See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/254045180 A New Grid-Connected PV System Based on Cascaded H-bridge Quasi-Z Source Inverter Article · May 2012 DOI: 10.1109/ISIE.2012.6237218 CITATIONS READS 74 759 6 authors, including: Baoming Ge 192 PUBLICATIONS 5,127 CITATIONS Fang Peng institute of atomic and molicular physics 474 PUBLICATIONS 35,998 CITATIONS SEE PROFILE SEE PROFILE Yushan Liu Texas A&M University at Qatar 78 PUBLICATIONS 2,159 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: high pressure, mechanism of solid phase transition, strong correlated electron systems View project novel topologies of the ac-ac converters View project All content following this page was uploaded by Baoming Ge on 19 November 2014. The user has requested enhancement of the downloaded file. A New Grid-Connected PV System Based on Cascaded H-bridge Quasi-Z Source Inverter Dongsen Sun 1, Baoming Ge 1,2, Fang Zheng Peng2, Abu Rub Haitham4, Daqiang Bi 3, Yushan Liu1,4 1 School of Electrical Engineering, Beijing Jiaotong University, Beijing, China Department of Electrical and Computer Engineering, Michigan State University, East Lansing, Michigan, USA 3 State Key Lab of Power Systems, Dept. of Electrical Engineering, Tsinghua University, Beijing, China 4 Department of Electrical and Computer Engineering, Texas A&M University at Qatar, Doha, Qatar E-mail: [email protected] INTRODUCTION As solar energy is one of the most promising renewable energy, the photovoltaic (PV) systems are becoming more and more popular. In recent years applying multilevel inverters to PV power systems is getting more and more attention due to the large power-scale demands. Three common multilevel inverters topologies are as follows: 1) diode clamped [1]-[3]; 2) capacitor clamped [4], [5]; and 3) cascaded H-bridge (CHB) inverter [6]–[11]. Among these topologies, the CHB inverter has unique advantages and is more widely used in PV system. Fig.1 shows a generic PV system using the CHB inverter. The system offers some advantages such as the independent maximum power point tracking (MPPT) of each string and the modulatity by cascading more H-bridge modules. In addition, the CHB output voltage can reaches medium voltage and has a high number of levels (2n+1, n=1, 2, …), which results in no step up transformer and much smaller scale filter [6]-[11]. However, with the PV string connecting to the inverter directly, a constant DC-link capacitor voltage of each inverter module is impossible in this system, because PV string voltage varies widely due to the changes of temperature and solar irradiation, or some serious conditions such as mismatch, partial shadows, etc. These cases will cause an imbalance DC-link voltage among different Hbridge modules [6]-[10]. Moreover, a variable DC-link voltage leads to a higher KVA rating for H-bridge module in practical applications. In [6], a control method is studied 978-1-4673-0158-9/12/$31.00 ©2012 IEEE Module 1 I. under the equal DC-link voltages through neglecting the PV string voltage differences. References [7]-[10] introduce a factor to express the different voltage and power of each PV string, but it cannot solve the DC-link imbalance problem. In [11] an additional DC/DC converter is added to compensate the imbalance of the DC-link voltage, but causing the whole system complex and expensive. Nowadays, the Z-source inverter (ZSI) and the quasi-Z source inverter (qZSI) have been widely applied for renewable energy power generation system due to some unique features [12]-[16]. They can implement voltage boost and power conversion simultaneously in a single stage, and improve the reliability due to the shoot-through cases no longer destroying the inverter [12]-[14]. However, PV system based on qZS-CHB multilevel inverter has never been presented by now. Introducing a quasi-Z source (qZS) network into the CHB module, the system features several advantages, such as PV string voltage boost, independent tracking MPP of each PV string, and keeping an equal DClink voltage for each H-bridge inverter module. This paper proposes a new PV system based on qZS- CHB multilevel inverter. Its whole control scheme including independent MPPT control, independent DC-link voltage control, and the grid injected power control is studied in this paper. Simulation results verify the proposed system and the control