Grid-Connected Solar Micro inverter Reference DesignAbstract-In traditional grid-connected PV system, it’s hard to remove failure of individual PV panels. This paper presents a Solar PV Grid-Connected Micro-inverter which can be embedded in a single stand-alone photovoltaic panel to solve the problem of single point of failure. For a single photovoltaic panel, rated power of the Micro-inverter is 220W, using the topology of interleaved flyback converter.Keywords-Micro-inverter; interleaved flyback converter; grid-connected; PV panelI. INTRODUCTIONWith the draining of fossil fuel and increasingly serious pollution caused by traditional power generation methods across the world, renewable and pollution-free energy has gained much attention in economic and political fields. Renewable energy includes photovoltaic (PV) and wind power generation systems. Wide application of renewable energy is now impeded by cost and extensive researches shall be conducted in order to improve cost effectiveness. PV system, also known as solar converter, has gained popularity in recent years as a convenient renewable energy with good prospects. High production cost and low conversion efficiency of silicon solar panel are major defects of PV energy. Cost effectiveness of PV projects will become more reasonable with the application of new PV panel production technology and theimprovement of converter efficiency.ІІ. EVOLUTION OF PV SYSTEMA. Traditional Grid-connected SystemTraditional grid-connected PV system is comprised of:Urban Home System – multiple solar panels are connected in serial to supply 200-400V DC and output medium power (2-10KW) AC electricity. If certain PV panel in the series loop is damaged (i.e. single point of failure), the entire system will be crashed, i.e. the system is unstableSingle Inverter With Multiple DC/DC Converters –multiple PV panels are connected in parallel after DC/DC conversion to input total DC bus bar voltage to inverter and increase output power. Such circuit structure also has problem of single point of failure meanwhile fusion of DC power supply is very complex.Urban Home System With String Inverters – PV panel providing 200-400V input DC voltage is connected to multiple parallel inverters to generate medium power (2-10KW) 120/240V AC power supply. Multiple parallel inverters can boost output power and improve system reliability.B. PV Grid-connected Micro-inverterBased on the above advantages and disadvantages of PV system, the present paper proposes the design of PV grid-connected micro-inverter to enable all PV panels in PV system to be embedded in grid-connectedmicro-inverter, see figure 1 for system structure.Micro-inverter with such structure has following advantages:•replace central inverter with distributed inverter to optimize energy utilization;•Integrated PV panel reduces installation cost;•Power of micro-inverter is low (hundreds W), resulting n low internal temperature and longer system service life, meanwhile fan is not required.III. DESIGN OF PV GRID-CONNECTED INVERTERThe present paper designs a single stage PV grid-connected micro-inverter. A simple interleaved flyback converter is applied to output sine half-wave commutating voltage and current, pass through bridge inverter to output full wave sine voltage and current and make the current have the same frequency and phase of the power grid voltage. This PV grid-connected micro-inverter matches with any PV components with 220W power rating, 25-45V output voltage and up to 55V open-circuit voltage.A. System ChartPV grid-connected micro-inverter applies parallel interleaved flyback converter, see figure 2.Ipri1 is current of flyback1 converter MOSFET and Isec1 is currentof flyback1 output diode. Current of secondary diode (Isec1) generates sinusoidal output voltage after being filtered by output filter capacitor. The inverter makes output current synchronous with grid voltage with digital phase-locked loop (PLL) technology. The maximum power tracks and controls the output current. V oltage output by PV panel is converted into sine half-wave commutating voltage/current by interleaved flyback converter, inputs full-bridge inverter circuit where it is inverted into current with the same phase of grid voltage, and is connected to power grid after EMC/EMI filtration. Duty ratio flyback converter switch shall be controlled to make the output current have the same phase and frequency with grid voltage. Interleaved flyback converter improves service life of capacitor by reducing the effective value of input large electrolytic capacitor ripple current. Interlaced output reduces output current ripple so as to decrease total harmonic distortion of current.B. Circuit AnalysisPV panel DC voltage inputs interleaved flyback converter and drives flyback MOSFET to generate sine output voltage/current with HF sine PWN modulating signal. Phase difference between two interleaved flyback converter driving signals is 180 degree. Interleaved flyback topology structure works under two switch modes.•Mode 1: when flyback MOSFET is opened, HF flyback transformer primary magnetic inductor accumulates energy, diode is phase reversalblocked and the secondary transformer winding voltage is reverse biased. During that period, primary inductor of HF flyback transformer is like a power inductor, primary current (Ipri1/Ipri2) ascends linearly and load current is from output capacitor.Mode 2: when flyback MOSFET is closed, voltage of primary winding is in phase reversal and output diode is forward biased. Energy stored at primary winding is transferred to secondary winding and provide current to output capacitor and load. During that period, output voltage is from the secondary transformer winding directly and then linearity of diode current decreases.Sine PWM modulating signal drives MOSFET to generate primary current and then generate current at the secondary diode. Half-wave sine average current of secondary rectifier diode generates standard half-wave sine voltage /current through output capacitor filtration. Controllable silicon full bridge rectifier circuit is used to produce sine-based output half-wave sine voltage/current. Thereby, the controllable silicon switches frequency into power grid frequency.Input voltage/current waveform of input voltage and solar