A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Development and Validation of 

WECC Variable Speed Wind 

Turbine Dynamic Models for 

Grid Integration Studies 

M. Behnke 

BEW Engineering 

A. Ellis 

Public Service Company of New Mexico 

Y. Kazachkov 

Siemens PTI 

T. McCoy 

Global Energy Concepts 

E. Muljadi 

National Renewable Energy Laboratory 

W. Price and J. Sanchez-Gasca 

GE Energy

Presented at AWEA’s 2007 WindPower Conference 

Los Angeles, California 

June 4–7, 2007 

Conference Paper 

NREL/CP-500-40851 

September 2007 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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DEVELOPMENT AND VALIDATION OF WECC 

VARIABLE SPEED WIND TURBINE DYNAMIC MODELS 

FOR GRID INTEGRATION STUDIES 

Michael Behnke1, Abraham Ellis2, Yuriy Kazachkov3, Timothy McCoy4, Eduard Muljadi5, 

William Price6

, Juan Sanchez-Gasca 

Abstract-- With high wind penetration levels being planned in 

North America, the need for grid operators to quickly assess the 

impacts of wind generation on system stability has become 

critical. In the planning phase, this assessment is normally done 

with positive sequence phasor time-domain analysis tools such 

PSS/E or PSLF, which allow for simulation of the dynamic 

response of a power system to major disturbances (e.g., short 

circuits). The lack of suitable dynamic models for the wide 

variety of wind turbines available in the marketplace has been an 

obstacle in performing accurate analyses of this type, though 

efforts led by the Western Electricity Coordinating Council 

(WECC) to develop industry-standard wind turbine models are 

addressing this issue. Still, the level of model complexity that is 

appropriate and necessary for these studies, particularly with 

regard to the wind turbine rotor and electromechanical 

drivetrain dynamics, is a subject of debate. 

This paper describes reduced-order, simplified wind turbine 

models developed under the leadership of the WECC Modeling 

& Validation Working Group. These models were developed for 

analyzing the stability impact of large arrays of wind turbines 

with a single point of network interconnection. Dynamic 

simulations have been performed with these models, and 

comparisons made with results derived from higher-order 

models used in manufacturer-specific representations of aero 

conversion and drivetrain dynamics. The paper concludes with 

an assessment of whether the simplified models impact the 

accuracy of the electrical model outputs when viewed from the 

point of interconnection. 

I. INTRODUCTION

he WECC Modeling & Validation Working Group 

recently initiated an effort to develop and validate a series 

of generic dynamic models for wind turbine generators 

(WTG). The objectives of this effort are to 1) allow 

performance of transient stability studies in early stages of 

interconnection process when WTG manufacturer/model may 

be undetermined, 2) reduce WTG manufacturer 

confidentiality concerns with respect to proprietary aspects of 

dynamic models, and 3) improve the quality, portability 

(between simulation platforms), and usability of models, 

consistent with the level of accuracy expected in an initial 

system impact evaluation. 

Generic models are being developed for four major WTG 

topologies. The first topology, referred to as a Type 1 WTG, 

is shown in Figure 1. This machine is pitch-regulated, and 

 

1

 M. Behnke is with BEW Engineering, San Ramon, CA. 

2

 A. Ellis is with Public Service Co. of New Mexico, Albuquerque, NM. 3

 Y. Kazachkov is with Siemens PTI, Schenectady, NY. 

4

 T. McCoy is with Global Energy Concepts, Seattle, WA. 

5

 E. Muljadi is with National Renewable Energy Laboratory, Golden, CO. 

6

 W. Price and J. Sanchez-Gasca are with GE Energy, Schenectady, NY. 

drives a squirrel cage induction generator which is directly 

coupled to the grid. 

Fig. 1. Conceptual block diagram of WECC Type 1 WTG 

The Type 2 WTG shown in Figure 2 is a variation on the 

Type 1, operating with variable slip. It utilizes a wound rotor 

induction generator whose rotor winding is brought out via 

slip rings and brushes. An external rotor resistance is 

electronically modulated to effect dynamic changes in the 

machine’s torque-speed characteristics. 

