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 

Montreal, Canada 

June 27-29, 2012 

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC. 

Conference Paper 

NREL/CP-5000-54605 

March 2012 

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A Tutorial of Wind Turbine Control for Supporting
 

Grid Frequency through Active Power Control
 

Jacob Aho, Andrew Buckspan, Jason Laks, Paul Fleming, Yunho Jeong,
 

Fiona Dunne, Matthew Churchfield, Lucy Pao, Kathryn Johnson
 

Abstract—As wind energy becomes a larger portion of the 

world’s energy portfolio and wind turbines become larger 

and more expensive, wind turbine control systems play an 

ever more prominent role in the design and deployment of 

wind turbines. The goals of traditional wind turbine control 

systems are maximizing energy production while protecting 

the wind turbine components. As more wind generation is 

installed there is an increasing interest in wind turbines actively 

controlling their power output in order to meet power setpoints and to participate in frequency regulation for the utility 

grid. This capability will be beneficial for grid operators, as 

it seems possible that wind turbines can be more effective 

at providing some of these services than traditional power 

plants. Furthermore, establishing an ancillary market for such 

regulation can be beneficial for wind plant owner/operators 

and manufacturers that provide such services. In this tutorial 

paper we provide an overview of basic wind turbine control 

systems and highlight recent industry trends and research in 

wind turbine control systems for grid integration and frequency 

stability. 

I. INTRODUCTION 

The wind industry has experienced large growth rates over 

the past decade and wind turbines have been installed around 

the world in increasing quantities [1]. As wind energy becomes more prevalent there is growing interest in controlling 

wind turbines or wind plants (a cluster of wind turbines as 

seen in Fig. 1) in an intelligent manner to minimize the 

cost of wind energy. This can be done by controlling the 

turbines to extract more energy from the wind and reduce 

Jacob Aho is a doctoral student in the Dept. of Electrical, Computer, 

and Energy Engineering, University of Colorado, Boulder, e-mail: jacob.aho@colorado.edu. 

Andrew Buckspan is a doctoral student in the Dept. of Electrical, 

Computer, and Energy Engineering, University of Colorado, Boulder, e-mail: 

andrew.buckspan@colorado.edu. 

Jason Laks is a doctoral candidate in the Dept. of Electrical, Computer, and Energy Engineering, University of Colorado, Boulder, e-mail: 

jhlaks@colorado.edu. 

Paul Fleming is a research engineer at the National Wind Technology 

Center, Boulder, e-mail: paul.fleming@nrel.gov. 

Yunho Jeong was a graduate student in the Division of Engineering, 

Colorado School of Mines, Golden, e-mail: yunho3600@gmail.com. 

Fiona Dunne is a doctoral candidate in the Dept. of Electrical, Computer, and Energy Engineering, University of Colorado, Boulder, e-mail: 

fiona.dunne@colorado.edu. 

Matt Churchfield is a research engineer at the National Wind Technology 

Center, Boulder, e-mail: matt.churchfield@nrel.gov. 

Lucy Pao is the Richard & Joy Dorf Professor in the Dept. of Electrical, 

Computer, and Energy Engineering, University of Colorado, Boulder. 

Kathryn Johnson is an Assistant Professor at the Colorado School 

of Mines Dept. of Electrical Engineering and Computer Science, jointly 

appointed at the National Wind Technology Center, e-mail: kjohnson@mines.edu. 

Fig. 1. A row of turbines in a wind plant located in Alberta, CA. This 

wind plant contains 114 horizontal axis wind turbines (HAWTs), each rated 

at 660 kW with a rotor diameter of 47m and hub height approx. 50m. 

structural loads that can cause component failure and is 

the focus of ongoing research. Though wind energy makes 

up a relatively small amount of global energy production, 

there are certain regions that produce a significant portion 

of their energy from the wind, such as Spain, Ireland, and 

Denmark [1]. The increasing penetrations of wind energy 

in these countries has raised interest in a new branch of 

wind turbine control research and development that focuses 

on wind turbine participation in frequency regulation for the 

utility grid. 

Grid operators require conventional utilities to provide 

regulation in order to maintain the necessary balance between generation and load, which in turn regulates the grid 

frequency. Wind power has not historically been required 

to provide grid regulation services, as most modern wind 

turbines do not intrinsically provide any of the grid regulation services that are available with conventional generators. 

High wind penetration levels in the aforementioned countries 

have lead their transmission system operators to impose 

new requirements for future wind plant installations to be 

1
capable of providing power tracking and frequency regulation 

services [2] when there is ample wind resource available. 

The interest in the potential for wind turbines to provide regulation services has motivated new opportunities 

in control system research and development. Wind turbines 

do not inherently provide these services, but they can be 

synthesized through designed control actions. Services that 

involve varying the active power output of the turbine will 

be referred to as active power control (APC). The new 

requirements by aforementioned European grid operators 

have forced turbine manufacturers to develop and implement 

control methodologies to provide APC capabilities. Ongoing research is focused on determining the upper bound 

of frequency regulation capability of wind turbines, as it 

seems possible that wind turbines could be more effective 

at providing some of these services than traditional power 

plants. The possible benefits of continuing the development 

of these methods present good opportunities for both grid 

operators and wind plant owner/operators. The intention of 

this paper is to introduce the controls engineer to standard 

wind turbine control systems and provide a brief overview of 

the methodologies used to provide APC with wind turbines. 

This paper is organized as follows: Section II highlights 

the recent growth in the wind energy industry and provides 

a general overview of the wind turbine structure, standard 

control configurations, and an introduction to the interaction 

of wind turbines within a wind plant. Section III explains 

the basics of frequency regulation, provides the motivation 

for developing active power control in wind turbines, and 

overviews methodologies implemented by manufacturers thus 

far to meet these requirements. Section IV reviews the prior 

and ongoing research of enabling APC on wind turbines 

and wind plants. Finally, Section V provides concluding 

comments. 

II. THE BASICS OF HARNESSING WIND ENERGY 

In this section, we highlight the recent growth in the 

wind energy industry and provide an overview of the most 

common utility scale wind turbines, their operating regions, 

their standard control goals, and the interactions between 

turbines when grouped in a wind plant. 

A. Growth of the Wind Energy Industry 

Wind energy is a quickly growing alternative energy 

technology that can provide clean power. According to the 

World Wind Energy Association, the average growth rate 

of installed capacity around the world over the last decade 

has been 27.7% [1]. In 2010, worldwide capacity reached 

196,630 MW (megawatts) out of which 37,642 MW were 

added during 2010, for a growth rate of 23.6% [1]. During 

2010, the United States increased installed wind capacity 

from 35,159 MW to 40,180 MW. China almost doubled 

installed capacity in 2010, growing from 25,810 MW to 

44,733 MW, to pass Germany and the US and become the 

number one country in installed capacity [1]. 2010 brought 

a 59% capacity increase in offshore wind, bringing the total 

to 3,117 MW, all of which is located in Europe, Japan, and 

China [1]. As wind turbine technology continues to mature, 

wind energy is becoming a larger portion of the global energy 

profile. 

