CT ration for solidly grounded XFR's

[claghe]

Hi to all,
I have a power transformer MV/LV with its secondary solidly grounded. What would be the ration of the CT used in the grounding circuit for the grounding protection? Should this CT match the CT’s used in the secondary phase o/c protection?
As an example 2MVA, 13,8/0,48kV, solidly grounded
Thank you

 

[Marmite]

It depends. What do you mean by grounding protection? Are you using digital numeric relays? You will have to be more specific about the protection function you want to achieve. For example, if you meant a restricted earth fault scheme, then you would need to select CT's with the same ratio's and electrical characteristics for the protection to remain stable for through faults.
Regards
Marmite

 

[xxjohnh]

Standard practice is to use the same ratio as the main in the phase devices. The higher the ratio the higher the saturation value.

 

[zkz]

Hello,
As already pointed out, if you are meaning about Restricted Earth Fault protection (i.e., protection against the earthfaults occcuring in the portion between the phaseh CTs and Neutral CT), you have to use CT same ratio. Please note that there are other factors which are also to be considered while selecting neutral CT for REF protection.
If you want protection against earth faults not only in transformer but also in associated bus, line etc., then you can use a CT in the neutral side with a ratio different from those in Phase CTs. Because this neutral CT will feed current to the earthfault relay when an earth fault occurs and current flows through the neutral of the transformer. Please note that the earthfault relays will be set delayed so as to be graded with other earthfault relays in the downstream side. The ratio will be depended to how much will be the minimum earthfault current available for an earthfault. But if a very low ratio has been taken, then there is a chance of CT saturation for close in earthfaults. Therefore, you may use lower ratio than of Phase CT if there is no issue of saturation during such conditons.
regards,
zkz

 

[claghe]

Thank you all for the quick responses
I was referring to the EF protection against EFs also in the associated system components. I’m not very familiar with the protection relaying but I read in some book that the pick-up current of the EF relay should be in the range of 10-25% of full load current, thus my guess was that the CT should be sized accordingly.
The reason I asked the question is because last week I have noticed in one of our documents that the CT in the grounding circuit of the power transformer supplying the LV SWGR is rated the same as the CT in the secondary phase o/c.
Zkz’s answer is good enough for me.
Thank you all