Among all the renewable energy sources,the installed capacity of solar power generation is the fastest growing in recent years,so photovoltaic(PV)power generation still has great market potential.Compared with low-pow...Among all the renewable energy sources,the installed capacity of solar power generation is the fastest growing in recent years,so photovoltaic(PV)power generation still has great market potential.Compared with low-power systems,large-scale PV systems are more commercially attractive,because they can reduce the cost of the system per watt.The PV inverters with centralized and string structure have been applied in large-scale PV plant,but it is difficult to further increase the voltage and power levels for a single converter.In addition,the line-frequency isolation transformer requires a large amount of materials and has a large volume and weight.Therefore,it is a current trend for large-scale PV system to increase the voltage and power levels to directly connect to the medium-voltage power grid.Based on this,this paper investigates and compares several topologies of PV inverters without line-frequency transformer,including the MMC structure and the three-phase cascaded H-bridge(CHB)structure,which are able to directly connect to the 35kV medium-voltage power grid,and can not only make the voltage and power levels higher,but also further reduce the cost and volume of the whole system.展开更多
Substantial usage of electronic-based renewable energy resources has completely changed the dynamic behaviours and response time of power networks,which are now fundamentally different from traditional power networks ...Substantial usage of electronic-based renewable energy resources has completely changed the dynamic behaviours and response time of power networks,which are now fundamentally different from traditional power networks dominated by Synchronous Generators(SGs).This paper evaluates the dynamic response of small-scale Photovoltaic(PV)inverters,which dominate the distribution networks and influence the dynamics of the entire power grid.Recently,some critical events which occurred in Australia have shown that the dynamic responses of small-scale inverters do not always follow the inverter standards.Subsequently,these uncertainties make PV inverters’response unpredictable and have the potential to threaten the security of power networks.The detailed investigation of the dynamic response characteristics of small-scale PV inverters to grid disturbances is lacking in the current literature.This paper presents new findings from experimental testing under extensive network disturbance scenarios.Furthermore,a datadriven method is proposed to accurately describe the dynamics of solar PV subjected to various frequency disturbances.The results provide beneficial insight to the network operators in predicting power system response to extreme disturbances and avoiding potential grid instability issues,which will assist in achieving 100%penetration of power electronics-based renewable energy resources in the future.展开更多
This paper proposes a grid-tied photovoltaic(PV)inverter capable of low-voltage ride through(LVRT), reactive power support, and islanding protection. Unlike other LVRT inverters, the proposed inverter is independent o...This paper proposes a grid-tied photovoltaic(PV)inverter capable of low-voltage ride through(LVRT), reactive power support, and islanding protection. Unlike other LVRT inverters, the proposed inverter is independent of sag severity while maintaining the maximum power-point tracking(MPPT)under normal and faulty conditions. The addition of an energy storage buffer stage mitigates the DC-link voltage surge during sags. At the same time, the inverter injects the reactive power during back-to-back sags of variable depths. The control system of the inverter generates the appropriate reference signals for normal, LVRT, and anti-islanding modes while the MPPT continues running. The salient features of the proposed inverter are:(1) active power injection under normal grid conditions;(2)sag-depth independent LVRT with reactive power support;(3)no DC-link fluctuations;(4) continuous MPPT mode;and(5) simultaneous LVRT and anti-islanding support during a grid outage. The inverter demonstrates an uninterrupted operation and seamless transition between various operating modes. Simulations and the experimental prototype have been implemented to validate the efficacy of the proposed PV inverter.展开更多
Under the partial shading conditions(PSC)of Photovoltaic(PV)modules in a PV hybrid system,the power output curve exhibits multiple peaks.This often causes traditional maximum power point tracking(MPPT)methods to fall ...Under the partial shading conditions(PSC)of Photovoltaic(PV)modules in a PV hybrid system,the power output curve exhibits multiple peaks.This often causes traditional maximum power point tracking(MPPT)methods to fall into local optima and fail to find the global optimum.To address this issue,a composite MPPT algorithm is proposed.It combines the improved kepler optimization algorithm(IKOA)with the optimized variable-step perturb and observe(OIP&O).The update probabilities,planetary velocity and position step coefficients of IKOA are nonlinearly and adaptively optimized.This adaptation meets the varying needs of the initial and later stages of the iterative process and accelerates convergence.During stochastic exploration,the refined position update formulas enhance diversity and global search capability.The improvements in the algorithmreduces the likelihood of falling into local optima.In the later stages,the OIP&O algorithm decreases oscillation and increases accuracy.compared with cuckoo search(CS)and gray wolf optimization(GWO),simulation tests of the PV hybrid inverter demonstrate that the proposed IKOA-OIP&O algorithm achieves faster convergence and greater stability under static,local and dynamic shading conditions.These results can confirm the feasibility and effectiveness of the proposed PV MPPT algorithm for PV hybrid systems.展开更多
