Patent GB-286775-A (GB)
Title: Improvements in or relating to apparatus and methods for the comparison of high alternating voltages
Filed: 1926-12-08
Abstract: 286,775. Churcher, B. A. G., and Metropolitan-Vickers Electrical Co., Ltd. Dec. 8, 1926. Potentiometers; phase - differences, mensuring. - The ratio and phase displacement of the voltages across the high and low tension windings 2, 3, Fig. 1, of a potential transformer 1 are measured by a capacity potentio meter consisting of a fixed condenser 5 and a variable condenser 9 connected in series across the windings 2, 3 their junction point being connected to the earthed junction point 4 of the winding 2, 3 by a bridge circuit including a vibration galvanometer 16 and the secondary winding 14 of a. fixed or variable mutual inductance 13, the primary winding 15 of which is connected in series with a variable resistance 19 across the low tension winding 3 of the transformer 1. The condenser 5 is provided with a guard-ring 8, to which is connected an electrostatic screen, shown in broken lines, enclosing the variable condenser 9, connecting lead 20, vibration galvanometer 16, mutual inductance 13, and a two-way switch 18 by means of which the galvanometer 16 may be connected either to the lead 20 or to the screening system. The screening system is connected to the junction 4 of the windings 2, 3 through the secondary winding 23 of a mutual inductance 21, the primary winding 22 of which is connected in series with the primary winding of the main mutual inductance 13. An alternating voltage is applied to the high tension winding 2 of the transformer 1 to be tested, its value and frequency being measured on a voltmeter 26 and frequency meter 27 on the low-tension side of the transformer. With the switch 18 connecting the galvanometer 16 to the lead 20, the variable condenser 9 and the mutual inductance 13, which injects a quadrature voltage into the bridging circuit, are adjusted until zero deflection is obtained on the galvanometer 16. The switch 18 is then changed to connect the galvanometer to the screen and the mutual inductance 21 is adjusted to obtain balance. The switch 18 is again returned to its first position and the mutual inductance 13 and variable condenser 9 are adjusted until balance is again obtained. The screening system and the lead 20 are then at the same potential, and the galvanometer and associated apparatus are screened from external electric fields. The ratio and phase displacement of the voltages across the windings 2, 3 may be calculated from the known values of the capacities of the condensers 5, 9, the value of the main mutual inductance 13, and the inductance and resistance of its primary circuit. Instead of using a mutual inductance to obtain balance for phase displacement, resistances may be employed in conjunction with either the fixed or the variable condenser. Fig. 2 shows a. modification for testing a transformer 30 having a tapping point 31 on the high-tension winding near the earthed end for tapping off a small portion 32 for connection to a voltmeter &c., it being required to determine the ratio and phase displacement between the voltage of the full winding 30 and the voltage of thetapped-oft portion 32. The galvanometer 16 is connected to the tapping point 31, while the screening system is connected through the secondary winding 23 of the mutual inductance 21 to the tapping point 33 of a resistance potential divider supplied from the lowtension winding 35 of the transformer under test, so that the potential of the screen may be raised to that of the galvanometer. Fig. 3 shows a modification for determining the ratio and phase displacement of a resistance potential divider 40 having a tapping point 41 which divides it into sections 42, 43. This is connected across the secondary winding 44 of a supply transformer having a primary winding 45, these windings being connected to earth at their junction point. It is assumed that owing to earth capacity the voltage across the section 42 will lag behind that of the section 43, and it is desired to measure this phase displacement. The galvanometer 16 is connected to the tapping point 41, and the section 43 is shunted by a variable condenser 47. Balance is obtained by adjusting the condensers 9, 47, the screening system being adjusted to the correct potential by means of the mutual inductance 21.