by Thomas Ackermann

Wind power use has grown significantly in recent years. As power system planners, investors, and other stakeholders evaluate the development of wind power plants, many questions about wind power plant operational characteristics have arisen. These questions range from asking whether wind power plants require storage, to how fast can wind power plants’ output fall to zero. Recent experience in operating power systems with increasing levels of wind power, along with a significant body of analysis, has provided answers to many of these questions. In this chapter, we discuss many of these key questions and answers drawing on international experience and analysis of wind power plant operations.

by Michael Milligan, Kevin Porter, Edgar DeMeo, Paul Denholm, Hannele Holttinen, Brendan Kirby, Nicholas Miller, Andrew Mills, Mark O’Malley, Matthew Schuerger, Lennart Söder

Part A THEORETICAL BACKGROUND

by Thomas Ackermann

by Lennart Söder, Thomas Ackermann

This chapter presents a comprehensive survey of the generator and power electronic concepts used by the modern wind turbine industry. A state of the art of wind turbines, from an electrical point of view, with focus on topologies and control strategies is provided. An overall perspective on both contemporary and new potentially promising wind turbine concepts, classified with respect to both their speed control ability and to their power control type is presented. A detailed investigation of the market penetration and share of different wind turbine concepts over years is also performed based on supplier’s market data and concept evaluation for each individual wind turbine type sold by the Top Ten suppliers over years.

by Anca D. Hansen

This chapter focuses on the power system impacts of large-scale wind power. The chapter is subdivided into sections covering basic principles of power system operation, main characteristics of wind power production and finally the different effects that wind power has on power systems.
Wind power will impact the grid planning (transmission grid adequacy). The variability and uncertainty of wind power will impact the reserves that power systems carry as well as scheduling and efficiency of other power plants. Wind power will reduce the fossil fuel consumption and emissions in power systems and also add reliability (capacity value). Large-scale wind power still lies in the future for many systems and there are long-term trends that can include the impact of wind power on the system, such as increasing flexible gas-powered power plants and the use of demand side management.

by Hannele Holttinen, Ritva Hirvonen

The aim of the power plants in a power system is to supply the load in an economical, reliable and environmentally acceptable way. Different power plants can fulfil these requirements in different ways. In order to select the right sources it is important to compare the value of the different sources using an objective approach. The aim of this chapter is describe the different needs of a power system and how these needs can be met with wind power, that is, the value of wind power in a certain system. The values are operating cost value, capacity value, control value, grid loss reduction value and grid investment value. The values can be calculated for different types of power plants, they can be both positive and negative, and they can be calculated both as a physical cost value and a market value.

by Lennart Söder

Part B TECHNICAL REGULATIONS AND GRID CODE VALIDATION

Injection of wind power into an electric grid affects the voltage quality. As the voltage quality must be within certain limits to comply with utility requirements, the effect should be assessed prior to installation. To assess the effect, knowledge about the electrical characteristics of the wind turbines is needed or else the result could be an inappropriate design of the grid connection. Characteristics specified according to IEC 61400-21 provide a basis for such assessment. The characteristics are explained and a case with a small wind farm on a distribution feeder is applied as an example showing step by step relevant calculations considering slow voltage variations, flicker and harmonics. It is shown that possible voltage quality problems due to a wind turbine installation in many cases may be overcome simply by selecting an appropriate wind turbine type, and/or by adjusting wind turbine control parameters. It is noted that IEC 61400-21 has developed very significantly from the first edition, having now focus on grid code compliance. Modern wind farms may be considered power plants with capabilities for grid support.

by John Olav Tande

by Fritz Santjer

This chapter focuses on the actual power quality measurements of different types of wind turbines connected to various types of grids. The chapter is subdivided into five sections covering voltage variations, flicker, harmonics, transients and frequency.
Voltage variations are caused by the power produced by the turbine while flicker is caused by continuous operation and by switching operations. Flicker during continuous operation is caused by power fluctuations from variations in the wind speed, the tower shadow effect and mechanical properties of the wind turbine.
Fixed-speed wind turbines are not expected to produce harmonics but they do cause transients. Variable-speed wind turbines equipped with a converter produce harmonic currents during continuous operation.
Wind power does not normally cause any frequency problems except for autonomous power systems. The spinning reserve is small in an autonomous grid and wind power may cause frequency fluctuations in case of a sudden wind rise or wind drop.

by Åke Larsson

This chapter provides a brief discussion and analysis of the current status of interconnection regulations for wind turbines and wind turbines in Europe. The chapter starts with a short overview of the relevant technical regulation issues, which includes a brief description of the relevant interconnection regulations considered in this chapter. This is followed by a detailed comparison of the different interconnection regulations. The discussion also includes the capabilities of wind turbines to comply with these requirements. Finally, issues related to international interconnection practice are briefly discussed.