The ‘penetration’ of wind energy in the local utility grid, 

which refers to the percentage of electrical energy generation 

that comes from wind energy sources, is an important metric 

to measure. Though wind energy provided only 2.5% of the 

global electrical energy supply in 2010, several countries 

have a relatively high percentage of their electrical energy 

produced by wind power. The countries with the highest 

percentage of electrical energy generated from wind in 2010 

were Denmark, Portugal, Spain, and Germany with 21%, 

18%, 16%, and 9%, respectively [1]. It should be noted 

that these percentages are annual averages. At times the 

instantaneous percentage of total power provided by wind 

can be much higher. Wind energy achieved a maximum 

instantaneous penetration level of 59.6% in Spain in 2011 

[3]. The high wind penetrations in these countries have been 

achieved not only from having good wind resources available, 

but also by aggressive national policies to produce more 

energy from renewable sources. 

Wind turbines have increased in size to take advantage of 

economies of scale. The turbines installed in the U.S. during 

2010 had an average rated power of 1.79 MW with average 

hub heights and rotor diameters of 79.8 and 84.3 meters, 

respectively [4]. The average rated power of turbines installed 

in the US has not increased significantly during the past 

3 years due to the challenges associated with transporting 

extremely large turbines over land and the popularity of 

a particular 1.5 MW turbine model [4]. The installation 

of turbines is also subject to economies of scale, as it is 

more profitable to cluster wind turbines together to reduce 

the cost of installation, maintenance, and transmission line 

construction. These clusters of wind turbines are often laid 

out in a grid-like pattern and are commonly referred to as 

“wind farms” or “wind plants,” the latter being the preferred 

term which is used in this paper. Fig. 1 shows a single row 

of turbines in a wind plant. 

Fig. 2. Cp curves for an example 5 MW wind turbine. The dotted lines 

represent the collective blade pitch β∗ and the tip-speed ratio λ∗ at which 

Cp is a maximum. 

2
Fig. 3. Wind power, turbine power, and operating regions for an example 5 MW turbine. 

B. Wind Turbine Overview 

A turbine with rotor axis of rotation that is horizontal 

to the ground is called a HAWT (Horizontal Axis Wind 

Turbine). HAWTs are representative of the majority of all 

large scale wind turbines today. These turbines are operated 

in an upwind manner, where the rotor plane is actively 

positioned to be directly upwind of the tower through the 

use of a yaw motor that rotates the entire nacelle (housing 

for all components located at the top of the tower). Wind 

passing over the turbine blades produces lift and this then 

induces a rotational torque. 

The available power in the wind is P = 1ρAv3, where 2

P is the power [W] passing through the rotor disk, ρ is 

the air density [kg/m3], A is the swept area of the rotor 

disk perpendicular to the wind direction [m2], and v is the 

wind speed [m/s]. The wind turbine rotor cannot extract all 

of the energy from the wind stream, as this would require 

the wind to become stationary on the downwind side of 

the rotor. The fraction of available power that a turbine 

does harvest is its power coefficient Cp(β, λ), which is a 

function of the collective blade pitch β and the tip-speed 

ratio (TSR) λ. The TSR is the tangential speed of the blade 

tips divided by the wind speed perpendicular to the rotor 

plane. A characterization of a wind turbine’s Cp is shown as 

a contour plot in Fig. 2. The theoretical upper limit for Cp 16 is the Betz Limit of [5]. 27

The aerodynamic torque captured by the blades is transferred to the hub, which connects the blades to a drivetrain 

and then a generator. Typically, the drivetrain includes a 

gearbox to scale rotational speed and torque to levels that are 

suitable for the generator configuration. Although gearboxes 

are still used in the majority of turbines, direct-drive wind 

turbines have been developed to directly connect the hub 

to the generator with a single shaft to increase reliability 

and reduce maintenance costs that are largely associated with 

gearbox failures [6]. 

The wind turbine’s generator converts the mechanical 

power of the drivetrain to electrical power which is either 

directly injected to the grid or first converted to the grid frequency via power electronics. Most large scale wind turbines 

installed during the 1980’s and 1990’s used gearboxes and 

fixed speed generators that produced voltage synchronous 

with the utility grid [6]. The wind turbine industry has 

since moved to using variable speed wind turbines that 

can maximize below-rated power production by matching 

blade tip-speeds against prevailing wind speeds to maximize 

aerodynamic efficiency, as described in Section II-C1. 

Variable speed operation is typically achieved by using 

one of two different configurations. The first employs a 

synchronous generator that spins at variable speeds and uses 

a full power converter to ensure the produced power matches 

in frequency and phase to that of the utility grid and is 

known as a ‘type 4’ wind generator [7]. The second, and 

most common way of achieving variable speed operation is 

to use a doubly-fed induction generator (DFIG), known as a 

‘type 3’ wind generator [7]. The stator of a DFIG is directly 

connected to the grid while the electromagnets of the rotor 

are excited by a time-varying waveform that is produced by 

power electronics that need to only convert roughly 30% 

of the turbine’s rated power [7]. Almost all commercially 

available large scale wind turbines use either type 3 or 4 

generators, both of which effectively decoupled from the grid 

via their power electronics. 

C. Standard Control Configurations 

The control of wind turbines is a complex problem and 

spans multiple fields of research, including materials, aerodynamics, and power systems. As the turbine structures 

become larger, their components become more expensive. 

Wind turbine manufacturers may attempt to counteract the 

increase in costs by using lighter weight components that 

can be more flexible. These large, expensive, and flexible 

components can be more susceptible to failure from fatigue 

and extreme loads that arise from the turbulent nature of the 

wind. Control system optimization to prevent extreme loads 

3
and to reduce fatigue load cycles becomes important to avoid 

component failure. 

Wind turbine control is typically divided into four primary 

regions, as seen in Fig. 3. Region 1 spans operation from 

startup to the ‘cut-in’ wind speed where the generator is 

turned on and starts producing power. When wind speeds are 

above cut-in, but still too low to produce maximum power, 

the turbine is said to be in Region 2. In this below rated 

region the objective is to maximize aerodynamic efficiency to 

capture as much energy as possible from the wind stream. In 

Region 3, wind speeds are high enough to drive the generator 

at its rated power output; in this case, the goal is to regulate 

speed and power safely at rated levels. Region 4 occurs when 

the turbine shuts down due to high wind speeds to prevent 

damage to the turbine. 

