Multi-terminal hybrid high-voltage direct current(HVDC)systems have been developed quickly in recent years in power transmission area.However,for voltage-source converter(VSC)stations in hybrid HVDC systems,no direct ...Multi-terminal hybrid high-voltage direct current(HVDC)systems have been developed quickly in recent years in power transmission area.However,for voltage-source converter(VSC)stations in hybrid HVDC systems,no direct current(DC)filters are required.In addition,the DC reactor is also not installed at the line end because the DC fault can be limited by the converter itself.This means that the boundary element at the line end is absent,and the single-ended protections used in line commutated converter(LCC)based HVDC(LCC-HVDC)systems or VSC-HVDC systems cannot distinguish the fault line in multi-terminal hybrid HVDC systems.This paper proposes a novel singleended DC protection strategy suitable for the multi-terminal hybrid HVDC system,which mainly applies the transient information and active injection concept to detect and distinguish the fault line.Compared with the single-ended protections used in LCC-HVDC and VSC-HVDC systems,the proposed protection strategy is not dependent on the line boundary element and is thus suitable for the multiterminal hybrid HVDC system.The corresponding simulation cases based on power systems computer aided design(PSCAD)/electromagnetic transients including DC(EMTDC)are carried out to verify the superiority of the proposed protection.展开更多
The close proximity and the necessity of coordination between multiple high-voltage direct currents(HVDCs)raise the issue of grid partitioning in multi-infeed HVDC systems.A multi-objective partition strategy is propo...The close proximity and the necessity of coordination between multiple high-voltage direct currents(HVDCs)raise the issue of grid partitioning in multi-infeed HVDC systems.A multi-objective partition strategy is proposed in this paper.Several types of relationships to be coordinated and complemented are analyzed and formulated using quantitative indices.According to the graph theory,the HVDC partition is transformed into a graph-cut problem and solved via the spectral clustering algorithm.Finally,the proposed method is validated for a practical multi-HVDC grid,confirming its feasibility and effectiveness.展开更多
A hybrid of line commutated converters(LCCs)and modular multi-level converters(MMCs)can provide the advantages of both the technologies.However,the commutation failure still exists if the LCC operates as an inverter i...A hybrid of line commutated converters(LCCs)and modular multi-level converters(MMCs)can provide the advantages of both the technologies.However,the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system.In this paper,the system behavior during a commutation failure is investigated.Both halfbridge and full-bridge MMCs are considered.Control strategies are examined through simulations conducted in PSCAD/EMTDC.Additionally,commutation failure protection strategies for multi-terminal hybrid LCC/MMC systems with AC and DC circuit breakers are studied.This paper can contribute to the protection design of future hybrid LCC/MMC systems against commutation failures.展开更多
Voltage source converter high-voltage direct current （VSC-HVDC） is a new power transmission technology pref- erable in small or medium power transmission. In this paper we discuss a new control system based on space...Voltage source converter high-voltage direct current （VSC-HVDC） is a new power transmission technology pref- erable in small or medium power transmission. In this paper we discuss a new control system based on space vector modulation （SVM） without any voltage line sensors. Using direct power control （DPC） SVM and a new double synchronous reference frame phase-locked loop （DSRF-PLL） approach, the control system is resistant to the majority of line voltage disturbances. Also, the system response has accelerated by using a feed forward power decoupled loop. The operation of this control strategy was verified in a SIMULINK/MATLAB simulation environment. To validate this control system, a 5 kV.A prototype system was constructed. Compared to the original controllers, the current total harmonic distortion （THD）, the active and reactive deviations and the DC voltage overshoot were lowered by 2.5%, 6.2% and 8%, respectively. The rectifier power factor in the worst condition was 0.93 and the DC voltage settling time was 0.2 s.展开更多
Wind energy sources have different structures and functions from conventional power plants in the power system.These resources can affect the exchange of active and reactive power of the network.Therefore,power system...Wind energy sources have different structures and functions from conventional power plants in the power system.These resources can affect the exchange of active and reactive power of the network.Therefore,power system stability will be affected by the performance of wind power plants,especially in the event of a fault.In this paper,the improvement of the dynamic stability in power system equipped by wind farm is examined through the supplementary controller design in the high voltage direct current(HVDC)based on voltage source converter(VSC)transmission system.In this regard,impacts of the VSC HVDC system and wind farm on the improvement of system stability are considered.Also,an algorithm based on controllability(observability)concept is proposed to select most appropriate and effective coupling between inputs-outputs(IO)signals of system in different work conditions.The selected coupling is used to apply damping controller signal.Finally,a fractional order PID controller(FO-PID)based on exchange market algorithm(EMA)is designed as damping controller.The analysis of the results shows that the wind farm does not directly contribute to the improvement of the dynamic stability of power system.However,it can increase the controllability of the oscillatory mode and improve the performance of the supplementary controller.展开更多
