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Service Experience with Composite Line Insulators: EPRI Perspective

Service Experience  with   Composite  Line  Insulators:   EPRI  Perspective   2013  INMR  World  Congress  September  8-­11,  2013,  Canada.   Andrew,  J.  Phillips,  Director,  Transmission,  EPRI,  USA;   Christiaan  S.  Engelbrecht,  Consultant,  EPRI,  The  Netherlands  

Introduction Since  2006,  some  utilities  have  experienced  an  increasing  number  of  polymer  insulator  failures  on   115  kV  and  138  kV  transmission  lines.  Investigations  have  shown  that  these  failures  can  be  attributed   to  high  electric  fields  (E-­‐fields)  occurring  close  to,  or  on  the  high-­‐voltage  end  fittings  of  these   insulators.  These  findings  suggest,  contrary  to  common  practice,  that  it  might  be  necessary  to   consider  the  application  of  corona  (also  called  grading)  rings  on  polymer  insulators  on  transmission   lines  with  a  system  voltage  below  161  kV.  Transmission  line  reliability  can  be  affected  if  utilities  do   not  have  measures  in  place  to  minimize  the  effect  of  corona  discharges  on  the  rubber  material.   EPRI  utilized  their  experience  and  research  results  to  help  the  affected  utilities  develop  a  strategy  to   address  premature  aging  of  polymer  insulators  on  115  kV  and  138  kV  transmission  lines  due  to  high   E-­‐fields.  This  included  providing  reference  information,  technical  support  and  recommendations  to   assess  existing  insulator  populations  and  to  specify  polymer  insulators  for  new  or  replacement  units.   In  this  paper  an  overview  is  given  of  these  insulator  failures  and  the  strategy  that  was  followed  to   manage  the  situation.  

Technical Background   EPRI  is  credited  for  being  one  of  the  first  in  identifying  high  E-­‐fields  and  the  resulting  discharge   activity  as  an  important  cause  of  premature  aging  of  polymer  insulators.  Based  on  the  results  from   the  multi-­‐stress  aging  chambers  and  testing  at  the  EPRI  lab  in  Lenox,  Massachusetts,  this   phenomenon  was  identified  as  a  primary  aging  mechanism  on  230  kV  and  500  kV  insulators,  and   appropriate  E-­‐field  limits  for  polymer  were  established.  The  insulator  failures  at  115  kV  and  138  kV   suggest  that  the  same  phenomena  are  present  on  these  lower  system  voltages.   Aging  of  the  insulator  sections  subjected  to  high  localized  electric  fields  is  usually  the  result  the   stresses  associated  with  one  or  more  of  the  following  types  of  discharge  activity:     • • •

Continual corona  activity  from  metallic  end-­‐fittings  or  grading  rings  under  dry  conditions Discharges  due  to  non-­‐uniform  wetting  of  the  polymer  rubber  material Internal  discharges:  e.g.  along  the  interface  between  the  core  and  rubber  housing material,  or  within  the  core  itself.

Continual corona  activity  from  the  metal  end  fittings  may  be  energetic  enough  to  directly  cause   rubber  erosion  and  a  loss  of  galvanization  of  the  metal  end  fitting.       On  hydrophobic  insulators  individual  drops  or  water  patches  of  relatively  limited  extent  may  enhance   the  local  E-­‐field  due  to  the  high  permittivity  of  water  (εr  =  80)  with  a  factor  of  up  to  12  times.    In  the   high  E-­‐field  regions  if  the  insulator  this  enhancement  may  be  sufficient  to  result  in  corona  activity   from  the  edge  of  the  water.  Research  indicates  that  it  is  unlikely  that  water  drop  corona  alone  will   result  in  significant  degradation  of  the  polymer  housing,  as  the  temperature  increases  due  to  this   type  of  corona  is  minimal.  There  is  however  significant  evidence  to  suggest  that  the  chemical  by-­‐ products  of  the  corona,  together  with  moisture,  may  result  in  significant  material  degradation.    In   this  respect  the  formation  of  Nitric  acid  is  considered  important.    It  was  found  that  the  pH  on  the   surface  of  the  insulator  drops  from  an  initial  value  of  about  7  to  3.4  after  15  minutes  of  corona   activity  on  a  wet  insulator  surface  Error!  Reference  source  not  found..    Furthermore  it  was  found   that  some  silicone  rubber  formulations  may  be  particularly  vulnerable  to  deterioration  when   exposed  to  nitric  acid.   The  available  evidence  suggest  that  water  drop  corona  may  just  be  the  initial  phase  of  the  following,   more  severe,  degradation  mechanism  that  affects  the  long-­‐term  performance  of  the  insulator.     Present  understanding  of  this  process  is  as  follows:   1.

