Risk assessment of 170 kV GIS connected to combined cable/OHL network

420 kV

ADL

MOS

DYB

HVV

60 kVABØ

NVV

VHA

60 kV

FER

SBA

THØ

HVO

60 kV

60 kV

C11 km C2

6 km

C1

14,7 km

C1

8,7 km

C2

1,8 km

C2

10,6 km

T1

T1

T2

T3

R1

R2

C2

3,5 km

C2

20 km

S1 S1 S1 S1 S1

S1 S2

S1

S1

S2

S2

S2

S2S2

S2

S2

S2

T1

T1

S3

JER

S1 S1S1

T2

NVV

1 km UC

1,5 km OHL

60 kV busbar

s.a.

170 kV busbar

ABØ - Indoor GIS

TF2

TF1

Busbar

breaker

VHA

6 km

ADL

14,7 km

Transmission system around

city of Aalborg

Planned 170 kV network 2014

Simulation taking into account:-Lightning parameters-Lightning current magnitude and rate of rise- ÅBØ substation layout- Underground cable ÅBØ-NVV- Limfjord high tower crossing- Surge arrester precise model-Dynamic grounding impedance-Voltage-time characteristics of insulators-Implemented in PSCAD/EMTDC

Transformer LIWL:650 kV

Shielding failure and back flashover

considered for Limfjord high tower

crossing

Introduction

IEC safety factor 15% makes admissible overvoltage level for transformers 565 kV. This is seen to be exceeded!

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.

Risk Assessment

MTBF vs. front time, SF, open breaker.

MTBF vs. soil resistivity, SF, open breaker.

MTBF vs. soil resistivity, BFO, open breaker.

MTBF vs. soil resistivity, BFO, closed breaker.

MTBF vs. distance, SF, open breaker.

MTBF vs. distance, BFO, open breaker.

Conclusion

•MTBF above the acceptable limit of 1.000 years was obtained for all cases.•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.

Simulations

Front time[ µs]

Time to half[ µs]

Crest magnitude[kA]

Soil Resistivity[ Ωm]

SF 1,4 350 -41,8 92,5

BFO 10 350 -200 92,5

Simulation of a -41,8 kA 1,4/350 µs Shielding Failure Simulation of a -200 kA 10/350 µs Back Flashover

Varying lightning front time, SF, closed breaker.

Varying lightning front time, SF, open breaker.

Varying soil resistivity, SF, closed breaker.

Varying soil resistivity, SF, open breaker.

Varying cable length to transformer, SF, closed breaker. Varying cable length to transformer, SF, open breaker.

Surge front time Soil resistivity

Cable length between GIS busbar and transformer

Effects of surge front time, soil resistivity and length of cable leading to the transformer are investigated:

25.0front

11

24

−⋅

=

P

I

t

Soil resistivity

Front time Cable length

•Claus Leth Bak, Institute of Energy Technology,

Aalborg University, Denmark

•Jakob Kessel, Energinet.dk, Denmark

•Víðir Atlason, Landsnet, Iceland

•Jesper Lund, NV Net, Denmark

NVV

ADL

FER

HVV

5 km

ABØ

VHA

Overhead lines

Underground cables

170 kV outdoor substation

170 kV gas insulated substation

To HVOTo SBA

To DYB

To MOSTo THØ

To BDK

Aalborg

Limfjord

Area 3

Area 2

Area 1

Area 4

Denmark