Wind-farm-side modular multilevel converters(WFMMCs) used in modular multilevel converter based highvoltage direct current(MMC-HVDC) transmission systems must be able to control the AC grid voltage in offshore wind fa...Wind-farm-side modular multilevel converters(WFMMCs) used in modular multilevel converter based highvoltage direct current(MMC-HVDC) transmission systems must be able to control the AC grid voltage in offshore wind farms. Different AC voltage control strategies can significantly affect the dynamic characteristics of WFMMCs. However, existing studies have not provided a general methodology of controller parameter design, and few comparative studies have been conducted on control performance under varying operating conditions as well as the effects of different AC voltage control modes(AVCMs) on the stability of MMC-HVDCs with offshore wind farms. This paper provides a controller parameter design method for AVCMs, which is tested in various operating scenarios. Sequence impedance models of offshore wind farms and WFMMCs under different AVCMs are then developed. The effects of AVCMs on the small-signal stability of the interconnected system are then analyzed and compared using the impedance-based method. Finally, case studies are conducted on a practical MMC-HVDC system with offshore wind farms to verify the theoretical analysis.展开更多
In the development of hybrid HVDC and HVAC transmission lines,the study of radio interference is an important issue.Positive corona current pulses from high voltage transmission lines are the main source of radio inte...In the development of hybrid HVDC and HVAC transmission lines,the study of radio interference is an important issue.Positive corona current pulses from high voltage transmission lines are the main source of radio interference.In this paper,the design of a wire-cylinder gap electrode system is presented to study the influence of AC voltage on the characteristics of positive corona current pulses.The study shows that the mode of the current pulses is different from that of either DC or AC corona discharge.Waveform parameters of the pulses,such as rise time,half wave time,duration time,repetition rates,average amplitude,and time intervals of secondary pulses are all statistically analyzed in this study.The empirical formulas for the repetition rates with different AC voltages are presented.A theoretical explanation based on an ion cloud model is given to reveal the mechanism behind the influence of AC voltage on positive corona discharge.The experimental results could provide some references for the prediction of radio interference from hybrid AC/DC transmission lines.展开更多
A new method for preparing effective inhibition film on copper has been developed. Phenylthiourea (PT) was first absorbed to copper surface to form a monolayer. 1-Dodecanethiol (DT) was then assembled on the surface...A new method for preparing effective inhibition film on copper has been developed. Phenylthiourea (PT) was first absorbed to copper surface to form a monolayer. 1-Dodecanethiol (DT) was then assembled on the surface for modification. Finally, AC voltage was loaded on copper covered the mixed film to improve it further. After these processes, an effective inhibition film was gained because of its high charge transfer resistance and low corrosion current density shown in electrochemical impedance spectra and polarization. The inhibition efficiency was more than 97%.展开更多
The generation and propagation of a streamer is a significant physical process of air gap discharge. Research on the mechanism of streamers under low-pressure conditions is helpful for understanding the process of lon...The generation and propagation of a streamer is a significant physical process of air gap discharge. Research on the mechanism of streamers under low-pressure conditions is helpful for understanding the process of long-gap discharge in a high-altitude area. This paper describes laboratory investigations of streamer discharge under alternating current(AC) voltage in a low pressure test platform for a 60 cm rod–plane gap at 30 kPa, and analyzes the characteristics of streamer generation and propagation. The results show that the partial streamer and breakdown streamer all occur in the positive half-cycle of AC voltage near the peak voltage at 30 kPa. The partial streamer could cause the distortion of current and voltage waveform, and it appears as the branching characteristic at the initial stage. With the extension of the streamer, the branching and tortuosity phenomena become gradually obvious, but the branching is suppressed when the streamer crosses the gap. The low-pressure condition has little influence on the tortuosity length and the tortuosity number of the streamer, but affect the diameter of streamer obviously.展开更多
