What are the benefits and risks of using a neutral-grounding resistor (NGR) for generator protection?

Neutral-grounding resistors (NGRs) play a crucial role in generator protection by limiting fault currents and providing a stable reference point for the system. Benefits: • Fault Current Limitation • Stabilizes Voltage • Selective Coordination • Reduced Mechanical Stresses • Ground Fault Detection Risks: • Loss of Sensing During High-Impedance Faults • Potential for Overvoltage • Resistor Heating • System Stability Concerns • Complexity in Testing and Maintenance • Cost Considerations The decision to use an NGR should be based on a comprehensive understanding of the system requirements, potential risks, and the need for fault protection

1. Neutral Grounding resistor are connected to the neutral of the generator winding or the neutral of the power transformer. There are many forms of grounding (High impedance and Low impedance grounding) results in the reduction in Ground Fault current to the desired values to mitigate the destruction of the stator core and winding for the generator and the winding and the core for the transformer 2. High impedance grounding the reduced fault current ranges from 1A-25A and for low impedance grounding the reduced fault current is up to 400A (mostly for industrial generators and transformers). The drawback is casual over-voltages can result.

The inputs from the protective elements like CTs and PTs are fed to the Stator Earth Fault (SEF) relay located in the control room. This further actuates the 95% SEF relay to safely isolate the healthy section from the faulty section using the master relay (86). It is an amazing piece of engineering to protect the expensive electrical equipment.

A Neutral Grounding Resistor (NGR) is a device used to limit the current flowing from the neutral point of a generator or transformer to the ground during a ground fault. It introduces resistance into the neutral-ground connection to control the fault current, preventing damage to equipment like generators and cables. By managing fault current, NGRs protect electrical systems from overheating, arcing, and other risks, ensuring safer operation during ground faults.

An NGR limits fault currents in power systems, enhancing safety by preventing equipment damage and reducing arc flash risks. It stabilizes voltage during ground faults, ensuring continuous operation. However, improper sizing can lead to inadequate protection or excessive fault currents, posing safety hazards.

What's the function of having the NGR as a switchable device instead of just bolting it between the neutral and grounding conductors? For the above contraption to work, the PT that measures P-G voltage across the NGR, the CT that measures fault current through the NGR, and the contactor with its associated control circuitry all need to work, otherwise the ground fault goes uncleared. This seems to introduce needless points of failure. Why not just bolt the NGR between the neutral and grounding conductors and have one CT monitor the circuit for fault current?

An NGR works by connecting a resistor between the neutral point of a generator or transformer and the ground. During normal operation, the NGR remains isolated, allowing minimal leakage current. When a ground fault occurs, the voltage and current in the neutral-ground connection increase. The NGR's control unit senses this change and activates a contactor to insert the resistor into the circuit, limiting the fault current to a safe level. This enables protective relays to detect the fault and trip circuit breakers, isolating the affected section of the system.

Note that the NGR only protects from phase to ground faults, not phase to phase faults. You need separate overcurrent protectivie devices such as relays, fuses and breakers for them.

An NGR is recommended or required in several situations, such as when generators are connected to distribution systems with multiple power sources or high-impedance grounding. It's also needed in systems prone to transient overvoltages or to comply with specific electrical codes and standards. NGRs help prevent high fault currents that could damage equipment, provide a low-impedance path for fault currents, reduce insulation stress, and ensure faster, more selective operation of protective relays.

The benefits of using a Neutral Grounding Resistor (NGR) include enhanced safety for personnel and equipment by limiting fault current and voltage levels. NGRs improve the reliability and availability of generators by reducing the risk of damage during faults. They also facilitate better fault detection and isolation, providing measurable current that protective relays can sense. Additionally, NGRs help minimize zero-sequence currents, improving overall power quality and reducing harmonics in the system, which can enhance communication signals as well.

A failure in a NGR system does not prevent the generator from working. An open system just "floats" the neutral (a number of portable systems operate without bonding the neutral in practice). A shorted resistor just bonds the neutral to ground as you typically would in a generator setup. I have not seen a mechanical contactor for an NGR system, outside of a disconnect on permanently installed systems for maintenance purposes. An NGR becomes an issue only when you have an actual ground fault, which should be dealt with promptly regardless.

The risks of using a Neutral Grounding Resistor (NGR) include introducing a single point of failure, which can compromise system reliability. NGRs add complexity and cost due to additional components and maintenance requirements. There's also a potential fire hazard from resistor heating, which can ignite nearby materials. Furthermore, the resistance introduced can lead to voltage drops and power loss, affecting generator performance and efficiency. If the mechanical contactor malfunctions, it may fail to operate correctly during a fault, leaving the system vulnerable.