CT verification 2

[AMBMI]

I was wondering which CT verification formula you are using.
I have to verify the CTs for many substations and I was going to use the formula showed in the attachment.

The "magic" Ktd factor seems to be:

Ktd = 1 for over current relays
Ktd = 1.2 for line differential relays
Ktd = 3 for trasf differential relays
Ktd = 5 for motor or generator differential relays

(I wonder also why CTs for generator differential relays seems to be more critical than the CTs for the transformer differential relays)

 

[veritas]

Hi AMBMI

First of all what class CT are you intending to use? The formulae given in your attachment are only applicable to Class P CT's. I always tend to go for Class PX CT's for diff applications especially for trfr and gen diff. Why? because class PX is more accurate and thus leads to increased differential sensitivity. Class P is okay for motor diff in my opinion.

But irrespective of whether you are using Class P or PX I always use Ktd = (X/R + 1) (where X/R = primary system reactance to resistance ratio)for diff applications. I have seen many manufacturers stating special formulae for Ktd which result in smaller CT's. However, they have not entirely convinced me that their relays will be 100% stable undr worst case throughfault conditions considering maximum DC offset. Also some relays have problems when CT's come out of saturation especially when the diff relay is not located at the electrical midpoint of the secondary diff system. Using X/R + 1 is the most conservative way of CT sizing I know and it gives me best peace of mind.

With regards to why are gen diff ct
s more critical? It is not that they are more critical but gen diff CT's must handle DC as well as AC offset whilst trfr diff rarely have significant AC offset to contend with. Motor diff also has AC decay but this is assumed to be the same as the locked rotor current. DC offset is a function of the primary system up to the point of fault. AC offset depends on the rate of rise of the back emf in a generator and is typically quantified by its time constant td", td' and td. What I do when I have to size gen CT's is to determine the worst case throughfault current based on the subtransient reactance, Xd". I thus cater for the worst case AC offset. The Ktd factor then again takes care of the DC offset.

Hope this helps.

Regards.

 

[slavag]

Hi.
In additional to Veritas's post.
DC offset is depend on Tp/Ts of system.
Of course possible use X/R+1.
Please see Ktd calculation according to IEC 44-6.
In all cases, I'm strongly recommend for the diff/distance
protection check mnf. requerements to the CT performance.
Regards.
Slava

 

[AMBMI]

Thanks for your answers!

Veritas: we are going to use class P CTs and we have motor differentials and a bunch of over current devices. I think it should be fine. Your explanation is very clear about the reason why the gen and the motor diff can be (are) more critical. Thanks!

Slavag:
Is there any way to find on Internet the IEC 44-6 guidelines?

 

[slavag]

Hi.
No way, to find IEC44-6 at the Internet.

Class P is fine for diff trafo/motor/generator/BB protection and distance protection too.

Yes, 44-6 and value X/R+1 are good for common practic.
But, you must, must check CT perfomance according to relay mnf. recommendation.
Maybe you calculate CT with 5P30 80VA or 120VA, so, you lot of time loocking for this size of CT ( for MV and HV too), but CT mnf dive you only 5P30 40-50VA.
Today newer relay have option for reconstraction current graph at CT saturation situation too.
Regards.
Slava