by Julija Matevosyan, Sigrid M. Bolik, Thomas Ackermann

Wind generated power now constitutes a noticeable percentage of the total electrical power consumed and in some utility areas it even exceeds the base load in the network. This indicates that wind is becoming a major factor in both electricity supply and balancing consumer demand with a flexible power production. However one major drawback to further integration of wind power into the grid is its variability.
Due to the dependence on the weather, the forecasted output cannot be guaranteed at any particular time. This makes planning the overall balance of the grid difficult and therefore utilities are often biased against using wind power.
One solution to improve the image of wind power could be, more accurate forecasting of power inputs from wind farms into the grid which in turn would reduce network operation issues caused by fluctuating wind power input. Meteorological and research and development (R&D) institutes initiated and developed the short-term prediction wind power tools. They have used detailed, area-specific, three-dimensional weather models and have worked with numerical weather prediction (NWP) models. The systems use either of physical equations, statistics or a combination of both to transform wind speed into electrical power and provide forecasts for single wind farms up to larger areas. Forecasts horizon vary from a few minutes or hours ahead, sometimes called nowcasting, up to several days. Some systems use ensemble forecasts or a combination of several NWP models to improve forecast quality.
So far wind power forecasts has help essentially to integrate wind power into the power systems during the last years. However, with increasing amount of wind power installed further improvements on forecasting skills will be needed. The number of research institutes and commercial initiative perform R&D on this field shows that the necessity for this is know and further developments can be expected within the next years.

by Martin Schellschmidt, Stephan Adloff, Markus Fischer

Part C WIND POWER PLANT AND TRANSMISSION ISSUES

by Nicholas Miller, Reigh Walling, Richard Piwko

by Thomas Ackermann, Antje Orths, Krzysztof Rudion

In this chapter, the grid connection of large-scale wind power plants or wind power plant clusters by means of high phase order transmission systems, or more profoundly bipolar HVAC cable systems is explained. In this HVAC bipolar system, the phase order is increased by appropriate transformer groups from three phases to either four or six phases. After a short description of these transformer groups, submarine cables for the bipolar transmission system are explained. Beside single-core submarine cables, a new cable type with four cable-cores is described. After the global description, application examples for bipolar HVAC offshore transmission systems are given. As the last part of the transmission chain, bipolar land cable systems are analysed with respect to their current rating as well as the magnetic inductions, also in comparison to three-phase cable systems.

by Heiner Brakelmann, Jan Brüggmann

By using a permanent magnet induction machine as wind generator, the gearbox and converter can be omitted, and the total number of parts reduced leading to a low maintenance and reliable turbine for offshore application.
The rotation speed of the turbine however cannot be matched to the wind speed, reducing the energy yield at part load. To overcome this drawback, a central converter can be used, which adjusts the frequency of the local grid in the wind park; this is the so-called park-variable concept.
This concept has been compared with respect to energy yield with constant speed and variable speed turbines.
Overall, the differences in energy yield of the investigated concepts are so small that other criteria, such as reliability or cost, may be relevant for the selection of one or the other approach. Above all, the park-variable concept represents an interesting alternative to today’s common concepts.

by Eckehard Tröster

Part D INTERNATIONAL STUDIES

by Hannele Holttinen

The chapter describes the findings of two studies looking at required improvements in the European transmission system to enable large scale integration of wind power. The TradeWind study of the European wind industry looks at the time horizon of 2030 integrating 400 GW of wind power capacity. The EWIS study of the European TSO’s looks at the time horizon of 2015, integrating 140-185 GW of wind power capacity. A common observation from the studies is that the European transmission system can be extended such that the wind power capacities needed to meet the European energy and climate targets up to the year 2030 can be integrated, by means of a gradual but ambitious extension programme of the network, mainly on the cross border links. In addition the EWIS study identified a whole series of operational and grid security measures at so called network pinch-points to complement the infrastructure reinforcements. The studies have quantified the large macro-economic benefits that the identified reinforcements will bring.

by Frans van Hulle

This chapter provides an overview of major transmission planning activities and related wind integration studies in the US. Transmission planning for energy resources such as wind is different from planning for capacity resources. Those differences are explained, and illustrated with examples from each of the three major interconnections in the US. Transmission planning for wind is becoming an iterative process consisting of generation expansion planning, economic-based transmission planning, state policy requirements, system reliability analysis, and wind integration studies. A brief look at the policy and regulatory environment in which this activity is taking place is provided, along with a brief summary of the two major recent wind integration studies and a short discussion of ongoing interconnection-wide planning efforts.

