Utility company power plants have large generators typically generating at medium voltages of, for example, 13,800 volts to 34,000 volts and current ratings of 5,000 amps to 30,000 amps. Typically this voltage is stepped up by transformers to much higher voltages in order to transmit the energy over long distances. The connection between the generator and the step-up power transformer is usually made by isolated phase bus duct. The bus duct usually comprises three phase conductors each comprising an inner conductor and a outer conductive housing surrounding, and electrically insulated from, the inner conductor.
A circuit breaker is typically provided in the isolated bus duct to protect the generator by isolating the generator in the event of a short circuit condition or a fault in the step-up transformer. To service the generator, the circuit breaker is opened and then the disconnect switch is opened to isolate the generator.
The disconnect switch utilized to isolate the generator may comprise a telescopic switch. The telescopic disconnect switch also finds application in a pump storage plant as a phase reversal switch. The telescopic switch comprises two fixed or stationary conductors and a hollow moveable cylindrical conductor which resides in a first one of the fixed conductors and telescopes between first and second positions to engage and disengage, respectively, contact fingers on a second one of the fixed conductors. Contact between both of the fixed conductors and the moving cylindrical conductor is made by contact fingers mounted on the circumference of both fixed conductors. The moving conductor has a smaller diameter than the two fixed conductors and moves along the same axis as the fixed conductors to slide inside one of the fixed conductors until it reaches the fully opened position. The contact fingers have two flat contact surface portions. The first flat contact surface portion contacts one of the fixed cylindrical conductors along a point of contact engagement and the flat second contact portion extends beyond the fixed conductor for sliding wiping engagement and contact along another point of contact engagement with the movable cylindrical conductor. The contact points of engagement provide an effective electrical engagement however the points also limit the surface making electrical contact between the telescopic conductor and the fixed conductor.
These existing telescopic disconnect switches with the contact fingers having the flat contact surface portion are able to operate between open and closed positions for cycles of about 500 operations before major maintenance of the telescopic switch is required. There is now a market driven requirement that these telescopic switches operate for more than 500 operations before requiring maintenance servicing. Accordingly, any improvements in the telescopic switch that enhances the number of operating cycles would be advantageous.