1. Assessing the Magnetic Circuit of a Transformer
Knowledge Is PowerSM Apparatus Maintenance and Power Management for Energy Delivery
Assessing the Magnetic Circuit of a Transformer
Jill Duplessis Doble Engineering Company
2002 Regional Seminar - Denver

2. Magnetic Circuit of a Transformer
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.
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.
2002 Regional Seminar - Denver

3. Fundamental Principle of Operation
A Transformer
Fundamental Principle of Operation
Energy Transfer from one electrical circuit to another.
Not perfect:
Some energy is lost and dissipated as heat.
Some energy is temporarily stored.
2002 Regional Seminar - Denver

4. Equivalent Circuit of an Ideal Transformer
If the energy transfer process was perfect, we’d be talking about an ideal transformer.
Assuming a 1:1 turns ratio, the equivalent circuit of an Ideal Transformer looks like this:
Energy In
Energy Out
Since there are no losses in an ideal transformer, Energy In = Energy Out
2002 Regional Seminar - Denver

5. Equivalent Circuit of a Transformer
From an energy transfer point-of-view, the elements in this circuit represent the imperfections in a transformer.
Primary winding dc resistance measurement
Secondary winding dc resistance measurement L m C
UST R L- 1 2
DC-1
DC-
Exciting Current and Loss measurement, Zm
Leakage Reactance and Loss measurement, ZL
Dielectric loss (measured in overall tests is lumped in with Rm)
2002 Regional Seminar - Denver

6. Losses in a Power Transformer
Practical Transformer vs. Ideal Transformer
Losses occur due to the following imperfections in a transformer:
Windings have resistance
DC resistance tests
Real and reactive losses exist in the core
Exciting current tests
Physical cores have a finite permeability; exciting current is required to produce magnetic flux
Leakage reactance tests
There is magnetic flux leakage
Losses in the dielectric circuit
What we measured in the overall tests on a xfmr.
2002 Regional Seminar - Denver

7. Losses in a Power Transformer
Good News
Losses in a transformer are specified & controlled.
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.
2002 Regional Seminar - Denver

8. A History Lesson in Magnetism:
Before we get started...
A History Lesson in Magnetism:
Hans Christian Oersted discovered that when an electric current flows through a wire, it causes a compass needle to rotate.
i.e. he discovered that an electric current produces a magnetic field.
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.
2002 Regional Seminar - Denver

9. (but only when relative movement is taking place)
History of Magnetism
Faraday demonstrated this principle of induction in 1831 with the following experiment:
He moved a coil of wire relative to a magnet & discovered that a voltage was induced in the coil.
(but only when relative movement is taking place)
Michael Faraday demonstrated the phenomenon of electromagnetism in a series of experiments.
Responsible for the principles by which electric generators and transformers work.
2002 Regional Seminar - Denver

10.  No voltage is produced if the magnetic field strength is constant.
History of Magnetism
For example  We apply Faraday’s discovery to the arrangement where a magnetic field is associated with the turns of a winding.
(This time - we are not moving the winding, it stays stationary. Instead, we vary the magnetic field  same effect.)
 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.
2002 Regional Seminar - Denver

11. Next  consider a winding through which a current is passed.
History of Magnetism
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.
2002 Regional Seminar - Denver

12. History of Magnetism
 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.
2002 Regional Seminar - Denver

13. How can we improve the linking between windings?
History of Magnetism
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.
2002 Regional Seminar - Denver

14. Electromagnetism Background
Recall that one of the properties that differentiates an ideal transformer from an actual transformer sitting in a substation is that ...
Physical cores have a finite permeability  exciting current is required to produce magnetic flux in the core
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.
2002 Regional Seminar - Denver

15. Example of Core Characteristics
Permeability (slope) - the ability of a material to conduct flux
Illustrates affect of core construction on magnetizing/ hysteresis effects.
2002 Regional Seminar - Denver

16. 1:1 Iex E1 E2 Exciting Current Theory
When the Secondary Winding Is Open E1
Iex
1:1 + - E2  
The current that flows in the primary winding should be sufficient to excite the core.
2002 Regional Seminar - Denver

