Note: Descriptions are shown in the official language in which they were submitted.
2~82112
CURRENT RATIO DEVICE AND TOROIDAL CORE ASSEMBLY THEREFOR
FIELD OF THE INVENTION -
The invention relates to a current ratio device that can
be used to construct an improved current transformer, and more
particularly a current transformer capable of accurately
scaling high alternating and direct currents down to usable
metering levels, for use in the measurement of electric power
and energy.
The invention also relates to a toroidal core and winding
assembly for use with such a current ratio device.
BACKGROUND OF THE INVENTION
Petar N. Miljanic et al in an article "An Electronically
Enhanced Magnetic Core for Current Transformers" published in
IEEE Transactions on Instrumentation and Measurement, Vol 40,
No. 2, pp 410 - 414, April 1991 (see also U.S. patent
4,841,236 issued June 20, 1989) have described a technique
that increases the apparent permeability of the magnetic core
of a current transformer by electronically providing the
magnetizing current for the core, whereby the ratio between
the primary and secondary currents is rendered virtually
without error.
There are two basic limitations in the various devices
that have been disclosed by Miljanic et al for carrying this
technique into practice.
First, while the above disclosure provided for current
transformers that can operate with pure ac, or with a mixture
of ac and dc, operation with dc alone was not possible.
Second, the devices were not structurally adapted for
easy use with busbars. For on-site measurement of high
~lc
2 2~2~ 1~
currents in a busbar that is part of an already installed
system, it is inconvenient to have to dismantle the busbar in
order to thread a toroidal core onto it from one end, as is
necessary if the busbar is to form a single turn primary to
generate currents in a secondary winding formed on the core.
This problem has been addressed in the past in some hand-held
metering devices by making the transformer core in two parts
that are pivoted together and can be opened to be placed over
the busbar from a side thereof and then closed when in place
to surround the busbar. The problem with "openable" cores of
this type, however, has been the loss of accuracy that results
from the increased reluctance in the core that arises from the
unavoidable air gap where the two parts of the core come
together. No matter how smoothly polished the mating end
faces of such a core are made, the result is always a
significant increase in reluctance relative to that of an
uninterrupted toroidal core.
SUMMARY OF THE INVENTION
The present invention has the objective of overcoming at
least one of the foregoing limitations. The preferred
embodiments of the invention serve to overcome both of such
limitations simultaneously, namely to provide a device that is
openable to embrace a fixed busbar from its side without loss
of measurement accuracy, while also being capable of measuring
direct current alone, as well as continuing to be able to
measure alternating current alone or mixtures of alternating
and direct currents.
According to the invention, this latter requirement is
met by adding to a circuit, such as disclosed by Miljanic et
al, a magnetic modulator plus an associated demodulator that
act together to generate and detect even harmonics that result
from dc in the primary winding. The demodulator generates a
dc signal that is amplified to provide a feedback dc current
2~21~2
which is returned to the transformer windings while also
flowing in the burden.
More specifically, in this aspect, the invention relates
to a current ratio device for use in constructing a current
transformer for measuring alternating current alone, direct
current alone, or a mixture of alternating and direct currents ~-
simultaneously, the device comprising a first magnetic core, a
second magnetic core, a measuring winding electromagnetically
coupled with the first core but not with the second core, an
alternating current amplifier having an input connected to
receive an alternating waveform that appears across the
measuring winding, and outer winding means electromagnetically
coupled with both the first and second cores, the outer
winding means including a secondary winding for connection
across a measuring burden. The output of the amplifier is
connected to the outer winding means to provide a magnetizing
current required to magnetize the second core when a primary
winding is electromagnetically coupled with the first and
second cores and an alternating current flows in such primary
winding. The structure so far described is known from the
Miljanic et al disclosure.
