1 an investigation into the breakdown mechanisms of a triggered water gap switch mohsen saniei...
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1
An investigation into the breakdown mechanisms of a triggered water gap switch
Mohsen Saniei
Institute for Energy and EnvironmentUniversity of Strathclyde
Glasgow G1 1XW
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Introduction
1- Water has a- High dielectric constant (=81)
b- High dielectric strength (1MV/cm)
c- High energy storage density
2- Water is used as the dielectric in water gap switches for pulsed power applications
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Aims and objectives
1-Investigating the effect of triggering on the breakdown of a water gap
2- Investigating the breakdown mechanism in triggered water gaps
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Electric set-up Schematic
• Main voltage is generated by the discharge of the 80nF high voltage capacitor by activating the trigatron switch
• Trigger pulse for the water gap is generated by a 4X Blumlein generator
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Triggering effect on the time lag to breakdown
Self-breakdown
Triggered-breakdown
Triggering decreases the time lag to breakdown
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Triggering effect on water gap breakdown
Triggering has decreased the time lag to breakdown and the minimum breakdown voltage
0
10
20
30
40
50
60
70
80
0 10 20 30 40Gap Voltage(kV)
t( s
ec)
non-triggered
triggered
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Triggering effect on water vapourisation
• Trigger pulse with trigger pulse energies of 1-2J and a pulse duration of 500ns means available power of 2-4MW
•This energy could vaporise water and generate a bubble
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Methods of examining bubble generation
1- Measurement of time lags to the main gap breakdown, when a delay time was applied between the trigger pulse and the main voltage
2- Optical procedure using a photo-detector and He-Ne laser
3- Using a conventional camera working in the open-shutter mode
4- Using a high speed digital camera
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0.01
0.1
1
10
100
1000
0 500 1000 1500
(delay time)/us
(tim
e lag)u
s
Time lag to breakdown vs. the delay time ,Plane-plane triggered water gap with a trigger ring, gap voltage=10kV, trigger pulse energy=1J
1- Time lag to the main breakdown measurement, when a delay time was applied between the trigger pulse and the main voltage
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Water/Gas Interface
Water
Gas Laser beam
Detector
2-Schematic of reflection and refraction of laser beam due to the presence of a gas bubble
Refractive index differences [gas phase 1.006 water 1.333] means bubble acts as a divergent spherical lens.Laser beam diverges reducing transmitted light intensity at the detector.
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Results from optical procedure using a photo-detector and He-Ne laser
Intensity of a light beam transmitted through a triggered plane-plane water gap as a function of time after the application of a voltage pulse to the trigger-pin
3.24J
0.8J
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3- Optical procedure using a conventional camera working in the open-shutter mode
High Voltage Electrode
Earth Electrode
Bubble
Trigger Pin
(a) (b)Still Pictures taken by an open shutter camera with a trigger pulse, but without the main gap voltage Trigger pulse energy: (a) 1.44J (b) 3.26J
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4-An optical method using a high speed digital camera
Successive frames at times of 200, 400, 600, and 800 sec showing the development of a bubble produced in the electrode gap after the application of a trigger pulse, energy 2.56J
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Simulated Equipotential lines in the parallel plane water gap including a hemisphere bubble
StrengthE (V/m)
1.720
1.548
1.376
1.204
1.032
0.860
0.688
0.516
0.344
0.172
0.000
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Electric field simulation in the water gap
containing a bubble using Quickfield software
0
0.5
1
1.5
2
2.5
0 20 40 60 80 100
Electrode separation, arbitary units
E/E
0
Gas/liquid boundary
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Pre-breakdown streamer images captured by the high-speed digital camera
Pre-breakdown streamer at the plane-plane triggered water gap with a trigger pin, the main gap spacing=8.7mm and the main voltage=16kV, trigger pulse energy=1.44J
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Still picture from the conductive channel after the main breakdown
Generated Bubble
Conductive Channel
Breakdown pictures taken by an open shutter camera technique, with a main gap voltage of 16kV, gap spacing of 8.7mm and trigger pulse energy=1.44J
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•Bubble generation as a result of a trigger pulse
• Electric field intensification within the bubble
• Initiation of an electric avalanche within the bubble
• Propagation of the electric streamer toward the high voltage electrode
• Final breakdown in the water gap
Summary
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Acknowledgements
I would like to thank Dr R A Fouracre and Professor S J MacGregor for their guidance and supervision, and also from Professor G Woolsey for his advice and assistance during this research.
I would like to thank the Ministry of Research, Science and Technology of Iran for their financial support.
