1. Abstract—Distribution substations feed power to the actual
consumers through distributors and service lines. The main
equipments are generators and transformers. To protected these
equipments and for stability purpose, over-voltages and over currents
protection are important to consider. Lightning is one of the most
serious causes of over-voltage. If the power equipments especially at
outdoor substation are not protected, the over-voltage will cause
burning of insulation. Lightning arrester can protect the damages of
equipments. This paper describes the arrester type, lightning terminal
and earthing plan of Dagon East substation in Myanmar. DynaVar
station class and intermediated arrester (Vrated = 72kV and I charge
(max) = 10kA) are used in this substation. Most of substation
equipments are designed to match with the insulation coordination. If
the insulation equipments are higher, the cost is also high. So, to
relax this, the lightning arrester must be put in front of the protected
equipments and protected zone. For this purposes, this paper
specially indicates the safety and saving cost of equipments for over-
voltage protection in distribution substation.
Keywords—Lightning arrester, Earthing plan, DynaVar station,
Intermediated arrester.
I. INTRODUCTION
UBSTATION design involves more than installing
apparatus, protective devices and equipment. The
significant momentary investment and required reliable
continuous operation of the facility requires detailed attention
to preventing surges from entering the substation facility. The
effects of disturbances with limiting in a power system, which
if allow to persist, may damage plant and interrupt the supply
of electrical energy. Lightning is one of the most serious
causes of over voltage. If the power equipment especially at
outdoor substation is not protected the over-voltage will cause
burning of insulation. Thus it results into complete shutdown
of the power and the loss may run into cores of kyat. Electrical
equipment can be damage due to over-voltage such as
switching surge over-voltage, Lightning surge over-voltage,
transient recovery voltage and power frequency temporary
over-voltage in transmission line and receiving end of
substation. It is important to protect power equipment against
them wherever possible, consistent with sound economic.
Lightning Arrester can protect the damages of electrical
equipments. So, Lightning Arrester needed to install in the
terminal end of the transmission line, substation, high voltage
Miss NayKyiHtwe is a student of Mandalay Technological University.
(e-mail :naykyihtwe08@ gmail.com).
transformers and low voltage transformer. The analysis of
electromagnetic transient is depended on operating voltage,
lengths of the lines and contactor configuration. So, it can be
chosen correctly the technical specifications of the apparatus
of Lightning Arrester base on the amounts of receiving over-
voltage.
II. PHYSICAL PHENOMENON OF LIGHTNING
Lightning is a huge spark caused by the electrical discharge
taking place between the clouds, within the same cloud and
between the clouds and the earth. The turmoil that is apparent
inside a thundercloud is most impressive to the viewer. It
shape changes continually, and one notes especially the
development of the towering ‘thunderhead’. It is very easy to
imagine the fierce updrafts with the cloud and the downdrafts
near it surface which are matters of practical experience for
aviators. It is generally accepted that the updraft is responsible
for charge separation within the cloud, like some gigantic
electrostatic generator, which leads to the creation of electric
fields within and around the cloud and ultimately to the
electric breakdown that is called lightning.
III. WAVE SHAPE OF LIGHTNING STROKE
The lightning stroke current rises to crest value very quickly
and then starts decaying at a low rate as illustrated in Figure.1.
The generalized wave shape can be characterized as:
1. Crest or peak value and it have been observed that
the maximum value of this current is 400 kA.
2. The wave front line varies from 1 t 10 sec.
3. The time at which the stroke current reduces to 50
percent value of that crest value and it has been
estimated that the time varies from 10 to 100 sec.
Figure.1 Generalized wave shape of lightning stroke
Analysis and Design Selection of Lightning
Arrester for Distribution Substation
Nay Kyi Htwe
S
World Academy of Science, Engineering and Technology 48 2008
174

2. IV. STATIC CHARGING OF THE CONDUCTOR DUE TO A
CHARG CLOUD
Suppose that the cloud is positively charged, then the line
will be charged to a negative potential by the electrostatic
induction. This negative charge will be present right under the
cloud and the portions of the line away from this point will be
charge positively as illustrated in Figure.2.
