The phase-locked loop(PLL)plays an essential role for synchronizing renewable power generation to the grid.However,as per the grid-code compliance for reactive current support,the PLL output frequency fluctuates signi...The phase-locked loop(PLL)plays an essential role for synchronizing renewable power generation to the grid.However,as per the grid-code compliance for reactive current support,the PLL output frequency fluctuates significantly and exceeds the limitation,which seriously threaten the safe supply of electricity.In this paper,the underlying theoretical mechanism and dominant force behind the maximum PLL frequency deviation are revealed.Accordingly,two feasible approaches are proposed to enhance the PLL frequency stability with validations in experimental results.展开更多
Voltage source converter based high voltage direct current transmission(VSC-HVDC)is considered one of the most suitable technologies to integrate renewable energies.However,connecting VSC to a weak grid is challenging...Voltage source converter based high voltage direct current transmission(VSC-HVDC)is considered one of the most suitable technologies to integrate renewable energies.However,connecting VSC to a weak grid is challenging since traditional vector control tends to become unstable under high power demand conditions.In this paper,an improved vector control method is proposed wherein a feed forward branch based on steady state and small signal analysis of the VSC system is added under weak grid situations.The feed forward branch promotes faster reactive power response,thus enhancing the stability of the VSC system.Since the improved vector control uses the same inner loop as traditional vector control,the proposed method allows for the ability to retain fault current suppression capabilities.Furthermore,the control parameters of the outer loop of the improved vector control need not vary according to the variation of the operating points,which makes it easy to implement.The feed forward branch is implemented by solving a nonlinear equation or through use of a look-up table.The influence of the estimation errors of short circuit ratio(SCR)on the control performance is also studied.The effectiveness of the improved vector control is demonstrated through small signal model analysis and time domain simulations.展开更多
Voltage source converter based high-voltage direct current(VSC-HVDC)transmission technology has been extensively employed in power systems with a high penetration of renewable energy resources.However,connecting a vol...Voltage source converter based high-voltage direct current(VSC-HVDC)transmission technology has been extensively employed in power systems with a high penetration of renewable energy resources.However,connecting a voltage source converter(VSC)to an AC weak grid may cause the converter system to become unstable.In this paper,a phase-shift phaselocked loop(PS-PLL)is proposed wherein a back electromotive force(BEMF)observer is added to the conventional phaselocked loop(PLL).The BEMF observer is used to observe the voltage of the infinite grid in the stationaryαβframe,which avoids the problem of inaccurate observations of the grid voltage in the dq frame that are caused by the output phase angle errors of the PLL.The VSC using the PS-PLL can operate as if it is facing a strong grid,thus enhancing the stability of the VSC-HVDC system.The proposed PS-PLL only needs to be properly modified on the basis of a traditional PLL,which makes it easy to implement.In addition,because it is difficult to obtain the exact impedance of the grid,the influence of shortcircuit ratio(SCR)estimation errors on the performance of the PS-PLL is also studied.The effectiveness of the proposed PSPLL is verified by the small-signal stability analysis and timedomain simulation.展开更多
A control strategy of repetitive control without inductorance decoupling was proposed to address the problem of high total harmonic distortion(THD)rate of the network-side current caused by the reduced stability of th...A control strategy of repetitive control without inductorance decoupling was proposed to address the problem of high total harmonic distortion(THD)rate of the network-side current caused by the reduced stability of the rectifier module of the DC charging pile under weak grid as well as the dead zone and nonlinearity of switching devices during charging.Firstly,the parallel repetitive control was constructed in the inner current loop,and the proportional-integral(PI)+repetitive controller based on parallel structure was designed.For system compensation,a second-order low-pass filter was selected to correct the system,and the network-side current harmonics were actively suppressed without increasing the filtering device,which effectively improves the quality of grid-connected current.Secondly,based on the synthetic vector method,the controller parameters were designed to realize the elimination of main pole by establishing two synchronous rotation coordinate system vector differential equations,so as to realize the inductanceless decoupling to cope with the influence of network-side inductance fluctuation on the stability of the control system under weak grid.By theoretical analysis and simulation,the proposed control strategy was embedded into the self-developed digital signal processor for the rectifier module of DC charging pile,simulated dynamic and steady-state operation experiments were conducted,and comparative analysis was performed to prove the feasibility of the proposed control strategy.展开更多
Obvious resonance peak will be generated when parallel photovoltaic grid-connected inverters are connected to the weak grid with high grid impedance, which seriously affects the stability of grid-connected operation o...Obvious resonance peak will be generated when parallel photovoltaic grid-connected inverters are connected to the weak grid with high grid impedance, which seriously affects the stability of grid-connected operation of the photovoltaic system. To overcome the problems mentioned above, the mathematical model of the parallel photovoltaic inverters is established. Several factors including the impact of the reference current of the grid-connected inverter, the grid voltage interference and the current disturbance between the photovoltaic inverters in parallel with the grid-connected inverters are analyzed. The grid impedance and the LCL filter of the photovoltaic inverter system are found to be the key elements which lead to existence of resonance peak. This paper presents the branch voltage and current double feedback suppression method under the premise of not changing the topological structure of the photovoltaic inverter, which effectively handles the resonance peak, weakens the harmonic content of the grid current of the photovoltaic grid-connected inverter and the voltage at the point of common coupling, and improves the stability of the parallel operation of the photovoltaic grid-connected inverters in weak grid. At last, the simulation model is established to verify the reliability of this suppression method.展开更多
