DC Superconducting Power Transmission Line Research Articles

Low voltage and large current DC superconducting power cable designs for 10 km to 100km transmission line using experimental data of Ishikari project

Main merit of the DC superconducting power transmission line is low loss even in low voltage power transmission. It should be realized when the transmission length is long. Therefore, we discuss and analyse an aspect of thermo-hydrodynamic performance of Ishikari project, and show one design of 100 km transmission line. At the same time, we also discuss the cable design and the rated voltage of the cable is lower than 50 kV for 500MW transmission. This is one of the most important advantages to make the DC power grid because we can omit an ultra-high voltage substation, the primary and the secondary substations and we can connect the power stations to the distribution stations directly. Finally, we may be able to reduce the construction and operation costs of the power grid.

Construction and the Circulation Test of the 500-m and 1000-m DC Superconducting Power Cables in Ishikari

A demonstration project of the dc superconducting power transmission line using the BSCCO wires was started in Ishikari, Japan as a national project (Ishikari project) from 2013FY. In the project, two power lines have been installed. Line 1 connects a photovoltaic power plant and an Internet data center by a 500-m underground cable for practical use, whereas line 2 with a 1000-m cable will be used for verification tests. In this project, we developed a new design of an adiabatic double tube with reduced heat leaks, i.e., <;1.5 W/m. We have also adopted the Peltier current lead that drastically cuts heat transfer at the cable terminal below 35 W/kA. To control the thermal contraction and expansion of the cable to avoid its breakage, we adopted two techniques, namely, a movable cryostat and a helical deformation technique. The construction was started in the field in June 2014 and completed at the end of April 2015 for line 1. Then, we performed the first cooling test for line 1 from May to June in 2015, during which we have confirmed that the thermal performance is as good as what we designed, and the current of 5 kA was successfully transported.

Iron-Steel Cryogenic Pipe for DC Superconducting Power Transmission Line

The 200-meter DC cable test facility was constructed in Chubu University. The cryogenic pipe is composed of two pipes, and the material of the inner pipe is stainless steel (SS304). The outer pipe is the straight pipe, and made from the iron steel because of high strength, low cost and not low temperature. If the cable structure is co-axial, the magnetic field is not generated outside the cable, and the ferromagnetism of the iron steel is not affected to the electric circuit. However, if the single core cable is used, the ferromagnetic effect of the outer pipe should be considered. Therefore, we measured the inductances of the cable after the construction of the experimental device. The measured inductance of the co-axial cable is matched with the analytical value, and it is very small. But the measured inductance of the single core cable configuration, it is 0. 4 mH. This is a large value, and therefore we can save the inductor in the power converter. Moreover, the energy storage can be available in the grid power line if large current operation is used.

Recent Progress of Experiment on DC Superconducting Power Transmission Line in Chubu University

A test stand of a DC superconducting power transmission cable was finished to construct in October 2006 in Chubu University, Japan, and three cooling cycles were carried out to measure the properties of the cable. Critical current of HTS tapes in the cable was measured at every cooling cycle and shows the similar temperature dependence; conclusively, the HTS tapes suffered no damage after cooling and heating process between the each cycle. Peltier current leads were partly installed in the test stand, and it was measured that the temperatures of the feedthrough near to the liquid nitrogen were decreased in spite of a current feeding.

Research activities of DC superconducting power transmission line in Chubu University

A experiment of a DC Superconducting power transmission line using HTS conductor was started in Chubu University, Japan in autumn 2006. The first cooling down began in October 2006. The coolant is liquid nitrogen, and the cooling system used a cryogenic cooler and cold pump. The operation temperature is the range of 72 K - 80 K. The power cable has a total length of 20 m, and composed of thirty-nine Bi-2223 HTS tape conductors with critical current approximately100 A. The power cable achieved a 2.2 kA. The insulation voltage of the cable is 20 kV. In order to reduce the heat leakage and to avoid the current imbalance in the HTS tapes, we installed the Pelteir Current Lead (PCL) for each of the nineteen HTS tapes of the cable, with remaining twenty HTS tapes connected individually by the usual copper leads (CCL). Depending on the visual observation, and the measurement of temperatures of the current leads, the heat leakage of the PCL is lower than that of CCL. We installed a current transformer for each individual HTS tape conductor circuit, and measured the current of each HTS tape conductor. We measured the critical current of each HTS tape after the cable was installed into the cryostat, and degradation was not observed. Since the variation of the current in each tape is less than 10%, we eliminated the problem of current imbalance. Computational Fluid Dynamics is used to estimate the pressure drop, showing that the straight-tube cryostat has the advantage against the bellows- and corrugated-tube cryostats to reduce the pressure drop of the circulation of coolant. We proposed further to use the siphon for circulating the coolant in order to reduce the circulation losses and costs. We proposed that the voltage of the system be kept below 30 kV in order to use low cost power inverters. This choice can increase the storage energy of the power transmission line itself if we do not use a co-axial cable system because of it allows use of large current. And the magnetic energy of power grid is estimated to 4.5MJ/km for ±30 kA.

Cryogenic System for DC Superconducting Power Transmission Line

Due to the recent developments of the electric power devices and the HTS power cables, DC superconducting power transmission system is considered to be practical. As one of the R & D issues of the DC superconducting power transmission system, we measured the radiation heat of the thermally-isolated stainless steel pipes with different surfaces. As the result, one sheet of aluminum foil pasted on the inner pipe surface reduced the heat radiation to 1.98 m/W between LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and the room temperature of 300 K.

