Currently, most of the voltage sourced converter (VSC) based high voltage direct current (HVDC) projects are point to point cable connections. However the market for VSC based HVDC projects using overhead lines is expected to grow rapidly in the near future. Depending on the requirements for DC fault clearing and grid topology there are different protection concepts for these HVDC applications. The application of full bridge technology or the application of DC circuit breakers (DCCB) are the main approaches being discussed today for providing a fast DC fault clearing with auto-reclosure. The main focus in this study is on the protection of HVDC systems by means of DCCBs and the challenges being faced with this approach. The first part of the study focuses on the description of hybrid DCCBs for the application with half bridge converter technology. The second and main part of the study presents the factors influencing the design of the DCCB: AC network, travelling waves, DC reactor and DC line inductance. The results from the given examples confirm the wide range of the energy to be absorbed (5.3 up to 50 MJ!) and the significant impact that these factors can have on the design of the DCCBs.
The large‐scale integration of remote renewable energy resources into existing AC grids is one of the main drivers for the construction of multi‐terminal direct current (MTDC) systems. Today, a few MTDC systems are already in execution stage or even in operation. However, with a growing number of extended MTDC systems, the probability of high‐voltage DC (HVDC) insulation faults is likely to increase and therefore, reliable solutions for handling of such disturbances are crucial in order to maintain high availability. Since most HVDC converters are realised by modular multilevel converters (MMC) converters are based on half‐bridge (HB) modules, they lack an inherent capability to control DC fault currents. Therefore, additional equipment like HVDC breakers needs to be installed as well. Among the various breaker types, the so‐called ‘hybrid’ DC circuit breaker (DCCB) is considered most popular. This contribution provides a more holistic view on the overall system behaviour, represented AC grid, MMC HVDC converter, DC reactor and DCCB altogether. It discusses the physical interactions of these main components in MTDC systems. In particular, the influence of the electrical parameters of the hybrid DCCB and its required DC reactors on the MMC converter's internal current and voltage conditions is outlined.
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