Impedance analysis is an effective method to analyze the oscillation issue associated with grid-connected photovoltaic systems.However,the existing impedance modeling of a gridconnected photovoltaic inverter usually o...Impedance analysis is an effective method to analyze the oscillation issue associated with grid-connected photovoltaic systems.However,the existing impedance modeling of a gridconnected photovoltaic inverter usually only considers the effect of a single perturbation frequency,ignoring the coupling frequency response between the internal control loops of a grid-connected inverter,which severely affects the accuracy of the stability analysis.Hence,a method of impedance modeling and stability analysis for grid-connected photovoltaic inverters considering cross-coupling frequency is proposed in this paper.First,the generation mechanism of frequency coupling in gridconnected photovoltaic inverters,and the relationship between the coupling frequency and perturbation frequency are analyzed.Secondly,a sequence impedance model of grid-connected photovoltaic systems considering the coupling frequency is established by using the harmonic linearization method.The impact of DC bus voltage control strategy on frequency coupling characteristics of a grid-connected photovoltaic system is evaluated,and the impact of a coupling frequency term on system stability is quantitatively analyzed.Finally,the advantages of the proposed method are verified by several simulations.The results show that the proposed impedance model can accurately predict the potential resonance points of the system,and the coupling frequency characteristics will become much stronger with smaller DC bus capacitance or larger bandwidth of the DC bus controller.展开更多
Photovoltaic(PV)inverters have been widely used in large-scale PV power generation systems to reduce the system mismatch and increase the output power.With the increasing installed capacity of PV power stations,the nu...Photovoltaic(PV)inverters have been widely used in large-scale PV power generation systems to reduce the system mismatch and increase the output power.With the increasing installed capacity of PV power stations,the number of string PV inverters increases,which brings about risk of inverter resonance and influences the quality and stability of multi-parallel inverters grid-connected system.Thus,the problems of multi-parallel inverters resonance have become a hot research topic.This paper is focused on the case of grid-connected string PV inverter systems,and the grid-connected PV inverter resonance where resonance suppression strategy will be analyzed and discussed.Firstly,the structure of string PV inverter and multi-parallel inverters system are presented.Secondly,discussions are made about the model of multi-parallel inverters system and its resonance characteristics based on the multi-input-multi-output(MIMO)system,Norton equivalent circuit and carrier synchronization.Thirdly,two strategies of^inverter level"and"system level"are provided to summarize the multi-parallel inverters resonance suppression strategy.Finally,the development tendency of resonance suppression strategy and conclusions are prospected.展开更多
The integration of photovoltaic (PV) systems into weak-grid environments presents unique challenges to the stability of grid-connected inverters. This review provides a comprehensive overview of the research efforts f...The integration of photovoltaic (PV) systems into weak-grid environments presents unique challenges to the stability of grid-connected inverters. This review provides a comprehensive overview of the research efforts focused on investigating the stability of PV grid-connected inverters that operate under weak grid conditions. Weak grids are characterized by a low short-circuit capacity and low inertia, making it essential to explore strategies that enhance the stability and performance of inverters in such challenging environments. This review covers various aspects, including control strategies and advanced technologies implemented to address stability problems. The research findings related to the impact of weak grid conditions on PV inverters, modeling techniques, and analysis results are discussed. Additionally, this review highlights emerging trends, identifies gaps in the current research, and suggests potential avenues for future investigations aimed at improving the stability of PV grid-connected inverters in weak grid scenarios.展开更多
基金This work was supported by National Natural Science Foundation of China(51937003).
