Power Systems Analysis & Troubleshooting Flashcards

During sweep frequency response analysis (SFRA) of a power transformer, a significant deviation in the mid-frequency range (1 kHz–100 kHz) compared to the reference fingerprint most likely indicates which condition?

Answer: Winding deformation, conductor movement, or inter-turn short circuits

SFRA mid-frequency deviations (1 kHz–100 kHz) are primarily associated with winding deformation, conductor movement, or inter-turn faults because these alter the distributed inductance and capacitance within the winding structure. Low-frequency deviations (100 kHz) suggest lead/bushing problems. Mid-frequency shifts are a hallmark of mechanical winding damage often caused by through-fault events.

A 13.8 kV feeder protection relay is set with a time-overcurrent characteristic. During a bolted three-phase fault at the end of the feeder, the fault current is measured at 4,200 A primary. The CT ratio is 600:5 and the relay pickup is set at 5 A secondary with a time dial of 3 using an IEEE Moderately Inverse curve. Approximately how long will the relay operate?

Answer: 1.14 seconds

First, determine the secondary current: 4,200 A ÷ 120 (CT ratio 600:5) = 35 A secondary. The multiples of pickup (M) = 35 A ÷ 5 A = 7× pickup. For IEEE Moderately Inverse: t = TD × [0.0515 / (M^0.02 – 1) + 0.114]. With TD=3 and M=7: 7^0.02 = e^(0.02×ln7) ≈ 1.0390; denominator = 0.0390; 0.0515/0.0390 = 1.321; 1.321 + 0.114 = 1.435; t = 3 × 1.435 ÷ (not divided — formula already gives seconds per TD=1, multiply by TD) ≈ 3 × 0.380 = 1.14 s. The IEEE Moderately Inverse formula yields approximately 1.14 seconds at TD=3.

When performing a power factor (dissipation factor) test on a 138 kV shunt capacitor bank, technicians observe that individual capacitor unit power factors are trending consistently 0.08% higher than factory nameplate values across the entire bank. The most technically accurate interpretation of this finding is:

Answer: A systematic error exists in the test setup, most likely stray capacitance coupling from adjacent energized equipment

A uniform shift across an entire bank — rather than isolated unit deviations — strongly suggests a systematic measurement error rather than actual dielectric degradation. At 138 kV substations, stray capacitance from adjacent energized buses or equipment can introduce parallel current paths that skew the capacitance and power factor readings uniformly. NETA and IEEE 18 guidelines emphasize that individual unit comparisons within the bank (tip-up analysis) and proper test setup shielding are critical. Actual aging or PD failures manifest as outliers within the bank, not uniform bank-wide shifts.

A technician is performing a ground resistance test on a substation ground grid using the fall-of-potential method. The test current electrode is placed 100 m from the grid edge and the potential probe is moved to multiple positions. The measured resistance values plateau at 55 m from the grid but then increase again near the current electrode. What is the correct interpretation and action?

Answer: The test electrodes are too close together — their resistance zones overlap, invalidating the plateau, and the test must be repeated with greater electrode spacing

In a valid fall-of-potential test, the resistance zone (Wenner sphere) of the current electrode must not overlap with that of the ground grid under test. When the potential probe readings plateau and then rise again near the current electrode, this is a classic signature of overlapping resistance zones — the two electrodes are not sufficiently separated. IEEE Std 81 requires that the distance between the grid edge and current electrode be at least twice the largest grid diagonal. The test must be repeated with the current electrode moved farther away, often 5–10× the grid diagonal for large substation grids.

During thermographic inspection of a 34.5 kV underground cable termination, a technician identifies a hot spot 18°C above the ambient-corrected reference termination on the same circuit. The load at the time of inspection is 35% of ampacity. Per NETA MTS severity criteria, this finding should be classified as:

Answer: Severity Level 3 — repair as soon as possible within 30 days

NETA MTS thermographic severity guidelines classify temperature differences relative to a reference component under the same load. A ΔT of 18°C at only 35% load is particularly significant because thermal severity scales approximately with the square of the current (I²R losses). At full load, this termination could exceed 100°C above reference — a critical condition. NETA classifies ΔT of 15°C–40°C above reference (under comparable load conditions) as Severity Level 3, requiring repair as soon as possible. The low load factor makes this finding even more serious than the absolute temperature difference suggests, as extrapolation to full load dramatically increases the projected temperature rise.

A 2,000 kVA, 13.8 kV–480V delta-wye transformer has a differential protection relay with CT ratios of 100:5 on the high side and 2,000:5 on the low side. During commissioning, with the transformer energized at no-load, the technician observes a continuous 10% differential current in the relay operating coil that persists beyond the inrush blocking period. No fault exists. Which of the following is the most likely root cause?

Answer: The high-side CT is connected in delta configuration to compensate for the transformer's winding phase shift, but the compensation introduces a √3 magnitude error if CTs are improperly ratio-matched

For a delta-wye power transformer, differential protection requires the CTs on the delta (high) side to be connected in wye and the CTs on the wye (low) side to be connected in delta (or vice versa via relay internal compensation) to correct the 30° phase shift between windings. When CTs on the delta side are connected in delta to compensate for phase shift, the secondary current magnitude is multiplied by √3 compared to a wye connection. If the relay tap settings or CT ratios do not account for this √3 factor, a persistent 15.4% (≈1/√3 × 100%) circulating differential current appears — matching the symptom of ~10–15% continuous differential at no-load unrelated to inrush. This is a commissioning/configuration error in CT connection compensation, not a fault.