Chen, G

Lewin, P L

Swaffield, D J

Swingler, S G

Tian, Y

English

Southampton (e-Prints Soton)

at 5000 frames per second (fps) using a high frequency pulse laser  with  a  pulse  duration  of  30ns  to  stroboscopically backlight the  needle  sample.    A  microscope lens with long working  distance  was  used  to  magnify  the  density  charge streamer  images.    Images  captured  are  512  by  512  pixels, which with a pixel viewing an area of 2.42µm square equates to a full frame size of 1.24mm square.  Since  it  is  not  possible  to  measure  the  true  charge  at  a discharge site the electrical circuit was calibrated prior to use.  This  was  performed  by  employing  a  standard  technique; injecting pulses of known charge and measuring the output signal seen on the pd detector and recorded on the dso.  This allows a measure of “apparent charge” for each density charge streamer recorded.  A typical approach when attempting to calculate the electric stress at the needle tip is to model the electrode geometry as a hyperboloid to plane.  The calculation is simplified by two assumptions;  1)  that  the  needle  geometry  is  regular  and  is close to a hyperboloid in geometry.  2) That there are no space charge  effects.    This  may  be  the  case  prior  to  the  initial discharge but is unlikely to be correct after the generation of vapour and deposition of charge into the liquid and onto the solid insulation surface.  The calculation of stress is further complicated  for  this  sample  by  the  presence  of  the  solid insulation  plates  with  additional  space  and  surface  charge effects.  Analysis, j j j j q n plotting  The pd data captured have been analysed using j q n plotting, Figures 4 to 7, to provide a useful visual representation of the results  relating  phase,  cumulative  number  and  charge magnitude of pd.  Raw data are recorded as phase locations and  amplitudes.    The  50Hz  power  cycle,  period  20ms,  is divided into 200 phase  windows  and 100  charge  amplitude divisions.    Cumulative  numbers  of  discharges  falling  into these  windows  are  calculated  and  plotted  to  provide  a diagrammatical representation of partial discharge and allows comparison of activity under different test conditions.  Since the zero crossing detection circuit has a delay that may be up to 11µs and a single discharge pulse may have a pulse width of up to 40µs at the dso.  The combined error may therefore be as  much  as  51µs,  equating  to  a  0.918degrees  phase  shift.  However  this  is  less  than  one  phase  window  width  of  1.8 degrees.    Therefore,  over  the  whole  cycle  errors  will  not significantly influence the shape of the j q n plot.  RESULTS  Two types of density change streamers have been observed, classified  as  “bush-like”  and  “filamentary”.    Bush-like streamers occur during the positive half cycle, i.e. when the needle  is  at  a  lower  potential  with  respect  to  the  plane electrode. 0 30 60 90 120 150 180 210 240 270 300 330 360-550-450-350-250-150-5050150250350450550Signal before processingapparent charge(pC)phase in degrees280 290 300 310 320 330 340 350 360 370 38050150250350450550Signal after peak detectionapparent charge(pC)corresponding frame numbers Figure 1, partial discharge signal at 18.28 kV, 76.0 ± 1 K  Typical  bush-like  streamers are  shown  in  figure  2.    In  this half-cycle it is usual that multiple discharges occur, the vapour from  one  discharge  often  remains  in  the  gap  while  a subsequent  discharge  event  occurs.    Filamentary  streamers appear in shape as single or multi-strand discharges with well defined branches along the discharge path from the needle tip toward the plane; these are only seen in the negative half-cycle i.e. when the needle tip is at higher potential than the plane electrode.  Figure  3  shows  a  typical  filamentary  streamer. Bush-like streamers have been observed in the negative half-cycle, but filamentary streamers have never been seen in the positive half cycle.         