All bushings should be power factored before they are inserted into the transformer. If a Power Factor set is not available when a new transformer is being assembled, a capacitance bridge should, at the very least, be used to measure the bushing tap capacitance values. Measure the values for both C1 and C2 (especially if they are specified on the bushing nameplate). A proper capacitance test could indicate whether a serious internal problem with a bushing exists prior to insertion and whether a power factor test would be advisable. Megger the bushing and its tap at 2500 volts if no Power Factor set is available. Check the bushing tap insulation rating before applying 2500 volts; small bushings may be able to withstand no more than 500 or 1000 volts at the tap.
The transformer itself should be power factored soon after the drying process is complete and the tank is filled with oil. All bushings should again be power factored at this time because their readings will change slightly after assembly. A complete set of Power Factor data should include winding-to-winding, winding-to-ground, and bushing tests. If a 10-kV power factor set is available, a Winding Excitation test should be performed. A Winding Excitation test on very large transformers may not be possible due to insufficient capacity of the Power Factor set for supplying required excitation current.