DEVELOPMENT OF A HIGH EFFICIENCY GRID-TIED THREE-PHASE INVERTER FOR SOLAR SYSTEM ______
A Thesis
Presented to the
Faculty of
California State University, Fullerton ______
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
in
Electrical Engineering ______
By
Mohammad Alsemaan
Thesis Committee Approval:
Professor Maqsood Chaudhry, Chair Professor Karim Hamidian , Department of Electrical Engineering Professor Mostafa Shiva, Department of Electrical Engineering
Spring, 2016
ABSTRACT
Solar energy is probably the most abundant renewable energy which is of great importance to fundamentally solve the energy crisis problem if can be harvested extensively and efficiently. This dissertation reviews the solar energy conversion systems and focuses on analyzing and developing high efficiency single-stage three-phase solar inverter system.
Firstly, the recently developed material for solar cell is reviewed, and the solar model is derived, as well as solar string model. Based on the output put characteristic of solar string, the current state-of-art MPPT technologies are discussed and the P&O method is adopted for simplification purpose. Then the system design of solar inverter is fully is performed. The hardware design provides the detail design guidelines for most key components and provide suggested component in the market for prototype construction. The latest GaN device is adopted to improve efficiency and six-channel compact driving chip is used to reduce converter size. After that, the control loop design for three-phase inverter is detailed, with current loop and voltage loop illustrated respectively.
Finally, the simulation results for control loop design and MPPT control are provide to verify the feasibility of the previous design. Based on the structure of selected system and new semiconductor device adopted, potential high efficiency solar conversion system can be expected.
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TABLE OF CONTENTS
ABSTRACT ...... ii
LIST OF TABLES ...... v
LIST OF FIGURES ...... vi
ACKNOWLEDGMENTS ...... ix
Chapter 1. INTRODUCTION ...... 1
1.1 Background ...... 1 1.2 Objective ...... 2 1.3 Outline of the Thesis ...... 4
2. REVIEW OF SOLAR ENERGY CONVERSION SYSTEM ...... 5
2.1 Stand-Alone and Grid-Tied Solar System ...... 5 2.2 Classification of Solar Inverter Based on Inverters Configurations ...... 7 2.2.1 Micro-Inverter, String Inverter and Centralized Inverter ...... 7 2.2.2 Single-Phase and Three-Phase Inverter ...... 9 2.2.3 High Frequency Isolation and Low Frequency Isolation Inverter ....10 2.2.4 One-Stage and Multi-Stage Inverter ...... 10 2.3 Commercialized Products of Solar Inverters ...... 12 2.4 Standards for Solar Inverters ...... 14 2.4.1 Ungrounded PV System ...... 19 2.4.2 Grounded PV System ...... 19
3. SOLAR STRING DESIGN AND MPPT TRACKING TECHNOLOGY ...... 20
3.1 Introduction of Solar Cell ...... 20 3.1.1 Solar Cell Materials ...... 20 3.1.2 Solar Cell Model ...... 22 3.2 Solar String Model and Design ...... 27 3.3 Maximum Power Point Tracking Technology ...... 29
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4. HARDWARE DESIGN OF 1KW THREE-PHASE SINGLE-STAGE SOLAR INVERTER ...... 33
4.1 DC Voltage Selection ...... 33 4.2 Selection of Switch ...... 35 4.3 Design of Output Filter ...... 36 4.4 Design of DC Capacitor ...... 38 4.5 Controller and Driving Circuit Selection ...... 40 4.6 Sensors Design ...... 41
5. CONTROL SYSTEM DESIGN OF 1KW THREE-PHASE SINGLE-STAGE INVERTER ...... 43
5.1 Modeling of Three-Phase Single-Stage Inverter ...... 43 5.2 Close Loop Design of Three-Phase Single-Stage Inverter ...... 47 5.2.1 Current Loop Design...... 47 5.2.2 Voltage Loop Design ...... 49
6. SIMULATION VERIFICATION OF THREE-PHASE SINGLE-STAGE SOLAR SYSTEM ...... 53
6.1 Solar String Test Simulation ...... 54 6.2 Simulation for Inner Current Loop ...... 55 6.3 Simulation for Outer Voltage Loop ...... 57 6.4 Simulation for Whole System with MPPT Control ...... 59
7. CONCLUSIONS AND FUTURE WORK ...... 63
APPENDIX: MPPT CODE IN PSIM ...... 64
REFERENCES ...... 65
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LIST OF TABLES
Table Page
1. Basic Requirements of the Solar System ...... 3
2. Typical Solar Inverters in Industrial Area ...... 13
3. Basic Requirements of the Solar System ...... 15
4. Interconnection System Response to Abnormal Voltages ...... 15
5. Interconnection System Response to Abnormal Frequencies ...... 16
6. Comparison for Solar Materials ...... 22
7. Parameter Definition and Typical Values ...... 24
8. Model Parameters of KM50(6) ...... 29
9. MPPT Methods ...... 30
10. Performance of TPH3206PD ...... 36
11. Requirement of Sensors ...... 42
12. Simulation Parameters...... 53
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LIST OF FIGURES
Figure Page
1. Desirable Single-stage three-phase solar system ...... 2
2. Stand-along PV system ...... 5
3. Grid-tied PV systems ...... 6
4. Classifications of solar inverter ...... 7
5. Inverter system based on power: a, Micro-inverter system; b, String inverter system; c, Centralized inverter system...... 8
6. single-phase PV system ...... 9
7. three-phase PV system ...... 9
8. HFI system ...... 10
9. LFT system ...... 10
10. Single-stage system ...... 11
11. Two-stage system...... 11
12. Multistage system ...... 11
13. Voltage dip up to 140ms condition ...... 17
