ENERGY I

SECURITY ASSESSMENT OF POWER SYSTEMS INCLUDING ENERGY STORAGE AND WITH THE INTEGRATION OF WIND ENERGY

Progress Report, October 1 -December 31,1979

BY D. P. Carroll P. C. Krause

January 1980

Work Performed Under Contract No. EC-77-S-02-4206 \

Energy Sources and Systems Simulation Laboratory School of Electrical Engineering Put due University West Lafayette, Indiana , -

> " I. - :Fa* - 4 - .s

U. S. DEPARTMENT OF ENERGY

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER

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DISCLAIMER

''This book was prepared as an account of work sponsored by an agency of the United States Covernment Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implted, or dg8UlheB any legal liability or responsibility for the accuracy, completeness, or usefulness of any informatian, apparatus, product, or process disclosed, or represents that its luie would not infringe privately owned rights. Reference hereifi to any speclflc oommercial producl, pruws, or mrvics by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof."

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SECURITY ASSESSMENT OF POWER SYSTEMS INCLUDING ENERGY STORAGE AND WITH THE INTEGRATION OF WIND ENERGY

Progress Report f o r P e r i o d 0ctobGr 1, 1979 - December 31, 1979

D. P. C a r r o l l and P. C. ~ r a u s e

Energy Sources and Systems S i m u l a t i o n Labo ra to r y School o f E l e c t r i c a l Eng. ineer ing

Purdue U n i v e r s i t y West Lafayet te , I n d i a n a 47907

January 1980

Prepared For

THE UNITED STATES DEPARTMENT OF ENERGY UNDER CONTRACT NO. DE-AS02-77ET 29 100

(FORMERLY EC-77-5-02-4204)

w BTI.STRIBUTlrS.N OF THIS CIOGUMENT IS.UP(:L.Ihti.TEj

CONTRACT ACTIVITY

Dur ing t h i s quar ter , S. Abd El-Hamid (33%), G. E. Gareis (25%), P. C . Krause

(17%), C . M. Ong ( l o % ) , J. P. S u l l i v a n (5%) and 0. Wasynczuk (67%) have been

invo lved i n t h i s cont rac t .

INTRODUCTION

The p rogress o f t h e f i r s t t h r e e t asks o f t h i s c o n t r a c t i s repor ted . A t

a meeting, c h a i r e d by D r . J. H. Rumbaugh, t h e program manager, seve ra l o f

t h e o b j e c t i v e s o f t h i s c o n t r a c t were mod i f ied . The proceedings o f t h i s De-

cember 4, 1979 meet ing a t DOE a r e g i v e n i n a l e t t e r t o M r . Ronald Thomas o f

NASA.from D r . J. H. Rumbaugh. T h i s l e t t e r i s Appendix A o f t h i s r e p o r t .

WIND MODELING

The impor tance of t h e wind f l u c t u a t i o n s upon t h e performance o f any

wind t u r b i n e genera to r i s no t f u l l y understood. The model o f t h e wind

se lec ted as t h e "des ign model o r des ign gus t " e f f e c t s t h e des ign o f t h e wind

t u r b i n e system, p a r t i c u l a r l y t h e des ign o f t h e p i t c h c o n t r o L l e r t o p reven t

system i n s t a b i l i t y and t h e des ign of v o l t a g e r e g u l a t o r o r e x c i t a t i o n syqtem

t o prevent v o l t a g e f l i c k e r . E a r l y i n t h i s research t h e importance o f t h e

wind model i n i n f l u e n c i n g t h e des ign o f t h e wind t u r b i n e genera to r became

apparent. I n an e f f o r t t o encourage DOE t o address t h e problem o f wind

model ing i n t h e wind c h a r a c t e r i s t i c s program a p o s i t i o n paper was w r i t t e n by

Pro fessor T r iezenberg and forwarded t o M r . G . P. Tennyson o f DOE on J u l y 10,

1979. A copy o f t h i s r e p o r t i s g i v e n i n Appendix B. Therein, t h e i n f l u e n c e

of t h e wind model upon t h e o p e r a t i o n o f t h e MOD-2 i n b o t h t h e hi-gh and low

modes i s c l e a r l y demonstrated.

Dur ing t h i s q u a r t e r two areas have been pursued:

1.) Wind t u r b u l e n c e models and t h e i r e f f e c t

on unsteady s h a f t t o rque o f h o r i z o n t a l

machines.

2.) Rrdrrced q i d e r aerodynamic models o f wind

machines.

