Enhanced Virtual-Oscillator Controlled Inverter: A Complex-Coefficient Filter Approach

In this article, the virtual oscillator (VOC) controlled grid-forming converter is analyzed from complex-coefficient filter (CCF) point of view. It reveals that the conventionalnoninertialVOC is actually an adaptive-gain frequency-shifted low-pass filter; i.e., a first-order CCF. Inspired by the inertia emulation technique in virtual synchronous machine (VSM), the proposedinertialVOC is realized by a second-order CCF. It can be interpreted as a 50/60 Hz-shifted VSM running in the stationary frame. We further modify the nonlinear block of the VOC, hence linear voltage droop and better voltage supporting can be achieved. Since VOC is highly nonlinear, the conventional discretization methods generate considerable errors in digital implementation, especially for high-power rated converter working at low switching frequency. Two discretization techniques are investigated: The forward-backward Euler method and the complex repetitive controller approach. The proposed algorithm is verified by experiments both in grid-connected and island modes.