Study On Related Technologies Of Grid-connected Photovoltaic Power System

The world photovoltaic (PV) industry and market have been accessing into arapid period under the pressure of severe energy and human ecology environmentsituation. With the increasing of the PV battery production year by year, PV powergeneration has become the new tendency of the usage of renewable energy. Therefore,the study of related technologies of grid-connected PV power generation has a veryimportant theoretical significance and practical utility. In this paper, grid-connectedPV power generation system is taken as the research object, and deep study has beenmade in the following aspects: the build and control of PV grid-connected systemmodel and its influence and countermeasure of distribution network voltage quality,the fault characteristics and protection scheme of the inverters in PV and otherdistributed generation (DG) grid-connected system, the forecast of the short-termgeneration power of PV power system, and the out-of-step protection in the PV andother DG grid-connected system.The method of external characteristic equivalence is used to deduce themathematical model of high capacity PV source. Aiming to the problems of step fixedand power loss of traditional perturbation and observation (P&O) algorithm, animproved P&O algorithm of the maximum power point tracking (MPPT) is proposed.The design process of LCL filter are introduced and compared with the performanceof L and LC filters. Gird-connection control of the monopolar three-phase voltagesource bridge type inverter is realized by using the synchronous PI dual-loop controlstrategy. On the basis, the high capacity grid-connected PV generation system modelis built to provide an effective platform for the research of other chapters. At the sametime, the voltage fluctuation, voltage distribution and harmonics is deeply researchedunder the condition of distribution network with DG accessed.The grid-connected inverter, whose operating condition affects not only its ownservice life but also the stability, security, and high efficiency of the whole PVgrid-connected system, is the key apparatus of energy transfer and control when PV orother DG is connected to power system. In this paper, switch function is used toanalysis the direct current (DC) side current fault features of grid-connected inverterwhen short circuit or open circuit occurs in the inner power switch tube, orasymmetric short-circuit happens at the outlet of the grid-connected AC side.Changing regularity of the base wave and second harmonics magnitude in DC sidecurrent under different faults is deeply studied. After the above work, a new inverter protection scheme is proposed which adds base wave over current protection andsecond harmonics over current protection to the existing inverter protection device.The new scheme needs no extra hard ware cost and can reflect all kinds of faults ingrid-connected inverter comprehensively, agilely and quickly.Accurate prediction of the PV source generated power in a specified period hasfundamental significance for the power network planning, economic dispatch,electricity trade and optimal combination of DG operation and management. In thispaper, influence of weather elements to generated power is analyzed. Furthermore, PVgeneration power forecasting model based on BP neural network and recurrent neuralnetwork (RNN) is proposed, and the design methods of neural network nodes numberin each layer, model assessment, learning algorithm and other aspects are introduced.On the basis, considering the problem that randomness of neural network initialweights and threshold values, slow convergent ability, and local optimum, theglobally convergent ability and strong robustness of chaos adaptive particle swarmoptimization (CAPSO) algorithm are used in the initial design of RNN, andgenerating power forecasting model based on the combination of CAPSO algorithmand RNN is proposed. Forecasting results of the former three models are analyzed andcompared respectively.More and more PV or other DG sources have connected to power grid, whichbring certain effect to the operation stability of power system. Aiming to theout-of-step problem exists in PV and other DG grid-connected system, in this paper,the difference between inverter-based distributed generation out-of-step andtraditional rotating generation out-of-step is analyzed, and the apparent impedanceangle deviation of common coupling point is used to judge the out-of-step condition.Accordingly, a new out-of-step protection theory based on apparent impedance angledeviation is proposed for DG grid-connected system. Simulation results suggest thatthe proposed out-of-step criterion has a clear principle and is insensitive to theinfluence of three-phase short circuit.