XLPE Cables Research Articles

A Study on the Space Charge Characteristics of AC Sliced XLPE Cables

Cross-linked polyethylene (XLPE) cables are widely used in power systems due to their excellent electrical and mechanical properties. Space charge is an important factor in cable aging and breakdown. At present, XLPE cables in the power system are mainly ac XLPE cables, so it is necessary to study the space charge characteristics under ac voltage. In this paper, an electro-thermal aging platform for XLPE cable was built and a 10-kV cable was aged. XLPE slices were obtained from the aging cable and ac space charge experiments were carried out on the XLPE slices at different stages of aging. The trap energy levels were calculated and the effects of different electric fields on the ac space charge characteristics were compared. The experimental results show that the trap energy level increases with the aging process and the non-averaged electric field is a necessary condition for charge migration.

Numerical Analysis of Thermo-Electric Field for AC XLPE Cable in DC Operation Based on Conduction Current Measurement

With rapid development of renewable energies and dc loads in distribution grids, converting ac distribution grids into dc operation has become a promising method to utmost utilize the present ac distribution grid in large cities. This paper reports a numerical investigation of thermo-electric fields of a three-core 10-kV ac cross-linked polyethylene (XLPE) cable and cable joint in the dc operation. DC conduction current for the XLPE and silicon rubber (SiR) was evaluated at different temperatures and electric fields. The thermo-electric field simulation was performed by COMSOL Multiphysics according to the measured data. The obtained result showed that the threshold of XLPE at 65 °C is 3.1 MV/m, whereas the threshold of SiR at 60 °C is 2.2 MV/m. It was found that the maximum electric field of the cable operated in dc voltage up to 12.5 kV was below the threshold of 3.1 MV/m. The maximum electric field in the cable joint appeared at the high voltage screen tube, whereas the maximum electric field at the high voltage screen tube was less than the threshold of SiR at a temperature difference of 25 °C. Moreover, the maximum transmission power of the three-core 10-kV AC XLPE cables running at the ±10-kV bipolar dc operation was 1.36 times of the power in the ac operation. It was elucidated that the three-core ac 10-kV XLPE cable could be operated reliably at ±10 kV in bipolar dc topology with sufficient safety margin. It is beneficial to upgrade the ac distribution grid to the dc distribution grid.

Space charge characteristics in 160 kV DC XLPE cable under temperature gradient

A temperature gradient (TG) will be formed across a high voltage cable insulation resulting from Joule heat in the conductor. Therefore, it is important to measure space charge profiles in a full-scale cable under TG. In this study, an improved pulsed electro-acoustic system used for coaxial cables was established by injecting nanosecond pulses into the outer semi-conductive layer. An induced current heating and a water circulation system was used to regulate the temperatures of a cable conductor and the the outer semi-conductive layer. By this method, a TG between the cable conductor and the outer semi-conductor layer was controlled and set at various temperatures and the TG was formed across the radial insulation so as to avoid the influence of heat from the cable conductor. Space charge profiles in a 160 kV DC coaxial XLPE cable was measured under 10 kV/mm stress with polarity reversal. According to CIGRE TB496, the load cycle operation of the cable under 15 kV/mm with temperature difference of 40°C was performed for 48 and 72 h, including cable conductor cooling and heating. The results show that there is little space charge injection in the bulk at room temperature, but some remnant hetero-charges accumulates near the outer semi-conductive layer during voltage reversal. Under TG, hetero-charges appear near the outer semi-conductive layer on the low temperature side and hetero-charges increased with increasing TG. In addition, the hetero-charges increase during cable conductor heating and decrease during cooling.

Electric Field and Current Density Characteristic of Contaminated Solid Insulator

The performance of an insulator may degrade over a time period. One of the main factor is due to presence of contamination on the insulator which leads to flashover, corona and damages the insulator. This study focuses on overhead XLPE cables used in distribution system with voltage rated 33kV. The contaminants are varied in order to study the behaviour of electric field and current density of the XLPE insulator. Contaminant used in this study is sodium chloride, lead and rain water. Quickfield software was used to draw and simulate the contaminated cable. The electrical conductivity of cable and contamination was used to represent every layer of the drawing. From the result produce, analysis on the electric field and current density of a contaminated and non-contaminated insulator was made. Analysis shows that the contaminated insulator has higher electrical field and current density compared to non-contaminated insulator. When the electrical conductivity is high, the electric field is the lowest and the current density is the highest in the insulator. Whereas, the XLPE insulator with sodium chloride contamination has the highest current density followed by rain water and lead because the conductivity of sodium chloride is the highest. The electric field strength of lead is the highest followed by rain water and sodium chloride.

