Development of a protection strategy for future DC networks based on low-speed DC circuit breakers
Development of a protection strategy
for future DC networks based on low-speed DC circuit breakers
Multi-terminal high voltage direct current (MTDC) grids are considered to be an interesting solution to integrate bulk renewable energies located far away the consumption areas. One of the most challenging issues for the development of such MTDC grids remains the protection against DC faults. Although several types of HVDC breaker technologies have already been proposed and demonstrators tested by manufacturers, the coordination of such devices to ensure a proper protection system has not yet been proved. Difficulties encountered during the development of such DC grid protection system address, among others, the reliability of the protection scheme and its ability to ensure the AC and DC systems stabilities during and after the fault clearance. Therefore not only a reliable primary protection scheme, but also a robust backup scheme, have to be developed. This paper presents a novel protection scheme based on converter breaker; its key element, namely a low-speed mechanical DC breaker, is located at each DC converter output. This strategy belongs to the non-selective fault clearing philosophy whose protection priority is given to the fault current clearance by opening the converter circuit breakers without any discrimination. The selectivity of the protection is thereafter ensured by the opening of mechanical DC breakers located at each end of faulty line. To validate the protection strategy, primary protection scheme as well as back-up solutions in case of main component failures have been considered. Protection schemes are validated through detailed EMT simulations for a 3-terminals DC grid in bipolar configuration and with cable links. Technical requirements of the main components such as DC circuit breakers are highlighted in the paper. Moreover, an optimized pre-insertion resistor for a fast grid voltage restoring after faulty line isolation is also described. To demonstrate the adequacy of the proposed protection strategy a particular emphasis is given to the impact of a DC fault on the AC transient stability. Benchmark cases of a future MTDC grid with high renewable energy source penetration will be presented to identify advantages and limits of such protection scheme.