Modulation-controlled clamped-three-level inverters supplied by series unbalance battery-banks under asymmetric SOCs

摘要

Energy storage systems can be utilised to counteract unreliability of renewable energy sources. This study proposes a novel modulation for clamped-three-level inverters when the dc link of inverter contains two series-connected battery banks. By proposed modulator matched with the considered topology, inverter is free from over-modulation even under unbalance neutral-point voltage. In addition, by appropriate zero-sequence common mode voltage injection, neutral-point current is controlled and batteries' state of charge (SOC) are dealt with so that power generation in discharge mode or power absorption in charge mode is properly shared between batteries. Due to the high capability of the proposed method, output voltage and current of inverter are almost free of low-order harmonic distortion even under severe unbalance condition and/or great asymmetric SOCs. After analysing and rejecting previous modulation-based control strategies for the considered topology, simulation study (charge and discharge modes) and experimental validations (discharge mode) are provided to verify the proposed method in grid-connected and islanded systems under different power factors.