Patent GB-543091-A (GB)
Title: Improvements in or relating to electric protective arrangements
IPC: H02H3/30
Filed: 1940-07-05
Abstract: 543,091. Protective cut-out systems. ELECTRICAL IMPROVEMENTS, Ltd., and MITCHELL, G. W. B. July 5, 1940, No. 11304. [Class 38 (v)] A sectionalized alternating current circuit is protected by high frequency currents transmitted on fault over the section from each end, the frequencies being different and, in the case of an external fault, setting up beats which actuate a receiver at each end to prevent tripping. With an internal fault, the signals are no longer synchronized and no beats are set up, so that tripping occurs. The equipment for one end is shown in Fig. 1, in which each phase A<SP>1</SP> .. A' contains two sets of current transformers, one of which operates a sensitive starting device D in the secondary circuit C<SP>1</SP> of a summation transformer energized from the other set. The secondary C' which has a harmonics filtering condenser C<SP>2</SP>, supplies a further transformer G<SP>3</SP> having a battery J applying a bias to the grid of a gas-filled triode valve E, the anode circuit of which includes a battery J'. During the first negative half-wave after C' overcomes the bias of battery J, valve E instantaneously charges a condenser E<SP>1</SP> which renders operative the first transmitter amplifying stage G<SP>1</SP> as shown at the point e in Fig. 3. The secondary C<SP>1</SP> supplies a transformer F' which is arranged to saturate at the critical voltage of C<SP>1</SP> and produce by resonance effects a voltage-peak f<SP>1</SP>, Fig. 3, which is applied to the grid of a further valve F, the resonance point being adjustable by a resistance F' in the circuit of the saturing winding F<SP>8</SP>. The anode circuit of valve F includes the battery J<SP>1</SP>, a condenser F<SP>2</SP> which is charged by a pentode F<SP>1</SP> at a constant current determined by an adjustable resistance F<SP>3</SP> and a resistance F<SP>3</SP>, the voltage across which renders operative the second amplifying stage G<SP>2</SP> of the transmitter G which thereby becomes operative. When, therefore, in the succeeding positive half-wave of secondary voltage, valve F becomes conducting, transmission begins at the point f, Fig. 3, and when the condenser F<SP>2</SP> is sufficiently charged by the constant current flowing, a further triode K in the trip circuit becomes conductive at point k, Fig. 3. The anode circuit of K contains a further triode K' which is normally unbiassed and would not therefore prevent tripping. In the event of an external fault, the far transmitter operates in synchronism as shown by the lower curve of Fig. 3, and the beat frequency received at L is impressed through transformer L<SP>1</SP>, L<SP>2</SP> and rectifier L<SP>4</SP> in the form of a bias on the grid of K<SP>1</SP> which is, at the point l, Fig. 3, rendered inoperative, so preventing tripping. With an internal fault, however, the lower curve would be displaced to one side, Fig. 4 (not shown), or of different magnitude, Fig. 5 (not shown), so that the narrow bands f-k of the two curves would not coincide in point of time and tripping would occur, back contacts A<SP>6</SP> of the breakers opening and de-energizing K, K'. In order to render the stabilizing operative during succeeding cycles, the transmitter is disabled by discharging condenser E<SP>1</SP> through a triode M energized through rectifier L<SP>5</SP> from the secondary L' of the transformer L', L<SP>2</SP>. The latter, when the receiver L is operative, charges a condenser L<SP>6</SP> which prevents tripping by maintaining the bias on K' during the interval l-k, Fig. 3, after which the condensers F<SP>2</SP>, L<SP>6</SP> are discharged by valves M', M<SP>2</SP> at points m<SP>1</SP>, m<SP>2</SP>, respectively, in Fig. 3. In a modification, Fig. 2 (not shown), the output of C<SP>1</SP> is fed to an oscillograph, the beam of light from which, oscillating in synchronism with the line frequency, falls on a mask provided with one slot on one side and two on the other side of its mean position. The former corresponds to the critical voltage in the negative half-wave and the latter is, respectively, the critical voltage and a voltage in excess thereof, in the positive half-wave, so as to prevent operation in case only one transmitter receives critical voltage. Behind the slits are photo-electric cells which, in turn, render the successive amplifying stages operative and finally apply the operating bias to the tripping valve. This bias is swamped, however, by a stabilizing bias supplied from the receiver on external faults. For this purpose, the receiver is connected over a time-circuit to the grid of a triode the anode circuit of which includes the grid of the tripping valve.