Cross-linked polyethylene (XLPE) is commonly used within high voltage cable insulation. It has improved thermal and mechanical resistance compared to normal low density polyethylene (LDPE). However, the cross-linking process may also vary the electrical characteristics of the material. This paper investigates changes in electrical properties of one type of LDPE before and after cross-linking. The effective lightning resistance is also considered, as the application of repetitive lightning impulse overvoltages can be a factor in insulation material ageing of high voltage cables. The material was cross-linked using trigonox-145 peroxide with controlled concentration. Samples were moulded to have a Rogowski profile and gold coated to make sure that they are evenly electrically stressed. Obtained results show that there are reductions in both space charge injection and the permittivity of the material after it is cross-linked. The breakdown strength of the material was also improved. However, the samples studied are more susceptible to ageing due to lightning impulses

Dao, N L

Lewin, P L

Swingler, S G

English

Southampton (e-Prints Soton)

EFFECT OF CROSS-LINKING ON THE ELECTRICAL PROPERTIES OF LDPE AND ITS LIGHTNING IMPULSE AGEING CHARACTERISTICS N.L. Dao1*, P.L. Lewin1, S.G. Swingler1 1 University of Southampton, the United Kingdom *Email: nld06r@ecs.soton.ac.uk  Abstract: Cross-linked polyethylene (XLPE) is commonly used within high voltage cable insulation. It has improved thermal and mechanical resistance compared to normal low density  polyethylene  (LDPE).  However,  the  cross-linking  process  may  also  vary  the electrical  characteristics  of  the  material.  This  paper  investigates  changes  in  electrical properties  of  one  type  of  LDPE  before  and  after  cross-linking.  The  effective  lightning resistance  is  also  considered,  as  the  application  of  repetitive  lightning  impulse  over-voltages can be a factor in insulation material ageing of high voltage cables. The material was  cross-linked  using  trigonox-145  peroxide  with  controlled  concentration.  Samples were moulded to have a Rogowski profile and gold coated to make sure that they are evenly electrically stressed. Obtained results show that there are reductions in both space charge injection and the permittivity of the material after it is cross-linked. The breakdown strength  of  the  material  was  also  improved.  However,  the  samples  studied  are  more susceptible to ageing due to lightning impulses.   1  INTRODUCTION Over  last  two  decades,  oil  impregnated  paper cable  insulation  has  been  replaced  by  polymeric insulation  material.  Cross-linked  polyethylene (XLPE) is used within high voltage cable insulation. It has improved thermal and mechanical resistance compared  to  normal  low  density  polyethylene (LDPE). LDPE can be cross-linked using peroxides at  elevated  temperatures.  However,  the  cross-linking  process  may  also  vary  the  electrical characteristics of the material due to the physical change in material itself and also the accumulation of residual by-products [1]. Impulse over-voltages are  very  common  phenomena  in  electric  power systems.  A  switching  impulse  is  created  by  a switching  surge  or  local  fault  while  a  lightning impulse is due to  lightning strike to an overhead line. The application of repetitive lightning impulses can  lead  to  accelerated  ageing  of  extruded polymeric cables [2-4]. The results show that there may well be a reduction in electric field strength of the insulation of a power cable that experiences a lot of lightning impulse over-voltages. The effect of a lightning impulse is dependent on the molecular structure  of  material.  This  paper  highlights  some changes  in  electrical  properties  of  LDPE  before and after crosslinking by peroxide. The analysis of the  behaviour  of  the  materials  that  experience lightning impulses will give a better understanding of  the  impulse  ageing  process.  In  this  work, moulded discs with a Rogowski profile have been manufactured  in  both  LDPE  and  XLPE.  The samples  then  have  been  electrically  aged  using standard  lightning  impulses.  A  real-time  software based monitoring tool has been designed to control the  impulse  wave-shape  and  monitor  the  ageing process. The electrical properties of materials have been  considered  by  breakdown  measurements, dielectric  spectroscopy  and  space  charge measurements. 2  MATERIALS USED AND SAMPLE PREPARATION The material used is the low density polyethylene LD100 from Exxon Mobil Chemical for blown film applications with a melting temperature from 160oC to  180oC.  The  material  has  MFI  of  2g/10min (ASTM  D  1238)  and  a  transition  temperature  of 103oC.  