Two-channel control of three-phase inverters for distortion reduction

A new two-phase, independent control strategy for three-phase three-wire static inverters to reduce unbalance in output voltage is proposed. The three-phase voltage and current signals are processed by the ideal Clarke transform block to yield two single-phase signals, which contain information regarding unbalance and harmonics which may be present in the three-phase signals. If the two single-phase voltage signals could be controlled so that they become equal in magnitude with a phase shift of 90 degrees (ideal alpha beta signals), then the corresponding terminal three-phase voltage signals would become balanced. Single-phase observers are employed to control the fundamental components in the two identical d-q frames created. Simultaneously, when the harmonics are eliminated in the two single-phase channels, the corresponding three-phase voltage signals would become distortion free. Feed-forward compensation can be carried out using the equivalent series-impedance parameters, estimated on-line using a simple identification algorithm. This procedure is not sensitive to frequency and can take care of the voltage drop in the distribution transformer. The simulation and experimental results for a field programmable gate array (FPGA) controlled 60 V table top inverter fabricated in the laboratory are provided.