Medium Voltage Cables Research Articles

Novel distribution transformer design for data center applications resilient to transient voltage stresses generated by VCB operation

Dry-type distribution transformers in modern data centres are usually protected by medium voltage (MV) vacuum circuit breakers (VCB). The interaction between VCB, MV cables, and dry-type transformers were studied extensively due to some catastrophic failures of the transformer primary windings, which have had a significant impact on data center up-time. It was observed that VCB switching events can initiate series and parallel resonances in transformer windings, as well as prestrike/restrike overvoltages at transformer terminals. In this work, a novel transformer design concept is presented, based on the shifting of natural transformer series resonance frequencies above the range of the expected frequencies common to operation of VCBs. This novel design has been confirmed by standard manufacturing tests, as well as novel special testing procedures replicating the real-life equipment installations in data centres.

ESTIMATING THE LIFETIME OF MEDIUM VOLTAGE CABLES AND CABLE ACCESSORIES IN AN URBAN ENVIRONMENT

Vital criteria in the renewal programs of the MV (medium voltage) network are the present condition and the estimation of the lifetime of MV cables and cable accessories. In renewal programs, cables and cable accessories are replaced alongside, thus in the paper they are highlighted together. The novel contribution of this paper is that it regards the MV cable network as an asset entity, including both cables and accessories. The paper presents a review of the lifetime estimations of MV cables and cable accessories. As a conclusion, the paper sets a technical lifetime and also estimates the techno-economical lifetime of the MV cable network in Helsinki.

A SCALABLE APPROACH TOWARDS REAL-TIME CABLE RATINGS FOR UNDERGROUND MEDIUM VOLTAGE CABLES

In this paper we describe an implementation of a scalable numerical model to calculate real-time cable ratings for underground medium voltage cables. Input data such as dynamic soil and weather data are automatically retrieved. The model can accommodate different cable types (both single-core and three-core cables) and realistic scenarios such as soil drying and mutual heating of cables.We found agreement within 3°C between model and measured temperatures on a cable circuit connecting a wind park, and agreement within 0.5°C for steady state temperatures calculated according to IEC standard 60287 and our model.

Influence of Thermal Aging and Water Adsorption on XLPE Cables Insulation Mechanical and Physico-chemical Properties

This paper presents an experimental study of the effects of thermal aging on the insulation characteristics of cross-linked polyethylene (XLPE) medium voltage cables. These cables are installed in Algeria where they are designed to withstand high temperatures and humidity levels. Therefore, the main objective of this work is to report experimental results concerning the effects of accelerated thermal aging and water adsorption on the XLPE insulation properties. For this purpose, mechanical characterization and visual observation analysis are performed. In addition, complementary diagnostic measurements, including Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) and Differential Scanning Calorimetry (DSC), are carried out. Furthermore, a correlation between XLPE mechanical, physical, and physico-chemical changes due to thermal and water adsorption is deduced.

Simulating the electric field distribution in medium-voltage cables of cross-linked polyethylene/Cu nanocomposites irradiated by E-beam with reference to the XLPE market

ABSTRACT In this paper, the distribution of the electric field of medium voltage (MV) underground cable insulation made of polyethylene/copper nanoparticles (XLPE/Cu) is investigated. Three methods were established for copper nanoparticles (CuNPs) synthesis such as the alkaline reduction process, electrolysis process and metal displacement reaction. Ultraviolet–visible spectra, X-ray diffraction, and transmission electron microscopy (TEM) were used to classify the obtained CuNPs. The XLPE/Cu films with different CuNP weight content (0%, 1%, 3% and 5%) were subjected to a 3 MeV electron beam (EB) with different doses (0, 15, 20 and 25 kGy). The optimum AC/DC conductivity (AC 2 × 10−3 S m−1, DC 4.63 × 10−2 S m−1) in minimum relative permittivity (2.05) achieved for (XLPE/5-Cu) 25 kGy that is has a CuNP content of 5 wt-% and irradiated at 25 kGy. The electric field distribution in the middle of the (XLPE/5-Cu) 25 kGy sample is maximum uniform than at the top or bottom.

Thermal Monitoring of Underground Medium Voltage Cables Based on Machine Learning Techniques

In this paper a monitoring system for Medium Voltage (MV) cables is presented and simulated. It consists of a classification tool capable of identifying the working temperature of several sections of underground cables starting from the Frequency Response Analysis (FRA). This means that the monitoring procedure can be applied during normal network operation with a low intrusive level and allows the localization of the cable section in the worst working conditions. Starting from this information it is possible to organize the maintenance operations and avoid the occurrence of catastrophic failures. The monitoring method shown in this paper focuses on the relationship between cable temperature and conductor resistance. The online classification of the working temperature is carried out through two different machine learning techniques: Support Vector Machine (SVM) and Complex Neural Network (CNN). Both the single-fault hypothesis and the multiple-fault hypothesis are studied; in the first case the classification results are always higher than 95%, while in the more complex situation the classification rate decreases slightly. To make the simulation procedure as general as possible, Simulink-Simscape blocks, called “Three-phase AC power cable”, are used in this paper.

