The hybrid dc circuit breaker(HCB)has the advantages of fast action speed and low operating loss,which is an idealmethod for fault isolation ofmulti-terminal dc grids.Formulti-terminal dc grids that transmit power thr...The hybrid dc circuit breaker(HCB)has the advantages of fast action speed and low operating loss,which is an idealmethod for fault isolation ofmulti-terminal dc grids.Formulti-terminal dc grids that transmit power through overhead lines,HCBs are required to have reclosing capability due to the high fault probability and the fact that most of the faults are temporary faults.To avoid the secondary fault strike and equipment damage that may be caused by the reclosing of the HCB when the permanent fault occurs,an adaptive reclosing scheme based on traveling wave injection is proposed in this paper.The scheme injects traveling wave signal into the fault dc line through the additionally configured auxiliary discharge branch in the HCB,and then uses the reflection characteristic of the traveling wave signal on the dc line to identify temporary and permanent faults,to be able to realize fast reclosing when the temporary fault occurs and reliably avoid reclosing after the permanent fault occurs.The test results in the simulation model of the four-terminal dc grid show that the proposed adaptive reclosing scheme can quickly and reliably identify temporary and permanent faults,greatly shorten the power outage time of temporary faults.In addition,it has the advantages of easiness to implement,high reliability,robustness to high-resistance fault and no dead zone,etc.展开更多
This paper proposes a frequency domain based methodology to analyse the influence of High Voltage Direct Current(HVDC) configurations and system parameters on the travelling wave behaviour during a DC fault. The metho...This paper proposes a frequency domain based methodology to analyse the influence of High Voltage Direct Current(HVDC) configurations and system parameters on the travelling wave behaviour during a DC fault. The method allows us to gain deeper understanding of these influencing parameters. In the literature, the majority of DC protection algorithms essentially use thefirst travelling waves initiated by a DC fault for fault discrimination due to the stringent time constraint in DC grid protection. However, most protection algorithms up to now have been designed based on extensive time domain simulations using one specific test system. Therefore, general applicability or adaptability to different configurations and system changes is not by default ensured, and it is difficult to gain in-depth understanding of the influencing parameters through time domain simulations. In order to analyse the first travelling wave for meshed HVDC grids, voltage and current wave transfer functions with respect to the incident voltage wave are derived adopting Laplace domain based component models. The step responses obtained from the voltage transfer functions are validated by comparison against simulations using a detailed model implemented in PSCADTM. Then, the influences of system parameters such as the number of parallel branches, HVDC grid configurations and groundings on the first travelling wave are investigated by analysing the voltage and current transfer functions.展开更多
With wider applications of power electronic devices in modern power systems,simulation using traditional electro-mechanical and electromagnetic simulation tools suffer from low speed and imprecision.Multi-rate technol...With wider applications of power electronic devices in modern power systems,simulation using traditional electro-mechanical and electromagnetic simulation tools suffer from low speed and imprecision.Multi-rate technologies can greatly improve simulation efficiency by avoiding simulating the entire system using a small time-step.However,the drawbacks of the current synchronization mechanisms is that they introduce numerical errors and numerical instabilities in multi-rate parallel simulations.An improved multi-rate parallel technology,node splitting interface(NSI),is proposed to reduce errors and enhance simulation stability.A new synchronization mechanism is used to avoid prediction and signal delays.Theoretical analyses are carried out to prove the convergence and absolute stability of the proposed NSI algorithm.This algorithm is particularly suitable for simultaneously investigating long term dynamics of DC grids and fast transients of power electronic converters.展开更多
Symmetrical monopolar configuration is the prevailing scheme configuration for modular multilevel converter based high-voltage direct current(MMC-HVDC) links, in which severe DC overvoltage or overcurrent can be cause...Symmetrical monopolar configuration is the prevailing scheme configuration for modular multilevel converter based high-voltage direct current(MMC-HVDC) links, in which severe DC overvoltage or overcurrent can be caused by the DC faults. To deal with the possible asymmetry in the DC faults and the coupling effects of the DC lines, this paper analyzes the DC fault characteristics based on the phase-mode transformation. First, the DC grid is decomposed into the common-mode and the differential-mode networks. The equivalent models of the MMCs and DC lines in the two networks are derived, respectively. Then, based on the state matrices, a unified numerical calculation method for the fault voltages and currents at the DC side is proposed. Compared with the time-domain simulations performed on PSCAD/EMTDC, the accuracy of the proposed method is validated. Last, the system parameter analysis shows that the grounding parameters play an important role in reducing the severity of the pole-to-ground faults, whereas the coupling effects of the DC lines should be considered when calculating the DC fault currents associated with the pole-to-pole faults.展开更多
