Understanding power transformer_factory_test_data
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Transcript of Understanding power transformer_factory_test_data
Understanding
Power Transformer
Factory Test Data
Mark F. Lachman
Doble Engineering Company
©Doble Engineering Company
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
©Doble Engineering Company
Class I includes power transformers with
high-voltage windings of 69 kV and below.
Class II includes power transformers with
high-voltage windings from 115 kV through
765 kV.
SYSTEM VOLTAGE CLASSIFICATION
©Doble Engineering Company
Routine tests shall be made on every
transformer to verify that the product
meets the design specifications.
Design tests shall be made on a
transformer of new design to determine
its adequacy.
Other tests may be specified by the
purchaser in addition to routine tests.
GENERAL CLASSIFICATION OF TESTS
©Doble Engineering Company
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
©Doble Engineering Company
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.)
©Doble Engineering Company
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/PD IEEE C57.12.90-2010 clause 10.7, 10.8, 10.9
IEEE C57.12.00-2010 clauses 5.10, 8.2 IEEE C57.113-2010; IEEE C84.1
No-load losses and excitation
current
IEEE C57.12.90-2010 clause 8 IEEE C57.12.00-2010 clauses 5.9, 8.2, 9.3, 9.4
SEQUENCE OF TESTS
©Doble Engineering Company
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.)
©Doble Engineering Company
TEST* REFERENCE
Insulation PF/C and
resistance
IEEE C57.12.90-2010 clauses 10.10, 10.11 IEEE C57.12.00-2010 clause 8.2
Single-phase exciting
current
IEEE C57.12.00-2010 clause 8.2 Lachman, 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 circuits IEEE C57.12.00-2010 clause 8.2
CT polarity/ratio/saturation IEEE C57.13.1-2006
Control and cooling losses IEEE C57.12.00-2010 clauses 5.9, 8.2
Operation of all devices IEEE C57.12.00-2010 clause 8.2
Core-to-ground insulation
resistance
IEEE C57.12.90-2010 clause 10.11 IEEE C57.12.00-2010 clause 8.2
SEQUENCE OF TESTS (cont.)
*Discussion of tests listed on this slide and DGA is not included in this presentation.
©Doble Engineering Company
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
©Doble Engineering Company
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.
©Doble Engineering Company
RATIO, POLARITY, PHASE
RELATION (Routine) ©Doble Engineering Company
Definition: The turns ratio of a transformer is the ratio of
the number of turns in the high-voltage winding to that in
the low voltage winding.
Objective: The turns ratio polarity and phase relation test
verifies the proper number of turns and internal
transformer connections (e.g., between coils, to LTC, to
various 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 the
actual system requirements. Therefore, it is important
that the nameplate drawing is approved by the customer
at the design stage.
RATIO, POLARITY, PHASE RELATION:
DEFINITION AND OBJECTIVE
©Doble Engineering Company
3T 2T 2V
VR = 3V/2V = 1.5
TR = 3T/2T = 1.5
In 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 the measurement and the voltage drop in the high-voltage winding. ©Doble Engineering Company
Ratio = N1/N2 = R1/R2
Polarity is determined via phase angle between two measured waveforms.
Phase relation is confirmed by testing the corresponding pairs 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
Balance
indicator
N2 N1
R1
H1 X0
H2
X2
Transformer in test
RATIO, POLARITY, PHASE RELATION:
SETUP AND TEST METHODOLOGY
©Doble Engineering Company
RATIO, POLARITY, PHASE RELATION:
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 of
all 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. ©Doble Engineering Company
RATIO, POLARITY, PHASE RELATION:
ABNORMAL DATA
To appreciate significance of 0.5% limit, it is instructive to
recognize the inherent errors this limit accommodates.
Actual turns RATIOTURN
Nameplate voltages
RATIONP
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 error
keeps RATIOmeas within
0.5% of RATIONP.
RATIOTURN
RATIONP
RATIOMEAS
©Doble Engineering Company
RATIO, POLARITY, PHASE RELATION:
RECOURSE IF DATA ABNORMAL
If deviation exceeds 0.5% for any of the measurements the
result is not acceptable.
The following steps should be considered:
Check if V/T exceeds 0.5% of nameplate voltage. If yes,
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 voltages selected by designer create a ratio that is too far (b 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.
©Doble Engineering Company
RATIO, POLARITY, PHASE RELATION:
COMPARISON WITH FIELD DATA
In verifying compliance with 0.5% deviation from the NP
voltages, the following should be recognized:
Older analog instruments produce results much closer to
the actual turns ratio than modern digital instruments.
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 with
external capacitor is to stress turn-to-turn insulation of both
windings for diagnostic purposes and not necessarily to
verify the 0.5% limit. In some cases, the latter could be
exceeded due to the loading effect of the test capacitor.
©Doble Engineering Company
WINDING DC RESISTANCE (Routine)
©Doble Engineering Company
WINDING DC RESISTANCE:
DEFINITION AND OBJECTIVE
Definition: Winding DC resistance is always defined as the
DC resistance of a winding in Ohms.
Objective: The measurement of winding resistance
provides the data for:
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 crimped
connections, problems with terminations and tap
changer contacts.
©Doble Engineering Company
WINDING DC RESISTANCE:
PHYSICS
i R
Domain
External
field
©Doble Engineering Company
WINDING DC RESISTANCE:
PHYSICS (cont.)
dy/dt
F = y/N
vmeas = iR + dy/dt R=vmeas / i
dy/dt
dy/dt
dy/dt
dy/dt
dy/dt
dy/dt
©Doble Engineering Company
WINDING DC RESISTANCE:
PHYSICS (cont.)
Time to stabilize resistance reading: On some units with closed
loops (e.g., GSU with two LV deltas or units with parallel
windings), it may take a long time for the reading to stabilize*; it
reduces with intermediate stability levels. This phenomenon is
not related to core saturation, which is saturating in a
reasonable time. However, as the core is being magnetized the
changing flux in the core induces voltage and sets up
circulating 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 currents
continues creating a changing flux in the core, inducing voltage
in the tested winding and thus changing the measured
resistance reading. Opening these loops, when possible,
reduces the time to stability.
* Personal communications with Bertrand Poulin, ABB, Quebec, Canada.
©Doble Engineering Company
WINDING DC RESISTANCE:
SETUP AND TEST METHODOLOGY
Data must be taken only when 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
Idc Vdc
+
+
Current
output
Voltage
input ©Doble Engineering Company
WINDING DC RESISTANCE:
ACCEPTANCE CRITERIA
Standards give no acceptance criteria; however, a deviation
from average of three phases of 0.5% for HV and 5% for LV
could serve as practical guideline.
As important as deviation is the assurance that test data is
credible:
No excitation with no pumps - 3h and with pumps - 1h,
TTO variation 2C for 1h, and TTO-TBO 5C. This assures
that 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.
©Doble Engineering Company
WINDING DC RESISTANCE:
ACCEPTANCE CRITERIA (cont.)
T stability: Experience* in the industry suggests that
relying on the T stability requirements given in the IEEE
standard does not produce a needed thermal equilibrium
and, consequently, an accurate measurement of the
winding dc resistance. To have a reliable 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 not
unreasonable 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.
©Doble Engineering Company
WINDING DC RESISTANCE:
ABNORMAL DATA
DETC H1-H3 H2-H1 H3-H2
1 3.7350 3.7352 3.7378
2 3.6470 3.6468 3.65023 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 97.720.4889 20.97440 97.719.9932 20.46944 97.719.6873 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 99.70.5855 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-X016 0.16537 0.16521 0.16499N 0.1566 0.1564 0.1562
Low-voltage winding
Comparison of each measurement with the average along with design data identifies an abnormal reading in H3-H2 with DETC in 4. This potentially can be caused by a problem with DETC contacts.
©Doble Engineering Company
WINDING DC RESISTANCE:
RECOURSE IF DATA ABNORMAL
If requirements associated with transformer thermal stability,
dc test current, influence of series unit or stability of the
reading are not met, a retest under different conditions
should be requested.
If acceptance criteria is exceeded, a justification from the
manufacturer should be requested. Potential problems may
include: bad crimping or brazing, incorrect conductor cross
section, loose connection, wrong design calculations.
