Harmonic waves

Since you are asking about the 'neutral point' you must be talking about either a 120/208 V or 277/480 volt generator.

Assuming the 120/208 VAC power, the line to line voltage between any of the 3 phase wires is 208 VAC and the line to neutral voltage between any one of the phase wires and neutral is 120 VAC.

If the load on all 3 phases is balanced, 15 amps for example, and the load is resistive with NO harmonics, the current flow in the neutral will be zero.

Conversely, if you have a balanced load of all complex circuits like computers, copy machines, electronic ballasts and other non-linear loads, the current in the neutral can be up to 1.414 x the current in one of the line conductors. This would amount to up to 21.2 amps for the 15 amp balanced loads above.

As should be obvious, a generator designed to supply resistive loads would have an overloaded neutral that would fail prematurely. This problem also exists in transformers that supply electricity to complex loads. (Loads high in harmonic content.)

Harmonics themselves do not affect the neutral of one phase because they have lower amplitude than the system voltage and distort the sine wave in perfect symmetrical shapes (to neutral) due to their non-zero phase in respect to the main AC voltage. An oscilloscope can show one or two saddles on each half-cycle). However, the presence of harmonics can unbalance a power system by being absorbed differently on the (loads) three phases and circulation currents can occur.

Harmonics can be generated continuously or randomly by transients such as those produced when a highly inductive load is disconnected.

That can be a spike of very high voltage that usually brakes the insulation or burns some component (according to Murphy, the most expensive one ).

Typically the strongest harmonic seen is the 3rd harmonic. The problem with this particular harmonic in 3 phase systems is that it combines in neutrals, instead of cancelling as the the guest above explained for normal currents. High harmonic loads, such as switching power supplies present in a lot of electronic components, generate a large 3rd harmonic component.

This, however, generally does not effect generators significantly. The greatest affect seen is the voltage drop imposed on the wiring by the flow of the 3rd harmonic current. The typical solution for this is a double sized neutral coming from the generator, or the use of Delta to Wye isolation transformers.