This paper deals with the stability problem at the inverter end of a HVDC link with STATCOM (Static Compensator), when connected to a weak AC system which has the stability enhancement for power instability and commut...This paper deals with the stability problem at the inverter end of a HVDC link with STATCOM (Static Compensator), when connected to a weak AC system which has the stability enhancement for power instability and commutation failures. The HVDC stability problem is tackled with a STATCOM which not only provides a rapid recovery from power, harmonic stability and commutation failures but also offers a lower cost filter design for the HVDC system. PSCAD/EMTDC simulations are presented to demonstrate the robust performance and to validate the proposed topology.展开更多
In multi-infeed HVDC system, the interactions and influences between DC systems AC systems are complex as the electrical distances among DC converter stations which are relatively short. Multi-infeed interaction facto...In multi-infeed HVDC system, the interactions and influences between DC systems AC systems are complex as the electrical distances among DC converter stations which are relatively short. Multi-infeed interaction factor (MIIF) can effectively reflect the interaction among DC systems. The paper theoretically analyzes the impact factors of MIIF like the electrical distances between two DC converter stations and the equivalent impedance of the receiving end AC system. By applying the Kirchhoff’s current law on the inverter AC bus, the paper deduces the analytical expressions for MIIF. From the expression, it is clear how the equivalent impedance of AC system and coupling impedance can affect MIIF. PSCAD simulations validate the effectiveness and the correctness of the proposed expression and some useful conclusions are drawn.展开更多
The interaction mechanism between AC and DC systems in a hybrid AC-DC transmission grid is discussed with PSS/E software. Analysis shows that receiving-end AC faults may cause much more damage on the HVDC system opera...The interaction mechanism between AC and DC systems in a hybrid AC-DC transmission grid is discussed with PSS/E software. Analysis shows that receiving-end AC faults may cause much more damage on the HVDC system operation than the sending-end AC faults in a multi-infeed HVDC system, and the damage severity depends on the power recovering rate of the HVDC systems. For HVDC systems with slow power recovering rate, the receiving-end AC faults may probably be a critical factor to constrain power transfer limits. Larger capacity of HVDC system means not only higher power transfer-limit of the parallel connected AC-DC transmission grid, but also more expensive stabilizing cost.展开更多
More and more high voltage direct current(HVDC) converters are being located in a load area in the Yangtze River Delta Region in China. It's large transmission capacity and intensive placement are hardly seen in o...More and more high voltage direct current(HVDC) converters are being located in a load area in the Yangtze River Delta Region in China. It's large transmission capacity and intensive placement are hardly seen in other countries. Accurate and reliable study tools and methods are extremely needed for power system engineers and researchers to deal with such problems which were never met before and otherwhere. A novel approach on electromagnetic modeling of alternating current / direct current(AC/DC) system is proposed and simulation is carried out to replay a real AC 3-phase to ground fault on a multi-infeed high voltage direct current(MIHVDC) system. AC system is specially simplified. Dynamic models and real parameters are adopted in main AC/DC equipments concerned and retained. Modeling and simulation are based on the power system computer aided design/electro-magnetic transient in direct current(PSCAD/EMTDC) system software package.Comparisons between simulation results and the records from the fault recorders are studied including both AC and DC quantities.展开更多
High Voltage Direct Current (HVDC) electric power transmission is a promising technology for integrating offshore wind farms and interconnecting power grids in different regions. In order to maintain the DC voltage, d...High Voltage Direct Current (HVDC) electric power transmission is a promising technology for integrating offshore wind farms and interconnecting power grids in different regions. In order to maintain the DC voltage, droop control has been widely used. Transmission line loss constitutes an import part of the total power loss in a multi-terminal HVDC scheme. In this paper, the relation between droop controller design and transmission loss has been investigated. Different MTDC layout configurations are compared to examine the effect of droop controller design on the transmission loss.展开更多
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