©Doble Engineering Company
WINDING DC RESISTANCE:
COMPARISON WITH FIELD DATA
Typically, a deviation of <5% from the factory value is
considered acceptable.
A factory value is often reported as a sum of three phase
readings at rated T. For field comparison, the per-phase
values at corresponding DETC/LTC positions should be
requested from the factory.
Comparison should be performed for readings referred to the
same 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 is
measured via thermocouples and in the field the T gauge is
frequently the best option).
©Doble Engineering Company
NO-LOAD LOSSES AND
EXCITATION CURRENT (Routine) ©Doble Engineering Company
NO-LOAD LOSSES AND EXCITATION CURRENT:
DEFINITION AND OBJECTIVE
Definition: No-load losses include core loss, dielectric
loss, and conductor loss due exciting current, including
current circulating in parallel windings. Excitation current
is flowing in any winding exciting the transformer with all
other windings open-circuited.
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.
©Doble Engineering Company
NO-LOAD LOSSES AND EXCITATION CURRENT:
PHYSICS
I R
F
V
B
H
Hysteresis losses
Ph = f(Bmax)
Bmax = f(Vave) F
Ieddy
Eddy
losses
Pe = f(V2rms)
PNL = Pe + Ph
Domain
rotation ©Doble Engineering Company
NO-LOAD LOSSES AND EXCITATION CURRENT:
SETUP AND TEST METHODOLOGY
Start with 110% on N. As unit demagnetizes, 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 same voltage as seen by xfmr.
PNL not corrected for T if TTO-TBO 5C and 10TO_ave30C
Iexc=aver. of 3 phases in % of Irated
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
©Doble Engineering Company
NO-LOAD LOSSES AND EXCITATION CURRENT:
SETUP AND TEST METHODOLOGY (cont.)
Frequency control of motor-generator sets at GE large transformer plant in Pittsfield, MA during the early 1900. Since the primary function of these generators was to provide power for no-load loss tests, they were often referred to as magnetizers.
Historical perspective
Courtesy IEEE Power & Energy Magazine
©Doble Engineering Company
NO-LOAD LOSSES AND EXCITATION CURRENT:
ACCEPTANCE CRITERIA
Measured no-load losses should not exceed the
guaranteed value by more than 10% and the total losses by
more than 6%.
Assurance that test data is credible:
Test voltage is set based Vave
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 the
voltage waveform, one should look at the following
criteria of the applied voltage waveform: THD < 5%, 3rd
and 5th harmonics <10% and waveform should not have
any visible distortions.
* Personal communications with Bertrand Poulin, ABB, Quebec, Canada.
©Doble Engineering Company
NO-LOAD LOSSES AND EXCITATION CURRENT:
ACCEPTANCE CRITERIA (cont.)
Test in parallel and series configurations, if present.
If PA is present, compare the loss difference between
non-bridging and bridging positions (max turns) with
loss measured in PA out-of-tank. If SU unit is present,
compare the loss difference between N 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.
©Doble Engineering Company
NO-LOAD LOSSES AND EXCITATION CURRENT:
ABNORMAL DATA
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 or
series transformer or series autotransformer
Example: guaranteed no-loss - 28 kW, measured – 35 kW
©Doble Engineering Company
NO-LOAD LOSSES AND EXCITATION CURRENT:
RECOURSE IF DATA ABNORMAL
Failure to meet the no-load test loss tolerance should
not warrant immediate rejection but shall lead to
consultation between purchaser and manufacturer
regarding further investigation of possible causes and
the consequences of the higher losses.
The acceptance criteria of 10% does not replace the
manufacturer’s guarantee of losses for economic loss
evaluation purposes.
©Doble Engineering Company
NO-LOAD LOSSES AND EXCITATION CURRENT:
COMPARISON WITH FIELD DATA
Factory no-load losses and excitation test is performed
at rated voltage and three-phase excitation. Since the
open-circuit magnetizing impedance of a transformer is
non-linear, i.e., it is changing with applied voltage, a
comparison of exciting current and losses test results
obtained at low-voltage (e.g., 10 kV) and single-phase
excitation with results of the factory no-load losses and
excitation test is not possible.
©Doble Engineering Company
DIELECTRIC TESTS
©Doble Engineering Company
DIELECTRIC TESTS:
DEFINITION AND OBJECTIVE
Definition: Tests aimed to show that transformer is
designed and constructed to withstand the specified
insulation levels are referred to as dielectric tests. They
include:
high-frequency tests: lightning and switching 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.
©Doble Engineering Company
HIGH-FREQUENCY:
LIGHTNING IMPULSE (Class I - design or other,
Class II - routine) ©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
OBJECTIVE
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 transmission line, travels to substation and, after reaching the crest of the surge, causes arrester operation or flashover across an insulator near transformer terminals - chopped wave (a.k.a. tail-chopped).
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).
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
PHYSICS
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 simulated by discharging capacitor while chopped wave by the operation of a gap triggered to flashover at required time.
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
PHYSICS (cont.)
*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
H1 HV
to DETC
H0
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
PHYSICS (cont.)
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.
VT CW
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
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY
Full Wave Parameters
Magnitude FW = 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
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
Increase of series (front) resistor Rs
increases the time of voltage rise - T1.
0% change from given Rs
Cg CT
Rs
Rp
Data courtesy Reto Fausch, Haefely
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
Increase of parallel (tail) resistor Rp increases
the time of voltage decline to half value - T2.
Data courtesy Reto Fausch, Haefely
0% change from given Rp
Cg CT
Rs Rp
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
Data courtesy Reto Fausch, Haefely
Cg CT
Rs
Rp
Increase of series (front) resistor Rs decreases
the voltage trace overshoot - .
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
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
2.0 45÷75 1.5
95 1.8 110 2.0 125 2.3 2.3
150 3.0 TC < 6.0 30% 1
See C57.12.90-2010 for instances when could be >30% and >1s. It also permits adding resistors in chopping gap circuit to limit .
All times in the table are in s.
0.9
0.3
1.0
T1 TC
t
V
1.0
0.7
0.1
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
Front-of-Wave Parameters
Magnitude C57.12.00-2010 Annex A TC
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
©Doble Engineering Company
Current shunt and
meas. circuit
i(t)
RG
LG Glaninger: T2
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
T2
Impulse
control &
measuring
system
Voltage divider
and measuring
circuit
v(t)
Cg
Rs
Rp
Impulse
generator
LT, CT
xfmr
Very high di/dt induces difference
of potential. Hence, it is very
important for all return and
grounding leads to be made as
short as possible, with a minimum
R and L.
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.
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
Line terminal in Y
i(t)
Line terminal in
i(t)
Neutral terminal in Y
i(t)
i(t)
LV line terminal in Auto
HV line terminal in Auto
i(t) i(t)
Neutral terminal in Auto ©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
Test sequence and trace comparison
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 2
RFW 3 @ RFW2 voltage
RW 4
Neutral:
RFW@ 50-70% BIL)
FW1
FW2
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA
If test equipment and tested transformer were perfectly linear, the
traces of repeated impulses, when overlaid, would perfectly
match. However, due to noise, setup imperfections or insulation
failure, discrepancies occur. Identifying their nature is the
objective of impulse data analysis.
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.
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
450 kV BIL, RFW on
HV winding – voltage
450 kV BIL, RFW on
HV winding – current
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
450 kV BIL, CW1 on
HV winding – voltage
450 kV BIL, CW2 on
HV winding – voltage
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
Overlay of 450 kV BIL
CW1 and CW2 - voltage
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
450 kV BIL, FW on
HV winding – voltage
450 kV BIL, FW on
HV winding – current
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
Overlay of 450 kV BIL
RFW and FW - voltage
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
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.
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
450 kV BIL, FOW1 on
HV winding – voltage
450 kV BIL, FOW2 on
HV winding – voltage
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
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.
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
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.
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ABNORMAL DATA
In general, whenever discrepancies occur the normal test procedure
need to be stopped and investigation performed. If the cause is found to
be external to the transformer, the corrections are made before the test
can continue.
If there is any doubt as to the cause of the discrepancies, additional
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.
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ABNORMAL DATA (cont.)
Overlay of 550 kV BIL RFW and FW
voltage traces – turn-to-turn failure
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ABNORMAL DATA (cont.)
