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In this study, the characteristics of non-breakdown and breakdown streamers have been investigated through the effects of dissolved gases/air, carbon particles, additives, reduced pressure, liquid chemistry and voltage polarities on streamers. Two types of white oil (Marcol 52 and Exxsol D140) considered base liquids are examined. A point-plane electrode system is employed to form a high divergent field with the electrode gap of 8 cm. A “step” voltage (0.5/1700 µs) is applied to the electrode system. The voltage magnitude is varied in steps up to 540 kV maximum. Streamer characteristics are observed with stopping length, velocity, shape, breakdown and acceleration voltages. Current and light emission pulses are recorded. Both still and streak images are captured. The properties of positive streamer channel are investigated more closely. The electric field at the channel tip is determined by using the finite element method with COMSOL Multiphysics program, and mechanisms for streamer propagation are also discussed. Dissolved gases/air has an insignificant effect on streamer propagation while carbon particles largely facilitate it. Carbon particles were seen to accelerate and markedly reduce inception, breakdown and acceleration voltages of streamers of both polarities. However, carbon particles have a stronger influence on negative streamers than positive ones. It seems that a small amount of carbon particles in the stagnant oil in the electrode gap can switch the slow 2nd mode[1] to fast 3rd mode streamers of negative polarity. Such a phenomenon cannot be observed in positive streamers. Two kinds of additives are employed. These are a low ionization potential, N,N-dimethylaniline (DMA), and an electron scavenger, trichloroethene (TCE). DMA accelerates positive non-breakdown streamers of Marcol oil but seems to decelerate those of Exxsol oil due to the fact that DMA makes streamers either more filamentary for Marcol oil or more branched for Exxsol oil. Thus, DMA reduces the breakdown voltage of Marcol oil and increases that of Exxsol oil. DMA always decelerates positive breakdown streamers since a reduction in macroscopic field resulting from increasing shielding effect formed by more branching. Therefore, acceleration voltage is significantly increased. On the other hand, DMA does not have any significant impacts on negative streamers in both types of oil. TCE increases the velocity of both polarities because it makes streamers either less branched (positive polarity) or more filamentary (negative polarity). This leads to a decrease of both breakdown and acceleration voltages. However, TCE affects negative streamers much more than positive streamers. It is proposed that streamer propagation involves electronic processes in front of streamer channel tips. Reduced pressure makes streamers more branched, but it still largely facilitates streamer propagation, i.e. a dramatic increase in the stopping length, thus significantly reducing the breakdown voltage. Reduced pressure seems not to influence the velocity of positive streamers while it significantly changes that of negative streamers. While slightly decreasing the acceleration voltage of positive streamers, reduced pressure decreases that of negative streamers by a factor of about two. Consequently, gaseous processes are involved in streamer propagation. Although, the general characteristics of streamers are similar in Exxsol oil and Marcol oil, streamers of these oils still show some differences. The 2nd mode of positive streamers has multi-filament shape for Exxsol oil whereas more bushy shape can be observed in Marcol oil. In Exxsol oil, the 1st mode of negative streamers (bush-like shape) has to switch to the 2nd mode streamers (tree-like shape) to cross the electrode gap and induce breakdown. On the other hand, the 1st mode streamers (bush-like shape) can propagate across the gap in Marcol oil. The ratio of acceleration voltage to breakdown voltage is higher than for positive streamers in Exxsol oil since the content of aromatics in Marcol oil is lower. Positive streamers are about ten times faster than negative streamers. This is due to the fact that positive streamers are more filamentary whereas negative streamers are bushier. In addition, the breakdown voltage of negative streamers is about twice of that of positive streamers, and the acceleration voltage behaves in the similar way. It seems that a weakly ionized plasma state is present inside low conductive channels of the 2nd mode streamers of positive polarity, and critical space charges accumulate at channel tips. Gas discharge is considered to be responsible for the appearance of reilluminations. Nevertheless, reilluminations do not have any significant effects on the propagation of streamers. For the 4th mode, channels of positive streamers are highly conductive possibly resulting from highly ionized plasma state, and critical space charges are still present at the channel tips. The similar characteristics are suggested for corresponding modes of negative streamers. It is proposed that tip processes governed by electric field, formed by the space charges, will control streamer propagation. The electric field at the channel tip is calculated. The values of 2.4 MV/cm and 7-20MV/cm are for the 2nd and 4th modes of positive streamers, respectively. The corresponding values are 1.3-2 MV/cm and 11-20MV/cm for negative streamers. Therefore impact ionization is possibly one main mechanism producing new charges for maintaining streamer propagation. PhD i elkraftteknikk PhD in Electric Power Engineering