ABB Excount II User Manual page 56

6. Surge arrester monitoring theory
56
The maximum continuous operating voltage of an arrester (the Uc
according to IEC, or the MCOV according to ANSI) usually cor-
responds to a voltage stress in the range 0,7-0,85 p.u. of the rated
voltage.
In normal arrester applications, the operating voltage stress usually
ranges from 0,5 to 0,8 p.u. of the rated voltage. In this range, the
resistive leakage currents at +20°C may vary from 10 to 600 µA
depending on the size and make of the varistors.
Harmonics in the total leakage current
The non-linear voltage-current characteristic of a metal-oxide arrester,
illustrated in figure 6.3.2, gives rise to harmonics in the total leakage
current when the arrester is energized with a sinusoidal voltage. The
harmonic content depends on the degree of non-linearity, which is a
function of voltage stress, temperature and make of the arrester. As
an example, the third harmonic content of the total leakage current is
typically 10-40% of the resistive current.
The harmonic content of the total leakage current can, therefore, be
used as an indicator of the resistive leakage current. Using harmonics
for measuring the resistive leakage current is advantageous compared
to other methods, since no voltage reference is needed to determine
the resistive part of the total leakage current. The third order harmonic
is of special interest in this respect, since it has the largest magnitude
of the current harmonics.
The actual resistive leakage current level can be readily determined
from measurements of the third harmonic, provided the appropriate
information is available regarding the third harmonic content of the
resistive current at the prevailing voltage stress and temperature. This
information is specific to the arrester make and type, and must there-
fore be supplied by the arrester manufacturer.
1HSA 801 080-15en EXCOUNT-II Users Manual