POWER TRANSFORMER AGEING ASSESSMENT IN SERVICE BY MEANS OF THE POLARISATION AND DEPOLARISATION CURRENT (PDC) ANALYSIS

Size: px

Start display at page:

Download "POWER TRANSFORMER AGEING ASSESSMENT IN SERVICE BY MEANS OF THE POLARISATION AND DEPOLARISATION CURRENT (PDC) ANALYSIS"

Melinda Bridges
5 years ago
Views:

1 POWER TRANSFORMER AGEING ASSESSMENT IN SERVICE BY MEANS OF THE POLARISATION AND DEPOLARISATION CURRENT (PDC) ANALYSIS T. Lebfred & A.J. Kachle D. Pollaro Transformatorenwerk Nurnberg Power Transmsson and Dstrbuton Semens AG, Germany Semens AG, USA ABSTRACT Deregulaton and strong nternatonal competton are forcng utltes throughout the world to cut back drastcally the electrc energy generaton and transmsson costs. Well-establshed, tme-based mantenance by experenced mantenance staff as well as conservatve replacement plannng s beng sacrfced now. Condton based mantenance by hred staff and onlne montorng as an early warnng system are ganng mportance n nsulaton ageng assessment of power transformers n servce. Insulaton ageng s a four dmensonal problem due to delectrc, chemcal, thermal and electromechanc stresses, whch are hghly dependent on operatonal condtons. All these ageng processes lead to the formaton of water molecules. Therefore, water plays a key role for the ageng of the ol and paper nsulaton systems snce water acts as a catalyst for the ageng process. Due to the complex nature of mosture mgraton a mu lttude of dfferent analytcal dagnostc procedures s requred [1]. To date ageng and mosture can only be relably detected by paper sample shavng at crtcal locatons (leads, outer wndng) and analysng these samples n the laboratory by Karl-Fscher ttraton and determnaton of degree of depolymersaton. Ths paper presents the Polarsaton and Depolarsaton Current analyss (PDC analyss) as an excellent and non-destructve method for determnaton of the mosture content n the sold nsulaton materal of power transformers. On the bass of ths relable nformaton one can decde about further actons lke on-ste dryng of the actve part of a power transformer. INTRODUCTION Numerous transformers, whch are currently n servce, have been nstalled 30 or even more years ago. They mght be close to ther nomnal end of lfe. Today, utltes and other electrcal power equpment operators are under pressure to reduce costs for mantenance and replacement. On the other hand, there are ndustres, whch requre a hgh level of energy supply qualty wth respect to avalablty and qualty of voltage. In addton envronmental aspects such as consequental damages, fre and polluton are of hgh rsk. These are the man reasons why nsulaton dagnostcs on power transformers s an mportant part of a modern power equpment mantenance strategy. Ageng of the ol paper nsulaton system of power transformers s determned by varous stresses, namely delectrc, thermal, electromechancal and chemcal stresses. Delectrc and thermal stresses lead to degradaton processes of ol and cellulose. Cellulose conssts of molecular chans, whch are charactersed by ther degree of Depolymersaton (DP). The chans of molecules break apart as the cellulose degrades whch decreases the DP. For new cellulose the DP value s about The molecule length of degraded cellulose s reduced to a DP value of about 200. In ths condton cellulose s brttle and the durablty aganst mechancal stresses s strongly reduced. Ths degradaton or breakng process of cellulose molecule chans produces water n the sold nsulaton, whch acts as a catalyst. Further, the breakdown voltage of the nsulatng ol s reduced wth ncreasng mosture content n the ol. Thus, knowledge about the water content both n the ol and n the sold nsulaton materal s an mportant bass for the decson about any further acton lke on-ste dryng. The PDC analyss s a non-destructve method for determnng the mosture content n the sold nsulaton materal lke paper and pressboard [2, 4]. Accordng to Fg. 1a a DC voltage step of 500 V s appled between the nsulaton system of HV and LV wndngs durng the polarsaton duraton T P. Thus, a pulse-lke polarsaton (chargng) current of the transformer capactance flows. The current decreases durng the polarsaton duraton to a fnal value, whch s determned by the conductvty of the nsulaton system (Fg. 1b). After elapsng, the polarsaton duraton T P, the swtch S goes nto the other poston and the delectrc s short-crcuted va the ammeter. Thus, agan a pulse-lke dschargng current of negatve polarty flows, whch goes gradually towards zero. Usually, the polarsaton tme T P and the depolarsaton tme T D are equal. Then, a model, whch descrbes the delectrc behavour of the transformer s man nsulaton system, s parametersed. The behavour of an arbtrary delectrc can be descrbed by an R-C network model shown n Fg. 1e, 1

