The contribution of Renewable Energy Resources(RER)in the process of power generation is significantly high in the recent days since it paves the way for overcoming the issues like serious energy crisis and natural con...The contribution of Renewable Energy Resources(RER)in the process of power generation is significantly high in the recent days since it paves the way for overcoming the issues like serious energy crisis and natural contamination.This paper deals with the renewable energy based micro-grid as it is regarded as the apt solution for integrating the RER with the electrical frameworks.As thefixed droop coefficients in conventional droop control approaches have caused various limitations like low power-sharing and sudden drops of grid voltage in the Direct Current(DC)side,the Harmonized Membership Fuzzy Logic(MFL)droop control is employed in this present study.This proposed droop control for the hybrid PV-wind-battery system with MFL assists in achieving proper power-sharing and minimizing Total Harmonic Distortion(THD)in the emer-gency micro-grid.It eradicates the deviations in voltage and frequency with itsflexible and robust operation.The THD is reduced and attains the value of 3.1%compared to the traditional droop control.The simulation results of harmo-nized MFL droop control are analogized with the conventional approaches to vali-date the performance of the proposed method.In addition,the experimental results provided by the Field Programmable Gate Array(FPGA)based laboratory setup built using a solar photovoltaic(PV)and wind Permanent Magnet Synchro-nous Generator(PMSG)reaffirms the design.展开更多
System frequency must be kept very close to its nominal range to ensure the stability of an electric power grid.Excessive system frequency variations are able to result in load shedding,frequency instability,and even ...System frequency must be kept very close to its nominal range to ensure the stability of an electric power grid.Excessive system frequency variations are able to result in load shedding,frequency instability,and even generator damage.With increasing wind power penetration,there is rising concern about the reduction in inertia response and primary frequency control in the electric power grid.Converter-based wind generation is capable of providing inertia response and primary frequency response;nevertheless,the primary frequency and inertia responses of wind generation are different from those of conventional synchronous fleets;it is not completely understood how the primary frequency and inertia responses affect the given system under various disturbances and available kinetic energy levels.Simulations are used to investigate the influences of inertia and droop control strategies on the dynamic frequency responses,particularly the index of the second frequency drop under various disturbance and wind conditions.A quantitative analysis provides insight into setting of inertia and droop control coefficients for various wind and disturbance conditions to facilitate adequate dynamic frequency responses during frequency events.展开更多
As there exists sorts of distributed generators in microgrid, an integrated control strategy containing different control methods against corresponding generators should be applied. The strategy in this paper involves...As there exists sorts of distributed generators in microgrid, an integrated control strategy containing different control methods against corresponding generators should be applied. The strategy in this paper involves PQ control and droop control methods. The former aims at letting generators like PV output maximum power. The latter stems from inverter parallel technique and applies to controlling generators which can keep the network voltage steady to make the parallel system reach the minimum circulation point. Due to the unworthiness of droop control applied in low-voltage microgrid of which the impedance ratio is rather high, the paper adopts the droop control introducing virtual generator and virtual impedance. Based on theoretical analysis, simulation in Matlab is also implemented to verify the feasibility of the strategy.展开更多
High Voltage Direct Current (HVDC) electric power transmission is a promising technology for integrating offshore wind farms and interconnecting power grids in different regions. In order to maintain the DC voltage, d...High Voltage Direct Current (HVDC) electric power transmission is a promising technology for integrating offshore wind farms and interconnecting power grids in different regions. In order to maintain the DC voltage, droop control has been widely used. Transmission line loss constitutes an import part of the total power loss in a multi-terminal HVDC scheme. In this paper, the relation between droop controller design and transmission loss has been investigated. Different MTDC layout configurations are compared to examine the effect of droop controller design on the transmission loss.展开更多
Different droop control methods for PV-based communal grid networks (minigrids and microgrids) with different line resistances (R) and impedances (X) are modelled and simulated in MATLAB to determine the most efficien...Different droop control methods for PV-based communal grid networks (minigrids and microgrids) with different line resistances (R) and impedances (X) are modelled and simulated in MATLAB to determine the most efficient control method for a given network. Results show that active power-frequency (P-f) droop control method is the most efficient for low voltage transmission networks with low X/R ratios while reactive power-voltage (Q-V) droop control method is the most efficient for systems with high X/R ratios. For systems with complex line resistances and impedances, i.e. near unity X/R ratios, P-f or Q-V droop methods cannot individually efficiently regulate line voltage and frequency. For such systems, P-Q-f droop control method, where both active and reactive power could be used to control PCC voltage via shunt-connected inverters, is determined to be the most efficient control method. Results also show that shunt-connection of inverters leads to improved power flow control of interconnected communal grids by allowing feeder voltage regulation, load reactive power support, reactive power management between feeders, and improved overall system performance against dynamic disturbances.展开更多
