Note: Descriptions are shown in the official language in which they were submitted.
-- 1 --
1 This applica-tion is a divisional applica-tion o~
Canadian patent application serial number 374,277 filed on
March 31, 1981.
BACKGROVND 0~ THE INVENTION
__. __
This invention is in the field of electric
power ~neration, and more particularly, relates to
induction generator systems.
Virtually all electric power generators in
current use are ~ynchronous machines. Such generators
are typically connected together to forrn an electric
power grid, In other cases synchronous generators are
operated as autonomous electric power yenerators. While
such synchronous mac~ines do effectively perform in the
requ:ired electrical power generating applications, those
machines are relatively high cost compared with other
known genera~ors, such as induction machi~es adapted for
o~erati~n in a power generation mo~e.
However, in sp;te of the relatively low C05t
of induction rnachines, the prior art autonomous induc-
tion generator ~ystems have been relatively costly due
to the nècessary electronics or magnetics required to
establish a regulated voltage and frequency. UnliXe a
~ynchronou~ generator, an in~uction generator operatec~
at fixed voltc~ge and fre~uency does not allow its recll
current and reactive current~ to independently vary. ~t
~ixed voltage and frequency, the real current from an
induction ~enerator can vary from ~ero to maximum with
th~ variation o the slip frequency (i.e. diference
between the electrical frequency and mechanical frequen-
cy). 'rhe reactive current required at fixed voltage and
frequency remains ~agginy and of significant magnitude
throughout the generator power range~ becoming maximum
at the maximum output power~ Consequently, an external
~ource of leading rea~tive current is req~ired to
e~tablish an ou~put voltage in an autonomous induction
generator. ~his reactive source must be controllable or
variable if the output voltage is to be xegulated below
~aturation of the generator.
. . .
.' ' '~
. ~, . . ..
.. . . _ , .. . . .
8~2
1The principle disadvantage of the prior art
autonomous induction generators has been the high cost
of the power electronics and associated mag~etics which
are required to perform the necessary regulation. In
addition, the quality of the output waveform of prior
art autonomou~ induction genera~or systems has req~ired
rel~tively expensive power filters to meet desired
spectral requirements.
The prior art grid-connected induction genera-
10tors have been infrequently used because of low power
factor and current surges during start up. Where an
induction generator is to be connected to a power grid,
the power grid fixes the induction generator voltage and
requency and acts as a sink for real power and as a
source for reactive power, During generator operation,
the induction generator shaft is rotated slightly faster
than synchronous speed by a mec~.~anical engine, or other
prime rnover. The resulting negative slip of t~e induc-
tion machine imposes a torque load on the mechanical
2~engine and causes real electrical power to be generated
and delivered to the grid. In such induction genera-
tors, the reactive current required to maintain the ~lux
of the induction generator is suppl ed by the grid,
esulting in ~ less than optimum power factor.
~ ,S. Patent No. 3,$29,75~ (Studtmann)
illu~trates one ~orm of induction generator which uses a
vc-ltage mode inverter ~or excitiny an induction genera-
to~. A second known form i8 disclosed by Abbondanti andBrennen in "~tatic Exciters ~or Induction Generators",
`IEEE IAS Transactions, Vol. L~-13, No. 5,
September/October 19770 In both of the~e prior art
approaches, a large fixed capaci~or is utilized across
the output power lines to provide leading reactive
current. According to the Studtmann patent, in this
. . .. . .. .
--3--
1 form, forced commuted SCR switches reconnect the capaci-
tor from phase to phase such that a nominally constant
DoC~ voltage appears across the capacitor. In con~rast,
the Abbondanti and Brennen papex teaches the control of
the reactive current by using fixed capacitors on each
phase in combination with large controllable or non-
linear inductors which "bleed" or "steal ~way" ~he
excessive leading reactive current which is not required
by the induction machine or load. A switched inductor
network is used in conjunction with a network for modu-
lating the 10ngth of times which the various inductors
are in the circuit. This approac~ minimi~es the number
o~ switches, but the cost o~ reactances is relatively
high.
In alternative prior art configurations, U.S.
Patent Nos. 3,043,1lS (Harter) 2,871,439 (Shaw) and
~,881,3-16 (Shaw) disclose a switched capacitor control
for induction machinesO However, those systems do not
perform voltage regulation but rather permit the induc-
tion machine to saturate. There is no voltage regula-
tion which was independent of ~he machine speed.
It is also known in the prior art to ~se
either a binary capacitor array or an arithmetic capaci-
tor array or controlling the reactive current in an
inductlon generator. Binary capacitor arrays use a
s~itchable seqUence oE capacitors having binary weighted
~alues (e~g. lC, 2C, 4C, BC. . ~) and arithmetic array
uses switchable capacitors having ~he same values (e~g D
lC, lC, lC. . .). With either of these two systems, any
inteyer value of capacitance may be attained by ~elect~
ively switching in the appropriate ones of the capaci-
tor~ to reach the desired value. However, for the
arithmetic array, a relatively larga number of capaci-
tox~ i8 required to attain a wide ran~e of capacitance
1 values. In the binary array, a smaller number of capa-
cit~rs is required, but the exponential nature of the
required values or the capacitors requires relatively
large capacitances to be used, contributing to system
error due to the tolerance values asscciated wit~l known
forms of power capacitors~
Accordingly, it is an object of this invention
to provide an improved induction generating system with
a controlled reactance networX.
~ Another object is to provide an improved
induction generating system that is selectively adapt-
able for grid-connected or autonomous operation.
Yet another object is to provide an improved
induction generating ~ystem that is selectively adap-
table for grid-connected operation while providing a
~ubstantially uity power factor under unbalanced line-
to-line or line~to-neutral loads.
Still another object is to provide a power
network including two or more parallel connected induc-
tion generators.
Another object i5 to provide an improv~d power
~actor correction ~ystem with a controlled reactance
n~twork.
$UMMAR~ OF THE INVFNTION
Brie1y, the present invention is an electri-
cal power generation eystem comprising an induction
machine. In one form of the invention, the induction
machine is configured in a genexator mode and may be
~electively adapted for autonomous operation with
controlled reactive excitation provided by an electron-
.
-5-
1 ically switchea capacitor array, or for grid-connected
operation with power fac~or correc~ion using ~he same
switched capacitor array. In the autonomous mode, ~he
~ystem delivers real and reactive power at a regula~ed
voltage and frequency to variable loads, or in the grid-
connected mode, delivers real and reactive power to the
power grid with a unity power factor at the nominal
voltage and frequency of the grid.
The induction generator ~yste~ in accordance
with the present invention includes an n-phase induction
machine having a rotatable input shaft and at least n
output lines, where n is an integer, e.g. as one or
three~ In various configurations, the machine may have
n differently phased output lines and, in addition, a
neutral output line. The input shaft iæ driven at a
controlled ~requency by a prime mover~ In practice, t'he
prirne mover may be, for example, an internal combustion
engine in a torque loop, so that the output torque from
the engine ~as applied to the input ~haft of the induc-
tion machine) is controlled in response to the detected
electrical frequency of the machine.
An N~stage, switched capacitor array provides
a controlled reactive current to the output lines o~ the
~nduction'~achine. Each fitage of the array includes a
capacitor network associated with each o~ the per-
m~t~ations of pairs of the n output lines~ The capacitor
n~twor~s for each stage are characteri~ed by substan-
tially the same capacitance. Eac'h stage further inclu-
des an associated switch network associated with eac'h
capacitor network for 6electively coupling the capacitor
network across its associated pair of outpu lines.
