A novel distributed control scheme for the parallel operation of digital controlled UPS

A novel distributed control scheme is proposed for the parallel operation of single phase digital controlled UPS (uninterrupted power supply). In this scheme, all the UPS modules that are interconnected only by two common lines are exactly the same. So the real redundancy is achieved. One of the two common lines provides a square wave. Each reference voltage of the paralleled UPS traces the square waveform in order to synchronize the output voltages. The other line is for average output current and current sharing is achieved by average current method. Both the two common lines are low pass lines in order to restrain the possible electromagnetic interference. The theoretic analysis and simulation results show that the gain of current is the main factor that influences the current sharing precision. Experiments are carried out to verify the control method with three 1 kVA on-line UPS modules. The results show that the control scheme is simple and feasible and the current sharing is precise in dynamic response and static state.