thermal performance of high voltage power cables james pilgrim 19 january 2011
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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
– Urban areas
– Wide river crossings
– Areas of natural beauty
Buried HV Cables
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
• Provides a better end to end rating
400kV Joint in Tunnel
Tunnel Air Velocity Contours
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
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