Analysis of A Transformer Neutral Resistor Damage On An 11kv Bus Fault At National Grid

Power outage minimization and correct and rapid fault isolation are of concern to utilities. Protection schemes shall work properly to identify and isolate faults quickly and minimize the impact on interrupting service to customers. This paper presents an analysis of a transformer neutral resistor damage on a low side bus fault utilizing fault records from digital fault recorders and digital relays to determine what and why happened. The fault records captured by the fault monitoring equipment and Sequence of Events provided valuable information which gave an insight into the nature of this event. The analog and digital data of fault records facilitate an efficient investigation and accurate analysis of this event. On 10/21/2014 at 10:06:57, an animal contact caused a C-phase-to-ground fault outside the Section 3 of 11.5kV #2 Bus between 115/11.5kV #3 transformer 11.5kV side 3T11 breaker and 3T11-1 and 3T11-2 disconnects at S St. During the fault, both #1 and #2 Bus Section 3 bus differential ground (BDG) relays operated and picked up the bus lockout relays, but none of the #2 Bus Section 3 normally closed breakers at S St tripped via the BDG lockout relay. Only the 11.5kV 66 and 68 cable breakers tripped by ground overcurrent relays (G) and the rest of #2 Bus breakers opened by Control Center remotely and O&M locally. At the moment of the fault, current (both phase and ground) on the 115kV side of #3 transformer was about 100 amperes primary. 20 seconds later, the current jumped to over 1800 amperes primary, which matches the #3 transformer 115kV overcurrent (OC) relay’s record and short circuit simulation without the neutral resistor. The OC relay then operated correctly and the fault was finally isolated. By taking a close look at the SER records, the fault lasted for 40 some seconds. Due to the long duration of the fault, the neutral resistor for #1, #2 and #3 transformers at S St and some 11.5kV underground cables in the area network got damaged. Three following questions were raised by the investigation team as: • Why did both #1 and #2 BDG relays sense the fault? • And why did the #2 Bus breakers open manually? • How did the 115kV fault current increased in 18 times and if resistor got damaged due to the long duration of the fault? Based on a thorough disturbance analysis, it was determined that: • The fault was inside the #1 BDG differential zone and outside the #2 BDG differential zone. Therefore, the #1 Bus Section 3 BDG operated per design but the #2 Bus Section 3 BDG operation was because of CT(s) saturation due to some low accuracy CTs in the BDG circuit. Furthermore, a design flaw on the #2 Bus BDG relaying was discovered. • Test trips were performed on the #2 Bus Section 3 via the BDG scheme, which confirmed that the BDG lockout relays (HEA) did not operate to the full trip position since they were mechanically bound. • The #3 transformer 115kV OC relay should not have sensed the 11.5kV fault if the neutral resistor on #1, #2 and #3 transformers at S St connects and works as per design. The 115kV OC relay operation was due to the neutral resistor being electrically by-passed. After going through the paper, the reader will understand what the root cause was for the slow tripping of the fault and why the neutral resistor was damaged.