After writing previous post, the RenewEconomy article kept going through my mind. The author of the article suggested that the response of the Hornsdale Power Reserve to a tripping coal fired power unit was extraordinary, when in reality it was insignificant in the grand scheme of things. I wondered why on earth the author was so lyrical about what was in fact a poor performance…
Then it suddenly struck me. It might well be a misinterpretation of how the event was represented.
Let me explain.
There were two graphs presented in the article. The first one is the frequency versus the response of the Hornsdale Power Unit and it stood central in previous post. There is however a second graph in the article and it is this graph that could easily lend itself to misinterpretation. It shows the sharp decline of the tripped coal unit combined with the response of the Hornsdale battery:
It is most probably created to show the overlap of the two graphs, indicating that the Hornsdale responded fast, even before the output of the Loy Yang unit went to zero.
I can certainly understand why this graph can be useful, but it also could easily be misinterpreted. Inattentive readers may be fooled into believing that the Hornsdale battery was more than strong enough to stabilize the frequency drop.
This is of course not possible. The Hornsdale battery only has a capacity of 100 MW and can store 129 MWh, so it could never fill in that gap. It also only provided on average about 5 MW during its three minute discharge. That is just a mere drop on a hot plate compared to the shortage created by the tripping coal unit.
What is the catch? Well, just look at the y-axis …
… I mean both y-axes.
This is by no means an apples-to-apples comparison. The AEMO used the primary axis (left) for the capacity loss of Loy Yang unit and a secondary axis (right) for the response of the Hornsdale Power Reserve. The primary axis goes from 0 to 600 MW and the secondary axis from 0 to … 9 MW.
What would it look like in a fair comparison between the two? Wanting to see this graphically, I digitized the graph and tried to recreate the graph. The final result looks something like this:
Maybe not exactly identical, but more than fit enough for the purpose that I want to use it for. It shows the same capacity decline of the Loy Yang unit from 560 MW to 0 and it also shows the initial 7.3 MW response of the Hornsdale Power Reserve, even before the Loy Yang unit output reached zero.
Now I have the graph lines in digital form, it is pretty easy to change the scale of the secondary y-axis to match the scale of the primary y-axis. Et voilà:
Where is the response of the Hornsdale battery? It is that tiny light blue streak that seemed to be glued to the x-axis…
That puts the cheered performance of the Hornsdale battery in another light. Now it is pretty clear that the Hornsdale Power Reserve could never ever do what the author of the article claims it did. The response was much too weak to make much of a difference.
I can also change the scale of the primary axis to match the scale of the secondary axis and then I get this:
The steepness of the Loy Yang unit capacity drop compared to the response of the battery shows how underwhelming the response of the Hornsdale Power Reserve was. This is only 1/62th of the height of the complete graph. To put it into perspective. This graph is about 7.5 cm on the screen of my computer. If my screen would been able to show the whole graph, than the top of the graph would roughly be near the ceiling of the next floor…
However, if I would just glance at the original graph as it is published in the article and I didn’t look too deep into the matter, then it might be tempting to glorify the ability of that shiny Hornsdale Power Reserve to save the grid from a blackout due to a dirty old tripping coal unit in another state 1000 km away… That could explain why the author (and his many sympathizers) cheered the performance of the battery. They might well have been fooled by the seemingly huge response of the battery, suggested by the use of two vastly different scales.
Looking at the bigger picture tells a different story. No matter how you look at it, there is no way that this minuscule response of the Hornsdale Power Reserve could play a significant role in stopping, let alone starting to reverse that steep drop.