Roland Eriksson

Dielectric spectroscopy for diagnosis of water tree deterioration in XLPE cables

An HV dielectric spectroscopy system has been developed for diagnostics of water tree deteriorated extruded medium voltage cables. The technique is based on the measurement of nonlinear dielectric response in the frequency domain. Todays commercially available systems are capable of resolving low loss and small variations of permittivity as a function of frequency and voltage. Experience from more than 200 field measurements was combined with laboratory investigations. Small samples were used in an accelerated aging test to elucidate the correlation between water tree growth and dielectric response. Furthermore, field aged cables were investigated in the laboratory. It has been shown that the dielectric response of water tree deteriorated crosslinked polyethylene (XLPE) cables can be recognized and classified into different types of responses related to the aging status and breakdown strength. The influence of termination and artifacts such as surface currents was investigated. The measurement method enables us to separate the response of the cable from the influence of accessories. Finally, two different field studies of the implementation of the diagnostic method are presented. The field studies show that the fault rate decreased significantly when replacement strategy was based on the diagnostic criteria formulated.

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.

Wide-Area Measurement System-Based Subspace Identification for Obtaining Linear Models to Centrally Coordinate Controllable Devices

The contribution of this paper is the application of subspace system identification techniques, to derive a low-order black-box state-space model of a power system with many controllable devices using global signals. This model is a multiinput, multioutput open system model describing the power oscillatory behavior of the power system. The input signals are the controllable setpoints of the controllable devices, the output signals are the speed of selected generators measured by a wide-area measurement system. This paper describes how to achieve and preprocess the data to use subspace techniques to estimate and validate to finally assign an accurate model. This new approach can be used directly to design a central coordinating controller for all of the relevant controllable devices, with the aim to increase the damping of the modes in the system. Previously presented methods use local measurements or output signals dependent on the actual operational point. The benefit of the presented method is that the used output signals are independent of the system state. This makes it possible to use state-feedback control to combine the controllable devices to coordinately damp the modes. The presented method is applied in the CIGRÉ Nordic 32-bus system including two HVDC links. The case study demonstrates that accurate low-order state-space models can be estimated and validated by using the described method to accurately model the systems power oscillatory behavior.

Calibration for time domain propagation constant measurements on power cables

A method has been developed for characterising the wave propagation properties of medium voltage (MV) power cables. The technique is based on time and frequency domain analysis of short pulse propagation. Particular attention is paid to the measurement calibration, as samples with arbitrary characteristic impedances cause significant multiple reflections in the test set-up.

Determination of moisture content in mass impregnated cable insulation using low frequency dielectric spectroscopy

The variations in the low frequency dielectric spectra of mass impregnated insulation depending on its moisture content have been investigated. For the calibration purposes a number of the impregnated paper samples was kept in the desiccators where different relative humidities were maintained. Thus obtained moisture content in the samples ranged from 1% up to 6%. The variations of the complex capacitance in a frequency range of 0.001-1000 Hz were measured on these samples as well as on paper previously dried at elevated temperatures under vacuum. The moisture content of the dried samples determined was in a range 0.2-0.5%. Moreover the study of water penetration in a 12 kV mass impregnated cable sample was carried out. Continuous measurements of the dielectric spectra revealed the dynamics of water ingress in the cable insulation.

Measurement technique for high frequency characterization of semiconducting materials in extruded cables

Knowledge on the dependence of wave propagation characteristics on material properties and cable design is important in establishing diagnostic methods for cable insulation. In this study, a high frequency measurement technique to characterize the semi-conducting screens in medium voltage cross-linked polyethylene (XLPE) cables has been developed. The frequency ranges from 30 kHz to 500 MHz. The influence of the experimental set-up, sample preparation methods, pressure and temperature are investigated. A dielectric function is developed for the semiconducting screens and this is incorporated into a high frequency model for the cable. The propagation characteristics obtained from the high frequency cable model are compared with those obtained from measurements made on the same cables.

Diagnosis of moisture in oil/paper distribution cables - Part I: Estimation of moisture content using frequency-domain spectroscopy

Dielectric spectroscopy (dielectric response measurements) has been applied for nondestructive estimation of humidity in oil-paper cable insulation. The experiments have been based upon two field-aged cables, 20 and 50 years old. Paper samples from these cables have been characterized and subjected to environments with different relative humidity. Dielectric loss and capacitance have been measured in a frequency range 1 MHz to 1 kHz and related to the moisture content determined by Karl Fisher titration. A method has been verified where the moisture content is correlated to the minimum value of loss tangent (tan/spl delta/). A number of field measurements have been performed where the method has been applied to estimate the moisture content in the distribution cables.

Estimation of disconnector contact condition for modelling the effect of maintenance and ageing

In order to optimize the maintenance of electric power equipment, models of equipment reliability as a function of age and maintenance are necessary. As a first step towards the development of these kinds of models, this paper proposes a quantitative method for establishing the condition of disconnector contacts by the use of thermography, which is a well- established diagnostic practice used in preventive maintenance programmes. Results from two sets of thermography measurements carried out in the Swedish transmission grid are presented. Statistical distributions for the uncertainties when using the method are estimated from the measurements. A method for converting temperature rises measured at non steady state loading conditions to a corresponding steady state temperature rise have been devised using a simple thermal model. The probability of sending an unaged contact to maintenance is estimated using Monte Carlo simulations based on the statistical distributions of the uncertainties. Finally, it is shown that by using the method proposed in this paper it is possible to estimate the contact condition of disconnectors, including Monte Carlo simulated confidence intervals.

Comparing two measurement techniques for high frequency characterization of power cable semiconducting and insulating materials

Understanding the high frequency characteristics of the materials that make up medium voltage extruded cables is important in establishing diagnostics schemes based on electromagnetic pulse propagation methods. Two measurement techniques have been developed and used to characterize the high frequency material properties of semi-conducting screens and cross linked polyethylene (XLPE) insulation up to 100 MHz. The experimental details, parameter extraction and limitations of the two measurement techniques are presented

Relation between preventive maintenance and reliability for a cost-effective distribution system

This paper describes a procedure for obtaining the relationship between preventive maintenance and the reliability of distribution systems. This procedure centers on deducing the components which have a critical impact on system reliability and identifying causes by which the components fail. This is followed by identifying which of these causes can be reduced or eliminated by preventive maintenance. These procedures are at the heart of reliability centred maintenance (RCM). The approach is illustrated by application to an urban distribution system in the Stockholm city area. The results clearly demonstrate the need to identify the critical components, to understand comprehensively the causes of failures, and not to create preventive maintenance programmes by considering components in isolation but always as part of the system in which it is being used.