An investigation of the thermal performance of a medium-voltage vacuum circuit-breaker

Higher current-ratings and improved thermal performance are being sought for existing medium-voltage vacuum circuit-breakers in order to meet market needs. At the same time, it is desired to maintain existing frame sizes and to minimize major design changes. Increases in current-ratings have an even greater effect on the ohmic heat dissipation inside the breaker. The thermal design must account for this increase in heat produced, in order for the breaker to meet various temperature-rise limits imposed by industry standards. To speed up the design cycle of these breakers with the higher current-rating, a thermal analysis has been performed to evaluate several design variations. A commercial computational package, ICEPAK, was used for the computations. Results from the computational model were validated against actual temperature-rise measurements obtained from experiments. The analysis showed the necessity for installing heat sinks on the Vacuum Interrupter (VI) stem and the bus extensions, so that the temperature-rise requirement of less than 65°C above ambient at any location on the breaker could be met. Different heat sink designs were separately analyzed prior to their incorporation into the system-level ICEPAK model, and appropriate ones chosen on the basis of thermal performance, cost and ease of manufacture. Design variations on other system components such as conductors, buses, VI, and venting were also analyzed. The predicted results provide a basis for the selection of the components to satisfy system constraints.