CA1120538A

CA1120538A - Multi-mode control system for wind turbines

Info

Publication number: CA1120538A
Application number: CA000330730A
Authority: CA
Inventors: Joseph M. Kos; John P. Patrick; Kermit I. Harner
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1978-08-17
Filing date: 1979-06-27
Publication date: 1982-03-23
Also published as: DE2961688D1; FI792440A; EP0008584B1; IN151737B; ZA793946B; NO792639L; FI76627B; FI76627C; US4193005A; KR830001519A; DK344079A; NO158557B; NO158557C; BR7904968A; IL57944A0; JPS6345509B2; AU4963079A; AU526931B2; IL57944A; KR840002220B1

Abstract

U. S. Ser. No. 934,65 MULTI MODE CONTROL SYSTEM FOR WIND TURBINES

ABSTRACT OF THE DISCLOSURE
A multi-mode electronic wind turbine control system establishes a reference blade angle for a two-bladed, horizontal axis, variable pitch wind turbine rotor, the blade angle being regulated by a hydraulic pitch change mechanism. The rotor, via appropriate shafts and a gear-box, is coupled to a synchronous generator to produce electrical energy which is fed to a power utility grid.
The control system provides closed loop pitch control for rotor acceleration rate during start-up, for rotor de-celeration rate during shutdown, for speed control when the synchronous generator is off-line, and for power con-trol when the synchronous generator is on-line. A single control integrator is used for all closed loop operating modes, with a rate limiter circuit in front of the control integrator to prevent integrator overtravel. The inte-grator has maximum and minimum blade angle stops, the minimum stop being variable as a function of rotor speed and wind speed. When on-line, power control is provided by a unique shaft torque control using proportional plus integral plus derivative controls in combination.

Description

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BACI~GRO ND OF THE INVENTION
Field of Inve~tion - This invention relates to a con-trol system for a variable pitch horizontal axis wind energy conversion system which utilizes wind energy to produce electrical power, and specifically to a closed loop control system which automatically mod~lates the pitch angle of the wind turbine blades to provide safe operation and optimize the energy capture from the wind.
The wind turbine rotor drives a synchronous generator which is connected to supply elec;~rical power to a power utility grid, the control system being adapted to rapidLy modulate the blade pitch angle to min~mize the effects of wind gusts and turbulence to maintain desired electrical frequency and phase, and to reduce stress on the rotor ~ ;
blades and other mechanical components.
Description o~ the Prior Art - The recent energy .
crisis and the high cost of fossil fuels has resulted in renewed interest in and development o large scale wind twrbines, in e~fect large windmills 7 which rotate and produce energy in response to natural wind currents. The ; rotor blades of the wind turbines are up to 300 feet in diameter, and the output power when converted into elec-trical energy is measured in megawatts. While in some wind turbines the output power is used directly to power mechanical devices, in most installations the rotary energy is converted into electrical power which may be stored, used to power isolated instalLations directly, or ~k ~2~

fed to power utility grids for ultimate distribution.
A major problem with wind turbines is that of controlling the electrical output phase and frequency in the presence of unpredictable wind gusts and turbulence, particularly when the wind turbine is driving a synchron-ous generator connected to a large electrical network. At high wind velocities, even mild wind turbulence conditions can create shaft torque fluctuations of sufficient magni-tude to cause the synchronous generator to pull out of phase synchronization and be disconnected from the grid.
In an attempt to overcome this problem, variable pitch angle rotor blades have been provided for use in wind turbines with a rapid acting control system to position the blade angle in response to selected operat-ing parameters such as wind velocity, rotor speed and output power. A representative control sy~tem of thi~
type is disclosecl and claimecl in copendin~ Canaclian applicatio~s 5erial Nos. 328,839 and 327,869, to which reference may be made for a complete e~planation of such control systems. Briefly, the prior art control systems, referenced supra, control the rotor blade pitch angle in - a closed loop manner to maintain constant speed or power depending on whether the generator is off-line or on-line, and in an open loop manner during start-up and shutdown.
The present invention is an improvement of the control system referenced supra, and discloses a wind tur-bine rotor blade pitch angle control which provides im-proved dynamic performance and transient response charac-teristics. The improved operation is obtained by closed loop control of rotor acceleration rate and deceleration rate during start-up and shutdown respectively; by use of a single control integrator for all closed loop operating ; modes~ with a rate limiter in front of the integrator to prevent integrator overtravel, the integrator having maxi-mum and minimum blade angle stops, the minimum stop being variable as a function of rotor speed and wind speed; and by a closed loop shaft torque control for on-line power control, the torque control having proportional, integral and derivative control paths in which rate of change of shaft torque is synthesized from the di~erence between rotor speed and generator speed.
It is there~ore an o~ject of the present invention to provide an improved pitch angle control for a wind turbine which modulates the wind turbine blade angle in ; 20 response to a number of selected operating conditions.
Another object of this invention is an electronic pitch angle control for wind turbines which schedules , blade angle in a closed loop manner to minimize blade stress and shaft torque variations during start~up and shutdown transients~
A further object o~ this invention is a closed loop electronic pitch angle control for wind turbines 3~

