Understanding Power Transformer Factory Test Data

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(1)Understanding Power Transformer y n a p m o C g n i FactoryinTest Data r e e ng E le. b o D ©. Mark F. Lachman Doble Engineering Company.

(2) OVERVIEW OF PRODUCTION TESTS CTs on cover: polarity, ratio, saturation. PA: loss, sound, core-to-gnd. Core/coil: ratio, Iex, core-to-gnd. le b o ©D. C g n i r e e. y n a mp. o. in g n E. Core/coil after VP: Iex, core-to-gnd Tanking: ratio, core-to-gnd, in-tank CTs - polarity, ratio, saturation. SU: ratio, Rdc, Iex, no-load/load loss, sound, core-to-gnd.

(3) SYSTEM VOLTAGE CLASSIFICATION. Class I includes power transformers with high-voltage windings of 69 kV and below. y n a p m Co Class II includes powerintransformers with g r e e in from 115 kV through high-voltage windings g n E e l b 765 kV. ©Do.

(4) GENERAL CLASSIFICATION OF TESTS. Routine tests shall be made on every transformer to verify that the product meets the design specifications. y n a p m o C g n i Design tests shall be made on a r e e n i g n transformer leofEnew design to determine b o D © its adequacy. Other tests may be specified by the purchaser in addition to routine tests..

(5) OVERVIEW OF TESTS TEST TYPE. PERFORMANCE. DIELECTRIC. MECHANICAL. Winding resistance. Winding insulation resistance (Other). Leak. Ratio/polarity/phase relation. No-load losses and excitation current. le b o Operation ©D of all. C g n i r e e. y n a mp. o. Dielectric withstand of control in g n E and CT sec. circuits (Other). Load losses and Impedance voltage Routine. Core insulation resistance (Other) Class I in red if Insulation PF/C different from Class II (Other). devices. Lightning impulse (Design and Other). Control and cooling losses (Other). Switching impulse  345 kV (Other). Zero-phase sequence impedance (Design). Low frequency test (Applied and Induced/Partial Discharge). DGA (Other). Class II < 345 kV is also Other. PD is Other for Class I only.

(6) OVERVIEW OF TESTS (cont.) TEST TYPE. PERFORMANCE. DIELECTRIC. MECHANICAL. Temperature rise. Design/ Other Audible sound level Short-circuit capability Other. oble. ©D Design. in g n E. C g n i r e e. y n a mp. o. Single-phase excitation current Front-of-wave impulse. Lifting and moving Pressure.

(7) SEQUENCE OF TESTS TEST. REFERENCE. DGA Ratio/polarity/phase relation. IEEE C57.12.90-2010 clauses 6, 7 IEEE C57.12.00-2010 clauses 8.2, 8.3.1, 9.1. Winding resistance. IEEE C57.12.90-2010 clause 5 IEEE C57.12.00-2010 clause 8.2. Lightning impulse. IEEE C57.12.00-2010 clauses 5.10, 8.2 IEEE C57.12.98-1993; IEEE Std. 4-1995. Applied voltage. IEEE C57.12.90-2010 clause 10.5, 10.6 IEEE C57.12.00-2010 clauses 5.10, 8.2. Induced voltage/PD. IEEE C57.12.90-2010 clause 10.7, 10.8, 10.9 IEEE C57.12.00-2010 clauses 5.10, 8.2 IEEE C57.113-2010; IEEE C84.1. No-load losses and excitation current. IEEE C57.12.90-2010 clause 8 IEEE C57.12.00-2010 clauses 5.9, 8.2, 9.3, 9.4. y n a p IEEE C57.12.90-2010 clause 8 No-load losses and excitation m o C IEEE C57.12.00-2010 clauses 5.9, 8.2, 9.3, 9.4 current g n i r C57.12.90-2010 clauses 10.1, 10.2 e IEEE e in IEEE C57.12.00-2010 clauses 5.10, 8.2 Switching impulse Eng IEEE C57.12.98-1993; IEEE Std. 4-1995 le b o IEEE C57.12.90-2010 clauses 10.1, 10.3 ©D.

(8) SEQUENCE OF TESTS (cont.) TEST. REFERENCE. DGA Load losses and impedance voltage. IEEE C57.12.90-2010 clauses 9.1-9.4, Annex B2 IEEE C57.12.00-2010 clause 5.8, 5.9, 8.2, 8.3.2, 9.2-9.4. ONAN temperature rise. IEEE C57.12.90-2010 clause 11 IEEE C57.12.00-2010 clause 8.2 IEEE C57.91-1995 Table 8 (with 2002 corrections). DGA. le b ONAF temperature rise o ©D. y n a mp. o C g nIEEE PC57.130/D17 i r e e. in g n E. IEEE C57.12.90-2010 clause 11 IEEE C57.12.00-2010 clause 8.2 IEEE C57.91-1995 Table 8 (with 2002 corrections). DGA. IEEE PC57.130/D17. Zero-phase sequence impedance. IEEE C57.12.90-2010 clause 9.5 IEEE C57.12.00-2010 clause 8.2. Audible sound level. IEEE C57.12.90-2010 clause 13, Annex B5 IEEE C57.12.00-2010 clause 8.2 NEMA TR1-1993. Core demagnetization DGA.

(9) SEQUENCE OF TESTS (cont.) TEST*. REFERENCE. Insulation PF/C and resistance. IEEE C57.12.90-2010 clauses 10.10, 10.11 IEEE C57.12.00-2010 clause 8.2. Single-phase exciting current. Lachman, M. F. “Application of Equivalent-Circuit Parameters to Off-Line Diagnostics of Power Transformers,” Proc. of the SixtySixth Annual Intern. Confer. of Doble Clients, 1999, Sec. 8-10.. Sweep frequency response analysis. IEEE C57.12.00-2010 clause 8.2. in g n Dielectric withstand of control E e l b and CT secondary circuits o D ©. C g n i r e e. y n a mp. o. IEEE PC57.149™/D8, November 2009. IEEE C57.12.00-2010 clause 8.2. CT polarity/ratio/saturation. IEEE C57.13.1-2006. Control and cooling losses. IEEE C57.12.00-2010 clauses 5.9, 8.2. Operation of all devices. IEEE C57.12.00-2010 clause 8.2. Core-to-ground insulation resistance. IEEE C57.12.90-2010 clause 10.11 IEEE C57.12.00-2010 clause 8.2. *Discussion of tests listed on this slide and DGA is not included in this presentation..

(10) DISCUSSION OUTLINE Tests to be discussed:  Ratio/polarity/phase relation  Winding DC resistance. y n a p m o C  No load losses and excitation current g n i r e e n i g n  Dielectric tests E le b o D ©  Load losses and impedance voltage.  Temperature rise  Zero-phase sequence impedance.  Audible sound level.

(11) DISCUSSION OUTLINE (cont.). For each test discussion includes:  Definition and objective  Physics. y n a p m o  Setup and test methodology C g n i r e e n i g  Acceptance criteria* n E le b o ©D data  Abnormal.  Recourse if data abnormal  Comparison with field data (if relevant) *This discussion is based on requirements of referenced standards. If customer test specification contains requirements different from those in standards, more stringent requirements prevail..

(12) y n a p RATIO, POLARITY, PHASE m o C g n i r e e RELATION n i g n E le (Routine) b o ©D.

(13) RATIO, POLARITY, PHASE RELATION: DEFINITION AND OBJECTIVE Definition: The turns ratio of a transformer is the ratio of the number of turns in the high-voltage winding to that in the low voltage winding. Objective: The turns ratio polarity and phaseyrelation test nand internal a p verifies the proper number of turns om C g transformer connections (e.g.,ribetween coils, to LTC, to n ee series auto- or series n various switches, to gPA, i n E transformer) and le serves as benchmark for later b o assessment © ofDpossible damage in service. The transformer nameplate voltages should reflect the actual system requirements. Therefore, it is important that the nameplate drawing is approved by the customer at the design stage..

(14) RATIO, POLARITY, PHASE RELATION: PHYSICS Volts per turn = 3V/3T = 1V/T. VR = 3V/2V = 1.5 TR = 3T/2T = 1.5 In ideal transformer: TR = VR. F 3V. In actual transformer Turns ratio  Voltage ratio due to accuracy of the measurement and the voltage drop in the highle b o voltage winding. ©D. 3T. C g n i r e e. 2T. 2V. y n a mp. o. in g n E. Volts per turn = 2.95V/3T = 0.98V/T F. 0.05V. VR = 3V/1.96V = 1.53 TR = 3T/2T = 1.5. 3V. 3V.  = 100(1.5 – 1.53)/1.5 = –2%. 2.95V. 3T. 2T. 1.96V.

(15) RATIO, POLARITY, PHASE RELATION: SETUP AND TEST METHODOLOGY Transformer in test H1. X0.  Polarity is determined via phase angle between two measured waveforms. y n a  Phaseprelation is confirmed m o N1 N2 C by testing the corresponding g n i r R2 e pairs of windings. e n i R 1 ng  Tests shall be made E X2 e l b o 1. at all positions of DETC D © with LTC on the rated voltage position Balance H2 2. at all positions of LTC with indicator DETC on the rated voltage position Ratio = N1/N2 = R1/R2 3. on every pair of windings.

(16) RATIO, POLARITY, PHASE RELATION: ACCEPTANCE CRITERIA With the transformer at no load and with rated voltage on the winding with the least number of turns, the voltages of all other windings and all tap connections shall be within 0.5% of the nameplate voltages.. y tolerance n For three-phase Y-connected windings, this a p m o When the phase-toapplies to the phase-to-neutral voltage. C g nmarked on the nameplate, i r e neutral voltage is not explicitly e n i g n voltage shall be calculated by the rated phase-to-neutral E le b o dividing the phase-to-phase voltage markings by 3. ©D H2 138. X2. 13.2 X1. Voltage ratio = VH2-H1/VX2-X0 =. X0. 138/(13.2/3) = 18.108 H1. H3. X3.

