Gas-solid interfaces stressed with HV impulses : surface flashover behaviour and control

In pulsed power engineering, solid spacers are used to insulate high voltage parts from extraneous metal parts, providing electrical insulation as well as mechanical support. The breakdown/flashover voltage, at which a discharge process initiates across the gas-solid interface, is important in the design process, as it informs designers of specific threshold ‘failure’ voltages of the insulation system. In this thesis, a method to potentially increase the failure voltage, tested under multiple environmental conditions, without increasing the length of the solid spacer, was investigated. Three dielectric materials: High-Density Polyethylene (HDPE), Polyetherimide (Ultem) and Polyoxymethylene (Delrin), were tested under 100/700 ns impulse voltages. Cylindrical spacers made of these materials were located in the centre of a plane-parallel electrode arrangement in air, which provided a quasi-uniform field distribution. Breakdown and flashover tests were performed in a sealed container at air pressures of −0.5, 0 and 0.5 bar gauge, with varying relative humidity (RH) level of 90%. The materials were tested under both, negative and positive, polarity impulses. Additionally, the surfaces of a set of solid spacers were subjected to a ‘knurled’ finish, where ~0.5 mm indentations are added to the surface of the materials, prior to testing, to allow comparison with the breakdown voltages for samples with ‘smooth’ (machined) surface finishes. The results show that the flashover voltage is controlled by the physical insulation system and environmental parameters, where the multiple test conditions yielded results where the V50 breakdown voltage for samples with a smooth surface finish was higher than for knurled, by up to ~55 kV; where there were similar V50 breakdown voltages for each type of surface finish; and where the knurled spacer resulted in a higher (by up to ~66 kV) hold-off voltage than the corresponding smooth spacer. Each of these results is discussed herein, particularly in terms of the location of the discharge channel at breakdown, where changing the physical and environmental test parameters was shown to affect the discharge path, and therefore the flashover voltage of the insulation system. The results and discussion will inform designers and operators of outdoor pulsed power systems on the design of air-solid insulation systems, and the control of the flashover characteristics, under varying environmental conditions.In pulsed power engineering, solid spacers are used to insulate high voltage parts from extraneous metal parts, providing electrical insulation as well as mechanical support. The breakdown/flashover voltage, at which a discharge process initiates across the gas-solid interface, is important in the design process, as it informs designers of specific threshold ‘failure’ voltages of the insulation system. In this thesis, a method to potentially increase the failure voltage, tested under multiple environmental conditions, without increasing the length of the solid spacer, was investigated. Three dielectric materials: High-Density Polyethylene (HDPE), Polyetherimide (Ultem) and Polyoxymethylene (Delrin), were tested under 100/700 ns impulse voltages. Cylindrical spacers made of these materials were located in the centre of a plane-parallel electrode arrangement in air, which provided a quasi-uniform field distribution. Breakdown and flashover tests were performed in a sealed container at air pressures of −0.5, 0 and 0.5 bar gauge, with varying relative humidity (RH) level of 90%. The materials were tested under both, negative and positive, polarity impulses. Additionally, the surfaces of a set of solid spacers were subjected to a ‘knurled’ finish, where ~0.5 mm indentations are added to the surface of the materials, prior to testing, to allow comparison with the breakdown voltages for samples with ‘smooth’ (machined) surface finishes. The results show that the flashover voltage is controlled by the physical insulation system and environmental parameters, where the multiple test conditions yielded results where the V50 breakdown voltage for samples with a smooth surface finish was higher than for knurled, by up to ~55 kV; where there were similar V50 breakdown voltages for each type of surface finish; and where the knurled spacer resulted in a higher (by up to ~66 kV) hold-off voltage than the corresponding smooth spacer. Each of these results is discussed herein, particularly in terms of the location of the discharge channel at breakdown, where changing the physical and environmental test parameters was shown to affect the discharge path, and therefore the flashover voltage of the insulation system. The results and discussion will inform designers and operators of outdoor pulsed power systems on the design of air-solid insulation systems, and the control of the flashover characteristics, under varying environmental conditions

