Page 70 - Energize October 2022

TECHNICAL

Transformer failures are costly

The Hillside aluminium smelter is located in Richards Bay, 200 km north of Durban, KwaZulu-Natal

province, South Africa. The operation is fully owned and operated by BHP Billiton. Construction of
Hillside Aluminium began in 1993 and Hillside’s two potlines cast their first metal in June 1995.

by Ian Gray, Oilwatch Transformer Services

n February 2003, Hillside was expanded with a further half oil sample is taken, DGA can detect intermittent faults. Also,
potline. First metal was poured from this half potline during because fault gases can be detected at very low levels, the DGA
IOctober 2003. This increased production at Hillside from 535 technique is very sensitive and eminently suitable for detecting
000 to more than 700 000 tons per annum, making it the largest faults at an early stage. Most guides for interpreting DGA results
aluminium smelter in the southern hemisphere and South include, and indeed concentrate on, schemes for diagnosing
Africa’s major producer of primary aluminium. It is one of the faults, usually by analysing the relative concentrations of the
worlds most advanced and efficient AP30 smelters and produces various fault gases, so the technique can also be described
T-bars and primary aluminium ingots. as discriminating and contributing to diagnosis as well as the
The Hillside smelter consumes 1100 MW of electrical power, detection of faults.
with approximately 147 installed transformers in 1995. The unit’s The main difficulty in making use of DGA results, which arises
capacity ranges from 90,8 MVA regulators/93,5 MVA rectifiers/35 from its very good sensitivity, is that it is not easy to draw the
MVA auxiliaries on the 132 kV system: 6,3 to 1,6 MVA on the 22 line between normal and abnormal results, i.e., to be sure that
kV system and 600 to 200 kVA on the 3,3 kV systems. a fault really exists. Most, but not all, interpretation schemes
Dissolved gas analysis (DGA) has been a widely accepted include a normal condition as one of the diagnostic outcomes
preventive maintenance tool for the electric power industry but have not been particularly effective in reliably identifying a
for over thirty years. Though DGA continues to be a vital normal condition.
component of assessing transformer condition, the demands This article provides a summary of a fault in a furnace
imposed by the increased loading of transformers and the rectifier transformer that was detected by dissolved gas analysis
ageing of the transformer population require new assessment at early life and will act as an aid and guide to the power
tools and diagnostic approaches. It has been suggested that engineer. “When the transformer should be removed from
over 70% of transformer condition information is contained service” (See Figure 2).
within the insulating fluid and that many transformer failures
are attributable to manageable problems. Many of these
problems are identified only after a thorough understanding
of the complex relationships that exist between DGA data and
information obtained from analysing the insulating fluid in
transformers.
Dissolved gas analysis (DGA) of oil samples is probably the
most effective means of monitoring the condition of oil-filled
electrical equipment such as transformers, for several reasons.
Firstly, nearly every possible fault generates one or more Figure 2: Transformer life cycle model
gases arising from the consequential increased degradation of
adjacent oil or cellulosic insulation, so DGA can be said to be Transformer design and construction
comprehensive in responding to many faults. Furthermore, since Transformers are normally very reliable items of electrical
in the early stages these ‘fault’ gases dissolve in the oil and can equipment, but when faults occur, they can lead to the loss of
then be detected at some subsequent point in time when an what is usually the most expensive item of equipment in the
substation. In addition, some faults can develop catastrophically,
with the potential to cause substantial collateral damage to
nearby equipment and posing a risk to personnel.
The modern power transformer is designed with far less
insulation material and electrical clearances due to the pressure
of driving down costs. This factor needs to be considered with
the failure rate at the Hillside smelter. See Table1 comparing
Figure 1: Transformer distribution on primary voltage transformers between the 1970s and 1980s.

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