By Phil Kreveld
There are prognostications aplenty about the developments necessary in the NEM, for example, batteries versus gas versus coal and, yes, even nuclear. New projects hit the pages of the newspapers such as Katter’s CopperString transmission line project connecting Mt Isa with Powerink’s grid. In all this noise, there is one clear note that is not being sounded—and therefore this article. In the confusion of new transmission link projects, new batteries, and new schemes to contain solar rooftop energy contributions, etc., etc., the control of the vast machine that is the NEM doesn’t rate a mention. So let’s establish some engineering home truths that cannot be ignored or putting it another way, if resilience and stability in the nation’s electricity system are deemed necessary, then now is the time to implement a national control scheme.
Ah, our readers might well say, “a national control scheme”; that’s just another shout in the noisy discussions about the future of the NEM. So let’s review first why we are, as things stand, heading for serious challenges to resilience and stability.
Synchronous generation provided by coal is heading out the door and synchronous gas generation projects have not firmed up and even if some, like the Hunter project take shape, there will still be a large net loss of synchronous generation. Snowy 2.0 might be, if thought through, the last bastion of synchronicity on the mainland of Australia.
Can the new renewable projects that are taking us to zero-emission fill the gap? The answer is ‘no’ unless we consider new control schemes for the NEM that can preserve a strong synchronicity in the national grid. The alternative would be a series of ‘back to the future’ state grids, unconnected to each other or even mini and microgrids. We are wedded to alternating current systems and these, by their very nature, require synchronicity.
Whether we are talking inverter-based resources (IBR) for wind, solar or batteries, their basic technologies utilise internal oscillators that control the switching patterns of large semiconductor switches, providing a simulated AC output. It is critically important to take this into account when considering renewable grids because in order to have synchronicity, not only must these internal oscillators all operate at 50 Hz, but they must also maintain their phase stability between one another, i.e., without that latter requirement, the network could sustain disruption and serious damage.
Up to the present, phase stability has been assured because all IBR resources, be they wind farms, the Hornsby battery in South Australia and similar installations, and the millions of home and business solar inverters by employing ‘grid following’ IBR. This requires the presence of 50 Hz voltage at IBR connection points. The IBR internal oscillators latch onto the 50 Hz voltage to provide this essential phase stability.
So what would happen in the absence of such synchronising voltage, currently provided by traditional generation. In short, it wouldn’t operate as a grid unless the IBR, instead of being grid followers would, at least for some, be turned into ‘grid masters’ it. The basic technology for IBR grid masters, technically termed ‘voltage forming’ is in existence. However, to remove any doubts, not one of the so-called battery firming installations planned, utilise voltage forming IBR. There is a very good reason why, what appears an obvious solution to disappearing conventional synchronous generation, is not being implemented. It is this; the NEM complete resilience and stability structure is based on existing, conventional protection technology of circuit breakers, reclosers, under frequency relays, and requirements of generators to ‘stay on line’ for prescribed time periods when network faults occur. Furthermore, the protection and control scheme is still based on inertia, a natural property of synchronous generators, and something that IBR do no naturally possess (although synthetic forms can be engineered).
Does the foregoing imply that we will never be able to relinquish old fashioned generation? Of course, not! Engineering and control solutions are available with the only note of caution being that the necessary ‘baby steps’ are taken as we transition from conventional synchronous sources to new age synchronous sources.
Let’s come back to the information and control system mentioned at the outset and its relevance to IBR generation. Let’s form a picture of the vast machine that is the NEM. At all the connection points of transmission lines to generators and to load centres, frequency (50 Hz) and phase as well as power flow information are available—we simply have to have that information available on a synchronised basis. Think of it this way, basically a series of second-by-second snapshots across the entire NEM network. OK, so why? Power flow probably seems fairly obvious, being the one quantity already controlled by AEMO (although imperfectly) but what about ‘phase’? In the foregoing part of this article we found that all IBR have internal 50 Hz oscillators and that phase control between them is essential if they are to be part of an AC grid. Phase control means that that the peaks and troughs of their alternating current (and voltage for ‘voltage forming’ IBR) bear predictable time differences between each other, and in cases where there are deviations, that these be contained!
Now that doesn’t sound so difficult as a concept, but should we wait with the implementation of such a NEM-wide information system as it is certainly another major investment, until we have further developments in IBR are available? No, we shouldn’t—we should already have had this in place. If we start now with its implementation, we will at least be in a position to adopt new IBR developments because one thing appears a certainty in examining these new designs—they will only be of real service in providing stability and resilience on the basis of comprehensive synchronous data being available. In other words, these second-by-second network network snapshots will provide the type of feedback options incorporated in the newer technology IBR.
It’s high time to take heed, the more so because of the various uncoordinated projects that are taking place and notwithstanding the Energy Security Board, nothing but nothing will be ‘secure’ without complete transparency of the NEM, second-by-second!