![Advanced tools and techniques of Condition Monitoring
Introduction Condition monitoring is a process of continuously monitoring operational characteristics of a machine to predict
the need for maintenance before a deterioration or breakdown occurs. Condition Based Maintenance (CBM) differs from
earlier used method of preventive maintenance by centering the maintenance based on the actual condition of the machine
rather than on some preset schedule. The need of condition monitoring arises from the fact that in a power plant or a power
utility any unexpected fault or a shutdown may result in a fatal accident or huge loss of output. Condition monitoring solves
these problems by providing useful information for utilizing the machines in an optimal fashion. The recent development in
computer and transducer technologies coupled with the advances in signal processing and artificial–intelligence (AI)
techniques has made it possible to implement CBM more effectively on electrical equipment making it a more reliable and
intelligent approach which can be used at various levels of power generation and distribution. This study shall present a
review of various condition monitoring techniques for electrical equipment.
Vibration signature analysis Vibration is a cyclic or pulsating motion of a machine or machine component from its
point of rest[1]. Vibration of a machine can be represented in time domain in terms of its phase and amplitude (which can be
measured as displacement, velocity or acceleration), and in frequency domain by its dominant frequencies, harmonics, etc.
Vibration signature analysis (VSA) is a widely used condition monitoring technique to determine the overall condition of a
machine, which is based on measurement of vibration severity of the machine under test. Every machine in its working
condition produces vibration and this vibration is a characteristic signature of the machine which does not change over time.
However, in cases of structural or functional anomaly or failure, the dynamic characteristics of the machine changes National
Seminar & Exhibition on Non-Destructive Evaluation, NDE 2014, Pune, December 4-6, 2014 (NDE-India 2014) Vol.20 No.6
(June 2015) - The e-Journal of Non-destructive Testing - ISSN 1435-4934 www.ndt.net/?id=17849 which is reflected in its
vibration signals[2]. The nature of the developing fault has unique vibration characteristics which can be compared with the
vibration signatures of the machine working under normal operating condition. By using various signal analysis techniques
one can determine the exact category/type of fault.](https://image.slidesharecdn.com/unit-iipart3-241012072955-1c62d5b1/85/Condition-monitoring-CM-is-a-maintenance-technique-that-uses-sensors-and-software-to-predict-the-health-and-safety-of-machines-33-320.jpg)
![Signal analysis: Vibration signals encountered in rotary machine systems, such as machine tools, wind turbines or
electric motors can be broadly classified as stationary or non-stationary. Stationary signals are characterized by time-
invariant statistic properties like periodic vibrations caused by a worn out bearing etc. Such signals can be adequately
analyzed using spectral techniques based on the Fourier Transform. In contrast, non-stationary signals are transient in
nature, with duration generally shorter than the observation interval. Such signals are generally generated by the sudden
breakage of a drilling bit, flaking of the raceway of a rolling bearing, or a growing crack inside a work piece. For analysis
of such non-stationary vibration signals, time-frequency techniques like Short-Time Fourier Transform for fault detection
during impulse testing of power transformers (STFT), wavelet transform and Hilbert-Huang Transform (HHT)are popularly
used.
Acoustic emission testing:
Acoustic Emission Testing (AET) is a condition monitoring technique that is used to analyze emitted sound waves caused
by defects or discontinuities. These acoustic emissions (AE) are transient elastic waves induced from a rapid release of
strain energy caused by small deformations, corrosion or cracking, which occur prior to structure failure. In electric
machines sources of AE include impacting, cyclic fatigue, friction, turbulence, material loss, cavitations, leakage, etc.
