Transient phenomena in power transformers theoretical and practical approach
In recent decades the theoretical and experimental study of transient phenomena regarding power transformers has experienced a great development. Results of these studies allowed the elucidation of the main theoretical and practical aspects of the transient phenomena due to propagation of impulse voltages inside the transformers. These studies increased the safe operation of power transformer and led to the introduction of rational insulation usage and also helped to develop efficient protection measures.
Calculation methods for lightning impulse voltage distribution in power transformers
This paper aims to develop and update the methodology for calculating the stress to which the internal insulation of power transformer is subjected during lightning impulse test. Power transformer internal isolation consists of insolating intervals of oil impregnated paper, transformer board and oil. In whole, the transformer's insulation must withstand the voltage testing levels prescribed by the standards. One of the methods for determining the internal stress of various parts of the power transformer's insulation is by using mathematical models.
New Sweet-Tasting C21-Pregnane Glycosides from Pericarps of Myriopteron extensum.
Ten novel C21 pregnane glycosides, extensumside C-L (1-10), were isolated as highly sweet-tasting substances from the edible pericarps of Myriopteron extensum (Wight) K. Schum by sensory-guided fractionation and purification. Their structures were determined through 1D and 2D NMR, such as HSQC, HMBC, 1H-1H COSY, HSQC-TOCSY, and ROESY, as well as other spectroscopic analysis combined with chemical evidence. These compounds shared the same aglycone, 3β,16α-dihydroxy-pregn-5-en-20-one, and contained the deoxysugar chain and the glucose chain which were linked to C-3 and C-16 of the aglycone, respectively. The sweetness potency was evaluated by a human sensory panel test and preliminary structure-taste relationship was discussed. The sweetness intensities of these compounds are between 50 and 400 times greater than that of sucrose. Furthermore, quantitation analyses of compounds 1, 3, 4, and 6 in different parts of M. extensum indicated that the concentrations of these sweet components in the pericarps are obviously higher than those in stems and roots.