The wind v e l o c i t y i s g i v e n by a mean component i n t h e X - d i r e c t i o n t h a t

v a r i e s w i t h h e i g h t p l u s a f l u c t u a t i n g component t h a t v a r i e d w i t h a l l t h r e e

space d imensions and t ime. That i s :

+ - * V wind = Vm (z)Cx + U (x,y,z,t)

Note t h a t t h e mean component vw(z) con ta i ns t h e mean wind shear. The s p a t i -

a l v a r i a t i o n s o f fi(x,y,z,t) p l a y a p a r t i c u l a r l y impo r tan t r o l e i n h o r i z o n t a l

a x i s wind machiries s i n c e t h e v e l o c i t y o f t h e b l ade i s much h i g h e r t han t h e

wind speedl, so t h e b l a d e s e c t i o n s c u t th rough t h e s p a t i a l v a r i a t i o n s as i f

t h e t u rbu lence was s t a t i o n a r y . T h i s model o f " f rozen" t u r b u l e n c e i s b e i n g

exp lo red f o r use, e s p e c i a l l y f o r t h e h i ghe r f requency f l u c t u a t i o n s .

-+ '

The unsteady s h a f t ' t 0 r q u e . i ~ found by i n s e r t i n g , V wind, i n t o an aero-

dynamic model o f t h e wind machine. A t t h e p resen t time, a b l a d e element

model and a G o l d s t e i n model a re be ing exp lo red . . E i t h e r t h e l lnsteady s h a f t

t o r q u e can be c a l u l a t e d d i r e c t l y o r t h e average, R.M.S., and spectrum o f t h e

-* t o r q u e can be found i n te rms o f v a r i o u s s t a t i s t i c a l moments o f U .

The model b e i n g developed above w i l l be t o o complex and r e q u i r e t o o

much computat ion t i m e t o be used i n a system s imu la t i on . Methods t h a t a r e

used i n p r o p e l l e r theory, such as c o n c e n t r a t i n g t h e aerodynamic loads a t

three-quar ' ter r a d i u s a r e b e i n g cons idered t o reduce t h e comp lex i t y o f t h e

aerodynamic model.

MODELING OF SINGLE-UNIT, WIND POWERED GENERATORS

I n t h e meet ing w i t h DOE on December 4, 1979 (Appendix A), i t was de-

c i d e d t o d i s c o n t i n u e work on t h e MOD-1 under t h i s con t rac t . T h i s work i s t o

be rep laced w i t h an i n v e s t i g a t i o n o f t h e a p p l i c a t i o n o f v a r i a b l e speed, con-

s t a n t frequency, and d i s c r e t e speed systems t o wind power. I n add i t i on , i t

appears t h a t da ta f o r t h e Shackle machine, now b e i n g manufactured by Rendix,

- 3 -

w i l l be de layed f o r some t ime.

I n accordance w i t h t h e mee t i ng .o f December 4, 1979, t h e prob lem o f

r e s y n c h r o n i z a t i o n i s t o be i n v e s t i g a t e d f o r t h e MOD-2 (Appendix A, I t e m 5).

I n o r d e r t o accommodate t hese and o t h e r t r a n s i e n t s t a b i l i t y s tud ies, t h e

MOD-2 s i m u l a t i o n has been m o d i f i e d t o i n c l u d e a more accu ra te r e p r e s e n t a t i o n

o f t h e power c o e f f i c i e n t . M o d i f i c a t i o n s a r e s t i l l underway so as t o i n c l u d e

t h e a c t i o n o f t h e synchron ize r .

A l s o dec ided a t t h e December 4, 1979 meet ing was t h e d e c i s i o n t o ex-

p l o r e v a r i a b l e r o t o r speed approaches t o g e n e r a t i n g 60 HZ power. The s tudy

t o be conducted on t h e computer i s t o be d i r e c t e d toward s o l v i n g t h e problem

o f r esynch ron i za t i on .

STABILITY INVESTIGATIONS OF SINGLE-UNIT, WIND POWERED GENERATORS

As a r e s u l t o f ou r s t u d i e s a t NASA/Lewis Research Center and a t Purdue,

i t was d iscovered t h a t t h e b l a d e p i t c h c o n t r o l o f t h e MOD-2 cou ld cause a

system i n s t a b i l i t y f o l l o w i n g a "down gus t " i n wind.. T h i s i n s t a b i l i t y oc-

cu r red due t o a con t inuous s w i t c h i n g between t h e h i g h and low modes. of.

o p e r a t i o n o f t h e p i t c h c o n t r o l . l e r - Accord ing t o Boeing t h i s i n s t a b i l i t y has

been e l i m i n a t e d by i n c r e a s i n g t h e p i t c h r a t e l i m i t f rom 4 deg/sec t o 8

deg/sec. A s tudy was made a t Purdue and a r e p o r t was w r i t t e n and forwarded

t o D r . J. H. Rumbaugh.. A copy o f t h i s r e p o r t i s g i v e n i n Appendix C.

