8.6.1 Multi-winding transformer protection2
Although we consider multi-winding transformers in this section, which deals with three-phase
transformers, similar considerations hold for single-phase transformers as well. Consider the threewinding
transformer shown in Figure 8.13. One winding is assumed to be delta connected, while
the other two are assumed to be wye connected. The CTs must of course be connected in wye
on the delta side and in delta on the wye side of the power transformer. This will ensure that the
phase shifts created in the currents of the power transformer are compensated by the CTs, so that
the secondary currents are once again in phase. The CT ratios are chosen so that when any two
windings are in service, equal secondary currents are produced. The third winding is assumed to be
open-circuited under these conditions. The ratios of all three CTs are chosen in this fashion. The
phasing of the CT and relay connections is checked as in Example 8.4.
It is interesting to note that under certain conditions a two-winding differential relay can be used
to protect a three-winding transformer. If the transformer is connected to a source only on one