Insights into testing and validation of DC offset removal filters in numerical distance protection

Distance relay maloperates due to various reasons, like dc offset in current waveforms, capacitive voltage transformer transients, remote-end in-feed in the presence of fault resistance, power swings and loading. dc offset in current waveforms results in overreach, causing distance relay to operate for faults outside its protected zone. There are many offset removal filters proposed by researchers in literature, but it is found, that the same ideas fail to penetrate as a filter into today's Intelligent Electronic Devices (IED). The objective of this paper is not to propose a new filter, but to provide insights into testing and validating those DC offset removal filters, which are not focused till now. This paper provides the additional factor to be considered for validating filters, which are proposed for dc offset removal and this is done by comparing the performance of three dc offset removal filters, digital mimic filter (dmf), patented filter (pf) to remove dc offset and double differentiator filter (ddf) which are currently used by relay manufacturers.