Mona Ghassemi

Coupled Computational Fluid Dynamics and Heat Transfer Modeling of the Effects of Wind Speed and Direction on Temperature Increase of an Ice-Covered FRP Live-Line Tool

A coupled computational fluid dynamics (CFD) and heat-transfer model for an ice-covered fiberglass-reinforced plastic (FRP) hot stick, elaborated in a previous study, could well explain why the flow of partial-discharge current could be sufficient to raise the temperature of an iced pollution layer just below freezing, where the cold-fog flashover mechanism prevails. However, the ice-covered hot stick was modeled as a solid “ice rod” having an equivalent cross section of ice, meaning that the exposed ice surface is smaller in the model compared to reality. In addition, the simulations were performed for a relatively low wind speed of 1 m/s, while average wind speeds of 6.1-14.4 m/s were reported for the two Manitoba flashovers. Both of these problems are addressed in this paper to deal with the site incident conditions. The ice cover is considered as a thin layer having a thickness of 1 mm on the FRP hot stick. The effects of wind speeds of 0.1-15 m/s and wind direction as parallel and perpendicular to the ice-covered FRP hot stick are studied. This paper also presents experimental investigations on the most reliable reproduction of four separate FRP hot stick flashover incidents in Canada achieved at CIGELE laboratories.

A coupled computational fluid dynamics and heat transfer model for accurate estimation of temperature increase of an ice-covered FRP live-line tool

Controlled laboratory tests validated a hypothesis that extremely light (~2-3 μg/cm2) levels of Equivalent Salt Deposit Density (ESDD) with no Non-Soluble Deposit (NSDD) can reduce voltage withstand capability of Fiberglass-Reinforced Plastic (FRP) hot sticks under cold-fog conditions near the freezing point, where the tool surfaces are fully wetted by the environment or alternately surrounded by fog. However, the source of moisture in flashovers at temperatures below -13 °C was not established. The mechanism of tool surface wetting was explored through coupled Computational Fluid Dynamics (CFD) and Heat Transfer mathematical equations for a FRP hot stick modeled in Commercial software, COMSOL Multiphysics. The results show that the flow of partial discharge current could be sufficient to raise the temperature of an iced pollution layer just below freezing, where the cold-fog flashover mechanism prevails.

Effects of tower, phase conductors and shield wires on the electrical field around a tower window during live-line work

The three-dimensional FEM electric field calculation model of a fiber glass-reinforced plastic (FRP) hot stick during live-line work, which was elaborated in a previous study, could well explain some features of the flashovers that occurred during a series of cold fog tests at CIGELE. These tests have achieved the most reliable reproduction of four separate FRP hot stick flashover incidents in Canada at a voltage stress of 105 kV/m at -1.04°C, Relative Humidity (RH) of 109 % with visible fog and 2.8 μg/cm2 Equivalent Salt Deposit Density (ESDD). However, at the incident site, the geometry is different from that of the laboratory tests. In this paper, a three-dimensional electric field calculation model of a FRP hot stick during live-line work based on the finite element method is proposed to account for the geometry of the Manitoba site incidents at 500-kV. Moreover, the influence of the tower, phase conductors and shield wires on the potential and electric field distribution around an FRP hot stick during live-line work is studied.

A thermo-electrodynamic electric field dependent molecular ionization model to realize positive streamer propagation in a wet-mate DC connector

Complete subsea factory concept, an equivalent of the full topsides processing facility to be operated on the seabed, is envisaged to power longer, deeper and colder subsea oil and gas fields in the future. This concept has been envisioned through a modular stacked subsea DC transmission and distribution system whose subsea umbilical cables and electrical power component on the seabed can be interfaced with each other by wet-mate (WM) DC connectors. Laboratory and theoretical investigations have been carried out to assess various electrical insulation systems and electrode geometries for a WM DC connector which should operate in the steady state as well as switching transients in a corrosive environment for high reliability and minimum maintenance in its lifetime. In this paper, the electrical insulation performance of a needle-sphere electrode geometry defined by IEC 60897 under a positive step voltage is studied. To approach the complicated solid-liquid insulation system envisaged in a WM DC connector after mating, the electrodes are covered by a dielectric solid and oil is enclosed by the dielectric solid as well. A full thermo-electrodynamic electric field dependent molecular ionization Multiphysics model was developed for the simulation of streamer initiation and growth in the oil while dielectric solid is modeled as a perfect insulator. It is shown that stabilization methods, mesh strategies and time step have a great influence on simulation results and guidelines to choose them properly are presented. Based on simulation results, it was found that the higher relative permittivity of the solid insulation the slower streamer propagation in the oil and the less electrical stress on the solid insulation.