Overlay of 550 kV BIL RFW and FW
current traces – turn-to-turn failure
©Doble Engineering Company
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ABNORMAL DATA (cont.)
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
Voltage drop to ground
indicates one of the leads
was at ground potential
Fault to ground diverts
current around winding,
reducing measured current.
©Doble Engineering Company
HIGH-FREQUENCY:
SWITCHING IMPULSE (Class I – other,
Class II <345 kV – other,
Class II 345 kV - routine) ©Doble Engineering Company
HIGH-FREQUENCY - SWITCHING IMPULSE:
OBJECTIVE
kV
s
FOW
CW
FW
SW
Demonstrate performance under transient high-frequency conditions
created by switching operations or network disturbance.
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 than those of lightning impulses.
©Doble Engineering Company
HIGH-FREQUENCY - SWITCHING IMPULSE:
PHYSICS
length
V
Comparing to lightning impulse, the switching impulse has a much longer duration and lower frequency, resulting in voltage approaching a uniform distribution of the low-frequency steady-state voltages, i.e., voltage distributes as per turns ratio.
V
Switching impulse test consists of applying or inducing a SW between each HV line terminal and ground. Similar to a lightning wave, the switching wave can be simulated by discharging a capacitor.
©Doble Engineering Company
HIGH-FREQUENCY - SWITCHING IMPULSE:
PHYSICS (cont.)
*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 D H1 HV
Region D – phase-to-ground and phase-to-phase insulation is stressed the most; stress imposed by SW is 1turns*. Charging and discharging processes are similar to those described for lightning impulse.
H1
H0
D D
D
To another
phase
©Doble Engineering Company
HIGH-FREQUENCY - SWITCHING IMPULSE:
SETUP AND TEST METHODOLOGY
Full Wave Parameters
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
Tp Td
t
V
Crest voltage
First zero crossing T0
Virtual
origin
>90% of crest
©Doble Engineering Company
HIGH-FREQUENCY - SWITCHING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
Xfmr
Impulse
control &
measuring
system
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.
©Doble Engineering Company
HIGH-FREQUENCY - SWITCHING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
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
E/2
ELV E
E/2 -ELV/2
ELV
E
-ELV/2
ELV
-E/2
©Doble Engineering Company
HIGH-FREQUENCY - SWITCHING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
SW can saturate the core, creating an air-core conditions, i.e.,
drastically reducing impedance faced by impulse. This rapidly
decays the tail of the voltage waveform to zero, making T0<1000
s. To extend the time to saturation, prior to start of each test,
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.
©Doble Engineering Company
HIGH-FREQUENCY - SWITCHING IMPULSE:
ACCEPTANCE CRITERIA
Tp, Td, T0, and voltage magnitude must meet requirements.
Failure detection is done primarily by scrutinizing voltage
traces for recognizable indications of failure. The test is
successful if there is no sudden collapse of voltage as
indicated on the trace.
Although overlaying RSW and SW traces in totality may
not be practical, the traces should match until the point
where 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.
©Doble Engineering Company
HIGH-FREQUENCY - SWITCHING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
650 kV BIL, RSW on
HV winding – voltage 650 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 HV winding; for 650 kV BIL, the BSL, i.e., the required test voltage, is 540 kV.
©Doble Engineering Company
HIGH-FREQUENCY - SWITCHING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
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
©Doble Engineering Company
HIGH-FREQUENCY - SWITCHING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
Overlay of 650 kV BIL
SW1 and SW2 - voltage
Slight deviation due to the
difference in core magnetic
state.
©Doble Engineering Company
HIGH-FREQUENCY - SWITCHING IMPULSE:
ABNORMAL DATA
In general, whenever discrepancies occur the normal test
procedure need to be stopped and investigation performed. If
the cause is found to be external to the transformer, the
corrections are made before the test can continue.
If there is any doubt as to the cause of the discrepancies,
additional impulses may be applied.
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. ©Doble Engineering Company
HIGH-FREQUENCY–LIGHTNING AND SWITCHING IMPULSE:
RECOURSE IF DATA ABNORMAL
If visual confirmation (e.g., carbon, bubbles) is obtained
or the data convincingly reveals a failure, the oil is
drained and internal inspection is performed.
If necessary, the unit is un-tanked. This is followed by a
thorough and well-documented investigation.
The user’s involvement in this process enhances the
quality of the investigation and that of the final product.
©Doble Engineering Company
LOW-FREQUENCY:
APPLIED VOLTAGE (Routine) ©Doble Engineering Company
LOW-FREQUENCY – APPLIED VOLTAGE:
OBJECTIVE
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 min
duration, with the expectation to demonstrate a long-term
capability to operate at the rated voltage.
The high-frequency tests (lightning and switching
impulse) always precede the low-frequency tests (applied
and induced voltage). This sequence is rooted in the fact
that due to a longer duration, the low-frequency tests
serve to stress further and to detect the damage caused
by the high-frequency tests.
©Doble Engineering Company
LOW-FREQUENCY – APPLIED VOLTAGE:
PHYSICS
Region D – major winding -to-ground and winding-to-winding insulation are stressed the most. LV
D
HV
D
HV
LV
Shorting
lead
D
©Doble Engineering Company
LOW-FREQUENCY – APPLIED VOLTAGE:
SETUP AND TEST METHODOLOGY
Applied Voltage Parameters
Magnitude C57.12.00-2010 Duration 1 min
Test is performed at low frequency (<500 Hz), normally, power 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
©Doble Engineering Company
LOW-FREQUENCY – APPLIED VOLTAGE:
ACCEPTANCE CRITERIA
The test is a pass/fail test and is considered
passed if during the time the voltage is applied no
evidence of possible failure is observed.
The indications to monitor include unusual sound
such as thump, sudden increase in the test circuit
current and collapse in the test voltage.
©Doble Engineering Company
LOW-FREQUENCY – APPLIED VOLTAGE:
ABNORMAL DATA
If unusual sound, sudden increase in the test
circuit current or circuit tripping occur, these
events should be carefully investigated by:
• observation, e.g., presence of carbon
and/or bubbles in the oil
• repeating the test
• other tests
to 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.
©Doble Engineering Company
LOW-FREQUENCY – APPLIED VOLTAGE:
RECOURSE IF DATA ABNORMAL
If visual confirmation (e.g., carbon, bubbles) is obtained
and/or repeating of the test and/or other tests reveal the
failure, the oil is drained and internal inspection is
performed.
©Doble Engineering Company
LOW-FREQUENCY:
INDUCED VOLTAGE/PD
Induced:
7200 cycles
Induced:
1 hour + PD
Class I
Class II
Routine
Routine
©Doble Engineering Company
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
OBJECTIVE
The induced voltage test demonstrates the strength of
internal insulation in all windings as well as between
windings and to ground. A combination of prolonged stress
and a very sensitive PD measurement makes it a very severe
and searching test. It must be the last dielectric test to be
performed.
©Doble Engineering Company
LOW-FREQUENCY - INDUCED VOLTAGE/PD:
PHYSICS
To stress turn-to-turn insulation to the required level, the winding needs to be excited to a level approaching twice rated voltage. At power frequency, this would overexcite the core.
Therefore, the test is performed as a higher frequency, which allows to obtain the needed volts/turn at a lower flux magnitude (v/t = dF/dt).
At higher frequency, transformers become capacitive with dangers of M-G set overexciting. This is addressed by using a variable reactor. The latter provides an additional benefit of reducing the load on MG set.
R C M G
L
Lv
xfmr
Variable reactor Lv is adjusted to reduce output from generator.
IG
VT
ILv
IT
VT
IG
IT
ILv
©Doble Engineering Company
LOW-FREQUENCY - INDUCED VOLTAGE/PD:
PHYSICS (cont.)
Region E – with voltage distributing per turns ratio, the most stress is present in the turn-to-turn insulation of each winding as well as in winding-to-winding and winding-to-ground insulation.
LV
E
HV
E
E
E
E
E
©Doble Engineering Company
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
PHYSICS (cont.)
From the physics point of view, self-sustaining electron avalanches
may occur only in gases. Hence, discharges in dielectrics may
only be ignited in gas-filled cavities, such as voids or cracks in
solid materials and gas bubbles or water vapor in liquids.
