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The Shocking Truth About Power Harmonics.

Harmonics play a crucial role in the efficiency (or otherwise) of a building's power consumption. Ignore the issue at your peril, warns Geoff Pile.

Planning for harmonics in your power supply is a bit like dealing with the Titanic. Underestimate the sheer scale of the impact of modern technology on building loads, or misunderstand the basic relationships behind cause and effect and you could find yourself with a crisis on your hands in the form of major equipment failures or unexpected power outtages.

In an ideal situation, power system generators should produce a clean, sinusoidal waveform resulting in a power factor of Unity or 1.0. In reality, the power supply to a building will be compromised, not only by the effects of magnetisation but increasingly, non-linear loads generated by modern semiconductor technology used in everyday plant and office equipment. The net effect of the distortion to the power factor is to increase energy consumption, with the unused power being fed back into the neutral supply.

The most common culprits are single-phased equipment such as PC's, photocopiers, faxes, printers, lighting and telecoms systems. Specialist equipment used in high precision applications such as hospitals and laboratories can also be a source of harmonics pollution.

Behind the scenes, building system elements may contribute to non-linear loads, typically air conditioning, UPS and rectifier systems. In industrial applications, electronic process control equipment adds to the problem.

The presence of harmonics can remain undetected for a long time. Eventually, however the build-up in the neutral current flowing back to the power supply will manifest itself in the form of poor system performance and overheating. This can lead to fuse breakers being tripped; resulting in unexpected downtime while the area of the fault is located. In especially sensitive areas, the generation of harmonics may also give rise to parallel problems with neighbouring plant and equipment.

In addition to the day on day inconvenience caused by interruptions to the power supply, excessive loads can add significantly to the life-cycle costs of a building, with unexpected repair costs and premature ageing of the building systems infrastructure and general equipment.

Utility companies faced with costly network upgrades to cope with the problem of feedback into the National grid have introduced severe penalties to prompt companies to take action. Under the G5/4 regulations, which stipulate a Power Factor of 0.92-0.99, additional levies of 10% or more could be imposed.

The answer argues Geoff Pile lies in the implementation of a planned control strategy to tackle the underlying problem, rather than ‘firefighting' or compensating with heavier duty transformers, switchgear and cabling, which just adds to the up-front construction costs.

The increasing use of variable speed drives aimed at cutting energy consumption in HVAC can help limit the potential fault contribution caused by motors and fans, delivering a near unity power factor. Typically, the inverter drives fitted in new or existing buildings will be of the 6-pulse connection type. Where harmonics are expected to be a problem, upgrading to 12-or 24-pulse drives, which offer a higher resistance, may be an option but can prove costly where a number of units are required.

The alternative is to install drives specifically designed to filter out harmonics from the power supply system as a whole.

Least costly is the passive filter, which is suitable for smaller buildings or where the extent of the harmonics disruption can be precisely calculated. For larger organisations, where unforeseen harmonics go undetected when for example, a new item of plant is added to the system, potential damage through overloading or an emergency power-down could still occur. In addition, passive filters may in themselves introduce new resonances causing additional harmonic problems. The filtration capacity of the passive filter operates in a fixed relationship to other impedances in the network that may change over time. This can result in reduced effectiveness and a deterioration of the filter.

Increasingly, active filters are being used to address harmonics issues for large-scale applications such as commercial complexes and environments with heavy-duty equipment or fluctuating requirements e.g. hospitals and hotels. Unlike passive filters, active filters are programmable and it is possible to install a single device to effectively cancel all harmonics whatever their source. This makes for a simpler installation and can reduce initial investment requirements at little extra cost.

First published in H & V News May 2004

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