Transformers
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. This is the purpose of the earthing transformer, which could consist of a zig-zag winding, or a two winding star delta transformer where the star winding of correct voltage supplies an accessible neutral point when connected to the supply system
Earthing transformers are used to create a neutral point in a three Phase system, which provides possibility for neutral earthing. The earthing can be through an arc suppression reactor, a neutral earthing reactor or resistor or directly. 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. 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 It is possible to provide the earthing transformer with a secondary winding for continuous auxiliary station supply.
Earthing transformers are usually oil immersed and may be installed outdoor. In cases where a separate reactor is connected between the transformer neutral and earth, the reactor and the transformer can be incorporated in the same tank. 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.
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.Most of the time in service the loading of earthing transformers is very low. It is the randomly occurring short –duration currents that cause any heating of significance. Ageing of the cellulose.
Uses of Earthing transformer:
The primary use of earthing transformers is to provide a low-impedance path to ground during fault conditions in electrical distribution systems. This serves to protect equipment and personnel by effectively dissipating fault currents. Additionally, earthing transformers are utilized to establish a neutral point for grounding in ungrounded systems or to supplement grounding in solidly grounded systems. They play a crucial role in industrial facilities, commercial buildings, and utility substations, ensuring electrical safety and system reliability.
Working of Earthing Transformer
The working of an earthing transformer involves several key steps:
- Normal Operation: During normal operation, the primary winding of the earthing transformer is connected to the system’s phase conductors, while the secondary winding is grounded. In this state, the transformer operates similar to a conventional transformer, stepping voltage up or down as required.
- Fault Condition: When a fault occurs in the system, such as a line-to-ground fault, fault currents flow through the earthing transformer’s secondary winding to ground. This creates a low-impedance path for fault currents to safely dissipate, preventing damage to equipment and minimizing the risk of electrical hazards.
- Limiting Fault Currents: The impedance of the earthing transformer, along with any additional grounding resistors or reactors, limits the magnitude of fault currents flowing through the system. By controlling fault currents, the earthing transformer helps maintain system stability and protects sensitive equipment from damage.
- Safety and Protection: By providing a reliable path to ground, the earthing transformer ensures the safety of personnel and equipment within the electrical system. It helps prevent electrical shocks, fires, and other hazards associated with fault conditions, contributing to a safer working environment and improved system reliability.
Design and Construction of Dry-Type Transformers:
The components used in making an earthing transformer typically include:
- Core: The core is typically made of high-permeability magnetic materials such as laminated silicon steel. It provides a path for the magnetic flux generated by the primary and secondary windings.
- Primary Winding: The primary winding is typically made of copper or aluminum conductors and is connected to the system’s phase conductors. It carries the primary current and induces a voltage in the secondary winding.
- Secondary Winding: The secondary winding is also made of copper or aluminum conductors and is connected to the ground. It provides a low-impedance path for fault currents to dissipate safely to ground.
- Winding Insulation: Insulating materials are used to insulate the primary and secondary windings from each other and from the transformer core to prevent electrical breakdown.
- Grounding Resistor or Reactor (Optional): In some cases, grounding resistors or reactors may be included in the design to limit the magnitude of fault currents and ensure proper grounding performance.
- Enclosure: The transformer may be housed in an enclosure for protection against environmental factors such as moisture, dust, and physical damage. These components work together to create a safe and effective grounding system in electrical distribution systems, providing protection against electrical faults and ensuring the safety of personnel and equipment.
