There are three types of current transformers which are used for various application.
1. Metering Purpose
2. Protection Purpose
3. Special Protection Purpose
The particular class of CT is used for specific application. if one class of CT is used for other application the protection or measuring reliability is lost.If the metering class CT is used for protection class, the CT will saturate if large fault current flow through it. If the protection class CT is used for metering purpose there will be large measuring error. For zone protection the PS class CT must be used.
The protection class CT is widely used in the protection application and, the protection CT does not saturate if the large fault current flows through it. The protection class CT is designed to carry the fault current of the electrical network and the selection of CT is paramount for reliable operation of protection system.
What Does it mean 5 P20 CT ?
The letter ‘P’ indicates it is a protection class CT. The number ’20’ indicates the magnitude of the fault current which can be safely flow through the primary of CT, without saturating the CT. For example if CT ratio is 1000/5 amperes, the current flowing through the primary of CT can be up to 20 x 1000=20 Kilo-Ampere. The number ’20’ is also called Accuracy Limit Factor(ALF) of CT. The Accuracy Limit Factor of 20 indicates that the protection class CT will sense the current with rated accuracy when the 20 times current of the primary rated current flows through the CT. The common Accuracy Limit Factor of the CT is 10,15,20,30.
The Number ‘5’ indicates the accuracy of the CT. When the current flows through the 5P20 protection class CT is 20 times of the rated primary current, the CT reads within the composite error of 5 %.
If the current is above 20 times then the CT will saturate.
Related Post: Accuracy Limit Factor(ALF) of CT
For 5P10 Class CT:
If the primary current is 10 times to the rated primary current of the CT, the CT will function perfectly, within the rated composite error of 5%.
A current transformer of CTR 500/5,5P10 accuracy class will function perfectly if the primary current 5000 Ampere or secondary current is 50 Ampere.
Marking of CT
The accuracy class of CT is written after the rated power- 10 VA 5P10,15 VA 10 P10,30 VA 5P20.
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