CT sizing formulae
CT sizing formulae
(OP)
I was reviewing an ABB document sizing CTs and VTs for a 230 kV GIS and noticed that the designer had used variation for maximum external circuit e.m.f. based on type of relaying.
The basic secondary emf formula is : Ikmax*(Isn/Ipn)*(Rct+RL+Sr/Ir^2).
For Breaker failure protection the following formula is provided:
5*Ioperate*(Isn/Ipn)*(Rct+RL+Sr/Ir^2) considering the Ioperate=10Iprimary_nominal
for differential protection, two cases are considered. the formulae for Ikamx are as follows:
30*Ipower_trasformer_primary_nominal for internal fault and 2*I_through_fault for through fault.
ABB relays application guides have quotes these formulae however I couldn't locate any derivation and got the feeling of something "empirical" here!.
Any clue about the basis of these equations?
The basic secondary emf formula is : Ikmax*(Isn/Ipn)*(Rct+RL+Sr/Ir^2).
For Breaker failure protection the following formula is provided:
5*Ioperate*(Isn/Ipn)*(Rct+RL+Sr/Ir^2) considering the Ioperate=10Iprimary_nominal
for differential protection, two cases are considered. the formulae for Ikamx are as follows:
30*Ipower_trasformer_primary_nominal for internal fault and 2*I_through_fault for through fault.
ABB relays application guides have quotes these formulae however I couldn't locate any derivation and got the feeling of something "empirical" here!.
Any clue about the basis of these equations?






RE: CT sizing formulae
"Selection of Current Transformers & Wire Sizing in Substations" Sethuraman Ganesan (ABB Inc.) -- Google this and you can find a non IEEE download.
"Sizing Current Transformers for Line Protection Applications" SEL Document has all the same equations and derivations as the ABB document.
https://www.selinc.com/workarea/downloadasset.aspx...
"Relaying Current Transformer Application" WECC
https://www.wecc.biz/Reliability/White%20Paper%20o...