Understanding Power Transformer Factory Test Data

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Understanding

Power Transformer

Factory Test Data

Mark F. Lachman

Doble Engineer ing Company



OVERVIEW OF PRODUCTION TESTS

Core/coil: ratio,

Iex, core-to-gnd

Core/coil after VP:

Iex, core-to-gnd

SU: ratio, Rdc, Iex,

no-load/load loss,

sound, core-to-gnd

Tanking: ratio, core-to-gnd,

in-tank CTs - polarity, ratio,

saturation

CTs on cover: polarity,

ratio, saturation

PA: loss, sound,

core-to-gnd



Class I includes power transformers with

high-voltage windings of 69 kV and below.

Class II includes power transformers withhigh-voltage windings from 115 kV through

765 kV.

SYSTEM VOLTAGE CLASSIFICATION



Routine tests shall be made on every

transformer to verify that the product

meets the design specifications.

Design tests shall be made on atransformer of new design to determine

its adequacy.

Other tests may be specified by thepurchaser in addition to routine tests.

GENERAL CLASSIFICATION OF TESTS



TEST TYPE PERFORMANCE DIELECTRIC MECHANICAL

Routine

Winding resistance Winding insulation resistance(Other )

Leak

Ratio/polarity/phase

relation

Core insulation resistance

(Other )

No-load losses and

excitation current

Insulation PF/C

(Other)

Load losses and

Impedance voltage

Dielectric withstand of control

and CT sec. circuits (Other )

Operation of all

devices

Lightning impulse

(Design and Other )

Control and cooling

losses (Other )

Switching impulse

345 kV (Other )

Zero-phase sequence

impedance (Design)

Low frequency test

(Applied and Induced/Partial

Discharge)

DGA (Other )

Class I in red if

different from Class II

Class II < 345 kV

is also Other

OVERVIEW OF TESTS

PD is Other for

Class I only



TEST TYPE PERFORMANCE DIELECTRIC MECHANICAL

Design/

Other

Temperature rise

Audible sound level

Other

Short-circuit

capability

Single-phase

excitation current

Front-of-wave

impulse

Design

Lifting and

moving

Pressure

OVERVIEW OF TESTS (cont.)



TEST REFERENCE

DGA

Ratio/polarity/phase relation IEEE C57.12.90-2010 clauses 6, 7

IEEE C57.12.00-2010 clauses 8.2, 8.3.1, 9.1

Winding resistance IEEE C57.12.90-2010 clause 5

IEEE C57.12.00-2010 clause 8.2

No-load losses and excitation

current

IEEE C57.12.90-2010 clause 8

IEEE C57.12.00-2010 clauses 5.9, 8.2, 9.3, 9.4

Switching impulse IEEE C57.12.90-2010 clauses 10.1, 10.2

IEEE C57.12.00-2010 clauses 5.10, 8.2

IEEE C57.12.98-1993; IEEE Std. 4-1995

Lightning impulse IEEE C57.12.90-2010 clauses 10.1, 10.3

IEEE C57.12.00-2010 clauses 5.10, 8.2

IEEE C57.12.98-1993; IEEE Std. 4-1995

Applied voltage IEEE C57.12.90-2010 clause 10.5, 10.6

IEEE C57.12.00-2010 clauses 5.10, 8.2

Induced voltage/PDIEEE C57.12.90-2010 clause 10.7, 10.8, 10.9

IEEE C57.12.00-2010 clauses 5.10, 8.2

IEEE C57.113-2010; IEEE C84.1

No-load losses and excitation

current

IEEE C57.12.90-2010 clause 8

IEEE C57.12.00-2010 clauses 5.9, 8.2, 9.3, 9.4

SEQUENCE OF TESTS



TEST REFERENCE

DGA

Load losses and

impedance voltage

IEEE C57.12.90-2010 clauses 9.1-9.4, Annex B2

IEEE C57.12.00-2010 clause 5.8, 5.9, 8.2, 8.3.2,

9.2-9.4

ONAN temperature rise IEEE C57.12.90-2010 clause 11

IEEE C57.12.00-2010 clause 8.2

IEEE C57.91-1995 Table 8 (with 2002 corrections)

DGA IEEE PC57.130/D17

ONAF temperature rise IEEE C57.12.90-2010 clause 11

IEEE C57.12.00-2010 clause 8.2

IEEE C57.91-1995 Table 8 (with 2002 corrections)

DGA IEEE PC57.130/D17

Zero-phase sequence

impedance

IEEE C57.12.90-2010 clause 9.5

IEEE C57.12.00-2010 clause 8.2

Audible sound level

IEEE C57.12.90-2010 clause 13, Annex B5

IEEE C57.12.00-2010 clause 8.2

NEMA TR1-1993

Core demagnetization

DGA

SEQUENCE OF TESTS (cont.)



TEST* REFERENCE

Insulation PF/C andresistance

IEEE C57.12.90-2010 clauses 10.10, 10.11IEEE C57.12.00-2010 clause 8.2

Single-phase exciting

current

IEEE C57.12.00-2010 clause 8.2Lachman, M. F. “Application of Equivalent-Circuit Parameters to

Off-Line Diagnostics of Power Transformers,” Proc. of the Sixty-

Sixth Annual Intern. Confer. of Doble Clients, 1999, Sec. 8-10.

Sweep frequency response

analysis IEEE PC57.149™/D8, November 2009

Dielectric withstand of control

and CT secondary circuitsIEEE C57.12.00-2010 clause 8.2

CT polarity/ratio/saturation IEEE C57.13.1-2006

Control and cooling losses IEEE C57.12.00-2010 clauses 5.9, 8.2

Operation of all devices IEEE C57.12.00-2010 clause 8.2

Core-to-ground insulation

resistance IEEE C57.12.90-2010 clause 10.11

IEEE C57.12.00-2010 clause 8.2

SEQUENCE OF TESTS (cont.)

*Discussion of tests listed on this slide and DGA is not included in this presentation.



Tests to be discussed:

Ratio/polarity/phase relation

Winding DC resistance

No load losses and excitation current

Dielectric tests

Load losses and impedance voltage

Temperature rise

Zero-phase sequence impedance

Audible sound level

DISCUSSION OUTLINE



For each test discussion includes:

Definition and objective

Physics

Setup and test methodology

Acceptance criteria*

Abnormal data

Recourse if data abnormal

Comparison with field data (if relevant)

DISCUSSION OUTLINE (cont.)

*This discussion is based on requirements of referenced standards. If customer test specification

contains requirements different from those in standards, more stringent requirements prevail.



RATIO, POLARITY, PHASE

RELATION(Routine)

RATIO POLARITY PHASE RELATION:



Definition: The turns ratio of a transformer is the ratio of

the number of turns in the high-voltage winding to that inthe low voltage winding.

Objective: The turns ratio polarity and phase relation test

verifies the proper number of turns and internal

transformer connections (e.g., between coils, to LTC, tovarious switches, to PA, series auto- or series

transformer) and serves as benchmark for later

assessment of possible damage in service.

The transformer nameplate voltages should reflect theactual system requirements. Therefore, it is important

that the nameplate drawing is approved by the customer

at the design stage.

RATIO, POLARITY, PHASE RELATION:

DEFINITION AND OBJECTIVE

RATIO POLARITY PHASE RELATION:



3T 2T 2V

VR = 3V/2V = 1.5

TR = 3T/2T = 1.5In ideal transformer:

TR = VR

3T 2T 1.96V

Volts per turn = 3V/3T = 1V/T

F

3V

Volts per turn = 2.95V/3T = 0.98V/T

2.95V

F

VR = 3V/1.96V = 1.53

TR = 3T/2T = 1.5

= 100(1.5 – 1.53)/1.5 = –2%

0.05V

3V


PHYSICS

3V

In actual transformer

Turns ratio Voltage ratio

due to accuracy of themeasurement and the

voltage drop in the high-

voltage winding.




Ratio = N1 /N2 = R1 /R2

Polarity is determined via

phase angle between two

measured waveforms.

Phase relation is confirmed

by testing the correspondingpairs of windings.

Tests shall be made

1. at all positions of DETC

with LTC on the rated

voltage position

2. at all positions of LTC with

DETC on the rated voltage

position

3. on every pair of windings

R2

Balanceindicator

N2

N1

R1

H1 X0

H2

X2

Transformer in test


SETUP AND TEST METHODOLOGY




, ,

ACCEPTANCE CRITERIA

X1

H1

H2

H3

X2

X3

X0

Voltage ratio =

VH2-H1 /VX2-X0 =

138/(13.2/3) = 18.108

13.2

138

With the transformer at no load and with rated voltage on

the winding with the least number of turns, the voltages ofall other windings and all tap connections shall be within

0.5% of the nameplate voltages.

For three-phase Y-connected windings, this tolerance

applies to the phase-to-neutral voltage. When the phase-to-neutral voltage is not explicitly marked on the nameplate,

the rated phase-to-neutral voltage shall be calculated by

dividing the phase-to-phase voltage markings by 3.




, ,

ABNORMAL DATA

To appreciate significance of 0.5% limit, it is instructive to

recognize the inherent errors this limit accommodates.

Actual turns RATIOTURN

Nameplate voltages

RATIONP

Deviation

100(RATIONP - RATIOTURN)/RATIONP =

Rounding off

NP voltages

creates error

Measurement RATIOMEAS

Deviation

100(RATIONP - RATIOMEAS)/RATIONP 0.5%

Measurement

introduces

error

NP voltages need to be

selected to keep well

within 0.5% (e.g., 0.2-

0.4). This assures that

measurement errorkeeps RATIOmeas within

0.5% of RATIONP.

RATIOTURN

RATIONP

RATIOMEAS




RECOURSE IF DATA ABNORMAL

If deviation exceeds 0.5% for any of the measurements the

result is not acceptable. The following steps should be considered:

Check if V/T exceeds 0.5% of nameplate voltage. If yes,

under these conditions the standard allows for deviation

from the NP voltage ratio to exceed the 0.5% limit.

Check if transformer is a duplicate of a legacy unit.

Review design data to determine if the NP voltagesselected by designer create a ratio that is too far ( is

too high) from true turns ratio. Discuss possibility of

changing nameplate voltages for relevant tap positions.

Review results of production ratio tests and, if applicable,

consider retesting with analog instrument.

Exciting current reported by turns ratio instrument is a

useful diagnostic indicator.




COMPARISON WITH FIELD DATA

In verifying compliance with 0.5% deviation from the NP

voltages, the following should be recognized: Older analog instruments produce results much closer to

the actual turns ratio than modern digital instruments.

Even within 8-200 V range, the results vary somewhat

with voltage and between different instruments.

Initial field test should be performed at the same test

voltage as the factory test with results compared with the

NP voltages and for all subsequent tests the comparison

should be made with the initial test.

The objective of the high-voltage (e.g., 10 kV) test withexternal capacitor is to stress turn-to-turn insulation of both

windings for diagnostic purposes and not necessarily to

verify the 0.5% limit. In some cases, the latter could be

exceeded due to the loading effect of the test capacitor.



WINDING DC RESISTANCE(Routine)

WINDING DC RESISTANCE:




Definition: Winding DC resistance is always defined as the

DC resistance of a winding in Ohms.

Objective: The measurement of winding resistance

provides the data for:

Calculation of the I2R component of conductor losses.

Calculation of winding temperatures at the end of a

temperature rise test.

Quality control of design and manufacturing processes.

Benchmark used in field for detection of open circuits,

broken strands, deteriorated brazed and crimpedconnections, problems with terminations and tap

changer contacts.


PHYSICS



PHYSICS

i R

Domain

External

field


PHYSICS ( )



PHYSICS (cont.)

d /dt

F = y N

v meas= iR + d /dt

R=v meas

/ i

d /dt

d /dt

d /dt

d /dt

d /dt

d /dt


PHYSICS ( t )



PHYSICS (cont.)

Time to stabilize resistance reading: On some units with closed

loops (e.g., GSU with two LV deltas or units with parallelwindings), it may take a long time for the reading to stabilize*; it

reduces with intermediate stability levels. This phenomenon is

not related to core saturation, which is saturating in a

reasonable time. However, as the core is being magnetized the

changing flux in the core induces voltage and sets upcirculating currents in closed loops. After the core is saturated,

there is no more induced voltage to sustain them, and the

currents begin to subside. This process, however, is associated

with LC oscillations with long time constant and may take up to

45 min to dissipate the energy. The flow of these currentscontinues creating a changing flux in the core, inducing voltage

in the tested winding and thus changing the measured

resistance reading. Opening these loops, when possible,

reduces the time to stability.

