Control of UPS inverters

Many applications of UPS systems require sinusoidal outputs at frequencies ranging from 50 Hz to 400 Hz. Important performance specifications for such systems include voltage regulation, total harmonic distortion, output impedance, transient response and operation with nonlinear loads. The most significant factors affecting the overall performance of the system are the power converter topology and the control or modulation strategy used. The most popular approach for realising such converters is through the use of voltage source DC link inverters operated with an LC output filter. The inverter is typically operated with a pulse width modulated strategy under feedback control to realise the desired output waveform. The output power rating and the output distortion specifications are generally used as the basis for the choice of switching frequency and LC filter size. The regulation quality of the output voltage is arguably the most important specification of the UPS system. Regulation quality will depend on the control strategy used. This paper reviews the analogue and digital control strategies used for realising sinusoidal ouput UPS systems