How Axpo power plants prevented a blackout

Fine, we admit it. That was pure clickbait. But when the frequency in the European interconnected grid dropped from one second to the next, it really was thanks to Axpo power plants (and, yes, many others too) that a blackout didn’t occur.

In order to understand the concept of grid frequency, it’s essential to know that the electricity grid in continental Europe is linked together, from Istanbul to Lisbon – including Switzerland along the way. The electricity in this interconnected grid is constantly changing direction, going back and forth 50 times a second. This is what we refer to as an ‘alternating current’. In order for the electricity supply to function, the rhythm, or frequency, of the electricity grid has to be the same everywhere – that is, 50 hertz. If there is too much electricity in the grid, the frequency increases; too little and it decreases.

On 8 January, the grid frequency in north-western Europe dropped by 250 millihertz – an unusually large drop. How did it happen? An investigation by the European Network of Transmission System Operators (ENTSO-E) suggests that the chain of events began with the failure of a substation in Ernestinovo, Croatia. At the time, a large amount of electricity from south-eastern and north-western Europe was flowing through this substation. After the substation failed, the electricity tried to find its way through other power cables, which then overloaded and deactivated, one after the other.

Split European electricity grid

With all connections between the north-east and the south-west now cut off, the European electricity grid was split in two. The south-east suddenly had an electricity surplus of about 6,300 megawatts – precisely the amount missing from the north-west. This led to a dramatic increase in frequency in the south-east and a drop in frequency in the north-west, which naturally included Switzerland.

Over-production is an issue that can be solved relatively quickly by simply shutting down power plants. But north-western Europe was suddenly missing 6,300 megawatts (or six times the output of the Gösgen nuclear power plant). The grid operators had to compensate for this missing output within a few seconds. France and Italy automatically removed numerous large industrial consumers from the grid (1,700 megawatts’ output) and an emergency boost of 480 megawatts was fed in from the UK and the interconnected Scandinavian grid. But to prevent the frequency from falling further and triggering a blackout, power plants in north-western Europe had to quickly make up the remaining 4,100 megawatts missing from the grid.

Axpo power plants deliver within seconds

A completely automatic process known as ‘primary control’ works throughout Europe. When the frequency in the grid declines, power plants are ramped up as required. Axpo power plants keep output in reserve for situations like this and are compensated for it. On 8 January, Axpo back-up power plants pumped about 300 megawatts of primary control power into the grid within 11 seconds. This was much more than expected. Axpo made up a good 7% of north-western Europe’s missing output within a very short time, and the largest supplier was the Limmern pumped storage plant.

So with their 300 megawatts, Axpo power plants provided vital support for grid stability in Europe. As a comparison, Swissgrid has just 60 megawatts in reserve for frequency equalisation throughout Switzerland. It was certainly fortunate that at that moment power stations in Switzerland and throughout north-western Europe were online and could step in and supply significantly more than the reserved capacity – and thus prevent the electricity grid from collapsing.

Number 1 risk: power shortage

If we look at the continent of Europe as a whole, the incident in January was not a power shortage. Overall, there was sufficient capacity. However, the means to transport electricity from the south-east to the north-west at short notice was missing, and that led to the shortage in this case. According to a risk report by the federal government, power shortages pose the greatest risk to Switzerland. In the event of a longer term electricity shortage – and thus blackouts – the damage would be enormous. In fact, the risk report rates this as an even bigger threat than a pandemic.