CT Ratio for Generator 51V Protection

[devilskm]

Emergency Diesel Generator 1000kVA, 0.38kV, FLA 1519Amps, CT Ratio 2000/5.
As per IEEE C37.102, the pick-up for 51V shall be 150% of the generator FLA. In my case, the 150% of 1519 is 2279Amperes. Now this value is much higher the the CT installed. When I do the setting, I cannot set the pick-up more then 130%.
1. Is it OK to set the pick up at 130% for 51V-R function.
2. My Relay curve for 51V-R is intersecting the Generator Field force curve (although its above the knee of the decrement curve). Is it OK that it is intersection or my Relay curve shall fall to the right of the field force limit.

Thanks

 

[dpc]

The CT ratio is based on continuous current, not fault current. There is no problem with that.

The 51V is intended to be a backup for phase faults. Where does the 130% limitation come from? Normally these relays are set between 150% and 200% of full load amps.

You have to adjust the relay curve based on the voltage during the fault. That is what makes the 51V such a PITA to set. The relay curve at 100% is not very useful.

David Castor

http://www.cvoes.com

 

[devilskm]

You mean that when I set the relay curve, I shall reduce the system nominal voltage for the curve less then the normal operating voltage?

 

[devilskm]

I cannot set my relay at 150 to 200% of FLA because the max. set point for my relay is 5, which corresponds to 2000A. So I cannot go beyond that. What should I do?

 

[davidbeach]

If the maximum setting is 5A on a relay intended for use with 5A secondary CTs you have either not read the manual correctly or you need to get a different relay. I've never seen such a thing and would never accept it if proposed.

 

[devilskm]

Please find the below link

http://cid-0a0494be47f56008.photos.live.com/browse.aspx/Public

There are two files, one which shows that the relay range limit is 5A.

The second file is for Generator Curve. Please tell me that when we talk about setting the pick up at 150% -200%, is it after applying the Voltage Restrain value or without applying the voltage restrain.

e.g in this case if I set the pickup at 150%, it will be 2278.5A (for FLA of 1519). so my pick-up is without applying the voltage restrain. Lets say this is option-1.

For option-2, I use Voltage restrain as 0.75 and OC setting as 150%, which will result as (FLA x 1.5 x 0.75), so the pickup will be 1709A.

So on which option 51V pick-up workds, Option-1 or 2?

 

[ScottyUK]

More likely that is a plug setting, not a CT secondary current parameter, in which case the available adjustment equates to 10% - 500%. That's quite a generous range of adjustment, especially compared to the old E-M relays.

dpc is right, 51V relays are awkward things to set. If you aren't confident in what you are doing then get some experienced help until your knowledge grows - badly set protection tends to result in plant burning down and people getting killed. A high price for misplaced pride and unwillingness to ask for assistance.


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