Research On Three-port Photovoltaic System Based On Z-source Network

Solar energy becomes one of the most important new energy resources that cansolve energy shortages, the problem of environment pollution and ecologicaldamage because of its abundant reserves and no-pollution. With a growingawareness of environmental protection and a gradual decline in photovoltaic price,countries all over the world are devoting to development and utilization of solarphotovoltaic(PV).However, influenced greatly by the external environment, solarPV may result in great fluctuation in the output of the PV module, which makes itdiffficult to provide a stable and continuous energy for the load or grid. Toovercome the problem, three-port PV systems with storage capacity are widely usedin independent users, space station, schedulable grid-connection. It not onlyenhances the flexibility in the use of solar photovoltaic, but also improves theoutput power quality of the inverter. In order to further improve the utilization ofsolar photovoltaic modules and extend the service life of the storage devices, in thispaper, the three-port photovoltaic inverter topology, energy management, grid-connected control strategy and perfect islanding detection was researched in detail.The main contents are as follows:Three-port photovoltaic inverter is the core part of the photovoltaic generationsystem. Because of the application of high frequency transformers, high conversionseries and more switching devices, traditional topology not only increases the sizeand the weight of the system, but also increases losses of the PV system. Toovercome these shortcomings, keep the process of charging and discharging of thebattery safe and to extend the life of battery, this paper proposes a novel three-portphotovoltaic topology based on Z-source network, which has a combined functionof buck and boost, inverter, charge and discharge control, maximum power pointtracking and so on. The simple and compact structure and efficient working modehave not only reduced the cost, volume and weight of the system, but also improvedthe utilization efficiency of PV module.and power quality of system.Simple and easy to implement, Z-source inverter partly of the novel three-portphotovoltaic system using the single-stage control strategy that inserting shoot-through duty in zero state of SPWM, respectively achieve the goal of boosting DClink and controlling the current of the inverter by adjusting the shoot-through dutyand modulation factors. However, the inverter’s output contains lots of distortionand harmonic component in the case of inductive load. This paper presents aimproved SPWM control method, withnot increasing the external device, which opens upper bridge legs to provide freewheeling path on the commutation,eliminates the distortion of inverter output, reduces the harmonic component of theoutput current, improves the power quality of the inverter output. In this paper, wealso establish a small-signal model of three-port PV system based on Z-source, andanalyse the impacts of the various parameters and direct duty cycle of Z-sourcenetwork, load and input voltage on the characteristics of output.Changing with the external environment, the output power of solar PV module,fluctuating greatly, would even affect the normal operation of the load or the powerquality of the injecting grid current. By controlling the charge-discharge of thebattery, three-port PV system based on Z-source can eliminate the effect of theweather. As a result, power management of the three-port system is critical forensuring the efficient and stable running of the PV module, battery and load. In thispaper, based on the supply-demand relationship between load and PV module, anovel power management method suitable for three-port PV system based on Z-source is proposed. On the premise of stable running of load, this powermanagement method improved the utilization efficiency of PV module.andprolonged the life of battery.The inverter’s output of three-port PV system based on Z-source can not onlydirectly provide power to the load of the Stand-alone Photovoltaic System, but alsoconstitute a photovoltaic grid power system which can be dispatched by connectingwith the grid. Its advantage lies in that the current injected into the power grid isconstant, which avoids the fluctuation of the input power and contributes to theregulation of the power grid peak during night. This paper adopts the proportionresonance control method to control the current injected into the grid, whichincreases the dynamic response speed of the system and reduces the fluctuation ofthe grid current. This paper further improves the power quality through correctingthe modulation factor and feed-forward control to eliminate the influence that thefluctuation of the capacitance voltage of Z source network and grid makes on theoutput voltage.Besides the function of keeping the current injected into the grid having thesame frequency and phase with the voltage of grid, photovoltaic grid-connectedsystem must have the ability of perfect islanding detection. Once the islandphenomenon occurs and wasn’t cut off from the photovoltaic grid-connected systemin time, it may result in damage of the equipment and threaten personal safety.Considering the severity of island phenomenon, the islanding detection should berapid and accurate, as well as taking into account of the power quality. This paperproposes a novel feedback active frequency drift method (AFD), which is based onpositive feedback of the change of active power. This method adds the change of active power into the frequency deviation, thus, the frequency deviation of point ofcommon coupling is accelerated, and the time of islanding detection is shortened.On the basis of the work above, a set of experiment device of three-port PVsystem based on Z-source is developed. This device takes TMS320F2812as theprimary control chip and the software and hardware is designed. Results of theexperiment verify the validity and feasibility of the proposed novel three-portphotovoltaic system and control method.