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P.O.Box 87
Mayfield, NSW, 2304
Tel 02 4928 1511
Fax 02 4928 1511
Mob 0425 326 541
Power Control Engineers Pty Ltd
Specialist Electrical Engineers
ABN 50 103 684 466
TRANSFORMER FAILURE
Review and Investigation
of
Transformer Failure
Rev No Description Originator Checked Date
0 Issued KB MS 23/01/09
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Table of Contents
1 INTRODUCTION ........................................................................................................................ 3
2 EXECUTIVE SUMMARY ............................................................................................................ 3
2.1 Recommendations .............................................................................................................. 3
3 REVIEW OF REPAIRERS REPORT ......................................................................................... 4
3.1 Repairer Inspection Findings............................................................................................... 4
4 INVESTIGATION OF FAULT AND FAILURE MODE .................................................................. 5
4.1 Fault Current Determination ................................................................................................ 5
4.1.1 Current waveform analysis. .......................................................................................... 5
4.1.2 Voltage Waveform Analysis. ........................................................................................ 5
4.2 Analysis of Fault Recording ................................................................................................ 6
4.3 Review of Protection Operation ........................................................................................... 8
4.4 Review of Tap changer Mechanism .................................................................................... 9
4.5 Review of Winding Physical Layout..................................................................................... 9
4.6 Winding Open Circuit Voltage ........................................................................................... 11
4.7 Detailed Failure Mechanism Sequence ............................................................................. 12
5 CONCLUSION .......................................................................................................................... 13
6 APPENDICES .......................................................................................................................... 14
6.1 Appendix 1 Calculations ................................................................................................. 14
6.2 Appendix 2 Photographs of Failed Transformer ............................................................. 15
6.3 Appendix 3 Tap Changer Data ....................................................................................... 17
6.4 Appendix 4 Notice of Failure .......................................................................................... 18
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1 INTRODUCTION
An industrial site experienced a failure of a 7.5/10MVA 33000V/6600V power transformer. The
transformer was sent to a repairer for inspection and repair.
This report reviews the findings of the inspection by the repairer and fault data gathered on site.
2 EXECUTIVE SUMMARY
The review of the repairers report and investigation of fault data verify that the transformer failure
was due to the failure of a connection to the transition resistor in the transformer tap changer. The
failure caused an open circuit in the delta HV winding leading to high voltages, internal arcing and
severe damage to the winding.
Existing protection schemes operated correctly and without delay but were unable to contain the
damage. No change is recommended to these systems.
It is possible to install some additional monitoring of the tap changers but direct detection and
prediction of this particular fault is difficult to achieve. Additional monitoring should be considered.
The recommended solution is early detection of potential problems through regular planned
maintenance according to manufacturers recommendations
2.1 Recommendations
Recommendation
1. Highlight this mode of failure and the inspection required to detect it. to
maintenance personnel
2. Consider installing additional tap changer monitoring equipment
3. Ensure tap changer maintenance is carried out at recommended number of
operations
4. Ensure transition resistor components are replaced as per manufacturers
recommendations
5. Carry out additional offline non-intrusive testing of the tap changers at
shorter intervals than tap changer maintenance intervals. Testing such as
contact resistance is included.
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3 REVIEW OF REPAIRERS REPORT
The data reviewed includes the following
Repairer Fault Investigation Report
Fault Disturbance Recording from power monitoring equipment
Fault records and notes by site personnel
3.1 Repairer Inspection Findings
The findings of the inspection report by the repairer are summarised below
The lead to the tap changer transition resistance contact of HV Winding-A was burned off.
Flash marks were evident on the tap changer fixed and change over contacts for Winding-A.
The top of HV Winding-A had failed due to interturn fault and flashover.
The bottom of HV Winding-A was damaged mechanically.
The LV winding B showed signs of slight distortion
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4 INVESTIGATION OF FAULT AND FAILURE MODE
PCE investigations included
Review of the fault recordings captured by the substation Power Monitor to further verify the
mode of failure.
Gathering additional fault data from site personnel.
Review of repairer report
Literature search
Calculations
Available literature indicated that that failure of tap changers is the second most common cause of
failure of transformers, second only to insulation deterioration and failure. The type of failure which
occurred with this transformer is fairly common and typical of this type of OLTC failure.
4.1 Fault Current Determination
The fault current was initially of the order of 2100A (3 x 700A) for the first two cycles and then it
increased beyond the range of the power monitor. However, the fault currents in this range could be
determined from the data available as follows.
