Medium Voltage Cables Research Articles

Electromagnetic sensing for predictive diagnostics of electrical insulation defects in MV power lines

Insulation degradation is one of the most frequent causes of the failure of electrical components. Partial discharge (PD) has been proven to be a reliable indicator whose early diagnostic can avoid the complete breakdown of the affected component. This paper proposes an online system for PD diagnostic system in a distribution network. A comprehensive diagnostic system is presented by developing a cascaded organization of the detection, location and quantization features. Rogowski coil (induction sensor) is employed for non-intrusive electromagnetic measurement of PD signals. Experimental evaluation is made for over-head covered conductor (CC) lines and medium voltage cables. A scheme is proposed for integration of the developed diagnostic system into the cable network. The explored diagnostic features are equally applicable for cable and CC line based regions of the electricity network. The favorable operating features of the Rogowski coil sensor and simplicity of the applied scheme make it easily adoptable for developing an efficient condition diagnostic system for the distribution networks.

Temperature dependent signal propagation velocity: possible indicator for mv cable dynamic rating

Insulation temperature is the critical parameter for dynamic rating of underground power cables. Power cables on medium voltage (MV) level are too numerous and widespread to economically justify solutions like optical fibers as temperature sensors, often employed for high voltage power cables. This paper proposes to utilize the propagation velocity of high frequency signals as indicator for MV cable dynamic rating. Laboratory scale tests are performed for both PILC and XLPE cables. Test results show that the high frequency signal propagation velocity for XLPE insulation increases with the temperature rise while PILC cable shows opposite behavior. The variation of propagation velocity with temperature is consistent with the variation of permittivity measured on test samples of both materials. The variation of propagation velocity of XLPE is confirmed by data of a power cable in service subjected to strong load cycling monitored over a week. Field data on propagation velocity also matches load variation observed for a PILC cable for load cycling recorded over one week. From laboratory tests and field measurements, it seems feasible to extract thermal information from MV cables for dynamic rating by means of high frequency propagation velocity.

Comparación de la formación de carga eléctrica en aislamientos de XLPE con antioxidantes sólido y líquido mediante la técnica TSDC

(Recibido: 2014/10/29 - Aceptado: 2014/12/17)En este trabajo se comparó la presencia de carga eléctrica en dos aislamientos de polietileno reticulado (XLPE) de cables de media tensión mediante la técnica de las Corrientes de Despolarización Estimuladas Térmicamente (TSDC). A uno de los aislamientos se le añadió antioxidante en estado sólido y al otro en estado líquido. Hemos comprobado mediante el análisis de las medidas TSDC que el uso de antioxidante líquido es mejor puesto que el sólido crea defectos que actúan como centros de atrapamiento de la carga de espacio. En estos centros se acumula la carga ionizada en el aislante por la influencia combinada de la temperatura y el campo eléctrico además de la carga inyectada desde los electrodos semiconductores por el efecto de campos elevados.

Partial discharge diagnostic system for smart distribution networks using directionally calibrated induction sensors

Asset management and optimization is becoming increasingly adaptable for distribution utilities with the help of developments in the digital and wireless technology. Medium voltage cables, cable accessories (joints and terminations) and distribution transformers are most prone to the insulation defects. This paper presents an online partial discharge (PD) diagnostic system to detect and locate the weak insulation spots developed in the network components spread over the distribution area. The PD pulses originating and propagating from the defect site arrive at feeder joints, terminals and T-splices and are reflected from there. This makes the interpretation of the measured signals ambiguous in order to find the location of PD faults, while using conventional location techniques. An improved technique is introduced to monitor multi-section multi-branched cable network. Identification of the faulty location is carried out by using direction of arrival (DOA) of PD signals. The DOA of PD signals is obtained by using directionally calibrated Rogowski coil induction sensors. Polarity of captured PD signals with reference to supply voltage, determines their DOA. Rogowski coil along with its directional sensing feature is simulated in ATP-EMTP. The DOA technique is integrated over a medium voltage (MV) cable network and its performance is evaluated using ATP-EMTP environment. Frequency-dependent JMarti cable model is used for simulation. An intelligent algorithm is proposed for practical implementation of DOA technique for PD monitoring and automated location. Proposed scheme can be adopted in distribution automation system to improve proactive diagnostic capabilities of the network.