Throughout these regions, the speed and power of the 

turbine are controlled by varying the generator load torque 

and the blade pitch angles based on measurement of the 

generator shaft speed. The generator torque is induced by 

power electronics onto the load side of the drivetrain, and 

actuation is sufficiently fast that it is considered as occurring 

with negligible delay in comparison with the dynamics of the 

rotor and structural loads. The blades are actuated with pitch 

motors which are often modeled as low-pass filters with a 

cutoff frequency on the order of 1 Hz, saturation limits, and 

slew-rate limits on the order of 10○/sec [8]. The generator 

shaft speed is typically measured using an encoder, and the 

signal is often fed through a low-pass filter to avoid high 

frequency actuation. Yaw control is also employed during 

turbine operation to keep the rotor perpendicular to the 

primary wind direction, typically based on 10-second-average 

wind direction measurements, with a yaw rate on the order 

of 0.5○/sec [9]. 

1) Region 2 (Below-Rated): In Region 2, the primary 

goal is to capture as much power as possible. The power 

coefficient Cp changes with both blade pitch and TSR, as 

shown in Fig. 2, and is largely determined by the geometry 

of each specific blade design. In standard Region 2 control, 

blade pitch is typically held constant at the value β∗ that 

produces the peak Cp. The goal is then to maintain the TSR 

at the optimal level λ∗; hence, the tip-speed, and therefore 

rotor speed, must vary proportionally to the wind speed. This 

is achieved by varying the generator torque. 

The commanded generator torque τg is set according to 

1 τg = kτΩ2

r Ngear

1 Cp(β∗, λ∗) kτ = ρAR3

2 λ∗

3

where Ngear is the high-speed to low-speed gearbox ratio 

(i.e. a constant greater than 1), Ωr is the rotor speed (measured generator speed divided by Ngear), ρ is air density, A

is rotor swept area, R is the rotor radius, and (β∗, λ∗) are 

the blade pitch and TSR, respectively where the maximum 

power coefficient Cp occurs. It can be shown that this 

generator torque control law will balance the aerodynamic 

Fig. 4. An example of the generator torque control in different operating 

regions, as described in [8]. 

and load torques to regulate the speed of the turbine to the 

optimal TSR in steady-state conditions [10]. Fig. 4 shows 

an example of generator torque command versus generator 

speed measurement, following this law in Region 2, and also 

showing transition Regions 1.5 and 2.5, as found in [8]. 

The torque controller may deviate from the optimal TSR 

at particular speeds to avoid tower resonances, and may also 

include drivetrain and/or tower damping by adding feedback 

at the appropriate resonant frequency or frequencies [11]. 

Though these features and the generator shaft speed measurement filter will not allow the turbine to perfectly track the 

optimal tip-speed ratio, the peak of the Cp curve is relatively 

flat and the power capture performance is acceptable. 

2) Region 3 (Above-Rated): In Region 3, the primary goal 

is to regulate generator speed at rated by shedding extra 

aerodynamic power. This is done using blade pitch control, 

typically pitching to feather which decreases the aerodynamic 

angle of attack. The blade pitch angle capturing the most 

power is β∗, which occurs at 0○ in Fig. 2. The standard convention is that pitching to feather corresponds to increasing, 

or positive pitch angles. Pitching to stall (increasing angle of 

attack) can decrease the rotor speed more rapidly, but pitching 

to feather decreases aerodynamic torque smoothly, is quieter, 

and typically induces smaller blade structural loads [5]. 

The blade pitch control system typically uses proportionalintegral (PI) control on the generator speed error signal to 

regulate the generator speed at rated. The control system also 

includes anti-wind up or saturation limits on the integrator to 

account for the physical limits placed on the pitch angle of 

the blade (e.g., not allowing the blade pitch to go below β∗). 

Gain scheduling is often used to adjust the PI gains based on 

the current blade pitch angle. This is done to address the nonlinear sensitivity of the Cp curve to blade pitch angle [8]. This 

PI speed controller pitches all blades to feather collectively. 

Individual pitch variations and higher frequency collective 

pitch variations may also be added to the PI pitch commands 

in order to reduce structural loads [11]. 

Because the blade pitch regulates generator speed at rated 

4
in Region 3, the generator torque can be held constant at 

rated torque. However, due to the stochastic turbulent nature 

of the wind and the slow response of the pitch actuators, at 

times the generator speed may vary more than 5% from the 

rated speed. It is common to maintain constant power output 

in above rated winds by commanding variations from the 

rated torque that are inversely proportional to fluctuations in 

rotor speed, as shown in Fig. 4. 

D. Wind Plants and Turbine Wake Interactions 

One area of research still very nascent is wind plant control 

to address turbine wake interactions. Wind turbines are often 

installed in wind plants to reduce the cost of installing transmission lines and operations and maintenance costs. As a 

byproduct of the extraction of energy from the wind, turbines 

create wakes that extend downstream [12, 13]. Wakes are 

characterized by lower speed and higher turbulence than that 

of the surrounding wind. With enough downstream distance, 

the wake mixes with the surrounding winds so that the wind 

speed and turbulence become approximately the same as that 

of the freestream. Turbines operating on the interior of a 

wind plant are often subject to wakes of upstream turbines 

as shown by Jensen et al. [14]. Wakes are significant because 

a turbine operating in the wake of an upstream turbine 

has less available energy to extract from the wind and is 

subject to increased mechanical loads-inducing turbulence. 

Wake effects translate to less revenue due to decreased power 

generation and increased maintenance costs. 

Wake effects pose a significant challenge for advanced 

wind plant-level control systems. One method that has been 

studied [15, 16, 17] involves operating the upwind rows of 

turbines in a plant at a decreased efficiency to reduce wake 

effects imposed upon downstream turbines, increasing overall 

plant power capture and reducing loads. Such a scheme 

could be tuned to the current atmospheric stability conditions 

at a particular site, since wakes persist more strongly in 

a more stable (nighttime) atmosphere. Wind farm control 

and wake interactions are the subject of numerous papers, 

though not explicitly mentioned here, this research can aid in 

determining how wind plant level controls can further enable 

active power control. 

III. OVERVIEW OF APC 

In this section, we present a brief overview of how grid 

frequency control is provided via conventional sources. We 

also explain the motivation for wind energy to provide active 

power control by presenting current frequency support requirements by some transmissions systems operators (TSOs). 

We then review industry activity to develop solutions that 

provide active power control capabilities that meet the new 

requirements. 

A. Frequency Regulation Basics 

In order for the grid to maintain a desired frequency, 

it is necessary that the total power generated be equal to 

power consumed by system loads and electrical losses on 

the grid. This balance must be preserved in the face of 

fluctuations in load and uncontrolled generation. If generation 

exceeds load, then the grid frequency will go up, whereas 

if total load exceeds generation, grid frequency will fall. 

Therefore, regulation provided by capable utilities is used 

by grid operators to compensate for these fluctuations and 

unplanned events such as sudden loss of generation [18]. 

Such capable utilities include regulation-providing generators 

or grid-responsive loads and storage. Fig. 5 provides an 

illustration of the utility grid’s power generation (blue), load 

(green), and the regulation required (red) to match supply 

and demand (from Kirby [18]). 

