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
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
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
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.
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
OVERVIEW OF TESTS (cont.) TEST TYPE
PERFORMANCE
DIELECTRIC
MECHANICAL
Temperature rise
Design/ Other Audible sound level Short-circuit capability Other
oble
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in g n E
C g n i r e e
y n a mp
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Single-phase excitation current Front-of-wave impulse
Lifting and moving Pressure
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
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
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.
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
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.
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
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.
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
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
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
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
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.
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.
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
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.
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
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
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.
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 Tave_rated_rise + 20C, e.g., 65+20= 85C and as total of 3 phases.
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-TBO 5C. This assures n i TTO variation 2C 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.
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.
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.
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
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).
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
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.
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
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 5C and 10TO_ave30C Iexc=aver. of 3 phases in % of Irated
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.
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.
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 +/-1.5C for T. gincurrent,
b o D ©
n E le
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
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
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
y n a mp
o C g DIELECTRIC n TESTS i r e e n i g n E le b o ©D
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.
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)
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
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.
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 >10turns**.
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 510turns. C
Region C – insulation across taps is tested by FW & CW impulse, with stress 510turns.
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.
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.
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 = 1.67Tny
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.
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
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
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
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 >1s. 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
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.
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)
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
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.
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.
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
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
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
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
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
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
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
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
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.
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.
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
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
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
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)
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
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
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 1turns*. 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.
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 = 0.83BIL +/- 3% RSW=(50-70%)0.83BIL >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.
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.
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
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
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.
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
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
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
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
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
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
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.
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
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
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
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.
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
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
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
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.
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.
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 << 1s. 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.
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
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
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.
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
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.
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 2.0nominal voltage for 7200 cycles with no PD (RIV) recordings. For class II units rated 115 ÷ 500 kV, the test includes applying to HV winding 1.8nominal voltage for 7200 cycles and 1.58nominal 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).
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.
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.
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
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.
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.
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.
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 ©
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.
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
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.
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).
LOAD LOSSES AND IMPEDANCE VOLTAGE: PHYSICS IIexc rated
R
FM
Note: Resistance R and short circuit of LV is not shown.
FL I2R lossesT 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 losses1/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.
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
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
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.
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 TLL_test Trated 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 Trated from Tin TR_test TLL_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 TLL_test losses at Trated converted to rated T. (stray + I2R)
Calculate stray losses at TLL_test (W - I2R)
Correct V from TLL_test Trated
Calculate %Vsc (V / Vrated)100 = %Zsc
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: TTO-TBO 5C. Average of T readings (Tave_oil) before and after the test should be used as test T. Their difference must be 5C. Frequency is within +/-0.5% of rated. Test system accuracy should be within +/-3% for loss, +/-0.5% for voltage, current and RDC, and +/-1.5C for T.
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
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
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
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.
y n a p TEMPERATURE RISE m o C g nother) i r e (Designinand e g n E le b o ©D
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.
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.
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.
10Tamb40C 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)
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 1C 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
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.
TEMPERATURE RISE: SETUP AND TEST METHODOLOGY (cont.)
Comparison with guaranteed values, e.g., Tto-a and Tw_ave-a 65C; Ths-a 80C GRAD correction for localized hot spot eddy currents
Tw_ave-a
Tw_hot To_ave_sd
*GRAD
GRAD
in g n E
g n i r ee
le b o ©D**To_ave_cb-a
any
p m Co GRAD
Tto-a
***Ths-a Value used for setting winding T monitors
Ta *GRAD is corrected for difference between required and actually used load current (it must be 15%). **To_ave_cb-a is corrected for difference between required and actually used total losses (it must be 20%) and altitude. ***This a simplified representation of Ths_a determination; actual design calculation is more involved.
TEMPERATURE RISE: SETUP AND TEST METHODOLOGY (cont.) During shutdown at the time of the first Rdc reading, the flux must be stabilized so that resistance change is caused only by reduction in temperature. It’s true in most cases, unless series autoxfmr is present.
vLV
X0
idc
i2
Series autoxfmr windings with LTC in N Series autoxfmr core
F1
le b o ©D
F2
idc
i1
C g n i r e e
y n a mp
o
in g n E
vLV idc
LV winding
idc Main unit core
F i1
X0
X2
idc i2
X2
TEMPERATURE RISE: SETUP AND TEST METHODOLOGY (cont.) 0.0047
0.00468
At t = 4 min, data 0.00467 y n collection a begins X 0 -X 2 p m o in main withCflux 0.00466 g ncore stable while i r e e flux in series core 0.00465 n i g n E le still changing. 0.00464 b o D © H 1 -H 2 0.00463
0.32
0.31
0.3
0.29
0.28
0.27
0.26
0.00462 0.25 0:00:00 0:00:43 0:01:26 0:02:10 0:02:53 0:03:36 0:04:19 0:05:02 0:05:46
Time [min]
Voltage removed t=0
Time remaining for stabilization = 2.5 min
HV circuit Rdc [Ohm]
Setup 1.5 min
LV circuit Rdc [ohm]
0.00469
Series autoxfmr should be excluded from both cold and hot Rdc measurements.
TEMPERATURE RISE: ACCEPTANCE CRITERIA The winding average T rise over ambient for all tested windings should not exceed the guaranteed value, e.g., 65 or 55C. The top-oil T rise over ambient should not exceed the guaranteed value, e.g., 65 or 55C. The winding hot spot T rise over ambient for all tested windings y for 65C rise n should not exceed the guaranteed value, e.g., 80C a p m o units and 65C for 55C rise units. C g n results of winding average i r If shutdown is performed on each phase, e e n i g rises should be comparable (rule of thumb: 4C difference, n E le in the standard). b presently, there is no limit o D © DGA results (after heatrun) should be normal. It is always useful to perform and review thermal scanning of all tank walls and the cover in search for excessive overheating (100C rise). Request image files to be provided with the certified test report and have software to view them. If agreed with manufacturer, the heatrun is a good time to check the performance of temperature controllers (using a preliminary winding T gradient) and turns ratio of CTs.
