J. Electromagnetic Analysis & Applications, 2009, 2: 97-101
doi:10.4236/jemaa.2009.12015 Published Online June 2009 (www.SciRP.org/journal/jemaa)
1
Analysis of Flashover Characteristics under
Nanosecond Pulsed Coaxial Electric Field
W. L. Huang1, J. F. Cui1, G. S. Sun2
1School of Mechatronics Engineering, Zhengzhou Institute of Aeronautical Industry Management, Zhengzhou, China; 2Institute of
Electrical Engineering, Chinese Academy of Sciences, Beijing, China.
Received February 17th, 2009; revised March 24th, 2009; accepted April 2nd, 2009.
ABSTRACT
Under nanosecond pulsed coaxial electric field, surface flashover voltage over the interfaces between nylon 1010 and
transformer oil increases almost linearly with gap length, and the steeper rising edge of applied pulse, the higher flash-
over voltage. Surface flashover properties are closely related to the electric field at the triple junctions of
solid-liquid-electrode and the field gradient along the interfaces. Although the increased difference between inner and
outer electrode radii will enhance electric field strength at the triple junctions and nonuniformity degree of potential
distribution along interfaces, it reduces simultaneously terribly the surface field strength of coaxial inner electrode, so
that flashover voltage doesn’t descend, but ascends almost linearly with gap length. The average flashover strength in
coaxial electric field can be estimated by that in uniform electric field for large enough difference between inner and
outer electrode radii, which is useful to practical engineering design for coaxial pulsed power apparatuses.
Keywords: Pulsed Power Technology, Coaxial Electrodes, Gap Length, Surface Flashover, Triple Junctions, Surface
Potential
1. Introduction
Although coaxial pulsed power apparatuses are wide-
spread, researches on discharge characteristic over trans-
former oil/solid interfaces under nanosecond pulsed co-
axial field, as opposed to discharge characteristic over
gas/solid or vacuum/solid interfaces under uniform field,
are less, which can’t meet the rapid development of
pulsed power technology [1-7]. Surface flashover,
whether applied dc, ac or impulse voltages, are closely
relevant to potential distribution gradient along interfaces
between different dielectrics. Trinh [8] and Menju [9],
respectively, pointed out the relationship between flash-
over field strength and different parameters of coaxial
experimental electrodes might be speculated on through
corresponding states of potential distribution along the
interfaces and field strength at triple junctions. Liu [10]
plotted the varying curve of breakdown voltage vs. non-
uniformity degree of electric field in electrode gap in
transformer oil, discovered that breakdown voltage in
uniform field was about three times higher than that in
nonuniform field under applied impulse voltage of
pulse-width of 40 µs, and achieved that change of uni-
formity degree of electric field in gap worked on break-
down voltage more prominent under impulse voltage than
under ac voltage. This contribution analyzed flashover
characteristic over transformer oil/nylon 1010 interfaces
under coaxial nanosecond pulsed field in the case of
changing radius difference between inner and outer elec-
trodes from the viewpoint of potential distribution over
the interfaces and field strength at triple junctions, and
compared flashover properties in coaxial field with ones
in uniform field, thus draw conclusions useful to studying
flashover over interfaces between different dielectrics
and to the developments of pulsed power technology.
2. Experimental Setup and Methods
The entire structures of electric connection of experi-
mental setup are shown in Figure 1. The high voltage
nanosecond pulsed power supply is SPG200 based on
Semiconductor Opening Switch (SOS) in Northwest In-
stitute of Nuclear Technology, whose amplitude of output
voltage which can reach maximum 350 kV is modulated
by adjusting the resistor of cycling salt water. A resistor
of 200 Ω is in series with electrode gap in order to avoid
excessive current of gap breakdown impairing SPG200
[11,12]. An impulse of pulse-width of about 30 ns, rising
edge of about 10 ns, is produced by SPG200. Accord-
ingly, the waveforms of breakdown voltage and current

