Assignment
# 1
concept of condition
monitoring:-
Condition
monitoring:-
Condition monitoring (or, colloquially, CM) is the process
of monitoring a parameter of condition in machinery (vibration, temperature
etc.), in order to identify a significant change which is indicative of a
developing fault. It is a major component of predictive maintenance. The use of
condition monitoring allows maintenance to be scheduled, or other actions to be
taken to prevent failure and avoid its consequences. Condition monitoring has a
unique benefit in that conditions that would shorten normal lifespan can be
addressed before they develop into a major failure. Condition monitoring
techniques are normally used on rotating equipment and other machinery (pumps,
electric motors, internal combustion engines, and presses), while periodic
inspection using non-destructive testing techniques and fit for service (FFS)
evaluation are used for stationary plant equipment such as steam boilers,
piping and heat exchangers.
Degradation of
insulation:-
Degradation of
insulating materials under electrical stress. Abstract: An avalanche of
physical and physicochemical phenomena, often self-sustained, leads to the end
of life of an insulating material. The collapse of the insulating role of a
material is mostly due to the electrical field stress.
When your plant electrical system and equipment are new, the
electrical insulation should be in top notch shape. Furthermore, manufacturers
of wire, cable, motors, and so on have continually improved their insulations
for services in industry. Nevertheless, even today, insulation is subject to
many effects which can cause it to fail – electrical stress, mechanical damage,
vibration, excessive heat or cold, dirt, oil, corrosive vapors, moisture from
processes, or just the humidity on a muggy day.
In various degrees, these enemies of insulation are at work
as time goes on – combined with the electrical stresses that exist. Electrical
stresses, particularly sustained overvoltages or impulses caused by faults will
lead to discharges in voids which will thereby expand and can develop into
electrical treeing. The aging of insulation is a slow process of degradation as
these factors interact with each other in a gradual spiral of decline. At some
point, dependent on both original and operating conditions the decline can
speed up significantly. As pin holes or cracks develop, moisture and foreign
matter penetrate the surfaces of the insulation, providing a low resistance
path for leakage current. Once started, the different enemies tend to aid each
other, permitting excessive current through the insulation. Sometimes the drop in insulation resistance
is sudden, as when equipment is flooded. Usually, however, it drops gradually,
giving plenty of warning, if checked periodically with insulation testing using
a unit like a Megger MIT525 or MIT1025. Such checks permit planned
reconditioning before service failure. If there are no checks, a motor with poor
insulation, for example, may not only be dangerous to touch when voltage is
applied, but also be subject to burn out. What was good insulation has become a
partial conductor.
Where critical, high-capital equipment is involved, the
introduction of new and improved insulating materials is re-writing the book on
insulation testing. Equipment with operating voltages above 1 kV requires
commensurately higher test voltages. Modern materials, when new or early in
their life cycles, can have insulation values into ranges that were previously
unmeasured. Your old insulation tester may not be fully adequate to meet the
demands of a rigorous and thorough program of preventive/predictive maintenance
on modern equipment. To be fully in conformance with the most modern testing
requirements, Megger offers a family of the highest quality insulation testers
at voltages above 1 kV. Download the
entire article on “The Complete Guide to Electrical Insulation Testing”
Megger 5-kV and 10-kV Insulation Resistance Testers – MIT525,
MIT1025ggh
·
Industry best guard terminal accuracy
·
Compact and lightweight for easy transport and
use
·
PI, DAR, DD, SV and ramp test
·
Unique dual case design provides additional user
protection
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Lithium-ion battery – extended capacity, rapid
charge
·
Advanced memory with time/date stamp
·
CAT IV 600 V safety rating on all terminals
DESCRIPTION:-
The new range of
Megger insulation resistance testers consists of three models: two 5 kV units
(MIT515 and MIT525) and a 10 kV unit (MIT1025). Resistance measurement is
available up to 10 TΩ for the 5 kV models and 20 TΩ for the 10 kV model. The
new instruments are smaller and lighter than previous models yet offer advanced
features and rapid charge capability. A key productivity feature is the ability
to take measurements when connected to line power/mains with a dead battery.
Intelligent battery charging ensures the optimum charge rate as a function of
battery level, resulting in minimum charge times.
The rugged, unique dual case design provides the ultimate
protection for a portable instrument and a clip-on lead pouch ensures that
leads remain with the instrument at all times. The case lid is removable for
improved terminal access. IP rating is IP65 with the case closed preventing
water/dust ingress. High reliability and safety are built in; all models are
safety rated to CATIV 600 V and are double insulated.
Five preset voltage ranges are provided in insulation test
mode, plus a user settable lock voltage range. Any selectable test voltage may
be locked and restored via the selector switch, thereby increasing efficiency
of commissioning and repetitive tests. Preconfigured diagnostic tests include
Polarization Index (PI), Dielectric Absorption Ratio (DAR), dielectric
discharge (DD), Step Voltage (SV) and ramp test.
