Patent GB-531477-A (GB)
Title: Improvements in apparatus for measuring the loss angle of dielectrics
IPC: G01R27/26
Filed: 1939-06-27
Abstract: 531,477. Loss angle measurements; valve voltmeters. WARREN, T. R. June 27, 1939, No. 18708. [Class 37] Loss angle of dielectrics is measured by matching the E.M.F. across an earthed measuring circuit connected to the low voltage end of the secondary winding of a high voltage transformer with the E.M.F. between a tapping on the secondary winding and the low voltage end, and ensuring that the current entering the dielectric under test is identical with that in the measuring circuit, the readings obtained in the measuring circuit after matching has been effected enabling the required loss angle to be calculated. One side of a bushing A whose loss angle is to be measured is earthed and the high voltage conductor F passing through the bushing is connected through a cable G to the high voltage terminal B2 of the secondary winding of a transformer B. The low voltage terminal B1 of the secondary winding is connected to earth through a variable resistance E and a variable condenser D, forming a standard or measuring circuit. The cable G is provided with two concentric screens G1, G2, of which the inner, G1, is connected to the low-voltage end B1 of the secondary winding and the outer, G2, is earthed. The capacity current of the cable is thereby diverted from the measuring circuit. The secondary winding of the testing transformer B is provided with two tappings B3, B4, which may be chosen so that the voltage between the points B1, B3 is half that between the points B1, B4. The tapping B3 is connected to earth through a thermionic detector H capable of measuring minute E.M.F.'s. The tapping B4 is connected to earth through a variable condenser J and a variable resistance K forming a compensation circuit. The measurement is effected by comparing the E.M.F. between the points B3, B1 with that across the measuring circuit and varying the condenser D and resistance E so as to equalize these E.M.F.'s in magnitude and phase. In order to ensure that the current entering the conductor F of the bushing is identical with that in the measuring circuit the measuring circuit condenser D is first set to zero so that no current flows in that circuit. Then with the testing cable G energized from the transformer B but disconnected from the bushing conductor F, the condenser J and resistance K in the compensation circuit are so adjusted that the detector H gives a zero reading, indicating that the point B3 is at earth potential. The current flowing ; to earth through the compensation circuit returns to the point B1 through the capacity formed by the two screens G1, G2. The testing cable G is then connected to the bushing conductor F and the condenser D and resistance E in the measuring circuit are adjusted until the detector H again gives a zero reading, indicating that the point B has been restored to earth potential. The currents in the bushing and the measuring circuit are then identical and the loss angle is calculated from the values of the capacity D and resistance E. The detector H comprises. a transformer S across. the primary winding, S1 of which the E.M.F. to be detected is applied through an inductance T. The winding S1 is tuned by a condenser U to resonate at the supply frequency. The secondary winding S2 is connected to the grid of a triode L through a resistance V. The sensitivity is varied by means of a potentiometer X, across which is a condenser Y which in conjunction with the resistance V reduces the harmonic content of the E.M.F. between the terminal H1 and earth. Between the anode and cathode of the valve L is the primary winding P1 of a transformer P, a condenser Q being connected in parallel with this winding so as to form a resonant circuit at the supply frequency. The secondary winding P2 is connected to a milliammeter R. An inductance M and condenser N in parallel are inserted between the anode and the positive D.C. supply in order to eliminate harmonics between the anode and cathode.