Evolution of positive streamers in air over non-planar dielectrics: Experiments and simulations

We study positive streamers in air propagating along polycarbonate dielectric plates with and without small-scale surface profiles. The streamer development was documented using light-sensitive high-speed cameras and a photo-multiplier tube, and the experimental results were compared with 2D fluid streamer simulations. Two profiles were tested, one with 0.5 mm deep semi-circular corrugations and one with 0.5 mm deep rectangular corrugations. A non-profiled surface was used as a reference. Both experiments and simulations show that the surface profiles lead to significantly slower surface streamers, and also reduce their length. The rectangular-cut profile obstructs the surface streamer more than the semi-circular profile. We find quantitative agreement between simulations and experiments. For the surface with rectangular grooves, the simulations also reveal a complex propagation mechanism where new positive streamers re-ignite inside the surface profile corrugations. The results are of importance for technological applications involving streamers and solid dielectrics.Comment: 15 pages, 12 figure

A stochastic model for contact surfaces at polymer interfaces subjected to an electrical field

Morphology of the contact area between solid insulation materials ultimately determines the short- and long-term electrical properties of the complete insulation system. The main purpose of this paper is to propose a statistical model to examine the real area of contact between solid dielectric surfaces and secondly to verify and correlate the model outputs with experiments. The model computes the real area of contact, number of contact spots and average cavity size at the interface as a function of elasticity, contact force, and surface roughness. Then, using the average cavity size and the Paschen's law, the discharge inception field of the cavity (CDIE) is estimated. AC breakdown strength (BDS) testing of solid-solid interfaces was carried out, where cross-linked polyethylene (XLPE) samples with four different surface roughnesses were tested at various contact pressures. Following the increased contact force, the calculated average cavity size decreased by a factor of 4.08-4.82 from the roughest to the smoothest surface, corresponding to increased CDIEs by a factor of 2.01-2.56. Likewise, the experimentally obtained BDS values augmented by a factor of 1.4-1.7 when the contact pressure was elevated from 0.5 MPa to 1.16 MPa. A linear correlation between the CDIE and BDS was assumed, yielding a correlation coefficient varying within 0.8-1.3. When the 90% confidence intervals were considered, the range reduced to 0.86-1.05. This correlation suggests that interfacial breakdown phenomenon is strongly related to the interfacial cavity discharge. Hence, the proposed model is verified with experiments.A stochastic model for contact surfaces at polymer interfaces subjected to an electrical fieldsubmittedVersio

Review of Water Treeing in Polymeric Insulated Cables

Since discovering the water treeing phenomenon (WT) in polymeric cables in the early 1970s, water treeing has been presumed to be the main cable degradation mechanism for polymeric cables and has been extensively researched up to the late 90s. Historically, different theories were proposed to describe this phenomenon's mechanism. The two most prominent theories link the initiation and growth of WTs to (i) Environmental stress cracking (ESC) and (ii) Stress-induced chemical degradation (SIED). Additionally, different experimental investigations were conducted to highlight the correlation between different operating conditions and the initiation and growth of WTs. Despite that it has been challenging to determine or agree upon a particular water treeing mechanism, it is generally accepted that there are main factors that can influence the inception and growth of water trees and can possibly steer the water tree growth mechanism to adhere ESC or SIED theories. This paper presents a comprehensive review of different selected papers and technical reports between 1969-2020 on the topic of water treeing in polymeric cables with an emphasis on the development of standing theories and operation factors that influence the initiation and growth of WTs.Review of Water Treeing in Polymeric Insulated CablespublishedVersio

Ageing of Technical Air and Technical Air with 7.5% C5-Fluoroketone by Free-Burning Arcs

This paper reports on the effect of ageing by free-burning arcs in 7.5% C5-fluoroketone (C5-FK) with 92.5% technical air in comparison to that in technical air (80% N2, 20% O2) at 1.3bar absolute pressure. The gases are aged by applying a series of arcs dissipating an accumulated energy of around 315kJ. It is found that the arc voltages in technical air and technical air with C5-FK are in the same range and do not vary significantly as a function of ageing or current amplitude (∼40-900 A). Contact erosion in both mediums is found to be similar if the discharge procedure is same. However, erosion increases significantly if ageing is performed in a short contact gap that needs more arcing operations to achieve similar level of arcing energy accumulation. Furthermore, gas decomposition by-products are analysed using gas chromatography coupled with mass-spectrometry

Streamer Propagation along Profiled Insulator Surfaces under Positive Impulse Voltages