Characterisation of a corona-stabilised switch in alternative gas mixtures

Sulphur hexafluoride (SF6) has traditionally been used as a switching medium within corona-stabilised switches (CSS). Due to its high global warming potential (GWP), however, other gases are under test in order to find a suitable alternative that can be used within CSS, without compromising on switching performance. Design changes may have to be made in order for the switch to remain at the high level of performance achieved when filled with SF6. This poster reports preliminary results obtained using a CSS operated with the refrigerant 1,3,3,3-tetrafluoropropene, known as HFO-1234ze as the basis of the operating gas. The electronegativity of HFO-1234ze makes it an attractive option to SF6 for switching applications. Additionally, the global warming potential (GWP) of this gas is 6 in a 100-year time horizon, compared to SF6 with a value of 23900. The performance of the switch has been characterized in terms of triggering range, delay time and jitter over a range of pressures when filled with air as a reference, as well as with HFO-1234ze in various mixtures with buffer gas nitrogen (N2) of the order of >80%. The results presented provide data on the feasibility of the approach of using HFO-1234ze as the operating gas in corona stabilised switches. It will also provide the initial basis for work refining the use of buffer gases and for the development of optimised switch configurations

Characterisation and analysis of a corona-stabilised switch filled with environmentally-friendly gas mixtures

Tests have been completed with the intention of finding an alternative switching medium to SF6 due to environmental concerns. For this purpose, a testing procedure was designed in order to test a novel gas in pulsed power, HFO-1234ze, in mixtures with N2, used as a buffer gas. Thus, decreasing the global warming potential (GWP) of the switching medium from 23900 in SF6 to 6 in HFO-1234ze. The performance of the gas was measured in a specific corona-stabilised switch geometry, characterising triggering range and delay time experimentally, and calculating jitter.;Over the testing phase it was shown that N2/HFO-1234ze mixtures were very promising in terms of breakdown strength, with self-breakdown voltages for a 80% N2 / 20% HFO-1234ze mixture at 3 bar gauge reaching up to 32.2 kV and 36 kV for positive and negative polarity, respectively, giving an increase of 191% and 306% breakdown increase from using 100% N2. Furthermore, the triggering ranges recorded reached a maximum of 13.4 kV for positive polarity and 13.6 kV for negative polarity, giving increases of 837% and 174% compared to 100% N2. The delay time and jitter increased accordingly as the total pressure of each individual mix increased.;Von Laue statistical analysis was conducted on the delay time data, in order to estimate the relative contributions of the statistical and formative times to the overall delay time. Statistical times were found to increase slightly with pressure, with no clear polarity effect. The formative times were found to form the majority of the overall delay times, increasing with increasing pressure for both polarities.;In terms of being a viable replacement for SF6, the gas HFO-1234ze showed positive characteristics in terms of self-breakdown voltage, triggering range and delay time/jitter, in the tested geometry. Although, after testing completion, the electrodes were found to have a layer of carbon which had formed during the testing process; this requires further investigation.Tests have been completed with the intention of finding an alternative switching medium to SF6 due to environmental concerns. For this purpose, a testing procedure was designed in order to test a novel gas in pulsed power, HFO-1234ze, in mixtures with N2, used as a buffer gas. Thus, decreasing the global warming potential (GWP) of the switching medium from 23900 in SF6 to 6 in HFO-1234ze. The performance of the gas was measured in a specific corona-stabilised switch geometry, characterising triggering range and delay time experimentally, and calculating jitter.;Over the testing phase it was shown that N2/HFO-1234ze mixtures were very promising in terms of breakdown strength, with self-breakdown voltages for a 80% N2 / 20% HFO-1234ze mixture at 3 bar gauge reaching up to 32.2 kV and 36 kV for positive and negative polarity, respectively, giving an increase of 191% and 306% breakdown increase from using 100% N2. Furthermore, the triggering ranges recorded reached a maximum of 13.4 kV for positive polarity and 13.6 kV for negative polarity, giving increases of 837% and 174% compared to 100% N2. The delay time and jitter increased accordingly as the total pressure of each individual mix increased.;Von Laue statistical analysis was conducted on the delay time data, in order to estimate the relative contributions of the statistical and formative times to the overall delay time. Statistical times were found to increase slightly with pressure, with no clear polarity effect. The formative times were found to form the majority of the overall delay times, increasing with increasing pressure for both polarities.;In terms of being a viable replacement for SF6, the gas HFO-1234ze showed positive characteristics in terms of self-breakdown voltage, triggering range and delay time/jitter, in the tested geometry. Although, after testing completion, the electrodes were found to have a layer of carbon which had formed during the testing process; this requires further investigation