Impulse voltage testing of phase conductor models

• Main Author: Harid, N.
• Published: Cardiff University 1990
• Subjects: 621.31
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.527124

The attenuation and distortion of surges by corona discharge on transmission lines is relevant to high-voltage insulation coordination. The predetermination of these aspects is desirable in terms of surge shape and conductor geometry , so that realistic situations can be accurately reproduced. In this thesis, an experimental investigation of the corona discharge on line conductors is described which, with the help of numerical models of corona and space charge, explains some aspects of the discharge under surge voltages. In simulating the corona characteristics, a numerical model is developed which relies on the physical properties of corona to give the non-linear variations with voltage of corona charge and corona capacitance, at present frequently used to simulate corona losses in surge attenuation calculations. The procedure results in a set of inter-related generalised equations whose solution requires the knowledge of the conductor geometry, the corona inception voltage and the geometrical capacitance of the system, together with the per-unit distributions of potential and electric field in the electrode configuration. These can be obtained numerically where analytical solution is not available. The model predictions are compared with both published test data and present laboratory measurements of charge and voltage on single and bundle conductors. New field filter probes incorporable in conductors of cylindrical cross-section are developed for measuring unipolar and bipolar field changes in impulse corona. The first type includes multiple-orifice filters for fields with radial symmetry, and filters with a single row of circular orifices for fields with no radial symmetry. The other type, which is of different design, is used with impulses of oscillatory shape, which give rise to bipolar variations in the surface field. Electric field measurements o n the surface of a twin-cond uctor assembly confirm the validity of the Deutsch-Popkov approximation and of the field boundary condition on the conductor used in the corona model. Charge and field measurements with oscillatory impulses show that the main corona charge is injected during the first voltage cycle, and that this charge is larger for impulses with higher frequency. On the second cycle, corona takes place at high overvoltages but is significantly weaker than on the first cycle. This has an important implication on the surge per formance of overhead lines. The importance of the surge steepness on corona inception voltage and corona injected charge is investigated by conducting experimental measurements of corona charge flow at various rates of voltage rise. The results show that minimum corona charge is obtained for impulse fronts intermediate between the standard lightning and switching shapes. This result is associated with the interdependence of corona charge and the statistical time lag of corona inception, and with the time required to clear the corona space charge. Time-lag effects are simulated by applying the concept of critical volume to present line conductor geometry.