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Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System

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Seminar report on transformerless PV system using NPC inverter.

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Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System

  1. 1. TRANSFORMER LESS FPGA CONTROLLED 2-STAGE ISOLATED GRID CONNECTED PV SYSTEM A seminar done in partial fulfillment of the requirements for the award of MASTER OF TECHNOLOGY IN POWER SYSTEM AND CONTROL - ELECTRICAL ENGINEERING OF UNIVERSITY OF KERALA BY ANOOP S (14401005) DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING GOVERNMENT ENGINEERING COLLEGE BARTON HILL
  2. 2. GOVERNMENT ENGINEERING COLLEGE, BARTON HILL, THIRUVANANTHAPURAM-695035 DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING 2014-2016 CERTIFICATE This is to certify that the report entitled “Transformer Less FPGA Controlled 2- Stage Isolated Grid Connected PV System” is a bonafide record of seminar presented by ANOOP S (14401005) towards partial fulfillment of the requirements for the award of the Master of Technology in Power System and Control – Electrical Engineering of the University of Kerala. Dr.K N Pavithran Adjunct Professor in EEE Prof. K L Sreekumar Assistant Professor in EEE Prof.Sheela S Professor & Head of Dept. EEE
  3. 3. ACKNOWLEDGMENT First of all I thank the Almighty God for the blessings showered upon me during the presentation. I would like to express my sincere gratitude to Prof. Sheela S, Professor and Head of the Dept. Electrical and Electronics Engineering for her continuous support in accomplishing the presentation. I express my heartfelt thanks to Professors K L Seekumar , Dr. K N Pavithran , Dr. J S Savier ,Dept. of EEE for their valuable suggestions ,advice and guidance throughout the preparation of seminar . Last but not the least, I place on record my gratefulness to my parents, friends and classmates for their suggestions, criticisms and assistance towards the improvement and successful completion of the report. Anoop S
  4. 4. ABSTRACT The grid-connected photovoltaic systems are an important part of renewable energy sources and their integration is getting more and more widespread. In order to improve the efficiency, practicality and reliability of the PV systems, many kinds of new inverter topologies have been proposed to avoid using a grid isolation transformer. The neutral point clamped inverter topology is discussed. In this topology no common mode voltage is generated, thus changes in the behavior of the inverter in terms of high efficiency and ensures that no DC will be injected into the load. Constant voltage MPPT charge controller is designed based on small signal analysis of converter. After this charge controller output is fed to multilevel inverter for the conversion of dc to ac. Proposed neutral point clamped inverter is offering very low line voltage THDs compared with conventional inverter; offering less size and cost of the filter.
  5. 5. CONTENTS 1. INTRODUCTION....................................................................................................1 2. PROPOSED PV SYSTEM TOPOLOGY ................................................................2 3. PV MODULE...........................................................................................................3 4. BOOST CONVERTER BASED MPPT CHARGE CONTROLLER .....................5 4.1 MODES OF OPERATION....................................................................................5 4.1.1 Charging Mode................................................................................................5 4.1.2 Discharging Mode...........................................................................................6 4.2 MAXIMUM POWER POINT TRACKING .........................................................6 4.2.1 Methods for MPPT..........................................................................................6 5. NEUTRAL POINT CLAMPED INVERTER..........................................................7 5.1 TOPOLOGY..........................................................................................................8 5.2 OPERATION.........................................................................................................9 5.3 FIRING SCHEME...............................................................................................11 6. CONCLUSION ......................................................................................................12 7. REFERENCES.......................................................................................................13
