Masahiko Nakade

Development of on-site diagnostic method for XLPE cable by harmonics in AC loss current

Water tree degradation in XLPE cables used for under-ground distribution or transmission lines is a serious problem. Some diagnostic methods to detect water tree degradation have been developed, but the sensitivity of them is not sufficient for 22 kV/66 kV class XLPE cables. The reason is that conventional methods are based on detecting signals from water trees bridged over insulation layer. In case of 22 kV/66 kV class cables, even unbridged water trees will be harmful because of their higher operating stress, so that there are demands on highly reliable new diagnostic methods for such class cables. We found that water tree in XLPE insulation generates harmonics in the AC loss current because of the non-linear voltage-current characteristics of itself. And, we found that the harmonics in AC loss current are closely correlated to water tree growth, break down strength, and the other aspects of the degradation. We made the on-site diagnostic system based on those results, and have applied to the actual 66 kV XLPE cable lines more than 22. As the result, we confirmed that it was suitable for the diagnostic method for 22 kV/66 kV class cables.

On-site application and improvement of water tree deterioration diagnosis for XLPE cable by residual charge method

ldquoResidual charge methodrdquo, which has been developed for detection of non-bridged water tree, is well known as a high reliability water tree deterioration diagnosis technique for XLPE cable, and it has been applied for on-site cable system diagnosis. Traditional residual charge method measures the amount of electrical charge, and the information of cable length is required to evaluate the result. In this paper, we would like to introduce our development of the new technique of residual charge method which does not need the information of cable length to evaluate the deterioration degree.

Life estimation of water tree deteriorated XLPE cables by VLF (very low frequency) voltage withstand test

The authors investigated a new life estimation method for service-aged, water tree deteriorated 22-77 kV XLPE cables by voltage withstand test. At first, they selected more suitable waveform for testing voltage from damped oscillating wave (OSW) and very low frequency wave (VLF) voltage as the instead of AC and DC. It was found that VLF voltage has high ability to detect of water tree, while was less harmful for water tree deteriorated cables. Breakdown tests under AC and VLF were carried out, using model cables and service aged XLPE cables. Based on the breakdown tests, they calculated optimal test voltage for remaining life estimation. Recently, they had onsite testing and confirmed the effectiveness of VLF testing.

The current status of superconducting fault current limiter development

Abstract Demand for electric power in Japan has been steadily increasing in recent years.To meet this demand, power networks have been more robustly connected with a view to improving their reliability. However, this has been accompanied by a trend toward a greater incidence of fault currents in these power networks. If a fault current limiter is used to connect these systems, electric power utilities will be able to accommodate electric power through the superconducting limiter from a system with excess capacity to another system in which the available power is insufficient, while the limiter protects systems from the influence of faults in a system. The authors have developed a 6.6 kV/2.0 kA class superconducting fault current limiter used for power distribution substations, as the preliminary stage of development of current for trunk power systems. In order to reduce impedance under normal operating conditions and quickly switch to high impedance in a fault condition, the limiter consists of a pair of double-layer non-inductive superconducting windings connected in series. Each winding is wound with a 36-stranded AC superconducting wire having ultra-fine NbTi filaments in a high-resistivity matrix. The device successfully limited a 6.9 kA short-circuit current to 3.4 kA and transmitted continuous power at 2.0 kArms.

Development of a new program to estimate the conductor temperature of underground transmission cables in the buried ducts

Tokyo Electric Power Co. and the Japan Research Institute, Ltd. have jointly developed a new conductor temperature estimation program that makes use of the finite element method, as a tool for calculating at high precision the transmission capacity of a cable installed in a buried duct. This program newly incorporates seasonal changes in air and ground temperature, ground-surface boundary conditions for sunlight and radiation, and temperature changes in the depth direction. Heretofore it has been impossible to vary the load current, but this time it was possible to obtain more realistic simulation results than by the previous stationary-current analysis, by allowing transient-current analysis to which daily and seasonal fluctuations are added. And by employing the finite element method, it has become possible to calculate the temperature even for locations other than the cable conductor temperature (example: the cable surface temperature).

