Wind Power in the German System—Research and Development for the Transition Toward a Sustainable Energy Future
Matthias Luther1, Kurt Rohrig2, Wilhelm Winter3, 1Friedrich-Alexander-University of Erlangen-Nuremberg, Erlangen, Germany, 2Fraunhofer-Institut für Windenergie und Energiesystemtechnik IWES, Kassel, Germany, 3Tennet TSO GmbH, Bayreuth, Germany Email: 1[email protected], 2[email protected], 3[email protected]
Abstract
As a result of developments within the last 30 years, wind energy has become a fundamental pillar for the electrical energy supply in Germany. As of today and beside other renewable energy sources, wind power generation is at the center of the ongoing transition process of the German energy system.
The chapter describes the state of play as well as future challenges with respect to large-scale wind energy integration into the German and European power system. Based on existing facts and figures and a long-term outlook, the developments in the onshore and offshore wind power technology together with the necessary grid development plans and relevant network codes are provided.
Recent experiences with respect to wind integration in Germany call for complex interoperability analyses between transmission and distribution including the sector coupling of generation and storage plants. Furthermore, ongoing research activities concerning new intersystem phenomena are addressed.
Keywords
Wind power integration; turbine technology; network operation; grid development; GDP; O-GDP; TYNDP; network codes; forecast systems; wind cluster management
6.1 Integration of Renewables in Germany and Europe
The use of renewable energy sources (RESs) and especially wind energy in Germany is based on both the considerations to achieve long-term independence of primary energy imports and the absolute need to reduce CO2 emissions to a minimum by 2050 at the latest. The political goals set by the German Federal Government in 2011 are given in Table 6.1. Beside an ambitious reduction of the CO2 targets, 80% of the electrical consumption is expected to be covered by renewables by 2050. Another important boundary condition for the transition of the German electricity system is the ongoing nuclear power phaseout by 2022.
Table 6.1
Energy Policy Goals in Germany
| Nuclear Energy | CO2 Targets (Basis 1990) | Renewable Energy | Reduction of Consumption | |||||
| Gross End Energy | Electricity Generation | Primary Energy | Building Heating | End Energy Traffic | Electricity Consumption | |||
| 2015 | −47% | |||||||
| 2017 | −54% | |||||||
| 2019 | −60% | |||||||
| 2020 | −40% | 18% | 35% | −20% | −20% | −10% | −10% | |
| 2021 | −80% | |||||||
| 2022 | −100% | |||||||
| 2030 | −55%, –40%a | 30%, 27%a | 50% | 27%a | ||||
| 2040 | −70% | 45% | 65% | |||||
| 2050 | −80%/−95% | 60% | 80% | −50% | −80% | −40% | −25% | |
Treibhausgas-Emissionsprojektionen bis zum Jahr 2020 für das BMU und UBA (Öko-Institut, 2011).
aClimate and energy framework until 2030 by the European Council as of 24 October 2014.
A possible long-term development of the installed wind and photovoltaic (PV) capacity in Germany by 2050 is described in Ref. [1] and shown in Fig. 6.1. The current installed capacity of renewables meanwhile exceeds the annual peak load of about 80 GW of the German electricity system. Furthermore and due to the regional domination of wind generation in Northern Germany during low load conditions—approximately 35 GW—the regional wind generation is temporarily three to five times higher than the regional load.
Additionally, the total installed capacity of PVs with a significant higher in-feed in Southern Germany has meanwhile surpassed the installed amount of wind capacity.
During the past three decades, the technological achievements in research and development have made wind energy in Germany a powerful and sustainable building block of the energy industry. From both economic and environmental aspects, the transformation of the energy system to a renewable-based, decentralized energy supply, electricity will take over the central role and will also serve the heat and traffic sectors. From today’s perspective, the three key phases of the transformation of the energy system are:
1. In the first completed phase, key technologies—in particular wind turbines (WTs) and PV and key techniques of biomass utilization—have been substantially developed, achieved significant cost reductions, and significantly promoted the launch to the energy markets. Today, over one-third of German electricity production is from renewable sources with 14% from wind energy.
2. The main task for the forthcoming next phase of energy transition is the complete integration of renewable energies. Volatile renewables require further development for their complete integration—both technical and economical—in an increasingly flexible overall system, covering all sectors of consumption: electricity, heat, and transport.
3. The third phase is characterized by a high degree of coverage of the country’s total energy needs with renewable energy.
The technology involved in wind energy is the most developed among the renewable energies today. The resources are also seen worldwide more than adequate and they show the lowest electricity generation costs. The weakness of wind energy is the (regional) volatility, which is smoothed by wide-area use [2]. The reason for this is the strong spatial–temporal variability of the wind in Europe; local or regional occurring calms or high wind situations are never or very rarely recorded over long periods or for over large regions. Fig. 6.2 provides the study results for the regional in-feed of an individual wind cluster (“Pixel”) as well as for different geographical regions normalized on the total installed capacity for 2030 [3]. Therefore, energy deficits are often balanced at a certain time in one place by simultaneously occurring energy surpluses at a different location. This positive effect can be used only when the installed capacity covers more than the maximum power consumption in these regions and the energy surplus can be transported over long distances. This however requires the strengthening of existing and the construction of new electric transmission grids across Europe as well as regional grid reinforcements on the distribution level.
Offshore wind power generation provides huge advantages in terms of full-load hours and therefore can be considered as reliable power source. However, the deployed wind farms have to be widely spread over Europe seas and cost lines. Taking into account such a distributed offshore wind generation, wind fluctuations occurring over several timescales and locations can be reduced considering the power in-feed into the power system of Europe. Such a system would introduce reliable and sustainable energy to a highly integrated European energy system.
