Hybrid DC circuit breakers using gas-discharge tubes for high-voltage switching

摘要

Although DC circuit breakers suitable for interrupting DC fault currents have been proposed as long ago as the 1970s, it is only since the first publication of the concept of a "hybrid" DC circuit breaker in 2011 that DC circuit breaker designs have started to appear with adequately low losses to be economic in a commercial HVDC system. Since 2011, hybrid DC circuit breakers have been proposed by several manufacturers but the commercial up-take of such breakers has been slow, mainly because of their relatively large cost, complexity and footprint. A new development in high-voltage switching devices has the potential to change this picture. Early HVDC schemes used mercury-arc valves (a type of gas-discharge tube) for the AC-DC conversion process. With mercury-arc valves it was possible to construct single switches with voltage ratings of hundreds of kilovolts, and the self-restoring liquid mercury cathode gave the valve a long operating life and robustness to faults that cannot be emulated by semiconductor-based switches. High maintenance requirements eventually led to the replacement of mercury arc valves by thyristor valves in the 1970s. However, since then, great advances have been made in the practical technology of microwave power tubes and x-ray tubes, with the result that a new generation of gas-discharge tubes for HVDC is possible, offering a much more compact solution than either thyristor- or mercury-arc based valves and - crucially - the ability not only to turn on but also turn off current. An ideal first application for such a gas-discharge tube in HVDC is to replace the complex and bulky high-voltage semiconductor switch of the DC circuit breaker by a single gas-discharge tube. Since the DC circuit breaker operates infrequently, the operating life of the cathode material is not a concern and the resulting DC circuit breaker can more compact than today's solution, such that an outdoor, containerised, factory-tested solution becomes feasible.