Analysis of Flashover Characteristics under Nanosecond Pulsed Coaxial Electric Field 98
Figure 1. Schematic diagram of experimental setup
Figure 2. Voltage and current waveforms with no gap
Figure 3. Breakdown voltage and current waveforms
for the direct short gap and for flashover, respectively,
are shown in Figure 2 and Figure 3. Breakdown voltage
and current signals, whose data acquisition and process-
ing were executed by an oscilloscope of Tektronic684C
combined with a computer, were attained by a capacitor
divider and a resistor divider respectively [13].
The coaxial brass testing electrodes shown in Figure 4
should be characteristic of carefully coaxial structures,
should be ensured to contact tightly with experimental
solid samples and to replace solid samples conveniently,
should be long enough in axis to avoid the influence of
edge-effect of coaxial field on flashover over solid/liquid
interfaces [14]. Solid samples of nylon 1010 with axial
length of 6 mm were given shape to round ring. The liq-
uid dielectric was transformer oil, marked 45 numbers,
with industrial desiccating, filtrating and degassing.
Flashover voltage was described as the voltage applied
between inner and outer electrodes while gap breakdown
happened at the front edge of impulse voltage whose am-
plitude was about 135 kV, waveform of 10/20 ns, and
two kinds of steepness of rising edge of 7.8 kV/ns and 8.6
kV/ns. The change of gap length of 1, 2, 3, 4 mm was
achieved through the according change of diameter of
outer electrode of 8, 10, 12, 14 mm, at the same time re-
taining the diameter of inner electrode of 6 mm. The total
number of solid samples in same size and shape was five,
and every solid sample was tested four times between
them an interval of 10 min was introduced [15,16]. It is
very necessary to replace solid sample, transformer oil
after flashover for four times and experimental electrodes
as etched spots have shown on the surface of electrodes.
3. Results and Discussion
3.1 Flashover Voltage and Gap Length
For two different kinds of steepness of rising edge of
applied voltage impulse, experimental data for flashover
voltage Uf of nylon 1010 vs. gap length D were shown in
Figure 5 in which every datum point was the average of
15 times tests and was associated with a standard devia-
tion. It is observed from Figure 5 that Uf of nylon 1010
increases almost linearly with D. As a result of surface
flashover of rising edge of applied impulse, Uf is hardly
Figure 4. Brass electrodes

Analysis of Flashover Characteristics under Nanosecond Pulsed Coaxial Electric Field 99
Figure 5. Flashover voltage according to variable gap length
affected by the amplitude of applied impulse, whereas
remarkably by the rising edge of applied impulse. For
example, Uf of nylon 1010 is enhanced about 15 kV with
the increase of 1 kV/ns in the steepness of rising edge of
applied impulse at the gap length of 3 mm.
3.2 Simulation Analyses of Experimental Results
3.2.1 Field Strength at Triple Junctions
The factor
 of field strength improvement at triple junc-
tions is defined by
 = Ej / Em, (1)
where Ej is the field strength at triple junctions, Em is the
average field strength along solid/transformer oil inter-
faces and denoted by
Em = U0 / D, (2)
where U0 is the amplitude of applied static voltage, D is
gap length. A 
i for field strength improvement factor at
triple junctions at inner electrode surface as well as a

o
for that at outer electrode surface varying with gap length
D is shown in Figure 6 in whose legend are 
i and

