New Publications are available for Organic insulation
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New Publications are available now online for this publication.
Please follow the links to view the publication.An innovative oil immersed power stack for wind power conversion using series connected GCTS
http://dl-live.theiet.org/content/conferences/10.1049/cp.2012.0242
A novel 450 kW DC wind power generator has been built using Active Stator technology, with an integrated oil immersed power electronic converter. The converter utilises unpackaged series connected Reverse Blocking Gate Commutated Thyristors (RB-GCT), operating at unity gain, to achieve high power density within a sealed `i-CAN' stack enclosure. The stack is cooled with synthetic ester oil which is also utilised as a liquid dielectric to provide 3 kV isolation in a small converter footprint.. The Active Stator machine is capable of operating in four quadrants for either motoring or generating. (5 pages)Learning from success and failure in transformer fault gas analysis and interpretation
http://dl-live.theiet.org/content/conferences/10.1049/cp.2011.0522
The importance of transformer fault gas analysis and interpretation is well recognised and documented. However, unlike the diagnosis of human blood where rules have been well established, the diagnosis of faults in transformer oil is not so clearly defined. Whilst some transformer faults are straightforward to diagnose, with dissolved gas analysis (DGA) results pointing to the type and severity of fault, in many other cases it is often difficult to diagnose what type of transformer fault is involved and even harder to determine the cause. In some cases the transformer may be damaged but this may not be obvious from DGA results. This paper introduces the causes of transformer faults and failure; presents an overview of development of DGA diagnostics and discusses the merits and limitation of DGA diagnostic method; and describes five case examples, showing that knowledge from forensic inspection of faults and failures in transformers are of vital importance in understanding results from DGA of oil samples, and an in-depth understanding of the transformer design and life-limiting faults enable better interpretation of DGA results. (6 pages)Investigating the effects of repetitive transients on paper insulation
http://dl-live.theiet.org/content/conferences/10.1049/cp.2010.0338
Repetitive transient aging is not a well understood phenomena in the high voltage world. For this reason, an aging waveform has been designed to understand what the scale of the impact of these pulses is on the paper insulation of a transformer. The times to breakdown between AC plus transients and clean AC are analysed and a clear trend is identified showing that transients are damaging to the insulation. The experimental method is will be described in detailed and also how these transients are achieved using a simple MOSFET and high frequency transformer. (5 pages)Effect of electro-thermal, load cycling and thermal transients stresses on high voltage XLPE and EPR power cable life
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.1818
All insulation systems life is definite and analysis of insulations life is very important. Power cables designed for 30 years life, but electrical, thermal, load cycling and other stresses caused reduced it. In this paper, a procedure is presented for the life estimation of high-voltage cables subjected to electro-thermal stress due to applied voltage and load cycles. Thermal transients due to load cyclic current are modeled by means of the well-known two-loop thermal network analog. The effect of the relevant cyclically varying electro-thermal stress is accounted for via the cumulative damage law of Miner. The life fractions lost during each step of the load cycle are evaluated by a proper combined electro-thermal life model holding for cable insulation. The procedure is applied to high voltage AC XLPE and EPR insulated cables, subjected to stepwise constant daily load cycles. (6 pages)HVAC test on 132 kV XLPE cable circuits with special switching arrangement
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.1778
This paper describes CLPP's experience on conducting HVAC tests on 132 kV transmission XLPE cable installations, the testing standards adopted and the special site arrangements for setting up the HVAC test sets. A summary of the HVAC results and the testing arrangement for new XLPE cable jointed to the old cable system are also included. (6 pages)Research of service condition impacts on cable polymer insulation
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.0712
During last decades the intensive increase of using of polymer insulation materials in high voltage electric equipment such as cables, apparatus, surge arresters, transformers, and insulators takes place. With all wide well-known advantages the challenge of polymer insulation material deterioration and conditions of defects formation under influence of different service factors in full-scale are poorly studied. Water trees formation and its growing are concerned with humidity impact on the reduction of polymer insulation electric strength under non-uniform electric field. Water diffuses in polymer structure under electric field intensity and concentrates in volumes with high local heterogeneity, where water trees have a tendency to grow. Despite unanimous agreement that the combination of water and electric field in polymer insulation affects its dielectric properties, there is no clear understanding of the water-tree aging process. Besides proposed deterioration physical models or registered service failure statistics have not been useful for calculation of insulation degradation level or remaining life. In paper the results of laboratory research of polymer material dielectric losses over high frequency range are represented. The dielectric losses measured by using of non-destructive dielectric response measurement techniques showed a good correlation with polymer insulation deterioration. On the basis of obtained results the mathematical model and rapid method of cable remaining life assessment is offered. Thermal deterioration of polymer cable insulation (especially polyethylene insulation) under service condition cannot be ruled out. Within the framework of the project the investigations of thermal deterioration of polyethylene insulation with using of method of X-ray structure analysis are carried out. During thermal deterioration investigations the correlations between dielectric losses and temperature load at different ageing time were defined. (4 pages)Water trees in medium voltage XLPE cables: very short time accelerated ageing tests
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.1140
