| PEA
Space Charge Measurement System |
INTROUCTION
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 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.
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