4.1.1 Current waveform analysis.
Inspection of the steady fault current waveform indicated a ratio along the x axis of a half cycle to
the truncated section of the waveform equal to 52:35 where 52 equates to 180 degrees. The
truncated level of the current waveform was 1250A and thus the peak value of the Sine wave is
calculated to be approximately 2545A. This equates to an RMS current of 1799A. The fault
recording is for one of three feeders supplying the bus to which the failed transformer was
connected and thus the transformer fault current would have been of the order of 5397A
4.1.2 Voltage Waveform Analysis.
The level to which the voltage waveform collapsed provides a second means of estimating the level
of fault current. Knowing the supply impedance at the bus to which the transformer is connected it is
possible to calculate the current flowing which would result in the voltage dropping to the level
recorded. The recording shows that the voltage collapsed from 17700V to 1180V. The fault current
required to cause this collapse is calculated to be 7030A (See Appendix 1 Calculations)
The known Supply Utility fault levels at the 33kV busbar are 8kA line to line and 3.8kA line to
ground. Based on these magnitudes, the initial fault could have been a single line-to-ground fault.
As the fault current eventually exceeded the 3.8kA line to ground fault level, the final fault is
confirmed as comprising a line-to-line fault or line-to-line-to-ground fault.
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4.2 Analysis of Fault Recording
The fault recording is shown below:
Figure 1 Fault Trace from Power Monitor
An analysis of this recording verifies the failure mode of the transformer.
Prefault conditions at point A indicate the system operating at a voltage of 25kV(peak) =
17.7kV(rms) line to ground and 100A pk (approx 210A rms total - 3 feeders)
At point B the fault is initiated. The fault is not a direct short circuit but develops as
evidenced by the recorded initial fault current peak of 650A developing to a steady state
peak of 2500A after 2 cycles (Note these are the fault currents seen by 1 of 3 feeders).
After one cycle the fault current has increased to a level which causes the voltage to
collapse as seen at point C.
Points C and D on the recording show a number of spikes on the voltage waveform. These
are probably due to instability of the developing arcing fault across the HV Winding-A of the
transformer with some arcing to the tank of the transformer. At this stage the fault current
reaches its maximum level.
Point E which is 3 to 4 cycles from the start of the fault is where the fault has developed to a
full phase to phase fault as a result of the interturn failure and arcing across the top of the
HV Winding-A.
At point F the vacuum circuit breaker feeding the transformer clears the fault. The fault
current is cleared and there is indication of a recovery voltage transient. The time from the
A B C D E F
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start to the clearing of the fault is approximately 90ms or 4.5 cycles. The transformer
differential protection would have initiated a trip signal in 20 to 30ms and the breaker
clearing time would be in the order of 60ms. This confirms the correct operation of the
protection scheme.
The current and voltage levels for the duration of the fault captured in Figure 1 Fault Trace from
Power Monitor above are shown in Figure 2 - Transformer Fault Current and Voltage Levels below
Figure 2 - Transformer Fault Current and Voltage Levels
0.0
1000.0
2000.0
3000.0
4000.0
5000.0
6000.0
7000.0
0 50 100 150 200 250
Cu
rre
nt
(Am
ps)
Time (ms)
Amps
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
0 50 100 150 200 250
Vo
lta
ge
(k
V)
Time (ms)
kV
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4.3 Review of Protection Operation
Site personnel advised that the following protection operated:
1. O/C Instantaneous on phase 1 and 2 (alarm flag #4) (SPAJ140C on Transformer 33kV
feeder with a trip recorded at 24x setpoint, or 6480A)
2. Bucholz on main tank (2 stage type with oil surge and gas detection though it is not known
which operated)
3. Oil explosion vent (rupture disk) failed on main tank expelling oil
4. Oil vent on tap changer tank remained intact
5. Differential relay type 4C21 with A & B phases flagged
The settings for the SPAJ140C on Transformer 33kV feeder are as follows:
Feeder Transformer
Relay SPAJ140C
CT Ratio 200 / 1
Overcurrent Settings Earth Fault Settings
Curve Very Inverse Curve Definite Time
Plug I>/In 1.35 (270A) Io>/In 0.20 (40A)
Time Dial t> 0.21 to> 0.10
Inst I>>/In 9.00 (1800A) Io>>/In Set off
Inst t>> 0.04 to>> - - - -
No earth fault was flagged on this relay. This is because the instantaneous operating time t>> of
0.04s is faster than the earth fault definite time of 0.1s.
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4.4 Review of Tap changer Mechanism
Figure 3 Transformer Tap Changer
4.5 Review of Winding Physical Layout
There was observed mechanical and flashover damage to the top of the HV winding and
mechanical movement at the bottom of the winding. The leads to the tap changer come out at the
top of the winding.
Figure 4 below shows diagrammatically one phase of the HV winding and tap take-offs 2 15.
