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									PEA Space Charge Measurement System                                   Seminar Report „04



                               INTRODUCTION

       The pulsed electro acoustic analysis (PEA) can be used for space charge
measurements under dc or ac fields. The PEA method is a non-destructive technique for
profiling space charge accumulation in polymeric materials. The method was first
proposed by T.Takada et al. in 1985. The pulsed electro acoustic (PEA) method has
been used for various applications. PEA systems can measure space charge profiles in
the thickness direction of specimen, with a resolution of around 10 microns, and a
repetition rate in the order of milliseconds. The experimental results contribute to the
investigation of the charge transport in dielectrics, aging of insulating materials and the
clarification of the effect of chemical properties on space charge formation. PEA
method can measure only net charges and does not indicate the source of the charge.


       Various space charge measurement techniques are thermal step, thermal pulse,
piezoelectric pressure step, and laser induced pressure pulse, the pulse electro acoustic
method. In the thermal step method, both electrodes are initially in contact with a heat
sink at a temperature around –10 degrees Celsius. A heat source is then brought into
contact with one electrode, and the temperature profile through the sample begins to
evolve towards equilibrium consistent with the new boundary conditions. The resulting
thermal expansion of the sample causes a current to flow between the electrodes, and
application of an appropriate deconvolution procedure using Fourier analysis allows
extraction of the space charge distribution from the current flow data. This technique is
particularly suited for thicker samples (between 2 and 20 mm). Next is the thermal pulse
technique. The common characteristic is a temporary, non –destructive displacement of
the space charge in the bulk of a sample created by a traveling disturbance, such as a
thermal wave, leading to a time dependent change in charge induced on the electrodes
by the space charge. Compression or expansion of the sample will also contribute to the
change in induced charge on the electrodes, through a change in relative permittivity.
The change in electrode charge is analyzed to yield the space charge distribution.
Thermal pulse technique yields only the first moment of the charge distribution and its
first few Fourier coefficients. Next is laser induced pressure pulse. A temporary
displacement of space charge can also be achieved using a pressure pulse in the form of


Dept. of EEE                                1                      MESCE, Kuttippuram
PEA Space Charge Measurement System                                      Seminar Report „04

a longitudinal sound wave. Such a wave is generated, through conservation of
momentum, when a small volume of a target attached to the sample is ablated following
absorption of energy delivered in the form of a short laser pulse. The pressure pulse
duration in laser induced pressure pulse measurements depends on the laser pulse
duration and it can be chosen to suite the sample thickness, ie, thinner the sample the
shorter should be the laser pulse.


        Space charge measurement has become a common method for investigating the
dielectric properties of solid materials. Space charge observation is becoming the most
widely used technique to evaluate polymeric materials for dc-insulation applications,
particularly high-voltage cables. The presence of space charges is the main problem
causing premature failure of high-voltage dc polymeric cables. It has been shown that
insulation degradation under service stresses can be diagnosed by space charge
measurements. The term” space charge” means uncompensated real charge generated in
the bulk of the sample as a result of (a) charge injection from electrodes, driven by a dc
field not less than approximately 10 KV/mm, (b) application of mechanical/thermal
stress, if the material is piezoelectric/ pyroelectric (c) field-assisted thermal ionization of
impurities in the bulk of the dielectric.




Dept. of EEE                                  2                       MESCE, Kuttippuram
PEA Space Charge Measurement System                                   Seminar Report „04



                                 PEA METHOD




                     Figure shows the principle of the PEA method.


When a pulsed electric field is applied to a specimen that has internal charge, the pulsed
electric field and charges (internal charges and induced charges on the electrodes)
generate forces, resulting in acoustic waves. The acoustic waves propagate in the
specimen and are detected by a piezoelectric sensor made with poly vinylidene fluoride
(PVDF) or lithium niobate (LiNbO3) attached to the electrode; an oscilloscope observes
them. The amplitude of the signal is proportional to the charge quantity, and the delay
indicates the distance from the sensor, ie, the position of the charge. In this manner, the
space charge distribution is measured quantitatively and nondestructively.




