This procedure is discussed here:
http://www.pyrometallurgy.co.za/InfaconXIII/0443-Saevarsdott...A small excerpt:
For a three phase AC furnace the current in each electrode should be equal. In rare cases of
extreme asymmetry in operation, electrodes are in the top or bottom position, there is no room for
adjusting the impedance and the furnace goes off balance. In this paper the feasibility of separately
adjusting the voltage provided by three single-phase tap transformers in order to compensate for
the furnace unbalance. The transformers are connected in L1 and normally operate with the same
tap position. In order to adjust the voltage for one electrode the tap number must be lowered I
raised on nro transformers. During this skew tapping circulating currents arise in the L1 connected
circuit, at a risk of overloading the transformers.