The stability of a voltage source converters(VSC)system based on phase-locked loop(PLL)is very important issue during asymmetric grid faults.This paper establishes a transient synchronous stability model of a dual-seq...The stability of a voltage source converters(VSC)system based on phase-locked loop(PLL)is very important issue during asymmetric grid faults.This paper establishes a transient synchronous stability model of a dual-sequence PLL-based VSC system during low voltage ride-through by referring to the equivalent rotor swing equation of syn-chronous generators.Based on the model,the synchronization characteristics of the VSC system under asymmetric grid faults are described,and the interaction mechanisms,as well as the transient instability phenomena of positive and negative sequence PLL during asymmetric faults are explained.Using the equal area criterion,the influences of sequence control switching action,detection delay,and interaction between the positive and negative sequence PLL on the transient synchronous stability of the VSC system are analyzed,respectively.In addition,a transient stabil-ity assessment criterion based on the critical fault clearance angle and time and an enhancement control strategy based on the improved positive and negative sequence PLL are proposed.Finally,the analytical results are validated through simulation and experiments.展开更多
A power-transmission collection line is connected to each wind turbine of a wind farm and then connected to the incoming switchgear at the low-voltage side of the booster station at a certain distance.Therefore,the gr...A power-transmission collection line is connected to each wind turbine of a wind farm and then connected to the incoming switchgear at the low-voltage side of the booster station at a certain distance.Therefore,the grouping of the wind turbines in the wind farm determines the layout of the lines and affects the line impedance.The line structure and composition of the wind farm are analyzed,and the relationship between the impedance of the collection line and reactive power generated by the wind turbine at low-voltage ride through is derived.We conclude that the smaller the equivalent impedance of the wind farm collection line structure is,the greater the reactive power at the wind farm collection line point is.In addition,the economic aspect of the collection line needs to be considered in the design.The economic aspect and impedance values have contrasting characteristics.Therefore,according to the condition of minimum impedance and optimized economic aspect,an optimization model of a wind-farm collector-circuit structure is proposed.The optimal structure of the wind farm collector circuit is calculated using the Monte Carlo method,and the theoretical analysis is verified by simulation.展开更多
Large integration of doubly-fed induction generator(DFIG)based wind turbines(WTs)into power networks can have significant consequences for power system operation and the quality of the energy supplied due to their exc...Large integration of doubly-fed induction generator(DFIG)based wind turbines(WTs)into power networks can have significant consequences for power system operation and the quality of the energy supplied due to their excessive sensitivity towards grid disturbances.Under voltage dips,the resulting overcurrent and overvoltage in the rotor circuit and the DC link of a DFIG,could lead to the activation of the protection system and WT disconnection.This potentially results in sudden loss of several tens/hundreds of MWs of energy,and consequently intensifying the severity of the fault.This paper aims to combine the use of a crowbar protection circuit and a robust backstepping control strategy that takes into consideration of the dynamics of the magnetic flux,to improve DFIG’s Low-Voltage Ride Through capability and fulfill the latest grid code requirements.While the power electronic interfaces are protected,the WTs also provide large reactive power during the fault to assist system voltage recovery.Simulation results using Matlab/Simulink demonstrate the effectiveness of the proposed strategy in terms of dynamic response and robustness against parametric variations.展开更多
For doubly-fed induction generator(DFIG)-based wind turbines(WTs),various advanced control schemes have been proposed to achieve the low voltage ride through(LVRT)capability,whose parameters design is significantly re...For doubly-fed induction generator(DFIG)-based wind turbines(WTs),various advanced control schemes have been proposed to achieve the low voltage ride through(LVRT)capability,whose parameters design is significantly reliant on the rotor electromotive force(EMF)of DFIG-based WTs.However,the influence of the rotor current on EMF is usually ignored in existing studies,which cannot fully reflect the transient characteristics of EMF.To tackle with this issue,this study presents a comprehensive and quantitative analysis of EMF during grid faults considering various control modes.First,the DFIG model under grid faults is established.Subsequently,the transient characteristics of EMF are analyzed under different control modes(that is,rotor open-circuit and connected to converter).Furthermore,the EMF transient eigenvolumes(that is,accessorial resistance item,transient decay time constant,and frequency offset)are quantitatively analyzed with the typical parameters of MW-level DFIG-based WT.The analysis results contribute to the design of the LVRT control scheme.Finally,the analysis is validated by the hardware-in-the-loop experiments.展开更多
