Thermal rating of HV cable accessories

We had discussions about the need for thermal rating of HV cables accessories. To conclude this we decided to summarize in a post the findings from the Cigré Task Force TF 21(B1)-10 who investigated this question and published an article in Elektra 212, 2003.

Posted 2021-10-16
Categories: Theory

It is common practice to base the calculation of the current carrying capacity of HV/EHV underground transmission lines on the thermal ratings of the cable, taking into account the losses in the cable and the heat transfer to its environment. International standards as IEC specifications 60840 and 62067 have defined the thermal ratings of extruded HV/EHV cables by their maximum cable conductor temperatures for different insulations. The thermal ratings of accessories are not explicitly mentioned as they were generally considered to have equal or better levels than those of the cable.

Questions have been raised, whether and how the thermal and thermo-mechanical ratings of accessories, i.e. terminations and joints, should be defined and how these should be taken into account in the design of cable systems. In 2001 a Task Force TF 21-10 was established with the following scope:

  • to specify the terms 'thermal and thermo-mechanical ratings' of accessories
  • to review existing test specifications with regard to appropriate verification of thermal and thermo-mechanical performance of accessories
  • to consider, if applicable, improved or new test procedures for thermal and thermo-mechanical verification of accessories
  • to prepare recommendations to IEC, whether and how specific thermal and thermo-mechanical tests should be combined with and/or integrated into existing test specifications for extruded HV and EHV cables (e.g. IEC 60840 and IEC 62067)
  • to prepare guidelines how to apply the findings of basic laboratory tests to the multitude of practical configurations.

We will state here the scope of this work and the conclusions with respect to thermal rating, excluding mechanical aspects and tests.

Thermal ratings of accessories

The thermal rating of an accessory is defined as 'the maximum temperature of the conductor or conductor connector contained within the accessory (whichever is the higher) allowed in normal operation'.

The conclusions were:

  • Thermal ratings of accessories need not be specified separately from cables, as they are considered identical due to the presence of cable inside the accessory.
  • Common test specifications can only assess the basic thermal performance of accessories, rather than compliance with rated cable conductor temperatures.
  • Many joints have worse heat dissipation characteristics than cables, thus developing during thermal IEC tests higher internal conductor temperatures than the remote cables.
  • Higher than rated conductor (insulation) temperatures are not allowed inside accessories during service operation.
  • For type/prequalification tests it is acceptable for maximum temperature in accessories to be higher than in the cable. The successful completion of IEC thermal tests on a complete cable system can be considered as simultaneous verification of the adequate thermal design of both cables and accessories, provided that comparable or higher conductor temperatures as rated for the cable are achieved inside joints.
  • The thermal performance of terminations in normal operation is not considered critical; therefore they do not have to reach the rated temperature for the cable during test.
  • Conductor heating by means of current should be applied during type and prequalification tests. No external means of heating or sheath current heating should be applied, as this will tend to reduce the conductor temperature within a joint in comparison to the temperature of the remote cable conductor.
  • Some cable systems with additional thermal insulation may require extended heating (and cooling) cycles.

Thermal ratings of accessories are considered identical to the cable and the thermal performance of terminations in normal operation is not to be considered critical.

Thermo-mechanical ratings of accessories

Not considered in this post

Systems design aspects

Not considered in this post

Annex: Thermal calculations in HV and EHV cables and joints

Joints and cables have different thermal properties. The basic differences can be expressed in thermal resistances and thermal time constants.

To verify the differences in the joint and cable temperature during one heating cycle, dynamical calculations were made for an 800mm2 132kV XLPE cable. First, the heating current is calculated to obtain a stable cable conductor temperature of 95 °C. Based on the heating current, the temperature was calculated inside the joint and cable for a period of 10h after switching on the current. The results are given in Figure 3 at a conductor heating current of 1800A.

Model for calculations, © CigréHeating curve, © Cigré
Model for calculation Heating curve

In stationary conditions, the joint reached a higher temperature than the cable, as a result of higher thermal resistance of the joint. In the first 6 hours of the heating cycle, the temperature in the joint is lower than in the cable due to the longer thermal time constant of the joint.