It’s system strength, stupid!

Wind turbine with solar panels against beautiful sunny orange sky (renewable locally)
Image: Shutterstock

With coal and gas synchronous generation shutting down, system strength is severely impacted by wind, solar and battery generation having only a fifth to a quarter of the reactive power capacity of synchronous generation, writes Phil Kreveld.

It’s the economy, stupid” was a phrase coined by James Carville in 1992, when he was advising Bill Clinton in his successful run for the White House. And here it is paraphrased to focus on the quintessential feature of system strength for the national grid. It was always thus but now, as the nation transitions to very high renewables, it doesn’t earn a guernsey with the state and Commonwealth energy ministers.

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Of course, AEMO publications on system strength are available to one and all but they make for sleep-inducing reading. And to all intents and purposes, its tortuously worded, highly technical language insulates the decision makers in the national grid from the important message and the risks to system strength the transition to renewables is posing. High or low electrical energy prices, nuclear, hydro, more or less wind or solar considerations pale into insignificance when compared to system strength. Without system strength, we do not have a reliable national asset. In essence, whatever the weather conditions, whatever instantaneous power demand, we need our electricity at the correct voltage, not at some other value where things burn up, or motors stall—and we need it 24/7, i.e., we require system strength. The media is filled with hyped-up arguments put up by both proponents and detractors of solar, wind, coal, gas, etc., and the stern warning “no transition without transmission” but not a word about system strength.

In a nutshell, the national problem is that we have no market for system strength. We do have one for energy—but system strength? System strength evades us like drops of mercury when it comes to a marketable commodity. Yes, there are complicated rules as to who forks out for synchronous condensers on REZ buses, but there is so much more that simply ‘falls off the desk’. Many electrical engineers as well as many in the public at large worry about coal-fired generation shutting down while RenewEconomy and its readers shout hurrahs with every announcement of a big battery, new wind farm project, and so on. Oh, system strength being impaired as a possible result? That’s someone else’s problem. However, sheeting the culpability for waning system strength to the transition to renewables is to trivialise the problem statement. There is hard yakka to be done engineering-wise that could ensure both large penetration of renewables AND system strength. That said, persisting with the requirement that 82% MUST be renewable generation by 2030 is, well, stupid!

What is necessary is an engineering review, in particular, of the south-eastern grid ‘plugging in’ inverter-based resources, be they voltage-forming or grid-following IBR. Obviously that process is to be done in an engineering model of the grid and involves extensive computer-based studies in PSSE. For the cognoscenti reading this article, the afore studies are not to be confused with AEMO’s PSCAD-version 5, which seems basically used to make connection applications of new generation as hard as possible. The erstwhile Energy Security Board, now replaced by a clique of energy ministers, tried in vain to warn AEMO about its Integrated Systems Plan two years ago. It didn’t use the words ‘systems strength’ but said in its critique of the ISP that the planned withdrawal speed of synchronous generation was too fast. Too fast for the preservation of system strength, the ESB could have said, but it didn’t come down to using electrical engineering language.

AEMO uses complex descriptions of system strength. However, in plain language sufficient system strength in addition to the availability of voltages within specifications, everywhere in the grid, requires that no section of the grid be ‘cast off’ unless it can take care of its own power demand and can smoothly reconnect again to the bulk of the grid system. The key ingredient in system strength, given the humongous transmission line lengths already in existence and being planned, is the availability of sufficient reactive power. The illustration and accompanying text in Figure 1 provide an explanation of what reactive power is.

Graph depicting real or useable power
Real, or usable power, is illustrated by the red curve (sinusoid), flowing from generator to load. In a 50Hz AC system, the power frequency is 100Hz. Reactive power is illustrated by the blue sinusoid and also has a frequency of 100Hz in a 50Hz AC system. Notice, however, that rather than always being positive, the blue sinusoid swings from positive excursions to negative ones, indicating that there is no net power transfer from generator to load, i.e., reactive power is bounced back and forth between generator and load. Real power is measured in watts (volts x amperes). Reactive power is measured in volt-amperes reactive (VAr). Long transmission lines require significant VAr contribution in addition the requirements of the load at the receiving end. Creation of reactive power, like real power, is the task of the generator. In the event that the generator cannot supply the reactive power, other sources such as synchronous condensers must supply this essential component. Insufficient reactive power capacity heads an electricity system towards voltage collapse.

Without the ability to supply reactive power, the grid experiences voltage collapse, and breaks up! Renewable sources of energy are light-on in being able to provide enough reactive power whereas conventional synchronous generation has little or no problems by comparison. The longer the transmission line, the more reactive power in addition to real or usable power required by energy consumption centres at the far end, has to be delivered. Plainly put, we have relied on the remaining synchronous generation capacity to do the reactive power heavy lifting and there is no ‘plan B’ to have this capacity in place for the renewable transition. The principal reason is that there is no reactive power capacity market! We have some markets that kind of work in that direction like the Reliability and Reserve Trader scheme, but problems aren’t solved by nibbling the edges around it.

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The strong point to be made is that, with the exception of hydro, solar and wind by virtue of their IBR-connections to the grid have only a fifth to a quarter of the reactive power capacity of synchronous generation. What should we do? First, draw up a central plan scenario in engineering terms and then think about the markets that will be needed to provide system strength. The national engineering plan would be based on the present network, plugging in future power and reactive power capacities based on renewable sources—and then repeating the exercise based on future energy scenarios as per AEMO’s ISP or whatever else might become our national renewable energy goal. This has to be accomplished through the use of engineering models, and not in real life until we are absolutely sure we have a proper grasp of the system strength challenge. That way, we would ensure that transmission links AND connected generation capacity match the power as well as reactive power requirements of distant load buses—in other words, an integrated design with system strength as the central requirement.

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