Frequency regulation of voltage source converter-based multi-terminal high-voltage direct current(VSC-MTDC)system with offshore wind farms enhances the frequency stability by compensating the power for a disturbed AC ...Frequency regulation of voltage source converter-based multi-terminal high-voltage direct current(VSC-MTDC)system with offshore wind farms enhances the frequency stability by compensating the power for a disturbed AC system.However,it is difficult to reasonably allocate frequency-regulation resources due to a lack of coordination mechanisms between wind farms and the MTDC system.Moreover,it is difficult for the frequency control of the wind farms to manage changes in wind speed;and the risk of wind-turbine stalls is high.Thus,based on the kinetic energy of wind turbines and the power margin of the converters,the frequency-regulation capability of wind turbines is evaluated,and a dynamic frequency-support scheme considering the real-time frequency-support capability of the wind turbines and system frequency evolution is proposed to improve the frequency-support performance.A power adaptation technique at variable wind speeds is developed;the active power in the frequency-support stage and restoration stage is switched according to the wind speed.A hierarchical zoning frequency-regulation scheme is designed to use the frequency-regulation resources of different links in the MTDC system with wind farms.The simulation results show that the novel frequency-regulation strategy maintains frequency stability with wind-speed changes and avoids multiple frequency dips.展开更多
Mid-and high-frequency resonance(MHFR)is highly likely to occur at the sending end of voltage source con-verter-based ultra-high voltage direct current(VSC-UHVDC)for large-scale renewable energy transmission.It is of ...Mid-and high-frequency resonance(MHFR)is highly likely to occur at the sending end of voltage source con-verter-based ultra-high voltage direct current(VSC-UHVDC)for large-scale renewable energy transmission.It is of great im-portance to investigate the resonance characteristics and the corresponding suppression strategies.Firstly,this paper intro-duces the overall'control scheme of VSC-UHVDC for large-scale renewable energy transmission.Then,the impedance mod-els of VSC under grid-forming control with AC voltage coordi-nated control are established.The mid-and high-frequency im-pedance characteristics of VSC-UHVDC are analyzed.The key factors affecting the impedance characteristics have been re-vealed,including the AC voltage control,the voltage feedfor-ward,the inner current loop,the positive-sequence and nega-tive-sequence independent control(PSNSIC),and the control de-lay.The MHFR characteristics at the sending-end system are analyzed in the whole operation process,including the black start and the normal power transmission operation.An integrat-ed control scheme is proposed to address the MHFR problems.Finally,extensive case studies are conducted on a planned VSC-UHVDC project to verify the theoretical analysis.展开更多
During the power modulation process of line com-mutated converter-based high-voltage direct current(LCC-HVDC),the transient power mismatch between the fast-change converter station and the slow-response reactive power...During the power modulation process of line com-mutated converter-based high-voltage direct current(LCC-HVDC),the transient power mismatch between the fast-change converter station and the slow-response reactive power compen-sators(RPCs)can cause transient voltage disturbances at the weak sending end of the AC grid.To mitigate such voltage dis-turbances,this paper proposes a coordinated feedback power control method for the hybrid multi-infeed HVDC(HMI-HVDC)system comprising an LCC-HVDC and voltage source converter-based HVDC(VSC-HVDC)systems.The mechanism of the disturbance caused by transient power mismatch is quan-titatively analyzed,and the numerical relationship between the instantaneous unbalanced power and the AC voltage is derived.Based on the numerical relationship and considering the time-varying relationship of reactive power between converter sta-tions,the unbalanced power is set as the feedback and coordi-nately distributed among the inverter stations of VSC-HVDC,and the rectifier and the inverter stations of LCC-HVDC.Simu-lation results verify that the proposed method can effectively suppress voltage disturbance without relying on remote commu-nication,thus enhancing the operation performance of the HMI-HVDC system.展开更多
In the presence of an MMC-HVDC system,current differential protection(CDP)has the risk of failure in operation under an internal fault.In addition,CDP may also incur security issues in the presence of current transfor...In the presence of an MMC-HVDC system,current differential protection(CDP)has the risk of failure in operation under an internal fault.In addition,CDP may also incur security issues in the presence of current transformer(CT)saturation and outliers.In this paper,a current trajectory image-based protection algorithm is proposed for AC lines connected to MMC-HVDC stations using a convolution neural network improved by a channel attention mechanism(CA-CNN).Taking the dual differential currents as two-dimensional coordinates of the moving point,the moving-point trajectories formed by differential currents have significant differences under internal and external faults.Therefore,internal faults can be identified using image recognition based on CA-CNN.This is improved by a channel attention mechanism,data augmentation,and adaptive learning rate.In comparison with other machine learning algorithms,the feature extraction ability and accuracy of CA-CNN are greatly improved.Various fault conditions like different net-work structures,operation modes,fault resistances,outliers,and current transformer saturation,are fully considered to verify the superiority of the proposed protection algorithm.The results confirm that the proposed current trajectory image-based protection algorithm has strong learning and generalizability,and can identify internal faults reliably.展开更多
