Wind Energy Conversion Systems

In the recent years, many wind turbine generation systems (WTGS) have been installed in many countries. However the electric power obtained from wind generators is not constant due to wind speed variations. The generated electric power and the loss in WTGS change corresponding to the wind speed variations, and consequently the efficiency and the capacity factor of the system also change. In this chapter, methods to evaluate the losses and output power of wind generator systems with Squirrel-Cage Induction Generator (IG), Permanent Magnet Synchronous Generator (PMSG), and Doubly-fed Induction Generator (DFIG) are explained. By using the presented methods, it is possible to calculate the generated power, the losses, total energy efficiency, and capacity factor of wind farms quickly.

This chapter contains a discussion of the advantages and challenges of introducing superconducting generators in future wind turbines. A special focus is given to the European offshore wind turbine marked, because this is the most mature and because the European Union (EU) has decided on a 20% renewable energy share of the electricity by 2020. Thus there are already scenarios of how the offshore wind power capacity is expected to develop in EU over the next two decades and this is used as the framework for a discussion of the advancements needed to make the superconducting drive trains feasible. The text is organized in a section first outlining the EU offshore plans; a section on the different drive trains; a section on the materials used to produce and shape the magnetic field in the generators and finally a section on the superconducting, vacuum, cryostat and cooling challenges of the superconducting direct drive technology.

Power electronics is an enabling technology found in most renewable energy generation systems. In a wind turbine system, it plays an important role in system integration, power quality, and reliability control. Moreover, the fast growth of wind energy poses the increasing need for high-power, low-loss, and fast-switching power electronic devices in order to reduce the system complexity and cost, and improve reliability and compactness. Among the technologies addressing this need, silicon carbide (SiC) power electronics as the most-recent technology stands out because of its superior voltage blocking capabilities and fast switching speeds. As the research samples of SiC power switches become available, it is possible to discuss the design of a wind turbine system using SiC devices and estimate its performance based on the characteristics of practical devices. Therefore, considering the high-power density and high voltage capability of SiC power devices and the recent trend on wind turbine converters, this chapter focuses on the studies of the application of SiC power devices in a full-scale wind turbine converter. First, the characteristics of the most recent devices are obtained through tests. Then, wind turbine system modeling including models for the major electrical components such as generator, power converter, etc., is discussed in detail. Next, the potential benefits from the use of SiC devices in a wind turbine system are explored by a comparison study of the SiC converter and its Si counterpart. Results are presented and analyzed at different wind speeds, temperatures, and switching frequencies. The conclusions drawn from these studies verify that the application of SiC converters in wind turbine systems can improve the wind system power conversion efficiency and reduce system size and cost due to the low-loss, high-frequency, and high-temperature properties of SiC devices even for one-for-one replacement for Si devices. It is also pointed out that the application of SiC devices may enable medium converter technology for wind turbine applications when such devices become available. In this way, substantial improvement can be achieved. The chapter is organized as follows: Section 4.1 introduces the present status of wind energy and power electronics. It briefly reviews the electrical technologies used in wind turbine systems such as generator, power converter technology, and power electronics suitable for wind turbine applications. It also summarizes the future trends on wind turbine systems. Section 4.2 focuses on studies of the application of SiC power devices in a full-scale wind turbine converter, including discussions on the present SiC device characteristics, system modeling, simulations of two wind turbine systems with the same components expected for the power converters (One is with SiC power converter, and the other is with a Si power converter). Section 4.3 draws the conclusions and discusses the future work.

In this chapter a new interconnecting method for a cluster of wind turbine/generators is discussed, and some examples of the basic characteristics of the integrated system are shown. This method can be achieved with a wind turbine generating system using a shaft generator system. A group of wind turbine generators can be interconnected easily with the proposed method, and high reliability and electric output power with high quality are also expected. Moreover, since this method enables transmission of the generated power through a long-distance DC transmission line, the optimum site for wind turbines can be selected so as to acquire the maximum wind energy.