The utilization of renewable energy in sending-end power grids is increasing rapidly,which brings difficulties to voltage control.This paper proposes a coordinated voltage control strategy based on model predictive co...The utilization of renewable energy in sending-end power grids is increasing rapidly,which brings difficulties to voltage control.This paper proposes a coordinated voltage control strategy based on model predictive control(MPC)for the renewable energy power plants of wind and solar power connected to a weak sending-end power grid(WSPG).Wind turbine generators(WTGs),photovoltaic arrays(PVAs),and a static synchronous compensator are coordinated to maintain voltage within a feasible range during operation.This results in the full use of the reactive power capability of WTGs and PVAs.In addition,the impact of the active power outputs of WTGs and PVAs on voltage control are considered because of the high R/X ratio of a collector system.An analytical method is used for calculating sensitivity coefficients to improve computation efficiency.A renewable energy power plant with 80 WTGs and 20 PVAs connected to a WSPG is used to verify the proposed voltage control strategy.Case studies show that the coordinated voltage control strategy can achieve good voltage control performance,which improves the voltage quality of the entire power plant.展开更多
Distinction of weak and strong AC grids for emerging hierarchical-infeed LCC-UHVDC systems is important for planning and operation departments. However, accuracy of earlier distinction methods is limited as they were ...Distinction of weak and strong AC grids for emerging hierarchical-infeed LCC-UHVDC systems is important for planning and operation departments. However, accuracy of earlier distinction methods is limited as they were developed by empirical reasoning without rigorous theoretical analysis. Hence in this letter, hierarchical-infeed interactive effective short-circuit ratio (HIESCR) index is first used for strength evaluation of HIDC systems with complex inter-inverter interactions considered. Boundary HIESCR (BHIESCR) is also introduced in the proposed distinction method of weak and strong AC grids. That is, weak (or strong) AC grids are, respectively, identified when HIESCR is less (or greater) than BHIESCR. Second, it is shown BHIESCR remains almost unchanged as 3.0 versus various system parameters and rated operation variables based on rigorous theoretical analysis. This salient feature makes the proposed method more accurate than earlier methods. Finally, the proposed method is validated by simulations based on the PSCAD/EMTDC program.展开更多
The grid-connected converter with grid-following control (GCC-GFL) for renewable energy sources has a risk of instability when integrated into the weak grid. This paper aims to investigate the dynamic interactions and...The grid-connected converter with grid-following control (GCC-GFL) for renewable energy sources has a risk of instability when integrated into the weak grid. This paper aims to investigate the dynamic interactions and stability characteristics of the GCC-GFL system. From a control system perspective, the mechanism of small-signal instability in the system is revealed through dynamic interaction analysis between the GCC-GFL and the weak grid. Meanwhile, a novel stability evaluation index is proposed based on the real and imaginary parts of the equivalent loop gain in a multi-loop control system. On this basis, the dominant loop of the control system leading to system instability is identified. Furthermore, quantitative analyses are conducted to investigate the stability region of the GCC-GFL, considering the influence of AC grid strength, steady-state operating points, and converter control parameters. Finally, the correctness and effectiveness of the proposed methods are verified by the impedance analysis method, the time-domain simulations, and the experiments, respectively.展开更多
Grid impedance and phase-locked loop(PLL)are critical factors for the stability of the grid-connected inverters(GCIs)in a weak grid.They are the positive feedback control loops formed by PLL in the GCI with grid imped...Grid impedance and phase-locked loop(PLL)are critical factors for the stability of the grid-connected inverters(GCIs)in a weak grid.They are the positive feedback control loops formed by PLL in the GCI with grid impedance.It is prone to GCI instability,especially in the case of the higher PLL bandwidth.A novel impedance-phase and magnitude control strategy is proposed to improve stability of GCI with different grid impedance.Moreover,a detailed design of control loop and parameter calculation for the impedance-phase and magnitude control strategy are introduced.First,PLL output impedance is reshaped to broaden the frequency range of the GCI phase-frequency characteristic curve above the−90°line towards the low-frequency band.In addition,current loop output impedance is reshaped to maintain the phase margin(PM)of the GCI near to 45°.Meanwhile,the magnitude of GCI output impedance is also increased significantly.Stability of the GCI in a weak grid is enhanced by adopting the proposed control strategy.Simulation and experimental results verify the analysis and the proposed method.展开更多
