This paper describes de sign and implementation principles of a microprocessor - based synchronizer for the synchronous generators that are to be switched into a running power system A 87C5 1 micro controller is the main processor element of this instrument and it has been, programmed so as to check first the correct phase sequence of the incoming generator and then the equality, with the required resolution , of the amplitudes and frequencies of the voltages belonged to the incoming generator and the running power system If the required conditions of voltage level and frequency found not to be satisfy ying , the correcting control signals will be sent to change the set points of the exciting and governor systems Finally, it closes the circuit breaker at the moment of zero phase difference taking into account the closing time of circuit reaker. This synchronizer with the advantages of having simple circuit , high resolution and reliability, flexibility and low cost could be considered instead of the conventional synchronizer which are not appropriately reliable and could hardly be repaired in case of any failure.

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