Research On Directional Protection Of Series Compensated Transmission Line

In China, it is a prominent characteristic that energy bases and load centers is reversely distributed. For optimal allocation of energy resources, long distance and large capacity power transmission needs the support of high-power ultra-high voltage (extra-high voltage) transmission technology. The number of high-voltage transmission projects in our country is increasing in recent years. So the structure of power grid is becoming more and more complex. It is the developing direction of power system how to make full use of the existing transmission lines, so as to increase transmission capacity, and effectively save energy, and improve the economic efficiency, reliability, stability and flexibility of system operation. Series compensated technology is an economical and effective method to improve the transmission capacity and stability of transmission line. It is widely used in power system. However, look from protection installation place, the series compensated capacitor and the over voltage protective device of capacitor will bring some problems to the fast and effective identification faults of transmission line. The main factors affecting the series compensated line protection are current reverse, voltage reverse, capacitive fault impedance and the nonlinear characteristics of over voltage protection component of capacitor. Therefore, it is of great theoretical value and practical significance to study the directional protection algorithm for series compensated lines.Firstly, two typical series capacitor compensated devices are introduced, which respectively are fixed series compensated device and controllable series compensated device. Their structure, the operating characteristics of MOV, behaviors of series compensated device before and after the failure and different dynamic processes of protection of series compensated device in different fault types are introduced. This part is the basis for two kinds of series compensated models and the analysis of line directional protection.Then, the voltage and current phase relations are changed in the series compensated line. The generation conditions and the influencing factors of voltage reverse and current reverse are studied. The relationship between them and the influence on the traditional direction protection are analyzed.Because of voltage reverse, current reverse of series compensated lines, the traditional directional protection is unable to correctly identify the fault direction. A directional protection algorithm, which is suitable for the series compensation lines based on the phase relationship of the power frequency voltage and current fault component is proposed. The algorithm is not affected by the reverse voltage and current, and is not affected by asymmetric and nonlinear operation after fault. The sensitivity of voltage fault components is not guaranteed in the case of some fault conditions. So it is introduced a impedance compensation on voltage fault component of compensation, to ensure positive failure protection sensitivity.Finally, according to the single trip characteristics, fault types (single-phase fault, polyphase fault) are identified by comparing the mode value of voltage fault component. Combined with the proposed fault direction identification algorithm, directional relay algorithm which both can identify fault direction and fault types is proposed. And three series compensated models of FSC, TCSC and FSC+TCSC are established. The effectiveness of the proposed algorithm is verified in three lines by a lot of simulation and calculation. At the same time, the influence of fault initial angle, system impedance, transition resistance, installation position of the series compensated on the protection algorithm are studied.