medium voltage reduced voltage autotransformer

1

Medium Voltage Reduced Voltage Autotransformer

Starter Failures– Explaining The Unexplained –

Medium Voltage Reduced Voltage AutotransformerMedium Voltage Reduced Voltage Autotransformer

Starter FailuresStarter Failures–– Explaining The Unexplained Explaining The Unexplained ––

Larry Farr, Senior Member IEEE Principal Engineer

Eaton ElectricalAsheville NC

Technical Advisor TC-17A IECChair of CANENA TC-17A SC-3

Arthur J. Smith III, Member IEEEVice President

Waldemar S. Nelson and Co. Inc.New Orleans, LA

•• BackgroundBackground





TODAY’S AGENDATODAYTODAY’’S AGENDAS AGENDA

•• InvestigationInvestigation

•• Testing 1988Testing 1988

•• FindingsFindings

••Testing 2002Testing 2002--0303


•• SolutionSolution•• ConclusionsConclusions

2

• Background•• BackgroundBackground





•• For the past century the AutoFor the past century the Auto--transformer or transformer or ““KorndorferKorndorfer””Starter has been a standard in Starter has been a standard in the electrical industry.the electrical industry.

BackgroundBackground

3

•• In the late 1970s, unexplained In the late 1970s, unexplained ““High Voltage StressHigh Voltage Stress”” failures failures started occurring.started occurring.


4

Failures Were Occurring WorldwideFailures Were Occurring WorldwideFailures Were Occurring Worldwide•• North Sea Platform 12kV starters failed multiple North Sea Platform 12kV starters failed multiple

times. Problems solved with single phase times. Problems solved with single phase autotransformers. autotransformers.


•• 20,000 hp20,000 hp--15kV starter in British Columbia failed 15kV starter in British Columbia failed four times until surge arresters from the Zero tap to ground four times until surge arresters from the Zero tap to ground were installed.were installed.

•• South America 2400 volt failed multiple times.South America 2400 volt failed multiple times.

•• Gulf of Mexico Gulf of Mexico –– Multiple 5kV starter failuresMultiple 5kV starter failures

•• IEC 60470 Clause 6.102.7 ChangeIEC 60470 Clause 6.102.7 Change--over Ability Testover Ability Test

•• Off the West Coast of Africa Off the West Coast of Africa –– Multiple 5kV starter failuresMultiple 5kV starter failures

3 Inches !!

What Happened?What Happened?Zero Tap Circuit to GroundZero Tap Circuit to Ground


5

High Voltage Flashover

Telescoping CoilsLayer to Layer

Failure ModesFailure ModesBackgroundBackground

High Voltage FlashoverHigh Voltage FlashoverTap to Tap Over the SurfaceTap to Tap Over the Surface


6

High Voltage FlashoverHigh Voltage FlashoverCoil to GroundCoil to Ground


High Voltage FlashoverHigh Voltage FlashoverLayer to Layer FailureLayer to Layer Failure


7

High Voltage FlashoverHigh Voltage FlashoverZeroZero--tap Circuit to Groundtap Circuit to Ground


Telescoping CoilsTelescoping Coils


•• S and R Conducting SimultaneouslyS and R Conducting Simultaneously

•• Flashover of S with R ClosedFlashover of S with R Closed

8


Flashover of S with R ClosingFlashover of S with R Closing






•• Investigation Investigation •• Testing 1988Testing 1988

9

TestingTesting

Initial TestingInitial Testing

TestingTesting


10

TestingTesting


TestingTesting

11








Design ModificationsDesign Modifications

•• Changed Design to Three Coil Three Changed Design to Three Coil Three Legged AutotransformerLegged Autotransformer

FindingsFindings

•• Changed Control Circuit to Include a Changed Control Circuit to Include a Transition on Current Below 125% Transition on Current Below 125% FLAFLA

•• HOWEVER, Failures Continued at HOWEVER, Failures Continued at a Rate of Two to Three a Year !a Rate of Two to Three a Year !

