Grid-Connected Solar Microinverter Reference
中华器官移植杂志Design
Xifei Cao
Institute of Information Engineering &Technical
Ningbo University
Ningbo, China
e-mail:bjb034@126
Weiqiang Zhang
Institute of Information Engineering &Technical
Ningbo University
Ningbo, China
e-mail: zhangweiqiang@nbu.edu
Abstract-In trad itional grid-connected PV system, it’s hard to remove failure of ind ivid ual 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 panel
I.INTRODUCTION
With 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 m氯化铁
ajor defects of PV energy. Cost effectiveness of PV projects will become more reasonable with the application of new PV panel production technology and the improvement of converter efficiency.
II.EVOLUTION OF PV SYSTEM
A.Traditional Grid-connected System
Traditional 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 unstable.
Single Inverter With Multiple DC/DC Converters – multiple PV panels are connected in parallel after DC/DC conversion to input total DC busbar 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 su
pply. Multiple parallel inverters can boost output power and improve system reliability.
B.PV Grid-connected Micro-inverter
Based 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-connected micro-inverter, see figure 1 for system structure.
Figure 1 Module incorporated inverter
Micro-inverter with such structure has following advantages:
•replace central inverter with distributed inverter to
Sponsor: Cultivation of Practical Talents for Non-adaptive Embedded System granted under No. JD090506
___________________________________
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optimize energy utilization;
•Integrated PV panel reduces installation cost;
•Power of micro-inverter is low (hundreds W), resulting in low internal temperature and longer system service life, meanwhile fan is not required.
III.
DESIGN OF PV GRID -CONNECTED INVERTER
The 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 Chart
PV grid-connected micro-inverter applies parallel玩具兵大战3
interleaved flyback converter, see figure 2.
Figure 2 Interleaved flyback converter diagram
Ipri1 is current of flyback1 converter MOSFET and Isec1is current of 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. Voltage 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 Analysis
PV panel DC voltage inputs interleaved flyback converter and drives flyback MOSFET to generate si
ne 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 reversal blocked 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.
Figure 3 Flyback and SCR bridge I/O voltage/current wavefrom
C.Control Circuit
PV grid-connected micro-inverter control system consist of following 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 SPWN carrier frequency and initial phase are regulated according the calculation results to enable PV inverter system output 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 circuit
Current 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 I AC follows reference current I ACref.
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.
Figure 4 BUCK-BOOST converter
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 not
change instantly, load current can be calculated with the
following formula.
I LOAD=I L(1-D)
(1)
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.
国际刑警组织
V x =sLI L (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.
V x = G(I L*-I L)
i
p
K
G K
s
*1load load I I Vx G
D (3)
According to basic power electronics theory, V x = V in * D -(1-D) * V o
Output 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.
*(1)()load load o
in o in o
I I V D G
D V V V V c
(4)
Formula 5 is the relation expression between input voltage and output voltage of buck/boost converter.
˄Vin +Vo ˅˄1-D ˅= Vin
(5)
Desired duty ratio can be calculated with formulae 4 and 5 as shown in formula 6, where I load * is modified sine wave.
*load load o
in in o I I V D G
V V c V
(6)
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 wave without control.
IV.
SYSTEM SIMULATION
A. Simulink Simulation Model
Based 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.
Figure 5 Simulink simulation model of the system
中兴u980Fig. 5 Simulation model of interleaved flyback converter
Figure 6 Control loop simulation model
B. Simulation Result and Analysis
Based on the above simulation mode, when PV input voltage is set to V in 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 m
agnitude with reference current and the same phase with grid voltage is obtained after the said current passes through full bridge inverter.
Figure 7 Simulation results
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 reference current signal; correctness of the control method is validated.
V.CONCLUSION
This 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 c
ontrol method and current control method in response to the inverter topology structure and establishes simulation model to validate the correctness of the design.
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Grid-Connected Inverters for Photovoltaic Modules[J],IEEE Trans. on
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淡漠如烟夫妻店2812[J].Power Electronics Technology, 2008, 42 (2): 75-77. [3]Wang Haining.Research of photovoltaic power conditioning
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[4]Liang Yongchunˈ Sun Linˈ Gong Chunyingˈet alˊResearch
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