Accurate Fault Diagnosis in Transformers Using an Auxiliary Current-Compensation-Based Framework for Differential Relays

This article proposes an auxiliary framework to address the challenges of transformer differential protection for single-phase transformers or three-phase transformer banks. This framework enables the differential scheme to: 1) work properly if transformers or current transformers (CTs) saturate; 2) detect internal and cross-country faults; 3) detect internal faults while energizing transformers; and 4) detect inrush currents. Unlike the existing methods in the literature, this framework addresses the above-mentioned challenges without sacrificing the sensitivity and/or the speed of differential relays. The proposed method models a transformer and its CTs with linear parameter varying (LPV) state-space equations, and uses the polytopic form of these equations and LPV observers to estimate the states of the transformer and its CTs. To address the CT saturation problem, it accurately estimates primary currents of CTs using their secondary currents. Thus, the differential scheme uses the estimated primary currents of its CTs instead of their distorted secondary currents. Additionally, the proposed framework detects inrush currents of transformers and differentiates them from internal faults by estimating the primary current of the transformer and comparing the estimated and measured primary currents. A discrepancy between the measured and estimated primary currents signifies an internal fault. The results of electromagnetic transient simulations in Electromagnetic Transient Program (EMTP) platform corroborate the effectiveness of the proposed method.