Fission reaction in the nuclear power plants(NPPs) is controlled by Control Element Assembly(CEA). Thus, the stability and reliability of Motor Generator Set(MG Set), which is a power supply system to Control Element Drive Mechanism(CEDM), is considered to be very important in the viewpoint of safety of NPPs.
However, MG Sets of Korean Standard NPPs had repeatedly experienced that the temperature and vibration in their generator-side bearing were abnormally increased. The inspection showed that the abnormal temperature and vibration were induced by a wear damage of bearing wheel and roller, whichwas caused by introducing small chips of cage and V-ring into bearing. In order improve the operational reliability of MG Set by resolving the high temperature and vibration of MG Set bearing, type of bearing was changed, i.e., N type bearing changed into NU Type bearing, and also V-rings that are sealing system were removed and Felt rings were installed on the inner and outer caps of MG Set. It showed that such modifications reduced the temperature and vibration level of MG Set bearings.
Installing protection relay panels and revising sudden stop signals were improved the protection and monitoring systems to new power plants level. Rewiring reverse power relay and reverse power auxiliary relay was prevented unexpected shutdown and ensured a monitoring channel at Main Control Room.
Harmonics generated by Control Element Drive Mechanism Control System(CEDMCS) affected the MG Set operation such as increased voltage waveform distortion and instantaneous maximum voltage. It was verified decreasing voltage Total Harmonic Distortion(THD) and instantaneous maximum voltage by analyzing harmonics effect, installing RC filter, and measuring voltage waveform.