The influence of magnitude and rise time of applied voltage and the type of oil on streamer growth in a wet-mate DC connector

For the safe design and operation of wet-mate (WM) DC power connector, a time-dependent full thermo-electrodynamic model comprised of Poissons equation, three charge continuity equations-one each for the positive and negative ions and one for the electrons-, and a thermal diffusion equation is developed to study the streamer initiation and propagation in the oil portion of a WM DC chamber. The electric field dependent molecular ionization mechanism accounts for the source term for free charge carriers, and positive ion/electron recombination, positive/negative ion recombination and electron attachment represent sink terms in the oil section. The solid portion of the WM DC connector is modeled as a perfect insulator. Considering a needle-sphere electrode geometry with electrodes covered by a dielectric solid and oil enclosed by the dielectric solid, it is approached the complicated solid-liquid insulation system envisaged in a WM DC connector after mating. By using the model, the influence of three parameters including magnitude and rise time of applied voltage as well as the type of oil on streamer initiation and propagation is investigated. It is found that the shorter rise time the more prominent streamer growth in the oil portion. For oil comprising only aromatics, an electric field magnitude larger than about 2×108 V/m is needed to propagate streamers, while this value for the oil comprising naphthenics/paraffinics will be exceeding 4×108 V/m.

Modeling a liquid-solid insulation system used in a DC wet-mate connector

Subsea DC transmission and distribution system is a promising technology for powering subsea oil and gas fields with high power, long distance and ultra-deepwater depth. DC connectors as the interface between cables and electrical power component and loads in such a system play a key role. An electrodynamic model is developed in this paper to study charge transport phenomena in a DC wet-mate subsea connector. The oil-solid insulation system of WM chamber contains synthetic transformer oil enclosed by dielectric solid and the electrodes covered by dielectric solid. This complicated hybrid insulation system placed in a cylindrical electrode geometry is simulated in this paper. It is shown that the oil can be susceptible to the streamer initiation and development. Moreover, it will be discussed that the free space charge carriers traveled to the interface between the oil and dielectric solid and converted to the surface charges may increase the electric field magnitude across the dielectric solid.

Nanostructured insulation for high torque density electric propulsion motors

It has been identified in ONR Next Generation Integrated Power System (NGIPS) Roadmap that fundamental research in dielectric insulation research enables payload efficiency and affordable high power density of integrated electric propulsion motors. The objective of this study is to revolutionize electrical insulation in the manufacturing of NGIPS motors for military electric propulsion with game-changing torque density and payload efficiency through the development of nanostructured insulation with significant improvements in both electrical and thermal performance. This paper presents the progress of nanostructured insulation innovation, high field characterization, performance and insulation integrity validation under high voltage, high frequency multi-stresses. The thermal, dielectric and voltage endurance properties of novel nanocomposite insulation based on 2D-nanostructured platelet fillers were investigated. It was demonstrated that nanostructured insulation could offer significant improvement over conventional insulation system in electrical, dielectric, thermal and mechanical properties. A Design of Experiment was employed to study the effects of various 2D fillers and their interplay, and more importantly to identify the optimal nanostructured formulation with high thermal conductivity of >0.8 W/mK, low dielectric constant of less than 5, low dissipation factor of less than 3% at 150°C and high breakdown strength of >1000V/mil. Furthermore, disk samples with optimal formulation from Design of Experiment were fabricated for voltage endurance tests in accordance with IEC 60343 standard for evaluation of their long-term endurance life.

Characterization of solid-liquid interface for wet-mate subsea HVDC connectors

Wet-mate connectors are essential components in a submarine HVDC system. Solid-liquid insulation systems play an important role in the electrical insulation performance of a WM chamber. In this paper, the electric field distribution in synthetic ester oil of a solid-oil insulation system is studied by using Kerr electro-optic technique where the measurements were carried out with the presence of moisture and ionic contaminants. Moreover, effect of voltage polarity on the electric field distribution in the oil is investigated. For two solid liquid interface geometries studied, the electric field norm decreases gradually from the ground electrode to the HV electrode under positive DC while it remains almost constant for an applied negative DC voltage. The deviation of magnitudes of electric fields particularly near the electrodes suggests the presence of positive ions in the oil under positive and negative testing conditions.

A study on performance of a wet-mate DC connector under overvoltages

Due to diminishing oil and gas reservoirs in existing brownfield sites, offshore drilling activity continues to migrate into new oil and gas sites located in deeper water and further from the shore. For technical and economic reasons, processing of hydrocarbons is preferred on the seabed in ultra-deepwater sites instead of doing it on a host platform at the surface. Since the use of AC subsea transmission and distribution system to supply subsea process becomes impractical once beyond the critical length limit of AC submarine cables due to their capacitive charging current, a modular stacked DC (MSDC) subsea transmission and distribution system emerges as a promising technology alternative. Allowing only the faulty module to be retrieved for repair, wet-mate (WM) DC connectors are key to the reduction of downtime for the complete MSDC system. While there is no commercially available WM DC power connector, a time-dependent full thermo-electrodynamic model comprised of Poissons equation, three charge continuity equations-one each for the positive and negative ions and one for the electrons-, and a thermal diffusion equation is developed to study the streamer initiation and propagation in the oil portion of a WM DC chamber. The envisaged geometry of a WM DC connector containing oil-solid insulation systems located in a cylindrical electrode geometry is considered for simulation. It is shown that the model developed can be used as an engineering tool to study the performance of compact designs of WM connector under overvoltages. In this regard, for the geometry considered for simulations, it is shown that a dielectric solid layer with a thickness of 1 mm is needed between moving contact and dielectric oil after mating to prevent streamer growth in the oil for a 4 p.u. overvoltage.