Leakage Current Measurements - ABB Excount II User Manual

6. Surge arrester monitoring theory
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6.3 Leakage current measurements

Apart from the brief occasions when a surge arrester is functioning
as an overvoltage-limiting device, it is expected to behave like an
insulator. The insulating properties are essential for the length of life
of the arrester and for the operation reliability of the power system.
Any deterioration of the insulating properties of a metal-oxide arrester
will cause an increase in the resistive leakage current, at given values
of voltage and temperature. Therefore, the resistive leakage current
in service can be used as a diagnostic tool to check the condition
of a surge arrester. Leakage current measurements for diagnostic
purposes are usually made on temporary basis at regular intervals.
Repeated measurements may be necessary for closer investigations,
if significant changes in the condition of an arrester are revealed by
temporary measurements.
Leakage current of metal-oxide arresters
The total leakage current of a metal-oxide arrester can be divided into
capacitive and resistive parts, see figure 6.3.1, with a predominant
capacitive component and a significantly smaller resistive part (5 to
20% of the total current). The capacitive leakage current is caused by
the permittivity of the metal-oxide varistors, by the stray capacitances
and by internal grading capacitors (if applied).
A large increase in the resistive leakage current is needed before a
noticeable change occurs in the total leakage current level. Therefore,
the total leakage current is unsuitable for arrester diagnostic purposes.
Instead, it may be used for other diagnostic or maintenance purposes,
e.g. to estimate the prevailing amount of insulator surface pollution
and the associated need for insulator washing, greasing etc. of the
insulators in the substation.
The resistive component of the leakage current, on the other hand,
is a sensitive indicator of any changes in the voltage-current cha-
racteristic of a metal-oxide arrester. The EXCOUNT-II is equipped for
measurement of the total leakage current and, optionally, for measure-
ment of the resistive leakage current. In this way, the EXCOUNT-II may
be equipped to fulfil different diagnostic needs in addition to surge
counting.
1HSA 801 080-15en EXCOUNT-II Users Manual