2 where R and C result from the conductvty and the capacty of the delectrc and the Rp-Cp-seres-crcuts represent the long term polarsaton effects. The complete R-C-model for a transformer (Fg. 1f) can be drectly derved from the smplfed geometry model of the man nsulaton system (Fg. 1d) and the knowledge of the model of the used delectrcs. The parameters of ths model (Fg. 1f) can be determned usng already measured characterstcs of pressboard materal samples wth certan water content, the ol parameters and the geometry of the man nsulaton system. The best ft between calculated relaxaton currents for dfferent mosture contents and measured currents provdes valuable nformaton about the transformer s condton such as the mosture content n the sold nsulaton materal, tan δ n a low-frequency range, polarsaton ndex (e. g. R 60 /R 15 ) and DC conductvty of the ol. 1U 1V 1W 1N S HV u T P U 0 T D a. U 0 A LV 2V 2U PDC-Analyser W b. POL DEPOL Pressboard LV spacers ol barrers pressboard barrer Ol duct c. pressboard spacer HV R O d. Pressboard I R S,1 C O R S C S C S,1 S: spacers B: barrers O: ol R S,n C S,n C R R p1 R pn C pn C p1 e. f. U L R B,1 R B C B C B,1 R B,n C B,n a. Measurement of the relaxaton currents usng the Semens measurng system PDC-Analyser-3205 [2, 3] b. Prncple waveform of relaxaton currents c. Part of the cross-secton of a power transformer man nsulaton system between HV and LV wndngs d. Smplfed geometry model for the man components ol, barrers and spacers e. Model for the behavour of a delectrc wth arbtrary polarsaton characterstc and conductvty f. Model for the delectrc behavour of nsulaton system of power transformers FIGURE 1 INTERPRETATION OF PDC MEASUREMENTS In Fg. 2 the prncpal effect of dfferent ol conductvtes and mosture contents n the sold nsulaton materal on the polarsaton current s shown. The conductvty of the ol changes the ampltude of the polarsaton current typcally n a tme range t < 100 s, whereby an ncreasng ol conductvty results n an ncreasng current. A hgher water content n the pressboard and paper nsulaton affects the polarsaton characterstc manly n the tme range t > 1000 s as t s clearly vsble by an ncreasng dfference of relaxaton currents beyond about s. The evaluaton of relaxaton currents n the tme doman allows to use ths characterstc of ol-paper nsulaton systems for separatng the effects of ol qualty and mosture content n the sold nsulaton materal from each other. Fg. 2b shows the PDC analyss of a new 392 MVA power transformer. The polarsaton currents have been calculated for mosture contents of 0.5 % and 1.0 % n the sold nsulaton materal. The measured polarsaton current s n the 2

3 tme range t > 1000 s n between the calculated currents for mosture contents of rm = 0.5 % and rm = 1.0 %. Thus, the concluson s: The mosture content n the sold nsulaton materal of ths transformer s n between 0.5 % and 1.0 %. In the case of new transformers the PDC result can be compared wth results of the Karl-Fscher ttraton and the dew pont measurement of the ntrogen fllng whch allows to determne the mosture n the sold nsulaton materal snce these measurements are carred out n the Nuremberg power transformer factory as routne qualty checks. For the 392 MVA transformer, the Karl-Fscher result was 0.61 % and the dew pont measurement value s 0.45 %. Thus, t can be stated that there s a good match between PDC analyss and other mosture determnaton methods A pol A 10-8 pol 10-9 σ = S/m σ = S/m s a A 10-8 t 10-9 Polarsaton Currents rm = 1 % rm = 0.5 % s t rm = 1.0 % 10-9 Measurement Calculatons rm = 0.5 % s b. t a. Affect of ol conductvty and mosture content n the sold nsulaton materal on the polarsaton current ( pol ) b. PDC analyss of a 392 MVA transformer FIGURE 2 1 0,8 PDC Analyss Karl-Fscher Ttraton Dew Pont Measurement rm n % 0,6 0,4 0,2 0 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 Transformator # Comparson of the PDC analyss wth other methods for mosture content determnaton n the sold nsulaton materal of power transformers FIGURE 3 Such comparsons between the PDC analyss and results from the Karl-Fscher-Ttraton and the dew-pont measurement have been carred out on numerous transformers wth dfferent ratngs and desgns. Ths proves the applcablty and relablty of the PDC method for determnng the mosture content n the sold nsulaton materal of power transformers (Fg. 3). 3

ON-SITE PDC MEASUREMENTS ON TRANSFORMERS IN SERVICE The ol of a 300 MVA transformer manufactured n 1978 was found sludged after 23 years of unnterrupted operaton.

