Thermal performance of high voltage power cables James Pilgrim 19 January 2011
Mar 28, 2015
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