Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to a current-
transformer arrangement Eor a static electricity meter,
including a primary conductor carrying the alternating
current to be measured and a secondary winding consisting of
at least two coils connected in series, the output voltage
from which is passed to an electronic integrating stage in
order to produce a measuring signal which is independent of
frequency.
The measuring of large currents for the
determination of energy consumption by means of static
electricity meters requires the use of current-transformers,
the output signals from which must be suitable for further
processing in electronic measuring instruments. The
currents to be measured amount to more than 100 amperes
which must be picked up in the milliampere range with little
deviation in linearity. Arrangements of this kind must be
largely insensitive to direct current componen-ts of the
current being measured. In addition, as little auxiliary
power as possible should be needed to operate the arrange-
ment.
Furthermore, the requirements set forth ln IECPublication 521 must be met, more particularly galvanic
separation with high insulating strength, resistance to
short-circuiting, insensitivity to external magnetic
interference fields and retention of frequency effects.
The arrangements according to German
~uslegunsschrift No. 1,079,192, which is in the form of a
magnetic volt-meter, consists of -two secondary coils
connected in series and surrounding a busbar. The ends of
the secondary part-windings are short-circuited with a
magnetic material. This provides a closed magnetic circuit
(a Rogowski coil) which behaves as-tatically -towards
lntexfering fields if the winding density of the part-
windings is suEficiently high and the distribution of -the
windings is uniform. In the case of high current densities
in the primary conductor, the secondary coils must be at a
cer-tain distance therefrom in order to ensure satisfactory
integration of the part voltages of the irregularly
distributed winding.
Known current-transformer arrangements are
equipped with an electronic integrating stage, the frequency
response of which, regarding its input signal to output
signal, compensates for the proportional
frequency-dependency of the voltage induced into the
secondary winding of the current being measured, and which
rotates the input signal, in relation to the opposite phase
position of its 011tpUt signal, through a 90 phase angle.
As a result, the measuring signal at the output from the
integrating stage is independent of the measuring frequency
and is in opposing phase to the measuring current, with
direct proportionality between the amplitudes.
One disadvantage of such a current--transformer
arrangement is that the requirement for substantial
insensitivity to external magnetic interference fields is
not completely met by the use of ferromagnetic materials.
Furthermore, the arrangement delivers very weak output
signals since only very slight coupling exists between the
fields of the primary conductor and of the secondary coils.
This arrangement is therefore unsuitable for measuring
current intensities of less than about l kiloampere.
'7
It is therefore an object of the invention to
provide a current-transformer arrangement of the -type
mentioned hereinabove featuring hiyh insensitivity to
external magnetic interference fields, and a high secondary
side output signal, combined with a compact design and
simultaneous use of inexpensive structural elements.
In accordance with the present invention, there
is thus provided a current-transformer arrangement for a
static electricity rneter, including a primary conductor
carrying the alternating current to be measured and a
secondary winding consisting of at least two coils connected
in series, the output voltage from which is passed to an
electronic integrating stage to produce a measuring signal
independent of frequency, characterized in that the primary
conductor is formed into a loop to produce maximal field
strength, the secondary winding is designed astatically wi~h
electrically identical coils and, in order to produce
maximal magnetic coupling, at least one of the secondary
coils, in the absence of ferromagnetic materials, takes up
with the primary conductor, as completely as possible, the
magnetic flux of the current flowing in the primary
conductor, such secondary coil ex-tending axially over the
smallest possible partial length of the magnetic flux lines
produced by the current in the primary conductor.
In the arrangement according to the invention,
the coil carrier exhibits a permeability substantially
independen-t of the magne-tic field of the primary conductor.
The secondary winding consists of two coils connected in
series, the axes thereof running parallel with each other.
The direction in which the coils are wound corresponds to
that of a solenoid buckled spatially in the middle through
~i ~J~
180. This astatic arrangement of the coils leads to a
seconda.ry winding which is independent of external
hornogeneous magnetic alternating interference fields, since
the part-voltage induced in the two coils by the i.nterfe-
rence fields cancel each other out. In the case of known
coils consisting of part-windings, the part-windings are
always placed together to form a closed integration path
corresponding to a Rogowski coili in the case of the
invention, the seconclary coils, for the purpose of achieving
small dimensions, each extends only over a partial length of
less than 50~ oE the magnetic field lines produced in the
primary conductor so that no closed integration path is
formed. In this case, the second secondary coil serves
mainly to compensate for the effect of external fields. In
order to ensure the best possible compensation, the two
secondary coils are of small spatial dimensions and are
arranged as closely together as possible.