                     Figure shows basic set-up of the PEA method.


Dept. of EEE                                3                      MESCE, Kuttippuram
PEA Space Charge Measurement System                                    Seminar Report „04




                 Figure shows example of profiles obtained by the PEA.


Here shows an example of charge, electric field and potential distributions obtained by
applying a dc bias voltage to a polystyrene sheet. Because the specimen does not
contain internal charge, only induced charges on the electrode were observed as shown
by red line q. By integrating the charge profile, the electric field distribution is obtained
as the blue line E; by integrating the field, the potential distribution is obtained as the
green line P. these three profiles give the fundamental performance of the individual
system.




Dept. of EEE                                 4                       MESCE, Kuttippuram
PEA Space Charge Measurement System                                 Seminar Report „04



                              3-D PEA SYSTEM


       A 3-D PEA system can observe space charge distribution in 3-D of dielectric
materials safely and nondestructively. There are two types of 3-D PEA system. They are
acoustic lens method and multisensor method. In the case of acoustic lens method the
measurement time was reduced, and the resolution was significantly improved. In the
case of multisensor method it can use a sensor array to collect space charge profiles in
the thickness direction from many points at once.




Dept. of EEE                               5                     MESCE, Kuttippuram
PEA Space Charge Measurement System                                    Seminar Report „04



                       ACOUSTIC LENS METHOD




               Figure shows schematic diagram of the acoustic lens method.


       With this lens, only acoustic waves generated at the focus can be detected by the
piezoelectric sensor. By moving the acoustic lens unit, the space charge profiles in the
thickness direction can be collected in sequence. After collecting all of the profiles in
the observation area, signals are displayed as lateral distributions. It takes about 2 sec to
observe a reasonable profile from one point. The measurement time is determined by
the number of detection points. Obviously, if the charge profile changes rapidly, the
lateral profile is affected by the time dependence.




                Figure shows example of 3-D space charge measurement.



Dept. of EEE                                 6                       MESCE, Kuttippuram
PEA Space Charge Measurement System                                 Seminar Report „04


       Here is an example of 3-D space charge measurement obtained by two
polystyrene sheets with an evaporated electrode „letter C‟ on one sheet. Positive charge
(red) appeared at the interface of the two polystyrene sheets having the shape of the
letter C. negative-induced charges (blue) were observed at the positions of the two
electrodes. Because there is no internal charge, the other areas were observed as neutral
(green).

       These profiles revealed that the resolution in the lateral direction was
approximately 200 micrometer. The improvement of resolution is ongoing.




            Figure shows ion migration monitoring of printed circuit boards.


       The specimen was a metal-base printed circuit board with an insulation layer of
epoxy resin on a thick aluminum base plate. The anode was a copper electrode and is
known to cause ionic migration under dc electric field at high temperature and high
humidity. Space charge profiles were measured and compared with the copper
distributions observed by electron probe microanalysis (EPMA) during an aging test.
The space charge profile after aging showed that a conductive region was formed near
the anode; EPMA analysis clearly showed the copper distribution in the same region.
These results prove that the PEA can monitor ion migration.




                 Figure shows ion migration observed by the 3-D PEA.


Dept. of EEE                               7                      MESCE, Kuttippuram
PEA Space Charge Measurement System                                Seminar Report „04


When a dc voltage was applied to a specimen, a positive (red) signal appeared at the
anode as well as inside the insulation, particularly at the fringe of the anode, and not
below the center part of the electrode. This suggests that the migration occurs
dominantly at the edge of the anode. Negative charge (blue) appeared at the ground
electrode, and the other part was neutral (green).


       Although the ion migration phenomenon has been studied for a long time, it is
becoming a more serious problem because of the severe requirements from the
electronics and telecommunication industries. The 3-D space charge measurement
should provide useful information for examining the ion migration phenomenon and
should contribute to the investigation of its mechanism.




