kV XLPE Cables Research Articles

Investigation on Fire Damage Caused by Breakage of Shielding Layer in Extruded Three-layer 6.6kV XLPE Cables

Investigation on Fire Damage Caused by Breakage of Shielding Layer in Extruded Three-layer 6.6kV XLPE Cables

Measurements of electrical resistances and investigations of factors affecting corrosion and breakages of shielding layers in extruded three‐layer 6.6 kV XLPE cables

AbstractCorrosion and breakages of shielding layers are one of the factors for degradation of extruded three‐layer (E‐E type) 6.6 kV cross‐linked polyethylene (XLPE) cables. Tohoku Electric Power Network has been measuring electrical resistances of shielding layers in E‐E type 6.6 kV XLPE cables, and recently has found breakages of shielding layers. The authors performed an accelerated degradation test for shielding layers of E‐E type 6.6 kV XLPE cables by taking account of aqueous solution, temperature, heat cycles, bending stress, frequency of an applied voltage, and water‐proof layer in the test conditions, and investigated factors affecting corrosion and breakages of shielding layers. Corrosion, cracks, and breakages were found on shielding layers in the cables without water‐proof layer, but no crack was found in the cables with water‐proof layer, when heat cycles were applied to the cables, and shielding layers were supplied with artificial sea water. We investigated the suppressive effect of the cables with water‐proof layer on cracks and breakages of shielding layers by measuring tensile strengths of copper tapes obtained from the shielding layers of the cables with and without water‐proof layers.

Measurements of Electrical Resistances and Investigations of Factors Affecting Corrosion and Breakages of Shielding Layers in Extruded Three-layer 6.6kV XLPE Cables

Measurements of Electrical Resistances and Investigations of Factors Affecting Corrosion and Breakages of Shielding Layers in Extruded Three-layer 6.6kV XLPE Cables

Influence of Combined Electrothermal Aging on Dielectric and Thermal Properties of HVAC XLPE Cable

To explore the influence of electrothermal aging on the properties of HVAC XLPE cable during its normal operation, two circuits of 110 kV XLPE cable were subjected to a combined electrothermal accelerated aging test for 180 days. The test voltage was set at 95 kV, and the test temperature was set at 90 °C. The constant temperature aging mode was adopted for one circuit, and the periodic thermal cycle aging mode was adopted for the other circuit. The dielectric property of the cable was diagnosed with dielectric spectroscopy measurement, and the thermal property was diagnosed with thermal parameters (thermal capacity and thermal resistance) measurement. Combined with Fourier infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) measurements, morphological changes in the insulation under different electrothermal aging modes were analyzed to construct the correlation with the dielectric and thermal properties. The results show that the short-term electrothermal constant temperature aging mode effectively enhances the dielectric and thermal properties of the cable by modifying spherulite morphology and migrating polar compounds; the short-term electrothermal cycle aging mode also enhances the thermal properties of the cable due to the drop in small polar compounds. However, different distributions of polar compounds and crystal interfaces in the insulation lead to extreme distortion in the electric field distribution.

Annealing effect on the crystallinity characteristics of XLPE from High-voltage Cables

The thermal effects during the cable operation brought various changes in cable insulation, strongly affecting the cable’s long-term behavior. In this study, a new and seven-year serviced 110 kV XLPE cable was taken, and the insulation layer was peeled and thermally aged at 115 for various 60, 120, and 180 days. Then, they were annealed at 90 to 115°C with a gap of 5°C, respectively. The gel content and differential scanning calorimetry (DSC) were measured. The results show that the 60 and 120-day aging tests promote the crystallization of XLPE and show a higher recrystallization point and crystallinity, while the 180 days aging test brought an opposite change. After the annealing process, melting point and crystallinity increase with the increasing annealing temperature. Before they reached the highest values, 60-day-aged peels showed higher the two factors than the fresh peels, with a slight difference to 120-day-aged peels, but 180-day aged showed the lowest values. However, the annealing process results in a lower melting point and crystallinity when the temperature exceeds the optimum point. Finally, it was found that the crystallization characteristics of the aged XLPE peels could be improved if the annealing was done at proper temperatures.

Study on Ultrasonic Propagation Characteristics of Partial Discharge in 10 kV XLPE Cable

Study on Ultrasonic Propagation Characteristics of Partial Discharge in 10 kV XLPE Cable

Analysis of Influence Factors on Space Charge Accumulation Characteristics of 320 kV XLPE Cable

Analysis of Influence Factors on Space Charge Accumulation Characteristics of 320 kV XLPE Cable

Aging Detection of 110 kV XLPE Cable for a CFETR Power Supply System Based on Deep Neural Network

To detect the aging of power cables in the TOKAMAK power supply systems, this paper proposed a deep neural network diagnosis model and algorithm for power cable aging, based on logistic regression according to the characteristics of different high-order harmonics generated by different aging parts of the power cable. The experimental results showed that the model has high diagnostic accuracy, and the average error is only 2.35%. The method proposed in this paper has certain application potential in the CFETR power cable auxiliary monitoring system.

