CONTROL STRATEGY OF FLYBACK MICROINVERTER WITH HYBRID MODE FOR PV AC MODULES USING FUZZY LOGIC ACKNOWLEDGMENTS I dedicate this work to my project guide Dr. Xingguo Xiong who supported me to work in the field of my interest and has guided me all along. I would like to thank University of Bridgeport for providing a platform to pursue my Masters’ program and helping me through every step of it and for providing all the infrastructure I have ever needed. I wish to express my gratitude to my parents without whose support my idea of pursuing a Masters’ degree would not have been possible. I would also like to thank my friends and family who have been a strong support system. 1 ABSTRACT A New fuzzy control strategy of a flyback microinverter with hybrid operation mode for photovoltaic (PV) AC modules is proposed in this paper. In this paper, we are using the fuzzy controller. Because, it has many advantages comparing to another controller. The fuzzy controller is the most suitable for the human decision-making mechanism, providing the operation of an electronic system with decisions of experts. The proposed control strategy consists of two components: the proportional-resonant (PR) controller with the harmonic compensator (HC), and the hybrid nominal duty ratio. The PR controller with HC provides higher system gain at the fundamental and harmonic frequencies of the grid without using high proportional gain in both operation modes. In FLC, a set of linguistic rules determines basic control action. The system determines these rules Then, it enhances tracking speed and disturbance rejection performances satisfying the desired stability. Moreover, by applying the hybrid nominal duty ratio yielded from the proposed operation mode selection, the disturbance rejection is achieved more effectively, and the control burden is reduced. By using the fuzzy controller for a nonlinear system allows for a reduction of uncertain effects in the system control and improve the efficiency. By using the simulation results we can verify the tracking speed and disturbance rejection performances of the proposed control strategy. 2 LIST OF TABLES TABLE 5.a. Page FUZZY RULES 9.b PARAMETERS AND COMPONENTS OF THE PROTOTYPE Bookmark not defined. 3 4 Error! LIST OF FIGURES FIGURE Page 2.1.1. Buck Converter .............................................................................................. Error! Bookmark not defined. 2.1.2 Voltage and current changes ......................................................................... Error! Bookmark not defined. 2.2.1 Buck Converter at Boundary ......................................................................... 3 2.3.1 6 Buck Converter - Discontinuous Conduction ............................................... 2.3.2 Output Voltage vs Current............................................................................. Error! Bookmark not defined. 2.4.1 Fig Boost Converter Circuit ........................................................................... Error! Bookmark not defined. 2.4.2 Voltage and current waveforms (Boost Converter) ...................................... 3 2.5.1. schematic for buck-boost converter............................................................... Error! Bookmark not defined. 2.5.2 Waveforms for buck-boost converter ............................................................ Error! Bookmark not defined. 2.6.1 Comparison of Voltage ratio ......................................................................... 3 2.7.1. CUK Converter .............................................................................................. Error! Bookmark not defined. 2.7.2 CUK “ON-STATE” ...................................................................................... 6 2.7.3 CUK “OFF-STATE” ..................................................................................... Error! Bookmark not defined. 2.9.1 Buck-Boost Converter .................................................................................. 3 2.9.2. Replacing inductor by transformer ................................................................ Error! Bookmark not defined. 4 2.9.3 Flyback converter re-configured .................................................................... Error! Bookmark not defined. 2.10.1 Forward converter ......................................................................................... 3 2.10.2 Forward converter with tertiary winding ....................................................... Error! Bookmark not defined. 3.0 Solar farm...................................................................................................... 6 3.1.1 Diagram of the possible components of a photovoltaic system .................... Error! Bookmark not defined. 3.2.1 Schematics of a typical residential PV system ............................................. 3 4.0 Schematic of a flyback converter .................................................................. Error! Bookmark not defined. 4.2.1. Two Configurations of a flyback converter in operation on state and off stateError! Bookmark not defined. 4.2.2 Waveform - using primary side sensing techniques - showing the 'knee point' 6 4.5.1 PID controller ................................................................................................ Error! Bookmark not defined. 4.6.1 Plot of PV vs time, for three values of K (K and K held constant) ............. p i d 3 4.7.1 Plot of PV vs time, for three values of K (K and K held constant) ............ Error! Bookmark not defined. ............................................................................................ d 5.1.1 p i Fuzzy logic controller ................................................................................... 3 5.1.a Input error as membership functions ............................................................ Error! Bookmark not defined. 5.1.b Change as error membership functions......................................................... 3 5.1.c Output variable Membership functions ......................................................... 6 5.3.1 fuzzy description .......................................................................................... 3 5.6.1 The FIS Editor .............................................................................................. Error! Bookmark not defined. 5.6.2 ‘Save to workspace as...’window ................................................................. Error! Bookmark not defined. 5 5.6.3 The updated FIS Editor ................................................................................ 6 6.0 Circuit diagram of the flyback micro-inverter ............................................. Error! Bookmark not defined. 6.1.1 Key waveforms in DCM and CCM .............................................................. 3 6.2.1 Operation regions of the flyback inverter during a half-cycle of the grid voltageError! Bookmark not defined. 7.1.1. Equivalent circuit of the grid-connected flyback microinverter……………..Error! Bookmark not defined. 7.1.2 Bode plots of PI controller (a) In DCM. (b) In CCM………………………… 8.1 6 Open-loop Bode plots of the compensated system by the proposed controller. (a) In DCM. (b) In CCM. 8.1.2 The block diagram of the proposed system ................................................. Error! Bookmark not defined. ............................................................................................ 9.1.1 Block diagram of simulation......................................................................... 