6.2 Onshore and Offshore Wind Development
Wind energy in Germany is making great strides. The year 2015 saw 63 GW of wind capacity newly installed across the globe—never before has the figure been so high. In 2014, 186.3 TW h, 3.3% of worldwide power consumption was covered by wind energy [3].
In 2015, German wind power plants produced 88 TW h, 53% more energy compared to 2014 (with 57.4 TW h). Wind energy thus represents a contribution of 14.5% of the total power consumption and 45% of the German renewable energy production [3].
In total, wind power plants with a capacity of about 6000 MW were installed in 2015 and put into operation. This leads to 45 000 MW capacity of wind power. The new installed onshore capacity in 2015 reached 3579 MW, so far the second highest figure after the record year of 2014 with 5188 MW. By 2015, offshore wind energy increased as well. At the end of the year 3283 MW (2280 MW installed in 2015) were already connected to the grid and contributed nearly 10% of the total wind power production.
For the first time in Germany, all renewable energies together provide the highest share of gross power consumption—32.6%. Renewables have now overtaken nuclear power plants (2012) and also the previously most important energy source, brown coal.
The development of the largest wind generators between 1985 and 2015 is shown in Fig. 6.3. Since the largest block sizes increased almost by a factor of 100, the corresponding annual energy production per generator was growing by a factor of 150 within the last 30 years.
For the analysis of turbines technology development, WT types are divided into category DD (direct drive), DD-PMSG (direct drive with permanent magnet generator), EESG (gear mechanism with external excitation synchronous generator), PSMG (gear mechanism with permanently excited generator), DFIG (gear mechanism with doubly fed asynchronous generator), IG (gear mechanism with asynchronous generator), CS (fixed speed WT), and others (turbine types with other concepts or inadequate level of detail). The name of a category is based upon the most concise feature of the concept. While previously the turbine market was characterized by fixed speed WTs, variable speed concepts are only being deployed nowadays. At 46%, direct-driven WTs (from the market leader Enercon) are the dominant type (Fig. 6.4). Also strongly represented are gear mechanisms with doubly fed induction generator (28%) and permanent magnet generator (18%).
The trend toward more powerful WTs is continuing—see Fig. 6.5. The 3–4 MW turbine class, with 48%, fell just short of the figure for the 2–3 MW turbine class which has been dominant for 10 years. This class reached a figure of 49% last year, presumably the last time it will experience the largest level of expansion. The 1–2 MW turbine class has for 5 years previously dominated the market. The WT capacity > 5 MW class continues to be limited to the E-126 model from the manufacturer Enercon—which is erected only occasionally. The average nominal capacity of WTs erected in 2014 was 2.68 MW, with 50% of WTs having a capacity of 2.3–3.1 MW. The total range of nominal capacity installed was between 0.5 and about 7.6 MW (Fig. 6.6).
The last few years have seen a total of 44 different turbine types and configurations with different rotor diameters and hub heights erected in Germany. While the capacity level of turbines, barring a few exceptions, is restricted to an interval between 2.3 and 3.1 MW, adaptation to location-specific conditions takes place by varying rotor diameter and hub height. Fig. 6.6 shows that the rotor diameters of turbines added in 2014 are in the 40–154 m range, with an average of 99 m. The largest rotor diameters on turbines designed for the onshore market are on the Enercon E-126 and Vestas V126 turbines, with rotor diameters of 127 and 126 m, respectively. Only the Siemens SWT6.0-154 and Senvion 6.2M152 are larger—but these are prototypes for the offshore market erected onshore.
The rotor diameter plays a key role in the capacity and the yield of a WT—this is because the rotor area determines the magnitude of available wind flow and amount of wind energy that can be converted into electrical energy. The variation options in turbine design can be seen very well by comparing the Enercon E-126 (7.6 MW) and Vestas V126 (3.3 MW). The nominal capacity differs by a factor of 2.3 for a virtually identical rotor diameter.
The most important interim goals for renewable energy have been met, but there are still a few challenges to be overcome. These include fluctuating feed-in levels, the concentration of wind energy in the north and PVs in the south, and the overall increasing share of renewable energies providing power require structural adaptation of the entire energy supply system. Furthermore, issues related to grid expansion, increase of storage capacities, flexibility of complementary power plants, the moving and capping of consumption and capacity peaks, and the continued expansion of renewables need to be addressed.
6.3 Network Operation and Grid Development
The European Energy Supply for Electricity is undergoing fundamental changes. This includes strong moves away from a heavy reliance on fossil fuels as the primary energy source mainly provided by large synchronous generators connected to the transmission systems, toward a decarbonized future supply relying increasingly on variable RESs using nonsynchronous generation predominantly connected to the network via power electronics (PE) and extensively connected and deeply embedded in distribution networks. Some countries in Europe have already experienced times in which at some periods the national demand for electricity has been exceeded by the RES production alone. As this phenomenon continues to extend, the development of Europe-wide markets and system operation will facilitate greater sharing of resources. This new and varied generation mix changes power system characteristics leading to major system technical challenges in normal operation as well as during disturbed or even emergency operation.
Figs. 6.7 and 6.8 provide some operational snapshots for the German transmission system for January and May 2016 extracted from the database of the ENTSO-E Transparency Platform [4]. A comparison of both figures underlines the volatility of wind and PVs due to different weather conditions in different seasons.
