Review of space charge measurements in dielectrics

In the last two decades, a significant effort has been directed to understand the internal charge distributions in dielectrics. This has resulted in the development of a number of methods which give detailed information about space charge distributions. This review presents the evolution of several experimental approaches for the determination of spatial charge and field distributions in dielectrics, including the nondestructive methods of direct probing such as the thermal-pulse methods and the methods using pressure waves that propagate through the charged sample. Detailed information is also provided on the principles used in each technique, typical resolution, limitations and advantages associated with each method. The applications of the methods to several insulating materials charged with various mechanisms are described. The paper concludes with a discussion on a number of quite interesting techniques which seem most promising     

Demonstrating a threshold for trapped space charge accumulation in solid dielectrics under DC field

This paper presents a discussion of the concept of threshold for trapped space charge accumulation in solid dielectrics submitted to a DC field. The starting point is the fact that it is often possible to define a critical field separating an ohmic type of conduction from a nonlinear regime in the current-voltage characteristic of solid dielectrics. In the space charge limited conduction theory, this critical field corresponds to the onset of space charge accumulation. However, other conduction processes, such as hopping conduction for example, can also explain nonlinearity in the current-voltage characteristic, which does not involve space charge. It is proposed to check for the existence of a critical field for space charge accumulation using complementary techniques, i.e., space charge detection and electroluminescence techniques. Polyethylene, polyester and polycarbonate were investigated as being representative of three different families of polymers. It is shown that similar values of thresholds are found for a given material using the three above-mentioned techniques, lending support to the physical explanation of a threshold for trapped space charge accumulation.     

Pulsed electro-acoustic technique applied to in-situ measurement of charge distribution in electron-irradiated polymers

Dielectric materials are frequently used in satellite structures for substantial savings in weight. Their dielectric nature and the effect of different forms of radiation encountered in space combine to accumulate electrical charges resulting in the occurrence of electrostatic discharges which cause harmful interference with the electronic parts of the satellite, leading to its malfunction or to a total loss of equipment control. Therefore, the behavior of dielectrics under irradiation must be investigated before being used in space. A pulsed electroacoustic (PEA) device was mounted in an irradiation chamber to monitor the spatial distribution of electrons implanted into polymer films during electronic irradiation. Internal charge accumulation was clearly identified. Penetration depth versus energy was double checked by comparing PEA with surface potential data.     

Numerical methods in the simulation of charge transport in solid dielectrics

This paper introduces some numerical methods pertaining to the simulation of charge transport in solid dielectrics that can be of interest to the electrical engineering community. These methods are generally scattered over numerical analysis and computing literature making difficult for the non-specialist to select the appropriate computing schemes. In the first part we consider an example of a set of physical parameters needed to describe charge transport and we introduce mathematical equations that must be solved. A series of numerical schemes are compared for one-dimensional convection problems in a second part. Numerical results show that a third order upwind scheme named QUICKEST, combined with a flux limiter (ULTIMATE) is competent for the resolution of the transport equation in solid dielectrics, avoiding numerical oscillations and numerical diffusion. The third part deals with numerical resolution of the Poisson's equation comparing two different methods.     

Novel numerical methods for measuring distributions of space charge and electric field in solid dielectrics with deconvolution algorithm

The deconvolution algorithm for measuring the distribution of space charge under DC by the pressure wave propagation (PWP) method is studied in this paper. A new Fredholm integral equation of the first kind, including a space charge distribution without a partial differential operator is presented. Numerical methods based on Tikhonov regularization for solving this integral equation and the original PWP equation are studied. Numerical simulation is studied for the effect of signal-to-noise ratio (SNR), and comparison with other algorithms is discussed. The numerical solution of an electric field distribution from measurements of a LDPE specimen is obtained successfully.     