scheme. Module 2 Abstract- A new scheme for grid-connected photovoltaic (PV) interface by combination of a quasi-Z source inverter (qZSI) into cascaded H-bridge (CHB) is proposed in this paper. The proposed scheme enables PV string voltage boost to a higher level, and solves the imbalance problem of DC-link voltage in traditional CHB inverters. A multilevel voltage waveform of inverter output is generated by an improved phase shifted sinusoidal pulse width modulation (PS-SPWM) algorithm, which introduces shoot-through states into the conventional zero states to control qZS-CHB module. The effective control schemes are proposed to regulate the maximum power point tracking (MPPT) of each string, and control the DC-link voltage of each H-bridge, respectively. Grid injected power is controlled corresponding to the proportionality factors of each PV string output power. A 1.5 kW system is built in MATLAB/SIMULINK, and the simulation results verify the proposed novel multilevel PV interface inverter and its control principles. Module 3 2 Fig. 1. Generic CHB multilevel inverter PV system. 951 II. OPERATING PRINCIPLE OF THE PROPOSED SYSTEM A. Topology Description The configuration of the PV system based on qZS-CHB multilevel inverter is illustrated in Fig. 2. The system is composed of three PV strings, three qZS H-bridge modules, filtering inductance, and the distribution grid. Each PV string’s output connects to a qZS H-bridge module. Comparing to the conventional H-bridge module, an inductor-capacitor impedance network is introduced. The output voltage of the qZS-CHB multilevel inverter is made of the sum of all module output voltages, and it has seven levels. The number of levels increases when increasing the series number of the qZS H-bridge modules. The details of operating principle have been illustrated in many literatures [12]-[16]. Assuming T is one switching cycle, T0 is the interval of the shoot-through state, and T1 is the interval of non-shoot-through states. Their relationship is T0+T1=T, and the shoot-through duty ratio is D=T0/T. In steady state the following derivations can be obtained. Two capacitor voltages vC1 and vC2, and the peak voltage of DC-link v̂PN1 can be obtained as 1− D ⎧ ⎪vC1 = 1 − 2 D vpv1 ⎪ D ⎪ vpv1 . ⎨vC2 = 1 − 2D ⎪ 1 ⎪ ⎪ vˆPN1 = vC1 + vC2 = 1 − 2 D vpv1 ⎩ where vpv1 is the PV string output voltage. Two inductor currents iL1, iL2 can be calculated as B. Quasi-Z Source Inverter iL2 = iL1 = P vpv1 . Take one module in the proposed system as an example to analyze the qZSI operating principle. The qZSI can be operated in two states, i.e., the non-shoot-through state and the shoot-through state [12]-[15]. Fig. 3 shows the qZSI equivalent circuits operating in the two states and defines the polarities of all voltages and currents. C. Improved PS-SPWM Algorithm The modulation method used in this proposed system is an improved phase shifted sinusoidal pulse width modulation (PS-SPWM) algorithm shown in Fig. 4. As an qZS network introduced into the conventional H-bridge, two shoot-through modulation reference, which are denoted as 1-Dn and Dn-1, are introduced into the conventional zero states. If the triangular carrier signal is bigger than Dn or smaller than 1-Dn, two switches of one leg in H-bridge module are turned on simultaneously. But the PWM output voltage is still kept at zero, the same as the conventional zero states. (b) Fig. 3. Equivalent circuit of the qZSI. (a) Non-shoot-through state. (b) Shoot-through state. (2) where P is the PV string output power. Fig. 2. Configuration of the proposed system. (a) (1) Fig. 4. Modulation scheme for the proposed system. 952 With the PWM shown in Fig. 4, each H-bridge module is a 3-level inverter. We take the H-bridge module 1 as an example, S1 and S2 (dashed line) are carriers for two legs (left leg and right leg of H-bridge), respectively, which have a phase shift in 180° each other. The output voltage va1 of Hbridge module 1 is a 3-level PWM signal. The carriers of different H-bridge modules are shifted in 60° each other to produce the multilevel stepped voltage waveform. By applying this method, 7-level PWM signal are generated as shown in the Fig. 4. III. B. Independent DC-link Voltage Control CONTROL SCHEME Each qZS H-bridge module presents two independent control freedoms: shoot-through duty ratio Dn and modulation index mn. Dn is independently