Fig. 2. Conceptual block diagram of WECC Type 2 WTG 

The doubly fed induction generator (DFIG), or partial 

conversion, topology of Figure 3 is designated as WECC 

Type 3. The turbine is pitch-regulated and features a wound 

rotor induction generator with an AC/DC/AC power converter 

connected between the rotor terminals and grid. The 

generator stator winding is directly coupled to the grid. The 

power converter in the rotor circuit allows for independent 

control of generator torque and flux, providing fast active and 

reactive power control over a wide range of generator speeds. 

T

1
Fig. 3. Conceptual block diagram of WECC Type 3 WTG 

Finally, the full conversion topology of Figure 4 is 

designated as WECC Type 4. The turbine is pitch-regulated 

and features an AC/DC/AC power converter through which 

the entire power of the generator is processed. The generator 

may be either induction or synchronous type. As with the 

Type 3 WTG, the power converter allows for independent 

control of quadrature and direct axis output currents at the 

grid interface, providing fast active and reactive power control 

over a wide range of generator speeds. 

itional (linear) 

tu

linearization of the power coefficient around initial operating 

Fig. 4. Conceptual block diagram of WECC Type 4 WTG 

In this paper, specific subsystem model development and 

validation exercises for two particular WTG types are 

described. Section II details the scope of the investigation. In 

section III, existing models are examined and validated 

against higher order models. Finally, conclusions are 

summarized in Section IV. 

II. SCOPE OF INVESTIGATION 

The objective of this study was to determine if simplified 

and generic aerodynamic and mechanical drive train models 

could be applied to a range a commercially available Type 3 

and Type 4 WTGs. These topologies have the highest priority 

to the WECC TSO membership based on the present state of 

interconnection queues. Due to the fast dynamics of the 

power electronics, it is relatively straightforward to generalize 

the generator/converter models in these machine types. 

However, in the currently available manufacturer-specific 

models, the WTG aerodynamic and mechanical drive train 

representations are quite diverse. 

The applicability of the models is limited to positive 

sequence transient stability studies related to normally cleared 

transmission system faults. These faults have durations of 150 

to 200 ms, with stable events recovering to new steady states 

in 20 to 30 seconds. Constant wind speed during the transient 

is assumed. 

It is recognized that generalization and simplification of the 

representation of the WTG dynamics has a cost with respect to 

accuracy. However, it is WECC’s objective that the level of 

complexity for the generic models be appropriate to the level 

of accuracy needed in an initial System Impact Study. 

Sensitivity analysis with manufacturer-specific models can 

follow later in the interconnection process, if necessary. 

III. MODEL DEVELOPMENT AND VALIDATION

A. Generic WTG Aerodynamic Model 

The primary motivations for developing generic 

aerodynamic models are that: a) WTG airfoil characteristics 

are manufacturer-specific and proprietary, and b) the 

coefficients of aero power and torque are highly non-linear 

functions of pitch angle and rotor tip speed ratio. Current 

practices on modeling WTG aerodynamics for transient 

stability studies generally involve the use of Cp(λ,θ) curves 

such as those shown in Figure 5. The coefficient of power, 

Cp, on the vertical axis represents the fraction of mechanical 

power that may be extracted from the available power in the 

free stream wind velocity by the WTG rotor. The horizontal 

axis is the rotor tip speed ratio, λ, the ratio of the speed of the 

tip of the rotor to the wind speed. As the pitch angle, θ, 

increases, the fraction of power transferred from the wind to 

the rotor decreases. For initial conditions with wind speeds 

producing less than rated power, θ will be close to zero 

degrees, and the turbine operates at a constant tip speed 

corresponding to peak Cp. Above rated wind speed, θ is 

increased to shed mechanical power. 

0 2 4 6 8 10 12 14 16 18 20

0

0.1

0.2

0.3

0.4

0.5

λ

Cp

θ=1o

θ=3o

θ=5o

θ=7o

θ=9o

θ=11o

θ=13o

θ=15o

Fig. 5. Typical Cp curves for variable-speed, variable-pitch WTG 

Under current industry practices, these manufacturerspecific Cp curves are typically fitted to high-order 

polynomial approximations which require dozens of 

coefficients to characterize due to their extreme non-linearity. 

