Understanding
Power Transformer
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©
Mark F. Lachman
Doble Engineering Company
OVERVIEW OF PRODUCTION TESTS
CTs on cover: polarity,
ratio, saturation
PA: loss, sound,
core-to-gnd
Core/coil: ratio,
Iex, core-to-gnd
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in
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Core/coil after VP:
Iex, core-to-gnd
Tanking: ratio, core-to-gnd,
in-tank CTs - polarity, ratio,
saturation
SU: ratio, Rdc, Iex,
no-load/load loss,
sound, core-to-gnd
SYSTEM VOLTAGE CLASSIFICATION
Class I includes power transformers with
high-voltage windings of 69 kV and below.
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Co
Class II includes powerintransformers
with
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in from 115 kV through
high-voltage windings
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765 kV. ©Do
GENERAL CLASSIFICATION OF TESTS
Routine tests shall be made on every
transformer to verify that the product
meets the design specifications.
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Design tests shall be
made
on
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transformer leofEnew
design to determine
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©
its adequacy.
Other tests may be specified by the
purchaser in addition to routine tests.
OVERVIEW OF TESTS
TEST TYPE
PERFORMANCE
DIELECTRIC
MECHANICAL
Winding resistance
Winding insulation resistance
(Other)
Leak
Ratio/polarity/phase
relation
No-load losses and
excitation current
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Operation
©D of all
C
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Dielectric
withstand of control
in
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E and CT sec. circuits (Other)
Load losses and
Impedance voltage
Routine
Core insulation resistance
(Other)
Class I in red if
Insulation PF/C different from Class II
(Other)
devices
Lightning impulse
(Design and Other)
Control and cooling
losses (Other)
Switching impulse
 345 kV (Other)
Zero-phase sequence
impedance (Design)
Low frequency test
(Applied and Induced/Partial
Discharge)
DGA (Other)
Class II < 345 kV
is also Other
PD is Other for
Class I only
OVERVIEW OF TESTS (cont.)
TEST TYPE
PERFORMANCE
DIELECTRIC
MECHANICAL
Temperature rise
Design/
Other
Audible sound level
Short-circuit
capability
Other
oble
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Design
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Single-phase
excitation current
Front-of-wave
impulse
Lifting and
moving
Pressure
SEQUENCE OF TESTS
TEST
REFERENCE
DGA
Ratio/polarity/phase relation
IEEE C57.12.90-2010 clauses 6, 7
IEEE C57.12.00-2010 clauses 8.2, 8.3.1, 9.1
Winding resistance
IEEE C57.12.90-2010 clause 5
IEEE C57.12.00-2010 clause 8.2
Lightning impulse
IEEE C57.12.00-2010 clauses 5.10, 8.2
IEEE C57.12.98-1993; IEEE Std. 4-1995
Applied voltage
IEEE C57.12.90-2010 clause 10.5, 10.6
IEEE C57.12.00-2010 clauses 5.10, 8.2
Induced voltage/PD
IEEE C57.12.90-2010 clause 10.7, 10.8, 10.9
IEEE C57.12.00-2010 clauses 5.10, 8.2
IEEE C57.113-2010; IEEE C84.1
No-load losses and excitation
current
IEEE C57.12.90-2010 clause 8
IEEE C57.12.00-2010 clauses 5.9, 8.2, 9.3, 9.4
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p
IEEE C57.12.90-2010
clause 8
No-load losses and excitation
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IEEE C57.12.00-2010
clauses 5.9, 8.2, 9.3, 9.4
current
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r C57.12.90-2010 clauses 10.1, 10.2
e
IEEE
e
in IEEE C57.12.00-2010 clauses 5.10, 8.2
Switching impulse Eng
IEEE C57.12.98-1993; IEEE Std. 4-1995
le
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o
IEEE C57.12.90-2010 clauses 10.1, 10.3
©D
SEQUENCE OF TESTS (cont.)
TEST
REFERENCE
DGA
Load losses and
impedance voltage
IEEE C57.12.90-2010 clauses 9.1-9.4, Annex B2
IEEE C57.12.00-2010 clause 5.8, 5.9, 8.2, 8.3.2,
9.2-9.4
ONAN temperature rise
IEEE C57.12.90-2010 clause 11
IEEE C57.12.00-2010 clause 8.2
IEEE C57.91-1995 Table 8 (with 2002 corrections)
DGA
le
b
ONAF temperature
rise
o
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nIEEE PC57.130/D17
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IEEE C57.12.90-2010 clause 11
IEEE C57.12.00-2010 clause 8.2
IEEE C57.91-1995 Table 8 (with 2002 corrections)
DGA
IEEE PC57.130/D17
Zero-phase sequence
impedance
IEEE C57.12.90-2010 clause 9.5
IEEE C57.12.00-2010 clause 8.2
Audible sound level
IEEE C57.12.90-2010 clause 13, Annex B5
IEEE C57.12.00-2010 clause 8.2
NEMA TR1-1993
Core demagnetization
DGA
SEQUENCE OF TESTS (cont.)
TEST*
REFERENCE
Insulation PF/C and
resistance
IEEE C57.12.90-2010 clauses 10.10, 10.11
IEEE C57.12.00-2010 clause 8.2
Single-phase exciting
current
Lachman, M. F. “Application of Equivalent-Circuit Parameters to
Off-Line Diagnostics of Power Transformers,” Proc. of the SixtySixth Annual Intern. Confer. of Doble Clients, 1999, Sec. 8-10.
Sweep frequency response
analysis
IEEE C57.12.00-2010 clause 8.2
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Dielectric withstand of control
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b
and CT secondary
circuits
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IEEE PC57.149™/D8, November 2009
IEEE C57.12.00-2010 clause 8.2
CT polarity/ratio/saturation
IEEE C57.13.1-2006
Control and cooling losses
IEEE C57.12.00-2010 clauses 5.9, 8.2
Operation of all devices
IEEE C57.12.00-2010 clause 8.2
Core-to-ground insulation
resistance
IEEE C57.12.90-2010 clause 10.11
IEEE C57.12.00-2010 clause 8.2
*Discussion of tests listed on this slide and DGA is not included in this presentation.
DISCUSSION OUTLINE
Tests to be discussed:
 Ratio/polarity/phase relation
 Winding DC resistance
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 No load losses and excitation
current
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 Dielectric tests
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 Load losses and impedance voltage
 Temperature rise
 Zero-phase sequence impedance
 Audible sound level
DISCUSSION OUTLINE (cont.)
For each test discussion includes:
 Definition and objective
 Physics
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 Setup and test methodology
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 Acceptance criteria*
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 Abnormal
 Recourse if data abnormal
 Comparison with field data (if relevant)
*This discussion is based on requirements of referenced standards. If customer test specification
contains requirements different from those in standards, more stringent requirements prevail.
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RATIO, POLARITY,
PHASE
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RELATION
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RATIO, POLARITY, PHASE RELATION:
DEFINITION AND OBJECTIVE
Definition: The turns ratio of a transformer is the ratio of
the number of turns in the high-voltage winding to that in
the low voltage winding.
Objective: The turns ratio polarity and phaseyrelation test
nand internal
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p
verifies the proper number of turns
om
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transformer connections (e.g.,ribetween
coils, to LTC, to
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ee series auto- or series
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various switches, to gPA,
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transformer) and
le serves as benchmark for later
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assessment ©
ofDpossible damage in service.
The transformer nameplate voltages should reflect the
actual system requirements. Therefore, it is important
that the nameplate drawing is approved by the customer
at the design stage.
RATIO, POLARITY, PHASE RELATION:
PHYSICS
Volts per turn = 3V/3T = 1V/T
VR = 3V/2V = 1.5
TR = 3T/2T = 1.5
In ideal transformer:
TR = VR
F
3V
In actual transformer
Turns ratio  Voltage ratio
due to accuracy of the
measurement and the
voltage drop in the highle
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voltage winding. ©D
3T
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2T
2V
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Volts per turn = 2.95V/3T = 0.98V/T
F
0.05V
VR = 3V/1.96V = 1.53
TR = 3T/2T = 1.5
3V
3V
 = 100(1.5 – 1.53)/1.5 = –2%
2.95V
3T
2T
1.96V
RATIO, POLARITY, PHASE RELATION:
SETUP AND TEST METHODOLOGY
Transformer in test
H1
X0
 Polarity is determined via
phase angle between two
measured waveforms.
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 Phaseprelation is confirmed
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N1
N2
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by testing the corresponding
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R2
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pairs of windings.
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ng
 Tests shall be made
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X2
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1. at all positions of DETC
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©
with LTC on the rated
voltage position
Balance
H2
2. at all positions of LTC with
indicator
DETC on the rated voltage
position
Ratio = N1/N2 = R1/R2
3. on every pair of windings
RATIO, POLARITY, PHASE RELATION:
ACCEPTANCE CRITERIA
With the transformer at no load and with rated voltage on
the winding with the least number of turns, the voltages of
all other windings and all tap connections shall be within
0.5% of the nameplate voltages.
y tolerance
n
For three-phase Y-connected windings,
this
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o When the phase-toapplies to the phase-to-neutral voltage.
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nmarked on the nameplate,
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neutral voltage is not explicitly
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n voltage shall be calculated by
the rated phase-to-neutral
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dividing the phase-to-phase
voltage markings by 3.
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H2
138
X2
13.2
X1
Voltage ratio =
VH2-H1/VX2-X0 =
X0
138/(13.2/3) = 18.108
H1
H3
X3
RATIO, POLARITY, PHASE RELATION:
ABNORMAL DATA
To appreciate significance of 0.5% limit, it is instructive to
recognize the inherent errors this limit accommodates.
Actual turns  RATIOTURN
Nameplate voltages 
RATIONP
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Rounding off
NP voltages
creates error 
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Deviation le
b)/RATIO = 
o
100(RATIONP - RATIO
D
NP
© TURN
Measurement  RATIOMEAS
Measurement
introduces
error
Deviation
100(RATIONP - RATIOMEAS)/RATIONP  0.5%
NP voltages need to be
selected to keep  well
within 0.5% (e.g., 0.20.4). This assures that
measurement
error
keeps RATIOmeas within
0.5% of RATIONP.
RATIONP

