Neutral Point Voltage Balance in Three-Level Voltage Inverter Based on Space-Vector PWM with Seven-Stage Switching Sequence

Abstract

The voltage imbalance of the neutral point (NP) of the dc link is an inherent limitation in using three-level autonomous voltage inverters with a fixed neutral point in electric-drive systems. To ensure the NP voltage balance within acceptable limits, a control algorithm based on space-vector PWM with a seven-stage switching sequence (SS) is proposed. Compared with the known control methods, the new alternative significantly improves the NP voltage balance by adjusting the fill factor of distributed base vectors in each PWM period based on phase load currents. The algorithm also eliminates the NP voltage imbalance caused by external factors, for example, by the asymmetry of dc capacitors or three-phase load and other factors due to the feedback signal on voltage deviations in two dc link capacitors. A system for calculating the turn-on lengths of basic vectors in a seven-stage SS is presented. The efficiency of the proposed algorithm is confirmed by computer simulation in the MATLAB Simulink suite. The proposed algorithm reduces the average maximal relative error in the NP voltage by 86% over the entire variation range of the modulation coefficient compared to the known method of balancing the NP voltage based on a feedback signal on voltage deviations in two dc link capacitors. This allows efficiently balancing the NP voltage and, as a result, improves the operational reliability of the inverter.