Tutorial Of Wind Turbine Control For Supporting Grid .

capable of providing power tracking and frequency regulationservices [2] when there is ample wind resource available.The interest in the potential for wind turbines to pro vide regulation services has motivated new opportunitiesin control system research and development. Wind turbinesdo not inherently provide these services, but they can besynthesized through designed control actions. Services thatinvolve varying the active power output of the turbine willbe referred to as active power control (APC). The newrequirements by aforementioned European grid operatorshave forced turbine manufacturers to develop and implementcontrol methodologies to provide APC capabilities. Ongo ing research is focused on determining the upper boundof frequency regulation capability of wind turbines, as itseems possible that wind turbines could be more effectiveat providing some of these services than traditional powerplants. The possible benefits of continuing the developmentof these methods present good opportunities for both gridoperators and wind plant owner/operators. The intention ofthis paper is to introduce the controls engineer to standardwind turbine control systems and provide a brief overview ofthe methodologies used to provide APC with wind turbines.This paper is organized as follows: Section II highlightsthe recent growth in the wind energy industry and providesa general overview of the wind turbine structure, standardcontrol configurations, and an introduction to the interactionof wind turbines within a wind plant. Section III explainsthe basics of frequency regulation, provides the motivationfor developing active power control in wind turbines, andoverviews methodologies implemented by manufacturers thusfar to meet these requirements. Section IV reviews the priorand ongoing research of enabling APC on wind turbinesand wind plants. Finally, Section V provides concludingcomments.to 3,117 MW, all of which is located in Europe, Japan, andChina [1]. As wind turbine technology continues to mature,wind energy is becoming a larger portion of the global energyprofile.The ‘penetration’ of wind energy in the local utility grid,which refers to the percentage of electrical energy generationthat comes from wind energy sources, is an important metricto measure. Though wind energy provided only 2.5% of theglobal electrical energy supply in 2010, several countrieshave a relatively high percentage of their electrical energyproduced by wind power. The countries with the highestpercentage of electrical energy generated from wind in 2010were Denmark, Portugal, Spain, and Germany with 21%,18%, 16%, and 9%, respectively [1]. It should be notedthat these percentages are annual averages. At times theinstantaneous percentage of total power provided by windcan be much higher. Wind energy achieved a maximuminstantaneous penetration level of 59.6% in Spain in 2011[3]. The high wind penetrations in these countries have beenachieved not only from having good wind resources available,but also by aggressive national policies to produce moreenergy from renewable sources.Wind turbines have increased in size to take advantage ofeconomies of scale. The turbines installed in the U.S. during2010 had an average rated power of 1.79 MW with averagehub heights and rotor diameters of 79.8 and 84.3 meters,respectively [4]. The average rated power of turbines installedin the US has not increased significantly during the past3 years due to the challenges associated with transportingextremely large turbines over land and the popularity ofa particular 1.5 MW turbine model [4]. The installationof turbines is also subject to economies of scale, as it ismore profitable to cluster wind turbines together to reducethe cost of installation, maintenance, and transmission lineconstruction. These clusters of wind turbines are often laidout in a grid-like pattern and are commonly referred to as“wind farms” or “wind plants,” the latter being the preferredterm which is used in this paper. Fig. 1 shows a single rowof turbines in a wind plant.II. T HE BASICS OF H ARNESSING W IND E NERGYIn this section, we highlight the recent growth in thewind energy industry and provide an overview of the mostcommon utility scale wind turbines, their operating regions,their standard control goals, and the interactions betweenturbines when grouped in a wind plant.A. Growth of the Wind Energy IndustryWind energy is a quickly growing alternative energytechnology that can provide clean power. According to theWorld Wind Energy Association, the average growth rateof installed capacity around the world over the last decadehas been 27.7% [1]. In 2010, worldwide capacity reached196,630 MW (megawatts) out of which 37,642 MW wereadded during 2010, for a growth rate of 23.6% [1]. During2010, the United States increased installed wind capacityfrom 35,159 MW to 40,180 MW. China almost doubledinstalled capacity in 2010, growing from 25,810 MW to44,733 MW, to pass Germany and the US and become thenumber one country in installed capacity [1]. 2010 broughta 59% capacity increase in offshore wind, bringing the totalFig. 2. Cp curves for an example 5 MW wind turbine. The dotted linesrepresent the collective blade pitch β and the tip-speed ratio λ at whichCp is a maximum.2

