ANALYSIS OF CORONA DISCHARGE AND EARTH FAULT 0N 33KV

OVERHEADLINE
S.K.Vjjithanantha, S.Vengadesan, S. Venkatraakavan, R.M.H.L.Thushara.
Supervised By: Prof. J. R. Lucas

ABSTRACT 1. Copper wire is present between cross arm and

This paper presents the findings of the analysis of ground.
Corona discharge and Earth fault on 33kV overhead 2. No copper wire is present between cross arm and
distribution line. ground.
Two high voltage experiments have been carried out to
find out the influence of concrete covering on flashover 2. LOCATION OF DISTRIBUTION LINE
and voltage and current variation along the pole. The identified places are located in the Moratuwa area
Possible solutions have been analyzed and presented. and comes under the Colombo district. Of these, Uyana
Suggestions have been as to how to avoid corona. Road starts from the Galle Road close to the Moratuwa
town and runs towards the sea. The overhead lines that
run along Uyana Road have been taken in to
1. INTRODUCTION consideration for the study. The location is very close to
the sea (no more than 1km from the sea).
Corona discharge and Earth faults are problems faced
by the Lanka Electricity Company (LECO) on some 33
kV distribution lines in the Moratuwa area. The 3. OBSERVATION OF CORONA
electricity supply and services to customers along 1. On almost all the poles along the Uyana road,
Uyana Road comes under LECO. There are 13m the hissing noise was heard on the 33kV
concrete poles on which the 33kV overhead line runs at overhead line.
the top and the 11kv line few meters below. The low 2. At certain poles only, it was observed that
voltage distribution line runs under the 11kV line. visual corona occurred and sometimes
Especially in the months of July and August, near to flashover on the 33kV overhead line.
almost all the poles, a hissing noise is heard during Generally not on all three conductors were
night. At some poles it was observed that Corona involved but the middle conductor and pole
flashover occurred on the 33kV line. What is special top.
here is that the flash over occurs not between the
conductors, but between the middle line conductor and 3.2 Damage observation
the pole top. Due to this flashover, the middle line
insulator gets damage and has to be changed several The following damages were observed in the flashover
times. One pole also got damaged at the top. poles:
1. Insulator damage
During this flashover, a significant current passes 2. Pole damage
through the concrete pole and affects the low voltage 3. Low voltage distribution line snapped
distribution line. The unwanted voltage rise due to the 4. LV line supporter got damaged.
passage of current through the pole affects the low
voltage distribution line and causes damages to the 3.3 Disruptive voltage (E0)
consumers electrical equipment. The disruptive voltage E0.rms is the minimum phase to
neutral voltage at which Corona occurs (hissing noise
A main aim of this paper is to present the reasons for
occurring voltage). This is given by
this flashover and to make suggestions to prevent or
reduce this effect. E 0.rms = 21.2δmorln (d/r) kV.

In the concrete poles, the cross arm of the 33kV To analyze the particular situation, it is necessary to
overhead line is generally connected with the ground introduce another correction factor for salt air condition
through green covered copper wire. However in some and wind effect. With this correction, if E0 reaches a
places somebody has snapped these wires to obtain the value that is enough for visual corona inception, this
copper. corona could be seen along all three phases of the 33kV
line. Sometimes during a particular period, flash could
The fault current for 33kV overhead line has been occur between conductors, but more often, not between
determined considering two cases. lines between conductor and pole top. So it clearly
indicates poles are playing major role in this problem.

