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minu
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Join Date: Oct 2008
Location: baroda, india
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Overcurrent Coordination Between TRF Primary and Secondary Relays

12/09/2013 7:23 AM

This is an odd question maybe. But i had a heated argument with my boss on the same.

Here goes:

There is a transformer with protective relays on its primary and secondary ends. The CTI (coordination time interval) selected for this project is 350 ms (client requirement). I coordinated the system from the load end to the transformer secondary using this CTI.

Now comes the coordination of xfr primary relay with the secondary relay, which i did as follows:

1. Calculated the secondary fault current that is reflected on the primary side of the xfr.

2. calculated the relay settings for the primary relay using reflected fault current and keeping the time interval as around 150 ms since as per my understanding we do not need to maintain the CTI of 350 ms in this case since our primary requirement here is to ensure that the primary relay does not mal-operate for any fault on the secondary side.

Am i correct here?

Thanks in advance.

PS: I apologise for the lengthy question
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LongintheTooth
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#1

Re: Overcurrent Coordination Between TRF Primary and Secondary Relays

12/09/2013 10:03 AM

What is the OTHER side of the argument ?

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minu
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#2
In reply to #1

Re: Overcurrent Coordination Between TRF Primary and Secondary Relays

12/09/2013 11:22 PM

other side of the argument is that we maintain the CTI of 350ms as it is.
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SHOCKHISCAN
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#3
In reply to #2

Re: Overcurrent Coordination Between TRF Primary and Secondary Relays

12/10/2013 9:02 AM

If your transformer is not oversized and it is design loaded for maximum efficiency at 100-125% :

If the transformer primary protection CTI is set at 150 ms and the secondary (load) side CTI is set at 350 ms, the primary will trip well brfore the secondary thereby defeating the secondary protection scheme.

Power in = power out

All fault current generated in the secondary will also be reflected in the primary with a magnitude value proportional to transformer ratio and on/at the same time interval.

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4wsilver
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#4

Re: Overcurrent Coordination Between TRF Primary and Secondary Relays

12/10/2013 9:07 AM

I am not a coordination engineer, (I had someone to hand that to) but your contention sounds wrong, and so does you bosses. What you want to have happen is for the secondary relay to operate faster than the primary relay, since it is downstream. If a fault occurs on the secondary the relay should take out the secondary loads first before the primary relay operates. That way the transformer is protected from the full effect of a thru fault. The transformer impeadance will reduce the fault but the time will be equal. The way we worked, the time value was for all faults to be cleared and that usually included the clearing time of breakers operating, not just the relays. The time given usually does not mean that the relays are all set for 350 ms. It usually means that the total time, coordinated properly for all items to clear, is 350 ms. The question and both you and your bosses positions sound to me like you do not really understand what a coordination study is all about or the goals you want to reach. I think your organization should turn the project over to someone who understands the concepts and has the capability. I would say to ask questions of the customer, but then he would quickly know that you are lacking, although he may also be, since he chose you. Sorry to be so blunt.
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Wal
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#5
In reply to #4

Re: Overcurrent Coordination Between TRF Primary and Secondary Relays

12/11/2013 12:11 AM

....not a coordination engineer, (I had someone to hand that to)

OP and his boss may not have that luxury.

At least they are recognising that a scientifically based decision needs to be made and are discussing the options. Heated is good. Makes you think and reflect on doubts.

Asking is good too.

Even though you admit you aren't a coordination engineer (first time I've heard of this specialisation) you seem to have a good grasp of what's required and could most likely make the right, soundly based decisions if you were put on the spot.

There is always a first time for everything we do.

GA by the way. (a bit harsh at the end...)

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ramvinod
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#6

Re: Overcurrent Coordination Between TRF Primary and Secondary Relays

12/11/2013 10:44 PM

I have gone thru all the suggestions. Many have not understood the CTI.It is only time interval. Both 150 ms and 350 ms can be considered. Since you have been maintaining 350 ms for this project, you can maintain same 350 ms for direct fault on secondary side of the transformer. However I hope there will be instantaneous setting on the primary side relay.

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4wsilver
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#7
In reply to #6

Re: Overcurrent Coordination Between TRF Primary and Secondary Relays

12/11/2013 11:06 PM

You cannot maintain 350 ms on both primary and secondary and still be by definition coordinated. Cordination means the the various components will clear in the order intended to promote safety of humans, equipment, and power supply in that order. In the end, it should take down the minimum amount of service. That usually means taking the devices out closest to the loads first. If the fault is further up the line, then of course that will not happen, but the first relay closest to the fault should operate.
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minu
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#8
In reply to #6

Re: Overcurrent Coordination Between TRF Primary and Secondary Relays

12/11/2013 11:10 PM

Mr. Ramvinod is correct. CTI is the time interval and not the operating time of the relay. My primary relay wil operate after the secondary relay only as a back-up.

Maybe i didnt put up my question clearly enough. I would try it with a sketch or something if someone can guide me how to upload sketches/drawings here!
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1capybara
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#9

Re: Overcurrent Coordination Between TRF Primary and Secondary Relays

12/11/2013 11:42 PM

Minu, one of the answers above is already rated GA good answer. Generally 2ndary should trip before primary. Not the other way around.

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minu
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Location: baroda, india
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#10
In reply to #9

Re: Overcurrent Coordination Between TRF Primary and Secondary Relays

12/12/2013 3:31 AM

I know that!! Read my question clearly please. Nowhere is it mentioned in my question that primary relay operates before secondary!
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Wal
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#11
In reply to #10

Re: Overcurrent Coordination Between TRF Primary and Secondary Relays

12/12/2013 4:44 AM

Settle down.

What we have here is a failure to communicate is all.

What do you want then?

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1capybara (1); 4wsilver (2); LongintheTooth (1); minu (3); ramvinod (1); SHOCKHISCAN (1); Wal (2)

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