In the case of HVDC cables, there are two limitations that must be observed. One is the maximum conductor temperature, similarly as in the AC case and the other is the limitation of the electric field strength in the insulation. The latter can be shown to be equivalent to limiting the temperature drop across the insulation.
In an HVDC cable, the electrical field is determined by the resistive properties (governed by the specific electric resistivity $\rho_i$) of the insulation material, contrary to a HVAC cable where this is determined by capacitive properties (governed by the dielectric permittivity $\epsilon_i$). The specific thermal resistivity $\rho_i$ is significantly dependent on temperature, and because this temperature is not constant throughout the insulation material, the electrical field is not constant and changes with changing cable loading situations. In order to ensure that the electrical fields in the insulation material remain within limits, the temperature difference over the insulation material is limited. This limit may be more stringent than the limit on the maximum conductor operating temperature with typical values between 10 and 15 K.
It is noted that the maximum temperature difference over the insulation material of a specific cable is an important parameter during type testing, and should not be surpassed during cable operation. To take this additional limitation into account, the Cigre TB 880 guidance point 9 may be followed.
Cableizer provides a method where the losses generated in the dielectic are considered and the resulting
