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
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.
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
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
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
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
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.
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.
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!
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.
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).
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
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
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
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?
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?
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.