* Personal communications with Bertrand Poulin, ABB, Quebec, Canada.






Data must be taken onlywhen reading is stable.

The time to stabilize the

reading depends on the

unit, varying from

seconds to minutes. Standard requires

measurements of all

windings on the rated

voltage tap and at the tap

extremes of the first unit

of a new design.

The measured data is

reported at Tave_rated_rise +

20C, e.g., 65+20= 85C

and as total of 3 phases.

Transformer in test

H2

H1

H3

H0

IdcVdc

+

+

Currentoutput

Voltage

input


ACCEPTANCE CRITERIA



ACCEPTANCE CRITERIA

Standards give no acceptance criteria; however, a deviation

from average of three phases of 0.5% for HV and 5% for LVcould serve as practical guideline.

As important as deviation is the assurance that test data is

credible:

No excitation with no pumps - 3h and with pumps - 1h,

TTO variation 2C for 1h, and TTO-TBO 5C. This assuresthat oil T represents conductor T; without reference T resistance data has a limited value.

Test current 10% of maximum rated load current.

Voltage test leads must be placed as close as possible to

winding terminals.

Test data should be recorded only when reading is stable.

Measuring system accuracy +/-0.5% of reading with

sufficient current output to stabilize the flux.


ACCEPTANCE CRITERIA (cont )



ACCEPTANCE CRITERIA (cont.)

T stability: Experience* in the industry suggests that

relying on the T stability requirements given in the IEEEstandard does not produce a needed thermal equilibrium

and, consequently, an accurate measurement of the

winding dc resistance. To have a reliable data, the unit

should be subjected to no excitation for 2-3 days. Hence, if

the time to begin testing is of essence, it is notunreasonable to agree to using resistance data available at

that time (assuming the IEEE T requirements have been

met), but request that resistance is re-measured later

(including cold resistance for heatrun), when the T is

stable. Obviously, the load loss and the heatrun results

should be then recalculated with the latest T.



ABNORMAL DATA



ABNORMAL DATA

DETC H1-H3 H2-H1 H3-H2

1 3.7350 3.7352 3.7378

2 3.6470 3.6468 3.6502

3 3.5590 3.5580 3.5622

4 3.4714 3.4698 3.5746

5 3.3838 3.3814 3.3870

High-voltage winding

Tested Calc. % of calc.

20.9832 21.47937 97.7

20.4889 20.97440 97.7

19.9932 20.46944 97.7

19.6873 19.96448 98.6

19.0065 19.45952 97.7

Average

3.7360 0.03% 0.02% -0.05%

3.6480 0.03% 0.03% -0.06%

3.5597 0.02% 0.05% -0.07%

3.5053 0.97% 1.01% -1.98%

3.3841 0.01% 0.08% -0.09%

Deviation from average

0.6185 0.03842 99.7

0.5855 21.47937 100.6

0.16519 -0.11% -0.01% 0.12%

0.15637 -0.12% 0.00% 0.12%

LTC X1-X0 X2-X0 X3-X0

16 0.16537 0.16521 0.16499

N 0.1566 0.1564 0.1562

Low-voltage winding

Comparison of each measurement with theaverage along with design data identifies

an abnormal reading in H3-H2 with DETC in

4. This potentially can be caused by a

problem with DETC contacts.






If requirements associated with transformer thermal stability,

dc test current, influence of series unit or stability of thereading are not met, a retest under different conditions

should be requested.

If acceptance criteria is exceeded, a justification from the

manufacturer should be requested. Potential problems may

include: bad crimping or brazing, incorrect conductor crosssection, loose connection, wrong design calculations.






Typically, a deviation of <5% from the factory value is

considered acceptable. A factory value is often reported as a sum of three phase

readings at rated T. For field comparison, the per-phase

values at corresponding DETC/LTC positions should be

requested from the factory.

Comparison should be performed for readings referred to thesame T.

The field measurement should be performed at the same test

current as the factory one.

Field tests are the subject to the same thermal stability

requirements as the factory test (note that at the factory T ismeasured via thermocouples and in the field the T gauge is

frequently the best option).



NO-LOAD LOSSES AND

EXCITATION CURRENT(Routine)

NO-LOAD LOSSES AND EXCITATION CURRENT:DEFINITION AND OBJECTIVE



Definition: No-load losses include core loss, dielectric

loss, and conductor loss due exciting current, includingcurrent circulating in parallel windings. Excitation current

is flowing in any winding exciting the transformer with all

other windings open-circuited.

Objective: No-load losses and excitation current, measured at specified voltage and frequency, provide the

data for:

Verification of design calculations.

Demonstration of meeting the guaranteed performance

characteristics. Since these parameters have often an

economic value attached to them, the accuracy of the

measurement becomes significant.

No-load losses are used as test parameter during the

temperature rise test.

NO-LOAD LOSSES AND EXCITATION CURRENT:PHYSICS



I R

F

V

B

H

Hysteresis losses

Ph = f(Bmax)Bmax = f(Vave)

F

Ieddy

Eddy

losses

Pe = f(V

2

rms)

PNL = Pe + Ph

Domain

rotation

NO-LOAD LOSSES AND EXCITATION CURRENT:SETUP AND TEST METHODOLOGY



Start with 110% on N. As unitdemagnetizes, losses drop.

Test at 100% Vrated on N, max turn

bridging position with inductive

LTC, and 16R if LTC with series

unit. Vave gives same Bmax as Vrms when

wave-shape is a perfect sin; set

based on Vave average of 3 phases

Pe is corrected for rated Vrms

Voltmeters should measure samevoltage as seen by xfmr.

PNL not corrected for T if TTO-TBO

5C and 10TO_ave30C

Iexc=aver. of 3 phases in % of Irated

CT

VT

Vrms

W

I

V

Transformer in test

X0

H2 X1

X2

X3

H1

H3

Vave

A

*

*

*Vrms and Vave (calibrated in rms) will show

the same voltage if perfect sine wave.

3

NO-LOAD LOSSES AND EXCITATION CURRENT:SETUP AND TEST METHODOLOGY (cont.)



Frequency control of motor-generator sets at GE large

transformer plant in

Pittsfield, MA during the

early 1900. Since the

primary function of thesegenerators was to provide

power for no-load loss

tests, they were often

referred to as magnetizers.

Historical perspective

Courtesy IEEE Power & Energy Magazine

NO-LOAD LOSSES AND EXCITATION CURRENT:ACCEPTANCE CRITERIA



Measured no-load losses should not exceed the

guaranteed value by more than 10% and the total losses bymore than 6%.

Assurance that test data is credible:

Test voltage is set based Vave

If oil T is not within limits, correction is applied

Frequency is within +/-0.5% of rated Distortion 5%. The 5% limit that standard allows for

distortion of the voltage waveform is too liberal.* The

limit applies to the difference between the measured kW

and kW corrected for eddy loss due to the difference

between Vrms and Vave. To monitor the quality of thevoltage waveform, one should look at the following

criteria of the applied voltage waveform: THD < 5%, 3rd

and 5th harmonics <10% and waveform should not have

any visible distortions.


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 betweennon-bridging and bridging positions (max turns) with

loss measured in PA out-of-tank. If SU unit is present,

compare the loss difference between N and 16R with

loss measured in SU out-of-tank.

Test system accuracy should be within +/-3% for loss,+/-0.5% for voltage and current, and +/-1.5C for T.

NO-LOAD LOSSES AND EXCITATION CURRENT:ABNORMAL DATA



Potential reasons for exceeding the guaranteed values may

include:

Variability in core steel characteristics

Different core steel

Oversights in design

Production process related factors or mistakes

Problems with windings (e.g., s. c. turn)

Wrong connection of preventative autotransformer orseries transformer or series autotransformer

Example: guaranteed no-loss - 28 kW, measured – 35 kW

NO-LOAD LOSSES AND EXCITATION CURRENT:RECOURSE IF DATA ABNORMAL



Failure to meet the no-load test loss tolerance should

not warrant immediate rejection but shall lead toconsultation between purchaser and manufacturer

regarding further investigation of possible causes and

the consequences of the higher losses.

The acceptance criteria of 10% does not replace the

manufacturer’s guarantee of losses for economic loss

evaluation purposes.

NO-LOAD LOSSES AND EXCITATION CURRENT:COMPARISON WITH FIELD DATA



Factory no-load losses and excitation test is performed

at rated voltage and three-phase excitation. Since theopen-circuit magnetizing impedance of a transformer is

non-linear, i.e., it is changing with applied voltage, a

comparison of exciting current and losses test results

obtained at low-voltage (e.g., 10 kV) and single-phase

excitation with results of the factory no-load losses andexcitation test is not possible.



DIELECTRIC TESTS

DIELECTRIC TESTS:DEFINITION AND OBJECTIVE



Definition: Tests aimed to show that transformer is

designed and constructed to withstand the specified

insulation levels are referred to as dielectric tests. They

include:

high-frequency tests: lightning and switching impulses

low-frequency tests: applied and induced/PD tests

Objective: Dielectric tests demonstrate:

compliance with users specification

compliance with applicable standards

verification of design calculations assessment of quality and reliability of material and

workmanship

Note : Unless agreed otherwise, all dielectric tests must be performed with

bushings supplied with the transformer.



HIGH-FREQUENCY:

LIGHTNING IMPULSE(Class I - design or other,

Class II - routine)

HIGH-FREQUENCY - LIGHTNING IMPULSE:OBJECTIVE



Demonstrate performance under transient high-frequency

conditions caused by lightning.

Surge of energy, from lightning striking

transmission line, travels to substation

and enters a transformer - full wave.

kV

s

Surge of energy, from lightning striking transmissionline, travels to substation and, after reaching the

crest of the surge, causes arrester operation or

flashover across an insulator near transformer

terminals - chopped wave (a.k.a. tail-chopped).

Surge of energy, from lightning striking transmission line, travels to

substation and operates gapped silicon-carbide arrester at

transformer terminals - front-of-wave (a.k.a. front-chopped).

HIGH-FREQUENCY - LIGHTNING IMPULSE:PHYSICS



length

V

Due to impulse front high frequency, the

initial voltage distribution is determined by

the capacitive network, with higher voltage

gradients towards the impulsed end of the

winding. The higher is , the steeper are the

gradients at the impulsed end of the winding.

As the front passes, the distribution changes

as determined by the tail of the wave.

Cg /Cs

Cg Cs

V

Full wave can be simulatedby discharging capacitor

while chopped wave by the

operation of a gap triggered

to flashover at required time.

HIGH-FREQUENCY - LIGHTNING IMPULSE:PHYSICS (cont.)



* Assumption that FOW stresses mostly the first few turns at the impulse end is not always true; it depends on

winding type and configuration, e.g., when the interleaved winding (one with high series capacitance) is in

series with RV, the impulse goes through the main winding and hits RV (Personal communications with

Bertrand Poulin, ABB, Quebec, Canada.)

**From W. McNutt 1989 Doble tutorial: Turns in this context mean the voltage that would have been present

if the applied voltage was distributed according to turns ratio.

Region A* - turn-to-turn

insulation at line is tested by

FOW impulse, with stress

>10turns**.

Region B – disk-to-disk, and

layer-to-layer insulation (and

turn-to-turn) is tested by FW& CW impulse, with stress 5-

10turns.

Region C – insulation across

taps is tested by FW & CW

impulse, with stress 5-

10turns.

LV H1

A

B

C

A

B

B

B

B

C

H1HV

to DETC

H0

HIGH-FREQUENCY - LIGHTNING IMPULSE:PHYSICS (cont.)



Cg CT

Rs

Rp

Charge of Cg – generator

capacitors are charged from

external DC source.

*CT includes preload capacitor.

Discharge into Rp – energy

from xfmr is discharged into

generator, reducing voltage

at tested terminal.

VT FW

Cg CT

Rs

Rp

Discharge at chop –

energy from xfmr is

discharged into chopping

gap, reducing voltage at

tested terminal to zero.

VTCW

Cg CT

Rs

Rp

FOW

Discharge into C*T –

energy from generator

capacitors is discharged into

xfmr, raising V at tested

terminal to crest level.