These acoustic emissions propagate on the surface of the material as Rayleigh waves[6] and the displacement of these
waves is measured by AE sensors which are almost always a piezoelectric crystal, commonly made from a ceramic such
as lead zirconatetitanate (PZT).](https://image.slidesharecdn.com/unit-iipart3-241012072955-1c62d5b1/85/Condition-monitoring-CM-is-a-maintenance-technique-that-uses-sensors-and-software-to-predict-the-health-and-safety-of-machines-34-320.jpg)
![Data acquisition and analysis:
For the purpose of data acquisition sensors are placed on the material surface, the information collected by each of the
sensors is monitored. If defects exist in some areas, the signal characteristics from the sensor attached nearest to the
discontinuity appears in different way. By analyzing the discontinuity, it is possible to ascertain the defect position and
suspect area of the structure. Broadly the data analysis can be done by two approaches. The first one is parameter based
approach which is based on the analysis of basic signal parameters such as the rate, energy and amplitudes etc. In
parameter based analysis only some of the parameters of the AE signal are recorded, but the signal itself is not recorded,
this minimizes the amount of data stored and enables faster analysis. However sometimes these parameters lose massive
information which makes characterizing of defects very difficult. The other approach is waveform analysis technique which
is based on the complete waveform rather than on the parameters. The waveform based approach offer better data
interpretation capability than parameter based approach by allowing the use of signal processing methods like Wavelet-
based acoustic emission characterization, second generation wavelet transform, wavelet envelopment spectrum analysis.
and also provides better noise discrimination.
Ultrasound condition monitoring:
Ultrasound is defined as “sound waves having a frequency above the limits of human hearing, or in excess of 20,000 cycles
per second”[14]. Many physical events cause sound at audible and/or ultrasonic frequencies, analysis of these frequencies
can frequently indicate correct or incorrect operation[15]. Ultrasonic condition monitoring (UCM) is a technique that uses
airborne(non-contact) and structure borne(contact) ultrasound instruments to receive high frequency ultrasonic emissions
produced by operating equipment, electrical emissions and leaks etc. to monitor the condition of equipment under test[16].
Ultrasound transducers electronically translate ultrasound frequencies through a process called heterodyning, down to the
audible range while maintaining the sound quality during the transition. These signals are observed at intensity and/or dB
levels for analysis.](https://image.slidesharecdn.com/unit-iipart3-241012072955-1c62d5b1/85/Condition-monitoring-CM-is-a-maintenance-technique-that-uses-sensors-and-software-to-predict-the-health-and-safety-of-machines-35-320.jpg)
![Active and passive ultrasound monitoring techniques:
In passive techniques ultrasound detected by airborne or structure borne instruments is produced by a
physical process i.e. by the component being analyzed. Passive ultrasound is used mainly for contact methods
of monitoring such as bearing faults, lubrication issues, gear damage and pump cavitations and non-contact
methods of monitoring like leaks in boilers, condensers, and heat exchangers, electrical discharge and corona
in high voltage equipment etc. Airborne ultrasound detects high frequency sound produced by mechanical
equipment, electrical discharges and most leakages which is extremely short wave in nature. These short wave
signal tends to be fairly directional and localized which make them very easy to separate from background
plant noises and to detect their exact location[16]. On the other hand active ultrasound is an approach where
a precisely guided beam of ultrasound is transmitted to a physical structure to analyze both surface and
subsurface discontinuities like delaminations, disbonds, cracks and porosity at early stages. The guided wave
interacts with the structural discontinuity which causes reflection from a particular depth in material or
scattering of guided waves in all directions, both results in transmission loss. These transmission losses can be
detected by mapping the transmitted signal over the whole structure, known as a Through-transmission C-
scan. From various characteristics of the received ultrasonic signal, such as the time of flight, amplitude,
frequency content, etc., the information about the depth of the damage is assessed.](https://image.slidesharecdn.com/unit-iipart3-241012072955-1c62d5b1/85/Condition-monitoring-CM-is-a-maintenance-technique-that-uses-sensors-and-software-to-predict-the-health-and-safety-of-machines-36-320.jpg)
Condition monitoring (CM) is a maintenance technique that uses sensors and software to predict the health and safety of machines.
- 1. AIRCRAFT CONDITION MONITORING UNIT-II Insulation stressing factors, insulation deterioration, polarization index, dielectric absorption ratio. Insulation ageing mechanisms, Insulation failure modes, Definition of terms, Concept of condition monitoring of electrical equipments. Overview of Advanced tools and techniques of condition monitoring, Condition monitoring by thermography.