On-line PD detection and localization in cross-bonded HV cable systems

This paper addresses the detection and localization of partial discharge (PD) in crossbonded (CB) high voltage (HV) cables. A great deal has been published in recent years on PD based cable insulation condition monitoring, diagnostics and localization in medium voltage (MV) and high voltage (HV) cables. The topic of pulse propagation and PD source localization in CB HV cable systems has yet to be significantly investigated. The main challenge to PD monitoring of CB HV cables is as a result of the interconnectedness of the sheaths of the three single phase cables. The cross-bonding of the sheaths makes it difficult to localize which of the three phases a PD signal has emanated from. Co-axial cables are used to connect cable sheaths to cable link boxes, for ease of installation and protection against moisture. A second challenge is, therefore, the coupling effect when a PD pulse propagates in HV cable joints and the co-axial cables, making PD detection and localization more complex. The paper presents experimental investigations into PD pulse coupling between the cable center conductor and the sheath and the behavior of PD pulse propagation in CB HV cables. It proposes a model to describe PD pulse propagation in a CB HV cable system to allow monitoring and localization, and also presents the knowledge rules required for PD localization in CB HV cable systems.

Diagnosis of Medium Voltage Cables for Nuclear Power Plant

Most accidents of medium-voltage cables installed in nuclear power plants result from the initial defect of internal insulators or the initial failure due to poor construction. However, as the service years of plants increase, the possibility of cable accidents is also rapidly increases. This is primarily caused by electric, mechanical, thermal, and radiation stresses. Recently, much attention is paid to the study of cable diagnoses. To date, partial discharge and Tanδ measurements are known as reliable methods to diagnose the aging of medium-voltage cables. High frequency partial discharge measurement techniques have been widely used to diagnose cables in transmission and distribution systems. However, the on-line high frequency partial discharge technique has not been used in the nuclear power plants because of the plant shutdown risk, degraded measurement sensitivity, and application problems. In this paper, the partial discharge measurement with a portable device was tried to evaluate the integrity of the 4.16kV and 13.8kV cable lines. The test results show that the high detection sensitivity can be achieved by the high frequency partial discharge technique. The present technique is highly attractive to diagnose medium voltage cables in nuclear power plants.

A study on three dimensional assessment of the aging condition of polymeric medium voltage cables applying very low frequency (VLF) tan δ diagnostic

Recently, diagnostic on Medium Voltage Cables using tan δ measurement that measures dissipation factor with 0.1Hz VLF (Very Low Frequency) high voltage source has emerged as an efficient way to assess water tree aging of MV (Medium Voltage) cables. This study verified the validity diagnostic evaluation criteria defined by IEEE400.2-2013 and it?s pervious drafts of DTD (Delta tan δ) and STDEV (Standard Deviation) - indicating voltage-stability and time-stability of tan δ respectively - by applying tan δ measurement to KEPCO (Korea Electric Power Corporation)'s power distribution system and suggested a new assessment factor being defined as Skirt, and its new formula in order to identify the precursor of insulation breakdown with patterns of tan δ. By visualizing complex correlations between various assessment factors and proposing a 3-dimensional assessment standard that can normalize the deterioration condition of a cable into a uniform value, this study has established a foundation to calculate the remaining life time of a cable through VLF tan δ diagnostic.

Time-Domain Modeling and Simulation of Partial Discharge on Medium-Voltage Cables by Vector Fitting Method

In this paper, a transmission line model is developed based on lumped element representation. A simple and efficient procedure is presented for modeling medium-voltage (MV) power cables directly in time domain. The proposed methodology, considering both the skin effects and dielectric losses, considers the frequency-dependent parameters of the line. To incorporate both the skin effects and dielectric losses into the state matrices, a fitting method is applied. With this method, a precise model of the MV power cables is constructed. Using the estimated propagation speed of the presented model, the experimental results show that the accurate partial discharges location on MV power cables can be achieved.