The variable speed wind turbine generator system (WTGS) has recently become more popular than the fixed speed system. In 2004, the worldwide market share of variable speed WTGS was more than 60%. Doubly fed induction generator (DFIG), wound field synchronous generator (WFSG), and permanent magnet synchronous generator (PMSG) are currently being used as variable speed wind generators. Besides the aforementioned classical machines used in variable speed operation of WTGS, the switched reluctance machine (SRM) has some superior characteristics suitable for wind power application. In this chapter, the construction and operation of switched reluctance generator (SRG) are presented. The static characteristics of SRG are involved. The power inverter circuits which can be used in SRG operation are presented. Furthermore, the control of a grid-connected variable speed wind turbine driving SRG is studied. Finally, the dynamic characteristics of variable speed wind turbine driving a switched reluctance generator are analyzed.

This chapter studies the dynamics and control methods of a wind turbine system. Although, there are various types of wind turbines, a general argument applicable to all types of wind turbines is attempted. Since the knowledge of the dynamics of each system is very essential to design a proper controller for that system, therefore, we first develop the dynamic equations of each part of a wind turbine system. The overall model will be the integration of all subsystem models. Next, the control algorithms of a wind turbine will be discussed. Since a wind turbine generator has several levels of control, these levels will be explained in detail. All subsystem control concept and implementation methods are explained after that.

In the last years the use of wind farms has drastically increased. The question is how those new generation systems will affect to the whole grid. In principle, wind energy can be considered a risky source in terms of power quality that must be certified on the basis of measurements performed according to international standards and guidelines. The IEC 61400-21 standard is the reference normative for the certification of the power quality of wind turbines. The first edition was published in 2001 and it specifies the main relevant features of power quality that should be measured in a wind turbine as well as the procedures for their measurement and assessment. According to it, measurements should be performed for harmonic content, flicker, voltage drops as well as active and reactive power, during normal and switching operations. Among those disturbances, flicker has the most complex and sensitive testing procedure.

Power fluctuations of the wind turbine generate voltage fluctuations causing changes in the luminous flux from light sources. Such changes may produce a very unpleasant visual sensation, known as flicker, leading to complaints from utility customers. The IEC 61000-4-15 standard describes the functional specifications of a flicker measurement device and provides a short-term indicator, \(P_{\rm st},\) to characterize the discomfort. This chapter demonstrates the sensitivity of the procedure defined by the IEC 61400-21 standard to asses the flicker by means of different signal processing techniques.

Wind energy resources, unlike dispatchable central station generation, produce power dependable on an external irregular source, the incident wind speed, which does not always blow when electricity is needed. This results in the variability, unpredictability, and uncertainty of wind resources. Therefore, the integration of wind facilities to utility electrical grid presents a major challenge to power system operators. Such integration has significant impact on the optimum power flow, transmission congestion, power quality issues, system stability, load dispatch, and economic analysis. Due to the irregular nature of wind power production, accurate prediction represents the major challenge to power system operators. This chapter investigates the usage of Grey predictor rolling models for hourly wind speed forecasting and wind power prediction.

We discuss in this chapter the salient issues related to lightning protection of large wind turbine blades. Lightning protection of modern wind turbines presents a number of new challenges due to the geometrical, electrical and mechanical particularities of turbines. Wind turbines are high structures and, like tall towers, they not only attract downward flashes but initiate upward flashes as well. The proportion between these types of flashes depends on many factors such as the structure height and the local terrain elevation. The rotation of the blades may also trigger lightning and result in considerable increase in the number of strikes to a wind turbine unit. Since wind turbines are tall structures, the lightning currents that are injected by return strokes into the turbines will be affected by reflections at the top, at the bottom, and at the junction of the blades with the static base of the turbine. This is of capital importance when calculating the protection of internal circuitry that may be affected by magnetically induced electromotive forces that depend directly on the characteristics of the current in the turbine. The presence of carbon reinforced plastics (CRP) in the blades introduces a new set of problems to be dealt with in the design of the turbines’ lightning protection system. One problem is the mechanical stresses resulting from the energy dissipation in CRP laminates due to the circulation of eddy currents. The thus dissipated energy is evaluated and recommendations are given as to the number of down conductors and their orientation with respect to the CRP laminates so that the dissipated energy is minimized. It is also emphasized that the high static fields under thunderclouds might have an influence on the moving carbon fiber parts. Representative full scale blade tests are still complex since lightning currents from an impulse current generator are conditioned to the electrical characteristics of the element under test and return paths. It is therefore desirable to complement laboratory tests with theoretical and computer modeling for the estimation of fields, currents, and voltages within the blades.

