Patent GB-529332-A (GB)
Title: Apparatus for determining insulating values
IPC: G01R31/12
Filed: 1939-05-30
Abstract: 529,332. Resistance and power-factor measurements. DOBLE, F. C., and BROWNING, G. H. May 30, 1939, No. 15854. [Class 37] Apparatus for measuring the insulation resistance and power factor of dielectrics such as bushings, operating rods, and oil in oil circuit breakers, bushings and windings of power and instrument transformers &c. is so arranged that the charging current and watts loss are measured from the input side, and that the same apparatus may be employed with test specimens taking a large range of charging current. As shown in Fig. 1 a variable inductance or resistance 3 is connected to an A.C. power source and supplies the primary winding P<1> of a step-up transformer T<1> through a reversing switch 4 by means of which the phase of the voltage may be varied by 180 degrees. The high-tension terminal of the secondary winding S<1> of the transformer T<1> is connected, through the primary winding P<2> of a transformer T<2>, a tapping switch 6, and a conductor 7, to one terminal of the insulation to be tested, represented by a capacitance Cx and resistance Rx. The secondary winding S2 of the transformer T2 is connected to an ammeter 10 and the current coil CC of a wattmeter, 11, and is also earthed. The tapping switch 6 enables the secondary current to be adjusted in accordance with the capacitance of the test specimen so that the same ammeter and wattmeter may be employed with test specimens of widely varying capacitances. A third transformer T3 has its primary winding P 3 connected across the high tension and its secondary winding S3 connected through a variable resistance 13 to the potential coil pc of the wattmeter 11, a condenser 14 being connected across the potential coil. A voltmeter 15 is connected across the secondary winding S3. Condensers 16, 17 may be connected across the primary and secondary windings P2, S2 of the transformer T2 in order to change the phase of the primary current in relation to the secondary voltage and to correct the phase of the current in the current coil of the wattmeter. A shield 18 surrounds the primary winding P<2> and is connected to the high potential terminal 19 and to a shield 20 surrounding the conductor 7. A shield 21 surrounds the secondary winding S2 and is connected to one terminal of the winding and to earth. An outer earthed shield 22 surrounds the shield 20. Earthed shields 23, 24, 25 surround the instruments 10, 11, 15 and the primary and secondary windings P3, S3 of the transformer T3.A neon tube 27 connected to the secondary winding S2 protects the instruments 10, 11 from damage due to excess voltages. Guard wires 29, 34, Fig. 2, connected to the shield 20, prevent surface leakage from affecting the instrument readings. Alternative measuring means shown in Fig. 2 comprise a resistance 36 connected across the secondary winding S2, a resistance 37 and variable condenser 38 connected in parallel across the secondary winding S3 and a mill detector instrument 39. The loss through the test specimen and its charging current can be calculated from the known relationships in phase and magnitude between the voltages across the resistances 36,37 and the test voltage, and the amounts of these resistances that are tapped off when a null indication is obtained. Induced currents in the conductor 7 due to adjacent currentcarrying conductors may be corrected for by taking two sets of readings with the phase of the test voltage varied 180 degrees by the reversing switch 4 and averaging these readings. U.S.A. Specification 1,945,263 is referred to.