TEMPERATURE RISE: ACCEPTANCE CRITERIA (cont.) To assure test data is credible, verify that: T and current requirements for measuring winding cold Rdc were complied with. Test is performed using maximum load loss and in corresponding DETC/LTC positions. y n a Test instrument type and setup used formcold and hot resistance p o C was the same, e.g., if two-channel measurement is used it must g n i r e be used for both hot and cold resistance tests. e n i ng unless it is shown that RDC can be If series auto is present, E le b o measured within shutdown time constrains, the auto is excluded D © from the resistance measurement*. During shutdown, fans are turned off right after transformer is deenergized. The first value of winding hot Rdc was recorded not later than 4 min after shutdown. *Lachman, M. F., et al “Impact of Series Unit on Transformer Winding DC Resistance Measurement During Heatrun”, Proc. of the Seventy-Sixth Annual Intern. Confer. of Doble Clients, 2009, Sec. T-4.
TEMPERATURE RISE: ACCEPTANCE CRITERIA (cont.) To assure test data is credible, verify that: Winding hot Rdc fits reasonably into the cooling curve. Final T rises are properly corrected: GRAD for actual test currents, Tto-a and To_ave_cb-a for actual total losses and altitude. Test system accuracy should be within +/-3% for loss, +/-0.5% for yand +/-1.5C for n a voltage, current and winding resistance, p m o C temperature. g nrated frequency, the results are i r e If the test could not be done at e n i g nto rated frequency (see C57.12.90-2010, converted from tested E lethe fans/pumps should be operated at the power b Annex B). However, o D frequency to© be used when unit is in service.
TEMPERATURE RISE: ABNORMAL DATA Example: guaranteed Tw_ave-a – 65C, measured – 67C Potential reasons for exceeding the guaranteed values may include: Oversights in design Testing/setup mistakes y n Presence of series auto-transformer a p LV_hot
0.00454
0.00535
0.00452
0.0045
0.00448 0.00446 0.00444
Without series auto-xfmr
Resistance [ohms]
Resistance [ohms]
y = 6.962E-08x2 - 5.974E-06x + 4.534E-03
2
0.0053
0.00525
0.0052
0.00515 0.0051 0.00505
With series auto-xfmr
0.005
0: 0 0: 0 3 1: 0 0 1: 0 3 2: 0 0 2: 0 3 3: 0 0 3: 0 3 4: 0 0 4: 0 3 5: 0 0 5: 0 3 6: 0 0 6: 0 3 7: 0 0 7: 0 3 8: 0 0 8: 0 3 9: 0 0 9: 0 10 30 :0 0
0.00442
Time [min]
0: 0 0: 0 30 1: 0 1: 0 3 2: 0 0 2: 0 30 3: 0 3: 0 3 4: 0 00 4: 30 5: 0 5: 0 3 6: 0 0 6: 0 30 7: 0 7: 0 3 8: 0 0 8: 0 30 9: 0 9: 0 10 30 :0 0
TLV_hot
m o C =[(234.5+30)4.534/4.081]-234.5=59.4ºC T =[(234.5+30)5.362/4.621]-234.5=72.5ºC g n i r e e in g y = 8.152E-07x - 3.235E-05x + 5.362E-03 n E le b o ©D
Time [min]
Note: The example shows a quadratic function, the suitability of which was confirmed via direct fiberoptic measurements and other methods, e.g., Blume. Different functions may be used if they fit the winding behavior.
TEMPERATURE RISE: RECOURSE IF DATA ABNORMAL Failure to meet the temperature rise test criteria should not warrant immediate rejection but shall lead to consultation between purchaser and manufacturer regarding an investigation of possible causes and solutions to address the problem. ny
le b o ©D
in g n E
g n i r ee
a p m Co
ZERO-PHASE SEQUENCE y n a p m o C g IMPEDANCE n i r e e in g n E (Class I - design e l b ©Do Class II - routine)
ZERO-PHASE SEQUENCE IMPEDANCE: DEFINITION AND OBJECTIVE Definition: The zero-phase sequence impedance is impedance to the single-phase current simultaneously present all three phases. It is measured from a wye or a zig-zag connected winding between three phase terminals connected together and the neutralyterminal.
n a p om
C Objective: The zero-phase sequence impedance serves g n i r e e as input in analysis of gunbalanced three-phase system n i n E using symmetricalblcomponents method. e ©Do
ZERO-PHASE SEQUENCE IMPEDANCE: PHYSICS Ia
Balanced
Ia1
Ib
Ic
Ia
Positive
In symmetrically loaded 3-phase system, only one phase needs to be analyzed since in other phases values have the same magnitudes and only have to be shifted by 120.
3-phase system, each phase are g n i r different and each phase needs e e in g to be analyzed separately. n E
Ic1
Ib1
Ib2 Negative
Ia2
le b o ©D
Ic2 Iao Ibo Ico
Ib
Unbalanced
Ic Zero
y n a In unbalanced p m Co impedances in
Method of symmetrical components converts any unbalanced system into 3 balanced systems, namely positive, negative and zerophase sequence systems.
After these are defined, the voltages and currents in the original unbalanced system are reconstructed.
ZERO-PHASE SEQUENCE IMPEDANCE: SETUP AND TEST METHODOLOGY For xfmr, the Z1 = Z2 = Zsc is known from impedance/load losses test. In zerophase sequence system, the phase currents are in-phase with each other and flow through the xfmr only if there is a path to return to the grounded source or to circulate while satisfying the Kirchhoff’s current law.. Therefore, this test applies only to transformers with one or more windings with a physical neutral brought out for external connection. any
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p m Co 1
2 Z0 N
1
2
ZERO-PHASE SEQUENCE IMPEDANCE: SETUP AND TEST METHODOLOGY (cont.) Z1Ns
Z1No
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Z1Ns, Z1No, Z2No are used to calculate Z1, Z2 and Z3.
in g n E
g n i r ee
If delta winding is not present, the currents y n ashown in delta are p m Co circulating in the tank.
