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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.

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    \$\begingroup\$ Be very aware that MOSFETs only have a nice temperature cofficient when operated close to the triode region. In the saturation region thay can have a self-destructive temperature coefficient. \$\endgroup\$ – Andy aka Dec 11 '18 at 11:40
  • \$\begingroup\$ @Andy JPL found the older MOSFETs were safe. But newer styles are not (this switchover was about 1990) \$\endgroup\$ – analogsystemsrf Dec 11 '18 at 15:20
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    \$\begingroup\$ @analogsystemsrf Aye, and here's the JPL report aka the spirito effect. \$\endgroup\$ – Andy aka Dec 11 '18 at 16:11
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HVDC design engineer by trade here. You don't want to use MOSFETs for the simple reason of cost (chip area) per A of current and since MLC concept was published, everyone is switching at low speed. Paralleling both IGBTs and MOSFETs becomes a challange at kA levels due to parasitics and fault cases. Series connecting is a major challange. Don't do it unless you absolutley have to and be prepared for several issues with high losses in snubbers, avalanche and so on.

My question is why is there a limit to how many devices can be used in series?

Losses in snubbers, timeing requirement on your gate drive, avalanche capability or ns response time in local gate drives with collector voltage slope regulation.

I would also like to know how the parallelism is affected in this case.

Your simple small MOSFET SMPS on your bench will usually work fine with parallelled MOSFETs as long as you have individual gate resistors. Moving up into kV and kA territory, parasitics will have far more to say and all work against you. Matched chips from the same wafer is something you won't be able to do at home and the kV of withstand will make sure you have a long way to go from your gate driver to each chip too, so you are in for a challange.

Bottom line: Don't series nor parallel any transistors unless you absolutley have to!

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  • \$\begingroup\$ My question now is, how do these MMC converters that operate at 330kV work, when IGBT are rated at 6.5kV, aren't they connected in series? \$\endgroup\$ – Cezar Chirila Dec 11 '18 at 19:59
  • \$\begingroup\$ No, they are capacitor clamped at each cell level and the cells are in turn connected in series. This very reason alone is what pushed VSC technology up in voltage level while decreasing losses at the cost of complexity but DSP cost << IGBT cost. \$\endgroup\$ – winny Dec 11 '18 at 20:09
  • \$\begingroup\$ Got it now! I was confused, but MMC sub-module in series is not the same thing as simple IGBT series connection. Thank you! Also I now understand why MOSFETs are not used, because of MMC, they do not have to commute fast, as the drive signal for each MMC signal is at the same frequency (50/60Hz) but just phase shifted. \$\endgroup\$ – Cezar Chirila Dec 11 '18 at 20:19
  • \$\begingroup\$ Almost yes, it’s not hard synced to the grid. You need a pulse number above 1, but not by much. If you feel this answered your question, feel free to accept my answer by using the green arrow. \$\endgroup\$ – winny Dec 11 '18 at 21:28
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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.

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  • \$\begingroup\$ Thank you for your answer. I selected winny s answer because his also had the answer to the extra question, but your answer helped me too. Thank you! \$\endgroup\$ – Cezar Chirila Dec 11 '18 at 22:52

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