International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 8 - October 2015
Composite insulation in Parallel under DC and AC Voltages
Khalaf Y. Al-Zyoud
Al-Balqa Applied University
Faculty of Engineering Technology
P.O.Box. 15008 Jordan , Amman- Marka ashamalyia
Abstract: The phenomenological aspects of the electrical
properties of Solid-liquid dielectrics are discussed with
special relevance to the utilization of such systems at high
electric stresses. Because modern society is strongly
dependent on a reliable power supply, there is an urgent need
to develop more reliable high voltage insulators. This paper
provides a brief description of composite insulators, including
a discussion of their benefits, the paper also discusses the
benefits of using insulators as a composite insulation in
parallel (solid and liquid), and identifies four broad issues
that are being addressed by the present research on same type
of insulation.
and dc voltages using the circuit of Fig. 1 which resulted in
the application or the waveform shown in Fig. 2 to the test
sample, Fig. 2 includes the definition of ripple factor.
Standard lightning and switching type impulse voltages were
derived from a 8 stage 800 kV, 10 kJ impulse generator.
Keywords: Insulation, DC, AC, Voltage.
Vdc = dc supply
R1 = 2.3 M Ω
R2, R3 = voltage divider
I. INTRODUCTION
The use of composite solid-liquid insulation gives rise to
parallel interfaces in which the solid-liquid boundary is
parallel to the field. Instinctively, one is tempted to conclude
that the presence of the interface lowers the breakdown
strength of the gap, that the presence of a parallel interface
may either increase or decrease the breakdown voltage, in the
dc case, depending upon the test geometry which influences
liquid flow under electric stress. In the ac case, while electro
hydrodynamic effects are important other factors 4 come into
play as well. From a practical standpoint it is important to
know the strength of the interface not only under ac and dc
stresses but also under lightning and switching impulse
voltages and dc voltage with ripple this last- type of waveform
occurs in converter transformers and smoothing reactors [7].
In this paper a practical electrode configuration has been used
to investigate the effect of introduction of a parallel type
interface using ac, dc, impulse type voltages and dc voltage
with ripple. The strength of the gap in the presence of the
interface is compared with that obtained in the absence of the
interface.
Fig .1. Test circuit for producing dc voltage with ripple
V ac = potential transformer
Rp = 25 k Ω
C = 1oo pF gas capacitor
Rm = 2.86 k Ω
II. SAMPLE EXAMINATION
Ring shaped brass electrodes of outer diameter 199.2 mm.
Inner diameter 157 mm. Width 10 mm were fitted snugly
around cylindrical pressboard samples of wall thickness 2.3
mm and inner diameter 152.4 mm. The test sample was
impregnated with transformer oil using proper procedures.
III. EXAMINATION C I R C U I T S
Alternating and direct voltage tests were performed using a
138 kV PT and a 220 kV, 10 mA dc supply; dc voltage
with· ripple was realized ·by simultaneous application of ac
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Fig . 2 Ripple factor, VAC / VDc
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 8 - October 2015
IV. EXAMINATION WAYS
AC: The test voltage was raised and automatically at the rate
of 2kV/s until breakdown occurred, all tests were conducted
with pressure in the test chamber at atmospheric. After
breakdown the test gap was examined to ascertain the location
of breakdown. Breakdown on the surface caused surface
damage which necessitated prolonged application of vacuum
for removal of bubbles. Following suffice damage a new gap
was used for subsequent testing. A similar procedure was
followed when breakdown occurred away from the interface
in this case a shorter time sufficed for removal of all bubbles
by application of vacuum and the same gap was. Used for
another test. In all cases the breakdown voltage and the
location of the breakdown was noted. For each gap at least ten
readings of breakdown voltage were obtained and the mean
breakdown voltage and the standard deviation computed.
DC: These tests were performed employing the procedures
described
in
section 4.1. The test voltage was raised
manually at an approximate rate of 2 kV /s until breakdown
occurred.
Hypothesis of voltage tests
The test circuit of Fig. 1 was used to apply a voltage
waveform such as that in fig.2 across. The test gap. The
procedure employed was essentially the same as described in
section 4.1. First, a dc voltage level was chosen and applied;
next the ac voltage was raised uniformly and automatically at
rate of 2 kV/s until breakdown occurred. The ac breakdown
voltage was recorded. From the results of at least ten
breakdown readings, the mean value of the ac peak voltage
and the standard deviation was computed; from this the peak
breakdown voltage (dc component + ac peak component) and
the ripple computed. By application of different dc levels it
was possible to vary the ripple factor in the range of (0.4 2.5).
