The sub-module of the modular multilevel converter (MMC) in practical engineering mainly adopt half-bridge sub-module (HBSM), which does not have the capability to clear dc fault current. Based on the analysis of the dc fault current path of traditional HBSM, this paper proposes a novel MMC sub-module with dc fault clearance capability. It adds a bidirectional switch and two diodes to the HBSM. When the dc fault occurs, the bypass absorption circuit is introduced into the arm through the diodes and the freewheeling effect of diodes is eliminated by the sub-module capacitor voltage. Thereby, the fault current can be extinguished rapidly and the requirement for the consistency of the trigger pulse is reduced. By comparing with the operation principle of the HBSM, it can quickly block the dc fault current without changing the original control and modulation strategy. Then, the fault clearance principle of the sub-module and the voltage stress of the key power devices are analyzed. The simulation results in MATLAB/SIMLINK have verified the validity and feasibility of the proposed sub-module topology in the dc fault current clearance. INDEX TERMS DC fault, fault clearance, HVDC transmission, modular multilevel converter (MMC), sub-module topology.
In order to solve the contradiction between energy supply and carbon emissions, using P2G to combine oxy-fuel combustion power plant with gas turbines. According to the laws of mass conservation and energy conservation, carbon and oxygen elements are introduced into the model in the form of gases to construct an energy hub model for the carbon oxygen cycle system. To solve the problem of time imbalance between interconnected devices, a flexible operation mode using storage devices as connection hubs is proposed. Based on the constraints of the system’s operating mechanism, study the variation range of the operating range and establish an objective function with the optimal operating cost. Optimize the operation of the combined system, study the change interval of its operation range, and analyze the scheduling model under three different operation models. Low-Carbon economic model of combined system is established in regional integrated energy system. The results show that the basic carbon oxygen cycle model can save 9.14% economic cost and reduce 44.05% carbon emission. The carbon oxygen cycle capacity increasing mode can save 13.05% economic cost and reduce 59.91% carbon emission. Carbon and oxygen circulation system in improving renewable energy consumption and reducing carbon dioxide emissions is verified by an example.
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