2. 3. 4.


6. 7.

Water drop  corona  in  the  high  E-­‐field  regions  results  in  localized  loss  of  hydrophobicity.   Regions  affected  have  E-­‐field  magnitudes  above  the  water  drop  corona  onset  threshold  –   see  Figure  1.   Under  wetting  conditions,  patches  of  water  form  in  the  regions  of  lower  hydrophobicity.   These  surface  water  patches  are  separated  from  each  other  by  dry  regions  or  bands.     Localized  arcs  form,  bridging  the  gaps  between  the  water  patches  Error!  Reference   source  not  found..     The  energy  and  temperature  of  these  localized  arcs  are  significantly  higher  than  that  of   water  drop  corona,  stressing  the  rubber  surface  further  Error!  Reference  source  not   found..   Over  time,  as  the  affected  regions  lose  hydrophobicity,  and  completely  wet  out,  the  E-­‐ field  in  the  adjacent  regions  is  enhanced  above  the  water  drop  corona  onset  threshold   under  wetting  conditions.     The  aging  mechanism  is  then  initiated  in  the  previously  unaffected  regions.  In  this   manner,  the  region  affected  is  increased.         The  by-­‐products  formed  by  corona  in  combination  with  water,  notably  nitric  acid,  may  be   aggressive  to  the  housing  resulting  in  cracks  in  the  material,  or  corrosion  of  the  end   fittings.  

Wetting corona  activity  

Loss of  hydrophobicity  

Figure 1: Water induced corona activity and the resulting loss of hydrophobicity on a hydrophobic composite insulator.

During wetting  conditions,  the  rubber  surface  of  hydrophilic  composite  insulators  (such  as  EPDM)  is   more  likely  to  be  covered  with  patches  of  water,  rather  than  distinct  droplets.  Dry  regions  separate   these  patches,  and  due  to  E-­‐field  enhancement,  sparking  may  occur  between  patches.  These   discharges  are  more  energetic  than  corona  and  may  degrade  the  rubber  material.    Although  this   activity  may  also  occur  away  from  the  high  E-­‐field  region,  casual  observation  in  aging  tests  indicates   that  it  is  more  prevalent  in  the  high  E-­‐field  regions.  

Ultraviolet image  

Infrared image  

Figure 3: Infrared and ultraviolet images of non-uniform wetting discharge activity on a hydrophilic composite insulator.

Sufficiently high  E-­‐field  magnitudes  may  result  in  discharge  activity  in  internal  defects  –  such  as  voids   inclusions  and  poor  bonding  between  sheath  and  core.    This,  in  turn,  may  eventually  lead  to  the   failure  of  the  insulator  either  by  destruction  of  rod  by  discharge  activity  or  by  flash-­‐under  –  see   Figure  4  for  examples.    

Destruction of  rod  by  discharge  activity  


Figure 4: Examples of insulators that failed due to destruction of rod by discharge activity and flash-under.

Research has  shown  that  not  all  insulators  are  equally  affected  by  high  electric  fields.    Important   factors  that  influence  the  rate  and  level  of  degradation  are:   • • •

The type  of  rubber  and  the  makeup  of  the  weather  shed  system     The  makeup  of  the  end  fitting  seal   The  level,  location  and  type  of  discharge  activity,  which  is  determined  to  a  large  extent  by   the  E-­‐field  field  along  the  insulator,  the  type  and  intensity  of  wetting,  the  presence  of   contaminants  and  the  level  of  hydrophobicity  of  the  material.  

