Patent GB-752438-A (GB)
Title: Improvements in or relating to the determination of the distance of faults in electric cables and conductors
IPC: G01R31/08
Filed: 1954-10-22
Abstract: 752,438. Measuring fault currents and voltages and distance to faults in electric cables and conductors. SIEMENS-SCHUCKERTWERKE AKT.-GES. Oct. 22, 1954 [Oct. 22, 1953], No. 30563/54. Class 37. In apparatus for measuring cable or line conductor fault currents and voltages and distance to faults the field magnets of moving-coil instruments are magnetized dependently on the fault currents and voltages and the degree of magnetization is measured. Measuring fault current.-In Fig. 1 a highremanent field magnet 1 co-operates with a moving coil 2 and when a fault current occurs the protection relay responds to trip the circuitbreaker and close switch 4 whereby a current i drawn from a line current transformer flows during the short response time of the protection relay (e.g. 0.1 second). When switch 4 opens a switch 7 is closed to apply from source 8 a predetermined direct current through moving coil 2 and the position attained by the instrument pointer of moving coil 2 indicates the value of the fault current. After each fault current measurement the field magnet 1 is demagnetized by applying A.C. via switch 11 to field winding 3 (or to an additional winding) and gradually reducing the A.C. to zero. The air gap in which coil 2 rotates may be closed by a magnetic shunt except during the fault reading- off time. Measuring distance to fault.-In Fig. 2, two Fig. 1 arrangements are utilized with one moving coil instrument 1, 2, 3 responsive to fault current drawn from a line current transformer and the other 1<SP>1</SP>, 2<SP>1</SP>, 31 responsive to fault voltage drawn from a line voltage transformer. The two moving coils 2, 2<SP>1</SP> are in series and when a fault has occured and magnetized field magnets 1, 1<SP>1</SP> the D.C. fed to the moving coils from source 8 is adjusted through variable resistor 10 so that moving coil 2 always deflects to a calibration mark (see Fig. 2a) corresponding to 100 per cent fault current (i.e. i= 1 and Z the line resistance or impedance is therefore proportional to fault voltage V). The position of the moving coil 21 pointer (see Fig. 2b) then indicates Z as a percentage of line length or may directly indicate the distance to the fault. When the circuit breaker trips to isolate the faulty line or at the cessation of fault magnetization of field magnets 1, 1<SP>1</SP> a motor may be automatically energized to drive variable resistor 10 (with switch 7 closed) so that D.C. from source 8 increases through the moving coils 2, 21 until the pointer of moving coil 2 reaches maximum fault current position at which point the pointer opens a circuit to stop the driving motor. The fault distance may be automatically recorded. In a modification, Fig. 3, for more accurately indicating shortdistance faults the fault current i also magnetizes field magnet 1<SP>1</SP> through winding 3<SP>11</SP> and this magnetization opposes that produced by the fault voltage winding 3<SP>1</SP> so that moving coil 21 rotates from a scale " zero " of 100 per cent (see Fig. 3b), i.e. where the i and e effects balance; when 2<SP>1</SP> stays at zero with 2 rotated to 100 per cent fault current (see Fig. 3a) the fault is therefore at the remote end of the line. In order to compensate for arc resistance of the fault a 2-primary, 1-secondary winding transformer is provided, Fig. 4, so that the fault voltage e drawn from the secondary winding is reduced by an amount ki. One primary winding is the fault voltage e winding and the other the fault current i winding tapped to the required amount ki. The current directions are opposite in the two primaries (see arrows).