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Electrical and electronics dept,TKMCE kollam Page 1
FLASH OVER PREVENTION ON HIGH ALTITUDE
HVAC TRANSMISSION LINE INSULATOR STRINGS
SEMINAR REPORT
Presented By
GILJITH M
07 403 024
DDeeppaarrttmmeenntt ooff EElleeccttrriiccaall && EElleeccttrroonniiccss EEnnggiinneeeerriinngg
TThhaannggaall KKuunnjjuu MMuussaalliiaarr CCoolllleeggee ooff EEnnggiinneeeerriinngg
KKoollllaamm –– 669911000055
UNIVERSITY OF KERALA
2011
Electrical and electronics dept,TKMCE kollam Page 2
TThhaannggaall KKuunnjjuu MMuussaalliiaarr CCoolllleeggee ooff EEnnggiinneeeerriinngg
KKoollllaamm,, KKeerraallaa
DDeeppaarrttmmeenntt ooff EElleeccttrriiccaall && EElleeccttrroonniiccss EEnnggiinneeeerriinngg
CERTIFICATE
TThhiiss iiss ttoo cceerrttiiffyy tthhaatt tthhee sseemmiinnaarr rreeppoorrtt ffoorr tthhee eennttiittlleedd
FLASH OVER PREVENTION ON HIGH ALTITUDE
HVAC TRANSMISSION LINE INSULATOR STRINGS
iiss aann aauutthheennttiicc rreeppoorrtt pprreesseenntteedd bbyy
GILJITH M
during the year 2011 in partial fulfillment of the requirements
for the award of Degree of Bachelor of Technology in Electrical
& Electronics EEnnggiinneeeerriinngg ooff UUnniivveerrssiittyy ooff KKeerraallaa.
Co-Ordinator Head of the Department
Dr. Bijuna Kunju Prof. N Prathapachandran Asst. Professor Professor
Department of Electrical & Electronics Engg. Department of Electrical & Electronics Engg.
T.K.M C.E T.K.M C.E
Kollam. Kollam
Electrical and electronics dept,TKMCE kollam Page 3
CONTENTS
ABSTRACT .........................................................................................................................1
ACKNOWLEDGEMENTS ......................................................................................................2
1.INTRODUCTION ..............................................................................................................3
2.NUMERICAL ANALYSIS OF VOLTAGE DISTRIBUTION ........................................................4
2.1 NUMERICAL ANALYSIS ...............................................................................................4
2.2 CALCULATION MODEL ...............................................................................................6
3.ONSITE AND LABORATORY MEASUREMENTS ..................................................................7
3.1 ONSITE MEASUREMENTS ...........................................................................................7
3.2 LABORATORY MEASUREMENTS .................................................................................8
4.COMPARISON AND ANALYSIS OF RESULTS ......................................................................9
4.1 EFFECT OF GRADING RINGS,YOKE PLATES,SUBCONDUCTORS AND TOWERS............... 13
5.FOG CHAMBER MEASUREMENTS AND EXPERIMENTS .................................................... 14
5.1 TEST RESULTS AND ANALYSIS ................................................................................... 15
5.2 ANALYSIS AND AMELIORATION METHODS ............................................................... 17
6.CONCLUSION ................................................................................................................ 18
BIBLIOGRAPHY ................................................................................................................ 19
Electrical and electronics dept,TKMCE kollam Page 4
ABSTRACT
Numerical analysis, on-site measurements, and laboratory experiments are used in this
paper to analyse and solve the flashover problems of 330 kV ac high altitudes transmission lines
middle phase glass suspension insulator strings. A sub-model approach based on a finite element
method (FEM) applied in calculating the potential and electric field distribution along the
insulator strings under clean and dry conditions. Using this approach, 3-dimensional electrostatic
models (taking into account grading rings, sub-conductors, tower framework, and yoke plates)
are setup and investigated. On-site and laboratory measurements were also carried out to make a
comparison.
