Contents I. Introduction
In Recent years, brushless excitation technology has been widely used in large-capacity turbo-generator sets. The magnetic pole on brushless exciters is stationary, and the armature rotates with the generator rotor at a synchronous speed. The armature winding cuts the main magnetic field and induces electromotive force, which provides the excitation current for the generator by rotating rectifier [1]. Compared with the static excitation mode, brushless excitation has a variety of advantages. These include low noise, a low failure rate, etc. but the rotating diodes are often damaged at high speeds and under high loads. When a large number of diodes are damaged, the generator may not excite normally, causing some serious consequences. In 2014, the No. 3 main transformer C phase failure at Liaoning Hongyanhe Nuclear Power Plant in China resulted in negative sequence current on the stator winding of the No. 1 generator, as well as 100 Hz voltage induced by rotor winding. This damaged a diode in the rotary rectifier of the exciter, and the downtime of the generator set was more than one month. Here, the loss of generation energy in RMB was more than 300 million yuan, with a daily loss of about 10 million yuan. This means significant net economic losses for nuclear power plants that replace nuclear fuel regularly and quantitatively [2]. Therefore, it is of great economic importance to improve the fault detection levels for rotating diodes in brushless exciters and reduce the outage time [3], [4].
