The work investigates Current Transformers (CTs) transient response and saturation effects on Microprocessor-Based (MPB) relays. The aim is to improve CT transient response and prevent saturation as these leads to relay mal-operation. Magnetic hysteresis loop test setup of conventional CTs core construction using ISA T2000 model and CT Analyzer tests were performed. Design algorithm for proposed Air-gap core CT parameters were presented. Results shows that hysteresis loop of the magnetic circuit is affected by the presence of air gap. Air-gap in the magnetic circuit increase the relucta nce of the core from 30315.43 to 4.54729 .?10 5 [A/Wb] and lowered the inductance of the winding from 1.9 to 0.1357 Henry, consequently the secondary time constants is reduced. Remanent flux in the core reduces from 80% of saturation flux to 10% and increases the useful flux density as a result of its lower magnetizing impedance. Errors in current transformation are thereby significantly reduced when compared with conventional close core CTs. CTs operate linearly under transient response without saturation when a given maximum current flows in the winding. Thus air-gap core CT improved transient response and prevent saturation.
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