The metering class current transformer is used for metering the energy for revenue purposes. Therefore, the metering class current transformer must be as accurate as possible for correct metering of the energy. The metering class current transformer is used for tariff metering application.
As per IEC/AS standard, the metering class current transformer is available in the different accuracy classes. The accuracy of the CT depends on the load, therefore IEC/AS have developed different standards to define accuracy under different load conditions, known as accuracy class.
IEC/AS Standard 62053-11
The standard covers Accuracy Class 0.5, 1.0 & 2 for electromechanical meters for active energy (watt-hours) . The measurement accuracy depends on full load conditions and unity power factor. However, the accuracy lowers under lower load conditions and power factor less than unity along with the presence of harmonics.
IEC/AS Standard 62053-21
The standard covers Accuracy Class 1.0 & 2 for static/electronic meters for active energy (watt-hours). The accuracy depends on the percentage loading of under unity power factor. However the accuracy lowers under lower load conditions and poor power factor less than unity in presence of harmonics.
IEC/AS Standard 62053-22
The standard covers a higher Accuracy Standard of 0.2S and 0.5S for static/electronic for active energy (watt-hours) providing a higher “Accuracy Standard” under full load conditions and unity power factor in addition to better accuracy readings at much lower load current, power factor conditions less than unity along with the presence of harmonics.
Difference between 0.2 and 0.2S class CT
0.2 and 0.2S refers to the accuracy of the Current Transformer. A 0.2S CT has a much higher accuracy than 0.2. Accuracy class of 0.2 means +/- 0.2 % error. But, the declared accuracy is guaranteed only between 100% & 120% rating. And, with some increased error, the CT performance can be guaranteed from 5% loading. Below this load, the error is not guaranteed. It can be anything.0.2S class special CTs guarantee the declared accuracy +/- 0.2%, even with 20% loading. The 0.2S class CT gives declared accuracy from 20 to 100 % rating. And with some definite error 0.2S class CT can be defined even with a load as low as 1%. Thus 0.2S & 0.5S class current transformers are used for tariff metering purposes.
Limit of Error for 0.2 and 0.2S Class CT
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