Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to balanced phase convertors
and more particularly to a static convertor for converting a
single phase, variable AC load to a balanced three phase load
for connection to a three phase power supply.
In many applications o~ single phase loads it is
necessary to connect the load to a three phase power supply.
In some instances, such as lighting, it is possible to
distribute the load evenly over the three phases; in other cases
this is not possible and an unbalanced situation occurs.
This situation can arisefrom the use of equipment such as
induction heating furnaces, various types of welders, electric
traction equipment such as locomotives in the railway industry,
and various other kinds of equipment.
An unbalanced load i3 a serious problem since it can
cause, among other things, a negative sequence current and
thereby produce unwanted heating and possible damage in AC
motors and generators.
At this time it should be pointed out that by a
"balanced" load, it is meant that the currents in each of the
phases of the three phase power supply, that supplies the load,
are of equal magnitude and they are each separated by a phase
angle of 120 degrees.
In order to transform a single-phase load into a
balanced three-phase load, a phase balancer is used. Phase
balancers of various descriptions are well known in the art.
Previous phase balancers have included methods such as
transformer convertors as shown by U.S. Patent 3,375,429 dated
March 26, 1968 to F. Pagano; methods involving solid state
switching and incorporating frequency conversion such as that
shown by Canadian Patent 825,775 dated October 21, 1969 to
P.P. Biringer and methods involving switching inductors
and capacitors as indicated by U.S. Patent 3,053,920 dated
September 11, 1962 to J~P. Seitz.
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lOBS9ZO
m ese previous methods of phase balancing have had
many limitations and drawbacks. The use of transformers for
phase balancers is expensive, gives low power factors and requires
mu~h physical space if the loads used are to be large.
me use of inductors acro~s a first phase and
capacitors across a second phase of a three phase ~ystem in order
to balance the load, connected across the remaining phase, is ~-
well known in the prior art. However, with these previous
methods, moving parts such as contactors or tap changers have
been employed which are subject to problems such as contact
corrosion, etc. In addition, only discrete reactance changes
are possible, and the reactance in each leg must be predeter-
mined as being either inductive or capacitive. Since these
methods make use of discrete switched components, they cannot
provide accurate phase balancing of loads having a variable
magnitude or a variable power factor.
Methods using solid state switching elements~have
also been disclosed. These methods, however, have been based
upon freguency multipliers, which limits their use to special
frequency equipment.
The present invention (in the preferred embodiment)
consists of connecting the single phase load across one phase
of the three phase supply and connecting, acrO#S each of the
two remaining phases, a circuit whose impedance can be varied,
both as to magnitude and as to reactance (capacitive or inductive).
This variable reactance circuit comprise# a pair of solid
state switching devices, such as SCR's, in an inverse-parallel
configuration and connected in series to an inductor; this
combinatisn of SCR's and inductor being in parallel relationship
with a capacitor. With the SCR's turned off, the capacitor ~`
alone is in the active AC circuit. With the SCR's switched
fully on, the capacitor is paralleled by the inductor, and
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10~9~ Case 2265
if the impedance of the inductor is made sufficiently small,
(so that the magnitude of the current is greater through the
inductor than through the capacitor) the inductor will be the
main impedance determining element and will thus effectively
cancel the capacitor's effects and there will be a predominantly
pure inductance in the AC circuit. By controlling how long
the SCR's are conducting one can determine the magnitude of
the reactance and whether it will be capacitive or inductive.
The magnitude and types of impedances required for balancing
the load will be determined according to the size of the
single phase load and its power factor.
Stated in another way, the present invention is an
apparatus for producing an approximately balanced, three phase
load from a single phase load, the apparatus comprising:
terminals A, B, and C for connection to a three phase, alternating
current, electric power supply; the single phase load is
connected across terminals A and C; a first reactance device is
connected across t0rminals A and B: a second reactance device is
connected across terminals B and C: at least one of the first
and second reactance devices being a variable reactance device:
and control circuitry to regulate the variable reactance devices.
