To reduce the maximum demand of electrified railways and improve the energy efficiency of power supply, this paper proposes a series-parallel hybrid maximum demand control system used in phase-separating section. This system was constructed based on multiple independent AC-DC-AC converters and single-phase transformers. It significantly reduces the capacity configuration requirements for the active devices and improves the overall reliability and cost-effectiveness of the system. Based on application scenarios for demand regulation and the parameter constraints of the main components, a key parameter design method was derived for the demand control system. Additionally, a device-level grid-connected control strategy and a system-level energy dispatching algorithm were developed to achieve fast and precise control of the maximum demand in traction power supply systems. Results from joint commissioning and simulation experiments show that the maximum demand control system can effectively optimize demand control in traction power supply systems.

SiC power MOSFETs are becoming increasingly widely used due to their superior performance, and their reliability is crucial for critical equipment such as power supplies. However, there are still unresolved issues that need to be addressed. Therefore, an experimental study was carried out to investigate the variation of static characteristics of SiC MOSFETs in reliability tests. Based on experimental data, the degradation trend of key parameters such as on resistance and threshold voltage of SIC MOSFET was summarized. The mechanism of temperature stress on parameters was analyzed, and strategies to cope with parameter drift were proposed, providing reference for promoting the more mature application of SiC MOSFET.

In this study, the broadband dielectric spectrum of polyimide composites was examined using the homogenization theory. The findings demonstrate that the addition of boron nitride nanosheets (BNNSs) and silver nanoparticles (AgNPs) causes an increase in conductivity, dielectric constant, and dielectric loss of the composites but does not result in a surge in the three parameters; the results also demonstrate that an increase in temperature causes an increase in dielectric constant and dielectric loss of the composites. In addition, temperature has a bigger impact on the dielectric loss than it does on the dielectric constant. An increase in the amount of polarized charges and a shallower bulk trap depth can result from the addition of nanomaterials, according to research on thermally stimulated current in composite materials. The presence of the interfacial region, which made it simple to cause charge accumulation and simple to form conduction current, is primarily responsible for the significant increase in bulk conductivity of the composites at 50 Hz that occurred with the addition of more nanomaterials. The large rise in bulk conductivity of the composites at 50 Hz that happened with the addition of more nanomaterials is mostly due to the existence of the interfacial region, which made it simple to produce charge buildup and simple to form conduction current. The variation in volume conductivity when the doping ratio is less than 10 vol% is not significant due to AgNPs’ low concentration and ambiguous contribution to the conduction current. As the polarization temperature rises, the amount of polarization charges increases, but it has minimal effect on the bulk trap depth of the composites.

采用可自关断功率的功率半导体器件主动换相换流器(ACC)相对于传统的电网换相换流器(LCC)能够抑制换相失败的风险,相对于电压源型的模块化多电平换流器(MMC)拓扑简单,无需大量的储能元件,体积小、重量轻,因此,ACC在高压直流输电场合有广泛的应用前景.本文针对由两台ACC直流侧串联、交流侧并联组成的组合型ACC,提出一种开关频率为50 Hz的基频开关高功率因数运行方法.与LCC不同,基频开关ACC的触发时刻不受外部电路条件约束,具备触发角超前和触发角滞后两种工作模式,使得组合型基频开关ACC具备调节有功功率和无功功率的能力,能够工作在高功率因数运行状态.此外,本文分析了基于电网电压定向时ACC的功率传输特性,在此基础上,为了进一步提高网侧功率因数,引入功率因数补偿角对基频开关ACC的触发角进行调整,使组合型基频开关ACC的网侧运行在单位功率因数附近.仿真和实验结果均验证了本文理论分析及所提出的功率因数控制方法的正确性和有效性.

The use of ionic liquids (ILs) for biomass pretreatment to produce cellulose-rich materials (CRMs) has been well proven. In this research, due to the wide range of applications and ease of using artificial intelligence procedures, on the basis of the algorithm of stochastic gradient boosting (SGB) decision tree, an artificial intelligence approach is proposed to estimate the properties of cellulose-rich materials (CRMs). That being the case, the dataset of the empirical output values was gathered and was randomly broken down into datasets for testing and training. These results show that the best forecasting tool for calculating the properties of CRMs is the developed model. Furthermore, the accuracy of the databank of the biodiesel target values has been examined. In contrast, the influences of model contributed variables on the output have been examined as a new issue. It reveals that the most influencing variable in determining the properties of CRMs is the cellulose enrichment factor. Therefore, this research provides an innovative and accurate tool for predicting the properties of CRMs and sensitivity investigation on effective parameters to help investigators developing the optimized process.

