Rapid and large variation of photovoltaic (PV) power may incur frequency variation in a power system with high PV penetration. In such a case, much more reserve capacity of automatic generation control (AGC) for frequency regulation will be needed. In this paper, the energy storage station (ESS) is used to decrease the ramp rate of the total PV power and consequently decrease the AGC reserve capacity requirement of the power grid. However, as an expensive option, the ESS capacity should be optimally determined considering the balance between ESS cost and the equivalent benefit from the reduction of AGC payment. Therefore, an optimal sizing scheme for ESS in a power system with high PV penetration is proposed to strike a balance between the ESS cost and the reduction of the AGC payment. Firstly, a control scheme of ESS is proposed for smoothing PV power variation, during which the ramp rate of PV power can be finitely controlled with a size-limited ESS. Afterwards, an object function aiming to achieve the tradeoff between the investment cost of ESS and the reduction of AGC payment for frequency regulation is presented. Finally, case studies are conducted with actual PV power data to verify the effectiveness of the presented sizing scheme.

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Subgrade and foundation soils undergo freeze and thaw cycles in seasonally frozen regions that leads to changes in its mechanical properties, causing the degradation of its service performance. This study aims to investigate the effects of freeze-thaw cycles on the dynamic mechanical properties of compacted silty clay. Specifically, the effects of number of freeze-thaw cycle, confining pressure, initial compaction degree, and initial water content on the dynamic shear modulus and damping ratio were studied by using resonant column tests. The test results showed that both maximum dynamic shear modulus and maximum damping ratio of the compacted silty clay sample had a decreasing trend with the increasing number of freeze-thaw cycles. In addition, the maximum dynamic shear modulus was significantly increased with the initial compaction degree and it was decreased with the initial water content, while the maximum damping ratio was decreased with the initial compaction degree and it was increased with the initial water content. Finally, the empirical models of maximum dynamic shear modulus and maximum damping ratio were proposed to consider the varied parameters.@en

A novel terahertz modulator based on graphene is proposed and designed. The device consists of a silicon ridge covered by a graphene sheet. The transmission properties of the proposed structure demonstrate that the introduction of graphene can improve the switching and filtering performance of modulators, and enhance its field confinement capability. In the case of the number of layers, a blue shift can be observed in the center wavelength with increase of graphene layers. Meanwhile, the relationship between the number of layers and the peak of reflection spectra is an inverse proportion function. In addition, we find that the center wavelength is almost unchanged with respect to the different chemical potential, thus the effective refractive indices of the cross section of light propagation direction can be well preserved. These findings will contribute to the research and development of graphene based THz waveguide modulators.

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