Evaluation And Modeling Of High-Voltage Cable Insulation Using A High-Voltage Impulse

Failure of underground cable on San Diego Gas & Electric\u27s electric underground distribution system is an ever increasing problem. While there are a great number of cable diagnostic techniques available, none lend themselves to both an averaged and location specific, on-line implementation. This dissertation demonstrates the development of an on-line suitable technique that utilizes transients and Fast Fourier Transforms to determine a cable section?s impedance magnitude and phase angle as a function of frequency. Simultaneously a theoretical model was developed to simulate various scenarios that an in-service cable might experience. Significant effort was expended developing and optimizing the measurement and data analysis technique. This includes a statistical approach for comparing performance of different cable samples. Both the preliminary and final tests demonstrated the superiority of the frequency domain analysis over comparisons in the time domain. With the effort to date, there appears to be three distinct results: good cable, degraded cable and damaged cable. These differences are statistically significant at the 95% confidence level. Additionally, there appears to be good agreement between the theoretical model and actual test results. Consequently, this measurement methodology continues to hold promise for future practical development

AN EXAMINATION OF METHODS TO DETERMINE THE FLAMMABILITY CHARACTERISTICS OF ELECTRICAL CABLES EXPOSED TO EXTENDED PERIODS OF RADIATION IN NUCLEAR FACILITIES

It is well known that prolonged low-level radiation compromises the flame spread ratings of insulated electrical cables, particularly those used in nuclear power plants, fuel production, and research facilities. This dissertation addresses the effects that thermal aging and irradiation have on the flammability characteristics of cable jackets (insulation around the conductor). It also seeks to develop a passive method to predict flame spread ratings for these aging cables. Changes in the inductive and capacitive reactance can alter the resonant frequency of the cable. These changes in reactance can be quantified as the cable is irradiated or thermally aged. Various testing methods are used to quantify the flammability characteristics of the cables. These tests are used to develop a complete understanding of how the irradiation or thermal aging impact the jacket and conductor of the cable

Effect of annealing on conductivity in XLPE mid-voltage cable insulation

A new study of the electrical conductivity of crosslinked polyethylene (XLPE) mid-voltage (MV) cable insulation is presented. Its main objective is to show the effect of annealing treatments on MV cables under actual service conditions. Complementary time domain (absorption/resorption currents) and frequency domain (dynamic electrical analysis) techniques are applied on different laboratory samples containing XLPE insulations: sections of XLPE insulated cable (with and without semiconducting screens) in the case of absorption/resorption currents and, in the case of dynamic electrical analysis, a thin ribbon obtained from the cable insulation by mechanical procedures. For annealing temperatures below a certain critical temperature, conductivity decreases both for XLPE cylinders (cable sections from which inner and outer semiconducting screens have been removed) and for real cables (sections of cable with semiconducting screens) but its value is smaller in the case of cables. If the annealing temperature is higher than the critical temperature, the behaviour of conductivity is more complex. In XLPE, cylinders conductivity initially decreases with the annealing time but after some annealing time it begins to increase, it passes over a maximum and eventually it decreases monotonically. In the case of real cable sections, conductivity grows, tending to a saturation value, which is noticeably higher than the corresponding value of the maximum obtained for XLPE cylinders. The experimental results are explained satisfactorily by means of the Mott equation that takes into account hopping conduction assisted both by temperature and electric field