Figure 4- Schematic of A Phase Winding
Open Circuit
occurs here
momentarily
These wires connect the contacts
to the transition resistor. A failure
of one of these wires resulted in
the open circuiting of the
transformer HV winding during a
tap changing operation
Transition Resistor
Barrier Plate
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Figure 5 below shows an approximate physical representation of the HV winding viewed horizontally
as a cross section of half the winding.
there are approximately 10 such layers.
photos and the nameplate data.
Figure 5- Physical Represen
The tap take-offs are at approximately mid winding (from nameplate diagram). Between tap 3 and
tap 13 is approx 10% of the winding (based on known tap range of 13%). Each layer is
approximately 10% of the winding (since there are 10 laye
offs are in one layer with tap leads brought out the top of the winding, so possibly passing close to
the top of layer 4 (at tap takeoff position 2).
A 2 3 4
5
6
7
8
9
10
11
12
13
To B
Tap changer
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Transformer Failure due to OLTC Fault
below shows an approximate physical representation of the HV winding viewed horizontally
as a cross section of half the winding. Each vertical line represents a layer of turns. From photos
there are approximately 10 such layers. This layout is not known for certain but is deduced from
Physical Representation of Winding
offs are at approximately mid winding (from nameplate diagram). Between tap 3 and
tap 13 is approx 10% of the winding (based on known tap range of 13%). Each layer is
approximately 10% of the winding (since there are 10 layers). This could mean most of the tap take
offs are in one layer with tap leads brought out the top of the winding, so possibly passing close to
the top of layer 4 (at tap takeoff position 2).
To C
14
15
A1
Tap changer Open Cct
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Investigation into Transformer Failure due to Failure of
below shows an approximate physical representation of the HV winding viewed horizontally
Each vertical line represents a layer of turns. From photos
This layout is not known for certain but is deduced from
offs are at approximately mid winding (from nameplate diagram). Between tap 3 and
tap 13 is approx 10% of the winding (based on known tap range of 13%). Each layer is
rs). This could mean most of the tap take-
offs are in one layer with tap leads brought out the top of the winding, so possibly passing close to
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4.6 Winding Open Circuit Voltage
With the transformer operating normally delivering load, the supply L-L voltage appears across the
HV windings distributed across the windings. If the tap changer fails open circuit and there is no
current flowing in the HV winding then the full Line voltage is seen between the open circuited
sections and is no longer distributed evenly across the whole winding.
In addition if there is a residual current and voltage in the secondary this will be transformed to
corresponding voltages in the open circuited sections of the primary winding and may add to the L-L
voltage further increasing the overall voltage which may appear between adjacent turns and tap
changer leads. (Note that the load current in the secondary winding may continue to flow for a
number of cycles after the primary winding is open circuited due to inductance and the load
effectively becomes a source to this phase with the secondary voltage transforming back to the
primary windings.) This is shown in Figure 6 Illustration of voltages across open circuited HV
windings .
Figure 6 Illustration of voltages across open circuited HV windings
HV Amps LV Amps
Source Load
Source HV2
HV Winding LV Winding
HV1 LV
Transformer Healthy Condition - No OC in HV winding Voltage Vectors
HV1
HV1 Winding LV Amps
Source Load
Source
HV2 Winding
LV
HV2 Voltage Vectors
Transformer with OC HV Winding with residual current in secondary
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4.7 Detailed Failure Sequence
This review and investigation verifies that the transformer failed as a result of a failure in the tap
changer open circuiting a high voltage winding.
The following sequence of events fits all the known facts.
1. There was a possible pre-existing poor connection on a lead to the tap changer transition
resistor in HV Winding-A.
2. Constant tap changing caused the connection wire to the transition resistor to fatigue and fail
strand by strand at the poor connection near the lug.
3. Eventually, as a tapping occurs the last strand(s) break or burn off due to the transition
current (from the momentarily shorted turn).
4. The HV load current immediately arcs across the open circuit tap changer contacts as the
induced voltage and supply voltage keep the load current flowing. The load current is
relatively low and this loacalised arcing is not severe.
5. The arc extinguishes at the first current zero so that there is no current flowing in the HV
winding at the next AC cycle.
6. With no current flowing in the HV winding the voltage between sections of the winding
separated by the open circuit in the tap changer increase to line voltage. This voltage may
increase beyond line voltage due to the superimposed transformation voltages from the LV
side of the transformer back to the open circuited sections of the HV winding. The voltage
may reach twice line voltage.
7. The level of voltage between open circuited sections of the winding (including the tap take
off leads) exceeds the interturn insulation level resulting in failure of the insulation in the
winding and subsequent flashover.
8. The failure and arcing across the section of winding results in the current through the
affected winding rising to a level high enough to saturate the core.