Study on drying performance of high voltage cable buffer layer material

In order to study the failure mechanism of high-voltage cable buffer layer caused by moisture and the performance of the material after immersion and drying, according to the structure and size of 110 kV XLPE cable, the requirements for the separation distance between corrugated aluminum sheath and insulating outer shield before and after the change of buffer layer resistivity caused by moisture were calculated, the influence of moisture on ablation failure was analyzed. The test platform of buffer layer material was built, the properties of two kinds of materials without water immersion and after water immersion drying were tested, including semi-conductive buffer water-blocking tape and semi-conductive buffer tape. The results show that when the moisture content was large, the resistivity of the buffer layer increases, which reduces the requirements of the allowable separation distance of the cable, and was more likely to cause the discharge and ablation of the buffer layer. The water in the buffer layer was removed by drying at 70 °C, with thickness, weight, the water content, elongation at break, breaking strength and volume resistivity can be recovered, which verifies that the heating drying method is feasible for the removal of water from the buffer layer.

Electrical Tree Driven Breakdown of XLPE Under Step Stress Breakdown Test at Multiple Frequencies

The electrical treeing of a 33 kV XLPE cable is investigated on a pin-plane electrode system in the frequency range of 50–650 Hz. Cable specimens are created with needle electrodes having a tip radius of 1 µm and a tip distance of 0.75 ± 0.1 mm. Step stress tests are conducted at a step increase of 1 kV peak voltage for step times of 15 seconds and 75 seconds. The breakdown instances of the specimens are recorded to obtain the time to breakdown (TTB), breakdown voltage and the polarity of the breakdown voltage. In addition, the breakdown path is observed using a stereo microscope. It is found that the normalized TTB and the breakdown voltage exhibit a decreasing trend with respect to the frequency for step stress tests. A high percentage of samples are broken down in the negative half cycle of the applied voltage. Microscopic imaging shows that the corresponding electrical trees are branch type trees for all the frequencies. The observations for normalized TTB and voltage variation is attributed to the phenomenon of space charge limiting electrical field variation with frequency.

Effect of Cavity Position, Size and Geometry on Partial Discharge Behaviour inside 18/30 kV XLPE Cables

Effect of Cavity Position, Size and Geometry on Partial Discharge Behaviour inside 18/30 kV XLPE Cables

Effect of Cavity Position, Size and Geometry on Partial Discharge Behaviour inside 18/30 kV XLPE Cables

Effect of Cavity Position, Size and Geometry on Partial Discharge Behaviour inside 18/30 kV XLPE Cables

A Comparative Study on the Insulation Ageing of 10 kV XLPE Cable via Accelerated Electrical Test and Accelerated Water Tree Test

A Comparative Study on the Insulation Ageing of 10 kV XLPE Cable via Accelerated Electrical Test and Accelerated Water Tree Test

Extended thermal rating calculations of 400 kV XLPE cables for urban grid applications based on long-term experimental data

Conventional rating calculations for power cables are based on the analytical methods given by IEC standards. These methods lead to rather conservative results though, which has been acceptable in the past since high-voltage cables are normally not loaded to their thermal limits and are rather operated in cold conditions, relatively speaking. However, the power system is undergoing a transition towards increasing and more dynamic loads and, therefore, towards the need for a more flexible and resilient grid. This applies in particular to cable systems in urban areas where high-voltage cables are used due to the infeasibility of overhead lines and where the renewal or expansion of those systems is attached to high installation costs. An optimized use of existing and future cable systems is a crucial feature in order to cope with the increasing requirements. Therefore, an attempt is made to combine the advantages of the different calculation methods in order to improve performance and efficiency. Furthermore, there is high potential to improve cable calculations with the application of artificial intelligence (AI). First, however, these procedures must be brought together to be evaluated, and experience in the application of AI in cable calculations must be gathered. Therefore, an actual 400 kV cable system with a typical urban laying profile was set up in a cooperation project between Vienna’s grid operator Wiener Netze GmbH and the High Voltage Test Laboratory Graz Ltd. to develop and validate calculation methods. In addition, it is investigated which input parameters are necessary, what effect do they have on the result and which accuracy and efficiency can be achieved.

Development of Severity and Location Indices Based Condition Monitoring Scheme for Underground Cables by Impedance Spectroscopy

This work presents the development of a condition monitoring scheme for detection, severity assessment and localization of small cavities in cable insulation by impedance measurement in frequency domain. A high frequency model of cable incorporating the frequency dependence of cable parameters is developed to analyze the impedance changes during the occurrence of cavities in the insulation. The simulation and analysis of impedance with cavities in a 11 kV, 95 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> cable reveals the parameters such as Severity Index (SI) and Location Index (LI) which shows gradual changes with change in size and location of void and have potential scope to be used for severity assessment and localization of cable deformations. Thus based on the dataset of SI and LI, the condition of cables while in service can be predicted. It is experimentally validated for deformation such as cavities at the outer surface of insulation in 11 kV XLPE cables having conductor areas of 95 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 150 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The reliability of the proposed method for detection and localization of cavities inside the insulation near cable joints is validated by measurement and analysis on 11 kV, 95 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> XLPE cables with artificially created voids at the insulation region near cable joints.

