It is believed that space charge buildup in cross-linked polyethylene (XLPE) insulation is the main cause for premature failure of underground power cables. The space charge activities in XLPE depend on many factors such as additives, material treatment, ambient temperature, insulator/electrode interface, etc. Degassing is one of the material treatment process commonly employ in cable manufacturing to improve insulation performance. In this paper, investigation on the effect of degassing period has on the morphology and space charge was carried out. Planar XLPE samples of the same composite were subjected to different degassing time. It is discovered that apart from removing volatile by-products, degassing also anneal XLPE material; changing the morphology as a result

Chen, G

Chong, Y L

Ho, Y F F

English

Southampton (e-Prints Soton)

 The Effect of Degassing on Morphology and Space charge  Y.L. Chong*, G. Chen and Y. F. F. Ho, Department of Electronics & Computer Science University of Southampton Highfield, Southampton SO17 1BJ United Kingdom *E-mail:ylc@soton.ac.uk   Abstract:  It is believed  that  space  charge  buildup  in cross-linked polyethylene (XLPE) insulation is one of the  main  cause  for  premature  failure  of  underground power  cables.    The  space  charge  activities  in  XLPE depend  on  many  factors  such  as  additives,  material treatment,  ambient  temperature,  insulator/electrode interface,  etc.    Degassing  is  one  of  the  material treatment  processes  commonly  employed  in  cable manufacturing  to  improve  insulation  performance.     In this paper, investigation on the effect of degassing period on the morphology and space charge was carried out.    Planar  XLPE  samples  of  the  same  composite were  subjected  to  different  degassing  time.    It  is discovered  that  apart  from  removing  volatile by-products,  degassing  also  anneals  XLPE  material; changing the morphology as a result.  INTRODUCTION  Polyethylene  (PE)  based  materials  are  replacing oil/paper  as  the  main  insulating  medium  for underground  high  voltage  power  cables.    The  main advantage  of  polyethylene  lies  in  its  high  intrinsic electrical breakdown strength.  PE has an upper operational temperature of about 70 oC. This limits its application as an insulating material for underground cables since it is not uncommon for the cables  to  operate  at  temperature  as  high  as  90 oC.   This problem can be largely overcome by cross-linking PE to form cross-linked polyethylene (XLPE).  Cross-linking  is  the  process  whereby  the  polymer structure is modified through the formation of chemical bonds  between  the  polymer  chains.    The  most common method of chemical cross-linking is to trigger a thermally activated catalyst, such as dicumyl peroxide, by subjecting the polymer to high temperatures.      The thermal and mechanical properties of the polymer were  reported  to  be  significantly  enhanced  by  the cross-linking process [1].    However, the additives and by-products  of  the  cross-linking  reaction  can  act  as impurities  and  significantly  affect  the  electrical properties of the insulation.    One of such effects is the formation of space charge. CROSS-LINKED POLYETHYLENE  As mentioned earlier, the main advantages of XLPE is its good electrical, mechanical and thermal properties.   However,  when  subjected  to  high  electric  stress  over prolong period, charges may be trapped in the material.   This is known as space charge.  Space charge formations result in local enhancement of electric stress which in turn alters the internal electric field distribution from the anticipated value. This may lead to breakdown of the insulation at stress level below the  designated  values.    Indeed,  stress  enhancement level as high as 3 times the applied electric stress has been reported for polyethylene based materials [2].  XLPE may have good thermal performance but along with it comes the problem associated with the residual crossed-linked by-products.    Under high electric stress, they  can  be  dissociated  into  ionic  pairs,  forming heterocharge at the semi-con/insulator interfaces which enhance the electric stress.    This is a main cause for concern since majority of electric breakdown in cables occurs  at  the  interfaces.    There  are  also  reports  that the cross-linking could act as localized charge trapping centres [1].  One of the most common material treatments adopted by cable manufacturer to improve the performance of cable  is  degassing.    