Transformer Neutral CT sizing for REF protection

[NikolaSuc]

Hello everyone,

has anyone encountered the following issue:

when sizing CT in the neutral of grounded Y side of transformer, for Restricted Earth Fault protection of transformer using Siemens Siprotec 4 7UT6 relay, does the neutral CT have to be sized so that ALF' of the CT meets some additional requirements in order to be used in the differential scheme?

For example, when sizing phase CTs for application in transformer differential protection scheme, it is requested for them to meet some required ALF (ALF'>ktd*Ifault/In,ct). It is than stated that if phase CTs are adequate for differential protection application, they are ok for application with REF.

Does the same, or some similar rule apply to the neutral CT for application in REF scheme, regarding ALF of neutral CT?

Kind regards,

Nikola

 

[powerpi]

Hello Nikola

This depends of the type of the network. The Siemens CT requirements are presupposing a symetrical ISCC fault current with some margin. This is the case if you have a strong network with little to no DC component. If you use an iron core CT's as a rule of thumb, the answer is No. Meaning that the same CT sizing can be used also for the neutral CT. In case you have larger DC component present, you should calculate the CT parameters using a dynamic simulation or perhaps use some older fault records in the protected area where you should search for the ISCC fault current peak value as a reference.

Hope this helps

Regards