Although vegetable oil is the best substitute for mineral oil, it exhibits numerous disadvantages, including a high dielectric loss factor, low volume resistivity, and high viscosity. Nanoparticle modification may improve the electrical properties of vegetable oil considerably. In this study, vegetable oil infused with Fe3O4, TiO2, and Al2O3 nanoparticles was investigated. Thermal aging experiments were performed at 100 °C, 130 °C, and 150 °C. The aged vegetable insulating oil was periodically sampled and tested. The results revealed that the nanoparticles with a mass fraction of 0.001% exhibited the best modification effect. The electrical properties of the vegetable insulating oil modified with magnetic nanoparticles were pronounced, whereas the modification effect of nonmagnetic nanoparticles was mild. The general trend of the modification effect of the nanoparticles on vegetable oil became obvious with the increase in the temperature. At the same content, the modification effect of nano-Fe3O4 was superior to that of nano-TiO2 and nano-Al2O3. The results of this study provide experimental and theoretical support for the mechanism of long-term stability of vegetable insulating oil with nanoparticles and can be used to improve the long-term stability of vegetable insulating oil.

This article reports the evaluation system of the insulating properties based on the pulsed electro-acoustic (PEA) method and the current integration charge Q(t) method. According to the basic principles of the methods, the relationship between the charge ratio and dielectric relaxation time in Q(t) data is first derived, by which a new method is proposed to calculate the material conductivity. By further analyzing the charge results of different materials, it is pointed out that the PEA method focuses on the related characteristics of the net charge distribution captured by deep traps, while the Q(t) method reflects the changing process of the total charge amount and related characteristics. No single method can completely analyze all the charge characteristics of the material, and thus the necessity of the combination of the two methods is determined. Meanwhile, based on the theoretical derivation and experimental results, it is indicated that when the charge ratio of Q(t) data exceeds 2, the electrical state inside the sample basically stabilizes. At this stage, the distorted electric field can be analyzed by the PEA method, and the Q(t) method can calculate the leakage current, dielectric relaxation time, and conductivity. Therefore, the combination of the two methods can provide a more reasonable assessment for the insulation property and state.

Different types of lightning protection systems (LPSs) have been developed for wind turbines to protect the blades from lightning strikes. However, severe damages caused by lightning strikes still happen frequently, which creates huge costs. Experiments using a 5-m blade specimen with tip receptors from 1.5-MW wind turbine blades under 3-m air gap were conducted to investigate the lightning attachment manner to the wind turbine blade with LPS in different situations. Factors including the polarity of the lightning strikes, the orientation of the blade, and the lateral distances (LDs) between the wind turbine blade and the lightning downward leader were taken into account. It was found that the types of discharge paths under positive and negative lightning strikes are quite different, and the positive discharges are much more dangerous to wind turbine blade than the negative ones. The LD between the downward leader and the wind turbine blade is a key factor that influences the interception efficiency. Three types of receptor interception failures were discovered. Multiple upward leaders may incept from the blade body as to intercept the downward leader. However, the protection range of the tip receptor is quite limited, and the connection of the tip receptor and the blade body is the most vulnerable position hit by the lightning strikes. The results present the useful reference to the optimal design of the wind turbine blade LPS.