Fig. 6.8 underlines the fact that during certain periods of the year the total in-feed of wind and PVs amounts to 85%–90% of the total load. Under consideration of the different regional distribution of the generation, the vertical network load becomes negative in specific network areas. Since the capacity of the German transmission system is limited in several areas, the daily operational practice by the German transmission system operators (TSOs) is to perform congestion management mainly by re-dispatching measures for conventional and renewable generators. In 2015 the total cost for congestion management in the German transmission system amount to €1.1×109 (1.1 billion Euros), which is nearly four times higher than in 2014 [5]. The dedicated energy volume for re-dispatching and countertrading increased from 5200 GW h in 2014 to 16 000 GW h in 2015.
Earlier investigations about wind integration from the system stability point of view have been analyzed and addressed [6–11]. From earlier experience and from today’s system performance, maintaining a stable operation becomes a challenge in the face of an increasingly varied array of both conventional and emerging power systems. These future challenges already require a new framework and all participants of the energy market will have to face significant changes. This requires that traditional practices be reevaluated, new approaches be found, and that there must be much greater levels of cooperation and coordination.
The current network situation and the ongoing integration of renewables requires the development of an extended coordinated transmission system operation and network as well as the setting up of various network codes at European level.
The long-term planning of a transmission system in Germany is mainly based on three pillars:
The GDP 2025 deals with the expansion requirements of the German onshore transmission network. As stipulated by the German Energy Management Act, the four German TSOs are planning, developing, and building the grid of the future. The GDP is used to show how power generation in Germany can be successfully restructured and renewable energy can be integrated within 10 and 20 years. The corresponding O-GDP 2025 describes the expansion measures required to the offshore grid over the next 10 and 20 years. Both plans are analyzed, supervised, and finally approved of the German regulator (“Bundesnetzagentur”). Further information about the scenario framework, the methodology, and the detailed results from the analysis of the GDP can be taken from Ref. [12].
GDP and O-GDP have proposed several network reinforcements to be implemented by 2025. In addition to the strengthening of the 380 kV AC network, the GDP proposes high-voltage direct current (HVDC) connections in order to tackle the long-distance transmission requirements from north to south and to further strengthen the interconnectors with neighboring countries. Fig. 6.9 provides a scheme of the control areas of the German TSOs (Amprion, TenneT, 50Hertz and TransnetBW) including the interconnected offshore areas in the North Sea and the Baltic Sea, and a scheme of the planned HVDC links with a capacity of 8–10 GW.
The GDP and O-GDP are an integrated part of the TYNDP prepared commonly by the European TSOs and endorsed by ENTSO-E every 2 years. The TYNDP 2014 describes how ENTSO-E proposes to integrate by 2030 up to 60% of renewable energy, respecting cost-efficiency and security through the planned strengthening of Europe’s electricity power grid [13].
Another important cooperation at European level is the North Seas Countries Offshore Grid Initiative (NSCOGI) [14]. Established in 2010 by signing a Memorandum of Understanding between 10 Governments, the initiative has established three intergovernmental working groups on the design of a future grid, market mechanisms, and to support regional cooperation and ways to improve the planning process in the North Sea region.
6.3.1 Innovative Methods to Plan and Operate the Power System
In the future, a growing amount of PE will lead to a transition of the system to a structure with very low synchronous generation [15]. Due to large transit power flows and uncertainties, transmission systems are being operated under increasingly stressful conditions and are close to their stability limits. Together with the integration of large amounts of renewable generation with PE interfaces and addition of HVDC links into the power system, these challenges will necessitate a review of the operation and control of transmission networks.
6.3.1.1 The Challenge for the Operation of the Transmission Grid in a New Power System Environment
Tools, currently in use by TSOs for operational planning [16] and system operation, have to evolve in order to work in an environment that is characterized by large-scale integration of RESs with low predictability and limited controllability as well as a system operation close to its stability limits. The inclusion of new control equipment such as phase shifters and HVDC lines, and the development of a European Integrated Electricity Market with huge power flows over large distances, will bring further challenges [17].
6.3.1.2 Dynamic Line Rating—The Way to Increase OHL Utilization
The utilization of overhead lines (OHLs) has increased due to the increased transmission of electrical energy in Europe as well as a growing power production from regenerative energy producers [18]. The transmission capacities of OHLs are limited and in many cases there are already bottlenecks which restrict the flow of power.
A method to increase the OHL transmission capacity, depending on ambient weather conditions, will be described in the chapter. A dynamic rating system has been implemented into the operating process of a transmission system. The usability and the effect on the ampacity of OHLs have been verified for 380 kV lines.
Usually worst-case estimations for ambient weather conditions are used for OHL design. In many central European countries, a temperature of 35°C, a wind speed of 0.6 m s−1 and full sun radiation are assumed. At ambient conditions which are more favorable than the worst-case scenario, the conductors in a transmission line can carry more current without exceeding clearance limits.
For a flexible AC power system, dynamic line rating [19] is a useful approach in order to provide temporary additional transmission capacity while ensuring (n−1) security with higher loading of transmission lines. (The (n−1) criterion is a planning rule for transmission systems. For (n−1) security, the system must be able to sustain any single outage, including disconnection of any single generator, busbar or transmission element, and remains within performance limits as defined in the System Code concerned.) Good experience has already been gained by dissemination projects in Germany and Slovenia in order to improve reliability and safety of operating networks, especially in cases of increased power flows through existing transmission infrastructure.
6.3.1.3 Impact of Reduced Inertia on Power System Frequency
Frequency in a power system must lie within a predefined range, and not deviate too far from the frequency for which it was designed, so that operational security is not compromised. If the frequency is not held near its nominal value, protection systems begin to active in order to protect machinery, and to keep the power system operational.