The modular multilevel converter(MMC)based DC grid is considered as a future solution for bulk renewable energy integration and transmission.However,the high probability of DC faults and their rapid propagation speed ...The modular multilevel converter(MMC)based DC grid is considered as a future solution for bulk renewable energy integration and transmission.However,the high probability of DC faults and their rapid propagation speed are the main challenges in the development of DC grids.Existing research primarily focuses on the DC fault clearance methods,while the fault current suppression methods are still barely researched.Additionally,the coordination method of fault current suppression and clearance needs to be optimized.In this paper,the technical characteristics of the current suppression methods are studied,and the coordinated methods of fault current suppression and clearance are proposed.At last,a cost comparison of these methods is presented.The research results show that the proposed strategies can reduce the cost of the protection equipment.展开更多
High voltage DC grids are developing in more terminals and with larger transmission capacity,thus the re-quirements for DC circuit breakers(DCCB)will continue to rise.Conventional methods only use the faulty line DCCB...High voltage DC grids are developing in more terminals and with larger transmission capacity,thus the re-quirements for DC circuit breakers(DCCB)will continue to rise.Conventional methods only use the faulty line DCCB to withstand the fault stress,and therefore this paper presents a coordination method of multiple DCCBs to protect the system.As many adjacent DCCBs are tripped to interrupt the fault current,the fault energy is shared,and the requirement for the faulty line DCCB is reduced.Moreover,the adjacent DCCBs are actively controlled to help system recovery.The primary protection,backup protection,and reclosing logic of multiple DCCBs are studied.Simulations confirm that the proposed control reduces the energy dissipation requirement of faulty line DCCB by approximately 30%-42%,the required current rating for IGBTs is reduced,and the system recovery time is also reduced by 20-40 ms.展开更多
To ensure their sound and continuous operation to the greatest extent,VSC-based DC girds have extremely stringent requirements for transmission line relay protection.In terms of guaranteeing their reliability,accurate...To ensure their sound and continuous operation to the greatest extent,VSC-based DC girds have extremely stringent requirements for transmission line relay protection.In terms of guaranteeing their reliability,accurate identification of lightning strikes on DC transmission lines is one of the urgent key problems to be solved.An effective ultra-high-speed identification scheme of lightning strikes suitable for the VSC-based DC grid is proposed in this paper.First,an 1-mode reverse voltage traveling wave(RVTW)is constructed applying the pole-mode transformation theory.Next,fault traveling wave propagation characteristics along the DC transmission line are analyzed in depth utilizing Peterson's law.Then,differences of time-frequency electromagnetic transient characteristics of 1-mode RVTWs between disturbances and faults caused by lightning strikes are distinguished in detail by means of the classical wavelet transformation multi-resolution analysis theory.Finally,extensive simulations are carried out to evaluate the performance of the proposed identification scheme,and by which its excellent rapidity,reliability and robustness are validated.Index Terms-Lightning-strike identification,Multi-resolution analysis,Relay protection,Traveling-wave protection,VsC-based DC grid,Wavelet transformation.展开更多
With the increase of dc based renewable energy generation and dc loads,the medium voltage dc(MVDC)distribution network is becoming a promising option for more efficient system integration.In particular,large-capacity ...With the increase of dc based renewable energy generation and dc loads,the medium voltage dc(MVDC)distribution network is becoming a promising option for more efficient system integration.In particular,large-capacity photovoltaic(PV)-based power generation is growing rapidly,and a corresponding power conversion system is critical to integrate these large PV systems into MVDC power grid.Different from traditional ac grid-connected converters,the converter system for dc grid interfaced PV system requires large-capacity dc conversion over a wide range of ultra-high voltage step-up ratios.This is an important issue,yet received limited research so far.In this paper,a thorough study of dc-dc conversion system for a medium-voltage dc grid-connected PV system is conducted.The required structural features for such a conversion system are first discussed.Based on these features,the conversion system is classified into four categories by series-parallel connection scheme of power modules.Then two existing conversion system configurations as well as a proposed solution are compared in terms of input/output performance,conversion efficiency,modulation method,control complexity,power density,reliability,and hardware cost.In-depth analysis is carried out to select the most suitable conversion systems in various application scenarios.展开更多
The fault current level analysis is important for bipolar direct current(DC)grids,which determines the operation and protection requirements.The DC grid topology significantly impacts the current path and then the fau...The fault current level analysis is important for bipolar direct current(DC)grids,which determines the operation and protection requirements.The DC grid topology significantly impacts the current path and then the fault current level of the grid,which makes it possible to limit the fault current by optimizing the grid topology.However,the corresponding discussion in the literature is indigent.Aiming at this point,the impact of grid topology,i.e.,the connecting scheme of converters,on the pole-to-ground fault current in bipolar DC grids,is investigated in this paper,and the ground-return-based and metallic-return-based grounding schemes are considered,respectively.Firstly,the decoupled equivalent model in frequency domain for fault current analysis is obtained.Then,the impacts of converters with different distances to the fault point on the fault current can be analyzed according to the high-frequency impedance characteristics.Based on the analysis results,a simplified fault current index(SFCI)is proposed to realize the fast evaluation of impact of grid topology on the fault current level.The SFCI is then applied to evaluate the relative fault current level.Finally,the simulation results validate the model,the analysis method,and the SFCI,which can effectively evaluate the relative fault current level in a direct and fast manner.展开更多
