1,720,973 research outputs found
Life-based Geometric Design of HVDC cables. Part 1: Parametric Analysis
No full textThis study, in two parts, investigates the cable design of HVDC cables depending on life modelling of the insulating material. This paper is the first part, in which a parametric analysis is carried out, illustrating the way in which different parameters affect the lifebased feasible designs of an HVDC cable with known nominal voltage and ampacity. An
ad hoc Matlab code has been developed for this study. Results show great sensitivity of the lifetime to insulation geometry, as a slight variation in inner and outer insulation radii leads to a significant variation of cable life due mainly to the related electric field variation. The effect of temperature can be hardly observed in the life map due to the
predominant effect of electric field. Increasing the maximum conductor temperature may not necessarily extend the feasible design area, which is also limited by the maximum temperature drop across insulation thickness. The higher the applied voltage, the greater the cable dimensions. This study also emphasizes the need to keep a low enough value of
soil thermal resistivity, which considerably reduces the cable dimensions. Temperature coefficient of conductivity shows a nonlinear (mainly drastic) effect on the life. On the contrary, Field coefficient of conductivity shows a slight positive effect on the life. Part 2 of this study will investigate the effect of electrical and thermal transients on the lifebased geometric design of HVDC cables
The Effect of Insulation Characteristics on Thermal Instability in HVDC Extruded Cables
Full text linkThis paper aims at studying the effect of cable characteristics on the thermal instability of 320 kV and 500 kV Cross-Linked Polyethylene XLPE-insulated high voltage direct-current (HVDC) cables buried in soil for different values of the applied voltages, by the means of sensitivity analysis of the insulation losses to the electrical conductivity coefficients of temperature and electric field, a and b. It also finds the value of dielectric loss coefficient bd for DC cables, which allows an analytical calculation of the temperature rise as a function of insulation losses and thermal resistances. A Matlab code is used to iteratively solve Maxwell’s equations and find the electric field distribution, the insulation losses and the temperature rise inside the insulation due to insulation losses of the cable subjected to load cycles according to CIGRÉ Technical Brochure 496. Thermal stability diagrams are found to study the thermal instability and its relationship with the cable ampacity. The results show high dependency of the thermal stability on the electrical conductivity of cable insulating material, as expressed via the conductivity coefficients of temperature and electric field. The effect of insulation thickness on both the insulation losses and the thermal stability is also investigated
Preliminary Experimental Investigation of the Effect of Superimposed Switching Impulses on XLPE-insulated HVDC Cables
No full textSwitching impulses superimposed onto rated DC voltage are a particularly-challenging kind of overvoltage which is applied to HVDC cable systems both in service and during qualification tests in the laboratory. This paper aims at a preliminary experimental study of the effect of Superimposed Switching Impulses on XLPE-insulated HVDC cables. It studies the effect of aging 0.15-mm XLPE specimens by applying a sequence of Superimposed Switching Impulses, defined according
to CIGRÉ technical Brochure 496. As illustrated in this paper, only preliminary results have been obtained so far from these tests. Such results show an increase in the conductivity in the aged specimens compared to non-aged specimens. An interfacial relaxation phenomenon is also noticed at low frequencies; however, no significant dipolar polarization can be noticed under the above-mentioned aging conditions
Deeper Insight into the Relationship between Experimental Expressions of Conductivity and DC Electric field in cables
No full textThis paper aims at a deeper insight into analytical relationships between experimental expressions of electrical
conductivity and electric field in DC cables. An analytical relationship was found in a previous paper focusing on one model of electrical conductivity only. Here the analysis is broadened to another more complex model used sometimes in the technicalscientific literature, and the derivation of the analytical relationship between the experimental conductivity expression and the electric field is carried out parametrically. A case study for DC-XLPE cable insulation is shown for the sake of illustration. The results agree with the literature values. While the temperature coefficient of conductivity a is weakly dependent on temperature, the stress coefficient of conductivity b is strongly dependent on electric field, especially for typical fields of HVDC cables
D and Qualification Tests on Power Cables
No full textThis paper presents the results of experimental tests on HVDC power cables and prototypes to validate the idea that the
real current absorbed by the power cable can be different from the current given by the HVDC generator, depending on different variables, as the test voltage, the test clearances, the ambient conditions and so on. The impact of all the testing features and conditions is evaluated. Experimental conditions and methods utilized are shown in the paper as well
Bayes Inference of Structural Safety under Extreme Wind Loads Based upon a Peak-Over-Threshold Process of Exceedances
Full text linkIn the present paper, the process of estimating the important statistical properties of extreme wind loads on structures is investigated by considering the effect of large variability. In fact, for the safety design and operating conditions of structures such as the ones characterizing tall buildings, wind towers, and offshore structures, it is of interest to obtain the best possible estimates of extreme wind loads on structures, the recurrence frequency, the return periods, and other stochastic properties, given the available statistical data. In this paper, a Bayes estimation of extreme load values is investigated in the framework of structural safety analysis. The evaluation of extreme values of the wind loads on the structures is performed via a combined employment of a Poisson process model for the peak-over-threshold characterization and an adequate characterization of the parent distribution which generates the base wind load values. In particular, the present investigation is based upon a key parameter for assessing the safety of structures, i.e., a proper safety index referred to a given extreme value of wind speed. The attention is focused upon the estimation process, for which the presented procedure proposes an adequate Bayesian approach based upon prior assumptions regarding (1) the Weibull probability that wind speed is higher than a prefixed threshold value, and (2) the frequency of the Poisson process of gusts. In the last part of the investigation, a large set of numerical simulations is analyzed to evaluate the feasibility and efficiency of the above estimation method and with the objective to analyze and compare the presented approach with the classical Maximum Likelihood method. Moreover, the robustness of the proposed Bayes estimation is also investigated with successful results, both with respect to the assumed parameter prior distributions and with respect to the Weibull distribution of the wind speed values
