The Interaction between Electric Field and Partial Discharges Simultaneously Detected in a HVDC cable under operating conditions

The reliability assessment of HVDC systems greatly depends on the insulating materials aging level and, among these, the cable insulation layer plays a fundamental role. With this in mind, in this work, the correlation between the electric field distribution and the triggering and evolution of PD in a DC cable containing an internal air void defect and subjected to normal operating conditions has been investigated. The aim is to demonstrate that the PD activity under DC depends on the electric field distribution which, in turn, is related to the conductivity gradient that varies with load. In a previous paper, the field distribution in a cable specimen was simulated. Here, instead, the field profiles have been experimentally obtained starting from the measured space charges detected simultaneously with the PD activity. For the charge detection and for the PD monitoring, an innovative PEA cell and a PD acquisition system have been used, respectively. Measurement results highlight that the PD behavior depends on both the electric field distribution and the time constant tau. Specifically, during the first 10 minutes of the beginning of the test, the field in the outer cable radius passes from 12 to 17 kV/mm and it is maintained around this last value until 30 minutes. PD are triggered after 1 minutes from the start of the test, corresponding to a PDIEF of 12.6 kV/mm calculated in the healthy cable section. The detected PD activity shows a PDRR with maximum value equal to 110 pulse/min at the beginning of the test. Whereas, after 15 minutes, the electric field variation dE/dt is approximately zero and the PDRR, that is maintained only by dE/dt results around 10 pulse/minute

Contributed Review: Review of thermal methods for space charge measurement

The space charge accumulation phenomenon has garnered great interest over the last two decades because of the increased use of direct current in high voltage electrical systems. In this context, a significant relevance has been achieved by the thermal methods, used for solid dielectrics. This paper presents a review of this non-destructive measurement system used for the measurement of space charge. The thermal pulse method, the thermal step method, and the laser intensity modulation method are described. For each configuration, the principle of operation, the thicknesses analyzed, and the spatial resolution are described, reporting also the main related application

Modified Hierarchical Clustering Algorithm for Partial Discharge Separation

To date, one of the main tools for evaluating the reliability of an insulation system is the continuous monitoring of those phenomena which, by interacting with the elements of the system, can induce aging processes or failures. For power grids, a signal that identifies possible aging or improper use of the component is Partial Discharge (PD) activity. Generally, the evaluation of the PD phenomenon is carried out through a two-step procedure: measurement and data analysis. To optimize the PD analysis process, increasingly sophisticated PD separation/classification algorithms are needed. Especially for the measurements carried out in HVDC systems for which the absence of a phase reference makes more difficult to identify the different types of discharge. The purpose of this article is to investigate the possibility of optimizing the input data to a hierarchical clustering algorithm in order to obtain a subdivision of the dataset more faithful to the real behavior of the phenomena. Specifically, the proposed approach is based on the use of the cross-correlation matrix to carry out the clustering operation. This matrix replaces the matrix of the distances among the points distributed in the map used for the representation of the data. Results show that with this modification it is possible to separate phenomena that present partially or completely overlapping patterns. Moreover, the algorithm turns out to be automatic and does not require the choice of references or thresholds to define the similarity among pulses