which regulates speed when the synchronous generator is off~line and regulates torque when the synchronous gen-erator is on-line.
AnothPr object of this invention is the use of a single con~rol integrator for all closed loop control modes of operation.
A further object o this invention is the use of a rate limiter circuit in front of the control integrator to prevent integrator overtravel.
Another object of this invention is the use of a variable minimum blade angle stop on the electronic con-trol integrator scheduled as a function of rotor speed and wind speed.
A further object of this invention is the use o;
proportional, integral and derivative compensa~ion ~or on line torque control, and proportional plus Lntegral compensation for off-llne speed control.
SUMMARY OF_THE INVENTION
In accordance with the present invention, a two-bladed horizontal axis, variable pitch wind turbine rotor is connected via a low speed coupling shaft, a step-up gearbox, and a high speed shaft to a synchronous generator which converts the rotational energy to a.c. electric power supplied to a power utility grid. An electronic control establishes a reference blade angle ~or a closed ! loop hydraulic pitch change mechanism which varies the pitch of the rotor blades in response thereto. During start-up and shutdown of the wind turbine, a cLosed loop integral control schedules blade angle to maintain the rotor acceleration and deceleration rates at a sa~e level.
During off line operation, a closed loop speed control modulates the blade angle to maintain the desired speed.
During an-line operation a closed loop shaft torque con-trol modulates the blade angle to maintain the desired level of power being supplied by the synchronous generator.
The torque control uses proportional plus derivative plus integral compensation, and the rate of change of shaft torque is synthesized from the difference between rotor speed and generator speed. The speed control uses pro-portional plus integral compensation.
A unique feature of the present invention is the use of a common elec~ronic integrator for each o~ the eLectron-ic closed loop controls which eliminates the need ~or ln-tegrator tracking circuits in each loop. Another important ; feature is the use of a rate limiter placed in front of the control integrator with rate limits set to be equal to or slightly less than the slew rate of the pitch change mechanism to prevent integrator overtravel which can lead to instability. The control integrator is provided with maximum and minimum blade angle stops, the minim~m blade angle stop being variable and scheduled as a function of velocity ratio, which is equal to rotor speed divided by wind speed.
The control system is specifically adapted ~or irnplementation using ~igital electronics, although analog electronic circuitry may also be used In accordance with a particu~ar embodiment of the invention, there is provided, in a control system for a wind turbine, said wind turbine having a wind driven rotor with variable pitch angle blades, the impxovement comprises:
closed loop control means responsive to system operat.ing conditions for producing a control signal indicative of desired rate of change of blade pitch angle, rate limiter means receiving said control signal for maintainlng said control signal within predetermined limits, electronic integrator means for producing from said control signal a signal indicative of desired blade pitch angle, said integrator means including pitch angle limiter means for maintaining said desired blade pitch angle signal within predetermined maximum and minimum limits, and blade pitch change means responsive to said desired blade pitch angle signal for varyi.ng the blade pitch angl~.
In accoxdance with a further ernbodi.ment of the invention, there is provided, in a wind turbine having a wind driven rotor with a plurality of variable pitch angle blades, means for maintaining a substantially constant rate of change of rotor speed during start up and shut down of said wind turbine comprises: means for sensing the actual speed of said rotor and producing a rotor speed signal indi-cative thereof, rate sensor means for producing from said rotor speed signal a signal indicative of the actual rate of change of rotor speed, means producing a reference signal proportional to a rotor speed rate of change limit, comparator means comparing said actual rate of change of 3~

rotor speed signal with said reference signal and producing an error signal proportional to the difference therebetween, and means responsive to said error signal for varying the pitch angle of said rotor blades.
In accordance with a still further embodiment of the invention, there is provided, in a wind turbine having a wind driven rotor with a plurality of variable pitch angle blades, said rotDr being adapted to be connected to a generator via a shaft means, means for controlling the power output from said generator comprises: means for sensing the actual speed of said rotor and producing a rotor speed signal indicative thereof, means for sensing -~
the actual speed of said generator and producing a generator speed signal indicative thereof, means for comparing said rotor speed signal with said generator speed signal and producing a speed error signal variable with the difference therebetween, signal compensatiny means re~ponsive to said speed error signal for producing therefrom a signal propor-tional to the rate of change of speed error, means for measuring the torque of said shaft means and prodhcing a shaft torque signal indicative thereof, means for producing a reference signal proportional to a desired shaft torque, means for comparing said shaft torque signal with said reference signal and producing a shaft torque error signal variable with the difference therebetween, means for summing said rate of change of speed error signal with said shaft torque error signal to produce a rate of change of shaft toreque signal, and means responsive to said rate of change of shaft torque signal for varying the pitch angle of said rotor blades.

_ 7a -In accordance with a still further embod:imen~ of the invention, there is provided, in a power generating system including a wind turbine driven generator adapted to be selectively connected to a load to supply power thereto, said wind turbine having a wind driven rotor with a plurality of variable pitch angle blades, a control for adjusting the pitch angle of sald blades:comprises: speed control means adapted to produce a first signal indicative of rate of change of blade pitch angle when said generator is not supplying power to said load, accelerationcontrol means adapted to produce a second signal indicative of rate of change of blade pitch angle during start up of said wind turbine, deceleration control means adapted to produce a third signal indicative of rate of change of blade pitch angle during shut down of said wind turbine, power control means adapted to produce a fourth signal indicative of rate of change of blade pitch angle when said generator is supplying power to sa:id load, mean~ for selecting one of said blade pitch angle rate signals, integrator means receiving said selected blade pitch angle rate signal and producing therefrom a desired blade pitch angle signal, and actuator means receiving said desired blade pitch angle signal and controlling the pitch angle of said blades in response thereto.

