Research On The New Distance Relaying Algorithm For Series Compensated Transmission Lines

The power system in our country must have the capacity of large scale and long distance transmission which is decided by the characteristics of the energy distribution and the energy demand in China. The series capacitor compensation technology has been used widely in the power system as a kind of scheme to improve the transmission capacity of transmission lines because of its mature technology and low cost. In addition, the access of series capacitor can improve the voltage quality and reduce the transmission losses. What is more important, it shortens the electrical distance and improves the stability of the system. On the other hand, it brings negative effect on the distance protection of transmission lines which makes the traditional distance relaying algorithm inapplicable as the access of series capacitor and the nonlinear characteristics of metal oxide varistor (MOV) changes the characteristics of uniform distribution of the impedance parameters of the transmission line, such as overreach or under-reach. But there is no protection algorithm that can solve the problem above effectively. Therefore, it is urgent to develop a new protection algorithm suitable for series compensated transmission lines.It analyses the stable effect on the distance relay of the acess of series capacitor, including the influence of the different series position on distance protection with different principle. Secondly, a brief analysis is made on the series compensation device. The operating principle of fixed series compensation (FSC) and thyristor controlled series compensation (TCSC) are introduced and the structure of fixed series compensation device and the different working state of metal oxide varistor (MOV) based protection system are discussed emphatically. Then, the fixed series compensation device are classified and modeled equivalently. It gives the KVL equation of the fault loop based on the lumped parameter model of the transmission lines and the linearized impendence model of the fixed series compensation device, and the fault impedance can be calculated accurately by solving the 3-order fault loop equations consisting of fault phase voltage, current, the differential of current and the integral of current. PSCAD/EMTDC and MATLAB are utilized for the series compensated system simulation and algorithm calculation, respectively. Numerical simulations have been done to verify the accuracy and reliability of the new algorithm, and the other influential factors such as fault resistance, shunt capacitance and system impedance ratio (SIR) effect are also investigated.A new algorithm of distance relaying for series-compensated transmission lines has been developed in this paper. It can calculate the fault impendence from the relay point to fault point without requiring the parameters of series capacitor device. What is more, The overreach and under-reach of distance relaying caused by MOV conduction are eliminated. Another advantage of the algorithm is that its calculation is simple and non-iterative. The protected zone can be 85%-90% of the transmission line length and the new algorithm proposed in this paper has great potential for engineering applications.