Hans Edin

Extraction of high frequency power cable characteristics from S-parameter measurements

A technique is developed for extraction of the wave propagation properties of power cables from S-parameter measurements. The method extracts the complex propagation constant and the characteristic impedance, as well as the LCRG telegraphers equation parameters. The extraction process is developed after clarifying the effect of the connection between the measurement port and the power cable. It is concluded that treating the connection solely as a characteristic impedance change could lead to considerable errors in the parameter extraction. Furthermore, the method corrects for electrical lengths, which are not accounted for by the standard network analyzer calibration. The extraction is demonstrated for a medium voltage cross linked polyethylene (XLPE) cable over the frequency range 300 kHz to 300 MHz. The results are compared to a time domain short pulse propagation method for cable characterization. Both measurement methods are evaluated against a cable model.

Partial discharges in a cavity at variable applied frequency part 2: measurements and modeling

In this paper partial discharges (PD) in a disc-shaped cavity are measured at variable frequency (0.01 - 100 Hz) of the applied voltage. The measured PD phase and magnitude distributions, as well as the number of PDs per voltage cycle, changed with the varying frequency. A charge consistent model is presented and used to dynamically simulate the sequence of PDs in the cavity. The simulation results show that the properties of the cavity surface, mainly the surface conductivity and the surface emission of electrons, change with the varying applied frequency. This is interpreted as an effect of the difference in time between consecutive PDs at different applied frequencies. This is the second of two papers addressing the frequency dependence of PD in a cavity. The first paper described how the PD frequency dependence changes with the applied voltage amplitude, the cavity size and the cavity location.

High-Frequency Model of the Rogowski Coil With a Small Number of Turns

In this paper, the Rogowski coil is modeled as a distributed-element transmission line. The elements in the distributed transmission line are modeled using physically based equations. It allows self and transfer impedances of the Rogowski coil to be modeled using the physical dimensions and the material properties of the coil. The models of the impedances were verified by measurements in the frequency domain on three Rogowski coils. The coils mounted on the power cable were modeled and simulated in the time domain and afterward compared with the measurements.

Partial discharges in a cavity at variable applied frequency part 1: measurements

In this paper partial discharges (PD) in disc-shaped cavities in polycarbonate are measured at variable frequency (0.01-100 Hz) of the applied voltage. The advantage of PD measurements at variable frequency is that more information about the insulation system may be extracted than from traditional PD measurements at a single frequency (usually 50/60 Hz). The PD activity in the cavity is seen to depend on the applied frequency. Moreover, the PD frequency dependence changes with the applied voltage amplitude, the cavity diameter, and the cavity location (insulated or electrode bounded). It is suggested that the PD frequency dependence is governed by the statistical time lag of PD and the surface charge decay in the cavity. This is the first of two papers addressing the frequency dependence of PD in a cavity. In the second paper a physical model of PD in a cavity at variable applied frequency is presented.

Change in partial discharge activity as related to degradation level in oil-impregnated paper insulation: effect of high voltage impulses

High voltage (HV) transients in electrical power systems are mainly caused by lightning strikes and switching operations. The dielectric properties of a particular electrical insulation at the instant of interaction with a high voltage transient would determine a level of the degradation of an electrical insulation. This work investigates how high voltage impulses would change Phase Resolved Partial Discharge (PRPD) patterns in test samples consisting of a cavity deliberately introduced between the sheets of oil-impregnated paper used in power transformer bushings. It also investigates how the change in PRPD patterns is related to the degradation level of oil-impregnated paper. In order to accomplish these objectives, the experiments were set in such a way that the effect of HV impulses and an early stage PD activity at an elevated AC stress, HV impulses in combination with a prolonged PD activity at an elevated AC stress, and a prolonged PD activity alone at an elevated AC stress could be investigated separately. The experimental results presented in this paper indicate that HV impulses below the impulse breakdown stress following an early stage AC PD activity would not cause a significant change in PRPD patterns, and would not damage oil-impregnated paper to a level which can be noticed by a visual observation. On the other hand, a prolonged AC PD activity alone can cause the change in PRPD patterns, but cannot quickly damage the oil-impregnated paper as it would do when it is combined with HV impulses. However, the combination of both, HV impulses and a prolonged AC PD activity caused a high drop in the PD parameters (total PD charge and a repetition rate). The study found that the decrease of the PD parameters can be considered as a sign of severe degradation of oil-impregnated paper.

