| In recent years,along with the development of economic and the advancement of industrial revolution,energy and environmental issues are also increasingly highlighted,all countries in the world are facing increasingly serious energy and environmental pressures,the utilization of the clean and green new energy has become the key breakthrough and the only way to solve the contradiction between development and energy problems.Solar energy is acknowledged as the mainstream direction of the development of the 21st century new energy because of its large reserves,installation convenience,and flexible size.The quantity of distributed photovoltaic system and grid-connected operation scale is increasing.With the continuous development of photovoltaic systems and photovoltaic component technology innovation,the photovoltaic inverter is also been put forward higher requirement.The three-level inverter has become the research focus in the experts and scholars because of its many advantages such as the good waveform quality and small switch stress,which has been widely used in the field of high voltage power,however,it still exist in the neutral voltage balance problems of insufficient.Hybrid clamped three-level photovoltaic inverter is a combination of the advantages of diode-clamped and the flying-capacitor topology,which is not influenced by load,do not need to test the capacitor voltage and closed-loop balance control and can realize the midpoint voltage self-balance.It can be applied to the active and reactive power transformation.In view of the hybrid clamped three-level inverter,this paper carry on research and system design from the following several aspects:Firstly,the topological charge and discharge circuits of capacitance voltage of the hybrid clamped three-level inverter are been analyzed comprehensively and the clamping capacitance charging and discharging rules are been summarized systematically,then a kind of hybrid clamped topology modulation strategy based on capacitance self-balancing mechanism is put forward,realizing the capacitor voltage of hybrid clamped three-level inverter balance,improving the quality of the output waveform.And then,considering the particularity and simplification of hybrid clamped three-level topology on the neutral voltage balance control,Z source network is combined with hybrid clamped three-level inverter.Problems between the addition of shoot-through state and the characteristics of clamping capacitance voltage control are been analyzing,what' s more,a kind of control strategy which can realize Z source booster and neutral voltage self-balancing control is been put forward.When combining the hybrid clamped three-level topology with the circulation of inverter in parallel,it can avoid the coupling problems between three-level neutral voltage balance and circulation control in small vector selection,realizing the decoupling control of both.Simulation results validate the theoretical analysis.Furthermore,this paper introduces the working principle of three-phase inverter to connect to the gird,analyzes the principle of digital phase-locked loop,the acquisition of power grid frequency and phase to achieve the grid-connected operation at unit power factor.The concept and the mechanism of island phenomenon in the photovoltaic power generation system are introduced.Several typical passive and active islanding detection methods are analyzed and their advantages and disadvantages are discussed.To solve the contradiction between island detection effect and the quality of waveform,an improved active frequency offset island detection strategy,the improved strategy has the advantages of low harmonic distortion rate,and small detection blind area and quickness.Finally,a hybrid clamped three-level inverter based on TMS320F28335DSP and EP4CE15F17C8FPGA is designed,including hardware circuit and software program.Using this platform the reliability and validity of the control strategy proposed in this paper are verified through the experiment. |
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