International Journal of Electrical Power and Energy Systems Engineering 3:1 2010

Understanding Charge Dynamics in Elastomers

Adopting Pulsed Electro Acoustic (PEA) Technique
R. Sarathi, M. G. Danikas, Y. Chen, and T. Tanaka

even raising the electric field much more than the breakdown
AbstractIn the present work, Pulsed Electro Acoustic (PEA) strength of the insulation causing earlier failure) which can
technique was adopted to understand the space charge dynamics in enhance the process of degradation thereby reducing the life
elastomeric material. It is observed that the polarity of the applied of the insulation. Hence it is essential to know the dynamics
DC voltage voltage and its magnitude alters the space charge of charge movement and charge trap sites in the insulation
dynamics in insulation structure. It is also noticed that any addition structure to enhance the reliability of the insulation material
of compound to the base material/processing technique have
characteristic variation in the space charge injection process. It could
and the world over researchers are trying to mitigate the space
be concluded based on the present work that the plasticizer could charge formation in insulation material. Earlier studies on
inject heterocharges into the insulation medium. Also it is realized space charge identification carried out through thermally
that space charge magnitude is less with the addition of plasticizer. In stimulated current to understand the magnitude of injected
the PEA studies, it is observed that local electric field in the charge in the insulation and by depolarization technique,
insulating material can be much more than applied electric field due which can provide information about trap depth distribution
to space charge formation. One of the important conclusions arrived [2].The two most feasible techniques adopted recently to
at based on PEA technique is that one could understand the safe measure the space charge distribution in solid insulation
operating electric field of an insulation material and the charge trap material were the pressure wave propagation (PWP) and
sites.
Pulsed Electro Acoustic (PEA) process. Takada provided
KeywordsPulsed electro acoustic technique, space charge, DC complete information about the principle of operation of PEA
voltage, elastomers, Electric field, high voltage. system in detail [3]. In the present study, the PEA technique
was adopted to understand the charge dynamics in elastomeric
I. INTRODUCTION material. Elastomeric material identified as suitable material
for outdoor insulation and also for cable insulation at
H IGH voltage DC power transmission has acquired
considerable prominence in the recent times for bulk
power transmission. The power loss in the insulation is
cryogenic temperatures. In this paper, the charge dynamics in
acrylic elastomer and acryl nitryl elastomer are presented.
These insulating material straind by the Maxwell stress at high
minimum under DC voltages. The major problem of failure applied electric field. These elastomers have potential
under DC voltages is due to the space charge formation. The application as actuators and soft robots [4,5]. Masuya et al.
space charge formed in the medium can cause insulation carried out space charge injection studies with different
degradation leading to its failure. The space charge injection elastomeric material and concluded that homo and
formed in the insulation medium can be homo/hetero charge heterocharge formation occurs in the elastomeric materials
in nature. In general the buildup of injected charge, which is due to applied DC voltage [6]. The polymer actuator devices,
observed near to the injecting electrode were called as in general, can exhibit their movement at electric field of
homocharges. Hetero charges are the one which are formed about 10kV/mm. Hence a methodical experimental study was
due to chemical species degradation or due to contaminant carried out to understand the charge dynamics in the
(conducting or non conducting in nature), appear to the elastomeric materials, adopting the PEA technique.
electrodes or in the bulk volume of insulation different from
polarity of the applied voltage. In recent times, it is observed II. EXPERIMENTAL STUDIES
hetero charges appear near to the electrodes [1]. This Fig. 1 shows typical PEA system. The PEA measurement
accumulated space charge can form non-uniform electric field device (Five Lab Make, Japan) was used for the space charge
distribution in the bulk volume of insulation (at some point measurement. It includes high DC voltage source, pulsed
generator of variable frequency, high voltage electrodes,
sensor for detection of acoustic signal generated due to
R. Sarathi is with Department of Electrical Engineering, IIT Madras,
Chennai- 600 036, India and he is now Visiting Researcher with Waseda
applied voltage pulse, amplifier and an oscilloscope. In the
University, Kitakyushu, Japan (corresponding author; e-mail: present study, high voltage DC generated using high voltage
sarathi@ee.iitm.ac.in). amplifier, which can generate voltage up to 12 Kv. The
M. G. Danikas is with Department of Electrical and Computer Engineering, applied DC voltage measured using a high voltage
Democritus University of Thrace, Xanthi, Greece and is now Visiting Scholar
measurement probe.
with Waseda University, Kitakyushu, Japan.
Y. Chen and T. Tanaka are with IPS Graduate School, Waseda University,
Kitakyushu, Japan.