3 9.1.2 Simulation results for conventional control system ...................................... Error! Bookmark not defined. 6 9.2.1 Block diagram of proposed system ............................................................... 3 9.2.2 Simulation results for the proposed system .................................................. 6 TABLE OF CONTENTS CHAPTER-1: INTRODUCTION CHAPTER-2: DC-DC CONVERTER 2.1 Buck Converter Step-Down Converter 2.2 Transition Between Continuous and Discontinuous 2.3 Voltage Ratio of Buck Converter (Discontinuous Mode) 2.4 Boost Converter Step-Up Converter 2.5 Buck-Boost Converter 2.6 Converter Comparison 2.7 CUK Converter 2.8 Isolated Dc-Dc Converters 2.9 Flyback Converter 2.10 7 Forward Converter CHAPTER-3: PHOTOVOLTAIC (PV) 3.1 Overview Of PV 3.2 Grid Connection CHAPTER-4: FLYBACK CONVERTER 4.1 Structure and Principle 4.2 Operations 4.3 Controllers 4.4 Concept of PI Controller 4.5 Tuning PI Controller 4.6 Proportional Term 4.7 Integral Term CHAPTER-5: FUZZY LOGIC CONTROLLER 5.1 Fuzzification 5.2 Interference Method 5.3 What Is Fuzzy Logic 5.4 Why Use Fuzzy Logic? 5.5 When not to Use Fuzzy Logic? 5.6. The FIS Editor CHAPTER-6: OPERATION PRINCIPLE 6.1 Steady-State Analysis of DCM And CCM Operations 6.2 Flyback Microinverter With Hybrid Mode CHAPTER-7: OPERATIONAL ANALYSIS (HYBRID MODE) 7.1 Control Issues CHAPTER-8. PROPOSED CONTROL STRATEGY 8 CHAPTER-9. SIMULATION RESULTS 9.1. Case:1 When Both PI and PR Controllers Are Present 9.2. Case:2 When Both Fuzzy and PR Controllers Are Present CHAPTER-10. CONCLUSION CHAPTER-11. REFERENCES 1. INTRODUCTION Nowadays, in renewable energy sources the PV energy has been widely utilized with the increasing demand of power. The PV power systems can be categorized into 3 systems such as, centralized, string, and ac module systems. Among them, the ac module system has many advantages. it can track the individual maximum power point (MPP), and so reduces power losses by PV module mismatch and partial shading. Moreover, the ac module system has higher reliability and easier maintenance than those of other PV systems. The ac module PV system has been used mostly with these advantages, as a trend of the future PV power systems. The worth of the microinverter is evaluated by its power conversion efficiency, the shape of output current, power density, reliability, and cost. Due to its simple circuit structure and potential for high efficiency and reliability, a single-stage flyback inverter topology has been adopted. Moreover, the flyback inverter topology has both step-down and step-up functions; for the PV applications, this characteristic is suitable where the inverter should operate in wide voltage range. 9 Under the constant switching frequency conditions the flyback inverter operates, the operation modes can be classified as the discontinuous current mode (DCM) and continuous current mode (CCM). The PV inverter called as the CCM flyback inverter has both operation modes; it inevitably operates in DCM at the low instantaneous power level or low solar irradiation level although it operates in CCM at all instantaneous power levels for rated average power. Then, it can be regarded that the flyback inverter has hybrid operation mode over whole ac-line period. Compared to a flyback inverter only with DCM, the flyback inverter with hybrid mode has the numerous merits such as higher efficiency with lower current stress, higher power capability, and easier filter design. However, the control input-to-output current transfer function of the flyback inverter in the CCM region has a right half-plane (RHP) zero which results in the limitations of increasing the system gain and controller bandwidth. Since the operating point varies widely in the PV inverter applications, especially, the controller should cover the minimum RHP zero. When a conventional PI controller is applied to the flyback inverter with hybrid mode, the proportional gain is designed to be relatively low for ensuring stability in all operating points. The system gain of the flyback inverter in the DCM region is inherently much low. To achieve fast reference tracking and disturbance rejection performances, the high gain feed-back controller is required in the DCM operation. However, when the conventional PI controller is applied, the control gain is limited by the RHP zero in CCM. As a result, it causes unacceptable power quality and high total harmonic distortion (THD) by the poor control performance in DCM. This is the reason, the use of the flyback inverter with hybrid mode is limited despite its many advantages. To avoid the mentioned problem, some previous studies in control the primary current instead of controlling the output current because there is no RHP zero in the transfer function for the control input to the primary current. The control approach bypasses the difficulties posed by the RHP zero. However, the power quality is low because this approach controls the output current indirectly. The PR controller is an alternative of the PI controller. It provides an infinite gain without using high proportional gain at a selected resonant frequency. Moreover, for compensating the harmonics of the selected fundamental frequency because the controller has 10 flexibility of selecting the resonant frequency, adding multiple PR controllers such as the harmonic compensator is possible The current control strategy of the flyback microinverter with PR and fuzzy controller is proposed in this paper. The proposed control strategy consists of two components: the PR controller with HC and the hybrid nominal duty ratio. At the fundamental and harmonic frequencies of the grid the PR controller with HC provides high gain and achieves the zerotracking error in both operation modes. The proposed operation mode selection the hybrid nominal duty ratio performs as a feedforward control input and is determined. According to proper operation region, it can achieve more effective disturbance rejection and faster dynamics by applying the hybrid nominal duty ratio. By using the simulation results the proposed control strategy gives the higher tracking performance and better disturbance rejection in both operation modes and strengthens the many advantages of the flyback inverter with hybrid mode. 2. DC-DC CONVERTER A DC-to-DC converter is a electrical device it will accepts the input voltage as DC and produces a DC output voltage. At a different voltage levels of input the output will be produced. And, to provide noise isolation, power bus regulation, etc. DC-to-DC converters are used. 2.1 Buck Converter Step-Down Converter In this circuit two modes of operation will be considered. Firstly, the transistor turning ON will put voltage Vin on one end of the inductor. This voltage will tend to cause the inductor current to rise. Secondly, When the transistor is OFF, the current will continue flowing through the inductor but now flowing through the diode. We initially assume that the current through the inductor does not reach zero, thus the voltage at Vx will now be only the voltage across the conducting diode during the full OFF time. The average voltage at Vx will depend on the average ON time of the transistor provided the inductor current is continuous. 