In the islanded operation of distribution networks,due to the mismatch of line impedance at the inverter output,conventional droop control leads to inaccurate power sharing according to capacity,resulting in voltage a...In the islanded operation of distribution networks,due to the mismatch of line impedance at the inverter output,conventional droop control leads to inaccurate power sharing according to capacity,resulting in voltage and frequency fluctuations under minor external disturbances.To address this issue,this paper introduces an enhanced scheme for power sharing and voltage-frequency control.First,to solve the power distribution problem,we propose an adaptive virtual impedance control based on multi-agent consensus,which allows for precise active and reactive power allocation without requiring feeder impedance knowledge.Moreover,a novel consensus-based voltage and frequency control is proposed to correct the voltage deviation inherent in droop control and virtual impedance methods.This strategy maintains voltage and frequency stability even during communication disruptions and enhances system robustness.Additionally,a small-signal model is established for system stability analysis,and the control parameters are optimized.Simulation results validate the effectiveness of the proposed control scheme.展开更多
This paper proposes virtual impedance adaptation of the lower-limb exoskeleton for human performance augmentation(LEHPA) based on deep reinforcement learning(VIADRL) to mitigate reliance on model accuracy and address ...This paper proposes virtual impedance adaptation of the lower-limb exoskeleton for human performance augmentation(LEHPA) based on deep reinforcement learning(VIADRL) to mitigate reliance on model accuracy and address the ever-changing human-exoskeleton interaction(HEI) dynamics. The classical sensitivity amplification control strategy is expanded to the virtual impedance control strategy with more learnable virtual impedance parameters. The adjustment of these virtual impedance parameters is formalized as finding the optimal policy for a Markov Decision Process and can then be effectively resolved using deep reinforcement learning algorithms. To ensure safe and efficient policy training, a multibody simulation environment is established to facilitate the training process, supplemented by the innovative hybrid inverse-forward dynamics simulation approach for executing the simulation. For comparison purposes, the SADRL strategy is introduced as a benchmark. A novel control performance evaluation method based on the HEI forces at the back, thighs, and shanks is proposed to quantitatively evaluate the performance of our proposed VIADRL strategy. The VIADRL controller is systematically compared with the SADRL controller at five selected walking speeds. The lumped ratio of HEI forces under the SADRL strategy relative to those under the SADRL strategy is as low as 0.81 in simulation and approximately 0.89 on the LEHPA prototype. The overall reduction of HEI forces demonstrates the superiority of the VIADRL strategy in comparison to the SADRL strategy.展开更多
With the rapid increase of wind farms,the grid code needs to be improved to meet the requirement of wind farms and enhance grid stability.Doubly-fed induction generators are largely used in wind turbines,but they are ...With the rapid increase of wind farms,the grid code needs to be improved to meet the requirement of wind farms and enhance grid stability.Doubly-fed induction generators are largely used in wind turbines,but they are very sensitive to grid disturbances.The voltage swell can be caused by switching on capacitor banks or switching off large loads,which may result in the reversal of the power flow in the grid convertor;the current may flow from the grid into the DC link,which may step up DC voltage,and result in large faults of rotor currents and instantaneous power oscillation.The grid reactive compensation devices can not have the automatic swithing function after the low voltage fault,which will result in local reactive power surplus,so some wind power generators will retreat from the grid under high voltage protection.展开更多
The high proportion of nonlinear and unbalanced loads results in power quality issues in islanded microgrids.This paper presents a novel control strategy for harmonic and unbalanced power allocation among distributed ...The high proportion of nonlinear and unbalanced loads results in power quality issues in islanded microgrids.This paper presents a novel control strategy for harmonic and unbalanced power allocation among distributed genera-tors(DGs)in microgrids.Different from the existing sharing strategies that allocate the harmonic and unbalanced power according to the rated capacities of DGs,the proposed control strategy intends to shape the lowest output impedances of DGs to optimize the power quality of the microgrid.To achieve this goal,the feasible range of virtual impedance is analyzed in detail by eigenvalue analysis,and the findings suggest a simultaneous adjustment of real and imaginary parts of virtual impedance.Because virtual impedance is an open-loop control that imposes DG to the risk of overload,a new closed-loop structure is designed that uses residual capacity and absorbed power as feedback.Accordingly,virtual impedance can be safely adjusted in the feasible range until the power limit is reached.In addi-tion,a fuzzy integral controller is adopted to improve the dynamics and convergence of the power distribution,and its performance is found to be superior to linear integral controllers.Finally,simulations and control hardware-in-the-loop experiments are conducted to verify the effectiveness and usefulness of the proposed control strategy.展开更多