1 Z1
Z2 2 Z3
Z2No N
ZERO-PHASE SEQUENCE IMPEDANCE: SETUP AND TEST METHODOLOGY (cont.) Transformer in test
CT 1
VT
le b o ©D
A
V
W
If delta winding is present, the applied ysuch that current in n voltage should be a p m delta winding Co Irated.
g n i r ee For Y/
V I
If no delta winding is present, applied voltage should be 30% of rated Vphase_gnd and measured current Irated.
in g n E
or /Y impedance in % is determined as:
Z0 = 300(Vmeas / V r) (Ir / Imeas) For Y/Y and autoxfmr with or without tertiary , the elements of the equivalent circuit are further determined as:
Z1 = Z1No - Z3 Z2 = Z2No - Z3 Z3 = Z2No ( Z1No - Z1Ns)
ZERO-PHASE SEQUENCE IMPEDANCE: ACCEPTANCE CRITERIA The standard does not provide an acceptance criteria for the zerophase sequence values. However, the following general guidelines can be useful (typical for 230 kV, 200 MVA core type units): For /Y, Z0 Zsc or slightly less. Example: 50 MVA, 161/69GndY kV, Zsc = 21.9%, Z0 = 21.8%
y n a p Example: 48 MVA, 235.75GndY/13.8 kV, Zsc = 9.9%, Z m 0 = 8.5% o C g For Y/Y/ or autoxfmrs with ridelta, Z1 (0.7-1.0)Zsc; with n e e<0. n typically <1.0% or sometimes i g n E Example: Auto, 18 MVA, 230GndY/60GndY/21 kV, Zsc = 4.9%, Z1 = 3.6%, e l b Z2 = 0.84%, ©DoZ3 = 10% For Y/ units, Z0 (0.8-1.0)Zsc.
Z2
50 MVA, 69GndY/34.5GndY/13.2 kV, Zsc = 7.8%, Z1 = 6.7%, Z2 = -0.16%, Z3 = 4.6%
For Y/Y and autoxfmrs without delta (rare occasion), magnetic flux has a strong coupling to the tank, making, in general, the relationship between voltage and current non-linear and the above observations not relevant. Example: Auto, 75 MVA, 115GndY/34.5GndY kV, Zsc = 12.7%, Z1 = -9.9%, Z2 = 27.4%, Z3 = 205.6%
ZERO-PHASE SEQUENCE IMPEDANCE: ABNORMAL DATA If unusual zero-phase sequence impedance data is obtained the test process should be reviewed (paying particular attention to voltages and currents used) along with comparing the measured data with the calculated design values. ny
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ZERO-PHASE SEQUENCE IMPEDANCE: RECOURSE IF DATA ABNORMAL Unusual zero-phase sequence impedance data does not warrant a unit rejection but should lead to a consultation between purchaser and manufacturer to understand the possible causes and consequences.
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C g n i r e e
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o C g AUDIBLE SOUND LEVEL n i r e e n i g n and other) (Design E le b o ©D
AUDIBLE SOUND LEVEL: DEFINITION AND OBJECTIVE Definition: The audible sound level test is the measurement of the sound pressure level around a fully assembled transformer under the rated no-load conditions with cooling equipment operating as appropriate for the power rating being tested.
y n a mp
o Objective: To protect the population from noise C g nrequired to operate within i r inconveniences transformers are e e n i g n audible sound level test provides specified noise limits. The E le data for: b o the sound pressure level ©D Verification of design calculations. Demonstration of meeting the guaranteed performance characteristics. The test also serves as a quality control tool as the sound, driven by the vibratory motion of the core, is transmitted to the tank through direct mechanical coupling as well as is produced by pumps and fans of the cooling system.
AUDIBLE SOUND LEVEL: PHYSICS Most of xfmr sound is generated by the core. When the core steel magnetized/demagnetized twice each cycle, the steel elongates and shortens due to a property called magnetostriction.
Tank Dielectric fluid
y n a mp
Core
le b o ©D Direction of dimensional change
C g n i r e e
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F Magnetostriction caused by domain rotation
o
This produces a vibratory motion in the core transmitted to the tank through the core mechanical support and the pressure waves in the dielectric fluid. At the tank this motion radiates as an airborne sound. The vibration magnitude depends on the flux density and magnetic property of the steel. The frequency spectrum of the sound contains mainly the even harmonics of the power frequency, i.e., 120, 240, 360, etc. The audible sound also includes a contribution emitted by pumps and fans, containing a broadband spectrum of frequencies.
AUDIBLE SOUND LEVEL: SETUP AND TEST METHODOLOGY
Transformer in test X0 H1 X1 H2 X2
3
y On certain tap an positions, xfmr may p produce sound om levels greater than at the C g principal tap, e.g., engaging PA and/or n i r H X3 3 e eseries autoxfmr. Test will be performed in n i g n E these positions if specified by customer. e
l b o ©D
VT
Vave
Xfmr is energized with no load, at rated (for the tap used) voltage and frequency, with tap changer on principal tap and pumps/fans operated as appropriate for the tested rating.
The voltage should be set as during noload loss test, based on Vave.
At least one test should be performed at the cooling stage for the min rating and one test at the cooling stage for max rating. Measurements begin when xfrm reaches steady-state conditions, i.e., to allow magnetic bias to decay.
AUDIBLE SOUND LEVEL: SETUP AND TEST METHODOLOGY (cont.) Microphone location 3 ft
6 ft
Fan cooled surface
Radiator Tank
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Drain valve
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1 ft Measurement surface
Microphones are located on the measurement surface at shown distance from reference ny surface. asound-producing
#1 Reference sound-producing surface is a vertical surface following the contour of a taut string stretched around xfmr periphery.
p m Co Xfmr is placed so that no LTC
acoustically reflecting surface is within 10 ft of the microphone.
If transformer H<7.9ft, measurements are made at H/2; if H7.9 ft, at H/3 and 2H/3. First measurement is made at drain valve proceeding clockwise.
AUDIBLE SOUND LEVEL: SETUP AND TEST METHODOLOGY (cont.)
The sound power rating of a transformer is determined using one of the following three measurement methods:
A-weighted sound pressure level (most frequent) ny One-third specified)
octave
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a p m Co
g sound rinpressure
e e n i Eng
level
(when
Narrowband sound pressure level (when specified)
AUDIBLE SOUND LEVEL: SETUP AND TEST METHODOLOGY (cont.) A-weighted sound pressure level Human ear can hear sounds in 20÷20000 Hz
range. However, it detects some frequencies much easier than others. This uneven frequency response needs to be considered when the annoyance of unwanted sounds is to be evaluated.
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To account for human’s greater sensitivity to noise at some frequencies relative to other, the measured data is passed through a weighting filter. Aweighting is most commonly used to allow for a broad peak between 1÷6 kHz but very strongly discriminating against low frequencies.
C g n i r e e
o
in g n E
As a result, when the average sound pressure level is calculated, the influence of frequencies not impacting the human hearing perception is minimized.
