Control Strategy And Stability Analysis For Grid-connected Inverters In Large-scale Photovoltaic Power Plants

The large capacity of photovoltaic grid power generation has become one of themain direction. As the scale of the photovoltaic power plants is more and more big, andfrom dozens to hundreds of megawatt, many problems must be considered. Due to theintrinsic characteristics of photovoltaic power generation and the different methods ofacceptance, large scale photovoltaic power plants will produce a great impact on thepower grid planning, safe and stable operation, power quality, etc. Based on the abovebackground, This paper mainly studied the following problem: First, the steady-stateanalysis, modeling and control technology of large photovoltaic power plants wasdesigned. Second, control strategy of grid-connected inverter under the non-ideal gridcondition for the large photovoltaic power plants was analyzed. Third, reactive powerand voltage control strategy of grid-connected inverter for the large photovoltaic powerplants was studied. Finally, stability analysis of large photovoltaic power plants for theeffect of grid impedance was considered. At the same time, the corresponding controlstrategy and analysis method have been proposed.Firstly, the research is based on the40MWp photovoltaic power plants, andtopological structure and working principle was analyzed. In addition, the equivalentcircuit model was eatablished through the analysis of the photovoltaic array, three-phasegrid-connected LCL inverter, and transmission network,. On this basis, an optimizedcurrent loop control scheme was designed based on the characteristics and requirementsof the three-phase grid-connected LCL inverter, such as the resonance problem of LCLfilter, unit or constant power factor interconnection,low switching frequency, and so on.These work laid a good foundation for subsequent issue.After that, the control strategy of grid-connected inverter under the non-ideal gridcondition was analyzed. Under grid distorted and unbalanced conditions,This paperanalysed the contradiction between the traditional positive sequence current controlscheme and active constant control scheme, and the coordinated control strategy wasproposed. In addition to, a new positive and negative sequence voltage detector waspresented. For the shortcoming of current controller under the condition of the gridfundamental frequency offset, the improved current controller was also presented. Onthis basis, the coordinated control strategy for three-phase grid-connected LCL inverterwithout a phase-locked loop was proposed. Through theoretical analysis, it is indicated that a high-quality grid current can be produced by introducing the new control strategy.In addition, the coordinated control of grid current amplitude and instantaneous activefluctuations can be achieved by adjusting the weights of negative sequence currentcomponent. Finally, the simulation and experimental results verify the effectiveness ofthe proposed control strategy.For the problem of reactive power and voltage control in the large photovoltaicpower plants, The influence of active output changes on the grid voltage stability wasfirstly analyzed, and the voltage fluctuation and the deviation caused by line impedancewas analyzed in detail. Taking an example of the standard of the photovoltaic powerplants reactive compensation technology specification, the voltage deviation, reactivepower, the control mode of grid-connected inverter, voltage regulation and the voltageregulation priority was regulated. In addition, reactive power capacity of grid-connectedinverter under different control mode was sure, and the reactive power capacityrelationship between large scale photovoltaic power plants and reactive powercompensation device was analyzed. On this basis, a perfect reactive power and voltagecontrol strategy for the large scale photovoltaic power plants was proposed based on thebasic thought of reactive power and voltage control strategy.Finally, harmonic problem of the grid-connected inverters was detected in manylarge-scale photovoltaic plants and the harmonic problem may occurred even if all thePV inverters do individually satisfy the standards. For these reasons, the effect of gridimpedance is considered, and the equivalent circuit model of three-phase grid-connectedparallel inverters with LCL filter is established based on the basic structure andoperating principle of the PV plants. According to the established equivalent circuitmodel, the influence of grid impedance in the current control loop is analyzed.Furthermore, for the effect of grid impedance in the current control loop, somepreliminary suggestions was given.