Zhongliu Zhou

Dramatically enhanced electrical breakdown strength in cellulose nanopaper

Electrical breakdown behaviors of nanopaper prepared from nanofibrillated cellulose (NFC) were investigated. Compared to conventional insulating paper made from micro softwood fibers, nanopaper has a dramatically enhanced breakdown strength. Breakdown field of nanopaper is 67.7 kV/mm, whereas that of conventional paper is only 20 kV/mm. Air voids in the surface of conventional paper are observed by scanning electron microscope (SEM). Further analyses using mercury intrusion show that pore diameter of conventional paper is around 1.7 μm, while that of nanopaper is below 3 nm. Specific pore size of nanopaper is determined to be approximately 2.8 nm by the gas adsorption technique. In addition, theoretical breakdown strengths of nanopaper and conventional paper are also calculated to evaluate the effect of pore size. It turns out that theoretical values agree well with experimental data, indicating that the improved strength in nanopaper is mainly attributed to the decreased pore size. Due to its outstanding...

Partial discharge characteristics of uniform gap in oil-impregnated paper insulation under switching impulse voltage

In comparison with the AC and DC working voltages, the effect of switching impulse voltage on insulation is very different because of its fast-rising rate and large amplitude. This paper mainly focuses on the characteristics of PD in oil-impregnated paper insulation under switching impulse voltage. First, a robust experimental platform was set up including the impulse power source, test samples with uniform gas and oil gap, and the signal acquisition and processing system. PD characteristics in the air and oil gap are analyzed and compared involving the time-domain waveforms, the variation tendency of the average amplitude and numbers of PD pulses, the occurrence time and the instantaneous value of impulse corresponding to the first PD. Based on the dielectric barrier discharge (filament) theory, a parallel microcircuit model was established to explain the similarities and dissimilarities, and to evaluate the gap discharge intensity. The occurrence and development of PD under switching impulse voltage of typical oil-impregnated paper defect were studied systematically with simulation, experiments, and modeling.

Modeling of the effective permittivity of insulating presspaper

Effective permittivity model of insulating presspaper is built on the basis of the microstructure of the material. Due to the essentially layered structure in z-direction of presspaper, air voids inside the mixture can be treated as right prismatic inclusions. Analytical formula for the prediction of the effective permittivity of insulating presspaper is derived. Interestingly, the derived formula equals to the mixing equation applied for dielectrics in series. Numerical simulation was used to validate the analytical results by considering the air voids as cubical inclusions. Results show a good agreement between the analytically and numerically calculated effective permittivity values. Furthermore, dielectric permittivity results of commercial kraft paper and laboratory-made presspaper at 50 Hz were measured and compared with modeled data. It turns out that the deduced results give a good accuracy for the effective permittivity determination.

Predicting the Dielectric Properties of Nanocellulose-Modified Presspaper Based on the Multivariate Analysis Method

Nanocellulose-modified presspaper is a promising solution to achieve cellulose insulation with better performance, reducing the risk of electrical insulation failures of a converter transformer. Predicting the dielectric properties will help to further design and improvement of presspaper. In this paper, a multivariable method was adopted to determine the effect of softwood fiber on the macroscopic performance of presspaper. Based on the parameters selected using the optimum subset method, a multiple linear regression was built to model the relationship between the fiber properties and insulating performance of presspaper. The results show that the fiber width and crystallinity had an obvious influence on the mechanical properties of presspaper, and fiber length, fines, lignin, and nanocellulose had a significant impact on the breakdown properties. The proposed models exhibit a prediction accuracy of higher than 90% when verified with the experimental results. Finally, the effect of nanocellulose on the breakdown strength of presspaper was taken into account and new models were derived.

Enhancing mechanical strength and breakdown behavior of insulating presspaper by introduction of nanofibrillated cellulose

This study explores the possibility of enhancing both mechanical and breakdown properties of insulating presspaper by introduction of an organic nano additive. Two different concentrations of nanofibrillated cellulose (NFC) were taken into account, namely 0.5 wt. % and 10 wt. %. Presspaper containing no NFC was also prepared as a reference. Obtained samples were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). Mechanical properties and breakdown behaviors were also measured. Results show that the addition of 10 wt. % NFC to softwood fibers can achieve better performance. Tensile strength of reference presspaper is 109 MPa, whereas that of presspaper modified by 10 wt. % NFC is 136 MPa, resulting in a 25% increase. The improved tensile strength can be attributed to the increased density and inter fiber bond strength. More importantly, presspaper reinforced by 10 wt. % NFC can also achieve increased AC and DC breakdown strengths, which are 19% and 21% higher than those of the reference presspaper. It is concluded that NFC is probably a promising nano additive for cellulose insulation.

Effect of pulp refining on mechanical and electrical properties of insulating presspaper

In this study, softwood pulps with Schopper Riegler (SR) freeness of 10 °SR, 17 °SR, 31 °SR, 40 °SR, 48 °SR were used to prepare presspaper samples. Morphological properties of fibers and presspaper were characterized by optical microscope (OM) and scanning electron microscope (SEM). Mechanical and electrical performances of obtained samples were evaluated. Results show that cellulose fibers become flat and flexible after refining. Presspaper made from refined pulp has fewer voids between fibers. With the increasing of refining degree, relative permittivity of presspaper first increases, and then keeps almost constant. Presspaper made from pulp with 17 °SR has a lower dielectric loss at frequency below 100 Hz. Both mechanical strength and electrical breakdown field can be reinforced by pulp refining. Tensile strength and AC breakdown field of presspaper made from pulp without refining are 51 MPa and 4 kV/mm, respectively. Whereas those of presspaper prepared from pulp with 31 °SR are 118 MPa and 11.1 kV/mm. When refining degree is higher than 31 °SR, mechanical and electrical strengths, however, are not significantly improved with further refining.