JPH07255122A - Overcurrent protective circuit - Google Patents

Abstract

PURPOSE:To provide an overcurrent protective circuit which prevents the burning of a load apparatus and makes it unnecessary to change current-limiting fuses frequently. CONSTITUTION:An overcurrent detected by a current transformer 8 on the secondary side of a transformer 1 is inputted to an overcurrent relay 7, and the overcurrent relay 7 opens a load switch 3 only when the current is in a range from the intersection of the minimum cut-off current of the current- limiting fuse 2 with the cut-off limit of the load switch 3 up to the outside of a normal current region.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent protection circuit protected by a current limiting fuse and a load switch provided on the primary side of a transformer.

[0002]

2. Description of the Related Art Since a load switch does not have the ability to interrupt a short circuit current by itself, an economical facility is obtained by combining a current limiting fuse to protect the short circuit current.

Generally, there are two types of current limiting fuses, a wide area fuse and a backup fuse. Since the minimum fusing current is guaranteed for each fuse, the current to be protected by the current limiting fuse must be selected so that it will be greater than or equal to the minimum breaking current of the current limiting fuse in use.
A wide-area fuse is a fuse that can interrupt the current in all areas from the minimum fusing current to the rated breaking current. The backup fuse is a fuse that can interrupt all currents from the rated minimum breaking current to the rated breaking current. Here, the relationship between the minimum fusing current and the rated minimum breaking current is the relationship of the minimum fusing current <the rated minimum breaking current, and the fusing current is a region where the current continues to flow even if the fuse is blown and cannot be broken. Of current.

As a load protection circuit using a current limiting fuse, for example, the issue date ... April 20, 1993, the issue place ... The Japanese Standards Association, "JIS Handbook Electric",
Page 972 The C4620 PF.S type distribution circuit includes a load protection circuit that opens the load switch after the current limiting fuse on the primary side of the transformer is blown. In the load protection circuit, when an overcurrent flows to the secondary load side of the transformer, the current limiting fuse is blown due to the magnitude of the overcurrent.
Or not.

[0005]

That is, since the high-voltage or extra-high-voltage current limiting fuse is a self-extinguishing method using arc energy, there is a limit to the current value (minimum breaking current close to the rated current) that can be interrupted. For this reason, when the fuse element reaches the end of its life, it may be blown out during current flow.
When the fusing current is small and the generated arc energy is small, self-extinguishing cannot be performed, and the arc lasts for a long time, and in the worst case, a short circuit occurs outside the fuse, and for example, the load device motor may be burned.

An object of the present invention is to provide an overcurrent protection circuit which prevents a load device from being burnt out and prevents frequent replacement of a current limiting fuse.

[0007]

The overcurrent protection circuit of the present invention inputs the overcurrent detected by the current detector on the secondary side of the transformer to the overcurrent relay, and the overcurrent is the minimum fusing current of the current limiting fuse. The overcurrent relay issues an open command to the load switch only when it is between the value and the rated minimum breaking current value.

[0008]

As a result, since the overcurrent between the minimum fusing current value and the rated minimum breaking current value can be interrupted by the load switch, the overcurrent does not flow into the load circuit, and not only can the load circuit be prevented from burning. Also, it has become possible to prevent frequent replacement of the current limiting fuse.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiment of the overcurrent protection circuit shown in FIG. 1 of the present invention is used for equipment in a substation, and will be described below.

The primary side of the transformer 1 is connected to a current limiting fuse 2 and a load switch 3. After the current limiting fuse 2 is blown by the short circuit current, the fuse switch mechanism 4 opens the load switch 3. Current limiting fuse 2 and load switch 3
The time characteristic diagram of the above will be described with reference to FIG.

The secondary side of the transformer 1 has a plurality of circuit breakers 5 for wiring.
And a load such as a motor 6 are connected. Between the secondary side of the transformer and the wiring breaker 5, a wiring breaker 7 and a current transformer 8 are provided.
Are connected. The overcurrent relay 9 is connected to the secondary side of the current transformer, the ammeter 10 and the trip coil 11. The trip coil 11 normally closes the load switch 3, and when an overcurrent having a timed characteristic flows, the trip coil 11 is excited to open the load switch 3. The opening time is determined by the time characteristic diagram of the overcurrent protection cooperation including the current limiting fuse 2 and the load switch 3 in FIG.