BRIEF DESCRIP~ION OF THE DRAWINGS
Fig. 1 is a diagram of a representative wind turbine.
Fig. 2 is a schematic diagram of the wind turbine power generating system including the blade pitch angle control ~ 7b -S3~
Fig. 3 is a s¢hematic diagram showing the details of the blade pitch angle control of Fig. 2.
Fig. 4 is a schematic diagram of the rotor speed control of Fig. 3, Fig. 5 is a schematic diagram of the acceleration and deceleration controls of Fig. 3.
Fig. 6 is a schematic diagram of the shaft torque control of Fig. 3.
Fig, 7 is a schematic diagram of the mode selector of Fig. 3, Fig, 8 is a schematic diagram of the integrator of Fig. 3.
DESCRIPTION OF THE PREFERRED_EMBODIMENT
Fig. 1 shows a representative wind turbine con-struction consisting of two diametrically opposed identical rotor blades 10, typically totaling 100 ko 300 feet in diameter, mounted on a tower 12. The mechanical co~npon-ents, control devices and electrlcal generating eqUiplnent are contained in a nacelle 140 The construction of wind turbines is well known and need not be described in detail.

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Also, yaw control or orienting the blades into the pre-vailing wind, although necessary for eficient operation, is well known and will not be described.
In Fig 2 the turbine rotor blades are shown mounted on a hub 1~ connected through a low speed coupling shaft 18, a gearbox 20, and a high speed shaft 22 to a synchro-nous generator 24. The gearbox 20 steps up the rotary motion of shaft 18 in a ratio dependent upon the number of pairs of poles in the synchronous generator 24 and the desired a.c. output frequency of the synchronous gene-;; rator. The electrical output from the generator 24 i5 I fed via conductor 26, swi~ch 28 and co~ductor 30 to the - load which may be the power grid of an electrical utility through which the power is ed via transmission lines to xemote locations. Switch 28 i9 closed by a si~naL appear~
ing on a signal line 32 produced ~y a conventional phase and frequency synchronizing circuit~ when the output from generator 24 is matched to that of the power grid to con-nect the generator on-line. A signal indicative of the state of switch 28 and thus the on-line/off-line condi-tion of the system LS fed via signal line 34 to a blade pitch angle control 36. The signal on line 34 may be digital, or an analog voltage or current level, and will be denoted 0 when the generator is off-line and 1 when the generator is on~line.
The blade pitch angle control 36 is the heart o-f the wind turbine control system, and will be described in detail in subsequent Figures. Briefly, control 36 receives input signals indicative of desired wind turbine operating levels, and signals from sensors which measure actual wind turbine operating parameters, and using closed loop elec~ronic controls modulates the pitch angle of the blades via an electro-hydraulic closed loop pitch change mechanism 38 Con~rol 36 produces a desired or reference blade angle signal BR which is fed to pitch change mechanism 38 via line 40. The electro-hydraulic pitch change mechanism converts the electrical blade angle reference signal BR to the actual blade angle de-noted Bp.
In order to provide the necessary operatlng data to control 36, feedback sensors are located at selected lo-cations in the wind turbine. A transducer 46 is connect-ed to hub 16 and provides a signal on a line 48 indicative of ro~or speed, N . A slmilar transducer 50 is connected to a shaft in synchronous generator 24 to provide a signal on a line 52 indicative of generator speed, NG. A trans-ducer 54 such as conventional strain gages is connected to shaft 22, or alternately to shaft 18 or a shaft in gearbox 20, to provide a signal on a line 56 indicative of shaft torque Q . While not shown, generator electrical power or generator current may be sensed in an analogous manner in place of shaft torque. Each of the signals on lines 48, 52 and 56 is fed as an input to control 36.
Also fed to control 36 are a plurality of fixed or _ g _ variable reference signals, either voltage levels inanalog format or digital words in digital format. The reference signal sources are a rotor speed reference source~ ~R REF, shown in block 58, and rotor acceleration and deceleration rate limits NR AC LIM and NR DC LDM re-spectively shown in blocks 60 and 62, the reference sig-nals being fed to control 36 via signal lines 64, 66 and ~`~ 68. A startistop switch 70 feeds a signal to control 36 via a signal line 72, the signal being a 1 to start the wind turbine, and a 0 to stop the turbine. The ~ REF
signal may be supplied from another portion of the system, not shown.
Wind velocity Vw is sensed by a wind velocity sensor 74 and a signal indicative thereof supplied to control 36 via a signaL line 76. The dynamic characteristics of the wind velocity sensor 74 are such that the wind velocity signal on line 76 represents the average rather than the instantaneous wind velocity.
While not shown in the drawings, other functions may be performed by the wind turbine control system such as the sensing and displaying of essential parameters includ~
ing line voltage, hydraulic pressure, oil level, oil tem-perature, bearing temperature, vibration and generator voltage. Emergency shutdown devices responsive to mal-functions may also be provided as part of the system. A
control using digital electronics is more readily adapted to these additional functions, and may easily be programmed 5~