VT

Cg CT

Rs

Rp

HIGH-FREQUENCY - LIGHTNING IMPULSE:SETUP AND TEST METHODOLOGY



Full Wave Parameters

MagnitudeFW = BIL +/- 3%

RFW = 50-70% BIL

T1 = 1.67T

1.2 s +/- 30%

0.84 ÷ 1.56 s

T2

50 s +/- 20%

40 ÷ 60 s

5%

Applied test waves are of negative polarity to reduce risk of erratic

external flashover.

See C57.12.90-2010 when for line terminals T1 is allowed to be >1.56

s and T2<40 s. For neutral bushing T1<10 s and T2 could be <40 s.

If the T2<40 s, it should be addressed at the bidding stage.

0.9

0.3

1.0

T1

0.5

T2

t

V

Crest voltage

Half voltage

Virtual

origin

T

HIGH-FREQUENCY - LIGHTNING IMPULSE:SETUP AND TEST METHODOLOGY (cont.)



Increase of series (front) resistor Rs

increases the time of voltage rise - T1.

0% change from given Rs

CgCT

Rs

Rp

Data courtesy Reto Fausch, Haefely


Increase of parallel (tail) resistor R increases



Increase of parallel (tail) resistor Rp increases

the time of voltage decline to half value - T2.


0% change from given Rp

CgCT

RsRp


Increase of series (front) resistor R decreases




CgCT

Rs

Rp

Increase of series (front) resistor Rs decreases

the voltage trace overshoot - .


Ch d W P t



Chopped Wave Parameters

Magnitude CW = 1.1BIL+/- 3%

T1

1.2 +/- 30%

0.84 ÷ 1.56

TC

BIL [kV] Class I Class II

30 1.0

2.045÷75 1.595 1.8

110 2.0

125 2.3 2.3

150 3.0

TC < 6.0

30%

1

See C57.12.90-2010 for instanceswhen could be >30% and >1s. It

also permits adding resistors in

chopping gap circuit to limit .

All times in the table are in s.

0.9

0.3

1.0

T1 TC

t

V

1.0

0.7

0.1


1 0



Front-of-Wave Parameters

Magnitude C57.12.00-2010

Annex ATC

30%

C57.12.90-2010 permits adding resistors in chopping gap circuit to limit .

With improved arrester technology, front-of-wave tests may not be necessary

and were removed as a requirement from C57.12.00. Annex A in that standard

includes the last published table of front-of-wave test levels from C57.12.00-

1980, for historical reference.

0.9

0.3

1.0

TC

t

V


Very high di/dt induces difference



Currentshunt and

meas. circuit

i(t)

RG

LG

Glaninger: T2

T2

Impulsecontrol &

measuring

system

Voltage divider

and measuring

circuit

v(t)

Cg

Rs

Rp

Impulse

generator LT, CT

xfmr

y g

of potential. Hence, it is very

important for all return and

grounding leads to be made asshort as possible, with a minimum

R and L.

Chopping gap

and preload

capacitor Chopping gap should not be connected in series

with voltage divider no matter how convenient it is

for the test department to have a permanent setup.


Line terminal in Y Line terminal in Neutral terminal in Y



Line terminal in Y

i(t )

Line terminal in

i(t )

Neutral terminal in Y

i (t )

i (t )

LV line terminal

in AutoHV line terminal

in Auto

i (t ) i (t )

Neutral terminal

in Auto


Test sequence and trace comparison



q p

Test is performed with minimum effective turns in the winding under

test, e.g., DETC = 5, LTC = 16L.

Standard:

RFW@ 50-70% BIL

CW 1

CW 2

FW

With FOW:

RFW@ 50-70% BIL

FOW 1

FOW 2

CW 1

CW 2

FW

With non-linear

protective devices:

RFW 1

RFW 2 @ 75-100% of BIL

to demonstrate growing

sensitivity to V

FW 1

CW 1

CW 2

FW 2RFW 3 @ RFW2 voltage

RW 4

Neutral:

RFW@ 50-70% BIL)

FW1

FW2

HIGH-FREQUENCY - LIGHTNING IMPULSE:ACCEPTANCE CRITERIA

If test equipment and tested transformer were perfectly linear, the



If test equipment and tested transformer were perfectly linear, the

traces of repeated impulses, when overlaid, would perfectly

match. However, due to noise, setup imperfections or insulationfailure, discrepancies occur. Identifying their nature is the

objective of impulse data analysis.

T1, T2, Tc, voltage magnitude, , must meet requirements.

RFW and FW voltage and current traces should compare; request

to zoom in on any areas of concern. If available, comparison of Transfer Function (TF) for RFW and FW

is used as additional diagnostic criteria. It removes sensitivity to

wave shape variations caused by impulse generator jitter (TF

should be considered only in frequency ranges where sufficient

data is present in the time domain impulse trace*). For chopped wave test, segments of CW1 and CW2 traces prior to

moment of chop are compared. While traces after chop may be

shift, they oscillate around zero with the same frequency.

Verify that DGA results (after dielectrics) are normal.

* IEEE PC57.98TM/D07, September 2011, Draft Guide for Transformer Impulse Tests.

HIGH-FREQUENCY - LIGHTNING IMPULSE:ACCEPTANCE CRITERIA (cont.)



450 kV BIL, RFW on

HV winding – voltage

450 kV BIL, RFW on

HV winding – current




450 kV BIL, CW1 on


450 kV BIL, CW2 on





Overlay of 450 kV BIL

CW1 and CW2 - voltage




450 kV BIL, FW on


450 kV BIL, FW on

HV winding – current





RFW and FW - voltage





RFW and FW - current

High-frequency oscillations at

the beginning of current trace

are acceptable deviations,

reflecting the test setup.




450 kV BIL, FOW1 on


450 kV BIL, FOW2 on





Influence of non-linear protective

device on overlay of RFW and FW

350 kV BIL voltage traces

illustrates the need for comparing

traces of the same voltage level.




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



g , p p

need to be stopped and investigation performed. If the cause is found to

be external to the transformer, the corrections are made before the test

can continue.

If there is any doubt as to the cause of the discrepancies, additional

impulses need to be applied, including several FW. If the deviation

increases in magnitude, it indicates progressive dielectric failure in the

transformer. Unusual sounds, emanating from inside the tank, should be noted; these

sounds may be helpful in locating general location of the fault.

Removing manhole covers and observing presence of gas bubbles

and/or carbon, serves as confirmation of failure and provides some

indication of the fault location. Occasionally, the damage caused but not detected by impulse is only

detected by tests that follow: applied or induced/PD voltage tests, DGA.

HIGH-FREQUENCY - LIGHTNING IMPULSE:ABNORMAL DATA (cont.)



Overlay of 550 kV BIL RFW and FW

voltage traces – turn-to-turn failure





current traces – turn-to-turn failure


Voltage drop to ground

indicates one of the leads




traces – lead-to-lead failure

between RV and main LV windings

RFW voltage

FW voltage

FW current

RFW current

was at ground potential

Fault to ground diverts

current around winding,

reducing measured current.



HIGH-FREQUENCY:

SWITCHING IMPULSE

(Class I – other,

Class II <345 kV – other,

Class II 345 kV - routine)

HIGH-FREQUENCY - SWITCHING IMPULSE:OBJECTIVE

Demonstrate performance under transient high-frequency conditions

created by switching operations or network disturbance.



kV

s

FOW

CW

FW SW

y g p

Surge of energy from equipment

switched on or disturbance on the power

system. The time to reach the crest

amplitude and the total time duration of

switching impulses are much longer thanthose of lightning impulses.

HIGH-FREQUENCY - SWITCHING IMPULSE:PHYSICS

Switching impulse test consists

of applying or inducing a SW



length

V

Comparing to lightning impulse, the

switching impulse has a much

longer duration and lowerfrequency, resulting in voltage

approaching a uniform distribution

of the low-frequency steady-state

voltages, i.e., voltage distributes as

per turns ratio.

V

of applying or inducing a SW

between each HV line terminaland ground. Similar to a

lightning wave, the switching

wave can be simulated by

discharging a capacitor.

HIGH-FREQUENCY - SWITCHING IMPULSE:PHYSICS (cont.)

LV HVD D



*From W. McNutt 1989 Doble tutorial: Turns in this context mean the voltage that would have been present

if the applied voltage was distributed according to turns.

LV

DH

1

HV

Region D – phase-to-

ground and phase-to-

phase insulation is

stressed the most; stress

imposed by SW is1turns*.

Charging and discharging

processes are similar to

those described for

lightning impulse.

H1

H0

D

To another

phase

HIGH-FREQUENCY - SWITCHING IMPULSE:SETUP AND TEST METHODOLOGY

Full Wave Parameters

SW = 0 83BIL +/ 3%Crest voltage



Magnitude

SW = 0.83BIL +/- 3%

RSW=(50-70%)0.83BIL

Tp >100 s

Td 200 s

T0 1000 s

LV windings shall be designed to withstand stresses from SW

applied to HV side.

Applied test waves are of negative polarity to reduce risk of

erratic external flashover.

0.9

1.0

TpTd

t

V

First zero crossingT0

Virtual

origin

>90% of crest

HIGH-FREQUENCY - SWITCHING IMPULSE:SETUP AND TEST METHODOLOGY (cont.)



Xfmr

Impulse

control &

measuringsystem

Voltage divider

and measuring

circuit

v(t)

Cg

Rs

Rp

Impulse

generator

Note : The shown setup is for SW being applied to the HV winding. The

test can also be performed with SW being induced.


E ELV E ELV



ELV /2

Note: The choice of tap connections for all

windings is made by the manufacturer.

Test sequence and trace

comparison:

RSW@ 50-70% SW

(+) RSW - bias

SW1

(+) RSW - bias

SW2

RFW@ 50-70% BIL

CW 1

CW 2

FW

E/2 E/2-ELV /2

E

-ELV /2

ELV

-E/2


SW can saturate the core, creating an air-core conditions, i.e.,

drastically reducing impedance faced by impulse. This rapidly



drastically reducing impedance faced by impulse. This rapidly

decays the tail of the voltage waveform to zero, making T0<1000

s. To extend the time to saturation, prior to start of each test,

the core is magnetized in opposite direction by applying RSW (or

small dc current) of opposite polarity .

t

V

When core saturates, the

voltage collapses drastically

reducing time to zero crossing.

Bias in the core in

direction opposite

to that created by

test SW extends

time to saturation

and T0.

HIGH-FREQUENCY - SWITCHING IMPULSE:ACCEPTANCE CRITERIA

Tp, Td, T0, and voltage magnitude must meet requirements.



Failure detection is done primarily by scrutinizing voltagetraces for recognizable indications of failure. The test is

successful if there is no sudden collapse of voltage as

indicated on the trace.

Although overlaying RSW and SW traces in totality may

not be practical, the traces should match until the pointwhere the difference in the core magnetic state becomes

obvious. Normally, these differences can be easily

distinguished from drastic voltage reduction caused by a

failure.

Verify that DGA results (after dielectrics) are normal.

HIGH-FREQUENCY - SWITCHING IMPULSE:ACCEPTANCE CRITERIA (cont.)



650 kV BIL, RSW on

HV winding – voltage650 kV BIL, SW1 on


650 kV BIL, SW2 on


Typical reduced and full

switching impulse voltage

traces as measured on the HVwinding; for 650 kV BIL, the

BSL, i.e., the required test

voltage, is 540 kV.





RSW and SW - voltage

Beginning of traces deviating

due to the difference in core

magnetic state. This is

typically more pronounced in

the overlay of reduced and full

switching waveforms





SW1 and SW2 - voltage

Slight deviation due to the

difference in core magnetic

state.

HIGH-FREQUENCY - SWITCHING IMPULSE:ABNORMAL DATA

In general, whenever discrepancies occur the normal test

procedure need to be stopped and investigation performed. If



the cause is found to be external to the transformer, thecorrections are made before the test can continue.

If there is any doubt as to the cause of the discrepancies,

additional impulses may be applied.

Removing manhole covers and observing presence of gasbubbles and/or carbon, serves as confirmation of failure and

provides some indication of the fault location.

HIGH-FREQUENCY –LIGHTNING AND SWITCHING IMPULSE:RECOURSE IF DATA ABNORMAL

If visual confirmation (e.g., carbon, bubbles) is obtained

or the data convincingly reveals a failure, the oil is



drained and internal inspection is performed.

If necessary, the unit is un-tanked. This is followed by a

thorough and well-documented investigation.