Environmental Life-Cycle Assessment of a 10 Kv High-Temperature Superconducting Cable System for Energy Distribution

This study conducts a life-cycle assessment for the use of superconducting medium voltage cables based on the AmpaCity cable in Germany. The results are compared with the use of a conventional high voltage cable. For the conventional cable, additional transformer losses are included, while for the superconducting cable the operation of the necessary cooling system is considered. The annual transmitted energy represents the functional unit. The annual loss energy of the HTS cable system is lower than that of conventional cable from a load factor of m <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> = 0.43. This load factor also represents the ecological break-even point. For a load factor of m <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ≥ 0.43, the greenhouse gas emissions and cumulative energy demand of the superconducting cable system are lower than those of the conventional cable system. Thus, if the load is sufficiently high, a superconducting cable can thus be a more environmentally friendly alternative for a future energy system.

Causes of Deterioration in XLPE MV Cables: A Review

Background: Power failure and its related challenges is the cause of low economic development in most developing countries. Power failure is sometimes caused by the constant technical problems experienced by power network components which are being used for electric power transmission and distribution. : Most distribution and transmission substations operate at medium voltages. Medium Voltage (MV) cables provide an efficient means of power distribution to customers. In recent times, medium voltage cables with cross-linked Polyethylene are most commonly used in modern power systems compared to the classical or paper-insulated cables due to their high resistance to Partial Discharge (PD), good electrical properties, and ability to operate at high temperature levels. However, during the process of transmission the deterioration of a cable is inevitable. This reduces the reliability and efficiency of the transmission and distribution process, which is of course a serious concern to power engineers and consumers. : Owing to the challenges experienced from the degradation of MV XLPE cables, it has become highly imperative to investigate the causes of failure in power network cables used for the transmission and distribution of electric power. Methods: A comprehensive review was carried out to understand the fundamental causes of MV XLPE degradation. This was pivotal in adopting the right monitoring technique for the MV XLPE cable. Results: The results show that the major causes of MV network cable faults are attributed to either material related or external causes. It was also discovered that there are different fault rates for different MV cable types. Further discoveries show that the XLPE cables failure rate is minimal compared to the Nordic Oil-paper cables which are said to have an average faults occurrences detection of about 4-5 times higher than that of the XLPE cables. Oil paper cables experience high failure rates and in addition experience substantial defects in their cable joints. Conclusion: This paper presented a comprehensive review of the causes of MV XLPE cable degradation and proffers solutions to the lingering challenges associated with MV XLPE cables.

Lightning Overvoltage Protection of Step-Up Transformer Inside a Nacelle of Onshore New-Generation Wind Turbines

This paper presents an electromagnetic transient analysis of lightning-initiated overvoltage stresses of the step-up transformers installed inside a nacelle of onshore, multi-megawatt, new-generation wind turbines. The increase in the wind turbine (WT) nominal power output, necessitated introducing the step-up transformer into the nacelle. A transformer installed inside a nacelle is subjected to completely different overvoltage stresses from those present if it were installed at the base of the WT tower. This has serious repercussions on its overvoltage protection (i.e., selection and installation of surge arresters) and insulation coordination. Furthermore, the overvoltage protection of medium-voltage cables (inside the tower) is also problematic when considering their length, proximity to the tower wall, and their screen grounding practices, and needs to be tackled in conjunction with that of the step-up transformer. This paper presents detailed models for the various components of the latest-generation WTs, intended for fast-front transient analysis and assembled within the EMTP software package. We further present the comprehensive results of the lightning-transient numerical simulations, covering both upward and downward (first and subsequent) strikes, their analysis, and recommendations for the optimal selection of medium-voltage surge arresters for the step-up transformers installed inside a nacelle.

Assessment and Verification of the Impedance-Frequency Method of Transverse Fault Location in Medium Voltage Cables

The paper presents the results of the assessment and verification of an impedance-frequency method of transverse fault location in medium voltage cables with a local weakening of insulation. A device cooperating with Hameg HM8118 LCR bridge was designed for this purpose. The objective of the method is to measure the input impedances for two different frequencies at one end of the cable with an open state at its other end as well as determining the transverse fault resistance and the distance to the fault location. The proposed method was compared to reflectometry. The effectiveness of the method was confirmed, by numerical and experimental verifications.