With the rapid development of power-electronicsenabled power systems,the new converter-based generators are deteriorating the small-signal stability of the power system.Although the numerical differentiation method ha...With the rapid development of power-electronicsenabled power systems,the new converter-based generators are deteriorating the small-signal stability of the power system.Although the numerical differentiation method has been widely used for approximately calculating the eigenvalue sensitivities,its accuracy has not been carefully investigated.Besides,the element-based formulation for computing closed-form eigenvalue sensitivities has not been used in any commercial software due to the average efficiency,complicated formulation,and errorprone characteristics.Based on the matrix calculus,this paper proposes an easily manipulated formulation of the closed-form eigenvalue sensitivities with respect to the power generation.The distinguishing feature of the formulation is that all the formulas consist of vector and matrix operations,which can be performed by developed numerical algorithms to take full advantages of architectural features of the modern computer.The tests on WSCC 3-machine 9-bus system,New England 10-machine 39-bus system,and IEEE 54-machine 118-bus system show that the accuracy of the proposed formulation is superior to the numerical differentiation method and the efficiency is also greatly improved compared to the element-based closed-form formulation.The proposed formulation will be helpful to perform a more accurate and faster stability analysis of a power grid with converter-based devices.展开更多
When a renewable energy station(RES)connects to the rectifier station(RS)of a modular multilevel converterbased high-voltage direct current(MMC-HVDC)system,the voltage at the point of common coupling(PCC)is determined...When a renewable energy station(RES)connects to the rectifier station(RS)of a modular multilevel converterbased high-voltage direct current(MMC-HVDC)system,the voltage at the point of common coupling(PCC)is determined by RS control methods.For example,RS control may become saturated under fault,and causes the RS to change from an equivalent voltage source to an equivalent current source,making fault analysis more complicated.In addition,the grid code of the fault ride-through(FRT)requires the RES to output current according to its terminal voltage.This changes the fault point voltage and leads to RES voltage regulation and current redistribution,resulting in fault response interactions.To address these issues,this study describes how an MMC-integrated system has five operation modes and three common characteristics under the duration of the fault.The study also reveals several instances of RS performance degradation such as AC voltage loop saturation,and shows that RS power reversal can be significantly improved.An enhanced AC FRT control method is proposed to achieve controllable PCC voltage and continuous power transmission by actively reducing the PCC voltage amplitude.The robustness of the method is theoretically proven under parameter variation and operation mode switching.Finally,the feasibility of the proposed method is verified through MATLAB/Simulink results.展开更多
基金supported by the National Key R&D Program of China(No.2022YFB2402700).
文摘Frequency regulation of voltage source converter-based multi-terminal high-voltage direct current(VSC-MTDC)system with offshore wind farms enhances the frequency stability by compensating the power for a disturbed AC system.However,it is difficult to reasonably allocate frequency-regulation resources due to a lack of coordination mechanisms between wind farms and the MTDC system.Moreover,it is difficult for the frequency control of the wind farms to manage changes in wind speed;and the risk of wind-turbine stalls is high.Thus,based on the kinetic energy of wind turbines and the power margin of the converters,the frequency-regulation capability of wind turbines is evaluated,and a dynamic frequency-support scheme considering the real-time frequency-support capability of the wind turbines and system frequency evolution is proposed to improve the frequency-support performance.A power adaptation technique at variable wind speeds is developed;the active power in the frequency-support stage and restoration stage is switched according to the wind speed.A hierarchical zoning frequency-regulation scheme is designed to use the frequency-regulation resources of different links in the MTDC system with wind farms.The simulation results show that the novel frequency-regulation strategy maintains frequency stability with wind-speed changes and avoids multiple frequency dips.
基金This work was supported by National Key Research and Development Program of China(No.2023YFB2405900)the Technology Project of China Southern PowerGrid(No.ZBKJXM20232198).
文摘Mid-and high-frequency resonance(MHFR)is highly likely to occur at the sending end of voltage source con-verter-based ultra-high voltage direct current(VSC-UHVDC)for large-scale renewable energy transmission.It is of great im-portance to investigate the resonance characteristics and the corresponding suppression strategies.Firstly,this paper intro-duces the overall'control scheme of VSC-UHVDC for large-scale renewable energy transmission.Then,the impedance mod-els of VSC under grid-forming control with AC voltage coordi-nated control are established.The mid-and high-frequency im-pedance characteristics of VSC-UHVDC are analyzed.The key factors affecting the impedance characteristics have been re-vealed,including the AC voltage control,the voltage feedfor-ward,the inner current loop,the positive-sequence and nega-tive-sequence independent control(PSNSIC),and the control de-lay.The MHFR characteristics at the sending-end system are analyzed in the whole operation process,including the black start and the normal power transmission operation.An integrat-ed control scheme is proposed to address the MHFR problems.Finally,extensive case studies are conducted on a planned VSC-UHVDC project to verify the theoretical analysis.
