Vacuum Technology in Electrical Switches

Vacuum Technology in Electrical Switches

Zhenxing Wang

Xi’an Jiaotong University

Now at University of Helsinki

September 2nd, 2015

7 September 2015 2

Content

I Background of Vacuum Switch

II Vacuum Breakdowns in 126kV

Vacuum interrupter

III Vacuum Arc and Its Effect

IV Post-arc Breakdowns

V Conclusion

Zhenxing Wang @ Xi’an Jiaotong University

7 September 2015 Zhenxing Wang @ Xi’an Jiaotong University 3

I Background

Vacuum technology is one good solution for

electrical insulation, and the environment-

friendly 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

Vacuum

Interrupter

7 September 2015 4

I Background: The Interrupting Processes

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.

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.


7 September 2015 5

I Background: Three Major Problems

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?

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?

The breakdown mechanism in low-pressure metal vapor on the destructed surfaces.How to give a more reliable

estimation to dielectric

recovery strength?

Problem I Problem II Problem III


7 September 2015 6

To Impulse

Generator Gap Spacing

Adjuster (0~50mm)

Insulation Gas SF6

Vacuum InterrupterPorcelain

Envelope

To Earth

d=10~50mm

15mm

Contact Diameter:

60mm or 75mm

Radius of Contact Edge:

R2mm or R6mm

Contact Material:

CuCr40

Surface Roughness:

3.2um or 1.6um

VI Radius of Contact Edge(mm) Roughness(μm) Contact

Radius(mm)

No.1 6 1.6 60

No.2 2 1.6 60

No.3 6 3.2 60

No.4 6 1.6 75

Adopting 126kV vacuum interrupters to study the behaviors

of breakdowns with a contact gap

of 10~50mm

Voltage type:1min AC voltages

impulse voltages

II Vacuum BDs in 126kV VIs: Experimental Setup


7 September 2015 7

Gap Length(mm)

AC

Bre

ak

dow

n V

olt

age (

kV

) UB=89d0.25

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%.

AC Voltage breakdowns

Impulse Voltage breakdowns

The Effect of roughness

The Effect of Contact Diameter

II Vacuum BDs in 126kV VIs: Results


7 September 2015 8

Materials

Region I %

Region II %

Before %

Cr 31 18 25

Cu 69 82 75

Composition of Melt Layer in Different Regions

III High Current Vacuum Arc: Experiments

Results from Electron

Scanning Microscope


7 September 2015 9

Physical Process: Melting/Solidification,

Free Surface,

Heat Flux from Arc Column,

Arc pressure.

Arc ColumnAnode Region

Mathematical ModelPhysical Model

III High Current Vacuum Arc: Simulation Model

d( ) 0

d

F FV F

t t

Boundary ConditionAdopting pressure and heat

from arc calculation as the

boundary of anode surface

Free Surface


7 September 2015 10

III High Current Vacuum Arc: Simulation Results

Velocity Pressure

Current Density Temperature

Evolution of Temperature and Surface

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.Zhenxing Wang @ Xi’an Jiaotong University

7 September 2015 11

2D3V PIC-MCC model of

post-arc breakdown

e

Cathode Anode

e

eI

e

10mm

Negative

Voltage Ground

Cu e Cu e *Cu e Cu e

Cu Cu Cu Cu

Cu Cu Cu Cu

Postarc cathode

Postarc anode

U = 0

U = UR(t)

positive

space

sheath

Neutral

plasma

ds

A 1D3V PIC-MCC model of

sheath development

Physical Process:

Plasma transportation under TRV.

The effect of existing background

neutral vapor.

Physical Process:

Breakdowns in a low density metal

vapor.

The effect of destructed surface.

IV Post-arc BDs : Simulation Model


7 September 2015 12

The distribution of electron

Sheath thickness

0 1 2 3 4 5

5.0x1016

1.0x1017

1.5x1017

2.0x1017

2.5x1017

150ns300ns450ns600ns750ns

Postarc CathodePostarc Anode

Ele

ctr

on

Den

sit

y(/

m3)

Axial Position(mm)

0 1 2 3 4 5

0.0

2.0x1016

4.0x1016

6.0x1016

8.0x1016

1.0x1017

1.2x1017

150ns300ns450ns600ns750ns

Postarc Anode Postarc Cathode

Ion

De

ns

ity

(/m

3)

Axial Position(mm)

0 1 2 3 4 5

-1400

-1200

-1000

-800

-600

-400

-200

0

150ns

300ns

450ns

600ns

Postarc Cathode

Vo

ltag

e(V

)

Axial Position(mm)Postarc Anode

750ns

The distribution of ion

The distribution of voltage across gap

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

1

2

3

4

5

n0=1018

n0=1020

n0=1021

Sh

eath

Th

ickn

ess(m

m)

Time(s)

n0=1022

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.

IV Post-arc BDs : Sheath Development


7 September 2015 13

The evolution of particles during a

breakdown

Paschen curve of copper

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.

IV Post-arc BDs : Metal Vapor BD


0 2 4 6 8 10 12 140

20

40

60

80

100

(pd=3.61Pam, Vb=30V)

(pd) / Pam

Bre

ak

do

wn

Vo

lta

ge

/ V

(pd=2.45Pam, Vb=44V)

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.0 0.5 1.0 1.5 2.0 2.5 3.00.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 0.5 1.0 1.5 2.0 2.5 3.00.00

0.02

0.04

0.06

0.08

0.10

Ion Current

Absorbed by Cathode

Cu

rre

nt

/ A

0.0 0.5 1.0 1.5 2.0 2.5 3.00.0

0.5

1.0

1.5

2.0

2.5

3.0

Ion Energy

Absorbed by Cathode

Time / s

Time / s

En

erg

y (

10

-11)

/ J

C

urr

en

t / A

Electron Current

Absorbed by Anode

Electron Energy

Absorbed by Anode

Time / s

En

erg

y (

10

-11)

/ J

0.0 0.5 1.0 1.5 2.0 2.5 3.0

7 September 201514

E0

0

Vacuum

Liquid metal

Height of apex

Surface tension force

Electric stress

Initial shape

d

Viscosity force

IV Post-arc BDs : Micro Tip Induced by Electric Field

Tip Formed Electric Field Enhanced

Current Emission Increased

The existence of micro tip can reduce the

BD voltages significantly.


0ms 0.7ms 0.9ms

1.1ms 1.16ms 1.21ms

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.51

2

3

4

5

6

7

Fie

ld e

nh

ace

me

nt

facto

r

Time (ms)

34MV/m

30MV/m28MV/m

27MV/m

26MV/m

0 1 2 3 4 5 610

-3

10-1

101

103

105

107

Th

erm

o-f

ield

Ele

ctr

on

em

issi

on

(A

/m2)

Time (s)

260MV/m 240MV/m 220MV/m

The evolution process of field

enhancement factor in the

micrometer level

0.0 0.4 0.8 1.2 1.60

50

100

150

200

250

Ele

ctr

ic F

ield

In

ten

sity

(M

V/m

)

Time (ms)

2300K 1700K 1356K

The electric field increased

further in the nanometer level

The evolution process of

electron emission

15

Conclusion

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.

Thanks For Your Attention!

Documents

Vacuum Technology in Electrical Switches