Sheath Currents Research Articles

Analysis of excessive sheath currents in multiple cable circuits bonded to the same grounding grid

AbstractIn urban power networks, a common practice is to bond numerous high‐voltage cable circuits to a single grounding grid located in underground tunnels, primarily for reasons of installation convenience. In these situations, excessively high levels of sheath currents were often detected during routine inspections, but no electrical faults were found to be responsible. Previous publications ignored the currents flowing through the shared grounding points into the closed sheath loops of different circuits. In this paper, a mathematical model is established for the current circulating among sheath loops of different circuits, and the factors influencing the circulating current were analysed. The abnormal excessive sheath current is demonstrated to be an increase in the circulating current due to the combined effect of electromagnetic coupling and the shared grounding grid. The circulating current depends on the induced voltages which, in turn, depends on the cable layouts and load currents. The effects of these factors are evaluated in various scenarios. The increase of the circulating current is verified in a field case where four electrically healthy cable circuits sharing the same grounding points were found to have abnormal excessive sheath currents.

Simulation of High-Voltage Cable Sheath Current with a Panoramic Digital Tunnel Model

Monitoring cable sheath current is crucial for guaranteeing the secure and efficient operation of high-voltage cable tunnels. In this study, a three-dimensional (3D) panoramic digital model of a tunnel in Beijing is modeled, a parallel cable sheath current simulation model is proposed, and a 3D panoramic visualization system with which the cable sheath current can be simulated online is developed in order to generate early warnings on the sheath current situation. The accuracy of the simulation model is verified and the results are as follows: The difference between the simulated sheath current and the actual measurement is within a maximum margin of 3.71%. A phase sequence optimization strategy can be obtained after calculations based on the simulation model, and an average sheath current reduction of 47.2% can be achieved following the optimization strategy. This study introduces a new approach by which cable sheath current dynamics and real-time warning against anomalies are visualized on a 3D tunnel model. In addition, a sheath current simulation model is integrated in the 3D visualization system developed in this study, and phase sequence optimization strategies for cables with abnormal sheath currents are automatically suggested, which may provide scientific support for auxiliary decision-making in real-time cable operation.

Important insights into steady-state calculations of voltages and currents in complex sheath bonding configurations of single-core cables

Ensuring effective sheath bonding holds paramount importance within underground single-core cable systems, as a misjudged selection can precipitate cable failures or jeopardize human safety. The foundation of a sound bonding design rests upon a meticulous examination of induced voltages and currents on the cable sheaths. To address this, guidelines and numerical techniques have been introduced, applicable to both handheld calculators and sophisticated software tools. However, their precision hinges on specific features and solving techniques, aspects that remain unclarified in current literature. This paper delves into a comprehensive exploration of sheath current and voltage calculations at steady-state in single-core cable systems. A rigorous benchmarking exercise is undertaken, comparing bonding-design simulation results derived from various computation methods. Moreover, the study scrutinizes the impact of several parameters, shedding light on crucial insights. Building upon this, the paper proposes modelling recommendations for the design and assessment of sheath bonding configurations in complex power cable installations.

Time-domain fault location method for medium voltage cables using sheath currents

Time-domain fault location method for medium voltage cables using sheath currents

Analysis of Increased Induced Voltages on the Sheath of Double-Circuit Underground Transmission Lines Guaranteeing Ampacity

This paper quantifies and discusses the increase in induced voltage on a sheath due to changes in duct banks in terms of type and dimensions along an underground transmission line, guaranteeing the ampacity required for a project. The four most common duct banks in double-circuit underground transmission lines with phase transposition were considered in this study, along with two special cross-bonding techniques: continuous cross-bonding (CCB) and sectionalized cross-bonding (SCB). These techniques aim to reduce sheath currents and enhance the distribution of the induced voltage on the sheath. The analysis considers two distinct scenarios in which the profile of the induced voltage is calculated: the first one accounts for underground obstructions, intersections with important traffic avenues, and ground with high excavation costs that force changes in the duct bank dimensions and configuration, which is the most exact and realistic case. The second one solely considers one typical configuration of a duct bank along the route. This last scenario is normally applied to calculate the induced voltage when an underground transmission design is required. The results show that when installing cables at a greater depth, it is imperative to increase the distance between them to guarantee the ampacity. The induced voltage on the sheath will rise as the distance increases. Furthermore, the results reveal that instead of calculating the induced voltage by considering the scenario that is exact and most like a real case, it is enough to calculate following the second scenario and then add a scaling factor according to each duct bank configuration.

