Nonlinear Neutral Grounding Equipment

Overview

The nonlinear neutral grounding resistor cabinet uses zinc oxide (ZnO) valve elements at its core, which dynamically change resistance according to current. It is suitable for 10 kV / 35 kV high-voltage distribution networks as well as 0.66 kV low-voltage circuits in coal mines and similar enterprises. Under normal conditions, it presents a high-resistance state, isolating the neutral point and preventing leakage current. When a single-phase ground fault occurs, the resistance drops sharply to a low-resistance state, allowing fault current to flow (facilitating protective device detection), suppressing arcs, and maintaining a small current so the system can operate briefly under fault conditions.

Its advantages are significant: it adapts automatically to faults while balancing supply continuity and safety, can dissipate overvoltagesto protect equipment insulation, and has a compact size—only 1/3 to 1/2 that of traditional arc-suppression coils—reducing operationand maintenance costs. It is compatible with multiple voltage levels, making it suitable for urban distribution networks, underground coal mines, chemical plants, and other scenarios. Additionally, it can be combined with grounding selector devices to greatly shorten fault handling time.

Operating Principle and Product Features

The core component of the OR-NNG is a high-energy zinc oxide (ZnO) varistor assembly, which is connected between the system neutralpoint and ground, forming a modern nonlinear resistance grounding scheme. This design fully utilizes the excellent volt-ampere characteristics of ZnO varistors, their large thermal capacity, and fast response speed. The operating threshold is set at the system phase voltage level, ensuring reliable protection and effective performance during operation.

Wiring Diagram of OR-NNG Nonlinear Neutral Grounding Resistor:

Operating Principle of OR-NNG Nonlinear Neutral Grounding Resistor

Normal Operation
During normal operation, the neutral point voltage remains very low. The nonlinear resistor exhibits high resistance, allowing only a minimal current to flow, effectively behaving as an open circuit. In this state, the system operates as if it were ungrounded, providing high supply reliability and enhanced safety.

Overvoltage During Faults
When a system fault induces overvoltage, the neutral point voltage rises. The nonlinear resistor responds immediately, conducting current and limiting the voltage to a predefined range. Typically, system overvoltages are restricted to below 2 times the phase voltage. This ensures low overvoltage stress on equipment and accurate fault line identification, offering advantages over traditional low-resistance grounded systems.

Resonance Suppression
If resonance occurs, the large energy capacity of the nonlinear resistor quickly absorbs the resonant energy. With overvoltages maintained at a low level, transformers and other equipment remain in their linear operating region, exhibiting high reactance and effectively damping the resonance.

Single-Phase Arc-to-Ground Fault
During a single-phase arc-to-ground fault, the nonlinear resistor absorbs the ground charge energy and limits the recovery voltage across the arc. This prevents restriking after the current passes through zero, causing the arc to extinguish rapidly. This performance generally surpasses that of conventional arc suppression coils.

Permanent Grounding Fault
In the event of a permanent ground fault, the nonlinear resistor maintains the phase voltage level while allowing a small current to flow, enabling continued system operation. Faulted feeders can be isolated either with a delayed or fast trip, accommodating different operational requirements.

System Protection and Reliability
By grounding through the nonlinear resistor, system overvoltages are limited within the insulation withstand range, reducing the likelihood of equipment insulation failure and preventing minor faults from escalating into major faults. This significantly enhances power supply reliability and safety.

Varistor Assembly Design
The ZnO varistor assembly consists of multiple parallel nonlinear resistors with identical V–I characteristics. The assembly is designed at 300% rated capacity, ensuring normal operation even if more than 50% of the varistor elements fail. Each varistor branch includes a dedicated fuse to guarantee overall functionality. Advanced heat dissipation and fully sealed construction provide excellent thermal management and moisture protection. The device automatically records operating events, including action count, duration, peak current,and voltage, supporting system technical analysis.

Model Description and Technical Specifications of OR-NNG:

Operating Environment

• Ambient Temperature: -20 ~ +40℃

• Operating Frequency: 50 ~ 60 Hz

• Altitude: < 2000 m (Plateau version: < 4000 m)

• Ambient Humidity: 90% (at 25℃), 50% (at 40℃)

• Seismic Intensity: Not exceeding 8 degrees

• Surrounding Air: Free from corrosive or flammable gases, water vapor, or significant pollution; no severe contamination

• or frequent violent vibration.

Installation, Operation, and Maintenance

The cabinet dimensions and parameters can be designed according to user requirements. Our company is responsible for the 

production, packaging, and transportation of the equipment. Upon request, our technical personnel can be dispatched to the site 

for installation and commissioning guidance, with necessary cooperation from the user.

Before commissioning, DC reference voltage and leakage current tests must be performed:

• Voltage: Not less than 90% of the factory value

• Leakage Current: Not exceeding 50 μA

The equipment requires no routine maintenance under normal operation. However, after operation, the fuses should be checked, 

and fault records reviewed. The equipment can continue operating if up to 30% of fuses are damaged, but the company must be 

notified. Users must not reinstall damaged fuses.

A routine inspection should be conducted annually, including dust cleaning, withstand voltage testing, and measurement of DC 

reference voltage and leakage current of the nonlinear resistor.

Our company provides lifelong maintenance and long-term supply of spare parts.