In order to study the insulation state of the cable joint with dampness, and to accurately evaluate the dampness of the cable joint, this paper measured the time-frequency domain dielectric constant of the XLPE cable joint using the PDC (polarized depolarized current, PDC) method. And the K-means algorithm is adopted to analyze it. The classification and state interval division are carried out, and the damp process of the cable joint is divided into four stages. Then, the damp and aging insulation state evaluation model of the cable joint is established according to the dynamic Bayesian network, and the network parameters are determined to compare comprehensive judgment is made on the degree of dampness of cable joints. Finally, the 2-CV cross-validation of the proposed damp and aging evaluation model is carried out, and the accuracy rate is as high as 90.065%, which shows that the model has good resolution and effectiveness in evaluating the dampness of cable joints.
Partial discharges (PD) measurements provide an estimation of the severity of the insulation degradation; additionally, it is used as a unit of standardization. Up to now, only the IEC 62478 briefly mentions partial discharge measurements using electromagnetic sensors in gas-insulated substations (GIS). The IEC60270 standard provides a method for PD charge measurement when the test object approximates to a lumped element; PD in SF 6 are in the range of nanosecond, and given the GIS length, it behaves as a transmission line. This work compares different sensors (commercially available and developed by the authors) used for measuring PD charge magnitude in GIS. The sensors’ sensitivity, time resolution, and charge estimation accuracy are tested in a full-scale 420 kV GIS. A nanosecond rise time pulse was connected to the GIS through a transition cone to provide a good PD representation. The pulse was measured by electric and magnetic sensors installed in mounting holes located in different sections of the GIS, and a directional coupler was used as a reference to the injected pulse. This work demonstrates that the charge magnitude can be extracted using different sensors, harmonizing the reading from different measuring systems. The results show the possibility of a standardized method for on-line PD measurements and routine and after-installation tests.
The UHF antenna is a key component for partial discharge detection of GIS equipment, whose performance directly determines the effectiveness of the detection. In this paper, a novel UHF antenna for partial discharge detection of GIS equipment is designed based on fractal theory, and the antenna is modeled and simulated in a three-dimensional electromagnetic software to explore the influence of different structural parameters on the performance of the antenna. The working frequency band of the optimized antenna is 500~1060 MHz with an average gain of 3.8 dB. To verify the detection performance of the antenna, partial discharge tests are designed and carried out based on the needle-plate discharge model and column-plate model. Experimental results show that the antenna can be used for partial discharge detection of GIS equipment.
Wet-design high voltage AC power cables are cost-effective alternatives to traditional lead-sheath cables, but the lack of a metallic water barrier can lead to water absorption in the cable insulation system. Intermittent load patterns can induce rapid cable temperature variations, which can induce supersaturation and then condensation of water during cooling at impurities in the insulation material. The main purpose of this paper is to study the effects of temperature cycling on water tree ageing in a homo-polymer high voltage AC XLPE insulation intended for wet-design cables. Rogowski test objects with 1 mm thick XLPE insulation were subjected to 3 or 10 kV/mm and 60 thermal cycles using a cooling rate of 1 °C/min. Sodium chloride particles were included at one of the semiconductor-insulation interfaces. The results show that the water trees appeared to be only affected by the level of applied electric stress when growing from the sodium chloride particles, suggesting that the level of applied electric field mainly affects water tree growth from very large ionic contaminants.
The rheological behavior, mechanical and electrical properties of the XLPE obtained by the new Buss AG LSHC ® (Linear Short Hyper Clean) XLPE process to manufacture hyper clean PE have been compared using a standard commercially available product for these applications, in order to determine the suitability to HV-AC and HV-DC cables.
Mixed anhydride is an effective method to regulate the crosslinked structure of epoxy resins, which is expected to achieve an improvement in the dielectric properties of the epoxy curing samples and thus improve the insulation performance of the insulation core of the UHV bushing. In this paper, five epoxy resin samples with different acid anhydride curing agent ratios are prepared by hot pressing method. The thermal and dielectric properties of the samples are tested, the best anhydride curing agent ratios were selected, and the steric effects caused by methyl substitution are analyzed. The results show that the mixed anhydride curing sample with the molar ratio of methylhexahydrophthalic/hexahydrophthalic anhydride of 8:2 reaches the highest glass transition temperature and the electrical strength is improved with essentially constant dielectric loss, which is closely related to the increased rigidity of the system.
This contribution introduces an adapted test set-up, which will be used to develop new methods for the determination of voltage endurance tests of insulating materials subjected to high DC voltages. The set-up combines the voltage endurance test for approximately 80 – 100 kV/mm of five 1 mm thick samples simultaneously and the space charge measurement up to the break down voltage of several polymeric insulating materials by using the LIPP-method. All components are designed to measure the space charges during the aging process until the breakdown of the sample. The set-up is also designed for examination under different temperatures from 20 to $80^{\circ}\mathrm{C}$ and different contact pressures. The components are described in detail to provide a template for constructing further test set-ups and developing optimization with using the LIPP-method.
