Recent demand of renewable energy generation has placed new requirements on the flexibility of transmission systems. These requirements together with technical advancements of high-voltage direct-current (HVDC) technology have resulted in the ambition to build large-scale multi-terminal DC grids (MTDC). These large-scale MTDC grids are expected to require fast DC breakers which allow for selective fault clearing. Selective fault clearing strategies avoid decreased availability during faults because only the faulted part can be isolated whereas the healthy part continues operation. For successful operation of a MTDC grid with multiple breakers, a protection system with selective fault detection is thus required such that the DC breakers achieve correct fault isolation in the event of a fault. Selective fault detection in MTDC grids is however not trivial because of low impedance characteristics and short required isolation times. This paper summarizes previously suggested selective fault detection methods for protection of a MTDC transmission system using DC breakers.

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Integrating multiterminal high-voltage direct-current (HVDC) connections into the existing alternating current (AC) power transmission system is a possible solution for transport of large amounts of renewable energy. Protection is considered a key enabler for multiterminal HVDC (MTDC) grids. Designing such a protection system is a challenge, in particular in a multivendor setup. Fault detection during transients might be required to achieve the stringent speed requirements for HVDC protection. This in turn requires knowledge about the expected system behaviour during faults and involves knowledge from all vendors supplying equipment in MTDC grid. Traditionally, HVDC projects are, however, supplied as turn-key solutions and the control and protection systems are the intellectual property of the vendors and not open for the HVDC endusers. This paper aims to provide a starting point on the discussion which information has to be shared between vendors and HVDC end-users when designing MTDC grid protection. Simulations results show that missing information about certain aspects can lead to a failure of the protection system. A discussion on organization of the available information is added. Open specifications of the used equipment are useful to design safe and reliable MTDC grid protection.

Future multiterminal high-voltage direct-current (HVDC) grids are considered an enabling technology to efficiently integrate large amounts of renewable energy into the existing grid. However, already in today's existing point-to-point HVDC links, harmonic interaction issues and instabilities related to the control and protection system of the converters have been reported. The converter control software is usually black-boxed and problems are therefore solved in close cooperation with the HVDC vendor. This paper aims to provide a starting point for a discussion on an open-source HVDC control system. In particular, it covers the control design including technical and non-technical aspects. The open-source approach can be useful to solve current as well as future control-related problems, both in point-to-point links as well as in multiterminal and multivendor HVDC grids.