In order to evaluate the insulation of two-phase immersion cooling in the HV power electronic package,the insulation degradation of the dielectric interface induced by bubbles is investigated.In this paper,a test stra...In order to evaluate the insulation of two-phase immersion cooling in the HV power electronic package,the insulation degradation of the dielectric interface induced by bubbles is investigated.In this paper,a test strategy with 50 Hz unipolar DC and AC combined voltage for partial discharge(PD)at boiling interface of AlN ceramic is proposed.The insulation threshold of an AlN ceramic surface is acquired in several dielectric environments,such as air,FC-72 liquid(FC-72,a Fluorinert^(TM) from 3^(TM)),FC-72 vapor,and boiling state of FC72.This reveals the deterioration of boiling on the insulation of the surface immersed in the dielectric refrigerant.To investigate the mechanism of the PD feature at the boiling interface,the PD patterns of the unrestricted bubble and the accumulated bubble are acquired and contrastively analyzed.Combined with the feature of the back discharge and the bubble behavior,the charged vapor-ceramic interface is relatively stable due to the accumulated vapor layer.This stability of the charged vaporceramic interface is broken if the bubble is unrestricted.Besides,it is discovered that the vapor-liquid interface inside the bubble may be another charged interface,which can also trigger a back discharge.展开更多
The feasibility of the formation of a liquid plasma catalysis system through micro arc oxidation(MAO) under AC power with titanium-aluminum alloy electrodes was investigated.In the decolorization of organic dyeing w...The feasibility of the formation of a liquid plasma catalysis system through micro arc oxidation(MAO) under AC power with titanium-aluminum alloy electrodes was investigated.In the decolorization of organic dyeing wastewater simulated with Rhodamine B,Ti-Al alloy electrodes were superior over Ti electrodes and Al electrodes.The optimal molar percentage of Ti in alloy electrodes was 70%and the optimal decolorization rate was up to 88.9%if the additive suitable for Al was added into the solution to be treated.The decolorization rates were the same in the case of the alloy-alloy electrodes and alloy-Al electrodes.The proportion of the effects of plasma,TiO2 catalyzer during MAO and H2O2 after MAO in decolorization has been obtained.With the catalysis of TiO2 formed on the electrodes,the reaction rate was improved by a maximum of 95%and the decolorization rate was improved by a maximum of 71.6%.Based on the spectral analysis,the plasma catalysis mechanism has been studied.展开更多
As the effect of climate plays a significant role in corona discharge under combined voltages,the variation of corona inception voltage with different air pressure and humidities is studied.An experimental platform,ba...As the effect of climate plays a significant role in corona discharge under combined voltages,the variation of corona inception voltage with different air pressure and humidities is studied.An experimental platform,based on a rod-to-plane electrode,is constructed with adjustable air pressure from 0.06 MPa to 0.10 MPa and with a relative humidity(RH)from 20%to 90%.The variation of ultraviolet(UV)photon count and corona inception voltage is obtained at various climate conditions under different applied voltages:the single AC,single DC,and combined AC-DC.It turns out that all the corona inception voltages decline with the drop of air pressure and the rise of humidity under different applied voltages.The influence between different voltages primarily relies on space charge,as more AC components make it easier to accumulate positive ions.The existence of AC makes ions move forward and backward while the existence of DC dominates the polarity of corona and general drift directions.At last the fitting formula of hybrid corona inception voltage combining air pressure and relative humidity is given.展开更多
This paper is concerned with power reduction control which is used to avoid DC over-voltage for multiterminal HVDC transmission of offshore wind power.Voltages and frequencies of offshore AC wind farm networks are use...This paper is concerned with power reduction control which is used to avoid DC over-voltage for multiterminal HVDC transmission of offshore wind power.Voltages and frequencies of offshore AC wind farm networks are used for transmitting control signals for the power reduction control.These methods do not require fast communication.Power reduction sharing among the offshore wind farms using the different control signals is analysed.The control systems are also compared against the DC chopper method to prevent a DC overvoltage.Simulation and experiments are carried out to evaluate the control systems.展开更多
