VFT （variable frequency transformer） has been recently used as art alternative to HVDC （high voltage direct current） to control power flow between asynchronous networks. VFT consumes less reactive power than a bac...VFT （variable frequency transformer） has been recently used as art alternative to HVDC （high voltage direct current） to control power flow between asynchronous networks. VFT consumes less reactive power than a back-to-back HVDC system, provides faster initial transient recovery, and has better natural damping capability. VFT is simply a DFIM （doubly-fed induction machine） where the machine torque controls the power flow from stator to rotor and vice versa. The main disadvantage of this VFT is the slip rings and brushes required for the rotor circuit, especially in bulk power transmission. The BDFM （brushless doubly-fed machine） with nested cage rotor machine is proved to be a comparable alternative to conventional DFIM in many applications with the advantage that all windings being in the stator frame with fixed output terminals. In this paper, the BDFM is used as a BVFT （brushless variable frequency transformer）. A prototype machine is designed and simulated to verify the system validity.展开更多
A circuit configuration and a circuit topologic family of the novel forward mode AC/AC converters with high frequency link are presented. The circuit configuration is constituted of input cycloconverter, high frequenc...A circuit configuration and a circuit topologic family of the novel forward mode AC/AC converters with high frequency link are presented. The circuit configuration is constituted of input cycloconverter, high frequency transformer, output cycloconverter, input and output filters. The circuit topologic family includes eight circuit topologies, such as full-bridge-full-wave mode, etc. The bi-polarity phase-shifted control strategy and steady principles are thoroughly investigated. The output characteristics are obtained. By using the bi-polarity phase-shifted control strategy with phase-shifted control between the output cycloconveter and the input cycloconverter, commutation overlap period of the output cycloconverter, and polarity selection of the output filtering inductance current and the input voltage, the leakage inductance energy and the output filtering inductance current are naturally commutated, and surge voltage and surge current of the cycloconverters are overcome. The converters have such advantages as simple topology, two-stage power conversions(LFAC/HFAC/LFAC), bi-directional power flow, high frequency electrical isolation, good output waveforms, and strong ability to stabilize voltage. The converters lay key technical foundation on a new-type of regulated sinusoidal AC power supplies and electronic transformers. The correction and advancement of the converters are well verified by a principle test.展开更多
To avoid stray current and maintain the benefit of no phase-split in the DC traction power supply system, an AC traction power supply system was proposed for the urban public transport such as metro and light rail tra...To avoid stray current and maintain the benefit of no phase-split in the DC traction power supply system, an AC traction power supply system was proposed for the urban public transport such as metro and light rail transit. The proposed system consists of a main substation （MSS） and cable traction network （CTN）. The MSS includes a single-phase main traction transformer and a negative-se- quence compensation device, while the CTN includes double-core cables, traction transformers, overhead catenary system, rails, etc. Several key techniques for the proposed system were put forward and discussed, which can be summarized as （1） the power supply principle, equivalent circuit and transmission ability of the CTN, the cable-catenary matching technique, and the selection of catenary voltage level; （2） the segmentation technology and status identification method for traction power supply network, distributed and centralized protection schemes, etc.; （3） a power supply scheme for single-line MSS and a power supply scheme of MSS shared by two or more lines. The proposed industrial frequency single-phase AC traction power supply system shows an excellent technical performance, good economy, and high reliability, hence provides a new alternative for metro and urban rail transit power supply systems.展开更多