B r i e f l y , t h e i n s t a b i l i t y w i t h a 4 deglsec r a t e l i m i t i s demonstrated d u r i n g

a down gust . Next, t h e performance w i t h a 10 deg/sec r a t e l i m i t i s i l l u s -

t r a t e d . Therein, i t i s shown t h a t l a r g e t o rques occur on t h e b lades and hub

f o l l o w i n g t h e down gust even though t h e system i s s t ab le . The p o s s i b i l i t y

o f system i n s t a b i l i t y and/or l a r g e t o rques i n t h e gear t r a i n due t o t h e

p i t c h c o n t r o l cou ld be a s e r i o u s problem i n o p e r a t i o n o f t h e MOD-2.

- 4 -

Another aspect o f t h e MOD-2 i s t h a t i t rep resen t s a c l a s s o f wind t u r -

b i n e gene ra to r s o f t h e megawatt s i z e which a re t o be i n t e g r a t e d i n t o power

systems i n perhaps l a r g e numbers t o supp ly a s i g n i f i c a n t amount o f power.

Common i n t h e p resen t d e s i g n o f t h i s t y p e o f wind t u r b i n e i s t h e so f t , low

speed shaf t , p l aced i n t h e mechanica l d r i v e t r a i n p r i m a r i l y t o p r o t e c t t h e

gears and b lades f rom t o r q u e v a r i a t i o n s o r i g i n a t i n g i n t h e e l e c t r i c a l system

and t o decouple t h e e l e c t r i c a l system f rom v a r i a t i o n s i n wind torque.

Viewed f rom t h e blades, t h i s s o f t s h a f t appears as a f i l t e r t o t o r q u e \ / a r i a -

t i o n s caused by wind f l u c t u a t i o n s . The s i g n i f i c a n c e o f t h e s o f t s h a f t as

v iewed f rom t h e e l e t i i c a l system came t o l i g h t i n a recen t d i s c u s s i o n w i t h

pe rsonne l a t Tennessee V a l l e y A u t h o r i t y (TVA). I n t h e case o f t h e MOD-2,

t h e s o f t s h a f t t w i s t s app rox ima te l y 3 . 6 O under f u l l load. I f , f o r example,

t h e wind t u r b i n e gene ra to r i s o p e r a t i n g under f u l l l oad and if t h e feeder

l i n e i s opened ( l o a d r e j e c t i o n ) t h e s o f t s h a f t w i l l u n t w i s t . The u n t w i s t i n g

o f t h i s low speed s h a f t causes t h e h i g h speed s h a f t t o r o t a t e . Wi th a gear

r a t i o o f 100:1, which i s common i n t h i s c l a s s o f wind tu rb ines , t h e h i g h

speed s h a f t w i l l r o t a t e 360°. The genera to r i s a 4-pole machine, hence t h e

r n t o r ang le w i 11 r o t a t e 720°, i n fac t , approx imate ly 1300' irl L t~e F i r s t

t r a n s i e n t overswing o f t h e mechanica l system. T h i s advance o f r o t o r ang le

i s rap id , o c c u r i n g i n l e s s t h a n one second. Boeing does n o t recommend re -

c l o s i n g o f t h e t r a n s m i s s i o n l i n e connec t ing t h e wind t u r b i n e genera to r t o

t h e e l e c t r i c a l network. T h i s i s t o p reven t l a r g e t o r q u e v a r i a t i o n s on t h e

gears and b lades d u r i n g p o s s i b l e out-of-phase rec l .os ing- T o s t ~ a d , t h e wind

t u r b i n e i s s t a l l ed , r e s t a r t e d and resynchron ized. The u n t w i s t i n g o f t h e

s o f t s h a f t i s c e n t r a l t o t h e i s sue of t r a n s i e n t s t a b i l i t y . A s tudy o f t h e

t r a n s i e n t s t a b i l i t y o f t h e MOD-2 i s b e i n g performed. The r e s u l t s o f t h i s

s tudy w i l l be i n c l u d e d i n t h e nex t q u a r t e r l y r e p o r t .