Experimental Investigation of Different Characterisitcs of PVC and XLPE Cables

Experimental Investigation of Different Characterisitcs of PVC and XLPE Cables

Space Charge and Electric Field Analysis on Contaminated XLPE Insulator

<p>Nowadays, XLPE cable has been widely used because it has better resistance than other cables. XLPE insulation has unique features including a high dielectric strength and high insulation resistance. A lot of researches based on hardware and software have been conducted to prove the effectiveness of XLPE cable such as AC and DC applications and Space Charge Distribution measurement under HVDC at High Temperature. This research focused on analysis of space charge and electric field on XLPE cable with effect of non-uniform contamination layer by using Quickfield Software. Non-uniform contaminations have been applied along XLPE cable using Arsenic Tribromide (AsBr3), Boron Bromide (BBr3), Ethylene Dichloride (CH2C1), Formic Acid (CH1O2), Formamide (CH3NO) and Alcohol element. Presence of these contamination elements represent of underground contamination. The size and layer of the contamination were non-uniform type. From the results, it is shown that lower dielectric constant of contamination will affect more on charge of XLPE insulation. As a conclusion, it can be seen lower dielectric constant value of contamination element greatly affecting the performance of XLPE insulation. Furthermore, size of contamination also influences the content of charge in contamination where the bigger the contamination size, the more charge contained in the contamination.</p>

Study of Breakdown Factors of XLPE Cables

Study of Breakdown Factors of XLPE Cables

DC Voltage and AC Voltage Characteristics of Current Flowing in Water Tree Degraded XLPE Cable

DC Voltage and AC Voltage Characteristics of Current Flowing in Water Tree Degraded XLPE Cable

Research on aging parameters of XLPE cable based on isothermal relaxation current

Dielectric response is closely related to the aging status of XLPE cables, and the isothermal relaxation current (IRC) has been applied to find out the change trend of cable insulation as a non-destructive method. In this paper, 4 groups of XLPE cable samples (16 cables in total) are artificially acceleratedly aged with different time interval at 120, 140 and 160°C, respectively, and their IRC curves are measured by a micro-current device. The results show that with the increasing aging time, IRC curves decrease slower and the absolute values of steady current become larger. In order to quantify the aging characteristics of XLPE cables, the trap parameters calculation theory is employed to work out the peak trap level of different polarization types. The experiment results show that the peak trap level of bulk polarization (type 1) remains stable and is around 0.76eV which is considered to be independent of aging degree. As the aging time increases, the peak trap level of amorphous crystalline relaxation (type 2), which is from 0.785eV to 0.82eV, increases at low temperature (120°C) due to the breaking of molecular chain and decreases at high temperature (more than 140°C) due to the over cross-linking. The peak trap level of impurities interface relaxation (type 3) is highly influenced by aging temperature, which is most sensitive to the aging degree of power cable. It is considered that IRC can be used as a practical way to judge cable insulation in the future.

Compact 30kV 0.1 Hz cosine square wave testing system for PD detection on XLPE cables

To provide a more convenient and efficient method for detecting partial discharge (PD) on XLPE cables, this paper describes a 0.1 Hz cosine-square wave testing system, which can output peak voltage up to 30 kV. Due to L-C series resonance principle, the wave of discharge process is in cosine form which can be equivalent to AC voltage. Therefore, PD detection can be conducted under the rising and falling edges. The vital component of the proposed system is high-voltage semiconductor switch which consists of several series connected IGBTs. Based on the idea of compactness, a prototype system is integrated in the lab. Withstand test and PD detection test are also carried out to validate the proposed system. The experiment result shows that the system is effective to detect PD signal in power cables and has certain development prospects on insulation condition assessment of XLPE cables.

Sudden Short-Circuit Test of 22-kV YBCO Triaxial Superconducting Cable

For the power line between a power generator and a step-up transformer, isolated power bus (IPB) is generally used. The present IPB has a normal cylindrical conductor made of aluminum or copper at the center of the outer jacket and generates very huge Joule loss. To decrease it, superconducting buses have been developing. In this study, a 22-kV 3-kA triaxial superconducting bus was designed and fabricated with YBa2Cu3O7-d (YBCO) tapes and the various kinds of properties were investigated. Previously, the ac and lightning impulse withstand voltage test, critical current Ic test, and ac loss measurement were conducted and it was shown that the developed triaxial superconducting cable met the designed specifications. In this paper, a sudden short-circuit test of a 2-m-long triaxial cable was carried out by using a 200-MVA short-circuit generator. For protection, each phase of the cable was connected to a XLPE cable with a length of 3 m and a cross-sectional area of 250 mm 2 in parallel. Three-phase short-circuit current of 25 kA was applied to every phase of the cable bath-cooled with liquid nitrogen at 77 K for 2 s. As a result, the following tremendous phenomenon was demonstrated. Any phase of the triaxial cable kept a flux-flow state and did not quench. Short-circuit current was shared between the superconducting cable and the connected XLPE cable, and the branch current ratio varied even during one period of 60 Hz. The numerical simulation was also conducted to explain the phenomenon theoretically. That should contribute to the instantaneous recovery to a normal operation after a short-period interrupt of the circuit.