In flexible DC grids, the rapid rise of fault current requires that the line protection must complete the fault identification within a few milliseconds. Dynamic state estimation based protection(DSEBP) provides a new...In flexible DC grids, the rapid rise of fault current requires that the line protection must complete the fault identification within a few milliseconds. Dynamic state estimation based protection(DSEBP) provides a new idea for flexible DC line protection with good performance. However, the operating frequency in the DC grid is 0 Hz. When the DC grid is operating normally, it is difficult to identify the line parameters online to improve the performance of the protection method. This paper proposes a method to identify the frequency-dependent parameters of flexible DC grids based on the characteristic signal injection of half-bridge modular multilevel converter(HB-MMC). The characteristic signal is extracted by the Prony algorithm to calculate the line parameter under different frequencies. Afterwards, the number and position of residues and poles of frequency-dependent parameters are determined using the vector fitting method. Finally, an improved DSEBP is proposed. The simulation shows that the frequency-dependent parameters obtained by the proposed parameter identification method can be used in the improved DSEBP normally, and the identified parameters have better precision.展开更多
With the continuous development of power electronic devices,intelligent control systems,and other technologies,the voltage level and transmission capacity of voltage source converter (VSC)-high-voltage direct current ...With the continuous development of power electronic devices,intelligent control systems,and other technologies,the voltage level and transmission capacity of voltage source converter (VSC)-high-voltage direct current (HVDC) technology will continue to increase,while the system losses and costs will gradually decrease.Therefore,it can be foreseen that VSC-HVDC transmission technology will be more widely applied in future large-scale renewable energy development projects.Adopting VSC-HVDC transmission technology can be used to overcome issues encountered by large-scale renewable energy transmission and integration projects,such as a weak local power grid,lack of support for synchronous power supply,and insufficient accommodation capacity.However,this solution also faces many technical challenges because of the differences between renewable energy and traditional synchronous power generation systems.Based on actual engineering practices that are used worldwide,this article analyzes the technical challenges encountered by integrating large-scale renewable energy systems that adopt the use of VSC-HVDC technology,while aiming to provide support for future research and engineering projects related to VSC-HVDC-based large-scale renewable energy integration projects.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
Over the last few years, smart grids have become a topic of intensive research, development and deployment across the world. This is due to the fact that, through the smart grid, stable and reliable power systems can ...Over the last few years, smart grids have become a topic of intensive research, development and deployment across the world. This is due to the fact that, through the smart grid, stable and reliable power systems can be achieved. This paper presents a fuzzy logic control for dual active bridge series resonant converters for DC smart grid application. The DC smart grid consists of wind turbine and photovoltaic generators, controllable and DC loads, and power converters. The proposed control method has been applied to the controllable load＇s and the grid side＇s dual active bridge series resonant converters for attaining control of the power system. It has been used for management of controllable load＇s state of charge, DC feeder＇s voltage stability during the loads and power variations from wind energy and photovoltaic generation and power flow management between the grid side and the DC smart grid. The effectiveness of the proposed DC smart grid operation has been verified by simulation results obtained by using MATLAB and PLECS cards.展开更多
Strong DC coupling with weak AC and large-scale renewable energy integration are the two significant characteristics of ultra-high-voltage AC/DC(UHVAC/DC)hybrid power grids in China.Strong coupling between AC and DC g...Strong DC coupling with weak AC and large-scale renewable energy integration are the two significant characteristics of ultra-high-voltage AC/DC(UHVAC/DC)hybrid power grids in China.Strong coupling between AC and DC grids and the different integration performance of renewable energy sources have profoundly changed the stability characteristics of the power system.The traditional stability control system is inadequate for the stability control of UHVAC/DC power grids.This paper analyzes the requirements for constructing an integrated defense system in a UHVAC/DC hybrid power grid(i.e.power system protection).The definition,connotation,and designing principles of power system protection are put forward.The relationship between the power system protection and the traditional three-defense lines is investigated.The design principles,general hardware structure and main functions of a power system protection are presented.Key problems and technologies are specified in the construction of the power system protection.展开更多