文摘Among all the renewable energy sources,the installed capacity of solar power generation is the fastest growing in recent years,so photovoltaic(PV)power generation still has great market potential.Compared with low-power systems,large-scale PV systems are more commercially attractive,because they can reduce the cost of the system per watt.The PV inverters with centralized and string structure have been applied in large-scale PV plant,but it is difficult to further increase the voltage and power levels for a single converter.In addition,the line-frequency isolation transformer requires a large amount of materials and has a large volume and weight.Therefore,it is a current trend for large-scale PV system to increase the voltage and power levels to directly connect to the medium-voltage power grid.Based on this,this paper investigates and compares several topologies of PV inverters without line-frequency transformer,including the MMC structure and the three-phase cascaded H-bridge(CHB)structure,which are able to directly connect to the 35kV medium-voltage power grid,and can not only make the voltage and power levels higher,but also further reduce the cost and volume of the whole system.
文摘Substantial usage of electronic-based renewable energy resources has completely changed the dynamic behaviours and response time of power networks,which are now fundamentally different from traditional power networks dominated by Synchronous Generators(SGs).This paper evaluates the dynamic response of small-scale Photovoltaic(PV)inverters,which dominate the distribution networks and influence the dynamics of the entire power grid.Recently,some critical events which occurred in Australia have shown that the dynamic responses of small-scale inverters do not always follow the inverter standards.Subsequently,these uncertainties make PV inverters’response unpredictable and have the potential to threaten the security of power networks.The detailed investigation of the dynamic response characteristics of small-scale PV inverters to grid disturbances is lacking in the current literature.This paper presents new findings from experimental testing under extensive network disturbance scenarios.Furthermore,a datadriven method is proposed to accurately describe the dynamics of solar PV subjected to various frequency disturbances.The results provide beneficial insight to the network operators in predicting power system response to extreme disturbances and avoiding potential grid instability issues,which will assist in achieving 100%penetration of power electronics-based renewable energy resources in the future.
基金supported by the Program Research Grant UMPEDAC-2020(No. MOHE HICOE-UMPEDAC)the Ministry of Education Malaysia (No.RU003-2020, RU002-2021)the University of Malaya。
文摘This paper proposes a grid-tied photovoltaic(PV)inverter capable of low-voltage ride through(LVRT), reactive power support, and islanding protection. Unlike other LVRT inverters, the proposed inverter is independent of sag severity while maintaining the maximum power-point tracking(MPPT)under normal and faulty conditions. The addition of an energy storage buffer stage mitigates the DC-link voltage surge during sags. At the same time, the inverter injects the reactive power during back-to-back sags of variable depths. The control system of the inverter generates the appropriate reference signals for normal, LVRT, and anti-islanding modes while the MPPT continues running. The salient features of the proposed inverter are:(1) active power injection under normal grid conditions;(2)sag-depth independent LVRT with reactive power support;(3)no DC-link fluctuations;(4) continuous MPPT mode;and(5) simultaneous LVRT and anti-islanding support during a grid outage. The inverter demonstrates an uninterrupted operation and seamless transition between various operating modes. Simulations and the experimental prototype have been implemented to validate the efficacy of the proposed PV inverter.
基金funding from the Graduate Practice Innovation Program of Jiangsu University of Technology(XSJCX23_58)Changzhou Science and Technology Support Project(CE20235045)Open Project of Jiangsu Key Laboratory of Power Transmission&Distribution Equipment Technology(2021JSSPD12).
