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1 Florida State University Libraries Electronic Theses, Treatises and Dissertations The Graduate School 2010 Study of the Cascaded Z-Source Inverter to Solve the Partial Shading for the Grid- Connected PV System Lei Wang Follow this and additional works at the FSU Digital Library. For more information, please contact

2 THE FLORIDA STATE UNIVERSITY COLLEGE OF ENGINEERING STUDY OF THE CASCADED Z-SOURCE INVERTER TO SOLVE THE PARTIAL SHADING FOR THE GRID-CONNECTED PV SYSTEM By LEI WANG A Thesis submitted to the Department of Electrical and Computer Engineering in partial fulfillment of the requirements for the degree of Master of Science Degree Awarded: Fall Semester, 2010

3 The members of the committee approve the Thesis of Lei Wang defended on Oct 12 th Hui Li Professor Directing Thesis Simon Y. Foo Committee Member Jim P. Zheng Committee Member Approved: Simon Y. Foo, Chair, Department of Electrical and Computer Engineering C. J. Chen, Dean, Dean, College of Engineering The Graduate School has verified and approved the above-named committee members. ii

4 ACKNOWLEDGEMENTS I would like to thank my advisor Dr. Hui Li, for her guidance and mentoring during my graduate research period at the Florida State University. Without her encouragement and endless support, it would be impossible for me to complete this master thesis. I want to thank Dr. Foo and Dr. Jim Zheng for serving as my committee and their valuable advice. Also I want to thank all my colleagues, staff and technicians at the Center for Advanced Power Systems (CAPS) for their great help when I encountered with the problems during these two years. Most importantly, I want to thank my parents and friends for their love and trust, which helps me to overcome the difficulties through my life. iii

5 TABLE OF CONTENTS Table of Contents... iv List of Tables... vii List of Figures...viii Abstract... x 1 INTRODUCTION Importance of the grid-connected PV system Partial shading effect STATE OF ARTS Structure based improvement Cascaded H-bridge inverter Micro-converter cascaded inverter MPPT algorithm based improvement Short-circuit current pulse method Fibonacci search method Compound MPPT method combined with conventional MPPT methods Summary and proposed method SINGLE STAGE Z-SOURCE INVERTER Z-Source inverter topology for the grid connected PV system Z-source inverter operating principle Z-source inverter PWM Modulation Control principle of the ZSI for the grid-connected system Grid current close loop control Inverter DC-link voltage control MPPT control algorithm and ZSI shoot through time control Simulation verification iv

6 3.5.1 System Specification Simulation condition and results: CASCADED Z-SOURCE INVERTER Introduction Cascaded Z-source inverter topology Z-source inverter PWM method Cascaded Z-source inverter control strategy Simulation verification System configuration Simulation condition and the results PV PANEL EMULATOR USING RTDS Introduction Different kinds of PV emulator Proposed RTDS based PV emulator RTDS based PV emulator topology Real time digital simulator (RTDS) Power hardware in the loop (PHIL) RTDS based PV emulator system configuration Simulation & Experiment verification PV model specification Controlled DC amplifier and electrical load specification Experiment result CONCLUSION AND FUTURE RESEARCH Conclusion Future research References v

7 Biographical Sketch vi

8 LIST OF TABLES Table 1 Single stage Z-source inverter topology PV panel specification Table 2 Grid connected PV system single stage Z-source network parameters Table 3 Cascaded Z-source inverter topology PV panel specification Table 4 Grid connected PV system cascaded Z-source network parameters Table 5 Comparison table between the traditional inverter topology and the proposed cascaded Z-source inverter topology Table 6 RTDS emulated Sanyo PV panel parameters Table 7 Controlled DC amplifier specification vii

9 LIST OF FIGURES Figure 1 Solar PV existing world capacity from Figure 2 Partial shading root cause... 4 Figure 3 Traditional grid-connected PV system... 6 Figure 4 Partial shading effect on the PV panel... 6 Figure 5 System configuration and partial shading pattern... 8 Figure 6 Different test scenarios experiment results... 9 Figure 7 Cascaded H bridge inverter topology Figure 8 Cascaded H-bridge inverter control scheme Figure 9 DC-link voltage for the partial shading condition Figure 10 Micro-converter cascaded inverter system configuration Figure 11 Control scheme for the dc-dc converter and the block diagram of autonomous control system Figure 12 Micro-converter cascaded inverter simulation result Figure 13 Configuration of current-controlled boost chopper with short-current pulse based MPPT Figure 14 Flowchart of the Fibonacci search based MPPT control diagram Figure 15 System configuration and the MPPT algorithm flowchart Figure 16 Z-source inverter circuit topology Figure 17 Equivalent circuit of the Z-source inverter viewed from the DC link Figure 18 Equivalent circuit of the Z-source inverter viewed from the DC link when the inverter bridge is in the shoot through state Figure 19 Equivalent circuit of the Z-source inverter viewed from the DC link when the inverter bridge is in the non-shoot through state...25 Figure 20 Bipolar PWM modulation for the Z-source inverter Figure 21 Common mode current in the non-transformer grid connected system Figure 22 Control system diagram...29 Figure 23 P&O MPPT algorithm for the PV panel Figure 24 Grid voltage and grid current for the simulated scenario one viii

10 Figure 25 MPPT controller reference and PV output voltage for the simulated scenario one Figure 26 3KW PV panel output power curve for the simulated scenario one Figure 27 DC-link voltage for the simulated scenario one Figure 28 Grid voltage and grid current for the simulated scenario two Figure 29 MPPT controller reference and PV output voltage for the simulated scenario two Figure 30 3KW PV panel output power curve for the simulated scenario two Figure 31 DC-link voltage for the simulated scenario two Figure 32 Cascaded Z-source inverter for the grid-connected PV system Figure 33 Modified unipolar PWM method Figure 34 Cascaded Z-source inverter control strategy Figure 35 Simulated scenario of the proposed topology Figure 36 Grid current and grid voltage Figure 37 (a) proposed system output power and (b) each PV panel output power Figure 38 Proposed cascaded Z-source inverter topology PV output characteristic curve Figure 39 Traditional series connected PV system Figure 40 Traditional series connected PV system output power characteristic curve Figure 41 RTDS based PV emulator system diagram Figure 42 Power hardware in the loop (PHIL) diagram Figure 43 RTDS based PV emulator system configuration Figure 44 (a) April 12th, 2010 day irradiation profile (b) April 12th, 2010 PV panel surface temperature profile Figure 45 PV panel performance emulation under real weather condition of April 12th, Figure 46 RTDS based PV emulator switching transient experiment ix

11 ABSTRACT Recently there have been increasing interests in developing the renewable energy resources for the future energy need. Solar energy generation has shown its great potential to serve as a clean distributed energy source. While one drawback of the solar source is that when partial shading happens, which means the PV panels receive the non-uniform irradiation, the PV system output power would reduce depending on the partial shading scenario. Under this circumstance, the cascaded Z-source inverter for the grid-connected PV system is proposed to solve the partial shading problem. In order to demonstrate the proposed technology, the single stage Z-source inverter for the grid-connected PV system is introduced first. The designed control diagram could grantee the PV panel output its maximum power and the DC-link voltage could be stabilized. Then the cascaded Z-source inverter topology is proposed, this topology could enable each PV panel to output its maximum power even when partial shading problem happens to the PV system. And one 3 KW inverter simulation module is built in the Matlab/Simulink software to verify the proposed technology performance under partial shading condition. At last, the PV array emulation using RTDS and controlled DC amplifier under different weather conditions is described to demonstrate the PV emulator response under varying condition. x

