HV Cables Research Articles

Effect of Extremely Cold Weather and Shrinkage Stress on Interfacial Discharge Between Epoxy and Silicone Rubber in HV Cable Terminations

Gas insulated switchgear (GIS) cable terminations have higher failure rates at low temperatures. In this study, a 110-kV full-size GIS cable termination model is developed to investigate this failure mechanism. The coupling characteristics of the thermal and mechanical stress distributions in the termination are analyzed. A pressure-controlled interfacial discharge experimental setup is established, and the partial discharge characteristics and damage features of tracking are investigated for different interface pressures. At temperatures as low as -40 °C, pressure distortion is observed at the interface between the stress cone and epoxy bushing, such that the interface pressure can drop below 0.1 MPa. The tracking at low interface pressures has a higher growth rate and fewer and finer branches than at high interface pressures. The accumulated damage area tends to develop from the epoxy resin towards the silicone rubber. As the interfacial pressure drops, the characteristics of the partial discharge change from those for internal discharge type to those for coexistence between internal discharge and surface discharge. Finally, a scheme for tracking development and damage under different interface pressures is proposed to explain the failure process for cable termination in an extremely cold environment, which is highly consistent with the characteristics of actual failure.

Effects of Mechanical Stress on Insulation Structure and Performance of HV Cable.

Mechanical stresses generated during manufacturing and laying process of high voltage cables can result in degradation of insulation properties, affecting the stable operation of the transmission system. Traditional test methods for testing the effect of mechanical stress on the insulation properties of polyethylene still have some shortcomings to be explored and it is able to explain the changes of the insulation properties of polyethylene under mechanical stress from a microscopic perspective. In order to further study the effect of stress on the insulation properties of polyethylene, microstructural changes, the breakdown field strength, conductivity and charge distribution of polyethylene at different elongation rates are investigated by a combination of experimental and molecular dynamics simulations. The results show that the increase in stress leads to a decrease in crystallinity and microcrystalline size of the material decrease. The untwisting and orientation of the polyethylene molecular chains during the stretching process can create cavities, resulting in an uneven sample distribution and thickness reduction, leading to a reduction in the breakdown field strength. Meanwhile, some crystal regions are transformed into amorphous regions. The loose amorphous regions facilitate the directional migration of carriers, resulting in the increase of conductivity. When the elongation ratio is smaller, the distance between the molecular chains increases and the trap depth of the specimen becomes shallower. This facilitates the migration of ions and electrons and increases the rate of decay of the surface potential. When the stretch is further increased, new traps are created by broken molecular chains to limit the movement of charges, decreasing the decay rate of the surface potential and reducing the insulation properties of the polyethylene. Meanwhile, the molecular dynamics model of semi-crystalline polyethylene was developed to observe the microstructure and energy changes during the stretching process. The conclusions in terms of tensile tests were verified from a microscopic perspective.

FoSSA Optimization-Based SVM Classifier for the Recognition of Partial Discharge Patterns in HV Cables

In order to enhance the classification accuracy and the generalization performance of the SVM classifier in cable partial discharge (PD) pattern recognition, a firefly optimized sparrow search algorithm (FoSSA) is proposed to optimize its kernel function parameters and penalty factors. First, the Circle-Gauss hybrid mapping model is employed in the population initialization stage of the sparrow search algorithm (SSA) to eliminate the uneven population distribution of random mapping. Sparrows tend to fall into local extremums during the search process, while the firefly algorithm has a fast optimization speed and strong local search ability. Thus, a firefly disturbance is added in the sparrow search process, and the fitness value is recalculated to update the sparrow position to enhance the sparrow's local optimization ability and accuracy. Finally, based on the SSA, a dynamic step-size strategy is adopted to make the step size dynamically decrease with the number of iterations and improve the accuracy of convergence. Six benchmark functions are employed to evaluate the optimization performance of the FoSSA quantitatively. Experiment results show that the recognition accuracy of the PD patterns using the SVM optimized by the FoSSA could reach 97.5%.

