Grid-connected Self-synchronizing Cascaded H-Bridge Inverters with Autonomous Power Sharing

Cascaded inverters are widely applied in applications where elevated ac voltages are required while using semiconductor devices with lower voltage ratings. Here, we focus on structures that require localized power transfer between low-voltage sources/loads dispersed across inverter dc links and the inverter ac-sides are series-connected across a three-phase medium-voltage ac grid. To date, decentralized controllers that allow for bi-directional power transfer in such systems are limited. To fill this gap, we propose a virtual oscillator controller which modulates the power processed by each inverter in a purely decentralized manner. The proposed controller uses only locally measured current, provides communication-free synchronization of the inverter modules, and enables control of power transfer in both directions. Moreover, it is implemented purely in time domain as opposed to phasor-domain based droop controllers. Stability is analyzed and a design procedure for the oscillator parameters is provided alongside relevant simulation and experimental results for a system of five series-connected inverters.