STEADY-STATE AND TRANSIENT AMPACITY OF BUS BAR

This paper presents a thermal model that can be used to calculate both the steady-state and transient electrical current carrying capacity ( ampacity) of bus bar. Using a thermal model, a computer program has be en formulated that can calculate the ampacity of bus bar for any time- varying current and any variation in environmental conditions. Relatio ns for the convection heat transfer coefficient for bus bar with circu lar, rectangular and angular cross-sections are presented. The steady state values for ampacity predicted by the program are compared to acc epted values. The transient capabilities of the program are illustrate d by calculating the variation in bus bar temperature when it is subje cted to a step change in current. The predicted temperature histories of typical bus designs are compared to temperatures measured in the la boratory using thermocouples attached to the bus material. The tempera ture data is also used to estimate thermal time constants for common b us cross-sections. An analytical expression for the time constant of t he bus is derived and values determined from this analytical expressio n are shown to be close to measured time constants. Analytically predi cted and experimentally measured values both show that typical bus bar designs have thermal time constants between 15 and 35 minutes. The co ncept of a time constant is very valuable, because it can be used in a simple temperature-time expression to estimate the thermal behavior o f bus material without resorting to a complex mathematical model which requires the solution of a non-linear differential equation.