Analysis of different topologies of multilevel inverters

Abstract

This thesis compares three different topologies of inverters (one level inverter, Diode clamped inverter, Flying capacitor clamped inverter and Cascaded H-bridge inverter). The multilevel inverters are 5-level and 9-level inverters. This comparison is done with respect of power losses, cost, weight and THD. The switching pattern for inverters is explained as well. These inverters are connected to a 400V, 75kW asynchronous motor. For each inverter, IGBTs and MOSFETs are used as switching devices to make the comparisons more accurate. The switches that are used for different inverters are the same for all of the inverters. (There is no control on inverter; also for loss calculation loading distribution is assumed.) If the THD is important, the 9-level inverters should be used, since it has a lower THD than the 5-level and the two-level inverter. The 9-level multilevel inverters have the lowest THD when filters are not used. Their THD is about 7%. If the cost is important the two-level inverter should be used, since it has the lowest cost between all of the inverter topologies. If the power losses are important, the 5-level diode clamped is the best choice since it has the lowest power losses between all other inverter topologies. If the weight is important the two-level inverter is the best choice since it has the lowest weight between all other inverter topologies. Its weight is about 5Kg. If the power losses are important, the 5-level flying capacitor is the best choice, since it has the lower power losses between all the other inverter topologies. To select a multilevel inverter is a tradeoff between cost, complexity, losses and THD. The most important part is to decide which one is more important.