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Contract No. DE-AC36-08GO28308

Simulation Tool to Assess 

Mechanical and Electrical 

Stresses on Wind Turbine 

Generators

Preprint 

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

and J. Jonkman 

National Renewable Energy Laboratory

Presented at the IEEE Energy Conversion Congress and Exposition

Denver, Colorado

September 15–19, 2013

Conference Paper

NREL/CP-5500-58962 

October 2013
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Simulation Tool to Assess Mechanical and Electrical 

Stresses on Wind Turbine Generators

Mohit Singh, Eduard Muljadi, Vahan Gevorgian, and Jason Jonkman

National Renewable Energy Laboratory

Golden, Colorado

Abstract—Wind turbine generators (WTGs) consist of 

many different components to convert kinetic energy of the 

wind into electrical energy for end users. Wind energy is 

accessed to provide mechanical torque for driving the shaft

of the electrical generator. The conversion from wind 

power to mechanical power is governed by the 

aerodynamic conversion. The aerodynamic-electricalconversion efficiency of a WTG is influenced by the 

efficiency of the blades, the gearbox, the generator, and the 

power converter.

This paper describes the use of MATLAB/Simulink to 

simulate the electrical and grid-related aspects of a WTG 

coupled with the FAST aeroelastic wind turbine computeraided engineering tool to simulate the aerodynamic and 

mechanical aspects of a WTG. The combination of the two 

enables studies involving both electrical and mechanical 

aspects of a WTG. For example, mechanical engineers can 

formulate generator control that may preserve the life of 

the gearbox or mitigate the impact of transient events 

occurring on the transmission lines (faults, voltage and 

frequency dips, unbalanced voltages, etc.). Similarly, 

electrical engineers can study the impact of high-ramping 

wind speeds on power systems, as well as the impact of 

turbulence on the voltage and frequency of a small 

balancing authority area. This digest includes some 

examples of the capabilities of the FAST and MATLAB 

coupling, namely the effects of electrical faults on the blade 

moments.

Index Terms—aerodynamic control, electromechanical 

interaction, variable speed, wind turbine, wind power generation

I. INTRODUCTION

ind energy deployment has grown substantially during 

the last decades. In the past, wind turbine generators 

(WTGs) utilized a very simple wind turbine with stall control 

and a fixed-speed directly-connected induction generator 

(Type 1). The affordable cost of power converters, advances in 

modern control and aeroelasticity, and the availability of fastcomputing microprocessors enabled wind turbine engineers to 

design very sophisticated, modern WTGs capable of delivering

high-quality output power while at the same time enhancing 

power system operations.

To perform a holistic design, all aspects of a WTG need to 

be considered. In this paper, we attempt to demonstrate a 

holistic WTG model by using the NREL-developed FAST [1] 

software to simulate the detailed aerodynamics and mechanical 

aspects of a WTG, and MATLAB/Simulink [2] to simulate the 

electrical generators, converters, collector systems, and grid 

aspect of a grid-connected WTG. The references [1–9] cover 

the basic equations used in FAST as well as in drivetrain and 

generator models in more detail.

Mechanical load and stress in a wind turbine drivetrain is 

influenced by the torque and nontorque loads applied between 

the input shaft at the blade side and the output shaft at the 

generator side. For example, unwanted loads entering the input 

shaft are caused by things such as wind turbulence, tower 

shadow effect, uneven loading of the blades, and sudden 

changes in the wind direction. Examples of unwanted 

disturbances on the output power (and thus torque) include 

transmission line disturbances (voltage and/or frequency dips, 

unbalanced voltage, under- or overvoltage). These differences 

between input and output torque manifest in the stresses, loads, 

and losses of the components (gearbox, shaft, bearing, etc.) in 

different parts of a WTG. Figure 1 illustrates the interfacing of 

FAST and MATLAB/Simulink.

Fig. 1. Hybrid simulation performed with the detailed aerodynamic and 

mechanical model of a wind turbine within FAST, and the detailed electrical 

and grid model in the Matlab/Simulink environment.

Using FAST in conjunction with MATLAB/Simulink 

allows us to examine loading of different components under 

grid transients or wind turbulence, and also to design 

controllers to mitigate the effects of these unusual conditions 

on a turbine structure and components. If we do not have any 

control to influence the torque difference between input and 

output, there is very little we can do to influence the lifetime 

and operations and maintenance of these components, which 

eventually affects the cost of the energy for the life of a WTG.

