JPH02159927A - High temperature superconducting current limiter - Google Patents

Abstract

PURPOSE:To protect a current limiting element from thermal impact, in a high temperature superconducting current limiter employing a current limiting element composed of such material as exhibiting superconductivity at temperature higher than that of liquid nitrogen, by suppressing temperature rise below transition temperature to film boiling and resetting to superconducting state within a predetermined time after interruption of shortcircuit current. CONSTITUTION:A current limiting element 1 is cooled by liquid nitrogen 2. Liquid nitrogen 2 evaporates due to intrusion of heat or heating during current limitation, but it is liquified again through a cooler 3. A safety valve 5 is provided in order to protect a container 4 from explosive evaporation of liquid nitrogen. The container 4 is surrounding by a vacuum layer 6. A fan 7 is provided in order to circulate the liquid nitrogen forcibly.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Field of Application) The present invention relates to a high-temperature superconducting current limiting device that uses a substance that becomes superconducting above the temperature of liquid nitrogen as a current limiting element for suppressing short circuit current.

(Prior Art) A current limiting device is a device for suppressing short-circuit current and protecting equipment in power transmission and distribution systems.

Various systems have been developed as current limiting devices, and current limiting devices using a material that becomes superconducting at a temperature higher than liquid nitrogen temperature as a current limiting element are already known.

Y-Ba is a substance that becomes superconducting above liquid nitrogen temperature.
-Cu-0, Ba-ca-8r-o, Tn-Ba-Ca
Many compositions such as -Cu-0 have been discovered. Even below the critical temperature, these superconductors transition to a normal conductive state when a certain amount of current flows through them, and become resistors. This state is called quench. In addition, this current value is calculated as the critical current (1
o).

Current limiting devices utilize these properties of high-temperature superconductors. In other words, when the normal current value is set below the critical current (Ic) and a short circuit current flows, the critical current (Ic) is applied to the current limiting element.
Make sure that a current that exceeds the current flows. In this way, the current limiting element is normally in a superconducting state and has no resistance, but when a short circuit accident occurs, it becomes a resistor and suppresses the short circuit current.

FIG. 4 shows a configuration diagram of the current limiting device. In FIG. 4, 1 is a current limiting element placed in liquid nitrogen 2. The liquid nitrogen 2 evaporates due to heat intrusion or heat generation during current limiting, but is liquefied again by the cooler 3.

In order to prevent the container 4 from being destroyed due to explosive evaporation of the liquid nitrogen 2, the safety valve 5 opens when the pressure exceeds a predetermined value to reduce the pressure inside the container 4. This container 4 is surrounded by a vacuum layer 6 for heat insulation.

(Problem to be Solved by the Invention) However, in power transmission and distribution systems, there is a restriction that the short-circuit current must continue to flow for a certain period of time or more in order for the protective relay system to detect the short-circuit current.

This time is currently 0.3 seconds at the shortest. As the short circuit current continues to flow through the current limiting device, the current limiting element is under development and its temperature rises. In power transmission and distribution systems that use current-limiting devices, if a short-circuit condition is quickly restored, such as an instantaneous short-circuit caused by lightning, normal conditions must be restored immediately after the short-circuit current is interrupted (approximately 0.2 seconds later). . However, with conventional current-limiting elements using high-temperature superconductors, the temperature becomes too high and not only does it not return to the superconducting state within 0.2 seconds after the short-circuit current is interrupted, but the current-limiting element breaks down after several current-limiting operations. There was a problem that it could be done.

FIG. 5 shows the temperature change of a conventional current limiting element using a high temperature superconductor. FIG. 5 shows the temperature change of the current limiting element when a short circuit current is passed for 0.1 seconds and then cooled for 02 seconds.

The temperature of the current limiting element increases rapidly after the temperature rise ΔT exceeds 40K. This is due to the fact that the heat transfer from the current limiting element to the liquid nitrogen changes significantly at this temperature.

FIG. 6 shows the temperature change of heat transfer from an object in liquid nitrogen to liquid nitrogen. Heat transfer decreases significantly when the temperature difference between the object and liquid nitrogen exceeds 40 degrees. This is called nucleate boiling when the temperature difference is about 40°C, in which liquid nitrogen is almost in contact with the object, but when the temperature difference exceeds this, it is called film boiling, and a film of gaseous nitrogen forms on the surface of the object. This is because heat transfer is inhibited.

Further, the temperature of liquid nitrogen is 77, and the critical temperature of Y-Ba-Cu-0 among high-temperature superconductors, for example, is about 90, so that the superconducting state will not be restored unless 8T is reduced to about 10 or less. In the example shown in Figure 5, even if the current limiting element is cooled down for 0.2 seconds after interrupting the short circuit current, the current limiting element does not return to the superconducting state because the ΔT is approximately 100, and therefore the normal state will be restored even if the short circuit accident is resolved. do not.

