To enhance power flow regulation in scenarios involving large-scale renewable energy transmission via high-voltage direct current(HVDC)links and multi-infeed DC systems in load-center regions,this paper proposes a hyb...To enhance power flow regulation in scenarios involving large-scale renewable energy transmission via high-voltage direct current(HVDC)links and multi-infeed DC systems in load-center regions,this paper proposes a hybrid modular multilevel converter–capacitor-commutated line-commutated converter(MMC-CLCC)HVDC transmission system and its corresponding control strategy.First,the system topology is constructed,and a submodule configuration method for the MMC—combining full-bridge submodules(FBSMs)and half-bridge submodules(HBSMs)—is proposed to enable direct power flow reversal.Second,a hierarchical control strategy is introduced,includingMMCvoltage control,CLCC current control,and a coordinationmechanism,along with the derivation of the hybrid system’s power flow reversal characteristics.Third,leveraging the CLCC’s fast current regulation and theMMC’s negative voltage control capability,a coordinated power flow reversal control strategy is developed.Finally,an 800 kV MMC-CLCC hybrid HVDC system is modeled in PSCAD/EMTDC to validate the power flow reversal performance under a high proportion of full-bridge submodule configuration.Results demonstrate that the proposed control strategy enables rapid(1-s transition)and smooth switching of bidirectional power flow without modifying the structure of primary equipment:the transient fluctuation ofDC voltage from the rated value(UdcN)to themaximumreverse voltage(-kUdcN)is less than 5%;the DC current strictly follows the preset characteristic curve with a deviation of≤3%;the active power reverses continuously,and the system maintains stable operation throughout the reversal process.展开更多
The introduction of fully controlled devices to build hybrid line commutated converter(H-LCC)has become a new idea to solve the commutation failure.However,existing H-LCC has not considered the implementation of a tar...The introduction of fully controlled devices to build hybrid line commutated converter(H-LCC)has become a new idea to solve the commutation failure.However,existing H-LCC has not considered the implementation of a targeted firing angle control strategy during AC faults,with the objective of enhancing their power transmission and fault response performance.For this reason,this paper proposes an optimized control method for firing angle of H-LCC,designated as flexible virtual firing(FVF).This method first analyzes the influence of alterations in firing angle on reactive power,commutation process and associated action paths.By combining prediction and dynamic search,it optimizes the natural commutation process through the utilization of dynamic boundary and minimum commutation area difference.This can mitigate the impact of AC faults on H-LCC and DC system,thereby improving power transmission and defense to commutation failure,which is beneficial for improving the stability of AC/DC power grids.Finally,the simulation results in PSCAD/EMTDC verify the effectiveness of the proposed method.展开更多
This paper systematically investigates the multidegree-of-freedom(multi-DoF)control framework of high voltage direct current(HVDC)converters based on their topological characteristics and control requirements.First,th...This paper systematically investigates the multidegree-of-freedom(multi-DoF)control framework of high voltage direct current(HVDC)converters based on their topological characteristics and control requirements.First,this study reveals that its topology determines the control degrees of freedom(CDoF)of a converter.Specifically,a line-commutated converter(LCC)possesses only one CDoF,a voltage source converter(VSC)has two,and a modular multilevel converter(MMC)can theoretically achieve up to eight.However,only six CDoF are practically necessary for MMC.Despite this,existing MMC control strategies utilize only four CDoF,limiting control flexibility.To overcome this issue,this paper proposes a six-degree-offreedom(6 DoF)control framework for MMC,which enables asymmetric and precise regulation of DC voltage and SM capacitor voltage compared to the existing MMC control.Based on this framework,corresponding control strategies are designed for MMCs in HVDC systems with different control objectives.Finally,a two-terminal MMC-HVDC system is built on the PSCAD/EMTDC to verify the effectiveness of the proposed 6 DoF control.展开更多
In this study,a novel parallel converter-based hybrid high-voltage direct current(HVDC)system is proposed for the integration and delivery of large-scale renewable energy.The rectifier uses the line commutated convert...In this study,a novel parallel converter-based hybrid high-voltage direct current(HVDC)system is proposed for the integration and delivery of large-scale renewable energy.The rectifier uses the line commutated converter(LCC)and low-capacity modular multilevel converter(MMC)in parallel,while the inverter uses MMC.This configuration combines the economic advantages of LCC with the flexibility of MMC.Firstly,the steady-state control strategies are elaborated.The low-capacity MMC operates in the grid-forming mode to offer AC voltage support.It also provides active filtering for the LCC and maintains the reactive power balance of the sending-end system.The LCC efficiently transmits all active power at the rectifier side,fully exploiting its bulk-power transmission capability.Secondly,the fault ride-through strategies of both the AC faults at two terminals and the DC fault are proposed,in which the MMCs at both terminals can remain unblocked under various faults.Thus,the proposed system can mitigate the impact of the faults and ensure continuous voltage support for the sending-end system.Finally,simulations in PSCAD/EMTDC verify the effectiveness and performance of the proposed system.展开更多
This paper presents a thermal management framework for 120 kV hybrid commutated converter(HCC)valves,addressing critical cooling challenges in multi-hundred-MW power conversion systems.Power loss calculations under ra...This paper presents a thermal management framework for 120 kV hybrid commutated converter(HCC)valves,addressing critical cooling challenges in multi-hundred-MW power conversion systems.Power loss calculations under rated(1.0 p.u.)and overload(1.2 p.u.)conditions demonstrate that HCC valves achieve comparable loss levels to line commutated converter counterparts while enabling active turn-off control.Comparative analysis of radiator configurations identifies 2-parallel branch connections as optimal.Integrated thermal-fluid models combining 3D finite element analysis and computational fluid dynamics reveal significant temperature gradients and flow maldistribution in baseline designs.On this basis,this paper modifies the flow from equal flow resistance allocation to heat-based allocation and it reduces maximum integrated gate-commutated thyristor temperature rise by 7.3%at 1.2 p.u.with minimal pressure drop variation.Experimental validation confirms the proposed cooling strategy enhances valve safety margins through improved heat dissipation balance,providing a validated theoretical foundation for high-power converter thermal design.展开更多
