A generator's output voltage is not 100% pure sinusoidal, there are harmonics present. If there are third and triplin (odd numbered multiples of three) harmonic voltages present, these will produce third and triplin harmonic currents. The third and triplin harmonic currents in each phase are in phase with each other and therefore summate via the starpoint of any load and return to the generator via the connection to the generator starpoint.
If you have two or more generators connected in parallel then there is a path for third and triplin harmonic currents to circulate from one generator to the other even if no load is connected. If the generators are identical and produce the same third and triplin harmonic voltages there will be no net third or triplin hrmonic voltage difference between the generators and hence no circulating third and triplin current between the generators. If the generators are different (as they can easily be because they may be of different capacity, by different manufacturers and of different winding pitch) the chances are that you will get a net third and triplin harmonic voltage and one machine will drive third and triplin harmonic currents through the other generators with the current returning to the starpoint of the driving generator.
The circulating third harmonic current can be unexpectedly high and can upset protective relaying (lots of modern relays have third harmonic filters for this reason). In one extreme case I was told about by a commissioning engineer, there was enough circulating third harmonic current for neutral earthing resistors to be getting decidedly hot.
You can stop circulating currents by breaking the circuit to all generator neutrals except for one. You must have one generator neutral earthed so that you can provide earth fault current for the earth fault protective schemes to work.
Another way to prevent third and triplin harmonic currents is to use generators that are two thirds pitch wound. Such generators produce essentially zero third and triplin harmonic voltages.
I'm sure there are lots of ways to do neutral switching. I will suggest one (expensive) way of doing it but it has the advantage of using standard equipment. It is as follows:
You could use a conventional low capacity 11kV switchboard as a neutral bus, call it the neutral switchboard. On the neutral switchboard, short the three phase busbars together (this becomes the neutral bus) and connect the neutral bus to the neutral earthing resistor which you then connect to earth. At the neutral switchboard, provide a (3-pole) circuit breaker for each generator neutral. At each circuit breaker, short the three "circuit" side connection points together other and connect each generator neutral to its neutral circuit breaker. Install logic to ensure that only one neutral circuit breaker can be closed at any time. The neutral circuit breakers are used as automatic switching devices only, not as protective devices. All would be open when no generators were in service. When the first generator was being run up, its neutral circuit breaker would close in. If that generator should trip, its neutral circuit breaker would also trip. If other generators were in service, another generator's neutral circuit breaker would automatically close in. You would have a permissive in each generator's main circuit breaker that it could not close in unless a neutral circuit breaker was closed.
Also, just a note on my previous posting. If you had a generator whose full load current was less than 400A, you would almost certainly size its neutral earthing resistor to allow generator full load current, not 400A.
Finally, just to re-emphasise that there are many considerations regarding generator neutral earthing. It is well and good to make generalisations (as my postings on the subject have been) but it is important to realise that each installation must be considered as a unique case.
