Similarities between Virtual Oscillator Controlled and Droop Controlled Three-Phase Inverters

Virtual oscillator control (VOC) and droop control are two inverter control methods that can realize load sharing between parallel inverters in an islanded microgrid. Compared to the droop control method, which requires measurement of inverter output current and voltage to compute average power with the aid of low-pass filters, VOC appears to react directly to the measured current, without the need for voltage measurement, power computation and associated filters. Here the two approaches are compared in terms of transient and steady-state responses of a single-inverter system feeding an RL load, considering the effect of load power changes. The analytical comparison of transients is aided by small-signal modeling. Small-signal models of the complete inverter systems governed by VOC and droop are obtained by linearizing around operating points obtained from simulations in MATLAB/SIMULINK. The linearized models are analysed in terms of the system eigenvalues (modes). Participation analyses examine the sensitivities of the modes to system states. By determining controller parameters that ensure similar droop characteristics at steady state, the dominant modes of linearized virtual oscillator and droop controlled models are shown to have similar distributions, even though mode numbers and sensitivities of the small-signal models are different. This indicates that the single-inverter systems can be made to have similar transient responses. The analytical results are verified by simulating a three-phase inverter supplying an RL load, with a step change in the load.