With large-scale integration of renewable energy

novel power systems feature frequent high-frequency disturbances and complicated electromagnetic behaviors. Insulation of transformers and other power apparatus endures persistent high-frequency electrical stresses

which readily initiates electrical tree degradation and jeopardizes operational safety. We develop an in-situ characterization method for surface and internal insulation damage based on electric-field-induced autofluorescence. Layered fluorescence slices of electrical trees inside epoxy resin are acquired via laser confocal layer scanning

followed by three-dimensional high-precision morphological reconstruction using automatic path tracking and equivalent circular cross-section approximation. With the reconstructed model

experiments are performed under high-frequency voltages to extract global and local branching parameters and their statistical distributions

so as to clarify frequency-dependent evolutionary rules of electrical trees. The results reveal that increasing frequency remarkably facilitates spatial expansion and volumetric growth of electrical trees at the macro scale. Locally

frequency effect presents staged characteristics: long-branch forward propagation dominates from 1 kHz to 3 kHz; branch proliferation becomes predominant within 3~5 kHz; thicker channels and dense multi-branch growth with widened branch diameter distribution occur at 5~10 kHz. This study supplies fundamental data for insulation degradation analysis and lifetime assessment of transformer insulation.