A feedback network is coupled betwe~n ~le out-
put lines and the capacitor array to adaptively control
.
... . . . . , .. . , .. . . . , . ~ .. .. . . ... . ..... ... . .. . . . . . . . . . . .. . . ..
--6--
1 the switching of the various N-stages in and ~ut of
operation.
In embodiments of the invention adapted for
grid-connected operation, the feedback ~etwo~k includes
a power fae~or detector for producing a signal represen-
tative of the power factor at the output lines of the
induction machine. The feedback network uses this power
factor si.gnal to control the switched capacitor array to
adaptively vary that net capacitance across the outp~t
lines of the induct.ion generator BO that the generator
presents a unity power factor to the grid, in spite of
unbalanced line--to-line or line-to-neutral local l~ads
coupled to the generator. In one form, the power factor
detector generates a power factor signa]. which
corresponds to detected reactive power, and the feedback
network is adapted to minimize the reac,ive power so
that the power factor optimized at or near unity. In
other forms of the invention, sensed current and voltage
signals may be combined to form a power factor signal,
~0 which may be optimized to unity. For the purposes of
the invention, all of these forms are considered to pro-
vide signals repre6entative of the power factor at t~le
output lines. In all of these configurations, the power
factor detector may be connected in an "open loop" con-
~igllration (which minimizes reactive current drawn by
lc~cal loads coupled to the induction generator), or in a
"closed loop" con~iguration (which minimizes reactive
current ~r~wn by the grid).
In embodiments of the invention adapted for
autonomous operation, the feedback network is adapted to
contr~l ~or regulate) both ~he voltage and frequency at
the o~tput lines of the anduc~ion machine. In this
case, the feedback network includes a detector w~ich
produces one or more signal~ repre~entativP of the
(
8'~
--7--
1 amplitude of the voltage at the output lines of the
induction machine. This feedback network uses the
amplitude signals to control the switched capacitor
array to ~daptively vary net capacitance across the out-
put lines of the induction yenerator. With this con-
figuration, if more capacitance is a~ded than is
required to balance the la~ging reactive power of the
autonomous generator and its loads, the generator
voltage increases in a ramp fashion. Voltage similarly
decreases if less than the required capacitance is
switched across the output lines. In steady state
operation at the desired operating voltage, the average
capacitance added provides leading reactive power to
exactly balance the net lagging reactive power of the
autonomous system.
In the autonomous mode, frequency is regulated
b~ a feedback loop which compares the output electrical
frequency to a reference and uses the xesultant error
signal to adjust the prime mover (e~g. the throttle of a
mechanical engine).
Generally, in the autonomous mode, khe feed-
back network compares the voltage on the output lines of
the induction machine ac3ainst a reference and the
samples and holds the resultant error signal.
Therea~ter, a capacitance proportional to the error is
~witched across -the output lines during the next cycle.
~or three-phase systems, improved bandwidth and voltage
regulation can be achieved by adding the required capa-
citance once each cycle per phase, resulting in an
effective rate of three times per cycle.
In an N-~tage switched capacitor array, where
at least X of the stages have binary weighted capaci-
tance values from ~tage to ~tage, at least 2x different
--8--
1 values of capacitance are available for switching across
the output lines per phase. Where X of the stages have
binary weighted capacitances and the remainin~ N-X
stages have identical capacitances corresponding to the
maximum binary value, the number of different capaci-
tance valeus which rnay be switched across the output
lines per phase is (N-(X-1)2X-l ~ 2X-1 -13. With such
configurations, each cycle or two, the amoun~ of capaci-
tance across the output lines may be dithered between
adjacent values with the appropriate duty cycle, such
that on the average, the exact amount of capacitAnce
required is on line. Step size of the reactive current
quantization is proportional to the smallest capacitor
ln the array. The small cycle-to-cycle variation in the
capacitor array reactive current caused by the finite
number of capacitance steps does not 6ignificantly
a~fect the output line voltage since the air gap flux,
and thus, the voltage, ~f the in~uction machine responds
relatively slowly to variations in reactive current
~0 excitation. The time constant o~ the voltage response
to a reactive current step is approximately equal to the
rotor time constant, which typically is~ hundreds of
milliseconds, or tens o cycles. Thus, the induction
machine inherently filters out most of the ef~ect o~
small dithering excita~ion current steps caused by the
lnite capac:itor quantization.
In one form of the invention, X of the N-
stages of the capacitor array are characteri~ed by
binary weighted capacitance values from stage to s~age
(e.g. lC, 2C, 4C, ~C,. . ,, where C is a reference capa-
citance value), and the capacitors of N-X of the N-
stages are characteri~ed by zubstantially equal
capacitance values ~rom stage to stage (e.g. lC', lC',
lC';. ~ ., where C' i~ a reference capacitance value and
typicaily C'=2XC~ Wi~h this hybrid binary/arithmetic
-
1 weighted capacitor configuration, relatively fine grada-
tions of capacitance may be adaptively switched in and
out of the network (using the binary weighted portion of
the array), while the arithmetic portion of the array
contributes relatively large units, when necessary.
Thus, the present invention combines the best of the
attributes of the binary and arithmetic array con-
figurations in that relatively small quantization errors
may be achieve~, while no capacitances are required to
be so large that tolerance values are a problem.
Moreover, a modular expandable system may be provided by
just adding another large value capacitor 6tage, rather
than having to re-scale the entire capacitor array as in
a straight binary weighted array systemO
In another form of the invention, the feedback
network includes both a power factor detector and an
amplitude detector for the volta~es on the output lines
of the induction machineO In this form, the feedback
network includes a two state controller, ~r switch,
whi~h is switchable to select between these two detec-
tors, in conjunction with a switch which 6electively
couples the output lines of the induction machine either
in or out of an external power grid. When the feedback
network is in one state, the induction generator sy6tem
is coupled to the external power grid, while providing
unit~ power actor at the output lines. In the second
6tate, the generator system is connected for autonomous
ope~ation with control of frequency and voltage at the
output lines.
` In another ~orm of the invention, two or more
induction generator 6ystems may be coupled in p~rallel,
where tha feedback network for ~he total system includes
a voltage detector coupled between the ~utput lines and
the capacitor array so that ~he ~ystem adaptively
-10-
1 controls the values of the capacitors swi~ched across
the output lines of the combined system.
