Types and Uses of Earthing Transformer

Types of Earthing Transformer

Basically two types of earthing is preferred in case of transformer.

Body and secondary is earthed separately.

Body of the transformer consists of the oil tank , insulation, tapped changer , conservator , breather etc . During any fault condition leakage current can flow through the body and then if any one touch the body then there is a chance of receiving severe shock.

Neutral grounding is done in case of small KVA transformers in case of power transformer resistance grounding is preferred under faulty condition or unbalanced loading condition current flows through the neutral wire that current should be grounded. Grounding is done by GI rod or Galvanized iron rod . In resistance grounding three phase resistors are connected to the respective phases and it creates a voltage drop thus reduce the grounding current.

Arcing ground is also preferred . It reduce the phase current by increasing the phase voltage √3 times of its normal value.

Note:- Don't use inductor or capacitor in grounding purpose it will seriously effect the stability of the transformer.

Uses of Earthing Transformer

A good power system should be capable of dealing with all kinds of abnormal situations including;

(1) Presence of harmonics due to peaky magnetising current drawn by transformers and due to non-linear loads.

(2) Presence of zero sequence currents due to load unbalance.

(3) Unbalanced current due to line-to-ground faults.

All the above situations call for neutral grounding because a wire connected between neutral (generator neutral or transformer neutral) and earth provides a path for third harmonic currents, zero sequence currents and ground fault currents. Absence of neutral ground path leads to abnormal or unbalanced voltages or waveform distortion.

There are situations when grounding is needed but a neutral is just not available e.g. if the winding is delta connected. Under such conditions, an earthing transformer is installed at appropriate location to create a neutral which is suitably grounded.

Though called a transformer, construction wise an earthing transformer is basically a three phase zig zag connected reactor. Each phase winding has two parts; each part wound on a different limb of a three limb core. The two parts of each phase coil are connected in such a way that the earthing transformer offers very low impedance to zero sequence currents and high impedance to positive and negative sequence currents. Thus it offers a good solution for neutral grounding where a neutral is otherwise not available.