Inverting Technology Research On Digital Grid Connected Photovoltaic System

The global energy issue is increasingly serious, each country is increasing thedevelopment and utilization of renewable energy. Grid connected photovoltaic generatingelectricity is being favored gradually when various policies is imposed in China forencouraging and supporting the development of photovoltaic energy. The key role of gridconnected PV system i.e. solar inverter has been obtained extensive research. This dissertationdeals with the digitized photovoltaic generating electricity system as the study goal, focusingon the photovoltaic converting technology which leads to developing DSP controlledsingle-phase two-section non-isolation/isolation grid-connected solar inverter.Firstly an introduction is given of the background and development of PV generatingelectricity and analysis of its basic principles, then research is proposed on digital controlledgrid-connected photovoltaic inverting technology based on dual-core of DSP and ARM,design is carried on the system main circuit, key device parameters, signal conditioningcircuits and auxiliary power supply etc., in which the400VDC is obtained from solar panel byDC/DC(Boost or phase shift full bridge converter) and then converted to sinusoidal current byDC/AC, which is finally fed into grid tie while being in synchronized with unity power factor,simulation and experiments verify theoretical analysis.Cascaded converters of more than two-section have to guarantee their stability anddynamics etc., so do two-section photovoltaic inverters. Using the EMI filter cascaded withDC/DC converter as an example, an introduction is given of EMI filter from the noisesuppression viewpoint and how to avoid the system performance degradation and instabilitywhen being applied to DC/DC such as Boost converters under PCMC. Via the state-spaceaveraging method, the small-signal model of Boost converters is constructed and the design ofthe PI controller used in the converter leading to its control bandwidth of4kHz and phasemargin of52.6°, as well as its simulation model is set up. EMI filter is designed on a Matlabplatform in respect to the noise suppression stipulated by the European standards whileremaining stability and dynamic performance of Boost converters cascaded. The interactionbetween the converters is the key problem of constructing cascade system, a graphical impedance ratio criterion with Nyquist stability criterion is presented to evaluate the stabilityof cascade system, simulation and modeling for the closed-loop converter is constructed.Simulation and experiment results prove that not only the EMI filter cascaded with Boostsuppresses the noise current and obtains an acceptable level complying with Europeanstandard but also remains stability and dynamics of the cascaded system, thus providing asolution seamless implemented the DC/DC of two-section solar inverter applications as wellas the reference of stable determination for the cascades between other levels.To feed solar DC into grid necessitating the inverter to convert DC to AC, the invertermust control its output current in synchronized with grid voltage of the same frequency andphase as well as unity power factor. The dissertation proposes current tracking control, gridvoltage feed-forward control and two close loop control strategy in which PI constant controlthe DC side voltage in front of inverting, compares and analyzes the external characteristics ofproportional resonant control and PI control as regulating current inner loop, according to thesimulation results PR control is adopted as realizing the output current inner loop withnon-static error and reduces effect of the grid frequency displacement. The grid voltagefeed-forward may eliminate the output current distortion resulting from grid voltage distortionor fluctuation, thus sharpening the power quality and anti-disturbing ability of grid connectedPV. The mathematical model of the control system is built. Related loop parameters designprocess is given, also single-phase inverter feeding grid simulation model established whichkeeps output current with the grid of the frequency/phase synchronism realized by using theDSP DPLL. The simulation results show that ideal sinusoidal output current is producedwhile avoiding the actual grid voltage distortion and spike, abrupt changing of solar arrayvoltage etc. Based on the grid connecting control strategy on an experimental prototype viaDSP digital control the inverter can generate electricity to the grid with a high power factor,having fast dynamic response, strong robustness with a high tracking precision, fast response,small THD of less than3%. Both simulation results and experimental waveforms verify thefeasibility and correctness of the control scheme.Finally the essential common technology of grid connected photovoltaic generation isstudied on issues of instability and controlling the electricity quality when generating electricity at present, an novel vary step plus perturbation and observation method as theMPPT algorithm is analyzed and verified through the experiment, which indicating thedesigned MPPT implementation realizes the fast tenacious tracking the maximum power point.The principle of islanding detection method is analyzed in detail, a combined scheme isproposed of the over/under-voltage, over/under-frequency of the passive form islandingdetection, active one and the phase shifting one, uses DSP as numerical control leading to thebig simplification of realizing the inverter islanding detection, which is completed on DSPcontrol including the island protect electric circuit and software design, experimental resultsshow that rapid, reliable islanding detection scheme in this dissertation brings out themaximum detection time required being far less than the international standards, the NDZ isvery small. Based on the above theoretical analysis and research, the1500Wnon-isolation/isolation grid-connected solar inverter is carried on, and testing experimentplatform built, the software design procedure of the system in CCS development environmentis given, the direct component examination and EMC experiment are conducted and obtainedthe authentication catering for international standards, the abundant practical experimentalresults obtained confirm the feasibility and correctness of theoretical analysis and design inthis dissertation.