Understanding Power Transformer Factory Test Data
Short Description
Power Transformer Factory Test Data...
Description
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
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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
©D Design
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
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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 1.56 s and T2 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 (
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