Influence of power quality on the performance of digital protection relays

The performance of protection relays depends on many factors and needs to be carefully evaluated. One of the significant factors impacting protection system behaviour is power quality. Depending on which of the power quality parameters is distorted, the influence on the performance of digital protection relays will be different. This paper analyses the influence of power quality deterioration on digital protection relays. In the first part of the paper, the historical limitations related to electromechanical relays and the signal processing algorithms used in digital relays are outlined. In the second part, the influence of distorted waveforms on the behaviour of digital relays is analyzed. Two aspects of power quality are taken into account: harmonics and frequency variations. Additionally, several simulation examples supported by laboratory measurements are presented in order to illustrate practical implications of poor quality of power.