EARTHING TRANSFORMERS

EARTHING TRANSFORMER AND ITS APPLICATION TO THE POWER SYSTEMByG.PRIYADHARSHNI

What is earthing transformers?Earthing transformers are used to create a neutral point in a three-phase system, which provides possibility for neutral earthing.

Purpose of earthing transformer:An earthing transformer is usually associated with three phase supply systems.Earthing of any electrical system at the source is considered by most countries to be the safer practice with regard to personnel and equipment safety.

On a three phase system, the neutral would be earthed either directly or through some limiting impedance / resistance. When the neutral point is not available or does not exist with a delta secondary winding of the transformer, a neutral point needs to be created.

Design Aspects:1.Zig-Zag connection 2.Star/delta connection.

Zig-Zag connection:The design can be a transformer with just one winding, which is zigzag connected. The zero sequence impedance of such a winding is normally quite low, but it can be increased if the purpose is to limit the current through the transformer in case of an earth fault somewhere in the system.

During undisturbed system operation with balanced (symmetrical) voltages the current through the earthing transformer is small & of the same size as the magnetizing current. Unbalanced voltages will cause some higher currents flowing through the earthing transformer, which it must be capable to carry.

Star/delta connection:An alternative connection to the zigzag is star/delta connection where the delta connected winding will compensate the zero sequence magnetic fields so it will be confined to a leakage field between the star and the delta winding and make the zero sequence impedance of the transformer relatively small.

However, if it is desired to increase the zero sequence impedance, this can be achieved by opening the delta connection and insert a reactor or resistor.

Earthing can be done through:Arc supprestion reactor.Neutral grounding reactors.Current limiting reactors.

Arc suppression reactor:When the earthing transformer is going to be used together with an arc-suppression reactor, the rated current (and its duration) of the earthing transformer will be determined by the data for the arc-suppression reactor.

Neutral grounding reactors:Neutral Grounding Reactors are single phase reactors generally connected between ground and neutral of transformers.Neutral Grounding Reactors are used in order to limit maximum fault current to a value which will not damage the equipment in the power system, yet allow sufficient flow of fault current to operate protective relays to clear the fault.

Current limiting reactor:If the earthing transformer is used for directly earthing or through a current limiting reactor, the neutral current through the transformer will be high but the duration is limited to a few seconds. Earthing transformer must be designed to withstand the thermal and mechanical effects of the rated neutral current. The characteristic of earthing transformer should be selected to match the property of the system.

Functions of earthing transformers:Apart from providing the easy path to ground current during the earth fault,the following additional features are also be achievedHolding the neutral shift within the limits.Permitting the circulation of unbalanced load current in the neutral.To limit the current during line to earth fault.

To earth the system.To provide single phase line to ground load.Can be used wit resistance, reactance and arc suppresion coil.

Protection of earthing transformers:The system earth fault protection relay may not be effective at low currents. To prevent damage to earthing transformer by such current, oil thermometer with alarm/trip contacts is recommended. Alternatively, the value of maximum continuous earth fault current may be declared and sensitive protection provided.

Application of earthing transformers:Earthing transformers are used in utility distribution networks and in some power electronic converters in order to provide a neutral point in a three-wire system.

An application example: occasionally engineers use a combination of Y (wye or star), delta, and zigzag windings to achieve a vectorphase shift. For example, an electrical network may have a transmission network of 110 kV/33 kV star/star transformers, with 33 kV/11 kV delta/star for the high voltage distribution network.

If a transformation is required directly between the 110 kV/11 kV network the most obvious option is to use 110 kV/11 kV star/delta. The problem is that the 11 kV delta no longer has an earth reference point. Installing a zigzag transformer near the secondary side of the 110 kV/11 kV transformer provides the required earth reference point.

Uses of earthing transformer:A earthing transformer is used to provide a ground path to either an ungrounded "Y" or a delta connected system **Grounding transformers are typically used to:Provide a relatively low impedance path to ground, thereby maintaining the system neutral at or near ground potential

Limit the magnitude of transient over voltages when re-striking ground faults occurProvide a source of ground fault current during line-to-ground faultsPermit the connection of phase to neutral loads when desired

A typical example is the wind farm. They utlize earthing transformers for fault protection on ungrounded lines.When ground fault occurs on a collector cable causes the circuit breaker to open, the wind turbine string becomes isolated.Turbines do not detect the fault and the generators continue to energise the cable.

Voltage between un-faulted cable and the ground rise by 173%.The transformer, placed on the turbine string, provides the ground path.

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