9. With the core saturated, the HV Winding-A impedance is drastically reduced allowing very
high currents to flow in the winding damaging it further by distorting the top and bottom turns
due to the high interturn magnetic forces.
10. In addition, the arcing across sections of the winding propagates to adjacent layers of the
winding as the insulation is damaged by the combined effects of arcing, voltage stress and
mechanical distortion.
11. The tap changer closes onto the faulted winding resulting in fault currents flowing through
the tap changer contacts causing burning and damage to these contacts
12. The fault effectively develops into a line to line fault. (Refer Figure 1 Point D) 13. The distortion of the winding under high fault currents results in some of the leads to the tap
changer breaking off and arc from these leads to the transformer tank. 14. The fault current and arcing continue until cleared by the transformer circuit breaker.
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5 CONCLUSION
The problem occurs because an open circuit in the tap changer results in high voltages between
sections of the HV windings and connection leads to the tap changer. This results in insulation
break down in the HV windings.
In conclusion,
1. A failure of a connecting lead in the transformer tap changer resulted in an open circuit of the
HV Winding-A
2. The open circuit of the HV winding resulted in voltages of at least line voltage (and possibly
up to 2x line voltage) between the open circuited sections of the winding, (ie effectively
interturn) causing winding insulation failure and flash over of the winding.
3. The initial fault developed in to a full phase to phase fault due to arc fault propagation,
saturation of the transformer core and mechanical distortion of the windings.
4. The HV Winding-A and the tap changer were both severely damaged by the fault
5. Circuit breaker protection is not fast enough to limit this damage once this occurs. There
appears to be no practical way to monitor during operation the onset of this particular
condition.
It is recommended that tap changers should be inspected when transformer maintenance is
undertaken to ensure that other similar problems do not occur. Manufacturers replacement
recommendations should be followed especially for transition resistor components.
Possible non intrusive testing or monitoring of main power transformers should also be investigated
such as:
Tap changer Motor current monitoring
Tap change speed of operation (offline test)
Tap contact resistance monitoring (offline test)
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6 APPENDICES
6.1 Appendix 1 Calculations
33kV POWER TRANSFORMER FAILURE ANALYSIS
Fault Current Calculation from Current Waveform
Half cycle units (180 degrres) 52
Units of truncated sinewave 35
Equivalent degrees of truncated section 121.2 deg
Phase angle at start of truncation 29.4 deg 0.514 radians
Truncated current level 1250 Amp
Peak Value of sinewave 2545 Amp
RMS fault current (3 feeders) 5399 Amp
Conclusion Fault was Line-Line not SLG (SLG FL =3.8kA only)
Fault Current Calculation from Voltage Sag
Voltage Sag L-N V sag 16520 V
Base voltage L-L Vb 33000 V
Base MVA Pb 292 MVA
Source Imped (pu) Zsource pu 0.1137+0.6198i pu
Base Imped Zb 3.73 (Vb2 / Pb)
Source Imped (complex) Zsource 0.424038698630137+2.31151438356164i Ohm Z=Zpu*Zb
Source Imped Zsource 2.35 Ohm
Fault Current (complex) Ifault 1268.37525723914-6914.15113840647i A
Fault Current Ifault 7030 A
Conclusion Fault was Line-Line not SLG (SLG FL =3.8kA only)
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6.2 Appendix 2 Photographs of Failed Transformer
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6.3 Appendix 3 Tap Changer Data
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6.4 Appendix 4 Notice of Failure
Notice to Site Electrical Personnel
10MVA 33/6.6kV Transformer Failure
This site has experienced a failure of a 7.5/10MVA 33000V/6600V power transformer .
An inspection of the transformer and investigation into the failure has revealed that the root cause of
the failure was due to a connecting lead in the tap changer failing to an open circuit condition
resulting in an open circuit near the middle of one of the delta connected HV windings.
The failure occurred as follows
1. A failure of a connecting lead in the transformer tap changer resulted in an open circuit of the
HV winding.
2. The open circuit of the HV winding resulted in voltages of at least line voltage (and possibly
up to 2x line voltage) between the open circuited sections of the winding, (ie effectively
interturn) causing winding insulation failure and flash over of the winding.
3. The initial fault developed in to a full phase to phase fault due to arc fault propagation,
saturation of the transformer core and mechanical distortion of the windings.
4. The HV winding and the tap changer were both severely damaged by the fault
5. Circuit breaker protection is not fast enough to limit this damage once this occurs.
6. There appears to be no practical way to monitor during operation the onset of this particular
condition, and the only remedy is regular and thorough maintenance.
The transformer has been sent for a rewind and will probably be out of service for 3 months.
This failure highlights the importance of regular planned maintenance according to manufacturers
recommendations to prevent similar failures in the future.