The present invention is characterized by the addition of
modulation magnetic core means, and modulation winding means
that are electromagnetically coupled with the modulation core
means but not with the first and second cores, the outer
winding means also being electromagnetically coupled with the
modulation core means. Modulating means are connected to
drive the modulation core means alternately into saturation,
and demodulating and amplifying means are provided to detect
even harmonics in the modulation core means that result from
direct current in the primary winding and to generate a
corresponding direct current output that is connected to the
outer winding means to form therewith a direct current series
circuit that also includes the burden. This circuit can also
in~lude means for measuring the direct current therein, such
as a resister, the voltage across which provides such
measurement, or an ammeter.
20~21~ 2
In a structural aspect, the invention comprises a
toroidal core assembly having at least one winding wound on
it, each such winding being divided into a pair of separate
portions, with each portion extending around the core assembly
S for a major part of a respective opposite half thereof in such
a way as to define between the portions a pair of diamet-
rically opposite "unwound" core sections, i.e. sections
without any windings thereon.
This construction can be adapted for convenient use with
an already installed busbar by cutting through both the
unwound core sections along a plane that extends across the
toroidal assembly while containing its axis, the result being
to separate the assembly into a pair of sub-assemblies that,
after being placed over a single turn primary, e.g. a busbar,
can be clamped back together to reform the assembly.
In another structural aspect, the invention provides a
toroidal core assembly for use with the above described
current ratio device, such assembly comprising a toroidal
magnetic outer core having a cross-section that consists of a
closed peripheral portion and a central bridging portion
interconnecting opposite regions of the peripheral portion in
such a manner as to define a pair of cavities surrounded by
the core. A first inner core is located in one of these
cavities and a first inner winding is wound around this inner
core to couple electromagnetically with it but not to couple
electromagnetically with the outer core. A pair of further
inner cores is located in the other one of the cavities, each
such further inner core having a further inner winding wound
around it to couple electromagnetically with it but not to
couple electromagnetically with any of the other cores.
Finally, outer winding means are wound around the peripheral
portion of the outer core so as to couple electromagnetically
with all the cores.
20~21~2
s
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a prior art circuit diagram and is
substantially identical to Figure 1 of the Miljanic et al U.S.
patent referred to above;
Figure 2 is a modification of the diagram of Figure 1
illustrating a first embodiment of the present invention in a
circuit aspect;
Figures 3 to 6 are variations of Figure 2, that
respectively show additional embodiments of the circuit aspect
of the invention;
Figure 7 is a fragment showing an alternative;
Figure 8 is a small scale plan view of a toroidal
assembly of cores and windings for implementing the circuit of
Figure 2, according to an embodiment of a structural aspect of
the present invention;
Figure 9 shows the assembly of Figure 8 cut into a pair
of sub-assemblies;
Figure 10 is a fragment of the assembly of Figure 8 seen
on a larger scale and as a section on the line 10-10 in Figure
11; and
Figure 11 is a cross-section taken on the line 11-11 in
Figure 10.
DETAILED DESCRIPTION OF THE PRIOR ART
The prior art circuit of Figure 1 shows a current
2S transformer with two cores Cl and C2. While these cores can
be located side-by-side, as shown in Figure 3 of an earlier
Miljanic U.S. patent 3,534,247 issued October 13, 1970, it is
preferred to employ a configuration in which the core C2 is a
hollow toroid that encloses and shields the core C1, as shown
in Figure 4 of such earlier patent and described in more
detail below. Such a transformer structure had already been
described in principle in United States patent 3,153,758
issued October 20, 1964 to N.L. Kusters et al.
2 a ~ 2
The outer core C2 couples electromagnetically with three
windings that are outside it, namely a primary winding Wp, a
secondary winding Ws and an auxiliary or "compensating"
winding Wc, but does not couple electromagnetically with a
fourth, sensing or "measuring" winding Wm that is wound on the
core C1 inside the core C2. The inner core Cl couples
electromagnetically with all four windings. In the normal
manner of a current transformer, an input or primary current
Ip flows in the primary winding Wp, and an output or secondary
current Is flows in the secondary winding Ws and a burden B
which will be a very low impedance, current measuring
instrument.