К вопросу создания усовершенствованных огнестойких кабелей среднего напряжения и методов их испытаний

The article addresses problems faced in designing medium voltage fire resistant power cables. These cables must operate reliably during 30–40 years under the influence of relatively strong electric field, long-term heating up to 90 оC, and short-term heating up to 130 оC. These cables must also retain serviceability for a limited period of time when subjected to a flame source with a temperature of (830±40) оC. The article presents the key state-of-the-art electrical characteristics of these cables (dielectric strength, dielectric loss angle tangent in the delivery state, etc.), the thermal ageing resistance test procedure, the results of studies of long-term electrical strength (life curves) of fire-resistant cable models, and the fire resistance proper of results test. The article also proposes a mathematical model of the processes that take place in performing fire-resistance tests of medium voltage cables. The obtained modeling results are in qualitative agreement with the experimental data.

Failure Prevention and Malfunction Localization in Underground Medium Voltage Cables

A smart monitoring system capable of detecting and classifying the health conditions of MV (Medium Voltage) underground cables is presented in this work. Using the analysis technique proposed here, it is possible to prevent the occurrence of catastrophic failures in medium voltage underground lines, for which it is generally difficult to realize maintenance operations and carry out punctual inspections. This prognostic method is based on Frequency Response Analysis (FRA) and can be used online during normal network operation, resulting in a minimally invasive tool. In order to obtain the good results shown in the simulation section, it is necessary to develop a lamped equivalent circuit of the network branch under consideration. The standard π-model is used in this paper to analyse sections of a medium voltage cable and the parameter variations with temperature are used to classify the state of health of the line. In fact, the variation of the electrical parameters produces a corresponding variation in the frequency response. The proposed system is based on the use of a complex neural network with feedforward architecture. It processes the frequency response, allowing the classification of the cable conditions with an accuracy higher than 90%.

Estimation of Moisture Content in XLPE Insulation in Medium Voltage Cable by Frequency Domain Spectroscopy

This paper presents a technique to estimate moisture content in XLPE cable insulation through frequency domain spectroscopy. Tan δ over the frequency range 0.2 mHz to 110 Hz is correlated with moisture in the samples. The Havriliak-Negami (H-N) model is applied to investigate the changes in relaxation characteristics due to moisture and the moisture content is estimated through the correlation, which is used to cross validate the results. The comparative study shows that the estimated moisture content through the H-N model provides better accuracy than using tan δ.

Development of a compact very low frequency cosine‐rectangular and damped alternating current voltage generator for insulation testing of medium‐voltage power cables

This study describes the research and design of a novel and compact very low frequency cosine-rectangular (VLF-CR) and damped alternating current (DAC) voltage generator that can be used to test the insulation condition of the medium-voltage power cables. Further, a complete system based on a voltage multiplier (VM), which can be cascaded to increase the output voltage, is proposed to simultaneously realise VLF-CR and DAC voltages for the withstand voltage and offline partial discharge (PD) tests, enabling the test system to be lightweight and efficient. The system principle and operating characteristics associated with the generation of two different voltages are analysed in detail according to the new circuit topology of the proposed generator. Furthermore, oscillation suppression and multiple output units are considered to improve the voltage waveform and provide power supply to the VM module, respectively. A laboratory prototype is also developed and tested. Finally, the integrated generator is used for the PD testing on cables with artificial defects, verifying the practicability of the proposed system.

Influence of heating dynamics of polymer insulation of medium voltage power cables on their performance

Currently, global trends in the development of medium voltage cable networks are aimed at the introduction of cables with cross-linked polyethylene insulation. Such cables are innovative products of domestic cable manufacturers. Most often, single-core cables are laid in parallel in one plane. The article analyzes a number of significant features of cables with cross-linked polyethylene insulation, which affect the thermal regime both in working condition and in emergency operation, among which - the dependence of thermal conductivity and heat capacity of polymer insulation on temperature. Cable throughput, which is determined by regulatory models and average environmental parameters, provides only preliminary information about its heating, useful for comparing its throughput with other cables of the same type. For certain requirements and for innovative cable designs, it is necessary to determine the capacity according to thermal constraints in specific operating conditions. Thermal and electrical asymmetry must also be taken into account in the test conditions of cables intended for laying three single-core cables in the plane. The heating of medium voltage cables with crosslinked polyethylene insulation was studied [1], an experimental assessment of the influence of the distance between the cables on the asymmetry of currents and temperatures was performed using three single-phase single-core cables АПвЭгаПу 1х70/16–35with the same current direction. It is established that in the absence of longitudinal currents in electrically conductive screens, the current in the middle core can be practically taken as a basis. It is proposed to use the method of concentrated heat capacities for the analysis of cable heating by overload current, which allows to significantly reduce the test time in production conditions. The study aims to create a method for determining the allowable current of medium voltage cables in specific operating conditions.

