Electrical Aging Effect on PD Characteristics of Cellulose Pressboard Modified by Nano-TiO2

Cellulose pressboard has been widely used as internal insulation in oil-immersed transformers. Partial discharge (PD) is the main cause of the degradation of the insulation properties for electrical equipment. Nano-modification is one of the methods to improve the electrical properties of the oil-paper insulation. In this article, the effects of the oil-immersed pressboards modified by nano-TiO2 particles on PD characteristics were studied. The pressboards modified by different mass fractions of nano-TiO2 (10-nm particle size) were prepared. The electrical aging experiment was carried out on the oil-immersed pressboard. Experimental platforms for partial discharge inception voltage (PDIV) and relative permittivity were established to measure the dielectric performance of the pressboard. Phase-resolved partial discharge (PRPD) spectrograms and PD current pulses of the samples under different electrical aging states were obtained for different PD time. The surface morphology of the pressboards was observed by scanning electron microscope (SEM) to verify the effects of nano-TiO2 particles. The results showed that the values of electric field strength for the samples were 4.58 and 3.6 kV/mm, respectively, when the PD occurred in the samples modified by nano-TiO2 with 3 wt% and the unmodified ones. Compared with the unmodified pressboard, the modified one had a lower value of relative permittivity. The pulse current of the pressboard modified by 3 wt% nano-TiO2 particles under different electrical aging states was smaller than that of the other samples. The addition of 3 wt% nanoparticles was the optimum ratio to suppress the PD of the samples in the ac electrical field.