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Floating or grounded neutrals on motors and supplies 1

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Younsi

Electrical
Mar 12, 2010
13
Hello
I am interested in learning how industrial motors (1000HP to 50,000 HP MV to HV)and their supplies are connected. More specifically I would like to know if the neutral of the motors and the newtral of the supply side are grounded. Also how does the neutral connection/grounding approaches differ from Asia to Europe and North/South America ? Your help and opinion is much appreciated
Best regards
Karim
 
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To summarize: If we wanted to improve sensitivity of differential protection there should be some gains possible if we had more accurate CTs. But not unlimited as you mention, since there are other errors (like capacitive current) to consider.

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(2B)+(2B)' ?
 
Dear Bill and Pete,
Many thanks for coming back to this topic and thread again and take time to help. I did obtain a lot of information from our field, service, installation and marketing guys on this. There is no standard way of connecting these motors even though this may sound very risky in some situations. Grounding of MV to HV neutral supplies is not always there (suprising). The more we learn about this the more surprized we are at how diverse approaches and practices are.
Thanks Pete for voting for me on the IEEE DEIS run. I am a rotating machine guy and I agree, focus is on other topics even if WE produce the MVAs ! All assests are important though of course.
The discussion on the differential CT sensitivity, capacitive and resitive insulation current values is the heart of the thread. I would just say that if we have a diff CT precise enough to give C and DF online then we would have IEEE286 ONLINE possible. A big deal I think.
Cheers
Karim
 
Interesting. So the small primarily capacitive leakage current that Bill mentioned is what you would intend to measure accurately and compare its angle to the voltage to ground. Sounds challenging to achieve that level of magnitude and phase accuracy, but I agree it would be a big reward.

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(2B)+(2B)' ?
 
The capacitive currents in measuring C & DF are extremely small, typically in milliamps range. That's the reason expensive test kits like as ac schedring bridges are used. I don't see how CT's can come anywhere close to those measurement levels.

Muthu
 
It may be worth looking into Ground Fault Interruption devices.
A typical GFI receptacle in North America will be rated at 15 or 20 Amps but will trip at 15 milli-amps. There is your 1000:1 ratio.
I don't suggest using a GFI but the measuring methods may be scalable.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
There is also the matter of discriminating the phase of the ground current (differential current). 1 degree error would be a huge error in a power factor measurement.

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(2B)+(2B)' ?
 
Just thinking out loud, I'm guessing gfci probably sends the main current directly through an operating and restraint coil (without any transformation). Would be a challenge with higher voltages and currents since the amount of copper and insulation in that coil would be large. But you never know.

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(2B)+(2B)' ?
 
Actually I have no idea how a gfci works but I doubt it has 2 coils as I suggested, so I withdraw my comment about gfci. Sorry.

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(2B)+(2B)' ?
 
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