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.