Investigation of a hybrid travelling wave/impedance relay principle

The paper describes how the fast fault detection capability of travelling wave protection schemes and proven impedance measurement techniques can be used in parallel in a single protection scheme to develop a high-speed, reliable distance relay. The high frequency transient components of the voltage and current waveforms contain valuable information about the fault location and the fault-inception angle. A correlation method is used to find an estimate of the fault position using the transient components of the signals. The line impedance at the relay location is calculated at the same time. Depending on the signal condition, the scheme decides whether to use the travelling wave information or the impedance measurement in its decision to trip the breaker. The algorithm was tested for different fault conditions through simulations using PSCAD and the results for A few cases am included.