Discharges are generally ignited if the electrical field strength
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 or
may not occur adjacent to a conductor. In
insulation, the PD events are the consequence
of local field enhancements due to dielectric
imperfections.
Gaseous
inclusion
Conductor
Dielectric
©Doble Engineering Company
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
PHYSICS (cont.)
To model the PD process, capacitance of the active void CC can be
viewed as part of a larger capacitive network. In that, CB is the
remaining capacitance of the immediate region in series with CC
and CA is the rest of the dielectric connected in parallel. Two
requirements must be fulfilled to initiate PD: 1) local field stress
exceeds the void’s breakdown voltage Vbd and 2) free electrons are
available.
CC
CB
CA
CC
CB CA
Vbd
©Doble Engineering Company
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 come
out of adjacent insulation.
CC
CB
CA
V
CC
CB
CA
Q
Q
Q Q Vcc= 0
V
Q Q Q
Q
Vcc= 0
CC
CB
CA
Q
Q Q
Q
Vcc
V
Buildup: As V , charges
move to and collect on
the surface of the void,
building potential stress
Vcc across the void.
CC
CB
CA Q Q Q
V
Vcc
CC
CB
CA
Q
Q
Vcc
V
Relaxation: Charges continue
to flow at a decreasing rate
with balance restoring. Vcc
as charges collect back on
the void’s surface.
CC
CB
CA Q
V
Vcc
Q
©Doble Engineering Company
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
PHYSICS (cont.)
We cannot measure the real charge. However, as the void discharges, the
charge redistribution creates a dip* in the terminal voltage. This minute voltage
drop causes a high-frequency current to flow through a coupling capacitor
connected to a measuring system. Putting it differently, the charge movements
appear, in part, in C1 connected in parallel with CT. The integration of these high-
frequency current pulses over time produces the reported apparent charge.
Terminal
voltage
Voltage
across void PD current
Dip in terminal
voltage
C1
C2 Z
M
CT
*The detectable voltage dip is in the mV
range, while that at the void may be in
the kV range.
©Doble Engineering Company
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
PHYSICS (cont.)
Measurement of partial discharge is
like trying to weigh a butterfly that
alights momentarily on scales
designed for an elephant (sometimes
during an earthquake). by Karl Haubner, Doble Australia
©Doble Engineering Company
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
SETUP AND TEST METHODOLOGY
M G
Lv
Xfmr in
test
X0
H2 X1
X2
X3
H1
H3
C1
C2
C1
C2
C1
C2 M
Step-up
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.
©Doble Engineering Company
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
SETUP AND TEST METHODOLOGY (cont.)
Induced Voltage/PD
Parameters
Voltage magnitude
C57.12.00-2010 clause 5.10
C84.1
Timing C57.12.90-2010
clauses 10.7, 10.8
PD/RIV criteria
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 voltage
for 7200 cycles and 1.58nominal voltage for 1 h, recording PD (RIV) data.
For windings other than HV, when possible, taps should be selected so that
voltages on other windings are as per ANSI C84.1 and C57.12.90 clause
10.8.1 (e.g., for 115÷345 kV units , the voltage on other windings should be
1.5 times their maximum operating voltage).
Ambient Ambient
100%
Enhanced level
7200 cycles 1h level, 5 min
recordings
100%
t
V
Hold as needed
until stable (min
60 sec)
1h
©Doble Engineering Company
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
>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 factory
machinery
- removing air bubbles from bushing gas space
- applying pressure to suppress bubbles in the main tank.
©Doble Engineering Company
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
ACCEPTANCE CRITERIA
Results are acceptable if:
Nothing unusual associated with sound, current, or voltage
is observed (see abnormal data for details).
The PD (RIV) results during 1h test period have 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.
©Doble Engineering Company
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
ACCEPTANCE CRITERIA (cont.)
V1 PD1 RIV1 Time1 V2 PD2 RIV2 Time2 V3 PD3 RIV3 Time3
1 0.2 kV 15.4 pC 4.5 µV 00:00:03 0.3 kV 14.6 pC 5.3 µV 00:00:11 0.2 kV 138. pC 4.5 µV 00:00:20 Ambient
2 30.4 kV 24.7 pC 4.5 µV 00:00:49 30.6 kV 26.3 pC 5.3 µV 00:00:58 30.5 kV 27.2 pC 4.2 µV 00:01:07 100%
3 37.8 kV 27.1 pC 4.4 µV 00:01:51 37.6 kV 38.3 pC 5.6 µV 00:02:00 37.7 kV 29.7 pC 4.6 µV 00:02:09 125%
4 42.4 kV 36.7 pC 5.2 µV 00:03:33 42.0 kV 29.2 pC 7.1 µV 00:03:42 42.1 kV 30.7 pC 4.7 µV 00:03:51 1hr level
5 55.5 kV 31.9 pC 4.9 µV 00:04:03 54.5 kV 33.5 pC 13.5 µV 00:04:12 54.7 kV 33.9 pC 6.2 µV 00:04:21 Enhanced
6 42.3 kV 27.1 pC 4.6 µV 00:00:03 41.9 kV 29.3 pC 5.6 µV 00:00:35 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
©Doble Engineering Company
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
ABNORMAL DATA
Results are not acceptable if the pC (or V) data exceeds any
of the required criteria, and no reasonable/acceptable
justification for the source/cause is provided.
Other tests, e.g., acoustic PD, DGA, can provide 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. ©Doble Engineering Company
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
RECOURSE IF DATA ABNORMAL
If pC (or V) data exceeds the limits, and all the attempts
to identify and eliminate external PD sources are not
successful, a longer standing time, long duration PD
test, degassing of oil, refilling transformer under
vacuum or a heatrun test (if one is specified) are often
successfully bring the PD data within limits.
A failure to meet the partial discharge acceptance
criterion 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 the
failure, the oil is drained and internal inspection is
performed.
©Doble Engineering Company
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.
©Doble Engineering Company
NO-LOAD LOSSES AND EXCITATION CURRENT after dielectrics:
OBJECTIVE
Objective: No-load loss and excitation current, measured
at 100% and 110% of the specified voltage and frequency
after all dielectric tests are completed, provide additional
confirmation that no damage, created by dielectric tests, is
present in the transformer. If this is the last power test to
be performed, it also serves to demagnetize the core for
subsequent low-voltage tests, e.g., 10-kV exciting current
and sfra.
©Doble Engineering Company
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
to:
Changes in the inter-laminar insulation
Temperature
Sometimes the change after initially exceeding 5% goes
away with time.
Failure to meet before and after dielectrics comparison
criteria should not warrant immediate rejection but shall
lead to consultation between purchaser and manufacturer
regarding further investigation of possible causes and
consequences.
©Doble Engineering Company
LOAD LOSSES AND
IMPEDANCE VOLTAGE (Routine) ©Doble Engineering Company
LOAD LOSSES AND IMPEDANCE VOLTAGE:
DEFINITION AND OBJECTIVE
Definition: The load losses of a transformer are losses
associated with a specified load and include:
windings I2R losses due to load current
stray losses due to eddy currents induced by leakage flux in
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.
©Doble Engineering Company
LOAD LOSSES AND IMPEDANCE VOLTAGE:
DEFINITION AND OBJECTIVE (cont.)
Objective: The impedance and load losses test provides the
data for:
Verification of design calculations.
Demonstration of meeting the guaranteed performance
characteristics. Since these parameters have often an
economic value attached to them, the accuracy of the
measurement becomes significant.
Maximum load losses are used as test parameter during
the temperature rise test.
Impedance voltage is an essential input parameter in power
system studies (e.g., load flow, transformer parallel
operation, short-circuit calculations).
©Doble Engineering Company
LOAD LOSSES AND IMPEDANCE VOLTAGE:
PHYSICS
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 then
dominated by the leakage channel where the eddy losses in
various conducting components in the FL path are induced.
R
LV HV
FM
Vrated
Iexc R
LV HV
Irated
I2R lossesT
Eddy currents
creating losses1/T
FL
Vsc
Note: Resistance R and short
circuit of LV is not shown.
©Doble Engineering Company
LOAD LOSSES AND IMPEDANCE VOLTAGE:
PHYSICS (cont.)