The self-synchronizing voltage source inverter(SS VSI)is widely studied because of its grid-forming capability.However,the slow response of the active power control loop(APCL)under the weak grid makes it difficult for...The self-synchronizing voltage source inverter(SS VSI)is widely studied because of its grid-forming capability.However,the slow response of the active power control loop(APCL)under the weak grid makes it difficult for the SSVSI to quickly support the frequency of a low-inertia grid.In this pa per,a grid framework is established to analyze the frequency support service process of the SSVSI,and the shortcomings of the regulation of the damping coefficient and virtual inertia co efficient for frequency support are analyzed.Then,an adaptive additional damping control method is proposed to optimize the ability of SSVSI to support the grid frequency.The proposed control method adjusts the damping of the APCL without affect ing the system steady-state characteristics,which improves the active power response speed of the SSVSI.Besides,the pro posed control method adaptively adjusts the additional damp ing coefficient based on the active power response without mea suring the grid parameters.Compared with other forms of con trol,the proposed control method excels in minimizing the rate of change of frequency(RoCoF)and the frequency deviation(FD)within the grid,without succumbing to the constraints posed by unknown grid parameters.Furthermore,the analysis of the system stability is also presented.Finally,the experimen tal hardware results obtained from a miniaturized grid proto type are presented,corroborating the effectiveness of the pro posed control method.展开更多
The maximum power transfer capability(MPTC)of phase-locked loop(PLL)-based grid-following inverters is often limited under weak-grid conditions due to passivity violations caused by operating-point-dependent control l...The maximum power transfer capability(MPTC)of phase-locked loop(PLL)-based grid-following inverters is often limited under weak-grid conditions due to passivity violations caused by operating-point-dependent control loops.This paper reveals and compares the mechanisms of these violations across different control strategies.Using admittance decomposition and full-order state-space models for eigenvalue analysis,MPTC limitations from control loops and their interactions are identified.The small-signal stabilities of different control loops are compared under varying grid strength,and both static and dynamic MPTCs for each control mode are examined.This paper also explores how control loop interactions impact the MPTC,offering insights for tuning control loops to enhance stability in weak grids.For example,fast power control improves the MPTC when paired with a slow PLL,while power control has minimal effect when the PLL is sufficiently fast.The findings are validated through frequency scanning,eigenvalue analysis,simulations,and experiments.展开更多
Under weak grid conditions,grid impedance is coupled with a control system for voltage source converter based high-voltage direct current(VSC-HVDC)systems,resulting in decreased synchronization stability.Unfortunately...Under weak grid conditions,grid impedance is coupled with a control system for voltage source converter based high-voltage direct current(VSC-HVDC)systems,resulting in decreased synchronization stability.Unfortunately,most studies are based on the assumption that impedance ratio(R/X)is sufficiently small to ignore the effects of grid impedance.In this study,we establish a dynamic coupling model that includes grid impedance and control loops,revealing the influence mechanism of R/X on synchronization stability from a physical perspective.We also quantify the stability range of R/X in the static analysis model and introduce a sensitivity factor to measure its effect on voltage stability.Additionally,we utilize a dynamic analysis model to evaluate power angle convergence,proposing a corresponding stability criterion.We then present a method of synchronous voltage reconstruction aimed at enhancing the grid strength.Theoretical analysis shows that this method can effectively mitigate the effects of coupling between grid impedance and the controller under weak grid conditions,ensuring stable operation even under extremely weak grid conditions.Experiments validate the accuracy and effectiveness of the analysis and method.展开更多
This paper proposes a robust dichotomy-based model predictive control(DS-MPC)with a fixed switching frequency for the grid-connected inverter(GCI).The proposed fast dichotomy algorithm can select and deduce the optima...This paper proposes a robust dichotomy-based model predictive control(DS-MPC)with a fixed switching frequency for the grid-connected inverter(GCI).The proposed fast dichotomy algorithm can select and deduce the optimal voltage vector dynamically through the space vector plane.Therefore,the proposed DS-MPC strategy could ensure dynamic performance and steady-state performance as well.Also,the current control robustness can be improved through DS-MPC with disturbance observer(DO)based on the extended Kalman filter(EKF).The novelty of this control is that the current control with fast dynamic response can be realized in the weak grid,even if the grid voltages are greatly distorted.Simulation and hardware experiments on the weak grid validate the effectiveness of the proposed DS-MPC with the EKF observer approach.展开更多
In multi-fed grid-connected systems,there are complex dynamic interactions between different pieces of equipment.Particularly in situations of weak-grid faults,the dynamic coupling between equipment becomes more prono...In multi-fed grid-connected systems,there are complex dynamic interactions between different pieces of equipment.Particularly in situations of weak-grid faults,the dynamic coupling between equipment becomes more pronounced.This may cause the system to experience small-signal instability during the fault steady-state.In this paper,multi-paralleled doubly fed induction generator(DFIG)-based wind farms(WFs)are taken as an example to study the dynamic coupling within a multi-fed system during fault steady-state of symmetrical low voltage ride-through(LVRT)in a weak grid.The analysis reveals that the dynamic coupling between WFs will introduce a damping shift to each WF.This inevitably affects the system’s dynamic stability and brings the risk of small-signal instability during fault steady-state in LVRT scenarios.Increasing the distance to fault location and fault severity will exacerbate the dynamic coupling between WFs.Because of the dynamic coupling,adjusting the control state of one WF will affect the stability of the remaining WFs in the system.Hence,a cooperative control strategy for multi-paralleled DFIG WFs is proposed to improve dynamic stability during LVRT.The analysis and the effectiveness of the proposed control strategy are verified by modal analysis and simu-lation.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
