Volume 2 wind energy 2 18 – wind power integration

Volume 2 wind energy 2 18 – wind power integration Volume 2 wind energy 2 18 – wind power integration Volume 2 wind energy 2 18 – wind power integration Volume 2 wind energy 2 18 – wind power integration Volume 2 wind energy 2 18 – wind power integration Volume 2 wind energy 2 18 – wind power integration

JA Carta, Universidad de Las Palmas de Gran Canaria, Las Palmas de Gran Canaria, Spain

© 2012 Elsevier Ltd All rights reserved

2.18.2 Overview of Conventional Electrical Power Systems

2.18.2.1 Structure of an Electrical Power System

2.18.2.1.2 Electrical networks

2.18.2.2 Operational Objectives of an Electrical Power System

2.18.2.3 Operating States of an Electrical Power System

2.18.2.3.1 Active power–frequency control

2.18.2.3.2 Voltage control

2.18.3 The Distinctive Characteristics of Wind Energy

2.18.3.1 The Unpredictability and Variability of Wind

2.18.3.2 The Variability of Electrical Energy from Wind Sources

2.18.3.2.1 Effect of wind turbine aggregation on wind power variability

2.18.3.2.2 Effect of the geographical distribution of wind farms on wind power variability

2.18.4 Wind Power and Power System Interaction

2.18.4.1 Comparison between Conventional and Wind Generation Technologies

2.18.4.2 Potential Disturbances in the Interaction of Wind Turbines with the Electrical Network

2.18.4.2.1 Frequency variations

2.18.4.2.2 Voltage variations

2.18.4.2.3 Voltage flicker

2.18.4.2.4 Phase voltage imbalance

2.18.4.2.5 Voltage dips and swells

2.18.5 Planning and Operation of Wind Power Electrical Systems

2.18.5.1 Repercussions of Wind Power for Power System Generation

2.18.5.1.1 Repercussions of wind power for generation reserve capacity

2.18.5.1.2 Repercussions of wind power for energy storage needs

2.18.5.1.3 Repercussions of wind power for generation capacity

2.18.5.2 Impact of Wind Power on the Power Transmission and Distribution Networks

2.18.5.2.1 Electrical power transmission from remote onshore wind farms

2.18.5.2.2 Electrical power transmission from offshore wind farms

2.18.5.2.3 Integration of wind power in distribution networks

2.18.6 Integration of Wind Energy into MGs

2.18.6.2 Benefits of Wind Energy Integration into MGs

2.18.6.3 Problems Associated with Wind Energy Penetration in MGs

2.18.7 Questions Related to the Extra Costs of Wind Power Integration

2.18.8 Requirements for Wind Energy Integration into Electrical Networks

2.18.9.2 Statistical and Data Mining Models

2.18.9.2.1 Statistical models

2.18.9.2.2 Data mining techniques

2.18.9.3 Currently Implemented Forecasting Tools

2.18.1 Introduction

The main application of the first wind turbines that were built at the end of the nineteenth century to convert the wind’s kinetic energy into electricity was in stand-alone systems [1–3] That is, these wind generators were connected to small stand-alone electrical networks and operated in parallel with electrical generators coupled to diesel engines, or incorporated some type of energy storage system which often consisted of a battery bank The main purpose was to provide electricity in remote areas where the installation of transmission and distribution lines from the power generation stations was prohibitively expensive Today, on the other hand, most wind installations fundamentally comprise installation, transmission, and distribution at a low cost

These wind turbine clusters, known as wind parks or wind farms [4], are interconnected to the main network, operating in parallel with it in such a way that they are both feeding power into and consuming power from that network While the first wind farms were installed in the 1980s in the United States and then in Europe [3, 4], it was not until the final years of the twentieth century that the numbers of wind farms connected to electrical networks began to rise spectacularly throughout the world [5–10] Wind farms were initially installed onshore (Figure 1), and indeed this trend continues However, in some northern European countries, a combination of the scarcity of suitable onshore sites with exploitable wind resources and certain favorable characteristics presented by the sea has led to the installation of offshore wind farms, as shown in Figure 2, which first began to be developed from

1991 onward [3, 8, 11] The initiative to install offshore wind farms was taken by Denmark, followed by Sweden, Ireland, the United Kingdom, and The Netherlands By the end of 2010, the 27 member states of the European Union (EU) were benefiting from a total installed wind power capacity of 84 278 MW, of which 2946 MW corresponded to offshore installations [5] According to the World Wind Energy Association (WWEA), the installed wind power capacity worldwide as of the end of 2009 amounted to 196 630 MW [6]

However, mean wind energy penetration, that is, the percentage of the demand for electricity that is covered over a long time period (normally 1 year) in a particular region by electrical energy derived from wind resources, can vary considerably from one country to another In some countries this penetration was less than 1%, while in Denmark a figure of around 21% was obtained [6] For 2008, the mean penetration throughout the EU was 4.2% [8] However, according to the European Wind Energy Association (EWEA) and its reference scenario for 2030, 300 GW of wind power will be installed by that year It is estimated that this power will produce 935 TWh of electricity, half of which will come from offshore installations, and will cover somewhere between 21% and 28% of the electricity demand of the EU [8], depending on the evolution of future power consumption

Parallel to the growing numbers of onshore and offshore wind farms which are connected to the high-voltage (HV) electrical network, there has been an increasing interest in proposals for the installation of ‘embedded’ or ‘distributed’ generation (DG), given the benefits such a system can offer [12–19] This type of generation refers to small generators that are normally connected to a distribution network at medium (MV) or low voltages (LV) rather than to a transmission network at high voltages, which is the normal situation in centralized generation (CG) systems The use of DG systems rules out the possibility of including large wind farms or large hydroelectric plants, but does provide the possibility of using generators powered by renewable energy sources, emergency generators, and combined heat and power (CHP) cogeneration systems Among the various options that have been proposed for DG are the subsystems known as microgrids (MGs) [12, 20]

MGs are small-scale, low-voltage networks which aim to supply energy to small communities An MG’s generation system is normally hybrid That is, it usually comprises generators powered by a variety of energy sources [21–26], both renewable and conventional, and energy storage devices [27–30] Such a power system supplies energy to loads that are located in the vicinity through intelligent coordination of the whole system These MGs can be designed in such a way that they can normally operate interconnected to the main network [31] or can operate in stand-alone mode [32]

There are a number of advantages to the integration of wind energy into already existing networks or into those under development This chapter will examine these advantages, along with the consequences that the unusual characteristics of this energy source (i.e., its unpredictable nature and the fluctuations in the power generated) can give rise to in the network to which it is connected, as well as the effects such integration has on the network’s operational strategies A presentation is also made of distributed systems together with an explanation of the benefits of the integration of wind energy into normal interconnected MGs and stand-alone MGs

2.18.2 Overview of Conventional Electrical Power Systems

Most existing electrical energy systems in the world have very similar structures regardless of the country in question They are basically industrial systems geared toward the generation, transmission, distribution, and supply of electricity [33–38] (Figure 3) Historically, the generation of electricity has been undertaken at large power production stations Often, this type of centralized station is located some distance away from the areas of major consumption, and the energy is supplied to these areas via electrical networks Large wind farms and renewable DG systems are connected to these networks by feeding energy into and extracting energy from them The resulting power flow caused by these installations can affect both the installations themselves and the power systems to which they are connected For a better understanding of the problems associated with wind energy integration, an introduction to a number of questions related to electrical power systems is given below

