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Understanding
Power Transformer
Factory Test Data
Mark F. Lachman
Doble Engineer ing Company
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OVERVIEW OF PRODUCTION TESTS
Core/coil: ratio,
Iex, core-to-gnd
Core/coil after VP:
Iex, core-to-gnd
SU: ratio, Rdc, Iex,
no-load/load loss,
sound, core-to-gnd
Tanking: ratio, core-to-gnd,
in-tank CTs - polarity, ratio,
saturation
CTs on cover: polarity,
ratio, saturation
PA: loss, sound,
core-to-gnd
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Class I includes power transformers with
high-voltage windings of 69 kV and below.
Class II includes power transformers withhigh-voltage windings from 115 kV through
765 kV.
SYSTEM VOLTAGE CLASSIFICATION
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Routine tests shall be made on every
transformer to verify that the product
meets the design specifications.
Design tests shall be made on atransformer of new design to determine
its adequacy.
Other tests may be specified by thepurchaser in addition to routine tests.
GENERAL CLASSIFICATION OF TESTS
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TEST TYPE PERFORMANCE DIELECTRIC MECHANICAL
Routine
Winding resistance Winding insulation resistance(Other )
Leak
Ratio/polarity/phase
relation
Core insulation resistance
(Other )
No-load losses and
excitation current
Insulation PF/C
(Other)
Load losses and
Impedance voltage
Dielectric withstand of control
and CT sec. circuits (Other )
Operation of all
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 I in red if
different from Class II
Class II < 345 kV
is also Other
OVERVIEW OF TESTS
PD is Other for
Class I only
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TEST TYPE PERFORMANCE DIELECTRIC MECHANICAL
Design/
Other
Temperature rise
Audible sound level
Other
Short-circuit
capability
Single-phase
excitation current
Front-of-wave
impulse
Design
Lifting and
moving
Pressure
OVERVIEW OF TESTS (cont.)
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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
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
Switching impulse IEEE C57.12.90-2010 clauses 10.1, 10.2
IEEE C57.12.00-2010 clauses 5.10, 8.2
IEEE C57.12.98-1993; IEEE Std. 4-1995
Lightning impulse IEEE C57.12.90-2010 clauses 10.1, 10.3
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/PDIEEE 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
SEQUENCE OF TESTS
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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 IEEE PC57.130/D17
ONAF 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 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.)
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TEST* REFERENCE
Insulation PF/C andresistance
IEEE C57.12.90-2010 clauses 10.10, 10.11IEEE C57.12.00-2010 clause 8.2
Single-phase exciting
current
IEEE C57.12.00-2010 clause 8.2Lachman, M. F. “Application of Equivalent-Circuit Parameters to
Off-Line Diagnostics of Power Transformers,” Proc. of the Sixty-
Sixth Annual Intern. Confer. of Doble Clients, 1999, Sec. 8-10.
Sweep frequency response
analysis IEEE PC57.149™/D8, November 2009
Dielectric withstand of control
and CT secondary circuitsIEEE 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
SEQUENCE OF TESTS (cont.)
*Discussion of tests listed on this slide and DGA is not included in this presentation.
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Tests to be discussed:
Ratio/polarity/phase relation
Winding DC resistance
No load losses and excitation current
Dielectric tests
Load losses and impedance voltage
Temperature rise
Zero-phase sequence impedance
Audible sound level
DISCUSSION OUTLINE
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For each test discussion includes:
Definition and objective
Physics
Setup and test methodology
Acceptance criteria*
Abnormal data
Recourse if data abnormal
Comparison with field data (if relevant)
DISCUSSION OUTLINE (cont.)
*This discussion is based on requirements of referenced standards. If customer test specification
contains requirements different from those in standards, more stringent requirements prevail.
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RATIO, POLARITY, PHASE
RELATION(Routine)
RATIO POLARITY PHASE RELATION:
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Definition: The turns ratio of a transformer is the ratio of
the number of turns in the high-voltage winding to that inthe low voltage winding.
Objective: The turns ratio polarity and phase relation test
verifies the proper number of turns and internal
transformer connections (e.g., between coils, to LTC, tovarious switches, to PA, series auto- or series
transformer) and serves as benchmark for later
assessment of possible damage in service.
The transformer nameplate voltages should reflect theactual system requirements. Therefore, it is important
that the nameplate drawing is approved by the customer
at the design stage.
RATIO, POLARITY, PHASE RELATION:
DEFINITION AND OBJECTIVE
RATIO POLARITY PHASE RELATION:
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3T 2T 2V
VR = 3V/2V = 1.5
TR = 3T/2T = 1.5In ideal transformer:
TR = VR
3T 2T 1.96V
Volts per turn = 3V/3T = 1V/T
F
3V
Volts per turn = 2.95V/3T = 0.98V/T
2.95V
F
VR = 3V/1.96V = 1.53
TR = 3T/2T = 1.5
= 100(1.5 – 1.53)/1.5 = –2%
0.05V
3V
RATIO, POLARITY, PHASE RELATION:
PHYSICS
3V
In actual transformer
Turns ratio Voltage ratio
due to accuracy of themeasurement and the
voltage drop in the high-
voltage winding.
RATIO, POLARITY, PHASE RELATION:
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Ratio = N1 /N2 = R1 /R2
Polarity is determined via
phase angle between two
measured waveforms.
Phase relation is confirmed
by testing the correspondingpairs of windings.
Tests shall be made
1. at all positions of DETC
with LTC on the rated
voltage position
2. at all positions of LTC with
DETC on the rated voltage
position
3. on every pair of windings
R2
Balanceindicator
N2
N1
R1
H1 X0
H2
X2
Transformer in test
RATIO, POLARITY, PHASE RELATION:
SETUP AND TEST METHODOLOGY
RATIO, POLARITY, PHASE RELATION:
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, ,
ACCEPTANCE CRITERIA
X1
H1
H2
H3
X2
X3
X0
Voltage ratio =
VH2-H1 /VX2-X0 =
138/(13.2/3) = 18.108
13.2
138
With the transformer at no load and with rated voltage on
the winding with the least number of turns, the voltages ofall other windings and all tap connections shall be within
0.5% of the nameplate voltages.
For three-phase Y-connected windings, this tolerance
applies to the phase-to-neutral voltage. When the phase-to-neutral voltage is not explicitly marked on the nameplate,
the rated phase-to-neutral voltage shall be calculated by
dividing the phase-to-phase voltage markings by 3.
RATIO, POLARITY, PHASE RELATION:
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, ,
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
Deviation
100(RATIONP - RATIOTURN)/RATIONP =
Rounding off
NP voltages
creates error
Measurement RATIOMEAS
Deviation
100(RATIONP - RATIOMEAS)/RATIONP 0.5%
Measurement
introduces
error
NP voltages need to be
selected to keep well
within 0.5% (e.g., 0.2-
0.4). This assures that
measurement errorkeeps RATIOmeas within
0.5% of RATIONP.
RATIOTURN
RATIONP
RATIOMEAS
RATIO, POLARITY, PHASE RELATION:
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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,
under these conditions the standard allows for deviation
from the NP voltage ratio to exceed the 0.5% limit.
Check if transformer is a duplicate of a legacy unit.
Review design data to determine if the NP voltagesselected by designer create a ratio that is too far ( is
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:
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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.
Even within 8-200 V range, the results vary somewhat
with voltage and between different instruments.
Initial field test should be performed at the same test
voltage as the factory test with results compared with the
NP voltages and for all subsequent tests the comparison
should be made with the initial test.
The objective of the high-voltage (e.g., 10 kV) test withexternal 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.
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WINDING DC RESISTANCE(Routine)
WINDING DC RESISTANCE:
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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:
Calculation of the I2R component of conductor losses.
Calculation of winding temperatures at the end of a
temperature rise test.
Quality control of design and manufacturing processes.
Benchmark used in field for detection of open circuits,
broken strands, deteriorated brazed and crimpedconnections, problems with terminations and tap
changer contacts.
WINDING DC RESISTANCE:
PHYSICS
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PHYSICS
i R
Domain
External
field
WINDING DC RESISTANCE:
PHYSICS ( )
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PHYSICS (cont.)
d /dt
F = y N
v meas= iR + d /dt
R=v meas
/ i
d /dt
d /dt
d /dt
d /dt
d /dt
d /dt
WINDING DC RESISTANCE:
PHYSICS ( t )
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PHYSICS (cont.)
Time to stabilize resistance reading: On some units with closed
loops (e.g., GSU with two LV deltas or units with parallelwindings), 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
reasonable time. However, as the core is being magnetized the
changing flux in the core induces voltage and sets upcirculating currents in closed loops. After the core is saturated,
there is no more induced voltage to sustain them, and the
currents begin to subside. This process, however, is associated
with LC oscillations with long time constant and may take up to
45 min to dissipate the energy. The flow of these currentscontinues 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
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SETUP AND TEST METHODOLOGY
Data must be taken onlywhen reading is stable.
The time to stabilize the
reading depends on the
unit, varying from
seconds to minutes. Standard requires
measurements of all
windings on the rated
voltage tap and at the tap
extremes of the first unit
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.
Transformer in test
H2
H1
H3
H0
IdcVdc
+
+
Currentoutput
Voltage
input
WINDING DC RESISTANCE:
ACCEPTANCE CRITERIA
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ACCEPTANCE CRITERIA
Standards give no acceptance criteria; however, a deviation
from average of three phases of 0.5% for HV and 5% for LVcould serve as practical guideline.
As important as deviation is the assurance that test data is
credible:
No excitation with no pumps - 3h and with pumps - 1h,
TTO variation 2C for 1h, and TTO-TBO 5C. This assuresthat oil T represents conductor T; without reference T resistance data has a limited value.
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 )
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ACCEPTANCE CRITERIA (cont.)
T stability: Experience* in the industry suggests that
relying on the T stability requirements given in the IEEEstandard does not produce a needed thermal equilibrium
and, consequently, an accurate measurement of the
winding dc resistance. To have a reliable data, the unit
should be subjected to no excitation for 2-3 days. Hence, if
the time to begin testing is of essence, it is notunreasonable to agree to using resistance data available at
that time (assuming the IEEE T requirements have been
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
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ABNORMAL DATA
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 3.5622
4 3.4714 3.4698 3.5746
5 3.3838 3.3814 3.3870
High-voltage winding
Tested Calc. % of calc.