W
2

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The intent of this paper is to provide a brief demonstration 

of the capabilities of the coupled FAST and 

MATLAB/Simulink models. Section II briefly discusses some 

results from a model of the simplest type of WTG, Type 1. In 

Section III, we highlight results from simulating voltage sags 

on a Type 3 (doubly-fed induction generator, or DFIG) turbine 

to illustrate the flexibility of variable-speed WTGs as well as 

illustrate that the strength of the grid connection can have an 

effect on mechanical transients experienced by a turbine 

during grid events. Section IV provides conclusions.

II. FIXED-SPEED WTGS

Broadly, four electrical topologies are in use in commercial 

megawatt-scale WTGs today. In a fixed-speed (Type 1) WTG 

(Figure 2a), there is no control input that can be used to 

influence the output of the generator except for the pitch 

controller, which is usually deployed when the power in the 

wind exceeds the rated power, and some form of yaw control 

would also be needed to follow the wind direction. On the 

other hand, in Type 2 (variable-slip), Type 3 (DFIG), and 

Type 4 (full-converter) WTGs, there is some controllability 

that can be implemented to affect the behavior of a WTG 

below and above rated wind speed. The characteristic of a 

Type 1 WTG is that it operates at nearly fixed speed with rated 

speed approximately 1% above synchronous speed; thus, the 

power versus speed characteristic (Figure 2b) is very steep.

a) Physical diagram

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.)

b) Power versus speed characteristic

Fig. 2. Type 1 WTG physical diagram and power versus speed characteristic

Another characteristic of this type of turbine is that the 

induction generator absorbs reactive power in the motoring 

mode or in the generating mode. A Type 1 WTG is normally 

equipped with a switched capacitor bank to compensate for the 

reactive power. As the wind speed varies, output level varies, 

and the slip varies between 0% and -1%. The reactive power 

requirement also varies with the slip, and the size of the 

capacitance providing reactive power to the induction 

generator is also varied, so that the output of the induction 

generator is maintained at unity power factor.

Figure 3 shows a single-line diagram describing the 

common layout of a wind power plant connected to a power 

system network. The turbine is electrically connected to the 

power system through a network of cabling, transformers, and 

overhead lines. The interaction between the wind turbine and 

the power system network is very important to the stability of 

the power system, especially when the size of the wind power 

generation is very large (e.g., high penetration levels of wind 

power generation). The interaction occurs both ways. Any 

transients occurring at the transmission line will affect the 

generator and its components. Similarly, any perturbation in 

the wind speed at the turbine site will be reflected in the utility 

grid.

Fig. 3. Single-line diagram for a small system for a Type 1 WTG.

Using FAST and MATLAB/Simulink, a model of the AOC 

AWT-27 turbine [10], a Type 1 WTG, was developed. A brief 

subset of the results is presented here. Because Type 1 WTGs 

are installed infrequently today, the bulk of this paper is 

devoted to modeling the Type 3 WTG behavior (described in 

the next section). However, the modeling of Type 1 turbines 

may be useful in academic settings because these models can 

illustrate many basic principles of wind turbines as well as 

show how far the technology has progressed and how many 

initial problems faced by these turbines have been resolved in 

more-modern designs.

Figure 4 shows that under steady wind conditions, the 

generator torque and the speed of this WTG are affected by the

tower shadow effect caused by the aerodynamic torque 

reduction every time one of the blades passes through the wake 

of the tower, thus creating torque pulsation (three times per 

full rotational angle). Figure 4 also demonstrates the unique 

capabilities of the FAST and MATLAB/Simulink coupling. 