A study is conducted to evaluate 1.2 kV silicon-carbide(SiC)MOSFETs in a cascaded H-bridge(CHB)three-phase inverter for medium-voltage applications.The main purpose of this topology is to remove the need for a bulky 6...A study is conducted to evaluate 1.2 kV silicon-carbide(SiC)MOSFETs in a cascaded H-bridge(CHB)three-phase inverter for medium-voltage applications.The main purpose of this topology is to remove the need for a bulky 60 Hz transformer normally used to step up the output signal of a voltage source inverter to a medium-voltage level.Using SiC devices(1.2-6.5 kV SiC MOSFETs)which have a high breakdown voltage,enables the system to meet and withstand the medium-voltage stress using only a minimal number of cascaded modules.The SiC-based power electronics when used in the presented topology considerably reduce the complexity usually encountered when Si devices are used to meet the medium-voltage level and power scalability.Simulation and preliminary experimental results on a low-voltage prototype verifies the nine-level CHB topology presented in this study.展开更多
Electrical power generation from wind technology is the most rapidly growing technology due to its ample characteristics.Nevertheless,because of its stochastic feature,it has the unnecessary impact on the operations a...Electrical power generation from wind technology is the most rapidly growing technology due to its ample characteristics.Nevertheless,because of its stochastic feature,it has the unnecessary impact on the operations and stability of the power grid system.The fluctuation of the grid frequency problem,for example,is more pronounced.The fluctuation of the frequency in turn impacts even the collapse of the power system.To minimize such problems,a droop-vector control strategy applied on a doubly-fed induction machine based(DFIM)variable speed pumped storage(VSPS)system is proposed in this paper.This method is should be used as a wind power fluctuation compensation solution in the wind farm-grid integration system.The system model is made on the basis of the technique called a phasor model.The frequency spectrum analysis approach is used in the VSPS plant for determining the dynamic performances of the grid in case of contingencies including wind power fluctuation compensation.The software platform MATLAB/Simulink is used for verifying the performance of the proposed system.The results show that the method of the frequency spectrum analysis technique is effective for determining the wind power fluctuation and stability requirements in large power networks.The control strategy proposed in this paper implementing the VSC-DFIM based VSPS plant integrated with the power gird and wind farm network achieves a well-controlled power flow and stable grid frequency with the deviations being in acceptable ranges.展开更多
基金supported by the National Natural Science Foundation of China (No. 51907125)。
文摘Wind-farm-side modular multilevel converters(WFMMCs) used in modular multilevel converter based highvoltage direct current(MMC-HVDC) transmission systems must be able to control the AC grid voltage in offshore wind farms. Different AC voltage control strategies can significantly affect the dynamic characteristics of WFMMCs. However, existing studies have not provided a general methodology of controller parameter design, and few comparative studies have been conducted on control performance under varying operating conditions as well as the effects of different AC voltage control modes(AVCMs) on the stability of MMC-HVDCs with offshore wind farms. This paper provides a controller parameter design method for AVCMs, which is tested in various operating scenarios. Sequence impedance models of offshore wind farms and WFMMCs under different AVCMs are then developed. The effects of AVCMs on the small-signal stability of the interconnected system are then analyzed and compared using the impedance-based method. Finally, case studies are conducted on a practical MMC-HVDC system with offshore wind farms to verify the theoretical analysis.
基金supported by National Basic Research Program of China(973 Program)under Grant 2011CB209402.
文摘In the development of hybrid HVDC and HVAC transmission lines,the study of radio interference is an important issue.Positive corona current pulses from high voltage transmission lines are the main source of radio interference.In this paper,the design of a wire-cylinder gap electrode system is presented to study the influence of AC voltage on the characteristics of positive corona current pulses.The study shows that the mode of the current pulses is different from that of either DC or AC corona discharge.Waveform parameters of the pulses,such as rise time,half wave time,duration time,repetition rates,average amplitude,and time intervals of secondary pulses are all statistically analyzed in this study.The empirical formulas for the repetition rates with different AC voltages are presented.A theoretical explanation based on an ion cloud model is given to reveal the mechanism behind the influence of AC voltage on positive corona discharge.The experimental results could provide some references for the prediction of radio interference from hybrid AC/DC transmission lines.