This paper describes a pragmatic method to assess the resistance of polymeric insulation cables to water treeing. Water trees are produced in 12 days in the polyethylene insulation of a real cable by use of a high frequency, high voltage power supply. The first objective was to provide a source of information for asset management (remaining lifetime estimation of installed cables) but it is also applicable for the assessment of new products and allows a rapid comparison between different materials. (4 pages)Characterization of partial discharges behavior in XLPE distribution cable joints
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.1076
This paper discusses research conducted to characterize medium voltage power cables PD activity which will be used to develop an on-line monitoring system. Data of the PD characteristics that take place in different models of joints performed on 20 kV XLPE cables are presented. Furthermore, a measurement system that enables the on-line partial-discharge detection and location based on Rogowski coils is discussed and designed. (4 pages)Changes in insulation condition of dry cured XLPE cables over a 6-year period
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.0523
The service experience with dry cured XLPE cables is good and their internal failure rate is much lower than is the case for the older steam cured cables. However, water tree aging also occurs in dry cured cables, unless the insulation is protected against moisture by water barriers. Since 2001, a series of diagnostic measurements has been carried out on in total 58 dry cured XLPE cable sections without water barrier. The cables were installed in 10 kV to 20 kV networks in the period between 1979 and 1996. The objective of these measurements has been to get an assessment of the insulation condition and the rate of change in insulation condition for these cable sections and the results are presented in the paper. (4 pages)Water absorption by oil in MV switchgear
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.0583
In non-hermetically sealed oil-insulated MV switchgear the insulating oil will adopt the relative humidity of the ambient air within a limited period of time (weeks). This requires a different approach for maintenance and requires additional procedures for operating the switchgear. For this situation, determining the oil quality is only feasible when oil temperature and water content are standardised to values in the testing room. Otherwise the oil analysis results may merely indicate the properties of saturated oil. Further due to condensation oil-insulated MV switchgear may even suffer from an internal fault (explosion) if the oil temperature drops significantly within several hours. (4 pages)Thermal response of a three core belted PILC cable under varying load conditions
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.0621
The most important indicator of the health of electrical plant items is the condition of their insulation. In the case of underground cables an important issue is the operating temperature and indeed the thermal history of the cable. There are several factors which will determine the thermal behaviour of a given cable installation. These include the assumed ampacity, the cable construction and circumstances of installation, the thermal properties of the surrounding soil and the ambient temperature. The work presented in this paper involves the use of COMSOL multiphysics finite element software to develop an integrated electrical, thermal and mechanical model of buried single or multiphase cables that simulates the behaviour occasioned by a varying load. The principal heat source in the problem is the Joule heat dissipated in the conductor(s). The transfer of this heat to the surroundings is governed by the geometry and material properties of the conductor, insulation, screening, sheathing and trench fill material as well as the ambient conditions. The thermal and electrical systems are coupled via the temperature dependence of the resistivities of the conductor and sheath materials. The input data to the model is provided in the form of time series describing the variations in load over a 24-hour cycle and the output takes the form of the thermal response of the cable for given installation and ambient conditions. (4 pages)Requirements for different components in cables for offshore application
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.0531
The installation of offshore wind-farms and the increase in electricity trading between countries has resulted in an increased number of offshore submarine cable installations as an efficient way to solve many electricity supply issues. Although offshore cables with XLPE insulation are similar to land based cables, the offshore environment adds to the challenges of very long lengths, much more onerous installation conditions and an aggressive environment. This paper discusses how these issues are confronted in distribution cable designs. In addition, the necessities for long-lasting medium voltage cables up to the lower high voltage level in wet conditions plus the increase in performance needs are discussed. It also covers the different requirements of 3 Core installation and water tree retardency tests in sea water applications. This paper will focus on medium and high voltage cables as they are used extensively for offshore applications connecting wind farms, for lake crossings and for inter- island connections. (4 pages)New generation of outdoor vacuum circuit breakers with rated voltage up to 40.5 kV
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.0647
The paper describes design peculiarities of the novel compact vacuum circuit breaker with rated voltage 40.5 kV. Design incorporates several novel technical solutions: polycarbonate support insulation, mono-stable magnetic actuator, labyrinth pulling insulator, core-type flexible contact and new compact vacuum interrupter (VI). For current and voltage sensing Rogowsky coil and capacitive dividers are applied. The module is encapsulated into silicon rubber providing required creepage distance and excellent tracking resistance. These novelties together with the extensive modeling of the mechanical and electrical fields followed by design optimization allowed reducing weight by more than 50% compared with alternatives available in the market. (5 pages)Advanced solution for on-site diagnosis of distribution power cables
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.0495
This paper describes important aspects of partial discharge (PD) diagnostics of distribution power cables. PD parameter were discussed under consideration of attenuation and dispersive effects of cable characteristics. Field experiences with PD diagnosis on paper insulated cables (PILC) and cables with cross linked polyethylene insulation (XLPE) by energizing with damped AC voltage demonstrate relevant demands and the effective outcome for asset management decision support. The main difficulty is to evaluate the rise of PD occurrences on the reliability of the cable system. (4 pages)The condition estimate of power transformer based on rough set theory