The user’s involvement in this process enhances the

quality of the investigation and that of the final product.



LOW-FREQUENCY:

APPLIED VOLTAGE(Routine)

LOW-FREQUENCY – APPLIED VOLTAGE:OBJECTIVE

The high-frequency tests (lightning and switching



The applied voltage test is a simple overvoltage test. The

early transformer engineers apparently took cues from

mechanical engineers. This is how a mechanical structure

would be tested, by applying stress that demonstrates a

safety factor of two. The applied voltage test has a 1 minduration, with the expectation to demonstrate a long-term

capability to operate at the rated voltage.

impulse) always precede the low-frequency tests (appliedand induced voltage). This sequence is rooted in the fact

that due to a longer duration, the low-frequency tests

serve to stress further and to detect the damage caused

by the high-frequency tests.

LOW-FREQUENCY – APPLIED VOLTAGE:PHYSICS

D LV



Region D – major winding

-to-ground and winding-to-

winding insulation arestressed the most.

LV

D

HV

HV

Shorting

lead

D

LOW-FREQUENCY – APPLIED VOLTAGE:SETUP AND TEST METHODOLOGY

Applied Voltage Parameters

Magnitude C57.12.00-2010

Test is performed at low frequency

(<500 Hz), normally, power

f



Duration 1 min frequency. All terminals of tested winding are

connected together; all other

terminals (including all cores,

buried windings with one terminal

brought-out and the tank) are

grounded. A sphere-gap, set for 10% above

test voltage, may be connected for

protection.

Test voltage (1-phase) is

determined by terminal with the

lowest BIL (e.g., Neutral). The voltage is raised from 25% or

less, held for 1 min and reduced

gradually.

Each winding or set of windings

(e.g., in auto) is tested.

Note : On grounded-wye transformers with

reduced Neutral BIL the test has a limited

significance; it inly tests insulation in the

vicinity of the Neutral.

E

1.1E

v

LOW-FREQUENCY – APPLIED VOLTAGE:ACCEPTANCE CRITERIA

The test is a pass/fail test and is considered

d if d i th ti th lt i li d



passed if during the time the voltage is applied noevidence of possible failure is observed.

The indications to monitor include unusual sound

such as thump, sudden increase in the test circuit

current and collapse in the test voltage.

LOW-FREQUENCY – APPLIED VOLTAGE:ABNORMAL DATA

If unusual sound, sudden increase in the test

circuit current or circuit tripping occur these



circuit current or circuit tripping occur, theseevents should be carefully investigated by:

• observation, e.g., presence of carbon

and/or bubbles in the oil

• repeating the test

• other teststo determine whether the failure has occurred.

Due to a significant energy being released during

applied voltage test, the test is repeated (if at all) to

confirm the failure a limited number of times (1, 2

max). The energy released is usually sufficient to

mark the location making it possible to find the

failure after un-tanking.

LOW-FREQUENCY – APPLIED VOLTAGE:RECOURSE IF DATA ABNORMAL


and/or repeating of the test and/or other tests re eal the



and/or repeating of the test and/or other tests reveal the

failure, the oil is drained and internal inspection is

performed.

LOW-FREQUENCY:

INDUCED VOLTAGE/PD



INDUCED VOLTAGE/PD

Induced:7200 cycles

Induced:1 hour + PD

Class I

Class II

Routine

Routine

LOW-FREQUENCY – INDUCED VOLTAGE/PD:OBJECTIVE

The induced voltage test demonstrates the strength of

internal insulation in all windings as well as between



internal insulation in all windings as well as betweenwindings and to ground. A combination of prolonged stress

and a very sensitive PD measurement makes it a very severe

and searching test. It must be the last dielectric test to be

performed.

LOW-FREQUENCY - INDUCED VOLTAGE/PD:PHYSICS

To stress turn-to-turn insulation to the

required level, the winding needs toL

xfmr IG

V

IT



required level, the winding needs tobe excited to a level approaching

twice rated voltage. At power

frequency, this would overexcite the

core.

Therefore, the test is performed as ahigher frequency, which allows to

obtain the needed volts/turn at a

lower flux magnitude (v/t = dF/dt).

At higher frequency, transformers

become capacitive with dangers of M-

G set overexciting. This is addressedby using a variable reactor. The latter

provides an additional benefit of

reducing the load on MG set.

R C M G

L

Lv

Variable reactor

Lv is adjusted to

reduce output

from generator.

VT

ILv

VT

IG

IT

ILv

LOW-FREQUENCY - INDUCED VOLTAGE/PD:

PHYSICS (cont.)

E



Region E – with voltage

distributing per turns ratio, the

most stress is present in the turn-

to-turn insulation of each windingas well as in winding-to-winding

and winding-to-ground insulation.

LV

E

HV

E

E

E

E

E

LOW-FREQUENCY – INDUCED VOLTAGE/PD:

PHYSICS (cont.)

From the physics point of view, self-sustaining electron avalanches

may occur only in gases. Hence, discharges in dielectrics may

only be ignited in gas-filled cavities, such as voids or cracks in

solid materials and gas bubbles or water vapor in liquids.



solid materials and gas bubbles or water vapor in liquids.Discharges are generally ignited if the electrical field strength

inside the inclusion exceeds the intrinsic field strength of the gas.

They can appear as pulses having a duration of << 1s.

Partial discharges are defined as localized

electrical discharges that only partially bridge

the insulation between conductors and may ormay not occur adjacent to a conductor. In

insulation, the PD events are the consequence

of local field enhancements due to dielectric

imperfections.

Gaseousinclusion

Conductor

Dielectric


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



A p

requirements must be fulfilled to initiate PD: 1) local field stress

exceeds the void’s breakdown voltage Vbd and 2) free electrons are

available.

CC

CB

CA

CC

CBCA

Vbd


PHYSICS (cont.)

Strike: As Vcc>Vbd, breakdown

occurs, charges move across

shorting the void, Vcc= 0 and

discharge stops. To make up

for imbalance, charges comet f dj t i l ti

Buildup: As V , charges

move to and collect on

the surface of the void,

building potential stressV across the void

Relaxation: Charges continue

to flow at a decreasing rate

with balance restoring. Vcc

as charges collect back onthe void’s surface



out of adjacent insulation.

CC

CB

CA

V

CC

CB

CA

Q

Q

Q Q

Vcc= 0

V

QQQ

Q

Vcc= 0

CC

CB

CA

Q

QQ

Q

Vcc

V

Vcc across the void.

CC

CB

CA Q Q Q

V

Vcc

CC

CB

CA

Q

Q

Vcc

V

the void s surface.

CC

CB

CA Q

V

Vcc

Q


PHYSICS (cont.)

Terminal

voltage

Dip in terminal

voltage

C1



We cannot measure the real charge. However, as the void discharges, thecharge redistribution creates a dip* in the terminal voltage. This minute voltage

drop causes a high-frequency current to flow through a coupling capacitor

connected to a measuring system. Putting it differently, the charge movements

appear, in part, in C1 connected in parallel with CT. The integration of these high-

frequency current pulses over time produces the reported apparent charge.

Voltageacross void

PD current

C2Z

M

CT

*The detectable voltage dip is in the mV

range, while that at the void may be in

the kV range.


PHYSICS (cont.)

Measu rement of part ial discharge isl ik t i t i h b t t f l th t



Measu rement of part ial discharge isl ike try ing to weigh a butterf ly that

al ights momentar i ly on scales

designed fo r an elephant (somet imesdu r ing an earthquake ).

by Karl Haubner, Doble Aus tral ia



Xfmr in

testX H

C1

C2

CStep-up



M G

Lv

X0

H2 X1

X2

X3

H1

H3

C1

C2

C1

C2 M

p p

xfmr V

pC

and/or

V

Before test commences, several important steps take place:

Transformer is connected for open-circuit conditions. Voltage is raised to verify that variable (Lv) setting allows

to reach the required test voltage.

Measuring system (M) is calibrated for PD, RIV and

voltage.


SETUP AND TEST METHODOLOGY (cont.)

Induced Voltage/PD

Parameters

Voltagemagnitude

C57.12.00-2010

clause 5.10C84 1

Enhanced level

7200 cycles1h level, 5 min

recordings

100%

V

Hold as needed

until stable (min

60 sec)



gC84.1

Timing

C57.12.90-2010

clauses 10.7, 10.8

PD/RIVcriteria

C57.12.90-2010

clause 10.8/ Annex A

Voltage is gradually raised, recording pC, V and kV.

For Class I units, the test includes applying to HV winding 2.0nominal

voltage for 7200 cycles with no PD (RIV) recordings. For class II units rated

115 ÷ 500 kV, the test includes applying to HV winding 1.8nominal voltagefor 7200 cycles and 1.58nominal voltage for 1 h, recording PD (RIV) data.

For windings other than HV, when possible, taps should be selected so that

voltages on other windings are as per ANSI C84.1 and C57.12.90 clause

10.8.1 (e.g., for 115÷345 kV units , the voltage on other windings should be

1.5 times their maximum operating voltage).

Ambient Ambient

100%

100%

t1h



PD (pC) measurements are performed using 100 ÷ 300 kHz and RIV

(V) using 0.85 ÷ 1.15 MHz frequency ranges.

For units with windings that have multiple connections (e.g., series-

parallel or delta-wye) with each connection having system voltage



>25 kV, two induced tests are performed, one in each connection. If

more than one winding has such multiple connection, then the

connections in each winding shall change between tests. In all

cases, the last test shall be for connection with highest test voltage.

To minimize the effects of external factors and stray capacitances,

the following steps are often relied on:

- filters on the power supply line

- shielding all sharp edges including those at ground potential

as well as the energized and grounded bushings

- turning off solid state power supplies, cranes and other factorymachinery

- removing air bubbles from bushing gas space

- applying pressure to suppress bubbles in the main tank.


ACCEPTANCE CRITERIA

Results are acceptable if:

Nothing unusual associated with sound, current, or voltageis observed (see abnormal data for details).



( )

The PD (RIV) results during 1h test period have shown:

- Magnitude 500 pC ( 100 V).

- Increase during 1 h 150 pC ( 30 V).- No steadily rising trends during 1 h

- No sudden sustained increase during the last 20 min.

Judgment should be used on the automatically recorded 5-min

readings so that momentary excursions caused by cranes or

other ambient sources are not recorded. Also, the test may be

extended or repeated until acceptable results are obtained. DGA results (after dielectrics) are normal.



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%


4 42.4 kV 36.7 pC 5.2 µV 00:03:33 42.0 kV 29.2 pC 7.1 µV 00:03:42 42.1 kV 30.7 pC 4.7 µV 00:03:51 1hr level

5 55.5 kV 31.9 pC 4.9 µV 00:04:03 54.5 kV 33.5 pC 13.5 µV 00:04:12 54.7 kV 33.9 pC 6.2 µV 00:04:21 Enhanced



6 42.3 kV 27.1 pC 4.6 µV 00:00:03 41.9 kV 29.3 pC 5.6 µV 00:00:35 42.1 kV 29.5 pC 4.9 µV 00:01:07 1 hr level

7 42.2 kV 27.3 pC 4.6 µV 00:05:03 41.9 kV 28.0 pC 6.1 µV 00:05:35 41.9 kV 29.8 pC 4.8 µV 00:06:10











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



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


ABNORMAL DATA

Results are not acceptable if the pC (or V) data exceeds any

of the required criteria, and no reasonable/acceptable justification for the source/cause is provided.



Other tests, e.g., acoustic PD, DGA, can provide confirmation

that a source of excessive partial discharge is present.

The presence of smoke and bubbles rising in the oil, audible

sounds such as thump, sudden increase in test current or

voltage collapse may all serve as a confirmation that

abnormal PD results are associated with a failure.

LOW-FREQUENCY – INDUCED VOLTAGE/PD:RECOURSE IF DATA ABNORMAL

If pC (or V) data exceeds the limits, and all the attempts

to identify and eliminate external PD sources are not

successful, a longer standing time, long duration PDtest, degassing of oil, refilling transformer under



test, degassing of oil, refilling transformer under

vacuum or a heatrun test (if one is specified) are often

successfully bring the PD data within limits.

A failure to meet the partial discharge acceptancecriterion shall not warrant immediate rejection, but it

shall lead to consultation between purchaser and

manufacturer about further investigations.


and/or repeating of the test and/or other tests reveal thefailure, the oil is drained and internal inspection is

performed.

