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CONDITIONING MONITORING & MAINTENANCE ( ASSIGNMENT )

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

·        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

·        Wind farm and solar generation operators

·        Power generation and distribution companies

·        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.