11 Fig: 2.1.1 Buck Converter Fig: 2.1.2 Voltage vs Current To analyze the voltages of this circuit let us consider the changes in the inductor current over one cycle. From the relation di Vx − V0 = L dt………………. (1) the change of current satisfies i = ∫ON(Vx − V0 )dt + ∫OFF(Vx − V0 )dt ………. (2) For steady state operation, the current at the start and end of a period T will not change. To get a simple relation between voltages we assume no voltage drop across transistor or diode while ON and a perfect switch change. Thus, during the ON time Vx=Vin and in the OFF Vx=0. Thus t t 0 = di = ∫0 on(Vin − Vo )dt + ∫t on on 12 +toff (−Vo )dt…………………. (3) This simplifies to (Vin − Vo )t on − Vo t off = 0 …………… (4) or Vo Vin = ton T …………… (5) and defining "duty ratio" as D= ton T ………. (6) The voltage relationship becomes Vo=D Vin Since the circuit is lossless and the input and output powers must match on the average Vo* Io = Vin* Iin. Thus, the average input and output current must satisfy Iin =D Io These relations assume that the inductor current does not reach zero. 2.2 Transition between continuous and discontinuous When the current in the inductor L remains always positive then either the transistor T1 or the diode D1 must be conducting. For continuous conduction, the voltage Vx is either Vin or 0. If the inductor current ever goes to zero then the output voltage will not be forced to either of these conditions. At this transition point the current just reaches zero as seen in Figure (buck booster boundary). During the ON time Vin-Vout is across the inductor thus IL (peak) = (Vin − Vout ). tout (7) L The average current which must match the output current satisfies IL (average at transition) = 13 IL (peak) 2 dT = (Vin − Vout ) 2L = Iout (transition) (8) Fig: 2.2.1 Buck Converter at Boundary If the input voltage is constant the output current at the transition point satisfies Iout (transition) = Vin (1 − d)d T 2L (9) 2.3 Voltage Ratio of Buck Converter (Discontinuous Mode) As for the continuous conduction analysis, we use the fact that the integral of voltage across the inductor is zero over a cycle of switching T. The transistor OFF time is now divided into segments of diode conduction ddT and zero conduction doT. The inductor average voltage thus gives (Vin − V0 )DT + (−Vo )id T = 0 14 (10) Fig: 2.3.1 Buck Converter - Discontinuous Conduction Vout d = Vin d + δd for the case . To resolve the value of (11) consider the output current which is half the peak when averaged over the conduction times Iout = IL (Peak) d + δd 2 (12) Considering the change of current during the diode conduction time V0 (δo T) L (13) V0 δ0 T(d + δ0 ) L (14) Vin dδd T 2L (15) IL (Peak) = Thus from (6) and (7) we can get Iout = using the relationship in (5) Iout = and solving for the diode conduction δd = 2LIout Vin dT (16) The output voltage is thus given as V0 d2 = Vin d2 + (2LIout ) Vin T (17) defining k* = 2L/(Vin T), we can see the effect of discontinuous current on the voltage ratio of the converter. 15 Fig: 2.3.2 Output Voltage vs Current As seen in the figure, once the output current is high enough, the voltage ratio depends only on the duty ratio "d". At low currents the discontinuous operation tends to increase the output voltage of the converter towards Vin. 2.4 Boost Converter Step-Up Converter The schematic below shows the basic boost converter. This circuit is used when a higher output voltage than input is required. Fig: 2.4.1 Boost Converter Circuit While the transistor is ON Vx =Vin, and the OFF state the inductor current flows through the diode giving Vx =Vo. For this analysis, it is assumed that the inductor current always remains flowing (continuous conduction). The voltage across the inductor is below and the average must be zero for the average current to remain in steady state Vin t on + (vin − Vo )t off = 0 This can be rearranged as 16 V0 T 1 = = Vin t off 1 − D and for a lossless circuit the power balance ensures I0 = 1−D IIN Fig: 2.4.2 Voltage vs Current (Boost Converter) Since the duty ratio "D" is between 0 and 1 the output voltage must always be higher than the input voltage in magnitude. The negative sign indicates a reversal of sense of the output voltage. 2.5 Buck-Boost Converter 17 Fig: 2.5.1 Schematic for buck-boost converter With continuous conduction for the Buck-Boost converter Vx =Vin when the transistor is ON and Vx =Vo when the transistor is OFF. For zero net current change over a period the average voltage across the inductor is zero. Vin t ON + V0 t OFF = 0 which gives the voltage ratio V0 D = − Vin 1−D and the corresponding current I0 1−D =− IIN D Since the duty ratio "D" is between 0 and 1 the output voltage can vary between lower or higher than the input voltage in magnitude. The negative sign indicates a reversal of sense of the output voltage. 18 Fig: 2.5.2 Waveforms for buck-boost converter 2.6 Converter Comparison The voltage ratios achievable by the DC-DC converters is summarized in Fig. 10. Notice that only the buck converter shows a linear relationship between the control (duty ratio) and output voltage. The buck-boost can reduce or increase the voltage ratio with unit gain for a duty ratio of 50%. Fig: 2.6.1 Comparison of Voltage ratio 2.7 CUK Converter 19 The buck, boost and buck-boost converters all transferred energy between input and output using the inductor, analysis is based of voltage balance across the inductor. The CUK converter uses capacitive energy transfer and analysis is based on current balance of the capacitor. From DUALITY principle on the buck-boost converter the circuit in Fig. below (CUK converter) is derived. Fig: 2.7.1 CUK Converter If we assume that the current through the inductors is essentially ripple free we can examine the charge balance for the capacitor C1. For the transistor ON the circuit becomes Fig: 2.7.2 CUK "ON-STATE" and the current in C1 is IL1. When the transistor is OFF, the diode conducts and the current in C1 becomes IL2. 20 Fig: 2.7.3 CUK "OFF-STATE" Since the steady state assumes no net capacitor voltage rise, the net current is zero IL1 t ON + (−IL2 )TOFF = 0 which implies IL2 1 − D = IL1 D The inductor currents match the input and output currents, thus using the power conservation rule V0 D = − Vin 1−D Thus, the voltage ratio is the same as the buck-boost converter. The advantage of the CUK converter is that the input and output inductors create a smooth current at both sides of the converter while the buck, boost and buck-boost have at least one side with pulsed current. 2.8 Isolated DC-DC Converters In many DC-DC applications, depending on the application multiple outputs are required and output isolation may need to be implemented. Additionally, to meet safety standards and / or provide impedance matching input to output isolation may be required. To provide isolation between input and output DC-DC topologies can be adapted. 