Neutral Point Voltage Balancing in Three Level Neutral Point Clamped (NPC) Multilevel Inverter Using Carrier Based Offset Addition PWM

Abstract: Three-level Neutral Point Clamped (NPC) converters are widely applied in high power medium voltage electrical drive systems. A significant problem related to the three-level NPC converters is the fluctuation of the neutral-point voltage. In this paper, a neutral point voltage balancing technique is proposed that injects an offset voltage into the sinusoidal modulating signals of the conventional Carrier-Based Pulse Width Modulation (CBPWM) method. Furthermore, when the injected offset voltage is maximized, it is not only balance the dc-link capacitors voltages, but also reduce output voltage distortions and switching losses. And also, the CBPWM offset addition method has reduced the switching commutations number to the lesser values. The performances of the strategies were verified by simulation tests. Modulation (PWM) strategies developed with continuous offset addition is demonstrated by MATLAB/SIMULINK based simulation presented in this paper.

Keywords: NPC inverter; 3 phase induction motor; offset addition;diode clamped inverter; multilevel inverter; carrier based PWM; dq theory; pi control.