2011 International Conference on Communication Systems and Network Technologies
APPLICATION OF MODERN TECHNOLOGY FOR FAULT DIAGNOSIS IN POWER TRANSFORMERS ENERGY MANAGEMENT #1
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Hasmat Malik , R.K. Jarial , Abdul Azeem , Amit Kr Yadav # Electrical Engineering Department, NIT Hamirpur (HP)-177005, India 1
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of all key performance affecting parameters of the transformer by using latest trends in the area of pre fault assessment & monitoring of key parameters affecting its useful life [ 2-7]. Any departure from its normal value indicates a fault condition which needs utmost attention to circumvent faults of all sorts & help prevent catastrophic failures. This paper analyses the causes of failures of substation transformers and present methods for preventive maintenance. The proper operational procedure to increase the life of the transformer has also been suggested.
Abstract— A transformer is the most important equipment for power supply to consumers. For uninterrupted power supply to consumers proper maintenance particularly preventive maintenance is very much necessary. The failure in magnetic, electric and dielectric circuits as well as structural failure may cause extensive damage to the equipment and surroundings. Proper operation and maintenance procedure may help to prevent failure and extend life of operation of the transformer. Continuous monitoring to health of the transformer and analyzing the history of past failures may indicate the type of maintenance required & facilitate identifying incipient faults. An overview of recently developed mythologies has been covered in the present paper along with planned strategy for curbing catastrophic failures creeping In service- transformers. The methods suggested in the paper are helpful in reducing failure and extending life of both the power transformers & distribution transformers.
II. FAILURE Failure of the transformer is defined as a non function of its rated performance so that the unit must be taken out of service to be repaired. A. Type Of Failure The data obtained from various state electricity boards with regards to normal types of failure of distribution transformers can be given as follows: A.1. H.V. Winding Failure 1. Winding open circuited 2. Winding burst out 3. Spacers found looses 4. Winding compressed and bulged out A.2. Failure in L.V. Winding 1. Loose terminals 2. Insulation failure between winding and core 3. Failure of winding termination near bushing A.3. Other Type Of Failure 1. Over fluxing 2. Oil oozing 3. Transformer tank bulged on sides 4. Radiator & tank punctured 5. Lightning 6. Damage to tap change switches and broken bushing 7. Ageing. 8. Moisture contaminated oil. 9. Other mixed failures.
Keywords—Transformer, Fault, Diagnostics, Chromatograph, Spectroscopy, Analyser
I. INTRODUCTION The high rate of failure of distribution transformers has been a matter of grave concern to power utilities in developing countries like India where it is now reported to be as high as 15-20% every year whereas in developed countries it is low as 1-2% [1]. This aspect is now engaging serious attention of all electricity boards and other organizations associated with power sector management. State electricity boards invest huge amount of the repair of distribution transformers. If the revenue loss due to interrupted power supply is also added as a result of transformer failure, the direct over-all loss to the country and the indirect loss to the consumers would run into corers of rupees that needs serious issue of grave concern amongst the utilities globally. Power Transformers are expected to give satisfactory services over a period of 25 to 30 year but in the practice this is rarely achieved and many transformers fail prematurely causing huge financial loss on account of repairs and supply interruptions and also causing inconvenience to the consumers [1]. It has been observed that protection of transformer against any internal and external fault with minimum time delay not only saves it against immediate damage but also improves life expectancy. Proper coordination of universally used backup over current and earth fault relay with minimum time delay against eternal fault is to be used. For protection with fuses only HRC fuses with consistent time current characteristics should be used. The basic notion calls for either continuous or periodic monitoring
978-0-7695-4437-3/11 $26.00 © 2011 IEEE DOI 10.1109/CSNT.2011.84
III. CAUSES OF FAILURES During operation of a distribution transformer failure may be occur in the primary and secondary coils, terminals insulation, cooling and insulating media, tap changing equipment, core etc. these failures may be divided into the following categories: (a) Failure in the windings. 376
V. OPERATING PROCEDURE
(b) Failure in the dielectric circuits (c) Failure in the magnetic circuits. (d) Structural failures in the tanks, bushings, clamping structure etc. These failures may be due to: (a) Improper operation and maintenance of transformers (b) Faulty manufacture comprising poor design, poor quality of material used, bad workmanship etc. (c) Damage in storage, transit and handling (d) Other causes like ageing meddling etc.
