Abstract: The power plant auxiliary machine united adjustment power source system is non-utility frequency and small current grounding system. Conventional protection device cannot detect single phase grounding fault in the system correctly. Operating conditions of united adjustment power source system is analyzed, and minimum capacitive ground fault current value is calculated in single busbar low frequency operating condition. Based on calculation, technical requirements of non-utility frequency low current grounding fault line selection device is proposed. Then a high precision fault line selection device which adaptive to system frequency changes is developed. A special functionality simulation test circuit is also built to test developed device. The testing results show that proposed device satisfies design requirements, works well after commissioning and solves practical engineering problem.

Key words: non-utility frequency, low current grounding, fault line selection