This  is  a  low  density  (0.923g/mm3) branched  polyethylene.  Trigonox145-E85  is  the mixture  of  85%  2,5-Dimethyl-2,5-di(tert-butylperoxy)hexyne-3  and  mineral  oil  (Figure  1) from  AkzoNobel  .  The  mixture  is  liquid  at  room temperature and was stored in the fridge. It can be used as the initiator for the crosslinking process of polyethylene.  This  peroxide  is  more  stable  with increasing  temperature  than  Dicumyl  peroxide (DCP).  The  half-life  of  trigonox145  at  150oC  is about 45 minutes and less than 1 minute at 200oC where as the half-life of DCP is 1 minute at 170oC. The  LD100  pellets  were  soaked  with  the Trigonox145 to create cross-linkable polyethylene (trigonoxLD100). This was performed under room temperature for 5 hours and followed by an hour for  drying  out  at  70oC  in  an  oven.  This  process results  in  a  concentration  of  1.1%  trigonox145 (XL1). Under 70oC for 5 hours, the process results in a 3.3%   trigonox145 concentration (XL3). The trigonoxLD100 was safely heated at 150oC for 10 minutes before the pressure was applied (normally 1.5 to 2 tons) for about 1minute. After releasing the pressure, the mould was moved to another press at 200oC to cross-link sample for 10 more minutes (this ensures more than 70% of gel-content would be  formed).  The  decomposition  products  formed during the cross-linking (Figure 1) are gaseous so XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011no  pressure  was  applied.  After  the  cross-linking reaction, the mould was quench cooled using tap water  and  the  sample  released  from  the  mould. The XL1 sample was made from LD100 with 1% trigonox  addition  and  crosslinked  for  5  min  at 200oC. For LDPE, the pellets were just melted at 180oC  before  a  pressure  of  2  tons  was  applied. The  mould  was  then  quenched  cooled  with  tap water.   Figure  1:  Trigonox-145  and  cross-linking  by-products The  moulded  samples  have  the  shape  shown  in Figure 2. The top surface of the sample follows a Rogowski profile, while the bottom surface is flat. Samples  for  breakdown  and  dielectric spectroscopy were gold sputter coated to improve the sample-electrode contact. The used for space charge measurement were left uncoated to prevent flashover.  Figure 2: Samples and mask for coating 3  EXPERIMENTAL PROCEDURES The samples after moulding experience an impulse ageing  process.  A  set  of  3000  negative  lightning impulses  with  front/tail  times  of  1.2/40µs respectively  were  applied  to  create  a  peak electrical  field  of  85kV/mm  across  the  samples. The  ageing  process  is  controlled  by  a  Labview program which manages both the lightning impulse parameter  calculation  and  ensures  safety  if breakdown occurs. Samples were aged in silicon oil  under  room  temperature.  An  external  optical trigger was implemented to provide a fixed impulse generation  rate.  Approximately  33  impulses  were generated  each  minute.  Samples  after  impulse ageing  were  used  in  either  breakdown measurements,  dielectric  spectroscopy  or  space charge  measurements.  Sample  breakdown strength  was  measured  using  a  step  up transformer  which  generates  AC  voltages  with  a ramp rate of 100V/s [5]. Dielectric spectroscopies of  samples  were  measured  using  the  Solartron system  including  1296  dielectric  interface  and 1260  impedance  analyser.  The  measurements were taken over the frequency range from 10-1 to 106Hz. A pulse electro-acoustic (PEA) experiment was used to measure the space charge profile [6]. Samples  were  stressed  with  an  electric  field  of 34kV/mm.  The  average  charge  density  (without sign)  in  the  material  during  charge  decay  was measured  and  results  curve  fitted  using  a  power law. 4  RESULTS The  breakdown  strengths  of  aged  and  un-aged LDPE samples show no significant difference from a  statistical  point  of  view  (Figure  3).  The  90% confidence  regions  overlap  over  the  whole spectrum. The mean value for breakdown strength of virgin LDPE is about 98.8kV/mm, whereas after 3000  lightning  impulses  the  aged  LDPE  mean value is 98.4kV/mm. Therefore, it can be said that LD100 is not sensitive to lightning impulses.  Figure 3: Breakdown strength of LDPE The XL3 has the same LD100 as the base material but  provides  a  much  better  performance  with  a mean  breakdown  strength  of  129kV/mm  (Figure XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 20114).  It  suggests  that  the  added  peroxide  trigonox-145  or  the  cross-linking  process  improves  the electrical strength. However, the material becomes more sensitive to lightning impulse ageing as the aged  samples  have  lower  mean  breakdown strength (121kV/mm).   