1 kArms Bi-2223 Superconducting Current Lead System for the Current Limiter

A 1 kA rms current lead system for the superconducting current fault limiter has been designed and tested. The lead system is composed of powder sintering (Bi,Pb)2Sr2Ca2Cu3O10+x tube lead (80-4 K), Cu lead (300-80 K), a liquid helium vessel and refrigerator which cools the junction of the Bi-2223 and Cu lead to 80 K. The Bi-2223 parts are placed in a vacuum and cooled by heat conduction. This lead system has successfully transported 1 kA rms for 3 hours under steady-state temperature distribution, with a total 4.2 K heat load, 3.5 W, which was a much smaller value than that obtainable with an all Cu lead. AC loss for the tubular Bi-2223 lead was also confirmed to be small.

Feasibility Study of High Tc Superconducting Materials to AC Downlink by Computer Simulation

The feasibility of applying high-temperature superconductors to AC downlinks was studied by computer simulation. The analysis was applied to three cases, i. e., a high-temperature superconductor only, a high-temperature superconductor combined with a gas-cooled copper lead, and a gas-cooled high-temperature superconductor combined with a copper lead. Two geometrical forms of high-temperature superconductive downlinks were considered, i. e., a circular configuration and a tubular configuration. The results revealed that the use of a high- temperature superconductor would permit a reduction of approximately 1/10 in the flow of heat into the cryostat as compared with a copper lead. As regards geometry, the tubular configuration was found most suitable for AC use, and gas cooling would permit an even further reduction in the inflow of heat.

Evaluation of AC Loss in High-Tc Superconductors for Power Cable Application

An important parameter which should be evaluated in detail for the design of a compact high-Tc superconducting cable is the AC loss associated with the alternative transport current. We have commenced the quantitative evaluation of the AC loss in high-Tc sintered bulks and Ag-sheathed wires. In order to clarify the differences in AC losses which are dependent on the frequency of applied magnetic fields, two methods have been used. One method is to measure magnetization curves by applying 50 Hz AC magnetic fields (AC method). The other is to measure magnetization curves by means of a SQUID susceptometer (DC method).

An Evaluation of Impulse Characteristics by Water Tree Growth in XLPE Cables

The relationship of AC breakdown performance and water tree has been reported. This time, using aged XLPE cables with some protrusion on the outer semiconductor layer, we evaluated the relationship of water tree growth and impulse breakdown performance. As a result, In case of XLPE cables without waterproof layer, it was confirmed that the impulse breakdown performance decreased because of water tree, and we got possibility the impulse breakdown performance depended on the progress of water tree growth. In addition, it might decrease to the around LIWV. On the other hand, in case of XLPE cables with waterproof layer, though water tree progressed slowly, the impulse breakdown performance was maintained for a long time. In other words, the effect of waterproof was proved. Even if XLPE cables with waterproof layer have big protrusion, most of water tree started from the protrusion does not grow up. But, if water tree grows up, it may largely reduce the impulse breakdown performance of XLPE cables with waterproof layer.

Dielectric Loss Characteristics of Oil Filled Cable Insulation Impregnated by Extremely Bad Tanδ Oil

In spite of being extremely large value of oil tandelta (several ten % of tandelta for example) in some oil filled (OF) cable splice box etc, there has been no such an event as a thermal breakdown due to dielectric loss up to the present. Therefore we examined the tandelta characteristics of OF cable insulation impregnated by extremely bad tandelta oil in detail. As a result, it turned out that the tandelta suppression effect by the following two reasons had acted, and was verified in the real use OF cable splice box insulation flowed by bad tandelta oil. (1) Tandelta decrease in high electrical stress (including operating stress of OF cable) region by so-called Garton effect which is related with the ion mobility of oil. (2) Tandelta decrease due to the absorption of ionic substance in oil to the insulating paper. The two above-mentioned effects are larger in the mineral oil than in the alkyl-benzene oil