  6. 6. LIST OF FIGURES Figure 1: Block diagram of proposed system 2 Figure 2: Equivalent circuit of solar cell 4 Figure 3: P-V I-V curve of a solar cell at given temperature and solar irradiation 4 Figure 4: Boost converter 5 Figure 5: Topology of neutral point clamped inverter 8 Figure 6: Line voltage waveforms 10 Figure 7: Gate pulses generated from FPGA 11
  7. 7. Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System DEPT. OF EEE Page 1 SEMINAR REPORT 1. INTRODUCTION A grid-connected photovoltaic power system or grid-connected PV system is electricity generating solar PV system that is connected to the utility grid. A grid-connected PV system consists of solar panels, one or several inverters, a power conditioning unit and grid connection equipment. They range from small residential and commercial rooftop systems to large utility-scale solar power stations. Unlike off-grid systems, a grid- connected system rarely includes an integrated battery solution, as they are still very expensive. When conditions are right, the grid-connected PV system supplies the excess power, beyond consumption by the connected load, to the utility grid. Two main topologies have been stated in the photovoltaic system i.e. with and without the galvanic isolation. The main aim of the galvanic isolation is to offer safety for the user, but this decreases the overall efficiency of the system. In the case of the transformer less system the efficiency of the system raises up. The most important advantage of the Transformer less system is that it offers higher efficiency, smaller in size and petite in weight as compared to system with transformer. PV inverter, which is the heart of a PV system, is used to convert dc power obtained from PV modules into ac power to be fed into the grid. Improving the output waveform of the inverter reduces its respective harmonic content and, hence, the size of the filter used and the level of Electromagnetic Interference (EMI) generated by switching operation of the inverter. In recent years, multilevel inverters mainly neutral point clamped inverter is being used. They offer improved output waveforms, smaller filter size and lower EMI, lower Total Harmonic Distortion (THD). The use of multi-level inverters also eliminates the use of grid isolation transformers that are usually used for providing personal protection and avoiding leakage currents between the PV system and the ground.
  8. 8. Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System DEPT. OF EEE Page 2 SEMINAR REPORT 2. PROPOSED PV SYSTEM TOPOLOGY Figure shows the block diagram of the proposed transformer less system which consists of two stages. In this system a solar array has been constructed using solar cells by combining it into series and parallel combinations. The output of solar cell is variable so we have deployed a DC-DC converter which converts variable DC into fixed DC, this is done in first stage. In second stage DC is converted into AC which will be utilized by the appliances. Figure 1: Block diagram of proposed system
  9. 9. Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System DEPT. OF EEE Page 3 SEMINAR REPORT 3. PV MODULE The basic building block of a photovoltaic module is the photovoltaic cell; these convert solar energy into electricity. The power output will depend on the amount of energy incident on the surface of the cell and the operating temperature of the photovoltaic cell. The power output of a single cell can supply small loads like calculators or watches, but in order to be useful for high energy demand projects these cells must be arranged in series and parallel connections. A photovoltaic module is an array of photovoltaic cells pre-arranged in a single mounting mold. The type of module is therefore determined by the cells that compose the module itself. There are three dominating cell technologies:  Monocrystalline: As the name implies, these are cells that are grown from a single crystal. The production methods are difficult and expensive. These tend to be more efficient (more power in less area) and more expensive.  Multicrystalline: The production process allows multiple crystalline structures to develop within the cell. It is easier to implement in a production line. It is relatively cheaper than monocrystalline at the expense of lower efficiency.  Thin-film: Uses less silicon to develop the cell allowing for cheaper production costs. It tends to be less expensive but also lower efficiency An ideal solar cell may be modelled by a current source in parallel with a diode; in practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model.