基金This work was supported in part by the State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources(No.LAPS23020).
文摘During the power modulation process of line com-mutated converter-based high-voltage direct current(LCC-HVDC),the transient power mismatch between the fast-change converter station and the slow-response reactive power compen-sators(RPCs)can cause transient voltage disturbances at the weak sending end of the AC grid.To mitigate such voltage dis-turbances,this paper proposes a coordinated feedback power control method for the hybrid multi-infeed HVDC(HMI-HVDC)system comprising an LCC-HVDC and voltage source converter-based HVDC(VSC-HVDC)systems.The mechanism of the disturbance caused by transient power mismatch is quan-titatively analyzed,and the numerical relationship between the instantaneous unbalanced power and the AC voltage is derived.Based on the numerical relationship and considering the time-varying relationship of reactive power between converter sta-tions,the unbalanced power is set as the feedback and coordi-nately distributed among the inverter stations of VSC-HVDC,and the rectifier and the inverter stations of LCC-HVDC.Simu-lation results verify that the proposed method can effectively suppress voltage disturbance without relying on remote commu-nication,thus enhancing the operation performance of the HMI-HVDC system.
基金supported in part by the Fundamental Research Funds for the Central Universities under Grant 2022JCCXJD01in part by Training Program of Innovation and Entrepreneurship for Undergraduates of China University of Mining and Technology(Beijing)under Grant 202204009.
文摘In the presence of an MMC-HVDC system,current differential protection(CDP)has the risk of failure in operation under an internal fault.In addition,CDP may also incur security issues in the presence of current transformer(CT)saturation and outliers.In this paper,a current trajectory image-based protection algorithm is proposed for AC lines connected to MMC-HVDC stations using a convolution neural network improved by a channel attention mechanism(CA-CNN).Taking the dual differential currents as two-dimensional coordinates of the moving point,the moving-point trajectories formed by differential currents have significant differences under internal and external faults.Therefore,internal faults can be identified using image recognition based on CA-CNN.This is improved by a channel attention mechanism,data augmentation,and adaptive learning rate.In comparison with other machine learning algorithms,the feature extraction ability and accuracy of CA-CNN are greatly improved.Various fault conditions like different net-work structures,operation modes,fault resistances,outliers,and current transformer saturation,are fully considered to verify the superiority of the proposed protection algorithm.The results confirm that the proposed current trajectory image-based protection algorithm has strong learning and generalizability,and can identify internal faults reliably.
基金supported by National Natural Science Foundation of China(No.51967001,No.51967002)Guangxi Provincial Natural Science Foundation of China(No.2018JJA160164)。
文摘With the rapid development of power-electronicsenabled power systems,the new converter-based generators are deteriorating the small-signal stability of the power system.Although the numerical differentiation method has been widely used for approximately calculating the eigenvalue sensitivities,its accuracy has not been carefully investigated.Besides,the element-based formulation for computing closed-form eigenvalue sensitivities has not been used in any commercial software due to the average efficiency,complicated formulation,and errorprone characteristics.Based on the matrix calculus,this paper proposes an easily manipulated formulation of the closed-form eigenvalue sensitivities with respect to the power generation.The distinguishing feature of the formulation is that all the formulas consist of vector and matrix operations,which can be performed by developed numerical algorithms to take full advantages of architectural features of the modern computer.The tests on WSCC 3-machine 9-bus system,New England 10-machine 39-bus system,and IEEE 54-machine 118-bus system show that the accuracy of the proposed formulation is superior to the numerical differentiation method and the efficiency is also greatly improved compared to the element-based closed-form formulation.The proposed formulation will be helpful to perform a more accurate and faster stability analysis of a power grid with converter-based devices.
基金supported in part by the National Key Research and Development Program of China(No.2020YFF0305800)State Grid Science Technology Project(No.520201210025)。
文摘When a renewable energy station(RES)connects to the rectifier station(RS)of a modular multilevel converterbased high-voltage direct current(MMC-HVDC)system,the voltage at the point of common coupling(PCC)is determined by RS control methods.For example,RS control may become saturated under fault,and causes the RS to change from an equivalent voltage source to an equivalent current source,making fault analysis more complicated.In addition,the grid code of the fault ride-through(FRT)requires the RES to output current according to its terminal voltage.This changes the fault point voltage and leads to RES voltage regulation and current redistribution,resulting in fault response interactions.To address these issues,this study describes how an MMC-integrated system has five operation modes and three common characteristics under the duration of the fault.The study also reveals several instances of RS performance degradation such as AC voltage loop saturation,and shows that RS power reversal can be significantly improved.An enhanced AC FRT control method is proposed to achieve controllable PCC voltage and continuous power transmission by actively reducing the PCC voltage amplitude.The robustness of the method is theoretically proven under parameter variation and operation mode switching.Finally,the feasibility of the proposed method is verified through MATLAB/Simulink results.
