Self-Synchronizing Cascaded Inverters With Virtual Oscillator Control

In this article, we introduce a decentralized communication-free control strategy to synchronize and control an arbitrary number of series-connected inverters delivering power to islanded loads. The proposed approach reduces wiring complexity, streamlines installation, and eliminates single points of failure in cascaded inverter systems. We focus on three-phase power stages that are composed of three full-bridge inverters on the ac side. The proposed controller at each module takes the form of a digitally implemented virtual oscillator controller (VOC), which uses the inverter current as a feedback signal and in turn modulates the ac voltage amplitude and frequency. Power and voltage sharing among modules can be adjusted on-the-fly via oscillator setpoints at each converter controller. We take advantage of the superior transient performance of VOC over the conventional droop-based controllers, which in turn yields a high-performance series-connected inverter system with fast responsiveness. A system model is put forward to analyze small-signal stability and provide a control design framework. Finally, experiments on a system of five series-connected inverters substantiate the proposed approach.