Carrier Transition Techniques for Parallel Connected VSCs Using Cross-Coupled Inductors

Parallel inverters can be connected using cross-coupled inductors (CIs) to produce high-quality line voltages in three-phase ac systems. Since no fundamental flux is present in the inductor cores, their size can be made considerably smaller than alternative inductor configurations. Interleaved switching and phase-shifted carriers (PS-PWM) can be used to produce multilevel three-phase line voltages. A modified approach (MPS-PWM) improves the quality of the line voltages by changing the phase of the carriers,
carrier swapping
, based upon the magnitude of the modulating signals. Since carrier swapping in MPS-PWM can cause the inductor circulating currents and flux patterns to jump and have dc offsets, two carrier transition techniques are presented to eliminate these jumps:
carrier manipulation
and
pulse injection
. These techniques represent open-loop PWM control, dependent only on the carrier phase change being used, and can easily be applied to an arbitrary number of parallel-connected inverters. The circulating currents and inductor flux patterns follow predictable patterns, allowing the CIs to be designed more precisely. The relative merits of using MPS-PWM over PS-PWM are assessed using measurements of the load current total harmonic distortion and the line voltage harmonic volt–seconds. The feasibility of the parallel inverter system described is verified using simulations and results from an experimental laboratory prototype.