To achieve the goal of carbon neutrality,renewable energy integration through a voltage source converter based multi-terminal direct current(VSC-MTDC)system has been identified as a promising solution.To tackle the si...To achieve the goal of carbon neutrality,renewable energy integration through a voltage source converter based multi-terminal direct current(VSC-MTDC)system has been identified as a promising solution.To tackle the significant DC voltage over-limit problem in a VSC-MTDC system during disturbances,this paper proposes a mode-switching strategy of droop control considering maximum DC voltage regulation capability.The close relationship between node injection powers and node DC voltages in the MTDC system is elaborated,and the most effective regulation approach of local injection power for limiting DC voltage deviation is presented.The operating point trajectories of different droop control explains that the DC voltage deviation can be minimized by fully utilizing the capacity of converters.Therefore,the mode-switching strategy with the maximum DC voltage regulation capability is realized by the switching between the voltage droop control and the constant maximum power control.In addition,a mode recovery process and a smooth switching method are developed to make converters regain the capability of maintaining DC voltage and reduce power fluctuation during mode switching,respectively.Furthermore,three cases are investigated to verify the effectiveness of the proposed mode-switching strategy.Compared with simulation results of the conventional droop control and the DC voltage deviation-dependent droop control,better performance of transient and steady-state DC voltage deviation is achieved through the proposed strategy.展开更多
The unbalanced state of charge(SOC)of distributed energy storage systems(DESSs)in autonomous DC microgrid causes energy storage units(ESUs)to terminate operation due to overcharge or overdischarge,which severely affec...The unbalanced state of charge(SOC)of distributed energy storage systems(DESSs)in autonomous DC microgrid causes energy storage units(ESUs)to terminate operation due to overcharge or overdischarge,which severely affects the power quality.In this paper,a fuzzy droop control for SOC balance and stability analysis of DC microgrid with DESSs is proposed to achieve SOC balance in ESUs while maintaining a stable DC bus voltage.First,the charge and discharge modes of ESUs are determined based on the power supply requirements of the DC microgrid.One-dimensional fuzzy logic is then applied to establish the relationship between SOC and the droop coefficient R,in the aforementioned two modes.In addition,when integrated with voltage-current double closed-loop control,SOC balance in different ESUs is realized.To improve the balance speed and precision,an exponential acceleration factor is added to the input variable of the fuzzy controller.Finally,based on the average model of converter,the system-level stability of microgrid is analyzed.MATLAB/Simulink simulation results verify the effectiveness and rationality of the proposed method.展开更多
Transient angle stability of inverters equipped with the robust droop controller is investigated in this work.At first,the conditions on the control references to guarantee the existence of a feasible post-disturbance...Transient angle stability of inverters equipped with the robust droop controller is investigated in this work.At first,the conditions on the control references to guarantee the existence of a feasible post-disturbance operating point are derived.Then,the post-disturbance equilibrium points are found and their stability properties are characterized.Furthermore,the attraction regions of the stable equilibrium points are accurately depicted by calculating the stable and unstable manifolds of the surrounding unstable equilibrium points,which presents an explanation to system transient stability.Finally,the transient control considerations are provided to help the inverter ridethrough the disturbance and maintain its stability characteristics.It is shown that the transient angle stability is not a serious problem for droop controlled inverters with proper control settings.展开更多
Featuring low communication requirements and high reliability,the voltage droop control method is widely adopted in the voltage source converter based multi-terminal direct current(VSC-MTDC)system for autonomous DC vo...Featuring low communication requirements and high reliability,the voltage droop control method is widely adopted in the voltage source converter based multi-terminal direct current(VSC-MTDC)system for autonomous DC voltage regulation and power-sharing.However,the traditional voltage droop control method with fixed droop gain is criticized for over-limit DC voltage deviation in case of large power disturbances,which can threaten stable operation of the entire VSCMTDC system.To tackle this problem,this paper proposes an adaptive reference power based voltage droop control method,which changes the reference power to compensate the power deviation for droop-controlled voltage source converters(VSCs).Besides retaining the merits of the traditional voltage droop control method,both DC voltage deviation reduction and power distribution improvement can be achieved by utilizing local information and a specific control factor in the proposed method.Basic principles and key features of the proposed method are described.Detailed analyses on the effects of the control factor on DC voltage deviation and imbalanced power-sharing are discussed,and the selection principle of the control factor is proposed.Finally,the effectiveness of the proposed method is validated by the simulations on a five-terminal VSC based high-voltage direct current(VSC-HVDC)system.展开更多
Hybrid AC/DC distribution networks are promising candidates for future applications due to their rapid advancement in power electronics technology.They use interface converters(IFCs)to link DC and AC distribution netw...Hybrid AC/DC distribution networks are promising candidates for future applications due to their rapid advancement in power electronics technology.They use interface converters(IFCs)to link DC and AC distribution networks.However,the networks possess drawbacks with AC voltage and frequency offsets when transferring from grid-tied to islanding modes.To address these problems,this paper proposes a simple but effective strategy based on the reverse droop method.Initially,the power balance equation of the distribution system is derived,which reveals that the cause of voltage and frequency offsets is the mismatch between the IFC output power and the rated load power.Then,the reverse droop control is introduced into the IFC controller.By using a voltage-active power/frequency-reactive power(U-P/f-Q)reverse droop loop,the IFC output power enables adaptive tracking of the rated load power.Therefore,the AC voltage offset and frequency offset are suppressed during the transfer process of operational modes.In addition,the universal parameter design method is discussed based on the stability limitations of the control system and the voltage quality requirements of AC critical loads.Finally,simulation and experimental results clearly validate the proposed control strategy and parameter design method.展开更多
