Assesment of health of transformer: a survey

Transcription

1 Volume 114 No , ISSN: (printed version); ISSN: (on-line version) url: ijpam.eu Assesment of health of transformer: a survey Ravi Kumar Jalli 1 Saptarshi Roy 2 1EEE Dept.,GMRIT, Andhrapradesh, rk.eagle.60@gmail.com 2EEE Dept., NIT Warangal, Telengana, 2saptarshi.roy.ju@gmail.com April 14-15, 2017 Abstract Condition monitoring of transformer 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 its expected values. Transformers are the most critical assets electrical transmission and distribution system. Transformer failures can 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 hotspot, electrical disturbances, deterioration of insulation, 281

2 lightning, inadequate maintenance, loose connections, overloading, failure of accessories such as OLTC, Bushings etc. In this work it is shown with case studies and examples how health is assessed and precautions can be taken to increase the life time of a transformer. AMS Subject Classification: Key Words and Phrases: Condition Monitoring, Tan delta, Degree of Polymerization, Transformer failures, dielectric breakdown, inadequate maintenance of transformer. 1 Introduction The life span of any equipment decreases with time[1]. So, to increase the longevity of any equipment the condition of the equipment should be assessed and preventive measures should be taken during its operation time. Condition Monitoring of Transformers is the process of collecting and processing of data related to various parameters or indicators of transformers so as to predict and prevent the failure of a transformer. This is executed by observing the anomaly of the parameters from their desired values. Transformers are the very important components of power system. Failures of transformers could cause power outages various hazards or purchase of power from other suppliers. Failures of transformer can happen due to several causes. E.g-insulating medium breakdown, winding distortion, hot spot in transformer windings, electrical disturbances, degradation of insulation and paper insulators, lightning, poor maintenance, bad-improper connections, overloading, failure of supporting equipment s such as OLTCs, bushings, etc [2-3]. Several works has been done based on various condition monitoring approaches of transformer from the past. [4] describes distribution transformer s condition monitoring of mechanical parts by using Sweep frequency response analysis.[5] describes a online condition monitoring approach for power transformer s bushing, tap changers and insulation system.[6] demonstrates a technique for condition monitoring of transformer oil using thermal analysis.[7] 282

3 describes a method for condition monitoring of transformer s insulation system.[8] describes condition monitoring of transformer insulation system by polarization and depolarization of current measurement. In this work it is shown with case studies and examples how health is assessed and precautions can be taken to increase the life time of a transformer. 2 Condition based monitoring methodologies 1)Tan delta on winding. 2)Test of oil samples. 3)Polarization Index. 4)Dissolved Gas Analysis 5)Furan Analysis 6)Degree of Polymerisation 7) Partial Discharge Test. 8)Acoustic Monitoring 9) High performance liquid chromatography 10) Moisture Monitoring Fig 1: Automatic Ratio meter Fig 1 shows an automatic ratio meter which is an associated test kit during the work. 3 Polarization Index Test The insulation resistance readings change w.r.t time because of alignment of dielectrics. Actually, the dielectric system contains polarized atoms with dipoles and the polarized atoms aligns (polarize) with the applied dc voltage. As they polarized, the dielectric resistance will increase and after sometimes it gets saturated. The direct ratio of two certain times can indicate the probable health condition of the insulation in the system. The ratio is called the Polarization Index. 283

4 Therefore,P.I = R( t R( t R(t1) = Insulation resistance after t1 times of applying d.c voltage. R(t2) = Insulation resistance after t2 times of applying d.c voltage. Generally we use t1 =10 sec t2 =60 sec P.I gives an idea of the cleanliness of the insulation system. The several interpretations obtained from the polarization indexed test are described as follows : P.I < 1.3 not good. 2 < P.I <5 - permissible P.I < 2- dirty or moist insulation. P.I > 5- Very dry, brittle insulation system. Exceptions : HV Vacuum pressure impregnated system may yield a P.I between 1&2,for a perfectly good winding. 3.1Case Study : The results shown are tested on a 16 MVA make transformer, Voltage Class 6.6 KV /433V in Table-1 Table-1 : P.I test results Connection I.R at 10 sec(mega Ohm) 2 1 ) ) I.R at 10 sec(mega Ohm) Ph- Earth Ph-Earth Ph-Ph Recommended Action : 1)The insulation is okay. 2)But little bit of oil purification will make its condition better. If even after oil purification the condition of the insulation is not improved then it is suggested to take the help of other methods like nitrogen purging or induction heating etc. If P.I value is very high, even if >5,then it is not a matter of concern.but it is really harmful, if its value is very low,specially below 1. It indicates the insulation contains moisture. Moisture content make it conducting and thus degrade its insulation property. 4 DGA (Dissolved Gas Analysis) P.I 284

