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Current Transformer

Current Transformer

Current Transformer

(OP)
How to measure the resistance of the secondary winding of a current transformer at site.

RE: Current Transformer

  If you mean internal secondary winding resistance, its value is normally not relevant and difficult (impossible I gess)to meassure under operation. What is relevant is the resistance value or load fed by CI trafo which must not exceed the class burden limit.

Julian

RE: Current Transformer

Measure the secondary resistance with the CT de-energized and the primary open circuited.  This value is needed to calculate CT performance, along with the loop resistance of the external secondary circuit.

RE: Current Transformer

  Peterb, please allow me a small discrepance with your view. Secondary burden only corresponds to external loop impedance, the maximmum value of which is limited by the "precision load". This load is indirectly specified by the apparent power, in VA, consumed by that max. impedance when supplied with nominal secondary current. Manufacturers take into account internal secondary impedance, reactance included, to cop whith specs. I think

Julian

RE: Current Transformer

Julian -
I fully agree with what you say, in terms of the defined external burden that a CT must be able to supply within its accuracy rating (for CTs rated on this basis).  
However, there are instances where knowledge of the CT secondary winding resistance is needed.  Examples of this are high impedance protection schemes and any application where the transient performance of the CT, as against its steady state performance, must be evaluated.
See Thread238-6198 for more discussion, or check http://www.geindustrial.com/pm/notes/ger3973.pdf

RE: Current Transformer

   Thanks Peterb. One always learn from colleagues

Julian

RE: Current Transformer

If you measure the resistance, make sure you short the leads of the CT first as an extremely high potential will exist if the CT is disconnected with current still going through the CT window. The VA rating of a CT is a measure of much load can be hooked to the CT leads as they come from the manufacturer. This includes any wires between the CT and wherever the signal needs to be. As a rule, the larger wire is better. Probably more than you needed to know or knew already but oh well. Thanks

RE: Current Transformer

  A minor objection: I dont see why primary winding sould be short circuited as say Peterb and Buzzp. That wolud be right if meassurement were performed in AC, but as far as I know, resistance meassurement of trafo windings needs to be made in DC; otherwhise you will obtain no load or short circuit impedances in which reactances are not negligeables at all. Am I right?
   By the way. Most of CT, exceeding 100 A as primary nominal current, have not primary winding as they are of the pass-through type.

Julian

RE: Current Transformer

Buzzp - I think that everyone needs to know quite clearly that you can NEVER EVER open a CT secondary circuit when it is carrying current; the CT can develop extermely high voltages under this condition - severe danger to all concerned.  Having said that, it wouldn't be possible to measure the CT secondary winding resistance with it loaded.

230842/Julian - I actually said that the CT primary should be OPEN circuited, precisely for the above reason.

RE: Current Transformer

(OP)
Thank you all, what I need to know is a practical method to measure the internal resistance of the secondary winding of a current transformer at site and it is not caryying any current in the primary winding ( totaly disconnected), this value of the secondary winding resistance will be used for the calculations of CT performance against saturation.

RE: Current Transformer

Use a digital multimeter if you don't have a precision DC resistance tester.  Sorry, I thought that was understood.

RE: Current Transformer

(OP)
I have already use a digital multimeter to measure the secondary winding resistance for a known resistance value, but the value is different from the value given in name plate!

RE: Current Transformer

If that is the case, you may have a problem with the CT.  Evaluate the degree of discrepancy in the resistance readings - correct for temperature if needed.
You can also do an magnetizing curve check to verify whether or not the CT is operating correctly.  

RE: Current Transformer

Just to double check... is it possible that the "resistance" you are reading off the nameplate is the maximum burden for which the ct will meet accuracy requirements?

(That max burden is required to be on the nameplate of metering class CT'S per IEEE C57.13 section 6.8.i.1.... the winding resistance is not).

RE: Current Transformer

Suggestion: The CT secondary winding resistance is needed during the CT design to determine its VA.
E.g. 5A CT 6 Ohm burden leads to 150VA with resistance of the secondary winding of about 0.2 Ohms
1A CT with 6 Ohm burden leads to 6VA with resistance of the secondary winding of about 5 Ohms. Hence, the total burden will be 1**2 x (5+6) = 11VA. For mechanical reasons, the 1A CT conductor is not proportionally scalled down for 1A so that its  secondary winding resistance may be 1.5 Ohms instead of 5 Ohms.
Reference:
A.R. Van C. Warrington, "Protective Relays Their Theory and Practice," Volume II, John Wiley & Sons, New York, 1978, par. 7.2.1 page 176

RE: Current Transformer

   Elhouni: Reading your answer of Dec 5 I feel quite probable that resistance given in CT name plate, correspond to that of max. burden to be connected to secondary for keeping accuracy range, as electricpete answered by Dec 8, and not to the winding resistance. On the other hand, saturation performance of CT's depends not only on secondary resistance but on its flux leakage reactance, and, mainly, on external burden. That performance is fixed by CT specs. (IEC 185, DIN 57414, UNE 21088 or BS 3938) and depends on CT type. Manufacturers do normally provide curves for different overcurrentes and overloads.

   As regards your answer of Dec. 6, my opinion is that CT secondary winding resistance is very low and do normally not exceed 0.1 ohms. Conventional digital polimeters (200 ohms) cannot meassure such a value without serious errors. These are due to resistance of connecting leads from meter to object and to pin contacts as well, which increases the total meassured resistance, in an erratic value amounting some tenths of ohms.
   That is why small resistance meassuements requires specific equipments (milliohmmeters) performing four wires.