Line commutated converter based high-voltage direct-current(LCC-HVDC)transmissions are prone to harmonic oscillation under weak grids.Impedance modeling is an effective method for assessing interaction stability.First...Line commutated converter based high-voltage direct-current(LCC-HVDC)transmissions are prone to harmonic oscillation under weak grids.Impedance modeling is an effective method for assessing interaction stability.Firstly,this paper proposes an improved calculation method for the DC voltage and AC currents of commutation stations to address the complex linearization of the commutation process and constructs an overall harmonic state-space(HSS)model of an LCC-HVDC.Based on the HSS model,the closed-loop AC impedances on the LCC-HVDC sending and receiving ends are then derived and verified.The impedance characteristics of the LCC-HVDC are then analyzed to provide a physical explanation for the harmonic oscillation of the system.The effects of the grid strength and control parameters on system stability are also analyzed.To improve the impedance characteristics and operating stability of the LCC-HVDC system,a virtual impedance based stability enhancement control is proposed,and a parameter design method is considered to ensure satisfactory phase margins at both the sending and receiving ends.Finally,simulation results are presented to verify the validity of the impedance model and virtual impedance based stability enhancement control.展开更多
Multi-paralleled bidirectional power converters(BPCs)are commonly used to improve the power capacity and reliability in an AC/DC hybrid microgrid.However,circulating current through multi-BPCs has been one of the chal...Multi-paralleled bidirectional power converters(BPCs)are commonly used to improve the power capacity and reliability in an AC/DC hybrid microgrid.However,circulating current through multi-BPCs has been one of the challenges and it can be aggravated in the presence of non-ideal operating conditions,such as unbalanced AC voltages,and the mismatch of hardware parameters.In order to suppress the circulating current,this paper proposes a distributed method based on adaptive virtual impedance,which also employs positive sequence power droop control and voltage deviation compensation control.The traditional positive sequence power droop control is adopted to only regulate the positive components of the BPCs output voltage.The negative sequence power term is fed to an adaptive virtual impedance generator to modify the damping characteristics of the BPCs.Also,an adaptive virtual impedance-based voltage deviation compensation method is proposed to recover the fluctuated output voltage of the BPCs due to droop action and the power fluctuations.The fully distributed regulation of adaptive virtual impedance enables the load power to be shared accurately among BPC modules and thus the circulating current can be effectively suppressed.The proposed control strategy does not require an additional communication system and the precise parameters of hardware equipment and line impedance.Furthermore,the effectiveness of the proposed method is verified by the experimental results.展开更多
This paper proposes a joint limiting control strategy for suppressing DC fault current and arm current in modular multilevel converter-based high-voltage direct current(MMC-HVDC) systems, which includes two target-ori...This paper proposes a joint limiting control strategy for suppressing DC fault current and arm current in modular multilevel converter-based high-voltage direct current(MMC-HVDC) systems, which includes two target-oriented current limiting controls. To limit the DC fault current in the early fault stage, an equivalent modular multilevel converter(MMC) impedance is obtained, and its high-frequency part is reshaped by introducing virtual impedance, which is realized by adjusting the inserted submodules adaptively. Following the analysis of MMC control characteristics, the arm current limiting strategy is investigated, with results showing that the inner-loop control has significant effects on arm current and that a simple low-pass filter can reduce the arm current in the fault period. Finally, by combining the virtual impedance shaping and innerloop control, the fault currents of DC lines and MMC arms can be suppressed simultaneously, which can not only alleviate the interrupting pressure of the DC circuit breaker, but also prevent the MMC from being blocked by the arm overcurrent. Theoretical analysis conclusions and the proposed strategy are verified offline by a digital time-domain simulation on Power Systems Computer Aided Design/Electromagnetic Transients including DC platform, and experiment on a real-time digital simulator platform.展开更多
The effects of nonlinear loads on voltage quality represent an emerging concern for islanded microgrids.Existing research works have mainly focused on harmonic power sharing among multiple inverters,which ignores the ...The effects of nonlinear loads on voltage quality represent an emerging concern for islanded microgrids.Existing research works have mainly focused on harmonic power sharing among multiple inverters,which ignores the diversity of different inverters to mitigate harmonics from nonlinear loads.As a result,the voltage quality of microgrids cannot be effectively improved.To address this issue,this study proposes an adaptive harmonic virtual impedance(HVI)control for improving voltage quality of microgrids.Based on the premise that no inverter is overloaded,the main objective of the proposed control is to maximize harmonic power absorption by shaping the lowest output impedances of inverters.To achieve this,the proposed control is utilized to adjust the HVI of each inverter based on its operation conditions.In addition,the evaluation based on Monte Carlo harmonic power flow is designed to assess the performance of the proposed control in practice.Finally,comparative studies and control-in-the-loop experiments are conducted.展开更多