Understanding harmonic propagation in high voltage transmission lines and cables is important for the design and operation of power systems. The presence of harmonics and other transients in transmission and distribution system pose significant risks to insulation coordination aspects and to the safe and reliable operation of various power system components such as transformers, cables, reactors and filters. In this paper simulations are carried out to study the performance of both classical Pi-section and frequency dependent high voltage cable models under different harmonic distortion conditions. Harmonics of different amplitude and frequencies are superimposed on a 100 kV DC voltage and the frequencies evaluated at both the sending and receiving ends of the cable. Total Harmonic Distortion (THD) values are compared for different levels of harmonic distortion and loading. Current and voltage source harmonics are introduced at the input and the output voltage harmonics are analyzed. The simulation results show individual harmonic amplitudes change for the two models resulting in different THD values. The results presented in this paper will help improve understanding of the potential harmonic propagation within HVDC cables and the harmonic impact on HVDC cable insulation.
With a more precise knowledge of the lifetime condition of individual components, distribution system operators (DSOs) can optimize load flows and their maintenance strategies. To assist the DSO in this important task, a project was initiated to gather the continuous ageing process of paper-insulated lead covered cables (PILC) through an accelerated ageing system. As part of this empirical approach, a comprehensive selection of new and pre-aged cables of the type NKBA 3x120 SM/ 6/10 kV are stressed with specific load flows and are expected to provide information on the ageing process of this cable type. In the paper, the initial measurements of the accelerated ageing system are introduced. Measurements at two temperatures (10 °C and 40 °C) and test voltage levels from 0.5 U o to 2.5 U o are regarded. The considered cables are three pre-aged NKBA cables with around 60 years of operation in the field and varying load histories, a NKBY cable with a moderate load history and a new NKBA cable as reference. At low temperatures, the samples show less volatility and a moderate increase in dissipation factor across voltage, whereas at medium temperatures a more significant spread in dissipation factor between samples and a decreasing value is evident.
Special magnet wires enamels have been developed to resist film degradation due to partial discharges activity in the presence of fast rise-time voltage surges as those ones generated by inverter fed drives. The behavior and the properties of corona resistant wires enamelled with nano-structured polyesterimide and polyamideimide set up by Elantas are described, with specific reference to wire build up. In particular, lately developed corona resistant polyamideimide Allotherm 602 CR FLAT, suitable for round and flat wires, showed outstanding resistance even when used as topcoat in typical wire base/top ratios.
This paper reports insulation properties and lifetime estimation (or V-t characteristics) of twisted-pair made of polyvinyl formal (PVF) wires undergoing thermal stress at 180 °C for two months in mineral oil. Assuming the worst case, an attempt is made to evaluate the insulation performance in PVF wire with artificial pinhole defect using a cross sample in which two simple straight lines of PVF wire crossed each other. Using a pulsed laser, a pinhole simulated defect was fabricated on the surface of PVF wire. The cross sample was prepared in a way that the defective portions faced each other. Measurements of partial discharge inception voltage (PDIV) and breakdown voltage (BDV) were performed in mineral oil with a water content of10 to 15 ppm. The same measurement was conducted in air as comparison. The applied voltage rise rate was 100 V/s and 200 V/s at 60 Hz in air and mineral oil, respectively. The threshold for PD detection was 3 pC. Same measurements were also performed for cross samples with sound(normal)-defect and sound(normal)-sound pairs for comparison. We measured PDIV and BDV in mineral oil for each sample configuration, respectively. It was found that the pinhole defect simulated sample with defect-defect facing each other exhibited 60% lower BD voltage in mineral oil than the sound-sound sample, respectively. However, due to the effect of mineral oil, the dielectric strength of the worst case was 3 kV, which was still higher than that of the operating voltage.
The electrical insulation of small power electrical machines is a critical element especially when they are powered by means of electronic power supplies using techniques such as PWM. Power supplies based upon pulsed voltage trains can introduce, under certain conditions, overvoltage and this overvoltage can produce the inception of partial discharge phenomena. Enameled wires, even in case of high insulating thermal classes, cannot resist for a long time to the erosive action of the partial discharges and therefore, inevitably, reaching the total discharge conditions. In addition, the erosion of the enamel causes a decrease of the inception voltage, aggravating the situation and quickly leading to the total breakdown. In this study enameled wires thermal class $\mathrm{W}- 240^{\circ}\mathrm{C}$ have been considered. These wires have been used to prepare twisted-pair specimen following the IEC 60851-5 standard. They have been subjected to thermo-electric ageing using a sinusoidal test voltage waveform, varying its amplitude and frequency. The obtained results show that, during the degradation, the PDIV values continuously decrease following an inverse power law relationship. Furthermore, the obtained data evidence that the PDIV derivatives do not depend on the test voltage amplitudes and frequencies but rather seems to depend on the temperature.
