WWW.IJITECH.ORG ISSN 2321-8665 Vol.05,Issue.07, July-2017, Pages:1240-1245 Fuzzy Logic Control of Single-Phase PV Cascaded H-Bridge Multilevel Grid Connected Inverter A. YAMINI 1, K. RAMA MOHAN REDDY 2, M. REDDY PRASANNA 3 1 PG Scholar, Dept of EEE (PE&ED), KORM, Kadapa, AndhraPradesh, India. 2 Associate Professor, Dept of EEE, KORM, Kadapa, AndhraPradesh, India. 3 Assistant Professor & HOD, Dept of EEE, KORM, Kadapa, AndhraPradesh, India. Abstract: This project presents a Fuzzy logic control of single-phase PV cascaded H-bridge multilevel grid connected inverter. The truly individual MPPT block for each H-bridge converter realizes independent MPPT, and the total of every voltage reference gives the system voltage reference for the voltage loop. And the total of each PV voltage is the system voltage input. The output of the voltage controller delivers the peak value of the sinusoidal current reference. The cascaded multilevel inverter have only one central double loop controller, and the PWM generation will be decoupled from the voltage controllers. To improve the dynamic response of the voltage loops, a sample and hold block at double line frequency is included in every voltage loop. Therefore, the voltage loop bandwidth will increase to 25~30 Hz and the MPPT speed will be much faster as well. The power-based duty-ratio feed forward technique is eliminate the output voltage variations and improves current loop performance, but it is not completely eliminate the current distortions. To eliminate the current distortions the proposed model is used as Fuzzy logic controller in the place of PI controller. Finally, the simulation results are provide to verify the improvement of the current lop performance of the proposed control approaches. Keywords: Cascaded Multilevel Inverter; Decoupled PWM Generation; Power Weighting Factor; S&H; Duty-Ratio Feed- Forward. I. INTRODUCTION As of late, cascaded H-bridge multilevel inverter appeared in Fig.1 has drawn more interests because of its high-quality voltage waveform, smaller filter, less switching power loss because of the lower switching frequency. Another imperative motivation to utilize this topology is the possibility of independent voltage control and MPPT for each PV unit which can be a short string or a single panel [1~2]. The individual MPPT capacity can largely reduce the power loss due to the irradiance mismatch between various PV units which is urgent to the PV system. For this multiple input and multiple output system, the existed control methodologies can be characterized into two classifications: The first is the master-slave type. which can't achieve truly independent control for every voltage loop, on the grounds that the system execution will be determined by the irradiation of the master unit; The other one is the energy-balance control which can achieve independent control for every voltage loop, however it makes the PWM generation dependent on the voltage controller s dynamics, because the PWM control signal is assigned by the elements created from the voltage controller's outputs [1~2]. Fig 1. The topology of cascaded H-bridge multilevel grid connected inverter In order to simplify the control structure, this paper proposes a novel control strategy in section II. It enables the PWM generation based on the known PV energy of each PV unit. It is now decoupled from the voltage controllers as the double loop control for single H bridge-extension inverter. In other words, the PWM generation will not be dependent on the voltage controller's dynamics like energy-balance control. In this proposed strategy, the system just has the central double loop controller (external voltage loop and internal current loop). II. PROPOSED GENERAL CONTROL STRATEGY FORCASCADED MULTILEVEL INVERTER A. Conventional Dual Loop Control Structure for the Single 1. H-bridge Inverter The traditional dualloop control structure for the single H- bridge inverter appeared in Fig. 2 contains a MPPT block, a central dualloop controller and a PWM generation block Copyright @ 2017 IJIT. All rights reserved.
