Characterisation and statistical analysis of breakdown data for a corona-stabilised switch in environmentally-friendly gas mixtures

Characterisation of a corona-stabilised switch in the single-shot regime, including triggering range, delay times and jitter is reported, over the pressure range 0-3 bar gauge, as a continuation of work from similar characterisation with this switch filled with SF6 with different gap spacings. When filled with mixtures of HFO-1234ze and N2, the breakdown voltage can be increased by up to ~306% and ~191% under negative and positive polarity, respectively, of that using 100% N2. These results were achieved with gas mixtures consisting of 80% N2 and 20% HFO-1234ze, by pressure. The maximum negative polarity triggering range was 13.6 kV, comparable to that achieved previously using SF6. The measured delay time and calculated jitter was generally found to increase with increasing pressure, and with increasing percentage (from 5% to 20%) of HFO-1234ze in the gas mixtures. Von Laue statistical analysis of time-to-breakdown data showed that both the formative time and statistical time increased with increasing pressure, and with increasing percentage of HFO-1234ze in the gas mixtures. The formative time under negative polarity was significantly longer than that for positive polarity. The results indicate that HFO-1234ze may be considered as a suitable candidate to replace SF6 for switching applications, although there are some operational observations that require further investigation

A Python-based adaptive mesh solver for drift-diffusion modelling of streamer discharges

Streamer discharges are fast-moving plasma fronts which can be formed in gases stressed with a sufficiently high electric field and represent a crucial stage in the evolution of an electrical breakdown. Recently, the investigation of streamer discharges has regained significant interest due to the numerous basic processes and practical applications that require their understanding. These include geophysical processes such as sprite development; gas-insulated system design for power and pulsed power equipment; and a growing number of industrial and environmental applications. Computational advances have provided deep insights into some critically important properties and characteristics of streamers, yet simulations remain highly nontrivial due to the multiscale nature of the phenomena. In the present study, the drift-diffusion approximation for the computational modelling of streamers in gases has been implemented using the open-source finite-element platform FEniCS. Equipped with a Python interface to a high-speed C++ backend, the use of FEniCS greatly improves usability by requiring less computational expertise, yet with little compromise on solver efficiency. The accuracy of the code has been verified through comparison with six other codes from a recently published benchmarking study. Thus, we conclude that the FEniCS platform may be a highly suitable alternative for furthering the study of streamer discharges

Simulation of streamer discharges across solid dielectric surfaces using the open-source fenics platform

Modelling the development of streamer discharges across gas-solid dielectric interfaces is of increasing importance to the power and pulsed power industries. Deeper understanding of the dynamics and morphology of fast ionisation fronts in gases and at gas-solid interfaces underpins insulation coordination and optimization in high voltage power and pulsed power systems. However, many physical mechanisms behind gas-solid flashover remain to be fully understood. In this work, an adaptive mesh drift diffusion solver implemented on the open-source finite element framework FEniCS is used to perform computational streamer modelling across gas-solid interfaces. In this study, we verify our implementation by performing studies under a published configuration of a positive streamer initiating and propagating along a flat dielectric surface in a 2D domain. FEniCS is proved fully capable of performing streamer simulations in complex gas-solid topologies, with our results providing further evidence of accelerated surface streamers and other observed effects under various initial seed positions, surface permittivity, and applied voltage magnitudes

Conduction characteristics of MIDEL eN 1204 insulating liquid under DC non-uniform conditions

In the continued search for environmentally friendly mineral oil alternatives for industrial applications, the present study expands on previous studies as in [1,2], with the addition of rapeseed based dielectric fluid MIDEL eN-1204. The conduction characteristics of MIDEL eN-1204, both fresh and thermally aged, have been measured and compared with well-known synthetic ester liquid MIDEL 7131 and mineral oil Diala D. The mobility of charge carriers under high voltage in a point-plane electrode topology, and the temperature dependence of conductivity under low-field conditions for each fluid sample have been obtained. Charge carrier mobility in the tested liquids was found through I1/2-V curves in the method described in [3]. I-V curves were obtained in the standard cylindrical test cell with electrode separation of 1 mm, energised with voltages up to 70 V at room temperature (20 Co), 45 Co and 75 Co to analyse the conductivity increase with temperature. The above was also repeated with thermally aged (with presence of copper) fluid samples. Obtained results agree well with theoretical conduction models and existing studies. It is therefore found that MIDEL eN 1204 natural ester fluid possesses a similar carrier mobility in the 0-10 kV range when compared to MIDEL 7131 synthetic ester liquid, but exhibits a much higher mobility with aging. In the low-field region, it is found that MIDEL eN possesses a lower conductivity across the temperature ranges, and a lower conductivity increase with aging compared with the other two fluids. The conduction data for Diala D are found to demonstrate a transient character under low-field conditions, with potential viscous and electrohydrodynamic processes being dominant, rendering the obtained data uncertain and difficult to analyse as a means of comparison, and opening a definite opportunity for further study

Modelling of streamer discharges in air-filled sub-millimeter needle-plane electrode gaps under fast-rising field conditions