Pulse width modulated current-source converter(PWM-CSC)has great prospects in high voltage direct current transmission system(HVDC)due to its attractive features,such as flexible control characteristics,ability to avo...Pulse width modulated current-source converter(PWM-CSC)has great prospects in high voltage direct current transmission system(HVDC)due to its attractive features,such as flexible control characteristics,ability to avoid commutation failure,and lower cost.However,valve voltage of the PWM-CSC is the jump value between filter capacitors line-voltage and zero voltage due to its bypass operation,resulting in high peak voltage withstand by the converter for a rated HVDC system and high voltage ripper ratio.In order to solve these issues,an improved modulation method called specific carrier frequency of SPWM is proposed.After adopting the improved modulation method,the number of the reverse blocking integrated gate commutated thyristor(RB-IGCT)in series can reduce by 31.5%under unity power factor operation and the value of the dc choke can reduce by about 78.6%compared to traditional modulation methods,improving practicability of engineering application.Moreover,active power and reactive power operating range are derived under different modulation methods and the relationship of the power factor,modulation in q-axis and DC current are studied.Finally,effectiveness of the improved modulation method and comparisons of power operating range are verified in PSCAD/EMTDC.展开更多
Multiterminal hybrid HVDC systems,combining the advantages of conventional and flexible DC systems,are considered important development trends for future power networks.With the ability to suppress DC fault current,hy...Multiterminal hybrid HVDC systems,combining the advantages of conventional and flexible DC systems,are considered important development trends for future power networks.With the ability to suppress DC fault current,hybrid HVDC systems eliminate the requirement of current limiting reactors at both ends of DC lines.Thus,non-unit protection based on line boundaries is no longer applicable.In this paper,a non-unit protection scheme in cooperation with converter control is proposed to ensure protection speed,reliability,and sen-sitivity for multiterminal hybrid HVDC systems without line boundaries.In the proposed protection scheme,the protection criterion based on the transient energy of the DC voltage and the direction criterion based on the travelling wave are used to identify whether the fault occurs on the DC line.When the fault is detected as a DC-line fault,the relevant converter station is quickly blocked.After a specified hold-off duration,the converter station interfacing with multiple DC lines is prioritized for restart,while the remaining converter stations are restarted once the DC voltage is stabilized.Simulations based on PSCAD/EMTDC validate the feasibility of the proposed protection scheme and its superiority over competing approaches.展开更多
This study analyzes the stability and reactive characteristics of the hybrid offshore wind farm that includes gridforming(GFM)and grid-following(GFL)wind turbines(WTs)integrated with a diode rectifier unit(DRU)based h...This study analyzes the stability and reactive characteristics of the hybrid offshore wind farm that includes gridforming(GFM)and grid-following(GFL)wind turbines(WTs)integrated with a diode rectifier unit(DRU)based high-voltage direct current(HVDC)system.The determination method for the proportion of GFM WTs is proposed while considering system stability and optimal offshore reactive power constraints.First,the small-signal stability is studied based on the developed linear model,and crucial factors that affect the stability are captured by eigenvalue analysis.The reactive power-frequency compensation control of GFM WTs is then proposed to improve the reactive power and frequency dynamics.Second,the relationship between offshore reactive power imbalance and the effectiveness of GFM capability is analyzed.Offshore reactive power optimization methods are next proposed to diminish offshore reactive load.These methods include the optimal design for the reactive capacity of the AC filter and the reactive power compensation control of GFL WTs.Third,in terms of stability and optimal offshore reactive power constraints,the principle and calculation method for determining the proportion of GFM WTs are proposed,and the critical proportion of GFM WTs is determined over the full active power range.Finally,case studies using a detailed model are conducted by timedomain simulations in PSCAD/EMTDC.The simulations verify the theoretical analysis results and the effectiveness of the proposed determination method for the proportion of GFM WTs and reactive power optimization methods.展开更多