(17) RATIO, POLARITY, PHASE RELATION: ABNORMAL DATA To appreciate significance of 0.5% limit, it is instructive to recognize the inherent errors this limit accommodates. Actual turns  RATIOTURN Nameplate voltages  RATIONP. y n a mp. Rounding off NP voltages creates error . C g n i r e e. o. in g n E. Deviation le b)/RATIO =  o 100(RATIONP - RATIO D NP © TURN Measurement  RATIOMEAS. Measurement introduces error. Deviation 100(RATIONP - RATIOMEAS)/RATIONP  0.5%. NP voltages need to be selected to keep  well within 0.5% (e.g., 0.20.4). This assures that measurement error keeps RATIOmeas within 0.5% of RATIONP.. RATIONP  RATIOMEAS. RATIOTURN.

(18) RATIO, POLARITY, PHASE RELATION: RECOURSE IF DATA ABNORMAL  If deviation exceeds 0.5% for any of the measurements the result is not acceptable.  The following steps should be considered:  Check if V/T exceeds 0.5% of nameplate voltage. If yes, ny for deviation under these conditions the standard p allows a om from the NP voltage ratio to exceed the 0.5% limit. C g n i r e eduplicate of a legacy unit.  Check if transformer is ia n g n E  Review designbledata to determine if the NP voltages o selected by create a ratio that is too far (b is ©Ddesigner too high) from true turns ratio. Discuss possibility of changing nameplate voltages for relevant tap positions.  Review results of production ratio tests and, if applicable, consider retesting with analog instrument.  Exciting current reported by turns ratio instrument is a useful diagnostic indicator..

(19) RATIO, POLARITY, PHASE RELATION: COMPARISON WITH FIELD DATA.  In verifying compliance with 0.5% deviation from the NP voltages, the following should be recognized:  Older analog instruments produce results much closer to the actual turns ratio than modern digital instruments. yvary somewhat  Even within 8-200 V range, the results n a p m oinstruments. with voltage and between different C g n eriperformed  Initial field test shouldine be at the same test ngtest with results compared with the E voltage as the factory le b o NP voltages ©Dand for all subsequent tests the comparison should be made with the initial test.  The objective of the high-voltage (e.g., 10 kV) test with external capacitor is to stress turn-to-turn insulation of both windings for diagnostic purposes and not necessarily to verify the 0.5% limit. In some cases, the latter could be exceeded due to the loading effect of the test capacitor..

(20) y n a mp. o C g WINDING DC RESISTANCE n i r e e n i g n (Routine) E le b o ©D.

(21) WINDING DC RESISTANCE: DEFINITION AND OBJECTIVE Definition: Winding DC resistance is always defined as the DC resistance of a winding in Ohms. Objective: The measurement of winding resistance provides the data for: y n a p  Calculation of the I2R component ofoconductor losses. m C g n i r  Calculation of winding temperatures at the end of a e e in g temperature rise test. n E e l b  Quality control ©Doof design and manufacturing processes.  Benchmark used in field for detection of open circuits, broken strands, deteriorated brazed and crimped connections, problems with terminations and tap changer contacts..

(22) WINDING DC RESISTANCE: PHYSICS. i. R. le b o ©D. C g n i r e e. o. y n a mp. External field. in g n E. Domain.

(23) WINDING DC RESISTANCE: PHYSICS (cont.). R=vmeas / i. /dt y/dt /dt vmeas = iR + ddydy dy/dt. dy/dt. dy/dt. le b o ©D. F = y/N. dy/dt. in g n E. C g n i r e e. o. y n a mp. dy/dt.

(24) WINDING DC RESISTANCE: PHYSICS (cont.) Time to stabilize resistance reading: On some units with closed loops (e.g., GSU with two LV deltas or units with parallel windings), it may take a long time for the reading to stabilize*; it reduces with intermediate stability levels. This phenomenon is not related to core saturation, which is saturating in a y n a reasonable time. However, as the core is m being magnetized the p ovoltage and sets up C changing flux in the core induces g n i r e e circulating currents in closed loops. After the core is saturated, n i g n voltage to sustain them, and the E there is no more induced le b o currents begin to subside. This process, however, is associated ©D with LC oscillations with long time constant and may take up to 45 min to dissipate the energy. The flow of these currents continues creating a changing flux in the core, inducing voltage in the tested winding and thus changing the measured resistance reading. Opening these loops, when possible, reduces the time to stability. * Personal communications with Bertrand Poulin, ABB, Quebec, Canada..

(25) WINDING DC RESISTANCE: SETUP AND TEST METHODOLOGY. Current + output. Voltage input + Vdc.  Data must be taken only when reading is stable. Transformer in test The time to stabilize the reading ydepends on the H2 n varying a unit, from p m oseconds to minutes. C g n i H1 r e  Standard requires e n i H ng 0 measurements of all E e l Idcb o windings on the rated D © voltage tap and at the tap extremes of the first unit H3 of a new design.  The measured data is reported at Tave_rated_rise + 20C, e.g., 65+20= 85C and as total of 3 phases..

(26) WINDING DC RESISTANCE: ACCEPTANCE CRITERIA  Standards give no acceptance criteria; however, a deviation from average of three phases of 0.5% for HV and 5% for LV could serve as practical guideline.  As important as deviation is the assurance that test data is credible: y n a p m  No excitation with no pumps - 3h C and with pumps - 1h, o g TO-TBO 5C. This assures n i TTO variation 2C for 1h, and T r e e n i g that oil T represents T; without reference T nconductor E le a limited value. b resistance data has o D ©  Test current 10% of maximum rated load current.  Voltage test leads must be placed as close as possible to winding terminals.  Test data should be recorded only when reading is stable.  Measuring system accuracy +/-0.5% of reading with sufficient current output to stabilize the flux..

(27) WINDING DC RESISTANCE: ACCEPTANCE CRITERIA (cont.) T stability: Experience* in the industry suggests that relying on the T stability requirements given in the IEEE standard does not produce a needed thermal equilibrium and, consequently, an accurate measurement of the winding dc resistance. To have a reliable ndata, the unit y a p m should be subjected to no excitationCfor 2-3 days. Hence, if o ngof essence, it is not i the time to begin testing eis r e n i g unreasonable to agreeEto using resistance data available at n lethe IEEE T requirements have been b that time (assuming o D © met), but request that resistance is re-measured later (including cold resistance for heatrun), when the T is stable. Obviously, the load loss and the heatrun results should be then recalculated with the latest T.. * Personal communications with Bertrand Poulin, ABB, Quebec, Canada..

(28) WINDING DC RESISTANCE: ABNORMAL DATA High-voltage winding % of calc.. Average. Deviation from average. 20.9832 21.47937. 97.7 97.7 97.7. 0.03% 0.03%. 0.02% -0.05% 0.03% -0.06%. 3.5622. 20.4889 20.97440 19.9932 20.46944. 3.7360 3.6480 3.5597. 0.02%. 0.05% -0.07%. 3.4698. 3.5746. 19.6873 19.96448. 98.6. 3.5053. 0.97%. 1.01% -1.98%. 3.3814. 3.3870. 19.0065 19.45952. 0.01%. 0.08% -0.09%. DETC. H1-H3. H2-H1. H3-H2. 1. 3.7350. 3.7352. 3.7378. 2. 3.6470. 3.6468. 3.6502. 3. 3.5590. 3.5580. 4. 3.4714. 5. 3.3838. Low-voltage winding LTC 16 N. Tested. Calc.. o C g n i r e e. in g n E 0.03842 0.16537 0.16521 0.16499 0.6185 e l b 0.1566 0.1564 0.1562 ©Do 0.5855 21.47937 X1-X0. X2-X0. X3-X0. y n a mp3.3841. 97.7. 99.7 100.6. 0.16519 -0.11% -0.01% 0.12% 0.15637 -0.12% 0.00% 0.12%. Comparison of each measurement with the average along with design data identifies an abnormal reading in H3-H2 with DETC in 4. This potentially can be caused by a problem with DETC contacts..

(29) WINDING DC RESISTANCE: RECOURSE IF DATA ABNORMAL  If requirements associated with transformer thermal stability, dc test current, influence of series unit or stability of the reading are not met, a retest under different conditions should be requested.  If acceptance criteria is exceeded, a justification from the y n a p m manufacturer should be requested.CPotential problems may o gincorrect conductor cross n i include: bad crimping or brazing, r e e n i section, loose connection, Eng wrong design calculations.. le b o ©D.

(30) WINDING DC RESISTANCE: COMPARISON WITH FIELD DATA  Typically, a deviation of <5% from the factory value is considered acceptable.  A factory value is often reported as a sum of three phase readings at rated T. For field comparison, the per-phase values at corresponding DETC/LTC positions should be y n a p m requested from the factory. o C g n for readings referred to the i  Comparison should be performed r e e n i g same T. n E le should be performed at the same test b  The field measurement o D © current as the factory one.  Field tests are the subject to the same thermal stability requirements as the factory test (note that at the factory T is measured via thermocouples and in the field the T gauge is frequently the best option)..

(31) y n a p NO-LOAD LOSSES AND m o C g n i r e e CURRENT EXCITATION n i g n E le (Routine) b o ©D.