5 It is very important for assessment of health of transformer. Generally oil samples from Buchlozz relay is collected, gas extracted, then they are separated, then analyzed, then we make interpretations from collected data. Some general interpretationsof DGA analysis is described below : 4.1 Some interpretations from DGA Free Gas i)colourless and odourless : Trapped air. ii)greyish with pungent smell: non-flamable overheating of insulation and insulation board. iii) Yellowish inflammable : decomposition of wood. iv)dark Grey : Flash over in oil / Overheat. The results are shown on a 50MVA Transformer, 132/33 KV in Table-2 Table-2: DGA Test results Test Values Top Sample Bottom Sample-I Bottom Sample-II BDV(KV) Water content(ppm) DGA Results TGC(%) N2(%) O2(%) H2(ppm) CH4(ppm) Recommended Action : i)content of H2,CH4 indicates partial discharge,c2h2 indicates arcing,content of C2H4 and C2H6 indicates decomposition of oil (Kitone formation). ii) The DGA shows high temperature arcing problem of the transformer. Some part of oil is also decomposed. Need proper internal inspection of the transformer, whether internal condition of winding insulation is o.k or not. 5 Tan Delta Test Tan delta test indicates the dielectric strength, dielectric loss and the moisture content of the various insulations. Delta is the angle between ideal capacitive current and lossy capacitive current. Tan delta test is very important to know 285

6 the condition of insulation of transformer bushing and other cables. The various interpretations obtained from the Tan Delta values are described in section Limit of Tan Delta Values: For good transformer ==< 0.5% Acceptable ==0.5%-2.0% Unacceptable ==2.0% 5.2Case Study : Results shown on a 138/11.5 KV Transformer, 16.5/33/82.5 MVA,Ynd11 in Table-3 and Table-4. Table3:Tan Delta measurement of the winding : Test Condition Voltage(KV) % Tan Delta Capacitance(Pf) HV-LV+Gnd(GST) LV-Gnd (GSTg) HV-Gnd(GSTg) HV-LV(UST) UST indicates Ungrounded specimen test. GST indicates Grounded specimen test. GSTg indicates Grounded Specimen test with guard. Table-4:Tan Delta measurement of the Bushing : Bushing Voltage(KV) % Tan Delta Capacitance(Pf) 1U V W Diagnosis : 1)Tan Delta value of LV-Gnd indicates wet insulations. 2) Tan Delta value of HV bushing ( 1V)slightly exceeds the limits. It may contain little moisture but within acceptable limit. 6.SFRA ( Sweep Frequency Response Analysis ) Test : It assesses the structural health of core and winding. It indicates the response of transformer winding to variable supply frequency. The frequency response from a newly made transformer given by original equipment manufacturer( OEM) and the frequency response at the time of the condition monitoring of a transformer is compared and 286

7 then analyzed by seeing the anomaly. Then desired prevention is taken. Fig 2 and Fig 3 shows the results. Fig 2: Frequency Response of newly made transformer Fig 3: Frequency Response at the time of condition based monitoring Recommendations : The 2 nd picture shows distortion with original figure, indicates displacement of winding with respect to original place of core coil assembly at first.it indicates that it needs liftingof core coil assembly from the structure and refit. 7 Furan Analysis Furan analysis is the paper insulation degradation test.furan is derivative of an aromatic compound formed due to paper degradation.derivatives of Furan soluble in oil are : Furaldehyde, Acetyl Furans,Methyl Furaldehyde, Furfurylalchohol,Hydroxy Methyl Furaldehyde. Limit: Acceptable : Total Furan < 100 ppb Questionable : 100ppb < Total Furan < Case Study : Transformer Details : 138/11.5 KV,16.5/33/82.5 MVA. Test 287

