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| 4. |
- Johansson, Anette, 1977, et al.
(författare)
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A Versatile System for Electrical Treeing Tests under AC and DC Stress Using Wire Electrodes
- 2011
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Ingår i: 8th International Conference on Insulated Power Cables, Jicable’11, 19 – 23 June 2011, Versailles - France.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- An alternative method to evaluate electrical treeing in solid dielectric materials is presented. Here the test sample utilises a wire electrode for creating the necessary high divergent electric field. This introduces a different preparation method and several of the disadvantages with the traditional needle sample can be avoided. Together with the test object a versatile test setup for both AC and DC measurements is illustrated. This alternative method has proven successful and an evaluation thereof as well as a comparative study between the two electrode systems is included.
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| 5. |
- Johansson, Anette, 1977, et al.
(författare)
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Wire-Plane Electrode System for Electrical Tree Initiation Exposed to AC and DC Voltage Stress
- 2011
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Ingår i: The Proceedings of 17th International Symposium on High Voltage Engineering, s. C-025. - 9783800733644
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Electrical treeing in XLPE exposed to DC (of both polarities) and 50 Hz AC voltages has been analysed in a wire-plane electrode geometry using real-time optical detection and microscopic observations. It has been found that treeing mechanisms differ between AC and DC stresses. For AC voltage, the tree initiation and growth is easily observable in real time during the testing. In the case of DC stress a complete breakdown of the XLPE occurs before electrical trees can be discerned with the use of CCD camera. However, in the microscopic observations of the specimens, very fine tree structures can be distinguished. The effect of voltage ramping rate was also analysed. For AC stress, the faster ramping results in a higher tree inception voltage. The opposite trend is true for DC stress, where the faster ramp results in a lower breakdown voltage level. When comparing the effect on polarity of the DC breakdown voltage, negative polarity results in higher voltage levels than positive polarity.
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- Nilsson, Susanne, 1981, et al.
(författare)
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Influence of Morphology Effects on Electrical Properties in XLPE
- 2011
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Ingår i: Journal of Applied Polymer Science. - : Wiley. - 1097-4628 .- 0021-8995. ; 121:6, s. 3483-3494
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Tidskriftsartikel (refereegranskat)abstract
- This study elucidates the influence of differences in crosslink structure and crosslink density on water tree growth, electrical tree growth, and electrical breakdown strength. One reference LDPE was compared with a LDPE containing a higher number of vinyl groups, introduced via the copolymerisation with a diene. Because of the chemical difference, the initially formed crosslink structure will be different for the materials. A range of various crosslink densities was considered for the two materials. It was found that along with a noticeable change in morphology after crosslinking, significant changes in particularly water tree growth, electrical tree growth, and electrical breakdown strength could be observed.
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- Sonerud, Björn, 1979
(författare)
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Arbitrary Waveform Impedance Spectroscopy for Characterization and Diagnostics of High Voltage Insulation
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In high voltage engineering the dielectric response technique is a widely used tool both for characterization of insulation materials and for diagnostics of components such as transformers and cables. Currently available techniques are not capable of measuring on-line which would be advantageous; when characterizing a material and evaluating the effect of different voltage magnitudes, waveforms and temperatures it is desirable to be able to observe the continuous development of the dielectric properties. Present diagnostic techniques based on dielectric response need to disconnect the high voltage component from the grid in order for measurements to be performed, which is a major drawback. Apart from the inconvenience of removing a part of the power system from operation, measurements are performed with voltages and for temperatures unlike those experienced in operation which can provide misleading results. In addition, the possibility of continuous online monitoring would also provide increased opportunities for condition based maintenance (CBM). In this thesis an approach towards dielectric response measurements utilizing the harmonic content of any applied repetitive voltage is introduced. This measurement technique can utilize voltage waveforms already present in the system which makes external test voltage sources superfluous. By measuring the applied voltage and the corresponding current response, the voltage and current spectra can be obtained using the fast Fourier transform (FFT). With the use of calibrations and a detailed circuit model, the dielectric response for many harmonics can be obtained simultaneously. This approach towards dielectric response measurements has been evaluated and compared with commercial instruments operating in the frequency domain. The agreement is good and shows that the technique as implemented here has the same accuracy level as conventional techniques. In addition, some examples of applications are shown which illustrate possible studies that previous techniques were unable to perform. The technique can be used for low voltage, high voltage and online measurements, each with its own limitations and specific set of requirements necessary for obtaining accurate results.