Circulating sheath currents in flat formation underground power lines

Three-phase underground power lines can induce voltages and currents in their recover sheaths. The sheath induced currents are undesirable and generate power losses and reduce the cable ampacity whereas the induced voltages can generate electric shocks to the workers that keep the power line. This means that when dealing with three-phase underground power lines, it is very important to know the sheath currents (called circulating currents) that can circulate throughout the sheath of the cables. It is very useful to know their values. The values of the circulating currents depend on different parameters, such as the sheath grounding system, the geometry of the cables, the gap between them, etc. In this work, different geometries of flat configuration underground three-phase power lines have been studied. For each geometry it has been computed the sheath circulating currents in each cable of the power line

Effects of the circulating sheath currents in the magnetic field generated by an underground power line.

In this paper the influence of the metallic sheath in the magnetic field generated by underground single core power cables is studied. When dealing with underground power cables, sheath circulating currents can be induced. These currents produce power losses in the sheaths and decrease the ampacity (capacity of carrying current) of the cables. The circulating sheath currents generate a magnetic field that adds to the cable magnetic field. In this paper the modification of the total magnetic field is studied.

Design of fault location algorithm based on online distributed travelling wave for HV power cable.

To tackle the challenge of localization failure due to traveling wave dispersion during a mid-line fault in a long high-voltage cable, this study conducts an in-depth analysis of the refraction characteristics of the cumulative three-phase sheath currents at cross commutation points and direct grounding points, facilitated by detailed theoretical derivation. We introduce the novel concept of a 'first traveling wave attenuation index,' which quantifies the peak of the initial faulted traveling wave. We strategically place distributed traveling wave detection devices solely in the first direct grounding box of each cross-interconnected main section, effectively segmenting the line into five distinct zones. These zones are identified based on the unique transient polarities exhibited by the first traveling wave of fault current at each impedance mismatch point along the cable. To overcome the issue of weak traveling wave signals collected at the first end measurement point, we propose an innovative peak detection method for the first wave. This method harnesses the power of empirical wavelet transform (EWT) and multi-resolution singular value decomposition (MRSVD), providing a significant boost in ranging accuracy compared to traditional wavelet methods. Simulation results validate the efficacy of our proposed fault location method, which accurately pinpoints the fault section by contrasting the polarity of the first wave peak of the sheath current detected at each measurement point. Notably, our method ensures safety and convenience as the equipment does not require direct contact with high voltage.

Steady-state electrical properties and loss reduction measures analysis for 220 kV tunnel cable system

This paper aims to reduce the tunnel cable systems’ power loss using different grounding and loss reduction methods. The submarine tunnel power cable project at Ningbo–Zhoushan in China is considered here as a case study. The cable is divided into micro-element segment, and their nodal admittance matrices are established using the telegraph equation. This approach helps to avert the prerequisite of diagonalizing the propagation matrix. For the analysis of induced voltage and circulating current in the cable sheath, the cascading algorithm is used while considering the presence of grounding node. It is found that the mixed cable laying methods and different lengths between minor sections result in a high circulating sheath current. This study finds that coordinating the amplitude and phase angle of series impedance inserted at the cross-bonded joint can reduce the cable system's power loss.

A 2D FEM Model for Impedance and Loss Calculation of Armored Three-Core Cables With Inclusion of 3D Pitching Effects

A method is introduced for impedance and loss calculation of three-core power cables with steel armoring, based on 2D finite element method (FEM) computations. The pitching effect of the cores and armor wires is taken into account by 1) enforcing identical net current on the wires, and by 2) introducing a fictitious non-conductive material between the wires having a complex-valued permeability. The permeability is obtained by considering the effect of the pitching on the total energy in a volume slab around each wire, which involves the solving of a small 2D FEM problem in an optimization loop. The method permits to calculate the positive-sequence and zero-sequence impedance on cables, induced sheath currents, and losses in cores, sheaths, and armor. The method is validated against published results using a full 3D FEM model. The calculations are fast, requiring only a few seconds of computation time. The procedure is simplified by use of pre-calculated look-up tables for the fictitious material permeability value.

An Online Data-Efficient HV Cable Fault Localization Approach Via an AI-Enabled Recognition of Modal Wavefront in Sheath Current

Precise permanent fault localization is an important task for fast power restoration in underground HV cable systems. Among online fault localization methods, the fault localization accuracy strongly relies on the type and volume of measurement, and there is always a tradeoff between localization accuracy and measurement cost. To balance the tradeoff, a data-efficient HV cable fault localization framework is proposed in this paper. First, the fault characteristics of sheath currents are analyzed in modal mode, and compared with the conventional core conductor measurements. It is discerned that the sum of three phases sheath currents has similar characteristics, which can be measured by fewer sensors in a lower rating as the replacement of the conventional core conductor measurements. Second, the challenges of recognizing the wavefront arrival in the sheath are presented, and a Convolution Neural Network is introduced for the localization purpose. The proposed approach can realize high localization accuracy with low-cost measurement, and keep consistent performance under various scenarios through limited training datasets. A case study has been carried out using PSCAD/EMTDC platform to validate the effectiveness and feasibility of the proposed approach, followed by a discussion on the results.