This modeling process is overly burdensome to the 

transmission planner who is unfamiliar with wind turbine 

technology and accustomed to simulating traditional turbinegenerators. Thus, it is desirable to find a replacement for 

these Cp(λ,θ) curves in the form of a trad

rbine-governor model. 

MW-class WTG have rotor inertia constants on the order of 

several seconds. Thus, the rotor speed change, and hence Δλ, 

is relatively small for the disturbances of interest in a typical 

transient stability study. This suggest a possibility of 

2
points. This approach has been shown to be practical with 

one manufacturer’s Cp curves in a previous paper [1]. 

In this study, the power coefficients for rotors from three 

commercially available variable speed turbines and one paper 

design were analyzed by taking the partial derivatives of Pmech

with respect to θpitch and ωrotor. The relationship between 

dPmech/dθpitch and θpitch for each rotor is shown in Figure 6. 

The resulting approximately linear relationship of 

dPmech/dθpitch to θpitch for each rotor is consistent with that of 

Reference 1. 

Partial Derivative of Aerodynamic Power w.r.t. Pitch

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0 5 10 15 20 25 30

Blade Pitch, deg

pu/deg

Turbine 1

Turbine 2

Turbine 3 - WindPact 2.5 MW

GE 1.5 MW

 

Fig. 6. dPmech/dθpitch as a function of θpitch for example Type 3 and 4 WTGs 

Next, the partial derivatives of Pmech with respect to ωrotor 

were taken. The relationships between dPmech/d ωrotor and 

θpitch for three of the rotors are shown in Figure 7. Over most 

of the pitch range, small changes in ωrotor result in even 

smaller changes in Pmech. 

Partial Derivative of Aerodynamic Power w.r.t. Speed

-2.00

-1.50

-1.00

-0.50

0.00

0.50

0 5 10 15 20 25 30

Blade Pitch, deg

Power pu/Speed pu

Turbine 1

Turbine 2

Turbine 3

Fig. 7. dPmech/dωrotor as a function of θpitch for example Type 3 and 4 WTGs 

Finally, a MATLAB® simulation of a grid fault using a twomass drive train model and complete pitch system model for 

one of the WTGs was performed. The change in mechanical 

power applied to the low speed shaft of the drive train with 

respect to the initial power was the sum of the partial 

derivatives with respect to pitch angle and rotor speed derived 

in Figures 6 and 7. Time domain plots of mechanical power 

and rotor speed for this disturbance are shown in Figures 8 

and 9. In each plot, two cases are considered: 1) dPmech/dωrotor

corresponding to Figure 7, and 2) dPmech/dωrotor = 0. It can be 

seen that very little accuracy is sacrificed by ignoring the 

dPmech/dωrotor term. This may be attributed to the small 

change in tip speed that occurs during the disturbance due to 

the long inertia constant (several seconds) of the WTG rotor. 

Fig. 8. Time response of Pmech with and without dPmech/dωrotor term 

Fig. 9. Time response of ωrotor with and without dPmech/dωrotor term 

B. Generic WTG Drivetrain Model 

The motivations for the development of generic WTG 

drivetrain models are similar to those for the generic 

aerodynamic models. A wide variance exists in the level of 

detail provided by individual WTG manufacturers (e.g., rigidmass versus multi-mass representations) in current models. 

The fast torque response of the power electronics in Type 3 

and Type 4 machines allows for damping of various natural 

frequencies present in the drivetrain to prevent WTG damage, 

but these damping algorithms are highly manufacturerspecific and proprietary. None of this bodes well for the 

transmission planner unfamiliar with wind turbine technology. 