Service Experience   The  occurrence  of  five  insulator  failures  between  June  2006  and  August  2007  on  115  kV  and  138  kV   lines  prompted  three  utilities  in  the  United  States  initiate  a  study  to  better  understand  the  aging   mechanisms  on  insulators  of  this  voltage  class.    As  is  evident  from  EPRI’s  failure  database,  see  Figure   5,  these  failures  were  not  isolated  incidences,  but  rather  part  of  an  increasing  trend  of  failures   reported  to  EPRI  on  115  kV  to  138  kV  insulators.  The  data  in  Figure  5  show  that  since  1998  on   average  7  failures  per  year  were  reported  to  EPRI.  

Figure 5: Numbers of 115 kV to 138 kV polymer insulator failures recorded by EPRI.

A breakdown  of  the  failure  modes  of  the  140  failures  recorded  in  the  EPRI  database  for  of  115  kV  to   138  kV  insulators  is  presented  in  Figure  6.  From  this  figure  it  can  be  seen  that  the  dominant  failure   modes  were  stress  corrosion  cracking  (brittle  fracture)  and  flash-­‐under.  Examples  of  a  stress   corrosion  failure  are  shown  in  Figure  7.    A  large  proportion  of  these  failures  were  on  the  same   insulator  design  and  on  units  manufactured  between  1993  and  1999.  

Figure 6: A breakdown of the failure mode of 115 kV to 138 kV polymer insulator failures in the EPRI failures database.

Figure 7: Example images of the fracture surfaces and one of the failed insulators in-situ.

During the  failure  investigations  it  was  shown  that  all  these  failures  could  directly  be  attributed  to   continual  discharge  activity  from  the  end  fitting  under  dry  conditions.    This  continual  exposure  to   corona  resulted  in  cracks  in  the  rubber  sheath  and  degradation  of  the  end  fitting  seal.    Once  the  seal   is  compromised,  moisture  can  come  into  contact  with  the  rod,  leading  to  a  brittle  fracture  of  the   fiberglass  rod.    Brittle  Fracture  is  a  mechanical  failure  of  the  fiberglass  rod  due  to  acid  attack  where   the  fracture  exhibit  one,  or  more,  smooth,  clean  planar  surfaces,  mainly  perpendicular  to  the  axis  of   the  fiberglass  rod,  giving  the  appearance  of  the  rod  being  cut  –  as  is  shown  in  Figure  7.   As  consequence  of  these  failures,  utilities  were  forced  to  reexamine  the  use  of  corona  rings  (or  lack   thereof)  on  115/138  kV  polymer  insulators.    Utilities,  in  cooperation  with  EPRI,  have  therefore   initiated  a  number  of  specific  activities  during  in  2007  and  2008  to  assess  the  risk  of  115  /  138  kV   polymer  insulators  to  premature  ageing  due  to  high  electric  fields.    These  included:   • • •

Daylight Discharge  Inspections,   Detailed  examinations  of  insulators  taken  from  service,  failure  investigations  and   E-­‐field  Calculations.  

It should  be  noted  that  these  activities  focused  on  the  particular  insulator  design  that  suffered  the   failures.      

Daylight Discharge  Inspections   EPRI  and  5  utility  members  together  performed  daytime  discharge  inspections  on  twelve  115  and   138kV  transmission  lines  to  determine  whether  continuous  discharge  activity  is  occurring  from  the   end  fittings  under  dry  conditions.      These  inspections  were  primarily  directed  towards  one  particular   insulator  design,  but  there  were  also  opportunities  to  inspect  a  limited  number  of  other  insulator   designs.    Some  examples  of  corona  observations  are  presented  in  Figure  8.  

Insulator Type  A  

Insulator Type  B  

Insulator Type  C  

Insulator Type  D  

No Corona Observed

Figure 8: Examples of discharge activity observed from two different designs of insulator.