Figure 4: Breakdown strength of XL3 The obtained results for dielectric spectroscopy of the  samples  are  shown  in  Figure  5  and  6.  For LDPE  (Figure  5),  the  loss  factor  of  the  aged sample is higher than the un-aged sample over a certain frequency range. However, there is no clear difference at power frequencies which were used for the breakdown measurements. The real parts of the relative permittivity are also constant with no difference between samples. The measured value of real relative permittivity is about 2.6, this is high compared  to  normal  LDPE  (about  2.3-2.4).  This may  be  the  consequence  of  the  quenching operation  during  sample  preparation,  the  smaller molecular  weight  and  the  presence  of  more branches on the main chains. All these lead to a less  dense  structure  and  molecules  can  move more easily under the application of an electrical field.  The  insensitivity  of  impulse  ageing  of  this LDPE  may  be  because  of  the  disorder  in  the amorphous  regions  after  quenching,  which dominates  the  properties  of  the  material  and overshadows  any  effect  of  impulse  ageing.  After cross-linking  by  peroxide,  the  real  part  of permittivity reduces from 2.6 to 2.3 (Figure 6). The cross-linking  limits  the  movement  of  chains  by linking  chains  to  create  a  bigger  structure.  The slope of loss factor in the low frequency region of XL3  is  a  bit  steeper  as  the  conduction  of  by-products may contribute to losses. The losses for aged XL3 are very much the same as the  virgin sample  except  at  higher  values  of  the  low frequency region. Therefore, ionic conduction can occur  and  increase  losses.  The  real  part  of  the permittivity increases after 3000 lightning impulses. With  no  appearance  of  oxidation,  this  increase could be due to chain scission and molecules can then move more easily within the bulk.  Figure 5: Dielectric spectroscopy of LDPE  Figure 6: Dielectric spectroscopy of XL3 Figure  7  and  Figure  8  show  the  space  charge profiles  of  LDPE  samples.  There  is  a  significant amount  of  positive  hetero-charges  formed  in  the LDPE  near  the  cathode.  Previous  work  showed similar formation of hetero-charges in LDPE if the material  is  quenched  cooled  [7].The  obtained results  indicate  that  hetero-charge  forms  within about  10  minutes.  After  that,  the  charge  profile remains  constant  for  the  next  50  minutes.  The charge gradually decays after the applied field is removed. After an hour, a lot of charges still exist in  the  bulk.  The  lightning  impulse  aged  LDPE shows a similar charge profile pattern as the virgin LDPE.  The  difference  in  peak  charges  for  two samples can be ignored. The amount of charge in XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011aged and virgin samples after 60 minutes poling is the  same  but  the  charges  in  the  aged  sample decay more slowly (Figure 12)  Figure 7: Space charge profile of LDPE  Figure  8:  Space  charge  profile  of  impulse  aged LDPE Charge  profiles  of  XL1  and  XL3  are  shown  in Figure 9 and 10. The result shows bipolar charge injection takes place with significant positive space charge in XL1. The positive charges are attracted to the cathode but limited by the negative charges from cathode. With the same electrical field stress of 34kV/mm as used with LDPE samples, very little positive  charge  formed  near  the  cathode  in  the bulk of XL3 sample.  A small amount of negative charge was observed near the anode. As shown in Figure  12,  the  increase  in  amount  of  peroxide content  for  crosslinking  from  0  to  3%  reduces charge  accumulation  in  XLPE  but  the  rate  of charge decay also decreases. It suggests that the effect  of  crosslinking  process  includes  both changes  of  morphology  and  existence  of  by-products in the bulk of the material. There is a difference in the space charge patterns of the impulse aged XL3 and virgin XL3. Positive charge formation is more consistent and there is a deeper  penetration  into  the  bulk  of  the  impulse aged  sample  (Figure  11).  The  shape  of  bulk charge is similar to results reported previously for impulse  aged  HDPE  [5].  The  amounts  of  charge formation  are  quite  small  for  both  virgin  and impulse aged XL3. The charges in virgin XL3 are concentrated  near  to  the  electrodes  where  the charges  spread  through  the  whole  sample.  The average  total  charge  in  Figure  12    shows  that there is a larger amount of charge in the impulse aged  XL3  and  this  charge  decays  slower  than virgin XL3.  