The invention will now be described in more detail
with reference to the accompanying drawings, in which:
Figure 1 is a simplified schematic diagram sh~wing
the preferred embodiment of the invention for phase balancing;
Figure 2A shows a simplified schematic of Figure 1
when the single phase load has unity power factor;
Figure 2B is a vector diagram relationship of the
voltages and currents of Figure 2A:
Figure 3A shows a simplified schematic of Figure 1
when the single phase load has a 0.8 power factor leading;
Figure 3B is a vector diagram relationship of the
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of the voltages and currents of Figure 3A;
Figure 4A shows a simplified schematic of Figure 1
when the ~ingle phase load has a 0.8 power factor lagging;
Figure 4B is a vector diagram relationship of the
voltage and currents of Figure 4A; and
Figure 5 shows a simplified embodiment as in Fig. 1,
but power factor control has been added.
Figure 1 shows a variable single phase load 10
connected, according to the invention, to produce a balanced
three phase load at terminals A, B and C. The load 10 is shown
connected across terminals A and C. Solid state switching devices,
such a~ Silicon Controlled Rectifiers (SCR's) 11 and 12, are
connected in an inverse-parallel relationship as shown in the
Figure. Inductor 13 is connected in series with the SCR 11 ~ ~
and 12 combination, and the whole combination of SCR's 11 and ~ -
12 and inductor 13 is connected across terminals A and B as
shown in the Figure. Capacitor 16 parallels this circuit and
is likewi6e conne~ted to terminal~ A and B. -~
A similar arrangement, cons~oting of SCR~s 14 and
15, inductor 18 and capacitor 17, is connected between the
terminals B andC as shown in Figure 1.
To balance a single phase load connected to a three
phase power supply it is well known in the art to connect the
load to one phase, to connect a capacitor across a second phase
and to connect an inductor across the remaining phase. The
value of the capacitive and inductive elements are determined
by the magnitude and power factor of the single phase load;
different values are necessary for different loads~
me present invention incorporates elements which
can be controlled to produce a variable effective reactance
and thereby provide the proper value of inductance or capacitance
to maintain the load in a balanced state regardless of the
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magnitude or power factor of the single phase load.
One such set of elements in the present invention
which serves to vary the reactance is indicated as Variable
Reactance Device 24 in Figure 1 and consists of the combination
of inverse-parallel SCR's 11 and 12, the inductor 13 and the
capacitor 16. A~ i8 readily apparent from Figure 1, if the
SCR's were kept in the non-conductive state, the inductor would
be eliminated as an active component in the AC circuit and
the capacitor 16 would be the only reactive component remaining
active in the AC circuitry between terminals A and B.
Alternatively, if the SCR's 11 and 12 were left in
the conducting~state continuously, the inductor 13 and capacitor
16 would be in a parallel relationship between terminals A and
B. However, if we make the Lmpedance of inductor 13 much smaller
than the impedance of capacitor 16, the effect of inductor 13
will dominate the effect of capacitor 16, and we will have
essentially a pure inductance means between the terminals A
and B. This is due to the fact that the magnitude of the
current conducted through inductor 13 will be much larger than
the magnitude of the current conducted through capacitor 16.
Additionally, by controlling how long a period the SCR's 11
and 12 conduct, we can obtain any value of reactance between :
our two extremes of essentially all capacitance and essentially
all inductance. The same effect is produced across terminals
B and C by the elements located therebetween; namely the SCR's
14 and 15, the inductor 18 and the capacitor 17.
The SCR's can be controlled manually or by any method
known in the art; the method indicated herein is but one of many
and is sho~n solely to exemplify the operation of the invention.
Current transformers 21, 22 and 23 are connected to the three
lines of the balanced load as shown in Figure 1. The current
transformers are used to measure the current flowing in each
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pha~e of the balanced three phase load and to feed this informa-
tion to the Current Detection Circuitry 20. The Current Detection
Circuitry 20 feeds the Control Circuitry 19 which controls the
firing of the SCR's, and accordingly the impedance across
phases A-B and B-C, in order to maintain the balanced load
in a balanced state.
The effect of varying the impedance in two legs of
the load can best be seen by studying an example. Accordingly,
Figure 2A shows a simplified version of the invention shown in
Figure 1. The load 30 is fixed and is purely resistive, inductor
32 is used across one phase of the three phase supply and
capacitor 31 is used across the remaining phase as shown in
Figure 2A. m e voltages (E) and currents (I) used in this
example are shown on the same Figure.
Figure 2B i8 a vector diagram showing vectorially
the relationship between the voltages and currents of Figure
2A and how the addition of capacitor 31 and inductor 32 results
in a balanced load at terminals A, B and C.