(32) NO-LOAD LOSSES AND EXCITATION CURRENT: DEFINITION AND OBJECTIVE Definition: No-load losses include core loss, dielectric loss, and conductor loss due exciting current, including current circulating in parallel windings. Excitation current is flowing in any winding exciting the transformer with all other windings open-circuited. y. n a p om. C Objective: No-load losses iand excitation current, g n r e and frequency, provide the e measured at specified voltage n i g n E data for: le b o D design calculations.  Verification©of  Demonstration of meeting the guaranteed performance characteristics. Since these parameters have often an economic value attached to them, the accuracy of the measurement becomes significant.  No-load losses are used as test parameter during the temperature rise test..

(33) NO-LOAD LOSSES AND EXCITATION CURRENT: PHYSICS. F. Eddy losses. Hysteresis losses Ph = f(Bmax) Bmax = f(Vave). PNL = Pe + Ph. Ieddy Pe = f(V2rms). I. V. le b o ©D R. C g n i r e e. in g n E F. y n a mp. B. o. H. Domain rotation.

(34) NO-LOAD LOSSES AND EXCITATION CURRENT: SETUP AND TEST METHODOLOGY Transformer in test. CT. X0 H1 X1 H2 X2. VT. X3 H3. 3. V. le b o D. I. © A. E. Vrms. *. Vave. *. and Vave (calibrated in rms) will show the same voltage if perfect sine wave. rms.  Test at 100% Vrated on N, max turn bridging position y with inductive n a p16R if LTC with series LTC,oand m C unit. g in. r e e ngin  Vave gives same Bmax as Vrms when. W. *V.  Start with 110% on N. As unit demagnetizes, losses drop.. wave-shape is a perfect sin; set based on Vave average of 3 phases.  Pe is corrected for rated Vrms  Voltmeters should measure same voltage as seen by xfmr..  PNL not corrected for T if TTO-TBO 5C and 10TO_ave30C  Iexc=aver. of 3 phases in % of Irated.

(35) NO-LOAD LOSSES AND EXCITATION CURRENT: SETUP AND TEST METHODOLOGY (cont.) Historical perspective. le b o ©D. Courtesy IEEE Power & Energy Magazine. in g n E. g n i r ee. Frequency control of motorgenerator sets at GE large transformer plant in Pittsfield, y MA during the n a p early 1900. Since the m Coprimary function of these generators was to provide power for no-load loss tests, they were often referred to as magnetizers..

(36) NO-LOAD LOSSES AND EXCITATION CURRENT: ACCEPTANCE CRITERIA  Measured no-load losses should not exceed the guaranteed value by more than 10% and the total losses by more than 6%.  Assurance that test data is credible:  Test voltage is set based Vave y n a p  If oil T is not within limits, correction m o is applied C grated  Frequency is within +/-0.5%riof n e e n i  Distortion  5%. The 5% limit that standard allows for g n le E distortion ofob the voltage waveform is too liberal.* The ©D to the difference between the measured kW limit applies and kW corrected for eddy loss due to the difference between Vrms and Vave. To monitor the quality of the voltage waveform, one should look at the following criteria of the applied voltage waveform: THD < 5%, 3rd and 5th harmonics <10% and waveform should not have any visible distortions. * Personal communications with Bertrand Poulin, ABB, Quebec, Canada..

(37) NO-LOAD LOSSES AND EXCITATION CURRENT: ACCEPTANCE CRITERIA (cont.)  Test in parallel and series configurations, if present.  If PA is present, compare the loss difference between non-bridging and bridging positions (max turns) with loss measured in PA out-of-tank. If SU unit is present, compare the loss difference between N yand 16R with n loss measured in SU out-of-tank. ompa C g  Test system accuracy should be within +/-3% for loss, n i r e e +/-0.5% for voltage and and +/-1.5C for T. gincurrent,. b o D ©. n E le.

(38) NO-LOAD LOSSES AND EXCITATION CURRENT: ABNORMAL DATA. Example: guaranteed no-loss - 28 kW, measured – 35 kW  Potential reasons for exceeding the guaranteed values may include:  Variability in core steel characteristics any p m o C  Different core steel g n i r e  Oversights in design gine. n E le.  Production process related factors or mistakes ob. ©D.  Problems with windings (e.g., s. c. turn)  Wrong connection of preventative autotransformer or series transformer or series autotransformer.

(39) NO-LOAD LOSSES AND EXCITATION CURRENT: RECOURSE IF DATA ABNORMAL  Failure to meet the no-load test loss tolerance should not warrant immediate rejection but shall lead to consultation between purchaser and manufacturer regarding further investigation of possible causes and the consequences of the higher losses. ny . a p m o not replace the The acceptance criteria of 10% does C g nlosses for economic loss i r e manufacturer’s guarantee of e n i g n evaluation purposes. E le b o ©D.

(40) NO-LOAD LOSSES AND EXCITATION CURRENT: COMPARISON WITH FIELD DATA Factory no-load losses and excitation test is performed at rated voltage and three-phase excitation. Since the open-circuit magnetizing impedance of a transformer is non-linear, i.e., it is changing with applied voltage, a comparison of exciting current and losses y test results n a p m obtained at low-voltage (e.g., 10 C kV) and single-phase o ng no-load losses and i excitation with results of theefactory r e n i g excitation test is not possible. En. le b o ©D.

(41) y n a mp. o C g DIELECTRIC n TESTS i r e e n i g n E le b o ©D.

(42) DIELECTRIC TESTS: DEFINITION AND OBJECTIVE Definition: Tests aimed to show that transformer is designed and constructed to withstand the specified insulation levels are referred to as dielectric tests. They include:  high-frequency tests: lightning and switching y impulses n a p m  low-frequency tests: applied and induced/PD tests Co. g n i r ee. in g Objective: Dielectric tests demonstrate: n E e l b  compliance with ©Do users specification  compliance with applicable standards  verification of design calculations  assessment of quality and reliability of material and workmanship. Note: Unless agreed otherwise, all dielectric tests must be performed with bushings supplied with the transformer..

(43) HIGH-FREQUENCY: y n a p m o C g n LIGHTNING IMPULSE i r e e in g n E (Class I - design or other, e l b ©Do Class II - routine).

(44) HIGH-FREQUENCY - LIGHTNING IMPULSE: OBJECTIVE Demonstrate performance under transient high-frequency conditions caused by lightning. kV. Surge of energy, from lightning striking transmission line, travels to substation and operates gapped silicon-carbide arrester at transformer terminals - front-of-wave (a.k.a. front-chopped). y. n a p om. le b o ©D. C g n energy, from lightning striking i r e Surge of e n i g En transmission line, travels to substation and enters a transformer - full wave.. Surge of energy, from lightning striking transmission line, travels to substation and, after reaching the crest of the surge, causes arrester operation or flashover across an insulator near transformer terminals - chopped wave (a.k.a. tail-chopped).. s.

(45) HIGH-FREQUENCY - LIGHTNING IMPULSE: PHYSICS. Full wave can be simulated by discharging capacitor while chopped wave by the operation of a gap triggered to flashover at required time. V. le b o ©D. Cg. V. C g n i r e e. in g n E. Cs. o. y n a mp.   Cg/Cs length. Due to impulse front high frequency, the initial voltage distribution is determined by the capacitive network, with higher voltage gradients towards the impulsed end of the winding. The higher is , the steeper are the gradients at the impulsed end of the winding. As the front passes, the distribution changes as determined by the tail of the wave..

(46) HIGH-FREQUENCY - LIGHTNING IMPULSE: PHYSICS (cont.). A. HV. LV. H1 B. H1. A. B. B B B. le b o ©D. g n i r ee. in g n E. C to DETC. Region A* - turn-to-turn insulation at line is tested by FOW impulse, with stress >10turns**.. yB – disk-to-disk, and Region n a p m layer-to-layer (and Co turn-to-turn) isinsulation tested by FW & CW impulse, with stress 510turns. C. Region C – insulation across taps is tested by FW & CW impulse, with stress 510turns.. H0 *Assumption that FOW stresses mostly the first few turns at the impulse end is not always true; it depends on winding type and configuration, e.g., when the interleaved winding (one with high series capacitance) is in series with RV, the impulse goes through the main winding and hits RV (Personal communications with Bertrand Poulin, ABB, Quebec, Canada.) **From W. McNutt 1989 Doble tutorial: Turns in this context mean the voltage that would have been present if the applied voltage was distributed according to turns ratio..

(47) HIGH-FREQUENCY - LIGHTNING IMPULSE: PHYSICS (cont.) Charge of Cg – generator capacitors are charged from external DC source.. Rs Cg. Rp. CT. VT Rs Cg. Rp. CT. le b o R ©D Rs. Cg. p. g n i r ee. in g n V E T. CT. FOW. Rs VT Cg. Rp. *CT includes preload capacitor.. FW. CT. CW. Discharge into C*T – energy from generator y n is discharged into capacitors a p Comxfmr, raising V at tested terminal to crest level. Discharge into Rp – energy from xfmr is discharged into generator, reducing voltage at tested terminal. Discharge at chop – energy from xfmr is discharged into chopping gap, reducing voltage at tested terminal to zero..

(48) HIGH-FREQUENCY - LIGHTNING IMPULSE: SETUP AND TEST METHODOLOGY Full Wave Parameters. Crest voltage 1.0. Magnitude. 0.9. FW = BIL +/- 3% RFW = 50-70% BIL. T1 = 1.67Tny. V. g n i r ee. 0.5. Half voltage. 0.3. T. Virtual origin. T1. le b o D. ©. in g n E. t. a p m Co. 1.2 s +/- 30% 0.84 ÷ 1.56 s. T2. 50 s +/- 20% 40 ÷ 60 s. . 5%. T2.  Applied test waves are of negative polarity to reduce risk of erratic external flashover.  See C57.12.90-2010 when for line terminals T1 is allowed to be >1.56 s and T2<40 s. For neutral bushing T1<10 s and T2 could be <40 s.  If the T2<40 s, it should be addressed at the bidding stage..

