PX CT specification
PX CT specification
(OP)
For once we've got a project where I get to specify the CTs rather than just making do with whatever is already there.
The CTs will supply a low-impedance transformer diff relay and I want to specify class PX so that I can ask for a high enough knee point to avoid saturation as much as possible.
I've worked out the fault level, X/R, secondary lead resistance, relay burden etc, but then I get stuck. To specify a suitable Vk for the CT I need to know what the CT secondary resistance is. How do I know what is a reasonable maximum secondary resistance to specify?
I've checked out test certificates from some similar older 5A CTs and it works out to be about 8 mOhm per turn - would this be a reasonable value for a 1A CT or would they use thinner wire to fit in more turns?
Also - is it still common practice to install separate cores for separate protection relays (we're also got a directional relay) or is it acceptable to now supply everything from the same core?
Can anyone recommend a good book on this?
The CTs will supply a low-impedance transformer diff relay and I want to specify class PX so that I can ask for a high enough knee point to avoid saturation as much as possible.
I've worked out the fault level, X/R, secondary lead resistance, relay burden etc, but then I get stuck. To specify a suitable Vk for the CT I need to know what the CT secondary resistance is. How do I know what is a reasonable maximum secondary resistance to specify?
I've checked out test certificates from some similar older 5A CTs and it works out to be about 8 mOhm per turn - would this be a reasonable value for a 1A CT or would they use thinner wire to fit in more turns?
Also - is it still common practice to install separate cores for separate protection relays (we're also got a directional relay) or is it acceptable to now supply everything from the same core?
Can anyone recommend a good book on this?






RE: PX CT specification
Stan Zocholl has a book called Analyzing and Applying Current Transformers
RE: PX CT specification
http://ww
It tells you how to specify CT's, although it has a European slant.
Regards
Marmite
RE: PX CT specification
For my pinion, today, with new transformer protection you don't need PX class of CT, it's good for high impedance protection.
I assume, that you in IEC land. Check with relay mnf. what is requirements for CT, I think in lot of case 5P30 30VA
is more from enough, more for the, if you order 1A secondary,you don't have any problem.
Actually, relay mnf. write in the manuals what is n'> ( overcurrent factor) or EMF for the differential, distance protection,etc.
For secondary resistance you can use 0.6-1 Ohm for 5A
and abou 5-7Ohm for 1A.
Regards.
Slava
RE: PX CT specification
I mean, internal resistance of CT.
Regards.
Slava
RE: PX CT specification
Very often we see the Vkp specified as a function of the Rct (CT winding resistance), i.e. Vkp = x * Rct. In fact, it's a good way to do it, as it allows the CT manufacture to explore options to keep the overall cost down.
RE: PX CT specification
I think Scottf's idea sounds like a good plan - leaves designing the CT up to the experts.
Already had the trusty Cahier technique thanks but I've just ordered a copy of Zocholl's book.
RE: PX CT specification
Of course Skottf are right.
I sent to you typical internal resistance ( CT winding resistance) for first calculation of n'>.
Skottf, we've never specified to CT factory what is Rin of CT we need. You know what is a better and cheaper.
Protection designer always have option change cross section of current wiring.
about 1A.
provide some small calculation.
secondary burden for 5A Rin=0.5Ohm, cable 6mm^2, lenght 100m
S= 0.5*25+0.019*200*25/6=28.3VA
for 1A Rin=8Ohm same cable and lengt
S= 8*1+0.019*200*1/6=8.6VA
That means 1A "more better".
I assume 5P20 20VA will enough.
Regards.
Slava