Lock-in amplifiers are used to detect and measure very small alternating current(AC)signals down to the range of nVs.Accurate measurements can be made even when the small signals are buried by noise thousands of times...Lock-in amplifiers are used to detect and measure very small alternating current(AC)signals down to the range of nVs.Accurate measurements can be made even when the small signals are buried by noise thousands of times larger.With the digital signal processing(DSP)technology involved in modern instrumentation,a lock-in amplifier is more versatile in sensing and recovering small signals.Combining the virtual instrumentation technology,we reorganize the functional blocks of a programmable lock-in amplifier and build it as a virtual spectrum analyzer,virtual impedance meter,virtual network analyzer,virtual semiconductor parameter analyzer,signal generator,etc.A 4 layer model is used to implement these virtual instruments.The same virtual instrument can also be implemented on a general purpose FPGA developing board.展开更多
Conventional droop control in multi-parallel grid-forming inverters exhibits poor reactive power sharing accuracy due to line impedance mismatches.In this study,we proposed a coordination control strategy integrating ...Conventional droop control in multi-parallel grid-forming inverters exhibits poor reactive power sharing accuracy due to line impedance mismatches.In this study,we proposed a coordination control strategy integrating adaptive virtual impedance with dynamic Q-V droop regulation to overcome this issue.We established a coupling model between the line impedance and power allocation to determine the quantitative relationship between reactive power deviation and impedance difference and to analyze the mechanism of reactive power deviation formation.Based on this,we proposed a transformer neural network-based online identification method for adaptive virtual impedance and dynamic droop coefficients.The self-attention mechanism dynamically characterizes the spatial distribution features of the impedance parameters considering the real-time voltage/reactive power time-series data as inputs to realize the dynamic impedance compensation without communication interaction.The contradiction constraint between the voltage drop and distribution accuracy caused by the introduction of conventional virtual impedance is improved by dynamic droop coefficient reconstruction.Lastly,we established a hardware-in-the-loop simulation platform to experimentally validate the operational efficacy and dynamic performance of the proposed control strategy under various grid scenarios.展开更多
In this paper,inspired by the concept of virtual inertia in alternating current(AC)systems,a virtual impedance controller is proposed for the dynamic improvement of direct current microgrids(DCMGs).A simple and inexpe...In this paper,inspired by the concept of virtual inertia in alternating current(AC)systems,a virtual impedance controller is proposed for the dynamic improvement of direct current microgrids(DCMGs).A simple and inexpensive method for injecting inertia into the system is used to adjust the output power of each distributed generation unit without using additional equipment.The proposed controller consists of two components:a virtual capacitor and a virtual inductor.These virtual components can change the rate of change of the DC bus voltage and also improve the transient response.A small-signal analysis is carried out to verify the impact of the proposed control strategy.Numerical simulation studies validate the effectiveness of the proposed solution for increasing the inertia of DCMGs.展开更多
This paper proposes an active islanding method based on impedance measurement for DC microgrids with multiple distributed generators(DGs).In the various presented methods,impedance-based islanding detection faces chal...This paper proposes an active islanding method based on impedance measurement for DC microgrids with multiple distributed generators(DGs).In the various presented methods,impedance-based islanding detection faces challenges in DC microgrids with multiple DGs due to the randomness of renewable energy.In the proposed method,the virtual impedance controls are designed to reshape the output impedance of DGs and the input impedance of loads at the detection frequency to ensure a clear distinction between grid-connected conditions and islanding conditions.Thus,the islanding events can be effectively detected under conditions that lead to a distinct decrease in system impedance.In this way,the accuracy of islanding detection is improved.Moreover,a dynamic allocation of the role algorithm is proposed to organize the orderly disturbance injection of DGs without communication,avoiding disturbance dilution.In addition,the selection principle of the frequency of disturbance current is proposed by studying the sensitivity of the point of common coupling(PCC)voltage to the disturbance.As a result,the negative influence of multiple DGs,loads,and disturbance dilution can be avoided compared to the existing methods.Finally,the effectiveness of the proposed method has been verified by both simulation and experimental results.展开更多
