Transformer Testing Procedures Flashcards

During a power factor (dissipation factor) test on a 138 kV transformer, the C1 (HV winding-to-ground) reading is 0.38% and the C2 (HV bushing tap) reading is 0.41%. The manufacturer's nameplate lists C1 = 0.35% and C2 = 0.39%. Which interpretation is most accurate?

Answer: The delta between field and nameplate values is within warning thresholds but should be trended at the next outage

NETA MTS and IEEE C57.152 use both absolute values and deviation from nameplate (or prior tests) to assess insulation condition. Both C1 and C2 are below the 0.5% warning threshold in absolute terms, but the deltas (C1: +0.03%, C2: +0.02%) are small and not alarming on their own. The correct action is to flag for trending—rising power factor over successive tests is a stronger diagnostic signal than any single reading. Neither deviation alone justifies replacement or immediate drying.

A technician performs a sweep frequency response analysis (SFRA) on a 3-phase transformer after transport. The HV winding traces for phases A and C show good correlation, but phase B exhibits a significant deviation in the mid-frequency range (1 kHz–100 kHz). What is the most likely cause?

Answer: Localized winding deformation or displacement on the phase B coil

SFRA compares phase-to-phase traces and baseline (factory or pre-transport) traces. Mid-frequency deviations (1 kHz–100 kHz) are most diagnostic of mechanical changes in winding geometry—coil displacement, conductor buckling, or hoop deformation—which alter the distributed inductance and capacitance of that winding. Core grounds affect the low-frequency region; bushing PD would show up in high-frequency resonances; an open LV circuit would cause a gross deviation across all frequencies, not a mid-band shift.

When performing a turns ratio test on an autotransformer with a tertiary delta winding, the technician measures the ratio between H1 and X1 as expected, but notices an anomalous reading when testing the tertiary terminals. Which condition most specifically explains a significantly low ratio reading on the tertiary?

Answer: A shorted turn within the tertiary delta winding

A shorted turn in a delta tertiary creates a circulating current path that significantly reduces the effective impedance and distorts the turns ratio measurement—a shorted turn acts like a transformer within the winding, drastically pulling down the measured ratio. High contact resistance would increase, not decrease, an impedance-based reading but wouldn't cause a low ratio on a ratio bridge. An open breaker isolates the tertiary externally but doesn't affect the wound turns. Residual flux may cause slight excitation asymmetry but not a gross ratio error.

A 230 kV/34.5 kV GSU transformer undergoes dissolved gas analysis (DGA). The results show: H2 = 420 ppm, CH4 = 180 ppm, C2H6 = 95 ppm, C2H4 = 310 ppm, C2H2 = 48 ppm, CO = 680 ppm, CO2 = 4,200 ppm. Applying the Duval Triangle method, what fault type is most indicated?

Answer: T3 — High-temperature thermal fault (>700°C) with cellulose involvement

The Duval Triangle uses the percentage contributions of CH4, C2H4, and C2H2. Here: total of three = 180+310+48 = 538. %CH4 ≈ 33%, %C2H4 ≈ 58%, %C2H2 ≈ 9%. This combination plots squarely in the T3 zone (high-temperature thermal >700°C). The elevated CO (680 ppm) and CO2 (4,200 ppm) confirm cellulose (paper insulation) involvement at high temperature. D2 arcing would require C2H2 dominance; PD would show H2 dominance with minimal heavier hydrocarbons; T2 would show lower C2H4 proportions.

During insulation resistance testing of a large power transformer, a technician takes a 10-minute reading and a 1-minute reading in a high-humidity environment (85% RH, 28°C). The polarization index (PI) calculates to 1.12. What is the most appropriate conclusion?

Answer: The PI is inconclusive; the test must be repeated after drying the transformer to below 60% RH or correcting to a standard temperature

IEEE 43 and NETA MTS explicitly caution that PI interpretation is unreliable when surface leakage currents are elevated by high humidity—the moisture creates conductive surface paths that suppress the time-based rise in resistance, yielding artificially low PI values that mimic poor insulation. A PI of 1.12 would normally indicate questionable insulation (threshold is typically 2.0 for Class A insulation), but in 85% RH the reading is not meaningful. The test must be repeated under controlled humidity (<60% RH) or at a corrected temperature, or the transformer surface must be dried before a valid diagnosis can be made.

A technician is conducting a short-circuit impedance test on a 3-winding transformer (HV, LV, TV). The leakage impedances measured are: Z(HV-LV) = 8.2%, Z(HV-TV) = 5.6%, Z(LV-TV) = 3.1%. Compared to nameplate values of Z(HV-LV) = 8.0%, Z(HV-TV) = 5.5%, Z(LV-TV) = 3.0%, what is the significance of a 2.5% deviation in Z(HV-LV)?

Answer: A 2.5% deviation from nameplate in leakage impedance can indicate winding movement and warrants correlation with SFRA results

While a 2.5% deviation from nameplate does technically fall within the ANSI/IEEE C57.12.00 tolerance band (±7.5% for most power transformers), NETA MTS and IEEE C57.152 treat any impedance deviation—especially when combined with post-through-fault or post-transport testing—as a signal warranting further investigation. Winding displacement changes the leakage flux path and therefore the measured impedance. The correct diagnostic action is to correlate with SFRA data: if SFRA also shows mid-frequency deviations on the same winding pair, mechanical deformation is likely. Answering that it is 'not significant' ignores the diagnostic value of trending and multi-test correlation.