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).
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?
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.
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 )
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.
