CT for differential relay

[power2020]

Dear Folks,
CT Knee voltage for differential protection shall have more than two times of Vk

Vk=If (secondary) x (Rct+ 2xRL)

What is the above fault current - 1/2 Asymmetrical rms / 3-cycle Asymmetrical rms / 3 cycle symmetrical rms.

Advanced thanks for your replies

 

[slavag]

Hi Power2020.
Small correction.
two times Vk it's requrements for the HI diff protection, not for LI bias protection.
Same about fault current.
Each diff relay have some CT requrements.
actually, it's n'> overcurrent ot oversizing factor.
this parameter is function of CT parameters ( class, Pn, Ri) and secondary burden ( CT wiring,relay burden, etc.).
Regards.
Slava

 

[power2020]

Thank you Slavag for your reply.
What is the fault current If?
Is it 1/2 cycle Asymmetrical rms / 3-cycle Asymmetrical rms / 3 cycle symmetrical rms?

 

[slavag]

Hi Power2020.
I'll explain, as I remember, last time I calculated it before sevral years and haven't now papers.

Uk=2xUs (stabilazing voltages).
Uk=0.8xALFxInx( Rin + R2) ( secondary value)
ALF--- accuaracy limit factor of CT.
In--- secondary current
Rin CT internal resistance
and R2---resistance of current wiring loop ( max).
now Us also secondary
Us=Ikmax X (Rin + R2)/n ( ratio of CT)(secondary value).
Q's, what is a Ikmax.
Actually it's max trought-fault current.
Calculation by some "rules"--- for the small xfr
Ikmax=16 x Ixfr nom
for the big 12 x Ixfr nom.
From here we see, that stability of HI REF basis on the same ratio of all CT's, if it's not same CT ( prefer same batch also) we haven't stability.
Regards.
Slava

 

[power2020]

Thank you Slavag for detail explanation.
My doubt is regarding through-fault current- Is it sub-transient or transient fault current (1/2 cycle or 3 cycle fault current) ?
Your reply is highly appreciated.

 

[nightfox1925]

As I understand, a differential relay has no inherent time delay. The CT should be able to absorb the maximum asymmetrical current (1/2 cycle) without saturation inorder to drive the relay.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[davidbeach]

nightfox1925, it's not quite that simple. Bus and machine differentials may be able to operate with no inherent time delay, but transformer differentials are always a bit slower to allow the relay to determine if it is looking at a fault or at inrush.

 

[power2020]

Thank you for your replies.
Can we take 3-cycle max Asymmetrical fault current as Ikmax for calculating Us (stabilizing voltage)for transformers?

 

[slavag]

Hi Power2020.
If we told about HI ( high impedance protection)87T or REF you use for calculation only Ikmax=(12-16)*I rated of power transformer. You don't use in calculation any sym. or asym.
fault current.
If we told about LI( low impedance) 87T protection, will write us and I try prepare other type of calculation.
Please pay attention on the David's remark.
It's not so simple. You use for check CT requeremnets 3ph
sym current fault and you need also calculate transient dimensioning factor ( is depend on primary , secondary system time constant and fault clearence time).
Regards.
Slava.
Formulas, formulas...

 

[RRaghunath]

power2020,

1/2 cycle asymmetrical current is to be considered for any high impedance protection. I think it is clear from nightfox1925 post.
The calculation and parameter won't apply to transformer differential protection which is always Biased type (Low impedance type), the CT sizing formulas are generally provided by the relay vendor in the literature.