A control strategy for grid-connected distributed energy resources in the presence of unbalanced and harmonically polluted loads

This paper presents a multi-input multi-output (MIMO) control strategy based on dq model of electronically coupled distributed energy resources (DERs). The DER represents a dispatchable distributed generation (DG) which is connected to the main grid by means of three phase voltage source converter (VSC). The control scheme uses Phase-Locked Loop (PLL) to estimate the frequency of the grid. Therefore, the controller can be implemented in dq frame. The DER is assumed to be in a microgrid with local loads which have different topologies such as unbalanced and harmonically polluted loads. The objective is to control the real and reactive powers that DER delivers to the main grid by current-mode control of VSC in the presence of different topologies for local loads. The control strategy uses the theory of repetitive controller which guarantees that the DER controller generates pulse width modulation (PWM) signals so that the real and reactive powers output of the microgrid track their corresponding references in the presence of sinusoidal reference inputs. The effectiveness of the proposed approach is verified through case studies, carried out on an electronically based DER in a grid-connected microgrid.