This article reviews recent technological trend in power cable systems used for power transmission in domestic and overseas. Especially, this article deals with the history of the power cables and accessories, the feature and technology of typical projects of the power transmission cable system including underground power cable, submarine power cable, offshore wind power cable, DC power cable, etc.
It also describes power transmission equipment, aging degradation diagnosis methods, maintenance support systems, and future issues.
Dry-cured and extruded three-layer (E-E type) 6.6kV cross-linked polyethylene (XLPE) cables was developed to reduce water tree degradation, and they were introduced into electric power systems more than 30 years ago. Three kinds of insulation thickness are used at this voltage class depending on the conductor size of XLPE cables, and it is possible that the insulation thickness affects residual AC breakdown voltage and degradation diagnostic data. Also, water immersion into XLPE is one of important factors for water tree degradation, and it could affect those electrical insulation characteristics. In this study, the authors measured AC breakdown voltages for two kinds of insulation thickness of dry-cured and E-E type 6.6kV XLPE cables removed after 12 to 30 years of operation and accelerated water-tree degraded XLPE cables. As a result, it was found that the AC breakdown voltages for the smaller insulation thickness were smaller than those for the larger insulation thickness, and the AC breakdown voltages with water immersion were smaller than those without water immersion. Taking account of these data, effects of the insulation thickness and water immersion on the lifetime of this type of cable were discussed. It was also found that effect of insulation thickness on DC leakage current and AC superposition current was negligible. To detect harmful water trees before they bridge cable insulation, we applied the current-integrated method to some of the cables, where water tree degradation was accelerated under an AC voltage at 3.8kV and 1000Hz and under several thermal conditions including the heat-cycles between room temperature and 60°C, for measuring DC leakage currents. As a result, it was found that two types of degradation degree due to non-bridged water trees could be distinguished using the current-integration method, but it was difficult using the conventional one.
Generally, the conductors of the long span transmission lines crossing a strait or a river are designed to have a special design different from the normal span. Various investigations and considerations are required beforehand for replacement of these conductors. In this paper, we show a new kind of high-strength and high-corrosion-resistant conductors enhanced conductivity and accessories that we have developed for the long span transmission lines.
We investigated effects of AC electrical stress, the oil pressure and the insulation construction on the partial discharge (PD) characteristics of an oil-impregnated paper insulation system for oil-filled (OF) cables. The oil-impregnated paper insulation system with an oil gap was composed of oil-impregnated papers with a hole and papers without any hole. They were sandwiched between parallel plane electrodes. We observed PD signal under the conditions of the AC electrical stress from 8 to 13kV/mm and the oil pressure from 0.00 to 0.25MPaG, and the insulation thickness from about 1 to 2mm or the oil gap thickness from 0.125 to 0.625mm. As a result, it is firstly revealed that the AC electrical stress, the oil pressure and the oil gap thickness affect the continuity of PD occurrence. Then, the maximum PD charge magnitude and the PD repetition rate increased with the oil gap thickness. The lead time from PD ignition to the breakdown was not much influenced by the insulation thickness.
The waveform immediately after the start of measurement has been used as a calibration waveform in the space charge measurement for HVDC cables. However, space charge accumulation may occur at high temperature during the acquisition of the calibration waveform, which may affect the accuracy of signal processing. In this study, we proposed a method to apply the waveform at the room temperature for the correction of the waveform at high temperature, for the space charge is unlikely to be accumulated at room temperature. A correction method of signal distortion under temperature was studied. As a result, space charge profiles considered to be reflected the temperature were obtained, and it is supposed that it improves the reliability of space charge profiles at high temperature.
Power cables are often laid in ducts buried underground of roads. In such installation, there is a possibility of so called cable creepage phenomenon in which cable moves in a direction of traffic flow. This phenomenon may result in abnormal stress on the cable, leading to damage. In order to take effective measures, it is important to accurately evaluate the cable movement force and amount generated by this phenomenon. So far, the cause of this phenomenon has been considered to be the vertical bend of the duct. The authors concluded that the cable creepage phenomenon is essentially caused by the horizontal displacement of the duct due to car loading. In the process of the passing of a car, the impulse due to the friction force acting on the cable is larger in traveling direction of the car than in the opposite direction. This impulse difference is considered to be the cause of the cable creepage phenomenon. Based on this theory, a cable ceepage computing system that can be used with conventional PCs was constructed.
Silicone rubber is widely used as the insulator for outdoor electrical equipment because of it's advantages such as light weight, compactness and easy to handle compared with porcelain one. However it is still not clear about their long-term performance under heavy contaminated conditions. To evaluate the long-term performance, exposure test under voltage application for 7 years were carried out using Outdoor cable termination made of silicone rubber. The results showed that the Silicone terminations approximately maintain the initial properties in electrical and physical characteristics.
