Evaluation of overshoot rate of lightning impulse withstand voltage test waveform based on new base curve fitting methods - study on overshoot waveform in an actual test circuit
In a lightning impulse withstand voltage test for large-sized electric power equipment, voltage waveforms sometimes contain overshoot due to the capacitance of the test equipment and the residual inductance of the test circuit including the power source. The authors have been studying fitting methods for extracting a reasonable base curve from this overshoot waveform. In the previous study, each of the fitting methods was applied to the overshoot waveform simulated using a mathematical equation to evaluate the respective merits and demerits. As the test waveform, the present paper used the overshoot waveform actually generated using a large-scale test circuit. This actual waveform was determined as one over which an oscillation waveform of several hundred kHz, which was actually at issue, was superimposed. The correct base curve of the actual waveform was derived through detailed analysis of the equivalent circuit of the test circuit and the overshoot rate was calculated based on this correct value. This correct overshoot rate was compared with that calculated using each of the fitting methods to evaluate which was an optimum fitting method. Consequently, it was found that an overshoot rate close to the correct value could be derived using the new base curve extraction method for the actual waveform. Meanwhile, it should be noted that the kfactor filtering scheme was implemented after the application of various fitting methods, and then the shape parameters of the test voltage waveforms, such as the crest value, wavefront duration, and wavetail duration, were almost identical, which confirmed that fitting methods had only a minor influence on the test voltage waveform.
K-factor value and front time related characteristics of UHV-class air insulation for positive polarity lightning impulse test
The authors are conducting a study on the lightning impulse voltage test standard for UHV-class equipment, including the application of the evaluation method using the Kfactor (test voltage) function. Previously, the actual measurement results of K-factor values assuming UHV-class power transformers and UHV-class gas insulated switchgear were reported. The present paper reports the evaluation results of the influence of LI voltage test waveforms on the breakdown characteristics in the largest possible bushing model, assuming actual UHV-class air insulation. According to the test of the breakdown voltage characteristics with respect to the overshoot rate, it was indicated that the 50% breakdown voltage tended to rise with increasing overshoot. Subsequently, the evaluation using the K-factor function is considered useful for UHV-class air insulation. The evaluation result of the K-factor value based on the experimental results was lower in value than the present K-factor function and the result of the European Project, which is considered attributable to the significant formative time lag of a discharge in the long air insulation distance. Consequently, the K-factor function might be reviewed downward in these cases. Furthermore, according to the experimental results of the breakdown characteristics, when the front time is extended, its extension up to about 3.6 μs might possibly be allowed under a standard assuming UHV-class air insulation.
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.