It is know~ that an att2mpt ~o flux energize
an unexcited induction generator from an existing
voltage line tends to instantaneously collapse the
voltage of that line (e.g. to one half the nominal value
where the added generator is identical to the already
running generator), causing "blink" or "flicker". In
the various forms of the invention adapted for parallel
interconnection of induction generators, ei~her in a
grid or autonomously, at least one of the induction
geTlerators may include a power thermistor network
coupled in at least one of its output lines. The ther-
mistor networX includes a thermistor device which may be
selectively switched in and out of that output line. In
opera~ion, the thermistor network acts as a buffer
between tlle output line of the generator to be magneti-
cally excited and the corresponding output line of the
excited generator. When a non-excited, but mechanically
~pinning (near synchronous speed) induction generator is
to be coupled in parallel to an already operating induc-
tion generator, with the thermistor network coupled in
one of the output lines, the thermistor initially
provides a relatively high resistance in the output
line ~reventing overload of the system. ~is initial
current to the unexcited but spinning generator cau6es
the b~lild-up of ~lux ~voltag~) across that machine~ The
curr~nt i~ maintained ~ubs~antially constant ~by sizing
the t~ermistor so that it decreases in resistance as it
3n ~elf heats at a rate tracking ~he voltage build-up)3
Consequently, the time constant associat~d with the
voltage build up would be approximately equal to the
rotor time constant. In the preferred form of the
invention, the current ls approximately equal to ~he
~teady state magnetizing current which, for ~ingle phase
1~8~ 8~
1 excitation of a three phase machine, is approximately
three times the "n~ load" magnetization current drawn by
a machine driven from a balanced source. When the ini-
tially unexcited induction generator is fully excited,
the voltage drop across the then relatively high tem~
perature thermistor is negligible and that device is
then switched out of the line by a switch that b~passes
the thermistor. With this configuration, magnetic
energy is built up in the initially non-excited genera-
tor in a controlled manner so that the generator is
brought on line without a significant current fiurge
(i.e. in a "blinX" free manner). In alternate embodi-
ments, separate thermis~ors are used for each phase,
requiring one nth the current necessary for exciting the
~enerator.
In yet another form of the invention, the
feedback network may include a voltage profiling network
for controlling the output voltage at times of relative
high loading. For example, when an AC electric motor
~0 load is started on line, it creates a substantial real
load on the prime mover, causing a reduction in the fre-
quency of rotation of the input 6haft of the induction
generator ~particularly if the prime mover is torque
limited). l~e voltage profiling ne~work detects when
such ~requ~ncy changes occur, and provides an offsetting
~ignal to cause the induction machine to provide a rela-
tivQly low output line vQltage (e~g. .707 times the
nominal voltage) for a range of frequencies just below
the nominal ope~ating frequency. As a result, at times
of high load, the output line voltage is reduced, pro-
viding less load to the prime mover. As a consequence,
the ~rime mover may continue to operate at its hi~h
power level close to the nominal system frequency. The
electrical frequency in the feedback loop of the
~witchea capacitors remains relatively high 60 that ~he
1 capacitor array can provide the required reactive
current (which is also high in a transient range) with a
minimwn of capacitors~ A second ~enefit of this con-
figuration is that the prime mover can operate at a
higher ~peed than would otherwise ~e possible and 50
provide higher power into -the in~uction machine.
In yet another form of the invention, addi-
tional leadin~ reactive current can be provided during
times of relative high reactive ~oaaing (for example,
line starting an induction motor) by insertion into part
of the capacitor array, A.C. electrolytic capacitors.
Normally most of the capacitors in the array are
desiyned for continuous A.C. operation. However, an
economical ~pproach to providing addi~ional reactive
current for use during intermittent overloads is to
include in the array A.C. electrolytic capacitors
(sometimes called "motor start capacitors") which are
designed for intermittent duty.
In all of the above embodiments, the feedback
network may include a switch control for the vario~ls
capacitor stages of the capacitor array. This switch
control network monitors the line-to-line voltages o~
th~se ]ines. Such vol~ages may include transients (such
as produced by rectifier loads) which cross zero. In
one ~orm of the invention, the switch control network
incorporates a first zero crossiny detector coupled to
the output lines. This first zero crossing detector is
coupled in turn to an integrator which in turn is
coupled to a second zero crossing detector. The output
from the second zero crossing detector provides a switch
control ~ignal w~ich is optimally adapted to switch the
capacitors in the array at such times when the capaci-
tors are fully charged to the line voltage, thereby eli-
minating one source of transient error6 on the line.
.
~? ~
- 12a -
I To this end, in one of its aspects/ the invention
provides a power yrid network comprising two or more induction
generator systems, each induction generator system comprising:
A. an n-phase induction machine having an input
shaft and at least n output lines, where n is
an integer, wherein each output line is
coupled to an associated output terminal,
B. means for generating a frequency control
signal representative of the difference be-
tween the fre~uency of the voltage of at least
1~ one of said output terminals and a reference
value,
C. torque generating means responsive to sai.d
frequency control signal for applying a torque
to said input shaft, said applied torque being
related to said frequency control signal,
D. an N-s-tage switched capacitor array, where N
is an integer, each stage including n capaci-
tor networks, each network being associated
with a pair o:E said output lines, wherein the
capacitor networks within each stage are each
characterized by a predeterm:inecl capacita.nce
or that stage, and wherein each of said capa-
citor networks includes an associated capaci-
tor switch means, each switch means being
responsive to a trigger signal for selectively
coupling said capacitor network across its
associated pair of output lines,
E. feedback means coupled to said output lines
and including trigger means for generating
said trigger signals,
. .. . ~ .
- 12 b -
1 wherein the output terminals of said induction generator
systen~s are coupled to each other, and
wherein said feedback means for each system includes a voltage
detection means Eor generati.ng an amplitude signal represen-tative
o:E the difference between the amplitude of the voltage of said
output terminals and a reference value, and for coupli.ng said
amplitude signal to said trigger means, whereby said induction
machine maintains a predetermined voltage to loads coupled to
said output terminals.
1 _RIEF DESCRIPTION OF THE DRAWINGS
The foregoing and o~her objects of t~is inven-
tio~, the various features thereof, as well ~s the
invention itself, may be more fully understood from the
following description, when read together with the
accompanying drawings in which:
Fig~ 1 shows in block diagram forrn, an induc-
tion generator system in accordance with the present
invention
~ Fig. 2 shows in schematic form, an embodiment
of the switched capacitor array of the system of ~ig. l;
Fig. 3 shows in schematic form, a capacitor
network and associated switch network of the array of
Fig. 2;
Fig. 4 shows exemplary waveforms illustrating
the operation of the array of Fig. 2;
Fig. 5 shows in block diagram form, the feed-
back sensor of the system of Fig. l;
Fig. 6 shows in block di~gram form, the
~0 trigger signal generator oE the ~ystem of Fig. l;
Fig. 7 shows in block diagram form, the filter
and zero cross detector of the trigger signal generator
o~ Fig. ~:
Fig. 8 ~hows in block diagram form the fre-
q~ency controller of ~he system of Fig. l;
~ ig~ 9 shows in ~loek diagram form, an
exemplary voltage profiling n~twork for ~ wi~h the
system of Fig. 1;
. . .
.. . . . . . .. . .. .... ..... . ... . .. . . .. .... . ...
1 -14-
Fig. 10 shows in bloc~ dia~ra~ form, an
exemplary thermistor network or use wi~h the system of
~ig. 1:
Fig. 11 shows an overload capacitance array
network for the system of Fiq. l;
Fig. 12 shows in schematic form, a branch net-
work for the network of Fig. 11; and
Figs. 13-18 show embodiments of the system of
Fig~ 1 adapated for correction of power factor ~or un-
balanced lo~ds.
F.ig. 18 appears on the same page as
~igur~s 13 and 14.
.,
DESCRIPTION OF THE PREFERRED EMBODIMENT
Fig. 1 shows an induction generator system 10
which i~cludes a three phase induction machine 12 having
three output lines coupled to an associated set of three
output terminals ~indicated collectively by reference
designation 14). In alternate embodiments, a fourth (or
neutral) line may be provided in addition to the three
output lin~s 14. In tha pre~ent embodiment, t~e output
terminals 14 rnay be selectively controlled by a switch
16 so that the kerminals 14 may be coupled to an ex-
ternal power grid or supplying and receiving real ~ncl
xeactive power from such a grid, or alternatively may be
de-coupled rom that grid for autonomous opera~ion. A
local load is indicated by`block 18 coupled to the out~
put lines of the induc~ion machine 12. In other forms
of the invention, different phase induction machines
(e~g~ ~ingle phase) may be similarly configured.