Julian

RE: Current Transformer

I have said all along that the VA rating on the CT is for max load and has little to do with the resistance measurement of the secondary windings of the CT(although it does have some). The CT winding resistance will reflect on the secondary current rating (5 amp, 1 amp or other)because of the current it needs to carry, as well as play a ROLE in the overall burden rating of the CT(impedance of windings). With this said, a better question might be what effects the saturation level of a CT?
The way I understand things the following should be considered when considering the max load on a CT:
1. the VA rating of the device (the load connected to it)
 
The impedance does play a role in this calculation by the mfg. Also, the core will play a role amongst others. But for the people using them, I see no reason any other information is necessary as long as the CT is used within the range of currents specified by the mfg. Thanks Buzzp

RE: Current Transformer

buzz - if nothing else it seems like possibly a quick easy diagnostic tool to check for gross ct anomaly (winding open or short). Although certainly a secondary excitation current test and/or ratio test are preferable... those tests are a little more difficult.

Elhouni - out of curiosity... why are you measuring resistance?

RE: Current Transformer

Buzzp - as pointed out above, the VA rating is fine for general applications, but there are cases where you need to know more than just this basic bit of information.  
These could include cases where you have to calculate time to saturation of a CT to evaluate whether the relays have enough time to react before the CT output is reduced due to saturation - I agree that this is not your everyday consideration, but we don't really know what Elhouni's application is.  Other considerations are given in the references cited above, which show that there are a number of factors that need to be considered when specifying or evaluating CT performance.
By the way, Elhouni, in addition to answering Electricpete's question above, could you advise details of the CT nameplate for our information?

RE: Current Transformer

To all contributors,

Good knowledge base ......

To Elhouni,

With all due respect, I think you should state what it is you are attempting to acheive.  The data from the CT Manufacturer should be all you need, unless you are trying to incorparate this CT into a bus voltage differential application, where all the CT's must be of matched impedances, otherwise it would be a standard overcurrent application ....

Having said that here is a possible way of figuring out the impedance/resistance of a CT.

Get yourself a current source (power supply, hopefully with a digital display, wired to supply current)

Hook up your CT to the power supply and adjust to a preferred current value i.e. 1 Amp, 5 Amps, etc

Take the voltage reading off the power supply or use a meter.

You now have currrent, voltage ... from this you can calculate the total resistance/impedance ... this will include the power supply internal resistance ...

To get the power supply internal resistance, you get a known resistance (hopefully a 1% resistor), do the same as with the CT.  Once you get the total resistance, subtract the known resistance ... you now have the power supply internal resistance.

Substract the power supply internal resistance from the first resistance reading you got from the CT resistance/impedance setup ... you now have the CT resistance

This should do it .....

NOTE - this procedure is valid for both DC and AC therefore resistance/impedance.

RE: Current Transformer

Alex - you lost me on the power supply resistance. If I'm measuring the power supply output current and voltage, then I don't really give a hoot what the power supply internal impedance is.  

As previously mentioned, for a sensitive dc resistance test you would want to use the four-terminal resistance measurement principle... connect the voltage sensing leads as close as possible to the ct (and ensure they are not carrying the test current).  For example if you connected voltage sensing leads to the output of the power supply, you would be measuring resistance of leads plus ct.... if you connect voltage sensing leads direct to the ct you are measuring ct winding resistance with no extra lead resistance. (lead resistance of voltage sensing circuit is insignificant due to low current in the high-impedance voltmeter circuit).

RE: Current Transformer

electricpete,

In this case you would want to take into account the power supply internal resistance since the CT resistance will be as low or lower than the power supply resistance throwing off the final result.

.... this is just a POSSIBLE way of doing CT measurement, it gets you into the ballpark .... there are errors that would need to be considered, as you mentioned lead length, etc ..... but Elhouni is out in the field ... without lab equipment ... if I want the most accurate I would definetely use the 4-wire method you described ....

... another possible way would be to use a DUCTOR ... but it can be a real pain holding down the contact points until you get a stable reading .... and you can only get a DC reading at that ....

.... but these are issues of accuracy that only Elhouni can answer .... how critical is the accuracy?  Are ballpark values good enough for his calculations?

as for the the application ... to quote "this value of the secondary winding resistance will be used for the calculations of CT performance against saturation" ... the response by 230842 on Dec, 10 pretty much nailed that one ...
 
which brings it back to accuracy ..... how accurate does he want his calculation?

... or maybe I am already out in left field .....

RE: Current Transformer

Alex
I agree that requirements for accuracy are in the eye of the beholder.  My comments on 4-wire measurement were not directed at you.

I'm still at a loss to see the relevance of power supply internal impedance/resistance.  

I compute the CT winding dc resistance as V/I where V is measured voltage across its terminals and I is measured current. I don't need to know internal impedance to calculate either of these measured quantities.

RE: Current Transformer

electricpete,

duuuuhhhh .... you are right .... if you have voltage and current .... that is all you need ....

.... I'll just go right back out to left field

RE: Current Transformer

(OP)
As I said before, what I'm interested in is the value of the secondary winding resistance not the resistance or impedance of the burden connected to the CT terminals.

I need to know this value for modeling of CT using the Alternative Transient Program (ATP) to perform some tests like effects of X/R, burden,….etc on CT saturation.

What is given in name plate are, ratio, VA and class e.g. ( 5P20 )

RE: Current Transformer

OK, now we know what the data will be used for.  I suggest that you do one of two things here -
1. Check the manufacturer's standard CT data sheet for resistance data for the CT
OR
2. Do an accurate bridge measurement of the actual value (see many posts above).

Note that in addition to the nameplate data, you will also need the kneepoint voltage of the CT in order to model the transient behaviour.  You will probably have to determine this by test as well.

RE: Current Transformer

Yes, the meat of the problem! A simple 4 wire ohmmeter will produce the winding resistance with Kelvin connections. Buzzp

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