knowledge is power sm apparatus maintenance and power management for energy delivery assessing the...

Knowledge Is PowerSM

Apparatus Maintenance and Power Management for Energy Delivery

Assessing the Magnetic Circuit of a TransformerJill DuplessisDoble Engineering Company

2002 Regional Seminar - Denver

Objectives•To provide an explanation of what we are learning about a transformer when we perform an exciting current test and a leakage reactance test.

Magnetic Circuit of a Transformer


•To review which tests Doble recommends that you should be performing & when.•To review how one goes about analyzing results from these tests.•Finally, to reinforce what we learn by reviewing case studies together.

Fundamental Principle of Operation

Energy Transfer from one electrical circuit to another.

A Transformer


Not perfect:

•Some energy is lost and dissipated as heat.•Some energy is temporarily stored.

Assuming a 1:1 turns ratio, the equivalent circuit of an Ideal Transformer looks like this:

Equivalent Circuit of an Ideal Transformer


Energy In

Energy Out

Since there are no losses in an ideal transformer, Energy In = Energy Out

If the energy transfer process was perfect, we’d be talking about an ideal transformer.

Equivalent Circuit of a Transformer


From an energy transfer point-of-view, the elements in this circuit represent the imperfections in a transformer.

Exciting Current and Loss measurement, Zm

Leakage Reactance and Loss measurement, ZL

Dielectric loss (measured in overall tests is lumped in with Rm)

Primary winding dc resistance measurement

Secondary winding dc resistance measurement

Lm CUST

Rm

R L-1 L 1 L 2 R L-2R DC-1 R DC-2


Practical Transformer vs. Ideal Transformer

•Losses occur due to the following imperfections in a transformer:

Losses in a Power Transformer

•Windings have resistance•Real and reactive losses exist in the core•Physical cores have a finite permeability; exciting current is required to produce magnetic flux•There is magnetic flux leakage•Losses in the dielectric circuit

What we measured in the overall tests on a xfmr.

DC resistance tests

Exciting current tests

Leakage reactance tests

Good News

Losses in a transformer are specified & controlled.

Losses in a Power Transformer


Manufacturer bases price on guaranteed losses.Manufacturer designs adequate cooling for a transformer based on losses.even though losses represent a cost to the user, from a diagnostic perspective, we can use loss info to verify the integrity of the unit.We are looking for evidence of a change in the known losses of the transformer.

A History Lesson in Magnetism:

Before we get started...


1820 - Hans Christian Oersted discovered that when an electric current flows through a wire, it causes a compass needle to rotate.

Michael Faraday - his ideas about conservation of energy led him to believe that since an electric current could cause a magnetic field, a magnetic field should be able to produce an electric current.

i.e. he discovered that an electric current produces a magnetic field.

Faraday demonstrated this principle of induction in 1831 with the following experiment:

History of Magnetism


•He moved a coil of wire relative to a magnet & discovered that a voltage was induced in the coil.

Michael Faraday demonstrated the phenomenon of electromagnetism in a series of experiments.

(but only when relative movement is taking place)

•Responsible for the principles by which electric generators and transformers work.

For example We apply Faraday’s discovery to the arrangement where a magnetic field is associated with the turns of a winding.



Any variation in the strength of the magnetic field will induce a voltage between the terminals of the winding.

No voltage is produced if the magnetic field strength is constant.

(This time - we are not moving the winding, it stays stationary. Instead, we vary the magnetic field same effect.)

Next consider a winding through which a current is passed.



(Remember that Oersted proved that a current flowing through a winding will produce a magnetic field within the winding & in the space surrounding the winding).So, when the current is varied (as by applying an a.c. source), the strength of the magnetic field produced by the winding will vary.If we place a 2nd winding near this 1st winding, the 2nd winding will enclose some of the magnetic field produced by the 1st.



Since the varying magnetic field produced by the 1st winding is “linked” by the 2nd winding, a voltage will be produced between the terminals of the 2nd winding.

A Word about Linking:

If windings are only in close proximity to each other, linking or coupling between them is not very effective.

a considerable amt. of the magnetic field produced by the 1st wdg. does not link the 2nd wdg.

How can we improve the linking between windings?



By arranging the windings relative to each other using a structure of magnetic material (the core).The core uses suitable magnetic material (usually silicon-iron) that allows a very high degree of coupling between windings.

UNFORTUNATELY...The core material is not perfect.

Recall that one of the properties that differentiates an ideal transformer from an actual transformer sitting in a substation is that ...

