Page 16 - Issue 2 2023

TECHNICAL

Non-invasive tests for OLTCs The primary test for OLTC diagnostics and condition
Dielectric breakdown voltage assessment is that of dissolved gas-in-oil, as this detects most
The oil in an OLTC should maintain a minimum dielectric of the problems. There are three main types of OLTCs:
breakdown voltage. In recent years on-line filters have been • Reactive with arcing contacts in oil
used for compartments containing arcing contacts in oil to • Resistive with arcing contacts in oil
better maintain the dielectric breakdown strength of the • Arcing contacts in a vacuum bottle
insulating materials. This approach has been effective in
pushing out maintenance cycles and reducing the rate of There should be differences in the gassing behaviour
contact wear. The dielectric breakdown voltage is a function of between resistive and reactive types, as the shorter time of
the relative saturation of water in oil and the amount, size and arc extinction of the resistive type (5-6 mins after contact
type (conductivity) of particles. separation) should lower the concentrations of gases
generated. However, the gassing behaviour of different models
Water content of OLTCs is so different that generic rules for reactive and
Excessive water reduces the dielectric breakdown strength of resistive OLTCs are inadequate.
the oil and can accelerate the ageing of the contacts.
The primary diagnostic gases used to develop condition
Neutralisation number codes are methane, ethylene and acetylene. In addition, the
As oils and cellulosic materials age, they will deteriorate following ratios are used:
and form ageing by-products, including acids. Eventually the • Ethylene/acetylene: Distinguishes between thermal and
aging by-products will begin to polymerize and form sludges. electrical discharge activity in oil
Increasing acidity can be used as a guide to the ageing rate • Methane/acetylene: Distinguishes between thermal and
of the oil. When high values are reached, the oil should be electrical discharge activity in oil and can also detect partial
replaced or reclaimed. Acid by-products, particularly in the discharge activity as a predominant gassing pattern. For
presence of water, are corrosive. example, localised overheating of contacts or the reversing
switch will generally show increasing combustible gas
Total metals in oil generation, with ratios of gassing going from an arcing
The metals test, consisting of both particulate metals and pattern to characteristics of high temperature overheating
those dissolved in the oil, is an extremely meaningful test. of oil.
It provides an indication of the amount of material that has
been worn or sublimated from the moving and/or stationary Excessive arcing between contacts is most likely to develop
contacts and is now present in the oil. It also provides a high gas concentrations until the later stages when heating
quantitative analysis as to composition of the metals found in occurs (causing the combustible gas ratios to change).
the oil.

Particle count and qualitative analysis
The total number of particles by size groupings is used to
detect abnormal quantities of by-products and wear materials.
The ratio(s) of the size groupings provides information as to
the extent that a detrimental condition has progressed.

DGA diagnostics
It is undisputed that DGA plays a primary diagnostic role.
The interpretation protocol for applying DGA is empirical
in nature. In the case of OLTCs it is generally accepted that Table 3: Diagnostic tools
fault gas interpretation will be most useful if it is model-
specific.
Individual gas concentrations-based diagnostics for OLTCs
are not useful as they are operation count and breathing
configuration dependent. DGA ratios of fault gases are fairly
independent of operation count. The various gas ratios are
excellent diagnostic tools.
Table 4 lists the gas ratios proposed by Weidmann-ACTI (F
Jakob, K Jakob, S Jones). Table 4: Diagnostic gas ratios

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