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Transformer Fault a d Detection

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In order to maximize the lifetime and efficacy of a transformer, it is important to be aware of possiblefaults that may occur and to know how to catch them early. Regular monitoring and maintenance canmake it possible to detect new flaws before much damage has been done.

The four main types of transformer faults are:

1. Arcing, or high current break down2. Low energy sparking, or partial discharges3. Localized overheating, or hot spots4. General overheating due to inadequate cooling or sustained overloading

These faults can all lead to the thermal degradation of the oil and paper insulation within the transformer.One way to detect them is by evaluating the quantities of hydrocarbon gases, hydrogen and oxides ofcarbon present in the transformer. Different gases can serve as markers for different types of faults. Forinstance,

Large quantities of hydrogen and acetylene (C2H2)can indicate heavy current arcing. Oxides of carbonmay also be found if the arcing involves paper

insulation. The presence of hydrogen and lower order

hydrocarbons can be a sign of partial discharge

Significant amounts of methane and ethane maymean localized heating or hot spots.

CO and CO2 may evolve if the paper insulationoverheats; which can be a result of prolongedoverloading or impaired heat transfer.

Techniques to Detect Faults

Techniques to detect transformer faults include the Buchholz Relay safety device, dissolved gas analysis(DGA) tests and a range of tests for detecting the presence of contaminants in the oil, as well as formeasuring indicators of oil quality such as electric strength and resistivity.

Buchholz Relay

A Buchholz Relay is also called a gas detection relay. It is a safety device generally mounted atthe middle of the pipe connecting the transformer tank to the conservator. A Buchholz Relay maybe used to detect both minor and major faults in the transformer.

This device functions by detecting the volume of gas produced in the transformer tank. Minorfaults produce gas that accumulates over time within the relay chamber. Once the volume of gasproduced exceeds a certain level, the float will lower and close the contact, setting off an alarm.

Major faults can cause the sudden production of a large quantity of gas. In this case, the abruptrise in pressure within the tank will cause oil to flow into the conservator. Once this is detectedthe float will lower to close the contact, which causes the circuit breaker to trip or sets off thealarm.

Dissolved gas analysis (DGA)

Dissolved gas analysis, or DGA, is a test used as a diagnostic and maintenance tool formachinery. Under normal conditions, the dielectric fluid present in a transformer will not

Techniques for finding faults:

Buchholz Relay safety

device

Dissolved gas analysis

Tests to detect oil

contaminants and oil quality

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decompose at a rapid rate. However, thermal and electrical faults can accelerate thedecomposition of dielectric fluid and solid insulation. Gases produced by this process are all oflow molecular weight, and include hydrogen, methane, ethane, acetylene, carbon monoxide andcarbon dioxide. These gases will dissolve in the dielectric fluid. Analyzing the specific proportionsof each gas will help in identifying faults. Faults detected in such a way may include processessuch as corona, sparking, overheating and arcing.

Abnormal functioning within a transformer can be caught early by studying the gases thataccumulate within it. If the right countermeasures are taken early on, damage to equipment canbe minimized.

Other oil tests

Other oil tests used to detect faults include acidity tests, electric strength tests, fiber estimationtests, color tests, water content tests, Polychlorinated Biphenyl Analysis (PCB) tests,furfuraldehyde analysis tests, metal in oil analysis tests and resistivity tests.

o Acidity test: The acidity of transformer fluid should be monitored regularly. High aciditiescan hasten the degradation of paper insulation and cause steel tanks to corrode.

o Electric Strength: The electric strength of an insulating fluid is its capacity to withstandelectrical stress without failing. The lower the dielectric strength of a fluid, the less it willbe able to insulate. Transformer failure can result if the dielectric strength drops too low.

o Fiber estimation: If fibers or other contaminants are present in a transformer's oil, theymay reduce the oil's electric strength. Wet fibers in particular can be drawn into anelectrical field, resulting in arcing. Passing polarized light through an oil sample canmake fibers and other sediments visible, making it possible to estimate the fiber contentof the sample. Sampling should be done carefully, since both fibers and moisture may bepicked up during the process of sampling itself.

o Color: Obvious changes in oil color (for instance, light oil abruptly growing dark) mayindicate deeper changes within the oil itself that need to be examined further.

o PCB Test: A Polychlorinated Biphenyl Analysis (PCB) test calculates the concentration orpresence of polychlorinated biphenyl within the oil. Capillary column chromatography

can be used for this process. While the presence of PCBs is not an indication of oilquality, PCBS are a banned substance, no longer allowed in new liquid filledtransformers.

o Metal in oil analysis: The concentrations of various metals in a transformer's oil can becalculated by using methods such as atomic absorption spectroscopy (AA) and inductivecoupled plasma spectrometry (ICP).

o Furfuraldehyde Analysis: The concentration of furfuraldehyde in an oil sample can beused as a measure of paper degradation. Furfuraldehyde is one of the byproducts ofpaper degrading and growing weaker, a process which sets a natural limit on atransformer's life. Monitoring its concentration levels can help determine the remainingservice life of a transformer.

o Moisture: Excess moisture in the oil can cause the oil's electric strength to plummet,leading to transformer failure. It is therefore very important to monitor moisture levels in

the transformer.o Resistivity Test: High resistivity indicates low levels of free ions and ion-forming particles,as well as low levels of conductive contaminants. Resistivity tests are generally carriedout at ambient temperature. It can also be useful, however, to carry out tests at muchhigher temperatures, the results of which can be compared to results at ambienttemperature.