An equivalent source-load MTDC system including DC voltage control units,power control units and interconnected DC lines is considered in this paper,which can be regarded as a generic structure of low-voltage DC micro...An equivalent source-load MTDC system including DC voltage control units,power control units and interconnected DC lines is considered in this paper,which can be regarded as a generic structure of low-voltage DC microgrids,mediumvoltage DC distribution systems or HVDC transmission systems with a common DC bus.A reduced-order model is proposed with a circuit structure of a resistor,inductor and capacitor in parallel for dynamic stability analysis of the system in DC voltage control timescale.The relationship between control parameters and physical parameters of the equivalent circuit can be found,which provides an intuitive insight into the physical meaning of control parameters.Employing this model,a second-order characteristic equation is further derived to investigate system dynamic stability mechanisms in an analytical approach.As a result,the system oscillation frequency and damping are characterized in a straight forward manner,and the role of electrical and control parameters and different system-level control strategies in system dynamic stability in DC voltage control timescale is defined.The effectiveness of the proposed reduced-order model and the correctness of the theoretical analysis are verified by simulation based on PSCAD/EMTDC and an experiment based on a hardware low-voltage MTDC system platform.展开更多
Stability of grid-connected VSCs in DC voltage control(DVC)timescales(i.e.,the frequency range of dynamics covering converter outer controls)has recently caught increased attention,while the existing approaches,such a...Stability of grid-connected VSCs in DC voltage control(DVC)timescales(i.e.,the frequency range of dynamics covering converter outer controls)has recently caught increased attention,while the existing approaches,such as eigenvalue analysis and dq-domain impedance analysis,have respective limitations on addressing these types of stability issues.This paper proposes an alternative net damping criterion dedicated for analyzing the DVC timescale stability in a multi-VSC system.This criterion is strictly mapped from the Nyquist stability criterion utilizing the gain margin concept,which preserves the advantages of the classical positive net damping criterion suggested by Canay[20]–allowing for decomposition analysis of a subsystem’s contribution to the closed-loop stability in a single-input single-output(SISO)framework,but overcomes its deficiency of possibly erroneous prediction of system dynamic behaviors.Case studies show that the proposed criterion can correctly predict some unstable conditions(e.g.,monotonic divergence)which cannot be identified by the classical net damping criterion.Additionally,the condition for when the classical criterion is available is also pointed out,the proposed criterion can also act as a complement of the classical criterion for stability examination.展开更多
This paper presents sliding mode technique associated to the direct torque control （DTC） for an isolated-loaded permanent magnet synchronous generator （PMSG）. The machine delivers an active power to a DC-load via ...This paper presents sliding mode technique associated to the direct torque control （DTC） for an isolated-loaded permanent magnet synchronous generator （PMSG）. The machine delivers an active power to a DC-load via a converter connected to a single capacitor on the DC side. Since the converter/capacitor model is nonlinear, the sliding mode technique constitutes a powerful tool to ensure the DC-bus voltage regulation. The computer simulations are provided to verify the validity of the proposed control algorithm.展开更多
Multi-terminal direct current(MTDC)grids provide the possibility of meshed interconnections between regional power systems and various renewable energy resources to boost supply reliability and economy.The modular mul...Multi-terminal direct current(MTDC)grids provide the possibility of meshed interconnections between regional power systems and various renewable energy resources to boost supply reliability and economy.The modular multilevel converter(MMC)has become the basic building block for MTDC and DC grids due to its salient features,i.e.,modularity and scalability.Therefore,the MMC-based MTDC systems should be pervasively embedded into the present power system to improve system performance.However,several technical challenges hamper their practical applications and deployment,including modeling,control,and protection of the MMC-MTDC grids.This paper presents a comprehensive investigation and reference in modeling,control,and protection of the MMC-MTDC grids.A general overview of state-of-the-art modeling techniques of the MMC along with their performance in simulation analysis for MTDC applications is provided.A review of control strategies of the MMC-MTDC grids which provide AC system support is presented.State-of-the art protection techniques of the MMCMTDC systems are also investigated.Finally,the associated research challenges and trends are highlighted.展开更多