RATIOMEAS
RATIOTURN
RATIO, POLARITY, PHASE RELATION:
RECOURSE IF DATA ABNORMAL
 If deviation exceeds 0.5% for any of the measurements the
result is not acceptable.
 The following steps should be considered:
 Check if V/T exceeds 0.5% of nameplate voltage. If yes,
ny for deviation
under these conditions the standard p
allows
a
om
from the NP voltage ratio to exceed
the 0.5% limit.
C
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eduplicate of a legacy unit.
 Check if transformer is ia
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 Review designbledata to determine if the NP voltages
o
selected by
create a ratio that is too far (b is
©Ddesigner
too high) from true turns ratio. Discuss possibility of
changing nameplate voltages for relevant tap positions.
 Review results of production ratio tests and, if applicable,
consider retesting with analog instrument.
 Exciting current reported by turns ratio instrument is a
useful diagnostic indicator.
RATIO, POLARITY, PHASE RELATION:
COMPARISON WITH FIELD DATA
 In verifying compliance with 0.5% deviation from the NP
voltages, the following should be recognized:
 Older analog instruments produce results much closer to
the actual turns ratio than modern digital instruments.
yvary somewhat
 Even within 8-200 V range, the results
n
a
p
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oinstruments.
with voltage and between different
C
g
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eriperformed
 Initial field test shouldine
be
at the same test
ngtest with results compared with the
E
voltage as the factory
le
b
o
NP voltages
©Dand for all subsequent tests the comparison
should be made with the initial test.
 The objective of the high-voltage (e.g., 10 kV) test with
external capacitor is to stress turn-to-turn insulation of both
windings for diagnostic purposes and not necessarily to
verify the 0.5% limit. In some cases, the latter could be
exceeded due to the loading effect of the test capacitor.
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C
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WINDING DC RESISTANCE
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(Routine)
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WINDING DC RESISTANCE:
DEFINITION AND OBJECTIVE
Definition: Winding DC resistance is always defined as the
DC resistance of a winding in Ohms.
Objective: The measurement of winding resistance
provides the data for:
y
n
a
p
 Calculation of the I2R component ofoconductor
losses.
m
C
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n
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 Calculation of winding temperatures
at the end of a
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e
in
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temperature rise test.
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l
b
 Quality control
©Doof design and manufacturing processes.
 Benchmark used in field for detection of open circuits,
broken strands, deteriorated brazed and crimped
connections, problems with terminations and tap
changer contacts.
WINDING DC RESISTANCE:
PHYSICS
i
R
le
b
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©D
C
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mp
External
field
in
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E
Domain
WINDING DC RESISTANCE:
PHYSICS (cont.)
R=vmeas / i
/dt
y/dt
/dt
vmeas = iR + ddydy
dy/dt
dy/dt
dy/dt
le
b
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©D
F = y/N
dy/dt
in
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C
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mp
dy/dt
WINDING DC RESISTANCE:
PHYSICS (cont.)
Time to stabilize resistance reading: On some units with closed
loops (e.g., GSU with two LV deltas or units with parallel
windings), it may take a long time for the reading to stabilize*; it
reduces with intermediate stability levels. This phenomenon is
not related to core saturation, which is saturating in a
y
n
a
reasonable time. However, as the core is m
being
magnetized the
p
ovoltage and sets up
C
changing flux in the core induces
g
n
i
r
e
e
circulating currents in closed
loops.
After the core is saturated,
n
i
g
n voltage to sustain them, and the
E
there is no more induced
le
b
o
currents begin to
subside. This process, however, is associated
©D
with LC oscillations with long time constant and may take up to
45 min to dissipate the energy. The flow of these currents
continues creating a changing flux in the core, inducing voltage
in the tested winding and thus changing the measured
resistance reading. Opening these loops, when possible,
reduces the time to stability.
* Personal communications with Bertrand Poulin, ABB, Quebec, Canada.
WINDING DC RESISTANCE:
SETUP AND TEST METHODOLOGY
Current +
output
Voltage
input + Vdc
 Data must be taken only
when reading is stable.
Transformer in test
The time to stabilize the
reading ydepends on the
H2
n varying
a
unit,
from
p
m
oseconds to minutes.
C
g
n
i
H1
r
e
 Standard
requires
e
n
i
H
ng 0
measurements of all
E
e
l
Idcb
o
windings on the rated
D
©
voltage tap and at the tap
extremes of the first unit
H3
of a new design.
 The measured data is
reported at Tave_rated_rise +
20C, e.g., 65+20= 85C
and as total of 3 phases.
WINDING DC RESISTANCE:
ACCEPTANCE CRITERIA
 Standards give no acceptance criteria; however, a deviation
from average of three phases of 0.5% for HV and 5% for LV
could serve as practical guideline.
 As important as deviation is the assurance that test data is
credible:
y
n
a
p
m
 No excitation with no pumps - 3h C
and
with
pumps - 1h,
o
g TO-TBO 5C. This assures
n
i
TTO variation 2C for 1h, and
T
r
e
e
n
i
g
that oil T represents
T; without reference T
nconductor
E
le a limited value.
b
resistance data
has
o
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©
 Test current 10% of maximum rated load current.
 Voltage test leads must be placed as close as possible to
winding terminals.
 Test data should be recorded only when reading is stable.
 Measuring system accuracy +/-0.5% of reading with
sufficient current output to stabilize the flux.
WINDING DC RESISTANCE:
ACCEPTANCE CRITERIA (cont.)
T stability: Experience* in the industry suggests that
relying on the T stability requirements given in the IEEE
standard does not produce a needed thermal equilibrium
and, consequently, an accurate measurement of the
winding dc resistance. To have a reliable ndata,
the unit
y
a
p
m
should be subjected to no excitationCfor
2-3
days. Hence, if
o
ngof essence, it is not
i
the time to begin testing eis
r
e
n
i
g
unreasonable to agreeEto
using
resistance data available at
n
lethe IEEE T requirements have been
b
that time (assuming
o
D
©
met), but request that resistance is re-measured later
(including cold resistance for heatrun), when the T is
stable. Obviously, the load loss and the heatrun results
should be then recalculated with the latest T.
* Personal communications with Bertrand Poulin, ABB, Quebec, Canada.
WINDING DC RESISTANCE:
ABNORMAL DATA
High-voltage winding
% of calc.
Average
Deviation from average
20.9832 21.47937
97.7
97.7
97.7
0.03%
0.03%
0.02% -0.05%
0.03% -0.06%
3.5622
20.4889 20.97440
19.9932 20.46944
3.7360
3.6480
3.5597
0.02%
0.05% -0.07%
3.4698
3.5746
19.6873 19.96448
98.6
3.5053
0.97%
1.01% -1.98%
3.3814
3.3870
19.0065 19.45952
0.01%
0.08% -0.09%
DETC
H1-H3
H2-H1
H3-H2
1
3.7350
3.7352
3.7378
2
3.6470
3.6468
3.6502
3
3.5590
3.5580
4
3.4714
5
3.3838
Low-voltage winding
LTC
16
N
Tested
Calc.
o
C
g
n
i
r
e
e
in
g
n
E 0.03842
0.16537 0.16521 0.16499 0.6185
e
l
b
0.1566 0.1564 0.1562
©Do 0.5855 21.47937
X1-X0
X2-X0
X3-X0
y
n
a
mp3.3841
97.7
99.7
100.6
0.16519 -0.11% -0.01% 0.12%
0.15637 -0.12% 0.00% 0.12%
Comparison of each measurement with the
average along with design data identifies
an abnormal reading in H3-H2 with DETC in
4. This potentially can be caused by a
problem with DETC contacts.
WINDING DC RESISTANCE:
RECOURSE IF DATA ABNORMAL
 If requirements associated with transformer thermal stability,
dc test current, influence of series unit or stability of the
reading are not met, a retest under different conditions
should be requested.
 If acceptance criteria is exceeded, a justification
from the
y
n
a
p
m
manufacturer should be requested.CPotential
problems may
o
gincorrect conductor cross
n
i
include: bad crimping or brazing,
r
e
e
n
i
section, loose connection,
Eng wrong design calculations.
le
b
o
©D
WINDING DC RESISTANCE:
COMPARISON WITH FIELD DATA
 Typically, a deviation of <5% from the factory value is
considered acceptable.
 A factory value is often reported as a sum of three phase
readings at rated T. For field comparison, the per-phase
values at corresponding DETC/LTC positions
should be
y
n
a
p
m
requested from the factory.
o
C
g
n for readings referred to the
i
 Comparison should be performed
r
e
e
n
i
g
same T.
n
E
le should be performed at the same test
b
 The field measurement
o
D
©
current as the factory one.
 Field tests are the subject to the same thermal stability
requirements as the factory test (note that at the factory T is
measured via thermocouples and in the field the T gauge is
frequently the best option).
y
n
a
p
NO-LOAD LOSSES
AND
m
o
C
g
n
i
r
e
e CURRENT
EXCITATION
n
i
g
n
E
le (Routine)
b
o
©D
NO-LOAD LOSSES AND EXCITATION CURRENT:
DEFINITION AND OBJECTIVE
Definition: No-load losses include core loss, dielectric
loss, and conductor loss due exciting current, including
current circulating in parallel windings. Excitation current
is flowing in any winding exciting the transformer with all
other windings open-circuited.
y
n
a
p
om
C
Objective: No-load losses iand
excitation current,
g
n
r
e and frequency, provide the
e
measured at specified voltage
n
i
g
n
E
data for:
le
b
o
D design calculations.
 Verification©of
 Demonstration of meeting the guaranteed performance
characteristics. Since these parameters have often an
economic value attached to them, the accuracy of the
measurement becomes significant.
 No-load losses are used as test parameter during the
temperature rise test.
NO-LOAD LOSSES AND EXCITATION CURRENT:
PHYSICS
F
Eddy
losses
Hysteresis losses
Ph = f(Bmax)
Bmax = f(Vave)
PNL = Pe + Ph
Ieddy
Pe = f(V2rms)
I
V
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b
o
©D
R
C
g
n
i
r
e
e
in
g
n
E
F
y
n
a
mp
B
o
H
Domain
rotation
NO-LOAD LOSSES AND EXCITATION CURRENT:
SETUP AND TEST METHODOLOGY
Transformer in test
CT
X0 H1
X1
H2
X2
VT
X3 H3
3
V
le
b
o
D
I
©
A
E
Vrms
*
Vave
*
and Vave (calibrated in rms) will show
the same voltage if perfect sine wave.
rms
 Test at 100% Vrated on N, max turn
bridging position
y with inductive
n
a
p16R if LTC with series
LTC,oand
m
C
unit.
g
in
r
e
e
ngin  Vave gives same Bmax as Vrms when
W
*V
 Start with 110% on N. As unit
demagnetizes, losses drop.
wave-shape is a perfect sin; set
based on Vave average of 3 phases
 Pe is corrected for rated Vrms
 Voltmeters should measure same
voltage as seen by xfmr.
 PNL not corrected for T if TTO-TBO
5C and 10TO_ave30C
 Iexc=aver. of 3 phases in % of Irated
NO-LOAD LOSSES AND EXCITATION CURRENT:
SETUP AND TEST METHODOLOGY (cont.)
Historical perspective
le
b
o
©D
Courtesy IEEE Power & Energy Magazine
in
g
n
E
g
n
i
r
ee
Frequency control of motorgenerator sets at GE large
transformer
plant
in
Pittsfield,
y MA during the
n
a
p
early
1900. Since the
m
Coprimary function of these
generators was to provide
power for no-load loss
tests, they were often
referred to as magnetizers.
NO-LOAD LOSSES AND EXCITATION CURRENT:
ACCEPTANCE CRITERIA
 Measured no-load losses should not exceed the
guaranteed value by more than 10% and the total losses by
more than 6%.
 Assurance that test data is credible:
 Test voltage is set based Vave
y
n
a
p
 If oil T is not within limits, correction
m
o is applied
C
grated
 Frequency is within +/-0.5%riof
n
e
e
n
i
 Distortion  5%. The
5% limit that standard allows for
g
n
le E
distortion ofob
the
voltage waveform is too liberal.* The
©D to the difference between the measured kW
limit applies
and kW corrected for eddy loss due to the difference
between Vrms and Vave. To monitor the quality of the
voltage waveform, one should look at the following
criteria of the applied voltage waveform: THD < 5%, 3rd
and 5th harmonics <10% and waveform should not have
any visible distortions.
* Personal communications with Bertrand Poulin, ABB, Quebec, Canada.
NO-LOAD LOSSES AND EXCITATION CURRENT:
ACCEPTANCE CRITERIA (cont.)
 Test in parallel and series configurations, if present.
 If PA is present, compare the loss difference between
non-bridging and bridging positions (max turns) with
loss measured in PA out-of-tank. If SU unit is present,
compare the loss difference between N yand 16R with
n
loss measured in SU out-of-tank. ompa
C
g
 Test system accuracy should
be
within +/-3% for loss,
n
i
r
e
e
+/-0.5% for voltage and
and +/-1.5C for T.
gincurrent,
b
o
D
©
n
E
le
NO-LOAD LOSSES AND EXCITATION CURRENT:
ABNORMAL DATA
Example: guaranteed no-loss - 28 kW, measured – 35 kW
 Potential reasons for exceeding the guaranteed values may
include:
 Variability in core steel characteristics any
p
m
o
C
 Different core steel
g
n
i
r
e
 Oversights in design gine
n
E
le
 Production process
related factors or mistakes
ob
©D
 Problems with windings (e.g., s. c. turn)
 Wrong connection of preventative autotransformer or
series transformer or series autotransformer
NO-LOAD LOSSES AND EXCITATION CURRENT:
RECOURSE IF DATA ABNORMAL
 Failure to meet the no-load test loss tolerance should
not warrant immediate rejection but shall lead to
consultation between purchaser and manufacturer
regarding further investigation of possible causes and
the consequences of the higher losses. ny

a
p
m
o not replace the
The acceptance criteria of 10%
does
C
g
nlosses for economic loss
i
r
e
manufacturer’s guarantee
of
e
n
i
g
n
evaluation purposes.
E
le
b
o
©D
NO-LOAD LOSSES AND EXCITATION CURRENT:
COMPARISON WITH FIELD DATA
Factory no-load losses and excitation test is performed
at rated voltage and three-phase excitation. Since the
open-circuit magnetizing impedance of a transformer is
non-linear, i.e., it is changing with applied voltage, a
comparison of exciting current and losses
y test results
n
a
p
m
obtained at low-voltage (e.g., 10 C
kV)
and
single-phase
o
ng no-load losses and
i
excitation with results of theefactory
r
e
n
i
g
excitation test is not possible.
En
le
b
o
©D
y
n
a
mp
o
C
g
DIELECTRIC
n TESTS
i
r
e
e
n
i
g
n
E
le
b
o
©D
DIELECTRIC TESTS:
DEFINITION AND OBJECTIVE
Definition: Tests aimed to show that transformer is
designed and constructed to withstand the specified
insulation levels are referred to as dielectric tests. They
include:
 high-frequency tests: lightning and switching
y impulses
n
a
p
m
 low-frequency tests: applied and induced/PD
tests
Co
g
n
i
r
ee
in
g
Objective: Dielectric tests
demonstrate:
n
E
e
l
b
 compliance with
©Do users specification
 compliance with applicable standards
 verification of design calculations
 assessment of quality and reliability of material and
workmanship
Note: Unless agreed otherwise, all dielectric tests must be performed with
bushings supplied with the transformer.
HIGH-FREQUENCY:
y
n
a
p
m
o
C
g
n
LIGHTNING
IMPULSE
i
r
e
e
in
g
n
E
(Class
I - design or other,
e
l
b
©Do Class II - routine)
HIGH-FREQUENCY - LIGHTNING IMPULSE:
OBJECTIVE
Demonstrate performance under transient high-frequency
conditions caused by lightning.
kV
Surge of energy, from lightning striking transmission line, travels to
substation and operates gapped silicon-carbide arrester at
transformer terminals - front-of-wave (a.k.a. front-chopped).
y
n
a
p
om
le
b
o
©D
C
g
n energy, from lightning striking
i
r
e
Surge
of
e
n
i
g
En transmission line, travels to substation
and enters a transformer - full wave.
Surge of energy, from lightning striking transmission
line, travels to substation and, after reaching the
crest of the surge, causes arrester operation or
flashover across an insulator near transformer
terminals - chopped wave (a.k.a. tail-chopped).
s
HIGH-FREQUENCY - LIGHTNING IMPULSE:
PHYSICS
Full wave can be simulated
by discharging capacitor
while chopped wave by the
operation of a gap triggered
to flashover at required time.
V
le
b
o
©D
Cg
V
C
g
n
i
r
e
e
in
g
n
E
Cs
o
y
n
a
mp
  Cg/Cs
length
Due to impulse front high frequency, the
initial voltage distribution is determined by
the capacitive network, with higher voltage
gradients towards the impulsed end of the
winding. The higher is , the steeper are the
gradients at the impulsed end of the winding.
As the front passes, the distribution changes
as determined by the tail of the wave.
HIGH-FREQUENCY - LIGHTNING IMPULSE:
PHYSICS (cont.)
A
HV
LV
H1
B
H1
A
B
B
B
B
le
b
o
©D
g
n
i
r
ee
in
g
n
E
C
to DETC
Region A* - turn-to-turn
insulation at line is tested by
FOW impulse, with stress
>10turns**.
yB – disk-to-disk, and
Region
n
a
p
m
layer-to-layer
(and
Co turn-to-turn) isinsulation
tested by FW
& CW impulse, with stress 510turns.
C
Region C – insulation across
taps is tested by FW & CW
impulse, with stress 510turns.
H0
*Assumption that FOW stresses mostly the first few turns at the impulse end is not always true; it depends on
winding type and configuration, e.g., when the interleaved winding (one with high series capacitance) is in
series with RV, the impulse goes through the main winding and hits RV (Personal communications with
Bertrand Poulin, ABB, Quebec, Canada.)
**From W. McNutt 1989 Doble tutorial: Turns in this context mean the voltage that would have been present
if the applied voltage was distributed according to turns ratio.
HIGH-FREQUENCY - LIGHTNING IMPULSE:
PHYSICS (cont.)
Charge of Cg – generator
capacitors are charged from
external DC source.
Rs
Cg
Rp
CT
VT
Rs
Cg
Rp
CT
le
b
o
R ©D
Rs
Cg
p
g
n
i
r
ee
in
g
n
V
E
T
CT
FOW
Rs
VT
Cg
Rp
*CT includes preload capacitor.
FW
CT
CW
Discharge into C*T –
energy
from
generator
y
n is discharged into
capacitors
a
p
Comxfmr, raising V at tested
terminal to crest level.
Discharge into Rp – energy
from xfmr is discharged into
generator, reducing voltage
at tested terminal.
Discharge at chop –
energy from xfmr is
discharged into chopping
gap, reducing voltage at
tested terminal to zero.
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY
Full Wave Parameters
Crest voltage
1.0
Magnitude
0.9
FW = BIL +/- 3%
RFW = 50-70% BIL
T1 = 1.67Tny
V
g
n
i
r
ee
0.5
Half voltage
0.3
T
Virtual
origin
T1
le
b
o
D
©
in
g
n
E
t
a
p
m
Co
1.2 s +/- 30%
0.84 ÷ 1.56 s
T2
50 s +/- 20%
40 ÷ 60 s

5%
T2
 Applied test waves are of negative polarity to reduce risk of erratic
external flashover.
 See C57.12.90-2010 when for line terminals T1 is allowed to be >1.56
s and T2<40 s. For neutral bushing T1<10 s and T2 could be <40 s.
 If the T2<40 s, it should be addressed at the bidding stage.
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
0% change from given Rs
Co
C
g
n
i
r
e
e
y
R
n
a
mp
g
s
Rp
CT
in
g
n
E
e
l
b
Increase
of series (front) resistor Rs
©Do
increases the time of voltage rise - T1.
Data courtesy Reto Fausch, Haefely
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
Increase of parallel (tail) resistor Rp increases
the time of voltage decline to half value - T2.
le
b
o
©D
C
g
n
i
r
e
e
in
g
n
E
0% change from given Rp
Cg
Rp
Rs
Data courtesy Reto Fausch, Haefely
CT
o
y
n
a
mp
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
Increase of series (front) resistor Rs decreases
the voltage trace overshoot - .
Rs
Cg
le
b
o
©D
Data courtesy Reto Fausch, Haefely
in
g
n
E
C
g
n
i
r
e
e
o
y
n
a
mp
Rp
CT
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
Chopped Wave Parameters
1.0
0.9
Magnitude
CW = 1.1BIL+/- 3%
T1
1.2 +/- 30%
0.84 ÷ 1.56
1.0
V
0.7
0.3
T1
TC
le
b
o
©D
BIL [kV] y Class I
n
a
p
m 30
o
C
g
n
i
45÷75
r
e
e
in
g
n
E
TC 
0.1

 t
 See C57.12.90-2010 for instances
when  could be >30% and >1s. It
also permits adding resistors in
chopping gap circuit to limit .
 All times in the table are in s.
1.0
1.5
95
1.8
110
2.0
125
2.3
150
Class II
2.0
2.3
3.0
TC <
6.0

