Power flow control effect with TCSC on operation margin of zone 3 impedance relays

Desirable series compensation schemes should control the power flow while not deteriorating performance of backup transmission protections, such as zone 3 impedance relays (mho relays) which are sensitive to power flow condition due to the overreaching design. In this paper, the relay’s operation margin between the apparent and operation impedance is given by a function of bus voltages. With the help of Jacobian matrix of power flow equations, the controlled parameters of TCSC, i.e. line reactance, line current, and line flow, are also defined as function of bus voltages. A sensitivity model is newly proposed for direct relationship between the operation condition of mho relays and the controlled parameters of TCSC, to verify the power flow control effect of TCSC on the relay performance, without repeatedly executing power flow schemes. Numerical results on IEEE RTS test system are provided to validate the accuracy of the proposed model, showing positive or negative effect of TCSC on mho relays at different locations.