Current transformers (CTs) are vital components in power system protection and monitoring. They are responsible for providing a proportional current output to the protection relays, making it possible to detect any fault or abnormality in the power system. However, the accuracy of CTs is critical to ensure the correct operation of protection relays.
CT accuracy is determined by its certified accuracy class which is stamped on the nameplate. eg, CT accuracy class of 0.3 means that CT is
certified to be accurate within 0.3 % of its rated ratio value for a primary current of 100 percent of the rated ratio.
CT with a rated ratio of 200/ 5 with an accuracy class of 0.3 would operate within 0.45 % of its rated ratio value for a primary current of
100 amps i.e. for a primary current of 100A it is certified to produce a secondary current between 2.489 amps and 2.511 amps.
Accuracy is specified as a %age of range and is given for maximum burden as expressed in VA. The total burden includes the input resistance of the meter and the loop resistance of the wire and connections between CT and the meter. Example: Burden = 2.0 VA. Maximum Voltage drop = 2.0 VA / 5 Amps= 0.400 Volts.
Maximum Resistance = Voltage / Current = 04.00 Volts / 5 Amps=0.080 Ohms.
If input resistance of meter is 0.010Ω, then 0.070Ω is allowed for loop resistance of wire, and connections between CT and meter.
It is essential to remember that the accuracy of CTs can be affected by several factors, such as the CT's temperature, the frequency of the input current, and the quality of the primary winding insulation. Therefore, regular calibration and maintenance of CTs are necessary to ensure their accuracy.