\documentclass[final,10pt,twocolumn]{elsarticle} \usepackage{amssymb} \usepackage{graphicx} \usepackage{gensymb} \usepackage{multirow} \usepackage{tabularx} \renewcommand{\tablename}{Table.} \renewcommand{\figurename}{Fig.} \journal{Journal of Power Technologies} \begin{document} \begin{frontmatter} \title{Verification of the magnetic method in testing of the modified composite core of overhead conductors} \author{AUTHORS} \address{AFFILIATION, ADDRESS} \begin{abstract} ACCC/TW conductors are a modern solution that increases effectivity of transfer of electrical energy. They have many advantages, however their composite cores may succumb to cracking under excessive bending. The article presents a concept of modification to its construction and a testing method that allows a verification of a technical state of a core of the conductor after installation in place of operation. The technique uses an existing method of magnetic testing of steel ropes. Tests were conducted on a series of conductor models with cores containing epoxy resin with an addition of a ferromagnetic FeSi 15 powder. The measurements showed a capability of detection of small crosswise gaps and cracking simulated in the core. Accuracy of the method is determined by the concentration a of magnetic component in the core. \end{abstract} \begin{keyword} composite core conductor\sep magnetic testing \sep power transmission \end{keyword} \end{frontmatter} \section{Introduction} \label{} Modern power transmission technologies are being increasingly often applied to minimize losses and increase effectivity of a power grid. Innovative types of electrical conductors, based on new constructional and material solutions, are an important aspect of developments in this field. Despite that, there is not much progress in matters of controlling a technical condition and safety of power lines directly before the beginning of their exploitation. The following work presents an experimental method of controlling the state of composite cores of ACCC/TW type conductors using existing and widely used methods of magnetic testing of steel ropes. ACCC/TW (aluminium composite core conductor/trapezoidal wire) type electric conductors (Fig. 1) are a modern solution in the field of electric energy transfer, developed to improve key properties of transmission lines, in comparison with conventional products. It is a design that is utilizing a composite core, composed of glass and carbon reinforcing fibres in an epoxy resin matrix, manufactured using methods of pultrusion. This method allows achieving a higher tensile strength with a much lower density than in case of steel cores, widely used in conventional ACSR (aluminium conductor steel reinforced) type conductors. It also permits using trapezoidal, annealed, 99.7\% pure aluminium wires, which results in a bigger share of aluminium in overall cross-section of the conductor and also a higher maximum work temperature of up to 250\degree C \citep{1}. Along with low coefficient of linear expansion, those properties qualify this design to HTLS (high temperature – low sag) group of conductors \citep{2}. \begin{figure} \includegraphics[width=\linewidth]{fig1.png} \caption{Sample of an ACCC/TW type conductor} \label{fig1} \end{figure} The structure of the ACCC conductor determines a number of its advantages: low weight, low sag and low electrical resistance. Because of that, power lines based on this solution feature higher effectivity of exploitation, lower transfer losses and safety of work during an overload. Those features are especially significant for realization of country-wide obligations in fields of energetic effectivity and limiting of CO2 emissions. Their main disadvantage is price (even 3 times higher than of its ACSR equivalent), but in a light of a possibility of quick and easy power line modernization it is often an economically justified investment. Because of this, as of year 2016 around 33 000 km of ACCC type conductors were installed world-wide \citep{2,3}. The described technology possesses a limiting factor, which is a higher allowed bend radius than this of its ACSR counterparts \citep{4}. Series of conducted tests and simulations show, that subjecting ACCC conductors to excessive bending may cause damage to their structure, resulting