A UPS with Inverter Specially Designed for Non Linear Load

The different blocks (rectifier, inverter, static bypass switch) in the three phase UPS model B312 are briefly discussed. The input characteristics of the commonly used diode/capacitor rectifier are discussed by analysing the input current waveform with and without a small inductor in the circuit. The source impedance influence on the total performance is stressed separately. In a three phase system with this type of rectifiers connected between each phase and the neutral, the phase current will be added up, not cancelled out, in the Neutral. In the last part of the paper, the input/output current waveforms in a Y/D/Y connected transformer, supplying rectifier load, is shown, and RMS, average and harmonics in the input and output current are calculated. It is shown that with this type of load, the transformer output kVA load will be about 30 % higher than the kVA supplied to the input. The closed loop control block diagram of the B312 inverter is presented, showing the common (dc) slow regulator controlling three individual (ac) fast regulator loops.