Dept. of EEE                                 8                   MESCE, Kuttippuram
PEA Space Charge Measurement System                                  Seminar Report „04



                          MULTI-SENSOR METHOD


       The multi-sensor technique for 3-D space charge measurement is „the normal
PEA‟ with many acoustic sensors. The set up has 10 sensors attached to the bottom
electrode, and signals are detected by the sensors at the same time when a voltage pulse
is applied. Each sensor is a PVDF film (2 mm wide, 3mm long, 9 micrometer thick),
and the sensors are placed at 3mm intervals. A coaxial switch is used to record signals
from the sensors separately. The resolution in the lateral direction is determined by the
distance between sensors, and the distance was about 3mm. Because the sensors are
placed in a line, the current multi sensor PEA gives space charge profiles in two
dimensions. By placing sensors at the nodes of a grid, 3-D profiles will be obtained.


       The acoustic lens method can measure 3-D space charge profiles precisely when
the profiles do not change rapidly, so that it would be suitable for monitoring long-time
aging phenomena such as ion migration. At the same time, there are users who
appreciate the rapid measurement capability of the multi sensor method though it results
in a rough 3-D profile.




Dept. of EEE                                9                     MESCE, Kuttippuram
PEA Space Charge Measurement System                                Seminar Report „04



                       TRANSIENT PEA SYSTEM


       To observe transient space charge behavior, such as charge dynamics at
insulation breakdown, it is essential to measure signals at a high repetition rate. The
pulsed electric field can be supplied by a pulse generator with a fast commercial
semiconductor switch, and the performance of oscilloscopes has progressed in recent
years. The obstacle was posed by signal progressing, ie, recording output signals at a
high repetition rate. Two outstanding systems have been developed recently. They are
3-H PEA system and high repetition rate, high temperature PEA system.




Dept. of EEE                              10                    MESCE, Kuttippuram
PEA Space Charge Measurement System                                 Seminar Report „04



                              3-H PEA SYSTEM


       The 3-H PEA system can measure space charge profiles under high voltage
application (-60KV to +60KV), at a high repetition rate (2khz, 0.5ms interval) with high
spatial resolution (10micrometer). The voltage application unit was molded in epoxy
resin, and silicone rubber was placed around the high voltage electrode to prevent
surface breakdown. The disadvantage of it is that the observation period was limited
because of the memory size of the oscilloscope. To observe space charge dynamics
through out long time aging tests, the new system stores 1000 signals during 500ms,
and then transfers them to a computer in 130ms. By collecting data repeatedly, space
charge behavior throughout a break down test can be observed reasonably accurately.




Dept. of EEE                              11                     MESCE, Kuttippuram
PEA Space Charge Measurement System                                  Seminar Report „04



 HIGH REPETITION RATE, HIGH TEMPERATURE PEA


       It can apply dc bias voltage as well as impulse voltage to perform impulse
breakdown tests. Silicone oil is used to prevent surface and partial discharges around
the high voltage electrode, and the temperature of the specimen can be controlled by
circulating the oil. The pulse generator used in this system supplies the pulse at 100khz;
so that the space charge profiles are observed every 10microseconds.the observation
time was limited to 1ms. Space charge behavior was observed by applying a dc voltage
of 25KV to a 0.1mm thick LDPE film. Experiments were performed at 90degree
Celsius. The breakdown occurred at 0.833ms after the voltage application. The effect of
dc pre-stressing on the impulse breakdown was investigated using this system. It was
revealed that dc pre-stressing causes charge accumulation and enhance charge injection
by the impulse voltage application.




Dept. of EEE                               12                     MESCE, Kuttippuram
PEA Space Charge Measurement System                                 Seminar Report „04



      HIGH AND LOW TEMPERATURE PEA SYSTEM


       There has been a demand for space charge measurements of insulating materials
used at extremely high temperatures, such as ceramics used in power plants. The
position of the profile shifted to the right-hand side along the horizontal axis with the
increasing temperature. This shift was a result of the decrease in the sound velocity in
the lower aluminum electrode, resulting in the longer delay time for the signal to reach
the sensor. The profile itself was also broadened with the increasing temperature;
because of the decrease in sound velocity in the specimen. Dielectric properties at
cryogenic temperature have been of interest for the development of cable systems using
superconductors. N.Hozumi developed a PEA system for cryogenic temperature to
investigate insulating materials for cable systems using a superconductor.




Dept. of EEE                               13                     MESCE, Kuttippuram
PEA Space Charge Measurement System                               Seminar Report „04



            PORTABLE/MOUNTABLE PEA SYSTEM


       With the increase of PEA users, more practical applications have been required,
such as on-site monitoring of charge accumulation in a laminated polymer sheet. The
size of the electrode unit has been reduced and developed a portable PEA system and a
mountable PEA unit. These can be divided into three. They are mini-PEA unit, portable
PEA system and mountable PEA unit.




Dept. of EEE                             14                    MESCE, Kuttippuram
PEA Space Charge Measurement System                                   Seminar Report „04



                                MINI-PEA UNIT




                         Figure shows diagram of mini-PEA unit.




                      Figure shows the photograph of mini-PEA unit.


All components used in the conventional PEA electrode unit are enclosed in aluminum
casing, including a 9-micrometer thick PVDF film and a laboratory built amplifier. The
performance of the mini-PEA is as good as that of the conventional PEA. The resolution
is about 12 micrometer, although it can be improved by the use of a thinner
piezoelectric film.




Dept. of EEE                               15                     MESCE, Kuttippuram
PEA Space Charge Measurement System                                 Seminar Report „04



                       PORTABLE PEA SYSTEM




                     Figure shows the entire portable PEA system.


       The mini-PEA was equipped with a pair of pliers. The large red box is a battery
and can be exchanged for a smaller one. The small box in front of the battery is a pulse
generator that supplies a modified pulse voltage. The pulse voltage was designed to
enable observation of a charge profile directly on the portable oscilloscope without the
need for any signal processing such as deconvolution. Thus no personal computer is
required for a measurement.




Dept. of EEE                              16                     MESCE, Kuttippuram
PEA Space Charge Measurement System                                Seminar Report „04



                          MOUNTABLE PEA UNIT


       A challenging problem for the space industry is to measure space charge
accumulation in materials in a space environment to investigate charging and
discharging phenomena on spacecraft. On earth, the space environment is reproduced in
an irradiation chamber.




                   Figure shows the diagram of mountable PEA unit.




                 Figure shows the photograph of mountable PEA unit.


       The configuration was modified to avoid covering the irradiation surface of the
specimen. The specimen has a metalized (evaporated) electrode, so that the voltage
pulse is applied from the voltage application unit to the specimen through the metalized
electrode. The propagation of acoustic waves with this configuration was explained by
numerical simulation.


Dept. of EEE                              17                     MESCE, Kuttippuram
PEA Space Charge Measurement System                                 Seminar Report „04




     Figure shows space charge profiles observed under electron beam irradiation.


       In fact, early publications on the PEA method described charge accumulation in
PMMA irradiated by an electron beam measured in air after irradiation. The specimen
shown here was a 125-micrometer thick polyamide film irradiated by the electron beam
of 95 keV, 220pA/cmsquare from the right-hand side of the specimen. Negative charges
accumulated inside the specimen were clearly observed, and the amount of the charge
increased with time.


       The irradiation chamber used in this work has surface charge measurement
facilities, including a potential probe and a discharge current moniter. The simultaneous
measurement of surface and internal charges should contribute to understanding the
electrostatic phenomena of materials used in the space environment. Moreover, this
work proved that the mini-pea could be mounted on various apparatus such as a high
vacuum chamber used in the plasma processing research field.




Dept. of EEE                               18                     MESCE, Kuttippuram
PEA Space Charge Measurement System                                  Seminar Report „04



                                 ADVANTAGES


       While deconvolution was initially required to extract the space charge
distribution from the transducer output, even in thin samples, the use of thin transducers
based on polyvinylidene fluoride has now largely eliminated the need for
deconvolution. Assuming no instrumentation limitations, the spatial resolution of the
acoustic techniques is the product of the pressure pulse duration and the speed of sound.




