Development of a new range of e.h.v. heavy-duty airblast circuit breakers

The paper outlines the design and development of a new range of heavy-duty high-pressure airblast circuit breakers. The interdependence of the design concept, the specifications, the testing-station capability, and testing techniques are examined. A description of the construction of the circuit breaker is given and the operating characteristics are discussed. Direct coupling of the contacts to the operating mechanism allows faster operating times to be achieved. The positioning of the blast-valves, relative to the nozzles, allows a relatively small volume of air to be discharged, and this improves the effectiveness of the integral multistage silencers which reduce the noise level to acceptable limits. The decision to operate at 6 MPa [900 lb/in2(g)] enables full advantage to be taken of the increased testingstation output which results from the establishment of synthetic-testing techniques. Type-test specifications and the type-test programme are considered, with particular attention to the short-circuit development work. The synthetic test circuits which introduced a power-frequency recovery voltage, as well as the necessary transient recovery conditions are illustrated. The value of this technique was shown by the discovery of a late-breakdown phenomenon, which would not necessarily have been apparent had the simpler d.c. synthetic testing techniques been used. The characteristics of the ancillary components and their effects on the performance are also discussed. Aspects of the insulating properties of high-pressure compressed air are discussed, together with the dielectric characteristics of the solid insulation.