A discussion of current-voltage and surface potential measurements to test stress grading materials

Current-voltage measurements are the most widely used technique for testing properties of grading materials. However, surface potential measurements seem to be more representative of these materials' actual behaviour. In order to discuss this issue, a wide range of stress grading materials, loaded with both SiC and ZnO at various fractions, are tested in dc and in 50 Hz ac, both with current-voltage and with surface potential measurements. The latter are compared with simulations performed with analytical and numerical techniques. The three methods are in good agreement. Moreover, on the one hand, simulations show that, when the grading effect is observed, the field that the material withstands is far below the field required for non-linear behaviour. Hence, a nonlinear relationship of the resistivity versus the electrical field does not seem to be necessary for achieving grading optimization. On the other hand, measurements show that all the materials studied here are capacitive at 50 Hz ; they are not suitable for grading purposes at that frequency, although they have a grading behaviour in dc. Since there is no direct link between current-voltage characteristics and surface potential measurements, the latter seem to provide much better insight into grading materials' actual performances.