in cracking of the epoxy matrix, kinked carbon fibres and delamination at the glass/carbon interface \citep{4,5,6}. Examples of damage analysed by Burks were presented in Fig. 2. \begin{figure} \includegraphics[width=\linewidth]{fig2.png} \caption{SEM imaging of damage to the composite core subjected to bending [4, 6]} \label{fig2} \end{figure} In that research it was found that the bend radius causing significant damage to the core, manifested externally as series of acoustic emissions, is around 280 mm, and a critical bend radius, resulting in breaking of the reinforcing fibres and breaking of the core – around 233 mm. In case of four point bending, being an equivalent of 80\% and 90\% of cores flexure strength (respectively 292 mm and 259 mm bend radius), it comes to damaging of the matrix in way of micro cracking. Such bending can occur as a result of errors in the conductor installation process or usage of an inappropriate equipment. As established by Burks, damage to the matrix of the tested cores has no direct influence on their tensile strength, although such discontinuities can have a major impact on performance of the conductor during exploitation and its mechanical and fatigue characteristics \citep{6}.\\ Magnetic rope testing (MRT) method is based on a phenomenon of a dissipation of magnetic field in areas of damage in a steel rope, previously magnetized in a constant magnetic field, registered by inductive or hallotronic sensors. The measurement is conducted using dedicated testing heads and registered on tape charts or digitally by a defectograph. This technique has been used since 1940s to control technical condition of steel ropes used in lifts, cableways, drilling equipment and many other structures. It allows defining places of wire breakage, attrition, corrosive damage and also quantitive calculations of a loss of a cross section of the rope \citep{7}. A MRT head along with a defectograph is shown in Fig.3. \begin{figure} \includegraphics[width=\linewidth]{fig3.png} \caption{MRT testing head with a defectograph} \label{fig3} \end{figure} A typical MRT head consists of strong permanent magnets, an inner inductive sensor (used to detect material loss), an outer inductive sensor (allowing to estimate its depth), a hallotronic sensor (simplifying detection of losses with elongated characteristic) and also a gauge of rope movement relative to the testing head \citep{8}. An example of a MRT measurement graph is shown in Fig. 4. \begin{figure} \includegraphics[width=\linewidth]{fig4.png} \caption{Example of a MRT chart} \label{fig4} \end{figure} The purpose of this work was to verify a testing potential of a new method of controlling the continuity of composite cores of electric conductors. The method consists of: \begin{itemize} \item Adding a ferromagnetic agent to the composition of the core in form of a FeSi powder with granulation of 0,063 mm or similar by introducing it into epoxy resin that constitutes an entirety or a part of the matrix of the composite, \item Applying or adapting existing methods of magnetic rope testing (MRT) to detect damage and discontinuities in the structure of the core. \end{itemize} For this goal a series of models of a core were made, containing epoxy resin with an addition of FeSi powder, and a series of MRT measurements were performed, with models as a substitute of cores of a ACSR conductor sections. To asses an accuracy of the method, various amounts of ferromagnetic powder were used in preparation of the models. \section{Methodology} \label{} To model composite cores containing ferromagnetic particles, 4 test samples (Fig. 5) were made in a form of PVC tubes with inner diameter of 5 mm filled with a mix of Epidian 5 epoxy resin, Z1 hardener and various amounts of MFeSi 15 powder with particle size of up to 0,063 mm. Nominal specific density of the powder was 7,0 g/cm\textsuperscript{3} and this value was used in calculations. \begin{figure} \includegraphics[width=\linewidth]{fig5.png} \caption{Model of the composite core containing FeSi powder} \label{fig5} \end{figure} In each mix, 60 cm\textsuperscript{3} (70,8 g) of Epidian 5 resin with 5 cm\textsuperscript{3} of Z1 hardener was used. FeSi additions for each of the samples are listed in Table 1. To achieve elasticity of the models that would allow eventual bending during handling and testing, lowered amounts of Z1 hardener were used in comparison to recommended by the producer. \begin{table}[] \centering \caption{FeSi content in test resin mixes} \begin{tabularx}{\linewidth}{cXX} \hline \multirow{2}{*}{Sample No.