A grounding transformer or earthing transformer it is a special – purpose transformer with a Zigzag or interconnected star winding. Transformer used in three-phase electric power systems to provide a ground path to either an ungrounded wye or a delta-connected system. Provide a source of ground fault current during line-to-ground faults.
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The grounding of the system can be done in the following way...
In a case of delta-star grounding transformer, the delta side is closed to provide a path for zero-sequence current. The star winding must be of the same voltage rating as the circuit that is to be grounded, whereas the delta voltage rating can be chosen to be any standard voltage level.
The selection of the type of grounding depends on the type of the system and its voltage levels. The following considerations are made for the selection of the grounding.
Generally, solid grounding is used for a low-voltage system up to 600V. For voltages up to 11KV resistance grounding is used.
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. Zigzag transformer terminates the harmonics of the power system. It also protects the power system by reducing the stress of the voltage under a fault condition.
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.