effect of line configuration changes on its insulation coordination[1]

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: [email protected]

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



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)



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.

effect of line configuration changes on its insulation coordination[1]

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