21 2.9 Flyback Converter The flyback converter can be developed as an extension of the Buck-Boost converter. Fig (a) shows the basic converter; Fig (b) (replacing inductor by transformer) replaces the inductor by a transformer. The buck-boost converter works by storing energy in the inductor during the ON phase and releasing it to the output during the OFF phase. With the transformer, the energy storage is in the magnetization of the transformer core. To increase the stored energy a gapped core is often used. In Fig (c) removal of the common reference of the input and output circuits clarify the isolated output. Fig:2.9.1 Buck-Boost Converter Fig: 2.9.2 Replacing inductor by transformer 22 Fig: 2.9.3 Flyback converter re-configured 2.10 Forward Converter The concept behind the foward converter is that of the ideal transformer converting the input AC voltage to an isolated secondary output voltage. For the circuit in Fig. (forward converter), when the transistor is ON, Vin appears across the primary and then generates Vx = N1 V N2 in The diode D1 on the secondary ensures that only positive voltages are applied to the output circuit while D2 provides a circulating path for inductor current if the transformer voltage is zero or negative. Fig:2.10.1 Forward Converter 23 The problem with the operation of the circuit in Fig above (forward converter) is that only positive voltage is applied across the core, thus flux can only increase with the application of the supply. The flux will increase until the core saturates when the magnetizing current increases significantly and circuit failure occurs. The transformer can only sustain operation when there is no significant DC component to the input voltage. While the switch is, ON there is positive voltage across the core and the flux increases. When the switch turns OFF we need to supply negative voltage to rset the core flux. The circuit in Fig. below shows a tertiary winding with a diode connection to permit reverse current. Note that the "dot" convention for the tertiary winding is opposite those of the other windings. When the switch turns OFF current was flowing in a "dot" terminal. The core inductance act to continue current in a dotted terminal. Fig:2.10.2 Forward converter with tertiary winding 24 3. PHOTOVOLTAIC (PV): A photovoltaic device, also sun PV power system, or PV system, is a strength system designed to supply usable solar strength via photovoltaics. It includes an arrangement of numerous additives, along with solar panels to soak up and convert daylight into electricity, a sun inverter to exchange the electric modern-day from DC to AC, in addition to mounting, cabling and other electric add-ons to install an operating gadget. it could also use a sun tracking machine to improve the system's typical overall performance and encompass an integrated battery solution, as costs for garage gadgets are expected to say no. A solar array simplest encompasses the ensemble of sun panels, the seen a part of the PV system, and does now not include all of the different hardware, regularly summarized as balance of system (BOS). Moreover, PV structures convert mild without delay into energy and should not be confused with different technologies, which includes concentrated sun energy or solar thermal, used for heating and cooling. PV systems variety from small, rooftop- mounted or building-incorporated structures with capacities from some to several tens of kilowatts, to big software-scale energy stations of hundreds of megawatts. These days, maximum PV systems are grid-linked, even as off-grid or stand-on my own systems handiest account for a small part of the market. Operating silently and without any shifting elements or environmental emissions, PV structures have advanced from being area of interest marketplace packages right into a mature era used for mainstream energy generation. A rooftop gadget recoups the invested power for its production and set up within zero.7 to 2 years and produces about ninety five percent of internet clean renewable strength over a 30-year provider lifetime. 25 Fig:3.0 solar farm 3.1 Overview of photovoltaic: Diagram of the viable components of a photovoltaic system. A photovoltaic system converts the sun's radiation into usable electricity. It incorporates the sun array and the stability of system additives. PV systems may be labeled by way of numerous elements, along with, gridconnected vs. stand-alone systems, constructing-included vs. rack-mounted structures, residential vs. utility structures, disbursed vs. centralized systems, rooftop vs. ground-established systems, tracking vs. fixed-tilt structures, and new built vs. retrofitted systems. Fig:3.1.1 Diagram of the possible components of a photovoltaic system 26 3.2 Grid-connection: Grid-related photovoltaic strength system Schematics of a normal residential PV device A grid related system is connected to a larger impartial grid (usually the public power grid) and feeds power without delay into the grid. This strength can be shared by way of a residential or commercial constructing before or after the sales size factor. The distinction being whether or not the credited energy manufacturing is calculated independently of the purchaser's strength consumption (feed-in tariff) or most effective at the distinction of power (net metering). Grid related systems range in length from residential (2–10 kWp) to sun strength stations (as much as 10s of MWp). this is a form of decentralized strength technology. The feeding of energy into the grid calls for the transformation of DC into AC with the aid of a unique, synchronising grid-tie inverter. In kilowatt-sized installations the DC facet device voltage is as high as accredited (commonly 1000V except US residential 600 V) to restrict ohmic losses. most modules (60 or 72 crystalline silicon cells) generate 160 W to three hundred W at 36 volts. it is once in a while important or proper to connect the modules partially in parallel rather than all in series. One set of modules related in series is referred to as a 'string'. Fig:3.2.1 Schematics of a typical residential PV system 27 4. FLYBACK CONVERTER: The flyback converter is utilized in both AC/DC and DC/DC conversion with galvanic isolation among the enter and any outputs. The flyback converter is a buck boost converter with the inductor split to form a transformer, so that the voltage ratios are accelerated with a further benefit of isolation. When driving as an instance a plasma lamp or a voltage multiplier the rectifying diode of the boost converter is left out and the device is known as a flyback transformer. Fig:4.0 Schematic of a flyback converter 4.1 Structure and principle: The operation of storing energy in the transformer before transferring to the output of the converter allows the topology to easily generate multiple outputs with little additional circuitry, although the output voltages should be able to match each other through the turns ratio. Also, there is a need for a controlling rail which should be loaded before load is applied to the uncontrolled rails, this is to allow the PWM to open up and supply enough energy to the transformer. 4.2 Operations: The flyback converter is an isolated power converter. The two prevailing manage schemes are voltage mode manage and current mode control (usually current mode manipulate 28 needs to be dominant for balance all through operation). It requires a signal related to the output voltage. There are 3 common methods to generate this voltage. The primary is to use an optocoupler at the secondary circuitry to send a signal to the controller. The second is to wind a separate winding at the coil and depend upon the go law of the layout. At the primary side, it composed on sampling the voltage amplitude, throughout the discharge, referenced to the standing primary DC voltage. The first method related to an optocoupler has been used to reap tight voltage and current regulation, while the second one approach has been developed for cost- sensitive applications where in the output does not need to be as tightly managed, but up to 11 components inclusive of the optocoupler will be eliminated from the overall layout. also, in programs wherein reliability is crucial, optocouplers can be unfavorable to the MTBF (suggest Time among Failure) calculations. The 0.33 approach, primary-facet sensing, can be as correct as the first and more within your budget than the second one, yet calls for