Hz
63
125
250
500
1000
2000
4000
8000
A-filter
-26
-16
-19
-3
0
1
1
-1
dB (measured)
67
76
73
70
65
66
62
52
dB (A-weighted)
41
60
64
67
65
67
63
51
AUDIBLE SOUND LEVEL: SETUP AND TEST METHODOLOGY (cont.) The following two methods are used when a more detailed investigation into the sources of noise is required: In one-third octave sound pressure level measurement, each octave band in the spectrum (i.e., 63, 125, 250, 500, ywith each “1/3 n 1000, 2000 and 4000 Hz) is split into three, a p m o 200, 250 Hz, etc.) sub-band” (e.g., 63, 80, 100, 125,C160, g n i r being evaluated individually. e e n i g n pressure level measurement is E The narrowband lsound e b o performed © atDthe power frequency (e.g., 60 Hz) and at least at each of the next six even harmonics (120 Hz, 240 Hz, 360 Hz, 480 Hz, 600 Hz, and 720 Hz). Once again, each frequency is evaluated individually.
AUDIBLE SOUND LEVEL: SETUP AND TEST METHODOLOGY (cont.) The sound power rating is determined using the following steps: Measure ambient sound pressure levels. This is established as an average of measurements at a min of four locations immediately preceding and immediately following the sound measurements with the unit energized. Measure combined transformer and ambient sound y pressure level. n a p Measurements are made if ambient levelois at least 5 dB or more m C g ambient sound pressure below the combined transformerrinand e e level. n i g n E Compute ambient-corrected sound pressure levels. For e l b Do 7 in C57.12.90-2010. corrections see ©Table Compute average sound pressure levels [in dB(A)]: 𝑵 𝑳𝒊 𝟏 𝑳𝒑 = 𝟏𝟎𝒍𝒐𝒈𝟏𝟎 𝟏𝟎𝟏𝟎
𝑵
𝒊=𝟏
Li is the sound pressure level measured at ith location by one of the 3 measuring methods. Sound power levels are calculated when requested.
AUDIBLE SOUND LEVEL: ACCEPTANCE CRITERIA Computed average sound pressure level should not exceed the audible sound levels as listed in NEMA TR1-1993, Tables 0-2 and 0-3 or as requested in customer test specification. Rectifier, railway, furnace, grounding, and mobile transformers are not covered by these tables. y Assurance that test data is credible: n a p m The sound pressure measuring instrument should meet the o C ng1 meters. i requirements of ANSI S1.4 for e Type r e instrument should be calibrated n i The sound pressure measuring g n E le set of measurements. If calibration change before and afteroeach b ©D >1dB, sound measurements shall be declared invalid, and the test repeated. Verify that microphones were positioned at required distances/heights, pumps/fans were operated as required for tested power rating and voltage set based on Vave. Verify that the ambient level was at least 5 dB or more below the combined transformer and ambient sound pressure level. If rated frequency is not used, 50/60 Hz conversion is applied.
AUDIBLE SOUND LEVEL: ABNORMAL DATA Example: guaranteed sound pressure level per NEMA – 75/77/78 dB(A), measured – 77/78/79 dB(A) Potential reasons for exceeding the guaranteed values may include: y
n a p m ambient oe.g.,
Problems with measurement,C noise, ng instrument calibration, i positions of microphones,esound r e n i g voltage adjustment, surrounding reflecting surfaces, n E le b etc. o ©D Variability in core steel characteristics Different core steel Oversights in design Assembly related factors or mistakes
AUDIBLE SOUND LEVEL: RECOURSE IF DATA ABNORMAL Failure to meet the audible sound test criteria should not warrant immediate rejection but shall lead to consultation between purchaser and manufacturer regarding an investigation of possible causes and solutions to address the problem. ny
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g n i r ee
a p m Co
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o C g CORE DEMAGNETIZATION n i r e e n i g n (Routine*) E le b o ©D
*This procedure is not required by standards but is a wildly recognized as standard practice and performed as routine.
CORE DEMAGNETIZATION: DEFINITION AND OBJECTIVE Definition: The core demagnetization is the process of removing the magnetic bias in the core through a series of steps, with each subsequent step creating magnetic field of opposite direction and lower intensity. The first step must bring the core to the main hysteresis loop with y n a the last step, upon removal, leaving no residual p m o C magnetism in the core. g in
r e e ngin
E Objective: The core demagnetization creates conditions e l b o low-voltage exciting current and loss for obtaining©D the test as well as sfra benchmark data not affected by residual magnetism .
CORE DEMAGNETIZATION: PHYSICS Br
If in the presence of residual magnetism Br, the voltage is increased from zero, the flux varies around minor hysteresis loops. The negative H tip of these loops lies on the main y voltage, the loop. The greateran the p of the minor loop m smaller is the ooffset C g along the n B axis. The bias is removed i r e e the main loop, symmetrical n i when g n E ble around the origin, is reached.
B
Main loop B ©Do Br = 0
H
If after reaching the main hysteresis loop, the voltage is gradually reduced, each minor loop will lie inside the previous larger loop. Reduction of voltage to zero brings working point to the center of these loops resulting in a demagnetized transformer.
CORE DEMAGNETIZATION: SETUP AND TEST METHODOLOGY The core demagnetization can be performed by one of the following: Applying rated 3-phase voltage (holding for 5-10 min) and reducing gradually to zero. y n a Applying DC voltage (e.g., 12 V), waiting until current p m opolarity and holding C stabilizes, then switching voltage g n i r e until current reachesginae lower value; this process n level is zero E continues until b current le o ©D Without ammeter, the above approach can be applied but a lower level of current is reached by applying alternate polarities of DC voltage for progressively shorter periods of time. If no-load losses or sound level tests are the last power tests to be performed, they serve the function of the core demagnetization process.
CORE DEMAGNETIZATION: RELATIONSHIP WITH LV DIAGNOSTIC DATA Field Factory
Data movement with no excitation applied between measurements
Field
Factory
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Controlled experiments showing data movement*
6 hr 3 hr
When xfmr is de-energized, the core is constantly looking for a state of lower energy, i.e., it relaxes, changing its magnetic state and moving away from the condition immediately following demagnetization*. This is obvious in the low-frequency range of the sfra trace but not in the low-voltage excitation current data. These sfra changes are normal and diagnostically insignificant.
in g n E
C g n i r e e
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o
72 hr
24 hr
9 hr
Factory
Field
mA
W
mA
W
20.5
128
20.5
126
9.3
61
9.6
58
20.7
131
21.6
131
1 hr 30 min dm_init
*Lachman, M. F., et al “Frequency Response Analysis of Transformers and Influence of Magnetic Viscosity”, Proc. of the Seventy-Seventh Annual Intern. Confer. of Doble Clients, 2010, Sec. TX-11.
