Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CONTROLLING THE RESIDUAL CHARGE
ON A THYRISTOR-SWITCHED CAPACITOR
BACKGROUND OF THE INVENTION
This invention relates, generally, to VAT goner-
atoms and more specifically to controlling the residual
charge on switched capacitors employed in static VAT
generators.
The use of switched capacitors in static VAT
generators is known in the art and is used in systems such
as may be found in United States Patent No. 4,307,331
"hybrid Switched-Capacitor Controlled-Inductor Static VAT
Generator and Control Apparatus" issued December 22, 1981
to Judge and United States Patent No. 4,234,843 "Static
JAR Generator with Discrete Capacitive Current Levels"
issued November 18, 1~80 to Judge et at. In These types
ox static VAT generators a number of capacitor banks are
employed in series with a bidirectional thruster switch
which may be used in conjunction with a surge current
limiting inductor. In schemes such as may be found in the
above-me~tioned patents the thruster switches are normal-
lye fired in response to a VAT demand signal at the time
when capacitor voltage and the AC network voltage are
equal, that is when the voltage across the thruster
switches is zero. however, the disconnection of the
capacitor banks takes place at the instant when the volt
tare across the capacitor bank is equal to the peak of the
I AC network voltage. Therefore, the capacitor bank remains
charged to that voltage after disconnection. Since the
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capacitor bank will remain charged to the peak of the AC
voltage applied the voltage presented across the Theresa-
ion switch will be the sum of the applied AC voltage and
the capacitor voltage thereby reaching a maximum value of
twice the peak AC voltage once in each cycle with the
necessity that the thruster switch must be able to with-
stand or block this voltage.
This will not normally prevent a problem in
maintaining thruster switch integrity. However, under
some conditions of the AC supply network, the AC voltage
may transiently increase well above it nominal values to
excessively high voltage levels. Should the capacitor
banks be disconnected when this high level voltage is
present, the thrusters would be subjected to excessively
high voltage. One solution which is known in the art is
to utilize a non-linear clamping device connected across
the thruster switch. Because the break over voltage level
of present day non-linear clamping devices is approxi-
mutely twice as high a the normal peak operating voltage
they are subjected to, the maximum residual voltage across
a capacitor bank would remain high and is typically twice
the voltage level encountered in normal operation. There-
fore, under severe over voltage conditions utilizing the
present art such as m y by found in the above-mentioned
patents, both the capacitor bank and the thruster switch
typically would be subjected to twice the normal operating
voltage stresses. Further, if a thruster is fired either
intentionally or unintentionally, a heavily overcharged
capacitor could be reconnected to the AC network which may
result in a very large surge current through the thruster
switch and a substantial transient disturbance in an AC
network.
It is desirable to provide a device so as to
ensure unrestricted operation for thruster switches under
over-voltage conditions by limitation ox the residual
voltage on the associated capacitor banks to, or below,
the level encountered in normal operation. It is also
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desirable to be able to reduce the required surge rating
of a thruster switch thereby providing cost as well as
size benefits. It is also desirable to provide a device
which will limit the residual charge on the capacitor to a
S low value without requiring a impracticably low clamping
voltage level.
Briefly stated, a VAT generator of the type
which supplies reactive power to an electrical system for
regulation thereof is taught. A capacitive reactive
device is interconnected with an electrical system for
supplying reactive power to the electrical system during a
predetermined interval of time. At least a controllable
bidirectional witch device which may be composed of a
pair of unidirectional devices interconnected in series
circuit relationship with the capacitive reactive device
and is for connecting the capacitor reactive device in
reactive circuit relationship with the electrical system
during this same interval of time. At least a pair of
non-linear clamping devices each of which is connected in
parallel circuit relationship with each of the control
fable switch devices for conducting current which is a
capacitive discharge current while the switch device is in
an off state, but only when the voltage across the switch
device is above a maximum predetermined allowable value
and during a period of time which is subsequent to the
earlier interval of time to thus limit the voltage across
the capacitive reactive device and the controllable switch
devices to a predetermined safe level.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference is now made to the description of the
preferred embodiment, illustrated in the accompanying
drawings, in why ah:
Figure 1 is a schematic diagram of a portion of
a static VAT generator showing the circuit arrangement of
a thruster switch with respect to a capacitor;
Figures pa through Ed is a waveform diagram of
the voltage and current characterizing the connection and
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disconnection of a capacitor bank under different switch-
in conditions;
Figure 3 is a schematic diagram of a typical
thruster capacitor circuit;
Figure pa is a waveform diagram of the current
and voltage associated with the circuit shown in Figure 3;
figure 4 is a schematic diagram of a capacitor-
thruster switch combination utilizing a non-linear clamp-
in device with associated firing pulse generators of the
present invention;
Figure 5 is a waveform diagram of the voltage
and current associated with the schematic of Figure 4
during operating conditions; and
Figure 6 is a waveform diagram drawn from the
schematic of Figure 4 showing waveform conditions during
transient and/or overvoltage conditions in the AC network.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to Figure 1 there is shown a
schematic diagram of a VAT generator as used in the prior
art such as may be found in United States Patent No.
