Investigations on Five Level Inverter Schemes for Extending the Linear Modulation Range for Induction Motor Drives

Abstract

Multilevel inverters (MLI) are the most preferable alternatives for medium to high voltage applications. It finds in many applications like traction drives, fans, blowers, HVDC and FACTs etc. It offers many attractive features over a conventional 2-Level inverter like lower voltage stress across the power switches, lower dv=dt across the load terminals, lesser EMI, lesser current and voltage total harmonics distortion (THD) etc. In contrast to a 2-Level inverter, the output of a MLI produces many discrete steps which makes the output voltage nearer to an ideal sine wave. This allows to reduce the switching frequency in a MLI which in turn control the losses in the power switches in a MLI. All these features make MLI superior compared to a 2-Level inverter in every aspect. In modern days, the application of MLI in the field of medium to high power AC motor drives becomes indispensable. There are many basic MLI schemes like Neutral Point Clamped (NPC), Flying Capacitor (FC) and Cascaded H-Bridge (CHB) inverters. These basic inverter scheme have many advantages and disadvantages over one another. To mitigate the disadvantages of the basic MLI schemes, hybrid MLI schemes have been presented in the literature. Also, the multilevel operation can be achieved using dual fed inverter schemes where two inverters are being fed from either side of the open-end load. With the increase in levels, basic MLI schemes require more number of components like clamping diode (in case of NPC inverters), floating capacitors (in case of FC inverters) and isolated DC-sources (in case of CHB inverter). The main objective behind developing a new MLI scheme is to optimize the component counts and also increasing the reliability. In this thesis, 5-Level reduced component count MLI schemes are proposed using a single DC link. Usage of a single DC-link in a MLI scheme enables easy four quadrant operation. Furthermore, the proposed schemes are more reliable compared to the basic MLI schemes. In case of a fault in any of the floating bridges...