A Novel Analysis of Voltage Distribution in Zinc Oxide Arrester using ...

POSTER PAPER
International Journal of Recent Trends in Engineering, Vol. 1, No. 4, May 2009
A Novel Analysis of Voltage Distribution
in Zinc Oxide Arrester using
Finite Element method
R.Karthik
Lecturer-EEE Dept, National Engineering College, Kovilpatti, Tamilnadu,India
E-mail: hvekarthik@gmail.com
Abstract
Necessity to preserve the continuity of service and electric
supply demands special attention towards protection of
transmission lines and power apparatus from over voltages.
Lightning arresters are devices used at sub-stations and at
line terminations to discharge these over voltages, more
particularly lightning surges and switching surges. The
arresters installed today are almost all metal- oxide (MO)
arresters without gaps. The distinctive feature of MO resistors
is its extremely non-linear voltage characteristics, rendering
unnecessary the disconnection of the resistors from the line
through serial spark–gaps earlier used with Sic resistors.
“Ref. [1]”The voltage distribution in a zinc oxide arrester
under normal operating conditions has been observed to be
non-uniform. The disc at the top are subject to higher voltage
and hence thermal stresses leading to faster thermal ageing of
these highly stressed disc’s, therefore efforts are made to make
the voltage distribution as uniform as possible.
Lightning arresters for 110 kV, 220 kV, 380 kV systems were
considered on application of maximum continuous operating
voltage. For 220 kV, 380 kV system arresters, observation of
voltage distribution showed that voltage level to be high at the
top discs of the arrester, which will lead to damage of
arresters. For stress to be relieved and voltage to be distributed
uniformly through out the discs of arrester ‘Grade Rings’ are
employed .These grade rings control the voltage distribution
from top to bottom of the arrester and uniform voltage
distribution is achieved. In this paper an analysis of voltage
distribution in lightning Arrester is done with the help of state
of art of technology ANSYS7.1 which analysis through
analytical method with Finite element as basics
Index terms- Arrester,Finite Element method,Grade rings.
I.INTRODUCTION
High voltage power systems in general,
experience over voltages that originate from the
system instability/fault conditions, switching
operations and lightning surges. The duration of the
over voltages vary from micro seconds to seconds,
depending upon the type of surges. Similarly the
magnitude of the over-voltages vary from 1.15 to 4
times the normal operating system voltage. Under
these over voltage conditions, the insulation of the
power system/equipments could undergo stresses that
lead to catastrophic failure. Hence, it is essential that
the power system equipments are protected from these
over voltages at the time of occurrence .The protection
of power system equipments from the over- voltages is
possible by the use of protective devices that posses
variable impedance as a function of voltage
© 2009 ACADEMY PUBLISHER
1
magnitude. The over- voltage protective device is
usually connected in parallel to the
equipment/system to be protected.
Under normal operating voltage, the
impedance of the over- voltage protective device is
high and, hence allows the power system
/equipments to perform the respective function.
Whenever the over-voltage appears in the system,
the impedance of the protective device reduces
drastically to such an extent that the equipments
will not experience the over-voltages due to
potential drop. As soon as the over-voltage
disappears the protective equipment regains the
impedance and allows the power system
equipments to perform the respective functions.
II.SURGE DIVERTERS
The following are the basic requirements of a
surge diverter: “Ref. [5]”
• It should not pass any current at normal or
abnormal (normally 5% more than the
normal voltage) power frequency voltage.
• It should break down as quickly as
possible after the abnormal high frequency
voltage arrives.
Fig 1- Arrangement of Lightning Arrester with Grade Rings
• It should not only protect the equipment
for which it is used but should discharge
the surge current without damaging itself.
• It should interrupt the power frequency
follow current after the surge is discharged
to ground.

