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Lightning protection for a OHL/UC connected GIS
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1
Lightning Simulation of a Combined Overhead Line/Cable Connected GIS
by
Jakob Kessel, Víðir Már Atlason and Claus Leth BakAalborg University,
Institute of Energy Technology
and
Jesper LundNV Net A/S
2
Introduction
∙ 170 kV transmission system for year 2014▫ Mainly underground cable and GIS
∙ Follow up on 9th semester project
3
Introduction
∙ Only Area 3 showed overvoltages
∙ Modelling▫ Lines, cables and outdoor busbars
◦ Transmission lines
▫ GIS busbars and transformers ◦ Equivalent capacitances
▫ Tower model▫ Grounding resistance▫ Surge arrester
SA
4
Modelling
∙ Tower model▫ Tower surge impedance▫ Insulator model▫ Grounding resistance
Cite: Fast Front Task Force of the IEEE, and Sargent et al.
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Modelling
∙ Insulator model▫ Voltage-time characteristic
75,02
1tv tK
KU
Cite: Fast Front Task Force of the IEEE, and Yadee et al.
6
Modelling
∙ Grounding resistance▫ Dynamic grounding resistance
▫ Where:◦ R0 is low current grounding resistance
◦ Ig is the critical current causing ionization of the soil
◦ IR is the current to ground
∙ Surge arrester model ▫ Simplified IEEE model
Cite: Fast Front Task Force of the IEEE, and Crisholm et al.
Cite: Pinceti et al.
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Simulation
∙ Simulation parameters▫ Shielding failure▫ Back flashover
▫ The lightning surge is estimated with double exponential function
∙ Parameter investigation▫ Lightning front time
◦ Only for shielding failure
▫ Soil resistivity (at the overhead line/cable transition point)▫ Cable length (between transformer and surge arrester in GIS)
Front time Time to half Crest magnitude Soil resitivity
[µs] [µs] [kA] [Ωm]
Shielding failure 1,4 350 -41,8 92,5
Back flashover 10 350 -200 92,5
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Results
Utf
∙ Simulation results ▫ Varying lightning
front time, SF, closed breaker
▫ Varying lightningfront time, SF, open breaker
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Results
∙ Simulation results▫ Varying soil
resistivity, SF, closed breaker
▫ Varying soil resistivity, SF, open breaker
Utf
10
Results
∙ Simulation results▫ Varying cable
length, SF, closed breaker
▫ Varying cable length, SF, open breaker
Utf
11
Results
∙ Evaluation of critical lightning current▫ Varying front time
◦ Only evaluated for shielding failure◦ No overvoltages with closed breaker
▫ Varying soil resistivity◦ No overvoltage for SF with closed breaker
12
Results
∙ Evaluation of critical lightning current▫ Cable length
◦ No overvoltages with closed breaker
▫ The MTBF is found based on the lightning current
13
Modelling
∙ Risk assesment▫ Back flashover
◦ MTBFclosed = (P(closed) P(current) Nflashes)-1
◦ MTBFopen = (P(open) P(current) Nflashes)-1
◦ P(open) ≈ 1/365
◦ P(closed) = 1 - P(open)
▫ Shielding failure◦ MTBFclosed = (P(closed) P(sf) P(current) Nflashes)-1
◦ MTBFopen = (P(open) P(sf) P(current) Nflashes)-1
∙ Mean Time Between Failure
14
Conclusion & Discussion
∙ Conclusion
▫ The steepness of the lightning surgeLimited effect on the overvoltages.
▫ The soil resistivity at the overhead line/cable transition pointGreat effect on the overvoltages.
▫ The cable length between the transformer and the surge arrester in the GISIncreased cable length yielding increased voltage magnitude at the transformer.
▫ MTBF > 2000 yearsThe surge arrester at the overhead line/cable transition point provides adequate protection for the substation.Further protection in form of a surge arrester at the GIS busbar is not necessary.