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.

Contract No. DE-AC36-08GO28308

Fixed-Speed and Variable-Slip 

Wind Turbines Providing 

Spinning Reserves to the Grid

Preprint

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

To be presented at the IEEE Power and Energy Society 

General Meeting

Vancouver, British Columbia

July 21–25, 2013

Conference Paper

NREL/CP-5500-56817

November 2012
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1 

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 reserve, 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 fixed-speed (also known as Type 1) and variableslip (also known as Type 2) 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]. WPPs with fixed-speed 

(Type 1) wind turbines contribute to system frequency support 

because each wind turbine generator (WTG) is directly 

connected to the power grid. The material reported in [3–9] 

laid the groundwork for understanding inertia and frequency 

issues related to wind. The work reported here seeks to 

develop techniques to improve individual wind turbine

response to frequency events. Individual wind turbine reserveholding capability scales up to provide significant reserveholding capability at the WPP level. Control modifications 

needed to operate 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. 

This paper is arranged as follows. Section II presents basic 

aerodynamic characteristics of wind turbines. The behavior of 

squirrel-cage induction generators (used in Type 1 wind 

turbines) and wound-rotor induction generators with external 

rotor resistance (used in Type 2 wind turbines) is explored 

based on steady-state characteristics. Section III describes

dynamic simulations to investigate different scenarios applied 

to the WPP with the implementation of reserve power. 

II. BASIC WIND TURBINE OPERATION

A. Type 1 and Type 2 Wind Turbine Generators (Induction 

Generators) 

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. For any wind turbine, a set of curves known as the CP

versus tip speed ratio (TSR) describes the aerodynamic 

characteristics of the wind turbine rotor. CP is the performance

coefficient—i.e., the proportion of power in the wind that is 

extractable. The TSR is the ratio of the speed of the tips of the

blades to the incoming wind speed. An example of Cp versus 

TSR curves can be seen in Figure 2, which shows that an 

optimum TSR exists for each value of pitch. Consider the 

simplest type of wind turbine. This wind turbine has a stallcontrolled induction generator without a pitch controller. The 

power limit is set by the aerodynamic stall instead of by the 

pitch control. The pitch angle can be considered zero.

(a) Type 1 WTG

(b) Type 2 WTG

Figure 1. Physical diagram of a (a) Type 1 and (b) Type 2 WTG

 1) Stall-Operated Wind Turbine

The wind turbine is started as an induction motor as the 

wind speed reaches cut-in. The rotor speed is driven by the 

available wind and is driven above synchronous speed at a 

very small slip (1% at most). As the wind speed increases at 

constant rotational speed, the TSR decreases and the operating 

CP travels along ABCD when the rated power is reached at the 

wind speed designed for the wind turbine. As the wind speed 

increases further, the operating CP decreases further and the 

TSR gets even smaller.

Fixed-Speed and Variable-Slip Wind Turbines

Providing Spinning Reserves to the Grid

W
2 

Figure 2. CP versus tip speed ratio

 2) Pitch Control

Modern wind turbines are equipped with pitch controllers to 

limit the aerodynamic power driving the generator. The blade 

pitch control is activated in the high wind speed region to limit

the stresses imposed on the mechanical components, to limit 

the output power generated, and to avoid a runaway condition 

if the wind turbine loses connection to the grid. Limiting the 

aerodynamic torque driving the generator is important to 

ensure that the mechanical components of the turbine 

(gearbox, generator shaft, low-speed shaft, etc.) will not be 

stressed due to overloads caused by wind fluctuations and 

turbulence.

Figure 3. Output power limiting with pitch control

Figure 3 shows the power curve of a pitch-controlled fixedspeed wind turbine at 15 rpm. The mechanical components 

can be designed to withstand the power as limited by the pitch 

controller. In this case, the output of the wind turbine is 

limited to 1.5 MW. Figure 2 shows the effect of changing the 

pitch angle on the CP-TSR curve. Increasing the pitch angle 

shrinks the CP-TSR curve.

