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