A relatively good agreement was obtained among the calculated, the on-site, and
the laboratory measured results, which demonstrated that insulator disk nearest to the high-
voltage end is required to withstand relatively high voltage. In addition, the insufficient dry arc
distance of the insulator strings is also proposed. Subsequently, the long insulator string flashover
tests under dry, wet, and artificial contaminated conditions, considering the middle (flashover)
and side phase (never flashover), respectively are carried out to validate the proposed insufficient
dry arc distance and discuss the measures for solving the flashovers. It has been observed that
lowering the grading rings position and adding a unit of insulator are considered to lengthen the
dry arc distance of middle phase insulator string in the tests. The experimental results are
compared and discussed, and it has been concluded that adding a unit of insulator is an effective
and economical way to solve flashover problems. The methods for analysing the flashover
problems are effective and can be used for other voltage level transmission lines
Electrical and electronics dept,TKMCE kollam Page 5
ACKNOWLEDGEMENTS
I would like to extend my sincere thanks to Prof. N. Prathapachandran, Head of Department,
Department of Electrical & Electronics Engineering for his encouragement and guidance.
I express my sincere gratitude to our co-ordinator Dr. Bijuna Kunju , Department of
Electrical & Electronics Engineering for his whole hearted support.
I also thank all other staff members of the department and my friends who encouraged
and helped me during the presentation of this paper and in the preparation of this report
Giljith M
Electrical and electronics dept,TKMCE kollam Page 6
CHAPTER 1
INTRODUCTION
Suspension insulator strings are widely used in power systems to provide electrical insulation
and mechanical support for HVAC transmission lines. Their insulation strength and the onset
of surface discharges are influenced by various uncontrollable meteorological and
environmental parameters such as pollution, altitude, humidity, temperature, ice and snow.
Several flashovers such as light flashovers moistened by high sustained humidity causes
heavy damage to insulator strings. There were also fog flashovers on 230 kV transmission
lines. Flashover damage was seen on the first or second unit at the high voltage end of the
insulator strings, on the grading rings, or on other hardware fittings. There was no trace of
discharge on the other insulators, the cross arms or the towers. These flashovers were initially
referred to as „unknown reason flashovers‟, now they are known as „fog flashovers‟.
These flashovers under light pollution and humidity and at high altitudes have not been
studied in depth we are focused on effective and economical measures to solve the flashover
problems where methods for analysing the flashover problems are also effective for other
voltage level transmission lines.
Potential distribution is non-uniform along insulator strings because of the capacitance effects
of the conductors, tower, and other hardware fittings to the insulators. The electric stress field
is highest across units near the line end. This can cause corona, surface discharges, audible
noise, especially for composite insulators. Consequently, the determination of potential and
electric field distributions along insulator strings is very important for the design operation
and maintenance of insulators.
The potential distribution along insulator string is determined by the geometry of towers,
hardware fittings and insulator strings, as well as the phase conductor configuration and can
be estimated by numerical calculation. The actual structure of the tower, insulator strings,
phase conductor configuration, hardware fittings, onsite pollution and high altitude are of
course included in the onsite potential measurements. However, these take considerable time
and resources and the measured results are greatly influenced by the environmental factors.
Potential distribution measurements and flash over experiments can be carried out flexibly
and with relatively less cost in the laboratory, and the effect of high humidity and pollution
considered.
Electrical and electronics dept,TKMCE kollam Page 7
It is thus worthwhile to use all three methods together and compare and analyse their results.
Numerical calculation, onsite measurements, and laboratory experiments were therefore used
to determine potential and electric field distribution along the insulator strings. The results are
compared and discussed in this paper.
CHAPTER 2
NUMERICAL ANALYSIS OF VOLTAGE DISTRIBUTION
The type, number of insulators, grading rings, tower framework, yoke plates, conductors,
neighbouring phase conductors and ground wires influence the potential and electric field
distribution along the insulator strings. In numerical analysis, we consider the problem of
potential and electric field distribution calculation along insulator strings as a 3-D open
boundary electrostatic problem.
2.1 NUMERICAL ANALYSIS
Numerical calculation is an economical and efficient way to evaluate the potential and
electric field distributions and the results have sufficient precision as well. Here we are using
sub-model method based on finite element method (FEM) where a finer sub-model of the
whole structure is made for calculations.