A Comparison of Special Bonding Techniques for Transmission and Distribution Cables Under Normal and Fault Conditions

In this article, a review of the existing special bonding techniques for medium voltage and high-voltage cables is presented. Special bonding techniques have the purpose of reducing sheath currents, thereby limiting copper losses and the reduction of the ampacity of cables. The literature review shows various bonding techniques and how these have evolved over the years thanks to new technologies. Simulations of each technique are performed in MATLAB/Simulink, to compare their strengths and drawbacks both under normal conditions and in the presence of a single-line-to-ground fault.

Incomplete Cross-Bonding in the MV Line. Experience from the Operation of MV Single Cable Lines

Cable lines are one of the basic components of power systems. Medium and high voltage cables mainly comprise a metallic sheath, which is concentric to the main core conductor. There are several operating schemes of such cable lines, which differ in the place of earthing of sheaths and the possible use of the sheaths and/or conductors crossing. The sheaths cross-bonding is typically done in two places of one cable line section, and it allows to reduce power losses. Nevertheless, the use of incomplete sheaths crossing—only in one place on cable route may have economic justification. The paper presents an incomplete sheaths cross-bonding analysis of an existing medium voltage cable line. The results obtained by the mathematical model are validated by measurements taken on 30 October 2019 on an existing cable line. Measurements recorded on a real object for various systems of crossing sheaths are presented. The influence of incorrect sheaths crossing on the measured quantities was shown. In addition, the risk of excess voltage on the sheaths during short-circuits has been verified using a mathematical model.

BPL-PLC Voice Communication System for the Oil and Mining Industry

Application of a high-efficiency voice communication systems based on broadband over power line-power line communication (BPL-PLC) technology in medium voltage networks, including hazardous areas (like the oil and mining industry), as a redundant mean of wired communication (apart from traditional fiber optics and electrical wires) can be beneficial. Due to the possibility of utilizing existing electrical infrastructure, it can significantly reduce deployment costs. Additionally, it can be applied under difficult conditions, thanks to battery-powered devices. During an emergency situation (e.g., after coal dust explosion), the medium voltage cables are resistant to mechanical damage, providing a potentially life-saving communication link between the supervisor, rescue team, paramedics, and the trapped personnel. The assessment of such a system requires a comprehensive and accurate examination, including a number of factors. Therefore, various models were tested, considering: different transmission paths and types of coupling (inductive and capacitive), as well as various lengths of transmitted data packets. Next, a subjective quality evaluation study was carried out, considering speech signals from a number of languages (English, German, and Polish). Based on the obtained results, including both simulations and measurements, appropriate practical conclusions were formulated. Results confirmed the applicability of BPL-PLC technology as an efficient voice communication system for the oil and mining industry.

From the editors

This issue of the Magazine presents articles related to cable diagnostics and technology. Two of them, written by authors from Canada and Australia, summarize experiences and research activities on the use of dielectric response spectroscopy for diagnoses of insulation condition in medium voltage cables. The third article presents work performed in the USA on development of superconducting gas insulated cable system for power transmission at voltage level above 10 kV. Finally, the fourth article, from Norway, raises the importance of mesoscale material effect on the performance of cable joints.

Thermal Distortion of Signal Propagation Modes Due to Dynamic Loading in Medium-Voltage Cables

Temperature variation from dynamic cable loading affects the propagation characteristics of transient signals. The distortion of modal signal components as a function of temperature in a three-phase medium-voltage cable is investigated. The temperature influence arises mainly through the complex insulation permittivity, which has a non-linear relationship with temperature. Near the maximum operating temperature of the cross-linked polyethylene insulation, the propagation velocity increases by 0.56% per degree centigrade but is an order of magnitude less sensitive at ambient temperature. The paper presents modeling results based on cable impedance and admittance matrices obtained from electromagnetic field simulation, taking into account the time-varying temperature distribution in the cable cross-section. The results are verified by applying Rayleigh–Schrödinger perturbation analysis. In the time domain, signal patterns shift when the modal propagation velocities change upon cable loading. Moreover, separation of degenerate modes is observed when the cable phase conductors carry an unbalanced current. The perspectives for exploiting the temperature dependency of signal propagation for pinpointing cable defects and for dynamic rating of underground power cables are discussed.