2.18.2.1 Structure of an Electrical Power System

Electrical energy systems can basically be structured into three main blocks: generation, energy transmission/distribution networks, and loads A brief presentation of each of these aspects, as well as of control and protection systems, will be made in the following sections

2.18.2.1.1 Generation

Power stations traditionally convert the energy stored in a primary energy source like coal, oil, nuclear fuel, gas, or a volume of water

at a certain height into electrical energy The most commonly used technologies are hydro, thermal, and nuclear power plants The generation of hydroelectric power (Figure 4) entails the conversion of the potential energy of a volume of water located at a certain height into kinetic energy in a hydraulic turbine and the conversion of mechanical energy into electrical energy in an electrical generator So, hydroelectric power stations require a flow of water and a difference of level in that flow [23, 28, 38–42] Depending on the method used to control the flow of water, hydroelectric plants can be classified into two basic types: run­of-river power plants and reservoir power plants Run-of-river power plants take part of the flow of a river and direct it toward the turbines There are a variety of possible configurations, but this type of plant can only allow small controlled flows of water through the turbines Reservoir power plants have the capacity to store water by means of a dam and thus control the flow of water through the turbines and, consequently, the production of electricity

Thanks to the storage capacity of reservoir power stations, these power stations can be combined with pumping stations to make pumped storage plants The pumping stations are responsible for returning to the dam, or the upper reservoir, the water that has been discharged from the turbines into a reservoir constructed in the lower part of the station In this way, the surplus energy

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of just a few seconds Moreover, they can maintain a practically constant efficiency throughout the power output range, which can

be widely controlled So, hydroelectric power plants constitute a highly flexible source of electrical generation that can be adapted to variations in demand Other advantages include the cost of the fuel and the absence of atmospheric contamination However, drawbacks of such plants include the high investment costs, a degree of randomness and constraints in terms of the amount of primary energy available, and the need to flood extensive areas with its consequent environmental impact

Conventional thermal power plants convert the primary energy that comes from a fossil fuel (coal, oil, or natural gas) into electrical energy [23, 34–38, 42–46] In these power stations, the fuel is burned in a steam boiler Here, the primary energy is converted into the thermal energy contained in the steam and gas emissions which are produced in the combustion process and escape into the atmosphere A steam turbine (Figure 5) converts the thermal energy stored in the water vapor into mechanical energy, namely, the rotational movement of a shaft This mechanical energy is then turned into electrical energy by means of a generator coupled to the turbine shaft (Figure 6)

As a result of the thermal inertia of the boiler, these power stations are somewhat inflexible in terms of connection and disconnection Moreover, control of conventional units is not possible over the entire power output range In order to guarantee power generation availability and to improve the system’s flexibility, a large number of units need to be permanently connected covering the loads, regardless of the power demand This means that a number of turbines operate under partial load, which is uneconomical The use of conventional thermal units to cover demand peaks results in efficiency losses and an important restriction

to optimizing the use of the primary energy source

One of the advantages of thermal power stations is that, in principle, enough primary energy can be stored to guarantee its availability for continuous use during a reasonably long period of time Drawbacks of the system, in addition to those outlined above, include the cost of the fuel and its contribution to atmospheric contamination

Conceptually, the turbines employed in gas-fired thermal power stations (Figure 7) are basically the same as steam turbines The fundamental difference lies in the fact that the stream which strikes the turbine blades is a mixture of the gases that result from the combustion of the fuel used, rather than water vapor (steam) The system uses a compressor which introduces air at high pressure (thereby raising its temperature) into a combustion chamber where it is mixed with the fuel, which ignites without a large increase

in pressure, raising the temperature of the air The gases thus generated are directed toward the blades of a turbine, causing its shaft

to rotate This shaft is coupled to an electrical generator (Figure 8) The exhaust gases, usually at a temperature range of 400–600 °C, are emitted into the atmosphere

Modern gas turbines used for electrical power generation employ axial compressors and multistage turbines in order to achieve high levels of efficiency A number of strategies aimed at improving efficiency have been proposed

Gas turbines are a little behind hydroelectric plants in terms of start-up times, interruptions, and operational load range They are suitable for covering power demand peaks In just a few minutes, they are able to achieve full output and can respond rapidly to variations in load demand or unforeseen output losses on the part of other generators Most gas turbines burn natural gas, which is a relatively clean fuel In addition to the relatively low contamination produced, investment costs are lower than those for coal-fired thermal power plants

Gas turbines in themselves are not very efficient This is partly due to the fact that the exhaust gases are still very hot In other words, the exhaust gases contain a significant amount of energy which is not being exploited One strategy to combat this is to capture the heat of the exhaust gases in a steam boiler and produce steam to drive a turbine and generate additional electricity (Figure 9) This is the principle behind combined cycle power plants (Figure 10) These plants are capable of achieving efficiencies

of up to 60% This advantage, along with their modularity and the relatively reasonable investment costs, makes them useful for covering base demand and peaks

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In conventional power stations, generating units based on reciprocating engines or piston engines are also used Reciprocating internal combustion engines are devices that convert the chemical energy contained in a hydrocarbon into mechanical energy (rotation of a shaft with a certain speed and torque) and into the thermal energy of the waste gases that escape into the atmosphere These engines can generate electricity if an electrical generator is coupled to its output shaft, while the waste heat can also be exploited for thermal applications (cogeneration) These piston engines can be classified into three categories: high-, medium-, and low-speed engines The choice will depend on the application Large engines of low (Figure 11) or medium (Figure 12) speed are generally the most suitable when it comes to covering the base load However, high-speed engines are more effective, economically speaking, for use as a backup service, where there is no requirement to operate over many hours throughout the year Internal combustion engines can work well under partial-load conditions Diesel engines in particular work very well when dealing with falls

in load from 100% to 50%

Modern-day nuclear power plants (Figure 13), of which there are several types, generate electricity by utilizing the enormous amounts of energy released when the nucleus of certain heavy elements, like uranium, splits after being bombarded by neutrons in a process known as nuclear fission [23, 35, 38, 47–51] The means of exploiting this type of energy is exclusively through the production of heat, which raises the temperature of a substance such as water, carbon dioxide, or sodium, converting it into steam or

a high-pressure gas and driving a turbine with this pressure to transform the thermal energy into mechanical energy Connection of electrical generators to the turbine shafts enables the production of electricity

Figure 11 Low-speed diesel generation unit at Jinámar power station, Gran Canaria Island, Spain Courtesy of ENDESA

Nuclear plants are more capital-intensive than plants which use fossil fuels, but the cost of the fuel is much less As a result of the high investment costs and the problems that can arise when changing the reactor’s cooling conditions, it is advisable to use these plants

to cover the base demand operating at maximum availability in order to achieve a high utilization factor of the nuclear energy The main drawbacks of nuclear power arise as a result of the dangers associated with breakdowns and the problems involved in the disposal of radioactive waste As a consequence, nuclear power plants are the most controversial of all power generating systems

2.18.2.1.2 Electrical networks

Conventional electrical power generation plants produce power at a voltage between 6 and 25 kV These relatively low voltages are not suitable for the transmission of power over long distances due to the losses that occur during transmission Longitudinal losses are proportional to the resistance of the medium and the square of the intensity of the current that is circulating Therefore, the intensity and/or resistance need to be reduced in order to reduce the losses [34, 35]