The cluster DC voltage balancing control adopting zero-sequence voltage injection is appropriate for the starconnected cascaded H-bridge STATCOM because no zerosequence currents are generated in the three-phase three-...The cluster DC voltage balancing control adopting zero-sequence voltage injection is appropriate for the starconnected cascaded H-bridge STATCOM because no zerosequence currents are generated in the three-phase three-wire system.However,as the zero-sequence voltage is expressed in trigonometric form,traditional control methods involve many complicated operations,such as the square-root,trigonometric operations,and inverse tangent operations.To simplify cluster voltage balancing control,this paper converts the zero-sequence voltage to the dq frame in a DC representation by introducing a virtually orthogonal variable,and the DC components of the zero-sequence voltage in the dq frame are regulated linearly by proportional integral regulators,rather than being calculated from uneven active powers in traditional controls.This removes all complicated operations.Finally,this paper presents simulation and experimental results for a 400 V±7.5 kvar star-connected STATCOM,in balanced and unbalanced scenarios,thereby verifying the effectiveness of the proposed control.展开更多
Impedance analysis is an effective method to analyze the oscillation issue associated with grid-connected photovoltaic systems.However,the existing impedance modeling of a gridconnected photovoltaic inverter usually o...Impedance analysis is an effective method to analyze the oscillation issue associated with grid-connected photovoltaic systems.However,the existing impedance modeling of a gridconnected photovoltaic inverter usually only considers the effect of a single perturbation frequency,ignoring the coupling frequency response between the internal control loops of a grid-connected inverter,which severely affects the accuracy of the stability analysis.Hence,a method of impedance modeling and stability analysis for grid-connected photovoltaic inverters considering cross-coupling frequency is proposed in this paper.First,the generation mechanism of frequency coupling in gridconnected photovoltaic inverters,and the relationship between the coupling frequency and perturbation frequency are analyzed.Secondly,a sequence impedance model of grid-connected photovoltaic systems considering the coupling frequency is established by using the harmonic linearization method.The impact of DC bus voltage control strategy on frequency coupling characteristics of a grid-connected photovoltaic system is evaluated,and the impact of a coupling frequency term on system stability is quantitatively analyzed.Finally,the advantages of the proposed method are verified by several simulations.The results show that the proposed impedance model can accurately predict the potential resonance points of the system,and the coupling frequency characteristics will become much stronger with smaller DC bus capacitance or larger bandwidth of the DC bus controller.展开更多
基金This work was supported in part by the National Natural Science Foundation of China under Grant No.51977142.
文摘An equivalent source-load MTDC system including DC voltage control units,power control units and interconnected DC lines is considered in this paper,which can be regarded as a generic structure of low-voltage DC microgrids,mediumvoltage DC distribution systems or HVDC transmission systems with a common DC bus.A reduced-order model is proposed with a circuit structure of a resistor,inductor and capacitor in parallel for dynamic stability analysis of the system in DC voltage control timescale.The relationship between control parameters and physical parameters of the equivalent circuit can be found,which provides an intuitive insight into the physical meaning of control parameters.Employing this model,a second-order characteristic equation is further derived to investigate system dynamic stability mechanisms in an analytical approach.As a result,the system oscillation frequency and damping are characterized in a straight forward manner,and the role of electrical and control parameters and different system-level control strategies in system dynamic stability in DC voltage control timescale is defined.The effectiveness of the proposed reduced-order model and the correctness of the theoretical analysis are verified by simulation based on PSCAD/EMTDC and an experiment based on a hardware low-voltage MTDC system platform.
基金This work was supported in part by the Research Grants Council of Hong Kong under Grant GRF 17207818the National Natural Science Foundation of China under Grant 51677160the Themebased Research Scheme(TRS)under T23-701/14-N.
文摘Stability of grid-connected VSCs in DC voltage control(DVC)timescales(i.e.,the frequency range of dynamics covering converter outer controls)has recently caught increased attention,while the existing approaches,such as eigenvalue analysis and dq-domain impedance analysis,have respective limitations on addressing these types of stability issues.This paper proposes an alternative net damping criterion dedicated for analyzing the DVC timescale stability in a multi-VSC system.This criterion is strictly mapped from the Nyquist stability criterion utilizing the gain margin concept,which preserves the advantages of the classical positive net damping criterion suggested by Canay[20]–allowing for decomposition analysis of a subsystem’s contribution to the closed-loop stability in a single-input single-output(SISO)framework,but overcomes its deficiency of possibly erroneous prediction of system dynamic behaviors.Case studies show that the proposed criterion can correctly predict some unstable conditions(e.g.,monotonic divergence)which cannot be identified by the classical net damping criterion.Additionally,the condition for when the classical criterion is available is also pointed out,the proposed criterion can also act as a complement of the classical criterion for stability examination.