Degassing  is  the  process  of subjecting  the  XLPE  to  moderately  high  temperature thereby removing volatile residual by-products.  In this paper, the effect of degassing on space charge and  morphology  is  investigated.    The  implication  of the various degassing parameters on the morphology of the material and their influence on space charge is also explained.  SAMPLE DETAILS  All  samples  used  in  the  study  was  made  using  cable grade  cross-linked  polyethylene  (Borealis  XL4201S) with semi-con electrodes made from the same grade of polyethylene material, but loaded with carbon black to 2004 International Conference on Solid Dielectrics, Toulouse, France, July 5-9, 2004 0-7803-8348-6/04/$20.00 ©2004 IEEE. increase its conductivity (Borealis LEO592).    Both the front and the rear electrodes were hot pressed into the plaque sample about 200 OC for 10 minutes to ensure cross-linking [3].    All degassing treatments were done in a vacuum oven, after which the samples were cooled at  room  temperature  for  15  minutes.    Details  of  the samples are shown in Table1.    Thickness mm Degassing Temp. Degassed period Applied Stress kVdc/mm A  3.40  -  0 hour B  3.40  60 OC  6 hours C  3.40  60 OC  12 hours D  3.40  60 OC  24 hours E  3.40  60 OC  48 hours F  3.40  90 OC  48 hours G  1.60  90 OC  48 hours 33.33 34.15 34.48 34.48 34.90 30.09 30.81 Table 1. Sample details  EXPERIMENT  The  effect  of  degassing  time  and  temperature  on  the morphological  structure  was  examined  by  the differential  scanning  calorimetry  (DSC).    DSC  is  a tool  commonly  used  for  thermal  analysis  of polymeric-based  material.    The  instrument  used, Perkin Elmer DSC7, was calibrated using high purity Idium each time it is used.    This is required to prevent possible  errors  as  a  result  of  instrumentation  errors such as drift.    The temperature  was  held  constant  at 25oC  for  1  min  and  the  melting  endotherms  were collected at a heating rate of 10 oC/min.  Next, space charge measurements on the samples were performed  using  the  Laser  Induced  Pressure  Pulse (LIPP)  technique.    Utilising  a  short  pulse  laser  to ablate the front semi-con electrode, an acoustic wave is generated acting as a virtual probe sensitive to charge as  it  transverse  through  the  material.    Details  and principle  behind  its  operation  can  be  found  in numerous literatures [4-5].  RESULTS AND DISCUSSION  Weight loss  Figure  1  shows  the  normalised  weight  of  samples  of different  thickness  subjected  to  different  degassing temperatures to degassing durations.  It  can  be  seen  that  both  degassing  temperature  and sample thickness directly effects the amount of volatile by-products that can be removed.    Broadly speaking, it is  more  difficult  to  remove  volatile  additives  and by-products  in  thicker  samples.    This  is  evident  by comparing line 2 with line 3.    It is worth pointing out that the weights of both 3.4 mm thick samples (line 1 & 2) continue to reduce even after 48 hours of degassing.   It is perhaps safe to assume most of the by-products that are  remaining  are  located  in  the  middle  of  the  bulk since it is farthest from surfaces.  The  weight  of  the  1.6  mm  thick  sample  (line3) stabilises after about 6 hours when degassed at 90 OC, suggesting that the most of the volatile by-products has already been removed.    Comparing with line 2 and 3, it can be seen that more by-products is removed in the thinner sample.  Figure  1:  Weight  to  degassing  duration  ratio  of different samples thickness and degassing temperature  DSC results  From the previous section, it is obvious that subjecting the  samples  to  moderate  heat  removes  volatile by-products from the samples.    What is less apparent, however, is that the morphology of the sample structure may also be modified as a result of degassing.    Figure 2 compares the DSC melting behaviour of five 3.4mm thick samples.    All samples were degassed at 60 oC  for  the  duration  indicated,  with  0  hour representing undegassed sample (sample A).  Apart  from  sample  A,  which  is  dominated  by  the quenched  polymer,  the  other  4  samples  exhibit transition,  i.e.  a  broad  feature  at  low  temperature,  a smaller  peak  extending  from  73   oC  to  78 oC  and  a major peak at about 103 oC.  It  was  observed  that  the  temperature  at  which  the smaller  peak  occurs  tends  to  increase  with  length  of degassing.    