During refurbishment, AC/DC converters or rectifier transformers undergo insulation resistance tests, polarization index, winding resistance, TTR, and often dielectric tests (applied and induced voltage). Oil-filled units get DGA and oil breakdown tests. Special attention is paid to DC stress patterns on insulation and rectifier connections. Visual inspection checks for thermal damage, loose connections, and contamination. Based on results, insulation drying, re-gasketing, and component replacement may be performed before final high-potential and functional tests.