- 13. Failure of Transformer Insulation & its Maintenance
- 14. Failure of Transformer Insulation & its Maintenance
- 28. Factors Contributing to Ageing Ageing of transformer insulation can take place due to one or a combination of several factors that include electrical, thermal, chemical, mechanical, and environmental ageing mechanisms. These ageing factors may either act independently, or there may be direct interactions between the stresses. The ageing of a practical insulation system can be complex and failure is usually caused by a combination of ageing mechanisms, even though there may be only one dominant ageing factor. For consideration of transformer insulation, the following factors are important. Thermal Ageing: The most dominant mechanism responsible for ageing of transformer insulation is thermal ageing, which involves chemical and physical changes in the insulation. Such ageing is brought about as a consequence of chemical degradation reactions, polymerization, de-ploymerization, diffusions, etc. Thermo-mechanical effects caused by forces due to thermal expansion and/or contraction are also a major factor responsible for insulation degradation. Both of these effects of chemical change and thermo-mechanical stress are highly influenced by the operating temperature.
- 29. Electrical Ageing : Electrical ageing, either under operating AC stress or impulse, also leads to insulation degradation over years of operation. These factors involve the influence of partial discharges in the liquid or gaseous dielectric adjacent to the discharge source, or the effects of tracking and treeing in the solid and liquid insulation under high electrical stresses. In addition, high dielectric losses and effects of space charges also contribute to insulation degradation. A combination of electrical and chemical degradation effects also takes place in the form of electrolysis of the liquid insulation, especially when it is contaminated with polar impurities. Mechanical Ageing : Mechanical ageing of the insulation structure can originate from low levels of electro-mechanical or thermo-mechanical stress cycles, but can gradually escalate to even rupture the solid insulation under external or internal forces. Components involving moving parts, such as tap changers, may have abrasive wear take place in the insulation parts. Environmental Ageing In addition to the chemical and thermal degradation processes highlighted above, other external environmental factors include the influences of dust and other contamination on electrical behavior of the equipment.
- 30. Condition monitoring of electrical machines can be used for different reasons. These include: 1) Preventing catastrophic failures and significant damage of the machines; 2) Avoiding loss of life, environmental harm and economic losses; 3) Stopping unscheduled outages; 4) Optimization of machine performance; 5) Reducing repair time and spare parts inventory; 6) Lengthening of the maintenance cycle; 7) Reducing price and raw material consumption; 8) Increasing product quality. MAIN FAULTS IN ELECTRICAL MACHINES Electrical machines are critical components in many commercially available equipment and industrial processes. Furthermore, they are often used in critical duty drives where sudden failures can cause safety risks and large economic expenses. Different failures can occur in electrical machines, some of which are listed below.
- 31. A. Rotor Bar Faults: One of the most common rotor faults in induction motors is the breaking of the rotor bars. The main reasons for such faults is poor manufacturing, such as defective casting and poor jointing. Another common reason is over current e.g. due to jam condition of the rotor, but there can be various reasons that will lead to cracking or broken rotor bars : 1) Thermal stress due to over-load, non-uniform heat distribution, hot spot and arc; 2) Magnetic stresses due to electromagnetic forces, magnetic asymmetry forces, noises and electromagnetic vibrations; 3) Residual stress from the fabrication process; 4) Dynamic stress due to rotor axial torque and centrifugal forces; 5) Circumferential stress due to wearing and pollution of rotor material by chemical materials and humidity; 6) Mechanical stress due to mechanical fatigue of different parts, bearing damage, loosened laminations etc. B. Bearing Faults: Damage of bearings is the most common cause of failures in squirrel-cage induction motors.Two types of bearing faults are usually distinguished. The first are single point defects and the second is generalized roughness In case of single point defects, the characteristic spectral components in vibration signal can be predicted for inner ring, outer ring, rolling element and cage fault. These frequencies can also appear in stator current around the fundamental harmonic . Although they are usually clearly visible using vibration analysis, in case of stator current it is difficult to observe them due to their low amplitude and noise disturbance. In contrary to single point defects, generalized roughness does not produce characteristic frequency, but rather specific frequency bands. Therefore, methods for diagnosing generalized roughness problems are usually based on removing the non-bearing faults components from diagnostic signal and utilizing the residuum or on seeking and utilizing the frequency bands with high probability of presence of bearing faults components. Since the generalized roughness is the most often type of bearing fault these methods are hardly investigated .