LAST SLIDE
C g n i r e e
o
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THE END le b o ©D
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y n UNDERSTANDING T HE a p om C g n i r TRANSFORMER T EST D ATA e e n gi
b o D ©
n E le
Barry M. Mirzaei – P.Eng. Hydro One September 2012 – Chicago
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September 2012
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Understanding The Transformer Test Data
2
No Load Test y n a mp
Test object is supplied from one side of the transformer (L.V.), the other side (H.V.) is left open circuit. Test voltage to be adjusted to the pre‐determined value(s)
o C ng
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g n E90% ‐ 100% and 110% of the rated Typical test voltage is e l voltage Dob © Characteristics of the No Load Test: “Low Current – High Voltage” September 2012
Understanding The Transformer Test Data
3
Induced Voltage Test Test object is supplied from one side of the transformer (L.V.), the other side (H.V.) is left open circuit. Test voltage to be adjusted to the pre‐ determined value(s)
o C ng
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i r e e n i Twice the rated voltage is applied for 7200 cycles g n E for transformers with uniformly insulated le b o windingsD ©
Characteristics of the Induced Voltage Test:
“Low Current – High Voltage and Frequency > 60” September 2012
Understanding The Transformer Test Data
4
Load Loss Test Test object is supplied from one side (H.V.), the other side (L.V.) is short‐circuited. Test voltage is ny a p adjusted to apply the rated current to the test m o C object g
n i r e ine
g n ER e ‐Resistive losses or l b o D ‐Eddy© current losses in the windings Load Loss:
‐Stray losses in leads, core plates and tank
Characteristics of the Load Loss Test: “High Current – Low Voltage” September 2012
Understanding The Transformer Test Data
5
g n le E
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Understanding The Transformer Test Data
6
Hysteresis Loss o C ng
i r e Proportional to theinfrequency e g n the area of and dependent on E le b o the hysteresis loop, which, in ©D turn, is a characteristic of the material and a function of the peak flux density September 2012
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Understanding The Transformer Test Data
7
Eddy Current Loss o C ng
i r e e Dependent on the square n i g n E of frequency but is also le b o D © directly proportional to
y n a mp
the square of the thickness of the material September 2012
Understanding The Transformer Test Data
8
4.44
(a)
(b)
Voltage
o C r ing ee
n i g n E
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PNL No Load Losses le b o DHysteresis Loss ©= = Eddy Current Loss , = Coefficients
= = Exponent with induction
September 2012
Understanding The Transformer Test Data
9
Minimizing hysteresis loss thus depends on the development of a material having a minimum area of hysteresis loop.
o C ng
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i r e e loss is achieved by n i Minimizing eddy current g n E lecore from a stack of thin building upothe b ©D laminations and increasing resistivity of the material in order to make it less easy for eddy currents to flow.
September 2012
Understanding The Transformer Test Data
10
g n le E
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STATEMENT OF THE ISSUE:
September 2012
Understanding The Transformer Test Data
11
During the No Load test of a rebuilt 3 phase 135 kV transformer in the factory, loud noises inside the tank were reported. Not a Hydro One Asset
The noises were described as similar to “release of large amounts of air bubbles pany om inside the oil”, started at around the 25% C g n i r e e of the test voltage. n gi
n E le
b o D ©
Deflection in the readings on metering devices (watt meters, …) were reported with the noise. September 2012
Understanding The Transformer Test Data
12
Solutions? What test data are available? What those test data really mean? g n le E
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Understanding The Transformer Test Data
13
Criteria & constraints for addressing the issue Un‐necessary activities to be y n a p avoided, delivery date was critical om C g n i r e e n Un‐tanking the transformer is costly and i g n E should be avoided if there is no clear e l b o D understanding about the issue © Insulation tests should not be repeated, if there is no need to do so
September 2012
Understanding The Transformer Test Data
14
1 Oil Sample 6
OK
2
Apply reduced “Induced Voltage”
Repeat TTR & DC Resistance
Not Convinced to apply
No Load Test
y n a mp OK
o C Investigation g n i r e Procedure e n i 5 g n E 3 e l Insulation b o D Observe The Test? © 4
PROBLEM
Apply Load Test OK September 2012
Understanding The Transformer Test Data
15
The Induced Voltage Test y n a stresses all parts of othe p m C g n including i r insulation system, e e n i g n E turnDoto bleturn, phase to phase © and winding to ground.
September 2012
Understanding The Transformer Test Data
16
4.44
(a)
Concept of customized Induced test: y n a p om
C g n i r e e
n i g By applying induced voltage up to n E le b o ©D rated voltage, basically the no load test is being repeated with reduced induction in the core September 2012
Understanding The Transformer Test Data
17
. .
x
x x x
. .
x
x x x
x x x x
g n le E
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September 2012
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Understanding The Transformer Test Data
18
1 Oil Sample OK 6
2
Apply reduced “Induced Voltage”
Repeat TTR & DC Resistance OK
Investigation Procedure 5
3
Insulation Test? Not Convinced to apply
Eddy Current Loss Dependent on the square of frequency but is also directly proportional to the square of the thickness of the material
y n a Hysteresis Lossp om C g and dependent on Proportional to thein frequency r e the area of the hysteresis loop, which, in turn, is a e n i characteristic of the material and a function of the g n E flux density le peak Observe The No Load Test
4 Apply Load Test
OK
b o D ©
PROBLEM
Load Loss: ‐Resistive losses or R ‐Eddy current losses in the windings ‐Stray losses in leads, core plates and tank September 2012
Understanding The Transformer Test Data
19
g n le E
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September 2012
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Understanding The Transformer Test Data
20
Core bolts are inserted through the core for the y n a p m o C g n i purpose of clamping the r e e n i g n E le b core laminations. o ©D
September 2012
Understanding The Transformer Test Data
21
During “Core Stacking Process” – Holes built for Core Bolts, used for proper core stacking
g n le E
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Understanding The Transformer Test Data
22
Core Plates
Core Bolts
g n le E
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Photo belongs to another transformer September 2012
Understanding The Transformer Test Data
23
Core Plate Core Bolt Weld
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Understanding The Transformer Test Data
24
g n le E
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25
Fiberglass insulation
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g n le E
Metal Washer
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Round Head Carriage Bolt
September 2012
Understanding The Transformer Test Data
26
g n le E
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Understanding The Transformer Test Data
27
In this case, the low impedance path formed by the bolts and the core clamping plates causes a local short circuit path which produces intense local eddy currents. The amount of heat generated by this phenomenon is sufficient to considerably damage the adjacent anyareas.