  10. 10. Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System DEPT. OF EEE Page 4 SEMINAR REPORT Figure 2: Equivalent circuit of solar cell Figure 3: P-V I-V curve of a solar cell at given temperature and solar irradiation
  11. 11. Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System DEPT. OF EEE Page 5 SEMINAR REPORT 4. BOOST CONVERTER BASED MPPT CHARGE CONTROLLER DC-DC Boost converter is used to magnify the voltage from PV to a suitable form of energy accepted by the load. Boost converter is a second order system consists of an inductor, a capacitor, a diode, and with the load resistance connected in parallel with the capacitor. As the output from PV is not constant due to the ambient temperature and environmental condition, the modeling of such converter is crucial. Figure 4: Boost converter 4.1 MODES OF OPERATION There are two modes of operation of a boost converter. Those are based on the closing and opening of the switch. The first mode is when the switch is closed; this is known as the charging mode of operation. The second mode is when the switch is open; this is known as the discharging mode of operation. 4.1.1 Charging Mode In this mode of operation; the switch is closed and the inductor is charged by the source through the switch. The charging current is exponential in nature but for simplicity is assumed to be linearly varying. The diode restricts the flow of current from the source to the load and the demand of the load is met by the discharging of the capacitor.
  12. 12. Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System DEPT. OF EEE Page 6 SEMINAR REPORT 4.1.2 Discharging Mode In this mode of operation; the switch is open and the diode is forward biased . The inductor now discharges and together with the source charges the capacitor and meets the load demands. The load current variation is very small and in many cases is assumed constant throughout the operation. 4.2 MAXIMUM POWER POINT TRACKING The efficiency of a solar cell is very low. In order to increase the efficiency, methods are to be undertaken to match the source and load properly. One such method is the Maximum Power Point Tracking (MPPT). This is a technique used to obtain the maximum possible power from a varying source. In photovoltaic systems the I-V curve is non-linear, thereby making it difficult to be used to power a certain load. This is done by utilizing a boost converter whose duty cycle is varied by using a mppt algorithm. Few of the many algorithms are listed below. 4.2.1 Methods for MPPT There are many methods used for maximum power point tracking a few are listed below: • Perturb and Observe method • Incremental Conductance method • Parasitic Capacitance method • Constant Voltage method • Constant Current method
  13. 13. Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System DEPT. OF EEE Page 7 SEMINAR REPORT 5. NEUTRAL POINT CLAMPED INVERTER Inverters are used to transform dc current to ac currents. The voltage and current waveforms produced by inverters are never perfect sinusoids, therefore some harmonic currents are expected during normal operation. Total harmonic distortion (THD) is a measure of the harmonic content in current and voltage waveform in order to improve the output voltage waveform and to eliminate the use of isolation transformers multilevel inverter topologies are used. The most commonly used multilevel topology is the diode clamped inverter, in which the diode is used as the clamped device to grip the dc bus voltage so as to attain steps in the output voltage. A neutral point clamped (NPC) inverter system has a DC power and an NPC inverter having a neutral point connected to the positive and negative poles of the DC power source to convert the DC voltage into AC voltage characterized in that first and second branch means having a switching element provided between the positive and negative poles sides of the DC power source and the neutral point of the NPC inverter, and control means for turning the switching elements of the second and first branch on when short-circuit current of the NPC inverter flow through the neutral point of the NPC inverter.