14. Voltage dip more than 140ms to 3 minutes condition ...... 17
15. Voltage dip withstanding requirements ...... 18
16. Reactive current injection requirements ...... 18
17. Equivalent circuit of a solar cell ...... 22
18. Solar module in PSIM ...... 23
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19. I-V and PV curve of a single solar cell under reference condition S=1000, T=25 ...... 25
20. I-V and P-V curve of a single solar cell: a, S=2000, T=25; b, S=1000, T=50 ...... 25
21. P-V curve of a solar cell under effect of environment ...... 26
22. Equivalent circuit of: a, Solar Panel; b, Solar String ...... 27
23. KM50(6) made by Komaes ...... 28
24. Principle of P&O Method ...... 31
25. The flowchart of P&O MPPT method for single-stage three-phase inverter ...... 32
26. Three-phase Inverter Hardware ...... 33
27. Minimum DC voltage selection condition ...... 34
28. Vector diagram for DC voltage selection ...... 34
29. TPH3206PD from transphorm ...... 36
30. Output filter design ...... 36
31. Imbalanced system: a, Phase Voltage; b, Positive sequence of three-phase current; c, Negative sequence of three-phase current ...... 38
32. Power fluctuation due to imbalanced load ...... 39
33. Three-phase inverter driver using IR2233 ...... 41
34. SCD05PUN and SCD10PUR ...... 42
35. LV 25-P ...... 42
36. Three-phase system for modeling analysis ...... 43
37. Small signal model equivalent circuits for D axis and Q axis ...... 46
38. Current loop control block ...... 47
39. Current loop PI parameter design ...... 49
40. Voltage loop control block ...... 50
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41. Voltage loop PI parameter Design ...... 52
42. Solar String P-V and I-V curves Tested for simulation ...... 54
43. Current loop control test ...... 56
44. Key waveforms for current control test ...... 57
45. Voltage loop control test ...... 58
46. Key waveforms for voltage loop control test ...... 59
47. Whole system simulation with MPPT control ...... 61
48. Key waveforms for MPPT control test ...... 62
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ACKNOWLEDGMENTS
Special appreciation goes to Professor Maqsood Chaudhry, for his supervision and constant support. His invaluable help of constructive comments and suggestions throughout the course of this thesis, while not forgetting his patience, motivation, and immense knowledge.
A special thanks to my family. Words cannot express how grateful I am to my mother, and my father for all of the sacrifices that they have made on my behalf. Your prayer for me was what sustained me thus far. At the end I would like to express appreciation to my beloved wife Sarah for her support, encouragement, quiet patience and unwavering love.
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CHAPTER 1
INTRODUCTION
1.1 Background
Because of the energy crisis that arose in recent years and deterioration of the environment caused by use of fossil fuels that seems to have reached the tipping point, renewable energy has drawn more and more attention both in academic and industrial circles.
Among the current renewable energies, photovoltaic provides the most possibilities for different uses due to the reason that it is suitable in different power range occasions. Solar energy utilization has the following advantages[1]: 1) Power ranges from microwatt for feeding a pocket calculator to megawatt for supplying public electricity. 2) It is the most resourceful energy and can be taken everywhere for free. 3) Quiet and noise-free. 4) Easy maintenance. 5) The output is DC source, safer and easier to control.
Solar energy utilization generally falls into two categories: Solar thermal[2] and solar photovoltaic energy. This thesis will focus on the key issues of technologies about the photovoltaic (PV) energy utilization. In the past 20 years, the market for PV grew rapidly and the cost of each watt of electricity generated by solar energy decreases continuously.
Germany and Spain are pioneers in PV system installation. In 2008, the PV demand from
Germany and Spain were 1.86GW and 2.46GW respectively, occupying 72.6% of the global market. In recent years, the market in the United States, China, South Korea, Japan, as well as Europe have increased rapidly. However, in Germany, the popularity of grid-connected
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2 photovoltaic systems did not begin until the 1000-Roof program launched in 1990. And in
2000, the enacting of the Renewable Energy Law which regulated the compensation policy introduced a new impetus to the PV market. With this law, a photovoltaic boom was successfully triggered in 2004, which led to Germany leading in solar photovoltaic utilization from then on. Germany provides a good example about solar energy development.
Globally, solar photovoltaic utilization is still undergoing many technical issues which may prevent it from mass usage, especially in many counties without government support. It is becoming increasingly important to develop high efficiency and low cost solar systems to promote the massive generation of solar energy.
1.2 Objective
GaN Switches
Vgs1 Vgs3 Vgs5 L Grid
Vgs2 Vgs4 Vgs6 C
Ia, Ib, Ic Solar Array Ipv
Vgs1 Vgs2 Vref MPPT Inverter Driver Vgs3 Algorism Control Vgs4 Vgs5 Vgs6 phase All-in-one driver Va, Vb, Vc PLL Algorism
High performance Digital Controller
Figure 1. Desirable Single-stage three-phase solar system.