Fig. 5. Regulation: the green line depicts total system load, while the blue 

line represents load-following generation. The red line represents regulation 

required to keep generation balanced with load (reprinted with permission 

from [18]). 

Conventionally, grid frequency response to a large disturbance is divided into separate control regimes: inertial, primary frequency response, and secondary frequency response 

or automatic generation control (AGC). These classifications 

are based on methods for providing each service with conventional generators: synchronous generator inertia for inertial 

response, generator governors for primary response, and 

finally output control responding to system operator power 

level demands for AGC response. For further information and 

exact definitions of inertial, primary, and secondary response, 

see [19] and [20]. It should be noted that some references 

refer to inertial control as a subset of primary response. 

Fig. 6 provides a plot of these regimes in the case where 

there is a sudden loss in total generation connected to the 

grid, and is depicted in terms of a time-domain plot of 

grid frequency following the loss of generation. The inertial 

response immediately follows the frequency event, primary 

control usually occurs within the timeframe of 20 - 30 seconds, and secondary control occurs within the timeframe 

of 5 - 10 minutes. The inertial response is conventionally 

determined by the physical inertia of large synchronous 

generators as they decelerate in response to the increased 

electrical load during a loss in total grid generation capacity. 

The frequency decline is arrested or stabilized in the second 

time frame, which is characterized as the primary response, or 

governor response, as it is normally automatically controlled 

by generator governors responding directly to changes in 

grid frequency. The final response, in which the frequency 

is restored to nominal, is called AGC and is performed 

5
Fig. 6. Inertial, primary frequency controls, and AGC (secondary) response 

(figure courtesy Pouyan Pourbeik of EPRI). 

by individual power plants adjusting their power levels in 

response to requests from the systems operator. 

B. Motivation for Active Power Control in Wind Turbines 

Active power control (APC) is the purposeful control of 

the real power output of a wind turbine or collection of wind 

turbines in order to assist in balancing total power generated 

on the grid with total power consumed. Real (also known as 

“active”) power is the portion of energy flow which results 

in a net transfer of energy; this is in contrast to imaginary 

or reactive power flow, which reverses direction over each 

cycle with no net energy transfer. While reactive power 

may be controlled through the turbine’s power electronics 

(even when the turbine is offline), real power is regulated by 

changing the actual amount of power delivered to the utility 

grid. In doing so, APC may allow wind power to provide 

important ancillary services to the electrical grid and assist 

in maintaining an acceptable frequency. 

Intrinsically, wind turbines do not provide APC services 

and they have not historically been required to provide such 

responses [21]. Most modern turbine generators are decoupled from grid frequency through power electronics (type 3 

or 4), as described in Section II-B. Therefore, the inertia 

of the generator and the turbine rotor do not automatically 

participate in the grid inertial response as would traditional 

synchronous generators. Further, because of this decoupling, 

changes in grid frequency do not elicit an automatic governor response that is common with conventional generation 

sources. 

Increasingly, there are two main motivations for why APC 

should be provided by wind turbines. The first motivation 

is that regulation is essential for maintaining grid frequency 

and as wind penetration increases it can provide key support in maintaining the required balance. A FERC/LBNL 

(Federal Energy Regulatory Commission/Lawrence Berkeley 

National Laboratory) study discusses a recent decline in grid 

frequency response, and states that although increasing wind 

penetration is not the cause, frequency response could be 

improved through the expanded use of frequency control 

capabilities in wind turbines [22, 23]. Further, a recent study 

by the IPCC (Intergovernmental Panel on Climate Change) 

found that although low to medium wind penetrations (up to 

20% of annual demand) poses no “insurmountable technical 

barriers,” higher levels could require additional flexibility 

options such as greater use of wind power curtailment and 

output control [24]. As wind energy penetration increases, it 

is potentially displacing regulation-providing generators. At 

very high wind penetration levels it becomes necessary for 

wind turbines to also provide these services [21] as it has for 

several European grid operators. 

In countries and regions with relatively isolated grids and 

relatively large levels of wind penetration, participation in 

grid frequency regulation by wind turbines and wind plants is 

crucial. A 2010 report issued by Project UpWind [2] indicates 

that replacing conventional generation sources with a large 

percentage of wind power not capable of active power control 

can potentially have significant impacts on stability of grid 

frequency. This effect is pronounced on island grids with low 

levels of interconnectivity to other grids, such as many of the 

Greek isles [25]. 

The necessity of participation in grid frequency regulation 

by wind turbines is reflected in the requirements and regulations put on wind plants by TSOs in regions with high levels 

of wind penetration or relatively isolated grids. For example, 

the Irish grid code requires that wind plants have active 

power curtailment capabilities, and outlines specific active 

power generation set-points as a function of available power 

in the event of a frequency deviation [26]. This code further 

specifies a minimum response rate for individual turbines 

of 1 % of rated power per second. Elsewhere, Denmark’s 

TSOs Eltra and Elkraft require that wind plants be able 

to track a reserve power offset and track reference power 

levels generated by the system operator [27]. In Canada, 

Hydro-Quebec requires that wind plants rated above 10 MW  ́

have the ability to modify their active power output for at 

least 10 seconds in response to grid frequency deviations 

greater than 0.5 Hz [28]. Additionally, the Spanish TSO, Red 

Electrica, mandates that wind plants respond to frequency 

deviations with proportional control of active power output 

within a range of percentages of rated power [29]. 

The second motivation for active power control by wind 

turbines is the potential to increase the profitability of wind 

plants by enabling participation in ancillary service markets. 

A recent study by Kirby demonstrates a potential for wind 

plants to “increase their own profits by providing regulation” 

[30]. 

C. Industry Activity in Support of APC 

The various active power control requirements laid out 

by grid operators have motivated technological development 

by wind turbine manufacturers. These developments have 

been made at both individual turbine and wind plant scales. 

Technologies to provide response in all of the inertial, primary, and secondary time scales have been developed at the 

individual turbine level. For example, Siemens has patented 

a method for dynamically modifying an individual turbine’s 

active power output in response to grid frequency deviations 

6
Fig. 7. A block diagram that shows the general interconnection for APC commands. The wind plant controller can measure the frequency of the utility 

grid and receive a power command signal from the grid operator and in turn produce a power reference for each turbine in the wind plant. 

on time scales similar to inertial and fast primary responses 

[31]. Methodologies for monitoring power available in the 

wind and maintaining a specified amount of active power in 

reserve are patented by both Mitsubishi [32] and Vestas [33], 

respectively. In another patent, participation in primary and 

secondary regulation is achieved by using blade pitch control 

to specify a percentage of available power as an active power 

reserve, which can be changed up or down in response to grid 

frequency fluctuations [34]. 