In Type 1 wind turbines, the power output is dictated by the 

characteristic of the induction generator. There is no power 

converter connected to the stator winding or the rotor winding. 

In Type 2 wind turbines, there is an external rotor resistance 

that can potentially be used to control the output power of the 

induction generator; however, using the rotor resistance 

control may result in additional losses. In the approach 

proposed in this paper, the pitch controller is used to allocate 

part of the available aerodynamic power as reserve power and 

the turbine delivers the remainder to the power system grid.

 3) Pitch Control to Allow Reserve Power Capability

With the pitch controller, it is possible to operate wind 

turbines at different pitch angles (or scheduled pitch angles).

This allows the WTG to set aside the reserve power to be 

called when needed—for example, when a large generator 

drops from the grid because of fault occurring in the 

transmission lines, the reserved power may be called to 

substitute the loss of generated power; thus, the power system 

frequency will not drop significantly.

Figure 4 shows an example of a WTG operating at a 

reduced output power. The operation follows the path 

ABCDE. The shaded area represents the reserve power 

available that can be retrieved. The disadvantage of this 

algorithm is that during productive winds, the output power 

has to be shed; thus, there is some productivity loss.

A

B

C D E

Reserve Power

Figure 4. Scheduled reserve power with pitch controller

 4) Type 1 Wind Turbine Generator—Power Speed 

Characteristic

Type 1 WTGs use squirrel-cage induction generators and 

operate in a very narrow slip range (about 1% rated slip). The 

real power versus the rotational speed is shown in Figure 4.

Both the power and the rotor speed are given in per-unit 

quantities. The output current versus the rotational speed is 

given in Figure 5. Note that the induction generator is always 

absorbing reactive power from the line; thus, reactive 

compensation is usually implemented by switched capacitor 

banks, and the size of the capacitance is controlled so that the 

wind turbine generates power at unity power factor.

As shown in Figure 4, the normal operating point at rated 

wind speed is at point A, where the output power is at 1.0 per 

unit. The aerodynamic power driving the generator fluctuates 

with wind speed; thus, the pitch is continuously controlled to 

limit the aerodynamic power developed by the blades, which

also limits the aerodynamic torque driving the induction 

0 10 20

0

1

2

15 rpm

15 rpm w pitch

 

Wind speed (m/s)

Power (MW)

0 5 10 15 20

0

0.2

0.4

0 deg pitch

5 deg pitch 

8 deg pitch

Cp - TSR characteristic of a typical WTG

 

Cp (Performance Coefficient)

A

Cp-max

D

C

B

TSRtgt
3 

generator. Instantaneously, individual WTGs may generate 

more or less at 1.0 per unit with small variation (indicated by 

the two-sided arrow); however, the average power will always 

be limited to 1.0 per unit. At the point of interconnection, the 

average output from hundreds of wind turbines will smooth 

out to an almost flat output when the wind speed is at or 

higher than rated wind speed.

To allow power to be held in reserve, the wind turbine is 

operated with an output power set-point lower than what is 

actually available, e.g., at high wind speeds, the pitch can be 

controlled to generate 80% of rated power although it is 

capable of generating 100% of rated power. The operating 

point moves from A to B, as shown in Figure 4.

Figure 4. Output power versus rotor speed (Type 1 WTG)

 5) Type 2 Wind Turbine Generator—Power Speed 

Characteristic

The Type 2 WTG uses a wound-rotor induction generator 

instead of the squirrel-cage generator as in the Type 1 WTG.

The wound rotor is connected to an adjustable external rotor 

resistance. 

Figure 5. Output power versus rotor speed (Type 2 WTG)

Below rated wind speed, the external rotor resistance is 

shorted; thus, the behavior of a Type 2 WTG is the same as a 

Type 1 WTG when the wind speed is below its rated value. 

