New Alternating current De-excitation for Large Hydraulic Generators

Abstract

De-excitation with varistor parallel connected to field winding for large generators has been widely used. However, field current and voltage become quite large for larger generators, such as 700 MW generator for Three-Gorge power plant, China. In order to put varistor into operation during de-excitation requirement for opening arc-voltage of dc field breaker is high and high. This causes big size, structure complexity, high cost and difficult maintenance of field breaker. That is why an alternating current de-excitation (ACDE) is adopted, which acts on ac side of thyristor rectifier-bridge for de-excitation. But up to now ACDE is not reliable enough, so ACDE in Three-Gorge power plant is used as an auxiliary for de-excitation. New ACDE proposed in the paper uses capacitor parallel connected to varistor, the former with inductance and resistance of field winding is able to cause serial resonance of RLC[1,2] and produce high voltage during de-excitation, which may force varistor to operate and fast de-excitation can be implemented. The field breaker on dc side of field may be omitted at all. Simulation proved that the new ACDE overcomes shortcoming of origin ACDE, and has high reliability.