Accurate Condition Monitoring of Semiconductor Devices in Cascaded H-Bridge Modular Multilevel Converters

This article presents an online condition monitoring (CM) scheme for semiconductors used in modular multilevel converters (MMCs) that comprise cascaded H-bridge submodules. The CM algorithm is based on detecting changes in the on-state resistance of the semiconductors over time. The proposed method is shown to successfully perform a curve tracing of semiconductors in MMCs while the semiconductor junction remains at a temperature that is readily measurable and undergoes minute changes during the measurement process. The on-state resistance value is estimated from the measured on-state voltage drop of the semiconductors and the measured arm current. Measuring the on-state resistance at known temperatures allows for separating temperature-dependent variations of the on-state resistance from age-dependent variations of this parameter. Suitable methods for reducing the effect of noise on the curve-traced data are proposed, and a recursive least square estimator is used to extract the optimum on-state resistance from the traced vCE-iC curve. Simulation results show that the proposed scheme can accurately determine the on-state resistance of semiconductors at a known temperature and under various levels of measurement noise. Moreover, experimental results on a low-power prototype show that the proposed scheme is applicable in practice, and provides similar online curves to what a commercial curve tracer can produce offline. The experimental verification has been conducted under constant load conditions; however, the proposed methods can be used under any variable load condition as well.