Electrical treeing has been widely studied in a range of polymeric materials. In these investigations, the morphology and PD patterns associated with the growth of electrical trees in a model transparent silicone rubber were investigated using a new system recently developed at Southampton. With increasing voltage the trees became more complex in appearance but nevertheless grow more rapidly. As the tree evolves the PD pattern becomes more intense which may provide a method of monitoring the extent of treeing in opaque samples. Raman studies indicate that treeing and breakdown channels are hollow, carbonaceous entities, a finding consistent with other studies

Freebody, N A

Hosier, I L

Moss, G

Swingler, S G

Vaughan, A S

English

Southampton (e-Prints Soton)

ELECTRICAL TREEING IN SILICONE RUBBER I. L. Hosier1*, N. A. Freebody1, A. S. Vaughan1, S. G. Swingler1 and G. Moss2 1University of Southampton, Highfield, Southampton, SO17 1BJ, UK, 2National Grid, Warwick, CV34 6DA, UK *Email: ilh@ecs.soton.ac.uk  Abstract: Electrical treeing has been widely studied in a range of polymeric materials.  In these  investigations,  the  morphology  and  PD  patterns  associated  with  the  growth  of electrical  trees  in  a  model  transparent  silicone  rubber  were  investigated  using  a  new system recently developed at Southampton.  With increasing voltage the trees became more complex in appearance but nevertheless grow more rapidly.  As the tree evolves the PD pattern becomes more intense which may provide a method of monitoring the extent of treeing in opaque samples.  Raman studies indicate that treeing and breakdown channels are hollow, carbonaceous entities, a finding consistent with other studies.   1  INTRODUCTION Electrical treeing has been widely reported in many polymeric  materials  such  as  polyethylene  [1], polypropylene  [2],  synthetic  resin  bonded  paper [3],  epoxies  [4-6],  unfilled  silicone  rubber  [7]  and cross-linked  polyethylene  (XLPE)  [8].    Electrical trees  form  through  localised  electrical  discharge events  which  cause  localised  erosion  of  the material,  leading  to  a  fractal  like  network  of channels within the dielectric [1-2, 8] and eventual breakdown  of  the  insulation.    A  needle-plane geometry is used in laboratory tests to localise tree growth [1-8], a process which is accompanied by partial  discharge  (PD)  activity  [3-6].    In  unfilled silicone rubber, electrical trees are white in colour, non-conducting and propagate rapidly [7] whereas in  polypropylene,  polyethylene  and  XLPE,  the trees  are  often  black  in  colour,  electrically conducting and propagate relatively slowly [1-2, 8, 9].  In epoxy resins both types of tree have been reported depending on the temperature [4-6]; this distinction has led to the widely used terminology “non-conducting” or “conducting” tree respectively [1, 8, 9].  Furthermore it has been reported that the morphology  of  an  electrical  tree  depends  on  the applied  voltage,  with  rather  simple  filamentary structures  giving  way  to  complex  bush  shaped objects at higher voltages [4, 7, 9]. In  these  initial  investigations  on  silicone  based systems, a transparent, unfilled silicone resin was selected.    This  has  the  advantages  of  optical clarity,  and  enabled  electrical  trees  to  be  grown over sensible timescales using reasonable voltage levels  [7].    To  do  the  experimental  work,  a  new apparatus was constructed incorporating an optical microscope  with  real  time  video  recording,  a  PD free  high  voltage  power  supply  and  a  PD monitoring  system.    We  have  considered  the effects of applied voltage on tree growth and have measured  both  the  morphology  and  PD  patterns as the tree evolves.   The kinetics  of tree growth from initiation to eventual failure was followed.  2  EXPERIMENTAL 2.1  Materials and sample preparation A commercial silicone resin kit (Sylgard 184) was obtained from Farnell and was used as supplied.  The  resin/hardener  was  mixed  in  the recommended  10:1  ratio,  degassed  at  room temperature under dynamic vacuum and then cast into  a  6 mm  deep  polished  mould.    