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Seamless transfer scheme for parallel PV inverter system

Most grid-connected inverters can operate in both grid-connected and islanded modes with usually different control schemes adopted in each mode. The control schemes, if noticeably complex and different, can result in inrush currents and voltage distortions at instants of mode transfer. An existing grid-connected form of droop control has hence been developed from the usual islanded droop control to better streamline the transfer. However, the grid-connected droop control does not regulate the grid current directly, causing it to distort by grid voltage harmonics and respond slower during a transient. The motivation here is thus to propose another highly unified droop control scheme that can regulate the grid current directly with a transfer dynamic lasting only two fundamental cycles. This is in addition to the usual targets of seamless transfer and proper power sharing among the islanded inverters. The hypothetical approach followed is to reformulate the grid-connected and islanded control schemes, so that mode transfer can simply be treated as a normal change of voltage reference, rather than an inner change of control structure. The final unified scheme has been tested in simulation and experiments with results apparently confirming its precise grid-current regulation, islanded voltage regulation and seamless mode transfer within two fundamental cycles.