Georgios D. Peppas

Ultrastable Natural Ester-Based Nanofluids for High Voltage Insulation Applications

Nanofluids for high voltage insulation systems have emerged as a potential substitute for liquid dielectrics in industrial applications. Nevertheless, the sedimentation of nanoparticles has been so far a serious barrier for their wide and effective exploitation. The present work reports on the development and in-depth characterization of colloidally ultrastable natural ester oil insulation systems containing iron oxide nanocrystals which lift the problem of sedimentation and phase separation. Compared to state-of-the-art systems, the final product is endowed with increased dielectric strength, faster thermal response, lower dielectric losses (decreased dissipation factor: tan δ), and very high endurance during discharge stressing. The developed nanofluid was studied and compared with a similar system containing commercial iron oxide nanoparticles, the latter demonstrating extensive sedimentation. Herein, the dielectric properties of the nanofluids are analyzed at various concentrations by means of breakdown voltage and dissipation factor measurements. The characterization techniques unequivocally demonstrate the high performance reliability of the reported nanofluid, which constitutes a significant breakthrough in the field of high voltage insulation technologies.

Induced Voltage on a above-ground natural gas/oil pipeline due to lightning strike on a Transmission line

This paper studies the effect of a High Voltage transmission line on a nearby above-ground natural gas/oil pipeline, under lightning conditions affecting the transmission line itself. A detailed description of the models used to simulate the transmission line - natural gas/oil pipeline layout, is presented and specific information concerning the calculations of model parameters and their implementation in ATP-Draw of EMTP are given. Thereafter, the coupling between the 400kV transmission line and the nearby natural gas/oil pipeline is examined and simulated. Finally simulation results are presented and an attempt to deduce useful conclusions based on the overall study is carried out.

Analysis of lightning impacts in Greece

In this paper, data analysis concerning human and animal deaths and injuries from lightning in Greece, is presented, for a time period of eleven years, from 2000 to 2010. In addition lightning strikes in factory facilities, buildings, aircrafts and lightning-ignited fires are recorded and presented. Lightning death data have been distributed in years and months of occurrence, and an effort has been paid to connect the casualties with the places they occurred and the actions of the victims. Furthermore in this paper, the relative lightning death indicator Dr is calculated for different, places in Greece to express the relative number of lightning deaths in a country (or province) taking into account its population, its area and the lightning activity Ng. This indicator is useful for comparison purposes and could merely be a criterion for the effectiveness of lightning protection measures, and for the results of educational and informational campaigns about protection from lightning danger.

Lightning protection of Airport Traffic Control Towers (ATCT)

In this paper effort has been made for improvement of the design of Lightning Protection Systems (LPS) for Airport Traffic Control Towers (ATCT) in Greece. The improvement was limited to the effective placement of down-conductors and grounding systems in order to increase the level of protection against direct lighting strike. Technical parameters for implementation of external lightning protection have been analyzed and constructional instructions for the proper installation of a LPS is presented according to the international norms and patterns. Additionally a simplified P-spice simulation is presented that indicates performance improvements.

Investigation of grounding resistance effect on the MV grid of Hellenic electromotive railway during lightning strikes

The paper discusses the effect of the grounding resistance on the transient response of MV railway grid during lightning strikes. For study purposes were modelled the electrified railway MV grid (ERMV) that the Hellenic electromotive suburban railway makes use, including the masts, consoles, the rails and the overhead medium-voltage transmission grid of 25kV dedicated for this purpose. For the simulation of the understudied model time-domain solver was adopted in ATP-EMTP software. The model Volt-Time Curve is used for the modeling of the insulator, but it was necessary to create a new equation for the specific model, because of the short length of the insulator in such projects. Two types of lightning are used for the simulation purposes, for fast and slow signal, with current value in 100kA and one measurement in 30kA. For fast signal, the values of front and tail time are 1.2/50μs and for the slow signal 10/350μs. In addition, for the ground resistance: 10Ω, 70Ω and 245Ω are used.

