Grounding Transformer or Earthing Transformer

Earthing

transformer on the Delta Side is outsides the Zone of protection the Earth

Fault(E/F)in the delta system outside Current Transformer(CT)

locations would produce current distributions as shown

which circulate within the differential CT secondaries and is kept out of

operating coils.

Zig-Zag

or inter connected star grounding transformer has

normal magnetising impedance of high value but for E/F, currents flow in

windings of the same - core in such a manner that the ampere turn cancel and

hence offer lower impedance.

In

cases where the neutral point of three phase system is not

accessible like the system connected to the delta connected side of a 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.

1st

half of Red phase winding is wound on the 1st limb of the core and 2nd half of

same Red phase is wound on 3rd limb.

1st

half of Yellow phase winding is wound on the 2nd limb of the core and 2nd half

of same Yellow phase is wound on 1st limb.

1st

half of Blue phase winding is wound on the 3rd limb of the core and 2nd half of

same Blue phase is wound on 2nd limb.

End point of all three winding 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/√3 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 flux 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 change of dφ/dt means no

concluded like that, the zigzag type earthing or grounding

choking effect occurs to impede the flow of fault current. So it can be

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 30 second.