Above rated wind speed, the external rotor resistance is 

controlled to allow the effective rotor resistance to be varied to 

maintain the output power constant even as the wind speed 

varies; thus, although the wind speed changes, the external 

rotor resistance can be adjusted so that the output power stays 

constant. This is indicated by A and A’ in Figure 5. The 

adjustable rotor resistance is implemented by using simple 

power electronics and resistors. Note that although the rotor 

resistance can be varied, in practice the pitch controller is still 

used to control the speed to minimize the external rotor 

resistance because deployment rotor resistance generates heat 

that must be dissipated. To minimize the heat generated, the 

pitch control is used. To allow power to be held in reserve, the 

wind turbine is operated with an output power set-point lower 

than available power as in the Type 1 WTG. The operating 

point moves from A to B, as illustrated in Figure 5. 

 6) Two Types of Reserve Power

Figure 6 shows a block diagram of the proposed controller 

for providing reserve-power capability. There are several 

groups of control blocks performing different functions. Block 

1 computes the input to the pitch controller to limit the 

rotational speed at rated value when the wind speed is high to 

avoid a runaway condition.

Figure 6. Pitch controller used to set reserve power for Type 1 

and Type 2 wind turbines

Block 2 computes the input to the pitch controller to limit 

the average output power of the wind turbine based on the 

target power Ptgt. The target power Ptgt varies with wind speed 

and the level of reserved power defined. In Block 2 there are 

two paths used: one path uses wind speed as an input to 

provide a feed-forward value by pre-computing the steadystate values of the pitch angle under normal situations 

1 1.01 1.02 1.03 1.04 1.05

0

1

2

3

P - Output Power

Real Power vs Rotor Speed

Rotor Speed (p.u.)

P (p.u.)

1 1.01 1.02 1.03 1.04 1.05

0

1

2

3

P - fpu=1.0

Real Power vs Rotor Speed

Speed (p.u.)

P (p.u.)

A

B

Small 

Wind

Variations 

Reserve power is implemented 

by pitch controller

Reserve power is implemented 

by pitch controller

A

B

A’

External rotor 

resistance control
4 

(without reserve power) with the help of a lookup table to map 

higher wind speeds versus blade pitch angle; the other path 

takes the commanded target power Ptgt as an input to control 

the desired output power, which may include controlling the 

reserve power below rated wind speed and limiting the rated 

power above rated wind speed. The P-I controller helps to 

fine-tune the power controller to follow the target power.

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

controller in adjusting the output power from the WTG. The 

input to Block 3 is the filtered wind speed that is be translated 

to aerodynamic power using the power curve of the turbine 

(similar to the one shown by the solid line curve in Figure 3) 

via a lookup table. The output of the lookup table is the actual 

output of the turbine without accommodation for reserve 

power. Block 3 also has a provision for users to select the 

method for holding reserve. These methods are explained in 

the next subsection. If governor droop control to help the grid 

by decreasing or increasing the reserve power is implemented, 

the additional inputs from the droop control can be accounted 

for in the proposed controller via the auxiliary output power 

Paux. 

 7) Two Types of Reserve Power

To set aside some reserve power, a constant value of 

reserve power (constant reserve - ∆P) or a constant proportion 

of the available aerodynamic power (proportional reserve—

refer to the switch available in Block 3) may be selected. 

Figure 7 (a) shows wind turbine power curves if the reserve 

power to be held is a constant value (20% and 40% of rated

power); Figure 7 (b) shows the power curves if the reserve 

power to be held is a constant percentage (20% and 40% of 

target power—i.e., maximum CP operation).

(a) Constant ∆P 

(b) Proportional reserve

Figure 7. The reserve power held using two different methods

III. DYNAMIC SIMULATION

The performance of the proposed controller was

investigated through dynamic simulations. The induction 

generator is represented by typical fifth-order dynamic 

representation. Dynamic models were developed in 

PSCAD/EMTDC and tested using 150s of real wind speed 

data. In each case, the rated power of the wind turbine is 3 

MW.