Commutation failure(CF)is an inherent problem faced by line commutated converter high voltage direct current(LCC-HVDC)technology.To completely solve the problem of CF,we have proposed a novel hybrid commutated convert...Commutation failure(CF)is an inherent problem faced by line commutated converter high voltage direct current(LCC-HVDC)technology.To completely solve the problem of CF,we have proposed a novel hybrid commutated converter(HCC)technology based on reverse blocking integrated gate commutated thyristor,which can utilise two methods for commutation:enhanced grid voltage commutation and active turn-off forced com-mutation.In this paper,the topology and operating principle of HCC are proposed.Then,the control and protection strategy is designed based on the current variation trend under AC faults.To verify the effectiveness of HCC in mitigating CF,a 120-kV/360-MW HCC-HVDC model is built in PSCAD/EMTDC,adopting LCC at the rectifier side and HCC at the inverter side.Based on this model,HCC steady-state and fault transient stresses are analysed.Various AC faults are simulated and the performance of HCC-HVDC is compared with LCC-HVDC.Finally,the results show that the HCC topol-ogy and proposed control strategy can solve CF under all fault conditions with almost the same attributes as LCC,such as large capacity,low cost,low loss and high reliability,which is meaningful for the application of HCC to the HVDC transmission system.展开更多
To reduce the probability of commutation failure(CF)of a line commutated converter based high-voltage direct current(LCC-HVDC)transmission,a DC chopper topology composed of power consumption sub-modules based on thyri...To reduce the probability of commutation failure(CF)of a line commutated converter based high-voltage direct current(LCC-HVDC)transmission,a DC chopper topology composed of power consumption sub-modules based on thyristor full-bridge module(TFB-PCSM)is proposed.Firstly,the mechanism of the proposed topology to mitigate CF is analyzed,and the working modes of TFB-PCSM in different operation states are introduced.Secondly,the coordinated control strategy between the proposed DC chopper and LCC-HVDC is designed,and the voltage-current stresses of the TFB-PCSMs are investigated.Finally,the ability to mitigate the CF issues and the fault recovery performance of LCC-HVDC system are studied in PSCAD/EMTDC.The results show that the probability of CF of LCC-HVDC is significantly reduced,and the performances of fault recovery are effectively improved by the proposed DC chopper.展开更多
This paper proposes a novel AC filter system for a line commutated converter high voltage DC(LCC-HVDC)transmission system.Through the coordination of the hybrid active power filters(APF)and the existing reactive compe...This paper proposes a novel AC filter system for a line commutated converter high voltage DC(LCC-HVDC)transmission system.Through the coordination of the hybrid active power filters(APF)and the existing reactive compensation devices,the proposed filter system can not only enhance the suppression performance for LCC-HVDC harmonics,but also optimize the AC yard layout with reduced reactive power subbanks,reducing the cost of HVDC projects.The novel filter system adopts a serial passive resonance topology obtained by careful comparison of different APFs.A proper control scheme is then designed integrating the control strategy of the APF and impedance characteristics of the HVDC system,which is able to realize harmonic suppression and dynamic reactive power support simultaneously.In addition,a novel self-adaption digital low-pass filter algorithm is presented,which is used in the APF harmonic detecting step,enhancing both high precision and fast dynamic response.On the basis of a real HVDC project,the advantages of proposed filter system in harmonic suppression,reactive power regulation,and sub-banks reduction are simulated and demonstrated.展开更多
The hybrid cascaded HVDC system employs a line commutated converter(LCC)as the rectifier and an LCC in series with multiple paralleled modular multilevel converters(MMCs)as the inverter.MMC arms are susceptible to ove...The hybrid cascaded HVDC system employs a line commutated converter(LCC)as the rectifier and an LCC in series with multiple paralleled modular multilevel converters(MMCs)as the inverter.MMC arms are susceptible to overcurrent following a severe AC fault at the receiving end,however,its fundamental mechanism has not been totally revealed.Therefore,this article explores the overcurrent characteristics on MMC arms,in terms of both the DC and AC components.Apart from the DC overcurrent component induced by the commutation failure(CF)of the inverter LCC,the AC overcurrent component is also significant.It dramatically depends on the coupling effects among the AC systems of the inverter side.Further,corresponding suppression strategies are proposed,which are applicable to different receiving-end AC fault scenarios.Eventually,the time-domain simulation results from PSCAD/EMTDC validate the effectiveness of the proposed overcurrent suppression control.It is also demonstrated that the presented methods can not only suppress overcurrent for MMC arms,but also reduce the imbalanced power between two sides,as well as improve the dynamic performances of the entire system.展开更多
The negative-sequence voltage is often caused by the asymmetrical fault in the AC system,as well as the harmonics after the symmetrical fault at the AC side of inverter in line commutated converter based high-voltage ...The negative-sequence voltage is often caused by the asymmetrical fault in the AC system,as well as the harmonics after the symmetrical fault at the AC side of inverter in line commutated converter based high-voltage DC(LCC-HVDC).The negative-sequence voltage affects the phase-locked loop(PLL)and the inverter control,thus the inverter is vulnerable to the subsequent commutation failure(SCF).In this paper,the analytical expression of the negative-sequence voltage resulting from the symmetrical fault with the commutation voltage is derived using the switching function and Fourier decomposition.The analytical expressions of the outputs of the PLL and inverter control with respect to time are derived to quantify the contribution of the negative-sequence voltage to the SCF.To deal with the AC component of the input signals in the PLL and the inverter control due to the negative-sequence voltage,the existing proportional-integral controls of the PLL,constant current control,and constant extinction angle control are replaced by the linear active disturbance rejection control against the SCF.Simulation results verify the contributing factors to the SCF.The proposed control reduces the risk of SCF and improves the recovery speed of the system under different fault conditions.展开更多