Inverter-based resources(IBRs)are expected to support black start in case of blackout events,as they can significantly speed up restoration process.This paper proposes a coordinated control scheme to improve frequency...Inverter-based resources(IBRs)are expected to support black start in case of blackout events,as they can significantly speed up restoration process.This paper proposes a coordinated control scheme to improve frequency and voltage stability for hybrid black start resource(HBSR)participated restoration.HBSR includes a line-commutated converter(LCC)-based high-voltage direct-current(HVDC)system and a self-starting generator.In the proposed scheme,outputs of different resources are controlled in a coordinated manner to suppress disturbances caused by inrush loads,where an active power error controller(PEC)and a reactive power error controller(QEC)are developed based on an analytical model.The analytical model calculates desired DC current reference considering the coupling effects between active power and reactive power under weak grid conditions.PEC is designed as a multi-variable system,which regulates active power of the LCC HVDC line so that frequency variation can be damped and eliminated properly.QEC observes reactive power related security margin and regulates the inverter extinction angle to reduce the risk of commutation failure.Simulations are executed on an HBSR integrated test network based on the CIGRE Benchmark model.Results show that in HBSR dominated black start,frequency deviations and voltage fluctuations can be suppressed more speedily and stably by the proposed control method,and the extinction angle is adapted for more secure commutation in case of insufficient reactive power.展开更多
Diode-rectifier-based high-voltage direct current(DR-HVDC)systems are considered an attractive solution for integrating offshore wind farms(OWFs).Grid-forming(GFM)control with a rational reactive power allocation capa...Diode-rectifier-based high-voltage direct current(DR-HVDC)systems are considered an attractive solution for integrating offshore wind farms(OWFs).Grid-forming(GFM)control with a rational reactive power allocation capability is crucial for the safe operation of numerous wind turbines(WTs).Most typical GFM controls aim to share surplus reactive power of the system equally among WTs,easily rendering capacity overloads for WTs that are outputting high levels of active power.In this paper,a novel GFM control for OWFs is proposed,allowing for adaptively allocating the reactive power according to the actual active power output of WTs.Firstly,the reactive power characteristics of the AC collection networks and WTs are analyzed across a wide wind power range.Then,combining the positive correlation of WT active power with the output AC voltage,a Q-θtype GFM control for WTs is presented.The adaptive reactive power allocation mechanism and the parameter design of the Q-θbased reactive power controller are elucidated,ensuring that WTs with lower active power output contribute more reactive power to the system than WTs with higher active power output.The AC impedance models of WTs under various GFM controls are established to evaluate the impact of different reactive power controllers.Finally,the feasibility of the proposed control is validated in PSCAD/EMTDC,accompanied by stability analysis.展开更多
This paper constructs a synthetic framework for the operational reliability evaluation and risk mitigation of asynchronous grids coupled through flexible high-voltage DC(HVDC)systems(AGs-FDCSs).First of all,an analyti...This paper constructs a synthetic framework for the operational reliability evaluation and risk mitigation of asynchronous grids coupled through flexible high-voltage DC(HVDC)systems(AGs-FDCSs).First of all,an analytical model for the unavailability of DC units is reformulated to refine and facilitate the reliability modeling of such flexible HVDC systems considering their time-dependent features as well as the impacts of converter station configurations.Subsequently,the operational risk associated with the redispatch procedure is extended to the reliability evaluation of composite power system,and the risk is mitigated through an optimal power flow(OPF)based short-term state assessment model.In addition,some new reliability indices like expected DC transmission power(EDCTP)and DC terminal outage probability(DCTOP)are defined to quantify the impact of the reliability of flexible HVDC systems on the entire grid.The effectiveness of the proposed framework on a modified IEEE RTS-79 system is validated with the elaborate discussions on the time-dependent reliability of AGs-FDCSs as well as the impacts of the converter station configurations.展开更多
This paper proposes a dual vector control scheme based on the sequence component separation strategy(SCSS)for second harmonic suppression in DC power.Firstly,an analytical model is established to depict the positive a...This paper proposes a dual vector control scheme based on the sequence component separation strategy(SCSS)for second harmonic suppression in DC power.Firstly,an analytical model is established to depict the positive and negative sequences of the controllable current source converter(CSC)under unbalanced grid conditions.Moreover,the generation mechanism of the DC power second harmonic component in the hybrid VSC-CSC HVDC system is demonstrated.This paper summarizes the impact of the modulation index(MI)and power factor(PF)on the operating characteristics of the modular multilevel converter(MMC)during the variation of the DC voltage.Based on the above analysis,the proposed control scheme extracts the sequence components of the CSC capacitor voltage and directly computes the sequence voltage reference value using instantaneous power.This approach effectively suppresses the second-harmonic component in DC power and significantly reduces the range of capacitor-voltage fluctuations in MMC submodules.Finally,an electromagnetic transient model of the hybrid HVDC system is constructed to validate the accuracy of the theoretical analysis and the effectiveness of the proposed control scheme.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