Methods for providing active power control of entire wind 

plants have also been developed. Performing active power 

control collectively across a wind plant is intended to provide 

faster response and recovery to grid frequency deviations 

than can be achieved by performing active power control 

on individual turbines separately. These plant scale technologies, like those patented by Mitsubishi [35], Ingeteam [36], 

and General Electric [37], have focused on monitoring and 

forecasting available power in the wind, so that total active 

power generation by the wind plant can be controlled. Additionally, these patents claim to develop intelligent strategies 

for communicating wind, power, and frequency conditions 

between individual turbines. An example of such a strategy 

can be seen in [36], where a subset of the turbines on a wind 

plant are used as observers to estimate power available, and 

the aggregate active power output of the wind plant is set to 

a percentage of the estimated available power. 

Two Danish offshore wind plants, Horns Rev and Nysted, 

serve as examples of the current state-of-the-art of the industry. Both facilities are outfitted with active power control 

systems capable of responding to TSO power set-point commands and automatically responding to fluctuations in grid 

frequency [2]. They have several available operating modes 

to provide APC, as described in Section IV. 

Patent literature is rich with technologies that are designed 

to meet performance demands set forth by TSOs and allow 

for control of active power output. It appears that current 

industry standards for maintaining an active power set-point 

is performed using pitch control on a relatively slow time 

scale, leaving inertial and primary response to be handled by 

controlling the generator and power electronics [2]. However, 

it would appear that much of this development, especially 

specific controller topologies, is proprietary information. 

IV.	 IMPLEMENTATION AND ONGOING RESEARCH OF 

APC ON WIND TURBINES 

In this section we present an overview of the existing 

implementation and the ongoing research of APC on wind 

turbines. These two topics are combined into one section 

because the current methods of implementation of APC are 

not made explicitly clear by turbine manufacturers. Fig. 7 

shows an example of a standard interconnection of a wind 

plant controller with the TSO or grid operator, utility grid, 

and the individual turbines. The individual turbine can follow 

the power reference from the wind plant controller by altering 

the standard generator torque and blade pitch feedback loops, 

which are described in Section II-C. Increasing the regulation performance of a wind turbine control system can be 

complicated due to a number of factors, including coupling 

with existing control loops, a desire to limit actuation usage 

and structural loading, and wind variability. 

As described in Section III, several European TSOs now 

have requirements for new wind plants to provide inertial 

response emulation to frequency events and to track either a 

delta or absolute power reference signal at a required slew 

rate. The new requirements by the TSOs have lead to the 

development of APC on wind turbines to meet two different 

goals. The first is ‘automatic frequency control’ which is 

designed to emulate the inertial response of conventional 

generators over a limited time window immediately following 

a frequency event. The second goal of APC controllers is 

met by tracking a power reference from the TSOs over 

longer time scales as a secondary, or AGC response. The 

minimum requirements have been met by manufacturers, 

but ongoing research is being performed to improve the 

implementation of such controllers and to determine the 

extent of the regulation capabilities that can be provided 

7
Fig. 8. Inertial response emulation showing sudden electrical power increase 

with a long recovery period. 

by wind turbines. These areas of on-going research include 

variations of control methodologies to implement inertial or 

AGC response, advanced controllers that combine torque and 

pitch control to implement primary control during frequency 

events, and analysis of impacts on turbine lifespan induced 

by implementing active power control strategies. 

A. Inertial and Primary Response 

Inertial response emulation, which is also referred to as 

inertial control or kinetic energy control, is now required 

by several European TSOs (Section III) to help regulate the 

grid frequency when events occur such as that depicted in 

Fig. 6. This response is implemented in a wind turbine or 

wind plant by measuring the frequency of the utility grid 

and using a control algorithm to vary the output power to 

compensate for deviations in grid frequency. The inertial 

control is performed over a short time scale typically by 

the generator torque control to emulate the built-in response 

of a conventional generator [2]. Wind turbines have the 

capability of providing more inertial frequency regulation 

than conventional generators per unit of spinning inertia, due 

to the speed at which the power electronics can actuate the 

torque command signal [7]. 

Inertial response emulation typically provides fast increases (or decreases) in the power production through sudden increases (or decreases) in the generator torque. It is 

more common to have a conventional generator fail than 

for a large load to drop from the grid, so here we look at 

the case where there is a demand for a sudden increase in 

turbine power. A rapid increase in generator torque will cause 

the rotor to decelerate while power is produced above the 

power set-point, as shown in Fig. 8. The downside of this 

method is a danger of the turbine stalling and possible added 

loading on the mechanical parts. This response is used to 

increase power in the short term and often requires a period 

of recovery as depicted in Fig. 8. This recovery period can be 

problematic in that it can cause the wind turbine to produce 

less power after the inertial response, which could adversely 

affect grid frequency. However, research in [38] shows that 

inertial response emulation can sometimes be more effective 

than inertial response from conventional generators in coal 

power plants, covering a wider frequency range. 

There are multiple approaches to extract additional power 

from the spinning rotor mass and still handle recovery. 

Both [39] and [21] use the previously mentioned method 

of quickly increasing torque in response to grid frequency 

falling, which thus causes a deceleration of the rotor speed 

and is called “Kinetic Energy Control I (KEC I)” and “GE 

WindINERTIA,” respectively. While some variation of KEC I 

is the most intuitive method for inertial response emulation, 

[39] goes on to propose another method called “Kinetic 

Energy Control II (KEC II),” which allows the rotor to 

accelerate initially before applying the higher torque. The 

benefit of KEC II is that the turbine power and rotor speed 

do not fall below normal operation speed throughout the 

whole course of power extraction, eliminating the recovery 

period and preventing turbine stall. However, a temporary 

over-speed can be caused when operating at rated power, 

and may not be a desirable solution. Another concern with 

KEC II is that the response speed could be delayed due to 

the speed-up process, and the magnitude of frequency drop 

can be more severe than with KEC I. 

While conventional synchronous generators automatically 

provide inertial control and have governors for primary control, the distinction between inertial and primary responses 

is less clear for wind turbines without these governors. 

While patents for such technologies apparently exist, the 

methods by which primary response is achieved are generally 

not explicitly outlined. Consequently, primary APC of wind 

turbines is an ongoing area of research. 

Another method of extracting additional power from the 

rotor inertia while enabling a primary response is covered in 

[40] and [41]. Both of these studies operated the turbine at 

higher than optimal tip-speed ratios to maintain a reserve of 

available wind power. With an overhead of un-utilized power, 

in an under frequency event, the controller can increase the 

generator torque to extract inertia from the turbine, causing 

the turbine to slow down to the optimal tip-speed ratio. 

These controllers therefore implement inertial response and 

a primary response as they produce a higher power output in 

response to the detection of under frequency events. This can 

be seen in Fig. 9, where if derated by 10% at 10 m/s wind 

speed and commanded to ramp up to rated power, the turbine 

would follow a trajectory from point A to point B. Pitch 

control is also used in [40] to derate the turbine and add to 

the primary response, but there is no consideration of stability 

issues arising from the interaction between the torque and 

pitch control loops, and simulations were only performed 

under constant, uniform winds. The fundamental concept in 

these works is worth noting, but further research in this area 

is required as neither [40] or [41] address the persistent overspeeding of the turbine while running in above-rated winds 

or the controller’s effect on the structural loads of the turbine. 