Rm Xm
XHV RHV XLV RLV
VSC
XL RL
Irated
VR_L VX_L
Irated VR_L
VX_L
VSC
Corresponds
to load loss
Corresponds
to leakage-flux
linkages of the
windings
Measured
ZSC
For most power transformers, VX_L >> VR_L.
Angle is close
to 90, requiring
high accuracy
test systems.
Compensating variable
capacitor Cc is adjusted
to reduce the input
current. VSC Iinput
IC
Irated
IC
Iinput
CC
©Doble Engineering Company
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 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.
Transformer
in test CT
VT
W
I
V
A
X0
H2
X1
X2
X3
H1
H3
V
If taps are present, the following
combinations of voltage ratings are tested:
3
DETC rated rated rated max max max min min min
LTC N max min N max min N max min
©Doble Engineering Company
LOAD LOSSES AND IMPEDANCE VOLTAGE:
SETUP AND TEST METHODOLOGY (cont.)
For 3-wdg units, three sets of measurements are performed using three pairs of windings, 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.
1
2
3
1 2
3
Z1
Z2
Z3
Z12 = Z1 + Z2
Z13 = Z1 + Z3
Z23 = Z2 + Z3
Z1 = (Z12 + Z13 – Z23)/2
Z2 = (Z12 + Z23 – Z13)/2
Z3 = (Z13 + Z23 – Z12)/2
©Doble Engineering Company
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 measured losses, individually converted from test T to rated T before combined again in reported load losses. V is also converted to rated T.
Convert Rdc from
TR_test TLL_test
Measure A, V, W, T
Correct W and V
from measured
amps to rated
Calculate I2R losses
at TLL_test
Calculate stray
losses at TLL_test
(W - I2R)
Convert I2R losses
from TLL_test Trated
Convert stray
losses from
TLL_test Trated
Calculate total
losses at Trated
(stray + I2R)
Calculate %Vsc
(V / Vrated)100 = %Zsc
Correct V from
TLL_test Trated
©Doble Engineering Company
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%.
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 duplicates
units 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.
©Doble Engineering Company
LOAD LOSSES AND IMPEDANCE VOLTAGE:
ABNORMAL DATA
Potential reasons for exceeding the guaranteed values may
include:
Oversights in design
Production process related factors or mistakes
Influence of temperature was not properly accounted for
Accuracy of measurements
Example: guaranteed load loss - 94 kW, measured – 110 kW
©Doble Engineering Company
LOAD LOSSES AND IMPEDANCE VOLTAGE:
RECOURSE IF DATA ABNORMAL
Failure to meet the load losses and impedance test
criteria should not warrant immediate rejection but shall
lead to consultation between purchaser and
manufacturer regarding further investigation of possible
causes and consequences.
The acceptance criteria of 6% for total losses does not
replace the manufacturer’s guarantee of losses for
economic loss evaluation purposes.
©Doble Engineering Company
LOAD LOSSES AND IMPEDANCE VOLTAGE:
COMPARISON WITH FIELD DATA
Factory and field results
cannot be compared
Factory losses are measured
under 3-phase excitation, at
rated current and reported as
sum of three phases I2R and
stray losses.
Field losses are measured
under 1-phase excitation, at
current much lower than rated
and reported as per-phase I2R
and stray losses.
©Doble Engineering Company
LOAD LOSSES AND IMPEDANCE VOLTAGE:
COMPARISON WITH FIELD DATA (cont.)
*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 the
measured value, results normally are considered acceptable. The initial
per-phase test should serve as a benchmark for future testing with
acceptable difference from the initial field test being <2%.
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.
©Doble Engineering Company
TEMPERATURE RISE (Design and other)
©Doble Engineering Company
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
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 instruments
and cooling control.
©Doble Engineering Company
TEMPERATURE RISE:
PHYSICS
Measured:
Tto, Tt_rad, Tb_rad, Ta.
Tto Tt_rad
Tb_rad
Rad
Tb_rad
Oil
Winding
Tt_rad
Tto-a
Tw_ave*
Ta
Tw_ave-a
Tto
LV HV
Ta
T
To_ave
GRAD
Main tank
Core
Tw_hs
Ta
Ta
height
Calculated: Tto-a, Tw_ave-a, Ths-a, GRAD
*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.
Needs NL+LL
losses Need rated
current
Ths-a
Located at 3 locations around xfmr at mid-height level.
©Doble Engineering Company
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 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)
Transformer
in test CT
VT
W
I
V
A
X0
H2
X1
X2
X3
H1
H3
V ©Doble Engineering Company
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
Tto, Tt_ rad
Tb_ rad
Ta_ ave
Tto-a
t [h]
T[C]
Preceding ONAN
Cutback
ONAF shutdown
Xfmr
energized
for ONAF
ONAN
shutdown
Rhot
measurement
begins
Rated current run
Steady-state oil
T rise (change of Tto-a in 3h
1C or 2.5%
whichever is greater)
Itest
Ptotal
Irated
Measurement before
cutback determines *Tto-a
*Tto-a is corrected for difference between required and
actually used total losses (it must be 20%) and for altitude.
Total loss run
1h
©Doble Engineering Company
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
Rhot
t [min]
Rhot as function of time in
presence of decreasing
temperature is recorded
t 4 min
Tw_hot = Rhot/Rcold(234.5 + Tw_cold) – 234.5
Rhot calculated at t = 0
*If two windings are tested simultaneously in series,
the Idc is selected based on the lowest rated current.
*Instrument
connected
Instrument output
current reached
pre-selected level Flux
stabilized
t = 0
Voltage
removed
Objective: resistance of
winding at the time when
load current is still present
©Doble Engineering Company
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
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
Tw_ave-a
Comparison with guaranteed values,
e.g., Tto-a and Tw_ave-a 65C; Ths-a 80C
*GRAD To_ave_sd
*GRAD is corrected for difference between required and actually used load current (it must be 15%).
Tto-a
GRAD
GRAD correction for
localized hot spot
eddy currents ***Ths-a
Value used for
setting winding
T monitors
***This a simplified representation of Ths_a determination; actual design calculation is more involved.
©Doble Engineering Company
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
Series autoxfmr
windings with LTC in N LV winding
idc
vLV
idc
vLV X2 X0
i2
i1
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.
X2 X0 idc
i1
i2
idc
idc
Main unit
core
Series autoxfmr
core
F F1 F2
©Doble Engineering Company
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
0.00462
0.00463
0.00464
0.00465
0.00466
0.00467
0.00468
0.00469
0.0047
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]
LV
cir
cu
it R
dc
[o
hm
]
0.25
0.26
0.27
0.28
0.29
0.3
0.31
0.32
HV
circ
uit R
dc
[Oh
m]
X 0-X 2
H1-H2
Setup
1.5 min
Voltage
removed
t = 0
Time remaining
for stabilization
= 2.5 min
At t = 4 min, data
collection begins
with flux in main
core stable while
flux in series core
still changing.
Series autoxfmr should be
excluded from both cold
and hot Rdc measurements.
©Doble Engineering Company
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
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 certified
test report and have software to view them.
If agreed with manufacturer, the heatrun is a good time to check the
performance of temperature controllers (using a preliminary winding
T gradient) and turns ratio of CTs.
©Doble Engineering Company
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
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 be
measured 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 4
min after shutdown.
*Lachman, M. F., et al “Impact of Series Unit on Transformer Winding DC Resistance Measurement During Heatrun”, Proc. of the Seventy-Sixth Annual Intern. Confer. of Doble Clients, 2009, Sec. T-4.
©Doble Engineering Company
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
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. ©Doble Engineering Company
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
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
0:00
0:30
1:00
1:30
2:00
2:30
3:00
3:30
4:00
4:30
5:00
5:30
6:00
6:30
7:00
7:30
8:00
8:30
9:00
9:30
10:0
0
Time [min]
Re
sis
tan
ce
[o
hm
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
0:00
0:30
1:00
1:30
2:00
2:30
3:00
3:30
4:00
4:30
5:00
5:30
6:00
6:30
7:00
7:30
8:00
8:30
9:00
9:30
10:0
0
Time [min]
Re
sis
tan
ce
[o
hm
s]
Without series auto-xfmr
Note: The example shows a quadratic function, the suitability of which was confirmed via direct fiberoptic measurements and other methods, e.g., Blume. Different functions may be used if they fit the winding behavior.
