You're on the right track thinking about the relationship between NGRs and current imbalance. Here's a breakdown of how they interact: NGRs and Ground Faults

NGRs primarily limit the ground fault current. This is the current that flows when a phase conductor comes into direct contact (or near-direct contact through a low impedance) with ground. This is their primary purpose.

Current Imbalance (Unbalance)

Current imbalance, or unbalance, refers to unequal currents flowing in the three phases of a three-phase system under normal operating conditions. This is not a ground fault. Sources of current imbalance include:

Unequal Loading

Different loads connected to each phase. For example, more single-phase loads on one phase than the others.

Impedance Asymmetries

Slight differences in impedance in the phases of the power system, such as from long line lengths, transformers, or other equipment.

Single-Phasing

A blown fuse or open connection in one phase of the supply.

Neutral Current and Current Imbalance

In a balanced three-phase system (with a neutral connection), the neutral current is ideally zero. However, with an unbalanced system, the neutral conductor carries the residual current. This is the vector sum of the three-phase currents. Mathematically: `In = Ia + Ib + Ic` (where `In` is the neutral current, and `Ia`, `Ib`, and `Ic` are the phase currents)

NGR Response to Imbalance vs. Ground Fault



Ground Fault

The NGR actively limits the ground fault current. When a ground fault occurs, the voltage of the faulted phase is pulled close to ground potential. The NGR resistor provides an impedance to ground, limiting the fault current to a pre-determined value. This reduces the severity of the fault, limits equipment damage, and makes protection easier.

Current Imbalance

The NGR does not actively limit the current imbalance itself. The NGR's resistance will affect the neutral current resulting from the imbalance, but it is a passive effect. Here's why:

Neutral Voltage

With a current imbalance, the neutral point of the source will shift slightly away from ground potential. The magnitude of this shift depends on the degree of imbalance and the impedance of the neutral path (including the NGR).

Neutral Current Magnitude

The NGR will limit the magnitude of the neutral current due to the impedance it introduces. However, the source of the neutral current is the imbalance of the three phases. The NGR just changes how much of that current will flow.

Possible Imbalance

Yes, a phase imbalance greater than the limited neutral current is absolutely possible. The NGR doesn't "fix" the imbalance; it simply limits the resulting neutral current. The phase currents can still be significantly different, even with a limited neutral current. The imbalance is defined by the difference between phases, not the neutral current.

Analogy

Imagine a bucket (the neutral point) filled with water from three pipes (the phases).

Balanced System

All three pipes pour in the same amount of water, and the bucket doesn't overflow.

Imbalanced System

One pipe pours in more water than the others. The bucket fills up until it overflows (neutral current).

NGR

You poke a small hole in the bottom of the bucket. The hole lets some water drain out (limited neutral current), but doesn't change how much water each pipe is pouring into the bucket (phase imbalance). The imbalance is still there.

In Summary

NGRs are primarily for limiting ground fault currents. They do not directly prevent or correct current imbalances. Current imbalances cause neutral current. The NGR passively limits the magnitude of the neutral current caused by the imbalance. A significant phase imbalance is still possible, even with a neutral grounding resistor.

What Can Be Done About Current Imbalance?

To address current imbalance, you need to: Balance Loads

Redistribute single-phase loads as evenly as possible across the three phases.

Improve System Design

Use delta-connected transformers when possible (they don't pass zero-sequence currents, which contribute to imbalance).

Use Power Electronics

Active power filters (APFs) can be used to inject currents that compensate for the imbalance.*

Regular Inspections: Check for damaged equipment, loose connections, or other issues that can cause impedance imbalances.I hope this clarifies the relationship! Flag for review