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Thermal performance
of high voltage power
cables
James Pilgrim
19 January 2011
HV Transmission Cable
• Vast majority of transmission
grid route length uses OHL
• National Grid has ~335 km of
cable
• In some instances cable is the
only option
Buried HV Cables
– Urban areas
– Wide river crossings
– Areas of natural beauty
HV Cables in a Tunnel
Importance of Ratings
• Rating defines maximum allowable power transfer and is
limited by dielectric maximum temperature (XLPE 90 °C)
• Rating needs to be accurate
– Pessimistic? Poor asset utilisation, higher costs
– Optimistic? Risk of premature asset ageing/failure
Buried Cables
• Normally rated using analytical calculation of IEC 60287
• A reliable “pen and paper” method, but not hugely flexible
• Proven to give optimistic ratings in some cases – for
instance shallow buried cables which suffer from moisture
migration in the soil
– Solution? Use FEA to model coupled heat/moisture
Buried Cables
• Using dynamic backfill model implemented in FEA it is
possible to explicitly model moisture migration
• Requires characterisation of soil properties and thorough
benchmarking in the lab
• Can’t easily be modelled by pen and paper methods
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Buried Cable Results
• Possible to model cable ratings under different
soil/environmental conditions
• Dry zone can be clearly seen forming around cable group
• IEC 60287 uses somewhat arbitrary technique to identify
this can give incorrect results
Tunnel Ratings
• Rated using numerical Electra 143 method which forces
some assumptions
– Constant tunnel cross section
– Cables considered to be of the same construction,
operating voltage and load
– No consideration of cables in riser shafts
– No consideration of cable joints/accessories
• New, more complex tunnels often require these restrictions
to be removed – hence use of FEA/CFD techniques
Tunnel Rating Improvements
• Better modelling of convective heat
transfer through use of CFD
• Verification with experimental data
• Redesigning thermal networks on
which models are based
• Incorporating FEA analysis of cable
joint temperatures
Tunnel Air Velocity Contours
• Provides a better end to end rating
400kV Joint in Tunnel
Tunnel Example Results
• Example tunnel with multiple
independent cable circuits
installed
• Possible to trade-off load ratings
between cables
• Maximise utilisation of cable
assets without risking excessive
temperatures
Conclusions
• Using modern numerical analysis techniques cable ratings
can be calculated much more accurately
• This maximises asset utilisation while minimising risk of
premature failure and loss of supply
• An important component of the smart grid concept –
provide better operational flexibility from our existing
power infrastructure