©Doble Engineering Company
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.
©Doble Engineering Company
ZERO-PHASE SEQUENCE
IMPEDANCE (Class I - design
Class II - routine) ©Doble Engineering Company
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
as input in analysis of unbalanced three-phase system
using symmetrical components method.
©Doble Engineering Company
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 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.
Ia
Ib
Ic
Unbalanced
Ia
Ib Ic
Balanced
Ia1
Ib1
Ic1
Positive
Ib2 Ia2
Ic2 Negative
Iao
Ibo
Ico
Zero
©Doble Engineering Company
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 test applies only to transformers with one or more windings with a physical neutral brought out for external connection.
Z0
1 2
N
1 2
©Doble Engineering Company
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
Z1 Z2
Z3
1 2
N
Z1No
Z2No
©Doble Engineering Company
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.
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)
Transformer in
test
CT
VT
W
I
V
A V
1
©Doble Engineering Company
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, Zsc = 21.9%, Z0 = 21.8%
For Y/ units, Z0 (0.8-1.0)Zsc. Example: 48 MVA, 235.75GndY/13.8 kV, Zsc = 9.9%, Z0 = 8.5%
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, Zsc = 4.9%, Z1 = 3.6%,
Z2 = 0.84%, Z3 = 10%
50 MVA, 69GndY/34.5GndY/13.2 kV, Zsc = 7.8%, Z1 = 6.7%,
Z2 = -0.16%, Z3 = 4.6%
For Y/Y and autoxfmrs without delta (rare occasion), magnetic
flux has a strong coupling to the tank, making, in general, the
relationship between voltage and current non-linear and the
above observations not relevant. Example: Auto, 75 MVA, 115GndY/34.5GndY kV, Zsc = 12.7%, Z1 = -9.9%,
Z2 = 27.4%, Z3 = 205.6%
©Doble Engineering Company
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.
©Doble Engineering Company
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.
©Doble Engineering Company
AUDIBLE SOUND LEVEL (Design and other)
©Doble Engineering Company
AUDIBLE SOUND LEVEL:
DEFINITION AND OBJECTIVE
Definition: The audible sound level test is the measurement
of the sound pressure level around a fully assembled
transformer under the rated no-load conditions with cooling
equipment operating as appropriate for the power rating
being tested.
Objective: To protect the population from noise
inconveniences transformers are required to operate within
specified noise limits. The audible sound level test provides
the 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.
©Doble Engineering Company
AUDIBLE SOUND LEVEL:
PHYSICS
Most of xfmr sound is generated by the core. When the core steel magnetized/demagnetized twice each cycle, the steel elongates and shortens due to a property called magnetostriction.
This produces a vibratory motion in the core transmitted to the tank through the core mechanical support and the pressure waves in the dielectric fluid. At the tank this motion radiates as an airborne sound. The vibration magnitude depends on the flux density and magnetic property of the steel.
The frequency spectrum of the sound contains mainly the even harmonics of the power frequency, i.e., 120, 240, 360, etc. The audible sound also includes a contribution emitted by pumps and fans, containing a broadband spectrum of frequencies.
Core
Dielectric
fluid
Tank
Direction of
dimensional
change Magnetostriction
caused by
domain rotation
F
©Doble Engineering Company
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 for the tested rating.
On certain tap positions, xfmr may produce sound levels greater than at the principal tap, e.g., engaging PA and/or 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 max rating.
Measurements begin when xfrm reaches steady-state conditions, i.e., to allow magnetic bias to decay.
VT
Vave
Transformer
in test
X0
H2 X1
X2
X3
H1
H3
3
©Doble Engineering Company
AUDIBLE SOUND LEVEL:
SETUP AND TEST METHODOLOGY (cont.)
Microphones are located on the measurement surface at shown distance from reference sound-producing surface.
Xfmr is placed so that no acoustically reflecting surface is within 10 ft of the microphone.
If transformer H<7.9ft, measurements are made at H/2; if H7.9 ft, at H/3 and 2H/3.
First measurement is made at drain valve proceeding clockwise.
Reference sound-producing
surface is a vertical surface
following the contour of a taut
string stretched around xfmr
periphery.
Tank
LTC Drain valve
Radiator
6 ft 3 ft
1 ft
#1
Fan cooled surface
Measurement
surface
Microphone
location
©Doble Engineering Company
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
specified)
Narrowband sound pressure level (when specified) ©Doble Engineering Company
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-
weighting is most commonly used to allow for a broad peak between 1÷6 kHz but very strongly discriminating 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
A-weighted sound pressure level
©Doble Engineering Company
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.
The narrowband sound pressure level measurement is
performed 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.
©Doble Engineering Company
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.
Compute ambient-corrected sound pressure levels. For
corrections see Table 7 in C57.12.90-2010.
Compute average sound pressure levels [in dB(A)]:
𝑳𝒑 = 𝟏𝟎𝒍𝒐𝒈𝟏𝟎𝟏
𝑵 𝟏𝟎
𝑳𝒊𝟏𝟎
𝑵
𝒊=𝟏
Li is the sound pressure level measured at ith location by one of the 3
measuring methods. Sound power levels are calculated when requested.
©Doble Engineering Company
AUDIBLE SOUND LEVEL:
ACCEPTANCE CRITERIA
Computed average sound pressure level should not exceed the
audible sound levels as listed in NEMA TR1-1993, Tables 0-2 and 0-3
or as requested in customer test specification. Rectifier, railway,
furnace, grounding, and mobile transformers are not covered by
these tables.
Assurance that test data is credible:
The sound pressure measuring instrument should meet the
requirements of ANSI S1.4 for Type 1 meters.
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.
©Doble Engineering Company
AUDIBLE SOUND LEVEL:
ABNORMAL DATA
Potential reasons for exceeding the guaranteed values may
include:
Problems with measurement, e.g., ambient noise,
positions of microphones, sound instrument calibration,
voltage adjustment, surrounding reflecting surfaces,
etc.
Variability in core steel characteristics
Different core steel
Oversights in design
Assembly related factors or mistakes
Example: guaranteed sound pressure level per NEMA –
75/77/78 dB(A), measured – 77/78/79 dB(A)
©Doble Engineering Company
AUDIBLE SOUND LEVEL:
RECOURSE IF DATA ABNORMAL
Failure to meet the audible sound test criteria should not
warrant immediate rejection but shall lead to consultation
between purchaser and manufacturer regarding an
investigation of possible causes and solutions to address
the problem.
©Doble Engineering Company
CORE DEMAGNETIZATION (Routine*)
*This procedure is not required by standards but is a wildly
recognized as standard practice and performed as routine.
©Doble Engineering Company
CORE DEMAGNETIZATION:
DEFINITION AND OBJECTIVE
Definition: The core demagnetization is the process of
removing the magnetic bias in the core through a series
of steps, with each subsequent step creating magnetic
field of opposite direction and lower intensity. The first
step must bring the core to the main hysteresis loop with
the last step, upon removal, leaving no residual
magnetism in the core.
Objective: The core demagnetization creates conditions
for obtaining the low-voltage exciting current and loss
test as well as sfra benchmark data not affected by
residual magnetism .
©Doble Engineering Company
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
around the origin, is reached.
Main
loop
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 of
voltage to zero brings working point
to the center of these loops resulting
in a demagnetized transformer.
Br = 0
Br
H
B
©Doble Engineering Company
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
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.
©Doble Engineering Company
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 the
condition immediately following
demagnetization*. This is obvious
in the low-frequency range of the
sfra trace but not in the low-voltage
excitation current data. These sfra
changes are normal and
diagnostically insignificant.
Factory
Field
Field
Factory
Data movement
with no excitation
applied between
measurements
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_init
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 Doble Clients, 2010, Sec. TX-11.
