Aging Characteristics of the Insulation Layer of HVDC Cables and Effect of Space Charge Accumulation on Electric Field Distribution

The aging of the insulation sample of high voltage direct current (HVDC) cables is one of the main reasons that aggravates space charge accumulation. The surface morphology, infrared spectrum, relative permittivity, and breakdown strength of the insulation layer samples of cable slices with different aging degrees are measured and analyzed. Further, the electric field simulation model of HVDC cables is established combined with Maxwell-Wagner polarization, and the charge accumulation characteristics of the insulation layer and the effects of temperature and the aging time on the charge accumulation are studied. The experimental results show that with the increase of aging days, the cross-linked polyethylene (XLPE) molecular chain segments are damaged, the size of spherical crystals gradually decreases, and the polar groups such as carbonyl and carboxyl groups gradually increase. Besides, the dielectric constant of XLPE gradually decreases from inside to outside and is positively correlated to the aging days. The conductivity of XLPE increases by 1–2 orders of magnitude, and the breakdown strength gradually decreases with the increase of the aging days. Substituting the experimental parameters into the simulation model, the maximum distortion electric field is 31.5 and 29.3 kV/mm, respectively, for the positions of the inner-middle interface and the middle-outside interface. When the conductor temperature is
$70~^{\circ }$
C, the maximum distortion electric field increases by 61.8% and 43.7%, respectively, at the inside-middle interface and the middle-outside interface. The charge accumulation at the interface of the insulation layer increases gradually with the increase of aging days and electric field distortion becomes serious.