Since a component of the input current Ip is required to
magnetize the outer or second core C2, the secondary current
Is deficient by a corresponding amount, and the theoretical
equality of ampere turns between the primary and secondary
circuits contains an error, such error being expressed as the
magnetizing current. The flux in the core C2 corresponding to
this magnetizing current is sensed by the measuring winding Wm
which acts with the inner or first core Cl to generate a
current Im in the winding Wm that is equal to the magnetizing
current.
The ends of the winding Wm are connected to input
terminals 1, 2 of an inverting amplifier Al so that such input
receives the current in this measuring winding Wm. The
amplifier Al has an external power supply PS and a gain that
is such that no appreciable voltage is permitted to remain
across the terminals 1, 2. The output terminals 3, 4 of the
amplifier A1 form a series circuit through windings Wm and Wc
so that the current Im passing in one direction through the
winding Wm is equal to the current Ic passing in the other
direction through the winding Wc. Hence, in this case, the
compensating current Ic equals the measuring current Im and
becomes equal to the magnetizing current, thus supplying the
necessary magnetizing current for the core C2. This
arrangement eliminates the need for this magnetizing current
to be supplied from the primary current Ip. As a result, the
2Ul~21 ~ 2
true equality of the ampere turns in the primary and secondary
windings is not upset by the need to supply a magnetizing
current. It should be noted that this is a compensation
circuit rather than a typical feedback circuit.
It should also be noted that the amplifier input
terminals 1, 2 need not necessarily be connected directly
across the measuring winding Wm. There could be an interposed
transformer, as in the arrangement shown in Figure 2 of the
earlier Miljanic patent No. 3,534,247 referred to above. The
important concern is that the amplifier input receives the
measuring winding waveform.
Figure 6 of the Miljanic et al patent 4,841,236 referred
to above discloses a further modification of this current
ratio device, that can simultaneously measure any dc current
that is mixed with the ac current in the primary winding Wp,
but, as indicated above, none of the circuits disclosed by
Miljanic et al can measure direct current alone, i.e. in the
absence of some ac in the primary winding, because there would
then be no electromagnetic coupling between the windings and
the cores.
DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE
PRESENT INVENTION
This limitation of the prior art is overcome by the
embodiment of the present invention shown in Figure 2 which
adds to the circuit of Figure 1 two additional modulation
cores C3 and C4 each with a respective associated modulation
winding W3, W4. These latter cores and windings are located
inside the main core C2 in the same manner as the core C1 and
the winding Wm, as shown in Figures lo and 1l. The windings
W3, W4 are energized by a magnetic oscillator or "modulator" M
that in the present embodiment takes the form of a transformer
T the primary winding Tp of which is connected to an
alternating power supply (not shown), and the secondary
winding of which is divided into two halves Tsl and Ts2 at a
center point Tc, these two halves of the secondary winding
8 2~211~
being respectively connected across each of the windings W3,
W4, the junction J between windings W3 and W4 being connected
to ground. The cores C3, C4 are made of a material of
sufficiently high permeability to be able to be driven into
saturation twice per cycle by the transformer T. If a direct
current flows in the primary winding Wp, a signal is generated
by the cores C3, C4, that contains even harmonics of the
modulation frequency of these cores. This signal is converted
to dc by a demodulator DM which in the present embodiment
takes the form of a demodulating circuit embodied in a dc
feedback amplifier A2, the input terminal 5 of which is
connected to the center point Tc.
The magnetic modulation and demodulation performed by
this portion of the circuit is a known technique, having been
explained in detail in "A Self-Balancing Direct Current
Comparator for 20,000 Amperes" by M.P. MacMartin et al,
published in IEEE Transactions on Magnetics, Vol. Mag-1,
No. 4, pp 396-402 (1966).