基金Subsidized with the Special Funds for the Major State Basic Research Projects G19990650 and the Chinese National Science Fund (No. 20173033).
文摘A new method for preparing effective inhibition film on copper has been developed. Phenylthiourea (PT) was first absorbed to copper surface to form a monolayer. 1-Dodecanethiol (DT) was then assembled on the surface for modification. Finally, AC voltage was loaded on copper covered the mixed film to improve it further. After these processes, an effective inhibition film was gained because of its high charge transfer resistance and low corrosion current density shown in electrochemical impedance spectra and polarization. The inhibition efficiency was more than 97%.
基金supported by National Natural Science Foundation of China (Grant No.51277063)
文摘The generation and propagation of a streamer is a significant physical process of air gap discharge. Research on the mechanism of streamers under low-pressure conditions is helpful for understanding the process of long-gap discharge in a high-altitude area. This paper describes laboratory investigations of streamer discharge under alternating current(AC) voltage in a low pressure test platform for a 60 cm rod–plane gap at 30 kPa, and analyzes the characteristics of streamer generation and propagation. The results show that the partial streamer and breakdown streamer all occur in the positive half-cycle of AC voltage near the peak voltage at 30 kPa. The partial streamer could cause the distortion of current and voltage waveform, and it appears as the branching characteristic at the initial stage. With the extension of the streamer, the branching and tortuosity phenomena become gradually obvious, but the branching is suppressed when the streamer crosses the gap. The low-pressure condition has little influence on the tortuosity length and the tortuosity number of the streamer, but affect the diameter of streamer obviously.
基金supported by the National Natural Science Foundation of China-State Grid Corporation Joint Fund for Smart Grid(No.U1766219)。
文摘In order to evaluate the insulation of two-phase immersion cooling in the HV power electronic package,the insulation degradation of the dielectric interface induced by bubbles is investigated.In this paper,a test strategy with 50 Hz unipolar DC and AC combined voltage for partial discharge(PD)at boiling interface of AlN ceramic is proposed.The insulation threshold of an AlN ceramic surface is acquired in several dielectric environments,such as air,FC-72 liquid(FC-72,a Fluorinert^(TM) from 3^(TM)),FC-72 vapor,and boiling state of FC72.This reveals the deterioration of boiling on the insulation of the surface immersed in the dielectric refrigerant.To investigate the mechanism of the PD feature at the boiling interface,the PD patterns of the unrestricted bubble and the accumulated bubble are acquired and contrastively analyzed.Combined with the feature of the back discharge and the bubble behavior,the charged vapor-ceramic interface is relatively stable due to the accumulated vapor layer.This stability of the charged vaporceramic interface is broken if the bubble is unrestricted.Besides,it is discovered that the vapor-liquid interface inside the bubble may be another charged interface,which can also trigger a back discharge.
基金National Natural Science Foundation of China(No.11675031) for their support of this research
文摘The feasibility of the formation of a liquid plasma catalysis system through micro arc oxidation(MAO) under AC power with titanium-aluminum alloy electrodes was investigated.In the decolorization of organic dyeing wastewater simulated with Rhodamine B,Ti-Al alloy electrodes were superior over Ti electrodes and Al electrodes.The optimal molar percentage of Ti in alloy electrodes was 70%and the optimal decolorization rate was up to 88.9%if the additive suitable for Al was added into the solution to be treated.The decolorization rates were the same in the case of the alloy-alloy electrodes and alloy-Al electrodes.The proportion of the effects of plasma,TiO2 catalyzer during MAO and H2O2 after MAO in decolorization has been obtained.With the catalysis of TiO2 formed on the electrodes,the reaction rate was improved by a maximum of 95%and the decolorization rate was improved by a maximum of 71.6%.Based on the spectral analysis,the plasma catalysis mechanism has been studied.
基金supported by the National Natural Science Foundation of China(51877082)Fok Ying-Tong Education Foundation(151058)the Fundamental Research Funds for the Central Universities(2019MS011).