Conclusions from  these  inspections  are:   •

• •

Corona discharge  activity  under  dry  conditions  was  observed  on  the  end  fittings  of  the   composite  insulators  installed  on  all  twelve  115  kV  and  138  kV  transmission  lines   inspected.      Not  all  insulators  on  the  lines  had  corona  activity  though.   Corona  discharges  are  more  likely  to  occur  on  dead-­‐end  strings  and  least  likely  on  brace   post  configuration.    This  is  not  completely  unexpected  as  it  is  known  from  previous   calculations  that  the  E-­‐field  is  generally  higher  for  dead-­‐end  insulators  than  it  is  for   suspension  units  or  brace  post  configurations.   To  date,  corona  activity  has  been  observed  on  three  out  of  the  four  insulator  designs  (i.e.   different  manufacturers)  inspected,  as  shown  in  Figure  8.   In  one  case  daylight  corona  observations  were  made  before  and  after  the  installation  of  a   corona  ring.  This  confirmed  that  the  addition  of  a  ring  eliminated  corona  from  the   insulator  end  fitting.  

Detailed Inspections   EPRI  has  worked  with  5  utilities  in  evaluating  the  degradation  on  over  200  115  kV  and  138  kV   insulators  removed  from  service.    All  of  these  insulators  were  installed  without  corona  rings  and   were  of  the  same  design.  The  units  were  installed  between  1994  and  2006.       74  of  the  insulators  removed  from  service  were  subjected  to  a  detailed  examination  comprising  a  (1)   visual  inspection,  (2)  Hydrophobicity  measurement,  (2)  dye  penetration  test,  (3)  dissection  and  in   some  cases  (4)  mechanical  testing.    The  remaining  units  were  evaluated  only  by  performing  a  visual   inspection.      

Some examples  of  the  degradation  observed  are  presented  in  Figure  9.    In  all  cases  it  was  found  that   the  most  severe  degradation  was  observed  in  the  same  areas  where  dry  corona  activity  was  seen   during  the  daylight  discharge  inspections.    On  some  units  it  was  found  that  the  degradation  of  the   sheath  and  end  fitting  seal  progressed  so  far  that  the  rod  was  exposed  to  the  environment.    These   latter  units  are  considered  as  high  risk  units  where  failure  is  considered  inevitable.  

Loss of  hydrophobicity  

Degradation of  the  end  fitting  seal  

Cracking of  shed  

Cracking of  the  sheath  

Figure 9: Examples of discharge activity observed from two different designs of insulator.

E-­field Calculations   EPRI  performed  extensive  3-­‐D  E-­‐field  calculations  for  four  utilities  at  both  115  and  138  kV  to  obtain  a   better  understanding  of  the  E-­‐field  distribution  that  can  be  expected  on  115  and  138  kV  insulators,   the  parameters  that  influence  it  and  to  evaluate  some  remedial  measures.    Importantly  the  E-­‐field   calculations  accounted  for  the  presence  of  all  three  phases,  and  in  some  cases  adjacent  circuits,  on   the  transmission  structure.  The  calculations  focused  on  those  structures  where  failures  occurred   previously  or  where  corona  has  been  observed.       These  calculations  considered  both  the  E-­‐field  on  the  end-­‐fittings  –  to  indicate  the  likelihood  of  dry   corona  –  that  along  the  insulator  sheath  –  to  indicate  the  likelihood  for  water  induced  corona.   The  following  conclusions  were  drawn  from  the  E-­‐field  calculation  results:   • • •

Dead-­‐end insulators  have  higher  E-­‐field  magnitudes  than  suspension  insulators   Single  dead-­‐end  insulators  have  higher  E-­‐field  magnitudes  than  double  dead  end   insulators.     The  addition  of  a  hot  line  link  results  in  a  slightly  higher  E-­‐field  magnitude  on  the   insulator.  

• • •

Insulator A  

There is  a  significant  difference  in  the  E-­‐field  levels  between  different  insulator  designs  –   see  Figure  10.    Small  and  slender  end  fittings  tend  to  have  higher  E-­‐fields  in  the  region  of   the  end  fitting  seal.    The  shape  of  the  end  fitting  dictates  where  the  highest  field  occur   and  accordingly  whether  or  not  the  dry  corona,  if  present,  will  be  in  contact  with  the   housing  material.   E-­‐field  magnitudes  exceed  the  EPRI  recommended  limits  on  all  designs  of  115  kV  and  138   kV  polymer  insulators  when  installed  without  corona  rings.       The  addition  of  8”  corona  rings  at  the  live  end  of  the  insulator  is  in  most  cases  sufficient   to  reduce  the  E-­‐field  magnitudes  to  an  acceptable  level.   The  E-­‐field  modeling  results  together  with  the  DayCor  inspection  confirmed  that  the   failures  that  occurred  on  115  kV  and  138  kV  insulators,  and  the  observed  degradation,   can  be  associated  with  a  high  E-­‐field  levels  on  the  insulators.   E-­‐field  limits  need  to  be  adjusted  downward  for  insulators  installed  at  high  altitude  i.e.   above  3300  ft  (1000  m).  

