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Transformer Faults and Detection

In order to maximize the lifetime and efficacy of a transformer, it is important to be

aware of possible faults that may occur and to know how to catch them early. Regular

monitoring and maintenance can make 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 down
2. Low energy sparking, or partial discharges
3. Localized overheating, or hot spots
4. 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 of carbon present in the transformer.  Different

gases can serve as markers for different types of faults. For instance,

* Large quantities of hydrogen and acetylene (C2H2) can indicate heavy current

arcing. Oxides of carbon may 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 may mean localized heating or hot spots.
* CO and CO2 may evolve if the paper insulation overheats; which can be a result of

prolonged overloading or impaired heat transfer.

Techniques to Detect Faults

Techniques for finding faults:

* Buchholz Relay safety device
* Dissolved gas analysis
* Tests to detect oil contaminants and oil quality

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 for measuring 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 at the middle of the pipe connecting the transformer tank to the

conservator. A Buchholz Relay may be 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.  Minor faults produce gas that accumulates over time within the relay chamber. Once

the volume of gas produced 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 abrupt rise in pressure within the tank will cause oil to flow into the

conservator.  Once this is detected the float will lower to close the contact, which

causes the circuit breaker to trip or sets off the alarm.
* Dissolved gas analysis (DGA)

Dissolved gas analysis, or DGA, is a test used as a diagnostic and maintenance tool

for machinery. Under normal conditions, the dielectric fluid present in a transformer

will not decompose at a rapid rate. However, thermal and electrical faults can accelerate

the decomposition of dielectric fluid and solid insulation. Gases produced by this

process are all of low molecular weight, and include hydrogen, methane, ethane,

acetylene, carbon monoxide and carbon dioxide. These gases will dissolve in the

dielectric fluid. Analyzing the specific proportions of each gas will help in identifying

faults.  Faults detected in such a way may include processes such as corona, sparking,

overheating and arcing.

Abnormal functioning within a transformer can be caught early by studying the gases

that accumulate within it.  If the right countermeasures are taken early on, damage to

equipment can be minimized.

*

Other oil tests

Other oil tests used to detect faults include acidity tests, electric strength

tests, fiber estimation tests, 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 acidities can 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 withstand electrical stress without failing. The lower the dielectric

strength of a fluid, the less it will be 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, they may reduce the oil's electric strength.   Wet fibers in

particular can be drawn into an electrical field, resulting in arcing.  Passing polarized

light through an oil sample can make fibers and other sediments visible, making it

possible to estimate the fiber content of the sample. Sampling should be done carefully,

since both fibers and moisture may be picked up during the process of sampling itself.

o Color: Obvious changes in oil color (for instance, light oil abruptly growing

dark) may indicate deeper changes within the oil itself that need to be examined further.

o PCB Test: A Polychlorinated Biphenyl Analysis (PCB) test calculates the

concentration or presence 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 oil quality, PCBS are a banned substance, no longer allowed in new liquid

filled transformers.

o Metal in oil analysis:  The concentrations of various metals in a

transformer's oil can be calculated by using methods such as atomic absorption

spectroscopy (AA) and inductive coupled plasma spectrometry (ICP).

o Furfuraldehyde Analysis: The concentration of furfuraldehyde in an oil sample

can be used as a measure of paper degradation. Furfuraldehyde is one of the byproducts of

paper degrading and growing weaker, a process which sets a natural limit on a

transformer's life.  Monitoring its concentration levels can help determine the remaining

service 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 carried out at ambient temperature. It can also be useful, however, to

carry out tests at much higher temperatures, the results of which can be compared to

results at ambient temperature.

*

Other oil tests

Other oil tests used to detect faults include acidity tests, electric strength

tests, fiber estimation tests, 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 acidities can 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 withstand electrical stress without failing. The lower the dielectric

strength of a fluid, the less it will be 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, they may reduce the oil's electric strength.   Wet fibers in

particular can be drawn into an electrical field, resulting in arcing.  Passing polarized

light through an oil sample can make fibers and other sediments visible, making it

possible to estimate the fiber content of the sample. Sampling should be done carefully,

since both fibers and moisture may be picked up during the process of sampling itself.

o Color: Obvious changes in oil color (for instance, light oil abruptly growing

dark) may indicate deeper changes within the oil itself that need to be examined further.

PCB Test: A Polychlorinated Biphenyl Analysis (PCB) test calculates the

concentration or presence 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 oil quality, PCBS are a banned substance, no longer allowed in new liquid

filled transformers.

o Metal in oil analysis:  The concentrations of various metals in a

transformer's oil can be calculated by using methods such as atomic absorption

spectroscopy (AA) and inductive coupled plasma spectrometry (ICP).

o Furfuraldehyde Analysis: The concentration of furfuraldehyde in an oil sample

can be used as a measure of paper degradation. Furfuraldehyde is one of the byproducts of

paper degrading and growing weaker, a process which sets a natural limit on a

transformer's life.  Monitoring its concentration levels can help determine the remaining

service 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 carried out at ambient temperature. It can also be useful, however, to

carry out tests at much higher temperatures, the results of which can be compared to

results at ambient temperature.

To know more about <a href="http://www.pacificcresttrans.com/home.html">Liquid filled

distribution transformers</a> check out Pacific crest transformers website.

http://www.pacificcresttrans.com/home.html

About the Author

mike dikinson-contributing writer for Pacific crest transformers.

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