The Line-Interactive Concept, an Optimal Approach to UPS

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.