I was looking at HVDC systems out of curiosity and I have a question that I have not exactly found an answer for.
In HVDC converter, because of the high voltage (Tens/Hundreds of kV) they have to use so-called IGBT valves, which is multiple devices in series in order to increase the voltage blocking capability. My question is why is there a limit to how many devices can be used in series? Is it because the conduction losses are too great? Is this also why they can't use let's say 10 MOSFETs in series instead of a big IGBT?
I would also like to know how the parallelism is affected in this case. I know that for MOSFETs this isn't a big problem, as they conduct less at a higher temperature, thus no positive feedback leading to the destruction of the device. I'm not necessarily looking for a straight answer, I am happy to research this myself if I am pointed in the right direction.