1. EMTP-RV USER GROUP MEETING, DUBROVNIK, 30 APRIL 2009
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 1

2. OUTLINE OF PRESENTATION
 INTRODUCTION
 SIMULATION OF LIGHTNING STROKES TO
TRANSMISSION LINE
 MODELING PROCEDURE FOR TRANSIENT
SIMULATIONS
 SIMULATION RESULTS
 CONCLUSIONS
2

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).
3

4. SIMULATION OF LIGHTNING STROKES TO TRANSMISSION LINE
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:
P - probability of occurrence of lightning current amplitude
higher than I.
4

5. SIMULATION OF LIGHTNING STROKES TO TRANSMISSION LINE
The general expression for the striking distance:
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.
5

6. SIMULATION OF LIGHTNING STROKES TO TRANSMISSION LINE
6,7 m
6,7 m
4,5 m
A 4,5 m
6,0 m
6,0 m
6,5 m
B 6,5 m
6,0mm
6,0
C 5,0 m
5,0 m
17,10m
19,50m
22,00m
24,60m
27,30m
30,00m
3D model of the part of the studied transmission line
6

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,
7
- critical current: 47.30 kA.

8. SIMULATION OF LIGHTNING STROKES TO TRANSMISSION LINE
Distribution of lightning currents striking phase conductors
0,1
0,09
0,08
0,07
0,06
Probability
0,05
0,04
0,03
0,02
0,01
0
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)
Distribution of lightning currents striking phase conductors 8

9. SIMULATION OF LIGHTNING STROKES TO TRANSMISSION LINE
Distribution of lightning currents striking top of towers or shielding wire
0,25
0,2
0,15
Probability
0,1
0,05
0
0 -20 -30 - 40 -50 -60 -70 -80 -90 0 0 0 0 0 0 0 0 0 0
1-1 10 20 30 40 50 60 70 80 -10 -11 -12 -13 -14 -15 -16 -17 -18 -19
90 1 00 1 10 1 20 1 30 1 40 1 50 1 60 1 70 1 80
Classes (kA)
Distribution of lightning currents striking top of towers or shielding wire
9

10. 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:
10

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:
t
k
U U0 d DE
T0
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.
11

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 .
12

13. MODELING PROCEDURE FOR TRANSIENT SIMULATIONS
EMTP-RV Model of footing resistance ionization
13

14. MODELING PROCEDURE FOR TRANSIENT SIMULATIONS
The model of gapless type LSA includes non-linear and dynamic
behaviour of the arrester.
800
700
600
500
Voltage (kV)
400
300
200
100
0
0,1 1 10 100
Current (kA)
U-I characteristic of surge arrester for the 220 kV line (Ur=210 kV) 14

15. Shield wire
lower
upper
middle
Transmision line
10 km
1
Z=139.3 Ω
Transmision line
276 m
Z=139.3 Ω 2
Transmision line
343 m
3
Z=135.2 Ω
Transmision line
289 m
4
Z=139.3 Ω
Transmision line
402 m
5
Model of 220 kV double-circuit line
Z=139.3 Ω
1000 Ω
Transmision line
295 m
6
Z=139.3 Ω
Stroke to tower
Transmision line
213 m
31 kA, 72 kA,
96 kA, 138 kA
7
Z=135.2 Ω
Transmision line
MODELING PROCEDURE FOR TRANSIENT SIMULATIONS
459 m
8
Z=139.3 Ω
Transmision line
350 m
9
Z=139.3 Ω
15
Transmision line
10 km

16. 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.
16

17. SIMULATION RESULTS
Back-flashovers in relation to the lightning current magnitude and the
footing resistance of a tower
17

18. SIMULATION RESULTS
Back-flashovers in relation to the lightning current magnitude and the
footing resistance of a tower (LSA in the upper phase)
18

19. SIMULATION RESULTS
Back-flashovers in relation to the lightning current magnitude and the
footing resistance of a tower (LSA in the middle phase)
19

20. SIMULATION RESULTS
Back-flashovers in relation to the lightning current magnitude and the
footing resistance of a tower (LSA in the lower phase)
20

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)
21

22. SIMULATION RESULTS
7
NO LSA
LSA in B
Number of Back-flashover Rates (annual, per 100km of line)
LSA in C
6 LSAs in B and C
5
4
3
2
1
0
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Tower resistance (Ohms)
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)
22
and LSAs in lower and middle phases (B and C)

23. SIMULATION RESULTS
8
NO LSA
LSA in B
7 LSA in C
Number of Flashover Rates (annual, per 100km of line)
LSAs in B and C
6
5
4
3
2
1
0
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Tower resistance (Ohms)
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) 23

24. 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).
24

25. EMTP-RV USER GROUP MEETING, DUBROVNIK, 30 APRIL 2009
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 25