The stability problem of weak grid connected converter interfaced generation(CIG)cannot be ignored.For multiple weak grid connected CIGs with different parameters,the system oscillation characteristics and equivalence...The stability problem of weak grid connected converter interfaced generation(CIG)cannot be ignored.For multiple weak grid connected CIGs with different parameters,the system oscillation characteristics and equivalence methods still need to be further studied.This paper first discusses the oscillation characteristics when CIGs are perfectly coupled,perfectly decoupled and their general conditions respectively.Based on the Monte Caro simulation,the number of critical eigenvalues,the participation of each CIG to critical eigenvalues and the correlative parameters to participation are analyzed.Then the single-CIG and multi-CIG equivalence methods are proposed for stations containing nonidentical CIGs.The CIG parameters of a single-CIG equivalent model are identified based on the consistency of the output admittance characteristics.According to the different participations of CIGs with critical eigenvalues,the station is equivalent to a multi-CIG model.Results of large simulation samples show that the two equivalent models can both preserve the critical eigenvalues very well,and can be used for stability analysis.Furthermore,the multi-CIG equivalent model can also very well reflect the participation of CIGs in detailed models,and can be used for damping control study.展开更多
Faced with the wave of global energy structure transition,it is imperative to promote the application of renewable power generators(RPGs),such as wind energy,in power systems.Thus,wind turbine generation(WTG)has been ...Faced with the wave of global energy structure transition,it is imperative to promote the application of renewable power generators(RPGs),such as wind energy,in power systems.Thus,wind turbine generation(WTG)has been gradually promoted.To improve the inertia response(IR)and primary frequency regulation(PFR)capability of WTG in power systems,the gridforming control strategy of WTG(GFM-WTG),which displays an independent IR ability,has shown great potential in PFR,especially in weak grids.展开更多
This paper investigates and discusses the interaction stability issues of a wind farm with weak grid connections,where the wind turbines(WTs)are controlled by a new type of converter control strategy referred to as th...This paper investigates and discusses the interaction stability issues of a wind farm with weak grid connections,where the wind turbines(WTs)are controlled by a new type of converter control strategy referred to as the voltage source(VS)control.The primary intention of the VS control method is to achieve the high-quality inertial response capability of a single WT.However,when it is applied to multiple WTs within a wind farm,its weakgrid performance regarding the stability remains concealed and needs to be clarified.To this end,a frequency domain model of the wind farm under the VS control is first developed.Based on this model and the application of a stability margin quantification index,not only the interactions between the wind farm and the weak grid but also those among WTs will be systematically assessed in this paper.A crucial finding is that the inertial response of VS control has negative impacts on the stability margin of the system,and the dominant instability mode is more related to the interactions among the WTs rather than the typical grid-wind farm interaction.Based on this knowledge,a stabilization control strategy is then proposed,aiming for stability improvements of VS control while fulfilling the demand of inertial responses.Finally,all the results are verified by time-domain simulations in power systems computer aided design/electromagnetic transients including DC(PSCAD/EMTDC).展开更多
The high penetration of renewable energy has led to the widespread emergence of weak grids,under which grid-following(GFL)inverters suffer from dynamic instability induced by phase-locked loops(PLL).Although reducing ...The high penetration of renewable energy has led to the widespread emergence of weak grids,under which grid-following(GFL)inverters suffer from dynamic instability induced by phase-locked loops(PLL).Although reducing the bandwidth of the PLL can enhance system stability,it significantly degrades the ability of the inverter to reject power interaction disturbances.The inherent trade-off between the bandwidth of the PLL and its disturbance suppression capability are investigated.A cooperative strategy through a grid-forming(GFM)inverter to improve the robustness of GFL inverters without requiring hardware modifications is introduced.The simulation results validate that the proposed strategy effectively suppresses power interaction disturbances and enhances system stability under weak-grid scenarios.展开更多
Based on subsynchronous oscillation(SsO)caused by the connection of a direct drive wind farm to a weak alternating current(AC)grid,a complex torque model is proposed.By dividing the wind farm into a direct current cap...Based on subsynchronous oscillation(SsO)caused by the connection of a direct drive wind farm to a weak alternating current(AC)grid,a complex torque model is proposed.By dividing the wind farm into a direct current capacitor and electrical control part,the analytical expression of damping torque for the electrical control part is deduced and the mechanism of system subsynchronous oscillation is analyzed from the perspective of damping.The system stability is determined based on a damping coefficient less than O,and the influence laws of grid strength,operating conditions,and control parameters on system damping characteristics are analyzed,based on additional subsynchronous oscillation damping control(SSDC)to suppress SsO.Furthermore,the complex torque model of the parallel access system for multiple sub-wind farms is established,and the dynamic interaction between multiple sub-wind farms is preliminarily explored.The time-domain simulation model is built in EMTDC/PSCAD to verify the correctness of the theoretical analysis.展开更多
基金supported by the National Natural Science Foundation of China under Grant 52407069the Science and Technology Project of Zhejiang Province under Grant 2024C01254the China Postdoctoral Science Foundation under Grant 2024T170766 and 2024M762824。
文摘The phase-locked loop(PLL)plays an essential role for synchronizing renewable power generation to the grid.However,as per the grid-code compliance for reactive current support,the PLL output frequency fluctuates significantly and exceeds the limitation,which seriously threaten the safe supply of electricity.In this paper,the underlying theoretical mechanism and dominant force behind the maximum PLL frequency deviation are revealed.Accordingly,two feasible approaches are proposed to enhance the PLL frequency stability with validations in experimental results.
