Oil is an important part in power transformers. It serves as both the electrical insulation and coolant and is in contact with metals and the paper insulation. Contaminants such as metal filings or cellulosic residual can be formed in the oil, especially for the transformers with aged paper insulation. These contaminants could form a bridge under the influence of the applied electric field. The bridge may potentially act as a conducting path between two different potentials within the transformer structure, leading to partial discharges or insulation failure. Experimental studies of contaminants motion under both dc and ac voltages will be done in the project. In addition to live optical observation and capturing of bridging phenomena between two electrodes in oil under different voltages, contamination levels and oil and paper insulation conditions, electrical conduction currents and partial discharges will also be measured simultaneously during bridging. These experimental results should allow one to establish a good understanding of contamination and its relation to electrical performance and pre-breakdown phenomena. To aid the understanding of bridging dynamics in the contaminated oil, a numerical model of particle movements and their accumulation at high field regions will be developed. It will be based on the hydrodynamic drift-diffusion approximation for the particles motion under dielectrophoresis force.