(49) HIGH-FREQUENCY - LIGHTNING IMPULSE: SETUP AND TEST METHODOLOGY (cont.). 0% change from given Rs. Co C g n i r e e. y R n a mp. g. s. Rp. CT. in g n E e l b Increase of series (front) resistor Rs ©Do increases the time of voltage rise - T1.. Data courtesy Reto Fausch, Haefely.

(50) HIGH-FREQUENCY - LIGHTNING IMPULSE: SETUP AND TEST METHODOLOGY (cont.). Increase of parallel (tail) resistor Rp increases the time of voltage decline to half value - T2.. le b o ©D. C g n i r e e. in g n E. 0% change from given Rp Cg. Rp. Rs. Data courtesy Reto Fausch, Haefely. CT. o. y n a mp.

(51) HIGH-FREQUENCY - LIGHTNING IMPULSE: SETUP AND TEST METHODOLOGY (cont.). Increase of series (front) resistor Rs decreases the voltage trace overshoot - . Rs Cg. le b o ©D. Data courtesy Reto Fausch, Haefely. in g n E. C g n i r e e. o. y n a mp. Rp. CT.

(52) HIGH-FREQUENCY - LIGHTNING IMPULSE: SETUP AND TEST METHODOLOGY (cont.) Chopped Wave Parameters 1.0 0.9. Magnitude. CW = 1.1BIL+/- 3%. T1. 1.2 +/- 30% 0.84 ÷ 1.56. 1.0. V. 0.7. 0.3. T1. TC. le b o ©D. BIL [kV] y Class I. n a p m 30. o C g n i 45÷75 r e e. in g n E. TC . 0.1. .  t.  See C57.12.90-2010 for instances when  could be >30% and >1s. It also permits adding resistors in chopping gap circuit to limit .  All times in the table are in s.. 1.0. 1.5. 95. 1.8. 110. 2.0. 125. 2.3. 150. Class II. 2.0. 2.3 3.0. TC <. 6.0. . 30%. . 1.

(53) HIGH-FREQUENCY - LIGHTNING IMPULSE: SETUP AND TEST METHODOLOGY (cont.) 1.0 0.9. Front-of-Wave Parameters Magnitude. V. p m Co. TC. 0.3. le b o ©D TC.  g n i eer. in g n E. C57.12.00-2010 anyAnnex A 30%. t .  C57.12.90-2010 permits adding resistors in chopping gap circuit to limit .  With improved arrester technology, front-of-wave tests may not be necessary and were removed as a requirement from C57.12.00. Annex A in that standard includes the last published table of front-of-wave test levels from C57.12.001980, for historical reference..

(54) HIGH-FREQUENCY - LIGHTNING IMPULSE: SETUP AND TEST METHODOLOGY (cont.). LG. Very high di/dt induces difference of potential. Hence, it is very important for all return and grounding leads to be made as short as possible, with a minimum R and L.. Glaninger:  T2. Rs xfmr Impulse generator. Rp RG Cg. C g n i r e e. LT, CT. le b o ©D. in g n E. y n a mp. oVoltage divider and measuring circuit v(t).  T2. Chopping gap and preload capacitor. Impulse control & measuring system. Current shunt and meas. circuit Chopping gap should not be connected in series with voltage divider no matter how convenient it is for the test department to have a permanent setup. i(t).

(55) HIGH-FREQUENCY - LIGHTNING IMPULSE: SETUP AND TEST METHODOLOGY (cont.) Line terminal in Y. Neutral terminal in Y. C g n i r e e. i(t). i(t). le b o ©D. HV line terminal in Auto. in g n E. Line terminal in . y n a mp. o. LV line terminal in Auto. i(t). i(t). i(t). i(t). Neutral terminal in Auto.

(56) HIGH-FREQUENCY - LIGHTNING IMPULSE: SETUP AND TEST METHODOLOGY (cont.) Test sequence and trace comparison. Standard: RFW@ 50-70% BIL CW 1 CW 2 FW. With non-linear With FOW: protective devices: RFW@ 50-70% BIL RFW 1 FOW 1 RFW 2 y @ 75-100% of BIL n FOW 2 a p to demonstrate growing m o C CW 1 g sensitivity to V n i r e CW 2gine FW 1 n E e l FW b CW 1 o. Neutral: D RFW@ 50-70% © BIL) FW1 FW2. CW 2 FW 2 RFW 3 @ RFW2 voltage RW 4. Test is performed with minimum effective turns in the winding under test, e.g., DETC = 5, LTC = 16L..

(57) HIGH-FREQUENCY - LIGHTNING IMPULSE: ACCEPTANCE CRITERIA  If test equipment and tested transformer were perfectly linear, the traces of repeated impulses, when overlaid, would perfectly match. However, due to noise, setup imperfections or insulation failure, discrepancies occur. Identifying their nature is the objective of impulse data analysis. y n  T1, T2, Tc, voltage magnitude, ,  must meet requirements. a p m oshould compare; request  RFW and FW voltage and current traces C g n i r to zoom in on any areas of concern. e e n i g  If available, comparison E ofnTransfer Function (TF) for RFW and FW lediagnostic criteria. It removes sensitivity to b is used as additional o D © wave shape variations caused by impulse generator jitter (TF should be considered only in frequency ranges where sufficient data is present in the time domain impulse trace*).  For chopped wave test, segments of CW1 and CW2 traces prior to moment of chop are compared. While traces after chop may be shift, they oscillate around zero with the same frequency.  Verify that DGA results (after dielectrics) are normal. * IEEE PC57.98TM/D07, September 2011, Draft Guide for Transformer Impulse Tests..

(58) HIGH-FREQUENCY - LIGHTNING IMPULSE: ACCEPTANCE CRITERIA (cont.). le b o ©D. in g n E. C g n i r e e. y n a mp. o 450 kV BIL, RFW on HV winding – voltage. 450 kV BIL, RFW on HV winding – current.

(59) HIGH-FREQUENCY - LIGHTNING IMPULSE: ACCEPTANCE CRITERIA (cont.). y n a mp. le b o ©D. o450 kV BIL, CW1 on C g n i HV winding – voltage r e e. in g n E. 450 kV BIL, CW2 on HV winding – voltage.

(60) HIGH-FREQUENCY - LIGHTNING IMPULSE: ACCEPTANCE CRITERIA (cont.). Overlay of 450 kV BIL CW1 and CW2 - voltage. le b o ©D. in g n E. C g n i r e e. o. y n a mp.

(61) HIGH-FREQUENCY - LIGHTNING IMPULSE: ACCEPTANCE CRITERIA (cont.). y n a mp. le b o ©D. in g n E. o C g n 450 kV BIL, FW on i r e e. HV winding – voltage. 450 kV BIL, FW on HV winding – current.

(62) HIGH-FREQUENCY - LIGHTNING IMPULSE: ACCEPTANCE CRITERIA (cont.). le b o ©D. C g n i r e e. y n a mp. o. in g n E. Overlay of 450 kV BIL RFW and FW - voltage.

(63) HIGH-FREQUENCY - LIGHTNING IMPULSE: ACCEPTANCE CRITERIA (cont.). High-frequency oscillations at the beginning of current trace are acceptable deviations, reflecting the test setup.. le b o ©D. C g n i r e e. y n a mp. o. in g n E. Overlay of 450 kV BIL RFW and FW - current.

(64) HIGH-FREQUENCY - LIGHTNING IMPULSE: ACCEPTANCE CRITERIA (cont.). y n a mp. o BIL, FOW1 on 450CkV g erinHV winding – voltage. le b o ©D. e n i g En. 450 kV BIL, FOW2 on HV winding – voltage.

(65) HIGH-FREQUENCY - LIGHTNING IMPULSE: ACCEPTANCE CRITERIA (cont.). Influence of non-linear protective device on overlay of RFW and FW y n a 350 kV BIL voltage traces mp o C illustrates the need for comparing g n i r level. traces of the same voltage e e. le b o ©D. in g n E.

(66) HIGH-FREQUENCY - LIGHTNING IMPULSE: ACCEPTANCE CRITERIA (cont.). le b o ©D. C g n i r e e. y n a mp. o. in g n E. Influence of non-linear protective device on overlay of RFW and FW 350 kV BIL current traces illustrates the need for comparing traces of the same voltage level..

(67) HIGH-FREQUENCY - LIGHTNING IMPULSE: ABNORMAL DATA  In general, whenever discrepancies occur the normal test procedure need to be stopped and investigation performed. If the cause is found to be external to the transformer, the corrections are made before the test can continue.  If there is any doubt as to the cause of the discrepancies, additional y nFW. If the deviation a impulses need to be applied, including several p m o C increases in magnitude, it indicates progressive dielectric failure in the g n i r e transformer. e in g n  Unusual sounds, emanating E from inside the tank, should be noted; these e l bin locating general location of the fault. o sounds may be helpful D ©  Removing manhole covers and observing presence of gas bubbles and/or carbon, serves as confirmation of failure and provides some indication of the fault location.  Occasionally, the damage caused but not detected by impulse is only detected by tests that follow: applied or induced/PD voltage tests, DGA..

(68) HIGH-FREQUENCY - LIGHTNING IMPULSE: ABNORMAL DATA (cont.). Overlay of 550 kV BIL RFW and FW voltage traces – turn-to-turn failure. le b o ©D. in g n E. C g n i r e e. o. y n a mp.

(69) HIGH-FREQUENCY - LIGHTNING IMPULSE: ABNORMAL DATA (cont.). Overlay of 550 kV BIL RFW and FW current traces – turn-to-turn failure. le b o ©D. in g n E. C g n i r e e. o. y n a mp.

