ground current pick-up - CBCT for 3 phase motor

[alexcao]

Hello - at our power plant, we have motor starters that use a CBCT (core balance current transformer) to measure zero sequence current on the motor phases. It's one of those window-type CTs with all three phase of the AC motor passing through the center. The CT ratio is 50:0.025 and it has a ge multilin relay connected to it. I am a bit clueless as to how to set the primary current pickup level as the relay setting can range from 0.5 to 15 A. I've looked through the Blackburn book for protective relaying and in the IEEE 242 buff book and both call out a primary sensitivity of 5-10 A, but nothing I've found has given me a good grasp of how to really calculate or determine a good threshold for ground current. Should the ground current really be that high? I thought its typically in the sub-amp level?

Was wondering if anyone could help me with the following questions I have?

1. Does the ground current pickup for a sensitive ground CT (in this case CBCT) depend on the motor size?

2. How do you calculate or determine accurately a good ground current pickup level for a motor with a 50:0.025 CT to prevent from nuisance tripping but still adequately protecting the motor from a ground fault?

Any help is appreciated

 

[alexcao]

anyone have any thoughts about this? really looking for some help in determining a setting for this relay. what do i need to consider???

 

[waross]

RTFM
There are three main sources of ground current.
1> Resistive leakage. This depends on the resistance to ground through the cable insulation and the motor insulation. This may be ignored.
2> Capacitive charging currents. These are caused by the capacitance of the motor windings and the motor circuit conductors. The cable capacitance depends on the cable insulation composition and thickness, the conductor orientation the proximity of the conductors to ground planes or grounded shields. This varies with the cable length.
3> Harmonic currents.

4> Ground faults in the motor or the motor circuit conductors. This may be several orders of magnitude greater than normal ground currents.
The best way to determine the normal ground leakage is to measure it. Use the existing CT if possible.
Choose a safety factor of several times normal ground leakage for you setting. A large motor may have more capacitive current than a small one.
A long cable will have more ground leakage than a short one.
I would suggest using this criteria to determine the motor circuit with the greatest leakage and select a setting. Then use this setting plant wide.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

1. No, not really. Any current detected is theoretically fault current, regardless of motor size.

2. Consult the relay instruction manual. You will likely want a short time delay to deal with asymmetrical inrush current and the small amount of error that will occur during starting.

5 A is a reasonable setting in my experience. The key is being able to get the motor started without nuisance tripping.

David Castor

http://www.cvoes.com

 

[mayanees]

I'd go with 20% of the phase pickup.
John M

 

[vandal06]

How is the system grounded?

 

[vandal06]

Sorry for the double post. Assuming there are ground fault sources you can set it as low as dpc said, which is generally how I set motor ground protection.

 

[alexcao]

@dpc - i believe a core balance transformer really only looks at zero-sequence currents (ground current). i remember reading somewhere that a CBCT would not detect asymmetrical/unbalance phase currents. so virtually if you have no connection or leakage to ground, you shouldn't see anything. so my assumption from this is that any imbalance wouldn't be detected by the ground CT using flux summation. Let me know if you disagree.

@mayanees - wouldn't that mean that the size would depend on the motor size? see dpc's answer to 1.). from my understanding motor size should not matter. J Lewis Blackburn says this in his book Protective Relaying: Principles and Applications

@vandal06 - system is solidly grounded. i hear its best to have a high resistive grounding when using these flux summation CTs but should still work with solidly grounded systems. i currently agree with dpc as well in terms of setting but looking for other opinions.

 

[RRaghunath]

alexcao,
With CBCTs, we can have much more sensitive pickup setting than that with residual current input. Typically, 5-15% of motor full load current is OK and no time delay is warranted.
If the system earthing (low resistance earthing, high resistance earthing etc.) allows sufficient fault current, you can have same pickup settings for all the motors (irrespective of the individual motor rating). The only required check is that the pickup shall not be over 10% of the available earth fault current in the system.
To my knowledge, 50/0.025A CBCT seems to be special purpose CT and not commonly specified. Looking at the CBCT ratio indicated by you - 50/0.025A, it appears that the CBCT and the relay might have been tested together with the setting recommended by relay manufacturer!

 

[dpc]

alexcao,

Asymmetrical current contains a dc component. Dc current will wreak havoc on the response of any CT.

Also, no CT is perfect. With inrush currents as high as 13x full load amps, it should not be surprising that even small flux leakages could create enough secondary current to trip the relay. It happens fairly frequently.

You can do what you want. From experience, I have learned that these flux summation CTs can cause nuisance trip during motor starting if cranked down to a very sensitive setting with no time delay.

You can always try calling Multilin tech support

David Castor

http://www.cvoes.com