1 an investigation into the breakdown mechanisms of a triggered water gap switch mohsen saniei...

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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.

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THANK YOU FOR YOUR ATTENTION AND PARTICIPATION

Msaniei@eee.strath.ac.uk