Output terminals 7, 8 of the amplifier A2 are connected
across a series circuit consisting of an additional dc winding
W2 (located outside the core C2), the burden B and a resistor
R across which there appears a voltage V that can be measured
to determine the magnitude of the dc current in the winding
W2. A capacitor Ca is connected in series with the secondary
winding Ws to avoid direct current from the winding W2 flowing
back to the amplifier A2 through the winding Ws.
The ampere-turns of the direct current Idc in the winding
W2 and the burden B is theoretically equal and opposite to the
ampere-turns of the dc component of the input current Ip in
the primary winding Wp, and the higher the gain of the
amplifier A2, the more closely this equality is approached.
The total current Is + Idc in the burden B is now a
measure of the combined ac and dc in the input current Ip,
while the current in the resistor R (voltage V) is a measure
of the dc component of this combined current. This direct
current appears in the resistor R even when the ac component
is zero.
-- 9 20821 12
Figure 2 includes a broken line L that signifies how the
circuit can be viewed functionally as divided into an ac
measuring portion (above the line L) and a dc measuring
portion (below the line L).
The circuit of Figure 3 is similar to that of Figure 2,
except that the windings Ws and W2 have been combined into a
single composite secondary winding W2s.
Figure 4 shows another alternative in which the
compensating winding Wc of Figures 1 - 3 has been omitted, its
function having been taken over by the measuring winding Wm,
since the output 3, 4 of the amplifier Al now passes a current
Im equal to the magnetizing current through the winding Wm and
the secondary winding Ws. The lower portion of the circuit of
Figure 4 is the same as the corresponding portion of the
circuit of Figure 2, so that the direct current Idc generated,
as before, by the amplifier A2 flows in a series path through
the secondary winding Ws, the burden B and the resistor R, a
second capacitor Cb being used to block direct current into
the circuit of the amplifier Al.
As an alternative, the lower portion of the circuit of
Figure 4 can correspond to the lower portion of the circuit of
Figure 3, i.e. with the windings Ws and W2 combined into a
single winding W2s. This basic alternative, i.e. two separate
windings or a combined winding, applies to all the circuits.
In the circuit of Figure 2 it is necessary for the
windings Ws and Wc to have the same number of turns as each
other. If it is preferred to avoid this requirement, which in
some instances necessitates a larger number of turns than is
convenient, the more general circuit of Figure 5 can be used.
In this case, the input terminals 1, 2 of the amplifier Al are
still connected across the winding Wm, and a current Im flows
in that winding from the amplifier output terminals 3, 4 and
through a resistor R1. This results in a voltage Vl at
terminal 4 equal to ImR1, which voltage acts through a second
resistor R2 to inject into the compensating winding Wc a
current Ic that magnetizes the core C2, i.e. provides the
magnetizing current as before. The resistance ratio R1/R2
20~2~2
must be equal to the turns ratio of the windings Wm/Wc. As
before, the lower portion of the circuit of Figure 5 can be
the same as Figure 2, as shown, or the same as Figure 3 using
a combined secondary winding.
The further alternative to the upper portion of the
circuit is shown in Figure 6 and corrects for the influence of
the impedance of the compensating winding Wc. The connections
to the amplifier A1 are unchanged from those of Figure 5, but
a further amplifier A3 is provided to act as a voltage
doubler. This amplifier A3 receives its input at terminals 9,
10 connected between a center point 14 between a pair of
series-connected resistors R3, R4 of equal value, and a
junction point 15 between the compensating winding Wc and the
resistor R2. This serves to generate a voltage 2VB across the
output terminals 16, 17 of the amplifier A3, where VB is the
voltage at point 15. The current provided by the amplifier Al
and flowing through the winding Wc and the resistor R2 equals
V1-VB. Due to the second amplifier A3 there will now be
R2
an additional current that flows through the winding Wc and a
further resistor R5 that is equal in value to the resistor R2.
Such additional current will have the value 2VB-VB. Since
R5
R5 = R2, the total compensating current in the winding Wc
becomes 2VB-VB + V1-VB = Vl, i.e. a current that is
R2 R2 R2
independent of the voltage VB across the winding Wc, and thus
independent of the impedance of such winding.