Investigations of Effects of Cable Insulation Thickness and Water Immersion on AC Breakdown Voltage and Degradation Diagnostic Data and Possibility of Detection of Non-bridged Water Trees of Dry-cured and Extruded Three-layer (E-E Type) 6.6kV XLPE Cables

Investigations of Effects of Cable Insulation Thickness and Water Immersion on AC Breakdown Voltage and Degradation Diagnostic Data and Possibility of Detection of Non-bridged Water Trees of Dry-cured and Extruded Three-layer (E-E Type) 6.6kV XLPE Cables

Research on Mechanical, Physicochemical and Electrical Properties of XLPE-Insulated Cables under Electrical-Thermal Aging

The stabilization of cross-linked polyethylene (XLPE) cables is crucial to the safe working of power systems. In order to investigate the aging characteristics of cable insulation in the process of operation, 10 kV XLPE cables were electrically-thermally aged at an AC voltage of 26.1 kV and three temperatures: 103°C, 114°C, and 135°C. Cable samples at five aging stages were cut into slices and tested to determine their mechanical, physicochemical properties and dielectric strength. The changes in these properties were analyzed in terms of aging time. The mechanical strength and oxidation induction time have the same changing trend of decrease, because the chemical bonds of the cable insulation material are fractured when the XLPE cable is aged. The AC space charge presented a trend of gradual accumulation with aging time. The breakdown field strength after AC voltage application decreases monotonically with aging time, which could be a characteristic index to evaluate the degradation degree of the cable insulation. This research contributes to the comprehension of degradation mechanism and discovery of aging assessment criteria, which is of great significance in assessments of insulation properties and fault detection on power cables.

A comparison of space charge behaviors in coaxial cable and film sample under temperature gradient

Space charge in DC cables affect the long-term reliability of the cable during operation. A temperature gradient (TG) develops across the cable insulation due to Joule heat in the conductor. Extensive researches on space charge in film insulations under TG have been reported. However, up to date, it is still questionable whether the measurement results of film samples can directly reflect the space charge characteristics in an actual coaxial cable. In this study, the space charge behavior in a 10 kV XLPE cable was investigated under different temperatures and electrical stresses using an improved pulse electro-acoustic (PEA) measurement system. The results show that the coaxial geometry and the TG both enhance hetero-charge accumulation near the cable outer semi-conductive layer. Space charge in film samples sliced from the same cable under the same conditions were also measured and compared to that in the cable. It is found that the space charge behaves similarly in terms of charge variation laws and distributions under different electrical stresses and TGs regardless of the type of sample structure, the two space charge measurement methods show well equivalence.

Space charges analysis on insulator with uniform layer contamination effect

High voltage direct current (HVDC) transmission provides an attractive alternative for bulk power transfer. However, HVDC transmission may have loss about half per unit length of high voltage alternating current (HVAC) at the same amount of power carried. This is due to the space charge formation around the conductor in HVDC cables. It is known that the presence of space charge inside an insulator may distort the local electric field and surface energy. This paper investigates the effect of electrostatics for space charge, electric field and surface energy in the HVDC cable in clean and contaminated conditions. The effect of uniform layer contamination from oil, sandstone and fresh water was conducted on 11 kV XLPE cable using finite element software under electrostatics study. The contamination layer was created around the XLPE cable by multifarious the radius of layer contamination from the conductor. The simulation results show that enlargement of contamination layer radius by 1.0 mm (light), 1.5 mm (medium) and 2.0 mm (heavy) resulted in the reduction of surface energy by 20% and electric field by 22% but increase the space charge amplitude by 76%. The study also found that fresh water can be considered as the worst contamination compared to oil and sandstone.

Thermo‐electric field analysis of AC XLPE cable in monopole, bipole, and tripole DC operation modes

This paper presents a numerical analysis of thermo-electric fields of 10 and 35 kV AC XLPE cables in monopole, and tripole DC operating modes based on conduction current measurements. DC conductivities and electric field thresholds of the space charge accumulation for XLPE materials were estimated at different temperatures and electric fields. The result showed that the electric field threshold of the space charge accumulation for the materials reduced with an increase of the temperature. DC currents of the cables were increased with the increase of the operating temperatures in the three DC operation modes. The maximum DC operation voltage of the cables increased with the decline of the operation temperature in the three DC operation modes. The maximum transient electric fields were below the threshold of the space charge accumulation for the cables in the tripole DC operation. The maximum transmission power of the 10 kV XLPE cable in the DC operation modes was at 60°C, whereas the maximum transmission power of the 35 kV XLPE cable was at 50°C. It was elucidated that the 10 kV AC XLPE cable could have more advantages than the 35 kV AC XLPE cable in the three DC operation modes.