Arc marks and cut wires on an ground wire are mainly checked through by a helicopter. When the helicopter cannot be used, a machine that incorporate a video camera is used. The machine attached wheels runs on the ground wire and takes a video of ground wire. After recoding videos, a worker check whether or not, there is an arc mark and cut wire in the video. There are few faults in the video. The task is very bored for the worker, therefore, it is required to reduce the amount of the video that the worker has to check. We have developed a new method that extracts images that could include those faults and discards other images. The method detects an arc mark, cut wire and corrosion product that appears on the surface of the ground wire due to inner corrosion, based on color feature histogram. The features are learned by one of machine learning method, which is called Support Kernel Machine (SKM). To verify the method, 100 images including arc marks and 186 images including corrosion products are used. 89 arc marks images are detected, 169 images that corrosion products appear are detected. Through the verification, the effectiveness of the proposed method was presented.
60kV class XLPE cable has been widely applied for power grids since 1964. In its early stage, its production technologies had been developed drastically including dry-curing method for cross linking process of cable insulation layer to avoid water-tree degradation. The developed XLPE cable has widely introduced into power grids in 1980's and 1990's and some of them are still under operation.
In order to clarify the electrical insulation performance of such highly aged XLPE cable body, this paper reports some results of the pre-breakdown discharge detection test for 60kV class decommissioned XLPE cables. The experimental investigation revealed that the electrical insulation performance in some specimen degraded to around 10% of their initial electrical insulation performance and some of them kept high performance as high as their initial condition. It is also depicted that the degradation cause was water trees even for the dry-cured XLPE cable.
This paper considers a future electric power system that is based on renewable energy resources and storage batteries. It proposes a control method of an inverter for the storage battery that adjusts the supply-and-demand balance of the future power system. A large overcurrent occurs when a short circuit fault happens on the power system side. The authors propose the strategy that control so as not to stop the inverter by the overcurrent. And it is verified by simulations. The authors also examines the influence on protection relay from the analysis of the fault current when a system fault occurred.
Recently, renewable energy has been penetrated into power supply in Japan. However, the high penetration of renewable energy causes a surplus of electricity. Reducing the surplus, the control methods of demand using household equipment have been proposed. In this study, we focused on heat pump water heaters (HPWHs). We simulated operation of HPWHs controlled to reduce the surplus. we simulated the supply-demand balancing of power system with the power demand of HPWHs. We evaluated the effect of contribution to power system to control HPWHs to reduce the surplus electricity from the viewpoint of the amount of surplus, CO2 emission, and fuel costs.
To realize larger-capacity “Hybrid-core transformer: HBT”, this research has assembled a 30-MVA three-phase trial HBT and evaluated the loss performances. The hybrid-core consisting of the wound amorphous- and the stacked silicon steel-cores, is expected having advantages of lower iron loss of the iron-based amorphous material and the higher mechanical strength and saturation magnetic flux density of the silicon steels simultaneously. Three-dimensional finite-element method analysis revealed that the silicon steel core prevented over-saturation of the amorphous core. The hybrid configuration enabled the design to have an approximately 10% higher magnetic flux density than configurations with only an amorphous core. We then assembled a prototype 30-MVA three-phase HBT designed on the basis of our investigation results. Operation tests demonstrated that the HBT has 62%-reduced iron loss and 0.10%-higher 50%-loaded power efficiency from the conventional silicon steel core transformer.
Regarding a PTFE (Polytetrafluoroethylene) nozzle ablation due to arc radiation exposure in SF6 circuit-breaker, the ablated mass of the nozzle was formulated as a function of an arc current and an internal radius of the nozzle surface considering the radiation power absorbed on the surface. Furthermore, this formulation contributed to subsequent development of calculation method for evaluating gas pressure rise process during current interruption. And, verification experiments of the calculation method were performed with a model SF6 circuit-breaker equipped a puffer chamber. Calculation results derived from the developed calculation method showed good agreement with measured gas pressure rise process. On the basis of the verified results, a discussion was made to explain pressure rise mechanism.
The series resistance of a transmission line in high frequency phenomena such as lightning surge becomes high due to the skin effect, and affects switching and lightning overvoltages in an electric power system. A number of transmission line models have been proposed to consider the frequency dependence of line constants. It is difficult to denote surge characteristics of a transmission line in time domain due to the existence of the series resistance. This paper proposes approximate formulas for terminal voltages on the transmission line in time domain. The approximate formulas are derived on the basis of the lattice diagram method. The expression of the approximate formulas is given by exponential functions, and is very simple. The formulas are useful for engineers to discuss many parameters and to understand simulation results for transmission line having high series resistance.
The R&D Steering Committee is working in planning and Steering of the Research and development of Power and Energy Society. In this article, activities of the committee of the last term are reported, and recent trend and future problems are also discussed. The process of planning and steering of the research and development, and the challenges to activation of Power and Energy Society are shown.