30%

1
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
1.0
0.9
Front-of-Wave Parameters
Magnitude
V
p
m
Co
TC
0.3
le
b
o
©D
TC

g
n
i
eer
in
g
n
E
C57.12.00-2010
anyAnnex A
30%
t

 C57.12.90-2010 permits adding resistors in chopping gap circuit to limit .
 With improved arrester technology, front-of-wave tests may not be necessary
and were removed as a requirement from C57.12.00. Annex A in that standard
includes the last published table of front-of-wave test levels from C57.12.001980, for historical reference.
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
LG
Very high di/dt induces difference
of potential. Hence, it is very
important for all return and
grounding leads to be made as
short as possible, with a minimum
R and L.
Glaninger:  T2
Rs
xfmr
Impulse
generator
Rp RG
Cg
C
g
n
i
r
e
e
LT, CT
le
b
o
©D
in
g
n
E
y
n
a
mp
oVoltage divider
and measuring
circuit
v(t)
 T2
Chopping gap
and preload
capacitor
Impulse
control &
measuring
system
Current
shunt and
meas. circuit
Chopping gap should not be connected in series
with voltage divider no matter how convenient it is
for the test department to have a permanent setup.
i(t)
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
Line terminal in Y
Neutral terminal in Y
C
g
n
i
r
e
e
i(t)
i(t)
le
b
o
©D
HV line terminal
in Auto
in
g
n
E
Line terminal in 
y
n
a
mp
o
LV line terminal
in Auto
i(t)
i(t)
i(t)
i(t)
Neutral terminal
in Auto
HIGH-FREQUENCY - LIGHTNING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
Test sequence and trace comparison
Standard:
RFW@ 50-70% BIL
CW 1
CW 2
FW
With non-linear
With FOW:
protective devices:
RFW@ 50-70% BIL
RFW 1
FOW 1
RFW
2
y @ 75-100% of BIL
n
FOW 2
a
p
to
demonstrate growing
m
o
C
CW 1
g
sensitivity to V
n
i
r
e
CW 2gine
FW 1
n
E
e
l
FW
b
CW 1
o
Neutral:
D
RFW@ 50-70% ©
BIL)
FW1
FW2
CW 2
FW 2
RFW 3 @ RFW2 voltage
RW 4
Test is performed with minimum effective turns in the winding under
test, e.g., DETC = 5, LTC = 16L.
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA
 If test equipment and tested transformer were perfectly linear, the
traces of repeated impulses, when overlaid, would perfectly
match. However, due to noise, setup imperfections or insulation
failure, discrepancies occur. Identifying their nature is the
objective of impulse data analysis.
y
n
 T1, T2, Tc, voltage magnitude, ,  must meet requirements.
a
p
m
oshould compare; request
 RFW and FW voltage and current traces
C
g
n
i
r
to zoom in on any areas of concern.
e
e
n
i
g
 If available, comparison E
ofnTransfer Function (TF) for RFW and FW
lediagnostic criteria. It removes sensitivity to
b
is used as additional
o
D
©
wave shape variations caused by impulse generator jitter (TF
should be considered only in frequency ranges where sufficient
data is present in the time domain impulse trace*).
 For chopped wave test, segments of CW1 and CW2 traces prior to
moment of chop are compared. While traces after chop may be
shift, they oscillate around zero with the same frequency.
 Verify that DGA results (after dielectrics) are normal.
* IEEE PC57.98TM/D07, September 2011, Draft Guide for Transformer Impulse Tests.
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
le
b
o
©D
in
g
n
E
C
g
n
i
r
e
e
y
n
a
mp
o 450 kV BIL, RFW on
HV winding – voltage
450 kV BIL, RFW on
HV winding – current
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
y
n
a
mp
le
b
o
©D
o450 kV BIL, CW1 on
C
g
n
i
HV winding – voltage
r
e
e
in
g
n
E
450 kV BIL, CW2 on
HV winding – voltage
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
Overlay of 450 kV BIL
CW1 and CW2 - voltage
le
b
o
©D
in
g
n
E
C
g
n
i
r
e
e
o
y
n
a
mp
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
y
n
a
mp
le
b
o
©D
in
g
n
E
o
C
g
n 450 kV BIL, FW on
i
r
e
e
HV winding – voltage
450 kV BIL, FW on
HV winding – current
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
le
b
o
©D
C
g
n
i
r
e
e
y
n
a
mp
o
in
g
n
E
Overlay of 450 kV BIL
RFW and FW - voltage
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
High-frequency oscillations at
the beginning of current trace
are acceptable deviations,
reflecting the test setup.
le
b
o
©D
C
g
n
i
r
e
e
y
n
a
mp
o
in
g
n
E
Overlay of 450 kV BIL
RFW and FW - current
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
y
n
a
mp
o BIL, FOW1 on
450CkV
g
erinHV winding – voltage
le
b
o
©D
e
n
i
g
En
450 kV BIL, FOW2 on
HV winding – voltage
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
Influence of non-linear protective
device on overlay of RFW and FW
y
n
a
350 kV BIL voltage traces mp
o
C
illustrates the need for comparing
g
n
i
r
level.
traces of the same voltage
e
e
le
b
o
©D
in
g
n
E
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
le
b
o
©D
C
g
n
i
r
e
e
y
n
a
mp
o
in
g
n
E
Influence of non-linear protective
device on overlay of RFW and FW
350 kV BIL current traces
illustrates the need for comparing
traces of the same voltage level.
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ABNORMAL DATA
 In general, whenever discrepancies occur the normal test procedure
need to be stopped and investigation performed. If the cause is found to
be external to the transformer, the corrections are made before the test
can continue.
 If there is any doubt as to the cause of the discrepancies,
additional
y
nFW. If the deviation
a
impulses need to be applied, including several
p
m
o
C
increases in magnitude, it indicates progressive
dielectric failure in the
g
n
i
r
e
transformer.
e
in
g
n
 Unusual sounds, emanating
E from inside the tank, should be noted; these
e
l
bin locating general location of the fault.
o
sounds may be helpful
D
©
 Removing manhole covers and observing presence of gas bubbles
and/or carbon, serves as confirmation of failure and provides some
indication of the fault location.
 Occasionally, the damage caused but not detected by impulse is only
detected by tests that follow: applied or induced/PD voltage tests, DGA.
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ABNORMAL DATA (cont.)
Overlay of 550 kV BIL RFW and FW
voltage traces – turn-to-turn failure
le
b
o
©D
in
g
n
E
C
g
n
i
r
e
e
o
y
n
a
mp
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ABNORMAL DATA (cont.)
Overlay of 550 kV BIL RFW and FW
current traces – turn-to-turn failure
le
b
o
©D
in
g
n
E
C
g
n
i
r
e
e
o
y
n
a
mp
HIGH-FREQUENCY - LIGHTNING IMPULSE:
ABNORMAL DATA (cont.)
Voltage drop to ground
indicates one of the leads
was at ground potential
FW voltage
C
g
n
i
r
e
e
RFW y
voltage
n
a
p
om
in
g
n
E
le
b
o
ground
©D diverts
Fault to
current around winding,
reducing measured current.
Overlay of 200 kV BIL RFW and FW
traces
–
lead-to-lead
failure
between RV and main LV windings
FW current
RFW current
HIGH-FREQUENCY:
y
n
a
p
m
SWITCHING IMPULSE
o
gC
n
i
r
e
e – other,
n
(Class
I
i
g
n
E
le II <345 kV – other,
b
o
Class
©D
Class II  345 kV - routine)
HIGH-FREQUENCY - SWITCHING IMPULSE:
OBJECTIVE
Demonstrate performance under transient high-frequency conditions
created by switching operations or network disturbance.
kV
FOW
CW
le
b
o
D
©
Surge of energy from equipment
switched on or disturbance on the power
ythe crest
n
system. The time to reach
a
p
m
o time duration of
amplitude and theC
total
g
n are much longer than
i
switching impulses
r
e
e
n
i
those
of
lightning
impulses.
g
n
E
SW
FW
s
HIGH-FREQUENCY - SWITCHING IMPULSE:
PHYSICS
Switching impulse test consists
of applying or inducing a SW
between each HV line terminal
and ground. Similar to a
lightning wave, the switching
wave can be simulated by
discharging a capacitor.
V
le
b
o
©D
y
n
a
mp
V
C
g
n
i
r
e
e
o
in
g
n
E
Comparing to lightning impulse, the
switching impulse has a much
longer
duration
and
lower
frequency, resulting in voltage
approaching a uniform distribution
of the low-frequency steady-state
voltages, i.e., voltage distributes as
per turns ratio.
length
HIGH-FREQUENCY - SWITCHING IMPULSE:
PHYSICS (cont.)
LV
D
HV
D
D
H1
H1
D
g
n
i
r
ee
To another
phase
le
b
o
©D
in
g
n
E
Region D – phase-toground y and phase-ton insulation is
a
phase
p
Comstressed the most; stress
imposed
by
SW
is
1turns*.
Charging and discharging
processes are similar to
those
described
for
lightning impulse.
H0
*From W. McNutt 1989 Doble tutorial: Turns in this context mean the voltage that would have been present
if the applied voltage was distributed according to turns.
HIGH-FREQUENCY - SWITCHING IMPULSE:
SETUP AND TEST METHODOLOGY
Full Wave Parameters
Crest voltage
1.0
Magnitude
>90% of crest
0.9
Tp
Virtual
origin
Tp
le
b
o
©D
Td
gT0
n
i
r
ee
in
g
n
E
T0
any
p
m
Co
Td
V
SW = 0.83BIL +/- 3%
RSW=(50-70%)0.83BIL
>100 s
200 s
1000 s
t
First zero crossing
 LV windings shall be designed to withstand stresses from SW
applied to HV side.
 Applied test waves are of negative polarity to reduce risk of
erratic external flashover.
HIGH-FREQUENCY - SWITCHING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
Rs
Xfmr
Impulse
generator
C
g
n
i
r
e
e
Rp
Cg
le
b
o
©D
in
g
n
E
o
y
n
a
mp
Voltage divider
and measuring
circuit
v(t)
Impulse
control &
measuring
system
Note: The shown setup is for SW being applied to the HV winding. The
test can also be performed with SW being induced.
HIGH-FREQUENCY - SWITCHING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
ELV
E
E/2
le
b
o
©D
E/2
ELV/2
C
g
n
i
r
e
e
in
g
n
E
ELV
E
ELV
E
y
n
a
mp
-ELV/2
o Test sequence and trace
comparison:
RSW@ 50-70% SW
(+) RSW - bias
SW1
-ELV/2
-E/2
Note: The choice of tap connections for all
windings is made by the manufacturer.
(+) RSW - bias
SW2
RFW@ 50-70% BIL
CW 1
CW 2
FW
HIGH-FREQUENCY - SWITCHING IMPULSE:
SETUP AND TEST METHODOLOGY (cont.)
SW can saturate the core, creating an air-core conditions, i.e.,
drastically reducing impedance faced by impulse. This rapidly
decays the tail of the voltage waveform to zero, making T0<1000
s. To extend the time to saturation, prior to start of each test,
y
the core is magnetized in opposite direction by
applying
RSW (or
n
a
p
small dc current) of opposite polarity . Com
V
©
le
b
o
D
g
n
i
r
ee
in
g
n
E
When core saturates, the
voltage collapses drastically
reducing time to zero crossing.
Bias in the core in
direction opposite
to that created by
test SW extends
time to saturation
and T0.
t
HIGH-FREQUENCY - SWITCHING IMPULSE:
ACCEPTANCE CRITERIA
 Tp, Td, T0, and voltage magnitude must meet requirements.
 Failure detection is done primarily by scrutinizing voltage
traces for recognizable indications of failure. The test is
successful if there is no sudden collapse of voltage as
y
n
indicated on the trace.
a
p
m
Cotraces in totality may
 Although overlaying RSW andinSW
g
r
e
e
not be practical, the traces
in should match until the point
g
n
Ein the core magnetic state becomes
where the difference
e
l
b
obvious. Normally,
these differences can be easily
©Do
distinguished from drastic voltage reduction caused by a
failure.
 Verify that DGA results (after dielectrics) are normal.
HIGH-FREQUENCY - SWITCHING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
650 kV BIL, RSW on
HV winding – voltage
le
b
o
©D
in
g
n
E
o
C
g
n
i
r
e
e
Typical
reduced
and
full
switching
impulse
voltage
traces as measured on the HV
winding; for 650 kV BIL, the
BSL, i.e., the required test
voltage, is 540 kV.
y
n
a
mp
650 kV BIL, SW1 on
HV winding – voltage
650 kV BIL, SW2 on
HV winding – voltage
HIGH-FREQUENCY - SWITCHING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
Overlay of 650 kV BIL
RSWnand
y SW - voltage
a
p
m
Co
g deviating
ntraces
i
Beginning eof
r
e
dueng
tointhe difference in core
E
magnetic
state.
This
is
e
l
b
©Do typically more pronounced in
the overlay of reduced and full
switching waveforms
HIGH-FREQUENCY - SWITCHING IMPULSE:
ACCEPTANCE CRITERIA (cont.)
y to the
Slight deviation adue
n
p
difference o
inm
core magnetic
C
g
state.
rin
le
b
o
©D
e
e
n
i
Eng
Overlay of 650 kV BIL
SW1 and SW2 - voltage
HIGH-FREQUENCY - SWITCHING IMPULSE:
ABNORMAL DATA
 In general, whenever discrepancies occur the normal test
procedure need to be stopped and investigation performed. If
the cause is found to be external to the transformer, the
corrections are made before the test can continue.
ny discrepancies,
 If there is any doubt as to the cause ofpathe
additional impulses may be applied. Com
g
n
i
r
e observing presence of gas
 Removing manhole covers
and
e
n
i
g
nserves
E
bubbles and/or carbon,
as confirmation of failure and
e
l
b
o
provides some
indication
of the fault location.
©D
HIGH-FREQUENCY–LIGHTNING AND SWITCHING IMPULSE:
RECOURSE IF DATA ABNORMAL
 If visual confirmation (e.g., carbon, bubbles) is obtained
or the data convincingly reveals a failure, the oil is
drained and internal inspection is performed.
 If necessary, the unit is un-tanked. This is followed by a
y
n
a
thorough and well-documented investigation.
mp

o
C
g
n process enhances the
i
The user’s involvement inerthis
e
n
i
g
quality of the investigation
and that of the final product.
n
E
le
b
o
©D
y
n
a
p
LOW-FREQUENCY:
m
o
C
g
n
i
r
e
eVOLTAGE
APPLIED
n
i
g
n
E
le (Routine)
b
o
©D
LOW-FREQUENCY – APPLIED VOLTAGE:
OBJECTIVE
The high-frequency tests (lightning and switching
impulse) always precede the low-frequency tests (applied
and induced voltage). This sequence is rooted in the fact
that due to a longer duration, the low-frequency tests
y
n
a
serve to stress further and to detect the
damage caused
p
m
o
C
by the high-frequency tests.
g
n
i
r
e
e
n
i
g
En
The applied voltage
letest is a simple overvoltage test. The
b
o
©D engineers apparently took cues from
early transformer
mechanical engineers. This is how a mechanical structure
would be tested, by applying stress that demonstrates a
safety factor of two. The applied voltage test has a 1 min
duration, with the expectation to demonstrate a long-term
capability to operate at the rated voltage.
LOW-FREQUENCY – APPLIED VOLTAGE:
PHYSICS
D
LV
HV
D
LV
le
b
o
©D
HV
in
g
n
E
C
g
n
i
r
e
e
o
y
n
a
p
D – major winding
mRegion
-to-ground and winding-towinding
insulation
are
stressed the most.
Shorting
lead
D
LOW-FREQUENCY – APPLIED VOLTAGE:
SETUP AND TEST METHODOLOGY
 Test is performed at low frequency
(<500 Hz), normally, power
Magnitude
C57.12.00-2010
frequency.
Duration
1 min
 All terminals of tested winding are
connected together; all other
terminals (including
all cores,
1.1E
y
n
a
p
buried windings
with one terminal
m
o
C
brought-out
and the tank) are
E
g
n
i
r grounded.
e
e
in  A sphere-gap, set for 10% above
g
n
E
v
e
l
b
test voltage, may be connected for
o
D
©
protection.
 Test
voltage
(1-phase)
is
determined by terminal with the
lowest BIL (e.g., Neutral).
 The voltage is raised from 25% or
Note: On grounded-wye transformers with
less, held for 1 min and reduced
reduced Neutral BIL the test has a limited
gradually.
significance; it inly tests insulation in the
 Each winding or set of windings
vicinity of the Neutral.
(e.g., in auto) is tested.
Applied Voltage Parameters
LOW-FREQUENCY – APPLIED VOLTAGE:
ACCEPTANCE CRITERIA
 The test is a pass/fail test and is considered
passed if during the time the voltage is applied no
evidence of possible failure is observed.
ny sound
 The indications to monitor include p
unusual
a
om
such as thump, sudden increase
in the test circuit
C
g
rintest voltage.
current and collapse in ethe
e
n
le
b
o
©D
i
g
n
E
LOW-FREQUENCY – APPLIED VOLTAGE:
ABNORMAL DATA
 If unusual sound, sudden increase in the test
circuit current or circuit tripping occur, these
events should be carefully investigated by:
• observation, e.g., presence of carbon
y
n
a
p
and/or bubbles in the oil
m
o
C
• repeating the test
g
n
i
r
e
e
n
• other tests
i
g
n
E
to determinebwhether
the failure has occurred.
le
o
 Due to a©D
significant energy being released during
applied voltage test, the test is repeated (if at all) to
confirm the failure a limited number of times (1, 2
max). The energy released is usually sufficient to
mark the location making it possible to find the
failure after un-tanking.
LOW-FREQUENCY – APPLIED VOLTAGE:
RECOURSE IF DATA ABNORMAL
If visual confirmation (e.g., carbon, bubbles) is obtained
and/or repeating of the test and/or other tests reveal the
failure, the oil is drained and internal inspection is
y
n
performed.
a
p
le
b
o
©D
in
g
n
E
om
C
g
n
i
r
ee
LOW-FREQUENCY:
INDUCED VOLTAGE/PD
y
n
a
mp
o
C
g
n Induced:
i
r
e
Routine
e
n
i
g
n
E
le
7200 cycles
Class IDob
©
Class II
Routine
Induced:
1 hour + PD
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
OBJECTIVE
The induced voltage test demonstrates the strength of
internal insulation in all windings as well as between
windings and to ground. A combination of prolonged stress
and a very sensitive PD measurement makes ity a very severe
n
a
p
and searching test. It must be the last
dielectric
test to be
m
o
C
g
performed.
rin
le
b
o
©D
e
e
n
i
Eng
LOW-FREQUENCY - INDUCED VOLTAGE/PD:
PHYSICS
xfmr
IG
IT
VT
M
Lv
G
ILv
le
b
o
reactor ©D
IT
Variable
Lv is adjusted to
reduce
output
from generator.
L
R
C
g
n
i
r
ee Therefore, the test is performed as a
in
g
n
E
VT
IG
ILv
 To stress turn-to-turn insulation to the
required level, the winding needs to
be excited to a level approaching
twice rated voltage. At power
y overexcite the
nwould
a
frequency, p
this
m
core.Co
higher frequency, which allows to
obtain the needed volts/turn at a
lower flux magnitude (v/t = dF/dt).
 At higher frequency, transformers
become capacitive with dangers of MG set overexciting. This is addressed
by using a variable reactor. The latter
provides an additional benefit of
reducing the load on MG set.
LOW-FREQUENCY - INDUCED VOLTAGE/PD:
PHYSICS (cont.)
E
LV
HV
E
E
E
leE
b
o
©D
E
Region E –ny with voltage
a turns ratio, the
p
distributing
per
m
o is present in the turnC
most
stress
g
n
i
r
e
to-turn
insulation of each winding
e
n
i
as well as in winding-to-winding
Eng
and winding-to-ground insulation.
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
PHYSICS (cont.)
From the physics point of view, self-sustaining electron avalanches
may occur only in gases. Hence, discharges in dielectrics may
only be ignited in gas-filled cavities, such as voids or cracks in
solid materials and gas bubbles or water vapor in liquids.
Discharges are generally ignited if the electrical field strength
y
inside the inclusion exceeds the intrinsic field strength
of the gas.
n
a
p
They can appear as pulses having a duration
Comof << 1s.
Dielectric
Conductor
Gaseous
inclusionle
b
o
D
©
g
n
i
r
ee
in
g
n
E
Partial discharges are defined as localized
electrical discharges that only partially bridge
the insulation between conductors and may or
may not occur adjacent to a conductor. In
insulation, the PD events are the consequence
of local field enhancements due to dielectric
imperfections.
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
PHYSICS (cont.)
To model the PD process, capacitance of the active void CC can be
viewed as part of a larger capacitive network. In that, CB is the
remaining capacitance of the immediate region in series with CC
and CA is the rest of the dielectric connected in parallel. Two
requirements must be fulfilled to initiate PD: 1) local field stress
y electrons are
exceeds the void’s breakdown voltage Vbd and 2)
free
n
a
p
m
available.
Co
g
n
i
r
ee
in
g
n
E
le
b
o
CB
©D

