A three-phase grid-connected inverter with improved ramptime current control in programmable logic

An improved variation of ramptime current control, implemented in programmable logic, is applied to a bidirectional three-phase "voltage source inverter". Named "polarized" ramptime, it is a zero average current error (ZACE) method with a more controlled and narrow switching frequency band. Only the polarity of the decoupled current error signal and the desired switching frequency are used in this current regulator to determine switching instants. Switching signals are adjusted immediately and automatically for variations in system parameters without manipulation, prediction or direct measurement. Polarized ramptime is applied in each of two current regulators in a balanced three-phase voltage source inverter operating with a standing phase. Simulation and prototype inverter results are presented showing negligible low order AC current harmonics. The synchronization of the two polarized ramptime current regulators is controlled in the prototype inverter so as to minimize the AC current ripple, with a similar result to that achieved with symmetrical pulse patterns in voltage space vectors.