Barking up the wrong tree—an engineer’s perspective

transmission towers against pretty evening sky (supernormal profit)
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By Phil Kreveld, electrical engineer

There is a lot of kerfuffle about nuclear energy at present with enthusiasts and detractors quoting all kinds of tech arguments to each other, and often politicising their ‘for or against’ stance.

Let’s agree, stripping away emotion, that nuclear generation is established technology and that it is basically a zero CO2 emitter. As to the CSIRO report, it is reasonable to assume that it was subject to professionalism—but does the CSIRO, or for that matter anyone else, really know how the grids of the future are going to behave, stability-wise, or energy versus capacity market-wise?

Related article: Large-scale nuclear included for first time in GenCost report

Ask the Electrical Power Research Institute, or the National Renewable Energy Laboratory, both USA-based, or the members of the Electrical Network Transmission System Operators of Europe (ENTSOE) and you’ll get ‘definite maybe’ answers. That’s because we are all learning as we go along the renewable path, and Australia is no exception. However, Australia does have a unique feature—domestic solar penetration is, percentage-wise, the highest in the world.

We are kings of the duck curve! And here is where a lot of the public forum discussions, including the politicians, are barking up the wrong tree. It is the duck curve, increasingly deeper and wider (think of domestic batteries) that causes long periods of close to zero energy import at zone substations. This is spooking the energy market for large scale renewables and other forms of generation—including nuclear. One might quibble with the CSIRO capacity factors for nuclear generation but one thing is for sure—baseload is disappearing at speed.

The other thing that has got transmission system operators seriously worried is transient stability, i.e., the loss of a transmission line, line faults cleared outside time limits imposed by inertia availability, voltage stability as transmission lines are maxed, and black start procedures. The seriousness of the situation as inertia goes down the tubes is setting us up for instability.

Engineers who want to refresh their knowledge of the equal area stability criterion, are encouraged to grab their Kundur Power System Stability and Control, pages 831 to 835, and to dwell on the fact that the physics of transient stability hasn’t conveniently changed to suit newer forms of generation. That’s where the discussion should be taking place—transient stability. Its most important ingredient is inertia.

There is more barking up the wrong tree. The electricity worrywarts are on, 24/7, about energy sufficiency. The handwringing about what to do when the ‘sun doesn’t shine, or the wind doesn’t blow’ should be replaced with a halt to the independent, silo-like, state-based planning. In spite of AEMO’s best efforts, this is a terrible way to assure that rate of change of frequency (RoCoF) is contained because without that, kiss reliability goodbye!

It goes like this:

RoCoF is directly proportional to power imbalance between generation and load requirements, and inversely proportional to inertia in the electricity system.

Related article: Report: Nuclear SMRs ‘too expensive, too slow, and too risky’

Power imbalance opportunities are on the increase and inertia is on the decrease. AEMO is well aware of this but it is essentially the operator, not the designer of the national grid. Frequency control ancillary services notwithstanding, the time of zero inertia is approaching if a system comprising entirely of inverters, whether wind or solar as imagined by the ‘climate change purists’ comes about. In that regard, and not a moment too soon, gas-fired generation is being welcomed back but its inertia contribution is small compared to coal plant.

There will those who have read this far down who will point out that forms of inertia, basically damping of the response speed of electronic control circuits of inverters, will take over from rotationally stored energy. But let’s be clear, mathematical modelling is not sufficient assurance—physical operational experience is the only assurance. And as to ‘barking’, there is far too little barking in the public space from the engineering community.

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