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HV Motor Cable Earthing

HV Motor Cable Earthing

HV Motor Cable Earthing

(OP)
Hi

I would like to invite comments as to the preferred method of earthing the screen of an HV motor XLPE cable please. Motor windings are typically star connected and unearthed.

I presume it is best to earth one side only as earthing both ends could lead to induced currents in the screen adding to cable heating.

I have seen applications where the cable was earthed at both the source and motor ends but which is better?

In particular I have a 3.3kV application where there is a core balance CT at the source end. I'm inclined to go with earthing at the source end as earthing at the motor end only could lead to cable EF currents traveling along the screen and down the motor casing to earth, thus raising the motor frame potential above earth which implies a safety hazard.

Of course, the screen is taken back through the eye of the core balance CT and earthed on the load side of the CT.

Comments?

Thanks.  

RE: HV Motor Cable Earthing

For medium voltage power cable I would expect it grounded at stress cones on both ends.

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RE: HV Motor Cable Earthing

For shorter cables, we float the shields at the motor and ground them at the source to avoid circulating currents.  There are a lot of discussions on this topic here, try a search.

RE: HV Motor Cable Earthing

Here's another recent related thread: thread238-261957: Stress Cones on 5kV cables
 

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

RE: HV Motor Cable Earthing

(OP)
electricpete - if grounded both ends the core balance EF protection cannot work properly as the relay will only see a fraction of the total EF current.

rcwilson - what do you do for longer cables? leave one end unearthed and protected with a surge arrester? what voltage levels are you talking about? Also, do you apply core balance protection at all?

I am interested to hear any comments about leaving the motor end unearthed as cable fault current flowing through the screen to earth at the motor end could result in a touch potential hazard at the motor frame.

Thanks in advance.

RE: HV Motor Cable Earthing

Look at the Okonite or other cable maunfacturer's website for details on how to calculate induced voltage on the shield (screen).  The shield voltage is approximately proportional to length of cable and varies with cable size, type and spacing. Okonite uses 25 volts as a conservative maximum shield voltage. This limits a motor run to about 3,000 -4,000 feet per Okonites tables. Our "long" motor runs are about 700 feet (200 meters), well within the suggested length.

The cable is terminated at the motor with a stress relief termination (stress cone) and the ground leads for the stress cone and shields are insulated and taped back on the cable. They are not left free to short out.

Zero sequence CT's are used for ground fault protection at the switchgear end.  Shield wires are brought back through the ZCT for proper sensing of all fault currents.  If a cable fault occurs, phase-to-ground voltage is impressed on the shield (high-resistance grounded systems) momentarily until the GF relay trips the breaker, but the shield end is isolated.

A cable fault somehow impressing system voltge on the motor frame is no different than a motor winding fault impressing the same voltage on the frame. If you believe you have a hazard from a cable fault, then you also have a hazard from the motor winding.   

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