POSTER PAPER
International Journal of Recent Trends in Engineering, Vol. 1, No. 4, May 2009
A.BASIC FEATURES OF ZINC OXIDE
ARRESTERS
The basic constituent of ZnO gapless arresters is
ZnO poly crystalline element. The elements are
housed in a hollow porcelain insulator and
hermetically sealed. The number and size of ZnO
elements vary depending upon the system voltage and
energy class requirements.
“Ref. [2]”The resistivity of ZnO grain is 1 to 10 ohm
cm, whereas the grain boundary resistivity is 10 10
ohm cm.All the voltage applied to the element is
concentrated across the high resistivity inter granular
layer. The resistance of the inter granular layer
decreases suddenly at a certain threshold voltage as the
applied voltage rises and so acts as a voltage
dependent switching device.
B.PROTECTIVE LEVELS
Lighting impulse protective level of an
arrester is the maximum residual voltage at the
nominal lightning discharge current.
“Ref. [3]”Switching impulse protective level is
the maximum residual voltage at the specified
switching impulse current.
Steep current impulse protective level is the
maximum residual voltage corresponding to a current
impulse possessing a virtual front time of one micro
second with a peak value equal to the nominal
discharge current.
C.APPLICATION OF GRADE RINGS:
Grade rings are added to modify the voltage
distribution across arresters at operating voltage to
approach the ideal condition where voltage would be
linearly distributed between arrester valve elements.
“Ref. [1]” An arrester energized at operating
voltage can be represented conceptually as a vertical
column of a dielectric material interspersed with
metallic masses. when the through (from end-to-end)
capacitance is not overwhelmingly larger than the
stray capacitance to ground of the metallic masses ,the
voltage distribution along the vertical length of the
column will be determined by the interaction of the
through capacitance and the capacitance of the
metallic masses. conceptually, the metallic masses will
be capacitively coupled (through stray capacitance)to
ground .Thus any metallic mass will tend to be at a
lower voltage than would have been the case if
there were there were no stray capacitance. The
effect is to increase the voltage gradient at the line
end to reduce it at the ground end. In severe cases,
(tall arresters with low through capacitance) the
arrester elements near the line end of the arresters
can be energized at such high voltage that the
elements risk deterioration by thermal instability.
The solution is to modify the voltage
distribution by external means so that the risk of
2
© 2009 ACADEMY PUBLISHER
thermal instability of these elements is reduced to
an acceptable level .Adding stray capacitance to
the line potential can tend to nullify the effect of
stray capacitance to ground, therefore tend to
redistribute the voltage to approach the ideal
uniform distribution. The grade ring configuration
is empirically determined by selecting various
grading ring configurations and observing the
voltage across strategically located arrester
elements.
III.ANALYSIS OF LIGHTNING ARRESTER
WITH AND WITH OUT GRADE RINGS
“Ref. [4]”Analysis of Voltage Distribution in Zinc
Oxide Arrester is done with the help of state of art
technology, (i.e.) ANSYS 7.1 software package,
which analyses through analytical method, with
Finite Element Method as basics.
A.SOLIDLY EARTHED NEUTRAL 220 Kv
SYSTEM.
1. With Out Grade Ring
Voltage in kV
160
140
120
100
ring
2. With Grade Ring
Voltage in kV
80
60
40
20
Voltage Distribution in 220kV System
(without Grade Ring)
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
Disc Number from Top of Arrester
Voltage Distribution in 220kV System
(with Grade Ring)
160
140
120
100
80
60
40
20
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
Disc Number from Top of Arrester
Fig 3- 220 kV with Grade ring
Fig 2- 220 kV
with out Grade

POSTER PAPER
B.SOLIDLY EARTHED NEUTRAL 380 Kv
SYSTEM
1. With Out Grade Ring
300
250
200
150
100
50
0
Voltag e in kV
Fig 4- 380 kV with out Grade ring
2. With Grade Ring
© 2009 ACADEMY PUBLISHER
International Journal of Recent Trends in Engineering, Vol. 1, No. 4, May 2009
Voltage Distribution in 380kV System
(without Grade Ring)
0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63
Disc Number from Top of Arrester
Fig 5- 380 kV with Grade ring
3
From the above analysis it is observed that the
disc at the top of the arrester are subject to
higher voltage and hence thermal stresses leading
to faster thermal ageing of these highly stressed
discs, therefore after implementing
grade rings the voltage stresses at the top of the
arrester disc are relieved and uniform voltage
distribution is achieved.
IV.CONCLUSION
The Finite Element analysis of the voltage
distribution in ZnO arrester using ANSYS-7.1
Software Package reveals the following results,
1. The voltage distribution is dependent on the
geometry of the Arrester.
2. Grade rings control the voltage distribution from
top to bottom of the arrester and uniform voltage
distribution is achieved and establishes that grade
rings are required for system voltages greater than
110 kV.
Therefore by using the proper design of Lighting
Arrester, using grade rings voltage distribution can
be obtained in a uniform manner. Hence, the
power lines and sub-stations can be operated and
protected against over voltages such that the
numbers of failures are as few as possible
REFERENCE
[1]. Voltage distribution in ZnO Arresters- IEEE Proceedings
Generation, Transmission and Distribution, Vol149, July2002
[2]. Transient current distribution in ZnO arrester blocks.
Cardiff University, Wales, UK
[3]. Methods for analyzing the performance of gapless metal
oxide surge arresters, Ontario Hydro Research Div.Canada.
[4].The Response of Metal Oxide Surge Arresters to Steep
Fronted Current Impulses, IEEE Transactions on Power
Delivery, Vol.PWRD-1, NO-1, Jan-1986
[5].High performance triggered lightning current arresters for
international power supply systems
Wetter, M.; McCurdy, P.;
Telecommunications Energy Conference, 2004. INTELEC
2004. 26th Annual International
19-23 Sept. 2004 Page(s):676 – 679
[6]. Dielectric Stimulated Arcs in Lightning-Arrestor
Connectors
Brainard, J.; Andrews, L.;
Components, Hybrids, and Manufacturing Technology, IEEE
Transactions on
Volume 2, Issue 3, Sep 1979 Page(s):309 – 316