17. Exciting Current Theory
If Load was Connected to the Secondary
Iex E2
1:1 1 ~ E1 + - f2
+ I2 ZL f2 I2
The primary current increases by the value of the secondary current.
2002 Regional Seminar - Denver

18. Exciting Current Theory
When We Have a Turn-to-Turn Fault on the Secondary During the Exciting Current Test
1:1 1 H1 H0 + E1 - HV LV
Iex ff
+ If ff If
The primary current increases by the value of the current through the short-circuited turns.
2002 Regional Seminar - Denver

19. 1:1 Exciting Current Theory
Detection of Winding to Ground Fault in the Secondary During Exciting Current Test?
1:1 f H0 H1 + - HV LV
Iex ff
+ If ff If
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.
2002 Regional Seminar - Denver

20. 1:1 Exciting Current Theory
How Do We Detect Fault in the Preventive Autotransformer During Exciting Current Test?
1:1 H1 H0 fa
Iex
+ Ia fa HV LV Ia
When autotransformer is connected across two taps it acts as a load and the primary current goes up.
2002 Regional Seminar - Denver

21. Exciting Current Tests
Useful in Detecting:
Turn-to-turn winding failure
1 or more turns completely short-circuited.
2 or more parallel strands of different turns are short-circuited.
LTC problems
Open circuit, shorted turns or high resistance connections in the LTC P.A., series auto or series transformer
misalignment, mechanical problems, coking and wear of LTC & DETC contacts
2002 Regional Seminar - Denver

22. Exciting Current Tests
Useful in Detecting (cont):
Manufacturing defects.
Abnormal (multiple) core grounds.
Changes in the core characteristics.
2002 Regional Seminar - Denver

23. 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)
Guard
Point
UST Mode
Note: For Exciting Current tests performed with the M4000,
the charging current (mA) and watts-loss are recorded.
2002 Regional Seminar - Denver

24. Exciting Current Test Procedure -Wye
H-V Test Cable
L-V Lead
I&W Meter
Guard Point H2 H1 H3
Ie (1-0) H0
Guard
Point
UST Mode
2002 Regional Seminar - Denver

25. Test Measurement Recommendations
Important!!!!
Test Measurement Recommendations
2002 Regional Seminar - Denver

26. Test Measurement Recommendations
Especially Important!!!!
Test Measurement Recommendations
2002 Regional Seminar - Denver

27. LTC and phase patterns should be analyzed
Analysis
TEST RESULTS ANALYSIS
LTC and phase patterns should be analyzed
It is useful to know whether specimen is capacitive or inductive
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?
2002 Regional Seminar - Denver

28. 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
2002 Regional Seminar - Denver

29. Capacitive or Inductive Specimen?
To understand what makes a specimen capacitive or inductive, we revisit the equivalent circuit of a transformer.
I2R loss is much lower than loss in the core L m C
UST R L- 1 2
DC-1
DC-
Exciting Current and Loss measurement, Zm ~
we can neglect the energy storage and loss in the leakage channel.
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.
2002 Regional Seminar - Denver

30. Capacitive or Inductive Specimen?
Equivalent Circuit of the Open-Circuit Test reduces to: I I V Q q L C R I ex V C I C R R I ex I
L - Magnetizing Inductance
C - Turn-to-turn Capacitance
R - Resistance associated with losses in the
core & turn-to-turn insulation L
2002 Regional Seminar - Denver

31. Capacitive or Inductive Specimen?
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.
2002 Regional Seminar - Denver

32. Analysis for Inductive Specimens
For an inductive specimen (majority of xfmrs):
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.
The phase pattern at each tap position should be confirmed.
This pattern should be identical at every tap position, for both mA & Watts measurements.
2002 Regional Seminar - Denver

33. Analysis for Inductive Specimens
Possible phase patterns:
H-L-H Characteristic of:
Phase Pattern A
3-legged core-type transformer
5-legged core or shell-type transformer that has a delta-connected secondary winding
L-H-L Characteristic of:
Phase Pattern B
3-legged core-type transformer that has a wye-connected winding with an inaccessible neutral.
3-legged core-type transformer with a delta-connected winding if testing two phases of the winding in parallel.
2002 Regional Seminar - Denver