In a DC grid with dedicated metallic return(DMR),the coupling effects among the positive pole,negative pole,and DMR conductors must be considered,which makes fault identification particularly difficult.In addition,the...In a DC grid with dedicated metallic return(DMR),the coupling effects among the positive pole,negative pole,and DMR conductors must be considered,which makes fault identification particularly difficult.In addition,the identification of high-impedance faults remains a major challenge for DC grid protection.To address these issues,this study proposes an adaptive single-end protection method for DC grid based on the transient mean value of the current limiting reactor(CLR)modal voltage.First,a fault analysis model of the DC grid with DMR is established using the Clarke transformation.The characteristics of CLR modal voltage are then clarified.A fault pole-selection method based on a novel modulus phase plane is next proposed.A threshold scaling factor based on the differential of DC bus voltage is then constructed to enhance the sensitivity and rapidity of the protection,which can adaptively modify the threshold according to the fault severity.Finally,a simulation model of a four-terminal DC grid with DMR is developed in PSCAD/EMTDC.The speed and reliability of the proposed protection method are verified by simulations and experiments.展开更多
The investigation of the pole-to-pole(p2p)DC short-circuit fault current in a modular multilevel converter(MMC)based high-voltage direct current(HVDC)grid is of vital importance for protection design and parameter opt...The investigation of the pole-to-pole(p2p)DC short-circuit fault current in a modular multilevel converter(MMC)based high-voltage direct current(HVDC)grid is of vital importance for protection design and parameter optimization.Existing calculation methods for the p2p fault current primarily rely on differential equations based on the RLC equivalent of the MMC station and the RL model of DC transmission lines.Some of them also take the AC in-feed currents into account.However,these approaches all carry heavy burdens for the complex formation and solving processes of equations,and they can only obtain the numerical solution of the fault current.The analytic solution of explicit physical meaning cannot be acquired at present because of the complex coupling relationship among MMC terminals.To address these issues,this paper builds a simplified and generalized fault equivalence model for the DC grid under a p2p fault.This is due to the fact that the terminals having long electrical distances from the fault point have less impact on the fault current.Next,not only the efficient and accurate fault current calculation is achieved,but also the approximate superposition-based analytical solution of the fault current is derived.The component analysis of the fault current and the sensitivity analysis of the components are provided as well.The calculation method and the analysis are both validated in PSCAD/EMTDC.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
High Voltage Direct Current(HVDC) grids are the most effective solutions for collection, integration and transmission of large scale remote renewable resources to load centers. A HVDC grid test model can provide a com...High Voltage Direct Current(HVDC) grids are the most effective solutions for collection, integration and transmission of large scale remote renewable resources to load centers. A HVDC grid test model can provide a common reference and study platform for researchers to compare the performance and characteristics of a DC grid with different DC control functions and protection strategies. It can also provide reference cases for testing of simulators and digital programs. This paper proposes a comprehensive HVDC grid test model and the associated four sub test models for system studies to meet the research purposes and requirements for different DC grid application scenarios. The design concept, topologies, configurations and functions of the test models are described in detail and their basic system data for load flow studies are provided. Finally load flow simulation studies with PSS/E(Power System Simulator/Engineering) program for each of the models are undertaken and the corresponding results are presented and analyzed in the paper.展开更多
Due to their high controllability and flexibility,DC power grids have broad application prospects in the fields of networking of renewable energy and the power supply for oceanic archipelagos and future cities.This pa...Due to their high controllability and flexibility,DC power grids have broad application prospects in the fields of networking of renewable energy and the power supply for oceanic archipelagos and future cities.This paper describes the system topology,control strategy,DC breaker configuration,and research,development and testing of converter valves and DC breaker equipment of the Zhoushan multi-terminal DC transmission system.Zhejiang Zhoushan has unique geographical and developmental features to demonstrate the irreplaceable technical advantages of a DC power grid for providing reliable power supply.The experience gained from the Zhoushan DC power grid project can offer valuable insights into the development and utilization of this technology worldwide.展开更多
High voltage direct current (HVDC) transmission is an economical option for transmitting a large amount of power over long distances. Initially, HVDC was developed using thyristor-based current source converters (CSC)...High voltage direct current (HVDC) transmission is an economical option for transmitting a large amount of power over long distances. Initially, HVDC was developed using thyristor-based current source converters (CSC). With the development of semiconductor devices, a voltage source converter (VSC)-based HVDC system was introduced, and has been widely applied to integrate large-scale renewables and network interconnection. However, the VSC-based HVDC system is vulnerable to DC faults and its protection becomes ever more important with the fast growth in number of installations. In this paper, detailed characteristics of DC faults in the VSC-HVDC system are presented. The DC fault current has a large peak and steady values within a few milliseconds and thus high-speed fault detection and isolation methods are required in an HVDC grid. Therefore, development of the protection scheme for a multi-terminal VSC-based HVDC system is challenging. Various methods have been developed and this paper presents a comprehensive review of the different techniques for DC fault detection, location and isolation in both CSC and VSC-based HVDC transmission systems in two-terminal and multi-terminal network configurations.展开更多
