Three methods for gravity field recovery are mainly studied from the views of observation equation, (1) numerical differential method, (2) energy balance approach, (3) numerical integration method.

The effects of differential speed and the concentration of sludge on the solid content of cake as well as the effect of flocculant dosage on the recovery yield of solid have been discussed.

Furthermore, the digital simulation results and dynamic simulation data indicate that transformer recovery inrush itself cannot result in mal-operation of differential protection.