Abstract: A DC grid (100) comprising a plurality of AC/DC converters (105) which are interconnected via DC lines (115) is provided, wherein, in order to limit the effects of a fault in the DC grid, the DC grid is divided into at least two zones (200) by means of at least one current limiter (205) in a manner so that a current limiter is connected in each of the DC line(s) (115z) by which two zones are interconnected. A method of limiting the effects of a fault in a DC grid by dividing the DC grid into at least two zones is also provided.

Abstract: A breaker failure protection system for a high voltage direct current, HVDC, circuit breaker is provided. The circuit breaker is arranged for interrupting a DC circuit upon reception of a trip signal. The protection system includes a current sensor, at least one inductor, and a breaker failure detection unit. The current sensor is arranged for measuring a current I(t) through the DC circuit. The at least one inductor is connected in series with the DC circuit. The breaker failure detection unit is arranged for assessing, whether the circuit breaker has failed, and sending, if the circuit breaker has failed, a trip signal to an adjacent circuit breaker. The assessment is based on the measured current. The stability of HVDC grids may be improved by sending, in case of a breaker failure, a trip signal to adjacent circuit breakers. Further, a method of breaker failure protection for an HVDC circuit breaker is provided.

Abstract: A current-limitation system for limiting a current through an DC connection in case of a fault occurring in a DC grid of which the DC connection forms a part is provided, as well as a method of operating a current-limitation system for limiting a current through an DC connection in case of a fault occurring in a DC grid of which the DC connection forms a part.

Abstract: The present invention relates to an HVDC switchyard arranged to interconnect a first part of a DC grid with a second part of the DC grid. By means of the invention, a first part of the DC grid is connected to the busbars of the HVDC switchyard via a fast DC breaker, while further part(s) of the DC grid are connected to the busbars of the HVDC switchyard by means of switchyard DC breakers of lower breaking speed. By use of the inventive HVDC switchyard arrangement, the cost for the HVDC switchyard can be considerably reduced, while adequate protection can be provided. In one embodiment, the fast DC breaker is an HVDC station DC breaker forming part of an HVDC station. In another embodiment, the fast DC breaker is a switchyard DC breaker.

Abstract: A protection system for high voltage direct current, HVDC, transmission lines is provided. The protection system includes a direct current, DC, bus, a device configured to connect an HVDC transmission line to the bus, at least one DC circuit breaker arranged for disconnecting the transmission line from the bus upon reception of a trip signal, current and voltage sensors, an inductor arranged such that the current through the transmission line passes the inductor, and a fault detection unit. The fault detection unit is arranged for assessing, on the basis of current and/or voltage measurements, whether a fault exists on the transmission line, and sending, if a fault is detected on the transmission line, a trip signal to the circuit breaker. The additional inductance limits the rise in fault current and facilitates a selective fault detection. Further, a method of fault protection for an HVDC transmission line is provided.

Abstract: A method for controlling power flow within a multi-terminal DC power transmission system including two or more converter stations and the corresponding multi-terminal DC power transmission system are provided. The method includes the steps of: controlling the power flow to a steady state reference operating point for operating points within a control dead band defined for each respective converter station, and controlling the power flow by means of droop control in at least one of the converter stations upon detection of exceeding of an end point of one or more of the control dead bands.

Abstract: A current-limitation system for limiting a current through an DC connection in case of a fault occurring in a DC grid of which the DC connection forms a part is provided, as well as a method of operating a current-limitation system for limiting a current through an DC connection in case of a fault occurring in a DC grid of which the DC connection forms a part.

Abstract: A DC grid (100) comprising a plurality of AC/DC converters (105) which are interconnected via DC lines (115) is provided, wherein, in order to limit the effects of a fault in the DC grid, the DC grid is divided into at least two zones (200) by means of at least one current limiter (205) in a manner so that a current limiter is connected in each of the DC line(s) (115z) by which two zones are interconnected. A method of limiting the effects of a fault in a DC grid by dividing the DC grid into at least two zones is also provided.