This  is  because  the  longer  the  degassing duration, the time for crystallization also increases.     The addition of the smaller peak suggests that another population  of  crystal  is  formed  and  it  becomes  more perfect  with  degassing  duration.    We  can,  therefore conclude that annealing of the samples has occurred.  Figure 3 shows the DSC melting behaviour of samples F  and  G.    They  are  samples  degassed  at  90 oC.   Similar to samples B to E, the DSC scan of samples F and  G  also  exhibits  2  transitions.    However,  the smaller peak occurs at a higher temperature of about 91 and 87 oC respectively.    The temperature at which the major peak occurs also shifted to about 106 and 105 oC respectively.    This is because increasing the degassing temperature  also  increases  the  crystallization temperature.  These higher  transition  temperatures  suggest  that  the crystals formed within samples F and G are of higher quality  as  a  higher  quality  crystal  generally  has  a higher melting temperature.      In  conclusion,  the  length  and  temperature  of  which degassing  is  carried  out  has  a  direct  effect  on  the morphology of the XLPE material.  Figure 2: DSC melting traces of 3.4mm thick samples degassed at 60 oC for different duration  Figure 3: DSC melting traces of sample F & G  Space charge results  All the samples were electrically aged for 24 hours and space  charge  measurements  were  taken  at  various interval.    According to findings in [2], samples A to E indicate heterocharge at both electrodes throughout the stressing period.    This suggests that ionic dissociation and  not  charge  injection/extraction  is  the  dominant process.    The ageing results indicate no homocharge formation observed  in  samples  A  to  E  within  the  24  hours  dc ageing period.    With reference to [2], it was suspected that this may be due to a combination effect of the bulk insulation  and  semi-con  material.    However,  exact reason  for  this  wasn’t  clear  at  this  time  and  further investigation is required. One possible reason for the absence of homocharge in sample A to E may be due to the fact that there are still quite  a  substantial  amount  of  by-products  left  in  the samples despite degassing at 60 oC for 48 hours.      One  way  of  removing  more  by-products  from  the sample  is  to  increase  the  degassing  temperature.   Sample F is a 3.4 mm thick sample degassed at 90 oC for  48  hours.    The  voltage-off  space  charge  profiles are shown figure 4.  Figure 4: Space charge profiles and charge integration in sample F  The  weight  loss  ratio  (line  2  of  figure  2)  and  space charge profiles of sample F indicate that by degassing the sample at higher temperature, more by-products can be removed, resulting in homocharge formation at the electrodes.    In such a case, charge injection/extraction is dominant over ionic dissociation.  It  is  noted  that  the  amount  homocharge  at  both electrodes decreases after 8 hours of dc ageing.    It is probably due to increase  in  bulk  stress  as  a  result  of homocharge formation during the first 8 hours.    This increase  in  bulk  stress  will  in  turn  ionise  the by-products that may still remain in the middle of the bulk, which as mentioned earlier is probably the area least  susceptible  to  degassing.    The  heterocharge produced by ionic dissociation will, therefore,  de-trap and cancel off the homocharge at the electrodes.  Even more volatile by-products can be  removed  from the sample by reducing its thickness. Thinner (1.6 mm) sample was made using the same type of materials for the XLPE bulk and semi-con electrodes.  The weight loss ratio of sample G (line 3) indicates that more by-products can be removed when a 1.6 mm thick sample is degassed at 90 oC for a duration of 48 hours.      Obvious similarities can be seen when comparing the space charge profiles of sample F and sample G (Figure 5).    Like sample F, sample G exhibits homocharge at both  electrodes.    Another  noticeable  similarity  of sample F and G is the net charge is negative throughout, in other words, more negative charge is trapped than positive  charge.  One  possible  explanation  for  this  is that it  is  easier  to  trap  negative  charge  than  positive charge.    This  is  evident  by  looking  at  the  relatively higher rate of increase of negative space charge in each sample before they start to be de-trapped by the positive charge.    