PART WINDING RESONANCE

PART WINDING RESONANCE

Reference: Margolis et al., IEEE Transactions on Power Apparatus and Systems, PAS- 94, no. 4, July/August 1975

Case History

During late 1960s and early 1970s, four transformer failures on the American Electric Power System, involving the dielectric breakdown of no-load tap changers of 500/345-kV and 765/500-kV units were encountered.

Case 1:

• The first of a series of transformer failures occurred within the no-load tap changer of a single-phase, 200-MVA, 500/345-kV auto-transformer, with a 13.2-kV tertiary.
• The bank was tripped by differential relays during a single-phase to ground fault, caused by lightning on Phase 1 of the 345-kV line. The location of the fault was determined to be approximately 145 miles from the auto-bank.
• It was found that a flashover had occurred between taps 2 and 6 of the tap changer of the transformer on Phase 1.

Case 2:

• On April 5, 1971 there was a double phase-to-ground fault on Phases 1 and 3 of the 500-kV line 340 miles distant from the auto-bank.
• When the fault was cleared in 5-1/2 cycles, the current in the Broadford auto-bank was only 20 percent of its rating. However, a current continued to persist within the tertiary even after the 500-kV fault was cleared, and it required an additional 117 cycles for the current to diminish to zero.
• Twelve minutes after this incident the auto-bank was tripped off the line by an overvoltage relay on the 13.8-kV delta winding. The 15-kV lightning arrester protecting Phases 1 and 2 of the tertiary failed, and the fault was cleared in 2-1/2 cycles.
• There was also an operation of the Phase 2 transformer pressure relief device. Subsequent examination of the Phase 2 transformer showed a flashover across the tap changer as well as a flashover from a crossover connection in the common winding to the tertiary winding.

Root cause analysis:

• Since all the failures occurred at winding locations remote from the HV and LV terminals, it was thought that the overvoltages might be caused by part-winding resonance.
• Distinct series type resonances were observed at 8kHz and 4kHz for the 500/345 kV and 765/500 kV transformers respectively when the low voltage terminals were energized through surge impedance and the high voltage terminal was grounded through surge impedance.
• These resonances were found to be very sensitive to terminal conditions. When the tertiary of the 765/500-kV transformer was shorted, the amplification factor at the critical taps was reduced by a factor of two. When the high voltage terminal of the 500/345-kV transformer was open circuited the resonance disappeared altogether.

Remedies:

• Capacitor banks across the tertiary windings of 765/500 kV transformers were installed to simulate a shorted tertiary for resonant and higher frequencies.
• In the case of 500/345-kV transformers, voltage stress on the tap changer could not be reduced to an acceptable level by connecting capacitors across the tertiary windings. Therefore, an externally mounted lightning arrester on a section of the 500/345 kV winding (internal crossover lead) was installed.