Study On Protective Relay Technology Of The New Type Transmission System

With the acceleration of modernization process in our country, power load keeps increasing, in order to reduce loss and meet demands of security, efficiency, and environmental protection, multi-circuit lines on the same tower, series compensation and other new types of transmission technology and modes have been widely applied in the power system of our state. However, due to a variety of line structures and complex fault characteristics, it brings many adverse effects to the operation performance of relay protection. Therefore, this paper focuses on the current differential protection for complex multi-circuit lines on the same tower which has been less rarely studied and the unresolved issues in distance protection for series compensated transmission lines, including the overreach, underreach of distance protection and contradiction of backup protection setting coordination. To solve the existing problems, some improvement measures and implementation scheme are proposed. The research on protective relay technology of the new type transmission has important theoretical and practical significance to improve the safety performance of the smart grid.To improve the performance of current differential protection for multi-circuit lines on the same tower, it is necessary to compensate distributed capacitive current for the lines. However, the current distributed capacitance current compensation scheme for differential protection is mainly limited to the structure of complete-journey double-circuit lines, and it is hard to adapt to various operation modes of the power system, which makes compensation precision difficult to meet the demand of the engineering application. This paper focuses on the main protection of current differential protection for complex multi-circuit lines on the same tower, the influence of complex line structure and operation modes on current differential protection is analyzed and the corresponding capacitance current compensation improvement scheme of local coupling double-circuit lines in complex structure and different voltage levels is proposed, based on PSCAD software, lumped parameter π model of local coupling double-circuit lines is established, which effectively improves the action performance of differential protection by simulation verification. To be further satisfied with actual operation of local coupling double-circuit lines, the process positive, the negative and zero sequence capacitance characteristics under different operation modes are analyzed, including two circuits are operated, one circuit is operated and the other circuit is cut off or grounded for the maintenance, and there are two capacitance current compensation schemes proposed based on the positive and negative sequence current and zero sequence current, which can significantly improve the accuracy of capacitive current compensation on various operating modes and avoid the protection malfunction, and the implementation method is simple and effective. The simulation results show that these two new protection schemes are adapt to different operation modes of multi-circuit lines on the same tower.Series compensated transmission technology can effectively increase the power transmission capacity, enhance the stability of power system, but the existence of series compensation device destroys the uniformity of line impedance, which brings adverse effect to the safe and stable operation of series compensated transmission system. This paper analyzes the fault characteristic of series compensated lines and adaptability of the existing improvement scheme for distance protection, and elucidates the problems of distance protection, such as the protection overreach and difficulty of setting coordination. Considering the effect of transient low-frequency components, transient processes of capacitive voltage transformer(CVT) and distributed capacitance of lines on the protection algorithm accuracy, a new distance protection improvement scheme of series compensated line based on parameter identification is proposed, the simulation model of RLC series equivalent circuit with complete series compensation device is established, The method is based on algorithm of solving RLC differential equations, the series capacitor is eliminated in the solving process, and the inductance parameter of the fault line is identified to accurately calculate the fault location without judging the relative position of series capacitor compensation and fault point, which can effectively overcome the adverse effects of series capacitor on distance protection, avoid overreach of distance section I protection, and solve the problem of backup protection setting coordination, has obvious advantages compared to the traditional distance protection algorithm by shortening the protection range to avoid malfunction. Through digital simulation, the influence of system running conditions(fault location, fault type, fault resistance, load current) and protection algorithm implementation ways(filter cutoff frequency, sampling rate, length of data window) on the operation characteristics of the distance protection are analyzed and summarized, and the effectiveness and accuracy of the algorithm proposed in this paper is verified by digital simulation.Finally, the research work and results are summarized and the further work to be accomplished is pointed out.