Thyristor switches to lower cost power control

The cost of many power control systems for megawatt switching could fall by 30% thanks to technology developed by Swiss-Swedish group ABB. The company claims that its new silicon integrated gate commutated thyristor (IGCT) will provide a more efficient switching solution than existing systems at medium voltage levels between 2.3 and 6.9kV. Possible applications include […]

The cost of many power control systems for megawatt switching could fall by 30% thanks to technology developed by Swiss-Swedish group ABB.

The company claims that its new silicon integrated gate commutated thyristor (IGCT) will provide a more efficient switching solution than existing systems at medium voltage levels between 2.3 and 6.9kV.

Possible applications include rail and marine power supplies, and pump and fan drives for the chemical, oil and power industries.

ABB has created IGCTs by adapting the traditional gate turn-off (GTO) thyristor to deliver some of the competing advantages of the insulated gate bipolar transistor (IGBT) and to overcome the disadvantages of both technologies.

The main savings achieved with IGCTs are through reductions in conduction losses and the total of amount of circuitry used in the final unit.

A GTO consists of thousands of individual switching elements on a silicon wafer. Until now, the standard GTO has been considered comparatively cheap to fabricate, but prices are increased because the thyristors also require snubber circuits which take up more than half of the area of the finished equipment. These circuits counterbalance a GTO’s inability to switch homogeneously, but involve the trade-off of higher conductivity losses.

IGBTs can switch homogeneously, but have higher conduction losses and have not yet been developed for direct medium voltage applications. Instead, a 4.16kV converter requires a series connection of four 1.8kV IGBTs, increasing conductivity losses and reducing reliability.

The IGCT combines the advantages of both: the switching properties of an IGBT with the conductivity and low fabrication costs of a GTO.

With the ICGT ABB has removed the need for snubber circuits in GTOs by using a low inductive housing for gate control.

GTO snubbers are used because of the cathode’s influence on switching in a typical GTO thyristor, during which it causes instability and filamentation – or wear and tear – in the unit.

The IGCT housing designed by ABB allows the cathode to be turned off for a microsecond, converting the thyristor momentarily into a transistor, allowing fast, simultaneous switching and preventing incremental damage to the unit.

Thus, the IGCT can operate like an IGBT but at higher voltage levels.

In addition, a buffer layer allows IGCT switching losses to be reduced by a factor of between 2-2.5 in comparison with a GTO.

ABB says it can also adopt the GTO production template for the new IGCT, driving costs down further.