Prediction of mechanical properties of XLPE cable insulation under thermal aging: neural network approach

The widespread use of Cross-linked Polyethylene (XLPE) as insulation in the manufacturing of medium and high voltage cables may be attributed to its outstanding mechanical and electrical properties. However, it is well known that degradation under service conditions is the major problem in the use of XLPE as insulation in cables. In order to reduce the aging experiments time, we have used Artificial Neural Networks (ANN) to predict the insulation properties. The proposed networks are supervised and non supervised neural networks. The supervised neural network was based on Radial Basis Function Gaussian (RBFG) and was trained with two algorithms: Backpropagation (BP) and Random Optimization Method (ROM). The non supervised neural network was based on the use of Kohonen Map. All these neural networks present good quality of prediction.

A combined experimental–numerical rheometric and mechanical characterization of EPM/EPDM rubber for medium voltage cable applications vulcanized with peroxides

ABSTRACTThe vulcanization of medium and high voltage (M‐H V) cables is an important industrial application where manufacturers use principally EPM/EPDM, crosslinked polyethylene (XLPE) and now also thermoplastic rubbers. In the present article, an EPDM compound for medium voltage cables—normally distributed to producers in pellets and ready to be cured—is considered and several experimental tests (rheometer curves and mechanical characterizations) are conducted changing controlled curing temperature, peroxide type and peroxide concentration. In particular, tests are replicated in a temperature range between 160 and 200°C, with data provided every 20°C, using two different peroxides, a mixture of three commercial peroxides at five different concentrations. A huge amount of experimental data (cure curves) is obtained (one for each temperature, peroxide used and concentration) and results are critically compared one each other, to have a quantitative insight into the most effective temperature and peroxide to be used during this kind of vulcanization, with the aim of maximizing vulcanization velocity, final torque reached and mechanical properties of cured rubber. The comprehensive experimental study proposed is finally assessed from a numerical point of view, utilizing a complex kinetic scheme constituted by reactions occurring in series and parallel, which allows estimating numerically the vulcanization degree by means of a single second order nonlinear differential equation, with unknown parameters determined through non‐linear least squares fitting, with target data represented by experimental rheoemeter curves. The experimental fitting is almost perfect for all the cases analyzed and is aimed at predicting (without an expensive experimentation) the most suitable production parameters (e.g., curing time and temperature) to guarantee improved mechanical properties and a good aging resistance of the items. In addition, the numerical approach could allow finding the most suitable recipe, also in presence of mixtures of peroxides, able to provide items with a uniform vulcanization level. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40075.

Simulation of partial discharges in the insulation of medium-voltage cables

Mathematical modeling of partial discharges in polyethylene cable insulation on the average voltage was performed. The results are in agreement with the experimental data obtained at the cable lines by using an Oscillating Wave Test System device.

Long Term Analysis of Cables Problems in Egyptian Distribution Network

In the last few decades the main focus of utility companies has been on developing diagnostic, location and pin pointing techniques for medium and high voltage cables, despite the fact that most of the installed cable assets on the network consist of low voltage cable. Egyptian distribution network is very complex network and has large lengths of underground cables, which mostly oil paper cables and XLPE cables. The network suffers from an increasingly faults at cable joints, where the main cause is the difficulty of oil paper cable joints which required a skilful technician. Large efforts are spent to categorize the types of faults happen at the cable joints to describe the avoiding methods of these faults. Benefits will be saving of operation cost, time and improvement of system availability and performance to avoid service outage which is a big lost for company and customer. Keyword s: underground cable, testing devices, location methods, fault detection

The use of power frequency current transformers as partial discharge sensors for underground cables