Compact supercapacitors(SCs)have attracted attention for their great potential to replace bulky aluminum electrolytic capacitors(AECs)in alternating current(AC)line filtering applications.Herein,the fabrication of a h...Compact supercapacitors(SCs)have attracted attention for their great potential to replace bulky aluminum electrolytic capacitors(AECs)in alternating current(AC)line filtering applications.Herein,the fabrication of a high-frequency SC is reported using Ketjen black(KB)nanoparticles doped with phosphorus(P)to achieve a high areal capacitance of up to 2.26 mF cm^(-2)along with a high-rate capability,with a phase angle of-80.2°at 120 Hz.The high performance of the phosphorus-doped KB(designated PKB)SC with a 6 M KOH aqueous electrolyte is associated with its increased surface wettability and additional capacitive sites provided by the P-doping.Density functional theory(DFT)calculations further indicate that the P-doping enhances the interactions between the electrolyte ions and the carbon surface,thus leading to an improved electrochemical performance.These results suggest that the P-doped carbonbased SC could be highly favored in replacing conventional AECs in various high-frequency electronic devices.展开更多
High-frequency supercapacitors are being studied with the aim to replace the bulky electrolytic capacitors for current ripple filtering and other functions used in power systems. Here, 3 D edge-oriented graphene(EOG)w...High-frequency supercapacitors are being studied with the aim to replace the bulky electrolytic capacitors for current ripple filtering and other functions used in power systems. Here, 3 D edge-oriented graphene(EOG)was grown encircling carbon nanofiber(CNF) framework to form a highly conductive electrode with a large surface area. Such EOG/CNF electrodes were tested in aqueous and organic electrolytes for high-frequency supercapacitor development. For the aqueous and the organic cell, the characteristic frequency at-45° phase angle was found to be as high as 22 and 8.5 k Hz, respectively. At 120 Hz, the electrode capacitance density was 0.37 and 0.16 m F cm^(-2) for the two cells. In particular, the 3 V high-frequency organic cell was successfully tested as filtering capacitor used in AC/DC converter, suggesting the promisingpotential of this technology for compact power supply design and other applications.展开更多
The effect of frequencies of AC power supply on the quality of the electroslag-melted ingot is studied. The results show that with a decrease in the frequency, electromagnetic force becomes more violent, and the tempe...The effect of frequencies of AC power supply on the quality of the electroslag-melted ingot is studied. The results show that with a decrease in the frequency, electromagnetic force becomes more violent, and the temperature in the slag bath becomes more homogeneous, and therefore, the depth of molten metal pool is decreased; electrochemical reactions occur with the decrease in the frequency, and the atomic oxygen electrolyzed dissolves in the molten metal pool; the nonmetallic inclusions, which are distributed dispersively in the ingot, have an increased content, and their size is approximately in the range of 2--3 μm.展开更多
The electrical and thermal characterization of near-surface electrical discharge plasma driven by radio frequency voltage are investigated experimentally in this paper. The influences of operating pressure, electrode ...The electrical and thermal characterization of near-surface electrical discharge plasma driven by radio frequency voltage are investigated experimentally in this paper. The influences of operating pressure, electrode distance, and duty cycle on the discharge are studied. When pressure reaches 60 Ton. （1 Torr= 1.33322 x 102 Pa） the transition from diffuse glow mode to constricted mode occurs. With the operating pressure varying from 10 Tort to 60 Torr, the discharge energy calculated from the charge-voltage （Q-V） Lissajous figure decreases rapidly, while it remains unchanged between 60 Torr and 460 Torr. Under certain experimental conditions, there exists an optimized electrode distance （8 mm）. As the duty cycle of applied voltage increases, the voltage-current waveforms and Q-V Lissajous figures show no distinct changes.展开更多
