What factors affect the CT’s accuracy

- Mar 30, 2019-

The accuracy of a CT is affected by a number of factors including:

· Burden

· Burden class/saturation class

· Rating factor

· Load

· External electromagnetic fields

· Temperature

· Physical configuration

· The selected tap, for multi-ratio CTs

· Phase change

· Capacitive coupling between primary and secondary

· Resistance of primary and secondary

· Core magnetizing current

Accuracy classes for various types of measurement and at standard loads in the secondary circuit (burdens) are defined in IEC 61869-1 as classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1 and 3. The class designation is an approximate measure of the CT's accuracy. The ratio (primary to secondary current) error of a Class 1 CT is 1% at rated current; the ratio error of a Class 0.5 CT is 0.5% or less. Errors in phase are also important, especially in power measuring circuits. Each class has an allowable maximum phase error for a specified load impedance. [1]

Current transformers used for protective relaying also have accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays during system faults. A CT with a rating of 2.5L400 specifies with an output from its secondary winding of twenty times its rated secondary current (usually 5 A × 20 = 100 A) and 400 V (IZ drop) its output accuracy will be within 2.5 percent.www.ctsensorducer.com