«Project management: Deutsche Energie-Agentur GmbH (dena) – German Energy Agency Project partners: 50Hertz Transmission GmbH, ABB AG, Amprion GmbH, ...»
For stable operation of the power supply system, the power fed must correspond with the electricity consumption in the grid at all times, taking the import/export balance into account. The balancing group managers should ideally ensure fully balanced planning and react in the event of deviations within their respective balancing group.
In the event of deviations between generation and consumption, the frequency increases or decreases.
The transmission system operators must ensure that the balance is restored immediately so that the target frequency of 50 Hz is maintained again.
The instantaneous reserve and the balancing energy are particularly important and are analysed in detail in this study. To maintain the frequency, transmission system operators can use contractually agreed flexible loads, or demand further adjustments from electricity suppliers and consumers in emergencies. Frequency-dependent load shedding, i.e. automatic gradual disconnection of loads from the grid, is used as a final security measure in the event of insufficient frequency. In the event of excess frequencies, electricity feed-in is throttled.
3.1 Instantaneous reserve.
Before balancing energy is technically fully available to equalise generation and consumption due to the activation times, rapid frequency changes are attenuated in the short term due to the inertia of the rotating masses of generators in the conventional power plant fleet. The capability of counteracting frequency changes by absorbing or feeding kinetic energy is referred to as instantaneous reserve.
In order to check whether there is sufficient kinetic energy in the system as an instantaneous reserve, a load or generation step of 3,000 MW is assumed as the design case for the current European integrated grid. In such cases, the instantaneous reserves must attenuate the resulting frequency change sufficiently before primary regulation sets in, so that the permitted frequency range of 50 Hz +/- 0.8 Hz (shortterm/dynamic) or 50 Hz +/- 0.2 Hz (stationary) is not exceeded.
Page 7 of 21 dena Ancillary Services Study 2030: Summary of the results of the project steering group.
Development of the need for instantaneous reserve until 2030.
In terms of the development of the demand for instantaneous reserve by 2030, it can be assumed that the currently standard design case for grid support with a capacity change of 3,000 MW (equivalent to the failure of a double power plant block) will remain adequate.
The introduction of more renewable energy systems with generally smaller system sizes will not reduce the design case of 3,000 MW by 2030. The design criterion applies in the entire synchronous integrated grid of ENTSO-E. In 2030, there will still be a sufficient number of large-scale power plants (which determine this design criterion) in operation – based on the assumed generation scenario for Germany and Europe. Even taking the connection of offshore wind farms, construction of HVDC lines in the integrated grid and the existence of electricity distribution grids with a high installed capacity from renewable energy sources, there will be no need to increase this design criterion. This is based on the assumption that the planning principle for grid design, i.e. that no capacity steps of over 3,000 MW can occur during failures, will be retained in future.
As the renewable energy systems which feed in via inverters cannot contribute to the instantaneous reserve without additional technical measures, Germany’s contribution to system support in the integrated grid would be far lower in 2030 in situations with a high RE feed-in unless countermeasures are taken.
Germany’s involvement in the instantaneous reserve and the need for alternative provision of instantaneous reserve to keep the contribution constant until 2030 is summarised in Figure 1. For 2011, the model calculations in this study show a contribution of the German balancing zones to a capacity step of 3,000 MW with a braking power of 372 MW and a kinetic energy of 0.95 MWh. Without the provision of instantaneous reserves from alternative sources, this contribution would reduce to roughly one third by 2030 during certain hours of the year. Until the limit value of the maximum dynamic frequency deviation of 49.2 Hz, there remains a sufficient safety margin of 0.25 Hz. To operate the power supply system as stably as in 2011 in future, i.e. to keep Germany’s contribution to the instantaneous reserve constant, in 2030 at times of high RE feed-in or low conventional generation, a capacity difference of roughly 254 MW and a kinetic energy of 0.68 MWh must be provided for the instantaneous reserve via suitable alternative technologies.
Figure 1 - Provision of the German share of instantaneous reserve.
Alternatives for provision of instantaneous reserve.
Renewable energy sources – especially wind turbines and large ground-mounted solar power plants – as well as battery storage capacities can already be technically equipped to contribute to the instantaneous reserve. In this case, the power electronics of the systems’ feed-in inverters emulates the inertial properties of an electromechanical synchronous generator.
Inverters must be able to absorb and output energy in order to provide instantaneous reserve. The main technical solutions which could potentially be used for this are throttling wind turbines or photovoltaic systems, using battery storage capacities or the inertia of wind turbines (emulation of instantaneous reserve). As throttling fluctuating renewable energy would lead to a long-term loss of active power, and additional investments would be required to build battery storage for instantaneous reserve 2, using the inertia of wind turbines is the most efficient alternative. The studies assume that a wind turbine with an average system capacity of 2 MW can provide a braking capacity of up to 0.2 MW, and thus kinetic energy of up to 0.55 kWh by using the inertia of the wind turbine. In 93 percent of all hours studied in 2030, the instantaneous reserve provided in this way by wind turbines would be sufficient to keep Germany’s contribution to the instantaneous reserve in the European integrated grid constant at the present level. In the remaining 7 percent of hours, there are sufficient power plants connected to the grid to provide the required braking capacity and kinetic energy for the stability of the electricity grids missing due to the lack of wind feed-in.
