Space Charge Accumulation Characteristics in HVDC Cable under Temperature Gradient

One of the main issues that affect the development of high-voltage direct-current (HVDC) cable insulation is the accumulation of space charge. The load operation of an HVDC cable leads to the formation of a radially distributed temperature gradient (TG) across the insulation. In this study, the space charge accumulation in a cross-linked polyethylene (XLPE) cable is measured under a DC electric field and TG using the pulsed electro-acoustic (PEA) method, and the effect of the TG on the space charge behavior is investigated. In addition, the bipolar charge transport (BCT) model and the conductivity model based on an improved cylindrical geometry are used to simulate the charge behavior in the HVDC XLPE cable under TG, and the experimental and simulated results are compared. The results show that the higher temperature of the cable conductor promotes the accumulation of homocharge near the side of high temperature. Additionally, with the increase of the TG, not only does more heterocharge accumulates adjacent to the side of low temperature, but more space charge also extends into the bulk of the cable insulation. More attention should be paid to the conductor shield layer and the insulation shield layer in HVDC cables. Moreover, the BCT model can more accurately describe the experimental results than the conductivity model

Estimation of Horton infiltration equation parameters and field-averaged roughness coefficient by surface irrigation advance

Based on the analyses of water stream and irrigation advance, two functions were given to describe irrigation advance and water depth profile. A method for estimation of Manning roughness coefficient was developed through adopting Manning equation for water flow. Combining with Horton infiltration equation, the solution for estimation of infiltration properties was established based on the water volume balance. The method was evaluated by the field experimental data, and the results indicate that the predicted irrigation advance and recession of SRFR4.06 with the parameters obtained by the new method go well with the observed

Sediment and solute transport on soil slope under simultaneous influence of rainfall impact and scouring flow

Soil erosion and nutrient losses with surface runoff in the loess plateau in China cause severe soil quality degradation and water pollution. It is driven by both rainfall impact and runoff flow that usually take place simultaneously during a rainfall event. However, the interactive effect of these two processes on soil erosion has received limited attention. The objectives of this study were to better understand the mechanism of soil erosion, solute transport in runoff, and hydraulic characteristics of flow under the simultaneous influence of rainfall and shallow clear-water flow scouring. Laboratory flume experiments with three rainfall intensities (0, 60, and 120 mm h(-1)) and four scouring inflow rates (10, 20, 30, and 40 l min(-1)) were conducted to evaluate their interactive effect on runoff. Results indicate that both rainfall intensity and scouring inflow rate play important roles on runoff formation, soil erosion, and solute transport in the surface runoff. A rainfall splash and water scouring interactive effect on the transport of sediment and solute in runoff were observed at the rainfall intensity of 60 mm h(-1) and scouring inflow rates of 20 l min(-1). Cumulative sediment mass loss (Ms) was found to be a linear function of cumulative runoff volume (Wr) for each treatment. Solute transport was also affected by both rainfall intensity and scouring inflow rate, and the decrease in bromide concentration in the runoff with time fitted to a power function well. Reynolds number (Re) was a key hydraulic parameter to determine erodability on loess slopes. The Darcy-Weisbach friction coefficients (f) decreased with the Reynolds numbers (Re), and the average soil and water loss rate (M(1)) increased with the Reynolds numbers (Re) on loess slope for both scenarios with or without rainfall impact. Copyright (C) 2010 John Wiley & Sons, Ltd

Effect of surface stone cover on sediment and solute transport on the slope of fallow land in the semi-arid loess region of northwestern China

In the semi-arid loess region of northwestern China, use of stone and gravel as mulch has been an indigenous farming technique for improving crop production for over 300 years. However, systematic studies on the effects of stone covers on soil and water conservation have been rarely conducted, except for a few investigations and documentations on the stone cover effects on erosion and solute transport in such a highly erodible loess region. Materials and methods We experimentally examined the effects of surface stone cover on sediment erosion and solute transport using the water-scouring method on sloping land in a semi-arid region in China, which had been left fallow with alfalfa (Medicago sativa) for 3 years. All covered stones rested on the soil surface, and none were partly or completely embedded in the soil surface layer. Stone cover percentages were classified into three groups: 0% (no stone cover, the control treatment), 5.1%, and 20.8%. Two sizes of stones, SCA (7.6 x 7.6 cm) and SCB (18.4 x 18.4 cm), were used in the treatment of 5.1% stone cover. A dye method was used to measure flow velocities in the experiments. Each stone treatment had one replicate. Results and discussion The surface cover by stones influenced soil erosion processes, runoff generation, and solute transport. Runoff rate and sediment yield decreased as stone cover percentages increased from zero (no stone cover) to 20.8%. The effect of stone sizes on the runoff was not significant, whereas stone size type SCA caused lower sediment yield than SCB at the same stone cover percentage of 5.1%. Likewise, water flow velocity and the Froude numbers also decreased with increasing stone cover percentage. The Manning roughness increased with increasing stone cover percentage, ranging from 0.0296 to 0.0579 m(-1/3) s. But the Reynolds numbers among different stone cover percentages and sizes remained nearly the same with a small variation from 483 to 486. Conclusions The study implied that stone cover percentage and size have important influences on sediment and solute concentration in runoff. Surface-covering stones reduced the velocity of runoff, increased surface roughness, decreased sediment yield in runoff, and consequently reduced the quantities of solute release from soil surface