After further degassing,  the  mould  was  transferred  into  a  fan oven  maintained  at  100 oC  for  a  period  of  30 minutes to cure the resin.  Samples approx. 10 x 10 mm were cut from the prepared sheets which were  then  mounted  into  a  Perspex  test  cell incorporating an earth plate and a needle holder.  The  sample  was  placed  firmly  against  the  earth plate  and  a  fresh  hypodermic  needle  (Becton Dickenson  Microlance  19,  38  mm  length)  was inserted into the opposite side.  The gap between the needle and earth plate was then adjusted to 2 mm  (±  0.5  mm)  [7]  with  the  aid  of  an  optical microscope, taking care not to introduce voids. 2.2  Treeing apparatus The apparatus (Figure 1) is based around a Prior Scientific Zoom 65 optical microscope which was equipped  with  a  GXCAM-1.3  digital  camera  (GT Vision).    The  PD  measurement  system  was comprised of a wideband LCR probe constructed in  accordance  with  IEC60270:2000  [4,  10] (resonance  200  kHz,  coupling  capacitor  33  pF) along  with  a  1:1000  resistive  divider  reference probe.      Both  probes  were  connected  to  a Picoscope 4224 PC oscilloscope for data collection using  a  1  MHz  sampling  rate.    Prior  to  use  the system  was  calibrated  with  a  Robinson  PD calibrator  and  a  calibrated  high  voltage  meter.   The test cell was placed into a small plastic oil bath containing silicone fluid (Dow Corning 20/200 CS) which was then backlit for observation.  The whole apparatus  was  enclosed  in  an  interlocked  metal box to ensure electrical safety and to guard against any external electrical noise. XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011 Figure 1: Schematic of the treeing apparatus. 3  RESULTS 3.1  Morphology and applied voltage A  number  of  electrical  trees  were  grown  at voltages  between  8  and  18  kV  rms  and  some examples are shown here.  At low voltages (8-10 kV)  single  or  doubly  branched  trees  are  formed      Figure 2: Morphologies of trees grown at (a) 8 kV, (b) 10 kV.  Earth plate bottom, needle at top. (Figure 2), whilst at intermediate voltages (Figure 3a),  more  complex  multiply  branched  structures are  formed.    At  the  highest  voltages,  complex structures  with  multiple  sub-branches  are  formed (Figures 3b, 3c).  The progression from simple to complex objects with increasing voltage has been reported elsewhere [4, 7, 9].    Figure 3: Morphologies of trees grown at (a) 12 kV,  (b)  16  kV,  (c)  18  kV.    Earth  plate  bottom, needle at top. XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 20113.2  Growth dynamics After  an  initiation  period,  which  can  range  from several seconds to several tens of minutes, where no  structure  is  formed  and  no  PD  activity  was recorded, the structure grows rapidly through the dielectric [9] before it becomes necessary to turn off  the  power  to  avoid  a  breakdown.    Growth  is rapid in the seconds following initiation (Figure 4a),        Figure 4: Progressive growth steps of a tree grown at 14 kV, (a) 3 s after initiation, (b) 10 s, (c) 20 s.     Figure 5: Growth rate plots for a typical selection of trees grown at (a) 8 – 12 kV, (b) 14 – 18 kV.  after  which  a  period  of  slower  growth  ensues (Figure  4b)  until  finally  a  “leader”  (Figure  4c, arrowed)  rapidly  approaches  the  earth  electrode requiring  the  power  to  be  shut  off  to  avoid  a breakdown which would otherwise destroy the tree.  It is interesting that the structure is tree-like even at the earliest stages of growth (Figure 4a). Figure  5  shows  some  representative  growth  rate plots, the length shown is that measured from to the furthest tree tip to the end of the needle (hence some  measurements  are  >  2  mm when  the tree grows  at  an  oblique  angle  to  the  earth  plate).  Whilst  over  20  structures  were  grown  for  these investigations,  many  which  showed  similar characteristics have been removed here for clarity whilst  preserving  some  data  to  illustrate  the variability  between  trees  grown  at  the  same voltage level.  