This paper presents a systematic analysis of DC voltage stability of a multi-terminal VSC-HVDC(MTDC)system,with the emphasis on a comparative study of the most ubiquitous droop control configurations.The paper introdu...This paper presents a systematic analysis of DC voltage stability of a multi-terminal VSC-HVDC(MTDC)system,with the emphasis on a comparative study of the most ubiquitous droop control configurations.The paper introduces a general framework for the analysis of various droop control configurations employed in MTDC systems.This framework is then used to compare leading droop control configurations in terms of their impact on the relative stability,performance and robustness of the overall MTDC system.A generalized analytical MTDC model that contains detailed models of AC and DC system components is derived.Limitations imposed by DC power flow,DC inductor,cable modeling and AC network impedance on DC system stability are identified.Classical and multivariable frequency response analysis and eigenvalue analysis are applied to open-loop and closed-loop models to compare the stability and robustness of five leading droop controllers,with the focus on feedback signal selection and controller parameterization.This paper also proposes an active stabilizing controller,which takes the form of a modified constant power control,to enhance the controllability and robustness of the DC voltage control.展开更多
When the line impedance is considered in the microgrid, the accuracy of load sharing will decrease. In this paper, the impact of line impedance on the accuracy of load sharing is analyzed. A robust droop control for a...When the line impedance is considered in the microgrid, the accuracy of load sharing will decrease. In this paper, the impact of line impedance on the accuracy of load sharing is analyzed. A robust droop control for a highvoltage microgrid is proposed based on the signal detection on the high-voltage side of the coupling transformer. For a high-voltage microgrid, the equivalent impedance of coupling transformer connecting distributed generator with the grid is usually the dominate factor. Compared with the conventional droop control strategy, the proposed control method in this paper detects the feedback signal from the high-voltage side of the coupling transformer. The impact of line impedance on the load sharing accuracy can be mitigated significantly. The proposed droop control only changes the detection point of the feedback signal, thus it is easy to be implemented. The PSCAD/EMTDC simulation results show the effectiveness of the proposed robust droop control concept in load sharing and voltage regulation with highly accuracy.展开更多
In this paper,a VSG(virtual synchronous generator)-based method with adaptive active power and DC voltage droop is proposed for the control of VSC stations in the multi-terminal DC(MTDC)system.This control strategy ca...In this paper,a VSG(virtual synchronous generator)-based method with adaptive active power and DC voltage droop is proposed for the control of VSC stations in the multi-terminal DC(MTDC)system.This control strategy can improve the inertial level of the AC networks and attenuate the rate of change of frequency when a disturbance occurs.In addition,the droop control of the active power and DC voltage is implemented to make the AC networks share the unbalanced power in the MTDC.The droop coefficients are adaptively adjusted depending on the frequency margin of every AC network,which makes the allocation of unbalanced power among AC networks more reasonable from the frequency variation perspective.The control strategy is evaluated in the scenarios of sudden load change and wind turbine tripping,and the results are presented to demonstrate its effectiveness.展开更多
With the popularity of electric vehicles(EVs),a large number of EVs will become a burden to the future grid with arbitrary charging management.It is of vital significance to the control of the EVs charging and dischar...With the popularity of electric vehicles(EVs),a large number of EVs will become a burden to the future grid with arbitrary charging management.It is of vital significance to the control of the EVs charging and discharging state appropriately to enable the EVs to become friendly to the grid.Therefore,considering the potential for EVs seen as energy storage devices,this paper proposes a multiport DC-DC solid state transformer topology for bidirectional photovoltaic/battery-assisted EV parking lot with vehicle-to-grid service(V2G-PVBP).Relying on the energy storage function of EVs,V2G-PVBP is able to not only satisfy the normal requirements of EVs’owner,but also provide the function of load shifting and load regulation to the microgrid.In this paper,EVs are categorized into limited EV and freedom EV.Limited EVs are always kept in charging state and freedom EVs can take part in the load regulation of the microgrid.The proposed adaptive bidirectional droop control is designed for freedom EVs to make them autonomously charge or discharge with certain power which according to each EV’s state of charge,battery capacity,leaving time,and other factors to maintain the stability of the future microgrid.Eventually,the simulation and experiment of the adaptive bidirectional droop control based V2G-PVBP are provided to prove the availability of V2G-PVBP.展开更多
A new cost-based droop control method based upon generation cost and demand side cost management of the microgrid is proposed in this paper.At present,many droop control methods have been developed based on either the...A new cost-based droop control method based upon generation cost and demand side cost management of the microgrid is proposed in this paper.At present,many droop control methods have been developed based on either the power rating or the generation cost of the distributed generation(DG)unit,without consideration of the demand side participation in the operation and control.This exclusion might not be appropriate,if different types of consumers are connected in the micro-grid systems.This study proposes a droop control method considering both DG and load operating cost characteristics in order to minimize the generation cost of the micro-grid.展开更多
