Patent GB-585278-A (GB)
Title: Improvements in electric measuring arrangements
IPC: G01R19/165
Filed: 1945-02-12
Abstract: 585,278. Current and voltage measurements by vacuum or low-pressure discharge apparatus. COLLINS, G. C., MONAHAN, T. F., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd.. Feb. 12, 1945, No. 3490. [Class 37] An arrangement for measuring impulse voltages comprises a number of voltage stages each constituted by a screen-grid valve coupled to a gas-filled triode, which triodes are triggered at different voltage values. Fig. 1 shows five voltage stages V1 . . . V5 and a " chopping " stage, Co. Each voltage stage includes a pentode p, Fig. 2, the cathode and suppressor grid of which are earthed while the control grid is connected through a condenser c to an input terminal v, whereby a positive voltage can be applied to the control grid, and through a resistance r to a terminal b by which negative grid bias is applied. The anode is connected through the primary winding of a phase-reversing pulse transformer t to the high tension supply h, and the auxiliary grid, which is capacity-coupled to the cathode, is connected to a tapping on a resistance d.' The secondary winding of the transformer t is connected in series with a resistance e between the control grid of an argon-filled triode a and a grid-bias terminal g, the cathode being connected to a terminal s and the anode through a resistance f to the high tension supply h. A neon lamp n is arranged to become illuminated when the triode a passes current. When there is applied to the terminal v a positive impulse voltage in excess of the negative bias applied to the terminal b the pentode p passes current and develops a negative voltage pulse across the primary winding of the transformer t. Across the secondary winding is developed a very steep-fronted positive voltage pulse which fires the triode a and causes the neon lamp n to become illuminated. The chopping stage, Fig. 3, is similar to the voltage stage except for a resistance j which is included in the discharge circuit of the triode a. The voltage stages V1 ... V5, Fig. 1, are each constituted as shown in Fig. 2 and have their terminals v connected to input terminals Vo, Vp, VQ, VR, VT. The chopping stage Co has its terminal v connected to an input terminal I. The terminals b for the voltage stages have applied to them negative grid-bias voltages V1, V2, V3, V4, V5 of which V1 is the lowest value and each succeeding voltage is higher than the preceding one so that when a positive voltage wave is applied to' the input terminals Vo, VP, VQ, VR, VT the triodes a of the voltage stages VI ... V5 fire successively and indicate the points in the voltage wave at which the voltages attain values corresponding to the grid-bias voltages for the successive stages. The terminal s of the chopping stage Co is connected to the terminal s of each voltage stage V1 ... V5, and the terminal b of the chopping stage is connected to negative grid-bias voltage Vo. The triode a of the chopping stage fires and illuminates the neon lamp n when a positive voltage wave applied to the terminal I attains a value in excess of the grid-bias voltage Vo. The circuit constants are so selected that the voltage developed across the resistance j when the triode a fires is a steepfronted impulse of magnitude greater than that of the impulse delivered by any of the transformers t of the voltage stages to the control grid of the corresponding triode a, and also of a duration in excess of that of the voltage wave applied to that stage. The triode a of the chopping stage fires when the voltage applied to the terminal I attains a predetermined value and nullifies the firing impulses of all the voltage stages that have not fired without affecting those that have fired, so that the value of the voltage from which is derived the voltages applied to the terminals v, which voltage corresponds to the predetermined voltage applied to the terminal I, is represented by the number of voltage stages for which the triode has fired. Fig. 4 shows a test circuit for measuring under high voltage conditions the value of voltage corresponding to a particular value of current passing through a resistor having a non-linear characteristic. The non-linear resistor X is included in circuit with an inductance L, sparkgap G, and resistance R across a high voltage H shunted by a condenser C, so that when the spark-gap G breaks down a current impulse is passed through the resistor X and causes a proportional voltage to appear across a non- inductive resistance S at the terminal Io. The voltage across the resistor X is measured in terms of the voltage developed between earth and terminals Vo, VP, VQ, VR, VT by means of potential dividers O, P, Q, R, T. The terminal Io is connected to the input terminal I, Fig. 1, and the terminals Vo, VP, VQ, VR, VT to the corresponding terminals in Fig. 1. The value of the voltage across the resistor X corresponding to the particular value of the current through it is greater than the grid bias for the voltage stage last to be fired but less than that for the next stage. For measuring the times to breakdown of samples of insulating material when subject to high impulse voltage a time-base circuit is employed to give a voltage proportional to the time interval from a predetermined instant. Voltages representative of time are applied to the terminals Vo, VP, VQ, VR, VT, Fig. 1, the voltage pulse to breakdown being applied to the terminal I. The time to breakdown is indicated by the number of voltage stages that have fired.