Figure.2 Static charging of the line due to a cloud
The charge on the line will not flow since it is a bound
charge. The positive charge on the far ends of the line will
however leak to the earth slowly through insulators metallic
parts etc, thus leaving only the negative charge on the line.
Due to a direct discharge occurring between this cloud and
another passing by cloud the charge on the cloud is neutralized
then the charge on the line is no more a bound charge and is
free to travel in both directions in the form of traveling waves.
V. OVERVOLTAGE DUE TO LIGHTNING STROKE
In case of direct strokes, a line having a surge impedance of
Zs and the discharge current be Id, then the over-voltage due to
a direct stroke is
sdd ZIV ×= (1)
When the traveling waves flow in one direction, the
equation is true. However, when they travel in both directions,
the current is halved and the over-voltage is
2
ZI
V sd
d
×
= (2)
When the lightning stroke is on the earth wire or top of a
tower, the over-voltage is
t
i
ccdd
d
d
lZIV +×= (3)
Where Z c is the impedance of the earth conductor and l c is
the inductance of the line conductor.
VI. INTERACTION BETWEEN LIGHTNING AND THE POWER
SYSTEM
When lightning strikes a power line, a current is injected
into the power system. This is very useful concept. This
current will give rise to depend upon its wave shape and the
impedances through which it flows. If a tower is struck, the
impedance of the tower will be of concern. The voltage drop
down the tower will appear across the line insulation. If this is
excessive, flashover of the insulation will occur and a fault will
be placed on the system.
The current comings into a tower have been postulated by
lightning stroke from a cloud, and then disappear into the
ground. A useful concept is to think of the cloud and earth as
forming a vast capacitor which is being discharge by the
stroke. The return circuit would be completed by displacement
current in the electric field. This is suggested by Figure.3.
Stroke
Earth
cloud
Figure.3 Lightning stroke from cloud to earth discharges a
vast Capacitor
VII. LIGHTNING ARRESTER
Lightning arresters are the most effective means of
protecting an electrical apparatus against traveling voltage
waves caused by lightning and switching. Lightning arresters
are connected across and apparatus to provide a Low-
resistance path to ground, thus limiting the transient voltages
below the Basic Impulse Level of the apparatus. There are four
different classes of arrester.
1. Station
2. Intermediate
3. Distribution, and
4. Secondary
The functions of a lightning arrester are
1. To act like an open circuit during the normal
operation of the system i.e., to hold off the system
voltage,
2. To limit the transient voltage to a safe level with
the minimum delay and fitter, and
3. To bring the system back to its normal operation
mode as soon as the transient voltage is
suppressed, i.e., to interrupt the power-follow
current and to reseal itself.
The normal operation or operational mode includes the
system under faulted condition. Under several types of system
faults, such as the single line-to-ground faults, the voltage to
ground across the unfaulted phases will rise above the normal
voltage level. The arrester must not go into conduction during
this fault condition. It should also be able to interrupt the
power-follow current and reseal itself under system fault
conditions when the power-frequency voltage across it rises.
VIII. SELECTION OF LIGHTNING ARRESTER
The lightning arresters are designated by the crest
magnitude of the discharge current having 10×20 second
wave shape which the arrester can safely pass without damage.
The lightning arresters are designated as 8, 10, 20 KA. They
World Academy of Science, Engineering and Technology 48 2008
175

3. can safely discharge these current crests. As the arrester is a
protective device, if is a general impression that if should be
rated for most severe conditions of discharge currents says 20
A. The discharge current from the arrester varies from a few
hundred amperes to kilo-amperes and sometimes if is even 20
KA. Maximum discharge voltage and discharge factor for the
arrester is defined the maximum value of voltage which
appears across the arrester terminals at the time of discharging
if rated current determines its impulse level of protection. The
discharge factor if is,
DF=
arrestertheofvalue)(RMSvoltageRated
arrestertheofvalue)(crestvoltagedischarge
(5)
In the early designs of arresters, this discharge factor was
quite high (about 5.6) but due to the advent of better material
and Modern research it has been reduced varying to 2.4 to 3.0.