12 CHAPTER ONE INTRODUCTION 1.1 Importance of the grid-connected PV system Nowadays, the current used energy sources people have depended on the fossil fuels and nuclear fission- have apparently to be one easy solution to the ever-growing need for the electric power. While endless issues popped out when these energy sources are been utilized to generate the power. For example, burning oil, coal and natural gas pumps nitrogen oxide, sulfur dioxide and other toxic metal into the atmosphere, which cause increasing incidents of lung disease, polluting earth soil and waters, damaging the corps and rendering the food unsafe enough to eat in quantity. Nuclear fission produces radioactive waste, which would remain deadly for thousands of years and there is still no better solution for them to storage yet. As the inter-connectedness of all the ecosystems cycle, the poisonous results of the various pollutants creating by the use of the fuels are becoming more difficulty to measure. While at the same time, besides the issues mentioned above, money cost would be another serious crisis. According to the American Lung Association (ALA) report, the air pollution from electricity production costs the whole nation $20 billion per year in the health care aspect. The National Academy of Sciences estimates that damage from acid rain causes $ 6 billion per year of damage to crops, forest, lakes and building in the United States. Also global warming resulting from increased atmospheric carbon produced by burning fossil fuels, is melting arctic and Antarctic icecaps and changing the global weather patterns, this would be another potential threaten to the human surviving. [1] Based on this serious situation, researchers and scientists worldwide are struggling to fully use the clean renewable energy, such as sun irradiation, wind and waves. Because every bit of the clean energy generated would reduce the amount of pollution people add to the environment, reduce the burgeoning hidden cost of this pollution in health care dollars and food dollars and at the same time act as a stimulant to the economy growth by providing new technology jobs. Thus the renewable energy sources are playing a more and more important role to the meet the global energy demands. Among all the renewable energy sources, the 1

13 Photovoltaic (PV) generation system is evolving rapidly and showing a great potential environmentally and economically due to its unique advantages. PV is known as one method for generating electrical power by converting solar radiation into direct current electricity using semiconductors which exhibit the photovoltaic effect. The photovoltaic effect refers to photons of light knocking electrons into a higher state of energy to create electricity. PV panel is one module composed of several PV cells connected in series and parallel. Nowadays, more and more attention has been paid to the grid-connected PV system. As early as in 2006, former President George W. Bush had proposed the solar America Initiative to accelerate widespread commercialization of solar energy technologies. The goal was to achieve market competitiveness for PV electricity by It was estimated that, by 2015, the initiative would result in deploying 5-10 gigawatts (GW) of PV (enough to power 1-2 million homes), avoiding about 10 million metric tons of annual CO2 emissions, and employing 30,000 new workers in the PV industry. Also at the beginning of this year, current President Barack Obama proposed one new plan named New Energy for America. This plan s goal is to help create five million new jobs by strategically investing $150 billion over the next ten years to catalyze private efforts to build a lean energy future; ensure 10 percent of the electricity comes from renewable sources by 2012, and 25 percent by 2025 and implement an economy-wide cap-and-trade program to reduce greenhouse gas emissions 80 percent by Thus such circumstances, the grid-connected PV systems are being installed in unprecedented numbers in the United States and worldwide. Solar PV generates electricity in well over 100 countries and continues to be the fastest growing power-generation technology in the world. Between 2004 and 2009, grid-connected PV capacity increased at an annual average rate of 60 percent. An estimated 7 GW of grid-tied capacity was added in 2009, increasing the existing total by 53 percent to some 21 GW. This was the largest volume of solar PV ever added in one year; the PV development was shown as Fig.1 [2] 2

14 Figure 1 Solar PV existing world capacity from Currently the United States added an estimated 470 MW of solar PV in 2009, including 40 MW of off-grid PV, bringing cumulative capacity above the 1 GW mark. California accounted for about half of the total, followed by New Jersey with 57 MW added; several other states are expected to pass the 50 MW per year mark in the near future. Residential installations came to 156 MW, a doubling from Thus, renewable energy sources, especially the grid-connected PV system is extremely important for both the public health issue as well as the overall consideration for the human society s continuous developing. 1.2 Partial shading effect Although grid-connected PV system has several remarkable advantages, there are still some problems which could not be avoided. Typically when the generated energy from the PV system was transmitted into the grid, its efficiency index could not reach a satisfaction value, because certain energy losses would occur during the power generating process. These losses 3

15 are mainly caused by the PV panel module mismatch loss, partial shading loss and Maximum Power Point Tracking (MPPT) efficiency loss. All these issues would reduce the PV panel output power and could cause the thermal stress as well. While among all these power losses, the partial shading problem on the PV panel would be the one easily encountered scenario especially in the building-integrated PV system and could cause significant power loss for the PV system. [3] As mentioned above, usually the partial shading on the PV panel is the one of the most common scenarios occurred in the daily life. The shadow on the PV panel could be caused by the cloud fronts, snow and other light-blocking obstacles. For the residential point of view, another cause of the partial shading on the PV panel could be the improper design for the roof utilization. Because usually the PV panels are installed on the residential building roof, while due to the roof s irregular shapes and additional obstructions (air- conditioning unit, vents, chimneys etc), the difficulty in selecting the best position for the PV panel to generate the maximum power would greatly increase. The partial shading situation shown as Fig.2 could not be avoided. Figure 2 Partial shading root cause After reviewing the related papers, influence of the partial shading of the PV system would be cataloged into two aspects. One is the PV partial shading would cause the reverse voltage across the PV modules on the panel, which would led to the overheating situation to certain module, then at last destroy the PV panel, and prevent the whole PV system from properly operating. And the other effect of the partial shading would cause the whole PV system 4

16 characteristic curve different from the standard test condition, which would greatly reduce PV system output power. The detail explanations are following. [4] As described in the Impact of shading on PV systems presented by National Semiconductor report in 2008, for the traditional PV generating system, by-pass diodes are parallel connected to each PV module. Otherwise without these by-pass diodes, partial shading would cause the reverse voltage across the shaded PV cells on the PV panel. This issue would causes the shaded PV cell overheated, which would greatly reduce the life-span of the PV panel or got module burnt finally. The whole PV system was shown in the Fig.3. PV cells are connected in series and parallel, and then connected to the MPPT controller, which is used to track the maximum output power from the PV system. After the MPPT, DC to AC inverter is used to convert the generated DC current into AC current, and then feed this generated power to the grid or the load. When partial shading happened, almost no power could be extracted from the shaded PV cell; the string current would flow through the by-pass diode instead of the shaded PV cell. In this same report, the experiments about the different partial shading effect of the PV system output power were also implemented. The system configuration is three PV cell connected in series form one PV string then three PV string connected in parallel as one PV panel. Different shading pattern scenarios were simulated as the left table shown in Fig.4. And the experiment result of the different partial shading patterns were also shown in the table. From the list table, the conclusion can be derived as it is not necessary that large shading would leads to the high percent power loss, certain small partial shading pattern also could cause large percent output power loss. 5

