Why 11kV Networks are Rapidly Transitioning to Low Resistance Grounding (LRG)？

Writer： admin Time：2026-04-23 08:54:10 Browse：35℃

1. Background and Operating Conditions

In a substation utilizing an 11kV single-bus sectionalized configuration, the measured capacitive current (I_c) on two bus sections is approximately 35A. As the number of connected users grows, this capacitive current is expected to rise and potentially exceed 50A. The selection and verification of the grounding resistor are primarily based on the most demanding operating conditions: one main transformer undergoing maintenance while the other carries the full load, including future system expansions. Based on these scenarios, this proposal adopts a single-phase-to-ground model with an estimated total capacitive current of 100A.

2. Selection of Grounding Method

When the total capacitive current reaches 100A, maintaining an ungrounded neutral system poses significant risks. Although small-current grounding selection devices can be used, the lack of damping means that intermittent arc grounding can trigger transient overvoltages exceeding 3.5 times the phase voltage. This can easily escalate into phase-to-phase or three-phase short circuits. Therefore, following international power grid trends, it is recommended to adopt Low Resistance Grounding (LRG) to provide a clear discharge path and protect system insulation.

3. Key Considerations for 11kV Low Resistance Grounding

(1) Control of Distribution Network Overvoltage

According to the principles of IEEE Std 142 (Green Book), the current flowing through the grounding resistor (I_R) must be sufficient to suppress transient overvoltages. When I_R ≥ I_c, the risk of arcing overvoltage is eliminated; when I_R ≥ 3I_c, the overvoltage level is stabilized within a safe range. To ensure adequate suppression and accommodate future expansion, it is standard practice to set the rated resistor current at I_R = 400A.

(2) Relay Protection Sensitivity and Resistance Calculation

Per IEEE Std C37.91 (Guide for Protective Relay Applications), the grounding resistor must ensure that zero-sequence protection devices can operate reliably even in the presence of fault transition resistance.

• Calculation Logic: For an 11kV system (Phase Voltage U_ph≈ 6.35kV), to achieve a design fault current of 400A, the resistance is calculated as: R = U_ph / I_R ≈5.8 Ω.

• Practical Application: Based on global engineering experience, selecting a 15Ω resistance (corresponding to approx. 423A) provides an optimal balance, ensuring a Sensitivity Ratio (K_sen) greater than 1.5.

(3) Fault Current Limits and Safety Standards

In accordance with IEC 60364 regulations, fault currents exceeding 100A must be isolated immediately via circuit breakers. A 400A rated current allows for rapid identification by residual overcurrent protection, ensuring fault clearance within the rated time (typically 10 seconds or less) as specified by IEEE Std 32, thereby preventing cable terminal burnout.

(4) Personnel Safety and Ground Potential Rise (GPR)

During a ground fault, the current entering the earth grid causes a Ground Potential Rise. Referring to IEEE Std 80 (Guide for Safety in AC Substation Grounding), with a properly engineered grid, a 400A current cleared within 0.5 seconds ensures that step and touch voltages remain well below human tolerance thresholds.

(5) Communication Interference and EMC

High-magnitude zero-sequence currents can induce electromagnetic interference in nearby communication lines. According to ITU-T recommendations and operational data from major global hubs like London and Dubai, 400A fault currents—when used with shielded cables and standardized metallic enclosures—stay well below international safety thresholds for induced voltage (typically 430V to 650V).

(6) Risk Mitigation through Independent Grounding

To minimize secondary risks, it is critical to implement a separation of grounding systems. The optimal strategy, as per IEC 60364-4, is to keep the 11kV high-voltage system protection ground independent from the low-voltage (400V/230V) neutral ground. This physical decoupling prevents high-voltage fault potentials from migrating to the low-voltage consumer side through the neutral conductor.

4. Practical Reference Case: International Data Center Project

Project Overview: A 11kV/0.4kV substation project for a Tier III data center in Southeast Asia, with a measured capacitance of 42A per bus section.

Solution:

• To accommodate future IT load expansion, the engineering team specified an NGR rated for 400A/10 seconds.

• The resistor elements were constructed from high-grade stainless steel with a resistance value of 15.8Ω.

  Result: During a 2025 cable joint failure, the system triggered a zero-sequence protection trip within 150ms. The NGR successfully suppressed the transient overvoltage, ensuring that the precision IT equipment on the same grid remained completely unaffected, preventing catastrophic downtime.

In summary, 11kV Low Resistance Grounding is a vital technology for ensuring the safety and reliability of modern urban grids. By scientifically selecting resistance parameters in accordance with international standards, operators can effectively suppress overvoltage, guarantee protection sensitivity, and safeguard both personnel and communication infrastructure.