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 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 Cant easily be modelled by pen and paper methods

Buried Cable Results Possible to model cable ratings under different soil/environmental conditions Dry zone can be clearly seen forming around cable group IEC 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