Note: Descriptions are shown in the official language in which they were submitted.
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METH~D AND CIRCUITRY FO-R SINE
WAVE RECONSTRUCTION
.~ .
BACKGROUND_OF: T~E I-N~ENTION
The present invention reIates generally to sine
wave reconstruction techniques and more particularly to a
method and circuitry for providing an outpu-t signal,
representative of a sine wave source voltage, from,
corrupted voltages which'appear on the lines connecting
the source to a load.
There are a number of instances in which it is
desirable'to have an accurate representation of a sine
wave source ~oltage. One of the more common of these is
in the power conversion/motor control art where
semiconductor (e.g., thyristor) converter bridges are
used to power and control an electric motor. In these
applications it is necessary to have an accurate
representation of the source voltage in order to
properly synchronize the rendering of the bridge
semiconductors conductive with'respect to the source
voltage. This is normally done by detecting when the
~, voltage crosses the zero axis.
If no disturbances occur on the lines, such
detection poses no problems~ Such,' however, is seldom
the case. This is particularly true where semiconductor
power bridges are used and the transfer of current from
one bridge thyristor to anothér causes periodic short
circuits across the power lines' which in turn causes
rather sevexe disturbances' on those lines. These
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disturbances become even more pronounced when plural
converters are connected to the lines with the attendant
increase in the number of disturbances. ~ number of
systems are known and have been employed to reconstruct
the sine wave representation of the source voltage. In
U.S. Patent No. 3,976~68 "~oltage Synthesization" by
L.J. Lane, issued August 24, 1976, signals proportional
to the instantaneous values of the voltage at the
terminals of the voltage supply and to the rate of
change, with respect to the time, of the current
supplied to a load are developed and utilized to
synthesize the voltage. An eLaboration of this scheme
involves providing the output of the 3,876,~6~ patent
to an overdriven ampli~ier, the output of which is then
furnishecL to a self-tuning filter such as that described
in U.S. Patent No. 3,978,420 'ISelf-Tuning Filter" by
L.~. I.ane, issued August 31, 1976.
These known methods are very satisfactory and
properly employed furnish excellent results. They are,
however, fairly complex and hence expensive and in those
applications requiring a high degree of precision can be
Eully justified. There are, however, a number of
situations in which this hiyh degree of accuracy is not
necessary and in which it is difficult to justify the
expense of these systems.
S`UMMARY OF T~E INVENTION
_ _ .
It is, therefore, an object of the present
invention to provide a method and circuitry for
reconstructing a representation of a sine wave source
voltage from corrupted line voltages.
It is a further object to provide a sine wave
reconstruction technique which requires the sensing of
only line to line voltages.
It is still a ~urther object to provide a sine
wa~e reconstruction technique which is relatively simple
and inexpensive and ye~ sufficiently accurate ~or many
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applications.
It is an additional object to provide a sine
wave recQnstruction technique which can be implemented
in a relatively simple manner using relatively in-
expensive components.
The foregoing and other objects are achieved
in accordance with the present invention by first
developing a signal which is representative of the
line-to-line voltage between two phases of the line
voltayes. In response to the thusly developed signal,
there is developed a second signal which is representative
of only the high frequency components o~ the first signal.
These two signals are then combin~d and this combined
signal is filtered to provide an output signal which is
representative of the sine wave source voltage.
BRIEF DESCR;IPTION OF THE D~AWING
While the present invention is described in
particularity in the claims annexed to and :Eorming a
part of this specification, a better unders-tanding of
the invention can be had by reference to the following
description taken into conjunct:Lon with the accompanying
drawing in which:
Figure 1 is a schemat~c drawing illustrating
the circuitry of the present invention in its preferred
embodiment; and
Figure 2 illustrates wave shapes helpful in
understanding the present invention.
DETAILED~DESCRIPTION
Referring now to Figure 1, which shows the
circuitry of the present invention in its preferred
embodiment, it is seen that a source voltage represented
y terminalS Tl! T2 and T3 is connected to a load 10 by
way of lines Ll, L2 and L3. The nature of load 10 is not
important to the present invention but it would,
typically, comprise one or more power converters each
supplying one or more eIectric motors. In accordance
with the depiction of Figure l, the vol~ages on lines
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Ll and L3 (which designation will also be used for the
voltage) serve as inputs to a differential amplifier
12 which includes an operational amplifier 14 having
its non-inverting input connected to ground by way of
a suitable resistor 16 and a feedback resistor 18
connected between its output and its inverting input.
In order to properly scale the voltage levels, the
phase voltage L3 is furnished to the inverting input
of the operational amplifier 1~ by way of a vol~age
divider comprised of serie~ connected resistors 20
and 22. Similarly, th~ phase voltage Ll is applied as
the non-inverting input to operational amplifier 14
by way of the voltage divider comprised of resistors 24
and 26. Reference is now made to the upper trace of
Figure 2 which shows the line to neutral voltages as
they might appear on lines ~1' L2 and L3, including
disturbances caused by a single converter supplying
a motor load. The illustrated distortions shown of the
sine wave are what are commonly known as commutation
notches. The wave shapes shown in this trace are those
which occur when the firing angle of the thyristors plus
one-half of the commutation angle equals 90.
