Transformer Testing Procedures Flashcards

During a turns ratio test on a three-phase delta-wye transformer, the measured TTR on one phase deviates by 0.6% from the nameplate ratio while the other two phases are within 0.1%. After verifying meter calibration, what is the MOST likely cause of this single-phase deviation?

Answer: Shorted turns in the winding corresponding to the deviant phase

A TTR deviation of 0.6% on a single phase while the other two phases read within 0.1% of nameplate is a classic indicator of shorted turns in the affected winding. Shorted turns reduce the effective number of turns, lowering the measured ratio. Manufacturing tolerances in a single unit are consistent across phases (typically <0.5% per IEEE C57.12.90). Polarity reversal would invert the reading dramatically, not cause a subtle 0.6% shift. Residual magnetism affects excitation current and losses, not the turns ratio reading itself.

A NETA technician performs a power factor (tip-up) test on a 138 kV transformer bushing at 10 kV and again at 2 kV. The power factor at 10 kV is 0.42% and at 2 kV is 0.38%. What does the tip-up value of 0.04% indicate?

Answer: The bushing insulation is acceptable; tip-up values below 0.1% are generally considered normal

The tip-up test (difference between high-voltage and low-voltage power factor) detects internal ionization (partial discharge) within voids. A tip-up of 0.04% is well below the NETA and IEEE concern threshold of approximately 0.1%. Values above 0.1% suggest ionization activity warranting investigation. A 0.04% tip-up reflects normal dielectric behavior without significant void activity. Moisture ingress would elevate the absolute power factor value at both voltages substantially, not just alter the differential. Test lead leakage would affect both measurements proportionally and not typically produce a clean low differential like 0.04%.

When performing a Doble M4100 excitation current test on a 230/115 kV autotransformer, the technician finds that the excitation current on the center phase (B-phase) reads approximately 3.5 times higher than the A and C phases. No abnormalities were found on the turns ratio or winding resistance tests. What is the CORRECT interpretation?

Answer: This is a normal result for a three-phase core-form transformer where the center limb has a shorter magnetic path

On a three-phase core-form transformer, the center phase (B-phase) magnetic flux path is shorter and more direct than the outer limbs (A and C phases), which must traverse a longer core path. This geometric asymmetry causes the center phase to draw significantly higher excitation current — commonly 3 to 5 times that of the outer phases — and is entirely normal. This phenomenon is well-documented in IEEE C57.12.90 and NETA testing standards. Since TTR and winding resistance are normal, an inter-turn fault is ruled out (inter-turn faults would alter resistance). Core delamination would affect all phases, not isolate to the center limb.

A technician measures the DC winding resistance of a 25 MVA ONAN transformer and finds the high-voltage winding resistance is 15% higher than the factory test report value after temperature correction. The low-voltage winding resistance matches the factory value within 1%. Which diagnostic step should be performed NEXT before concluding there is a winding defect?

Answer: Verify that the high-voltage tap changer is on the same tap position used during the factory test

A 15% increase in HV winding resistance with an accurate temperature correction is significant, but before condemning the winding, the technician must confirm the tap changer is on the same position as during the factory test. De-energized tap changers (DETCs) add or remove sections of the HV winding; if the tap is two positions higher than the factory test position, the measured resistance will legitimately include more turns and appear elevated. This is one of the most common sources of apparent winding resistance discrepancy in the field. Increasing injection current does not improve the measurement accuracy for a resistive discrepancy. Impedance testing and DGA are follow-on steps if the tap position is confirmed identical.

During frequency response analysis (FRA/SFRA) of a transformer following transportation, the high-frequency response (above 500 kHz) on one phase shows significant deviation from the fingerprint, while the low- and mid-frequency regions match closely. What type of mechanical deformation is MOST consistent with this pattern?

Answer: Localized bushing lead or internal connection movement

SFRA frequency regions correspond to different transformer components: low frequency ( 500 kHz) is sensitive to the lead connections, bushing tap capacitances, and localized conductor movement. A deviation only in the high-frequency band, with normal low- and mid-frequency response, is characteristic of bushing lead movement or internal connection displacement rather than bulk winding deformation. Radial buckling and axial displacement produce distinctive mid-frequency deviations. Core loosening affects low-frequency response primarily.

A NETA technician performs an insulation power factor test on a 69 kV transformer using the grounded specimen test (GST) mode and then the ungrounded specimen test (UST) mode. The GST reading is 1.8% and the UST reading on the same winding is 0.3%. What is the CORRECT conclusion?

Answer: The bushing(s) associated with that winding are the primary source of power factor loss, not the winding insulation itself

In Doble/power factor testing, GST (grounded specimen test) measures the total power factor of the specimen plus all other grounded parallel paths — including bushings. UST (ungrounded specimen test) isolates only the winding-to-core/tank insulation by removing the bushing contribution. A high GST (1.8%) combined with a low UST (0.3%) on the same winding demonstrates that the winding insulation itself is in good condition (UST is clean), but the parallel path — the bushings — is the source of elevated loss seen in the GST reading. This is a standard Doble test interpretation used to separate bushing from winding insulation problems without physically disconnecting the bushings.