The Research Of Fuzzy Control Strategy And Digital Design Technology For The Photovoltaic Grid-connected Inverter System

With the development of industry and economic globalization the traditional fossil sources have been increasing depletion.At the same time,the consumption of those inevitably results in green-house effect and environment pollution.Solar energy has a huge capacity and is green in one of renewable energy sources.Photovoltaic(PV)power generation is used to extract the solar energy.And the PV system can be set up easily and maintained simply without noise pollution.The social and economic benefits can be achieved well during the rapid development of PV industry,which has huge market space and application sites.How to improve PV array efficiency,output the maximum power,produce grid-connected power quality and reduce the cost of PV system as are involved in the PV technology issues.There are three points to develop the core technology in photovoltaic(PV)inverter system.The PV engineering model has been created practically and the PV inverter should work around the maximum power point(MPP).Furthermore,PV inverter needs a reasonable grid-connected control method.As to the related points,a new fuzzy control strategy and digital integrated-circuit(IC)design technology are provided for the PV inverter system.Since the model of solar cells from the physical equation is difficult to balance the accuracy and the engineering practice by the analyzing solution,a fractional order PV model combined with fuzzy logic controller(FOFLC)can be used to overcome this issue and work in the PV maximum power point tracking(MPPT).The stable conditions of PV system are given by Lyapunov's method.The diffusion current of PV cells can be derived by the photocurrent crossing the semiconductor PN energy.And,it can be descripted by fractional order differentiation behavior for solar cells.The FOFLC fuzzification interface converts the error input variables of the output power respect to voltage and its error ratio to linguistic values.Based on the Grünwald–Letnikov(GL)derivative,the reasonable approximations can be set up by the fractional order equation.The FOFLC algorithm can combine the fractional order accuracy with fast and robust fuzzy advantages in PV system.To verify the progress and reliability of this algorithm,various simulations and experiments are carried out to evaluate the PV system performances of different environment conditions.The commonly used methods such as perturbation & observation(Perturb & Observ),hill climbing,general incremental conductance(Gen Inc Cond)incur temporary loss of power and error behavior.Compared with the conventional controllers,a scalable fuzzy logic controller(SFLC)is designed for MPPT because it can provide a better accuracy performance and avoid nonconvergence result under variable atmospheric conditions.Using linear combinations of the fuzzy basis functions,a distributed,scalable and flexible fuzzy logic controller(FLC)by fuzzy accuracy factor is proposed in MPPT.The circuit structure of PV inverter is also studied.The digital real-time measuring solution is used in Boost circuit.PV voltage & current parameters are sampled by AD7656 chip.The digital sampling data is obtained and used for SFLC on field programmable gate array(FPGA)platform.The SFLC algorithm of the MPP is verified on FPGA digital system experiment.The physical characteristic of PV array and its MPP are affected by nonlinear conditions such as irradiation,temperature,electricity conduction and so on.It is difficult to track the MPP.The traditional MPPT methods(Perturb & Observ,Gen Inc Cond,general FLC)have some disadvantages of oscillation,power loss and error behavior.A fast,scalable and robust FLC is integrated in the FPGA.And,the scalable fuzzy standards are provided for MPPT.MPPT should speed the dynamic performance at initial or variable weather conditions.And,it should achive high-precision error of stable state.SFLC and digital current loop in MPPT are schemed for PV inverter.Based on FPGA and digital control model,the Boost converter is simply controlled in real-time.The current loop by SFLC can improve dynamic performance,reduce the tracking error,overcome the load disturbance and enhance the efficiency of generating energy.Under the changes of the weather(irradiation,temperature)and load conditions,the performances of grid-connected stability and power quality are affected by PV inverter control method.The grid-connected PV inverter is based on FPGA.And the synchronization by FOFLC is created due to the robustness and high accuracy.FOFLC can keep the voltage stable at the coulped input port.Based on FPGA the digital sampling filter of integrated-circuit design can decrease the stable error of the system and the disturbance of the alternating current(AC)voltage amplitude.And,the current loop by FOFLC can also improve the control accuracy,output the high quality of power energy and maximize the output power and efficiency.The test and sample platform are built on Altera Cyclone V FPGA.The FOFLC is implemented by digital integrated-circuit(IC)design.The results of experiment show that FOFLC can be progressive and reliable in PV inverter system.The current single-phase inverter is studied further.The FOFLC can also be combined with grid-connected control techqiue.The FOFLC and digital FPGA control system(circuit and software)are designed for PV inverter.The digital design method of integrated-circuit(IC)and single-chip FPGA implementation will make the inverter smaller scale,better accuracy,denser integration and higher efficient convertion.The development trend of the inverter is shown in the future.