Grid-forming(GFM)converters are recognized for their stabilizing effects in renewable energy systems.Integrating GFM converters into high-voltage direct current(HVDC)systems requires DC voltage control.However,there c...Grid-forming(GFM)converters are recognized for their stabilizing effects in renewable energy systems.Integrating GFM converters into high-voltage direct current(HVDC)systems requires DC voltage control.However,there can be a conflict between GFM converter and DC voltage control when they are used in combination.This paper presents a rigorous control design for a GFM converter that connects the DC-link voltage to the power angle of the converter,thereby integrating DC voltage control with GFM capability.The proposed control is validated through small-signal and transient-stability analyses on a modular multilevel converter(MMC)-based HVDC system with a point-to-point(P2P)GFM-GFM configuration.The results demonstrate that employing a GFM-GFM configuration with the proposed control enhances the stability of the AC system to which it is connected.The system exhibits low sensitivity to grid strength and can sustain islanding conditions.The high stability limit of the system with varying grid strength using the proposed control is validated using a system with four voltage source converters.展开更多
文摘Multi-terminal hybrid high-voltage direct current(HVDC)systems have been developed quickly in recent years in power transmission area.However,for voltage-source converter(VSC)stations in hybrid HVDC systems,no direct current(DC)filters are required.In addition,the DC reactor is also not installed at the line end because the DC fault can be limited by the converter itself.This means that the boundary element at the line end is absent,and the single-ended protections used in line commutated converter(LCC)based HVDC(LCC-HVDC)systems or VSC-HVDC systems cannot distinguish the fault line in multi-terminal hybrid HVDC systems.This paper proposes a novel singleended DC protection strategy suitable for the multi-terminal hybrid HVDC system,which mainly applies the transient information and active injection concept to detect and distinguish the fault line.Compared with the single-ended protections used in LCC-HVDC and VSC-HVDC systems,the proposed protection strategy is not dependent on the line boundary element and is thus suitable for the multiterminal hybrid HVDC system.The corresponding simulation cases based on power systems computer aided design(PSCAD)/electromagnetic transients including DC(EMTDC)are carried out to verify the superiority of the proposed protection.
基金supported by the Science and Technology Project of State Grid Corporation of China:“Control Strategy Optimization Technology for Large-Scale Photovoltaic Power Generation on the Sending-end and Receiving-end of DC Power System”(4000-201934198A-0-0-00)
文摘The close proximity and the necessity of coordination between multiple high-voltage direct currents(HVDCs)raise the issue of grid partitioning in multi-infeed HVDC systems.A multi-objective partition strategy is proposed in this paper.Several types of relationships to be coordinated and complemented are analyzed and formulated using quantitative indices.According to the graph theory,the HVDC partition is transformed into a graph-cut problem and solved via the spectral clustering algorithm.Finally,the proposed method is validated for a practical multi-HVDC grid,confirming its feasibility and effectiveness.
基金supported by the Science and Technology Project of the State Grid Corporation of China,HVDC Systems/Grids for Transnational Interconnections(Project number:SGTYHT/16-JS-198).
文摘A hybrid of line commutated converters(LCCs)and modular multi-level converters(MMCs)can provide the advantages of both the technologies.However,the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system.In this paper,the system behavior during a commutation failure is investigated.Both halfbridge and full-bridge MMCs are considered.Control strategies are examined through simulations conducted in PSCAD/EMTDC.Additionally,commutation failure protection strategies for multi-terminal hybrid LCC/MMC systems with AC and DC circuit breakers are studied.This paper can contribute to the protection design of future hybrid LCC/MMC systems against commutation failures.
文摘Voltage source converter high-voltage direct current （VSC-HVDC） is a new power transmission technology pref- erable in small or medium power transmission. In this paper we discuss a new control system based on space vector modulation （SVM） without any voltage line sensors. Using direct power control （DPC） SVM and a new double synchronous reference frame phase-locked loop （DSRF-PLL） approach, the control system is resistant to the majority of line voltage disturbances. Also, the system response has accelerated by using a feed forward power decoupled loop. The operation of this control strategy was verified in a SIMULINK/MATLAB simulation environment. To validate this control system, a 5 kV.A prototype system was constructed. Compared to the original controllers, the current total harmonic distortion （THD）, the active and reactive deviations and the DC voltage overshoot were lowered by 2.5%, 6.2% and 8%, respectively. The rectifier power factor in the worst condition was 0.93 and the DC voltage settling time was 0.2 s.
