Electrical Testing Procedures & Equipment Operation Flashcards

During a power factor (dissipation factor) test on a 138 kV shunt reactor, the technician observes that the corrected power factor at 10 kV test voltage is 0.42%, but rises to 0.61% when tested at 2 kV. What does this voltage dependence most likely indicate?

Answer: Ionization activity within voids or delaminations in the insulation system

A power factor that increases at lower test voltage (reverse voltage dependence) is a classic signature of partial discharge or ionization within voids in the insulation. Normally, a healthy insulation system shows stable or slightly decreasing power factor as voltage increases. The reverse pattern — higher PF at lower voltage — indicates that ionization activity commences at a voltage below the lower test level and the energy dissipated in the voids is proportionally greater at reduced voltage. Moisture typically causes power factor to increase with voltage, not decrease. Calibration drift would produce a consistent offset, not a voltage-dependent change.

A technician performing a time-resistance (polarization index) test on a 4160V motor winding obtains a 1-minute IR of 8,200 MΩ and a 10-minute IR of 9,100 MΩ at 40°C. Applying the standard temperature correction to 40°C reference, what is the PI and what is the most appropriate conclusion?

Answer: PI = 1.11; the result is indeterminate because absolute IR exceeds 5,000 MΩ, rendering the PI ratio meaningless per IEEE 43

IEEE Std 43-2013 explicitly states that when the 1-minute insulation resistance value exceeds 5,000 MΩ, the polarization index ratio is not a reliable diagnostic indicator and the test result should be considered indeterminate or 'not applicable.' At extremely high IR values, the absorption current is negligible relative to the leakage current floor of the instrument, so the ratio loses statistical meaning. The PI of 1.11 is mathematically correct (9,100 ÷ 8,200), but the correct interpretation per IEEE 43 is that the test is inconclusive due to the very high absolute values — not that the motor fails. NETA MTS does not provide a blanket pass based solely on IR magnitude; PI must be evaluated when IR is in the meaningful range.

When performing a transformer turns ratio (TTR) test on a delta-wye transformer with a delta primary, a technician measures a ratio of 57.735 on the H1-H2 winding pair excited against the X1-X2 secondary terminals. The nameplate lists a 100:1 turns ratio (line-to-line). Which statement best explains the measured value?

Answer: The measured ratio of 57.735 equals the nameplate ratio (100) divided by √3, correctly reflecting that the winding-to-winding ratio on a delta side is the line ratio divided by √3

On a delta-connected primary, the line-to-line voltage equals the winding voltage. However, when a TTR bridge excites one winding pair (e.g., H1-H2) of the delta and reads across the corresponding secondary terminals of a wye-connected secondary, the voltage ratio measured represents the actual winding ratio. For a delta-wye transformer with a nominal line-to-line ratio of 100:1, the winding ratio is 100/√3 ≈ 57.735:1 because the wye secondary line voltage is √3 times the winding voltage. The TTR instrument reads the true winding-to-winding ratio, which is the expected value. Technicians unfamiliar with this relationship incorrectly flag it as a fault.

A technician is performing a sweep frequency response analysis (SFRA) on a 230 kV autotransformer following a nearby lightning strike. Comparing post-event traces to baseline, the technician observes a frequency shift in a resonant null between 200 kHz and 2 MHz in the common winding LV measurement, but no deviation is visible in the series winding HV-to-LV measurement. Which failure mode does this pattern most specifically suggest?

Answer: Radial winding deformation of the common winding conductors, altering local inductance and capacitance at mid-frequency

In SFRA diagnostics, frequency shifts in mid-frequency resonant nulls (roughly 200 kHz to 2 MHz) are characteristic of changes in winding inductance and capacitance resulting from mechanical deformation — specifically radial buckling or winding displacement. In an autotransformer, the common winding forms the LV section; a resonant shift isolated to the common winding LV measurement with no corresponding change in the series winding HV-to-LV trace indicates that the deformation is localized to the common winding. Core movement typically manifests at lower frequencies (below 20 kHz) as shifts in the inductive region. Bushing capacitance changes would appear in both measurements. Turn-to-turn shorts increase leakage inductance and typically affect higher frequency correlation.

A technician measures contact resistance on a 15 kV vacuum interrupter using a DLRO at 100 A DC. The result is 68 µΩ, which exceeds the manufacturer's maximum of 50 µΩ. Before condemning the interrupter, which procedure should the technician perform first, and why?

Answer: Perform five manual close-open operations and retest; oxide film on contact faces can be disrupted by mechanical wiping, and post-operation resistance often returns within specification

NETA MTS and IEEE C37.09 both recognize that vacuum interrupter contact resistance can be temporarily elevated due to surface oxide films that form on the contact faces during storage or inactivity. Performing several close-open operations mechanically wipes and disrupts the oxide layer through contact face motion and the localized heat of contact friction. This is a standard, accepted conditioning procedure before condemning a vacuum interrupter. Increasing test current beyond the instrument's specified range risks damaging the DLRO, the connections, or the interrupter itself and is not a recognized procedure. Contact enhancer compounds are contraindicated inside sealed vacuum bottles. Replacement without conditioning would result in unnecessary equipment replacement costs.

During a very low frequency (VLF) withstand test at 0.1 Hz on a 15 kV XLPE cable rated 133% insulation level, a technician observes that the charging current measured by the VLF set increases by 18% over the 60-minute test duration at constant voltage. Leakage current remains below the trip threshold throughout. What is the most technically accurate interpretation of this observation?

Answer: The cable insulation is in good condition; a gradual increase in charging current at 0.1 Hz is caused by space charge accumulation in XLPE that temporarily increases the dielectric constant

Space charge injection and accumulation in extruded XLPE insulation is a well-documented phenomenon at very low frequencies. At 0.1 Hz, unlike 60 Hz, the period is long enough for homocharge and heterocharge to build up at electrode interfaces and insulation defect sites. This accumulated space charge modifies the local electric field distribution and can temporarily alter the effective dielectric constant, causing the instrument to measure an apparent increase in charging current over time even though the applied voltage is constant. This is a recognized characteristic of XLPE behavior at VLF and does not necessarily indicate water treeing, imminent failure, or instrument malfunction. NETA and ICEA literature note this behavior when interpreting VLF test results on extruded dielectric cables as distinct from EPR or paper-insulated cables.