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The objective of this thesis is to design and analyze a 1kW High Efficiency Grid-tied
Three-phase Inverter for a solar system as shown in Figure 1 by employing cutting-edge components and chips along with the advancement in the semiconductor industry. The finalized solar system is a one-stage three-phase inverter with maximum power point tracking (MPPT) control. This system has two basic parts for the main circuit: Photovoltaic
Array and a DC/AC inverter. The control part includes the MPPT algorism, the Phase Loop
Lock (PLL) algorism and the inverter control which will be achieved all in a high performance digital microcontroller. The driving circuit will be realized by a simple six channel IGBT driver chip.
The single-stage system can reduce the weight and cost, and achieve high efficiency for solar energy conversion. The basic requirement of the solar system is given in Table 1.
Table 1. Basic Requirements of the Solar System
DC Voltage(PV) 280V-580V
MPPT tracking included
Power Rating 1kW
AC voltage(Grid) 110V(Three-phase) efficiency 98%
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1.3 Outline of the Thesis
Chapter 2 of this thesis gives an overview of latest technologies and development in
PV systems with different inverter configurations, focusing on the most efficient way of converting solar energy with the lowest cost.
Chapter 3 introduces the operation mechanism of solar cells and discusses the design of solar array. Moreover, the most popular MPPT control techniques are reviewed. The design of solar array for this thesis and MPPT control adopted are analyzed in detail.
In Chapter 4, the hardware design of the single-stage three-phase system is fully discussed. The design requirements of most key components of main circuits are all covered.
Finally, a hardware selection for prototype construction is suggested.
In Chapter 5, the mathematical model of the three-phase inverter is discussed. The control system design based on a mathematical model and with assistance of MATLAB is investigated. The double loop control of three-phase inverter is explained. The inner current loop and outer voltage loop are designed respectively.
Chapter 6 gives the simulation verification of the designed solar system. A number of simulation results provided the analysis of circuit and control design in previous sections.
Finally, the conclusions and future work of this thesis are given in Chapter 7.
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CHAPTER 2
REVIEW OF SOLAR ENERGY CONVERSION SYSTEMS
In order to feed various applications’ demand, different PV systems were developed.
The functionalities, cost, and efficiency can be different, based on different configurations.
2.1 Stand-Alone and Grid-Tied Solar Systems
According to the power flow direction and its control scheme, the PV system can be classified into a stand-alone system or a grid-tied system, shown in Figure 2 andFigure 3. For the stand-alone system, the AC output is directly connected to AC load. It means the inverter has to generate a certain voltage with a proper frequency output to meet the load demand.
Mostly this kind of system needs storage to guarantee a stable power delivery.
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6
DC DC Road DC AC
Storage
Figure 2. Stand-alone PV system.
DC DC DC AC
DC DC
Figure 3. Grid-tied PV systems.
The grid-tied inverter has a totally different control mechanism with stand-alone system due to the disparity of the control goal. As the grid-tied inverter is connected to the grid directly, there is no need for the inverter to control the output voltage and frequency, which are already determined by the utility. What a grid-tied inverter needs to do is to generate the output current in phase with the grid voltage and keep the DC input voltage within a designed voltage range. Stand-alone and grid-tied systems both have wide applications in both residential and commercial areas. Sometimes, these two systems refer to two different working modes of one PV inverter system, which could be interchanged seamlessly, according to the working conditions. In high power applications, such as solar stations, grid-tied inverter systems are mainly used because they have to collect this large amount of power to deliver to the customers which may be far away. Grid is a best medium to realize this power delivery.
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2.2 Classification of Solar Inverter Based on Inverters’ Configurations
A more detailed classification of a solar inverter is given as Figure 4. Since solar energy is widely exploited from residential applications to commercial and utility applications, the associated power electronics converters adopted can be various in power range, stage, phase number, isolation equipment, and so on[3]. Therefore, different solar inverter systems can be identified to satisfy different application requirements.
Figure 4. Classifications of solar inverters.
2.2.1. Micro-Inverter, String Inverter, and Centralized Inverter
Currently, there are generally three types of solar array configurations[4][5], AC module technology, string technology, and Centralized technology, as shown in Figure 5.
Accordingly, the inverters employed in different configurations are called micro inverter, string inverter, and centralized inverter.
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8
The micro-inverter is usually connected between a solar panel with the grid, which can also be called an AC module. It handles the power of hundreds of watts with efficiency around 95-96%[6][7]. The string inverter has a number of solar panels cascaded at DC input to increase the DC voltage. Therefore, single-stage inverters may be enough for grid connection. They usually handle kW or tens of kW power conversion. For centralized inverters, the input is connected with multi solar strings in parallel. It usually adopts single- stage three-phase configuration, which is able to deal with several MW solar power conversion. The efficiency can reach around 97-
DC DC AC AC
DC AC
.
.
.
.
.
.
DC AC
(a) (b) DC AC ......
. .
. .
. . ...
(c) Figure 5. Inverter system based on power (a) Micro-inverter system (b) String inverter
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system (c) Centralized inverter system.
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2.2.2. Single-Phase and Three-Phase Inverter
PV systems can also be defined by phases. The most commonly used systems are single phase and three-phase. Single phase systems need fewer switches and more simplified control schemes. However, the natural output fluctuation power requires the single phase system to contain a large electrolytic capacitor which will lower the life span and increase the size of the system. The voltage ripple will still be considerably large. Thus, single phase single-stage MPPT control is hard to realize[8]. Usually another stage is added before the
DC-AC converter to stabilize the output voltage of the solar panel to achieve more accurate
MPPT. For the three-phase inverter, can handle much larger power than a single phase, which make it widely used in middle and high power applications. Another feature is that its output power is constant at balanced condition. That means it does not need a larger capacitor at the DC side. So three-phase single-stage system with MPPT function is achievable and has been widely used.