- 32. C. Air-gap Eccentricity Air-gap eccentricity can be introduced due to manufacturing imperfections or during operation and the inherent level of static or dynamic eccentricity is typically within 10% of the air-gap. Static eccentricity is a condition where the position of the minimum radial air-gap is fixed. It can be caused by stator core ovality, or incorrect positioning of stator core or bearing at commissioning or following a repair, and its level usually does not change over time . Dynamic eccentricity is a condition where the center of the rotor is not at the center of rotation, and the position of the minimum radial air-gap rotates with the rotor. This can be produced by worn bearings, a bent shaft, asymmetric thermal expansion of the rotor, or by high level of static eccentricity. Eccentricity causes unbalanced magnetic pull which results in vibration, acoustic noise, bearing wear, and/or rotor deflection. This increases the risk of stator-rotor rub, which can cause serious damage in the motor, stator or rotor core, and/or insulation. D. Stator Faults Winding turn faults are one of the most common problems that arise in stator. Other common group of stator faults is the inter laminar short circuits. Stator winding failure usually starts with a short circuit between the adjacent turns in the stator winding. Winding insulation damages may cause faults which in turn produce high currents and winding overheating. This overheating can quickly result in severe faults between windings of different phases or between winding and ground, producing then permanent and irreversible damages both in windings and stator core.Early detection of such winding faults is needed to prevent serious damage for the motor. The use of variable-speed drives increases these problems due to the high rates of voltage changes produced by inverter switching .
- 33. Advanced tools and techniques of Condition Monitoring Introduction Condition monitoring is a process of continuously monitoring operational characteristics of a machine to predict the need for maintenance before a deterioration or breakdown occurs. Condition Based Maintenance (CBM) differs from earlier used method of preventive maintenance by centering the maintenance based on the actual condition of the machine rather than on some preset schedule. The need of condition monitoring arises from the fact that in a power plant or a power utility any unexpected fault or a shutdown may result in a fatal accident or huge loss of output. Condition monitoring solves these problems by providing useful information for utilizing the machines in an optimal fashion. The recent development in computer and transducer technologies coupled with the advances in signal processing and artificial–intelligence (AI) techniques has made it possible to implement CBM more effectively on electrical equipment making it a more reliable and intelligent approach which can be used at various levels of power generation and distribution. This study shall present a review of various condition monitoring techniques for electrical equipment. Vibration signature analysis Vibration is a cyclic or pulsating motion of a machine or machine component from its point of rest[1]. Vibration of a machine can be represented in time domain in terms of its phase and amplitude (which can be measured as displacement, velocity or acceleration), and in frequency domain by its dominant frequencies, harmonics, etc. Vibration signature analysis (VSA) is a widely used condition monitoring technique to determine the overall condition of a machine, which is based on measurement of vibration severity of the machine under test. Every machine in its working condition produces vibration and this vibration is a characteristic signature of the machine which does not change over time. However, in cases of structural or functional anomaly or failure, the dynamic characteristics of the machine changes National Seminar & Exhibition on Non-Destructive Evaluation, NDE 2014, Pune, December 4-6, 2014 (NDE-India 2014) Vol.20 No.6 (June 2015) - The e-Journal of Non-destructive Testing - ISSN 1435-4934 www.ndt.net/?id=17849 which is reflected in its vibration signals[2]. The nature of the developing fault has unique vibration characteristics which can be compared with the vibration signatures of the machine working under normal operating condition. By using various signal analysis techniques one can determine the exact category/type of fault.