With the increasing capacity of wind farm, HVDC technology has become a promising transmission scheme for long distance transportation of large-scale wind power. However oscillation caused by this system will have a g...With the increasing capacity of wind farm, HVDC technology has become a promising transmission scheme for long distance transportation of large-scale wind power. However oscillation caused by this system will have a great influence on the security and stability of power system operation. In this paper, the oscillation of a doubly-fed induction generator(DFIG)-based wind farm interfaced with line commutated converter(LCC) based HVDC is discussed. Low-frequency oscillation and subsynchronous oscillation(SSO) are studied since these two oscillations are the particularly concerned oscillations in the stability study of power system in recent years. The model of a DFIG-based wind farm interconnected with LCC-HVDC is developed. The impact of drive train model's structure and parameters on the oscillation characteristics is analyzed. Eigenvalue and participation factor analysis are used to identify the three main modes, which include controller mode, electromechanical mode, and shaft mode. The effects of DFIG controller's parameters, wind speed and operating conditions of HVDC on those modes are studied. Electromagnetic transient simulations are performed to verify the results of the eigenvalue analysis.展开更多
Multi-terminal high-voltage DC(MTDC)technology is a promising way to transmit large amounts of offshore wind power to the main grids.This paper proposes a hybrid MTDC scheme to integrate several offshore wind farms in...Multi-terminal high-voltage DC(MTDC)technology is a promising way to transmit large amounts of offshore wind power to the main grids.This paper proposes a hybrid MTDC scheme to integrate several offshore wind farms into the onshore power grids at different locations.A hybrid four-terminal HVDC system comprising two onshore line commutated converters(LCCs)and two voltage source converters(VSCs)connecting an offshore wind farm is constructed in PSCAD/EMTDC.A coordination control scheme based on the VSCs’AC voltage control and the LCCs’DC voltage droop control is designed to ensure smooth system operation and proper power sharing between onshore AC grids.The operational characteristics of the system are analyzed.In addition,a black start-up method without any auxiliary power supply for the VSCs is proposed.The transmission scheme is tested through simulations under various conditions,including start-up,wind speed variation,and the disconnection of one VSC or of one LCC.展开更多
The hybrid-HVDC topology,which consists of line-commutated-converter(LCC)and voltage source converter(VSC)and combines their advantages,has extensive application prospects.A hybrid-HVDC system,adopting VSC on rectifie...The hybrid-HVDC topology,which consists of line-commutated-converter(LCC)and voltage source converter(VSC)and combines their advantages,has extensive application prospects.A hybrid-HVDC system,adopting VSC on rectifier side and LCC on inverter side,is investigated,and its mathematic model is deduced.The commutation failure issue of the LCC converter in the hybrid-HVDC system is considered,and a novel coordinated control method is proposed to enhance the system commutation failure immunity.A voltage dependent voltage order limiter(VDVOL)is designed based on the constant DC voltage control on the rectifier side,and constant extinction angle backup control is introduced based on the constant DC current control with voltage dependent current order limiter(VDCOL)on the inverter side.The hybrid-HVDC system performances under normal operation state and fault state are simulated in the PSCAD/EMTDC.Then,system transient state performances with or without the proposed control methods under fault condition are further compared and analyzed.It is concluded that the proposed control method has the ability to effectively reduce the probability of commutation failure and improve the fault recovery performance of the hybrid-HVDC system.展开更多
This paper investigates the small-signal stability of the hybrid high-voltage direct current(HVDC)transmission system.The system is composed of line commutated converter(LCC)as rectifier and modular multi-level conver...This paper investigates the small-signal stability of the hybrid high-voltage direct current(HVDC)transmission system.The system is composed of line commutated converter(LCC)as rectifier and modular multi-level converter(MMC)as inverter under weak AC grid condition.Firstly,the impact of short-circuit ratio(SCR)at inverter side on the system stability is investigated by eigen-analysis,and the key control parameters which have major impact on the dominant mode are identified by the participation factor and sensitivity analysis.Then,considering the quadratic index and damping ratio characteristic,an objective function for evaluating the system stability is developed,and an optimization and configuration method for control parameters is presented by the utilization of Monte Carlo method.The eigenvalue results and the electromagnetic transient(EMT)simulation results show that,with the optimized control parameters,the small-signal stability and the dynamic responses of the hybrid system are greatly improved,and the hybrid system can even operate under weak AC grid condition.展开更多
Line-commutated converter based high-voltage direct-current(LCC-HVDC)transmission systems are prone to subsequent commutation failure(SCF),which consequently leads to the forced blocking of HVDC links,affecting the op...Line-commutated converter based high-voltage direct-current(LCC-HVDC)transmission systems are prone to subsequent commutation failure(SCF),which consequently leads to the forced blocking of HVDC links,affecting the operation of the power system.An accurate commutation failure(CF)identification is fairly vital to the prevention of SCF.However,the existing CF identification methods cause CF misjudge or detection lag,which can limit the effect of SCF mitigation strategy.In addition,earlier approaches to suppress SCF do not clarify the key factor that determines the evolution of extinction angle during system recovery and neglect the influence.Hence,this paper firstly analyzes the normal commutation process and CF feature based on the evolution topology of converter valve conduction in detail.Secondly,the energy in the leakage inductance of converter transformer is presented to characterize the commutation state of the valves.Then a CF identification method is proposed utilizing the leakage inductance energy.Thirdly,taking the key variable which is crucial to the tendency of extinction angle during the recovery process into account,a fault current limiting strategy for SCF mitigation is put forward.Compared with the original methods,the proposed methods have a better performance in CF identification and mitigation in terms of detection accuracy and mitigation effect.Finally,case study on PSCAD/EMTDC validates the proposed methods.展开更多