The seeondary eoils may be in the form of
cylindrical or flat coils having axes running parallel with
each other, at least one of the two coils being located
spatially at a point where the primary current produces the
highest possible field strength. The high field strength
necessary to obtain a strong secondary side output signal
from the arrangement is obtained by shaping the primary
conductor in the form of a loop. The secondary part-coils
thus piek up the magnetic field of the primary conduetor
only locally at a point, the sum of the voltages induced
into the two secondary coils being proportional to the
primary current to be picked up.
One particular characteristic of -the novel
current-transformer arrangement is its high magnet:ic
coupling between the primary conductor and the secondary
coil. This produces strong secondary side output signals
which allows the arrangement to be used for the linear
detection of currents, having current intensities of down to
a few milliamperes, and this is accomplished without the use
of ferromagnetic ma-erials. This provides a spatially
compact design resulting in favourable production costs~
According to a preferred embodiment, -the primary
conductor which is in the form of a loop surrounds the
secondary coil peripherally as closely and as completely as
possible. Since, in this case, the secondary coil is
arranged within a primary conductor designed as an eye,
there is provided an optimal magnetic coupling with
correspondingly strong output signals.
According to another preferred embodiment, the
primary conductor encloses two secondary coils with two
windings connected in series. However, it is also possible
for the primary conductor to consist of part-conductors
connected in parallel, each winding enclosing one of the
coils. In this case, the primary current to be measured is
divided between two windings so that, in the case of a pri-
mary conductor preferably stamped out of copper and having a
rectangular cross-section, there will be no folds where the
conductor parts intersect.
According to a particularly advantageous
embodiment, the primary conductor is designed as a flat
conductor and formed into current-loop comprising opposed
conductor sections each having at least one recess, the
recesses facing the conductor sections forming a primary
--5--
'7
winding with a winding area approximately parallel with the
plane of the flat conductor. The coils of the secondary
winding are arranged with their winding areas substantially
parallel with the flat conductor and are traversed, at least
partly, by the primary magnetic flux running substantially
at right angles to the plane of the flat conductor. In this
case, the primary conductor, in the form of a flat
conductor, is folded about a transverse axis through an
angle of 180 so that the outgoing and return conductors lie
at a short distance above each other. This distance can be
designed, at least sectionally, in such a manner that the
resulting space may be used to accommodate the secondary
winding. In this design, the influence of magnetic
:interference fields upon the measurement results is
practically eliminated, even without magnetic materials.
Fully automatic production is rendered possible, very
simply, by the shape and the small dimensions of the
arrangement.
It is also desirable for the recesses to extend
in opposite directions approximately from the centreline to
the edge of the primary conductor. In this way, the
electrical current flowing in the longitudinal direction of
the flat primary conductor is deflected to the middle
thereof so that the path of the current becomes a loop.
According to a further preferred embodiment,
opposing sections of the primary conductor each comprise two
recesses arranged in opposite directions and staggered in
parallel with each other, thus forming two windings lying
side by side in the longitudinal direction of the primary
conductor, the magnetic flux of each winding traversing one
of the coils of the secondary winding. In this case, it is
desirable for the coils of the secondary winding to be
located between sections of the primary conductor. Since
-the configuration of the primary conductor leads to two
windings lying side by side, the axes of which may be formed
by the facing ends of the recesses, each coil of the
secondary winding may be associated with a primary winding,
thus providing optimal flux-linkage.
A further advantageous embodiment is obtained
when the coils o the secondary winding, as in
planar-technology design, are applied in one or more layers,
if possible as spirals on both sides of a substrate. This
plate-like substrate may be inserted between the spaced
apart conductor sections. The substrate with the two coils
oE the secondary winding may also be arranged externally of
the space between the conductor sections above the effective
winding areas of the primary conductor.
In addition to this, it is possible for the
substrate to contain other electronic components of the
electricity meter, for example electronic components of the
integrating and multiplying stages.
According to yet another preferred embodiment,
one conductor section comprises two recesses running in
opposite directions and extending to the edge of the primary
conductor on a common longitudinal axis. Arranged in
parallel with these recesses, on the other
conductor-section, is a central recess. With this arrange-
ment of recesses, the current paths run in such a manner as
to form two windings connected in parallel, each of which is
linked with the magnetic flux of a coil of the secondary
winding.
