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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to static VAR generators and more
particularly to static VAR generators employing controlled inductance
means switched in combination with capacitance means into an AC network to
regulate power factor and terminal voltage.
2. Description of the Prior Art
The basic function of a static VAR generator is to supply controlled
reactive power for an electrical network to reduce terminal voltage
variation and to improve load power factor. Inherent in all static VAR
generator designs utilizing anti-parallel switching thyristors is a delay
in response resulting from the requirement that a sampling of the systems
reactive demand must be made plus computations determining the reactive
compensation prior to any corrective action. Additionally, after the
computations are complete the output current for the VAR generator can be
adjusted only once in each half cycle. Consequently, after the first
sampling and computation time elapses, if the reactive demand should
change suddenly, the VAR generator cannot further adjust its output
current until the next half cycle. It would be desirable for a VAR
generator design to have the capability to allow further corrective action
to be taken in response to any reactive demand change that may occur after
the first sampling and computation time interval for each half cycle.
SUMMARY OF THE INVENTION
There is provided by this invention a static VAR generator arrangement that
employs two three-phase thyristor controlled inductor banks. One inductor
bank operates from a set of voltages obtained from a phase shifting
transformer that leads AC network voltage from which the other inductor
bank is operated. A control signal is derived for each thyristor
controlled inductor synchronized to the voltage source at the inductor
bank. This control signal contains the information for the required
reactive compensation to the AC network. With this arrangement the output
current of the VAR generator can be changed twice in each half cycle.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a functional block diagram of a static VAR generator connected to
an AC network and a rapidly varying load incorporating the principles of
this invention;
FIG. 2 are wave forms of the three-phase voltages for the reactor banks of
the VAR generator shown in FIG. 1 with respective phase firing angles.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1 there is shown a static VAR generator connected to an
AC network and a rapidly varying load such as an arc furnace 5. The VAR
generator is generally comprised of a fixed capacitor bank C connected in
a wye configuration to the AC network. A first inductor bank 10 having
delta connected inductors L.sub.1 is disposed to be switched into the AC
network by means of anti-parallel thyristors THR-1. Also connected to the
AC network is the input terminals of an auto-transformer 11. A second
inductor bank 12 is connected to the output terminals of the
auto-transformer 11 having inductors L.sub.2 in a delta-configuration in
such a manner that the inductors L.sub.2 are operated at voltages
phase-shifted by 30 electrical degrees. The inductors L.sub.2 of the
inductor bank 12 are also disposed to be switched into the AC network by
means of anti-parallel thyristors THR-2.
The inductors in the two inductor banks 10 and 12 are inserted into the AC
network to control reactive power by phase angle firing the thyristors
THR-1 and THR-2. Two separate control channels for each inductor bank is
employed. Timing pulse generators 14 and 15 provide timing pulses P.sub.T
and P.sub.T ' synchronized to the voltage supply of the respective
inductor bank. The timing pulses P.sub.T obtained from pulse generator 14
are synchronized to the AC network. Similarly, timing pulses P.sub.T '
from pulse generator 14 are synchronized to the voltages phase shifted 30
degrees by the auto-transformer 11. The signals P.sub.T and P.sub.T ' are
inputted into an inductor current computing circuits 16, 17 and current to
firing angle converters 18, 19. Two sets of desired inductor currents
I.sub.L12, I.sub.L23, I.sub.L31 and I.sub.L12 ', I.sub.L23 ', I.sub.L31 '
are computed during the computation intervals by circuits 16 and 17 for
the inductor banks 10 and 12 respectively. Because of the phase shifted
voltages V.sub.12 ', V.sub.23 ' and V.sub.31 ' inductor currents I.sub.L12
', I.sub.L23 ', I.sub.L31 ' are computed 30 electrical degrees earlier
than the currents I.sub.12, I.sub.23 and I.sub.31 of the inductor bank 10.
