Transformer Loading with a Contractor's Arithmetic
A housing society in Multan operates a single 500 kVA, 11 kV/400 V ONAN distribution transformer serving 96 apartments and a small commercial block. In June, with ambient temperature peaking at 47 degrees Celsius, the top-oil alarm logged 96 degrees Celsius and the transformer twice tripped on the winding-temperature trip at 105 degrees Celsius. The recorded maximum demand is 380 kW at a displacement power factor of 0.78 lagging. The society now wants to connect 150 kW of additional air-conditioning load for a new block, and the electrical contractor argues that the transformer is 500 kVA and the load is only 380 kW, so there is 120 kW spare. The utility's distribution code requires the consumer to maintain a power factor of at least 0.90, and the nameplate rating assumes a 40 degrees Celsius ambient. No forced cooling is fitted, and there is no space at the plinth for a second transformer without relocating the metering cubicle.
1. Assess the present and the proposed loading against what the transformer can actually deliver.
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The contractor is comparing kW with kVA, which is the first error.
- Present load. 380 kW at 0.78 power factor is 380 / 0.78 = 487 kVA, about 97 percent of nameplate, with a line current of roughly 703 A at 400 V. The transformer is already effectively fully loaded, which is consistent with the oil and winding alarms.
- Ambient derating. The nameplate assumes 40 degrees Celsius. At 47 degrees Celsius the capacity falls by roughly 1 percent per degree, so about 465 kVA. Against that, 487 kVA is already an overload of around 5 percent, and the measured temperatures confirm it.
- Proposed load. Total becomes 530 kW. At the code-minimum 0.90 power factor that is 589 kVA; even corrected to 0.95 it is 558 kVA. Either figure exceeds the 500 kVA nameplate, let alone the derated 465 kVA.
- Headroom after correction. Power-factor correction to 0.95 at the existing load brings it to 400 kVA. The spare capacity at the derated 465 kVA then corresponds to only about 60 kW of real load, not 150 kW.
Conclusion: the additional 150 kW cannot be connected on this transformer in any configuration of capacitors. Capacity must increase, or the load must be reduced and managed.
2. Recommend and justify the least-cost compliant option, having regard to the site constraint.
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Compare options on life-cycle cost, not capital cost alone.
- Option A, replace with 630 kVA. Derated capacity at 47 degrees Celsius is roughly 586 kVA. With power factor corrected to 0.95, the post-expansion load is 558 kVA, which fits with a small margin. It is a like-for-like swap, needs no second plinth, and can reuse the existing 11 kV switchgear if the fault level and protection settings are rechecked. Usually the lowest-cost compliant route, but the margin is thin for future growth.
- Option B, second transformer. A 400 kVA unit would require the metering cubicle relocation and a split of the LV board, plus duplicate protection and earthing. Higher capital and outage cost, but gives N-1 security.
- Option C, load management only. Interlocking the new air-conditioning with existing load can hold demand below the derated rating, but it will not deliver the cooling the new block was built for and is unlikely to satisfy the utility's power-factor requirement on its own.
Recommendation: fit a detuned or tuned capacitor bank of roughly 175 to 200 kvar for correction to about 0.95, then replace the transformer with a 630 kVA unit, and check the LV cables, main breaker, fault level, earthing and protection settings for the new through-fault and load currents. The capacitor bank alone is not a solution and must not be sold to the society as one.
3. What must appear in the design note sent to the client and the utility, and what mistakes must it avoid?
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The note has to be defensible line by line, because it will be read by a utility engineer looking for a reason to refuse.
- State the basis. Ambient 47 degrees Celsius, derating method and its source, load survey dates, and the measured maximum demand rather than an assumed figure.
- Show both quantities. Real power in kW, apparent power in kVA and power factor, at present and after expansion; never a kVA rating compared with a kW reading.
- Show the measured evidence. Oil and winding temperature logs and the trip records, because these corroborate the calculation and cannot be argued with.
- Show the correction. Capacitor rating, target power factor, and the resulting kVA, with the calculation attached.
- State assumptions and limits. Diversity and demand factors used, the load growth allowed for, and the fact that summer ambient is the governing case.
- Avoid: mixed units, unstated assumptions, presenting nameplate kVA as if it were available capacity, and certifying adequacy for the utility connection before protection settings and cable ratings have been checked. A note that overstates capacity to ease approval is a misrepresentation and will be traced back to the signatory.