The need for reliability in power supplies has obviously been with us for many years. With the increased use of electrical power and our dependence on an electrical supply, reliability has become an increasing concern. Initially the market for uninterruptible power supply equipments was of low demand and was led by users who sought clean and secure electricity supplies from their various engineering groups.

Within the family of rotary UPS systems are those which, instead of using battery banks as the short term energy store, use a form of kinetic energy storage and an integral diesel engine. These systems provide uninterrupted or no-break power, utilising the kinetic energy store for a matter of seconds and the diesel engine for long term energy supply.

The ever increasing use of computer systems, especially in the medium power range (10 MIPS, 5 Gbytes), results in an ever increasing requirement of associated UPS systems generally with ratings of up to 100 kVA. In many of these applications the reliance placed on the system dictates that a secure clean continuous power supply is available. They are often installed in existing buildings in which there is no dedicated plant room. The alternative is to locate these systems within the computer room. Obviously the traditional large system, producing in excess of 70 dBA noise level with high heat losses, is not suitable for operation in this type of environment. There are many aspects of the design of these systems which differ from the plant room systems. Obviously the first criterion with any system must be reliability, the noise emitted must be less than the noise level in the computer room, and the heat losses reduced by designing a high efficiency system. With the module located in the vicinity of the computer equipment much more consideration must be given to radio frequency interference (RFI) either conducted or radiated, both of which can result in data corruption.

This chapter reviews the application of uninterruptible power supply systems (UPS) used at airports, by airlines and by air traffic control.

A non-linear load may be defined as a load which, having a sinusoidal voltage applied to it, passes a non-sinusoidal current. Many everyday loads are non linear but the non-linearity is often unimportant: an unloaded transformer is non-linear owing to magnetic saturation. Significant non-linear loads associated with UPS equipments include rectifiers and switched mode power supplies. Any regular non-sinusoidal waveform may be regarded as a compound wave made up from a fundamental component and harmonic components: the composition of such waveforms may be established mathematically by applying Fourier's analysis. As triplen harmonic currents have zero phase sequence they cannot exist in a three-phase balanced form unless there is a neutral return path. Even harmonics indicate a lack of symmetry between positive and negative half cycles.

It may seem a trite statement, but before a specification for a UPS equipment can be written, it is necessary to consider carefully what is expected of the equipment. It is the author's opinion that a weak original specification is frequently responsible for the failure of an equipment to perform as expected. Important items are sometimes omitted from the specification either from oversight or because they are too difficult to express; it is the purpose of this Chapter to assist purchasers to decide their functional requirements, and to include them in a specification. The list is divided into the following sections: location of equipment; type of equipment; earthing the neutral of the UPS output; input power-ratings and quality; type of load; output power-ratings and quality; battery and battery charger (if applicable); service conditions and installation requirements.