OLTC Failure

Power Control Engineers Pty Ltd Specialist Electrical Engineers

P.O.Box 87 Mayfield, NSW, 2304 Tel 02 4928 1511 Fax 02 4928 1511 Mob 0425 326 541 [email protected]

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TRANSFORMER FAILURE

Review and Investigation of Transformer Failure

Rev No

Description

Originator

Checked

Date

0

Issued

KB

MS

23/01/09

PCE Transformer Failure due to OLTC Fault

23/01/09 Page 2 of 18

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Table of Contents

1

INTRODUCTION ........................................................................................................................3

2

EXECUTIVE SUMMARY ............................................................................................................3 2.1

3

REVIEW OF REPAIRER’S REPORT ......................................................................................... 4 3.1

4

Recommendations ..............................................................................................................3

Repairer Inspection Findings............................................................................................... 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

�

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 t hat the transformer failure was due to the failure of a connection to the transition r esistor 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 REPAIRER’S REPORT

The data reviewed includes t he 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 t he 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 t ype 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 t wo cycles and then it increased beyond the range of the power monitor. However, t he 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 t he 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 t ransformer 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 c alculated 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:

A

B

C

D

E

F

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 W inding-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 fr om the

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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 7000.0 6000.0 � 5000.0 � � � � 4000.0 � � � 3000.0 � � � � � 2000.0

��

1000.0 0.0 0

50

100

150

200

250

��

45.00 40.00 35.00 � � � � � � � � � � �

30.00 25.00 20.00 ��

15.00 10.00 5.00 0.00 0

50

100

150

200

250

��

Figure 2 - Transformer Fault Current and Volt age Levels

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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 Inst I>>/In Inst t>>

t>

0.21 9.00 (1800A) 0.04

t o> Io>>/In to>>

0.10 Set off ----

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

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 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. Open Circuit occurs here momentarily

Figure 4- Schematic of A Phase Winding

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--------------------------------------------------------------------------------------------------------------Figure 5 below shows an approxi ate physical representation of the HV winding iewed horizontally as a cross section of half the winding. Each vertical line represents a layer of turn s. From photos there are approximately 10 such l yers. This layout is not known for certain but is deduced from photos and the nameplate data.

Tap changer O en Cct To B

A

To C

2

3 4 5 6 7 8 9 10 11 12 13

14

A1

15

Figure 5- Physical Representation of Winding

The tap take-offs are at approximately mid winding (from nameplate diagram). Be tween tap 3 and tap 13 is approx 10% of the winding (based on known tap range of 13%). Each la er is approximately 10% of the winding (since there are 10 layers). This could mean m ost of the tap takeoffs 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).

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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 t ap changer leads. (Note that the load curr ent in the secondary winding may continue to flow f or a number of cycles after the primary winding is open circuited due to inductance and the load effectively becomes a source to t his phase with the secondary voltage transforming back to t he primary windings.) This is shown in Figure 6 Illustration of voltages across open circuited HV windings .

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Figure 6 Illustration of voltages across open circuited HV w indings

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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 t urn). 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 t op and bottom turns due to the high interturn m agnetic 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 f lowing 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 t o 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 t he 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 t o monitor during operation the onset of t his particular condition. It is recommended that tap changers should be inspected when transformer maintenance is undertaken to ensure that other similar problems do not occ ur. Manufacturer’s replacement recommendations should be followed especially for tr ansition 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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APPENDICES 6.1 Appendix 1 – Calculations

33�� (180 ��) 52 �� 35 �� 121.2 �� 29.4 �� 1250 �� 2545 �� (3 ��) 5399 �� (�� =3.8�� )

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16520

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33000

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292

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0.424038698630137+2.31151438356164�

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2.35

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1268.37525723914�6914.15113840647�

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7030

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0.1137+0.6198�

0.514

�� 2

(�� / � �)

3.73

�� (�� =3.8�� )

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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 t he 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 m anufacturers’ recommendations to prevent similar failures in the future.

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OLTC Failure

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