Page 6 - Energize Issue 1 2023

COVER STORY

If the values are Good, continue PROPERTY CATEGORY GOOD FAIR POOR
with the current sample interval action Breakdown voltage (kV) O, A, D >60 50 to 60 <50
plan. B, E >50 40 to 50 <40
If the values are Fair, more
frequent sampling is recommended C >40 30 to 40 <30
in collaboration with other parameter F <30 kV for OLTC in star point application
results like the water content, DDF <40 kV for OLTC in delta or line-end application
(dielectric dissipation factor) and G <30
acidity. Table 3: Breakdown voltages
If the values are Poor, immediate
action is required. Excess water in
the transformer system decreases apply to transformers with operating might move into the transformer or
transformer life significantly; an temperatures between 40 and 70°C. electrical equipment through leaks. This
extremely high-water content can If it is found that the unit’s operating problem might be more severe if the
cause flashover in the unit, resulting temperature is outside this range, it is transformer or electrical equipment is
in loss of the asset. The oil should best to refer to Annex A of the standard. outside and not in a covered area.
be reconditioned. If alternative tests When the value obtained through
indicate severe ageing, the oil can analyses is Good, normal sampling Please click here the read the full
be replaced with new or reclaimed interval can be maintained. version which includes discussion on the
oil. Another option would be to When the value returns a Fair following tests:
perform on-site oil reclamation using a result, more frequent sampling is • Acidity (mgKOH/g oil) neutralisation
reclamation plant. recommended. It is also helpful to number
Reclamation of oil has the consider other parameters like the • Dielectric dissipation factor at 40 Hz
advantage that the colour of the oil is breakdown voltage, particle content and to 60 Hz at 90°C
restored, and the polar components are DDF/resistivity, and acidity to decide on • Resistivity (GΩm) at 20°C or 90°C
removed. This process removes acid the action to be implemented. • Inhibitor content %
and water as well as other compounds. A Poor result will require immediate • Passivator content
Another advantage is that the oil can be action from the asset manager. This • Sediment and sludge
re-used, and, mostly, this can be done might include taking another sample to • Interfacial tension
without switching off the unit, which confirm the results. If high water content • Corrosive sulphur
contributes to cost-saving. is confirmed, the oil can be filtered; this • Particle counting and sizing
This is a routine test for all classes of process should remove a large portion • Flashpoint (°C)
electrical equipment, except class G. of the moisture from the oil. Follow-up • PCB (Polychlorinated Biphenyls)
These test results should always samples need to be taken to ensure • DGA (dissolved gas analysis)
be considered in conjunction with that the moisture content is still within
the breakdown strength. If it is found the required limits. The reason is that Conclusion
that the water content is high, and the most significant portion of the Transformer condition monitoring is an
the breakdown strength is low, further water is caught up in the paper system interlaced, highly exciting field of study.
action is needed. It is recommended in the transformer. This moisture will In this article, we focused on the types of
that a second sample from the same move from the paper into the oil under tests to determine the condition of the
unit is tested to confirm the results. conditions that favour this movement. It transformer, the critical values, and the
In the case of switching equipment, might be found later that the oil in the recommended actions.
where there is no paper present, the water has increased again without any The Health Index indication
breakdown voltage is the determining apparent reason, but the source would makes it possible to see the supposed
factor. be the paper in the transformer. reliability of a specific unit at a specific
It should be noted that the limits A visual inspection is also date and time. This makes it possible
indicated by IEC 60422 Edition 4 recommended to determine if any water to ensure best practice application and
optimised maintenance. It also makes
it easier to draw up a maintenance and
PROPERTY CATEGORY GOOD FAIR POOR action plan.
Water content (mg/kg O, A <15 15 to 20 >20
at transformer B, D <20 20 to 30 >30 Click here to find the references
operating temperature) C, E <30 30 to 40 >40
F Action necessity >40 Contact WearCheck,
Phone 031 700-5460,
G Not a routine test marketing@wearcheck.co.za,
Table 4: Water content (mg/kg at transformer operating temperature) www.wearcheck.co.za

energize | Issue 1 2023 | 6