What limits the number of MOSFETs/IGBTs in Series / Parallel
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?


My (limited) knowledge is in the use of thyristors for use in giga watt DC links as supplied by GE, Siemens, ABB etc. I'm not expert but I do have some knowledge. So my answer is about using thyristors but, the same would apply with MOSFETs and IGBTs (except those devices need series diodes when handling the incoming AC). 

Let's take an example of the Infineon T2871N80TOH. It is rated at maximum voltages of 8 kV and max current of about 5000 amps. When conducting it has a forward volt drop of about 3 volts at 5000 amps. This means a power dissipation of 4.5 kW and, if (say) 100 are used in series to achieve a peak handling voltage of 800 kV then the power lost is about 0.5 MW.

Compare that with the throughput power of such a converter. Let's say it controls an RMS voltage of 400,000 volts and average current (say) 2000 amps RMS. That is a load power of 800 MW.

So does wasting 0.5 MW look such a bad deal when controlling a load of 800 MW? Does it look like there is some problem with having more series thyristors if the required HVDC link voltage needed to be (say) doubled? It doesn't to me.