(70) HIGH-FREQUENCY - LIGHTNING IMPULSE: ABNORMAL DATA (cont.) Voltage drop to ground indicates one of the leads was at ground potential. FW voltage. C g n i r e e. RFW y voltage. n a p om. in g n E. le b o ground ©D diverts. Fault to current around winding, reducing measured current. Overlay of 200 kV BIL RFW and FW traces – lead-to-lead failure between RV and main LV windings. FW current. RFW current.

(71) HIGH-FREQUENCY: y n a p m SWITCHING IMPULSE o gC n i r e e – other, n (Class I i g n E le II <345 kV – other, b o Class ©D Class II  345 kV - routine).

(72) HIGH-FREQUENCY - SWITCHING IMPULSE: OBJECTIVE Demonstrate performance under transient high-frequency conditions created by switching operations or network disturbance. kV FOW CW. le b o D. ©. Surge of energy from equipment switched on or disturbance on the power ythe crest n system. The time to reach a p m o time duration of amplitude and theC total g n are much longer than i switching impulses r e e n i those of lightning impulses. g n. E. SW FW. s.

(73) HIGH-FREQUENCY - SWITCHING IMPULSE: PHYSICS Switching impulse test consists of applying or inducing a SW between each HV line terminal and ground. Similar to a lightning wave, the switching wave can be simulated by discharging a capacitor. V. le b o ©D. y n a mp. V. C g n i r e e. o. in g n E. Comparing to lightning impulse, the switching impulse has a much longer duration and lower frequency, resulting in voltage approaching a uniform distribution of the low-frequency steady-state voltages, i.e., voltage distributes as per turns ratio.. length.

(74) HIGH-FREQUENCY - SWITCHING IMPULSE: PHYSICS (cont.). LV. D. HV. D. D H1. H1. D. g n i r ee. To another phase. le b o ©D. in g n E. Region D – phase-toground y and phase-ton insulation is a phase p Comstressed the most; stress imposed by SW is 1turns*. Charging and discharging processes are similar to those described for lightning impulse.. H0. *From W. McNutt 1989 Doble tutorial: Turns in this context mean the voltage that would have been present if the applied voltage was distributed according to turns..

(75) HIGH-FREQUENCY - SWITCHING IMPULSE: SETUP AND TEST METHODOLOGY Full Wave Parameters Crest voltage 1.0. Magnitude. >90% of crest. 0.9. Tp. Virtual origin. Tp. le b o ©D Td. gT0 n i r ee. in g n E. T0. any. p m Co. Td. V. SW = 0.83BIL +/- 3% RSW=(50-70%)0.83BIL >100 s 200 s 1000 s t First zero crossing.  LV windings shall be designed to withstand stresses from SW applied to HV side.  Applied test waves are of negative polarity to reduce risk of erratic external flashover..

(76) HIGH-FREQUENCY - SWITCHING IMPULSE: SETUP AND TEST METHODOLOGY (cont.). Rs Xfmr Impulse generator. C g n i r e e. Rp Cg. le b o ©D. in g n E. o. y n a mp. Voltage divider and measuring circuit. v(t). Impulse control & measuring system Note: The shown setup is for SW being applied to the HV winding. The test can also be performed with SW being induced..

(77) HIGH-FREQUENCY - SWITCHING IMPULSE: SETUP AND TEST METHODOLOGY (cont.) ELV. E. E/2. le b o ©D. E/2. ELV/2. C g n i r e e. in g n E ELV. E. ELV. E. y n a mp. -ELV/2. o Test sequence and trace comparison:. RSW@ 50-70% SW (+) RSW - bias SW1. -ELV/2 -E/2. Note: The choice of tap connections for all windings is made by the manufacturer.. (+) RSW - bias SW2 RFW@ 50-70% BIL CW 1 CW 2 FW.

(78) HIGH-FREQUENCY - SWITCHING IMPULSE: SETUP AND TEST METHODOLOGY (cont.) SW can saturate the core, creating an air-core conditions, i.e., drastically reducing impedance faced by impulse. This rapidly decays the tail of the voltage waveform to zero, making T0<1000 s. To extend the time to saturation, prior to start of each test, y the core is magnetized in opposite direction by applying RSW (or n a p small dc current) of opposite polarity . Com V. ©. le b o D. g n i r ee. in g n E. When core saturates, the voltage collapses drastically reducing time to zero crossing.. Bias in the core in direction opposite to that created by test SW extends time to saturation and T0. t.

(79) HIGH-FREQUENCY - SWITCHING IMPULSE: ACCEPTANCE CRITERIA  Tp, Td, T0, and voltage magnitude must meet requirements.  Failure detection is done primarily by scrutinizing voltage traces for recognizable indications of failure. The test is successful if there is no sudden collapse of voltage as y n indicated on the trace. a p m Cotraces in totality may  Although overlaying RSW andinSW g r e e not be practical, the traces in should match until the point g n Ein the core magnetic state becomes where the difference e l b obvious. Normally, these differences can be easily ©Do distinguished from drastic voltage reduction caused by a failure.  Verify that DGA results (after dielectrics) are normal..

(80) HIGH-FREQUENCY - SWITCHING IMPULSE: ACCEPTANCE CRITERIA (cont.). 650 kV BIL, RSW on HV winding – voltage. le b o ©D. in g n E. o C g n i r e e. Typical reduced and full switching impulse voltage traces as measured on the HV winding; for 650 kV BIL, the BSL, i.e., the required test voltage, is 540 kV.. y n a mp. 650 kV BIL, SW1 on HV winding – voltage. 650 kV BIL, SW2 on HV winding – voltage.

(81) HIGH-FREQUENCY - SWITCHING IMPULSE: ACCEPTANCE CRITERIA (cont.). Overlay of 650 kV BIL RSWnand y SW - voltage. a p m Co. g deviating ntraces i Beginning eof r e dueng tointhe difference in core E magnetic state. This is e l b ©Do typically more pronounced in the overlay of reduced and full switching waveforms.

(82) HIGH-FREQUENCY - SWITCHING IMPULSE: ACCEPTANCE CRITERIA (cont.). y to the Slight deviation adue n p difference o inm core magnetic C g state. rin. le b o ©D. e e n i Eng. Overlay of 650 kV BIL SW1 and SW2 - voltage.

(83) HIGH-FREQUENCY - SWITCHING IMPULSE: ABNORMAL DATA  In general, whenever discrepancies occur the normal test procedure need to be stopped and investigation performed. If the cause is found to be external to the transformer, the corrections are made before the test can continue. ny discrepancies,  If there is any doubt as to the cause ofpathe additional impulses may be applied. Com g n i r e observing presence of gas  Removing manhole covers and e n i g nserves E bubbles and/or carbon, as confirmation of failure and e l b o provides some indication of the fault location. ©D.

(84) HIGH-FREQUENCY–LIGHTNING AND SWITCHING IMPULSE: RECOURSE IF DATA ABNORMAL.  If visual confirmation (e.g., carbon, bubbles) is obtained or the data convincingly reveals a failure, the oil is drained and internal inspection is performed.  If necessary, the unit is un-tanked. This is followed by a y n a thorough and well-documented investigation. mp. . o C g n process enhances the i The user’s involvement inerthis e n i g quality of the investigation and that of the final product. n E le b o ©D.

(85) y n a p LOW-FREQUENCY: m o C g n i r e eVOLTAGE APPLIED n i g n E le (Routine) b o ©D.

(86) LOW-FREQUENCY – APPLIED VOLTAGE: OBJECTIVE. The high-frequency tests (lightning and switching impulse) always precede the low-frequency tests (applied and induced voltage). This sequence is rooted in the fact that due to a longer duration, the low-frequency tests y n a serve to stress further and to detect the damage caused p m o C by the high-frequency tests. g n. i r e e n i g En. The applied voltage letest is a simple overvoltage test. The b o ©D engineers apparently took cues from early transformer mechanical engineers. This is how a mechanical structure would be tested, by applying stress that demonstrates a safety factor of two. The applied voltage test has a 1 min duration, with the expectation to demonstrate a long-term capability to operate at the rated voltage..

(87) LOW-FREQUENCY – APPLIED VOLTAGE: PHYSICS. D. LV HV. D LV. le b o ©D. HV. in g n E. C g n i r e e. o. y n a p D – major winding mRegion -to-ground and winding-towinding insulation are stressed the most.. Shorting lead. D.

(88) LOW-FREQUENCY – APPLIED VOLTAGE: SETUP AND TEST METHODOLOGY  Test is performed at low frequency (<500 Hz), normally, power Magnitude C57.12.00-2010 frequency. Duration 1 min  All terminals of tested winding are connected together; all other terminals (including all cores, 1.1E y n a p buried windings with one terminal m o C brought-out and the tank) are E g n i r grounded. e e in  A sphere-gap, set for 10% above g n E v e l b test voltage, may be connected for o D © protection.  Test voltage (1-phase) is determined by terminal with the lowest BIL (e.g., Neutral).  The voltage is raised from 25% or Note: On grounded-wye transformers with less, held for 1 min and reduced reduced Neutral BIL the test has a limited gradually. significance; it inly tests insulation in the  Each winding or set of windings vicinity of the Neutral. (e.g., in auto) is tested. Applied Voltage Parameters.

(89) LOW-FREQUENCY – APPLIED VOLTAGE: ACCEPTANCE CRITERIA.  The test is a pass/fail test and is considered passed if during the time the voltage is applied no evidence of possible failure is observed. ny sound  The indications to monitor include p unusual a om such as thump, sudden increase in the test circuit C g rintest voltage. current and collapse in ethe e n. le b o ©D. i g n E.

