Nested Neutral Point Clamped Converter Based DSTATCOM with Mixed-Sequence Reactive Current Compensation Capability

Abstract

This research deals with the low-voltage ride-through (LVRT) in a distribution grid STATCOM (DSTATCOM) based on nested neutral point clamped (NNPC) inverter. A NNPC-DSTATCOM with a LVRT control scheme is introduced through which positive-sequence (PS) and negative-sequence (NS) reactive currents are handled. NNPC based DSTATCOM contains two kinds of capacitors, i.e., dc-bus capacitor(s) and flying capacitors (FLCs) placed in the legs. Since in DSTATCOM, the dc-bus is not fed by a dc supply, a hierarchical balancing strategy is presented to stabilize the voltages of capacitors. In this strategy, first, the common dc-bus voltage is stabilized at the desired reference and then the voltages of FLCs are adjusted. NNPC-DSTATCOM has a single dc source and circulating current paths through the converter legs. Taking this advantage, the LVRT scheme is able to independently manage both PS and NC reactive currents when an unbalanced grid condition appears while maintaining the capacitors voltages balanced. Simulation results are extracted in MATLAB/SIMULINK environment proving the correspondent operation of the introduced system.