Atypical PWM for maximizing 2L-VSI DC-bus utilization in inverter-based microgrids with ancillary services

Three-phase microgrid-interactive inverters are expected to provide a range of ancillary services, with negative-sequence current compensation being particularly prominent. However, unbalanced pulse width modulation (PWM) references may be generated when providing these services. Since references used by carrier-based PWM techniques cannot exceed the dc-bus voltage defined limits without causing over-modulation, the maximum positive-sequence voltage microgrid-interactive inverters can generate may therefore be diminished. This will in turn reduce the maximum active-and reactive-power that the inverter could otherwise provide, unless PWM references are adjusted. In this paper, an atypical PWM method is proposed for maximizing dc-bus utilization of microgrid-interactive two-level voltage source inverters (2L-VSI) operating under unbalanced conditions. Through the injection of a common-mode component computed based on instantaneous reference magnitudes, the proposed method increases the maximum active-and reactive-power that a 2L-VSI can deliver when providing negative-sequence current compensation. The validity of the proposed technique is verified through simulation, as well as experimental data obtained using a 208V three phase grid-connected 2L-VSI.