In the light of user-side energy power control requirements, a power control strategy for a household-level EPR based on HES droop control is proposed, focusing on the on-grid, off-grid and seamless switching process....In the light of user-side energy power control requirements, a power control strategy for a household-level EPR based on HES droop control is proposed, focusing on the on-grid, off-grid and seamless switching process. The system operating states are divided based on the DC bus voltage information with one converter used as a slack terminal to stabilize the DC bus voltage and the other converters as power terminals. In the on-grid mode, the GCC and the HES are used as the main control unit to achieve on-grid stable operation, whereas in the off-grid mode, the PV, HES and LC are used as the main control unit at different voltages to achieve stable operation of the island network. Finally, a DC MG system based on a household-level EPR is developed using the PSCAD / EMTDC simulation platform and the results show that the control strategy can effectively adjust the output of each subunit and maintain the stability of the DC bus voltage.展开更多
This work investigates the problem of controller design for the inverters in an islanded microgrid.Robust-synthesis controllers and local droop controllers are designed to regulate the output voltages of inverters and...This work investigates the problem of controller design for the inverters in an islanded microgrid.Robust-synthesis controllers and local droop controllers are designed to regulate the output voltages of inverters and share power among them,respectively.The designed controllers alleviate the need for additional sensors to measure the states of the system by relying only on output feedback.It is shown that the designed-synthesis controller properly damps resonant oscillations,and its performance is robust to the control-loop time delay and parameter uncertainties.The stability of a droop-controlled islanded microgrid including multiple distributed generation(DG)units is analyzed by linearizing the nonlinear power flow model around the nominal operating point and applying theorems from linear algebra.It is indicated that the droop controller stabilizes the microgrid system with dominantly inductive tie-line impedances for all values of resistive-inductive loads,while for the case of resistive-capacitive loads the stability is conditioned on an upper bound on the load susceptances.The robust performance of the designed-synthesis controller is studied analytically,compared with the similar analysis in an control(benchmark)framework,and verified by simulations for a four DG benchmark microgrid.Furthermore,the robustness of the droop controllers is analyzed by Monte Carlo simulations in the presence of local voltage fluctuations and phase differences among neighboring DGs.展开更多
In general,the power distribution of a parallel inverter is achieved by the use of droop control in a microgrid system,which consists of PV inverters and non-regeneration energy source inverters without energy storage...In general,the power distribution of a parallel inverter is achieved by the use of droop control in a microgrid system,which consists of PV inverters and non-regeneration energy source inverters without energy storage devices in an islanded mode.If the shared load power is no more than the available maximum PV inverter output power,then there is a power waste for the PV inverter.In addition,due to the intermittency of PV sources,the system may become unstable if the shared load power is more than the available maximum power output of the PV(MPO-PV)inverter.Therefore,in order to avoid power waste and potential instability caused by insufficient PV power by traditional droop control,this paper recommends an improved droop control scheme to maximize the power output of PV units.As required by the load,the remaining power is composed of the other inverters,which can effectively improve the utilization rating of renewable energy sources and system stability.At the same time,according to the system stability analysis based on small signal modeling,it has been designed around the droop coefficients of the improved droop control loop.In the end,the simulation and experimental results show that the suggested scheme has a varied validity and robustness.展开更多
To obtain a larger controllable range of output/input power of droop-control sources,a multi-objective optimization segmented droop control suitable for economic dispatch for a DC microgrid is proposed.According to th...To obtain a larger controllable range of output/input power of droop-control sources,a multi-objective optimization segmented droop control suitable for economic dispatch for a DC microgrid is proposed.According to the small-signal analysis,the worst point of the stability in the droop-control curve is determined through the analysis of a simplified model with multiple droop-control sources.By considering the worst points of stability as constraints,an elitist non-dominated sorting genetic algorithm is used to search the better turning points of the proposed droop-control curves after obtaining the new rated operation points from the system-layer economic dispatch.Simultaneously,optimization objectives,including the influence of eliminating the line resistance and capacity matching,are considered in the search process.Finally,the simulation results of the DC microgrid simulation model based on RT-Lab are presented to support the stability conclusion and proposed droop control.展开更多
