By Phil Kreveld
An alternating current electricity grid is a high wire act. Everything is fine until wobbles make it lose its balance.
There are two states in an electricity grid: one, where the sum of all active and reactive power, whether generated or consumed, is a zero sum (Tellegen’s theorem); the other stable state is the blackout—nothing being generated or consumed, and also a zero sum.
The first state mentioned is the equivalent of the high wire act. And like a high wire performance, wobbles in generation and consumption can cause an electricity grid to crash. As to those wobbles, these are growing and are more difficult to contain than three decades and longer ago.
Consumer energy resources—principally rooftop solar PV, and the vagaries of wind and sun—generally make it so. Blaming renewables is unwise: wobbles in voltage, frequency and power have always been part of the fun of electrical engineers. But now things are more challenging, i.e., wobbles in the face of increasing inverter-based generation, both large scale and small scale in distribution networks have altered the challenge. Getting back on the high wire, once fallen off, is now a very different and more difficult exercise than ever before.
Related article: Milliseconds for decisions—or is it hasta la vista?
Behind the Australian Financial Review article of 28 June quoting the Australian Energy Market Operator’s concerns about black-start procedures, should blackouts occur, is a cloudy body of concepts on how to deal with ‘non-credible contingencies’ in the national grids. This is a term that describes any breakdown in the electricity system no one thought likely to occur.
Obviously planned maintenance of generators and transmission lines are ‘credible contingencies’. The ‘non-credible’ variety is the stuff of nightmares and causes the blame games. They are about to erupt in Spain, and we remember the blackout of South Australia in 2016, and then prime minister Malcolm Turnbull blaming the state’s premier, Jay Weatherill, for his irresponsible love affair with renewables.
In truth, non-credible contingencies can easily be defined following major breakdowns. Investigations then reveal ‘learnings’ that might form the basis of ‘credible contingencies’ in future planning. A cold-eyed view of major electricity crashes around the world reveals that notwithstanding forethought, being bitten on the bum is a highly credible risk. Thinking up the various ways grid failures might occur and what black-start cranking paths (defining local ‘islands where generation and limited consumption are first established) can be availed of requires intensive engineering research.
It is far easier to either conclude that we have chosen the wrong technologies, or to express ill-founded confidence that we have matters in hand. Either way, we are warming up debates without achieving practical solutions. Meanwhile we are nationally ‘rumbling our worry stones’ but to no avail because we do not have a centralised, national engineering authority charged with the analysis and design of maintaining stability on the ‘high wire’.
So, let’s get back to Bernard Tellegen’ theorem. All active and reactive power, whether generated or absorbed must add to zero at any and all instants. Therefore, power, voltage and frequency control are forced to fit in with the theorem; not the other way around. Whether we do a good job or a bad job on the control of those variables doesn’t affect the outcome other than that inappropriate measures have us falling off the high wire. Our national challenge is therefore ‘herding the cats’ into one place; one where power-frequency, reactive power-voltage stabilities—and reestablishing the grid after regional or blackouts are the basis of one engineering solution.
Hiding behind the limitations of particular technologies, for example; limited reactive power support of inverter-based resources, therefore wanting to retain synchronous machines in the grid, or attempting to create yet another new-fangled market, i.e., for inertia to solve power-frequency stability, is not the way of eliminating non-credible contingencies. If anything, those events become more ‘credible’ because we have refused to assess the stability requirement on a ‘whole of system’ basis.
Related article: Reactive power, voltage regulation, and other lessons from Spain’s grid failure
It is too early to assess what went wrong for Spain on 28 April but what does emerge is a worrying unpreparedness for the limitations imposed by IBR, as evidenced by the requirement for synchronous generators to provide reactive power support during hours of darkness.
We face headaches in our electricity landscape: long transmission lines in a radial topology without tie-lines. This results in high charging currents, thus stretching reactive power capacity for black start and more of a challenge in containing frequency spread. A large part of distribution networks with very high ratios of solar PV are unable to assist in restarting because stable voltage for household and business solar inverters must first be established. Until then, being voltage-following, they have to remain switched off.
The task of identifying cranking paths for black start, by region, by jurisdiction, and nationally is in the ‘too hard basket’. Thus, we are a target for patchwork ‘solutions’ that sooner or later will end in tears and recriminations. The headlines may well read something like “Crash landing because of wind, solar and batteries”. Small consolation, indeed!