(90) LOW-FREQUENCY – APPLIED VOLTAGE: ABNORMAL DATA  If unusual sound, sudden increase in the test circuit current or circuit tripping occur, these events should be carefully investigated by: • observation, e.g., presence of carbon y n a p and/or bubbles in the oil m o C • repeating the test g n i r e e n • other tests i g n E to determinebwhether the failure has occurred. le o  Due to a©D significant energy being released during applied voltage test, the test is repeated (if at all) to confirm the failure a limited number of times (1, 2 max). The energy released is usually sufficient to mark the location making it possible to find the failure after un-tanking..

(91) LOW-FREQUENCY – APPLIED VOLTAGE: RECOURSE IF DATA ABNORMAL. If visual confirmation (e.g., carbon, bubbles) is obtained and/or repeating of the test and/or other tests reveal the failure, the oil is drained and internal inspection is y n performed. a p. le b o ©D. in g n E. om C g n i r ee.

(92) LOW-FREQUENCY: INDUCED VOLTAGE/PD y n a mp. o C g n Induced: i r e Routine e n i g n E le 7200 cycles. Class IDob ©. Class II. Routine. Induced: 1 hour + PD.

(93) LOW-FREQUENCY – INDUCED VOLTAGE/PD: OBJECTIVE. The induced voltage test demonstrates the strength of internal insulation in all windings as well as between windings and to ground. A combination of prolonged stress and a very sensitive PD measurement makes ity a very severe n a p and searching test. It must be the last dielectric test to be m o C g performed. rin. le b o ©D. e e n i Eng.

(94) LOW-FREQUENCY - INDUCED VOLTAGE/PD: PHYSICS xfmr. IG. IT VT M. Lv. G ILv. le b o reactor ©D IT. Variable Lv is adjusted to reduce output from generator.. L. R. C. g n i r ee Therefore, the test is performed as a. in g n E VT. IG. ILv.  To stress turn-to-turn insulation to the required level, the winding needs to be excited to a level approaching twice rated voltage. At power y overexcite the nwould a frequency, p this m core.Co higher frequency, which allows to obtain the needed volts/turn at a lower flux magnitude (v/t = dF/dt)..  At higher frequency, transformers become capacitive with dangers of MG set overexciting. This is addressed by using a variable reactor. The latter provides an additional benefit of reducing the load on MG set..

(95) LOW-FREQUENCY - INDUCED VOLTAGE/PD: PHYSICS (cont.). E. LV. HV. E. E. E. leE b o ©D E. Region E –ny with voltage a turns ratio, the p distributing per m o is present in the turnC most stress g n i r e to-turn insulation of each winding e n i as well as in winding-to-winding Eng and winding-to-ground insulation..

(96) LOW-FREQUENCY – INDUCED VOLTAGE/PD: PHYSICS (cont.) From the physics point of view, self-sustaining electron avalanches may occur only in gases. Hence, discharges in dielectrics may only be ignited in gas-filled cavities, such as voids or cracks in solid materials and gas bubbles or water vapor in liquids. Discharges are generally ignited if the electrical field strength y inside the inclusion exceeds the intrinsic field strength of the gas. n a p They can appear as pulses having a duration Comof << 1s. Dielectric. Conductor. Gaseous inclusionle. b o D ©. g n i r ee. in g n E. Partial discharges are defined as localized electrical discharges that only partially bridge the insulation between conductors and may or may not occur adjacent to a conductor. In insulation, the PD events are the consequence of local field enhancements due to dielectric imperfections..

(97) LOW-FREQUENCY – INDUCED VOLTAGE/PD: PHYSICS (cont.) To model the PD process, capacitance of the active void CC can be viewed as part of a larger capacitive network. In that, CB is the remaining capacitance of the immediate region in series with CC and CA is the rest of the dielectric connected in parallel. Two requirements must be fulfilled to initiate PD: 1) local field stress y electrons are exceeds the void’s breakdown voltage Vbd and 2) free n a p m available. Co. g n i r ee. in g n E. le b o CB ©D. . CA CC. CA. CB CC. Vbd.

(98) LOW-FREQUENCY – INDUCED VOLTAGE/PD: PHYSICS (cont.) Buildup: As V , charges move to and collect on the surface of the void, building potential stress Vcc across the void.. Strike: As Vcc>Vbd, breakdown occurs, charges move across shorting the void, Vcc= 0 and discharge stops. To make up for imbalance, charges come out of adjacent insulation.. V. V CB. Q. CA. Q. CB. Q Q. Vcc. CC. le b o D. ©. g n i r ee. Q QQ Q. n i g C n E A. Relaxation: Charges continue to flow at a decreasing rate with balance restoring. Vcc  as charges collect back on the void’s surface.. V. any. p m Co. CB Q. Q. CA. Q. Q. CC. QQ. Vcc= 0. CC. Vcc. Q. V CB CA. Q QQ. CC. V. V. CB. CB Vcc. CA. CA CC. Vcc= 0. Q. CC. Vcc.

(99) LOW-FREQUENCY – INDUCED VOLTAGE/PD: PHYSICS (cont.) Dip in terminal voltage. Terminal voltage C1. PD current. le b o ©D. C g n i r e e. Voltage in g n Eacross void. y n C a mp. o. 2. Z. M. CT. *The detectable voltage dip is in the mV range, while that at the void may be in the kV range.. We cannot measure the real charge. However, as the void discharges, the charge redistribution creates a dip* in the terminal voltage. This minute voltage drop causes a high-frequency current to flow through a coupling capacitor connected to a measuring system. Putting it differently, the charge movements appear, in part, in C1 connected in parallel with CT. The integration of these highfrequency current pulses over time produces the reported apparent charge..

(100) LOW-FREQUENCY – INDUCED VOLTAGE/PD: PHYSICS (cont.). Measurement of partial discharge is like trying to weigh a butterfly that y n a p alights momentarily Comon scales g n i r designed for anngelephant (sometimes inee E e l b o during an©Dearthquake). by Karl Haubner, Doble Australia.

(101) LOW-FREQUENCY – INDUCED VOLTAGE/PD: SETUP AND TEST METHODOLOGY C1. Lv M. Step-up xfmr. G. Xfmr in test. C2. X0 H1 X1 H2 X2. C1. le b o ©D. C2. C g n i r e e. X3 H3. in g n E. p m o. any. C1. M. V pC and/or V. C2. Before test commences, several important steps take place:  Transformer is connected for open-circuit conditions.  Voltage is raised to verify that variable (Lv) setting allows to reach the required test voltage.  Measuring system (M) is calibrated for PD, RIV and voltage..

(102) LOW-FREQUENCY – INDUCED VOLTAGE/PD: SETUP AND TEST METHODOLOGY (cont.) Enhanced level 7200 cycles. V. 1h level, 5 min recordings. Hold as needed until stable (min 60 sec). 100%. C g n i r e t e. 100%. Ambient. ©. 1h le b o D. in g n E. Ambient. Induced Voltage/PD Parameters Voltage magnitude. C57.12.00-2010 clause 5.10 C84.1. y n a Timing p m. C57.12.90-2010 clauses 10.7, 10.8. PD/RIV criteria. C57.12.90-2010 clause 10.8/ Annex A. o.  Voltage is gradually raised, recording pC, V and kV.  For Class I units, the test includes applying to HV winding 2.0nominal voltage for 7200 cycles with no PD (RIV) recordings. For class II units rated 115 ÷ 500 kV, the test includes applying to HV winding 1.8nominal voltage for 7200 cycles and 1.58nominal voltage for 1 h, recording PD (RIV) data.  For windings other than HV, when possible, taps should be selected so that voltages on other windings are as per ANSI C84.1 and C57.12.90 clause 10.8.1 (e.g., for 115÷345 kV units , the voltage on other windings should be 1.5 times their maximum operating voltage)..

(103) LOW-FREQUENCY – INDUCED VOLTAGE/PD: SETUP AND TEST METHODOLOGY (cont.)  PD (pC) measurements are performed using 100 ÷ 300 kHz and RIV (V) using 0.85 ÷ 1.15 MHz frequency ranges.  For units with windings that have multiple connections (e.g., seriesparallel or delta-wye) with each connection having system voltage >25 kV, two induced tests are performed, one in each connection. If ny more than one winding has such multiple pconnection, then the a om between tests. In all connections in each winding shall change C g with highest test voltage. n i r cases, the last test shall be for connection e e n i g  To minimize the effects E ofnexternal factors and stray capacitances, leoften relied on: b the following steps are o D © - filters on the power supply line - shielding all sharp edges including those at ground potential as well as the energized and grounded bushings - turning off solid state power supplies, cranes and other factory machinery - removing air bubbles from bushing gas space - applying pressure to suppress bubbles in the main tank..

(104) LOW-FREQUENCY – INDUCED VOLTAGE/PD: ACCEPTANCE CRITERIA Results are acceptable if:  Nothing unusual associated with sound, current, or voltage is observed (see abnormal data for details).  The PD (RIV) results during 1h test period have y shown: n a p m - Magnitude  500 pC ( 100 V).Co g n i r e pC ( 30 V). - Increase during 1 h in150 e g n E - No steadily rising trends during 1 h e l b o D © - No sudden sustained increase during the last 20 min. Judgment should be used on the automatically recorded 5-min readings so that momentary excursions caused by cranes or other ambient sources are not recorded. Also, the test may be extended or repeated until acceptable results are obtained.  DGA results (after dielectrics) are normal..

