Note: Descriptions are shown in the official language in which they were submitted.
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INPUT ~WITCHING IN ELECTRONIC WATTHOUR METER
BACKGROUND OF THE INVENTION
The present invention relates to electronic
watthour metering and, more particularly, to
techniques for overcoming offset voltages at an input
of an amplifier serving as part of an active-feedback
element in a current transformer feeding an
electronic watthour meter.
One type of electronic watthour meter disclosed,
for example, in U.S. Patents Nos. 3,947,736 and
3,955,138 employs a multiplier using a pulse-width-
modulated signal having pulse widths ratios related
to an instantaneous value of one of line voltage or
load current to modulated the instantaneous value of
the other thereof. The resulting modulated product
signal is proportional to the product of line voltage
and line current; that is, to the instantaneous power
consumption of the load. The product signal is
integrated to produce a sawtooth waveform which
triggers an output pulse when its amplitude attains a
predetermined value. Upon the production of the
output pulse, the direction of integration is
reversed until an opposite threshold is attained.
Each cycle of the output pulse indicated the
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consumption of a predetermined quantum of electric
energy, conventionally measured in units of watthours
or kilowatthours.
The load current is typically many times the
value of a current signal appropriate for use in an
electronic watthour metering device. In some
systems, the load current is as much as 10,000 times
larger than the desired current signal. It is
conventional to employ a current transformer wherein
a small number of turns (for example, one or two)
about a toroidal core serve as a primary
current-transformer winding carrying the load
current. A secondary winding of many turns has
induced in it a current proportional to the load
current but reduced by the primary-to- secondary
turns ratio of the current transformer.
Current transformers are prone to core
saturation in the presence of large primary currents.
Core saturation conventionally is avoided by using
large cores and making the cores of high-quality
materials. Both large size and high-quality
materials invoke high cost.
One solution for core saturation disclosed, for
example, in Canadian Patent Application Serial No.
561,728, filed March 17, 1988, Bullock, and the
references cited therein, includes providing a
feedback winding on the core carrying an inverse
current signal just sufficient to maintain the core
flux near zero. Limiting the core flux near zero
permits using smaller cores and cheaper core
materials. As the load current increases, the
inverse current signal also increases just enough to
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maintain the core flux near zero, whereby all levels
of load current can be accommodated without
experiencing core saturation of the current
transformer. The inverse current may, itself, be
employed as the output signal from the current
transformer.
The active feedback employed in the above-
referenced Canadian Application Serial No. 561,728 is
produced by an operational amplifier receiving the
output of the secondary winding of the current
transformer. The conventional high gain of an
operational amplifier produces an inverse current
easily capable of maintaining near zero flux in the
core. The high gain of the operational amplifier,
however, leads to a further complication. That is,
coupling between the feedback windlng and the
secondary winding of the current transformer is only
for alternating current (AC). There is no DC
feedback coupling to the input of the operational
amplifier. Thus, DC offset voltages of, for example,
a fraction of a millivolt may appear at the input of
the operational amplifier. Operational amplifiers
conventionally have DC gains on the order of several
million. As a consequence, any offset voltage, even
a fraction of a millivolt at the input of the
operational amplifier is capable of driving the
operational amplifier to saturation.
One technique for offset-voltage compensation is
disclosed in Canadian Application Serial No. 569,668,
filed June 16, 1988, Milkovic. This device
momentarily short-circuits the input of an
operational amplifier and stores a sample of any
output voltage as a measure of the offset voltage.
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During subsequent operation, the sample is applied to
the input of the operational amplifier along with the
input signal. This technique appears to have little
value in solving DC offset problems in an operational
amplifier producing a negative current signal for
maintaining the flux in a current transformer core
near zero.
U.S. Patent No. 4,066,960 discloses compensating
for offset voltages by integrating the product signal
first in one direction and then in the opposite
direction during the half cycles of the output pulse
of an electronic watthour meter. Any influence of
offset voltages on the length of a half cycle of the
output signal is exactly compensated by an equal and
opposite effect on the length of the adjacent half
cycle. Although useful for avoiding errors in an
integrator, this technique is not helpful in the
present application since any integration taking
place is well downstream of the point at which offset
voltage acts.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide an
electronic watthour meter which overcomes the
drawbacks of the prior art.