One mode present in all MW-class WTGs is the torsional 

mode of the low speed shaft connecting the rotor to the 

gearbox (or to the generator in a direct-drive machine). The 

two-mass representation of the drivetrain shown in Figure 10 

captures these dynamics. Mechanical torque Tmech is applied 

to the rotor inertia to produce rotor acceleration. Likewise, 

electrical torque Telec is applied to the generator inertia to 

resist generator acceleration. The resulting difference in rotor 

and generator speed causes wind-up of the mainshaft in 

proportion to torsion coefficient K. The natural frequency is a 

function of this torsion coefficient and the rotor and generator 

inertias. 

Fig. 10. Two-mass, compliant-shaft drivetrain representation 

Some natural damping (Dshaft) acts to resist this speed 

difference, but design of an efficient drivetrain causes this 

damping to be negligible in MW-class variable speed WTGs. 

Instead, this mode (and possibly others) are damped via a 

component of torque (Tdamp) produced by the power 

electronics. It is desirable to find a way to replace the 

Rotor Speed

0.95

0.97

0.99

1.01

1.03

1.05

0 1 2 3 4 5 6 7 8 910

Tim (seconds)

Speed (pu)

e

Non zero dP/dOmega

dP/dOmega = 0

Rotor (Aero) Power

1.005

1.010

1.015

1.020

0 1 2 3 4 5 6 7 8 910

Ti (seconds)

Power (pu)

Non zero dP/dOmega

0.980

0.985

0.990

0.995

1.000

dP/dOmega = 0

me

3
manufacturer-specific damping algorithm with a generic 

damping function, or to eliminate it altogether by replacement 

of the multi-mass model with a single-mass model. 

To determine the feasibility of generalizing the damping of 

the mainshaft mode, time-domain simulations of the response 

of a commercially available Type 4 WTG to a grid fault were 

performed. The WTG model included a two-mass 

representation of the drivetrain per Figure 10, as well as the 

manufacturer’s actual algorithm for generating the damping 

torque Tdamp. Time domain plots of rotor speed, generator 

speed, terminal voltage and electrical power for three cases 

are shown in Figure 11. In the top plot, there is no passive 

damping (i.e., Dshaft = 0), and the active damping term Tdamp 

is driven to zero, as well. It can be seen that mainshaft mode 

continues ringing even 10 seconds after the fault has been 

cleared. In the center plot, Dshaft remains zero, but the 

manufacturer’s damping algorithm is enabled, resulting in the 

mainshaft mode being damped within about four cycles of the 

natural frequency. Finally, in the lower plot, active damping 

has been disabled, and Dshaft is increased from zero to 0.5 pu. 

While far from exact, it can be seen an approximate response 

of manufacturer-specific active damping may be obtained 

through proper selection of a passive damping coefficient. 

Fig. 11. Time domain response of two-mass, compliant-shaft drivetrain with 

three different mainshaft mode damping models 

A further investigation undertaken as part of this study was 

to determine the diversity impact on mainshaft mode power 

oscillations when using an equivalent model of a plant 

consisting of many Type 3 or Type 4 WTGs and a single point 

of interconnection with the transmission network. It is 

generally accepted that power transients occurring at the 

individual WTG output terminals due to wind turbulence and 

tower shadow effects are effectively statistically filtered in a 

plant with a large number of spatially distributed WTGs [2]. 

Mainshaft wind-up due to these effects can be assumed to 

have poor correlation from WTG to WTG, thus the equivalent 

model of the plant approximates a single mass under these 

pseudo steady-state conditions. 

To determine whether diversity of mainshaft wind-up due to 

these pseudo steady-state effects impacts the response of the 

plant at the point of interconnection for transmission system 

faults, a plant consisting of 12 individual WTGs was modeled 

using the WTG manufacturer’s active damping algorithm. 

Rather than assuming constant wind speed, which would 

result in no mainshaft wind-up as an initial condition on each 

WTG, a series of sinusoidal wind speed disturbances of 

varying magnitude and frequency was superimposed on a 1.0 

pu base (DC) wind speed. For each wind speed disturbance, 

power at the point of interconnection was monitored for two 

different cases: 1) with the same wind speed disturbance 

phase angle at each of the 12 WTGs, and 2) with the wind 

speed disturbance phase angle randomly distributed between 

zero and 360 degrees amongst the 12 machines. 