20.9832 21.47937 97.7
20.4889 20.97440 97.7
19.9932 20.46944 97.7
19.6873 19.96448 98.6
19.0065 19.45952 97.7
Average
3.7360 0.03% 0.02% -0.05%
3.6480 0.03% 0.03% -0.06%
3.5597 0.02% 0.05% -0.07%
3.5053 0.97% 1.01% -1.98%
3.3841 0.01% 0.08% -0.09%
Deviation from average
0.6185 0.03842 99.7
0.5855 21.47937 100.6
0.16519 -0.11% -0.01% 0.12%
0.15637 -0.12% 0.00% 0.12%
LTC X1-X0 X2-X0 X3-X0
16 0.16537 0.16521 0.16499
N 0.1566 0.1564 0.1562
Low-voltage winding
Comparison of each measurement with theaverage 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
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RECOURSE IF DATA ABNORMAL
If requirements associated with transformer thermal stability,
dc test current, influence of series unit or stability of thereading are not met, a retest under different conditions
should be requested.
If acceptance criteria is exceeded, a justification from the
manufacturer should be requested. Potential problems may
include: bad crimping or brazing, incorrect conductor crosssection, loose connection, wrong design calculations.
WINDING DC RESISTANCE:
COMPARISON WITH FIELD DATA
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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
requested from the factory.
Comparison should be performed for readings referred to thesame T.
The field measurement should be performed at the same test
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 ismeasured via thermocouples and in the field the T gauge is
frequently the best option).
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NO-LOAD LOSSES AND
EXCITATION CURRENT(Routine)
NO-LOAD LOSSES AND EXCITATION CURRENT:DEFINITION AND OBJECTIVE
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Definition: No-load losses include core loss, dielectric
loss, and conductor loss due exciting current, includingcurrent circulating in parallel windings. Excitation current
is flowing in any winding exciting the transformer with all
other windings open-circuited.
Objective: No-load losses and excitation current, measured at specified voltage and frequency, provide the
data for:
Verification of design calculations.
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
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I R
F
V
B
H
Hysteresis losses
Ph = f(Bmax)Bmax = f(Vave)
F
Ieddy
Eddy
losses
Pe = f(V
2
rms)
PNL = Pe + Ph
Domain
rotation
NO-LOAD LOSSES AND EXCITATION CURRENT:SETUP AND TEST METHODOLOGY
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Start with 110% on N. As unitdemagnetizes, losses drop.
Test at 100% Vrated on N, max turn
bridging position with inductive
LTC, and 16R if LTC with series
unit. Vave gives same Bmax as Vrms when
wave-shape is a perfect sin; set
based on Vave average of 3 phases
Pe is corrected for rated Vrms
Voltmeters should measure samevoltage as seen by xfmr.
PNL not corrected for T if TTO-TBO
5C and 10TO_ave30C
Iexc=aver. of 3 phases in % of Irated
CT
VT
Vrms
W
I
V
Transformer in test
X0
H2 X1
X2
X3
H1
H3
Vave
A
*
*
*Vrms and Vave (calibrated in rms) will show
the same voltage if perfect sine wave.
3
NO-LOAD LOSSES AND EXCITATION CURRENT:SETUP AND TEST METHODOLOGY (cont.)
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Frequency control of motor-generator sets at GE large
transformer plant in
Pittsfield, MA during the
early 1900. Since the
primary function of thesegenerators was to provide
power for no-load loss
tests, they were often
referred to as magnetizers.
Historical perspective
Courtesy IEEE Power & Energy Magazine
NO-LOAD LOSSES AND EXCITATION CURRENT:ACCEPTANCE CRITERIA
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Measured no-load losses should not exceed the
guaranteed value by more than 10% and the total losses bymore than 6%.
Assurance that test data is credible:
Test voltage is set based Vave
If oil T is not within limits, correction is applied
Frequency is within +/-0.5% of rated Distortion 5%. The 5% limit that standard allows for
distortion of the voltage waveform is too liberal.* The
limit applies to the difference between the measured kW
and kW corrected for eddy loss due to the difference
between Vrms and Vave. To monitor the quality of thevoltage 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.)
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Test in parallel and series configurations, if present.
If PA is present, compare the loss difference betweennon-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 and 16R with
loss measured in SU out-of-tank.
Test system accuracy should be within +/-3% for loss,+/-0.5% for voltage and current, and +/-1.5C for T.
NO-LOAD LOSSES AND EXCITATION CURRENT:ABNORMAL DATA
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Potential reasons for exceeding the guaranteed values may
include:
Variability in core steel characteristics
Different core steel
Oversights in design
Production process related factors or mistakes
Problems with windings (e.g., s. c. turn)
Wrong connection of preventative autotransformer orseries transformer or series autotransformer
Example: guaranteed no-loss - 28 kW, measured – 35 kW
NO-LOAD LOSSES AND EXCITATION CURRENT:RECOURSE IF DATA ABNORMAL
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Failure to meet the no-load test loss tolerance should
not warrant immediate rejection but shall lead toconsultation between purchaser and manufacturer
regarding further investigation of possible causes and
the consequences of the higher losses.
The acceptance criteria of 10% does not replace the
manufacturer’s guarantee of losses for economic loss
evaluation purposes.
NO-LOAD LOSSES AND EXCITATION CURRENT:COMPARISON WITH FIELD DATA
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Factory no-load losses and excitation test is performed
at rated voltage and three-phase excitation. Since theopen-circuit magnetizing impedance of a transformer is
non-linear, i.e., it is changing with applied voltage, a
comparison of exciting current and losses test results
obtained at low-voltage (e.g., 10 kV) and single-phase
excitation with results of the factory no-load losses andexcitation test is not possible.
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DIELECTRIC TESTS
DIELECTRIC TESTS:DEFINITION AND OBJECTIVE
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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 impulses
low-frequency tests: applied and induced/PD tests
Objective: Dielectric tests demonstrate:
compliance with 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.
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HIGH-FREQUENCY:
LIGHTNING IMPULSE(Class I - design or other,
Class II - routine)
HIGH-FREQUENCY - LIGHTNING IMPULSE:OBJECTIVE
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Demonstrate performance under transient high-frequency
conditions caused by lightning.
Surge of energy, from lightning striking
transmission line, travels to substation
and enters a transformer - full wave.
kV
s
Surge of energy, from lightning striking transmissionline, 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).
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).
HIGH-FREQUENCY - LIGHTNING IMPULSE:PHYSICS
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length
V
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.
Cg /Cs
Cg Cs
V
Full wave can be simulatedby discharging capacitor
while chopped wave by the
operation of a gap triggered
to flashover at required time.
HIGH-FREQUENCY - LIGHTNING IMPULSE:PHYSICS (cont.)
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* 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.
Region A* - turn-to-turn
insulation at line is tested by
FOW impulse, with stress
>10turns**.
Region B – disk-to-disk, and
layer-to-layer insulation (and
turn-to-turn) is tested by FW& CW impulse, with stress 5-
10turns.
Region C – insulation across
taps is tested by FW & CW
impulse, with stress 5-
10turns.
LV H1
A
B
C
A
B
B
B
B
C
H1HV
to DETC
H0
HIGH-FREQUENCY - LIGHTNING IMPULSE:PHYSICS (cont.)
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Cg CT
Rs
Rp
Charge of Cg – generator
capacitors are charged from
external DC source.
*CT includes preload capacitor.
Discharge into Rp – energy
from xfmr is discharged into
generator, reducing voltage
at tested terminal.
VT FW
Cg CT
Rs
Rp
Discharge at chop –
energy from xfmr is
discharged into chopping
gap, reducing voltage at
tested terminal to zero.
VTCW
Cg CT
Rs
Rp
FOW
Discharge into C*T –
energy from generator
capacitors is discharged into
xfmr, raising V at tested
terminal to crest level.
VT
Cg CT
Rs
Rp
HIGH-FREQUENCY - LIGHTNING IMPULSE:SETUP AND TEST METHODOLOGY
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Full Wave Parameters
MagnitudeFW = BIL +/- 3%
RFW = 50-70% BIL
T1 = 1.67T
1.2 s +/- 30%
0.84 ÷ 1.56 s
T2
50 s +/- 20%
40 ÷ 60 s
5%
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.
0.9
0.3
1.0
T1
0.5
T2
t
V
Crest voltage
Half voltage
Virtual
origin
T
HIGH-FREQUENCY - LIGHTNING IMPULSE:SETUP AND TEST METHODOLOGY (cont.)
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Increase of series (front) resistor Rs
increases the time of voltage rise - T1.
0% change from given Rs
CgCT
Rs
Rp
Data courtesy Reto Fausch, Haefely
HIGH-FREQUENCY - LIGHTNING IMPULSE:SETUP AND TEST METHODOLOGY (cont.)
Increase of parallel (tail) resistor R increases
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Increase of parallel (tail) resistor Rp increases
the time of voltage decline to half value - T2.
Data courtesy Reto Fausch, Haefely
0% change from given Rp
CgCT
RsRp
HIGH-FREQUENCY - LIGHTNING IMPULSE:SETUP AND TEST METHODOLOGY (cont.)
Increase of series (front) resistor R decreases
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Data courtesy Reto Fausch, Haefely
CgCT
Rs
Rp
Increase of series (front) resistor Rs decreases
the voltage trace overshoot - .
HIGH-FREQUENCY - LIGHTNING IMPULSE:SETUP AND TEST METHODOLOGY (cont.)
Ch d W P t
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Chopped Wave Parameters
Magnitude CW = 1.1BIL+/- 3%
T1
1.2 +/- 30%
0.84 ÷ 1.56
TC
BIL [kV] Class I Class II
30 1.0
2.045÷75 1.595 1.8
110 2.0
125 2.3 2.3
150 3.0
TC < 6.0
30%
1
See C57.12.90-2010 for instanceswhen could be >30% and >1s. It
also permits adding resistors in
chopping gap circuit to limit .
All times in the table are in s.
0.9
0.3
1.0
T1 TC
t
V
1.0
0.7
0.1
HIGH-FREQUENCY - LIGHTNING IMPULSE:SETUP AND TEST METHODOLOGY (cont.)
1 0
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Front-of-Wave Parameters
Magnitude C57.12.00-2010
Annex ATC
30%
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 C57.12.00-
1980, for historical reference.
0.9
0.3
1.0
TC
t
V
HIGH-FREQUENCY - LIGHTNING IMPULSE:SETUP AND TEST METHODOLOGY (cont.)
