High frequency properties of shielded power cable part 2: Sources of error in measuring shield dielectric properties

The types of errors that are likely to occur during the measurement of high frequency cable shield properties are presented. The primary source of loss in shielded solid dielectric cable is the high frequency capacitive current, which is determined by the impedance of the cable dielectric passing through the complex impedance of the shielded layers. RC grading structures, which are common in high voltage applications can take the form of a resistive element that is grounded at one end and capacitively coupled to an electrode over its length. The type of electrode used in the measurements can be very important as the surface resistivity of the electrode can cause a voltage drop along the surface, particularly at higher frequencies when there are larger capacitive currents. Thus, regions of the sample further away from the measuring contact will be at a reduced potential and will affect the overall results.