Accuracy Limit Factor of Current Transformer is a very important factor while selecting a CT for protection purpose. A current transformer chosen for protection purpose are defined by protection class ‘P’. The whole and sole requirement of a protection class CT is to maintain its measurement accuracy during fault as well as abnormal condition. Such CT should not get saturated under the condition of heavy current flow through the primary of CT during fault. Unlike metering CT, where the CT get saturated at comparatively lower current to protect connected meters, protection class CT saturate at higher value of current. It should be noted that Instrument Safety Factor is defined for Metering CT.
If you see the specification or name plate of a protection class CT, you will find that it is given like 5P10. This CT can be interpreted as a protection class CT having an accuracy of 5% over a current range of 10 times of normal primary current rating. Let us elaborate this by taking an example.
Let us consider a CT of ratio 200/5 A, 5P10. This CT will maintain its measurement accuracy of 5% over 10xrated primary current i.e. 2 kA. This factor of 10, is called Accuracy Limit Factor.
“Thus accuracy Limit Factor can be defined as the ratio of limiting primary current up to which accuracy of CT is maintained to the rated primary current.”
Since a protection class CT secondary is connected to some protection relay, therefore it expected that the CT should not get saturated at least up to its setting value. Let us consider this statement by assuming over current protection. Let us assume that over current setting is 8 times the nominal current.
Is = Setting of Over Current = 8In
So, the CT should not get saturated at least till 8In. Therefore the Accuracy Limit Factor (ALF) of CT must be more than 8. While choosing ALF, a safety factor of 2 is considered, hence the ALF of suitable CT should be 2×8 = 16. This CT will maintain its measurement accuracy up to 16 times of rated current. This does not mean that CT will get saturated beyond the ALF rather its measurement will not be within measurement accuracy limit (5% in our example).