The Effect of Insulation Characteristics on Thermal Instability in HVDC Extruded Cables

Abstract

This paper aims at studying the effect of cable characteristics on the thermal instability of 320 kV and 500 kV Cross-Linked Polyethylene XLPE-insulated high voltage direct-current (HVDC) cables buried in soil for different values of the applied voltages, by the means of sensitivity analysis of the insulation losses to the electrical conductivity coefficients of temperature and electric field, a and b. It also finds the value of dielectric loss coefficient βd for DC cables, which allows an analytical calculation of the temperature rise as a function of insulation losses and thermal resistances. A Matlab code is used to iteratively solve Maxwell’s equations and find the electric field distribution, the insulation losses and the temperature rise inside the insulation due to insulation losses of the cable subjected to load cycles according to CIGRÉ Technical Brochure 496. Thermal stability diagrams are found to study the thermal instability and its relationship with the cable ampacity. The results show high dependency of the thermal stability on the electrical conductivity of cable insulating material, as expressed via the conductivity coefficients of temperature and electric field. The effect of insulation thickness on both the insulation losses and the thermal stability is also investigated.

Electric Field Distribution and Dielectric Losses in XLPE Insulation and Semiconductor Screens of High-Voltage Cables

“…Many articles have described the effect of various factors on the electrical conductivity γ of cross-linked polyethylene [1,[4][5][6][7][9][10][11][12]18,. These factors include electric field stress, percentage of nanoparticles (most commonly Al 2 O 3 ), distance from the high-voltage conductor, temperature, aging time, degassing time, frequency, and insulation thickness.…”

“…In article [5], the authors presented the voltage distribution in pure XLPE insulation and semiconductor screens for 320 kV and 500 kV high-voltage cables. The distribution showed that the authors made a simplification, assuming that the distributions of the potential, and thus the electric field stress, in both semiconductor screens and pure XLPE, were similar.…”

“…In papers [5,15,58,59], the authors, determining the losses in cable insulation, analyzed only the losses in XLPE insulation, omitting losses in semiconductor screens. In publication [15], the authors presented the results of the analysis of dielectric losses for a 30 kV cable.…”

“…In publication [59], the authors presented the results of calculations of dielectric losses for a 110 kV cable, which amounted to 239 W•km −1 . In publication [5], the authors presented the results of calculations of dielectric losses in the insulation of 320 kV and 500 kV cables. These losses amounted to 60 and 100 W•km −1 , respectively.…”

“…As you can see, there are significant discrepancies in the presented values of dielectric losses. For example, for a 110 kV cable, the dielectric losses exceed 200 W•km −1 [59], and for a 500 kV cable, the losses are only 100 W•km −1 [5].…”