flash over prevention on high altitude transmission lines

Electrical and electronics dept,TKMCE kollam

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


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


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


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


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


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.


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.


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


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.


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.


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.


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.


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.


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.


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.


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,


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


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.


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.


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.


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.


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

http://www.ieee.org/


IF YOU WANT ANY HELP PLEASE FEEL FREE TO CONTACT ME

GILJITH M

TKMCE

KOLLAM

[email protected]

9633111535

mailto:[email protected]

flash over prevention on high altitude transmission lines

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