[3~6]. In this single central loopsystem, the PWM generation is truly decoupled from the voltage controller. This feature guarantees that the duty ratio is only determined by the current controller's output. A. YAMINI, K. RAMA MOHAN REDDY, M. REDDY PRASANNA utilized here. For the Power-Weighting (PW) PWM generation, the central control signal v c from the output of the current controller is distributed to each PWM generation block based on the PV power weighting factor wk as equation (3). wk is directly calculated with the PV unit's powerp pvk and voltage reference v refk as in Equation (4~5). vok=vcwk (3) mk=p pvk v refk (4) Fig 2. The topology and control strategy for single H- bridge string inverter In this traditional double loop control structure, the bandwidth of the outer voltage should be much less than the bandwidth of the current loop. The PWM generation is straightforwardly determined by the current loop. Therefore, the dynamic of the current loop will not be affected by the voltage loop. In other words, the current loop can be decoupled with the voltage loop. This includes makes the single H-bridge string inverter has a simple design and a good control execution. B. Proposed Dual Loop Control Structure for the Cascaded Multilevel PV Inverter For the cascaded multilevel inverter, the energy balance control [1~2] makes the PWM generation determined by the coordinated outputs of the currentloop and voltage loops. As a result, the dynamic of the currentloop should be affected by the voltage loops. Be that as it may, the proposed control scheme in this paper as shown in Fig. 3 doesn't have this issue. It can make the current loop decouple with the voltage loop. This structure firstly contains N independent MPPT blocks which can achieve individual MPPT function for each PV unit. At that point for the voltage loop, the system voltage reference v ref is the sum of the each MPPT block. v ref = v refi (1) The feedback voltage v pv is the sum of each PV voltage. v pv = v pvi (2) The output of the voltage loop's compensator G cv is the most extreme estimation of the present reference i pk. The current reference i ref will be controlled by i pk and the PLL result. As far, the double loop controller has no difference with the conventional dual loop control structure for the single H- bridge inverter. In this structure, all the valid MPPT algorithms, G cv and G ci for H-bridge inverter [3~17] can be wk=mk/ (5) This power weighting factor wkis derived from the following algorithms. One H-bridge s output power is P ok. It equals to the output voltage v ok multiplied by the inductor current i L. Pok=voki L (6) If neglecting the power loss of each H-bridge, p ok can be expressed as the PV power. Pok=Ppvk (7) v ok is determined by the distributed control signal v ck and the input voltage v ink. Fig 3. The proposed control strategy for single-phase PV cascaded H-bridge multilevel grid-connected inverter vok=v ink v ck (8) Then the following equation can be got. Ppvk=v ink v ck i L (9) As a result, the distributed control signal v ck can be expressed by the following equation. v ck = Ppvk /v ink i L (10) In other words, v ck should be proportional to the PV power and inversely proportional to the input voltage v ink. v ck Ppvk / v ink (11) To make v ck decouple from the voltage feedback loop, input voltage v ink is replaced by the input voltage reference v refk which should be the same with v ink in steady state. Based on this equation, there is a conclusion that the central control signal can be distributed directly based on each PV power P pvk and v refk. v ck Ppvk v refk (12) In view of the above derivation, the coefficient m k can be formed as Equation (4). At that point the power weighting factor should be got in view of this coefficient m k as Equation (5).
Fuzzy Logic Control of Single-Phase PV Cascaded H-Bridge Multilevel Grid Connected Inverter This novel criteria for the control signal assignment is a kind of feed-forward technique. The PV unit's power and input voltage reference is the feed- forward value which is decoupled from the voltage loop. In other words, the dynamics of the current loop will not be affected by the voltage loop's dynamics. The Phase-Shift PWM understands the multilevel voltage waveform with individual signal v ck. With w k, the output voltage v ok of every H-bridge wouldbe quick and accurately determined by PV power and input voltage reference while independent of the voltage controllers' dynamics. In addition, this decoupling technique largely simplifies the system design. In summary, this proposed control structure is simplified one compared to the energy balance control strategy. Really, it is quite similar to the control diagram for the single H-bridge inverter because they both have only one central double loop controller. The difference between them is that the single H-bridge only has one MPPT block and one PWM generation due to the centralized topology, then this proposed control structure has N individual MPPT blocks and N feed-forward controlled PWM generation blocks. Thus, the design complexity of this system will still be similar to the single H-bridge inverter. compensator for G ci [3~6]. So as to improve the execution of the current loop, this paper proposes to add the estimated instantaneous duty-ratio signal to each PWM generation block as shown in Fig. 5. The estimated output voltage V'ok which is 120Hz sinusoidal wave shape and in phase with Vg can be calculated based on the PV power as appeared in Equation (14). The estimated signal of the duty-ratio d k is expressed as Equation (14). d k =v refk / V'ok (13) V'ok= Vg(ppvk/ ppvk ) (14) vck = v c Wk + d k =v c (mk/ )+(v refk/ V'ok) (15) The last control signal vck is the sum of the feedback and the feed-forward values appeared in Equation(15). This duty ratio feed-forward strategy effectively eliminates the impact of the output voltage variation, and the feedback loop only needs to guarantee the stability of the system and minimization of the steady state error. III. PERFORMANCE IMPROVEMENTS SCHEMES FOR CASCADED MULTILEVEL INVERTER A. Voltage Loop with Fast Dynamic Response For the single stage single phase inverter, the input voltage has a large ripple at double line frequency due to the output power fluctuation. Therefore, the conventional voltage loop design would be required to keep the bandwidth only 5~10Hz. So as to avoid the significant output current distortion. Then the poor transient response of the input voltage would limit the MPPT speed when using common algorithms such as P&O and Inc-Cond because of the restricted reference changing speed. Referring to the procedures to improve the output voltage response while maintaining a power factor in AC/DC PFC system [18~20], this paper proposes to put a sample and hold (S&H) in the voltage loop as shown in Fig. 4. By sampling v ref - v pv at the zero crossing point of output voltage, this approach could extract the average input voltage information accurately deprived of 120 Hz ripple component. With this S&H block, a higher bandwidth (25~30Hz) of the voltage loop provides much faster dynamic response and eliminates the output current distortion simultaneously. Hence, a considerably quicker MPPT can be achieved for the system. Fig5. Power based feed forward. Fig6. io IV. CONVENTIONAL RESULTS Fig4. Fast Voltage control loop. B. Duty-ratio Feed-forward for Current Control Due to the LCL filter, it is difficult for the output current to follow the 60Hz sinusoidal reference with only PI Fig7. Vpv1.
