CT parameters 1

[trbartlett]

Hello all,

I do not have manufacturers (South Wales Electric??)information on diff cts 2.5P30 F20 (600:2.89). I cannot find rated burden (ohms or VA) on nameplate of switchgear - cannot access HV cabinet to check ct's themselves. I want to work out actual ALF - am i completely stuck not having a rated ct burden value. Also, what is the F20 all about - just a guess - fault current 20 x rated? perhaps a kneepoint? 10%error?

thanks in advance.

Tim

 

[trbartlett]

C'mon guys, help me out here. I've checked other ct threads and i know there is a bunch of guys out there that could answer this straight up or at least point me in the right direction.

 

[ThePunisher]

Patience is a Virtue.

I'm sure they will be coming to you pretty soon. You have to consider time zones and work loads.

 

[Ohmly]

What standard is the CT made to ?

In the Australian standard AS1675, this CT would have a 2.5 % error, when the CT is driving 20× Nominal current (F20 is the accuracy limit factor in AS1675) by producing 30 Volts at the secondary terminals.

In the IEC standard, this would be 4.335VA 2.5P20

But check that it is to AS1675 first, or confirm this is correct through testing.

 

[trbartlett]

Thanks Ohmly,

What you speak of is what I am confused about. I am not an old boy at this stuff so have little experiance to draw on.

In the beginning I believed that 2.5P was percent error @ F20 fault current 20 x flc and that the 30 was the volts accross the terminals - which is what you seem to have explained as being AS1675. I did some calculations for burden and was hoping to find an actual ALF for the purpose of working out the accuracy of power monitoring in normal conditions. (does this make sense?)

There are new multilin dif relays (745's - 3 transformers!) being installed and i was hoping to be able to network the power monitoring data of these new relays and am trying to establish the accuracy of the data. THEN i started to read some things that seemed to indicate what i had calculated was not correct which is what i think you (Ohmly) have indicated as IEC standard. All i know is that I have no burden value VA or ohms.

Unfortunately, it requires an order at the hightest level to isolate/access this HV installation. That is what happens when you are dealing with the country's piggy bank.

The cts are manufactured by "South Wales Electric(AUST.)P/L" and i believe the installation is 30+ years old. I was hoping someone may be able to help out with the identification of the cts with respect to a standard and be able to confirm the nameplate values. The markings are "600/2.89" "2.5P30 F20" turns ratio 1/207. I just managed to find on a nameplate on a second piece of switchgear that the CTs are compliant to AS1675 - however the first piece of switchgear states AS-C388. The last version of this was 1968 and it is no longer available.

Any additional comments would be appreciated.

I would also like some guidence with respect to calculating accuracy at currents up to 400Amp which is max capacity of the tx's.

Cheers, have a good week all.

Tim

 

[Ohmly]

AS-C388 is roughly the same as AS1675, the main difference as is AS-C388 is in imperial units, AS1675 is metric.

I think I can follow your question, you are looking for accuracy up to the rated current. Accuracy Limit Factor is only relevant for fault currents.

I don't have a copy of C388, but for AS1675 the accuracy at rated primary current is 1% for burdens up to 0.04 ohms. I can only assume this was the same in C388.

If the burden is above this value I am not sure of the accuracy ! It would probably still be OK, but it is just not guaranteed.

P class CTs are OK for indication purposes where accuracy is not essential (eg readings of Amps for the SCADA).

If metering accuracy is required, we wouldn't use a P class CT, nor a numercial protection relay.

 

[trbartlett]

Interesting Ohmly,

It is predominantly for monitoring for 'green' compliance which is +-5% but the client wants better acuracy than that.

I am interested in your last comment "nor a numericial protection relay". The multilin 745 that is being installed - is this what you refer to as a numerical relay? - are you indicating that monitoring acuracies of this type of relay may not be good? - they are quoted as:-

Phase current input
Accuracy at <4 x CT - +-0.25% of 4 x CT (+-0.01 x CT) at 50/60 Hz nominal frequency
Accuracy at >=4 x CT - +-0.5% of 46 x CT (+-0.2 x CT) at 50/60 Hz nominal frequency
(how do i correctly interperate this information?)
(Multilin burden <0.2VA)

For the other installations on site, satec PM175's are being installed which i believe are purpose built for power and power quality monitoring with quoted accuracies of 0.2%-0.3%. Satec burden <0.1VA.

My simplistic way of thinking about it was that the class of the ct (2.5P30) F20 ment that the accuracy was 2.5% when the secondary terminal voltage was 30 volts and this would be at 20 FLC. Then i figured that this would indicate its capacity in terms of burden which could be assumed as its rated burden and then if the actual burden was less, then the accuracy would be better. I believe i read somewhere that a actual ALF could be worked out if this was the case. (IEEE std C37.110-1996?)

comments and corrections appreciated

Tim

 

[Ohmly]

Yes an SR745 is a numerical relay.

If the application was for accurate metering (say for billing purposes) then an SR745 (or any other relay) and a P Class CT is not suitable in our organisation.

However they are OK for indications of Amps/Volts/Power etc to SCADA, and we use these relays and CTs for this purpose all the time.