La Scienza e l'immaginario

L’attività di divulgazione della cultura scientifica ha un ruolo fondamentale sulla società, sia in termini di applicazioni innovative che di pianificazione dell’ambiente. I ricercatori dell’IAS-CNR di Capo Granitola operano da anni nell’ambito della diffusione della cultura scientifica, attraverso processi complessi e percorsi di divulgazione in partnership con istituti scolastici del territorio, realizzando attività seminariali, convegni direttamente nelle scuole, nonché visite didattiche guidate degli alunni nei laboratori dell’Istituto ed esperimenti interdisciplinari sull’ambiente marino. Tali processi divulgativi si sono sviluppati creando numerosi percorsi, in maniera per certi aspetti analoga a quella per cui dalla mescolanza dei tre colori fondamentali si è in grado di ottenere un numero pressoché illimitato di tinte diverse. Lo scopo di questa “mescolanza” è stato quello di ottenere un ventaglio di competenze e strumentazioni che consentissero di indagare i differenti aspetti dell’ecosistema marino da diversi punti di vista ed in maniera sinergica, tale da restituire un quadro il più ricco possibile di “tinte” e particolari. (Scienza e arte di Salvatore Mazzola) La Scienza e l'immaginario di Angela Cuttitta. Il progetto “La Scienza e l’Immaginario” nasce dalla collaborazione tra l’IAS - CNR di Capo Granitola e l’Accademia di Belle Arti di Palermo, che attraverso un approccio multidisciplinare ha voluto sperimentare l’unione tra il mondo scientifico e quello artistico, mettendo i giovani artisti, attraverso proiezioni e seminari scientifici, nelle condizioni di scoprire il mondo dell’ambiente marino e degli ecosistemi in esso presenti. Il progetto è nato dalla consapevolezza di come sia necessario operare sul piano della diffusione e divulgazione della cultura scientifica nei più vasti contesti sociali, a partire dall’ambito scolastico. Le azioni divulgative mirano, infatti, a diffondere la conoscenza dei processi geologici, chimico-fisici, climatici e biologici in modo pervasivo, non limitato a singole categorie/settori. La funzione strategica di tali azioni è quella di stimolare idee ed iniziative nonché di sviluppare una maggiore sensibilità nei confronti dei fenomeni che ci circondano, quale presupposto essenziale per una corretta programmazione politico-gestionale. Lo spirito che ha mosso tutte gli attori del progetto è stato quello di sensibilizzare gli studenti nei confronti della tutela delle risorse marine proprie del loro territorio e di sviluppare e promuovere la cultura come volano dello sviluppo sostenibile, della pace e dell’integrazione sociale, in armonia con quanto indicato dal Consiglio Europeo di Lisbona 2000. Grazie al lavoro di docenti e di ricercatori, l’arte come forma espressiva si è rivelata uno strumento valido e innovativo di divulgazione della cultura scientifica e ha portato alla creazione di suggestioni sui ragazzi che hanno percepito e realizzato forme e armonie espresse in questa mostra. L’impegno per questa manifestazione rappresenta, quindi, un appuntamento importante con le forze vive siciliane nel campo delle scienze del mare segnatamente ad esperti di biologia, chimica, fisica ed al mondo fantastico dell’arte, al fine di esprimere con le varie tecniche pittoriche un momento di riflessione culturale

Review of the PEA Method for Space Charge Measurements on HVDC Cables and Mini-Cables

This review takes into account articles and standards published in recent years concerning the application of the Pulsed Electro Acoustic (PEA) method for space charge measurement on High Voltage Direct Current (HVDC) cables and mini-cables. Since the 80s, the PEA method has been implemented for space charge measurements on flat specimens in order to investigate space charge phenomena and to evaluate the ageing of dielectrics. In recent years, this technique has been adapted to cylindrical geometry. Several studies and experiments have been carried out on the use of the PEA method for full size cables and HVDC cable models. The experiments have been conducted using different arrangements of the measurement setup and focusing attention on different aspects of space charge phenomena. In this work, the importance of space charge measurement is highlighted and the state-of-the-art PEA method application to full size cables and mini-cables is described. The main aim of this paper is to offer a complete and current review of this technique. In addition, limits on the use of PEA method are examined and main possible directions of research are proposed in order to improve the applicability, reliability, and replicability of this method

