GB701862A - The measurement of di-electric dispersion - Google Patents

Abstract

701,862. Dielectric dispersion measurements ; determining moisture content. BRITISH ELECTRICAL & ALLIED INDUSTRIES RESEARCH ASSOCIATION. Sept. 24, 1952 [Oct. 9, 1951], No. 23487/51. Class 37 The effective value of the dielectric dispersion of an insulating material, i.e. the relative variation of instantaneous dielectric constant over any selected period of time from a time T 1 seconds to a time T 2 seconds after the application of the charging voltage, is measured by measuring a voltage applied between electrodes to charge the material, isolating the electrodes, and measuring the voltage subsequently established between the electrodes consequent upon the change of instantaneous dielectric constant. Two pulse generators PI, P2 are provided to give positive and negative rectangular voltage pulses. The top level of the pulse given by the generator PI is adjusted to be equal to the desired charging voltage, e.g. +200 volts, the duration a, b of each pulse being 300 m.sec. The lower level of each pulse given by the generator P2 is zero volts and the top level is higher than the charging volts, e.g. + 400 volts, the duration of each pulse being 3 m.sec. The initiation of the discharge pulse generator P2 is controlled by the back edge e of the pulse from the generator P 1 so that the pulse from P2 starts after a time delay of the order of 1 m.sec. Four diodes D1, D2, D3, D4, are arranged as shown and when the pulse generators PI, P2 are inoperative remain insulating for all values of voltage on the test specimen S between 0 and + 200 volts, because the cathode of the diode D4 has a cathode bias of +200 volts. Before beginning the test the switch Sw 1 is placed on its contact s for a time long compared with 300 m.sec. in order to remove any charge from the specimen S by short-circuiting its terminals. The switch is then placed on its upper contact r to connect the cathode of the diode D2 to the pulse generator P2. The pulse generator PI is then made operative and the positive pulse is applied through the diode D 1 to the specimen S for 300 m.sec., current flowing into the specimen until it reaches the charging voltage, whereupon the diode D 1 cuts off. The diode D4 acts as a clipper and discharges to the earth line E if its anode receives a voltage from the specimen greater than +200 volts, thus balancing the bias at the terminals t1. t2. After a short interval the pulse generator P2 operates and the cathode of the diode D2 is brought to zero for 3 m.sec. The diode D2 conducts and current flows through it from the specimen S to earth until the voltage across the specimen is reduced to zero, when the diode cuts off. To prevent overshooting the diode D3 is provided as a clipper. During the recovery period and all other periods of isolation all the diodes D1, D2, D3, D4 are cut off because the recovery and isolation voltages must lie between zero and 200 + volts. The specimen S is thus isolated from the pulse generators. The voltage on the specimen S is read on a voltmeter V through a circuit consisting of a pentode V1 and a triode V2 connected as a cathode follower. The lead 1 has an earthed outer screen OS and an inner screen IS connected to a screening box B, which is also connected to guard rings G on the outside of the envelope of each of the diodes D1...D4, and to a screen N surrounding the leads from the transformer T and a screen M surrounding the secondary winding. This winding is screened from the core by an earth screen K. The test of dielectric dispersion may be employed as a guide to the moisture content of insulating materials such as paper, cotton, &c. used in bushings, cables, and transformers.