Large deviations in frequency are often caused by the tripping of large production units, which result in sudden imbalances in active power. Frequency can then drop by unacceptable amounts, resulting in the disconnection of production units and loads, producing a cascade effect which may lead to widespread power outages. As production units become large and larger, greater imbalances result, with larger possible frequency deviations.
Large synchronous generators help the power system to resist such system frequency deviations. All machines contribute to this resistance with their inertia. However, as renewable generation begins to replace conventional generation, the ability of the system to resist these deviations decreases.
In the near future it is expected that electricity will be produced more and more by wind and solar power plants. Additionally, aging thermal power plants will spend less time connected to the network. Therefore new control concepts such as direct voltage control of electronic power equipment will replace today’s current source control mechanisms [20].
In order to control the power systems as efficiently as possible, more real-time information of inertia is needed. This real-time information needs to be sufficiently accurate. Possible countermeasures have been investigated in the Nordic countries such as real-time measurements of power plants by mitigating the impact of future production and consumption on inertia and frequency. However, further research and development work with a broader perspective will be necessary in the development of the project entitled “inertia, control, and protection of large power systems with a large amount of inverter-based components” within the framework of horizon 2020. This project is expected to cover all major issues related to the change in future power systems.
The suite of network codes which has been developed [21] will play a central role in creating the conditions which will allow a competitive, secure, and low carbon energy sector to develop and thrive.
Network codes are sets of rules which apply to one or more part of the electricity sector. The need for these codes was identified during the course of developing the third legislative package and Regulation 714/2009/EC and sets out the areas in which network codes are to be developed and the process for developing them. Based on Framework Guidelines written by the Agency for the Cooperation of Energy Regulators (ACER), ENTSO-E, in close cooperation with stakeholders, they have allocated 12 months in which to draft a network code on a particular subject. These network codes are assessed by ACER to ensure that they are in line with the Framework Guidelines and, once this is the case, they are submitted to the European Commission. Finally the network codes go through the comitology procedure before becoming fixed in legislation. Network codes are intended to complement existing national rules by tackling cross-border issues in a systematic manner and, by creating a coherent and coordinated framework, represent a practical way of driving forward the adaptations in the energy system. Network codes are being developed in three areas.
6.3.2 The System Operation Network Codes
The purpose of the system operation network codes is to build on the close cooperation which exists today to provide a solid basis for coordinated and secure real-time system operation across Europe. Clearly system operation practices need to be developed in light of the connection requirements established in related network codes. The purpose of the system operation network codes is to define common pan-European operational standards for the existing and a future European electricity system, particularly as the penetration of renewable energy generation increases. The system operation network codes define the general operational criteria and procedures to be applied in and across all of Europe’s synchronous areas. At the highest level, the system operation network codes aim to ensure security of supply and support for the efficient functioning of electricity markets.
FACTS (SVC, STATCOM, and TCSC among others) are PE-based technologies that are being installed on the power system for reactive power compensation and power flow control. It is worth noting that some of them (e.g., SVC and TCSC) have been installed with other objectives such as power oscillation damping. The reactive power regulation capability allows them to participate in ancillary services with the provision of being an active part of the system. In a system based on PE, it is important to determine their interaction with all the other devices and their impact on system stability.
Currently, energy storage is used for power balancing (mainly hydro) and in some cases for fast frequency response (as in the case of fly-wheel technologies). However, among those main time frames, there exists a gap that currently is filled by thermal or gas power plants. In a 100% RES-based power system, this gap must be provided by other generation (renewable) or by energy storage systems. Potential energy storage devices for providing ancillary services to the power system at different timescales must be researched and analyzed. Moreover, since energy storage systems tend to be integrated to the grid with a power converter, these systems may also provide reactive power regulation.
6.3.3 The Market-Related Network Codes
The market-related network codes, which cover the different time frames in which capacity is allocated and the balancing time frame (when the TSO is the only party active in the market), as well as rules for calculating cross-border capacity in a coordinated manner and defining bidding zones, aim to introduce a standard set of market rules across Europe and promote the implementation of a competitive pan-European market (building on the very significant progress made by regional market integration projects). The aim is to create a relatively simple set of market rules which can promote effective competition, to minimize risks for all parties (particularly renewable generators who will benefit from markets close to real time) and to give incentives for market players to act in a way which is supportive to the efficient operation of the system and minimize costs.
6.3.4 The Connection-Related Network Codes
Investment decisions taken now will affect the power system for the next decades. The European energy system of 2020 is being built today and the foundations of the European energy system of 2050 are being conceived. As such, there is a need to make sure that all users are aware of the capabilities which their facilities will be required to provide—recognizing both the need for all parties to make a contribution to security of supply and the high cost of imposing requirements retrospectively. The grid connection codes therefore seek to set proportionate connection requirements for all parties connecting to transmission networks (including generators, demand customers, and HVDC connections). A stable set of connection rules also provides a framework within which operational and market rules can be developed.
One of main principles in the development of the Network Codes for Grid Connections (NCs RfG, DCC, and NC HVDC) is the goal of a consistent set of connection requirements for new generators, demand, and DC links, which take into account local system needs and inherent technical capabilities. Whereas this code details requirements for capabilities, it does not provide answers to operational or market-related issues. These rules can be found within the operational/market network codes, notably NCs Operational Security, Load Frequency Control and Reserves, Electricity Balancing and Emergency and Restoration, or appropriate national rules. It is also emphasized that whereas operational and market codes often reflect present needs, connection codes need to ensure future operational/market rules can be facilitated as well. The Network Code for “HVDC Connections and DC connected Power Park Modules” (NC HVDC) [22,23] contributes to a clear and nondiscriminatory way to connect large-scale RES, by means of HVDC links, to the European electricity grid, in order to sustain a reliable, efficient, sound, and secure transmission system that can make use of the technical opportunities of HVDC technology. The NC HVDC contains significant new requirements for some regions based on best practice in industry. Technical specifications are required to meet certain performance criteria necessary for this role in order to make the technologies capable of todays and future system needs.