Analysis of corona discharge and earth fault 0n 33kV overhead line 1

3.4 Some past records * The distance between reinforcement and electrode
was kept as constant.
The following past records were taken from LECO
* Concrete covering thickness (x mm) above
Moratuwa area office for the line under consideration.
reinforcement was varied.
They give information on the date, type of damage and
pole number.
Secondary voltage (kV) = (100/220)*32 = 14.55kV
¾ 07:07:2000 – Insulator damage due to flashover on From the following table we can say when if the
the pole 73/1/1 concrete covering thickness is high flashover voltage
also high.
¾ 10:08:2001- Low voltage neutral wire snapped on
the pole ALM81 In 33kV line voltage is constant when concrete
covering reduces it will easily lead to flashover
¾ 23:08:2001- Insulator damage due to flashover on
the pole ALM87 Thickness Voltmeter Secondary
¾ 20:07:2003- Low voltage neutral wire snapped on ( X mm ) Reading (V) Voltage (kV)
the pole 73/1/1
0 32 14.55
3.5 The factors affecting the corona formation 5 38 17.27
1. Atmospheric condition – corona depends on it 10 40 18.18
since it occurs mainly in July and August
months. 4. EARTH FAULT
2. Physical condition of the conductor – not a
major factor. Corona is seen to occur only on The earth fault situation can be analyzed in two
middle conductor at the poles. If it is a major different manners. These are by considering
factor visual corona should occur on other • Earth fault without earthing the cross arm by
conductors also. copper wire.
• Earth fault with earthing the cross arm by
3. Spacing between conductor – It is sufficient
copper wire.
(1100mm)
4. Line voltage – it influences because observation 4.1 Earth fault without earthing the cross arm.
was made on 33kVline.
To identify the behavior of the earth fault without
3.6 Experiments carried out earthing the cross arm the following test was carried out
in the field (figure 4.1).
Objective: To show influence of concrete covering on
flashover. Pole Top
Water
resistance
Conductor

V meter

To variac
220V/100kV
Transformer
Transformer
400V/11kV
P.T

Wood stand
Fig. 4.1 Application of test voltage to pole
Fig 3.6 Equipment set up in the high voltage
laboratory to observe the influence of In this experiment, high voltage is applied to the pole
concrete covering on flashover. top.

Analysis of corona discharge and earth fault 0n 33kV overhead line 2

The voltage is gradually increased in six steps using an 1. Special insulator (height is high) can be used which
autotransformer. The voltage distribution along the pole can increase clearance between conductor and pole
was measured with the corresponding current. top.

4.11 Observations 2. Clean the insulators frequently.

* Voltage distribution is almost a constant along the 3. Keep better insulation covering between conductor
pole 10m from the ground. and pole top.

* Almost all the applied voltage appears on the top 4. Replace the poles.
region of the pole.
In consumers point of view, we observed that electrical
* Concrete layer of the pole top burn out at the 840V. appliances like refrigerators, electric kettles, electric
fans and trip switches have got damaged. This occurs
Using above voltage distribution one can find out the during the time of flashover. If we avoid the flashover
pole impedance behavior at different voltages and this we can automatically correct it.
experiment reveals a equivalent model for a ideal
reinforced pole.
6. ACKNOWLEDGEMENT
Actually this situation can be nearly modeled as a single
line to ground fault with an impedance (pole This project has been completed with the help of many
impedance) between the line and the ground. The persons. We regret not being able to thank all the
relevant fault current is given by; persons who helped us to complete the project

If = 3Ef At the top of the list is Mr. Frank Perera Operational

{Z1+Z2+Z0 +3 (Zp+Ze)} Engineer of the LECO without whose contributions; we
would never have been able to get this kind of
Z1: positive sequence impedance. opportunity.
Z2: negative sequence impedance.
Z0: zero sequence impedance. Among many individuals we like to pay our heartiest
Zp: pole impedance. gratitude to Professor Rohan Lucas, Head of the
Ze: impedance of the earth. department.

But the rather severe fault current occurs having with We want to say our special thanks to
the earth wire. Mr.H.N.Gunasekara, System Engineer of the LECO
who spent lot of his valuable time with us and directed
4.2 Earth fault with earthing the cross arm. us in the correct direction.

This situation can be modeled as single line to ground It is not good to leave out Moratuwa area Engineers and
fault. If we do not neglect the earth resistance, the fault Electrical superintend of the LECO who helped us in
current is given by, various ways.
If = 3Ef
In this occasion we cannot forget Mr. Kosala
(Z1+Z2+Z0 +3Ze) Gunawardana, Angulana factory manager and Mr.
Rukshaika Pathberiya Electrical engineer of the Lanka
In this case Pole impedance lies parallel with the earth Transformers Ltd who helped us to obtain a
wire and as it is very high compare with the resistance transformer.
of the copper wire it can be neglected.
Finally we thank all the other persons those helped us to
complete this project.
5. CONCLUSION
The major factor for the flashover is not sufficient
thickness of the concrete covering. Experiment also 7. REFERENCE:
clearly shows it. Following suggestion can be made to “Principles of Power Systems”[V.K Mehta & Rohit
avoid it. Mehta], Revised 3rd Edition (2002)

Analysis of corona discharge and earth fault 0n 33kV overhead line 3