Reactive power sharing cannot be achieved using many existing microgrid(MG)control methods,but the convergence speed of these methods is slow.To solve these problems,a finite-time distributed control approach is propo...Reactive power sharing cannot be achieved using many existing microgrid(MG)control methods,but the convergence speed of these methods is slow.To solve these problems,a finite-time distributed control approach is proposed in this paper,which is based on the hierarchical control structure.The hierarchical control structure consists of a dual loop control,a droop control used as a primary control and a secondary control.First,the secondary controller is modeled,and the MG system composed of distributed generators(DGs)is considered as a multi-agent system.The secondary controller consists of a frequency regulator,voltage regulator and power regulator.Secondly,the adaptive virtual impedance module is established,using the output of the reactive power regulator as its input.Thirdly,a dual loop controller is combined with a primary controller and secondary controller to generate a pulse width modulation(PWM)signal to control the power and voltage of the MG.In order to reduce the fluctuation of the MG,a damping module is introduced when the structure of the system changes.Finally,the stability of the proposed control strategy is proved by the related theorems.A simulation system is established in the Matlab environment,and the simulation results show that the proposed method is effective.展开更多
In recent years,the hybrid AC-DC microgrid has been well accepted as it combines the advantages of both AC and DC systems.As the microgrid contains both DC sub-grids and AC sub-grids,interlinking DC-AC converters are ...In recent years,the hybrid AC-DC microgrid has been well accepted as it combines the advantages of both AC and DC systems.As the microgrid contains both DC sub-grids and AC sub-grids,interlinking DC-AC converters are essential.Meanwhile,considering the nonlinear AC loads may deteriorate the voltage quality of the AC bus,embedding an ancillary harmonic compensation function to the interlinking converters is promising.However,the conventional harmonic control methods used for active power filters(APFs)may not be suitable for the interlinking converters due to the main purpose of it is to exchange real and reactive power between the DC and AC sub-grids.The switching frequency is preferred to be lower than the APFs when the capacity of the microgrid is large.At low switching frequency,harmonic compensation performance or even the system stability may be affected.In this paper,a harmonic compensation approach suitable for hybrid AC-DC interlinking converters at low switching frequency is proposed.Through feeding the PWM reference signal with the harmonic compensation component directly to avoid the multi-loop control path of the fundamental component,the proposed method can achieve the effective harmonics compensation without being limited by the closed-loop control bandwidth.The proposed method,modeling approaches,stability analysis,as well as detailed virtual impedance design are presented.Experimental verification is also provided.展开更多
This paper investigates integration of distributed energy resources(DERs)in microgrids(MGs)through two-stage power conversion structures consisting of DC-DC boost converter and DC-AC voltage source converter(VSC)subsy...This paper investigates integration of distributed energy resources(DERs)in microgrids(MGs)through two-stage power conversion structures consisting of DC-DC boost converter and DC-AC voltage source converter(VSC)subsystems.In contrast to existing investigations that treated DC-link voltage as an ideal constant voltage,this paper considers the non-ideal dynamic coupling between both subsystems for completeness and higher accuracy,which introduces additional DC-side dynamics to the VSC.The analysis shows parameters of the boost converter’s power model that impact stability through the DC-link.Carefully selecting these parameters can mitigate this effect on stability and improve dynamic performance across the DC-link.Hence,an optimization framework is developed to facilitate in selecting adequate boost converter parameters in designing a stable voltage source converter-based microgrid(VSC-MG).The developed optimization framework,based on particle swarm optimization,considers dynamic coupling between both subsystems and is also effective in avoiding inadequate boost converter parameters capable of propagating instability through the DC-link to the VSC.Simulations are performed with MATLAB/Simulink to validate theoretical analyses.展开更多
The coordinated control of parallel three-phase fourwire converters in autonomous AC microgrids is investigated in this paper.First,based on droop control,virtual impedance is inserted in positive-,negative-and zero-s...The coordinated control of parallel three-phase fourwire converters in autonomous AC microgrids is investigated in this paper.First,based on droop control,virtual impedance is inserted in positive-,negative-and zero-sequences to enhance system damping and imbalance power sharing.Then,to facilitate virtual impedance design,small signal models of the three-sequence equivalent circuits are established respectively.Corresponding indexes are proposed to comprehensively evaluate the impact of sequence virtual impedance on current sharing accuracy,voltage quality at the point of common coupling(PCC)and system stability.In addition,constraint of DClink voltage is also considered to avoid over modulation when subjected to unbalanced loads.Furthermore,to address the PCC voltage degradation resulting from virtual impedance,a voltage imbalance compensation method,based on low-bandwidth communication,is proposed.Finally,simulation and experimental results are provided to verify the correctness of the theory model,indicating that the proposed method can achieve PCC voltage restoration while guaranteeing the current sharing accuracy with desirable dynamics.展开更多
基金supported by the National Natural Science Foundation of China(52007009)Natural Science Foundation of Excellent Youth Project of Hunan Province of China(2023JJ20039)Science and Technology Projects of State Grid Hunan Provincial Electric Power Co.,Ltd.(5216A522001K,SGHNDK00PWJS2310173).