Very high di/dt induces difference
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Currentshunt and
meas. circuit
i(t)
RG
LG
Glaninger: T2
T2
Impulsecontrol &
measuring
system
Voltage divider
and measuring
circuit
v(t)
Cg
Rs
Rp
Impulse
generator LT, CT
xfmr
y g
of potential. Hence, it is very
important for all return and
grounding leads to be made asshort as possible, with a minimum
R and L.
Chopping gap
and preload
capacitor 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.
HIGH-FREQUENCY - LIGHTNING IMPULSE:SETUP AND TEST METHODOLOGY (cont.)
Line terminal in Y Line terminal in Neutral terminal in Y
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Line terminal in Y
i(t )
Line terminal in
i(t )
Neutral terminal in Y
i (t )
i (t )
LV line terminal
in AutoHV line terminal
in Auto
i (t ) i (t )
Neutral terminal
in Auto
HIGH-FREQUENCY - LIGHTNING IMPULSE:SETUP AND TEST METHODOLOGY (cont.)
Test sequence and trace comparison
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q p
Test is performed with minimum effective turns in the winding under
test, e.g., DETC = 5, LTC = 16L.
Standard:
RFW@ 50-70% BIL
CW 1
CW 2
FW
With FOW:
RFW@ 50-70% BIL
FOW 1
FOW 2
CW 1
CW 2
FW
With non-linear
protective devices:
RFW 1
RFW 2 @ 75-100% of BIL
to demonstrate growing
sensitivity to V
FW 1
CW 1
CW 2
FW 2RFW 3 @ RFW2 voltage
RW 4
Neutral:
RFW@ 50-70% BIL)
FW1
FW2
HIGH-FREQUENCY - LIGHTNING IMPULSE:ACCEPTANCE CRITERIA
If test equipment and tested transformer were perfectly linear, the
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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 insulationfailure, discrepancies occur. Identifying their nature is the
objective of impulse data analysis.
T1, T2, Tc, voltage magnitude, , must meet requirements.
RFW and FW voltage and current traces should compare; request
to zoom in on any areas of concern. If available, comparison of Transfer Function (TF) for RFW and FW
is used as additional diagnostic criteria. It removes sensitivity to
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.)
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450 kV BIL, RFW on
HV winding – voltage
450 kV BIL, RFW on
HV winding – current
HIGH-FREQUENCY - LIGHTNING IMPULSE:ACCEPTANCE CRITERIA (cont.)
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450 kV BIL, CW1 on
HV winding – voltage
450 kV BIL, CW2 on
HV winding – voltage
HIGH-FREQUENCY - LIGHTNING IMPULSE:ACCEPTANCE CRITERIA (cont.)
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Overlay of 450 kV BIL
CW1 and CW2 - voltage
HIGH-FREQUENCY - LIGHTNING IMPULSE:ACCEPTANCE CRITERIA (cont.)
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450 kV BIL, FW on
HV winding – voltage
450 kV BIL, FW on
HV winding – current
HIGH-FREQUENCY - LIGHTNING IMPULSE:ACCEPTANCE CRITERIA (cont.)
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Overlay of 450 kV BIL
RFW and FW - voltage
HIGH-FREQUENCY - LIGHTNING IMPULSE:ACCEPTANCE CRITERIA (cont.)
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Overlay of 450 kV BIL
RFW and FW - current
High-frequency oscillations at
the beginning of current trace
are acceptable deviations,
reflecting the test setup.
HIGH-FREQUENCY - LIGHTNING IMPULSE:ACCEPTANCE CRITERIA (cont.)
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450 kV BIL, FOW1 on
HV winding – voltage
450 kV BIL, FOW2 on
HV winding – voltage
HIGH-FREQUENCY - LIGHTNING IMPULSE:ACCEPTANCE CRITERIA (cont.)
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Influence of non-linear protective
device on overlay of RFW and FW
350 kV BIL voltage traces
illustrates the need for comparing
traces of the same voltage level.
HIGH-FREQUENCY - LIGHTNING IMPULSE:ACCEPTANCE CRITERIA (cont.)
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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
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g , p p
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
impulses need to be applied, including several FW. If the deviation
increases in magnitude, it indicates progressive dielectric failure in the
transformer. Unusual sounds, emanating from inside the tank, should be noted; these
sounds may be helpful in locating general location of the fault.
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.)
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Overlay of 550 kV BIL RFW and FW
voltage traces – turn-to-turn failure
HIGH-FREQUENCY - LIGHTNING IMPULSE:ABNORMAL DATA (cont.)
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Overlay of 550 kV BIL RFW and FW
current traces – turn-to-turn failure
HIGH-FREQUENCY - LIGHTNING IMPULSE:ABNORMAL DATA (cont.)
Voltage drop to ground
indicates one of the leads
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Overlay of 200 kV BIL RFW and FW
traces – lead-to-lead failure
between RV and main LV windings
RFW voltage
FW voltage
FW current
RFW current
was at ground potential
Fault to ground diverts
current around winding,
reducing measured current.
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HIGH-FREQUENCY:
SWITCHING IMPULSE
(Class I – other,
Class II <345 kV – other,
Class II 345 kV - routine)
HIGH-FREQUENCY - SWITCHING IMPULSE:OBJECTIVE
Demonstrate performance under transient high-frequency conditions
created by switching operations or network disturbance.
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kV
s
FOW
CW
FW SW
y g p
Surge of energy from equipment
switched on or disturbance on the power
system. The time to reach the crest
amplitude and the total time duration of
switching impulses are much longer thanthose of lightning impulses.
HIGH-FREQUENCY - SWITCHING IMPULSE:PHYSICS
Switching impulse test consists
of applying or inducing a SW
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length
V
Comparing to lightning impulse, the
switching impulse has a much
longer duration and lowerfrequency, resulting in voltage
approaching a uniform distribution
of the low-frequency steady-state
voltages, i.e., voltage distributes as
per turns ratio.
V
of applying or inducing a SW
between each HV line terminaland ground. Similar to a
lightning wave, the switching
wave can be simulated by
discharging a capacitor.
HIGH-FREQUENCY - SWITCHING IMPULSE:PHYSICS (cont.)
LV HVD D
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*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.
LV
DH
1
HV
Region D – phase-to-
ground and phase-to-
phase insulation is
stressed the most; stress
imposed by SW is1turns*.
Charging and discharging
processes are similar to
those described for
lightning impulse.
H1
H0
D
To another
phase
HIGH-FREQUENCY - SWITCHING IMPULSE:SETUP AND TEST METHODOLOGY
Full Wave Parameters
SW = 0 83BIL +/ 3%Crest voltage
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Magnitude
SW = 0.83BIL +/- 3%
RSW=(50-70%)0.83BIL
Tp >100 s
Td 200 s
T0 1000 s
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.
0.9
1.0
TpTd
t
V
First zero crossingT0
Virtual
origin
>90% of crest
HIGH-FREQUENCY - SWITCHING IMPULSE:SETUP AND TEST METHODOLOGY (cont.)
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Xfmr
Impulse
control &
measuringsystem
Voltage divider
and measuring
circuit
v(t)
Cg
Rs
Rp
Impulse
generator
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.)
E ELV E ELV
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ELV /2
Note: The choice of tap connections for all
windings is made by the manufacturer.
Test sequence and trace
comparison:
RSW@ 50-70% SW
(+) RSW - bias
SW1
(+) RSW - bias
SW2
RFW@ 50-70% BIL
CW 1
CW 2
FW
E/2 E/2-ELV /2
E
-ELV /2
ELV
-E/2
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
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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,
the core is magnetized in opposite direction by applying RSW (or
small dc current) of opposite polarity .
t
V
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.
HIGH-FREQUENCY - SWITCHING IMPULSE:ACCEPTANCE CRITERIA
Tp, Td, T0, and voltage magnitude must meet requirements.
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Failure detection is done primarily by scrutinizing voltagetraces for recognizable indications of failure. The test is
successful if there is no sudden collapse of voltage as
indicated on the trace.
Although overlaying RSW and SW traces in totality may
not be practical, the traces should match until the pointwhere the difference in the core magnetic state becomes
obvious. Normally, these differences can be easily
distinguished from drastic voltage reduction caused by a
failure.
Verify that DGA results (after dielectrics) are normal.
HIGH-FREQUENCY - SWITCHING IMPULSE:ACCEPTANCE CRITERIA (cont.)
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650 kV BIL, RSW on
HV winding – voltage650 kV BIL, SW1 on
HV winding – voltage
650 kV BIL, SW2 on
HV winding – voltage
Typical reduced and full
switching impulse voltage
traces as measured on the HVwinding; for 650 kV BIL, the
BSL, i.e., the required test
voltage, is 540 kV.
HIGH-FREQUENCY - SWITCHING IMPULSE:ACCEPTANCE CRITERIA (cont.)
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Overlay of 650 kV BIL
RSW and SW - voltage
Beginning of traces deviating
due to the difference in core
magnetic state. This is
typically more pronounced in
the overlay of reduced and full
switching waveforms
HIGH-FREQUENCY - SWITCHING IMPULSE:ACCEPTANCE CRITERIA (cont.)
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Overlay of 650 kV BIL
SW1 and SW2 - voltage
Slight deviation due to the
difference in core magnetic
state.
HIGH-FREQUENCY - SWITCHING IMPULSE:ABNORMAL DATA
In general, whenever discrepancies occur the normal test
procedure need to be stopped and investigation performed. If
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the cause is found to be external to the transformer, thecorrections are made before the test can continue.
If there is any doubt as to the cause of the discrepancies,
additional impulses may be applied.
Removing manhole covers and observing presence of gasbubbles and/or carbon, serves as confirmation of failure and
provides some indication of the fault location.
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
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drained and internal inspection is performed.
If necessary, the unit is un-tanked. This is followed by a
thorough and well-documented investigation.
The user’s involvement in this process enhances the
quality of the investigation and that of the final product.
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LOW-FREQUENCY:
APPLIED VOLTAGE(Routine)
LOW-FREQUENCY – APPLIED VOLTAGE:OBJECTIVE
The high-frequency tests (lightning and switching
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The applied voltage test is a simple overvoltage test. The
early transformer engineers apparently took cues from
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 minduration, with the expectation to demonstrate a long-term
capability to operate at the rated voltage.
impulse) always precede the low-frequency tests (appliedand induced voltage). This sequence is rooted in the fact
that due to a longer duration, the low-frequency tests
serve to stress further and to detect the damage caused
by the high-frequency tests.