g n i r ee
p m o C
n i g The problem was noticeable in No‐Load test since there n E le b was higher induction to create higher current in the o D © through bolts when compared to reduced induced test. Increase in the Load Loss increased the probability of “Core Plates” related issues. September 2012
Understanding The Transformer Test Data
28
g n le E
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This picture shows the correct insulation of the core bolts Photo belongs to another transformer .
September 2012
Understanding The Transformer Test Data
29
y n a mp
Thank You i r e gine n E le
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September 2012
Understanding The Transformer Test Data
30
Understanding any p m o C Transformer g n i r e e n i g n Factory Testing E le
b o D ©
September 30, 2012
Transformer Temperature Tests
On some occasions additional methods must be employed to determine the suitability of tested transformer.
o C ng
y n a mp
i These techniques may include calculated corrections r e e n i g or multiple tests atn different loading conditions, etc. E e l ob
©D
Lets look at two actual factory cases: Case 1: Good Test Results – Bad Data Case 2: Bad Test Results – Good Transformer
Understanding Transformer Factory Testing
2
Transformer Temperature Tests
AVERAGE WINDING TEMP.
Core
le b o D
©
n i g En
WINDING HOTTEST SPOT
TEMPERATURE DISTRIBUTION
i r e e
Coils
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Top Oil Temp.
o C ng
Gradient . x H.S.F
Top Oil
Hot Spot Distance
Cooling
TOP OIL TEMP.
Average Oil
Gradient
Avg. Wdg. Temp.
Bottom Oil
Oil
Ambient
Temperature
Understanding Transformer Factory Testing
3
Case 1: Good Results – Bad Data UAT 39/52/65 MVA; 230 - 6.9 (XV) & 4.16 (YV) kV 60Hz (+15-5% LTC for YV) • •
y
Heat run test was performed accordingpa tonANSI/IEEE m ospecification. Standards and the clients technical C g
n i r e ine
g n Temperature results E were well below Standard limits e l and according Dob to client’s specification. ©
•
Very Clean DGA Results.
•
Test Results did not match design data?
Understanding Transformer Factory Testing
4
Case 1: Good Results – Bad Data
~
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Measurement System
Understanding Transformer Factory Testing
XV YV
HV
SHORT CIRCUIT
L3
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SHORT CIRCUIT
Short-Circuit Method – Three Phase, 3-Winding:
Unit Under Test
5
Case 1: Loading Cycle
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Understanding Transformer Factory Testing
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6
Case 1: Heat Runs Results Total Losses (kW ) Tap Position Average Oil Rise Top Oil Rise Winding Gradient, YV Winding Gradient, XV Winding Gradient, HV HS over TOR, YV HS over TOR, XV HS over TOR, HV Hot Spot Factor, YV Hot Spot Factor, XV Hot Spot Factor, HV Average Winding Rise, YV Average Winding Rise, XV Average Winding Rise, HV Hot Spot Rise, YV Hot Spot Rise, XV Hot Spot Rise, HV
Test 172.600 1R 42.9 49.6 10.2 2.3 3.3 11.2 2.5 3.6 1.10 1.10 1.10 53.05 45.13 46.18 60.8 52.1 53.2
©D
oble
Corrections 228.464 1R 51.2 59.2 10.2 2.3 3.2 11.2 2.5 3.5 1.10 1.10 1.10 61.3 53.4 54.4 70.4 61.7 62.7
g n E
Limit
65.0
Winding HV XV YV
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Understanding Transformer Factory Testing
Winding Current for Individual Gradient Runs
65.0
80.0
Test (A) 99.5 1757.0 2483.0
y n a mp
Rated (A) 97.9 1757.0 2483.0
Ratio 0.984 1.000 1.000
Winding Current for Oil Rise Run
Winding HV XV YV
Test (A) 97.9 2149.1 1791.5
Rated (A) 97.9 1757.0 2483.0
Ratio 1.000 1.223 0.722
Exponents 0.63 n: 0.80 m:
7
Case 1: Temp Rise not Expected Heat Run Result vs Design Data 39.0 MVA ONAN Cooling Mode Tested 228.682 Losses (kW) 59.2 Top Oil Rise 51.2 Average Oil Rise 39.8 Bottom Oil Rise YV Winding 10.2 Gradient 61.4 Average Winding Rise 11.2 Hot Spot Gradient 70.4 Hot Spot Rise XV Winding 2.3 Gradient 53.5 Average Winding Rise 2.5 Hot Spot Gradient 61.7 Hot Spot Rise HV Winding 3.2 Gradient 54.4 Average Winding Rise 3.5 Hot Spot Gradient 62.7 Hot Spot Rise
Design 228.464 51.6 39.7 27.9 12.8 52.5 14.1 65.7
Guar. 65.0
65.0
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11.1 50.8 12.2 63.8 10.5 50.2 11.6 63.2
80.0
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Why so far off?
