DC grid and a method of limiting the effects of a fault in a DC grid

1. A DC grid comprising:

a plurality of AC/DC converters which are interconnected via DC lines, wherein

the DC grid further comprises at least one current limiter which is capable of breaking the current at at least the rated voltage;

the DC grid is divided into at least two zones by the at least one current limiter in a manner so that at least one of the at least one current limiter is connected in each of the DC line(s) by which two zones are interconnected; and

the at least one current limiter dividing the at least two zones is adapted for limiting, but not breaking, a fault current that flows into a fault in a zone from neighbouring zones during a fault-on period; and

in connection with a current limiter interconnecting a first and a second zone, a control system for controlling the current-limiting strength of the current limiter, the control system being operable to instruct, if a fault occurs in the first zone, the current limiter to limit the current flowing into the first zone from the second zone through the at least one current limiter, wherein

the control system comprises a current measuring device arranged to measure the current through the current limiter; and

the control system is operable to detect a fault, and if a fault has been detected, to adjust the current-limiting strength of the current limiter in a manner so that:

if the current through the breaker exceeds a first current threshold (I max ), the current-limiting strength is increased; and

if the current through the breaker falls below a second current threshold (I min ), the current-limiting strength is decreased.

2. The DC grid of claim 1 , wherein

the at least one current limiter comprises a series connection of independently controllable breaker sections, wherein a breaker section comprises a parallel connection of a non-linear resistor and a semi-conductor switch of turn-off type.

3. The DC grid of claim 1 , further comprising

a self-protective control system operable to generate a damage indication if the current limiter is at risk for thermal damage, and to generate, in response to such damage indication, a tripping signal instructing the current limiter, or a breaker protecting the current limiter, to break the current.

4. The DC grid of claim 1 , wherein

the control system further comprises a voltage measuring device arranged to measure the voltage on at least one side of the current limiter; and

the control system is operable to detect a fault by checking if the voltage on at least one side of the current limiter has fallen below a first voltage threshold (U low ).

5. The DC grid of claim 1 , wherein

the control system is operable to detect a fault by checking if the current through the current limiter has risen above the first current threshold.

6. The DC grid of claim 1 , wherein

at least one connection connecting an AC/DC converter to the DC grid is provided with a current-limitation mechanism.

7. The DC grid of claim 1 , wherein

at least one object in at least one zone is protected by an AC breaker connected to an AC/DC converter in said zone.

8. A DC grid comprising:

a plurality of AC/DC converters which are interconnected via DC lines, wherein

the DC grid further comprises at least one current limiter which is capable of breaking the current at at least the rated voltage;

the DC grid is divided into at least two zones by the at least one current limiter in a manner so that at least one of the at least one current limiter is connected in each of the DC line(s) by which two zones are interconnected;

the at least one current limiter dividing the at least two zones is adapted for limiting, but not breaking, a fault current that flows into a fault in a zone from neighbouring zones during a fault-on period;

the at least one current limiter comprises a series connection of independently controllable breaker sections, wherein each of the breaker sections comprises a parallel connection of a non-linear resistor and a semi-conductor switch of turn-off type; and

a control system for controlling the current-limiting strength of the current limiter, wherein

the control system is further operable to estimate the present energy-absorbing capacity of the non-linear resistors; and

the control system is operable to select which breaker section(s) should be opened or closed, if any, in dependence of the different energy absorbing capacities of the non-linear resistors.

9. A method of limiting the effects of a fault in a DC grid comprising a plurality of AC/DC converters which are interconnected via DC lines, the method comprising the steps of:

dividing the DC grid into at least two zones by series-connecting, in each of the DC line(s) by which two zones are interconnected, at least one current limiter capable of breaking the current at at least the rated voltage; and

limiting but not breaking, by the at least one current limiter, a fault current that flows into a fault in a zone from neighbouring zones during a fault-on period, wherein the DC grid comprises, in connection with the at least one current limiter dividing the at least two zones, a control-system for controlling the current-limiting strength of the current limiter;

detecting, in the control system, a fault; and

in response to detecting the fault, adjusting the current-limiting strength of the current limiter in a manner so that

if the current through the breaker exceeds a first current threshold (I max ), the current-limiting strength is increased; and

if the current through the breaker falls below a second current threshold (I min ), the current-limiting strength is decreased.

10. The method of claim 9 , wherein

at least the second current threshold lies below the rated current of the transmission.

11. The method of claim 9 , wherein

the second current threshold lies above the rated current of the transmission.

12. The method of claim 11 , wherein

the detecting of the fault is performed by checking whether the current through the current limiter exceeds the first current threshold.

13. The method of claim 9 , wherein

the detecting of the fault is performed by checking whether the voltage on at least one side of the current limiter has fallen below a first voltage threshold (U low ).

14. The method of claim 9 , wherein

the current-limiting strength in the step of adjusting if the current exceeds the first current threshold is set in dependence of the difference between the present current level and the first current threshold; and

the current-limiting strength in the step of adjusting if the current falls below the second threshold is set in dependence of the difference between the present current level and the second current threshold.

15. The method of claim 9 , wherein a current limiter comprises a number of individually controllable, series connected breaker sections, each comprising a parallel connection of a non-linear resistor and a semi-conductor switch of turn-off type, wherein

increasing the current-limiting strength comprises sending a blocking signal to at least one closed breaker section, if any breaker section is closed; and

decreasing the current-limiting strength comprises sending a firing signal to at least one open breaker section, if any breaker section is open.

16. The method of claim 15 , further comprising

estimating the present energy-absorbing capacity of the non-linear resistors; and

selecting which breaker section(s) should be open or closed, if any, in dependence of the different energy absorbing capacities of the non-linear resistors.

17. The method of claim 9 , wherein

the current limiter comprises a transition switch connected in parallel with a main switch;

said step of adjusting the current-limiting strength is conditional on the transitional switch being open; and

the method further comprises:

opening the transition switch, thus commutating the current to the main switch, if:

the current through the current limiter exceeds a third current threshold (I arm ), wherein the third current threshold is above the expected current level during normal operation, or

the voltage at a side of the current limiter goes below a second voltage threshold, wherein the second voltage threshold is below the expected voltage level during normal operation.

18. The method of claim 9 , further comprising

generating a damage indication in dependence of estimations of the present energy-absorbing capacity of the current limiter indicating that the current limiter is at thermal risk; and

tripping the current limiter to break the current in response to said damage indication.