基金supported by the Science and Technology Project of State Grid Corporation of China under Grant 520201210025。
文摘The hybrid dc circuit breaker(HCB)has the advantages of fast action speed and low operating loss,which is an idealmethod for fault isolation ofmulti-terminal dc grids.Formulti-terminal dc grids that transmit power through overhead lines,HCBs are required to have reclosing capability due to the high fault probability and the fact that most of the faults are temporary faults.To avoid the secondary fault strike and equipment damage that may be caused by the reclosing of the HCB when the permanent fault occurs,an adaptive reclosing scheme based on traveling wave injection is proposed in this paper.The scheme injects traveling wave signal into the fault dc line through the additionally configured auxiliary discharge branch in the HCB,and then uses the reflection characteristic of the traveling wave signal on the dc line to identify temporary and permanent faults,to be able to realize fast reclosing when the temporary fault occurs and reliably avoid reclosing after the permanent fault occurs.The test results in the simulation model of the four-terminal dc grid show that the proposed adaptive reclosing scheme can quickly and reliably identify temporary and permanent faults,greatly shorten the power outage time of temporary faults.In addition,it has the advantages of easiness to implement,high reliability,robustness to high-resistance fault and no dead zone,etc.
基金funded by Horizon 2020 PROMOTioN(Progress on Meshed HVDC Offshore Transmission Networks)project under Grant Agreement No.691714funded by a research grant of the Research Foundation-Flanders(FWO)
文摘This paper proposes a frequency domain based methodology to analyse the influence of High Voltage Direct Current(HVDC) configurations and system parameters on the travelling wave behaviour during a DC fault. The method allows us to gain deeper understanding of these influencing parameters. In the literature, the majority of DC protection algorithms essentially use thefirst travelling waves initiated by a DC fault for fault discrimination due to the stringent time constraint in DC grid protection. However, most protection algorithms up to now have been designed based on extensive time domain simulations using one specific test system. Therefore, general applicability or adaptability to different configurations and system changes is not by default ensured, and it is difficult to gain in-depth understanding of the influencing parameters through time domain simulations. In order to analyse the first travelling wave for meshed HVDC grids, voltage and current wave transfer functions with respect to the incident voltage wave are derived adopting Laplace domain based component models. The step responses obtained from the voltage transfer functions are validated by comparison against simulations using a detailed model implemented in PSCADTM. Then, the influences of system parameters such as the number of parallel branches, HVDC grid configurations and groundings on the first travelling wave are investigated by analysing the voltage and current transfer functions.
基金This work was supported in part by the People Programme(Marie Curie Actions)of the European Union’s Seventh Framework Programme FP7/2007-2013/under REA grant agreement(No.317221)project title MEDOW,in part supported by the Project of National Science Foundation for Distinguished Young Scholars of China(No.51407164).
文摘With wider applications of power electronic devices in modern power systems,simulation using traditional electro-mechanical and electromagnetic simulation tools suffer from low speed and imprecision.Multi-rate technologies can greatly improve simulation efficiency by avoiding simulating the entire system using a small time-step.However,the drawbacks of the current synchronization mechanisms is that they introduce numerical errors and numerical instabilities in multi-rate parallel simulations.An improved multi-rate parallel technology,node splitting interface(NSI),is proposed to reduce errors and enhance simulation stability.A new synchronization mechanism is used to avoid prediction and signal delays.Theoretical analyses are carried out to prove the convergence and absolute stability of the proposed NSI algorithm.This algorithm is particularly suitable for simultaneously investigating long term dynamics of DC grids and fast transients of power electronic converters.
文摘Symmetrical monopolar configuration is the prevailing scheme configuration for modular multilevel converter based high-voltage direct current(MMC-HVDC) links, in which severe DC overvoltage or overcurrent can be caused by the DC faults. To deal with the possible asymmetry in the DC faults and the coupling effects of the DC lines, this paper analyzes the DC fault characteristics based on the phase-mode transformation. First, the DC grid is decomposed into the common-mode and the differential-mode networks. The equivalent models of the MMCs and DC lines in the two networks are derived, respectively. Then, based on the state matrices, a unified numerical calculation method for the fault voltages and currents at the DC side is proposed. Compared with the time-domain simulations performed on PSCAD/EMTDC, the accuracy of the proposed method is validated. Last, the system parameter analysis shows that the grounding parameters play an important role in reducing the severity of the pole-to-ground faults, whereas the coupling effects of the DC lines should be considered when calculating the DC fault currents associated with the pole-to-pole faults.
