1.
Causes of stator winding insulation
deterioration
There are a
lot of reasons causing the winding insulation deterioration. They can be categorized
into thermal, electrical, mechanical, and environmental stress.
Thermal stress)
Thermal aging process and thermal overloading are two most significant stresses
relating thermal effect. The aging process is accelerated through the increase
in temperature, and as a result, reduces the lifetime of the insulation. As a
rule of thumb, a 10oC increase in temperature decreases the insulation life by
50%. The other stress is thermal over- loading, which occurs due to voltage
variations, supply voltage imbalances, cycling, overloading, obstructed
ventilation, or ambient temperature. As a rule of thumb, the temperature in the
phase with the highest current will increase by 25% for a voltage imbalance of
3.5% per phase.
Electrical
stress) Regarding electrical stress, the most common causes would be from the
dielectric material, the phenomena of tracking and corona, and the transient
voltages. The type of dielectric material under the influence of the voltage
stresses applied to the insulating materials can decrease the lifetime of the
insulation significantly. Tracking and corona are phenomena that only occur at
operating voltages above 600 V and 5 kV, respectively, but have noticeable
influence as well. Relating voltage transients, they cause either deterioration
of the insulation or even inter-turn or turn-to-ground failures.
Mechanical stress)
Two main mechanical stresses are due to coil movement and strikes from the
rotor. Under magneto-motive force, one force is exerted on the winding coils
and causes the coils to move and vibrate. As a result, under some
circumstances, this movement can lead to severe damage to the coil insulation
or conductors. Several common causes of strikes from the rotor to stator would
be bearing failures, shaft deflection, and rotor-to-stator misalignment. Under
the strikes, the contact between rotor and stator can be made during the start
or at full speed of the motor, and both situations can result in a grounded
coil.
Environmental
stress) Contamination, high humidity, aggressive chemicals, radiation in nu-
clear plants, are among other kind of tresses in this environmental or ambient
stress. Some can lead to significant consequence through reduction in the heat
dissipation such as the presence of foreign material by contamination. Other
stress such as aggressive chemicals can directly deteriorate the insulation and
reduce the insulation resistance to mechanical stresses. Radiation is a stress
in rage of nuclear power plants or nuclear powered ships and its aging process
is comparable to thermal aging. Stator winding inter-turn fault is usually
considered as the one occurring first and leading to further severe faults such
as phase-to-phase and phase-to-ground. When the stator winding inter-turn fault
involving even only a few turns occurs, the current in the fault loop winding
can be of the order of twice locked-rotor current.
2.
Tests and diagnosis methods
The methods
for fault diagnosis of stator winding inter-turn fault can be categorized into
different groups based on various criteria. They can be categorized into
offline testing and online monitoring groups in which the former requires the
motor to be removed from service and the latter still works while the machine
is in operation.
…
Firstly, a
group of stator winding insulation testing methods is discussed. The methods
can be offline or online and the term “testing” implies that they require
additional devices/signals to test whether or not there is a winding insulation
fault. Usually, these test methods can provide accurate and reliable diagnosis
results. Insulation resistance (IR) and polarization index (PI), voltage surge,
and partial discharges (PD) tests are among the most common testing methods in
industry. The two formers are suitable for machines rated 400 V and above while
the latter is only relevant for those rated at least 4 kV]. In the insulation resistance
test, a DC voltage is applied between the winding copper and ground and the
insulation resistance is calculated as the ratio between this voltage and the
resultant current. The calculated IR should be high and the specific values of
the applied voltage and the resistance threshold for evaluating insulation
condition are followed several different standards such as IEEE 43-2000 and
NEMA MG-1-1993. This test has simple procedure. Nonetheless, it strongly
depends on temperature at test condition. The PI can be used to overcome this
drawback of insulation resistance test. This index basically generates a
quantity to measure the time required for molecules of insulation to polarize
to resist the flow of current and hence evaluate the ability of groundwall
insulations to polarize. It is calculated as the ratio of the insulation
resistance at one minute moment and ten minutes moment. One again, the specific
value of PI for testing is available at several standards such as IEEE 43-2000.
Due to it easy implementation, this testing method is widely used but mainly
for testing phase-to-ground insulation.
The voltage
surge test can probe inter-turn fault by applying a high voltage between the
tested turns. The principle of this test is to create a voltage surge so that a
capacitor is charged and subsequently discharged. Since the capacitor and the
motor present an RLC-series circuit, if the inter-turn insulation is
deteriorated, a change in the frequency and the magnitude of the impulse
response can be observed. Even though this method can test inter-turn fault, it
can reduce significantly the lifetime of the tested machine.
The PD test
can be offline and online depending on whether or not an AC external voltage is
applied between the winding and ground. This method requires an additional PD
detector device to measure the PD. Because the test can be implemented online
and provide a reliable result, it is commonly used as well. However, it is only
applicable to medium- and high-voltage machines since the voltage level in
low-voltage may not be sufficient to create PD indicator. IR and PI test,
voltage surge test, and PD test have been standardized for medium- and
high-voltage machines.
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