Aalborg Universitet Design of Energy Storage Control Strategy to Improve the PV System Power Quality Lei, Mingyu; Yang, Zilong; Wang, Yibo; Xu, Honghua; Meng, Lexuan; Quintero, Juan Carlos Vasquez; Guerrero, Josep M. Published in: Proceedings of IECON 26: 42nd Annual Conference of the IEEE Industrial Electronics Society DOI (link to publication from Publisher):.9/IECON.26.77947 Publication date: 26 Document Version Early version, also known as pre-print Link to publication from Aalborg University Citation for published version (APA): Lei, M., Yang, Z., Wang, Y., Xu, H., Meng, L., Quintero, J. C. V., & Guerrero, J. M. (26). Design of Energy Storage Control Strategy to Improve the PV System Power Quality. In Proceedings of IECON 26: 42nd Annual Conference of the IEEE Industrial Electronics Society (pp. 222-227). IEEE Press. DOI:.9/IECON.26.77947 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.? You may not further distribute the material or use it for any profit-making activity or commercial gain? You may freely distribute the URL identifying the publication in the public portal? Take down policy If you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from vbn.aau.dk on: maj 3, 28
Design of Energy Storage Control Strategy to Improve the PV System Power Quality Mingyu Lei, Zilong Yang, Yibo Wang,Honghua Xu Institute of Electrical Engineering Chinese Academy of Sciences Beijing, China Lexuan Meng, Juan C. Vasquez, Josep M. Guerrero Research Programme on Microgrids Department of Energy Technology Department Aalborg University, Demnark www.microgrids.et.aau.dk Abstract- Random fluctuation of PV power is becoming a more and more serious problem affecting the power quality and stability of grid as the PV penetration keeps increasing recent years. Aiming at this problem, this paper proposed a control strategy of energy storage system based on Model Predictive Control (MPC). By the continuous optimizing of MPC, we can obtain the system parameters accurately, and then calculate the energy storage power. At the same time, this control took state of charge (SOC) and other parameters into account to ensure the health and stability of the energy storage units. Finally, simulation results proved the proposed control strategy had good performance and robustness. Keywords- engrgy storage; power fluctuation; power quality; model predictive cotnrol; I. INTRODUCTION In recent years, more and more grid-connected photovoltaic systems are being in operation around the world. As a result, the fluctuation and randomness of photovoltaic power is an important factor affecting the power quality of grid []. As the maximum power point tracking (MPPT) control is used in most of the PV systems, the output power necessarily varies with the change of illumination intensity and some other enviromnental factors [2]. In addition, the algorithm of the PV converter controllers and the parameters may also result in the variation of the output power [3]. This problem would be more serious when photovoltaic power systems access to low voltage distribution network, whose capacity is relatively small. Many researches have proved application of energy storage units is an effective method to reduce the impact of PV power fluctuation. Obviously, the control strategy of energy storage systems is one of the most important aspects in related researches. In [4] a power management method for a hybrid system of PV and energy storage was proposed, which will keep power balance inside the a micro grid and keep the energy storage system always operating at a healthy state. Paper [5] examined aggregated and distributed connection topologies of the ESS technologies, and found the aggregated ESS could have a lower power rating due to smoothing effect in a wind farm. Paper [6] presented an energy manager for energy