Inertial and primary control can be implemented and 

simulated at the wind plant level. Patent literature, such 

as [36], suggests that intelligent, distributed response of an 

entire wind plant to a frequency disturbance could potentially minimize overall grid frequency deviation and improve 

8
Fig. 9. Various steady-state power capture curves for given wind speeds. 

The ‘Max Power’ line is the trajectory of the turbine that achieves maximum 

power capture for each wind speed and the ‘90% Power’ line is the trajectory 

that leaves 10% overhead power via rotor speed control. The axes are 

normalized to the rated values. 

recovery speed. Primary grid response to frequency disturbances with varying levels of penetration by wind plants 

with active power controllers was simulated in [42], with 

results suggesting that participation in frequency response by 

wind plants can greatly increase grid robustness and reduce 

the maximum frequency deviation from nominal during a 

disturbance. This simulation consisted of the grid represented 

as an electrical bus and analyzed the effects of the wind plant 

APC controller when a conventional generator goes offline. 

While these simulation results are promising, the simulations 

themselves are limited by their use of a number of turbines 

much smaller than a typical wind plant and simplified models 

of turbines and turbine wake interactions. 

B. Secondary (AGC) Response 

A secondary (AGC) response consists of power plants 

raising and lowering power output in response to commands 

from the TSO. Conventional generators typically receive an 

absolute power reference from the TSO since the power plant 

can vary the fuel input to meet the power demand up to the 

plant’s rated output. AGC capable wind turbines and plants 

also receive their power command signals from the TSO. 

These commands may be in terms of a ‘balance control’ 

or absolute power command signal (so long as that level 

of power is available from the wind) or a ‘delta control’ 

command which specifies the percentage of the available 

wind power to be captured. Using delta control ensures that 

there is a percentage of available wind power that is kept 

as overhead in case the TSO demands more power. There is 

also often a ‘power gradient limiter’ that is specified by the 

TSO which is a lower limit on the rate of change of wind 

plant power production to meet the TSO’s setpoint [2]. 

1) AGC at the Turbine Level: The most popular methods 

for reference power tracking by wind turbines appear to be 

pitch angle control and rotor speed control. These methods 

intentionally achieve sub-optimal operation with lower Cp

(Fig. 2) to track the power set-point provided by the TSO. 

Both [40] and [41] show that pitch angle and rotor speed 

controls are effective methods for primary and secondary 

controls of wind turbines. In addition, a similar control 

scheme called “GE WindCONTROL” [21] shows that the 

total plant level power output can be controlled to provide a 

desired response. 

A recent study developed several novel combined torquepitch controllers to respond as quickly as possible to a 

change in power reference command [43]. Though this study 

assumes an absolute power reference AGC signal is provided 

to the turbine control system, a primary response can be 

achieved from this controller if the power reference signal 

is augmented to respond to variations in grid frequency (i.e. 

through a droop curve). Metrics for the speed of power 

tracking and turbine structural loads were included in the 

analysis of the designed controllers found in [43]. This is an 

important consideration, as much of the literature on APC 

controllers neglects the loads induced on the turbines by the 

control system. In order to determine structural loads induced 

on the turbine, simulations must be performed with a high 

fidelity turbine model under various operating conditions. An 

example of such simulations can be found in [43] which uses 

the FAST turbine simulation code developed by NREL [44]. 

It is also useful to perform field tests to validate a designed control system. The NREL National Wind Technology 

Center has the ability to test developed controllers on the 3­

bladed Controls Advanced Research Turbine (CART3). The 

testing of one of the primary/secondary APC controllers 

found in [43] has been conducted on the CART3. The APC 

controller modifies CART3’s actuator commands to meet the 

required power reference. Data from an example field test is 

shown in Fig. 10. This experiment tests an initial control 

design that adjusts the maximum rotor speed to track a 

power reference. The second subplot in Fig. 10 compares 

the measured and demanded output power of the turbine, 

and it can be observed that the control successfully tracks 

with the desired power reference. It would appear that this 

initial design can be improved, since excessive tower load 

fluctuations can be observed near 600 and 925 seconds, where 

the controller may be tracking changes in the power reference 

too aggressively. 

2) AGC at the Wind Plant Level: As was discussed in 

Section III, basic grid support capabilities are currently 

implemented on some wind plants, such as Horns Rev and 

Nysted. Many TSOs also require that wind plants be able to 

modify total active power output. Correspondingly, previous 

and on-going research has investigated the ability for active 

power control to be implemented on a plant-scale level. 

Simulations using simplified wind plant models have been 

used to verify the performance of some of these control 

schemes. 

A significant portion of the research into implementing 

AGC with wind energy has been performed at the wind plant 

level and has focused on plant-wide tracking of a reference 

power signal to achieve an overall active power reserve. For 

9
Fig. 10. Field-test data of power reference tracking on CART3 shows that sudden changes in the power reference induce increased tower bending moment.
 

Further, note how the simultaneous change of wind speed and the power reference affect the power output. Figure from [43] (reprinted with permission)
 

example, Hansen et al. [45] propose using a plant-wide PI 

control loop to distribute power reference signals to individual turbines based on a TSO command for overall plant 

active power generation. Each of the turbines use a combined 

blade pitch/generator torque control strategy to track their 

individual power reference. Another method for wind plant 

active power tracking is proposed in [46]. Here, two plant 

scale controllers are used to respond to reference power 

commands by the TSO: a high level controller monitors mean 

wind speed and sets the overall plant operating point, while a 

lower level controller monitors local wind effects and adjusts 

the operating point based on power available to each turbine. 

Although sophisticated methods for power reference tracking 

and controlling active power output of wind plants have been 

developed, research of control methodologies specifically for 

concerted inertial and primary response to deviations in grid 

frequency appears to be sparse. 

Simulations have been used to demonstrate active power 

controller capabilities of wind plants, or small groups of wind 

turbines. In [45], simulations demonstrate the robustness of 

a wind plant active power control strategy as the requested 

operation method of the wind plant is quickly switched 

between maximum power, balance control, and delta control 

modes. Long time scale simulations, on the order of multiple 

hours, have been performed to demonstrate active power 

forecasting and reserve capabilities of a wind plant [47]. 

The full potential and impact of APC controls at the wind 

plant level calls for more advanced simulation capability. 