Proper operation of a distribution transformer increases its life to a large extent. Over voltage and under frequency operation should be avoided. During of load period the input voltage is often more than rated voltage. Proper setting of tapchanger in the primary side during this period does not allow operation against over voltage. During peak period the supply frequency is often less than the rated frequency and if the supply voltage is maintained at rated value, exciting current may be very high due to high V/f ratio. This problem may be avoided by adjusting primary side toppings in the transformer. Since unbalance loading in the transformer decreases its life due to overheating and development of unbalanced electro-mechanical force. Unbalance loading may be avoided by using load balancing switches connected to distributed line. All component parts like cooling tubes, conservator tank etc. connected to main tank should be bolted sacredly to reduce induced vibration due to magnetostiction. The induced secondary vibration not only increases noise but also creates mechanical fatigue with adventitial reduction in life of transformer. Moisture should never be allowed to enter the transformer particularly during breathing. Ingress of moisture into oil quickly damages its dielectric property. Periodic heating and moisture removal from silica gel in the breather helps to maintain the dielectric property of oil.
IV. SALIENT FEATURES The study of failures has brought out the following features: 1. Induced vibrations in the core structure due to bad clamping and tightening of bolts may cause damage to the insulation between laminations and also around the core. This causes local short circuits and hot spots which may ultimately be the reason of transformer failure. It can be easily prevented by preventive maintenance. 2. Cutting and punching processes if not done correctly may create burrs in the core and yoke laminations. Therefore local short circuit in the iron laminations may be produced which may create abnormal heating of core. It can be prevented during manufacturing. 3. Frequent short circuits due to various reasons may damage H.V & L.V. winding due to wide temperature changes and sudden electromagnetic forces acting axially and radically. Assembly of coils spacers and end plates should be properly compressed to withstand axial forces. Gaps may also be produced between spacers and coils and may be avoided by tightening nuts and bolts. 4. The failure of the windings may also be due to improper design and fabrication. 5. Moisture entry in the insulation of the coil due to improper impregnation may cause failure. It should be stop using activated silicagel in the breather. 6. Insulation of L.V. winding is damage due to indiscriminate over loading of transformer which is not provided with the correct size of fuses on L.V. side. it leads to undue heating and burning out of coil. 7. Tap changer failure may be due to improper fabrication of handle and mishandling. Contact pressure should be adequate and contact area should be large to reduce contact resistance. 8. H.V & L.V. bushing should be properly designed and connections should be firmed and secured. 9. Failure in dielectric are due to oil leakage, improper closing of bottom valve and damage to tank walls, inadequate clearance of oil and moisture entry. This failure can be prevented by continuous monitoring of oil providing diagnostic remedies. Data recorded by various electricity boards have shown that nearly 30 to 40% failures occur due to improper maintenance of transformer oil and protective devices [1]. A large number of transformers can be saved from damaged by following preventive maintenance and keeping its record.
VI. THERMOGRAPHY Thermography is a technique to extend the human vision in to infrared region of electromagnetic spectrum. Normally infrared radiations are visible to human eyes for temperature bellow 800k. Thermography widens the total field of human vision and permit presentation of thermal range in real time of the object at temperature range of 250k to 2000k. The system consists of a scanner and display unit. The infrared radiation emitted by a source are received by scanner and transmitted through its optical system to detector. The detector cooled by thermoelectric couple (100K) converts the infrared radiation to video signal which is proceed and displayed as a line B/W picture on screen of the display unit. VII. THERMO VISION SCANNING The routine periodical thermo vision scanning of transformer in substation is carried out by use of a portable thermovision system. The working of the system can be easily understood with the help of fig 1. It has facility of temperature read out computer (TRC) and video cassette (VCR). The temperature of joint scan can be directly with the help of TRC after feeding data of object distance and it emissivity. This scanning is carried out during peak load period so that most of the hot spot are detected. This scanner is also used to check the internal condition of equipment. After hot spot is detected the hot line staffs carry out the necessary diagnostics and may again scan to check the effect of diagnostic applied. This type of thermoscanning is also done on other equipments used in power system.
