(iv) a frequency and temperature-dependent capacitance also associated with polarisation with losses
(v) a capacitance to account for polarisation without losses. An equivalent circuit based on laboratory test data included the series grain resistance and inductance and contains a three-branch parallel circuit [128]:
(i) a non-linear resistance which is a function of temperature
(ii) a voltage and temperature dependent capacitance which is also affected by the rate of change of the voltage
(iii) a turn on element, which will account for the dynamic charge distribution at the grain boundary; this is a function of voltage rate of rise of voltage and the time constant t to reach the equilibrium of electrons and holes at the grain boundary. Recent research on zinc oxide surge arresters has concentrated on the very fast transient characterisation [156, 157], and two main equivalent circuits were proposed. In addition to the above elements, these circuits include parallel branches
(Figures 5.25b and 5.25c) with inductances to account for the fast transient measurement data.
5.8.5.2 Determination of the equivalent circuit main components
Low voltage tests and/or computation [158] can be used to estimate the parallel capacitance Cig, and high current impulse data can be used to calculate the ZnO grain Rg and intergranular layer Rig resistances. The voltage Vig across the intergranular layer resistance is related to current by: