Enhanced electrification charge densities due to recirculatory flow

A model is developed for volume charge accumulation in recirculatory flow between a charge source and several charge relaxation regions. According to the model, large volume charge densities result if the residence time in the relaxation volumes is small compared to the oil dielectric relaxation time. Under these conditions, charge can accumulate in the oil with each pass through the flow loop, until charge leakage just balances charge generation. To stimulate the volume charge accumulation in a transformer, a flow loop experiment was developed in which transformer oil was circulated at volume flow rates of 2-20 GPM between a 55-gallon reservoir and a paper cellulose filter. The charge density entering and exiting the filter was simultaneously measured using absolute charge sensors. Bypassing the reservoir resulted in increased volume charge densities, in agreement with the model. It is concluded that charge densities can be enhanced by recirculated flow if the charge is not allowed to significantly relax before re-entering a paper filter charge source