Condition Monitoring of Equipment

Condition Monitoring Condition Monitoring of Transformer

Condition Monitoring of Transformers is the process of acquisition and processing of data related to various parameters of transformers so as to predict and prevent the failure of a transformer. This is done by observing the deviation of the transformer parameters from their expected values. Transformers are the most critical assets of electrical transmission and distribution system. Transformer failures could cause power outages, personal and environmental hazards and expensive rerouting or purchase of power from other suppliers. Transformer failures can occur due to various causes. Transformer in-service interruptions and failures usually result from dielectric breakdown, winding distortion caused by short-circuit withstand, winding and magnetic circuit hot spot, electrical disturbances, deterioration of insulation, lightning, inadequate maintenance, loose connections, overloading, failure of accessories such as OLTCs, bushings, etc. Integrating the individual cause monitoring allows for monitoring the overall condition of transformer. The important aspects of condition monitoring of transformers are:

1. Thermal Modelling

2. Dissolved Gas Analysis

3. Frequency Response Analysis

4. Partial Discharge AnalysisThermal ModelingThe useful life of a transformer is determined partially by the ability of transformer to dissipate the internally generated heat to its surroundings. The comparison of actual and predicted operating temperatures can provide a sensitive diagnosis of the transformer condition and might indicate abnormal operation. The consequences of temperature rise may not be sudden, but gradual as long as it is within break down limit. Among these consequences, insulation deterioration is economically important. Insulation being very costly, its deterioration is undesirable. Thermal modeling is the development of a mathematical model that predicts the temperature profile of the power transformer using the principle of thermal analysis. The thermal model is used to determine the top oil temperature and hot spot temperature (maximum temperature occurring in the winding insulation system).Dissolved Gas AnalysisGases are produced by degradation of the transformer oil and solid insulating materials. Gases are generated at a much more rapid rate whenever an electrical fault occurs. Normal causes of fault gases are classified into three categories: Corona or partial discharge, thermal heating and arcing. These faults can be detected by evaluating the quantities of hydrocarbon gases, hydrogen and oxides of carbon that are present in the transformer. Different gases can serve as markers for different types of faults. The concentration and the relation of individual gases allow a prediction of whether a fault has occurred and what type it is likely to be.

Frequency Response AnalysisWhen a transformer is subjected to high currents through fault currents, the mechanical structure and windings are subjected to severe mechanical stresses causing winding movement and deformations. It may also result in insulation damage and turn-to-turn faults. Frequency response analysis (FRA) is a non-intrusive very sensitive technique for detecting winding movement faults and deformation assessment caused by loss of clamping pressure or by short circuit forces. FRA technique involves measuring the impedance of the windings of the transformer with a low voltage sine input varying in a wide frequency range.Partial Discharge AnalysisPartial discharge (PD) occurs when a local electric field exceeds a threshold value, resulting in a partial breakdown of the surrounding medium. Its cumulative effect leads to the degradation of insulation. PDs are initiated by the presence of defects during its manufacture, or the choice of higher stress dictated by design considerations. Measurements can be collected to detect these PDs and monitor the soundness of insulation. PDs manifest as sharp current pulses at transformer terminals, whose nature depends on the types of insulation, defects, measuring circuits and detectors used.Condtion Monitoring of Rotating EquipmentThe most commonly used method for rotating machines is called a vibration analysis. Measurements can be taken on machine bearing casings with accelerometers (seismic or piezo-electric transducers) to measure the casing vibrations, and on the vast majority of critical machines, with eddy-current transducers that directly observe the rotating shafts to measure the radial (and axial) displacement of the shaft. The level of vibration can be compared with historical baseline values such as former start ups and shutdowns, and in some cases established standards such as load changes, to assess the severity.

Interpreting the vibration signal obtained is an elaborate procedure that requires specialized training and experience. It is simplified by the use of state-of-the-art technologies that provide the vast majority of data analysis automatically and provide information instead of raw data. One commonly employed technique is to examine the individual frequencies present in the signal. These frequencies correspond to certain mechanical components (for example, the various pieces that make up a rolling-element bearing) or certain malfunctions (such as shaft unbalance or misalignment). By examining these frequencies and their harmonics, the CM specialist can often identify the location and type of problem, and sometimes the root cause as well. For example, high vibration at the frequency corresponding to the speed of rotation is most often due to residual imbalance and is corrected by balancing the machine. As another example, a degrading rolling-element bearing will usually exhibit increasing vibration signals at specific frequencies as it wears. Special analysis instruments can detect this wear weeks or even months before failure, giving ample warning to schedule replacement before a failure which could cause a much longer down-time. Beside all sensors and data analysis it is important to keep in mind that more than 80% of all complex mechanical equipment fail accidentally and without any relation to their life-cycle period.