--7--
~;~6~'7
Further features and advantages of the present
invention will become more readily apparent from the
following description of preferred embodiments as illus-
trated by way of examples in the accompanying drawings, in
which:
Fig. 1 is a front view of two astatically
designed secondary coils, one of which is enclosed in a
primary conductori
Fig. 2 is a front view of two astatically
designed coils enclosed in primary-conductor windings
connected in series;
Fig. 3 shows an arrangement of secondary coils
according to Fig. 2, but with the primary conductor windings
connected in parallel;
Fig. 4 is a perspective view of a primary
conductor in the form of a flat conductor, in which one of
the astatic secondary coils is arranged between opposed
sections of the flat conductor, while the other secondary
coil is arranged externally thereof;
Fig. 5 is a perspective view of a primary
conductor which has been modified as compared with that in
Fig. 4;
Fig. 6 is a perspective view of astatic coils of
the secondary winding with a baseplate adapted to be
inserted into the primary conductor according to Fig. 5;
Fig. 7 is a cross-sectlon to a reduced scale of
the arrangement according to Figs 5 and 5, in the operative
conditioni
Fig. 8 is a perspective view of another primary
conductor which has been modified as compared with Figs 4
and 5;
Fig 9 is a perspective v:Lew of astatic flat coils
as a secondary winding upon a baseplate, adapted to be
inserted into the primary conductor according to Fig. 8;
Fig. 10 is a perspective view of a primary
conductor comparable to that shown in Fig. 8, with secondary
winding coils arranged therein;
Fig. 11 is a plan view of a primary conductor in
the form of a folded-open flat conductor;
Fig. 12 is a plan view of the primary conductor
according to Fig. 11 in the folded condition;
Fig. 13 is a plan view of a baseplate with
astatic flat coils in a design comparable with that shown in
fi~
Fig. 14 is a plan view of the folded primary
conductor according to Fig. 11 on the side facing Fig. 12;
and
Fig. 15 is a cross-section of the arrangement
according to Fig. 14.
Fig. 1 shows two cylindrical astatic coils 1 and
2 of a secondary winding 3, which are arranged at a distance
from each other. The coils which are retained by a spacer 4
are geometrically and electrically identical and the
cylinder axes thereof are parallel with each other. They
are arranged in insulating cylinders 5 and 6, respectively.
Coil 1 is enclosed in a winding 7a of primary conductor 7
through which the current to be measured Il flows in the
direction of the arrows. The voltages induced by the
alternating current in coils 1 and 2 by the magnetic field
of the alternating current flowing in primary conductor 7
add up to a signal proportional to alternating current Il to
be measured. Voltages induced by homogeneous external
lnterference fields have different signs and cancel each
other out. This arrangement largely eliminates the effect
of external magnetic alternating fields upon the correct
functioning of the current-transformer arrangement. The
effect of external fields may be still further reduced by
magnetic screening material surrounding the coils.
In Fig. 2~ secondary coils 8 and 9 corresponding
to those shown in Fig. 1 are surrounded consecutively by the
common primary conductor 10. Connecting primary windings
lOa and lOb in series produce a stronger measuring signal as
compared with Fig. 1.
In Fig. 3, coils 11 and 12 correspond to coils 8
and 9 shown in Fig. 2. primary conductor 13 b~anches out
into two part-conductors each of which is shaped into a
winding 13a vr 13b enclosing coils 11 and 12. Current Il is
branched to the part-conductors with windings 13a and 13b,
the sum of the voltages induced in coils 11 and 12 being
proportional to current Il to be measured. As compared with
the design in Fig. 2, the advantage of the arrangement
according to fig. 3 is that, in the case of a primary
conductor 13, preferably stamped out of copper and having a
rectangular cross-section (a flat conductor), folds may be
avoided at the intersection of the conductor parts.
In the embodiment showing in Fig. 4, a primary
conductor 14 is in the form of a flat conductor which has a
rectangular cross-section and which is folded in such a
manner that opposed conductor sections 14a and 14b produce a
rectangular cavity 15. Externally of cavity 15, opposite
sections of primary conductor 1~ are separated from each
other by an insulating layer 16. Conductor section 14a is
provided with a slot-like recess 17 extending approximately
--10--
from the middle to the edge. A recess 18 running in the
opposite direction to the edge, is provided in the opposed
conductor section 14b. Recesses 17 and 18 influence the
geometrical position of the current paths of the current to
be measured, indicated by arrows 19 and 20, in such a manner
as to form a winding for the primary current. Arranged in
the magnetic field of this winding is secondary coil 21,
shown in dotted lines. A second secondary coil 22 fo
compensating externa1 magnetic fields lS located externatlly
of the primary conductor.