- 5 -

A s tudy o f a v a r i a b l e speed wind powered gene ra to r i s underway. The

system b e i n g s t u d i e d concerns a doub ly f e d i n d u c t i o n genera to r w i t h s l i p

power b e i n g f e d t o t h e r o t o r by means o f a DC l i n k n a t u r a l l y commutated con-

v e r t e r . A f i r s t - c u t a t tempt a t c o n t r o l o f t h e DC l i n k conve r t e r s w i l l use

conven t i ona l HVDC c o n t r o l ph i losophy. A l so underway i s an i n v e s t i g a t i o n o f

p o t e n t i a l approaches t o a d j u s t i n g t h e speed o f ' t h e wind genera to r by

' d i s c r e t e speed changes. A p a r t i c u l a r l y a t t r a c t i v e approach concerns a na-

t u r a l l y commutated i n t e g r a l c y c l e cyc l oconve r t e r . T h i s method i s p r e s e n t l y

undergo ing a n a l y t i c a l i n v e s t i g a t i o n .

- 6 -

APPENDIX A

U.S. Department of Energy Washington, D.C. 20585

M r . Ronald L.' Thomas N a t i o n a l A e r o n a u t i c s and Space A d m i n i s t r a t i o n Lewis Research C e n t e r C l e v e l a n d , Ohio 44135

Dear Ron:

I n r e s p o n s e t o your l e t te r o f December 3 , 1979 which invo lved t h e LeRC g r a n t t o Purdue and t h e Purdue c o n t r a c t w i t h t h e D i v i s i o n o f E l e c t r i c Energy Systems, I h e l d two i n d i v i d u a l m e e t i n g s and one j o i n t mee t ing o n December 4 , 1979 t o a d d r e s s t h e LeRC r e q u e s t s .

The f : i r s t mee t ing was a t t e n d e d by m y s e l f , D r . Rober t J . Thomas o f C o r n e l l who i s working w i t h m e t h i s y e a r w h i l e on s a b b a t i c a l l e a v e , Dr. Thomas W. Reddoch, M r . P a t r i c k M. F innegan, and D r . Leonard J . G i l b e r t . T h i s mee t ing was t o d i s c u s s t h e LeRC r e q u e s t s based on your c u r r e n t p r i o r i t i z e d n e e d s and l a s t e d a p p r o x i m a t e l y two h o u r s .

The second group t h a t met excluded t h e two LeRC r e p r e s e n t a t i v e s and i n c l u d e d D r . P a u l C. Krause o f Purdue. I n t h i s m e e t i n g , i t was agreed t h a t based upon t h e LeRC r e q u e s t s , D r . Krause would per fo rm t h e f o l l o w i n g t a s k s . I n o t e t h a t t h e t ime e s t i m a t e s f o r t h e s e t a s k s a r e "off t h e t o p o f t h e head" p r o j e c t i o n s by D r . Krause and shou ld n o t be judged a s a b s o l u t e .

Develop a reduced o r d e r t r a n s i e n t s t a b i l i t y model (on t h e o r d e r o f two o r t h r e e c y c l e s ) f o r t h e Mod-2 machine. D r . Krause e s t i m a t e s abou t two c a l e n d a r months f o r t h e comple t ion o f a f i n i s h e d p roduc t . S i n c e I b e l i e v e t h a t t h i s a n a l o g s i m u l a t i o n \<ill c o n s i s t o f two masses , a s p r i n g , and a g e n e r a t o r ; t h e two month time frame shou ld p r o b a b l y i n c l u d e t h e v a l i d a t i o n of t h i s reduced o r d e r model a g a i n s t t h e d e t a i l e d m o d e l - ~ r . Krause h a s a l r e a d y s i m u l a t e d . Also this w f l l p r o b a b l y . i n c l u d e t h e use o f t h e wind model d e s c r i b e d i n (2) below.

2. Examine t h e impact o f wind f l u c t u a t i o n s on t o r q u e c o n t r o l and t h e gearbox t h r o u g h t h e new PNL wind model. Mr. D a n i e l F. Ancona, upon h e a r i n g t h i s , s u g g e s t e d a f u r t h e r comparison u s i n g t h e NASA wind model which h e c o n s i d e r s t o be q u i t e s i m i l a r .