CA
CC
CA
CB
CC
Vbd
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
PHYSICS (cont.)
Buildup: As V , charges
move to and collect on
the surface of the void,
building potential stress
Vcc across the void.
Strike: As Vcc>Vbd, breakdown
occurs, charges move across
shorting the void, Vcc= 0 and
discharge stops. To make up
for imbalance, charges come
out of adjacent insulation.
V
V
CB
Q
CA
Q
CB
Q
Q
Vcc
CC
le
b
o
D
©
g
n
i
r
ee
Q
QQ Q
n
i
g
C
n
E A
Relaxation: Charges continue
to flow at a decreasing rate
with balance restoring. Vcc 
as charges collect back on
the void’s surface.
V
any
p
m
Co
CB
Q
Q
CA
Q
Q
CC
QQ
Vcc= 0
CC
Vcc
Q
V
CB
CA
Q QQ
CC
V
V
CB
CB
Vcc
CA
CA
CC
Vcc= 0
Q
CC
Vcc
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
PHYSICS (cont.)
Dip in terminal
voltage
Terminal
voltage
C1
PD current
le
b
o
©D
C
g
n
i
r
e
e
Voltage
in
g
n
Eacross void
y
n
C
a
mp
o
2
Z
M
CT
*The detectable voltage dip is in the mV
range, while that at the void may be in
the kV range.
We cannot measure the real charge. However, as the void discharges, the
charge redistribution creates a dip* in the terminal voltage. This minute voltage
drop causes a high-frequency current to flow through a coupling capacitor
connected to a measuring system. Putting it differently, the charge movements
appear, in part, in C1 connected in parallel with CT. The integration of these highfrequency current pulses over time produces the reported apparent charge.
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
PHYSICS (cont.)
Measurement of partial discharge is
like trying to weigh a butterfly that
y
n
a
p
alights momentarily Comon
scales
g
n
i
r
designed for anngelephant
(sometimes
inee
E
e
l
b
o
during an©Dearthquake).
by Karl Haubner, Doble Australia
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
SETUP AND TEST METHODOLOGY
C1
Lv
M
Step-up
xfmr
G
Xfmr in
test
C2
X0 H1
X1
H2
X2
C1
le
b
o
©D
C2
C
g
n
i
r
e
e
X3 H3
in
g
n
E
p
m
o
any
C1
M
V
pC
and/or
V
C2
Before test commences, several important steps take place:
 Transformer is connected for open-circuit conditions.
 Voltage is raised to verify that variable (Lv) setting allows
to reach the required test voltage.
 Measuring system (M) is calibrated for PD, RIV and
voltage.
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
SETUP AND TEST METHODOLOGY (cont.)
Enhanced level
7200 cycles
V
1h level, 5 min
recordings
Hold as needed
until stable (min
60 sec)
100%
C
g
n
i
r
e
t
e
100%
Ambient
©
1h
le
b
o
D
in
g
n
E
Ambient
Induced Voltage/PD
Parameters
Voltage
magnitude
C57.12.00-2010
clause 5.10
C84.1
y
n
a
Timing
p
m
C57.12.90-2010
clauses 10.7, 10.8
PD/RIV
criteria
C57.12.90-2010
clause 10.8/
Annex A
o
 Voltage is gradually raised, recording pC, V and kV.
 For Class I units, the test includes applying to HV winding 2.0nominal
voltage for 7200 cycles with no PD (RIV) recordings. For class II units rated
115 ÷ 500 kV, the test includes applying to HV winding 1.8nominal voltage
for 7200 cycles and 1.58nominal voltage for 1 h, recording PD (RIV) data.
 For windings other than HV, when possible, taps should be selected so that
voltages on other windings are as per ANSI C84.1 and C57.12.90 clause
10.8.1 (e.g., for 115÷345 kV units , the voltage on other windings should be
1.5 times their maximum operating voltage).
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
SETUP AND TEST METHODOLOGY (cont.)
 PD (pC) measurements are performed using 100 ÷ 300 kHz and RIV
(V) using 0.85 ÷ 1.15 MHz frequency ranges.
 For units with windings that have multiple connections (e.g., seriesparallel or delta-wye) with each connection having system voltage
>25 kV, two induced tests are performed, one in each connection. If
ny
more than one winding has such multiple pconnection,
then the
a
om between tests. In all
connections in each winding shall change
C
g with highest test voltage.
n
i
r
cases, the last test shall be for connection
e
e
n
i
g
 To minimize the effects E
ofnexternal
factors and stray capacitances,
leoften relied on:
b
the following steps
are
o
D
©
- filters on the power supply line
- shielding all sharp edges including those at ground potential
as well as the energized and grounded bushings
- turning off solid state power supplies, cranes and other factory
machinery
- removing air bubbles from bushing gas space
- applying pressure to suppress bubbles in the main tank.
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
ACCEPTANCE CRITERIA
Results are acceptable if:
 Nothing unusual associated with sound, current, or voltage
is observed (see abnormal data for details).
 The PD (RIV) results during 1h test period have
y shown:
n
a
p
m
- Magnitude  500 pC ( 100 V).Co
g
n
i
r
e pC ( 30 V).
- Increase during 1 h in150
e
g
n
E
- No steadily rising
trends during 1 h
e
l
b
o
D
©
- No sudden sustained increase during the last 20 min.
Judgment should be used on the automatically recorded 5-min
readings so that momentary excursions caused by cranes or
other ambient sources are not recorded. Also, the test may be
extended or repeated until acceptable results are obtained.
 DGA results (after dielectrics) are normal.
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
ACCEPTANCE CRITERIA (cont.)
V1
PD1
RIV1
Time1
V2
PD2
RIV2
Time2
V3
PD3
RIV3
Time3
1
0.2 kV
15.4 pC
4.5 µV
00:00:03
0.3 kV
14.6 pC
5.3 µV
00:00:11
0.2 kV
138. pC
4.5 µV
00:00:20
Ambient
2
30.4 kV
24.7 pC
4.5 µV
00:00:49
30.6 kV
26.3 pC
5.3 µV
00:00:58
30.5 kV
27.2 pC
4.2 µV
00:01:07
100%
3
37.8 kV
27.1 pC
4.4 µV
00:01:51
37.6 kV
38.3 pC
5.6 µV
00:02:00
37.7 kV
29.7 pC
4.6 µV
00:02:09
125%
4
42.4 kV
36.7 pC
5.2 µV
00:03:33
42.0 kV
29.2 pC
7.1 µV
00:03:42
42.1 kV
30.7 pC
4.7 µV
00:03:51
1hr level
5
55.5 kV
31.9 pC
4.9 µV
00:04:03
54.5 kV
33.5 pC
13.5 µV
00:04:12
54.7 kV
33.9 pC
6.2 µV
00:04:21
Enhanced
6
42.3 kV
27.1 pC
4.6 µV
00:00:03
41.9 kV
29.3 pC
5.6 µV
00:00:35
1 hr level
7
42.2 kV
27.3 pC
4.6 µV
00:05:03
41.9 kV
28.0 pC
6.1 µV
00:05:35
8
42.1 kV
27.8 pC
4.5 µV
00:10:03
41.7 kV
29.4 pC
5.2 µV
00:10:35
9
41.8 kV
27.1 pC
4.5 µV
00:15:03
41.6 kV
28.4 pC
6.0 µV
10
42.1 kV
28.8 pC
4.6 µV
00:20:03
41.7 kV
29.7 pC
11
42.3 kV
28.0 pC
4.3 µV
00:25:03
42.0 kV
12
42.1 kV
28.0 pC
4.8 µV
00:30:03
13
41.9 kV
31.3 pC
5.1 µV
14
41.8 kV
28.2 pC
15
42.1 kV
16
y
n
a
mp
42.1 kV
29.5 pC
4.9 µV
00:01:07
41.9 kV
29.8 pC
4.8 µV
00:06:10
41.8 kV
30.6 pC
4.9 µV
00:11:07
00:15:35
41.8 kV
30.1 pC
5.1 µV
00:16:07
6.0 µV
00:20:35
41.8 kV
30.9 pC
4.9 µV
00:21:07
29.5 pC
6.2 µV
00:25:35
42.1 kV
31.3 pC
5.0 µV
00:26:09
41.7 kV
29.0 pC
5.8 µV
00:30:35
41.8 kV
30.1 pC
4.9 µV
00:31:07
00:35:03
41.7 kV
28.8 pC
6.0 µV
00:35:35
41.8 kV
29.7 pC
5.0 µV
00:36:07
4.8 µV
00:40:03
41.6 kV
29.5 pC
5.4 µV
00:40:35
41.6 kV
31.1 pC
4.8 µV
00:41:07
27.8 pC
4.8 µV
00:45:03
41.7 kV
29.4 pC
5.8 µV
00:45:35
41.8 kV
30.8 pC
5.2 µV
00:46:07
42.0 kV
27.8 pC
4.6 µV
00:50:03
41.7 kV
28.0 pC
5.9 µV
00:50:35
41.8 kV
30.6 pC
4.6 µV
00:51:07
17
41.8 kV
29.4 pC
4.7 µV
00:55:03
41.6 kV
30.5 pC
5.6 µV
00:55:35
41.6 kV
31.9 pC
4.7 µV
00:56:07
18
41.8 kV
28.0 pC
4.6 µV
01:00:03
41.6 kV
29.1 pC
5.1 µV
01:00:35
41.6 kV
30.3 pC
4.8 µV
01:01:07
1 hr level
19
37.9 kV
27.5 pC
4.5 µV
01:02:50
37.7 kV
30.1 pC
5.2 µV
01:03:01
37.7 kV
30.6 pC
4.8 µV
01:03:09
125%
20
30.7 kV
24.7 pC
4.6 µV
01:04:02
30.7 kV
26.4 pC
5.1 µV
01:04:11
30.7 kV
27.7 pC
4.7 µV
01:04:20
100%
21
0.3 kV
18.2 pC
4.6 µV
01:04:38
0.3 kV
11.6 pC
5.2 µV
01:04:47
0.3 kV
12.0 pC
4.9 µV
01:04:56
Ambient
oble
©D
o
C
g
n
i
r
e
e
in
g
n
E
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
ABNORMAL DATA
 Results are not acceptable if the pC (or V) data exceeds any
of the required criteria, and no reasonable/acceptable
justification for the source/cause is provided.
 Other tests, e.g., acoustic PD, DGA, can provide
y confirmation
n
a
p
that a source of excessive partial discharge
is present.
m
o
C
g rising in the oil, audible
n
i
r
 The presence of smoke and ebubbles
e
n
i
ngsudden increase in test current or
sounds such as thump,
E
le all serve as a confirmation that
b
voltage collapse
may
o
D
©
abnormal PD results are associated with a failure.
LOW-FREQUENCY – INDUCED VOLTAGE/PD:
RECOURSE IF DATA ABNORMAL
 If pC (or V) data exceeds the limits, and all the attempts
to identify and eliminate external PD sources are not
successful, a longer standing time, long duration PD
test, degassing of oil, refilling transformer under
vacuum or a heatrun test (if one is specified)
y are often
n
a
p
m
successfully bring the PD data within
limits.
Co

g
n
i
r
e
A failure to meet theinpartial
discharge acceptance
e
ng
E
criterion shall not
warrant
immediate rejection, but it
e
l
b
shall lead©D
too consultation between purchaser and
manufacturer about further investigations.
 If visual confirmation (e.g., carbon, bubbles) is obtained
and/or repeating of the test and/or other tests reveal the
failure, the oil is drained and internal inspection is
performed.
NO-LOAD LOSSES aAND
y
n
p
m
o
C
EXCITATIONerCURRENT,
g
n
i
e
n
i
g
ndielectrics
E
after
oble
©D
(Routine*)
*The test is not required by standards and no test type is
assigned to it; however, it is a wildly recognized as
standard practice and performed as routine.
NO-LOAD LOSSES AND EXCITATION CURRENT after dielectrics:
OBJECTIVE
Objective: No-load loss and excitation current, measured
at 100% and 110% of the specified voltage and frequency
after all dielectric tests are completed, provide additional
confirmation that no damage, created by dielectric tests, is
ypower test to
n
present in the transformer. If this is the last
a
p
m
o
C
be performed, it also serves to demagnetize
the core for
g
n
i
re.g., 10-kV exciting current
e
subsequent low-voltage tests,
e
in
g
n
and sfra.
le E
b
o
D
©
NO-LOAD LOSSES AND EXCITATION CURRENT after dielectrics:
ACCEPTANCE CRITERIA AND RECOURSE IF DATA ABNORMAL
No-load losses measured after dielectric tests are
compared with the results obtained before dielectric tests.
The 5% difference is often used as an acceptable criteria.
Difference between the before and after data could be due
y
to:
n
a
p
m
 Changes in the inter-laminar insulation
o
C
g
n
i
r
 Temperature
e
e
n
i
g
Sometimes the change
nafter initially exceeding 5% goes
E
away with time.Doble
©
Failure to meet before and after dielectrics comparison
criteria should not warrant immediate rejection but shall
lead to consultation between purchaser and manufacturer
regarding further investigation of possible causes and
consequences.
y
n
a
p
LOAD LOSSES
AND
m
o
C
g
n
i
r
e
e VOLTAGE
IMPEDANCE
n
i
g
n
E
le (Routine)
b
o
©D
LOAD LOSSES AND IMPEDANCE VOLTAGE:
DEFINITION AND OBJECTIVE
Definition: The load losses of a transformer are losses
associated with a specified load and include:
 windings I2R losses due to load current
 stray losses due to eddy currents induced by leakage flux in
ytank walls, and
the windings, core clamps, magnetic shields,
n
a
p
m
omay also be caused by
other conducting parts. Stray losses
C
g
n
i
r
currents circulating in parallel
windings
or strands.
e
e
n
i
Load losses do not include
control and cooling losses.
Eng
le
b
o
D
The impedance ©
voltage
of a transformer is the voltage required
to circulate rated current through two specified windings with
one winding short-circuited.
LOAD LOSSES AND IMPEDANCE VOLTAGE:
DEFINITION AND OBJECTIVE (cont.)
Objective: The impedance and load losses test provides the
data for:
 Verification of design calculations.
 Demonstration of meeting the guaranteed performance
y often an
n
characteristics. Since these parameters
have
a
p
m
othe accuracy of the
economic value attached to them,
C
g
n
i
r
measurement becomes significant.
e
e
n
i
g
nare used as test parameter during
E
 Maximum load losses
le
b
o
the temperature
©D rise test.
 Impedance voltage is an essential input parameter in power
system studies (e.g., load flow, transformer parallel
operation, short-circuit calculations).
LOAD LOSSES AND IMPEDANCE VOLTAGE:
PHYSICS
IIexc
rated
R
FM
Note: Resistance R and short
circuit of LV is not shown.
FL
I2R lossesT
Vrated
Vsc
HV LV
in
g
n
E
C
g
n
i
r
e
e
le
b
o
conditions
©D when
o
y
n
a
mp
Eddy currents
creating losses1/T
To create
losses are limited to I2R and stray
losses, and applied voltage is equal to the voltage drop across a
loaded transformer, one winding is short-circuited and voltage is
raised until rated current is reached. The flux path is then
dominated by the leakage channel where the eddy losses in
various conducting components in the FL path are induced.
LOAD LOSSES AND IMPEDANCE VOLTAGE:
PHYSICS (cont.)
For most power transformers, VX_L >> VR_L.
ZSC
Iinput
R
HV
Irated
X
HV
RL
XLV
XL
RLV
VX_L
Measured
Corresponds
to leakage-flux
linkages of the
windings
VSC
VSC
IC
VR_L
CCRm
le
b
o
D
©
Compensating
variable
capacitor Cc is adjusted
to reduce the input
current.
y
n
a
mp
VX_L
Xm
o
C
g
n is close
i
r
Angle
e
e to 90, requiring
n
i
g
En
IC
Iinput
Irated
high accuracy
test systems.
VSC