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 International Journal of Electrical Power and Energy Systems Engineering 3:1 2010

III. METHODOLOGY ADOPTED FOR IDENTIFICATION OF

ELECTRODES IN PEA STUDIES
Typical PEA signal measured as reference is shown in Fig.
2. It is possible to evaluate the position of the electrodes and
from where the injection sites and charge dynamics could be
evaluated. The positive peak and the negative peaks
correspond to the anode and the cathode of the electrode gap.
Once we know the thickness of the sample (d) measured by
vernier and the time difference between the positive and
negative peak (t), then it is possible for one to evaluate
thevelocity of the signal in the sample specimen under test as
distance by time as the velocity of the signal in the material. In
Fig. 1 Typical Pulsed electro acoustic system the present work, the samples are elastomers and once the top
electrode is placed over the sample then due to the dead
The output voltage of pulsed generator used in the present weight of the top electrode the thickness gets reduced. Hence
study could generate voltage up to 600V operating at a fixed in the present work, the thickness is evaluated using the
frequency of about 400Hz. The rise time of the pulse is about following procedure. First the sample is placed between the
5ns. The PTFE material is used as sensor material. The high electrodes as it is used for testing. After 10 minutes, the
electrodes is of parallel plane configuration. The top electrode sample is removed and thickness of the specimen where the
coated with semiconducting (carbon loaded) material and the semi-conducting electrode was in contact, at that location the
bottom ground electrode is aluminium material. The thickness thickness is measured using a vernier. Such measurements
of the materials used was measured using a vernier. The were made at three different locations and the average of it
oscilloscope used for measurement Lecroy 500 MHz, with was treated as thickness of the specimen. For thin specimens,
sampling rate of 4GS/s. The insulating material is of flat type identifying the the electrode position is a cumbersome process.
sheet. In the present work, the charge dynamics were studied The first positive peak and the adjacent negative peak were
by applying different electric fields up to 50 kV/mm. In the treated as the electrode separation.
present study three different materials were studied, which
includes acrylic elastomeric material (AEM), Acryl Nitryl IV. RESULTS AND DISCUSSION
Butadiene Rubber (ANBR), which is called as TYPE-I
material and the third material is ANBR with addition of
plasticizer, which is identified as TYPE-II material. The
composition of Acryl Nitryl Butadien rubber has 69%
Butadiene and Acryl Nitryl of 31%.

Fig. 2 Typical PEA reference signal used for analysis of velocity of

signal in the specimen

Fig. 3 Variation in charge injection magnitude in acrylic elastomers

at different electric fields (i) +DC (ii)-DC

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 International Journal of Electrical Power and Energy Systems Engineering 3:1 2010

Fig. 3 shows typical variation in magnitude of charge

injection into the insulation material at different electric fields
under positive and negative DC voltage. It is observed that the
magnitude of charge injection is high when the material is
subjected to negative DC voltage. The cause for it could be
due to application of negative DC voltage and positive nano
second pulse for the measurement of space charge, which
would have allowed higher magnitude of charges to get
injected into the insulation material. This allows one to
confirm that any transient voltages of opposite polarity can
cause higher space charge formation in to the insulation
material. Also it is observed that when the applied electric
field is kept constant, it allows space charge injection to occur
in the material and with time the material could fail. For
instance in Fig. 3(ii), at 25 kV/mm electric field, the specimen
has failed during its operation. This confirms that charge
movement in the insulation material could cause failure of the
insulation material.
Fig. 4 shows a two dimensional variation of space charge in
the insulating material, at different electric fields. In this the
material stressed for definite time instants at different electric
fields. It could be realized that adjacent to the high voltage
electrode, the heterocharges gets injected in to the insulating
material. Tanakas model provides clear visulisation on to the
mechanism of hetero charge injection adjacent to the high Fig. 4 Two dimension variation of space charge in Acrylic elastomers
voltage electrode [1]. In the present study, with AEM material at different electric fields .Fig 4A (i) 2kV/mm (ii) 5 kV/mm (iii) 20
kV/mm (iv) 20 kV/mm (v) 30 kV/mm (vi) 40 kV/mm (vii)50 kV/mm.
such phenomena could be observed in both positive and
Fig. 3B. Fig 4A (i) 5kV/mm (ii) 10kV/mm (iii) 15 kV/mm (iv) 20
negative DC voltages. In the AEM material no hetero charge kV/mm (v) 25 kV/mm . (A) +DC (B) DC.
injection is observed in the bulk volume of the insulation
material, irrespective of polarity of the applied DC voltage.
Recently Delphino et al. studied the high space charge
dynamics in EVA nano composite material and concluded that
high space charge dynamics is a positive property of the
material, which helps to reduce the charge transient followed
with voltage polarity reversal [7].
Fig. 5 shows electric field variation in the AEM material
operated at different voltages. The electric field could be
obtained by integrating the charge profile [8]. It is observed
that even though the material is the same, the characteristic
variation of electric field in the material at same operating
electrical stress is different. This variation could be due to
polarity of applied nano second pulse or could be due to the
inherent characteristics of magnitude of charges that gets
injected into the bulk volume of the insulating material. In
general, it could be realized that local electric field in the bulk
volume of the insulating material, is much higher than the
theoretically calculated electric field, assuming parallel plane
configuration. This indirectly confirms that the point of higher
stress can cause ageing of material and initiate the process of
damage in the insulation leading to earlier failure. Thus the
PEA technique, not only provides the magnitude and position
of space charge, it also provides information to understand the
local electric field variation in the bulk volume of insulating
material and could provide information to the end user to
identify the safe operating voltage.
Fig. 5 Variation in Electric field in the bulk volume of the Acrylic
elastomer insulation material measured through PEA process (a)
+DC (b) -DC

