This document discusses risk assessment of connecting a 170 kV gas insulated substation (GIS) to a combined cable and overhead line network in Denmark. Simulations were conducted to analyze the effects of shielding failures and back flashovers on the network under varying conditions of surge front time, soil resistivity, and cable length between the GIS and transformers. The simulations showed transformer overvoltages exceeding safe levels set by standards in some scenarios, requiring further investigation into mitigation approaches.

1. Risk assessment of 170 kV GIS connected to combined
cable/OHL network
•Claus Leth Bak, Institute of Energy Technology,
Aalborg University, Denmark
•Jakob Kessel, Energinet.dk, Denmark
Simulations
•Víðir Atlason, Landsnet, Iceland Front time Time to half Crest magnitude Soil Resistivity
•Jesper Lund, NV Net, Denmark [ µs] [ µs] [kA] [ Ωm]
SF 1,4 350 -41,8 92,5
Introduction BFO 10 350 -200 92,5
Danish power system has been decided to be cabled
fully up to and including 170 kV. This makes planning
of new network and GIS an urgent matter.
Transmission system around
city of Aalborg Simulation of a -41,8 kA 1,4/350 µs Shielding Failure Simulation of a -200 kA 10/350 µs Back Flashover
To HVO To BDK
To SBA
IEC safety factor 15% makes admissible overvoltage level
NVV
Area 1
for transformers 565 kV. This is seen to be exceeded!
Area 3 VHA
To DYB
Area 2
Effects of surge front time, soil resistivity and length of cable leading to
ABØ
the transformer are investigated:
Aalborg
HVV Surge front time Soil resistivity
ADL
Limfjord
Varying lightning front time, Varying lightning front time, Varying soil resistivity, SF, Varying soil resistivity, SF,
SF, closed breaker. SF, open breaker. closed breaker. open breaker.
Denmark
Cable length between GIS busbar and transformer
Area 4 FER 170 kV outdoor substation
170 kV gas insulated substation
To MOS
To THØ Overhead lines
5 km Underground cables
Planned 170 kV network 2014 Varying cable length to transformer, SF, closed breaker. Varying cable length to transformer, SF, open breaker.
SBA
C2
3,5 km DYB
420 kV
JER Risk Assessment
HVO
S1
NVV
S1 S1
S1
S1
S1
S1
S1
VHA
S1
S1
S2
Soil resistivity
60 kV
HVV ADL 60 kV
T2 T2
S2
S2 S2
T1 T1
S3 ABØ 60 kV
C2
1,8 km T1 T1
C2 C1 C1
10,6 km 8,7 km 1 km
C1 C2
14,7 km 6 km
R1 S2
S2
Transformer LIWL:
S1 T3
S2
S2
60 kV MTBF vs. soil resistivity, BFO, MTBF vs. soil resistivity, SF, MTBF vs. soil resistivity, BFO,
650 kV open breaker. closed breaker.
R2
C2 open breaker.
20 km
S2 FER
MOS
THØ
Front time Cable length
Simulation taking into account:
-Lightning parameters
-Lightning current magnitude and rate of rise I
tfront =
- ÅBØ substation layout 1 
24 ⋅  − 1
0.25
- Underground cable ÅBØ-NVV P 
- Limfjord high tower crossing
ABØ - Indoor GIS
60 kV busbar 170 kV busbar
- Surge arrester precise model
TF2
s.a. NVV
-Dynamic grounding impedance
Busbar 1 km UC
breaker 1,5 km OHL
MTBF vs. front time, SF, open MTBF vs. distance, SF, open MTBF vs. distance, BFO, open
TF1
VHA breaker. breaker. breaker.
-Voltage-time characteristics of insulators
6 km
ADL
14,7 km
-Implemented in PSCAD/EMTDC
Conclusion
Shielding failure and back flashover •MTBF above the acceptable limit of 1.000 years was obtained
considered for Limfjord high tower for all cases.
crossing •The steepness of the lightning surge did not prove to be a
parameter of significance for this system.
•Improved grounding resulted in a decrease of the voltage
appearing at the transformer terminals.
•Increased cable length yielded increased voltage magnitude to
appear at the transformer terminals, for cable lengths up to 50 m.