ACCURACY CLASS OF CURRENT TRANSFORMER: 101 COMPREHENSIVE DISCUSSION

The accuracy class of Current Transformers used in the industry is designated by the highest permissible percentage ratio error at rated primary current and rated output.

The RATIO ERROR in the accuracy class of current transformer

The ratio error plays a major role in determining the accuracy class of current transformer. In a CT the primary current must be equal to the secondary current multiplied by the turns ratio. That is, I_p=k* I_s, where I_p is the primary current and I_s is the secondary current. But, this is an ideal case scenario, in actual practice there is always an error. It means the actual primary current is never equal to the secondary current multiplied by the turns ratio,k. This error or difference is known as the ratio error.

Mathematically, the ratio error of CT, e = (k * I_s – I_p) / I_p * 100%

The output of a CT normally has some error and this is due to the ratio error and the phase displacement between the primary and the secondary current. As per IEC-61869-2, the accuracy class of current transformer is designated as 0.1 – 0.2 – 0.2S – 0.5 – 0.5S – 1 – 3 – 5.

Out of these 0.1 accuracy class of current transformer is used in the labs for high precision measurement. 0.2, 0.2s, 0.5, 0.5s, 1 are popularly used in the transmission and distribution sector. The 0.2 class CT has an error of 0.2% of the rated secondary current at the rated primary current.

This means if we have a CT rated as 200/1-1-1A, with an accuracy class of 0.2, then it will have a ratio error of 0.75% at 5% of the rated primary current that is 10Amps and only have 0.2% ratio error in the rated primary current or 120% of rated that is 200 or 240 Amps.

Similarly, for station class CTs, the ratio error, however, is less at 5% of the rated primary current. These CTs can also measure at 1% of the rated primary current with standard ratio error explained in the figure.

5A vs 1A secondary rating of CT

The rated secondary current of CT generally used are 1A and 5A. 5Amp Secondary CT is mostly used where the relay or energy meter is very near, mostly in the same panel. Where a certain distance is associated between the CT and the relay/meter, 1Amp rated secondary is chosen.

This is because, during a fault, the primary current will be in Kilo-amps, accordingly the secondary too will follow 5 to 10 times the rated secondary current depending on the ISF of the CT. At this high secondary current, the I²R will become more in the case of a 5Amp secondary-rated CT. It is because of this reason that 1Amp-rated secondary CT are mostly preferred in the HV application/ substations.

S or Special/Station class CT

This is also a standardization of the accuracy class of current transformer. according to the IEC, these types of CT can measure 1% of the rated primary current. S-class CTs are more accurate than their counterparts. They can measure with the rated accuracy from 20-120% of the rated primary current.

For example, a 0.2 class CT ensures 0.2% accuracy from 100-120% of the rated primary current but a 0.2s class can measure with 0.2% accuracy from 20-120% of the rated primary current. 0.2 class CT cannot measure below 5% of the rated primary current, the accuracy of measurement is very poor below 5% but for a station class CT, it can measure 1% of the rated primary current without losing much of its accuracy.

Station class CT is mostly preferred for tariff and revenue metering purposes and other than these the normal class can be taken into use.

Selection of accuracy class of current transformer: