Fault location in DC networks

A current limiting apparatus for a DC power transmission network comprising:

• a current limiting element that can be selectively connected and disconnected to the network, wherein, in a current limiting mode of operation, the current limiting element is selectively connected to the network to limit a current through the apparatus;
• a controller for controlling the current limiting element, the controller being configured to:
• in the event of a fault, activate the current limiting mode of operation so as to selectively connect the current limiting element to the network and limit a fault current through the current limiting apparatus to less than a first current level;
• maintain the fault current at a non-zero level;
• and
• selectively connect and disconnect the current limiting element to apply a predetermined modulation to the fault current.

A method of fault location in a DC power transmission network comprising:

• in the event of a fault, controlling a plurality of current limiting elements at first different parts of the network to:
• limit a fault current flowing through the network to less than a first current level, wherein the first current level is within an expected current operating range of the network in normal operation;
• maintain the fault current flowing through the network at a non-zero level;
• and
• applying a plurality of modulations to the fault current at second different parts of the network, wherein each of the plurality of modulations has a different characteristic frequency of a plurality of characteristic frequencies and each of the plurality of characteristic frequencies is associated with an associated one of the second different parts of the network;
• determining a plurality of fault currents, wherein each of the plurality of fault currents is the fault current at an associated one of third different parts of the network;
• and
• determining a location of the fault based on characteristic frequencies found in the plurality of fault currents.