Neutral Earthing Resistors (NER) and Reactors: Application Guide
Overview: Neutral Earthing Resistors (NER) and Neutral Earthing Reactors are devices used in electrical power systems to limit fault currents that arise during ground faults. These components serve critical roles in maintaining system stability, minimizing damage, and improving safety. Both have distinct applications, and their selection depends on system requirements such as fault current limitation, insulation levels, and system grounding practices.
Neutral Earthing Resistors (NERs)
Purpose: NERs are used to limit the magnitude of fault current during a single line-to-ground fault. They offer a high level of control over ground fault currents in medium- and high-voltage power systems. By limiting the fault current, NERs help prevent damage to equipment such as transformers, generators, and cables, reduce voltage dips during faults, and improve overall system reliability.
Applications:
- Industrial Power Systems: NERs are widely used in industrial networks to control ground fault currents in generators and transformers.
- Substations and Distribution Networks: In substations and distribution systems, NERs are applied to protect transformers and reduce the severity of ground faults.
- Protection and Safety: NERs improve safety by limiting touch and step potential during faults, reducing the likelihood of arc flash incidents.
Key Features:
- Fault Current Limitation: NERs are designed to limit the fault current to safe levels (typically 50–400A), reducing the stress on equipment.
- Thermal Management: NERs are designed to handle thermal stresses during short-duration faults. They are rated to withstand faults for a few seconds.
- Material Construction: They are typically made from stainless steel or special alloys to withstand high temperatures and offer corrosion resistance.
Sizing Considerations:
- Fault Duration: The resistors must be sized based on fault duration (e.g., 1 second, 10 seconds, etc.).
- System Voltage: The resistor is selected based on the system’s operating voltage, typically in medium-voltage (1kV to 36kV) or high-voltage systems.
- Fault Current Rating: The resistors are chosen to limit the fault current to a specific level based on the protection requirements.
Neutral Earthing Reactors (NERs)
Purpose: Neutral Earthing Reactors limit the ground fault current by adding inductive impedance to the system. They are used in high-voltage systems where fault current levels need to be controlled, but the use of resistors may not be appropriate or desirable due to thermal or operational constraints.
Applications:
- High-Voltage Transmission Systems: Neutral Earthing Reactors are commonly used in high-voltage transmission networks to limit the magnitude of ground fault currents while maintaining system stability.
- Generator Protection: In power generation plants, neutral reactors are used to limit fault currents without the excessive heating issues associated with resistors.
- Capacitor and Reactor Banks: When used in conjunction with reactor banks, they help manage resonant conditions that could amplify fault currents.
Key Features:
- Inductive Reactance: Reactors provide a reactance that limits the fault current based on the system’s operating frequency.
- Continuous Operation: Unlike resistors, reactors can remain in continuous service, as they do not produce excessive heat during faults.
- Harmonic Filtering: In some cases, reactors also help in controlling harmonics in the system.
Sizing Considerations:
- System Voltage: Reactors must be rated for the system’s operating voltage.
- Inductive Reactance: The value of the reactor’s inductive reactance is selected to limit the fault current to the desired level.
- System Configuration: The system's grounding arrangement (e.g., solidly grounded, resonant-grounded, or impedance-grounded) influences the choice and design of the reactor.
Comparison of NERs and Reactors
| Feature | Neutral Earthing Resistor (NER) | Neutral Earthing Reactor |
|---|---|---|
| Fault Current Limitation | Provides precise control of fault current by limiting the magnitude. | Provides moderate limitation by adding inductive reactance. |
| Thermal Impact | Produces heat during faults; rated for short-term fault duty. | Minimal heat generation; can remain in service continuously. |
| Use Case | Best for medium-voltage systems and applications where precise control over fault current is needed. | Best for high-voltage systems and continuous service with moderate fault current limitation. |
| Harmonic Control | Does not control harmonics. | Can help control harmonic resonance in some applications. |
Selection Criteria for NERs and Reactors:
- System Voltage: For medium-voltage systems, resistors are more common, while reactors are more prevalent in high-voltage networks.
- Fault Current Magnitude: For systems that need precise fault current control, resistors are preferred. Reactors provide a more moderate limitation.
- Thermal Requirements: Resistors have limitations due to heat generation, while reactors can operate continuously.
- System Type: Systems with continuous service requirements or harmonic considerations may favor reactors, while systems with specific fault current limitations may opt for resistors.
Conclusion:
Both Neutral Earthing Resistors and Reactors are essential components in electrical power systems for controlling ground fault currents. Their selection depends on factors such as system voltage, fault current requirements, and operational constraints. While resistors offer precise control over fault current, reactors are advantageous for high-voltage applications and continuous service. Proper sizing and selection of these components are critical to ensure system reliability, protection, and safety.
Neutral Earthing Resistors (NER) or Reactors Calculation
Neutral Earthing Resistors (NER) and Neutral Earthing Reactors are used to limit fault currents in electrical power systems. Proper calculation and design of these devices are essential for protecting equipment and ensuring system stability during ground faults. Below, we explain their functions and methods for calculating the required values for these devices.
1. Neutral Earthing Resistors (NER) Calculation
Neutral Earthing Resistors are connected between the neutral point of a transformer or generator and earth (ground). Their primary function is to limit the ground fault current to safe levels, thus minimizing equipment damage and improving personnel safety.
Key Design Parameters:
- System Voltage (V_L-L): The line-to-line system voltage.
- Fault Current (I_F): The desired fault current limit. This is typically set based on system design, safety, and protection needs.
- Fault Duration (t): The time for which the fault is allowed to persist before protection systems isolate the fault (usually in seconds).
- Temperature Rise: The maximum allowable temperature rise of the resistor during the fault period.
- Resistor Rating: Continuous and short-time (e.g., 10 seconds) ratings for current and voltage.
2. Neutral Earthing Reactors Calculation
Neutral Earthing Reactors, like resistors, are used to limit ground fault currents, but they function by adding inductive reactance rather than resistance to the circuit. Reactors are typically used in high-voltage systems to limit fault currents while allowing some level of fault current for reliable fault detection.
Key Design Parameters:
- System Voltage (V_L-L): The line-to-line voltage.
- Fault Current (I_F): The desired fault current during a single-line-to-ground (SLG) fault.
- Reactance (X_L): The inductive reactance to limit the fault current.
- Frequency (f): The system frequency (typically 50 Hz or 60 Hz).
Inductance (L):
The inductance of the reactor can be calculated using:Where:
- is the inductance (in Henry),
- is the system frequency in Hz (50 Hz or 60 Hz).
3. Comparison of NER and Reactors
- Resistors dissipate energy as heat and limit the current by a fixed value during faults.
- Reactors limit the current by adding inductive reactance, which provides a lower loss method of fault current limiting but can allow slightly higher fault currents.
- Reactors are generally used in high-voltage transmission systems, while resistors are more common in lower voltage and medium-voltage systems.
4. Considerations for Selection
- System Voltage and Fault Current: Determines whether a resistor or reactor is best suited for the application.
- Thermal and Mechanical Strength: The chosen resistor or reactor must withstand the thermal and mechanical stress during fault conditions.
- Ground Fault Protection Schemes: The method of neutral earthing affects the choice, especially whether fault currents are solidly grounded or impedance grounded.
Conclusion:
Accurate calculation of Neutral Earthing Resistors or Reactors is critical for controlling fault currents in electrical systems, ensuring protection for both equipment and personnel. Each system's voltage, desired fault current limit, and protection requirements dictate the size and type of NER or reactor used.
