There are myriad ways of revamping the power grid. New power lines, more powerful lines, buried power lines, cross-border lines – this blog has documented them extensively in the Grid Expansion series. However, there’s also an entirely different approach to increasing the scarce capacity of our networks: a technique that doesn’t involve re-laying lines or linking them up to one another. This method is designed to simply increase the throughput of existing power lines. The point of departure is referred to as the ‘n-1 criterion’.
It happened over a decade ago, but it’s still surprisingly fresh in our minds: Early in November of 2006, the mammoth cruise ship Norwegian Pearl built by the Meyer Wharf in Papenburg is ready to set off on the Ems. To ensure smooth sailing, the ultra high-voltage line above the river is turned off. But the entire undertaking is poorly planned. All of a sudden, millions of people are in the dark for 2 hours starting at around 10 p.m. on Saturday night. The effects can be felt even as far away as Morocco. The engineers had violated a cardinal rule of electricity transmission, known as the ‘n-1 criterion (pronounced ‘n minus one’). The consequence was one of the biggest blackouts in modern history.
The principle of n-1 security in grid dispatch is understood by experts to mean that a fully loaded grid made up of n components must remain functional even if one of the components fails (e.g. a transformer or power circuit). Such events – unlike the November 2006 disaster – should not result in supply interruptions or increased failure. A buffer sees to it that street lights stay on, machines run without interruption and homes are kept warm with electric heaters. Highly sensitive power users such as the chemical industry, for which an outage could pose a danger, apply a rule that is even stricter than the n-1 criterion: the grids to which they are connected are designed in such a way that grid security is not curtailed by the deactivation of a component concurrent to the failure of a second one (n-2 event).
Security of supply is a special good – especially in Germany. The country’s utilities repeatedly and justifiably underscore how seldom it fails. From Zingst in the high north and Zugspitze in the south, the nation’s population sits in the dark for an average of a mere twelve minutes a year – an act that only Switzerland can follow. Average annual outage time in the United Kingdom and France is a full 50 minutes, with the USA clocking nearly double that. No one wishes to second-guess Germany’s outstanding track record or the capacity buffer necessary in the grid. Only last year, the Head of the German Network Agency, Jochen Homan, refused abandoning the n-1 criterion. Period.
However, network bottlenecks and the sluggish construction of new lines begs the question whether one can eke out more of existing power lines. Or – to use a quote by the German Association of Energy and Water Industries (BDEW) – “Making optimal use of the existing network is something that will definitely require our attention in the next few years.” He adds that one could initially continue to develop the n-1 criterion purely theoretically and then introduce the new elements into the system once they have been researched and passed suitability tests. BDEW believes that placing the securement of the power grid on several shoulders is an important approach.
Facilities such as the huge battery stores of the Eastern German regional utility Wemag are helpful in this regard. Wemag built Europe’s largest commercial storage facility in Schwerin in cooperation with tech firm Younicos. Its mission: Offset sudden fluctuations in the grid that can occur when feeding wind and solar power into the system. Following the most recent expansion, the battery features twice its original output, which now totals ten megawatts, and nearly triple its former capacity, which now amounts to 14.5 megawatt hours. “The energy transition will only succeed if system services such as frequency and voltage stability build on new technologies,” underscores Wemag.
However, it is equally true that this 30 percent capacity is hardly every needed. It’s more of a reserve for the highly unlikely event of a malfunction. Christoph Maurer, Consentec GmbH
Someone dealing with the intelligent continued development of the n-1 criterion is Christoph Maruer. He holds a post-doctorate degree in industrial engineering and heads up Aachen-based Consentec GmbH, a consultancy specialised in the energy business. The point of departure of his considerations and those of other experts is that modern power grids can usually run at roughly 70 percent of their maximum capacity. The remaining 30 percent allows the electricity to be distributed over other lines or spread within the grid, ensuring that the lights never go out. “However, it is equally true that this 30 percent capacity is hardly every needed. It’s more of a reserve for the highly unlikely event of a malfunction,” explains Maruer. The one million dollar question is thus whether this ample buffer can be used after all. Can system capacity utilisation be increased without jeopardising security?
The means to this end is more intelligent control. For starters, this requires automated grid management. This involves not giving electricity free reign, forcing it to take a specific route. The advantage is that power lines with capacity to spare jump in for lines with heavy loads caused by system malfunction, reducing the risk of a crash. This intelligent mechanism works like a policeman at an intersection, who directs traffic away from the backup on street A to street B where it flows better. The increased control thus enables electricity to flow more evenly.
The second means is referred to as ‘reactive plant management.’ Today, this mechanism is preventive. This means that the grid cannot unleash its full potential since precautions are taken to cover possible malfunction. Wouldn’t it be marvellous, then, if there were means that were highly available and very rapidly deployable in reacting to network faults? Here’s an example: The wind is blowing so much in the north that the networks are at capacity. The unforeseen failure of a power line would force offshore turbines to be switched off because their output could not be placed on the system. (Sidebar: Grid problems in Germany are often connected to (strong) winds.) At the same time, energy is needed in the south, but there isn’t enough time to ramp up any more power plants. An alternative would be huge super-batteries that put out the fire. They could be as big as a football field, trouncing the Wemag battery in Schwerin.
“A power line can handle an overload for a very short period,” Consentec boss Maruer remarks. However, if one can rely on this type of turbo booster balancing things out in this timeframe, the line can run at much more than 70 percent on a daily basis. “This approach is roughly between the research and test stages,” the expert analyses. He adds that this technique can prove in practice in the next decade whether it is capable of keeping voltage up permanently.
Photo credits: WR.lili, shutterstock.com