A. YAMINI, K. RAMA MOHAN REDDY, M. REDDY PRASANNA Fig8. Vref1. Fig13. Without S&H. Fig9. Vref2. Fig14. With S&H. VI. PROPOSED RESULTS Fig10. Vpv2. Fig15. io. Fig11. Ppv1. Fig16. Vpv1. Fig12. Ppv2 Fig17. Vref1.
Fuzzy Logic Control of Single-Phase PV Cascaded H-Bridge Multilevel Grid Connected Inverter Fig18. Vref2. Fig19. Vpv2 Fig20. Ppv1. Fig21. Ppv2. Fig22. Without S&H. Fig23. With S&H. VII. CONCLUSION The proposed Fuzzy logic control of single phase PV cascaded H-bridge multilevel grid connected inverter is based on the typical dual loop control for H-bridge inverter and just extend one MPPT block to N independent MPPT blocks are designed and simulated. This feed forward technique uses the connection between every H-bridge's Duty-ratio cycle data, the PV power and input voltage. As a result, this strategy decouples the PWM generation from the voltage controller's dynamics. At that point the current loop's performance can be decoupled from the voltage loop. The simplified control structure makes this multiple input and multiple output system design much easier. Also, the S&H part in the voltage loop realizes both the quick voltage dynamic response and low current distortion. Besides, the current distortions are eliminated by fuzzy logic controller. And the current loop performance is improved by the power based duty-ratio feedforward technique. VIII. REFERENCES [1] Villanueva, Elena, Pablo Correa, José Rodríguez, and Mario Pacas."Control of a single-phase cascaded H-bridge multilevel inverter for grid-connected photovoltaic systems." Industrial Electronics, IEEE Transactions on 56, no. 11 (2009): 4399-4406. [2] Chavarria, Javier, Domingo Biel, Francesc Guinjoan, Carlos Meza, and Juan J. Negroni. "Energy-balance control of PV cascaded multilevel grid-connected inverters under levelshifted and phase-shifted PWMs." Industrial Electronics, IEEE Transactions on 60, no. 1 (2013):98-111. [3] Teodorescu, Remus, FredeBlaabjerg, UffeBorup, and Marco Liserre."A new control structure for grid-connected LCL PV inverters with zero steady-state error and selective harmonic compensation." In Applied Power Electronics Conference and Exposition, 2004. APEC'04.Nineteenth Annual IEEE, vol. 1, pp. 580-586. IEEE, 2004. [4] Shen, Guoqiao, et al. "A new feedback method for PR current control of LCL-filter-based grid-connected inverter." Industrial Electronics, IEEE Transactions on 57.6, pp 2033-2041, 2010. [5] Papavasiliou, A., S. A. Papathanassiou, S. N. Manias, and G. Demetriadis. "Current control of a voltage source inverter connected to the grid via LCL filter. "In Power Electronics Specialists Conference, 2007. PESC 2007. IEEE, pp. 2379-2384. IEEE, 2007.
A. YAMINI, K. RAMA MOHAN REDDY, M. REDDY PRASANNA Author's Profile: A. Yamini, has received the B.Tech (Electrical and Electronics Engineering) degree from RVPECW, Kadapa in 2013 and persuing M.Tech (PE&ED) in KORM, Kadapa, AP, India. K. Rama Mohan Reddy, received his master degree in Jawaharlal Nehru technological university, Anantapuramu. He is presently working as Associate professor in K.O.R.M college of Engineering, Kadapa. M. Reddy Prasanna, received her Master Degree in KSRMCE, Kadapa, Affiliated to Jawaharlal Nehru Technological University, Anantapuramu. She is presently working as Assistant Professor & HOD in K.O.R.M. College of Engineering, Kadapa.