It is a further object of the invention to
provide offset voltage compensation in an amplifier
producing a negative current signal for maintaining a
core of a current transformer.
It is a still further object of the invention to
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provide DC feedback to an input of an operational
amplifier used as part of an active feedback element
of a current transformer. The DC feedback means
employs a time constant that is long relative to a
line frequency and normal current spikes.
It is a still further object of the invention to
provide current sensing means having a current
transformer with primary, secondary and feedback
windings. A current signal induced in the primary
winding is fed to an input of an amplifier. The
output of the amplifier is fed through the feedback
winding in a sense effective for maintaining the core
flux near zero. The input connections to the
operational amplifier and connections to the feedback
windings are rever~ed periodically, whereby an AC
signal, related to the DC offset voltage is fed back
to the input o~ the operational amplifier and the
effects of DC of~set voltages are eliminated.
It is a still further ob;ect of the invention to
provide an electronic watthour metering system
wherein a current sensor includes a current
transformer with primary, secondary and feedback
windings. A current signal induced in the primary
winding is fed to an input of an amplifier. The
output of the amplifier is fed through the feedback
winding in a sense effective for maintaining the core
flux near zero. The input connections to the
operational amplifier and connections to the feedback
windings are reversed by a pulse-width modulated
signal having a duty ratio related to an
instantaneous voltage. The output of the feedback
winding is proportional to the instantaneous power
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consumption. In addition, the switching produces an
AC signal, related to the DC offset voltage fed back
to the input of the operational amplifier, whereby
the effects of DC offset voltages are eliminated.
Briefly stated, the present invention provides
an electronic metering circuit including a current
transformer having active feedback for maintaining
core flux near zero. In one embodiment of the
invention, DC offset-voltage compensation in an
operational amplifier providing the active-feedback
signal is accomplished using an integrator having a
long time constant compared to the time constant of
periodic signals. A further embodiment of the
invention integrates the functions of current
sensing, DC offset-voltage compensation, and part of
the switching function required in the electronic
metering circuit.
According to an embodiment of the invention,
there is provided a current sensor comprising: a
current transformer, the current transformer
including a primary winding, a secondary winding and
a feedback winding, means for magnetically coupling
together the primary winding, secondary winding and
feedback winding, feedback generating means
responsive to an AC signal in the secondary winding
for producing a feedback signal for application to
the feedback winding, the feedback signal having an
amplitude and a phase effective for maintaining a
flux in the means for magnetically coupling near
zero, and means for producing a signal effective for
compensating a DC offset voltage in the feedback
generating means.
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According to a feature of the invention, there
is provided an electronic watthour metering system
comprising: means for producing a current signal
having an amplitude proportional to a load current,
means for producing a voltage signal having an
amplitude proportional to a voltage applied to the
load, means for producing a pulse-width-modulated
signal having a duty ratio related to and amplitude
of the voltage signal, the means for producing a
current signal including: a current transformer, the
- current transformer including a primary winding, a
secondary winding and a feedback winding on a common
core, an operational amplifier responsive to an AC
signal in the secondary winding for producing a
fQedback signal for application to the feedback
winding, the feedback signal having an amplitude and
a phase effective for maintaining a flux in the means
for magnetically coupling near zero, a first switch
assembly, the first switch assembly including a first
pair of switches effective when closed to connect
first and second ends of the secondary winding to
first and second inputs, respectively, of the
operational amplifier, the first switch assembly
including a second pair of switches effective when
closed to connect the first and second ends of the
secondary winding to second and first inputs,
respectively, of the operational amplifier, a second
switch assembly, the second switch assembly including
a third pair of switches effective when closed to
connect an output of the operational amplifier to a
third end of the feedback winding and to connect a
fourth end of the feedback winding to a succeeding
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circuit, the second switch assembly including a
fourth pair of switches effective when closed to
connect the output of the operational amplifier to
the fourth end of the feedback winding and to connect
the third end of the feedback winding to the
subsequent circuit, and means responsive to the
pulse-width-modulated signal for alternately closing
the first and third pairs of switches while opening
the second and fourth pairs of switches and for
opening the first and third pairs of switches while
closing the second and fourth pairs of switches
whereby an AC signal related to a DC offset voltage
is coupled to the operational amplifier through the
means for magnetically coupling.