Electric Field and Temperature Simulations of High-Voltage Direct Current Cables Considering the Soil Environment

For long distance electric power transport, high-voltage direct current (HVDC) cable systems are a commonly used solution. Space charges accumulate in the HVDC cable insulations due to the applied voltage and the nonlinear electric conductivity of the insulation material. The resulting electric field depends on the material parameters of the surrounding soil environment that may differ locally and have an influence on the temperature distribution in the cable and the environment. To use the radial symmetry of the cable geometry, typical electric field simulations neglect the influence of the surrounding soil, due to different dimensions of the cable and the environment and the resulting high computational effort. Here, the environment and its effect on the resulting electric field is considered and the assumption of a possible radial symmetric temperature within the insulation is analyzed. To reduce the computation time, weakly coupled simulations are performed to compute the temperature and the electric field inside the cable insulation, neglecting insulation losses. The results of a weakly coupled simulation are compared against those of a full transient simulation, considering the insulation losses for two common cable insulations with different maximum operation temperatures. Due to the buried depth of HV cables, an approximately radial symmetric temperature distribution within the insulation is obtained for a single cable and cable pairs when, considering a metallic sheath. Furthermore, the simulations show a temperature increase of the earth–air interface above the buried cable that needs to be considered when computing the cable conductor temperature, using the IEC standards.

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.

Energization of windfarm infrastructure as part of microgrid based restoration

Studies on windfarms have focussed on their integration under normal and short duration weak grid conditions. With their increasing penetration at the same time that there is increasing disaster related blackouts, there is need to investigate their role during restorative conditions assuming a bottom-up approach in which local generation is used to re-energize the network. This is commonly referred to as microgrid based restoration. Wind Energy Conversion Systems (WECSs) are not blackstart units thus they require cranking from designated blackstart units in order to play a role in re-energizing the network. Prior to the WECSs being cranked up, the windfarm infrastructure, which includes underground cables, reactors (applicable to long HV cables), WECS transformers and yaw-pitch motor system, needs to be re-energized. This paper investigates the energization of different components within the windfarm considering availability of a single medium sized (5 MVA) blackstart synchronous source. Firstly, diesel based generation (DBG) will be used to study the different challenges associated with the energization of the different components considering different re-energization sequences after which thermal (TBG) and hydro (HBG) based generation will be used for comparison. High frequency modelling has been considered for static network components such as transformers, cables and reactors while dynamic models have been considered for rotating network components such as generators and motors. Excitation limiters have been included in the generation systems in order to obtain a near accurate response on energization of the different components. Discussion is provided on the different factors that influence the success of energization. Analysis is also provided on the effect of size of the excitation source and influence of the synchronous condenser which has been considered as part of the windfarm. Resonance, using a simple empirical formula, is briefly discussed as a factor in influencing and determining the energization sequence. A typical windfarm layout has been considered in this study with MATLAB/Simulink being used as the simulation platform due its flexibility in modelling.

A Low-Cost DAC System for On-Site PD Testing of HV Cables Based on ACRF and Capacitor Bank

Damped AC (DAC) technique has proven to be one of the most acceptable techniques for on-site partial discharge (PD) detection. However, the expensive high-voltage switches, used in the DAC system, impede the further spread of the technique. This letter presents a low-cost DAC system for PD test of high-voltage cables. The DAC voltage is generated by the frequency-tuned series resonant technique and two cost-effective switches which have relatively low rated voltage. Since the system uses a capacitor bank, high input power is not necessary. This letter describes the design and implementation of the system. An experimental study and an on-site test validate the effectiveness of the proposed system.