Figure 2B will now be described in more detail with
Figure 2A being used for definition of symbols and general
background reference. The vectors El, E2 and E3 represent
the voltages of each of the phases of the power supply. They
are equal in magnitude and are separated in phase by 120
degrees; since they are produced by a constant voltage source
(i.e. the power supply) they will maintain this relationship
whether the load is balanced or not, and accordingly they
provide our frame of reference for the rest of the discussion.
Since lo~ad 30 is a pure resistance, the current I1 passing
through it will be in phase with the voltage El across the
load 30. Accordingly, Il is shown as being colinear to, and
in the same direction as~ El in the vector diagram of Figure 2B.
The current I2 passing through the inductor 32 will
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lag, by 90 degrees, the voltage E2 across the inductor 32
Accordingly, the vector I2 in the vector diagram i9 shown
90 degrees clockwise of the vector E2.
The current I3 through the capacitor 3I will lead,
by 90 degrees, the voltage E3 across the capacitor 31. Therefore,
the vector I3 in the diagram is shown 90 degrees counterclockwise
of the vector E3.
Referring to Figure 2A, the currents flowing towards
the terminals A, B and C are Il-I2, I2-I , and I -Il
respectively. These currents, shown vectorially in Figure 2B,
can be obtained by vector subtraction of the currents involved.
As can be seen in Figure 2B, the currents Il-I2, I2-I3, and
I3-Il are all of equal magnitude and are all separated by
angles of 120 degrees. We have obtained a balanced three
phase load.
Figure 3A shows another simplified version of Figure
1. This time, however, load 40 has a leading power factor of
0.8. Accordingly, capacitor 41 and 42 are connected across -~ -
the remaining two phases to balance the three phase supply as
shown.
Figure 3B is a vector diagram showing vectorially -~
the relationship between the voltages and currents of Figure 3A
and how the addition of capacitors 41 and 42 results in a
balanced load at terminals A, B and C. As before, the vectors
El, E2 and E3 represent the voltages of each of the phases~
They are equal in magnitude and are separated in phase by
120 degrees; they are produced by a constant voltage source
and thus will be constant, and accordingly, provide our frame
of reference for the remainder of the discussion. Since the
load 40 has a leading power factor of 0.8, the current I4 passing
t~rough it will lead the voltage El across the load. Accordingly,
I4 is shown in Figure 3B counterclockwise of the vector El.
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10 ~ 9 ~ Case 2265
The current I5 passing through the capacitor 41
wi:Ll lead, by 90 degrees, the voltage E2 across the capacitor
41. Accordingly, the vector I5 in the vector diagram i8 ~hown
90 degrees counterclockwise of the vector E2. ~-
The current I6 through the capacitor 42 will lead,
by 90 degrees, the voltage E3 across the capacitor 42.
Therefore, the vector I6 in the diagram is shown 90 degrees
counterclockwise of the vector E3.
Referring to Figure 3A, the currents flowing towards
the terminals A, B and C are I4-I5, I5-I6 and I6-I
respectively. These currents, shown vectorially in Figure 3B,
can be obtained by vector subtraction of the current involved.
As can be seen from Figure 3B these currents are all of equal
magnitude and are all separated by angles of 120 degrees.
Figures 4A and 4B show a simplified version of
Figure 1 and the associated vector diagram, respectively, for
a load with a power factor of 0.8 lagging. It can be shown
that this example also produces a balanced load condition by
reference to Figures 4A and 4B and by using the same logic
a9 was applied in the preceding examples for unity and leading
power factor loads.
Accordingly, the present invention (in the preferred
embodiment) controls the effective impedance across two phases
of a three phase supply in order to produce a balanced load
condition regardless of how the single phase load across the
remaining phase of the supply varies in magnitude or power
factor.
The foregoing has been a description at the preferred
embodiment of the invention as envisioned by the inventor. In
one variation of the preferred embodiment, the capacitors 16
and 17 [as well as the capacitor in variable reactance device
35, yet to be described) can be combined in series with tuning
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1~920 Case 2265
reactors to produce low impedance filters to minimize the flow
of harmonic currents from the load 10 or the phase balancer to
the power system.
A further variation, not shown in any of the Figures,
is to replace one of the variable reactance devices with a fixed
value reactance device. For example, the variable reactance
device 24 of Figure 1 could be replaced by a capacitor (or an
inductor, depending upon the particular circumstances), leaving
the remaining variable reactance device across terminals B and
C to compensate for changes in load 10.
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