The above ratio for arresters manufactured by different firms
varies from 1.5 to 1.7, so, the average value may be taken as
1.6 E is the rated arrester voltage KV (R.M.S) and LS is the
minimum impulse insulation level in kV (crest valve) its value
after allowing 10% as tolerance factor and 25% as margin
factor can be obtained.
LS= E
0.8
21.61.11.25 ×××
=3.88E (6)
In case Extra High Voltage system LP in kV,
LP = 2.3 × power frequency withstand voltage in kV (RMS)
= 2.3 × EL (7)
For 75% arrester,
LP = 2.37 EL (8)
For 80% arrester,
LP=2.53EL (9)
IX. EARTHING SYSTEM
The frame of every generator, stationary motor, and so far
as particable, portable motor, and metallic parts of all
transformer and regulating and controlling apparatus
connected with supply shall be earthed by the owner by
separate and distinct connection with earth.
Every conductor used on earthing shall be of stranded as
solid copper or suitable copper alloy, and shall be protected
wherever liable to mechanical damage and also, where
necessary, against corrosion, particular attention being given in
these respects to the earthing leads at its point of connection
with the earth electrode. The coefficient of earthing is below
80 percent. On four wire distribution systems, with solidly
ground transformer neutral at every voltage level, coefficient
of earthing is generally less than 80%. On high voltage
transmission systems the coefficient of earthing does not
exceed 75%.
In resistance of Common Types of Earth-electrode, there are
three types.
1. Plates
2. Pipes and rods
3. Strip or conductor electrodes
X. INTERMEDIATE CLASS DYNAVAR ARRESTER
Application is based upon the maximum continuous
operation voltage, line to neutral, at the arrester location. For
grounded neutral systems, this is computed as maximum
system voltage divided by √3. For historical comparison, the
maximum continuous operating voltage is 81% of the
conventional 71% arrester installed on an affectively grounded
neutral system.
Figure.4 Section view of typical unit
A. Lightning Arrester Design
At Dagon East substation, DynaVar station class and
intermediate surge arresters are used. The system voltage is 66
kV and maximum continuous operation voltage is 48 kV rms.
The duty cycle rating is 60 kV rms and maximum discharge
current is 10 kA.
1. Maximum 0.5µs discharge voltage = 163.5 kV
2. Maximum switching surge protective level=116.4 kV
3. Maximum discharge voltage using an 8/20=148.8 kV
Current wave-kV
The maximum discharge voltage for a10kA impulse current
wave produces a voltage wave cresting in 0.5µs.
B. Structure of Lightning Arrester
In Figure.5, PVN 48.0kV maximum continuous operation
voltage arrester is shown. It has 0.56 diameter holes, clamp
type terminal, arrester name plate. Its internal diameter is 9.3
diameters.
Figure.5 PVN 48.0 kV maximum continuous operation
voltage Arrester
9.3
1.7
Clamp Type Terminals Suitable
for Use with CU DR AL
Conductor 0.25 to 0.81 Dia
0.56 Dia 3 Holes at
120˚ On 10.0 Dia BC
1.750.56
Diameter 4
Holes
Arrester Nameplate
32.
3
World Academy of Science, Engineering and Technology 48 2008
176

4. C. Specification of Lightning Arrester for Incoming Side
Incoming side of Dagon East substation, the
specifications of lightning arrester are as follow.
Figure.6 One Line Diagram of East Dagon 30MVA
Substation
System nominal voltage = 66kV
Rated normal Voltage = 66 × 1.1 = 72.6kV
Continuous Operating Voltage (kV) rms = 48.0kV
Normal Discharge Current (8 /20µs) kA = 10kA
1/50 Impulse Spark over Voltage = 163.5kV
Frequency (Hz) = 50Hz
Type = outdoor
D. Specifications of Lightning Arrester for Outgoing Side
The followings are the specifications of lightning arrester
for outgoing side of Dagon East substation.