FIG. 2 is a time characteristic graph determined by the relationship between the time t on the vertical axis and the current i on the horizontal axis. A time characteristic chart X1 is a fusing characteristic chart of the current limiting fuse 2. The time characteristic X2 is an operation characteristic diagram of the load switch 3 and the overcurrent relay 9. i1 is a normal current region, i2 is a current region from outside the normal current region of the current limiting fuse 2 to the minimum fusing current i3, and i4 is a rated minimum breaking current. Further, if an intersection A is obtained between the rated minimum breaking current i4 of the current limiting fuse 2 and the breaking limit of the load switch 3, the left side of the intersection A can be protected by the load switch 3, and the right side of the intersection A is limited. It can be protected by the current fuse 2 and is perfectly coordinated. That is, on the left side of the intersection A, the load switch 3 is cut off earlier than the current limiting fuse 2,
On the right side of the intersection A, the current limiting fuse 2 is the load switch 3
It will shut off sooner. Therefore, the overcurrent relay 9
From the normal current region to the rated minimum breaking current i4, or from the minimum fusing current i3 to the intersection A or the rated minimum breaking current i.
Each overcurrent value up to 4 and time t proportional to this overcurrent value i
The load switch 3 is instructed to be released by and.

Now, in the motor 6 of the overcurrent protection circuit, for example, when dust is present between the shaft and the bearing, that is, the rotor does not rotate and is locked, the current flowing to the armature is from the normal current region i1. In the overcurrent region i2, the overcurrent relay 9 works to open the load switch 3, so that the burnout of the motor 6 and the damage of the load switch 3 can be prevented.

When the operating characteristic X3 of the load switch 3 and the overcurrent relay 9 is indicated by a chain double-dashed line, the intersection A with the current limiting fuse blowing characteristic is the minimum current interruption impossible region of the current limiting fuse 2. It is located at B, and there is no coordination. That is,
When an overload current between the intersection A1 and the rated minimum breaking current i4 of the current limiting fuse 2 flows, the current limiting fuse 2 causes the load switching device 3 to operate despite the current in the unbreakable range of the load switching device 3. It melts faster, and the load switch 3 operates for a time t to be cut off. The current limiting fuse 2 cannot be reused, and the current limiting fuse 2 needs to be replaced frequently, resulting in uneconomical equipment. In the present invention, the intersection point A
When an overcurrent from 1 to the rated minimum breaking current i4 flows,
Since the overcurrent relay 9 operates and opens the load switch 3, the above-mentioned drawbacks can be eliminated and frequent replacement of the current limiting fuse 2 is no longer necessary. When a plurality of transformers are connected with one current limiting fuse, the same effect as that of the present invention can be obtained by adding a device that divides the current on the secondary side of each transformer.

[0015]

As a result, the minimum breaking current and load switch 3
The crossing point A of the breaking limit or the overcurrent in the current range between the rated minimum breaking current i4 and the outside of the normal current area does not flow to the load circuit, so that the load circuit can be prevented from being burned and the current limiting fuse 2 must be replaced frequently. You can now prevent it from doing so.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an overcurrent protection circuit of a power distribution system as an embodiment of the present invention.

FIG. 2 is a time characteristic diagram illustrating overcurrent protection coordination of FIG.

[Explanation of symbols]

1 ... Transformer, 2 ... Current limiting fuse, 3 ... Load switch, 6 ...
Motor, 8 ... Current transformer, 9 ... Overcurrent relay.

Claims (4)

[Claims] 1. An overcurrent flowing to a secondary load side of a transformer,
In the load protection circuit that opens the load switch after the current limiting fuse on the transformer primary side is blown, the overcurrent detected by the current detector on the transformer secondary side is input to the overcurrent relay to limit the overcurrent. An overcurrent protection circuit in which an overcurrent relay issues an open command to a load switch only when it is between a minimum fusing current value of a fuse and a rated minimum breaking current value. 2. The overcurrent relay is designed to open the load switch only in the current range from outside the normal current of the current limiting fuse to the intersection of the breaking limit of the load switch and the rated minimum breaking current of the current limiting fuse. The overcurrent circuit according to claim 1, which is characterized in that: 3. The overcurrent protection circuit according to claim 1, wherein the load switch is opened by a wiring breaker provided on the secondary side of the transformer and opened by an overcurrent. 4. An overcurrent flowing through the secondary load of the transformer,
In the load protection circuit that opens the load switch after the current limiting fuse on the primary side of the transformer is blown, the overcurrent detected by the current detector on the secondary side of the transformer is input to the overcurrent relay, and the load is switched by the overcurrent relay. Overcurrent protection circuit characterized by opening the device.