Water-Tree Modelling and Detection for Underground Cables

In recent years, aging infrastructure has become a major concern for the power industry. Since its inception in early 20th century, the electrical system has been the cornerstone of an industrial society. Stable and uninterrupted delivery of electrical power is now a base necessity for the modern world. As the times march-on, however, the electrical infrastructure ages and there is the inevitable need to renew and replace the existing system. Unfortunately, due to time and financial constraints, many electrical systems today are forced to operate beyond their original design and power utilities must find ways to prolong the lifespan of older equipment. Thus, the concept of preventative maintenance arises. Preventative maintenance allows old equipment to operate longer and at better efficiency, but in order to implement preventative maintenance, the operators must know minute details of the electrical system, especially some of the harder to assess issues such water-tree. Water-tree induced insulation degradation is a problem typically associated with older cable systems. It is a very high impedance phenomenon and it is difficult to detect using traditional methods such as Tan-Delta or Partial Discharge. The proposed dissertation studies water-tree development in underground cables, potential methods to detect water-tree location and water-tree severity estimation. The dissertation begins by developing mathematical models of water-tree using finite element analysis. The method focuses on surface-originated vented tree, the most prominent type of water-tree fault in the field. Using the standard operation parameters of North American electrical systems, the water-tree boundary conditions are defined. By applying finite element analysis technique, the complex water-tree structure is broken down to homogeneous components. The result is a generalized representation of water-tree capacitance at different stages of development. The result from the finite element analysis is used to model water-tree in large system. Both empirical measurements and the mathematical model show that the impedance of early-stage water-tree is extremely large. As the result, traditional detection methods such Tan-Delta or Partial Discharge are not effective due to the excessively high accuracy requirement. A high-frequency pulse detection method is developed instead. The water-tree impedance is capacitive in nature and it can be reduced to manageable level by high-frequency inputs. The method is able to determine the location of early-stage water-tree in long-distance cables using economically feasible equipment. A pattern recognition method is developed to estimate the severity of water-tree using its pulse response from the high-frequency test method. The early-warning system for water-tree appearance is a tool developed to assist the practical implementation of the high-frequency pulse detection method. Although the equipment used by the detection method is economically feasible, it is still a specialized test and not designed for constant monitoring of the system. The test also place heavy stress on the cable and it is most effective when the cable is taken offline. As the result, utilities need a method to estimate the likelihood of water-tree presence before subjecting the cable to the specialized test. The early-warning system takes advantage of naturally occurring high-frequency events in the system and uses a deviation-comparison method to estimate the probability of water-tree presence on the cable. If the likelihood is high, then the utility can use the high-frequency pulse detection method to obtain accurate results. Specific pulse response patterns can be used to calculate the capacitance of water-tree. The calculated result, however, is subjected to margins of error due to limitations from the real system. There are both long-term and short-term methods to improve the accuracy. Computation algorithm improvement allows immediate improvement on accuracy of the capacitance estimation. The probability distribution of the calculation solution showed that improvements in waveform time-step measurement allow fundamental improves to the overall result

Dielectric Characterization and Conduction Modelling of a Water Tree Degraded LDPE

Distribution of electric energy by extruded polymer insulated cables continues to be a subject of outstanding relevance in modern industrialized countries all over the world. Dielectric characterization, conduction modelling and finally diagnostics of polymeric insulations are necessary steps towards the development of reliable and less expensive robust technologies of electric power distribution. This paper is devoted to a detailed experimental / theoretical study of the conductive properties of LDPE affected by different levels of degradation by water trees. Water tree layers of different lengths were grown in accelerated conditions and were characterized by water tree kinetics, time-dependent permittivity and polarization current. The polarization current was found to obey a Curie-von Schweidler law whose parameters were used to characterize the effect of ageing time. A new conduction model that takes into account dipole interactions and was obtained from a two-wells Debye model is presented which allows us to give an interpretation of the effect of ageing. This laboratory study was intended to improve the characterization of service power cables aged by water trees

Prediction of insulation degradation of distribution power cables based on chemical analysis and electrical measurements

This thesis deals with the prediction of medium voltage cable insulation condition. Different kinds of electrical measurements and chemical analyses are tested to find out the most representative combination. A large scale test program was carried out on field aged XLPE-insulated and oil-paper insulated cables. Cable samples were collected from different utility companies in Finland. Degradation of XLPE-insulation will change insulation material properties and it should be possible to detect these changes with chemical analysis. These chemical changes can lower the voltage withstand level of XLPE-cables. Degradation of paper insulation will decrease the mechanical properties of the paper. Increased moisture content will affect the electrical performance of the oilpaper insulation and will also speed up degradation processes. Methods such as dielectric response measurement and FTIR-analysis were used to determine the degree of degradation. It was found that XLPE-cables used in friendly environments are still in good condition after thirty years of service. Cables can be ranked clearly into different condition classes, but overall the XLPE-cables were in good condition. The FTIR-analysis results had good correlation to the voltage withstand levels of the cable samples. The field aged oilpaper insulated cables were also in good condition, even after more than fifty years of service life. Insulations were dry and degree of polymerisation (DP-value), related to mechanical strength, were high. FTIR-analysis results showed good correlation to the moisture content in the insulation. Since increased moisture content and high temperature will increase degradation rate significantly, FTIR-analysis can be used to estimate oilpaper insulation condition. Based on the research project results, FTIR-analysis can be used to estimate cable insulation condition. Results of FTIR-analysis can be linked to the voltage withstands levels of XLPE-cables and the moisture content of oilpaper cables. FTIR-analysis results can be converted to the condition classes of XLPE and oilpaper insulated cables

Wave Propagation on Power Cables with Special Regard to Metallic Screen Design

The high frequency properties of coaxial power cables are modeled using time- and frequency-domain numerical simulations. This is required due to the complex helical structure of the outer metallic screen. The finite element (FEM) and finite difference time domain methods (FDTD) have been employed to study the effect of screen spiralization. It is established that this screen design causes a dependence of the cable high frequency characteristics on the surrounding medium. Analytical model based on modal analysis of wave propagation in coaxial cables confirms the numerical observations