It was observed that in most cases, tree growth is most rapid in the moments just after initiation  and  also  at  the  final  stages  of  growth when  a  “leader”  approaches  the  earth  electrode.  Despite  the  inherent  variability  between  trees grown at the same voltage, there is a general trend that  trees  grow  faster  at  higher  voltages  despite their increasing complexity. XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 20113.3  Partial discharge patterns 3.2.1 Effects of voltage   Over the 20 or so trees considered,  it  was  observed  that  the  intensity  of the associated PD activity increased with applied voltage.    Figure  6  shows  a  selection  of  phase resolved  PD  patterns  obtained  over  a  5  second sampling window near the midpoint of tree growth, each dot representing a single PD event.  The PD activity is centred in the 1st and 3rd quadrants, that is, where the AC voltage is rising to its maximum     Figure  6:  Phase  resolved  PD  patterns  collected over 5 s intervals for trees grown at (a) 8 kV, (b) 14 kV, (c) 18 kV. positive  or  negative  value  respectively  [3-6,  8].  Trees grown at 8 kV (Figure 5a) are accompanied by “low level” PD activity (<200 pC) with relatively few  “high  level”  (>1000  pC)  discharge  events occurring during the sampling period.  These high level  discharges  predominantly  occur  during  the negative  going  half  cycle  (i.e.  quadrant  3).  Increased  voltage  then  results  in  an  increase  in both  the  low  level  activity  and  the  high  level discharge  events,  some  of  which  clearly overwhelm the detector (> 5 nC, Figure 5c). 3.2.2  Time  dependence      Above,  we  have deliberately chosen PD patterns obtained near the midpoint of tree growth, this was because it was also noted that the intensity and frequency of PD events  increased  as  the  tree  structure  became larger and more complex.  A typical example of this behaviour is shown in Figures 7 and 8 for a tree grown  at  12  kV  (see  Figure  3a  for  morphology).  Initially (Figure 7a) only a few low level discharges (< 200 pC) per AC cycle occur, these then intensify to a level of ~400 pC at a rate of 3-5 per AC cycle (Figure  8a).    Finally  many  more  high  level discharges  (>1000  pC)  occur  as  the  tree approaches the earth electrode (Figure 8b).  It is     Figure  7:  Phase  resolved  PD  patterns  collected over 5 s intervals for a tree grown at 12 kV (a) 5 s after initiation, (b) 10 s after initiation. XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011  Figure  8:  Phase  resolved  PD  patterns  collected over 5 s intervals for a tree grown at 12 kV (a) 20 s after initiation, (b) 30 s after initiation. clear from these patterns that they are related to the extent of tree growth; the high level activity in particular is associated with the later stages of tree growth and it may be possible to use this type of activity to diagnose items of plant for the presence of damaging electrical trees. 3.4  Analysis of tree structure Two  samples  were  taken  for  further  analysis.  Sample 1 had undergone treeing and breakdown at  18  kV,  whereas  sample  2  had  undergone treeing at 14 kV without breakdown.  Both samples were  cut  open  to  reveal  breakdown  and  tree channels  respectively,  using  a  RMC  ultra-cryomicrotome  operated  at  -100 oC.    They  were then  examined  in  a  Renishaw  Ramascope  1000 operated in non-confocal mode with an excitation wavelength of 785 nm provided by a 25 mW diode laser.    Scans  taken  at  various  points  near  the breakdown channel in sample 1 are numbered in Figure 9.  Well away from the breakdown channel (area 5), the Raman spectrum shows characteristic peaks  associated  with  silicones.    Visibly  darker areas of the breakdown channel (area 3) show the D and G bands (arrowed) of graphitic carbon [1]    Figure 9: Photograph and Raman of sample 1.  Figure 10: Photograph and Raman of sample 2. XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011whereas,  in  lighter  areas  (area  2),  the  dominant feature is the rising fluorescent background.  