301(a)  302  303(b)         307(c)  311(d)  318(e)  Figure 2, Typical negative tip, bush-like discharges, with frame numbers, apparent charge and phase relates to figure 1. a c d b f e        352(f)  353  354  Figure 3, Typical positive tip filamentary discharge, with frame numbers, with apparent charge and phase related to figure 1.  PTFE results  The j q n plots for two voltages applied to the PTFE/LN2 composite sample are shown in Figures 4 and 5.  These plots show the character of the discharges over 250 power cycles.  It is evident that there is more pd activity in with the needle at lower potential than for higher, i.e. discharge is more readily initiated from a negative tip.  It is also clear that there are higher  maximum  charge  values  for  a  positive  filamentary streamer  compared to the negative  tip  bush-like  discharges, 600 pC and 460 pC respectively.  For the case at an applied voltage of 20.56 kV the pd activity is  grouped  and  centred  on  105˚  and  280˚,  with  bands stretching from 45˚ to 150˚ and 255˚ to 340˚ respectively.  As the voltage is increased to 26.78 kV the activity advances to 90˚ and 260˚ for respective half cycles.  The positive tip band also narrows to 250˚ to 290˚, a closer examination shows that less, large discharges occur late in the cycle.  The average and maximum  charge  magnitudes  increase  for  both  half  cycles.  Average magnitudes remain at approximately 200 pC for the negative tip and rise from 500 pC to 550 pC for the positive.  Maximum  magnitudes  rise  from  460 pC  to  580 pC  for  the negative tip and 600 pC to 720 pC for the positive.    0100200300400500045901351802252703153600246Phase (degree) Charge (pC)Number Figure 4, φ q n plot with an applied voltage of 20.56 kV at 77.3 ± 1 K, atmospheric pressure  0200400600045901351802252703153600510Phase (degree) Charge (pC)Number Figure 5, φ q n plot with an applied voltage of 26.78 kV at 77.3 ± 1 K, atmospheric pressure The  most  significant  increase  is  in  cumulate  discharge numbers.    For  cumulative  numbers  by  phase  window  an increase in the peak from 50 at 20.56 kV to a peak of 140 at 26.78 kV is seen, this represents a large increase in activity considering the increase across the whole activity band.  It is noted  that  a  similar  trend  was  also  seen  for  an  increase  in voltage for the 6F/40 GRR.  A more dramatic phase advance and  increase  in  the  average  and  maximum  discharge magnitudes was seen for the 10G/40 sample set with a closer gap  of  2.6 mm,  further  detail  from  this  experiment  can  be found in [6].  6F/40 results  Figures 6 and 7 present the effect of increasing the applied pressure on the pd activity for a fixed voltage, in this example for the 6F/40 sample. 01002003004005006000459013518022527031536002468Phase (degree) Charge (pC)Number Figure 6, φ q n plot with an applied voltage of 20.71 kV at 77.4 ± 1 K, atmospheric pressure 010020030040050004590135180225270315360051015Phase (degree) Charge (pC)Number Figure 7, φ q n plot with an applied voltage of 20.73 kV at 77.2 ± 1 K, 0.4 MPa,  As the pressure is increased the grouped activity is slightly retarded of the order of 5˚ to 10˚ in phase as the pressure is increased, this is consistent with the result for 10G/40.  It is unclear if the distribution is skewing or retarding, whichever is the case it would indicate less charge storage in the sample.  However  the  effect  of  pressurising  to  this  level  has  less dramatic  effect than  was seen  for the  10G/40  sample  for  a decrease in temperature [6].  Cumulative numbers by phase window  are  reduced  for  both  half-cycles,  Figure  7.    The increase in pressure causes the bush-like streamers from the negative  tip  to  reduce  in  average  and  maximum  charge magnitudes.  The average charge drops from around 150 to 90 pC  for  the  negative  tip  and  the  maximums  from  500  to 200 pC.  It has also been observed that an increase in applied voltage to 28 kV at 0.4 MPa shows the same trends as for an increase in volts at a lower pressure.  For the positive tip the activity band narrows, this remains unchanged for the negative tip.  At various stages in the experimental run the pd inception and extinction  voltage  was  measured.    