Search

The Office of Hydrologic Development (OHD) of the U.S. National Oceanic and Atmospheric Administration's (NOAA) National Weather Service (NWS) conducted the two phases of the Distributed Model Intercomparison Project (DMIP) as cost-effective studies to guide the transition to spatially distributed hydrologic modeling for operational forecasting at NWS River Forecast Centers (RFCs). Phase 2 of the Distributed Model Intercomparison Project (DMIP 2) was formulated primarily as a mechanism to help guide the U.S. NWS as it expands its use of spatially distributed watershed models for operational river, flash flood, and water resources forecasting. The overall purpose of DMIP 2 was to test many distributed models forced by high quality operational data with a view towards meeting NWS operational forecasting needs. At the same time, DMIP 2 was formulated as an experiment that could be leveraged by the broader scientific community as a platform for the testing, evaluation, and improvement of distributed models.DMIP 2 contained experiments in two regions: in the DMIP 1 Oklahoma basins, and second, in two basins in the Sierra Nevada Mountains in the western USA. This paper presents the overview and results of the DMIP 2 experiments conducted for the two Sierra Nevada basins. Simulations from five independent groups from France, Italy, Spain and the USA were analyzed. Experiments included comparison of lumped and distributed model streamflow simulations generated with uncalibrated and calibrated parameters, and simulations of snow water equivalent (SWE) at interior locations. As in other phases of DMIP, the participant simulations were evaluated against observed hourly streamflow and SWE data and compared with simulations provided by the NWS operational lumped model. A wide range of statistical measures are used to evaluate model performance on a run-period and event basis. Differences between uncalibrated and calibrated model simulations are assessed.Results indicate that in the two study basins, no single model performed best in all cases. In addition, no distributed model was able to consistently outperform the lumped model benchmark. However, one or more distributed models were able to outperform the lumped model benchmark in many of the analyses. Several calibrated distributed models achieved higher correlation and lower bias than the calibrated lumped benchmark in the calibration, validation, and combined periods. Evaluating a number of specific precipitation-runoff events, one calibrated distributed model was able to perform at a level equal to or better than the calibrated lumped model benchmark in terms of event-averaged peak and runoff volume error. However, three distributed models were able to provide improved peak timing compared to the lumped benchmark. Taken together, calibrated distributed models provided specific improvements over the lumped benchmark in 24% of the model-basin pairs for peak flow, 12% of the model-basin pairs for event runoff volume, and 41% of the model-basin pairs for peak timing. Model calibration improved the performance statistics of nearly all models (lumped and distributed). Analysis of several precipitation/runoff events indicates that distributed models may more accurately model the dynamics of the rain/snow line (and resulting hydrologic conditions) compared to the lumped benchmark model. Analysis of SWE simulations shows that better results were achieved at higher elevation observation sites.Although the performance of distributed models was mixed compared to the lumped benchmark, all calibrated models performed well compared to results in the DMIP 2 Oklahoma basins in terms of run period correlation and %Bias, and event-averaged peak and runoff error. This finding is noteworthy considering that these Sierra Nevada basins have complications such as orographically-enhanced precipitation, snow accumulation and melt, rain on snow events, and highly variable topography. Looking at these findings and those from the previous DMIP experiments, it is clear that at this point in their evolution, distributed models have the potential to provide valuable information on specific flood events that could complement lumped model simulations. © 2013.

The partial discharge (PD) pulse burst characteristics of a transformer type synthetic organic ester fluid were compared with those of an equivalent transformer type mineral oil. Wide and narrow bandwidth measurements were carried out to determine simultaneously the apparent charge transfers associated with the overall PD pulse burst as well as that of the individual discrete pulses within the PD pulse burst. The PD pulse burst behavior was found to be similar in both insulating liquids. The discharge cavity formation times ranged from 0.5 to 1.5 [micro]s, while the charge transfers associated with the PD pulse bursts in the liquids were found to extend from approximately 5 to 30 pC. Index Terms-Partial discharges, mineral oil, synthetic ester.

An identification technique is described, based on a developed adaptive fuzzy logic network, that enables the recognition of partial discharges (PD) generated by different defects in heat-shrinkable joints and terminations of XLPE insulated power distribution cables. It is shown that different sources of PD can be identified on the basis of fuzzy rules applied to a selection of parameters derived from PD-pulse phase and amplitude distributions. A comparison with other PD pattern recognition techniques based on traditional neural networks is presented and discussed. Index Terms--Joints, neural networks, terminations, XLPE.

Discrete partial discharge (PD) pulse occurrence times within the PD pulse bursts in transformer oils were found to extend from approximately 0.42 to 2.9 [micro]s, with the discrete pulse separation times evincing a weak dependence on the inverse of the oil viscosity. Many PD pulse burst patterns exhibited substantial deviation from the classical behavior, which is normally characterized by successive discrete pulses of ascending amplitude with well defined increases in the pulse separation times. The average elapsed time from the incipient formation for the cavity to the onset of the first partial discharge event extended downwards from 0.62 to 0.42 [micro]s with increasing oil viscosity.