This approach relies on Saint- Venant‟s principle which states that effects of stress are
localized around the stress concentration. Therefore if boundaries of the sub-model are at
sufficient distance from the stress concentration accurate results can be calculated.
From the sub-model, large scale course model is created to determine the potential
distribution near the insulator strings. Finally a refinement sub-model is created. The sub-
model boundaries are called cut boundaries and the potential at those nodes along the cut
boundaries can be obtained by interpolating the potential results from the course model at
these locations.
In the numerical analysis, we are considering a 330 kV transmission line situated at an
altitude of 1500m above sea level. The tower dimensions and its various components are
shown in figure. Three insulator strings are installed in the tower, each with 21 units.
Electrical and electronics dept,TKMCE kollam Page 8
2.2 CALCULATION MODEL
The 3D electrostatic calculation model was set up on the assumption that there was no corona
or leakage currents and that the insulators were clean and dry. Using the symmetry of the
transmission tower, half and quarter models were used in the numerical analysis: the half
Electrical and electronics dept,TKMCE kollam Page 9
model to analyse the effects with all three phases included and the quarter model, which
greatly reduces the scale of the calculation model, for the middle phase alone.
The curved surface AEBCFD models the open boundary by using infinite elements, and the
plane ABCD models the earth plane. Figure 4b and 4c indicate the method used to analyse the
middle phase insulator string and the side phase insulator string respectively.
Electrical and electronics dept,TKMCE kollam Page 10
To reduce the calculation scale, the quarter model was set up. The cap and its attached pin are
modeled as a single entity as they are at the same potential. It has been seen that the sub-
model is more finely meshed compared to the coarse model. The course model of the
insulator mesh consists of about 900 elements, the finer sub-model some 1500 elements. So
any defects can be easily detected .
CHAPTER 3
ONSITE AND LABORATORY MEASUREMENTS
3.1 ON-SITE MEASUREMENTS
Potential distribution along the insulator strings of the three phases were obtained by the live
line measurements of the 330 kV transmission line under dry conditions.
An optical fiber electric field and voltage meter was used to measure the potential distribution
and field along the insulator strings. The voltage on each unit was measured three times and
the average value was taken as the measured value. The optical fiber meter , having no metal
parts does not influence the electromagnetic field around insulator strings and tower.
3.2 LABORATORY MEASUREMENTS
To validate the numerical approach, potential distribution measurements were carried out in
the laboratory for comparison. The insulator string, grading rings, sub-conductors, yoke plates
in the laboratory measurements model have the same dimensions as that of the tower under
our study and the insulators were suspended at the correct tower height in the high voltage
hall.
Electrical and electronics dept,TKMCE kollam Page 11
The potential distribution across the middle phase insulator string was measured using metal
pellet electrode gap method. The capacitive effects on the potential distribution is negligible
because of the small dimension of the pellet. The voltage across each insulator unit was
measured three times and average of their values were taken.
CHAPTER 4
COMPARISON AND ANALYSIS OF CALCULATED AND MEASURED
RESULTS
The voltage distribution curves from the on-site measurements and the half model of the three
phases are shown in the following figures.
Electrical and electronics dept,TKMCE kollam Page 12
It can be seen from figures 8 and 9 that the voltage distribution along the middle phase
insulator string is significantly greater than that of the side phase strings at the high voltage
end, which is critical for flashover initiation .So it is sufficient to analyse the middle phases
alone.
Figure 9 demonstrates that there is not much difference between the voltage distributions
under three phase and single phase excitation. On the other hand, the voltage difference
between the middle phase and side phase for on-site measurements is smaller than that of
numerical analysis. This may be due to some degree of pollution.
Electrical and electronics dept,TKMCE kollam Page 13
The quarter model equipotential lines of the middle phase insulator string and its vicinity
inside the tower head are shown in the figure
The potential magnitude is based on unity phase voltage and the actual values can be
obtained by multiplying by phase-to-earth voltage (190.53 kV). The potential of the sub-
conductors , yoke plates, and grading rings were set to 1.00 and the potential of the tower
framework to zero as shown in the figure 10.