The thrust loading and power output are shown under steady 

wind. As shown, the thrust loading varies because of the tower 

shadow as well. This result would not be obtainable from a 

dedicated power system transient modeling software. The plot 

on the right shows the power also varies because of the tower 

shadow, as expected from the variations seen in the speed and 

torque plots. This tower shadow effect is barely seen in morerecent, advanced turbines, such as Type 3 and Type 4.
3

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5 5.5 6 6.5 7 7.5 8 8.5 9 1400

1500

1600

1700

1800

1900

2000

Time (s)

Gen Torque (Nm)

5 5.5 6 6.5 7 7.5 8 8.5 9 126

126.1

126.2

126.3

126.4

126.5

126.6

126.7

126.8

126.9

127

Time (s)

Elec Rot Speed (rad/sec)

5 5.5 6 6.5 7 7.5 8 8.5 9 25

25.5

26

26.5

27

27.5

28

28.5

29

29.5

30

Time (s)

Thrust load (kN)

5 5.5 6 6.5 7 7.5 8 8.5 9 180

190

200

210

220

230

240

250

Time (s)

Power (kW)

Fig. 4. Simulation results showing the impact of tower shadow on the 

generator torque, speed, thrust load, and power output for a Type 1 WTG.

Figure 5 shows another interesting result. A 2% voltage 

unbalance on Phase A is shown to generate a torque pulsation 

at 120 Hz because of the negative sequence voltage during the 

unbalanced voltage event. Note that the small unbalanced 

voltage creates a large torque pulsation because the negative 

sequence impedance is very low (negative sequence slip ~ 

200%) compared to the positive sequence impedance (positive 

sequence slip ~ 1%).

12.9 13 13.1 13.2 13.3 13.4 1250

1260

1270

1280

1290

1300

1310

1320

1330

1340

1350

Time (s)

Phase A RMS Voltage (Vrms)

12.9 13 13.1 13.2 13.3 13.4 1800

1850

1900

1950

2000

2050

2100

2150

2200

Time (s)

Gen Torque (Nm)

Voltage on Phase A drops

to 0.98 pu

Fig. 5 Simulation results showing the impact of voltage unbalance with 2% 

voltage drop on Phase A for a Type 1 WTG.

These results illustrate the ability of the coupled FAST and 

MATLAB/Simulink models to provide insights into combined 

electrical and mechanical behaviors that would be difficult to 

extract with other software that is solely dedicated to electrical 

modeling or aero/mechanical modeling.

III. VARIABLE-SPEED WTGS

Type 3 WTGs are variable-speed wind turbines with 

doubly-fed induction generators. The DFIG is operated in 

variable-speed mode using a partial-size power converter 

connected to the rotor winding of the wound-rotor induction 

generator (WRIG). The stator winding of the WRIG is 

connected to the grid at a frequency of 60 Hz.

This turbine type is probably the most popular type 

available in the market, and it has been deployed in very large 

quantities. A WTG is normally operated between 30% slip 

(subsynchronous speed) and -30% slip (supersynchronous 

speed), and the converter is typically at about 30% of rated 

power output.

a) Physical diagram of a Type 3 wind turbine

0 0.2 0.4 0.6 0.8 1 0

0.333

0.667

1

Real Power vs Rotor Speed

Rotor Speed (p.u.)

P (p.u.)

b) Power versus speed characteristic for Type 3 WTG

Fig. 6 Type 3 WTG physical diagram and power versus

speed characteristic

Figure 6a shows a schematic of a Type 3 WTG. The power 

converter performs a back-to-back AC-DC-AC conversion 

using two pulse-width modulation–switched voltage-source 

inverters coupled with a DC link. A crowbar circuit is also 

provided as protection, to allow the shorting of the rotor circuit

if necessary. 

A Type 3 WTG has a torque characteristic that is a 

quadratic function of the rotational speed. Type 3 WTGs allow 

maximal extraction of wind power because their power output 

can be electronically controlled to follow the optimal power 

curve. The optimal power curve is a cube function of the 

rotational speed (refer to Figure 6b). If the rotor speed exceeds 

its rated value, the pitch controller must be deployed to limit 

the rotational speed at its rated speed. If the pitch controller 

could not control the aerodynamic power of the wind turbine, a 

WTG may experience a runaway event. Note that the speed 

range of a Type 3 WTG is much larger than the speed range of 

a Type 1 WTG; thus, the kinetic energy stored in the rotating 

blades and other components within the wind turbine is 

sufficiently large, and the output of the generator is not 

impacted as much by the wind fluctuations and turbulence 

because some of the energy is stored and restored in the kinetic

energy of the rotating mass.
4

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9 10 11 12 13 14 15

1600

1700

1800

1900

2000

HSS Torque (NM)

