Advantages of Low-Resistance Grounding in Medium-Voltage Distribution Systems

Writer： admin Time：2025-11-21 17:37:40 Browse：116℃

In modern 11 kV urban distribution networks, low-resistance grounding of the neutral point has become an important method for improving system safety, protection performance, and communication quality. Its advantages can be summarized as follows:

1. Reduction of System Overvoltage

The internal overvoltage level of the system decreases as the rated current IR flowing through the neutral grounding resistor during a single-phase-to-ground fault increases. However, when IR > 4IC (where IC is the total system capacitive current to ground), the reduction effect becomes marginal while investment costs rise significantly.

From a practical design perspective, considering both overvoltage suppression and the possibility of two bus sections sharing one neutral grounding resistor, IR is typically selected as:

IR ≈ K × 2 × 2IC

where K is a margin coefficient (usually 1.0–1.5) that accounts for future network expansion.
For typical medium-voltage systems where each bus section contributes about 50 A of capacitive current, the rated neutral resistor current is set around 400 A under single-phase fault conditions. This ensures effective overvoltage limitation without excessive cost.

2. Improved Relay Protection Sensitivity

If the grounding resistor value is excessively high, the neutral current IR becomes too small, thereby reducing the sensitivity of ground fault relays. From the standpoint of protection reliability, the resistor current must be sufficiently large so that the fault current through the neutral point is well above the capacitive current of each feeder.

This ensures that zero-sequence current protection and directional earth-fault relays can operate accurately and selectively.
In practical system design, both the voltage limitation and relay sensitivity must be balanced by selecting an optimal grounding resistor current value—large enough for reliable protection, yet not so large as to compromise safety or increase equipment stress.

3. Improved Power-Line Communication Performance

According to international standards such as IEC 60479 and IEEE Std 80, excessive ground potential rise (GPR) and induced voltages can interfere with telecommunication circuits that share common corridors with power cables.
When no protective discharge gap is installed between the communication cable and ground, the permissible steady-state induced voltage is generally limited to below 430 V, and for critical circuits, below 600 V.

Therefore, the fault current through the neutral resistor must be moderate—large enough for protection operation but not high enough to cause unacceptable interference.
Each city or utility should determine the optimal grounding current according to network topology, load density, and electromagnetic compatibility (EMC) considerations to ensure communication quality.

4. Enhanced Personnel Safety

The larger the current flowing through the grounding electrode, the greater the potential rise and risk of step or touch voltage hazards. Hence, the neutral resistor current rating must be carefully limited to maintain safety under fault conditions.

Based on IEEE 80 and IEC 60479 guidelines, acceptable step and touch voltage limits are typically verified at a fault current level of about 1 kA.
In many medium-voltage systems, a neutral resistor rated for approximately 400 A strikes an appropriate balance—sufficient for protection while keeping step and touch voltages within safe limits.

Fault currents above 10 A in solidly grounded or impedance-grounded systems should be promptly cleared by protection devices, preventing prolonged fault conditions that could endanger personnel or equipment.
When selecting the grounding resistor, designers must ensure that the system withstand capacity, insulation level, and permissible voltage rise are all adequately coordinated.

5. Enabling Technological Development

Although medium-voltage grounding technology has advanced, further innovation is needed to adapt to multi-neutral and mixed grounding configurations that exist in complex urban networks.
Future designs must aim for compact, reliable, and safe low-resistance grounding systems that integrate intelligent control and monitoring features.

With the rapid progress of power system automation, SCADA, and smart grid communication, many 11 kV distribution networks are already being upgraded for remote monitoring and automatic fault isolation.
As technology evolves, these developments will continue to enhance the performance, safety, and adaptability of resistance-grounded distribution systems in modern cities.

 Summary

Low-resistance grounding combines the benefits of controlled overvoltage, sensitive protection, safe operation, and improved communication compatibility.
It represents the preferred grounding method for modern medium-voltage (typically 11 kV or 13.8 kV) urban distribution systems, balancing reliability, safety, and cost-effectiveness.