Patent GB-548931-A (GB)
Title: Improvements in and relating to protective apparatus for electric circuits
IPC: H02H3/40
Filed: 1941-09-08
Abstract: 548,931. Protective cut-out systems. BRITISH THOMSON-HOUSTON CO., Ltd. Sept. 8, 1941, No. 11443. Convention date, Sept. 6, 1940. [Class 38 (v)] In a distance-protective system employing, as reactance-dependent protective devices, electric devices which avoid the delay in operation inherent in mechanical relays, the inherent inability of electronic devices to take into account the phase-angle between the quantities on which the measurement is based, is overcome by operating the electronic devices at the zero of the current cycle, when the reactance to be measured, E sin #/I is proportional to the ratio of a voltage dependent on the system-voltage e and a voltage dependent on the first timederivative of the system-current, di/dt. The mathematical considerations underlying the invention are set out in the Specification. The electronic device comprises a pair of valves 19, 20, preferably of the gas-discharge type, connected in series with the trip coil 16, Fig. 1, (not shown) the valve 19 being operated in accordance with a minimum reactance value, whereas valve 20 is rendered conductive only at one of the zero-points of the current cycle. Thus, the grid of valve 19 is acted upon by the opposing voltages set up (1) in the secondary 34' of a transformer 32' energized by the linevoltage and in a restraining sense, and (2) in the primary 27 of a transformer 26 energized by the line current through transformer 28, and in an operating sense, whereby the voltage set up in winding 27 leads the current therein, and hence the line-current, by 90'. Transformer 26 has its secondary 29 connected to the gridcathode circuit of valve 20 and is of such character that it produces a voltage-peak as described in Electrical Engineering of Nov. 1932 at p.803, each time the line-current passes through zero, as shown at epi, -epi in Fig. 4. At these points di/dt = I and e = E sin #, but only the positive values epi affect the valve 20. If the value of E sin #/I (or e/(di/dt) is below or equal to the minimum, the voltage of 27 equals or overcomes that of 34<SP>1</SP> and produces tripping. The arrangement is made direction-responsive by the addition of an electronic starting unit which, in operation, supplies the anode-cathode potential of the series-connected valves 19, 20 by means of a resistor 38 in the trip circuit. A directional effect comparable with that E/I cos # usually obtainable with mechanical relays is obtained at the instant of the supply-voltage cycle when the voltage is a maximum, at which point i = Icos#. In order to operate at this instant, a pair of valves 39, 41, one for each half-wave are fed from the secondary 47 of a transformer similar to that 26 for the ohm-unit, the primary 46 of which is energized from the secondary 34" of transformer 32<SP>1</SP>. Thus the current in 46 lags the voltage in 34", and hence the systemvoltage, by 90 degrees, so that the peaked voltage of secondary 47 occurs at systemvoltage maxima, when e = E or - E. A further valve 50 in series with 39, and measuring the quantity E/I cos #, has its grid-cathode circuit energized in series with winding 34" and a winding 56 energized in accordance with the system-current, so that it only becomes conducting when the quantity decreases to a given amount. In this way a voltage appears across the ristor 38 to operate the ohm-unit and produce tripping. In a simple form, Fig. 2 (not shown), the trip circuit is-completed by the valves 19, 20, there being no directional restraint.