Harmonics study for a data-centre UPS feed on utility and generator

Blue Ridge Colo is building Hall 4: 2 MW of IT load behind four 750 kVA double-conversion UPS modules, plus about 900 kW of cooling fans and pumps on VFDs. The hall is fed from the utility at 13.2 kV through a 3,000 kVA, 13.2 kV/480 V transformer with 5.75% impedance. The utility gives a fault level of 250 MVA at the point of common coupling. Two 2,000 kW diesel generators back up the hall, with a subtransient reactance of 0.15 per unit on their own rating.

Two things worry the operator: - The utility's interconnection agreement asks the site to meet IEEE 519 at the point of common coupling, and the utility engineer has asked for a harmonics study before energising. - At another campus, generators tripped on transfer and a UPS stayed on battery for 11 minutes because voltage distortion on generator power was too high for its input to accept.

The design team has two UPS options. One is a lower-cost unit with a 6-pulse rectifier and a passive input filter. The other is a unit with an active, IGBT front end that draws nearly sinusoidal current. They also want to know whether the 300 kvar power-factor capacitor bank planned for the mechanical board is a good idea.

You will build the system model, represent each load as a harmonic current source, run a frequency scan to find resonances, calculate voltage distortion and current distortion against the limits on both utility and generator supply, and compare mitigation options on cost. Everything is done in Python from first principles, and LTspice is optional for checking one case. No site data is needed beyond what is given here.

Done looks like a study report the utility engineer and the design team can both use: system data, source models, impedance scans, compliance tables for every operating case, a clear answer on the UPS choice and the capacitor bank, and a mitigation recommendation with cost.