65.0 80.0
65.0 80.0
Understanding Transformer Factory Testing
8
Case 1: Heat Runs Temp. Log Time Measured kW Measured Amps Upper Radiator 1 Upper Radiator 2
CH 14:00 171.0 101.6 2 77.43 7 77.58
Average Upper Rads Lower Radiator 1 Lower Radiator 2
1 3
Average Lower Rads Ambient # 1 Ambient # 2 Ambient # 3 Average Ambient Top Oil Temp Top Oil Temp Average of Top Oil Averge Oil Rise @ 3300' Top Oil Rise @ 3300' Bottom Oil Rise @ 3300'
15:00 173.2 102.0 78.66 78.78
16:00 172.0 103.0 79.56 79.78
17:00 171.1 101.5 80.10 80.30
18:00 173.0 102.0 81.55 81.88
19:00 172.9 102.0 82.66 82.68
20:00 173.6 100.0 83.83 84.65
21:00 172.0 99.0 84.18 85.03
22:00 172.0 99.1 84.57 85.42
23:00 172.3 99.4 84.73 85.51
0:00 172.5 99.6 84.92 85.58
1:00 172.6 99.5 85.09 86.10
2:00 Take HV 84.98 85.70
3:00 Take XV 82.27 82.91
5:00 Take YV 83.61 84.36
y n a mp
77.51 78.72 79.67 80.20 81.72 82.67 84.24 84.61 85.00 85.12 85.25 85.60 85.34 82.59 83.99 67.51 68.24 69.22 70.25 71.47 72.57 73.53 74.16 73.77 72.81 73.61 73.88 74.69 70.88 71.83 63.81 64.85 65.87 66.88 67.85 68.83 69.83 69.93 70.26 69.60 70.42 70.41 69.66 66.67 68.78
65.66 4 37.86 11 38.79 12 35.78 37.48 6 81.21 DV 78.00 79.61 36.21 42.13 28.18
g n le E
66.55 37.96 38.95 35.88 37.60 81.46 80.00 80.73 37.05 43.13 28.95
b o D ©
67.55 38.00 39.08 36.88 37.99 82.00 81.00 81.50 37.45 43.51 29.56
68.57 40.00 38.90 36.20 38.37 83.25 82.80 83.03 38.84 44.66 30.20
i r e ine
o C ng
69.66 40.75 39.20 37.88 39.28 84.52 83.00 83.76 38.46 44.48 30.38
70.70 41.27 39.56 38.95 39.93 85.88 83.50 84.69 38.78 44.76 30.77
71.68 41.64 39.72 39.81 40.39 87.95 84.00 85.98 39.31 45.59 31.29
72.05 41.94 39.52 39.78 40.41 89.16 85.00 87.08 40.39 46.67 31.63
72.02 41.70 38.11 39.33 39.71 89.98 84.00 86.99 40.79 47.28 32.30
71.21 41.35 37.85 39.06 39.42 90.47 83.95 87.21 40.83 47.79 31.79
72.02 40.92 37.25 38.81 38.99 91.07 85.00 88.04 42.42 49.04 33.02
72.15 40.53 37.40 38.41 38.78 91.36 85.40 88.38 42.88 49.60 33.37
72.18 0.00 0.00 0.00 0.00 91.12
68.78 0.00 0.00 0.00 0.00 88.60
70.31 0.00 0.00 0.00 0.00 89.87
91.12 84.54 91.12 72.18
88.60 81.69 88.60 68.78
89.87 83.03 89.87 70.31
∆T (3 Hr) > 2 ºC
Understanding Transformer Factory Testing
9
Case 1: Heat Runs Results Total Losses (kW ) Tap Position Average Oil Rise Top Oil Rise Winding Gradient, YV Winding Gradient, XV Winding Gradient, HV HS over TOR, YV HS over TOR, XV HS over TOR, HV Hot Spot Factor, YV Hot Spot Factor, XV Hot Spot Factor, HV Average Winding Rise, YV Average Winding Rise, XV Average Winding Rise, HV Hot Spot Rise, YV Hot Spot Rise, XV Hot Spot Rise, HV
Test 172.600 1R 42.9 49.6 10.2 2.3 3.3 11.2 2.5 3.6 1.10 1.10 1.10 53.05 45.13 46.18 60.8 52.1 53.2
©D
oble
Corrections 228.464 1R 51.2 59.2 10.2 2.3 3.2 11.2 2.5 3.5 1.10 1.10 1.10 61.3 53.4 54.4 70.4 61.7 62.7
g n E
Limit
65.0
Winding HV XV YV
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Understanding Transformer Factory Testing
Winding Current for Individual Gradient Runs
65.0
80.0
Test (A) 99.5 1757.0 2483.0
y n a mp
Rated (A) 97.9 1757.0 2483.0
Ratio 0.984 1.000 1.000
Winding Current for Oil Rise Run
Winding HV XV YV
Test (A) 97.9 2149.1 1791.5
Rated (A) 97.9 1757.0 2483.0
Ratio 1.000 1.223 0.722
Exponents 0.63 n: 0.80 m:
10
Case 1: Summary • Stable oil temperatures must be met to achieve reasonably accurate winding gradient measurements.
y • Accurate cold resistance temperature measurements n a prises. m are critical in determining the winding o C g rin 1R – Cold resistance not YV hot resistance onee Tap n i g measured, used En Tap 1N Not valid. le b o ©D
• Simulated load losses should be close to the expected load losses for the transformer during operation. Actual winding currents not measured during simultaneous loading. Understanding Transformer Factory Testing
11
Case 2: Good Unit – Bad DGA GSU 820 MVA 362 / 25 kV DETC(±5%) 60Hz • • •
Heat run test was performed according to IEEE/ANSI y n a Standards and the clients technical specification. p
om C ng
i r e Temperature results were e below limits and according to n i g n clients requirements. E le b o ©D DGA performed after heat run test found gas generation above client and the manufacturer’s acceptance limits.
Understanding Transformer Factory Testing
12
Case 2: Bad DGA Results
Outside Lab In House Lab Sample # 1 2 3 Before Heat 4 hours after 4 hours after Description Run [ppm] Heat Run [ppm] Heat Run [ppm] H2 - Hydrogen 4 22 17 O2 - Oxygen 3989 2314 300 N2 - Nitrogen 11045 12181 9050 CO - Carbon Monoxide 10 62 50 CO2 - Carbon Dioxide 82 392 250 CH4 - Methane 0 3 14.4 C2H4 - Ethylene 0 5 4 C2H6 - Ethane 0 27 22 C2H2 - Acetylene 0 0 0
g n le E
b o D ©
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Change Gas Evolution [ppm] 13 40 168 14.4 4 22 0
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Client Limits Gas Evolution [ppm] 10 25 200 5 2 2 0
All measured temperature rises within calculated tolerances. Understanding Transformer Factory Testing
13
Case 2: Possible Causes
1. Bad DGA Sample
o C ng
5. TX Hot Spot
i r e ine
g n Why ? E le
b o D ©
4. Stray Gassing
Understanding Transformer Factory Testing
y n a mp
2. Pump Problem
3. Improper Testing
14
Case 2: Possible Causes 1.
2.
