Acoustic and optical methods for measuring electric charge distributions in dielectrics

Abstract

Much progress had been made during the last two decades in acoustic and optical methods for measuring charge distributions in dielectrics. A review on this topic is given, which mainly covers the past research activities associated with that at our laboratory. For acoustic methods, we will discuss and compare the pulsed electroacoustic (PEA) and pressure wave propagation (pwP) methods and present some of the results that enabled us to gain physical insights into the charge dynamics within solid plate samples and coaxial cables. For optical methods, we will discuss the Pockels effect technique that is used for the dynamic measurement of surface charge distributions, and the Kerr effect technique that is developed for measuring electric field distributions within liquid dielectrics. 1 INTRODUCTION T HE Poynting vector represents a flow of electric power equivalent to EXH [W 1m2], where E [V 1m] represents electric field inten sity and H [Aim] the magnetic field intensity. To facilitate the transmission of large amounts of electric energy, which has its equivalent in a large Poynting vector, researchers must strive to increase either the magnetic field jj or the electric field E. An increase of H can be ac complished by using superconducting transmission lines, while that of if can be achieved by increasing the transmission voltage. However, at present, the development of a practical, usable superconducting trans mission system has not been fully realized. Therefore, we have to con tinue our efforts in obtaining high E through a HV transmission system.. To this end, much research on the properties of dielectric and insulating materials will have to be conducted. This is one of the goals of the present workThe advancement of excellent electric insulating materials, in which electric power flows in the form of a Poynting vector, has made a sig nificant contribution to the development of electric power engineer ing, electronics engineering and electronic communication systems in present everyday life, Many researches have been conducted on de voltage-current characteristics by electric conductivity (r; [S/m]) mea surement, and on ac voltage�current characteristics by permittivity (c [F/m]) measurement in the past years. However, that research con sidered only the case where the current flowing through the materials had reached the steady state and the electric field across thematerial was uniform and independent of time.During the last 20 years, because electric power apparatus has be· come more compact, and electronic devices have become much smaller, the insulating materials used are highly stressed to > 1 MV I cm, which is near the breakdown strength of the insulating materials. Under such high electric field application, electric charges may be injected fromelec trodes or be dissociated from impurities that occur in the material. These electric charges accumulated in the insulating material distort the orig· inal internal electric field distribution. The process may cause the insu lating material to degrade and som...

Decay of electric charge on corona charged polyethylene

“…The acoustic waves traverse across the material and are converted into an electrical signal by a piezo-electric transducer, amplified by one or two amplifiers depending on the signal magnitude and finally captured with a digital oscilloscope. The principle diagram is shown in figure 2 and the detailed description of the PEA method can be found in [8,9]. If the deposited charge on the surface corona charged film can inject into the bulk then the PEA technique will capture the signal and help in the analysis of the decay mechanism.…”

Decay of space charge in a glassy epoxy resin following voltage removal

“…The samples were produced in the form of thin films with a thickness varying between 250 and 300 µm. When placed under DC voltage space charge developed in the samples and this was measured using a pulsed electro-acoustic (PEA) system [12]. In the measurements the lower electrode was aluminium while the upper electrode through which the high voltage was applied was a carbon black loaded conducting polymer, which in some cases was covered with a conducting layer of aluminium.…”

Space Charge Measurement Technologies and Their Potential Applications

“…(13,14) 3.3 Applications of optic methods to insulation system for power equipment A combination of optic methods and the PWP/PEA methods make it possible to measure the distribution of space charge in liquid/solid and gas/solid insulation systems that are often used in power equipment, such as in cables, transformers, bushings, reactors, capacitors, circuit breakers, motors, and generators. The Kerr effect method has been used to measure the electric field distribution in liquids, such as in transformer mineral oils (26)(27)(28)(29)(30) (Fig. 6).…”

“…The pressure pulse techniques have also been used for the measurement of space charge in composite oil/cellulose insulating systems. (21)(22)(23)(24)(25) For the measurement of electric fields and space charge in liquid dielectrics and surface charge on solids in air, the electrooptic Kerr effect technique (26)(27)(28)(29)(30) and Pockels effect technique (31)(32)(33) have been developed. In comparison with the earliest slice method, (34) these latest techniques can be used to measure quantitatively the distribution of space charge in an insulating material nondestructively, and thus are valuable tools in helping to understand the electrical behaviors of an insulation system in the development of power equipment, such as high-voltage direct-current (HVDC) cables ( Fig.…”