Some Anomalous Phenomena Ubserved in Dielectric Loss Measurements Using a Three-Electrode System

The physical parameters of the guard gap between the guard and the guarded electrodes of the three-electrode system generally used for dielectric loss measurements can be modelled by a simple T-network. Theoretical analysis shows that under certain conditions these parameters can cause great errors or even make the apparent dissipation factor negative, when measured with a Schering bridge. The sharp edges of the guard and the guarded electrodes may result in the creation of high local fields and give rise to electrical discharges under high voltage conditions. Local discharges near the sharp edges directly modify the physical parameters of the guard gap. Such anomalous phenomena have been experimentally observed and they can be attributed to the change in physical parameters in the guard gap. Experimental results are in good agreement with the theoretical analysis based on the T-network model. Methods for suppressing or minimizing these anomalous effects are also discussed.