4,234,843 (mentioned previously). Here it can be seen
that a number of capacitor banks may vary with the tray-
into characteristics and/or VAT problems upstream and/or
downstream ox the circuit. Therefore, a number of keeps-
ions Of, C2 through ON art utilized in series circuit relationship with a bidirectional thruster switch Sol,
SUE through SWAN and possibly a surge current limiting
inductor Lo, Lo through LO. As is usual in the art,
thruster switches Sol through SWAN are fired or grated in
response to a VAT demand signal at a time when the keeps-
ion voltage Vc and the AC network voltage V1 are equal.
Therefore, the voltage across the thruster switches Sol
through SWAN is zero. The disconnection of the capacitor
banks takes place at an instant when the current crosses
zero in the thruster switch SUE through SWAN. At these
instants, the voltage across the capacitor bank TV is
equal to the peak or crest of the AC network voltage V1,
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thereby, the voltage across the capacitor banks after they
are disconnected is equal to the AC network peak voltage
at V1.
Referring to Figures pa through Ed a graphical
representation of the voltage and current waveforms char-
acterizing the connection (switching-in) and disconnection
(switching-out) of a capacitor bank under different per-
milted switching conditions is illustrated. At Figure pa
a condition for switching at the zero crossing of AC
network voltage Al is illustrated. In this situation when
the AC network voltage Al is zero the capacitor such as
Of (of Figure 1) for instance is switched in allowing
current IT to flow and allowing voltage Vc which is Essex-
tidally equal to the AC network voltage Al to be impressed
across the capacitor Of. This condition typically exists
at start-up or when the capacitor bank Of through ON is
allowed to be completely discharged. At figure 2b and 2c
the switching of positively and negatively charged keeps-
ion banks, respectively, at the peak of the applied volt
tare Al is illustrated. Note that in Figure 2b the switching occurs on a positive peak of line voltage V1 and
in figure 2c, the switching occurs at a negative peak of
line voltage Al. Note also that switching off occurs at
the appropriate positive and negative peaks respectively.
At Figure Ed a condition when a discharging capacitor is
switched in is illustrated. Note that switching in occurs
where the capacitor voltage Vc is less than the maximum
value ox the line voltage V1. Switching out occurs at the
positive peak in this instance.
Referring now to Figures 3 and pa there is shown
a schematic diagram of a capacitor thruster switch comb-
nation without a surge current limiting inductor and the
associated waveforms. As mention d earlier, since the
capacitor C3 will remain charged to the peak of the AC
applied voltage V3, a thruster switch which may be used
such as SUE must be rated to be able to block twice this
voltage. This is because the voltage across the thruster
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switch VSW3 is the sum of the applied AC voltage V3 and
capacitor voltage VC3 which reaches a maximum value of
twice the peak AC voltage V3 once in each cycle. This is
graphically illustrated in figure pa. Note that the first
maximum of the voltage across the thruster switch VSW3 is
reached a half of a cycle after the capacitor C3 has been
disconnected and which is at the peak of the applied AC
voltage V3 following the first change of polarity.
Under some conditions of the AC supply network
such as, for example, removal of a short circuit or load
rejection, the AC voltage V3 may transiently increase well
above its nominal crest value thereby charging the con-
netted capacitor C3 and associated capacitor banks to high
voltage levels. During this overvoltage condition keeps-
ion VAT compensation of the AC network is undesirable Andes such it is well known that the capacitor bank should be
disconnected. However, if this were to be done the thy-
wrester switch SUE would be expose to high overvoltage
during the text half cycle when the AC voltage reverses.
However, to protect the thruster switch SUE against high
voltage stress caused by an overcharged capacitor C3. The
prior art protection arrangement typically would be to
inhibit the disconnection of the capacitor bank under high
AC network voltage conditions by keeping the thruster
switch SUE conductive). However, to do so is disadvanta-
genus in that the connected capacitor C3 will increase the
already high network voltage (due to the leading current
they may draw through the basically inductive AC network)
and con also create dangerous oscillatory conditions in
the network which may further aggravate overvoltage probe
lets. Therefore, a contradiction between the requirements
of the AC supply network and the safe operation of the
thruster switch SUE exists. While the former would
necessitate the rapid disconnection of the capacitor C3,
the latter would require that the thruster switch SUE be
conductive until such time such time as the AC network
voltage V3 decreases to a normal level.