The effects of crosslinking byproducts on the electrical properties of low density polyethylene

Crosslinked polyethylene (XLPE) is widely used for high voltage insulation in power transmission systems. However, it has been found that, after crosslinking with Dicumyl Peroxide (DCP), the crosslinking byproducts such as acetophenone, ?-methylstyrene and cumyl alcohol have a significant influence on electrical properties of XLPE power cables. This thesis distinguished the individual contribution of the crosslinking byproducts on space charge formation, dielectric properties, dc conductivity as well as the ac breakdown strength. Percentage weight increases as well as the Fourier Transform Infrared (FTIR) spectrum were used to monitor the chemical level in the soaked samples. Despite high concentration of byproducts in the LDPE film compared to practical, the measurement results have successfully reveal the contribution of each byproduct on the electrical properties. It should be noted that some consideration should be taken when taking the quantitative value from the result obtained. Space charge accumulation was measured using the pulsed electroacoustic (PEA) technique. Homocharges are observed in acetophenone and ?-methylstyrene soaked LDPE. Meanwhile heterocharge formed in cumyl alcohol soaked LDPE. From the charge decay profile in dc condition, these chemicals are observed to assist the transportation of the charges in the sample bulk due to shallow traps from the byproducts. These shallow traps assist the trapping process into deep traps when ac field is applied to the byproduct soaked LDPE. As a result, more charges trapped in deep traps were found in soaked LDPE compared to clean LDPE. In addition, from the space charge measurement in ac condition, it is proved that the amount of charge trapped in deep traps also depends on the population of shallow traps in the polymer which is contradicted to the literature where the byproducts are normally associated to the deep traps. Permittivity values of acetophenone, ?-methylstyrene soaked LDPE and cumyl alcohol are slightly higher than permittivity value of the clean untreated LDPE. Cumyl alcohol soaked LDPE has higher dielectric loss at lower frequency due to Maxwell-Wagner-Sillars polarisation as well as space charge polarisation effect. In contrast, acetophenone does not change the dielectric loss value and ?-methylstyrene gives very little effect. These byproducts have very high dc conductivity values. It is also proposed that the chemicals provide shallow traps that aid the charge movement and this is consistent with the mobility values that calculated from the conduction current result. The ac breakdown results however show no significant difference from the breakdown strength of clean LDPE. Based on ac space charge results and ac breakdown test results, it is concluded that the byproducts have little effects in ac conditio

Study on conduction mechanismes of mediun voltage cable XLPE insulation in the melting range of temperatures.