For the safe and fast recovery of line commutated converter based high-voltage direct current(LCC-HVDC)transmission systems after faults,a DC current order optimization based strategy is proposed.Considering the const...For the safe and fast recovery of line commutated converter based high-voltage direct current(LCC-HVDC)transmission systems after faults,a DC current order optimization based strategy is proposed.Considering the constraint of electric and control quantities,the DC current order with the maximum active power transfer is calculated by Thevenin equivalent parameters(TEPs)and quasi-state equations of LCC-HVDC transmission systems.Meanwhile,to mitigate the subsequent commutation failures(SCFs)that may come with the fault recovery process,the maximum DC current order that avoids SCFs is calculated through imaginary commutation process.Finally,the minimum value of the two DC current orders is sent to the control system.Simulation results based on PSCAD/EMTDC show that the proposed strategy mitigates SCFs effectively and exhibits good performance in recovery.展开更多
Line commutated converter based high-voltage direct-current(LCC-HVDC)transmissions are prone to harmonic oscillation under weak grids.Impedance modeling is an effective method for assessing interaction stability.First...Line commutated converter based high-voltage direct-current(LCC-HVDC)transmissions are prone to harmonic oscillation under weak grids.Impedance modeling is an effective method for assessing interaction stability.Firstly,this paper proposes an improved calculation method for the DC voltage and AC currents of commutation stations to address the complex linearization of the commutation process and constructs an overall harmonic state-space(HSS)model of an LCC-HVDC.Based on the HSS model,the closed-loop AC impedances on the LCC-HVDC sending and receiving ends are then derived and verified.The impedance characteristics of the LCC-HVDC are then analyzed to provide a physical explanation for the harmonic oscillation of the system.The effects of the grid strength and control parameters on system stability are also analyzed.To improve the impedance characteristics and operating stability of the LCC-HVDC system,a virtual impedance based stability enhancement control is proposed,and a parameter design method is considered to ensure satisfactory phase margins at both the sending and receiving ends.Finally,simulation results are presented to verify the validity of the impedance model and virtual impedance based stability enhancement control.展开更多
The commutation failure(CF) mitigation effectiveness is normally restricted by the delay of extinction angle(EA)measurement or the errors of existing prediction methods for EA or firing angle(FA). For this purpose, th...The commutation failure(CF) mitigation effectiveness is normally restricted by the delay of extinction angle(EA)measurement or the errors of existing prediction methods for EA or firing angle(FA). For this purpose, this paper proposes a CF mitigation method based on the imaginary commutation process. For each sample point, an imaginary commutation process is constructed to simulate the actual commutation process.Then, the imaginary EA is calculated by comparing the imaginary supply voltage-time area and the imaginary demand voltage-time area, which can update the imaginary EA earlier than the measured EA. In addition, the proposed method considers the impacts of commutation voltage variation, DC current variation, and phase angle shift of commutation voltage on the commutation process, which can ensure a more accurate EA calculation. Moreover, the DC current prediction is proposed to improve the CF mitigation performance under the single-phase AC faults. Finally, the simulation results based on CIGRE model prove that the proposed method has a good performance in CF mitigation.展开更多
In line commutated converter based high-voltage direct current(LCC-HVDC)transmission systems,the transformer saturation can induce harmonic instability,which poses a serious threat to the safe operation of the power s...In line commutated converter based high-voltage direct current(LCC-HVDC)transmission systems,the transformer saturation can induce harmonic instability,which poses a serious threat to the safe operation of the power system.However,the nonlinear characteristics of the power grids introduced by the transformer saturation considerably limit the application of the conventional analysis methods.To address the issue,this paper derives a linear model for the transformer saturation caused by the DC current due to the converter modulation.Afterwards,the nonlinear characteristics of power grids with the transformer saturation is described by a complex valued impedance matrix.Based on the derived impedance matrix,the system harmonic stability is analyzed and the mechanism of the transformer saturation induced harmonic instability is revealed.Finally,the sensitivity analysis is conducted to find the key factors that influence the system core saturation instability.The proposed impedance model is verified by the electromagnetic transient simulation,and the simulation results corroborate the effectiveness of the proposed impedance model.Index TermsLine commutated converter based high voltage direct current(LCC-HVDC),transformer saturation,harmonic instability,impedance model.展开更多
The mitigation of commutation failure(CF)depends on the accuracy of CF prediction.In terms of the large error of the existing extinction angle(EA)calculation during the fault transient period,a method for CF predictio...The mitigation of commutation failure(CF)depends on the accuracy of CF prediction.In terms of the large error of the existing extinction angle(EA)calculation during the fault transient period,a method for CF prediction and mitigation is proposed.Variations in both DC current and overlap angle(OA)are considered in the proposed method to predict the EA rapidly.In addition,variations in critical EA and the effect of firing angle(FA)on both DC current and OA are considered in the proposed method to obtain the accurate FA order for the control system.The proposed method can achieve good performance in terms of CF mitigation and reduce reactive consumption at the inverter side when a fault occurs.Simulation results based on the PSCAD/EMTDC show that the proposed method predicts CF rapidly and exhibits good performance in terms of CF mitigation.展开更多
基金supported by Science and Technology Project of the headquarters of the State Grid Corporation of China(No.5500-202324492A-3-2-ZN).