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.0677
A transformer is one of the most important units in power networks; thus, condition estimate of transformers is quite significant. Rough set theory has been successfully applied to many areas including pattern recognition, machine learning, decision support, process control and predictive modeling. Due to incompleteness and complexity of condition estimate for power transformer, a specific model based on rough set theory is presented in this paper. After the statistic analysis on the collected fault examples of oil-immersed power transformer and using rough set theory to reduce result, estimate rules are acquired and they could be used to improve the condition assessment of power transformer. The condition estimate inference model was built based on the advantage of effectively simple decision rules and easy reality of rough sets. The significant advantage of the new method is that it can discriminate the indispensable alarm signals from dispensable ones that would not affect the correctness of the estimate results even if they are missing or erroneous. (4 pages)High voltage pulse transformer employing magnetic self-insulation
http://dl-live.theiet.org/content/conferences/10.1049/ic_20060091
High-voltage pulse transformers are key elements of many pulsed power systems, where they provide an attractive alternative the Marx generators for providing short rise time voltage pulses. Unfortunately, the commonly used forms of insulation cannot be used above 500 kV, without considerable added complexity. Immersion in oil is effective, but leads to a heavy and bulky unit. Magnetic insulation provides the only practical form of insulation for much of the equipment used in ultrahigh voltage pulsed-power systems, including high power transmission lines and plasma opening switches. Although its adoption in transformers would bring significant advantages in both size and weight, a number of seemingly insurmountable problems have so far prevented this. Experiments on a 0.5 MV prototype transformer in which the magnetic field required for magnetic insulation is provided by the transformer current (ie magnetic self-insulation) have shown that the electromagnetic and mechanical design adopted can remove the previous difficulties. Details of the design are presented including theoretical predictions made using filamentary modelling, ANSOFTreg; software and MAGICreg; software, which are all valuable tools for use in the design of future transformers.First major 275 kV XLPE cable system in the UK
http://dl-live.theiet.org/content/conferences/10.1049/cp_20060022
Commissioned in 2004, a strategic EHV XLPE cable system installed in Glasgow represents three significant advances at the 275 kV level: 1) first 275 kV XLPE long route project in the UK. 2) first significant use of XLPE joints in a UK 275 kV system. 3) use of pre-installed unfilled ducting, which hitherto has been applied only at lower voltage levels. This paper describes the background to and implementation of the project.Application of multivariable grey model in power transformer gas-in-oil prediction
http://dl-live.theiet.org/content/conferences/10.1049/cp_20061633
The concentration prediction of gas-in-oil for power transformer is an effective way to evaluate the transformer insulation on-line and prevent the transformer from further deterioration. However most conventional methods, such as <sub xmlns="http://pub2web.metastore.ingenta.com/ns/">GM(1,1)</sub> model consider each index separately. To make up for the disadvantage, the multivariable grey model (<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">MGM(l,n)</sub>) is introduced to allow for the change in several different indices described simultaneously. The <sub xmlns="http://pub2web.metastore.ingenta.com/ns/">MGM(1,7)</sub> model for the seven characteristic gases monitored on-line in the power transformer oil is then developed. In the built model the interaction among these gases is comprehensively considered. The effectiveness and advantages of the proposed approach is demonstrated by the actual prediction case. (4 pages)Quantitative analysis of FTIR used in transformer monitoring
http://dl-live.theiet.org/content/conferences/10.1049/cp_20061815
The application of FTIR in online monitoring of dissolved gas in transformer oil is studied in this paper. Different concentration and ratio of gases are sampled with FTIR spectrum instrument and "Flow gas method". The absorbing peak characteristic of 6 fault gases is analyzed. The problem of cross disturbance on C<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">2</sub>H<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">6</sub> is effectively solved with character subtraction method. Quantitative analysis of all key gases is made by quantitative features selection and non-linear calibration. (5 pages)Oil-filled cable system pressure monitoring - a 21<sup xmlns="http://pub2web.metastore.ingenta.com/ns/">st</sup> century solution
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050054
This paper describes the development and key features of an integrated telemetry system for the monitoring of oil usage and hydraulic pressures in supertension cable systems. The telemetry system provides data to a pre-defined number of users. This allows the integrity of a pressurised cable system to be monitored by alerting the user to potential operational problems.Modelling the ageing of cellulose insulation in power transformers
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050059
Ageing degradation of cellulose insulation in power transformers increases with operating cycles in a cumulative manner and may lead to the failure of power transformers. In this study a cumulative degradation model for the ageing of cellulose insulation is proposed. The concept of degradation accumulation is applied to predict the life of cellulose insulation; a degradation evaluation function, called the degree of degradation, is introduced to represent the ageing of cellulose paper insulation. The model has been applied to explain the experimental data of Kraft paper over a wide ageing temperature range from 60°C to 160°C. The results indicate that the model gives a good description of the evolution process of ageing degradation of cellulose insulation.Recognition of discharge signals from a transformer bushing using signal processing techniques
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050060