NO LOAD LOSSES AND



NO-LOAD LOSSES AND

EXCITATION CURRENT,

after dielectrics(Routine*)

*The test is not required by standards and no test type is

assigned to it; however, it is a wildly recognized as

standard practice and performed as routine.



NO-LOAD LOSSES AND EXCITATION CURRENT after dielectrics:

ACCEPTANCE CRITERIA AND RECOURSE IF DATA ABNORMAL

No-load losses measured after dielectric tests are

compared with the results obtained before dielectric tests.

The 5% difference is often used as an acceptable criteria. Difference between the before and after data could be due



to:

Changes in the inter-laminar insulation

Temperature

Sometimes the change after initially exceeding 5% goesaway with time.

Failure to meet before and after dielectrics comparison

criteria should not warrant immediate rejection but shall

lead to consultation between purchaser and manufacturerregarding further investigation of possible causes and

consequences.



LOAD LOSSES AND

IMPEDANCE VOLTAGE(Routine)

LOAD LOSSES AND IMPEDANCE VOLTAGE:


Definition: The load losses of a transformer are losses

associated with a specified load and include:

windings I 2 R losses due to load current stray losses due to eddy currents induced by leakage flux in



y y y g

the windings, core clamps, magnetic shields, tank walls, and

other conducting parts. Stray losses may also be caused by

currents circulating in parallel windings or strands.

Load losses do not include control and cooling losses.

The impedance voltage of a transformer is the voltage required

to circulate rated current through two specified windings with

one winding short-circuited.




PHYSICS

RFM

Vrated

Iexc

RIrated

I2R lossesT

FL

V

Note: Resistance R and short

circuit of LV is not shown.



To create conditions when losses are limited to I2R and stray

losses, and applied voltage is equal to the voltage drop across a

loaded transformer, one winding is short-circuited and voltage is

raised until rated current is reached. The flux path is thendominated by the leakage channel where the eddy losses in

various conducting components in the FL path are induced.

LVHV LVHV

Eddy currents

creating losses1/T

Vsc





Applied voltage is adjusted

until rated current is present in

the excited winding.

After data is recorded, if

necessary, correction for losses

Transformer

in testCT

VT

V

X0

H2

X1

X2

X3

H1

H3

3



necessary, correction for losses

in external circuit is made.

If three line currents are not

balanced the average RMS

value should correspond to the

desired value.

The duration of the test should

be kept to a minimum to avoid

heating up winding conductors.

W

I

A V

If taps are present, the following

combinations of voltage ratings are tested:

DETC rated rated rated max max max min min min

LTC N max min N max min N max min



For 3-wdg units, three sets of

measurements are performedusing three pairs of windings,

d i Z Z Z d P

1

2

31

2

3

Z1

Z2

Z



producing Z12, Z13, Z23 and P12,

P13, P23. Solving shown

equations, determines Zi and

Pi of each branch. For test, the current is set

based on capacity of the

winding with lowest MVA in the

pair.

When results are converted to%, all data is given based on

MVA of HV winding.

Z3

Z12 = Z1 + Z2

Z13 = Z1 + Z3

Z23 = Z2 + Z3

Z1 = (Z12 + Z13 – Z23)/2

Z2 = (Z12 + Z23 – Z13)/2Z3 = (Z13 + Z23 – Z12)/2



Since stray and I2R

losses have different

dependencies on T,each need to be

obtained from

Measure A, V, W, T

Correct W and Vfrom measured

amps to rated

Convert straylosses from

TLL test Trated



obtained from

measured losses,

individually converted

from test T to ratedT before combined

again in reported load

losses. V is also

converted to rated T.

Convert Rdc from

TR_test

TLL_test

p

Calculate I2R losses

at TLL_test

Calculate stray

losses at TLL_test

(W - I2R)

Convert I2R losses

from TLL_test

Trated

LL_test rated

Calculate total

losses at Trated

(stray + I2R)

Calculate %Vsc

(V / Vrated)100 = %Zsc

Correct V from

TLL_test Trated


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%



at the same time is +/-7.5%.

For 3-wdg units, autotransformers or units having a zigzag

winding, tolerance for measured impedance is +/-10% of the

guaranteed value. The tolerance for comparison of duplicatesunits produced at the same time is +/-10%.


Thermal stability prior to test: TTO-TBO 5C.

Average of T readings (Tave_oil) before and after the test

should be used as test T. Their difference must be 5C. Frequency is within +/-0.5% of rated.

Test system accuracy should be within +/-3% for loss, +/-0.5%

for voltage, current and RDC, and +/-1.5C for T.


ABNORMAL DATA

Potential reasons for exceeding the guaranteed values mayinclude:

O i ht i d i

Example: guaranteed load loss - 94 kW, measured – 110 kW




Production process related factors or mistakes

Influence of temperature was not properly accounted for

Accuracy of measurements



Failure to meet the load losses and impedance test

criteria should not warrant immediate rejection but shall

lead to consultation between purchaser andmanufacturer regarding further investigation of possible

causes and consequences.



q

The acceptance criteria of 6% for total losses does not

replace the manufacturer’s guarantee of losses for

economic loss evaluation purposes.



Factory losses are measuredunder 3-phase excitation, at

rated current and reported as

Field losses are measuredunder 1-phase excitation, at

current much lower than rated



Factory and field results

cannot be compared

rated current and reported as

sum of three phases I2R and

stray losses.

current much lower than rated

and reported as per-phase I2R

and stray losses.


COMPARISON WITH FIELD DATA (cont.)

Factory short-circuit impedance is

reported as average of three

phases, obtained at rated current*

under 3-phase excitation.

Field leakage reactance is reported

as per-phase reactive component of

the short-circuit impedance,

obtained at current* much lower than

rated under 1-phase excitation.



*Since test is confined to leakage channel (where reluctance is determined by air/oil) the

leakage inductance (L= /I), remains the same regardless of the current level.

Experience shows that a combined influence of different instrumentation

and test setups, difference in flux distribution under 3- and 1-phase

excitation, presence of the resistive component and averaging of factory

data can result in differences ranging from nearly perfect (<1%) to up to

6% (of the measured value).

However, the differences between factory and field test conditions

notwithstanding, the ZNP can serve as a useful guideline for evaluating

the initial value measured in the field. If, during initial test, the field per-

phase tests deviate from average (of three readings) by <3% of themeasured value, results normally are considered acceptable. The initial

per-phase test should serve as a benchmark for future testing with

acceptable difference from the initial field test being <2%.

TEMPERATURE RISE



TEMPERATURE RISE(Design and other)

TEMPERATURE RISE:


Definition: The temperature rise is a test that verifies

transformer thermal performance through determination

of winding and oil temperature rises over ambient.

Objective: The temperature rise test provides the top-oil

rise winding average rise and winding hot-spot rise over



rise, winding average rise and winding hot spot rise over

ambient for:

Verification of design calculations. Demonstration of meeting the guaranteed performance

characteristics.

Provides data for calculation of potential MVA margin.

Setup of various temperature monitoring instrumentsand cooling control.

TEMPERATURE RISE:

PHYSICS

Measured:

Tto, Tt_rad, Tb_rad, Ta.

Tto Tt_rad

Tt rad

Tto-a Tto

Main tank

T

Ta

height

Calculated: Tto-a, Tw_ave-a, Ths-a, GRAD

Needs NL+LL

lossesNeed rated

current

T



Tb_rad

Rad

Tb_rad

Oil

Winding

t_rad

Tw_ave*

Ta

Tw_ave-a

LV HV

TaT

To_ave

GRAD

Core

Tw_hs

Ta

*The term “winding average T rise”, Tw_ave-a, is not the T at any given point in a winding

nor is it an arithmetic average of results determined from different terminal pairs. It refers

to the value determined by measurement on a given pair of winding terminals.

Ths-a

Located at 3 locations around xfmr at mid-height level.

TEMPERATURE RISE:


Total losses (NL+LL) and winding

cold resistance data should be

available.

Test is performed for min and max

MVA and in a combination of

Transformer

in testCT

VT

X0

H2

X1

X2

H1



MVA, and in a combination of

DETC/LTC positions, producing

highest load losses.

10Tamb40C and measured in

containers with liquid, having a time

constant as per C57.12.90-2010.

Test contains 3 key segments:

- total loss run (to include 3 hr of

thermal stability)- rated current run (1 hr)

- hot resistance measurement

(e.g., 10-20 min after shutdown)

W

I

V

A

X3 H3

V

TEMPERATURE RISE:


Tto, Tt_ rad

Tb_ rad

T[C]

Cutback

ONAF shutdown

Xfmrenergized

for ONAF

Rhot

measurement

begins

Measurement before

cutback determines *Tto-a



Ta_ ave

Tto-a

t [h]

PrecedingONAN

ONAN

shutdown

Rated currentrun

Steady-state oil

T rise

(change of Tto-a in 3h 1C or 2.5%

whichever is greater)

Itest

Ptotal

Irated

*Tto-a is corrected for difference between required and

actually used total losses (it must be 20%) and for altitude.

Total loss run

1h

TEMPERATURE RISE:


Rhot

Rhot as function of time in

f d i

Rhot calculated at t = 0

*Instrument

t d

Objective: resistance of

winding at the time when

load current is still present



t [min]

presence of decreasing

temperature is recorded

t 4 min

Tw_hot = Rhot /Rcold(234.5 + Tw_cold) – 234.5

*If two windings are tested simultaneously in series,

the Idc is selected based on the lowest rated current.

connected

Instrument output

current reached

pre-selected level Flux

stabilized

t = 0

Voltageremoved

TEMPERATURE RISE:


TTw ave-a

Comparison with guaranteed values,

e.g., Tto-a and Tw_ave-a 65C; Ths-a 80C

GRAD correction for

localized hot spot

eddy currents ***Ths-a



Tw_hot

**To_ave_cb-a is corrected for difference between required and actually used total losses (it must be 20%)

and altitude.

Ta

**To_ave_cb-a

GRAD

_

*GRAD To_ave_sd

*GRAD is corrected for difference between required and actually used load current (it must be 15%).

Tto-a

GRAD Value used for

setting winding

T monitors

***This a simplified representation of Ths_a determination; actual design calculation is more involved.

TEMPERATURE RISE:


i

vLV v

LVX2

X0 i

1

During shutdown at the time of the first Rdc reading, the flux must be

stabilized so that resistance change is caused only by reduction in

temperature. It’s true in most cases, unless series autoxfmr is present.



Series autoxfmr

windings with LTC in N LV winding

idc

idc

i2

X2X

0idc

i1

i2

idc

idc

Main unit

core

Series autoxfmr

core

F

1 F2

TEMPERATURE RISE:


0.00467

0.00468

0.00469

0.0047

R d c [ o h m ]

0.29

0.3

0.31

0.32

HV ci r c ui t X 0-X 2

At t = 4 min, data

collection begins

ith fl i i

Series autoxfmr should be

excluded from both cold

and hot Rdc measurements.



0.00462

0.00463

0.00464

0.00465

0.00466

0:00:00 0:00:43 0:01:26 0:02:10 0:02:53 0:03:36 0:04:19 0:05:02 0:05:46

Time [min]

L V c

i r c u i t R

0.25

0.26

0.27

0.28

tR d c [

Oh m ]

H1-H 2

Setup 1.5 min

Voltage

removed

t = 0

Time remaining

for stabilization

= 2.5 min

with flux in main

core stable while

flux in series corestill changing.

TEMPERATURE RISE:

ACCEPTANCE CRITERIA

The winding average T rise over ambient for all tested windings

should not exceed the guaranteed value, e.g., 65 or 55C.

The top-oil T rise over ambient should not exceed the guaranteed

value, e.g., 65 or 55C. The winding hot spot T rise over ambient for all tested windings

should not exceed the guaranteed value, e.g., 80C for 65C rise

it d 65 C f 55 C i it



units and 65C for 55C rise units.

If shutdown is performed on each phase, results of winding average

rises should be comparable (rule of thumb: 4C difference,presently, there is no limit in the standard).

DGA results (after heatrun) should be normal.

It is always useful to perform and review thermal scanning of all

tank walls and the cover in search for excessive overheating

(100C rise). Request image files to be provided with the certifiedtest report and have software to view them.

If agreed with manufacturer, the heatrun is a good time to check the

performance of temperature controllers (using a preliminary winding

T gradient) and turns ratio of CTs.

