T-type three-level power converters find extensive application in various fields, including microgrids, low-voltage motor drives, and renewable energy conversion systems, etc. Swift and precise fault diagnosis is vital for enhancing the reliability of T-type three-level converters in afore-mentioned applications. At the outset, we create a rapid and straightforward open-circuit diagnosis technique based on Tellegen’s Theorem. We first introduce the "quasi-power" concept, which solely depends on switch positions and statuses, ensuring robustness to parameter changes and operating point variations. Furthermore, we offer a simple and effective open-circuit fault diagnosis method based on "quasi-power" and current direction, enabling accurate identification of IGBT failures within one sampling period. Finally, HiL data confirms the effectiveness of the proposal.