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Anonymous Poster #1

Calculating the Stabilizing Resistance Value

11/09/2013 6:39 AM

We have a number of 11 KV /433 V, Delta/ Star transformers of 2000 KVA rating. The Primary side CT is 120/5, 5P10. The relays provided are Schneider P142 make over current and earth fault feeder protection relays on Primary side. The earth fault on the delta side is measured by residual connection of the CT secondaries. Here the panel designer had provided a Stabilising resistor in the CT circuit to avoid spurious trippings. However we observe that the transfomers trip during motor starting (big motors) or during energisation on earth fault protection. Now how can i find out whether the stabilising resistance is sufficient or needs changing. Kindly provide me details of how to calculate the value of the stabilising resistance in this case. Many of the sites offer the means of calculating for REF or motor earthfault protection, but no details could be found for the case mentioned above. Request to kindly provide the details
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electricalexpert65
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#1

Re: Calculating the Stabilising Resistance value

11/09/2013 8:54 AM

Please visit ABB website. They have published an application note on "stabilising resistor in motor earth fault protection" in which a formula is given to calculate the Rstab value with respect to the motor starting current.

In case, you are unable to locate the same, kindly mail me your mail ID thro the forum's mailbox so that I can mail you the same.
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7anoter4
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#4
In reply to #1

Re: Calculating the Stabilising Resistance value

11/10/2013 5:11 AM

I agree.See:

http://www05.abb.com/global/scot/scot229.nsf/veritydisplay/1f4988c88d48a7d3c2256feb002dca41/$file/applicationstabil_resistor%20in%20motor%20efprotena.pdf

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Zvi
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Join Date: Jun 2012
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#5
In reply to #1

Re: Calculating the Stabilising Resistance value

11/10/2013 5:15 AM

Good source! Here's the link:

http://www05.abb.com/global/scot/scot229.nsf/veritydisplay/1f4988c88d48a7d3c2256feb002dca41/$file/applicationstabil_resistor%20in%20motor%20efprotena.pdf
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ramvinod
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Location: chennai,India
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#8
In reply to #1

Re: Calculating the Stabilising Resistance value

11/14/2013 10:51 AM

Dear sir

I have a doubt on this. The request was how to prevent spurious tripping of REF relay provided for the transformer, when the motor starts.The notes on ABB gives the calculations for the stabilizing resistance for the relay to be connected with the motor circuit.

Does this solve if so how?

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lyn
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Join Date: Oct 2008
Posts: 42376
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#2

Re: Calculating the Stabilising Resistance value

11/09/2013 9:15 AM

Request you to kindly do your own work.

[Protection Relay] MiCOM P141 / P142 / P143 - Documents ...

mt.schneider-electric.be
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Zvi
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#3
In reply to #2

Re: Calculating the Stabilising Resistance value

11/10/2013 4:03 AM

"Kindly provide me details of how to calculate the value of the stabilising resistance in this case. Many of the sites offer the means of calculating for REF or motor earthfault protection, but no details could be found for the case mentioned above."

Your reply didn't offer him any solution to his problem. It confirmed that manufacturer's (Scneider's) documentation was lacking as he indicated.

It's no trick to come up with fat, fancy links. But you need to study them at least a bit to realize they're of no use.
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PWSlack
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United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

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#6
In reply to #3

Re: Calculating the Stabilising Resistance value

11/12/2013 3:00 AM

Please explain why Anonymous Poster #1 would be unable to follow the links in order to pick up the phone and talk to someone at the other end?

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Zvi
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#7
In reply to #6

Re: Calculating the Stabilising Resistance value

11/14/2013 9:53 AM

Anonymous Poster #1 sought "...details of how to calculate the value of the stabilising resistance in this case." --- The case being spurious tripping of his MiCOM feeder management relays when starting big motors.

The MiCOM relays were provided by Schneider Electric and the stabilizing resistors were selected by whoever designed the panels.

Evidently he's familiar with the calculations given in the MiCOM Technical Manual for selecting resistor values. However they don't include compensation for his case. The Manual only suggests purchasing an optional variable resistor. That means solving his problem by (1) trial and error and (2) only after making modifications to his panels. Let's not forget, this guy is playing with a bunch of 11 KV / 433 V, 2000 KVA transformers!

Yeah, he could call MiCOM or Schneider or whoever it was that designed the panels and didn't get it right the first time. Could waste a lot of time and get nowhere.

But, isn't he better off coming up with the proper values for these resistors and then just swapping them out? So, he posts a request on CR4. And low and behold: within a few hours he has a reply with the link he was searching for. Chalk one up for CR4!!! Plus, he saved himself a nickel for the phone call.

BTW, why doesn't electricalexpert65 have his GA yet?
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Users who posted comments:

7anoter4 (1); electricalexpert65 (1); lyn (1); PWSlack (1); ramvinod (1); Zvi (3)

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