Search

QC 20150225

In the planning of the electrical transmission system it is of greatest concern to quantify the security margin for unwanted conditions in the system. This paper proposes an approach based on quantifying the risk of insufficient transmission capacity in bottlenecks in the system. Stresses in these critical transfer sections (CTS) provide a potential risk to corrective actions, or worst, load curtailments. The proposed method provides a general screening of component outages in order to find potential risk events for the CTS. Furthermore, the severity of each risk event is quantified based on the likelihood of the event and the consequence on the section's transmission capacity. The components' contribution to the risk of insufficient capacity in the CTS is then based on these risk events' severity. The method investigates several forecasted load levels during the year and consequently gives an input to a scheme for a risk based transmission system planning. The method is demonstrated on the reliability test system RBTS., QC 20101101

The results from an extensive study of SF6 and minimum-oil circuit breaker failures in the Swedish and Finnish transmission systems are presented. The study includes 1546 breakers with a total operating history of 16 384 years. In the study, a detailed analysis of a specific SF6 type of breaker revealed which parts caused the failures and an estimation of their hazard rates was made. A model of complete reliability for this type of breaker is presented, illustrated by presenting the operating and maintenance history of a shunt reactor breaker in the Swedish transmission system., QC 20100823. Uppdaterad från submitted till published (20100823).

QS 20120319

The high frequency characteristics, the complex permittivity in particular, of water-tree degraded XLPE insulation in power cables are determined using a two port measurement method. The insulation samples are obtained from both laboratory made samples and those from ex-service cables. The electrical measurements are related to the water tree content of the samples which is estimated by an optical method. An effective materials model is derived to help understand the observed characteristics. From the model it is possible to estimate the water content of the water tree and the conductivity of the water. There is a considerable difference in the water tree characteristics between laboratory and service aged cables. The conductivity of the water in the water trees of service aged cables is much higher than that found in laboratory aged samples., QC 20100525

The paper presents a methodology for successful localization of degraded insulation in medium voltage cables. Extensive work has been performed on development of cable models and characterization of new and degraded cable material to support the diagnostic technique. A non-destructive technique has been developed using differential Time Domain Reflecometry (TDR) measurements. The main application has been localization of water treed sections of XLPE cables where the voltage dependence of the permittivity of water treed insulation has been used as diagnostic criteria.

This paper introduces a first step towards establishing the effect of maintenance on component reliability. The concept of relative age is introduced and defined as well as a method to establish this relative age of electric power system components for the purpose of maintenance optimization. The proposed method is demonstrated by modeling the relative age of a sub-component for a circuit breaker (CB). The modeling is based on the results from a major CB reliability study and a survey of all CB failures in the Swedish National Grid during the period from January 1(st), 1999 to July 31(st), 2003. The results from this application study show that it is possible to model the reliability of power system components by using results from development tests and calculations from the design process along with failure and maintenance data., QC 20100823

This paper proposes a method for comparing the effect of different maintenance strategies on system reliability and cost. This method relates reliability theory with the experience gained from statistics and practical knowledge of component failures and maintenance measures. The approach has been applied to rural and urban distribution systems. In particular, a functional relationship between failure rate and maintenance measures has been developed for a cable component. The results show the value of using a systematic quantitative approach for investigating the effect of different maintenance strategies., QC 20100525

QS 2010 QS 20120319

In order to optimize the maintenance of electric power equipment, models of equipment reliability as a function of age and maintenance are necessary. As a first step towards the development of these kinds of models, this paper proposes a quantitative method for establishing the condition of disconnector contacts by the use of thermography, which is a well-established diagnostic practice used in preventive maintenance programmes. Results from two sets of thermography measurements carried out in the Swedish transmission grid are presented. Statistical distributions for the uncertainties when using the method are estimated from the measurements. A method for converting temperature rises measured at non steady state loading conditions to a corresponding steady state temperature rise have been devised using a simple thermal model. The probability of sending an unaged contact to maintenance is estimated using Monte Carlo simulations based on the statistical distributions of the uncertainties. Finally, it is shown that by using the method proposed in this paper it is possible to estimate the contact condition of disconnectors, including Monte Carlo simulated confidence intervals., QC 20100823

Extruded polyethylene cables were first introduced on the Swedish market in the late 1960s. A specific degradation of the insulation, called “water treeing” was soon discovered. This type of ageing substantially reduces the electric breakdown strength of the insulation and is the cause for cable failures. In Sweden a significant amount of the cross-linked polyethylene (XLPE) cables from the 1970s are still in service and these cables represent a valuable asset to the utilities. Some cables are still “as good as new”, and to replace them would be an unnecessary and costly option. There is hence a need for a non-destructive diagnostic technique to assess the quality of the cables., QC 20110715