1 Hz wth a transportable on-ste measurng system, have been carred out. Table 1 gves a summary of the results. Furthermore, a paper sample was shaved at a lead of a tap wndng.

1 Hz, from PDC analyss 0.901 0.191 conductvty σ ol of the ol, from PDC analyss n 1/Ωm 4.5 10-11 3.5 10-12 Mosture n the sold nsulaton materal, from PDC analyss 3 % 2.

11 Depolymerzaton degree 352 Results of dagnostc measurements of the 300 MVA transformer carred out pror and after the ol exchange TABLE 1 The PDC analyss provdes some nformaton n form of so-called

4 ON-SITE PDC MEASUREMENTS ON TRANSFORMERS IN SERVICE The ol of a 300 MVA transformer manufactured n 1978 was found sludged after 23 years of unnterrupted operaton. Therefore, the utlty decded to exchange the ol. Pror and after the ol change dagnostc measurements namely PDC, RVM as well as tan δ at 0.1 Hz wth a transportable on-ste measurng system, have been carred out. Table 1 gves a summary of the results. Furthermore, a paper sample was shaved at a lead of a tap wndng. The degree of depolymerzaton s DP = 352, whch ndcates a normal thermal ageng of the paper. Measurements Pror to ol change After ol change tan δ at 0.1 Hz, drectly measured tan δ at 0.1 Hz, from PDC analyss conductvty σ ol of the ol, from PDC analyss n 1/Ωm Mosture n the sold nsulaton materal, from PDC analyss 3 % 2.7 % Mosture n the sold nsulaton materal, from RVM measurements 3.45 % 2.48 % Polarsaton ndex, R 60 /R Depolymerzaton degree 352 Results of dagnostc measurements of the 300 MVA transformer carred out pror and after the ol exchange TABLE 1 The PDC analyss provdes some nformaton n form of so-called fngerprnts or ntal state characterstcs. These fngerprnts are tan δ n a frequency range from about 10 Hz down to 10-5 Hz, the ol conductvty σ ol, and the polarsaton spectrum and polarsaton ndexes, e. g. R 60 /R 15. Fg. 4a/b shows a comparson of the polarsaton currents measured pror and after the ol change as well as the tan δ calculated from these currents. The currents show dfferences n the whole tme range. However, the dfferences are more sgnfcant for short measurement tmes. Ths ndcates a much lower conductvty of the new ol, n fact the conductvty σ ol s reduced by about one decade as shown n Table 1. Furthermore, the tan δ value s drastcally reduced after the ol change. The comparson of the tan δ values obtaned by the transportable measurng system and the results from the PDC analyss shows almost the same values. However, the determnaton of tan δ usng the PDC method s much easer and has the advantage to get tan δ over a wde frequency range. a. b. c. d. PDC measurements on a 300 MVA transformer, measured wth the PDC-Analyser-3205 b. Polarsaton currents measured pror and after the ol change c. tan δ calculated from measured polarsaton currents measured and calculated d. polarsaton currents for rm = 2.5 %, rm = 3 % and rm = 3.5 % e. pror to the ol change f. after the ol change FIGURE 4 4