Progress in space charge measurement of solid insulating materialsin Japan

The space charge measurement technique may be applied to diagnostic technology for high-voltage machines in the near future. The authors review the progress in the various methods of space charge measurement in Japan     

Luminescence and space charge in polymeric dielectrics - [whitehead memorial lecture (2008)]

Polymers are extensively used as insulating material in high voltage devices, such as underground power cables, power capacitors and transformers. During normal operation the polymeric insulation of a power device is not only subjected to electrical stresses, but could also be subjected to other stresses that can cause the degradation and ultimately lead to insulation failure. It has been well established that electroluminescence and charge injection, which gives rise to space charge in the polymeric insulation, occur at ac, dc and impulse field above a certain threshold value. Space charge can cause dissipative energetic processes such as photon and phonon emission, increase the local electric field and reduce the withstand voltage of the insulation. This paper describes the characteristics of electroluminescence in polymeric insulation subjected to ac voltage and shows its relevance to space charge injection in the material. It is shown that insulation subjected to high voltage could emit various types of light but electroluminescence emission which is related to space charge injection can be clearly distinguished from these other types of light emission. The electroluminescence technique is a valuable tool to evaluate the dielectric properties of novel insulating materials, such as nanodielectrics.     

Influence of space charge buildup on the transition to electricaltreeing in PE under ac voltage

Our investigation is focused on the understanding of electrical aging in polymeric materials under 50 Hz ac voltage. In this report, a needle electrode molded into the insulant to simulate defects producing local field enhancement is used. Special emphasis is given to low density polyethylene. The transition between the discharge-free electroluminescent state to micropartial discharge (PD) state (early electrical tree propagation phase) is studied with a sensitivity reaching 0.01 pC. Optical diagnosis is carried out simultaneously. At the moment of tree initiation, electrical discharges from 0.04 to 0.1 pC occur in the positive half cycle. A very small electrical tree of 5 μm was observed. Using the phase angle of the first measurable PD with respect to the applied voltage offers additional information helping to understand tree initiation. Then a correlation between the magnitude of electrical discharges and the characteristic traces of local breakdown (electrical tree) is reported. We proposed an interpretation based on the similarity with grounding tree experiments in which the initiation of a local breakdown is mainly due to a strong Poisson field radiated by a space charge region in the vicinity of the needle tip     

Space charge effect for streamer initiation and propagation in water subjected to reciprocal traveling wave voltage pulse

This paper deals with initiation and propagation processes of streamer discharges in water subjected to high-speed repetitive voltage pulse. The 50%-duty repetitive pulses were produced by the reciprocal traveling wave in a pulse forming cable which was connected with a point-plane electrode gap in a water-cell. Laue plot of time lag distributions for the streamer initiation indicated that the initiation probability during the second pulse application became much higher than that during the first pulse application. We concluded that the change in space charges distribution due to internal field during the rest-time after the first pulse enhanced the streamer initiation probability at the second pulse application. Temporal development in the positive streamers during the repetitive pulse application was observed using a gated image intensifies. When the pulse was relatively high, the streamer discharges started the propagation at the first pulse and then the discharge channel became long step by step at the subsequent pulse applications. It was also found that the periodical emission due to excitation of OH radicals was detected for a long period after the development of the streamer discharge in a spectroscopic measurement.     

Charging characteristics of a solid insulator in vacuum under AC voltage excitation

We have investigated charging and flashover characteristics of a polymeric or glass insulator exposed to AC voltage in vacuum in order to develop compact and reliable high voltage VCBs (vacuum circuit breakers). This paper focuses on charging characteristics of a cylindrical model insulator. The charging of an insulator is investigated using an electrostatic probe that measures the electric field near the triple junction on the grounded electrode. This method allows a time-resolved measurement of the charging process. The insulator was made of borosilicate, fused quartz or polymetyl methacrylate, and was in the shape of a right cylinder with 10 mm in thickness. It has been clarified that the charging is characterized by three sequential states; initiation, quasi-stable and stable states, and that the polarity of the charge is positive for these states irrespective of the voltage phase. The charging characteristics with AC voltage are compared to our previous results with DC voltage excitation. We find that the charge magnitude at the stable state coincides with that obtained by DC. The electric field on the grounded electrode, and therefore the charge magnitude, decreases with the surface roughness, and decreases as the insulation strength is increased. A computer simulation has been conducted to investigate the quasi-stable state, which clarifies that the transition in surface charge distribution being synchronous to the voltage phase is responsible for causing the quasi-stable state.     