Typical results are shown in Figures 12 and 13. In this 

case, the wind speed disturbance has a frequency of one hertz 

and a magnitude of 0.2 pu. This magnitude corresponds to a 

turbulence intensity of 0.14, classified as a “medium” level of 

turbulence for a WTG with a rated wind speed of 15 m/s 

according to IEC 61400-1. 

In Figure 12, the upper plot shows generator speed, rotor 

speed and power output for one WTG. The lower plot shows 

the collective power output of all 12 machines. The phase 

angle of the wind speed disturbance is the same at all WTGs, 

so the one hertz power disturbance is reflected directly to the 

point of interconnection. Thus, the initial condition on 

mainshaft wind-up is identical on all 12 WTGs. 

No damping 

Active damping 

Passive damping 

Fig. 12. Single and multiple WTG power output, same mainshaft wind-up 

In Figure 13, the phase angle of the wind speed disturbance 

is randomly distributed amongst WTGs, so the one hertz 

power disturbance is nicely filtered from the plant output, both 

before and after the grid disturbance. In this case, however, 

the mainshaft wind-up from WTG to WTG has poor 

correlation. Despite this, there is practically no statistical 

filtering of the power oscillations associated with the 

mainshaft mode frequency (~ 4 hertz in this case). From these 

results, it may be concluded that the mainshaft windup 

variations associated with typical wind turbulence levels are 

insignificant with respect to the change in windup associated 

with grid fault. Thus, the diversity effect is negligible, and a 

two-mass model is necessary if the mainshaft mode is of 

interest in the particular transmission system under study. 

4
5

Fig. 13. Single and multiple WTG power output, random mainshaft wind-up

IV. CONCLUSIONS 

A generic simplified aerodynamic model for WECC Type 3 

and Type 4 WTGs, with sufficient accuracy for initial 

transient stability analysis, has been developed. The model 

has been validated against a MATLAB® state-space 

simulation. In a future task, WECC will investigate the 

applicability of these results to Types 1 and 2 WTGs. 

A passive damping term for the two-mass drive train model 

has been verified to reasonably approximate the response of 

manufacturer-specific mainshaft mode damping controls in 

Types 3 and 4 WTGs. 

The diversity effect of mainshaft windup has been 

demonstrated to be an insufficient filtering mechanism of the 

mainshaft mode in Types 3 and 4 WTGs. A two-mass 

representation (rather than single rigid mass) is necessary if 

modes in the 2-5 Hz range typical for MW-class WTGs are 

relevant to the system impact being evaluated in the 

simulation. 

These conclusions are being incorporated in the 

documentation being produced by the WECC Modeling and 

Validation Working Group for the generic models of these 

variable speed WTGs [3, 4]. 

V. REFERENCES

[1] Price, W.W., Sanchez-Gasca, J.J., “Simplified wind turbine generator 

aerodynamic models for transient stability studies” 

Power Systems Conference and Exposition, 2006. PSCE '06. 2006 IEEE 

PES,Oct. 29 2006 - Nov. 1, 2006, pp. 986-992. 

[2] E. Muljadi and B. Parsons, “Comparing Single and Multiple Turbine 

Representations in a Wind Farm Simulation”, presented at the European 

Wind Energy Conference, Athens, Greece, Feb. 27 – Mar. 2, 2006. 

[3] Generic Type-3 Wind Turbine-Generator Model for Grid Studies, 

WECC Wind Generator Modeling Group, Version 1.1, Sept. 14, 2006. 

[4] Generic Type-4 Wind Turbine-Generator Model for Grid Studies, 

WECC Wind Generator Modeling Group, Version 1.0, Sept. 15, 2006. 
F1147-E(09/2007) 

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Development and Validation of WECC Variable Speed Wind Trubine 

Dynamic Models for Grid Integration Studies 

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This paper describes reduced-order, simplified wind turbine models for analyzing the stability impact of large arrays 

of wind turbines with a single point of network interconnection developed under the leadership of the WECC 

Modeling & Validation Working Group. 

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