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Contract No. DE-AC36-08GO28308

Variable-Speed Wind Power 

Plant Operating With Reserve 

Power Capability

Preprint

M. Singh, V. Gevorgian, E. Muljadi, and E. Ela

National Renewable Energy Laboratory

Presented at the IEEE Energy Conversion Congress and Exposition

Denver, Colorado

September 15–19, 2013

Conference Paper

NREL/CP-5500-58961 

October 2013
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Variable-Speed Wind Power Plant Operating 

With Reserve Power Capability

Mohit Singh, Vahan Gevorgian, Eduard Muljadi, and Erik Ela

Transmission and Grid Integration Group

National Renewable Energy Laboratory

Golden, Colorado

Abstract—As the level of wind penetration increases, wind 

turbine technology must move from merely generating power 

from wind to taking a role in supporting the bulk power system. 

Wind turbines should have the capability to provide inertial 

response and primary frequency (governor) response. Wind 

turbine generators with this capability can support the frequency 

stability of the grid. To provide governor response, wind turbines 

should be able to generate less power than the available wind 

power and hold the rest in reserves, ready to be accessed as 

needed. In this paper, we explore several ways to control wind 

turbine output to enable reserve-holding capability. The focus of 

this paper is on doubly-fed induction generator (also known as 

Type 3) and full-converter (also known as Type 4) wind turbines.

Index Terms—wind turbine generator, variable speed, 

induction generator, governor response, inertial response, 

renewable energy

I. INTRODUCTION

ind power generation may reach 300 GW by 2030, 

achieving a level of penetration of 20% total energy 

production [1]. Recent advances in wind turbine technology 

allow efficient and rapid deployment of wind power plants 

(WPPs). As more WPPs are integrated into the bulk utility 

power system, the adverse effects of wind power uncertainty 

and variability on the power system are expected to become 

more noticeable. The issue of frequent and significant 

frequency excursions from nominal value is of particular 

concern, especially in a synchronous power system with high 

wind power penetration [2]. The material reported in [3–9] 

laid the groundwork for understanding inertia and frequency 

issues related to wind. WPPs with doubly-fed induction 

generator (Type 3) wind turbines as well as full-converter 

(Type 4) wind turbines typically do not contribute to system 

frequency support because each wind turbine generator 

(WTG) is indirectly connected to the power grid. The work 

reported here seeks to develop a control method to improve 

individual wind turbine response to frequency events. 

Individual wind turbine reserve-holding capability scales up to 

provide significant reserve-holding capability at the WPP 

level. Control modifications needed to operate Type 3 and 

Type 4 WTGs with reserves are presented in this paper. 

Detailed simulations in the time domain have been conducted 

to demonstrate the efficacy of the proposed control 

modifications. The unique feature of this proposed controller 

is that the wind turbine is always operated at the optimal 

(rated) value of the tip speed ratio (TSR).

Different wind turbine types use different energy conversion 

systems (generator, power converter, and control algorithms). 

The strategies used to control the prime mover are generally 

similar. Common elements include mechanical brakes and 

blade pitch control to avoid a runaway condition and reduce

stresses on the mechanical components of the wind turbine. 

Type 3 and Type 4 turbines have power converters as well to 

allow additional control and conditioning of the output real 

and reactive power. The topologies of the Type 3 and Type 4 

turbines are shown in Figure 1.

This paper is arranged as follows. Section II presents basic 

characteristics of wind turbines as well as the proposed reserve 

power controls. Section III describes dynamic simulations to 

investigate different scenarios applied to a WPP with the 

implementation of reserve power controls.

a) Type 3 WTG

b) Type 4 WTG 

Fig. 1. Physical diagram of variable-speed WTGs

II. BASIC WIND TURBINE OPERATION

A. Type 3 and Type 4 WTGs

Type 3 and Type 4 wind turbines are converter-based 

turbines that offer maximum power point tracking as well as 

independent real and reactive power control. The generatorpower converter is controlled to operate at maximum 

aerodynamic efficiency (Cpmax). The complete paper will 

W
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at www.nrel.gov/publications. 

contain a more detailed background of variable-speed 

operation for both Type 3 and Type 4 turbines. Modern wind 

turbines are usually equipped with pitch controllers to limit the 

aerodynamic power driving the generator. With the pitch 

controller, the blade pitch angle is activated in the high-windspeed region to limit the stresses imposed on the wind turbine 

mechanical components, limit the output of generation, and 

avoid a runaway condition when the WTG loses connection to 

the grid. The controller design described here uses both power 

converter control as well as pitch control to achieve the 

holding and releasing of reserve power. 