©Doble Engineering Company
THE END
LAST SLIDE
©Doble Engineering Company
UNDERSTANDING THETRANSFORMER TEST DATA
Barry M. Mirzaei – P.Eng.Hydro One
September 2012 – Chicago
©Doble Engineering Company
2Understanding The Transformer Test DataSeptember 2012
©Doble Engineering Company
3
No Load TestTest object is supplied from one side of the transformer(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 ratedvoltage
Characteristics of the No Load Test:“Low Current – High Voltage”
Understanding The Transformer Test DataSeptember 2012
©Doble Engineering Company
4
Induced Voltage Test
Test object is supplied from one side of thetransformer (L.V.), the other side (H.V.) is left opencircuit. Test voltage to be adjusted to the pre‐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
©Doble Engineering Company
5
Load Loss TestTest object is supplied from one side (H.V.), theother side (L.V.) is short‐circuited. Test voltage isadjusted 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
©Doble Engineering Company
6Understanding The Transformer Test DataSeptember 2012
©Doble Engineering Company
7
Hysteresis Loss
Proportional to the frequencyand dependent on the area ofthe hysteresis loop, which, inturn, is a characteristic of thematerial and a function of thepeak flux density
Understanding The Transformer Test DataSeptember 2012
©Doble Engineering Company
8
Eddy Current Loss
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
©Doble Engineering Company
9
4.44 (a)
(b)
Voltage
PNL NoLoadLosses= HysteresisLoss= EddyCurrentLoss, = Coefficients
= = Exponentwithinduction
Understanding The Transformer Test DataSeptember 2012
©Doble Engineering Company
10Understanding The Transformer Test DataSeptember 2012
Minimizing hysteresis loss thus depends onthe development of a material having aminimum area of hysteresis loop.
Minimizing eddy current loss is achieved bybuilding up the core from a stack of thinlaminations and increasing resistivity of thematerial in order to make it less easy foreddy currents to flow.
©Doble Engineering Company
STATEMENT OF THE ISSUE:
11Understanding The Transformer Test DataSeptember 2012
©Doble Engineering Company
12
Deflection in the readings on meteringdevices (watt meters, …) were reportedwith the noise.
During the No Load test of a rebuilt 3phase 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 bubblesinside the oil”, started at around the 25%of the test voltage.
Understanding The Transformer Test DataSeptember 2012
©Doble Engineering Company
13
Solutions?What test data are available?
What those test data really mean?
Understanding The Transformer Test DataSeptember 2012
©Doble Engineering Company
14
Criteria & constraints for addressing the issue
Un‐necessary activities to beavoided, 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
©Doble Engineering Company
15
Investigation Procedure
1Oil Sample
OK 2Repeat TTR & DC Resistance
OK
3Observe The No Load TestPROBLEM
4Apply Load
TestOK
5Insulation Test?Not
Convinced to apply
6Apply reduced “Induced Voltage”
Understanding The Transformer Test DataSeptember 2012
©Doble Engineering Company
16Understanding The Transformer Test DataSeptember 2012
The Induced Voltage Teststresses all parts of theinsulation system, includingturn to turn, phase to phaseand winding to ground.©Doble Engineering Company
17
4.44 (a)
Concept of customized Induced test:
By applying induced voltage up to rated voltage, basically the no loadtest is being repeated with reducedinduction in the core
Understanding The Transformer Test DataSeptember 2012
©Doble Engineering Company
18Understanding The Transformer Test DataSeptember 2012
.
.x x x x
.
.x x x x
x x x x
©Doble Engineering Company
19Understanding The Transformer Test DataSeptember 2012
Investigation Procedure
1Oil Sample
OK2
Repeat TTR & DC Resistance
OK
3Observe The No Load TestPROBLEM
4
Apply Load TestOK
5Insulation Test?
Not Convinced to apply
6Apply reduced
“Induced Voltage”
Load Loss:‐Resistive losses or R‐Eddy current losses in the windings‐Stray losses in leads, core plates and tank
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
Proportional to the frequency and dependent onthe area of the hysteresis loop, which, in turn, is acharacteristic of the material and a function of thepeak flux density
©Doble Engineering Company
20Understanding The Transformer Test DataSeptember 2012
©Doble Engineering Company
21Understanding The Transformer Test DataSeptember 2012
Core bolts are inserted through the core for the purpose of clamping the core laminations. ©Doble Engineering Company
22Understanding The Transformer Test DataSeptember 2012
During “Core Stacking Process” –Holes built for Core Bolts, used for proper core stacking
©Doble Engineering Company
23Understanding The Transformer Test DataSeptember 2012
Core Plates Core Bolts
Photo belongs to another transformer
©Doble Engineering Company
24Understanding The Transformer Test DataSeptember 2012
Core Plate
Core Bolt
Weld
©Doble Engineering Company
25Understanding The Transformer Test DataSeptember 2012
©Doble Engineering Company
26Understanding The Transformer Test DataSeptember 2012
Fiberglass insulation
Round Head Carriage Bolt
Metal Washer©Doble Engineering Company
27Understanding The Transformer Test DataSeptember 2012
©Doble Engineering Company
28Understanding The Transformer Test DataSeptember 2012
In this case, the low impedance path formed by thebolts and the core clamping plates causes a local shortcircuit path which produces intense local eddy currents.The amount of heat generated by this phenomenon issufficient to considerably damage the adjacent areas.
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.
©Doble Engineering Company
29Understanding The Transformer Test DataSeptember 2012
This picture shows the correct insulation of the core boltsPhoto belongs to another transformer .
Insulation Material©Doble Engineering Company
30Understanding The Transformer Test DataSeptember 2012
Thank You©Doble Engineering Company
Understanding Transformer
Factory Testing
September 30, 2012
©Doble Engineering Company
Understanding Transformer Factory Testing 2
On some occasions additional methods must be employed to determine the suitability of tested transformer.
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
Transformer Temperature Tests
©Doble Engineering Company
Understanding Transformer Factory Testing 3
Core
Coils
Oil
AVERAGE WINDING
TEMP. WINDING HOTTEST
SPOT
TOP OIL
TEMP.
Cooling
Dis
tanc
e
Temperature
AverageOil
BottomOil
Avg. Wdg. Temp.
Hot Spot
Top Oil Temp.
Gradient
TopOil
Ambient
Gradient . x H.S.F
TEMPERATURE DISTRIBUTION
Transformer Temperature Tests
©Doble Engineering Company
Understanding Transformer Factory Testing 4
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/IEEE Standards and the clients technical specification.
• Temperature results were well below Standard limits and according to client’s specification.
• Very Clean DGA Results.
• Test Results did not match design data?
Case 1: Good Results – Bad Data
©Doble Engineering Company
Understanding Transformer Factory Testing 5
Short-Circuit Method – Three Phase, 3-Winding:
Measurement System Unit Under Test
YV
HV
XVL3
L2
L1
~
SHO
RT
CIR
CU
ITSH
OR
T C
IRC
UIT
Case 1: Good Results – Bad Data
©Doble Engineering Company
Understanding Transformer Factory Testing 6
Case 1: Loading Cycle
©Doble Engineering Company
Understanding Transformer Factory Testing 7
Test Corrections Limit Total Losses (kW ) 172.600 228.464Tap Position 1R 1R Average Oil Rise 42.9 51.2Top Oil Rise 49.6 59.2 65.0Winding Gradient, YV 10.2 10.2Winding Gradient, XV 2.3 2.3Winding Gradient, HV 3.3 3.2HS over TOR, YV 11.2 11.2HS over TOR, XV 2.5 2.5HS over TOR, HV 3.6 3.5Hot Spot Factor, YV 1.10 1.10Hot Spot Factor, XV 1.10 1.10Hot Spot Factor, HV 1.10 1.10Average Winding Rise, YV 53.05 61.3Average Winding Rise, XV 45.13 53.4Average Winding Rise, HV 46.18 54.4Hot Spot Rise, YV 60.8 70.4Hot Spot Rise, XV 52.1 61.7Hot Spot Rise, HV 53.2 62.7
65.0
80.0 n: 0.63m: 0.80
Exponents
Case 1: Heat Runs Results
Winding Test (A) Rated (A) RatioHV 99.5 97.9 0.984XV 1757.0 1757.0 1.000YV 2483.0 2483.0 1.000
Winding Current for Individual Gradient Runs
Winding Test (A) Rated (A) RatioHV 97.9 97.9 1.000XV 2149.1 1757.0 1.223YV 1791.5 2483.0 0.722
Winding Current for Oil Rise Run
©Doble Engineering Company
Understanding Transformer Factory Testing 8
MVACooling Mode
Tested DesignLosses (kW) 228.682 228.464 Guar.Top Oil Rise 59.2 51.6 65.0Average Oil Rise 51.2 39.7Bottom Oil Rise 39.8 27.9
Gradient 10.2 12.8Average Winding Rise 61.4 52.5 65.0Hot Spot Gradient 11.2 14.1Hot Spot Rise 70.4 65.7 80.0
Gradient 2.3 11.1Average Winding Rise 53.5 50.8 65.0Hot Spot Gradient 2.5 12.2Hot Spot Rise 61.7 63.8 80.0
Gradient 3.2 10.5Average Winding Rise 54.4 50.2 65.0Hot Spot Gradient 3.5 11.6Hot Spot Rise 62.7 63.2 80.0
Heat Run Result vs Design Data
YV Winding
HV Winding
39.0ONAN
XV Winding Why so far off?