The voltage distribution curves for the numerical calculation and the on-site and laboratory
measurements are shown in figure 11. They are seen to be very similar. However, the on-site
results are less smooth and diverge from others particularly near the ends of the string which
may be due to some amount of pollution.
Electrical and electronics dept,TKMCE kollam Page 14
Comparison o the calculated results for the quarter model and the coarse model indicates that
the potential distribution results are very close. However the electric field distributions are not
in close agreement, particularly along the axis where the coarse model has higher and sharper
maxima as indicated in figure 12a.
Electrical and electronics dept,TKMCE kollam Page 15
The voltage stresses across six insulators of the middle phase insulator string are shown in
table1. The numbering increases from the high-voltage end to the ground end of the insulator
string. The first insulator unit of the half model has a voltage across it which is about 0.8%
and 0.5% higher for both three phase and single phase than that of the quarter model.
However the laboratory measured results are about 0.7% lower than that of the quarter model.
Electrical and electronics dept,TKMCE kollam Page 16
The calculated and laboratory measured results in table1 indicate that the voltage across the
first unit of the insulator string is higher than other insulator units. So these insulator units are
likely to flashover under any unwanted condition such as pollution, humidity, etc.
4.1 EFFECT OF GRADING RINGS, YOKE PLATES, SUB-
CONDUCTORS AND TOWER
The voltage distribution curves for the quarter full model, for the insulator without sub-
conductor and without the tower are shown in figure13. It has been seen that the grading
rings, tower and sub-conductors do influence the voltage distribution along the insulator
strings significantly. Hence 3d calculation model is necessary and the grading rings, tower,
Electrical and electronics dept,TKMCE kollam Page 17
sub-conductors and yoke plates should be considered especially for the calculation of voltage
across the first insulator unit.
CHAPTER 5
FOG CHAMBER MEASUREMENTS AND EXPERIMENTS
Artificial flashovers under dry, wet, and contaminated conditions for the middle phase and
side phase were carried out to validate the proposed reasons for fog flashovers.
The artificial fog chamber is 24m x 24m x 26m. The test voltage was supplied from a 3 x 750
kV, 4 A transformer cascade. Each transformer was 3000 kVA, 750 kV unit with a rated
current of 4 A and a short-circuit impedance of 5.38 %. As shown in the figure, the voltage
was supplied through suitable metering and an 800 kV wall bushing to the fog chamber.
The insulators were tested under the following conditions:
Clean and dry condition.
Clean with >90% humidity but no rain.
Medium pollution with >90% humidity but no rain.
Artificial pollution were given to the fog chamber and the insulator are dried, hung and
moistened by the fog in the chamber. To get high humidity, the artificial fog chambers was
rapidly filled with a large amount of water vapour for about 10 minutes and then the supply
Electrical and electronics dept,TKMCE kollam Page 18
voltage was applied after all the fog has disappeared. Initially about 75% of anticipated
flashover voltage „u‟ was applied. Later the voltage was increased in 2% steps until flashover
occurred. The flashover voltage was taken as the average from five flashovers.
5.1 TEST RESULTS AND ANALYSIS
The flashover voltage can be estimated from the following equation ;
u = ∂d u0 / Hhn
where u is the estimated flashover voltage for the given altitude and humidity ,u0 is the
flashover voltage under standard atmospheric and humidity , ∂d is the relative air density for
the given atmospheric pressure and humidity , Hh is the air humidity factor , and n is a factor
which is a function of the insulation length l.
∂d = (1- αHa / T0 )4.26
where α is the air temperature factor(0.0065 0
C/m) , Ha is the given altitude in m , and T0 is
the standard temperature (293 K).
Hh for ac voltage can be estimated from the following equation:
Hh = 1+0.0125 (11-h) , where h is the absolute humidity at the given altitude in g / m3.
The measured flashover voltages and estimated flashover voltages after correction for
altitude, for a 21 unit insulator string are given in the table.
