Earthing Transformer

In cases where the neutral point of three phase system is not accessible like the system connected to the delta connected side of an electrical power transformer, an artificial neutral point may be created with help of a zigzag connected Earthing Transformer. This is a core type transformer with three limbs. Every phase winding in zigzag connection is divided into two equal halves. One half of which is wound on one limb and other half is wound on another limb of the core of transformer. End point of all three windings ultimately connected together and forms a common neutral point. Now if any fault occurs at any of the phases in delta connected system, the zero sequence fault current has close path of circulating through earth as shown in the figure. In normal condition of the system, the voltage across the winding of the Earthing Transformer is 1/V3 times of rated per phase voltage of the system.

But when single line to ground fault occurs on any phase of the system, as shown in the figure, zero sequence component of the earth fault current flows in the earth and returns to the electrical power system by way of earth star point of the Earthing Transformer. It gets divided equally in all the three phases. Hence, as shown in the figure,    the currents in the two different halves of two windings in the same limb of the core flow in opposite directions.  And therefore the magnetic (lox set up by these two currents will oppose and neutralize each other. As there is no increase in flux due to fault current, there is no extra dcp/dt means no extra voltage induced across the winding   and no choking effect occurs to impede the tow of fault current. So it can be concluded like that, the zigzag type Earthing or grounding transformer maintains the rated supply voltage at normal current as well as when a solid   single line to ground fault current flows through it.

The rated voltage of an Earthing or grounding transformer is the line to line voltage on which it is intended to be used. Current rating of this transformer is the maximum neutral current in Amperes that the transformer is designed to carry in fault condition for a specific time. Generally the time interval, for which transformer designed to carry the maximum fault current through it safely, is taken as 9 second.