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# Univ Of Zagreb Uglesic Prevention Of Flashover

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### Univ Of Zagreb Uglesic Prevention Of Flashover

1. 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. 2. OUTLINE OF PRESENTATION  INTRODUCTION  SIMULATION OF LIGHTNING STROKES TO TRANSMISSION LINE  MODELING PROCEDURE FOR TRANSIENT SIMULATIONS  SIMULATION RESULTS  CONCLUSIONS 2
3. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 13. MODELING PROCEDURE FOR TRANSIENT SIMULATIONS EMTP-RV Model of footing resistance ionization 13
14. 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. 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. 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. 17. SIMULATION RESULTS Back-flashovers in relation to the lightning current magnitude and the footing resistance of a tower 17
18. 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. 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. 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. 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. 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. 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. 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. 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