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Thermal performance of high voltage power cables

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

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Thermal performance of high voltage power cables

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  1. Thermal performance of high voltage power cables James Pilgrim 19 January 2011

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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 Tunnel Air Velocity Contours 400kV Joint in Tunnel

  9. 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

  10. 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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