Electromagnetism Background


We see this...During an open-circuit measurement in which we observe that a small (usually inductive) current is drawn at the primary terminals even though the secondary terminals are open.

•Physical cores have a finite permeability exciting current is required to produce magnetic flux in the core

Example of Core Characteristics


Permeability (slope) - the ability of a material to conduct flux

Illustrates affect of core construction on magnetizing/ hysteresis effects.

When the Secondary Winding Is Open

Exciting Current Theory


E1

Iex

1:1

+

-E2

The current that flows in the primary winding should be sufficient to excite the core.

If Load was Connected to the Secondary



IIexex

EE22

1:11:1

11

EE11

++

--

The primary current increases by the value of the secondary current.

ZZLL

II22

22

+ I+ I2222

When We Have a Turn-to-Turn Fault on the Secondary During the Exciting Current Test



1:11:111

HH11

HH00

++EE11

--

HVHV

LVLV

IIff

ff

ffIIexex + I+ Iff

The primary current increases by the value of the current through the short-circuited turns.

Detection of Winding to Ground Fault in the Secondary During Exciting Current Test?



1:1

H0

H1

+

-

HV

LV

Iex

If

f

+ If ff

If secondary winding is and one of the windings develops a fault to ground, the primary current will increase by the value of current circulating through the secondary winding and two grounds.

How Do We Detect Fault in the Preventive Autotransformer During Exciting Current Test?



1:1

H1

H0

HV

LV

Iex

Ia

a

+ Iaaa

When autotransformer is connected across two taps it acts as a load and the primary current goes up.

Useful in Detecting:

Exciting Current Tests


•Turn-to-turn winding failure

•LTC problems

•1 or more turns completely short-circuited.

•Open circuit, shorted turns or high resistance connections in the LTC P.A., series auto or series transformer

•2 or more parallel strands of different turns are short-circuited.

•misalignment, mechanical problems, coking and wear of LTC & DETC contacts

Useful in Detecting (cont):

Exciting Current Tests


•Manufacturing defects.

•Abnormal (multiple) core grounds.

•Changes in the core characteristics.

Exciting Current Test Procedure - Delta


H-V Test Cable

L-V Lead

GND Lead

I&W Meter

Guard Point

H2

H3

Ie (1-2)

Ie (1-3)

L-V Lead

GND Lead

I&W Meter

GuardPoint

H2H3

Ie (1-2)

Ie (1-3)

UST Mode

Note: For Exciting Current tests performed with the M4000, the charging current (mA) and watts-loss are recorded.

Exciting Current Test Procedure -Wye


H-V Test Cable

L-V LeadI&W Meter

Guard Point

H2

H1

H3

Ie (1-0)

H0

L-V Lead

I&W Meter

GuardPoint

H2H1H3

Ie (1-0)

H0

UST Mode

Important!!!!


Test Measurement Recommendations

Especially Important!!!!


Test Measurement Recommendations

TEST RESULTS ANALYSIS

Analysis


•It is useful to know whether specimen is capacitive or inductive

•LTC and phase patterns should be analyzed

•Watts loss is always determined by the core

So...•What do we mean by LTC & phase pattern?•What makes a specimen inductive rather than capacitive or vice-versa?

LTC and Phase Pattern


Nomenclature

•LTC pattern•The relationship between exciting current (or loss) measurements recorded within a phase as the LTC is moved from one position to another.•12 LTC patterns

•Phase Pattern•The relationship between exciting current (or loss) measurements recorded for all three phases at a single tap position.•3 Phase patterns

To understand what makes a specimen capacitive or inductive, we revisit the equivalent circuit of a transformer.

Capacitive or Inductive Specimen?


Lm CUST

Rm

R L-1 L 1 L 2 R L-2R DC-1 R DC-2

Exciting Current and Loss measurement, Zm

Practically all of the magnetic flux is confined to the core. the impedance encountered by the current is predominantly determined by the reluctance of the core.

we can neglect the energy storage and loss in the leakage channel.

I2R loss is much lower than loss in the core

Equivalent Circuit of the Open-Circuit Test reduces to:



I C R

IC

IL

R

ex

I

I

I

I

V

Q

L C R

L I I

ex

V

L - Magnetizing InductanceC - Turn-to-turn CapacitanceR - Resistance associated with losses in the core & turn-to-turn insulation



•Inductive LTC pattern•Magnetizing current capacitive current in each tap position, so that the resultant measured current is always inductive in nature.