storage system in micro-grids. A battery lifetime model that uses the proposed Peukert lifetime energy throughput based on the workload of the battery is developed. The objectives of the energy manager are focused on improving the energy efficiency and extending the life expectancy of ESS while ensuring constraints of energy storage modules are complied with. However, conventional control methods often use the data of past sampling periods, resulting in obvious delays in the output power of energy storage systems [7]. The delays will waste a large amount of energy of the storage units and increase the capacity requirements. This paper proposed a control strategy of the energy storage system based on model predictive control (MPC). This control helps to maintain the power grid voltage stability and reduce voltage fluctuations when the PV system is working at the maximum power point. Another target of it is to avoid the energy storage units over charged or over discharged. MPC can provide a control command to the energy storage system without a delay by using prediction. This improvement can save a large part of the energy storage capacity. In this paper, the values predicted in the MPC are the grid impedance and other parameters. As a result, we can obtain the relatively accurate predictive value by rolling optimization, and the real value won't change over time. So this control strategy has better accuracy and robustness. Finally, simulation results proved the effectiveness and stability of the proposed control strategy. Line Impedance Transformer Energy Storage Converters PV Modules Fig.l Hybrid system model of photovoltaic generator and energy storage units www.microgrids.et.aau.dk 222
I I. S Y S T E M M O D E L T h e h y b r i d s y s t e m o f p h o t o v o l t a i c g e n e r a t o r a n d e n e r g y s t o r a g e u n i t s a r e s h o w n i n F i g. l. I n t h i s s y s t e m, t h e m a x i m u m p o w e r o f P V i s k W a n d t h e c a p a c i t y o f e n e r g y s t o r a g e i s 2 k W h. : h e y a r e c o n n e c t e d t o a 3 8 V t h r e e -p h a s e A C b u s, t o g e t h e r w I t h a l o c a l l o a d. T h i s h y b r i d s y s t e m i s r e g a r d e d a s a? e n e r a t i n g u n i t c o n n e c t e d t o t h e g r i d, a n d i t s t o t a l o u t p u t p o w e r I S e x p e c t e d t o b e s m o o t h. B y c o n t r o l l i n g t h e e n e r g y s t o r a g e c h a r g i n g a n d d i s c h a r g i n g p o w e r, w e c a n r e d u c e t h e e f f e c t o f P V p o w e r fl u c t u a t i o n a n d i m p r o v e t h e p o w e r q u a l i t y, s u c h a s v o l t a g e s t a b i l i t y a, t t h e p o i n t o f c o m m o n c o u p l i n g ( P C C ). I n t h i s s y s t e m, t h e e n e r g y s t o r a g e u n i t s a r e c o n n e c t e d t o t h e A 9 b u s t o g e t h e r w i t h t h e P V. T h e r e a r e s e v e r a l a d v a n t a g e s o f t h I S s tr u c t u r e : fi r s t l y, w e d o n ' t n e e d t o r e p l a c e t h e P V i n v e r t e r s a n d t r a n s f o r m e r s t o l a r g e r o n e s. S o w e c a n i n s t a l l t h e e n e r g y s t o r a g e s y s t e m i n a b u i l t P V s y s t e m. S e c o n d l y, w e d o n ' t n e e d t o s t op t h e P V g e n e r a t o r s w h e n r e p a i r i n g o r r ep l a c i n g a n e n e r g y s t o r a g e u n i t. A s w e k n o w, a g i n g i s a c o m m o n p r o b l e m o f e n e r g y s t o r a g e u n i t, s u c h a s b a t t e r y. S o t h i s s t r u c t u r e c o n tr i b u t e s t o t h e s t a b i l i t oy f t h e h y b r i d s y s t e m. T h i r d l y, t h i s s t r u c t u r e i s l e s s c o m p l e x a n d e a s i l y c o n tr o l l e d. 9 8 7 6 5 4 3 2 2 3 4 5 6 7 7 6 5 4 3 2 F i g. 2 3 - d a y s o u t p u t p o w e r c u r v e o f a l O O k W p P V s y s t e m 2 3 4 5 6 7 F i