Electrical and electronics dept,TKMCE kollam Page 19
Test Conditions Flashover Voltage
( kV )
Flashover
Status
Flashover Path
Measured Estimated
Middle phase
Clean & Dry
470
390
Did not
flashover
_
Middle phase
clean & high
RH
385
319
Flashover
Yoke plate
to
Cross arm
Middle phase
Pollution &
high RH
259
214
Flashover
Yoke plate
to
Cross arm
Side phase
Pollution &
High RH
298
247
Flashover
Yoke plate
to
Cross arm
Here RH indicates medium pollution with >90% humidity but no rain.
Electrical and electronics dept,TKMCE kollam Page 20
The phase-to-earth voltage of a 330 kV transmission line should normally be 191 kV but the
maximum allowed phase-to-earth voltage is 210 kV. Thus from the table it is clear that at sea
level there should be sufficient safety margin to avoid flashovers. The altitude correction
factor is about 0.83 and this has been applied to measured flashover voltages to obtain the
estimated equivalent test results at an altitude of 1500 m.
From the results it was clear that for the insulator in the middle phase under clean, dry
conditions should never flashover and clean insulators under very humid (>90% humidity)
conditions have a 34% safety margin. But under conditions of high pollution and humidity,
flashover should be a distinct possibililty as estimated as it has only 2% safety margin. For
insulaor strings in the side phases flashovers are most unlikely.
5.2 ANALYSIS OF AMELIORATION METHODS
To prevent the occurrence of flashovers need to be economical, effective, and convenient
without requiring major reconstruction of the towers. Two measures have been proposed to
increase the dry arc distance. One is by lowering the height of the grading rings by one
insulator spacing and the other is to add another insulator unit to the insulator string.
Electrical and electronics dept,TKMCE kollam Page 21
Test conditions
Flashover voltage
( kV )
Flashover
path
Measured Estimated
Lowering grading
rings with one
spacing
290
240
Yoke plate to
Cross arm
Adding a unit of
Insulator
362
300
Yoke plate to
Cross arm
It is observed from the table that the effect of adding an insulator unit is much better than that
of lowering the grading rings with one insulator spacing. The flashover voltage is about 43%
higher than the critical voltage of 210 kV. Hence the insulation level of the 22 insulator
strings should easily be sufficient to avoid flashovers. Furthermore adding an extra unit is
convenient and inexpensive.
Electrical and electronics dept,TKMCE kollam Page 22
CHAPTER 6
CONCLUSION
Flashovers of 330 kV transmission lines middle phase glass cap and pin insulator strings were
analysed by using numerical analysis, on-site measurements, and laboratory experiments. A
localised fine mesh „sub-model‟ was used to calculate the potential distributions along HVAC
transmission line insulator strings which was found to be very accurate. The calculation
model considered the geometry of the tower and various hardware fittings. It has been found
that the potential distribution of the middle phase is less uniform than the side phases.
Therefore electrical strength co-ordination is more critical in these phases. The first insulator
at the high voltage end has the highest potential drop across it and is more prone to
flashovers. An effective and economical way to solve the fog flashovers was shown
artificially which is to add another insulator unit to the middle phase insulator strings. The
altitude was shown to be a major factor in reduced flashover voltages under conditions of
humidity and medium pollution. The methods used and results obtained may usefully be
applied to the design, operation and maintenance of HVAC transmission line insulators
BIBLIOGRAPHY
IEEE transaction on Dielectrics and Electrical Insulation, Volume 16, No:1, pp
88-97, February 2009.
L.Hu, C.Sun, X.Jiang, Z.Zhang and L.Shu on ” performance of pre-contaminated
and ice-covered composite insulators to be used in 1000 kV UHV AC
transmission lines”, IEEE Trans. Dielectric Electric Insulators, Vol 14, pp 1347-
1356, 2007.
I.W.McAllister, “Electric fields and electrical insulation” , IEEE
Trans.Dielectr.Electr.Insu, Vol.9, pp 672-696, 2002.
www.ieee.org
Electrical and electronics dept,TKMCE kollam Page 23
IF YOU WANT ANY HELP PLEASE FEEL FREE TO CONTACT ME
GILJITH M
TKMCE
KOLLAM
9633111535