A double signal PWM scheme for neutral point voltage balancing in three level NPC converters

A Carrier Based Double Signal PWM (CB-DSPWM) scheme in conjunction with a zero sequence signal to mitigate voltage unbalances at neutral point (NP) for three-level NPC converters is proposed. The scheme compensates the dc link capacitor voltage unbalances and eliminates the low frequency voltage oscillations at NP that appear under certain operating conditions of the converter. The voltage balancing is achieved by adding a zero sequence current component with the zero sequence voltage. Injection of the modified zero sequence with the modulation signal produces substantial average neutral current for all power factor (PF) angles. By incorporating the proposed PWM scheme, improved NP voltage balancing for full PF range and at high modulation indices are achieved. The dynamic performances of the proposed scheme are evaluated through simulation in MATLAB/SIMULINK and the practicability of the proposed algorithm is validated through experimental results.