文摘In the islanded operation of distribution networks,due to the mismatch of line impedance at the inverter output,conventional droop control leads to inaccurate power sharing according to capacity,resulting in voltage and frequency fluctuations under minor external disturbances.To address this issue,this paper introduces an enhanced scheme for power sharing and voltage-frequency control.First,to solve the power distribution problem,we propose an adaptive virtual impedance control based on multi-agent consensus,which allows for precise active and reactive power allocation without requiring feeder impedance knowledge.Moreover,a novel consensus-based voltage and frequency control is proposed to correct the voltage deviation inherent in droop control and virtual impedance methods.This strategy maintains voltage and frequency stability even during communication disruptions and enhances system robustness.Additionally,a small-signal model is established for system stability analysis,and the control parameters are optimized.Simulation results validate the effectiveness of the proposed control scheme.
文摘This paper proposes virtual impedance adaptation of the lower-limb exoskeleton for human performance augmentation(LEHPA) based on deep reinforcement learning(VIADRL) to mitigate reliance on model accuracy and address the ever-changing human-exoskeleton interaction(HEI) dynamics. The classical sensitivity amplification control strategy is expanded to the virtual impedance control strategy with more learnable virtual impedance parameters. The adjustment of these virtual impedance parameters is formalized as finding the optimal policy for a Markov Decision Process and can then be effectively resolved using deep reinforcement learning algorithms. To ensure safe and efficient policy training, a multibody simulation environment is established to facilitate the training process, supplemented by the innovative hybrid inverse-forward dynamics simulation approach for executing the simulation. For comparison purposes, the SADRL strategy is introduced as a benchmark. A novel control performance evaluation method based on the HEI forces at the back, thighs, and shanks is proposed to quantitatively evaluate the performance of our proposed VIADRL strategy. The VIADRL controller is systematically compared with the SADRL controller at five selected walking speeds. The lumped ratio of HEI forces under the SADRL strategy relative to those under the SADRL strategy is as low as 0.81 in simulation and approximately 0.89 on the LEHPA prototype. The overall reduction of HEI forces demonstrates the superiority of the VIADRL strategy in comparison to the SADRL strategy.
文摘With the rapid increase of wind farms,the grid code needs to be improved to meet the requirement of wind farms and enhance grid stability.Doubly-fed induction generators are largely used in wind turbines,but they are very sensitive to grid disturbances.The voltage swell can be caused by switching on capacitor banks or switching off large loads,which may result in the reversal of the power flow in the grid convertor;the current may flow from the grid into the DC link,which may step up DC voltage,and result in large faults of rotor currents and instantaneous power oscillation.The grid reactive compensation devices can not have the automatic swithing function after the low voltage fault,which will result in local reactive power surplus,so some wind power generators will retreat from the grid under high voltage protection.
基金supported by the Science and Technology Project of SGCC under grant 5400-202219417A-2-0-ZN.
文摘The high proportion of nonlinear and unbalanced loads results in power quality issues in islanded microgrids.This paper presents a novel control strategy for harmonic and unbalanced power allocation among distributed genera-tors(DGs)in microgrids.Different from the existing sharing strategies that allocate the harmonic and unbalanced power according to the rated capacities of DGs,the proposed control strategy intends to shape the lowest output impedances of DGs to optimize the power quality of the microgrid.To achieve this goal,the feasible range of virtual impedance is analyzed in detail by eigenvalue analysis,and the findings suggest a simultaneous adjustment of real and imaginary parts of virtual impedance.Because virtual impedance is an open-loop control that imposes DG to the risk of overload,a new closed-loop structure is designed that uses residual capacity and absorbed power as feedback.Accordingly,virtual impedance can be safely adjusted in the feasible range until the power limit is reached.In addi-tion,a fuzzy integral controller is adopted to improve the dynamics and convergence of the power distribution,and its performance is found to be superior to linear integral controllers.Finally,simulations and control hardware-in-the-loop experiments are conducted to verify the effectiveness and usefulness of the proposed control strategy.
