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Generator loss of excitation protection

          A loss-of -magnetism relay with static or asynchronous boundaries as a criterion can identify normal operation and loss of excitation faults. However, in the external generator short circuit, system shocks, charging long lines, self-synchronization and the voltage circuit disconnection, etc., loss of magnetic relay may malfunction. Therefore, other features must be used as auxiliary criteria. Additional auxiliary components, in order to ensure the protection of the selectivity. In the generator loss of magnetic protection , commonly used auxiliary criteria and blocking measures are as follows:  

(1) When the generator loss of excitation, the excitation voltage drops. In the external circuit, the system shock process, the excitation DC voltage will not decline, but because of forcing excitation and rise. However, in the system oscillation, the external circuit, the excitation circuit will appear alternating component voltage, it is superimposed on the DC voltage, the excitation circuit voltage sometimes zero. In addition, during the asynchronous operation after demagnetization, the excitation circuit will also generate a large induced voltage. Thus, the excitation voltage is a variable parameter, usually its change as a loss of magnetic field protection criteria. 

(2) In the event of a loss of excitation, the voltage and current of the three-phase stator loop are symmetrical and have no negative sequence component. During the short circuit or short circuit induced oscillation, the negative sequence component always appears for a short time or during the whole process. Therefore, the negative sequence component can be used as a secondary criterion to prevent the loss of field protection during short circuit or short circuit during the oscillation. 

(3) In the process of system oscillation, the locus of the terminal-side measuring impedance may only pass through the action zone of the demagnetization relay for a short time without staying in the action zone for a long time. Therefore, the loss of magnetic protection with delay can avoid the impact of shock. 

          Since the same period of the process is the inverse of loss of magnetism. When the outlet circuit breaker is closed, the end of the measured impedance of the terminal is within the bound of the asynchronous impedance, and the demagnetization relay malfunctions irrespective of the positive definite conditions. With the slip of the decline and the growth of synchronous torque, and gradually exit the action zone, and finally into the first quadrant of the complex plane, the relay returns. Since the same period belongs to the normal operation, therefore, can be taken in the same period from the loss of magnetic protection device contact to prevent it from malfunction.

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