Electrodes and charge injection in low-density polyethylene usingthe pulsed electroacoustic technique

The effects of electrode materials on space charge formation in low-density polyethylene (LDPE) have been investigated experimentally using the pulsed electroacoustic (PEA) technique. Common electrode materials used in either the laboratory or power cable industry were selected, i.e. aluminum, gold and carbon loaded crosslinked polyethylene (XLPE), and space charge accumulation after the application of high electric stress was monitored. Experimental results demonstrated that charge injection processes take place in all cases once the applied stress has exceeded a threshold. However the amount of charge, and the polarity of the dominant injected charges showed a significant dependence on the electrode materials under the same applied electric stress     

Mechanism of high field electroluminesence and determination of the space charge limited field in polymeric dielectrics

The mechanism of electroluminescence under inhomogeneous field conditions is investigated, as is the use of electroluminescence for determination of the space charge limited field. The latter investigation indicates that if the volume of polymer within the space charge limited field region is too small, the photon flux can be too low to permit accurate determination of the space charge limited field. Further, no means is available to assure that the photon flux is sufficient to assure accuracy.     

Creepage discharge propagation in air and SF6 gasinfluenced by surface charge on solid dielectrics

Influence of surface charging on creepage discharge propagation in air and SF₆ gas was investigated. The authors have employed a surface charging system using corona discharges, which permitted the deposition of surface charge on solid dielectrics with a given density distribution. By applying a lightning impulse across the charged surface of polyester film, they measured the discharge extension length as a function of surface potential generated by the surface charge. It was found that the impulse creepage discharges along the charged surface under a potential of 2 to 3 kV, extended a distance which was maximum 6× longer than the case of the uncharged surface both in air and SF₆ gas. Experimental results were also discussed from the viewpoint of the specific capacitance, permitting their application to actual HV apparatus such as GIS     

Basic study on calibration using waveform variation in space charge measurement by pulsed electroacoustic method

The pulsed electroacoustic method determines the distribution of space charge in an insulation system by applying a pulse voltage and detecting the acoustic response. It estimates the location and magnitude of charges from time-domain signals. For quantitative analysis, a bias voltage is applied to a specimen with no space charge, and the acoustic signal generated from the electrodes only is used as a reference waveform for calibration. However, a full-scale, extra-high voltage insulation system often already contains space charges in it. Therefore, a method of acquiring a reference signal under such a condition was studied. We studied methods to compare voltage variations and response signal variations. We proposed a method that measures the signal difference at different voltages. In addition, we proposed a method of correlating voltage and signal variations that may allo for more rapid signal acquisition. The feasibility was proved using an 11 mm-thick mimic polyethylene specimen.     

1000kV特高压输电技术在我国的开创与应用

阐述了我国第一条1 000 kV特高压输电示范工程的概况,论述特高压工程的意义、特点、必要性及发展前景。     

Relation between space charge accumulation and partial discharge activity in enameled wires under PWM-like voltage waveforms

It has been observed that voltage waveforms generated by power electronic converters may affect significantly the reliability of electric motor insulation. Since partial discharges are considered to be the main cause of the reliability loss, new enamel insulations for magnet wires are being developed in order to withstand better stress amplification. The electrical characterization of these insulating materials is often carried out through aging tests which may provide estimation of life under different stress levels and conditions. However, deeper investigation of aging phenomena due to supply voltage waveforms is needed, especially regarding the relation between aging factors and stress conditions. This paper deals with this topic, showing experimental evidences of relation between partial discharge quantities (e.g., inception voltage, repetition frequency, amplitude) and electrical properties, associated with charge accumulation, which can be directly evaluated through space charge measurements. Characterization of insulating materials and comparison of materials candidate for application in power electronic waveform environment can be carried out resorting to the methodology proposed here. This approach can provide, therefore, a useful feedback to wire manufacturers regarding, e.g., the choice of additive nature and enamel components for magnet wires in power-electronic controlled motors.     

Dynamic behavior of interconnected channels in water-treedpolyethylene subjected to high voltage

While increasing a 500 Hz voltage from 300 V to 2000 V, the charging and ac loss currents of a water-treed XLPE are observed to increase more with voltage than those of a non-degraded XLPE. The fundamental and the third harmonic of the ac loss current have been numerically analyzed based on a model of water-treed insulation. In the model, a nonlinear voltage dependence of the conductance of a water-filled channel region is assumed. Magnitudes and phase angles of the fundamental and 3rd harmonic components of the ac loss current calculated from the model are compared with the observed ones. It was found that the length of water-filled channel region of a water tree grows with increasing voltage, and that this length approaches that of the water-treed region estimated by the pulsed electroacoustic (PEA) method