Verification of Transformer Restricted Earth Fault Protection by using the Monte Carlo Method

Abstract

The results of a comprehensive investigation of the influence of current transformer (CT) saturation on restricted earth fault (REF) protection during power transformer magnetization inrush are presented. Since the inrush current during switch-on of unloaded power transformer is stochastic, its values are obtained by: (i) laboratory measurements and (ii) calculations based on the input data obtained by the Monte Carlo (MC) simulation. To make a detailed assessment of the current transformer performance the uncertain input data for the CT model were obtained by applying the MC method. In this way, different levels of remanent flux in CT core are taken into consideration. By the generated CT secondary currents, the algorithm for REF protection based on phase comparison in time domain is tested. On the basis of the obtained results, a method of adjustment of the triggering threshold in order to ensure safe operation during transients, and thereby improve the algorithm security, has been proposed. The obtained results indicate that power transformer REF protection would be enhanced by using the proposed adjustment of triggering threshold in the algorithm which is based on phase comparison in time domain.

Keywords

• power transformer
• power system protection
• transformer earth fault protection
• current transformer saturation
• measurements
• Monte Carlo method