In this paper, an analysis of experimental results on the behaviour under impulse currents of various grounding electrodes: rod and horizontal electrodes, a ground grid and tower footings is presented. The parameters used for analysing transient performance are reviewed and differences highlighted based on test data. The analysis is extended to (i) quantify the effect of impulse shape, (ii) quantify the effect of current magnitude, (iii) compare low-frequency and impulse performances, (iv) compare impulse and high frequency performances, and (v) examine the effects of the test-set up on measured results, e.g. in the case of field tests, the effect of current return leads, and proximity and extent of return electrodes. A generalized impulse index is introduced to help elucidate the differences between different parameters used for the analysis of transient test results on ground electrodes. It is found that the analysis of test data based on different parameters may lead to different assessment of impulse performance. The results also show that the impulse parameters used for the analysis of test data can be influenced by several factors such as electrode length, impulse current rise time and the experimental set up. In addition, variable-frequency test results are analysed by introducing a "harmonic coefficient" which quantifies the deviations of the harmonic impedance from the low-frequency resistance over different frequency ranges covering the entire lightning frequency spectrum. Significant variations of the harmonic coefficient with frequency were observed, highlighting the importance of taking the frequencydependence of soil properties when modelling impulse and highfrequency behaviour of grounding systems.

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An approach to high field control, particularly in the areas near the high voltage (HV) and ground terminals of an outdoor insulator, is proposed using a nonlinear grading material; Zinc Oxide (ZnO) microvaristors compounded with other polymeric materials to obtain the required properties and allow easy application. The electrical properties of the microvaristor compounds are characterised by a nonlinear fielddependent conductivity. This paper describes the principles of the proposed fieldcontrol solution and demonstrates the effectiveness of the proposed approach in controlling the electric field along insulator profiles. A case study is carried out for a typical 11 kV polymeric insulator design to highlight the merits of the grading approach. Analysis of electric potential and field distributions on the insulator surface is described under dry clean and uniformly contaminated surface conditions for both standard and microvaristor-graded insulators. The grading and optimisation principles to allow better performance are investigated to improve the performance of the insulator both under steady state operation and under surge conditions. Furthermore, the dissipated power and associated heat are derived to examine surface heating and losses in the grading regions and for the complete insulator. Preliminary tests on inhouse prototype insulators have confirmed better flashover performance of the proposed graded insulator with a 21 % increase in flashover voltage.

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