An earthing transformer, also known as a grounding transformer or neutral grounding transformer, is a specialized type of transformer used in electrical power systems to provide a low-impedance path for the flow of fault currents to the earth ground. Its primary purpose is to facilitate the grounding or earthing of the neutral point of a power system.
Here are some key points about earthing transformers:
Neutral Grounding:
In a power system, the neutral point grounded to maintain system stability and safety. Grounding helps limit overvoltages, provides a return path for fault currents, and protects equipment and personnel from dangerous voltages during faults.
Zigzag or Wye-Delta Connection:
Earthing transformers are commonly constructed with a zigzag or wye-delta winding configuration. The primary winding is connected in a zigzag or wye configuration, while the secondary winding is typically connected in a delta configuration.
Voltage Level:
Earthing transformers usually rated for the system voltage level and installed between the neutral point and the earth ground. They are designed to handle the fault currents and maintain system voltage stability during ground faults.
Neutral Grounding Methods:
Earthing transformers can be used for different neutral grounding methods, including solid grounding, resistance grounding, and reactance grounding. The choice of grounding method depends on the system requirements, fault current levels, and operational considerations.
Fault Current Limitation:
During a ground fault, the earthing transformer provides a low-impedance path for the fault current to flow. By limiting the fault current magnitude, it helps protect equipment from thermal and mechanical stress and reduces the potential for damage.
Voltage Regulation:
Earthing transformers also play a role in voltage regulation by providing a path for zero-sequence currents. They help control system voltage levels and mitigate issues such as unbalanced voltages and excessive neutral currents.
Resonance Control:
In systems with high zero-sequence impedance or ungrounded systems, the earthing transformer can be used to control or mitigate the risk of resonance conditions that can occur due to the interaction of system capacitance and inductance.
Auxiliary Services:
Earthing transformers may also be used to provide auxiliary services, such as supplying power to auxiliary loads, providing voltage for metering, or isolation for sensitive equipment.
Size and Ratings:
The size and ratings of the earthing transformer depend on the system parameters, fault current levels, and neutral grounding. Earthing transformers designed to handle the expected fault currents and provide adequate impedance to limit fault current magnitude.
Residual Voltage Compensation:
Earthing transformers can be used to compensate for residual voltages that may occur in ungrounded or impedance-grounded systems. By connecting the neutral point to the earthing transformer, the residual voltage can be reduced or eliminated, improving system operation and reducing the risk of insulation failure.
Ground Fault Detection:
Earthing transformers play a crucial role in ground fault detection and protection systems. By providing a low-impedance path for fault currents, they enable the operation of protective relays and devices that detect and isolate ground faults.
Size and Impedance:
The size and impedance of an earthing transformer are determined based on various factors, including the fault current level, system voltage, and the desired level of fault current limitation. The impedance of the transformer shall be carefully selected to ensure proper operation and coordination with protective devices.
Installation Considerations:
Earthing transformers are typically installed at the neutral point of the power system, often in a substation or at the generator’s neutral point. Proper installation and connection are essential to ensure effective grounding and fault current flow.
Transient Behavior:
During fault conditions, earthing transformers may experience transient phenomena such as inrush currents, ferro resonance, or overvoltage. These transient behaviors should be considered during the design and selection of earthing transformers to ensure their reliable operation.
Earthing Transformer in Arc Furnaces:
In arc furnace applications, where high fault currents and severe operating conditions are present, earthing transformers are commonly used for grounding the furnace transformer secondary winding. They ensure the reliable operation of the arc furnace and provide a safe path for fault currents during faults.
Remote Earth Fault Detection:
Earthing transformers can be equipped with additional devices or schemes for remote earth fault detection. These systems monitor the neutral current or voltage and provide an indication or alarm when an earth fault occurs in a remote part of the power system.
Testing and Maintenance:
Regular testing and maintenance of earthing transformers are essential to ensure their proper functioning. This may include insulation resistance testing, winding resistance measurements, oil analysis (if applicable), and visual inspections.
International Standards:
Earthing transformers are designed and manufactured in accordance with international standards, such as IEC 60076 and IEEE C57.12.00. These standards provide guidelines for the design, testing, and installation of transformers, including earthing transformers.
Safety Considerations:
When working with earthing transformers or any electrical equipment, safety should be a top priority. Proper safety procedures, such as wearing personal protective equipment, following lockout/tagout procedures, and working with qualified personnel, should be observed to minimize the risk of electrical hazards.
Earthing transformers are an essential component in power systems, particularly in high-voltage and medium-voltage applications. They help maintain system stability, protect equipment, and ensure the safety of personnel by providing a controlled and reliable path for fault currents to the earth ground. The design, installation, and operation of earthing transformers should comply with relevant standards, codes, and regulations to ensure effective grounding and system performance.