An adaptive algorithm to prevent distance relay overreach during CCVT transient

In this paper, the impact of coupling capacitor voltage transformer (CCVT) subsidence transients on the performance of distance relay is investigated. The reach accuracy of distance relay is not maintained during CCVT subsidence transient and for this the responsible factors are system source-impedance-ratio (SIR), CCVT secondary burden, and fault inception angle. Relay generally overreaches during CCVT subsidence transient and in order to mitigate this problem an integrated adaptive logic is proposed. The adaptive zone-1 algorithm uses two criteria: (1) online SIR calculation using prefault and fault positive sequence voltage and current measured at relay location and (2) the maximum value of the transient monitor function obtained from the three-phase voltages to discriminate the in-zone fault from out-of-section fault. For accurate zone discrimination and to avoid relay overreach issue during end zone fault, the importance of adaptive threshold is crucial and can be set based on the online computed SIR value. The simulation study is performed using EMTDC/PSCAD. As the CCVT available in master library (ML) is not able to develop sufficient subsidence transient, a 500 kV, 60 Hz CCVT is modelled. Distance relay performance is analysed for different critical cases using the developed CCVT model and also the response of proposed method is observed. Results for different critical cases prove that the proposed method is able to maintain proper selectivity between in-zone and end-zone faults and also using the method the time-delay operation can be avoided during subsidence transient in the secondary side of CCVT