Case Study: Detection of Turn-To-Turn Faults In Load Tap Changer Mechanism

A 138 kV/13.8 kV/13.8 kV delta-wye-wye distribution transformer experienced a turn-to-turn fault in the load tap changer (LTC) tank. This fault evolved to involve multiple tap leads feeding the same phase at the bulkhead of the LTC, effectively creating a turn-to-turn fault. While the phase differential relay, set with typical values based upon the base MVA rating, operated correctly for the fault, further investigation of the oil and subsequent damage revealed that the fault had likely been present for some time. Analysis of the LTC shoes and contacts indicated that the cause of the fault is believed to have been the result of inadequate contact within the mechanism, which led to an increase in heat and insulation breakdown. Turn-to-turn faults are known to challenge phase differential element sensitivity. Event analysis shows that the initial fault was below the minimum sensitivity of the phase differential element until evolution of the fault led to an increase in current magnitude allowing the protective relay to operate.

This paper provides analysis of the operation and performance of the existing restrained and unrestrained phase differential protection elements. In addition, analysis of the recorded fault data before the relay operation is provided to show how a protection element, such as negative-sequence current differential, can supplement the phase differential protection for increased sensitivity for low-grade faults that may otherwise go undetected and result in significant equipment damage.