Streamer discharges are fast ionisation fronts generated under intensive electrical stress. They are a crucial stage in the evolution of an electrical breakdown in gas, and also important to a range of industrial applications. In this work, streamers in sub-millimeter needle-plane gaps in atmospheric air and under fast-rising ramp voltages are modelling using a hydrodynamic approach, using the an open-source finite-element framework. The plasma model used in this study is considerably more advanced than before, now including 7 species partaking in 18 total reactions, including photoionisation. The local mean energy approximation was additionally used to ensure validity over a wider range of electric field. The study of streamers in short gaps and fast-rising voltages helps to inform the design and understanding of HV pulsed power systems, including: the design of plasma-closing switches, understanding electrical breakdown in gas-insulated systems in divergent fields, design of HV diagnostics and discharge detection equipment, and the study of the impact of impulsive fields on plasma composition for chemical processing applications

Electrostatic precipitation efficiency for a multi-needle plane electrode topology under DC excitation

Recent years have seen major governmental and humanitarian organisations place great emphasis on the issue of air quality and pollution. Both national and international strategies such as those detailed in [1] and [2] have set ambitious goals regarding air quality, most notably on the reduction of fine and ultrafine particulate matter (PM). The present work revisits the principles of Electrostatic Precipitation (ESP) using a novel multineedle-plane electrode topology under both positive and negative DC excitation, results of which are potentially of interest with regards to precipitation of contaminants on HVDC power lines in various atmospheric conditions. Using the GRIMM Laser Aerosol Spectrometer, concentrations of PM 0.265 nm and above in laboratory air were measured for a DC voltage range of 0 – 21 kV. The measurements were again repeated for humid air, by injection of atomised water via the means of an ultrasonic humidifier. All tests conducted ultimately achieved over 95% precipitation efficiency above 15 kV, where dry air proved to be much more stable with less variance in the recorded PM count. The fluctuation and noise seen in humid air tests however, may be attributed to inconsistencies in water injection and poor control of condensation within the test cell. Negative energisation was shown to result in significantly better precipitation performance than positive energisation, exhibiting a much higher precipitation efficiency in the 0 – 10 kV range, and also achieving close to 100% precipitation efficiency beyond 15 kV. The measured relationship between the precipitation efficiency and voltage was found also to closely match predicted behaviour derived from theoretical particle charging mechanisms as described in [3]. A proposed future line of investigation is to repeat the study under a pulsed regime to compare performance to a single point-plane electrode topology

Electric field inside a gas cavity formed at a solid-solid dielectric interface stressed with HV impulses

Interfaces between solid dielectric materials may exhibit lower breakdown strength compared to that associated with bulk breakdown of solid materials. A reason for such reduction is the presence of gas cavities which are formed at the interface. When the solid-solid interface is subjected to impulsive electrical stress, the enhanced electric field inside cavities may result in the development of initial (partial) discharges. This may ultimately lead to breakdown across the interface, resulting in the catastrophic failure of the entire insulation system. Therefore, it is paramount to understand the field distribution and ionisation processes within interfacial cavities, such that the behavior and strength of the insulation system can be fully predicted. The present paper considers a gas-filled cavity formed between poorly-conducting solids, subjected to a transient external electric field. The corresponding boundary value problem is defined and analytically solved, obtaining closed form solutions for the electric field distribution inside and around an isolated cavity. Results from model validation and intra-cavity field enhancement are presented, as well as brief discussion of other possible applications and future model extensions

Investigation of pulsed micro-discharges and ozone production by dielectric barrier discharges

In this work, pulsed micro-discharges produced by dielectric barrier discharges (DBDs) with a sub-millimeter gap were electrically-characterized under ac voltage at 100 Hz and at 5 kHz. Ozone production was investigated for different discharge gap lengths and pressures. The aim of the work was to understand the statistics of filamentary current pulses and their relationship to the reduced electric field and the ozone production efficiency. A transient sinusoidal voltage of 200 cycles was employed to reduce the heating effects in the ozone-synthesis process. It was shown that the amplitude of the filamentary current pulses measured over 200 voltage cycles conformed to a Gaussian distribution. The mean filamentary current and ozone production efficiency measured at 100 Hz and at 5 kHz were almost the same. The ozone production efficiency was found to increase with increasing pressure from 1 bar to 2 bar, and the gap length from 0.2 mm to 0.5 mm. The maximum ozone production efficiency achieved in the work was 217 g/kWh, with a gap length of 0.5 mm, 2.0 bar absolute pressure, and an applied voltage of 5.5 kV at 5 kH

Hydrodynamic parameters of air-bubble stimulated underwater spark discharges

Considerable amount of research work has been focused on investigation and optimization of strong acoustic waves generated by spark discharges in water. There are several methods to achieve and to stimulate underwater spark breakdowns, including free-discharges, wire-guided and gas-bubble stimulated discharges. In the present work, air bubbles are injected into water in order to achieve spark discharges in relatively long inter-electrode gaps. This paper reports on hydrodynamic and acoustic characteristics of spark discharges stimulated by air bubbles and presents the functional relationships between the hydrodynamic and electrical parameters of such discharges, including breakdown voltage, spark plasma resistance and energy available in the discharge. A hydrodynamic analytical model has been developed and used to calculate the acoustic efficiency of the underwater spark discharges