3. Add a d e t a i l e d Mod-2 model t o t h e two a l r e a d y i n e x i s t e n c e a t Purdue. D r . Krause f e e l s t h a t h e c a n t i e t h i s t h i r d d e t a i l e d model

n i n w i t h t h e o t h e r s i n a few d a y s .

M r . Ronald L. Thomas 1 3 DEC 19i9-

4 . Compare t h e o p e r a t i o n of t h e s imulated t h r e e d e t a i l e d Mod-2s wi th t h e o p e r a t i o n of t h e t h r e e reduced o r d e r models u s ing t h e Goodnoe H i l l s system t o develop a f i n i s h e d t h r e e machine reduced o r d e r model. This is t h e f i r s t s t e p toward a t h r e e machine aggrega t ion and u l t i m a t e l y an aggrega te model f o r a l a r g e c l u s t e r . D r . Krause e s t ima te s s i x months f o r completion of t h i s t a s k .

5. I f , a s p r o j e c t e d , t h e t r a n s i e n t s t a b i l i t y problems a s s o c i a t e d with the Mod 2 a r e minimal o r non-exis ten t ; examine t h e problem of r e synchron iza t ion us ing both t h e complex and reduced o rde r models. Also, on a rudimentary b a s i s , examine system p r o t e c t i o n schemes.

6. Examine v a r i a b l e speed, cons t an t f requency and v o l t a g e conf igura t i .ons . D r . Lipo i s i n i t i a t i n g work i n t h i s a r e a by looking a t :

( a ) a doubly f ed induc t ion machine w i t h a r e c t i f i e r i n v e r t e r o r a c y c l e c o n v e r t e r ;

(b) .. a d i s c r e t e two speed machine; ( c ) o t h e r v a r i a b l e speed d r i v e s .

It w a s agreed t h a t a l l s imu la t ion work on t h e E.lod-1 would be te rmina ted a t Purdue i n l i e u of i t em ( 6 ) above. Any a d d i t i o n a l work on t h e Mod-1 w i l l b e handled by LeRC s i n c e , i t i s germane t o them and probably w i l l no t be a product ion machine.

I 1001: forward t o a s u c c e s s f u l program w i t h a . h i g h degree of coopera t ion . To keep t h e program on t r a c k , I w i l l schedule q u a r t e r l y review meet ings. The f i r s t meeting should bc held i n tho approximate t ime frame of January 1 5 - Fe'bruary 1 5 , 1980.

S i n c e r e l y ,

J. H. Rumbaugh Program Manager D iv i s ion of E l e c t r i c Energy Systems

cc : Dan Ancona Pa t Finnegan Len G i l b e r t Paul Krause Gerry Park Tom Reddoch Bob Thomas

Ilr. G.P. Tcnnyson Dcpartr~cnt o f Ellorgy \ttnd Sy stcns Rtanch : l ~ s t r I n ~ t o n , D.c.' 20585

CrlcloscJ 1s a shor t papar rrrlttcn by Dr. D.tl. Trlczctlbcrg f o r t ! ~ c purposc of convcylng tfrc nccd for wttrd v c l o c l t y f l u c t u a t f o ; ~ data at lllc~tt s i h ~ o ~ i ~ l f r ~ y r a t e s . For c ~ a n p l c , a t lcirst 2 to It sc.con:ls fn t l ~ c case of I133 2. Ye ~ r o u l J wolcome t h e opportunl ty to df scusl; tl11 s \ r f th D3E acid/or Cotto1 l c ~ ~ c r s o r ~ n c l . We feel I t ni:ccssary to Inecirir yo11 ,of tt~c t r . ~ ~ c r t n r ~ c c of \rind c ' ~ i ~ r a c t t : r l s t l c s In thc cfcsi2n of rrirrd tur:~lnc.s old

t l ~ r ? l r approi)riiltc Intcgratlotr It~to p w r systc-~s.

cc: I. .L. \Ilcrr:!c:lI, G a t t c l l c J.11. K1~r.i5augli, P3E

The S i g n i f i c a n c e o f the V ind F l u c t u a t i o n Model i n

Assessing Wind Turbine Dynamic Performance

D. M. Tr iezenberg

School o f E l e c t r i ca l Eng ineer i ng Purdue Un i ve r s i t y

West La faye t te , Ind iana 47307

A cons ide ra t i on i n the i n s t a l l a t i o n o f w ind t u r b i n e genera tors i n con-

ven t i ona l e l e c t r l c power networks i s the constancy o f t he power generated.

