On the Power Sharing Dynamics of Parallel-Connected Virtual Oscillator-Controlled and Droop-Controlled Inverters in an AC Microgrid

Accurate active and reactive power sharing is of utmost importance in an inverter-interfaced AC microgrid. Power droop-control has been proven as a good design tradeoff and communication less technique to achieve load sharing. Implementation of power droop-control requires both the voltage and current tapping at the inverter output to calculate the power and achieve synchronization with rest of the system. On the other hand, recently proposed virtual oscillator controller (VOC) features inherent synchronization using only the current measurement. However, VOC being a non-linear oscillator introduces harmonic distortion in the output. This paper investigates the power sharing capability, dynamic response and harmonics profile for a system consisting of parallel-connected (i) droop-controlled inverter and (ii) virtual oscillator-controlled inverter. It has been demonstrated that power sharing between these two different control techniques can be achieved by properly designing both the droop-controller and VOC-parameters. The harmonic distortion present in the output of VOC results in the harmonic current to flow between the two inverters. The harmonic distortion can be reduced by designing a tightly regulated VOC inverter. However, this results in a slow dynamic response. Several scenarios including the plug-n-play capability, step change in load, and the design trade-off in between dynamic response and harmonic content present in the output is demonstrated through the simulation results.