The neutral or star point is usually available at every voltage level form generator or transformer neutral .However if no such point is available due to delta connections or if neutral point is desired on bus-bars,a Zig-Zag transformer is most commonly used .Zig-Zag transformer has no secondary .Zig-Zag transformer is core type transformer having three limbs.The connections of such Transformer is illustrated below.It is seen that the currents in the two halves of the winding on each limb are in opposite directions.So there will not be any undesirable harmonic prevailing in the circuit and so stresses on the insulation of the transformer are considerably reduced.
The fault current is quite high as the impedance of earthing transformer is quite low.The resistance or resistances are inserted either in the neutral circuit or in the windings of earthing transformer in order to limit the magnitude of the fault current.
Under normal conditions,only iron losses will be continuously occurring at the time of fault copper losses will also in addition,because of heavy fault current,in the earthing transformer.The duration of occurrence of copper losses is quite short which is usually 30-60 second duration.
As compared to the power transformer the rating of a Zig-Zag transformer is quite different.A grounding transformer is usually specified by the single-phase fault current that it handles.The two most common intervals specified for the fault current are 30 and 60 s. A 60 s grounding transformer will be more expensive as compared to a 30 one.
In case if a Zig-Zag transformer is not available, a star-delta transformer can be used without loading the delta side.Star delta transformer is a step-down transformer.The star connected primaries are connected to the bus-bars and its neutral is grounded.The secondaries are delta-connected and generally do not supply any load but provide a closed path for triple harmonic currents to circulate in them.Under normal operation the current in the grounding transformer is only its own exciting current.However,in the event of a single-line-to -ground fault condition,large current may flow.So sufficient rating is necessary to handle the fault current.
Fig:Star-delta transformer grounding