•Characteristic of the vast majority of exciting current test results reported for transformers.

•Capacitive LTC pattern•Capacitive current magnetizing current, at several tap positions.

Analysis for Inductive Specimens


For an inductive specimen (majority of xfmrs):

•The phase pattern at each tap position should be confirmed.•This pattern should be identical at every tap position, for both mA & Watts measurements.

•The LTC pattern should be identified by comparing the behavior of the test data with one of the 12 documented LTC patterns.•The LTC pattern should be the same in each of the 3 phases, for both the mA results & the Watts results.



•3-legged core-type transformer with a delta-connected winding if testing two phases of the winding in parallel.

•3-legged core-type transformer that has a wye-connected winding with an inaccessible neutral.

L-H-L Characteristic of: Phase Pattern B

•5-legged core or shell-type transformer that has a delta-connected secondary winding

•3-legged core-type transformerH-L-H Characteristic of: Phase Pattern A•Possible phase patterns:

Wye Winding with No H0 Bushing


All 3 Readings Dissimilar (H-M-L)•May be indicative of a magnetized core



•May actually be “capacitive” specimen - not inductive after all

•Characteristic of four and five legged core-type transformers and shell-type transformers with non-delta secondary windings

All 3 Readings Similar Phase Pattern C

FACTORS OTHER THAN DEFECTS THAT MAY INFLUENCE TEST RESULTS:

Analysis


•UST capacitance

•Test voltage

•Residual magnetism

•Design and position of LTC

•Test Connections

If capacitance inductive component, you have a capacitive specimen & analysis changes.

Test results are voltage dependent so data can only be compared if performed at identical voltages.

Experience with Capacitive LTC Patterns

•Effects documented as early as 1972

Capacitive LTC Patterns


•1996 - 3rd Component of Exciting Current discussed in detail

•Negligible in Low-voltage Transformers

•Traditionally, IC Im in High-voltage Transformers

•Today, IC may be of same order of magnitude or Im

•Due to Reduced losses & magnetizing power of xfmr cores•Due to high capacitance windings

Effects of a Strong Capacitive Presence

•PHASE PATTERN IS AFFECTED

Capacitive LTC Patterns


•Depends on Relative Magnitudes of Im and IC in Each Phase•Typically all 3 Phases are Capacitive

•CAN RESULT IN ANY PHASE PATTERN

• Measured Phase Pattern Accepted as Benchmark

•Phase Pattern for Current may Differ from Phase Pattern for Loss

Test Voltage

Factors other than defects that can influence test results.


C, I [mA] ICL

5 10 V [kV]

I

IL

IL < IC

IL > IC

RESIDUAL MAGNETISM

Factors other than defects that can influence test results.


•Always present, but in most cases has no significant effect on test results.

•Majority of problems have a much larger effect (> 50%) on test results than residual magnetism would have

•Increases current if specimen is inductive•Increases or decreases current if specimen is capacitive

Example of an LTC Pattern


Test results for all non-bridging positions are equal. Test results for all bridging positions are equal.

Pattern 1:

I1

N 4R 8R 12R 16R

Example of an LTC Pattern


Test results for all non-bridging positions are equal. Test results for all bridging positions are equal, except in one or several positions, with all readings in these positions being equal as well.

Pattern 2:

N 4R 8R 12R 16R

Exciting Current Testing


Case Studies

Case Number 02-06

Unit Tested:

U.S. Transformer, 3Φ two-winding transformer

Exciting Current Case Study 1


•20 MVA

•69/12.47 kV

•Rewound in 2000

•1978 - vintage

•Δ-Y connected

Testing Circumstances:

•Tested upon receipt from the manufacturer’s repair facility where it had been completely rewound.

Case Study 1 (# 02-06)


•overall insulation tests - acceptable

•bushing tests - acceptable

•field power factor test on an oil sample from the main tank - acceptable

Case Study 1 - Exciting Current Results


Tap Position A-phase(mA)

B-phase(mA)

C-phase(mA)

A-phase (W) B-phase (W) C-phase (W)

N 20.06 8.72 19.05 149.6 66.5 142.81R 82.54 72.04 81.98 161.7 75.91 155.32R 20.16 8.94 19.14 150.1 69.06 143.33R 82.7 72.2 82.14 162.7 80.92 156.2

4R 20.37 9.61 19.36 151.4 76.53 144.75R 82.94 72.45 82.37 164.5 90.76 158.26R 20.69 10.72 19.68 153.5 89.04 1477R 83.26 72.86 82.68 166.8 106.2 161.18R 21.09 12.32 20.28 156.3 106.5 152

