Why Do Transformers Need NGR?

The core reason that transformer requires neutral grounding resistors is to limit single phase grounding fault current, prevent overload and damage of equipment, and keep power system running steadily. Here are some details:
1.Limit short-circuit current and Protect Equipment safety
Fault scenario: when transformer neutral point is grounded directly, if single-phase grounding fault occurs in the system (such as single-phase conductor touching ground), the fault current will increase sharply, far more than the rated load capacity of the equipment.
For example, when the cable temperature exceeds 760°C, damage to other cables between 100 ° C and 100 ° C is effectively prevented, and when the resistor exceeds 100A ° C, cable damage is effectively prevented.
2.Maintain voltage stability and minimize power outages
Comparison of neutral grounding methods:
Direct grounding: Fault currents is high, may trigger the protection device to jump, leading to widespread power outages. Grounding via NGR: After a limited fault current, the system can operate for a period of time (say minutes to hours) during a fault, buy maintenance time, prevent voltage from rising during a non-fault phase, and minimize impact on other devices.
Application Scenario: NGR is widely used to limit ground fault currents and improve power supply reliability in a medium voltage distribution network of 10 kV to 35 kV.
3.Prevent Resonant Overvoltages and protect insulation
Problem: When the neutral point is ungrounded or grounded through an arc suppression coil, the interaction between the capacitor current and inductive current can generate resonant overvoltages, threatening the insulation of the device.
NGR Solution: introduce a resistive component, interrupt resonant condition, suppress overvoltage amplitude, prolong equipment life.
4. Compliance with international standards and safety regulations
Standard Requirements: IEEE, IEC and other international standards stipulate that the neutral-point grounding resistor must meet specific resistance and temperature requirements. (For example, Vishay's NGR series complies meets the IEEE-32 standard and can withstand a 8kV phase voltage and a 13.8kV system voltage.) Safety certification: NGR products typically require certifications such as CSA and UL to ensure reliability in extreme operating conditions.
V. Flexible Adapt to Different Application Scenarios
Resistor Value Selection: NGR resistor values can be calculated based on system capacity, voltage level and fault current.
Structural advantages: NGR adopts flexible bundling design, eliminates load voltage during assembly, and is easy to install and maintain. They also support high current ratings (e.g., 100A-1000A) to make them suitable for high-capacity transformer applications.