The primary conductor 23 which is shown in Fig. 5
differs from the design according to Fig. 4 in that two
slot-like recesses 24,25 and 26,27, running in opposite
directions, are provided in each of the opposite conductors
sections 23a and 23b. Each of the laterally open recesses
24 to 27 runs approximately to the middle of conductor
sections 23a and 23b. Recesses 24 and 26 are located in the
same plane at right angles to primary conductor 23 when this
is folded through 180 into the operative condition, i.e.
conductor sections 23a and 23b running parallel with each
other. In a similar manner, recesses 25 and 27 are arranged
in a common plane at right angles to primary conductor 23.
As a result of the foregoing design of recesses
24 to 27, the primary current flows along the paths
indicated by arrows 29a -to 29g. Thus, primary windings
connected in series are formed in the planes of conductor
sections 23a and 23b, i-t being possible to arrange secondary
coils in the magnetic fields thereof.
Located upon baseplate 30 shown in Fig. 6 are two
astatically arranged secondary coils 31 and 32. In the
operative condition, baseplate 30 is located with coils 31
D~
and 32 between sections 23a and 23b of the primary conductor
according to Fig. 5. The position of coil 31 in Fig. 6 is
indicated on conductor section 23b in fig. 5 by dotted
circle 33, while the position of coil 32 in fig. 6 is
indicated by dotted c:ircle 60 in Fig. 5.
Fig. 7 shows the current-transformer arrangement
with the primary side part according to Fig. 5 and the
secondary side part according to Fig. 6 in the operative
condition. In this case, the upper and lower sections of
primary conductor 23 are separated from each other by an
insulating layer 61.
Primary conductor 34 in Fig. 8 is comparable with
the primary conductor 23 shown in Fig. 5. However, the
distance between upper conductor section 34a and lower
conductor section 34b is less and corresponds to the
thickness of insulating layer 35.
The baseplate 36 which is shown in Fig. 9 and
carried secondary coils 38 and 39 is located, in the
operative condition of the current-transformer arrangement,
between conductor sections 34a and 34b of primary conductor
34 according to Fig. 8. Coils 38 and 39 in Fig. 9 are in
the form of spirals produced by planar technology, so that
the small space between conduc-tor sections 34a and 34b
according to Fig. 8 is sufficient. in the operative
condition, the centre of coil 38 is located approximately at
the middle end of slot-like recess 40 in Fig. 8. Similarly,
the centre of coil 39 coincides approximately with the
middle end of recess 41.
Fig. 10 shows a primary conductor 42 which
corresponds substantially to the primary conductor 34 shown
in fig. 8~ However, the ends of recesses 44 and 45 in the
J'~
middle of primary conductor 42 are in the form oE holes in
which astatically designed secondary coils 46 and 47 are
mounted.
In the embodiment according to Fig. 11, a primary
conductor 48 is shown open, i.e. before folding about the
line 49. In -the folded condition, a conductor section 48a
is located over a conductor section 48b. Conductor section
48b comprises recesses 50 and 51 which run in opposite
directions along a common longitudinal axis in parallel with
fold line 49. Conductor section 48a carries a recess 52
which extends only over the central area of conductor
section 48 and is at the same distance from fold line 49 as
recesses 50 and 51. The locations of the secondary coils
are indicated by dotted circles 53 and 54. Dotted circles
55 and 56~ located in mirror symmetry to fold line 49,
indicate the locations of the secondary coils.
Fig. 12 shows primary conductor 48 according to
Fig. 11 in the folded condition, with conductor sections 48a
and 48b lying one above the other. Only recesses 50 and 51
are shown. Circles 53 and 54 indicate the locations for the
secondary coils.
Fig. 13 shows astatic secondary coils 56 and 57
secured to a baseplate 55. Although this corresponds in
principle to the arrangement shown in Fig. 9, it differs in
that coils 56 and 57 in Fig. 13 are at the same distance
from fold line 49. Coils 56 and 57 may also be produced by
planar technology. In order to expose the two coils to the
corresponding primary magnetic flux, they may also be
arranged externall.y of the space between folded conductor
sections 48a and 48b according to Fig. 11, as long as the
magnetic coupllng is sufficient for a strong output signal.
Again in this case, the circles shown in fig. 11 and 12 are
the corresponding locations for the secondary coils.
Fig. 14 shows primary conductor 48 according to
Fig. 11 in the folded condition, from the opposite side as
compared with Fig. 12. Thus, only central recess 52 is
visible.
Primary conductor 48 is also shown in the folded
condition in Fig. lS, the distance between upper conductor
section 48a and lower conductor section 48b being determined
by an insulating layer 57. The direction of the primary
current to be measured is indicated by arrows 58 and S9.
Baseplate SS according to Fig. 13 is inserted into space 50.