9R 83.62 73.42 83.39 169.9 126 171.910R 21.57 14.42 22.4 159.7 128.9 17611R 84.07 74.18 84.27 173.6 150.5 194.712R 22.13 17.04 23.93 163.9 156.3 190.513R 84.54 75.17 85.17 178.1 180.4 213.6

14R 22.78 20.18 25.8 168.6 188.7 21115R 85.09 76.45 86.21 183.1 215.3 234.616R 23.49 23.85 27.88 173.9 226.2 232.9

LTC Pattern 4


Test results represent a series transformer or autotransformer exciting current superimposed on pattern 1. This current changes according to increments in the tap winding.

Pattern 4:

I1

N 4R 8R 12R 16R

LTC Pattern - Phase A


Phase A

0

12

24

36

48

60

72

84

N (1R) 2R (3R) 4R (5R) 6R (7R) 8R (9R) 10R (11R) 12R (13R) 14R (15R) 16R

LT C Posit ion

[mA

] B

ridg

ing

0

6

12

18

24

[mA

] N

on-b

ridg

ing

Bridging Positions Non-bridging Positions

LTC Pattern - Phase B


Phase B

0

12

24

36

48

60

72

84


LT C Posit ion

[mA

] B

ridg

ing

0

5

10

15

20

25

[mA

] N

on-b

ridg

ing

Bridging Positions Non-bridging Positions

LTC Pattern - Phase C


Phase C

0

14

28

42

56

70

84


LT C Posit ion (Non-bridging superimposed on Bridging)

[mA

] B

ridg

ing

0

6

12

18

24

[mA

] N

on-b

ridg

ing

Briding Posit ions Non-bridging Posit ions

Phase Pattern (bridging positions) - Current


60

65

70

75

80

85

90

1R 3R 5R 7R 9R 11R 13R 15R

[mA

]

A-phase b. B-phase b. C-phase b.

Phase Pattern (non-bridging positions) - Current


0

5

10

15

20

25

30

N 2R 4R 6R 8R 10R 12R 14R 16R

[mA

]

A-phase n.b. B-phase n.b. C-phase n.b.

Phase Pattern (non-bridging positions) - Watts


0

50

100

150

200

250

N 2R 4R 6R 8R 10R 12R 14R 16R

[W]

A-phase n.b. B-phase n.b. C-phase n.b.

Phase Pattern (bridging positions) - Watts


0

50

100

150

200

250

1R 3R 5R 7R 9R 11R 13R 15R

[W]

A-phase b. B-phase b. C-phase b.

Problem Found


Investigation

•Manufacturer electrically isolated the series transformer for tests; exciting current tests indicated a definite problem on the center phase.

The short was at the bottom of the coil between the first turn and the bottom lead. At the location where the lead enters the coil and bends, the insulation on the top strand of the lead and the bottom strand of the first turn was cut, allowing the two strands to come into contact with each other. The factory’s normal practice includes taping a NOMEX pad in between the lead and adjacent strands for added protection since there is a risk of damaging the insulation in this area by moving the leads around. In this case, the pad was missing.

•Problem: a short between turns in the outer coil of the center phase of the series transformer.

Comments


The insulation failure that caused the strands to short affected the current circulating through the series transformer in all LTC positions.

•partial turn-to-turn short circuit acted as a load on the transformer.

in-phase, or loss, component of the exciting current exciting current magnitude

Why, in the bridging tap positions, was the problem not noticeable in the exciting current measurements while it was in the loss readings?

Capacitive LTC patterns

Unit Tested:

General Electric, 3Φ two-winding transformer

Case Study 2 (# 02-09)


•7.5 MVA

•67/12.5 kV

•G.E. Type LRT-200A LTC

•1982 - vintage

•Δ-Y connected

Testing Circumstances:

•Concern from utility owner that the protection circuitry for the vacuum bottles in the LTC was not working properly.

Case Study 2


•X1 LTC lead “S” was twisted, which caused the X1 bypass switch to be out of synchronization with the other two phases.