As mentioned above, current limiting elements using high-temperature superconductors reduce the temperature rise during current limiting, maintain a nucleate boiling state with high cooling efficiency, and return to a superconducting state within a predetermined time when short-circuit current is interrupted. This is necessary. This also prevents the element from being destroyed by thermal shock.

By focusing on the boiling characteristics of liquid nitrogen, the present invention makes it possible to return to the superconducting state within a predetermined time after interrupting the short circuit current by sufficiently reducing the temperature rise of the current limiting element, thereby preventing destruction due to thermal shock. The objective is to obtain a high-temperature superconducting current limiting device that can

(Structure of the Invention) (Means and Effects for Solving the Problems) In order to solve the above problems, the present invention proposes to design a current limiting element so that the temperature rise is saturated in the nucleate boiling region of liquid nitrogen. Features.

(Example) An example of the present invention will be described with reference to FIGS. 1, 2, and 3.

The temperature rise ΔT of the current limiting element depends on the composition of the element and, if the critical current density is the same, the length of the element. FIG. 1 shows the relationship between element length and maximum humidity increase. When the element length becomes less than the length of the element, the maximum temperature rise increases rapidly. This is because, as shown in Figure 6, when the temperature rise exceeds the film boiling transition temperature of liquid nitrogen (40K in this example), heat transfer rapidly decreases.In other words, as the element length becomes shorter, the temperature increases. However, when the length exceeds 40K, the cooling by liquid nitrogen decreases rapidly, so the temperature rises rapidly.If the element length is designed appropriately in this way, the maximum temperature rise will be below the reduction temperature of liquid nitrogen to film boiling. It can be done.

FIG. 2 shows the temperature change of the current limiting element according to the present invention. Second
Similar to FIG. 5, the figure shows the change in T as the temperature of the current-limiting element rises when the short-circuit current is passed for 0.1 seconds, the current is immediately cut off, and the current is then cooled for 0.2 seconds. In this example, ΔK reaches saturation at about 20, and after that, ΔK reaches almost 0 in 0.2 seconds.
The current limiting element is cooled down to K and returns to the superconducting state.

The reduction temperature to film boiling varies greatly depending on the cooling conditions. For example, by forcibly circulating liquid nitrogen using a fan or the like, the temperature at which it returns to film boiling can be increased. It also depends on the wire diameter of the current limiting element; the smaller the wire diameter, the higher the reflux temperature and the higher the cooling efficiency.

FIG. 3 shows a configuration diagram of a forced circulation type current limiting device using a fan. In Figure 3, 1 is a current limiting element and liquid nitrogen 2
is cooled by. The liquid nitrogen 2 evaporates due to heat intrusion and heat generation during current limiting, but is liquefied again by the cooler 3. A safety valve 5 is installed to prevent destruction of the container 4 due to explosive evaporation of liquid nitrogen. Also container 4
is surrounded by a vacuum layer 6. A fan 7 is installed to forcefully circulate liquid nitrogen.

〔Effect of the invention〕

As described above, according to the present invention, in a high-temperature superconducting current limiting device that uses a material that becomes superconducting above the liquid nitrogen temperature as a current limiting element, short circuits can be achieved by suppressing the temperature rise to below the reflux temperature to film boiling. After the current is interrupted, the superconducting state can be restored within a predetermined time, and damage to the current limiting element due to thermal shock can be prevented.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the length of the current-limiting element and the maximum temperature rise, and Figure 2 is a temperature change characteristic diagram of the current-limiting element in the present invention. Fig. 4 is a block diagram of a conventional current limiting device using a high-temperature superconductor, and Fig. 5 is a temperature change characteristic diagram of a conventional current limiting device. FIG. 6 is a temperature change characteristic diagram of heat transfer from an object in liquid nitrogen to liquid nitrogen. 1... Current limiting element 2... Liquid nitrogen 3...
Cooler 4... Container 5... Safety valve
6...Vacuum layer 7...Fan agent Patent attorney Nori Chika Ken Yudo Daikomaru Ken! Length CP, u) of Ding-yang element

Claims (1)

[Claims] (1) In a high-temperature superconducting current limiting device that uses a substance that becomes superconducting above the liquid nitrogen temperature as a current limiting element to suppress short-circuit current, the maximum temperature rise of the current limiting element is in the nucleate boiling region of liquid nitrogen or equivalent to that temperature. A high-temperature superconducting current limiting device characterized in that the temperature is below a temperature range in which conductivity is obtained.