An ON-State Voltage Calculation Scheme of MMC Submodule IGBT

The degradation of submodule (SM) insulated gate bipolar transistors (IGBTs) in a modular multilevel converter (MMC) can be monitored through their on-state voltages. This paper proposes a scheme to calculate the IGBT on-state voltages at rated current and normal junction temperature. The equations of on-state resistances are established based on Kirchhoff voltage laws (KVL) in one MMC arm for positive and negative current directions separately. The arm current, arm voltage, capacitor voltages, gate signals, and junction temperatures are utilized in the equations, and most of the signals are known for the control system. The dimension of the equation set is reduced to make it solvable based on an assumption that all the on-state resistances of diodes remain equal. In addition, the Kalman filter (KF) is used to improve the stability and accuracy of the calculation results. The calculated results are converted to the normal condition based on the current-voltage (I-V) and resistance-temperature (R-T) characteristics of SM IGBTs and diodes. The scheme is verified by using the simulation of a five-level three-phase MMC model. The errors of the calculated VCE are within 0.3%, and the calculation accuracy is good enough for the estimation of IGBT degradation. Furthermore, the influences of the measurement accuracy and the equal diode voltage assumption on the calculation accuracy are analyzed. The proposed scheme is expected to be applicable in online condition monitoring of SM IGBTs.