If a neutral point is
required or not available in case of delta connections and bus bar
points, a zig-zig transformer is used. Earthed transformer are used for
providing the neutral pint for such cases. It is a core-type transformer
having three limbs built-up in the same manner as that of a power
transformer. Each limb accommodates two equally-spaced windings and the
way they are connected is shown in the Fig. 1. It will be seen that the
current in the two halves of the winding on each limb acts in opposite
directions. These currents do not allow undeserving harmonics to prevail
in the circuit, and thereby, the stresses on the insulation of the
transformer are considerably reduced.
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Fig. 1 Representation of an earthing transformer |
The impedance of the earthing transformers is quite low, and
therefore, the fault current will be quite high. The magnitude of the
fault current is limited by inserting resistance either in the neutral
circuit as shown in Fig. 2 or in the windings of the earthing
transformer. Components of various currents flowing under the conditions
are also shown therein.
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Fig. 2 Insertion of resistance in the neutral circuit |
The terminals of the earthing transformers are soldered to the
power transformer for obtaining a solid connection between them. The
capacity of the earthing transformer is denoted by the fault current it
is capable of handling. Under normal operating conditions, it is only
iron losses that are continuously present; copper losses are present
only when the fault occurs. These copper losses are present only for
short periods due to the short duration of fault (in the order of a few
seconds).
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