g. 3 l O - m i n u t e l a r g e s t c h a n g e c u r v e o f 3 - d a y s P V p o w e r W e a n a l y z e t h e fl u c t u a t i o n c h a r a c t e r i s t i co s f t h e P V o u tp u t p o w e r. C h a n g e o f e n v i r o n m e n t i s t h e m a i n c a u s e o f fl u c t u a t i o n o f P V o u t p u t p o w e r. T h e fl u c t u a t i o n m a y a f f e c t t h e p o w e r q u a l i t y o f g r i d b a d l y w h e n n a t u r a l f a c t o r s c h a n g e s h a rp l y [ 8 ]. F i g. 2 s h o w s a 3 - d a y s o u tp u t p o w e r c u r v e o f a k W p P V s y s t e m. T o a n a l y z e i t s c h a r a c t e r i s t i c s w, e c a l c u l a t e d t h e m a x i m u m p o w e r c h a n g e o f - m i n u t e h o r i z o n, a s s h o w n i n F i g. 3. W e c a n s e e t h e m a x i m u m v a l u e o f - m i n u t e l a r g e s t p o w e r c h a n g e i s S 9. S k W, a c c o u n t i n g f o r n e a r l y 6 % o f t h e n o m i n a l p o w e r o f t h e P V s y s t e m. I n t h i s s i t u a t i o n t, h e v o l t a g e c h a n g e w o u l d b e p r e t t y g r e a t, e s p e c i a l l y w h e n t h e n o m i n a l p o w e r o f a P V s y s t e m i s l a r g e o r i t c o n n e c t e d i n t o t h e l o w v o l t a g e d i s tr i b u t i o n n e t w o r k s. I n v i e w o f t h i s s i t u a t i o n, w e p r o p o s e d a n o v e l c o n tr o l m e t h o d b a s e d o n M P C. T o s o l v e t h e p o w e r fl u c t u a t i o n a n d p o w e r q u a l i t y p r o b l e m s, w e u s e l a r g e - c ap a c i t y e n e r g y s t o r a g e s y s t e m t o s m o o t h t h e p o w e r a n d i m p r o v e t h e p o w e r q u a l i t y. T h e e n e r g y s t o r a g e t e c h n o l o g y u s e d i n a d i s tr i b u t e d g e n e r a t i o n s y s t e m s h o u l d h a v e f o l l o w i n g a d v a n t a g e s : T h e e n e r g y s t o r a g e u n i t s m u s t h a v e h i g h d e g r e e o f s a f e t y a n d s t a b i l i t y T. h e y s h o u l d b e g u a r a n t e e d t o w o r k w e l l w i t h o u t m a i n t e n a n c e [ 9 ]. S e c o n d l y, t h e e n e r g y s t o r a g e t e c h n o l o g y s h o u l d h a v e g o o d e c o n o m y, s i n c e t h e s t o r a g e c a p a c i t y i rs a t h e r l a r g e t o e n s u r e l o n g - t e rm c h a r g i n g o r d i s c h a r g i n g. T h i r d l y, t h e e n e r g y s t o r a g e s y s t e m s h o u l d a l s o b e i n s t a l l e d e a s i l y b e c a u s e m a n y d i s t r i b u t e d g e n e r a t i o n s y s t e m s a r e l o c a t e d i n r e m o t e a r e a s [ ]. A ft e r c o n s i d e r i n g t h e p e r f o r m a n c e a n d c h a r a c t e r i s t i cos f v a r i o u s c o n v e n t i o n a l e n e r g y s t r a g e t e c h n o l o g i e s, w e c h o o s e b a t t e r i e s a s t h e s t o r a g e u n i t s i n t h I s p p e r. R e c e n t y e a r s, e n e r g y s t o r a g e b a t t e r i e s a r e w i d e l y u s e d il l-i p o w e r s y s t e m s a n d r e n e w a b l e g e n e r a t i o n s y s t e m s, o w i n g t o i t s t e c h n o l o g i c a l m a t u r i t y a n d e c o n o m i c a d v a n t a g e. S o w e s h o u l d c o n s i d e r t h e o p e r a t i n g c h a r a c t e r i s t i cws h e n w e a r e d e s i g n i n g t h e c o n t r o l s tr a t e g i e s. I I I. C O N T R O L S T R A T E G Y O F E N E R G Y S T O R A G E S Y S T E M M P C i s w i d e l y u s e d i n p r o c e s s c o n tr o l, a s w e l l h a v e b e e n u s e d t o c o n tr o l p o w e r c o n v e r t e r s a n d s o m e s w i t c h i n g p o w e r s u p p l i e s [ ]. M P C c a n m a k e a g r e a t c o n t r i b u t i o n t o t h e a c c u r a c y a n d t i m e l i n e s s o f t h e c o n tr o l l e d s y s t e m b e c a u s e i t h a s t h e a b i l i t y t o p r e d i c t t h e fu t u r e c h a n g e o f t h e c o n t r o l l e