Other  reasons  for  the  negative  net  charge could be that more negative charge is injected into the samples  than  positive  charge  or  it  could  also  be  the combination of both.  Figure  5.  Profiles  and  charge  integration  of  space charge in sample G.  The main difference of sample F and G is that in the case of the latter, the amount of negative homocharge only starts to decrease after 12 hours of dc ageing.    On the  other  hand,  the  amount  of  positive  charge  very much stabilized after 8 hours of dc ageing.  The decrease in negative homocharge is most probably due to de-trapping by positive charge.    The question to address  is  the  origin  of  the  positive  charge.    One possible  source  is  ionization  of  by-products  in  the middle  of  the  bulk.    While  this  may  be  possible  in sample F, it is however believed to have minimal effect in sample G as that its weight loss ratio stabilizes after 6  hours  of  degassing,  suggesting  that  most  of  the by-products has been removed.      Another possible source of positive charge may be the migration of charge from the anode to the cathode.    It is shown that positive charge is more difficult to trap; therefore,  significantly  more  positive  charge  injected will  not  be  trapped  at  the  vicinity  of  the  anode  but rather  attracted  to  the  cathode  hence  cancel  or recombine  with  the  negative  charge,  as  the  space charge measurement technique only gives the resultant net charge density profile across the sample.  CONCLUSION  In  this  paper,  the  influence  of  degassing  has  on  the morphological development and space charge of XLPE smaples was reported.    Parameters such as degassing duration, degassing temperature and sample thickness were investigated.  More volatile by-products can be removed by increasing the  length  and  temperature  of  degassing.    It  is  also proven that it is easier to remove the by-products from thinner samples.  Apart  from  removing  volatile  by-products,  degassing also  has  annealing  effect  on  the  sample.    Likewise, factors  that  affect  the  morphological  development include degassing duration, temperature and thickness of the sample.  Space  charge  distribution  is  highly  dependent  on  the amount of by-products in the samples.    Complication of  space  charge  behavior  is  attributed  to  ionic dissociation,  de-trapping  and  perhaps  difference  in morphology of each sample.  ACKNOWLEDGMENT  The authors would like to thank Dr. Alun Vaughan and Mr.  Chris  Martin  of  University  of  Southampton  for provision  of  the  sample  material  and  help  with  the Perkin Elmer DSC7 respectively.  REFERENCES  [1]  G.A. Cartwright, PhD thesis, “The measurement of   space  charge  and  its  effect  on  the  breakdown   strength  of  solid  polymeric  insulation,  1994,   University of Southampton.  [2]  Y. F. F. Ho, Y. L. Chong, G. Chen, “Effect of the   length  of  degassing  period  on  the  space  charge               dynamic  in  XLPE  insulation  under  DC  stressing               condition”, pp. 160-163    (ICMEP 2003)  [3]  A.S. Vaughan, Y. Zhao, L.L. Barre, S.J. Sutton ,   S.G.  Swingler  “On  additives,  morphological   evolution and    dielectric  breakdown  in  low   density  polyethylene”,  Eur.  Polym.  J.  39(2),   2003, pp.355-365  [4]  P. Laurenceau, G. Dreyfus and J. Lewiner, “New             Principle  for  the  determination  of  potential               distributions  in  dielectrics”,  Physical  Review   Letters,  Volume  38,  Number  1,  3  January  1977,   pp. 46-49.  [5]   C. Alquié, T. Ditchi anf J. Lewiner, “Measurement   of  charge  density  in  insulating  material  for  high   voltage cables”, Proc. 2nd Conf. on Properties and   application  of  dielectric  materials,  IEEE,  1988   pp.434-437 

Effect of the length of degassing period on the space charge dynamic

Measurement of charge density in insulating material for high voltage cables”,

New Principle for the determination of potential distributions in dielectrics”,

PhD thesis, “The measurement of space charge and its effect on the breakdown strength of solid polymeric insulation,

Swingler “On additives, morphological evolution and dielectric breakdown in low density polyethylene”,

The effect of degassing on morphology and space charge

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The effect of degassing on morphology and space charge

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