Patent ID: 8749933

Claim:
A high voltage direct current, HVDC, fault protection system comprising: an HVDC unit comprising: a direct current, DC, bus, means for connecting at least one HVDC transmission line to the bus, and at least one DC circuit breaker being arranged for disconnecting the transmission line from the bus upon reception of a trip signal, and a protection unit comprising: a current sensor being arranged for measuring a current I(t) through the transmission line, a voltage sensor being arranged for measuring a voltage U(t) between the transmission line and ground, an inductor or inductors being arranged such that the current through the transmission line passes the inductor or inductors, and a fault detection unit arranged for: assessing whether a fault exists on the transmission line by deriving at least one fault measure from the voltage and/or current and comparing the fault measure with a predetermined value, and sending, if a fault is detected on the transmission line, a trip signal to the circuit breaker, and wherein the fault detection unit is further arranged for assessing whether a fault exists on the transmission line by: evaluating S=[U(t 2 )−U(t 1 )]×[I(t 2 )−I(t 1 )], where t 1 and t 2 are consecutive sampling times, and deciding that a fault exists on the transmission line if S<−S 0 , where S 0 is a predetermined positive value.