34. Wye Winding with No H0 Bushing
2002 Regional Seminar - Denver

35. Wye Winding with No H0 Bushing
2002 Regional Seminar - Denver

36. Analysis for Inductive Specimens
All 3 Readings Similar
Phase Pattern C
Characteristic of four and five legged core-type transformers and shell-type transformers with non-delta secondary windings
All 3 Readings Dissimilar (H-M-L)
May be indicative of a magnetized core
May actually be “capacitive” specimen - not inductive after all
2002 Regional Seminar - Denver

37. FACTORS OTHER THAN DEFECTS THAT MAY INFLUENCE TEST RESULTS:
Analysis
FACTORS OTHER THAN DEFECTS THAT MAY INFLUENCE TEST RESULTS:
If capacitance  inductive component, you have a capacitive specimen & analysis changes.
UST capacitance
Test voltage
Test results are voltage dependent so data can only be compared if performed at identical voltages.
Residual magnetism
Design and position of LTC
Test Connections
2002 Regional Seminar - Denver

38. Capacitive LTC Patterns
Experience with Capacitive LTC Patterns
Effects documented as early as 1972
rd 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
2002 Regional Seminar - Denver

39. Capacitive LTC Patterns
Effects of a Strong Capacitive Presence
PHASE PATTERN IS AFFECTED
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
2002 Regional Seminar - Denver

40. Test Voltage 5 10 V [kV] I , I [mA] I I < > I
Factors other than defects that can influence test results.
Test Voltage L 5 10
V [kV] I C
, I
[mA] I I L
< C
> I
2002 Regional Seminar - Denver

41. Increases current if specimen is inductive
Factors other than defects that can influence test results.
RESIDUAL MAGNETISM
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
2002 Regional Seminar - Denver

42. Example of an LTC Pattern
Test results for all non-bridging positions are equal. Test results for all bridging positions are equal. I 1 N 4R 8R
12R
16R
2002 Regional Seminar - Denver

43. 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
2002 Regional Seminar - Denver

44. Exciting Current Testing
Case Studies
2002 Regional Seminar - Denver

45. Exciting Current Case Study 1
Case Number 02-06
Unit Tested:
U.S. Transformer, 3Φ two-winding transformer
Δ-Y connected
20 MVA
69/12.47 kV
vintage
Rewound in 2000
2002 Regional Seminar - Denver

46. Testing Circumstances:
Case Study 1 (# 02-06)
Testing Circumstances:
Tested upon receipt from the manufacturer’s repair facility where it had been completely rewound.
overall insulation tests - acceptable
bushing tests - acceptable
field power factor test on an oil sample from the main tank - acceptable
2002 Regional Seminar - Denver

47. Case Study 1 - Exciting Current Results
2002 Regional Seminar - Denver

48. LTC Pattern 4
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. I 1 N 4R 8R
12R
16R
2002 Regional Seminar - Denver

49. LTC Pattern - Phase A
2002 Regional Seminar - Denver

50. LTC Pattern - Phase B
2002 Regional Seminar - Denver

51. LTC Pattern - Phase C
2002 Regional Seminar - Denver

52. Phase Pattern (bridging positions) - Current
2002 Regional Seminar - Denver

53. Phase Pattern (non-bridging positions) - Current
2002 Regional Seminar - Denver

54. Phase Pattern (non-bridging positions) - Watts
2002 Regional Seminar - Denver

55. Phase Pattern (bridging positions) - Watts
2002 Regional Seminar - Denver

56. Problem Found Investigation
Manufacturer electrically isolated the series transformer for tests; exciting current tests indicated a definite problem on the center phase.
Problem: a short between turns in the outer coil of the center phase of the series transformer.
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.
2002 Regional Seminar - Denver

57. 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?
2002 Regional Seminar - Denver

58. General Electric, 3Φ two-winding transformer
Case Study 2 (# 02-09)
Capacitive LTC patterns
Unit Tested:
General Electric, 3Φ two-winding transformer
Δ-Y connected
7.5 MVA
67/12.5 kV
vintage
G.E. Type LRT-200A LTC
2002 Regional Seminar - Denver

