Using your transformers at less than a third of their capacity? Mike Lowenstein, president of Harmonics Limited, examines whether under-loading can lead to problems.

Q. Does under-loading of a transformer at as low as 25% to 30% of the rated capacity increase harmonics and adversely affect power quality in a local distribution system? The transformer in question is K-13 rated and supplies audio/video (A/V) power supply loads. Does power quality affect performance of A/V equipment so drastically that the transformer should be changed to a smaller one?

Lowenstein's answer: In general the A/V loads served by your transformer will be nonlinear and powered by switch-mode power supplies. These loads draw harmonic currents, particularly the 3rd. The amount of harmonic current drawn by such loads depends on the impedance of the system that serves them and particularly on the impedance of the distribution transformer. Since a large under-loaded transformer has a lower impedance than a correctly sized transformer relative to the size of the load, harmonic currents will be greater. In addition, a k-rated transformer will often have a lower impedance than an equivalent standard transformer, which further increases harmonic currents.

Harmonic currents have very little effect on nonlinear loads because they're caused by load operation. Harmonic voltage distortion, however, will increase as harmonic currents increase. In your situation, a lower system impedance results in lower voltage distortion and thus offsets harmonic current increase.

In any case, harmonic voltage distortion also has little effect on the operation of nonlinear loads. (See “Harmonic Current and Voltage Distortion” in the November 2002 issue of EC&M for more detailed information.) The major effect of your under-loaded transformer is an increase in energy waste and, therefore, operating costs. The energy required to energize a transformer is proportional to the size of the transformer, all other factors (impedance, temperature rise, etc.) being equal. To better understand the costs of under-loading a transformer — in terms of energy and dollars — consider the results of a comparison of the two following fictitious transformers:

  • 75kVA, 150°C temperature rise, 480V-208/120V, 60 Hz

  • 30kVA, 150°C temperature rise, 480V-208/120V, 60 Hz

Using a transformer efficiency calculator developed for the EPA, the transformers were “tested” at the same load to determine which would be more efficient and cost effective. To relate to your system, the 75kVA transformer was loaded to 30% during the business day with a reduction at night and weekends, while the 30kVA transformer was loaded to 75% during the day with proportional loading reduction at night and on weekends. Energy costs were assumed to be $0.10 per kWhr. The Table above lists the data calculated in the comparison.

Another source of energy waste is the flow of harmonic currents throughout your system, which causes heating of the transformer, distribution equipment, and wire. The k-rated transformer is specifically designed to survive this extra heat. However, this heat represents wasted energy and can range from 3% to 5% of the energy consumed by nonlinear loads. For a 24/7 facility that's heavily populated by computers, this energy waste can be several thousand dollars per year, far exceeding the energy wasted by an under-loaded transformer. In facilities that are lightly loaded and don't operate continuously, the energy waste is obviously less.

In the calculated example, though, the annual savings in energy by changing to a smaller, more heavily loaded transformer is only $28.02. So it would appear that you would gain little economic benefit by changing your transformer. Also, changing to a smaller, more heavily loaded transformer without mitigating harmonic currents could overheat it.

Unless you're suffering equipment malfunction, there's little financial or operational reason to change to a smaller transformer.

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