The power f l ow a t any p o i n t . i n a network i s no t a c t u a l l y constant ; i t va r ies

as the system load va r ies . But convent ional thermal c y c l e power genera t ion

u n i t s a re most e f f i c i e n t a t cons tant load. Thus one o f t he major c o n t r o l

cons idera t ions i n a power system i s ma in ta in ing vo l tages and frequency w i t h -

i n to le rances w h i l e m in im iz ing thermal c y c l e genera tor l oad ing changes. I f

the wind tu rb ines t h a t may be i n s t a l l e d i n the f u t u r e do no t have a r e l a -

' t i v e l y constan't ou tpu t , t h e i r presence wi.11 make o p e r a t i o n o f the t o t a l poker

system more d i f f i c u l t t o manage, as the power o u t p u t f l u c t u a t i o n s w i l l appear

t o the thermal c y c l e genera tors as i nc reased ' l oad f l u c t u a t i o n . E i t h e r , l o o s e r

to lerances on network u o l tage and frequency w i 1 1 have t o be accepted o r lower

e f f i c i e n c y o f t h e thermal c y c l e u n i t s (due t o l oad ing changes i n i t i a t e d by

c o n t r o l ac t i on ) w i l l r e s u l t .

Two f a c t o r s determine the ou tpu t power f l u c t u a t i o n s o f a w i n d turbiine.

One i s the f l u c t u a t i o n s o f w ind v e l o c i t y a t the t u r b i n e and the o t h e r i s the

dynamic e lect romechanical response o f t he t u r b i n e i t s e l f . O f these two

fac to rs , the second i s probably b e t t e r understood. Mathematical model i ng o f

e l e c t r i c gen'erators, r o t a t i n g i n e r t i a s , and p i t c h c o n t r o l systems i s f a i r l y

w e l l developed. I t appears, however, t h a t data on w ind ve loc i t y f l u c t u a t i o n s

- 10 - a t a s u f f i c i e n t l y h i g h frequency (perhaps 0.1 t o 1.0 rad ian per second) i s

lack ing .

P a r t o f t he problem i s t h a t t he e f f e c t i v e wind v e l o c i t y change over the

swept area o f t h e wind t u r b i n e blades may n o t be the same as those seen on the

ou tpu t reco rd o f a smal l anemometer. I t i s thought t h a t the l a r g e b lades tend

t o i n t e g r a t e the smal le r l o c a l i z e d tu rbu lence centers t h a t would be seen by

a smal l anemometer. T h i s cons ide ra t i on suggests t h a t when a s t o c h a s t i c w ind

f l u c t u a t i o n model i s developed, i t should encornpass s p a t i a l as w e l l as

temporal f l u c t u a t i o n s . The s p a t i a l aspect of the model would be e s p e c i a l l y

h e l p f u l i n the e v a l u a t i o n o f dynamic p e r f o n a n c e o f a w i n d m i l l "farm".

The IdASA/Boeing MOD-2 wind t u r b i n e has two b a s i c o p e r a t i n g nodes. When

wind v e l o c i t y i s l ess than 28 mph, the t u r b i n e i s unable t o develop f u l l o u t -

p u t power. Blade p i t c h i s h e l d cons tant so t h a t outp,ut power i s maximized. .

However, f o r wind v e l o c i t i e s above. 28 .mph, the second ope ra t i ng mode i s

empl.oyed t o 1 i m i t the t u r b i n e ou tpu t t o the genera tor thermal r a t i n g . I n

t h i s mode, b lade p i t c h i s cont inuous ly ad jus ted by a p r o p o r t i o n a l - i n t e g r a l

c o n t r o l l e r work ing o f f o f power e r r o r from a f u l l l o a d s e t p o i n t . The analog

computer s i m u l a t i o n repo r ted here represents bo th ope ra t i ng modes.

Computer s t u d i e s o f t h e t u r b i n e connected t o a l a r g e power system

have been made us ing two d i f f e r e n t models f o r the w ind f l uc tua t . i on . For

b o t h models, a d i s c r e t e ' i n t e r v a l b i n a r y n o i s e (i .e., a s igna l t h a t . swi tches

randomly between two l e v e l s ) i s passed through a s imp le f i r s t o r d e r l a g low

pass f i . l t e r . The two models d i f f e r i n t he f i l t e r t ime constant , howeve,r,

f o r b o t h models the f i l t e r g a i n i s ad jus ted so . that s tandard d e v i a t i o n o f

t h e f i l t e r ou tpu t i s 2 mph i n v e l o c i t y .

