DC-Link Bus active balancer/filter for a half-bridge three-phase transformerless UPS

This paper proposes an active method to balance the voltage and to filter low-order current harmonics on the DC-link Bus capacitors of a three-phase half-bridge transformerless UPS. These low-frequency current components occur due to utility unbalanced voltage conditions or even under the connection of non-linear and/or unbalanced loads. They increase the DC bus capacitors losses and also may cause supply line harmonic currents to flow by the neutral conductor, since they are fed back in the voltage loop, distorting the current reference and deteriorating the PFC performance. According to the results from the proposed approach, the active balancer/filter quite reduces the low-order current ripple, reducing losses and guaranteeing a suitable behavior of the voltage across the DC-Link Bus capacitors.