A DISCUSSION OF CURRENT-VOLTAGE AND SURFACE-POTENTIAL MEASUREMENTS TOTEST STRESS GRADING MATERIALS

Current-voltage measurements are the mast widely used technique for te sting properties of grading materials. However, surface potential meas urements seem to be more representative of these materials' actual beh aviour. In order to discuss this issue, a wide range of stress grading materials, loaded with both SiC and ZnO at various fractions, are tes ted in do and in 50 Hz sc, both with current-voltage and with surface potential measurements. The latter are compared with simulations perfo rmed with analytical and numerical techniques. The three methods are i n good agreement. Moreover, on the one hand, simulations show that, wh en the grading effect is observed, the field that the material withsta nds 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 a t that frequency, although they have a grading behaviour in do, Since there is no direct link between current-voltage characteristics and su rface potential measurements, the latter seem to provide much better i nsight into grading materials' actual performances.