Power Electronics / Power Management


Designing an effective power alternative

10 November 2010 Power Electronics / Power Management

Against the backdrop of uncertain utility power supplies in South Africa, companies are turning to alternative power sources in order to maintain up-time and remain competitive.

While solar, wind and even wave generated power might satisfy the ‘green lobby’, the reality is there are very few alternatives to mains power to satisfy the demands of most organisations, particularly in the manufacturing sector.

These companies will have to employ, for the foreseeable future at least, the backup products and technologies with which many South Africans are now familiar; generators (either petrol or diesel driven) and battery-powered uninterruptible power supply (UPS) systems. The challenge for all companies with a power requirement in excess of 10 kVA will be the optimisation of the generator and UPS in a comprehensively designed alternative power solution.

For these companies, there is no longer a choice between generator and battery-pack when it comes to standby power. Ideally they should be specified as a unit and installed as such, particularly in organisations using sensitive electronic equipment. Here, the transition from utility to standby power must be absolutely seamless. This is facilitated by the UPS system which will be able to carry the load (depending on size) for a limited period of time, during which the generator will be given an opportunity to take over, powering the UPS and its battery pack until utility power is restored.

For the combination to function effectively, the compatibility between generator and UPS system must be ensured. Compatibility is not always easily achieved and the services of a specialist are often required to define operational characteristics and load interaction, and to control the design of generator and UPS devices to ensure reliability. For example, many UPS systems on the market make use of a thyristor rectifier to charge their battery packs. The rectifier can cause ‘notching’ on the power feed which can damage the control systems of some generators when the two devices are linked in a common alternative power solution.

What is more, harmonic distortion in the current can cause generators to overheat. Some UPS manufacturers resolve problems linked to line notches and harmonic distortion with the addition of a passive filter designed to reduce harmonic feed-back.

The sizing of the UPS and generator is often critical. When a generator is sized too closely to a line interactive UPS and there is little or no load on the generator when it is called to action, the problem of voltage rise can occur. Unless checked by a UPS able to disconnect its filter when its charger is in standby mode, the power may spike uncontrollably to around 120% of the desired voltage with consequent damage to electrical equipment. The alternative is to attach a preload voltage regulator to the generator to obviate the problem.

Other common problems associated with frequency fluctuations include an inability to closely control generator and UPS responses to changing power loads, often exacerbated by slow governor response. On the other hand, if the UPS battery charger is unable to accurately regulate its power needs, voltage fluctuations will also occur.

At the system design stage, the generator’s governor should be correctly sized and, once commissioned, adjusted to meet the dynamic demands of the system. In addition, the generator’s voltage regulator must complement that of the governor to prevent incompatibility with the UPS battery charger. And, finally, the UPS system should be fitted with a control mechanism designed to respond to – and dampen – fast line frequency fluctuations.

For more information contact Philip Hampton, Powermode, +27 (0)11 235 7750, philiph@powermode.co.za, www.powermode.co.za





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