Power transformer failure
results in production loss, unavailability of critical services, and loss of
revenue. Routine testing and performing diagnostics can minimize loss and down
time.
Reliable and continual
performance of power transformers is the key to beneficial generation and
transmission of electric power.
Generally, reasons for failure
include external factors such as lightning strikes, system overload, short
circuits, and internal factors such as insulation deterioration, winding
failure, overheating, and the presence of oxygen, moisture, and solids in the
transformer oil.
To minimize unexpected
outages, periodic transformer testing and diagnostics is necessary.
Three categories can be
defined for transformer testing:
○ Performance acceptance test after
installation and commissioning of the transformer.
○ Predictive maintenance plan-based test
during normal operation of the transformer to verify that electrical properties
have not changed from design specifications.
○ Failure test for identify breakdown cause
of the transformer.
These tests are required to
determine electrical, thermal, and mechanical characteristics.
A daily checklist procedure
should be established to perform the visual routine test. It should contain oil
temperature, winding temperature, oil level, humming (noisy operation), and oil
leakage checks. An annunciation window (an indicator that announces which
electrical circuit has been active) displays alarm and trip signals generated
from the load.
Buchholz Relay
A Buchholz relay is a safety
device normally mounted at the middle of the pipe connecting the transformer
tank to the conservator. It is a gas detection relay used to detect minor and
major faults in the transformer. A Buchholz relay operates by detecting the
volume of gas generated in the transformer tank. Gas produced by faults accumulates
over time within the relay chamber. Whenever the volume of gas exceeds a
certain safe level, the float moves lower, closes the contact, and generates an
alarm. The fault alarm can be displayed on an annunciation window and the
master trip relay will cause the circuit breaker to open.
Thermal imagers capture images
of infrared energy or temperature. They can detect heat patterns or temperature
changes in equipment. The engineer can determine problems prior to an expensive
down time by analyzing these temperature changes. Conveniently, one can measure
and compare heat readings for each part of the equipment without disrupting the
transformer's operation.
Prevention, diagnosis, and
repair benefits can be obtained for transformers by introducing Infrared
thermography into your predictive maintenance plan.
Insulation ages and
deteriorates because of moisture, dust, and electrostatic stress. Insulation
should be monitored continually to avoid sudden failure of the equipment.
An insulation resistance test
detects insulation quality within the transformer. The conductive impurities or
mechanical flaws in the dielectric can be analysis based on this test. The
instrument used to measure insulation resistance is known as the "megger."
Normally meggers have a test voltage of 500V, 2500V, or 5000V.
Each winding should be short
circuited at the bushing terminals. The resistance value should be measured
between each winding and with respect to ground also. The winding should be
discharged after the test is completed by connecting to the ground.
The insulation resistance
value measured is usually in the order of mega-ohms. Generally the value should
be greater than 1 megohm for every 1kV rating of the equipment.
Insulation resistance values
decrease with increase in the temperature. Therefore the values should be
normalized for a standard temperature. It is necessary to have the insulation
resistance as high as possible.
Each winding of a transformer
contains a certain number of turns of wire. The "transformer turns
ratio" is the ratio of the number of turns in the high voltage winding to
that in the low voltage winding. The ratio is calculated under no-load
conditions.
The transformer ratio can
change due to several factors like physical damage because of faults,
deteriorated insulation, contamination of oil etc. If a transformer ratio
changes more than 0.5 percent from the rated voltage ratio, it needs immediate
attention.
The turns ratio will establish
the proper relationship between the primary and secondary winding impedances.
The turns ratio is the square root of the impedance ratio, i.e.
iZpri/Zsec = (Npri/Nsec)2
Zpri = Primary Impedance
Zsec = Secondary Impedance
Npri = Number of turns on the
primary coil
Nsec = Number of turns on the
secondary coil
Transformer overloading,
overheating, corona, sparking, and arcing can cause thermal degradation of the
oil and paper insulation within the tank. Thermal and electrical faults can accelerate
the decomposition of dielectric fluid and solid insulation. Gases generated by
this process include hydrogen, methane, ethane, acetylene, carbon monoxide, and
carbon dioxide, all which will dissolve in the transformer oil.
The DGA test involves extracting
the gases from the oil and injecting it into a gas chromatograph. Gas
concentrations are detected using a flame ionization detector and a thermal
conductivity detector.
Diagnostic and analysis of the
specific proportions of each gas shall help to identify the fault type (thermal
conditions involving the oil or the paper, partial discharge, sustained arcing,
etc.).
A DGA test study can minimize
damage by taking precautionary actions at an early stage.
The magnetic balance test is
conducted on transformers to detect inter-turn faults and magnetic imbalance.
It gives an idea about the flux distribution in the core. It is only an
indicative test and does not reduce the need for other tests in diagnostics.
The magnetic balance test is
carried out on a three phase transformer by applying a two phase supply across
the phases (i.e. one winding say U and V) with a lower than rated voltage.
Other phases should be kept open circuit. The sum of voltage measured between
V-W and U-W should be equal to U-V. A voltage measured in the secondary side
will also be proportional to the voltage measured at the primary.
A very low voltage will induce
in defected winding because it will not allow flux to pass in the magnetic path
around the core. It may result in the sum of the two voltages not being equal
to the applied voltage.
Degradation of the insulation
takes place because of mechanical vibration, over temperature operation, and
gaseous and metallic impurities in the transformer. This may cause insulation
ageing and breakdown. It is very important to study the insulation quality of
the machine. The dissipation factor Tan or Power Factor Cos Ø is considered to
indicate the quality of insulation. It is also known as the loss angle test or
the dissipation factor test.
A clean insulation acts as a
capacitor. The current should lead the voltage by 90 degrees in a pure
capacitor. The pure insulation should also conduct similarly. If the insulation
is deteriorated, the current will also have resistive factor. This will cause
the angle of the current to be less than 90 degrees. This measured difference
in the angle is described as the loss angle. The tangent of the angle (i.e.
opposite/adjacent) indicates the condition of the insulation. A greater loss
angle value points to a high contamination of the insulation.
The BDV test measures the
dielectric strength of the oil using an oil tester. During this test, spherical
electrodes having a 2.5 mm gap shall be gradually applied voltage until the oil
loses its dielectric strength. This test should be performed for one minute,
and the breakdown voltage displayed on the oil tester meter should be
considered the BDV. Normally it may be 60 kV and over for one minute or as per
ASTM D877-82 or IS-335.
A low value in this test
indicates the presence of impurities in the oil. In this case it should be
filtered to remove impurities and moisture.
Followings are other tests
that can be used to detect oil based faults:
○ Acidity test
○ Electric strength test
○ Color test
○ Polychlorinated Biphenyl Analysis (PCB)
test
○ Fiber estimation
○ Furfuraldehyde analysis test
○ Metal in oil test
○ Resistivity test
○ Furan analysis
○ Frequency Response Analysis