电力电子变压器是交直流混合配电系统中实现交直流电能变换与管控的核心设备,该文以应用于苏州同里交直流混合配电系统中10kV-3MV·A四端口电力电子变压器样机为对象,考虑示范工程中多电压等级的交直流设备接入需求,针对交直流混合配电接入的复杂应用场景,在电力电子变压器样机的电路拓扑设计和分布式控制技术方面开展研究,提出一种适用于多端口、直流真双极的电路拓扑及其端口协调控制策略.示范工程现场测试结果验证了该文中电力电子变压器样机设计能够满足交直流混合配电系统复杂应用需求,对于电力电子变压器在交直流混合配电网中推广应用及发展具有重要意义.

Due to the power interaction between the phase-shifted dual active bridges (PSDABs) and series resonant dual active bridges (SRDABs) in the hybrid modular dc Transformer (HMDCT), it is difficulty to optimize current stress based on the transferred power. An extended-state-observer based model predictive control (ESO-MPC) strategy with the extended phase shift (EPS) is proposed to optimize the current stress of the PSDABs in the HMDCT. The MPC is employed to calculate the optimal transferred power of the PSDABs. Moreover, a first-order ESO is introduced to observe the uncertain output current of the PSDABs. Based on the calculated transferred power, the optimal phase-shifted duty ratios of each PSDAB under EPS, which achieves optimal current stress, are obtained by the Lagrange multiplier method. Because of the ESO-MPC strategy, it enables to achieve the current sensorless control of the HMDCT. Simulations on a four-cell 3kV/500V HMDCT and the extensive experiments of a downscale laboratory platform, using DSP OMPL138 -EZW + FPGA XC7A75T as the core controller, validate the feasibility of the proposed control strategy.

The energy storage requirements of full-bridge submodule (FBSM)-based modular multilevel converters (FB-MMCs) can be reduced more than 50% compared to conventional half-bridge SM-based MMCs (HB-MMCs) when negative voltage states of FBSMs are used. However, under single-line-to-ground (SLG) faults conditions, the FBSM voltages ripples increase significantly, which is quite different from HB-MMCs. This article analyzes the characteristics of the FBSM voltage fluctuations under both steady-state and SLG faults conditions, and the impact of modulation index, power factor angle and inductance at the grid side on the FBSM voltage fluctuations is also analyzed. Results indicate that the peak-to-peak value of the FBSM voltage fluctuations has strong correlation with the modulation index. It is also found that the fundamental frequency components become dominant when SLG faults occur, and the ripples are almost twice of the steady-state operations, which may threaten safe operation of FB-MMCs. Based on these analysis, a modulation index design method is proposed for the purpose of balancing the FBSM voltage fluctuations of the FB-MMC under SLG faults conditions. The validity of the proposed modulation index design method is verified by simulation results on a ±200 kV/800 MW FB-MMC.

高压直流(high voltage direct current,HVDC)输电技术相对于高压交流(high voltage alternating current,HVAC)输电技术具有线路损耗低、输电走廊占地少、不存在交流电网同步问题等突出优势,特别适用于大容量、长距离、高效率的电能输送场合.相较于传统的电网换相换流器(line commutated converter,LCC)和以模块化多电平换流器(modular multi-level converter,MMC)为典型代表的电压源型换流器(voltage source converter,VSC),主动换相换流器(actively commutated converter,ACC)具有电路拓扑简单、功率因数可控、不存在换相失败风险、具备黑启动的能力、直流侧无需配置储能电容等优点,在高压直流输电领域尤其是海上风电汇集等场合有广泛的应用前景.本文提出了一种大陆端和离岸端换流站均选用ACC作为换流器的海上风电场背靠背高压直流输电系统,大陆端换流站采用开关频率为50 Hz的高功率因数运行的工作模式,离岸端换流器选取脉冲宽度调制(pulse width module,PWM)的工作模式,并分别建立了大陆端换流站和离岸端换流站的数学模型,分析了基于ACC的海上风电场高压直流输电系统的黑启动方法和稳态功率传输特性,并提出了一种大陆端换流站采用直流电流控制、离岸端换流站选取交流侧电压控制的控制策略.在PSCAD/EMTDC中搭建了500 kV/1000 MW的基于ACC的海上风电场背靠背高压直流输电系统仿真模型,仿真结果验证了本文理论分析以及所提出控制策略的正确性和有效性.

