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By 2010: 460 km
By 2015: a further 390 km
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Diagram: Extension of the German electricity grid until 2015. Source: dena Grid Study |
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The research paper “Planning of the Grid Integration of Wind Energy in Germany Onshore and Offshore” (the dena Grid Study) analyses the integration of renewable energy sources (RES), and particularly wind energy, into the electricity grid and the impact of this integration on the power pool in the period 2003 to 2015. According to the scenario presented in the paper, renewable energies will already contribute 20% to the supply of electricity by 2015. The contribution made by wind energy will be 13%. Using models provided by the network operators participating in the study, analyses of both statistical and dynamic network stability were carried out in respect of the further development of onshore and offshore wind energy. In a follow-on study (dena Grid Study II), a further increase in the contribution of renewable energy sources to the generation of electricity will be examined, the time horizon being 2025.
The German electricity grid (at the 220 / 380 kV extra high voltage level) is designed to connect the conventional power stations (and particularly large coal-fired and nuclear power plants) with high demand areas. The geographic concentration of wind energy development in northern Germany, due particularly to the offshore wind parks planned for the North and Baltic Seas, results in huge variations in the power flow. In the dena Grid Study model calculations were carried out to determine where bottlenecks can be expected in the grid.
As a result of the priority regulation set out in the Renewable Energy Sources Act (EEG), wind energy supplants part of the electricity produced by conventional power stations. The result, particularly in periods of strong winds, is a regional shortage of reactive power, which in contrast to active power cannot be carried over long distances. Reactive power must therefore be replaced locally. The problem is aggravated by the closure of conventional power stations either due to age or under the agreement to phase out nuclear power stations. Modern wind turbines (WT) can produce both active and reactive power. Their contribution towards the reactive power required in the grid is limited, however. The dena Grid Study concludes that up to 7,350 Mvar of additional, regionally distributed reactive power must be provided.
If the electricity grid is to be adapted to the above demands resulting from the further development of wind energy, the overhead network must be upgraded and extended and additional grid equipment installed. The following measures will be necessary over the next few years (see Diagram 2):
These grid construction measures would extend the existing interconnected network by a total of 850 km or around 5% by the year 2015.
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By 2010: 460 km
By 2015: a further 390 km
Caption:
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Diagram: Extension of the German electricity grid until 2015. Source: dena Grid Study |
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The extension of the electricity grid will also affect the geographic distribution of any new conventional power stations. As wind energy dominates the transmission capacities in northern Germany in periods of strong winds, these are only available to a minor degree for the transmission of electricity from other power stations at such times. This must be taken into account when planning new, fossil-fuelled power stations in new locations in northern Germany and in the replacement of nuclear power stations which are being closed down in the North.
The cost of adapting the electricity grid to the increased amount of wind energy being fed into the system by 2015 amounts to a total of Euro 1.1 billion. Investments in the renewal and extension of the distribution networks are, however, not only necessary for the integration of renewable energies into the system, but also under the scope of technical modernization, the improvement of East-West transmission and the growing electricity trading within the European electricity market. Integrated planning is therefore necessary where upcoming investment in the distribution networks is concerned. Moreover, political backing (for example in the guise of an acceleration law) could provide considerable support to the extension of the grid within the target period.
Carrying large amounts of electricity over long distances is technically feasible, but must also be reconciled with environmental demands. The successful development of offshore wind energy is therefore inextricably bound up with the coordinated planning of a technically, economically, and ecologically optimized offshore network. Solutions must therefore be found which make it possible to transport energy from wind parks in the North and Baltic Seas to the grid connection point in a both technically secure and environmentally compatible fashion.
A systems model has been created for the extension of wind parks in the North Sea in the period after 2015 which avoids a large number of parallel undersea cables and wide cable corridors in the coastal area. It consists of four offshore collection stations, to which several wind parks could be connected. Wind power could then be transmitted to shore from these collection stations by use of only one common undersea cable. More research is necessary for this model, and it is therefore recommended that coordinated action be initiated by the wind park operators in the near future.
An increase in the use of renewable energies to generate electricity and developments resulting from the liberalisation of the energy markets result in alterations in the way electricity generation is structured, which in turn affect the dynamic stability of the electricity grid (performance of the grid at times of fault-based fluctuations in voltage or frequency). The dena Grid Study examined these effects, identifying critical situations and suggesting solutions.