17 Figure 3 Traditional grid-connected PV system Figure 4 Partial shading effect on the PV panel 6

18 Similarly, the report SolarEdge architecture overview from SolarEdge company also mentioned that, traditional systems produce anywhere form 5% to 25% less than optimal power because of conditions like module mismatch, partial shading and MPPT inefficiencies, and to the extremely case, one completely shaded cell can reduce solar module s output power by 40% to 95%. [5] In addition some experiments were also implemented in the academic area. One paper named On the impact of partial shading on PV output power presented by Dezso Sera on the 2 nd WSEAS/IASME international conference on renewable energy sources (RES 08), also cited that partial shading indeed impose serious problem on the grid-connected PV system [6]. As shown in the Fig.5, two identical PV panel were used in the experiment with different partial shading pattern, one pattern was two PV cells shaded in one sub-module, and the other pattern was one PV cells shaded in each sub-module. While the experiment result was shown in the Fig.6, these two Figures showed the PV system output current- voltage (I-V) characteristic curve and power-voltage (P-V) characteristic curve separately. In these Figures, blue line are the typically I-V and P-V characteristic curve without partial shading situation, the rest curves were the different test cases results separately. Similar to the results of the Fig.4, the partial shading power loss varied as the shading pattern changes, to the worst partial shading case, it is possible that nearly no output power could generate from the PV system. As described in this paper, although only 2 out of 36 cells are shadowed (which is less than 6% of the total area), the maximum power reduction in case a, case b are 50% and 70%, respectively. 7

19 Figure 5 System configuration and partial shading pattern 8

20 Figure 6 Different test scenarios experiment results To sum up the above chapter, the conclusion about the partial shading effect can be expressed as following. First, partial shading is a most encounter case for the PV system caused by various reasons, it could not be avoided. Second, partial shading would greatly reduce the PV system output power; the partial shading pattern would become another key factor to determine the generated output power from the PV system besides the solar irradiation and temperature. 9

21 CHAPTER TWO STATE OF ARTS As described in the above sections, the partial shading issue would greatly influence the power generated from the PV panel. Under this research motivation, more and more researches trying to solve the partial shading issue are under implemented. Scientists are still trying to transmit the output power as much as possible even when partial shading happens to the PV panel. After reviewing the related papers, current solutions for the partial shading situation could be categorized into two main aspects. One solution method is emphasized on the PV system topology; the other solution is emphasized on the MPPT algorithm. All these solutions would be explained in detail as following. 2.1 Structure based improvement The structure base improvement is one of the most used methods nowadays to minimize the partial shading effect of the PV system output power. By modifying the PV system circuit topology, the maximum output power from the whole PV system can be granted even the partial shading condition happened. For the reason that, the whole PV system are divided into several individual parts, the MPP of the unshaded PV cell can be realized, thus this structure based improvement could ensure the maximum output power from the PV system. There are two main improving topologies, one is called cascaded H-bridge inverter; the other is called micro converter cascaded inverter. In the following sections, these two topologies are going to be introduced separately Cascaded H-bridge inverter One related paper named Control of a single-phase cascaded H-Bridge Multilevel Inverter for Grid-Connected Photovoltaic Systems was presented by Elena Villanueva in the IEEE transactions on industrial electronics on Nov It had emphasized the importance of this cascaded H-bridge inverter topology. Its system configuration was shown in the Fig.7; the cascaded multilevel converter topology consists of n H-bridge converters connected in series. 10

22 Each dc link is fed by a short string of PV modules. By considering cells with the same dc-link voltage, the converter can synthesize an output voltage V HT with n level. This high-quality voltage waveform enables the reduction of the harmonics in the generated current, reducing the filtering effort at the input. And the adopted control scheme permits the independent control of each dc-link voltage, enabling the tracking of the maximum power point for each string of PV modules. Thus the MPP of the whole grid-connected PV system could be tracked. Figure 7 Cascaded H bridge inverter topology The cascaded H-bridge inverter control scheme was shown in the Fig.8. This control system has two main control loops; one is the grid side current control loop, which was used to generate of a sinusoidal input current with the unity power factor; the other control voltage control loop was used to control the PV cell output voltage to be maintained at its maximum power point voltage, which value was calculated by the MPPT controller using Perturb and Observe (P&O) algorithm. 11

23 Figure 8 Cascaded H-bridge inverter control scheme The simulation results were also shown in the Fig. 9, which was carried out in the Matlab/Simulink. In the simulation, only two PV panels were emulated, and the simulation operation were described as following, during the first 3 second, both temperature and solar radiation were equal for the two PV panels under the standard condition (with 25 and 1 kw/m 2 ). At the third second, the solar radiation over the second panel decrease to 0.6w/m 2. And at the fifth second, the temperature in the second panel increase form 25 to 35. During these three different operating conditions, the MPPT controllers gave references around the optimum point in only three levels according to the P&O algorithm result. The dc-link voltage followed the reference after a short transient. Whereas the MPP voltage of the first cell did not change because the operating point during the whole simulation process was the same. A lower radiation in the second PV module affected the current generated from the cell, as could be observed in the lower ripple of the dc-link between the third and fifth second. Moreover, the change in temperature also affected the mean value of the MPP voltage, which was can be observed from the simulation result between the fifth and seventh second. 12

24 Figure 9 DC-link voltage for the partial shading condition As the simulation result showed above, this cascaded H-bridge inverter could the minimized the power loss caused by the partial shading, and thanks to its circuit topology, it also could reduce voltage stress for circuit devices; reduce the current ripple and the enable the PV system to operate at a lower switching frequency (which would also reduce the filter sizing). While it still has a few disadvantages, for example, the dc-bus voltage needs to be increase for realizing the wide PV input range; when serious partial shading happens to one PV panel, probably no output power would be generated from the PV system and the control method would become more complicated when the dc-bus voltage for each PV panel is unequal. [7] Micro-converter cascaded inverter Besides the cascaded H-bridge inverter topology, there is another topology named microconverter cascaded inverter. Comparing with the first topology method, this method is more commonly used in the industry field, because it could realize wide PV input voltage range and 13

25 save more energy from the power loss due to the partial shading effect. This was mentioned in one paper named Improved Energy Capture in Series String Photovoltaics via smart distributed Power Electronics Leonor Linares on the applied power electronics and exposition (APEC) The system configuration was shown in the Fig.10; the difference between this topology with the cascaded H-bridge inverter is that one dc-dc converter, which is usually one buck-boost converter, operated with autonomous control for the tracking the maximum power of the solar panel was integrated to each PV module. Each controller is completely autonomous and the module maximum power point trackers are decoupled for each other. [8] Figure 10 Micro-converter cascaded inverter system configuration The control scheme for the DC-DC converter was shown in the Fig.11. This topology would have three operating modes, which are buck mode, boost mode and pass-through mode. When the generated PV current is less than the string current, the DC-dc converter would work under the buck mode; when generated current is greater than the string current, the converter would work under the boost mode; when the generated current is equal to the string current, which means the PV panel is already working under the maximum power point and no additional control is necessary, the converter would work under the pass-through mode. The whole system control method can be described as, first step to decouple individual MPPTs for each PV modules; second step to control the DC-DC voltage mode and the last step is to control the PV 14