Cryogenic Fluid Dynamics for DC Superconducting Power Transmission Line

Distributions of the velocity, the temperature and the pressure of a liquid nitrogen (LN2) flow are analyzed at hydraulic cross-sections on two types of superconducting power transmission lines (SC PT). One is on a DC-SC PT, the other is on a three phase AC-SC PT. Inflow heat from the pipe wall and the cables to LN2 are evaluated, however the heat flux from the DC cable is zero. The channels are straight, and hydraulic cross-sections are 3.77 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 4.27 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> on the DC and AC-SC PT lines, respectively. The increased temperature and the pressure drops on the DC-SC PT lines are lower than those of the AC-SC PT. The fluid velocity dependence on the distance between two cooling stations and on the pump power are estimated. When cooling stations are installed every 10 km, the flow velocity of the DC and AC system are 0.10 and 0.25 m/s, respectively. The pump power of the DC system is dramatically lower compared with the AC system. Therefore, minimizing the SC PT by using DC systems is much more practical.

Some Characteristics of Dielectric Materials at Cryogenic Temperatures for HVDC Systems

This dielectric study was performed as part of the dc Superconducting Power Transmission Line Program. During a screening test program, the dc breakdown strength of 12 dielectric materials, in sheet form, was determined at 12 K under 1.38 MPa of helium pressure, the operating conditions of the cable design. Tests were also made with four of these materials impregnated with mineral oil. Further dc breakdown tests were performed on five of these dielectric materials at various temperatures (12 to 298 K) and pressures (0.69 and 1.38 MPa) of helium. Two sets of experiments were performed on cellulose paper impregnated with distilled water at 83 K and 1.38 MPa of helium pressure. Five dielectric materials were then selected for tests as cable samples with dc and impulse voltages at 12 K and 1.38 MPa of helium. Data are presented on these tests, as well as on those of the electrical conductivity of four dielectric materials at cryogenic temperatures, and of the surface flashover strength of an epoxy bushing at 12 K and 298 K under 0.69 MPa and 1.38 MPa of helium pressures. The characteristics of the dielectric behavior at cryogenic temperatures are discussed. Impregnation of cellulose and copaco papers and PP/C with mineral oil improved their dc withstand strengths significantly, even at cryogenic temperatures. The dc breakdown field (uniform field con figuration) of cellulose and copaco papers (with and without oil impregnation) and PP/C varied as the exponential of the inverse of temperature.

Current test of a DC superconducting power transmission line

A test of a prototype dc superconducting electric power transmission line (dc SPTL) has been performed at 5 kA and 12 K. A supercritical helium refrigeration system was used to cool the test system which included a cable with a hollow central core so that the go and return refrigerant streams could countercurrently cool the inner and outer surfaces of the cable. Multifilamentary Nb 3 Sn superconducting wire was developed for the dc SPTL application and extensively tested to establish the optimum fabrication techniques and the expected response of the wire to cabling, operating, and installation stresses.

Bulk power transmission by superconducting DC cable

Abstract It is estimated that the total electric system load in the U.S.A. will increase five- to seven-fold by the year 2000. It is reasonable to assume that the transmission line capacities will also increase at least by the same amount. For energy conservation, future transmission lines must be more efficient than those of today. There will also be mounting public pressure to transmit electric power underground because of increasing awareness of environmental quality. There are considerable research activities to develop new technologies to transmit large blocks of power more efficiently by underground cables. The goal of the DC superconducting power transmission line (SPTL) program at the Los Alamos Scientific Laboratory is to develop such a cable. The DC SPTL has the advantage that it has very high power capability (10 GW and more) and no conductor or dielectric losses. The only “loss” of a DC SPTL is in the refrigeration requirement. It has no systems constraints, such as the stability or reactive compensation requirements of an AC system. The paper discusses the conceptual Los Alamos design, some of the technical problems associated with the development and the approaches to their solution.

Electrical, cryogenic and systems design of a DC superconducting power transmission line

The electrical, cryogenic, and systems design concepts for a dc superconducting power transmission line are discussed. The line consists of a hollow-core flexible coaxial cable, using multifilamentary or tape Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn superconductor and wrapped tape dielectric, within a rigid cryogenic envelope. Cryogen flows through the core, is expanded in a turbine and returns over the outer cable surface. The concept is applied to power levels of 1 to 10 GW and for a range of line voltages and maximum dielectric stresses. Systems considerations define capacity ratings and optimum operating voltages. Cost estimates for the various components and cost optimizations are presented.

Investigations on potential conductors for a D. C. superconducting power transmission line in a 20 meter test bed

The cryogenic and electrical aspects of a program to evaluate superconductors for a dc superconducting power transmission line are presented. Results for conductors in lengths of 20 meters operated at currents up to 10 kA are described. A battery driven power supply which is capable of delivering and controlling currents upto 50 kA is discussed. Specific experiments are discussed which have been designed to probe the response of the conductors to a variety of thermal and electrical conditions, such as: thermal gradients over the length of the conductor, single and two phase flow of cryogen, loss of coolant flow, operation at temperatures near T c , a.c. ripple current, and high current ramping rates.

Critical current densities in Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Ge between 14 and 21 K

Self field critical current densities of thick Nb 3 Ge films have been measured in the liquid hydrogen temperature range 13.8 - 21 K. Bulk critical current densities J c above 1.6 × 106A/cm2were observed at 14 K falling linearly at higher temperatures. The behavior extrapolated to zero critical current gave a critical temperature T c ∼ 19.2 K which is ∼1.5 K lower than the onset T c determined from the a.c. magnetic susceptibility. The samples were long sections of copper tubes, which had been coated on the inside with Nb 3 Ge by a chemical vapor deposition process. J c for various high T c materials are compared including calculations of adiabatic stability. Nb 3 Ge holds promise for a dc superconducting power transmission line operating near 14 K.