Case Studies in Analysis of Transmission Line Faults: Part 2
This paper presents 8(Eight) cases of mal-operation / non-operation of line distance protection in the 132KV transmission network 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 depicting the core problem then a brief description of the protection mis-operation, discusses related protection philosophy, power system phenomena involved, root cause analysis of events, lessons learnt and remedial action taken during each occurrences. Wherever necessary, each case study has been substantiated with disturbance and event report extracts of operated relays and illustrated diagrams for better understanding. Event # 1A & 1B analyses the tripping of distance protection for reverse faults due to CT shorting direction error. It also discusses some easy field oriented methods to check the CT shorting direction without going into elaborate polarity testing. Event # 2 discusses the cause of non-operation and depiction of negative fault distances due to advancement of all current phasors in a distance relay by 120° due to CT wiring error while it’s VT connection was correct. The effects of shifting of CT currents w.r.t PT voltages on the apparent impedance of distance relay in both LG & LL fault loops have been theoretically derived to match the event data. Event # 3 discusses mal-tripping of both lines entering a radially fed substation due to reversal of open delta voltage used in zero sequence voltage polarised Directional Earth Fault (D/E/F) protection. Two easy methods have been discussed on understanding the open delta PT polarity without going to elaborate testing. Event # 4 talks about excess tripping of a line through Directional E/F protection during auto reclose attempt of parallel line on permanent fault from remote end due to choosing of wrong ‘Reset Characteristics’ of E/F protection. Event # 5 analyses the cause of different fault location depicted by relay for LG & LL fault loops during an actual fault at same location. The investigation led to discovery of missing neutral (4th CT) connection across distance relays of the entire substation causing wrong apparent impedance calculation in LG fault loops. Event # 6 critically analyses the instantaneous tripping of two transmission lines for a fault beyond the remote bus in a Carrier Blocking scheme (DCB Scheme) due to non-receipt of carrier blocking signal from remote end due to zone timer start being set from ‘General Fault Detection of Device’ and not from ‘Distance Zone Pick Up’. As in this case the fault had started as a distance zone 3 fault and then transited into zone 2, the carrier coordination delay timer had already elapsed by the time the impedance came within it’s zone 2 reach whence instantaneous tripping had occurred in both lines as per DCB scheme. Event # 7 discusses about overtripping of a line for out zone faults due to wrong choosing of Carrier Communication Logic Scheme in SEL 311C Distance Relay i.e. implementing DCB Scheme logic by choosing POTT scheme. Event # 8 analyses 3(three) incidences where excess tripping had occurred in ‘Carrier Blocking / DCB’ communication assisted trip scheme due to non-receipt of Carrier Blocking signal from remote end although the fault was beyond the remote bus. The paper is concluded with a synopsis of the most important lessons learnt in the course of analysis of these protection malfunctions.