20-cm-Long Fault-Current Limiter Element Using Superconducting Oxide Thin Film, Remarkably Improved

Contents:１) Improvements, 2) Brief summary, 3) Background description, 4) Principle description, and 5) New fault-current limiter element

1) Improvements
a) Increase of fault-current limiter element capacity
The research team succeeded in increasing the capacity of the new fault-current limiter to 500 V/200 A.
b) Remarkable cost reduction of the fault-current limiter element
Withstand voltage of the fault-current limiter element is significantly increased.
The fault-current limiter element can withstand the voltage 4 times as high as of the previous fault-current limiter element. In other words, the required length of the superconducting oxide thin film could be reduced to 1/4 or shorter, and this fact leads to remarkable cost -reduction of the current limiter element.

The improvements were made by a research team, led by Hirobumi Yamazaki, in superconductor technology group, Energy Technology Research Institute (RTRI), AIST (national institute of advanced industrial science and technology)

2) Brief summary
A new fault-current limiter element of only 20 cm long was constructed, and tested while being placed in liquid nitrogen. It was confirmed in the test that current of 237 A (effective value, rms) flowed through the fault-current limiter element, at zero resistance. A further test of the fault-current limiter element was conducted under high voltage of 500 V.
It was confirmed that the fault-current limiter element instantaneously limited fault-current to 1/3 or smaller. Unless the fault-current limiter element is used, the fault-current was 2.26 kA. Also in the test, current flow of five cycles was observed (also see Figs. 4 and 6). Connection of eight fault-current limiter elements could form one phase of a fault-current limiter element of a 3-phase 6.6 kV/200. “From the above experimental facts, it is confident to realize a practical superconducting fault-current limiter of several tens kV/several hundreds amperes, which is featured by low cost, compact, low loss, and quick response,” said AIST.

3) Background description
Wind power generators are increasing and will be more increasing in the near future.
If a fault, for example, a short-circuiting fault, occurs in one of the wind power generators, which are connected to the existing electric power distribution network,
short-circuit current flows into the power distribution network and will give rise to a serious problem, for example, damaging of the power facilities, in the network.
The fault-current limiter prevents such an overcurrent from flowing into the power distribution network. The superconducting thin film fault-current limiter element, developed this time, will be, for sure, one of the effective solutions to the problems which will occur when a number of the distributed power systems based on new energy#1 are connected to the existing electric power distribution network.
Actually, there is a strong demand for the superconducting fault-current limiter of low cost and high reliability. The reality is far from this demand, however.
A major reason for this is that the superconducting thin film is expensive.
There are many approaches to developing a fault-current limiter, which uses a superconducting tape wire having a metal base material.
The fault-current limiter needs the wire of which the length is 100 times the length of the superconducting thin film. From this it is readily seen that the current limiter has many problems to be solved, for example, high cost, large size and large loss (in the normal conducting state).

4) Principle description
Related diagrams = Figs. 1 and 2
The superconducting thin-film fault-current limiter element is made up of a superconducting oxide thin film, a gold-silver alloy layer, and a non-inductive winding shunt resistor.
The superconducting oxide thin film was formed by using the MOD process, originally developed by AIST. (The S - N transition property of the super conducting thin film is utilized for the fault-current limiter. S = superconducting and N = normal.)
The gold-silver alloy layer (having high resistivity) was vapor-deposited on a superconducting oxide thin film (having a large area).
The gold-silver alloy layer serves as a shunt protecting layer.

In terms of circuit connection, both ends of the superconducting oxide thin film is connected to the power source terminals. The shunt protecting layer and the non-inductive winding shunt resistor are connected in parallel with the superconducting oxide thin film (Fig. 1). The shunt protecting layer and non-inductive winding shunt resistor are provided to avoid the hot-spot phenomenon#2, which inherently occurs in the superconducting oxide thin film.
In the fault-current limiter element constructed above, with high resistance of the superconducting oxide thin film, the voltage (tolerable electric field) that is allowed to be applied to the superconducting oxide thin film per unit length of the film, could be increased to have a voltage value, which is about 4 times as large as that when the Au-Ag shunt protecting layer of high resistivity is not used. In other words, the necessary length of the superconducting thin film could be reduced to 1/4 or shorter, resulting in cost reduction of the current limiter element. This fault-current limiter element well operated where it is small in size. The current limiter element was proposed on November 2004.
[Copyright by FuelCell japan]
[Copyright by FuelCell japan]

5) New fault-current limiter element
In the electric power distribution network including the fault-current limiter elements coupled to the network, when the electric power handled by the network is large, the fault -current limiter element must be designed to have a large rated current.
Accordingly, the ampacity of the superconducting thin film of the fault-current limiter element must also be increased. The increase of the ampacity raises the level of the hotspot problem. The researchers have made a try of supplementing the overcurrent shunting function that the external shunt resistor has. Exactly, a capacitor of small capacitance, commercially available, was connected across the superconducting thin film (Fig. 2). The external resistor and the capacitor, together with the superconducting thin film, were placed in liquid nitrogen. ….
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Source: AIST’s press release

Tags: fault-current limiter, fault-current limiter element, superconducting oxide thin film, AIST, wind power generators, short-circuiting fault, electric power distribution network