by J. Charles Smith, Dale Osborn, Richard Piwko, Robert Zavadil, Brian Parsons, Lynn Coles, David Hawkins, Warren Lasher and Bradley Nickell

The best conditions for wind power integration are in the remote areas where the transmission system is weak and not dimensioned to accommodate large-scale power production. Thermal limits of the overhead lines or voltage stability problems limit transmission capability during the extreme situations. Various measures exist to increase transmission capacity. However, even if additional transmission capacity is built, it is not economical to dimension it for extreme situations that occur very seldom. The alternative is to curtail wind energy during the congestion problems or use coordination with hydropower to store the excess wind energy. This chapter discusses transmission limits, methods that European TSOs are using to determine transmission capacity. The chapter then presents various measures to increase transmission capacity, as well as alternatives to grid reinforcement such as wind energy curtailment and coordination between wind and hydropower. Methods to estimate wind energy curtailment are also presented and application is shown with the example of Swedish power system.

by Julija Matevosyan

This chapter examines electrical energy storage in systems with high amounts of wind power. Applications for energy storage and wind and storage technologies which could be used are outlined. A literature review is given on using storage to integrate wind. Storage is an expensive resource, and therefore hard to justify; ensuring studies capture the full range of value from storage is an area of active research. Results are given from a production cost study on the future Irish system with high (>30% by energy) levels of wind. It is shown that the main benefits storage can bring versus a conventional plant are the reduction of wind curtailment and management of uncertainty. At current storage costs, pumped storage is unlikely to justify itself on the Irish system until approx. 50% of energy is from wind; the flexibility of existing plant mix and other system factors are shown to be important factors in justifying storage. While storage has benefits in terms of quick response times and ability to reduce curtailment and congestion, it will be difficult to justify in many systems until high levels of wind power are seen.

by Aidan Tuohy, Mark O’Malley

by Poul Erik Morthorst, Thomas Ackermann

Part E POWER SYSTEM INTEGRATION EXPERIENCE

The Danish power system is known for being the first system with both, a very fast growth of wind energy providing today a significant share of electricity consumption. The massive expansion of wind power is the result of targeted political efforts by e.g. introducing extensive public subsidising schemes.
Simultaneously to the increase of wind power, the market was unbundled. Both facts have changed the TSO’s business substantially. Production, transport and trade of energy were separated which has lead to changes in the information flow.
The national Danish TSO is responsible for owning and operating the transmission grids for electricity and gas as well as providing well functioning markets by the respective suitable market design.
This chapter describes the main factors of success as well as coming challenges with further increase of wind- also in neighbouring countries.

by Antje Orths, Peter Børre Eriksen

Large capacities of wind power have already been accommodated in Germany with around 26 GW (May 2010). Yet further increases can be expected in order to achieve Europe’s 2020 targets for renewable energy. This poses big challenges for wind generation developers in terms of obtaining suitable sites and financing the associated investments. Such developments also impact the networks and especially in Germany with around 40% of the Europe’s current installed wind power capacity. Flexibility measures are used to provide a temporary relief but distances between generation and consumption will more and more increase. This gives rise to new challenges for the entire system in particular for transmission system operators as far as a safe and reliable electricity supply is concerned. By the year 2030 a total wind power capacity of more than 60 GW is expected for Germany, which is much higher than 50% of the German peak load. The immediate transmission related challenges for Germany embedded in a broader European context were already published by the German TSOs as a result of national and European wide investigations. The increase in onshore wind power combined with foreseeable offshore developments will require further action to guarantee the appropriate quality of supply.

by Matthias Luther, Wilhelm Winter

by Ana Estanqueiro

Wind power generation capacity in the Spanish peninsular power system will reach 20 GW by the end of 2010 and current national plans aim to nearly double that figure by 2020. Power system peculiarities and current and future challenges and solutions associated with wind power integration in the peninsular Spanish territory are described in this chapter:

by Juan Ma. Rodríguez García, Olivia Alonso García, Miguel de la Torre Rodríguez

by Jonathan O’Sullivan

This chapter provides a review of the factors that drove the rapid development of wind generation in the State of Texas, which currently has the most installed wind generation capacity in the United States. It addresses the unique circumstances that caused this rapid development, including ease of transmission access, availability of significant wind resources, the existence of a competitive electricity market and the passage of a Renewable Portfolio Standard. One section of the chapter deals with Texas’ pioneering concept of Competitive Renewable Energy Zones (CREZ) and the associated $5 billion transmission build out to accommodate over 18,000 MW of wind generation. The chapter also examines the various operational and market issues encountered by the grid operator as wind generation has increased as well as the solutions to those issues developed by the grid operator and market participants. Finally, the chapter discusses future issues that could impact additional wind generation development, such as reliability constraints and the potential use of storage technologies.

