Overloaded 100 kVA Distribution Transformer with Phase Imbalance
At a suburban 11/0.4 kV distribution transformer in Lucknow (100 kVA, ONAN, installed 2007), the feeder JE records a peak demand of 118 kVA at 20:30. Consumer currents are R = 192 A, Y = 171 A, B = 147 A. The tail-end voltage is 372 V while the transformer terminal voltage is 400 V. After two hours at this load, the top-oil temperature gauge reads 86 °C and ambient temperature is 41 °C. No fuse has blown, but three dim-light complaints were logged in the past week. The subdivision has 92 connected consumers, mostly single-phase domestic with 2 kW sanctioned load each, plus 4 kW of street lighting. The JE can either (i) redistribute service connections among phases and change the off-circuit tap, or (ii) request a 200 kVA augmentation, for which material lead time is 10 weeks. The subdivision has one spare 100 kVA transformer but no spare 200 kVA unit.
1. Quantify the overload, phase imbalance and neutral current. Does the 86 °C oil reading by itself establish that the transformer has failed or is about to fail? Show your calculation.
Read the worked answer
Rated secondary current = 100,000/(√3×400) = 144.3 A. The recorded phase currents sum to 510 A, giving an average of 170 A and an equivalent throughput of √3×400×170 = 117.8 kVA, i.e. 18% above rating. The hottest phase, R at 192 A, is 33% above rated current. Phase imbalance by maximum deviation is (192−170)/170 = 12.9%.
For resistive single-phase loads the neutral current is approximately √(192²+171²+147²−192×171−171×147−147×192) = √11,521 ≈ 107 A. That neutral current is high and adds heating.
The top-oil rise is 86−41 = 45 K, below a typical 55 K nameplate rise, so 86 °C alone is not proof of failure; the gauge may be inaccurate. But the transformer is operating outside its rating with a hot phase and high neutral current, so it is being aged rapidly. Check the gauge, oil level, oil test and thermography; do not rely on the oil gauge alone.
2. The 200 kVA unit has a 10-week lead time. Recommend what to do in the next 72 hours and over the next 12 weeks, including whether the spare 100 kVA transformer can be paralleled. Justify each step with the numbers.
Read the worked answer
Balancing alone cannot cure the overload: even with perfect balance each phase would carry 170 A against a 144 A rating. Immediate steps: (1) move single-phase services from R to B until the phase currents are within about 10% of each other, targeting about 170 A each; this cuts the neutral current and the hottest-phase temperature but leaves the unit at 118% load. (2) Transfer 15–20% of the load, about 25–30 consumers, to an adjacent transformer if its spare capacity permits, or use the spare 100 kVA unit as a separate LV source for a cut-out section, not simply paralleled. Paralleling two 100 kVA units is acceptable only if they have the same vector group, tap position and closely matched impedance, and the LV busbars can be paralleled; a temporary mobile transformer is a better option. (3) Check and record the tap position; the 372 V tail-end voltage is 7% low, but tap changing will not remove the overload and may raise load current. (4) Inspect oil level, oil filtration and connections; log top-oil and load every hour. (5) Start the 200 kVA augmentation process immediately because 92 consumers at 2 kW connected load plus street lighting already exceeds the 100 kVA unit's practical firm capacity. The spare 100 kVA can offload a section but cannot make the existing unit compliant. Do not raise the fuse rating or bypass the temperature protection.