59. Testing Circumstances:
Case Study 2
Testing Circumstances:
Concern from utility owner that the protection circuitry for the vacuum bottles in the LTC was not working properly.
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
2002 Regional Seminar - Denver

60. LTC PATTERN 2 Exciting Current Data: LTC Pattern Analysis
2002 Regional Seminar - Denver

61. Phase Pattern B Non-Bridging Tap Positions - Current Measurements
Exciting Current Data: Phase Pattern Analysis
Non-Bridging Tap Positions - Current Measurements
Phase Pattern B
2002 Regional Seminar - Denver

62. IC IL = Icore Current Measurements - Phase Pattern B IL IC IC IQ IQ
Explanation of Phase Pattern in N.B. Positions
Current Measurements - Phase Pattern B IL IC IC IC IQ IQ
IQ = Quadrature
Component of Exciting Current ~ Measured Exciting Current (L-H-L) IL IL
IL = Icore
2002 Regional Seminar - Denver

63. Phase Pattern A Non-Bridging Tap Positions - Loss Measurements
Exciting Current Data: Phase Pattern Analysis
Non-Bridging Tap Positions - Loss Measurements
Phase Pattern A
2002 Regional Seminar - Denver

64. Phase Pattern A Bridging Tap Positions - Loss Measurements
Exciting Current Data: Phase Pattern Analysis
Bridging Tap Positions - Loss Measurements
Phase Pattern A
2002 Regional Seminar - Denver

65. Phase Pattern A Bridging Tap Positions - Current Measurements
Exciting Current Data: Phase Pattern Analysis
Bridging Tap Positions - Current Measurements
Phase Pattern A
2002 Regional Seminar - Denver

66. IPA IPA IPA IC IC IC IQ Icore Icore Icore
Explanation of Phase Pattern in B. Positions IC IC IC IQ
Icore
Icore
Icore
IQ = IL + IC = Quadrature Component ~ Measured Exciting Current
(H-L-H) IQ
IL = Icore + IPA
IPA
IPA
IPA
2002 Regional Seminar - Denver

67. Exciting Current Measurements
2002 Regional Seminar - Denver

68. Watts Measurements
2002 Regional Seminar - Denver

69. Doble Transformer Turns Ratio
Doble Turns Ratio Test
Doble Transformer Turns Ratio
2002 Regional Seminar - Denver

70. Benefits of the Turns Ratio Test
Confirm nameplate ratios
Detect short-circuited turn-to-turn insulation
Detect open-circuited windings
2002 Regional Seminar - Denver

71. Doble TTR Test Procedure
UST
HV Lead
LV Lead
Doble TTR
Capacitor
CTRUE
CTRUE= I
V x w
2002 Regional Seminar - Denver

72. CTRUE/CApparent Doble TTR Test Procedure HV Lead UST V1
LV Lead
Doble TTR
Capacitor V1
Now, if we take the ratio:
CTRUE/CApparent
We obtain:
N - the turns ratio
2002 Regional Seminar - Denver

73. Doble TTR - DTA Screen
2002 Regional Seminar - Denver

74. General Electric, 3Φ two-winding transformer
TTR Case Study
Unit Tested:
General Electric, 3Φ two-winding transformer
Δ-Y connected
5 MVA
50.5/13.09 kV
vintage
2002 Regional Seminar - Denver

75. Exciting Current Test Results (4/24/02)
TTR Case Study
Exciting Current Test Results (4/24/02)
TTR Test Results (4/24/02)
Previous TTR Test Results (7/25/95)
2002 Regional Seminar - Denver

76. TTR Case Study
2002 Regional Seminar - Denver

77. TTR Case Study
2002 Regional Seminar - Denver

78. Doble Leakage Reactance Test
Leakage Reactance Tests
Doble Leakage Reactance Test
2002 Regional Seminar - Denver

79. Leakage Reactance Test
Iex E2
1:1 1 ~ E1 + - f2
+ I2 ZL f2 I2
2002 Regional Seminar - Denver

80. Leakage Flux
The combined action of both currents results in some of the flux being present in the unit permeability space. L
Flux that is not confined to the core for the entire length of its path. 
The unit permeability space includes the space between the windings, w/in the windings & between the windings and the tank.
2002 Regional Seminar - Denver