Controlling discharge growth on insulator surfaces is important in high voltage gaseous insulation systems. In this study, the effect of small-scale surface profiles on streamer discharge propagation is examined experimentally. The experimental test objects were 5x72x150 mm polycarbonate plates with and without machined surface profiles. One test object had a surface with 0.5 mm deep semi-circular corrugations, while the other profile had 0.5 mm deep rectangular corrugations. The semi-circular profile increased the surface area with 20 %, while the rectangular profile increased the area with 110 %. A plain surface was also examined as a reference. Positive impulse voltages were applied to a 1 mm thick disk electrode placed 2 mm above the insulator. The insulator was placed in a grounded aluminium casing. The streamer development was imaged with a light-sensitive high-speed camera. Surface charges left on the surface after the impulse were examined using an electrostatic probe and simulations of saturation charge. The rectangular surface profile reduced the streamer range significantly, which suggests an effect of added surface area. Imaging indicated that the wavelike surface streamers follow the profiles closely. Surface potential measurements showed a saddle-shaped distributions, with values in line with saturation charge computations.Streamer Propagation along Profiled Insulator Surfaces under Positive Impulse VoltagespublishedVersio

Charge accumulation in rod-plane air gap with covered rod

The focus of this work has been on hybrid insulation in inhomogeneous electric fields under lightning impulse voltage stress. The principal idea behind hybrid insulation is the intentional use of surface charges to re-distribute the electric field within an insulation system. This allows a significant part of the electric stress to be transferred from the dielectric weaker gas to the dielectric stronger solid insulation thus increasing the total electric strength of the insulation system. The concept has been theoretically and experimentally addressed by means of a hemispheric rod covered with a layer of solid insulation. Discharge activity and surface charge accumulation have been studied in an air gap by measuring the voltage and discharge current and recording the discharge activity using a high-speed digital camera. New methods have been introduced and evaluated for the evaluation of surface charge measurements. The experiments found that the increase in positive inception voltage was considerable compared to uncovered rods. This increase varied from 35% up to 100% depending on the electrode distance. The increase in breakdown strength is higher than the increase in inception voltage and dependent on the covered length of the rod. During the application of a lightning impulse, the discharge activity spreads upwards along the rod and out into the air gap. Positive discharges form numerous branches and bridge the air gap in most cases. Negative discharges are more diffuse, less light intensive and only form a few branches around the tip of the rod where the electric field is the strongest. Discharge activity along the insulating surface has been observed where the background field is lower than the critical electric field strength. Visible discharge activity is observed where the background field is higher than 2.3 kV/mm and 2.5 kV/mm for positive and negative impulses respectively. During the application of lightning impulses, discharge activity starts in the air gap around the tip where the electric field is highest and spreads upwards along the rod. As expected, negative charges accumulate on the surface in the case of positive impulse voltage and vice versa. However, after more powerful discharges during negative impulse voltage application, surface charges of both polarities have been observed. Accumulated surface charges decay exponentially with a time constant τ varying from micro-seconds to hours depending on the material properties of the solid insulation. The dominating relaxation mechanism is found to be conduction through the solid insulation. Improved methods to calculate surface charges based on probe response for a 2D axial symmetric case have been developed and evaluated. The method that is best suited for this purpose is the λ-method with truncated singular value decomposition (TSVD) as regularization. Surface charge calculations show that the accumulated surface charges for the used configuration typically have a maximum value of 0.6 to 1.5 µC/m² and 0.4 to 1 µC/m² after positive and negative impulses respectively. The surface charge density in the areas with the highest discharge activity is relatively uniform. Further upwards along the rod, the surface charge density is reduced relatively fast towards zero, and in some cases, it changes polarity before approaching zero

Electric Field Simulations of High Voltage DC Extruded Cable Systems

The market for high voltage DC (HVDC) transmission systems has increased dramatically the last few decades. This is mainly due to the economic, electrical, and environmental advantages for bulk power transmission over long distances. Also, in some cases, such as for long subsea cable links, HVDC is the only option. HVDC cables have traditionally used oil-paper for insulation, with the most common type being the mass-impregnated, nondraining cable. The oil or mass-impregnated cables have for a century shown to have a very high reliability and are currently used for voltages and power ratings up to 525 kV and about 1 GW. Today, it is still the preferred technology for the highest voltage levels [1]. However, in the last two decades, extruded polymeric insulation has become more popular, and the highest voltage level currently installed is +320 kV used for grid interconnections and for offshore wind farms. HVDC cable systems using cross-linked polyethylene (XLPE) are available for voltages up to +600 kV and 3 GW and polypropylene based for +600 kV and 3.5 GW

Surface charging of dielectric barriers under positive lightning impulse stress

acceptedVersio

Breakdown Mechanisms of Rod-Plane Air Gaps with a Dielectric Barrier Subject to Lightning Impulse Stress

Breakdown Mechanisms of Rod-Plane Air Gaps with a Dielectric Barrier Subject to Lightning Impulse StressacceptedVersio