A Single-Stage Z-source Inverter for Transformerless Grid Connection with a Proportional-Resonant Controller for DC Current Elimination

The ever-rising number of grid-connected inverters contributes to an increase of DC current injection into utility grid, resulting in the saturation of distribution transformers, metering errors and the corrosion of earthling conductors. A transformerless single-stage Z-source inverter uses only two switching devices in its converter circuit to generate sinusoidal voltage as that of a full-bridge inverter. However, this inverter has the problem of having a DC offset in the AC waveform due to the presence of steady-state error when the modulation index is varied. The paper proposes a Proportional-Resonant (PR) control scheme to eliminate this DC offset. By comparing Z-source inverter output voltage with the sinusoidal reference voltage obtained from the sinusoidal control signal, an error signal is obtained which is fed into the PR controller. An infinite gain at the fundamental frequency is introduced by the PR controller, thus achieving zero steady-state error resulting in the elimination of DC current injection into the utility grid. This method does not depend on high-precision current measurement or the use of coupled inductors. Also, this method can be used to improve power quality by providing reactive power compensation to the load at the point of common coupling. Simulation results are presented to confirm that this simple, cost-effective method can be used to eliminate DC current injection for different values of modulation index without compromising the dynamic response of the current feedback loop.