurately,discriminate different fault types of transmission lines effectively,but also is not affected by the transition resistance and fault distance.Furthermore,a pilot protection method based on generalized S-transform and high frequency component ratio of current on both sides of capacitor is proposed based on the characteristics of large capacitance on both ends of transmission line in parallel in VSC-HVDC system.Firstly,the fault of transmission line is determined according to the current increment of positive and negative cables.Secondly,the generalized S-transform is utilized to extract different frequency components of the fault current on both sides of capacitor.The criteria of the internal and external faults is constructed according to the different ratios of same frequency at both ends of the shunt capacitor.Finally,based on the low frequency component of fault current,the polarity of fault cable is separated.The simulation results show that the proposed method can quickly detect the fault and fault type under the characteristics of parallel capacitance of transmission line.Thus,the response time of DC circuit breaker is reduced and the overall performance of protection method is improved.