What is Earthing Transformer or Grounding Transformer ?

Stability on External Earth Fault(E/F) on Delta Side of Star-Delta Power Transformer

If the earthing transformer on the Delta side is outside the protection zone, an Earth Fault (E/F) in the delta system outside Current Transformer (CT) locations would create current distributions that circulate within the differential CT secondaries, avoiding the 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.

When the neutral point of a three phase system is not accessible, like in a delta-connected electrical power transformer, an artificial neutral point can be created using 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 choking effect occurs to impede the flow 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 30 second.

FAQ

What is a grounding or earthing transformer?

A grounding transformer or earthing transformer is a type of auxiliary transformer used in three-phase electric power systems to provide a ground path to either an ungrounded wye or a delta-connected system. Grounding transformers are part of an earthing system of the network

What is the purpose of the earth transformer?

Earthing transformers are also used in electrical systems to reduce voltage fluctuations and balance load distribution. They can be used in electrical systems where there are unbalanced loads or where there are large variations in load demand

What type of transformer is earthing?

Types of Earthing Transformers: There are various types of earthing transformers, including solidly grounded, impedance grounded, and resistance grounded transformers. The choice of the type depends on the specific requirements of the power system and the degree of fault protection needed

What is the earthing current transformer?

Grounding of Current Transformers
According to VDE 0414, current and voltage transformers should be secondary grounded from a series voltage of 3.6 kV. For low voltage, grounding can be omitted if the current transformers do not have large metal contact surfaces

What is the difference between grounding and earthing?

The earthing is for the connection of the non-current carrying part to the earth. Whereas, in grounding the current-carrying part directly connected to the ground. The grounding is responsible for load balancing and earthing is responsible for protection from electrical shock

Where are grounding transformers used?

Grounding transformers are essential for large multi-turbine wind farms, where the substation transformer frequently provides the sole ground source for the distribution system. A grounding transformer placed on the turbine string provides a ground path in the event the string becomes isolated from the system ground

How many earthing in transformer?

At least two earthing connections are required for a healthy distribution transformer system. Distribution transformers require earthing to ensure safety and proper functioning.At least two earthing co… 5 earthing, 3 for transformer body and 2 for neutral

Which side of transformer is earthed?

secondary terminal – The secondary terminal of a current transformer is typically grounded for safety reasons. Current transformers are commonly used to measure high currents in electrical power systems