Analysis of Phase Comparison Application in Power Transformer Protection Against Internal Faults

This paper investigates the application of phase comparison between primary and secondary currents for detecting internal short circuits in power transformers. The directional index, derived from a digital phase comparator, is used as an indicator of the phase shift between these currents. A methodology for internal fault detection is proposed, based on two sequential conditions: an identification phase triggered by an overcurrent condition and a directional index assessment for the fault decision phase. The methodology is tested using MATLAB/Simulink generated signals, where inter-turn faults affecting different percentages of transformer windings are simulated. The proposed approach is compared with conventional differential protection in terms of sensitivity and operating speed. The paper presents the algorithm and the results of the conducted tests. Key words. power transformer, internal fault detection, inter-turn faults, phase comparison, relay protection.