d s y s t e m [ 2 ]. T h e o v e r a l l p r o c e s s o f M P C i s c o n s i s t e d o f t h r e e s t ep s : p r e?i c t. 8 g d y n a m i c m o d e l s o f t h e c o n t r o l l e d s y s t e m, o p t l m l z m g m a n i p u l a t e d v a r i a b l e s, a n d c o r r e c t i n g d y n a m i c m o d e l s a c c o r d i n g t o t h e c o l l e c t e d d a t a. A. O p t i m i z a t i Co on s t F u n c t i o n T h e t a r g e t o f t h e p r o p o s e d m o d e l p r e d i c t i v e c o n tr o l i s t o r e d u c e v o l t a g e d e v i a t i o n c a u s e d b y P V p o w e r c h a n g e s, a t t h e s a m e 7i m e, t r y t o a v o i d e n e r g y s t o r a g e u n i t s o v e r c h a r g e d o r o v e r d I s c h a r g e d. T h e o p t i m i z a t i o n c o s t fu n c t i o n i s g i v e n b y ( I ). J m i n [ I :2= I - WI O * ( I S O C - S O C* I ) + "" k n- o -I 2 w 2 * ; "" k - l U t) - V*U ) ) 2 D m 2 D i = k - m2 V*U ) ] ( I ) W h e r e W I-I a n d W 2-I a r e t h e w e i g h t c o e ffi c i e n t s o f t h e t w o p a r t s o f c o s t fu n c t i o n s r e sp e c t i v e l y. S O C i s t h e s t a t e o f c h a r g e? fh te e n e r g y s t o r a g e s y s t e m, w h o s e i d e a l v a l u e i s S O C *. V ) I S t hẹ e a s u r e d v o l t a g e v a l u e o f p o i n t o f c o m m o n c o u p l i n g, a n d I t S I d e a l v a l u e i s V * O ), t h e n o m i n a l v o l t a g e o f t h e g r i d. n i s t h e h o r i z o n w i d t h o f r o l l i n g o p t i m i z a t i o n i n M P C s tr a t e g y. m i s t h e h o r i z o n w i d t h o f c a l c u l a t i n g t h e v o l t a g e m e a n s q u a r e v a l u e. T h i s c o s t fu n c t i o n c o n fi r m s t h e c o n t r o l t a r g e t s : s m o o t h i n g t h e 2 2 3
PCC voltage to improve the power quality, and keeping the energy storage units working in a healthy condition. The coefficients WI and W2 are determined by the fluctuation characteristics of the PV power and the capacity of the energy storage system. When the capacity of the storage system is rather large or the fluctuation of the PV power is not very dramatic, we can increase the proportion of W2 to improve the smoothing result. Otherwise, we should increase the proportion of wi to ensure the security and health of energy storage units. The state of charge of the batteries is another important factor to influence the control effect and security. It is defined as the available capacity expressed as a percentage of the whole capacity of a battery. The SOC is calculated by socu + ) = SOC(k) + [ lfl (k) + uu + ) ] *lo.t (2) Where Pbat is the energy storage power, lo.t is the width of the sampling horizon, and C is the designed capacity of the energy storage system. In every control cycle, we predict the future SOC value with the calculated energy storage power. B. Constraint of the Control Another advantage of MPC is that the constraint conditions can be involved in the control process. In this paper, the SOC is not only used to calculate the cost function at every step, but also a constraint of the control. When we use batteries as energy storage units, we must ensure SOC of any battery does not exceed the range [.2 ]. In addition, when we use the batteries as the energy storage units, the limitation of energy storage power should also be taken into account. Take lead acid battery for example, usually the maximum charge rate is.25c, and discharge rate is.5c. To summarize, the constraint function of this control is given by c At the same time, the health and service life are taken into account. Battery energy storage is widely used in micro grids and distributed generation systems. Therefore, we choose the batteries as the energy storage units in this paper. To avoid the batteries over charged or over discharged, the sampling SOC value is used as feedback to the energy storage power control. Based on above analysis, the state variable and output variable can be calculated by (5) and (6). x(k + ) x(k) = +,,)k) + u(k + ) + [Ppv!'