Optimized Controller for Converter Based DG

Abstract

Most of Distributed Generators (DGs) use Voltage Source Inverter (VSI) for the purpose of interfacing DG with the grid and producing electricity at the desired voltage and frequency. By applying different control strategies for the inverter, we can control the DG as a current source or as a voltage source depending on the controlled variable in the control strategy.

DG is controlled as a current source in the grid connected mode and the controlled variable in this mode is the grid current to control the active and reactive power injected to the grid. In contrast, DG is operated as a voltage source in the islanded mode, and the controlled variable will be the voltage across the local load to keep its amplitude and frequency within the allowable limits. Therefore, the DG operates in two steady state modes: grid connected mode and islanded mode, and two transient modes: transition from grid connected to islanded, and from islanded to grid connected. During these transient modes, the voltage and current of the critical local load connected to the DG suffer from destructive spikes, as we need to switch the control strategy of the inverter. To solve this problem a seamless control strategy for the inverter of the DG is used with no need for switching between different control schemes. The strategy is called the “Unified Control Strategy". The strategy is implemented in the Synchronously Rotating reference Frame (SRF), where AC quantities, that need to be regulated, are converted to DC quantities in this frame so conventional Proportional Integral (PI) controllers can be used which are simple and can achieve zero steady state error.