used to control the each PV string voltage and adjust the voltage to a desired reference given by the MPPT algorithm. While mn is used to control each DC-link voltage and the grid injected power. The following subsections describe the independent MPPT control, the independent DC-link voltage control, and the grid injected power control. A. Independent MPPT Control In this paper Perturb and Observe (P&O) method is used to fulfill the MPPT [16], [17]. The PV string in each module is composed by the series connection of four KD135GH-2P PV panels. Fig. 5 shows the P-V and I-V characteristics of the PV strings, which are corresponding to different environmental conditions, i.e., PV1: S=1.0 kW/m2, T=15 °C; PV2: S=1.0 kW/m2, T=35 °C; PV3: S=1.0 kW/m2, T=55 °C. From Fig. 5 we can see that the MPP voltage decreases with the environment temperature increasing. Ppv (W) 600 The DC-link voltage control scheme is illustrated in Fig. 6. As the system has three independent DC-links, each one should be controlled independently. The DC-link voltage vPN of qZS H-bridge changes between the peak value and zero. While, the peak voltage v̂PN1 of DC-link will be controlled in a constant value. As the expression shown in (1), vˆPNn is made of the sum of vC1 and vC2. So vC1 and vC2 are measured and added together to produce vˆPNn . A proportional and integral (PI) controller is employed to control vˆPNn by adjusting the output power reference P*n. Although the reference of vˆPNn is set at the same value, the reference P*n will be different according to each PV string output power. C. Grid Injected Power Control The total power injected to the grid is the sum of each PV string output power. So the total power reference is 3 P*total = ∑ P*n . n =1 * * = 2 P total . iˆgrid vˆgrid PV1 PV2 PV3 200 20 40 60 vpv (V) 80 100 (a) 10 5 0 0 40 60 vpv (V) 80 an = 100 P*n P*total . (5) Using SPWM modulation method, the modulation index mn for each module can be calculated by a v mn = n total . vˆPNn (b) Fig. 5. Characteristics of PV strings in different temperature. (a) P-V characteristics of three PV strings. (b) I-V characteristics of three PV strings. (4) To ensure unity power factor operation, the phase locked loop (PLL) is performed to measure the phase of grid voltage. The grid current is measured and fed back to the current control loop regulator. The qZS-CHB multilevel inverter output voltage vtotal is given by the current loop to produce the modulation index mn for each module. As each qZS H-bridge module provides different power, the proportionality factors can be calculated as follows [6], [11] PV1 PV2 PV3 20 (3) As the three qZS H-bridge modules are in the series connection, the grid-injected current drawing from each module is the same. So the peak value of the grid current is 400 0 0 ipv (A) The independent MPPT control of each module is shown in Fig. 6. The output voltage and current of each PV string are detected and inputted to the MPPT controller. The controller outputs a reference to the PV voltage close-loop regulator. The PV voltage reference is refreshed every 0.0002 s by using MPPT algorithm. By regulating the shoot-through duty ratio Dn the output voltage of each PV string will be controlled at different values according to the environment conditions. (6) With the modulation index mn and the shoot-through duty ratio Dn, the gate signals for three qZS H-bridge modules are 953 produced in PS-SPWM controller. The block diagram that performs the system power control is shown in Fig. 7. vˆPNn vˆPNn inductor currents of three qZS H-bridge modules, which illustrate the operating principle in (1) and (2). The DC-link voltage of each module is shown in Fig. 11. With the independent DC-link voltage control, all their peak values are kept at the reference 145 V. The output voltage of qZS-CHB multilevel inverter is shown in Fig. 12. The 7-level voltage verifies the correct operation of the improved PSSPWM techniques. Fig. 13 shows that the grid voltage vgrid and the grid current igrid are in phase, which ensures unity power factor operation. The modulation index of each module is shown in Fig. 14. Module 1 has a higher modulation index because it provides the most power. While module 2 and module 3 have a lower modulation index, corresponding to their lower output power. Fig. 6. Control scheme of qZS H-bridge module. vpv (V) 80 2 