8 results are shown on Table 5 Table 5: Test Report Analysis of Furanic Compounds Obtained Value (ppb) 5-Hydroxymethyl-2-Furfural ND 2-Furfuryl Alchohol ND 2-Furfural Acetyl Furan ND 5-Methyl-2-Furfural ND Total Furan content in Oil ND: Not Detected. Recommendations : The transformer is having furan level 240.8ppb, which indicates normal aging of transformer. It needs proper attention before effective life is getting expired. 8 Degree of Polymerisation Test It requires paper samples. Aging of paper leads to loss of tensile strength and reduced life of electrical equipments. Insulation is long chain of hydro-carbons. With respect to time, it will deteriorates. At the beginning of life the degree of polymerization is around and at the end of life it is around Transformer Details : 138 /11.5 KV MVA Class : 16.5 /33 /82.5 MVA Vector Group: Ynd Test Results : The insulation paper samples were taken for measurement of D.P from various locations.the sample details and result as below (ref. Table-6). Table6: D.P Results Paper Samples taken from D.P Value HV lead,top 650 HV lead,bottom 600 LV lead,center 563 LV lead,bottom 576 LV lead, top 580 HV lead, Center

9 The above results indicates the normal aging of transformer is already started. With the degree of polymerization the residual life of transformer is also can be assessed. For assessing the residual life of a transformer the degree of polymerization vs residual life curve is required which is to be provided by original equipment manufacturer ( OEM) at the time of purchasing of the transformer. 9 Partial Discharge Test Local dielectric breakdown of a nominal area of a solid /liquid electrical insulation system under HV stress. Normal range of partial discharge from a transformer is in terms of pico-coulomb. Partial discharge creates a hissing sound. Corona is a kind of partial discharge. 10 Acoustic Monitoring With the load of the transformer increases noise will also increase.hence several measurement of noise needed to monitor the transformer performances.e.gi) The noise level of new transformer. ii)changes of noise level over time at given load. iii)relative level of noise with the background noise. 11 Conclusion Finally, the condition monitoring of transformer is very effective in case of large size power transformers which is having rating in terms of 100,200,300 MVA or even more. A standard 315 MVA transformer costs in terms of crores, may be 5-6 crores with on an average life span yrs. Instead of replacing the whole transformer with erection, testing and commissioning costs after the effective life time, it is suggested to do condition monitoring and increase its effective life time up to its optimum usage.prone to damage spares are needed to be replaced and obsolete spares are needed to be replaced with spares with updated features. It is seen from the experiences if the user invests just 10% of the transformer s cost on the account of its condition monitoring the effective life time increment of transformer will be of about 25%, 30% or even more. So, condition monitoring of transformer is a very very important aspect 289

10 w.r.t its maintenance and it is expected that this article will be very helpful for all industry practitioners to create a general awareness about its different methodologies with practical examples. References [1]Saptarshi Roy, Performance assesment of SCADA based wind turbine : condition monitoring approaches, International Journal of Electrical Power System and Technology,vol.1, issue 2,pp: 1-9,September [2] Johnson &Philips, J & P Handbook of Transformers. Transformer, BHEL [3]Ghani S, Thayoob Y.H.M and Hanum Y, Condition Monitoring of Distribution Transformer s Mechanical Parts using Sweep Frequency Response Analysis ( SFRA),2013, The Malaysian International Tribology Conference [MITC2013],Elsevier, pp: [4] Stirl T, Vilaithong R and Tenbohelen S., On-line Condition Monitoring and Diagnosis for Power Transformers their Bushings,Tap Changer and Insulation System, ichungen/cmd2006tobias.pdf [5]Degeratu S, Rotaru P and Rizescu S, Condition Monitoring of Transformer Oil using Thermal Analysis and Other Techniques, Journal of Thermal Analysis and Caliometry,Vol.119, Issue : 3,pp ,March [6] Mehta A.K, Sharma R.N and Chauhan S, Condition Monitoring of Insulation System in Power Transformers, International Journal of Computer and Electrical Engineering,Vol.4,No. 2,pp , April [7] Saha T.K, Purakait P and Yao Z.T, Condition Monitoring of Transformer Insulation by Polarisation and Depolarisation Current Measurements, January 2002, onito.pdf 290

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