to accept and execute manual co~ands.
The control 36 for the wind turbine provides four distinct modes of operation. The first control mode is start-up, and occurs when the average wind velocity reaches a value where the wind turbine and generator can provide useful power. The blades must be moved from their feathered (90) position toward low pitch so that the blades can provide torque to accelerate the rotor and generator to rated speed conditions, without at the same time stalling the blade or producing large accelerating stresses~
The second control mode is off~line speed control.
As the speed of the generator increases to the value re-quired to provide the desired electrical output frequency7 a speed contrvl is utilized to maLntain the desired speed by modulating the blade angle. 'rhe speed control may al-80 be used to adjust the phase r~lationship between the synchronous generator and the power grid so that the syn-chronous generator can be connected to the network. The ; 20 NR REF signal may be varied for this purpose. A specific example of a speed control used for phase adjustment is described in copending application Serial No. 916,221.
In theory it is possible to control either generator vr rotor speed, but the control is easier to implement if rotor speed is regulated.
The third control mode is on line pow~r cvntrol.
Once the generator has reached a speed which produces the desired frequency, and the generator phase is adjusted to match that of the power grid, the generator is connected to the network. The pitch angle of the rotor blades is now modulated to maintain the desired level of power be-ing supplied by the generator. At high wind velocities ; the rotor is capable of developing many times rated power, Turbulent wind fluctuations in a high average velocity region present significant disturbances, and rapid pitch modulation is required to minimize power fluctuations during these turbulent wind conditions. Power control is -provided by measuring shaft torque. Since speed is essen-tially constant during on-line operation, shaft torque is a direct measure of power, Other parameters which may be used to control power are generator current or electrical power.
The fourth control mode is shutdown. If the s~stem i9 to be shutdown ~or any reason, the blade angle must be lncreased toward its 90 ~eathered posi~ion, Large de-celerating torques are developed if the blade angle is ;~
inereased too rapidly.
The blade pitch angle control 36 of Fig. 2 is shown ln block diagram form in Fig. 3. The rotor speed control shown as block 78 receives as inputs the average wind v~locity signal V on line 76, the rotor speed reference signal N REF on line 64, and the rotor speed signal N
on line 48, as well as the start/stop signal on line 72, The rotor speed control, described in detail in Fig. 4, produces a rotor speed blade angle rate signal BN which is fed via a signal line 80 to a maximum selector circuit 82.
The acceleration control shown as block 84 receives as inputs the rotor acceleration limit signal N~ AC LIM

on line 66, and the rotor speed signal N on line 48, and R
produces an acceleration blade angle rate signal BA which is fed via a signal line 86 to the maximum selector cir cuit 82 The acceleration control 84 is described in de-tail in Fig. 5.
The maximum selector circuit 82 se].ects the blade angle rate signal on lines 80 or 86 which will produce the slowest rate of decrease o~ blade angle, or highest rate of increase of blade angle, that is, the blade an~le rate which will maintain ths blade pitch angle at i~s ma~imum wi.th respect to ~eather, 90. The selected blade angle rate signal is ~ed via a signal line 88 to a minimum ; sel~ctor circuit 90 The selected blade angle signal on Line 88 is denoted BMX .

2~ Also fed as an input to minimum selector circuit 90 is the deceleration blade angle rate signal ~ on signal line 92 which is produced by the deceLeration control shown as block 94. The deceleration control 94 receives as input signals thereto the rotor decel ration limi~
signal N~ D~ L~M on line 68, and the rotor speed signal N
on line 48. The deceleration control 94 is described in greater detail in Figure 5.

The minimum selector circuit 90 selects the blade . angle rate signal on line 88 or 92 which calls ~or the gastest rate of decrease of blade angle. The output from the minimum select circuit 90 is the minimum rate signal B~N appearing on signal line 95 and is fed as an input to a mode selector circuit 96.
Also fed as an input to the mode selector circuit 96 is the shaft torque blade angle rate signal BQ on signal line 98. The signal is produced by a shaft torque con-trol shown as block 100 which receives as input signals thereto the average wind velocity signal Vw on line 75, the shaft torque signal Q on line 56, the rotor speed signal N on line 48, and the generator speed signal NG

: on line 52. The shaft torque control 100 is described in grea~er detail in Figure 6.
Also fed as an input to mode selector 96 is the off/on line signal on line 34 which indicates whether the synchronous generator 24 of Figure 2 is connected to the power grid via switch 28. The mode selector ~6, de-scribed in greater detail in Figure 7~ selects the mini-mum blade angle rate signal on line 95 when the synchro-nous generator is off-line, and the torque blade angle rate signal on line 98 when the synchronous generator is on-line. The selected signal is denoted as the blade angle reference rate signal, B , and is passed via signal line 102 to an electronic integrator 104. The integrator also receives as inputs thereto the rotor speed signal NR
on line 48, and the average wind velocity signal Vw ~
line 76. The integrator circuit 104, described in greater detail in Figure 8, integrates the output from the mode select circuit 96 and produces as an output signal on line 40 the blade angle reference signal ~ , also shown in Fig. 20 With reference to Figure 3, when the start-up com-mand is given a digital 1 appears on signal line 72 and the speed reference signal NR REF in rotor speed control 78 is stepped from zero ~o its nominal value. The speed control 78 then commands a large negative blade angle rate BN on line 80. The acceleration control 84 at this tlme also commands a negative blade angle rate BA which is algebraically larger. The maximum selector 82 selects BA ~ that i9, the blade angle rate calling for the slowest ra~e of decrease of blade angle. The deceleratlon control 94, during start-up, commands a positive blade angle rate and the minimum select circuit 90 also selects B , the acceleration blade angle rate. Since the system is oper-atlng of~-line, the mode selector 96 passes BA to the ; integrator 104.
As the rotor speed increases, the value of B on signal line 80 increases (becomes less negative) until it eventually exceeds the value of B . At this point, the maximum sel ct circuit 82 selects ~ . Since the N
value of BN is still lower than BD on line 92, the .~
minimum selector circuit 90 passes the value of BN through the mode select circuit 96 to the integrator 104. The system is now operating on rotor speed control, and in a steady state constant wind velocity condition, B would approach zero, B would be at some negative value and BD
at some positive value.
If now a command is given to stop the wind turbine~
the reference speed in the rotor sp~ed control is stepped from its nominal value to zero by virtue of the stop sig- -nal, a digital zero, on signal line 72. The rotor speed control 78 now commands a very large positive blade angle rate, much larger than the rate commanded by the accelera-tion control 84. The maximum selector circuit 82 now se-lects BN. Mowever, this value is also much larger than the rate commanded by the deceleration coatrol, BD~ The minimum selector circui~ 90 thereore selects BD and pa~es this signal through the mode seLect circuit 96 to the integrator 104. The wind turbine now stops under the control of the deceleration control 94.
2~ For on-line operation, the mode selector 96 passes the blade angle rate B commanded by the shaft torque con-trol 100 to the control integrator 104 by vir~ue of the signal on signal line 34.
The dynamic characteristics of the four control cir-cuits shown in Figure 3 as well as the mode selector 96 and rate limited integrator 104 will be described wi~h respect to subsequent figures.

