By Phil Kreveld
Imagine having all the water you need, never having to turn on the main tap—but they make you pay for your neighbour’s water pressure anyway.
The electrical equivalent is in prospect for rooftop solar owners. There’s no alternative because the Australian Energy Market Operator (AEMO) has long since figured out that it simply cannot run the south-eastern grid based on zero energy transfer to distribution networks, to which household and business solar are connected. And as distribution networks—at times of brilliant sunshine on the east coast—generate just about all the energy needed, there is very little contribution required from wind and solar farms, plus hydro, plus gas, plus the watts coming from Tassie.
The qualifier ‘at times’ is disappearing as the period now stretches from morning to …? AEMO knows only too well that the ‘question mark period’ must not be allowed to enlarge. With solar owners being encouraged to invest in batteries by way of very generous subsidies, energy independence at midday will creep into early evening and beyond. The water paradigm translates into enough home-grown energy not to have to extract energy from the network—but having to pay anyway for voltage (water pressure) at the inverter terminals.
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Enthusiastic solar owners with ‘in-line’ battery systems could, if brave enough, disconnect from their distribution network. But—if they lack confidence, or battery storage capacity, they will continue to have to pay for the presence of that voltage. The majority of household solar inverters require the presence of voltage at their terminals because like their interposed battery companions when in on-line mode, they are grid voltage following. All small-scale inverters utilise a phase locked loop, even the ones that can switch over to independent operation. The PLL-inverters cannot operate without line voltage at their terminals. Inverters that can operate offline as well, ‘form’ voltage in offline mode, but when connected to the grid, are voltage following.
The domestic and business solar inverters of Australia represent a mixed bag of equipment and the specs required as the march of rooftop solar continues is seeing to frequent changes in technical requirements. The most recent one, emergency backstop, mandatory for new installations since last year, is causing far sighted solar installers to play down its significance. Emergency backstop brings in DRM0, the remote instruction to switch off the inverter. It has been lying in wait amongst the dusty pages of Australian Standard AS/NZS 4755—but its time has come.
The backstop measure is presented as an unlikely event, i.e., in the event of ‘grid instability’, it might be invoked. Not mentioned, is that it is the very existence of all those Aussie solar systems basking in the sun that is responsible for grid instability. AEMO’s major headache is the operation of south eastern grid with stable transmission voltages but with minimum, or worse, zero power flow other than charging current. The only known way of running the south eastern grid is for electrical power to always be flowing from ‘big generation’ to distribution networks.
We have backed ourselves into a corner where all the encouragement to invest in rooftop solar and batteries has come home to roost. AEMO never figured on governments of Australia whipping up enthusiasm for doing the right thing by the planet, for handing out all kinds of financial assistance by way of subsidies, etc., to the extent that political expediency added extra momentum! In one corner, so to speak are the transmission networks and large-scale generation—in the other, the pip-squeaks, who in aggregate match the ‘big end of town’. The former are a highly controllable group—and come what may, must be kept in business. The latter? We will have to shut them off to keep ‘the system’ in operation—voltage and frequency stability-wise. But not only stability is at stake here. It is the very existence of transmission and large-scale generation that must be safeguarded—financially.
Whether we use the AEMO term ‘shaking off solar’, or DRM0, or some other wording, the inescapable reality is that we have allowed the rise and rise of rooftop solar, coming up from time to time with schemes to reward its owners, but the game is up. The virtual power plant schemes and the dynamic operating envelope schemes whereby distribution networks can selectively control export from inverters are in themselves good developments. However, we face a problem of scale: rooftop capacity is now so large as to interfere with the control of large generator-transmission grid technology. This is a uniquely Australian problem. Capital investment is almost solely directed to transmission, batteries, and wind and solar farms. Distribution network schemes trying to take a more active role in virtual power where they take charge directly are disapproved by the Australian Energy Regulator.
In sum, we are using the renewable technology afforded by solar, wind and batteries in a classical electrical engineering format. Most of the population may be ‘generating their own watts’ but in order to keep the classic system going, they must help in absorbing externally generated power so as to preserve grid stability.
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The current state of consumer energy resources energy-independence for increasingly longer periods, demands a rethink of the national electricity system. Worrying enough is the stranding of assets we are busily constructing in the ‘no transition without transmission’ policies being pursued. To turn distribution networks into microgrids is no mean exercise, engineering-wise and capital investment-wise. The benefits could grant us greater energy security because interconnected systems like our south eastern grid are always subject to concatenation of failures that can bring down the complete system. A series of autonomous networks do not face this potential problem—their voltage, voltage angle and frequency stabilities are independent of one another.
If AEMO has ever been tempted to think along the above lines, we are none the wiser, which is not to denigrate the operator because microgrid operation would be complex. But so is the operation of an essentially radial transmission grid with a generator topology providing an enormous challenge under reduced inertia conditions and significant weakening of system strength. Emergency backstop however, again puts the spotlight on the disjunction building up in the national electricity system, one in which mums and dads will pay for voltage whether they’re energy self-sufficient or not—and worse, where their investment in solar will wither because stability demands that they remain net importers of energy.
As things are, the cost benefits of solar, wind and batteries, will slip through our fingers. There is no way out of keeping the big end of town viable other than for us consumers of electrical energy to continue with the price levels we ‘enjoy’ at present, hoping they don’t rise further.