o.
From Figure 6, on the one hand the

i, far greater than 1,
increasing rapidly with D indicates the field strength Ei at
triple junctions at inner electrode surface is enhanced to a
certain extend and is, along with the increase of D, more
and more higher than the average field strength Em; on
the other hand the 
o, at all time less than 1, decreasing
nearly linearly with D indicates the field strength Eo at
triple junctions at outer electrode surface is always lower
than Em. The above opinions imply, under coaxial non-
uniform field, because of Ei far greater than Eo surface
flashover over solid/transformer oil interfaces should
begin from the triple junctions at the inner electrode sur-
face whose physical, chemical natures would character
flashover properties. As a result for a single reason, the
increase of D would enhance Ei, consequently reduce Uf
which contradicts the experimental phenomena.
3.2.2 Potential Distribution along the Interfaces be-
tween Nylon 1010/Transform Oil
The nonuniformity degree γ of potential distribution over
the interfaces between nylon 1010/transformer oil is de-
fined by
γ = Pmax /Pmin, (3)
where Pmax, Pmin respectively stand for the maximum,
minimum potential gradient in unit length along the in-
terfaces. That the γ increases almost linearly with D
should be the other reason for declining Uf, which also
contradicts the experimental phenomena.
3.2.3 Field Strength at the Surface of Inner Electrode
When flashover occurs, that the field strength Ein at the
inner electrode surface varies with gap length D is shown in
Figure 8, where Ein reduces nonlinearly rapidly with D.
This is the only positive reason for Uf to increase with D.
From Figure 6 and 7, Uf should decrease with D, but in-
versely from Figure 8. So under the combined effects of the
above three reasons Uf behaves to increase almost linearly
with D. It is sure that the positive effect of Ein on the surface
flashover shown in Figure 8 is intense enough to counteract
the negative effects of  and γ shown in Figure 6 and 7 re-
spectively, so that Uf increased almost linearly with D.
Figure 6. Field strength at triple junctions vs. gap length
Figure 7. Potential distribution along the interfaces vs. gap
length

Analysis of Flashover Characteristics under Nanosecond Pulsed Coaxial Electric Field 100
Figure 8. Field strength at the inner electrode surface vs.
gap length
Figure 9. Flashover comparison between coaxial and uni-
form field
The above statement indicates , in a general way, the
higher field strength at solid-liquid-electrode triple junc-
tions as well as the more nonuniform potential distribu-
tion over the interfaces between different dielectrics
should lessen the resultant flashover voltage, however,
under the actual circumstances, one should think over all
sorts of factors working on flashover, otherwise would
receive false results.
3.3 Comparing the Results under Coaxial Elec-
tric Field with Those under Uniform Field
The average flashover field strength Eav in the coaxial
field was compared with that in the uniform field in Fig-
ure 9. Eav equals to flashover voltage Uf divided by gap
length D in the coaxial and uniform field respectively, i.e.
Eav = Uf / D. (4)
The Eav in the coaxial field is lower than that in the
uniform field for the same solid samples, applied impulse
voltage and gap length; For example, the Eav in the coax-
ial field is about half of the one in the uniform field at the
gap length of 1 mm. With the increase of D, the differ-
ence of Eav between two types of electric fields is dimin-
ished and the decline of both Eav with D is also stabilized
gradually. That two curves of Eav vs. D in Figure 9 ap-
proaches each other after surpassing a relatively greater
gap length ( > 4 mm under this experimental conditions ,
for example) reveals it is sensible to speculate on flash-
over voltage under coaxial field by the one under uniform
field, which is valuable to the industrial designs of pulsed
power equipments of coaxial structure.
4. Conclusions
The flashover voltage along the interfaces between nylon
1010/transformer oil increases nearly linearly with gap
length under nanosecond pulsed coaxial electric field.
The composition effects of enhanced field strength at
triple junctions, heightened nonuniformity degree of po-
tential distribution over the interfaces between nylon
1010/ transformer oil and descended sharply field
strength at the inner electrode surface make flashover
voltage increase almost linearly with gap length whose
increase would lead the coaxial field to tend to the very
nonuniform field under point-plane electrodes.
Flashover natures are closely related to the electric
field strength at triple junctions of solid-liquid-electrode
and the potential distribution along the interfaces between
different dielectrics. It is, in a general way, sure that the
higher field strength at triple junctions and the more
nonuniform potential distribution along the interfaces
between different dielectrics would result in the lower
flashover voltage, whereas in practice one should take all
kinds of factors controlling flashover into account for
correct results.
While gap length of coaxial electrodes is large enough,
the average flashover field strength in coaxial field can be
estimated by means of that in uniform field, which is
rather significant to the design of coaxial structural
pulsed power apparatus.
5. Acknowledgments
The authors gratefully acknowledge the support and as-
sistance in experiment by Room 3 and 6 in Northwest
Institute of Nuclear Technology.
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