The ERDA/NASA 100 kW experimental wind turbine is modeled on a digital computer in order to study the performance of a wind turbine under operating conditions. Simulation studies of starting, overspeed, and shutdown performance were made. From these studies operating procedures, precautions, and limitations are prescribed

Gilbert, L. J.

English

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Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19760011584 2020-03-22T17:18:46+00:00Z
by Leonard J, Gilbert
Lewis Research Center
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NASA TECHNICAL	 NASA TM X-71864MEMORANDUM
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00	 (NASA — TM—X-71864)	 A 100 kW EXP E RIMENTAL
ti	 WIND TUPPINE: SIMULATION OF STARTING,
OVEPSPEED, AND SHUTLOWN CHARACTERISTICS
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A 100 KW EXPERIMENTAL WIND TURBINE SIMULATION OF
STARING :
 OVERSPEED. AND SHUTDOWN CHARACTERISTICS
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t1, Report No 2. Government Accession No. 3.	 Recipient's Catalog No
T M X -71804
4. Title and Subtnm 5. Rupert Date
A 100 KW EXPERIMENTAL WIND TURBINE: SIMULATION OF
6	 Performina Organization CodeSTARTING, OVERSPEED AND SHUTDOWN CHARACTERISTICS
7. Aug, - 1 0 8. Performing Organization Report No
by Leonard J. Gilbert E-8620
la Work unit No.,
0	 Performing Organization Name and Address
Lewis Research Center
National Aeronautics and Space Administration
11. Contract or Grant No
Cleveland, Ohio 44135
13. Typo of Report and Period Covered
Tec hnical Memorandum12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration 14. Sponsor ing Agency Code
Washington, D.C.	 20540
16. Supplementary Notes
19, Abstract
The ERDA/NASA 100 kW experi:uental wind turbine designed and built near Sandusky, Ohio, by
NASA-Lewis Research Center has been modeled on a digital computer in order to study the per-
formance of a wind turbine under operating conditions. 	 Simulation studies of starting, overspeed
and shutdown performance have been made,	 From these studies operating procedures, precau-
tions, and limitations have been prescribed.
17. Key Words (Suggested by Authorts)l 18. Distribution Statement
Unclassified - unlimited
19.	 Security Classil 	 (of this report) 20.	 Security Classif. (of this panel 21. No. of Pages 22.	 Price'
Unclassified Unclassified
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A 100 KW EXPERIMENTAL WIND TURBINE: SIMULATION OF
STARTING, OVERSPEED, AND SHUTDOWN CHARACTERISTICS
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by Leonard J, Gilbert
Lewis Research Center
SUMMARY
As part of the national wind energy program under the direction of
ERDA, the NASA Lewis Research Center has designed and built a large
experimental wind turbine near Sandusky, Ohio, This wind turbine is
a horizontal-axis, propeller-type machine with a two-bladed 125-foot
rotor, It is designed to gef orate 100 Iff power from a synchronous gen-
erator in an 18 mph wind. The system is intended to be used as a test
bed for the evaluation of various wind turbine component designs for the
national wind energy program,
The power system of the wind turbine has been simulated on a
digital computer using a Continuous System Modeling Program (CSMP)
on an IBM 360 Time Sharing System, Simulation studies of starting,
overspeed, and shutdown performance have been made in order to an-
ticipate the actual performance of the wind turbine. From these studies
operating procedures, precautions, and limitations have been prescribed,
This report describes the results of that analysis. The calculations
showed that feathering the blades in an 18 mph wind can stop rotor rota-
tion in 12 seconds, without causing unacceptably high blade root stresses,
INTRODUCTION
As part of the national wind energy program under the direction of
ERDA, the NASA Lewis Research Center (LeRC) has designed and con-
structed a 100 kW wind turbine at the Plum Brook Station of LeRC near
Sandusky, Ohio, The objective of this experimental v ind turbine project
is to provide, as soon as possible, engineering data for use as a base
r
/	
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2for the entire wind energy program and to serve as a test bed for com-
ponents and subsystems. This project and its statLs are described in
published reports (refs, 1, 2, 3, and 4),
In order to predict and analyze performance of the wind turbine, a
system model. designed for a digital computer has been developed. The
model facilitates analysis of both transient and steady-state performance
of the system for varied parameters and operating conditions, Among
the initial problems considered in the analysis of the turbine performance
are those involving starting, overspeeding, and shutdown. The results
obtained from studying these three areas with the computer model are
discussed in this report,
ANALYSIS
System Description
The Lewis Research Center wind turbine is a horizontal-axis,
propeller-type machine. A two-bladed, 125 -foot diameter rotor, mounted
on a 100 foot truss tower, drives a 125 kVA synchronous alternator through
a conventional step-up gear box, The rotor is downwind of the tower and
rotates at a constant speed of 40 rpm. The alternator operates at 1800 rpm
and provides 60 IIz three-phase power, The system is rated at 100 kW in
an 18 mph wind. Figure 1 is a sketch of the wind turbine. The details of
the drive train assembly and the yaw system are shown in fig. 2.