Partial discharge (PD) diagnostics can be used to diagnose insulation defects in medium voltage cables and thereby inform required maintenance planning to extend the lifespan of ageing assets. High frequency current transformers (HFCT) installed on the earth strap at the cable termination are widely used to detect the fast varying pulses resulting from PD. Due to constructional reasons earth strap accessibility in underground cables is often limited. Furthermore in three core cables, phase angle dependency of pulses resulting from PD sometimes fails to induce pulses in the earth screen. This paper investigates an alternative method of PD detection using conventional power frequency current transformers (PFCT) principally used for protection and measurement purposes. This investigation assesses the feasibility of this approach by conducting a magnetic study of the transformer core, frequency response analysis (FRA), and finally capturing and analyzing onsite PD measurements using conventional PFCTs and the developed PD data acquisition/interpretation system. Based on the laboratory and on-site measurement results, PFCTs can be considered for detecting PD. This approach does not require retrofitting additional PD sensors and can also be applied to older switchgear design without any outage. Hence this approach can be considered as a low cost and noninvasive method of PD detection in cables.

Conceptualization and Multiobjective Optimization of the Electric System of an Airborne Wind Turbine

Airborne wind turbines (AWTs) represent a radically new and fascinating concept for future harnessing of wind power. This concept consists of realizing only the blades of a conventional wind turbine (CWT) in the form of a power kite flying at high speed perpendicular to the wind. On the kite are mounted a turbine, an electrical generator, and a power electronics converter. The electric power generated is transmitted via a medium voltage cable to the ground. Because of the high flight speed of the power kite, several times the actual wind speed, only a very small swept area of the turbine is required according to Betz's Law and/or a turbine of low weight for the generation of a given electric power. Moreover, because of the high turbine rotational speed, no gear transmission is necessary and the size of the generator is also reduced. For takeoff and landing of the power kite, the turbines act as propellers and the generators as motors, i.e., electric power is supplied so that the system can be maneuvered like a helicopter. In the present work, the configuration of power electronics converters for the implementation of a 100 kW AWT is considered. The major aspect here is the trade-off between power-to-weight ratio (W/kg) and efficiency. The dependence of cable weight and cable losses on the voltage level of power transmission is investigated, and a comparison is made between low voltage (LV) and medium voltage (MV) versions of generators. Furthermore, the interdependence of the weight and efficiency of a bidirectional dual active bridge dc-dc converter for coupling the rectified output voltage of a LV generator to the MV cable is discussed. On the basis of this discussion, the concept offering the best possible compromise of weight and efficiency in the power electronics system is selected and a model of the control behavior is derived for both the power flow directions. A control structure is then proposed and dimensioned. Furthermore, questions of electromagnetic compatibility and electrical safety are treated. In conclusion, the essential results of this paper are summarized, and an outlook on future research is given. To enable the reader to make simplified calculations and a comparison of a CWT with an AWT, the aerodynamic fundamentals of both the systems are summarized in highly simplified form in an Appendix, and numerical values are given for the 100 kW system discussed in this paper.

Narrowband power line communication: Medium voltage cable modeling and laboratory experimental results

Power line communication (PLC) is a key component in the communication infrastructure of the future smart grid with applications in both medium voltage (MV) and low voltage (LV) networks. PLC channel characteristics are very important for the signal propagation and further analysis of their performance is needed. Scope of this paper is to investigate the propagation characteristics of MV cables by implementing experimental and simulation tests for different cable configurations and operational cases. A PLC channel simulation model is presented using proper cable models and is validated by measurements showing a very good agreement in the results. Investigations concerning the signal propagation are conducted and results are discussed.

