Relay pickup setting

[RBsai]

Whats the relation between relay pickup setting and turns ratio for CTs and PTs ? (With reference to CI 2#15 old version)

 

[iahim]

When you program the relay, you tell it what CT ratio you used. In the CI problem the ratio is 450:7 (BTW, that's nonsense - ratios are xxx:5 or xxx:1). So if you program the relay to trip at 450A, it will trip when it detects 7A at its input. If you set it for 420A, it will look for 420 x 7 / 450 A, or 6.53A.

 

[RBsai]

Does it mean I1/Ip = CT ratio ? (Where I1 - relay pickup setting, Ip is primary current) and what about PTs ?

 

[shsweet28]

The PT value is usually used in Distance Relaying on transmission systems, not on OC relays. You would use your CT ratio and PT ratio to calculate the secondary Impedance your relay is seeing as follows:

Zsec = Zpri × (CT Ratio)/(VT Ratio)