A Control Strategy With Seamless Mode Transition for a Low-voltage Distributed Generation System Without an Energy Storage System

The world energy demand is expected to rise in the coming decades. Renewable Energy Source Distributed Generation (RES-DG) is being used to meet this increased demand. RES-DG systems can serve local loads on an isolated system (autonomous operation) or connected to the power grid (grid-connected operation). Ideally, RES-DG systems should have the ability to transition smoothly between modes so that a continuous supply of power is achieved regardless of the grid state. This paper proposes a communication-less control strategy for a low-voltage RES-DG system capable of operating in autonomous as well in grid-connected mode and of achieving a smooth transfer between modes. An energy storage system is not required for operation in autonomous mode since the energy balance is maintained by acting in the power generated by the RES. The proposed control strategy is divided in three parts: (a) the VSI control, which constitutes the inner control loops of the Voltage Source Inverter (VSI) (current, voltage and power control loops), (b) mode-related control loops (DC link, voltage at the PCC and DG system angular frequency control loops) and (c) the synchronization and islanding detection methods. The control strategy is validated through simulation of a three-phase RES-DG system. The proposed strategy presents a smooth transition between modes, with little to no perturbation on the PCC voltage, and a fast synchronization process from autonomous to grid-connected mode. In addition, it also presents a fast DC link voltage control response time that is capable of maintaining the transitory small, even for large power imbalances during mode transition.