System nominal voltage = 33kV
Rated normal Voltage = 33 × 1.1 = 36kV
Continuous Operating Voltage (kV) rms = 24kV
Normal Discharge Current (8 /20µs) (kA) = 10kA
Frequency (Hz) = 50Hz
Type = outdoor
XI. LIGHTNING EARTHING
In earthing system, lightning arresters with PVC coated
wire and cable lug are used.
1. 66kV Lighting Arrester(70mm2
PVC Coated Wire)
= 50'
2. 33kV Lighting Arrester(70mm2
PVC Coated Wire)
= 100'
3. 70mm2
Cable Lug = 2 No
Figure.7 shows the earthing plan of Dagon East substation.
XII. DETAILED DESIGN DATA
For incoming side and outgoing side, the discharge
voltage, insulation level, minimum impulse insulation level
and power frequency withstand voltage base on 100% arrester
are as shown in Table.1. Ground voltage peak value and
switching surge withstand voltage are also shown in Table.1.
Table.1. Detailed Design Data Sheet
System
Voltage
66kV for
Incoming Side
33kV for
Outgoing Side
Rated Voltage (kV) 72.6 36
Discharge Voltage
on 100% Arrester (kV)
188.76 93.6
Insulation Level for
100% Arrester (kV)
284.78 164.4
Minimum Impulse
Insulation Level on
100%(kV)
732.389 363.168
Power Frequency
withstand Voltage on
100% (kV)
151.8 75.9
Ground Voltage
Peak Value(kV)
59.28 29.63
Require Switching
Surge Withstand
Voltage (kV)
215.6 107.8
XIII.CONCLUSION
In this paper, the basis theory of lightning, lightning
shielding and design of lightning arrester are presented.
DynaVar station class and intermediate arresters are used. The
type of arrester is outdoor type. The rated voltages of arresters
are 72kV and 36kV, the maximum discharge current is 10kA
and MCOV are 48kVrms and 24kVrms. The lightning arrester
in this paper is provided for overvoltage protection in
distribution substation. This paper will help and give the
electrical knowledge of the protection system in distribution
substation which coach to the technicians, the professional
engineers, the students who facing the overvoltage condition
and protection coordination of distribution substation.
ACKNOWLEDGMENT
Firstly, the author would like to express her indebtedness
and gratitude to her beloved parents, for their kindness,
support, understanding during the whole course of this work
and encouragement to attain ambition without any trouble. The
author is indebted to all her teachers who give her knowledge
from M.T.U and Y.T.U in Myanmar.
REFERENCES
[1] Allan Greenwood 1923 “Electrical Transients in Power System”
Second Edition, John Willey & Sons, Inc
[2] Anderson 1987 “Transmission Line Reference Book”, Second
Edition Substations Committee
[3] ANSI/ IEEE 1989 ” IEEE Standard for Gapped Silicon-Carbide
Surge Arresters for AC Power Circuit”
[4] Franked- Graham 1970 “New Electric Library” Vol-2
[5] IEEE Std 998- 1996 “Guide for Direct Lightning Stroke Shielding of
Substation”, IEEE Working Group D5,
[6] U Tin Swe “Power System Analysis Part 3”
World Academy of Science, Engineering and Technology 48 2008
177

5. Figure.7 Dagon East Sub-station Earthing Plan
66kV TR:
30 MVA
R = 2
66kV LA
66kV Switch
Yard
33kV LA
33kV TR:
10 MVA
100kVA
Transformer
Switch Gear Panel Earth Ring System
10MVA×R
Neutral Earth
100VA
Neutral Earth
Lightning
Arrester
Lightning
Arrester
Body Earth
LA Earth
LA Earth
DS Earth
R = 1.8
2 Pole DS LA PT CT CB CB CT PT DS
CB CT PT DS
10´ 10´ 10´ 10´ 10´ 10´ 10´ 10´ 10´ 10´ 10´10´ 10´10´ 10´
R = 2
Lightning Earth
R = 2 R = 2
Lightning Earth
30MVA
Body
Earth
30MVA×
R
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