Björn Sonerud

Dielectric properties and partial discharge endurance of polypropylene-silica nanocomposite

This paper presents the results of the dielectric properties and partial discharge endurance measurements conducted on polypropylene (PP)-silica nanocomposite. The material compounds were analyzed with micro-Raman spectroscopy, X-ray tomography and transmission electron microscopy (TEM). ac and dc breakdown strength of the materials was measured. Dielectric response, capacitance and loss factor of the film samples were measured as a function of temperature and frequency. Partial discharge (PD) endurance of the reference PP and PP Silica nanocomposite was studied as a function of ac voltage. Material surfaces were analyzed after PD stress with optical microscopy. All dielectric measurements were done for oriented thin films with a thickness of 11-23 μm. The results were analyzed statistically to determine the effects of the additive on the properties of PP. The paper discusses the potential of PP Silica nanocomposite with a view to high voltage applications, especially power capacitors.

Dielectric response measurements utilizing semi-square voltage waveforms

Dielectric response measurements belong to the group of diagnostic tools used for quality evaluation of high voltage insulation systems. A measurement system called arbitrary waveform impedance spectroscopy (AWIS) is presented here which is capable of utilizing the harmonic content of repetitive voltages with a semi-square waveform and the corresponding current response to perform dielectric response measurements. Extensive circuit modeling and calibrations are required in order to perform accurate measurements. AWIS could offer new possibilities, especially for continuous monitoring of the dielectric properties of high voltage components and systems. The accuracy of the technique in both low and high voltage applications is verified by comparison with results obtained by means of the frequency domain spectroscopy (FDS) technique.

Dielectric heating in insulating materials subjected to voltage waveforms with high harmonic content

Dielectric heating is one potential aging mechanism active below partial discharge inception voltage in materials used as high voltage insulation. When exposed to voltage waveforms containing high amount of harmonics, the heat generation will be larger due to increased power losses as compared with power frequency excitation. This may result in a decreased life or even failure of insulation due to the increased operating temperature or to thermal runaway. An analysis of the power developed due to dielectric heating in two different materials subjected to voltage waveforms with high harmonic content is presented in this paper. By expressing the non-sinusoidal loss as an enhancement factor to the sinusoidal one, a geometry-independent formalism is derived. From dielectric response measurements at low voltage and at several temperatures the dielectric power loss in the material can be calculated for different voltage levels and waveforms. Two important material parameters can be extracted from the calculated dielectric power loss: (i) non-sinusoidal loss compared with sinusoidal loss with the same fundamental frequency (pfact) and (ii) change of loss with changing temperature (dpfact/dT). These two parameters could potentially be used to indicate the suitability of materials for use in applications where voltage waveforms contain high harmonic content.

Nonlinear electrical properties and mechanical strength of EPDM with polyaniline and carbon black filler

The influence of filler loading of polyaniline (PANI) and carbon black (CB) on the nonlinear electrical properties as well as the mechanical properties (tensile strength and Youngs modulus) of ethylene propylene diene monomer (EPDM) was investigated. It has been found that low field conductivity was most sensitive to filler addition and in particular carbon black. Threshold field for nonlinearity onset was also influenced but the relationship between parameter values and filler content was less clear. For the nonlinearity coefficient, tensile strength and Youngs modulus there was no observable relation to filler content. The study performed here show that the polyaniline/EPDM is a composite with good possibilities for tailoring the low field conductive properties whereas the mechanical strength seems unaffected by the variation of filler content.

Noise and Aliasing Aspects in a Multiharmonic-Dielectric-Response-Measurement System

Dielectric-response measurements are commonly performed with frequency-domain spectroscopy, polarization/depolarization-current measurements, or return-voltage measurements. These techniques operate in a frequency or time domain, and all have high requirements on the voltage source in order to acquire accurate results. This limits dielectric-response measurements to offline applications. A new technique, which is called arbitrary-waveform-impedance spectroscopy, has been developed, which makes use of the harmonics of any voltage waveform to perform dielectric-response measurements. The technique provides possibilities for online measurements facilitating the monitoring of materials and components in high-voltage applications. Here, the different aspects of the measurement system are presented, including circuit modeling, normalization, and discussions on aliasing and noise; all of them are necessary to control in order to perform accurate measurements.