From the two sets of computed currents, current-to-firing angle converters
18 and 19 derive firing angles .alpha..sub.12, .alpha..sub.23,
.alpha..sub.31 and .alpha..sub.12 ', .alpha..sub.23 ', .alpha..sub.31 '
for the anti-parallel thyristors THR-1 and THR-2 respectively. The output
of the firing angle converters 18 and 19 are amplified by firing pulse
generators 20 and 21 which directly control the firing of thyrisors THR-1
and THR-2. Since the firing angles .alpha..sub.12 ', .alpha..sub.23 ' and
.alpha..sub.31 ' of inductor bank 12 lead those of inductor bank 10 by 30
electrical degrees, inductor bank 12 is operated in advance of inductor
bank 10 resulting in overall improvement of response and accuracy in
reactive control of the VAR generator.
Referring now to FIG. 2 to exploit the possibility that the inductor bank
12 connected to the phase shifting auto-transformer 11 can be operated in
advance of the inductor bank 10, it is necessary to compute the desired
inductor currents over two different computation intervals. This
requirement can be accomplished by using appropriate power quantities in
the computation of the desired inductor current. Consider FIG. 2 where the
two sets of line-to-line voltages V.sub.12, V.sub.23, V.sub.31 and
V.sub.12 ', V.sub.23 ' and V.sub.31 ' are shown together with the
corresponding sets of firing angles .alpha..sub.12, .alpha..sub.23,
.alpha..sub.31 and .alpha..sub.12 ', .alpha..sub.23 ', .alpha..sub.31 '.
The computation of the desired inductor currents, which define the firing
angles, is carried out between the two subsequent voltage peaks that
represent the two subsequent earliest firing points of thyristors THR-2
and THR-1 respectively.
The desired inductor currents of the three branches of the two delta
connected inductor banks 10 and 12 can be calculated in any half cycle
interval of the applied voltage preceding the earliest firing points by
the following equations.
##EQU1##
where I.sub.C.DELTA. is the amplitude of the fixed capacitor current,
referred to delta connection, that is
I.sub.C.DELTA. =(V/3).omega.C
V is the amplitude of the line-to-line voltage in both the AC supply and
auxiliary voltage sets
C is the capacitance of one branch of the (wye connected) capacitor bank
.omega. is the angular frequency of the AC supply
##EQU2##
v.sub.12, v.sub.23, v.sub.31 are the line-to-line voltages of the
three-phase AC supply
v.sub.12 ', v.sub.23 ', v.sub.31 ' are the line-to-line auxiliary voltages
obtained at the terminals of the phase shifting transformer
i.sub.l1, i.sub.l3, i.sub.l3 are the three load (line) currents
.alpha..sub.12,k, .alpha..sub.23,k, .alpha..sub.31,k are the firing angles
measured from the kth peak of voltage v.sub.12, v.sub.23, and v.sub.31,
respectively
.alpha..sub.12,k ', .alpha..sub.23,k ', .alpha..sub.31,k ' are the firing
angles measured from the kth peak of auxiliary voltage v.sub.12 ',
v.sub.23 ', and v.sub.31 ', respectively
.alpha..sub.12,k =0, a.sub. 23,k =0, .alpha..sub.31,k =0, .alpha..sub.12,k
'=0, .alpha..sub.23,k =0, .alpha..sub.31,k =0, define the earliest firing
points, which, in the present case coincide with the kth peak of the
corresponding voltages
From the desired inductor currents, the corresponding firing angles can be
determined from the following relationships:
##EQU3##
Referring again to FIG. 1 control circuits for the timing pulse generators
14, 15; the inductor current computing circuits 16, 17; the
current-to-firing angle convertor 18, 19; and the firing pulse generator
20, 21 are more specifically described in U.S. Pat. No. 3,999,117 issued
Dec. 21, 1976 to Laszlo Gyugyi et al., assigned to the assignee of the
present application and is incorporated herein by reference. The inductor
current computing circuits 16 and 17 further incorporates an improvement
to the resettable integrator RI that provides improved response based on
the equations 1-6 herein recited.
It can be readily seen that there is provided by this invention a new and
improved static VAR generator design that allows the reactive power to be
adjusted twice as often in any cycle that compensation is demanded
compared to prior art design.
Although there has been illustrated and described a specific embodiment, it
is clearly understood that the same were merely for purposes of
illustration and that changes and modifications may be readily made
therein by those skilled in the art without departing from the spirit and
scope of this invention.
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