Transformer, Generator and a Value-Engineering List
An Islamabad office client operates a 12,000 m², ten-storey building with a fully glazed curtain wall: 45% window-to-wall ratio, single 6 mm glass, no external shading. Two 350-TR water-cooled chillers and two 1,000-kVA transformers serve a connected load of 1,800 kW at 0.85 power factor. A 500-kVA standby generator is meant to carry essential loads, but during summer outages it trips when the chiller pumps start; tenants report voltage dips and afternoon temperatures of 27–29°C. The client's cost consultant proposes value engineering: delete the economizer, reduce roof insulation from 100 mm to 50 mm, replace double Low-E glazing with single glazing, and remove the building management system's metering. The client asks you to approve the VE list so tender can proceed next week and to confirm that the generator is adequate.
1. Check the transformer and generator capacity against the stated loads. What risks do you see and what information would you request?
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At 1,800 kW and 0.85 power factor, apparent power is 1,800 ÷ 0.85 = 2,118 kVA. Two 1,000-kVA transformers give 2,000 kVA, so the installation is already short at full connected load; it may run only because actual demand is below connected load, but there is no firm margin. The 500-kVA generator at 0.8 power factor gives about 400 kW. A 350-TR chiller at roughly 0.6 kW/TR draws about 210 kW; two chillers plus pumps and fans exceed 400 kW before starting current is considered. Direct-on-line starting at 5–6 times running current explains the voltage dip and trip. I would request the load schedule and demand factors, chiller starting method, motor starting kVA, generator subtransient reactance and ATS sequence. I would not confirm the generator as adequate; I would require sequential starting, soft starters or VFDs, and a revised essential-load schedule.
2. Evaluate the four value-engineering proposals from an energy and occupant-comfort standpoint, and recommend an alternative package.
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Each proposal moves in the wrong direction. Deleting the economizer increases chiller energy in cool months. Halving roof insulation increases heat gain through the largest exposed surface. Replacing double Low-E glazing with single glazing raises solar gain, glare, condensation risk and noise, and works against the 45% glazing ratio. Removing BMS metering destroys the evidence needed to commission and verify savings. I would reject all four as presented. An alternative package would keep the envelope, add external shading to the west and south, specify double Low-E glazing with a low solar heat gain coefficient, retain the economizer, fit VFDs on pumps and fans, use CO2-based demand ventilation, replace lamps with LEDs, and commission and meter by tenant. That addresses the summer discomfort and the generator problem without trading away long-term performance.