When a transformer is used for the purpose of providing a neutral point for grounding purpose in a system where the neutral point of a three phase system are not available or where the transformers or generators are delta connected, that type of transformer is called Earthing or neutral grounding transformer.
The Earthing or neutral grounding transformer may be two winding with a zig- zag connected primary and a star connected secondary or a single winding three phase auto-transformer with windings interconnected star or zig-zag. Earthing transformer is a three limbed core type transformer having two equally balanced windings on each core. One set of windings is connected in stare to provide the neutral point. The others ends of this set of windings are connected to the second set of windings as shown in figure.
The distribution of currents in the various windings of the earthing transformer, under the single line to a ground fault condition on phase B, is shown in fig. The Earth fault current flowing in the earth returns to the power system by way of the earth star point of the earthing transformer. It gets divided equally into all the three phases. It would be apparent from the figure that the currents in the two windings of the same limb flow in opposite directions. Consequently, the magnetic flux set up by the currents in the two windings will neutralize each other. No chocking effect occurs to hamper the flow of fault current.
Zig-Zag type earthing transformers are designed on the base of rated normal current flows when a solid single line to ground fault is applied at transformer terminals. It is common to select the current rating of the earthing transformer equal to the full load current rating of the largest generator or transformer unit. The KVA rating of a three phase earthing transformer is the product of normal line to neutral voltage (KV ) and the neutral current in amperes that the transformer is designed to carry under fault conditions for a specified time. The two most common time intervals specified for the duration of the earth fault current are 30 and 60 seconds. Unless the system conditions or protective system applied warrant it is quite adequate to specify the rating for 30 seconds only.