The voltage that leaves the power station has to reach the terminals of the consumers Since voltage drops occur across the impedance of the network, the impedance of the network or the intensity of current needs to be reduced in order to reduce the voltage drop For this purpose, large transformers are used, which raise the voltage to hundreds of thousands of volts, transmitting the same power but reducing the current [34, 35, 37, 38] On lowering the intensity and increasing the voltage, losses as a result of the Joule effect are reduced quadratically, while the voltage drop is linear [35]

Extra-high-voltage lines transmit the energy produced by the power generation stations to high-voltage substations – the starting point of the so-called subtransmission networks (Figure 3) These networks operate at a lower voltage than the transmission networks and, in turn, supply the local networks, known as distribution networks, via substations that reduce the high voltage to medium voltage The distribution networks usually have a distinctively radial configuration for the purpose of supplying energy to the medium- and small-sized consumers spread out throughout the area and connected, respectively, to medium and low voltages Medium-sized consumers are generally the large industries, while the small-sized consumers tend to be represented by domestic loads, businesses, and small factories In order to cover their particular energy requirements, large-sized consumers are usually connected to the high-voltage network (Figure 3)

Electrical networks, fundamentally comprising lines and substations, were initially used to connect the production and consumption centers of a particular region Gradually, however, from the end of the nineteenth century to the present day, these networks have found themselves hooked up to other networks of nearby areas until the networks have grown to cover the geography

of each country [35] Figure 14 shows an outline of the transmission network map of the Spanish mainland system According to Red Eléctrica de España (REE), the Spanish transmission system operator (TSO), Spain’s transmission system in 2009 was

34 754 km long, with 3385 substation positions and a transformation capacity of 66 259 MW Indeed, given the economic advantages presented by network interconnection, some continents now have network interconnections between several countries, with the consequent creation of enormous and complex supranational meshed networks

Most networks work with alternating current (AC), though there are a few exceptions in high-voltage energy transmission (e.g., direct current (DC) transmission may be more convenient when the aim is to connect production and consumption centers separated by considerable distances) [35] So, when operating at the same rated frequency and with all the synchronous generators

in phase, it is possible for the energy produced in an area of one country to be transmitted beyond its frontiers and shared with other areas of the country or with other countries One of the advantages offered by a robust and secure meshed network is supply continuity, since the various substations can be fed from a number of directions The feasibility of reducing the reserve capacity required to cover demand peaks also constitutes an added value Another advantage lies in the possibility of reducing the spinning reserve, namely, the power that has to be connected, but without a load, to cover unforeseen increases in demand The way that

Figure 15 High-voltage tower Courtesy of Iberdrola (http://www.iberdrola.es)

these interconnected networks function sees the reserve capacity shared out between different generation centers in such a way that there is mutual support against network disturbances Market systems can decide how the power injected into the network is to be shared out between the different power generation stations at any given moment

Aerial power lines (Figure 15), which generally consist of steel-reinforced, stranded aluminum alloy cables, are sized according

to the maximum operating voltage, the power to be transmitted, the distance involved, and the location of the starting/end points and other interconnection nodes [34, 35] The aim is to achieve a design that, from both an electrical and a mechanical point of view, generates the least longitudinal and transverse losses in the transmission of the energy and minimizes the investment, operating, and maintenance costs Longitudinal losses are primarily due to the Joule effect, while transverse losses are the result of the corona effect The Joule effect and the corona effect [37] are related to the operating current and voltage, respectively Fiber-optic cables of the telecommunications network can also be housed inside the high-voltage cables Spain’s TSO, respon­sible for the high-voltage network, currently has a network of more than 21 300 km of fiber-optic cable and around 19 000 devices

to provide telecommand, telecontrol, and teleprotection services [52]

The substations [53] act as network nodes and, in their most basic configuration, comprise thick bars to which the electrical lines are interconnected They can, however, be configured in very different ways, each one of which gives the substation different characteristics in terms of reliability, operational flexibility, cost, and so on

Substations (Figure 16) can be classified according to their use, and they fulfill a variety of functions One of those functions is to serve as an interconnection point for the power lines, directing the power flow toward different geographical regions in such a way

that the flow of power generated in the various power generation stations can be controlled in accordance with the country’s energy policy or market conditions Another function can be as a transformation center which carries out interconnections at different operating voltages This function also includes raising the outgoing voltage of the transformation centers A third function is that of housing the various elements of protection, cut-out, and switching

The equipment of a substation comprises power transformers, devices used to connect and disconnect electrical circuits, measuring and protection transformers, protection relays, and so on, depending on its configuration

Among the devices used for the connection and disconnection of electrical circuits are automatic switchgear and isolators Automatic switchgear is designed to open and close a line along which an abnormally high current circulates as a consequence of, for example, a short circuit The job of the isolators is not to cut off any current, but rather to electrically isolate in a visible way the damaged area once the current along the line has been cut off as a result of the activation of a switch Measuring and protection transformers, usually of current or voltage, are used to power the measuring instruments, relays, and other equipment, including the communications system Measuring devices detect faults, and the protection equipment decides what switch needs to be activated to clear the fault For this reason, protection systems need to be selective, reliable, and sensitive enough to operate under minimum-fault conditions in the system area for which it is responsible and must be able to act with a speed appropriate to the type of fault that needs to be cleared Selectivity in this case is related to the capacity to clearly recognize the type of fault and to minimize the size and extent of the interruption Reliability indicates the capacity to operate exclusively when a fault condition arises

Though the interconnection of networks does present economic advantages, the intensity of the current increases when a short circuit occurs in a system Disturbances caused by a short circuit can extend to systems that are interconnected to it So, for a network

to fulfill its mission and guarantee its operation in secure and safe conditions, it must be equipped with appropriate measuring, protection, and control equipment

The load curve for each consumer category usually has its own distinctive characteristics [18, 23] Some may display clearly noticeable demand peaks and troughs, while others may be flat For example, the maximum domestic sector consumption tends to occur when people are at home and are using their electrical household appliances This happens usually during mornings and evenings and at weekends By contrast, the domestic sector’s minimum consumption tends to be during the rest of the day and at night Electricity consumption is also extremely dependent on the weather and climate of a particular region, which affect the amount household appliances consume for cooling or heating purposes Industrial consumption tends to display a more stable profile than domestic or commercial consumption

By superimposing the different load curves of the various consumer types, the electrical system’s load curve is obtained These load curves can be represented for different time periods That is to say, they can be daily, weekly, monthly, or annually It should be mentioned that fluctuations in total demand are less than those corresponding to an individual load In other words, demand aggregation gives rise to a smoothing effect of the load curves This is due to the different load profiles of the various consumption sectors and the random component of demand As a result, the variance of total consumption is less than that of any one individual consumer type

Figure 17 shows the total demand curve for 14 and 15 February 2010 (a Sunday and Monday, respectively), in the Spanish mainland system The variability of demand over time can be observed together with the hourly intervals in which the peaks and troughs took place Figure 18 shows the evolution of the peaks and troughs of the daily demand curves recorded during 2009 The influence of weekends and holiday and seasonal periods can be observed

Meanwhile, each point of the so-called load–duration curves indicates, on the x-axis, the accumulated time in which the system’s power demand was higher than the value indicated on the ordinate axis (Figure 19) These curves, like the load curves, can be represented for different time periods: weekly, monthly, or yearly

Though it depends on the nature of the loads, that is, whether they are resistive or inductive, the overall power demand has an active and a reactive component Active power is the power converted into physical power Reactive power helps create the magnetic field which certain loads require So, there is a variation of these components in the daily evolution of power demand With regard

to reactive power, it should be mentioned that aerial power lines, depending on their load, generate or consume reactive power Transformers, on the other hand, always consume reactive power