Grid
DC AC
Figure 6. Single-phase PV system
Grid
DC AC
Figure 7. Three-phase PV system.
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2.2.3. High Frequency Isolation and Low Frequency Isolation Inverter
The PV inverter system has a different type of isolation. It should be considered in a different power rate. According to isolation, PV systems can be defined as a high frequency transformer isolated (HFT) system, a low frequency transformer isolated (LFT) system, and no isolated system. If the system should be connected to the grid, isolation is generally required for safety consideration. HFT systems usually use the high frequency transformer which is working in switching frequency to realize isolation. This kind of isolation is popular in low power AC module technology. LFT isolation mainly refers to inserting a large line frequency transformer between inverter and grid to realize isolation. This kind of isolation is easy and suitable in middle and high power applications. However, it’s bulky and costly.
DC AC AC AC
HFT
Figure 8. HFI system.
DC AC
LFT
Figure 9. LFT system.
2.2.4. One-Stage and Multi-Stage Inverters
According to the topologies popping up to be used in solar energy conversion, PV
system can be classified as one stage, two stage and multistage system. Different stage 10
11 systems have their advantages and limitations. For example, one stage could achieve high device efficiency for its simplicity of components. However, the high DC voltage requirement make it inevitable to connect bunches of PV panels in series, which results in harder realization for each panel to work at its own maximum power point. Thus, two stage and multistage topology appear with the ability to boost the voltage of single panel to a required DC voltage. And the multistage topology may achieve the isolation function at the same time. However, the major problem is the complex of circuit lowers the circuit efficiency.
DC AC
Figure 10. Single-stage system
DC DC DC AC
Figure 11. Two-stage system
DC AC DC AC DC AC
Figure 12. Multistage system
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2.3 Commercialized Products of Solar Inverters
The application of solar inverters in the areas of residential, commercial, and utility scale are already in mass production. Products are seen in many solar inverter supply companies Table 2 shows the solar inverters provided by four global top companies ranging in different power rate and application areas. Key parameters for the inverters are shown.
From the table, it can be seen that micro-inverters were recently commercialized though some companies still have not released their products despite micro-inverters being proposed several years ago. The efficiency problem prevents mass production. The latest technology development and market demand seems to show a promising future for micro-inverters.
Some inverter suppliers are making an effort to promote their corresponding projects.
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Table 2. Typical Solar Inverters in Industrial Area (Source: [7])
MPP Lif Company Rate Nominal AC Model Volta Max.effiency/ e (Time,2012 Powe AC Frequen Topology Name ge ECE efficiency Spa .7) r Voltage cy range n Sunny Boy 240 23V- 59.3- SMA Solar 240V HF transformer 95.5%/95% / Technolog 240- W 32V 60.5Hz y 96.9/95.5%, Sunny Boy 250V- 208,240,27 59.3- (Germany, 6kW LF transformer 96.8/95.5%, / 6000-US 480V 0V 60.5Hz ranked 97/96% largest Sunny 500k 330V- 59.3- External medium- solar Central 200V 98.6/98% / W 600V 60.5Hz voltage transformer inverter) 500HE-US Galvanically isolationg high Blueplanet 1.5k 125V- 59.3- frequency DC/DC 95.9/95.5%, 1502x(small 240,208V / W 400V 60.5Hz converter with 95.5/95% est power) downstream self- commutated inverter KACO 320V- Ungrounded, (Germany) Blueplanet 6.4k 550V, 59.3- 97.2/96.5%, 240,208V Transformerless / 6400M W 290V- 60.5Hz 96.9/96.5% inverter 550V Blueplanet Negative Ground XP100U- 100k 300V- 59.3- (optional positive 96.5/96%, 240,208V / H2 (largest W 600V 60.5Hz ground 96.5/95.5% power) configuration) MICRO- 250 20V- 59.3- Power- 0.25-I- 240,208V HF Transformer 96/95% / One (USA, W 50V 60.5Hz OUTD-US ranked PVI-6000- 200V- 240,208V, 59.3- Transformerless(Flo 97.1/96%, second 6kW / OUTD-US 530V 270V 60.5Hz ating Array) 97.1/96.5% largest 47- solar PVI-500- 500k 475V- 53Hz, inverter 320V / 98.50% / TL-CN W 900V 57- company) 63Hz 47- SG1KSTL 1.5k 200V- 52Hz, (smallest 230V / 95% 5y W 400V 57- power) 62Hz Yang Guang 45- 125V- 55Hz, Dian Yuan SG5KTL-M 5kW 230V / 97.60% 5y (China) 500V 55- 65Hz 47- 500k 450V- 52Hz, SG500KTL 270V / 98.70% / W 820V 57- 62Hz
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2.4 Standards for Solar Inverters
There are a number of standards developed for governing the operation of solar inverters to ensure the safety of people. The brief summarization is provided in Table 3.
Island is defined as an condition in which a portion of an area Electrical Power System (EPS) is energized solely by one or more local EPSs through the associated Point of Common
Coupling (PCCs) while that portion of the area EPS is electrically separated from the rest of the area EPS. Since island would cause significant hazards to personnel and equipment tied with the island, anti-islanding is desirable to prevent the continued existence of island[9].
