Earthing Transformer

When the transformers or generators are delta connected or if the neutral points are not accessible then artificially the neutral earthing point can be created with the use of star connected earthing transformer. Such transformer has no secondary. Each phase of primary has two equal parts. There are three limbs and each limb has two windings providing opposite flux during normal condition. Such a transformer is shown in the Fig.1.

Fig. 1

       One set of windings are connected in star providing the neutral point. The other ends of this set of windings are connected to the second set of windings as shown in the Fig. 1. The directions of the currents in the two windings on each limb are opposite to each other. The small exciting current is circulated in the windings during normal operation. Under faulty condition, the transformer offers a low impedance path to the flow of zero phase sequence currents. The value of fault current is limited in some cases by the use of a resistor in series with the neutral earthing connection. This is necessary in systems with operating voltage between 2.2 KV and 3.3 KV.

       The transformers are of short time ratings in the range of 10 seconds to 1 minute. Hence the size of these transformers is small as compared to power transformers of small rating. If the earthing transformer is not available then a star-delta transformer is used.