VR_L
Irated
Corresponds
to load loss
LOAD LOSSES AND IMPEDANCE VOLTAGE:
SETUP AND TEST METHODOLOGY
Transformer
in test
CT
3
X0
H1
VT
X1
H2 X2
 After data is recorded, if
necessary, correction for losses
y
n
a
in external
circuit
is made.
p
H3 X3
V
m
o
C
g If three line currents are not
n
i
r
balanced the average RMS
ee
I
A
oble
V
©D
W
 Applied voltage is adjusted
until rated current is present in
the excited winding.
in
g
n
E
value should correspond to the
desired value.
 The duration of the test should
be kept to a minimum to avoid
heating up winding conductors.
If taps are present, the following
combinations of voltage ratings are tested:
DETC
rated
rated
rated
max
max
max
min
min
min
LTC
N
max
min
N
max
min
N
max
min
LOAD LOSSES AND IMPEDANCE VOLTAGE:
SETUP AND TEST METHODOLOGY (cont.)
Z2
2
Z1
1
1
3
Z12 = Z1 + Z2
Z13 = Z1 + Z3le
b
o
D
©
2
 For 3-wdg units, three sets of
measurements are performed
3
using three pairs of windings,
Z3
producingaZn12y, Z13, Z23 and P12,
P13,omPp
Solving shown
23.
C
gequations, determines Zi and
n
i
r
e P of each branch.
e
n
i
i
g
En
Z23 = Z2 + Z3
Z1 = (Z12 + Z13 – Z23)/2
Z2 = (Z12 + Z23 – Z13)/2
Z3 = (Z13 + Z23 – Z12)/2
 For test, the current is set
based on capacity of the
winding with lowest MVA in the
pair.
 When results are converted to
%, all data is given based on
MVA of HV winding.
LOAD LOSSES AND IMPEDANCE VOLTAGE:
SETUP AND TEST METHODOLOGY (cont.)
Measure A, V, W, T
Since stray and I2R
losses have different
Correct W and V
Convert stray
dependencies on T,
from measured
losses from
each need to be
amps to rated
TLL_test Trated
obtained
from
y
n
a
p
measured
losses,
m
o
2
C
Convert I R losses
Convert Rdc from
g
individually converted
n
i
r
e Trated
from Tin
TR_test  TLL_test
e
from test T to rated
LL_test
g
n
E
T before combined
e
l
b
again in reported load
Calculate I2R losses
Calculate total
©Do
losses.
V is also
at TLL_test
losses at Trated
converted to rated T.
(stray + I2R)
Calculate stray
losses at TLL_test
(W - I2R)
Correct V from
TLL_test  Trated
Calculate %Vsc
(V / Vrated)100 = %Zsc
LOAD LOSSES AND IMPEDANCE VOLTAGE:
ACCEPTANCE CRITERIA
 The total losses (no-load + load) should not exceed the
guaranteed value by more than 6%.
 For 2-wdg units, if Zsc>2.5%, the tolerance for measured
impedance is +/-7.5% of the guaranteed value, otherwise, it is +/10%. The tolerance for comparison of duplicates units produced
at the same time is +/-7.5%.
y
n
a
p having a zigzag
 For 3-wdg units, autotransformers orom
units
C
g
nimpedance is +/-10% of the
winding, tolerance for measured
i
r
e
e for comparison of duplicates
n
i
guaranteed value. The tolerance
g
n
E
lesame time is +/-10%.
units produced at o
the
b
D data is credible:
 Assurance that©test
 Thermal stability prior to test: TTO-TBO 5C.
 Average of T readings (Tave_oil) before and after the test
should be used as test T. Their difference must be 5C.
 Frequency is within +/-0.5% of rated.
 Test system accuracy should be within +/-3% for loss, +/-0.5%
for voltage, current and RDC, and +/-1.5C for T.
LOAD LOSSES AND IMPEDANCE VOLTAGE:
ABNORMAL DATA
Example: guaranteed load loss - 94 kW, measured – 110 kW
 Potential reasons for exceeding the guaranteed values may
include:
 Oversights in design



y
n
a
mp
o
C
g
Production process related
factors
or mistakes
n
i
r
e
e
n
i
g
n
Influence of temperature
was not properly accounted for
E
e
l
b
o
D
Accuracy©of measurements
LOAD LOSSES AND IMPEDANCE VOLTAGE:
RECOURSE IF DATA ABNORMAL
 Failure to meet the load losses and impedance test
criteria should not warrant immediate rejection but shall
lead to consultation between purchaser and
manufacturer regarding further investigation of possible
causes and consequences.
ny

a
p
m
o losses does
The acceptance criteria of 6% for
total
C
g
nguarantee of losses
i
r
e
replace the manufacturer’s
e
n
i
g
n purposes.
economic loss evaluation
E
le
b
o
©D
not
for
LOAD LOSSES AND IMPEDANCE VOLTAGE:
COMPARISON WITH FIELD DATA
Factory losses are measured
under 3-phase excitation, at
rated current and reported as
sum of three phases I2R and
stray losses.
Field losses are measured
under 1-phase excitation, at
ylower than rated
current much
n
a
p
m
and reported
as per-phase I2R
o
C
g stray losses.
n
i
and
r
ee
in
g
n
E
le
b
o
©D Factory and field results
cannot be compared
LOAD LOSSES AND IMPEDANCE VOLTAGE:
COMPARISON WITH FIELD DATA (cont.)
Factory short-circuit impedance is
reported as average of three
phases, obtained at rated current*
under 3-phase excitation.
Field leakage reactance is reported
as per-phase reactive component of
short-circuit
impedance,
the
obtained at current* much lower than
rated under 1-phase excitation.
y
n
a
p
m
Experience shows that a combined influence of
different
instrumentation
o
C
g
under 3- and 1-phase
and test setups, difference in flux distribution
n
i
r
e
ecomponent and averaging of factory
excitation, presence of the resistive
n
i
g
nranging from nearly perfect (<1%) to up to
E
data can result in differences
le
b
o
6% (of the measured
value).
©D
However, the differences between factory and field test conditions
notwithstanding, the ZNP can serve as a useful guideline for evaluating
the initial value measured in the field. If, during initial test, the field perphase tests deviate from average (of three readings) by <3% of the
measured value, results normally are considered acceptable. The initial
per-phase test should serve as a benchmark for future testing with
acceptable difference from the initial field test being <2%.
*Since test is confined to leakage channel (where reluctance is determined by air/oil) the
leakage inductance (L=/I), remains the same regardless of the current level.
y
n
a
p
TEMPERATURE
RISE
m
o
C
g
nother)
i
r
e
(Designinand
e
g
n
E
le
b
o
©D
TEMPERATURE RISE:
DEFINITION AND OBJECTIVE
Definition: The temperature rise is a test that verifies
transformer thermal performance through determination
of winding and oil temperature rises over ambient.
Objective: The temperature rise test provides the top-oil
y rise over
n
rise, winding average rise and winding hot-spot
a
p
m
o
ambient for:
gC


n
i
r
e
ine
Verification of designncalculations.
g
E
e
l
b
Demonstration
of meeting the guaranteed performance
o
D
©
characteristics.
 Provides data for calculation of potential MVA margin.
 Setup of various temperature monitoring instruments
and cooling control.
TEMPERATURE RISE:
PHYSICS
Calculated: Tto-a, Tw_ave-a, Ths-a, GRAD
Measured:
Tto, Tt_rad, Tb_rad, Ta.
Needs NL+LL
losses
Ta
Main tank
Tto
Tt_rad
height
Tto-a
Need rated
current
Tto
y
Tn
t_rad
a
mp
Ths-a
Ta
LV
le
b
o
©D
HV
Core
in
Rad
g
n
E
Tb_rad
Ta
C
g
n
i
r
e
e
o
Oil
To_ave
GRAD
Tw_hs
Tw_ave*
Winding
Tw_ave-a
Tb_rad
Ta
T
Located at 3 locations around xfmr at mid-height level.
*The term “winding average T rise”, Tw_ave-a, is not the T at any given point in a winding
nor is it an arithmetic average of results determined from different terminal pairs. It refers
to the value determined by measurement on a given pair of winding terminals.
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY
Transformer
in test
CT
X0
H1 X1
VT
I
ng
E
le
Dob
©V
W
 Test is performed
for min and max
y
n
ain a combination of
p
MVA, m
and
H3 X3
o
C
DETC/LTC
positions, producing
g
n
i
r highest load losses.
e
e
in
H2 X2
V
A
 Total losses (NL+LL) and winding
cold resistance data should be
available.
 10Tamb40C and measured in
containers with liquid, having a time
constant as per C57.12.90-2010.
 Test contains 3 key segments:
- total loss run (to include 3 hr of
thermal stability)
- rated current run (1 hr)
- hot resistance measurement
(e.g., 10-20 min after shutdown)
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
Measurement before
cutback determines *Tto-a
T[C]
ONAF shutdown
Tto, Tt_ rad
Rhot
measurement
begins
Cutback
Xfmr
energized
for ONAF
Tb_ rad
ONAN
shutdown
y
n
a
mp
o
C
Steady-state oil
g
n
i
r
e
T rise
e
Tto-a ngin
le
b
o
D
©
E
Ta_ ave
(change of Tto-a in 3h
 1C or  2.5%
whichever is greater)
Ptotal
1h
Preceding
ONAN
Itest
Irated
Total loss run
*Tto-a is corrected for difference between required and
actually used total losses (it must be 20%) and for altitude.
t [h]
Rated current
run
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
Objective: resistance of
winding at the time when
load current is still present
Rhot
Rhot calculated at t = 0
y in
Rhot as function a
ofntime
p
presenceo
ofmdecreasing
C
g
temperature
is recorded
rin
*Instrument
connected
e
e
Instrument output
n
i
ng
current reached
E
le
pre-selected levelDob Flux
© stabilized
t=0
Voltage
removed
t [min]
t  4 min
Tw_hot = Rhot/Rcold(234.5 + Tw_cold) – 234.5
*If two windings are tested simultaneously in series,
the Idc is selected based on the lowest rated current.
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
Comparison with guaranteed values,
e.g., Tto-a and Tw_ave-a  65C; Ths-a  80C
GRAD correction for
localized hot spot
eddy currents
Tw_ave-a
Tw_hot
To_ave_sd
*GRAD
GRAD
in
g
n
E
g
n
i
r
ee
le
b
o
©D**To_ave_cb-a
any
p
m
Co GRAD
Tto-a
***Ths-a
Value used for
setting winding
T monitors
Ta
*GRAD is corrected for difference between required and actually used load current (it must be 15%).
**To_ave_cb-a is corrected for difference between required and actually used total losses (it must be 20%)
and altitude.
***This a simplified representation of Ths_a determination; actual design calculation is more involved.
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
During shutdown at the time of the first Rdc reading, the flux must be
stabilized so that resistance change is caused only by reduction in
temperature. It’s true in most cases, unless series autoxfmr is present.
vLV
X0
idc
i2
Series autoxfmr
windings with LTC in N
Series autoxfmr
core
F1
le
b
o
©D
F2
idc
i1
C
g
n
i
r
e
e
y
n
a
mp
o
in
g
n
E
vLV
idc
LV winding
idc
Main unit
core
F
i1
X0
X2
idc
i2
X2
TEMPERATURE RISE:
SETUP AND TEST METHODOLOGY (cont.)
0.0047
0.00468
At t = 4 min, data
0.00467
y
n
collection a
begins
X 0 -X 2
p
m
o in main
withCflux
0.00466
g
ncore stable while
i
r
e
e flux in series core
0.00465
n
i
g
n
E
le
still changing.
0.00464
b
o
D
©
H 1 -H 2
0.00463
0.32
0.31
0.3
0.29
0.28
0.27
0.26
0.00462
0.25
0:00:00 0:00:43 0:01:26 0:02:10 0:02:53 0:03:36 0:04:19 0:05:02 0:05:46
Time [min]
Voltage
removed
t=0
Time remaining
for stabilization
= 2.5 min
HV circuit Rdc [Ohm]
Setup 
1.5 min
LV circuit Rdc [ohm]
0.00469
Series autoxfmr should be
excluded from both cold
and hot Rdc measurements.
TEMPERATURE RISE:
ACCEPTANCE CRITERIA
 The winding average T rise over ambient for all tested windings
should not exceed the guaranteed value, e.g., 65 or 55C.
 The top-oil T rise over ambient should not exceed the guaranteed
value, e.g., 65 or 55C.
 The winding hot spot T rise over ambient for all tested windings
y for 65C rise
n
should not exceed the guaranteed value, e.g.,
80C
a
p
m
o
units and 65C for 55C rise units.
C
g
n results of winding average
i
r
 If shutdown is performed on each
phase,
e
e
n
i
g
rises should be comparable
(rule of thumb: 4C difference,
n
E
le in the standard).
b
presently, there is no
limit
o
D
©
 DGA results (after heatrun) should be normal.
 It is always useful to perform and review thermal scanning of all
tank walls and the cover in search for excessive overheating
(100C rise). Request image files to be provided with the certified
test report and have software to view them.
 If agreed with manufacturer, the heatrun is a good time to check the
performance of temperature controllers (using a preliminary winding
T gradient) and turns ratio of CTs.
TEMPERATURE RISE:
ACCEPTANCE CRITERIA (cont.)
 To assure test data is credible, verify that:
 T and current requirements for measuring winding cold Rdc were
complied with.
 Test is performed using maximum load loss and in corresponding
DETC/LTC positions.
y
n
a
 Test instrument type and setup used formcold
and hot resistance
p
o
C
was the same, e.g., if two-channel
measurement
is used it must
g
n
i
r
e
be used for both hot and cold
resistance
tests.
e
n
i
ng unless it is shown that RDC can be
 If series auto is present,
E
le
b
o
measured within
shutdown
time constrains, the auto is excluded
D
©
from the resistance measurement*.
 During shutdown, fans are turned off right after transformer is deenergized.
 The first value of winding hot Rdc was recorded not later than 4
min after shutdown.
*Lachman, M. F., et al “Impact of Series Unit on Transformer Winding DC Resistance Measurement
During Heatrun”, Proc. of the Seventy-Sixth Annual Intern. Confer. of Doble Clients, 2009, Sec. T-4.
TEMPERATURE RISE:
ACCEPTANCE CRITERIA (cont.)
 To assure test data is credible, verify that:
 Winding hot Rdc fits reasonably into the cooling curve.
 Final T rises are properly corrected: GRAD for actual test
currents, Tto-a and To_ave_cb-a for actual total losses and altitude.
 Test system accuracy should be within +/-3% for loss, +/-0.5% for
yand +/-1.5C for
n
a
voltage, current and winding resistance,
p
m
o
C
temperature.
g
nrated frequency, the results are
i
r
e
 If the test could not be done
at
e
n
i
g
nto rated frequency (see C57.12.90-2010,
converted from tested
E
lethe fans/pumps should be operated at the power
b
Annex B). However,
o
D
frequency to©
be used when unit is in service.
TEMPERATURE RISE:
ABNORMAL DATA
Example: guaranteed Tw_ave-a – 65C, measured – 67C
 Potential reasons for exceeding the guaranteed values may
include:
 Oversights in design
 Testing/setup mistakes
y
n
 Presence of series auto-transformer
a
p
LV_hot
0.00454
0.00535
0.00452
0.0045
0.00448
0.00446
0.00444
Without series auto-xfmr
Resistance [ohms]
Resistance [ohms]
y = 6.962E-08x2 - 5.974E-06x + 4.534E-03
2
0.0053
0.00525
0.0052
0.00515
0.0051
0.00505
With series auto-xfmr
0.005
0:
0
0: 0
3
1: 0
0
1: 0
3
2: 0
0
2: 0
3
3: 0
0
3: 0
3
4: 0
0
4: 0
3
5: 0
0
5: 0
3
6: 0
0
6: 0
3
7: 0
0
7: 0
3
8: 0
0
8: 0
3
9: 0
0
9: 0
10 30
:0
0
0.00442
Time [min]
0:
0
0: 0
30
1:
0
1: 0
3
2: 0
0
2: 0
30
3:
0
3: 0
3
4: 0
00
4:
30
5:
0
5: 0
3
6: 0
0
6: 0
30
7:
0
7: 0
3
8: 0
0
8: 0
30
9:
0
9: 0
10 30
:0
0
TLV_hot
m
o
C
=[(234.5+30)4.534/4.081]-234.5=59.4ºC T
=[(234.5+30)5.362/4.621]-234.5=72.5ºC
g
n
i
r
e
e
in
g
y = 8.152E-07x - 3.235E-05x + 5.362E-03
n
E
le
b
o
©D
Time [min]
Note: The example shows a quadratic function, the suitability of which was confirmed via direct fiberoptic
measurements and other methods, e.g., Blume. Different functions may be used if they fit the winding behavior.
TEMPERATURE RISE:
RECOURSE IF DATA ABNORMAL
Failure to meet the temperature rise test criteria should
not warrant immediate rejection but shall lead to
consultation between purchaser and manufacturer
regarding an investigation of possible causes and
solutions to address the problem.
ny
le
b
o
©D
in
g
n
E
g
n
i
r
ee
a
p
m
Co
ZERO-PHASE SEQUENCE
y
n
a
p
m
o
C
g
IMPEDANCE
n
i
r
e
e
in
g
n
E
(Class
I - design
e
l
b
©Do Class II - routine)
ZERO-PHASE SEQUENCE IMPEDANCE:
DEFINITION AND OBJECTIVE
Definition: The zero-phase sequence impedance is
impedance to the single-phase current simultaneously
present all three phases. It is measured from a wye or a
zig-zag connected winding between three phase
terminals connected together and the neutralyterminal.
n
a
p
om
C
Objective: The zero-phase sequence
impedance serves
g
n
i
r
e
e
as input in analysis of gunbalanced
three-phase system
n
i
n
E
using symmetricalblcomponents
method.
e
©Do
ZERO-PHASE SEQUENCE IMPEDANCE:
PHYSICS
Ia
Balanced
Ia1
Ib
Ic
Ia
Positive
 In symmetrically loaded 3-phase
system, only one phase needs to
be analyzed since in other
phases values have the same
magnitudes and only have to be
shifted by 120.
3-phase system,
each phase are
g
n
i
r different and each phase needs
e
e
in
g
to be analyzed separately.
n
E