HVDC is the preferred transmission technology for long distances and for connection of nonsynchronous systems. Several HVDC transmission systems have been developed worldwide mainly using line commutated converter technology (LCC-HVDC). Recent developments on voltage source converter technology (VSC-HVDC) may allow the power converters to provide additional services to the AC power systems where they are connected, including black-start capability (the process of restoring a power station to operation without relying on external electricity power), reactive power support, simple power reversal, and flexible operation. VSC-HVDC technology is the backbone technology for the development of a European Supergrid [24], to interconnect offshore power plants with different terrestrial systems. In addition, maintaining stable operation becomes a challenge in the face of an increasingly varied array of both conventional and emerging power system stability aspects. A major new challenge is coping with extremely weak power systems for some hours (during high RES production) followed a few hours later (during low RES production) with operation of a strong power system again supported by large centrally connected synchronous generators. Therefore, requirements for the new and varied generation mix are necessary to ensure that the power system characteristics can cope with the major system technical challenges in normal operation as well as during disturbed or even emergency operation. Demand connection code requires the capabilities for distribution systems and demand facilities in order to minimize deviations, for example, of frequency from its nominal value due to generation/load imbalances. In order to increase power system flexibility, demand side must be capable of providing flexibility. This flexibility may allow the system to maintain stability and operate in a more secure way. Demand flexibility may provide an additional way to ensure power balance by regulating the loads and consumptions as well as provide ancillary services (ASs). In order to provide these potential services, TSO–DSO interaction is needed. Demand-side response regardless of scale or connection voltage can contribute to correcting generation/load demand imbalance. Demand offered for demand-side management has already been selected by the demand facility owner and therefore is available for changes in demand. Adjustment of this demand in emergency situations for low-frequency demand disconnection (LFDD) and low-voltage demand disconnection (LVDD) purposes in advance of disconnection of other load demand is the most efficient, lowest consumer impact response in these circumstances.
6.4 Further Research and Development for Wind Power Integration
The necessary innovations concerning technology, infrastructure, and markets for the transformation of the power system can be reached only by common and coordinated research and development [25]. The most important objectives for wind energy integration research are:
• Improve wind power forecasts by wide-area aggregation, probabilistic forecasts, and data assimilation with wind farm data.
• Enhance wind power plant capabilities to enable higher wind power penetration levels while maintaining adequate security of supply and power quality.
• Support grid planning and expansion by development of knowledge and tools suitable for high levels of wind power generation.
• Research tools, market rules. and power system regulations that promote cost-efficient operation and investments in power systems with major contributions from variable generation.
• Develop robust and stable control algorithms for PE devices in PE-dominated power systems.
• Analyze system needs for the integration of offshore grids for the onshore and offshore transmission system, respectively.
• Interoperability and analysis of interactions between HVDC systems and other connections.
• How to operate meshed offshore grids connected to all synchronous areas in Europe.
• Appropriate operational measures in order to cope with given uncertainties (market, renewables), optimization by using HVDC and FACTS, dynamic risk assessment.
• RES integration and coordinated approach in transmission and distributed systems.
6.4.1 New Control Concepts for PE-Dominated Power Systems
The rapid increase of renewable energy-based power generation and the ongoing implementation of smart grid concepts including the installation of new VSC-based HVDC systems is changing the dynamic characteristics of the entire power system in significant ways. For instance, when connecting offshore wind farms via HVDC links, new network constraints (e.g., the presence of harmonics under steady-state conditions) result in the adjustment of the protection schemes of cables. Moreover, with the increased penetration of renewables and HVDC links, one might rapidly reach, during some specific intraday periods, 100% penetration of PE, thus resulting in no mechanical inertia in the network (significant PE penetration already occasionally occurs in specific areas in continental Europe, e.g., Iberian Peninsula or Germany). Finally, following a major incident, the network (a control zone) could be partitioned into several areas, some of them with no synchronous machines. Such high penetration levels of PE result in four main categories of problems already encountered:
TSOs are faced with the task of finding appropriate solutions for the dynamic problems brought about by these changes. The European codes for grid connection define the necessary performance requirements with the cross-border implications of these changes in characteristics in mind. The HVDC connections, for example, should not lead to the degradation of existing levels of system reliability. For national level implementations, more detailed concepts have to be developed.
New hierarchical control concepts for integrating the advanced features of VSC-HVDC for voltage and reactive power control of the AC network must be developed and analyzed. This is based on the analysis of the fundamental control requirements in steady-state and under contingency situations. Fault ride-through capability and fast reactive power injection to support network voltage belong to the category of dynamic requirements, whereas steady-state operational requirements such as set point tracking involve slow changes. PE devices are usually operated as current source equivalent. The analysis of the today’s voltage control approaches for PE devices shows that the current control concept in which the voltage control is performed via reactive current injection is not well suited for islanding operation or in case of PE-dominated power systems. However direct voltage control as already used for synchronous machines shift the characteristic of PE devices back to voltage source operation and is able to support in combination voltage and frequency stability issues.
