Power Systems HVDC - converting from DC to AC

[EEBlackSheep]

I live on an island and the undersea power cable uses HVDC to provide the island with 700MW of electricity.  It is old and obsolete, using Thyristor tubes for rectification, but still in operation.

The conversion to smooth DC seems to use roughly the same technology that we use in smaller systems (rectifier, smoothing reactors or capacitors).  However I cannot seem to find how they convert it back to 3 phase AC.  I looked at a great HVDC article from Siemens and nothing about inverting.  On some sites there is an offhand reference that IGBTs or Thyristors are used to convert to DC (rectification) as weil as converting DC to AC, but no details.

For high voltage DC power systems conversion  to AC is it just like the smaller car inverters using PWM and IGBTs?

thanks

[Inflex]

Surprisingly, Wikipedia is quite useful in this case;  key point being that the HVDC systems are supposedly bidirectional in most cases.

https://en.wikipedia.org/wiki/HVDC_converter

[T3sl4co1l]

Using thyristors or thyratrons (tubes), you use a large series inductor on the DC bus (and usually a resonant circuit to help out), and that allows the inverter to commutate (switch from one set of devices conducting, to the next).  This allows the grid to start up, and allows it to be bidirectional.

IGBTs of course don't need any help to switch (just a pile of drivers), and you can save some cost on reactive components (smaller inductors).  The voltage drop and semiconductor cost is higher, though.

It's most definitely not like those shitty little automotive inverters.

Tim

[Richard Head]

It is old and obsolete, using Thyristor tubes for rectification, but still in operation.
If it's old and still working it's probably a decent system. Most of the older generation equipment is so over-designed that it'll probably last forever.