LOW-FREQUENCY – APPLIED VOLTAGE:PHYSICS
D LV
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Region D – major winding
-to-ground and winding-to-
winding insulation arestressed the most.
LV
D
HV
HV
Shorting
lead
D
LOW-FREQUENCY – APPLIED VOLTAGE:SETUP AND TEST METHODOLOGY
Applied Voltage Parameters
Magnitude C57.12.00-2010
Test is performed at low frequency
(<500 Hz), normally, power
f
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Duration 1 min frequency. All terminals of tested winding are
connected together; all other
terminals (including all cores,
buried windings with one terminal
brought-out and the tank) are
grounded. A sphere-gap, set for 10% above
test voltage, may be connected for
protection.
Test voltage (1-phase) is
determined by terminal with the
lowest BIL (e.g., Neutral). The voltage is raised from 25% or
less, held for 1 min and reduced
gradually.
Each winding or set of windings
(e.g., in auto) is tested.
Note : On grounded-wye transformers with
reduced Neutral BIL the test has a limited
significance; it inly tests insulation in the
vicinity of the Neutral.
E
1.1E
v
LOW-FREQUENCY – APPLIED VOLTAGE:ACCEPTANCE CRITERIA
The test is a pass/fail test and is considered
d if d i th ti th lt i li d
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passed if during the time the voltage is applied noevidence of possible failure is observed.
The indications to monitor include unusual sound
such as thump, sudden increase in the test circuit
current and collapse in the test voltage.
LOW-FREQUENCY – APPLIED VOLTAGE:ABNORMAL DATA
If unusual sound, sudden increase in the test
circuit current or circuit tripping occur these
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circuit current or circuit tripping occur, theseevents should be carefully investigated by:
• observation, e.g., presence of carbon
and/or bubbles in the oil
• repeating the test
• other teststo determine whether the failure has occurred.
Due to a 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 re eal the
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and/or repeating of the test and/or other tests reveal the
failure, the oil is drained and internal inspection is
performed.
LOW-FREQUENCY:
INDUCED VOLTAGE/PD
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INDUCED VOLTAGE/PD
Induced:7200 cycles
Induced:1 hour + PD
Class I
Class II
Routine
Routine
LOW-FREQUENCY – INDUCED VOLTAGE/PD:OBJECTIVE
The induced voltage test demonstrates the strength of
internal insulation in all windings as well as between
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internal insulation in all windings as well as betweenwindings and to ground. A combination of prolonged stress
and a very sensitive PD measurement makes it a very severe
and searching test. It must be the last dielectric test to be
performed.
LOW-FREQUENCY - INDUCED VOLTAGE/PD:PHYSICS
To stress turn-to-turn insulation to the
required level, the winding needs toL
xfmr IG
V
IT
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required level, the winding needs tobe excited to a level approaching
twice rated voltage. At power
frequency, this would overexcite the
core.
Therefore, the test is performed as ahigher 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 M-
G set overexciting. This is addressedby using a variable reactor. The latter
provides an additional benefit of
reducing the load on MG set.
R C M G
L
Lv
Variable reactor
Lv is adjusted to
reduce output
from generator.
VT
ILv
VT
IG
IT
ILv
LOW-FREQUENCY - INDUCED VOLTAGE/PD:
PHYSICS (cont.)
E
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Region E – with voltage
distributing per turns ratio, the
most stress is present in the turn-
to-turn insulation of each windingas well as in winding-to-winding
and winding-to-ground insulation.
LV
E
HV
E
E
E
E
E
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.
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solid materials and gas bubbles or water vapor in liquids.Discharges are generally ignited if the electrical field strength
inside the inclusion exceeds the intrinsic field strength of the gas.
They can appear as pulses having a duration of << 1s.
Partial discharges are defined as localized
electrical discharges that only partially bridge
the insulation between conductors and may ormay not occur adjacent to a conductor. In
insulation, the PD events are the consequence
of local field enhancements due to dielectric
imperfections.
Gaseousinclusion
Conductor
Dielectric
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
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A p
requirements must be fulfilled to initiate PD: 1) local field stress
exceeds the void’s breakdown voltage Vbd and 2) free electrons are
available.
CC
CB
CA
CC
CBCA
Vbd
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
PHYSICS (cont.)
Strike: As Vcc>Vbd, breakdown
occurs, charges move across
shorting the void, Vcc= 0 and
discharge stops. To make up
for imbalance, charges comet f dj t i l ti
Buildup: As V , charges
move to and collect on
the surface of the void,
building potential stressV across the void
Relaxation: Charges continue
to flow at a decreasing rate
with balance restoring. Vcc
as charges collect back onthe void’s surface
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out of adjacent insulation.
CC
CB
CA
V
CC
CB
CA
Q
Q
Q Q
Vcc= 0
V
QQQ
Q
Vcc= 0
CC
CB
CA
Q
Q
Vcc
V
Vcc across the void.
CC
CB
CA Q Q Q
V
Vcc
CC
CB
CA
Q
Q
Vcc
V
the void s surface.
CC
CB
CA Q
V
Vcc
Q
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
PHYSICS (cont.)
Terminal
voltage
Dip in terminal
voltage
C1
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We cannot measure the real charge. However, as the void discharges, thecharge 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 high-
frequency current pulses over time produces the reported apparent charge.
Voltageacross void
PD current
C2Z
M
CT
*The detectable voltage dip is in the mV
range, while that at the void may be in
the kV range.
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
PHYSICS (cont.)
Measu rement of part ial discharge isl ik t i t i h b t t f l th t
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Measu rement of part ial discharge isl ike try ing to weigh a butterf ly that
al ights momentar i ly on scales
designed fo r an elephant (somet imesdu r ing an earthquake ).
by Karl Haubner, Doble Aus tral ia
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
SETUP AND TEST METHODOLOGY
Xfmr in
testX H
C1
C2
CStep-up
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M G
Lv
X0
H2 X1
X2
X3
H1
H3
C1
C2
C1
C2 M
p p
xfmr V
pC
and/or
V
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.)
Induced Voltage/PD
Parameters
Voltagemagnitude
C57.12.00-2010
clause 5.10C84 1
Enhanced level
7200 cycles1h level, 5 min
recordings
100%
V
Hold as needed
until stable (min
60 sec)
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gC84.1
Timing
C57.12.90-2010
clauses 10.7, 10.8
PD/RIVcriteria
C57.12.90-2010
clause 10.8/ Annex A
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 voltagefor 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).
Ambient Ambient
100%
100%
t1h
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., series-
parallel or delta-wye) with each connection having system voltage
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>25 kV, two induced tests are performed, one in each connection. If
more than one winding has such multiple connection, then the
connections in each winding shall change between tests. In all
cases, the last test shall be for connection with highest test voltage.
To minimize the effects of external factors and stray capacitances,
the following steps are often relied on:
- 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 factorymachinery
- 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 voltageis observed (see abnormal data for details).
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( )
The PD (RIV) results during 1h test period have shown:
- Magnitude 500 pC ( 100 V).
- Increase during 1 h 150 pC ( 30 V).- No steadily rising trends during 1 h
- 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
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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 42.1 kV 29.5 pC 4.9 µV 00:01:07 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 41.9 kV 29.8 pC 4.8 µV 00:06:10
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 41.8 kV 30.6 pC 4.9 µV 00:11:07
9 41.8 kV 27.1 pC 4.5 µV 00:15:03 41.6 kV 28.4 pC 6.0 µV 00:15:35 41.8 kV 30.1 pC 5.1 µV 00:16:07
10 42.1 kV 28.8 pC 4.6 µV 00:20:03 41.7 kV 29.7 pC 6.0 µV 00:20:35 41.8 kV 30.9 pC 4.9 µV 00:21:07
11 42.3 kV 28.0 pC 4.3 µV 00:25:03 42.0 kV 29.5 pC 6.2 µV 00:25:35 42.1 kV 31.3 pC 5.0 µV 00:26:09
12 42.1 kV 28.0 pC 4.8 µV 00:30:03 41.7 kV 29.0 pC 5.8 µV 00:30:35 41.8 kV 30.1 pC 4.9 µV 00:31:07
13 41.9 kV 31.3 pC 5.1 µV 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
14 41.8 kV 28.2 pC 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
15 42.1 kV 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
16 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
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.
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Other tests, e.g., acoustic PD, DGA, can provide confirmation
that a source of excessive partial discharge is present.
The presence of smoke and bubbles rising in the oil, audible
sounds such as thump, sudden increase in test current or
voltage collapse may all serve as a confirmation that
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 PDtest, degassing of oil, refilling transformer under
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test, degassing of oil, refilling transformer under
vacuum or a heatrun test (if one is specified) are often
successfully bring the PD data within limits.
A failure to meet the partial discharge acceptancecriterion shall not warrant immediate rejection, but it
shall lead to 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 thefailure, the oil is drained and internal inspection is
performed.
NO LOAD LOSSES AND
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NO-LOAD LOSSES AND
EXCITATION CURRENT,
after dielectrics(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.
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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
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to:
Changes in the inter-laminar insulation
Temperature
Sometimes the change after initially exceeding 5% goesaway with time.
Failure to meet before and after dielectrics comparison
criteria should not warrant immediate rejection but shall
lead to consultation between purchaser and manufacturerregarding further investigation of possible causes and
consequences.
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LOAD LOSSES AND
IMPEDANCE VOLTAGE(Routine)
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 I 2 R losses due to load current stray losses due to eddy currents induced by leakage flux in
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y y y g
the windings, core clamps, magnetic shields, tank walls, and
other conducting parts. Stray losses may also be caused by
currents circulating in parallel windings or strands.
Load losses do not include control and cooling losses.
The impedance voltage of a transformer is the voltage required
to circulate rated current through two specified windings with
one winding short-circuited.
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LOAD LOSSES AND IMPEDANCE VOLTAGE:
PHYSICS
RFM
Vrated
Iexc
RIrated
I2R lossesT
FL
V
Note: Resistance R and short
circuit of LV is not shown.
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To create conditions when 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 thendominated by the leakage channel where the eddy losses in
various conducting components in the FL path are induced.
LVHV LVHV
Eddy currents
creating losses1/T
Vsc
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LOAD LOSSES AND IMPEDANCE VOLTAGE:
SETUP AND TEST METHODOLOGY
Applied voltage is adjusted
until rated current is present in
the excited winding.