(105) LOW-FREQUENCY – INDUCED VOLTAGE/PD: ACCEPTANCE CRITERIA (cont.) V1. PD1. RIV1. Time1. V2. PD2. RIV2. Time2. V3. PD3. RIV3. Time3. 1. 0.2 kV. 15.4 pC. 4.5 µV. 00:00:03. 0.3 kV. 14.6 pC. 5.3 µV. 00:00:11. 0.2 kV. 138. pC. 4.5 µV. 00:00:20. Ambient. 2. 30.4 kV. 24.7 pC. 4.5 µV. 00:00:49. 30.6 kV. 26.3 pC. 5.3 µV. 00:00:58. 30.5 kV. 27.2 pC. 4.2 µV. 00:01:07. 100%. 3. 37.8 kV. 27.1 pC. 4.4 µV. 00:01:51. 37.6 kV. 38.3 pC. 5.6 µV. 00:02:00. 37.7 kV. 29.7 pC. 4.6 µV. 00:02:09. 125%. 4. 42.4 kV. 36.7 pC. 5.2 µV. 00:03:33. 42.0 kV. 29.2 pC. 7.1 µV. 00:03:42. 42.1 kV. 30.7 pC. 4.7 µV. 00:03:51. 1hr level. 5. 55.5 kV. 31.9 pC. 4.9 µV. 00:04:03. 54.5 kV. 33.5 pC. 13.5 µV. 00:04:12. 54.7 kV. 33.9 pC. 6.2 µV. 00:04:21. Enhanced. 6. 42.3 kV. 27.1 pC. 4.6 µV. 00:00:03. 41.9 kV. 29.3 pC. 5.6 µV. 00:00:35. 1 hr level. 7. 42.2 kV. 27.3 pC. 4.6 µV. 00:05:03. 41.9 kV. 28.0 pC. 6.1 µV. 00:05:35. 8. 42.1 kV. 27.8 pC. 4.5 µV. 00:10:03. 41.7 kV. 29.4 pC. 5.2 µV. 00:10:35. 9. 41.8 kV. 27.1 pC. 4.5 µV. 00:15:03. 41.6 kV. 28.4 pC. 6.0 µV. 10. 42.1 kV. 28.8 pC. 4.6 µV. 00:20:03. 41.7 kV. 29.7 pC. 11. 42.3 kV. 28.0 pC. 4.3 µV. 00:25:03. 42.0 kV. 12. 42.1 kV. 28.0 pC. 4.8 µV. 00:30:03. 13. 41.9 kV. 31.3 pC. 5.1 µV. 14. 41.8 kV. 28.2 pC. 15. 42.1 kV. 16. y n a mp 42.1 kV. 29.5 pC. 4.9 µV. 00:01:07. 41.9 kV. 29.8 pC. 4.8 µV. 00:06:10. 41.8 kV. 30.6 pC. 4.9 µV. 00:11:07. 00:15:35. 41.8 kV. 30.1 pC. 5.1 µV. 00:16:07. 6.0 µV. 00:20:35. 41.8 kV. 30.9 pC. 4.9 µV. 00:21:07. 29.5 pC. 6.2 µV. 00:25:35. 42.1 kV. 31.3 pC. 5.0 µV. 00:26:09. 41.7 kV. 29.0 pC. 5.8 µV. 00:30:35. 41.8 kV. 30.1 pC. 4.9 µV. 00:31:07. 00:35:03. 41.7 kV. 28.8 pC. 6.0 µV. 00:35:35. 41.8 kV. 29.7 pC. 5.0 µV. 00:36:07. 4.8 µV. 00:40:03. 41.6 kV. 29.5 pC. 5.4 µV. 00:40:35. 41.6 kV. 31.1 pC. 4.8 µV. 00:41:07. 27.8 pC. 4.8 µV. 00:45:03. 41.7 kV. 29.4 pC. 5.8 µV. 00:45:35. 41.8 kV. 30.8 pC. 5.2 µV. 00:46:07. 42.0 kV. 27.8 pC. 4.6 µV. 00:50:03. 41.7 kV. 28.0 pC. 5.9 µV. 00:50:35. 41.8 kV. 30.6 pC. 4.6 µV. 00:51:07. 17. 41.8 kV. 29.4 pC. 4.7 µV. 00:55:03. 41.6 kV. 30.5 pC. 5.6 µV. 00:55:35. 41.6 kV. 31.9 pC. 4.7 µV. 00:56:07. 18. 41.8 kV. 28.0 pC. 4.6 µV. 01:00:03. 41.6 kV. 29.1 pC. 5.1 µV. 01:00:35. 41.6 kV. 30.3 pC. 4.8 µV. 01:01:07. 1 hr level. 19. 37.9 kV. 27.5 pC. 4.5 µV. 01:02:50. 37.7 kV. 30.1 pC. 5.2 µV. 01:03:01. 37.7 kV. 30.6 pC. 4.8 µV. 01:03:09. 125%. 20. 30.7 kV. 24.7 pC. 4.6 µV. 01:04:02. 30.7 kV. 26.4 pC. 5.1 µV. 01:04:11. 30.7 kV. 27.7 pC. 4.7 µV. 01:04:20. 100%. 21. 0.3 kV. 18.2 pC. 4.6 µV. 01:04:38. 0.3 kV. 11.6 pC. 5.2 µV. 01:04:47. 0.3 kV. 12.0 pC. 4.9 µV. 01:04:56. Ambient. oble. ©D. o C g n i r e e. in g n E.

(106) LOW-FREQUENCY – INDUCED VOLTAGE/PD: ABNORMAL DATA  Results are not acceptable if the pC (or V) data exceeds any of the required criteria, and no reasonable/acceptable justification for the source/cause is provided.  Other tests, e.g., acoustic PD, DGA, can provide y confirmation n a p that a source of excessive partial discharge is present. m o C g rising in the oil, audible n i r  The presence of smoke and ebubbles e n i ngsudden increase in test current or sounds such as thump, E le all serve as a confirmation that b voltage collapse may o D © abnormal PD results are associated with a failure..

(107) LOW-FREQUENCY – INDUCED VOLTAGE/PD: RECOURSE IF DATA ABNORMAL  If pC (or V) data exceeds the limits, and all the attempts to identify and eliminate external PD sources are not successful, a longer standing time, long duration PD test, degassing of oil, refilling transformer under vacuum or a heatrun test (if one is specified) y are often n a p m successfully bring the PD data within limits. Co. . g n i r e A failure to meet theinpartial discharge acceptance e ng E criterion shall not warrant immediate rejection, but it e l b shall lead©D too consultation between purchaser and manufacturer about further investigations..  If visual confirmation (e.g., carbon, bubbles) is obtained and/or repeating of the test and/or other tests reveal the failure, the oil is drained and internal inspection is performed..

(108) NO-LOAD LOSSES aAND y n p m o C EXCITATIONerCURRENT, g n i e n i g ndielectrics E after oble ©D. (Routine*). *The test is not required by standards and no test type is assigned to it; however, it is a wildly recognized as standard practice and performed as routine..

(109) NO-LOAD LOSSES AND EXCITATION CURRENT after dielectrics: OBJECTIVE. Objective: No-load loss and excitation current, measured at 100% and 110% of the specified voltage and frequency after all dielectric tests are completed, provide additional confirmation that no damage, created by dielectric tests, is ypower test to n present in the transformer. If this is the last a p m o C be performed, it also serves to demagnetize the core for g n i re.g., 10-kV exciting current e subsequent low-voltage tests, e in g n and sfra. le E. b o D ©.

(110) NO-LOAD LOSSES AND EXCITATION CURRENT after dielectrics: ACCEPTANCE CRITERIA AND RECOURSE IF DATA ABNORMAL. No-load losses measured after dielectric tests are compared with the results obtained before dielectric tests. The 5% difference is often used as an acceptable criteria. Difference between the before and after data could be due y to: n a p m  Changes in the inter-laminar insulation o C g n i r  Temperature e e n i g Sometimes the change nafter initially exceeding 5% goes E away with time.Doble. ©. Failure to meet before and after dielectrics comparison criteria should not warrant immediate rejection but shall lead to consultation between purchaser and manufacturer regarding further investigation of possible causes and consequences..

(111) y n a p LOAD LOSSES AND m o C g n i r e e VOLTAGE IMPEDANCE n i g n E le (Routine) b o ©D.

(112) LOAD LOSSES AND IMPEDANCE VOLTAGE: DEFINITION AND OBJECTIVE Definition: The load losses of a transformer are losses associated with a specified load and include:  windings I2R losses due to load current  stray losses due to eddy currents induced by leakage flux in ytank walls, and the windings, core clamps, magnetic shields, n a p m omay also be caused by other conducting parts. Stray losses C g n i r currents circulating in parallel windings or strands. e e n i Load losses do not include control and cooling losses. Eng. le b o D The impedance © voltage of a transformer is the voltage required to circulate rated current through two specified windings with one winding short-circuited..

(113) LOAD LOSSES AND IMPEDANCE VOLTAGE: DEFINITION AND OBJECTIVE (cont.) Objective: The impedance and load losses test provides the data for:  Verification of design calculations.  Demonstration of meeting the guaranteed performance y often an n characteristics. Since these parameters have a p m othe accuracy of the economic value attached to them, C g n i r measurement becomes significant. e e n i g nare used as test parameter during E  Maximum load losses le b o the temperature ©D rise test.  Impedance voltage is an essential input parameter in power system studies (e.g., load flow, transformer parallel operation, short-circuit calculations)..

(114) LOAD LOSSES AND IMPEDANCE VOLTAGE: PHYSICS IIexc rated. R. FM. Note: Resistance R and short circuit of LV is not shown.. FL I2R lossesT Vrated Vsc HV LV. in g n E. C g n i r e e. le b o conditions ©D when. o. y n a mp. Eddy currents creating losses1/T. To create losses are limited to I2R and stray losses, and applied voltage is equal to the voltage drop across a loaded transformer, one winding is short-circuited and voltage is raised until rated current is reached. The flux path is then dominated by the leakage channel where the eddy losses in various conducting components in the FL path are induced..