According to the IEEE 1547 requirement[10], the distributed resources should detect the unintentional island and cease to energize the area EPS within two seconds of the formation of the island. The criteria is posted as Table 4 and Table 5.
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Table 3. Basic Requirements of the Solar System
Low voltage Reactive Islanding Grounding Standards Focus ride- capability capability issue through capability Design, Operation, and IEEE Std Integration of Distributed No No Yes / 1547/UL 1741 Resource Island Systems with Electric Power Systems NEC/article Grounding in 690 the (National Solar photovoltaic system / / / low/medium Electrical voltage system Code) ground rules for practical safeguarding of persons NECS during the installation, Grounding on (National operation, or maintenance of / / / the medium- Electrical electric supply and voltage system Safety Code) communication lines and associated equipment Sizing in stand-alone IEEE Std 1562 / / / / photovoltaic system The GC is a technical document containing the rules German that govern the operation, Yes Yes No / GC(Grid Code) maintenance & development of the system at the Point of Common Coupling (PCC).
Table 4. Interconnection System Response to Abnormal Voltages (Source:[10])
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Table 5. Interconnection System Response to Abnormal Frequency’s (Source:[10])
Low Voltage Ride Through (LVRT) is also described as Fault Ride Through (FRT) which requires that generating units continue to be connected to the EPS when there is a voltage dip at the grid for the purpose of enhancing the stability of the power system. The different degrees in voltage dip generally require different enduring time for the distributed resource[11]. Some standards not only demand that distributed resources continue to provide active power during a voltage dip, but also require them to inject certain reactive power to the grid when the voltage variation exceeds a certain value to mitigate the voltage fluctuation.
Low Voltage Ride Through requirements seem to contradict anti-islanding capbility.
However, LVRT capability is designed for generating Units, power park modules, or DC converters in large power stations such as wind plants or solar plants which have considerable high power rate compared to the the grid. But anti-islanding functions should be equiped by low power distributed generators for safty purposes in some residential applications.
Grid code[12] proposed by National Grid in Great Britain regulates the fault ride
through sepcifications as Figure 13 and Figure 14 show. Figure 13 gives the reqirements for 16
17 the condition that voltage dips caused by onshore transmission systems on the low voltage side of offshore platforms last up to 140ms in duration. Figure 14 gives the requirments for the condition that voltage dips occur on the low voltage side of offshore platforms have durations greater than 140ms and up to 3 minites. These requirements are designed for offshore generating units, offshore power park modules which should withstand the voltage dip for the corresponding time given in graphs.
Figure 13. Voltage dip up to 140ms condition (Source:[12])
Figure 14. Voltage dip more than 140ms to 3 minutes condition (Source:[12])
It is obvious that Low Voltage Ride Through requirements are mainly designed for offshore wind power plant application. In Gemany, another grid code proposed by E.ON
Nets GmH (ENE) gives an even more strict requriment for LVRT function, shown as Figure 17
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15 and Figure 16. This requriment is designed for the type two generating plant[13]. Type two generating plant refers to those plants which do not contain any synchronous generators.
Figure 15. Voltage dip withstanding requirements (Source:[13])
Figure 16. Reactive current injection requirements (Source:[13])
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National Electrical Code (ENC) article 690 has defined two categories of PV systems[14]: Ungrounded Photovoltaic Power Systems and Grounded Photovoltaic Power
Systems.
2.4.1. Ungrounded PV System
PV system could be ungrounded if the following requirements are met:
1. It has met the usual rules of overcurrent protection and disconnecting means.
2. PV arrays have GFP (Ground-Fault Protection) for fire protection and has nothing
to do with shock protection.
3. Wiring of PV source circuits must be in nonmetallic, jacketed cables or in
raceways.
4. Boxes and combiners should be labeled to prevent the ungrounded circuit from
being energized.
5. Inverters and charge controllers must be specifically listed for ungrounded
applications.
2.4.2. Grounded PV System
Although there are some special cases which could run without grounding, the grounding portion of a PV system is still of great importance in most applications. Proper grounding could protect personnel from unintentional shock and possible death. It also could minimize the fire possibility in the system which may leads to a disaster. A grounded PV system should have both equipment grounding and system grounding[15].
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CHAPTER 3
SOLAR STRING DESIGN AND MPPT TRACKING TECHNOLOGY
3.1 Introduction of Solar Cell
3.1.1. Solar Cell Materials
Materials used to manufacture solar panels become the bottle neck for mass usage of solar photovoltaic energy. The basic mechanism in a solar cell relies on the photoelectric effect and semiconductor physics[16].
According to the materials, solar cells can be classified into silicon, poly-compound, organic polymers, and nano-scale materials. Silicon solar cell refers to mono-crystalline silicon cell, polycrystalline silicon cell and amorphous silicon cell. Currently, more than 80% of solar cells are mono-crystalline solar cells, which has achieved the efficiency of 15% at the commercial level. The experimental results in the lab show that the efficiency can reach
24%. Polycrystalline silicon and amorphous silicon are materials for thin film solar cells. The polycrystalline silicon costs much less than mono-crystalline silicon and saves silicon materials, but the efficiency is lower. It is suitable to be used in airspace applications right now. Amorphous silicon cells cost even less, but it has lower efficiency and stability problems.