文摘To enhance power flow regulation in scenarios involving large-scale renewable energy transmission via high-voltage direct current(HVDC)links and multi-infeed DC systems in load-center regions,this paper proposes a hybrid modular multilevel converter–capacitor-commutated line-commutated converter(MMC-CLCC)HVDC transmission system and its corresponding control strategy.First,the system topology is constructed,and a submodule configuration method for the MMC—combining full-bridge submodules(FBSMs)and half-bridge submodules(HBSMs)—is proposed to enable direct power flow reversal.Second,a hierarchical control strategy is introduced,includingMMCvoltage control,CLCC current control,and a coordinationmechanism,along with the derivation of the hybrid system’s power flow reversal characteristics.Third,leveraging the CLCC’s fast current regulation and theMMC’s negative voltage control capability,a coordinated power flow reversal control strategy is developed.Finally,an 800 kV MMC-CLCC hybrid HVDC system is modeled in PSCAD/EMTDC to validate the power flow reversal performance under a high proportion of full-bridge submodule configuration.Results demonstrate that the proposed control strategy enables rapid(1-s transition)and smooth switching of bidirectional power flow without modifying the structure of primary equipment:the transient fluctuation ofDC voltage from the rated value(UdcN)to themaximumreverse voltage(-kUdcN)is less than 5%;the DC current strictly follows the preset characteristic curve with a deviation of≤3%;the active power reverses continuously,and the system maintains stable operation throughout the reversal process.
基金supported in part by the National Key Research and Development Program of China(No.2021YFB2400900)the Integration Projects of National Natural Science Foundation of China-State Grid Joint Fund for Smart Grid(No.U2166602)+1 种基金the National Natural Science Foundation of China(No.52207200)the Science and Technology Innovation Program of Hunan Province(No.2024RC3113).
文摘The introduction of fully controlled devices to build hybrid line commutated converter(H-LCC)has become a new idea to solve the commutation failure.However,existing H-LCC has not considered the implementation of a targeted firing angle control strategy during AC faults,with the objective of enhancing their power transmission and fault response performance.For this reason,this paper proposes an optimized control method for firing angle of H-LCC,designated as flexible virtual firing(FVF).This method first analyzes the influence of alterations in firing angle on reactive power,commutation process and associated action paths.By combining prediction and dynamic search,it optimizes the natural commutation process through the utilization of dynamic boundary and minimum commutation area difference.This can mitigate the impact of AC faults on H-LCC and DC system,thereby improving power transmission and defense to commutation failure,which is beneficial for improving the stability of AC/DC power grids.Finally,the simulation results in PSCAD/EMTDC verify the effectiveness of the proposed method.
基金supported in part by National Key R&D Program of China(2022YFB2405400)in part by National Natural Science Foundation of China(52237004).
文摘This paper systematically investigates the multidegree-of-freedom(multi-DoF)control framework of high voltage direct current(HVDC)converters based on their topological characteristics and control requirements.First,this study reveals that its topology determines the control degrees of freedom(CDoF)of a converter.Specifically,a line-commutated converter(LCC)possesses only one CDoF,a voltage source converter(VSC)has two,and a modular multilevel converter(MMC)can theoretically achieve up to eight.However,only six CDoF are practically necessary for MMC.Despite this,existing MMC control strategies utilize only four CDoF,limiting control flexibility.To overcome this issue,this paper proposes a six-degree-offreedom(6 DoF)control framework for MMC,which enables asymmetric and precise regulation of DC voltage and SM capacitor voltage compared to the existing MMC control.Based on this framework,corresponding control strategies are designed for MMCs in HVDC systems with different control objectives.Finally,a two-terminal MMC-HVDC system is built on the PSCAD/EMTDC to verify the effectiveness of the proposed 6 DoF control.
基金supported by the Science and Technology Projects of State Grid Corporation of China(No.5200-202356402A-2-4-KJ).
文摘In this study,a novel parallel converter-based hybrid high-voltage direct current(HVDC)system is proposed for the integration and delivery of large-scale renewable energy.The rectifier uses the line commutated converter(LCC)and low-capacity modular multilevel converter(MMC)in parallel,while the inverter uses MMC.This configuration combines the economic advantages of LCC with the flexibility of MMC.Firstly,the steady-state control strategies are elaborated.The low-capacity MMC operates in the grid-forming mode to offer AC voltage support.It also provides active filtering for the LCC and maintains the reactive power balance of the sending-end system.The LCC efficiently transmits all active power at the rectifier side,fully exploiting its bulk-power transmission capability.Secondly,the fault ride-through strategies of both the AC faults at two terminals and the DC fault are proposed,in which the MMCs at both terminals can remain unblocked under various faults.Thus,the proposed system can mitigate the impact of the faults and ensure continuous voltage support for the sending-end system.Finally,simulations in PSCAD/EMTDC verify the effectiveness and performance of the proposed system.