~ controlled-torque prime mover, or driver, 20
is adapted to drive the input sha~t of induction machine
12 at a frequency.relate~ to a frequency control ~ignal
~pplied by way of a line ~2. In the present embodiment,
.~ . .
.. . . .. ... ~ .... , .. , . . . . .... ., . . ~ . , . . . ,,, ,,,,, , , , ,,, , , , ,, _ , . . .
4&~i~
-15-
1 the prime mover 20 is an internal combustion engine 2~.
The speed of the engine 24 is controlled by a throttle
21 driven by the signal on line 22. Throttle 21
controls the fuel flow from a fuel supply 26, In alter-
native embodiments, the prime mover may be a windmill,
for example, with its output torque (speed) controlle~
by varying the pitch of the blades, In yet other forms,
the prim~ mover may be a d.c. motor with its output
speed controlled by a conventional motor speed c~ntrol
signal.
The frequency control signal on line 22 is
provided by a frequency controller 28 which is coupled
back to the output lines from machine 120
A switched capac tor array 30 is adapted to
provide a controlled reac~ive current to the various
output lines of the induction machine 1~. Array 30
includes N stages, each including a capacitor network
associated with the various permutations of the pairs of
the output lines of machine 12, In the present embodi-
ment where machine 12 is three phase, each stage of
array 30 includes three identical capacitor networks,
Each capacitor network includes one or more capacitors
providing a characteristic capacitance value ~or that
stac~e and has an associateA switch networkO l'he capaci-
tance values within each stage are characterizecl by
~ubstant.iall~ the same net capacitance, The swi.~ch net-
work is responsive to an applied trigger signal for
6electively coupling the capacitor networks o~ that
stage across the as~ociated pair of output lines of
machine 12.
qhus, in ~h2 prPferred embodiment, the
switched capacitor array ~0 includes N ~tages, where
each stage i5 in the "delta" configuration mode ~i~e~
.. . ...... . .. . .......... ...... . . . .. . .. .. . . ..... . . . .. . .. . .. . . . . . . . ...
. . ..
-16-
1 each stage includes a capacitor selectively coupled be-
tween an associa*ed pair of output lines of machine 12)D '
In an alternate, but equivalent configuration~ the array
30 may include N stages, where each stage is in the
"wye" configuration (i.e. each stage include~ a capaci-
tor selectively coupled between an associated output
line of machine 12 and a common potential on a neutral
line. The illustrated delta configurati~n generally
-permits the use o capacitors with higher voltage
ratings, and less capacitance ~and correspondingly less
expenqe), than its dual wye confiyuration.
In the present three-p~ase embodiment, there
are 3N triyger signals (on 3N trigger signal lines 34a)
applied to the N-stage array 30 for selectively
switching the various capacitors in and out of opera-
tion. The trigger signals are provided by trigger
signal generator 34, which in turn is driven by a feed-
back sensor 36~ In the presently described embodiment,
the feedback sensor 35 is coupled to the output lines
from ;nduction machine 12 by way of three voltage sense
lines 35a providing signals representa~ive of the
voltage on those output lines, and by way of three
current sense lines 35b providing signals representative
o the current through those output lines. Thus, the
system 10 is "closed loop". In alternate forms, the
system 10 may be "open loop" and current sense lines 35b
may sense current in line6 18a passing to the local load
1~. In the configuration o Fig. 1, when the switch 16
couples terminal 14 to an external power gridt the
6ensor 36 functions in a first sta~e to determine the
reactive current at terminals 14. When switch 16 de-
co-lples system 10 from ~he external grid, i.e. for
autonomous operation, ~ensor 36 unction in a second
state to determine the amplitude of the voltage on the
output lines from machine 12.
-17-
1 Fig. 2 shows the first and Nt~ ~tages of the
capacitor array 30, and the manner in which ~hose s~ages
are coupled to the output lines from induction machine
12. In Fig. 2 the three output lines from ~achine 12
are denoted A B and C. The first and Nth stages o~
array 30 are shown schematically to include a capacitor
(denoted C ~ith subscripts) and a switch (denoted S with
subscripts) coupled between the various pairs of output
lines A B and C. The subscripts for the respective
capacitors and switches in Fig. 2 are indicative of the
two output lines associated with those elements. In the
pr~sent embodiment, the first X of the stages of array
30 include capacitors which have binary weighted branch
capacitances from ~tage to stage. The remaining N-X
stages have equal value capacitances in the various
branches from stage to stage, as indicated in the
following Table. Fig. 2 ~hows 3N trigger signals each
being associated with one of the switch networks in the
stages.
. . .
- - - . ~. .. .. .
-18-
1 TABLE
BINARY WElGHT ARITHMETIc WEIG~T
CAB(l) -1 cAB(x+~
CBC(l) ~ C CBC~X+l) 2xc
CCA(l) J CCA(X+l)
CAB(2)
CBC(2) 3 2C
CCA(2)
CAB(N)
:L0 ' CBC~N~ ~ 2 C
. CCA(N) J
CAB(X)
CBC(X) J 2~-1C
~CA(X)
In one form of the invention, shown in Figs. 1
and 2, the capacitors in arxay 30 are in the "delta"
configuration, where the 3N trigger signals for network
array 30 permit independent control ~f the switching of
each delta capacitor in the various stages. In that
embodiment, a combination binary-linear weighted capaci-
tor array is utili~ed which switches ~tages in or out at
one time per cycle at only the posltive peaks of t~e
line volta~e. In general, switching off-line of the
capacitors of the various stages can occur at either
positive or negative ~zero current) voltage peaXs, iOe~
within 180 degrees of a desired time with correspondillg
turn-on ~a~ K x 360 degree6) from this turn-off point.
- Fig. 3 shows an exemplary form for the irst
stage line-to-line capacitor and associated ~witch net--
work for the array 30 between lines A and B on ~he
induction machine 12. In this form, the output lines
.
,.. , . . .. . . . - - - -
~19-
1 and B each provide ~igh current buses fc~ the ou~put
current of the various stages. The buses are indicated
in Fig. 3 by reference designations 40 and ~2. It will
be understood that the buses are particularly adapted to
provide highly efficient convective heat transfer so
that these buses act ~s heat sinks for the respective
components coupled thereto.
The capacitor network is coupled between the
bus elements 42 and 40 by semiconductor swi~ches SCR 46
and TRIAC 48, respectively. The capacitor network
includes a capacitor (denoted C) in series with an air-
core inductor (denoted L). The current through that
6eries capacitor-inductor combination is denoted by IAB.
In the present embodiment, the capacitors are A.C. capa-
cito,r~ type 520P or metallized polypropylene A.C. capa-
citors type 325P, manufactured by Sprayue~
The capacitor is coupled to the cathode of SCR
46 and the anode of an anti-parallel diode Dl. In the
present embodiment, SCR 46 has a T0-220AB package having
~0 its anocle connected in direct thermal and electrical
contact with bus elmeent 42. The diode Dl ;s a stud
mounted diode coupled having its cat'hode in direct ther-
mal and electrical contact with bus element 4~ The
trigger sign~l from generator 34 (as defined more fully
below) is applied by way of line 34a across the gat.e
c~thode terminals of SCR 46. In Fig. 3, the trigger
siynal line for the illustrated switch networX includes
our wires ~denoted collectively 34a)0 The wire 35a
running ~o the gate of SCR 46 has an associated return
30 wire 35b running.from the cathode of SC~ 46 back to
generator 34.