Computer Model
In order to predict the starting, overspeed, and shutdown character-
istics of the system, a computer model of the wind turbine was used. The
computer simulation provides a means of calculating the time-dependent
behavior of the wind turbine during rotor accelerations and decelerations
resulting from aerodynamic and inertial forces. In the model, the system
is partitioned into three subsystems; The rotor, the torque load, and the
pitch control. These subsystems are depicted in fig. 3.
Rotor, - The usable torque, Tin, developed at the rotor shaft is a
rpu
c 1,
r
1
t.a
3i
IJ
j
3
function of wind velocity, V`y , blade pitch 0, and rotational velocity, M
Tin = f(Vw' 0, 52)
Torque load, - The torque load on the rotor represents the system
inertial and damping torques as well as the torques produced by the
alternator electrical load and system losses. Thu, the torque balance
of the system can be expressed by a differential equation equating the
torque developed by the rotor to the sum of the torque loads. This gives:
Tin = Jeg6 + 13 egQ + NTL + L	 (1)
where,
`feq	 inertia of the complete rotating system
Beq	 damping coefficient of the complete rotating system
TL	 torque load of alternator
N	 gear ratio between high and low speed shaft
L	 torque losses of system
S2	 rotor rotational speed
.9
52	 rate of change of rotor ratational speed with time
Pitch control. - The rotational speed of the turbine is controlled by
varying the pitch of the rotor blades. The pitch control is effected by a
hydraulically powered control which varies the blade pitch in accordance
with a feedback signal from the output shaft. The controller used for
pitch control of the 100 kW wind turbine provides a proportional plus
integral output signal in response to an input. The model assumed for
the pitch control is described by the operational equation:
0(s) ^ CK1 + s2) CT s1+ 1) n e(s)	 (2)P
where
s	 Laplace operator
0(s)	 pitch angle of blade
's
t
)
't
1n
I	
.r y
4
T 	
control hydraulic actuator time constant
62 e(s)	 rotor rotational speed error
Kt	controller proportional gain
K2	controller integral gain
The control subsystem gains used for the analyses were K 1
 = 5. 0
and K2 = 0.5. These values were determined by the computer study
undertaken to determine gain values for satisfactory transient response
of the wind turbine to wind gusts. They are the controller gains used in
the wind turbine.
The dynamic response of the hydraulic subsystem was represented
by the factors .l in equation (2), where Tp = 0. 1 second. This
Tp
value was considered to be representative of the actual hardwares used
in the system.
Computer Program
To facilitate the analysis of this system on a digital computer, the
IBM System/360 Continuous System Modeling Program (CSMP) was used.
CSMP readily handles nonlinear and time-variant sets of differential
equations, Reference 5 describes CSMP.
The principal nonlinear feature of a wind turbine, as far as the
useful output is concerned, is the torque developed by thn, rotor. This
torque is a function of the blade pitch, wind velocity, and rotational speed
as well as the blade configuration, To generate values of the wind tur-
bine developed torque for varying input conditions, use has been made of
a computer program developed by Wilson and Lissaman (ref. 6). This
Fortran program, written a.l. Oregon State University, calculates wind
turbine performance relatively rapidly. It uses a Simpson's Rule/three
pass method of numerical integration. The program has been modified
to return only that output pertinent to the studies described herein, and
the computer time has been reduced appreciably Fora given propeller
v^
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Rgy,S	 ^^.
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J	 geometry, this program calculates the torque developed by the pro-
f 	 peller as a function of wind speed, rotor speed, and blade pitch.
I I
RESULTS AND DISCUSSION
Starting
li
A set of torque-pitch characteristics of the 100 kW wind turbine is
illustrated in fig. 4. Tins figure is a plot of the torque developed by the
rotor as a lunction of the blade pitch angle for a 10 mph wind speed.