An emerging multi-terminal looped DC(MTDC)collector system is now advocated for collecting and transferring large-scale renewable generation.However,it remains an open question as to improving the cooperative control ...An emerging multi-terminal looped DC(MTDC)collector system is now advocated for collecting and transferring large-scale renewable generation.However,it remains an open question as to improving the cooperative control capability of looped converter stations for flexible and robust response to renewable grid-connection fluctuation.This paper addresses this problem with a novel Power Margin Tracking(PMT)droop control and its corresponding system-level control strategy from the perspective of optimal dispatch of the power system.By introducing a power margin correction factor into the droop coefficient,the converter station can make self-adaptive regulations according to its actual available power margin.For operation verification,a multi-period optimal operation model and a four-terminal simulation model is built to provide optimal control parameters and real-time operation states of converter stations,where the power flow model of the looped MTDC grid with renewables generation is considered.The case results prove that the proposed control strategy can improve the cooperative operation capability of multiple converter stations,mitigating grid-connected power fluctuation.It can effectively reduce the DC voltage deviation to enhance the operation stability of the MTDC grid.The operational robustness of the proposed control strategy under“N−1”fault cases is verified as well.展开更多
文摘The contribution of Renewable Energy Resources(RER)in the process of power generation is significantly high in the recent days since it paves the way for overcoming the issues like serious energy crisis and natural contamination.This paper deals with the renewable energy based micro-grid as it is regarded as the apt solution for integrating the RER with the electrical frameworks.As thefixed droop coefficients in conventional droop control approaches have caused various limitations like low power-sharing and sudden drops of grid voltage in the Direct Current(DC)side,the Harmonized Membership Fuzzy Logic(MFL)droop control is employed in this present study.This proposed droop control for the hybrid PV-wind-battery system with MFL assists in achieving proper power-sharing and minimizing Total Harmonic Distortion(THD)in the emer-gency micro-grid.It eradicates the deviations in voltage and frequency with itsflexible and robust operation.The THD is reduced and attains the value of 3.1%compared to the traditional droop control.The simulation results of harmo-nized MFL droop control are analogized with the conventional approaches to vali-date the performance of the proposed method.In addition,the experimental results provided by the Field Programmable Gate Array(FPGA)based laboratory setup built using a solar photovoltaic(PV)and wind Permanent Magnet Synchro-nous Generator(PMSG)reaffirms the design.
基金This work was supported by the Natural Science Foundation of the Jiangsu Higher Education Institutions of China(20KJB470026)Key Project of Smart Grid Technology and Equipment of National Key Research and Development Plan of China(2016YFB0900601).
文摘System frequency must be kept very close to its nominal range to ensure the stability of an electric power grid.Excessive system frequency variations are able to result in load shedding,frequency instability,and even generator damage.With increasing wind power penetration,there is rising concern about the reduction in inertia response and primary frequency control in the electric power grid.Converter-based wind generation is capable of providing inertia response and primary frequency response;nevertheless,the primary frequency and inertia responses of wind generation are different from those of conventional synchronous fleets;it is not completely understood how the primary frequency and inertia responses affect the given system under various disturbances and available kinetic energy levels.Simulations are used to investigate the influences of inertia and droop control strategies on the dynamic frequency responses,particularly the index of the second frequency drop under various disturbance and wind conditions.A quantitative analysis provides insight into setting of inertia and droop control coefficients for various wind and disturbance conditions to facilitate adequate dynamic frequency responses during frequency events.
文摘As there exists sorts of distributed generators in microgrid, an integrated control strategy containing different control methods against corresponding generators should be applied. The strategy in this paper involves PQ control and droop control methods. The former aims at letting generators like PV output maximum power. The latter stems from inverter parallel technique and applies to controlling generators which can keep the network voltage steady to make the parallel system reach the minimum circulation point. Due to the unworthiness of droop control applied in low-voltage microgrid of which the impedance ratio is rather high, the paper adopts the droop control introducing virtual generator and virtual impedance. Based on theoretical analysis, simulation in Matlab is also implemented to verify the feasibility of the strategy.
文摘High Voltage Direct Current (HVDC) electric power transmission is a promising technology for integrating offshore wind farms and interconnecting power grids in different regions. In order to maintain the DC voltage, droop control has been widely used. Transmission line loss constitutes an import part of the total power loss in a multi-terminal HVDC scheme. In this paper, the relation between droop controller design and transmission loss has been investigated. Different MTDC layout configurations are compared to examine the effect of droop controller design on the transmission loss.
文摘Different droop control methods for PV-based communal grid networks (minigrids and microgrids) with different line resistances (R) and impedances (X) are modelled and simulated in MATLAB to determine the most efficient control method for a given network. Results show that active power-frequency (P-f) droop control method is the most efficient for low voltage transmission networks with low X/R ratios while reactive power-voltage (Q-V) droop control method is the most efficient for systems with high X/R ratios. For systems with complex line resistances and impedances, i.e. near unity X/R ratios, P-f or Q-V droop methods cannot individually efficiently regulate line voltage and frequency. For such systems, P-Q-f droop control method, where both active and reactive power could be used to control PCC voltage via shunt-connected inverters, is determined to be the most efficient control method. Results also show that shunt-connection of inverters leads to improved power flow control of interconnected communal grids by allowing feeder voltage regulation, load reactive power support, reactive power management between feeders, and improved overall system performance against dynamic disturbances.
基金supported in part by the National Natural Science Foundation of China under Grant 52377119 and U22B20109.
