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The majority of present day UPS systems is based on the traditional dual conversion topology. In this concept, all the energy for the critical loads is converted twice: first from AC power into some kind of intermediate energy that can be stored as kinetical or chemical energy, and than, in a second conversion, again into stabilised AC power for the loads. This concept is now widely used and appreciated, however, improvements to the complexity and losses of the dual conversion and to the relatively high harmonic content of the AC input current is possible. An alternative topology is proposed, in which there is only one bidirectional power converter between AC power and storage means, that is still capable to stabilise and filter the AC input power without the above stated disadvantages.UPS systems based on this 'line-interactive" concept are now available as three phase units ranging from 2okVA as static UPS systems with battery to over 10 MVA as rotary systems with kinetic energy storage and an integrated diesel engine.The line-interactive concept.

Rapid current control in highly inductive loads requires a regenerative convertor whose output voltage is a multiple of that determined by the obmic resistance. During the flat top of the pulse the convertor is phased back resulting in a relatively large ripple and a high reactive power. Controlled freewbeeling using thyristors between the d.c. terminals and the neutral of the transformer winding improves the output waveform and reduces the power drawn from the mains without reduction of the dynamic performance. Twelve-pulse rectification, a passive smoothing filter with low stored energy and an active filter ensure an extremely low ripple and a fast dynamic performance. Circuits and design principles are illustrated by describing a 220 V/6500 A pulsed power source for testing magnets. Control range 0-6500 A, max. di/dt 20 kA/s, stability 100 mA, p-t-p ripple 30 mA; magnet time constant between 0.05 s and 10 s. The current source is provided with a high-precision digital current programmer; current shaping is achieved witb 8 adjustable parameters.

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Many requirements for UPS systems for use in remote tele com locations are quite clear: they must be compact, easily transportable, flexible in U& and able to operate with minimum maintenance. Severe climatic and EMC requirements, both on emission and immunity, also are common. These requirements also apply to temporary UPS systems.Less clear are the special requirements that result from the generally long feeden and their associated high impedance. Any fluctuation in feeder current like surges, reactive power variations and harmonics, give larger than normal disturbances at the end of the feeder cable, and produce unacceptable dips or voltage distortion to ac loads in the telecom station, including to the UPS system itself.Many of these problems become significant at higher power ratings, above a few hundred kVA, when solutions like fiters and batteries are less effective or difficult to install at remote or temporary sites. An alternative in the form of a containerised, diesel-powered UPS is presented.