B. Reserve Power for Variable-Speed WTGs 

In variable-speed WTGs, the power versus rotational speed 

characteristic of a WTG is shaped by the power converter. The 

pitch controller adjusts the aerodynamic power. 

1) Two Types of Reserve Power 

If we want to set aside some reserve power, the portion of 

aerodynamic power that will be reserved should be included as 

a constant proportion of the rated power (constant reserve – 

P) or a constant proportion of the available aerodynamic 

power (proportional reserve). This spinning reserve capability 

can be used to implement “governor control” to help the grid 

by decreasing or increasing the reserve power held from or 

delivered to the grid. In Figure 2a, the amount of reserve 

power is a constant output power as a percentage of rated 

power, and in Figure 2b, the amount of reserve power is a 

fraction of the target power (Cpmax operation). 

0 10 20

0

1

2

Max Cp (k w^3)

k w^3 at 0.8 Gain

k w^3 at 0.6 Gain

Proportional Reserve for VS WTG

Wind speed (m/s)

Power (MW)

a) Constant P 

0 10 20

0

1

2

Max Cp (k w^3)

k w^3 at 20% Reserve

k w^3 at 40% Reserve

Delta P Reserve for VS WTG

Wind speed (m/s)

Power (MW)

b) Proportional reserve 

Fig. 2. The reserve power for a variable-speed WTG using two different 

methods 

2) Combining Control and Power Converter Control 

Figure 3 illustrates how the control strategy works. The 

WTG is operated at variable speed. The electrical output 

power is commanded to follow the thick red line in normal 

operation without the reserve requirement. With the reserve 

requirement enabled, the dashed green line (indicating the 

proportional reserve) is the path followed. This is done via 

converter control. The thin red line represents the aerodynamic 

power of the WTG at 8 m/s without pitch action, and the 

operating point of the WTG is at Point A for optimal 

operation. To fulfill the reserve requirement, the pitch is 

controlled to ensure that the aerodynamic operation of the 

WTG will be maintained at optimal tip speed ratio (TSR), and 

the aerodynamic power moves from the thin red line to the 

dashed green line after the pitch angle is adjusted. The new 

operating point is Point B. Moving the operating point from 

Point A to Point B requires both the aerodynamic adjustment 

via pitch angle control and power converter control. The 

purpose of keeping the TSR at the optimal value is to optimize 

the response of the turbine when the pitch is returned to 

normal (pitch angle =0o

); thus, the turbine will return to 

operating at optimum Cp right away. The operating point 

moves back from Point B to Point A when the reserve power 

of a WPP is recalled (i.e., reserve requirement is disabled) and 

it will return performance coefficient to Cpmax operation. 

3) Control Block Diagram 

The control block diagram for the proposed controller is 

presented in Figure 4. There are several groups of control 

blocks performing different functions: 

 Block 1 ensures that the operation of the WTG is 

maintained at constant TSR. The input to this block 

is the wind speed. The average (filtered) value of 

the wind speed is used to compute the 

corresponding rotor speed to keep the TSR 

constant at the optimal target value (TSRtgt). The 

reason we want to keep the TSR at TSRtgt is to 

ensure that the WTG will respond instantaneously 

and return to its original operating point quickly 

when the pitch angle is returned to normal. 

 Block 2 controls the pitch angle so that the rotor 

speed follows the target rotor speed (m-tgt) and the 

TSR is kept at TSRtgt. Another function of this 

block is to ensure that the rotor speed will never 

exceed the rated (upper limit) rotor speed (m < 

m-limit). Thus, it will prevent the runaway problem 

when the turbine gets disconnected from the grid. 

The output of this block will be limited (0o

 << 

30o

). 

 In Block 3, the target power is computed to guide the 

pitch controller in adjusting the output of the WTG. 

The input to Block 3 is the rotational speed that will 

be translated to the calculated power (Pcalc) 

deliverable at maximum Cp operation. From this 

calculated power, the reserve power must be 
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subtracted to obtain the electrical power, including 

the reserve power – Ptgt_rsv. Another input is Paux, to 

include the governor droop control capability if it is 

implemented (discussed in the next section). Finally, 

the output of the block is the target power. Note that 

the target power at lower-than-rated rotor speed must 

be checked to ensure the operating torque of the 

generator will not exceed the designed maximum 

torque (corresponding to output power at maximum 

rated torque PratedT ). This precaution will allow the 

mechanical torque limit to be observed to preserve 

the gearbox, the shaft, and other mechanical 

components of the WTG. Finally, the target power 

Ptgt will be used to command the power converter so 

that the total output of the generator will be equal to 

Ptgt. 

