to parallel all of the neutral CTs to be sure that, if at least one of the banks is in service, the necessary
polarization is provided. Three winding banks can be used as long as there is a delta winding and
one wye-grounded winding. If there are two wye-grounded windings, then both neutrals must have
their CTs paralleled with their ratios inversely proportional to the bank ratings. The grounded
neutral of an autotransformer with delta tertiary may or may not be a suitable polarizing source.
The autotransformer is different from the three-winding transformer in that some of the fault current
flows directly from one voltage system to the other while the rest of the fault current is transferred
through the transformer. It is the direct flow of current in the common winding to the neutral
that presents the problem. For a high-side fault, if IH0 represents the high-side zero-sequence fault
current and IL0 represents the low-side zero-sequence fault current (both currents in amperes), then
the neutral current = 3[IH0 − IL0], IL0 = IH0 × n × k, where n is the voltage ratio and k is the
current distribution factor which gives the zero-sequence current contribution of the low-voltage
system. This expression reduces to