It is not essential that a ratio of 2:1 be used for this
circuit. If, instead of being equal in value, the resistors
R4, R3 have a predetermined ratio n to each other, then the
resistors R2 and R5 must meet the requirement that
n = R4 = R5.
R3 R2
In Figures 3 - 6 the power supplies to the amplifiers
have been omitted for simplicity.
Figure 7 shows an alternative applicable to all the
-- 11 2#~112
circuits, in which the resistor R is replaced by an ammeter
AM.
Figures 8 - 11 show a preferred structural arrangement of
cores and windings for the circuit of Figure 2. Figure 8
shows a toroidal core and winding assembly 20 defining a
central hole 21 suitable for receiving a single turn primary
(not shown), such as a busbar, or alternatively for the
passage of convolutions of a multiple turns primary winding
therethrough. The toroidal assembly 20 consist of laminated
core members that in cross-section (Figure 11) consist of
inner and outer, spaced apart, side members 22 and 23 with a
central bridging member 24, and top and bottom closing members
25 and 26 to complete a closed periphery. Above the central
core member 24 there is a cavity 27 that is occupied by an
inner core member 28 that constitutes the core C1 of the
circuit diagrams, with the winding Wm wound around it and
occupying the remainder of the space in the cavity 27 except
for the usual insulation and epoxy (shown in black) that fill
all parts of the cavity 27 (and other cavities) that would
otherwise be void. Below the central core member 24 there is
a further cavity 29 that is occupied by a pair of further
inner core members 30 and 31 that constitute the modulation
cores C3 and C4, each with its respective winding W3 and W4
wound around it, and insulation and epoxy as before. The core
members 22-26 constitute the outer core C2 which, in addition
to its outer periphery, effectively provides two inner
magnetic circuits, one surrounding the core 28(Cl) and the
winding Wm, and the other surrounding the cores 30(C3), 31(C4)
and the windings W3, W4. Insulation ensures that these
magnetic circuits do not become short-circuited electrical
turns. Outside the outer core C2 there is an outer winding
assembly Wo consisting of various windings, namely the
compensating winding Wc, the dc winding W2 and the secondary
winding Ws when the Figure 2, 5 or 6 circuitry is used, or the
windings W2 and Ws when the Figure 4 circuitry is used or the
windings Wc and W2s when the Figure 3 circuitry is used.
Conventional copper shields 43 and 44 with respective
2~g21~2
12
interruptions 45, 46 are also located respectively outside the
outer core C2 and outside the outer winding assembly Wo.
While Figure 10 shows a section on line 10-10 in Figure
11 taken through the upper part of the toroidal structure 20,
a cross-section taken lower down, e.g. through the core C3 or
the core C4 would be structurally the same and hence such
extra sections have not been illustrated.
As Figure 8 shows, the outer winding assembly Wo will be
wound as two separate halves Wol and wo2. The inner windings
Wm, W3 and W4 will similarly each be divided into two portions
whereby between their ends to define a pair of diametrically
opposite, unwound core sections 32. After a complete toroidal
core and winding assembly 20 of this construction has been
formed, it is cut through along a diametrical plane 33 that
passes through the unwound sections 32 and contains the axis X
of the toroidal assembly. The result is a pair of separate,
semi-toroidal, sub-assemblies 20a and 20b, as shown in Figure
9. These separate sub-assemblies can readily be placed over a
busbar and clamped together by means of clamps (not shown)
that engage members 36 that will have been secured to the sub-
assemblies. Other members (not shown) can serve to support
terminal boards for the winding portions, so that these can be
series connected externally to effectively form continuous
windings extending substantially fully around the device.