基金supported in part by the Science and Technology project supported by the State Grid Corporation of China under Grant FX71-16-006.
文摘Voltage source converter based high voltage direct current transmission(VSC-HVDC)is considered one of the most suitable technologies to integrate renewable energies.However,connecting VSC to a weak grid is challenging since traditional vector control tends to become unstable under high power demand conditions.In this paper,an improved vector control method is proposed wherein a feed forward branch based on steady state and small signal analysis of the VSC system is added under weak grid situations.The feed forward branch promotes faster reactive power response,thus enhancing the stability of the VSC system.Since the improved vector control uses the same inner loop as traditional vector control,the proposed method allows for the ability to retain fault current suppression capabilities.Furthermore,the control parameters of the outer loop of the improved vector control need not vary according to the variation of the operating points,which makes it easy to implement.The feed forward branch is implemented by solving a nonlinear equation or through use of a look-up table.The influence of the estimation errors of short circuit ratio(SCR)on the control performance is also studied.The effectiveness of the improved vector control is demonstrated through small signal model analysis and time domain simulations.
基金supported by the National Natural Science Foundation of China(No.51677142)the National Key R&D Program of China(No.2016YFB0900600)。
文摘Voltage source converter based high-voltage direct current(VSC-HVDC)transmission technology has been extensively employed in power systems with a high penetration of renewable energy resources.However,connecting a voltage source converter(VSC)to an AC weak grid may cause the converter system to become unstable.In this paper,a phase-shift phaselocked loop(PS-PLL)is proposed wherein a back electromotive force(BEMF)observer is added to the conventional phaselocked loop(PLL).The BEMF observer is used to observe the voltage of the infinite grid in the stationaryαβframe,which avoids the problem of inaccurate observations of the grid voltage in the dq frame that are caused by the output phase angle errors of the PLL.The VSC using the PS-PLL can operate as if it is facing a strong grid,thus enhancing the stability of the VSC-HVDC system.The proposed PS-PLL only needs to be properly modified on the basis of a traditional PLL,which makes it easy to implement.In addition,because it is difficult to obtain the exact impedance of the grid,the influence of shortcircuit ratio(SCR)estimation errors on the performance of the PS-PLL is also studied.The effectiveness of the proposed PSPLL is verified by the small-signal stability analysis and timedomain simulation.
基金supported by National Natural Science Foundation of China(No.61903291)Shaanxi Province Key R&D Program(No.2022GY-134)。
文摘A control strategy of repetitive control without inductorance decoupling was proposed to address the problem of high total harmonic distortion(THD)rate of the network-side current caused by the reduced stability of the rectifier module of the DC charging pile under weak grid as well as the dead zone and nonlinearity of switching devices during charging.Firstly,the parallel repetitive control was constructed in the inner current loop,and the proportional-integral(PI)+repetitive controller based on parallel structure was designed.For system compensation,a second-order low-pass filter was selected to correct the system,and the network-side current harmonics were actively suppressed without increasing the filtering device,which effectively improves the quality of grid-connected current.Secondly,based on the synthetic vector method,the controller parameters were designed to realize the elimination of main pole by establishing two synchronous rotation coordinate system vector differential equations,so as to realize the inductanceless decoupling to cope with the influence of network-side inductance fluctuation on the stability of the control system under weak grid.By theoretical analysis and simulation,the proposed control strategy was embedded into the self-developed digital signal processor for the rectifier module of DC charging pile,simulated dynamic and steady-state operation experiments were conducted,and comparative analysis was performed to prove the feasibility of the proposed control strategy.