by Henry Durrwachter, Warren Lasher

The New Zealand power system provides some unique challenges to wind integration. It is a medium sized power system with diverse electricity fuel sources, a lightly regulated market, no interconnections due to its isolation, and wind penetration approaching 20% at times of high wind and light load. This chapter describes the development of utility scale wind generation in NZ, some of these challenges and provides some interesting case studies. One case study describes the solutions adopted to maximise the scale of a wind farm development in a very weak rural sub-transmission network (White Hill), and a another case study describes the lessons learnt connecting a wind farm to the transmission grid in the middle of New Zealand’s capital city and close to the HVDC link converter station (West Wind).

by Ray Brown

by Yongning Chi, Zhen Wang, Yan Li, Weisheng Wang

The market for using wind technology to support isolated power generation has recently progressed from a topic discussed by researchers to commercial operating systems. From Alaska to the Antarctic, from Spain to the Galapagos Islands, the integration of wind turbines with conventional isolated generation has become a commercial reality. Additionally, the rising costs, environmental regulations, and the carbon impacts of diesel fuel are driving remote communities dependent on diesel generation to look for alternatives, of which wind-diesel now represents a viable alternative. This chapter provides a detailed overview of the use of wind in isolated power systems with a designated focus on wind-diesel applications. Topics addressed are specifically power system design, addressing issues of grid stability in remote systems and a description of commonly occurring impacts surrounding the integration of large amounts of wind into isolated networks. Examples of wind-diesel systems are provided to demonstrate the variety of operating configurations.

by E. Ian Baring-Gould, Per Lundsager

by Poul Sørensen

Wind generated power now constitutes a noticeable percentage of the total electrical power consumed and in some utility areas it even exceeds the base load in the network. This indicates that wind is becoming a major factor in both electricity supply and balancing consumer demand with a flexible power production. However one major drawback to further integration of wind power into the grid is its variability.
Due to the dependence on the weather, the forecasted output cannot be guaranteed at any particular time. This makes planning the overall balance of the grid difficult and therefore utilities are often biased against using wind power.
One solution to improve the image of wind power could be, more accurate forecasting of power inputs from wind farms into the grid which in turn would reduce network operation issues caused by fluctuating wind power input. Meteorological and research and development (R&D) institutes initiated and developed the short-term prediction wind power tools. They have used detailed, area-specific, three-dimensional weather models and have worked with numerical weather prediction (NWP) models. The systems use either of physical equations, statistics or a combination of both to transform wind speed into electrical power and provide forecasts for single wind farms up to larger areas. Forecasts horizon vary from a few minutes or hours ahead, sometimes called nowcasting, up to several days. Some systems use ensemble forecasts or a combination of several NWP models to improve forecast quality.
So far wind power forecasts has help essentially to integrate wind power into the power systems during the last years. However, with increasing amount of wind power installed further improvements on forecasting skills will be needed. The number of research institutes and commercial initiative perform R&D on this field shows that the necessity for this is know and further developments can be expected within the next years.

Bernhard Ernst

Part F DYNAMIC MODELLING OF WIND TURBINES FOR POWER SYSTEM STUDIES

This chapter provides a general overview of aspects related to wind turbine modelling and to computer simulations of electrical systems with wind turbines.
An overview of wind turbine aerodynamic modelling is given together with basic descriptions of how to represent the physical properties of a wind turbine in different mathematical models. The main purpose of this section is to convey an intuitive understanding of the aerodynamic system of a wind turbine, and how this aerodynamic system can be modelled in various ways.
A block diagram of a generic wind turbine model is presented, and the various independent elements of a wind turbine are explained.
A general overview of p.u. systems is presented with a special emphasis on p.u. systems for the mechanical system of a wind turbine. Some basic, yet by experience often troublesome, considerations associated with per unit systems and mechanical data are presented in details. Typical mechanical data for a contemporary size wind turbine is given, and the conversion of these data from physical units to p.u. is carried out. These per unit data are representative for a wide range of sizes of wind turbines and therefore suitable for user applications in a number of electrical simulation programs.
Finally an overview of various types of simulations is presented together with a discussion what to include in a wind turbine model for each specific type of simulation.
The issues related to model implementation in various simulation programs are not addressed, except for some specific issues related to implementation in transient stability programs.

