An alternative protection strategy for multi-terminal HVDC

Development of multi-terminal HVDC has been held back by the lack of adequate dc breakers. This has led to a number of novel breaker designs being developed by manufacturers. However, perceived operating time constraints have resulted in complex solutions with associated cost and size penalties. For this reason system operators have yet take up any of the proposed designs for use within real systems. To realise any multi-terminal system the cost of the dc breakers must, therefore, be reduced. Given that the complexity of the breakers is driven by the requirements of their speed it is prudent to question why there is this requirement. A relaxation of the operational speed will enable simpler, cheaper solutions to appear, in turn assisting the economic case of multi-terminal systems. This paper re-evaluates the requirements for dc circuit breakers with an emphasis on the impact of dc faults on converter stations and ac connections. It is shown that ultra-fast dc breakers are not necessary in order to have adequate performance from the ac grid’s point of view. The converter is also shown to survive for the required period of time. This allows slower, less complex dc breakers to be used, which could lead to a swifter uptake within commercial systems and eventually multi-terminal system