A transformer bushing tap point can be used as a measurement point to detect partial discharges (PD) of an oil-filled power transformer. The partial discharges occurring around and within a transformer may include external discharges such as corona and surface discharges on the bushing, and internal discharges such as corona in oil, floating discharge and void discharges. These discharge signals have different characteristics in the time and frequency domains. For example, external corona and surface discharges are "glow" type of discharges, and have a bandwidth below 40 MHz. Internal discharge signals such as floating discharges and corona in oil (needle-plane arrangement), have a bandwidth up to 200 MHz. Discharge signals can also be displayed relating a 50 Hz test voltage waveform. This paper introduces the application of software tools to filter and recognize partial discharge signals obtained form a bushing tap point.Mechanisms of failure in oil and paper insulated low voltage distribution cables
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050032
Despite substantial research activity aimed at understanding degradation mechanisms in medium and high voltage cables, the nature of failure in low voltage (LV) systems is still not well understood. Early stages of an investigation into the role of moisture in the ageing and degradation of oil/paper insulated LV cables are reported in this paper. Experiments have been performed in which oil-filled paper has been exposed to moisture for varying periods and then time-to-failure at constant working voltage determined. It is shown that moisture plays a key role in determining the rate of cable failure, and that the ionic content of the moisture is critical. Pre-breakdown discharges show potential as signals with which to monitor the condition of these assets.Power transformer life management
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050902
Utility asset management and network reliability have, over recent times, become well researched topics. Electrical utilities, across the globe, recognize the fact that major power system equipment is reaching the end stage of its expected life. The age profiles of this equipment are very similar. Recent power failure events have highlighted how vulnerable electrical distribution networks really are. Events on the east coast of North America and Europe have emphasis the real value of electricity at much higher levels than its selling price. The ability to extend life expectancies by effective asset management can have a major impact on capital investment planning programs. Power transformers represent the bulk of the asset value for distribution and transmission utilities. Traditional asset management strategies for transformers revolves greatly on time based maintenance methodologies and transformer oil monitoring. With increased demands for electricity power transformers are operated at or beyond its nameplate value. The strategic nature of power transformers makes it highly critical items on the distribution network. It is therefore imperative to manage power transformers to attain optimum utilization and thereby maximising the return on investment without lowering their production and availability levels. The proposed paper details the life assessment process of power transformers and include criteria for subsequent recommended remedial action. (4 pages)Quality control of XLPE cables by means of impulse voltage test
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050981
High voltage cable lines are rated among the most expensive investments in the electric energy supply system. Due to economic and operational requirements these lines are to live up to the highest expectations as far as reliability and adequate life time are concerned. Therefore it is recommended to undertake impulse voltage tests with increased level apart from tests as provided in the relevant standards. Thanks to these examinations impurities in the insulation and in the interface between insulation and semi conductive layers can be recognized fast and easily for they cause lower levels of the impulse strength. Particularly so-called protrusions in the interface between insulation and semi conductive layers or inadequate adhesion of the semi conductive layers to the insulation, both defects that can occur during the production of the cable, are subjected to the test. During operation these impurities can cause increased local electrical field strength in the insulation as well as water trees and electrical trees. Additionally they speed up the aging process in the insulation of the cable and this can lead to early faults respectively a shorter life time. These production related defects are not detectable by standard tests as partial discharge measurement or voltage tests with power frequency. Due to our investigations and good experience in the field of quality control of XLPE cables we carry out this test for all Austrian and some German utilities. (3 pages)Time varying characteristics of φ-q-n patterns from natural voids in resin
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050975
The measurement and assessment of partial discharge (PD) activity is used both for condition assessment of high voltage plant (by operators) and for quality control (by manufacturers). Except for the online circumstances, the measurement usually lasts for a short period of time while higher than working stress is applied. The resulting pattern of discharge data, when compared with a knowledge base of PD patterns of different types of defects is a crucial factor in diagnosing the condition of the insulation system in the plant. In this paper the characteristics of φ-q-n patterns for a void-type defect is extensively investigated using an IEC60270 PD measurement system and a fast digital oscilloscope and processing methods. Spherical void defects are deliberately manufactured in epoxy resin samples in order to simulate natural defects as precisely as possible. As one of the vital factors causing variation in PD patterns, different stress levels are applied. Different sizes of void are also considered, to reveal the effect of void size on PD patterns. Measures are taken to ensure that PD results only from the manufactured defects rather than any other source. Several traditional criteria are calculated to characterise the different patterns. Many observations are achieved and presented with discussions. (5 pages)Factory testing of long submarine XLPE cables using frequency-tuned resonant systems
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050959
This paper addresses the principles of testing long submarine solid dielectric cables with AC-voltage, using up to date high voltage equipment with frequency tuning. It is shown that lengths up to 100 km or more is possible to test if all parameters affecting such tests are taken into account and treated accurately. Whether an AC or a DC solid dielectric cable is chosen, an successful AC factory test with enhanced phase to ground voltage, give high credibility to the XLPE technologies of today. (5 pages)The influence of the stress level on electro isolating oil state from electric power transformers
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050897
