Integration of Large Offshore Wind Power into Energy Supply

Transcription

1 Integration of Large Offshore Wind Power into Energy Supply Dipl.-Ing. Bernhard Ernst Dr.-Ing. Martin Hoppe-Kilpper Dipl.-Ing. Kurt Rohrig Institut für Solare Energieversorgungstechnik Königstor Kassel Germany Tel Fax Introduction By July 2003, more than Wind Turbines (WTs) with a capacity of 12,850 MW have been installed in Germany. Wind-generated power now provides a noticeable percentage of the total electrical power consumed. This indicates that wind is a significant factor in electricity supply, and in balancing consumer demand with power production. In the control areas of the German Transmission System Operators (TSO) E.ON Netz and Vattenfall Europe Transmission GmbH more than 100 % of the electricity consumption has been covered by wind power at times. This aspect will be distinctly strengthened when the offshore potential in the North and Baltic Seas is developed to the extent that is predicted in many scenarios. According to Federal Government s long-term planning, wind turbines with a total rated capacity of up to 25 GW shall be erected in the North and Baltic Seas, which would cover around 15 % of the German electricity consumption. For the integration of this immense electrical power into the German energy supply system, the wind turbines already installed on land have to be considered. This large amount of intermittent generation has growing influence on the load and security of the electrical network, the operation of thermal power plants, the electricity trading and on the overall efficiency of the German electrical supply system. It is today already foreseeable that these ambitious challenges must be reacted on with both technical and regulative measures [1]. Connection to the Interconnected Network The coastal areas on the German North and Baltic Seas are relatively lightly populated, with similarly low industry density and mainly weak electrical networks. Extra-high voltage power lines, with 400 kv, are only available in locations where large power plants are operated. An extension of the existing overhead power lines is judged to be extremely difficult due to environmental concerns [2]. The already existing extra-high voltage power line (400 kv) to the Danish Jutland serves for the power exchange within the UCTE control area and has no open capacity. 1

2 Offshore wind farms in the 1,000 MW range can only be connected to the extra-high voltage system at locations, where power plants are already operated. This means a restriction to the cities Brunsbüttel, Bremerhaven, Wilhelmshaven and Leer on the North Sea and Greifswald and Rostock on the Baltic Sea. Extensive load flow calculations, considering the wind capacity already installed on land and that to be installed on and off shore, will be required in order to indicate which amount of wind power can actually be integrated. However, an integration of 25 GW offshore wind energy would most likely result in an extension of the existing extra-high voltage power line system, or the development of a separate overlay wind energy network connecting the offshore wind farms with the main centres of consumption in the Rhine-Ruhr and Rhine-Main area of Germany. Furthermore, it has to be considered that the maximum power, to be connected to a single grid connection point, is 3,000 MW. This results from UCTE agreements, as the maximum reserve power has been limited to this value in the European interconnected network. Short Term Wind Power Prediction One task of transmission system operators (TSO) is the permanent grid balancing within it s control area. The grid load (sum of all feed-outs) can be predicted with great accuracy, the feed-in from conventional power plants is available in the form of power plant schedules. The need for regulation power arises, therefore, from the difference in the predicted feed-in from WTs and the actual feed-in values. Therewith, the accuracy of the wind power prediction has direct influence on the amount of regulated power to be procured. If the forecast is available early, the procuring is generally less costly than when dealing occurs immediately from necessity. Figure 1: Structure for Online Recording and Prognosis of Fed-in Wind Power In this context wind power forecast, which is as precise as possible, is also an important step in regard to the improvement of grid management of individual TSOs. In this way, they can plan their tasks and requirements involving the control and voltage scheduling in grid areas 2

3 more precisely, and prepare the necessary measures to preserve grid and supply security in the scope required (see Fig. 1). In the last few years ISET has developed a forecast model, which is in use at three TSOs covering 95% of the German wind power. It delivers the temporal course of the expected wind power for supply areas, for up to 72 hours in advance. To achieve this, representative wind farms, or wind farm groups, were determined and equipped with measurement technology. For these locations, the Deutsche Wetterdienst (DWD) provides predicted meteorological data in 1-hour intervals for a forecast period of up to 72 hours. The corresponding power is calculated with the aid of artificial neural networks (ANN). The ANNs are trained with predicted meteorological parameters and measured power data from the past, in order to learn physical coherence of wind speed (and additional meteorological parameters) and wind farm power output. This method is superior to other procedures, which calculate the relation between wind speed and power by the use of power curves of individual plants, as the actual relation between wind speed (and other meteorological parameters) and wind farm power output depends on a multitude of local influences and is therefore very complex, i.e. physically difficult to describe. The advantage of artificial neural networks over other calculation procedures is the learning of connections and conjecturing of results, also in the case of incomplete or contradictory input data. Furthermore, the ANN can easily use additional meteorological data like air pressure or temperature to improve the accuracy of the forecasts. The average error (RMSE) between predicted and actual occurring power currently is about 9% of the installed capacity [3]. Operational Control of Offshore Wind Farms Today, the regulation and operational control of individual wind turbines already allows a multitude of interventions in plant operation, according to the design of the turbine. These interventions essentially assist in secure plant operation (e.g. limitation of mechanical strain), the adherence to grid connection conditions (e.g. safe grid coupling and shut-down, limitation of power output to rated power) and also the maximisation of the energy yield (e.g. adaptation of the rotation speed), whereby one and the same intervention can often fulfil several functions simultaneously. Besides these intervention options, which can already be counted as standard, individual applications occur where individual WTs undertake additional tasks in the field of grid stability; namely, where WTs with appropriate technical properties for voltage control (reactive power provision) are utilised in the case of weak grids. Turbine concepts, which incorporate these increasingly demanded additional electrical characteristics, are granted better prospects in the market. This is also a reason for the fact that more and more modern MW turbines are equipped with (individual) adjustable rotor blades and speed variable drive train concepts (double-fed asynchronous generator or synchronous generator with IGBT inverters). In the meantime, the installed wind power in particular grid areas (and control areas) has already achieved such a scale that problems in grid control and grid operation management can occur in strong wind periods, caused by power fluctuations. Phases have already occurred in which the total grid load was covered by wind energy, for the complete control area of Vattenfall Europe Transmission (previously VEAG), and grid stability could only be maintained through interaction with other control zones. This aspect is particularly significant in conjunction with the erection of larger offshore wind farms, which supply power in the range of several hundred MW over one connection point. Here, the active contribution of wind farms in grid operational management is required (see Fig. 2). These tasks can, however, no longer be solved alone by the individual operational control of single WTs according to the criteria mentioned above, operational security, keeping of the 3