基金This work was supported by National Key Research and Development Program under Grant No.2018YFB0904600the National Natural Science Foundation of China under Grant No.51777072.
文摘The modular multilevel converter(MMC)based DC grid is considered as a future solution for bulk renewable energy integration and transmission.However,the high probability of DC faults and their rapid propagation speed are the main challenges in the development of DC grids.Existing research primarily focuses on the DC fault clearance methods,while the fault current suppression methods are still barely researched.Additionally,the coordination method of fault current suppression and clearance needs to be optimized.In this paper,the technical characteristics of the current suppression methods are studied,and the coordinated methods of fault current suppression and clearance are proposed.At last,a cost comparison of these methods is presented.The research results show that the proposed strategies can reduce the cost of the protection equipment.
基金the National Key R&D Program of China(Grant No.2018YFB0904600)the National Natural Science Foundation of China(Grant No.51777072)。
文摘High voltage DC grids are developing in more terminals and with larger transmission capacity,thus the re-quirements for DC circuit breakers(DCCB)will continue to rise.Conventional methods only use the faulty line DCCB to withstand the fault stress,and therefore this paper presents a coordination method of multiple DCCBs to protect the system.As many adjacent DCCBs are tripped to interrupt the fault current,the fault energy is shared,and the requirement for the faulty line DCCB is reduced.Moreover,the adjacent DCCBs are actively controlled to help system recovery.The primary protection,backup protection,and reclosing logic of multiple DCCBs are studied.Simulations confirm that the proposed control reduces the energy dissipation requirement of faulty line DCCB by approximately 30%-42%,the required current rating for IGBTs is reduced,and the system recovery time is also reduced by 20-40 ms.
基金supported by the National Natural Science Foundation of China(No.52277075)the State Key Laboratory of Advanced Power Transmission Technology(Grant No.GEIRI-SKL-2020-012).
文摘To ensure their sound and continuous operation to the greatest extent,VSC-based DC girds have extremely stringent requirements for transmission line relay protection.In terms of guaranteeing their reliability,accurate identification of lightning strikes on DC transmission lines is one of the urgent key problems to be solved.An effective ultra-high-speed identification scheme of lightning strikes suitable for the VSC-based DC grid is proposed in this paper.First,an 1-mode reverse voltage traveling wave(RVTW)is constructed applying the pole-mode transformation theory.Next,fault traveling wave propagation characteristics along the DC transmission line are analyzed in depth utilizing Peterson's law.Then,differences of time-frequency electromagnetic transient characteristics of 1-mode RVTWs between disturbances and faults caused by lightning strikes are distinguished in detail by means of the classical wavelet transformation multi-resolution analysis theory.Finally,extensive simulations are carried out to evaluate the performance of the proposed identification scheme,and by which its excellent rapidity,reliability and robustness are validated.Index Terms-Lightning-strike identification,Multi-resolution analysis,Relay protection,Traveling-wave protection,VsC-based DC grid,Wavelet transformation.
基金Supported by the National Natural Science Foundation of China(51811540405,52007096)National Key R&D Program of China(2016YFB0900205).
文摘With the increase of dc based renewable energy generation and dc loads,the medium voltage dc(MVDC)distribution network is becoming a promising option for more efficient system integration.In particular,large-capacity photovoltaic(PV)-based power generation is growing rapidly,and a corresponding power conversion system is critical to integrate these large PV systems into MVDC power grid.Different from traditional ac grid-connected converters,the converter system for dc grid interfaced PV system requires large-capacity dc conversion over a wide range of ultra-high voltage step-up ratios.This is an important issue,yet received limited research so far.In this paper,a thorough study of dc-dc conversion system for a medium-voltage dc grid-connected PV system is conducted.The required structural features for such a conversion system are first discussed.Based on these features,the conversion system is classified into four categories by series-parallel connection scheme of power modules.Then two existing conversion system configurations as well as a proposed solution are compared in terms of input/output performance,conversion efficiency,modulation method,control complexity,power density,reliability,and hardware cost.In-depth analysis is carried out to select the most suitable conversion systems in various application scenarios.