6.4.2 Wind Power Forecasts
Wind power plants will be sited more and more across broader and new geographic areas, increasing the wind power forecasting challenge; new spatially joint models and forecast types are needed to accommodate the larger European level scale, including offshore and addressing least modeled effects such as sea-land breezes. Second, high penetration means that variable and uncertain wind power and other renewable sources will dominate markets and control rooms. Therefore, new probabilistic forecast types need to be designed for the necessary market and power system operator requirements. Real-life forecasting with these new algorithms will require more data than has been collected before, often at higher spatial and time resolution and lower latencies. Defining what data is needed for these algorithms is also needed.
The short- to medium-term wind power forecasting, using numerical weather forecasts combined with statistical (artificial intelligence) or physical methods, has experienced enormous progress in the last few years and are an integral part of today’s energy supply [26].
The basis for a good wind power forecast is a reliable Numerical Weather Prediction (NWP) model. Unfortunately, the current NWP models do not satisfactorily fulfill the need in wind power forecast as wind speed prediction at WT hub height was not a primary aim of these models. Within the framework of the German project EWeLiNE [27–29], the aim of the NWP model of the German Weather Service COSMO-DE was to develop the needs of the renewable energy industry, namely wind power and PVs.
To improve a weather model, different strategies are possible: the parameterization of the model with respect to wind can be optimized by calibration of the model with wind measurements or the assimilation of measurements of wind speed and direction into the model.
All approaches have the same problem: wind speed measurements at hub height are required. In case of the assimilation, a new way is proposed in the project to directly include nacelle anemometer and wind power measurements from wind farms in the NWP-model assimilation procedure.
As wind power measurements are often more easily available as data from nacelle anemometers, the approach presented here includes in the first step, wind power measurements assimilated into the COSMO-DE model used by the German weather service.
The concept of integration of wind power forecasting into the NWP model, the construction of the forward operator with a new approach of fitting the power curve of a complete wind farm and the first results from the forecast of the NWP model are presented in Refs. [27–29].
Accurate forecasting of expected wind capacities is required in order to achieve improved integration of wind energy into transmission grids. Extreme values of wind power generation have strong impact on electricity prices (Fig. 6.10).
Precise wind power forecasts can reduce extreme price variations. Fig. 6.11 shows the graph of the error in the day-ahead forecast over recent years, based upon the disclosure obligations of TSOs for forecasted and projected current feed-in levels of wind energy (according to §17 Section 1 StromNZV (German Federal Ministry of Justice and Consumer Protection: Verordnung über den Zugang zu Elektrizitätsversorgungsnetzen (Stromnetzzugangsverordnung—StromNZV))). For Germany, the average root mean square error (RMSE) in relation to the average turbine capacity installed was 2.89% in 2014. The maximum positive discrepancy was 14.2%, and its negative equivalent −21.3%.
A decreasing tendency has been evident for the RMSE since 2010. The graph for the smallest German TSO, TransnetBW, represents an exception to this trend. The potential compensation effects mean the lower the forecast error is, the bigger the transmission grid is and the more WTs are installed in it (see Fig. 6.11). In addition to the higher forecast quality for TenneT and 50Hertz compared to Amprion and TransnetBW, it is reflected most of all in the comparison with the overall German error value. The small transmission grid and the low installed capacity consequently mean a far higher inherent susceptibility to errors at TransnetBW.
6.4.3 Wind Farm Clusters
Wind power already has proven capabilities of providing ASs, as it complies with an increasingly demanding set of grid code requirements. However, most of the times, providing those services implies costs, related to the lost energy, since it is required to operate in a down-regulated mode. Future research and development needs to demonstrate the participation of a wind power plant in a market for short-term reserves, and verify methods to understand how much the power from a wind power plant needs to be de-rated to guarantee a given firm reserve capacity for a given time horizon and with a given statistical significance.
For the provision of such services from wind power, a coordinated management of distributed wind farms aggregated to clusters will improve their capabilities in terms of system support. In such a system, controllability of nearly all components of the electrical equipment plays an important role in the operation of the power system. On one hand, this controllability gives the necessary flexibility to react on system responses over several timescales and events, but on the other hand, it introduces high complexity to the system and dedicated operation would be a hard task.
Aggregating distributed large wind farms to a cluster and controlling the cluster members is realized by the so-called wind cluster management system (WCMS). The system is able to allocate set points for distributed wind farms. While integrated in the control room of the system operator as well as in the energy management system (EMS), different control objectives can be achieved by means of the coordinated management of the cluster [30]. Possible beneficial operational aims are loss minimization, distinct reactive power exchange, reserve allocation, active power re-dispatch for higher voltage levels, fulfilling active power schedules, minimizing control actions of other devices like tap-changers, and the fulfillment of (n−1) security criterion. The structure of the system is shown in Fig. 6.12.
As shown in Fig. 6.12, the system contains forecast information of the wind farm clusters or neighboring clusters in the system as well, in order to achieve good coordination. The wind forecast itself requires the SCADA data of the wind farms. Also, in order to identify the condition of the power system, the WCMS has a connection to the state estimator of the control room and therefore is able to access measurement values that are received with the state estimators cycle time. The system borders to higher and lower voltage levels as well as neighboring systems areas can be sufficiently represented by electrical network equivalents.
The basic functions can be realized by adjusting the active power, or reactive power set points of the wind farm controllers within the cluster. Fig. 6.13 indicates the correction of an exemplary voltage violation during power system operation.
Based on the network equation as indicated in Fig. 6.13, a detected violation of the predefined voltage band is corrected by adjusting the reactive power set point of the cluster. The detection of violations takes place either within the limit check routine or the security assessment in the EMS of the control center, in which different outage scenarios are simulated and evaluated.