Poly-compound solar cells are also film polycrystalline solar cell[17][18]. There are different kinds of poly-compound materials like Copper-Indium-Selenide (CIS), Copper-
Indium-Gallium-Selenide (CIGS), Cadmium Telluride (GdTe), Cadmium Selenide (GdS)
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21 and gallium arsenide(GaAs). CIS and CIGS solar cell have similar efficiency with polycrystalline silicon and low cost. They have potential for further development. GdTe, GdS polycrystalline solar cells have higher efficiency than amorphous silicon and low cost, but will pollute the environment as they contain Ge. GaAs solar cells can reach an efficiency of
28%. However, the price is comparatively high.
Organic polymers solar cells[19], [20], are under development. Recent research shows that it can reach the efficiency of about 5%. It has the potential to be a low cost, lightweight, and flexible solar material in the future. Nano-scale materials solar cells are investigated to have higher efficiency and lower cost. The light spectrum of absorption is considered to be changed based the nano-crystal construction. Higher efficiency is desirable if solar cells could absorb more light waves.
Some attention has also moved from materials to the physical configuration of the solar panels. Concentrated solar cell technology focuses higher light intensity on the solar cells by the use of mirror and lens systems. This system tracks the sun, always using direct radiation. The solar cell is made by high efficiency and costly materials like GaAs. A comparison table of solar materials is shown as Table 6 [21].
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Table 6. Comparison for Solar Materials (Source:[21])
Max cell Max cell Typical module Surface require Cell material efficiency efficiency efficiency for 1kW (lab) (mass product) Monocrystalline silicon 24.70% 22.00% 15% 6.7m2 Polycrystalline silicon 20.30% 17.40% 14% 7.2 Amorphous silicon 12.10% 6.80% 6% 16.7 CIS/CIGS 20.00% 11.60% 10% 10 CdTe 16.50% 12% 7% 14.3
Concentrator cells 41.10% 36.50% 28% 3.6
3.1.2. Solar Cell Model
The solar cell has the equivalent circuit shown as Figure 17[22]. According to the solar module given PSIM software[23], the comprehensive output of V-I characteristics is depicted as equation (0.1). The cell photocurrent is depicted as (0.2). The diode current is presented as (0.3).
R Id s I I ph R sh Vd V
Figure 17. Equivalent Circuit of a Solar Cell
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VRI s IIIph d (0.1) Rsh
S IICTTph sc0 t() ref (0.2) S0
q(VR s I ) AkT Id Io ( e 1) (0.3)
Where:
qE 11 g () T AkT Tref T Io I so () e (0.4) Tref
TTKSas (0.5)
In PSIM, the solar module has two user-defined terminal inputs, the node with letter “S” refers to the light intensity while the node with the letter “T” refers to the ambient temperature input.
S
T
Figure 18. Solar moduel in PSIM
The detail definition of model parameters and typical values are given as Table 7.
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Table 7. Parameter Definition and Typical Values
Parameters Definitions Typical Values Units S light intensity input User defined W/m2 k Boltzmann constant 1.3806505x 10-23 / q electron charge 1.6 x 10-19 C
Ta Ambient temperature input User defined C
Tref Reference temperature 25 C Coefficient that defines how the light intensity K 0 s affects the solar cell temperature
Ct Temperature coefficient 0.0024 AC/ Ideality factor A of each solar cell, also called A emission coefficient. It is around 2 for crystalline 1.2 / silicon, and is less than 2 for amorphous silicon Band energy of each solar cell, It is around 1.12 for Eg crystalline silicon, and around 1.75 for amorphous 1.12 eV silicon. Diode saturation current of each solar cell at the I 2.16e-8 A s0 reference temperature Short circuit current of each solar cell at the I 3.8 A sc0 reference temperature
Rsc The shunt resistance of each solar cell 1000
Rs Series resistance of each solar cell 0.008 Light intensity under the standard test conditions, 2 S0 The value is normally 1000 W/m2 in manufacturer 1000 W/m datasheet
According to this model, the output characteristics could vary with the ambient temperature and light intensity change. If standard light intensity 1000W/m2 and ambient temperature 25
C are adopted, the output power and current curves can be derived as a function of terminal voltage, as shown in Figure 19. As seen in the model, with typical parameter values used, a solar cell has approximately 0.6V open circuit voltage and 3.8A short circuit current. The
Maximum Power Point (MPP) voltage is around 0.48V and the MPP power is around 1.68W.
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25
Isolar Psolar
4
2
0
0.1 0.2 0.3 0.4 0.5 Vsolar Figure 19. I-V and PV curve of a single solar cell under reference condition S=1000, T=25.
However, when the light intensity is changed from 1000W/m2 to 2000W/m2 with the
temperature maintained, a new graph is derived as Figure 20(a). When the temperature is
shifted from 25 C to 50C with light intensity unchanged, a new IV and PV graph is given
as Figure 20(b).
Isolar Psolar Isolar Psolar
8 4
6
2 4
2 0 0
0.2 0.4 0.6 0 0.2 0.4 Vsolar Vsolar
(a) (b)
Figure 20. I-V and P-V curve of a single solar cell (a) S=2000, T=25 (b)S=1000, T=50.
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Based on the solar cell test, the effect of light intensity and temperature on solar cells becomes clear, as depicted in Figure 21. The light intensity increase can increase the solar cell output power and slightly increase its open circuit voltage. Nevertheless, the increase of temperature will reduce the open circuit voltage of the solar cell and decrease the output power generated by the solar cell.