(k + ) - Ppv(k)] y(k + ) = _x_(k_+ ) _ - y(k) _P...c.c (5)!o=ad _(k_) * ZL + Uo (6) The manipulated variable u(k+ ) is the change value of energy storage system power. x(k) is the total power of photovoltaic and energy storage. y(k) is the voltage of PCC. The rolling optimization can be completed by taking (5) and (6) into the cost function (). Ppvf (k+ I) is the predicted value of PV power, and Ppv(k) and Pbat(k) are the measured PV power and energy storage power respectively. ZL is the grid impedance parameter, which is unknown and differs between different systems. When we obtain ZL close to the real value, we can model the system accurately and this parameter won't change over time. Taking ZL into expressions of state variable x and output variable y -. 25P* < [ 7f)k) + u(k + )] <.5P* (3) O. 2 SOC(k) [,,)k) + u(k + ) ] + *lo.t (4) c Correcting Parameter ZL Optimizing Cost Function J Calculating control command u(k+l) C. Model Expression Model prediction is necessary in energy storage power charge and discharge control, in this process, we need to predict the PV power, load power and energy storage power. Among them, The PV power can be obtained by short-term PV power forecasting. The predicted load power is considered the same with the sampling value of the last horizon because it changes slowly. The energy storage power is the manipulated variable of the control. Above power values are predicted with errors inevitably, but the control strategy proposed in this paper is able to offset forecasting error partly by optimizing the model parameters. In other words, the predicted parameters of the system model may not be the same with the real ones, but they can make sure the output variable of the control will be the same with the sampling result. Measuring the PCC voltage Vpcc Predicting PCC voltage V*pcc * < :cc=;:p:c.. --"';"'--_...J l Voltage prediction error Fig.4 Flow chart of the proposed control 224
I n t h i s p a p e r, t h e v a l u e o f Z L i s c o rr e c t e d a c c o r d i n g t o t h e e rr o r b e t w e e n t h e p r e d i c t e d a n d a c t u a l v o l t a g e o f p e e. W e c a l c u l a t e t h e p r e d i c t e d p e e v o l t a g e b y ( 5 ) a n d ( 6 ), a n d t h e n c o m p a r e i t w i t h t h e s a m p l i n g r e s u l t a nt e x t c o n tr o l p e r i o d t o o b t a i n t h e p r e d i c t i o n e rr o r, w h i c h i s u s e d t o c o rr e c t t h e m o d e l. A n a c c u r a t e m o d e l o f t h e h y b r i d s y s t e m c a n g u a r a n t e e t h e c o rr e c t n e s s o f t h e c o n tr o l. T h e fl o w c h a r t o f t h i s c o n tr o l s t r a t e g y i s s h o w n i n F i g A. 2 3 5-I I V. R E S U L T O F S I M U L A T I O N I n o r d e r t o p r o v e t h e p r o p o s e d c o n tr o l a l g o r i t h m i s e f f e c t i v e, s i m u l a t i o n s w e r e c ar r i e d o u t b y M A T L A B / S i m u l i n k. T h e d a t a a n d p a r a m e t e r s a r e g a i n e d fr o m t h e a c t u a l P V s y s t e m, w h o s e s t r u c t u r e i s s h o w n i n F i g. l. T h e p a r a m e t e r s o f t h i s h y b r i d s y s t e m a r e l i s t e d i n T a b l e I. W e u s e P V p o w e r d a t a fr o m a n a c t u a l s y s t e m i n t h i s s i m u l a t i o n, w h i c h i s s h o w n i n F i g. 5. T h e p o w e r fl u c t u a t e d g r e a t l y d u r i n g t h i s p e r i o d o f t i m e. Power (kw) 9 8 7 6 5 4 3 2 2 4 6 8 2 4 6 8 F i g. S P V p o w e r c u r v e o f 8 s 2 -I I-I I-I I-I I-I I-I o-i 2 -I 4 -I 6 -I 8 -I -I 2 -I 4 -I 6 -I 8 -I T i m e ( 5 ) F i g. 6 P C C v o l t a g e c u r v e b e f o r e s m o o t h e d b y e n e r g y s t o r a g