The above, and other ob~ects, features and
advantages of the present invention will become
apparent from the following description read in
conjunction with the accompanying drawings, in which
like reference numerals designate the same elements.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram of an electronic
metering system according to the prior art.
Fig. 2 is a block diagram of the apparatus of
Fig. 1 with the contents of the multiplier thereof
detailed.
Fig. 3 is a schematic diagram of a current sensor
according to the present invention.
Fig. 4 is a schematic diagram of a further
current sensor according to the present invention.
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Fig. 5 is a schematic diagram of the current
sensor of Fig. 4 in one of its switch configurations.
Fig. 6 is a schematic diagram of the current
sensor of Fig. 4 in the other of its switch
configurations.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Fig. 1, there is shown, generally at
10, a generalized representation of relevant parts of
an electronic watthour meter according to the
referenced prior patents. A current sensor 12
produces a signal proportional to a current consumed
by a load (not shown) for application on a line 14 to
a multiplier 16. A voltage sensor 18 produaes a
signal proportional to a line voltage for application
on a line 20 to multiplier 16. In the referenced
patents, multiplier 16 produces a pulse-width-
modulated signal having a duty ratio controlled by
one of the input signals and an amplitude controlled
by the other of the input signals. The pulse-width-
modulated product signal Vz, proportional toinstantaneous power consumption, is applied on a line
22 to an integrator 24. Integrator 24 produces an
output signal having a slope proportional to the
instantaneous power consumption for application on a
line 26 to a pulse generator 28.
As is conventional, pulse generator 28 includes a
threshold detector effective for producing
alternately a positive-going and a negative-going
transition in its output upon the magnitude of the
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integrated signal on pulse generator 28 reaching a
predetermined negative and positive value,
respectively. Each cycle of the resulting pulse
signal contains a predetermined significance in
energy consumption such as, for example, a
predetermined quantum of watthours of energy
consumption. The pulse signal is applied on a line
30 to a conventional counter (not shown) or external
circuits for accumulating the energy consumption for
billing or other purposes. The pulse signal is also
applied on a feedback line 32 to multiplier 16 for
reversing the sense of the product signal Vz, whereby
the integrated signal on line 26 is a sawtooth
waveform having positive and negative peaks of equal
magnitude.
Referring now to Fig. 2, multiplier 16 includes a
pulsewidth modulator 34 including a triangular-wave
generator 36 producing a fixed-frequency and
fixed-amplitude triangular wave for application on a
line 38 to an input of a threshold circuit 40. A
switch 42, reversed each time the pulse signal on
feedback line 32 completes a half cycle, alternately
applies a voltage signal X on line 20 or the inverse
voltage signal -X on line 20' to the second input of
threshold circuit 40. The output of threshold
circuit 40, a pulse-width-modulated signal having a
constant period T (or equivalently, a constant
frequency 1/T) and a duty ratio (ratio o~ ON time to
OFF time) related to the amplitude o~ the voltage
signal applied through switch 42 to threshold circuit
40, is employed to control a switch 44. The two
input terminals of switch 44 receive a current signal
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Y on line 14 and the inverse current signal -Y on
line 14'. The output of switch 44 is product signal
Vz, a pulse-width-modulated signal having a duty
ratio responsive to the voltage signal X or its
inverse -X, and an amplitude responsive to the
amplitude of the current signal Y and its inverse -Y.
In the prior art, current sensor 12 (Fig. 1) is
a center-tapped current transformer including a
primary winding (not shown) of a small number of
turns (typically one or two) and a secondary winding
of a large number of turns. The resulting current
transformation is required to produce a current
signal on line 14 on the order of milliamperes or
millivolts from a primary current on the order of
tens or hundreds of amperes. Maintaining the core of
such a current transformer in the unsaturated
condition requires a core of a large physical size
and of expensive material.
Voltage sensor 18 of the prior art (Fig. 1) is a
center-tapped voltage transformer with a transformer
ratio effective to produce voltage signals at a level
appropriate for the electronic circuits in multiplier
16.
Referring now to Fig. 3, there is shown,
generally at 46, a current sensor including means for
overcoming the problem of core saturation in a
prior-art current transformer. A current transformer
48 includes a primary winding 50, a secondary winding
52, and a feedback winding 54, all wound on a common
core 56. The two ends of secondary winding 52 are
connected to inputs of an operational amplifier 58.
