High voltage direct current (HVDC) technology has widely been used since its first commercial operation in 1954. It has been applied all over the world for bulk power transmission over long distances with low amount of losses. It is also applied in order to transmit power through submarine cables and to interconnect two asynchronous systems. The converter transformer can be considered as one of the most important components of HVDC transmission system. The durability of the converter transformer insulation system is one of the main concerns for electric power utilities all over the world in the present time. As a result of the wide increase in converter transformer usage, it is very important to make its insulation system as reliable as possible. Small gas bubbles can be formed in transformer insulation system due to aging problems, previous partial discharges (PD), or imperfect manufacturing process. The electric field strength inside the cavity is high compared to the surrounding insulation material when high voltage is applied. Because of the high electric field strength in the cavity and its low dielectric strength, PD activities take a place inside the insulation system due to this cavity. These PD activities will degrade the insulation system over the time and finally a breakdown can occur. PD characteristics in the presence of cavities in the oil-paper insulation system have been not fully studied especially when HVDC is applied. It is assumed that, the dielectric behaviour of insulating materials can be fully described according to their permittivity  and conductivity  (RC linear-models). Previous experimental results showed contradiction with these models as a base to describe the dielectric behaviour. It has been indicated that, the dielectric behaviour of oil-paper insulation system is depending on the charge carriers movement in the insulation materials. The aim of this study is to clarify the effect of these charge carriers on the oil-paper insulation system behaviour in the presence of small gas bubbles when HVDC is applied. This paper shows the difference in the behaviour of the gas bubble under the effect of DC stress compared to AC stress. The difference in partial discharge characteristics when either HVDC or HVAC is being applied is also clarified. The impact of the charge carriers on the behaviour of oil-paper insulation system in the presence of small gas bubbles under the effect of HVDC is also presented.