Most vibration analysis instruments today utilize a Fast Fourier Transform (FFT) which is a special case of the generalized Discrete Fourier Transform and converts the vibration signal from its time domain representation to its equivalent frequency domain representation. However, frequency analysis (sometimes called Spectral Analysis or Vibration Signature Analysis) is only one aspect of interpreting the information contained in a vibration signal. Frequency analysis tends to be most useful on machines that employ rolling element bearings and whose main failure modes tend to be the degradation of those bearings, which typically exhibit an increase in characteristic frequencies associated with the bearing geometries and constructions. Depending on the type of machine, its typical malfunctions, the bearing types employed, rotational speeds, and other factors, the CM specialist may use additional diagnostic tools, such as examination of the time domain signal, the phase relationship between vibration components and a timing mark on the machine shaft (often known as a keyphasor), historical trends of vibration levels, the shape of vibration, and numerous other aspects of the signal along with other information from the process such as load, bearing temperatures, flow rates, valve positions and pressures to provide an accurate diagnosis. This is particularly true of machines that use fluid bearings rather than rolling-element bearings. To enable them to look at this data in a more simplified form vibration analysts or machinery diagnostic engineers have adopted a number of mathematical plots to show machine problems and running characteristics, these plots include the bode plot, the waterfall plot, the polar plot and the orbit time base plot amongst others.

Handheld data collectors and analyzers are now commonplace on non-critical or balance of plant machines on which permanent on-line vibration instrumentation cannot be economically justified. The technician can collect data samples from a number of machines, and then download the data into a computer where the analyst (and sometimes artificial intelligence) can examine the data for changes indicative of malfunctions and impending failures. For larger, more critical machines where safety implications, production interruptions (so-called "downtime"), replacement parts, and other costs of failure can be appreciable (determined by the criticality index), a permanent monitoring system is typically employed rather than relying on periodic handheld data collection. However, the diagnostic methods and tools available from either approach are generally the same.

Recently also on-line systems have been applied to heavy process industries such as pulp, paper, mining, petrochemical and power generation. These can be dedicated systems like Sensodec 6S or nowadays this functionality has been embedded into DCS.

Performance monitoring is a less well-known condition monitoring technique. It can be applied to rotating machinery such as pumps and turbines, as well as stationary items such as boilers and heat exchangers. Measurements are required of physical quantities: temperature, pressure, flow, speed, displacement, according to the plant item. Absolute accuracy is rarely necessary, but repeatable data is needed. Calibrated test instruments are usually needed, but some success has been achieved in plant with DCS (Distributed Control Systems). Performance analysis is often closely related to energy efficiency, and therefore has long been applied in steam power generation plants. Typical applications in power generation could be boiler, steam turbine and gas turbine. In some cases, it is possible to calculate the optimum time for overhaul to restore degraded performance.Other techniques Often visual inspections are considered to form an underlying component of condition monitoring, however this is only true if the inspection results can be measured or critiqued against a documented set of guidelines. For these inspections to be considered condition monitoring, the results and the conditions at the time of observation must be collated to allow for comparative analysis against the previous and future measurements. The act of simply visually inspecting a section of pipe work for the presence of cracks or leaks cannot be considered condition monitoring unless quantifiable parameters exist to support the inspection and a relative comparison is made against previous inspections. An act performed in isolation to previous inspections is considered a Condition Assessment, Condition Monitoring activities require that analysis is made comparative to previous data and reports the trending of that comparison.

Slight temperature variations across a surface can be discovered with visual inspection and non-destructive testing with thermography. Heat is indicative of failing components, especially degrading electrical contacts and terminations. Thermography can also be successfully applied to high-speed bearings, fluid couplings, conveyor rollers, and storage tank internal build-up.

Using a Scanning Electron Microscope of a carefully taken sample of debris suspended in lubricating oil (taken from filters or magnetic chip detectors). Instruments then reveal the elements contained their proportions, size and morphology. Using this method, the site, the mechanical failure mechanism and the time to eventual failure may be determined. This is called WDA - Wear Debris Analysis.

Spectrographic oil analysis that tests the chemical composition of the oil can be used to predict failure modes. For example high silicon content indicates contamination of grit etc., and high iron levels indicate wearing components. Individually, elements give fair indications, but when used together they can very accurately determine failure modes e.g. for internal combustion engines, the presence of iron/alloy, and carbon would indicate worn piston rings.

Ultrasound can be used for high-speed and slow-speed mechanical applications and for high-pressure fluid situations. Digital ultrasonic meters measure high frequency signals from bearings and display the result as a dBuV (decibels per microvolt) value. This value is trended over time and used to predict increases in friction, rubbing, impacting, and other bearing defects. The dBuV value is also used to predict proper intervals for re-lubrication. Ultrasound monitoring, if done properly, proves out to be a great companion technology for vibration analysis.

Headphones allow humans to listen to ultrasound as well. A high pitched 'buzzing sound' in bearings indicates flaws in the contact surfaces, and when partial blockages occur in high pressure fluids the orifice will cause a large amount of ultrasonic noise. Ultrasound is used in the Shock Pulse Method of condition monitoring.

