201 research outputs found
Report on ICDL 2019. University of Roma "la Sapienza," Rome, Italy, June 23 27, 2019
No full textThe 20th IEEE International Conference on Dielectric
Liquids (ICDL 2019), chaired by Massimo Pompili, was held
from June 23 to 27, 2019, hosted by the Faculty of Engineering
of University of Roma "La Sapienza". The ICDL 2019 is fully
sponsored by the IEEE Dielectrics and Electrical Insulation
Society (DEIS), supported by Cargill, Fratelli Parodi, Midel,
Baur, Ekofluid, Ergon, Haefely Hipotronics, IET, Nynas,
Omicron and Sea Marconi. The conference was chaired by
Massimo Pompili in collaboration with Technical Program
Chair Luigi Calcara and the Italian CEI (Comitato Elettrotecnico
Italiano
Bibliography. Works by Aby Warburg and secondary literature (updated June 2018)
No full textN.B. This Bibliography is a compilation of the main editions of Aby Warburg’s works, essays and critical materials from 1930 to the present [updated: June 2018]. We invite scholars and readers to send suggestions for the inclusion of any additional bibliographic materials to the editorial team of Engramma
Standard evolution of Partial Discharge detection in dielectric liquids
No full textPartial Discharges (PD) detection in HV components has shown to be a very powerful diagnostic tool. Anyway, also the acquisition of the same signals in simple insulations, like the dielectric liquids, may give additional information for maintenance of HV components in which they are employed, as in the case of liquid insulated transformers. Among the standardized electrical tests suggested for the insulating liquids, the breakdown voltage (IEC 60156) and partial discharges determination (IEC 61294TR) at power frequency are not basic material properties but test procedures intended to indicate the presence of contaminants such as water and solid suspended matter and the advisability of carrying out drying and filtration treatment. Nevertheless, the PD detection method for insulating liquids actually standardized is based on superseded circuitry and is only addressed to measure the PD Inception Voltages (PDIV). The present paper shows the state of art in PD measurements in dielectric liquids to be taken into account when revising the actual IEC 61294TR or preparing new international Standards (ASTM)
MV/LV parallel transformers. An experimental study on circulating currents
No full textWhen a high value load is to be supplied and greater system reliability is required, the medium voltage power supply is generally used with a low voltage conversion by means of two or more MV/LV transformers. This configuration is also more suitable for maintenance or resilience aspects. In Europe, in case of use of these distribution transformers, the windings configuration is Delta (primary winding) and Star (secondary winding). To obtain a common ground reference for the three phases and neutral system, the star point is grounded. In case of parallel transformers, this ground point is common. Furthermore, when using transformers in parallel, a proportional reduction of the short-circuit impedance is obtained with the consequent increase in the short-circuit current during faults and the need to adopt cables and protections that consider this aspect. From the other hands, the use of parallel transformers may introduce an increasing of the no load losses. In transformers parallel configuration system, if the transformation ratios are not identical, a circulation current is generated. This current is proportional to the voltage difference divided by the sum of the impedances along the path comprising windings and ground connections of the star centers. In a distribution substation with two parallel transformers of identical characteristics, the circulating currents in each transformer are of equal amplitude and in opposite directions. Few studies have investigated what happens at the connection points between the star centers of the transformers and the earth bar of the secondary substations. This paper gives a contribution on the evaluation of the circulating currents based both on theoretical considerations and experimental measurements
Heatwaves and underground MV cable joints failures
No full textIn the last years, the summer meteorological conditions became much hotter and more arid as one of the consequences of the climate changes. These conditions have caused and are causing an increasing number of unexpected failures in underground medium voltage cables, mainly in correspondence of cable joints which are weak points of the system. The anomalous hot weather conditions during the summer period may last for several days (heatwaves), worsening appreciably the consequence of this phenomenon; the stress obviously increases as the number of the warmer days and lack of rain prolongs, heating cable and joint insulating materials with their internal defects and voids, wherein partial discharge phenomena may be easily triggered at this higher temperature under the action of the electrical field greatly increased in correspondence of such defects and voids. The present paper reports a study on the hotter day sequences in some Italian main cities with the aim to establish a correct definition for the term heatwave, to be used when dealing with underground cables failure topic
