Why Neutral Grounding Resistors (NGR) Are Essential in Power Systems?

1. Reason
2. Types
3. Configuration
4. Monitoring and Maintenance
5. Application

Why Neutral Grounding Resistors (NGR) Are Essential in Power Systems

A Neutral Grounding Resistor (NGR) is a key component in power systems, primarily used to control the fault current during a ground fault. Here’s why NGRs are essential for safe and reliable power system operation:

1. Limiting Fault Current :-

Technical Detail: NGRs are designed with a specific resistance value to limit the ground fault current to a predetermined level. The resistance is typically chosen based on the system’s voltage and desired fault current. For example, in a medium-voltage system (e.g., 11kV), an NGR might limit the fault current to 10A or 50A, depending on the design criteria.

Formula: –

If = VLL/ root 2 * R ngr

The fault current I_f during a ground fault can be calculated using Ohm’s Law:

Where VLL is the line-to-line voltage and RNGR is the resistance of the NGR.

2. Protecting Equipment

• Example: In an ungrounded or high-resistance grounded system, fault currents might be too low to trip overcurrent protection devices. An NGR ensures that the current is within a range that can be detected and acted upon by protective relays.

3. Reducing Voltage Stress

• Example: In systems with sensitive electronic equipment, controlling voltage transients is critical to prevent insulation breakdown and prolong equipment life.

3. Ensuring System Stability:-

• The presence of an NGR ensures that ground faults result in a controlled, detectable fault current, allowing protective devices such as relays to operate correctly. Without an NGR, a ground fault might go undetected, especially in ungrounded systems, leading to prolonged instability and the possibility of cascading failures.

4. Minimizing Arc Flash Hazards:-

• Arc flash is a serious hazard that occurs when an electrical fault causes an electric arc, resulting in intense heat, light, and pressure. The energy released during an arc flash is proportional to the fault current and the duration of the fault. By limiting the fault current, an NGR reduces the potential energy of an arc flash, thereby lowering the risk to personnel.

• Calculation: The incident energy E (in cal/cm²) during an arc flash can be estimated using:

Where k is a constant based on the system configuration, I_f is the fault current,

t is the duration of the fault, and D is the distance from the arc. By reducing I_f, the incident energy is significantly lowered.

• Impacts on System Design
  • System Neutral Management: The use of an NGR affects how the neutral point of a transformer or generator is handled. In systems with an NGR, the neutral is typically brought out and connected to ground through the resistor, which contrasts with solidly grounded or ungrounded systems.
  • Grounding Practices: The decision to use an NGR involves considerations of system grounding practices, fault current levels, and the need for continuity of service during faults. High-resistance grounding (HRG) is commonly employed in industrial systems where continuous operation is critical, and the NGR allows for a controlled, manageable fault response.

NGR Sizing and Selection

• Resistance Value Selection: The resistance of the NGR must be carefully selected based on the system voltage and the desired fault current. The goal is to ensure that the fault current is high enough to be detectable by protective relays but low enough to avoid damage to equipment.

• Formula: The resistance R_NGR is selected using:

where V_L−N is the line-to-neutral voltage, and I_fault is the desired fault current.

• Power Dissipation: The NGR must be capable of dissipating the energy associated with the fault current without overheating. The power rating is calculated using:

The resistor must be able to handle this power for the duration of the fault, which could be several seconds.

• Impact on Protection Schemes
  • Relay Sensitivity and Coordination: – NGRs affect the settings and coordination of protective relays. The fault current must be within the detection range of the ground fault relays (often a few amperes in high-resistance systems).
  • Ground Fault Relays: -These are typically set to detect currents that are slightly above the expected ground fault current. If the NGR limits the current to 10A, the relay might be set to operate at 5A to 8A to ensure reliable detection without false trips.
  • Time-Current Characteristics: -The time-current characteristics of the protective devices must be coordinated with the NGR to ensure that the fault is cleared quickly but without unnecessary tripping of the system. The NGR allows for more precise coordination by controlling the fault current magnitude.

Role in Different System Grounding Configurations

• High-Resistance Grounding (HRG): In HRG systems, the NGR is designed to limit the ground fault current to a very low value (e.g., 5-10A). This allows the system to continue operating with a single ground fault, which is crucial in processes where downtime is costly.

• Low-Resistance Grounding (LRG): In LRG systems, the NGR allows higher fault currents (e.g., 100-600A) to ensure prompt detection and isolation of the fault. This is typically used in systems where fault clearing is more important than continuous operation.

• Ungrounded Systems: In ungrounded systems, the line-to-ground voltages can become unbalanced during a fault, leading to potential over-voltage conditions. NGRs are used to convert these systems into high-resistance grounded systems, which stabilize the voltage and limit fault currents.

• Solidly Grounded Systems: These systems do not use NGRs, resulting in very high fault currents. While this allows for fast fault clearing, it also increases the risk of damage and requires robust equipment and protection systems.
• Transient Overvoltages and Resonance
  • Transient Overvoltage Control: During ground faults, especially in ungrounded or lightly grounded systems, transient overvoltages can occur due to system capacitance and inductance. An NGR helps dampen these oscillations, reducing the peak voltages that the system must withstand.
  • Resonance Conditions: In some systems, resonance can occur between the system capacitance to ground and the inductance of the NGR. This can lead to amplified voltages and currents. NGRs must be designed to avoid these resonant conditions, or additional damping resistors may be added to control resonance.
• Arc Flash Considerations
  • Arc Flash Energy Reduction: The energy released during an arc flash is proportional to the square of the fault current. By limiting the fault current with an NGR, the incident energy is reduced, thereby lowering the risk and severity of arc flash incidents.
  • Incident Energy Analysis: Electrical engineers perform incident energy analysis using software tools to calculate the potential energy during an arc flash event. By incorporating the NGR’s fault current limitation into these calculations, the required personal protective equipment (PPE) ratings can be reduced, improving safety and comfort for workers.

Grounding Resistor Design Considerations

1.Resistor Material: NGRs are typically constructed from stainless steel or other high-resistance alloys that can withstand the thermal and mechanical stresses associated with fault currents. The material choice affects the temperature rise and longevity of the resistor.

2.Cooling Methods: Some NGRs are naturally air-cooled, while others may require forced air or even liquid cooling in higher power applications. The cooling method must be adequate to prevent overheating during prolonged fault conditions.

3.Enclosure Ratings: NGRs are often installed outdoors or in harsh environments, so they must be housed in enclosures with appropriate ingress protection (IP) ratings. The enclosure also needs to provide adequate ventilation for heat dissipation while protecting the resistor from environmental conditions.

Monitoring and Maintenance

1.Continuous Monitoring: Modern NGR systems may include monitoring equipment to continuously measure the resistance value, temperature, and fault current. This helps in early detection of potential issues like resistor degradation or open circuits.

2.Periodic Testing: Regular testing of the NGR, including insulation resistance testing and thermal imaging, ensures that the resistor remains in good condition and capable of performing its function during a fault.

Case Studies and Applications

Industrial Applications: In industrial plants, especially in mining, oil & gas, and chemical industries, NGRs are critical for maintaining operational safety. Case studies have shown that NGRs prevent catastrophic equipment failures and reduce downtime, making them a valuable investment.