e s y s t e m A s e r i e s o f s i m u l a t i o n s w e r e c o n d u c t e d o n t h e P V d a t a a n d p a r a m e t e r s. F i r s t l y, w e d i d a s i m u l a t i o n w i t h o u t t h e e n e r g y s t o r a g e u n i t s, a n d t h e r e s u l t i s s h o w n i n F i g. 6. I n t h i s s i t u a t i o n, w e c a n fi n d v o l t a g e c h a n g e d g r e a t l y b e c a u s e o f t h e d r a m a t i c fl u c t u a t i o n o f P V p o w e r, w h i c h w i l l a f f e c t t h e p o w e r q u a l i t y o f t h e s y s t e m. W h e n w e c o n t r o l t h e b a t t e r y e n e r g y s t o r a g e s y s t e m c h a r g i n g a n d d i s c h a r g i n g b y t h e p r o p o s e d c o n tr o l s t r a t e g y, t h e p e e v o l t a g e c u r v e a n d t h e v o l t a g e d e v i a t i o n c u r v e d u r i n g t h i s p e r i o d o f t i m e a r e s h o w n i n F i g. 7. T h e P V p o w e r f o r e c a s t i n g r e s u l t u s e d i n t h i s s i m u l a t i o n h a d a l o w e r a c c u r a c y t h a n c u r r e n t a d v a n c e d l e v e l, i n o r d e r t o p r o v i n g t h e c o n t r o l m e t h o d i s s t i l l e f f e c t i v e w h e n t h e P V p o w e r f o r e c a s t i n g i s n o t a c c u r a t e e n o u g h. T h e f o r e c a s t i n g a v e r a g e e rr o r i s 5 %, a n d m e a n s q u a r e e rr o r i s 3 %. T h e w e i g h t c o e f fi c i e n t s o f c o s t fu n c t i o n s W I-I a n d W 2-I w e r e. 3 a n d. 7 r e s pe c t i v e l y. T h e i d e a l v a l u e o f s o e i s. 8, w h i c h i s l a r g e e n o u g h t o m a k e s u r e t h e b a t t e r i e s h e a l t h y a n d l e a v e s o m e r o o m t o c h a r g e w h e n t h e P V p o w e r h a d a d r a m a t i c r i s e. T A B L E I. T A B L E I. T H E P A R A M E T E R S O F-I T H E S I M U L A l i O N-I P a r a m e t e r s s y m b o l v a l u e u n i t 7 6 PV PV+Batteries N o m i n a l b u s v o l t a g e V * 3 8 V N o m i n a l fr e q u e n c y r 5 H z P V n o m i n a l p o w e r P p, k W T h e m a x i m u m l o a d p o w e r P a d 7 k W C a p a c i t y o f e n e r g y s t o r a g e s y s t e m C * 2 k W h L i n e R e s i s t a n c e R l i l l e. l 5 Q-I L i n e I n d u c t an c e L l i n e 2 m H Voltage Deviation (%) 5 4 3 2 2 4 6 8 2 4 6 8 F i g. 7 T h e v o l t a g e d e v i a t i o n c u r v e s b e f o r e a n d a ft e r s m o o t h e d b y e n e r g y s t o r a g e s y s t e m W e c a n f m d t h a t t h e p e e v o l t a g e fl u c t u a t i o n c a n b e s m o o t h e d e f f e c t i v e l y. T h i s r e s u l t s h o w s t h a t t h e c o n t i n u o u s r o l l i n g o p t i m i z a t i o n o f t h e M P e h e lp e d i m p r o v e t h e a c c u r a c y o f t h e c o n t r o l o v e r t i m e. W h e n t h e a c c u r a t e m o d e l w a s o b t a i n e d, t h e c o n tr o l r e s u l t w o u l d b e v e r y g o o d, a n d t h e i n fl u e n c e o f t h e p h o t o v o l t a i c p o w e r p r e d i c t i o n e r r o r w a s r e d u c e d o b v i o u s l y. D u r i n g t h e s a m e p e r i o d o f t i m e, t h e s o e c u r v e o f e n e r g y s t o r a g e s y s t e m i s s h o w n i n F i g. 8. W e c a n f m d s o e w a s c o n t r o l l e d i n a r e a s o n a b l e r a n g e o w i n g t o t h e c o s t fu n c t i o n a n d t h e c o n s tr a i n t fu n c t i o n s. 2 2 5
