Using Modular Multilevel Converter-Distribution Static Synchronous Compensator Integrated with Yy-Transformer to Compensate Reactive Power and Unbalanced Voltage in Distribution Grid

Unbalanced loads in the distribution grid can cause unbalanced supply voltage and reduce the power quality of the system. Besides, reactive power loads might create the voltage disturbances such as voltage sag/swell. In this paper, a four-wire distribution static synchronous compensator (DSTATCOM) integrated with Yy-transformer to compensate both the unbalanced voltage and reactive power is proposed. A DSTATCOM based on modular multilevel converter (MMC) connects to the group of taps in the primary windings of Yy-transformer to constitute an interconnected structure. The compensating currents flow through the taps. This structure is similar to an autotransformer. Compared to the conventional MMC-DSTATCOM, with proposed structure, The DSTATCOM connection point voltage decreases and one can trade-off between the capability of the power device's rated voltage and current. Therefore, it can be decreased the primary investment costs, the voltage stresses and the size of compensation system. Furthermore, a control algorithm based on measuring the point of common coupling (PCC) instantaneous voltage and current is presented that it is able to compensate the zero and negative sequence voltage due to unbalanced laod and grid. To compensate these components, voltage and current loops are used to track accurately the reference voltages of MMC-DSTATCOM by means of unbalanced algorithm. Simulation results verify the proposed method.