基金supported by National Natural Science Foundation of China (No. 61573303)Natural Science Foundation of Hebei Province (No. E2016203092)
文摘Obvious resonance peak will be generated when parallel photovoltaic grid-connected inverters are connected to the weak grid with high grid impedance, which seriously affects the stability of grid-connected operation of the photovoltaic system. To overcome the problems mentioned above, the mathematical model of the parallel photovoltaic inverters is established. Several factors including the impact of the reference current of the grid-connected inverter, the grid voltage interference and the current disturbance between the photovoltaic inverters in parallel with the grid-connected inverters are analyzed. The grid impedance and the LCL filter of the photovoltaic inverter system are found to be the key elements which lead to existence of resonance peak. This paper presents the branch voltage and current double feedback suppression method under the premise of not changing the topological structure of the photovoltaic inverter, which effectively handles the resonance peak, weakens the harmonic content of the grid current of the photovoltaic grid-connected inverter and the voltage at the point of common coupling, and improves the stability of the parallel operation of the photovoltaic grid-connected inverters in weak grid. At last, the simulation model is established to verify the reliability of this suppression method.
基金supported by National Natural Science Foundation Joint Key Project of China(2016YFB0900900).
文摘The utilization of renewable energy in sending-end power grids is increasing rapidly,which brings difficulties to voltage control.This paper proposes a coordinated voltage control strategy based on model predictive control(MPC)for the renewable energy power plants of wind and solar power connected to a weak sending-end power grid(WSPG).Wind turbine generators(WTGs),photovoltaic arrays(PVAs),and a static synchronous compensator are coordinated to maintain voltage within a feasible range during operation.This results in the full use of the reactive power capability of WTGs and PVAs.In addition,the impact of the active power outputs of WTGs and PVAs on voltage control are considered because of the high R/X ratio of a collector system.An analytical method is used for calculating sensitivity coefficients to improve computation efficiency.A renewable energy power plant with 80 WTGs and 20 PVAs connected to a WSPG is used to verify the proposed voltage control strategy.Case studies show that the coordinated voltage control strategy can achieve good voltage control performance,which improves the voltage quality of the entire power plant.
基金supported in part by the National Natural Science Foundation of China(51907067)in part by the Industrial Research Chair Program of the Natural Sciences and Engineering Research Councilof Canada。
文摘Distinction of weak and strong AC grids for emerging hierarchical-infeed LCC-UHVDC systems is important for planning and operation departments. However, accuracy of earlier distinction methods is limited as they were developed by empirical reasoning without rigorous theoretical analysis. Hence in this letter, hierarchical-infeed interactive effective short-circuit ratio (HIESCR) index is first used for strength evaluation of HIDC systems with complex inter-inverter interactions considered. Boundary HIESCR (BHIESCR) is also introduced in the proposed distinction method of weak and strong AC grids. That is, weak (or strong) AC grids are, respectively, identified when HIESCR is less (or greater) than BHIESCR. Second, it is shown BHIESCR remains almost unchanged as 3.0 versus various system parameters and rated operation variables based on rigorous theoretical analysis. This salient feature makes the proposed method more accurate than earlier methods. Finally, the proposed method is validated by simulations based on the PSCAD/EMTDC program.
基金supported by Science and Technology Project of State Grid Corporation of China(4000-202317083A-1-1-ZN).
文摘The grid-connected converter with grid-following control (GCC-GFL) for renewable energy sources has a risk of instability when integrated into the weak grid. This paper aims to investigate the dynamic interactions and stability characteristics of the GCC-GFL system. From a control system perspective, the mechanism of small-signal instability in the system is revealed through dynamic interaction analysis between the GCC-GFL and the weak grid. Meanwhile, a novel stability evaluation index is proposed based on the real and imaginary parts of the equivalent loop gain in a multi-loop control system. On this basis, the dominant loop of the control system leading to system instability is identified. Furthermore, quantitative analyses are conducted to investigate the stability region of the GCC-GFL, considering the influence of AC grid strength, steady-state operating points, and converter control parameters. Finally, the correctness and effectiveness of the proposed methods are verified by the impedance analysis method, the time-domain simulations, and the experiments, respectively.
基金supported in part by the Leading talents of scientific and technological innovation in Hunan Province under Grant(No.2019RS3014)the National Natural Science Foundation of China under Grant(No.51907057)。
文摘Grid impedance and phase-locked loop(PLL)are critical factors for the stability of the grid-connected inverters(GCIs)in a weak grid.They are the positive feedback control loops formed by PLL in the GCI with grid impedance.It is prone to GCI instability,especially in the case of the higher PLL bandwidth.A novel impedance-phase and magnitude control strategy is proposed to improve stability of GCI with different grid impedance.Moreover,a detailed design of control loop and parameter calculation for the impedance-phase and magnitude control strategy are introduced.First,PLL output impedance is reshaped to broaden the frequency range of the GCI phase-frequency characteristic curve above the−90°line towards the low-frequency band.In addition,current loop output impedance is reshaped to maintain the phase margin(PM)of the GCI near to 45°.Meanwhile,the magnitude of GCI output impedance is also increased significantly.Stability of the GCI in a weak grid is enhanced by adopting the proposed control strategy.Simulation and experimental results verify the analysis and the proposed method.
基金supported by the Natural Science Foundation of China(No.52077070)Young Scientists Fund of National Natural Science Foundation of China(No.52207201)+1 种基金Hunan Provincial Natural Science Foundation of China(No.2024JJ3012)Young Scientists Fund of Hunan Provincial Natural Science Foundation of China(No.2024JJ6172).
