Artículo de publicación ISIThe increasing interest in integrating intermittent
renewable energy sources into microgrids presents major challenges
from the viewpoints of reliable operation and control. In
this paper, the major issues and challenges in microgrid control
are discussed, and a review of state-of-the-art control strategies
and trends is presented; a general overview of the main
control principles (e.g., droop control, model predictive control,
multi-agent systems) is also included. The paper classifies microgrid
control strategies into three levels: primary, secondary, and
tertiary, where primary and secondary levels are associated with
the operation of the microgrid itself, and tertiary level pertains
to the coordinated operation of the microgrid and the host grid.
Each control level is discussed in detail in view of the relevant
existing technical literature
The modular multilevel converter (MMC) has become the most promising converter technology for high-voltage direct current (HVDC) transmission systems. However, similar to any other voltage-sourced converter-based HVDC system, MMC-HVDC systems with the half-bridge submodules (SMs) lack the capability of handling dc-side short-circuit faults, which are of severe concern for overhead transmission lines. In this paper, two new SM circuit configurations as well as a hybrid design methodology to embed the dc-fault-handling capability in the MMC-HVDC systems are proposed. By combining the features of various SM configurations, the dc-fault current path through the freewheeling diodes is eliminated and the dc-fault current is enforced to zero. Several MMC configurations based on the proposed hybrid design method and various SM circuits, that is, the half-bridge, the full-bridge, the clamp-double, and the five-level cross-connected SMs, as well as the newly proposed unipolar-voltage full-bridge and three-level cross-connected SMs, are investigated and compared in terms of the dc-fault-handing capability, semiconductor power losses, and component requirements. The studies are carried out based on time-domain simulation in the PSCAD/EMTDC software environment for various SM configurations and dc-fault conditions. The reported study results demonstrate the proposed hybrid-designed MMC-HVDC system based on the combination of the half-bridge and the proposed SM circuits is the optimal design among all evaluated systems in terms of the dc-fault-handing capability, semiconductor power losses, and component requirements.Index Terms-DC-side short-circuit fault, fault clearance, modular multilevel converter (MMC).
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