Abstract: In the switching over operation process between main and stand by excitation systems of a thermal power plant in Guangzhou, the reactive power fluctuated greatly, and the excitationloss accident of the unit was happened. Aiming at the above problems, the current switching process of parallel silicon controlled rectification and diode rectification was analyzed, and the mechanical characteristics of the voltage regulating device of the standby excitation system were analyzed too. The measures to increase the reliability of switching over operation between main and standby excitation systems were put forward, and the reliability of switching over operation between main and standby excitation systems was increased effectively.

Key words: excitation system, main excitation, standby excitation, automatic voltage regulator (AVR), silicon controlled rectification, diode rectification