Ic1
Ib1
Ib2
Negative
Ia2
le
b
o
©D
Ic2
Iao
Ibo
Ico
Ib
Unbalanced
Ic
Zero
y
n
a
In unbalanced
p
m
Co
impedances
in
 Method
of
symmetrical
components
converts
any
unbalanced system into 3
balanced
systems,
namely
positive, negative and zerophase sequence systems.
 After these are defined, the
voltages and currents in the
original unbalanced system are
reconstructed.
ZERO-PHASE SEQUENCE IMPEDANCE:
SETUP AND TEST METHODOLOGY
For xfmr, the Z1 = Z2 = Zsc is known from impedance/load losses test. In zerophase sequence system, the phase currents are in-phase with each other and
flow through the xfmr only if there is a path to return to the grounded source or
to circulate while satisfying the Kirchhoff’s current law.. Therefore, this test
applies only to transformers with one or more windings with a physical neutral
brought out for external connection.
any
le
b
o
©D
g
n
i
r
ee
in
g
n
E
p
m
Co
1
2
Z0
N
1
2
ZERO-PHASE SEQUENCE IMPEDANCE:
SETUP AND TEST METHODOLOGY (cont.)
Z1Ns
Z1No
le
b
o
©D
 Z1Ns, Z1No, Z2No are
used to calculate Z1,
Z2 and Z3.
in
g
n
E
g
n
i
r
ee
 If delta winding is not
present,
the currents
y
n
ashown in delta are
p
m
Co
circulating in the tank.
1 Z1
Z2 2
Z3
Z2No
N
ZERO-PHASE SEQUENCE IMPEDANCE:
SETUP AND TEST METHODOLOGY (cont.)
Transformer in
test
CT
1
VT
le
b
o
©D
A
V
W
 If delta winding is present, the applied
ysuch that current in
n
voltage should
be
a
p
m
delta winding
Co  Irated.
g
n
i
r
ee For Y/
V
I
 If no delta winding is present, applied
voltage should be 30% of rated
Vphase_gnd and measured current Irated.
in
g
n
E
or /Y impedance in % is
determined as:
Z0 = 300(Vmeas / V r) (Ir / Imeas)
 For Y/Y and autoxfmr with or without
tertiary , the elements of the
equivalent
circuit
are
further
determined as:
Z1 = Z1No - Z3
Z2 = Z2No - Z3
Z3 = Z2No ( Z1No - Z1Ns)
ZERO-PHASE SEQUENCE IMPEDANCE:
ACCEPTANCE CRITERIA
The standard does not provide an acceptance criteria for the zerophase sequence values. However, the following general guidelines can
be useful (typical for 230 kV,  200 MVA core type units):
 For /Y, Z0  Zsc or slightly less.
Example: 50 MVA, 161/69GndY kV, Zsc = 21.9%, Z0 = 21.8%
y
n
a
p
Example: 48 MVA, 235.75GndY/13.8 kV, Zsc = 9.9%,
Z
m
0 = 8.5%
o
C
g
 For Y/Y/ or autoxfmrs with ridelta,
Z1  (0.7-1.0)Zsc; with
n
e
e<0.
n
typically <1.0% or sometimes
i
g
n
E
Example: Auto, 18 MVA, 230GndY/60GndY/21
kV, Zsc = 4.9%, Z1 = 3.6%,
e
l
b
Z2 = 0.84%,
©DoZ3 = 10%
 For Y/ units, Z0  (0.8-1.0)Zsc.
Z2
50 MVA, 69GndY/34.5GndY/13.2 kV, Zsc = 7.8%, Z1 = 6.7%,
Z2 = -0.16%, Z3 = 4.6%
 For Y/Y and autoxfmrs without delta (rare occasion), magnetic
flux has a strong coupling to the tank, making, in general, the
relationship between voltage and current non-linear and the
above observations not relevant.
Example: Auto, 75 MVA, 115GndY/34.5GndY kV, Zsc = 12.7%, Z1 = -9.9%,
Z2 = 27.4%, Z3 = 205.6%
ZERO-PHASE SEQUENCE IMPEDANCE:
ABNORMAL DATA
If unusual zero-phase sequence impedance data is
obtained the test process should be reviewed (paying
particular attention to voltages and currents used) along
with comparing the measured data with the calculated
design values.
ny
le
b
o
©D
in
g
n
E
g
n
i
r
ee
a
p
m
Co
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.
le
b
o
©D
in
g
n
E
C
g
n
i
r
e
e
o
y
n
a
mp
y
n
a
mp
o
C
g
AUDIBLE SOUND
LEVEL
n
i
r
e
e
n
i
g
n and other)
(Design
E
le
b
o
©D
AUDIBLE SOUND LEVEL:
DEFINITION AND OBJECTIVE
Definition: The audible sound level test is the measurement
of the sound pressure level around a fully assembled
transformer under the rated no-load conditions with cooling
equipment operating as appropriate for the power rating
being tested.
y
n
a
mp
o
Objective: To protect the population
from noise
C
g
nrequired to operate within
i
r
inconveniences transformers
are
e
e
n
i
g
n audible sound level test provides
specified noise limits.
The
E
le data for:
b
o
the sound pressure
level
©D
 Verification of design calculations.
 Demonstration of meeting the guaranteed performance
characteristics.
The test also serves as a quality control tool as the sound,
driven by the vibratory motion of the core, is transmitted to
the tank through direct mechanical coupling as well as is
produced by pumps and fans of the cooling system.
AUDIBLE SOUND LEVEL:
PHYSICS
 Most of xfmr sound is generated
by the core. When the core steel
magnetized/demagnetized twice
each cycle, the steel elongates
and shortens due to a property
called magnetostriction.
Tank
Dielectric
fluid
y
n
a
mp
Core
le
b
o
©D
Direction of
dimensional
change
C
g
n
i
r
e
e
in
g
n
E
F
Magnetostriction
caused by
domain rotation
o
 This produces a vibratory motion
in the core transmitted to the tank
through the core mechanical
support and the pressure waves
in the dielectric fluid. At the tank
this motion radiates as an
airborne sound. The vibration
magnitude depends on the flux
density and magnetic property of
the steel.
 The frequency spectrum of the
sound contains mainly the even
harmonics
of
the
power
frequency, i.e., 120, 240, 360, etc.
The audible sound also includes
a contribution emitted by pumps
and fans, containing a broadband
spectrum of frequencies.
AUDIBLE SOUND LEVEL:
SETUP AND TEST METHODOLOGY
Transformer
in test
X0 H1
X1
H2
X2
3
y
 On certain tap an
positions,
xfmr may
p
produce sound
om levels greater than at the
C
g
principal
tap, e.g., engaging PA and/or
n
i
r
H
X3 3
e
eseries autoxfmr. Test will be performed in
n
i
g
n
E
these positions if specified by customer.
e
l
b
o
©D
VT
Vave
 Xfmr is energized with no load, at rated
(for the tap used) voltage and frequency,
with tap changer on principal tap and
pumps/fans operated as appropriate for
the tested rating.
 The voltage should be set as during noload loss test, based on Vave.
 At least one test should be performed at
the cooling stage for the min rating and
one test at the cooling stage for max
rating.
 Measurements begin when xfrm reaches
steady-state conditions, i.e., to allow
magnetic bias to decay.
AUDIBLE SOUND LEVEL:
SETUP AND TEST METHODOLOGY (cont.)
Microphone
location
3 ft
6 ft
Fan cooled surface
Radiator
Tank
le
b
o
©D
Drain valve
g
n
i
r
ee
in
g
n
E
1 ft
Measurement
surface
 Microphones are located
on the measurement
surface
at
shown
distance from reference
ny
surface.
asound-producing
#1
Reference sound-producing
surface is a vertical surface
following the contour of a taut
string stretched around xfmr
periphery.
p
m
Co  Xfmr is placed so that no
LTC
acoustically
reflecting
surface is within 10 ft of
the microphone.
 If transformer H<7.9ft,
measurements are made
at H/2; if H7.9 ft, at H/3
and 2H/3.
 First measurement is
made at drain valve
proceeding clockwise.
AUDIBLE SOUND LEVEL:
SETUP AND TEST METHODOLOGY (cont.)
The sound power rating of a transformer is determined
using one of the following three measurement methods:
 A-weighted sound pressure level (most frequent)
ny
 One-third
specified)
octave
le
b
o
©D
a
p
m
Co
g
sound rinpressure
e
e
n
i
Eng
level
(when
 Narrowband sound pressure level (when specified)
AUDIBLE SOUND LEVEL:
SETUP AND TEST METHODOLOGY (cont.)
 A-weighted sound pressure level
 Human ear can hear sounds in  20÷20000 Hz
range. However, it detects some frequencies much
easier than others. This uneven frequency
response needs to be considered when the
annoyance of unwanted sounds is to be evaluated.
le
b
o
©D
y
n
a
mp
 To account for human’s greater sensitivity to noise
at some frequencies relative to other, the measured
data is passed through a weighting filter. Aweighting is most commonly used to allow for a
broad peak between 1÷6 kHz but very strongly
discriminating against low frequencies.
C
g
n
i
r
e
e
o
in
g
n
E
 As a result, when the average sound pressure
level is calculated, the influence of frequencies not
impacting the human hearing perception is
minimized.
Hz
63
125
250
500
1000
2000
4000
8000
A-filter
-26
-16
-19
-3
0
1
1
-1
dB (measured)
67
76
73
70
65
66
62
52
dB (A-weighted)
41
60
64
67
65
67
63
51
AUDIBLE SOUND LEVEL:
SETUP AND TEST METHODOLOGY (cont.)
The following two methods are used when a more detailed
investigation into the sources of noise is required:
 In one-third octave sound pressure level measurement,
each octave band in the spectrum (i.e., 63, 125, 250, 500,
ywith each “1/3
n
1000, 2000 and 4000 Hz) is split into three,
a
p
m
o 200, 250 Hz, etc.)
sub-band” (e.g., 63, 80, 100, 125,C160,
g
n
i
r
being evaluated individually.
e
e
n
i
g
n pressure level measurement is
E
 The narrowband lsound
e
b
o
performed ©
atDthe power frequency (e.g., 60 Hz) and at
least at each of the next six even harmonics (120 Hz, 240
Hz, 360 Hz, 480 Hz, 600 Hz, and 720 Hz). Once again, each
frequency is evaluated individually.
AUDIBLE SOUND LEVEL:
SETUP AND TEST METHODOLOGY (cont.)
The sound power rating is determined using the following steps:
 Measure ambient sound pressure levels. This is established as an
average of measurements at a min of four locations immediately
preceding and immediately following the sound measurements
with the unit energized.
 Measure combined transformer and ambient sound
y pressure level.
n
a
p
Measurements are made if ambient levelois
at least 5 dB or more
m
C
g ambient sound pressure
below the combined transformerrinand
e
e
level.
n
i
g
n
E
 Compute ambient-corrected
sound pressure levels. For
e
l
b
Do 7 in C57.12.90-2010.
corrections see
©Table
 Compute average sound pressure levels [in dB(A)]:
𝑵
𝑳𝒊
𝟏
𝑳𝒑 = 𝟏𝟎𝒍𝒐𝒈𝟏𝟎
𝟏𝟎𝟏𝟎
𝑵
𝒊=𝟏
Li is the sound pressure level measured at ith location by one of the 3
measuring methods. Sound power levels are calculated when requested.
AUDIBLE SOUND LEVEL:
ACCEPTANCE CRITERIA
 Computed average sound pressure level should not exceed the
audible sound levels as listed in NEMA TR1-1993, Tables 0-2 and 0-3
or as requested in customer test specification. Rectifier, railway,
furnace, grounding, and mobile transformers are not covered by
these tables.
y
 Assurance that test data is credible:
n
a
p
m
 The sound pressure measuring instrument
should meet the
o
C
ng1 meters.
i
requirements of ANSI S1.4 for e
Type
r
e instrument should be calibrated
n
i
 The sound pressure measuring
g
n
E
le set of measurements. If calibration change
before and afteroeach
b
©D
>1dB, sound
measurements shall be declared invalid, and the
test repeated.
 Verify that microphones were positioned at required
distances/heights, pumps/fans were operated as required for
tested power rating and voltage set based on Vave.
 Verify that the ambient level was at least 5 dB or more below the
combined transformer and ambient sound pressure level.
 If rated frequency is not used, 50/60 Hz conversion is applied.
AUDIBLE SOUND LEVEL:
ABNORMAL DATA
Example: guaranteed sound pressure level per NEMA –
75/77/78 dB(A), measured – 77/78/79 dB(A)
Potential reasons for exceeding the guaranteed values may
include:
y
n
a
p
m ambient
oe.g.,
 Problems with measurement,C
noise,
ng instrument calibration,
i
positions of microphones,esound
r
e
n
i
g
voltage adjustment,
surrounding
reflecting surfaces,
n
E
le
b
etc.
o
©D
 Variability in core steel characteristics
 Different core steel
 Oversights in design
 Assembly related factors or mistakes
AUDIBLE SOUND LEVEL:
RECOURSE IF DATA ABNORMAL
Failure to meet the audible sound test criteria should not
warrant immediate rejection but shall lead to consultation
between purchaser and manufacturer regarding an