Fig. 3.Wind power, turbine power, and operating regions for an example 5 MW turbine.power of the drivetrain to electrical power which is eitherdirectly injected to the grid or first converted to the grid fre quency via power electronics. Most large scale wind turbinesinstalled during the 1980’s and 1990’s used gearboxes andfixed speed generators that produced voltage synchronouswith the utility grid [6]. The wind turbine industry hassince moved to using variable speed wind turbines thatcan maximize below-rated power production by matchingblade tip-speeds against prevailing wind speeds to maximizeaerodynamic efficiency, as described in Section II-C1.Variable speed operation is typically achieved by usingone of two different configurations. The first employs asynchronous generator that spins at variable speeds and usesa full power converter to ensure the produced power matchesin frequency and phase to that of the utility grid and isknown as a ‘type 4’ wind generator [7]. The second, andmost common way of achieving variable speed operation isto use a doubly-fed induction generator (DFIG), known as a‘type 3’ wind generator [7]. The stator of a DFIG is directlyconnected to the grid while the electromagnets of the rotorare excited by a time-varying waveform that is produced bypower electronics that need to only convert roughly 30%of the turbine’s rated power [7]. Almost all commerciallyavailable large scale wind turbines use either type 3 or 4generators, both of which effectively decoupled from the gridvia their power electronics.B. Wind Turbine OverviewA turbine with rotor axis of rotation that is horizontalto the ground is called a HAWT (Horizontal Axis WindTurbine). HAWTs are representative of the majority of alllarge scale wind turbines today. These turbines are operatedin an upwind manner, where the rotor plane is activelypositioned to be directly upwind of the tower through theuse of a yaw motor that rotates the entire nacelle (housingfor all components located at the top of the tower). Windpassing over the turbine blades produces lift and this theninduces a rotational torque.1The available power in the wind is P ρAv 3 , where2P is the power [W ] passing through the rotor disk, ρ isthe air density [kg/m3 ], A is the swept area of the rotordisk perpendicular to the wind direction [m2 ], and v is thewind speed [m/s]. The wind turbine rotor cannot extract allof the energy from the wind stream, as this would requirethe wind to become stationary on the downwind side ofthe rotor. The fraction of available power that a turbinedoes harvest is its power coefficient Cp (β, λ), which is afunction of the collective blade pitch β and the tip-speedratio (TSR) λ. The TSR is the tangential speed of the bladetips divided by the wind speed perpendicular to the rotorplane. A characterization of a wind turbine’s Cp is shown asa contour plot in Fig. 2. The theoretical upper limit for Cpis the Betz Limit of 16[5].27The aerodynamic torque captured by the blades is trans ferred to the hub, which connects the blades to a drivetrainand then a generator. Typically, the drivetrain includes agearbox to scale rotational speed and torque to levels that aresuitable for the generator configuration. Although gearboxesare still used in the majority of turbines, direct-drive windturbines have been developed to directly connect the hubto the generator with a single shaft to increase reliabilityand reduce maintenance costs that are largely associated withgearbox failures [6].The wind turbine’s generator converts the mechanicalC. Standard Control ConfigurationsThe control of wind turbines is a complex problem andspans multiple fields of research, including materials, aero dynamics, and power systems. As the turbine structuresbecome larger, their components become more expensive.Wind turbine manufacturers may attempt to counteract theincrease in costs by using lighter weight components thatcan be more flexible. These large, expensive, and flexiblecomponents can be more susceptible to failure from fatigueand extreme loads that arise from the turbulent nature of thewind. Control system optimization to prevent extreme loads3

and to reduce fatigue load cycles becomes important to avoidcomponent failure.Wind turbine control is typically divided into four primaryregions, as seen in Fig. 3. Region 1 spans operation fromstartup to the ‘cut-in’ wind speed where the generator isturned on and starts producing power. When wind speeds areabove cut-in, but still too low to produce maximum power,the turbine is said to be in Region 2. In this below ratedregion the objective is to maximize aerodynamic efficiency tocapture as much energy as possible from the wind stream. InRegion 3, wind speeds are high enough to drive the generatorat its rated power output; in this case, the goal is to regulatespeed and power safely at rated levels. Region 4 occurs whenthe turbine shuts down due to high wind speeds to preventdamage to the turbine.Throughout these regions, the speed and power of theturbine are controlled by varying the generator load torqueand the blade pitch angles based on measurement of thegenerator shaft speed. The generator torque is induced bypower electronics onto the load side of the drivetrain, andactuation is sufficiently fast that it is considered as occurringwith negligible delay in comparison with the dynamics of therotor and structural loads. The blades are actuated with pitchmotors which are often modeled as low-pass filters with acutoff frequency on the order of 1 Hz, saturation limits, andslew-rate limits on the order of 10 /sec [8]. The generatorshaft speed is typically measured using an encoder, and thesignal is often fed through a low-pass filter to avoid highfrequency actuation. Yaw control is also employed duringturbine operation to keep the rotor perpendicular to theprimary wind direction, typically based on 10-second-averagewind direction measurements, with a yaw rate on the orderof 0.5 /sec [9].1) Region 2 (Below-Rated): In Region 2, the primarygoal is to capture as much power as possible. The powercoefficient Cp changes with both blade pitch and TSR, asshown in Fig. 2, and is largely determined by the geometryof each specific blade design. In standard Region 2 control,blade pitch is typically held constant at the value β thatproduces the peak Cp . The goal is then to maintain the TSRat the optimal level λ ; hence, the tip-speed, and thereforerotor speed, must vary proportionally to the wind speed. Thisis achieved by varying the generator torque.The commanded generator torque τg is set according toτg kτ Fig. 4. An example of the generator torque control in different operatingregions, as described in [8].and load torques to regulate the speed of the turbine to theoptimal TSR in steady-state conditions [10]. Fig. 4 showsan example of generator torque command versus generatorspeed measurement, following this law in Region 2, and alsoshowing transition Regions 1.5 and 2.5, as found in [8].The torque controller may deviate from the optimal TSRat particular speeds to avoid tower resonances, and may alsoinclude drivetrain and/or tower damping by adding feedbackat the appropriate resonant frequency or frequencies [11].Though these features and the generator shaft speed measure ment filter will not allow the turbine to perfectly track theoptimal tip-speed ratio, the peak of the Cp curve is relativelyflat and the power capture performance is acceptable.2) Region 3 (Above-Rated): In Region 3, the primary goalis to regulate generator speed at rated by shedding

Tutorial of Wind Turbine Control for Supporting Grid Frequency through Active Power Control Preprint Jacob Aho, Andrew Buckspan, Jason Laks, Yunho Jeong, Fiona Dunne, and Lucy Pao University of Colorado Paul Fleming, Matt Churchfield, and Kathryn Johnson National Renewable Energy Laboratory To be presented at the 2012 American Control Conference