Univ Of Zagreb Uglesic Prevention Of Flashover

1

Prevention of Flashover on a Double-circuit 220 kV

Line

Ivo Uglešić*, A. Xemard**, V. Milardić*, B. Filipović-Grčić*

*Faculty of Electrical Engineering and Computing

University of Zagreb, Croatia

**EDF R&D, Paris, France

EMTP-RV USER GROUP MEETING, DUBROVNIK, 30 APRIL 2009

2

OUTLINE OF PRESENTATION

INTRODUCTION

SIMULATION OF LIGHTNING STROKES TO

TRANSMISSION LINE

MODELING PROCEDURE FOR TRANSIENT

SIMULATIONS

SIMULATION RESULTS

CONCLUSIONS

3

INTRODUCTION

- The case study is related to the improvement of the lightning

performance of a 220 kV double-circuit overhead line, which connects

the thermo power plant to the rest of the power system.

- Several double-circuit outages provoked by lightning caused the

interruption of power supply of the power plant.

- To avoid back-flashovers due to lightning strokes to tower or

overhead shielding wires, the tower footing resistance should be as

low as possible.

- Installation of unbalanced insulation on a double-circuit line is one of

possible solution for suppression of double-circuit simultaneous

faults.

- Installation of line surge arresters (LSA).

4

The goal of the simulation is to determine the distribution of lightning

current amplitudes which strike HV transmission line towers and shield

wires or the phase conductors directly.

The Monte Carlo method is used – reproducing numerically a

stochastic problem.

The log–normal distribution of lightning current amplitude:

SIMULATION OF LIGHTNING STROKES TO TRANSMISSION LINE

P - probability of occurrence of lightning current amplitude

higher than I.

5

The general expression for the striking distance:

SIMULATION OF LIGHTNING STROKES TO TRANSMISSION LINE

R - striking distance,

I - lightning current amplitude,

a - constant [3.3 – 10.6],

b - constant [0.5 – 0.85].

In calculations is used a=7.2, b=0.65.

6

SIMULATION OF LIGHTNING STROKES TO TRANSMISSION LINE

4,5 m

6,5 m

5,0 m

6,7

m6,0

m6

,0 m

A

B

C

30,00m

27,30m

24,60m

22,00m

19,50m

17,10m

4,5 m

6,5 m

5,0 m

6,7

m6

,0 m

6,0

m

3D model of the part of the studied transmission line

7

SIMULATION OF LIGHTNING STROKES TO TRANSMISSION LINE

-Total number of simulations 37932,

-1000 simulations finished with phase conductor strikes,

-25635 ground strikes,

-11297 with shielding wire and tower strikes,

-shielding failure occurs in 8.85% cases.

According to statistical calculation, the following characteristics of

the crest values of the current for lightning striking phase conductors

are calculated:

- average value: 15.40 kA,

- variance: 98.36 kA,

- standard deviation: 9.92 kA,

- maximal phase conductor strike current: 42.80 kA,

- critical current: 47.30 kA.

8

SIMULATION OF LIGHTNING STROKES TO TRANSMISSION LINE

Distribution of lightning currents striking phase conductors

0

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,09

0,1

1-3 3-5 5-7 7-9 9-11 11-13 13-15 15-17 17-19 19-21 21-23 23-25 25-27 27-29 29-31 31-33 33-35 35-37 37-39 39-41 >41

Classes (kA)

Pro

bab

ilit

y

Distribution of lightning currents striking phase conductors

9

SIMULATION OF LIGHTNING STROKES TO TRANSMISSION LINE

Distribution of lightning currents striking top of towers or shielding wire

Distribution of lightning currents striking top of towers or shielding wire

0

0,05

0,1

0,15

0,2

0,25

1-1010-20

20-3030-40

40-5050-60

60-7070-80

80-90

90-100

100-110

110-120

120-130

130-140

140-150

150-160

160-170

170-180

180-190

Classes (kA)

Pro

bab

ilit

y

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MODELING PROCEDURE FOR TRANSIENT SIMULATIONS

The lightning stroke hitting a tower or a phase conductor can be

replaced by a surge current generator and a resistor (Norton

generator).

The CIGRE Lightning Current Waveform model can approximate well

the concave form of the lightning current front.

The transmission line, conductors and earth wire is represented by

several multi-phase untransposed distributed parameter line spans at

both sides of the point of the lightning stroke impact.

Tower surge impedances are calculated using equation:

11

MODELING PROCEDURE FOR TRANSIENT SIMULATIONS

Phase voltages at the instant at which a lightning stroke impacts the

line must be included.

Insulators are represented using the area criterion that involves

determining the instant of breakdown using the formula:

DEdUU

t

T

k

0

0

U(τ) is the voltage applied at time t,

U0 is a minimum voltage to be exceeded before any breakdown

process can start or continue,

k and U0 and DE are constants corresponding to an air gap

configuration and overvoltage polarity,

T0 is the time from which U(τ) > U0.

