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The fractional frequency offshore wind power system (FFOWPS) is a core technology for the long‐distance transmission and integration of the large‐scale offshore wind power. However, the risk of wide‐band oscillation in the FFOWPS has become increasingly prominent due to the inevitable presence of the power electronic devices. The traditional admittance of the frequency converter, which is a crucial component in the FFOWPS, may contain right‐half‐plane (RHP) poles when operating under various control modes. To address this deficiency, this paper establishes a four‐port small‐signal admittance model of a back‐to‐back frequency converter and demonstrates its interconvertibility with the traditional model. On this basis, the wide‐band oscillation analysis method for the FFOWPS is proposed, which explicitly describes the influence of the industrial frequency system, the fractional frequency system, and their coupling interactions on system stability. Simulation results in MATLAB/Simulink verify the proposed stability analysis method, highlight stability misjudgements associated with the traditional admittance, and analyse the impact of the coupling terms on system stability. [ABSTRACT FROM AUTHOR]

By using the flexural strength and compressive strength, the effects of metakaolin contents and water-binder ratio on the mechanical properties of cement mortar were evaluated in this study. The results showed that under the same water-binder ratio, with 5%, 10%, and 15% metakaolin, 3 d and 7 d compressive strength and flexural strength of cement mortar did not significantly change, but the 28 d compressive strength and flexural strength were greatly improved. Under the same amount of metakaolin, 28 d compressive strength and flexural strength of the cement mortar showed a decreasing trend with the increase of water-binder ratio. The optimum ratio of water to binder ratio was determined to be 0. 3. [ABSTRACT FROM AUTHOR]

In order to study the freeze-thaw performance of ultra-high strength concrete with different mineral admixtures at water cement ratios of 0.21, 0.24 and 0.27, the mass loss rate and the relative dynamic elastic modulus were evaluated, which were combined with the pozzolanic effect. The results showed that the frost resistance of ultra-high strength concrete was closely related to the ratio of mineral admixtures and water binder ratio and there was an optimal dosage. Meanwhile, under the same water to binder ratio, the mass loss rate of the ultra-high strength concrete prepared by ternary complex-admixture with 20% fines, 20% fly ash and 10% silica fume was the smallest, and the relative dynamic elastic modulus was the highest. Therefore, there was an optimal dosage. When the mineral admixture was the same for 0.21 water binder ratio, the mass loss rate was the smallest and the relative dynamic elastic modulus was the highest. Under this condition, it has the highest frost resistance. [ABSTRACT FROM AUTHOR]

The characteristics of isolated grids lead island power systems to rely on stable conventional generation, which is not environmentally friendly and has high carbon emissions. This paper proposes a low-carbon transition model for island power systems considering large-scale offshore wind resources and the long-term uncertainty of carbon prices. First, a detailed model for offshore wind power systems is proposed considering the interaction of multiple wind farms and three transmission technologies. Second, a multi-stage power system planning model is established including the upgrading and decommissioning of thermal units. Meanwhile, an improved long-term carbon price uncertainty modeling method based on the binomial tree is proposed. Then, focusing on the uncertainty of carbon prices, a multi-stage stochastic optimization model is established, and the stochastic nested decomposition algorithm is used to solve the problem. Finally, numerical results show that the low-carbon transition develops plenty of offshore wind power as the primary power source and usually adopts a fractional frequency transmission system for offshore wind integration. The uncertainty of carbon prices has a significant impact on the transition: in the high carbon price realization, the annual carbon emissions are only 0.088 Mt, while in the low carbon price realization, the annual emissions are 0.255 Mt. • Low-carbon transition is modeled via multi-stage programming and solved by SND. • Improved binomial tree model for long-term carbon price uncertainty is proposed. • Detailed planning model of offshore wind systems is established. • Wake effect between wind farms are considered and linearized by wake factors. • Fractional frequency transmission system is adopted and analyzed. [ABSTRACT FROM AUTHOR]