Research On High-Quality Grid-Connected Modulation And Control Strategy Of Photovoltaic Inverter System Under Complicated Conditions

Solar photovoltaic(PV)power generation has broad development prospects because it is not limited by energy resources,raw materials and application environment.It is an effective way to solve the traditional fossil energy crisis and environmental pollution,and will gradually become the leading energy in the 21st century.As the core equipment of energy conversion and control in PV power generation system,inverter is very important for the efficient,safe and stable operation of the whole power generation system.Among all inverter topologies,the transformerless three-level PV inverter has the advantages of high output waveform quality,low switching loss,small device voltage stress and high system efficiency,which has great development potential in the field of PV grid-connected inverter.However,due to the high proportion of renewable energy and large-scale power electronic equipment access to the grid,the complex operating environment of PV panels and the connection form of PV array,the transformerless three-level PV inverter will encounter many complicated conditions during operation,such as power grid fault on the ac side,large leakage current during grid-connected operation without transformer,the unbalanced dc-links,etc.If no effective control measures are taken,the above complicated conditions will easily lead to grid failure,equipment damage,and even threaten personal safety,which will bring severe challenges to the safe and stable operation of inverter and power system.However,the relevant theory and key control technology of transformerless three-level PV inverter under the above complicated conditions are not well-solved,which seriously restricts its largescale application.Specifically,on the ac side,grid faults occur regularly,which lead to grid voltage dip.Therefore,the grid code requires that the inverter must have lowvoltage ride-through(LVRT)capability.But the conventional methods have many disadvantages,such as more control parameters,complex controller design or slow dynamic response,which is difficult to meet the LVRT requirements for fast and accurate grid-connected current.This will cause the inverter to fail to operate normally,or damage the inverter equipments,which is easy to result in LVRT failure.Furthermore,for the three-level PV inverter,its inherent neutral point(NP)voltage imbalance makes the output current distorted and the electromagnetic interference increased.Especially when the grid voltage dips occur,the NP voltage fluctuation intensifies with the decrease of power factor,which greatly increases the difficulty of NP balance control,and even damages the inverter equipment.On the dc side,a common-mode circuit is formed among the parasitic capacitance of PV panels,the inverter and the grid.The common-mode voltage output by inverter generates large leakage current in the common-mode circuit,which is very easy to exceed the leakage current limit specified by national standards and threatens the equipment and personal safety.In addition,the connection form of independent PV array is an ideal solution to improve the efficiency and increase the power output.However,the connection form will cause the upper and lower capacitor voltages to be unequal.When the upper and lower capacitor voltages on the dc side are not equal,the harmonic content of the output current will increase by using conventional modulation methods,which seriously affects the safe operation of the grid-connected system.These problems seriously restrict the development of three-level inverter and its large-scale application in PV power generation system.For these reasons,the goal of this dissertation is to improve the grid adaptability and system stability and security for the transformerless three-level PV inverter.Some high-performance modulation and control technologies are focused on,such as fast and accurate tracking control of grid current in LVRT operation,the NP voltage balance control in full power factor range,simultaneous suppression of leakage current and common-mode resonant current,and modulation method of low harmonic current under unbalanced dc-links.The expected performance indices of the system includes "high reliability,high efficiency,high safety and low harmonic".The main research contents and innovations of this dissertation are composed of the following four parts.1.In order to solve the problem of grid-connected current tracking in LVRT operation,the Euler-Lagrange model of three-level PV inverter system under grid faults condition is firstly established,and the influence mechanism of grid voltage imbalance on grid-connected current closed-loop control is revealed.Then,based on the damping injection principle,Lyapunov stability criterion,a hybrid passivity-based control method is proposed to realize the fast and accurate tracking control of gridconnected current in LVRT operation,which ensures the safe and stable operation of grid-connected inverter,and improves the grid adaptability.Finally,the system stability analysis is given to provide theoretical support for industrial application of the proposed method.2.In high-power PV power generation system,the selective harmonic elimination pulse width modulation(SHEPWM)method is the first choice to improve inverter efficiency and reduce system loss.However,the conventional optimization algorithms for solving harmonic elimination equations are computationally expensive,complex to implement,or easy to fall into the local optimal solution,leading to premature convergence.In addition,when the voltage dips occur,due to the influence of the inverter injecting reactive power into the grid,the difficulty of NP balance control increases sharply,and the grid-connected current distortion is caused.Therefore,the vectorization model of three-level selective harmonic elimination is established,and the influence mechanism of redundant small vectors and current direction on NP voltage is revealed.On this basis,the negative exponential inertia weight factor considering the current and optimal fitness values is introduced,and an improved particle swarm optimization algorithm with fast convergence speed and high accuracy is designed to solve the harmonic elimination equations and obtain the switching angles.At the same time,an enhanced NP balance control method suitable for full power factor range is proposed.By increasing the degree of freedom of output current direction,the NP voltage balance control in LVRT operation is realized,and high-quality grid-connected current is obtained.3.In transformerless three-level PV inverter system,the modified LC filter is adopted to suppress the leakage current,which is considered as a simple and effective method for the direct connection of the dc-side neutral point and the common point of filter capacitors.However,the common-mode resonant current is induced,which makes the output current of inverter seriously distorted and even affects the stability of the system.Therefore,the common-mode loop current model is established,and the internal mechanism of leakage current suppression and common-mode resonance current generation with the modified LC filter is revealed.On this basis,a secondorder time-domain passivity-based control method is proposed by using the passive control theory,zero-state detectable theory and Lyapunov stability criterion,which can effectively suppress the common-mode resonant current in the common-mode loop current.In addition,the global asymptotic stability analysis of the system is also given,which provides theoretical support for the practical application of the method.4.The connection form of independent PV array can effectively reduce the power loss caused by the mismatches of PV modules or partial shading,which is of great significance to improve system efficiency and increase power output.However,this connection form will cause the upper and lower capacitor voltages to be unequal,and the conventional modulation methods will cause serious output current distortion,which will threaten the safe operation of grid-connected system.Therefore,a new modulation method using large,medium and zero vectors to synthesize the reference voltage vector is proposed.By introducing the unbalanced factor of capacitor voltage,the position of the basic voltage vectors is corrected,and the new space voltage vector diagram and sector judgement criteria are obtained.Based on the volt-second balance principle,the duty cycle of basic voltage vectors under unbalanced dc-links is calculated,and the switching sequence is optimized to obtain low harmonic grid-connected current.To summarize,four contents of three-level PV inverter are studied in this dissertation,including the fast and accurate tracking control of grid-connected current in LVRT operation,the NP voltage balance control of three-level inverter system in LVRT operation,the simultaneous suppression of leakage current and commonmode resonance current during grid-connected operation without transformer,and the low harmonic modulation of grid-connected current under unbalanced dc-links.Some innovative research results have been obtained,which meet the high-quality grid connection requirements of PV power generation system,and provide reference for the development and large-scale application of transformerless three-level PV inverter.