1,721,007 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
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/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
Standardized methods for the determination of breakdown voltages of liquid dielectrics
No full textIEC 60156 and IEC 60897 were standardized with the purpose of evaluating the breakdown voltage of liquid dielectrics both under AC and impulse voltages. AC breakdown voltage determination is a simple method but only able to provide information on the presence of contaminants. The limit of this test is the large scatter in the results and, for this reason, this IEC standard has been recently revised. Impulse breakdown voltage determination allows discrimination of different dielectric liquids, also in terms of their chemical composition. This test has only limited application because of the dimensions of the high voltage generator. The present paper shows a comparison among breakdown voltages of various mineral oils and natural esters under both AC and impulse voltages. The paper presents also an effort to describe how these two tests may reveal the electrical performance of the dielectric liquid under test. © 1994-2012 IEEE
A microgrid control strategy to save energy and curb global carbon emissions
No full textThe proposed study concerns the implementation of new strategies aimed at saving energy and reducing the global warming through the reduction of carbon emissions. For this purpose, the implementation of synergic procedures, contemporarily applied to microgrid and prosumers, are proposed and validated in the paper. The main implemented methodologies allow the real time management of different energy flows present inside buildings and, at the same time at higher hierarchic level, enable the supervision and control of the energy managed by the microgrid. The proposed general strategy optimizes contemporarily energy exchanges with the public electricity system, local electrical generation, storage and controllable loads. The developed concepts are valid in general, but also for the port systems where the purpose aims at saving energy and reducing CO2 emissions
Standard evolution for the determination of the power frequency breakdown voltages in insulating liquids
No full textThe 2016-2018 ongoing revision of the lEC 60156 'Insulating liquids - Determination of the breakdown voltage at power frequency' was just approved and the revised standard will be published by the end of 2018. This document will include the recommended use of a stirring system with the main purpose to reduce the scatter in the ac breakdown voltage results. The present paper reports a comparison of breakdown voltage mean values and related scatters of two different insulating liquids (traditional mineral oils and natural ester liquids) in accordance with the tests methods of the actual and 2018 upcoming revision of IEC 60156. The results presented here confirm that the modification (stirring system) introduced in the lEC standardized test method, even a simple rearrangement, is able to reduce the scatter of the mean values of the breakdown voltage of the insulating liquids. Some considerations of an alternative test method procedure introduced by the 2018 upcoming revision of lEC 60156 are also reported. © 2018 IEEE
Influence of water contents on ester liquids partial discharge inception voltage
No full textPartial Discharge Inception Voltage (PDIV) and AC breakdown voltage in insulating liquids are important indicators used for representing their integrity. The test procedures for the determination of these indicators are described in IEC TR 61294 (Determination of the Partial Discharge Inception Voltage) and IEC 60156 'Determination of the breakdown voltage at power frequency - Test method' respectively. In this paper, PDIV and Partial Discharge (PD) activities in a mineral oil, a natural ester and a synthetic ester are considered, taking also into account the influence of added water in these insulating liquids. Furthermore, different PDIV determination criteria have been adopted and compared with the actual described in IEC TR 61294
Behavior of transformers interconnecting microgrid and prosumers
No full textThe recent evolution in electricity distribution systems has led to the development of new intelligent structures, better known as Smart Grids, Microgrids and Prosumers. The consequent advantages regard mainly both economy aspects and environmental safeguard. In a microgrid, producers and consumers can exchange energy in peer-to-peer way. Normally, the microgrid is connected to a Smart Grid in a single common point. In this case, the microgrid can be easily disconnected from the external grid and continue to work in 'island' mode. In this condition, local production must be equal to the energy consumed, unless local storage systems are installed on site. In case of a bidirectional management, the source may change in load and vice versa. In this situation, a traditional transformer may work in an overload condition and the protection system may trip, provoking, in the worst conditions, a blackout. In order to avoid this condition, some solutions are here proposed and commented. © 2019 IEEE
The New Prosumer Tasks in the Energy Management of Buildings
No full textA prosumer (energy producer-consumer) usually exchanges energy and information with a microgrid, taking into consideration at the same time also such economic aspects as buying or selling energy to the highest bidder based on current rates per kilowatt hour. The proposed study describes the implementation of a new approach that allows to manage in real time the different energy flows inside a building according to a general strategy of simultaneous optimization of electrical generation, storage and controllable loads. On a local level, management may also include more prosumer points, each connected to the microgrid. In this instance, the microgrid supervises and optimizes, at a higher hierarchical level, the various energy flows with consequent synergic, mutual advantages of the different systems, both private and public
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