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C. SFRA diagnostics D. Furan Analyses E. Dielectric Dissipation Factor (DDF) or Tan Delta (tan δ) F. Water in oil Analyser G. On line Transformer Temperature Monitor H. Routine Inspection Monitor & Indicator device using µP I. Thin film Capacitor Sensor J. Use of HTS Transformers The basic concept behind all above methods is the idea that calls for either continuous or periodic monitoring of all key performance affecting parameters of the transformer. Any departure from its normal value indicates a fault condition which needs utmost attention to circumvent faults of all sorts & help prevent catastrophic failures. The brief details of above methods are enumerated below:
Fig. 1: Thermo Scanning Block Diagram
VIII. NEW TECHNOLOGIES A. AMORPHOUS METAL DISTRIBUTION TRANSFORMERS(AMDTS) By using amorphous metal as core material, instead of CRGO steel, No-load losses are reduced by 75%. In AMDTs both hysteresis and eddy current losses are low compared to the CRGO steel. Amorphous metal, used in AMDTs has a crystalline atomic structure. The unique non crystalline or random atomic structure of amorphous alloys results in dramatic reduction of the hysteresis loss. Moreover, the electrical resistivity of amorphous alloy is higher than CRGO steel by a factor of 3, so the voltage induced across the core of an AMDT will have a lower eddy current associated with it. Further eddy currents are lower for thinner materials, and amorphous metal is approximately 1 ml. thick, where as silicon steel are produced in the 7 to 11 mil range. As per heat produced due to losses are reduced the chance of failure becomes less.
A. Online Dissolved Gas Analysis By Gas-Chromotograph The production of organic gases such as methane, ethane, ethylene, propane, acetylene, and propylene as well inorganic gases such as hydrogen, carbon dioxide and carbon monoxide, in the insulating system of the operating transformers in alarming proportions in indicative of fault occurrence. These faults may lead to the breakdown of the equipment. Some of the faults that can be indicated by the production of particular gas are given bellow: 1. Acetylene in major concentration indicates arcing 2. Ethylene indicates over heating 3. Hydrogen with considerable proportion indicates partial discharge. 4. Methane with considerable proportion indicates partial discharge. 5. Carbon dioxide and carbon monoxide indicates the involvement of solid insulation in the fault.
B. FAIL SAFE DISTRIBUTION TRANSFORMERS (FSDTS) FSDTS with built in protection of LTCB, H.V. fuse link and lightening arrester have maximum safety. Reliability and minimal maintenance requirement. This type of transformer provides protection from excessive overloading, dead short circuit, low oil level and high operating temperature. They provide auto reclose and in case of persistence fault, provide an indication of the nature of fault. They are innovative. Intelligent system using microprocessor base technology that surpasses existing protection schemes in terms of technical superiority and cost effectiveness. These transformers have shown remarkable low failure rates in western counties.
Now it has been found that the quantitative analysis of the above mentioned gases is more reliable & far more sensitive that hazards fault detection devices. Such as bucholtz relay & differential relay For quantitative gas analysis gas chromatography is one of the most sensitive commonly used techniques. Gas chromatography should be done online bases regularly while the transformer energized. The result of gas analysis provides an inside about incipient fault. Corrective action may be taken for preventive maintenance. This technology is however more useful in the case of power transformer but can be equally effective in the distribution transformer.
IX. RECENT ADVANCES IN MONITORING TRANSFORMER CONDITION
B. Oil Testing By UV-VIS Spectroscopy Apart from normal prevailing testing procedures with regards to ascertaining the electrical insulating quality of in use - insulating oil of the transformers i.e. Physical appearance, colour, density, electric strength, moisture content, gas content acidity, sludge inhibition content, flash point, power factor and insulation resistance that have been in practice by utility engineers for electric health assessment indicators, recently new techniques of UV-VIS Spectroscopy is also being suggested for knowing the degree of degradation of In–service oil filled transformers [4-7].