~ 16 -Figure 4 shows the details of the rotor speed control 78 of Figure 3. The start/stop signal on line 72 is fed to a multiplier 110 together with the speed reference signal ; N REF on line 64.
R
The start/stop signal is a multiplier for the speed reference signal, a digital L appearing on line 72 when it is desired to start the wind turbine, and a digital 0 when it is desired to stop the wind ~urbine. Assuming that a digital 1 appears on line 72, mult;plier 110 passes there-through the N REF signal on line 64 via output line 112 which is fed as one input to junction 114. Also fed as an input to junction 114 is the rotor speed signal, NR, on line 48. The rotor reference speed on line 112 is compared with the actual rotor speed on line 48 a-t junction 114, ; and any speed error appears on signal line 116. The speed error on line 116 ls then passed through the cc~mpensating circuit 118 and ~ed via signal line 120 to a multipller 122.
The compensating circwit 118 contains a lead-lag network shown in analog form. The gain K2 is cons~ant and repre-sen~s a manually adjustable control gain.
Also fed to the multiplier 122 is a variable gain on signal line 124 scheduled as a unction of average wind speed. The wind speed signal ~W on line 76 is fed to a function generator 126 which schedules the variable gain, K , as a function of wind speed. The use of a variable gain is desirable to compensate or the non-llnear rotor aerodynamic characteristic in which rotor torque changes ~ r~ 8 with blade angle. The variable gain on line 124 produced in function generator 126 is used to multiply the speed control signal on line 120 in multiplier 122. The output of rnultiplier 122 is the rotor speed blade angle ra~e signal BN on signal line 80.
Wind turbine speed control can be achieved either by directly controlling generator speed or rotor speed. The preferred choice is rotor speed control since in order to achieve a responsi~e control using generator speed, the conkrol must provide a compensating network not only to compensate for a long rotor lag, but also to provide quad-ratic lead compensation to compensate for the resonance creaked by the shaft stifness lnterconnecking the rotor and generator inertias~ For wind turbine applications, this quadratic lead compensation is automatically provided by sensing rotor speed rather than generator speed. Sens-lng rotor speed ~or speed control requires only Lead com-pensation for the rotor lag to achieve a responsive speed control.
Figure 5 shows the details of the combined accelera-tion and deceleration controls 84 and 94 of Figure 3.
The rotor speed signal NR on line 48 is fed to a deriva-tive network 128 with a transfer function as shown in the block. The output from the derivative network 128 is proportional to the rate of change of rotor speed, NR, and appears on signal line 130. The signal on line 130 is compared in junction 132 with the rotor speed acceleration limi~ signal NR AC LIM on line 66. Any d~f-ference therebetween, a speed acceLeration error signal, ; is fed via signal line 136 to an acceleration control gain 138 to produce the acceleration blade angle rate signal B on signal line 86. The rotor speed rate signal NR on signal line 130 is also compared at junction 134 with the rotor speed deceleration limit signal ~R DC LIM on signal line 68, any difference therebetween being fed via signal line 140 through gain 142 to produce the deceleration blade angle rate signal BD on signal line 92. During accelera-tion and deceleration the blade angle is modulated ~o main-tain a constant rotor acceleration or deceleration rate.
Acceleration and deceleration N limits are established by computing the acceleration resulting from an accelerating torque of approæimately 100% of rated torque. The ac-celeration and deceleration controls in e-Eect limit the value of rotor torq~e to a value of -~100% Eor start-up and -100% during shutdown, thereby mlnlmlzing blade stresses during ~hese modes of operation. Because there is no long lag in the transfer unction from aerodynamic torque change to rotor acceleration, a simple integral control is satis-factory, the time constant T3 in derivative network 123 being used for noise fiLtering.
Figure 6 shows the detai1s of the on-line shaft torque control 100 of Figure 3. A torque reference sig~al QREF is scheduled in function generator 144 as a function of the average wind velocLty signal, Vw appearing on line 76, the - 19 ~

