A zig-zag transformer is used for providing grounding on the transformer. It provides insulation between the ground and the component so that the system component may not be affected by the fault currents.
The zig-zag transformer has no secondary winding. It is a three-limbed (branched) transformer. Each limbed has got two identical windings. One set of windings is connected in the star to provide the neutral point. The other ends of this set of windings are connected to the second set of windings as shown in the figure below. The direction of current in the two windings on each limb is opposite to each other.
Under normal operating conditions the total flux in each limb is negligibly small. Therefore, the transformer draws very small magnetizing current. Under fault conditions, the impedance of the grounding transformer is very low.
In order to limit the fault current, a resistor is connected in series with the neutral grounding. It is designed for a short-time kVA rating and for carrying the rated current for a very short time of 10 seconds.