The electrical demand of the loads, unlike the other components of a conventional electrical generation system, cannot be controlled and displays a high degree of randomness This random behavior can be modified to a certain extent through the use of demand-side management techniques aimed at rationalizing electrical energy consumption [55–57] Some loads, normally known

as deferrable loads or interruptible loads, can postpone their connection to the network within certain time intervals Loads that can feasibly be temporarily interrupted include those that have thermal inertia (air conditioning, heating, etc.), certain water pumping applications, and sea or brackish water desalination plants [58–60] Likewise, the establishment of electricity tariffs that depend on

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the amount consumed and the time of consumption (via the use of time bands) can have the effect of persuading consumers to react

to the price of the energy used and alter their consumption habits accordingly

In order to be able to carry out both the planning and operational tasks of an electrical system, short-term demand has to be estimated in all situations The use of predictive models based on time series [61, 62] and models that employ data mining techniques, such as neural networks [63], has been proposed to estimate short-term demand and employ it in electrical system operational tasks These models take advantage of certain behavioral traits that are observed in the evolution of demand, which have been mentioned above, namely, the relationships that exist between the demand and parameters such as type of day, time of the day, climate and atmospheric conditions, type of user, and geographical location

Electricity operators have developed expert systems for the forecast of daily and hourly demand to help in their operational decision-making [52]

2.18.2.2 Operational Objectives of an Electrical Power System

The operational objectives of electrical power systems can be synthesized into two fundamental priorities, namely, guaranteeing continuity of supply so that demand is covered with the required quality and, within the restrictions imposed by this objective, ensuring that the system is run in the most economic way possible [62, 64, 65]

Guaranteeing continuity of supply requires the establishment of security criteria in the operation of the electrical system For this,

a series of parameters must be controlled that enable supervision of the status of the electrical system These parameters are the frequency, the voltages at the network nodes, and the load levels in the various elements of the transmission network that the system operator manages, namely, the lines and substations

The control centers (Figure 20) [9, 52] manage the information they receive in real time from the power generation centers and the network via an extensive telecommunications network Studies that guarantee security of the electrical system are carried out using this information

Security depends on the robustness of the system in the face of predefined contingencies For example, the contingencies that are normally considered are (1) single outage of any element in the system; (2) simultaneous outage of double-circuit lines that share towers over an extensive section of the line path; and (3) in special situations, the outage of the largest generator unit in the area and

of one of its interconnection lines with the rest of the system [33]

The system must have the capacity to maintain the control parameters within certain preestablished admissible limits in the face

of changes in demand and contingencies that may arise Given the inherent uncertainty in relation to some contingencies, when the term ‘security’ is used, it often refers to a measure of short-term operational reliability

The second objective of electrical power systems is related to what is known as the economic dispatch [33, 62], that is, the method of dispatching the available generation resources to supply the load on the system in such a way that the total generation cost is minimized

2.18.2.3 Operating States of an Electrical Power System

A balance must be achieved at all times in the operation of electrical systems between generation and demand without overloading the system The function of the system operator lies in ensuring that this balance is always maintained For this purpose, the operator needs

to be able to forecast demand and supervise the generation and transmission installations in real time If the system gets out of balance, the operator orders the production centers to adjust generation so that it can be adapted to the variations in demand However, depending on the conditions in which the power system finds itself, it can be run under different operating states [33, 66, 67]

In the normal operating state, in addition to the prerequisites that there exist a balance between generation and demand and that the restrictions imposed on certain variables be met, a specific security level is also contemplated That is, the normal operating state

is also characterized by reserve margins, in relation to both the transmission system and the power generation system Typically, a system will find itself under the normal operating state for a high percentage of the time [68] (Figure 21)

If the levels of security fall, whether as a result of an increase in load or due to the probability that some disturbance will occur, then the system reliability gets reduced When the level of security falls below a specified limit, even if the demand is covered and the operating limits imposed on the variables are not breached, the system finds itself in the state known as ‘alert’

In alert state, any disturbance caused by the evolution of demand or the presence of a contingency could result in the operating variables being outside their established ranges If such a situation arises, the system changes to emergency state

When a system is in emergency state, it has to make urgent use of corrective measures required to avoid the system losing its integrity and collapsing Among the measures that can be used to ensure that the values of the variables return to admissible operating intervals is the temporary suspension of the supply to the users That is, a load shedding is performed

In some countries, automatic load shedding is undertaken following the guidelines of the Union for the Coordination of Transmission of Electricity (UCTE), which has established the load percentages to be disconnected depending on the frequency of the system However, it must be pointed out that from 1 July 2009 onward, the European Network of Transmission System Operators for Electricity (ENTSOE) [69] took over all operational tasks of the six existing TSO associations in Europe, including the UCTE There are also nonautomatic, selective load shedding mechanisms designed to avoid loss of demand in alert or emergency situations Both types of shedding try to avoid the disconnection of highly sensitive loads, such as hospitals and radio and television services Likewise, as mentioned in Section 2.18.2.1.3, there can also exist load interruption services In Spain, this service can be offered by consumers who acquire the energy from the production market [52] The system operator, depending on the category of the load interruption service, can interrupt the energy supply to the consumers during certain time periods In practically all the service categories, the consumers must be given prior notification

Action must be taken to restore the system to the normal or alert status In problematic situations, operational objectives of a technical nature take priority over economic ones

From the consumer’s point of view, optimum quality of the electricity supply is determined by the uniformity of the voltage, with a pure sinusoidal wave at a constant frequency and effective value The quality of the supply will be affected by disturbances that modify these characteristics Sine wave purity is becoming increasingly relevant as a result of the demands of the recipients The waveform can be distorted by both the generation itself and the actual recipients of the electrical energy The problems related to waveform quality are usually tackled at a local level However, supervision of the frequency and effective value of the voltage is usually dealt with by a centralized and hierarchical control structure

In order to better understand how electrical energy systems function, something which will be useful when it comes to analyzing the impact of wind energy integration into the systems, the following sections will provide a brief description of (1) active power and frequency control and (2) reactive power and voltage control, though to a certain extent both types of control are interrelated

2.18.2.3.1 Active power–frequency control

The rated frequency used in European and African countries is 50 Hz This frequency is also used by the vast majority of countries in the Middle East, Asia, Australia, and the Pacific Islands However, in the United States, Canada, and most Central and South American countries, the frequency used is 60 Hz

The frequency of an electrical system is closely related to the equilibrium that exists between the power supplied by the generation system, the power consumed by the loads, and power losses When there occurs a deviation in this power balance, the control system acts to reestablish it following a hierarchical procedure organized into three stages: primary, secondary, and tertiary control [52, 65, 70] Each of these levels operates over different time margins and involves areas of greater or lesser size of the whole electrical system

If the power demand increases sharply or a generator fault occurs, the primary control stage must be immediately available to provide the generation power required to reestablish the system’s power balance This additional generation power is provided initially by making use of the kinetic energy stored in the rotating elements of the power generation units, which are able to provide full output power for a few seconds, and following this by intervention of the turbine and engine speed governors