Dielectric spectroscopy (dielectric response measurements) has been applied for nondestructive estimation of humidity in oil-paper cable insulation. The experiments have been based upon two field-aged cables, 20 and 50 years old. Paper samples from these cables have been characterized and subjected to environments with different relative humidity. Dielectric loss and capacitance have been measured in a frequency range 1 MHz to 1 kHz and related to the moisture content determined by Karl Fisher titration. A method has been verified where the moisture content is correlated to the minimum value of loss tangent (tan 6). A number of field measurements have been performed where the method has been applied to estimate the moisture content in the distribution cables., QC 20100525 QC 20111101

A technique is developed for extraction of the wave propagation properties of power cables from S-parameter measurements. The method extracts the complex propagation constant and the characteristic impedance, as well as the LCRG Telegrapher's Equation parameters. The extraction process is developed after clarifying the effect of the connection between the measurement port and the power cable. It is concluded that treating the connection solely as a characteristic impedance change could lead to considerable errors in the parameter extraction. Furthermore, the method corrects for electrical lengths, which are not accounted for by the standard Network Analyzer calibration. The extraction is demonstrated for a medium voltage crosslinked polyethylene (XLPE) cable over the frequency range 300 kHz to 300 MHz. The results are compared to a time domain short pulse propagation method for cable characterization. Both measurement methods are evaluated against a cable model., QC 20100914

Dynamics of water penetration in mass impregnated cable insulation has been studied. For experimental purposes, artificial damage has been inflicted to a 40-cm-long cable sample and water ingress has been continuously monitored by frequency response measurements. A similar experiment has been conducted on 2.8-m-long cable sample, where both frequency response and time-domain reflectometry (TDR) measurements have been performed. After termination of both experiments, actual moisture content has been measured radially and axially. Based on dielectric measurements, a model of water ingress has been developed and diffusion coefficients have been estimated for mass impregnated cable paper., QC 20100908

A technique to measure the high frequency characteristics of insulation materials has been developed. Using this technique, high frequency characteristics of known insulation materials have been obtained to check its accuracy. The test cell, measurement technique and the extraction of the relevant parameters from the measurements are discussed., QC 20101015

To support the maintenance optimization carried out within the framework of the reliability-centered asset management (RCAM) method this paper presents a feasibility study for the probabilistic modeling of the aging of circuit-breakers (CB). The aim of the study is to investigate the possibilities of the development of a reliability model for a CB, using failure data and maintenance records. A study of such statistics was carried out on all 565 CBS in the Swedish National Grid (transmission level) during a time period ranging from 1999-01-01 to 2003-06-30. A total of 53 CB failures were identified and studied with respect to faulty subcomponent, CB function, number of operating cycles to failure and time in operation to failure. Results from this study are presented in this paper., QC 20101209

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. It is established that this screen design causes a dependence of the cable characteristics on the surrounding medium., QC 20141210

QC 20100823

Time Domain Reflectometry (TDR) has been used for localisation of transmission line discontinuities in various applications. One of the challenges when applying TDR is to obtain knowledge of the high frequency characteristics of both the degraded and the undegraded sections of the cable. The wave propagation characteristics of water-treed XLPE cables are investigated in this study. The parameters are extracted from the cable S-parameters measured with a Network Analyzer (NA) in the frequency range from 300 kHz to 300 MHz. The cable samples are characterised at different temperatures and with different water content of the water trees. The effect of the application of nominal voltage for extended period on the cable wave propagation characteristics is also shown. High voltage dielectric spectroscopy is used as a reference method for estimation of the level of water tree deterioration., QC 20101012

In this study, a high frequency measurement technique to characterize the semi-conducting materials of the medium voltage XLPE cables in the frequency range of 30 kHz to 500 Mhz has been developed. The influence of the experimental set-up and the sample preparation methods are investigated. A dielectric response model is then developed for the semiconducting materials and this is incorporated into a model for the whole cable. The propagation characteristics obtained from the cable model are then compared with those obtained from measurements carried out on the actual XLPE cables., QC 20101015

A HV dielectric spectroscopy system has been developed for diagnostics of water tree deteriorated extruded medium voltage cables. The technique is based on the measurement of nonlinear dielectric response in the frequency domain. Today's commercially available systems are capable of resolving low loss and small variations of permittivity as a function of frequency and voltage. Experience from more than 200 field measurements was combined with laboratory investigations. Small samples were used in an accelerated aging test to elucidate the correlation between water tree growth and dielectric response. Furthermore, field aged cables were investigated in the laboratory. It has been shown that the dielectric response of water tree deteriorated crosslinked polyethylene (XLPE) cables can be recognized and classified into different types of responses related to the aging status and breakdown strength. The influence of termination and artifacts such as surface currents was investigated. The measurement method enables us to separate the response of the cable from the influence of accessories. Finally, two different field studies of the implementation of the diagnostic method are presented. The field studies show that the fault rate decreased significantly when replacement strategy was based on the diagnostic criteria formulated., QC 20100525

Cited By :7; Export Date: 3 April 2018; Conference Paper. QCR 20180412