A three-phase autotransformer has a star winding on both the primary and secondary sides. Suppose an imbalance arises in the three-phase current. It may happen on either the primary or the secondary side. In both cases, it produces zero-sequence current through the winding.
For example, suppose an earth fault occurs in any one of the phases. Then, obviously, strong zero-sequence currents start flowing through the winding in both the primary and secondary. This is because, in an autotransformer, both windings are common. Because of these zero sequence currents, the core may become temporarily saturated. As a result, the voltage waveforms of the other two phases will be disturbed and distorted.
Function of Tertiary Winding or Stabilizer Winding
Now, we shall place a delta-connected tertiary winding in the autotransformer core. Suppose, due to a through fault, a zero-sequence current flows through the transformer windings. That means the current, having the same phase and equal magnitude, flows through the winding of each of the three phases. As a result, zero-sequence flux is produced in the transformer. This means that the same flux, with the same phase, appears in the R-phase limb, Y-phase limb, and B-phase limb at the same time.
The tertiary winding is also placed on these limbs. So, the equal co-phasal currents are induced in all phases of the tertiary winding. Since the tertiary winding is delta-connected, it forms a closed path. Hence, the zero sequence currents start circulating within the winding. Obviously, this circulating current produces a counterflux (an opposing flux) in the three limbs of the transformer. Thus, the fluxes produced by the tertiary winding cancel the main zero sequence fluxes. Therefore, the net flux in each limb of the transformer core remains nearly unchanged even during the occurrence of an electrical fault.
As a result, the core does not enter the saturation region. The waveform of the phase voltages does not become distorted. Therefore, during a fault in one phase, the other two phases remain stable. That is why we refer to a tertiary winding as a stabilizer winding.