The existing HVDC transmission technologies for offshore wind farm is summarized and analyzed first. Based on these analyses, this paper presents a current-source actively commutated converter (ACC) based HVDC transmission system for offshore wind farm. This system employs fundamental switching frequency ACC for HVDC transmission with very-low-capacity AC network as the startup and auxiliary power supply for the wind farm. The operation principle of this system is analyzed and the startup and steady-state control strategy of this system is also proposed. Comparative analyses with the existing VSCHVDC solution show obvious advantages of the technology in this paper in terms of passive component requirement, DC side fault suppression capability and so forth. Simulation results of a ± 250kV/1000 MW ACC based wind farm HVDC system verified the proposed system with the control methods.

采用自关断功率半导体器件的电流源型主动换相换流器(actively commutated converter,ACC)具有有功与无功功率可解耦、不存在换相失败、无需大量储能电容等特点,在高压直流输电领域具有较好的应用前景.该文针对适用于高压直流输电(high voltage direct current,HVDC)的ACC功率半导体器件及其均压方法、电路拓扑、调制方法、功率特性、控制策略、故障及保护方法等进行调研和分析.结合具体实例,将ACC与现有HVDC的2种换流器,即电网换相换流器(line commutated converter,LCC)和模块化多电平换流器(modular multilevel converter,MMC)进行对比分析.同时,对ACC的潜在应用、存在的问题以及发展的方向进行总结和归纳.

针对以移相双有源桥为功率单元的多模块输入串联输出并联型直流变压器(ISOP-DCT),为提高系统面对输入电压变化及负载突变等复杂工况下的动态响应特性,提出一种基于扩张状态观测器(ESO)的模型预测控制(ESO-MPC)策略,分析并建立了考虑各功率单元能量均衡传输时系统简化离散2步预测模型,同时利用ESO计算了预测模型中由输出端口负载电流及内部死区效应所引起的功率偏差电流.所提出的ESO-MPC策略可有效提高ISOP-DCT动态响应速度及抗干扰能力,同时减少输出端口电流传感器的使用.MATLAB/Simulink中搭建的2.25 kV/750 V ISOP-DCT模型的仿真结果及实验样机测试结果验证了所提控制策略的正确性和有效性.

串联谐振型双有源桥变换器(DABSRC)由于具有控制简单及软开关范围宽的特点,可用在级联H桥型电力电子变压器隔离级DC-DC电能变换环节中.考虑电力电子变压器各相单元存在二倍电网工频瞬时功率波动,导致隔离级DABSRC高频电流幅值呈现相同波动,且过大波动将会增加隔离级开关器件导通损耗和电流应力.为解决该问题,文中以优化高频电流有效值为目标,提出一种零序电压注入控制策略,通过在交流侧级联H桥变换器中注入零序电压,降低了隔离级DABSRC开关器件电流应力.最后,利用苏州同里10 kV交流/750 V直流3 MVA级联H桥型电力电子变压器工程示范样机进行验证,仿真及样机测试结果证明了控制方法的有效性.

The actively commutation converter (ACC) has been regarded as one of many interesting circuit topologies in high-power systems. In this article, unit power factor operation control method of ACC with switching frequency of 50Hz is proposed. A 500 kV/400 MW back-to-back HVDC transmission-based ACC is built using PSCAD/EMTDC software to verify the correctness of the unit power factor operation control method.

