JPH0739218Y2 - Zero-phase current transformer shield structure - Google Patents

Description

[Detailed description of the device]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shield structure of a zero-phase current transformer used for detecting a ground fault current in a power distribution system.

[0002]

2. Description of the Related Art A zero-phase current transformer used to detect a ground fault current in a three-phase distribution line has a three-phase cable bundled together as a primary side, and a coil wound around this to form a secondary side. It has a structure. Ideally, the sum of the instantaneous values of the positive-phase currents of each of the three phases is always zero under normal conditions. Therefore, on the secondary side of the zero-phase current transformer, there are three currents proportional to the currents of the three wires. Is induced, and eventually no current flows on the secondary side.

[0003]

[Problems to be solved by the device] However, in reality,
The cable arrangement through the zero-phase current transformer is not arranged geometrically symmetrical with respect to the center of the coil. Due to this asymmetrical arrangement, the secondary side current of the zero-phase current transformer does not become zero even in the normal state, that is, the secondary residual current becomes large. Therefore, it has not been possible to increase the sensitivity for detecting the weak ground fault current that occurs in the power distribution system in the event of a ground fault.

Therefore, the problem to be solved by the present invention is to provide a structure in which an existing zero-phase current transformer can be installed without disassembling.

Shielding the non-equilibrium magnetic field generated by the asymmetry of the cable arrangement through the zero-phase current transformer.

Shielding the magnetic field by an external cable placed around the zero-phase current transformer.

[0007]

[0008]

In order to solve this problem, the shield structure of the zero-phase current transformer of the present invention has a flanged cylindrical inner part made of a plurality of circumferentially divisible magnetic materials. It is a shield whose height is almost the same as the height of the zero-phase current transformer.
Mounting an inner shield equal height in the circumferential cable insertion hole of the zero-phase current transformer, was assumed fixed.

In this structure, an outer shield with a collar that shields a magnetic field from an external cable arranged around the zero-phase current transformer may be further attached to the zero-phase current transformer.

[0010]

The three feedthrough cables of the zero-phase current transformer are not arranged geometrically symmetrically with respect to the feedthrough holes. This asymmetry causes non-equilibrium in the magnetic flux that links the coils of the zero-phase current transformer. In this case, if an internal shield is attached to the zero-phase current transformer, regardless of the asymmetrical arrangement of the feedthrough cables,
Since an equal magnetic surface is formed by the inner shield surface, the magnetic flux interlinking with the coil of the zero-phase current transformer located outside thereof is constant in the circumferential direction. That is, when a balanced three-phase alternating current is flowing in the cable, the alternating magnetic field having a phase difference of 120 ° is canceled and the secondary residual current of the zero-phase current transformer becomes zero. Actually there is some leakage magnetic flux and it does not become zero,
The secondary residual current can be effectively reduced.

The actual zero-phase current transformer installed in the switchboard of the substation is also affected by the magnetic fields generated by other external cables in the switchboard other than the through cables. In order to prevent the external magnetic field from interlinking with the coil of the zero-phase current transformer, an external shield is attached to the outer circumference and the side surface of the zero-phase current transformer. In this case, the outer shield surface becomes an isomagnetic surface, so
It is possible to prevent the external magnetic field from interlinking with the coil of the zero-phase current transformer.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on the embodiments shown in the drawings.

FIG. 1 is an exploded perspective view of a shield showing an embodiment of the present invention, and FIG. 2 is a perspective view showing a use state of the shield according to the present invention.

In these figures, the cable 5 is, of course, attached to the zero-phase current transformer 1 arranged in the switchboard of the substation.
The zero phase current transformer 1 cannot be removed from the panel for mounting the shield. In consideration of this, in the present invention, a split type shield structure is adopted, which is attached to the zero-phase current transformer 1 as it is installed.

Specifically, the split inner shields 2 and 3 are attached to the inner circumference of the cable through hole of the zero-phase current transformer 1. These inner shields 2 and 3 are each formed by combining three steel plate-made cylindrical pieces with flanges in this example, and in order to eliminate the influence of the gap of the seam of the first inner shield 2,
The second inner shields 3 are shifted by 60 ° and are arranged in double. The first and second shields 2 and 3 are connected to each other by bolts at the flange portion to be integrated.

Further, in order to shield the magnetic field from the external cable 6, the outer periphery and the side surface of the zero-phase current transformer 1 are covered with an external shield 4 formed of a steel plate-made cylindrical piece with a collar. The outer shield 4 is fixed by being connected to the first and second shields 2 and 3 by bolts.

The shield structure constructed as described above is
The test was carried out on a zero-phase current transformer with a window diameter of 190 mm installed in a 7.2 kV, 1200 A switchboard.

As a result, the through cable 5 of the zero-phase current transformer 1
The secondary residual current was 2.65 mA when a three-phase alternating current of 1200 A was applied to it, but the shield was not attached.
It could be reduced to A. In addition, when 1200 A was applied to the external cable 6 arranged near the zero-phase current transformer 1, the secondary residual current was 1.47 mA without the shield, but the secondary residual current was 1.45 mA.
Could be reduced to However, in each case, the secondary load was a power factor delay of 0.5 and 10Ω.

[0019]

As described above, according to the present invention, the following
Produce an effect . The inner shield of the present invention has a separable cylindrical shape with a collar.
And its height is almost equal to the height of the zero-phase current transformer,
Attach to the cable through hole of the zero-phase current transformer and change the collar part to zero-phase current
The existing zero-phase current transformer can be separated by simply fixing it to the transformer with bolts, etc.
The shield can be attached without understanding . Inserting an internal shield allows the penetration cable to be
Can shield the non-equilibrium magnetic field generated by the asymmetry of the position
I can . Also, by installing an external shield with a collar, the external cable
The magnetic field due to the cable can be shielded . This allows secondary residual current and external cable
The effect of the magnetic field generated can be greatly reduced, and it can be used in the distribution system during a ground fault.
It is possible to increase the sensitivity to detect the weak ground fault current that occurs.
Wear.

[Brief description of drawings]

FIG. 1 is an exploded perspective view of a shield showing an embodiment of the present invention.

FIG. 2 is a perspective view showing a use state of the shield according to the present invention.

[Explanation of reference numerals] 1 zero-phase current transformer, 2 first inner shield, 3 second inner shield, 4 outer shield, 5 through cable,
6 External cable

Claims (2)

[Scope of utility model registration request] 1. A cylindrical inner shield with a collar made of a magnetic material that is dividable in a plurality of circumferential directions , the height of which is
Is an internal shield with a height approximately equal to the height of the zero-phase current transformer.
Shield structure of the zero-phase current transformer, characterized in that the zero-phase current transformer of the cable insertion hole periphery to the mounting, and fixed. 2. The zero-phase current transformer is further provided with an outer shield with a collar that shields a magnetic field generated by an external cable arranged around the zero-phase current transformer.
Shield structure of the described zero-phase current transformer.