1,721,094 research outputs found
Crossing quantities: How to compare electrical strength performances of insulation compounds for power cables
No full textAn innovative theory—derived from the ‘‘enlargement law’’—about how to compare the breakdown
performances of power cable insulation compounds is illustrated and applied herein. This theory
enables the selection between two different compounds candidate for the insulation of power cables via
dielectric strength tests performed on cable models. In particular, compound performances are
investigated vs. cable length looking for the so-called crossing length, i.e. a crossover point between the
performances of the two compounds, such that one of the two performs better above this length, and
conversely the other below it. The application of this theory consists in a comparison between two EPR
compounds, based on lightning impulse breakdown tests realized on mini-cables, and shows that the
crossing length depends strongly on cable size and voltage rating. Therefore, the practical selection of
the best compound must consider the typical installation lengths of real cables.
Finally, the relationship between crossing quantities and failure probability is analyzed, highlighting
that while crossing length does depend on failure probability, crossing strength and breakdown voltage
do not. Thus, since crossing strength depends mainly on compounds Weibull parameters, it is the key
for comparing the intrinsic breakdown characteristics of competing compounds
Considerations about the application of the enlargement law to dielectric strength data relevant to power cables
No full textTheoretical considerations about the “enlargement law”, that were developed by one of the authors in a previous work, are employed here for the comparison between two different compounds candidate for the realization of full-size cables, on the basis of dielectric strength test results relevant to small-size cable models. In particular, compound performances are investigated as a function of cable length, searching for the existence of the so-called crossing length, i.e. a crossover point between the performances of the two compounds, so that one of the two performs better above such length, and conversely the other below such length. The practical application of this theory is then illustrated via a benchmark between two Ethylene-Propylene Rubber (EPR) compounds, based on lightning impulse strength tests relevant to cable models. Compound performances are investigated for different full-size cable voltage ratings, i.e. 20 kV and 150 kV. The application shows that the above-mentioned crossing length does exist, but it depends strongly on cable size and voltage rating, so that in practice the choice of the best compound requires a comparison between the value of crossing length and the typical installation lengths of real cables
Comparison of breakdown performances of extruded cables via the enlargement law
No full textIn this paper, previous theoretical developments by one of the authors - relevant to the “enlargement law” - are applied for a benchmark between two alternative compounds candidate for the realization of the solid-extruded insulation of power cables. By resorting to the results of dielectric strength tests relevant to cable models, breakdown performances of full-size cables are investigated as a function of cable length. In particular, the focus is on the existence of the so-called crossing length, such that one of the two compounds performs better for cables longer than crossing length, and conversely the other for cables shorter than this. As a practical case-study, the breakdown performances of two cross-Linked Polyethylene (XLPE) compounds are compared on the basis of lightning impulse strength tests relevant to cable models, by analyzing various power cable voltage ratings, i.e. 6 kV, 20 kV, 150 kV and 400 kV. The application shows that, though the crossing length in fact exists, it is a strong function of cable size and voltage rating. As a general consequence, the choice of the best compound should be based on a comparison between the value of crossing length and typical installation lengths of real cables
The effect of conductor radius and insulation thickness in the application of the enlargement law for comparing power cable breakdown performances
No full textPrevious investigations by the Authors of this paper showed that the so-called crossing quantities - i.e. cable length, strength and voltage at which a crossover in the breakdown performances of two compounds for power cable insulation occurs - are a strong function of cable size and voltage rating. Here, it is proved that also conductor radius and insulation wall thickness do play an important role, as witnessed by the functional expressions of crossing quantities. The investigation carried out in this paper takes advantage of a rigorous analysis of the mathematical formulae that express the influence of conductor radius and insulation thickness on crossing quantities. Moreover, a comparison is reported between two EPR-based, as well as between two XLPE-based, compounds for HV applications, for which the results of lighting impulse breakdown tests realized on mini-cables are available. In this framework, useful guidelines for the choice of the best compound for power cable insulation are derived, in the light of the dependence of cable breakdown performances on the dimensional parameters of the cable, i.e. cable line length, conductor cross section and insulation thickness
The enlargement law as a tool for comparing the breakdown performances of power cables: the role played by conductor radius and insulation thickness
No full textAs shown in previous papers by the authors, the
“enlargement law” can be used for selecting the best
among two compounds for power cable insulation, by
evaluating the so-called crossing quantities, namely the
values of length, dielectric strength and breakdown
voltage of the cable line at which a crossover in the
breakdown performances of the two compounds occurs.
This paper investigates the effect of conductor radius
and insulation thickness on crossing quantities. For this
purpose, a theoretical analysis of the mathematical
relationships between crossing quantities and cable
conductor radius plus insulation thickness is carried out.
Such analysis is supported by applicative examples
relevant to a comparison between two typical XLPE–
based compounds for cable insulation, subjected in
laboratory to lightning-impulse breakdown tests in the
form of mini-cables
Combining weather radar and raingauge data for hydrologic applications
No full textIn 2001 the paper titled “A Bayesian technique for conditioning RADAR precipitation estimates to raingauge measurements” (Todini, 2001) introduced a new technique based upon the use of block-Kriging and of Kalman filtering to combine, optimally in a Bayesian sense, areal precipitation fields estimated from meteorological radar to point measurements of precipitation, such as are provided by a network of raingauges. Block Kriging was used to estimate the average field over the radar pixels and its variance from the point rain gauge measurements, while a Kalman filter was taken to find the a posteriori estimates by combining the a priori estimates provided by the RADAR with the block Kriged measurements provided by the gauges, in a Bayesian framework. An original Block Kriging approach to the problem of spatial interpolation was introduced, which also included a new formulation of Kriging with uncertain point precipitation measurements. A Maximum Likelihood estimator was used at each step in time to estimate the semi-variogram parameters, while a new non negativity constraints were added to the Kriging system to prevent negative values in the Kriging weights. The paper summarizes the new potentialities of the new system and shows the results obtained in real world applications
A deeper insight into impulse levels selection for long DC extruded cable lines
No full textThe selection of both lightning and switching impulse test values for a power DC link are mainly based on numerical simulation and past experience on similar links, since well-established and standardized practices are missing. In this paper, as a continuation of a previous investigation, the selection of lightning and switching impulse voltage levels to be applied on short cable lengths in the prequalification and/or type tests is addressed for long DC cable lines taking into account the impulse withstand behaviour vs. cable length via the enlargement law and the effect of overvoltage attenuation for surges travelling along the cable. In particular, a deeper analysis into the algorithm of voltage impulse level selection is performed aiming at considering the typical range of variation of all the parameters that take part in this process. The analysis is performed via Montecarlo simulations considering all the variable involved in the estimation of the impulse withstand voltages. This approach is particular useful since at present standardized impulse voltage levels for HVDC cables are still left to an agreement between manufacturer and user
Lightning and switching impulse level selection for long DC cable lines
No full textThe impulse test levels play an even more important
role for the design of HVDC cables than for HVAC cables. Nevertheless,
the selection of both lightning and switching impulse
test values for a given power DC intertie are mostly based on
numerical simulation and on past experience on similar interties
rather than on well-established and standardized practices. In
this paper the selection of lightning and switching impulse voltage
levels to be applied on short cable lengths in the prequalification
and/or type tests is addressed for long DC cable lines (longer
than 50 km) taking into account the impulse withstand behaviour
with cable length via the enlargement law and the effect of overvoltage
attenuation for surges that travel along the cable
Analytical Expressions for Current Share by Means of a Lightning Protection System Model
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