文摘As the effect of climate plays a significant role in corona discharge under combined voltages,the variation of corona inception voltage with different air pressure and humidities is studied.An experimental platform,based on a rod-to-plane electrode,is constructed with adjustable air pressure from 0.06 MPa to 0.10 MPa and with a relative humidity(RH)from 20%to 90%.The variation of ultraviolet(UV)photon count and corona inception voltage is obtained at various climate conditions under different applied voltages:the single AC,single DC,and combined AC-DC.It turns out that all the corona inception voltages decline with the drop of air pressure and the rise of humidity under different applied voltages.The influence between different voltages primarily relies on space charge,as more AC components make it easier to accumulate positive ions.The existence of AC makes ions move forward and backward while the existence of DC dominates the polarity of corona and general drift directions.At last the fitting formula of hybrid corona inception voltage combining air pressure and relative humidity is given.
基金supported by the Research Councils UK,through the HubNet consortium,www.hubnet.org.uk(grant number:EP/I01363611)the Top and Tail Transformation programme,(grant number:EP/I031707/1)+1 种基金supported by the People Programme(Marie Curie Actions)of the European Union's Seventh Framework Programme FP7/2007-20131(grant number:317221,project title MEDOW)supported by the Joint Research Fund for Overseas Chinese,Hong Kong,and Macao Scientists of the National Natural Science Foundation of China(grant number:51128701).
文摘This paper is concerned with power reduction control which is used to avoid DC over-voltage for multiterminal HVDC transmission of offshore wind power.Voltages and frequencies of offshore AC wind farm networks are used for transmitting control signals for the power reduction control.These methods do not require fast communication.Power reduction sharing among the offshore wind farms using the different control signals is analysed.The control systems are also compared against the DC chopper method to prevent a DC overvoltage.Simulation and experiments are carried out to evaluate the control systems.
文摘A study is conducted to evaluate 1.2 kV silicon-carbide(SiC)MOSFETs in a cascaded H-bridge(CHB)three-phase inverter for medium-voltage applications.The main purpose of this topology is to remove the need for a bulky 60 Hz transformer normally used to step up the output signal of a voltage source inverter to a medium-voltage level.Using SiC devices(1.2-6.5 kV SiC MOSFETs)which have a high breakdown voltage,enables the system to meet and withstand the medium-voltage stress using only a minimal number of cascaded modules.The SiC-based power electronics when used in the presented topology considerably reduce the complexity usually encountered when Si devices are used to meet the medium-voltage level and power scalability.Simulation and preliminary experimental results on a low-voltage prototype verifies the nine-level CHB topology presented in this study.
基金supported by the State Key Laboratory of the Smart Grid Protection and Control of China and“111”project:Large Scale Power Grid Protection and Safety Defense 2.0(BP0820024)。
文摘Electrical power generation from wind technology is the most rapidly growing technology due to its ample characteristics.Nevertheless,because of its stochastic feature,it has the unnecessary impact on the operations and stability of the power grid system.The fluctuation of the grid frequency problem,for example,is more pronounced.The fluctuation of the frequency in turn impacts even the collapse of the power system.To minimize such problems,a droop-vector control strategy applied on a doubly-fed induction machine based(DFIM)variable speed pumped storage(VSPS)system is proposed in this paper.This method is should be used as a wind power fluctuation compensation solution in the wind farm-grid integration system.The system model is made on the basis of the technique called a phasor model.The frequency spectrum analysis approach is used in the VSPS plant for determining the dynamic performances of the grid in case of contingencies including wind power fluctuation compensation.The software platform MATLAB/Simulink is used for verifying the performance of the proposed system.The results show that the method of the frequency spectrum analysis technique is effective for determining the wind power fluctuation and stability requirements in large power networks.The control strategy proposed in this paper implementing the VSC-DFIM based VSPS plant integrated with the power gird and wind farm network achieves a well-controlled power flow and stable grid frequency with the deviations being in acceptable ranges.