Statistical approach to identify the discharge source in MV cables and accessories

Partial discharge (PD) analysis is a reliable tool to assess the integrity of electrical insulation. Representation and interpretation of the data, obtained from e.g. online PD monitoring, are key issues to reveal the discharge source, i.e. defect type, as well as the physical phenomena behind the occurrence. Analyses of various PD patterns such as discharge height distribution presented in this work provide useful statistical parameters to identify the discharge source. Research shows that the 2-parameter Weibull distribution is a reliable model to quantify the characteristics of the patterns of the defect. The model fits well to the charge-height distribution. In addition, trends in the discharge density pattern that occur over long times, can be used as complementary information to discover the discharge nature. It alerts for a possible failure and therefore assists in taking corrective measures to prevent failure. This paper presents the application of such statistical modeling to the area of on-line power cable diagnostics. Data obtained from laboratory experiments as well as field data have been studied

Technical developments and practical experience in large scale introduction of on-line PD diagnosis

On-line Partial Discharge (PD) detection and location systems for medium-voltage cables are at present being introduced in Dutch utilities and worldwide. The technical challenges now move from the development of the diagnostic technique itself to efficient implementation on alarge scale. In this paper we discuss several implementation related challenges and will propose adequate solutions. These challenges include robust algorithms to determine time of arrivals of distorted PD waveforms, signal propagation along cable types and configurations as three-core and cross bonded cables, and effect of ring main units or substations on signal propagation. Algorithms based on signal energy and on phase angle in frequency domain are preferred above e.g. threshold detection to determine PD arrival times. By introducing effective dielectric properties, cable parameters for accurate fault location as characteristic impedance and propagation velocity can be estimated also if data on semiconducting layers are unavailable. Models are proposed for cross-bonded connections and for three-core cables with common earth screen. A pulse injection circuit, already included in the PD equipment for time synchronisation, can be employed to extract a model for PDs passing ring main units or even entire substations

SupernetNL program: 3.4 km 110 kV AC underground superconducting cable in the Dutch grid

TenneT, a leading European electricity transmission system operator (TSO) is planning to install a 3.4 km long underground superconducting 110 kV cable as part of the Dutch electricity grid, in the city of Enschede. HTS cables have already been demonstrated on a relatively small scale in other countries, but they are usually not part of the meshed high-voltage grid and the length of the relevant cable section generally does not exceed 1 km. In 2009, a 600-meter section of HTS cable was installed in New York, and in 2014 a 1-km long section was taken in operation in Essen, Germany to replace a 10 kV AC medium-voltage line. In the Supernet NL program, TenneT is working together with several leading knowledge institutes including University of Twente, Delft University of Technology, the Institute of Science and Sustain- able Development (IWO), HAN University of Applied Sciences and RH Marine. These institutes have been investigating control engineering aspects and the requirements the cable must meet. In the meantime, the tender process has been started which consists of two phases. In the fi rst phase (summer 2017) appropriate candidates are selected directly followed by a call for tender in August. Receipt of the best and fi nal offer is scheduled for the end of November. In the presentation, the project will be introduced and requirements will be discussed, specifi cally focusing on the cryogenic aspects

Harmonic resonances due to transmission cables

Godkänd; 2014; 20141209 (matbol

Harmonic resonances due to transmission-system cables

This paper gives some examples of harmonicissues that can occur when long ac cables are connected in thetransmission grid. The main impact is that resonances can occurat much lower frequencies than when only overhead lines arepresent. Two illustrative case studies are presented: one for a275-kV cable, one for a 400-kV cable in combination with a 132-kV capacitor bank. A simple rule-of-thumb is given, to decide ifa detailed harmonic study is needed. Some guidelines for such astudy are given as well.Godkänd; 2014; 20140422 (matbol); Konferensartikel i tidskrift</p

Experiences with on-line PD measurement and interpretation for MV cable systems - field data analysis

On-line partial discharge (PD-OL) monitoring systems for medium voltage cable connections have recently been introduced in Dutch grids and also a few worldwide. The vast continuous stream of diagnostic data calls for a re-evaluation of statistical methods to be applied for the interpretation of the PD activity. In the present paper a variety of statistical parameters are discussed. These parameters include parameters for observing trends in PD activity over time. The methods are applied to data obtained from several continuous online field measurements collected over a year. The results of analyses on several weak spots which were detected before failure are presented. Results of failed accessories are discussed in relation to their prior PD behavior

Statistical approach to identify the discharge source in MV cables and accessories

Partial discharge (PD) analysis is a reliable tool to assess the integrity of electrical insulation. Representation and interpretation of the data, obtained from e.g. online PD monitoring, are key issues to reveal the discharge source, i.e. defect type, as well as the physical phenomena behind the occurrence. Analyses of various PD patterns such as discharge height distribution presented in this work provide useful statistical parameters to identify the discharge source. Research shows that the 2-parameter Weibull distribution is a reliable model to quantify the characteristics of the patterns of the defect. The model fits well to the charge-height distribution. In addition, trends in the discharge density pattern that occur over long times, can be used as complementary information to discover the discharge nature. It alerts for a possible failure and therefore assists in taking corrective measures to prevent failure. This paper presents the application of such statistical modeling to the area of on-line power cable diagnostics. Data obtained from laboratory experiments as well as field data have been studied

Continuous condition monitoring of MV cable connections and PD interpretation

Recently an on-line partial discharge (named PD-OL) for power cables is introduced in mainly Dutch medium voltage grids and first data has become available. This paper discusses the results on statistical analysis, including PD magnitude, PD repetition frequency and PD density distributions, employed to study the degradation of cable connections and their components. The potential of Weibull modelling, as a tool to identify the discharge origin, is investigated. PD fielddata is collected over one year from several connections including suspected weak spots in cables, joints and terminations. Continuous monitoring and trend watching of statistical parameters over time enabled us to detect ongoing aging processes, and several outages were prevented. Two cable circuits are specifically examined in more detail

Accuracy of switching transients measurement with open-air capacitive sensors near overhead lines

Application of open-air sensors is an attractive option for detecting switching over-voltages as their installation does not interfere with operation of the lines. Drawback is that coupling strengths are not known beforehand and cross-coupling between a sensor near one phase and other phase conductors or even to other circuits affects the recordings. Calibration of the coupling matrix between phase voltages and sensor signals can be realized by relating the power frequency component in the recorded signals to a priori assumed base of symmetric phase voltages. This paper establishes a method to quantify the different causes for measurement uncertainties. The coupling matrix cannot be completely reconstructed from measurements, enforcing model assumptions which introduce uncertainties. Uncertainties arising from not perfect symmetric phase voltages and measurement equipment are investigated as well, but play a minor role when judging over-voltages on a per unit base. The methodology is presented for measurements at an overhead line to underground cable transition point and to a substation. At transition points, the sensors could be positioned such that the cross-coupling remains low and the resulting uncertainty in the determination of maximum per unit over-voltages is typically 2%. Despite the installation at the substation resulting in relatively strong cross-coupling, per unit over-voltages could still be determined with an accuracy within 10%