Current transformer

Details Current transformer Current transformer

2002-10-29

2004-11-23

Mai, Anh (Department: 2832)

Inductor devices

Coil or coil turn supports or spacers

Printed circuit-type coil

C336S223000, C336S232000

Reexamination Certificate

active

06822547

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-330688 filed on Oct. 29, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a current transformer which is used to measure alternating current passing in, for example, a circuit of an electric power system, and especially related to its use in a Rogowski coil.
2. Description of the Related Art
Generally, a penetrated type current transformer is used in many cases for measuring alternating current passing in electric power distribution equipment and substation main circuits equipment and so on. In a conventional penetrated type current transformer, a secondary winding is wound around a toroidal former, that is a core, and a conductor in which a primary current passes penetrates a centered opening of the core. An iron core, or non-ferromagnetic material is used as the core of this penetrated type current transformer. Among these, a current transformer using a non-ferromagnetic material is called a air core coil type current transformer or a Rogowski coil, which can acquire excellent linearity characteristics without saturation.
FIG. 11
shows the structure of a common Rogowski coil. The Rogowski coil
1
shown in this figure is constituted by a conductor winding
2
coiled from point P to point Q on all over the periphery of the core
6
made of the non-ferromagnetic material, and returning a wire (return circuit line)
3
from point Q to point R in a direction opposite to a winding direction of the winding
2
along the core
6
. The return circuit line
3
usually returns between the core
6
and the winding
2
. Moreover, a conductor
5
of a main circuit of an electric distribution equipment or substation equipment penetrates an opening
6
a
of the core
6
.
In this situation, voltage is generated proportional to amount of time change of the primary current flowing in the conductor
5
between terminals
4
,
4
of the winding
2
and the return circuit line
3
. Accordingly, the above-mentioned primary current can be measured by integrating this voltage and multiplying a constant determined by a form of the coil. For an ideal Rogowski coil, the voltage between terminals
4
,
4
is not influenced by a gap of centered points of the core
6
and the conductor
5
, and by magnetic field of outside the Rogowski coil
1
. An ideal Rogowski coil satisfies the following conditions: (a) a winding interval (pitch) of the winding
2
is constant, (b) an area surrounded by the winding
2
is equal to an area surrounded by the return circuit line
3
, (c) cross-sectional area of the core
6
is fixed over the entire circumference and not influenced by temperature, and (d) the winding
2
is completely wound over the entire circumference of the core
6
without any missing portion.
However, when manufacturing the Rogowski coil
1
as shown in
FIG. 11
, it is technically difficult to satisfy the above-mentioned condition (a), that is, to wind the winding
2
to the core
6
while keeping a constant winding interval. Although a fixed winding interval can be maintained by preparing slots or projections to the core
6
for fixing position of the winding
2
, a special core and winding machine is necessary for this preparation, and thus increases the price of the Rogowski coil which becomes very expensive.
A Japanese Patent Disclosure (koukai) No. 6-176947, which is a counterpart of the U.S. Pat. No. 5,414,400, discloses a means for solving this problem.
FIG. 12
shows a conventional structure of the Rogowski coil indicated by this patent disclosure. In the Rogowski coil
1
shown in this figure, a metal foil
2
e
is formed on both sides of a printed circuit board
7
having an opening
9
penetrated by a conductor
5
at a central part so as to provide straight lines radially spreading from the center of the opening
9
. Moreover, the winding
2
and the return circuit line
3
is constituted so that the radially-arranged metal foils
2
c
of one side surface of the printed circuit board
7
and the metal foils of a reverse side surface thereof are electrically connected by plated holes which penetrate the printed circuit board
7
. In the example shown in
FIG. 12
, the return circuit line
3
is formed in the shape of winding, thus, the output voltage between the terminals
4
,
4
per unit current and unit frequency becomes large, and sensitivity of the Rogowski coil
1
improves. In addition, winding progress direction of the winding
2
is in a clockwise rotation, and that of the return circuit line
3
is in a counterclockwise rotation.
According to such conventional technology, by applying general technique of manufacturing printed circuit boards, the Rogowski coil
1
can be cheaply manufactured while keeping winding intervals of the winding
2
and the return circuit line
3
constant. Therefore, it becomes possible to realize the condition (a) mentioned above to a great degree.
By the way, also in the conventional Rogowski coil mentioned above, the condition (b) mentioned above, that is, the condition of making an area which the winding
2
surrounds and an area which the return circuit line
3
surrounds equal, cannot be fulfilled completely. This makes it easy for the Rogowski coil to be influenced by an external magnetic field, and this gives rise an error at the time of current measurement increases.
FIG. 13
is a pattern diagram showing a situation that magnetic flux &PHgr; due to an external magnetic field interlinks the winding
2
of the common Rogowski coil
1
as shown in FIG.
11
.
FIG. 14
is a pattern diagram showing a situation that the same magnetic flux &PHgr; due to an external magnetic field interlinks the return circuit line
3
of the common Rogowski coil
1
as shown in FIG.
11
.
Since the winding progress direction of the winding
2
is reverse of that of the return circuit line
3
, the voltage generated between the terminals
4
,
4
of the Rogowski coil
1
shown in
FIG. 11
is equal to difference of the voltage generated between the points P and Q shown in FIG.
13
and the voltage energized between the points P and Q shown in FIG.
14
. Assuming that the magnetic flux &PHgr; due to the external magnetic field is uniform over the whole surface of the Rogowski coil
1
, if the area A, designated by diagonal hatched lines in
FIG. 13
, which the winding surrounds is not equal to the area B, designated by diagonal hatched lines in
FIG. 14
, which the return circuit line surrounds, a voltage due to the exterior magnetic field is generated between the terminals
4
,
4
. Since this voltage is unrelated to the primary current which should originally be measured, it causes a measurement error.
Factors that give rise an external magnetic field are explained below. For example, the external magnetic field is generated when a bend exists in the conductor
5
or when a current flowing conductor
8
exists near the Rogowski coil
1
as shown in
FIG. 15
, or when the conductor
5
is arranged at an angle to the Rogowski coil
1
. When applying the Rogowski coil
1
to an actual electric power distribution main circuit equipment or substation main circuit equipment, it is impossible to completely eliminate the above-mentioned factors. In addition, usually, since an actual magnetic flux &PHgr; due to the external magnetic field is not uniform, the influence becomes still more complicated.
It is possible to reduce an error by completely making the area A which the winding surrounds and the area B which the return circuit line
3
surrounds equal, more preferably, by arranging the form of the winding
2
and the form of the return circuit line
3
to be completely identical. However, in the common Rogowski coil
1
shown in
FIG. 11
, it is difficult to manufacture while controlling the area the return circuit line
3
surrounds being constant, thus it is very difficult to avoid the influence of an external

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