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Degradation caused by humidity affects the blocking characteristic of an insulated gate bipolar transistor (IGBT) module, leading to an increased leakage current and/or a reduced voltage blocking capability. The proposed measurement system enables the in situ detection of decreased breakdown voltage and increased leakage current. Thus, it can assess the degradation level of power modules when installed in a converter. This test is intended primarily for wind power converters and can be part of the start-up procedure of the system. The working principle has been validated on predamaged power modules arranged in half-bridge configuration, using a demonstrator implementation of the proposed approach in laboratory experiments. Due to the simplicity of the circuit, this concept is easy to integrate in existing inverter systems at a comparatively low cost. [ABSTRACT FROM AUTHOR]