Electric field distribution in polymer electrical insulations under DC constraints is directly linked to the presence of charges within the material. Space charge generation is thought to mainly arise from injection at the electrodes, in polyethylene based materials. Hence, describing this injection is still a challenge. Surface roughness, increase of the energy levels linked to the physical interface, variation of these energy levels along the surface are among the physical hypotheses that impact the charge injection, and these hypotheses take place at the nanometric or micrometric scale. A bipolar charge transport model, developed in 2D, is presented, and accounts for these different physical hypotheses at the electrode in order to observe the impact of each process on the space charge behavior at the macroscopic scale.
This paper explores in detail tree structures grown in negative DC fields superimposed with AC ripples (previously named ‘slim bouquet’ structures). Fine channels which grow during negative DC ramp down are included in the study, and trees grown in purely 50 Hz AC fields are considered for comparison. A state-of-the-art 4K optical microscope was employed to observe tree structures with both transmitted and reflected light. With reflected lighting, dark stem channels and light yellow quasi-2D leaf-type structures were identified in AC trees. Raman spectroscopy suggested the presence of carbon decomposition products within dark stem channels. Bouquet structures were found to have similarities to leaf-type structures as both appeared as light yellow pixels under reflected lighting. Pictures comprising images taken at different focal planes are shown to be powerful tools in understanding tree morphology.
Fiber reinforced epoxy resin has the advantages of high strength, corrosion resistance, good insulation performance and light weight, and has been widely used in the production of insulation pull rod. Due to the complexity of the working environment of insulation pull rod, different fiber materials are usually used to enhance certain properties of epoxy resins. In this paper, electrical tree degradation experiments were carried out on glass fiber reinforced epoxy resin and polyester fiber reinforced epoxy resin under tensile stress. The needle electrode is close to the fiber cloth and is parallel to the cloth surface, the direction of the applied tension is parallel to the direction of the electric field, and the tension varies between 0 and 30 MPa. The results show that the electrical tree in glass fiber reinforced epoxy resin (GREP) is more dispersed than those in polyester fiber reinforced epoxy resin (PREP) under the condition of no applied stress. Under tensile stress, the electrical tree in both materials undergoes obvious morphological changes, and PREP is more significantly affected by stress.
This study aims at developing an analytical closed-form formula for electric field calculation during both fast and slow polarity reversal events in HVDC cables. For the sake of validation, a previously developed transient iterative code, which depends on the solution of Maxwell’s equations, is used to compare the results of the analytical model.
Mineral oil is generally used as a liquid insulating material. However, mineral oil has the problem of having a large environmental load. Therefore, in order to use pure water as an environmentally friendly liquid insulating material as an alternative to mineral oil, we have been researching aimed at improving the electrical properties of pure water. In this paper, we focused on the generating time of nitrogen fine bubbles (N 2 -FBs) and the standing time after generating N 2 -FBs, and investigated the effects of N 2 -FBs generating time and standing time on resistivity and negative lightning impulse breakdown voltage (NLI-BDV) of pure water in nitrogen or air atmosphere. As a result, the resistivity of pure water decreased as increasing of the generating time of N 2 -FBs and standing time after generating N 2 -FBs. However, the NLI-BDVs of each pure water were approximately same value, regardless of the generating time of N 2 -FBs and the standing time after generating N 2 -FBs. From the above results, it was suggested that N 2 -FBs affect the resistivity of pure water and does not affect NLI-BDV of pure water.
Polypropylene (PP), a typical insulating medium for metalized film capacitors, undergoes significant reduction in electric breakdown strength at high temperatures, which affects engineering applications. In this paper, the effect of gamma radiation on crosslinking modification of the material, and the enhancement of the high-temperature breakdown strength of PP were investigated. A 0.4 wt% sensitizer trimethylolpropane triacrylate (TMPTA) was added, and radiation exposure was conducted at $25^{\circ}\mathrm{C}$ with total radiation doses of 0, 2, 5, and 10 kGy, respectively. FTIR spectra and crosslinking degrees were measured to obtain quantitative and qualitative characterization of sample crosslinking. The breakdown strength and conductivity were measured at 25 and $115^{\circ}\mathrm{C}$. The crosslinking degree and breakdown strength were found to increase and then decrease with increasing radiation dosage, reaching a maximum at 2 kGy. The breakdown strength of this sample was 566.9 kV/mm at $115^{\circ}\mathrm{C}$, which was 13.8% higher than that of pure PP. The conductivity was also minimal at $1.47 \times \mathrm{e}^{-13}$ S/m. It is considered that gamma radiation can cross-link polypropylene and enhance the high temperature breakdown properties and conductivity.
