Thyristors – The heart of HVDC
Rather than being an endangered species, these semiconductor devices demonstrate how continual development can meet modern market needs.
High voltage direct current (HVDC) technology transmits very large amounts of electricity over very long distances with low energy losses.
The core elements of HVDC transmission are converter stations at either end of the transmission link, where a solid state valve – or thyristor – converts AC and DC current. This thyristor, which is a silicon-based power semiconductor device, is the heart of the converter station, and really the heart of HVDC transmissions, because without them, there are no conversions, and no HVDC transmissions.
Some would argue that thyristors are electronic dinosaurs, an endangered species with just a few years of useful life remaining. But consider that dinosaurs still ruled the earth for millions of years.
Let’s take a look at the details.
The exterior of HVDC thyristors is not so different from components of 20 years ago, but the size has increased considerably. Heavy copper electrodes on the top and bottom cope with enormous electrical currents, and the hermetic seal of the ceramic housing repels any corrosion. Nothing special – so has evolution really stalled for three decades?
At first glance, there is nothing special inside the thyristor either. Hidden between massive molybdenum disks lies the silicon device with a cathode that looks a bit different from older versions, featuring a snowflake-like structure. However, the weight of the active part is a bit surprising: it’s hard to believe that some 40 grams of silicon is sufficient to handle the 20 megawatts (MW) of nominal switching power. The secret is that for all their small size, these rugged devices easily endure prolonged and heavy use.
Caption: 6-inch thyristor with corresponding silicon wafer and cross-sectional structure.
Not so long ago, one of the first commercially viable high-power HVDC lines was serviced in Cahora Bassa, in Mozambique. After about 40 years, new switchgear was installed, new transformers, new controls – new everything, except the thyristor valves. There was not much to service – they just continued to operate. Maybe ants aren’t the only ones working so hard relative to their weight…
Cut more deeply into the device to find real changes: blocking junctions are now structured; towards the edges, they are substantially deeper than in the active part. Cathode patterns are finer, and have an elaborate distribution around the amplifying gate electrode. All of these changes in total add up to a reduction of the on-state voltage drop of up to 300 millivolts. Overall, this new generation of devices offers power savings in the range of 1 kilowatt per device at rated current. That is power savings, not consumption.
Rather than being old-fashioned technology doomed to extinction, thyristors are now being developed to safely convert larger and larger amounts of power. This means that next-generation ultrahigh-voltage direct current (UHVDC) links will be able to deliver even more power through existing transmission corridors.
ABB has just booked the first thyristor orders for a 10-gigawatt HVDC line in China – that is, 10,000,000,000 watts of power in a single installation, with a conversion efficiency beyond 99.6%. This is substantially more than the total energy consumption of Switzerland. Each device has the capacity to conduct 6,250 amperes, and will have blocking capability of 7,200 volts. Under normal operating conditions, a single component will switch more than 20 MW of power.
The end result is increased capacity of new HVDC power systems to feed the hunger for energy in China’s heavily industrialized southern and eastern provinces. Not bad for an apparent technological dinosaur.
- Web page: Thyristors