文摘Wind energy sources have different structures and functions from conventional power plants in the power system.These resources can affect the exchange of active and reactive power of the network.Therefore,power system stability will be affected by the performance of wind power plants,especially in the event of a fault.In this paper,the improvement of the dynamic stability in power system equipped by wind farm is examined through the supplementary controller design in the high voltage direct current(HVDC)based on voltage source converter(VSC)transmission system.In this regard,impacts of the VSC HVDC system and wind farm on the improvement of system stability are considered.Also,an algorithm based on controllability(observability)concept is proposed to select most appropriate and effective coupling between inputs-outputs(IO)signals of system in different work conditions.The selected coupling is used to apply damping controller signal.Finally,a fractional order PID controller(FO-PID)based on exchange market algorithm(EMA)is designed as damping controller.The analysis of the results shows that the wind farm does not directly contribute to the improvement of the dynamic stability of power system.However,it can increase the controllability of the oscillatory mode and improve the performance of the supplementary controller.
文摘This paper deals with the stability problem at the inverter end of a HVDC link with STATCOM (Static Compensator), when connected to a weak AC system which has the stability enhancement for power instability and commutation failures. The HVDC stability problem is tackled with a STATCOM which not only provides a rapid recovery from power, harmonic stability and commutation failures but also offers a lower cost filter design for the HVDC system. PSCAD/EMTDC simulations are presented to demonstrate the robust performance and to validate the proposed topology.
文摘In multi-infeed HVDC system, the interactions and influences between DC systems AC systems are complex as the electrical distances among DC converter stations which are relatively short. Multi-infeed interaction factor (MIIF) can effectively reflect the interaction among DC systems. The paper theoretically analyzes the impact factors of MIIF like the electrical distances between two DC converter stations and the equivalent impedance of the receiving end AC system. By applying the Kirchhoff’s current law on the inverter AC bus, the paper deduces the analytical expressions for MIIF. From the expression, it is clear how the equivalent impedance of AC system and coupling impedance can affect MIIF. PSCAD simulations validate the effectiveness and the correctness of the proposed expression and some useful conclusions are drawn.
文摘The interaction mechanism between AC and DC systems in a hybrid AC-DC transmission grid is discussed with PSS/E software. Analysis shows that receiving-end AC faults may cause much more damage on the HVDC system operation than the sending-end AC faults in a multi-infeed HVDC system, and the damage severity depends on the power recovering rate of the HVDC systems. For HVDC systems with slow power recovering rate, the receiving-end AC faults may probably be a critical factor to constrain power transfer limits. Larger capacity of HVDC system means not only higher power transfer-limit of the parallel connected AC-DC transmission grid, but also more expensive stabilizing cost.
文摘More and more high voltage direct current(HVDC) converters are being located in a load area in the Yangtze River Delta Region in China. It's large transmission capacity and intensive placement are hardly seen in other countries. Accurate and reliable study tools and methods are extremely needed for power system engineers and researchers to deal with such problems which were never met before and otherwhere. A novel approach on electromagnetic modeling of alternating current / direct current(AC/DC) system is proposed and simulation is carried out to replay a real AC 3-phase to ground fault on a multi-infeed high voltage direct current(MIHVDC) system. AC system is specially simplified. Dynamic models and real parameters are adopted in main AC/DC equipments concerned and retained. Modeling and simulation are based on the power system computer aided design/electro-magnetic transient in direct current(PSCAD/EMTDC) system software package.Comparisons between simulation results and the records from the fault recorders are studied including both AC and DC quantities.
文摘High Voltage Direct Current (HVDC) electric power transmission is a promising technology for integrating offshore wind farms and interconnecting power grids in different regions. In order to maintain the DC voltage, droop control has been widely used. Transmission line loss constitutes an import part of the total power loss in a multi-terminal HVDC scheme. In this paper, the relation between droop controller design and transmission loss has been investigated. Different MTDC layout configurations are compared to examine the effect of droop controller design on the transmission loss.
文摘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 in part by Science and Technology Project of State Grid Corporation of China(5500202058059A0000).
