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A Unified Control Approach with Inherent Islanding and Synchronizing Capabilities for Grid Interfaced Inverters Operating in Autonomous Mode

In this paper a unified control structure which would enable grid interfaced converters to inherently transfer between grid and autonomous modes is proposed. The proposed control structure is based on the instantaneous dynamics at the point of the common coupling. A reference space vector is continuously applied in the direction of the PCC space vector. The magnitude and the speed deviations of the PCC space vector with respect to the applied reference space vector is instantaneously corrected to meet the real and imaginary power demands. In the grid connected mode, this inherently transforms into a current controlled system where as in an autonomous mode, the control system behaves as a voltage - frequency controlled one. The proposed controller thus eliminates the need of critical islanding detection mechanisms thereby reducing the cost, complexity and infrastructure. The necessity for two separate control architectures and the problems associated with switch over across control architectures are also solved by the proposed controller. The controller being implemented in a synchronous reference frame with respect to the PCC space vector makes the implementation simpler and also ensures the improved dynamic responses during mode transfers. The proposed controller is verified and validated by simulation and experimentation.