A PROVEN LOSS PREVENTION TECHNIQUE
Transformer oil testing is a key part of any preventative maintenance programme
and should be conducted on a regular basis.
Consider it an early warning system that helps set priorities, schedule work, and
arrange for outside services and order parts and materials. Listed below are some
of the most common tests for transformer oil:
Dissolved Gas-in-Oil Analysis
A Dissolved Gas-In-Oil Analysis (DGA) is the most important test for transformer
oil and can give an early indication of abnormal conditions. This test analyses
the type and quantity of gases dissolved in the oil by using a small sample that
is examined in the laboratory. Certain quantities and combinations of gases typically
found in the oil can indicate insulation overheating/overloading, liquid overheating,
partial discharge (corona) or arcing in the transformer.
The screen test is a collection of physical, electrical and chemical tests for transformer
oil. Typically, a one-litre sample of oil is collected. No single test will indicate
the true condition of the liquid, so it is suggested that all tests be performed:
- Di-electric Breakdown
- Interfacial Tension
- Power Factor (dissipation factor)
The water content test detects the moisture content in parts per million of the
liquid insulation. A test to determine polychlorinated biphenyls (PCBs) on a percentage
value, or as a parts-per-million value, is widely available from commercial laboratories.
It is recommended that DGA is carried out every six months on critical transformers.
Screen tests should be carried out every 3 years.
Case In Point
As a regular part of an insurance contract, a HSB engineer was carrying out DGA
oil tests on large transformers at an electric utility. On the strength of the test
results the engineer recommended that one 156MVA transformer should be taken out
of service to permit an internal inspection. This revealed burned contacts on the
no load tap changer – a serious condition if left untreated. The transformer was
repaired on site and returned to operation.
This quick response to the machine’s own danger signal saved the utility the cost
of a replacement transformer, estimated at £600,000.