Analysis of Tan Delta Test Results for a 20 kV Incoming Cable of a 30 MVA Transformer at Keramasan Substation

Abstract

The 20 kV incoming cable connected to a power transformer is a critical asset for maintaining the continuity of power delivery. This study evaluates the insulation condition of the 20 kV incoming cable of a 30 MVA transformer at Keramasan Substation using tan delta test data. The data were obtained through offline testing with a B2 VLF unit at 0.1 Hz and test-voltage levels of 0.5 U0, 1.0 U0, and 1.5 U0. The analysis considers three indicators: the mean tan delta at U0, measurement stability, and the voltage-dependent increase in tan delta. The tan delta values at U0 for phases A, B, and C were 3.39 × 10⁻3, 3.93 × 10⁻3, and 2.75 × 10⁻3, respectively. All phases remained within the acceptable category. However, phase B recorded a delta tan delta of 4.84 × 10⁻3, the value closest to the 5.0 × 10⁻3 threshold. The cable remains suitable for service, but phase B should be prioritized for retesting, termination inspection, and partial discharge confirmation. These findings support the application of phase-specific, condition-based maintenance intervals.