Fig. 3. Operating points for the proposed control 

Fig. 4. Pitch controller and real power controller used to set reserve power for 

Type 3 and Type 4 WTGs 

III. DYNAMIC SIMULATION

1) Constant Reserve Power Implementation 

Type 3 and Type 4 turbines, including the controller 

described above, were developed in MATLAB/Simulink. 

Details of the model are presented in the complete version of 

the paper. Figure 5 shows a test wind speed time series, and 

the corresponding power output, for the cases with and without 

reserve requirement enabled. As shown in Figure 5, the 

requested reserve is 20% of the rated power output (constant 

ΔP). 

2) Proportional Reserve Power Implementation 

Figure 6 illustrates the result of proportional reserve power 

implementation. As shown in Figure 6, the requested reserve is 

10% of the available aerodynamic power (proportional 

reserve). For both operations, the plots show that the reserve 

controller functions as desired. 

Fig. 5. Constant reserve power implementation (Preserve = 20%) 
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at www.nrel.gov/publications.

Fig. 6. Proportional reserve power implementation ( reserve 10%)

3) Inertial response capability

The inertial response for variable speed WPP has been 

discussed earlier in reference [10]. This topic will not be 

presented in this paper. More detail discussion on this subject 

is not covered by this paper due to space limitation.

4) Governor response capability

To implement governor response capability, we will assume 

that the wind power plant is operated in the medium to rated 

wind speeds. A simple power system is constructed to 

simulate the power system behavior under sudden perturbation 

with a sudden load change. A comparison between system with 

and without governor response capability is compared.

With spinning reserve implemented in a WPP, nonsymmetric droop characteristics similar to one shown in Figure 

7 can be implemented in wind turbines.

As in the case of inertial response, the primary response 

parameters (dead bands, droops, reserve margin) can be tuned 

up for optimum system performance.

Fig. 7. Governor control implemented with a frequency droop on a wind 

power plant

The governor control will take the frequency of the grid as 

the input and the commanded additional power Po as the 

output. Note, that the WPP can respond to the system overfrequency by shedding the output power (droop+), and it can 

respond to the system under-frequency by deploying the 

reserve power (droop -). The Paux shown in Block 3 in Figure 

4 can be used with Paux = Po (output of the governor control) to 

accomplish the governor control.

Fig. 8. Small power system network

A small power system network is assembled to simulate the 

system with governor control capability as shown in Figure 8. 

The balance of real power generation and the load is perturbed 

by disconnecting one of the generators. As the frequency of 

the power system drops.

An example in Figure 9, shows sensitivity to different 

individual wind turbine droop settings (5, 4, 3 and 2 %). In this 

particular example, the difference in frequency trajectories are 

not significant and not affecting neither nadir nor recovery 

times. However, this picture may be different at higher levels 

of the wind penetration.
5 

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at www.nrel.gov/publications.

Fig. 91. Sensitivity to wind turbine droop characteristic

Fig. 10. Frequency response for different spinning reserve by wind power

The amount of spinning reserve available from wind 

turbines impacts the amount of power that wind turbine can 

inject into power system during the fault. The example in 

Figure 10 shows some improvements in minimum frequency 

for cases with no spinning reserve, 5% and 10% spinning 

reserves respectively. The higher spinning reserve capacity, 

the higher the frequency nadir and the sooner the frequency 

restoration to normal range.

The primary frequency control by wind turbines can be 

integrated into the rotor-side active power control loop for 

Type 3 WTG or directly to the full power converter for Type 4 

WTG and demonstrate behavior similar to conventional 

synchronous generators.

IV. CONCLUSION

This paper is written to illustrate the capability of WPPs to 

provide auxiliary functions such as spinning reserve. In this 

paper, variable-speed WTGs (Type 3 and Type 4) are 

considered. The control method to implement the reserve 

holding capability is described in detail. A controller is 

developed that controls the pitch angle and the power 

converter simultaneously to allow individual WTGs to hold 

power in reserve. The reserved power may be delivered to the 

grid in the event of a frequency event. The unique feature of 

this controller is that the turbine always remains at the optimal 

TSR, with or without the reserve requirement enabled.

ACKNOWLEDGMENT

This work was supported by the U.S. Department of Energy 

under Contract No. DE-AC36-08-GO28308 with the National 

Renewable Energy Laboratory.

REFERENCES

[1] S. Lindenberg, B. Smith, and K. O’Dell, “20% wind energy by 2030,” 

Work performed by Renewable Energy Consulting Services, Energetics 

Incorporated, National Renewable Energy Laboratory, U.S. Department 

of Energy, Golden, CO, NREL/TP-550-41869, 2008. 

[2] J. Eto at al., “Use of frequency response metrics to access the planning 

and operating requirements for reliable integration of variable renewable 

generation,” Ernest Orlando Lawrence Berkeley National Laboratory,

Berkeley, CA, LBNL-4142E, Dec. 2010.

