Hybrid modulation technique for neutral point clamped inverter to eliminate neutral point shift with minimum switching loss

Abstract

In neutral-point-clamped (NPC) multilevel inverter eliminating neutral point shift is a challenging task. The carrier based pulse width modulation (CBPWM) techniques proposed in literature for this works well for certain range of load power factor and modulation index. Though non-nearest vector (NNV) Space vector PWM (SVPWM) technique would solve this problem, it does it at the cost of increased switching loss. This paper proposes a hybrid technique by combining both CBPWM and SVPWM. In this technique CBPWM is used for modulating the output voltage and SVPWM is used to eliminate neutral point shift. SVPWM technique is activated for a small duration of one carrier cycle at every zero crossing of reference. This minimizes the switching transitions and thereby switching loss. The proposed technique is tested in MATLAB-Simulink under varying load power factor(pf), different initial capacitor voltages and for unbalanced loads. Simulation results yield better performance.