The kinetic energy depends on the inertia of the rotating masses and the rotational speed The rotational speed of the synchronous generators of an electrical system is proportional to the frequency of the voltage So, a deviation from the active power balance leads to changes in the rotational speed of the generators and, consequently, in the frequency of the system After a certain delay, while the system responds to the release of the stored kinetic energy, more significant amounts of energy can

be supplied to restore the active power balance This supplementary energy is achieved through the intervention of the systems that regulate the opening of the valves that control engine fuel input, the flow of water in hydroelectric plant turbines or the flow of gas

or steam in thermal power plant turbines (Figure 22) As the engines and turbines are mechanically coupled to the generators it is possible to regulate the power that the latter generate by controlling the mechanical power output of the former When a reference power level is either not reached or exceeded, the turbine and engine governors adjust the valves to increase or decrease mechanical power It should be mentioned that some loads are sensitive to frequency changes and vary their power demand accordingly The participation of generators in restoring the power balance depends on their power–frequency characteristic This character­istic depicts the variation in frequency when the power generated by a machine changes from zero to its rated value This relationship can be approximated to a straight line whose slope, of negative sign, is the constant of the governor This constant is what determines the characteristic of the governor in continuous operating conditions (steady state) and is known as the generator speed droop Speed droop is expressed in hertz per megawatts, and typical values are in the range of 4–6% The point where the straight line intercepts the frequency axis is known as the set point The governor allows the system operator to adjust the set point in such a way that the mechanical power output varies without modifying the frequency

Figure 23 shows the power–frequency characteristic which corresponds to a generator with a specific regulation constant, operating at two different set points However, the primary control does not modify the set point; rather, the variation in mechanical power is obtained by varying the frequency

Steam

Governor valve

Exhaust Speed

sensing device Synchronous machine

phase grid

Three-Substation

Turbine Flow

Valve control

Measurement device

Electrical power

Rotational speed Load frequency control Frequency

P

System frequency (fS)

PB, power supplied by the generator with set point B

f Unit A set point

Output unit B

When there are various generators of different rated powers working in parallel with different speed droops, the contribution each one makes will depend on the value of its speed droop A unit with a lower speed droop will contribute to the primary regulation a higher percentage of power with respect to its rated power, while a generator with a higher speed droop contributes

a lower percentage of power Figure 24 shows load allocation between two generators with different speed droops If several units in parallel have the same speed droop, all of them will contribute to the primary control proportionally to their rated power

Primary control, which acts at a local level, enables restoration of the balance between power generated plus losses and power demanded The primary control must complete its intervention within 15–30 s of the instant when the imbalance takes place This time will depend on the magnitude of the imbalance The primary control ensures that the frequency is never outside the acceptable range However, this control is unable to prevent the frequency from remaining deviated from the nominal or rated reference frequency In addition, the load increase allocation among the generators does not have to maintain the power flows that have been scheduled In some countries, like Spain, primary control is an obligatory and unpaid complementary service that must be provided by connected generators In Spain, primary control of generating units must allow a speed droop such that the generators can vary their load by 1.5% of their rated power [52]

The secondary power–frequency control acts on a zonal level, where the frequency in each of the neighboring production zones

is uniform This control level allows the defects of the primary control to be corrected, though it is slower-acting Commencement of secondary control intervention should not be delayed for longer than 30 s The automatic generation control (AGC) is responsible for this intervention level, and its actions are centralized

As has been mentioned, it is possible to modify the reference power of the generators That is, it is possible to change the set point

in the power–frequency characteristics This action entails vertical movement of the power–frequency characteristic, as can be seen

in Figure 23

Through determination of the reference power that has to be produced by each generator which intervenes in the secondary control, the frequency error can be corrected in a stable manner, thereby contributing to maintaining the frequency in that particular zone or area In addition to covering the demand of the area, secondary control, which is performed by the system operator, must maintain the scheduled energy exchanges The control strategy defines for each area an error signal known as the area control error (ACE) The reference power variation allocated to each of the generators that participate in the secondary control has to be proportional to the integral of the ACE That is, the error signal is an input variable of an integrator Its purpose is that the mechanical power variation of the generators is modified in such a way that the area error tends to zero in steady state The gains of the integrator are determined with control stability criteria

In the participation of the different generation systems in frequency control and demand variation control, several strategies can

be used based on speed of response, nondiscrimination of generator type, and so on The intervention time of the secondary control reserve is usually limited and should normally be no greater than 15 min

The reserve power that must be maintained to undertake secondary control is determined by the system operator In the Spanish mainland electrical system, the recommendation of the UCTE is normally taken as the point of reference In Spain, the secondary control service is optional and paid for through market systems [52]

Tertiary control operates over a more extensive range of the electrical system, simultaneously regulating the frequency and voltages following economic and security criteria [33] The purpose of tertiary control is restoration of the secondary control reserve

to make it available again In general, tertiary control works with generators that may or may not be coupled and must act within a time margin of between 10 and 15 min, and its reserves must be able to be maintained for some hours

In some countries, tertiary control is a complementary service which is optional and paid for through market systems

AC three-phase grid

Control

Reference reactive power

SCR

Inductance

Fixed capacitor

Variable inductance Fixed

capacitor

Equivalent model

2.18.2.3.2 Voltage control

The purpose of voltage control is maintenance of the effective value of the voltages in the network within acceptable limits [33, 52,

64, 65, 68] There is a close relationship between the reactive power flow between two network nodes and the difference between the effective voltage values at these nodes Reactive power variations entail voltage variations So, the control system has to be constantly working to correct voltage deviations There must also be reactive power reserves available to resolve any voltage incident that might take place

A variety of devices can be used to control the voltage These include static compensators, synchronous compensators, and synchronous generators [71]

Capacitor and/or reactor banks constitute static compensators that inject or consume reactive energy into/from the nodes where they are connected The voltage is discretely modified via connection and disconnection of these devices Mechanical connection is performed through relays and electronic connection through thyristors

Static compensators consisting of fixed-capacity condensers and adjustable coils can be used for continuous regulation of the reactive power injected or consumed into/from the network This device is known as a thyristor-controlled reactor-fixed capacitor (TCR-FC; Figure 25)

Thanks to advances in power electronics, the pulse-width modulation (PWM) technique is being used more and more commonly in power electronic systems [72–76] Indeed, it can be stated that the PWM technique has opened up a whole new field in reactive energy compensation methods

Static compensators (STATCOMs) normally use the PWM technique The STATCOM (Figure 26) is a voltage-sourced converter (VSC) system that generally uses gate turn-off thyristors (GTOs) or insulated gate bipolar transistors (IGBTs) combined with diodes enabling application of the PWM technique The frequency at which the switches operate can vary depending on the power of the system to which the STATCOM is connected Thus, the STATCOM behaves as if it were a synchronous capacitor, consuming or absorbing reactive power continuously, but without storing energy to compensate

Coupling transformer

AC three-phase grid

GTO Inductance

Variable controlled voltage source

Measurement device

Automatic Voltage Reference voltage voltage

Governor control valve

Turbine Excitation

Synchronous machine Substation Exhaust

Control of static compensators, in the specific case of voltage regulation in an electrical power system, is carried out by means of

a closed loop similar to that of the governors used in synchronous generators The bar voltage is compared with a reference voltage The error passes through a proportional–integral governor, which generates the firing angle required to obtain the desired voltage

A synchronous compensator, also known as a synchronous capacitor, is a synchronous machine whose shaft is not coupled to any mechanical load It consumes and generates reactive power when underexcited and overexcited, respectively There are a number of advantages to this device when compared with static compensation devices, including continuous voltage regulation without electromagnetic transients Another advantage is that they do not introduce harmonics into the network, nor are they affected by them Likewise, they do not generate problems as a result of electrical resonance