S P
P MPP T
O VMPP V
Figure 21. P-V curve of a solar cell under effect of environment.
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3.2 Solar String Model and Design
In this thesis, the string inverter configuration as shown in Figure 5(b) is adopted for circuit design. Therefore, a solar string with a certain amount of solar panels in cascade is required to provide a sufficient high DC voltage for the inverter to connect with the power grid.
A solar panel is composed of a certain number of solar cells. Its equivalent circuit is shown as Figure 22(a). A solar panel usually has terminal voltage around 20-40V.
Therefore, in order to build up more than 280V, a number of solar panels are required to build a solar string, with an equipped reversed diode for each panel. The final equivalent circuit is shown as Figure 22(b).
D1 Rs D1 Rs Rsh Iph1 Iph1 Rsh
Rs Rs
Rs D2 Rs D2 Iph2 Rsh Iph2 Rsh
Rs D3 Rsh Rs Iph3 D3 Rsh Iph3 ...... (a) (b)
Figure 22. Equivalent circuit of (a)Solar Panel (b)Solar String.
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For simplification purposes, the solar string can be regarded as a large amount of solar cells connected in series under the same environmental condition. Assuming the total solar cell number is n, the solar string will have same equivalent circuit model as Figure 17.. The circuit analysis equations are the same as (0.1)-(0.5) except for equation (0.3). Since the number of solar cell has become n, the equation (0.3) should be modified as following:
q(VR s I ) nAkT Ido I( e 1) (0.6)
Therefore, the MPP voltage of a solar string can be approximated as n time of the MPP voltage of a single cell.
In order to realize 1kW solar system with DC voltage about 280V-480V, a 50W solar panel KM50 made by Komaes Solar is selected, as shown in Figure 23. A total number of 20 solar panels is required. For each solar panel, the key parameters are provided in Table 8.
Figure 23. KM50(6) made by Komaes
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Table 8. Model Parameters of KM50(6)
Model No. KM50(6)
Peak Power Pmax 50W
Maximum Power Current IMPP 2.84A
Maximum Power Voltage VMPP 17.74V
Short-Circuit Voltage ISC 3.04A
Open-circuit voltage VOC 21.56V Weight 4.5kG Dimension 640*550*35mm
3.3 Maximum Power Point Tracking Technology
According to Figure 19, solar string has a maximum output power point under certain terminal voltage conditions. The goal of MPPT technology is to ensure that solar string terminal voltage is controlled at MPPT voltage by an external power electronics circuit.
In the past, various MPPT methods were developed by researchers[24], [25], as listed in Table 9. More methods can be found in papers mainly based on modification or a combination of listed methods. For example, variable step control could be applied to the
P&O method to minimize the power oscillation at the maximum power point. Fuzzy logic control can be combined with neural network theory to create an even better controller.
Regardless of the complexity of these methods, all MPPT methods have the same final goals:
(1) Being fastest to reach the MPP (2) Ultra accurate and stable at MPP (3) Fast response to changes of environmental conditions.
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Table 9. MPPT Methods(Source:[24])
Classification MPPT Simplicity Accuracy Principle
Incremental Good Excellent dP /dV = 0 Conductance According to at MPPT Perturbation & dI /dV = -I /V Observation (P&O) or Good Good According to at Classical MPPT control hill climbing Three-point weight Compare present pointe with the past Excellent Fair comparison two pointe V =V + K(T -T ) Temperature method Good Fair According to oc oc_st st Control Vmpp According to Constant voltage Excellent Poor datasheet Simple control Open circuit voltage Excellent Poor Control Vmpp equals to 80% of Voc Open circuit current Excellent Poor Control Impp equals to k*Isc According to fuzzy rule table and Fuzzy logic control Poor Excellent 6 6 D = d åWi fi(dn ) åWi i=1 i=1 Advanced Neural network with several layers Neural network control Poor Excellent control filled by trained neurons Sliding surface is created to fulfill Sliding mode control Poor Excellent the control target
Currently, Incremental Conductance and P&O methods are widely adopted with some moderation in most applications. The reason is that the algorism is not complicated and maintains high reliability and stability. In order to make the algorism more adaptive and intelligent, people do research in fuzzy logic control, neural network control[24] and sliding mode control[26]. However the methods are still not widely used due to complexity.
In this thesis, the simple P&O method is adopted for simulation. It works by giving a disturbance to the terminal voltage of the solar module to observe the changes in output power, as shown in Figure 24.
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P
Pk P(k-1) P(k-1) Pk
V(k-1) Vk Vmpp Voc V V(k-1) Vk
Figure 24. Principle of P&O Method.
By measuring the voltage and current of solar modules, the output power Pk can be calculated in every step. The calculated power at step k is compared to the previous step (k-1). If it is increased, then the disturbance direction of terminal voltage will continue. If the output power is reduced, then the direction of the disturbance should be reversed. The flowchart of
P&O MPPT method for single-stage three-phase inverter designs is presented in Figure 24.
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Start
Measure Vk，Ik
Calculate Pk
Yes Pk= （P k-1）？
No
Yes No Pk > P（k-1）？
Yes No Yes No Vk> V（k-1）？ Vk>V（k-1）？
VREF=VREF+ΔV VREF=VREF -ΔV VREF=VREF -ΔV VREF=VREF+ΔV
（P k-1）= Pk
Figure 25. The flowchart of P&O MPPT method for single-stage three-phase inverter.