- 34. Signal analysis: Vibration signals encountered in rotary machine systems, such as machine tools, wind turbines or electric motors can be broadly classified as stationary or non-stationary. Stationary signals are characterized by time- invariant statistic properties like periodic vibrations caused by a worn out bearing etc. Such signals can be adequately analyzed using spectral techniques based on the Fourier Transform. In contrast, non-stationary signals are transient in nature, with duration generally shorter than the observation interval. Such signals are generally generated by the sudden breakage of a drilling bit, flaking of the raceway of a rolling bearing, or a growing crack inside a work piece. For analysis of such non-stationary vibration signals, time-frequency techniques like Short-Time Fourier Transform for fault detection during impulse testing of power transformers (STFT), wavelet transform and Hilbert-Huang Transform (HHT)are popularly used. Acoustic emission testing: Acoustic Emission Testing (AET) is a condition monitoring technique that is used to analyze emitted sound waves caused by defects or discontinuities. These acoustic emissions (AE) are transient elastic waves induced from a rapid release of strain energy caused by small deformations, corrosion or cracking, which occur prior to structure failure. In electric machines sources of AE include impacting, cyclic fatigue, friction, turbulence, material loss, cavitations, leakage, etc. These acoustic emissions propagate on the surface of the material as Rayleigh waves[6] and the displacement of these waves is measured by AE sensors which are almost always a piezoelectric crystal, commonly made from a ceramic such as lead zirconatetitanate (PZT).
- 35. Data acquisition and analysis: For the purpose of data acquisition sensors are placed on the material surface, the information collected by each of the sensors is monitored. If defects exist in some areas, the signal characteristics from the sensor attached nearest to the discontinuity appears in different way. By analyzing the discontinuity, it is possible to ascertain the defect position and suspect area of the structure. Broadly the data analysis can be done by two approaches. The first one is parameter based approach which is based on the analysis of basic signal parameters such as the rate, energy and amplitudes etc. In parameter based analysis only some of the parameters of the AE signal are recorded, but the signal itself is not recorded, this minimizes the amount of data stored and enables faster analysis. However sometimes these parameters lose massive information which makes characterizing of defects very difficult. The other approach is waveform analysis technique which is based on the complete waveform rather than on the parameters. The waveform based approach offer better data interpretation capability than parameter based approach by allowing the use of signal processing methods like Wavelet- based acoustic emission characterization, second generation wavelet transform, wavelet envelopment spectrum analysis. and also provides better noise discrimination. Ultrasound condition monitoring: Ultrasound is defined as “sound waves having a frequency above the limits of human hearing, or in excess of 20,000 cycles per second”[14]. Many physical events cause sound at audible and/or ultrasonic frequencies, analysis of these frequencies can frequently indicate correct or incorrect operation[15]. Ultrasonic condition monitoring (UCM) is a technique that uses airborne(non-contact) and structure borne(contact) ultrasound instruments to receive high frequency ultrasonic emissions produced by operating equipment, electrical emissions and leaks etc. to monitor the condition of equipment under test[16]. Ultrasound transducers electronically translate ultrasound frequencies through a process called heterodyning, down to the audible range while maintaining the sound quality during the transition. These signals are observed at intensity and/or dB levels for analysis.
- 36. Active and passive ultrasound monitoring techniques: In passive techniques ultrasound detected by airborne or structure borne instruments is produced by a physical process i.e. by the component being analyzed. Passive ultrasound is used mainly for contact methods of monitoring such as bearing faults, lubrication issues, gear damage and pump cavitations and non-contact methods of monitoring like leaks in boilers, condensers, and heat exchangers, electrical discharge and corona in high voltage equipment etc. Airborne ultrasound detects high frequency sound produced by mechanical equipment, electrical discharges and most leakages which is extremely short wave in nature. These short wave signal tends to be fairly directional and localized which make them very easy to separate from background plant noises and to detect their exact location[16]. On the other hand active ultrasound is an approach where a precisely guided beam of ultrasound is transmitted to a physical structure to analyze both surface and subsurface discontinuities like delaminations, disbonds, cracks and porosity at early stages. The guided wave interacts with the structural discontinuity which causes reflection from a particular depth in material or scattering of guided waves in all directions, both results in transmission loss. These transmission losses can be detected by mapping the transmitted signal over the whole structure, known as a Through-transmission C- scan. From various characteristics of the received ultrasonic signal, such as the time of flight, amplitude, frequency content, etc., the information about the depth of the damage is assessed.
