The accurate DC system model is the key to fault analysis and harmonic calculation of AC/DC system. In this paper, a frequency domain analysis model of DC system is established, and based on it a unified fundamental f...The accurate DC system model is the key to fault analysis and harmonic calculation of AC/DC system. In this paper, a frequency domain analysis model of DC system is established, and based on it a unified fundamental frequency and harmonic iterative calculation method is proposed. The DC system model is derived considering the dynamic switching characteristic of converter and the steady-state response features of dc control system synchronously. And the proposed harmonic calculation method fully considers the AC/DC harmonic interaction and fault interaction under AC asymmetric fault condition. The method is used to the harmonic analysis and calculation of CIGRE HVDC system. Compared with those obtained by simulation using PSCAD/EMTDC software, the results show that the proposed model and method are accurate and effective, and provides the analysis basis of harmonic suppression, filter configuration and protection analysis in AC/DC system.展开更多
In modern ZnO varistors,traditional aging mechanisms based on increased power consumption are no longer relevant due to reduced power consumption during DC aging.Prolonged exposure to both AC and DC voltages results i...In modern ZnO varistors,traditional aging mechanisms based on increased power consumption are no longer relevant due to reduced power consumption during DC aging.Prolonged exposure to both AC and DC voltages results in increased leakage current,decreased breakdown voltage,and lower nonlinearity,ultimately compromising their protective performance.To investigate the evolution in electrical properties during DC aging,this work developed a finite element model based on Voronoi networks and conducted accelerated aging tests on commercial varistors.Throughout the aging process,current-voltage characteristics and Schottky barrier parameters were measured and analyzed.The results indicate that when subjected to constant voltage,current flows through regions with larger grain sizes,forming discharge channels.As aging progresses,the current focus increases on these channels,leading to a decline in the varistor’s overall performance.Furthermore,analysis of the Schottky barrier parameters shows that the changes in electrical performance during aging are non-monotonic.These findings offer theoretical support for understanding the aging mechanisms and condition assessment of modern stable ZnO varistors.展开更多
[Objectives]To assess the effects of endophytic bacteria on the growth,antioxidant responses,and the production of key secondary metabolites in Emilia prenanthoidea DC.[Methods]Three endophytic strains(H1,H2,L1)were i...[Objectives]To assess the effects of endophytic bacteria on the growth,antioxidant responses,and the production of key secondary metabolites in Emilia prenanthoidea DC.[Methods]Three endophytic strains(H1,H2,L1)were inoculated onto tissue-cultured seedlings and cultivated for 20 d under greenhouse conditions.Growth traits,reactive oxygen species(ROS)indicators,antioxidant enzyme activities,and the content of chlorogenic acid and quercetin were analyzed using one-way ANOVA followed by Tukey s test.[Results]Bacterial inoculation significantly enhanced plant performance.Treatment H2 increased plant height by 27%,chlorophyll content by 73%,and fresh weight by 31%.Levels of ROS(O^(-)_(2),H_(2)O_(2))and MDA decreased markedly,whereas the activities of POD and CAT increased.Additionally,the content of chlorogenic acid and quercetin increased by up to 67%and 64%,respectively,with both H2 and L1 treatments showing the most pronounced effects.[Conclusions]Endophytic bacteria markedly improve growth,redox balance,and phenolic accumulation in E.prenanthoidea.Strain H2 represents a promising bioinoculant for improving the medicinal quality of this species.展开更多
单级式双有源桥(dual active bridge,DAB)DC/AC变换器控制自由度多,变压器匝比、漏感、开关频率等参数相互耦合,导致变换器效率优化面临多重制约。该文分析变压器匝比、漏感与软开关范围、漏感电流有效值、漏感电流峰值之间的约束关系,...单级式双有源桥(dual active bridge,DAB)DC/AC变换器控制自由度多,变压器匝比、漏感、开关频率等参数相互耦合,导致变换器效率优化面临多重制约。该文分析变压器匝比、漏感与软开关范围、漏感电流有效值、漏感电流峰值之间的约束关系,提出直接表征DAB-DC/AC变换器效率的物理量:效率敏感因子。通过研究效率敏感因子对系统损耗的影响机理,实现变换器效率最优的硬件参数设计,为多参数耦合的单级式DAB-DC/AC变换器效率优化提供理论指导。最后通过一台450 W样机验证所提理论的正确性。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
