Lab 1 - Introduction

Diode and thyristor-based AC-DC converters are used in a number of applications. Diode AC-DC converters are also known as uncontrolled rectifiers because they present unidirectional power flow characteristics, only being able to operate as rectifiers, and do not allow the variation of the DC bus voltage for a constant AC side voltage. In electric utility applications that require either bidirectional power flow control, such as in HVDC transmission systems, or control of the output voltage, such as in static exciters for synchronous generators, thyristor AC-DC converters are used.

A three-phase full-wave diode and thyristor rectifier are illustrated in Fig. 1(a) and (b) respectively. The power supply is represented by a pure three-phase voltage supply (resistance and inductance of supply are neglected), the diodes/thyristors are considered ideals, and the load is represented by a pure DC current source (highly inductive load).

Figure 1: (a) Three-phase full-wave rectifier: Diode (b) Three-phase full-wave rectifier: Thyristor.

Figure 2 (a) shows the two operational quadrants of a thyristor rectifier in the voltage (e_d) current (i_d) plane. The variation of the DC side voltage (e_d) as a function of alpha (α) and the thyristor rectifier operational modes (rectification, inversion) are shown in Fig. 2 (b). Positive values of e_d and i_dimply rectification, whereas a negative value of e_d and positive i_dimplies inversion. The inverter mode of operation on a sustained basis is only possible if a source of power such as a battery is present on the DC side.

Figure 2: (a) Output I-V diagram thyristor rectifier (b) Output average DC voltage as a function of delay angle α.