•Power factor & TTR test results - normal

•Exciting current results - unusual

Exciting Current Data: LTC Pattern Analysis


LTC PATTERN 2

Non-Bridging Tap Positions - Current Measurements

Exciting Current Data: Phase Pattern Analysis


Phase Pattern B

Current Measurements - Phase Pattern B

Explanation of Phase Pattern in N.B. Positions


IQ = Quadrature

Component of Exciting Current ~ Measured Exciting Current (L-H-L)

IL = Icore

IC

IQIQ

ICICIL

IL

IL

Non-Bridging Tap Positions - Loss Measurements



Phase Pattern A

Bridging Tap Positions - Loss Measurements



Phase Pattern A

Bridging Tap Positions - Current Measurements



Phase Pattern A

Explanation of Phase Pattern in B. Positions


Icore

IC

IQ

IQ

ICIC

Icore

Icore

IPAIPAIPA

IL = Icore + IPA

IQ = IL + IC =

Quadrature Component ~ Measured Exciting Current

(H-L-H)

Exciting Current Measurements


N 1L 2L 3L 4L 5L 6L 7L 8L 9L 10L 11L 12L 13L 14L 15L 16L

H1-H2

H3-H10

10

20

30

40

50

60

[mA]

LTC Position

Watts Measurements


N 1L 2L 3L 4L 5L 6L 7L 8L 9L 10L 11L 12L 13L 14L 15L 16L

H1-H2

H3-H10

10

20

30

40

50

60

70

80

90

100

[W]

LTC Position

Doble Turns Ratio Test


Doble Transformer Turns Ratio

Benefits of the Turns Ratio Test


•Confirm nameplate ratios

•Detect short-circuited turn-to-turn insulation

•Detect open-circuited windings

Doble TTR Test Procedure


UST

HV Lead

LV Lead

Doble TTR Capacitor

CTRUE

CTRUE=I

V x

Doble TTR Test Procedure


UST

HV Lead

LV Lead

Doble TTR Capacitor

V1

N VV

1

2

CIV

apparent2

CI

V /N)apparent

1

(

Now, if we take the ratio:

CTRUE/CApparent

We obtain:

N - the turns ratio

Doble TTR - DTA Screen


TTR Case Study


Unit Tested:


•5 MVA•50.5/13.09 kV•1980 - vintage

•Δ-Y connected

TTR Case Study


Exciting Current Test Results (4/24/02)

TTR Test Results (4/24/02)

Previous TTR Test Results (7/25/95)

TTR Case Study


Leakage Reactance Tests


Doble Leakage Reactance Test

Leakage Reactance Test


IIexex

EE22

1:11:1

11

EE11

++

--

ZZLL

II22

22

+ I+ I2222

The combined action of both currents results in some of the flux being present in the unit permeability space.

Leakage Flux


The unit permeability space includes the space between the windings, w/in the windings & between the windings and the tank.

LLFlux that is not confined to the core for the entire length of its path.

•The primary winding is linked by almost all of the leakage flux in addition to the magnetizing flux, while the secondary winding is linked by the magnetizing flux but very little of the leakage flux.

Leakage Flux


the primary winding has a greater voltage induced in each of its turns under load than the secondary winding.

•We can account for this voltage drop by introducing a leakage reactance.

Leakage Reactance Equivalent Circuit


Lm CUST

Rm

R L-1 L 1 L 2 R L-2R DC-1 R DC-2

EE22EE11

21 1 2 L-R L- L L R 2R DC-1 R DC-

EE22EE11

Short-Circuit Impedance

Leakage Reactance

Leakage reactance for most xfmrs is constant & can be measured w/out the presence of the “full load” leakage flux that requires full load current.

Leakage Channel


Leakage channelOuter winding

Inner winding

Top yoke

Bottom yoke

Core leg

The leakage flux path includes the regions occupied by the windings. The leakage reactance may be sensitive to deformations in the windings.

•Confirm nameplate impedance

Benefits of the Leakage Reactance Test


•Investigate winding deformations•Due to through faults•Due to rough handling during transportation

•Easy to perform with the proper additions to the M4000 (M4110 Module)

Capacitance:•Sensitive to temperature & contamination•Normally involves all three phases

Capacitance versus Leakage Reactance


Leakage Reactance:•Not sensitive to temperature & contamination•Can be performed on a per-phase basis•Better sensitivity to winding deformations•Can compare results to N/P Impedance

Excitation Current Tests:•More sensitive to core problems than winding deformations

Initial test:•Perform Three-Phase Equivalent test for comparison to Nameplate•Perform Per-Phase tests to act as benchmark for future tests

Test Procedures


Subsequent tests:•Perform only Per-Phase tests for comparison to benchmark tests

Doble M4110 Leakage Reactance Test Set

Leakage Reactance Test


M4100 & M4110 L.R. Module Test Connections


Nameplate Data Required for LRT


3-Phase Equivalent Leakage Reactance Test

3-Phase Equivalent Leakage Reactance Test


Nameplate Data Required for LRT


Per-Phase Leakage Reactance Test

Per-Phase Leakage Reactance Test


•First, or benchmark test, should be within 3% of Nameplate.