- 37. Infrared thermography: Temperature is one of the most common indicators of the structural and functional health of equipment and components. Faulty machineries, corroded electrical connections, damaged material components, etc., can cause abnormal temperature distribution.Infrared thermography(IRT) is the process of using thermal imagers to capture infrared radiations emitted by an object to locate any abnormal heat pattern or thermal anomaly which indicate possible fault, defects or inefficiencies within a system or machine asset. The basic principle underlying this technique is based upon Planck’s law and Stefan-Boltzmann’s law which states that all objects with temperature above 0 K (i.e. -273˚C) emits electromagnetic radiation in the infrared region of electromagnetic spectrum i.e. wavelength in the range of 0.75–1000 µm and the intensity of this IR radiation is a function of temperature of body. Infrared thermography is generally classified in two categories, passive and active thermography.In passive thermography, the temperature gradients are naturally present in the materials and structures under test. However in some cases the thermal gradient is not so prominent in case of deeper and smaller defects and is not visible on the surface using passive thermography. This is overcomed by the use of active thermography where the relevant thermal contrasts are induced by an external stimulus. Passive thermography is mainly applied for condition monitoring of electrical and mechanical equipment, whereas active method has been widely applied in areas such as medicine, thermal efficiency survey of buildings, agriculture and biology, detection of gas leak etc.
- 38. Condition monitoring of machine using IRT: Condition monitoring of an electrical machine using IRT is a technique that relies majorly on temperature measurement of the equipment under test. There are two approaches for temperature measurement. The first one is quantitative, in which the exact temperature values of the objects are considered with ambient temperature as reference. The second approach is qualitative, in which the relative temperature values of a hotspot with respect to other parts of the equipment with similar conditions are considered. Qualitative analysis require a great understanding of variables influencing radiometric measurement including object’s emissivity, transmissivity, reflectivity, atmospheric conditions and machine. In qualitative analysis there is no need to have a finer knowledge about variables influencing the temperature measurement. However, this method fails to perform correctly when a fault occurs in all similar components or a systematic failure occurs affecting all three phases, it also does not provide information about whether the equipment temperature limits are actually exceeded. In terms of analysis condition monitoring of a machine using IRT can be done by two methods i.e. manual and automatic. In manual method a thermographer proposes potential faults and anomalies by manually analyzing the thermal images of equipment under test. In automated method of inspection artificial intelligence techniques use acquired temperature information as well as information available from previous tests to generate results. Automatic condition monitoring of machine in a broader view can be categorized as a three step process. The first step involves acquisition of infrared images(s) of the machine under test. The second step involves extracting the region of interest (ROI) containing the potentially faulty portion of the machine using techniques likes image segmentatio, thresholding. Finding ROI plays a very crucial role in IRT as the key success of decision making process depends on the correct ROI detection. The relevant information like Tmin, Tmax, Tavg and temperature gradient etc. are extracted from ROI and passed on to classifiers for final fault classification. The final stage of classification uses the extracted current features and compares them with a prior created database using advance learning methods like artificial neural networks(ANN), support vector machine (SVM)and fuzzy based decision making, etc.
- 39. Lubrication oil analysis : Lubrication oil is used in electrical and mechanical machines to reduce friction between moving surfaces. The lubrication oil is important source of information for early machine failure detection. In comparison with vibration based machine health monitoring techniques, lubrication oil condition monitoring can provide approximately 10 times earlier warnings for machine malfunction and failure. Lubrication oil analysis (LOA) includes fluid property analysis (fluid viscosity, additive level, oxidation properties and specific gravity), fluid contamination analysis (moisture, metallic particles, coolant and air) and wear debris analysis. Fluid property and contamination analysis is used to analyze the condition of oil to determine whether the oil itself has deteriorated to such a degree that it is no longer suitable to ful fill its primary function of reducing friction and preventing wear. Wear debris analysis is a technique which is used to monitor equipment’s operating condition (health) by analyzing the content of debris in the lubrication and hydraulic oil samples. For all these techniques methods like particle filtering, spectrographic oil analysis, Analytical ferrography and Radioactive tracer methods etc. are used to study the chemical composition of oil. However, it is an offline analysis requiring the sample to be taken from the machine and tested in laboratory.