- Earthing Transformer Definition: An earthing transformer, or grounding transformer, is used to create a neutral point in systems without one, such as delta-connected systems.
- Purpose and Function: It provides a return path for fault currents, ensuring safety and stability during ground faults.
- Zigzag Connection: Each phase winding is split and wound on different core limbs to form a neutral point and manage fault currents efficiently.
- Operation During Faults: During a ground fault, the fault current flows through the transformer’s neutral point, with opposing currents canceling magnetic flux to prevent impedance.
- Voltage and Current Ratings: The transformer’s rated voltage matches the system’s line-to-line voltage, and it can handle the maximum fault current for up to 30 seconds.
Stability on External Earth Fault(E/F) on Delta Side of Star-Delta Power Transformer
An earthing transformer creates a neutral point on a delta system so earth-fault current has a return path. If that transformer on the delta side is outside the protection zone, an earth fault in the delta system outside Current Transformer (CT) locations would create current distributions that circulate within the differential CT secondaries and miss the operating coils.
A zigzag or interconnected-star grounding transformer has high magnetising impedance in normal service, but for an earth fault the currents in windings on the same core cancel ampere-turns and the impedance is low.
When the neutral point of a three phase system is not accessible, as in a delta-connected electrical power transformer, an artificial neutral point can be created with 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 is wound on one limb and the other half is wound on another limb of the core of transformer.
The first half of the red phase winding is wound on the first limb of the core and the second half of the same red phase is wound on the third limb.
The first half of the yellow phase winding is wound on the second limb of the core and the second half of the same yellow phase is wound on the first limb.
The first half of the blue phase winding is wound on the third limb of the core and the second half of the same blue phase is wound on the second limb.
The ends of all three windings are connected together and form a common neutral point. If a fault occurs on any phase of the delta-connected system, the zero-sequence fault current has a closed path through earth as shown in the figure.
In normal system conditions, the voltage across the winding of the earthing transformer is 1/√3 times the rated per-phase voltage of the system.
When a single line-to-ground fault occurs on any phase of the system, as shown in the figure, the zero-sequence component of the earth-fault current flows in the earth and returns to the electrical power system by way of the earth star point of the earthing transformer. It divides equally in all three phases. As shown in the figure, the currents in the two different halves of two windings on the same limb of the core flow in opposite directions. 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, so no choking effect impedes the flow of fault current. A zigzag earthing or grounding transformer therefore holds rated supply voltage at normal current and also 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 a fault for a stated time. This page takes that interval, for which the transformer is designed to carry the maximum fault current safely, as 30 seconds.