v̂grid 2.1 2.2 2.3 2.4 2.5 600 vˆPNn 1 Ppv (W) ∑ Pn n =1 60 50 2 3 PV1 PV2 PV3 70 PV1 400 2.2 Ptotal PV2 500 PV3 2.25 2.3 2.35 t (s) 2.4 2.45 2.5 Fig. 7. Power control of the whole system. SIMULATION RESULTS In order to verify the proposed system and its control schemes, the simulations are performed by using MATLAB /SIMULINK. A 1.5 kW single-phase 7-level qZS-CHB multilevel inverter system, as shown in Fig. 2, is built for simulation. The simulation parameters are as follows: carrier frequency fc=2 kHz, L1=L2=3 mH, C1=C2=2 mF, C3=1 mF, Lf=1 mH, and the resistances of L1 and L2 are r1=r2=0.1 Ω. The grid voltage has a constant phase voltage of 220 V rms, with a frequency of 50 Hz. The PV string is modeled and simulated based on KD135GH-2P. To verify the proposed control scheme in the case of PV strings outputting different voltage and different power, we assume the three PV strings operating in different temperature conditions. The characteristics of the PV strings in three modules are shown in Fig. 5. The peak value of DC-link voltage of all three modules is controlled at 145 V. Fig. 8 shows the voltage and power of PV string in three modules. All of the three PV strings operate at their MPPs, and the voltage and power of PV 1, PV 2, and PV 3 are 71.5 V /545 W, 66 V/500 W, 60.5 V/455 W, respectively. And the total power injected into the grid is 1500 W, which corresponds to the grid-injected current with a peak value of 9.6 A. Fig. 9 and Fig. 10 show the capacitor voltages and Fig. 8. PV voltages and output powers of three modules. vC1 and vC2 (V) vC1 and vC2 (V) vC1 and vC2 (V) IV. 150 vC1 100 vC2 50 0 2.2 150 2.25 2.3 2.35 2.25 2.3 2.35 2.4 vC1 100 2.5 Module 2 2.45 2.5 Module 3 vC2 50 2.25 2.3 2.35 t (s) 2.4 Fig. 9. Capacitor voltages of three qZS H-bridge modules. 954 2.45 vC2 50 0 2.2 2.4 vC1 100 0 2.2 150 Module 1 2.45 2.5 10 2.25 2.3 2.35 2.4 2.45 2.5 2.25 2.3 2.35 2.4 2.45 2.3 2.35 2.4 2.45 2.5 2.25 2.3 2.35 t (s) 2.4 2.45 2.5 0 -20 2.2 2.5 Module 3 Fig. 13. Grid voltage and current of qZS-CBH multilevel Inverter. 10 0 2.2 2.25 20 Module 2 10 0 2.2 20 400 200 0 -200 -400 2.2 igrid (A) 0 2.2 20 vgrid (V) Module 1 2.25 2.3 2.35 t (s) 2.4 2.45 1 2.5 m1 iL1 and iL2 (A) iL1 and iL2 (A) iL1 and iL2 (A) 20 0 Fig. 10. Inductor currents of three qZS H-bridge modules. -1 2.2 1 vPN2 (V) 0 2.2 200 2.25 2.3 2.35 2.4 2.45 2.3 2.35 2.4 2.45 2.35 2.4 2.45 2.5 2.25 2.3 2.35 2.4 2.45 2.5 2.25 2.3 2.35 t (s) 2.4 2.45 2.5 0 2.5 -1 2.2 100 0 2.2 Fig. 14. Modulation index of each module. 2.25 2.3 2.35 t (s) 2.4 2.45 2.5 Fig. 11. DC-link voltages of three modules. 500 vH (V) 2.3 0 -1 2.2 1 2.25 2.25 2.5 100 0 2.2 200 vPN3 (V) m2 100 m3 vPN1 (V) 200 0 -500 2.2 2.25 2.3 2.35 t (s) 2.4 2.45 Fig. 12. Output voltage of qZS-CBH multilevel Inverter. 2.5 V. CONCLUSION A new multilevel inverter for PV power generation systems is proposed. It is a combination of qZSI and CHB multilevel topology, and has both advantages of them. This enables independent MPPT control and independent DC-link voltage control for each PV generation module. Moreover, the proposed system effectively overcomes the DC-link voltage imbalance problem. The control methods of the proposed system including independent MPPT control, independent DC-link voltage control, and the output power control are presented in detail. A 1.5 kW simulation model is built to test the performance of the system. Simulation results verify the operating principle and control strategies of the proposed system. 955 ACKNOWLEDGEMENTS Parts of this work, specifically Sections III and IV were supported by Education Development Program of Delta Environmental & Educational Foundation under grant No. DREG2010001 and the State Key Laboratory of Control and Simulation of Power System and Generation Equipments under grant No. SKLD11KM01, Tsinghua University, Beijing 100084, China. In addition, we also acknowledge support from NPRP grant No. 09-233-2-096 from the Qatar National Research Fund (a member of Qatar Foundation) for Section II, and from the Beijing Jiaotong University Foundation under grant No. 2009JBM093 for Section IV. The statements made herein are solely the responsibility of the authors. Annual Applied Power Electronics Conference and Exposition, APEC 2009, Washington DC, USA, pp. 918-924, 15-19 Feb. 2009. [15] U. Supatti, F.Z. 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