基金National Key Research and Development Program,Grant/Award Number:2023YFB2405900。
文摘This paper presents a thermal management framework for 120 kV hybrid commutated converter(HCC)valves,addressing critical cooling challenges in multi-hundred-MW power conversion systems.Power loss calculations under rated(1.0 p.u.)and overload(1.2 p.u.)conditions demonstrate that HCC valves achieve comparable loss levels to line commutated converter counterparts while enabling active turn-off control.Comparative analysis of radiator configurations identifies 2-parallel branch connections as optimal.Integrated thermal-fluid models combining 3D finite element analysis and computational fluid dynamics reveal significant temperature gradients and flow maldistribution in baseline designs.On this basis,this paper modifies the flow from equal flow resistance allocation to heat-based allocation and it reduces maximum integrated gate-commutated thyristor temperature rise by 7.3%at 1.2 p.u.with minimal pressure drop variation.Experimental validation confirms the proposed cooling strategy enhances valve safety margins through improved heat dissipation balance,providing a validated theoretical foundation for high-power converter thermal design.
基金National Natural Science Foundation of China-State Grid Corporation Joint Fund for Smart Grid,Grant/Award Number:U2166602。
文摘Commutation failure(CF)is an inherent problem faced by line commutated converter high voltage direct current(LCC-HVDC)technology.To completely solve the problem of CF,we have proposed a novel hybrid commutated converter(HCC)technology based on reverse blocking integrated gate commutated thyristor,which can utilise two methods for commutation:enhanced grid voltage commutation and active turn-off forced com-mutation.In this paper,the topology and operating principle of HCC are proposed.Then,the control and protection strategy is designed based on the current variation trend under AC faults.To verify the effectiveness of HCC in mitigating CF,a 120-kV/360-MW HCC-HVDC model is built in PSCAD/EMTDC,adopting LCC at the rectifier side and HCC at the inverter side.Based on this model,HCC steady-state and fault transient stresses are analysed.Various AC faults are simulated and the performance of HCC-HVDC is compared with LCC-HVDC.Finally,the results show that the HCC topol-ogy and proposed control strategy can solve CF under all fault conditions with almost the same attributes as LCC,such as large capacity,low cost,low loss and high reliability,which is meaningful for the application of HCC to the HVDC transmission system.
基金supported by National Natural Science Foundation of China(No.51877077)。
文摘To reduce the probability of commutation failure(CF)of a line commutated converter based high-voltage direct current(LCC-HVDC)transmission,a DC chopper topology composed of power consumption sub-modules based on thyristor full-bridge module(TFB-PCSM)is proposed.Firstly,the mechanism of the proposed topology to mitigate CF is analyzed,and the working modes of TFB-PCSM in different operation states are introduced.Secondly,the coordinated control strategy between the proposed DC chopper and LCC-HVDC is designed,and the voltage-current stresses of the TFB-PCSMs are investigated.Finally,the ability to mitigate the CF issues and the fault recovery performance of LCC-HVDC system are studied in PSCAD/EMTDC.The results show that the probability of CF of LCC-HVDC is significantly reduced,and the performances of fault recovery are effectively improved by the proposed DC chopper.
基金This work was supported in part by the National Natural Science Foundation of China(U1766210,51625702)Science and Technology Program of SGCC.
文摘This paper proposes a novel AC filter system for a line commutated converter high voltage DC(LCC-HVDC)transmission system.Through the coordination of the hybrid active power filters(APF)and the existing reactive compensation devices,the proposed filter system can not only enhance the suppression performance for LCC-HVDC harmonics,but also optimize the AC yard layout with reduced reactive power subbanks,reducing the cost of HVDC projects.The novel filter system adopts a serial passive resonance topology obtained by careful comparison of different APFs.A proper control scheme is then designed integrating the control strategy of the APF and impedance characteristics of the HVDC system,which is able to realize harmonic suppression and dynamic reactive power support simultaneously.In addition,a novel self-adaption digital low-pass filter algorithm is presented,which is used in the APF harmonic detecting step,enhancing both high precision and fast dynamic response.On the basis of a real HVDC project,the advantages of proposed filter system in harmonic suppression,reactive power regulation,and sub-banks reduction are simulated and demonstrated.
文摘The hybrid cascaded HVDC system employs a line commutated converter(LCC)as the rectifier and an LCC in series with multiple paralleled modular multilevel converters(MMCs)as the inverter.MMC arms are susceptible to overcurrent following a severe AC fault at the receiving end,however,its fundamental mechanism has not been totally revealed.Therefore,this article explores the overcurrent characteristics on MMC arms,in terms of both the DC and AC components.Apart from the DC overcurrent component induced by the commutation failure(CF)of the inverter LCC,the AC overcurrent component is also significant.It dramatically depends on the coupling effects among the AC systems of the inverter side.Further,corresponding suppression strategies are proposed,which are applicable to different receiving-end AC fault scenarios.Eventually,the time-domain simulation results from PSCAD/EMTDC validate the effectiveness of the proposed overcurrent suppression control.It is also demonstrated that the presented methods can not only suppress overcurrent for MMC arms,but also reduce the imbalanced power between two sides,as well as improve the dynamic performances of the entire system.
基金supported by National Natural Science Foundation of China(No.51877061).
文摘The negative-sequence voltage is often caused by the asymmetrical fault in the AC system,as well as the harmonics after the symmetrical fault at the AC side of inverter in line commutated converter based high-voltage DC(LCC-HVDC).The negative-sequence voltage affects the phase-locked loop(PLL)and the inverter control,thus the inverter is vulnerable to the subsequent commutation failure(SCF).In this paper,the analytical expression of the negative-sequence voltage resulting from the symmetrical fault with the commutation voltage is derived using the switching function and Fourier decomposition.The analytical expressions of the outputs of the PLL and inverter control with respect to time are derived to quantify the contribution of the negative-sequence voltage to the SCF.To deal with the AC component of the input signals in the PLL and the inverter control due to the negative-sequence voltage,the existing proportional-integral controls of the PLL,constant current control,and constant extinction angle control are replaced by the linear active disturbance rejection control against the SCF.Simulation results verify the contributing factors to the SCF.The proposed control reduces the risk of SCF and improves the recovery speed of the system under different fault conditions.
