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We herein report the synthesis of bis-(5,10-diphenyltripyrrin)nickel(II), which is the first meso-substituted tripyrrin metal complex. The use of tripyrrane as the starting material provides a new synthetic route to tripyrrins. The functionalization on the meso positions endows this type of compound a brighter perspective in synthetic chemistry. Meanwhile, the fluorescent property of the compound has been detected, indicating that this type of compound a strong candidate for fluorescent materials.

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The symmetrical bipolar configuration shows a great advantage in modular multilevel converter (MMC) based high-voltage dc-current (HVDC) transmission. In a bipolar MMC-HVDC system, station internal ac grounding faults cause sub-module (SM) capacitors to discharge violently. These faults do great harm to MMCs and require special protection schemes. In this paper, a single-phase-to-ground fault is taken as an example to investigate transient characteristics of station internal ac grounding faults in a bipolar MMC-HVDC system. A calculation theory of fault current based on the improving RLC model and second-order differential equation with variable coefficients is introduced. A novel transient zero-mode current based protection criterion with better reliability, quickness, and ability to endure fault resistances is proposed to detect internal ac grounding faults. The fault clearing strategy applicable to three types of grounding faults is also designed. The correctness of the proposed calculation theory and protection schemes is verified through simulations performed in PSCAD/EMTDC. INDEX TERMS MMC-HVDC, station internal ac grounding fault, second-order differential equation with variable coefficients, transient zero-mode current, protection scheme.

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