文摘To achieve the goal of carbon neutrality,renewable energy integration through a voltage source converter based multi-terminal direct current(VSC-MTDC)system has been identified as a promising solution.To tackle the significant DC voltage over-limit problem in a VSC-MTDC system during disturbances,this paper proposes a mode-switching strategy of droop control considering maximum DC voltage regulation capability.The close relationship between node injection powers and node DC voltages in the MTDC system is elaborated,and the most effective regulation approach of local injection power for limiting DC voltage deviation is presented.The operating point trajectories of different droop control explains that the DC voltage deviation can be minimized by fully utilizing the capacity of converters.Therefore,the mode-switching strategy with the maximum DC voltage regulation capability is realized by the switching between the voltage droop control and the constant maximum power control.In addition,a mode recovery process and a smooth switching method are developed to make converters regain the capability of maintaining DC voltage and reduce power fluctuation during mode switching,respectively.Furthermore,three cases are investigated to verify the effectiveness of the proposed mode-switching strategy.Compared with simulation results of the conventional droop control and the DC voltage deviation-dependent droop control,better performance of transient and steady-state DC voltage deviation is achieved through the proposed strategy.
基金supported by the National Natural Science Foundation of China(No.U22B20116).
文摘The unbalanced state of charge(SOC)of distributed energy storage systems(DESSs)in autonomous DC microgrid causes energy storage units(ESUs)to terminate operation due to overcharge or overdischarge,which severely affects the power quality.In this paper,a fuzzy droop control for SOC balance and stability analysis of DC microgrid with DESSs is proposed to achieve SOC balance in ESUs while maintaining a stable DC bus voltage.First,the charge and discharge modes of ESUs are determined based on the power supply requirements of the DC microgrid.One-dimensional fuzzy logic is then applied to establish the relationship between SOC and the droop coefficient R,in the aforementioned two modes.In addition,when integrated with voltage-current double closed-loop control,SOC balance in different ESUs is realized.To improve the balance speed and precision,an exponential acceleration factor is added to the input variable of the fuzzy controller.Finally,based on the average model of converter,the system-level stability of microgrid is analyzed.MATLAB/Simulink simulation results verify the effectiveness and rationality of the proposed method.
基金supported in part by National Natural Science Foundation of China (No.51877133)China Scholarship Council,and National Science Foundation (Award No.1810105)。
文摘Transient angle stability of inverters equipped with the robust droop controller is investigated in this work.At first,the conditions on the control references to guarantee the existence of a feasible post-disturbance operating point are derived.Then,the post-disturbance equilibrium points are found and their stability properties are characterized.Furthermore,the attraction regions of the stable equilibrium points are accurately depicted by calculating the stable and unstable manifolds of the surrounding unstable equilibrium points,which presents an explanation to system transient stability.Finally,the transient control considerations are provided to help the inverter ridethrough the disturbance and maintain its stability characteristics.It is shown that the transient angle stability is not a serious problem for droop controlled inverters with proper control settings.
基金supported by the Key Science and Technology Projects of China Southern Power Grid Corporation(No.090000KK52180116)National Natural Science Foundation of China(No.51807135)。
文摘Featuring low communication requirements and high reliability,the voltage droop control method is widely adopted in the voltage source converter based multi-terminal direct current(VSC-MTDC)system for autonomous DC voltage regulation and power-sharing.However,the traditional voltage droop control method with fixed droop gain is criticized for over-limit DC voltage deviation in case of large power disturbances,which can threaten stable operation of the entire VSCMTDC system.To tackle this problem,this paper proposes an adaptive reference power based voltage droop control method,which changes the reference power to compensate the power deviation for droop-controlled voltage source converters(VSCs).Besides retaining the merits of the traditional voltage droop control method,both DC voltage deviation reduction and power distribution improvement can be achieved by utilizing local information and a specific control factor in the proposed method.Basic principles and key features of the proposed method are described.Detailed analyses on the effects of the control factor on DC voltage deviation and imbalanced power-sharing are discussed,and the selection principle of the control factor is proposed.Finally,the effectiveness of the proposed method is validated by the simulations on a five-terminal VSC based high-voltage direct current(VSC-HVDC)system.
基金This work was supported by the National Key R&D Program of China(2018YFB0904700).
文摘Hybrid AC/DC distribution networks are promising candidates for future applications due to their rapid advancement in power electronics technology.They use interface converters(IFCs)to link DC and AC distribution networks.However,the networks possess drawbacks with AC voltage and frequency offsets when transferring from grid-tied to islanding modes.To address these problems,this paper proposes a simple but effective strategy based on the reverse droop method.Initially,the power balance equation of the distribution system is derived,which reveals that the cause of voltage and frequency offsets is the mismatch between the IFC output power and the rated load power.Then,the reverse droop control is introduced into the IFC controller.By using a voltage-active power/frequency-reactive power(U-P/f-Q)reverse droop loop,the IFC output power enables adaptive tracking of the rated load power.Therefore,the AC voltage offset and frequency offset are suppressed during the transfer process of operational modes.In addition,the universal parameter design method is discussed based on the stability limitations of the control system and the voltage quality requirements of AC critical loads.Finally,simulation and experimental results clearly validate the proposed control strategy and parameter design method.
