Because the conductivity of XLPE is dependent on temperature and electric field strength

it is necessary to take both thermal and electric field into account when investigating the electric field distribution of HVDC cable insulation under operating conditions.Furthermore

the insulation is exposed to not only steady-state DC voltages

but also transient voltage waveforms including continuous rising DC voltage

polarity reversal DC voltage and switching impulse superimposed on DC voltage.In this paper

the temperature distribution in XLPE insulation of 500 kV DC submarine cable is calculated by using Comsol Multi-physics

a finite element software firstly.Then the formula of XLPE conductivity changing with temperature and field strength is obtained by fitting the measured data.Based on this

the steady-state and transient field distribution in cable XLPE insulation are simulated.The results show that the heating power of XLPE insulation is much less than that of Joule loss power of cable conductor.The thermal bottleneck exists in the landing section of the submarine cable

which may cause the allowable current rating lower than the design requirements.The steady-state electric field of XLPE insulation is approximately linear along the radial direction

and its distribution is quite uniform under the rated current load.A high transient field will be generated in the inner insulation under the continuous rising DC voltage

whose strength depends on the rising rate of voltage and the relaxation time constant of insulation.The field distribution under polarity reversal DC voltage can be estimated by superposition of the resistive and capacitive fields.Under the composite voltages with same amplitude

the transient field in the inner insulation caused by a switching impulse superimposed on DC voltage with a reverse polarity is higher than that caused by a superposition with a same polarity.