TABLE 1
BREAKDOWN VOLTAGE OF 15 MM OIL GAP
UNDER AC AND DC VOLTAGE AT ROOM
TEMPERATURE
Interface present
Average BD.
voltage
kv
Interface absent
Deviation Average BD.
St.
voltage
kv
Deviation
St.
AC
130
7.8
138
3.5
DC
145
3.8
125
5.5
The presence of the interface increases the breakdown
voltage by 11.8 percent under dc stress, but decreases the
strength in the ac case by 3.9 percent. It appears that in the
dc case EHD phenomenon is responsible for initiating
breakdown; the insertion of the interface acts to dampen
liquid flow. In the ac case besides this consideration,
other factors such as mismatch of dielectric Constants
and the presence. Of minute particles and contaminants
also come into play. Figure.3. shows similar data for the
5 and 10 mm gaps tested under dc voltage with ripple. For
both gaps the strength increases in the presence of the
interface for the 5 mm gap this difference is larger for
smaller value of ripple. The opposite occurs with the 10
mm gap.
Lightning and switching impulse voltage tests
Impulse voltage of positive polarity was applied to the
gap and increased in steps starting from a suitable value until
breakdown of the test gap occurred. Each increment in voltage
was more than 3% of the impulse magnitude employed in the
previous step. The wait interval between two impulse
applications, when breakdown did not occur, was 3 minutes.
Other procedures remained as described in section 4.1.
V. RESULTS AND DISCUSSION
Table 1 compares the strength of 15 mm gap both in the
absence and presence of the interface under ac and dc applied
voltages.
ISSN: 2231-5381
Fig . 3 surface (or gap) peak breakdown voltage Ripple relationship; and 10 rn m gaps in the presence
arid absence of the interface.
Table 2 shows breakdown strengths under Impulse voltage
for the 15 mm gap. For switching and lightning impulses
the interface causes the strength of the gap to increase by
29. 2 and 11.9 Percent receptivity. This reduction is much
larger than that experienced under ac voltage and may be
explained by the hypothesis proposed in[8] that the
presence of a pressboard Interface causes the behaviour of
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 8 - October 2015
streamers to be quite different from that observed. in oil
gaps alone. In the former case the velocities are larger and
show a greater dependence thus enabling high velocity
steamers to form at Lower voltages which results in a
lowering of the breakdown voltage. Finally, in Table 3 the
impulse ratios are compared for the 15mm gap. The
lightning and switching impulse ratio are indicated by Kl
and ks respectively it is seen that the impulse ratios are
lower in the presence of the interface. In the presence
of the
interfaces under application of ac, dc and
composite· voltages most breakdowns occurred in the inter
electrode space away from the interface; when it did
occur on the interface , damage was only slight, In contrast,
most breakdowns under impulse type voltages occurred on
the surface. Switching impulses caused more damage than
lightning impulses.
TABLE. 2
BREAKDOWN VOLTAGES OF THE 15 MM OIL GAP UNDER
LIGHTNING AND SWITCHING IMPULSE VOLTAGES AT
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
ROOM TEMPERATURE
Interface present
Average
Breakdown
Voltage(kv)
Lightning
impulse
235
Switching
impulse
171.2
Interface absent
std.
Average
deviation std.
Breakdown
deviation
Voltage(kv)
18.3 270
7.1
10.3 250.1
11.3
[9]
[10]
[11]
[12]
[13]
[14]
[15]
TABLE. 3
IMPULSE RATIOS KL AND KS FOR THE 15 MM TEST
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GAP WITH AND WITHOUT AN INTERFACE
Interface present
kL
1.6
Interface
absent
1.88
ks
1.2
1.71
VI. CONCLUSIONS
For the test configuration considered the presence of the
parallel interface decreases the ac and impulse (lightning and
switching) strengths. The impulse ratio is also lowered thus
indicating that the cerise is greater under impulse voltages
especially with switching type impulses. Under dc applied
voltage the opposite occurs and
the interface causes an
increase in strength. When ripple is present in the dc voltage
the interface caused the strength to increase. However for the
two gap Lengths considered, opposite behaviour was observed
as far as variation of strength with ripple concerned.
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