Design of new control strategies for a four-leg three-phase inverter to eliminate the neutral current under unbalanced loads

This paper presents new control strategies for a four-leg three-phase UPS inverter which is used in AC power distribution system in data centers. The IT loads are distributed between the three phase terminals and the neutral terminal of the inverter. Due to the natural unbalance of the three-phase loads in the data center system, there exists a non-zero neutral current if the output voltage of the inverter is controlled to be three-phase balanced. The objective of this work is to eliminate the neutral current such that the thermal losses in the neutral conductor can be reduced and the power usage efficiency (PUE) in date centers can be improved. The elimination of the neutral current is achieved by controlling the inverter output voltages. Three different control methods are designed to dynamically adjust the amplitudes and the mutual phase differences of the three phase voltages, respectively. The effectiveness of the proposed strategies is validated through intensive simulations.