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Performing in-situ diagnosis is a crucial task during routine inspections of high-power converter systems, such as mine hoist. This paper presents an on-board offline condition assessment method for bond wire degradation of high-power IGBT modules within three-level neutral-point clamped (3L-NPC) converter. This method is based on the increment of the on-state voltage <inline-formula> <tex-math notation="LaTeX">$V_{\mathrm {on}}$ </tex-math></inline-formula> during series short-circuit (SSC). The SSC is formed by the conduction of the IGBT modules to be tested and the external constant current switch (CCS). In CCS circuits, a PI-controlled gate drive circuit is employed to achieve high constant short-circuit current and maintain the stability of the <inline-formula> <tex-math notation="LaTeX">$V_{\mathrm {on}}$ </tex-math></inline-formula>. The improved <inline-formula> <tex-math notation="LaTeX">$V_{\mathrm {on}}$ </tex-math></inline-formula> detection circuit takes the decrement of charging time <inline-formula> <tex-math notation="LaTeX">$t_{\mathrm {cp}}$ </tex-math></inline-formula> of the capacitor as the parameter for aging assessment. It simplifies the data processing of <inline-formula> <tex-math notation="LaTeX">$V_{\mathrm {on}}$ </tex-math></inline-formula>, while increasing the aging sensitivity. To validate the effectiveness of this method in practical applications, a single-phase 3L-NPC experimental platform was built to simulate the aging state of IGBTs by cutting off the bonding wires. The feasibility of the detection circuit, the influence of short-circuit current and ambient temperature on <inline-formula> <tex-math notation="LaTeX">$t_{\mathrm {cp}}$ </tex-math></inline-formula>, and the reliability of the CCS circuit are analyzed in detail.