Direct power control of voltage source inverter in a virtual synchronous reference frame during frequency variation and network unbalance

This study proposes a direct power control (DPC) scheme for grid-connected voltage source inverter (VSI) in a virtual synchronous reference frame during frequency variations and network unbalances. The proposed DPC scheme applies a virtual phase angle instead of the voltage phase angle acquired by the phase-locked loop (PLL) for coordinated transformations. This approach can eliminate the PLL and its low-frequency-oscillatory behaviours during the frequency variations. Meanwhile, two available control targets, that is, smooth active power and smooth reactive power, are defined for the VSI operation during network unbalance. In the proposed DPC scheme, the reduced-order vector integrator is employed to directly regulate the power pulsations caused by the voltage unbalances. Consequently, it can eliminate both the sequential separations of the voltages/currents and the complex calculations of the power compensating components. Then, based on the mathematical model of the VSI control system, the control performance, including the steady-state power errors and the system stability, is fully analysed. Finally, both the simulation and experimental results are provided to confirm the effectiveness of the proposed DPC scheme.