3.3kv Motor Earth Fault Setting

[UKEng]

Hi All,

I am starting to commission a new chemical plant and have come across the following.

Motor Volts - 3300v
Motor FLA - 106A
Motor Protection - Gemstart 4

Normally I would accept the Earth Fault Setting from betwwen 10% - 20% of FLA, in this case approximately 10 - 20A.
We primary injected the Starter and the relay trips at 1A which equates to a 1% setting. On checking the documentation it was designed to this figure. We are able to increase the setting to 1.6A but no further without changing components. Has anyone got experience of settings this low and should I expect any nuisance tripping. The sensing device is a Core Balance CT and not 3 CT's residually connected. My intention is to increase the setting to 10% by changing the burden resistor on the CBCT, however please could you share experiences or your own standards on Earth fault setting on motors.

regards

UKEng

 

[Kiribanda]

Hi UKEng,

From your posting I may raise two questions to clear some doubts.

1)Is your 3300 V system a High resistance grounded one or Solid Grounding?

2)If the C/T s are not residually connected, but connected in Core Balance(Zero seq) mode, how did you check it’s setting by a primary injection to the 3.3 kV starter ?

Regards!

Kiribanda

 

[UKEng]

Kiribanda,

The 3.3kv system is resistance earthed which limits the earth fault current to approx 400A.

We used a single phase injection set and passed the test lead through the centre of core balance CT and raised the current until the relay tripped.

The earth fault has a time delay also. The contactor has high voltage fuses fitted.

regards

UKEng

 

[jbartos]

Suggestion: Please, clarify who originated those components that you intend to upsize to avoid any nuisance trips. The 3.3kV system has a medium resistance system grounding that appears to be designed about right, (400A).
Question: How many motors are simultaneously "ON" on the 3.3kV power supply?

 

[Kiribanda]

Hi UKEng,

Thanks for the reply. (Since you have written that “we primary injected to the starter and the relay trips ------“ I wanted to make it clear). Your method is ok and also it is the standard.

From your reply you say that your 3.3 kV system is low resistance grounded with an allowable earth fault current of 400 A. Due to that in my opinion –also by my experience – I think 1% is too low and might give nuisance tripping. 1% seems to be ok for a high resistance grounding system where the allowable earth fault current is in the range of 1-5 Amps.

Therefore the easiest method is to change the c/t ratio. I cannot see any other method to change the setting by 10 times.

Regards!

Kiribanda

 

[UKEng]

jbartos,

After further ivestigation, please find the following.


The original design was done in Japan and approved in Japan. Apparently this is the standard earth fault setting over there. Alstom built and fitted these components as per design. I rang Alstom UK and their recommended setting is also between 10% - 20%. They recommend the changing of the resistor if we want to increase level of tripping.

The core balance CT drives current through a burden resistor in the secondary, the voltage across that resistor is the signal sent into the motor prtection relay.

The plant is not yet up and running but there will be apporximately 10 - 13 motors @ 3.3kv simultaneously on.

The plant was designed/approved in Japan but constructed in the UK.
As mentioned before, I have never seen earth fault setting set so low for motor circuits and was trying to get information on what other Engineers around the world do.

Many thanks for your replies.

UKEng

 

[sabap]

Hi UKeng,

At our plant which was designed by US Company, the earth fault setting is 1% of FLA. Our system is 6.6 KV, and earth fault current limited to 400 A through 9.95 ohm resistor connected in the neutral of aux transformer. We are using core balance CT for earth fault detection. GFCT is 50/5

We also had the same discussion on earth fault setting during last month when we changed our motor protection relay to 369 Multilin of GE. The available settings in the relay did not allow us to choose 2 A GF current setting with the CT ratio of 10. Motors FLA was 183 A. Min possible setting on their relay was 5 A in our case.

We consulted GE support section for this problem and they told us to either change the CT with other ratio or connect the existing CTs secondary on the 1 A input terminal of the relay. The thing to look for in this solution is the max rating of the relays terminal and available GF current on secondary.

I just wanted to tell u guys my experience.

I will be curious to know what the IEEE buff book or MG-1998 says on this. I dont have a copy of these standards.

Thanks,

SABAP.

 

[dpc]

A motor ground fault relay does not have to coordinate with anything downstream. So the lower the setting, the better, provided that nuisance tripping does not become a problem.

With a core balance CT, the 1% setting should not be a problem, provided that the conductors are properly installed throught the CT and the CT and relay are properly matched.

The ground fault element should also include a short time delay.

If there is to be nuisance tripping, it will likely occur when the motor is started. If it does occur, simply increase the pickup setting slightly.