High proportion of renewable energies and the installation of power electronic devices(PEDs)pose tough challenges to the operation of power systems.In this paper,the remote coordination adjustment(RCA)of PEDs in stoch...High proportion of renewable energies and the installation of power electronic devices(PEDs)pose tough challenges to the operation of power systems.In this paper,the remote coordination adjustment(RCA)of PEDs in stochastic scenarios is studied.The steady-state model for the AC/DC system with PEDs is first established,and the alternate iteration method based on linearization(AIML)is adopted,especially for efficient deterministic power flow calculation.Then,the RCA is proposed using a modular local sensitivity method combined with AIML,which can adjust the electrical variables by diverse PEDs with high efficiency.Additionally,the probabilistic power flow calculation using the quasi-Monte Carlo method with the adaptive sampling number(ASN-QMC)is introduced to keep the balance between the computational efficiency and accuracy,as well as demonstrating the positive impact of RCA by the PEDs in stochastic scenarios.The effectiveness of the proposed RCA is validated by a series of modified IEEE test systems.展开更多
As an emerging trend in power grid development,the AC/DC hybrid power grid presents the characteristics of multi-type DC links and large-scale AC/DC interconnection.However,existing research on AC/DC power flow calcul...As an emerging trend in power grid development,the AC/DC hybrid power grid presents the characteristics of multi-type DC links and large-scale AC/DC interconnection.However,existing research on AC/DC power flow calculation is mainly based on single-object models,ignoring the complex interactions and interdependencies between the AC system and various types of DC links in the hybrid power grid,such as LCCHVDC,MMC-HVDC,and LCC-MMC hybrid HVDC.Therefore,this paper proposes a power flow calculation algorithm for large-scale AC/DC systems with multi-type DC links.Firstly,a CIM/XML document conversion strategy applicable to the AC/DC system is proposed.Then,a unified modeling method is used to derive the power flow model of the AC/DC system,and a unified iterative algorithm for the large-scale AC/DC system with multi-type DC links is proposed.Finally,the algorithm’s correctness and effectiveness are verified by comparing it with actual measurements from the Southern Power Grid of China,with a voltage deviation of less than 1%.The research shows that the proposed algorithm has good convergence and high computational efficiency,which is applicable to power flow calculations in large-scale DC-embedded hybrid AC/DC grids in the future.展开更多
三相双有源桥双向AC/DC变换器具备高效双向变换和升降压运行的优势,适合应用于储能系统,但是该变换器在宽电压范围下运行时的参数设计与性能优化面临挑战。针对适用于该变换器在宽电压范围下实现零电压开关(zero voltage switching,ZVS...三相双有源桥双向AC/DC变换器具备高效双向变换和升降压运行的优势,适合应用于储能系统,但是该变换器在宽电压范围下运行时的参数设计与性能优化面临挑战。针对适用于该变换器在宽电压范围下实现零电压开关(zero voltage switching,ZVS)的混合调制策略,提出基于遗传算法的多目标参数优化方法;以开关管损耗和电感电流有效值作为优化目标,建立多目标优化模型,对电感值和变压器匝比进行求解;在实验室搭建一台1.5 kW的原理样机,实验结果表明,采用混合调制策略的变换器可在宽电压和宽功率范围下实现ZVS。此外,多目标优化参数设计方法能降低变换器的损耗,从而提升效率,为其性能提升提供了有效方案。展开更多
文摘The accurate DC system model is the key to fault analysis and harmonic calculation of AC/DC system. In this paper, a frequency domain analysis model of DC system is established, and based on it a unified fundamental frequency and harmonic iterative calculation method is proposed. The DC system model is derived considering the dynamic switching characteristic of converter and the steady-state response features of dc control system synchronously. And the proposed harmonic calculation method fully considers the AC/DC harmonic interaction and fault interaction under AC asymmetric fault condition. The method is used to the harmonic analysis and calculation of CIGRE HVDC system. Compared with those obtained by simulation using PSCAD/EMTDC software, the results show that the proposed model and method are accurate and effective, and provides the analysis basis of harmonic suppression, filter configuration and protection analysis in AC/DC system.
文摘In modern ZnO varistors,traditional aging mechanisms based on increased power consumption are no longer relevant due to reduced power consumption during DC aging.Prolonged exposure to both AC and DC voltages results in increased leakage current,decreased breakdown voltage,and lower nonlinearity,ultimately compromising their protective performance.To investigate the evolution in electrical properties during DC aging,this work developed a finite element model based on Voronoi networks and conducted accelerated aging tests on commercial varistors.Throughout the aging process,current-voltage characteristics and Schottky barrier parameters were measured and analyzed.The results indicate that when subjected to constant voltage,current flows through regions with larger grain sizes,forming discharge channels.As aging progresses,the current focus increases on these channels,leading to a decline in the varistor’s overall performance.Furthermore,analysis of the Schottky barrier parameters shows that the changes in electrical performance during aging are non-monotonic.These findings offer theoretical support for understanding the aging mechanisms and condition assessment of modern stable ZnO varistors.