Fault Diagnosis of HV Cable Metal Sheath Grounding System Based on LSTM

At present, the metal sheath of high voltage (HV) cables generally adopts the cross-bonded grounding method, which brings many types of faults and challenges the monitoring and diagnosis of the operation status of the cables. In order to effectively diagnose various types of faults in the metal sheath grounding system of HV cables, this paper proposes a fault diagnosis method for the metal sheath grounding system of HV cables based on long and short-term memory (LSTM). Firstly, the grounding system model of HV cable metal sheath is established. Secondly, the sheath currents of four faults are analyzed. Based on the sheath current amplitude ratio and phase difference of the same loop and the same grounding box, 14 feature vectors reflecting the operation state of the metal sheath grounding system are constructed. Then, the operation state of 18 kinds of metal sheath grounding systems is simulated, and the fault database is established. Finally, the LSTM algorithm is used to accurately identify the fault of HV cable grounding system. The results show that the LSTM algorithm can effectively diagnose and identify the faults of the HV cable metal sheath grounding system, and the accuracy rate is 100%.

Experimental Validation of Ultra-Shortened 3D Finite Element Models for Frequency-Domain Analyses of Three-Core Armored Cables

Recently, large offshore wind power plants have been installed far from the shore, using long HVAC three-core armored cables to export power. Its high capacitance may contribute to the appearance of unwanted phenomena, such as overvoltages or resonances at low frequencies. To adequately assess these problems, detailed and reliable cable models are required to develop time-domain/frequency-domain analyses on this type of cables. This paper presents, for the first time in the literature, an assessment on the performance of 3D finite element method-based (3D-FEM) models for developing frequency-domain analyses on three-core armored cables, confronting simulation results with experimental measurements found in the literature for three real cables. To this aim, a simplified ultra-shortened 3D-FEM model is proposed to reduce the simulation time during frequency sweeps, through which relevant aspects never analyzed before with frequency-domain 3D-FEM simulations are addressed, such as total losses, induced sheath current, magnetic field around the power cable, positive and zero sequence harmonic impedances, as well as resonant frequencies. Also, a time-domain example derived from the frequency-domain analysis is provided. Remarkable results are obtained when comparing computed values and measurements, presenting the simplified ultra-shortened 3D-FEM model as a valuable tool for the frequency-domain analysis of these cables.

FEM calculation of the sheath currents in 110Kv cables

Cross-bounding system is the most commonly used method to reduce sheath current. The sheath currents of cables are induced by currents in the cable cores, and sheath currents also increase when the loads increase gradually. The current carrying capacity of the cable is limited by sheath currents further, owing to their thermal effect, which is not favourable to the heat dissipation and operation of the cable. The sheath currents of single cable and the sheath currents of single loop cables in the case of cross-bounding interconnection are calculate by FEM including configurations of three phase cables in square triangle, horizontal and right angled triangle with the same distance between the cable cores. The results show that the right triangle arrangement should be preferred in order to minimize the sheath circulation.

Characteristic Analysis of the Outer Sheath Circulating Current in a Single-Core AC Submarine Cable System

The single-core alternating current (AC) submarine cable can be provided with an outer sheath that is firmly grounded on both ends of the cable. The circulating currents of the outer sheath are generated to be almost as large as the conductor current. The outer sheaths, which have different structures and properties, generate unwanted losses, asymmetric distribution of circulating current, and extra heat in the single-core AC submarine cables. The formation mechanism of the circulating currents in the submarine cable sheath and armoring is analyzed from the perspective of electromagnetic shielding using electromagnetic transient theoretical analysis, simulation calculation, and field experiments. Equations for calculating the circulating currents of the sheath and armoring are proposed, and influences of these relationships that include the different material characteristics of the sheath and armoring are analyzed. The influence factors, which include different levels of magnetic armoring permeability, resistivity, and ground resistance of the outer sheath, can affect the symmetrical distribution of the circulating current in the outer sheaths. We propose using the phase differences to determine the material properties of each metallic section in the submarine cable.

