## 1 Introduction  
Breakdown characteristics of transformer oil have been widely investigated for its important significance in the safe operation of power equipment [1; 2]. Breakdown phenomena in transformer oil are caused by the initiation and propagation of charged gaseous channels called \"streamer\" under the high local electric field [3]. Positive streamers have been proved to initiate at lower applied voltages and propagate faster and further than negative streamers [4; 5; 6], which constitute a greater risk to oil insulated high voltage electrical equipment than negative streamers. It has been found that the positive impulse breakdown strength of transformer oil depends on the prebreakdown phenomenon [7; 8; 9] and it can be considerably influenced by adjusting the propagation characteristic of prebreakdown streamers with the addition of additives [10; 11].  
Many efforts have been devoted to improve the positive breakdown voltage of transformer oil. In the past two decades, nanofluids (NFs) have attracted much attention due to their good insulating and thermal performance [12; 13]. A variety of nanoparticle (NP) materials have been used to prepare transformer oil-based nanofluids, exhibiting different modification effects. It is found that the positive lightning impulse breakdown voltage of mineral transformer oil can be improved by 82.6% with the addition of Fe\({}_{3}\)O\({}_{4}\) nanoparticles [13]. For TiO\({}_{2}\) nanofluid, the breakdown voltage is increased by 23.6% [14]. However, with the addition of Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\) nanoparticles, the breakdown voltage is decreased by 11.6% and 8.8%, respectively [15]. Recently, three types of nanoparticle materials with the same particle size were used to modify the switching impulse breakdown voltage of transformer oil. Based on the simulation of the potential well distribution on the nanoparticles, a possible mechanism was proposed to explain modification effects of the three kinds of nanoparticles [16]. However, there has been still dearth of experimental evidence that the type of nanoparticle materials has an effect on prebreakdown process in transformer oil, especially for nanoparticles with the same size, surface modification and concentration. The propagation process of prebreakdown streamer will give a valuable insight into the event leading to breakdown and provide a bridge between macroscopic breakdown phenomena and microscopic effect mechanism. Therefore, it is helpful to study the prebreakdown process in nanofluids for an improved understanding of the effect mechanism of nanoparticle materials.  
In this paper, three types of nanoparticle materials, including conductive Fe\({}_{3}\)O\({}_{4}\), semiconductive TiO\({}_{2}\) and insulating Al\({}_{2}\)O\({}_{3}\) nanoparticles, were prepared with the same size and surface modification. The breakdown strengths of transformer oil and nanofluids with the same nanoparticle concentration were measured. The propagation characteristics of positive streamers in the oil samples are studied by comparing their shapes, propagation lengths and velocities with the help of the shadowgraph technique. Moreover, the effects of nanoparticle materials on the breakdown and prebreakdown characteristics of nanofluids are discussed.