Online fault detection is one of the key technologies to improve the performance of cloud systems. The current data of cloud systems is to be monitored, collected and used to reflect their state. Its use can potential...Online fault detection is one of the key technologies to improve the performance of cloud systems. The current data of cloud systems is to be monitored, collected and used to reflect their state. Its use can potentially help cloud managers take some timely measures before fault occurrence in clouds. Because of the complex structure and dynamic change characteristics of the clouds, existing fault detection methods suffer from the problems of low efficiency and low accuracy. In order to solve them, this work proposes an online detection model based on asystematic parameter-search method called SVM-Grid, whose construction is based on a support vector machine(SVM). SVM-Grid is used to optimize parameters in SVM. Proper attributes of a cloud system's running data are selected by using Pearson correlation and principal component analysis for the model. Strategies of predicting cloud faults and updating fault sample databases are proposed to optimize the model and improve its performance.In comparison with some representative existing methods, the proposed model can achieve more efficient and accurate fault detection for cloud systems.展开更多
After the North China grid and the Central China grid get into connection with the UHVAC demonstration, a new phenomenon is discovered according to some simulations. That is, the faults at the remote end of the UHV in...After the North China grid and the Central China grid get into connection with the UHVAC demonstration, a new phenomenon is discovered according to some simulations. That is, the faults at the remote end of the UHV interconnected grid will result in significant power fluctuation and voltage drop on the UHV transmission line and even system splitting. But the faults near the UHV line only have marginal effects. Further, the simulation results also indicate that the short-circuit current of the buses near the UHV line is larger than that of the buses far away from the UHV line. This phenomenon is divergent from the traditional view. In this paper, the detail will be introduced, and the factors influencing the system stability after faults are presented and analyzed. The results indicate that transmission power of the UHV line and of the lines between the remote end and the major grid influence the fluctuation on UHV line. The load model and the grid structure of the remote end also have effect on it. Finally, corresponding control scheme is presented to improve the operation conditions of the UHV interconnected grid and ensure its security and stability.展开更多
Smart grid was proposed as a practical form of future power distribution system. Evaluating the reliability of smart grids was of great importance and significance. A revised fault tree model was proposed to distingui...Smart grid was proposed as a practical form of future power distribution system. Evaluating the reliability of smart grids was of great importance and significance. A revised fault tree model was proposed to distinguish and separate grid-connected operation mode and islanded operation mode of smart grids,focusing on the perspective of the consumers. A hierarchical Monte Carlo simulation method for reliability evaluation was also proposed based on the proposed fault tree model. A case of reliability evaluation for the future renewable electric energy delivery and management( FREEDM) system was carried out and analyzed. The proposed methods can be applicable to other forms of smart grids.展开更多
Nowadays, the DC distribution system has been suggested, as a replacement for the AC power distribution system with electric propulsion. This idea signifies a fresh approach of issuing energy for low-voltage installat...Nowadays, the DC distribution system has been suggested, as a replacement for the AC power distribution system with electric propulsion. This idea signifies a fresh approach of issuing energy for low-voltage installations. It can be used for any electrical application up to 20 MW and works at a nominal voltage of 1000 V DC. The DC distribution system is just an extension of the multiple DC links that previously available in all propulsion and thruster drives, which typically comprise more than 80% of the electrical power consumption on electric propulsion vessels. A fault detection and islanding scheme for DC grid connected PV system is presented in this paper. Unlike traditional ac distribution systems, protection has been challenging for dc systems. The goals of this paper are to classify and detect the fault in the PV system as well as DC grid and to isolate the faulted section so that the system keeps operating without disabling the entire system. The results show the measured values of power at PV panel and DC grid side under different fault condition, which indicates the type of fault that occurs in the system.展开更多
Fault management study in smart grid systems (SGSs) is important to ensure the stability of the system. Also, it is important to know the major types of power failures for the effective operation of the SGS. This pape...Fault management study in smart grid systems (SGSs) is important to ensure the stability of the system. Also, it is important to know the major types of power failures for the effective operation of the SGS. This paper reviews diverse types of faults that might appear in the SGS and gives a survey about the impact of renewable energy resources (RERs) on the behavior of the system. Moreover, this paper offers different fault detection and localization techniques that can be used for SGSs. Furthermore, a potential fault management case study is proposed in this paper. The SGS model in this paper is investigated using both of the Matlab/Simulink and the Real Time Digital Simulation (RTDS) to compute the fault management study. Simulation results show the fast response to a power failure in the system which improves the stability of the SGS.展开更多
基金supported in part by the National Natural Science Foundation of China under Grant 51977019in part by the Joint Research Fund in Smart Grid(U1966208)under cooperative agreement between the National Natural Science Foundation of China and State Grid Corporation of China.