文摘Under the partial shading conditions(PSC)of Photovoltaic(PV)modules in a PV hybrid system,the power output curve exhibits multiple peaks.This often causes traditional maximum power point tracking(MPPT)methods to fall into local optima and fail to find the global optimum.To address this issue,a composite MPPT algorithm is proposed.It combines the improved kepler optimization algorithm(IKOA)with the optimized variable-step perturb and observe(OIP&O).The update probabilities,planetary velocity and position step coefficients of IKOA are nonlinearly and adaptively optimized.This adaptation meets the varying needs of the initial and later stages of the iterative process and accelerates convergence.During stochastic exploration,the refined position update formulas enhance diversity and global search capability.The improvements in the algorithmreduces the likelihood of falling into local optima.In the later stages,the OIP&O algorithm decreases oscillation and increases accuracy.compared with cuckoo search(CS)and gray wolf optimization(GWO),simulation tests of the PV hybrid inverter demonstrate that the proposed IKOA-OIP&O algorithm achieves faster convergence and greater stability under static,local and dynamic shading conditions.These results can confirm the feasibility and effectiveness of the proposed PV MPPT algorithm for PV hybrid systems.
文摘Impedance analysis is an effective method to analyze the oscillation issue associated with grid-connected photovoltaic systems.However,the existing impedance modeling of a gridconnected photovoltaic inverter usually only considers the effect of a single perturbation frequency,ignoring the coupling frequency response between the internal control loops of a grid-connected inverter,which severely affects the accuracy of the stability analysis.Hence,a method of impedance modeling and stability analysis for grid-connected photovoltaic inverters considering cross-coupling frequency is proposed in this paper.First,the generation mechanism of frequency coupling in gridconnected photovoltaic inverters,and the relationship between the coupling frequency and perturbation frequency are analyzed.Secondly,a sequence impedance model of grid-connected photovoltaic systems considering the coupling frequency is established by using the harmonic linearization method.The impact of DC bus voltage control strategy on frequency coupling characteristics of a grid-connected photovoltaic system is evaluated,and the impact of a coupling frequency term on system stability is quantitatively analyzed.Finally,the advantages of the proposed method are verified by several simulations.The results show that the proposed impedance model can accurately predict the potential resonance points of the system,and the coupling frequency characteristics will become much stronger with smaller DC bus capacitance or larger bandwidth of the DC bus controller.
基金the key program of National Natural Science Foundation of China under Grant 51277051。
文摘Photovoltaic(PV)inverters have been widely used in large-scale PV power generation systems to reduce the system mismatch and increase the output power.With the increasing installed capacity of PV power stations,the number of string PV inverters increases,which brings about risk of inverter resonance and influences the quality and stability of multi-parallel inverters grid-connected system.Thus,the problems of multi-parallel inverters resonance have become a hot research topic.This paper is focused on the case of grid-connected string PV inverter systems,and the grid-connected PV inverter resonance where resonance suppression strategy will be analyzed and discussed.Firstly,the structure of string PV inverter and multi-parallel inverters system are presented.Secondly,discussions are made about the model of multi-parallel inverters system and its resonance characteristics based on the multi-input-multi-output(MIMO)system,Norton equivalent circuit and carrier synchronization.Thirdly,two strategies of^inverter level"and"system level"are provided to summarize the multi-parallel inverters resonance suppression strategy.Finally,the development tendency of resonance suppression strategy and conclusions are prospected.
基金Supported by the National Natural Science Foundation of China under Grants 52207211,U2166601,51937003,U23A20655by the Fundamental Research Funds for the Central Universities under Grants JZ2024HGTB0259.
文摘The integration of photovoltaic (PV) systems into weak-grid environments presents unique challenges to the stability of grid-connected inverters. This review provides a comprehensive overview of the research efforts focused on investigating the stability of PV grid-connected inverters that operate under weak grid conditions. Weak grids are characterized by a low short-circuit capacity and low inertia, making it essential to explore strategies that enhance the stability and performance of inverters in such challenging environments. This review covers various aspects, including control strategies and advanced technologies implemented to address stability problems. The research findings related to the impact of weak grid conditions on PV inverters, modeling techniques, and analysis results are discussed. Additionally, this review highlights emerging trends, identifies gaps in the current research, and suggests potential avenues for future investigations aimed at improving the stability of PV grid-connected inverters in weak grid scenarios.