The ramp function gradually increases voltage up to a
selected level while graphing current vs. voltage (graph downloadable). Graphs
can be compared to example curves in IEEE 95-2002 to reveal a variety of faults
difficult to detect otherwise. Small defects can be easily detected without
risking the sudden large voltage increments produced by a Step Voltage test.
Monitoring the developing graph during test enables the operator to terminate
prior to breakdown, thereby reducing the possibility of damage to already
flawed insulation. These units are particularly informative on polyester,
asphalt and epoxy-mica insulations. They can also test voltage suppression
devices.
Simplicity of operation is achieved with two rotary switches
and the large backlit display enables multiple results to be displayed
simultaneously. Advanced memory storage includes time/date stamping of results,
logging of data and recall of results to screen. A fully isolated USB device
interface (type B) is used for safe transfer of data to Megger’s PowerDB asset
management software.
Typical end users include:
·
Electrical contractors
·
Testing and service companies
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Wind farm and solar generation operators
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Power generation and distribution companies
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Industrial companies
·
Rail companies
Significance of
loss angle measurement:-
Measurement of loss
angle of a transformer insulating material and its winding capacitance between
primary and secondary windings or between windings and ground by bridge
technique may suffer from errors due to the effect of stray capacitance between
the bridge output lead wires as well as between the lead wires and ground. The
conventional Wagner-earth technique may be used to minimise this effect. But
one disadvantage of this technique may be the requirement of several repetitions
of bridge balance and Wagner earth balance in each observation. In the present
paper a modified operational amplifier based Schering bridge network has been
proposed, where the effect of stray capacitance may be assumed to be
negligible. Moreover, the bridge sensitivity may be adjusted by a linear
potentiometer. The experimental work was performed with 33/3.3 kV transformers
in the field at 33 kV substation. The dissipation factor for the adjustable
value of the bridge sensitivity factor potentiometer with the change of high
voltage at power frequency were obtained and reported. These experimental
results appear to reveal the satisfactory performance of the bridge network.
Insulation
resistance measurement:-
The insulation is
opposite from the conductor; it should resist current and keep the current on
its path along the conductor. The purpose of insulation around a conductor is
similar to a water hose carrying water, and Ohm’s Law of Electricity can be
more easily understood with a water hose comparison. Pressure on water from a
pump causes flow along the hose. If the hose were to spring a leak, you’d waste
water and lose water pressure, eventually causing the hose to be destroyed
completely. Similar to the loss of water, when there is a problem with the
integrity of the insulation of the wire, what results is a loss in the current,
affecting the capacity of the aircraft to fly properly. So then, what’s the
purpose of insulation resistance testing?
Insulation resistance testing is used as a quality control
measurement. The insulation resistance (IR) test (also commonly known as a
Megger) is a spot insulation test which uses an applied DC voltage (typically
either 250Vdc, 500Vdc or 1,000Vdc for low voltage equipment
We will pose this question to you again…. If your family
member or best friend’s life was at stake, would you settle for a company that
simply “beeps out” their wiring harnesses and panels, checking only for
continuity; or would you prefer a company that performs EXTENSIVE insulation resistance
testing on 100% of all electrical wired products, including military and
commercial wiring harnesses, aircraft
panels, and aerospace circuit breaker panels? We at
InterConnect Wiring recommend you not take that risk. Only buy your aircraft
wiring harnesses from a company like InterConnect, whose processes REQUIRE
extensive testing, 100% of the time, for continuity AND insulation resistance.
Click here to see an article about InterConnect Wiring in the Aerospace Testing
International Magazine
Recovery voltage
measurement:-
The primary
motivation in insulation diagnostic study is to understand the exact state of
insulation. Traditional methods using insulation resistance measurements,
polarization index measurements and capacitance, tan delta measurements serve
as good quality assurance criterion of acceptable values. However, for
obtaining a more clear and focussed information on insulation health, newer
testing and analytical techniques have to be adapted. This paper discusses two
such methodologies. Recovery voltage measurement (RVM) is a technique based on
measurement of increase of voltage across the transformer insulation after a
charge-discharge cycle. This method yields key insulation parameters like
moisture content and temperature withstand capability. A full cycle of recovery
voltage measurement often requires a large amount of time. The DC absorption
test aims at finding similar information as in the case of RVM with simple
instrumentation and in a lesser amount of time. This paper describes the
technique of RVM and later explores the possibility of using the DC absorption
test for determination of health of transformer insulation. Experimental
results to support the contention have also been included.
Interpretation of
result:-
Researchers should
describe their results clearly, and in a way that other researchers can compare
them with their own results. They should also analyse the results, using
appropriate statistical methods to try to determine the probability that they
may have been chance findings, and may not be replicable in larger studies. But
this is not enough. Results need to be interpreted in an objective and critical
way, before assessing their implications and before drawing conclusions.
Interpretation of research results is not just a concern for researchers.
Health professionals reading or hearing research results should be able
themselves to interpret them correctly, and to assess their implications for
their work. Policymakers should also be aware of the possible pitfalls in
interpreting research results and should be cautious in drawing conclusions for
policy decisions.