Like frequency control, voltage control is also undertaken at hierarchical levels [33] The primary control aims to maintain a voltage set point at a particular system node The synchronous generator constitutes the most characteristic element in primary control of voltage Synchronous generators, which can modify their active power as described in Section 2.18.2.3.1, are also capable

of modifying their reactive power Control of the latter is automatic and is achieved via regulation of the generator excitation current (Figure 27)

Automatic voltage regulation (AVR) acts within a very few seconds, and its objective is to control the generated reactive power and/or maintain the voltage at the generator terminals A sensor measures the voltage at the generator terminals, corrects it, and compares it with the desired voltage

While primary voltage control is local in scope since the information it uses is local, secondary control [33, 70, 77, 78] is responsible for the voltages of a set of representative nodes of an area or region, known as pilot nodes Secondary control coordinates the voltage governors of the area’s generators and, like primary control, functions automatically

Tertiary control, as has been mentioned, is a combined control of frequency and voltage It is generally nonautomatic and uses information from the whole system to determine the reference values of the pilot nodes

2.18.3 The Distinctive Characteristics of Wind Energy

Electrical energy generated using wind as the energy source displays some distinctive characteristics, which distinguish it from the power generated by the conventional energy sources described in Section 2.18.2 In this respect, the most notable features of wind energy are its temporal and spatial unpredictability and variability, the impossibility of directly storing the primary energy, the abundance of the resource in many places in the world, and its renewable and noncontaminating character

2.18.3.1 The Unpredictability and Variability of Wind

The power of wind on the Earth is a consequence of solar radiation [79–81] On a planetary scale and due, among other factors, to the shape and position of the Earth with respect to the Sun, there exist insolation differences between different areas of the planet Broadly speaking, the thermal differences that occur, combined with the rotation of the Earth with respect to its own axis, give rise to the displacement of masses of air Such displacement or the large-scale movement of air is known as atmospheric circulation These global winds are affected by the presence of the continents and water masses that shape the Earth’s surface and by the movement of the planet with respect to the Sun The circulation of these air masses will also be influenced by local thermal and climate effects as well as by the orography of the region Thus, wind characteristics depend markedly on geographical location Because of the influence of orographic features, the wind speed in one area can differ substantially from that in another just a few kilometers away

Mean (B) = 9.54 m s–1

Anemometer station A Anemometer station B

Years

Figure 28 shows, by way of example, the variation in daily mean wind speeds during the month of August, 2005, at two anemometer stations on the island of Gran Canaria (Canary Island Archipelago, Spain) installed 10 m above ground level and separated by a distance of approximately 34 km The mean wind speed for that month differs by about 36.6%

The considerable number of variables that can affect the movement of air masses makes the wind unpredictable and, over a wide range of scales, means that its behavior is both temporally and spatially variable The wind shows variations from a scale of seconds

to interannual timescales That is, the mean wind speed of a site can vary from one year to another Figure 29 shows, by way of example, the mean interannual wind speed variation over a 10-year period at an anemometer station installed 10 m above ground level on an island in the Canary Archipelago

Significant changes can also be observed from one season to another and, indeed, from one month to another during the same year Figure 30 shows the monthly wind variation at an anemometer station on the island of Gran Canaria It can be seen how, given its geographical location (between latitudes 27°37′ and 29°25′, subtropical, and longitude 13°20′ W of Greenwich), the frequency

of the trade wind regime is very high during the summer months, with the highest wind speeds occurring during that season Normally, the wind behaves differently in the Northern Hemisphere, with the summer months seeing the lowest wind speeds Similarly, wind speed can vary from one hour to another over the course of a day

Figure 31 shows, by way of example, the substantial variations observed in the mean hourly wind speed on a day timescale at an anemometer station installed near the coast of an island in the Canary Archipelago It can be seen how the wind displays very high-speed values during the day and lower ones at night This behavior is the result of the different effects of the heating of the surface of the land during the day and of the sea breezes

It is usually accepted that the random variations seen in periods ranging from 1 s to approximately 10 min represent turbulent wind speed variations Figure 32 shows the wind speed variation recorded over a 2 h period at an anemometer station located in the Canary Archipelago The values for each second and the means for each 10 min can be seen

2.18.3.2 The Variability of Electrical Energy from Wind Sources

The power that a wind turbine extracts from the wind is proportional to the air density, the rotor swept area, the power coefficient, the power transmission system efficiency, and the cube of the wind speed [1, 82–84] The power coefficient is a function of the tip speed ratio and the pitch angle [82–84] Air density varies with pressure, temperature, and humidity [85] However, given the more striking variations

in wind speed and its cubic influence, it is wind speed variations that can give rise to the most significant power fluctuations of a turbine

Rated wind speed

Cut-out wind speedCut-in wind speed

Full loadPartial load

Maximum power output

0

There are different control systems that can be used for wind energy conversion systems (WECSs) [3, 8–10, 82, 86–88] The intervention of these control systems enables regulation of the electrical power generated by the wind turbine

Figure 33 shows a typical power curve for a modern pitch-regulated wind turbine This curve represents the mean performance of the wind turbine Oscillations and deviations from the mean values will appear in the event of significant turbulence Likewise, the local orography and wake caused by nearby turbines will distort the power curve provided by the manufacturer Two clearly differentiated areas can be observed in Figure 33 One area lies between the cut-in wind speed and the rated wind speed and the other between the rated wind speed and the cut-out wind speed At the rated wind speed, the electrical generator of the wind turbine will produce its maximum power output When the wind turbine is operating in the first section of its power curve, it is said to be working in the partial-load range However, if the rated wind speed is exceeded, then the machine is said to be operating in the full-load range If the wind speed exceeds the cut-out wind speed for a few seconds, then the turbine stops and, therefore, no longer produces energy Likewise, the wind turbine has no output for wind speeds below the cut-in wind speed

Figure 34 shows the power produced by a wind turbine installed in a wind farm in the Canary Archipelago over a 2-day period It can be deduced from the figure that wind speed fluctuations in the partial-load range of a single wind turbine can give rise to large fluctuations in its electrical power output In the full-load range, the power curve displays a constant mean performance and is not affected much by wind speed variations

The timescale of wind speed variation has a significant effect on the power output fluctuation of the wind turbine If the spectra of wind fluctuations are analyzed at a particular site from micro- to macrometeorological range, it can be seen how the kinetic energy of the horizontal wind speed is distributed as a function of the variation frequency of that wind [89–93] Regardless of the site where the spectral analysis is conducted, a valley or spectral gap can typically be observed, which is bounded by one peak at around 1 min and

another peak at around 12 h There is normally a third peak in the region of several days The first peak is the consequence of turbulent wind fluctuations, the second corresponds to diurnal wind variations, and the third is normally attributed to the passage of anticyclones and depressions So, the observed spectral gap separates the daily wind speed variations from turbulent variations Within this spectral gap, the interval that ranges between 10 min and a few hours usually reflects a low wind energy content Turbulent wind fluctuations which occur on a scale of seconds or minutes can generate substantial deviations in the power output

of a single wind turbine with respect to its mean value These rapid fluctuations in the electrical energy generated by a wind turbine can have a harmful effect on the quality of the energy and, therefore, on the electrical system to which the turbine is connected It should also be mentioned that the energy production loss of a wind turbine as a result of the high-wind hysteresis effect [87, 93] depends, among other factors, on the level of turbulence High-wind hysteresis is basically the turbine’s control system lag between shutting down when the cut-out wind speed is exceeded and restarting The reconnection speed is normally 3–4 m s−1 lower than the cut-out speed The harm that wind turbulence can cause to the quality of the energy generated and its impact on losses due to the high-wind hysteresis effect depend to a large degree on the level of the technology installed in the wind turbine

