Control of Multi-Level Converter Using By-Pass Switches
Rasha G. Shahin, Hussien D. Al-Majali |Pages: 1-14 |

Abstract— 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 MatlabSimulink 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.