After data is recorded, if
necessary, correction for losses
Transformer
in testCT
VT
V
X0
H2
X1
X2
X3
H1
H3
3
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necessary, correction for losses
in external circuit is made.
If three line currents are not
balanced the average RMS
value should correspond to the
desired value.
The duration of the test should
be kept to a minimum to avoid
heating up winding conductors.
W
I
A V
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.)
For 3-wdg units, three sets of
measurements are performedusing three pairs of windings,
d i Z Z Z d P
1
2
31
2
3
Z1
Z2
Z
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producing Z12, Z13, Z23 and P12,
P13, P23. Solving shown
equations, determines Zi and
Pi of each branch. 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.
Z3
Z12 = Z1 + Z2
Z13 = Z1 + Z3
Z23 = Z2 + Z3
Z1 = (Z12 + Z13 – Z23)/2
Z2 = (Z12 + Z23 – Z13)/2Z3 = (Z13 + Z23 – Z12)/2
LOAD LOSSES AND IMPEDANCE VOLTAGE:
SETUP AND TEST METHODOLOGY (cont.)
Since stray and I2R
losses have different
dependencies on T,each need to be
obtained from
Measure A, V, W, T
Correct W and Vfrom measured
amps to rated
Convert straylosses from
TLL test Trated
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obtained from
measured losses,
individually converted
from test T to ratedT before combined
again in reported load
losses. V is also
converted to rated T.
Convert Rdc from
TR_test
TLL_test
p
Calculate I2R losses
at TLL_test
Calculate stray
losses at TLL_test
(W - I2R)
Convert I2R losses
from TLL_test
Trated
LL_test rated
Calculate total
losses at Trated
(stray + I2R)
Calculate %Vsc
(V / Vrated)100 = %Zsc
Correct V from
TLL_test Trated
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%
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at the same time is +/-7.5%.
For 3-wdg units, autotransformers or units having a zigzag
winding, tolerance for measured impedance is +/-10% of the
guaranteed value. The tolerance for comparison of duplicatesunits produced at the same time is +/-10%.
Assurance that test data is credible:
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
Potential reasons for exceeding the guaranteed values mayinclude:
O i ht i d i
Example: guaranteed load loss - 94 kW, measured – 110 kW
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Oversights in design
Production process related factors or mistakes
Influence of temperature was not properly accounted for
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 andmanufacturer regarding further investigation of possible
causes and consequences.
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q
The acceptance criteria of 6% for total losses does not
replace the manufacturer’s guarantee of losses for
economic loss evaluation purposes.
LOAD LOSSES AND IMPEDANCE VOLTAGE:
COMPARISON WITH FIELD DATA
Factory losses are measuredunder 3-phase excitation, at
rated current and reported as
Field losses are measuredunder 1-phase excitation, at
current much lower than rated
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Factory and field results
cannot be compared
rated current and reported as
sum of three phases I2R and
stray losses.
current much lower than rated
and reported as per-phase I2R
and stray losses.
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
the short-circuit impedance,
obtained at current* much lower than
rated under 1-phase excitation.
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*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.
Experience shows that a combined influence of different instrumentation
and test setups, difference in flux distribution under 3- and 1-phase
excitation, presence of the resistive component and averaging of factory
data can result in differences ranging from nearly perfect (<1%) to up to
6% (of the measured value).
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 per-
phase tests deviate from average (of three readings) by <3% of themeasured 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%.
TEMPERATURE RISE
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TEMPERATURE RISE(Design and other)
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
rise winding average rise and winding hot-spot rise over
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rise, winding average rise and winding hot spot rise over
ambient for:
Verification of design calculations. Demonstration of meeting the guaranteed performance
characteristics.
Provides data for calculation of potential MVA margin.
Setup of various temperature monitoring instrumentsand cooling control.
TEMPERATURE RISE:
PHYSICS
Measured:
Tto, Tt_rad, Tb_rad, Ta.
Tto Tt_rad
Tt rad
Tto-a Tto
Main tank
T
Ta
height
Calculated: Tto-a, Tw_ave-a, Ths-a, GRAD
Needs NL+LL
lossesNeed rated
current
T
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Tb_rad
Rad
Tb_rad
Oil
Winding
t_rad
Tw_ave*
Ta
Tw_ave-a
LV HV
TaT
To_ave
GRAD
Core
Tw_hs
Ta
*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.
Ths-a
Located at 3 locations around xfmr at mid-height level.
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY
Total losses (NL+LL) and winding
cold resistance data should be
available.
Test is performed for min and max
MVA and in a combination of
Transformer
in testCT
VT
X0
H2
X1
X2
H1
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MVA, and in a combination of
DETC/LTC positions, producing
highest load losses.
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)
W
I
V
A
X3 H3
V
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
Tto, Tt_ rad
Tb_ rad
T[C]
Cutback
ONAF shutdown
Xfmrenergized
for ONAF
Rhot
measurement
begins
Measurement before
cutback determines *Tto-a
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Ta_ ave
Tto-a
t [h]
PrecedingONAN
ONAN
shutdown
Rated currentrun
Steady-state oil
T rise
(change of Tto-a in 3h 1C or 2.5%
whichever is greater)
Itest
Ptotal
Irated
*Tto-a is corrected for difference between required and
actually used total losses (it must be 20%) and for altitude.
Total loss run
1h
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
Rhot
Rhot as function of time in
f d i
Rhot calculated at t = 0
*Instrument
t d
Objective: resistance of
winding at the time when
load current is still present
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t [min]
presence of decreasing
temperature is recorded
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.
connected
Instrument output
current reached
pre-selected level Flux
stabilized
t = 0
Voltageremoved
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
TTw ave-a
Comparison with guaranteed values,
e.g., Tto-a and Tw_ave-a 65C; Ths-a 80C
GRAD correction for
localized hot spot
eddy currents ***Ths-a
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Tw_hot
**To_ave_cb-a is corrected for difference between required and actually used total losses (it must be 20%)
and altitude.
Ta
**To_ave_cb-a
GRAD
_
*GRAD To_ave_sd
*GRAD is corrected for difference between required and actually used load current (it must be 15%).
Tto-a
GRAD Value used for
setting winding
T monitors
***This a simplified representation of Ths_a determination; actual design calculation is more involved.
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
i
vLV v
LVX2
X0 i
1
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.
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Series autoxfmr
windings with LTC in N LV winding
idc
idc
i2
X2X
0idc
i1
i2
idc
idc
Main unit
core
Series autoxfmr
core
F
1 F2
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
0.00467
0.00468
0.00469
0.0047
R d c [ o h m ]
0.29
0.3
0.31
0.32
HV ci r c ui t X 0-X 2
At t = 4 min, data
collection begins
ith fl i i
Series autoxfmr should be
excluded from both cold
and hot Rdc measurements.
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0.00462
0.00463
0.00464
0.00465
0.00466
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]
L V c
i r c u i t R
0.25
0.26
0.27
0.28
tR d c [
Oh m ]
H1-H 2
Setup 1.5 min
Voltage
removed
t = 0
Time remaining
for stabilization
= 2.5 min
with flux in main
core stable while
flux in series corestill changing.
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
should not exceed the guaranteed value, e.g., 80C for 65C rise
it d 65 C f 55 C i it
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units and 65C for 55C rise units.
If shutdown is performed on each phase, results of winding average
rises should be comparable (rule of thumb: 4C difference,presently, there is no limit in the standard).
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 certifiedtest 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.
Test instrument type and setup used for cold and hot resistance
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was the same, e.g., if two-channel measurement is used it must
be used for both hot and cold resistance tests.
If series auto is present, unless it is shown that RDC can bemeasured within shutdown time constrains, the auto is excluded
from the resistance measurement*.
During shutdown, fans are turned off right after transformer is de-
energized.
The first value of winding hot Rdc was recorded not later than 4min after shutdown.
*Lachman, M. F., et al “Impact of Series Unit on Transformer Winding DC Resistance MeasurementDuring 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
voltage, current and winding resistance, and +/-1.5C for
t t
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temperature.
If the test could not be done at rated frequency, the results are
converted from tested to rated frequency (see C57.12.90-2010,Annex B). However, the fans/pumps should be operated at the power
frequency to be used when unit is in service.
TEMPERATURE RISE:
ABNORMAL DATA
Potential reasons for exceeding the guaranteed values may
include:
Oversights in design Testing/setup mistakes
Presence of series auto-transformer
Example: guaranteed Tw_ave-a – 65C, measured – 67C
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TLV_hot=[(234.5+30)4.534/4.081]-234.5=59.4ºC
y = 8.152E-07x2
- 3.235E-05x + 5.362E-03
0.005
0.00505
0.0051
0.00515
0.0052
0.00525
0.0053
0.00535
0 : 0 0
0 : 3 0
1 : 0 0
1 : 3 0
2 : 0 0
2 : 3 0
3 : 0 0
3 : 3 0
4 : 0 0
4 : 3 0
5 : 0 0
5 : 3 0
6 : 0 0
6 : 3 0
7 : 0 0
7 : 3 0
8 : 0 0
8 : 3 0
9 : 0 0
9 : 3 0
1 0 : 0 0
Time [min]
R
e s i s t a n c e [ o h m s ]
TLV_hot=[(234.5+30)5.362/4.621]-234.5=72.5ºC
With series auto-xfmr
y = 6.962E-08x2
- 5.974E-06x + 4.534E-03
0.00442
0.00444
0.00446
0.00448
0.0045
0.00452
0.00454
0 : 0 0
0 : 3 0
1 : 0 0
1 : 3 0 2 : 0 0
2 : 3 0 3 : 0 0
3 : 3 0
4 : 0 0
4 : 3 0
5 : 0 0 5 : 3 0
6 : 0 0 6 : 3 0
7 : 0 0 7 : 3 0
8 : 0 0 8 : 3 0
9 : 0 0 9 : 3 0
1 0 : 0 0
Time [min]
R
e s i s t a n c e [ o h m s ]
Without series auto-xfmr
Note: The example shows a quadratic function, the suitability of which was confirmed via direct fiberopticmeasurements 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.
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ZERO-PHASE SEQUENCE
IMPEDANCE
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IMPEDANCE
(Class I - design
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 neutral terminal.
Objective: The zero phase sequence impedance serves
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Objective: The zero-phase sequence impedance serves
as input in analysis of unbalanced three-phase system
using symmetrical components method.