torque raference signal appearlng on signal line 146 and be~g fed as one input to summing junction 148. The torque reference signal is compared in th~ summing junction 148 with the actual tor~ue signal, Q , appearing on slgnal line 56. The output from the summing junction, a signal indicative of torque error, appears on signal line 150 and is passed through the dynamic compensation circuit 152 which provides a gain and a lag to the torque error signal.
Tha compensated torque error signal is then fed via a sign~l line 154 as one input to a summing junction 156.
The deriva~ive of shaft torque is given approximately by the equation (NR - N Generator Ratio), i.e., the tor-; sional spring ra~e between the rotor and the generator.
This equation is implemented in Figure 6. Th~ rotor speed~
NR, on Line 48 is fed as one input to summlng junction 158.
The generator speecl signal, N , on slgnal line S2 is ~ed to a divider circuit 160 where the generator speed is di-vided by a number equivalent to the gear ratio between the rotor and the generator. The signal appearing on line 162 is therefore equivalent to the rotor speed if there is no torque. The signal on line 162 is fed as the other inp~t to summing ~unction 158. The two signals are compared in summing junction 158 and produce on~signal line 161 a signal indicative of the diference between rotor speed and generator speed which is proportional to the deriva-tive of shaft torqueO This signal is then -fed through the dynamic compensation network 163 and via signal line ~l~VI.j38 16~ to summing junctLon 156 where the compensated shaft torque deriva~ive signal i~ added to the lagged torque error signal on line 15~. The lead compensation in block ; 163 makes its output a function of speed error and rate of change of speed error through the lag Tl. Since speed error is proportional to the derivative of shaft torque Q , then the output from block 163 is related ko the first and second derivatives of shaft torque ~S and QS' through a lag Tl. When the output from block 163 is summed with shaft torque error through the lag in block 152 via lines 164 and 15~ in summing junction 156, the output becomes ~ d Wh i t d i i t grator 104 (Fig. 3), this control provides proportional, integral and derivative control gains to the shaft torque signal.
The outpu~ ~rom th~ summlng ~unctlon 156 is ed vla ~ignal line 166 to multiplier 168 where it is multiplied by a variable gain signal, K , appearing on signal line 170. The variable gain is provided in function generator block 172 as a function of average wind velocity~ V , on line 76. The output from the multipller 168 is the shaft torque blade angle rate signal BQ on line 98. The use of a variable gai~ and multlplier 168 is deslrable to com-pensate for the non-linear rotor aerodynamic characteris-tics. The control mode described in Figure 6 in effect provides quadratic lead compensation which compensates for the on-line torsional resonance which is present in the power control loop. This resonance is de~ermined primariLy by the rotor inertia and the equivalent tor-sional spring rate between the rotor and power grid. The derivative gain provides improved damping on this resonance.
When the generator is connected to a large power grid network, generator speed is so nearly constant that the generator speed signal NG on line 52 can be repla ed by the fLxed synchronous value of generator speed with no signLficant effect on system performance.

Figure 7 shows the preferred implementation of the mode selector circuit 96 o Figure 3. The off/on line signal on line 34 is fed ~o a surnming junction 174 and a multiplier 176. The ofE/on line signal is produced in switch 28 of Figure 2, and ig O when the synchro~ous - generator is of l~ne, and ~ when the synchronous genera~or is onnected to the power grid on line. Also fed to sum;
ming junctlon 174 is a 1 on slgnal line 178. The output ~rom the summing junction 174 ls fed via signal line 180 to a multiplier 182. The minimum blade angle rate signal BM~ is also fed to multiplier 182 via line 94. The out put from the multiplier~l82 is fed via line 184 as one input to summing junction 186. Also-fed as an inpu~ to multiplier 176 is the torque blade angle rate signal, BQg on signal line 98. The output from multiplier 176 is fed via signal line 188 as the second input to ~umming j1mc~
tion 186. The output from the summing junction 186 ap~
pears as reference blade angle rate signal, BR, on line 102.

-~2 -:
If the control system Ls operating such that the synchronous ge~erator is o line, 0 appears on signal line 34 and is compared with the 1 in summing junction 174. The output from the summing junctio~ will be a 1 on ~ signal line 180, and the minimum blade angle signal BMM
: on line 94 is thereore multiplied by 1. The~efore the output from the multiplier 182 on line 184 is e~actly ~ equal to the minimum blade angle rate signal on line 94.
: At the same time the 0 on line 34 is fed to the multiplier ;

176 and the signal appearing on line 188 will be 0.
Therefore, the output signal on line 102 will be exactly equal to the signal on line 94.
: If the synchronous generator is connected on line, the 1 appearing on line 34 wilL be canceled by khe 1 on line L78 in summLng junction 174, and -the si.gnal applied : to muLtiplier 18Z vla line 1~0 will be 0, result~ng in a 0 appearing on signal line 184. At the sam0 time, the 1 on line 34 will cause the signaL on line 188 to be exactly the same as that on Line 98, and the output signal on line 102 wlll be equal to the tor~ue bLade angle rate signal B on line 98.
A single common integrator 104 as shown in Figure 3 is used for all modes of control. Figure 8 shows in block diagram form a preferred implementation of a rate limi~ed integrator with maximum and minimum position stops. Referring to Figure 8, the reference bl~de angLe rate signal BR on signal line 102 ls fed ~o a rate limiter ~3 -

3~

circuit 190 and then via signal line lg2 to a junction 194. The output from junction 194 is ~ed via signal line 196 to an in~egrator 198. Tntegration of the rate signaL
BR provides a blade angle reerence signal denoted BR on signal line 40. The rate limits in rate limiter circuit 190 are set to a value equal to or less than the slew rate capabilities of the hydraulic pitch change loop. Thus if the blade angle reerence rate signal BR is greater than the slew rate capability of the pitch change mechanism, the control rate limiter 190 will limit the output of the rate limiter to a value never larger than the slew rate capabilities of the pitch change mechanism. The rate limiting i~ important because it pre~ents the control in-tegrator 19~ from overtraveling. Without the rate limiter, a large wind disturbance which cvuld result in B exceed-ing the slew rat~ capability o the pLtch change mechanlsm would cause the control integrator to o~ertravel, that is, the error between the reference blade angle B~ and the actual blade angle would become large. In such cases a severe limit cycle instability could result, The rate limiter prevents this instability from occurring.
The control integrator 198 has maximum and minimum position stops, In a digital electronic control system these stops would be Lmplemented by digital words. The maximum stop corresponds to the eathered or 90 position, and the minimum stop of the control in~egrator is a variable which is scheduled as a function of ~ , where ~ is - 2~ -3~3 the ratio of rotor speed to wind speed. Referring again to Flgure 8, ~ is computed in a divider circuit 200 which divides the rotor speed signal ~ on line 48 by the average wind veloci~y signal ~W on line 76. The output from the divider circuit 200 is the ~elocity ra~io that appears on signal line 202. A function generator 204 receives the velocity ratio ~ on line 202 and schedules as a function thereof a minimum blade angle signal, M¢N B, on signal line 206. The minimum blade angle schedule shown in function generator 204 is obtained by establishing a blade angle corresponding to maximum rotor torqua.
The minimum blade angle signal on line 206 is ed to a circuit 210, Also fed as an input to circuit 210 is the blade angle reference signal BR on signal line 40.
Circuit 210, whlch ~mplements the maximum and minimum lntegrator stops, produces a high gain output signal on ; signal line 212 whenever the blade angle reference BR is greater than 90 or less than a minimum blade angle. The output of circuit 210 is zero when the blade angle is be-tween the minimum and maximum limits. The high gain feed-back signal on signal line 212 is compared with the signal on line 192 in summing junction 194 to produce the input to the integrator 198 which prevents the lntegrator out-put on line 40 from exceeding the limits.
As no-ted in Figure 8~ the minimum blade angle in circuit block 210 is variable as a function of the - 25 ~