Knowledge of the short-term behavior of the wind is of vital importance in the operation of electrical systems, whereas monthly, seasonal, and interannual behavior will have an impact on, among other questions, the planning of the electrical system

2.18.3.2.1 Effect of wind turbine aggregation on wind power variability

The power output fluctuations of a single wind turbine, as described in the previous section, could cause significant problems in terms of the integration of the wind power into an electrical power network if the energy generated by each of the various turbines that make up the wind farm behaved in an identical manner

The total power output of a wind farm is the summation of the output of each of the wind turbines that compose it However, since the wind speeds that strike the rotors of the different wind turbines on a wind farm are generally not identical, the power output fluctuations of each wind turbine are different As happens with demand aggregation [94], the total power output fluctuation of the wind farm is dampened or smoothed out That is, the overall fluctuation is less than the individual fluctuations [8, 93, 95]

Figure 35 shows the power output of a wind farm installed in the Canary Archipelago over a 2-day period The wind farm has nine wind turbines, and the power variation over this period was on the order of 31% Also shown in the figure is the power output

of the turbine with the highest variation (39%)

The extent of the dampening effect depends basically on the number of wind turbines that make up the wind farm and the degree

of correlation between the wind speeds that strike the corresponding rotors The number of wind turbines need not be very high However, the more uncorrelated the power outputs of the wind turbines are, the higher the level of power smoothing will be The degree of correlation depends, among other factors, on the topography and roughness of the terrain of the wind farm platform as well as on the downwind spacing between wind turbines [96]

As previously mentioned, the highest fluctuations in the power output of a wind turbine are produced when the wind turbine is operating in the partial-load range with high wind variability So, the beneficial effects of aggregation will be more pronounced under these operating conditions

Wind farm (nine wind turbines)

One wind turbine

make up the same wind farm

2.18.3.2.2 Effect of the geographical distribution of wind farms on wind power variability

Several studies [97–100] have shown that the correlations between wind speeds recorded at different geographical sites decrease with the distance between them (Figure 36) When the geographical distance is considerable, the winds can be affected by different microclimatic characteristics So, at such distances, wind speeds can be practically uncorrelated Given that wind farm power output fluctuations decrease when the correlation between the wind speeds that strike the various wind turbines is reduced, the geographical dispersion of wind farms over a wide area can entail a notable smoothing of the fluctuation of the total power fed into the interconnected networks [8, 101]

2.18.4 Wind Power and Power System Interaction

As mentioned in previous sections, the primary goal of electrical power systems is quality coverage of the electrical energy demand

[102–104] With this concept of quality in mind, the electrical system has to guarantee the continuity of power supply and ensure that the electrical energy that covers the demand does so with perfect quality of the voltage waveform

The quality of electricity supply depends not only on the energy source but also on the interaction of that energy source with the network to which it is connected Wind turbine connection to a network, with the particular operating characteristics the wind

turbines possess, can be the cause of a series of disturbances which may affect the quality of energy At the same time, wind turbines can find themselves affected by disturbances that originated in the network

2.18.4.1 Comparison between Conventional and Wind Generation Technologies

There are marked differences between conventional and wind generation technologies The most noteworthy of these include the primary energy source, the rated powers of the individual units, and the control and regulation strategies that the different generation systems use

As has been mentioned earlier, the primary energy of wind turbines is random and fluctuating and cannot be stored In comparison, conventional generation technologies use fuels that can be stored and are available at any given moment

The rated powers of large wind turbines have increased considerably over the past 10 years, but, even so, the rated power of the biggest wind turbine constructed to date is just 7.5 MW [8] Also the size of wind turbines is much smaller than that of conventional synchronous generators The main objective of the control strategies of conventional generation systems is to supply the power demanded by the load However, at the present time, the control strategies of wind turbines aim to maximize the power output, regardless of the load, in order to minimize costs

Various systems have been developed to convert the kinetic energy of the wind into electrical energy [3, 8–10, 82, 86–88] The architecture of these systems, like the control elements and strategies, is highly varied Different wind turbine configurations will depend on the type of generator involved, the variability or constancy of the rotational speed at which the system functions, and the power control devices installed Specific features of these configurations can have an influence on the characteristics of the electrical energy that is generated and affect to different degrees the response to disturbances caused by the network

Though variable-speed systems have a higher technological complexity and, perhaps, a lower reliability, they do offer a series of advantages over fixed-speed systems These include a greater capacity to smooth out wind speed fluctuations Variable-speed systems are also better able to capture the energy when operating in the partial-load range Due to the higher quality of the output power they inject into the network, pitch-regulated variable-speed systems are being implemented in most large wind turbines Various configurations are possible for variable-speed operation of wind turbines These configurations can be based on synchronous generators, induction generators, or asynchronous generators Power is generated at a variable frequency, and, by means of power electronic devices, this frequency is converted to the frequency of the network

Depending on the type of turbine, variable-speed turbines may be able to regulate the reactive power via the intervention of an electronic converter This control ensures that the voltage at the terminals of the generator remains within the established limits That is, a wind turbine can potentially operate in a similar way to synchronous generators at conventional power stations Research and development into a variety of power electronic devices is currently being intensified so that wind farms can enjoy a control capability of the same level afforded by the intervention characteristics of a conventional power plant [105, 106] In the same way as a wide range of strategies can be employed to acquire such characteristics, the points where power electronic devices can

be installed on a wind farm can also be diverse

It should be mentioned that the innovative trends that have followed the larger wind turbines that make up wind farms have not been copied by small wind turbines (Figure 37) Small wind turbines used in stand-alone systems, MGs, and DG systems have not

achieved the same technological level, and the costs per installed kilowatt are higher than those for large wind turbines [8] Evolution of the different systems that comprise small wind turbines has stagnated, and consequently, the differences between the electrical and control systems of small and large wind turbines are enormous

Conventional synchronous generators, as stated in Section 2.18.2, have control devices that use feedback information provided

by the network to regulate the system frequency and voltage At the present time, however, wind turbine control devices connected

to electrical networks do not use feedback information provided by these networks for the purpose of participating in the voltage and frequency control tasks However, modern variable-speed wind turbines can potentially regulate themselves taking into account the voltage and frequency of the network [3, 8–10, 82, 86–88, 105]

In addition to economic factors, one reason such control mechanisms have not been installed on a widespread scale is the random nature of wind The use of suitable forecasting tools for the short-term power output of a wind farm could help in this respect, but at the present time it is the network itself that is responsible for maintaining the frequency and voltage within the specified limits

The lack of feedback mechanisms in wind turbines has a number of different consequences depending on the type of network and the percentage of wind energy injected into it

2.18.4.2 Potential Disturbances in the Interaction of Wind Turbines with the Electrical Network

The quality of the voltage waveform produced by power generation stations can be disturbed by different faults that might occur in the network [82, 107–109] These disturbances can affect the correct operation of wind generation systems Likewise, as a consequence of the type of technology that some wind turbines use or as a result of wind variability and so on, wind farms can

be the source of some of the disturbances that occur in the electrical network

2.18.4.2.1 Frequency variations

As previously stated, frequency variations of an electrical system are essentially caused by alterations to the balance between generation and demand These imbalances are corrected in conventional electrical systems through the hierarchical intervention of the control systems [33, 65, 70]