文摘The self-synchronizing voltage source inverter(SS VSI)is widely studied because of its grid-forming capability.However,the slow response of the active power control loop(APCL)under the weak grid makes it difficult for the SSVSI to quickly support the frequency of a low-inertia grid.In this pa per,a grid framework is established to analyze the frequency support service process of the SSVSI,and the shortcomings of the regulation of the damping coefficient and virtual inertia co efficient for frequency support are analyzed.Then,an adaptive additional damping control method is proposed to optimize the ability of SSVSI to support the grid frequency.The proposed control method adjusts the damping of the APCL without affect ing the system steady-state characteristics,which improves the active power response speed of the SSVSI.Besides,the pro posed control method adaptively adjusts the additional damp ing coefficient based on the active power response without mea suring the grid parameters.Compared with other forms of con trol,the proposed control method excels in minimizing the rate of change of frequency(RoCoF)and the frequency deviation(FD)within the grid,without succumbing to the constraints posed by unknown grid parameters.Furthermore,the analysis of the system stability is also presented.Finally,the experimen tal hardware results obtained from a miniaturized grid proto type are presented,corroborating the effectiveness of the pro posed control method.
基金funded by the Australian Renewable Energy Agency(No.2023/ARP010).
文摘The maximum power transfer capability(MPTC)of phase-locked loop(PLL)-based grid-following inverters is often limited under weak-grid conditions due to passivity violations caused by operating-point-dependent control loops.This paper reveals and compares the mechanisms of these violations across different control strategies.Using admittance decomposition and full-order state-space models for eigenvalue analysis,MPTC limitations from control loops and their interactions are identified.The small-signal stabilities of different control loops are compared under varying grid strength,and both static and dynamic MPTCs for each control mode are examined.This paper also explores how control loop interactions impact the MPTC,offering insights for tuning control loops to enhance stability in weak grids.For example,fast power control improves the MPTC when paired with a slow PLL,while power control has minimal effect when the PLL is sufficiently fast.The findings are validated through frequency scanning,eigenvalue analysis,simulations,and experiments.
基金supported in part by the National Natural Science Foundation of China(No.52077037)in part by the Science and Technology Projects of Jiangsu Province(No.BE2022016).
文摘Under weak grid conditions,grid impedance is coupled with a control system for voltage source converter based high-voltage direct current(VSC-HVDC)systems,resulting in decreased synchronization stability.Unfortunately,most studies are based on the assumption that impedance ratio(R/X)is sufficiently small to ignore the effects of grid impedance.In this study,we establish a dynamic coupling model that includes grid impedance and control loops,revealing the influence mechanism of R/X on synchronization stability from a physical perspective.We also quantify the stability range of R/X in the static analysis model and introduce a sensitivity factor to measure its effect on voltage stability.Additionally,we utilize a dynamic analysis model to evaluate power angle convergence,proposing a corresponding stability criterion.We then present a method of synchronous voltage reconstruction aimed at enhancing the grid strength.Theoretical analysis shows that this method can effectively mitigate the effects of coupling between grid impedance and the controller under weak grid conditions,ensuring stable operation even under extremely weak grid conditions.Experiments validate the accuracy and effectiveness of the analysis and method.
文摘This paper proposes a robust dichotomy-based model predictive control(DS-MPC)with a fixed switching frequency for the grid-connected inverter(GCI).The proposed fast dichotomy algorithm can select and deduce the optimal voltage vector dynamically through the space vector plane.Therefore,the proposed DS-MPC strategy could ensure dynamic performance and steady-state performance as well.Also,the current control robustness can be improved through DS-MPC with disturbance observer(DO)based on the extended Kalman filter(EKF).The novelty of this control is that the current control with fast dynamic response can be realized in the weak grid,even if the grid voltages are greatly distorted.Simulation and hardware experiments on the weak grid validate the effectiveness of the proposed DS-MPC with the EKF observer approach.
基金the National Natural Science Foundation of China(NSFC)(No.51977019)in part by the Joint Research Fund in Smart Grid under Cooperative Agreement between the National Natural Science Foundation of China(NSFC)(No.U1966208)State Grid Corporation of China(SGCC).
文摘In multi-fed grid-connected systems,there are complex dynamic interactions between different pieces of equipment.Particularly in situations of weak-grid faults,the dynamic coupling between equipment becomes more pronounced.This may cause the system to experience small-signal instability during the fault steady-state.In this paper,multi-paralleled doubly fed induction generator(DFIG)-based wind farms(WFs)are taken as an example to study the dynamic coupling within a multi-fed system during fault steady-state of symmetrical low voltage ride-through(LVRT)in a weak grid.The analysis reveals that the dynamic coupling between WFs will introduce a damping shift to each WF.This inevitably affects the system’s dynamic stability and brings the risk of small-signal instability during fault steady-state in LVRT scenarios.Increasing the distance to fault location and fault severity will exacerbate the dynamic coupling between WFs.Because of the dynamic coupling,adjusting the control state of one WF will affect the stability of the remaining WFs in the system.Hence,a cooperative control strategy for multi-paralleled DFIG WFs is proposed to improve dynamic stability during LVRT.The analysis and the effectiveness of the proposed control strategy are verified by modal analysis and simu-lation.