12

MODELING PROCEDURE FOR TRANSIENT SIMULATIONS

Tower footing resistances are modelled taking into account ionization:

R0 - footing resistance at low current and low frequency, i.e. 50 Hz,

I - stroke current through the resistance,

Ig - limiting current to initiate sufficient soil ionization.

ρ - soil resistivity m ;

E0 - soil ionization gradient, recommended value: 400 kV/m .

13

MODELING PROCEDURE FOR TRANSIENT SIMULATIONS

EMTP-RV Model of footing resistance ionization

14

MODELING PROCEDURE FOR TRANSIENT SIMULATIONS

The model of gapless type LSA includes non-linear and dynamic

behaviour of the arrester.

0

100

200

300

400

500

600

700

800

0,1 1 10 100

Current (kA)

Vo

ltag

e (

kV

)

U-I characteristic of surge arrester for the 220 kV line (Ur=210 kV)

15

MODELING PROCEDURE FOR TRANSIENT SIMULATIONS

Model of 220 kV double-circuit line

8 9

Z=

13

9.3

Ω

Z=

13

9.3

Ω

Z=

13

5.2

Ω

6 7

Z=

13

9.3

Ω

5

Z=

13

9.3

Ω

4

Z=

13

9.3

Ω

3

Z=

13

5.2

Ω

2

Z=

13

9.3

Ω

1

Z=

13

9.3

Ω

upper

middle

lower

Tra

ns

mis

ion

lin

e

27

6 m

Tra

ns

mis

ion

lin

e

34

3 m

Tra

ns

mis

ion

lin

e

10

km

Tra

ns

mis

ion

lin

e

28

9 m

Tra

ns

mis

ion

lin

e

40

2 m

Tra

ns

mis

ion

lin

e

29

5 m

Tra

ns

mis

ion

lin

e

21

3 m

Tra

ns

mis

ion

lin

e

45

9 m

Tra

ns

mis

ion

lin

e

35

0 m

Shield wire

1000 Ω

31 kA, 72 kA,

96 kA, 138 kA

Stroke to tower

Tra

ns

mis

ion

lin

e

10

km

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SIMULATION RESULTS

When a lightning strikes the top of a 220 kV tower the occurrence of

the back-flashover depends on many parameters:

- peak current magnitude and maximal steepness,

- tower footing resistance,

- flashover voltage of insulation clearances,

- magnitude and phase angle of the voltage,

- atmospheric condition (rain, snow, pressure, temperature, humidity)

etc.

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SIMULATION RESULTS

Back-flashovers in relation to the lightning current magnitude and the

footing resistance of a tower

18

SIMULATION RESULTS

Back-flashovers in relation to the lightning current magnitude and the

footing resistance of a tower (LSA in the upper phase)

19

SIMULATION RESULTS

Back-flashovers in relation to the lightning current magnitude and the

footing resistance of a tower (LSA in the middle phase)

20

SIMULATION RESULTS

Back-flashovers in relation to the lightning current magnitude and the

footing resistance of a tower (LSA in the lower phase)

21

SIMULATION RESULTS

Back-flashovers in relation to the lightning current magnitude and the

footing resistance of a tower (LSAs in middle and lower phases)

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SIMULATION RESULTS

Back-flashover rate of one circuit of the 220 kV line: not protected by

LSAs, protected by LSA in middle phase (B), LSA in lower phase (C)

and LSAs in lower and middle phases (B and C)

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Tower resistance (Ohms)

Nu

mb

er

of

Bac

k-f

las

ho

ve

r R

ate

s (

an

nu

al, p

er

10

0k

m o

f li

ne

)

NO LSA

LSA in B

LSA in C

LSAs in B and C

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SIMULATION RESULTS

Total flashover rate (back and shield failure) of one circuit of the 220 kV

line: not protected by LSAs, protected by LSA in middle phase (B), LSA

in lower phase (C) and LSAs in lower and middle phases (B and C)

0

1

2

3

4

5

6

7

8

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Tower resistance (Ohms)

Nu

mb

er

of

Fla

sh

ov

er

Ra

tes

(an

nu

al,

per

10

0km

of

lin

e)

NO LSA

LSA in B

LSA in C

LSAs in B and C

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CONCLUSIONS

The following recommendations can be given for the purpose of

optimization of the number of LSAs:

-Improvement of footing resistances on towers if economically

justified.

-No LSA (tower footing resistance < 21 ).

-LSA in the lower phase (tower footing resistance > 21 and < 47 ).

-LSAs in the middle and lower phases (tower footing resistance > 47

< 150 ).

-Arresters installed in all 3 phases at selected towers with tower

footing resistance > 150 (the installation of three LSAs in one circuit

will only prevent back-flashover in that circuit on that tower and back-

flashovers could occur on neighboring towers).

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Prevention of Flashover on a Double-circuit 220 kV

Line

Ivo Uglešić*, A. Xemard**, V. Milardić*, B. Filipović-Grčić*

*Faculty of Electrical Engineering and Computing

University of Zagreb, Croatia

**EDF R&D, Paris, France

EMTP-RV USER GROUP MEETING, DUBROVNIK, 30 APRIL 2009