  14. 14. Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System DEPT. OF EEE Page 8 SEMINAR REPORT 5.1 TOPOLOGY Figure 5: Topology of neutral point clamped inverter Figure shows the main circuit of the NPC-PWM inverter. (A1,A4), (B1, B4), (C1, C4) are main transistors operating as switches for PWM; and (A2, A3), (B2, B3), (C2, C3) are auxiliary transistors to clamp the output terminal potentials to the neutral point potential, together with diodes. To this inverter, all conventional PWM techniques can be applied. Auxiliary transistors (A 3, A 2) are driven complementary to the main transistors (A1 , A4), respectively. With such control, each output terminal potential is clamped to the neutral potential in the off-periods of the PWM control. The phase outputs are the center point of a series connection of four IGBTs , and the DC bus input is connected to the top and bottom row of devices, Al, Bl, CI, and A4, B4, C4, R Y B
  15. 15. Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System DEPT. OF EEE Page 9 SEMINAR REPORT respectively. The center point of the DC bus is shown by a ground symbol and is connected between a pair of series connected diodes in each phase. These six clamping diodes connected to the neutral bus control the voltage distribution among the four IGBTs in each phase leg. A conventional inverter requires the switches to sustain the full voltage drop between the positive and negative DC buses. However, the voltage drop (stress) across each switch of the NPCI is one half of the voltage between the positive and negative bus since the switches on either side of the neutral bus are in series, and an actual neutral point exists. Each IGBT has an individual gate signal that must be referenced between the respective IGBT gate and emitter terminal. The diode shown between the collector and emitter of each IGBT is an internal "body diode" inherent to the IGBT device structure. 5.2 OPERATION This specific NPCI topology uses 3-level switching instead of 2-level switching used in conventional 3-phase inverters. The three levels correspond to the positive, negative, and neutral buses. Taking leg A of Figure 4 as an example, the phase output A is connected to the positive bus by turning on switches Al and A2. Turning switches A3 and A4 on connect the phase A output to the negative bus, and turning switches A2 and A3 on connects the phase A output to the neutral bus. The other two phases operate in the same manner, but with phase shifted results with respect to phase A. The phase and line voltages are given by Pole voltages VRO = 0, ± , VYO = 0, ± , VBO = 0, ± Line voltage VRY = 0, ± , ± Vdc Phase voltage VRN = 0, ± , ± , ± , ±
  16. 16. Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System DEPT. OF EEE Page 10 SEMINAR REPORT Figure 6: Line voltage waveforms
  17. 17. Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System DEPT. OF EEE Page 11 SEMINAR REPORT 5.3 FIRING SCHEME The output terminal voltages of the neutral point clamped inverter based on the on off position of the switches of one leg are given below From the table it can be observed that the switches A1 and A3 are complimentary and A2 and A4 are also complimentary. Hence we need to produce only to gating pulses i.e for A1 and A4. Gate signals for A2 and A3 can be obtained by complimenting firing pulses of A1 and A4 respectively Figure 7: Gate pulses generated from FPGA A1 A2 A3 A4 VRO 1 1 0 0 +𝑉𝑑𝑐 2 0 1 1 0 0 0 0 1 1 −𝑉𝑑𝑐 2
  18. 18. Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System DEPT. OF EEE Page 12 SEMINAR REPORT 6. CONCLUSION Transformer less inverters offers a better efficiency, compared to those inverters that have a galvanic isolation. In the proposed topology no common mode voltage is generated, thus changes in the behavior of the inverter in terms of high efficiency and insures that no DC will be injected into the load. After this charge controller output is fed to multi-level inverter for the conversion of dc to ac. The proposed multilevel inverter using neutral point clamped inverter is offering very low line voltage THDs compared with conventional inverter; offered less size and cost of the filter. Leakage current will be eliminated since midpoint of capacitors is connected to ground. It also ensures better waveform quality and harmonic elimination. The power devices and the DC-link capacitors have to stand only one half of the DC-link voltage and hence the switching losses will also be less.
  19. 19. Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System DEPT. OF EEE Page 13 SEMINAR REPORT 7. REFERENCES 1. Dogga Raveendhra, Saad Faruqui and Parvesh Saini,“Transformer Less FPGA Controlled 2-Stage Isolated Grid Connected PV System”, 2014 Power and Energy Systems: Towards Sustainable Energy (PESTSE 2014) 2. Y. Xue, K. e. Divya, G. Griepentrog, M. Liviu, S. Suresh, and M. Manjrekar, "Towards next generation photovoltaic inverters" ,in Froc. iEEE Energy Converso Congr. Expo., 2011, pp. 2467-2474 3. Raveendhra, D.; Pathak, M.K.; Panda, A, "Power conditioning system for solar power applications: Closed loop DC-DC convertor fed FPGA controlled diode clamped multilevel inverter," Electrical, Electronics and Computer Science (SCEECS), 2012 IEEE Students' Conference on, vol., no., pp.l,4, 1-2 March 2012

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