While the cut ends of the two sub-assemblies 20a, 20b
will be polished in order to minimize the effective air gaps
in the magnetic circuits when they are clamped together, it is
impossible to avoid the cutting of the cores having a major
impact on the magnetic properties of the cores, i.e.
increasing their reluctances considerably. As indicated above
in connection with openable cores that have been used in the
past, it has always been necessary to accept the loss of
accuracy consequent upon this increase in reluctance, for
which reason openable core devices have never been practicable
for precision measuring instruments, in particular for
measuring large currents.
13 20~ 211 2
The present invention has overcome this disadvantage by
providing circuitry that is effectively independent of the
core reluctances. This effect is achieved because the
magnetizing current is supplied separately and electronically
from an auxiliary circuit. Hence, any increase in this
magnetizing current resulting from an increase in core
reluctance can be tolerated, leaving the accuracy of the
device substantially unimpaired. Another advantage that flows
from this insensitivity of the circuitry to increases in the
reluctance of the cores, which insensitivity also extends to a
tolerance for any lack of uniformity of permeability of a core
throughout its length, is that all the cores Cl, C2, C3 and C4
can be made of a lower grade of material. For example, it is
possible to use relatively low permeability Hypersil for all
these cores, which is not only less expensive but less brittle
and hence more readily workable than the high permeability
Supermalloy that has hitherto been considered necessary, at
least for the inner core C1, if the desired accuracy was to be
achieved in a non-openable core.
2 ~ 2
CLAIM KEY
Claim
1 Independent Current ratio device that
combines basic Miljanic et al
circuit with modulating and :~
demodulating means broadly `~
claimed for detecting dc alone
2 Dependent on 1 Add means for measuring the dc
3 Dependent on 2 Refers to resistor or ammeter in
the direct current circuit
4 Dependent on 1 Specifies dc winding Ws (Figures
2, 4 and 5) and capacitor C
Dependent on 1 Specifies dc flows through
secondary winding W2s (Figure 3)
6 Dependent on 5 Specifies ac amplifier passes
magnetizing current through
secondary winding (Figure 3)
7 Dependent on 1 Adds compensating winding to pass
magnetizing current (Figures 2, 4
and 5)
8 Dependent on 7 Specifies compensating and
secondary windings in series and
have equal number of turns
(Figure 2)
9 Dependent on 7 Adds resistors to permit
compensating and measuring
windings to have unequal number
of turns (Figure 4)
Dependent on 9 Adds means (generally) for making
compensating current independent
of the impedance of the
compensating winding (Figure 5)
11 Dependent on 10 Specifics of these means
(Figure 5)
12 Dependent on 1 Specifics of modulation and
demodulation means (Figures 1-5)
13 Dependent on 1 Adds to the main claim the
division of the windings into a
pair of portions on a toroidal
core structure (Figure 6)
14 Dependent on 13 Adds the feature of cutting the
core into two parts (Figure 6a)
lS Dependent on 14 Adds the specific cross-section
of Figure 8
~ 2~32~
16 Independent Toroidal core assembly without
reference to the external
circuitry but specifies cross-
section as shown in Figure 8
17 Dependent on 16 Adds the circuitry of any one of
Figures 2-5
18 Dependent on 16 Specifies windings are each
divided into a pair of separate
portions (Figure 6)
19 Dependent on 18 The assembly is cut into two sub-
assemblies (Figure 6a)
Independent Core assembly in which the
windings are each divided into a
pair of separate portions (Figure
6) without specifying the cross-
section of Figure 8
21 . Dependent on 20 The assembly is cut into two sub-
assemblies (Figure 6a)
22 Dependent on 20 A pair of separate sub-assemblies
for forming an assembly as per
claim 20, each sub-assembly being
semi-toroidal and having a
winding portion.
23 Dependent on 22 Adds clamping and terminal means.
Summary of the claim structure based on
A = circuitry (Figs 2 - 7)
B = core cross-section (Fig 11)
C = openability (Figs 8 and 9)
Feature Claims
A 1 - 12
A + C 13 and 1
A + C + B 15
B 16
B + A 17
B + C 18 and 19
C 20 - 23