ZERO-PHASE SEQUENCE IMPEDANCE:
PHYSICS
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.
In unbalanced 3-phase system,
impedances in each phase are
Ia
Ia
Ib Ic
Balanced
Ia1
I
Positive
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impedances in each phase are
different and each phase needs
to be analyzed separately. Method of symmetrical
components converts any
unbalanced system into 3
balanced systems, namely
positive, negative and zero-
phase sequence systems.
After these are defined, the
voltages and currents in the
original unbalanced system are
reconstructed.
Ib
Ic
Unbalanced
Ib1
Ic1
Ib2 Ia2
Ic2 Negative
Iao
Ibo
Ico
Zero
ZERO-PHASE SEQUENCE IMPEDANCE:
SETUP AND TEST METHODOLOGY
For xfmr, the Z1 = Z2 = Zsc is known from impedance/load losses test. In zero-
phase 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 testapplies only to transformers with one or more windings with a physical neutral
brought out for external connection.
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Z0
1 2
N
1 2
ZERO-PHASE SEQUENCE IMPEDANCE:
SETUP AND TEST METHODOLOGY (cont.)
Z1Ns, Z1No, Z2No are
used to calculate Z1,
Z2 and Z3.
If delta winding is not
present, the currents
shown in delta are
circulating in the tank.
Z1Ns
Z
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circulating in the tank.
Z1 Z2
Z3
1 2
N
Z1No
Z2No
ZERO-PHASE SEQUENCE IMPEDANCE:
SETUP AND TEST METHODOLOGY (cont.)
If no delta winding is present, applied
voltage should be 30% of rated
Vphase_gnd and measured current Irated.
If delta winding is present, the applied
voltage should be such that current in
delta winding Irated.
Transformer in
test
CT
VT
V
1
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For Y/ 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)
W
I
AV
ZERO-PHASE SEQUENCE IMPEDANCE:
ACCEPTANCE CRITERIA
The standard does not provide an acceptance criteria for the zero-
phase 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, Z sc = 21.9%, Z 0 = 21.8%
For Y/ units, Z0 (0.8-1.0)Zsc. Example: 48 MVA, 235.75GndY/13.8 kV, Z sc = 9.9%, Z 0 = 8.5%
For Y/Y/ or autoxfmrs with delta Z (0 7 1 0)Z with Z
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For Y/Y/ or autoxfmrs with delta, Z1 (0.7-1.0)Zsc; with Z2
typically <1.0% or sometimes <0.
Example: Auto, 18 MVA, 230GndY/60GndY/21 kV, Z sc = 4.9%, Z 1 = 3.6%,
Z 2 = 0.84%, Z 3 = 10%
50 MVA, 69GndY/34.5GndY/13.2 kV, Z sc = 7.8%, Z 1 = 6.7%,
Z 2 = -0.16%, Z 3 = 4.6%
For Y/Y and autoxfmrs without delta (rare occasion), magnetic
flux has a strong coupling to the tank, making, in general, therelationship between voltage and current non-linear and the
above observations not relevant. Example: Auto, 75 MVA, 115GndY/34.5GndY kV, Z sc = 12.7%, Z 1 = -9.9%,
Z 2 = 27.4%, Z 3 = 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.
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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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AUDIBLE SOUND LEVEL
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U SOU
(Design and other)
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 ratingbeing tested.
Objective: To protect the population from noise
i i t f i d t t ithi
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inconveniences transformers are required to operate within
specified noise limits. The audible sound level test providesthe sound pressure level data for:
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 propertycalled magnetostriction.
This produces a vibratory motion
in the core transmitted to the tank
through the core mechanical
support and the pressure waves
Core
Dielectric
fluid
Tank
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in the dielectric fluid. At the tank
this motion radiates as anairborne 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.
Direction ofdimensional
changeMagnetostriction
caused by
domain rotation
F
AUDIBLE SOUND LEVEL:
SETUP AND TEST METHODOLOGY
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 forthe tested rating.
On certain tap positions, xfmr may
produce sound levels greater than at the
principal tap, e.g., engaging PA and/or
Transformer
in test
X0
H2 X1
X2
X
H1
H
3
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series autoxfmr. Test will be performed in
these positions if specified by customer.
The voltage should be set as during no-
load 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 maxrating.
Measurements begin when xfrm reaches
steady-state conditions, i.e., to allow
magnetic bias to decay.
VT
Vave
X3 H3
AUDIBLE SOUND LEVEL:
SETUP AND TEST METHODOLOGY (cont.)
Microphones are located
on the measurement
surface at showndistance from reference
sound-producing surface.
Xfmr is placed so that no
acoustically reflectingTank
Radiator
6 ft3 ft
Fan cooled surface
Microphone
location
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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 ismade at drain valve
proceeding clockwise.
Reference sound-producing
surface is a vertical surface
following the contour of a taut
string stretched around xfmr
periphery.
LTC
Drain valve
1 ft
#1Measurement
surface
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)
One-third octave sound pressure level (when
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specified)
Narrowband sound pressure level (when specified)
AUDIBLE SOUND LEVEL:
SETUP AND TEST METHODOLOGY (cont.)
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.
To account for human’s greater sensitivity to noise
at some frequencies relative to other, the measured
data is passed through a weighting filter. A-
A-weighted sound pressure level
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weighting is most commonly used to allow for a
broad peak between 1÷6 kHz but very stronglydiscriminating against low frequencies.
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,
1000, 2000 and 4000 Hz) is split into three, with each “1/3
sub-band” (e.g., 63, 80, 100, 125, 160, 200, 250 Hz, etc.)
being evaluated individually.
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being evaluated individually.
The narrowband sound pressure level measurement isperformed at the 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 pressure level.
Measurements are made if ambient level is at least 5 dB or more
below the combined transformer and ambient sound pressure
level
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level.
Compute ambient-corrected sound pressure levels. Forcorrections see Table 7 in C57.12.90-2010.
Compute average sound pressure levels [in dB(A)]:
=
L i is the sound pressure level measured at i th 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 bythese tables.
Assurance that test data is credible:
The sound pressure measuring instrument should meet the
requirements of ANSI S1.4 for Type 1 meters.
Th d i i t t h ld b lib t d
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The sound pressure measuring instrument should be calibrated
before and after each set of measurements. If calibration change
>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
Potential reasons for exceeding the guaranteed values mayinclude:
Problems with measurement, e.g., ambient noise,
positions of microphones, sound instrument calibration,
lt dj t t di fl ti f
Example: guaranteed sound pressure level per NEMA –
75/77/78 dB(A), measured – 77/78/79 dB(A)
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voltage adjustment, surrounding reflecting surfaces,
etc.
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 addressthe problem.
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CORE DEMAGNETIZATION
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(Routine*)
*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 firststep must bring the core to the main hysteresis loop with
the last step, upon removal, leaving no residual
magnetism in the core.
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Objective: The core demagnetization creates conditionsfor obtaining the low-voltage exciting current and loss
test as well as sfra benchmark data not affected by
residual magnetism .
CORE DEMAGNETIZATION:
PHYSICS
If in the presence of residual
magnetism Br , the voltage is increased
from zero, the flux varies around
minor hysteresis loops. The negative
tip of these loops lies on the main
loop. The greater the voltage, the
smaller is the offset of the minor loop
along the B axis. The bias is removed
when the main loop, symmetrical
Main
loop
Br
H
B
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when the main loop, symmetrical
around the origin, is reached.
H
B
If after reaching the main hysteresis
loop, the voltage is gradually reduced,
each minor loop will lie inside the
previous larger loop. Reduction ofvoltage to zero brings working point
to the center of these loops resulting
in a demagnetized transformer.
Br = 0
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.
Applying DC voltage (e.g., 12 V), waiting until current
stabilizes, then switching voltage polarity and holding
until current reaches a lower value; this process
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continues until current level is zero 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
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 thecondition 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
Factory
Field
Field
Factory
Data movement
with no excitation
applied between
measurements
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changes are normal and
diagnostically insignificant.
Factory Field
mA W mA W
20.5 128 20.5 126
9.3 61 9.6 58
20.7 131 21.6 131
72 hr
24 hr
9 hr
6 hr
3 hr
1 hr
30 min
dm_ini t
Controlled
experiments showing
data movement*
*Lachman, M. F., et al “Frequency Response Analysis of
Transformers and Influence of Magnetic Viscosity”, Proc.of the Seventy-Seventh Annual Intern. Confer. of DobleClients, 2010, Sec. TX-11.
LAST SLIDE
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THE END
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UNDERSTANDING THETRANSFORMER TEST DATA
Barry M.
Mirzaei
– P.Eng.
Hydro One
September 2012 – Chicago
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2Understanding The Transformer Test DataSeptember 2012
No Load
Test
Test object is supplied from one side of the transformer
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3
(L.V.), the other side (H.V.) is left open circuit. Test voltageto be adjusted to the pre‐determined value(s)
Typical test voltage is 90% ‐ 100% and 110% of the rated
voltage
Characteristics of the No Load Test:
“Low Current – High Voltage”
Understanding The Transformer Test DataSeptember 2012
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‐
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4
determined value(s)
Twice the rated voltage is applied for 7200 cycles
for transformers with uniformly insulated
windings
Characteristics of the Induced Voltage Test:
“Low Current – High Voltage and Frequency > 60”
Understanding The Transformer Test DataSeptember 2012
Load Loss
Test
Test object is supplied from one side (H.V.), the
other side (L.V.) is short‐circuited. Test voltage is
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5
adjusted to apply the rated current to the testobject
Load Loss:
‐Resistive losses or R
‐Eddy current losses in the windings
‐Stray losses in leads, core plates and tank
Characteristics of the Load Loss Test:
“High Current – Low Voltage”
Understanding The Transformer Test DataSeptember 2012
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6Understanding The Transformer Test DataSeptember 2012
Hysteresis Loss
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7
Proportional to the frequency
and dependent on the area of
the hysteresis loop, which, inturn, is a characteristic of the
material and a function of the
peak flux density
Understanding The Transformer Test DataSeptember 2012
Eddy Current
Loss
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8
Dependent on the square
of frequency but is also
directly proportional
to
the square of the
thickness of the material
Understanding The Transformer Test DataSeptember 2012
4.44 (a)
(b)
Voltage
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9
PNL No Load Losses
= Hysteresis Loss
= Eddy Current Loss
, = Coefficients
=
= Exponent with induction
Understanding The Transformer Test DataSeptember 2012
Minimizing hysteresis loss thus depends onthe development of a material having a
minimum area of hysteresis loop.
