Areva Micom P521 CT dimensioning 1

[AMBMI]

Hi to everybody,

we should verify the CT dimensioning for a line differential Areva Miccom P521 device. The critical point is the stability for external faults. We are using class PX Cts
Is anybody aware of any special value "suggested" by Areva for the "Ktd" value? We found that Siemens for instance is suggesting
VK > Isccmax(external fault )* (Rct + R'b) * Isn / 1.3
where Rct = secondary winding d.c. resistance at 75°C
and R'b = Rlead + Rrelay = connected resistive burden
in this case Ktd is 1
1.3 is nothing else that a magic number to pass from the class P to the class PX CT (according to BS 3938/IEC 60044-1).
I assume that also Areva should use Ktd = 1 but in this case the stability is very critical...basically impossible (CT with Vk = 220 V, 250/1 verification with 40 kA! )

 

[ZapSib]

Hi,

Kt = 1 to be use only for inst.
In case time delay is used, Kt shall be defined accoring to X/R of the power system.
1/ X/R less than 40 :
When delay less or equal than 0.15 sec. => Kt = 1-(6.2*time_delay)
When delay above 0.15 sec. => kt = 0.07
2/ X/R above or equal 40 :
kt = 1-(2.5*time_delay) with time_delay less or equal to 0.25 sec.


All those datas are from AREVA manuals.

It is stated that it is possible to use time_delay in case of retrofit to get CT matching with diff. stability.
Hope this will help.

 

[AMBMI]

Thanks ZapSib for your answer.
The main protective function we use is a line differential (87L) so I don't think we have any delay.
So I assume that we should consider Kt = 1.
Do you have any link to the Areva manual where you found the formulas you listed?
Thanks in advance!

 

[Marmite]

Have you tried here?

http://www.areva-td.com/solutions/US_447_MiCOM+P521.html

There's a 468 page technical guide to this relay which includes setting and commissioning.
Regards
Marmite

 

[AMBMI]

Marmite,

yes I took a look to this manual but I didn't find anything....
Perhaps I simply skipped the right page.
Anyway I think that for a low impedance line differential as the P521 Kt is really 1. Some exotic values are available for the high impedance differential devices but nothing else than the "standard" for the low impedance devices.
we will require an higher Vk value for the PX CT! At least 340 V with RCt = 2.5 Ohm...
Iscmax = 40 kA
Ct ratio = 250/1
Rrelay + Rwire = 0.15
so we get
(40000/250) * (2.5 + 0.15)/1.3 = 326 V (Ct Kv)

 

[slavag]

Hi
AMBMI.
Please only check yourself: Rwire recommended calculate as 2xRwire, in case of ground fault.
Best Regards.
Slava

 

[AMBMI]

Hi Slava,

yes Rwire is taking care of 2 times the wire length. It's correct in my calculation , anyway thanks for pointing that out; it was not clear in what I wrote.
By the way can somebody confirm that a IEC Class P CT can be approximately transfered into
the IEC Class PX (BS Class X) standard definition
by the following formula
Vk = (Rb + Rct)*Isn*Kssc /1.3
where Rct = secondary winding d.c. resistance at 75°C
and R'b = Rlead + Rrelay = connected resistive burden
??
It should be the reason of the "1.3" factor I used in my calculation.

 

[Marmite]

This Schneider Cahier technique provides information on converting between CT's.

http://www.schneiderelectric.com.tr/Schneider_tr/pdf/publications_ect/ECT195.pdf

Regards
Marmite

 

[slavag]

Marmite, you are very speedy .
Second time, I start write answer and see your post.
But I would like finished my post.
Class X old definition-- corresponding to TPS class.
B.S. 3938 --> IEC60044-6 ( please see TPS).
ALF of IEC class (5Px)
n= ((Uk x Insec)/(Pm+Pe)) x 1.3.
magic value 1.3 coressponding by induction level ( uised by CT mnf)
for example Uk=1.6Tesla
U ( for the 5P)=1.6 Tesla.
conversion 1.6/1.2=1.3.
Regards.
Slava

 

[slavag]

Sorry.
Uk=1.2 Tesla.

 

[AMBMI]

Ok Folks!
Thanks you for all your answers and suggestions!
I think I understood something more...now.
I think that the solution will be overrating the CT.
The choice should be a 500/1 CT.