The DC and AC insulating properties of magnetic fluids based on transformer oil

The AC‐dielectric breakdown was investigated in magnetic fluids based on transformer oil TECHNOL US 4000 for two orientations of external magnetic field ( B ‖ E and B ⊥ E ) and in B = 0. The found results were compared with those obtained formerly for the DC‐dielectric breakdown. The observations of the time development of the AC‐dielectric breakdown showed the presence of partial discharges long before the complete breakdown occurrence, while for DC‐dielectric breakdown a complete breakdown of the gap next to the onset of a measurable ionization was characteristic. The comparison of the AC‐dielectric breakdown strengths of pure transformer oil and transformer‐oil‐based magnetic fluid showed better dielectric properties of magnetic fluid in external magnetic field and comparable, but not worse, in B = 0. Regarding to the better heat transfer, provided by magnetic fluids, they could be used in power transformers as insulating fluids. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)