Accuracy Limit Factor of Current Transformer

What is accuracy limit factor(ALF) of CT?

Let us first understand what is the accuracy limit factor of  current transformer. There are three types of current transformer widely used in a electrical network for protection  and metering purpose. The metering class CT is used for the measurement of electrical parameters. The protection class current transformer "P class CT" is used for protection of electrical network. The protection class CT "PS class CT" is used for differential protection of transformer, motor and generator.

The metering class CT is intented for maintaining its measurement accuracy during normal operation of the electrical network, the accuracy during the fault condition does not matter for metering class CT. However, the metering class CT should have adequate instrument safety factor(ISF) so that the equipment connected to it remains safe during fault condition. The metering class CT saturates during fault and thus it protects the measuring instruments connected to it.

However, in the case of fault in a electrical network, the accuracy limit factor for protection class CT is of paramount importance. Therefore, before selection of protection class the network fault current should be calculated, and the protection class CT should be selected accordingly. The protection class CT should maintain the measurement accuracy during fault condition to protect the electrical network.

Definition of Accuracy LIMIT Factor(ALF)

The protection class CT saturates at higher secondary current as compared to the metering class CT. The primary function of the protection class CT is to provide protection during fault current. At fault current, the protection class CT  must not saturate,else or it will not provide protection to the electrical network. Also, the protection class CT should maintain its rated accuracy at the fault current to have intact protection.

The accuracy limit factor (ALF) of the CT is defined as the ratio of rated accuracy limit primary current to rated primary current. The protection class CT is marked as 5P10 or 5P20. Here, P stands for ptotection class, 20 is the accuracy limit primary current and 5 stands for composite error of the CT when accuracy limit current flows through the primary of the current transformer. The specifications of the metering and protection class CT are as given below.

In the above given data CT is 5P20 with CTR 200/1. The ALF is 20. CT accuracy class is 5. It means if 20 times of the rated primary current(4000 amperes) flows in the primary at the time of fault, the CT reads the current with 5 % accuracy. This factor (In x 20) 20 is called the accuracy limit factor(ALF).

If the current in the primary of CT exceeds above 20 times of the primary rated current, the secondary current will be distorted and the protection is not gauranted. Therefore, it is very important to select the ALF of CT correctly for reliable protection of electrical network.

The actual accuracy limit factor(ALF) of CT may differ from the rated ALF. The accuracy limit factor (ALF) depends on;

• Internal burden of CT secondary coil
• Actual burden of CT 

How to Calculate actual ALF of CT?

Actual accuracy limit factor of CT can be calculated using following mathematical formula.

Illustrative example:

A protection class CT is rated 200/5,5P20,15 VA. The CT internal secondary resistance is 0.08 Ω, the secondary burden (including wires and relay) is 0.118 Ω. Calculate actual accuracy limit factor of CT.

Effect of higher CT burden on ALF of CT

The actual ALF of CT is 68. The ALF of the CT changes with change in the burden. Let us take the same example with different burden. Let CT burden is 0.335 Ω. The actual ALF can be calculated as follows.

The actual ALF reduces with increase of CT burden. The smaller CT burden will lead to higher accuracy limit factor and the CT can give reliable reading at higher ALF even. 

Effect of lower CT burden on ALF

Now, let the CT burden is 0.005 Ω. The actual ALF of CT can be calculated as follows.

Thus, the CT ALF increases with decrease in CT burden. The graph between CT burden and actual CT ALF is as shown below.

Therefore, CT burden must be as low as possible for getting the better accuracy limit factor of CT.