The rapid expansion of wind power generation has brought problems involving lightning strikes to the fore. Many such incidents have damaged not only the wind turbine that was actually struck but also other turbines that were not, a phenomenon that is yet to be fully explained. In this chapter, the author presents a case study using a wind farm model with multiple wind turbines connected to a power system. The aim is to clarify the influence of the earthing system on surge propagation from a wind turbine that has been struck to others which have not, during a winter lightning strike. When one of the wind turbines in a wind farm is struck by lightning, the phenomenon of surge invasion to the collection system is categorised as “back-flow surge”. It has been reported that this back-flow surge sometimes burns out surge protection devices (SPDs) or breaks low-voltage circuits even far from the point where the lightning struck. In practice, many such incidents that have occurred not only involved the wind turbine that was actually struck but also other affected wind turbines that had not been struck. This chapter will analyse incidents of burnout to SPDs resulting from winter lightning at wind farms using ARENE and PSCAD/EMTDC. Calculations were performed to clarify the mechanism of how the back-flow surge propagates to other turbines from the directly struck wind turbine. The calculations also clarified that burnout incidents could easily occur even in a turbine that had not been struck by the lightning. It also became evident that burnout incidents can be reduced when interconnecting earthing wires are installed between wind turbines.

Grid integration of wind power is one of the prime concerns as wind power penetration level is increasing continuously. New grid codes are being set up to specify the relevant requirements for efficient, stable, and secure operation of power system and these specifications have to be met in order to integrate wind power into the electric grid. This chapter discusses several issues of advanced grid codes relating to the wind turbines integration into power system. New grid codes for wind power integration of different countries are presented and compared. In India, share of wind power as percentage of installed generation capacity has exceeded 10% in many states and grid connection standards for wind power are in the process of establishment. Grid code requirements for wind power for Indian power sector have been suggested. System operational aspects of wind power generation with Indian experience have discussed.

Pumped storage is generally viewed as the most promising technology to increase renewable energy source (RES) penetration levels in power systems and particularly in small autonomous island grids, where technical limitations are imposed by the conventional generating units. In this chapter, an operating policy is proposed for hybrid wind-hydro power stations (HPS) in island grids, in order to increase wind penetration levels, while at the same time minimizing the impact on the conventional generation system and ensuring the viability of the HPS investment. The proposed operating strategy is applied to different autonomous island systems using a dedicated logistic model, in order to evaluate the effect on the overall operation and economics of the island systems and to assess the feasibility of HPS investments. In addition, the real case study of the HPS in Ikaria island, Greece, which is currently in the construction stage and will be one of the first wind-hydro-pumped-storage hybrid stations in the world, is examined and the expected benefits from its operation according to the proposed policy are presented. The material presented in this chapter is based on publications Papaefthimiou et al. (IET Renew Power Gener 3:293–307, 2009, IEEE Trans Sustainable Eng 1:163–172, 2010) available in IET-Renewable Power Generation and IEEE Transaction on Sustainable Energy.

Considering the viewpoint of cost-effectiveness, a computational method to determine the SMES power rating needed to minimize the grid frequency fluctuation is analyzed in this chapter. Moreover, the required minimum energy storage capacity of SMES unit is determined. Finally, simulation results using pulse width modulation (PWM) based voltage source converter (VSC) and two-quadrant DC–DC chopper-controlled SMES system are presented. It is seen that the proposed SMES system with required minimum energy storage capacity can significantly decrease the voltage and output power fluctuations of wind farm, which consequently mitigate the grid frequency fluctuation.

To optimize the deployment of offshore wind farms for electric power generation, the geographical and seasonal distributions of strong wind over global oceans were examined using nine years of equivalent neutral winds measured by a space-based scatterometer. The relation between scatterometer measurement and surface wind vector is explained. The dependence of wind strength on height and stability is examined. Near-shore locations of strong wind are identified.

This chapter presents an effective control scheme using a line-commutated high-voltage direct-current (HVDC) link with a designed rectifier-current regulator (RCR) to simultaneously perform both power-fluctuation mitigation and damping improvement of four parallel-operated 80 MW offshore wind farms delivering generated power to a large utility grid. The proposed RCR of the HVDC link is designed by using modal control theory to contribute adequate damping to the studied four offshore wind farms under various wind speeds. A systematic analysis using a frequency-domain approach based on eigenvalue analysis and a time-domain scheme based on nonlinear model simulations is performed to demonstrate the effectiveness of the proposed control scheme. It can be concluded from the simulation results that the proposed HVDC link combined with the designed RCR can not only render adequate damping characteristics to the studied offshore wind farms under various wind speeds but also effectively mitigate power fluctuations of the offshore wind farms under wind-speed disturbance conditions (Wang et al., IEEE Trans Power Delivery 25(2):1190–1202, 2010).