minimum blade angle scheduled via line 206. The minimum blade angle schedule is encountered only during start~up in low wind speeds. When s~arting ~p in high wind speeds, there is more than enough power in the wind to accelerate the rotor at the rate desired by the acceleration control 84 of Figure 3. This acceleration rate is approximately 6.5% speed/second. Under these conditions 3 the accelera tion control governs the start-up operation and the mini-mum blade angle limit is not encountered. When starting in very low wind speeds, there is not sufflcient power in the wind to accelerate the rotor at the rate desired.
Under these conditions, the minimum blade angLe limit is encountered and a maximum but less than 100% accelerating torque is developed. The combi~ation of ~he closed loop acceleratlon control and the variable minimum blade ang~e stop in the electronlc integrator pre~ent stalli~g of the rotor during ~tart-up and insures that the rotor will ac-celerate at the maximum rate possible but never faster than the rate corresponding to an accelerating torque of 100% under all wind conditions.
While the wind turbine system has been described in a preferred embodiment thereof, it is apparent that modi-fications may be made thereto without departing from the scope of the invention as hereinafter claimedO

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. In a power generating system including a wind turbine driven generator adapted to be selectively con-nected to a load to supply power thereto, said wind tur-bine having a wind driven rotor with a plurality of varia-ble pitch angle blades, a control for adjusting the pitch angle of said blades comprising:
speed control means adapted to produce a first signal indicative of rate of change of blade pitch angle when said generator is not supplying power to said load, acceleration control means adapted to produce a second signal indicative of rate of change of blade pitch angle during start up of said wind turbine, deceleration control means adapted to produce a third signal indicative of rate of change of blade pitch angle during shut down of said wind turbine, power control means adapted to produce a fourth signal indicative of rate of change of blade pitch angle when said generator is supplying power to said load, means for selecting one of said blade pitch angle rate signals, integrator means receiving said selected blade pitch angle rate signal and producing therefrom a desired blade pitch angle signal, and actuator means receiving said desired blade pitch angle signal and controlling the pitch angle of said blades in response thereto.

2. A power generating system as in claim 1 in which said acceleration control means comprises:
means for sensing the speed of said rotor and producing a signal indicative thereof, means for producing from said rotor speed sig-nal a rate signal indicative of rate of change of rotor speed, means for producing a reference signal indica-tive of rotor acceleration limit, and comparator means for comparing said rotor speed rate signal with said rotor acceleration limit refer-ence signal to produce therefrom said second signal.

3. A power generating system as in claim 1 in which said deceleration control means comprises:
means for sensing the speed of said rotor and producing a signal indicative thereof, means for producing from said rotor speed sig-nal a rate signal indicative of rate of change of rotor speed, means for producing a reference signal indica-tive of rotor deceleration limit, and comparator means for comparing said rotor speed rate signal with said rotor deceleration limit reference signal to produce therefrom said third signal.

4. A power generating system as in claim 1 in which said speed control means comprises:
means for sensing the speed of said rotor and pro-ducing a signal indicative thereof, means for producing a reference signal indica-tive of desired rotor speed, comparator means for comparing said rotor speed signal with said reference rotor speed signal and produc-ing therefrom a rotor speed error signal, means for producing from said rotor speed error signal a signal indicative of the rate of change of rotor speed error, means for sensing average wind velocity and producing a signal indicative thereof, means responsive to said wind velocity signal for producing a gain signal as a function whereof, and multiplier means for multiplying said rate of change of rotor speed error signal by said gain signal to produce said first signal.

5. A power generator system as in claim 4 in which said means for producing a signal indicative of the rate of change of rotor speed error comprises signal compensation means providing proportional and integral compensation.

6. A power generating system as in claim 4 and further including switch means adapted to produce a first signal for initiating operation of said wind turbine and a second signal for stopping operation of said wind tur-bine, and means connected with said switch means and with said means for producing said reference rotor speed signal for passing said reference rotor speed signal to said comparator means when said switch means produces said first signal, and for passing a reference rotor speed signal of zero to said comparator means when said switch means produces said second signal.

