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How many grounding brushes should there be in the generator?

          Once in a, 35,000 air-cooled unit project found that the generator side has a grounding brush, but in its incentive side of the end of a two-shaft carbon brush, two-axis carbon brush leads were received a "RC" Small box of words, one of the terminals directly grounded, I do not understand the usefulness of the grounding, because this grounding is not to encourage the side ground, you do not have a shaft current, the other is the carbon brush for Rotor grounding protection used, so grounding can not cause protection malfunction? Please enlighten.

          The general group is a large shaft grounding brush, used to eliminate the shaft voltage. The other group is connected to the shaft brush, to the excitation system or transformer group protection , the connection shaft brush is set for the excitation circuit insulation monitoring.

          As for the connection of the two sets of carbon brush shaft current, I think so, according to different conditions of the unit, I observed that some units of the large shaft grounding brush is grounded, while the shaft brush is not grounded , So there is only one grounding point equivalent to the big shaft, and no shaft current loop will be formed. Of course, some of the units are indeed connected to the shaft brush is also grounded, but according to the shaft voltage shaft current hazards we can see that it is mainly damaged pad oil film insulation resulting in the final deterioration of oil-burning, and even if the shaft itself appears unequal potential , Two groups of grounding brush shaft current is also formed by the brush itself, the circuit will not be formed by the bearing film film circuit, the actual equivalent of the grounding brush is the short axis and the short axis, so at this time and a short Contacts, but not because of the shaft current burning tile.

          There rotor protection problems, if the measured rotor insulation on the ground need to retreat rotor protection, then naturally understand this truth. Connecting the carbon brush to the rotor ground protection provides only a ground reference point, as long as the big axis go to ground, it is impossible to cause the protection of misoperation. For example, the three-phase four-wire neutral grounding system, the three-phase line connected to the ground then trip, and if the zero line is grounded? Certainly not directly trip, but in order to ensure the neutral point to reduce the grounding resistance and to avoid the risk of zero line disconnection, in fact, we also advocate zero line repeat ground. I give this example just to show that the grounding wire to brush the ground protection to the line itself is equivalent to a ground for rotor ground protection reference, it is grounded again, and no significant impact, in turn, the large shaft grounding carbon Brush grounding, connecting shaft brush is also grounded, is the reference point repeated several times, naturally much more reliable. In other words, no matter how the brush connected, under normal circumstances is not caused by protection of misconduct, and only in poor brush contact, resulting in damage to the rotor capacitance or mutation when the rotor may also cause ground fault protection A similar example has appeared in individual power plants.

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