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Dielectrics for high temperature superconducting applications

Dielectrics for high temperature superconducting applications
Dielectrics for high temperature superconducting applications
This thesis is concerned with the development of condition monitoring for future design of high temperature superconducting (HTS) power apparatus. In particular, the use of UHF sensing for detecting PD activity within HTS has been investigated. Obtained results indicate that fast current pulses during PD in LN2 radiate electromagnetic waves which can be captured by the UHF sensor. PD during a negative streamer in LN2 appears in the form of a series of pulses less than 1 ?s apart. This sequence cannot be observed using conventional detection method due to its bandwidth limitation. Instead, a slowly damped pulse is recorded which shows the total amount of charge transferred during this period.

A study into PD streamer development within LN2 has been undertaken that reveals the characteristics of pre-breakdown phenomena in LN2. For negative streamers, when the electric field exceeds a threshold value, field emission from the electrode becomes effective which leads to the formation of initial cavities. Breakdown occurs within these gaseous bubbles and results in the development of negative streamers. For positive streamers, the process is much less well-understood due to the lack of initial electrons. However, from the recorded current pulses and shadow graphs, the physical mechanism behind positive streamer development is likely to be a more direct process, such as field ionisation, compared with the step-wise expansion in the case of negative streamers.

The mechanisms that cause damage to solid dielectrics immersed in LN2 have been investigated. Obtained results indicate that pre-breakdown streamers can cause significant damage to the solid insulation barrier. Damage is the result of charge bombardment and mechanical forces rather than thermal effects. Inhomogeneous materials, such as glass fibre reinforced plastic (GRP), tend to introduce surface defects which can create local trapping sites. The trapped charges when combined with those from streamers can create much larger PD events. Consequently, damage observed on GRP barriers is much more severe than that on PTFE barriers under similar experimental conditions. Thus, design of future HTS power apparatus must consider this degradation phenomenon in order to improve the reliability of the insulation system.
Truong, L.H.
9aaef951-d28e-4f6a-b5a2-669b92a426c0
Truong, L.H.
9aaef951-d28e-4f6a-b5a2-669b92a426c0
Lewin, Paul
78b4fc49-1cb3-4db9-ba90-3ae70c0f639e

(2013) Dielectrics for high temperature superconducting applications. University of Southampton, Faculty of Physical Sciences and Engineering, Doctoral Thesis, 204pp.

Record type: Thesis (Doctoral)

Abstract

This thesis is concerned with the development of condition monitoring for future design of high temperature superconducting (HTS) power apparatus. In particular, the use of UHF sensing for detecting PD activity within HTS has been investigated. Obtained results indicate that fast current pulses during PD in LN2 radiate electromagnetic waves which can be captured by the UHF sensor. PD during a negative streamer in LN2 appears in the form of a series of pulses less than 1 ?s apart. This sequence cannot be observed using conventional detection method due to its bandwidth limitation. Instead, a slowly damped pulse is recorded which shows the total amount of charge transferred during this period.

A study into PD streamer development within LN2 has been undertaken that reveals the characteristics of pre-breakdown phenomena in LN2. For negative streamers, when the electric field exceeds a threshold value, field emission from the electrode becomes effective which leads to the formation of initial cavities. Breakdown occurs within these gaseous bubbles and results in the development of negative streamers. For positive streamers, the process is much less well-understood due to the lack of initial electrons. However, from the recorded current pulses and shadow graphs, the physical mechanism behind positive streamer development is likely to be a more direct process, such as field ionisation, compared with the step-wise expansion in the case of negative streamers.

The mechanisms that cause damage to solid dielectrics immersed in LN2 have been investigated. Obtained results indicate that pre-breakdown streamers can cause significant damage to the solid insulation barrier. Damage is the result of charge bombardment and mechanical forces rather than thermal effects. Inhomogeneous materials, such as glass fibre reinforced plastic (GRP), tend to introduce surface defects which can create local trapping sites. The trapped charges when combined with those from streamers can create much larger PD events. Consequently, damage observed on GRP barriers is much more severe than that on PTFE barriers under similar experimental conditions. Thus, design of future HTS power apparatus must consider this degradation phenomenon in order to improve the reliability of the insulation system.

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Published date: July 2013
Organisations: University of Southampton, EEE

Identifiers

Local EPrints ID: 355538
URI: http://eprints.soton.ac.uk/id/eprint/355538
PURE UUID: 3ae12181-b710-4519-adf8-7d10a7544ec4

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Date deposited: 11 Nov 2013 14:31
Last modified: 18 Jul 2017 03:47

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