Insulator B  

Insulator C  

Insulator D  

Figure 10: Examples of the E-filed calculated on the insulator end fitting without corona rings. Blue corresponds to the lowest E-field magnitude and Red to the highest. The corona threshold corresponds approximately to orange

Population Assessment   In  the  previous  section  service  experience  is  presented  that  suggests  very  strongly  the  need  for   corona  rings  on  115  kV  and  138  kV  polymer  insulators  to  protect  them  from  premature  ageing  due  to   corona  activity.    Although  this  may  seem  simple,  the  implications  of  such  a  conclusion  may  be  quite   extensive,  especially  if  large  numbers  of  these  insulators  are  installed.    Utilities  are  then  faced  with   the  difficult  task  of  identifying  high-­‐risk  units,  and  to  decide  what  the  most  appropriate,  and  cost   effective  remedial  actions  is  to  undertake.  Fortunately,  the  deterioration  due  to  corona  discharge   activity  develops  slowly,  which  gives  Utilities  some  time  to  do  a  proper  condition  assessment.     EPRI  has  helped  Utilities  to  develop  a  population  assessment  strategy  to  address  premature  aging  of   polymer  insulators  on  115  kV  and  138  kV  transmission  lines  due  to  high  E-­‐fields.  A  key  in  the   development  of  this  strategy  is  the  set  of  tools  developed  and  maintained  by  them  that  includes  field   guides,  failure  databases,  E-­‐field  modeling  techniques,  corona  inspection  technologies,  and  relevant   accelerated  aging  test  results.      An  overview  of  the  process  is  given  in  Figure  12.  

Figure 12: An overview of a strategy to perform a population assessment on polymer insulators.

In addition  EPRI  has  an  ongoing  a  follow-­‐up  research  effort  that  includes  the  development  of  small   scale  accelerated  aging  tests  specifically  to  address  these  concerns.  

Conclusions Since  2006  there  have  been  an  increasing  number  of  polymer  insulator  failures  recorded  on  115  kV   and  138  kV  transmission  lines.  These  failures  were  seen  in  a  serious  light  as  they  occurred  mostly  on   the  more  critical  dead-­‐end  insulators  that  pose  a  threat  to  system  integrity  due  to  the  risk  of  a   downed  conductor.  Investigation  results  suggests  that  these  failures  are  due  to  high  electric  fields  (E-­‐ fields)  occurring  close  to,  or  on,  the  high  voltage  end  fittings  of  these  insulators.    Consequently   corona  or  grading  rings  may  also  be  necessary  for  polymer  insulators  installed  at  115  kV  and  138  kV.       Higher  levels  of  dry  corona  activity  from  the  end  fittings  occurred  in-­‐service  than  was  expected  based   on  laboratory  testing.    E-­‐field  modeling  showed  two  reasons:   1. At  115  kV  and  138  kV  the  close  proximity  of  the  nearby  phases  increases  the  surface  E-­‐field   magnitudes  by  a  significant  amount.         2. Most  laboratory  testing  is  done  on  suspension  configurations  while  the  E-­‐field  magnitudes  on   dead-­‐end  and  hard  angle  insulators  is  higher.   Prompted  by  these  developments  EPRI  initiated  a  supplemental  project  to  provide  participating   utilities  with  the  information  necessary  to  develop  a  strategy  to  address  premature  aging  of  polymer   insulators  due  to  high  E-­‐fields.      

Service Experience With Composite Line Insulators: EPRI Perspective  

INMR Article: Service Experience With Composite Line Insulators: EPRI Perspective