Design Challenge- PV, Battery, Grid, Generator all in one system

I am not sure I understand your question... The question can cover a wide range of issues. First, here is a discussion on the major types of AC inverters, how they work, and are connected:

battery-less Outback Radian (GT/Off Grid/Hybdrid Inverter options explained)

Next, the specific question of how to connect to a genset.... A "standard" off grid battery backed AC inverter would connect to the Genset or the Utility and simply use the AC power source for either directly powering the AC loads (pass through mode) and recharge the battery bank from the AC source. Or, if the AC source "goes away", then the AC source is disconnected and battery power is used to power the AC inverter and power the loads connected to the output of the AC inverter.

Then there is the GT inverter... The standard Grid Tied Inverter (solar panel -> GT inverter -> AC mains) takes 100% of the available energy from the solar array and "dumps" that energy into the AC mains. The actual power is either used "locally" and/or sent back upstream to the AC source to be used elsewhere (nearby utility customers, etc.). A GT Inverter does not "regulate" voltage/frequency/current--It (more or less) simply dumps energy into the AC mains and the AC source (utility generators) set your 230 VAC 50 Hz power quality. For all the world, it is very much like the alternator simply dumping energy into the car's electrical system. The car's battery "regulates" the 12 volts (absorb charging current or supplies current to power loads when the engine is not running). If the grid voltage is too high/low and/or the grid frequency is + 1.0 Hz too high/too low, the GT inverter simply turns off for 5 minutes waiting for the grid voltage/frequency to stabilize.

There is also a "hybrid AC inverter". It is basically an Off Grid inverter that can also behave like a GT inverter and feed excess solar panel energy back to the grid.

Now--Note that a GT inverter (and Hybrid Inverter) treats the AC power source (utility power) for all the world like a giant (infinite) AC Battery Bank.The "battery" (utility) can supply or accept all loads/"charging" current from the solar array (and turn the electric meter "backwards" when "recharging" the grid).

AC generators (really "alternators") cannot usually accept "excess" (or charging current) from a GT inverter. Generators are only designed to output energy. If they receive energy, that energy would have to "go somewhere" -- Like spinning the generator faster (genset actually "spins" the diesel motor faster than 50 Hz) or into "heating" the electric windings.

So, for a GT + Generator, the AC loads would always have to be larger than the GT Solar power so that there is never any energy "backfeeding" the generator. If backfeed does occur, either the genset spins "too fast" (GT inverter shuts down, Genset faults/shuts down) or you get damaging currents in the genset (windings overheat, etc.).

There can be some other issues... One is that the genset needs to be pretty accurate in its frequency/RPM -- 50 Hz + 1.0 Hz. Many smaller gensets (portable generators) are not accurate enough for a GT inverter to "lock on" to the AC input power (GT inverter will never "turn on" or "connect" to AC power).

You are talking about relatively large levels of power and you need a very good electrical engineering team with experience designing/connecting such systems. And even then, you can still have problems. For example, here is a discussion about a (relatively) large solar+genset installation for a large villa and the headaches that that owner/caretaker had:

http://forum.solar-electric.com/forum/solar-electric-power-wind-power-balance-of-system/off-grid-solar-battery-systems/17986-is-the-system-set-up-right

Having a good power engineer (check references) will be absolutely essential to make your system work. And if you are doing something that nobody has done before--You may have a lot of issues to address to get the system to work (or it may never work and you have to start over with new/different equipment).

-Bill

If the loads are always larger than the GT Inverter's output (i.e., 5kWatt of GT inverter and 6+ kWatts of load), then as long as the generator frequency/voltage is stable enough for the GT inverter to sync with--You should be OK.

Of course, what happens if you pop a circuit breaker to your 6+ kWatt of base loads and the GT now starts to (try) back feed the genset--Will it be damaged or will the GT inverter "fault" due to voltage/frequency regulation issues--That I do not know.

There is another mode for some Off Grid (/Hybrid) AC inverters--It is called generator support mode. More or less, the OG inverter behaves like a UPS (AC in, battery, AC out) but if there is excess solar energy available, the AC inverter uses less AC power from the Genset and uses Solar+Battery power to supply energy to the loads. [h=3]Demonstration of Generator Support [/h] It is a possible solution for you... Mixes Generator+AC Mains with solar.

This can be a complex subject--And not all inverters implement generator support the same way or how you might have wanted to do it.

-Bill

In theory, yes, you could add the genset+inverter capability... In practice, the AC inverter usually has a fix/limited maximum output current. And for a 6 kWatt inverter, it would normally be limited to 6kW/kVA and a ~12kW/kVA surge (few seconds to few minutes, depending on inverter specifications).

For many folks, it allows them to use a smaller genset (say 4kW) and run larger loads (like a well pump) for short periods of time with the larger inverter. A smaller genset running near maximum rated load (typically ~80% maximum for non-commercial rated gensets) for best fuel economy (running base loads, battery charging) and the inverter supports the high surge current/short term loads.

-Bill