5 Fg. 4c/d shows the PDC analyss of measurements taken pror and after the ol exchange. The results are wth rm = 3 % before the ol change and rm = 2.7 % after the ol change very close to each other. Due to the low mosture absorpton capablty of ol, t s not possble to decrease sgnfcantly the mosture n the sold nsulaton by exchangng the ol of a transformer. Assumng a weght of the sold nsulaton of kg and mosture content of 3 %, we have total water content of 300 kg. Assumng further an ol weght of kg and water n ol content of 30 ppm, whch s a hgh value, we get a water mass of only 1.5 kg stored n the ol. Thus, the water content n the transformer pror and after the ol exchange remans almost the same. These reflectons confrm once more that mosture n a transformer can not be extracted by exchangng the ol. In other words: Dryng of the ol durng a short perod s not an approprate method for dryng the actve part of transformers. Pror to the ol exchange the polarsaton ndex R 60 /R 15 s 3.16, whch s hgher than the value after the ol exchange, whch s Ths reducton can be well explaned by the shape of polarsaton currents before and after the ol exchange (Fg. 4a). The dfference of the polarsaton currents pror and after the ol change decreases contnuously n a tme range up to 100 s. Thus, the ncrease of R 15 measured before and after the ol change s hgher than for the R 60 values and thus the quotent R 60 /R 15 from current measurements after the ol change decreases. Obvously, the polarsaton ndex R 60 /R 15 s not only affected by the mosture n the sold nsulaton but also by the ol conductvty (ol qualty) to a certan degree. Therefore, the polarsaton ndex s not a good ndcator for the state of the sold nsulaton. The polarsaton spectrum can also be determned by RVM (Recovery Voltage Measurement). A DC voltage s appled to the delectrc durng a certan chargng tme T C. After elapsng a perod of 0.5 T C durng whch the delectrc s short crcuted the so-called recovery voltage s measured for such a cycle. The polarsaton spectrum s the maxmum recovery voltage over the chargng tme T C for chargng perods from e. g. T C = 1 s up to T C = s. The same polarsaton spectrum can be derved from polarsaton and depolarsaton current measurements usng the R-C model (Fg. 1f) for the entre transformer. Fg. 5 shows the polarsaton spectra determned by the RVM method and those calculated from PDC measurements before and after the ol exchange. a. b. Polarsaton spectra determned by PDC analyss and recovery voltage measurement (RVM) from measurements b. pror to the ol exchange c. after the ol exchange FIGURE 5 The polarsaton spectra determned by PDC analyss and the RVM method are farly dentcal. The maxmum values occur at T C,max = 5 s for the polarsaton spectra from measurements pror to the ol change and at T C,max = 50 s from measurements after the ol change. Ths s one more prove that the shft of the polarsaton spectrum s not due to dfferent mosture contents n the sold nsulaton but due to a dfference n ol qualty (conductvty). From RVM nterpretaton of the polarsaton spectra (based on the poston of ther maxma) results a bg dfference of the mosture content n the sold nsulaton before and after the ol exchange of about 1 % (Table 2). The reflecton on water content n ol and n sold nsulaton mentoned above shows that such a bg dfference s mpossble. Obvously, the mproved 5

6 ol qualty has a major mpact on the results provded by the RVM method. Ths outcome s n agreement wth the recent nvestgatons of CIGRE TF [5]. CONCLUSIONS The PDC measurng and analyss system s a non-destructve method, whch provdes relable nformaton about the condton of a transformer s nsulaton system, namely the mosture content n the sold nsulaton materal and the conductvty of the ol as well as other quanttes lke tan δ, polarsaton ndex and polarsaton spectra. Investgatons of numerous transformers n new and aged status of dfferent desgns, voltage levels and ratngs show a good correlaton between the PDC results, the results of Karl-Fscher ttraton and dew pont measurements. Therefore, t may be concluded that the PDC analyss provdes a valuable tool to assess the status of power transformer nsulaton systems. REFERENCES [1] Kachler, A. J.: "On-ste Dagnoss of Power and Specal Transformers, ISEI 2000, Proceedngs, pp , Anahem, USA, Aprl 2-5, 2000 [2] Alff, J.-J.; Der Houhanessan, V.; Zaengl, W. S. and Kachler, A. J.: "A novel, compact nstrument for the measurement and evaluaton of relaxaton currents conceved for on-ste dagnoss of electrc power apparatus. Conference record of the 2000 IEEE Int. Symposum on El. Insulaton (ISEI), Anahem, USA, Aprl 2-5, 2000, pp [3] PDC-Analyser-3205, Semens AG, Transformatorenwerk Nürnberg, Dept. PTD T MCS T, Katzwanger Strasse 150, Nürnberg, Germany, contact: Thomas.Lebfred@ptd.semens.de [4] Der Houhanessan, V.: Measurement and Analyss of Delectrc Response n Ol-Paper Insulaton Systems. Ph. D. dssertaton, ETH No , Zurch, 1998 [5] Zaengl, W. S.: Delectrc spectroscopy n tme and frequency doman for hv power equpment (transformers, cables, etc.), 12 th Intern: Symp. On Hgh Voltage Engneerng (ISH 2001), August 20-24, 2001, Bangalore, Inda (Key note speech, sesson 9, pp ) 6

Time-frequency Analysis Based State Diagnosis of Transformers Windings under the Short-Circuit Shock

Time-frequency Analysis Based State Diagnosis of Transformers Windings under the Short-Circuit Shock Tme-frequency Analyss Based State Dagnoss of Transformers Wndngs under the Short-Crcut Shock YUYING SHAO, ZHUSHI RAO School of Mechancal Engneerng ZHIJIAN JIN Hgh Voltage Lab Shangha Jao Tong Unversty