A Review on Wind Speed Extreme Values Modeling and Bayes Estimation for Wind Power Plant Design and Construction
No full textRapid growth of the use of wind energy calls for a more careful representation of wind speed probability distribution, both for identification and estimation purposes. In particular, a key point of the above identification and estimation aspects is representing the extreme values of wind speed probability distributions, which are of great interest both for wind energy applications and structural tower reliability analysis. The paper reviews the most adopted probability distribution models and estimation methods. In particular, for reasons which are properly discussed, attention is focused on the evaluation of an opportune “safety index” related to extreme values of wind speeds or gusts. This topic has gained increasing attention in recent years in both wind energy generation assessment and also in risk and structural reliability and safety analysis. With regard to wind energy generation, there is great sensitivity in the relationship between wind speed extreme upper quantiles and the corresponding wind energy quantiles. Concerning the risk and reliability analysis of structures, extreme wind speed value characterization is useful for a proper understanding of the destructive wind forces that may affect structural tower reliability analysis and, consequently, the proper choice of the cut off wind speed value; therefore, the above two kinds of analyses are somewhat related to each other. The focus is on the applications of the Bayesian inference technique for estimating the above safety index due to its effectiveness and usefulness
Forecasting the reliability of components subject to harmonics generated by power electronic converters
Full text linkThis paper aims at refining an experimentally based reliability model for the insulation of power components subjected to the randomly varying harmonics generated by power electronic converters. Compared to previous papers of the same authors and to the existing literature, here the model is re-formulated from the theoretical viewpoint focusing on the foremost role played by low percentiles of time to failure—in particular by the 1st percentile—selected as the rated life in the framework of modern probabilistic design of components. This is not only more correct from the viewpoint of component design, but also on the safe side as for the reliability of devices. Moreover, the application of the model is broadened to treat the whole sequence of odd voltage harmonics from the 5th to the 25th, i.e., those taken as the most significant in power systems according to international standards. The limits to voltage distortion set in Standard EN50160 are the reference for establishing parametrically a series of typical distorted voltage waveshape analyzed in the applicative part, which account for the possible phase-shift angles between voltage harmonics. The effect of current harmonics is also considered, from both the theoretical and applicative viewpoint. As a last, but not least novelty, the reliability model is used here for life and reliability estimates not only of Medium Voltage (MV)/Low Voltage (LV) capacitors and cables—already studied in the previous stages of this investigation—but also of induction motors and transformers in the presence of harmonics from power converters
Space Charge Characterization of Cross-linked Polyethylene and Polypropylene for HVDC Cable Insulation
Full text linkWith the introduction of HVDC links into the power grid, there has been a surge of interest in the use of polymeric cables for DC transmission. XLPE, a common insulation material for HVDC cables, has been central to this discussion. However, managing the issue of space charge accumulation in XLPE cables remains an ongoing challenge. To address this challenge, the exploration of new insulating materials is essential, provided that we have a deep understanding of their properties. In this study, we aim to investigate and characterize two distinct insulating materials: traditional XLPE and PP. The primary focus of this research is to gain insights into the properties of these materials. XLPE is subjected to full characterization at various temperatures and electric fields, PP samples are subjected to initial characterization at the design temperature and electric field.. Employing the PEA-method, we conduct space charge measurements on the samples, providing valuable information on charge behavior in the insulating material. The ensuing discussion delves into the advantages and disadvantages associated with these materials, offering a comprehensive view of their performance
Life Estimation of HVDC Cables Subjected to Qualification Test Conditions
Full text linkThe goal of the Master Thesis is estimating the life of HVDC XLPE-insulated cables subjected to the Qualification Tests conditions according to CIGRÉ Technical Brochure 496 and for different values of the coefficients (a) and (b).
During the Electrical Type Test (TT), a series of load cycles (LC) with DC voltage UT=1.85 U0 (rated voltage) are applied in three stages, i.e.:
• 12 cycles lasting 24 hours each with a negative polarity of the applied voltage (12 days).
• 12 cycles lasting 24 hours each with a positive polarity of the applied voltage (12 days).
• 3 cycles lasting 48 hours each with a positive polarity of the applied voltage (6 days).
according to CIGRÉ technical brochure 496, Load Cycles are of two types:
1. A 24-hour Load Cycle consists of 8 hours heating (with steady conductor temperature equal to the rated one during at least the last 2 hours), followed by 16 hours of natural cooling.
2. A 48-hour Load Cycle consists of 24 hours heating (with steady conductor temperature equal to the rated one during at least the last 18 hours), followed by 24 hours of natural cooling.
Results:
-The phenomenon called “Field Inversion” takes place only in the case of high values of “a” and “b” coefficients where the outer part of the insulation is stressed more than the inner part.
-In case of low “a” and “b” that the lower those values are, the more the inner part of the insulation is stressed.
-The life of the cable under the Type Test condition (around 90 days) is three times longer than the Type Test duration (30 days), considering the worst-case which corresponds to low values of “a” and “b”.
-The loss of life in one 48-hour Load Cycle (LC) is twice that in two 24-hour LC (equivalent to the same duration of 48 hours).
-For the same values of a and b, the inversion of the life curve over the insulation thickness in the Pre-Qualification Test is greater than that in the Type Test because of the High Load period in PQ test
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