A power panel in a typical office building has a number of circuits to duplex outlets throughout a business. A careful design arranges the circuits to draw nearly the same power from each phase of the three-phase power supply. This results in a low neutral current.
This current in the neutral conductor is not used by any machines or equipment, since it is either grounded, returned to the main service,
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or returned to the power-distribution grid. In either case, the neutral current is paid for by the utility customer, even though it’s not used. This usually isn’t a problem, unless there are a lot of computers or computerized equipment on-line.
One or two computers on a panel, or even a dozen, will not increase the neutral current appreciably. Two or three times that amount can cause problems, though. The culprit is harmonics, distortions in the sinusoidal waveform that propagate down the neutral conductor. In extreme circumstances—for example, with many computers, fax machines, or other semiconductor devices on the panel—the neutral current can exceed the systems’ capacity and overload the high-voltage transformer down-line of the panel. This does more than waste a lot of electricity. It generates excess heat for the air-conditioning to handle, causes long-term damage to the conductor-cable coating, and can even threaten fire.
These dangers extend all the way back to the service entrance, potentially damaging conductors along the whole path, jamming up power throughout the building with harmonic distortions, and wreaking havoc in sensitive computers and control systems.
The solution is to install a shielded transformer, serving a panel dedicated only to computers. This at least prevents the harmonic distortions from escaping into the remainder of the power distribution system. It is also advisable, where a large quantity of microprocessor-based equipment is powered, to oversize the neutral conductor and upgrade the shielded transformer to one that can handle the high temperatures that happen when a high neutral current is dissipated at the transformer.
In this scenario, the neutral current is gotten rid of in the transformer coils, dissipated as heat. A preferable alternative is to use the power more constructively, such as to operate a large-horsepower synchronous motor. Some manufacturers claim their motors not only make productive use of the distorted neutral current flow, but also improve the power factor at the service entrance. Since many utilities charge a power-factor penalty, this can save money as well as use normally wasted energy, perhaps to operate a generator to provide clean power back into the main electrical distribution system of the building.
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Excerpt from Energy Conservation Projects.