TEMPERATURE RISE:


To assure test data is credible, verify that:

T and current requirements for measuring winding cold Rdc were

complied with.

Test is performed using maximum load loss and in corresponding

DETC/LTC positions.

Test instrument type and setup used for cold and hot resistance



was the same, e.g., if two-channel measurement is used it must

be used for both hot and cold resistance tests.

If series auto is present, unless it is shown that RDC can bemeasured within shutdown time constrains, the auto is excluded

from the resistance measurement*.

During shutdown, fans are turned off right after transformer is de-

energized.

The first value of winding hot Rdc was recorded not later than 4min after shutdown.

*Lachman, M. F., et al “Impact of Series Unit on Transformer Winding DC Resistance MeasurementDuring Heatrun”, Proc. of the Seventy-Sixth Annual Intern. Confer. of Doble Clients, 2009, Sec. T-4.

TEMPERATURE RISE:


To assure test data is credible, verify that:

Winding hot Rdc fits reasonably into the cooling curve.

Final T rises are properly corrected: GRAD for actual test

currents, Tto-a and To_ave_cb-a for actual total losses and altitude.

Test system accuracy should be within +/-3% for loss, +/-0.5% for

voltage, current and winding resistance, and +/-1.5C for

t t



temperature.

If the test could not be done at rated frequency, the results are

converted from tested to rated frequency (see C57.12.90-2010,Annex B). However, the fans/pumps should be operated at the power

frequency to be used when unit is in service.

TEMPERATURE RISE:

ABNORMAL DATA

Potential reasons for exceeding the guaranteed values may

include:

Oversights in design Testing/setup mistakes

Presence of series auto-transformer

Example: guaranteed Tw_ave-a – 65C, measured – 67C



TLV_hot=[(234.5+30)4.534/4.081]-234.5=59.4ºC

y = 8.152E-07x2

- 3.235E-05x + 5.362E-03

0.005

0.00505

0.0051

0.00515

0.0052

0.00525

0.0053

0.00535

0 : 0 0

0 : 3 0

1 : 0 0

1 : 3 0

2 : 0 0

2 : 3 0

3 : 0 0

3 : 3 0

4 : 0 0

4 : 3 0

5 : 0 0

5 : 3 0

6 : 0 0

6 : 3 0

7 : 0 0

7 : 3 0

8 : 0 0

8 : 3 0

9 : 0 0

9 : 3 0

1 0 : 0 0

Time [min]

R

e s i s t a n c e [ o h m s ]

TLV_hot=[(234.5+30)5.362/4.621]-234.5=72.5ºC

With series auto-xfmr

y = 6.962E-08x2

- 5.974E-06x + 4.534E-03

0.00442

0.00444

0.00446

0.00448

0.0045

0.00452

0.00454

0 : 0 0

0 : 3 0

1 : 0 0

1 : 3 0 2 : 0 0

2 : 3 0 3 : 0 0

3 : 3 0

4 : 0 0

4 : 3 0

5 : 0 0 5 : 3 0

6 : 0 0 6 : 3 0

7 : 0 0 7 : 3 0

8 : 0 0 8 : 3 0

9 : 0 0 9 : 3 0

1 0 : 0 0

Time [min]

R

e s i s t a n c e [ o h m s ]

Without series auto-xfmr

Note: The example shows a quadratic function, the suitability of which was confirmed via direct fiberopticmeasurements and other methods, e.g., Blume. Different functions may be used if they fit the winding behavior.

TEMPERATURE RISE:


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.



ZERO-PHASE SEQUENCE

IMPEDANCE



IMPEDANCE

(Class I - design

Class II - routine)

ZERO-PHASE SEQUENCE IMPEDANCE:


Definition: The zero-phase sequence impedance is

impedance to the single-phase current simultaneously

present all three phases. It is measured from a wye or a

zig-zag connected winding between three phase

terminals connected together and the neutral terminal.

Objective: The zero phase sequence impedance serves



Objective: The zero-phase sequence impedance serves

as input in analysis of unbalanced three-phase system

using symmetrical components method.


PHYSICS

In symmetrically loaded 3-phase

system, only one phase needs to

be analyzed since in other

phases values have the same

magnitudes and only have to be

shifted by 120.

In unbalanced 3-phase system,

impedances in each phase are

Ia

Ia

Ib Ic

Balanced

Ia1

I

Positive



impedances in each phase are

different and each phase needs

to be analyzed separately. Method of symmetrical

components converts any

unbalanced system into 3

balanced systems, namely

positive, negative and zero-

phase sequence systems.

After these are defined, the

voltages and currents in the

original unbalanced system are

reconstructed.

Ib

Ic

Unbalanced

Ib1

Ic1

Ib2 Ia2

Ic2 Negative

Iao

Ibo

Ico

Zero



For xfmr, the Z1 = Z2 = Zsc is known from impedance/load losses test. In zero-

phase sequence system, the phase currents are in-phase with each other and

flow through the xfmr only if there is a path to return to the grounded source or

to circulate while satisfying the Kirchhoff’s current law.. Therefore, this testapplies only to transformers with one or more windings with a physical neutral

brought out for external connection.



Z0

1 2

N

1 2



Z1Ns, Z1No, Z2No are

used to calculate Z1,

Z2 and Z3.

If delta winding is not

present, the currents

shown in delta are

circulating in the tank.

Z1Ns

Z



circulating in the tank.

Z1 Z2

Z3

1 2

N

Z1No

Z2No



If no delta winding is present, applied

voltage should be 30% of rated

Vphase_gnd and measured current Irated.

If delta winding is present, the applied

voltage should be such that current in

delta winding Irated.

Transformer in

test

CT

VT

V

1



For Y/ or /Y impedance in % is

determined as:Z0 = 300(Vmeas / V r ) (Ir / Imeas)

For Y/Y and autoxfmr with or without

tertiary , the elements of the

equivalent circuit are further

determined as:Z1 = Z1No - Z3

Z2 = Z2No - Z3

Z3 = Z2No ( Z1No - Z1Ns)

W

I

AV


ACCEPTANCE CRITERIA

The standard does not provide an acceptance criteria for the zero-

phase sequence values. However, the following general guidelines can

be useful (typical for 230 kV, 200 MVA core type units):

For /Y, Z0 Zsc or slightly less. Example: 50 MVA, 161/69GndY kV, Z sc = 21.9%, Z 0 = 21.8%

For Y/ units, Z0 (0.8-1.0)Zsc. Example: 48 MVA, 235.75GndY/13.8 kV, Z sc = 9.9%, Z 0 = 8.5%

For Y/Y/ or autoxfmrs with delta Z (0 7 1 0)Z with Z



For Y/Y/ or autoxfmrs with delta, Z1 (0.7-1.0)Zsc; with Z2

typically <1.0% or sometimes <0.

Example: Auto, 18 MVA, 230GndY/60GndY/21 kV, Z sc = 4.9%, Z 1 = 3.6%,

Z 2 = 0.84%, Z 3 = 10%

50 MVA, 69GndY/34.5GndY/13.2 kV, Z sc = 7.8%, Z 1 = 6.7%,

Z 2 = -0.16%, Z 3 = 4.6%

For Y/Y and autoxfmrs without delta (rare occasion), magnetic

flux has a strong coupling to the tank, making, in general, therelationship between voltage and current non-linear and the

above observations not relevant. Example: Auto, 75 MVA, 115GndY/34.5GndY kV, Z sc = 12.7%, Z 1 = -9.9%,

Z 2 = 27.4%, Z 3 = 205.6%


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.





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.



AUDIBLE SOUND LEVEL



U SOU

(Design and other)

AUDIBLE SOUND LEVEL:


Definition: The audible sound level test is the measurement

of the sound pressure level around a fully assembled

transformer under the rated no-load conditions with cooling

equipment operating as appropriate for the power ratingbeing tested.

Objective: To protect the population from noise

i i t f i d t t ithi



inconveniences transformers are required to operate within

specified noise limits. The audible sound level test providesthe sound pressure level data for:

Verification of design calculations.

Demonstration of meeting the guaranteed performance

characteristics.

The test also serves as a quality control tool as the sound,

driven by the vibratory motion of the core, is transmitted to

the tank through direct mechanical coupling as well as is

produced by pumps and fans of the cooling system.


PHYSICS

Most of xfmr sound is generated

by the core. When the core steel

magnetized/demagnetized twice

each cycle, the steel elongates

and shortens due to a propertycalled magnetostriction.

This produces a vibratory motion

in the core transmitted to the tank

through the core mechanical

support and the pressure waves

Core

Dielectric

fluid

Tank



in the dielectric fluid. At the tank

this motion radiates as anairborne sound. The vibration

magnitude depends on the flux

density and magnetic property of

the steel.

The frequency spectrum of the

sound contains mainly the even

harmonics of the power

frequency, i.e., 120, 240, 360, etc.

The audible sound also includes

a contribution emitted by pumps

and fans, containing a broadband

spectrum of frequencies.

Direction ofdimensional

changeMagnetostriction

caused by

domain rotation

F



Xfmr is energized with no load, at rated

(for the tap used) voltage and frequency,

with tap changer on principal tap and

pumps/fans operated as appropriate forthe tested rating.

On certain tap positions, xfmr may

produce sound levels greater than at the

principal tap, e.g., engaging PA and/or

Transformer

in test

X0

H2 X1

X2

X

H1

H

3



series autoxfmr. Test will be performed in

these positions if specified by customer.

The voltage should be set as during no-

load loss test, based on Vave.

At least one test should be performed at

the cooling stage for the min rating and

one test at the cooling stage for maxrating.

Measurements begin when xfrm reaches

steady-state conditions, i.e., to allow

magnetic bias to decay.

VT

Vave

X3 H3



Microphones are located

on the measurement

surface at showndistance from reference

sound-producing surface.

Xfmr is placed so that no

acoustically reflectingTank

Radiator

6 ft3 ft

Fan cooled surface

Microphone

location



acoustically reflecting

surface is within 10 ft of

the microphone.

If transformer H<7.9ft,

measurements are made

at H/2; if H7.9 ft, at H/3

and 2H/3.

First measurement ismade at drain valve

proceeding clockwise.

Reference sound-producing

surface is a vertical surface

following the contour of a taut

string stretched around xfmr

periphery.

LTC

Drain valve

1 ft

#1Measurement

surface



The sound power rating of a transformer is determined

using one of the following three measurement methods:

A-weighted sound pressure level (most frequent)

One-third octave sound pressure level (when



specified)

Narrowband sound pressure level (when specified)



Human ear can hear sounds in 20÷20000 Hz

range. However, it detects some frequencies much

easier than others. This uneven frequency

response needs to be considered when the

annoyance of unwanted sounds is to be evaluated.

To account for human’s greater sensitivity to noise

at some frequencies relative to other, the measured

data is passed through a weighting filter. A-

A-weighted sound pressure level



weighting is most commonly used to allow for a

broad peak between 1÷6 kHz but very stronglydiscriminating against low frequencies.

As a result, when the average sound pressure

level is calculated, the influence of frequencies not

impacting the human hearing perception is

minimized.

Hz 63 125 250 500 1000 2000 4000 8000

A-filter -26 -16 -19 -3 0 1 1 -1

dB (measured) 67 76 73 70 65 66 62 52

dB (A-weighted) 41 60 64 67 65 67 63 51



The following two methods are used when a more detailed

investigation into the sources of noise is required:

In one-third octave sound pressure level measurement,each octave band in the spectrum (i.e., 63, 125, 250, 500,

1000, 2000 and 4000 Hz) is split into three, with each “1/3

sub-band” (e.g., 63, 80, 100, 125, 160, 200, 250 Hz, etc.)

being evaluated individually.



being evaluated individually.

The narrowband sound pressure level measurement isperformed at the power frequency (e.g., 60 Hz) and at

least at each of the next six even harmonics (120 Hz, 240

Hz, 360 Hz, 480 Hz, 600 Hz, and 720 Hz). Once again, each

frequency is evaluated individually.



The sound power rating is determined using the following steps:

Measure ambient sound pressure levels. This is established as an

average of measurements at a min of four locations immediately

preceding and immediately following the sound measurements

with the unit energized.

Measure combined transformer and ambient sound pressure level.

Measurements are made if ambient level is at least 5 dB or more

below the combined transformer and ambient sound pressure

level



level.