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A typical solution to solve this problem has been
to use a non-linear clamping device across the thyrlstor
switch SUE thereby reducing the residual overvoltage on the
capacitor C3. While this arrangement may enable the
capacitor C3 to be disconnected under overvoltage
conditions, the maximum residual voltage across the
capacitor VC3 remains high. This voltage VC3 is typically
twice the voltage level encountered in normal operation
because the break over voltage level of present-day
non-linear clamping devices is approximately twice as high
as the normal peak operating voltage stress they are
subjected to. Thus under severe overvoltage conditions,
both the capacitor C3 and the thruster switch SUE typically
would be subjected to twice the normal operating voltage
stress This condition for capacitor C3 would normally
extend for many seconds until the internal discharge
resistor (normally built into the capacitor as a unit) would
reduce the residual voltage. During this discharge period
time any accidental or purposeful reconnection of a heavily
overcharged capacitor to the AC network may result in very
large surge currents through the thruster switch So and may
propagate a substantial transient disturbance in the AC
network voltage V3.
Referring now to Figure 4 a schematic diagram of
the preferred embodiment of the present invention is shown.
The construction of the shown schematic allows a signal
which is received from a firing request control circuit (20)
to control the sequencing of the thruster switches SUE
and SUE. A suitable firing control circuit may be found
in United States Patent No. 4,274,135 "Grating Circuit for
High Voltage Thruster Strings" issued June 16, 1981 to
Rosa et at. The capacitor bank is part of a VAT generator
described in United States Patent No. 4,234~843 mentioned
previously. Therefore, only a brief description will be
found below.
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Accordingly, a signal from a firing request
control circuit is received at firing input 20. This
signal is received at the input of OR gates 30 and 32
which are, in the preferred embodiments of the present
invention, two input OR gates. The it it Saigon is
also sent to the input of flip-flop 22 which in the pro-
furred embodiment of the present invention is a D-type
flip-flop and to the input of pulse stretcher 24. the
output of pulse stretcher 24 is connected to each of one
of two inputs of AND gates 26 and 28. The Q output of
flip-flop 22 is then connected to the remaining input of
AND gate 26 and the Q output of flip-flop 22 is connected
to the remaining terminal of AND gate 28. The output of
AND gates 26 and 28 are connected to the remaining input
of OR gates I and 32 respectively. The output of the OR
gates 30 and 32 are connected to the input of the first
firing pulse generator 34 and the second firing pulse
generator 36 respectively. The outputs of the first
firing pulse generator 34 are connected to each ox the
gates of the thrusters in thruster switch SUE. Semi-
laxly the outputs of the second firing pulse generator 36
are connected to the gates of the thrusters contained in
thruster switch SUE. The thruster switches SUE and
SUE are, in the preferred embodiment of the present
invention, comprised of semiconductor thruster devices.
A relatively large number of series connected back-to-back
thruster device pairs may be utilized thereby forming two
alpha switches each of which is switch SUE or SUE.
Connected across each thruster switch SUE and SUE is
a non-linear device Al and R2 respectively. The non-
linear devices R1 and R2 in the preferred embodiment of
the present invention are conventional or zinc-oxide type
voltage surge arrestors having a volt/ampere character
fistic such that below a voltage level, which is known as a
clamping or break over voltage, they exhibit a very high
resistance white above that level a very low resistance
(ideally approaching zero). The serifs connected thy-
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wrester switches SUE and SUE are further serially
connected with capacitor C4 and inductor I which is then
connected across line terminals 38 and 39. It is to be
understood that a collection of capacitor-thyristor switch
inductors may be connected in parallel across line terming
awls 38 and 39 in a VAT generator.
To ensure unrestricted operation for thruster
switches Swahili and SUE under overvoltage conditions and
to limit the residual voltage on capacitor C4 or other
lo capacitors which may be in the VAT generator, an to
reduce the requited surge rating of theorizer switches
SUE and SUE or any other thruster switch contained in
a VAT generator, thruster pairs as mentioned earlier are
divided into "half" switches by the center tap Tl-T2. The
first firing pulse generator 34 and the second firing
pulse generator 36 allow independent control of the upper
and lower halves of the thruster switch SUE and SUE
no pectively. Each "half" of the thruster switch being
switch SUE or SUE is shunted by a non-linear clamping
pa device Al or R2 respectively. The clamping voltage level
of the non-linear devices Al and R2 is chosen to be higher
than the peak voltage appearing across WOW and SUE
respectively during normal operating conditions. Thus,
the clamping voltage level of the total thruster switch,
composed of the two switch "halves" SUE and SUE, is
determined by the sum of the clamping voltage levels of
the two series connected clamping devices Al and R2.