26 string output voltage at one constant value, which is in order to add modularity and flexibility for the further PV module. Figure 11 Control scheme for the dc-dc converter and the block diagram of autonomous control system In order to verify this topology s ability to reduce the partial shading effect, one prototype 3-module series-connected PV system was simulated. Each module was consisted by 36 PV cells and two by-pass diode. The simulation result was shown in the Fig.12, and the simulation result showed that an incidental shaded output power loss of nearly 20% with no dcdc converter in the system; this can be cut in half by use of smart converters at the by-pass diode level. With 25% of the panel shaded, both converters generate an approximate 22% increase in output power. With 50% shaded, the single smart converter per module yields a 30% increase, while two per module yield 45%. [8] 15

27 Figure 12 Micro-converter cascaded inverter simulation result As described above, comparing with the cascaded H-bridge inverter, the micro-converter cascaded inverter could realize MPPT for each PV model; meanwhile it also could grantee the wide operating input voltage range for the PV models, the power loss caused by the partial shading could greatly reduce. Moreover, its unique topology with autonomous control enables the system much easier for the further extension. While at the time, because the dc-dc converter stage for the PV system, the whole PV system cost would relatively high, and efficiency maybe low for certain voltage range. 2.2 MPPT algorithm based improvement The MPPT algorithm based improvement emphasizes on the control algorithm for the MPPT controller. Because of the climate change, the shadow left by the light-obstacle would cause the shading pattern varying by time. This situation would cause the PV system output power changing all the time, thus one accuracy and fast response enough control algorithm would be necessary to track the MPP for the whole PV system. Otherwise under the rapid atmospheric change the inefficient MPPT algorithm would cause extra power loss, and finally reduce the power transited to the grid. Typically there are two most commonly used MPPT algorithm, they are Perturb and observe method (P&O) and incremental conductance method (Inc). These two MPPT algorithms all belongs to the hill climbing method. By measuring the PV 16

28 output current and voltage, the MPPT controller would send the correct reference value for the PV system control loop. Because the PV characteristic curve would only have one peak power point under the standard condition, once the correct reference value was tracked, the whole PV system would transit the maximum power to the grid or the load. While when partial shading happened, the PV characteristic curve would have more than one peak value, only one belongs to the global peak, the rest are belongs to the local peak, under this situation one proper MPPT algorithm would be necessary for the PV system to track the global peak power point. Under this circumstance, MPPT algorithm base improvement methods would be another approach to reduce the partial shading power loss for the PV system Short-circuit current pulse method This MPPT algorithm was proposed by Noguchi Toshihiko on the International Society for the Industrial Ecology (ISIE) conference The paper title was Short-current pulse based adaptive maximum-power-point tracking for Photovoltaic power generation system. He proposed one novel MPPT method with simple algorithm for PV power generation system. The method was based on using of a short-current for the maximum output power and completely differs from conventional hill-climbing based method. In his proposed system, the optimum operating current was instantaneously determined by taking a product of the short-current pulse amplitude and a parameter k because the optimum operating current is exactly proportional to the short current under various conditions of irradiation and temperature. Also, the system offers an identification function of k by means of fast power-current curve scanning, which made the short-current pulse based MPPT adaptive to disturbances such as shades partially covering the PV panels. The above adaptive MPPT algorithm has been adopted to a converter system composed by PV-chopper modules. Various operating characteristic have been examined, and excellent MPPT performance has been confirmed through the experimental tests. [9] 17

29 Figure 13 Configuration of current-controlled boost chopper with short-current pulse based MPPT Fibonacci search method This algorithm was proposed by Masafumi Miyatake on International Power Electronics and Motion Control conference (IPEMC) The paper s title was Control characteristics of a Fibonacci-Search-based maximum power point tracker when a Photovoltaic array is partially shaded. In this paper, the author proposed the MPPT employing line search algorithm with Fibonacci sequence. Using this method, the global maximum under the partial shaded condition among the multi peak point could be found. The whole proposed MPPT algorithm was shown as below Fig.14. [10] 18

30 Figure 14 Flowchart of the Fibonacci search based MPPT control diagram Compound MPPT method combined with conventional MPPT methods This method was proposed by Kenji Kobayashi on IEEE Power Engineer Society (PES) 2003 conference, the paper title was A study on a two stage maximum power point tracking control of a photovoltaic system under partially shaded insolation conditions. In his paper, he proposed a two stage MPPT control method which enabled to track the real maximum power point on the I-V curve swiftly with relatively simple control process even under non-uniform irradiation conditions. The system configuration and the control algorithm are shown as following. [11] 19

31 Figure 15 System configuration and the MPPT algorithm flowchart 2.3 Summary and proposed method In this chapter, topology based improvement and the MPPT algorithm based improvement for the PV system to reduce the power loss caused by the partial shading effect were introduced. But these two methods focused on the different situation for the PV system. First, for the topology based improvement method, its purpose is to make sure each PV module of the PV panel would output its maximum power no matter how the partial shading problem affects the other modules; all the PV modules are decoupled with each other, then most of the times basic MPPT algorithm would be enough to track the whole PV system maximum output power. While when partial shading happens to one PV module, its output power maybe locked at local peak power point under the improper MPPT controller then the whole PV system would lose its ability to output maximum power. Second, for the MPPT algorithm base improvement, once the partial shading condition happened, the PV system characteristic curve would vary according to the shading pattern, one or more local peak power points would occur. Under this situation, the MPPT algorithm based improvement would be able the distinct the global peak power point from the local peak value. Then by sending the correct reference value, the PV system would be able the generated its maximum power point. While one serious disadvantage for this improvement is that it would be impossible for each PV module to work under its MPP, 20

32 it only can enable the maximum power point for the whole PV generating system. For the most cases, this MPP value would be less than the total sum value of each PV modules MPP. In summary, each improvement method would not be perfect, in order to enable the maximum power output from the PV system, these two methods need to be implemented together to achieve this goal. Based on this circumstance, this thesis proposed one novel technology. This proposed technology is focused on developing an effective technology based on improved power electronics circuits and control algorithms. Comparing with the previous solutions, the proposed technology could enable each PV module to work at its own MPP and the whole PV system to output the maximum power even when serious partial shading happened to the PV panel. Moreover it also could grantee the wide PV input range for each PV module under reasonable cost and efficiency. 21