文摘Pulse width modulated current-source converter(PWM-CSC)has great prospects in high voltage direct current transmission system(HVDC)due to its attractive features,such as flexible control characteristics,ability to avoid commutation failure,and lower cost.However,valve voltage of the PWM-CSC is the jump value between filter capacitors line-voltage and zero voltage due to its bypass operation,resulting in high peak voltage withstand by the converter for a rated HVDC system and high voltage ripper ratio.In order to solve these issues,an improved modulation method called specific carrier frequency of SPWM is proposed.After adopting the improved modulation method,the number of the reverse blocking integrated gate commutated thyristor(RB-IGCT)in series can reduce by 31.5%under unity power factor operation and the value of the dc choke can reduce by about 78.6%compared to traditional modulation methods,improving practicability of engineering application.Moreover,active power and reactive power operating range are derived under different modulation methods and the relationship of the power factor,modulation in q-axis and DC current are studied.Finally,effectiveness of the improved modulation method and comparisons of power operating range are verified in PSCAD/EMTDC.
基金supported by the National Natural Science Foundation of China(No.52207088)the Natural Science Foundation of Hebei Province(No.E2024202184).
文摘Multiterminal hybrid HVDC systems,combining the advantages of conventional and flexible DC systems,are considered important development trends for future power networks.With the ability to suppress DC fault current,hybrid HVDC systems eliminate the requirement of current limiting reactors at both ends of DC lines.Thus,non-unit protection based on line boundaries is no longer applicable.In this paper,a non-unit protection scheme in cooperation with converter control is proposed to ensure protection speed,reliability,and sen-sitivity for multiterminal hybrid HVDC systems without line boundaries.In the proposed protection scheme,the protection criterion based on the transient energy of the DC voltage and the direction criterion based on the travelling wave are used to identify whether the fault occurs on the DC line.When the fault is detected as a DC-line fault,the relevant converter station is quickly blocked.After a specified hold-off duration,the converter station interfacing with multiple DC lines is prioritized for restart,while the remaining converter stations are restarted once the DC voltage is stabilized.Simulations based on PSCAD/EMTDC validate the feasibility of the proposed protection scheme and its superiority over competing approaches.
基金supported by the Research Project of China Southern Power Grid Co.,Ltd.(No.030400KK52220008(GDKJXM20220327))。
文摘This study analyzes the stability and reactive characteristics of the hybrid offshore wind farm that includes gridforming(GFM)and grid-following(GFL)wind turbines(WTs)integrated with a diode rectifier unit(DRU)based high-voltage direct current(HVDC)system.The determination method for the proportion of GFM WTs is proposed while considering system stability and optimal offshore reactive power constraints.First,the small-signal stability is studied based on the developed linear model,and crucial factors that affect the stability are captured by eigenvalue analysis.The reactive power-frequency compensation control of GFM WTs is then proposed to improve the reactive power and frequency dynamics.Second,the relationship between offshore reactive power imbalance and the effectiveness of GFM capability is analyzed.Offshore reactive power optimization methods are next proposed to diminish offshore reactive load.These methods include the optimal design for the reactive capacity of the AC filter and the reactive power compensation control of GFL WTs.Third,in terms of stability and optimal offshore reactive power constraints,the principle and calculation method for determining the proportion of GFM WTs are proposed,and the critical proportion of GFM WTs is determined over the full active power range.Finally,case studies using a detailed model are conducted by timedomain simulations in PSCAD/EMTDC.The simulations verify the theoretical analysis results and the effectiveness of the proposed determination method for the proportion of GFM WTs and reactive power optimization methods.
基金supported by the National Natural Science Foundation of China(U22B2099).
文摘Inverter-based resources(IBRs)are expected to support black start in case of blackout events,as they can significantly speed up restoration process.This paper proposes a coordinated control scheme to improve frequency and voltage stability for hybrid black start resource(HBSR)participated restoration.HBSR includes a line-commutated converter(LCC)-based high-voltage direct-current(HVDC)system and a self-starting generator.In the proposed scheme,outputs of different resources are controlled in a coordinated manner to suppress disturbances caused by inrush loads,where an active power error controller(PEC)and a reactive power error controller(QEC)are developed based on an analytical model.The analytical model calculates desired DC current reference considering the coupling effects between active power and reactive power under weak grid conditions.PEC is designed as a multi-variable system,which regulates active power of the LCC HVDC line so that frequency variation can be damped and eliminated properly.QEC observes reactive power related security margin and regulates the inverter extinction angle to reduce the risk of commutation failure.Simulations are executed on an HBSR integrated test network based on the CIGRE Benchmark model.Results show that in HBSR dominated black start,frequency deviations and voltage fluctuations can be suppressed more speedily and stably by the proposed control method,and the extinction angle is adapted for more secure commutation in case of insufficient reactive power.