基金supported in part by the National Natural Science Foundation of China(No.U2166602)in part by the Major Special Project of Hunan Province(No.2020GK1010)in part by the Innovation Young Talents Program of Changsha Science and Technology Bureau(No.kq2107005).
文摘Line commutated converter based high-voltage direct-current(LCC-HVDC)transmissions are prone to harmonic oscillation under weak grids.Impedance modeling is an effective method for assessing interaction stability.Firstly,this paper proposes an improved calculation method for the DC voltage and AC currents of commutation stations to address the complex linearization of the commutation process and constructs an overall harmonic state-space(HSS)model of an LCC-HVDC.Based on the HSS model,the closed-loop AC impedances on the LCC-HVDC sending and receiving ends are then derived and verified.The impedance characteristics of the LCC-HVDC are then analyzed to provide a physical explanation for the harmonic oscillation of the system.The effects of the grid strength and control parameters on system stability are also analyzed.To improve the impedance characteristics and operating stability of the LCC-HVDC system,a virtual impedance based stability enhancement control is proposed,and a parameter design method is considered to ensure satisfactory phase margins at both the sending and receiving ends.Finally,simulation results are presented to verify the validity of the impedance model and virtual impedance based stability enhancement control.
基金This work was supported in part by the National Natural Science Foundation of China(51807130)the National key research and development program of China(2018YFB0904700)+1 种基金the Major Science and Technology Projects in Shanxi Province(20181102028)the Postgraduate Education Innovation Project of Shanxi Province(2019BY048)。
文摘Multi-paralleled bidirectional power converters(BPCs)are commonly used to improve the power capacity and reliability in an AC/DC hybrid microgrid.However,circulating current through multi-BPCs has been one of the challenges and it can be aggravated in the presence of non-ideal operating conditions,such as unbalanced AC voltages,and the mismatch of hardware parameters.In order to suppress the circulating current,this paper proposes a distributed method based on adaptive virtual impedance,which also employs positive sequence power droop control and voltage deviation compensation control.The traditional positive sequence power droop control is adopted to only regulate the positive components of the BPCs output voltage.The negative sequence power term is fed to an adaptive virtual impedance generator to modify the damping characteristics of the BPCs.Also,an adaptive virtual impedance-based voltage deviation compensation method is proposed to recover the fluctuated output voltage of the BPCs due to droop action and the power fluctuations.The fully distributed regulation of adaptive virtual impedance enables the load power to be shared accurately among BPC modules and thus the circulating current can be effectively suppressed.The proposed control strategy does not require an additional communication system and the precise parameters of hardware equipment and line impedance.Furthermore,the effectiveness of the proposed method is verified by the experimental results.
基金supported in part by the Fundamental Research Funds for the Central Universities (No.2022SCU12005)the General Project of Natural Science Foundation of Sichuan Province (No.2022NSFSC0262)。
文摘This paper proposes a joint limiting control strategy for suppressing DC fault current and arm current in modular multilevel converter-based high-voltage direct current(MMC-HVDC) systems, which includes two target-oriented current limiting controls. To limit the DC fault current in the early fault stage, an equivalent modular multilevel converter(MMC) impedance is obtained, and its high-frequency part is reshaped by introducing virtual impedance, which is realized by adjusting the inserted submodules adaptively. Following the analysis of MMC control characteristics, the arm current limiting strategy is investigated, with results showing that the inner-loop control has significant effects on arm current and that a simple low-pass filter can reduce the arm current in the fault period. Finally, by combining the virtual impedance shaping and innerloop control, the fault currents of DC lines and MMC arms can be suppressed simultaneously, which can not only alleviate the interrupting pressure of the DC circuit breaker, but also prevent the MMC from being blocked by the arm overcurrent. Theoretical analysis conclusions and the proposed strategy are verified offline by a digital time-domain simulation on Power Systems Computer Aided Design/Electromagnetic Transients including DC platform, and experiment on a real-time digital simulator platform.