D’ençà que el polietilè reticulat (XLPE) es va començar a utilitzar com aïllament elèctric per cables de subministrament elèctric, s’han destinat molts esforços a l’estudi de les propietats dielèctriques del polietilè i l’efecte que la càrrega d’espai té sobre el seu comportament. En aquest sentit, les corrents de despolarització estimulades tèrmicament (TSDC) s’han utilitzat extensament per estudiar les relaxacions de càrrega d’espai. Aquesta tècnica ha demostrat tenir prou resolució per distingir diferències en aïllaments de XLPE amb composicions o processos de fabricació diferents. En aquesta tesi, els mecanismes de conducció dels aïllaments XLPE de cables de mitjana tensió (MV) han estat estudiats per TSDC i diverses tècniques complementàries, com l’anàlisi dinàmica elèctrica (DEA), les corrents d’absorció/resorció (ARC), el pols electroacústic (PEA) i les corrents de despolarització isotèrmiques (IDC). Altres tècniques, com l’espectroscòpia d’infrarojos (FTIR) o la difracció de raigs X, han estat també utilitzades per caracteritzar el material. S’han obtingut espectres TSDC per diferents mostres de cable, les quals en condicions de servei treballen en un rang de tensió AC de 12 a 20kV i a temperatures al voltant dels 90ºC. D’altra banda, s’han realitzat mesures de la conductivitat per ARC i DEA en mostres de cable, en cilindres de XLPE i en films. Les mesures s’han dut a terme a temperatures pertanyents al rang de fusió del XLPE (50–110ºC), en mostres sotmeses a aquestes temperatures durant diferents períodes de temps. Els resultats mostren diferències importants entre el comportament de les propietats conductives de les mostres de cable amb pantalles semiconductores (SC) i sense (cilindres de XLPE). El comportament observat ha estat explicat mitjançant la coexistència de dos mecanismes de conducció. La difusió d’impureses des de les pantalles SC determina el comportament d’una d’aquestes contribucions a mig i llarg termini. Els resultats obtinguts per FTIR són consistents amb aquest model. Respecte la microestructura, tant les mesures DSC com la difracció per raigs X mostren que existeixen processos de recristal•lització quan les mostres són sotmeses a temperatures ubicades dins del rang de fusió. Els electrets formats mitjançant el mètode de la polarització per finestres (WP) mostren una descàrrega TSDC amb un ample pic heteropolar en el rang de fusió, amb el màxim al voltant dels 105ºC. En treballs previs, aquest pic es va associar a la fusió de la fracció cristal•lina. Tanmateix, en lloc de decréixer quan la temperatura de polarització augmenta, el pic presenta una temperatura de polarització òptima al voltant de 90-95ºC. Aquest comportament ha estat explicat tenint en compte els processos de recristal•lització que es produeixen quan el material es polaritza isotèrmicament. Durant la recristal•lització, una nova fracció cristal•lina creix en un estat polaritzat degut el camp aplicat, i origina una corrent de despolarització quan es fon durant la mesura TSDC. Amb l’objectiu de determinar l’origen d’altres pics que apareixen en l’espectre TSDC del XLPE, s’han emprat les IDC com a tècnica complementària. Les corrents IDC obtingudes de mostres no tractades es poden representar com la combinació de dues contribucions diferenciades: un terme que és una funció potencial del temps i un d’exponencial. La segona relaxació es correspon amb un pic TSDC que apareix a 95ºC. D’aquesta manera s’ha pogut determinar l’origen dipolar del pic. Finalment, mitjançant la tècnica PEA s’ha obtingut la distribució de la càrrega d’espai en mostres polaritzades que havien estat sotmeses a diferents tractaments tèrmics. S’ha observat un comportament transitori tant per PEA com per TSDC. Tanmateix no s’ha pogut establir cap relació directa entre les descàrregues TSDC i mesures PEA. En conseqüència, s’ha proposat una explicació per les corbes TSDC que considera mecanismes que no són detectables en els perfils de càrrega obtinguts per PEASince cross-linked polyethylene (XLPE) started to be used as electrical insulation for power cables, much research has been focused on polyethylene dielectric properties and the effect of the space charge on its behavior. In this sense, thermally stimulated depolarization currents (TSDC) have been widely used to study space charge relaxation. This technique has proved to have enough resolution to determine differences in charge trapping properties among XLPE insulations with different composition and manufacturing processes. In this thesis work, the conduction mechanisms of medium voltage (MV) cable XLPE insulation have been studied by TSDC and several complementary techniques, such as dynamic electrical analysis (DEA), absorption/resorption currents (ARC), pulsed electroacoustic (PEA) and isothermal depolarization currents (IDC). Other techniques, like Fourier transform infrared (FTIR) spectroscopy or X-ray diffractometry, have been used to characterize the material. TSDC spectra have been obtained for different cable samples, which in service conditions work under AC voltages ranging from 12 to 20kV and at temperatures around 90ºC. On the other hand, conductivity measurements by ARC and DEA have been performed in cable samples and in XLPE cylinders, as well as XLPE films. Measurements have been carried out at temperatures within the melting temperature range of XLPE (50–110ºC) on samples annealed at such temperatures during several annealing times. Results show significant differences in the behavior of the conductive properties of XLPE cable samples with semiconducting (SC) screens and without them (XLPE cylinders). The observed behavior has been explained by the coexistence of two conduction mechanisms. Diffusion of impurities from SC screens determines the medium and long-term behavior of one of these contributions and, hence, of cable conductivity. FTIR results are consistent with this model. With respect to microstructure, DSC and X-ray diffractometry results show that recrystallization processes exist when samples are annealed in the melting range of temperatures. Electrets formed by means of the windowing polarization method (WP) showed a TSDC discharge with a wide heteropolar peak in the melting temperature range, and with the maximum at about 105ºC. This peak was associated with the melting of the crystalline fraction in previous works. However, in spite of decreasing with the temperature of polarization, an optimal polarization temperature around 90–95ºC is found. This behavior has been explained by taking into account recrystallization processes when the insulation is isothermally polarized. During recrystallization, the new crystalline fraction grows in a polarized state due to the applied electric field, and it causes the depolarization current when it melts during the TSDC measurement. Results obtained from different experiments are consistent with this assumption. With the aim to find out the origin of other TSDC peaks present in the spectrum of XLPE cable samples, IDC has been used as complementary technique. IDC currents obtained from as-received cable samples at temperatures close to service conditions can be considered as the combination of two different contributions: a power law current and a stretched exponential contribution. The last relaxation process has been successfully associated with with a TSDC peak found at 95ºC. By this way the dipolar origin of the peak has been determined. Finally, PEA measurements have provided the space charge distribution profiles of polarized samples with different annealings. A transient behavior has been observed in both PEA and TSDC measurements. However, no straightforward relation between TSDC discharges and space charge detected by PEA can be established. Therefore, an explanation for TSDC curves has been proposed, which considers mechanisms that are not noticeable in charge profiles obtained by PEA.Postprint (published version