Qin Chen

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.

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.

Comparison of space charge behavior in XLPE and EPR with thermal gradient

A comparative study of the space charge behavior in XLPE and EPR in the presence of thermal gradient was conducted. For power cable under load conditions, a temperature gradient is established across the cable insulation. Due to the resistive grading under high voltage direct current, such thermal gradient will result in the accumulation of space charge. A modified pulsed-electroacoustic system was developed so that the space charge behavior in thin films under thermal gradient can be investigated at the development phase of new DC insulation materials. The study indicates the enhancement of the space charge injection and accumulation in XLPE and EPR under DC with the presence of the thermal gradient.

Effect of moisture and ionic contaminations on DC insulation for subsea applications

DC electrical insulation is critical for the reliable operation of subsea DC electrical systems. Contamination due to exposure to seawater may significantly affect the reliability of DC insulation. This paper reports the impact of seawater-related contamination on DC-related properties of epoxy insulation. Diffusion coefficients and saturation levels of seawater in the insulation were measured through water uptake experiments. DC conductivity and breakdown strength are significantly affected by seawater contamination. For the seawater-saturated samples, conduction current increases with time under sufficiently high DC field. Whereas the short-term DC breakdown strength is 23 kV/mm, a seawater-saturated sample breaks down after 130 min under only 12 kV/mm. Space charge measurement reveals different charge evolution behaviors under different electric field levels. Under average field of 12 kV/mm, much higher local field of 27 kV/mm exists near the positive electrode. A possible explanation is the interplay between heterocharge-induced field enhancement around the electrodes and the promotion of charge injection due to the enhanced field.

The effect of thermal gradient on space charge pattern in XLPE

This paper describes the comparison between the simulated results of thermal gradient using COMSOL and actual observed results of thermal gradient in modified thin film pulsed electroacoustic system, together with the effect of thermal gradient on space charge at room temperature and 50 °C. The results show the presence of thermal gradient facilitates the charge injection and electric field distortion, resulting in accelerated aging of insulation under HVDC.