Power Control of Virtual Oscillator Controlled Inverters in Microgrids

Abstract

This thesis focuses on power control of multiple parallel-connected virtual oscillator-controlled inverters in an islanded microgrid. The proposed power dispatch technique simultaneously regulates both the active and reactive power of multiple parallel-connected virtual oscillator-controlled inverters. The proposed technique enables better energy source utilisation, increased efficiency, and reduced line losses and stress on the distribution network. The control laws are derived, and power security constraints are presented to determine the feasible operating region. Moreover, a filter and line parameter design procedure is discussed for non-dispatched inverters to share the power demand proportionally. The local stability of the system with 1-D manifold of equilibria is also established.

The existing actual and averaged VOC models do not take into account the inverter non-linearities, including the voltage loss/gain due to the dead-time, and semi-conductor voltage-drop. Considering this, secondary voltage and current control loops are proposed to compensate for these unmodelled inverter non-linearities. Experimental results demonstrate that the proposed secondary control loops enable the virtual oscillator-controlled inverters to follow the desired droop-characteristics.

Moreover, in contrast to the existing literature, a new version of the averaged VOC model is derived for inverters with current feedback after the output LC/LCL filter. The corresponding VOC parameter design procedure is presented. Further, the proposed power dispatch technique is extended to this new version of averaged VOC dynamics. The updated control laws and power security constraints are derived to determine the feasible operating region. Simulation results demonstrate that the proposed new version of averaged VOC model more accurately predicts the actual VOC dynamics than the existing averaged VOC model for an inverter with current feedback after the output LC/LCL filter.

Finally, a system of heterogeneously controlled inverters with two different types of control techniques: i) virtual oscillator control and ii) droop control is considered. It is demonstrated that the two heterogeneous inverter controllers can be designed to share the power proportionally. Further, the effects of VOC design parameter epsilon on the system’s harmonic profile and transient response are investigated.

The proposed research work demonstrates the significance and potential of dispatchable inverter control techniques for microgrids powered by distributed energy resources.