Time (s)

Stiff grid

Weak grid

9 10 11 12 13 14 15

1825

1830

1835

1840

1845

HSS Speed (RPM)

Time (s)

Stiff grid

Weak grid

9 10 11 12 13 14 15

320

340

360

380

HSS Power (kW)

Time (s)

Stiff grid

Weak grid

a) HSS torque, speed, and power

5 6 7 8 9 10 11 12 13 14 15 -500

0

500

Edgewise Moment (kNM)

Time (s)

Stiff grid

Weak grid

5 6 7 8 9 10 11 12 13 14 15

240

260

280

300

320

Flapwise Moment (kNM)

Time (s)

Stiff grid

Weak grid

b) Edgewise and flapwise blade moments at the blade root

Fig. 7 Simulation results showing the impact of a single-phase voltage sag 

for a Type 3 WTG on high-speed shaft torque, speed, and power, as well as on

edgewise and flapwise blade moments at the blade root

Using FAST and MATLAB/Simulink, a model of the GRC 

750-kW turbine [11], a Type 3 WTG, was developed. The 

model was used to demonstrate that turbines connected in stiff 

grids will experience slightly different transient behavior than 

turbines connected in weaker grids. These demonstrations may 

have a value in determining turbine operations and 

maintenance schedules or turbine life. In the simulations, at 

t=10s, a voltage sag occurred. (Both single-phase unbalanced 

and three-phase balanced faults are modeled.) The sag 

dropped the grid voltage from 1 p.u. to 0.1 p.u. The sag 

persisted for nine cycles (150 ms) and then cleared. The 

simulations were carried out for relatively strong and weak 

grids, and the results were plotted together. Note that the weak 

grid was simulated by doubling the line impedance of the grid. 

The wind speed was held steady at 12 m/s, below the rated 

speed, so pitch control was inactive.

Figures 7a and 7b show some results from simulation of a 

single-phase sag. Figure 7a shows torque, speed, and power at 

the high-speed shaft. The sag caused an approximately 2.5-Hz 

oscillation to occur that persisted long after the fault cleared, 

indicating that some mechanical oscillation mode within the 

drivetrain was likely excited. The plots also show that the 

amplitude of the oscillation was greater for the weak-grid case, 

potentially leading to more damage to the high-speed shaft and 

drivetrain over the life of a turbine. Although Figure 7a is 

informative, Figure 7b shows the true potential of the FAST 

and MATLAB/Simulink coupling. Figure 7b shows the 

edgewise and flapwise blade moments at the blade root during 

the single-phase sag. As shown, the sag produced no 

noticeable responses between the turbine connected to the 

weak grid and when it is connected to a stiff grid. It is also 

shown that the strength of the grid connection had no impact 

on these moments. These insights would be difficult to obtain 

without the coupled FAST and MATLAB/Simulink model.

9 10 11 12 13 14 15

-1000

0

1000

2000

3000

4000

HSS Torque (NM)

Time (s)

Stiff grid

Weak grid

9 10 11 12 13 14 15 1700

1800

1900

2000

HSS Speed (RPM)

Time (s)

Stiff grid

Weak grid

9 10 11 12 13 14 15

-200

0

200

400

600

800

1000

HSS Power (kW)

Time (s)

Stiff grid

Weak grid

Fig. 8 Simulation results showing the impact of a three-phase voltage sag 

on a Type 3 WTG for the high-speed shaft torque, speed, and power
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Figure 8 shows results for the three-phase sag. In this case, 

the transient was much more severe. The torque swing was 

near an order of magnitude greater than in the single-phase 

case (compare Figure 8 to Figure 7a). Similar swings can be 

seen in the speed and torque, indicating the severity of the 

event. It is also shown that the level of grid stiffness affects the 

response both in frequency and damping. Note that this was a 

worst-case scenario: in this case, the crowbar was inactive; 

thus, the crowbar did not operate to limit the rotor currents and 

the turbine was exposed to the full intensity of the event. A 

noticeable difference can also be seen between the weak-grid 

and stiff-grid cases: a phase shift occurred in the weak-grid 

case for which the cause was unknown. Again, the oscillations 

had higher amplitude in the weak-grid case.