MV underground cables: effects of soil thermal resistivity on anomalous working temperatures
No full textOne of the parameters that has to be considered in designing underground cables is the thermal resistivity of the soil along their routes. In fact, the ampacity of the MV underground cables having different types of insulation is normally evaluated through the application of the IEC Standard 60287-x 'Electric cables - Calculation of the current rating'. This series of IEC Standard is formed of three parts as in the following: part 1: Formulae of ratings and power losses; part 2: Formulae for thermal resistance; part 3: Sections on operating conditions. In Italy, the current capacity of the same MV cables is established by the Italian Standard CEI-UNEL 35027 (2009) 'Power cables with rated voltages from 1 kV to 30 kV - Steady state current ratings: cables laid in air and in ground', which are based on the above mentioned IEC Standard. These thermal ampacities of the underground cables are function of ambient temperature and the thermal resistivity of the soil. The soil thermal resistivity is a parameter not easy to evaluate and for this reason, several times, only typical values are assumed just in function of the kind of the interested terrain (organic, loam, quartz sand, etc.). This assumption ignores the fact that the thermal resistivity varies continuously day by day especially in function of humidity soil content, being strongly dependent from this parameter. This paper describes as important differences of thermal resistivity may be present in consequence of seasonal variation in moisture content of the soil. It is also shown as such variations may cause important anomalous increases of cable maximum working temperature, which may be assumed to be at the origin of unexpected failures especially in correspondence of MV cable joints. In fact, the joints are, still today, the weakest points of the underground cables where the breakdown easily occurs due to poor workmanship, moisture ingress and thermal degradation. This thermal degradation may be also due to circulation of fault current in the metallic sheaths
Insulating liquids breakdown voltage determination: Test method efficiency
No full textThe 50-60 Hz breakdown voltage of insulating liquids is not an intrinsic property of the dielectric but strongly depends on the presence of contaminants, such as particles and suspended water. Both IEC and ASTM give tests procedures finalized to determine the 50-60 Hz breakdown voltages. This test is adopted as acceptance on new deliveries, testing of treated liquids prior to or during filling into electrical equipment, and monitoring and maintaining oil-filled apparatus in service. Following the IEC Standards, this test has to be performed following the IEC 60156, Ed. 2.0 published in 1995 which is presently under revision. The main reason for revising the standard for the 50-60 Hz breakdown voltage determination of the insulating liquids is the attempt to reduce the scatter in the results, the quantities of the sampling oils and the testing times. The future new edition of IEC 60156 is on the way to reconfirm the present standardized procedure but will recommend the use of a stirring system. Also in an informative annex a new advanced procedure will be included, claiming to be able to reduce the scatter in the determination of the 50-60 Hz breakdown voltages. With the aim to evaluate the mean value and related scatter of power frequency breakdown voltages of insulating liquids, testing time and quantities of samples, this paper presents a comparison of adopted test methods recommended by IEC 60156 and other methods, like the Up-and-Down- and Multiple-Level methods
MV Underground Power Cable Joints Premature Failures
No full textIn the last years, the frequency and the intensity of adverse meteorological conditions are on the rise and climate changes have the potential to affect negatively the electrical systems in many different ways. Local or extended electrical outages have been and are caused by extremely severe negative meteorological events of different nature, like strong wind, wet snow, heavy rain and heat waves. Of course, such interruptions of electrical system affect also critical infrastructures with a negative impact on all the human and industrial activities. To face this reality, the Italian Regulatory Authority for Energy, Networks and the Environment (ARERA), established new more stringent rules to maintain high standards of power quality services also through more resilient electrical system. For this reason, there have been established very ambitious benchmark values for the key performance indicators, normally used to evaluate the quality of electric service. With the aim to increase the resilience of the distribution electrical networks, the DSOs are studying and developing new grid operations with improved assets adopting also advanced electrical technologies. For this reason, in the last decades were developed medium voltage cable joints much easier and faster to install. These joints can make the system more vulnerable for possible imperfections introduced during the construction phase for unperfect manpower and for the complexity of the operations
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