effect of line configuration changes on its insulation coordination[1]
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7/28/2019 Effect of Line Configuration Changes on Its Insulation Coordination[1]
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EFFECT OF LINE CONFIGURATION CHANGES ON ITS INSULATION COORDINATION
M. P. Arabani A. F. Fathi
Moshanir Co.
Tehran-IRAN
Niroo Research Institute*
Tehran-IRAN
*P.O.Box: 14155-1655 - Email: affathi@nri.ac.ir
Summary: One of the most important factors that
must be considered in designing a transmission line is
insulation coordination. There exist many factors that
affect insulation coordination and inconsideration of this
would cause serious failures.
There are many lines that have different configuration
through their route. As in these lines the parameters of
various sections are different, the surge impedance of
these sections would not be the same and so the
traveling wave through these lines would have some
reflections on those parts of different surgeimpedances. It would be clear that because of this
phenomenon, the over voltage profile along the line
would be a function of the parameters of various parts
of the line and if the required attention doesn’t be
considered, the failures of insulators along the line
would be the result. In this paper the effect of the above
mentioned phenomena in one of Iranian 400 KV,
transmission lines has been described and analyzed,
and some practical points of view for implementation
of current method of insulation coordination in the line
has been presented.
Keywords: tower configuration, Insulators, traveling
wave, Insulation coordination.
Introduction
In an interconnected network we have many
transmission lines, which are connected together, and
each two buses are connected via different lines with
different tower configurations. Every change of tower
configuration in a transmission line route will make the
electrical parameters vary. This change may include
bundling of conductors, number of circuits on a tower or the tower shape. All of the above will make the
electrical parameters, R, L and C and therefore the
surge impedance, ZC change. So we have a transmission
line with different characteristics on its route. Any
disturbance like lightning or switching surges that may
appear on each side of the line will move from side to
side by traveling waves. Because of different
characteristics of the line, these traveling waves may
show unexpectable features and therefore unpredictable
overvoltages may take place. For example we consider
a lossless transmission line with two cascading surge
impedances ZC1 and ZC2 connected to a source via aswitch as can be seen in figure 1.
Figure 1: A sample network with two different
characteristics.
In this case and by assuming that ZC1<ZC2 we can
determine the voltage profile along the line.If the voltage of the source at t=0 is assumed to be E,
then current I1 will flow at that instant when the switch
is closed, where:
1C
1Z
EI = (1)
By approaching the junction of two sections, the
traveling waves will divide in two parts. One will
reflect by opposite sign:
1CZ
'E'I = (2)
And another which refract into second line will be:
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1C
2Z
EI = (3)
The refracted voltage wave can be achieved by the
following equation:
E2 =E+E’ (4)And the current is derived by:
I2 =I1 +I’ (5)In this case we have:
2c1c
2c2
ZZ
EZ2E
+= (6)
And
2c1c
2c1c
ZZ
ZZE'E
+−−= (7)
And so in each instant we have the sum of these two
waves that will cause unexpected overvoltages along
the transmission line.
If we assume:
2c1c
2c1c
ZZ
ZZa
+−
= (8)
And
2c1c
1c
ZZ
Z2 b
+= (9)
Then the lattice diagram of the above-mentioned line
can be obtained in fig. 2 and the voltage at junction
point can be seen in fig.3.
Figure 2: Lattice diagram of the sample network
Figure 3: voltage at the junction point
A case study in Iran
Gazvin-Rasht 400 KV Transmission line, is a 163 km
line that has connected the Gillan power plant on one
side and Rajaie plant from the other side. Fig. 4 shows
this network configuration.
Figure 4: Network under consideration
This line at the beginning has a double circuit vertical
configuration of bundled curlew conductors, which the
characteristics of this section of the line have been
summarized in table 1.
Table-1:Tower configuration Phase-Phase
Distance (m)
Phase to Ground
Distance (m)
Mid Span
Clearance (m)
9.7 37 19
8 28 10
8 28 10
Based on the tower configuration and by neglecting thefrequency dependent parameter of the ground, and
considering single layer ground model with receptivity
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of 100 Ω.m , the electrical parameters of the line
would be as presented in table 2.