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10Understanding The Transformer Test DataSeptember 2012
Minimizing eddy current loss is achieved by
building up the core from a stack of thin
laminations and increasing resistivity of the
material in order to make it less easy for
eddy currents to flow.
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STATEMENT OF THE ISSUE:
11Understanding The Transformer Test DataSeptember 2012
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
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12
Deflection in the readings on metering
devices (watt meters, …) were reportedwith the noise.
inside the oil”, started at around the 25%
of the test voltage.
Understanding The Transformer Test DataSeptember 2012
Solutions?What
test
data
are
available?
What those test data really mean?
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13Understanding The Transformer Test DataSeptember 2012
Criteria & constraints for
addressing the
issue
Un‐necessary activities to be
avoided delivery date was critical
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14
avoided, delivery date was critical
Un‐tanking the transformer is costly and
should be avoided if there is no clear
understanding about
the
issue
Insulation tests should not be repeated,
if there is no need to do so
Understanding The Transformer Test DataSeptember 2012
1
Oil Sample
OK 2
Repeat TTR & DC Resistance
OK
6
Apply reduced
“Induced Voltage”
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15
Investigation Procedure
3
Observe The No Load Test
PROBLEM4
Apply Load Test
OK
5
Insulation Test?
Not Convinced to
apply
Voltage
Understanding The Transformer Test DataSeptember 2012
The Induced Voltage Test
stresses all parts of the
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16Understanding The Transformer Test DataSeptember 2012
stresses all parts of theinsulation system, including
turn to turn, phase to phase
and winding to ground.
4.44 (a)
Concept of customized Induced test:
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17
By applying induced voltage up to
rated voltage,
basically
the
no
load
test is being repeated with reduced
induction in
the
core
Understanding The Transformer Test DataSeptember 2012
.
.x
x
x
x
.
.x
x
x
x
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18Understanding The Transformer Test DataSeptember 2012
x
x x x
Investigation Procedure
1
Oil Sample
OK2
Repeat TTR & DC Resistance
OK
3
Observe The No Load Test
PROBLEM
4Apply Load
5
Insulation Test?
Not Convinced to apply
6
Apply reduced “Induced Voltage”
Eddy Current Loss
Dependent on
the
square
of
frequency but is also directly
proportional to the square of
the thickness of the material
Hysteresis Loss
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19Understanding The Transformer Test DataSeptember 2012
pp y
Test
OK
Load Loss:
‐Resistive losses or R
‐Eddy current losses in the windings
‐Stray losses in leads, core plates and tank
Proportional to the frequency and dependent onthe area of the hysteresis loop, which, in turn, is a
characteristic of the material and a function of thepeak flux density
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20Understanding The Transformer Test DataSeptember 2012
Core bolts
are
inserted
through the core for the
f l i th
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21Understanding The Transformer Test DataSeptember 2012
purpose of clamping the
core
laminations.
During “Core Stacking Process” –
Holes built for Core Bolts, used for proper core stacking
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22Understanding The Transformer Test DataSeptember 2012
Core Plates Core Bolts
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23Understanding The Transformer Test DataSeptember 2012
Photo belongs to another transformer
Core Plate
Core Bolt
Weld
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24Understanding The Transformer Test DataSeptember 2012
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25Understanding The Transformer Test DataSeptember 2012
Fiberglass insulation
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26Understanding The Transformer Test DataSeptember 2012
Round Head Carriage Bolt
Metal Washer
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27Understanding The Transformer Test DataSeptember 2012
In this case, the low impedance path formed by the
bolts and the core clamping plates causes a local shortcircuit path which produces intense local eddy currents.
The amount of heat generated by this phenomenon is
sufficient to considerably damage the adjacent areas.
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28Understanding The Transformer Test DataSeptember 2012
The problem was noticeable in No‐Load test since there
was higher induction to create higher current in the
through bolts when compared to reduced induced test.
Increase
in
the
Load
Loss
increased
the
probability
of
“Core Plates” related issues.
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29Understanding The Transformer Test DataSeptember 2012
This picture shows the correct insulation of the core bolts
Photo belongs to another transformer .
Insulation Material
Thank You
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30Understanding The Transformer Test DataSeptember 2012
Thank You
Understanding
Transformer
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Factory Testing
September 30, 2012
On some occasions additional methods must be
employed to determine the suitability of tested
transformer.
These techniques may include calculated corrections
Transformer Temperature Tests
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Understanding Transformer Factory Testing 2
These techniques may include calculated corrections
or multiple tests at different loading conditions, etc.
Lets look at two actual factory cases:
Case 1: Good Test Results – Bad Data
Case 2: Bad Test Results – Good Transformer
Core
AVERAGE
WINDING
TEMP.WINDING
HOTTEST
SPOT
TOP
OIL
TEMP.
Cooling
Top Oil Temp.
Top
Oil
Gradient . x H.S.F
TEMPERATURE DISTRIBUTION
Transformer Temperature Tests
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Understanding Transformer Factory Testing 3
Coils
Oil
D i s t a n c e
Temperature
AverageOil
Bottom
Oil
Avg. Wdg.Temp.
Hot Spot
Gradient
Oil
Ambient
UAT 39/52/65 MVA; 230 - 6.9 (XV) & 4.16 (YV) kV 60Hz(+15-5% LTC for YV)
• Heat run test was performed according to ANSI/IEEEStandards and the clients technical specification.
Case 1: Good Results – Bad Data
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Understanding Transformer Factory Testing 4
• Temperature results were well below Standard limitsand according to client’s specification.
• Very Clean DGA Results.
• Test Results did not match design data?
Short-Circuit Method – Three Phase, 3-Winding:
L3S
H O R T C I R C U I T
Case 1: Good Results – Bad Data
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Understanding Transformer Factory Testing 5
Measurement
System Unit Under Test
YV
HV
XV
L3
L2
L1
~
S H O R T C I R
C U I T
S
Case 1: Loading Cycle
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Understanding Transformer Factory Testing 6
Test Corrections LimitTotal Losses (kW ) 172.600 228.464
Tap Position 1R 1R
Average Oil Rise 42.9 51.2
Top Oil Rise 49.6 59.2 65.0
Winding Gradient, YV 10.2 10.2
Winding Gradient, XV 2.3 2.3
Winding Gradient, HV 3.3 3.2
HS over TOR, YV 11.2 11.2
HS TOR XV 2 5 2 5
Case 1: Heat Runs Results
Winding es a e RatioHV 99.5 97.9 0.984
XV 1757.0 1757.0 1.000
YV 2483.0 2483.0 1.000
Winding Current for Individual Gradient Runs
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Understanding Transformer Factory Testing 7
HS over TOR, XV 2.5 2.5
HS over TOR, HV 3.6 3.5
Hot Spot Factor , YV 1.10 1.10
Hot Spot Factor , XV 1.10 1.10
Hot Spot Factor , HV 1.10 1.10
Average Winding Rise, YV 53.05 61.3
Average Winding Rise, XV 45.13 53.4
Average Winding Rise, HV 46.18 54.4
Hot Spot Rise, YV 60.8 70.4
Hot Spot Rise, XV 52.1 61.7
Hot Spot Rise, HV 53.2 62.7
65.0
80.0 n: 0.63
m: 0.80
Exponents
n ng est ate at o
HV 97.9 97.9 1.000
XV 2149.1 1757.0 1.223
YV 1791.5 2483.0 0.722
Winding Current for Oil Rise Run
MVA
Cooling Mode
Tested Design
Losses (kW) 228.682 228.464 Guar.
Top Oil Rise 59.2 51.6 65.0
Average Oil Rise 51.2 39.7
Bottom Oil Rise 39.8 27.9
Gradient 10.2 12.8
Average Winding Rise 61.4 52.5 65.0
Heat Run Result vs Design Data
YV Winding
39.0
ONAN
Case 1: Temp Rise not Expected
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Understanding Transformer Factory Testing 8
Average Winding Rise 61.4 52.5 65.0
Hot Spot Gradient 11.2 14.1
Hot Spot Rise 70.4 65.7 80.0
Gradient 2.3 11.1
Average Winding Rise 53.5 50.8 65.0
Hot Spot Gradient 2.5 12.2
Hot Spot Rise 61.7 63.8 80.0
Gradient 3.2 10.5
Average Winding Rise 54.4 50.2 65.0
Hot Spot Gradient 3.5 11.6
Hot Spot Rise 62.7 63.2 80.0
HV Winding
XV Winding Why so far off?