文摘The stability of a voltage source converters(VSC)system based on phase-locked loop(PLL)is very important issue during asymmetric grid faults.This paper establishes a transient synchronous stability model of a dual-sequence PLL-based VSC system during low voltage ride-through by referring to the equivalent rotor swing equation of syn-chronous generators.Based on the model,the synchronization characteristics of the VSC system under asymmetric grid faults are described,and the interaction mechanisms,as well as the transient instability phenomena of positive and negative sequence PLL during asymmetric faults are explained.Using the equal area criterion,the influences of sequence control switching action,detection delay,and interaction between the positive and negative sequence PLL on the transient synchronous stability of the VSC system are analyzed,respectively.In addition,a transient stabil-ity assessment criterion based on the critical fault clearance angle and time and an enhancement control strategy based on the improved positive and negative sequence PLL are proposed.Finally,the analytical results are validated through simulation and experiments.
基金Supported by Natural Science Foundation of Hunan Province of China(2019JJ50119)Educational Commission of Hunan Province of China(18C0510)Zhuzhou City Science and Technology Plan(Zhu Ke Fa[2017]No.68)。
文摘A power-transmission collection line is connected to each wind turbine of a wind farm and then connected to the incoming switchgear at the low-voltage side of the booster station at a certain distance.Therefore,the grouping of the wind turbines in the wind farm determines the layout of the lines and affects the line impedance.The line structure and composition of the wind farm are analyzed,and the relationship between the impedance of the collection line and reactive power generated by the wind turbine at low-voltage ride through is derived.We conclude that the smaller the equivalent impedance of the wind farm collection line structure is,the greater the reactive power at the wind farm collection line point is.In addition,the economic aspect of the collection line needs to be considered in the design.The economic aspect and impedance values have contrasting characteristics.Therefore,according to the condition of minimum impedance and optimized economic aspect,an optimization model of a wind-farm collector-circuit structure is proposed.The optimal structure of the wind farm collector circuit is calculated using the Monte Carlo method,and the theoretical analysis is verified by simulation.
文摘Large integration of doubly-fed induction generator(DFIG)based wind turbines(WTs)into power networks can have significant consequences for power system operation and the quality of the energy supplied due to their excessive sensitivity towards grid disturbances.Under voltage dips,the resulting overcurrent and overvoltage in the rotor circuit and the DC link of a DFIG,could lead to the activation of the protection system and WT disconnection.This potentially results in sudden loss of several tens/hundreds of MWs of energy,and consequently intensifying the severity of the fault.This paper aims to combine the use of a crowbar protection circuit and a robust backstepping control strategy that takes into consideration of the dynamics of the magnetic flux,to improve DFIG’s Low-Voltage Ride Through capability and fulfill the latest grid code requirements.While the power electronic interfaces are protected,the WTs also provide large reactive power during the fault to assist system voltage recovery.Simulation results using Matlab/Simulink demonstrate the effectiveness of the proposed strategy in terms of dynamic response and robustness against parametric variations.
基金Supported in part by the National Natural Science Foundation of China under Grant 51907072in part by the Fundamental Research Funds for the Central Universities under Grant 2021XXJS004。
文摘For doubly-fed induction generator(DFIG)-based wind turbines(WTs),various advanced control schemes have been proposed to achieve the low voltage ride through(LVRT)capability,whose parameters design is significantly reliant on the rotor electromotive force(EMF)of DFIG-based WTs.However,the influence of the rotor current on EMF is usually ignored in existing studies,which cannot fully reflect the transient characteristics of EMF.To tackle with this issue,this study presents a comprehensive and quantitative analysis of EMF during grid faults considering various control modes.First,the DFIG model under grid faults is established.Subsequently,the transient characteristics of EMF are analyzed under different control modes(that is,rotor open-circuit and connected to converter).Furthermore,the EMF transient eigenvolumes(that is,accessorial resistance item,transient decay time constant,and frequency offset)are quantitatively analyzed with the typical parameters of MW-level DFIG-based WT.The analysis results contribute to the design of the LVRT control scheme.Finally,the analysis is validated by the hardware-in-the-loop experiments.