基金supported by the Science and Technology Project of State Grid Corporation of China“Cloud energy storage framework-based AI dispatching strategy of renewable energy integration and contingency response” (No.5100-202199274A-0-0-00)。
文摘The fault current level analysis is important for bipolar direct current(DC)grids,which determines the operation and protection requirements.The DC grid topology significantly impacts the current path and then the fault current level of the grid,which makes it possible to limit the fault current by optimizing the grid topology.However,the corresponding discussion in the literature is indigent.Aiming at this point,the impact of grid topology,i.e.,the connecting scheme of converters,on the pole-to-ground fault current in bipolar DC grids,is investigated in this paper,and the ground-return-based and metallic-return-based grounding schemes are considered,respectively.Firstly,the decoupled equivalent model in frequency domain for fault current analysis is obtained.Then,the impacts of converters with different distances to the fault point on the fault current can be analyzed according to the high-frequency impedance characteristics.Based on the analysis results,a simplified fault current index(SFCI)is proposed to realize the fast evaluation of impact of grid topology on the fault current level.The SFCI is then applied to evaluate the relative fault current level.Finally,the simulation results validate the model,the analysis method,and the SFCI,which can effectively evaluate the relative fault current level in a direct and fast manner.
基金This work was supported by National Natural Science Foundation of China-State Grid Corporation Joint Fund for Smart Grid(No.U2066210).
文摘In flexible DC grids, the rapid rise of fault current requires that the line protection must complete the fault identification within a few milliseconds. Dynamic state estimation based protection(DSEBP) provides a new idea for flexible DC line protection with good performance. However, the operating frequency in the DC grid is 0 Hz. When the DC grid is operating normally, it is difficult to identify the line parameters online to improve the performance of the protection method. This paper proposes a method to identify the frequency-dependent parameters of flexible DC grids based on the characteristic signal injection of half-bridge modular multilevel converter(HB-MMC). The characteristic signal is extracted by the Prony algorithm to calculate the line parameter under different frequencies. Afterwards, the number and position of residues and poles of frequency-dependent parameters are determined using the vector fitting method. Finally, an improved DSEBP is proposed. The simulation shows that the frequency-dependent parameters obtained by the proposed parameter identification method can be used in the improved DSEBP normally, and the identified parameters have better precision.
基金State Grid Corporation of China Science and Technology Project: Research on Power Transmission of Largescale Renewable Energy Base by VSC-LCC hybrid HVDC(No. NY71-19-037)
文摘With the continuous development of power electronic devices,intelligent control systems,and other technologies,the voltage level and transmission capacity of voltage source converter (VSC)-high-voltage direct current (HVDC) technology will continue to increase,while the system losses and costs will gradually decrease.Therefore,it can be foreseen that VSC-HVDC transmission technology will be more widely applied in future large-scale renewable energy development projects.Adopting VSC-HVDC transmission technology can be used to overcome issues encountered by large-scale renewable energy transmission and integration projects,such as a weak local power grid,lack of support for synchronous power supply,and insufficient accommodation capacity.However,this solution also faces many technical challenges because of the differences between renewable energy and traditional synchronous power generation systems.Based on actual engineering practices that are used worldwide,this article analyzes the technical challenges encountered by integrating large-scale renewable energy systems that adopt the use of VSC-HVDC technology,while aiming to provide support for future research and engineering projects related to VSC-HVDC-based large-scale renewable energy integration projects.
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Over the last few years, smart grids have become a topic of intensive research, development and deployment across the world. This is due to the fact that, through the smart grid, stable and reliable power systems can be achieved. This paper presents a fuzzy logic control for dual active bridge series resonant converters for DC smart grid application. The DC smart grid consists of wind turbine and photovoltaic generators, controllable and DC loads, and power converters. The proposed control method has been applied to the controllable load＇s and the grid side＇s dual active bridge series resonant converters for attaining control of the power system. It has been used for management of controllable load＇s state of charge, DC feeder＇s voltage stability during the loads and power variations from wind energy and photovoltaic generation and power flow management between the grid side and the DC smart grid. The effectiveness of the proposed DC smart grid operation has been verified by simulation results obtained by using MATLAB and PLECS cards.
文摘Strong DC coupling with weak AC and large-scale renewable energy integration are the two significant characteristics of ultra-high-voltage AC/DC(UHVAC/DC)hybrid power grids in China.Strong coupling between AC and DC grids and the different integration performance of renewable energy sources have profoundly changed the stability characteristics of the power system.The traditional stability control system is inadequate for the stability control of UHVAC/DC power grids.This paper analyzes the requirements for constructing an integrated defense system in a UHVAC/DC hybrid power grid(i.e.power system protection).The definition,connotation,and designing principles of power system protection are put forward.The relationship between the power system protection and the traditional three-defense lines is investigated.The design principles,general hardware structure and main functions of a power system protection are presented.Key problems and technologies are specified in the construction of the power system protection.