Numerical study of influential factors on partial discharges in HVDC XLPE cables

PurposeFor the dramatically developed high voltage direct current (HVDC) power transmission, HVDC cables play a vital role in the power transmission across seas and connections with renewable power sources. However, the condition monitoring of HVDC cables is still a challenging research topic. This paper aims to understand the influence of external factors, namely, current, cavity location and material properties, on partial discharge (PD) characteristics in HVDC cable in a numerical way referring to the refined Niemeyer’s model.Design/methodology/approachThe influences of the three external factors are studied by a proposed numerical model for DC PDs based on the modification of a conventional PD model for AC voltage via a finite element analysis method.FindingsThe external factors can influence the discharge magnitude and discharge repetition rate via affecting the electrical conductivity of the material: DC PD is more frequent and with higher discharge magnitude when the cavity is closer to the conductor or the current through the conductor is higher. Both DC PD repetition rate and average discharge magnitude in long-term aged material are lower than virgin material. The effect of discharge on insulation degradation becomes decreasingly significant.Research limitations/implicationsThe current work is based on the numerical modelling of DC PDs. Further experimental validations and comparisons are essential for improving the model.Practical implicationsThe studies of the influence factors for PDs under HVDC voltage can benefit the research and practical power transmission on DC PDs, contributing the design and test of DC PDs in HVDC cables, exploring the understandings of the DC PDs’ mechanism.Originality/valueThis paper, to the best of author’s knowledge, first studies the influence factors on DC PDs based on the numerical modelling work.

Research on Water Content and Physicochemical Properties of Insulating Materials for Water-Soaked XLPE Cable

The buried environment of cross-linked polyethylene (XPLE) cable is relatively severe, and some ones are even immersed in water directly. In this paper, an XLPE cable was soaked in foul water to simulate the cable’s actual water area environment, lasting for three years. The water content in the insulation layers of water-soaked cable was measured by infrared spectrometer and moisture analyzer, and the crystal morphology and physicochemical properties of the insulation layers were investigated by differential Scanning Calorimeter (DSC), scanning electron microscopy (SEM) and tensile testing technology. The results showed that a small amount of water did enter into the cable insulation owing to the increasing of hydroxyl (-OH) content, and followed being verified by moisture analyzer. The degree of crystallinity of water-soaked cable insulating material is also increased, and the mechanical properties were deteriorated.

Analysis on the Performance of High Insulation Materials for Current Bending XLPE Cable

Cross-linked polyethylene (XLPE) as insulation material has been widely used in the manufacturing of medium and high voltage cables for its excellent physical, chemical, mechanical and electrical properties. However, as cable insulation, the XLPE will degradate under service conditions, such as thermal oxidative degradation, mechanical treatment, operating environment etc. In this paper, the closed cable loop had been heated for 7 days by induced current of 1000A, and several diagnostic measurements had been adopted to characterize the performance of XLPE. Firstly FTIR has been conducted on different bending degree cable samples in order to reveal the effect of mechanical treatment on the content changes which occur in XLPE insulation layers. DSC analysis showed the effect of cooling process on the shoulder melting peak temperatures. In the last part, microstructure of the insulation of high current bending XLPE cable was studied. The results showed that the properties of the bending cable after heating by the high current changed a lot and the degree of bending also has an effect on the performance of the cable.

Classification of partial discharge images within DC XLPE cables in contourlet domain

This paper presents a new method to improve the insulation defect classification rate of DC cross linked polyethylene (XLPE) cables. A partial discharge (PD) test platform is built in the laboratory and four DC cable defect models are designed. The PD signal is collected by a high frequency current transducer (HFCT) and the q-At-n image is constructed. The contourlet transform is performed on the PD image, and the Tsallis entropy feature of each subband coefficient is calculated. The error-correcting output codes (ECOC) classifier is optimized by the adaptive cuckoo search (ACS) algorithm optimization sparse random (SR) coding matrix to realize the defect classification. The experimental data show that the ACS-SR-ECOC classifier in this paper has shown promising performance in the classification of each defect and the Gaussian noise model. Compared with to four kinds of traditional ECOC classifiers, the proposed method has higher overall classification accuracy, which provides a new approach for the defect type identification of PD image in the DC cable.

