All geared up to cut cost

A small servo motor and drive is pointing the way to greatly improving the reliability of high voltage switchgear. Moreover, it is bringing reductions in footprint and energy losses at the same time. Circuit breakers have progressed from oil immersion through air blast types and minimum oil types to the current generation, based on contacts opening in SF6 gas inside a silicone rubber insulator. This has massively improved dependability, although there are now concerns because of the greenhouse gas potential of SF6, because its total inertness means it has a very long lifetime once it gets into the atmosphere. The other line of technical development that has improved reliability is progressing from pneumatic operation, through hydraulic and spring operation to the present servo driven mechanism, which operates what is now called a ‘Disconnector Circuit Breaker’, or DCB, because it does away with the need for the separate disconnectors on each side. “The circuit breaker used to be a sensitive creature, so we used to have disconnectors around it,” says Professor Carl Solver, technical marketing manager, ABB power products, and associate professor at Chalmers University in Gothenburg. This meant that each circuit breaker had to have a large mechanical device on each side, usually a large knife switch type device sitting in the open air, so that the breaker could be worked on safely when it required maintenance. According to Solver, the motor in question makes only half a revolution and is 20dB quieter than a conventional mechanical system, with its sudden loud reports, when it operates. Positional feedback is from a resolver on the motor. Power is a nominal 10kW. However, because it only operates briefly, sufficient energy can be derived from a capacitor bank to manage it. In addition to opening and closing the device, the motor drive also provides continuous self-supervision of the circuit breaker. This function is described as ‘Micro motion’ and involves activating the system at regular intervals and moving the contacts by less than 1mm. This test allows a range of diagnostic functions to be undertaken to check all units, cables, mechanisms and the motor. If any errors are detected, they are at once flagged to a central control system, so that the breaker can be serviced. None of this would be possible with any of the older mechanisms for operating circuit breakers. Maintenance is also possible, because once the contacts are in the open position, the device is disconnected from the circuit to which it is attached on both sides. Safety during maintenance is ensured because the locking bar can only be operated – and padlocked into place – when the DCB is in the open position. Similarly, the earthing switch can only be closed when the DCB is locked in the open position – again through electrical interlocking. Energy and resources are saved because typical installations take up 20-70% less space, reducing the size of foundations and enclosure, and also because they replace three sets of contacts – two in the open air and one in a sealed space full of protective gas. As a result, it is estimated that using a DCB in place of a conventional system, saves 1 million kWh or 700 tonnes of CO2 over its lifetime. Pointers * System replaces mechanical/pneumatic/hydraulic operation of circuit breakers, by using a servo motor instead * The servo motor can move the contacts by very small amounts from time to time in order to test everything * Since the system is much more reliable, and disconnects itself on both sides in the open position, it removes the need for conventional disconnectors on each side