Active Cable Sheath Current-Based Protection Scheme for an Offshore HVAC Wind Transmission System

Due to the complex sending terminal structure of offshore wind transmission systems, conventional on-line monitoring-based protection cannot work well. The electromagnetic transient faults are also difficult to locate due to a short time span with fast characteristics that are affected by various power electronic devices and control algorithms. To solve the aforementioned problems, a new offshore wind transmission line protection scheme based on active detection of submarine cable sheath current is proposed. This method uses a normalized coding cooperation to realize the risk levels and failure locations of the transient defects, and then the sheath currents become a characteristic input data source, which can build a clear system protection boundary. Case studies using MATLAB and ATP simulations are carried out, where three types of transient faults represented by sheath currents are studied, i.e., loss of electrical continuity of grounding device, short circuit of segmented metal sheath of cable joint, and water immersion of junction box. Testing results illustrated that the proposed method could achieve fast fault detection and precise fault location of the electromagnetic transients. Moreover, compared with conventional wind farm protection techniques, it only needs to add few signal injection modules with high sampling frequency into the submarine optical fibers.

Insulation Monitoring and Diagnosis of Faults in Cross-Bonded Cables Based on the Resistive Current and Sheath Current

To reduce the induction voltage in the metal sheath, cross-bonded grounding is a widely adopted method in long-distance high voltage power cables, but this introduces challenges to the online monitoring and diagnosis of cable insulation. An online monitoring and diagnosis method for cable insulation based on the analysis of resistive current is proposed in this paper. A theoretical formula for the separation between leakage current and sheath current was derived, and a theoretical formula for the separation between resistive current and leakage current was obtained. An equivalent circuit of a three-phase long distance cable was established using the MATLAB/Simulink platform. The simulation was used to model three typical faults: cable insulation deterioration, open sheath circuit faults, and the sheath breakdown of intermediate joint faults. The results show that the method can be used to monitor each cable’s insulation state under metal sheath cross-bonded, and it also can determine the open sheath circuit faults and fault joint location using the measured current values by the corresponding current sensors increases significantly. Based on the analyses, diagnostic criteria are established for different types of cable faults and fault locations.

Experimental validation of ultra-shortened 3D finite element electromagnetic modeling of three-core armored cables at power frequency

Due to recent advances, the numerical analysis of submarine three-core armored cables can nowadays be developed through the finite element method (FEM) in a small slice of the cable. This strongly reduces the computational burden and simulation time. However, the performance of this ultra-shortened 3D-FEM model is still to be fully assessed with experimental measurements. This paper focuses on this validation for an extensive variety of situations through the experimental measurements available in the specialized literature for up to 10 actual cables. In particular, it deals not only with relevant calculations at power frequency, like the series resistance and inductive reactance or the induced sheath current, but also with other aspects never analyzed before through 3D-FEM simulations, such as the zero sequence impedance, the magnetic field distribution around the power cable, as well as side effects due to the nonlinear properties of the armor wires. All this considering different armoring and sheath bonding configurations. Results show a very good agreement between measured and computed values, presenting the ultra-shortened 3D-FEM model as a suitable tool for the analysis and design of three-core armored cables, and opening the possibility to reduce the need of extensive experimental tests in the design stage of new cables.

Simulation Analysis of Current in Metal Sheath of Power Cable Considering Temperature Characteristics

This paper proposes a cable broadband circuit model considering the temperature characteristics. This paper establishes the equivalent circuit model of the cross-bonded power cable and proposes the broadband parameter calculation method considering the temperature. The cable parameters are calculated by the vector fitting method. Based on the operating current and cable temperature data of the cable on a certain day, the simulation analysis of the cable sheath current considering the temperature is carried out using Simulink. Finally, the simulation results show that the cable sheath current is significantly affected by temperature.

Identification of defects in HV cable sheath based on equivalent impedance spectrum characteristic coding

In recent years, on-line monitoring and condition-based maintenance of HV cable systems have been extensively investigated to prevent cable fault. By analyzing cable sheath current, sheath defects can be identified. A new method of cable sheath defect identification is proposed in this study on the basis of the coding of the equivalent sheath impedance spectrum characteristics. In the proposed method, the sheath defects are identified by calculating the equivalent sheath impedance spectrum on the basis of the detected currents of the on-line sheath monitoring system. The equivalent circuit model of cross-bonded (CB) HV cable for sheath defect identification is constructed. The three-phase circuit model is decoupled to solve each phase independently in accordance with the CB scheme of the cable sheath. The equations of solving the equivalent sheath impedance are deduced. The criteria of sheath defect identification are proposed using the coding of characteristic parameters of the equivalent sheath impedance spectrum. Finally, a simulation model of the HV cable is built in the Simulink software to simulate 12 types of sheath defects. The simulation results have proven the correctness and efficiency of the proposed method.