investigation of possible causes and solutions to address
the problem.
ny
le
b
o
©D
in
g
n
E
g
n
i
r
ee
a
p
m
Co
y
n
a
mp
o
C
g
CORE DEMAGNETIZATION
n
i
r
e
e
n
i
g
n
(Routine*)
E
le
b
o
©D
*This procedure is not required by standards but is a wildly
recognized as standard practice and performed as routine.
CORE DEMAGNETIZATION:
DEFINITION AND OBJECTIVE
Definition: The core demagnetization is the process of
removing the magnetic bias in the core through a series
of steps, with each subsequent step creating magnetic
field of opposite direction and lower intensity. The first
step must bring the core to the main hysteresis loop with
y
n
a
the last step, upon removal, leaving
no residual
p
m
o
C
magnetism in the core.
g
in
r
e
e
ngin
E
Objective: The core
demagnetization
creates conditions
e
l
b
o low-voltage exciting current and loss
for obtaining©D
the
test as well as sfra benchmark data not affected by
residual magnetism .
CORE DEMAGNETIZATION:
PHYSICS
Br
If in the presence of residual
magnetism Br, the voltage is increased
from zero, the flux varies around
minor hysteresis loops. The negative
H
tip of these loops lies on the main
y voltage, the
loop. The greateran
the
p of the minor loop
m
smaller is the ooffset
C
g
along the
n B axis. The bias is removed
i
r
e
e the main loop, symmetrical
n
i
when
g
n
E
ble around the origin, is reached.
B
Main
loop
B
©Do
Br = 0
H
If after reaching the main hysteresis
loop, the voltage is gradually reduced,
each minor loop will lie inside the
previous larger loop. Reduction of
voltage to zero brings working point
to the center of these loops resulting
in a demagnetized transformer.
CORE DEMAGNETIZATION:
SETUP AND TEST METHODOLOGY
The core demagnetization can be performed by one of the
following:
 Applying rated 3-phase voltage (holding for 5-10 min)
and reducing gradually to zero.
y
n
a
 Applying DC voltage (e.g., 12 V), waiting
until current
p
m
opolarity and holding
C
stabilizes, then switching voltage
g
n
i
r
e
until current reachesginae lower value; this process
n level is zero
E
continues until b
current
le
o
©D
 Without ammeter,
the above approach can be applied
but a lower level of current is reached by applying
alternate polarities of DC voltage for progressively
shorter periods of time.
 If no-load losses or sound level tests are the last power
tests to be performed, they serve the function of the
core demagnetization process.
CORE DEMAGNETIZATION:
RELATIONSHIP WITH LV DIAGNOSTIC DATA
Field
Factory
Data movement
with no excitation
applied between
measurements
Field
Factory
le
b
o
©D
Controlled
experiments showing
data movement*
6 hr
3 hr
When xfmr is de-energized, the
core is constantly looking for a
state of lower energy, i.e., it
relaxes, changing its magnetic
state and moving away from the
condition immediately following
demagnetization*. This is obvious
in the low-frequency range of the
sfra trace but not in the low-voltage
excitation current data. These sfra
changes
are
normal
and
diagnostically insignificant.
in
g
n
E
C
g
n
i
r
e
e
y
n
a
mp
o
72 hr
24 hr
9 hr
Factory
Field
mA
W
mA
W
20.5
128
20.5
126
9.3
61
9.6
58
20.7
131
21.6
131
1 hr
30 min
dm_init
*Lachman, M. F., et al “Frequency Response Analysis of
Transformers and Influence of Magnetic Viscosity”, Proc.
of the Seventy-Seventh Annual Intern. Confer. of Doble
Clients, 2010, Sec. TX-11.
LAST SLIDE
C
g
n
i
r
e
e
o
y
n
a
mp
THE END
le
b
o
©D
in
g
n
E
y
n
UNDERSTANDING
T
HE
a
p
om
C
g
n
i
r
TRANSFORMER
T
EST
D
ATA
e
e
n
gi
b
o
D
©
n
E
le
Barry M. Mirzaei – P.Eng.
Hydro One
September 2012 – Chicago
g
n
le E
b
o
D
©
September 2012
i
r
e
ine
o
C
ng
y
n
a
mp
Understanding The Transformer Test Data
2
No Load Test
y
n
a
mp
Test object is supplied from one side of the transformer
(L.V.), the other side (H.V.) is left open circuit. Test voltage
to be adjusted to the pre‐determined value(s)
o
C
ng
i
r
e
ine
g
n
E90% ‐ 100% and 110% of the rated
Typical test voltage
is
e
l
voltage Dob
©
Characteristics of the No Load Test:
“Low Current – High Voltage”
September 2012
Understanding The Transformer Test Data
3
Induced Voltage Test
Test object is supplied from one side of the
transformer (L.V.), the other side (H.V.) is left open
circuit. Test voltage to be adjusted to the pre‐
determined value(s)
o
C
ng
y
n
a
mp
i
r
e
e
n
i
Twice the rated voltage is applied for 7200 cycles g
n
E
for transformers with uniformly insulated le
b
o
windingsD
©
Characteristics of the Induced Voltage Test: “Low Current – High Voltage and Frequency > 60”
September 2012
Understanding The Transformer Test Data
4
Load Loss Test
Test object is supplied from one side (H.V.), the
other side (L.V.) is short‐circuited. Test voltage is ny
a
p
adjusted to apply the rated current to the test
m
o
C
object
g
n
i
r
e
ine
g
n
ER
e
‐Resistive losses
or
l
b
o
D
‐Eddy©
current losses in the windings
Load Loss:
‐Stray losses in leads, core plates and tank
Characteristics of the Load Loss Test:
“High Current – Low Voltage”
September 2012
Understanding The Transformer Test Data
5
g
n
le E
b
o
D
©
September 2012
i
r
e
ine
o
C
ng
y
n
a
mp
Understanding The Transformer Test Data
6
Hysteresis Loss
o
C
ng
i
r
e
Proportional to theinfrequency
e
g
n the area of
and dependent
on
E
le
b
o
the hysteresis
loop, which, in
©D
turn, is a characteristic of the
material and a function of the
peak flux density
September 2012
y
n
a
mp
Understanding The Transformer Test Data
7
Eddy Current Loss
o
C
ng
i
r
e
e
Dependent on the square n
i
g
n
E
of frequency but is also le
b
o
D
©
directly proportional to y
n
a
mp
the square of the thickness of the material
September 2012
Understanding The Transformer Test Data
8
4.44
(a)
(b)
Voltage
o
C
r ing
ee
n
i
g
n
E
y
n
a
mp
PNL NoLoadLosses
le
b
o
DHysteresisLoss
©= = EddyCurrentLoss
, = Coefficients
=
= Exponentwithinduction
September 2012
Understanding The Transformer Test Data
9
Minimizing hysteresis loss thus depends on
the development of a material having a
minimum area of hysteresis loop.
o
C
ng
y
n
a
mp
i
r
e
e loss is achieved by
n
i
Minimizing eddy current
g
n
E
lecore from a stack of thin
building upothe
b
©D
laminations and increasing resistivity of the
material in order to make it less easy for
eddy currents to flow.
September 2012
Understanding The Transformer Test Data
10
g
n
le E
b
o
D
©
i
r
e
ine
o
C
ng
y
n
a
mp
STATEMENT OF THE ISSUE:
September 2012
Understanding The Transformer Test Data
11
During the No Load test of a rebuilt 3
phase 135 kV transformer in the factory,
loud noises inside the tank were reported.
Not a Hydro One Asset
The noises were described as similar to
“release of large amounts of air bubbles pany
om
inside the oil”, started at around the
25%
C
g
n
i
r
e
e
of the test voltage.
n
gi
n
E
le
b
o
D
©
Deflection in the readings on metering
devices (watt meters, …) were reported
with the noise.
September 2012
Understanding The Transformer Test Data
12
Solutions?
What test data are available?
What those test data really mean? g
n
le E
b
o
D
©
September 2012
i
r
e
ine
o
C
ng
y
n
a
mp
Understanding The Transformer Test Data
13
Criteria & constraints for addressing the issue
Un‐necessary activities to be y
n
a
p
avoided, delivery date was critical
om
C
g
n
i
r
e
e
n
Un‐tanking the transformer is costly and i
g
n
E
should be avoided if there is no clear e
l
b
o
D
understanding about the issue
©
Insulation tests should not be repeated, if there is no need to do so
September 2012
Understanding The Transformer Test Data
14
1
Oil Sample
6
OK
2
Apply reduced “Induced Voltage”
Repeat TTR & DC Resistance
Not Convinced to apply
No Load Test
y
n
a
mp
OK
o
C
Investigation g
n
i
r
e
Procedure
e
n
i
5
g
n
E
3
e
l
Insulation b
o
D
Observe The Test?
©
4
PROBLEM
Apply Load Test
OK
September 2012
Understanding The Transformer Test Data
15
The Induced Voltage Test
y
n
a
stresses all parts of othe
p
m
C
g
n including
i
r
insulation system,
e
e
n
i
g
n
E
turnDoto
bleturn, phase to phase
©
and winding to ground.
September 2012
Understanding The Transformer Test Data
16
4.44
(a)
Concept of customized Induced test: y
n
a
p
om
C
g
n
i
r
e
e
n
i
g
By applying induced voltage up to n
E
le
b
o
©D
rated voltage, basically the no load
test is being repeated with reduced
induction in the core
September 2012
Understanding The Transformer Test Data
17
.
.
x
x x x .
.
x
x x x x x x x g
n
le E
b
o
D
©
September 2012
o
C
ng
i
r
e
ine
y
n
a
mp
Understanding The Transformer Test Data
18
1
Oil Sample
OK
6
2
Apply reduced “Induced Voltage”
Repeat TTR & DC Resistance
OK
Investigation Procedure
5
3
Insulation Test?
Not Convinced to apply
Eddy Current Loss
Dependent on the square of frequency but is also directly proportional to the square of the thickness of the material
y
n
a
Hysteresis Lossp
om
C
g and dependent on
Proportional to thein
frequency
r
e
the area of the
hysteresis loop, which, in turn, is a
e
n
i
characteristic
of the material and a function of the
g
n
E flux density
le peak
Observe The No Load Test
4
Apply Load Test
OK
b
o
D
©
PROBLEM
Load Loss:
‐Resistive losses or R
‐Eddy current losses in the windings
‐Stray losses in leads, core plates and tank
September 2012
Understanding The Transformer Test Data
19
g
n
le E
b
o
D
©
September 2012
i
r
e
ine
o
C
ng
y
n
a
mp
Understanding The Transformer Test Data
20
Core bolts are inserted through the core for the y
n
a
p
m
o
C
g
n
i
purpose of clamping the r
e
e
n
i
g
n
E
le
b
core laminations. o
©D
September 2012
Understanding The Transformer Test Data
21
During “Core Stacking Process” –
Holes built for Core Bolts, used for proper core stacking g
n
le E
b
o
D
©
September 2012
i
r
e
ine
o
C
ng
y
n
a
mp
Understanding The Transformer Test Data
22
Core Plates
Core Bolts
g
n
le E
b
o
D
©
i
r
e
ine
o
C
ng
y
n
a
mp
Photo belongs to another transformer September 2012
Understanding The Transformer Test Data
23
Core Plate
Core Bolt
Weld
g
n
le E
b
o
D
©
September 2012
i
r
e
ine
o
C
ng
y
n
a
mp
Understanding The Transformer Test Data
24
g
n
le E
b
o
D
©
September 2012
i
r
e
ine
o
C
ng
y
n
a
mp
Understanding The Transformer Test Data
25
Fiberglass insulation
b
o
D
©
g
n
le E
Metal Washer
o
C
ng
i
r
e
ine
y
n
a
mp
Round Head Carriage Bolt
September 2012
Understanding The Transformer Test Data
26
g
n
le E
b
o
D
©
September 2012
i
r
e
ine
o
C
ng
y
n
a
mp
Understanding The Transformer Test Data
27
In this case, the low impedance path formed by the
bolts and the core clamping plates causes a local short
circuit path which produces intense local eddy currents.
The amount of heat generated by this phenomenon is
sufficient to considerably damage the adjacent
anyareas.
g
n
i
r
ee
p
m
o
C
n
i
g
The problem was noticeable in No‐Load test since there n
E
le
b
was higher induction to create higher current in the o
D
©
through bolts when compared to reduced induced test.
Increase in the Load Loss increased the probability of “Core Plates” related issues.
September 2012
Understanding The Transformer Test Data
28
g
n
le E
b
o
D
©
i
r
e
ine
o
C
ng
y
n
a
mp
Insulation Material
This picture shows the correct insulation of the core bolts
Photo belongs to another transformer .
September 2012
Understanding The Transformer Test Data
29
y
n
a
mp
Thank You
i
r
e
gine
n
E
le
o
C
ng
b
o
D
©
September 2012
Understanding The Transformer Test Data
30
Understanding any
p
m
o
C
Transformer
g
n
i
r
e
e
n
i
g
n
Factory
Testing
E
le
b
o
D
©
September 30, 2012
Transformer Temperature Tests