Partial discharge (PD) diagnostics are utilised to examine the aging process of mineral oil-impregnated-high density kraft paper capacitor bushing models, subjected to repetitive voltage switching surges. It is demonstrated that the PD measurements of cumulative apparent charge transfer and discharge inception stress constitute an effective means for assessing aging and its reversibility due to gas evolution and re-absorption or solubility of a mineral oil-impregnated-paper insulation system in the presence and absence of switching surges. While the experimental results indicate that electrically overstressed solid-liquid insulation can recover in part its initial electrical properties following an electrical stress-free period, long term stability is only effectively achieved by a reduction in the switching surge overstress through an increase in the overall insulation thickness. The electrical stress levels, employed in the experiments, are found to be not sufficiently elevated to lead to any detectable partial discharge formation within the oil phase itself; the observed discharge behavior is rather characteristic of the type that is normally associated with macroscopic cavities occluded within the oil-paper insulation systems.

The calculation of voltages induced by indirect lightning on multiconductor lines has been the subject of several studies. The reported conclusions are not always in agreement with each other. In this paper, using a modeling procedure presented in a previous work, we study the shielding effect due to mutual coupling among the conductors of a three-phase line and between the ground wires and the line conductors. The results are compared with those published by other authors, and explanations of the disagreements are given. Additionally, a simplified method to evaluate lightning-induced voltages on a multiconductor line from the values obtained for the single conductor case is presented, and a simple formula which gives the magnitude reduction of the induced voltages due to the presence of other conductors is derived. It is shown that, for the examined case, the derived simple formula gives practically the same results as those obtained using the rigorous procedure. We have also compared results obtained using the simplified formula proposed by Rusck to evaluate the protective ratio of a ground wire with our results, and we have found that the values predicted by the Rusck formula are about 6% lower than ours.

The two coupling models most frequently used in the power-lightning literature for the calculation of lightning-induced overvoltages, namely the model by Chowdhuri and Gross [1] and the model by Agrawal, Price and Gurbaxani [2], are compared and discussed. It is shown that the Chowdhuri-Gross model is incomplete: the source term due to the contribution of the magnetic induction is missing. It is investigated how much the lack of the source term affects the accuracy of the calculated overvoltages. Experimental tests of the two models using a short line illuminated by a NEMP simulator support the theoretical conclusion.

It is a well known fact that the breakdown voltage of an insulating liquid increases nonlinearly with increasing electrode gap. Under nonuniform field dc conditions two breakdown voltages are determined depending on the electrode polarity. Using point-plane geometries, with gaps of 5 mm of larger, one finds that typical transformer oils have higher breakdown voltages when the point is negative than when it is positive. Recently, perfluorinated polyethers have been found to produce opposite results when using average gap sizes of 5 to 10 mm. To elucidate this situation a study of the effect of gap size on breakdown voltage has been undertaken. The results show that for the polyethers at larger gaps the sequence is reversed. These observations are explained in terms of the respective streamer developments in these liquids and the relative stability of their negative ions. In the case of the conventional hydrocarbon type oils, the positive streamer usually grows slightly faster than the negative one, but the streamer inception voltages of both are rather similar. In the case of perfluorinated polyethers the positive streamers move at least 10X faster than the negative ones, but they require much higher inception voltages than negative ones. At gaps [is greater than or approximately] 12 mm and the associated higher applied voltages this breakdown at negative polarity occurs at higher voltages than at the positive one. The negative ions of the perfluoro compounds are much more stable than those of conventional hydrocarbons. The discharge of the negatively charged perfluoro ions requires much higher fields than do those of conventional hydrocarbons. The implications of these observations are discussed in some detail.

At present several definitions of partial discharge inception voltage (PDIV) have been advanced by various interested groups. To establish the value of each of these definitions, four practical electrical insulating fluids were selected and their PDIV were determined according to three of these different specifications. The results of this study are presented and they suggest that the use of PDIV as a quality criterion generally is not warranted. The significance of these findings is discussed.

Many studies of partial discharge activity in solid dielectric systems have been published in recent years. Only a few limited reports have appeared in the literature on partial discharges in liquid dielectrics. For this reason an investigation was undertaken to study the partial discharge behavior of some well-defined dielectric fluids such as phenyl xylene ethane, ditolyl ether, mono and dibenzyl toluene and perfluoro polyether. The partial discharge inception and extinction voltages of these fluids were determined under uniform and nonuniform field conditions. The results of this study are presented and their significance is discussed in detail.

An investigation of partial discharge (PD) activity carried out in dielectric fluids is presented. It measured the PD distributions via a detector and a multichannel analyzer in terms of the applied alternating current stress. The results confirmed the relation of PD development to the prebreakdown and the electrical breakdown process as a random mechanism. The state wherein PD phenomena occurs was also proven to be confined to gases only.

Partial discharge pulse phase analysis carried out on a mineral oil and a perfluoropolyether with point-to-plane electrodes provided further and additional evidence that discharge epochs tend to be concentrated in the vicinity of the alternating voltage peaks at discharge inception in lieu of the voltage zeros characteristic of normal cavities occluded in solid and liquid-impregnated solid insulating systems. Relatively large positive polarity discharge pulses, having an associated charge transfer of [is greater than or equal to] 15 pC with a recurrence rate of at least one per every ten consecutive cycles, appear initially on the positive half-cycle; with further increases in applied voltage both their number and magnitude continue to exceed those of negative polarity over the negative half-cycle. This behavior is to be distinguished from that commonly observed with high sensitivity measurements, which clearly indicate that early discharge onset in liquids is characterized by the occurrence of minute highly intermittent negative polarity pulses. The increase in charge transfer of the discharge pulses with applied voltage is suggestive of the development of longer more intense streamers in the dielectric liquids at the more elevated electrical fields. Also the results infer that discharges or streamers form and propagate more readily in the mineral oil than in the electronegative perfluoropolyether.