基金supported by the Science and Technology Project of State Grid Corporation of China(No.5400-202219417A-2-0-ZN)。
文摘The effects of nonlinear loads on voltage quality represent an emerging concern for islanded microgrids.Existing research works have mainly focused on harmonic power sharing among multiple inverters,which ignores the diversity of different inverters to mitigate harmonics from nonlinear loads.As a result,the voltage quality of microgrids cannot be effectively improved.To address this issue,this study proposes an adaptive harmonic virtual impedance(HVI)control for improving voltage quality of microgrids.Based on the premise that no inverter is overloaded,the main objective of the proposed control is to maximize harmonic power absorption by shaping the lowest output impedances of inverters.To achieve this,the proposed control is utilized to adjust the HVI of each inverter based on its operation conditions.In addition,the evaluation based on Monte Carlo harmonic power flow is designed to assess the performance of the proposed control in practice.Finally,comparative studies and control-in-the-loop experiments are conducted.
文摘Lock-in amplifiers are used to detect and measure very small alternating current(AC)signals down to the range of nVs.Accurate measurements can be made even when the small signals are buried by noise thousands of times larger.With the digital signal processing(DSP)technology involved in modern instrumentation,a lock-in amplifier is more versatile in sensing and recovering small signals.Combining the virtual instrumentation technology,we reorganize the functional blocks of a programmable lock-in amplifier and build it as a virtual spectrum analyzer,virtual impedance meter,virtual network analyzer,virtual semiconductor parameter analyzer,signal generator,etc.A 4 layer model is used to implement these virtual instruments.The same virtual instrument can also be implemented on a general purpose FPGA developing board.
基金supported by the National Natural Science Foundation of China(No.62063016)the Science and Technology Plan of Gansu Province(No.25JRRA088).
文摘Conventional droop control in multi-parallel grid-forming inverters exhibits poor reactive power sharing accuracy due to line impedance mismatches.In this study,we proposed a coordination control strategy integrating adaptive virtual impedance with dynamic Q-V droop regulation to overcome this issue.We established a coupling model between the line impedance and power allocation to determine the quantitative relationship between reactive power deviation and impedance difference and to analyze the mechanism of reactive power deviation formation.Based on this,we proposed a transformer neural network-based online identification method for adaptive virtual impedance and dynamic droop coefficients.The self-attention mechanism dynamically characterizes the spatial distribution features of the impedance parameters considering the real-time voltage/reactive power time-series data as inputs to realize the dynamic impedance compensation without communication interaction.The contradiction constraint between the voltage drop and distribution accuracy caused by the introduction of conventional virtual impedance is improved by dynamic droop coefficient reconstruction.Lastly,we established a hardware-in-the-loop simulation platform to experimentally validate the operational efficacy and dynamic performance of the proposed control strategy under various grid scenarios.
文摘In this paper,inspired by the concept of virtual inertia in alternating current(AC)systems,a virtual impedance controller is proposed for the dynamic improvement of direct current microgrids(DCMGs).A simple and inexpensive method for injecting inertia into the system is used to adjust the output power of each distributed generation unit without using additional equipment.The proposed controller consists of two components:a virtual capacitor and a virtual inductor.These virtual components can change the rate of change of the DC bus voltage and also improve the transient response.A small-signal analysis is carried out to verify the impact of the proposed control strategy.Numerical simulation studies validate the effectiveness of the proposed solution for increasing the inertia of DCMGs.
基金supported by the National Key Research and Development Program of China(2022YFE0101900)the National Natural Science Foundation of China(52107214)the Provincial Key Research and Development Program of Inner Mongolia(2021D0026).
文摘This paper proposes an active islanding method based on impedance measurement for DC microgrids with multiple distributed generators(DGs).In the various presented methods,impedance-based islanding detection faces challenges in DC microgrids with multiple DGs due to the randomness of renewable energy.In the proposed method,the virtual impedance controls are designed to reshape the output impedance of DGs and the input impedance of loads at the detection frequency to ensure a clear distinction between grid-connected conditions and islanding conditions.Thus,the islanding events can be effectively detected under conditions that lead to a distinct decrease in system impedance.In this way,the accuracy of islanding detection is improved.Moreover,a dynamic allocation of the role algorithm is proposed to organize the orderly disturbance injection of DGs without communication,avoiding disturbance dilution.In addition,the selection principle of the frequency of disturbance current is proposed by studying the sensitivity of the point of common coupling(PCC)voltage to the disturbance.As a result,the negative influence of multiple DGs,loads,and disturbance dilution can be avoided compared to the existing methods.Finally,the effectiveness of the proposed method has been verified by both simulation and experimental results.
