Control of Multi-Level Converter Using By-Pass Switches

This paper describes a modification of the conventional bridge circuit under balanced conditions using two isolated-gate bipolar transistor (igbt) by-pass valves connected to tapping points on the secondary windings of the transformer. this scheme permits a wide range of voltage control with a reduction in both the harmonic generation and reactive volt-ampere absorption. this scheme should improve the efficiency of voltage control; it possibly eliminates the need for an on-load tap-changer on the converter transformer. the modified ac/dc converter is fully analyzed. a general mathematical model and the principle of operation for the modified converter have been determined. the characteristics for output dc voltage for the modified converter with two by-pass igbt valves are obtained; and the mathematical equations have been derived for different modes of operation. the supply current harmonic contents, reactive volt-ampere absorption and power factor have been compared with a conventional bridge for different converter bridge arrangements, modified bridge using one igbt by-pass valves and modified bridge with two igbt by-pass valves. simulation results are obtained using matlab-simulink for a model that has a three-phase bridge converter using conventional thyristors as main valves and two igbt as by-pass valves employing a controllable digital firing angles with pulse generators maintained on a small firing angle of the main thyristors.