{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,17]],"date-time":"2026-08-17T08:27:07Z","timestamp":1786955227099,"version":"build-2736575974"},"reference-count":24,"publisher":"MDPI AG","issue":"16","license":[{"start":{"date-parts":[[2026,8,15]],"date-time":"2026-08-15T00:00:00Z","timestamp":1786752000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Polymers"],"abstract":"<jats:p>Polyimide (PI) film is a widely used insulation material in high-frequency power equipment, yet its frequency\u2013lifetime relation and terminal discharge features have rarely been resolved together over a complete ageing trajectory. This paper reports three contributions. First, a condition-specific frequency\u2013lifetime relation for PI surface insulation is presented, in which the discharge inception voltage is frequency-invariant while the flashover voltage and the surface lifetime both fall with frequency. Second, a stage-resolved discharge morphology derived from phase-resolved patterns is proposed, in which a late-stage finger-like cluster serves as a candidate flashover precursor, complemented by a count-aware, time-normalised discharge activity proxy. Third, a space-charge interpretation linking the per-cycle charge injection budget growth at high frequency to the suppressed inter-cycle dissipation, jointly accounting for the lifetime relation and the non-dendritic damage morphology, is presented. The fitted lifetime relation provides a quantitative planning basis over the tested 10\u201340 kHz grid for insulation design of high-frequency power equipment, and the finger signature provides a candidate device-level morphological criterion for online monitoring.<\/jats:p>","DOI":"10.3390\/polym18161987","type":"journal-article","created":{"date-parts":[[2026,8,17]],"date-time":"2026-08-17T08:05:41Z","timestamp":1786953941000},"page":"1987","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Frequency-Dependent Surface-Discharge Lifetime and PRPD Failure Signatures of Polyimide Insulation Under High-Frequency Sinusoidal Voltage"],"prefix":"10.3390","volume":"18","author":[{"ORCID":"https:\/\/orcid.org\/0009-0009-9619-7867","authenticated-orcid":false,"given":"Bei","family":"Li","sequence":"first","affiliation":[{"name":"School of Engineering, The University of Edinburgh, Edinburgh EH9 3FB, UK"},{"name":"State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0008-3328-0287","authenticated-orcid":false,"given":"Tianrun","family":"Qi","sequence":"additional","affiliation":[{"name":"State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5049-3980","authenticated-orcid":false,"given":"Qingmin","family":"Li","sequence":"additional","affiliation":[{"name":"State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,8,15]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"186","DOI":"10.1109\/JESTPE.2013.2277917","article-title":"Review of solid-state transformer technologies and their application in power distribution systems","volume":"1","author":"She","year":"2013","journal-title":"IEEE J. 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