The second trace of Figure 2 illustrates the
output of the differential amplifier circuit 12 and it
is seen that it is essentially 30 electrical degrees
lagging with respect to the Ll to neutral voltage and
that the commutation notches are essentially in time
alignment of that phase to neutral voltage.
Referring again to Figure 1, the output of the
differential amplifier 12 serves as the input to a high
pass filter network 28. The high pass filter 23
consists of a pair of series connected input capacitors
30 and 36 through which the signal from the amplifier
12 isapplied to the inverting input of an operational
amplifier 37. The juncture of capacitors 30 and 36
(point 32) is connected to ground by way of a resistor
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34 and this same point i5 also connected by way of
capacitor 40 to the output of the operational amplifier
37. A feedback resistor 38 is connected between the
inverting input and the output of operational amplifier
37 and the non-inverting input of amplifier 37 is
connected to ground by way of a resistor 39. The high
pass filter within block 28 is essentially that which
is shown and described in greater detail in the
publication "Handbook of Operational Amplifier Active
RC Networks" by Burr-Brown, copyright 1966 (Reference
clrcuit number five on page 76). The frequencies to be
passed by the high pass filter network28 are somewhat
arbitrary but it is believed that satisfactory results
can be obtained when the filter network 28 is designed
to pass frequencies greater than approximately two and
one-half times the fundamental fre~uency of the source
voltage. Assuming this voltage were 60 hertz, the high
pass filter 28 would, therefore, be designed and its
component values appropriate to passing frequencies greater
than 150 hertæ.
Referencing once again Figure 2, the output
o:E ~ilter 28 is shown b~ the third trace from the top
and it is seen that the filter output is a series o~
spikes or narrow pulses which a:re in time synchronization
with the disturbances o~ the Ll-L3 voltage, equal the
magnitude and inverted with respect thereto.
The outputs of the differential amplifier 12
and the fiLter 28 are summed within a suitable summing
circuit represented within dashed line block 42. As
shown, the summing circuit includes an operational
amplifier ~6 having its non-inverting input connected
to ground by way of a resistor 47 and the two signals
from the circuits 12 and 28 are connected by respective
resistors 48 and 44 to the inverting input. A feedback
resistor 50 is connected between the output and the
inverting input of the operational amplifier 46. The
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The output of the summing circuit ~2 is supplied to a
filter circuit 52 which provides as its output on line
62, a signal which is a representation of the sine wave
source voltage. In the presently illustrated embodiment
of the invention, the filter network 52 takes on the
basic form of an integrating network which provides a
fixed amount of the phase shift with resepect to its
input. To this end the filter 52 includes an operational
amplifier 54 having its non-inverting input connected
to ground via resistor 55. The inverting input of the
operational amplifier receives the output of circuit
42 by way of an input resistor 56. A parallel connection
of a capacitor 58 and a resistor 60 is connected between
the output and inverting input of the amplifier 54. Those
familiar with operational amplifier integrators will
recognize that the existence of the resistor 60 precludes
this circuit from being a pure integrator and that,
therefore,the phase displacement will not be exactly
~ 90. The need or desirability for the resistor ~ is
apparent when it is recognized that a certain amount of
dc components will be present in the signal applied to
the integrator and were it not for the resistor 60 the
integrating circuit (filter) would have a tendency to
go to satur~tion and thus preclude the efficient use of
the present invention.
The bottom trace of Figure 2 illustrates the
improvement of the present invention ov~r a simple
integration of the line voltages. As shown by the solid
line in Figure 2, the integral of the Ll to L3 voltage,
it is seen that, for example, at times tl and t2 there
is a substantial flat spot within the wave shape. When
this i~ compared to the upper trace Figure 1, it is seen
that, in this instance, these flat spots correspond, in
time, to the zero crossings of the L2 line voltage.
Thus t these flat spots would render a pure integration
of the line voltages unsuitable since there are periods
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of time in which the zero crossing is not well defined.
~y supplying, in accordance with the present invention,
the combina~ion of the voltage Ll-L3 with the inverted
high frequency components of that voltage the dashed
line depiction of Figure 2 is achieved and it is seen
that this more closely approximates a sine wave and
does not exhibit the flat spot characteristics of the
solid line depiction.
Thus, by the design o~ the integrator filter
circuit 52, the phase displacement of the output signal
with respect to the line voltages is known and by the
use of any of the appropriate known zero crossing
detectors~ the present invention may be utilized to
provide synchronization with respect to the line voltages
regardless o~ the number or type of disturbances on the
lines.
While there has been shown and descrihed what
is at present considered to be the preferred embodiment
of the present invention, modifications thereto will
readily occur to those skilled in the ar~. It is not
desired, therefore, that the invention be limited to
this specific circuit and scheme shown and described
and it is i.ntended to cover in the appended claims
all such modifications as fall within the true spirit
and scope of the invention.