Effect of low amount of nanosilica on dielectric properties of polypropylene

This paper presents the results of the dielectric properties measurements conducted on Silica-Polypropylene (PP) nanocomposites. According to prior investigations by the authors silica nanoparticles have improved the performance of the dielectric material considering capacitor applications. Especially breakdown strengths with ac and dc voltages and resistance against surface degradation have increased. The relative permittivity and dielectric losses have also been comparable to reference PP. In this paper the results of the dielectric measurements conducted on composites with 1–2 wt-% silica are compared with reference PP. Silica dispersion in PP was confirmed with transmission electron microscopy (TEM). Weibull analysis was applied to the breakdown strength measurement results with dc voltage. Capacitance and loss factor measurements were conducted as a function of temperature to study thermal behavior. The measurements were conducted at the high voltage laboratories of Tampere University of Technology and Chalmers University of Technology. Statistical analysis was applied to the results to determine the significance of the differences between the materials.

Capacitance measurements and tree length estimation during electrical treeing in sub-picofarad samples

A method for measuring capacitance below 1 pF is presented, which is used to measure dielectric properties during electrical treeing in XLPE samples. The measured capacitance is compared with finite element computations and from these simulations a simple relationship between the capacitance increase and the tree length can be established which corresponds well to the tree length observed by optical microscopy.

Dielectric response measurements utilizing non-sinusoidal waveforms

In order to assess the effect non-sinusoidal high voltages impose on electrical insulation systems, new methods for material characterization and diagnostics are needed. The method developed in the work presented here utilizes arbitrary non-sinusoidal voltage waveforms. It thereafter makes use of the fast Fourier transform (FFT) analyses for extracting the harmonic contents of the applied voltages and the resulting currents. In this way the response spectrum for the harmonic frequencies is obtained. The method puts high demands on the measurement instrumentation as well as on the calibration and correction procedures for obtaining reliable results. One of the main advantages with the measurement system developed is that it does not require a precise and well controlled voltage source. This provides new possibilities for on-line monitoring and diagnostics, when any voltage appearing in the system can be utilized. It is shown that measurement results obtained by the new method are in good agreement with results obtained by means of the conventional frequency response techniques.

Continuous monitoring of dielectric properties of LDPE samples during electrical treeing

A technique capable of utilizing voltage waveforms containing harmonics for measurements of dielectric response over a range of frequencies is presented. It has proved useful in several applications and offers high accuracy and flexibility. This paper presents measurement of the changes imposed by electrical treeing in the dielectric properties of LDPE samples and shows that the capacitance increase monitored during the tree growth corresponds well with the optically measured tree length. There is also a change of dielectric loss associated with partial discharge activity during the tree growth, which could be correlated to different treeing modes. The method provides an attractive alternative to the traditional measurements of tree growth by means of optical observations, as it allows for investigations of nontransparent materials.

Measurement and analysis of partial discharge current during square voltage waveforms

Arbitrary Waveform Impedance Spectroscopy (AWIS) is used to measure the current generated by partial discharges in a twisted pair sample close to inception voltage. By subtracting the dielectric current resulting from the inherent complex capacitance of the insulating system from the total measured current, the additional current contribution from partial discharges can be obtained. The use of the AWIS system for partial discharge measurements provides integrated information of magnitude, phase position and changes of these properties during time. In addition a conductive current, believed to be related to transport of charge deposited by discharges, can be observed. For the twisted pair sample studied here, the frequency of discharge bursts during the intermittent stage seems to be related to the rate of conductive current decay. This indicates that the conductive current might be an important quantity of insulating systems operating close to partial discharge inception voltage.