HV Motor star point earthing

[THD]

Is the star point of an HV induction motor always earthed? Where is the star point normally brought out to earth? It is here that I need to mount a neutral CT for a motor differential protection scheme. The three line CTs need to be mounted as close as possible to the motor terminations to the studs (so that ONLY the motor is protected by the diff scheme)

Kevin Bosch
Rainbow Technologies

 

[ScottyUK]

The star point is never normally earthed. A true diff scheme usually uses a pair of CTs per phase, one on the line end and at the neutral end, so you have six CTs per motor.

The neutral of the source is usually earthed at the transformer, sometimes solidly, sometimes via a resistance or reactance.


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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

In addition to what Scotty said, if you can get both ends of the winding through the same CT- opposite directions - you can do motor differential with just 3 CTs and get better performance than with 6 CTs.

 

[electricpete]

Often you have access to all 6 leads at the term box (but may or may not have room for CT's there).

For a very few motors, the motor neutral is connected internally (in the endwindings at the connection end)... there are only 3 leads in the terminal box, and you cannot use a differential protection (also can't do insulation resistance test between phases). We have on family of 8000hp motors like that ...don't know why the OEM chose that... maybe he saved a few bucks and apparently we didn't specify otherwise.

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