Investigations on Generation of 30-Sided Polygonal Voltage Space Vector Structures Using a Single DC-link for Induction Motor Drives

Abstract

A conventional 2-level inverter is the most prevalent DC-AC converter used to drive an induction motor. A conventional 2-level inverter produces a hexagonal voltage space vector structure. To extract maximum voltage from a DC-link, the inverter has to operate in overmodulation region. During this region of operation, a hexagonal space vector structure generates undesirable lower order harmonics, which in turn produce torque pulsations in the motor shaft. To avoid this problem, the inverter can be operated in the linear modulation region of the space vector structure at a high switching frequency. Always to operate the inverter in linear modulation region, the inverter has to be supplied with a DC-link of higher voltage and hence the DC source is underutilized. In addition, higher frequency switching at linear modulation is not an accepted solution for a high power drive because of the high power dissipation in the switches due to the switching loss. Methods like passive filtering technique and selective harmonic elimination techniques are adopted to filter and eliminate the harmonics while operating at lower switching frequencies. These techniques limit the maximum fundamental extracted from the DC-link. One other elegant method to eliminate the lower order harmonics and to extract the maximum fundamental voltage from the DC-link is by modulating the inverter using higher sided polygonal structures. First part of the work proposes a novel polygonal voltage space vector structure having 30 sides using a single DC-link. The space vector structure eliminates the presence of harmonics up to 25th order from motor phase voltage throughout the entire modulation range, providing a torque profile devoid of lower order pulsations. Linear modulation is extended till 99.63% of base speed without exceeding the motor phase voltage rating. The topology consists of a DC-link fed primary inverter and two equal low voltage modular capacitor fed secondary inverters. Here the harmonics generated by the primary inverter is cancelled by the secondary inverter which acts as a switched capacitive filter. Further, second part of the work is obtaining, a multilevel inverter scheme generating a 30-sided space vector structure with congruent triangles. The scheme being multilevel reduces the dv/dt and further reduces the harmonic content in the output voltage. The proposed scheme is implemented using a single DC-link on an open end induction motor. The open end induction motor is fed with primary inverter from one end and secondary inverter from the other end. Primary inverter is fed from an active DC-link and supplies all the required active power for the scheme. Secondary inverter is a capacitor fed inverter which acts as the switched capacitive filter. In third research work, the generation of an even denser 30-sided multilevel space vector structure using a single DC-link for an open end induction motor drive is presented. The resultant space vector structure has 15-concentric 30-sided polygons. The proposed scheme also eliminates lower order harmonics till 25th order from motor phase voltage throughout the modulation range. The dv/dt stress in the phase voltage applied to the motor will also be highly reduced owing to the multilevel structure. The topology consists of an active DC-link fed 3-level primary inverter and a capacitor fed 5-level secondary inverter connected to either end of an open end induction motor. In fourth research work, a very high resolution multilevel voltage space vector structure having 117-concentric 30-sided polygons of different radii is proposed. In this work, non-aligned 30-sided polygons i.e. regular 30-sided polygons which are not symmetric with respect to alpha and beta axis are also considered for inverter operation, for the first time. The denseness of space vector structure allows to use nearest level switching, enabling further reduction of switching loss in the system. The feasibility of all the proposed scheme is proved by experimental results during open loop v/f and field oriented control. All the schemes used in this thesis requires only a single DC-link for its operation, which enables easy four quadrant operation by using a single active front end converter. All the above mentioned features make the schemes best suited for high power medium voltage applications.