Effect of thermal aging on AC and DC insulation characteristics of 10 kV AC XLPE cables

By comparing the AC and DC insulation characteristics of 10 kV AC cross linked polyethylene(XLPE)cable samples after accelerated thermal aging at 110 ℃, through broadband dielectric spectroscopy, AC and DC breakdown, DC conductivity, and space charge experiments at different temperatures, combined with finite element simulation, the AC and DC electrical characteristics of 10 kV AC XLPE cable insulation with different aging times, and the electric field distribution of cable insulation after changed to DC operation were studied. The results show that as the aging time increases, the real part of the complex permittivity of the XLPE sample increases first and then slows down, and the dielectric loss in the high-frequency region gradually increases. The current density of the sample first decreases and then increases, while the electric field threshold showed the opposite trend, and the space charge in the sample gradually changed from hetero-charge accumulation to homo-charge accumulation. With the increase of the test temperature, the real part of complex permittivity and the dielectric loss of the XLPE sample decreases and increases respectively. Meanwhile, the current density increases and the threshold field decreases, and the amount of space charge accumulation has increased. In this way, the AC and DC breakdown field strength decreases. Under DC voltage, the insulation layer has a higher electric field near the cable core, and the electric field difference increases as the decrease of the activation energy of the insulation samples. After long-term thermal ageing, the DC electric field of the cable insulation is more uniformly distributed and the problem of space charge accumulation is ameliorated, which contributes to the reliability of the 10 kV AC cable under DC operation. © 2022 Editorial Department of Electric Machines and Control. All rights reserved.