In a similar way, overloaded lines or other equipment-like transformers can be handled. In general, if the system operator can detect a congested line section within the limit check or the security assessment of the EMS, the system operator can perform re-dispatches as corrective or preventive measures. The active power set points of already dispatched (according to the market condition) conventional power plants are adapted in such a way as to de-load the congested line. In principle, the set point of one plant in front of the congested line is lowered, while the active power mismatch of the system is tracked back by another power plant raising its active power production at the end of the line segment. If distributed wind farms are aggregated to clusters, sufficient amount of active power can be controlled. In such a way, the WCMS can also provide the re-dispatch functionality. Fig. 6.14 shows the re-dispatch principle.
Similar to the reactive power correction measures, the active power re-dispatch is performed using the network equations by transforming bus and branch variables during the sensitivity analysis. Both methods are based on the power flow formulation of the system.
6.4.4 Virtual Power Plants
For the further transformation of the electricity sector and for an adapted energy supply system, in addition to the network expansion and reinforcement, measures and solutions are needed to reduce the short-term, regional fluctuations of wind power feed-in. Here, storage technologies, the interaction of various RESs, and load management, even including the transport sector, are needed.
The coordination of different components of the electricity system is performed by the aggregation and processing of operational data from all units and their superior control by an EMS. These so-called virtual power plants (VPPs) offer new opportunities for demand-driven energy supply and to improve the flexibility of the consumer. But it is not only required to meet the energy needs at any time, but also to support the reliable operation of the energy system. The energy industry calls this capability of RES a power plant property.
The term power plant property for renewable energy systems indicates that the generation can be planned, controlled, and reliable, as required from the power system needs and that the renewable power plants must support the electric grid even when faults occur. These capabilities are based on the control of active and reactive power of the units [33] as well as the behavior during grid faults like the fault ride-through behavior. These measures to maintain grid stability are referred to as system services or ASs.
Since in the future, often situations will arise where at high wind or PV feed-in periods only a few or no conventional power plants in operation, it is imperative that RESs need to provide the ASs, to ensure security of supply.
It has been shown that modern plants for generating renewable electricity are able to provide ASs. The fact that these options do not or are rarely used is mainly due to three obstacles:
1. The fact that RES can provide AS is, even in the group of experts, not generally or only superficially known.
2. To allow the provision of AS by RES, partly extensive changes to the legal and economic conditions are necessary (e.g., prequalification rules, tendering periods, and technical requirements).
3. To make AS provision reliable, the various producers need to be linked and coordinated by appropriate control systems.
Research and development in the field of energy supply with high share of RES will have to deal with these topics. It is mandatory to develop concepts and mechanisms and create legal foundations which allow the necessary measures to maintain security of supply by ASs, secured by VPPs.
The main services, which may be provided by RES VPP, include in addition to the control power supply (frequency stability, secondary reserve power, and minute reserve) the reactive power (voltage support) and the network congestion management. Other services as the black-start capability are indeed important, but not the focus of current research and development challenges.
6.4.5 Sector Coupling Concepts
The coupling of the sectors electricity, heating, and transport is the key component in the successful transformation of the energy system. The sector coupling thereby effects two significant changes in the system:
1. The heat supply and the traffic can be powered by the electricity sector with renewable energy and so substitute the use of fossil fuels.
2. The coupling of the sectors leads to new, controllable loads in the electricity sector, and so causes an increased flexibility of the overall system.
The further development of renewable energy in the medium term leads to situations with serious surpluses and deficits in the power sector. In the electricity industry, the temporal course of the surpluses and deficits in the power sector is known as the residual load. The greater the fluctuations of the residual load in a region, the more electrical energy needs to be exchanged with the upper level (transmission system) or the neighboring region. Today, PV systems require derating at lunchtime and wind farms are curtailed in order to avoid network congestion. This wasted energy can be ideally used for the operation of heat pumps and the loading of e-cars. Prerequisite for a meaningful coupling is a predictive mode for heat pumps and an intelligent charging management with electric vehicles. Concerted, coordinated management of consumers in the sectors can largely be done with electricity price signals. For this purpose it is also necessary that the forecasts for wind and PV power are used in all sectors for energy management.
Fig. 6.15 shows the supply of RES in 2050 in Germany and the associated courses of traditional and new consumers. Behind this view is a scenario for Germany, that assumes an 83% reduction in greenhouse gases [34]. The electricity demand in this scenario is growing to 793 (TW h) a−1, including heat pumps, e-cars, and OHL trucks. The consumption of e-cars is 110 TW h, including 45 TW h for OHL trucks. The maximum load is about 50 GW. The installed capacity for batteries is 15 and 12 GW for power-to-gas. The needed capacity for this scenario is 200 GW PV, 140 GW wind onshore, and 38 GW wind offshore.
In the load graph in the upper half of the picture, all producers and consumers are included that are not separately pictured in the lower half. The lower half shows mainly consumers, already existing today and expected to exist in future, taking some efficiency measures into account. The graphic represents the weeks 15 and 16 of the year.
6.4.6 European Wind Integration Projects and Studies
By 2020, the EU plans to produce 20% of its electricity from wind, with a much larger percentage beyond that. A few countries already have high wind penetration, but they have been able to do this largely by using conventional reserve power from neighbors with less ambitious renewable energy goals. However, in the future, it will no longer be possible to consider individual wind power plants in isolation. In order to efficiently provide services delivered by conventional generators, it is necessary to consider large clusters of wind farms—both onshore and offshore as well as in different countries—as a single power plant. Conventional generation across Europe will be scarce or expensive, and the grid integration problem will become qualitatively different. The biggest challenge is that the controllability currently provided by conventional generation will come from large numbers of variable wind power plants, coordinated across great geographic, political, and electrical distances. Such coordination will require new tools, data, and procedures—the subject of several European studies and projects. There are already two major initiatives on the road toward the future pan-European power system:
1. The project TWENTIES demonstrates how new solutions can be used to support wind power integration. The project is dealing with pan-European impact and large-scale demonstrations (six demonstrated solutions with analyzed effects). One of three task focuses is on ASs with a total focus on grid impact of existing technology.
