In a three-phase autotransformer a single star winding shares both the primary and secondary sides. Suppose an asymmetrical disturbance (unbalanced condition or fault) 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.
For example, suppose an earth fault occurs in any one of the phases. Then, obviously, a zero-sequence current starts flowing through the winding. The zero-sequence current may saturate the core temporarily. This flux saturation may distort the voltage waveforms of the other two phases.
Function of Tertiary Winding or Stabilizer Winding
Now, we shall place a delta-connected tertiary winding in the autotransformer core. Suppose, due to the asymmetrical fault, a zero-sequence current flows through the transformer winding. That means the current, having the same phase and equal magnitude, flows through the winding of each of the three phases. So, it produces zero-sequence flux in the transformer core. In other words, the same flux, with the same phase, appears in each limb of the winding.
We place the tertiary winding on these limbs. Here, we connect each limb of the tertiary winding in series with proper polarities. This arrangement forms a closed delta. The alternatively varying zero sequence flux causes equal co-phasal currents in all phases of the tertiary winding. The closed delta tertiary circulates these zero sequence currents in it. Obviously, these circulating currents produce a counter flux (an opposing flux) in all three limbs of the transformer. Thus, the flux of each limb produced by the tertiary cancels the main zero sequence flux of the same limb. Therefore, the net flux in each limb of the transformer core remains nearly unchanged.
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.
Also, in the same way a tertiary winding isolates the fault of one voltage level of the transformer to other voltage level.
Is Tertiary Winding Mandatory for a Star – Star Transformer?