A Line-Commutated Thyristor-Bridge Emulated Rotating Power Electronic Converter for Brushless Exciter Applications

Abstract

Conventional brushless exciter supplies field current to the wound-field synchronous machines through a diode bridge rectifier. Though these diode bridge based exciters are more reliable than the static exciters, which use brushes and slip rings, they can not produce a negative voltage across the field winding, making the rate of decay of the field current slow. In case of an internal fault in the synchronous machine, the field current needs to be removed rapidly. In this dissertation, a novel thyristor emulated rotating power electronic converter (RPEC) is proposed for brushless exciter (BLE) applications. In this proposed three-phase AC-DC converter, each phase comprises a diode in series with a MOSFET. This converter can produce both positive and negative voltage across the field winding. It also can operate at higher frequencies, and thus exciter will be more compact and energy-efficient. This topology also eliminates the need for a DC-link capacitor, which improves the brushless exciter system's reliability. Moreover, this converter needs only a single gate drive power supply as the MOSFETs have their source terminals connected together. Furthermore, as the exciter is operated at higher frequencies, there will be a lower torque ripple at the synchronous machine output. Mode change is detected using a higher frequency component superimposed on the rotating transformer's fundamental voltage, due to which the exciter system is more robust and reliable. The first part of the thesis describes the proposed rotating power electronic converter operation and gate pulse generation during rectification and inversion modes. The second part of the thesis analyzes the field winding current disruption issue while switching from rectification to inversion mode. A new control algorithm is proposed to avoid the problem of mode change from rectification to inversion. The proposed topology, along with the proposed control algorithm, is simulated using Matlab/Simulink. A hardware prototype has been fabricated and tested. The simulation and experimental results verify the operation and advantages of the proposed topology and control.