Research On Key Technologies Of Cascaded Photovoltaic Grid-Connected Inverter System

Cascade multilevel inverters have the advantages of easy modular expansion,low output voltage harmonic content,low switching frequency,and high conversion efficiency.Moreover,each H-bridge cell of the cascade system requires an independent DC power supply,it is in line with the characteristics of photovoltaic arrays power generation.However,cascaded photovoltaic grid-connected inverters still need to tackle several key technologies: power balance control technology of interphase/ interbridge,stable operation control technology under weak grid,and highly reliable,low-cost communication technology.Therefore,the main research work and innovation of this article are as follows:1)Aiming at the problem of interbridge power imbalance,three types of typical interbridge power balance control methods are first outlined,and the power balance range expressions of the three types of methods are mathematically derived.A power balance space(PBS)and power balance factor(PBF)are proposed and adopted for the qualitative and quantitative comparison of the power balance ranges of the three methods.On the basis,the advantages and disadvantages of the two hybrid pulse width modulation(HPWM)methods are further analyzed,and a power balance control method based on switched HPWM modulation is proposed.Simulation and experimental results show that the proposed method increases the power balance range while effectively suppressing the DC-side voltage fluctuations and increasing the power generation of the system.2)Aiming at the problem of interphase and interbridge power imbalance,six typical methods of interphase power balance control are first outlined,and their power balance range expressions are derived.Their power balance ranges are qualitatively and quantitatively compared by PBS and PBF.Then,the balance range of the six power balance control methods combining zero-sequence voltage injection(ZSVI)and active component modification of duty cycles(ACMDC)is further derived and compared through PBS and PBF.Based on this,six kinds of ZSVI + HPWM combined interphase and interbridge power balance control methods are proposed,and their power balance range expressions are derived.The twelve interphase and interbridge power balance control methods are compared by PBS and PBF.Simulation and experimental results show that the proposed methods greatly expand the power balance range of interphase and interbridge.3)Aiming at the problem of the stability of the grid-connected system under weak grid,the output impedance model of the cascaded grid-connected inverter including phase-locked loop and voltage feedforward is established.The influence of number of H-bridge cells,bandwidth of phase-locked loop,resonance feedforward coefficient and grid impedance on the stability of system is analyzed.Based on this,a phase angle compensation control method is adopted to enhance the stability of the grid-connected system under the weak grid.Simulation and experimental results show that the method can effectively suppress the background harmonics of the power grid while ensuring the stable operation of the system.4)Aiming at the communication problem of cascade system,the basic principles of power line communication(PLC)are first introduced,typical photovoltaic DC PLC schemes and their existing problems are analyzed,and a photovoltaic DC PLC scheme based on parallel resonant coupling technology is proposed.Experimental results show that the proposed scheme can effectively reduce the load of the coupling circuit and signal attenuation,and improve signal transmission efficiency.Based on this,a bridge-coupled AC PLC scheme for a cascaded grid-connected system is explored and researched.Simulation results show that the scheme can effectively separate PLC signals and suppress the full frequency band of noise signals.