                              DISADVANTAGES

            As the PEA method can measure only net charges and does not indicate the
source of the charge, the use of other measurements, such as conduction current,
thermally stimulated current, electro luminescence, and microscopic analyses, is useful
to avoid misleading data and to establish theoretical models.




Dept. of EEE                               19                     MESCE, Kuttippuram
PEA Space Charge Measurement System                                 Seminar Report „04



                               APPLICATIONS


       The pulsed electro acoustic (PEA) method has been used for various industrial
applications, especially for the evaluation of high voltage insulation. The pulse electro
acoustic analysis (PEA) can be used for space charge measurements under dc or ac
fields. It is clear that materials should be examined by various parameters. Among the
numerous issues that should be considered, the simultaneous observation of surface and
internal charge behavior is being pursued with vigorous effort.




Dept. of EEE                               20                     MESCE, Kuttippuram
PEA Space Charge Measurement System                                 Seminar Report „04



                                CONCLUSION


       With the increase of industrial applications of space charge measurements,
various new types of PEA systems have been developed in the past several years. Three
dimensional systems, transient systems, and high and low temperature systems have
been introduced in addition to a mini PEA unit with its application to a portable system
and to a vacuum irradiation chamber. These new PEA systems should expand the
applicable research field.




Dept. of EEE                              21                     MESCE, Kuttippuram
PEA Space Charge Measurement System                                Seminar Report „04



                                 REFERENCES


    “Innovative PEA Space Charge Measurement Systems                    for industrial
      applications” IEEE Electrical Insulation Magazine, page no: 18 to 25, vol.20,
      no.2, March/April 2004.
    “Space Charge Dynamics in LDPE Films Immediately Before Breakdown”
      IEEE Transactions on Dielectrics and Electrical Insulation, page no: 304 to 306,
      vol.8, no.2, April 2001.
    “High Repetition Rate for PEA Systems” IEEE Transactions on Dielectrics and
      Electrical Insulation, page no: 155 to 158, vol.11, no.1, February 2004.




Dept. of EEE                              22                     MESCE, Kuttippuram
PEA Space Charge Measurement System             Seminar Report „04



                      TABLE OF CONTENTS

      INTRODUCTION
      PEA METHOD
      3-D SYSTEM
      ACOUSTIC LENS METHOD
      MULTISENSOR METHOD
      TRANSIENT PEA
      3-H PEA
      HIGH REPETITION RATE, HIGH TEMPERATURE PEA
      HIGH AND LOW TEMPERATURE PEA SYSTEM
      PORTABLE/MOUNTABLE PEA STSTEM
      MINI-PEA UNIT
      PORTABLE PEA UNIT
      MOUNTABLE PEA UNIT
      FIGURES
      ADVANTAGES
      DISADVANTAGES
      APPLICATIONS
      CONCLUSION
      REFERENCES




Dept. of EEE                          23      MESCE, Kuttippuram
PEA Space Charge Measurement System                               Seminar Report „04



                                   ABSTRACT


       This report introduces new space charge measurement systems developed
in Japan using the pulsed electro acoustics (PEA) method. The new methods include
two types of 3-D systems, two types of transient systems for extremely high or low
temperature, a small PEA unit for a portable system, and a mountable PEA unit that can
be applied to another facility, such as a vacuum chamber. These new PEA systems
should expand the applicable research field.




Dept. of EEE                               24                  MESCE, Kuttippuram
PEA Space Charge Measurement System                                Seminar Report „04



                        ACKNOWLEDGEMENT


     I express my sincere gratitude to Dr. P.M.S Nambisan, Prof. & Head,

 Department of Electrical and Electronics Engineering, MES College of Engineering,

 Kuttippuram, for his cooperation and encouragement.


     I would also like to thank my seminar guide Ms. Jayasree N.R. (Lecturer,

 Department of EEE), Asst. Prof. Gylson Thomas. (Staff in-charge, Department of

 EEE) for their invaluable advice and wholehearted cooperation without which this

 seminar would not have seen the light of day.


     Gracious gratitude to all the faculty of the department of EEE & friends for their

 valuable advice and encouragement.




Dept. of EEE                              25                     MESCE, Kuttippuram

								
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