For dielectric barrier discharge(DBD)driven by AC voltage in helium at atmospheric,the relationship between the breakdown voltage and the driving frequency is experimentally investigated using a pair of parallel elect...For dielectric barrier discharge(DBD)driven by AC voltage in helium at atmospheric,the relationship between the breakdown voltage and the driving frequency is experimentally investigated using a pair of parallel electrodes.The gap between the electrodes is 1 mm,4 mm,7 mm,and 10 mm,respectively.Meanwhile with an increment of 2 kHz,the applied AC voltage varies from 12 kHz to 30 kHz.In each experiment,the driving voltage increases slowly,till the helium-filled gap breaks down.Based on a number of experimental results and further analyses,conclusions are obtained as follows.(1)For a small gap(1 mm),the voltage that triggers the first breakdown(Uf)is close to the one that sustains steady breakdowns(Us).However,in the larger gaps(4,7,and 10 mm),Uf is obviously larger than Us.(2)For a fixed gap,Uf does not change significantly with the driving frequency,whereas in the gaps except the 1 mm one,Us drastically decreases with the increase of driving frequency.(3)The motion of residual space charges and the dissipation of positive column,two reasonable factors that explains asymmetrical discharges,are also main reasons for the effect of the driving frequency on the breakdown voltages.展开更多
The systematic mathematical analysis of the high frequency soft-switched AC-AC converter is proposed for variable frequency induction machine load in this paper. Both PWM and square-wave modes of operation have been c...The systematic mathematical analysis of the high frequency soft-switched AC-AC converter is proposed for variable frequency induction machine load in this paper. Both PWM and square-wave modes of operation have been considered. The frequency relation and phase unbalance problem due to discrete time integral half-cycle switching has been discussed in the beginning. Then, generalized Fourier series have been derived for output voltage, output current and supply current in two modes.The analytical results help to understand tbe converter characteristics, design optimally a convertermachine system of arbitrary capacity considering the various trade-off parameters.展开更多
在两级式AC-DC变换器中,前级功率因数校正(power factor correction,PFC)固有的瞬时功率波动特性会造成母线电压存在二倍频纹波,影响后级CLLLC谐振变换器的输出电压质量。针对以上问题,该文提出了基于二阶广义积分(second order general...在两级式AC-DC变换器中,前级功率因数校正(power factor correction,PFC)固有的瞬时功率波动特性会造成母线电压存在二倍频纹波,影响后级CLLLC谐振变换器的输出电压质量。针对以上问题,该文提出了基于二阶广义积分(second order generalized integral,SOGI)的可变增益母线电压纹波前馈控制方法。采用SOGI提取母线电压纹波信息,基于品质因数Q与电压增益的关系和母线电压纹波对归一化频率的影响,解析了母线电压纹波对CLLLC谐振变换器输出电压的影响机理,得到Q值与前馈增益系数Ka的关系,采用仿真寻优加数据拟合的方法得到前馈可变增益系数曲线。仿真和实验结果表明,相比于无前馈控制,所提控制方法对CLLLC谐振变换器的输出电压纹波具有较好的抑制效果,输出电压纹波降低了72%,验证了所提算法的有效性。展开更多
Aluminum electrolyte capacitors(AEC)are widely used in AC filtering as traditional filter capacitors.However,their strong rigidity,high hardness,and high-risk factor have hindered the flexible development of filter ca...Aluminum electrolyte capacitors(AEC)are widely used in AC filtering as traditional filter capacitors.However,their strong rigidity,high hardness,and high-risk factor have hindered the flexible development of filter capacitors.Flexible supercapacitors,due to their high energy density and outstanding cycle stability,are becoming one of the main directions for the development of filter capacitors in the future.This paper self-assembles a MnO_(2)-based flexible supercapacitor with high area capacitance and fast frequency response,and preliminarily verifies the feasibility of using the capacitor for AC filtering through simulation tests in Multisim.Finally,the AC filtering test of the flexible supercapacitor using an oscilloscope demonstrates that the ripple factor in the range of 2.7%to 8%,which confirms that the MnO_(2)-based flexible supercapacitor has a great AC filtering function and can replace traditional aluminum electrolyte capacitors in AC filtering,promoting the flexible development of electronic products.展开更多
文摘VFT （variable frequency transformer） has been recently used as art alternative to HVDC （high voltage direct current） to control power flow between asynchronous networks. VFT consumes less reactive power than a back-to-back HVDC system, provides faster initial transient recovery, and has better natural damping capability. VFT is simply a DFIM （doubly-fed induction machine） where the machine torque controls the power flow from stator to rotor and vice versa. The main disadvantage of this VFT is the slip rings and brushes required for the rotor circuit, especially in bulk power transmission. The BDFM （brushless doubly-fed machine） with nested cage rotor machine is proved to be a comparable alternative to conventional DFIM in many applications with the advantage that all windings being in the stator frame with fixed output terminals. In this paper, the BDFM is used as a BVFT （brushless variable frequency transformer）. A prototype machine is designed and simulated to verify the system validity.