基金This work was supported by the UK Engineering and Physical Science Council Project(EP/L102463/1).
文摘This paper presents a systematic analysis of DC voltage stability of a multi-terminal VSC-HVDC(MTDC)system,with the emphasis on a comparative study of the most ubiquitous droop control configurations.The paper introduces a general framework for the analysis of various droop control configurations employed in MTDC systems.This framework is then used to compare leading droop control configurations in terms of their impact on the relative stability,performance and robustness of the overall MTDC system.A generalized analytical MTDC model that contains detailed models of AC and DC system components is derived.Limitations imposed by DC power flow,DC inductor,cable modeling and AC network impedance on DC system stability are identified.Classical and multivariable frequency response analysis and eigenvalue analysis are applied to open-loop and closed-loop models to compare the stability and robustness of five leading droop controllers,with the focus on feedback signal selection and controller parameterization.This paper also proposes an active stabilizing controller,which takes the form of a modified constant power control,to enhance the controllability and robustness of the DC voltage control.
基金supported by the National Natural Science Foundation of China(No.51207048)the National High Technology Research and Development of China(No.2014AA052601)Higher National Excellent Doctoral Dissertation of Special Funds(No.201441)
文摘When the line impedance is considered in the microgrid, the accuracy of load sharing will decrease. In this paper, the impact of line impedance on the accuracy of load sharing is analyzed. A robust droop control for a highvoltage microgrid is proposed based on the signal detection on the high-voltage side of the coupling transformer. For a high-voltage microgrid, the equivalent impedance of coupling transformer connecting distributed generator with the grid is usually the dominate factor. Compared with the conventional droop control strategy, the proposed control method in this paper detects the feedback signal from the high-voltage side of the coupling transformer. The impact of line impedance on the load sharing accuracy can be mitigated significantly. The proposed droop control only changes the detection point of the feedback signal, thus it is easy to be implemented. The PSCAD/EMTDC simulation results show the effectiveness of the proposed robust droop control concept in load sharing and voltage regulation with highly accuracy.
基金supported by the National Nature Science Foundation of China(51621065,51567021)Independent Research Program of Tsinghua University(20151080416)China Postdoctoral Science Foundation(2016M601025).
文摘In this paper,a VSG(virtual synchronous generator)-based method with adaptive active power and DC voltage droop is proposed for the control of VSC stations in the multi-terminal DC(MTDC)system.This control strategy can improve the inertial level of the AC networks and attenuate the rate of change of frequency when a disturbance occurs.In addition,the droop control of the active power and DC voltage is implemented to make the AC networks share the unbalanced power in the MTDC.The droop coefficients are adaptively adjusted depending on the frequency margin of every AC network,which makes the allocation of unbalanced power among AC networks more reasonable from the frequency variation perspective.The control strategy is evaluated in the scenarios of sudden load change and wind turbine tripping,and the results are presented to demonstrate its effectiveness.
基金This work was supported by National Key Research and Development Program of China(2018YFA0702200)National Natural Science Foundation of China(61773109,6143304)Major Program of National Natural Foundation of China(61573094).
文摘With the popularity of electric vehicles(EVs),a large number of EVs will become a burden to the future grid with arbitrary charging management.It is of vital significance to the control of the EVs charging and discharging state appropriately to enable the EVs to become friendly to the grid.Therefore,considering the potential for EVs seen as energy storage devices,this paper proposes a multiport DC-DC solid state transformer topology for bidirectional photovoltaic/battery-assisted EV parking lot with vehicle-to-grid service(V2G-PVBP).Relying on the energy storage function of EVs,V2G-PVBP is able to not only satisfy the normal requirements of EVs’owner,but also provide the function of load shifting and load regulation to the microgrid.In this paper,EVs are categorized into limited EV and freedom EV.Limited EVs are always kept in charging state and freedom EVs can take part in the load regulation of the microgrid.The proposed adaptive bidirectional droop control is designed for freedom EVs to make them autonomously charge or discharge with certain power which according to each EV’s state of charge,battery capacity,leaving time,and other factors to maintain the stability of the future microgrid.Eventually,the simulation and experiment of the adaptive bidirectional droop control based V2G-PVBP are provided to prove the availability of V2G-PVBP.
文摘A new cost-based droop control method based upon generation cost and demand side cost management of the microgrid is proposed in this paper.At present,many droop control methods have been developed based on either the power rating or the generation cost of the distributed generation(DG)unit,without consideration of the demand side participation in the operation and control.This exclusion might not be appropriate,if different types of consumers are connected in the micro-grid systems.This study proposes a droop control method considering both DG and load operating cost characteristics in order to minimize the generation cost of the micro-grid.
基金National Key R&D Program of China(2018YFB0905000)Science and Technology Project of State Grid Corporation of China(SGTJDK00DWJS1800232).
