NETA Transformer Testing Procedures 6

Question 1

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?

Accepted Answer

Shorted turns in the 'C' phase winding

Answer

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.

Question 2

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?

Accepted Answer

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

Answer

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.

Question 3

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?

Accepted Answer

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

Answer

In SFRA interpretation, different frequency ranges correlate to different components of the transformer's equivalent circuit. The low-frequency range (< 1 kHz) is predominantly governed by the core's magnetic inductance. Deviations in this region indicate changes to core permeability, core loss, or physical shifting of the core assembly. Mid-frequency deviations (1 kHz–100 kHz) indicate winding deformation or displacement. High-frequency deviations (> 100 kHz) reflect capacitive interchanges between windings and to ground. A shorted turn would appear as a mid-to-high frequency signature change.

Question 4

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?

Accepted Answer

3.600 Ω

Answer

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.

Question 5

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?

Accepted Answer

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

Answer

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.

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

NETA Transformer Testing Procedures 6