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DC transmission and distribution systems are more efficient when compared with traditional AC
transmission system, and therefore is gaining significantly market share in todayelectric power grid.
However, protection of the fault in the DC system is harder than AC system. In AC system, the
current has a zero crossing, allowing many protection devices be developed which interrupt the
current at the zero crossing. For DC system, there is no zero crossing. Moreover, since DC capacitors
are used in the DC system, it stores huge amount of energy, equal to 0.5*C*VDC^2, where VDC is
the DC bus voltage. For HVDC system, VDC is above 200kV. So large amount energy will be
discharged when a fault happens. This invention provides normal protections again DC system fault,
and will enable even wider adoption of the DC system.