ABSTRACT
Neutral point clamped (NPC) inverter faces the problem of fluctuating neutral-point potential (NPP), which leads to severe problems like increase in total harmonics distortion (THD) of output voltage and increase in voltage stress of the power semiconductor switches. This paper is focused on hybrid techniques used for reducing fluctuations in NPP by means of balancing the voltage across dc-link capacitors. A technique of providing offset in carrier waves using phase disposition pulse width modulation (PDPWM) is proposed to ensure the voltage balancing. The drawback of this scheme is it does not work satisfactorily for four-level and five-level mode of operations of the inverter. It works as intended up to three-level operation. Other approach to achieve the task is based on carrier less PWM technique. In this technique, the redundancy of switching states in voltage state phasor diagram is used. The limitation of this technique is that it provides dc-link balancing up to modulation index 0.5 due to its inherent characteristics. To provide dc-link voltage balancing above 0.5 modulation index, external hardware is required. Using hybrid combination of bidirectional buck-boost chopper and SVPWM scheme, the dc-link voltage balancing for the entire range of operation of inverter is obtained. MATLAB simulation of this technique is carried out on three-phase, 50 Hz, 4 pole, 400 V squirrel cage induction motor. For experimental verification of the proposed technique, three-phase five-level NPC inverter is used with three-phase star connected 100 Ω resistors.