The Dimensional Approach in the Design and Qualification Tests of AC and DC HV Cables: The Occhini Approach Revisited

The expression of the dimensional effect proposed by Enrico Occhini in the 70's is still commonly used to transform cable insulation breakdown and life data, obtained through laboratory accelerated testing on cable models to those relevant to full-size cables. However, the Occhini model does not take into account the dependence of dielectric strength on insulation thickness. Indeed, the balance between the insulation losses generated in the volume and the dissipated energy which occurs through cable insulation surface affect dielectric strength and this can have an impact in cable characterization and testing. This paper presents a modification of the statistical expression of dimensional effect, through the introduction of the dependence of dielectric strength on insulation thickness due to thermal instability. The modelling study carried out in the paper, and the preliminary validation of the revised model, show that the derived correction terms can influence both the design field and the life line estimation for cables having different insulation thickness and lengths. Eventually, it is highlighted how much and in which way the dimensional effect issues can impact on the results of prequalification and type tests made according to the present IEC and CIGRE specifications.

Optimization of the electrical connection topology of a tidal farm network

This article presents an approach to optimize the electrical connection topology of tidal energy converters in a tidal farm. The methodology is based on a genetic algorithm (GA). The main purpose is to present a technique of coding to find the best electrical connection topology of the tidal farm network. The optimization model takes into account the energy loss in the submarine cables. The model gives as its output the optimal number of turbine clusters connected to each offshore substation, the number of turbines in each cluster, the cross-section of MV and HV cables, the connection design for each cluster of turbines as well as the number and the locations of the offshore substations. A particle swarm optimization algorithm (PSO) is used to confirm the results obtained with the GA. The optimization approach is applied to the Fromveur Strait (France).

The Effects of Seasonal Factors on Life and Reliability of High Voltage AC Cables Subjected to Load Cycles

In previous articles by this author, a procedure was set up and validated for life and reliability evaluation of HV cables subjected to the time-varying electro-thermal stress associated with the typical daily cycles of conductor current. In this paper, the procedure is improved to account for the effects of seasonal factors, by taking for every season different daily cycles of cable current, as well as different values of thermal resistivity and temperature of the soil. The procedure is applied to a case-study HVAC XLPE-insulated cable. The results show that the fraction of life and reliability lost by cable insulation is strongly dependent on seasonal factors, in particular on soil resistivity and temperature, and the vast majority of cable life and reliability is lost within the hot (summer) season.

Characterization of Silicone Oil used in HV Cable Sealing Ends

Existing polymeric cable sealing end (CSE) typically contain silicone oil as an insulating liquid between the cable core and the outside of the CSE. In recent years, a number of performance issues have been identified with silicone oil filled CSEs. To understand the reasons for the observed behavior of silicone oil, a range of characterization methods are reviewed, to allow investigation of the influence of moisture, potential chemical contaminants and the compatibility with CSE materials. The water content of all the oils studied was above the limit given in IEC 60836 (2015) for new silicone oil. There is no significant difference between the moisture content, whether the oil is used in CSE or new. Neither did the Fourier infrared (FTIR) spectroscopic analysis show difference between the oils. Preliminary results from gas chromatography connected to mass spectrometer (GC/MS) clearly show structural differences between unused expired oil and new oil samples particularly that species suspected to be cyclic siloxane appear to be present in the expired sample.

Modelling of MV and HV Cable Lines

Modelling of MV and HV Cable Lines

Analytical Determination of Current Distributions in Shielded HV Cables and Ground Systems of Electric Automotive Power Trains

Shields are commonly used to reduce conducted and radiated emissions in automotive electrical drive applications. Concerning a cable shield, the nonideal parameters due to its structure determine the current distribution between the shield and the ground. These currents are decisive for magnetic field emissions. It is important to quantify the current distribution between the shield and the ground due to strict requirements for low-level field emissions. The geometrical arrangement of the cables with respect to other components and the ground plays a crucial role in the estimation of shield and ground currents. An analytical approach to obtain shield and ground currents as a function of inner conductor current is presented.

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.