On some occasions additional methods must be
employed to determine the suitability of tested
transformer.
o
C
ng
y
n
a
mp
i
 These techniques may include
calculated corrections
r
e
e
n
i
g
or multiple tests atn different loading conditions, etc.
E
e
l
ob
©D
 Lets look at two actual factory cases:
 Case 1: Good Test Results – Bad Data
 Case 2: Bad Test Results – Good Transformer
Understanding Transformer Factory Testing
2
Transformer Temperature Tests
AVERAGE
WINDING
TEMP.
Core
le
b
o
D
©
n
i
g
En
WINDING
HOTTEST
SPOT
TEMPERATURE DISTRIBUTION
i
r
e
e
Coils
y
n
a
mp
Top Oil Temp.
o
C
ng
Gradient . x H.S.F
Top
Oil
Hot Spot
Distance
Cooling
TOP
OIL
TEMP.
Average
Oil
Gradient
Avg. Wdg.
Temp.
Bottom
Oil
Oil
Ambient
Temperature
Understanding Transformer Factory Testing
3
Case 1: Good Results – Bad Data
UAT 39/52/65 MVA; 230 - 6.9 (XV) & 4.16 (YV) kV 60Hz
(+15-5% LTC for YV)
•
•
y
Heat run test was performed accordingpa
tonANSI/IEEE
m
ospecification.
Standards and the clients technical
C
g
n
i
r
e
ine
g
n
Temperature results
E were well below Standard limits
e
l
and according
Dob to client’s specification.
©
•
Very Clean DGA Results.
•
Test Results did not match design data?
Understanding Transformer Factory Testing
4
Case 1: Good Results – Bad Data
~
le
b
o
©D
L2
g
n
E
i
r
e
ine
L1
Measurement
System
Understanding Transformer Factory Testing
XV
YV
HV
SHORT CIRCUIT
L3
o
C
ng
y
n
a
mp
SHORT CIRCUIT
Short-Circuit Method – Three Phase, 3-Winding:
Unit Under Test
5
Case 1: Loading Cycle
g
n
le E
b
o
D
©
i
r
e
ine
o
C
ng
Understanding Transformer Factory Testing
y
n
a
mp
6
Case 1: Heat Runs Results
Total Losses (kW )
Tap Position
Average Oil Rise
Top Oil Rise
Winding Gradient, YV
Winding Gradient, XV
Winding Gradient, HV
HS over TOR, YV
HS over TOR, XV
HS over TOR, HV
Hot Spot Factor, YV
Hot Spot Factor, XV
Hot Spot Factor, HV
Average Winding Rise, YV
Average Winding Rise, XV
Average Winding Rise, HV
Hot Spot Rise, YV
Hot Spot Rise, XV
Hot Spot Rise, HV
Test
172.600
1R
42.9
49.6
10.2
2.3
3.3
11.2
2.5
3.6
1.10
1.10
1.10
53.05
45.13
46.18
60.8
52.1
53.2
©D
oble
Corrections
228.464
1R
51.2
59.2
10.2
2.3
3.2
11.2
2.5
3.5
1.10
1.10
1.10
61.3
53.4
54.4
70.4
61.7
62.7
g
n
E
Limit
65.0
Winding
HV
XV
YV
o
C
ng
i
r
e
ine
Understanding Transformer Factory Testing
Winding Current for Individual Gradient Runs
65.0
80.0
Test (A)
99.5
1757.0
2483.0
y
n
a
mp
Rated (A)
97.9
1757.0
2483.0
Ratio
0.984
1.000
1.000
Winding Current for Oil Rise Run
Winding
HV
XV
YV
Test (A)
97.9
2149.1
1791.5
Rated (A)
97.9
1757.0
2483.0
Ratio
1.000
1.223
0.722
Exponents
0.63
n:
0.80
m:
7
Case 1: Temp Rise not Expected
Heat Run Result vs Design Data
39.0
MVA
ONAN
Cooling Mode
Tested
228.682
Losses (kW)
59.2
Top Oil Rise
51.2
Average Oil Rise
39.8
Bottom Oil Rise
YV Winding
10.2
Gradient
61.4
Average Winding Rise
11.2
Hot Spot Gradient
70.4
Hot Spot Rise
XV Winding
2.3
Gradient
53.5
Average Winding Rise
2.5
Hot Spot Gradient
61.7
Hot Spot Rise
HV Winding
3.2
Gradient
54.4
Average Winding Rise
3.5
Hot Spot Gradient
62.7
Hot Spot Rise
Design
228.464
51.6
39.7
27.9
12.8
52.5
14.1
65.7
Guar.
65.0
65.0
g
n
le E
b
o
D
©
11.1
50.8
12.2
63.8
10.5
50.2
11.6
63.2
80.0
i
r
e
ine
o
C
ng
y
n
a
mp
Why so far off?
65.0
80.0
65.0
80.0
Understanding Transformer Factory Testing
8
Case 1: Heat Runs Temp. Log
Time
Measured kW
Measured Amps
Upper Radiator 1
Upper Radiator 2
CH 14:00
171.0
101.6
2 77.43
7 77.58
Average Upper Rads
Lower Radiator 1
Lower Radiator 2
1
3
Average Lower Rads
Ambient # 1
Ambient # 2
Ambient # 3
Average Ambient
Top Oil Temp
Top Oil Temp
Average of Top Oil
Averge Oil Rise @ 3300'
Top Oil Rise @
3300'
Bottom Oil Rise @ 3300'
15:00
173.2
102.0
78.66
78.78
16:00
172.0
103.0
79.56
79.78
17:00
171.1
101.5
80.10
80.30
18:00
173.0
102.0
81.55
81.88
19:00
172.9
102.0
82.66
82.68
20:00
173.6
100.0
83.83
84.65
21:00
172.0
99.0
84.18
85.03
22:00
172.0
99.1
84.57
85.42
23:00
172.3
99.4
84.73
85.51
0:00
172.5
99.6
84.92
85.58
1:00
172.6
99.5
85.09
86.10
2:00
Take
HV
84.98
85.70
3:00
Take
XV
82.27
82.91
5:00
Take
YV
83.61
84.36
y
n
a
mp
77.51 78.72 79.67 80.20 81.72 82.67 84.24 84.61 85.00 85.12 85.25 85.60 85.34 82.59 83.99
67.51 68.24 69.22 70.25 71.47 72.57 73.53 74.16 73.77 72.81 73.61 73.88 74.69 70.88 71.83
63.81 64.85 65.87 66.88 67.85 68.83 69.83 69.93 70.26 69.60 70.42 70.41 69.66 66.67 68.78
65.66
4 37.86
11 38.79
12 35.78
37.48
6 81.21
DV 78.00
79.61
36.21
42.13
28.18
g
n
le E
66.55
37.96
38.95
35.88
37.60
81.46
80.00
80.73
37.05
43.13
28.95
b
o
D
©
67.55
38.00
39.08
36.88
37.99
82.00
81.00
81.50
37.45
43.51
29.56
68.57
40.00
38.90
36.20
38.37
83.25
82.80
83.03
38.84
44.66
30.20
i
r
e
ine
o
C
ng
69.66
40.75
39.20
37.88
39.28
84.52
83.00
83.76
38.46
44.48
30.38
70.70
41.27
39.56
38.95
39.93
85.88
83.50
84.69
38.78
44.76
30.77
71.68
41.64
39.72
39.81
40.39
87.95
84.00
85.98
39.31
45.59
31.29
72.05
41.94
39.52
39.78
40.41
89.16
85.00
87.08
40.39
46.67
31.63
72.02
41.70
38.11
39.33
39.71
89.98
84.00
86.99
40.79
47.28
32.30
71.21
41.35
37.85
39.06
39.42
90.47
83.95
87.21
40.83
47.79
31.79
72.02
40.92
37.25
38.81
38.99
91.07
85.00
88.04
42.42
49.04
33.02
72.15
40.53
37.40
38.41
38.78
91.36
85.40
88.38
42.88
49.60
33.37
72.18
0.00
0.00
0.00
0.00
91.12
68.78
0.00
0.00
0.00
0.00
88.60
70.31
0.00
0.00
0.00
0.00
89.87
91.12
84.54
91.12
72.18
88.60
81.69
88.60
68.78
89.87
83.03
89.87
70.31
∆T (3 Hr) > 2 ºC
Understanding Transformer Factory Testing
9
Case 1: Heat Runs Results
Total Losses (kW )
Tap Position
Average Oil Rise
Top Oil Rise
Winding Gradient, YV
Winding Gradient, XV
Winding Gradient, HV
HS over TOR, YV
HS over TOR, XV
HS over TOR, HV
Hot Spot Factor, YV
Hot Spot Factor, XV
Hot Spot Factor, HV
Average Winding Rise, YV
Average Winding Rise, XV
Average Winding Rise, HV
Hot Spot Rise, YV
Hot Spot Rise, XV
Hot Spot Rise, HV
Test
172.600
1R
42.9
49.6
10.2
2.3
3.3
11.2
2.5
3.6
1.10
1.10
1.10
53.05
45.13
46.18
60.8
52.1
53.2
©D
oble
Corrections
228.464
1R
51.2
59.2
10.2
2.3
3.2
11.2
2.5
3.5
1.10
1.10
1.10
61.3
53.4
54.4
70.4
61.7
62.7
g
n
E
Limit
65.0
Winding
HV
XV
YV
o
C
ng
i
r
e
ine
Understanding Transformer Factory Testing
Winding Current for Individual Gradient Runs
65.0
80.0
Test (A)
99.5
1757.0
2483.0
y
n
a
mp
Rated (A)
97.9
1757.0
2483.0
Ratio
0.984
1.000
1.000
Winding Current for Oil Rise Run
Winding
HV
XV
YV
Test (A)
97.9
2149.1
1791.5
Rated (A)
97.9
1757.0
2483.0
Ratio
1.000
1.223
0.722
Exponents
0.63
n:
0.80
m:
10
Case 1: Summary
• Stable oil temperatures must be met to achieve
reasonably accurate winding gradient measurements.
y
• Accurate cold resistance temperature measurements
n
a
prises.
m
are critical in determining the winding
o
C
g
rin 1R – Cold resistance not
YV hot resistance onee
Tap
n
i
g
measured, used
En Tap 1N  Not valid.
le
b
o
©D
• Simulated load losses should be close to the expected
load losses for the transformer during operation.
Actual winding currents not measured during
simultaneous loading.
Understanding Transformer Factory Testing
11
Case 2: Good Unit – Bad DGA
GSU 820 MVA 362 / 25 kV DETC(±5%) 60Hz
•
•
•
Heat run test was performed according to IEEE/ANSI
y
n
a
Standards and the clients technical specification.
p
om
C
ng
i
r
e
Temperature results were
e below limits and according to
n
i
g
n
clients requirements.
E
le
b
o
©D
DGA performed after heat run test found gas
generation above client and the manufacturer’s
acceptance limits.
Understanding Transformer Factory Testing
12
Case 2: Bad DGA Results
Outside Lab
In House Lab
Sample #
1
2
3
Before Heat 4 hours after
4 hours after
Description
Run [ppm] Heat Run [ppm] Heat Run [ppm]
H2 - Hydrogen
4
22
17
O2 - Oxygen
3989
2314
300
N2 - Nitrogen
11045
12181
9050
CO - Carbon Monoxide
10
62
50
CO2 - Carbon Dioxide
82
392
250
CH4 - Methane
0
3
14.4
C2H4 - Ethylene
0
5
4
C2H6 - Ethane
0
27
22
C2H2 - Acetylene
0
0
0
g
n
le E
b
o
D
©
i
r
e
ine
o
C
ng
Change
Gas Evolution
[ppm]
13
40
168
14.4
4
22
0
y
n
a
mp
Client Limits
Gas Evolution
[ppm]
10
25
200
5
2
2
0
All measured temperature rises within calculated tolerances.
Understanding Transformer Factory Testing
13
Case 2: Possible Causes
1.
Bad DGA
Sample
o
C
ng
5.
TX Hot
Spot
i
r
e
ine
g
n
Why ?
E
le
b
o
D
©
4.
Stray
Gassing
Understanding Transformer Factory Testing
y
n
a
mp
2.
Pump
Problem
3.
Improper
Testing
14
Case 2: Possible Causes
1.
2.
Bad DGA Data?
• DGA results of the outside lab matched the results
obtained at factory.
y
n
a
mp
Bad Pump (s) ?
o
C
g
n
i
r
The most probable causeeof
pump overheating is the
e
n
i
g
n
pump running backwards.
E
e
l
bratings matched Nameplate
o
• Running
D
©
• No Noise
• Thermal Scan normal for pumps and oil flow
Understanding Transformer Factory Testing
15
Case 2: Possible Causes
3.
Improper Testing Method ?
•
•
•
•
•
•
Loading per IEEE/Expedited Heating
y
Fiber Optic Sensors in Coils – No high
temperatures
n
a
p
m
o
Thermocouples on structural C
metal parts – Normal
g
n
i
r
heating
e
e
n
i
g
n
E
Ambient temperature
was below 40 ºC
e
l
b
o
D
Total©heat load (kW) matched cooler rating
Maximum current was only 112% of rated/ Less
than 7 percent of allowable continuous overload
current.
Understanding Transformer Factory Testing
16
Case 2: Possible Causes
Only two possible causes left:
4.
An oil problem due to “thermal stray gassing”.
Or
5.
o
C
ng
ri spot.
An abnormal transformeree
hot
le
b
o
©D
n
i
g
En
Understanding Transformer Factory Testing
y
n
a
mp
An experiment
is needed!
17
Case 2: Experimental Loading
y
n
a
mp
How is the gassing influenced ?
•
•
r
e
Load dependent
e
in
i
o
C
ng
g
n
E
e
l
Oilotemperature
dependent
b
©D
Understanding Transformer Factory Testing
18
Case 2: Experimental Loading
Step A: Transformer Rated Conditions
y
n
a
mp
o
C
1. Test Floor Open and Ventilated
g
n
i
r
e
e
n
2. All Pumps & Fans
i On
g
n
EConditions of the transformer
e
l
3. Full Rating
ob
©D
Understanding Transformer Factory Testing
19
Case 2: Experimental Loading
Step B: Simulate Stray Gassing
y
n
a
mp
1. Test Floor Closed
o
C
g
n
i
r
e
2. Reduced Current
e
n
i
g
n
E adjusted to keep oil at ~ 90 ºC
3. All Pumps on/leFans
b
o
D
©
Understanding Transformer Factory Testing
20
Case 2: Experimental Method
Test
Number
Test #1
Test #2
Test #3
Test #4
Load
Condition
Oil
Comments
Temperature
y
n
a
p The source of gassing
This is the intial heat run
result.
High Oil
m
o
Overload
C
is indeterminate.
Temperature
g
n
i
r
e under this condition is most likely not from the
Normal Oil inGassing
e
Rated Load
g
n
oil.
Temperature
E
e
l
b High Oil Gassing under this condition is most likely not from the
o
D
© Load Temperature
Reduced
transformer.
Normal Oil
Gassing is most likely from the transformer.
Overload
Temperature
Understanding Transformer Factory Testing
21
Case 2: Experimental Method
Test
Duration
Number [Hours]
8.0
Test #1
1.0
Test #2
8.0
Test #3
8.0
Test #4
Criteria
Loading
Total Heat
Load
Rated
Per IEEE
Current
Rated
Current
Reduced
Curr.
Per IEEE
le
b
o
©D
n
i
g
En
Load
[%]
Top Oil
Temp.
Coil Oil
Temp.
Ambient
[ºC]
108.0
73.0
90.3
37.6
100.0
70.0
y
n
92.3
a
mp
38.4
78.8
24.0
o
C
g 53.5
100.0
n
i
r
ee
80.0
83.5
92.6
35.2
7.5
Per IEEE
Total Heat
Load
108.0
62.5
87.8
26.0
1.0
Per IEEE
Rated
Current
100.0
59.7
81.0
26.0
Understanding Transformer Factory Testing
22
Case 2: Experiment Results
Test #
H2 - Hydrogen
O2 - Oxygen
N2 - Nitrogen
CO - Carbon
CO2 - Carbon Dioxide
CH4 - Methane
C2H4 - Ethylene
C2H6 - Ethane
C2H2 - Acetylene
CO2/CO Ratio
1
Gas
Evolution
[ppm]
13
40
168
14.4
4
22
0
4.2
le
b
o
©D
2
Gas
Evolution
[ppm]
0
10
66
1.8
0.6
0
0
6.6
n
i
g
En
Understanding Transformer Factory Testing
3
Gas
Evolution
[ppm]
12
36
272
8
1.7
13.6
0
7.6
g
n
i
r
ee
p
m
o
C
4
Gas
Evolution
[ppm]
3
12
105
4.2
1.3
0
0
8.8
any
Criteria
10
25
200
5
2
2
0
<3
23
Case 2: Test #1 Results
• Most of the gas concentrations exceed the customer
limits.
y
n
a
pand Hydrogen
• Dominant gasses are Methane, Ethane
m
o
C
g
(low temperature gasses or rthermal
stray gassing).
n
i
e
e
n
• No cellulose decomposition.
ngi
E
e
l
ob
©D
The Source of the Excessive Gassing
is Indeterminate.
Understanding Transformer Factory Testing
24
Case 2: Test #2 Results
• Gassing, all gasses within acceptance limits.
y
n
a
• No dominant gasses.
omp
•
C
g
n
i
r
e
e
No cellulose decomposition.
n
i
g
n
E
le
b
o
©D
If there was excessive gassing it would
likely be from the transformer active
parts.
Understanding Transformer Factory Testing
25
Case 2: Test #3 Results
•
•
•
•
A gasses exceeding limits except Ethylene.
Dominant gasses are Methane, Ethane and
Hydrogen.
y
n
a
p
No cellulose decomposition.
om
C
gno gassing can be
n
i
In this test the load is reduced
r
e
e
n
i
correlated with the transformer.
g
n
E
e
l
b are Methane, Ethane and Hydrogen,
• Dominant D
gasses
o
this is an©indication of possible thermal stray gassing.
• The gassing results are similar Test #1.
This excessive gassing is likely from
the oil.
Understanding Transformer Factory Testing
26
Case 2: Test #4 Results
• All gasses within acceptance limits.
• Dominant gasses are Methane, Ethane and Ethylene.
Typical gasses for a heat run test without additional
stray
y
n
a
p
gassing.
om
C
g
n
i
• No cellulose decomposition.
r
e
e
n
i
g
n
• The absence of gasses
E confirm that gas generation is
e
l
b load or a transformer condition.
not related D
toothe
©
If there was excessive gassing it would
likely be from the transformer active
parts.
Understanding Transformer Factory Testing
27
Case 2: Summary
•
•
•
The transformer successfully passed the heat run test
according to ANSI/IEEE Standards.
y
n
a
mp
Test #3 results closely match with
oTest #1 and are
C
g
indicative that the source ofrigasses
during heat run
n
e
test is thermal stray gassing
gine of the oil.
n
E
le
b
o
D
Gasses©
generated during heat run test performed are
produced by thermal stray gassing of the oil used for
FAT.
•
The Doble Oil Lab confirmed the stray gassing
tendency of the oil used for the factory heat run.
Understanding Transformer Factory Testing
28
o
C
ng
i
r
e
e
CONCLUSION
n
i
g
n
E
oble
y
n
a
mp
©D
Understanding Transformer Factory Testing
29