Variable speed AC-generators in wind energy convertors

Abstract

S-412 96 Goteborg, Sweden SUMMARY T h i s paper d e s c r i b e s two v a r i a b l e speed a cd r i v e systems, a synchronous g e n e r a t o r w i t h f r e q u e n c y c o n v e r t o r f o r l a r g e speed r a n g e s and a r o t o r c a s c a d e i n d u c t i o n g e n e r a t o r f o r s m a l l v a r i a t i o n s of s p e e d . R e s u l t s o b t a i n e d w i t h t r a n s i e n t s t a t e c a l c u l a t i o n s u s i n g c o m p u t e r models a s w e l l a s v e r i f y i n g l a b o r a t o r y t e s t s a r e p r e s e n t e d . T h e synchronous g e n e r a t o r system t o g e t h e r w i t h a d i g i t a l c o n t r o l system i s o p e r a t i n g on a 4 0 k W w i n d m i l l , f o r t e s t p u r p o s e . The c o n v e r t o r s i n t r o d u c e h a r m o n i c s i n t o t h e c u r r e n t s i n t h e g e n e r a t o r s and t h e r e b y a l s o harmonics i n t h e e l e c t r i c a l t o r q u e and i n t o t h e s h a f t s of t h e g e n e r a t o r s . A t e m p e r a t u r e r i s e i s a l s o c a u s e d by t h e h a r m o n i c c u r r e n t s . B y c u r r e n t c o n t r o l o f t h e i n v e r t o r s t h e e l e c t r i c a l t o r q u e i s c o n t r o l l e d , and by t h i s c o n t r o l t h e maximum t o r q u e i n t h e d r i v e t r a i n of t h e w i n d m i l l i s l i m i t e d . O p t i m a l o p e r a t i o n can a l s o be achieved i n low wind c o n d i t i o n s . -V a r i a b l e speed o p e r a t i o n of a c machines has become i n c r e a s i n g l y f r e q u e n t i n m o t o r a p p l i c a t i o n s d u r i n g t h e l a s t d e c a d e . The r e a s o n s f o r t h i s a r e s e v e r a l , e . g . r e d u c t i o n of energy consumption, new power e l e c t r o n i c s t e c h n o l o g y a n d a p p l i c a t i o n s i n new i n d u s t r i a l p r o c e s s e s . Generator a p p l i c a t i o n s of a c machines, have u n t i l some y e a r s ago, been r e s t r i c t e d t o e s s e n t i a l l y c o n s t a n t speed o p e r a t i o n . However, with t h e i n c r e a s e d i n t e r e s t i n s m a l l -s c a l e e n e r g y r e s o u r c e s , v a r i a b l e speed o p e r a t i o n of g e n e r a t o r s h a s been d i s c u s s e d . I n wind e n e r g y c o n v e r t o r s , € o r example, v a r i a b l e s p e e d o p e r a t i o n would i n c r e a s e e n e r g y p r o d u c t i o n a n d r e d u c e m e c h a n i c a l s t r e s s e s and n o i s e i n low wind c o n d i t i o n s . A s a r e s u l t o f t h e s e i m p r o v e d c h a r a c t e r i s t i c s , t h e number of v a r i a b l e speed g e n e r a t o r s i n wind energy c o n v e r t o r s has i n c r e a s e d i n r e c e n t y e a r s . 2.With an e l e c t r i c a l g e n e r a t o r , i n a windmill, t h e m e c h a n i c a l power i s c o n v e r t e d i n t o e l e c t r i c a l power. The f r e q u e n c y o f t h e v o l t a g e and t h e c u r r e n t i n t h e g e n e r a t o r i s p r o p o r t i o n a l t o t h e r o t a t i o n speed of t h e m e c h a n i c ...

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