Voltage source converter-based HVDC systems have been proposed for connecting large offshore wind farms. Fault ride-through during onshore ac fault is one of the main grid code requirements and technical challenges. Due to the reduction of transmitted grid power during such faults, the wind farm generated power must be either reduced or damped to ensure continuous system operation. Three different strategies, i.e., telecommunication based, offshore frequency modulation, and DC damping resistor are investigated.

The development of off-shore wind farms located at a large distance from the coastline imposes a series of technical challenges. At distances larger than 50–70 km and installed powers larger than 500 MW, the use of HVDC links based on line commutated converters (LCC-HVDC) is the most advantageous solution for the connection to the on-shore transmission grid. At the same time, the market share of wind turbines with fully rated converters is increasing. Currently, manufacturers are offering either direct-drive, single-stage or double-stage gearbox multi-megawatt wind turbines for off-shore applications. The additional fault-ride through and control capabilities of wind turbines with fully rated converters can be exploited to control off-shore ac-grid voltage and frequency. Moreover, the wind turbine front-end converters can also be used to control the power transmited through the HVDC link. At this stage, the use of a controlled rectifier is no longer needed, and a diode-based rectifier can be used at the off-shore side of the HVDC link. This chapter shows how an adequate control system on each wind turbine can be used to enable the diode-based HVDC rectifier to be operated in voltage or current control mode, in a similar way as standard HVDC links. Moreover, the proposed control strategy includes adequate protection and fault ride-through capability against most common grid disturbances.

In this chapter, investigation of wind farms with line commutated converter (LCC) HVDC delivery in system frequency response participation is carried out. While LCC-based high power HVDC is a viable choice to deliver large-scale wind power, the consequent responsibility of such wind energy systems in frequency response should be assumed. A coordination control strategy for wind farms with HVDC delivery for participating in inertial response and primary frequency control is discussed in this chapter. The coordination philosophy is to feedback the grid frequency and its derivative and adjust the delivery power of the HVDC link according to the feedback signals. The feedback loop employing the derivative of the grid frequency aims to improve the inertial response while the feedback loop employing the grid frequency deviation introduces a droop at the rectifier control loop. When the grid frequency is too high or too low, active power flow through the HVDC link will be ramped down or up. In turn, the wind generation will increase or decrease the blade angles to reduce or increase the captured wind power through pitch control. A case study demonstrates the effectiveness of the inertial enhancement and frequency droop in HVDC control. Simulation results in TSAT are given.

The innovative renewable energy conversion system called “hybrid offshore-wind and tidal turbine power controller with bi-directional converter (HPB)” was proposed. The system was used to evaluate a tidal turbine (induction machine) with the ability to change the AC frequency reference to a bi-directional converter for optimal performance. The tidal power generation system consists of an induction generator/motor connected to a DC capacitor link through an insulated-gate bi-polar–transistor (IGBT) bi-directional converter. Using PWM control of the bi-directional converter, the speed of the rotating magnetic field (control by the AC voltage frequency) fed to the induction machine can be controlled. The generation power can be controlled quickly by only changing the frequency reference for the PWM converter.

Trends in growth of the wind energy is getting additional pace by offshore technology. This chapter investigates a suitable control strategy for a DC-based offshore wind farm to transmit bulk power to an onshore grid through a high voltage DC (HVDC) transmission line. The offshore wind farm is composed of variable-speed wind turbines driving permanent magnet synchronous generators (PMSG). Each PMSG is connected to the DC bus through a generator-side converter unit to ensure maximum power point tracking control. The DC voltage of the DC-bus is stepped up using a full-bridge DC–DC converter at the offshore HVDC station, and the wind farm output power is transmitted through the HVDC cable. The onshore HVDC station converts the DC voltage to a suitable AC grid voltage. Detailed modeling and control strategies of the overall system are presented. Real wind speed data is used in the simulation study to obtain a realistic response. The effectiveness of the coordinated control strategy developed for the proposed system is verified by simulation analyses using PSCAD/EMTDC, which is the standard power system software package.