Compute ambient-corrected sound pressure levels. Forcorrections see Table 7 in C57.12.90-2010.

Compute average sound pressure levels [in dB(A)]:

=

L i is the sound pressure level measured at i th location by one of the 3

measuring methods. Sound power levels are calculated when requested.


ACCEPTANCE CRITERIA

Computed average sound pressure level should not exceed the

audible sound levels as listed in NEMA TR1-1993, Tables 0-2 and 0-3

or as requested in customer test specification. Rectifier, railway,

furnace, grounding, and mobile transformers are not covered bythese tables.


The sound pressure measuring instrument should meet the

requirements of ANSI S1.4 for Type 1 meters.

Th d i i t t h ld b lib t d



The sound pressure measuring instrument should be calibrated

before and after each set of measurements. If calibration change

>1dB, sound measurements shall be declared invalid, and the

test repeated.

Verify that microphones were positioned at required

distances/heights, pumps/fans were operated as required for

tested power rating and voltage set based on Vave. Verify that the ambient level was at least 5 dB or more below the

combined transformer and ambient sound pressure level.

If rated frequency is not used, 50/60 Hz conversion is applied.


ABNORMAL DATA

Potential reasons for exceeding the guaranteed values mayinclude:

Problems with measurement, e.g., ambient noise,

positions of microphones, sound instrument calibration,

lt dj t t di fl ti f

Example: guaranteed sound pressure level per NEMA –

75/77/78 dB(A), measured – 77/78/79 dB(A)



voltage adjustment, surrounding reflecting surfaces,

etc.

Variability in core steel characteristics

Different core steel


Assembly related factors or mistakes



Failure to meet the audible sound test criteria should not

warrant immediate rejection but shall lead to consultation

between purchaser and manufacturer regarding an

investigation of possible causes and solutions to addressthe problem.



CORE DEMAGNETIZATION



(Routine*)

*This procedure is not required by standards but is a wildly

recognized as standard practice and performed as routine.

CORE DEMAGNETIZATION:


Definition: The core demagnetization is the process of

removing the magnetic bias in the core through a series

of steps, with each subsequent step creating magnetic

field of opposite direction and lower intensity. The firststep must bring the core to the main hysteresis loop with

the last step, upon removal, leaving no residual

magnetism in the core.



Objective: The core demagnetization creates conditionsfor obtaining the low-voltage exciting current and loss

test as well as sfra benchmark data not affected by

residual magnetism .


PHYSICS

If in the presence of residual

magnetism Br , the voltage is increased

from zero, the flux varies around

minor hysteresis loops. The negative

tip of these loops lies on the main

loop. The greater the voltage, the

smaller is the offset of the minor loop

along the B axis. The bias is removed

when the main loop, symmetrical

Main

loop

Br

H

B



when the main loop, symmetrical

around the origin, is reached.

H

B

If after reaching the main hysteresis

loop, the voltage is gradually reduced,

each minor loop will lie inside the

previous larger loop. Reduction ofvoltage to zero brings working point

to the center of these loops resulting

in a demagnetized transformer.

Br = 0



The core demagnetization can be performed by one of the

following:

Applying rated 3-phase voltage (holding for 5-10 min)and reducing gradually to zero.

Applying DC voltage (e.g., 12 V), waiting until current

stabilizes, then switching voltage polarity and holding

until current reaches a lower value; this process



continues until current level is zero Without ammeter, the above approach can be applied

but a lower level of current is reached by applying

alternate polarities of DC voltage for progressively

shorter periods of time.

If no-load losses or sound level tests are the last power

tests to be performed, they serve the function of the

core demagnetization process.


RELATIONSHIP WITH LV DIAGNOSTIC DATA

When xfmr is de-energized, the

core is constantly looking for a

state of lower energy, i.e., it

relaxes, changing its magnetic

state and moving away from thecondition immediately following

demagnetization*. This is obvious

in the low-frequency range of the

sfra trace but not in the low-voltage

excitation current data. These sfra

changes are normal and

Factory

Field

Field

Factory

Data movement

with no excitation

applied between

measurements



changes are normal and

diagnostically insignificant.

Factory Field

mA W mA W

20.5 128 20.5 126

9.3 61 9.6 58

20.7 131 21.6 131

72 hr

24 hr

9 hr

6 hr

3 hr

1 hr

30 min

dm_ini t

Controlled

experiments showing

data movement*

*Lachman, M. F., et al “Frequency Response Analysis of

Transformers and Influence of Magnetic Viscosity”, Proc.of the Seventy-Seventh Annual Intern. Confer. of DobleClients, 2010, Sec. TX-11.

LAST SLIDE



THE END



UNDERSTANDING THETRANSFORMER TEST DATA

Barry M.

Mirzaei

– P.Eng.

Hydro One

September 2012 – Chicago



2Understanding The Transformer Test DataSeptember 2012

No Load

Test

Test object is supplied from one side of the transformer



3

(L.V.), the other side (H.V.) is left open circuit. Test voltageto be adjusted to the pre‐determined value(s)

Typical test voltage is 90% ‐ 100% and 110% of the rated

voltage

Characteristics of the No Load Test:

“Low Current – High Voltage”

Understanding The Transformer Test DataSeptember 2012

Induced Voltage Test

Test object is supplied from one side of the

transformer (L.V.), the other side (H.V.) is left open

circuit. Test voltage to be adjusted to the pre‐



4

determined value(s)

Twice the rated voltage is applied for 7200 cycles

for transformers with uniformly insulated

windings

Characteristics of the Induced Voltage Test:

“Low Current – High Voltage and Frequency > 60”


Load Loss

Test

Test object is supplied from one side (H.V.), the

other side (L.V.) is short‐circuited. Test voltage is



5

adjusted to apply the rated current to the testobject

Load Loss:

‐Resistive losses or R

‐Eddy current losses in the windings

‐Stray losses in leads, core plates and tank

Characteristics of the Load Loss Test:

“High Current – Low Voltage”





Hysteresis Loss



7

Proportional to the frequency

and dependent on the area of

the hysteresis loop, which, inturn, is a characteristic of the

material and a function of the

peak flux density


Eddy Current

Loss



8

Dependent on the square

of frequency but is also

directly proportional

to

the square of the

thickness of the material


4.44 (a)

(b)

Voltage



9

PNL No Load Losses

= Hysteresis Loss

= Eddy Current Loss

, = Coefficients

=

= Exponent with induction


Minimizing hysteresis loss thus depends onthe development of a material having a

minimum area of hysteresis loop.




Minimizing eddy current loss is achieved by

building up the core from a stack of thin

laminations and increasing resistivity of the

material in order to make it less easy for

eddy currents to flow.



STATEMENT OF THE ISSUE:


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



12

Deflection in the readings on metering

devices (watt meters, …) were reportedwith the noise.

inside the oil”, started at around the 25%

of the test voltage.


Solutions?What

test

data

are

available?

What those test data really mean?




Criteria & constraints for

addressing the

issue

Un‐necessary activities to be

avoided delivery date was critical



14

avoided, delivery date was critical

Un‐tanking the transformer is costly and

should be avoided if there is no clear

understanding about

the

issue

Insulation tests should not be repeated,

if there is no need to do so


1

Oil Sample

OK 2

Repeat TTR & DC Resistance

OK

6

Apply reduced

“Induced Voltage”



15

Investigation Procedure

3

Observe The No Load Test

PROBLEM4

Apply Load Test

OK

5

Insulation Test?

Not Convinced to

apply

Voltage


The Induced Voltage Test

stresses all parts of the




stresses all parts of theinsulation system, including

turn to turn, phase to phase

and winding to ground.

4.44 (a)

Concept of customized Induced test:



17

By applying induced voltage up to

rated voltage,

basically

the

no

load

test is being repeated with reduced

induction in

the

core


.

.x

x

x

x

.

.x

x

x

x




x

x x x

Investigation Procedure

1

Oil Sample

OK2

Repeat TTR & DC Resistance

OK

3

Observe The No Load Test

PROBLEM

4Apply Load

5

Insulation Test?

Not Convinced to apply

6

Apply reduced “Induced Voltage”

Eddy Current Loss

Dependent on

the

square

of

frequency but is also directly

proportional to the square of

the thickness of the material

Hysteresis Loss




pp y

Test

OK

Load Loss:

‐Resistive losses or R

‐Eddy current losses in the windings

‐Stray losses in leads, core plates and tank

Proportional to the frequency and dependent onthe area of the hysteresis loop, which, in turn, is a

characteristic of the material and a function of thepeak flux density




Core bolts

are

inserted

through the core for the

f l i th




purpose of clamping the

core

laminations.

During “Core Stacking Process” –

Holes built for Core Bolts, used for proper core stacking




Core Plates Core Bolts




Photo belongs to another transformer

Core Plate

Core Bolt

Weld




Fiberglass insulation




Round Head Carriage Bolt

Metal Washer




In this case, the low impedance path formed by the

bolts and the core clamping plates causes a local shortcircuit path which produces intense local eddy currents.

The amount of heat generated by this phenomenon is

sufficient to considerably damage the adjacent areas.




The problem was noticeable in No‐Load test since there

was higher induction to create higher current in the

through bolts when compared to reduced induced test.

Increase

in

the

Load

Loss

increased

the

probability

of

“Core Plates” related issues.




This picture shows the correct insulation of the core bolts

Photo belongs to another transformer .

Insulation Material

Thank You




Thank You

Understanding

Transformer



Factory Testing

September 30, 2012

On some occasions additional methods must be

employed to determine the suitability of tested

transformer.

These techniques may include calculated corrections

Transformer Temperature Tests



Understanding Transformer Factory Testing 2

These techniques may include calculated corrections

or multiple tests at different loading conditions, etc.

Lets look at two actual factory cases:

Case 1: Good Test Results – Bad Data

Case 2: Bad Test Results – Good Transformer

Core

AVERAGE

WINDING

TEMP.WINDING

HOTTEST

SPOT

TOP

OIL

TEMP.

Cooling

Top Oil Temp.

Top

Oil

Gradient . x H.S.F

TEMPERATURE DISTRIBUTION

Transformer Temperature Tests




Coils

Oil

D i s t a n c e

Temperature

AverageOil

Bottom

Oil

Avg. Wdg.Temp.

Hot Spot

Gradient

Oil

Ambient

UAT 39/52/65 MVA; 230 - 6.9 (XV) & 4.16 (YV) kV 60Hz(+15-5% LTC for YV)

• Heat run test was performed according to ANSI/IEEEStandards and the clients technical specification.

Case 1: Good Results – Bad Data




• Temperature results were well below Standard limitsand according to client’s specification.

• Very Clean DGA Results.

• Test Results did not match design data?

Short-Circuit Method – Three Phase, 3-Winding:

L3S

H O R T C I R C U I T

Case 1: Good Results – Bad Data




Measurement

System Unit Under Test

YV

HV

XV

L3

L2

L1

~

S H O R T C I R

C U I T

S

Case 1: Loading Cycle




Test Corrections LimitTotal Losses (kW ) 172.600 228.464

Tap Position 1R 1R

Average Oil Rise 42.9 51.2

Top Oil Rise 49.6 59.2 65.0

Winding Gradient, YV 10.2 10.2

Winding Gradient, XV 2.3 2.3

Winding Gradient, HV 3.3 3.2

HS over TOR, YV 11.2 11.2

HS TOR XV 2 5 2 5

Case 1: Heat Runs Results

Winding es a e RatioHV 99.5 97.9 0.984

XV 1757.0 1757.0 1.000

YV 2483.0 2483.0 1.000

Winding Current for Individual Gradient Runs




HS over TOR, XV 2.5 2.5

HS over TOR, HV 3.6 3.5

Hot Spot Factor , YV 1.10 1.10

Hot Spot Factor , XV 1.10 1.10

Hot Spot Factor , HV 1.10 1.10

Average Winding Rise, YV 53.05 61.3

Average Winding Rise, XV 45.13 53.4

Average Winding Rise, HV 46.18 54.4

Hot Spot Rise, YV 60.8 70.4

Hot Spot Rise, XV 52.1 61.7

Hot Spot Rise, HV 53.2 62.7

65.0

80.0 n: 0.63

m: 0.80

Exponents

n ng est ate at o

HV 97.9 97.9 1.000

XV 2149.1 1757.0 1.223

YV 1791.5 2483.0 0.722

Winding Current for Oil Rise Run

MVA

Cooling Mode

Tested Design

Losses (kW) 228.682 228.464 Guar.