[3] P. W. Christensen and G. T. Tarnowski, “Inertia of wind power plants— 

state-of-the-art review year 2011,” 10th International Workshop on 

Large-Scale Integration of Wind Power, Aarhus, Denmark, Oct. 25–26, 

2011. 
6

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

[4] L. Ruttlege and D. Flynn, “Systemwide inertial response from fixedspeed and variable-speed wind turbines,” IEEE PESGM 2011, pp. 1–7,

2011.

[5] N. W. Miller, K. Clark, and M. Shao, “Impact of frequency responsive 

wind plant controls on grid performance,” 9th International Workshop on 

Large-Scale Integration of Wind Power, Quebec, Canada, Oct. 18–19, 

2010. 

[6] N. Miller, M. Shao, and S. Venataraman, “California ISO: Frequency 

response study,” Final draft, Nov. 9, 2011. Available at 

www.uwig.org/Report-FrequencyResponseStudy.pdf 

[7] S. Sharma, S. H. Huang, and N.D.R. Sarma, “System inertial frequency 

response estimation and impact of renewable resources in ERCOT 

interconnection,” in IEEE PES GM Proc., July 24–29, 2011, pp. 1–6. 

[8] L. Holdsworth, J. B. Ekanayake, and N. Jenkins, “Power system 

frequency response from fixed-speed and doubly-fed inductiongenerator-based wind turbines,” Wind Energy vol. 7, no. 1, pp. 21–35, 

2004. 

[9] J. Morren, J. Pierik, and S.W.H. de Haan, “Inertial response of variablespeed wind turbines,” Electric Power Systems Research, vol. 76, no. 11, 

pp. 980–987, 2006. 

[10] E. Muljadi, V. Gevorgian, M. Singh, S. Santoso, “Understanding Inertia 

and Frequency Response of Wind Power Plants” presented at the 2012 

IEEE Symposium on Power Electronics and Machines for Wind 

Applications (PEMWA 2012) on July 16-18, 2012 in Denver, Colorado

V. BIOGRAPHIES

Mohit Singh (M’11) received his M.S. and Ph.D. in electrical engineering 

from the University of Texas at Austin in 2005 and 2011, respectively. His 

research thesis focused on dynamic modeling of WTGs. 

Dr. Singh is a postdoctoral research engineer at the National Renewable 

Energy Laboratory in Golden, Colorado. His research interests include 

modeling and testing of various applications of WTGs and other renewable 

energy resources. He is member of IEEE and is involved in the activities of 

the IEEE PES.

Vahan Gevorgian (M’97) graduated from the Yerevan Polytechnic 

Institute in Armenia in 1986. During his studies, he concentrated on electrical 

machines. His thesis research dealt with doubly fed induction generators for 

standalone power systems. He obtained his Ph.D. in electrical engineering 

from the State Engineering University of Armenia in 1993. His dissertation 

was devoted to a modeling of electrical transients in large WTGs.

Dr. Gevorgian is a research engineer at the National Wind Technology

Center of the National Renewable Energy Laboratory in Golden, Colorado. 

His research interests include modeling and testing of various applications of 

wind generation in power systems.

Eduard Muljadi (M’82, SM’94, F’10) received his Ph.D. in electrical 

engineering from the University of Wisconsin at Madison. From 1988 to 

1992, he taught at California State University at Fresno. In June 1992, he 

joined the National Renewable Energy Laboratory in Golden, Colorado. His 

research interests are in the fields of electric machines, power electronics, and 

power systems, with an emphasis on renewable energy applications. He is a 

member of Eta Kappa Nu and Sigma X, and a fellow of the Institute of 

Electrical and Electronics Engineers (IEEE). He is involved in the activities 

of the IEEE Industry Application Society (IAS), Power Electronics Society, 

and Power and Energy Society (PES), and an editor of the IEEE Transactions 

on Energy Conversion. 

Dr. Muljadi is a member of various committees of the IAS, as well as a 

member of the Working Group on Renewable Technologies and the Task 

Force on Dynamic Performance of Wind Power Generation, both of the PES. 

He holds two patents in power conversion for renewable energy. 

Erik Ela (M ’05) received the B.S.E.E degree from Binghamton 

University and the M.S. degree in Power Systems at the Illinois Institute of 

Technology.

He joined the NREL grid integration team to work on different wind 

integration issues. His experience lies mostly in different topics relating to 

grid operations and market operations. Erik previously worked for the New

York ISO developing and improving products in the energy markets and 

operations areas. Erik is a member of IEEE and the Power and Energy 

Society.