~he inductor L is connected dir~ctly to ~he
MTl terminal of TRIAC 48. In the present embodiment,
.
. . , . . ,,,, ., . ,, . .. .... . . ,, ,, .. ... , . , . , , , , , , , , _ _ _ _
-
-20-
1 TRIAC 48 has a TO-220AB packase having its MT2 terminal
connected in direct therm~l and electrical contast wi~h
the bus element 40. A signal diode 50 has its cathode
connected to the gate of TRIAC 48. The an~de of diode
50 is connected to the bus 40. The trigger signal from
generator 34 for TRIAC 48 is applied by way of line 34a
across the gate-MT2 terminals of TRIAC 48. As with SCR
46, a first wire 35c provides the trigger signal ~o the
gate terminal of TRIAC 48, with a return wire 35d
running back to generator 34.
With this configuration, the various capacitor
net~orks may be selectively switchecl three times per
machine cycle in a manner so that the "off" or discon-
nected capacitors remain charged to the peak line-to-
line voltage. Current surges are avoiaed in normal
operation by triggering the semiconductor switches (SCR
~6 and TRIAC 48) of each phase at the peak line-to-line
voltage which occurs at the mid-point between the line-
to-line voltage zero crossings. Consequently, there is
nominally zero voltage across the semi-conductor
6witches, and no current surge when those switches are
triggered on.
Fig, ~ indicates the representative waveforms
of operation for the configuration of Fig, 3 for a
~ingle trigger ~ignal on line 34a. As shown, the nomi-
nal capacitor currQnt ramps from ~ero and has a sinu-
~oicdal shape. rrhe inductor L is an air-core induc~or
coupled in ~eries with t.he capacitor to accommodate
slight timing erroxs or error~ due to waveform distor-
tionsO The inductor limit6 the rate of change of
current with time. The inductors further serve to pro-
tect the switches during line faults by keeping the peak
current within the switch surge current ra~ingr
.
, .. . ~ . _ .. _ ..... . . . . . . . .. .. . . . . . . ... . . .. . . . .
- ~ (
-21-
1 In operation, the capacitors are switched off
the line by removing the trigger signals. The switches
have self (uncontrolled) gating in one polarity, ~o that
on the following half cycle, the switches naturally com-
mutate off at a current zero crossing. I~e switched-off
capacitor is left holding a charge proportional to the
line-to-line peak voltage. The self gating of the
switches in one polarity insures that the "off" capaci-
tors remain fully charged.
Since a capacitor held off the line is charged
to the peak sy~tem voltage, double the system line-to-
line voltage i5 seen by the 6emiconduc~0r ~witch or
switches in ~eries wiLh it. For example, the switches
must tolerate 1250 volts in a 440 volt, 60 Hertz system,
or 1080 volts in a 380 vol~ 50 Hertz system.
Accordingly, the embodiment of Fig. 3 is particularly
~dvantageous since two relatively low voltage (and low
cost) moderate current switches may be used in series
with each capacitor section.
The capacitor current is nomin~lly a ~ine
wave, but because the capacitor current is proportional
-to the derivative of voltage, in practice this ~ignal
can depart significan~ly from ~he ~ine wave. For this
rea~on, the trigger signals are provided (as described
more fully below) are relatively wide. In the preferred
form, the trigger command is provided whenever a switch
is desired to be on.
The particular configuration o~ Fig. 3
providès a relatively compact arrangement wherein the
TRIAC, SCR and anti-parallel diode all may ~e connected
~o the bus element~ forming the output line~, which in
turn function a~ electrically hot heat ~ink~, thereby
avoiding the need for individual electrical isolation ~f
the power semi~conductor~D
.. . ., . , . .. . . . . , . . . . . . . ~ . . .
-22-
1 Fig. 5 shows the fe~dback sensor 36 for the
present embodimen~ Sensor 36 includes a po~er ~actor
detector network 60 coupled to the voltage sense lines
35a and the current sense lines 35b from the o~tput
lines of machine 12. Detector 60 provides output
signals on lines 66 which are representative of the
reactive power at terminals 14, which in turn are
relate~ to the p~wer factors at terminasl 14. In alter-
nate embodiments, detector 60 may provide ~ignals
directly representa~ive of the power factor~ at ter-
minals 14.
Sensor 36 also include~ a rectifier network 68
coupled to the voltage sense lines 35a. Rectiier 68
provides signals on lines 70 representative of the
amplitudes of the voltages at the ~erminals 14~ A surn-
mation network 72 provides signals on lines 7~ represen-
tative Of the di fference in amplitude of the voltages at
terminals 14 and a reference signal. A switch 76 is
arranged to be selsctively operated in a manner coupling
the signals from lines 66 or lines 74 to output ]ines 78
of the ~ensor 36. The ~witch 76 may be operated in con-
junction with the switch 16, so tha~ during grid-
connected operation, the signals from power factor
detector 60 are coupled to lines 78 when switch 16 i 6 in
its closed position (coupling the system 10 to the power
grid). When the SWitc-l 16 is in its open position, i.e.
for autonomous operation, the switch 76 couples the
signals from lines 74 to lines 78.
Fig. 6 ~hows the trigger signal generator 34
in detailed form. Generator 34 includes an error
amplifier 82 coupled to ~ignal lines 78 and to timing
signal line~ 91. In ~ome embodimen~s, amplifier 82 may
include an input multiplexer and an output
demultiplexer~ The output ~rom amplifier 82 may have
-23-
1 its signal time modulated such that sampling in the
following latch 86 provides somewhat different capacitor
corrections to the individual phases of ~he system. In
this form, balanced voltages can be maintained in the
presence of unbalanced loads.
In the present embodiment, the output from
amplifier 82 is coupled to a binary A-to-D convertor 84,
which in turn is coupled to latch 86. A ~ilter and zero
cross network 90 is coupled to terminals 14 to provide a
sampling 9i gnal to the latch 86 at the system operatiny
frec~uency. l`he sampled signal from latch 86 is applied
to a triy~er networX 92. The filter and zero cross net-
work ~0 also provides appropriate timing signals to
generate t~e signals for switching the ~tages of array
30 in and out of operation. Switching "in" occurs at
such times when the fully charges capacitors in array 30
are co~3pled to the peak voltages at the lines o~ machine
12. Switching "out" occurs prior to a peak voltage with
actual turn off at naturally occurring æero capaci~or
current (which is normally at the voltage peak).
The trigger networX 92 is responsive to the
sampled values in latch 86 to select and activate the
appropriate ones of the 3N trigger signal lines for the
appropriate stages to adaptively modify the value of the
capacitances coupled across the outpu~ lines oE machine
12. In various forms of the invention, the trigger net-
work 92 may include a prograrnmed microprocessor, or some
otller sultable orm of computational network.
With the control of individual branches of the
various stages of array 30, both line-to-line and line~
to-neutral unbalanced loads may be accommodated, pro-
vided that the net loads (before correction) are
ind~ctive (since only capacitor~ are used for control).
.