基金supported by the National Hi-Tech Research and Development Program of China("863"Project)(Grant No.2011AA05A301)"111"Project of China(Grant No.B08013)
文摘With the increasing capacity of wind farm, HVDC technology has become a promising transmission scheme for long distance transportation of large-scale wind power. However oscillation caused by this system will have a great influence on the security and stability of power system operation. In this paper, the oscillation of a doubly-fed induction generator(DFIG)-based wind farm interfaced with line commutated converter(LCC) based HVDC is discussed. Low-frequency oscillation and subsynchronous oscillation(SSO) are studied since these two oscillations are the particularly concerned oscillations in the stability study of power system in recent years. The model of a DFIG-based wind farm interconnected with LCC-HVDC is developed. The impact of drive train model's structure and parameters on the oscillation characteristics is analyzed. Eigenvalue and participation factor analysis are used to identify the three main modes, which include controller mode, electromechanical mode, and shaft mode. The effects of DFIG controller's parameters, wind speed and operating conditions of HVDC on those modes are studied. Electromagnetic transient simulations are performed to verify the results of the eigenvalue analysis.
基金This study was supported by National Natural Science Foundation of China(No.50937002,No.51190104)National HI-Tech R&D Program of China(No.2011AA05A112).
文摘Multi-terminal high-voltage DC(MTDC)technology is a promising way to transmit large amounts of offshore wind power to the main grids.This paper proposes a hybrid MTDC scheme to integrate several offshore wind farms into the onshore power grids at different locations.A hybrid four-terminal HVDC system comprising two onshore line commutated converters(LCCs)and two voltage source converters(VSCs)connecting an offshore wind farm is constructed in PSCAD/EMTDC.A coordination control scheme based on the VSCs’AC voltage control and the LCCs’DC voltage droop control is designed to ensure smooth system operation and proper power sharing between onshore AC grids.The operational characteristics of the system are analyzed.In addition,a black start-up method without any auxiliary power supply for the VSCs is proposed.The transmission scheme is tested through simulations under various conditions,including start-up,wind speed variation,and the disconnection of one VSC or of one LCC.
基金supported by the National High Technology Research and Development Program of China("863" Program)(Grant No.2013AA050105)the National Natural Science Foundation of China(Grant No.51177042)+1 种基金the Fundamental Research Funds for the Central Universities(Grant No.13QN03)2012 science and technology projects of State Grid Corporation of China(Grant No.XT71-12-015)
文摘The hybrid-HVDC topology,which consists of line-commutated-converter(LCC)and voltage source converter(VSC)and combines their advantages,has extensive application prospects.A hybrid-HVDC system,adopting VSC on rectifier side and LCC on inverter side,is investigated,and its mathematic model is deduced.The commutation failure issue of the LCC converter in the hybrid-HVDC system is considered,and a novel coordinated control method is proposed to enhance the system commutation failure immunity.A voltage dependent voltage order limiter(VDVOL)is designed based on the constant DC voltage control on the rectifier side,and constant extinction angle backup control is introduced based on the constant DC current control with voltage dependent current order limiter(VDCOL)on the inverter side.The hybrid-HVDC system performances under normal operation state and fault state are simulated in the PSCAD/EMTDC.Then,system transient state performances with or without the proposed control methods under fault condition are further compared and analyzed.It is concluded that the proposed control method has the ability to effectively reduce the probability of commutation failure and improve the fault recovery performance of the hybrid-HVDC system.
基金This work was supported by the National Natural Science Foundation of China(No.51877077).
文摘This paper investigates the small-signal stability of the hybrid high-voltage direct current(HVDC)transmission system.The system is composed of line commutated converter(LCC)as rectifier and modular multi-level converter(MMC)as inverter under weak AC grid condition.Firstly,the impact of short-circuit ratio(SCR)at inverter side on the system stability is investigated by eigen-analysis,and the key control parameters which have major impact on the dominant mode are identified by the participation factor and sensitivity analysis.Then,considering the quadratic index and damping ratio characteristic,an objective function for evaluating the system stability is developed,and an optimization and configuration method for control parameters is presented by the utilization of Monte Carlo method.The eigenvalue results and the electromagnetic transient(EMT)simulation results show that,with the optimized control parameters,the small-signal stability and the dynamic responses of the hybrid system are greatly improved,and the hybrid system can even operate under weak AC grid condition.
基金supported by the National Natural Science Foundation of China(No.51977183).
文摘Line-commutated converter based high-voltage direct-current(LCC-HVDC)transmission systems are prone to subsequent commutation failure(SCF),which consequently leads to the forced blocking of HVDC links,affecting the operation of the power system.An accurate commutation failure(CF)identification is fairly vital to the prevention of SCF.However,the existing CF identification methods cause CF misjudge or detection lag,which can limit the effect of SCF mitigation strategy.In addition,earlier approaches to suppress SCF do not clarify the key factor that determines the evolution of extinction angle during system recovery and neglect the influence.Hence,this paper firstly analyzes the normal commutation process and CF feature based on the evolution topology of converter valve conduction in detail.Secondly,the energy in the leakage inductance of converter transformer is presented to characterize the commutation state of the valves.Then a CF identification method is proposed utilizing the leakage inductance energy.Thirdly,taking the key variable which is crucial to the tendency of extinction angle during the recovery process into account,a fault current limiting strategy for SCF mitigation is put forward.Compared with the original methods,the proposed methods have a better performance in CF identification and mitigation in terms of detection accuracy and mitigation effect.Finally,case study on PSCAD/EMTDC validates the proposed methods.
