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Vacuum Technology in Electrical Switches
Presented by Zhenxing Wang
From Xi’an Jiaotong University
Now at University of Helsinki
29 January, 2015
Content
23 May 2017
I
Background of Vacuum Switch
II
Vacuum Breakdowns in 126kV
Vacuum interrupter
III
Vacuum Arc and Its Effect
IV
Post-arc Breakdowns
V
Conclusion and Future Plan
Zhenxing Wang
2
I Background
Vacuum
Interrupter
 Vacuum technology is one good solution for
electrical insulation, and the environmentfriendly merit
makes it suitable for
substituting SF6 gas switches.
 Now vacuum switches dominate the medium
voltage level of power system(3kV - 40.5kV).
 We would like to develop a vacuum switch
can be used in the power system above
70.5kV - 126kV or above.
This is a 126kV vacuum circuit breaker
designed by my group in XJTU
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3
I Background: The Interrupting Processes
 Vacuum arc can destroy
the contact surfaces
severely.
 There are three stages in
the post –arc stage:
 Residual plasma dissipates
from the gap.
 Metal vapor dissipates
from the gap.
 The gap recovers to
vacuum.
 If the contacts can
withstand the transient
voltage and turn to be
vacuum again the current
is interrupted successfully.
Otherwise the contact gap
will restrike.
Schade, E. and E. Dullni, "Recovery of breakdown strength
of a vacuum interrupter after extinction of high currents".
Ieee Transactions on Dielectrics and Electrical Insulation,
2002. 9(2): p. 207-215.
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4
I Background: Three Major Problems
Problem I
Problem II
Problem III
Vacuum
Breakdown
Vacuum Arc
Interruption
Post-arc
Breakdown
The breakdown
mechanism in
long vacuum
gap (>10mm).
Does the same
mechanism dominate
breakdowns between
the processes in short
and long vacuum gap?
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The arc burning
process and
erosion of contact
material.
How to get a more precise
plasma arc model and
calculate the erosion of the
arc on the surfaces?
Zhenxing Wang
The breakdown
mechanism in lowpressure metal
vapor on the
destructed surfaces.
How to give a more reliable
estimation to dielectric
recovery strength?
5
II Vacuum BDs in 126kV VIs: Experimental Setup
To Impulse
Generator
Gap Spacing
Adjuster (0~50mm)
Contact Material:
CuCr40
d=10~50mm
Surface Roughness:
3.2um or 1.6um
Vacuum Interrupter
Porcelain
Envelope
15mm
Radius of Contact Edge:
R2mm or R6mm
Contact Diameter:
60mm or 75mm
Insulation Gas SF6
Adopting 126kV vacuum
To Earth
VI
Radius of Contact
Edge(mm)
Roughness(μm)
Contact
Radius(mm)
No.1
No.2
No.3
No.4
6
2
6
6
1.6
1.6
3.2
1.6
60
60
60
75
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interrupters to study the behaviors
of breakdowns with a contact gap
of 10~50mm
Voltage type:
1min AC voltages
impulse voltages
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6
 The relation between contact
gaps and AC breakdown
voltages can be expressed as
behaviors of UB=89d0.25
 The possibilities of impulse
voltage breakdowns in a
vacuum interrupter satisfy
Weibull distribution.
 The discrepancies between the
contact with roughness 1.6um
and the one with 3.2um are
within 3%.
 The discrepancies between the
contact with a diameter of
60mm and the one with 75mm
are within 10%.
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AC Breakdown Voltage(kV)
II Vacuum BDs in 126kV VIs: Results
UB=89d0.25
Gap Length(mm)
AC Voltage breakdowns
The Effect of roughness
Impulse Voltage breakdowns
The Effect of Contact Diameter
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7
III High Current Vacuum Arc: Experiments
Results from Electron
Scanning Microscope
Composition of Melt Layer in Different Regions
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Material
s
Region I
%
Region II
%
Before %
Cr
31
18
25
Cu
69
82
75
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8
III High Current Vacuum Arc: Simulation Model
Mathematical Model
Physical Model
Anode Region
Arc Column
Free Surface
Physical Process:
Melting/Solidification,
Free Surface,
Heat Flux from Arc Column,
Arc pressure.
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dF F

 (V ) F  0
dt
t
Boundary Condition
Adopting pressure and heat
from arc calculation as the
boundary of anode surface
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III High Current Vacuum Arc: Simulation Results
Velocity
Current Density
Pressure
Temperature
 This process reshapes the contact surface
and energy distribution.
 Pressure from arc can be a dominant force
to shape the surface of anode contact.
 The influence of the process has a
significant impact on the post-arc period.