7. A power generating system as in claim 1 in which said rotor is connected to drive said generator via a shaft means, and in which said power control means com-prises:
means for sensing the speed of said rotor and producing a rotor speed signal indicative thereof, means for sensing the speed of said generator and producing a generator speed signal indicative thereof, means for comparing said rotor speed signal with said generator speed signal and producing a speed error signal proportional to the difference therebetween, compensating means including integral, deri-vative and proportional compensation for producing from said speed error signal a speed error rate signal, means connected with said shaft means for sens-ing the torque of said shaft means and producing a shaft torque signal indicative thereof, means for sensing the average velocity of the wind driving the rotor and producing a wind velocity signal indicative thereof, first means responsive to said wind velocity signal for producing as a function thereof a shaft torque reference signal, means for comparing said shaft torque signal with said shaft torque reference signal and producing a shaft torque error signal proportional to the difference therebetween, means for summing said shaft torque error signal with said speed error rate signal, second means responsive to said wind velocity signal for producing as a function thereof a gain signal, and means for multiplying said summed shaft torque error signal and speed error rate signal by said gain signal to produce said fourth signal.

8. A power generating system as in claim 1 in which said means for selecting one of said blade pitch angle rate signals comprises:
a maximum selector means receiving said first and second signals and passing therethrough the signal adapted to maintain the blade pitch angle at its maximum relative to feather, a minimum selector means receiving the signal passed by said maximum selector means and also receiving said third signal, said minimum selector means passing therethrough the signal adapted to maintain the blade pitch angle at its minimum relative to feather, a mode selector means receiving the signal passed by said minimum selector means and also receiving said fourth signal, means for sensing the connection of said gene-rator to said load and producing a discrete signal indica-tive thereof, means for feeding said discrete signal to said mode selector means, said mode selector means passing therethrough said fourth signal when said discrete signal is present and passing therethrough the signal received from said minimum selector means when said discrete signal is not present, and means connecting the signal passed by said mode selector means to said integrator means.

9. A power generating system as in claim 1 in which said integrator means comprises:
rate limiter means receiving said selected blade pitch angle rate signal for limiting the magnitude of said blade pitch angle rate signal within a preselected range, and an electronic integrator receiving the blade pitch angle rate signal from said rate limiter means and producing therefrom said desired blade pitch angle signal.

10. A power generating system as in claim 9 and further including blade angle limiter means having maxi-mum and minimum blade angle limits and responsive to the magnitude of the desired blade pitch angle signal produced in said electronic integrator for maintaining said desired blade pitch angle signal within said maximum and minimum blade angle limits.

11. A power generating system as in claim 10 and further including:
means for sensing the average velocity of the wind driving said rotor and producing a wind velocity signal indicative thereof, means for sensing the speed of said rotor and producing a rotor speed signal indicative, means for dividing said rotor speed signal by said wind velocity signal to produce a velocity ratio signal, means responsive to said velocity ratio signal for producing a minimum blade angle signal variable as a function thereof, and means for varying the minimum blade angle limit in said blade angle limiter means in response to said minimum blade angle signal.

12. In a control system for a wind turbine, said wind turbine having a wind driven rotor with variable pitch angle blades, the improvement comprising:
closed loop control means responsive to system operating conditions for producing a control signal indica-tive of desired rate of change of blade pitch angle, rate limiter means receiving said control signal for maintaining said control signal within predetermined limits, electronic integrator means for producing from said control signal a signal indicative of desired blade pitch angle, said integrator means including pitch angle limiter means for maintaining said desired blade pitch angle signal within predetermined maximum and minimum limits, and blade pitch change means responsive to said desired blade pitch angle signal for varying the blade pitch angle.

13. A control system as in claim 12 and including means responsive to wind velocity and rotor speed for varying the minimum limit in said pitch angle limiter means.

14. A control system as in claim 12 in which said closed loop control means comprises means responsive to the difference between actual rate of increase of rotor speed during start up of said wind turbine and a limiting rate of increase of rotor speed during start up for pro-ducing said control signal.

15. A control system as in claim 12 in which said closed loop control means comprises means responsive to the difference between actual rate of decrease of rotor speed during shut down of said wind turbine and a limiting rate of decrease of rotor speed during shut down for pro-ducing said control signal.

16. In a wind turbine having a wind driven rotor with a plurality of variable pitch angle blades, means for maintaining a substantially constant rate of change of rotor speed during start up and shut down of said wind turbine comprising:
means for sensing the actual speed of said rotor and producing a rotor speed signal indicative thereof, rate sensor means for producing from said rotor speed signal a signal indicative of the actual rate of change of rotor speed, means producing a reference signal proportional to a rotor speed rate of change limit, comparator means comparing said actual rate of change of rotor speed signal with said reference signal and producing an error signal proportional to the differ-ence therebetween, and means responsive to said error signal for varying the pitch angle of said rotor blades.

17. In a wind turbine having a wind driven rotor with a plurality of variable pitch angle blades, said rotor being adapted to be connected to a generator via a shaft means, means for controlling the power output from said generator comprising:
means for sensing the actual speed of said rotor and producing a rotor speed signal indicative thereof, means for sensing the actual speed of said generator and producing a generator speed signal indica-tive thereof, means for comparing said rotor speed signal with said generator speed signal and producing a speed error signal variable with the difference therebetween, signal compensating means responsive to said speed error signal for producing therefrom a signal pro-portional to the rate of change of speed error, means for measuring the torque of said shaft means and producing a shaft torque signal indicative thereof, means for producing a reference signal pro-portional to a desired shaft torque, means for comparing said shaft torque signal with said reference signal and producing a shaft torque error signal variable with the difference therebetween, means for summing said rate of change of speed error signal with said shaft torque error signal to produce a rate of change of shaft torque signal, and means responsive to said rate of change of shaft torque signal for varying the pitch angle of said rotor blades.

18. A wind turbine control as in claim 17 and including means responsive to the velocity of the wind driving said rotor for producing said reference signal as a function thereof.