4 grid connection conditions and maximum energy yield, but require superordinate operational management in addition, with appropriate predetermined desired values. This superordinate operation management for individual wind farms must also be able to fulfil additional demands, e.g. from the perspective of the grid operator. TSO control unit Group Cluster Group Single Generator Requirements: Profile Based Operation Mode uninfluenced operation power limitation energy compliance constant power output supply of control energy Requirements: maximum power limitation (dynyamic threshold values) short circuit current emergency cut-off (disconnection) by network outages coordinated start-up and shut-down procedures (gradients limitation) Requirements: safe and reliable operation maximum energy yield Group 1 Group Cluster Group 2 Group N Gen 1.1 Gen 1.2 Gen 1.3 Gen 2.1 Gen 2.2 Gen n.n Figure 2: Structure for the Control of Larger Wind Farms and also for Wind Farm Clusters Large (offshore) wind farms will therefore require access from a central operational control centre in order to coordinate and control the operation of many individual wind turbines with correspondingly defined demands. This task and responsibility will presumably be assigned to wind farm operators, whereby the predetermined desired values (e.g. for active and reactive power output) must be defined in consultation with the grid operator, responsible for control. As the current operational data of large wind farms will be joined together by the grid operator and desired values are given, a type of control centre for clusters of large wind farms is established, which increasingly takes on the character of conventional power plants. The utilisation of operation management, to be newly developed, would provide a multitude of new applications to enable simple, flexible and uninterruptible reaction to the demands of participants (wind farm operators, TSOs and electricity traders). These operation control centres must facilitate energy and power control, as well as the provision of reactive power, in order to maintain wind farms comparable to conventional power plants. The implementation of the operating methods depicted in Figure 2 will significantly increase the economical use of wind energy. Until now, criticism has often been made that the positive contribution of wind energy in CO 2 reduction would be decreased through the regulation power reserve in thermal power plants. By the manner of operation outlined above, it would be possible to provide reserve power by rejection of production maximisation (see Fig. 3). Thereby, the wind farm power prediction would be reduced to the desired control band to 4

5 provide fast regulation power by the demand of the grid operator (this manner of operation is a central component in Danish wind farm projects). P Forecast P pot P max P min potential reserve power P P (t) t 1 t Figure 3: Provision of Reserve Power from Wind Farms with the aid of Wind Power Prognosis Further advantages of the described operation method could be achieved if several large offshore and onshore wind farms could be managed as one large, widely distributed wind farm. As the high amount of existing onshore wind power, and the planned offshore wind farms, is installed in the control areas of few grid operators, this operating method could be relatively easily realised. The superordinate operational control centres, to be developed for the control areas, would then have to perform the task of managing the widely distributed WTs for the economical optimised integration into central power plant scheduling. The foundations for this are laid with the existing systems for online monitoring and prediction, which are already installed in the affected control areas. Conclusion In the next few years the direct replacement of conventional power plant capacity by wind power is less in focus, but more important is an improved collaboration between renewable power production and thermal power plants, where operation should be as environmentally friendly as possible. Thereto, fast adjustable power plant units are necessary, which are already increasingly implemented today e.g. gas turbines or combined cycle power plants. From the conventional power plant s viewpoint, besides the dynamic behaviour of wind power is of particular interest. Despite the already diverse available knowledge and approaches to the improvement of the ability of large wind power to be integrated into the supply system, many questions remain open that must be clarified through further intensive investigations. It has been especially 5

6 demonstrated that a medium to long-term strategy for offshore wind energy use is required, in order to affect the necessary investment decisions for grid extension under the correct boundary conditions. It will be of significance to clarify the legal framework, which legislators wish to pass, or said differently, to what extent the state will (or is able to) intervene in regulating the opening of offshore potential. References [1] M. Durstewitz, B. Hahn, M. Hoppe-Kilpper, C. Nath, V. Köhne: Offshore- Windenergienutzung in der AWZ, Studie im Auftrag des Bundesministeriums für Wirtschaft und Technologie, Kassel, 2001 [2] Fichtner, Deutsches Windenergie- Institut: Von Onshore zu Offshore - Randbedingungen für eine ökonomische und ökologische Nutzung von Offshore- Windenergieanlagen in Deutschland, im Auftrag des VDMA, Fachverband Kraftmaschinen, Frankfurt, 2001 [3] C. Ensslin, B. Ernst, K. Rohrig, F. Schlögl: Online-Monitoring and Prediction of Wind Power in German Transmission System Operation Centers, European Wind Energy Conference, Madrid,