In traditional substation equipment, it is the mechanically complex circuit breaker that requires the most maintenance. Disconnectors are therefore installed on either side of the circuit breaker so that it can be disconnected for maintenance.
In recent years, ABB has pioneered significant advances in circuit breaker design, with SF6 replacing air and oil as the interrupting medium, combined with refinements in operating mechanism technology. As a result, ABB circuit breakers are now virtually maintenance-free, with service intervals increased to 15 years. However, because the primary contacts of the conventional disconnectors are still exposed to the environment, it is these devices that now require the most maintenance. The conclusion is that disconnectors now actually decrease substation availability rather than increase it.
ABB decided that, in most cases, circuit breakers no longer require separate disconnectors, and set about designing an alternative solution that would both simplify the substation layout and save space. The result is the disconnecting circuit breaker (DCB), for voltages from 72.5kV to 420kV, in which the breaker’s own contacts, protected in the SF6 breaking chamber, also provide the disconnecting function. The circuit breaker contacts in the open position are also open disconnector contacts, enabling the DCB to replace the conventional combination of circuit breaker and adjacent disconnectors. A reliable interlocking system, in combination with a motorised grounding switch and clear indication of the contact position, provide complete safety for maintenance personnel.
The DCB will enable designers to think along new lines when designing compact substations with simpler layouts for single or double busbars, or other configurations for transmission, primary distribution, distribution and industrial applications. In many cases double busbar systems, used previously to safeguard operation during circuit breaker and disconnector maintenance can now be replaced by single busbars with the same, or even improved, operational availability. Smaller foundations, lower material requirements and a smaller substation footprint – possibly one-quarter of the size of a conventional 145kV substation – also mean less impact on the environment.
IDEAL COMPANION
ABB’s new Motor Drive is the ideal companion to the DCB. It is a digital, directly controlled servomotor system capable of operating circuit breaker contacts with high precision and reliability. Studies by CIGRE have shown that most circuit breaker failures are mechanical in origin.Motor Drive increases availability by eliminating complex mechanical systems and springs, and reducing the number of moving parts in the operating mechanism to just one – the rotating motor shaft.
Motor Drive also provides continuous self-supervision of the circuit breaker. A key element of this process is the uniqueMicro-motion function that activates the system at regular intervals and moves the contacts by a very small amount – less than 1mm. This sub-test enables a range of diagnostic functions to be carried out to check all units, cables, mechanisms and the motor. If any errors are detected, they are flagged immediately to a central control system so that the breaker can be serviced. This approach to selfsupervision ensures maximum availability so that the substation operator can be sure that the DCB will function when needed.