In the recent years, several researchers & transformer experts have been able to achieve significant breakthrough in developing ways & means of assessing condition of the Inservice Power & Distribution Transformers that have proved a boon for detecting incipient faults in them. As a result, catastrophic failures can be easily detected & remedial actions can be taken timely to save both human life & other Capital losses in addition to preventing electric supply failures and economic loss to state electricity boards & utilities. Some of the recently developed techniques are: A. On line Chromatography B. Oil test by UV-VIS Spectroscopy
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• Spectroscopy is the study of the interaction between radiation and matter as a function of wavelength (λ) or frequency (ν). • Spectrophotometer: A spectrophotometer is device that being which measure the intensity of the light as a function of the light source wavelength. • Spectrophotometers use a Monochromators containing a diffraction grating to produce the analytical spectrum • UV/VIS analyser is used to the analysis of the relative content of dissolve decay in the insulation oil, according to ASTM standard. • UV Fluorescence is applicable for the sulphur analysis of insulation oil. The Beer-Lambert law is the linear relationship between absorbance and Concentration of an absorbing Species. The general Beer-Lambert law is usually written as: A = a*b*c Where, b: Sample Path length, c: Sample Concentration, a: Absorbance Constant Experimental measurements are usually made in terms of transmittance (T), which is defined as:
T = It
experience large number of short circuits during its service life. Due to short circuits, there could be winding moment which changes its winding inductance and capacitance. Other mechanical faults occur is in the form of displace winding, hoop buckling, winding moment, deformation and damaged winding. Winding movement may also result from stresses induced by electrical faults such as inter turns ,short circuits etc as a result of lightning strikes. It may also results in insulation damage. Another important factor is winding shrinkage which occurs due to aging, leading to a reduction in clamping pressure and short circuits withstand strength. The deformation can also be due to aging of paper. Winding deformation in transformers are difficult to establish by conventional methods of diagnostics test like ratio, impedance/inductance, magnetizing current etc. Winding deformation results in relative changes to the internal inductance and capacitance of the winding .These changes can be detected externally by low voltage impulse method or FRA method. It is therefore desirable to be able to check the mechanical condition of transformer periodically during their service life. In the FRA technique, a low amplifier swept frequency signal is applied at the end of one winding and response is measured at the other end of the winding with one phase at a time. The method is based on the fact that every transformer winding has a unique signature of its transfer function which is sensitive to change in resistance, inductance and capacitance. It consists of measuring the impedance of transformer over a wide range of frequency and comparing the results of this measurement with a reference set taken either during insulation or at any other point of time. Difference in signature of the responses may indicate damage to the transformer which can be investigated further using other techniques or by an internal examination. Several utilities have considered this test as benchmark for newly installed locations of Power transformer to ascertain mechanical integrity of the entire structure intactness before energizing at new location & prevent catastrophic failure [9].
Io
Where It is the light intensity after it passes through the sample and Io is the initial light intensity. The relation between A and T is: A = - log T = - log (It / Io). The basic concept behind all above methods is the idea that calls for either continuous or periodic monitoring of all key performance affecting parameters of the transformer. Any departure from its normal value indicates a fault condition which needs utmost attention to circumvent faults of all sorts & help prevent catastrophic failures. The brief details of above methods are enumerated below: Dissolved decay content of insulating oil
D. Furan Analyser The Cellulose insulation has a structure of long chain o molecules. The cellulosic paper contains about 90% cellulose, 6-7% of hemicelluloses and 3-4% of lignin processed by the Kraft chemical process (Kraft in German is used for strong). Cellulose is a polymer of alpha-D-glucose units linked to one another in a special manner. It may be represented simply as [C5H10O5], where n is the degree of polymerization (DP). The DP of paper can be determined using ASTM method D-4243. Generally, DP lie in the range of 1100-1600 for new paper but its value can drop by 10% after drying and oil impregnation [11]. The DP range of Kraft pulps varies from 110 to 1200, for mixed pulp fibers it varies from 1400 to 1600 [12]. Middle aged and old aged paper have DP around 500 and 2500