Large-scale integration of wind energy into an electrical network introduces new factors in the active power–frequency interac­tion Random wind speed fluctuations give rise to variations of the active power generated by wind turbines (Figure 38) Hence, the behavior of the active power generated by wind farms differs from the typical behavior of the power generated by conventional synchronous generator sets

Given the unpredictability and fluctuations of the power generated by wind farms, it can be said that its behavior presents a greater similarity to that of demand than to that of conventional generation

The variations in the power generated by wind farms will affect the frequency of the system in different ways depending on the size of the electrical system and the level of wind penetration

Such frequency variations can have a variety of effects on wind systems that use this variable as a reference signal in their operation In this sense, disturbances can originate from the operation of the power electronic devices, changes to the rotational speed of the wind turbines, and so on There are several measures that are usually adopted to avoid these frequency variations These include, among others, an increase of the spinning reserve, the use of energy storage systems, and the establishment of limitations to wind penetration levels

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Hours

REE (http://www.ree.es)

2.18.4.2.2 Voltage variations

Voltage variations normally occur when the effective value of the voltage wave finds itself altered Variations in the power injected into or consumed by wind farms at the connection node to the network can give rise to voltage variations at that node and, eventually, at other nearby nodes [108] The magnitude of these voltage fluctuations will depend on the technology and operating characteristics of the wind turbines and on the impedance of the network This last factor depends on the power demand of the loads, which is variable

Fixed-speed turbines equipped with asynchronous generators connected to a network inject active power into and consume reactive power from that network Depending on the magnitude of the ratio of the reactance to the resistance, different levels of voltage fluctuation take place The weaker the network is at the connection node of the wind turbine, the higher the voltage fluctuation levels will be Wind turbines that have synchronous generators and are connected to electrical networks can control the reactive power through generator excitation

Wind turbines are usually equipped with specific devices to regulate the power factor, that is, the ratio of active to apparent power Each country has its own regulations that lay down the minimum power factor values for the installations The regulations in Spain state that the power factor of installations equipped with asynchronous generators cannot be lower than 0.86 at rated power The power factor regulators supply in a controlled manner the reactive power that the turbine needs Capacitor banks can be used for this purpose, though it is also possible to employ so-called FACTS technology [71, 74], otherwise known as flexible AC transmission systems Included in this technology are, among others, the static var compensator (SVC) and the STATCOM

The flicker effect is also influenced by the change of generators in turbines with two generators, as well as by the aerodynamic behavior of wind turbine rotors

At high wind speeds, fixed-speed pitch-regulated turbines perform worse than the stall-regulated type This is due to the inertia in the response of the pitch regulation mechanisms

The shadow that a tower makes when a blade passes in front of it can also be the cause of flicker For this reason, some wind farms suffer from what is known as the 3P effect [113, 115] The name of this effect comes from the power and voltage fluctuations caused as a result of the shadow made by the tower each time a blade of a three-bladed wind turbine passes in front of it in a cycle Fluctuations due to this effect are normally on the order of 1 Hz

The severity of the flicker caused by a particular wind turbine depends, in addition to the technology employed in its operation,

on the wind characteristics at the site and on the characteristics of the network at the connection point The overall flicker effect of a wind farm is normally dampened as a result of aggregation However, cases of synchronization phenomenon at wind farms that have resulted in an increase in the flicker effect linear to the number of wind turbines have been known Wind turbines can be equipped with devices such as static compensators for the purpose of reducing these disturbances

2.18.4.2.4 Phase voltage imbalance

An imbalance (or mismatch) in a three-phase system is a condition in which the effective values of three voltages differ in amplitude, or their relative phase angles differ by 120°, or both circumstances happen at the same time

Phase voltage imbalances can be the consequence of the connection of single-phase loads to a low-, medium-, or high-voltage network These imbalances can cause excessive heating of the wind turbine generators as a result of the abnormal currents that circulate in the windings In some situations, this heating can result in significant generation losses Likewise, the imbalances can affect the operation of wind turbines equipped with power electronic systems

2.18.4.2.5 Voltage dips and swells

A voltage dip (or sag) is a sudden reduction in the supply voltage on one or more phases to a value between 90% and 1% of the reference voltage followed by a voltage recovery shortly afterward The reference voltage is the nominal voltage, the declared voltage,

or the pre-dip voltage Under Spanish regulation UNE-EN 50160/96, the lower limit is set at half a cycle (10 ms at 50 Hz), since this

is the minimum period of time for which the effective value of the voltage can be calculated Drops in voltage with duration of less than half a cycle cannot be characterized as a change in the effective value of the voltage and are considered transitory The aforementioned Spanish regulation sets the upper limit at 1 min

Voltage dips are commonly caused by network faults and load switching The operating procedures established by various countries require that no wind installation disconnection be undertaken for voltage dips at the network connection point with

Commencement of voltage restoration

Disturbance starting point

95%

Time (ms)

certain fault-ride-through profiles (Figure 39) There can also be no consumption of active or reactive power on the part of the installation at the network connection point while maintenance work on the fault is being carried out, nor during the period of voltage recovery after clearance of the fault

The necessary design and/or control measures have to be taken in the installations to avoid wind turbines disconnecting instantaneously during voltage dips associated with correctly cleared short circuits [116, 117]

One measure can consist of installing STATCOM devices at the node where the energy generated by the wind farm is injected into the network In this way, the probability of the failure of components located between the wind farm and the substation where it is connected is reduced

Though less common, connection of a turbine working with an induction generator can also cause voltage dips In this case, the STATCOM device installed in the substation cannot correct the disturbance and the wind turbines connected to the lines where the disturbance has taken place will be disconnected The installation of a TCR device in each wind turbine will enable it to remain connected to the network during the voltage dip, even if the origin of the disturbance was in the wind farm itself

Voltage swells are abrupt increases in the instantaneous value of the voltage, which can be several times higher than the nominal value Voltage swells may or may not be oscillatory and typically last less than a few milliseconds They can be caused by switchgear opening or closing, atmospheric discharges, the removal of large loads, or the energizing of a capacitor bank Such voltage swells can also be caused by wind farms themselves, as a result of wind turbine shutdowns and start-ups, capacitor bank connection, and so on

Depending on the magnitude and duration of the voltage swell, wind turbines that use power electronic systems for their operation can be affected if they are not equipped with overvoltage protection devices

The parameter used to measure the degree of waveform distortion is called the total harmonic distortion (THD) Regulations, such as the IEC 61000-3-6 of the International Electrotechnical Commission, give the reference levels for voltage harmonics that must not be exceeded

Appliances that have power converters constitute an important source of high-frequency harmonic currents The presence of harmonics can cause the incorrect functioning of protective devices, generate interferences with communication circuits, and provoke resonances in wind energy systems

Since variable-speed wind turbines normally contain electronic converters, harmonic distortions are associated with this type of turbine and must be specified [118–121] Fixed-speed wind turbines are not prone to causing significant harmonics or interhar­monics However, harmonics can be detected, fundamentally in the brief connection periods when the electronic devices begin operating, though it is also possible to detect harmonics with low THD at other moments probably as a result of resonances between power factor compensation equipment and the generator winding

In certain situations, the devices that produce harmonic distortion are required to have suitable passive filters for particular harmonic components [122]