Generator protection - 5A CTs 3

[RRaghunath]

There is a case (bidding stage) of 140MW generator with 8500/5A ratio CTs and protective relays to be located about 200meters away.

This is an outdoor located gas turbine generator in a combined cycle gas power plant. The generator relay panels are located in electrical / control building that is about 200meters away.

I know the 5A CTs and the relays located in the same building (in case of steam based plants and gas based indoor plants).

Has any one come across a similar situation??

With a primary current of 8500A, is it a practical solution to go in for CTs with 1A secondaries (size issue). The CTs are located in isolated phase busducts (usual practice for this magnitude of currents).

Thanks in anticipation.

 

[GTstartup]

This CT ratio for this size generator is OK (about 4A on the secondary at rated current). Most protective relays nominal current are 5 A. I am not sure what your concern is but seems ok to me. If you are worried about the 200M distance from CT to relay, it seems not unsual to me. However the burden should be calculated of course, but at the bidding stage, I would accept what they propose.

To answer your question more directly,

Yes I have come across a simlar situation (100's of times).
and..
No. - 5 amp secondaries are standard for this application. Also, since you mention size limitations, I would assume that 8500A to 1 A CT's would be physically bigger.

 

[RRaghunath]

Thanks GTstart up for informative reply.

 

[electricpete]

If I understaood correct -

The first post talked about CT's and relays being 200m from the machine. (I would think the issues would be a larger zone. And the ability to provide differential and ground protection... is the neutral accessible).

The 2nd post talked about CT's being 200m from the relays. (I would think the possible issue would be burden).

These are two different situations. Am I missing something?

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[scottf]

Strictly from a CT size perspective, the size will not change appreciably between a 1A and a 5A rated secondary. The 1A secondary has 5 times the turns of course, but smaller diameter wire is used and they normally cancel each other out.

 

[bacon4life]

A quick search on google gave me several CTs with options of 1A or 5A in the same dimension housing. However someone recently told me Europe uses 1A CTs because they can have a smaller core due to the lower VA output, and yet still avoid saturation. What are the benefits/ disadvantages of 1A CTs?

 

[ScottyUK]

The primary benefit for an application where the CTs are fairly remote from the relay is that the IR drop in the connecting cables is much lower, allowing lighter gauge cable to be used for a given volt-drop, or alternatively allowing the I²R loss in the cable, which has to be met by the CT, to be reduced. This might account for the reduced burden on the CT allowing a smaller core to be used.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[scottf]

The only savings for a 1A core that I can think of is possibly reduced cabling costs from the CT to the relay/meter. There is no real relaying advantage, as all of the parameters of current, resistance, VA equal out in the 5A vs. 1A question.

Note that the VA burden is less in a 1A circuit, but the internal CT resistance is also much higher (more turns of smaller diameter wire).

Possibly there could be some savings on the relay itself in that it has to handle smaller currents in a 1A circuit, but I doubt it.

 

[electricpete]

Am I the only one who is confused by the question I raised in my post above?

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[RRaghunath]

electricpete,

I am concerned about the higher VA of the CT that would be necessitated by the longer lead burden (with relays / meters located far away, over 200mtrs, from the CT) and possibly larger dimensions for the CT. The generator has neutral side CTs as well (neutral is formed outsidethe machine as is the practice with large generators).

Bacon4life,

You mentioned about search for 1A / 5A CTs with specific reference to dimensions and your observation that the dimensions are same for a given primary current is really interesting. Could you please inform whether the observation can be applied to 8500/1A (5A) CTs also!! Thanks in anticipation.

 

[scottf]

rraghunath-

As I mentioned above, there is typically no difference in physical size between a 1A and 5A rated secondary, regardless of primary current rating.

 

[bacon4life]

A C800 relaying CT with a 1A secondary would generate 4000V at the modified burden. Would 4000V cable be required or any special work practices?

 

[rcw retired EE]

1A secondary CT's will be smaller.

5A secondary CT's are the standard supply for gas turbines, becausu most are supplied from North American facilities. A 200 M run from CT's to relays is longer than we would normally do, but it can work if you pay attention to burden ratings. You may have to use #8 awg wire to keep the burdens down.

Using 5A secondary CT's everywhere is easier than getting the generator supplier to change fromn their standard 5A to 1A CT's.

 

[scottf]

rcwilson-

Why do you say 1A CTs will be smaller??

I agree about using 5A CTs everywhere.

 

[rcw retired EE]

ScottF - when I buy ANSI/NEMA 5A CT's they are larger than the IEC rated 1A CT's.

On the projects where we used 1 A CT's, the core cross section appeared to be smaller and the total package was smaller. Maybe it was because the 5A CT's were specified as C400 and the IEC CT's specifications require VA, knee point voltage and other parameters. Maybe I wasn't looking at apples and apples?

 

[scottf]

rcwilson-

You probably weren't comparing like CTs. IEC CTs tend to have much lower burden ratings than CTs designed for the IEEE market.

Given the same performance data, 1A and 5A CTs will be about the same size, as the number of turns and the size of wire used in the secondary balance the coil size out. The core design is the same, since the ampere-turns relationship is the same.

 

[RalphChristie]

Nice summary, Scott.
Strangely enough I did not find Generator Current Transformers with 1A secondary ratings, just some with 5A secondaries. Maybe you have to order them specifically?

http://www.hoytmeter.com/transducers/ct/genct/genct.html

 

[bacon4life]

https://www.kuhlman.com/navigation.asp?pathIDs=10,21,673&selectedID=673

These have options for 1A or 5A secondaries.

 

[RalphChristie]

Thx bacon

 

[scottf]

RalphChristie-

Since someone named a company, I'll shamelessly recommend Ritz as well

http://www.ritzusa.com.

Ritz can make GCTs with 1A and 5A secondaries as well.