The paper has three parts. The first part presents the characteristics of the power transformers that make the object of the analysis and their loading characteristics (load curve, over-voltages, over-currents). The second part shows the state characteristics of the electric insulating oil for each transformer under analysis, in terms of some essential state indicators: dielectric rigidity, tangent to the angle of losses and concentration of the seven essential gases (H<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">2</sub>, CH<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">4</sub>, C<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">2</sub>H<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">6</sub>, C<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">2</sub>H<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">4</sub>, C<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">2</sub>H<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">2</sub>, CO and CO<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">2</sub>). The third part of the paper makes an analysis of the level of correlation between the power transformer straining degree and the state characteristics of the electric insulating oil. Based on the results obtained proposals are made regarding the ways of reclaiming and replacing or the electric insulating oil in power transformers depending on the transformer degree of straining. (6 pages)Research and experimental characterization of alternative materials to reduce the environmental impact of some electrical distribution equipment
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050938
In the electric system the aim of a sustainable politics is to reconcile the rational use of the energy with the environment protection for an improvement of the life quality. This objective can be reached by optimising the functional, safety and environmental performances of the network components. This means to develop new products and new processes which, beyond to improve the quality of service to consumers or to anticipate the evolution in standards and the demands of the market, can also reduce energy and non renewable resources consumption, risks for human and ecosystem health and end-of-life waste. So a research project named MATEALT has started in CESI at the end of 2003 with a funding regulated by the Decree of the Italian Ministry for Industry and Trade dated 26 January 2000. The activity was concluded at the end of 2005. Its aim is to improve the eco-profile of some electrical components, studying and testing the following materials: carbon dioxide CO<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">2</sub> as an alternative to sulfur-exafluoride SF<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">6</sub> for dielectric applications and arc extinguishing medium in MV circuit breakers; ester-based oils for paper-oil transformers as an alternative to mineral oil for reducing soil and water pollution in the event of an accidental spill and providing enhanced fire safety; recycled polymeric materials to replace the metal enclosure of MV switchgears for MV/LV substation to reduce waste and the depletion of steel. In such a way the installation of these electrical components in town and natural land of the country can be supported in full agreement with environmental and safety regulations and can meet the public opinion. In this paper the results of breakdown voltage tests on CO<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">2</sub> and on its mixtures are presented and discussed for different electrode configurations and distances, gas pressure and service voltage stresses (AC, LI, SI). A comparison of the main chemico-physical properties of two ester-based oils and a naphtenic oil and the results of breakdown voltage tests on pressboard impregnated in each fluid are shown. Finally the main features of a polymeric material suitable for a MV switchgear enclosure are presented with the results of internal arc tests on PE, PP, PC and PET and the polymers and the geometry were studied and tested for enclosure panels are shown. (7 pages)Detection and localisation of water treeing for condition based replacement of medium voltage cables
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050960
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 crosslinked 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. Two diagnostic methods are described in this paper. The first method is based on the measurement of the cable low frequency dielectric losses and it detects the general level of water-tree deterioration. A second method is used for localisation of water-treed regions along the cable length. This method is based on the time domain reflectometry (TDR) technique. (4 pages)The performance of XLPE water tree resistant insulation systems against the requirements of DIN VDE 0276-605/A3
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050967
Since the introduction of polyethylene, there have been a number of significant advances in materials technology that have contributed to polyethylene's continued worldwide position as the material of choice for insulating cable. In North America the use of water tree resistant insulation (WTR) is predominant, the European market has diverged into the use of so-called homo-polymer systems and co-polymer systems. The use of co-polymer systems in Europe has been governed especially by the two-year wet aging requirements of VDE. The use of homo-polymers is preferably used in "dry designs", where the access of wafer to the cable core during the life time of the cable is prevented. The harmonized standard CENELEC HD605 SI HD 605 SI:1994/A3:2002 "Electric cables - Additional test methods" is in place now. It contains the two-year aging test in water and may drive a more uniform approach to the use of insulation materials. The current study evaluates the performance of WTR insulation materials against critical European test requirements. The paper also discusses the fundamental mechanism of the WTR technology in this material. (5 pages)The compact joint with integrated shear bolt connector; a new approach to function integration in medium voltage joints
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050945
The design of medium voltage joints for polymeric cables has been improved several times during the last decades. Remarkable development steps are represented by the tape joint, the heat-shrink joint and the cold-shrink joint. All these solutions have in common that the connector is installed separately, followed by the rebuilding of the insulation, the earthing system and the outer protection. A new step is done now by integrating a shear bolt connector into a silicone rubber insulated joint. In order to establish appropriate design criteria specific investigations on silicone rubber interfaces were carried out. (5 pages)Distribution transformers: a study of the relationship of their oil dielectric strength and their previous history