The behaviour of sample 2 (Figure 10) is comparable; within  the  tree  channel  (area  6)  the  associated spectra  shows  evidence  of  graphitic  carbon (arrowed) whereas near the tree tips (areas 7 and 8)  they  show  signs  of  silicone  and  some fluorescence.    It  is  clear  that  both  the  tree  and breakdown  channels  are  hollow  entities  with carbonaceous  walls  and  that  the  associated degradation  is  a  localised  phenomenon.    These findings are in agreement with Raman studies of electrical trees in polyethylene [1] despite the very different material used here. 4  CONCLUSIONS A  new  piece  of  apparatus  was  constructed  at Southampton University for the study of electrical treeing in solid and liquid samples.  In this paper we  have  evaluated  its  performance  and capabilities using a transparent silicone rubber and have studied the evolution of electrical trees as a function  of  both  voltage  and  time.    The  use  of voltages between 8 and 18 kV with a needle-plane geometry resulted in electrical trees being formed over reasonable timescales. With  increasing  voltage,  the  trees  became  more complex, ranging from single channels at 8 kV to complex,  multiply  branched  structures  at  18  kV.  Despite this increased complexity, the trees form more quickly at higher voltages with growth rates being  highest  in  the  moments  following  initiation and in the final stages of growth, where a leader is observed to rapidly approach the earth electrode. The intensity and frequency of the associated PD activity  increases  with  voltage  and  as  the  tree matures.    This  could  provide  a  useful  diagnostic tool  in  opaque  samples  where  electrical  trees cannot  be  imaged  optically  or  in  items  of  high voltage plant. The  tree  channels  were  shown  to  be  hollow, carbonised  structures  in  agreement  with  our previous studies of treeing in polyethylene.  Future work  will  consider  the  effects  of  inclusions  and voids on the growth of electrical trees in silicone rubber. 5  ACKNOWLEDGMENTS The authors would like to thank National Grid for financial  support  and  for  giving  permission  to publish this work.  6  REFERENCES [1]  A. S. Vaughan, I. L. Hosier, S. J. Dodd, S. J. Sutton,  “On  the  structure  and  chemistry  of electrical  trees  in  polyethylene”,  J.  Phys  D: Appl. Phys., Vol. 39, pp. 962-977, 2006. [2]  A. S. Vaughan, S. J. Dodd, A. M. Macdonald, "Tree  growth  in  a  propylene/ethylene copolymer:  Relationships  between  electrical activity  during  growth  and  the  structure, chemistry  and  properties  of  the  trees  that result",  In  Proc.  Conference  on  Electrical Insulation and Dielectric Phenomena, pp. 548-551, 2005  [3]  M. A. Brown, S. J. Dodd, B. Ahern, J Pettinger, F. White "Correlation of partial discharge and dissolved gas analysis results from discharge activity in SRBP", In Proc. 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An investigation of partial discharge energy dissipation and electrical tree growth in an epoxy resin&quot;,

Comparison of streamers in mineral and synthetic insulating oils with electrical treeing in solids”,

High voltage test techniques – Partial discharge measurements”, BSI copyright,

On the structure and chemistry of electrical trees in polyethylene”,

Phenomena and mechanism of electrical tree in silicone rubber”,

Statistical analysis of partial discharges from electrical trees grown in a flexible epoxy resin&quot;,

Study on electrical tree growth in XLPE by PD patterns&quot;,

The Influence of Absorbed Moisture on Partial Discharge Patterns Measured During Tree Growth in an Epoxy Resin&quot;, In

Tree growth in a propylene/ethylene copolymer: Relationships between electrical activity during growth and the structure, chemistry and properties of the trees that result&quot;,

White &quot;Correlation of partial discharge and dissolved gas analysis results from discharge activity in SRBP&quot;,

Electrical Treeing in Silicone Rubber

Electrical Treeing in Silicone Rubber

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