Page 15 - Issue 2 2023

TECHNICAL

Condition-based assessment of

on-load tap changers

By IAR Gray, Oilwatch

n-load tap changers (OLTCs) are a crucial element of Since gas data alone cannot provide sufficient information
utility networks, as they must operate in a precise to fully assess OLTC condition, new approaches were required
Ofashion in order to maintain a constant voltage output. for OLTC evaluations. The search for an effective new approach
This must be achieved regardless of variation on input or led to the development of Condition Codes, which provide
load. OLTCs have been a weak link in many networks, as they a condition assessment of the load path components. In
deteriorate over time due to mechanical problems or contact addition to revealing useful information for the maintenance of
wear from repeated operation. Erosion of the contacts over insulating fluid, fluid assessment tests are used in conjunction
time is expected due to the nature of their function. Coking with OLTC gas data to provide diagnostic information about the
of the contacts causes overheating, which can cause thermal condition of OLTCs.
runaway. Regular maintenance is necessary to ensure continued Keeping the oil free of water, arc decomposition products
proper functioning. Oil testing has long been recognised as an and other contaminants, is essential for proper operation of the
important tool for detecting incipient faults in the main tanks of OLTC. Particle profiling provides important information about
transformers and is being applied to load tap changers. the deterioration of materials that result in particle production.
This includes information about in-service processes such as
Oil testing: fluid degradation, contact deterioration and mechanical wear of
• Can usually be performed while equipment remains in moving parts and rust formation.
service Two of the most important fluid degradation processes to
• Can detect a wide range of problems in the early stages be evaluated are charring of the oil and coke formation.
• Can be used to ascertain a reasonable sense of the severity Table 1 shows the gas-producing processes that occur in
of the problem oil-filled electrical equipment. The gases that are produced
• Has been shown to be very cost effective by these processes are listed in Table 2. Recognition of these
differences between normal and abnormal gassing patterns
Oil testing is used to provide one of the most important early paved the way to diagnostic assessment of OLTCs.
warning diagnostics for on-load tap changers and, because of DGA and other non-invasive tests can be combined to
its effectiveness, has made condition-based maintenance a assess the OLTC’s condition. Diagnostic programmes have been
reality. successfully developed to achieve the goal of condition-based
Dissolved gas analysis (DGA) has been applied successfully maintenance with the consequent saving in costs.
for many years to noncurrent switching oil-filled power
equipment. The application of DGA to OLTCs has both
similarities and differences to the use of DGA in other oil-filled
power equipment. It is similar in that the same processes
produce the same gases. However, in terms of gas production,
OLTCs are far more complex than transformers. OLTCs may or
may not produce all of the so-called ‘fault gases’ in normal
operation and the gases that are produced may or may not be Table 1: Gas-producing process
lost through venting.
The first applications of DGA to evaluate the condition of an
OLTC were based on experiences with transformers. Threshold
limits were developed for the gases produced by overheating,
both individually and in combination. Many factors such as
design, operation, ventilation, and online filtration affect gas
levels. Consequently, this gas-threshold approach offered
limited success, but proved the potential usefulness of fluid
testing for OLTC condition assessment. Table 2: Gases produced

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