a minimum load so that the dischargeoccasion maintains happening, offering the possibilities to pattern the 1: N secondary voltage on the primary winding (while Tdischarge,) Fig:4.2.1 Two configurations of a flyback converter in operation on state and off state A variation in primary-side sensing technology is where the output voltage and current are regulated by monitoring the waveforms in the auxiliary winding used to power the control IC itself, which have improved the accuracy of both voltage and current regulation. The auxiliary primary winding is used in the same discharge phase as the remaining secondary’s, but 29 it builds a rectified voltage referenced commonly with the primary DC, hence considered on the primary side. Fig: 4.2.2 Waveform - using primary side sensing techniques - showing the 'knee point'. Previously, a measurement was taken across the whole of the flyback waveform which led to error, but it was realized that measurements at the so-called knee point (when the secondary current is zero, see Fig. allow for a much more accurate measurement of what is happening on the secondary side. This topology is now replacing ringing choke converters (RCCs) in applications such as mobile phone chargers. 30 4.3 Controllers Proportional & Integral Controllers Proportional + Integral (PI) controllers were developed because of the desirable property that systems with open loop transfer functions of type 1 or above have zero steady state error with respect to a step input. 4.4 Concept of PI Controller: The PI regulator is: 𝑈(𝑠) 𝐸(𝑠) = 𝐾𝑃 + 𝐾𝐼 𝑠 But can be realized easily in the following form: 4.5 Tuning PI Controllers General approach to tuning: 1. Initially have no integral gain (TI large) 2. Increase KP until get satisfactory response 3. Start to add in integral (decreasing TI) until the steady state error is removed in satisfactory time (may need to reduce KP if the combination becomes oscillatory) To eliminate the steady state error P-I controller is principally used. However, it's a negative impact in terms of the speed of the response and overall stability of the system. In areas wherever speed of the system isn't a problem this controller is usually used. To predict the future 31 errors of the system P-I controller has no ability it cannot decrease the increase time and eliminate the oscillations. If applied, any quantity of I guarantees set point overshoot. Fig: 4.5.1 PID controller In industrial control systems, a proportional–integral–derivative controller (PID controller) may be a control loop feedback mechanism(controller) usually used. A PID controller because the difference between a measured process variable and a desired setpoint continuously calculates an error value. By adjustment of an impact variable the controller makes an attempt to reduce the error over time, similar to the position of an impact valve, adamper, or the ability supplied to a component, to a new value determined by a weighted sum 𝒕 𝒅 . 𝒖(𝒕) = 𝑴𝑽(𝒕) = 𝑲𝒑 𝒆(𝒕) + 𝑲𝒊 ∫𝟎 𝒆(𝝉)𝒅𝝉 + 𝑲𝒅 𝒅𝒕 𝒆(𝒕) Equivalently, the transfer function in the Laplace Domain of the PID controller is 𝐿(𝑠) = 𝐾𝑝 + Where : complex number frequency 32 𝐾𝑖⁄ 𝑠 + 𝐾𝑑 𝑠 4.6 Proportional Term: The proportional term produces an output value that is proportional to the current error value. The proportional response can be adjusted by multiplying the error by a constant Kp, called the proportional gain constant. The proportional term is given by: 𝑃𝑜𝑢𝑡 = 𝐾𝑝 𝑒(𝑡) In a large change in the output for a given change in the error is the high proportional gain results. The system can become unstable, if the proportional gain is too high (see the section on loop tuning). In contrast, to a large input error a small gain results in a small output response, and a less responsive or less sensitive controller. when responding to system disturbances if the proportional gain is too low, the control action may be too small. Fig: 4.6.1 Plot of PV vs time, for three values of Kp (Ki and Kdheld constant) 4.7 Integral Term: The integral in a PID controller is the sum of the instantaneous error over time and gives the accumulated offset that should have been corrected previously To both the magnitude of the error and the duration of the error the contribution from the integral term is proportional.. The accumulated error is then multiplied by the integral gain ( 33 ) and added to the controller output. The integral term is given by: 𝑡 𝐼𝑜𝑢𝑡 = 𝐾𝑖 ∫ 𝑒(𝜏)𝑑𝜏 0 The integral term eliminates the residual steady-state error that occurs with a pure proportional controller and accelerates the movement of the process towards set point. However, since from the past the integral term responds to accumulated errors, to overshoot the setpoint value it can cause the present value. Derivative Term By determining the slope of the error over time and multiplying this rate of change the derivative of the process error is calculated by the derivative gain Kd. The magnitude of the contribution of the derivative term to the overall control action is termed the derivative gain, Kd. The derivative term is given by: 𝐷𝑜𝑢𝑡 = 𝐾𝑑 𝑑 𝑒(𝑡) 𝑑𝑡 Derivative action improves settling time and stability of the system. An ideal derivative is not casual, so that implementations of PID controllers include an additional low pass filtering for the derivative term,. Derivative action is seldom used in practice to limit the high frequency gain 34 and noise - by one estimate in only 25% of deployed controllers - because of its variable impact on system stability in real-world applications. Fig: 4.7.1 Plot of PV vs time, for three values of Kd (Kp and Kiheld constant) 35 5. FUZZY LOGIC CONTROLLER In FLC, a set of linguistic guidelines decides fundamental control action. These rules are determined by using the gadget. Since the numerical variables are transformed into linguistic variables, mathematical modeling of the gadget isn't required in FC. The FLC incorporates of three elements: fuzzification, interference engine and defuzzification. The FC is characterized as; i. seven fuzzy units for every enter and output. ii. Triangular club features for simplicity. iii. Fuzzification using non-stop universe of discourse. iv. Implication using Mandeni’s „min‟ operator. v. Defuzzification the usage of the „top‟ method. 5.1 Fuzzification: Membership function values are assigned to the linguistic variables, using seven fuzzy subsets: NB (poor large), NM (negative Medium), NS (bad Small), ZE (0), PS (Positive Small), PM medium (Positive Medium), and PB (Positive big). The partition of fuzzy subsets and the form of membership CE(K) E(K) feature adapt the shape as much as appropriate system. The cost of input errors and alternate in mistakes are normalized by way of an enter scaling component. Fig:5.1.1 Fuzzy logic controller In this system, the input scaling factor has been designed such that input values are between -1 and +1. The triangular shape of the membership function of this arrangement presumes that for any E(k) input there is only one dominant fuzzy subset. The input error for the FLC is given as 36 E(k) = 𝑃𝑝ℎ(𝑘) −𝑃𝑝ℎ(𝑘−1) 𝑉𝑝ℎ(𝑘) −𝑉𝑝ℎ(𝑘−1) CE(k) = E(k) – E(k-1) (a) (b) TABLE: FUZZY RULES . Fig:5.1. a input error as membership functions 37 Fig:5.1. b change as error membership functions Fig:5.1. c output variable Membership functions 5.1 Interference Method: There are various composition techniques which contents of Max–Min and Max-Dot are proposed in this literature. In this paper Min technique is used. The output membership feature have various rule are