What are the Common Mistakes Made When Sizing Neutral Grounding Resistors (NGRs)？

Writer： admin Time：2026-01-19 14:01:01 Browse：29℃

As critical equipment in power system earthing, the improper selection of a Neutral Grounding Resistor (NGR) not only leads to protection relay failure (maloperation or non-operation) but can also cause permanent damage to generator or transformer windings. Below is an analysis of core selection errors and practical case studies for 11kV and 33kV systems, along with mitigation logic.

I. Resistance Value Deviation: Failure to Match System Parameters

The resistance value (R) is the core parameter of an NGR. It must be precisely calculated based on the system voltage level, capacitive current (I_c), protection coordination, and fault tolerance requirements.

(A) Excessive Resistance: Relay Non-Operation due to Low Fault Current

To minimize resistor losses, some designers inadvertently select a resistance that is too high. This results in a ground fault current (I_r) lower than the pickup threshold of the protection relays (IEDs), leading to sustained faults, insulation aging, and escalation to phase-to-phase short circuits.

1.Case Study (11kV Distribution Network):

In an 11kV system (Rated Phase Voltage U_n≈ 6.35kV), the measured system capacitive current I_c was 15A. An NGR with R = 120Ω was incorrectly selected.

Calculation: I_r = U_n / R = 6350 / 120 ≈ 52.9A.

Failure Analysis: Although 52.9A > 15A, in complex industrial sites, high-impedance faults (e.g., tree branches touching lines) cause the current to decay further. If the relay margin is insufficient or the Zero Sequence CT (CBCT) ratio error is not considered at low currents, the protection will fail to trip. This leads to neutral insulation breakdown and flashover of insulators.

Mitigation Logic: According to IEEE Std 142, the resistor current I_r should be equal to or slightly greater than the system's total three-phase capacitive current I_c (i.e., I_r ≥ I_c) to effectively suppress transient overvoltages.

(B) Insufficient Resistance: Equipment Damage due to Excessive Fault Current

If the resistance is too low, the fault current may exceed the equipment's withstand capability, damaging the resistor elements, transformer/generator windings (deformation), and causing voltage dips that affect sensitive loads.

Case Study (11kV Generator - High Resistance Grounding/HRG):

A 20MW generator required the ground fault current to be limited to 15A (HRG for "Alarm-Only" operation). However, a Medium Resistance Grounding (MRG) logic was mistakenly applied, selecting R = 45Ω.

Consequences: Upon a single-phase-to-ground fault, the current reached 141A. Per Joule’s Law (Q = I²Rt), a 10-fold increase in current results in a 100-fold increase in heat. This caused localized melting of the stator core within seconds, turning a simple fault into a catastrophic overhaul involving stator rewinding.

Mitigation Logic: Distinguish between High Resistance Grounding (HRG) (typically ≤ 10A, alarm only) and Medium Resistance Grounding (MRG) (100A ~ 400A, immediate trip).

Case Study (11kV Generator - Medium Resistance Grounding/MRG):

In a 20MW captive power plant in Southeast Asia, the design required a fault current of 120A~180A. The designer ignored the large system capacitive current I_c (60A) and selected R = 45Ω.

Total Fault Current ( I_f ): 

Consequences: While 153.4A was within the 120A~180A range, the excessive electromagnetic force (I²) during frequent faults caused the stator winding end-turns to loosen. Furthermore, the resistor element suffered thermal collapse because the actual current exceeded the thermal rating design.

Correct Calculation: 

II. Improper Resistor Material: Ignoring Environmental Conditions

Resistor materials (Stainless steel, Cr-Ni alloys, Cast iron) determine temperature withstand and corrosion resistance.

(A) Environmental Mismatch: Accelerated Corrosion

Case Study (33kV Coastal Transformer):

A 33kV NGR in a Middle Eastern coastal area used standard SS304 elements. High salinity and humidity caused the oxide layer to peel and the resistor ribbons to snap within 14 months. This led to an open-circuit neutral, resulting in an arrester explosion during a subsequent fault.

Mitigation Logic: For coastal/saline environments, specify Hastelloy or Ni-Cr alloys with anti-corrosion coatings and IP54/55 enclosures with space heaters.

(B) Confusion of Duty Cycles: Short-Time vs. Continuous Rating

Case Study (11kV Generator with Unbalance):

A 15MW generator in Europe required the neutral to withstand a continuous 3-5A unbalance current. A short-time rated (10s) resistor was installed. Within 8 months, the insulation aged and cracked due to long-term overheating.

Mitigation Logic: Ensure compliance with IEC 62271. If continuous unbalance current exists, select a Continuous Duty rating or add forced-air cooling.

III. Control System Configuration Failures: Incompatibility and Logic Errors

(A) Insufficient Monitoring Functions

Case Study (33kV Distribution Substation):

To save costs, temperature and current monitoring were omitted. A loose terminal caused localized overheating, which went undetected until the NGR enclosure caught fire, damaging the transformer bushing and causing a 24-hour blackout.

Mitigation Logic: Standardize on monitoring units that include Over-temperature trips, Fault current recording, and Resistance continuity monitoring.

(B) Signal Interface Incompatibility

Case Study (Central Asia Project):

An NGR with a proprietary communication protocol could not interface with the plant's Modbus TCP DCS. A ground fault went unnoticed for 40 minutes, leading to severe stator insulation damage.

Mitigation Logic: Specify international protocols such as IEC 61850 or standard Modbus TCP for seamless SAS/DCS integration.

(C) Protection Logic Conflict

Case Study (Eastern Europe 33kV System):

The NGR trip delay was set to 6s, while the transformer's zero-sequence protection was set to 4s. During a fault, the transformer tripped before the NGR protection could coordinate, violating the local grid code.

Mitigation Logic: Coordinate the NGR protection curves with the primary equipment (Transformer/Generator) to ensure proper Time-Current Coordination (TCC).

IV. Other Common Errors

Grounding Mode Mismatch: Attempting to install an NGR in a system already using a Petersen Coil (Arc Suppression Coil) without proper calculation can cause resonance overvoltages.

Enclosure vs. Environment: In high-altitude regions (e.g., Africa, 3500m), standard NGRs fail due to lower air density and reduced dielectric strength. De-rating factors and increased clearances per IEC 60071 must be applied.