(115) LOAD LOSSES AND IMPEDANCE VOLTAGE: PHYSICS (cont.) For most power transformers, VX_L >> VR_L. ZSC Iinput R. HV. Irated X. HV. RL. XLV. XL. RLV. VX_L Measured. Corresponds to leakage-flux linkages of the windings. VSC. VSC. IC. VR_L. CCRm. le b o D. ©. Compensating variable capacitor Cc is adjusted to reduce the input current.. y n a mp. VX_L Xm. o C g n is close i r Angle e e to 90, requiring n i g En IC. Iinput. Irated. high accuracy test systems.. VSC. . VR_L. Irated. Corresponds to load loss.

(116) LOAD LOSSES AND IMPEDANCE VOLTAGE: SETUP AND TEST METHODOLOGY Transformer in test. CT 3. X0 H1. VT. X1. H2 X2.  After data is recorded, if necessary, correction for losses y n a in external circuit is made. p. H3 X3. V. m o C g If three line currents are not n i r balanced the average RMS ee. I. A. oble. V. ©D. W.  Applied voltage is adjusted until rated current is present in the excited winding.. in g n E. value should correspond to the desired value..  The duration of the test should be kept to a minimum to avoid heating up winding conductors.. If taps are present, the following combinations of voltage ratings are tested:. DETC. rated. rated. rated. max. max. max. min. min. min. LTC. N. max. min. N. max. min. N. max. min.

(117) LOAD LOSSES AND IMPEDANCE VOLTAGE: SETUP AND TEST METHODOLOGY (cont.) Z2 2. Z1 1. 1 3. Z12 = Z1 + Z2 Z13 = Z1 + Z3le. b o D ©. 2.  For 3-wdg units, three sets of measurements are performed 3 using three pairs of windings, Z3 producingaZn12y, Z13, Z23 and P12, P13,omPp Solving shown 23. C gequations, determines Zi and n i r e P of each branch. e n i i g. En. Z23 = Z2 + Z3 Z1 = (Z12 + Z13 – Z23)/2 Z2 = (Z12 + Z23 – Z13)/2 Z3 = (Z13 + Z23 – Z12)/2.  For test, the current is set based on capacity of the winding with lowest MVA in the pair..  When results are converted to %, all data is given based on MVA of HV winding..

(118) LOAD LOSSES AND IMPEDANCE VOLTAGE: SETUP AND TEST METHODOLOGY (cont.) Measure A, V, W, T. Since stray and I2R losses have different Correct W and V Convert stray dependencies on T, from measured losses from each need to be amps to rated TLL_test Trated obtained from y n a p measured losses, m o 2 C Convert I R losses Convert Rdc from g individually converted n i r e Trated from Tin TR_test  TLL_test e from test T to rated LL_test g n E T before combined e l b again in reported load Calculate I2R losses Calculate total ©Do losses. V is also at TLL_test losses at Trated converted to rated T. (stray + I2R). Calculate stray losses at TLL_test (W - I2R). Correct V from TLL_test  Trated. Calculate %Vsc (V / Vrated)100 = %Zsc.

(119) LOAD LOSSES AND IMPEDANCE VOLTAGE: ACCEPTANCE CRITERIA  The total losses (no-load + load) should not exceed the guaranteed value by more than 6%.  For 2-wdg units, if Zsc>2.5%, the tolerance for measured impedance is +/-7.5% of the guaranteed value, otherwise, it is +/10%. The tolerance for comparison of duplicates units produced at the same time is +/-7.5%. y n a p having a zigzag  For 3-wdg units, autotransformers orom units C g nimpedance is +/-10% of the winding, tolerance for measured i r e e for comparison of duplicates n i guaranteed value. The tolerance g n E lesame time is +/-10%. units produced at o the b D data is credible:  Assurance that©test  Thermal stability prior to test: TTO-TBO 5C.  Average of T readings (Tave_oil) before and after the test should be used as test T. Their difference must be 5C.  Frequency is within +/-0.5% of rated.  Test system accuracy should be within +/-3% for loss, +/-0.5% for voltage, current and RDC, and +/-1.5C for T..

(120) LOAD LOSSES AND IMPEDANCE VOLTAGE: ABNORMAL DATA. Example: guaranteed load loss - 94 kW, measured – 110 kW  Potential reasons for exceeding the guaranteed values may include:  Oversights in design   . y n a mp. o C g Production process related factors or mistakes n i r e e n i g n Influence of temperature was not properly accounted for E e l b o D Accuracy©of measurements.

(121) LOAD LOSSES AND IMPEDANCE VOLTAGE: RECOURSE IF DATA ABNORMAL  Failure to meet the load losses and impedance test criteria should not warrant immediate rejection but shall lead to consultation between purchaser and manufacturer regarding further investigation of possible causes and consequences. ny . a p m o losses does The acceptance criteria of 6% for total C g nguarantee of losses i r e replace the manufacturer’s e n i g n purposes. economic loss evaluation E le b o ©D. not for.

(122) LOAD LOSSES AND IMPEDANCE VOLTAGE: COMPARISON WITH FIELD DATA. Factory losses are measured under 3-phase excitation, at rated current and reported as sum of three phases I2R and stray losses.. Field losses are measured under 1-phase excitation, at ylower than rated current much n a p m and reported as per-phase I2R o C g stray losses. n i and r ee. in g n E. le b o ©D Factory and field results cannot be compared.

(123) LOAD LOSSES AND IMPEDANCE VOLTAGE: COMPARISON WITH FIELD DATA (cont.) Factory short-circuit impedance is reported as average of three phases, obtained at rated current* under 3-phase excitation.. Field leakage reactance is reported as per-phase reactive component of short-circuit impedance, the obtained at current* much lower than rated under 1-phase excitation.. y n a p m Experience shows that a combined influence of different instrumentation o C g under 3- and 1-phase and test setups, difference in flux distribution n i r e ecomponent and averaging of factory excitation, presence of the resistive n i g nranging from nearly perfect (<1%) to up to E data can result in differences le b o 6% (of the measured value). ©D. However, the differences between factory and field test conditions notwithstanding, the ZNP can serve as a useful guideline for evaluating the initial value measured in the field. If, during initial test, the field perphase tests deviate from average (of three readings) by <3% of the measured value, results normally are considered acceptable. The initial per-phase test should serve as a benchmark for future testing with acceptable difference from the initial field test being <2%. *Since test is confined to leakage channel (where reluctance is determined by air/oil) the leakage inductance (L=/I), remains the same regardless of the current level..

(124) y n a p TEMPERATURE RISE m o C g nother) i r e (Designinand e g n E le b o ©D.

(125) TEMPERATURE RISE: DEFINITION AND OBJECTIVE Definition: The temperature rise is a test that verifies transformer thermal performance through determination of winding and oil temperature rises over ambient. Objective: The temperature rise test provides the top-oil y rise over n rise, winding average rise and winding hot-spot a p m o ambient for: gC  . n i r e ine Verification of designncalculations. g E e l b Demonstration of meeting the guaranteed performance o D ©. characteristics.  Provides data for calculation of potential MVA margin.  Setup of various temperature monitoring instruments and cooling control..

(126) TEMPERATURE RISE: PHYSICS Calculated: Tto-a, Tw_ave-a, Ths-a, GRAD. Measured: Tto, Tt_rad, Tb_rad, Ta.. Needs NL+LL losses. Ta. Main tank Tto. Tt_rad. height. Tto-a. Need rated current. Tto. y Tn t_rad a mp. Ths-a. Ta. LV. le b o ©D HV. Core. in Rad g n E. Tb_rad Ta. C g n i r e e. o. Oil. To_ave GRAD. Tw_hs. Tw_ave* Winding. Tw_ave-a Tb_rad. Ta. T. Located at 3 locations around xfmr at mid-height level.. *The term “winding average T rise”, Tw_ave-a, is not the T at any given point in a winding nor is it an arithmetic average of results determined from different terminal pairs. It refers to the value determined by measurement on a given pair of winding terminals..

(127) TEMPERATURE RISE: SETUP AND TEST METHODOLOGY. Transformer in test. CT. X0 H1 X1. VT. I. ng E le. Dob. ©V W.  Test is performed for min and max y n ain a combination of p MVA, m and H3 X3 o C DETC/LTC positions, producing g n i r highest load losses. e e in. H2 X2. V. A.  Total losses (NL+LL) and winding cold resistance data should be available..  10Tamb40C and measured in containers with liquid, having a time constant as per C57.12.90-2010.  Test contains 3 key segments: - total loss run (to include 3 hr of thermal stability) - rated current run (1 hr) - hot resistance measurement (e.g., 10-20 min after shutdown).

(128) TEMPERATURE RISE: SETUP AND TEST METHODOLOGY (cont.) Measurement before cutback determines *Tto-a. T[C]. ONAF shutdown. Tto, Tt_ rad Rhot measurement begins. Cutback Xfmr energized for ONAF. Tb_ rad. ONAN shutdown. y n a mp. o C Steady-state oil g n i r e T rise e. Tto-a ngin. le b o D. ©. E. Ta_ ave. (change of Tto-a in 3h  1C or  2.5% whichever is greater). Ptotal 1h. Preceding ONAN. Itest. Irated. Total loss run. *Tto-a is corrected for difference between required and actually used total losses (it must be 20%) and for altitude.. t [h] Rated current run.

(129) TEMPERATURE RISE: SETUP AND TEST METHODOLOGY (cont.) Objective: resistance of winding at the time when load current is still present. Rhot. Rhot calculated at t = 0. y in Rhot as function a ofntime p presenceo ofmdecreasing C g temperature is recorded rin. *Instrument connected. e e Instrument output n i ng current reached E le pre-selected levelDob Flux © stabilized t=0 Voltage removed. t [min] t  4 min. Tw_hot = Rhot/Rcold(234.5 + Tw_cold) – 234.5. *If two windings are tested simultaneously in series, the Idc is selected based on the lowest rated current..

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