文摘This paper presents sliding mode technique associated to the direct torque control （DTC） for an isolated-loaded permanent magnet synchronous generator （PMSG）. The machine delivers an active power to a DC-load via a converter connected to a single capacitor on the DC side. Since the converter/capacitor model is nonlinear, the sliding mode technique constitutes a powerful tool to ensure the DC-bus voltage regulation. The computer simulations are provided to verify the validity of the proposed control algorithm.
基金funded by SGCC Science and Technology Program under project Research on Electromagnetic Transient Simulation Technology for Large-scale MMC-HVDC Systems.
文摘Multi-terminal direct current(MTDC)grids provide the possibility of meshed interconnections between regional power systems and various renewable energy resources to boost supply reliability and economy.The modular multilevel converter(MMC)has become the basic building block for MTDC and DC grids due to its salient features,i.e.,modularity and scalability.Therefore,the MMC-based MTDC systems should be pervasively embedded into the present power system to improve system performance.However,several technical challenges hamper their practical applications and deployment,including modeling,control,and protection of the MMC-MTDC grids.This paper presents a comprehensive investigation and reference in modeling,control,and protection of the MMC-MTDC grids.A general overview of state-of-the-art modeling techniques of the MMC along with their performance in simulation analysis for MTDC applications is provided.A review of control strategies of the MMC-MTDC grids which provide AC system support is presented.State-of-the art protection techniques of the MMCMTDC systems are also investigated.Finally,the associated research challenges and trends are highlighted.
基金supported by National Key R&D Program of China(No.2021YFB2401100)the Science and Technology Project of State Grid Corporation of China(No.5211DS22002C).
文摘The cluster DC voltage balancing control adopting zero-sequence voltage injection is appropriate for the starconnected cascaded H-bridge STATCOM because no zerosequence currents are generated in the three-phase three-wire system.However,as the zero-sequence voltage is expressed in trigonometric form,traditional control methods involve many complicated operations,such as the square-root,trigonometric operations,and inverse tangent operations.To simplify cluster voltage balancing control,this paper converts the zero-sequence voltage to the dq frame in a DC representation by introducing a virtually orthogonal variable,and the DC components of the zero-sequence voltage in the dq frame are regulated linearly by proportional integral regulators,rather than being calculated from uneven active powers in traditional controls.This removes all complicated operations.Finally,this paper presents simulation and experimental results for a 400 V±7.5 kvar star-connected STATCOM,in balanced and unbalanced scenarios,thereby verifying the effectiveness of the proposed control.
文摘Impedance analysis is an effective method to analyze the oscillation issue associated with grid-connected photovoltaic systems.However,the existing impedance modeling of a gridconnected photovoltaic inverter usually only considers the effect of a single perturbation frequency,ignoring the coupling frequency response between the internal control loops of a grid-connected inverter,which severely affects the accuracy of the stability analysis.Hence,a method of impedance modeling and stability analysis for grid-connected photovoltaic inverters considering cross-coupling frequency is proposed in this paper.First,the generation mechanism of frequency coupling in gridconnected photovoltaic inverters,and the relationship between the coupling frequency and perturbation frequency are analyzed.Secondly,a sequence impedance model of grid-connected photovoltaic systems considering the coupling frequency is established by using the harmonic linearization method.The impact of DC bus voltage control strategy on frequency coupling characteristics of a grid-connected photovoltaic system is evaluated,and the impact of a coupling frequency term on system stability is quantitatively analyzed.Finally,the advantages of the proposed method are verified by several simulations.The results show that the proposed impedance model can accurately predict the potential resonance points of the system,and the coupling frequency characteristics will become much stronger with smaller DC bus capacitance or larger bandwidth of the DC bus controller.