Top Oil Rise 59.2 51.6 65.0


Bottom Oil Rise 39.8 27.9

Gradient 10.2 12.8

Average Winding Rise 61.4 52.5 65.0

Heat Run Result vs Design Data

YV Winding

39.0

ONAN

Case 1: Temp Rise not Expected





Hot Spot Gradient 11.2 14.1

Hot Spot Rise 70.4 65.7 80.0

Gradient 2.3 11.1



Hot Spot Rise 61.7 63.8 80.0

Gradient 3.2 10.5



Hot Spot Rise 62.7 63.2 80.0

HV Winding

XV Winding Why so far off?

Time CH 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 0:00 1:00 2:00 3:00 5:00Measured kW 171.0 173.2 172.0 171.1 173.0 172.9 173.6 172.0 172.0 172.3 172.5 172.6 Take Take Take

Measured Amps 101.6 102.0 103.0 101.5 102.0 102.0 100.0 99.0 99.1 99.4 99.6 99.5 HV XV YV

Upper Radiator 1 2 77.43 78.66 79.56 80.10 81.55 82.66 83.83 84.18 84.57 84.73 84.92 85.09 84.98 82.27 83.61

Upper Radiator 2 7 77.58 78.78 79.78 80.30 81.88 82.68 84.65 85.03 85.42 85.51 85.58 86.10 85.70 82.91 84.36

Average Upper Rads 77.51 78.72 79.67 80.20 81.72 82.67 84.24 84.61 85.00 85.12 85.25 85.60 85.34 82.59 83.99

Lower Radiator 1 1 67.51 68.24 69.22 70.25 71.47 72.57 73.53 74.16 73.77 72.81 73.61 73.88 74.69 70.88 71.83

Lower Radiator 2 3 63.81 64.85 65.87 66.88 67.85 68.83 69.83 69.93 70.26 69.60 70.42 70.41 69.66 66.67 68.78

Average Lower Rads 65 66 66 55 67 55 68 57 69 66 70 70 71 68 72 05 72 02 71 21 72 02 72 15 72 18 68 78 70 31

Case 1: Heat Runs Temp. Log




∆T (3 Hr) > 2 ºC

Average Lower Rads 65.66 66.55 67.55 68.57 69.66 70.70 71.68 72.05 72.02 71.21 72.02 72.15 72.18 68.78 70.31

Ambient # 1 4 37.86 37.96 38.00 40.00 40.75 41.27 41.64 41.94 41.70 41.35 40.92 40.53 0.00 0.00 0.00

Ambient # 2 11 38.79 38.95 39.08 38.90 39.20 39.56 39.72 39.52 38.11 37.85 37.25 37.40 0.00 0.00 0.00

Ambient # 3 12 35.78 35.88 36.88 36.20 37.88 38.95 39.81 39.78 39.33 39.06 38.81 38.41 0.00 0.00 0.00

Average Ambient 37.48 37.60 37.99 38.37 39.28 39.93 40.39 40.41 39.71 39.42 38.99 38.78 0.00 0.00 0.00

Top Oil Temp 6 81.21 81.46 82.00 83.25 84.52 85.88 87.95 89.16 89.98 90.47 91.07 91.36 91.12 88.60 89.87

Top Oil Temp DV 78.00 80.00 81.00 82.80 83.00 83.50 84.00 85.00 84.00 83.95 85.00 85.40

Average of Top Oil 79.61 80.73 81.50 83.03 83.76 84.69 85.98 87.08 86.99 87.21 88.04 88.38 91.12 88.60 89.87

Averge Oil Rise @ 3300' 36.21 37.05 37.45 38.84 38.46 38.78 39.31 40.39 40.79 40.83 42.42 42.88 84.54 81.69 83.03

Top Oil Rise @ 3300' 42.13 43.13 43.51 44.66 44.48 44.76 45.59 46.67 47.28 47.79 49.04 49.60 91.12 88.60 89.87

Bot tom Oil Rise @ 3300' 28.18 28.95 29.56 30.20 30.38 30.77 31.29 31.63 32.30 31.79 33.02 33.37 72.18 68.78 70.31

Test Corrections LimitTotal Losses (kW ) 172.600 228.464

Tap Position 1R 1R


Top Oil Rise 49.6 59.2 65.0

Winding Gradient, YV 10.2 10.2

Winding Gradient, XV 2.3 2.3

Winding Gradient, HV 3.3 3.2

HS over TOR, YV 11.2 11.2

HS over TOR XV 2 5 2 5

Case 1: Heat Runs Results

Winding es a e RatioHV 99.5 97.9 0.984

XV 1757.0 1757.0 1.000

YV 2483.0 2483.0 1.000

Winding Current for Individual Gradient Runs




HS over TOR, XV 2.5 2.5

HS over TOR, HV 3.6 3.5

Hot Spot Factor , YV 1.10 1.10

Hot Spot Factor , XV 1.10 1.10

Hot Spot Factor , HV 1.10 1.10

Average Winding Rise, YV 53.05 61.3

Average Winding Rise, XV 45.13 53.4

Average Winding Rise, HV 46.18 54.4

Hot Spot Rise, YV 60.8 70.4

Hot Spot Rise, XV 52.1 61.7

Hot Spot Rise, HV 53.2 62.7

65.0

80.0 n: 0.63

m: 0.80

Exponents

n ng est ate at o

HV 97.9 97.9 1.000

XV 2149.1 1757.0 1.223

YV 1791.5 2483.0 0.722

Winding Current for Oil Rise Run

• Stable oil temperatures must be met to achieve

reasonably accurate winding gradient measurements.

• Accurate cold resistance temperature measurements

are critical in determining the winding rises.YV hot resistance on Tap 1R – Cold resistance not

Case 1: Summary




p

measured, used Tap 1N Not valid.

• Simulated load losses should be close to the expected

load losses for the transformer during operation.

Actual winding currents not measured during

simultaneous loading.

GSU 820 MVA 362 / 25 kV DETC(±5%) 60Hz

• Heat run test was performed according to IEEE/ANSIStandards and the clients technical specification.

• Temperature results were below limits and according to

Case 2: Good Unit – Bad DGA




Temperature results were below limits and according toclients requirements.

• DGA performed after heat run test found gasgeneration above client and the manufacturer’sacceptance limits.

Case 2: Bad DGA Results

Outside Lab Change Client LimitsSample # 1 2 3 - -

Description

Before Heat

Run [ppm]

4 hours after

Heat Run [ppm]

4 hours after

Heat Run [ppm]

Gas Evolution

[ppm]

Gas Evolution

[ppm]

H2 - Hydrogen 4 22 17 13 10O2 - Oxygen 3989 2314 300 - -N2 Nit 11045 12181 9050

In House Lab




N2 - Nitrogen 11045 12181 9050 - -

CO - Carbon Monoxide 10 62 50 40 25

CO2 - Carbon Dioxide 82 392 250 168 200

CH4 - Methane 0 3 14.4 14.4 5C2H4 - Ethylene 0 5 4 4 2

C2H6 - Ethane 0 27 22 22 2

C2H2 - Acetylene 0 0 0 0 0

All measured temperature rises within calculated tolerances.

5.

TX Hot

Spot

2.

Pump

Problem

1.

Bad DGA

Sample

Case 2: Possible Causes




4.

Stray

Gassing

3.

Improper

Testing

Why ?

1. Bad DGA Data?

• DGA results of the outside lab matched the results

obtained at factory.

2. Bad Pump (s) ?The most probable cause of pump overheating is the





pump running backwards.

• Running ratings matched Nameplate• No Noise

• Thermal Scan normal for pumps and oil flow


3. Improper Testing Method ?

• Loading per IEEE/Expedited Heating

• Fiber Optic Sensors in Coils – No high temperatures

• Thermocouples on structural metal parts – Normalheating




• Ambient temperature was below 40 ºC

• Total heat load (kW) matched cooler rating• Maximum current was only 112% of rated/ Less

than 7 percent of allowable continuous overload

current.

Only two possible causes left:

4. An oil problem due to “thermal stray gassing”.

Or 5. An abnormal transformer hot spot.


An experiment

is needed!




How is the gassing influenced ?

• Load dependent

Case 2: Experimental Loading




• Oil temperature dependent

Step A: Transformer Rated Conditions

1. Test Floor Open and Ventilated

2. All Pumps & Fans On





2. All Pumps & Fans On

3. Full Rating Conditions of the transformer

Step B: Simulate Stray Gassing

1. Test Floor Closed2. Reduced Current





3. All Pumps on/ Fans adjusted to keep oil at ~ 90 ºC

Case 2: Experimental Method

Test

Number

Load

Condition

Oil

TemperatureComments

Test #1 Overload High OilTemperature This is the intial heat run result. The source of gassingis indeterminate.

Test #2 Rated LoadNormal Oil Gassing under this condition is most likely not from the




Test #2 Rated LoadTemperature oil.

Test #3 Reduced LoadHigh Oil

Temperature

Gassing under this condition is most likely not from the

transformer.

Test #4 OverloadNormal Oil

TemperatureGassing is most likely from the transformer.

Case 2: Experimental Method

TestNumber

Duration[Hours]

Criteria Loadingoa[%]

Top OilTemp.

Coil OilTemp.

Ambient[ºC]

8.0 Per IEEE Total Heat

Load 108.0 73.0 90.3 37.6

1.0 Per IEEE Rated

Current 100.0 70.0 92.3 38.4

Test #2 8.0 - Rated

Current 100.0 53.5 78.8 24.0

Test #3 8 0 Reduced

80 0 83 5 92 6 35 2

Test #1




Test #3 8.0 -Curr.

80.0 83.5 92.6 35.2

7.5 Per IEEE Total Heat

Load 108.0 62.5 87.8 26.0

1.0 Per IEEE Rated

Current 100.0 59.7 81.0 26.0

Test #4

Case 2: Experiment Results

Test # 1 2 3 4Gas

Evolution

[ppm]

Gas

Evolution

[ppm]

Gas

Evolution

[ppm]

Gas

Evolution

[ppm]

H2 - Hydrogen 13 0 12 3 10

O2 - Oxygen - - - - -N2 - Nitrogen - - - - -

CO - Carbon 40 10 36 12 25

Criteria




CO2 - Carbon Dioxide 168 66 272 105 200

CH4 - Methane 14.4 1.8 8 4.2 5

C2H4 - Ethylene 4 0.6 1.7 1.3 2

C2H6 - Ethane 22 0 13.6 0 2

C2H2 - Acetylene 0 0 0 0 0

CO2/CO Ratio 4.2 6.6 7.6 8.8 < 3

• Most of the gas concentrations exceed the customerlimits.

• Dominant gasses are Methane, Ethane and Hydrogen(low temperature gasses or thermal stray gassing).

• No cellulose decomposition.

Case 2: Test #1 Results




p

The Source of the Excessive Gassing

is Indeterminate.

• Gassing, all gasses within acceptance limits.

• No dominant gasses.

• No cellulose decomposition.





If there was excessive gassing it would

likely be from the transformer active

parts.

• A gasses exceeding limits except Ethylene.

• Dominant gasses are Methane, Ethane and Hydrogen.

• No cellulose decomposition.• In this test the load is reduced no gassing can be

correlated with the transformer.





• Dominant gasses are Methane, Ethane and Hydrogen,

this is an indication of possible thermal stray gassing.• The gassing results are similar Test #1.

This excessive gassing is likely from

the oil.

• All gasses within acceptance limits.

• Dominant gasses are Methane, Ethane and Ethylene.

Typical gasses for a heat run test without additional stray

gassing.• No cellulose decomposition.

• The absence of gasses confirm that gas generation is





The absence of gasses confirm that gas generation is

not related to the load or a transformer condition.

If there was excessive gassing it would

likely be from the transformer active

parts.

• The transformer successfully passed the heat run testaccording to ANSI/IEEE Standards.

• Test #3 results closely match with Test #1 and areindicative that the source of gasses during heat runtest is thermal stray gassing of the oil.

Case 2: Summary




• Gasses generated during heat run test performed areproduced by thermal stray gassing of the oil used forFAT.

• The Doble Oil Lab confirmed the stray gassingtendency of the oil used for the factory heat run.

CONCLUSION

Understanding Power Transformer Factory Test Data

Documents

Transcript of Understanding Power Transformer Factory Test Data