Rotor rotational speed is the parameter, The blade pitch angle varies
from - 1900 toward 00 , representing the change from a feathered condition
to a power condition. Because the blade has twist, the pitch is referenced
to the pitch at the 3f4 radius point. In order to start rotation, it is neces-
sary to "pitch-up" - that is, move the blade out of the feathered position.
In the feathered position, no net lift is produced, and no torque is developed.
As the blade is ''pitched up", there is sufficient blade lift produced to
rotate the propeller. As the rotational speed increases, and the blade
pitch angle moves away from feather, the torque produced increases under
certain conditions of pitch and speed. Since the torque increases only
under these certain conditions, the rate at which the blade is "pitched up"
is not arbitrary, Examinatior of fig. 4 will show that there are conditions
of speed and angle fur which the torque developed is negative. These con-
ditions represent a deceleration of the rotor rather than an acceleration.
Too rapid an angle charge results in stalling the blades. Thus, a controlled
pitch change program for starting is necessary.
One manner of prograinmed start is to vary the pitch at a fixed rate
from the feathered position until a rotational speed of 40 mph (rated speed)
is reached. When 40 rpm is reached, the fixed speed controller is actuated,
providing closed loop speed control A rapid pitch rate is desirable in
order to avc;id lingering at rotational speeds that excite system resonances.
However, the pitch rate is limited b y the rotor aerodynamic characteristics.
Too rapid a pitch rate leads to blade stalling-
A parametric study was made to predict the time required for the
100 kW wind turbine rotor to accelerate from standstill to 40 rpm if the
Y
r	 d	 I_
6blade pitch is advanced at a steady rate from the feathered positions
Figure 5 shows the reduction in time to reach 40 rpm as the pitch-
ing rate is increased. The right: hand termination of the curves occurs
at the pitching rate at which the blades stall and the torque oil
	
rotor
would act to decrease rotor speed, Figure 3 shows time to reach 40 rpm
for 2 wind speeds of 10 mph and 18 mph, these times vary from 158 seconds
to 40 seconds respectively,.
Another nonlinear feature of the wind turbine torque characteristics
Is indicated in fig, 6. As the wind speed increases from 10 to 18 mph,
the startup time decreases with respect to the wind speed. This decrease
in time occurs more rapidly at the lower wind speeds than at the higher
wind speeds.
The variation of the startup rate with rotor inertia, Jeq, is shown in
fig. 7. As expected the higher inertia rotor takes longer to reach 40 rpm,
but the difference is relatively small at the higher wind speeds.
The acceleration of the rotor for this mode of constant pitching rate
starting is significant because this acceleration increases rapidly as the
rotational velocity increases. Figure 8 is indicative of the manner in
which the rotor can be expected to accelerate for a pitch rate of 10/sec
in an 18 -mph wind velocity. The rotor torque producing this acceleration
is shown in fig, 9.
Overspeed
Pitch control failure during operation of the wind turbine can result
in excessive overspeeding of the rotor. The torques produced by the wind
turbine in high wind and rotor overspeed conditions require a braking sys-
tem to prevent excessive overspeed and possible rotor damage.
In order to predict the performance of the wind turbine in these
critical circumstances, a severe failure mode was modeled on the com-
puter and the system behavior was studied It was assumed that the
pitch control failed and the pitch remained fixed at the rated power condi-
tion as the wind speed increased, Specifically, the conditions assumed
were a wind increasing in speed from rated speed of 18 mph to a speed of
f
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7
36 mph at a lmpli,see rate while the blades remainLd fixed and tnc sys-
tem was carrying rated load The oblcctivc of the investigation was to
t	 determine the braking torque required to prevent the rotor front accel-
erating to destructive speeds To simulate the bralce a step load torque
was applied to the systcni.