81. Leakage Flux
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.
 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.
2002 Regional Seminar - Denver

82. Leakage Reactance Equivalent Circuitm C
UST R L- 1 2
DC-1
DC- E2 E1
Short-Circuit Impedance 2 1 L- R L
DC-1
DC- E2 E1
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.
2002 Regional Seminar - Denver

83. Leakage Channel Top yoke Leakage channel Outer winding Core leg
The leakage flux path includes the regions occupied by the windings. The leakage reactance may be sensitive to deformations in the windings.
Top yoke
Leakage channel
Outer winding
Core leg
Inner winding
Bottom yoke
2002 Regional Seminar - Denver

84. Benefits of the Leakage Reactance Test
Confirm nameplate impedance
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)
2002 Regional Seminar - Denver

85. Capacitance versus Leakage Reactance
Sensitive to temperature & contamination
Normally involves all three phases
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
2002 Regional Seminar - Denver

86. Test Procedures Initial test:
Perform Three-Phase Equivalent test for comparison to Nameplate
Perform Per-Phase tests to act as benchmark for future tests
Subsequent tests:
Perform only Per-Phase tests for comparison to benchmark tests
2002 Regional Seminar - Denver

87. Leakage Reactance Test
Doble M4110 Leakage Reactance Test Set
2002 Regional Seminar - Denver

88. M4100 & M4110 L.R. Module Test Connections
2002 Regional Seminar - Denver

89. Nameplate Data Required for LRT
3-Phase Equivalent Leakage Reactance Test
2002 Regional Seminar - Denver

90. 3-Phase Equivalent Leakage Reactance Test
2002 Regional Seminar - Denver

91. Nameplate Data Required for LRT
Per-Phase Leakage Reactance Test
2002 Regional Seminar - Denver

92. Per-Phase Leakage Reactance Test
2002 Regional Seminar - Denver

93. First, or benchmark test, should be within 3% of Nameplate.
Analysis
First, or benchmark test, should be within 3% of Nameplate.
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.
2002 Regional Seminar - Denver

94. General Electric, 3Φ two-winding transformer
LRT Case Study
Unit Tested:
General Electric, 3Φ two-winding transformer
114 GR. Y/ kV
80/89.6 MVA
vintage
Background:
During a short outage for normal generator maintenance, the 13.8 kV generator bus PTs were replaced due to PCB contamination.
2002 Regional Seminar - Denver

95. 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.
2002 Regional Seminar - Denver

96. 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.
2002 Regional Seminar - Denver

97. LRT Case Study - 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.
2002 Regional Seminar - Denver

98. LRT Case Study - Investigation
Overall Test Results - Post-fault (2000)
2002 Regional Seminar - Denver

99. Observations from Overall Test Results
Insignificant change in the capacitance of the LV winding to ground insulation occurred from 1986 to (There were no results available for this transformer prior to 1986).
However, capacitance decreased from 42,371 pF in 1999 to a post-incident capacitance of pF, a change of 5.53%.
2002 Regional Seminar - Denver

100. Observations
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 HV LV
Inter-winding
Shield CL CH
CH-S
CL-S
Tank
and
Core
Changes in a capacitance measurement usually represent physical changes in the insulation system under measurement.
2002 Regional Seminar - Denver

101. Leakage Reactance Test Results
Three-Phase Equivalent
The average short circuit impedance listed on nameplate was 10.02%.
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%.
2002 Regional Seminar - Denver

102. Leakage Reactance Test Results
Per-Phase Tests:
The measured per-phase results were 11.92%, 10.79% and 9.12% for phases A, B and C, respectively.
The per-phase measurement deviated from the average by as much as 14%.
An internal inspection was scheduled.
2002 Regional Seminar - Denver

103. Internal Inspection
Upon entering the transformer it was obvious severe damage had occurred. Damage to top clamping plates and wedge clamps.
2002 Regional Seminar - Denver

104. 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.
2002 Regional Seminar - Denver

105. Observations from Internal Inspection
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.
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.
2002 Regional Seminar - Denver

106. Thank You!
QUESTIONS?
2002 Regional Seminar - Denver