Charge Transport in Full-Size HVDC Cable Joint with Modeling of XLPE/EPDM Interface

Charge transport in high-voltage direct-current (HVDC) cable joint with complicated structure has not been profoundly investigated. In this study, a two-dimensional electrothermal coupled charge transport model combining finite element method (FEM) and bipolar charge transport algorithm is proposed, and the effects of temperature gradient and stress cone tilt angle on charge transport and electric field distribution in a 320 kV HVDC cable joint is investigated by this model. In the simulation, the interfacial area between the cross-linked polyethylene (XLPE) cable insulation and the ethylene-propylene-diene monomer (EPDM) joint insulation is specially modeled with different trap parameters from XLPE and EPDM. To obtain the simulation parameters, simultaneous measurements of space charge and relaxation current are performed on XLPE/EPDM double-layer samples, XLPE single-layer samples, and EPDM single-layer samples. It is found that with the increase of temperature gradient, more positive charges are injected and migrate from the inner side of XLPE insulation to the interface, thus, the charges accumulated at the interface change from negative to positive under large temperature gradient. At the same time, the position of the maximum electric field changes from the inner side of XLPE insulation to the root area of the stress cone at the interface. With the increase of the stress cone tilt angle, both the positive charges in XLPE insulation and the negative charges near the stress cone become less, and the overall maximum electric field first decreases and then increases under 20°C temperature gradient.