An investigation of power transformer cooling with magnetic fluids

Experimental results are presented on the thermal cooling of power transformers with mineral oil based magnetic fluids. Extensive temperature mapping was performed on a simple laboratory model and on two small distribution transformers with disc and foil/wire windings representing two main designs used in practice. It is found that there is an upper limit to the magnetization of ferrofluid, similar to 50 G, beyond which the magnetically driven convection impedes the vertical buoyancy circulation and decreases the transformer cooling efficiency. With a proper magnetization of the fluid (<50 G). the thermal gradients across the windings and along the transformer height are reduced. Also, the maximum temperature inside transformers for both the active and passive cooling units is significantly lowered. Direct thermocouple measurements show that the cooling enhancement with ferrofluid is due to an intensified heat transfer from within the windings/magnetic core assembly while the Archimedes convection prevails outside the assembly. Limited test data acquired on dielectric properties of ferrofluids confirm their feasibility for transformer use. It was found, surprisingly, that the DC (impulse) breakdown strength in a needle positive test setup is much higher for ferrofluid than for oil.