Simulation tools that include high fidelity turbine models 

and controls capability like NREL’s FAST code, usually 

consider a single turbine rather than a plant. FAST includes 

aerodynamic modeling based on blade-element momentum 

theory, but this does not account for far downstream wake 

effects that are important in wind plant simulation. In addition, it is common practice in controls simulations that 

the inflow turbulence is “frozen” and does not evolve as it 

encounters the turbine or with downstream distance. Tools 

relying on frozen-wake methods are likely insufficient for 

simulation studies of the viability of APC at the wind plant 

level. An advanced simulation tool called SOWFA (Simulator 

for Offshore Wind Applications) allows the simulation of 

multiple FAST actuator line models coupled with a CFD 

(computational fluid dynamics) solver that is capable of computing an evolving turbulent inflow and wakes that propagate 

downstream and interact with other turbines [48]. Variations 

in control inputs to individual turbines will cause variations 

in the wakes created and propagated by the CFD solver. 

Hence, this type of tool can potentially reveal sensitivities 

of proposed APC schemes across an entire wind plant. 

V. CONCLUSION 

In this paper we provided an overview of modern turbine 

control and discussed the recent development of active power 

control (APC) through wind turbine control systems. As 

wind energy continues to reach higher penetration levels, 

the role of wind turbines and wind plants in grid frequency 

regulation is becoming more important. Several European 

transmission system operators in countries like Spain, Den10
mark and Ireland now require that new wind plants have a 

number of APC capabilities, including emulation of inertial 

frequency response and power reference tracking, with the 

aim of using wind plants to assist in regulation of grid 

frequency. The decline of grid frequency stability in the 

United States further motivates the implementation of APC 

on wind turbines. This service could be beneficial even in 

areas without high penetration levels, as previous research 

suggests, grid frequency robustness can be enhanced by even 

a small number of wind turbines providing frequency regulation services. Ongoing research investigates the possibility 

of implementing an ancillary market for these regulation 

capabilities, as this type of market could potentially increase 

the economic viability of wind energy resources. If it is 

possible for wind plant operators to provide the services 

required for grid regulation without significantly increasing 

fatigue damage to the turbines or other turbine costs, like 

O&M, then wind energy might be viewed as an enabling 

resource for grid regulation objectives, aiding in wind energy 

penetrations higher than 20%. 

While wind turbine manufacturers have developed the capabilities to meet the requirements set by European transmission system operators, ongoing research is being performed 

to determine the full capabilities of wind turbine APC. 

Various methodologies have been presented in literature for 

providing the different forms of regulation (inertial, primary, 

and secondary); however, further research on such methodologies is necessary. Areas of interest for future research include development of controllers that achieve faster response 

times to changing power demands, integration of controllers 

designed for inertial and secondary regulation, and optimal 

participation by wind plants over all regulation phases. Other 

areas of further research include development of strategies to 

balance aggressive power control against increased actuator 

usage and structural loads, and investigation into the potential 

coupling of active power control loops and conventional 

turbine control loops. Research into these areas can be 

augmented by simulations that demonstrate, for instance, the 

ability of a controller to provide one or all of the conventional capabilities while not preventing pre-existing turbine 

controllers from maintaining turbine safety and minimizing 

fatigue and extreme loading. Ongoing research will help 

determine the full benefit of widespread adoption of advanced 

APC services provided by wind turbines. 

REFERENCES 

[1] “World Wind Energy Report 2010,” in Proc. 10th World 

Wind Energy Conference. Cairo, Egypt: World Wind 

Energy Association, Nov. 2011. 

[2] A. D. Hansen, “Evaluation of power control with different electrical and control concepts of wind farms,” 

Project UpWind, Roskilde, Denmark, Tech. Rep., 2010. 

[3] R.	 Electrica. (2011, Nov.) An Historic Challenge  ́

for Red ectrica. Release. Red El El ́ Press ectrica.  ́

[Online]. Available: http://www.ree.es/ingles/sala 

prensa/web/notas detalle.aspx?id nota=207 

[4] R. Wiser and M. Bolinger, “2010 Wind Technologies 

Market Report,” US Department of Energy, Tech. Rep., 

2011. 

[5] J. F. Manwell, J. G. McGowan, and A. L. Rogers, Wind 

Energy Explained: Theory, Design and Application. 

Wiley, 2009. 

[6] H. Polinder, D.-J. Bang, H. Li, and Z. Chen, “Concept 

Report on Generator Topologies, Mechanical and Electromagnetic Optimization,” Project UpWind, Mekelweg, 

the Netherlands and Aalborg East, Denmark, Tech. 

Rep., 2007. 

[7] L. Greedy, “Review of Electrical Drive-train Topologies,” Project UpWind, Mekelweg, the Netherlands and 

Aalborg East, Denmark, Tech. Rep., 2007. 

[8] J. Jonkman, S. Butterfield, W. Musial, and G. Scott, 

Definition of a 5-MW Reference Wind Turbine for 

Offshore System Development. Golden, CO: National 

Renewable Energy Laboratory, Feb. 2009. 

[9] E.	 Hau, Wind turbines: fundamentals, technologies, 

application, economics. Springer, 2005. 

[10] K. Johnson, L. Pao, M. Balas, and L. Fingersh, “Control 

of variable-speed wind turbines: standard and adaptive 

techniques for maximizing energy capture,” IEEE Control Systems Magazine, vol. 26, no. 3, pp. 70 – 81, June 

2006. 

[11] E. Bossanyi, “Controller for 5mw reference turbine,” 

Project UpWind, Bristol, England, Tech. Rep., 2009. 

[12] L. Vermeer, J. Sørensen, and A. Crespo, “Wind Turbine 

Wake Aerodynamics,” Progress in Aerospace Sciences, 

vol. 39, pp. 467–510, 2003. 

[13] B. Sanderse, “Aerodynamics of Wind Turbine Wakes,” 

Energy Research Center of the Netherlands (ECN), 

ECN-E–09-016, Petten, The Netherlands, Tech. Rep., 

2009. 

[14] L. Jensen, C. Mørch, P.	 Sørensen, and K. Svendsen, 

“Wake Measurements from the Horns Rev Wind Farm,” 

in Proceedings of the 2004 European Wind Energy 

Conference and Exhibition, London, UK, 2004. 

[15] M. Steinbuch, W. de Boer, O. Bosgra, S. Peters, and 

J. Ploeg, “Optimal Control of Wind Power Plants,” 

Journal of Wind Energy and Industrial Aerodynamics, 

vol. 27, pp. 237–246, 1988. 

[16] G. Corten and P. Schaak, “More Power and Less Loads 

in Wind Farms: ‘Heat and Flux’,” Energy Research 

Center of the Netherlands (ECN), ECN-RX–04-122, 

Petten, The Netherlands, Tech. Rep., 2004. 

[17] K. E. Johnson and N. Thomas, “Wind Farm Control: 

Addressing the Aerodynamic Interaction Among Wind 

Turbines,” in Proc. American Control Conf., 2009. 

[18] B. Kirby, “Frequency Regulation Basics and Trends, 

ORNL/TM-2004/291,” Tech. Rep., 2004. 