Performance analysis, where the physical efficiency, performance, or condition is found by comparing actual parameters against an ideal model. Deterioration is typically the cause of difference in the readings. After motors, centrifugal pumps are arguably the most common machines. Condition monitoring by a simple head-flow test near duty point using repeatable measurements has long been used but could be more widely adopted. An extension of this method can be used to calculate the best time to overhaul a pump based on balancing the cost of overhaul against the increasing energy consumption that occurs as a pump wears. Aviation gas turbines are also commonly monitored using performance analysis techniques with the original equipment manufacturers such as Rolls-Royce plc routinely monitoring whole fleets of aircraft engines under Long Term Service Agreements (LTSAs) or Total Care packages.

Wear Debris Detection Sensors are capable of detecting ferrous and non-ferrous wear particles within the lubrication oil giving considerable information about the condition of the measured machinery. By creating and monitoring a trend of what debris is being generated it is possible to detect faults prior to catastrophic failure of rotating equipment such as gearboxes, turbines, etc.

Vehicle condition monitoring

Description/ObjectiveVehicle condition monitoring allows the driver, dispatchers and support personnel to become aware of defects or emerging problems on the vehicle. They can take proactive steps to repair the defect or to substitute the vehicle or the defective piece of equipment. Condition monitoring includes:

Engine diagnostics

Fuel, oil, brake fluid, water levels

Status of ITS equipment

Status of fare collection equipment

Other information provided by the driver

Technologies, data and resources

Monitoring of engine diagnostics and vehicle mechanical problems are provided as standard and are displayed on the drivers dashboard. No additional ITS is required for this.

The ITS-related content for vehicle condition monitoring, over and above what is provided as standard in the vehicle, includes:

Interface between the engine/vehicle diagnostic systems and the on-board computer of the CAD/AVM system or the fare collection system

Diagnostic and self-reporting capability included in the ITS, fare collection and customer-facing devices installed on the vehicles

Preset buttons or touchscreen options on the drivers display unit, allocated to fault or condition messages initiated by the driver

Inclusion of vehicle condition and fault messages in the data transferred during the polling cycle of the CAD/AVM system.

Advantages and Cautions

The primary advantages of Vehicle Condition Monitoring applications are to:

improve the response time to equipment defects and vehicle breakdowns

minimise occasions when service is interrupted due to detective vehicles or equipment

identify vehicles with critical conditions and intervening before serious damage occurs

The principal cautions in relation to Vehicle Condition Monitoring are:

procedures are required at the dispatch centre and at maintenance support to ensure that reported faults are acted upon promptly, and in a manner that minimises service disruption

Auto Maintenance

Auto maintenance describes the act of inspecting or testing the condition of car subsystems (e.g., engine) and servicing or replacing parts and fluids. Regular maintenance is critical to ensure the safety, reliability, drivability, comfort and longevity of a car. During preventive maintenance, a number of parts are replaced to avoid major damage or for safety reasons, e.g. timing belt replacement.SchedulingThe actual schedule of car maintenance varies depending on the year, make, and model of a car, its driving conditions and driver behavior. Car makers recommend the so-called extreme or the ideal service schedule based on impact parameters such as

number of trips and distance traveled per trip per day

extreme hot or cold climate conditions

mountainous, dusty or de-iced roads

heavy stop-and-go vs. long-distance cruising

towing a trailer or other heavy load

Experienced service advisors in dealerships and independent shops recommend schedule intervals, which are often in between the ideal or extreme service schedule. They base it on the driving conditions and behavior of the car owner or driver.Common maintenanceCommon car maintenance tasks include:

Car wash

check/replace the engine oil and replace oil filters

check/replace fuel filters

inspect or replace windshield wipers

check or refill windshield washer fluid

inspect tires for pressure and wear

Tire balancing

Tire rotation

Wheel alignment

check, clean or replace battery terminals and top up battery fluid

inspect or replace brake pads

check or flush brake fluid

check or flush transmission fluid

check or flush power steering fluid

check and flush engine coolant

inspect or replace spark plugs

inspect or replace air filter

inspect or replace timing belt and other belts

lubricate locks, latches, hinges

check all lights

tighten chassis nuts and bolts

check if rubber boots are cracked and need replacement

test electronics, e.g., Anti-lock braking system or ABS

read fault codes from the Engine control unit

Some tasks that have equivalent service intervals are combined into one single service known as a tune-up. In modern cars, where electronics control most of the car's functions, the traditional tune-up doesn't apply anymore. Maintenance jobs like a tune-up used to mean getting the engine's performance back on track. Today embedded software takes care of it by constantly checking thousands of sensor signals, compensating for worn-out spark plugs, clogged filters, etc. The so-called limp-home function allows driving on limited power when the engine is in trouble. In the old days this might have meant a breakdown. In some countries, the completed services are recorded in a service book which is rubber-stamped by the service center upon completion of each service. A complete service history usually adds to the resale value of a vehicle.