DC cable feature extraction based on the PD image in the non-subsampled contourlet transform domain

A new method to extract the features of direct current (DC) cross linked polyethylene (XLPE) cable partial discharge (PD) image based on non-subsampled contourlet transform (NSCT) is proposed in this paper. Four types of PD images are obtained from the artificially designed cable insulation defects. The features include Shannon entropy, Renyi entropy, Euclidean distance, and Mahalanobis distance of the optimized subband coefficients by NSCT. The optimized subband coefficients are selected by the immune algorithm optimized affinity propagation clustering method. The back propagating neural network, k-nearest neighbor, support vector machine and decision tree algorithms are used to classify the extracted features. The experimental results show that the proposed method can be applied to the feature extraction of PD signals in DC XLPE cables. Compared with the wavelet method, the recognition rate of the method is improved by more than 16.80% under the same classifier. It is found that the type of defect within insulation can be classified efficiently with the features extracted by the proposed method with a reasonable degree of accuracy.

Enhancement of Water Tree initiation due to residual and applied Mechanical Strain on XLPE Cables

This paper present results from laboratory water tree experiments performed on samples of commercial available 6/10 kV XLPE insulated cables. The main purpose of the investigation was to examine possible water tree enhancement, due to mechanical tension of the insulationDuring water tree ageing at 50 Hz AC voltage of 14 kV (E_max = 5.2 kV/mm) the cable samples were mechanically stretched at 1%, using both static and dynamic mechanical load varying at 0.1 Hz. In addition, thermal treatment indicated a total residual longitudinal mechanical strains of 6%, frozen in during th emanufacturing process.Both the density and growth rate of vented and bow-tie water trees was found to increase with increasing applied mechanical tension. During non-strained thermal treatment the axial length of cables was reduced by about 6%. The low density of vented trees originating from te outer insulation screen supports the assumption that frozen in compressive stresses at the outer insulation surface may balance the effect of applied strain.

Estimating Transmission Line Parameters of Three-core Power Cables with Common Earth Screen

The propagation modes of travelling wave in a threecore power cable (XLPE or PILC) with common earth screen can be uncoupled into three modes: one shield to phase (SP) mode and two phase to phase (PP) modes. A generic model approach is applied in this paper to estimate the transmission line parameters of three-core cables with common earth screen, which includes the effect of different layers on series impedance and admittance. The characteristic impedance, the propagation velocity and the attenuation for SP and PP modes of two cable types, PILC and XLPE are modeled and compared with test results. Calculated parameters match within 5% for the characteristic impedances of different modes and 5% for the velocity for the PILC cable. Larger deviations occurred for XLPE cable, due to difficulties in appropriate modeling the semiconducting insulation screens around each conductor. The attenuation for both PILC and XLPE is hard to predict because data on complex permittivity, being the dominant factor, is hardly known.

Axial Water Ingress MV XLPE Cable Designs with Watertight Barrier

Humidity can enter the insulation system when the outer sheath of a cable becomes damaged. The relative humidity can then increase above a critical value (70 % RH), facilitating initiation and growth of water trees. The purpose of this experimental and numerical work is to determine how fast water vapour will diffuse axially in a soaked cable with a damaged outer sheath.Before drying (evacuation) relative humidity and temperature sensors were placed within the outer sheath at different axial positions. After the dryingprocess a hole was cut at the cable end facilitating water ingress. Numerical calculations of axial water diffusion were performed using Comsol.The numerical calculations show that the axial water vapour diffusion in the cable is slow and dependent of the air gap close to the swelling tape. After 430 days the measured humidity at 0.5 m had increased by about 75 % (initial 20%), and the sensor at 1 m had increased by 40%. Numerical calculations show a slower increase in the relative humidity. The actual axial liquid water penetration extensions, is yet not determined. The numerical calculations show that this is an important factor, as the calculations seem to be in more agreement with measurements when adjusting the position of the water front.

Current integrated technique for insulation diagnosis of water-tree degraded cable

The diagnosis of insulation performance in the high-voltage alternating current (HVAC) and high-voltage direct current (HVDC) systems has always attracted many researchers in the field of electrical engineering. New diagnostic technology and condition monitoring apparatus are of prime importance for HVAC and HVDC developments. In this study, we introduce a valuable technique called direct current integrated charge (DCIC-Q(t)) for determining the degradation of insulating materials by means of charge accumulation. This work demonstrates the typical results of full-size cross-linked polyethylene (XLPE) cables by using this effective method. Firstly, we describe the basic measurement principle of DCIC-Q(t) technique and the evaluation details of the degradation. Then, the validity of this technique is confirmed by comparing with the pulsed electroacoustic (PEA) results. The similar charge behaviors of polystyrene and low-density polyethylene are presented via the DCIC-Q(t) and PEA methods, which indicates the good availability of this technique. Three underground, full-size, 6.6-kV XLPE cables were diagnosed using the DCIC-Q(t) method. Two cables were degraded by water and electric stress for a different time under controlled conditions. The degradation extent can be evaluated easily by the charge accumulation from the DCIC-Q(t) results. Furthermore, based on the results of charge dynamics related to the applied voltage, the properties of the degraded cable by water tree are discussed.