Design and evaluation of an adaptive distance protection scheme suitable for series compensated transmission feeders

Series capacitors (SC) are installed on long transmission lines to reduce the inductive reactance of the line. This makes it appear electrically shorter and increases the power transfer capability. SCs and their associated over-voltage protection devices (typically metal oxide varistors, and/or air gaps) create several problems for protection relays, these include voltage and/or current inversion, sub-harmonic oscillations, transients caused by the air-gap flashover and sudden changes in the operating reach. The paper will investigate how the apparent fault loop impedance seen by a distance relay changes during the period from fault occurrence to fault clearance. The relay, simulated on a PC using PSCAD/EMTDC, calculates the line impedance using the sampled voltage and current. The operating behaviour of the relay was assessed using a 400 km double-end fed simulated transmission line with solid and resistive faults at various location on the line. The data generated by PSCAD/EMTDC describes the voltage and current signals seen by the relay immediately before and during the fault. The data was applied to the relay simulator, which evaluates whether the impedance trajectory of the fault enters one or more of the operating zones. The results will be presented in graphical form using an R-X diagram.