Line fault protection unit for an electrical power system

Apparatus for monitoring current flow in a power supply line of a three phase electrical power system and for controlling circuit breakers to provided protection for the system, the system comprising:

• a ring bus assembly including a first bus and an auxiliary bus;
• a plurality of three phase power supply lines each connected to the first bus and to the auxiliary bus for communicate power current between the respective line and the ring bus assembly;
• a plurality of first circuit breakers each connected between a respective one of the lines and the first bus for disconnecting flow of current therebetween;
• a plurality of second circuit breakers each connected between a respective one of the lines and the auxiliary bus for disconnecting flow of current therebetween;
• a first plurality of current sensors each responsive to the three phase current flow between the first bus and a respective one of the lines to provide three outputs indicative of the A-phase, B-phase and C-phase respectively of the three phase current flow;
• a second plurality of current sensors each responsive to the three phase current flow between the auxiliary bus and a respective one of the lines to provide three outputs indicative of the A-phase, B-phase and C-phase respectively of the three phase current flow;
• and a plurality of voltage sensors each responsive to the voltage on a respective one of the lines to provide a voltage output indicative thereof;
• the apparatus comprising a monitoring and control unit associated with a respective one of the lines having:
• six current input terminals each for receiving a respective one of the three outputs of the first current sensor and the three outputs of the second current sensor associated with said respective one of the lines;
• a voltage input terminal for receiving the voltage output of the voltage sensor associated with said respective one of the lines;
• detector means in the unit responsive to the outputs from the terminals for generating data indicative of the instantaneous values of the voltage and of the six currents defined by the A-phase, B-phase and C-phase currents of the first sensor and the A-phase, B-phase and C-phase currents of the second sensor;
• summation means in the unit for summing the two A-phases to define an A-phase summed value, the two B- phases to define an B-phase summed value and the two C-phases to define an C-phase summed value;
• calculation means in the unit for effecting pre-determined algorithmic calculations on the voltage, the A-phase summed value, on the B-phase summed value and on the C-phase summed value to determine a fault condition;
• a trip output generator for generating a trip output in response to said fault condition for tripping both the circuit breakers associated with said respective line;
• recording means for recording data relating to said six current values in the event of a fault condition;
• said calculation means being arranged, in the event of a trip output, to detect current values defined by the A-phase, B-phase and C-phase currents of the first sensor which are greater than zero and to provide a first breaker fail output for tripping all the first circuit breakers;
• said calculation means being arranged, in the event of a trip output, to detect current values defined by the A-phase, B-phase and C-phase currents of the second sensor which are greater than zero and to provide a second breaker fail output for tripping all the second circuit breakers.