Research On Fast Protection For Ultra-high Voltage Direct Current Transmission Lines

UHVDC transmission with the characteristics of large transmission capacity, long distance transmission, low transmission loss, narrow transmission corridors, conveying from point to point directly and the ability of connecting the asynchronous or different frequency grids, takes on much more important task in energy transporting between large regions. There is a±660kV DC and five±800kV UHVDC transmission projects have been built in China with another two are under construction. China leads in UHVDC construction over other countries in the world. The characteristics of long distance, large capacity and passing through mountains and canyons make UHVDC transmission lines be vulnerable to suffer faults caused by lightning storms. Therefore, it requires rapid, reliable and sensitive line protection to ensure the safe and stable operation of the transmission system. Most of the core control and protection technology of HVDC transmission projects put into operation in China are controlled by foreign companies. Domestic research in this field is relatively backward, while the DC line protection used in practical project has the defects such as low sensitivity, poor reliability, high malfunction rate and so on. There are still many problems in HVDC line protection. Therefore, the further research on this project has important theoretical significance and engineering application value.Based on the analysis of transient characteristics generated by fault occurring on UHVDC transmission line, this paper proposes two kinds of rapid protection methods for UHVDC transmission lines. First, by using the reflective and refractive theory, an integral criterion is constructed to identify the fault direction according to the characteristic differences of the directional travelling wave under an internal or external fault. And then, a novel traveling wave based pilot directional protection method for UHVDC line is proposed. Theoretical analysis shows that if fault occurs in the forward direction of the relay, the ratio of the forward and backward travelling wave amplitude integral will be far less than1while it will be much larger than1 when fault occurs in the backward. Therefore, by the analysis of the fault direction recognition from relays on both sides of the line, it can be determined whether it is an internal fault. If the two relays at both ends of the line detect a forward direction fault, an internal fault is identified. In addition, according to the polarity difference between aerial mode component and zero mode component of the backward traveling wave, the fault line and fault type can be determined. The protection principle has absolute selectivity, and it is rapid, sensitive and reliable.Second, using the characteristics of fault component voltage and current during a period of time after fault occurred, a transient energy directional protection method is proposed. Supposing the positive current direction is from bus to line. When fault occurs in positive pole line, the fault component currents detected by the two relays on both sides of line are positive and the fault component voltages are negative, so the transient energy is negative. When fault occurs in negative pole line, the fault component currents detected are negative and fault component voltages are positive, then the transient energy direction is negative. If the transient energy direction is negative, the fault direction is positive. The fault line can be determined if the protection at two sides of the line detect a forward direction fault. This method can directly determine the fault pole and has high selectivity and anti-interference ability.According to the parameters of UHVDC transmission project put into operation in2014from Hami to Zhengzhou, a UHVDC transmission model of bipolar, double pulse is built using PSCAD/EMTD. The rated current is5kA. Upon the platform, a series of simulations is processed for the proposed protection method. The simulation results verify the feasibility and correctness of the proposed method.Finally, considering the traveling wave and transient energy protection for HVDC transmission line is easy-interfered by lighting, using the high frequency energy difference between lightning interference and lightning fault, a method for the identification of non-fault strike is proposed. The simulation results in PSCAD verify the validity of the proposed identification method.