The computer-aided design of voltage source inverters

D.Ing. (Electrical and Electronic Engineering)This thesis presents a series of analyses developed for several voltage source inverter topologies, which are indispensable in aiding the understanding and the design of these circuits. It describes the groundwork that is required for the development of custom design software for voltage source inverters. The different circuits that can be designed by using this program, are analyzed using conventional circuit analysis techniques. The equations are used to simulate the switching behaviour and to specify the required or recommended ratings of the main devices. The custom design program can be used to design three different topologies of voltage source inverters: a) hard switching (snubberless) six pulse bridge; b) hard switching (snubberless) six pulse bridge, equipped with a simple voltage clamp; and c) soft switching (snubbed) six pulse bridge, equipped with a conventional dissipative snubber circuit. The snubberless behaviour of a voltage source inverter leg is both experimentally and analytically investigated. Approximate equations are formulated which can be used to. determine the peak ratings of the main devices in the inverter, taking the reverse recovery of the freewheel diodes into account. The snubberless behaviour of an inverter leg, equipped with a simple voltage clamp across the dc link, is experimentally and theoretically investigated. Equations for design and determining the required peak ratings of the main devices are developed. It is shown that the simple voltage clamp can operate in two different modes, either as a fast discharging or as a slow discharging voltage clamp and that the ranges of applicability of thetwo modes of operation are different. Design rules for both modes of operation are presented. In order to develop the design equations for a snubbed inverter leg a suitable topology had to be identified first. A method for objectively comparing different inverter toplogies is proposed in this thesis, according to which the minimum on- and off-states of the main devices in the different topologies are compared for identical applications. This method is used to compare four different snubbed voltage source inverter topologies both theoretically and experimentally. As both methods indicate that the conventional snubber circuit allows the highest PWM switching frequency of the uncontrolled snubber circuits, this circuit is further investigated in its dissipative topology to develop the equations required for simulation and for inclusion in the program. The influence of snubber circuits on inductive inverter loads is also investigated. It is shown that for normal snubber sizes the effect of dead time forms the larger part of the output voltage distortion and that the effects of the snubber elements are normally of secondary importance. The effect of asymmetry of a specific snubber circuit on inverter loads is investigated and it is found that it introduces a de component in the output voltage. Finally the application of linear snubbers to switches that exhibit tail-forming characteristics is analyzed and optimized. The results allow the designer to minimize the total losses of the converter by identifying the optimal snubber size. The theoretical results are experimentally verifie