given with the aid of the minimal operator and most operator. Table.1 suggests rule base of the FLC. 38 5.2 Defuzzification: The plant normally needed a non-fuzzy value of control, which is defuzzification stage is required . To compute the output of the FLC, height method is used along with the FLC output modifies the control output. Fuzzy logic having completely different which means in 2 means they're in slender sense, symbolic logic may be a system of rules, which may able to expand of multivalve logic. a lot of over its employed in massive sense symbolic logic (FL) is sort of substitutable beside the speculation of fuzzy sets, a theory which may able to relates to the categories of objects on with un-sharp boundaries within which membership may be a matter of degree. during this perspective, symbolic logic in its slender sense may be a branch of Everglade State. Even in its a lot of slender definition, symbolic logic differs each in conception and substance from ancient multivalve logical systems. As the plant typically needs a non-fuzzy price of management, a defuzzification stage is required. To work out the output of the FLC, „height‟ technique is employed and therefore the FLC output modifies the management output. Further, the output of FLC controls the switch within the electrical converter. In UPQC, the active power, reactive power, terminal voltage of the road and electrical device voltage square measure needed to be maintained. so as to regulate these parameters, they're detected and compared with the reference values. to attain this, the membership functions of FC are: error, modification in error and output The set of FC rules are derived from u=-[α E + (1-α)*C] (6) Where α is self-adjustable factor which can regulate the whole operation. E is the error of the system, C is the change in error and u is the control variable. 39 5.3 What Is Fuzzy Logic? fig 5.3.1 fuzzy description Fuzzy logic is used in two ways they are old and new ,because from the modern and methodical science , fuzzy logic is the young concept ,this concept is related to the fuzzy logic relies on age-old skills of human reasoning. 5.4 Why Use Fuzzy Logic? Fuzzy logic may be a convenient thanks to map AN input house to AN output house. Mapping input to output is that the place to begin for everything. contemplate the subsequent examples: • With data concerning however smart your service was at a eating place, a symbolic logic system will tell you what the tip ought to be. • With your specification of however hot you wish the water, a symbolic logic system will change the tap valve to the correct setting. • With data concerning however remote the topic of your photograph is, a symbolic logic system will focus the lens for you. • With data concerning how briskly the automotive goes and the way exhausting the motor is functioning, a symbolic logic system will shift gears for you. 40 To determine AN acceptable quantity of tip needs mapping inputs to the proper outputs. Among the input and also the output, the previous verify suggests a recording equipment which will embody any amount of things: fuzzy structures, linear structures, knowledgeable systems, neural networks, differential equations, interpolated third-dimensional search tables, or maybe a spiritual consultant, merely to decision variety of the possible choices. Clearly the listing have to be compelled to cross on and on. 5.5 When not to Use Fuzzy Logic? Fuzzy logic is not a remedy-all. while should you not use symbolic logic? the foremost secure Announcement is that the initial one created during this advent: fuzzy logic may be a convenient manner to map AN enter area to an output area. If you discover it's not convenient, attempt some factor else. If a easier answer already exists, use it. symbolic logic is that the codification of commonplace sense — use commonplace feel once you place into result it and you may in all probability build the correct call. several controllers, as AN instance, do a good task while not victimization symbolic logic. However, just in case you are taking the time to grow to be accustomed to symbolic logic, you will see it is a really powerful device for dealing speedy and with success with impreciseness and nonlinearity. 5.5 The FIS editor: The following discussion walks you thru building a replacement fuzzy abstract thought system from scratch. If you wish to store time and adjust to on fast, you'll load the already made machine by means that of writing fuzzy tipper this can load the FIS associated with the report tipper.fis (the .fis is implied) and unharness the FIS Editor. but, just in case you load the pre-built device, you may not be building pointers and building club options. The FIS Editor shows most well-liked facts just about a fuzzy abstract thought machine. there is a simple diagram that shows the names of every input variable on the left, and people of every output variable at the proper. The sample membership capabilities shown inside the bins area unit simply icons and don't depict the particular shapes of the membership capabilities. 41 Fig:5.6.1 The FIS Editor We'd like to change the variable names to reflect that, though: • In the white edit field on the right, change input1 to service and press Return. • Click once on the left-hand (yellow) box marked input1 (the box will be highlighted in red). • Click once on the right-hand (blue) box marked output1. • In the white edit field on the right, change output1 to tip. • In the white edit field on the right, change input2 to food and press Return. • Click once on the left-hand (yellow) box marked input2 (the box will be highlighted in red). • From the File menu select Save to workspace as.. and a window appears as below • Enter the variable name tipper and click on ok. We can show the diagram updated to reflect the new names of the input and output variables. There is now a new variable in the workspace called tipper that contains all the information about this system. 42 Fig: 5.6.2 ‘Save to workspace as...’ Window By saving to the workspace with a new name, you also rename the entire system. Your window will look like as shown in Fig.5. Fig:5.6.3 The updated FIS Editor 43 6. OPERATION PRINCIPLE Below is a circuit diagram of the flyback microinverter; it consists of an input capacitor Cin, a flyback converter with turn ratio n (Ns / Np), a full-bridge type unfolding circuit (S2 − S5), and an output filter. Fig : 6.0 Circuit diagram of the flyback microinverter The flyback converter operates under the high switching frequency to convert PV power into rectified sinusoidal waveform. The unfolding circuit works under the grid frequency f to inject sinusoidal ac current into the grid; switches S2 and S5 are turned on during the positive half-cycle of the grid voltage vg, while S3 and S4 are turned on during the negative half-cycle. 6.1 Steady-state analysis of DCM and CCM operations : Under the constant switching frequency fs, the operation modes are classified into DCM and CCM. In DCM, the magnetizing current im becomes zero within each switching period Ts, and the transformer T is completely demagnetized as shown in Fig. 2. When S1 is turned on, the primary current Ipri is stored in the magnetizing inductance Lm, and its peak value is expressed as follows: 𝐼𝑝𝑟𝑖.𝑝𝑘−𝐷𝐶𝑀 (𝑡) = 𝑉𝑃𝑉 𝐿𝑚 𝐷𝐷𝐶𝑀 (𝑡)𝑇𝑠 (1) where DDCM is the duty ratio in DCM. The energy ELm stored in Lm is expressed as 44 1 (𝑉𝑃𝑉 𝐷𝐷𝐶𝑀 (𝑡)𝑇𝑠 )2 2 2𝐿𝑚 𝐸𝐿𝑚 = 𝐿𝑚 𝐼𝑝𝑟𝑖.