基金supported by the National Key Research and Development Program of China(No.2021YFB2400902)the Innovation Young Talents Program of Changsha Science and Technology Bureau(No.kq2107005)the Postgraduate Scientific Research Innovation Project of Hunan Province(No.QL20210101).
文摘For the safe and fast recovery of line commutated converter based high-voltage direct current(LCC-HVDC)transmission systems after faults,a DC current order optimization based strategy is proposed.Considering the constraint of electric and control quantities,the DC current order with the maximum active power transfer is calculated by Thevenin equivalent parameters(TEPs)and quasi-state equations of LCC-HVDC transmission systems.Meanwhile,to mitigate the subsequent commutation failures(SCFs)that may come with the fault recovery process,the maximum DC current order that avoids SCFs is calculated through imaginary commutation process.Finally,the minimum value of the two DC current orders is sent to the control system.Simulation results based on PSCAD/EMTDC show that the proposed strategy mitigates SCFs effectively and exhibits good performance in recovery.
基金supported in part by the National Natural Science Foundation of China(No.U2166602)in part by the Major Special Project of Hunan Province(No.2020GK1010)in part by the Innovation Young Talents Program of Changsha Science and Technology Bureau(No.kq2107005).
文摘Line commutated converter based high-voltage direct-current(LCC-HVDC)transmissions are prone to harmonic oscillation under weak grids.Impedance modeling is an effective method for assessing interaction stability.Firstly,this paper proposes an improved calculation method for the DC voltage and AC currents of commutation stations to address the complex linearization of the commutation process and constructs an overall harmonic state-space(HSS)model of an LCC-HVDC.Based on the HSS model,the closed-loop AC impedances on the LCC-HVDC sending and receiving ends are then derived and verified.The impedance characteristics of the LCC-HVDC are then analyzed to provide a physical explanation for the harmonic oscillation of the system.The effects of the grid strength and control parameters on system stability are also analyzed.To improve the impedance characteristics and operating stability of the LCC-HVDC system,a virtual impedance based stability enhancement control is proposed,and a parameter design method is considered to ensure satisfactory phase margins at both the sending and receiving ends.Finally,simulation results are presented to verify the validity of the impedance model and virtual impedance based stability enhancement control.
基金supported by the Science and Technology Innovation Major Project of Hunan Province (No. 2020GK1010)the Innovation Young Talents Program of Changsha Science and Technology Bureau (No. kq2107005)the Postgraduate Scientific Research Innovation Project of Hunan Province (No.QL20210101)。
文摘The commutation failure(CF) mitigation effectiveness is normally restricted by the delay of extinction angle(EA)measurement or the errors of existing prediction methods for EA or firing angle(FA). For this purpose, this paper proposes a CF mitigation method based on the imaginary commutation process. For each sample point, an imaginary commutation process is constructed to simulate the actual commutation process.Then, the imaginary EA is calculated by comparing the imaginary supply voltage-time area and the imaginary demand voltage-time area, which can update the imaginary EA earlier than the measured EA. In addition, the proposed method considers the impacts of commutation voltage variation, DC current variation, and phase angle shift of commutation voltage on the commutation process, which can ensure a more accurate EA calculation. Moreover, the DC current prediction is proposed to improve the CF mitigation performance under the single-phase AC faults. Finally, the simulation results based on CIGRE model prove that the proposed method has a good performance in CF mitigation.
文摘In line commutated converter based high-voltage direct current(LCC-HVDC)transmission systems,the transformer saturation can induce harmonic instability,which poses a serious threat to the safe operation of the power system.However,the nonlinear characteristics of the power grids introduced by the transformer saturation considerably limit the application of the conventional analysis methods.To address the issue,this paper derives a linear model for the transformer saturation caused by the DC current due to the converter modulation.Afterwards,the nonlinear characteristics of power grids with the transformer saturation is described by a complex valued impedance matrix.Based on the derived impedance matrix,the system harmonic stability is analyzed and the mechanism of the transformer saturation induced harmonic instability is revealed.Finally,the sensitivity analysis is conducted to find the key factors that influence the system core saturation instability.The proposed impedance model is verified by the electromagnetic transient simulation,and the simulation results corroborate the effectiveness of the proposed impedance model.Index TermsLine commutated converter based high voltage direct current(LCC-HVDC),transformer saturation,harmonic instability,impedance model.
基金supported by the National Natural Science Foundation of China(No.51907058)Project of Hunan Power Co.,Ltd.of the State Grid Corporation of China(No.SGTYHT/18-JS-206)Natural Science Foundation of Hunan Province(No.2020JJ5081)。
文摘The mitigation of commutation failure(CF)depends on the accuracy of CF prediction.In terms of the large error of the existing extinction angle(EA)calculation during the fault transient period,a method for CF prediction and mitigation is proposed.Variations in both DC current and overlap angle(OA)are considered in the proposed method to predict the EA rapidly.In addition,variations in critical EA and the effect of firing angle(FA)on both DC current and OA are considered in the proposed method to obtain the accurate FA order for the control system.The proposed method can achieve good performance in terms of CF mitigation and reduce reactive consumption at the inverter side when a fault occurs.Simulation results based on the PSCAD/EMTDC show that the proposed method predicts CF rapidly and exhibits good performance in terms of CF mitigation.
