Influence of Surface Charges on Alumina Dielectrics …...Influence of Surface Charges on Alumina...

Surface charge

60

50

40

30

20

10

0

Fla

sh

ove

r e

lectr

ic f

ield

-10 -8 -6 -4 -2 0 2 4 6Surface charging potential [kV]

str

en

gth

Ep [

kV

/mm

]

100

102

104

106

Se

co

nd

ary

ele

ctr

on

50403020100Distance from center of electrode [mm]

No charge

Negative charge (Vs = 9.5kV) Positive charge (Vs = +5.0kV)

en

erg

y E

n [

eV

]

10-4

10-3

10-2

10-1

100

Flig

ht

dis

tan

ce

S [

mm

]

50403020100Distance from center of electrode [mm]

No charge Negative charge (Vs = -9.5kV) Positive charge (Vs = +5.0kV)

Influence on development of surface discharge

Influence of Surface Charges on Alumina Dielectrics

on Impulse Flashover Characteristics in VacuumHidenori Kato1, Katsumi Kato1, Ayumu Morita2, Hitoshi Okubo1

1Nagoya University, Japan 2Hitachi, Ltd., Japan

In order to enhance the electrical insulation performance of vacuum circuit breakers (VCBs), the surface flashover mechanism on soliddielectrics in vacuum needs to be clarified. We investigated the surface flashover characteristics under the existence of surface charge onalumina ceramic insulator in vacuum. We investigated the dependence of the location and magnitude of surface charge on surface flashovercharacteristics. The experimental result revealed the influence of surface charges on surface flashover characteristics in vacuum.

Surface charge generation

Introduction

Influence on inception of surface discharge

Vacuum circuit breaker

2tan2cos4tancossin41

0nE

nE

tan

tan12sin02

zEn

ES

The surface charge is critical factors to the

surface flashover phenomena.

When surface charge located near the

cathode electrode (Case A), positive

charge made surface flashover voltage

lower and negative charge made surface

flashover voltage higher. We could explain

the results by the change of electric field

at flashover by surface charge.

When surface charge located at the way

of discharge path (Case B), either positive

or negative charge made surface flashover

voltage lower. We could explain the results

by the role of surface charge for making

the secondary electron energy higher for

satisfying the condition of secondary

electron emission avalanche.

Vacuum gap length

Back electrode(Grounded)

H.V. electrode

GND electrode

H.V. dc

(negative or positive)

Alumina(150mm×150mm×5mmt)

75

0

-7575-75 0

(a) Negative charge (b) Positive charge

-8

-6

-4

-2

0

Surface potential [kV]

75-75 0

-8.0 -4.0 0 0 1 2 3 4

75

0

-75

0 2.0 4.0Surface potential [kV]

H.V. electrode

Alumina

GND electrode

1.0mm

Negative impulse (1.2/50ms)

H.V. electrode

1.0mm

Negative impulse (1.2/50ms)

Alumina

Surface charge

Case B

Case A

H.V. electrode

GND electrode

Alumina

En0= 4 [eV]

Vacuum

En

S

Alumina

-Ex

-Ez

GND electrode

GND electrode

SEE coefficient is higher than 1

Charging area

Negative charge(relax electric field)

Positive charge(enhance electric field)

= 5°

Surface flashover generation

Conclusion

1.

2.

3.

Key points

on insulation designStationary rod

Electrodes

Contacts

Shield

Moving rod

Bellows

Insulation of

shield-electrode

and shield-rod

Insulation

between contacts

Creepage insulation

of solid insulator

Alumina

dielectrics

External insulation

Negative impulse (1.2/50ms)

Negative impulse (1.2/50ms)

Case A

-30

-25

-20

-15

-10

-5

Surf

ace fla

sh

ove

r voltage

[k

V]

-10 -8 -6 -4 -2 0 2 4 6Surface charging potential [kV]

Case A Case B

-30

-25

-20

-15

-10

-5

Su

rfa

ce

fla

sh

ove

r vo

lta

ge

[k

V]

-10 -8 -6 -4 -2 0 2 4 6Surface charging potential [kV]

Case A Case B

50mm

50mm

GND electrode