An Improved PLL Based Seamless Transfer Control Strategy

This paper proposes a seamless transfer control method for distributed generation units to solve the issue of hybrid voltage and current mode control method and overcome the limitation of direct current control strategy. Upon the occurrence of utility outage, the voltage controller is automatically activated to regulate the load voltage. The inverter can transfer from controlled current source to controlled voltage source automatically. Therefore, the quality of critical load voltage can be maintained during the transferring process and doesn't rely on the islanding detection method. The direct current control strategy can only operate without grid line impedance, but this proposed method can overcome this limitation. When the grid returns normal, the pre-synchronization can be realized automatically without extra infrastructure. The simulation results are provided to verify the proposed control method.