A new approach to partial discharge detection under DC voltage

The continuing development of HVDC power transmission systems presents many problems related to evaluation of the reliability of power system assets [1]-[5]. In this context the identification of insulation defects plays a key role in preventing unexpected failures of electrical components. Partial discharge (PD) measurement is a useful approach to assessing the condition of HV power apparatus and cables. Such measurements are also widely employed for HVAC systems. The inception mechanisms of PD in AC systems are well-known, and measurements are usually performed following the IEC 60270 standard [6]. PD measurements under DC voltage present complexities related to the nature of the phenomenon and the supply conditions [7]. In AC systems phase-resolved-partial-discharge (PRPD) patterns allow assessment of the insulation condition, and provide information on the types of defect present [8]. Such analysis cannot be performed under DC voltage, since each discharge event cannot be related to a phase value. The interpretation of the acquired data therefore requires a different approach

The industrial applicability of PEA space charge measurements, for performance optimization of HVDC power cables

: Cable manufacturing industries are constantly trying to improve the electrical performance of power cables. During the years, it was found that one of the most relevant degradation factors influencing the cable lifetime is the presence of space charge in the insulation layer. To detect the accumulated charge, the pulsed electro-acoustic (PEA) method is the most used technique. Despite the wide use of the PEA cell, several issues are still present. In particular, the PEA output signal is strongly disturbed by the acoustic waves reflections within the PEA cell. This causes the distortion of the output signal and therefore the misinterpretation of the charge profiles. This, in turn, may result in an incorrect cable characterization from the space charge phenomenon point of view. In 2017, due to the proved degradation effect of the space charge accumulation phenomenon, the IEEE Std 1732 was developed. This standard describes the steps to be followed for the space charge measurement in cables specimens during pre-qualification or type tests. Therefore, cable manufacturing industries started to take a particular interest in these measures. In the light of this, the aim of the present work is to highlight that the enacted standard is not easily applicable since various problems are still present in the PEA method for cables. In particular, in this work, the effect of multiple reflected signals due to the different interfaces involved, but also the effect of the signal attenuation due to cable dielectric thickness, as well as the effect of the PEA cell ground electrode thickness in the output charge profile, are reported. These issues have been demonstrated by means of an experimental test carried out on a full-size cable in the Prysmian Group High Voltage laboratory. To better understand the PEA cell output signal formation, a PEA cell model was developed in a previous work and it has been experimentally validated here. In particular, simulations have been useful to highlight the effect of the reflection phenomena due to the PEA cell ground electrode thickness on the basis of the specimen under test features. Moreover, by analyzing the simulation results, it was possible to separate the main signal from the reflected waves and, in turn, to calculate the suitable ground electrode thickness for the cable specimen under test

Partial Discharge Detection Using a Spherical Electromagnetic Sensor

The presence of a partial discharge phenomenon in an electrical apparatus is a warning signal that could determine the failure of the insulation system, terminating the service of the apparatus and/or the network. In this paper, an innovative partial discharge (PD) measurement instrument based on an antenna sensor is presented and analyzed. Being non-intrusive is one of the most relevant features of the sensor. The frequency response of the antenna sensor and the features to recognize different PD sources and automatically synchronize them with the supply voltage are described and discussed in details. The results show the performance of the instrument can make a fast and correct diagnosis of the health state of insulation systems

Reliability of PEA Measurement in Presence of an Air Void Defect

This paper deals with the reliability of the Pulsed Electro-Acoustic (PEA) technique in the case of a specimen containing an air layer. The first approach to this study has been proposed by the authors in previously published works. In these papers, the mathematical description, the PEA cell simulation model, and some experimental tests have been reported. In this work, a more accurate description of the acoustic wave behavior within the PEA cell and specimen with and without an air layer is given. Moreover, the comparison between simulation and experimental tests for both cases (specimen with and without air layer) allowed the validation of the previously developed PEA cell simulation model. The latter was previously validated only for a single layer specimen, here the good performances of the model have also been confirmed in the case of a multilayer specimen, also with an air layer. Experimental and simulation results show that the air layer acts as a barrier for the acoustic signal, due to the dierent acoustic impedance between the air and the solid dielectric material which constitute the specimen. Therefore, the aim of the present work is to demonstrate that in the case of a three-layers specimen, composed as dielectric-air-dielectric, the PEA cell is not able to provide the complete profile of the entire specimen under test