Water and Energy International

This paper presents a practical earth fault (EF) protection philosophy for resistance-grounded 6.6 kV systems, with a focus on low-resistance grounding (LRG) schemes commonly used in industrial plants and power stations. A detailed case study highlights challenges encountered during motor starting and Motor Bus Transfer (MBT), particularly in Fast Bus Transfer (FBT) mode, where multiple false EF trips raised concerns over protection system stability. These incidents were traced to residual-connected Current Transformer (CT) limitations, CT mismatches, and inadequate Neutral Grounding Resistor (NGR) sizing—especially under conditions of inrush and multiple motor reacceleration.

The study is embedded within the System Integrity Protection Scheme (SIPS) framework, specifically addressing two key SIPS processes: viz Protection Element Operation, and Protection System Performance Review. These processes ensure systematic validation of protection functions and are aligned with the Indian Electricity Grid Code 2023 and the CBIP Protection System Manual, thus providing assurance of protection system reliability.

The paper further discusses the implications of MBT schemes and LRG coordination and proposes a revised EF protection approach. Key recommendations include optimized NGR sizing relative to CT ratio, use of Core Balance CTs (CBCTs), permissive logic based on 3V0 and Neutral CT (NCT) inputs, and Definite Time (DT) characteristics for improved selectivity.