by Hans Knudsen, Jørgen Nygård Nielsen

Generic models are a viable alternative to represent wind turbine-generators and wind power plants in system planning and interconnection studies. Compared to manufacturer-specific models, generic models tend to be more accessible, easier to maintain, and more portable across simulation platforms. The non-proprietary nature of generic models is more compatible with traditional bulk system study practice and procedures. The Western Electricity Coordinating Council (WECC) has developed an initial set of generic models that are implemented in several commercial simulation platforms. It has been shown that generic models provide reasonable approximation of the characteristics of many wind turbine-generators. Generic models can also be used to represent wind power plants. The existing generic models provide a good basis for standardized models for wind power plants; however, they are a work in progress. WECC and other organizations continue to work with industry to refine and validate generic models.

by Abraham Ellis, Yuriy Kazachkov, Juan Sanchez-Gasca, Pouyan Pourbeik, Eduard Muljadi, Michael Behnke, Jens Fortmann, Slavomir Seman

In this chapter power system dynamics simulation (PSDS) is used to study the dynamics of large-scale power systems. It is necessary to incorporate models of wind turbine generating systems into PSDS software packages in order to analyse the impact of high wind power penetration on electrical power systems. These models need to match the assumptions and simplifications applied in this type of simulation.
This chapter presents models that can be used to represent wind turbines in PSDSs. We give a brief introduction to PSDS, and describe the three main wind turbine types and the assumptions on which they are modelled. We then present the models of the various subsystems of each of the most important current wind turbine types are then presented. The response of the models to a simulated wind speed sequence is then shown.

by Katherine Elkington, J.G. (Han) Slootweg, Mehrdad Ghandhari, Wil L. Kling

The chapter focuses on the modelling of Doubly-Fed Induction Generators (DFIGs).
The chapter presents a model of a doubly-fed induction generator on the most detailed level namely a 6th-order state variable model including both rotor and stator electromagnetic transients.
The Voltage Source Converter (VSC) of the rotor is not modelled with a detailed modulation scheme, rather it is assumed that the switching frequency is infinite. In spite of the later simplification, limitations in the voltage generation by the converter due to the dc-link are taken into account and are implemented as limitations to the voltage and torque controllers. The chapter describes a Sequencer with different modes of operation of the DFIG.
The DFIG-model is tested in a small power system simulated within instantaneous value mode. No verifications have been made to a real installation of a DFIG-model.
Finally it is described how to reduce the order of the DFIG-model.

by Eva Centeno López, Jonas Persson

by Vladislav Akhmatov

This chapter examines how wind power will impact the stability of power systems. It focuses on the three aspects of power system stability: voltage stability, rotor angle stability and frequency stability. It completes a detailed analysis as to how wind power in power systems will impact the short-term and long-term stability of the system. Utilizing fault analysis, power flow studies, frequency simulations, small-signal analysis, and transient analysis this chapter examines the spectrum of stability issues associated with power systems. As wind power continues to increase in power systems around the world, it will become critical to examine stability issues in power system in the same manner and traditional power system components. This chapter provides a basic outline on how to perform power system stability studies with large penetrations of wind power.

by Eknath Vittal, Andrew Keane, J.G. (Han) Slootweg, Wil L. Kling

by Vladislav Akhmatov, Bjorn Andresen

Part G Future Issues

by Goran Strbac, Predrag Djapić, Thomas Bopp, Nick Jenkins

by Thomas Ackermann, J.G. (Han) Slootweg

To keep the voltage within certain ranges is important for the stability and quality of electrical power systems. Decentralised power systems contribute to an increasing share in power generation and can offer additional benefits to secure a safe operation of the grid.
The provision of reactive power by decentralised generation systems allows to influence the electrical power system with regard to power flow and voltage regulation. The quality and benefit of a voltage regulation depends on the dynamic range of reactive power provision and the underlying control structure.
Although the generation of active power depends on the wind conditions and is therefore fluctuating, modern decentralised feeding systems, especially wind turbines with dynamic reactive power capabilities, are already able to substantial contribute to power system stability and voltage stability in particular; independent from the wind.
Further developments in wind turbine dynamics in combination with new control concepts and communication systems, will contribute to secure a safe electrical energy supply under a steady increasing share of renewable energy.

by Marcel Kruse, Alfred Beekmann, Volker Diedrichs

Fluctuating and meteorologically determined energy sources can only be absorbed into electricity transmission grids to a limited extent. From a share of 25% of wind energy contribution in the network load upwards, excess energy production will occur in most European electricity distribution systems. To avoid this, a storage medium needs to be incorporated in the wind power or generation system that allows a flexible usage of surplus power.
Hydrogen can offer such a medium and furthermore displays several further interesting characteristics:

by Robert Steinberger-Wilckens