For the assumed conditions, if the torque load (braking torque) is not
i	 large enough, the rotor will rapidly accelerate to lugh speeds. This per-
formance is illustrated in fig, 10. For the problem illustrated, the rated
load was 21,400 lb-ft at rated speed. As the wincl speed increased (with a
fixed pitched propeller), the rotor speed increased. When rite speed
reached 45 rpm, a simulated braking torque was applied to the output
shaft.: A braking torque of 3 tames rated (64, 200 lb-ft) was not adequate
to stop the acceleration (fig. 10), A h rque of 3 5 times rated (74,900 lb-ft)
was required to stall the blades and bring the rotation to a halt, As the
brake is applied and the rotational speed decreases, the blades stall because
the blade pitch angle is too flat for (lie slower speed, The inertial energy
of the rotor at the time the brake is applied must be absorber] by the brake
and its surroundings. In the example shown this energy is 1 11x106.ft-ibs,
This response is illustrated in fig, 10, The changing torque produced in
each instance is shown in fig. 11, Yawing the: rotor out of the wind, that is,
orienting the rotor so that the wind velocity is not perpendicular to the
plane of rotation of the blades, is also a means of rotor speed control.
However, yawing the 100 KW research wind turbine is not an effective means
of preventing overspeed in the event of a blade pitch failure because the
yawing rate available (1 0', see) is limited. The changing torque-producing
component of the wind velocity as the rotor is you cd is indicated in fig. 12.
There was virtually no effect oat the rotor speed performance when a yawing
procedure was included in the simulation.
Emergency Shutdown
In emergency conditions, the rotor blades are feathered (or pitched
to the no-power position) at a constant rate of S o, sec,
8This action produces reverse thrust on the rotor and higher (hall usual
blade root moments This rapid feathering was simulates( to predict the
rotor response.. Tile rotor performance shown in the computer traces of
fig, 13 is for the rapist feathering action from all 	 condition of
40 rpm, no load, in a 24 mph wind,
The rotor speed decreases is shown in fig, 13(a) as the blade pitch
moves toward feather at the fixed rate illustrated in fig, 13(b). As a
result of the increased pitch, the rotor develops the negative torque
(fig, 13(c)) which decelerates (lie rotor. The rapid slowing of the rotor
produces a corresponding negative thrust shown in fig. 13(d).
To summarize, these calculations have shown that the rapid feather-
ing can shut the rotor down in less than 12 seconds. Further, although
the blade root loads for this condition are higher lhai, usual, they are still
within the design load case limits
CONCLUDING REMARKS
The 100 kW wind turbine designed and built by NASA LeRC near
Sandusky, Ohio, has been modeled or, a digital computer in order to study
the performance of this kind of power system under operating conditions,
Simulation ;alculations of starting, overspeed, and shutdown have shown
the highly nonlinear characteristics of the wind turbine developed torque.
The following specific information has been determined from the simula-
tion calculations.
1. The wind turbine ' pitch up - the rate of pitching the blades toward
the maximum power position - must be c rintrolled and limited during start-
ing to avoid stalling the blades.
2- The braking torque required to stall the rots r under conditions of a
fixed blade pitch and all 	 winch speed is critical. For this particu-
lar 100 kW wins( turbine it must be al leas! 3.5 times rated torque.
3. Rapid feathering of the blades for an emergency shutdown results
in stopping the rotor in less than 12 seconds when operating initially at
40 rpm in a wind speed of 24 nipin. This condition produces the maximum
loads on blade roots, but these loads do not exceed the allowable blade
design loads,
x=y':
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9 a
REFERENCES
1, Thomas, R,; et al.: Plans and Status of the NASA-Lewis Research
Center Wind Energy Project. 	 NASA TM X-71701, 1975•
2. Puthoff, Richard L.; and Sirocky, Paul: The 100-kW Experimental
Wind Turbine Generator Project. 	 NASA TM X-71758, 1975.
3. Thomas, Ronald L,; and Sholes, John E.: Preliminary Results of the
r Large Experimental Wind Turbine Phase of the National Wind Energy6	 L
Program.	 NASA TM X-71796, 1975.
4. Puthoff, Richard L•; and Sirocky, Paul J.: Preliminary Design of a S
100 kW Wind Turbine Generator.	 NASA TM X-71585, 1974.
5. System/360 Continuous System Modeling Program User's Manual,
GI-I20-0367-4.	 5tli ed. , International Business Machines Corp., 1972.
6, Wilson, Robert E.; and Lissaman, Peter B. S.: Applied Aerodynamics
i
of Wind Power nachines.	 Oregon State Univ., 1974.
J r
s7.5 M (125 FT)
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WIND —mO
30 M (100 FT)
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Figure 1. -100-kilowatt experimental wind turbine,
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A 100 kW experimental wind turbine: Simulation of starting, overspeed, and shutdown characteristics

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