基金supported by the National Natural Science Foundation of China(61472005,61201252)CERNET Innovation Project(NGII20160207)
文摘Online fault detection is one of the key technologies to improve the performance of cloud systems. The current data of cloud systems is to be monitored, collected and used to reflect their state. Its use can potentially help cloud managers take some timely measures before fault occurrence in clouds. Because of the complex structure and dynamic change characteristics of the clouds, existing fault detection methods suffer from the problems of low efficiency and low accuracy. In order to solve them, this work proposes an online detection model based on asystematic parameter-search method called SVM-Grid, whose construction is based on a support vector machine(SVM). SVM-Grid is used to optimize parameters in SVM. Proper attributes of a cloud system's running data are selected by using Pearson correlation and principal component analysis for the model. Strategies of predicting cloud faults and updating fault sample databases are proposed to optimize the model and improve its performance.In comparison with some representative existing methods, the proposed model can achieve more efficient and accurate fault detection for cloud systems.
文摘After the North China grid and the Central China grid get into connection with the UHVAC demonstration, a new phenomenon is discovered according to some simulations. That is, the faults at the remote end of the UHV interconnected grid will result in significant power fluctuation and voltage drop on the UHV transmission line and even system splitting. But the faults near the UHV line only have marginal effects. Further, the simulation results also indicate that the short-circuit current of the buses near the UHV line is larger than that of the buses far away from the UHV line. This phenomenon is divergent from the traditional view. In this paper, the detail will be introduced, and the factors influencing the system stability after faults are presented and analyzed. The results indicate that transmission power of the UHV line and of the lines between the remote end and the major grid influence the fluctuation on UHV line. The load model and the grid structure of the remote end also have effect on it. Finally, corresponding control scheme is presented to improve the operation conditions of the UHV interconnected grid and ensure its security and stability.
文摘Smart grid was proposed as a practical form of future power distribution system. Evaluating the reliability of smart grids was of great importance and significance. A revised fault tree model was proposed to distinguish and separate grid-connected operation mode and islanded operation mode of smart grids,focusing on the perspective of the consumers. A hierarchical Monte Carlo simulation method for reliability evaluation was also proposed based on the proposed fault tree model. A case of reliability evaluation for the future renewable electric energy delivery and management( FREEDM) system was carried out and analyzed. The proposed methods can be applicable to other forms of smart grids.
文摘Nowadays, the DC distribution system has been suggested, as a replacement for the AC power distribution system with electric propulsion. This idea signifies a fresh approach of issuing energy for low-voltage installations. It can be used for any electrical application up to 20 MW and works at a nominal voltage of 1000 V DC. The DC distribution system is just an extension of the multiple DC links that previously available in all propulsion and thruster drives, which typically comprise more than 80% of the electrical power consumption on electric propulsion vessels. A fault detection and islanding scheme for DC grid connected PV system is presented in this paper. Unlike traditional ac distribution systems, protection has been challenging for dc systems. The goals of this paper are to classify and detect the fault in the PV system as well as DC grid and to isolate the faulted section so that the system keeps operating without disabling the entire system. The results show the measured values of power at PV panel and DC grid side under different fault condition, which indicates the type of fault that occurs in the system.
文摘Fault management study in smart grid systems (SGSs) is important to ensure the stability of the system. Also, it is important to know the major types of power failures for the effective operation of the SGS. This paper reviews diverse types of faults that might appear in the SGS and gives a survey about the impact of renewable energy resources (RERs) on the behavior of the system. Moreover, this paper offers different fault detection and localization techniques that can be used for SGSs. Furthermore, a potential fault management case study is proposed in this paper. The SGS model in this paper is investigated using both of the Matlab/Simulink and the Real Time Digital Simulation (RTDS) to compute the fault management study. Simulation results show the fast response to a power failure in the system which improves the stability of the SGS.