Abstract: A method for controlling power flow within a multi-terminal DC power transmission system including two or more converter stations and the corresponding multi-terminal DC power transmission system are provided. The method includes the steps of: controlling the power flow to a steady state reference operating point for operating points within a control dead band defined for each respective converter station, and controlling the power flow by means of droop control in at least one of the converter stations upon detection of exceeding of an end point of one or more of the control dead bands.

Abstract: A method for controlling power flow within a DC power flow within a DC power transmission network includes two or more interconnected converter stations. The method includes the steps of: establishing a common feedback signal (Ud,common, ?d,common) to the converter stations, the common feedback signal (Ud,common, ?d,common) being based on an overall voltage level in the DC transmission, power network; providing, in the converter stations, a control signal (S1, S2, S3, S4) based on the common feedback signal (Ud,common) and a drooped error signal (Pe,droop, Ue,droop); and controlling the power flow within the DC power transmission network towards set operating points (Udref, PCCref) by using the control signal (S1, S2, S3, S4). A control device and computer program product is also described.

Abstract: A method for controlling power flow within a DC power flow within a DC power transmission network includes two or more interconnected converter stations. The method includes the steps of: establishing a common feedback signal (Ud,common, ?d,common) to the converter stations, the common feedback signal (Ud,common, ?d,common) being based on an overall voltage level in the DC transmission, power network; providing, in the converter stations, a control signal (S1, S2, S3, S4) based on the common feedback signal (Ud,common) and a drooped error signal (Pe, droop, Ue, droop); and controlling the power flow within the DC power transmission network towards set operating points (Udref, PCCref) by using the control signal (S1, S2, S3, S4). A control device and computer program product is also described.

Abstract: The present invention relates to an HVDC switchyard arranged to interconnect a first part of a DC grid with a second part of the DC grid. By means of the invention, a first part of the DC grid is connected to the busbars of the HVDC switchyard via a fast DC breaker, while further part(s) of the DC grid are connected to the busbars of the HVDC switchyard by means of switchyard DC breakers of lower breaking speed. By use of the inventive HVDC switchyard arrangement, the cost for the HVDC switchyard can be considerably reduced, while adequate protection can be provided. In one embodiment, the fast DC breaker is an HVDC station DC breaker forming part of an HVDC station. In another embodiment, the fast DC breaker is a switchyard DC breaker.

Abstract: A protection system for high voltage direct current, HVDC, transmission lines is provided. The protection system includes a direct current, DC, bus, a device configured to connect an HVDC transmission line to the bus, at least one DC circuit breaker arranged for disconnecting the transmission line from the bus upon reception of a trip signal, current and voltage sensors, an inductor arranged such that the current through the transmission line passes the inductor, and a fault detection unit. The fault detection unit is arranged for assessing, on the basis of current and/or voltage measurements, whether a fault exists on the transmission line, and sending, if a fault is detected on the transmission line, a trip signal to the circuit breaker. The additional inductance limits the rise in fault current and facilitates a selective fault detection. Further, a method of fault protection for an HVDC transmission line is provided.

Abstract: A breaker failure protection system for a high voltage direct current, HVDC, circuit breaker is provided. The circuit breaker is arranged for interrupting a DC circuit upon reception of a trip signal. The protection system includes a current sensor, at least one inductor, and a breaker failure detection unit. The current sensor is arranged for measuring a current I(t) through the DC circuit. The at least one inductor is connected in series with the DC circuit. The breaker failure detection unit is arranged for assessing, whether the circuit breaker has failed, and sending, if the circuit breaker has failed, a trip signal to an adjacent circuit breaker. The assessment is based on the measured current. The stability of HVDC grids may be improved by sending, in case of a breaker failure, a trip signal to adjacent circuit breakers. Further, a method of breaker failure protection for an HVDC circuit breaker is provided.