F igu re 1 shows the t u r b i n e 'response when the mean wind v e l o c i t y i s ,

30 rnph so t h a t the p i t c h c o n t r o l system i s f u l l y a c t i v e . I n t h i s case the

- 11 - low pass f i l t e r i n the wind model cuts o f f a t 0.01 radian per second. The

l a s t trace, labe led 6, i s the r e l a t i v e angle between the generator r o t o r and

a constant ve loc i t y reference. I t can be seen t h a t the power f l uc tua t i ons

are a small p a r t o f the t o t a l output. The f u l l load ra t i ng o f the MOD-2

generator i s 2.5 megawatts. Contrast t h i s w i t h Fig. 2, where the mean wind

v e l o c i t y i s 20 mph so t ha t the blades stay a t f i x e d p i t ch . The absence o f

p i t c h con t ro l a1 lows the power output t o change as much as 0.7 MW i n as 1 i t t l e

as 5 seconds. Such a performance would make the wind tu rb ine an unwelcome

a d d i t i o n t o a conventional power system. It would be des i rab le from a con-

t r o l s tandpoint t o use p i t c h con t ro l a t outputs less than rated, b u t t h i s

would make wind t u rb i ne i n s t a l l a t f o n much less economically a t t r ac t i ve .

Since a large f r a c t i o n o f the t o t a l annual output i s achieved a t wind

v e l o c i t i e s t ha t are lower than the ra t ing , ac t i ve p i t c h con t ro l a t low wind

v e l o c i t y would reduce t h i s output.

Figures 3 and 4 are a repeat o f Figs. 1 and 2, bu t the low pass f i l t e r

i n the wind f l uc tua t i on model now has a c u t o f f a t 0.1 radian per second. The

output f l uc tua t i ons i n bo th operat ing modes are much greater. C lear ly the

performance depicted i n I n I- i g . 4 i s unecrsptahle.

On the bas i s o f t h i s study, the inves t iga to rs o f wind tu rb ine system

integr a t i o n can formulate several questions for the invest i qa to rs o f wind

charac te r i s t i cs . Which wind f l u c t u a t i o n i s more r e a l i s t i c , t ha t o f Fig. 1

o r t h a t o f Fig. 31 I f n e i t h e r i s reasonable, then what model i s appropr iate

f o r the e f f e c t i v e wind f l u c t u a t i o n on a s i n g l e turb ine? I f two wind turb ines

are 1000 feet apart, arc the i nd i v i dua l wind f l uc tua t i ons cor re la ted?

I n summary, i t i s f e l t t h a t adequate in format ion on wind character i s t 1 cs

i s not present ly i n the hands o f those who must p lan wind turb ine i n l c y r a t i o n

i ,,to convcntianal power sys tcms. I t would seem t h a t an appropr iate f 1 uctuat i on

model would be a w e l l def ined s tochas t i c process.

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- 16 -

APPENDIX C

Mr. Jeffrey H. l~unbaugh U t v i slon o f E lec t r i c Cngery Systems O f f i ce o f Energy Technology bcportmant of Energy 20 flassachusetts Ave., N.W. Wash i ngton, D.C. 20585

Dear Jeff:

Enclosed i s o report en t i t l o d "Possible Problcms w l t h the t10D-2 Blade P i tch Control," This report describes the problens wh Ich nay occur I n the tiOD-2 system as n resu l t o f p i t ch control act ion during a decrease and subsequent increaso i n wind velocity, As you may re- c a l l I described thfs to you and tlASA on Dccerber 3, 1979. A f te r reading the report you m y f i n d i t appropriate t o send copies t o Boeing and INSA.

Paul C, Krause Professor of E lac t r l ca l Engineering

POSS l BLE PROBLEMS W l TH THE

MOD-2 BLADE PITCH CONTROL

P.C. Krause

School o f E l e c t r i c a l Engineering Purdue Univers i ty

West Lafayette, l ndiana 47907

Since tlovember 1978 we have been involved i n the analysis and simulat ion

of the HOD-2 wind energy system. Our work began under a NASA grant and i s

now cont inuing under a DOE contract. Several reports and papers have been

w r i t t e n on the work thusfar [I-61. During the one year contract w i t h NASA

we implemented a hybr id computer s imulat ion o f two MOD-2's a t HASA/Lewis

Research Center and a t Purdue. I t was discovered tha t the blade p i t c h con-

t r o l could cause a system instabi 1 i t y fol lowing a "down gust" i n wind. I n

par t i cu la r , i f the system were i n i t i a l l y operating i n a wind ve loc i t y o f

s l i g h t l y above 28 mph so that the blade p i t c h contro l i s i n the high mode

and if a down gust occurred caus i l l w t h e blade p i t c h contro l t o switch t o the

lm mode and then back t o the high mode, an i n s t a b i l i t y could occur resu l t i ng

from continuous switching between the two nodes. This was demonstrated t o

Boeing by NASA a t which time i t was a lso pointed out that an increase i n the

ra te l i m i t o f the p i t c h control would prevent the instabi 1 i ty. Hence the

r a t e 1 i m i t has been increased from approximately 4 deg/sec t o 8 deg/sec.