基金supported by the Natural Science Foundation of Shandong Province of China(ZR2017MEE053).
文摘Reactive power sharing cannot be achieved using many existing microgrid(MG)control methods,but the convergence speed of these methods is slow.To solve these problems,a finite-time distributed control approach is proposed in this paper,which is based on the hierarchical control structure.The hierarchical control structure consists of a dual loop control,a droop control used as a primary control and a secondary control.First,the secondary controller is modeled,and the MG system composed of distributed generators(DGs)is considered as a multi-agent system.The secondary controller consists of a frequency regulator,voltage regulator and power regulator.Secondly,the adaptive virtual impedance module is established,using the output of the reactive power regulator as its input.Thirdly,a dual loop controller is combined with a primary controller and secondary controller to generate a pulse width modulation(PWM)signal to control the power and voltage of the MG.In order to reduce the fluctuation of the MG,a damping module is introduced when the structure of the system changes.Finally,the stability of the proposed control strategy is proved by the related theorems.A simulation system is established in the Matlab environment,and the simulation results show that the proposed method is effective.
文摘In recent years,the hybrid AC-DC microgrid has been well accepted as it combines the advantages of both AC and DC systems.As the microgrid contains both DC sub-grids and AC sub-grids,interlinking DC-AC converters are essential.Meanwhile,considering the nonlinear AC loads may deteriorate the voltage quality of the AC bus,embedding an ancillary harmonic compensation function to the interlinking converters is promising.However,the conventional harmonic control methods used for active power filters(APFs)may not be suitable for the interlinking converters due to the main purpose of it is to exchange real and reactive power between the DC and AC sub-grids.The switching frequency is preferred to be lower than the APFs when the capacity of the microgrid is large.At low switching frequency,harmonic compensation performance or even the system stability may be affected.In this paper,a harmonic compensation approach suitable for hybrid AC-DC interlinking converters at low switching frequency is proposed.Through feeding the PWM reference signal with the harmonic compensation component directly to avoid the multi-loop control path of the fundamental component,the proposed method can achieve the effective harmonics compensation without being limited by the closed-loop control bandwidth.The proposed method,modeling approaches,stability analysis,as well as detailed virtual impedance design are presented.Experimental verification is also provided.
基金supported by the U.S.National Science Foundation under Grant#2124849.
文摘This paper investigates integration of distributed energy resources(DERs)in microgrids(MGs)through two-stage power conversion structures consisting of DC-DC boost converter and DC-AC voltage source converter(VSC)subsystems.In contrast to existing investigations that treated DC-link voltage as an ideal constant voltage,this paper considers the non-ideal dynamic coupling between both subsystems for completeness and higher accuracy,which introduces additional DC-side dynamics to the VSC.The analysis shows parameters of the boost converter’s power model that impact stability through the DC-link.Carefully selecting these parameters can mitigate this effect on stability and improve dynamic performance across the DC-link.Hence,an optimization framework is developed to facilitate in selecting adequate boost converter parameters in designing a stable voltage source converter-based microgrid(VSC-MG).The developed optimization framework,based on particle swarm optimization,considers dynamic coupling between both subsystems and is also effective in avoiding inadequate boost converter parameters capable of propagating instability through the DC-link to the VSC.Simulations are performed with MATLAB/Simulink to validate theoretical analyses.
文摘The coordinated control of parallel three-phase fourwire converters in autonomous AC microgrids is investigated in this paper.First,based on droop control,virtual impedance is inserted in positive-,negative-and zero-sequences to enhance system damping and imbalance power sharing.Then,to facilitate virtual impedance design,small signal models of the three-sequence equivalent circuits are established respectively.Corresponding indexes are proposed to comprehensively evaluate the impact of sequence virtual impedance on current sharing accuracy,voltage quality at the point of common coupling(PCC)and system stability.In addition,constraint of DClink voltage is also considered to avoid over modulation when subjected to unbalanced loads.Furthermore,to address the PCC voltage degradation resulting from virtual impedance,a voltage imbalance compensation method,based on low-bandwidth communication,is proposed.Finally,simulation and experimental results are provided to verify the correctness of the theory model,indicating that the proposed method can achieve PCC voltage restoration while guaranteeing the current sharing accuracy with desirable dynamics.
