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This paper presents a new relaying scheme for the single ground fault in the main field winding of a brushless synchronous generator, The protection system consists of a Fault Detecting and Signal Transmitting (FDSl) module and a Signal Receiving and Alanning (SRA) module. The POST module is mounted on one end of the rotor shaftThe detector with a dual-balance bridge circuit can effectively detect any ground fault in the field winding. The signal transmitter transmits different Frequency Shift Keying (FSK) infrared mys depending on fault or normal conditions. The circuits between the detector and the transmitter are coupled optically. The SRA module includes a signal receiver and an alarming circuit It can determine whether a ground fault occurs in the field winding or the POST module is out of operation, according to the received signals. This protection system can also be used as AC synchronous motor field ground protection if the size or importance of the machine warrants it The results of laboratory test simulations are very satisfactory.

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