Figure 9: Space charge profile of XL1 XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011 Figure 10: Space charge profile of XL3  Figure 11: Space charge profile of impulse aged XL3   Figure 12: Average charge densities of samples 5  DISCUSSIONS It  is  thought  that  the  effect  of  lightning  impulses may be to produce hot electrons with high energy that enter the material bulk. These electrons collide with molecules of the sample damaging molecular bonds  and  disturbing  the  crystalline  structure  of material.  The  homogeneity  of  the  material  is reduced and this leads to a reduction in breakdown strength  of  the  material.  With  quench  cooled LDPE, the crystallinity of material is low. The small contribution  to  increasing  the  amorphous  region after  impulse  ageing  can  only  lead  to  a  small change in breakdown strength. The space charge results  for  LDPE  show  that  the  same  amount  of charge  was  obtained  in  the  case  of  the  impulse aged sample but the charges decay with a slightly lower rate. Therefore, the effect of impulse ageing seems to be negligible. The  peroxide  crosslinking  process  changes  the morphology  of  quenched  cooled  LDPE  and consequently  lead  to  increased  breakdown strength  of  the  material.  The  chains  are  linked together, molecular size increases and the mobility of  molecules  is  reduced.  The  effect  of  impulse ageing may cause bond breaking and ionization. It may lead to the increase in ionic conduction and subsequently  interfacial  polarization.  The movement of molecules under an electric field may also become easier as the molecule weights could be reduced after the ageing process. XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 20116  CONCLUSIONS It  has  been  shown  that  the  XLPE  formed  using trigonox-145  exhibits  a  higher  electrical  strength and  lower  space  charge  formation  compared  to quench cooled LDPE. However, the XLPE seems to  be  more  sensitive  to  the  effect  of  lightning impulse  ageing.  The  impulse  aged  XLPE  has  a reduced breakdown strength, higher losses due to conduction and more charge injection under a DC applied voltage. 7  ACKNOWLEDGMENTS Authors  wish  to  acknowledge  the  EPSRC  for funding this work.  8  REFERENCES [1]  M.  Fu,  G.  Chen,  A.  Dissado,  and  J.  C. Fothergill, "Influence of thermal treatment and residues  on  space  charge  accumulation  in XLPE for DC power cable application," IEEE Transactions  on  Dielectrics  and  Electrical Insulation, vol. 14, pp. 53-64, 2007. [2]  S.  Boev,  "Electric  aging  of  polyethylene  in pulsed  electric  field,"  in  IEEE  International Pulsed  Power  Conference,  Monterey,  CA, USA, 1999, pp. 1365-1368. [3]  G.  C.  Stone,  R.  G.  Van  Heeswijk,  and  R. Bartnikas,  "Electrical  aging  and electroluminescence in epoxy under repetitive voltage  surges,"  IEEE  Transactions  on Electrical  Insulation,  vol.  27,  pp.  233-244, 1992. [4]  R.  A.  Hartlein,  V.  S.  Harper,  and  H.  Ng, "Effects  of  voltage  surges  on  extruded dielectric  cable  life  project  update,"  IEEE Transactions  on  Power  Delivery,  vol.  9,  pp. 611-619, 1994. [5]  N. L. Dao, P. L. Lewin, I. L. Hosier, and S. G. Swingler, "A comparison between LDPE and HDPE  cable  insulation  properties  following lightning  impulse  ageing,"  in  2010  IEEE International Conference on Solid Dielectrics, ICSD  2010,  July  4,  2010  -  July  9,  2010, Potsdam, Germany, 2010, pp. 1-4. [6]  Y.  Li,  M.  Yasuda,  and  T.  Takada,  "Pulsed electroacoustic  method  for  measurement  of charge accumulation in solid dielectrics," IEEE Transactions  on  Dielectrics  and  Electrical Insulation, vol. 1, pp. 188-195, 1994. [7]  Y.  Tanaka,  G.  Chen,  A.  Vaughan,  and  T. Takada, "Space charge formation in quenched low  density  polyethylene,"  in  Proceedings  of the 7th International Conference on Properties and Applications of Dielectric Materials, June 1, 2003 - June 5, 2003, Nagoya, Japan, 2003, pp. 970-973.   XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011

A comparison between LDPE and HDPE cable insulation properties following lightning impulse ageing,&quot;

Effects of voltage surges on extruded dielectric cable life project update,&quot;

Electric aging of polyethylene in pulsed electric field,&quot;

Electrical aging and electroluminescence in epoxy under repetitive voltage surges,&quot;

Influence of thermal treatment and residues on space charge accumulation in XLPE for DC power cable application,&quot;

Pulsed electroacoustic method for measurement of charge accumulation in solid dielectrics,&quot;

Space charge formation in quenched low density polyethylene,&quot;

Effect of Cross-Linking on the Electrical Properties of LDPE and its Lightning Impulse Ageing Characteristics

Effect of Cross-Linking on the Electrical Properties of LDPE and its Lightning Impulse Ageing Characteristics

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