Response of power cables to fast transient loads
The most accepted standards for ampacity of power cables consider only steady-state loads. In "Cable Sizing for Fast Transient Loads," R.E. Henry, the transient temperature response of cables is addressed when installed in tray and conduits and undergoing fast transient heating. Herein, the fast transient work is extended by presenting how power cables respond to short heavy loads. Fast heating is usually found with variable frequency drives that deliver varying amounts of occasional non-periodic high torque, or with rectifiers that deliver occasional heavy currents to equipment, like melting of scrap in an arc furnace. Transient temperature rises are calculated for four sizes of cables subjected to spikes of kiloampere currents of short durations. These peak temperature rises are tabulated in a summary. With the rms value of a transient current profile and the NEC ampacity tables, this tabulation provides the means for a rapid cable size comparison, and cable size selection. The linearity and proportional responses of these peaks rises show that they are independent of how the cables are installed. Hence, this method of sizing feeders for fast transient loads applies to all installations of cables covered in the NEC ampacity tables.
Fast Varying Loads
The operation of Fast Varying Loads (FVLs) in distribution systems is associated with high variation in the active power and bursts of distortion. FVLs affect surrounding loads sensitive to voltage rms variation and distortion. Thus they affect quality of the supply. This paper presents an overview of some electric loads that behave as FVLs. FVLs are characterized in this paper in terms of the main two properties that distinguish them from other electrical loads, namely, the active power variation and the contribution to the distortion. When a FVL is the major load in a distribution system, the power variation and the burst of waveform distortion could reach unacceptable levels. Thus, compensation could be necessary. The requirements for this compensation are also presented in this paper.
Laminar heat or mass transfer in rectangular channels and in cylindrical tubes for fully developed flow: comparison of solutions obtained for various boundary conditions
Abstract A comparison is made of results obtained with various boundary conditions at the wall of the channels. By comparing blood Sherwood numbers for finite wall resistance with those for constant concentration boundary condition only a small variation is found for various points along the blood channel.