Patent GB-806572-A (GB)
Title: Electrical voltage measuring apparatus
Inventor: FOSTER GEORGE EDWARD
IPC: G01R19/252
Filed: 1955-06-07
Abstract: 806,572. Voltage measurement. BAILEYMETERCO. June 7,1955, No. 16397/55. Class 37. [Also in Group XXXVIII] An electronic device for measuring direct or rectified alternating voltages comprises a variable timebase circuit for producing a pulse of time length linearly proportional to the input voltage after an initial period of non-linearity, an oscillator which is stimulated into operation by the pulse, and circuit means responsive to a command pulse for simultaneously triggering the variable timebase circuit and rendering the oscillator inoperative for a time period corresponding to the initial period of non-linearity of the timebase circuit whereby the oscillator is operative only for the duration of the linear portion of the timebase pulse and generates a train of oscillations corresponding in number to the magnitude of the input voltage. In Fig. 2, a voltage-measuring operation, e.g. of a substation bus-bar voltage, is initiated by a command pulse (Fig. 3a) applied over input 1 to a differentiating resistance-capacitance network 22, 23, 24 deriving a negative-going pulse (Fig. 3 b) which is freed from false transients by filter resistance-capacitance network 25, 26 and drives non-conductive the left-hand section of double triode 27 whereby the right-hand section is driven conductive over common cathode resistance 28<SP>1</SP>, capacitance 36 and resistance 37, for the duration of the differentiated negative-going pulse, to develop at the right-hand anode a rectangular pulse of similar duration. (Fig. 3 c), which is differentiated by capacitance 38 and resistance 39 to a sharp negative followed by a sharp positive pulse (Fig. 3 d) of which the former is selected by series diode 40 for application to a time delay circuit 4, wherein a double triode 41 is connected as a mono-stable multivibrator whose left-hand section normally conducts and whose right-hand section is normally cut off but is driven to conduct by the negative pulse from diode 40 on its anode; while driving the left-hand section non- conductive over coupling capacitance 48. A positive potential is applied over resistancecapacitance networks 53, 54, 55 to the grid of the right-hand section to drive it conductive for a period determined by the time constant of capacitance 48 and resistance 56, after which the circuit reverts to normal. A resultant positive-going square pulse waveform at the anode of the left-hand section (Fig. 3 e) is differentiated by capacitance 58 and resistance 59 to a positive-going sharp pulse (Fig. 3 f coincidentally with the leading edge thereof, which is cathode follower amplified by the left-hand section of double triode 57 for application to the second and fourth grids of a heptode 66, which is connected as a phantastron waveform generator with its anode retroactively coupled over capacitance 77 to its first grid 72 and also energized through compensating circuit 14a and zero-setting circuit 14b from a measurement voltage applied to terminals 13a, 13b and bridge rectified at 4 if necessary. The first grid is also coupled to positive supply line 20 over resistances 78, 79, while the third grid is coupled through resistance 81 shunted by capacitance 80 to the second and fourth grids 69, 71. Normally the latter are positively biased from the supply -line over resistances 74, 75 and the third grid is negatively biased over resistance 82 so that no anode current flows, but when this grid is driven positive by the impulse from the left-hand section of tube 57 over 80, 81, anode current flows (limited by the values of capacitance 77 and resistances 78, 79 effective to negatively pulse the first grid over capacitance 77 and reduce the anode current to a low level at equilibrium. The anode potential falls until it no longer exerts control over the first grid and the current flow in the tube increases, to produce a voltage drop across resistors 74, 75 which is fed back as a negative pulse over resistance 81 and capacitance 80 to the third grid, to terminate the anode current flow. A negativegoing sawtooth waveform appears at anode 67 (Fig. 3 j) whose time duration t is linearly proportional to the measurement voltage at the anode plus a standing voltage derived from the slider of potentiometer 134 of zero-setting circuit 14b energized from the positive supply in series with resistances 133, 135; the standing voltage being adjusted to a level at which the corresponding sawtooth waveform duration is equal to the delay of timing circuit 4, so that any extension in duration of the waveform beyond this point is linearly proportional in duration t1, t2, t3 to the measurement voltage E1, E2, E3 (Fig. 3 k); measurement being prevented by the time-delay circuit until the point of equality is reached. At the second and fourth heptode grids, there appears a rectangular positivegoing waveform (Fig. 3 j) coupled over resistance 81 and capacitance 80 to the grid of the righthand section of double triode 57 operating as an amplifier with anode load resistance 85 in parallel with a neon tube 86 in series with resistance 87; the tube indicating visually the operation of the triode section. An oscillator control unit comprising a double triode 90 with parallelconnected anodes and cathodes has its grids biased from a - 150 V. source and respectively connected to the anode circuits of the left-hand section of double-triode tube 41 and the righthand section of double-triode tube 57, so that its left-hand section is normally cut off and its right-hand section conducts. Responsive to an increasing command pulse, the positive-going pulse derived from the delay circuit 4 (Fig. 3 e) drives the left-hand section conductive. Shortly afterwards the amplified negative pulse (Fig. 3 h) representing in duration the measured voltage cuts off the right-hand section. On termination of the positive pulse after a time determined by the constants of the delay circuit 4, the left-hand section is again cut off and pulsing oscillations are then generated by double triode 102 of which the left-hand section has its cathode returned to earth over variable-resistance chains 117, 118 and its anode resistance capacitance coupled to the grid of the right-hand section, whose anode is directly coupled to the grid of the left-hand section and whose cathode is earthed. The cathode of the left-hand section is biased from the common cathodes of the double triode 90 across capacitance 101, and normally the lefthand section is cut off while the right-hand section is conductive, and the former section also conducts when both sections of double triode 90 are cut off during the duration of the measurement pulse after the termination of the positive delay pulse (Fig. 3 c) so that a frequency pulse train is generated (Fig. 3 i) at output 19, which is applicable to a counter (not shown). The frequency is adjustable by variation of resis. tances 117, 118 or by closing switch 116 to insert variable resistance chains 114, 115. The time delay imposed before oscillations commence prevents measurement during initial energization of heptode tube 66 with possible non-linearity, while since network 133, 134, 135 injects a steady direct voltage corresponding in magnitude to that represented by the delay time interval, the number of pulses at output 19 is accurately and linearly proportional to the measurement voltage. The rectifier shunt circuit comprising diode 136, and resistances 137, 138 across network 133, 134, 135 corrects for non- linearity of bridge rectifier 4. The cathode of the right-hand section of double triode 57 is switchably earthed to disable the oscillator operation; the cathode being normally held positive by resistance 119 connected to the positive supply line. For direct voltage measurement, the rectifier bridge and connecting diode are eliminated. In a modification (Fig. 4, not shown) the apparatus is arranged to be switchably and sequentially connected to successive incoming sub-station power lines for voltage or current measurement; the results being displayed locally on a digital counter and coded for transmission to a printer at a central station. The measured voltage may be derived from transducer devices responsive to various unspecified physical conditions, positions and characteristics.