http://dl-live.theiet.org/content/conferences/10.1049/cp_20050879
This paper is concerned with the correlation of the oil dielectric strength and the previous history of a number of distribution transformers of the P. P. C. (Public Power Corporation) from the region of Eastern Macedonia and Thrace. Our samples come from distribution transformers from the Xanthi area and from the broader region of Macedonia and Thrace, Northern Greece. In the Xanthi area there are 1631 distribution transformers. In the broader region of Macedonia and Thrace 39963 distribution transformers are in the network. Our work is a continuation of the work reported in CIRED 2001. Our effort is concentrated on the establishment of a statistical model which would relate the dielectric strength with the electric stresses to which a distribution transformer is subjected as well as its previous history. It is to be noted that the measurements of the dielectric strength were carried out with the aid of a Foster test cell according to the BS 148/78 standard. (6 pages)Algorithm of the microprocessor thermal protection of oil power transformers
http://dl-live.theiet.org/content/conferences/10.1049/cp_20040142
This paper surveys the work on the improving of the thermal modelling of oil power transformers. The original algorithm for the hot-spot temperature calculation is exposed. The algorithm describes the physical phenomena better and provides higher calculation accuracy then the algorithms in the valid IEC standards and in the new draft. The algorithm is convenient to be practically realised on the microprocessor thermal relay. The research of the important practical problems of unknown starting hot-spot temperature and of thermal characteristics change during a long-term operation of a transformer is also overviewed. The prospect for future work is a further research of other configuration of the thermal observer. Also, the natural step after the research and extensive experimental verification of the algorithms is the application in commercial microprocessor thermal protection and monitoring systems.Electromagnetic radiation in buildings
http://dl-live.theiet.org/content/conferences/10.1049/ic_20020258
With the increasing use of electromagnetic radiation for communications and data transfer, there is a growing need to adapt the built environment to optimise electronic systems working within it. Legislation may also dictate that electromagnetic radiation needs to be controlled within buildings, and it will be necessary to develop solutions that meet these specifications. Different environments have different requirements, and building materials may be required to perform shielding, absorption, transmission or filtering functions. The mechanisms by which these functions can be achieved are different, but all are feasible with minimal changes to existing building materials. Shielding materials can be used to prevent interference between different operating systems, and this function is typically achieved by the use of metallic sheets or foils, or more flexible options such as metallised polymer films or fabrics. Absorbers can also be used to protect from interference, but can also prevent unwanted reflections - the radar absorbing cladding applied to the BA World Cargo Centre is a good example of this. Transmissive materials are used where signals needed to be transmitted into and out of buildings, for example the mock architectural facades made from glass fibre composite used to house mobile phone antennas. Filtration of radiation can also be achieved by deposition of frequency selective metal patterns on insulating substrates, producing materials that allow certain frequencies to pass through whilst reflecting others. The challenge of incorporating these types of functionality into building materials is being addressed. (4 pages)Insulation condition assessment of 11 kV paper cables using neural networks
http://dl-live.theiet.org/content/conferences/10.1049/ic_20010151
The increasing demand of energy and the cost associated with the installation of new power systems are intensifying the design and implementation of condition monitoring techniques, which are able to estimate the remaining active life of existing systems and, therefore, provide decision support for their maintenance or replacement. For paper insulated power cables, a common method for their condition assessment is the detection and analysis of partial discharges (PD). These discharges in the cavities of the paper insulation increase the size of the voids and cause the development of carbonaceous paths between the fibres of the paper layers. These defects may lead to fatal breakdown of the cable. Although this physical deterioration process is well known, the essential parameters required for the control of the different stages of the degradation remain uncertain. Therefore, a large number of variables have to be monitored to develop an efficient and reliable condition monitoring system for paper insulated cables. Self-organising maps (SOM), a type of artificial neural network, are a suitable technique for the processing and analysis of high dimensional problems involving large amounts of data, being particularly appropriate for the recognition of trends and feature clustering. They form the basis of the condition monitoring technique presented in this paper. (4 pages)Breakdown of small oil gaps bridged by pressboard under AC, DC and combined AC/DC voltages
http://dl-live.theiet.org/content/conferences/10.1049/ic_20010154
The spark discharges occurring on paper-oil insulation could cause deterioration of surface of pressboard, which ultimately results in the breakdown of the insulation. The use of paper-oil insulation in convertor transformers has led to new problems due to the appearance of both AC and DC voltages simultaneously. In the present work, the deterioration of paper-oil insulation based on the surface breakdown strength of the system across small gaps was investigated. The surface of the insulation was subjected to spark discharges and the time duration the material can withstand the spark discharge without forming a conductive path was considered to estimate the surface behavior of pressboard. In addition to this parameter, conventional AC, DC, combined AC/DC voltage studies are presented. The effect of thermal ageing of insulation system on break down of small oil gaps bridged by pressboard was studied. (4 pages)Dielectric response measurements in time and frequency domain of different stressed low pressure oil-filled cables
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010512