With the development of semiconductor, the current source actively commutated converter (ACC) provides a new solution for HVDC. Before the engineering application, the voltage and current stresses of ACC under different operational conditions should be tested first. Building a full-circuit test platform to test the valves is very resource-intensive, thus an operational test system of ACC with the low power requirement is proposed in this paper. It consists of a low-voltage high-current source, a low-current high-voltage source, a resonant circuit, and multiple auxiliary valves and is the improvements of thyristor valves test circuit. The simulation verification shows that the proposed method can better test the voltage and current stresses of ACC in various operational conditions with a low power rate. It can be applied to ACC-HVDC engineering construction.

In order to improve dynamic performance and robustness, a model predictive control (MPC) strategy is proposed for a hybrid dual active bridges (HyDABs) based DC transformer (DCT). Based on the power transmission characteristics, a dynamic first-order linear discretization model is established. Moreover, a two-step-ahead prediction algorithm is used to deal with one-sampling time delay. The traditional voltage closed-loop control (TVLC) scheme with load current feedforward and the proposed MPC scheme are compared on a downscale four-cell laboratory prototype. In addition, the PowerPC P2020 + FPGA_ 6SLX45 is used as the controller. The excellent dynamic performance with MPC algorithm is validated by extensive experimental results as suffering voltage fluctuation and power step.

Cascaded H-bridge (CHB) -based solid-state transformer (SST) consists of CHBs and dual active bridge (DAB) converters. Series-resonant DAB converters (SRCs) are widely used for its advantages of soft switching characteristic and simple control. However, the high-frequency-link (HFL) currents of SRCs contain double-line-frequency harmonic fluctuation in three-phase CHB-based SST, resulting in high current stress. In this article, the SRC is substituted with dc terminal dynamic equivalent circuit, and the analytical expression of the HFL current fluctuation is deduced. With consideration of third-order harmonic voltage injection (THVI), the HFL current fluctuation characteristics with this method are analyzed and the optimal amplitude and phase of the injected third-order harmonic voltage are derived, with which the HFL current stress can be minimized. Both the simulation and experiment results indicate that the HFL current stress of SRCs can achieve a reduction of about 20% with the optimized THVI method.

基于可关断功率半导体器件的电流源型主动换相换流器(actively commutated converter,ACC)具有电路拓扑简单、功率因数可控、支持黑启动、不存在换相失败风险、直流侧无需配置储能电容等优点,相比传统电网电压换相换流器(line commutated converter,LCC)可以提高可控性,相对于电压源型换流器(voltage source converter,VSC)可减小换流器的重量和体积,降低成本,具有广阔的应用前景.但是,ACC将功率半导体器件串联使用以提高电压等级,功率半导体器件的开关频率受到严重限制.为了实现低开关频率下的低谐波电流,该文在分析ACC约束条件及特定谐波消除法(selective harmonic elimination,SHE)工作原理的基础上,提出了通过"虚拟相电流"生成换流器触发信号的调制方法,可以在满足ACC开关约束的条件下显著简化SHE调制方法的实现过程.仿真和实验结果验证该文理论分析及所提出的SHE调制策略和零开关状态分配原则的正确性和有效性.

DC transformer (DCT) which is composed of dual active bridge (DAB) converters, is capable of fulfiling high-ratio voltage conversion between the medium-voltage dc distribution and low-voltage dc distribution. A model predictive control (MPC) strategy was proposed for the DAB-DCT to improve dynamic performance and achieve a proper power distribution, considering the bidirectional power of DCT. The traditional voltage closed-loop control scheme with the single-phase shift (SPS) and the MPC scheme with SPS are compared on a downscale DAB-DCT laboratory prototype, using PowerPC P2020 + FPGA_6SL X45 as the core controller. The excellent dynamic characteristics of DCT with proposed MPC strategy is verified by extensive experimental results.

针对三相级联H桥型电力电子变压器(PET),分析了输入侧瞬时功率波动特性,建立了其直流侧电容电压及中间级高频变压器原、副边电流的时域解析模型,并基于该模型提出一种PET电容值设计方法.基于MATLAB/Simulink仿真平台搭建三相级联H桥型PET仿真模型,并在功率模块测试平台进行实验验证.仿真及实验结果表明该解析模型可以准确地描述电容电压及变压器高频电流的波动特性,进而为主电路参数设计与控制系统设计提供研究基础.