Unbalanced loads cause the imbalance of the grid current and the resulting imbalance of the grid voltage when the imbalance rate of the grid current increases considerably. To cope with this problem flexibly at low cost, we modify the circuit structure from previously installed Δ-Y transformer 3-phase 3leg inverter to split dc-link capacitor 3-leg inverter eliminating the transformer. And then we propose two kinds of compensation algorithms for balancing the grid current that can basically utilize a preinstalled control board. It is divided into reactive compensation, where dc-link of the inverter does not supply effective power to the grid but compensates for the imbalance of grid current, and effective compensation, where dc-link of the inverter supplies effective power to the grid for compensating the unbalanced grid current. We design an additional power calculator board to obtain power information, and apply it to a 10kW prototype, and then verify its validity through PSIM simulation and experiments.INDEX TERMS energy storage system (ESS), power conditioning system (PCS), sinusoidal pulse width modulation (SPWM), split dc-link capacitor, 3-leg inverter, 3-phase 4-wire system, unbalanced current compensation.

In this paper, we analyzed the output voltage imbalance and the cause of the offset voltage in 3-phase 3-leg inverters by using Millman’s theory. Based on this result, we proposed a voltage imbalance compensation algorithm using the dc offset voltage that appeared at the neutral point voltage of the load. To apply the proposed imbalance compensation algorithm, it needs a circuit structure of 3-phase 4-wire such as split dc-link capacitor 3-leg inverter and 4-leg inverter. Therefore, the total harmonic distortion (THD) of the load phase current according to the imbalance rate of the load was analyzed for the two inverters. Then, PSIM simulation and experiments based on a 10 kW-prototype of split dc-link capacitor 3-leg inverter were implemented to verify the validity of the proposed imbalance compensation algorithm.