2. The European Energy Program for Recovery will support the offshore grid development subarea, i.e., projects in the “grid connection and power transmission.”
Furthermore, a couple of projects, studies, and other activities have worked out important solutions to foster wind energy integration in Europe:
• The main result of IEA Task XXV is a state-of-the-art report which is mainly a summary of national projects and experiences. The goal was to analyze and further develop the methodology to assess the impact of wind on power systems.
• The object of the TRADEWIND project was to assess the impacts of wind power in the European power systems. A new model was developed to analyze scenarios of up to 300 GW wind power in Europe. A trans-European power market and grid expansion will allow higher penetrations.
• The EWIS project emphasized that the integration of wind power is only realizable by joint activities and cooperation at European level. Therefore coordinated measures to reinforce the European grid as provided in EWIS are necessary. Apart from that, legal and market aspects have to be considered to maintain system security. Wind power control affords small wind integration costs compared to benefits.
• The WindGrid project attends to wind grid management, especially wind farm cluster systems and grid operation. The capabilities of wind farms were demonstrated by three zones with (1) high penetration, (2) low penetration, and (3) large power system with low wind energy penetration. The main goals are minimizing dispatching costs and modeling interconnected power systems to integrate large-scale wind farms into the network.
• The PEGASE project points out a pan-European network as a stochastic model operation including observation, planning, simulation, and improved security. It contains an interchange of information about large size networks. The study does not consider the capabilities of wind farms.
• The project SAVEWIND is a continuation of the ANEMOS and ANEMOS+ projects. The aim of SAFEWIND is to substantially improve wind power predictability in challenging or extreme situations.
• The EERA-DTOC project combines expertise to develop a multidisciplinary integrated software tool for an optimized design of offshore wind farms and clusters of wind farms.
• The e-highway 2050 project developed a top-down planning methodology to provide a first version of a modular and robust expansion of the Pan-European Electricity Network from 2020 to 2050, in line with the pillars of European energy policy.
• The NSON project will analyze and evaluate different market and network connection alternatives of Northern Seas regard to their impact on the German as well as the overarching European energy supply system. Use of energy-related simulations, novel mathematical optimization models, methods, and analysis of the European energy system are examined to demonstrate the feasibility and the energy-economic implications of different concepts.
• The IRPWIND project proposes a roadmap for coordinated steps to the transformation of the energy supply system. Besides the sustainable and well-coordinated grid extension and expansion on European level, the operation of the future supply system needs to be supported by precise and high-performance forecasts. This also requires a coordinated data and information exchange on European level, with high data security and reliability as well as fast accessibility. In the frame of IRPWIND, this proposal includes improving the prediction of wind power feed-in as the focus of activities for the integration of RES.
• In the framework of the Energy Concept 2050 by the German Government the large-scale federal research initiative, Kopernikus was launched in September 2016 by the Federal Ministry of Education and Research [35] with the aim to investigate and develop innovative technological and economical solutions for the transformation of the energy system. Within a period of 10 years, more than 230 partners from science, industry, and NGOs will work on the four research topics new grid structure, storage of renewable energy, redesign of industrial processes, and system integration. The results of the first phase are expected to be available by the end of 2019.
6.5 Summary
Since the turn of the millennium, the energy supply in Germany is undergoing a challenging transformation. The shortages of conventional resources as well as the limitation of CO2-emission account for new, sustainable, and affordable energy concepts. Currently, the changes in electrical energy systems (power plant grid storage usage) are at the center of the transition process.
During the last three decades, wind energy has developed into one of the fundamental pillars of the electricity market. The current annual peak load and low load of the German electricity system amounts to 80 and 35 GW, respectively. At the end of 2015, the installed wind capacity in Germany amounts to 45 GW including 3500 GW of offshore installations. Additionally, the installed solar capacity from PVs is 45 GW. Long-term forecasts anticipate tremendous growing rates for both renewable resources up to 300 GW by 2050.
The transition of the electricity system from the integrated and thermal-based generation to volatile RESs requires new challenges for the entire system and in particular for the German and European TSOs. The ongoing wind power and PV integration alters the capacity limits of the German transmission and distribution grid resulting in day-by-day congestions. Short-term measures for more flexible operation, i.e., power flow control, generation management, demand-side management, or flexible line management, are used to provide a temporary relief. However, from a long-term perspective, network reinforcements, according to the German Network Development Plans and the TYNDP by ENTSO-E, are the only sustainable solutions to further integrate an increased capacity of generation from renewables. The long-term development plans reveal new hybrid system structures and therefore an indispensable analysis of new interactions and intersystem phenomena.
During the past 30 years, the wind energy in Germany has developed from small regional installations to a major cornerstone of the today’s electricity generation. Earlier investigation concerning innovative generation concepts and system services of wind generators have paved the way for new technology concepts. Future research with respect to wind integration is dedicated to new tools and concepts of improved wind forecast systems, the integration of VPPs, as well as sector coupling concepts which will combine electricity, heat, and transport.
New grid concepts, the integration of further volatile RES and especially onshore and offshore wind power, require increasing reserve power as well as new economic energy conversion and storage concepts. Accordingly, current and future research activities need to be aligned to a global and interdisciplinary system planning and operation at the European level.