By Phil Kreveld, electrical engineer
The recently announced Capacity Investment Scheme (CIS) by the Commonwealth Government requires some forensic examination before we get all excited. There are problems associated with the transition to 82% renewables by 2030, which the CIS is meant to address, therefore requiring analysis. Some elementary notions in electrical engineering discussed in this article demonstrate the need for much more detail in the announced capacity scheme. It is reasonable to assume that the CIS has as little engineering underpinning as the previous Morrison Government’s Underwriting New Generation Investments Program (UNGI), which was designed to stop the demise of coal-fired generation.
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The hasty move to the CIS by the Commonwealth Government might keep catastrophic grid failure at bay—a little longer, perhaps! However, it is yet another brave attempt to short-circuit required electrical engineering effort with a new-fangled market scheme to fulfil the renewable dream. The scheme, announced on Thursday 23 November, will involve a series of six-monthly blind auctions between the June quarter of next year and 2027 to underwrite 18GW of new wind power, 5GW of solar and 9GW of battery storage. The subsidy scheme, basically contracts for difference, hinges on the government providing clean energy developers with a revenue guarantee that ensures payments to companies when electrical energy market rates fall below a certain threshold. Conversely, if prices surge the government will take a portion of the “super profit” upside.
Taxpayers will have to fund successful capacity bids from reverse auctions for supply of gigawatt capacity—from existing and potential investors with newly revived hopes of making a buck out of Australia’s transition to wind. solar, batteries and hydro—and ‘no gas’ please. In one sense, this mixture of commerce and faux engineering prescription of ruling out gas reinforced by the studied ignorance of the facts of (electrical) life is humorous but it is actually too serious a matter to make light of.
There are two critically important engineering aspects to any CIS; (a) Where are the CIS-attracted investments to be located, and (b) exactly what is understood by capacity. Therefore, nutting out the technology requirements for a stable, strong grid, not one subject to catastrophic voltage collapse and grid separations, should have been the first priority. The ‘where’ part relates to availability to transmission lines, and their capacity to transmit power stably; the ‘what is understood by capacity’ is all about prevention of the grid breaking up through voltage collapse. The questions of location and definition of capacity interact providing complexity. But we try to untangle it here in as clear a way as possible. To do this in an accessible manner, some gross simplifications are made in order to make clear the interrelationship between power flow, transmission line capacity and voltage stability, being the three ingredients for a stable, reliable and secure electricity system.
Let’s examine this hasty scheme to underpin energy reliability. Below is an illustrative, highly simplified equation, electrical engineers are very familiar with. In order to understand the dangers lurking behind the CIS, it is important to understand the electrical basics. The following remarks will explain how important the location of generation capacity viz a viz energy consumption centres is and why capacity is not just a matter of gigawatts!
P stands for power transmitted by a transmission line at some instant and
obviously will vary based on changes in electricity consumption with time; Pmax is the maximum power that can be transmitted by the transmission line, mainly determined by its length. The trigonometric function ‘sine delta’ (i.e., sin δ) refers to the lag in time between the voltage at the power sending end (i.e., where a generator or battery is connected), and at the receiving end where the power is needed to run motors, lights, etc. Alternating current (AC) has a frequency of 50 cycles per second, and each cycle therefore has a period in time of 1/50th of a second, equivalent to an angle of 360 electrical degrees.
A visualisation of alternating current transmission in a transmission line. If the generator is pictured to be the black trace on the LH side (sending end), then the orange trace on the RH side is the receiving end. The intermediate traces are relevant for shorter transmission lines. They have smaller electrical angle (separation in time).
At a delta (δ) of 90 degrees (i.e., 1/200th of second time difference between sending and receiving ends), the trigonometric function ‘sin(e) delta is 1.00 and therefore the maximum that can be sent down the transmission line. As a rule, the longer the transmission line is, the larger the time difference, or in other words, the electrical angle, and the lower is Pmax therefore limiting power capacity. At larger angles, i.e., more than 90 degrees, synchronicity is lost—and that causes the grid to separate into smaller parts no longer capable of transmitting power to each other as required by local energy consumption requirements. And that, for the national grid would be a calamity. The reason is that the smaller parts, like South Australia at present, may not have self-sustaining dispatchable capacity, and therefore already demonstrates the importance of the locations of generation capacity.
As to capacity, specifying this in gigawatts is missing the essence of what capacity is. First, capacity is not necessarily dispatchable and several critics of the CIS have already pointed this out. Here, however, we discuss the essence of capacity and its relevance to long transmission lines. As mentioned, the longer, the line, the more the power being transmitted (in megawatts, or gigawatts), the larger the voltage angle difference between generator and load, and the more additional power (reactive power) has to be provided by the generator to provide stable voltage for consumers of electrical energy. Reactive power does not provide usable energy. As an example, let’s assume for every gigawatt, 50% additional reactive power is needed—a very reasonable assumption. That requires from the generator 12% additional capacity. If the generator owner earns only when supplying usable power, there’s 12% of additional capacity necessary but for which there appears to be no financial reward. Alternately if the owner’s plant is limited in its total capacity output, the financially rewarding usable power capacity is reduced to 90%.
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The foregoing comments illustrate the need to actually design—engineering-wise—the renewable transition path and then, and only then, to figure out market mechanisms that are likely to attract the right investments. Chris Bowen’s CIS will most probably meet with quiet resistance from investors who already face transmission line congestion, likely to be exacerbated by inappropriately located new solar, wind and batteries. The propeller heads in AEMO are also going to break out in cold sweats as they are the guardians of grid stability, and therefore will impose all kinds of technical holdups for the connection of new capacity. Time for quiet reflection and a return to the drawing board to avoid ‘electrical storms’!