-2~
1 In the preferred for~ o~ the invention, the
filter and zero cross de~ector network 90 has the form
shown in Fig. 7 wherein a first zero crossing detector
network 94 is coupled to an integrator 96, which in turn
is coupled to a second zero cross detector network 98.
This form of filter and zero crossing detector 90 is
particularly advantageous where the line-to-line voltage
at terminals 14 includes transients (such as due to rec-
tifier loads) which may cross zero. In this con-
figuration, the network ~4 provides a binary signal
which has a state change for each zero crossing of the
input. The integrator 96 integrates this resultan-~
signal to provide a nominally triangle~waveform which
has zero crossing points nominally at the desired
switching times. The second zero cross detector 98 pro-
vides a trigger timing signal for controlling the
switching of the stages for the various line-to-line
pairs.
Fig. 8 shows the frequency controller for the
preferred embodiment. In this embodiment, the
controller 28 includes a filter and zero crossing detec-
tor network 100 coupled to terminals 14. The output of
network 100 is coupled to a ~umming network 102 which in
turn is coupled to an error ~nplifier 104 for driviny
line 22. In practice, the networX 100 may be the æame
as corresponding network 90 in generator 34. In such
cases, the output fxom generator 90 may be used directly
in controller 28 in place of that provided by network
100. ~he summing network 102 provides a Ereq~ency error
si~nal representative o the di~erence in frequency of
the voltage at terminals 14 and a reEerence re~uency.
This frequency error ~ignal is applied by way 3f error
ampliier 104 and line 22 to the variable speed prime
mover.
-~5-
1 In one form of the invention, the output from
the 6umming network 102 may be coupled by way of a
voltage profile network to an input of the summing net-
worX 72 of the feedback sensor 36. With this con-
figuration, the voltage profile network 106 modi~ies the
commanded system voltage on line 78 as a function oE the
system frequency error. In normal operation, the system
10 frequency error is small, and there is no significant
output from the voltage profile network 106. However,
in momentary overload situations, e.g. when the system
10 is called upon to start relatively large motor loads,
the resultant slow down at the prime mover 20 can be
dir~ctly sensed by detecting the reduced frequency on
output lines of machine 12. NetworX 106 detects times
when the requency at terminals 14 falls below a prede-
termined threshold, and or a range of frequencies below
that threshold, provides an appropriate signal to net-
~orX 72 to establish a relatively low output voltage
from machine 12, for example, by reducing the voltage to
.707 of the nominal voltage when a few percent slow-down
is detected. As a consequence of this operation, the
~ffective load seen by the prime mover 20 is substan-
~ially reduced and that element may continue to operate
near the normal system frequency where it can provide
more power and thus maintain the highest possible output
voltaye. This feature is particularly advantageous ln
preventing inadvertent cut outs when relays are used in
th~e system. This con~iguration may be utili~ed in the
situation where a single induction generat.or system 10
is operating, or where a ~lurality of such induction
generator systems are coupled in parallel at terminals
1~ .
It is well ~no~n ~hat during induction genera-
tor start-up, an initial remnant flux must either exist
in the machine or be placed in the machine 12~ In khe
,, . . , , . , . , ... , . . , . , . ,, , ., .. . ~, .. . . . .. .. .. . . . . .. . ... . .. . . .
(
-26-
1 prior art, this remnant flux may be placed in the
machine at ~ero mechanical speed with a D.C. bias
current in one winding of the generator, or alter-
natively a suficient remnant flux naturally exists in
the machine from the last time it was operated. For a
sinyle autonomous induction generator system, the
switched capacitor array may be used to creat~ voltage
build-up in the generator automatically when the machine
speed reaches some minimum value. The load is normally
disconnected during such flux initialization, and until
proper output voltage and frequency are established.
However, when a spinning but unexcited induction machine
is connected to an external grid, or another induction
generator, a very large current transient occurs until
the ~lux builds up in this machine~ For example, ~uch a
transient might well cause an instantaneous voltage drop
on the order of 50% if two identical machines are
paralleled in this manner. If the machine to be added
to the grid is initially excited by using a separate
capacitor bank, the transient would very likely be even
worse unless the frequencies are phase locked using con-
ventional synchronous machine line connection tech-
niques.
In accordance with the present invention, a
thermistor network, as shown in Fig. 10, may be used to
bxing an unexcited, but near synchronously turning
induction machine on-line wi~h a minimal transient. The
network of Fig. 10 includes a two terminal ~108a and
lO~b) network having a three phase switch 110 coupled
between those terminals 108a and 108b, and a series con-
nected single phase switch 112 and thermistor 11
coupled in parallel with one phase of the switch 110
The thermistor 114 has a temperature dependent
- resistance characteri~tic, providing a relatively high
r~sistance at low temperature~ and a relatiYely low
-27-
1 resistance at high temperatures~ An associated
controller 116 controls the operation of the switches
110 and 112. The network 108 is coupled between one of
the terminals 14 of la~ ~perating or grid-connected
induction machine and the corresponding output terminals
of the induction m~chine to be brought on line. By way
of example, to bring system 10 of Fig. 1 on line to the
external grid, network 108 mayb be coupled into one of
the output lines between terminals 14 and switch 16. In
other multiple systems, a single network 108 may be used
repetitively (after cooling down) to sequentially bring
the multiple systems on line. In alternative systems,
scparate thermistor branches similar to the branch
including 6witch 112 and thermistor 114 may be similarly
coupled in each of the output lines from the induction
machineO
In operation, witli the ~ystem 10 including
network 108 which is to be coupled to an external grid
(e.g. by switch 16) or another induction generator~ the
switches 11~ and 11~ are initially ~on~rolled by
controller 116 to be in their open positions. ~hen, the
un~xcited induction machine 12 is brought up to a speed
close to the desir~d line frequency. Frequency or phase
locking is not required. The switch 112 is then closed
by controller 116, bringing the thermistor 114 into one
of the output lines which connects the two generators in
parallel. With this configuration, the power dissipated
in the thermistor 114 causes its temperature to
increase, thereby lowering its resistatlce. By
appropriate thermistor device selection, it will be
under6tood that the thermistor ~or plurality of series
connected thermistors) is ~elected so that its
resistance-temperature characteristic is matched to t~e
rate of voltage build-up. Consequently, ~he current in
the thermistor increases and it6 resistance decreases
-28-
1 until the temperature and resistance reach such values
so that the current through there is essentially equiva-
lent to the steady state final value which is required
for the no-load magnetizing current. At this point,
controller 116 opens switch 112 while closing the three
phase switch 110. The system 10 is then fully on-line
without a transient. In practice, controller 116
changes the state of switches 110 and 112 by detecting
when the thermistor voltage falls below a predetermined
threshold, or alternatively may just provide a predeter-
mined time delay. The same thermistor 114 may be used
after cooling to provide nearly transient free excita-
tion for additional ~ystems as they are brouyht on-line.
The prior art induction generator systems have
a relativ~ly limited ability to start A.C. motor loads.
Typically, w~en an A.C. motor load is star~ed, that load
requires much more reactive current than during normal
(steady state) run operation. If insufficient capaci-
tance is available in the induction generator capacitive
array 30, the voltage provided by system 10 rapidly
collapses toward zero when a relatively large A.C. motor
is switched onto the output line 14. Motor starting
ability o~ the system is enhanced by switching in an
overload capacitance array network across the output
terminal 14 during overload conditions, sucll as durin~
start-up of a large A.C. motor.
