Current transformer

Details Current transformer Current transformer

1999-05-20

2001-02-20

Donovan, Lincoln (Department: 2832)

Inductor devices

With electric and/or magnetic shielding means

C336S174000, C336S175000, C336S178000, C336S08400R

Reexamination Certificate

active

06191673

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a current transformer for measuring an electric current flowing through three phase bus bar conductors.
FIG. 9
is a schematic illustration of a conventional current transformer disclosed in Japanese Patent No. 2600548 and
FIG. 10
is a cross-sectional view taken along line A—A of the current transformer of FIG.
9
. In the figures, reference numeral
1
is one of the three-phase bus bar conductors which is a line to be measured in terms of the electric current.
2
a
and
2
b
are iron cores for defining a magnetic path intersecting the bust conductor
1
, and
3
a
and
3
b
are secondary windings wound on the iron cores
2
a
and
2
b
for measuring the electric current flowing through the bus conductor
1
to be measured. The secondary windings
3
a
and
3
b
are not illustrated in FIG.
9
.
4
a
and
4
b
are shield windings, each of which comprises four coils having the equal number of windings wound on the iron cores
2
a
and
2
b
to extend over the equal circumferential distance of the iron core. The shield windings
4
a
and
4
b
respectively surround in intimate contact the current transformers
8
a
and
8
b
which are disposed in the direction of the longitudinal axis of the bus conductor
1
. The shield windings
4
a
and
4
b
are for alleviating the influence of the electric current flowing through the neighboring bus conductors
6
a
and
6
b
on the electric current flowing through the secondary windings of the bus conductor
1
.
5
is a connection line for connecting together the terminals of the same polarity of the shield winding
4
.
6
a
are two of three-phase bus conductors, which are adjacent to the bus conductor
1
to be measured.
6
b
are conductors connecting the bus conductor
1
and the bus conductors
6
a
to each other for providing a neutral point, which is a junction between the bus conductor
1
and the bus conductors
6
b
.
8
a
and
8
b
are current transformers (
8
a
and
8
b
are transformer units, which generally referred to as a transformer), which are composed of iron cores
2
,
2
a
and
2
b
, secondary windings
3
a
and
3
b
and shield windings
4
,
4
a
and
4
b
, respectively. L is a distance between the iron cores
2
a
and
2
b.
The operation of this current transformer will now be described. In
FIGS. 9 and 10
, the electric currents flowing through the secondary windings
3
a
and
3
b
are proportional to the current flowing through the bus conductor
1
to be measured, so that the electric current flowing through the bus conductor
1
can be measured by the secondary windings
3
a
and
3
b
. The shield windings
4
a
and
4
b
are wound around the iron cores
2
,
2
a
and
2
b
, respectively, divided into four along the circumference of the iron core and the same polarity of each coil is connected together by the connection line
5
, so that the electric current flowing through the bus conductors
6
a
and
6
b
induces an electric current in the shield windings
4
a
and
4
b
, thereby reducing the magnetic flux penetrating into the iron cores
2
,
2
a
and
2
b
. Also, it is possible to ensure that the magnetic flux generated by the induced current does not affect the current in the bus conductor
1
to be measured.
However, since there are two secondary windings and two shield windings between the iron cores
2
a
and
2
b
, generating a large amount of heat and since the heat dissipating surface area at this portion is small, the current transformer generates a large amount of heat.
Also, as for the current transformer
8
a
far from the bus conductor
6
b
constituting the neutral point, the mutual inductance between the bus conductor
6
b
and the shield winding
4
a
is small and the electric current induced in the shield current is small. On the other hand, as for the current transformer
8
b
close to the bus conductor
8
a
, the mutual inductance between the bus conductor
6
b
and the shield winding
4
b
is large and the current induced within the shield windings is large generating a large heat at the current transformer
8
b
close to the bus conductor
6
b
. Therefore, a problem has been posed that a material having a good heat resistivity and a winding having a large winding diameter must be used.
Also, when the distance L between the iron cores
2
a
and
2
b
is large, the magnetic flux from the bus conductors
6
a
can easily penetrate, so that a large current is induced in the shield windings
4
a
and
4
b
, generating a large heat in the transformer and the magnetic flux is apt to concentrate at the iron cores
2
,
2
a
and
2
b
, posing a problem that the cross-sectional area of the iron core must be made large for achieving the precise current measurement of the bus conductor
1
.
Further, the shield windings
4
a
and
4
b
are divided into the coil sections of an even number disposed around the transformer
8
and have the same polarity connected together, but since the mutual inductance between the respective divided coil sections and the bus conductors
6
a
and
6
b
are different, making the induced current imbalance, generating a local high temperature in the current transformer.
SUMMARY OF THE INVENTION
The chief object of the present invention is to provide a current transformer free from the above-discussed problems of the conventional current transformer.
Another object of the present invention is to provide a current transformer in which the heat generation from the shield winding is decreased and which can be made small in size and light in weight.
Another object of the present invention is to provide a current transformer in which the penetration of the magnetic flux into the iron core is decreased to allow a high accuracy measurement of the bus conductor.
A further object of the present invention is to provide a current transformer in which the inductance of the divided coils are made equal so that the local temperature elevation in the transformer may be suppressed.
With the above object in view, the current transformer of the present invention comprises a plurality of transformer units combined into a bundle, each of the transformer units including an annular iron core surrounding a bus conductor to be measured and a secondary winding wound around the iron core for measuring an electric current flowing through the bus conductor, and a shield winding wound around the bundle of the transformer units.
The bus conductor to be measured and a bus conductor neighboring to the bus conductor to be measured may be arranged in a common plane, and the current transformer unit is provided with a first air gap in which no secondary winding and no shield winding are wound at a portion of the current transformer unit located on a line extending perpendicularly to the plane from the bus conductor to be measured.
The current transformer of the present invention may comprise a plurality of bus conductors neighboring said bus conductor to be measured are arranged, and the transformer unit is provided with a first air gap in which no secondary winding and no shield winding are wound at a portion of the current transformer unit in a resultant vector direction of vectors perpendicular to a line connecting the bus conductors neighboring the bus conductor to be measured and passing through the bus conductor to be measured.
The shield winding is divided into two at the position of the air gap as well as at the position opposite to the first air gap relative to the bus conductor to be measured.
The current transformer may includes a second air gap in which no secondary winding and no shield winding are wound is provided at the position opposite to the first air gap of the transformer unit relative to the bus conductor to be measured.
The shield winding may be divided into two to have equal circumferential length.


REFERENCES:
patent: 3030573 (1962-04-01), Yamashita et al.
patent: 4471333 (1984-09-01), Stephanides
patent: 4806896 (1989-02-01), Shikano et al.
patent: 5659251 (1997-08-01), Wakamatsu
patent: 5764123 (1998-06-01), Waters et al.

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