33 CHAPTER THREE SINGLE STAGE Z-SOURCE INVERTER As mentioned above in the chapter two, both the topology based improvement as well as the MPPT algorithm based improvement can partial reduce the power loss caused by the partials shading effect. Their traditional topologies still have limitations which would be difficult to overcome, for example, traditional grid-connected inverter requires the input voltage higher than the grid side voltage; otherwise one boost converter would be necessary to step up the input voltage to connect to the grid. While this extra stage finally add the circuit complexity and more electronics components, which would cost more money as well as increase the power loss for the grid-connected PV system. Another disadvantage of the traditional inverter would be the upper and lower devices of each phase leg could not be gated on simultaneously considering the device safety or the EMI noise, in this case dead time to block both devices on one leg has to be employed in the inverter, which would cause the output waveform distortion. Based on this circumstance, the new technology is proposed. Its name is cascaded Z- source inverter for the grid-connected PV system. This new proposed cascaded Z-source inverter allows the both switching devices on the phase leg to be gated on at the same time; this unique feature would boost the inverter input voltage to the desired value by controlling the shoot through time. Then in this chapter, the single stage Z-source inverter topology, operating principle and control strategy and etc. are going to be introduced first. And in the next chapter the cascaded Z-source inverter would be explained in detail. 3.1 Z-Source inverter topology for the grid connected PV system The new circuit topology can be viewed as impedance-source (impedance-fed) power inverter (ZSI), which employs a unique impedance network consisted of a split-inductor L 1, L 2 and capacitor C 1, C 2 connected in X shape. With this unique structure, the Z-source inverter can gate on both switches on the phase leg of the ZSI, this shoot through state could boost the 22

34 voltage to required range then connect to the grid. [12] The Fig. 16 shows the single stage ZSI system configuration of the Grid-connected PV system, which is consist of four main parts: PV panel model, Z source network, an inverter bridge and AC inductor. Figure 16 Z-source inverter circuit topology 3.2 Z-source inverter operating principle For the Z-source network, its simplified circuit shows as Fig.17 from the DC-link side of view, and the inductors L 1, L 2 and capacitors C1, C2 are selected with same inductance and capacitance value, which makes the Z-source network become symmetrical. Its equivalent voltage equations could be derived as, V V L1 C1 = V = V L2 C 2 = V L = V C (1) Figure 17 Equivalent circuit of the Z-source inverter viewed from the DC link 23

35 During the Z-source inverter shoot through time interval period T 0, the equivalent circuit could be expressed as Fig. 18, where V d is the Z-source network input voltage after the diode, and V i is the DC-link voltage as well as equal to the inverter bridge input voltage after the Z- source network V d V L = 2V i = V C C V = 0 > V In this state, because the input voltage after the diode is larger than the PV panel input voltage, the diode would block the current flow into the PV source for protecting purpose. pv (2) Figure 18 Equivalent circuit of the Z-source inverter viewed from the DC link when the inverter bridge is in the shoot through state Similarly during the non-shoot through time interval period T 1, the equivalent circuit shown in the Fig.19 could be expressed as V i V = V C L = V V d V = V L pv V pv = 2V C C V pv (3) 24

36 L 1 V c1 V c2 C 1 C 2 V PV V d V i L 2 Figure 19 Equivalent circuit of the Z-source inverter viewed from the DC link when the inverter bridge is in the non-shoot through state Where T 0 + T 1 = T, T is the switching cycle time of the inverter. Then in one switching cycle, to the Z-source network inductors, the average voltage should equal to zero in the steady state, which leading to the following equations, V T 0 * VC + T1 * ( Vpv VC ) = 0, or V C PV T1 = T T, then peak DC-link voltage during the steady state can be rewritten as T V ˆ T 1 i = VC VL = 2V C VPV = V0 = BVPV, where B = = 1 T1 T0 T1 T T T B is the boost factor resulting from the shoot through time during one switching cycle. The equivalent peak DC-link value would be the input voltage for the inverter bridge, and then the output peak voltage of the grid side can be expressed as V ˆ = MVˆ = MBV ac i, where M is the sinusoidal modulation index, and 0 M 1. From the above equations, by choosing proper modulation index M and boost factor B, the output voltage of the Z-source inverter is controllable. Note that because the shoot through time was insert between the zero state, as shown in the Fig.18, in this case the equivalent voltage on the grid side still remain at PV

37 zero which means that the shoot through period would not affect the output waveform, low THD and power loss could be expected. [12] 3.3 Z-source inverter PWM Modulation As mentioned above, usually the conventional VSI PWM method would not allow the shoot through time for each phase leg, because it shoot time would short circuit the system, and cause damage to the switching devices. While for the Z-source inverter, due to its unique LC network, the shoot through state could exist. In this thesis, bipolar PWM modulation is implemented for the Z-source inverter. The bipolar PWM modulation principle is shown in the Fig.20. This modulation method uses different carrier triangular waveform to control the upper and lower switching devices for one phase leg. These two modified carrier triangular waveform are generated by the original carrier signal add and subtract one shift value, which is calculated by the shoot through time control. Moreover, from this Figure, the shoot through time interval T 0 is split into two parts, which are equally inserted into the gating signals for each phase legs. This shoot through period for the Z-source inverter would in turn give rise to an ac output voltage. 26

38 Figure 20 Bipolar PWM modulation for the Z-source inverter Although comparing with unipolar PWM, the bipolar PWM modulation would produce more power loss; it could avoid the leakage current between the PV panel and the ground. For the reason that, due to the large area surface of the PV generator, its stray capacity with respect to the ground reaches values that can be really high. Because no transformer was used in the thesis s grid-connected PV system, common-mode current caused by the common mode voltage can transited by the stray capacitor between the PV panel and ground. This current would be harmful to the PV panel and its operating technicians safety. Then from the safety and energy saving point of views, the tradeoff between the modulation method and the power loss, the bipolar PWM would be the best choice among the other solutions methods. Fig.20 shows the system configuration diagram. [13] 27

39 Figure 21 Common mode current in the non-transformer grid connected system 3.4 Control principle of the ZSI for the grid-connected system The purpose for the ZSI for the grid-connected PV system is to converter the maximum solar energy DC power exacted from the PV module into the AC power then feed it into the grid. To achieve this goal, three control loops are necessary; the first one is the grid current inner control loop, which would track the current fed into the grid and realized the unity power factor; the second one is the inverter DC-link voltage outer control loop, which maintains the input DC link voltage of the inverter at certain constant value for the further cascaded connection purpose. Meanwhile, by controlling the shoot through duty cycle of the Z-source network, the voltage across the PV output capacitor could also be controlled to track the MPP of the PV panel, this control loop would enable the PV panel output its maximum power.[14] The whole Z-source inverter control diagram is shown in the Fig

40 Figure 22 Control system diagram Grid current close loop control Typically the PV panel could be treated as current source, when connecting to the grid, Phase Lock Loop (PLL) is necessary to synchronize the phase and frequency between the input and its output. Then the grid sinusoidal current loop control was adopted to realize the unity power factor for this PV system. Theoretically the grid side current could be expressed in the Laplace form, shown as derived from the above equations, i i ac vac vgrid =, where v ac = Mvi L s ac ac Mvi vgrid =, where i ac is the grid-connect current, v ac is L S ac the inverter output voltage, v i is the inverter input DC-link voltage, v grid is the grid voltage, L ac is the AC inductance and S is the Laplace operator Inverter DC-link voltage control As mentioned above, the inverter input voltage, which is also the DC-link voltage after the Z-source network, its value is an important parameter. As mentioned in the last section, the grid connected current could be expressed as i ac Mvi vgrid =, in this ZSI grid-connected PV L S ac 29