In order to enable Germany to fulfil its system responsibility in the European integrated grid reliably and fully in future, suitable alternative technological solutions are required to provide the instantaneous reIf battery storage are already available in the grid for other reasons (e.g. to provide primary balancing capacity), they can be incorporated for the instantaneous reserve.
serve in future in parallel to the further expansion of renewable energy. To implement this, the regulatory framework conditions must be adapted such that decentralised energy units can contribute to the provision of instantaneous reserve in future. In particular, in a first step, the conditions required for a provision of instantaneous reserve via large-scale wind turbines (emulation of instantaneous reserve) must be created. In the longer term, the extent to which the integration of other alternative providers (throttling decentralised energy units, battery storage) is necessary/economically viable and must be examined.
3.2 Balancing energy.
In order to compensate the excess generation or load which occurs over all balancing groups, the transmission system operators use positive or negative balancing energy. They purchase the balancing energy in the three product qualities – primary and secondary control and minute reserve3 – via a regular marketbased auction process. Potential providers on the balancing energy market are subjected to a prequalification process before participating to prove that the planned generation units or flexible loads have the required availability, reliability and controllability.
Development of the demand for balancing energy until 2030.
Assuming the generation scenario in the 2013 Network Development Plan, the assessment of the demand for balancing energy reveals a significant increase in the secondary balancing energy and minute reserve to be provided. In particular, the effect of generation forecasting errors which grows with the installed renewable energy capacity affects the demand for balancing energy. Assuming a constant forecast precision for RE feed-in, the demand for negative minute reserve capacity will increase approximately 70 percent and the demand for positive minute reserve capacity will increase by approximately 90 percent. The demand for secondary balancing energy will increase to a lesser extent (approx. 10 percent for negative and 40 percent for positive secondary balancing energy), however the increased occurrence of major wind flanks leads to the assumption of more frequent activation of the secondary balancing energy.
The dimensioning processes used today measures the demand for balancing energy on a quarterly basis.
The increase of the average balancing energy demand required between now and 2030 can be restricted (see Figure 2) if in future an adaptive dimensioning process were used for balancing energy demand, e.g.
for the previous day, and calculated based on the actual forecasts for load and feed-in of renewable energy.
Note that even if the adaptive process is used, there will be individual days with high electricity feed-in from renewable energy sources with almost double the demand for minute reserve in 2030 compared with the present-day demand.
Primary control is used to stabilise the system where there is only a brief power deficit or surplus. It is provided in a way of solidarity by all synchronously connected TSOs inside the UCTE area and has to be activated within 30 seconds. The time period of availability per single incident is up to 15 minutes.
If a longer disturbance occurs, secondary control is automatically activated within 5 minutes. The time period of availability per single incident is between 30 seconds and 15 minutes.
If the power flow deviation lasts for an extended period (more than 15 minutes) secondary control gives way to minute reserve. The latter is activated by a telephonic or schedule-based request of the affected TSO at the respective suppliers. In case of a telephonic request, the minute reserve has to be activated within 15 minutes after the phone call.
Figure 2 - Estimating the future demand for balancing energy.
Alternative providers of balancing energy.
In the present-day power supply system, balancing energy is largely provided by conventional power plants including pumped-storage plants. Alternative providers, some of which already market their capacity on the balancing energy market, include balancing energy pools comprising biogas plants, emergency electricity generators and large-scale batteries, as well as particularly energy-intensive industrial companies with flexible loads. Other alternative providers which have the fundamental capability to provide balancing energy include remote-controlled wind turbines or photovoltaic systems, and smaller generation systems (e.g. small-scale CHP plants) and loads (e.g. connection of flexible electricity loads).
In future, there will be more periods when the electricity feed-in from renewable energy sources will exceed the consumption in Germany. Then, there will be very few or no conventional thermal power plants on the German grid due to market signals. The study reveals that in 2030, market forces will dictate that during certain hours, there will not be enough conventional power plants to provide balancing energy.
There are alternatives for the provision of all balancing energy products which can meet the demand, even in these hours.
Economic viability of alternative provision of balancing energy.
The study shows that there are technical options with sufficient potential. Compared with exclusive utilisation of conventional must-run capacity for balancing energy provision, using alternative providers is more economical. The study results indicate that large-scale batteries are the most economically viable alternative of those considered for primary balancing energy. There are a variety of possible alternatives for secondary balancing and minute reserve energy. The extent to which the respective alternatives can actually be developed and utilised to provide balancing energy must be derived from the supply and demand on the balancing energy market.
Page 11 of 21 dena Ancillary Services Study 2030: Summary of the results of the project steering group.
In order to avoid a conventional must-run capacity in order to provide balancing energy in the medium term, and thus also improve the system integration of renewable energy, the conditions for providing balancing energy from alternative sources should be improved. To do so, we must evaluate the extent to which product characteristics and pre-qualification requirements can be adapted to facilitate the market entry of new providers of balancing energy from e.g. renewable energy sources, flexible loads and electricity storage units, and meet the changing system requirements (e.g. steep flanks). In this context, a reduction of the tender periods for primary and secondary balancing energy must also be reviewed.
At the same time, technical and organisational solutions must be developed to permit coordination of increased provision of balancing energy via decentralised energy systems from the distribution grid, taking the local grid conditions into account.
In addition to this, the implementability of the adaptive assessment process must be reviewed, for example to determine and tender the probable balancing energy demand for the next day based on the previous day.
4 Voltage control in 2030.