micro-inverter in front of SCR bridge circuit and output voltage/current waveform of solar micro-inverter are shown in figure 3.C. Control CircuitPV grid-connected micro-inverter control system consist offollowing control circuits: digital phase-locked loop (PLL), current control circuit, maximum power tracking circuit and load balance control circuit. PLL and current control circuit related to grid-connected control are discussed in the present paper only.1) Digital phase-locked loop (PLL)PLL control system is a crucial component of control system to enable electric energy output by the system to be connected to power grid in unit power factor. PLL makes inverter output current have the frequency and phase angle synchronous with grid voltage.ADC channel of the software samples grid voltage and inverter output current signal and saves polarity of grid-connected voltage in register hence polarity of grid voltage is clear during each sampling period. Zero-voltage detection mark is set by the software when polarity of grid voltage varies. When grid voltage passes zero crossing point, the input timer interrupts and waits for the next zero crossing point, and count of the timer during the interruption between two zero crossing points is half of the period value of grid voltage. Period value can be used to express grid frequency and decides phase angle increment in citing of sine table reference values. Frequency and phase of grid voltage and inverter output current can be acquired with the above sampling parameters, and relevant SPWM carrier frequency and initial phase are regulated according the calculation results to enable PV inverter systemoutput current to track frequency and phase of grid voltage. The sine table covers 512 reference elements of sine 0 through 90 degree.2) Current control circuitCurrent control circuit applies PI controller and is the core of control system. Output control signal of current control circuit controls duty ratio of flyback MOSFET (D) to ensure that input current IAC follows reference current IACref.Equivalent non-isolated circuit of flyback converter acts as a buck-boost converter; therefore buck/boost converter can be used to establish model and calculate control circuit parameters. Like buck/boost converter is a highly nonlinear system like boost converter. Output voltage and current have nonlinear relation with the duty ratio when the system operates under continuous conduction mode. The current challenge is how to control the duty ration of flyback MOSFET D and generate a sinusoidal current. Circuit of buck/boost converter is shown in figure 4.Magnetic inductor of flyback is replaced by a buck-boost inductor. Giving duty ratio D to switch is to generate sinusoidal current passing load. The buck/boost topology structure generates reverse voltage. Therefore, average current through diode and load should be like a modified sine wave upside down. As the current of inductor does notchange instantly, load current can be calculated with the following formula.I LOAD represents the current of flyback inverter system; I AC, I L represents the current passing flyback current magnetic converter; I L* represents I ACref reference current; D represents duty ratio of flyback MOSFET; G is coefficient of control circuit compensation circuit K p and K i.Fundamental formula of inductor can be expressed by formula 2.It’s unlike to obtain current parameters directly in stead current error proportional to voltage is used to control current as shown in formula 3.According to basic power electronics theory, V x=V in*D -(1-D) * V oOutput voltage of flyback circuit V o is half-wave rectified sinusoidal voltage and is connected to power grid through thyristor full bridge inverter. Duty ratio D is calculated by formula 4 with input voltage V in and output voltage V o measured.Formula 5 is the relation expression between input voltage and output voltage of buck/boost converter.Desired duty ratio can be calculated with formulae 4 and 5 as shown in formula 6, where I load* is modified sine wave.The first item is the calculation result of PI compensator of which bandwidth is given by G/L. The second item is the result of open loop control with the purpose of enabling current to output in sine wavewithout control.IV. SYSTEM SIMULATIONA.A. Simulink Simulation ModelBased on the above analysis, the present paper establishes Simulink digital simulation model of PV grid-connected micro-inverter with the general flow chart shown in figure 5, where,Vin_ref-secondary1 is PV panel output voltage; Subsystem2 outputs reference current and grid voltage; Subsystem3 is model of flyback converter with internal flow chart as shown in figure 6; and controller1 is flow chart of control circuit and includes PLL control and average current tracking control as shown in figure 6.B. Simulation Result and AnalysisBased on the above simulation mode, when PV input voltage is set to Vin to 25V, reference input is set to 1A/50HZ sine current and grid voltage V grid is set to 220V, waveform of output current in front of SCR full bridge inverter acquired is as shown in figure 7, and output current with the same magnitude with reference current and the same phase with grid voltage is obtained after the said current passes through full bridge inverter.The upper part of the figure is the waveform of reference current and the lower part is the waveform of output current. According to simulation result, output current of the system has the same phase with referencecurrent signal; correctness of the control method is validated.V. CONCLUSIONThis paper presents an innovative PV grid-connected micro-inverter with 220W power rating and can be used by combining with individual PV panels into module so as to shoot trouble of single point of failure of individual PV panels in PV grid-connected power generation system and improves generating efficiency. This inverter applies interleaved flyback transformer topology falling into single-phase inverter structure which is simple and efficient. The paper also studies grid-connected control method and current control method in response to the inverter topology structure and establishes simulation model to validate the correctness of the design.太阳能光伏并网逆变器设计一、引言与排水的化石燃料，由传统的发电方式，在世界各地造成了日益严重的污染，可再生、无污染的能源在经济和政治领域备受关注。