𝑝𝑘 2 (𝑡) = (2) Assuming lossless operation in the inverter, the power balance equations can be obtained as 𝑉𝑔 𝐼𝑔 𝑉𝑃𝑉 𝐼𝑃𝑉 = = 𝑃0 (3) 2 𝑉𝑃𝑉 𝐼𝑃𝑟𝑖 (𝑡) = 𝑣𝑔 (𝑡)𝑖𝑔 (𝑡) = 𝑣𝑔 𝐼𝑔 𝑠𝑖𝑛2 𝜔𝑡 (4) where VPV and IPV are the average values of the voltage and current for a PV module. vg and ig are the grid voltage and grid current; Vg and Ig are the peak values of vg and ig, respectively. Po is the average output power. Ipri is the average primary current. ω is the angular frequency of the grid voltage. If there is no loss, the energy stored in Lm is equal to the energy transferred to the grid. Assuming |vg |≈vo, from (2)-(4), DDCM can be derived a 𝐷𝐷𝐶𝑀 (𝑡) = 2 𝑉𝑃𝑉 √𝑃0 𝐿𝑚 𝑓𝑠 |𝑠𝑖𝑛𝜔𝑡| = 𝑑𝐷𝐶𝑀.𝑝𝑘 |𝑠𝑖𝑛𝜔𝑡| (5) where dDCM,pk is the peak value of DDCM. In CCM, Lm is applied to VPV during the turn-on time of S1, while the voltage across Lm is reflected the output voltage during the turn-off time. Using the voltage-seconds law for Lm, the duty ratio DCCM in CCM is calculation as 𝐷𝐶𝐶𝑀 (𝑡) = 𝑉𝑔 |𝑠𝑖𝑛𝜔𝑡| 𝑛𝑉𝑃𝑉 +𝑉𝑔 |𝑠𝑖𝑛𝜔𝑡| (6) The duties DDCM and DCCM determine the relationship between the input voltage VPV and output voltage vo. 45 Fig:6.1.1 Key waveforms in DCM and CCM. As mentioned above, the peak value of the primary current in DCM is expressed as the equation (1). In case of CCM, as shown in Fig. 2, the peak value of the primary current Ipri, pk_CCM is equal to the sum of the average magnetizing current Im and the ripple component. Because the magnetizing current is the same as the primary current when S1 is turned on, its average value can be expressed using the average value of the primary current Ipri and the duty ratio DCCM as 𝐼𝑚 (𝑡) = 𝐼𝑝𝑟𝑖 (𝑡) (7) 𝐷𝐶C𝑀 (𝑡) Thus, from Fig. and the power relationship in (2), the peak value of the primary current Ipri, pk_CCM is calculated as 𝐼𝑝𝑟𝑖.𝑝𝑘−𝐶𝐶𝑀 (𝑡) = 46 𝐼𝑝𝑟𝑖 (𝑡) 𝐷𝐶𝐶𝑀 + (𝑡) 𝑉𝑃𝑉 𝐷𝐶𝐶𝑀 (𝑡)𝑇𝑆 2𝐿𝑚 = 𝑖0 𝑉𝑃𝑉 (𝑛𝑉𝑃𝑉 + |𝑣𝑔 (𝑡)|) + 2𝐿 1 𝑚 𝑓𝑠 𝑉𝑃𝑉 |𝑣𝑔 (𝑡)| 𝑛𝑉𝑃𝑉 +|𝑣𝑔 (𝑡)| (8) The peak value of the secondary current is the same as the peak value of ipri for each mode divided by the turn ratio n. 6.2 Flyback microinverter with hybrid mode : Under the DCM operation, the turn-off time toff is divided into the falling time tf and the zero time Δt. The time tf is constant and is given by 𝑡𝑓 = 𝑛𝑉𝑃𝑉 𝑑𝐷𝐶𝑀,𝑝𝑘 𝑣𝑔 𝑇𝑠 = 𝑛𝜆𝑑𝐷𝐶𝑀,𝑝𝑘 𝑇𝑠 (9) where VPV/Vg is denoted as λ. Because im is zero before end of each switching period Ts, the following condition (10) is satisfied in DCM 𝑡𝑜𝑛 (𝑡) + 𝑡𝑓 = 𝑑𝐷𝐶𝑀,𝑝𝑘 (|𝑠𝑖𝑛𝜔𝑡| + 𝑛𝜆)𝑇𝑠 ≤ 𝑇𝑠 (10) With an increase of the output power, DDCM also increases, and the sum of the turn-on time and falling time becomes Ts. Thus, from (5) and (10), the critical duty ratio Dcri can be obtained as follows: 𝐷𝑐𝑟𝑖 (𝑡) = t𝑜𝑛 (𝑡) 𝑇𝑠 = 𝑑𝐷𝐶𝑀.𝑝𝑘 𝑇𝑠 |𝑠𝑖𝑛𝜔𝑡| 𝑑𝐷𝐶𝑀.𝑝𝑘 (|𝑠𝑖𝑛𝜔𝑡|+𝑛𝜆)𝑇𝑠 = |𝑣𝑔 (𝑡)| 𝑛𝑉𝑃𝑉 +|𝑣𝑔 (𝑡)| (11) From (11), the fact that the duty ratio DDCM is equal to DCCM under boundary condition is verified, and the flyback operates in the DCM region when DDCM is smaller than DCCM. Fig. shows the operation regions of the flyback inverter in a half-cycle of the grid under conditions given in Table I. As shown in Fig. 3, the flyback inverter operates in DCM at the low instantaneous power level or low solar irradiation level although it operates in CCM region above a certain power level in ac line period. Because the flyback inverter has both operation modes over whole ac line period, it performs as the flyback inverter with hybrid mode. 47 Fig:6.2.1 Operation regions of the flyback inverter during a half-cycle of the grid voltage. The boundary between DCM and CCM regions varies according to the magnetizing inductance Lm. Lower Lm results in larger DCM region at the given output power. The critical magnetizing inductance Lm,cri is derived from (5) and (6) as follows: 𝐿𝑚,𝑐𝑟𝑖 = 1 ( 𝑉𝑃𝑉 𝑉𝑔 4𝑃0 𝑓𝑠 𝑛𝑉𝑃𝑉 +𝑉𝑔 2 ) (12) To make the flyback inverter only perform within the DCM region, Lm should to be lower than Lm,cri at a certain output strength. The flyback inverter only with the DCM region suffers from the high current strain which reasons high strength losses and bounds the boom of the electricity ability. As Lm increases, the CCM region increases, and the maximum cuttingedge stress step by step decreases. as a result, the high Lm complements the performance and electricity capability. But, while placing the cost of Lm, there is a alternate-off between the performance and transformer length; higher Lm offers the decrease current strain however a larger transformer size. Therefore, the design of Lm for the flyback inverter with hybrid mode must be above Lm,cri, and consider the desirable current stress and transformer size. 48 7. OPERATIONAL ANALYSIS (HYBRID MODE) 7.1 Control issues : In the flyback microinverter with hybrid mode, the current controller should ensure the reference tracking and disturbance rejection performances in both operation regions. Fig. shows the equivalent circuit of the grid-connected flyback micro- inverter. Fig:7.1.1 Equivalent circuit of the grid-connected flyback microinverter. Using the control input-to-output current transfer function introduced in the transfer function Gid_DCM in DCM can be expressed as follows: 𝐺𝑖𝑑−𝐷𝐶𝑀 = 𝑉𝑃𝑉 𝑉𝑔,𝑟𝑚𝑠 𝑃𝑜 √2𝐿 (13) 𝑚 𝑓𝑠 where Vg,rms is the rms value of the grid voltage. Eqn. (13) is noted that the system gain in DCM is constant and very low at all frequency ranges. Using a small signal modeling, the transfer function Gid_CCM in CCM can be represented as 𝐺𝑖𝑑−𝐶𝐶𝑀 = 49 𝐴𝑠+𝐵 2 𝑅𝑖𝑛 𝐶𝑖𝑛 𝐿𝑚 𝑠 2 +𝐿𝑚 𝑠+𝐷𝐶𝐶𝑀 𝑅𝑃𝑉 − 𝐼𝐿𝑚 𝑛 (14) Where (1 − 𝐷𝐶𝐶𝑀 ) (𝑉𝐶𝑖𝑛 + 𝑣𝑔 𝑛) 𝐴 = 𝑅𝑃𝑉 𝐶𝑖𝑛 𝑛 𝑣𝑔 1 − 𝐷𝐶𝐶𝑀 𝐵= (𝑉𝐶𝑖𝑛 + − 𝐷𝐶𝐶𝑀 𝐼𝐿𝑀 𝑅𝑃𝑉 ) 𝑛 𝑛 From (14), it is observed that the control input-to-output current transfer function in CCM has an RHP zero. The RHP zero varies according to the operating points, and its minimum value is at the peak of the grid voltage under maximum output power. Thus, the minimum RHP zero should be considered when the controller for the flyback inverter with hybrid mode is designed. In the conventional manipulate system [17], the PI controller is used to ensure the reference tracking and disturbance rejection performances. Fig. indicates open-loop Bode plots of the compensated systems by way of the traditional PI controller. Used parameters are listed in Table-2. The working factor in DCM is at the instant energy 25W below the rated common output energy. while, the point in the CCM is the height of vg beneath the rated average output power wherein it's miles the minimal RHP is zero. The proportional benefit kp of the PI controller is tuned to be low to make sure the steadiness within the operation factor with the minimum RHP 0. Accordingly, the profits of the conventional PI controller are set as follows: The proportional crucial benefit kp = 0.08 and ki = 64. As shown in Fig, the device advantage on the fundamental and its harmonic frequencies of the grid in DCM is lots decrease than that during CCM although the imperative action of the PI controller gives excessive DC benefit on the beginning. This makes the flyback inverter in the DCM area be unable to make sure monitoring the reference and rejecting disturbances by means of the PV and grid voltages effect. To increase the device advantage at the ones frequencies, a high proportional gain is needed. However, it raises the gadget gain in any respect frequencies and so ought to make the flyback inverter within the CCM region become unstable. Consequently, whilst applying the traditional PI controller to the flyback inverter with hybrid mode, there may be a trade-off among manipulate overall performance in DCM and stability in CCM. 50 (a) (b) Fig:7.1.2 Open-loop Bode plots of the compensated system by the conventional PI