The effect of nano-MGO addition on grounded DC tree in cross-linked polyethylene

In order to investigate the effect of nano-MgO addition on grounded DC tree in cross-linked polyethylene (XLPE), comparative tree experiments were performed using XLPE and 0.5wt% MgO/XLPE nanocomposite by periodic prestress-grounding in a needle-plane system. And the space charge measurement based on pulsed electro-acoustic (PEA) method was performed on both kinds of samples as a complement. The tree experiment shows that MgO/XLPE nanocomposite exhibits higher 50% tree inception voltage. Combined with the space charge measurement results that more homocharge is injected in XLPE, the possible reason may be that nanoparticles impede the homocharge injection and returning back to needle tip, and reduce the local electric strength and electric mechanical energy at grounding, which impedes the generation of electrical tree. And MgO/XLPE nanocomposite shows shorter average tree length and width when under positive voltages, which may because the carries generated from high stress region collide with nanoparticles and results in slower tree propagation. Besides, the average tree length-width ratio in MgO/XLPE nanocomposite decreases faster with the positive voltage increasing. This can be explained by the fact that electrical tree is hard to penetrate nanoparticles, they can only develop around surfaces of nanoparticles, which may result in the change of tree channel direction and emergence of many thin lateral branches.