what is the current mechanism for providing a base 50HZ for the non continuous supplies to tie to?
I guess if there are some large Hydro generators to provide the inertia it would be ok, but they sometimes need to be turned off due to lack of water.
An inverter operating as an islanded power system, that is without the grid, can of itself send out 50Hz with no external reference so long as it's designed to do so.
On a tiny scale the one I've got at home can do that. If the grid goes dead well then the house can be (is) isolated from it via relays and my little inverter will then put itself into operation as a stand alone power system in order to keep the load supplied. So long as the solar panels + battery can supply the required DC current to the inverter, and house load doesn't exceed 5kW, it'll keep running and I'll have 230V 50Hz.
On a larger scale the Dalrymple battery (SA) is at the end of a single transmission line supplying the region and is capable of operation as a stand alone system, that is without the transmission line being in service, if required. So long as the battery has charge in it, which may last quite some time if it's sunny or windy due to the large number of houses with rooftop solar and a wind farm in the area, then the local distribution network will remain live despite being disconnected from the rest of the grid.
On a larger scale though well it's somewhat more complex than that and it's most easily explained by noting what would happen if I drew more than 5kW, even momentarily, from my inverter at home whilst operating in islanded (without the grid) mode. The short answer is a complete loss of supply, it'll shut down. Not so bad if it's just my house but a rather big problem if it's the whole state.
The idea of running a grid without synchronous generation, so that is one based entirely on inverters, is an area where Australia is pushing the limits (globally) at present.
SA does at times generate more wind + solar than local consumption indeed so far as is known to AEMO and others in Australia it has the highest use of wind + solar (combined) of any large power system on the planet. Anywhere using more is a small island etc which doesn't have the same reliability requirements.
Due that situation, the "natural" outcome with respect to the operation of synchronous generating plant (which in SA's case is mostly gas-fired and the rest is using oil-based fuels) is problematic in that often there'd be effectively no such plant in operation, indeed it would literally go to zero. That causes a huge problem in terms of system strength and the ability of the system to withstand faults.
The workaround to the situation has thus far been to simply force the operation of a minimum number of synchronous units, regardless of the economics, so as to maintain system strength. That's expensive and means at times gas is burned whilst wind + solar are throttled back (ie wasted) but it does resolve the technical problem. The numbers have been crunched, there's an official list of acceptable combinations of synchronous plant online, but for the record the most common minimum arrangement in practice is one gas turbine + the steam turbine at Pelican Point power station plus any two (of four) 200 MW steam units at Torrens Island B. Many other combinations exist but that's the most commonly used, largely driven by economics.
A better workaround is the four synchronous condensers being installed, two each at Davenport (near Port Augusta) and Robertstown (about 100km north of Adelaide) substations. They'll significantly reduce, but not eliminate, the need to have synchronous plant online.
The other place of significance is Tasmania.
Between wind farms, rooftop solar and Basslink (since it's a DC link) Tasmania has exceeded 80% non-synchronous generation in the grid. Bearing in mind that unlike SA, Tasmania has no AC interconnection to anywhere else, it's a possible world record. Not confirmed but it plausibly is and even if not then it's certainly right up there pushing the limits in terms of what's been done, anywhere, for a power system of substantial size and complexity.
Making it work in Tasmania comes down to two key things:
1. The Basslink SPS (Special Protection Scheme) which is a bespoke system which gets around the problem that in import (Vic to Tas) mode Basslink represents what would otherwise be an unacceptably large single source of supply relative to the overall system size.
That poses a huge risk if it trips (which has actually occurred on multiple occasions since large inverters aren't particularly stable in operation, they're somewhat prone to random trips). Without the SPS it could very easily collapse the entire system but, thus far at least, the SPS has worked as intended and the lights have stayed on every time.
2. Many of the hydro stations have the ability to run without water as synchronous condensers, meaning there's been no need to build anything specifically for that purpose. Since the need to operate as a syn con occurs when not much synchronous plant is being run for generation, there's no conflict there, the same machine can be used for both purposes.
Water on for generation, water off and in the case of Francis turbines blow the tail water out (with compressed air) for use as a syn con. No need for that step with a Pelton turbine since they're not submerged in normal operation.
Three small open cycle gas turbines in Tas, which are rarely used for generation, are also set up to be run as syn cons. They don't offer the same degree of mechanical inertia as a hydro unit of the same capacity would but they're still sufficient to be useful. Again it's a case of putting the same equipment to multiple uses which saves $.
For the other states there aren't the same issues at a state wide level (though there are in some specific locations - eg at the connection point for wind and solar farms) thus far but Victoria's getting close enough that AEMO has identified and listed suitable combinations of synchronous (coal / gas / hydro) plant that must remain on as a minimum. Same at a regional level for North Queensland where it's a very definite constraint.
Looking ahead, nobody's running any major power system without synchronous plant at all so far as I'm aware. It's being done on a small scale at times, islands and so on, but they generally don't have the same reliability requirements plus their relatively simple power systems could be very quickly restarted if the need arises - just turn the diesel engines on.
It's much harder for a major system where there's cities, heavy industry, electric transport and so on all using it and where a full restart is a very major exercise that could end up taking a full 24 hours or even longer. Can't take so many risks there.
At some point running without any synchronous generation will happen but there's a lot more number crunching and modelling to be done before it happens on a large scale system.
Related to that is not simply how inverters perform but also how they could be made to perform differently. That is, essentially, how they could be made to replicate great big lumps of spinning metal which, whilst far lower tech, does have that inherent robustness and ability to deliver fault currents that anything based around electronics traditionally struggles to match.