Fig~ 11 shows an exemplary overload capaci~
tance array network 118, including three similar branch
networks 120, 12~ and 124, for connection in a "wye"
3Q configuxation to lines A, B and C and to a neutral (or
ground) line N of the system 10 of Fig. 1. Each of
branch netw~rks 120, 122 and 124 includes a capacitor
(denoted C with a correspondin~ sub-script~ and a switch
(denoted S with a corresponding ~ub-script). By way of
. , _ . . _ . .. . . . . . . . . . . . . . . . . . ... ..
-29-
1 example, Fig. 12 shows a particularly economical embodi-
ment of the branch network 120 which includes a high
current density A.C. electrolytic capacitor C120 coupled
in series with a semiconductor switch network
S120 between the output line A and ground. In the
illustrated embodiment, the capacitor C120 may be a
"motor start" capacitor, desi~ned for intermittent du~y,
such as the Sprague Type 9A, This capaci~or type
generally includes a pair of polarized capacitor con-
nected back-to-back in series.
The switch network S120 includes a pair oE
oppositely directed SCR's 126 and 128 connected in
parallel to form a bidirectional switch. The pair of
SCR's is connected in series with an air core inductor
130 between capacitor 120 and a common potential, such
as g~ound. l~e output of a trigger network 132 is con-
nected to the primary coils of trigger tran~formers
Tl and T2. The secondary coils of transformers Tl and
T2 are connected across the cathode and gate texminals
of SCR's 126 anc~ 128, respectively. A detector 134 pro-
vides an inhibit signal ~o the trigger network 132. The
trigcJer network input is coupled to A/D converter 84.
In operation, when extra capacitance is required (which
may be due to ~C motor start-up), the ~iynal from A~D 84
normally causes a gate 6ignal from network 132 to switch
SCR's 126 and 128 to their conductive state. However,
i the voltage across SCR's 126 and 128 is above a pre-
determined threshold, the inhibit signal from detector
134 prevents turn-on of SCR' 8 ~26 and 128 to their non-
3~ conductive states until a point in the waveform when
transients are minimal an arbitrary initial condition on
the capacitor voltage. With this configuration, the
network 118 i6 optimized to accommodate start-up of an
uncharged capacitor or re-~tart up if the relatively
poorer thermal and electrical capacitor voltage is
anywhere between zero and full voltageO
-30-
1 In the preferxed embodiment~ the motor start
capacitors are connected in a "wye" configuration to
allow use of available lower capacitor voltage ratings.
In lower voltage applications, a "del~a" configuration
may more economically be used. In all of these con-
figurations, capacitor ~hermal protection in situations
of inadvertent capacitor over use may be accommodated by
inhibiting the motor start array switches if the series
air core inductor exceeds a predetermined temperatureA
Fig. 13 shows, in block diagram form, an
alternate p~wer factor correction network 150 which may
replace trigger network 92 of Fig. 6. Network 140
includes a computer 142 and associated memory 144 and an
interface 146. When switch 76 of Flg. 5 connects line
66 to line 78, then network 140 operates as a closed
loop power factor correction ~ystem which provides power
factor correciton on a periodic basis for loads which
may be balanced ~r unbalanced.
During the first cycle and for all subsequent
cycles, the power factor correction network measures the
residual three reactive power terms (each quadrature
line curre~t times its corresponding line-toneutral
volta~e) during one cycle. The resultant residual or
error signals are representative of the change in reac~
cive power since that last correction. The sy~tem 140
then uses this error signal to determine the capacitance
to be added to or subtracted from the respective phases
of the array 30 duriny the next correction cycle. In
Fig. 13, the memory 144 provides storage for data repre-
sentative of the state of network 30l i.e. data which
defines the existing capacitors that are on-~ine~
Between power factor correction cycles, computer 142
mongtor~ the signals ~rom power factor detector 60 to
determine the three indep~ndent line-to-line capacitance
changes required to correct ~he power factor. Camputer
142 ~ums these incremental values with the previous
-31-
1 values as stored in memor 144 to compute the new desired
v~luesO At a c~rrection time, computer 142 generates
control signals representative of the new values which
are to be switched from the network 30. These control
signals are the trigger signal which are applied by way
of interface 146 to the various stages of array 30.
Thus, the computer 142 measures the residual
line-to-neutral reactive power. This value may be posi-
tive or neyative. In systems where array 30 is a wye
configuration, the complement of this reactive power is
the value required to compensate (i.en ~he corresponding
value capacitive increment, positive or negative, may be
switched into the system from line-to-neutral).
In the pxeferred embodiment, which u~ilizes a
three phase delta configuration capacitor array 30~ the
computer 142 first determines the required incremental
line~to-n~utral reactive power correction value for the
output line terminal of each line, and ~hen converts
that value to an equivalent reactive power delta correc-
tion. The incremental delta capacitor equivalent asso-
ci~ted with a determined incremental wye value is formed
f~om two equal incremental delta capacitors having one
terminal coupled to the associated wye terminal, with
each of those incremental delta capacitors h~ving the
same sign and one-third the capacitance of the incremen-
tal value oE the wye computed value. The third opposing
lag incremental delta capacitor has an opposite sign and
has t.he same one-third capacitance magnitude.
These above values for the various output
terminals are incremental values. ~he net required
delta capacitors are determined by the computer 142 by
addïng to the most recent c~rrective ~tate, t~e required
change, which in the algebraic sum of the three incre-
mental capacitance values for each terminal. Thus, ~he
~hree new capacitor~ for the delta network are 5btained
-32-
1 by adding appropriately transformed wye incremental
values to the previous delta value.
The computer 142 then generates the trigger
signals on line 34a which switch the desired total capa-
citor value across the various lines at the next cycle
during which power factor correction is made.
In cases where a computed delta capacitive
value for power factor correction is determined to have
a net negative value, the computer 142 modifies the
values in the following optimum manner before generating
the trigger signals. Computer 142 first subtracts one-
third of the magnitude of ~his negative value from each
o~ the other non-negative line-to-line capacitors to
speciy two new total values to be placed on line. The
terminal pair associated with the ~riginal desired nega-
tive capacitor compensation is left uncompensated.
Figs. 14-18 show an alternate configuration
Eor this power factor correction network for a three
phase system 10 having a delta configuration capacitor
array 30 and adapted to optimally compensate for
unbalanced line-to-line or line-to~neutral loads. In
this con~iguration, network 160 (Fig~ 14) replaces block
60, lines 66 and 78 of Fig. 5 and blocks 82 antl 84 of
~ig~ 6. Timing signals or the various sampling opera-
tions provided by network 160 are provided by line 91
from network 90. In network 160, a pulse width mod~1la-
tor ~PWM) type multiplier is used for the reactive power
computation to achieve accuracy and simplicity, although
other forms of multipliers would also provide the
necessary dataO The pulse width modulation represen-
tations of the line-to-neutral voltages are created by
comparing the line-to-neutral voltage against a triangle
reference. The6e digital representations allow for a
6impler, digital type multiplication implementation with
the integrated currents. For fixed voltage7 the reac-
-33~
1 tive power measurement translates to a capacitor compen-
sation value. If the voltage increases, the
cornpensation capacitors reactive power also increases.