41 system, the v grid and L ac are fixed parameters. While for the H bridge inverter topology, M is limited between certain values smaller than 1, in this case the DC-link voltage also needs to be limited between certain values, otherwise when the DC-link voltage is too low; even with the up limit of the modulation index, the modulation output still cannot produce a proper AC output voltage, which would lead to a non-sinusoidal current. Similarly when the DC-link voltage is too high, it would bring too much thermal stress for the devices. In this case, the inverter DC-link voltage should be controlled and stabilized, which required the DC-link voltage outer loop was adopted. Note that in the real experiment, this DC-link voltage could not be measured for the control purpose, because its value is changing too fast and varying between wide ranges. Then in this case, indirect control for the DC-link voltage is realized, from the equation 3, DC-link voltage could be expressed as V dc = 2 V V, in the experiment, these two voltage value can be c pv easily measured by the voltage sensor. Then the inverter DC-link voltage control loop could be adopted MPPT control algorithm and ZSI shoot through time control Nowadays, there are two main methods used to track the MPP of the PV panel, which are Perturbation & Observation method (P&O) and incremental conductance method (IncCond). To the fact that, under certain solar irradiation PV power-voltage curve would have only one peak power along with the whole voltage range. Based on this characteristic curve, both of these hillclimbing methods would be able to lock PV output power around its peak value. P&O method is an iterative approach which perturbs the operation point of the PV system in order to track the correct direction of change for maximum the PV output power. It is operated by comparing the PV panel output power with that of the previous cycle then according to the MPPT algorithm to perturb the PV terminal voltage. Although using this MPPT algorithm the peak power would oscillate around the MPP in steady state, it is easy to be implemented and widely used in the PV simulation related field. Thus in this thesis system, the P&O method was employed shown as Fig

42 Figure 23 P&O MPPT algorithm for the PV panel After this MPPT controller, the output reference value is compared with the real voltage detected by the PV panel output voltage sensor, then the difference between these two values through one PI controller to calculate the desired shoot through time reference for the further modulation use to control the switching devices on one phase leg for the Z-source inverter. 3.5 Simulation verification In order to verify the operating principle of the single stage Z-source inverter for the gridconnected PV system, one 3 KW PV panel model was built in the thesis; its specification was list in the Table.1. The entire model was built in the Matlab/Simulink software, first the detail analysis of the parameters would be introduced, and then the simulation result output waveforms were presented. 31

43 3.5.1 System Specification Table 1 Single stage Z-source inverter topology PV panel specification PV Parameters MPP voltage (Vm) MPP current (Im) Open circuit voltage (Voc) Short circuit current (Isc) Maximum power (Pm) Value V 7.84 A V 8.54 A 3035 W Table 2 Grid connected PV system single stage Z-source network parameters ZSI Parameters Switching frequency (fs) PV output capacitor (C PV ) Z-source inductor (L Z ) Z-source capacitor (C Z ) AC inductor (L ac ) Grid voltage (V grid ) Grid frequency Value 10k Hz 2000 uf 4 mh 1000 uf 5 mh 240 V(rms) 60 Hz Simulation condition and results: The system configuration is like described above, the first simulation scenario is carried out when the 3kw PV panel receive the same irradiation 1000 w/m2 during the whole time interval. The system grid side voltage and current in the steady state are shown in the Fig. 24. The green curve is the voltage value, which RMS value is 240V; peak value is around 340V. The grid connected current is around 18A by calculation. Fig.25 shows the MPPT controller command and the PV output voltage in the steady state. The green curve is the PV output voltage; 32

44 the red curve is the controlled reference value for the PV output voltage. Because the hillclimbing MPPT method algorithm is used in the thesis, the controller reference varying between five levels, and the PV output voltage could track the reference command with reasonable oscillation. Fig.25 shows the PV output power Figure during the steady state, the PV output power could reach its peak value around 3034W, and only about 1W power oscillation could be observable. Fig.26 shows the controlled DC-link voltage, for the consideration of the modulation index M, this value is controlled at 600V. Figure 24 Grid voltage and grid current for the simulated scenario one 33

45 Figure 25 MPPT controller reference and PV output voltage for the simulated scenario one 34

46 Figure 26 3KW PV panel output power curve for the simulated scenario one 35

47 Figure 27 DC-link voltage for the simulated scenario one Similarly other scenario of the single stage Z-source inverter for the grid-connected PV system is also simulated. In the simulated scenario two, the solar panel is given one constant 1000 W/m2 irradiation at the beginning, then at the 0.8 second, the solar irradiation decrease to 800 W/m2 with a ramp stage. Fig.27 shows the grid-connected voltage and the current for this scenario, the switching transient is also shown in the Figure. The generated current changes from about 18A to 14A when PV system reaches the steady state under the two different solar irradiations. Fig. 28 shows the MPPT controller output reference and PV output voltage curve for this simulated scenario, this Figure indicted that the MPPT controller could make the right decision and response fast enough when the solar irradiation changes. Fig.29 shows the PV panel output power curve under this changing weather condition. Its output power drops from 3035W 36

48 to 2336W from the steady state. Similarly, the DC-link voltage for the H-bridge inverter is shown in the Fig.30. Figure 28 Grid voltage and grid current for the simulated scenario two 37

49 Figure 29 MPPT controller reference and PV output voltage for the simulated scenario two 38

50 Figure 30 3KW PV panel output power curve for the simulated scenario two 39

51 Figure 31 DC-link voltage for the simulated scenario two To sum up this chapter, the single stage Z-source inverter for the grid-connected PV system is emulated in the Matlab/Simulink software. First the system configuration as well as the system control scheme are introduced. Second two simulation scenarios are emulated to test and verify the PV system performance under different conditions. From the simulated Figures above, the conclusion can be reached that the designed singe stage Z-source inverter for the gridconnected PV system is capable to output its maximum power under changing weather condition, the generated current feed into the grid can reach the unity power factor, and the DC-link voltage is controlled to be the desired value, which would be useful for the further cascaded purpose. 40

52 CHAPTER FOUR CASCADED Z-SOURCE INVERTER 4.1 Introduction The single stage Z-source inverter has been introduced in the previous chapter, and then the cascaded Z-source inverter topology would be described in this chapter. The cascaded topology has several advantages, such as the reduced device voltage stress, low electromagnetic emission. Thus the use of the cascaded schemes has been increased in the industrial applications which require the high voltage and the high output quality. [15] Usually for the traditional topology, several PV panel are needed to connect in series to meet the voltage requirement to connect to the grid. While when the PV panels operate under non-uniform irradiation, this stacked PV panels would result in partial shading effect. Under this situation, the MPPT controller maybe would fall into the local maximum point and the its output power would considerable decreased, which phenomenon is already talked about in the chapter one. However this problem can be reduced through the cascaded Z-source inverter topology. Using this topology, PV panels are divided by the number of the H-bridge modules, and each PV panel operating voltage can be controlled to its maximum power point using individual MPPT algorithm. In this chapter, one 3KW two level cascaded Z-source inverter for the grid-connected PV systems would be introduced. 4.2 Cascaded Z-source inverter topology The whole cascaded Z-source inverter topology for the grid-connected PV system is shown in the Fig.32. Similar to the single stage Z-source inverter, the PV panels are connected to the Z-source network through diodes and the Z-source network directly connected to the H- bridge of the inverters, and then these H-bridges are cascaded into one inverter system to connect to the grid. 41

