Earth Fault Protection

[vennivivi]

Hi,

I need to protect a 3,0 MW 11 k V electric motor. The 11 k V supply comes from a nearby power station, where a 11 k V cable is run to my 11 k V distribution panel, and the maximum earth fault current is limited to one amp at the power station. The protection relay I am using is limited to a minimum earth current of 20 amps. My question is : Is it technically possible to protect the motor againgst earth fault ?

Thanks.

Grundig

 

[dpc]

Because the ground fault current is so limited, your motor is already protected against ground faults, to a large degree. If a ground fault occurs within the stator, the nearby power station will see a ground fault. You need to find out what they do when a ground fault is detected - if they trip, then the next problem is how they will determine the location of the fault. If they don't trip, but only alarm, you may want to know when a ground fault has occurred so you can shut down your motor, just in case the fault is in your motor.

With 1 amp of current, motor damage will be very limited, but of course, you will still a coil or winding to fix or replace.

The big drawback to high-resistance grounding is locating the fault.

 

[Yuma]

It sounds a bit strange to me, that the relay cannot see a fault of less than 20 Amps...

 

[davidbeach]

It sounds a bit strange to me, that the relay cannot see a fault of less than 20 Amps...

Sounds like a bad relay/CT selection. Sounds like a 200:5 CT with a relay that has a 0.5A secondary minimum setting. That's not that uncommon, and trying to get there with a residual connection/calculation leaves you trying to distinguish fault current from CT mismatch and other unequal performance. A separate zero-sequence CT (around all three conductors) should provide better results.

 

[slavagar]

Hi.
i'm completely agree with David. In this case must use toroid CT ( zero-seguence CT). Numerical relays have setting
1% from In. 100/1A, 100/5 is good for this application.
From time to time possible use some other solution:
CT is 200/5A ,minimum setting is 2A, please connect secondary
of CT (5A) to 1A input of relay( if you have 5/1A inputs) and your minimum setting now is
0.4A.
We use this solution in networks with petersen coil or ungrounded networks,

 

[dpc]

For high-resistance grounding, fault detection is normally done a voltage relay at the grounding resistor.

With modern relays, a good zero-sequence CT might work for sensing the very small ground fault current, but it will require a very low setting and the relay must be properly filtered for third harmonic currents.

 

[vennivivi]

Hi,

Thanks for the various replies. I am using 200/1 CT and the MPPR 2000 motor protection relay. I have tried to get 30/1 or 50/1 zero sequence CT but to no avail.
Is it possible to get on the market ex-stock a good protection relay and zero sequence CT to do the job.
Thanks for any guidance.

Grundig

 

[RRaghunath]

Are you sure that the system in the power station limits the earth fault current to 1A. It is extremely unlikely as it precludes having much cable lengths.

Better to check, to be sure.

 

[slavagar]

Hi.
First of all, please check with power station:
1. What is protection them use for "your" cable?
2. What is CT them use?
3. What are protection setting: set-point and delay?
4. What is network grounding:
4.1 High resistance
4.2 Ungrounded.
5. Are you have in your 11kV panel VT with open delta connection?
6. What is a lenght of 11kV cable, between your panel and 3MW motor?
Please check again MPR-2000 setting, from my point of view setting are or 5% or 1% In. ( I know two companies what provide relays with same name).
Please pay attention on dpc recommendation :
the relay must be properly filtered for third harmonic currents.
Not all numerical relay have these functionality.
If you have open delta VT connection and will add toroid CT, best solution is directional earth fault protection 67N or 32N.

 

[vennivivi]

Hi,

First of all many thanks for the prompt replies. Unfortunately I am in the far Africa and it is a struggle to get the connection to this wonderful site.
The power station has designed their system for a mimum earth fault current of 10 amps. However, the earth fault protection on their generator is set to 0,5 amp 1 s. They have a 10/1 zero sequence CT on their machine.
They run a 200 metres 11 k V cable to my plant. The minimum earth fault setting that I can set for my 11 k V motor is 0,1 % of the In of my ct which is 200 Amps i.e 2 amps.
My concern is that the 11 k V motor could trip the power station on starting.

Thanks for replies.

Grundig

 

[KVuorio]

If you are not able to get smaller CT then you can just set the time delay of your eart fault relay to smaller value than 1 s for example 0,5s. This is not a good solution, the sensitivity of 200/1A CT you have is propably quite poor and it is very likely that you expereience unselectivity between their generator relay.

For more help try to contact VAMP distributor in South-Africa:
Vamp Solutions (Pty)Ltd
Rodney Bristow Unit 78, Van Dyk Road
Van Dyk Business Park
Boksburg East
1459
Tel: +27 11 914 2393
Fax: +27 11 914 2395
Email: bristowr@mweb.co.za
Email: info@vamp.co.za

http://www.vamp.co.za

Good luck!