Self Protection of Silent Diesel Generator Set under Low Frequency

How can a silent diesel generator set achieve self-protection at low frequencies? The protection function of a silent diesel generator set is very powerful, and a single stator turn to turn protection is composed of longitudinal zero sequence voltage and negative sequence fault direction criteria. PT disconnection blocking measures are set as the main protection for internal turn to turn and phase to phase short circuits and stator winding open welding in the silent diesel generator set. The fault component negative sequence direction criterion is achieved by detecting the negative sequence power flowing out of the silent diesel generator set, and the longitudinal zero sequence voltage criterion is achieved by detecting the longitudinal 3U0 output from the 3PT open delta winding directly connected to the neutral point of the silent diesel generator set but not grounded, and the protection action is not stopped.

If it is out of step protection, the protection adopts a three impedance component, which detects the number of slips and determines the position of the oscillation center through the trajectory change of impedance. In case of short circuit fault, system oscillation, voltage circuit disconnection, etc., the protection shall not malfunction. Protective action on signals; When the oscillation center is inside the transformer group of the silent diesel generator set, the protection section I starts and sends a trip command through t1 (0.5s), acting on the disconnection and demagnetization; When the oscillation center is outside the silent diesel generator set transformer group, the protection section II starts and sends a signal through t2 (2s). The protection device is equipped with a current blocking device to ensure that the current does not exceed the rated out of step breaking current of the circuit breaker when it is disconnected.

Finally, low-frequency accumulation protection appears, which reflects the cumulative effect of the system frequency reduction on the turbine. The protection is composed of sensitive frequency relays and counters, which are locked by the auxiliary contact of the outlet circuit breaker (i.e., low-frequency accumulation protection also exits operation when the silent diesel generator set exits). The accumulated system frequency is below the frequency setting value of 47.5Hz, and when the accumulated time reaches the set value of 3000 seconds, After a delay of 30 seconds, it acts to send a signal. The device can monitor in real-time during operation: display of fixed value, frequency f, and cumulative time.

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