Countermeasures of Zero-Missing Phenomenon in (E)HV Cable Systems

Zero-missing is a phenomenon in shunt-compensated cable systems, in which the current through the line breaker does not cross the zero point for several cycles. This paper deals with a thorough investigation on countermeasures of the zero-missing phenomenon in transmission systems and determines the requirements, benefits, and risks of applying each method. The effectiveness of countermeasures is studied on a simulated cable project with different cable lengths in an actual grid model of the Dutch 380 kV transmission system. Results are analyzed based on three criteria related to the IEC standards and the Dutch grid code. In addition, the switching sequence of circuit breakers is specified to maximize the effectiveness of the countermeasures. A statistical switching analysis is performed for the insulation coordination study since the application of some countermeasures increases the probability of high transient switching overvoltages. Moreover, the closing variation threshold of circuit breakers is calculated as a function of the circuit impedance and the shunt compensation degree.

Effect of Thermal Aging on the Electrical Characteristics of XLPE for HV Cables

This work highlights the effect of thermal aging on the commonly studied electrical properties of cross-linked polyethylene devoted to be used in the HV cables insulation. The obtained results show the great effect of thermal aging on the electrical properties of the polymer especially for aging temperatures situated above the melting temperature of the material. On the other part, it is believed that the degradation degree is related to the applied temperature. Hence, the degradation process has been occurred in fast way as the temperature is higher. The almost deterioration markers in the dielectric behavior can be attributed to both the chemical and physical changes. The main chemical changes are the bulk thermo-oxidation degradation leading to the enhancement of dipoles and carbonyl groups concentration which disturbs the dielectric properties leading to the failure of the material. The physical changes result from the changes in the ratio of crystalline to amorphous parts in the material. Both changes have something to do it together in the life time reduction of the XLPE insulation.

Online Monitoring and Diagnosis of HV Cable Faults by Sheath System Currents

Cross-bonded metal sheath connection is applied in sectioned single-core power cables to reduce or eliminate the voltages that are induced in the sheath over long distances. However, cross-bonded cables present an opportunity as well as a challenge for online measurement and diagnosis of cable conditions. In this paper, a methodology to identify cable sheath faults through analysis of the sheath system currents in a cross-bonded cable system is presented. First, a numerical model is established to simulate the sheath currents in cross-bonded cable systems. Second, analyses of several faults, which happen frequently with serious consequences, are presented on the basis of current measurement at the link cable. Simulations of normal and fault conditions are given to determine the feasibility of fault diagnosis. A case study using field data from a cable tunnel in China considering the normal condition is presented to verify the numerical model. Results in normal condition show good consistency with field data with error less than 5%. Simulation results of fault conditions show that analysis of readings from six current sensors can distinguish different fault types and fault positions using the method proposed. Based on the analyses, criteria are established for sheath loop fault type diagnosis.

Editorial: Partial discharge measurement and interpretation

In the past two decades I have been honored by the Editor-in-Chief, Professor Reuben Hackam, to serve as Guest Editor of the IEEE Transactions on Dielectrics and Electrical Insulation on the subject of partial discharges for three times and more precisely in 1998 (Vol. 5, Issue N0.3), 2008 (Vol. 15, Issue No. 6) and for the present Issue of February 2017 (Vol. 24, No. 1). These commitments, with a separation time of ten years, gave me the opportunity to follow with continuity the progress of scientific knowledge on the issue of great interest of partial discharges. In this progress, an increasingly shared element is represented by the fact that partial discharge (PD) detection, measurement and classification constitute today an important tool for quality assessment of insulation systems utilized in HV power apparatus and cables. In fact, the patterns obtained with PD detectors may contain important characteristic features useful to identify the defects of the faults involved.

Zasady doboru i stosowania ograniczników przepiec w liniach kablowych WN

Zasady doboru i stosowania ograniczników przepiec w liniach kablowych WN

Zagrozenia przepieciowe linii kablowych WN

Zagrozenia przepieciowe linii kablowych WN