Table-2: Electrical parameters of the line
Mode R
( / )
Ωmile
ωωωωL
( / )
Ωmile
Cωωωω
(Mho/mile)
0
1
2
0.017
0.011
0.015
0.75
0.101
0.13
8.6× 10-6
2.98× 10-6
2.33× 10-5
This line has been divided to two single circuit lines
after 34 Km from the beginning of the line, where the
configuration of each part has been changed to flat
configuration with parameters presented in table 3 and
electrical characteristics as presented in table 4.
Table-3:second route configuration Phase-Phase
Distance (m) Phase-Ground
Distance (m)
Mid Span
(m)
14.9 28 10
14.9 28 10
29.8 28 10
Table-4: Second route electrical parameters
Mode R
( / )Ω mile
ωωωωL
( / )Ω mile
Cωωωω
(Mho/mile)
0
1
2
0.0189
0.028
0.027
0.934
0.385
0.295
6.2× 10-6
8.29× 10-6
1.02× 10-5
On its path, the line has been changed to double circuit
transmission line at 39 Km from the beginning of theline, where the parameters of the towers and electrical
characteristics of the line has been presented in table 5
and 6 respectively.
Table-5: Third route configuration
Phase-Phase
Distance (m)
Phase to Ground
Distance (m)
Mid Span
Clearance
(m)
9.7 37 19
8 28 10
8 28 10
Table-6: Third route electrical parameters
Mode R
( / )Ω mile
ω L
( / )Ω mile
ω Cmho
mile
0
1
2
0.0172
0.013
0.017
0.78
0.12
0.15
8.09× 10-6
2.6× 10-6
2.02× 10-5
Just after the first energization of the line, three
consecutive flashovers caused the protective devices of
the line operate and de-energize the line. There were
100 towers in its path and the flashovers took place in
towers 25 twice and tower 27 once.
Determination of insulation coordination
In order to simulate these interruptions, the EMTP
software is used and the above-mentioned network isconsidered completely by its model. By using the
statistical method, which is used by EMTP, the V50%
and standard deviation of the line are calculated in four
equal length sections.
The results of the calculation can be seen in table 7.
Table-7: EMTP output
Physical distance V50%
(Pu)
Standard
Deviation
Tower 1-Tower 25 2.2 63%
Tower 26-Tower 50 2.18 36%
Tower 51-Tower 75 2.18 33%
Tower 76-Tower 100 2.2 11%
As it can be seen the results show the problem lies
within the first and second section in which the
flashovers took place.
The insulators that were used at first on this line were
24(254*146-290)-120KN II-string insulators. Base on
this and using equations 10 and 11, applying the gapfactor k=1.35 [4], the stress, strength and Risk of
Failure functions for each section can be calculated.
Figures 5, 6, 7 and 8 show these functions for section1
to section 4 of the transmission line respectively.
6.0
%50 kd500V = (10)
∫ ∞
=0
111 dV).V(G)V(FROF (11)
Where:
F(V1): The probability of occurrence of overvoltage
(V1)
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And
G(V1): probability of breakdown in that voltage. [3]
Figure. 5: Outage probability of section 1
Figure 6: Outage probability of section 2
Figure 7: Outage probability of section 3
Figure 8: Outage probability of Section 4
As it can be seen in the first section we have a highRisk of Failure (ROF) and it decreases when we move
to sections 2 to 4.
Due to the insulator failures and successive interruption
of the line, the insulators of the line path changed to
21(254*170-290)-210KN, I-string insulators and after
this replacement no other failures took place.
It would be noticeable that although we had acceptable
over voltage in first section but because of the
spreading stress function, which means a high value for
standard deviation, we would have a high outage
probability and so these failures would take place.
Conclusion
Inadequate attention to insulation coordination factors
like over voltage and standard deviation, which is
calculated, based on the topology of the network and
parameters of the transmission line, will cause a
relatively large stress function that strength of the
insulators used in the line cannot handle and this would
cause dangerous flashovers in transmission lines.In this article the above-mentioned problem has been
shown in one of Iran's 400 kV transmission lines and
the insulation coordination parameters calculation has
been carried out.
References
[1]. “Electric Power Systems”, B. M. Weedy, B. J.
Cory, John Wiley.
[2]. “High Voltage Engineering”, W. S. Zaengl,
Pergamon Press.
[3]. IEC. 71, “Insulation Coordination”.
[4]. “Insulation coordination for power systems”,
A.R.Hileman, Marcel Dekker inc. 1999.
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