| In order to deal with the energy crisis,environmental pollution and reduce carbon emissions,China has put forward the strategic goal of "carbon peak and carbon neutrality".In the energy field,the renewable energy power generation technology replacing the traditional coal power generation is the main way to achieve the "carbon peak and carbon neutrality" goal.Photovoltaic(PV)power generation system has become an important means to achieve this goal because of its clean energy properties.To this end,the T-type three-level photovoltaic grid-connected inverter is taken as the research object to solve the key technical problems such as grid-connected current control strategy,zero-sequence circulating current suppression strategy,adaptive resonance suppression strategy.The purpose is to meet the performance requirements of PV gridconnected inverter such as "high efficiency,modularization,low grid-connected current harmonics,strong adaptability",and lay a solid theoretical foundation for further promoting the progress of photovoltaic power generation technology.The main research contents consist of the following four points.1.The advantages and disadvantages of the existing grid-connected current control strategies of three-phase PV inverter are analyzed.In view of the separate processing of the grid-onnected current control and the LCL filter resonance suppression of the existing current control strategies,the whole control algorithm is more complex and requires a large amount of calculation.A DualDivision-Summation(D-D-Σ)based grid-connected current control strategy is proposed for LCLtype three-phase T-type three-level grid connected inverter in this thesis,which realized the integration of grid-connected current control and resonance suppression.Through the division and summation process of inductors current on both sides of LCL filter,the duty cycle expression considering grid-connected current tracking and resonance suppression is derived.The proposed D-D-Σ grid-connected current control strategy has the advantages of independent control of each phase,considering grid-connected current tracking and harmonic suppression,reducing the amount of calculation,and fast dynamic response.Through experiment results,the validity of LCL-type three-phase T-type three-level grid-connected inverter current control strategy based on D-D-Σ method is verified.2.The grid impedance cannot be ignored because of low short circuit in weak grid,which is changed with the load fluctuation.The changes of the grid impedance and the accuracy of the filter inductor will affect the resonant frequency offset,but the accuracy of the filter inductor is often ignored.In view of this,an adaptive resonance suppression strategy based on D-D-Σ gridconnected current control strategy for LCL-type three-phase T-type three-level grid-connected inverter is proposed in this thesis,which considering the resonance frequency offset caused by the changes of filter inductance and grid impedance.The proposed adaptive resonance suppression strategy uses an improved Routh Criterion with less computation to obtain the control gain range that meets the stability requirements.The results show that the control gain range is related to the filter inductance and the grid impedance.Therefore,this paper proposes an online estimation method of full inductance based on Extended Kalman Filter(EKF),and simultaneously estimates the changes of filter inductance and grid impedance.The feasibility of adaptive resonance suppression strategy based on D-D-Σ grid-connected current control strategy for LCL-type three-phase T-type three-level grid-connected inverter is verified through experiment results.3.Parallelled operation of multiple modular grid-connected inverters is an effective means to improve the capacity and power generation efficiency of the PV power generation system when a single grid-connected inverter does not meet the power generation capacity.Aiming at the problems such as high Total Harmonic Distortion(THD)of grid-connected currents of parallel grid-connected inverters system with the same capacity under low load rate,dynamic circulating current during inverter startup and shutdown and power mutation,and the influence of filter inductance accuracy on control effect,a Multiple Hybrid Power Parallel Inverters(MHPPIs)structure is proposed,which considers both the generation efficiency and the grid-connected current THD at low load rate.Based on MHPPIs,a stable and dynamic zero-sequence circulating current suppression strategy based on virtual impedance is proposed.a real-time online estimation of filter inductance based on Model Reference Adaptive System(MRAS)is introduced to eliminate the influence of filter inductance accuracy on control effect.The correctness of MHPPIs circuit structure,stable and dynamic zero-sequence circulating current suppression strategy based on virtual impedance and MRAS-based on-line estimation of filter inductance parameter are verified by experiments.4.The photovoltaic power station is assumed as an inverter cluster system with MHPPIs as the unit.At this time,a MHPPIs unit can be equivalently regarded as an inverter unit.When N inverter units with the same parameters are simultaneously incorporated into the Point of Common Coupling(PCC),the equivalent grid impedance of the inverter unit will be expanded by N times,which will lead to more serious resonance frequency offset.Using the derivative function of logarithmic amplitude frequency characteristic curve,this paper analyzes the resonant characteristics of inverter cluster under weak grid in detail.Aiming at the resonance characteristics after the equivalent impedance is enlarged by N times,a resonance suppression method based on D-D-Σ grid-connected current control for inverter cluster under weak grid is proposed.The control gain of the proposed resonance suppression strategy is obtained by using the improved Rouse criterion,which can consider the wider range of resonance frequency offset caused by grid impedance variation.The effectiveness of the resonance suppression strategy based on D-D-Σ grid-connected current control for inverter cluster under weak grid is verified by experiments.To summarize,this paper focuses on four aspects of in-depth research: grid-connected current control strategy of single inverter,adaptive resonance suppression strategy under weak grid,stable and dynamic zero-sequence circulating current suppression strategy under MHPPIs circuit structure,and resonance suppression strategy of inverter cluster under weak grid.Some fruitful research results have achieved to provide theoretical support for ensuring the safe and stable operation of the three-phase parallel grid-connected inverters system and promoting the goal of " carbon peak and carbon neutrality ". |
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