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Fig. 8 Variation in Electric field in the bulk volume of the Acryl

Nitryl rubber (Type-I) insulation material measured through PEA
Fig. 6 Variation in charge injection magnitude in Acryl Nitryl process (a) +DC (b) -DC
Rubber (Type-I) material at different electric fields (i) +DC (ii) -DC

Fig. 7 Two dimension variation of space charge in Acryl Nitrile

rubber (Type-I) insulation at different electric fields (i) 0.8kV/mm
(ii)1.6 kV/mm (iii)3 kV/mm (iv) 5 kV/mm (v)10 kV/mm (vi)15
kV/mm (vii)20 kV/mm (viii)25 kV/mm (ix)30 kV/mm (x) just above Fig. 9 Variation in charge injection magnitude in aryl rubber material
30 kV/mm . (A) +DC (B) DC. (Type-II) at different electric fields (i) +DC (ii)-DC

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Fig. 6 shows a typical variation in magnitude of charge

injection into NBR-Type1 material the insulation material at
different electric fields under positive and negative DC
voltage. It is also realized formation of heterocharges in the
bulk volume of insulation. The characteristics of heterocharge
injection injection into the bulk volume irrespective of
polarity of applied DC voltage. It is also observed that the
magnitude of injected charge is high with the negative DC
voltage. This could easily be realized in the two dimension
variation of space charge in the insulation material (Fig. 7).
The heterocharge injection in the bulk volume could be due to
charge trap sites in the insulating material. Natsui et al.,
carried out PEA studies in epoxy composites where they could
conclude that charge migrates in to the bulk volume of the
material [9].
Fig. 8 shows variation in the electric field in the insulation
material under DC stress measured through PEA technique. It
could be observed that operating electric field in the bulk
volume is not constant and vary accordingly with the injected
charge magnitude. Also it is observed from Fig. 8 that electric
field magnitude due to space charge could enhance local
electric field much higher especially under negative DC
voltage.
Fig. 9 shows variation in injected charge magnitude under
different electric fields in the Type-II insulating material.
Fig. 10 Two dimension variation of space charge in Acryl Nitryl
rubber (Type-II) insulation at different electric fields (I) 0.8 kV/mm Comparing Figs. 6 and 8, it is observed that Type-II material
(II)3 kV/mm (III) 5 kV/mm (IV) 10 kV/mm (V) 20 kV/mm (VI) 30 where the magnitude of charge injection is less compared to
kV/mm (VII)40 kV/mm (VIII) 50 kV/mm (a) +DC (b) DC. that in the Type-I material. This could be due to addition of
plasticizers. Hayase eat al studied space charge dynamics in
LDPE nanocomposite material and could conclude that
addition of nano material could reduce the space charge
injection [10].
Fig.10 shows two dimensional pattern of space charge
variation in the Type-II material. It is observe that
heterocharge formation occurs in the bulk volume of the
insulation. This formation could be formed because of the
plasticizers, which could activate the process of heterocharge
formation. Fig. 11 shows the variation in local electric field in
the TYPE-II insulation material. It could be easily realized
that the local electric field variation is less compared to Type-I
material, allowing one to conclude that addition of plasticizer,
eventhough it aids the formation of hetero charges in the bulk
volume of insulation, due to less charge magnitude, in the
volume, the electric field variation is also very much limited,
thus allowing the material to operate at high electric field
compare to TYPE-I.
Comparing Figs. 3, 6 and 9 the space charge magnitude is
less with Type-II material. In addition, it could be realized that
addition of any compound could alter the space charge
injection characteristics of the insulating material. Also it is
observed that addition of plasticizer could alter the charge
injection process and could reduce charge trap sites.