. 4 8-I U. 4 4-I o. 4 6 - - - - I-I - - - - - - I-I - - - T-I - - - j - - - - - - -. 4 2-I- - - - - - - --+-I - - - +- - - - - - - -- --I - - - +- - - -. 4-I. 3 8-I -I -I -I -I -I -I o-i 2 -I 4 -I 6 -I 8 -I -I 2 -I 4 -I 6 -I T i m e ( 5 ) F i g. 8 S O C c u r v e o f t h e M P C A t t h e s a m e t i m e, a s e r i e s o f s i m u l a t i o n s w e r e c a rr i e d o u t t o c o m p a r e t h e e f f e c t o f M P C w i t h t h e t r a d i t i o n a l L o w P a s s F i l t e r i n g ( L P F ) m e t h o d i n c o n t r o l l i n g t h e e n e r g y s t o r a g e i n a P V s y s t e m. T h e L P F a l g o r i t h m n o w i s w i d e l y u s e d i n c o n tr o l l i n g e n e r g y s t o r a g e s y s t e m t o s m o o t h t h e d i s tr i b u t e d g e n e r a t i o n o u t p u t p o w e r. T h e s i m u l a t i o n t i m e a n d c o n tr o l s t ep s i z e a r e b o t h t h e s a m e. T h e v o l t a g e d e v i a t i o n c u r v e s o f t h e s e t w o c o n t r o l m e t h o d s a r e s h o w n i n F i g. 9, a n d t h e S O C c u r v e o f L P F m e t h o d i s s h o w n i n F i g. O. W e c a n fi n d, a t t h i s t i m e, t h e S O C c h a n g e d m u c h g r e a t e r t h a n t h a t o f M P C, w h i l e t h e v o l t a g e d e v i a t i o n o f L P F w a s a l s o l a r g e r t h a n M P C. I f w e i n c r e a s e t h e t i m e c o e ffi c i e n t o f L P F t o m a k e t h e p o w e r s m o o t h e r, t h e b a t t e r i e s w o u l d b e o v e r c h a r g e d o r o v e r d i s c h a r g e d. I n o t h e r w o r d s, t h e p r o p o s e d M P C m e t h o d c a n i m p r o v e t h e p o w e r q u a l i t y m o r e e ffi c i e n t l y t h a n tr a d i t i o n a l m e t h o d s, o w i n g t o i t s a b i l i t y o f p r e d i c t i n g t h e fu t u r e e v e n t s..4.2.8.6.4.2. 5-I.4 5-I.4-I F i g. 9 V o l t a g e d e v i a t i o n c u r v e s o f L P F c o n t r o l a n d M P C I-I I-I I-I < -I _ L-I -I _. 3 5-I - - - - - - _ - ı-i -I I-I I-I I-I -I -I -I -I. 3-I - - - - - - --.l-i _ L-I -I _ ł-i -I.2 5-I.2 "- -- - 2 @ - 4 øo O - ß 8 -I - -, O O -"c @ - -" 2, -I- -, 4 @ - 6,-" -I - --" 8 -I "- --œ 6 -"c T i m e-i ( 5 ) F i g. I O S O C c u r v e s o f L P F c o n t r o l LPF MPC 2 4 6 8 2 4 6 8 W e c a n a l s o fi n d a p h e n o m e n o n : w h e n t h e P V p o w e r h a d a d r a m a t i c fl u c t u a t i o n, t h e v o l t a g e c u r v e o f L P F c o n tr o l c h a n g e d m o r e s h a rp l y t h a n t h a t o f M P C. S o t h e r o b u s t n e s s o f M P C i s b e t t e r. F o r t h e M P C t a k e s a p e r i o d o f t i m e t o f m i s h t h e r o l l i n g o p t i m i z a t i o n, t h e c o n v e r g e n c e o f M P C w i l l b e o b t a i n e d a b i t s l o w e r t h a n L P F c o n tr o l. B u t t h i s p r o c e s s i s p r e t t y s h o r t, a n d t h e a c t u a l m o d e l w i l l n o t c h a n g e a n y m o r e o n c e t h e c o n v e r g e n c e i s o b t a i n e d. S o t h i s m e t h o d h a s b e t t e r r o b u s t n e s s. V. C O N C L U S I O N I n t h i s p a p e r, w e p r o p o s e d a n o v e l e n e r g y s t o r a g e s y s t e m c o n tr o l s tr a t e g y b a s e d o n M P C, a i m i n g t o s m o o t h t h e p o w e r fl u c t u a t i o n a n d i m p r o v e t h e p o w e r q u a l i t y o f a P V g e n e r a t i o n s y s t e m. T h i s c o n tr o l m o d e l e d t h e h y b r i d s y s t e m a c c u r a t e l y b y c o n t i n u o u s o p t i m i z i n g t h e l i n e i m p e d a n c e a n d o t h e r p a r a m e t e r s, a n d t h e m o d e l w a s u s e d t o c a l c u l a t e t h e c h a r g i n g o r d i s c h a r g i n g p o w e r o f e n e r g y s t o r a g e u n i t s T. h e M P C h a d t h e a b i l i t y t o a n t i c ip a t e fu t u r e e v e n t s t o a v o i d t h e d e l a y o f c o n t r o l. I n t h e o p t i m i z a t i o n p r o c e s s, t h e e r r o r o f P V p o w e r f o r e c a s t i n g A s a r e s u l t, t h e c a p a c i t y o f e n e r g y s t o r a g e s y s t e m w a s r e d u c e d, a n d t h e e ffi c i e n c y i s h i g h e r. A t t h e s a m e t i m e, t h e c o n tr o l t o o k S O C a n d s o m e o t h e r c h a r a c t e r i s t i ci s n t o a c c o u n t t o e n s u r e t h e h e a l t h a n d s e c u r i t y o f t h e s y s t e m. 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