How to Select the Right NGR for a Power Transformer?

Writer： admin Time：2026-10-08 14:40:47 Browse：90℃

A neutral grounding resistor (NGR) is not selected simply by matching the power rating of a transformer. Its primary purpose is to control single-line-to-ground fault current while keeping the system grounding conditions suitable for protection and equipment insulation.

For transformer applications, the correct NGR selection should be based on the system voltage, required ground-fault current, fault duration, grounding method, protection scheme, and installation environment.

IEC 60076-25:2023 specifically covers neutral grounding resistors used with transformers and generators, including their characteristics, service conditions, tests, and marking requirements. IEEE C57.32 and its amendments provide the corresponding North American framework for neutral grounding devices.

1. Start With the Transformer Secondary Voltage

The first parameter is the line-to-line voltage of the transformer winding being grounded.

For a solidly grounded star point, the resistor sees approximately the line-to-neutral voltage:

Vₗₙ = Vₗₗ / √3

For example, a 13.8 kV transformer has:

Vₗₙ = 13.8 / √3 ≈ 7.97 kV

This 7.97 kV value—not 13.8 kV—is the starting voltage for calculating the NGR resistance.

The NGR must also be coordinated with the system insulation level, neutral terminal insulation, grounding transformer if used, and required clearances.

2. Determine the Required Ground-Fault Current

The most important engineering decision is the desired single-line-to-ground fault current.

A higher NGR current makes ground-fault protection easier to detect and operate, but increases thermal and mechanical stress on the resistor and other equipment. A lower current reduces fault energy but may make protection sensitivity more difficult.

For a resistance-dominated grounding circuit:

R ≈ Vₗₙ / I(GF)

where:

• R = NGR resistance

• Vₗₙ = line-to-neutral voltage

• I(GF) = specified ground-fault current

The final fault-current calculation should also consider transformer zero-sequence impedance, grounding-transformer impedance, system impedance, cable impedance, and fault resistance.

Case Study 1 — 13.8 kV Transformer, 400 A NGR

Consider a 13.8 kV industrial transformer requiring 400 A ground-fault current.

Vₗₙ = 13.8 / √3 = 7.97 kV

Therefore:

R = 7,970 / 400 ≈ 19.9 Ω

A practical specification would therefore be approximately:

13.8 kV system / 7.97 kV neutral-to-ground / 400 A / 19.9 Ω

If the required fault duration is 10 seconds, the resistor must be designed and tested for the specified 400 A, 10-second duty—not merely for its calculated resistance.

A published engineering example using the same 13.8 kV, 400 A, 10-second application arrives at approximately 20 Ω and identifies the short-time thermal duty as a critical part of the specification.

3. Specify the Fault Duration Correctly

NGRs are commonly specified for short-time duties such as 10 seconds, 30 seconds, or 60 seconds, depending on the protection philosophy.

The duration should be coordinated with:

fault detection + relay operating time + breaker operating time + safety margin

For example, if ground-fault protection normally clears a fault in less than 1 second, a 10-second NGR rating can provide substantial thermal margin. However, the actual project specification—not a generic "10-second standard"—must determine the required duty.

The instantaneous resistor power can be very high.

For the 13.8 kV, 400 A, 20 Ω example:

P = I²R = 400² × 20 ≈ 3.2 MW

This does not mean the resistor is a 3.2 MW continuous-load device. It represents the initial electrical power dissipated during the fault. The resistor heats rapidly, its resistance changes with temperature, and the specified short-time thermal design determines whether it can safely withstand the event.

IEC 60076-25 specifically recognizes the effect of resistance variation with temperature.

4. Check Whether a Grounding Transformer Is Required

Not every transformer provides a usable neutral point.

If the transformer secondary is delta-connected, or if the available neutral cannot be directly grounded, a grounding transformer may be required.

Common solutions include:

• Zig-zag grounding transformer

• Wye-delta grounding transformer

• Grounding transformer + NGR combination

The grounding transformer must be evaluated for the required zero-sequence current and short-time thermal duty. Its impedance also affects the actual ground-fault current, so the NGR resistance should not be calculated in isolation.

For example, a 13.8 kV, 400 A grounding system corresponds to approximately:

7.97 kV × 400 A ≈ 3.19 MVA

This figure is useful for evaluating short-time grounding-transformer duty, but it should not be interpreted as the continuous rating of the grounding transformer.

5. Make Sure the Protection System Can Detect the Fault

A technically correct NGR can still be poorly selected if the protection system cannot reliably detect its fault current.

The engineer should verify:

NGR current → CT ratio → relay pickup → relay operating time → circuit-breaker clearing time

For example, selecting a 10 A NGR when the protection CT and relay require substantially more current for dependable operation can create a coordination problem.

IEEE guidance on neutral grounding specifically addresses the relationship between grounding-device selection and ground-fault protection sensitivity.

6. Consider Continuous Current and Multiple Faults

An NGR normally carries little current during balanced operation, but the specification should state whether continuous leakage or zero-sequence current is expected.

Repeated faults are also important. A resistor that survives one 10-second fault may require additional thermal consideration if the project permits repeated fault events before sufficient cooling time.

For critical facilities, the specification should therefore include:

rated current + resistance + time rating + number of fault events + required recovery/cooling conditions.

7. Select the Enclosure and Environmental Rating

Outdoor transformer substations may expose the NGR to rain, dust, salt, UV radiation, freezing temperatures, or high altitude.

The enclosure should therefore be selected according to the installation environment rather than using a standard enclosure by default.

Typical project requirements may include:

• Outdoor or indoor installation

• IP rating

• NEMA rating where applicable

• Corrosion protection

• Ambient temperature

• Altitude

• Ventilation

• Cable entry arrangement

• Access and maintenance requirements

At high altitude, reduced air density can affect insulation clearances and heat dissipation, so the NGR design should be reviewed accordingly.

8. Case Study 2 — Why a "Correct Resistance" Can Still Be Wrong

Suppose two suppliers quote an NGR for a 10 kV system and both offer approximately 11.55 Ω for a 500 A application:

Vₗₙ = 10 / √3 = 5.77 kV

R = 5,770 / 500 ≈ 11.54 Ω

The resistance appears correct.

However, Supplier A offers 500 A for 10 seconds, while Supplier B offers 500 A for 1 second.

Electrically, both resistors have approximately the same nominal resistance. Thermally, they are not equivalent.

If the protection system requires several seconds to clear a fault, Supplier B's NGR is unsuitable even though its resistance calculation is correct.

This is why NGR selection must be treated as a complete fault-duty specification, not an ohmic-value calculation.

NGR Selection Checklist

Before approving an NGR for a power transformer, confirm these parameters:

System voltage → transformer winding connection → available neutral → target ground-fault current → NGR resistance → short-time duration → grounding-transformer requirement → zero-sequence impedance → CT/relay coordination → insulation level → enclosure/environment → applicable standards → FAT/type-test requirements.

The most important principle is simple: select the NGR from the protection and fault-current requirements of the complete power system, not from the transformer MVA alone.

A properly selected NGR limits ground-fault current while still providing sufficient current for reliable fault detection and clearing. That balance is what determines whether the grounding system is genuinely effective.