光伏直流升压接入系统在交流电网发生故障时应该具备故障穿越(fault ride through,FRT)的能力,然而交流电网中较大过渡电阻故障被清除后,直流变压器(DC transformer,DCT)控制策略难以自动恢复至最大功率点跟踪(maximum power point trac...光伏直流升压接入系统在交流电网发生故障时应该具备故障穿越(fault ride through,FRT)的能力,然而交流电网中较大过渡电阻故障被清除后,直流变压器(DC transformer,DCT)控制策略难以自动恢复至最大功率点跟踪(maximum power point tracking,MPPT)控制。提出基于模块化多电平换流器(modular multilevel converter,MMC)二次谐波注入的故障清除协调控制策略。故障清除时,通过修正逆变器控制内环负序分量,主动向系统注入短窗长二次谐波。DCT根据直流母线二次谐波电压幅值切换控制策略,实现光伏发电单元在故障清除后快速切换回MPPT控制。PSCAD仿真结果表明,通过所提方法,在电网故障时,光伏侧均能可靠且快速的切换控制策略,有效提高光伏资源的利用率。展开更多
We investigate two-photon transitions to the electron-hole scattering continuum in monolayer transition-metal dichalcogenides, and identify two contributions to this nonlinear optical process with opposite circularly ...We investigate two-photon transitions to the electron-hole scattering continuum in monolayer transition-metal dichalcogenides, and identify two contributions to this nonlinear optical process with opposite circularly polarized valley selection rules. In the non-interacting limit, the competition between the two contributions leads to a crossover of the selection rule with the increase of the two-photon energy. With the strong Coulomb interaction between the electron and hole, the two contributions excite electron-hole scattering states in orthogonal angular momentum channels, while the strength of the transition can be substantially enhanced by the interaction. Based on this picture of the two-photon transition, the second harmonic generation(SHG) in the electron-hole continuum is analyzed, where the Coulomb interaction is shown to greatly alter the relative strength of different cross-circular polarized SHG processes. Valley current injection by the quantum interference of one-photon and two-photon transition is also investigated in the presence of the strong Coulomb interaction, which significantly enhances the injection rate.展开更多
文摘光伏直流升压接入系统在交流电网发生故障时应该具备故障穿越(fault ride through,FRT)的能力,然而交流电网中较大过渡电阻故障被清除后,直流变压器(DC transformer,DCT)控制策略难以自动恢复至最大功率点跟踪(maximum power point tracking,MPPT)控制。提出基于模块化多电平换流器(modular multilevel converter,MMC)二次谐波注入的故障清除协调控制策略。故障清除时,通过修正逆变器控制内环负序分量,主动向系统注入短窗长二次谐波。DCT根据直流母线二次谐波电压幅值切换控制策略,实现光伏发电单元在故障清除后快速切换回MPPT控制。PSCAD仿真结果表明,通过所提方法,在电网故障时,光伏侧均能可靠且快速的切换控制策略,有效提高光伏资源的利用率。
基金supported by the Croucher Foundation(Croucher Innovation Award)the Research Grants Council(HKU17305914P,C7036-17W)
文摘We investigate two-photon transitions to the electron-hole scattering continuum in monolayer transition-metal dichalcogenides, and identify two contributions to this nonlinear optical process with opposite circularly polarized valley selection rules. In the non-interacting limit, the competition between the two contributions leads to a crossover of the selection rule with the increase of the two-photon energy. With the strong Coulomb interaction between the electron and hole, the two contributions excite electron-hole scattering states in orthogonal angular momentum channels, while the strength of the transition can be substantially enhanced by the interaction. Based on this picture of the two-photon transition, the second harmonic generation(SHG) in the electron-hole continuum is analyzed, where the Coulomb interaction is shown to greatly alter the relative strength of different cross-circular polarized SHG processes. Valley current injection by the quantum interference of one-photon and two-photon transition is also investigated in the presence of the strong Coulomb interaction, which significantly enhances the injection rate.
