Enhancing earth-fault protection in high impedance earthed networks with modern low-power passive voltage transformers and protection relays

In this paper, we examine the technology for measuring neutral point voltage (Uo), or residual voltage, in earth-fault (EF) protection for compensated and unearthed networks. First, traditional inductive voltage transformers (VTs) and modern low-power passive voltage transformers (LPVTs) are introduced, explaining their technical properties. Next, the practical performance of VTs and LPVTs are evaluated and compared using actual measurement data from field tests conducted in a 20kV compensated and unearthed network. Finally, the applicability of EF-protection using LPVT measurements from the perspective of commonly used EF-protection functions is assessed. Based on the results, LPVTs combined with modern protection relays offer several benefits, including improved measurement accuracy, protection performance, and mitigation of ferroresonance. The authors further believe that there are no obstacles to using the calculated Uo from phase voltage measurements with traditional voltage transformers, rendering the open-delta winding obsolete from pure measurement perspective.