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.
基金supported by the Applied Basic Research Program of Science and Technology Plan Project of Sichuan Province of China(No.2020YJ0252).
文摘The stability problem of weak grid connected converter interfaced generation(CIG)cannot be ignored.For multiple weak grid connected CIGs with different parameters,the system oscillation characteristics and equivalence methods still need to be further studied.This paper first discusses the oscillation characteristics when CIGs are perfectly coupled,perfectly decoupled and their general conditions respectively.Based on the Monte Caro simulation,the number of critical eigenvalues,the participation of each CIG to critical eigenvalues and the correlative parameters to participation are analyzed.Then the single-CIG and multi-CIG equivalence methods are proposed for stations containing nonidentical CIGs.The CIG parameters of a single-CIG equivalent model are identified based on the consistency of the output admittance characteristics.According to the different participations of CIGs with critical eigenvalues,the station is equivalent to a multi-CIG model.Results of large simulation samples show that the two equivalent models can both preserve the critical eigenvalues very well,and can be used for stability analysis.Furthermore,the multi-CIG equivalent model can also very well reflect the participation of CIGs in detailed models,and can be used for damping control study.
基金supported by the National Natural Science Foundation of China(Grant No.52325702)。
文摘Faced with the wave of global energy structure transition,it is imperative to promote the application of renewable power generators(RPGs),such as wind energy,in power systems.Thus,wind turbine generation(WTG)has been gradually promoted.To improve the inertia response(IR)and primary frequency regulation(PFR)capability of WTG in power systems,the gridforming control strategy of WTG(GFM-WTG),which displays an independent IR ability,has shown great potential in PFR,especially in weak grids.
基金supported in part by the National Key R&D Plan of China(Grant No.2018YFB1501300)by the Key Laboratory of Control of Power Transmission and Conversion(SJTU),Ministry of Education(2021AC03).
文摘This paper investigates and discusses the interaction stability issues of a wind farm with weak grid connections,where the wind turbines(WTs)are controlled by a new type of converter control strategy referred to as the voltage source(VS)control.The primary intention of the VS control method is to achieve the high-quality inertial response capability of a single WT.However,when it is applied to multiple WTs within a wind farm,its weakgrid performance regarding the stability remains concealed and needs to be clarified.To this end,a frequency domain model of the wind farm under the VS control is first developed.Based on this model and the application of a stability margin quantification index,not only the interactions between the wind farm and the weak grid but also those among WTs will be systematically assessed in this paper.A crucial finding is that the inertial response of VS control has negative impacts on the stability margin of the system,and the dominant instability mode is more related to the interactions among the WTs rather than the typical grid-wind farm interaction.Based on this knowledge,a stabilization control strategy is then proposed,aiming for stability improvements of VS control while fulfilling the demand of inertial responses.Finally,all the results are verified by time-domain simulations in power systems computer aided design/electromagnetic transients including DC(PSCAD/EMTDC).
基金Supported by the Smart Grid-National Science and Technology Major Project(2024ZD0801500)by the National Natural Science Foundation of China(52207211,U24B2085,U23A20655)by the Fundamental Research Funds for the Central Universities(JZ2024HGTB0259).
文摘The high penetration of renewable energy has led to the widespread emergence of weak grids,under which grid-following(GFL)inverters suffer from dynamic instability induced by phase-locked loops(PLL).Although reducing the bandwidth of the PLL can enhance system stability,it significantly degrades the ability of the inverter to reject power interaction disturbances.The inherent trade-off between the bandwidth of the PLL and its disturbance suppression capability are investigated.A cooperative strategy through a grid-forming(GFM)inverter to improve the robustness of GFL inverters without requiring hardware modifications is introduced.The simulation results validate that the proposed strategy effectively suppresses power interaction disturbances and enhances system stability under weak-grid scenarios.
基金supported by Integrated project of National Natural Science Foundation of China(No.U1866601)Science and Technology Project of State Grid Corporation of China(5230DK20004V)。
文摘Based on subsynchronous oscillation(SsO)caused by the connection of a direct drive wind farm to a weak alternating current(AC)grid,a complex torque model is proposed.By dividing the wind farm into a direct current capacitor and electrical control part,the analytical expression of damping torque for the electrical control part is deduced and the mechanism of system subsynchronous oscillation is analyzed from the perspective of damping.The system stability is determined based on a damping coefficient less than O,and the influence laws of grid strength,operating conditions,and control parameters on system damping characteristics are analyzed,based on additional subsynchronous oscillation damping control(SSDC)to suppress SsO.Furthermore,the complex torque model of the parallel access system for multiple sub-wind farms is established,and the dynamic interaction between multiple sub-wind farms is preliminarily explored.The time-domain simulation model is built in EMTDC/PSCAD to verify the correctness of the theoretical analysis.
