Page 52 - Energize January 2022

TECHNICAL

A full-spectrum analysis (DFR) with
a Megger IDAX300 (Figure 11) is the
definitive method to determine the
condition of solid and liquid insulations.
For a service-aged transformer, the
recommended maximum limit of moisture
is <2% and the recommended limit for
liquid insulation conductivity is <37 pS/m.
Surface contamination of the solid
insulation and tank walls must be analysed
in the laboratory. Sediments and particles
must be properly identified to better
understand the source of such material.
Be proactive, not reactive, and make
Figure 9: Dielectric Response after oil processing sure that you are equipped with the right
tools to make the right decisions.
core coil assembly several times to remove regions. T 1 and T 2 are part of the six basic
contamination. Samples were taken and types of faults or stresses in transformers Benefits
sent to the laboratory for analysis. identifiable with the Duval Triangle 1. T 1 The main concern is always safety. Proper
It is important to mention that corresponds to thermal faults < 300°C, and screening of the electrical assets that
historical DGA data (Figure 10) did not T 2 corresponds to thermal faults between personnel work around is paramount in
alarm the operator. In the Duval Triangle 300 and 700°C. reducing risk of unexpected failure(s)
1, values fluctuated within T 1 and T 2 and collateral damage. In this case,
Takeaways a thorough screening identified a
Line frequency PF (DF or tangent delta) transformer that may have suddenly
measurements by themselves may or failed due to dielectric breakdown,
may not reflect the true condition of the overheating or thermal runaway.
insulation system inside a transformer. While lab results must confirm, the likely
It is fundamental practice to record source of contamination is the compromised
a benchmark signature of the dielectric pump. Even flushing the transformer several
condition of a power transformer before times may not guarantee a residual-free
any maintenance work is carried out. environment for the liquid and paper
Look beyond the information provided insulation. The significantly better screening
by LF PF. 1 Hz (as demonstrated in this tool of 1 Hz PF and a full DFR test will
case study) and 500 Hz complement LF PF. provide some perspective about the success
Figure 10: DGA identified fault condition from
2009 to 2021 for the 1978 transformer of this Practical references are now available to of a flushing remedy. Should conductive
case study assess 1 Hz and 500 Hz PF test results. contaminants remain, however, the
transformer design is invalid, and the asset
owner assumes elevated risk of failure. In
this event, the transformer may need to be
replaced as soon as possible. Alternatively,
this transformer may be derated with some
manufacturer guidance, and, for example, be
used or relocated to an area where load will
not exceed 50% of rated capacity.

Reference
1. Diego Robalino: Transformers Magazine,
https://transformers-magazine.com/
magazine/2139-individual-temperature-
compensation-power-transformer/

Contact Kenneth Petroff, Megger,
Kenneth.petrof@megger.com,
Figure 11: Megger IDAX300 DFR insulation diagnostic instrument www.megger.com

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