53 Figure 32 Cascaded Z-source inverter for the grid-connected PV system 4.3 Z-source inverter PWM method The modified unipolar PWM method is used in this chapter. Comparing to the traditional unipolar PWM, the modified unipolar PWM employ two straight lines equal to or greater (smaller) than the peak value of the phase references to control shoot-through duty ratio shown as Fig.33. S1and S2 are two switching device gating signal for one phase leg of the inverter, S3 and S4 are the other phase leg switching devices. Then by controlling the position of the two shoot through lines, the shoot through duty cycle of the H-bridge inverter could be controlled to the desired value. While it would be another limitation of this PWM modulation method, which is that the maximum shoot through straight line position shift value is only 1-M. This means the two shoot through lines position are limited between 1-M and M. [16] 42

54 Figure 33 Modified unipolar PWM method 43

55 4.4 Cascaded Z-source inverter control strategy Figure 34 Cascaded Z-source inverter control strategy The whole system control scheme is shown in the Fig.34. Similar as the single stage Z- source inverter control strategy, the cascaded Z-source control strategy also has the DC-link voltage control loop, grid current control loop and the MPPT & shoot through control loop. The additional control part is the PV power distribution part which in charge of the power distribution process. Because in this cascaded Z-source topology, each DC-link voltage before the H-bridge inverter is controlled to be one constant value, in this case, by controlling the generated current of the PV panels, the purpose of distributing power could be realized. Thus the power distribution control part is circled out in the Fig.34. And the rest control blocks can reference to the chapter Three control strategy section. 4.5 Simulation verification System configuration Because two PV panel are cascaded together to form this cascaded Z-source inverter system, which is also designed for one 3 KW system, then the PV panels are scale down to meet the 44

56 requirement for the simulation purpose. From the listed tables below, the conclusion can be derived that only the voltage parameters for the PV panels are scale down to half of the parameters as before. By changing the parameters under this assumption, the designed 3 KW cascaded Z-source inverter requirement can be reached. Table 3 Cascaded Z-source inverter topology PV panel specification PV Parameters MPP voltage (Vm) MPP current (Im) Open circuit voltage (Voc) Short circuit current (Isc) Maximum power (Pm) Value V 7.84 A V 8.54 A W Table 4 Grid connected PV system cascaded Z-source network parameters ZSI Parameters Switching frequency (fs) PV output capacitor (C PV ) Z-source inductor (L Z ) Z-source capacitor (C Z ) AC inductor (L ac ) Grid voltage (V grid ) Grid frequency Value 10k Hz 1500 uf 1 mh 1200 uf 3 mh 240 V(rms) 60 Hz Simulation condition and the results The simulated scenario can be described as the Fig.35. In this system, at the beginning, both the PV panels of the proposed topology receive the uniform irradiation as 1000 w/m2. While after about 0.5 second, the partial shading happens to the first PV panel, which changes its 45

57 irradiation reduce from 1000 w/m 2 to 600 w/m 2. At the same time, the second PV panel is still receiving the same irradiation of 1000 w/m 2. Figure 35 Simulated scenario of the proposed topology The simulated result for the whole system under partial shading condition is shown in the Fig. 36. During the whole simulation process, one panel receives the varying irradiation at the time of 0.5 second, while the other panel still receives the same irradiation. The Figure shows that, the grid side current reduces from 17.8 A to 13.8 A, while the grid remains the same voltage RMS value of 240 V. 46

58 Figure 36 Grid current and grid voltage And the whole system output power and the power output for each PV panel is shown in the Fig.36. From the Figure, the whole system output power could be observed, during the first 0.5 second, the total power is 3034 W, and both the PV panels output the same power 1517 W. While after the first 0.5 second, when shading happens to the first PV panel, the irradiation reduces from 1000 w/m 2 to 600 w/m 2, the total output power of the proposed system is reduce to 2356 W. And the first PV panel output power drops from 1517 W to 840 W, the other PV panel remains the same output power 1517 W. 47

59 Figure 37 (a) proposed system output power and (b) each PV panel output power And the proposed PV system output power characteristic curve under the partial shading weather condition is shown in the Fig.38. In this Figure, the peak point power is located around 2356W. Then with proper MPPT control strategy, the total PV panel output power could be granted. 48

60 Figure 38 Proposed cascaded Z-source inverter topology PV output characteristic curve Furthermore, under the same simulation scenario, in order to compare the results between the proposed cascaded Z-source inverter system and the traditional series connected PV system. The traditional series connected PV system; the traditional series connected PV system is built shown as Fig.39. And its output power characteristic curve is shown in the Fig.40. Observed from this Figure, the PV panel output voltage-power characteristic curve has multi peak points because of the partial shading issue, which has already talked in the chapter one. While the peak power of the whole PV system is only 1923 W, which means even with the MPPT controller for the same PV panels, the maximum output power can just maintained at 1923 W. Comparing with proposed method, this traditional topology PV system maximum output power is about 23.48% less. The detail comparison table would refer to the table.5 49

61 Figure 39 Traditional series connected PV system 50

62 Figure 40 Traditional series connected PV system output power characteristic curve 51

63 Table 5 Comparison table between the traditional inverter topology and the proposed cascaded Z-source inverter topology Traditional inverter for the PV system Cascaded Z- source inverter system for the PV system Full irradiation output power When one panel shading output power Power loss due to the shades Output power improvement Recouped power loss 3035 W 1908 W 37.1% N/A N/A 3035 W 2356 W 22.3% 23.48% 60.25% The proposed method recouped the power loss by ( )/( )=60.25%, and improved the output power by ( )/1908= 23.48% 52

64 CHAPTER FIVE PV PANEL EMULATOR USING RTDS In this chapter, the Real time digital simulator (RTDS) based PV panel emulator is proposed, which would be used to verify the PV panel system performance under different weather conditions. This RTDS based PV emulator system configuration and the system experiment result would be described later in this chapter. 5.1 Introduction In order to analyze the PV system performance under certain conditions, usually there are two ways can be implemented for this purpose. One way is to emulate the whole PV system in the simulation software, for example Matlab/Simulink or PSim software; the other way is to install the real commercial PV panels in the lab to test its performance. While there is one disadvantage for the pure simulation in the software, the simulation running time would depends on the complicity of the simulated circuit topology, the more complicity the more time would be needed for the software to emulate the PV performance. It prevents this method from emulating the PV panel for long time duration. And the other way would be using PV emulator to replace the real PV panel Different kinds of PV emulator While sometimes due to the limitation of the lab environment or cost factors, the real PV panel would be replaced by the PV emulator to act as the PV source for the system research. Typically there are two kinds of PV emulator: One kind is using DC power supply series connected with one resistor to emulate the PV panel. And the other kind would be the integrated commercial PV emulator produced by the different manufactures. While both of PV emulators have their own disadvantages, for example, because the characteristic curves of the PV panel and the DC power supply connected with a resistor are 53

