© BMWi
What does "redispatching" actually mean?
How does the electricity get to the plug? Though consumers rarely spare this a thought, for the people behind the scenes it is a mammoth synchronisation task: they have to balance supply and demand and ensure that electric power is always available where and when it is needed. Increasing inputs of electricity from renewable sources are making this challenge even tougher. When the wind turbines off the coast of Helgoland are turning at full speed, the electricity flowing into the grid still has to reach the energy-intensive industry at the far end of the country in Bavaria that is working extra shifts to meet demand for its output. The electricity grids are more and more frequently coming up against their limits.
To prevent blackouts, the grid is under constant surveillance in the grid operators' control stations – and in case of doubt they have to intervene. Too much electricity feeding into line one? No alternative route available? Then the grid operators have to order a redispatch. The same applies in the opposite case, if there is not enough electricity in the grid.
"Dispatching": which power plant is supplying how much electricity when?
Redispatching means changing the operating schedule of power plants: the power plant operators tell the transmission grid operators in advance how they are planning to deploy their generating capacities the next day. Which power plant will be feeding how much electricity into the grid and when? If an analysis of this planning indicates that there is a risk of bottlenecks, or if an overload actually occurs at short notice, the transmission grid operators instruct the power plant operators to change their schedules; this is known as redispatching.
The aim is to keep the grid and the system stable – and so to ensure a highly reliable supply to the consumers – and to avert breakdowns in the grid: because too much incoming load stretches the lines to the limits of their technical capability. If those limits are exceeded, the sections of line affected automatically cut out as a security precaution to prevent costly damage. However, this increases the load on all the "alternative routes," which eventually also cut out. Figuratively speaking, it is just like a huge traffic jam: if everybody takes the detour, in the end nobody can move any more. In the long term, new by-passes are needed.
Redispatching has recently become more and more important
The situation with the transmission grids in Germany is similar: the need for redispatching has been increasing in recent years because of a shortage of high-capacity lines in some areas. According to provisional calculations by the national regulatory authority, the Federal Network Agency, the duration of redispatching interventions increased again slightly in 2014 compared to the previous year; by now, some adjustment to the operating schedules of the power plants is needed nearly every day. In all, the transmission grid operators reported interventions totalling 8,116 hours (provisional figures) in 2014 – in 2013 the figure was still 7,965. In 2014 these actions affected an estimated capacity of 5,131 gigawatt hours; in 2013 the figure was still 4,604 gigawatt hours, at the time equivalent to around one percent of overall generation (not including infeed of electricity from renewable energy sources remunerated under the Renewable Energy Sources Act) or the annual consumption of around 1.3 million average households.
The strain is greatest on the trunk transmission lines in the winter half-year: in the cold and windy months, the wind turbines in the north are feeding a particularly high input into the grid, while the industrial centres in the south are demanding even more energy – especially when it is cold and gets dark early. To prevent the lines from cutting out due to this "onslaught," two kinds of redispatching action are needed. The units in the north have to be run down, those in the south run up. At present, that is the only way to ensure stable operation of the grid. The data from the transmission grid operators reflect this quandary: while in the wind-power-producing regions in northern and eastern Germany (grid operated by 50Hertz) infeed frequently had to be cut back in 2013, the redispatching interventions in the south (grids operated by Amprion and Transnet-BW) went in the other direction. Here power plants were more often instructed to run up out of schedule.
The consequence is: the generating units in the north are not able to feed their output into the grid. And in the south, power plant capacity has to be kept on standby to be able to respond to bottlenecks at short notice. Since the winter of 2011/2012, the transmission grid operators in the south have more and more frequently had to fall back on foreign reserves. And in future the situation could be aggravated still further: in the north, more and more onshore and offshore wind farms are going on line, in the south the nuclear power plants are scheduled for decommissioning one by one. To prevent the ultimate jam from occurring, the Federal Requirement Plan approved by Parliament provides for the extension and expansion of the important north-south routes.
About "direct view"
The Energy Transition is not only changing our energy supply system, it is also bringing a lot of new buzzwords into play. Whether capacity market, HVDC lines or grid stability – technical terms that used to be only of interest to insiders are now becoming everyday concepts in the ongoing debate. In our new "direct view" column we will be taking a closer look at these terms: what do they actually mean? And what have they got to do with the Energy Transition? The series, which will be continued in random order, starts with the technical term "Redispatching."