5 6 7 8 9 10 11 12 13 14 15

-500

0

500

Edgewise Moment (kNM)

Time (s)

Stiff grid

Weak grid

5 6 7 8 9 10 11 12 13 14 15

240

260

280

300

320

Flapwise Moment (kNM)

Time (s)

Stiff grid

Weak grid

Fig. 9 Simulation results showing the impact of a three-phase 

voltage sag on a Type 3 WTG for the edgewise and flapwise blade 

moments at the blade root.

Figure 9 shows that in the three-phase fault case, in contrast 

to the single-phase case shown in Figure 7b, the severity of the 

three-phase fault actually led to noticeable oscillations in the 

edgewise and flapwise moments at the blade root. As shown in 

Figure 8b, the system with a stiff grid responded more 

favorably than the system connected to a weak grid. Although 

the models display the ability to provide valuable insights into 

turbine mechanical and electrical coupled transients, the 

reliability of the results cannot be confirmed yet. In further 

work, efforts will be made to validate results obtained from 

coupled FAST and MATLAB/Simulink models using real 

field measurements. Potential methods to mitigate stresses 

using advanced controls will also be studied [12,13].

Figure 10 shows the x-, y- and z-axis moments at the base of 

the tower during the same single-phase fault event as in Figure 

8. Here the x-axis is parallel to the wind direction, y-axis is 

perpendicular but in the same plane, while the z-axis is 

perpendicular out of the plane. It can be seen that the event 

does indeed cause some small variations in the moment forces 

on the tower. However, these variations are not significant in 

the sense that their amplitude is small and thus they do not 

appear to be a reliability issue. It can also be seen that the 

weakness or stiffness of the grid does not make a significant 

difference in the behavior of the turbine.

5 6 7 8 9 10 11 12 13 14 15

50

100

150

200

250

300

x moment (kNM)

Time (s)

Stiff grid

Weak grid

5 6 7 8 9 10 11 12 13 14 15

3750

3800

3850

y moment (kNM)

Time (s)

Stiff grid

Weak grid

5 6 7 8 9 10 11 12 13 14 15

0

5

10

15

z moment (kNM)

Time (s)

Stiff grid

Weak grid

Fig. 10 Simulation results showing the impact of a single-phase voltage sag 

on a Type 3 WTG for the x-, y- and z-axis moments at the tower base.

Figure 11 shows the x-, y- and z-axis moments at the base of 

the tower during the same three-phase fault event as in Figure 

9. It can be seen that the event does cause significant 

variations in the moment forces on the tower. The amplitude of 

the mechanical oscillations is significant and could potentially 

pose a reliability issue if often repeated. In this case, the 

oscillations have a higher amplitude when the grid is stiff as 

opposed to when the grid is weak. This is a contrary result 

from that observed in Figure 9 when it was seen that the 

oscillations in the edgewise and flapwise blade root moments 

were more pronounced when the grid was weak. This indicates 

that the coupled electrical and mechanical model can provide 

insights that may lead to superior turbine designs in the future 

or at the very least to better reliability analyses.
6

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5 6 7 8 9 10 11 12 13 14 15

-1000

-500

0

500

1000

1500

x moment (kNM)

Time (s)

Stiff grid

Weak grid

5 6 7 8 9 10 11 12 13 14 15

3750

3800

3850

3900

y moment (kNM)

Time (s)

Stiff grid

Weak grid

5 6 7 8 9 10 11 12 13 14 15

-40

-20

0

20

40

60

z moment (kNM)

Time (s)

Stiff grid

Weak grid

Fig. 11 Simulation results showing the impact of a three-phase voltage sag

on a Type 3 WTG for the x-, y- and z-axis moments at the tower base.

IV. CONCLUSIONS AND FUTURE WORK

This demonstration of the ability of the coupled FAST and 

MATLAB/Simulink models to provide insights about the 

effects of strong and weak grid connections is but one example 

of the potential of this coupling. The effects of electrical faults 

on mechanical components (gearbox thrust loading, tower 

dynamics, etc.); effects of mechanical oscillations on output 

power/current/voltage; effects of voltage unbalance; benefits 

of soft-starting and other power electronic–based damping 

mechanisms; converter-control effects on mechanical systems 

(and other controller interactions); and effects of wind gusts 

and turbulence on electrical systems will be studied as part of 

this research.

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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