文摘A circuit configuration and a circuit topologic family of the novel forward mode AC/AC converters with high frequency link are presented. The circuit configuration is constituted of input cycloconverter, high frequency transformer, output cycloconverter, input and output filters. The circuit topologic family includes eight circuit topologies, such as full-bridge-full-wave mode, etc. The bi-polarity phase-shifted control strategy and steady principles are thoroughly investigated. The output characteristics are obtained. By using the bi-polarity phase-shifted control strategy with phase-shifted control between the output cycloconveter and the input cycloconverter, commutation overlap period of the output cycloconverter, and polarity selection of the output filtering inductance current and the input voltage, the leakage inductance energy and the output filtering inductance current are naturally commutated, and surge voltage and surge current of the cycloconverters are overcome. The converters have such advantages as simple topology, two-stage power conversions(LFAC/HFAC/LFAC), bi-directional power flow, high frequency electrical isolation, good output waveforms, and strong ability to stabilize voltage. The converters lay key technical foundation on a new-type of regulated sinusoidal AC power supplies and electronic transformers. The correction and advancement of the converters are well verified by a principle test.
文摘To avoid stray current and maintain the benefit of no phase-split in the DC traction power supply system, an AC traction power supply system was proposed for the urban public transport such as metro and light rail transit. The proposed system consists of a main substation （MSS） and cable traction network （CTN）. The MSS includes a single-phase main traction transformer and a negative-se- quence compensation device, while the CTN includes double-core cables, traction transformers, overhead catenary system, rails, etc. Several key techniques for the proposed system were put forward and discussed, which can be summarized as （1） the power supply principle, equivalent circuit and transmission ability of the CTN, the cable-catenary matching technique, and the selection of catenary voltage level; （2） the segmentation technology and status identification method for traction power supply network, distributed and centralized protection schemes, etc.; （3） a power supply scheme for single-line MSS and a power supply scheme of MSS shared by two or more lines. The proposed industrial frequency single-phase AC traction power supply system shows an excellent technical performance, good economy, and high reliability, hence provides a new alternative for metro and urban rail transit power supply systems.
基金supported by grants from the National Research Foundation of Korea(NRF-2020R1A2C2008798)Korea University。
文摘Compact supercapacitors(SCs)have attracted attention for their great potential to replace bulky aluminum electrolytic capacitors(AECs)in alternating current(AC)line filtering applications.Herein,the fabrication of a high-frequency SC is reported using Ketjen black(KB)nanoparticles doped with phosphorus(P)to achieve a high areal capacitance of up to 2.26 mF cm^(-2)along with a high-rate capability,with a phase angle of-80.2°at 120 Hz.The high performance of the phosphorus-doped KB(designated PKB)SC with a 6 M KOH aqueous electrolyte is associated with its increased surface wettability and additional capacitive sites provided by the P-doping.Density functional theory(DFT)calculations further indicate that the P-doping enhances the interactions between the electrolyte ions and the carbon surface,thus leading to an improved electrochemical performance.These results suggest that the P-doped carbonbased SC could be highly favored in replacing conventional AECs in various high-frequency electronic devices.
基金supported by the National Science Foundation(1611060)
文摘High-frequency supercapacitors are being studied with the aim to replace the bulky electrolytic capacitors for current ripple filtering and other functions used in power systems. Here, 3 D edge-oriented graphene(EOG)was grown encircling carbon nanofiber(CNF) framework to form a highly conductive electrode with a large surface area. Such EOG/CNF electrodes were tested in aqueous and organic electrolytes for high-frequency supercapacitor development. For the aqueous and the organic cell, the characteristic frequency at-45° phase angle was found to be as high as 22 and 8.5 k Hz, respectively. At 120 Hz, the electrode capacitance density was 0.37 and 0.16 m F cm^(-2) for the two cells. In particular, the 3 V high-frequency organic cell was successfully tested as filtering capacitor used in AC/DC converter, suggesting the promisingpotential of this technology for compact power supply design and other applications.