Power transmission has become commercially relevant to an increasing degree. Therefore it is necessary to find out strategies for an assessment of the reliability of cable systems, especially after many years under operating conditions which have changed the ageing state of the insulation owing to electrical, thermal, environmental or mechanical influences. With the aim of gaining a better understanding of these problems from a technical point of view, the dielectric properties of low pressure oil-filled cable cores, which have been equally produced but differently used, are investigated. Results are presented and discussed in this paper and should lead to some new experiences of diagnostic tools for ageing mechanisms in oil-paper insulation.Cost-efficient XLPE cable system solutions
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010514
In this paper, different XLPE transmission system solutions are described in terms of technology, ecology and economy. Based on the growing experience of XLPE cable systems and their qualitative improvements during the years, a condensed reliability study between OH-lines and XLPE cable systems with pre-moulded accessories is presented. Other aspects are commented with regard to the following question: "Which consequences are present if another power transmission system solution is chosen?".Optimising the maintenance of 11 kV oil filled switchgear - an example of condition based decision making
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010702
Owners and operators of electricity networks are reviewing policies and procedures to ensure the optimum level of maintenance is carried out on network plant. EA Technology has worked closely with a number of UK RECs to assist with the introduction of maintenance regimes which are based on establishing the condition (and future performance) of assets using carefully designed programmes of sampling and inspection. These programmes are resulting in very significant cost savings while maintaining or improving the level of reliability and safety. This paper describes the application of a maintenance optimisation scheme related to 11 kV oil filled switchgear within UK electricity companies. It is believed the approach described can not only be applied more widely to 11 kV oil filled switchgear but many of the generic features are applicable, with the potential for very significant savings, to other assets on electricity networks and beyond. (3 pages)Economical 36 kV cable system for the Belgian network
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010673
In order to reduce the cost of the 36 kV cable, Electrabel redefined cables in terms of performance rather than construction regulations as in the past. This allowed cable to be designed with lower XLPE insulation and sheath thickness and reduced screen cross-section, which led to a cost reduction of 20% on cables. Tests on the new lean cable with accessories were performed successfully. Several 100 km of the new cables have been installed in the network since the end of 1998. The utilities intend to ask the Belgian Electrotechnical Committee to take the necessary steps to have the new construction of 36 kV cable included in European standards. (8 pages)Cable diagnosis. In-situ tests with returned voltage diagnosis method in Romania
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010721
It has been established that in service aged cable insulation, the polar components are directly related to the presence of water trees in the solid dielectric. R-C combinations, which represent water trees from an equivalent circuit point of view, have different time constants depending on the progression of the water tree. This paper describes an in-situ cable diagnosis method based on measurement of the return voltage. The cable is charged for several minutes by applying a DC high voltage. This step is followed by a soft discharge via a resistor for several seconds. During the third step the return voltage is measured, which results from the discharge of the polar components. The shape of the return voltage curve is characterized by the different time constants. While the applicability of the method seems to be proven in daily practice for PE and XLPE cables, many more tests and the creation of a database are necessary before a final decision regarding the applicability of the return voltage method for paper and PVC cables can be rendered. (10 pages)Condition based maintenance of MV power cable systems on the basis of advanced PD diagnostics
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010701
From the point of view of a power utility, insight in the condition of their medium voltage power cable network becomes more and more important to guarantee a reliable and secure power supply. For several years, detection and localisation of partial discharges (PD) using AC voltages is internationally accepted as a symptomatic, reliable diagnostic method for determination of the quality of insulation systems. In this paper, several ageing aspects in medium voltage power cables are discussed, supported by several diagnostic field examples. The following conclusions are drawn: studying fault statistic of components results in the identification of dominant faults; old 'diagnostics' are still very useful for some of these fault causes; using PD diagnostics on cables, important information of a cable section is collected in the PD 'fingerprint' of the measured cable section; standard interpretation rules are used for optimal decision-making for cable repair before failure occurs; the PD inception voltage, PD magnitudes, PD location, PD intensity and PD mapping together can give a good insight in the condition of a cable section; discharge defects can be related to different insulation systems of a cable section, either the cable accessories or the cable insulation itself; from PD mappings important information can be collected concerning PD source types in the paper insulated lead cable, e.g. either concentrated PD or scattered PD; and for determination of PD activity in XLPE cable insulation, combined analysis of PD quantities is necessary. (5 pages)Specialized polymer concrete composite material for in-situ cleansing/improvement of transformer oil
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010692
By the very nature of its composition, involving the presence of micron-size particles bridging the various aggregate fractions in the matrix, a very large specific surface becomes available for adsorptive mechanisms to come into play. The diffusion of fluids through the polymeric films on the filler materials in polymer concrete, coupled with the adsorptive nature of the matrix, together offer a beneficial opportunity of interest to the transformer industry. Specifically, this opportunity pertains to the in-situ cleansing/improving of transformer oil during the operational life of the transformer. The premise of this paper is that the purification ability of polymer concrete in an oil-filled transformer environment can render many economic benefits in both a specific and overall sense by reducing or eliminating oil-related maintenance chores. (3 pages)Evolution of the French MV cable system
