District-cooling plant: electrical demand, transformer checks and detuned power-factor correction

Al Qudra's Plant 3 has eight 2,000 TR centrifugal chillers on an 11 kV bus and a 400 V auxiliary system on four 2,500 kVA transformers. The 400 V side feeds condenser-water pumps, primary and secondary chilled-water pumps, 16 cooling-tower fans and plant building services. Many of the pumps and all the tower fans run on 6-pulse VFDs.

Two problems landed on the electrical engineer's desk this summer: - The supply agreement asks for a power factor of at least 0.95 lagging, but the 400 V buses run at about 0.86 at peak. The plant is paying a penalty. - A fixed 600 kvar capacitor bank fitted three years ago failed twice. Its capacitors bulged and its fuses blew. The maintenance team suspects harmonics.

Before buying anything, the plant manager wants a proper study. On a design day of 46 °C dry bulb, the cooling load climbs from about 9,000 TR at 04:00 to 15,200 TR at 15:00. In January it barely reaches 5,000 TR.

You will build the load list, model hourly electrical demand from the cooling load and chiller staging, check the auxiliary transformers including an N-1 case, size automatic power-factor correction for each 400 V bus, and check the resonance risk with the VFD harmonics before choosing a detuning ratio. Everything is calculation and simulation in Python or a spreadsheet; no plant access is needed.

Done looks like a report the plant manager can use to procure a new APFC panel: demand profiles for summer and winter, transformer loading, kvar per bus with step sizes, a resonance check that explains why the old bank failed, and a specification for the new detuned bank. If PF correction at 400 V cannot reach 0.95 at the utility meter on its own, say what else is needed.