文摘The effect of frequencies of AC power supply on the quality of the electroslag-melted ingot is studied. The results show that with a decrease in the frequency, electromagnetic force becomes more violent, and the temperature in the slag bath becomes more homogeneous, and therefore, the depth of molten metal pool is decreased; electrochemical reactions occur with the decrease in the frequency, and the atomic oxygen electrolyzed dissolves in the molten metal pool; the nonmetallic inclusions, which are distributed dispersively in the ingot, have an increased content, and their size is approximately in the range of 2--3 μm.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11472306,51407197,and 51507187)
文摘The electrical and thermal characterization of near-surface electrical discharge plasma driven by radio frequency voltage are investigated experimentally in this paper. The influences of operating pressure, electrode distance, and duty cycle on the discharge are studied. When pressure reaches 60 Ton. （1 Torr= 1.33322 x 102 Pa） the transition from diffuse glow mode to constricted mode occurs. With the operating pressure varying from 10 Tort to 60 Torr, the discharge energy calculated from the charge-voltage （Q-V） Lissajous figure decreases rapidly, while it remains unchanged between 60 Torr and 460 Torr. Under certain experimental conditions, there exists an optimized electrode distance （8 mm）. As the duty cycle of applied voltage increases, the voltage-current waveforms and Q-V Lissajous figures show no distinct changes.
基金Project supported by Fundamental Research Fund for the Central Universities of Ministry of Education of China(2011ZM0016)State Key Laboratory of Electrical Insulation and Power Equipment(EIPE10210)
文摘For dielectric barrier discharge(DBD)driven by AC voltage in helium at atmospheric,the relationship between the breakdown voltage and the driving frequency is experimentally investigated using a pair of parallel electrodes.The gap between the electrodes is 1 mm,4 mm,7 mm,and 10 mm,respectively.Meanwhile with an increment of 2 kHz,the applied AC voltage varies from 12 kHz to 30 kHz.In each experiment,the driving voltage increases slowly,till the helium-filled gap breaks down.Based on a number of experimental results and further analyses,conclusions are obtained as follows.(1)For a small gap(1 mm),the voltage that triggers the first breakdown(Uf)is close to the one that sustains steady breakdowns(Us).However,in the larger gaps(4,7,and 10 mm),Uf is obviously larger than Us.(2)For a fixed gap,Uf does not change significantly with the driving frequency,whereas in the gaps except the 1 mm one,Us drastically decreases with the increase of driving frequency.(3)The motion of residual space charges and the dissipation of positive column,two reasonable factors that explains asymmetrical discharges,are also main reasons for the effect of the driving frequency on the breakdown voltages.
文摘The systematic mathematical analysis of the high frequency soft-switched AC-AC converter is proposed for variable frequency induction machine load in this paper. Both PWM and square-wave modes of operation have been considered. The frequency relation and phase unbalance problem due to discrete time integral half-cycle switching has been discussed in the beginning. Then, generalized Fourier series have been derived for output voltage, output current and supply current in two modes.The analytical results help to understand tbe converter characteristics, design optimally a convertermachine system of arbitrary capacity considering the various trade-off parameters.
文摘Aluminum electrolyte capacitors(AEC)are widely used in AC filtering as traditional filter capacitors.However,their strong rigidity,high hardness,and high-risk factor have hindered the flexible development of filter capacitors.Flexible supercapacitors,due to their high energy density and outstanding cycle stability,are becoming one of the main directions for the development of filter capacitors in the future.This paper self-assembles a MnO_(2)-based flexible supercapacitor with high area capacitance and fast frequency response,and preliminarily verifies the feasibility of using the capacitor for AC filtering through simulation tests in Multisim.Finally,the AC filtering test of the flexible supercapacitor using an oscilloscope demonstrates that the ripple factor in the range of 2.7%to 8%,which confirms that the MnO_(2)-based flexible supercapacitor has a great AC filtering function and can replace traditional aluminum electrolyte capacitors in AC filtering,promoting the flexible development of electronic products.