} & \multicolumn{2}{c}{FeSi content} \\ & \hfil g& \hfil cm\textsuperscript{3} \\ \hline 1 & \hfil250 & \hfil35,7 \\ 2 & \hfil200 & \hfil28,6 \\ 3 & \hfil150 & \hfil21,4 \\ 4 & \hfil100 & \hfil14,3 \\ \hline \end{tabularx} \end{table} \begin{table}[] \centering \caption{FeSi content in 1m of tested cores} \begin{tabularx}{\linewidth}{cXX} \hline \multirow{2}{*}{Sample No.} & \multicolumn{2}{c}{FeSi content / m} \\ & \hfil g& \hfil cm\textsuperscript{3} \\ \hline 1 & \hfil48,7 & \hfil7,0 \\ 2 & \hfil42,0 & \hfil6,0 \\ 3 & \hfil34,1 & \hfil4,9 \\ 4 & \hfil24,8 & \hfil3,5 \\ \hline \end{tabularx} \end{table} Basing on a ratio of components of mixes, contents of the FeSi powder per one meter of each core were calculated (Table 2.). The mixes were introduced into the PVC tubes gravitationally or by using slight pressure and then left to harden. In this way 120 cm long samples were acquired. The longitude of core models possible to acquire was limited by a viscosity of introduced mix, especially in case of higher contents of the FeSi powder. Cores were then cut in half. From a piece of empty PVC tube spacers were made, 1 mm, 3 mm and 5 mm in width, to serve as a substitute of damage or discontinuities of the core. Role of the aluminium part of the conductor was fulfilled by layers of round aluminium wires, acquired by removing a steel core from 4 lengths of 236-AL1/40-ST1A conductor with a nominal diameter of 21,7 mm. During the tests, obtained core models, separated in the middle by a chosen spacer, were introduced into empty spaces left by steel cores. On such prepared conductor models (Fig. 6.) MRT tests were performed for every variant of the FeSi content and width of the spacer. \begin{figure} \includegraphics[width=\linewidth]{fig6.png} \caption{Model of the conductor with a composite core enriched with FeSi} \label{fig6} \end{figure} The testing system (Fig. 7) consisted of a GP-3ARH MRT testing head (Fig. 7a) equipped with a set of permanent magnets (Fig. 7b) and an inductive sensor (Fig. 7c). Tests were performed by passing 4 models of an electric conductor (Fig. 7d) through the testing head multiple times using a reciprocating motion with a speed of approximately 1 m/s, during which a core containing FeSi powder (Fig. 7e) was magnetized, and an introduced spacer (Fig. 7f) caused a dissipation of magnetic field proportional to the width of the defect. \begin{figure} \includegraphics[width=\linewidth]{fig7.png} \caption{Schematic of the testing head during measurement} \label{fig7} \end{figure} The signal generated by the testing head was registered digitally using a MD121 defectograph with a measurement sensitivity set to 1 mV/div and a sampling resolution of 5000 samples/s. The software provided with the device (Fig 8.) allowed to browse, view and analyse gathered data, including detecting peaks of the signal caused by the gap inserted into the cores. \begin{figure} \includegraphics[width=\linewidth]{fig8.png} \caption{MD121 View software interface} \label{fig8} \end{figure} The cores of the tested models were assembled from two identical parts, composed of an epoxy resin with an addition of various amounts of Fe Si 15 powder, separated by 1mm, 3mm, or 5mm spacer. Additional measurements of conductors with a cut core with no spacer (0mm) were also performed. Due to a dependence of the signal strength on the speed of movement of the conductor relative to the inductive sensor and also a lack of a possibility of using speed compensation function because of the dimensions of the samples, the test results should be primarily considered in a qualitative manner. \section{Results} \label{} \begin{figure} \includegraphics[width=\linewidth]{fig9.png} \centering \caption{Dependence of damage detection signal on gap width and FeSi content} \label{fig9} \end{figure} \begin{table*}[t!] \centering \caption{Results of testing of the conductor models using the MRT method.} \begin{tabularx}{\linewidth}{XXXXXX} \hline \multirow{3}{*}{Sample No.