文摘In the light of user-side energy power control requirements, a power control strategy for a household-level EPR based on HES droop control is proposed, focusing on the on-grid, off-grid and seamless switching process. The system operating states are divided based on the DC bus voltage information with one converter used as a slack terminal to stabilize the DC bus voltage and the other converters as power terminals. In the on-grid mode, the GCC and the HES are used as the main control unit to achieve on-grid stable operation, whereas in the off-grid mode, the PV, HES and LC are used as the main control unit at different voltages to achieve stable operation of the island network. Finally, a DC MG system based on a household-level EPR is developed using the PSCAD / EMTDC simulation platform and the results show that the control strategy can effectively adjust the output of each subunit and maintain the stability of the DC bus voltage.
文摘This work investigates the problem of controller design for the inverters in an islanded microgrid.Robust-synthesis controllers and local droop controllers are designed to regulate the output voltages of inverters and share power among them,respectively.The designed controllers alleviate the need for additional sensors to measure the states of the system by relying only on output feedback.It is shown that the designed-synthesis controller properly damps resonant oscillations,and its performance is robust to the control-loop time delay and parameter uncertainties.The stability of a droop-controlled islanded microgrid including multiple distributed generation(DG)units is analyzed by linearizing the nonlinear power flow model around the nominal operating point and applying theorems from linear algebra.It is indicated that the droop controller stabilizes the microgrid system with dominantly inductive tie-line impedances for all values of resistive-inductive loads,while for the case of resistive-capacitive loads the stability is conditioned on an upper bound on the load susceptances.The robust performance of the designed-synthesis controller is studied analytically,compared with the similar analysis in an control(benchmark)framework,and verified by simulations for a four DG benchmark microgrid.Furthermore,the robustness of the droop controllers is analyzed by Monte Carlo simulations in the presence of local voltage fluctuations and phase differences among neighboring DGs.
文摘In general,the power distribution of a parallel inverter is achieved by the use of droop control in a microgrid system,which consists of PV inverters and non-regeneration energy source inverters without energy storage devices in an islanded mode.If the shared load power is no more than the available maximum PV inverter output power,then there is a power waste for the PV inverter.In addition,due to the intermittency of PV sources,the system may become unstable if the shared load power is more than the available maximum power output of the PV(MPO-PV)inverter.Therefore,in order to avoid power waste and potential instability caused by insufficient PV power by traditional droop control,this paper recommends an improved droop control scheme to maximize the power output of PV units.As required by the load,the remaining power is composed of the other inverters,which can effectively improve the utilization rating of renewable energy sources and system stability.At the same time,according to the system stability analysis based on small signal modeling,it has been designed around the droop coefficients of the improved droop control loop.In the end,the simulation and experimental results show that the suggested scheme has a varied validity and robustness.
基金supported by the National Key R&D Program of China(No.2016YFC0600906)the National Natural Science Foundation of China(No.U1810124)+1 种基金the Xuzhou Promoting Science and Technology Innovation Project(No.KC18076)the Natural Science Foundation of Jiangsu Province(No.BK20160268)。
文摘To obtain a larger controllable range of output/input power of droop-control sources,a multi-objective optimization segmented droop control suitable for economic dispatch for a DC microgrid is proposed.According to the small-signal analysis,the worst point of the stability in the droop-control curve is determined through the analysis of a simplified model with multiple droop-control sources.By considering the worst points of stability as constraints,an elitist non-dominated sorting genetic algorithm is used to search the better turning points of the proposed droop-control curves after obtaining the new rated operation points from the system-layer economic dispatch.Simultaneously,optimization objectives,including the influence of eliminating the line resistance and capacity matching,are considered in the search process.Finally,the simulation results of the DC microgrid simulation model based on RT-Lab are presented to support the stability conclusion and proposed droop control.
基金supported in part by the National Key Research and Development Program of China(2016YFB0900100)the Smart Grid Joint Foundation Program of National Natural Science Foundation of China and State Grid Corporation of China(U1866204).
文摘An emerging multi-terminal looped DC(MTDC)collector system is now advocated for collecting and transferring large-scale renewable generation.However,it remains an open question as to improving the cooperative control capability of looped converter stations for flexible and robust response to renewable grid-connection fluctuation.This paper addresses this problem with a novel Power Margin Tracking(PMT)droop control and its corresponding system-level control strategy from the perspective of optimal dispatch of the power system.By introducing a power margin correction factor into the droop coefficient,the converter station can make self-adaptive regulations according to its actual available power margin.For operation verification,a multi-period optimal operation model and a four-terminal simulation model is built to provide optimal control parameters and real-time operation states of converter stations,where the power flow model of the looped MTDC grid with renewables generation is considered.The case results prove that the proposed control strategy can improve the cooperative operation capability of multiple converter stations,mitigating grid-connected power fluctuation.It can effectively reduce the DC voltage deviation to enhance the operation stability of the MTDC grid.The operational robustness of the proposed control strategy under“N−1”fault cases is verified as well.