12

The High Voltage Stress FailuresThe High Voltage Stress FailuresOccurred When:Occurred When:

•• Transformers were on the 80% tapTransformers were on the 80% tap

FindingsFindings

•• Transitions Were Forced at or Transitions Were Forced at or Near Locked Rotor CurrentNear Locked Rotor Current

Remaining Failures Required a Closer Look and Additional Testing

Remaining Failures Required a Closer Look Remaining Failures Required a Closer Look and Additional Testingand Additional Testing

FindingsFindings

13








•• Testing 2002Testing 2002--0303

Additional TestingAdditional Testing

•• However, The Sample Rate Was However, The Sample Rate Was only 50,000 Samples Per Seconds only 50,000 Samples Per Seconds and I suspect some level of and I suspect some level of smoothing.smoothing.

•• Confirmed the Test Results From Confirmed the Test Results From 1988 on Three Coil Three Legged 1988 on Three Coil Three Legged TransformersTransformers

(February 2002 High Power Lab in Pennsylvania)(February 2002 High Power Lab in Pennsylvania)

14

Dielectric WithstandDielectric WithstandTwo Layers of .005 Nomex 14kV rms for 60 Two Layers of .005 Nomex 14kV rms for 60 SecsSecs..


Partial DischargePartial DischargeAdditional TestingAdditional Testing

15

November, December and JanuaryNovember, December and JanuaryBelmont, North CarolinaBelmont, North Carolina


A Closer LookA Closer LookAdditional TestingAdditional Testing

16

FebruaryFebruaryArden, North CarolinaArden, North Carolina







•• InvestigationInvestigation•• Testing 1988Testing 1988


•• Testing 2002Testing 2002--0303


17

Issue Found !Issue Found !Issue Found !

FindingsFindings

Current Flat During Voltage EscalationCurrent Flat During Voltage EscalationFindingsFindings

18

When Reignition OccursWhen Reignition OccursFindingsFindings

Each Burst Each Burst 70,000 Volts/Micro Sec70,000 Volts/Micro Sec

Each Burst70,000 Volts/Micro Sec

Each BurstEach Burst70,000 Volts/Micro Sec70,000 Volts/Micro Sec

FindingsFindings

19

•• BackgroundBackground•• InvestigationInvestigation

•• Testing 1988Testing 1988•• FindingsFindings•• Testing 2002Testing 2002--0303•• FindingsFindings

•• SolutionSolution





New Circuit for Autotransformer StartersNew Circuit for Autotransformer Starters

SolutionSolution

20

With Surge ArrestersWith Surge Arresters1212--13,000 Volts at 1213,000 Volts at 12--18 amps for 800 Micro Sec18 amps for 800 Micro Sec

SolutionSolution

•• BackgroundBackground•• Investigation Investigation

•• Testing 1988Testing 1988•• FindingsFindings•• Testing 2002Testing 2002--0303•• FindingsFindings

•• SolutionSolution

•• ConclusionsConclusions





21

ConclusionsConclusions

•• When controllers are configured for When controllers are configured for transition on current sensing below 125% transition on current sensing below 125% FLA, high voltage potential is reduced but FLA, high voltage potential is reduced but not eliminated.not eliminated.

•• When autotransformer starters are forced When autotransformer starters are forced to transition before they reach near full to transition before they reach near full speed, high voltages are generated on the speed, high voltages are generated on the 0% taps0% taps


•• Vacuum contactor autotransformer Vacuum contactor autotransformer starters and starter retrofits require starters and starter retrofits require surge arresters even if no problems surge arresters even if no problems were encountered with air break were encountered with air break contactorscontactors

•• When 6 kV distribution surge arresters are When 6 kV distribution surge arresters are installed on a 4,160 volt circuit from the 0% installed on a 4,160 volt circuit from the 0% tap to ground, the voltage is clamped to 13 tap to ground, the voltage is clamped to 13 kV without the resultant high kV without the resultant high dv/dtdv/dt across across the transformer coil regardless of transition the transformer coil regardless of transition currentcurrent

22


•• Care must be taken so the Metal Oxide Care must be taken so the Metal Oxide arrestors do not conduct near normal arrestors do not conduct near normal operating voltages.operating voltages.

•• The 15kV Autotransformer Starter in The 15kV Autotransformer Starter in British Columbia was set on the 55% British Columbia was set on the 55% tap. The starter used MV Circuit tap. The starter used MV Circuit Breakers. Therefore all Breakers. Therefore all autotransformer starters need surge autotransformer starters need surge arrestors zero tap to ground or across arrestors zero tap to ground or across the VI,s. the VI,s.

medium voltage reduced voltage autotransformer

Engineering