Lightning protection of pipeline systems

In this paper a full scale simulation analysis is done for the interference of a High Voltage Transmission line and a underground/aboveground pipeline system. Herein, a coupled system of 400kV transmission line and a nearby pipeline is simulated by means of ATP-EMTP for the case of direct lightning strike to transmission line. Simulation results are presented and possible issues and suggestions are commentated. The necessity of a detailed integrated standard prescribing the lightning and surge protection of pipelines in combination with cathodic protection system is essential.

Effectiveness investigation of wind farm earthing system

Wind turbines are tall structures with heights that can exceed 80m. Their installation is usually done at high altitude and isolated areas causing these vulnerable to lightning strikes considering their total hub height and blade radius. The sites that wind farms are installed are usually of high lightning activity. Furthermore, in these areas the soil resistivity is high, leading to the design and construction of a reliable earthing system that shall ensure both lightning and earth fault current dissipation. Step and touch voltages shall be compatible with the safety limits as these are defined in IEEE80-2000. The earthing system is the main part of the wind turbine lightning protection system apart from the receptors and down conductors. It leads the lightning and the earth fault currents to the earth in a safe way both for the infrastructure and for the human beings. The earthing system of a wind turbine and thus the wind farm shall be designed on project specific conditions, considering the ground conditions, short-circuit analysis of the power plant collector system, international standards and also national requirements of the country that it shall operate if such exist. In the current paper will be discussed the effectiveness of a wind farm earthing system regarding the current dissipation. The evaluation will be executed based on project specific soil conditions which have been measured during a dry season and foundation earthing system of each wind turbine. The transient analysis will be performed via simulations in EMTP-ATP. The extracted results will be useful for other wind farms that are installed in similar soil and/or electrical conditions in order to reduce the project execution time by using a proven earthing solution.

Dielectric study of self assembled nanofluids by means of AC breakdown tests and dielectric relaxation spectroscopy

In this paper a detailed study of two types of nanofluids is performed by means of AC breakdown voltage tests (IEC 60156) and Dielectric Relaxation Spectroscopy. The first under tested nanofluid (pNF) is composed from natural ester oil matrix and purchased nanoparticles (Aldrich) ensuant to surface modification coating. Accordingly, the second nanofluid (colNF) aparted from colloidal hydrophobic self-assembled nanoparticles with in-situ surface coating technique, inserted adequately into natural ester oil matrix. The aforementioned nanofluids (pNFcolNF) are studied under AC breakdown voltage wherein an interesting increase of the dielectric strength is demonstrated. Accordingly the dielectric response of the aforesaid nanofluids has been investigated by employing dielectric measurements, wherein the nanofluids dielectric permittivity is studied isothermally, as a function of frequency in the range of 10−1 to 106 Hz at temperatures from 20–100 °C under dry nitrogen atmosphere. Finally a thermal response study is performed, wherein the response of the nanofluids is compared with the natural ester oil matrix.

Electrical and optical-emission parallel study of natural ester liquids stressed by impulse high voltage

Dielectric insulation liquids are employed in a diversity of high voltage equipment due to their unique properties. Especially, natural ester oils exhibit high bio-degrability, non-toxicity and fire resistant properties, just to refer a few, making them a promising alternate of mineral oil dielectrics. Hereby, an effort for better understanding of breakdown phenomena in natural ester oils is attempted. In-situ electrical and optical emission signals are recorded during oil stressing by impulse high voltage (standard switching waveform). Point-to-plane electrode configuration is used, with constructive features imposed by the IEC 60897 Standard for non-homogenous field production. Two gap distances are examined i.e. 1 cm and 2 cm. We specially focus on the pre-breakdown phenomena which are well distinguished from the optical emission signals in the wider gap separation.

Dielectric strength measurements in photovoltaic modules

The purpose of this study is to determine the dielectric strength of a photovoltaic module through laboratory measurements. The measurements were taken using the standard impulse voltage, according to IEC 61730-2. Measurements were taken before and after the voltage stress in order to investigate in which way the module is affected. These results appear in the IV and PI characteristic curves. Subsequently, after a period of six months, the stressed module was compared to a similar non-stressed to find out whether the first regains some of its properties. In the same way the IV and PI curves are quoted. In conclusion the results obtained are evaluated and confirmed by the standards with a view to approach an optimal design of lightning and surge protection systems for photovoltaic installations (PVIs).