V. CONCLUSION
Fig. 11 Variation in Electric field in the bulk volume of the Acryl The important conclusions obtained based on the present
Nitryl Rubber (Type-II) insulation material measured through PEA study are the following:
process (a) +DC (b) DC.

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 International Journal of Electrical Power and Energy Systems Engineering 3:1 2010

1. The PEA technique not only provides information about

space charge dynamics, it could allow one to understand
the safe operating electric field of the material.
2. Addition of any new compounds to the base material
could alter the the charge injection process in insulation
material.
3. Addition of plasticizers alters the charge injection
magnitude and the mechanism
4. The characteristics of charge injection mechanism is
almost the same under Positive or negative DC voltage
but magnitude of charge injection is always high when it
is operated with positive pulse and negative DC, which
can cause early failure of insulation.
5. In acryl elastomers the heterocharges occurs adjacent to
the operating electrode by fast charge injection process
6. In Acryl nitryl rubber insulation, heterocharge formation
occurs. Also the addition of plasticizer into the acryl
nitry rubber allows the material to operate at high electric
fields.
7. It is confirmed, based on the present study that space
charge injection can cause failure of insulating material
during operation.

REFERENCES
[1] T. Tanaka and K. Masuya, Peculiar space charge formation in some
elastomers, Proc. of the 2008 Int. Symp. on electrical insulating
materials, PP187-191, Sept7-11, 2008, Yokkaichi, Mie, Japan.
[2] D.Fabiani, G.C. Montanari, A. Dardano, G. Guastavino, L. Testa, M.
Sangermano, Space charge dynamics in nano structures epoxy resin,
2008 Annual report Conf. on Elec Insulation Dielectric Phenomena
(CEIDP), pp710-713, Quebec, Sept 2008, Canada
[3] T. Takada, Acoustic and optical methods for measuring electric charge
distributions in dielectrics, IEEE Trans. on Dielectrics and Electrical
Insulation, Vol-6, No.5, pp519-547, Oct1999
[4] T. Tanaka, M. Sato and M. Kozako, High Field Maxwell Stress-Strain
Characteristics of Conventional Polymers as Actuators, Ann. Rept.
IEEE-CEIDP, pp. 364-367, 2004.
[5] T. Tanaka, M. Sato and M. Kozako, Voltage-Strain Characteristics of
Cylindrical Polymer Actuators, Proc. ISEIM2005, No. A5-3, pp.780-
783, 2005.
[6] K. Masuya, Measurement of space charge distribution in various kinds
of dielectrics, M.Sc Thesis submitted to Waseda University, No.
44061077, 2007 (in Japanese).
[7] S. Delphino, D. Fabiani, G.C. Montanari, High space charge dynamics in
EVA based nanocomposites flat specimen, 2008 Annual Report Conf.
on Elec Insulation Dielectric Phenomena (CEIDP), pp137-140, Quebec,
Sept 2008, Canada
[8] Kaori Fukunaga, Innovative PEA space charge measurement systems for
industrial applications, IEEE Electrical Insulation Magazine, Vol-20,
No.2,pp18-25, 2004.
[9] N. Natsui, Y. Achigo, T. tanaka, Y. Ohki and T. Maeno, Detection of ion
migration in composites for printed circuit boards by PEA, Proc. Of the
2008 Int. Symp. On Electrical Insulating Materials, PP376-379, Sept7-
11, 2008, Yokkaichi, Mie, Japan.
[10] Y. Hayase, H. Aoyama, K. Matsui, Y. Tanaka, T. Takada, Y. Murata,
Space charge formation in LDPE/MgO nanocomposite films under high
DC electric stress, IEEJ Trans on Fundamental Science and Materials,
Vol-126, No.11, pp1084-1089,2006.

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