Analysis


•Subsequent tests should be within 2% of benchmark.

•If all three phases on Per-Phase tests agree, it is likely that there is no winding deformation.

LRT Case Study


Unit Tested:


•80/89.6 MVA

•1961 - vintage

•114 GR. Y/65.8 - 13.2 kV

Background:

•During a short outage for normal generator maintenance, the 13.8 kV generator bus PTs were replaced due to PCB contamination.

LRT Case Study - Background


•During the replacement, the secondary wiring on the potential transformers was mistakenly reversed.

•The reversed potential to the synchronizing equipment allowed the generator breaker to be closed, and connected the generator into the transmission system 180 degrees out-of-phase.

•The generator remained connected to the system for 3.06 seconds and operated in an out-of-step manner for the entire period.

LRT Case Study - Background


•The generator was closed into the system, 180 degrees out-of-phase, a second time.

•The generator operated in an out-of-step condition for 200 ms.

•The initial current was about 26,600 amps on the 13.8 kV bus and then declined until the trip occurred.

•The second trip command was initiated by the step-up transformer sudden pressure relay, which tripped into a lockout relay. No further attempts were made to close the generator breaker.

LRT Case Study - Inspection


Inspection:•An external visual inspection was made and no apparent problems were found.

•The secondary wiring on the 13.8 kV bus PT’s was checked and wiring errors were discovered, corrected and tested.•The sudden pressure relay that initiated the second trip was found to have welded contacts and was replaced.

•An oil sample was taken from the transformer and sent in for DGA analysis; no change in gas quantities was found.

LRT Case Study - Investigation


% Power FactorInsul. kV mA Watts

Meas Corr

Corr.Factor

Cap(pF)

RTG

CH + CHL 10 185.6 6.642 1.01 49246CH 10 184.5 6.646 0.36 0.36 1.01 48952 G

CHL (UST) 10 1.101 0 0 0 1.01 292.1 GCHL 1.100 -0.004 -0.04 -0.04 1.01 294 GCL + CHL 10 152 5.190 1.01 40322CL 10 150.90 5.196 0.34 0.34 1.01 40028 ICHL (UST) 10 1.100 0 0 0 1.01 291.90 GCHL 1.100 -0.006 -0.05 -0.05 1.01 294 G

Overall Test Results - Post-fault (2000)

•Insignificant change in the capacitance of the LV winding to ground insulation occurred from 1986 to 1999. (There were no results available for this transformer prior to 1986).

Observations from Overall Test Results


•However, capacitance decreased from 42,371 pF in 1999 to a post-incident capacitance of 40028 pF, a change of 5.53%.

Observations


HV

LV

Inter-winding Shield

CL

CH

CH-S

CL-S

TankandCore

•Due to a grounded shield in between the HV and LV windings, the L-G measurement is actually a combination of insulation systems: CL & CL-S

•Changes in a capacitance measurement usually represent physical changes in the insulation system under measurement.

•The average short circuit impedance listed on nameplate was 10.02%.

Leakage Reactance Test Results


•The 3-phase Leakage Reactance measurement using the M4110 was 11.26%.•The 3-phase equivalent test deviated from the average short circuit impedance by 12.35%.

Three-Phase Equivalent

•The measured per-phase results were 11.92%, 10.79% and 9.12% for phases A, B and C, respectively.

Leakage Reactance Test Results


•The per-phase measurement deviated from the average by as much as 14%.

•An internal inspection was scheduled.

Per-Phase Tests:

Upon entering the transformer it was obvious severe damage had occurred. Damage to top clamping plates and wedge clamps.

Internal Inspection


Internal Inspection


•Damage on phase #1 was somewhat worse.

•Photo 5 shows top end ring pushed up about three inches

•Following the internal inspection, arrangements were made to replace the failed step-up transformer.

•During the disassembly of the transformer, it was expected to see some deformation in the windings from the forces which caused the damage to the clamping plates, wedge clamps and end rings.

Observations from Internal Inspection


•Actual damage to the windings was very minimal with no noticeable deformation.

•The sudden pressure relay operated due to a shock wave in the oil created by mechanical forces. This is assumed due to lack of gas generation and no hot spots were found.

Thank You!


QUESTIONS?

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