Patent GB-483304-A (GB)
Title: Improvements in or relating to the testing of electrical networks in respect of fault characteristics
IPC: G01R31/28
Filed: 1936-10-14
Abstract: 483,304. Pressure meters. REYROLLE & CO., Ltd., A., LEESON B. H., HARLE, J. A., WILD, R. W., and TODD, J. R. Oct. 14, 1936, No. 27903. [Class 37] To determine the voltage transients which would occur in an A.C. network following the clearance of a fault therefrom, an artificial fault is produced by connecting a testing circuit to the appropriate point of the network while the latter is alive and in service, the testing circuit then being rapidly disconnected from the network whereby the transient voltages to be determined are set up and are measured, for example, by an oscillograph, the transient currents being limited by non-inductive resistances in the testing circuit to a value which will not disturb the network stability. To initiate a test, the testing circuit is connected at B to the network B<1>, B<2>, B3, and a pendulum contact J is set in operation to close a main switch C for a few cycles only. Switch C immediately connects current limiting resistances A<2>, A<3>, in two of the phases of the network, the current limiting resistance A<1> in series with a fusible cut-out D being connected in the third phase of the network at a predetermined point in the voltage cycle by a switch F controlled by a synchronizing device, which may, as shown, comprise a triode rectifier G supplied through a saturated transformer H from the network, the grid being automatically biassed so that, after closure of the anode circuit at C<1>, anode current commences to flow at the desired point in the voltage wave cycle, preferably in the neighbourhood of the peak of the wave, to energize the operating coil F<1> of switch F. The time of closure of F is chosen so that the cut-out D immediately operates and the current decays according to a straight-line law, the circuit being finally broken after about a quarter of a cycle. The required voltage transients appear across the cut-out D and are measured or indicated by a cathode ray oscillograph E in parallel therewith. Alternatively, the main switch C may be closed by a control switch and opened by a pendulum contact brought into operation by the control contact. Further, to show the transient wave form on the oscillograph as a standing wave the closing and breaking of the testing circuit may be repeated at timed intervals by providing a series of fusible cut-outs D brought into circuit seriation by cam-operated contacts controlled by a synchronous motor. The above arrangements simulate the transient conditions arising when the first phase clears in the event of a three-phase fault, but obviously single phase or other faults may be simulated. In another embodiment, Fig. 2 (not shown), the synchronising device is dispensed with, and the switch F is replaced by a spark gap which discharges at the voltage wave peak, means being provided to render the oscillograph inoperative until initiation of the test. In the case when a continuous record is required on the oscillograph the fusible cut-out is dispensed with, and the circuit periodically closed and opened by a disc having one or more contact segments rotated between the spheres of the spark gap, the arc being extinguished by an air blast.