Positive sequence voltage memory filter for numerical digital relaying applications

A positive sequence voltage memory filter for numerical digital relaying applications is proposed. It is designed based on converting an analogue first-order lowpass filter into an infinite impulse response digital filter using the backward difference method. Its combination with both recursive and non-recursive phasor estimation algorithms is discussed. The obtained results reveal the decaying time constant of the proposed filter transient response is easily set, providing a simple and stable mean to adaptively adjust the positive sequence voltage memory polarisation of numerical relays.