Updated: Aug 3
The different class of current transformers are used for various applications. According to applications of current transformers, CTs are categorized into three categories- Metering or revenue application,protection application and special protection application. It is desired that the metering class current transformer must accurately read for revenue calculation.
IEC/AS Standard 62053-11
This standard is applicable for the electromechnical type energy meters. The substantial error at lower loading and power factor less than unity may affect the accuracy of the meter. However, nowadays this technology of energy measurement through electromechanical is almost obsolete.
IEC/AS Standard 62053-21
This standard is applicable for 1 & 2 class static and electronic energy meters. The standard specifies the permissible error at different loading and power factor. The accuracy deteriorates at lower loading and poor power factor. The electronic loads connected to supply source introduces harmonics in the system, and the measuring accuracy further deteriorates if measurement is done in harmonic rich power supply system.
IEC/AS Standard 62053-22
This standard covers the accuracy of the microprocessor based meter/electronic meter of availability class tariff(ABT class) class where much higher accuracy class meters of 0.5S and 0.2S class meters are used, These class of meter gives accurate readings even at lower loading and poor power factor.These meters are designed to operate in harmonic rich supply network.
Difference between 0.2 and 0.2S class CT
0.2 and 0.2S are both indicates the accuracy of current transformer. However, the 0.2S class CT is more accurate than 0.2 class current transformer. 0.2 S class CT gives +/- 0.2 % error when loading on the CT is between 100 to 120 %. However, at lower loading the error in CT measurement is not guaranteed +/-0.2%, CT error is much more at 5,20,50,80 % loading. If the CT is operated below 5 % loading the error may be anything.
In view of more accuracy in measurement,0.2S class CT is used for revenue metering. Now, utility companies uses 0.2S class current transformer for revenue metering. The 0.2S class CT is much more accurate at lower loading than 0.2 class CT. The gauranteed error of 0.2 S class CT from 20 to 100% loading is +/- 0.2%. At 5 % loading CT error is +/-0.75 %. The CT can read up to 1 % loading with some increased error. Thus 0.2S class CT can be used from 1 to 120 % loading for energy measurement.
Limit of Error for 0.2 and 0.2S Class CT