Why Are NGRs Used in Transformers?

The core purpose of using a neutral grounding resistor (NGR) in transformers is to provide protection through resistance grounding balancing equipment, system safety and power continuity requirements, while addressing the limitations of traditional grounding methods such as direct grounding, ungrounding or arc cancelling. Here's a look at why:
1.Limit ground fault current and protect equipment safety.
Shortcomings of direct grounding:
If the transformer neutral point is grounded directly, the single-phase grounding fault (such as single-phase conductor touching the ground) can lead to a sharp increase in short-circuit current, which may burn transformer winding, iron cores or neutral insulators, or even cause a fire. In a directly grounded a 10kV system, for example, the fault current can reach thousands of amperes, far more than the device can tolerate.
The NGR solution:
By inserting a series of resistors between the neutral point and ground, the fault current is limited to a safe range (e.g., tens to hundreds to hundreds of amperes). For example:
For a 10kV system, the NGR resistor is usually in the dozens of ohms, limiting the fault current to 100-400A to prevent equipment damage. 35kV systems: High resistance value (e.g. hundreds of ohms), limiting current to low levels (e.g., 50-100A).
2.Overvoltage suppression, Overvoltage insulation and protection system
Overvoltage Risks of Ungrounded Systems:
When the neutral point is ungrounded, the single-phase grounding fault can cause the non-fault phase voltage to rise to the line voltage (μ3 times phase voltage), thus causing the insulation to breakdown. In addition, fault arc can be repeatedly extinguished and re-ignited, generating arc flashes several times the phase voltage (e.g. 6-8 times the phase voltage) to ground the overvoltage, threatening equipment insulation.
Limitations arc suppression coil Grounding:
Arc dissipation loop compensates capacitive current and extinguishes the arc by inductive current. However, they cannot limit the overvoltage amplitude and may induce ferroresonance overvoltages.
NGR Suppression Effects:
Residual Charge Discharge: After the fault arc extinguished, the NGR provides a discharge path for the residual charge in the system'sground capacitance, reducing the likelihood of arc reignition. Damping resonance: Resistance as damping element, in the condition of interference resonance, the amplitude of overvoltage is limited to 2.5 times the phase voltage, prolonging the service life of the equipment.
3. Increased continuity of power supply continuity and reduction in the scope of power outages
Shortcuts to strong grounding:
In solid-state grounding systems, single-phase grounding fault will immediately trigger the protection device tripping, causing a large area of power outages.
NGR compatibility:
NGR allows the system to operate briefly (say 1-2 hours) in the event of a single-phase grounding fault, which buys maintenance time. For example:
Urban distribution network: The introduction of NGR in 10kV cable networks increases fault line detection accuracy to over 95% by simply cutting fault lines while ensuring power supply in fault free areas.
Industrial users: petrochemicals, mines and other industries require high continuity of power supply continuity. NGR can prevent arcs of electricity from causing fires or explosions, and can prevent power outages across the plant.
4. Meeting relay protection requirements
Protective device operating requirements:
Relay protection devices must detect grounding currents to identify and eliminate faults. If the current is too low (e.g., no grounding system), the protection may not operate, and if the current is too high (e.g., direct grounding system), the protection may fail.
NGR's Balancing Function:
By adjusting resistance value, the fault current is controlled within the relay protection device's sensitivity range (e.g., 10-100A) to ensure fast and accurate operation. For example:
Zero-sequence current protection: The NGR provides a stable zero-sequence current signal for protection device to identify fault lines.
Differential protection: NGR does not affect the normal operation of differential protection in the event of internal transformer failure.
Adapt to different capacitance current requirements system.
The Impact of Capacitive Current:
System capacitance current (IC) increases with cable length. When IC exceeds a certain value (e.g. greater than30A in a a 10kV system), the arc is difficult to extinguish and requires NGR to limit the current.
NGR's Flexibility:
resistance value can be adjusted according the capacitive current of the system to meet the needs of different situations. For example:
low capacitive current system: Select a low resistance value to achieve both current limiting and overvoltage suppression. High-capacitance current systems: choose high resistance, overheating caused by overflow.
6. Compliance with international standards and safety regulations
Standard Requirements:
IEEE, IEC and other standards define the design, testing and installation requirements for NGR.
Temperature Rating: The resistor must be able to withstand the heat generated by fault currents (e.g., 760°C for 10 seconds).
Safety Certifications:
NGR products must be certified by CSA, UL and other organizations to ensure reliability in extreme operating conditions.
7. Practical Application Case Verification
Power plant case studies:
A 50MW generator set uses NGR at neutral points, limiting fault currents to 5-25A, protecting windings from insulation and preventing core saturation.
Case studies of urban distribution networks:
A 10kV cable network uses NGR technology to control arc-fault grounding overvoltages to less than 2.5 times the rated value, reducing fault rate by 60% and greatly improving the reliability of the power supply reliability.