Current Transformers, used in Sub-Station mainly serve two purposes:
- Measuring purpose: This type of CT used for measuring current. This Current Transformers step-down the current based on its ratio (like 1000-800-400 to 1 Amp.) so that reduced current can be measured in meter. During fault, CT primary current increase to kilo-Amp range (like 31.5 KA, 40KA etc.) but due to saturation of core secondary current remains same I.e., 1 Amp and meters/measuring devices stay protected.
- Protection purpose: Core of this type of CT does not saturate during fault/overloading and secondary current increases based on type of faults so that relay senses fault for tripping of circuit breaker. During rated primary current condition CT gives rated secondary current ( 1 amp).