El Sewedy Cables: expansion in Russia and Kazakhstan

PurposeThe purpose of this paper is to explain the process by which El Sewedy Company in Egypt decided to expand internationally by examining several modes of entry and choosing the best choice to expand in Russia and Kazakhstan.Design/methodology/approachThe authors did several market analyses for each company to discover the external and internal environments. In addition, they weighted the advantages and disadvantages of each mode of entry to decide on the best mode to enter Russia and Kazakhstan.FindingsThe results confirm that the expansion for El Sewedy is challenging whether in Russia or Kazakhstan. Currently, El Sewedy's strategy is to deal through distributors to minimize risk; these distributors are mostly working in Kazakhstan given their easy connections and access there. The best solution for El Sewedy to expand is to trade high voltage and medium voltage cables in Kazakhstan as well as the trade of high voltage cables in Russia. Whether it is trading, exporting or a joint venture, El Sewedy's intentions are clear in its willingness to access the Russian and Kazakhstani markets, due to their richness and most of all their location as a central trade area in the region.Research limitations/implicationsThe comparison between Russia and Kazakhstan may appear very similar since both countries share almost the same circumstances and environment. Therefore, some of the information may be repeated twice to analyze both countries.Practical implicationsThe results of this case study ensure the importance of analyzing the different modes of entry and doing a cost‐benefit analysis before deciding on how to expand in foreign market. Since circumstances and environments vary between markets, it is crucial to know all the details of the country in order to succeed in expanding in this foreign market.Originality/valueThis study explains a common case that happens in Egypt and every country whenever a company decides to expand. Usually, a company prefers a certain mode of entry over the other, but what is necessary is to follow what El Sewedy did, which is to evaluate all alternatives equally then decide on the best choice.

Diagnosis of MV Power Cables for Nuclear Power Plants

The diagnosis of MV (Medium Voltage) cable used in NPPs (Nuclear Power Plants) has become one of the most urgent issues. In accordance with PSR (Periodic Safety Review) results, continuous condition monitoring of cables is also continuously required. In this paper, HFPD (High Frequency Partial Discharge) measurement and an infrared imaging thermography test were performed to check the status of 13.8 and 4.16 kV MV cables installed in the domestic power plant. The diagnosis was found to be good for the target cable. The on-line diagnostic technology to assess the condition of MV cables during the operation of a power plant will contribute to enhance the safety of NPPs. In the future, these latest technologies will be applied to perform condition monitoring of HV (High Voltage) cables in NPPs.

The Evolution of Medium Voltage Power Cables

This article introduces a survey on the state-of-the-art and the technological progress in medium-voltage (MV) power cables for distribution systems. At present, MV cable insulation technology is characterized by a changeover from the conventional lapped-paper, paper-insulated laminated (PIL), polyvinyl chloride (PVC), crosslinked polyethylene (XLPE) to tree-retardant cross-linked polyethylene (TRXLPE), and ethylene propylene rubber (EPR). The former types have many disadvantages such as high dielectric loss, high operating cost, and low power handling capability.

Power Quality Solutions for Light Rail Public Transportation Systems Fed by Medium-Voltage Underground Cables

In this paper, the combination of a thyristor-switched shunt reactor and a current source converter-based active power filter has been proposed for mitigation of power quality (PQ) problems of Light Rail public Transportation Systems (LRTSs) fed by long medium-voltage underground cables. A case study has been carried out on a typical LRTS to assess the performance of the proposed solution for both capacitive reactive power compensation of underground cables and harmonic filtering of 12-pulse catenary rectifiers. It has been shown by extensive field tests carried out that this solution meets the requirements satisfactorily, thus constituting a complete solution to the PQ problems of LRTS. Conventional PQ solutions have been also assessed, and the corresponding theoretical results are given in comparison with the proposed system.

Stochastic Optimization Model for Electric Power System Planning of Offshore Wind Farms

In the coming years, an extensive development of large-scale offshore wind farms (OWFs) is foreseen in Europe. Electric power systems of OWFs need to be optimized in order to minimize investment and operational costs. This paper proposes a comprehensive decision support model that covers the three key factors that characterize the design of ac electric power systems of OWFs: investment costs of the components, system efficiency, and system reliability. Stochasticity of wind speed and reliability of the main system components are also considered. The main decisions concern the layout and the cross-sections of cable connections and the location of the central collection point for the medium voltage cables. GAMS language has been used to implement the model. It has been validated using a real OWF. An application example has also been carried out to explain the extension of the model to OWFs with dc power systems. The proposed model is useful for basic engineering design and as a decision tool for planning the electric power system of OWFs.