Time CH 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 0:00 1:00 2:00 3:00 5:00Measured kW 171.0 173.2 172.0 171.1 173.0 172.9 173.6 172.0 172.0 172.3 172.5 172.6 Take Take Take
Measured Amps 101.6 102.0 103.0 101.5 102.0 102.0 100.0 99.0 99.1 99.4 99.6 99.5 HV XV YV
Upper Radiator 1 2 77.43 78.66 79.56 80.10 81.55 82.66 83.83 84.18 84.57 84.73 84.92 85.09 84.98 82.27 83.61
Upper Radiator 2 7 77.58 78.78 79.78 80.30 81.88 82.68 84.65 85.03 85.42 85.51 85.58 86.10 85.70 82.91 84.36
Average Upper Rads 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
Lower Radiator 1 1 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
Lower Radiator 2 3 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
Average Lower Rads 65 66 66 55 67 55 68 57 69 66 70 70 71 68 72 05 72 02 71 21 72 02 72 15 72 18 68 78 70 31
Case 1: Heat Runs Temp. Log
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Understanding Transformer Factory Testing 9
∆T (3 Hr) > 2 ºC
Average Lower Rads 65.66 66.55 67.55 68.57 69.66 70.70 71.68 72.05 72.02 71.21 72.02 72.15 72.18 68.78 70.31
Ambient # 1 4 37.86 37.96 38.00 40.00 40.75 41.27 41.64 41.94 41.70 41.35 40.92 40.53 0.00 0.00 0.00
Ambient # 2 11 38.79 38.95 39.08 38.90 39.20 39.56 39.72 39.52 38.11 37.85 37.25 37.40 0.00 0.00 0.00
Ambient # 3 12 35.78 35.88 36.88 36.20 37.88 38.95 39.81 39.78 39.33 39.06 38.81 38.41 0.00 0.00 0.00
Average Ambient 37.48 37.60 37.99 38.37 39.28 39.93 40.39 40.41 39.71 39.42 38.99 38.78 0.00 0.00 0.00
Top Oil Temp 6 81.21 81.46 82.00 83.25 84.52 85.88 87.95 89.16 89.98 90.47 91.07 91.36 91.12 88.60 89.87
Top Oil Temp DV 78.00 80.00 81.00 82.80 83.00 83.50 84.00 85.00 84.00 83.95 85.00 85.40
Average of Top Oil 79.61 80.73 81.50 83.03 83.76 84.69 85.98 87.08 86.99 87.21 88.04 88.38 91.12 88.60 89.87
Averge Oil Rise @ 3300' 36.21 37.05 37.45 38.84 38.46 38.78 39.31 40.39 40.79 40.83 42.42 42.88 84.54 81.69 83.03
Top Oil Rise @ 3300' 42.13 43.13 43.51 44.66 44.48 44.76 45.59 46.67 47.28 47.79 49.04 49.60 91.12 88.60 89.87
Bot tom Oil Rise @ 3300' 28.18 28.95 29.56 30.20 30.38 30.77 31.29 31.63 32.30 31.79 33.02 33.37 72.18 68.78 70.31
Test Corrections LimitTotal Losses (kW ) 172.600 228.464
Tap Position 1R 1R
Average Oil Rise 42.9 51.2
Top Oil Rise 49.6 59.2 65.0
Winding Gradient, YV 10.2 10.2
Winding Gradient, XV 2.3 2.3
Winding Gradient, HV 3.3 3.2
HS over TOR, YV 11.2 11.2
HS over TOR XV 2 5 2 5
Case 1: Heat Runs Results
Winding es a e RatioHV 99.5 97.9 0.984
XV 1757.0 1757.0 1.000
YV 2483.0 2483.0 1.000
Winding Current for Individual Gradient Runs
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Understanding Transformer Factory Testing 10
HS over TOR, XV 2.5 2.5
HS over TOR, HV 3.6 3.5
Hot Spot Factor , YV 1.10 1.10
Hot Spot Factor , XV 1.10 1.10
Hot Spot Factor , HV 1.10 1.10
Average Winding Rise, YV 53.05 61.3
Average Winding Rise, XV 45.13 53.4
Average Winding Rise, HV 46.18 54.4
Hot Spot Rise, YV 60.8 70.4
Hot Spot Rise, XV 52.1 61.7
Hot Spot Rise, HV 53.2 62.7
65.0
80.0 n: 0.63
m: 0.80
Exponents
n ng est ate at o
HV 97.9 97.9 1.000
XV 2149.1 1757.0 1.223
YV 1791.5 2483.0 0.722
Winding Current for Oil Rise Run
• Stable oil temperatures must be met to achieve
reasonably accurate winding gradient measurements.
• Accurate cold resistance temperature measurements
are critical in determining the winding rises.YV hot resistance on Tap 1R – Cold resistance not
Case 1: Summary
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Understanding Transformer Factory Testing 11
p
measured, used Tap 1N Not valid.
• Simulated load losses should be close to the expected
load losses for the transformer during operation.
Actual winding currents not measured during
simultaneous loading.
GSU 820 MVA 362 / 25 kV DETC(±5%) 60Hz
• Heat run test was performed according to IEEE/ANSIStandards and the clients technical specification.
• Temperature results were below limits and according to
Case 2: Good Unit – Bad DGA
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Understanding Transformer Factory Testing 12
Temperature results were below limits and according toclients requirements.
• DGA performed after heat run test found gasgeneration above client and the manufacturer’sacceptance limits.
Case 2: Bad DGA Results
Outside Lab Change Client LimitsSample # 1 2 3 - -
Description
Before Heat
Run [ppm]
4 hours after
Heat Run [ppm]
4 hours after
Heat Run [ppm]
Gas Evolution
[ppm]
Gas Evolution
[ppm]
H2 - Hydrogen 4 22 17 13 10O2 - Oxygen 3989 2314 300 - -N2 Nit 11045 12181 9050
In House Lab
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Understanding Transformer Factory Testing 13
N2 - Nitrogen 11045 12181 9050 - -
CO - Carbon Monoxide 10 62 50 40 25
CO2 - Carbon Dioxide 82 392 250 168 200
CH4 - Methane 0 3 14.4 14.4 5C2H4 - Ethylene 0 5 4 4 2
C2H6 - Ethane 0 27 22 22 2
C2H2 - Acetylene 0 0 0 0 0
All measured temperature rises within calculated tolerances.
5.
TX Hot
Spot
2.
Pump
Problem
1.
Bad DGA
Sample
Case 2: Possible Causes
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Understanding Transformer Factory Testing 14
4.
Stray
Gassing
3.
Improper
Testing
Why ?
1. Bad DGA Data?
• DGA results of the outside lab matched the results
obtained at factory.
2. Bad Pump (s) ?The most probable cause of pump overheating is the
Case 2: Possible Causes
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Understanding Transformer Factory Testing 15
pump running backwards.
• Running ratings matched Nameplate• No Noise
• Thermal Scan normal for pumps and oil flow
Case 2: Possible Causes
3. Improper Testing Method ?
• Loading per IEEE/Expedited Heating
• Fiber Optic Sensors in Coils – No high temperatures
• Thermocouples on structural metal parts – Normalheating
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Understanding Transformer Factory Testing 16
• Ambient temperature was below 40 ºC
• Total heat load (kW) matched cooler rating• Maximum current was only 112% of rated/ Less
than 7 percent of allowable continuous overload
current.
Only two possible causes left:
4. An oil problem due to “thermal stray gassing”.
Or 5. An abnormal transformer hot spot.
Case 2: Possible Causes
An experiment
is needed!
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Understanding Transformer Factory Testing 17
How is the gassing influenced ?
• Load dependent
Case 2: Experimental Loading
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Understanding Transformer Factory Testing 18
• Oil temperature dependent
Step A: Transformer Rated Conditions
1. Test Floor Open and Ventilated
2. All Pumps & Fans On
Case 2: Experimental Loading
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Understanding Transformer Factory Testing 19
2. All Pumps & Fans On
3. Full Rating Conditions of the transformer
Step B: Simulate Stray Gassing
1. Test Floor Closed2. Reduced Current
Case 2: Experimental Loading
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Understanding Transformer Factory Testing 20
3. All Pumps on/ Fans adjusted to keep oil at ~ 90 ºC
Case 2: Experimental Method
Test
Number
Load
Condition
Oil
TemperatureComments
Test #1 Overload High OilTemperature This is the intial heat run result. The source of gassingis indeterminate.
Test #2 Rated LoadNormal Oil Gassing under this condition is most likely not from the
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Understanding Transformer Factory Testing 21
Test #2 Rated LoadTemperature oil.
Test #3 Reduced LoadHigh Oil
Temperature
Gassing under this condition is most likely not from the
transformer.
Test #4 OverloadNormal Oil
TemperatureGassing is most likely from the transformer.
Case 2: Experimental Method
TestNumber
Duration[Hours]
Criteria Loadingoa[%]
Top OilTemp.
Coil OilTemp.
Ambient[ºC]
8.0 Per IEEE Total Heat
Load 108.0 73.0 90.3 37.6
1.0 Per IEEE Rated
Current 100.0 70.0 92.3 38.4
Test #2 8.0 - Rated
Current 100.0 53.5 78.8 24.0
Test #3 8 0 Reduced
80 0 83 5 92 6 35 2
Test #1
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Understanding Transformer Factory Testing 22
Test #3 8.0 -Curr.
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
Test #4
Case 2: Experiment Results
Test # 1 2 3 4Gas
Evolution
[ppm]
Gas
Evolution
[ppm]
Gas
Evolution
[ppm]
Gas
Evolution
[ppm]
H2 - Hydrogen 13 0 12 3 10
O2 - Oxygen - - - - -N2 - Nitrogen - - - - -
CO - Carbon 40 10 36 12 25
Criteria
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Understanding Transformer Factory Testing 23
CO2 - Carbon Dioxide 168 66 272 105 200
CH4 - Methane 14.4 1.8 8 4.2 5
C2H4 - Ethylene 4 0.6 1.7 1.3 2
C2H6 - Ethane 22 0 13.6 0 2
C2H2 - Acetylene 0 0 0 0 0
CO2/CO Ratio 4.2 6.6 7.6 8.8 < 3
• Most of the gas concentrations exceed the customerlimits.
• Dominant gasses are Methane, Ethane and Hydrogen(low temperature gasses or thermal stray gassing).
• No cellulose decomposition.
Case 2: Test #1 Results
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Understanding Transformer Factory Testing 24
p
The Source of the Excessive Gassing
is Indeterminate.
• Gassing, all gasses within acceptance limits.
• No dominant gasses.
• No cellulose decomposition.
Case 2: Test #2 Results
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Understanding Transformer Factory Testing 25
If there was excessive gassing it would
likely be from the transformer active
parts.
• A gasses exceeding limits except Ethylene.
• Dominant gasses are Methane, Ethane and Hydrogen.
• No cellulose decomposition.• In this test the load is reduced no gassing can be
correlated with the transformer.
Case 2: Test #3 Results
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Understanding Transformer Factory Testing 26
• Dominant gasses are Methane, Ethane and Hydrogen,
this is an indication of possible thermal stray gassing.• The gassing results are similar Test #1.
This excessive gassing is likely from
the oil.
• All gasses within acceptance limits.
• Dominant gasses are Methane, Ethane and Ethylene.
Typical gasses for a heat run test without additional stray
gassing.• No cellulose decomposition.
• The absence of gasses confirm that gas generation is
Case 2: Test #4 Results
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Understanding Transformer Factory Testing 27
The absence of gasses confirm that gas generation is
not related to the load or a transformer condition.
If there was excessive gassing it would
likely be from the transformer active
parts.
• The transformer successfully passed the heat run testaccording to ANSI/IEEE Standards.
• Test #3 results closely match with Test #1 and areindicative that the source of gasses during heat runtest is thermal stray gassing of the oil.
Case 2: Summary
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Understanding Transformer Factory Testing 28
• Gasses generated during heat run test performed areproduced by thermal stray gassing of the oil used forFAT.
• The Doble Oil Lab confirmed the stray gassingtendency of the oil used for the factory heat run.
CONCLUSION
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Understanding Transformer Factory Testing 29
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