Accuracy Limit Factor (ALF) of Current Transformer

Accuracy Limit Factor (ALF) is the ratio of the rated accuracy limit primary current to the rated primary current. This factor is critical for ensuring accurate relay operation during fault conditions, safeguarding the integrity of the protective system.

ALF is used for protection class CTs. For metering class CTs there is similar term called Instrument security factor (ISF).

In simple terms, it is the number times rated current that the stated accuracy is good up to. For example, if a CT has rating of 5P20, it will have 5% accuracy upto 20 ALF. Meaning the CT will maintain 5% accuracy up to 20 times rated current with rated burden applied. In practice, it means that the CT won't saturate at upto 20 times rated current with rated burden applied. 

Equivalent circuit of CT:

I_P = Primary current

I_S = Secondary current

V_S = Secondary voltage

Z_E = Exciting impedance

I_E = Exciting current

R_S = Secondary resistance

X_L = Leakage reactance

Z_B = Burden impedance 

Effect of burden on ALF: ALF is specified for a certain burden. If actual burden is different from rated burden, ALF will change as below:

In practise, the actual accuracy limit factor differs from the rated accuracy limit factor (Fn) and is proportional to the ratio of the rated CT burden and the actual CT burden.

ALF at actual burden = ALF at rated burden x (Rated burden / Actual burden)

* Internal secondary winding resistance of CT to be added for calculating burden.

For example, for a CT with rated burden 20VA and ALF 5, if actual burden is 10VA, ALF will be =5 x (20/10) =10. I.e. accuracy will be maintained upto 10 times rated current.

End Fault Protection in bus bar relays

Bus bar protection is generally differential protection for which zone of protection is defined by location of CTs. However, location of isolation is defined by CBs. The area between CB and CT is always an issue for protection schemes. 

In above image, CT location in Bay-1 is on line side and CT location in Bay-2 is on bus side. It has been done intentionally to discuss both cases. However, in actual practice mostly CTs are placed on line side. In old schemes, without end fault protection enabled, this area is taken care by Breker Failure Relays with a time delay around 200ms. For Numerical bus bar relays, end fault protection is an additional feature to take care of the area between CB and CT.

Bay-1: CT location on line side

Case 1.1 When CB is open: Current flowing through 1CT (feed from remote end) is not included in busbar differential relay calculations. Therefore, busbar relay will not operate for fault between 152CB and 1CT. End fault protection relay has one additional over current element, which will operate in this condition when current through 1CT is above set value. This will send direct trip command to remote end through communication channel so that fault is cleared without any time delay.

Case 1.2 When CB is closed: Current flowing through 1CT is included in busbar differential relay calculations. Therefore, busbar relay will operate for fault between 152CB and 1CT. Depending on scheme, Busbar protection will send direct trip command to remote end through communication channel so that fault is cleared without any time delay or end fault protection will operate and send direct trip after CB is open.

Bay-2: CT location on bus side

Case 2.1 When CB is open: Current flowing through 2CT (feed from local end) is not included in busbar differential relay calculations. This causes extension of bus bar protection zone upto CB. Therefore, busbar relay will operate for fault between 252CB and 2CT. 

Case 2.2 When CB is closed: Current flowing through 2CT is included in busbar differential relay calculations. Since area between 252CB and 2CT is outside differential zone, busbar relay will not operate for this fault. This fault will be detected by line protection. After opening of CB, End fault protection relay will come in picture as mentioned in Case 2.1.