Two intentional neutral grounding points can create parallel zero-sequence paths. Depending on transformer connections and network impedance, ground-fault current may split between neutrals and normal unbalance may drive circulating current. Before allowing dual grounding, the study should map every zero-sequence path, grounding impedance and protection measurement zone for each bus-tie state.
Two equal resistors can change the total current
As a simplified illustration, two identical neutral-resistance paths supplied at the same displaced-neutral voltage act in parallel. Their equivalent resistance is half that of one path, so their combined resistive current can be twice the single-path value. Real transformer sequence impedances and network connections modify this result. The example shows why two individually correct grounding assemblies do not automatically preserve the original system fault-current target when their buses are interconnected.
Separate fault-current division from normal circulating current
During an earth fault, each neutral contributes according to its complete zero-sequence path. Under normal conditions, load unbalance, harmonic components or a voltage difference between connected systems may also drive current through available return paths. These are different studies and should not be merged into one maximum-current entry. Quantify continuous neutral heating separately from short-time fault withstand. A neutral CT reading alone cannot identify the mechanism without the simultaneous phase currents, bus state and grounding configuration.
Put grounding states into the interlocking design
The study matrix should show bus ties, source breakers and intentional neutral connections as a coordinated arrangement. Determine which states are permitted, which need a different protection group and which must be prevented. An interlock specification should describe the required condition and verified indication, not substitute for a site switching procedure. A commissioning review must then confirm that the implemented logic matches the approved grounding matrix, including loss of status feedback or control power.
Technical reference framework
[1] IEC 60076-8:1997 — Application guide.
[2] IEC 60076-1:2011 — Power transformers, General.
[3] Post Glover NG112-06 — Neutral grounding resistors: Technical information.
[4] Post Glover GT210-10 — Neutral grounding resistors with transformer.