CT Burden Test

[zaza123]

For the 750/1 CT in the CT burden Test We are injecting 1 Ampere in the CT secondary & the reading in Relay for that phase Shows 750A, the voltage measured at the Secondary terminals is 2 Volts. So the Burden measured is 2 VA.
the CT Name plate mentions 15 VA.
Is this type of Testing sufficient for CT burden Test.

 

[ScottyUK]

No, the CT could likely drive current through a higher impedance than that which you are using to short out the primary. This would increase the burden seen from the secondary side.

What are you actually trying to prove?


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If we learn from our mistakes I'm getting a great education!

 

[lz5pl]

zaza123, if this a site test and you are trying to verify where the CT-secondary load is within the limits, this test should be sufficient according to me.
But what the client will require from you depends on local regulations.
In my practice we just measure active resistance of the complete loop - CT-winding + cable cores + relay/measuring instruments resistance + etc. - and calculate active burden. For typical installations the error from measuring resistance, not impedance, is not very important.


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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[zaza123]

Hello Scotty & Iz5pl

This is a site test & I am the client. The Test engineer is doing CT burden test, for the CT 750/1 , he is injecting 1 Amp in the CT secondary wire, (at that time it is not connected to CT) , & we check in the relay 750 A ( the relay is 7UT6121), he is injecting 1 amp & voltage he gets is 2 v, so the burden= Volt * Amp
ie, 2 * 1 = 2 VA , he calculates, on CT 15 VA is marked.
I am not clear on the method of measuring the burden, in my opinion the resistance of the entire wires from the CT & relay coil should be considered as IZ5pl mentioned.
i need your views on this & how important is this type of test.

 

[ScottyUK]

lz5pl is correct that the CT winding needs to be included in the calculation, but from a practical perspective the CT is so hugely over-sized compared to the connected burden that the additional resistance of the CT winding is not likely to have a significant effect on the operation of the scheme. If your tests showed that the relay and wiring burden was 14VA and you were using a 15VA CT then you would have more cause for concern about using approximations.

What your test guy is doing is not strictly 'right' but it's not likely have a detrimental effect on the relaying scheme in this instance.


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If we learn from our mistakes I'm getting a great education!

 

[lz5pl]

ScottyUK, you are right about the particular values measured. Internal resistance of CT winding should be taken in account on design stage when cable cross-section is calculated. In zaza case it will add couple of VA's, what is not a problem.

By the way, last week I had the opposite problem. On a new installation 20 VA rated measuring core will be loaded during normal operation less than 1 VA. The reason is that cables are sized according to CT-rating (1 A), but actual load will be less than 100 mA. People from metering department asked us to replace secondary cable with one with less cross-section to increase the load, because their energy meters will not count correctly !

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[slavag]

Hi.
This kind of test is good practic and important thing. But isn't test of CT burden.
It's more test of secondary CT loop, contacts, connections, etc. 2V for 1A seems more or less OK, what is a wiring size and lenght of cable?
Best Regards.
Slava

 

[zaza123]

Hi! Slava,

the wire size is 2.5 sq mm & cable legth approx 130 meters( the CT is for differential relay which is in another subststion)
What do you mean by
" BUT isn't test of CT Burden"
For CT burden is there some thing else.

 

[ppedUK]

What Slava is saying is that your test engineer is actually carrying out a secondary injection test which is a standard test for protection relays. There is nothing wrong with what he is doing in measuring the injection set output volts and calculating cable and relay burden, but as the others have said, this is not taking into account CT impedance, so it's not a burden test.

Burden is normally calculated and CTs sized accordingly.

For a differential scheme you should be expecting him to check the mag curves of the CTs and ensure that all diff CTs in the circuit have matching knee points etc.

 

[scottf]

lz5pl-

Are you sure you typed this correctly?:

People from metering department asked us to replace secondary cable with one with less cross-section to increase the load, because their energy meters will not count correctly !

I think someone is confused. I'm guessing their concerns is that technically, CTs rated to the IEC standard are to maintain the stated accuracy class from 25% to 100% rated burden. Decreasing the wire size will increase the burden, but it won't raise the secondary current level.

 

[slavag]

OK.
zaza123, Im strongly recommend check with your designer or supplyer, if this CT with this cable are OK for the 87T protection. BUT CHECK according to Siemens requeremnts, not some code or burden.

130m of 2.5 sq mm is seems a wrong calculation/design.

(0.019x260)/2.5=1.976Ohm x 1A=2V, your loop is OK.

Of course CT is 5P10 ( as usually today :-( for diff protection).

Best Regards.
Slava

 

[lz5pl]

scottf, it is not a typing mistake ! They actually asked us to replace already installed 4 sq.mm control cable with 2,5 sq.mm to increase resistance of the loop and this way to increase the burden of CT's. But the actual load will be very small and even after full loading of the supplied power transformer secondary CT-burden will not be more than 1 VA, as it is required by the standard for class 0,2S.
There is nothing to do here, this is a specific of the particular installation. Even 2,5 sq.mm cables (minimum required by local regulations) will not help.


------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[alanlawand]

It is not so difficult, inject the current in primary side measure the voltage and current on secondary winding and using Burden (VA) = VxI you can find it.

 

[rhidoy2003]

I think you got a very good result. As your CT is sized for 12 VA burden but you got 2 VA . Oversized CT. its ok.