Thus, for the same reactive power at higher voltage, a
smaller cornpensation capacitor is appropriate, indi-
cating that the multiplier product (reactive power
measurement) should be voltage compensated before using
it to specify capacitance. These line vol~age
variations can be substantially compensated by
appropriately varying the amplitude Vp o~ ~he triangle
reerence, VSAw. This form o ~he invention will now be
described in detail.
Fig. 14 shows a general hlock diagram of net-
work 160, which includes Vp generator 162 (sho~l in
detail in Fiy. lS) coupled by way of voltage sense lines
35a to output lines A, B, and C of machine 12. Each o
lines 35a provides a sinusoidal signal representative of
the line-to-neutral voltage for that line (represented
irl Fig. 14 by VANsin wt, VBNsin(wt~l~oo) and
VcNsin~wt~40~ for lines A, B, and C, respectively).
Ge.nerator includes a full- lor half-) wave rec~ifier and
filter 164~ scallng networks 166 and 167, summing net-
work 168 and triangle generator 169~ For this block
diagram/ the signal VREF equals ~1 times the nominal
~ull wave output voltage ~or machine 12 and the nomis~al
triangle wave amplitude Vp (nom) equals VREF. With this
coni~uration, generator 162 provides a compensated
triangle output, VSAw on line 162a having a peak value
~, and a frequency fO. Vp thus corresponds to
[~2vL/vL(nom))-l~vREF~ where VL is ~le amplitude o the
signal on line 166a~ ~his linear first order compen-
sation substantially eliminates the scaling error due to
compensation capacitor aependence on voltage, which
improves the system dynamic response.
.. .. ... . .. . . .... .. .. . . . . .. .. . ... . .. . . . .. . ... .. . . . . . . . . . . .
.
-34-
1 Network 160 also include5 three ~imilar wye
value networks 174-176, where eac~ of ~hese networks is
coupled to line 162a, one of lines 35a, a~d an asso-
ciated one of lines 35b (which provide signals iA~ iB
and ic representative of the currents in lines A, B and
C, respectively). Network 174 is shown in detailed form
in Fig. 16. Network 174 i~cludes scaling network~ 177
and 178, ~lultiplier 180, summing network 1~2, zero cross
detector 184 and integrator 186 (which is reset once
during each compensation cycle). The networks 175 and
176 are similarly configured. With this configuration,
networks 174, 175 and 176 provide output signals on
lines 174a, 175a and 176a, respectively, representative
oE the incremental wye (line-to-neutral) capacitance
values ( ~CA~, ~CBN and ~CN~ respectively) for power
factor correction.
Thus, with this configuration, the line-to-
neutral power factor signals are generated by simulta-
neously integra~ing (after reset), over a 360~ interval,
the products of the line-to-neutral voltages for the
line pairs, and the integrals of the a.c. component of
the correspondinc~ line currents. As a result, the
system provides substantial harmonic reduction.
Moreover, the average products of the harmonics are
ne~ligible everl when both current and voltage waveform~
contain distortion~. The system al~o provides the 90
phase shift of the quadrature current so that the
product OUtpllt contains a d.c. term proportional to
reactive power only.
3n Lines 174a~ 175a and 176a are each coupled to
a wye/delta conversion network 180 (shown in detail in
Fig. 17)~ ~etwork 180 includ~s three scaling networks
179A, 179B and 179C and three sllmming networks 121-183
which provide incremental delta tline-to-line) capaci-
.... . . ., .. .. ~ , . . , ., . , ., ~, ... . ... . . . . . . . . . . .. . . . . . .. .
-35-
1 tance values ( ~C~B, ~CBc and ~CCA~ reSpectively)
lines 181a, 182a and 183a for power factor correction.
The signals on lines 181a, 182a and 183a are coupled to
ass~cia~ed ones of summing networks 186-188 where those
signals are summed with the respective ones of sommanded
capacitance signals CAB(comm), CBc(comm), and CcA(comm~
to provide ~ignals which are sampled and held in sample-
and hold (S/H) ne~works 190-192, respectively. The out-
puts from S/H networks 190-192 provide desi~ed
capacitance signals CAB(des), CBc(des), and CcA(des) on
lines 180a, 180b and 180c, respectively. The latter
siynals represent the capacitance already across the
various terminals of machine 12 ~from the next previous
me~surement cycle) plus the incremental value determined
during the current measurement cycle.
The lines 180a, 181a, and 182a are coupled to
negative capacitance value ~orrection network 196 (shown
in detail in Fig. 18). Network 196 includes three
summing networks 201~203 having an input coupled to a
~0 respective one of lines 180a, 180b and 180c. Each oE
networks 201-203 has its output coupled to one of three
networks 206-~08 having a contin~lous Vin/V~ut transfer
function which passes through ~0,0) ancl has a slope of 1
in the irst quadrant and output equal to zero in the
third quadrant. The output from each of networks ~06
208 is coupled by way of one of sample-and-hold ~S/H)
networks 212-214 to one o~ output lines 196a, 196b and
196c. Each o networks 201~203 also has it~ output
coupl~d to one of three networks 218-220 having a
VIN/VOUT transfer function which passes through ~0,0)
and has a slope equal to ~ in the first quadrank and a
810pe e~ual to 1/3 in the third quadrant- The output
rom each of network~ 218-220 is coupled to a summing
input of the two networXs 201 ~203 which are not c~upled
~o it6 input. With this conigur~tion, when one of the
-36-
1 desired capacitance signals is negative, co~nand capaci-
tance signals are generated which correct the command
values to provide op~imal power fac~or correction with
zero or positive capacitances only.
In summary, the system 10 using network 142
performs simultaneous three-p~lase reactive power sensing
during one 360 degree interval of the line ~requenc~ by
simultaneously integrating three signals, each being
proportional to the product of an integrated (90 degree
phase shift of ~undamental) line current and its
respective sinusoidal line-to-neutral voltage. The
three integrators are reset prior t~ initiation of a new
measurement cycle. As a res~lt, by integrating over 360
degrees, the reactive power without additional iltering
is determined during one cycle. In this configuration,
the integrator 176 provides harmonic reduction, gO
degree phase shift and fre~uency compensation (achieved
by integrating line current prior to multiplication by
line-to-neutral voltage~. The present system is a
~0 closed loop configuration in that a power fac~or correc-
tion value is alxeady present in parallel with the load
thus the reactive power error is measured and the
correction value is adap~ively modified. The syst~m 10
provides relatively high speed closed loop power fac~or
correction and can also accommodate ~mbalanced line-to~
line anA line-to-neutral inductive loads.
In general, the compensation capacitors are
not taken on (or off) line duriny the 3~0 degree
measurement interval to avoid measurement errors. The
new value~ of capacitance, computed after a measurement,
are placed on line at the ne~t opportunity consistent
w;th the transient-free 6witch-on.
~hi6 reactive power compensation ~pproa~h
minimizes the three-phase RMS reactive currents even
, .. .,, , . . ,, . .. ... .~, .... . . . . .. .. ...... .. .. .. . . . . . .
-37
1 when full compensation is not possible with delta
corrected capacitors only. This similar situation
arises, for example, during heavy unbalanced loading
such as a single phase line-to-neutral connected motor
load is present.
The invention may be embodied in other speci-
fic forms without departing from the spiri~ or essential
characteristics thereof. The present embodiments are
therefore to be considered in all respects as illustra-
tive and not restrictive, the scope of the invention
being indicated by the appended claims rather than by
the foregoing description, and all changes which come
within the meaning and range of equivalency of the
claims are therefore intended to be embraced therein.
