Patent GB-415926-A (GB)
Title: Improvements in protective systems for electric power lines
IPC: H02H1/00
Filed: 1933-12-06
Abstract: 415,926. Protective cut-out systems. SIEMENS - SCHUCKERTWERKE AKT.- GES., Siemensstadt, Berlin. Dec. 6, 1933, No. 34342. Convention date, Dec. 6, 1932. [Class 38 (v).] A protective system for a polyphase line, particularly one which can be fed from both ends, has a switch device operable automatically on the occurrence of a short-circuit to cause the current and voltage paths of the single resistance relay at each protecting point to be so connected that they are only energized respectively by the current of a conductor affected by the fault and the potential of the circuit which is established by the short-circuit. The current-transformers of the conductors R, S, T are connected as shown to overload relays 1, 2, the current coils 6, 5 of directional and impedance relays, and to a change-over contact 7 operated from the normal position shown by means of a coil 8 when 1 operates. In the position shown, coils 5, 6 are energized by the current in line T, but when 1 operates, they are energized by that of line R. Coils 1, 2 also control coils 9, 10 which operate contacts 11, 12 of the parallel-connected potential coils 13, 14 of the relays. In accordance with whether the short-circuit occurs between lines R, S, or S, T, or R, T, the relay 1 or relay 2 or both will operate and connect coils 13, 14 across rs, or st, or rt, the current being that of R or T according to whether the short-circuit affects R or not. When two lines develop earth faults, the leakage currents operate relay 17 which energizes relay 18 to change contact 16 over to earth. With double earth faults on lines RS, or ST, the relays will be energized by the current in R and the potential of r to earth, or by current in T and potential of t to earth. However, with a double earth fault on RT, contacts 19, 20 disconnect the coil 10 so that the relays cannot receive the potential rt, but instead receive the potential of r to earth. In order to render the time of operation on a three-phase fault equal to that of a two-phase fault, the " resistances " of which as measured by the relay are not normally the same, a resistance in series with the potential coil of the impedance relay may be arranged to be cut out on a three-phase fault, the control relay for which resistance being controlled by contacts of relays associated with each line, Fig. 2 (not shown). This equality may also be obtained by the arrangement in Fig. 3, in which only the impedance relay is shown, its potential coil 14 being connected with chokes D<1>, D<2> in an equal-resistance star and controlled by switches 11, 12, 33, of which 33 is controlled by a coil 22 in the lead associated with line S. The operation for two-phase faults is the same as Fig. 1 except that 14 receives half the linevoltage, but with a three-phase fault it receives the phase-voltage owing to the star-connection created by closure of 11, 12, 31, 33, thus producing the same time delay for each kind of fault. As in Fig. 1, relay 10 is locked against operation when earth current flows through two of the lines or through one line and an earthed neutral point, thereby ensuring that with an earth short the potential to earth is always used.