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- Sonerud, Björn, 1979
(författare)
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Characterization of Electrical Insulation Exposed to Arbitrary Voltage Waveforms
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The extensive use of power electronics in power systems today gives rise to other voltage waveforms than the conventional 50/60 Hz sinusoidal and consequently other types of electric stress act on high voltage insulating materials. Previous studies has shown that Pulse Width Modulated (PWM) voltages and similar waveforms increase stress in Adjustable Speed Drives (ASD) as compared with sinusoidal waveforms and it has also been shown that partial discharge (PD) activity is intensified during PWM voltages and that rise time of square like pulses affect PD inception voltage. In the light of these findings a need for new characterization and diagnostic methods which can be used to study the influence of different types of voltage waveforms on insulating materials and systems has arisen. In this project a method called Arbitrary Waveform Impedance Spectroscopy (AWIS) has been developed which is based on conventional dielectric response measurement techniques but capable of using any voltage waveform to perform measurements. By utilizing the harmonic content of the waveform the dielectric properties of an insulating system can be determined at several frequencies simultaneously. This provides an advantage compared to other dielectric response techniques where the applied voltage needs to be carefully controlled. With AWIS a test object can be stressed with any waveform and the influence on the dielectric properties studied. The accuracy level has been found to be in the same range as for commercially available instruments, but additional complications related to aliasing and spurious/harmonic distortion have to be taken into account. AWIS has been used to study PD behaviour in systems where different analysis methods have to be applied depending on the relationship between the magnitude of the discharge and dielectric current. A small discharge current compared to the dielectric current requires an approach which relies on the increase of noise levels during PD activity and the amount of information possible to extract is limited. For larger discharge currents PD behaviour over time can be monitored both in frequency and time domain and in certain objects it is also possible to detect a conductive current most likely related to charge relaxation. The measurement system has also been used to monitor the behaviour of dielectric properties during electrical tree growth in low density polyethylene samples having capacitance less than one picofarad. The tree length was found to be correlated with capacitance increase which can be modeled and used in order to estimate tree growth rate. This possibility appears especially useful if the samples are opaque as conventional CCD cameras used for such measurements are no longer applicable.In addition to the development of the measurement system and the application on PD and electrical treeing measurements, the influence of the voltage waveform on dielectric heating was studied. A method for estimating the increased amount of power generated in different insulating systems due to the harmonic content of the voltage waveform was developed and evaluated. AWIS was used to measure the dielectric properties of different insulating systems and based on these two different dielectric heating parameters was calculated using the proposed method.
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| 10. |
- Sonerud, Björn, 1979, et al.
(författare)
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Continuous monitoring of dielectric properties of LDPE samples during electrical treeing
- 2010
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Ingår i: 2010 IEEE International Conference on Solid Dielectrics, 4th - 9th July 2010, Potsdam,Germany. - 9781424479429 ; , s. 683-686
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Konferensbidrag (refereegranskat)abstract
- 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. ©2010 IEEE.
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| 12. |
- Sonerud, Björn, 1979, et al.
(författare)
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Dielectric response measurements during electrical treeing in sub-picofarad samples
- 2011
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Ingår i: Polymer Testing. - : Elsevier BV. - 0142-9418. ; 30:1, s. 43-49
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Tidskriftsartikel (refereegranskat)abstract
- A technique called Arbitrary Waveform Impedance Spectroscopy (AWIS) developed for measuring dielectric properties in insulating materials has been used to study capacitance and dielectric loss changes during electrical tree growth in LDPE samples. The sample capacitance is small, in the range of 0.1–0.15 pF, placing high demands on the measurement system. Simulations in FEM-software show that the measured capacitance is in agreement with simulated values and can be used to estimate tree growth rate. It also seems to be possible to differentiate between different regions of tree growth by analyzing changes in capacitance and loss of the studied samples.
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| 19. |
- Sonerud, Björn, 1979, et al.
(författare)
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Noise and Aliasing Aspects in a Multiharmonic-Dielectric-Response-Measurement System
- 2011
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Ingår i: IEEE Transactions on Instrumentation and Measurement. - 1557-9662 .- 0018-9456. ; 60:13, s. 3875-3882
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Konferensbidrag (refereegranskat)abstract
- 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. © 2011 IEEE.
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| 22. |
- Takala, M., et al.
(författare)
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Dielectric Properties and Partial Discharge Endurance of Polypropylene- Silica Nanocomposite
- 2010
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Ingår i: IEEE Transactions on Dielectrics and Electrical Insulation. - 1558-4135 .- 1070-9878. ; 17:4, s. 1259-1267
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Tidskriftsartikel (refereegranskat)abstract
- 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 capac itors. © 2010 IEEE.
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| 23. |
- Takala, M., et al.
(författare)
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Effect of low amount of nanosilica on dielectric properties of polypropylene
- 2010
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Ingår i: Proceedings of the 2010 IEEE International Conference on Solid Dielectrics, ICSD 2010. - 9781424479443
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- 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. ©2010 IEEE.
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