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Zhenxing Wang
Evolution of Temperature and Surface
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IV Post-arc BDs : Simulation Model
A 1D3V PIC-MCC model of
sheath development
2D3V PIC-MCC model of
post-arc breakdown
Cathode
Anode
Postarc anode
Neutral
plasma
U=0
U = UR(t)
ds
Postarc cathode
e
Negative
Voltage
positive
space
sheath
Cu  e  Cu  e
Cu  e  Cu*  e
Ground
10mm
Cu  Cu   Cu  Cu 
e
Cu  Cu   Cu   Cu
I
e
e
Physical Process:
Physical Process:
Plasma transportation under TRV.
The effect of existing background
neutral vapor.
Breakdowns in a low density metal
vapor.
The effect of destructed surface.
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11
IV Post-arc BDs : Sheath Development
0
17
2.5x10
17
1.2x10
-200
17
1.0x10
17
1.0x10
8.0x10
Voltage(V)
17
1.5x10
16
6.0x10
16
4.0x10
750ns
16
5.0x10
16
750ns
600ns
600ns
450ns 300ns 150ns
0.0
0
Postarc Anode
1
2
3
Axial Position(mm)
4
5
Postarc Cathode
0
Postarc Anode
1
2
3
Axial Position(mm)
4
450ns
-600
600ns
-800
750ns
-1000
2.0x10
450ns 300ns 150ns
300ns
-400
16
3
Ion Density(/m )
3
Electron Density(/m )
150ns
17
2.0x10
-1200
5
Postarc Cathode
-1400
0
The distribution of electron
The distribution of ion
1
Postarc Anode
2
3
4
5
Postarc Cathode
Axial Position(mm)
The distribution of voltage across gap
n0=1020
n0=1021
n0=1018
4
Sheath Thickness(mm)
Sheath development can last for
several microseconds.
The existing of metal vapor can
affect the development of residual
plasma only in a high density
situation.
5
n0=1022
3
2
1
0
0.0
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0.5
1.0
1.5
2.0
2.5
Time(s)
Sheath thickness
3.0
12
IV Post-arc BDs : Metal Vapor BD
4x1011
3x1011
2x1011
1x1011
0
The evolution of particles during a
breakdown
The Number of Electron
0
50
100
4x10
3x1011
2x1011
1x1011
0
0
200
250
300
50
100
150
200
250
300
Time(ns)
10
8
6
4
2
0
The Current Absorbed by Cathode
0
50
100
150
200
250
300
250
300
Time(ns)
0
-2
-4
-6
-8
-10
-12
The Current Absorbed by Anode
0
100
50
100
150
200
Time(ns)
Breakdown Voltage / V
The paschen curve for copper are
only limited available from
experiments.
PIC-MCC is helpful for
estimating the breakdowns in a
low-density metal vapor.
150
Time(ns)
The Number of Copper Ion
11
75
50
(pd=5.52Pam, Vb=30V)
25
(pd=1.01Pam, Vb=11.7V)
0
0
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2
4
6
8
10
12
14
(pd) / Pam
Paschen curve for copper
13
IV Post-arc BDs : Micro Tip Induced by Electric Field
Electric stress
Initial shape
Surface
tension force
Viscosity force
Height of
apex
(3)
(3)
(3)
(2)
(2)
16
9
10
3000
148
10
(1)
(1)
10
103
2000
10
2
108
1500
1
106
0
1000
10
4-1
10
500-2
102
0
Liquid metal
4
220MV/m (1)
220MV/m
220MV/m (1)
(1)
240MV/m (2)
240MV/m
240MV/m (2)
(2)
260MV/m (3)
260MV/m
260MV/m (3)
(3)
-3
100
00
11
22
33
Time
Time (us)
(us)
44
55
66
Electric field (MV/m)
2
Thermo-field
electron
emssion
Height
of apex
(um) (A/m )
d
167
10
3000
146
10
5
2500
10
12
Electric field (MV/m)
2
Thermo-field
electron
emission
(A/m )
Height
of apex
(um)
E0
Vacuum
7
1356K (1)
(1)
1356K
1700K
(2)
1700K (2)
2300K
(3)
2300K (3)
(3)
(3) (2)
(1)
(2) (1)
(3)
(2)
10
2500
126
(1)
10
105
10
2000
4
10
83
10
1500
2
106
1
10
1000
0
104
-1
500
102
-2
10
-3
100
0 0.5 11.0 1.5 2 2.0 2.53 3.0 3.54 4.0 4.5
5
0.0
Time
Time (us)
(us)
Tip Formed
Electric Field
Enhanced
Current Emission
Increased
The existence of micro tip can
reduce the BD voltages
significantly.
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14
Conclusion & Future Plan
 Breakdowns in vacuum and low density metal vapor are the
most fundamental issues in designing a high voltage
interrupter.
 The mechanism of vacuum breakdowns with a large contact
gap (10mm~60mm) still does not be understood.
 It is necessary to integrate the process of vacuum arcs and
post-arc breakdowns for the purpose of better
understanding the interrupting processes.
 We plan to model breakdowns with a several millimeters
contact gap and verify the model by observing the evolution
of vacuum breakdowns adopting a steak camera.
 A integrated post-arc breakdown model is being developed.
15
Thanks For Your Attention!