PEA Space Charge Measurement System


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.