 1) Reserve Power Implementation

Figure 8 shows the traces of output power and pitch angle 

for a Type 1 wind turbine. It also shows the traces for constant 

reserve operation. Comparison between normal operation and 

the operation with reserve is presented. In Figure 8, a constant 

reserve power is commanded (20% of rated power—i.e., 0.6 

MW). The output power shows both the available wind power 

(blue line) and the output power with reserve power held 

(green line). The pitch angle is shown to vary at a slower rate 

than the wind speed variation. The pitch controller is slower 

than the power electronics controller when used to limit the 

output power; thus, only the average power is limited to the 

rated output power. As shown in Figure 8, there are times 

when the peak of the output power briefly exceeds the rated 

power (3 MW) of the wind turbine.

Figure 8. Output power and pitch angle for constant reserve 

power (∆Preserve) implementation on a Type 1 wind turbine

As described previously, there is an external rotor resistance 

control that can be used in Type 2 wind turbines. In this 

implementation, the rotor resistance is used to limit the 

generator output current to a constant value, according to the 

percentage of the reserve power to be held. For example, if the 

reserve power to be held is 20% of rated power, the output 

current will be limited to 80% of rated current by controlling 

the external rotor resistance. Note that the rotor resistance 

control is much faster than the pitch controller; thus, the 

current regulation can be accomplished very effectively.

The effect of fast control of external resistance control can 

be shown by comparing the output power fluctuations between 

Type 1 wind turbines and Type 2 wind turbines, as shown in 

Figure 9. In Type 2 wind turbines, the power output is 

practically clamped at the maximum output power of the wind 

0 10 20

0

1

2

Max Cp (k w^3)

k w^3 at 20% Reserve

k w^3 at 40% Reserve

 

Wind speed (m/s)

Power (MW)

 

 

 

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

 

Wind speed (m/s)

Power (MW)

50 60 70 80 90 100 0

1

2

3

4

time (s)

Pout (MW)

No reserve

Delta reserve

50 60 70 80 90 100 0

2

4

6

8

time (s)

Pitch (deg)

No reserve

Delta reserve
5 

turbine, which is equal to the rated power (3 MW); whereas in

Type 1 wind turbines, the output power sometimes exceeds 

the rated value of 3 MW.

Figure 9. Output power comparison between the output power of 

a Type 1 wind turbine and a Type 2 wind turbine with ∆Preserve = 

20% of the rated power in time domain

The Type 2 WTG was simulated and the output was plotted 

against wind speed. Figure 10 and Figure 11 show the output 

power comparison between base case and the operation with 

reserved power. As expected, the actual output power was 

scattered. However, when we used the polynomial fitting of 

7th order, the output power characteristic was similar to the 

one predicted in Figure 7. The scattered points can be 

accounted to the changes of the kinetic energy stored in the 

rotating mass (blades, generator, gearbox, etc.) because the

rotational speed of the wind turbine changes with time.

Figure 10. Output power comparison between the base case and 

delta reserve power of a Type 2 wind turbine based on the 

dynamic simulation with ∆Preserve = 20% of the rated power

Figure 11. Output power comparison between the base case and 

proportional reserve power of a Type 2 wind turbine based on 

the dynamic simulation with ∆Preserve = 20% of the rated power

IV. CONCLUSIONS

This paper describes the reserve power implementation in a 

WPP for Type 1 and Type 2 wind turbines. We chose Type 1 

and Type 2 wind turbines because they are still available on 

the market and because the reserve power is more difficult to 

control with pitch control. The power converters in Type 3 

and Type 4 wind turbines make it easier to implement 

spinning reserves; however, as shown in this paper, spinning 

reserve power can be implemented successfully even with 

only the pitch controller. When scaled up to the plant level, 

these capabilities can offer significant frequency support to the 

bulk power system grid and should be considered in a high 

wind penetration scenario.

V. 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.

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[2] J. Eto et 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 

report, 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,” in Proc. 10th International Workshop 

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60 65 70 75 80 85 90

0

1

2

3

4

time (s)

Pout (MW)

Type 1 delta

Type 2 delta