65 similar, then this method could approximately to emulate the PV panel, while the issue would be this method s accuracy would not meets the requirement to perfectly emulate the actual PV panel performance under certain conditions. [14] To the other method, although it would emulate the PV panel in a higher accuracy, comparing with the other methods, its cost would be high, and its specification would hard to modify for the further research Proposed RTDS based PV emulator Based on the issues mentioned above, one novel method is proposed to emulate the PV panel performance. This new method is using RSCAD (RTDS simulation software) to build the PV panel model then using the controlled DC amplifier and power hardware in the loop (PHIL) concept to emulate the PV panel performance under different weather condition. It basically cut the PV panel system equivalent circuit into two parts, one part is implemented in the simulator in the RSCAD and the other part is realized in the real hardware configuration, both of these parts are connected by the power interface. For the proposed RTDS based PV emulator, the system diagram is shown in the Fig.41. Figure 41 RTDS based PV emulator system diagram 54

66 Compared with the methods above, this new RTDS based PV emulator would have the following advantages. First, because the PV panel mode is built in the simulator side, by choosing the proper PV mathematic model, the whole system accuracy would reach a satisfying index. Second, thanks to the RTDS unique advantage, real time simulation for the PV emulator under varying weather condition could be realized, which would be impossible for the other simulator software. Furthermore, real weather condition data can be imported into the RTDS system to test the simulated PV panel performance under real weather condition. And based on the fact that Center for Advanced Power Systems (CAPS) of the Florida State University (FSU) already have the whole RTDS equipment, no extra experiment components would be necessary to implement the proposed PV emulator, the total cost would reduce by certain percent. 5.2 RTDS based PV emulator topology Real time digital simulator (RTDS) The real time digital simulator (RTDS) is a combination of specialized computer hardware and software designed specifically for the solution of power system electromagnetic transients. An extensive power system component library together with a friendly graphical user interface, facilitate the assembly and study of a wide variety of ac, dc and integrated power systems. Because real time operation can be achieved and indefinitely sustained, the RTDS can be applied in areas traditionally reserved for analogue simulators. [17] Its main network is solved with a simulation time-step size of 50us. Moreover, it has the capability of simulating subnetworks modeling power electronics converters, which can be interfaced to the main network solution, with a time-step of about 2us Power hardware in the loop (PHIL) Nowadays, virtual prototyping is widely used in the design, development and testing of new systems. It uses the software-based model of a product, system, or component to explore, test, demonstrate and validate the design alternatives. One of the most common methods is called Hardware-in-the- loop (HIL), in which simulation based on mathematical models replaces part of the real system or components, and interacts with real hardware. This paper used the methodology called Power-Hardware-In-the-Loop, show as Fig.42. In PHIL simulation, the 55

67 interface point involves conservation of energy so that real power is virtually exchanged between the simulation software and the actual hardware. This methodology significantly extends the applicability of HIL to any electrical or mechanical device or system. Figure 42 Power hardware in the loop (PHIL) diagram RTDS based PV emulator system configuration As described above in the introduction section, the proposed RTDS based PV emulator system can be divided into three parts, which are simulator, power interface and the hardware under test. The detail of the system configuration is shown in the Fig.43. First the single index PV panel model is built by the RSCAD software, and then the reference signals are transferred by the power interface to the real hardware. In this system, A/D device, D/A devices and the controlled DC amplifier could be viewed as the power interface. Their function is to amplify reference voltage into the desired voltage value then connect it to the real resistor load. And the measured current value is processed by the A/D device to send the current signal back to the PV panel model for further calculation. 56

68 Figure 43 RTDS based PV emulator system configuration 5.3 Simulation & Experiment verification PV model specification In order to emulate the real PV panel performance under different weather condition, real commercial PV panel parameters are selected as below in the Table.6. In the proposed PV emulator, single index PV mathematic model is used. For this model, certain parameters of the PV panel would be enough to emulate the PV panel performance. These parameters are maximum power point voltage, maximum power point current, open circuit voltage and the short circuit current. The four parameters are usually provided in the PV panel data sheet for the commercial products. 57

69 Table 6 RTDS emulated Sanyo PV panel parameters Sanyo PV panel parameters MPP voltage (Vm) 55.8V MPP current (Im) 3.59A Open circuit voltage (Voc) 68.7V Short circuit current (Isc) 3.83A Maximum power (Pm) 200W Controlled DC amplifier and electrical load specification In the proposed experiment, LVC 5050 is used to work as the controlled DC amplifier. It has two control modes, which are constant current control and constant voltage control. Because the PV panel could be viewed as the current source, then the LVC 5050 is set to working under constant current mode. The LVC 5050 has two separate channels that can be operated independently or combined for greater maximum voltage or current. In Bridge-mono mode the available output voltage doubles. In Parallel-mono mode the amplifier operates with twice the available output current. This DC amplifier specification table is shown in the Fig.7. Also in this experiment, one 15 ohms electrical resistor is connected after the controlled DC amplifier to emulate the load of the PV system. 58

70 Table 7 Controlled DC amplifier specification Experiment result In order to study the PV panel performance under real weather condition, the actual data of the weather condition is necessary to be imported. Thanks to the PV panels system installed on the CAPS building roof, actual irradiation and the panel surface data could be detected. In this experiment, real weather condition data of April 12 th, 2010 is chosen to implement this test, shown as Fig

71 Figure 44 (a) April 12th, 2010 day irradiation profile (b) April 12th, 2010 PV panel surface temperature profile Based on given conditions, the PV performance for this day (April 12 th, 2010) could be emulated, the time duration is chosen from 6 am to 8 pm. Besides these hours, there is almost no solar irradiation, and then no power would generate from the PV panel. Under such circumstances, real irradiation and panel surface temperature data are imported into the system to emulate the PV panel performance under real weather condition. The result is shown in the Fig.45. From the simulation result, the maximum power is generated around the peak point of the solar irradiation, and this value is very close to the emulated PV specification. Based on these 60

72 reasons, the conclusion can be reached that the proposed PV emulator could represent the real PV panel very well during the whole simulation process. Figure 45 PV panel performance emulation under real weather condition of April 12th, 2010 Another experiment for testing the PV panel simulation accuracy is also implemented. This switching transient result is shown in Fig. 46. For the first second, the experiment is carried out in the RSCAD, and the simulation system uses on virtual load with the same resistance value as the real electrical load. Also its simulated output power value is proved to be the same one as the Matlab/Simulink simulation result. Then after the first second, this RTDS based PV emulator switches to the actual electrical load using PHIL method. From the simulation result, the output power using the PHIL method only has 0.3% difference with the theoretical value. Thus the proposed RTDS based PV emulator, which would be used to replace the real PV panel, is accuracy enough for the research purpose. 61

73 Figure 46 RTDS based PV emulator switching transient experiment 5.4 Conclusion In this chapter, the proposed RTDS-based PV emulator is implemented. Single index PV mathematical model with real weather condition data input is built in the RSCAD software to emulate actual PV panel performance under varying circumstances. Furthermore, PHIL methodology enables the whole system to response similarly to a real PV panel system and the experiment result has shown satisfying results, which is very close to the theoretical results with less than 1% difference. Moreover using this proposed experiment test bed, by inputting different parameters and weather condition data, the system is capable to emulate different PV response under varying weather conditions for certain time period, like one day, one month or even a whole year, which would be difficult for the other traditional PV emulators. 62