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010674
The 20 kV synthetic cable on the French distribution network has an excellent experience feedback; the very innovative design at the introduction of this cable on the network, has barely changed during 20 years of operation. Meanwhile, significant improvements of the synthetic insulation, on the material point of view as well as on the industrial process, have led to an extension of this technology to the highest voltage levels and to the development of more efficient accessories. Due to these technical achievements, EDF and the manufacturers of this product have considered that its optimisation was possible. The chosen way to directly draw benefit from progress of the synthetic insulation was to reduce the insulation thickness of the cables from 5.5 mm to 4.5 mm. These optimised cables were developed, and the corresponding connection accessories were also adapted. Taking into account the reductions of the cable diameters, and the increase in the electric field, the compatibility of these new cables with the accessories has been carefully studied. Accessories were validated according to the European document HD 629.1 the French document C 33-001, and additional breakdown tests. Field experimentation has been done and showed that some tools needed to be adapted. This project required good practice of the jointers in cable preparation. Since March 2000, Electricite de France, provisions exclusively this new cable and this has allowed a sensible reduction of the cost of 20 kV underground cables. (4 pages)Evaluation of on-site dielectric response methods for nondestructive testing of water treed MV XLPE cables
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010720
This paper presents results from a joint Nordic project for condition assessment of water tree degraded 12 and 24 kV XLPE cables. The results show that that the condition of old cables with a high density of vented water trees, are correctly assessed by all the test methods. To detect bad cables with water trees bridging the insulation, it is necessary to also measure the polarisation currents when using the time domain methods. In case of cables with very few but long vented water trees, the evaluation criteria need to be further developed. During diagnostic testing of cables in service, the response from the installed accessories and the air humidity should be known in order to avoid wrong interpretation of the cable condition. (5 pages)The durability and performance of polymer housed metal oxide surge arresters under impulse current stresses
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010707
Unexpectedly, many failures of modern MOA types have happened in some laboratory tests where arresters are stressed with either multipulses or series of impulse current stresses. The typical failure mode in these tests was an internal flashover over one or more metal oxide discs in an MOA. In this paper the effects of internal humidity content on the internal flashover failure tendency are reported. The polymer housed arresters tested were exposed to a humidity stress in such a way that internal humidity content in the arresters gradually increased. At the same time impulse current stress series were regularly applied to the specimens. The test results are discussed and compared to the results obtained in a similar impulse current stress series conducted earlier on arresters of the same types at Tampere University of Technology. In that test series only impulse current stresses were applied on arresters without any other stresses. (5 pages)Detecting and locating MV failure before it occurs. Experience with live line partial discharge detection on underground paper insulated 11 kV cables in London
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010712
Detecting and locating incipient failure in underground cables before electrical faults occur has significant benefits in terms of customer satisfaction and quality of supply statistics particularly for “worst served customers” and investment targeting. Timely location and targeted replacement of specific sections of unsatisfactory circuits enables the effective service life of the whole circuit to be economically extended. As some 98% of the supply interruptions in London arise from cable systems, London Power Networks (LPN) is seeking to develop techniques to eliminate cable faults before they occur. LPN have recently commenced installing live line multi-functional monitoring equipment on 300 higher fault rate feeder circuits. Preliminary results of live line condition monitoring have shown very encouraging results in being able to detect and locate failure before it occurs. (5 pages)Thermal endurance of high temperature fiber reinforced cellulose insulation [for transformers]
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010693
Advancements have been made in the thermal endurance of cellulose-based insulating papers by reinforcing the cellulose matrix with a high temperature synthetic fiber. This new insulating material is made by adding synthetic fibers to the cellulose pulp along with a binder material that bonds the synthetic fiber into the cellulose matrix. The high temperature fibers form a reinforcing backbone that gives the paper mechanical integrity, even after the cellulose component has thermally degraded. The authors describe how this new material has been used to improve the thermal durability of liquid-filled power transformers. (5 pages)An assessment on brittle fracture of composite insulators' rods and factors improving their properties
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010689
Different mechanisms of the brittle fracture of a composite insulator's rod, manufactured by a pultrusion process with a given percentage of fiberglass (70-75% by weight), have been investigated in this research. The epoxy-fiberglass composites made in this way show high tensile strengths under normal conditions, but they are sensitive to environmental factors while under electrical and mechanical stresses, and thus corrosive environments, temperature and stress can affect their properties with time. This is due to the fact that the composite consists of a large number of fiber-resin interfaces. The occurrence of failure in any of these interfaces will lead to the diffusion of moisture which in turn will be condensed to water. It can be concluded from the results that the following factors greatly influence the brittle fracture of the rod: type and chemical composition of resin and the fiberglass used, and fiber weight percentage in the rod. Thus by changing the chemical composition of fibers and also by increasing the fiber-resin compatibility, the long-term tensile properties of the rod can be significantly improved. (4 pages)