Current-limiting device

Details Current-limiting device Current-limiting device

1999-07-15

2002-01-01

Leja, Ronald W. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Impedance insertion

C361S093900

Reexamination Certificate

active

06335851

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a current limiting device. More specifically, the present invention relates to a current limiting device which is applied to a general AC circuit including all electric power systems (low voltage power systems to ultra-high voltage power systems), and is used to suppress over-current attributing to short-circuit accidents or in-rush current which may flow upon application of load.
2. Description of the Related Art
For instance, all power systems (low voltage power systems to ultra-high voltage power systems) have a current limiting device to suppress over-current attributing to short-circuit accidents or in-rush current which may flow upon application of load so as to reduce the interrupting capacity required for the circuit breaker. For example, as a current limiting device, there are a super-conductive current-limiting element utilizing the quench phenomenon of a super-conductor, or a thermal current-limiting element of a positive temperature characteristic (PTC) thermistor.
The super-conductive current-limiting element utilizing the quench phenomenon of a super conductor, such as a film type super-conductive current limiting element, is to increase the resistance instantaneously when over-current higher than the critical current occurs. That is, it is popularly employed as a high performance element because it is high in response, and of a self-operation type; that is, it is high in reliability. On the other hand, the thermal current-limiting device of the positive temperature characteristic (PTC) thermistor is formed with polymer material or barium titanate material. It generates heat when over-current is allowed to flow therein, so that it increases the resistance. Hence, it is also popularly employed being economical, and simple in construction.
The current-limiting device utilizing the super-conductive current-limiting element or the thermal current-limiting element is such that, as shown in
FIG. 7
, a current-limiting element S is series-connected between the system power source
1
of the power system and the load
2
. For instance, in the case where a short-circuit accident a occurs to allow the flow of over-current, the resistance is increased because of the quench phenomenon. In the case of the thermal current-limiting element, heat is generated to increase the resistance. That is, the increase in resistance of the current-limiting element S suppress the over-current flowing in the electric power system. While the current-limiting element S performs its current-limiting function, a circuit breaker
3
is opened thereby to open the circuit of the electric power system.
In the above-described current limiting device, the current limiting element S is applied to the system whose linkage can be opened. However, the current limiting element can be applied to the system whose linkage cannot be opened. In the latter case, as shown in
FIG. 8
, a current limiting reactor
4
is connected in parallel to the current limiting element S. Normally, a current is allowed to flow in the current limiting element S. When a short-circuiting current flows in the element S; that is, when the element S is increased in resistance, the current is transferred to the current limiting reactor
4
, and the switch S is operated to electrically disconnect the current limiting element S from the system, so that the system is ready for restoration.
A current limiting device utilizing the rectifying bridge circuit is as shown in FIG.
9
. The AC terminals
15
,
16
of the rectifying bridge circuit
14
, which is formed with four diodes D
1
through D
4
, are connected in series to the electric power system, and a DC reactor
19
is connected between the DC terminals
17
and
18
. In the case where a high-speed response type current limiting effect is required, the current limiting device is suitable. Thus, it is popularly employed.
The current limiting device functions as follows when a short-circuiting accident occurs. That is, the AC current exceeds the DC current flowing in the DC reactor
19
due to the occurrence of the short-circuiting accident a, the pair of diodes D
1
and D
4
, or D
2
and D
3
are stopped. If this is viewed from the AC terminals
15
and
16
, during this period, the DC reactor
19
is inserted in series in the AC circuit, and the generation of a voltage across the DC reactor
19
suppresses the over-current flowing in the electric power system. While the rectifying bridge circuit
14
limits the current, the circuit breaker
3
is opened thereby to open the linkage of the electric power system.
By the way, the current-limiting device having the conventional super-conductive current-limiting element or thermal current-limiting element has the following problems.
First, the super-conductive current-limiting element is poor in thermal conduction in the direction of length, and it is considerably difficult to arrange the quench phenomenon occurrence characteristic in the direction of length. The thermal current-limiting element is considerably difficult to make the resistance-temperature characteristic completely uniform in the direction of length.
That is, in the case of those current limiting elements, only the element limited in length can be used in which heat generated partially is quickly thermal-diffused so that the whole is uniformly increased in temperature. Therefore, its rated voltage is of the order of 100V per an element. Hence, when it is applied, for instance, to electric power systems of 200V, 400V, 6600V or the like, at the time of current limitation, voltages corresponding to system voltages should be assigned to current-limiting elements. As a result, a plurality of current-limiting elements are connected in series.
However, if a plurality of current-limiting elements are series-connected, a common short-circuiting current flows in each of the elements. Therefore, if the elements are not uniform in characteristic, a part of the plurality of current-limiting elements is firstly increased in resistance. Consequently, heat-generating power concentrates at the element to thermally break the elements, or over-current concentrates as the elements to break the insulation thereof. This is a fatal problem.
Furthermore, the conventional current limiting device having the thermal current limiting element S, and the conventional current limiting device having the rectifying bridge circuit
14
have the following problems.
In the case that the thermal current limiting element S is the thermal current limiting element of a positive temperature characteristic (PTC) thermistor, the thermal current limiting element itself is simple in construction and economical. It functions in such a manner that, for instance at the time of a short-circuiting accident, the over-current generates heat, thus limiting the flow of current. However, the current limiting action has a delay of several tens of milli-seconds (msec), so that it is impossible to suppress the sudden current.
On the other hand, in the case of the rectifying bridge circuit
14
, at the time of a short-circuit accident, the inductance of the DC reactor
19
positively limits the over-current. However, the DC current flowing in the DC reactor
19
increases every half cycle. Therefore, in order to expect the current limiting effect for a certain period of time, for instance 50 to 100 msec, both the cost and the loss are increased. Hence, in order to maintain the current limiting function for a relatively long period of time, for instance for more than 50 msec from the occurrence of a short-circuiting accident, it is necessary to provide a larger DC reactor. That is, the current limiting device using the rectifying bridge circuit is not economical.
In order to eliminate the above-described difficulties accompanying those devices, the following current limiting device has been proposed. That is, the thermal current limiting element S and the rectifying bridge circuit
14
are combined to mutually complement their characteristics. I

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