Case Studies in Analyzing Transformer Installation Faults – Part 1
This paper presents 8(Eight) cases of mal-operation of Transformer Differential, Restricted Earth Fault Protection and Earth Fault protection at various powerhouses and substations of Damodar Valley Corporation, a power utility dealing in power generation, and transmission in Eastern India. It shares the experiences gained while analyzing these mis-operations of protection system with the objective of reducing maltrippings due to commissioning errors, setting errors, scheme errors etc. Each case study starts with a short abstract highlighting the main aspect of the problem followed by a brief history of the event, system SLD with operated relay targets, in depth study of relay disturbance records [DR] & event records [ER] picked up, root cause analysis of the event and finally lessons learnt and remedial actions taken. The primary focus of the paper is on derivation of theoretical relationships and phasor diagrams to explain the unusual events and compare the derived results with real life relay picked up disturbance records. Event # 1 & 2 analyses the reason of mal-tripping of High Impedance REF Protection during out-zone faults due to wrong maximum through fault current assumptions and wrong CT cable lead length assumptions respectively in REF Stabilizing Resistance calculations. A general saturation voltage calculation methodology has been included for varying fault currents (for 1LG / 3LG fault type), varying distances of Neutral CT and Phase CT secondary to REF shorting point and varying CT resistances of Neutral & Phase CTs. Event # 3 and 4 scrutinizes the reason for mal-operation of a transformer differential protection during external system faults and during circuit breaker pole scattering respectively due to wrong neutral CT polarity connection which was creating wrong zero-sequence correction thereby excess tripping the differential protection. The theoretical phasor relationship between faulted phase and ground current during such external earth faults and single pole open conditions with correct CT polarity connections have been discussed with phasor diagrams. Event # 5 deals with mal-operation of transformer differential protection due to looseness in neutral return path in CT secondary. It depicts the distortion introduced in the phase current waveforms due to high voltages developed across CT secondary. Event # 6 examines the supposed excess tripping of a transformer through low impedance REF protection when the actual mal-tripping was occurring downstream due to LV board motors backfeeding into a fault. Event # 7 analyzes non-operation of low impedance REF protection for a fault within its Zone due to wrong polarity connection during paralleling of two Neutral CTs to provide one REF protection to two transformers having one protection panel. Event # 8 examines the maloperation of a transformer earth fault relay multiple times without any primary system fault due to circulating currents caused by intermittent multiple ground creation in CT circuit. The paper is concluded with a synopsis of the most important lessons learnt in the course of analysis of these protection malfunctions.