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.
Analysis and calculation of ampacity of 10kV cables in pipes under transient conditions
It is one of the most important that the cable ampacity correctly under transient conditions is defined to guarantee the safe operation of cable. In this paper, the calculation of cable ampacity is based on the method provided by IEC60287 and IEC60853 standard. An ampacity calculation program for screened 10kV XLPE cables, based on the thermal circuit technique, is developed in MATLAB language. By using a technique based on chord secant method, the calculation of ampacity under transient conditions also becomes available. We can better balance between the running speed and the calculation accuracy by this method when if the calculating step is tm/180 (tm is the cut-off time). As the results show, the longer the cut-off time, the smaller the difference between the calculation result and the ampacity under steady condition. In addition, the initial iteration current, the load current will also influence the transient calculation process. Calculation based on secant method of ampacity under transient conditions meet the engineering demands of real-time, accuracy and easy operating to the utmost extent, and greatly facilitated the engineering and technical personnel.
Stroking different body regions of dairy cows: effects on avoidance and approach behavior toward humans.
Understanding perception of dairy cows to common human contact such as stroking is important for improving the human-animal relationship, animal welfare, and production. We hypothesized that repeated stroking of body regions licked most during social grooming, especially the ventral neck, would reduce cows' avoidance of and increase their approach to humans. Sixty tethered dairy cows were randomly allocated to 4 treatment groups that received 5 min of daily human contact 5 d/wk during 3 consecutive weeks: 3 groups were stroked on different body regions. The first group was stroked on the ventral part of the neck (neck); the second group on the withers (both licked often in social grooming); the third group on the lateral side of the chest (chest, licked rarely); and the last group (control) was exposed to simple human presence. The reactions to the person who had provided the treatment were measured using 2 tests in the home tie-stall assessing avoidance from an approaching person who tried to touch the head (approaching person test) and avoidance/approach reactions to a stationary person (stationary person test). Approach behavior was recorded in a novel environment using a standard arena test. In the home tie-stall, cows stroked on the neck showed less avoidance (median avoidance score: 3.33) in the approaching person test compared with cows stroked on the chest and the controls (both: 4.00). That is, at least 75% of the animals stroked on the neck tolerated the touching of their heads (75th percentile <or= 3.75), whereas at least 50% of the cows in the other treatment groups did not accept it. The stationary person test did not reveal any differences between the treatment groups. In the arena test, the 3 stroked groups showed more approach behavior (median latencies to contact: from 145 to 240 s) compared with simple human presence (300 s), but stroking treatments did not differ from each other. Stroking, particularly the neck, reduced avoidance of and increased approach reactions to humans in both the home tie-stall and the arena. Increasing acceptance of being touched after being stroked on the neck suggests that this procedure should be adopted to improve routine handling of dairy cattle.