This  showed  a  gradual increase related to the time for which the sample had been subjected  to  applied  voltage.    This  indicates  the  gradual erosion of the needle tip with discharge activity.  After the experimental run was complete, the needles and samples were removed for inspection.  This showed that the needles were eroded by the discharges.  DISCUSSION AND CONCLUSIONS  The results show that the different types of discharge hold for all  samples  and  experimental  conditions  studied.    It  would appear  that  there  is  a  difference  in  the  mechanism  of  pre-discharge dependent on the polarity of the needle electrode.  This is further supported by the observation that the positive tip pre-discharge appears to be less pressure dependant than for the negative tip.  For the negative needle tip electrons could be readily supplied by field emission from the high electric stress at the needle tip; these  would  be  attracted  to  the  plane  electrode.    Electron avalanche  would  propagate  through  the  liquid  phase,  local heating resulting in the density change streamers seen in the images of Figure 2.  The electron paths would spread due to the  reducing  field  strength  moving  away  from  the  tip  and towards  the  plane  electrode.    This  would  also  allow  the charges to repel.  This along with a large area from which to initiate  could  be  responsible  for  the  bush-like  shape  of  the density change streamer.   This would also accounts for the observed  spreading  of  the  base  of  the  bush  structure  away from the needle tip as repetitive discharges continue and the initial electric field profile is distorted by the movement of charge  The  average  discharge  magnitude  is  higher  in  the  negative half-cycle when the needle tip is at a higher potential than the plane  electrode.    Electron  scavenging  must  occur  from  the liquid  nitrogen  and  free  electrons  in  the  liquid  are  not  so readily available.  With increasing potential eventually there is a breakdown to the needle tip, this results in space charge and leads  to  further  propagation  in  to  the  liquid.    With  the electrons  directed  toward  the  needle  tip  this  will  lead  to  a narrow conduction path, as seen in Figure 3.  REFERENCES  1.  Gerhold, J. "Dielectric breakdown of cryogenic gases and liquids", Cryogenics. 1979, pp. 571-584. 2.  Hayakawa, N., Sakakibara, H., Goshima, H., Hikita, M., Okubo, H. "Breakdown mechanism of liquid nitrogen viewed from area and volume effects", IEEE Transactions on Dielectrics and Electrical Insulation. Vol. 4 No. 1, 1997, pp. 127-134. 3.  Suehiro, J., Matsumoto, Y., Imasaka, K., Hara, M. "Partial discharge induced bubbles generated in subcooled liquid nitrogen at atmospheric pressure". 13th International Symposium on High Voltage Engineering (ISH), 2003. pp. 153. 4.  Takahasi, Y., Ohtsuka, K. "Corona discharges and bubbling in liquid nitrogen", Journal of Physics D (Applied Physics). Vol. 8, 1974, pp. 165-169. 5.  Swaffield, D. J., Lewin, P. L., Chen, G., Swingler, S. G. "Variable Pressure and Temperature Liquid Nitrogen Cryostat for Optical Measurements with Applied Electric Fields", Journal of Measurement Science and Technology. Vol. 15 No. 11, 2004, pp. 2325-2332. 6.  Swaffield, D. J., Lewin, P. L., Tian, Y., Chen, G., Swingler, S. G. "Characterisation of partial discharge behaviour in liquid nitrogen". International Symposium on Electrical Insulation, 2004. IEEE, pp. 135-138. 

Breakdown mechanism of liquid nitrogen viewed from area and volume effects&quot;,

Characterisation of partial discharge behaviour in liquid nitrogen&quot;.

Corona discharges and bubbling in liquid nitrogen&quot;,

Dielectric breakdown of cryogenic gases and liquids&quot;,

Partial discharge induced bubbles generated in subcooled liquid nitrogen at atmospheric pressure&quot;.

Variable Pressure and Temperature Liquid Nitrogen Cryostat for Optical Measurements with Applied Electric Fields&quot;,

Partial Discharge Characterisation in Liquid Nitrogen Composite Systems

Partial Discharge Characterisation in Liquid Nitrogen Composite Systems

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