Updated: Mar 2, 2021
Types of Current Transformers
There are three types of current transformer widely used for metering and protection. These are metering class CT, protection class CT and special protection class CT. The different class of CTs are used as per the application and, the protection class current transformers can not be used for metering application and the metering class current transformers can not be used for protection application.
Why can’t we use Metering Class CT for Protection Purpose?
The metering class CT is used to measure the energy accurately. The metering class CT is designed to have low measurement error even reading of low current. The class 0.5,1.0,2.0 class metering class CT are used for metering application. These CTs give accurate reading from 100-120 % loading with a permissible rated error. Even now, 0.2 and 0.2 s class CT is used to measure the current accurately from 20 -100 % with a measurement error of +/- 0.2% and can read up to 1 % loading with some more error.
The metering class CT has very low measurement error because CT functions in the linearity region of the BH curve of CT. The knee point voltage- The point at which CT saturates- of the metering class CT is very low and CT saturates at lower current. The size of the metering class current transformer is smaller than the protection class CT.
In the case of fault, the current through the current transformer may go up to 20 times. If metering class current transformer is used for protection class, the CT gets saturate and its output current drops to zero. In this condition, the metering class CT can not give the protection. Also, the metering class CT fails to protect the equipment because the protection relay will not output the fault. Under this condition, the CT will remain under saturation for longer period of time which will cause CT winding insulation failure.
Therefore. the metering class CT should never be used for protection application.
Why can’t we use Protection Class CT for Metering Purpose?
The protection class CT is deigned to operate near the saturation point of the BH curve of CT. The higher knee point voltage is desired feature for a protection class CT. The protection class CT must not saturate at lower current. 5 P10 protection class CT will not saturate up to 10 times of the rated primary current of CT flows through it. The composite error in this case is +/- 5 %. The protection class CT is designed for higher knee point voltage with lower measurement accuracy as compared to metering class CT.
If the protection class current transformer is used for metering application, the current transformer measurement error will be very high at lower loading, and energy reading is not accurate. The CT is connected to various instruments which have generally 10 to 12 times overloading capacity for finite time. In case of the fault, the protection class CT connected to measuring instruments will not saturate and the large current flowing through the instruments will damage the instruments.
The instrument safety Factor (ISF) of metering class CT is generally 3-5. It means if the current through the CT is 3 to 5 times CT gets saturated, and its output current drops to zero. Thus instruments connected to CT remains protected.
Therefore, the protection class CT can not be used for metering application.