基金supported in part by the Joint Funds of the National Natural Science Foundation of China(No.U22B6006)the National Natural Science Foundation of China(No.52207126)Natural Science Foundation of Sichuan Province(No.2023NSFSC0296).
文摘In a DC grid with dedicated metallic return(DMR),the coupling effects among the positive pole,negative pole,and DMR conductors must be considered,which makes fault identification particularly difficult.In addition,the identification of high-impedance faults remains a major challenge for DC grid protection.To address these issues,this study proposes an adaptive single-end protection method for DC grid based on the transient mean value of the current limiting reactor(CLR)modal voltage.First,a fault analysis model of the DC grid with DMR is established using the Clarke transformation.The characteristics of CLR modal voltage are then clarified.A fault pole-selection method based on a novel modulus phase plane is next proposed.A threshold scaling factor based on the differential of DC bus voltage is then constructed to enhance the sensitivity and rapidity of the protection,which can adaptively modify the threshold according to the fault severity.Finally,a simulation model of a four-terminal DC grid with DMR is developed in PSCAD/EMTDC.The speed and reliability of the proposed protection method are verified by simulations and experiments.
基金supported by the National Key R&D Program of China under Grant 2018YFB0904600。
文摘The investigation of the pole-to-pole(p2p)DC short-circuit fault current in a modular multilevel converter(MMC)based high-voltage direct current(HVDC)grid is of vital importance for protection design and parameter optimization.Existing calculation methods for the p2p fault current primarily rely on differential equations based on the RLC equivalent of the MMC station and the RL model of DC transmission lines.Some of them also take the AC in-feed currents into account.However,these approaches all carry heavy burdens for the complex formation and solving processes of equations,and they can only obtain the numerical solution of the fault current.The analytic solution of explicit physical meaning cannot be acquired at present because of the complex coupling relationship among MMC terminals.To address these issues,this paper builds a simplified and generalized fault equivalence model for the DC grid under a p2p fault.This is due to the fact that the terminals having long electrical distances from the fault point have less impact on the fault current.Next,not only the efficient and accurate fault current calculation is achieved,but also the approximate superposition-based analytical solution of the fault current is derived.The component analysis of the fault current and the sensitivity analysis of the components are provided as well.The calculation method and the analysis are both validated in PSCAD/EMTDC.
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 the State Grid Corporation of China,through the 1000-Plan project(No.[2014]264)
文摘High Voltage Direct Current(HVDC) grids are the most effective solutions for collection, integration and transmission of large scale remote renewable resources to load centers. A HVDC grid test model can provide a common reference and study platform for researchers to compare the performance and characteristics of a DC grid with different DC control functions and protection strategies. It can also provide reference cases for testing of simulators and digital programs. This paper proposes a comprehensive HVDC grid test model and the associated four sub test models for system studies to meet the research purposes and requirements for different DC grid application scenarios. The design concept, topologies, configurations and functions of the test models are described in detail and their basic system data for load flow studies are provided. Finally load flow simulation studies with PSS/E(Power System Simulator/Engineering) program for each of the models are undertaken and the corresponding results are presented and analyzed in the paper.
基金supported in part by the National Natural Science Foundation of China(No.51261130471)by the Science and Technology Research Project of the State Grid Corporation of China(No.SGRIPGKJ(2013)166).
文摘Due to their high controllability and flexibility,DC power grids have broad application prospects in the fields of networking of renewable energy and the power supply for oceanic archipelagos and future cities.This paper describes the system topology,control strategy,DC breaker configuration,and research,development and testing of converter valves and DC breaker equipment of the Zhoushan multi-terminal DC transmission system.Zhejiang Zhoushan has unique geographical and developmental features to demonstrate the irreplaceable technical advantages of a DC power grid for providing reliable power supply.The experience gained from the Zhoushan DC power grid project can offer valuable insights into the development and utilization of this technology worldwide.
文摘High voltage direct current (HVDC) transmission is an economical option for transmitting a large amount of power over long distances. Initially, HVDC was developed using thyristor-based current source converters (CSC). With the development of semiconductor devices, a voltage source converter (VSC)-based HVDC system was introduced, and has been widely applied to integrate large-scale renewables and network interconnection. However, the VSC-based HVDC system is vulnerable to DC faults and its protection becomes ever more important with the fast growth in number of installations. In this paper, detailed characteristics of DC faults in the VSC-HVDC system are presented. The DC fault current has a large peak and steady values within a few milliseconds and thus high-speed fault detection and isolation methods are required in an HVDC grid. Therefore, development of the protection scheme for a multi-terminal VSC-based HVDC system is challenging. Various methods have been developed and this paper presents a comprehensive review of the different techniques for DC fault detection, location and isolation in both CSC and VSC-based HVDC transmission systems in two-terminal and multi-terminal network configurations.
