IEEE 15th International Symposium on Power Electronics for Distributed Generation Systems (PEDG 2024)
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In recent years, high voltage DC (HVDC) systems have increasingly been utilized to connect offshore wind power plants to an onshore AC system. When connecting multiple offshore wind power plants to onshore AC systems, HVDC systems form a network, known as an HVDC grid. This presentation focuses on protection technologies for HVDC grids from DC faults. There are some existing methods to protect HVDC grids from DC faults: selective fault clearing using DC circuit breakers (DCCB) and non-selective fault clearing using AC circuit breakers.
In the selective fault-clearing method, rapid fault detection and current interruption within a few milliseconds are necessary to prevent the spread of faults to healthy sections. To achieve high speed, development is underway for protection relays capable of single-ended fault detection and hybrid DCCBs that combine mechanical contacts with semiconductor current interrupting units. However, these differ in operational principles and specifications from conventional equipment, lacking standardization, and thus making off-the-shelf products not readily available.
In the nonselective fault-clearing method, the fault current is interrupted by existing AC circuit breakers. Then, the faulted section is isolated by DC disconnectors. This method requires several hundred milliseconds, and power transmission halts during the period. Although fast equipment is unnecessary, the trip and restart of wind power plants are inevitable. Once the wind power plants trip, they take several minutes to restart, impacting the transient stability and frequency of the AC system.
As stated above, existing HVDC grids face challenges in simultaneously achieving continuous operation of wind power plants and a conventional relatively low-speed fault protection scheme. The research group of Dr. Sano aims to study the realization of an HVDC system that enables the continuous operation of wind power plants while using the controllability of existing modular multilevel converter (MMC) along with reliable double-ended protection relays and mechanical DCCBs.
In the presentation, Dr. Sano introduces a double-circuit HVDC system consisting of a backbone and branches. DC circuit breakers are placed at both ends of each circuit (transmission line), and a pair of the circuits forms a reliable backbone. The branches of offshore wind power plants are connected to the backbone through disconnectors. To construct this system using existing equipment, such as MMCs, mechanical circuit breakers, and disconnectors, several improvements are implemented, as listed below. Their combination enables the clearance of line faults with a delay of 50 milliseconds by the DCCB and resumption of transmission within 100 milliseconds. As a result, all offshore wind power plants can continue operation.
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