controller. (a) In DCM. (b) In CCM 51 8. PROPOSED CONTROL STRATEGY To satisfy the desired control performance and stability in both operation modes, the PR controller can be developed, and its transfer function is expressed as 𝐶𝑃𝑅 (𝑠) = 𝑘𝑝 + 𝑘𝑟 𝑠 2 𝑠 +𝜔2 (15) where kr is the resonant gain. The PR controller in (15) has an infinite gain at the grid frequency. However, the infinite gain would degrade the control performance and even cause the system to become unstable. In the practical implementation, the following form of the PR controller can be adopted as CPR (s) = k p + 2kr ωc s 2 s +2ωc s+ω2 (16) In which ωc is selected to widen the controller bandwidth and determines the -three dB cutoff frequency of the controller. That is, the value of the compensator will become kr / 2 at ωωc or ω+ωc. In the PR controller, kp is selected inside the same manner as that of a PI controller. this is, it is designed with the aid of the steadiness of the compensated structures thinking about the minimal RHP zero. The benefit kr is tuned notably high to decrease constant-kingdom error but it's far restricted by way of the stability. In case of the proposed PR controller, the gains kr and ωc are 20 and 16, respectively; To ensure the stability kr is selected to make the system have the segment margin above 45 degrees. Similarly, the harmonic compensator is able to alleviate errors for the selective harmonic frequencies, and its transfer function is represented as 2k ωc s 2 c s+ω CHC (s) = ∑h=3,5,7… s2 +2ωr 52 (17) (a) (b) Fig:8.1 Open-loop Bode plots of the compensated system by the proposed controller. (a) In DCM. (b) In CCM. where h is the harmonic order, and krh is the resonant gain for each harmonic frequency. The open-loop Bode plots of compensated system by way of the PR controller with the third to seventh harmonic compensators is shown in Fig. 6. The third, fifth, and seventh harmonics are the most prominent harmonics below the grid environment. Like the PR controller, HC affords the excessive advantage at selected harmonic frequency additives, which 53 allows eliminating consistent-kingdom errors and the disturbance by way of the chosen frequency components. Compared to the compensated machine by means of the conventional PI controller proven in Fig., it is apparent that the PR controller with harmonic compensators brings the higher benefit on the essential and its harmonic frequencies of the grid without excessive proportional advantage. in addition, the bandwidth in DCM is extended. Accordingly, it enhances the reference tracking velocity and disturbance rejection performances with satisfaction of the favored stability in both operation modes. Fig:8.1.2 The block diagram of the proposed control system. The general proposed control device for the flyback inverter with hybrid mode is proven in Fig .it includes the PR controller with HC and the nominal obligation ratio Dn. Ig * is the peak fee of the reference grid current (or output current). As a kind of the feedforward control inputs, the nominal duty ratio Dn gets rid of the disturbance results and reduces the load of the comments controller. the responsibility ratio Dccm in (6) is carried out to entire ac line period. In this situation, the duty ratio Dccm reasons the voltage mismatch in the DCM location, which increases the burden of the feedback controller. For the reason that system gain in DCM is relatively lower, this burden will become heavier. To overcome the mismatch in DCM, the duty ratio DDCM have to be carried out while the flyback inverter operates inside the DCM region; it method that the nominal duty ratio must be determined according to the operation region. To categorized the section of operation modes without an extra modern sensor, the vital duty ratio in may be used; it is stated that the flyback inverter operates within the DCM region while the subsequent condition is satisfied as DDCM (t) ≤ DCCM (t) 54 (18) Thus, the hybrid nominal duty ratio Dn in the proposed control strategy is determined as follows: Dn (t) = { DDCM (t) ifDDCM (t) ≤ DCCM (t) DDCM (t), ifDDCM (t) ≥ DCCM (t) (19) Finally, the proposed hybrid nominal duty ratio can significantly reduce the disturbance effect in both operation modes, and so improve the performance of the feedback controller. 55 9. SIMULATION RESULTS To verify the feasibility and performance of the proposed control strategy, the simulation by a simulator Psim and experiment using the prototype for the flyback microinverter shown in Fig. 1 were conducted. The nominal PV voltage and rated power were set up to 60V and 200W, respectively. The detail system parameters and parts are listed in Table II. TABLE II PARAMETERS AND COMPONENTS OF THE PROTOTYPE Parameters PV voltage Grid Voltage Grid frequency Rated average output power Switching frequency Primary winding turns Secondary winding turns Magnetizing inductance Leakage inductance Input capacitor Output capacitor Output inductor 56 Symbols VPV Vg f Po fs Np Np Lm Llk Cin Co Lo Value 40-80V 210Vrms 60HZ 200w 60HZ 14 turns 51turns 50µH 0.6 µH 6.6mF 0.68 µF 400 µF CASE:1 When both PI and PR controllers are present Fig:9.1.1 Block diagram of simulation 57 (a) (b) Fig:9.1.2 Simulation results for the grid current ig and its reference ig_ref when the conventional control system is applied. (a) quarter-load condition. (b) full-load condition. 58 Fig. shows the waveforms for the grid voltage and current when the proposed control strategy is applied. As shown in Fig , regardless of load conditions, the grid current has an almost perfect sinusoidal form and desired power level. The THD on the grid current is measured as 2.4% under full-load condition. Fig. shows the dynamic performance under the load variation. From Fig it is verified that the proposed control system makes output current well track its desired value under the load transient-state. 59 CASE:2 When both Fuzzy and PR controllers are present Fig:9.2.1 Block diagram of simulation of proposed system 60 Fig:9.2.2 Simulation results for the grid current ig and its reference ig_ref when the proposed control system is applied. (a) quarter-load condition. (b) full-load condition. 61 10.CONCLUSION In this paper proposes the current control strategy of the flyback microinverter with hybrid mode for PV ac module has been introduced and verified by simulation results. In the proposed control strategy, the PR controller with HC provides the high system gain at fundamental and harmonic frequencies in both operation modes without using high proportional gain. Here we are using the fuzzy controller compared to other controllers. The proposed hybrid nominal duty ratio yielded from the proposed operation mode selection eliminates the disturbance more effectively and reduces the burden of the feedback controller. In FLC, basic control action is determined by a set of linguistic rules. These rules are determined by the system. Since the numerical variables are converted into linguistic variables, mathematical modeling of the system is not required in FC. By using the simulation results we can verify the proposed method having faster reference tracking and better disturbance rejection than those of the conventional strategy. The proposed control strategy has the many advantages of the flyback inverter with hybrid mode and makes it to be used in the industrial field. By using the simulation results we can analyze the proposed method. 62 11. REFERENCES [1] Y. –H. Kim, J. –W. Jang, S. –C. Shin, and C. –Y. 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