Figures 1 and 2 show the behavior o f the MOD-2 during a down gust o f

1-cosine waveform from 30 mph t o 20 mph and back t o 30 nph i n 10 sec. Figure

1 i l l u s t r a t e s the sustained o s c i l l a t i o n which occurs w i t h a ra te l i m i t of

4 deg/sec, On the NASA simulat ion a n o s c i l l a t i o n , increasing i n amplitude,

was noted f o r s l i g h t l y d i f f e r e n t operat ing condit ions. Figure 2 shows the

behavior o f the HOD-2 w l t h a ra te l i m i t o f 10 deglsec. I n each f igure the

f o l lowlng system var iab les are p lo t ted.

= per u n l t wind torque on blade 1 (each blade i s

represented as a lumped mass).

- "B 1

= pe r cent change i n the speed o f blade 1.

TBH per un i t torque t ransmi t ted from blades t o hub.

VT = per u n l t magnitude o f the terminal voltage. - - r 1 , ,

-: *,--- . - - - -

" B % blade p i t c h angle ill deg.-- - - - . - - - - I i*

AuG per cent change i n generator speed.

6 = r o t o r angle i n e l e c t r i c a l deg.

Y ' T = pe r u n l t electromagnetic generator torque. - 1

e

Changing the r a t e l i m i t from 4 t o 10 deg/sec e l im ina ted the unstable con-

d l t ions, however, t h e l a rge torque on the blades and gears as we1 1 as the

v a r i a t i o n I n termina l vo l tage dur ing the down gust w i t h the 10 deg/sec r a t e

- ' - 1 i n1 t (Figure 2) are of concern. I n t h i s mode o f operat ion the s o f t sha f t - - i s not p ro tec t i ng t i le blades and gears from large torque var ia t ions.

T

7 Slnce i t i s d i f f i c u l t f o r us t o obta in the up-to-date design o f the blade

. - - 4

-) p i t c h con t ro l ou r representat ion may not prot ray the present operat ion o f t h i s - cont ro l system. Regardless o f possible inaccuracies, i t i s recommended tha t b:=

NASA and Boeing be made aware o f these resu l t s by means o f a copy o f t h i s

report . -

1. D.M. Triezenberg, "Wind Turbine-Generator Simulat ion Seminar," ( ~ r i t t e n Report), May 1979.

2. M.C. Reynolds, " S t a b i l i t y o f Wind Turbine Generators t o Wind Gusts," Purdue U n i v e r s i t y Technical Report, TR-EE 73-20, May 1979.

3. L.J. G i l b e r t and D.M. Triezenberg, Lewis Research Center, "Studies o f M u l t i p l e Large Wind Turbine Generators on a U t i l i t y Network," Workshop on Economic and Operational Requirements and Status o f Large Scale Wind Systems, 14on terey, Cal i fo rn i a, March 28-30, 1979.

4. L.J. Gf tber t and D.M. Trtezenberg, "Simulation StudIes of H u l t i p l e Large Wfnd Turblne Generators on a U t i 1 i t y Network," Workshops on Large Wind Torblne Design Character is t ics and R and D Requirements, Cleveland, Ohio, Apr i 1 24-26, 1979.

5. D.H. Trlezenberg, "The Signif icance of the Wfnd Fluctuat ion Model i n Assessing Wind Turbine Dynamic Performance," a pos i t ion paper, Ju ly 1979.

6 . P.C. Krause, t'Analysls and Simulation o f Wind Energy Systems," F ina l Report f o r NASA (In d r a f t form), November 1979.

P.C. Ksause December 12, 1979

Fig. 1.- Ten MPH Down Gust w i th 4 deglsec. Rate L i m i t i n Pitch - Controller.

- 21 -

~ l g * 2, Ten HPH Down Gust with 10 dcglsec. Rate L f m l t in P i t c h Controller.