}& \multirow{2}{*}{FeSi content,} &\multicolumn{4}{c}{Signal peak Umax, mV}\\ \cline{3-6} & & \multicolumn{4}{c}{Width of spacer, mm} \\ &\hfil g/m&\hfil0&\hfil1&\hfil3&\hfil5 \\ \hline \\[-1em] \hfil 1&\hfil 48,7&\hfil1,71&\hfil4,13&\hfil7,58&\hfil12,92 \\ \hfil 2&\hfil 42,0&\hfil1,15&\hfil3,06&\hfil5,36&\hfil7,77 \\ \hfil 3&\hfil 34,1&\hfil0,64&\hfil1,46&\hfil3,85&\hfil6,08 \\ \hfil 4&\hfil 24,8&\hfil0,50&\hfil1,08&\hfil1,60&\hfil2,27 \\ \hline \end{tabularx} \end{table*} Values of highest signal peaks Umax recorded during testing, sorted by the ferromagnetic powder content, are presented in Table 3. As the primary function of the proposed method is detecting discontinuities and damage to the composite core of the conductor it would be mostly based on the analysis of charts generated by the defectograph (Fig. 10). \begin{figure*} \includegraphics[width=\linewidth]{fig10.png} \centering \caption{Chart fragments registered during testing of samples} \label{fig10} \end{figure*} \section{Result analysis} \label{} Models of an electric conductor were tested using MRT methods. Due to model geometry, results of the tests allow to estimate accuracy of the proposed method in detecting discontinuities and damage perpendicular to the axis of the conductor. Sample 1, characterized by the highest content of FeSi powder of 48,7 grams per meter of the conductor, exhibited an easily distinguishable signal peak for all of the applied spacers. It also allowed for the detection of a crack in the core of the conductor, which indicates a potential of the method to indicate early fractures that could occur during improper installation. Slightly lower ferromagnetic content of 42,0 g/m in sample 2 caused the signal generated by the cut in the conductor to be practically lost in a signal noise of the measurement. Nevertheless, a gaps of 1mm and wider were easily detectable during MRT measurement. Further decrease of FeSi powder in the model causes the accuracy of the method to decrease and detecting a presence of the 1mm spacer in the sample 3 proved to be inconclusive with applied test parameters. Sample 4, containing lowest amounts of ferromagnetic additive of just 24,8 g/m, allowed sure detection of 3mm and 5mm gaps in the core of the conductor, which still guarantees detection of a critical failure of the core. Part of the background noise in the signals visible on the measurement charts is caused by air bubbles contained in the resin mix, which are the result of used method of production of the models. It is safe to assume that during continuous manufacture of conductor cores this problem would not occur, and their high homogeneity will in turn result in a higher accuracy of this testing method. The tests have shown a relationship between the width of the gap in the core, FeSi content in its epoxy matrix and strength of the signal registered by the defectograph in place of the simulated core damage. With higher contents of ferromagnetic powder raises the sensitivity and accuracy of the analysed method of verifying the continuity of the core, simultaneously causing overall weight of the conductor to increase. Results of testing allow to consider to be possible to use existing MRT methods to verify continuity of composite cores of electric conductors after enriching them with functional ferromagnetic particles. To evaluate full potential of this method further research is necessary, considering core damage of axial character, fully sized samples and fully functional prototypes of conductors. \section{Summary} \label{} The ACCC/TW type conductors exhibit many advantages and the range of their application still increases. Adding functional, magnetic elements to their construction would allow to equip them with a possibility of controlling a technical state of their cores after or before installation in a place of exploitation using existing methods of magnetic rope testing. Sensitivity of detection of damage and discontinuities of the core is dependent on the content of ferromagnetic elements and a way of conducting the measurement. \vspace{5mm} \textit{This work was funded by ****. } \textit{Conflict of Interest: The authors declare that they have no conflict of interest.} \bibliographystyle{elsarticle-num-names} \bibliography{references} \end{document} \endinput