Generator grounding errors can cause current flow in de-energized circuits. This can cause nuisance tripping of protective circuits, with costly and possibly lethal results.
Most electricians and engineers work little with grounding generators, and they certainly have little time to research the particulars of generator grounding. Specifications for a project often read "ground as per governing codes." Those codes include the National Electrical Code (NEC), which uses such terms as "separately derived," "grounding conductor," and "grounded conductor." These phrases have precise meanings, which require study and experience to grasp. Things get complex when you have generators located throughout a system, operating in parallel with the utility.
Where to begin. Let's look at system grounding requirements for the reasoning behind the NEC: Think with the Code instead of using it as a "cookbook" solution. Think of ground as a wire of earth (but not one you can use as a bonding jumper), and think in terms of eliminating the paralleled neutral/ground paths (what we refer to as "ground loops"). Then you can more easily reason with the Code. Keep in mind that a "by-the-Code" design isn't necessarily efficient, convenient, or adequate for good service or future expansion of electrical use (Art. 90-1). You should usually supplement Code-specified grounds with water pipe, reinforcing steel, and structural steel to provide a legal, low-impedance ground path.
The NEC requires you to ground the neutral at only one location in a system:usually at the utility service entrance. Two grounds on a grounded system (e.g., 1-phase, 3-wire; 3-phase, 4-wire; or grounded delta) would result in parallel neutral and ground paths. Current will divide between the two paths and be a safety/coordination problem. By definition, system grounds are not part of ungrounded systems.
Grounding gen-sets on grounded systems. Fig. 1 and Fig. 2 (in original article), show two approaches to grounding a generator on a grounded system. In Fig. 1, an automatic transfer switch (ATS) transfers the phases and the neutral of the generator, using 4-pole switching. Fig. 1 also shows that the ground/neutral loop stays open, as required, at the ATS. If your generators are on a grounded system, you should have the ATS transfer the neutrals.
In Fig. 2, the ATS does not transfer the neutral. However, you keep the ground/neutral open by not grounding the neutral terminal at the generator. The ground for the generator is the one system ground at the service entrance. Only the lightning arrester and case grounds are at the generator.
If you allow a second ground to exist, you'll end up looping the resulting parallel paths. Thus, neutral current could flow in a "de-energized" circuit and through ground relaying. Then, the main system breaker or the generator breaker could open via ground fault relaying without a ground fault. Because of the load balance inherent in generator designs, trouble may not immediately surface. Common problems that may not have immediate symptoms include such things as locating a generator on heavy harmonic circuits and not accounting for inrush. With an already compromised ground fault protection system, such a situation is a recipe for disaster.
The Fig. 2 solution has engineering and coordination problems whose impact varies depending on how close the generator is to the service ground plane. The 4-pole, neutral switching ATS in Fig. 1 is usually too restrictive. A better way to go is the Fig. 2 configuration. Both configurations require caution.
Grounding a gen-set on an ungrounded system. Fig. 3 (in original article) shows a generator and ATS on an ungrounded system. Notice it has no neutral/ground loops. Here, we ground the neutral on the generator for better generator protection. If you ground more than one generator neutral, however, you'll get "talk" between the generators during normal operations. During inrush, this talk is more like shouting. Grounding impedance can help, but getting it right is an engineering exercise.
A grounded generator on an ungrounded system requires a transformer interface. You can ground generator neutrals at the system ground, but only after careful engineering. Distance and impedance are major considerations here.
Grounding many gen-sets. If you have several generators dispersed throughout the total electrical distribution system, and each has its own ATS, treat each generator as an individual unit.
Suppose you have a group of generators in one generating plant, and it's possible to parallel them to supply one or more circuits through one or more ATSs. In this case, you can ground the generators to a common ground plane. That plane can be a Code-grounded bus in switchgear, a ground grid, concrete reinforcing bar (rebar), or structural steel.
The best place to ground generators is normally at their terminals. Locate lightning arresters on the generator terminals and route the ground wire directly to ground with as little length and as little bend as possible.
You normally use 3-pole ATSs for ungrounded systems and 4-pole ATSs for grounded systems. Be aware that parallel generators will "talk" to each other through their grounded neutrals because of harmonics, surges, and other events. Even generators with the same pitch can generate different harmonics that will cause interplay between parallel units. The talk can affect ground fault protection, depending on ground resistance and applied impedance.
You can add ground impedance in larger and higher voltage installations. The size and character of this impedance is a system decision and one you should engineer to fit the installation. Higher impedance will probably mean line-to-line rated lightning arresters and potential transformers.
Peak shaving, demand management and co-generation can add to the return available to a standby generator investment. However, the generator load at a certain time of day or any time on a standby unit is usually well below the generator capacity. Paralleling the generator directly into the system will use the capacity of the generator more fully. The problems in a standby transfer and those of paralleling differ. Accordingly, the paralleling equipment is different and you must interlock these so only one is in service at a time.
Fig. 4 (in original article) shows a combination standby and demand management scheme for a grounded system. Here, we have a 4-pole ATS paralleled with (and interlocked with) a wye/delta isolation transformer. The ATS synchronizes across the breaker. The transformer will isolate the generator neutral grounding from the system ground and allow you to parallel the circuit. The breaker (and therefore the potential transformers) will be on the wye side of the transformer and not subject to ferroresonance in system voltages over 2400V to ground. When the main system power fails, the paralleling breaker opens and the ATS transfers. When the paralleling breaker is in the closed position, the ATS is not in the circuit. The ATS, on standby, keeps the neutral/ground loop open. The wye/delta transformer isolates the loop when in demand management mode.
Fig. 5 (in original article) illustrates a combination standby and demand management scheme for an ungrounded system. It's similar to the grounded system, except that the ATS is 3-pole and the interfacing transformer is delta/delta in the demand management circuit. Whether to include the transformer or use a delta/wye unit is debatable. Since you have parallel generator grounds, the units will talk to each other. The transformer will reduce talk from other sources. In all cases, you must avoid ferroresonance.
Fig. 6 (in original article) looks at three ways of grounding the neutral. There's no one best way, but each way has an effect you must deal with in your overall design. Impedance in the ground loop reduces talk between generators, but canhave a negative effect on your ground fault protection system.
Sizing grounding conductors. The NEC is specific about the size of wire you must use for grounding. However, this is the minimum for "the practical safeguarding of persons and property from hazards arising from the use of electricity" (Art. 90-1). To improve the overall design and performance of the system, increase the size of the ground up to the size of the current-carrying conductor.