Bad DGA Data? • DGA results of the outside lab matched the results obtained at factory.
y n a mp
Bad Pump (s) ? o C g n i r The most probable causeeof pump overheating is the e n i g n pump running backwards. E e l bratings matched Nameplate o • Running D © • No Noise • Thermal Scan normal for pumps and oil flow
Understanding Transformer Factory Testing
15
Case 2: Possible Causes 3.
Improper Testing Method ? • • • • • •
Loading per IEEE/Expedited Heating y Fiber Optic Sensors in Coils – No high temperatures n a p m o Thermocouples on structural C metal parts – Normal g n i r heating e e n i g n E Ambient temperature was below 40 ºC e l b o D Total©heat load (kW) matched cooler rating Maximum current was only 112% of rated/ Less than 7 percent of allowable continuous overload current.
Understanding Transformer Factory Testing
16
Case 2: Possible Causes Only two possible causes left: 4.
An oil problem due to “thermal stray gassing”.
Or
5.
o C ng
ri spot. An abnormal transformeree hot le b o ©D
n i g En
Understanding Transformer Factory Testing
y n a mp
An experiment is needed!
17
Case 2: Experimental Loading
y n a mp
How is the gassing influenced ? • •
r e Load dependent e in
i
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g n E e l Oilotemperature dependent b ©D
Understanding Transformer Factory Testing
18
Case 2: Experimental Loading
Step A: Transformer Rated Conditions
y n a mp
o C 1. Test Floor Open and Ventilated g n i r e e n 2. All Pumps & Fans i On g n EConditions of the transformer e l 3. Full Rating ob ©D
Understanding Transformer Factory Testing
19
Case 2: Experimental Loading
Step B: Simulate Stray Gassing
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1. Test Floor Closed o C g n i r e 2. Reduced Current e n i g n E adjusted to keep oil at ~ 90 ºC 3. All Pumps on/leFans
b o D ©
Understanding Transformer Factory Testing
20
Case 2: Experimental Method
Test Number Test #1 Test #2 Test #3 Test #4
Load Condition
Oil Comments Temperature y n a p The source of gassing This is the intial heat run result. High Oil m o Overload C is indeterminate. Temperature g n i r e under this condition is most likely not from the Normal Oil inGassing e Rated Load g n oil. Temperature E e l b High Oil Gassing under this condition is most likely not from the o D © Load Temperature Reduced transformer. Normal Oil Gassing is most likely from the transformer. Overload Temperature
Understanding Transformer Factory Testing
21
Case 2: Experimental Method Test Duration Number [Hours] 8.0 Test #1 1.0 Test #2
8.0
Test #3
8.0
Test #4
Criteria
Loading
Total Heat Load Rated Per IEEE Current Rated Current Reduced Curr. Per IEEE
le b o ©D
n i g En
Load [%]
Top Oil Temp.
Coil Oil Temp.
Ambient [ºC]
108.0
73.0
90.3
37.6
100.0
70.0
y n 92.3 a mp
38.4
78.8
24.0
o C g 53.5 100.0 n i r ee 80.0
83.5
92.6
35.2
7.5
Per IEEE
Total Heat Load
108.0
62.5
87.8
26.0
1.0
Per IEEE
Rated Current
100.0
59.7
81.0
26.0
Understanding Transformer Factory Testing
22
Case 2: Experiment Results Test #
H2 - Hydrogen O2 - Oxygen N2 - Nitrogen CO - Carbon CO2 - Carbon Dioxide CH4 - Methane C2H4 - Ethylene C2H6 - Ethane C2H2 - Acetylene CO2/CO Ratio
1 Gas Evolution [ppm] 13 40 168 14.4 4 22 0 4.2
le b o ©D
2 Gas Evolution [ppm] 0 10 66 1.8 0.6 0 0 6.6
n i g En
Understanding Transformer Factory Testing
3 Gas Evolution [ppm] 12 36 272 8 1.7 13.6 0 7.6
g n i r ee
p m o C
4 Gas Evolution [ppm] 3 12 105 4.2 1.3 0 0 8.8
any
Criteria 10 25 200 5 2 2 0 <3
23
Case 2: Test #1 Results
• Most of the gas concentrations exceed the customer limits. y n a pand Hydrogen • Dominant gasses are Methane, Ethane m o C g (low temperature gasses or rthermal stray gassing). n i e e n • No cellulose decomposition. ngi
E e l ob
©D
The Source of the Excessive Gassing is Indeterminate.
Understanding Transformer Factory Testing
24
Case 2: Test #2 Results
• Gassing, all gasses within acceptance limits. y n a • No dominant gasses. omp •
C g n i r e e No cellulose decomposition. n i g n E le b o ©D
If there was excessive gassing it would likely be from the transformer active parts. Understanding Transformer Factory Testing
25
Case 2: Test #3 Results
• • • •
A gasses exceeding limits except Ethylene. Dominant gasses are Methane, Ethane and Hydrogen. y n a p No cellulose decomposition. om C gno gassing can be n i In this test the load is reduced r e e n i correlated with the transformer. g n E e l b are Methane, Ethane and Hydrogen, • Dominant D gasses o this is an©indication of possible thermal stray gassing. • The gassing results are similar Test #1.
This excessive gassing is likely from the oil. Understanding Transformer Factory Testing
26
Case 2: Test #4 Results • All gasses within acceptance limits. • Dominant gasses are Methane, Ethane and Ethylene. Typical gasses for a heat run test without additional stray y n a p gassing. om C g n i • No cellulose decomposition. r e e n i g n • The absence of gasses E confirm that gas generation is e l b load or a transformer condition. not related D toothe
©
If there was excessive gassing it would likely be from the transformer active parts. Understanding Transformer Factory Testing
27
Case 2: Summary
• •
•
The transformer successfully passed the heat run test according to ANSI/IEEE Standards.
y n a mp
Test #3 results closely match with oTest #1 and are C g indicative that the source ofrigasses during heat run n e test is thermal stray gassing gine of the oil.
n E le
b o D Gasses© generated during heat run test performed are
produced by thermal stray gassing of the oil used for FAT. •
The Doble Oil Lab confirmed the stray gassing tendency of the oil used for the factory heat run.
Understanding Transformer Factory Testing
28
o C ng
i r e e CONCLUSION n i g n E oble
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©D
Understanding Transformer Factory Testing
29