[19] Y. Rebours, D. Kirschen, M. Trotignon, and S. Rossignol, “A Survey of Frequency and Voltage Control 

Ancillary Services-Part I: Technical Features,” in IEEE 

Transactions on Power Systems, vol. 22, no. 1, 2007, 

pp. 350–357. 

11
[20] E. Ela, M. Milligan, and B. Kirby, “Operating Reserves 

and Variable Generation,” Tech. Rep., 2011. 

[21] N. W. Miller and K. Clark, “Advanced Controls Enable 

Wind Plants to Provide Ancillary Services,” in IEEE 

Power and Energy Society General Meeting, July 2010. 

[22] E. Ela, “Testing Active Power Control from Wind Power 

at the National Wind Technology Center, NREL/PR5500-51607,” in Windpower, 2011. 

[23] J.	 Eto, “Use of Frequency Response Metrics to 

Assess the Planning and Operating Requirements for 

Reliable Integration of Variable Renewable Generation. 

Lawrence Berkeley National Laboratory: Lawrence 

Berkeley National Laboratory. LBNL Paper LBNL4142E.” 2011. [Online]. Available: http://escholarship. 

org/uc/item/0kt109pn 

[24] R. Wiser, Z. Yang, M. Hand, O. Hohmeyer, D. Infield, 

P. H. Jensen, V. Nikolaev, M. O’Malley, and 

G. S. A. Zervos, “IPCC Special Report on Renewable 

Energy Sources and Climate Change Mitigation: Wind 

Energy,” IPCC, Cambridge, UK and New York, 

NY, USA, Tech. Rep., 2011. [Online]. Available: 

http://srren.ipcc-wg3.de/report 

[25] I.	 Margaris, “Power system requirements for high 

wind penetration,” Project UpWind, Roskilde, Denmark, Tech. Rep., 2007. 

[26] “Wind Farm	 Transmission Grid Code Provisions: A 

Direction by the Commission for Energy Regulation,” 

Commission for Energy Regulation, Dublin, Ireland, 

Tech. Rep., 2004. 

[27] “Wind	 Turbines Connected to Grids with Voltages 

Above 100kV: Technical regulation for the properties 

and the regulation for wind turbines,” Elkraft System 

and Eltra, Erritsø, Denmark, Tech. Rep., 2003. 

[28] “Technical Requirements for the Connection of Generation Facilites to the Hydro-Quebec Transmission Sys­  ́

tem: Supplementary Requirements for Wind Generation,” Hydro-Quebec, Montreal, Qu  ́ ebec, Canada, Tech.  ́

Rep., 2005. 

[29] “Technical Requirements for Wind Power	 and Photovoltaic Installations and Any Generating Facilities 

Whose Technology Does Not Consist of a Synchronous 

Generator Directly Connected to the Grid,” Red Electrica, Madrid, Spain, Tech. Rep., 2008. 

[30] B. Kirby, M. Milligan, and E. Ela, “Providing Minuteto-Minute Regulation from Wind Plants,” in 9th Annual 

International Workshop on Large-Scale Integration of 

Wind Power into Power Systems and Transmission 

Networks for Offshore Wind Power Plants, Quebec, 

Canada, 2010. 

[31] R. J. Nelson, “Frequency-responsive wind turbine output control,” U.S. Patent 0 001 318, Jan. 6, 2011. 

[32] A. Yasugi, “Wind turbine generator and method of 

controlling the same,” U.S. Patent 0 074 152, Mar. 31, 

2011. 

[33] A. Nyborg and S. Dalsgaard, “Power	 curtailment of 

wind turbines,” U.S. Patent 0 286 835, Nov. 11, 2011. 

[34] J. A. Sanchez, A. C. Mayor, J. M. Lusarreta, J. P.  ́

Barbachano, S. S. Segura, D. S. Lopez, M. Z. Maeztu,  ́

L. M. Palomo, and J. L. Taberna, “Method of operation 

of a wind turbine to guarantee primary or secondary 

regulation in an electric grid,” U.S. Patent 0 057 445, 

Mar. 10, 2011. 

[35] H. Ichinose and T.	 Wakasa, “Wind-turbine-generator 

control system, wind farm, and wind-turbine-generator 

control method,” U.S. Patent 0 057 445, Aug. 4, 2011. 

[36] J. Mayor, D. Sole, J. Acedo, A. Carcar, J. Barbachano, 

M. Zabaleta, and S. Simon, “Control of active power 

reserve in a wind-farm,” U.S. Patent 0 055 030, Feb. 26, 

2009. 

[37] M.	 A. Morjaria and M. A. Shah, “Intelligent active power management system for renewable variable 

power generation,” U.S. Patent 0 144 816, Jun. 16, 2011. 

[38] P. Keung, P. Li, H. Banakar, and B. Ooi, “Kinetic Energy of Wind-Turbine Generators for System Frequency 

Support,” IEEE Transactions on Power Systems, vol. 24, 

no. 1, Feb. 2009. 

[39] I. Erlich and M. Wilch, “Primary Frequency Control 

by Wind Turbines,” in IEEE Power and Energy Society 

General Meeting, July 2010. 

[40] H. Ma and B. Chowdhury,	 “Working Towards Frequency Regulation with Wind Plants: Combined Control Approaches,” Renewable Power Generation, IET, 

vol. 4, no. 4, pp. 308–316, July 2010. 

[41] X.	 Juankorena, I. Esandi, J. Lopez, and L. Marroyo, “Method to Enable Variable Speed Wind Turbine Primary Regulation,” in International Conference 

on Power Engineering, Energy and Electrical Drives, 

March 2009, pp. 495–500. 

[42] R. de Almeida and J. P. Lopes, “Participation of Doubly 

Fed Induction Wind Generators in System Frequency 

Regulation,” IEEE Transactions on Power Systems, 

vol. 22, pp. 944–950, 2007. 

[43] Y.	 Jeong, “Comparison and Testing of Active Power 

Control Strategies for Grid-connected Wind Turbines,” 

Master’s thesis, Colorado School of Mines, 2011. 

[44] J.	 Jonkman and M. L. Buhl, FAST User’s Guide. 

Golden, CO: National Renewable Energy Laboratory, 

2005. 

[45] A. Hansen, P. Sørensen, F. Iov, and F. Blaabjerg, “Centralised Power Control of Wind Farm with Doubly Fed 

Induction Generators,” Renewable Energy, vol. 31, pp. 

935–951, 2006. 

[46] V.	 Spudic, M. Jelavi  ́ c, M. Baoti  ́ c, and N. Peri  ́ c, “Hi­  ́

erarchical Wind Farm Control for Power-Load Optimization,” in The Science of making Torque from Wind 

(Torque2010), 2010. 

[47] A. Gesino, “Power reserve provision with wind farms,” 

Ph.D. dissertation, University of Kassel, 2010. 

[48] M.	 Churchfield and S. Lee. (2012, Mar.) NWTC 

design codes- SOWFA. [Online]. Available: http: 

//wind.nrel.gov/designcodes/simulators/sowfa 

12