Generator Protection Part 9
As seen uptill now, no protection scheme is in a position to give complete protection to the stator of generator against earth faults. The maximum protection achieved is upto 85 to 90% from the schemes discussed uptill now.
But in modern days it possible to provide 100% earth fault protection to the stator of the generator. It uses a coupling transformer and the coded single current. The scheme is shown in the Fig. 1.
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| Fig. 1 100% Earth Fault Protection |
A coupling transformer is connected between the earth and the earthing resistance R i.e. in neutral to ground circuit. The primary of the coupling transformer is excited by coded signal current source. This coded signal current has a frequency of 12.5 Hz. This current is continuously injected into the generator stator winding through the secondary of the coupling transformer.
During the normal condition the signal current injected into the stator flows through stray capacitance of the generator and directly connected system. But when earth fault occurs, the stray capacitance is bypassed. This increases the monitoring current. This increase is measured by a measuring device. Depending upon this measurement an immediate corrective action is taken.
This schemes gives the protection of 15 to 20% of stator winding from the neutral side, the portion which is unprotected by Merz-Price protection. The remaining portion is protected by Merz-Price protection. Overall 100% of stator winding gets protected against earth faults.
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