Case 1: Temp Rise not Expected
©Doble Engineering Company
Understanding Transformer Factory Testing 9
∆T (3 Hr) > 2 ºC
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 TakeMeasured 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 YVUpper 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 83.61Upper 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 83.99Lower 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 71.83Lower 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 70.31Ambient # 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 0.00Ambient # 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 0.00Ambient # 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 0.00Average 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 0.00Top 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 89.87Top 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 85.40Average 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 89.87Averge 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 83.03Top 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 89.87Bottom 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
Case 1: Heat Runs Temp. Log
©Doble Engineering Company
Understanding Transformer Factory Testing 10
Test Corrections Limit Total Losses (kW ) 172.600 228.464Tap Position 1R 1R Average Oil Rise 42.9 51.2Top Oil Rise 49.6 59.2 65.0Winding Gradient, YV 10.2 10.2Winding Gradient, XV 2.3 2.3Winding Gradient, HV 3.3 3.2HS over TOR, YV 11.2 11.2HS over TOR, XV 2.5 2.5HS over TOR, HV 3.6 3.5Hot Spot Factor, YV 1.10 1.10Hot Spot Factor, XV 1.10 1.10Hot Spot Factor, HV 1.10 1.10Average Winding Rise, YV 53.05 61.3Average Winding Rise, XV 45.13 53.4Average Winding Rise, HV 46.18 54.4Hot Spot Rise, YV 60.8 70.4Hot Spot Rise, XV 52.1 61.7Hot Spot Rise, HV 53.2 62.7
65.0
80.0 n: 0.63m: 0.80
Exponents
Case 1: Heat Runs Results
Winding Test (A) Rated (A) RatioHV 99.5 97.9 0.984XV 1757.0 1757.0 1.000YV 2483.0 2483.0 1.000
Winding Current for Individual Gradient Runs
Winding Test (A) Rated (A) RatioHV 97.9 97.9 1.000XV 2149.1 1757.0 1.223YV 1791.5 2483.0 0.722
Winding Current for Oil Rise Run
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Understanding Transformer Factory Testing 11
• 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
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.
Case 1: Summary
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Understanding Transformer Factory Testing 12
GSU 820 MVA 362 / 25 kV DETC(±5%) 60Hz
• Heat run test was performed according to IEEE/ANSI Standards and the clients technical specification.
• Temperature results were below limits and according to clients requirements.
• DGA performed after heat run test found gas generation above client and the manufacturer’s acceptance limits.
Case 2: Good Unit – Bad DGA
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Understanding Transformer Factory Testing 13
Case 2: Bad DGA Results
Outside Lab Change Client LimitsSample # 1 2 3 - -
DescriptionBefore 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 - Nitrogen 11045 12181 9050 - -CO - Carbon Monoxide 10 62 50 40 25CO2 - Carbon Dioxide 82 392 250 168 200CH4 - Methane 0 3 14.4 14.4 5C2H4 - Ethylene 0 5 4 4 2C2H6 - Ethane 0 27 22 22 2C2H2 - Acetylene 0 0 0 0 0
In House Lab
All measured temperature rises within calculated tolerances.
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Understanding Transformer Factory Testing 14
5.TX Hot Spot
4.Stray
Gassing
3. Improper Testing
2.Pump
Problem
1.Bad DGA Sample
Why ?
Case 2: Possible Causes
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Understanding Transformer Factory Testing 15
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 pump running backwards.
• Running ratings matched Nameplate• No Noise• Thermal Scan normal for pumps and oil flow
Case 2: Possible Causes
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Understanding Transformer Factory Testing 16
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 – Normal
heating• 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.
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Understanding Transformer Factory Testing 17
Only two possible causes left:
4. An oil problem due to “thermal stray gassing”.
Or5. An abnormal transformer hot spot.
Case 2: Possible Causes
An experiment is needed!
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Understanding Transformer Factory Testing 18
How is the gassing influenced ?
• Load dependent
• Oil temperature dependent
Case 2: Experimental Loading
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Understanding Transformer Factory Testing 19
Step A: Transformer Rated Conditions
1. Test Floor Open and Ventilated2. All Pumps & Fans On3. Full Rating Conditions of the transformer
Case 2: Experimental Loading
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Understanding Transformer Factory Testing 20
Step B: Simulate Stray Gassing
1. Test Floor Closed2. Reduced Current 3. All Pumps on/ Fans adjusted to keep oil at ~ 90 ºC
Case 2: Experimental Loading
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Understanding Transformer Factory Testing 21
Case 2: Experimental Method
Test Number
Load Condition
Oil Temperature Comments
Test #1 Overload High Oil Temperature
This is the intial heat run result. The source of gassing is indeterminate.
Test #2 Rated Load Normal Oil Temperature
Gassing under this condition is most likely not from the oil.
Test #3 Reduced Load High Oil Temperature
Gassing under this condition is most likely not from the transformer.
Test #4 Overload Normal Oil Temperature Gassing is most likely from the transformer.
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Understanding Transformer Factory Testing 22
Case 2: Experimental Method
Test Number
Duration [Hours] Criteria Loading
Load [%]
Top Oil Temp.
Coil Oil Temp.
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 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 #1
Test #4©Doble Engineering Company
Understanding Transformer Factory Testing 23
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 10O2 - Oxygen - - - - -N2 - Nitrogen - - - - -CO - Carbon 40 10 36 12 25CO2 - Carbon Dioxide 168 66 272 105 200CH4 - Methane 14.4 1.8 8 4.2 5C2H4 - Ethylene 4 0.6 1.7 1.3 2C2H6 - Ethane 22 0 13.6 0 2C2H2 - Acetylene 0 0 0 0 0CO2/CO Ratio 4.2 6.6 7.6 8.8 < 3
Criteria
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Understanding Transformer Factory Testing 24
• Most of the gas concentrations exceed the customer limits.
• Dominant gasses are Methane, Ethane and Hydrogen (low temperature gasses or thermal stray gassing).
• No cellulose decomposition.
Case 2: Test #1 Results
The Source of the Excessive Gassing is Indeterminate.
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Understanding Transformer Factory Testing 25
• Gassing, all gasses within acceptance limits. • No dominant gasses.
• No cellulose decomposition.
Case 2: Test #2 Results
If there was excessive gassing it would likely be from the transformer active parts.
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Understanding Transformer Factory Testing 26
• 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. • Dominant gasses are Methane, Ethane and Hydrogen,
this is an indication of possible thermal stray gassing. • The gassing results are similar Test #1.
Case 2: Test #3 Results
This excessive gassing is likely from the oil.
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Understanding Transformer Factory Testing 27
• 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
not related to the load or a transformer condition.
Case 2: Test #4 Results
If there was excessive gassing it would likely be from the transformer active parts.
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Understanding Transformer Factory Testing 28
• The transformer successfully passed the heat run test according to ANSI/IEEE Standards.
• Test #3 results closely match with Test #1 and are indicative that the source of gasses during heat run test is thermal stray gassing of the oil.
• Gasses generated during heat run test performed are produced by thermal stray gassing of the oil used for FAT.
• The Doble Oil Lab confirmed the stray gassing tendency of the oil used for the factory heat run.
Case 2: Summary
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Understanding Transformer Factory Testing 29
CONCLUSION
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