Transformer Testing Procedures Flashcards

During a turns ratio test on a three-phase transformer, the measured ratio on the 'C' phase is 0.5% outside the nameplate ratio while phases A and B are within tolerance. What is the MOST likely root cause?

Answer: Shorted turns in the 'C' phase winding

A turns ratio deviation isolated to a single phase while adjacent phases are within tolerance is a classic indicator of shorted turns in that winding. Shorted turns effectively reduce the active turns count, altering the ratio. A tap changer error would shift all phases equally if it's a common tap, or would show a discrete ratio change (not 0.5% drift) if per-phase. High lead resistance affects the voltage measurement magnitude but not the ratio calculation in a ratio bridge. Core saturation from residual flux affects excitation current, not the ratio measurement.

A power transformer's insulation power factor (Doble) is measured at 20°C and returns 0.42%. NETA acceptance criteria require ≤0.5%. Before accepting the transformer, what additional step is MANDATORY per NETA MTS?

Answer: Compare the value to the factory test report and assess the percentage change

NETA MTS specifies that power factor results must be compared to the manufacturer's factory test data, not just evaluated against an absolute threshold. A value of 0.42% may be acceptable in isolation but could represent significant insulation degradation if the factory baseline was 0.08%. The percentage increase (0.08% → 0.42% = 425% rise) would be cause for rejection or further investigation. Temperature correction is important but is performed before applying any criteria, not as an additional step after a passing result. PI is a DC test applied to rotating machinery more than power transformers.

During a swept-frequency response analysis (SFRA) on a large GSU transformer after a nearby lightning strike, the technician observes a significant deviation in the low-frequency range (below 1 kHz) of the transfer function compared to the factory fingerprint. What does this deviation most specifically indicate?

Answer: A change in core inductance, suggesting core damage or a shifted core assembly

In SFRA interpretation, different frequency ranges correlate to different components of the transformer's equivalent circuit. The low-frequency range ( 100 kHz) reflect capacitive interchanges between windings and to ground. A shorted turn would appear as a mid-to-high frequency signature change.

A technician performs a DC winding resistance test on a delta-connected winding. She measures terminal-to-terminal resistance of 2.400 Ω, 2.398 Ω, and 2.402 Ω. What is the calculated resistance of a single winding leg?

Answer: 3.600 Ω

For a balanced delta winding, measuring across any two terminals places one winding leg in series with the parallel combination of the other two legs. If each leg resistance = R, the measured terminal-to-terminal resistance = R × (2R)/(R + 2R) = R × (2R/3R) = 2R/3. Therefore: 2R/3 = 2.400 Ω → R = 3.600 Ω. This is a commonly misapplied formula; technicians who forget the parallel path through the other two legs will incorrectly calculate 1.600 Ω (using R = 1.5 × measured) or simply report the measured value.

A 138kV/13.8kV transformer passes its initial insulation resistance test. During acceptance testing, the technician applies the 10-minute dielectric absorption test and records IR values every minute. At 1 minute the IR is 9,200 MΩ and at 10 minutes it is 10,100 MΩ. The calculated Polarization Index (PI) is 1.10. What is the correct interpretation and recommended action?

Answer: The PI is inconclusive because the 1-minute IR exceeds 5,000 MΩ; no PI criterion applies

Per IEEE Standard 43-2013, when the 1-minute insulation resistance value exceeds 5,000 MΩ, the PI test results are considered inconclusive and the PI criterion should NOT be applied. At very high resistance levels, the absorption current becomes negligible compared to measurement system noise and leakage, making the PI ratio meaningless. The correct response is to document the 1-minute IR as satisfactory (9,200 MΩ is excellent), note that PI is not applicable per IEEE 43, and proceed based on other acceptance criteria. Applying a PI < 2.0 failure criterion to this result would be incorrect.

A technician is performing a no-load (excitation) loss test on a three-phase transformer energized from the LV side. The wattmeter readings using the two-wattmeter method are: W1 = +18.4 kW and W2 = −6.2 kW. What is the total no-load loss and what does the negative wattmeter reading indicate?

Answer: Total loss = 24.6 kW; the negative reading indicates the load power factor is less than 0.5

In the two-wattmeter method for three-phase power measurement, total power P = W1 + W2 = 18.4 + (−6.2) = 12.2 kW is incorrect here. The algebraic sum is P = 18.4 − 6.2 = 12.2 kW only for the net value, but the question asks about total no-load loss and interpretation. More critically: in the two-wattmeter method, W2 goes negative when the load power factor drops below 0.5 (i.e., angle > 60°). No-load excitation current is highly reactive (power factor typically 0.1–0.3), well below 0.5, which is why W2 reads negative. Total loss = W1 + W2 = 18.4 + (−6.2) = 12.2 kW. The negative wattmeter reading is expected and indicates PF < 0.5, not a wiring error — the leads on W2 must be reversed and its reading recorded as negative.