A Calculation Method of IGBT Insulation Voltages Based on Submodule Stray Capacitors in MMC

In practical modular multilevel converter (MMC) engineering, the dc voltage can be as high as tens to hundreds of kV, while the insulated gate bipolar transistors (IGBTs) isolation voltage is only several kV. The extreme high voltage of MMC can easily destroy the ceramic layers of the IGBT. To assess the IGBT insulation voltages and help design insulation protection measures in the early design stage of MMC, this article proposes a submodule stray capacitor based calculation method for IGBT insulation voltages. First, the switching-states and the stray capacitor model of submodules are introduced. Then, the IGBT insulation voltages are derived in detail, and the voltage characteristics under normal and fault conditions are analyzed. After that, the effect of the heatsink-to-ground capacitance on the maximum IGBT insulation voltages is discussed. The proposed method has a good accuracy with most relative errors less than 5% (calculation versus simulation) and 20% (calculation versus experiment). To protect the insulation safety of IGBTs, this article also gives a design example of the maximum heatsink-to-ground stray capacitance. The proposed method can not only guide the IGBT insulation protection design, but also provide the model reference and the heatsink-to-ground voltage estimation for the EMI researches of MMC.