| The solar energy has several salient features,such as wide distribution,large reserves,and clean and environmentally friendly characteristics.Therefore,it has become an important way for countries all over the world to deal with energy crisis and environmental pollution.In addition,it is an important measure for our country to promote the energy revolution and energy transition.The inverter acts as the central unit for power conversion,which connects the photovoltaic(PV)panels and the utility grid.Thus,it is the core equipment for grid-connected PV power generation system to realize high efficiency,safety,and stability.Among all inverter topologies,the non-isolated three-level inverter has salient advantages of less passive devices,high power density,low switching losses of power devices,high efficiency,and low harmonics of output waveforms.Therefore,it has great prospects in PV power generation system.However,there still exist several related theories and technologies for the non-isolated three-level PV grid-connected inverter that are not yet perfect.The dc-side of the three-level inverter connects the PV panels,and its parasitic capaeitance,the inverter,and the grid form a leakage current loop.The inherent high-frequency switching transitions of power devices result in large leakage current,which seriously threatens the equipment and personal safety.The output power of PV panels has intermittent and fluctuating characteristics,which decreases the system efficiency in light load.The parallel operation of three-level inverters is an effective way to realize power matching,which however causes large high-frequency and low-frequency circulating current,serious electromagnetic interference(EMI),and further reduces the system stability.The ac-side of the three-level inverter connects the utility grid.However,the grid conditions are complicated and changeable,and faults frequently occur,which causes serious nonlinear dynamic coupling between the inverter and the grid.It is difficult for the conventional control methods to realize precise tracking control of grid currents and effective suppression of neutral-point voltage oscillations,which easily cause fault ride-through failure and directly threatens the security of the grid.To this end,this thesis takes the non-isolated three-level PV grid-connected inverter as the research object,and the main goal is to increase the operation safety of the inverter system.Several key control technologies have been fulfilled,which include effective suppression of common-mode leakage current,accurate suppression of high-frequency and low-frequency circulating current,fast tracking control of grid current in grid fault operation,and suppression of neutral-point voltage oscillation.Several performance indexes of the inverter have been achieved,which include high safety,high efficiency,high reliability,and low harmonics.The main research contents and innovations consist of the following four points.1.In order to solve the problem of common-mode leakage current in non-isolated three-level PV inverter system,this thesis proposes a novel three-level inverter topology based on the modified LCL filter,which directly connects the common point of filter capacitors and dc-link neutral-point.Doing so,the high-frequency leakage current is effectively suppressed,and its magnitude is nearly zero.However,this new topology results in the problem of common-mode resonance current,which degrades grid current quality and even shuts down the inverter system for over current protection.However,the generating mechanism of common-mode resonance current resulted from this topology is not clear.Moreover,three control objectives are mutually coupled,which include common-mode resonance current,differential-mode resonance current and neutral-point voltage balance.It is difficult for conventional methods to solve these problems.To this end,this thesis establishes the mathematical model of the common-mode resonance current,which reveals that the common-mode resonance current is directly related to the common-mode voltage.Then,a common-mode current closed-loop control scheme is proposed,and the dwell time of redundant small vectors is adjusted to control the common-mode voltage,suppress the common-mode resonance current,guarantee the current quality,and improve the system security and stability.Additionally,the capacitor voltage feed-forwarded control strategy is used to suppress the differential-mode resonance,and the proportional controller is employed to control the neutral-point voltage balance.The grid current with good quality is guaranteed in this way.2.The parallel operation is a good solution to realize flexible power regulation and solve the problem of low efficiency in light load for the three-level inverter system.Moreover,it is convenient for modular design,and the production cycle and cost are also reduced.However,parallel operation of three-level inverters causes serious problems of high-frequency and low-frequency circulating currents,which further lead to distortion and resonance of grid currents and even damage the power switches.The parallel three-level inverters with modified LCL filters can effectively suppress the high-frequency circulating current,which however result in harmful resonance circulating current.To this end,this thesis establishes the circulating current model of parallel three-level inverters with modified LCL filters,and the mechanism of high-frequency circulating current,internal and external resonance circulating current is subsequently revealed,which indicate that the resonance circulating current is affected by the capacitor current and the common-mode voltage.On this basis,a novel control scheme for capacitor circulating current is proposed,which adjusts the magnitude of commons-mode voltage in resonant frequency through a closed-loop form,and the effective suppression of resonance circulating current is realized.What's more,the control schemes for low-frequency circulating current and differential-mode resonance are adopted,and the parallel system is operated with low harmonics,high safety,high reliability,and high stability.3.As large-scale power electronics equipments are connected to the utility grid,the inertia of the utility grid is significantly reduced,and the faults occur frequently,which seriously affects the stable operation of PV grid-connected inverters.In order to improve the performance of PV inverter,the national standard stipulates that the PV inverter must meet the requirements of low voltage ride through(LVRT)in case of grid faults.However,when grid faults occur,there exists nonlinear dynamic coupling between the three-level inverter and the utility grid,which causes serious consequences,such as failure for the fault ride-through of the system.To solve this critical problem,this thesis establishes the power model of three-level PV grid-connected inverter under grid faults.A new real-time calculation method for reference currents without phase locked loop(PLL)is proposed,and the reference values grid currents are optimized according to the fault types of grid voltages and the requirements of fault ride-through guideline.Besides,the deadbeat-based current tracking control strategy with fast responses is employed,which reduce the dynamic response time and improve the accuracy of grid current tracking.4.In LVRT operation,the three-level inverter must deliver reactive power to the utility grid,and the system power factor is extremely low.In addition,the output voltage vector of the inverter changes,and the conventional neutral-point voltage balance methods have uncontrolled areas.To address this critical problem,this thesis proposes a new concept named small vector force(SVF)and a selective space vector modulation(SSVM)scheme,which selects the effective small vector set to mitigate the neutral-point voltage oscillations according to the location of output voltage and the directions of three-phase output currents.Then the SVF values of effective small vectors are utilized to determine the optimal small vector,and the corresponding switching sequence is designed.Doing so,the neutral-point voltage oscillation is effectively suppressed in all regions.In addition,the control method to mitigate the dc offset in neutral-point voltage is adopted to actively control the neutral-point voltage balance.The proposed method improve the safty and reliability of inverter in LVRT operation.To summarize,four main research contents for the three-level PV inverter are carried out in this thesis,which include simultaneous suppression of leakage current and common-mode resonance current,suppression of high-frequency,low-frequency circulating current,and resonance circulating current,tracking control method for grid currents without PLL,and neutral-point voltage balance control scheme in fault ride-through operation.The research results with great innovations are achieved,which lay a solid foundation to guarantee the safe,high-efficient,stable operation,and large-scale applications of three-level PV inverters. |
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