The output of operational amplifier 58 is applied
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through feedback winding 54. The output of feedback
winding 54 is the desired current signal.
When poled as shown, the high gain of a typical
operational amplifier 58 produces a voltage across
feedback winding 54 sufficient to cancel
substantially all of the flux in core 56 and thereby
to avoid saturation of core 56, thus permitting the
use of cores of small size and reasonable cost.
A problem remains, however, in providing means
for avoiding saturation of operational amplifier 58
due to offset voltage. Insofar as it has been
described in the preceding, only AC feedback coupling
éxists between feedback winding 54 and secondary
winding 52. No means for DC feedback is yet
described. Thus, no means for DC offset voltage
compensation is yet discu~sed. Such DC offset
voltage compensation i8 provided by an integrator 60
consisting of an amplifier 62 receiving the output
voltage of operational amplifier 58 through a
resistor 64 and with an integrating capacitor 66
connected between its output and its input. The
output voltage of amplifier 62 is applied to the
inverting input of operational amplifier 58. The
time constant of integrator 60 is long relative to
the period of the primary current, and any other
periodic signal which may be present. A time
constant on the order of seconds may be used. With
such a long time constant, integrator 60 responds
essentially to the DC component in the output of
operational amplifier 58 and generally ignores the AC
component. This DC component, fed back to
operational amplifier 58 cancels DC offset voltages,
as is desired.
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The embodiment of the invention shown in Fig. 3
responds in an oscillatory manner to step changes in
load current as may be produced, for example, by the
starting or stopping of a large motor. In some
applications, such oscillatory response may be
undesirable.
~ A further improved embodiment of the invention
is shown in Fig. 4 wherein the functions of
preventing core saturation, DC offset-voltage
compensation, and part of the multiplier function are
integrated. The two ends of secondary winding 52 of
current transformer 48, corresponding to that of Fig.
3, are connected to inputs of operational amplifier
58 through a reversing switch assembly 68. Reversing
switch assembly 68 contains one pair of switches
labelled A and a second pair of switches labelled B.
Switchea A and B are driven by complementary
pulse-width-modulated signals responsive to the
voltage in a manner corresponding to that described
in connection with Fig. 2. That is, switches A are
driven directly by the pulse-width-modulated signal,
and switches B are driven by its inverse produced,
for example, by an inverter 70. One of switch pairs
A and B is closed, and the other is open.
The output of operational amplifier 58 is
connected to feedback winding 54, and the output of
feedback winding 54 is connected to an output
amplifier 72 through a second switch assembly 74.
Switch assembly 74 includés a pair of switches
labelled A, driven by the pulse-width-modulated
signal in common with corresponding A switches in
reversing switch assembly 68, and a pair of switches
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B driven by the inverse of the pulse-width-modulated
signal in common with corresponding B switches in
reversing switch assembly 68.
Output amplifier 72 includes an operational
amplifier 76 having an input resistor 78 connected
from its inverting input to ground, and a feedback
resistor 80 connected from its output to its input.
Output amplifier 72 maintains a virtual ground at its
non-inverting input, with its output voltage
increasing as necessary to maintain this condition.
The output of operational amplifier 76 is the desired
product signal Vz.
Referring now to Fig. 5, the condition of the
circuit of Fig. 4 is shown while all A switches are
closed, and all B switches are open and omitted from
the figure for purposes of description. It will be
noted that this configuration corresponds to that
shown in Fig. 3 with integrator 60 omitted. The
alternate configuration, with all B switches closed
and all A switches open and omitted from the drawing,
is shown in Fig. 6.
It will be clear to one skilled in the art that
any DC component in the output of operational
amplifier 58 is converted to a corresponding AC
signal by the switching between the configurations in
Figs. 5 and 6. The AC signal derived from DC offset
voltages is coupled through current transformer 48
back to the input of operational amplifier 58 which
thereupon produces a compensating component of output
to maintain the effect of DC offset close to zero.
Having described preferred embodiments of the
invention with reference to the accompanying
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drawings, it is to be understood that the invention
is not limited to those precise embodiments, and that
various changes and modifications may be effected
therein by one skilled in the art without departing
from the scope or spirit of the invention as defined
in the appended claims.
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