文摘[Objectives]To assess the effects of endophytic bacteria on the growth,antioxidant responses,and the production of key secondary metabolites in Emilia prenanthoidea DC.[Methods]Three endophytic strains(H1,H2,L1)were inoculated onto tissue-cultured seedlings and cultivated for 20 d under greenhouse conditions.Growth traits,reactive oxygen species(ROS)indicators,antioxidant enzyme activities,and the content of chlorogenic acid and quercetin were analyzed using one-way ANOVA followed by Tukey s test.[Results]Bacterial inoculation significantly enhanced plant performance.Treatment H2 increased plant height by 27%,chlorophyll content by 73%,and fresh weight by 31%.Levels of ROS(O^(-)_(2),H_(2)O_(2))and MDA decreased markedly,whereas the activities of POD and CAT increased.Additionally,the content of chlorogenic acid and quercetin increased by up to 67%and 64%,respectively,with both H2 and L1 treatments showing the most pronounced effects.[Conclusions]Endophytic bacteria markedly improve growth,redox balance,and phenolic accumulation in E.prenanthoidea.Strain H2 represents a promising bioinoculant for improving the medicinal quality of this species.
文摘单级式双有源桥(dual active bridge,DAB)DC/AC变换器控制自由度多,变压器匝比、漏感、开关频率等参数相互耦合,导致变换器效率优化面临多重制约。该文分析变压器匝比、漏感与软开关范围、漏感电流有效值、漏感电流峰值之间的约束关系,提出直接表征DAB-DC/AC变换器效率的物理量:效率敏感因子。通过研究效率敏感因子对系统损耗的影响机理,实现变换器效率最优的硬件参数设计,为多参数耦合的单级式DAB-DC/AC变换器效率优化提供理论指导。最后通过一台450 W样机验证所提理论的正确性。
基金supported by the National Natural Science Foundation of China(No.52177071).
文摘High proportion of renewable energies and the installation of power electronic devices(PEDs)pose tough challenges to the operation of power systems.In this paper,the remote coordination adjustment(RCA)of PEDs in stochastic scenarios is studied.The steady-state model for the AC/DC system with PEDs is first established,and the alternate iteration method based on linearization(AIML)is adopted,especially for efficient deterministic power flow calculation.Then,the RCA is proposed using a modular local sensitivity method combined with AIML,which can adjust the electrical variables by diverse PEDs with high efficiency.Additionally,the probabilistic power flow calculation using the quasi-Monte Carlo method with the adaptive sampling number(ASN-QMC)is introduced to keep the balance between the computational efficiency and accuracy,as well as demonstrating the positive impact of RCA by the PEDs in stochastic scenarios.The effectiveness of the proposed RCA is validated by a series of modified IEEE test systems.
基金supported by the Science and Technology Project of China Southern Power Grid(ZDKJXM20200052).
文摘As an emerging trend in power grid development,the AC/DC hybrid power grid presents the characteristics of multi-type DC links and large-scale AC/DC interconnection.However,existing research on AC/DC power flow calculation is mainly based on single-object models,ignoring the complex interactions and interdependencies between the AC system and various types of DC links in the hybrid power grid,such as LCCHVDC,MMC-HVDC,and LCC-MMC hybrid HVDC.Therefore,this paper proposes a power flow calculation algorithm for large-scale AC/DC systems with multi-type DC links.Firstly,a CIM/XML document conversion strategy applicable to the AC/DC system is proposed.Then,a unified modeling method is used to derive the power flow model of the AC/DC system,and a unified iterative algorithm for the large-scale AC/DC system with multi-type DC links is proposed.Finally,the algorithm’s correctness and effectiveness are verified by comparing it with actual measurements from the Southern Power Grid of China,with a voltage deviation of less than 1%.The research shows that the proposed algorithm has good convergence and high computational efficiency,which is applicable to power flow calculations in large-scale DC-embedded hybrid AC/DC grids in the future.
文摘三相双有源桥双向AC/DC变换器具备高效双向变换和升降压运行的优势,适合应用于储能系统,但是该变换器在宽电压范围下运行时的参数设计与性能优化面临挑战。针对适用于该变换器在宽电压范围下实现零电压开关(zero voltage switching,ZVS)的混合调制策略,提出基于遗传算法的多目标参数优化方法;以开关管损耗和电感电流有效值作为优化目标,建立多目标优化模型,对电感值和变压器匝比进行求解;在实验室搭建一台1.5 kW的原理样机,实验结果表明,采用混合调制策略的变换器可在宽电压和宽功率范围下实现ZVS。此外,多目标优化参数设计方法能降低变换器的损耗,从而提升效率,为其性能提升提供了有效方案。