Dynamic grid analyses have shown that certain faults can lead to large-scale voltage drops and critical grid situations. If, for example, a regional voltage drop of more than 20% were to occur as a result of the three-pole short-circuit of a busbar, those wind turbines which were taken into operation before 2004 would have to disconnect from the grid in accordance with the Grid Codes in force at that time. These additional disconnections would worsen the critical grid situation and could lead to a total short-term drop in voltage of over 3,000 MW. This value exceeds the primary control reserve level maintained by UCTE (Union for the Coordination of Transmission of Electricity) to compensate for short-term power station failure and could thus put the reliability of supply in the German and European interconnected network at risk. To prevent this, the regulations were altered for power stations joining the grid from 2004. According to the new terms, wind farms need not disconnect from the grid until the voltage drops by more than 80%.
Wind turbines installed before 2004 are, however, still ruled by the old grid regulations, thereby increasing the supply risk and as such endangering dynamic grid stability. In principle, technical instruments are available for the adaptation of the interconnected network and power stations, but their implementation still needs to be examined in detail and agreed between network and wind farm (WF) operators. Only if these solutions can be implemented in good time can the forecasted wind power development be realized by 2015 without lessening the security of supply. The measures include:
The installed capacity of conventional power stations and their electricity input is not only being displaced by renewable energy sources, but also by a phenomenon triggered by the liberalisation of the market, as a result of which an increasing number of companies are producing electricity for their own use which is not available for the provision of systems services in the grid. Here, too, technical solutions must be developed to ensure that the dynamic stability of the grid is not jeopardized as a result.
From 2010, an increase in electricity exports from Germany to neighbouring countries can be expected as a measure to maintain the energy balance, particularly at times of both strong winds and low loads. This should be considered a pro rata import in almost all neighbouring grids in the proportion of the relevant grid load (in total 7 to 9 GW). The additional electricity exports resulting from wind energy were accounted for in the dena Grid Study. Depending on the underlying assumption, these should amount to between 1 and 7 TWh/a in 2015. In comparison, the total energy exchanged by Germany in 2003 amounted to approx. 26 TWh/a. The development of wind energy will therefore make only a low contribution to the total volume of energy exchanged by Germany, and there will only be a few hours per annum of wind-based demand on the high transmission capacity at the interconnection points.
Large-scale power generation from wind energy converters in Germany considerably impairs the reliable operation of the grids in neighbouring countries. As a result, cross-border interconnectors and transmission lines in border areas are operated close to their n-1 reliability limit at weekends with strong wind conditions and during low-load night-time hours. Because of the increasing amount of energy being fed into the grid from wind farms, international regulations need to be introduced regulating the temporary, excessively high exchange of energy between UCTE member countriesFrom 2010, the short-term export of large amounts of (wind) energy when the load is low and/or the wind strong can only be avoided or reduced using the following three principle strategies:
Rules need to be agreed for WF production management to ensure that individual turbine operators are not placed at a disadvantage. Additional regulations must be agreed in respect of communication between the network operator and the wind park operator. The same applies to communication between consumer (load) and network operator in the case of load management.
The results of the dena Grid Study show that integrating wind and other renewable energies to contribute up to 20% of Germany’s electricity supply can be effected at an economically justifiable expense. To this end, alterations must be made in the next few years to the electricity grid. It will be necessary to upgrade 400 km of existing power lines, build a further 850 km of new lines and to install additional grid equipment by 2015. The new lines will extend the existing grid by 5%. Investment costs for the alterations required by 2015 will amount to a total of Euro 1.1 billion. Political support for the grid extension would be desirable if this is to be completed within the projected period.
The coordinated planning of a technically, economically, and ecologically optimized offshore network which ensures that offshore wind energy is transported to the grid in a both technically secure and environmentally compatible fashion is necessary if offshore wind energy development is to be successful.
The Grid Code valid until 2003 for wind turbines increases the supply risk and endangers dynamic grid stability if a fault should occur. The wind turbines affected should be brought in line as soon as possible with the Grid Code which has been valid since 2004.
Wind energy development will make the amount of short-term exchange power exchanged between Germany and its neighbours much higher in 2015 than it was in 2003. This can, however, be prevented by means of production and load management or the creation of additional storage capacities. Wind-based electricity exports will be low against the total amount of wind energy generated and also low in comparison to the total volume of electricity exchanged, as they will only become necessary for several hours each year.
