Motor Ground Setting 2

[chadwiseman]

Is there any recommendation for setting the ground pick up level for an induction motor. Our current setting is just based on co-ordination with upstream protection. However, outside of that, is there any standard setting to protect different motors from ground faults?

 

[dpc]

Generally for motor ground fault protection, there is no concern about coordination with anything downstream, so the pickup is generally set as low as possible. The problem is usually nuisance tripping on motor starting due to a variety of causes. If this occurs, the ground fault pickup is increased slightly and/or additional time delay is added.

 

[RalphChristie]

The setting would depend on the type of earth fault protection applied (residual connection, balancing core, etc.) and your system earthing arrangement. (solidly earthed, resistance earthed, etc.)

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[chadwiseman]

Thanks. In particular we are using a low resistance ground. It's a 4160V motor with a 6 ohm grounding resistor on the transformer (400A ground fault current). We have different arrangements of CT's (residual and core type). However, we are having nuisance tripping, but I don't want to increase the relay setting without understanding the maximum setting or consequences.

 

[RalphChristie]

First of all, for any real earth fault your earth fault current will be limited to 400A. Thus, 400A is your maximum earth fault current.

For earth fault protection in the residual connection:
Relay must be set above any false residual current that can result from the unequal motor starting currents. (and CT saturation during this time) This is difficult to predetermine - a rule of thump is 10% of motor FLA. A time delay can also be used until the starting offset has decayed, but this delays tripping for actual faults.

For earth fault protection with core-balance protection scheme:
This setting can be very low (high sensitivity) and settings of 1%-2% FLA (and sometimes even lower) can be obtained. You can use a definite time delay, normally 1-5 seconds are sufficient, but it can be increased if nuicance trips are experienced.

Read also:

http://www.areva-td.com/static/html...typid=1107432285131&productline=1079943326752

Download Chapter 19 (AC motor protection)






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[dpc]

I agree with Ralph's comments, but the 1-5 second time delay seems long to me. I generally start with delays of a few cycles and increase if necessary. I would expect upstream relays to operate with one second for a solid ground fault at the full 400A.

 

[chadwiseman]

Thanks for the advise. However, if I have a 500 hp 4160V motor, that would mean I wouldn't set the pickup above about 5 amps, and possibly less than one amp. We are struggling with a pickup of 10A. Any comments are appreciated.

 

[RalphChristie]

dpc:

Probably you are correct, but just in my opinion:

The reasons I'm stating an 1-5 second time delay on a core balance protection scheme are:
The earth-fault current is reduced so I assume you can have it longer on your system.
Normally we use a core balance scheme together with a residual scheme. The core balance scheme looks for the smaller earth faults that can be longer on the system while the residual scheme looks for bigger faults and normally act faster on big faults. If you have just the core balance scheme as earth protection I would also start with a much shorter time.
Depending on the start-up time of the motor we experienced nuicance trips on too short time delays with a core balance scheme.


86ranger:

On which protection scheme do you experienced the trips?
What is your CT-ratio?
What are your settings? (Pick-up, time delay and curve selection)
What are your motor start-up time and your motor starting method?

Many times the setting of a core balance sceme is a trial-and-error method.




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[chadwiseman]

My CT ratio is 50:5. Core type setup.
The GF setting is 20A, no time delay (50G).

Thanks.

 

[RalphChristie]

You do not provide much info.

The cause of your tripping is the instantaneous time setting. This is too short. Reduce your pick-up setting and increase your time setting.





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[dpc]

Yes, I would agree - put in some time delay and see if the problem goes away.

 

[rcw retired EE]

We have had nuisance trips on motors with surge arrestors or capacitors. The capacitor charging currents on starting were seen as a fault. Longer time delays cured the problem.

Cable shield ground wires not running back through the zero sequence CT caused false trips on other units. Cable terminations were on the line side of the CT so the CT saw the cable shield currents. Running the ground wires back through cancelled out shield currents.

If the cables are not centered in the CT, there can be nuisance trips on motor inrush due to asymmetrical saturation of the CT.

We have gotten by with 5 A pickups and 50 mSec time delays on similar installations to yours.