基金supported by the National Key Research and Development Program of China(No.2022YFB2405400)in part by the National Natural Science Foundation of China(No.52237004)。
文摘Diode-rectifier-based high-voltage direct current(DR-HVDC)systems are considered an attractive solution for integrating offshore wind farms(OWFs).Grid-forming(GFM)control with a rational reactive power allocation capability is crucial for the safe operation of numerous wind turbines(WTs).Most typical GFM controls aim to share surplus reactive power of the system equally among WTs,easily rendering capacity overloads for WTs that are outputting high levels of active power.In this paper,a novel GFM control for OWFs is proposed,allowing for adaptively allocating the reactive power according to the actual active power output of WTs.Firstly,the reactive power characteristics of the AC collection networks and WTs are analyzed across a wide wind power range.Then,combining the positive correlation of WT active power with the output AC voltage,a Q-θtype GFM control for WTs is presented.The adaptive reactive power allocation mechanism and the parameter design of the Q-θbased reactive power controller are elucidated,ensuring that WTs with lower active power output contribute more reactive power to the system than WTs with higher active power output.The AC impedance models of WTs under various GFM controls are established to evaluate the impact of different reactive power controllers.Finally,the feasibility of the proposed control is validated in PSCAD/EMTDC,accompanied by stability analysis.
基金supported by the National Natural Science Foundation of China(No.52177071)。
文摘This paper constructs a synthetic framework for the operational reliability evaluation and risk mitigation of asynchronous grids coupled through flexible high-voltage DC(HVDC)systems(AGs-FDCSs).First of all,an analytical model for the unavailability of DC units is reformulated to refine and facilitate the reliability modeling of such flexible HVDC systems considering their time-dependent features as well as the impacts of converter station configurations.Subsequently,the operational risk associated with the redispatch procedure is extended to the reliability evaluation of composite power system,and the risk is mitigated through an optimal power flow(OPF)based short-term state assessment model.In addition,some new reliability indices like expected DC transmission power(EDCTP)and DC terminal outage probability(DCTOP)are defined to quantify the impact of the reliability of flexible HVDC systems on the entire grid.The effectiveness of the proposed framework on a modified IEEE RTS-79 system is validated with the elaborate discussions on the time-dependent reliability of AGs-FDCSs as well as the impacts of the converter station configurations.
基金supported by Smart GridNational Science and Technology Major Project(2025ZD0803700).
文摘This paper proposes a dual vector control scheme based on the sequence component separation strategy(SCSS)for second harmonic suppression in DC power.Firstly,an analytical model is established to depict the positive and negative sequences of the controllable current source converter(CSC)under unbalanced grid conditions.Moreover,the generation mechanism of the DC power second harmonic component in the hybrid VSC-CSC HVDC system is demonstrated.This paper summarizes the impact of the modulation index(MI)and power factor(PF)on the operating characteristics of the modular multilevel converter(MMC)during the variation of the DC voltage.Based on the above analysis,the proposed control scheme extracts the sequence components of the CSC capacitor voltage and directly computes the sequence voltage reference value using instantaneous power.This approach effectively suppresses the second-harmonic component in DC power and significantly reduces the range of capacitor-voltage fluctuations in MMC submodules.Finally,an electromagnetic transient model of the hybrid HVDC system is constructed to validate the accuracy of the theoretical analysis and the effectiveness of the proposed control scheme.
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developed during the DICIT project sponsored by a public grant overseen by the French National Research Agency(No.ANR-20-CE05-0034 DICIT)。
文摘Grid-forming(GFM)converters are recognized for their stabilizing effects in renewable energy systems.Integrating GFM converters into high-voltage direct current(HVDC)systems requires DC voltage control.However,there can be a conflict between GFM converter and DC voltage control when they are used in combination.This paper presents a rigorous control design for a GFM converter that connects the DC-link voltage to the power angle of the converter,thereby integrating DC voltage control with GFM capability.The proposed control is validated through small-signal and transient-stability analyses on a modular multilevel converter(MMC)-based HVDC system with a point-to-point(P2P)GFM-GFM configuration.The results demonstrate that employing a GFM-GFM configuration with the proposed control enhances the stability of the AC system to which it is connected.The system exhibits low sensitivity to grid strength and can sustain islanding conditions.The high stability limit of the system with varying grid strength using the proposed control is validated using a system with four voltage source converters.
