Grid-related ancillary services
The energy reforms have fundamentally changed the power supply in Germany. With this, the demands have also changed forthe so-called ancillary services which are furnished by the grid operators for a stable operation of the power supply system. Frequency and voltage stability, restoration of supply and operational management are facing new challenges.
Frequency control ensures that power fed into the grid corresponds to power consumption (load) at all times. Deviations in the total balance - e. g. through projection error - can cause a change in frequency. The transmission system operator is then prompted to ensure that the required frequency is returned to 50 Hz again without delay with the help of ancillary service products (here: balancing energy). The rising amount of fluctuating generation capacity because of renewable energy has increased the demand for ancillary services on the grid.
For stable operation and to protect people, equipment and consumer devices, the voltage for the consumer has to be maintained by various means within the permissible voltage range of +/- 10 % of the nominal voltage. A sufficient supply of reactive power is necessary, among other things, to ensure voltage stability. Both in the transmission and the distribution grids, the requirements for voltage stability increase while at the same time the operating times of conventional power plants decrease. Therefore, more alternative sources, such as the reactive power supply from decentralised generation facilities or alternative concepts for voltage stability, must be used.
In the event of a complete or large-scale blackout in the European synchronous grid, restoring the supply would be implemented by means of a central concept of starting up black start capable large-scale power plants on transmission network level. Against the backdrop of the increasing share of renewable energies, in the future, it will have to be clarified to what extent specific control options for generation plants can or must be established in the case of a supply restoration.
Within the framework of operational management, the grid operators are tasked with monitoring the power grid and all of the connected generation units and loads, and if necessary to intervene to ensure the safe operation of the entire system. Due to the increasing complexity in grid operation, the need for information and control has increased in order to ensure system stability.
Interruptible loads
Certain companies consume power in significant quantities virtually around the clock (on the scale of a town or medium-sized city). However at the same time, they are able to reduce or almost completely stop their consumption at short notice without adversely interfering with their production process. Transmission system operators should take advantage of this opportunity to reduce electricity consumption (i. e. load shedding) in order to stabilize the power grids in certain situations. In the future, companies will have the opportunity to take part in tendering procedures for load shedding offered by transmission system operators. After winning a tender, the companies get a certain amount of compensation for staying ready to shut down every month, while on the other hand, there is an additional compensation for actually shutting down.
The tendering procedure is as follows: nationwide, the operators have to advertise 3,000 megawatts per month of so-called "stand-down capacity" on an IT platform. 1,500 megawatts of this must be able to be switched off immediately, thus be able to be taken off the grid within seconds. The other 1,500 megawatts has to be able to be taken off the grid quickly - within 15 minutes. The costs incurred by the grid operator may be apportioned evenly to all electricity consumers. For an average household, this should be approx. 1 to 2 euros per year (maximum approx. 4 euros).
The German government put in place a Ordinance on Interruptible Loads in 2012. In December 2012, a regulation that was initially limited to three years was adopted by the German Bundestag and went into effect on January 1, 2013.
Retrofitting of photovoltaic, wind energy, biomass, CHP and small hydroelectric power plants
Within the last decade, the feeding-in of decentralised generation plants has increased significantly. The feeding in of these decentralised plants takes place largely in the low or medium voltage grid.
Studies in 2011 have shown that decentralised generation plants can pose a system threat for the grid. An acute threat could create for example a large-scale disruption to the European synchronous grid if the mains frequency differs greatly from 50.0 Hz (e.g. over 50.2 Hz or below 49.5 Hz). In such a case, the frequency in the power grid could, under certain conditions, abruptly plummet by an automatic (frequency dependent) shutting down of these decentralised plants and could no longer be offset by the grid operators’ countermeasures.
This problem affects photovoltaic (PV), biomass, small cogeneration and hydroelectric power plants and wind farms.
The 50.2 Hertz problem
Until 2011, PV plants were equipped with a "circuit breaker" that caused the plant to be shut down during an overfrequency of 50.2 Hertz. A study prepared by consulting firm Ecofys within the framework of the Platform for Future-oriented Energy Grids has shown that under unfavourable circumstances, i. e. when the critical frequency of 50.2 Hz is reached at times of insolation, PV plants in Germany with a capacity of around 9 gigawatts would abruptly shut down (by comparison, that is equal to the capacity of approx. 9 to 13 large-scale power plants). The balancing reserve in the European synchronous grid can counter up to 3 gigawatts. A critical grid situation with far-reaching consequences - including a possible power failure - cannot then be ruled out. A retrofit of PV plants has become absolutely necessary to eliminate these system hazards.
The German government has therefore enacted the System Stability Ordinance (SysStabV). The ordinance requires that electricity distribution grid operators carry out retrofits and that the plant operators cooperate. The necessary costs for retrofitting will be financed by the grid operators in accordance with § 10 SysStabV and § 47 of the Renewable Energy Act (EEG).
The 49.5 Hertz problem
There is a need for action not only at PV plants but also at wind farms, biomass, CHP and small hydroelectric power plants. A study commissioned by the Federal Ministry for Economics and Energy ("Developing a Retrofitting Strategy for Generation Plants on Medium and Low Voltage Grids to Preserve System Security during Over and Under Frequency") shows that by a drop in the grid frequency to 49.5 Hz, plants with an installed capacity of a total of 27 GW will be automatically shut down. Should such a case occur, the grid operators could no longer stabilize the power grid - with far-reaching consequences including a possible complete power failure. The frequency security settings at these plants must therefore be changed.
The Energy Grids Platform worked closely together with the relevant business associations, the grid operators, the Federal Network Agency and the relevant federal ministries to discuss the retrofitting options and the legal and organisational shape of the retrofitting process.
An amendment to the System Stability Ordinance is now intended to resolve the 49.5 hertz problem and to oblige the operators of the approx. 21,000 facilities to retrofit the frequency protection settings of their facilities accordingly. The Federal Government adopted this amendment to the System Stability Ordinance in the cabinet on 17 December 2014. After being considered and adopted by the German Federal Council, the regulation will come into force during the first quarter of 2015.