According to the system block presented in Figure 1, MPPT algorism will provide a dynamic voltage reference Vref for inverter control loop to force the terminal voltage of the solar string to operate at maximum power voltage.
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CHAPTER 4
HARDWARE DESIGN OF 1KW THREE-PHASE SINGLE-STAGE SOLAR INVERTER
In this chapter, the hardware design and component selection for 1kW three-phase single-stage solar inverter are implemented. The key components of design criteria are given and corresponding commercialized products are suggested. The basic components are DC capacitor, switch, LC filter, and some voltage and current sensors, as shown in Figure 26.
Q1 Q3 Q5 L Grid Cdc
Q2 Q4 Q6 C
Sensor 3 Sensor 4 Sensor 1 Sensor 2
Figure 26. Three-phase inverter hardware.
4.1 DC Voltage Selection
The DC voltage should be maintained high enough to generate appropriate AC voltage to interface with the power grid. According to the circuit operation condition in
Figure 26, the minimum voltage of Vdc can be derived when the grid line to line voltage Vab reaches its peak.
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Q1 Vab ia Grid VLa VaN Cd N VbN N c VLb ib Q4
N Figure 27. Minimum DC voltage selection condition
VbN Vdc VLb VLb ib VLa VLa Vab ia VaN
Figure 28. Vector diagram for DC votlage selection
In order to understand the filter inductor effect on DC voltage selection, a vector diagram is provided in Figure 28. The phase voltage VaN and VbN has 120 degree phase difference while the phase current ia and ib is in phase with each phase voltage. Therefore, the voltage drop on inductor VLa and VLb can be derived respectively, which is 90 degrees ahead of each phase current. Based on the voltage vectors of inductors derived and the voltage vector of Vab, the minimum voltage of Vdc can be derived, which is the amplitude of
Vab.
The three-phase grid system has an effective phase voltage of 110V. Therefore, the minimum DC voltage can be calculated as:
VV110 2 3 269.4( ) dc(min) (0.7)
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However, considering the power grid fluctuation and the dead time effect of PWM modulation, 130% of minimum DC voltage is suggested for DC voltage selection. With regard to the proper solar string designed in previous chapter, the final selected DC voltage is
350V at rated power for inverter design and simulation purpose.
4.2 Selection of Switch
Gallium Nitride (GaN) Switches provide significant advantages over Silicon (Si)
Super Junction MOSFETs with lower gate charge, faster switching speeds, and smaller reverse recovery charge. According to the data sheet provided by Transform, GaN Switches can exhibit in-circuit switching speeds in excess of 150 V/ns and can be even pushed up to
500V/ns, compared to current silicon technology usually switching at rates less than 50V/ns.
Due to the fast switching capability of GaN, it can reduce current-voltage cross-over area leading to higher efficiency compared with traditional power switches.
For our 1kW three-phase inverter system design, Gallium Nitride switches can be used to achieve high efficiency under 30kHz switching frequency condition.
According to the MPPT the voltage range is 280V-580V. The minimum voltage rating of power switch can be selected as:
(1 3%)VVdc(max) 1.03 580 597.4( ) (0.8)
Flow each power switch, the peak current can be calculated as:
2Pm 2 1000 IAm 4.3( ) (0.9) 3U ph 3 110
Where Pm is the power rating of the inverter, Vph is the effective phase voltage.
Finally, we select the GaN power switch TPH3206PD as final design[27], as shown in Figure
29. The key performance characteristics of this power switch are given in Table 10. 35
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Figure 29. TPH3206PD from transphorm.
Table 10. Performance of TPH3206PD
Parameter Symbol KM50(6) Maximum Drain-Source Voltage VV() 600 DS
Drain-Source On-Resistance (TJ = 25 °C) RDS() on () 0.15
Reverse Recovery Charge Q() nC 54 rr
Continuous Drain Current @TC=100 °C IA() 12 DC100 Gate Threshold Voltage VA() 2.1 GS() th Input Capacitance C() pF 760 iss Rise Time t() ns 4.5 r Fall Time t() ns 4 f
4.3 Design of Output Filter
In a three-phase inverter, the output for each phase can be regarded as a circuit configuration shown in Figure 30 to facilitate inductor ripple calculation. The inductor current continuous conduction loop is not drawn.
Q1 L Vdc VaN 2 C Grid N
Figure 30. Output filter design. 36
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Assuming the top switch of each bridge has the duty cycle of d, and the corresponding phase voltage isVm sin( wt ) , the inductor current ripple can be calculated as:
V dT i(dc V sin wt ) s (0.10) Lm2 L
Because the LC filter serves the function of deriving the average voltage, it can be approximated that:
V Vsin wt dc d (0.11) m 2
Combining (0.10) and (0.11), it can be derived that:
Vdc 1 iL d(1 d ) (0.12) 2 Lfs
Therefore:
Vdc 1 iL(max) (0.13) 8 Lfs
We choose iL(max) 10% I L peak 10% I m , thus:
V 1 350 1 Ldc 3.4( mH ) (0.14) 8fIsm 10% 8 30000*0.1*4.3
In order to give enough attention of the ripple component of the switching frequency, the cutoff frequency of the LC filter is designed as 1/10 of the switching frequency[28], thus:
11 f *30 kHz 3 kHz (0.15) res 2