Dielectric properties of alumina-filled poly (ethylene-co-butyl acrylate) nanocomposites Part II- wet studies

The influence of moisture on the dielectric properties of different types of poly (ethyleneco- butyl acrylate) filled with alumina nanoparticles was systematically investigated by varying the type of aluminum oxide, particle surface treatment and filler content (2, 6 and 12 wt%). The nanoparticles were either unmodified or surface-treated with either aminopropyl triethoxysilane or octyltriethoxy silane. The complex permittivity was measured with an IDA200 dielectric spectroscopy analyzer at applied voltage of 200 Vpeak with frequencies varying between 1 mHz and 1 kHz. The measurements were performed at 25°C and relative air humidities of 24, 54 and 86%. In order to expand the frequency range the HP 4284A precision LCR meter was used for measuring sample impedance at the frequency range of 100 Hz to 1 MHz. From the frequency dependence of dielectric losses it can be seen that absorbed water plays a significant role in determining the dielectric properties of the nanocomposites. The magnitude of tan δ seems to be determined primarily by the particle size and filler content, while the peak frequency is mostly influenced by the amount of absorbed water and the type of particle coating.

Modeling partial discharges in a cavity at different applied frequencies

A model of partial discharges (PD) in an insulated disc-shaped cavity is presented. The flat cavity walls are covered with copper foil and each PD is assumed to affect the whole cavity. The discharge process in the cavity is simulated dynamically and the model is charge consistent. The model is used to simulate the sequence of PDs in the cavity at two different applied frequencies: 0.02 Hz and 100 Hz. The simulation results are compared with measurements and good agreement is found. As the applied frequency is increased from 0.02 Hz to 100 Hz, the mean apparent charge increases and the number of PDs per cycle of the applied voltage reduces. This is interpreted as an effect of the statistical time lag of PD.

Modelling of partial discharge spectra measured with variable applied frequency

Variable Frequency Phase Resolved Partial Discharge Analysis (VF-PRPDA) is a newly developed method for partial discharge measurements. In this method the frequency of the applied voltage is variable and not fixed at the power frequency as in the standard PRPDA implementation. In this paper, simulations illuminate the frequency dependence of PD of a discharging cavity embedded in an otherwise homogeneous insulation. It is shown how the frequency dependence is influenced by the conductivity of the bulk insulation, the surface conductivity of the cavity walls and the statistical time lag. The results of simulations are compared with result of measurements on an asphalt-mica insulated machine stator bar.

On-line time domain reflectometry measurements of temperature variations of an XLPE power cable

In this paper an on-line time domain reflectometry system and a four-days measurements sequence are presented. Detected pulse velocity variations are attributed to temperature variations of the cable caused by a load cycling. The changes in the pulse velocity are verified by propagation constant measurements of a power cable at different temperatures in 300 kHz-500 MHz frequency range. Also an influence of temperature dependent semiconducting screens and XLPE insulation to the changes in the cables capacitance is investigated.

Influence of cavity size and cavity location on partial discharge frequency dependence [polymer insulation]

Variable frequency phase resolved partial discharge analysis (VF-PRPDA) is a partial discharge measurement technique where the frequency of the applied voltage is variable. In this paper, the VF-PRPDA technique is applied to a test object containing a cylindrical cavity in an otherwise homogeneous insulation of polycarbonate. The frequency is varied in the range 10 mHz-100 Hz. It is shown how the frequency dependence of PD is influenced by the cavity diameter and the cavity location in relation to the electrodes. The PD frequency dependence is also numerically modeled for an isolated cylindrical cavity with diameter 1.5 mm using a field model.