Alternator neutral groundig

"function of neutral grounding ?"

"There is no impending doom for not connecting neutral and ground correctly. It is a potential safety issue, but not a guarantee of death and destruction. Like wearing the seatbelt in your car, leaving it off is not a death sentence, but wearing it is no guarantee of absolute safety either. We do things to try to ensure our safety, and proper electrical wiring and use is a good habit to follow."

"Ground is used as a safety return in case of failure in a circuit. It also ensures the chassis of equipment remains at zero volts potential and is safe to touch. Neutral is used as the normal return line for current flowing on the live line. It is bonded to ground at the main service entrance, and should be isolated from ground elsewhere."

http://members.rennlist.org/warren/genfaq.html

It must be determined if the transfer system includes a pole for connection of the neutral. If this is so, there is no need to ground the neutral at the generator and it is, in fact detrimental to ground it, particularly if the system includes a ground detection system to open the serfvice.

If the neutral is, via the transfer, or connection equipment, not connected to the system neutral, then the ground is necessary to handle the offset in phase balance, and should be sized to carry the maximum offset. To leave it un-connected in this case, invites circulating currents and perhaps a capacitive buildup of voltage that may perforate the insulation, while coming up to speed and synchronizing, before the connection to the grid or parallel unit.

If you speak about NGRs directly connected on the neutral point of the generators, and in none isolated neutral earthing system, in this case, the NGR will be used in order for 2 purposes:

- Fix the generator neutral point to the earth in order to detect a phase to earth fault.

- Limit this phase to eartrh fault current level

The NGR status will depend of different criteria:

- are the generator running // to the grid

- are the generator connected via a transformer.

In case of non Isolated neutral earthing system:

1/ In case of generator connected via a transformer.

In this case the transformer acts as a galvanic isolator, and thus the neutral earthing system has to be rebuilt. So the NGR will be continously connected to the earth.

2/ In case of generator connected directly.

In this case, and in the most of the application that i know, the neutral has to be connected only at one point of the network.

Generally, this will be done at the incomming step down transformer(s) of your installation. That means that the NGR has to be open when 1 or 2 or several generator are running // to the grid.

If the installation is not connected to the grid, but only by the generator(s), in this case only one of all the generator NGR running has to be connected. If the generator which hase the NGR close is shut down, the NGR of an other running generator has to be closed automatically.

The idea is to have usually one Grounding resistor connected (on the transformer or on the generator). If several neutral resistors would be connected at the same times, the earthing fault current would be dependant of the number of neutral resistance connected; it woulld not be possible to set the homopolar current protections.

Best regards