How to check CT ratio if CT nameplate not available

[Powerykh]

thread238-415974

Hi,

Reference to above thread.
If we have an unknown CT (without nameplate data), do we have any method to determine its correct ratio?

Let say we injecting 10A primary, and get 0.02A secondary on a bushing CT (installed inside a Transformer for winding temperature used).
The ratio after calculated is 500.
Ratio of 500 could be 500/1A, 1000/2A, 750/1.5A and many other ratio possibility.

Any one experienced this situation before?
Thanks in advance for reading this thread.

 

[davidbeach]

The ratio is obviously 500. What it seems that you might actually be looking for is the rated current rather than the ratio. Try finding the knee point. A 500:1 and a 2500:5 should have very different knee points.

 

[Powerykh]

Thanks for reply.
Sorry i think my question maybe misleading.

I mean, let say the ratio is 500. So anyway that we can say that this CT is probably 1000/2A or probably 750/1.5A or...etc.?

The transformer winding rated current is around 500Amp. ( but primary current of this unknown bushing CT maybe ranged from few hundred to 1000amp also .)

The purpose is to know the bushing CT primary and secondary current so that i can set/buy a Winding temperature indicator with suitable current input. (Winding temperature indicator have various current input model, such as 1 amp, 1.2amp, 1.5amp, 2.0amp....)

 

[thermionic1]

The DC resistance will give you a clue, 1A vs. 5A. Omicron makes a test set (CT Analyzer) that will "guess" pretty well. Maybe you can rent one. With a WTI CT it may be a bit trickier as you mentioned there can be some odd secondary ratings.

 

[beyond86]

thermionic1, can you explain please, how information about DC resistance can help if I don't have any information about CT?

 

[thermionic1]

Hi beyond86,

From the CT Analyzer manual:

"Decision 1A/5A CT If the guesser function is active, the device uses the
measured winding resistance to decide whether the
CT is a 1A or 5A CT. This value specifies the decision
threshold.
Possible values: 0.5 to 2Ω. Default: 1Ω.
If the measured winding resistance is higher than the
defined value, the guesser function decides that the
CT’s nominal secondary current is 1A. Otherwise the
secondary current is 5A."

My company owns a CT Analyzer and I've used the guesser function a few times. There is a little more to the guessing than just DC resistance, such as knee point, etc. What I found is that if certain things are learned and confirmed (Protection vs Measurement via kneepoint), then rerun the test, it gets more accurate in its assessment.

 

[beyond86]

thermionic1, but in case when CT with secondary current 5A has resistance more that 1Ω, CT Analyzer will make incorrect decision.
Or all CTs with current 5A have DC resistance less that 1Ω? I'm not sure about it.

 

[scottf]

The CT analyser considers the ratio in the secondary resistance range in the black box mode (guesser mode). So for an effective ratio of 500:1, along with the measured knee-point one range of secondary resistance would indicate a 500:1 A ratio and another would indicate a 2500:5 A ratio.

 

[NCTHAI]

Powerykh, CTs installed in bushings and used for winding temperature indicator has very typical rations based on selection of WTI. If you know check the ratings of WTI installed, it will specify how much current is required for biasing of Oil temperature reading at rated capacity of the transformer. Typically this will be your secondary current for CT. Because of non standard ratios, normally these CTs are hand wound. If you do not have information for existing installation, you may have buy new CT and along with new known WTI. However, for installtion if CT, you may have lift the bushing - depends of construction.

 

[scottf]

NCTHAI-

I'm not following why a non-standard ratio would require the CT's to be hand-wound.

 

[argotier]

There is no need to know the actual rated currents of the CT for the case you mention. You only need the CT ratio.

You already know the primary current (of the power transformer), with the CT ratio you can obtain the corresponding secondary current and check if it suits your WTI.

First you will need to check for the maximum continuous transformer current, so you don't overload the WTI.

If you are below the limit, then check the Δ temperature rise vs input current curve of the WTI to see if your CT secondary current is enough to simulate the winding hot-spot temperature you need.

If some of these values don't match your WTI or you can't find a suitable WTI alternative, you can also consider a multi-ratio transformer unit to convert the CT current to an useful value. These units are sold by the same WTI manufacturers like this one from Messko. Qualitrol has something similar.

Hope it helps.

 

[scottf]

argotier-

From a practical perspective, you are probably correct. However, technically, you really do need to confirm the current rating of the CT against the maximum current of the power transformer.

As an example from above, we know the CT has an effective ratio of 500:1. What if the power transformer max current is 1500A. If the CT is 500:1A with RF 3.0, then it might be ok, but you have no good way to guess at the rating factor.

If the CT is 2500:5A, then you know it's ok in this example.

 

[argotier]

You are right scottf, I was focusing on the WTI and forgot to account for the CT itself. My bad, sorry.

 

[Powerykh]

Thank you everyone for your reply.

Hi argotier,

I only can obtain the following info:
Tx HV winding rated current: 525A (obtained from power Tx nameplate)
HV WTI(winding temperature indicator) is of very old version type, no info of the CT current input can be obtained.

Let said we obtained the WTI CT ratio is 500:1, then there may be still many possibility of the rating of this
WTI CT, i.e.500/1A or 1000/2A or 750/1.5A or 600/1.2A and etc..(many possibilities of the CT secondary current rating)

 

[argotier]

Can you give us more information about this unknown CT? Where it came from?

1- It was a CT used for WTI from a different transformer and you want to re-use it on a new one?
2- or the CT was always installed in the transformer but it's data went lost and you only want to replace the WTI?

If 1 then you need to know not only the CT ratio but the actual current ratings in order to avoid CT overloading like the guys have said above.

If 2 then I stand correct with my first post: the CT is already capable of managing the TRX current and you only need the CT ratio to check the possibility for adapting a new WTI, with or without an intermediate adapter.

Let said we obtained the WTI CT ratio is 500:1, then there may be still many possibility of the rating of this
WTI CT, i.e.500/1A or 1000/2A or 750/1.5A or 600/1.2A and etc..(many possibilities of the CT secondary current rating)

There are no many possibilities for the CT secondary current.

For a given primary current and CT ratio, lets say 500:1 (with any ratings like 500/1 A; 1000/2 A.. and more), you will always have the same secondary current:

CT secondary current = trx HV winding current / CT ratio = 1.05 A (for the HV rated current of 525 A).

Hope it helps.

 

[Powerykh]

Hi argotier,

The CT was always installed in the transformer but it's data went lost, i only want to resetting the WTI or replace it if found faulty.

After read your last reply, i suddenly understand that the CT ratio is good enough for resetting/replace the WTI.

I actually haven't conduct any CT ratio test yet, cause i can't get my head around these things (my mind stuck on this).
As i thinking that to get bushing CT ratio by using primary injection method (need effort and time) also cannot serve the purpose.

Thank you very much for your help.



I also thank you others who reply this thread (sorry as some was confused by my writing style)

 

[DTR2011]

It’s very unlikely to test transformer bushing without a big set up.

In ANSI/IEEE it’s perfectly acceptable to use the voltage method, injecting on CT secondary and measuring the CT primary via the transformer Windings.

There are numerous CT test sets that test this way, along with instructions & connection diagrams