Therefore by essentially shorting out one of the clamping
devices Al or R2 during the time interval when the kapok-
it or C4 is being discharged the effective clamping voltage level is reduced to that of Al or R2. This will therefore
limit the residual charge on the capacitor C4 to a low
value without requiring an impracticably low clamping
voltage level for a single clamping device during normal
operating voltage. Since it is the clamping voltage level
of a jingle clamping device which determines the residual
charge on the capacitor C4, while two clamping devices in
series support the normal operating voltage.
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In the preferred embodiment illustrated in
Figure 4, the control technique is such that the gate
drive signal received at the firing input terminal 20 is
extended for an additional half cycle after the firing
request from the firing request control circuitry (not
shown) has stopped, by pulse stretcher 24 for one-half of
the thruster switches SUE and SUE. Therefore, the
thruster switches SUE and SUE would alternately carry
out the capacitor C4 discharge thereby insuring that both
clamping devices R1 and R2 are subjected on the average to
the same number of current surges.
Referring now to Figure 5 there is illustrated a
graphical representation of the operation of the thruster
switched capacitor in conjunction with the clamping de-
vices Al and R2 which are assumed to be identical. Assume
in that the network voltage V4 is relatively constant
prior to time to and that the AC network rewires keeps-
live compensation prior to time to the capacitor bank C4
is switched in and voltage and current are in a normal
steady-state as illustrated. At time to the AC network
voltage V4 is suddenly increased due to, for example,
transient disturbances or load switching. This increased
voltage V4 would generally necessitate the reduction of
the capacitive VATS supplied, by blocking the firing
request control signal 20 (Figure 4) to the thruster
switch thereby disconnecting the capacitor bank C4.
However, in the preferred embodiment of the present invent
lion the grating signal is blocked to only one of the two
halves of the thruster switch. Therefore, for example,
grating signal to thruster switch SUE (GSSW4-1) would be
blocked before the current crosses zero at time instant if
as illustrated in Figure 5. The signal GSSW4-2 would be
applied to the other half of the thruster switch, that is
thruster switch SUE for an additional half cycle as
illustrated in Figure 5. This signal GSSW4-2 would be
blocked just before time instant to. Since the drive to
thruster SUE is blocked, it turns off at if at the zero
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current crossing. At that instant in time the capacitor
voltage VC4 will reach the peak overvoltage value with the
voltage across the total thruster switch SUE plus SUE
equal to zero. As the AC network voltage starts to change
polarity the voltage across the entire thruster switch
will begin to increase. Further, at time to when the AC
network voltage V4 crosses the zero axis, the voltage
across the blocking thruster switch (in this case SUE
will reach the clamping level of the non-linear device Al.
It is to be understood that in the preferred embodiment of
the present invention the clamping level of Al is chosen
so as to be twice the normal peak AC voltage V4. After
the clamping level of the non-linear device R1 is reached
the non-linear device will break down and become conduct
live offering a low resistance. As the AC network voltageV4 further increases, a discharge current IT flows from
capacitor C4 via R1, thruster switch SUE and inductor
h. The discharge of capacitor C4 is completed by time
instant To at which time the AC network voltage V4 has
reached its peak. Prior to this time the signal to the
thruster switch SUE will have been blocked and as it
illustrated in Figure 5 both the capacitor residual volt
tare V4 and the thruster switch voltage VOW are settled
into their normal values.
Referring now to Figure 6 a similar operation as
in Figure 5 is illustrated for the case when a severe
sustained overvoltage condition was established in the AC
network at time to. As can be seen, the thruster voltage
is again limited to the peak value of the AC network
voltage V4 and the residual capacitor voltage VC4 is
reduced to zero. In the situation where, for limited
times, the AC network voltage may vary between one and two
times the normal value, the capacitor residual voltage
VC4, which would correspondingly vary between one and two
times the AC network voltage V4, is limited to a maximum
value of thy normal AC network voltage V4. The peak
thruster switch voltage, which could vary between two and
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four times the normal AC network voltage V4 is reduced to
or below twice the AC network average voltage level V4.
It is to be understood that many variations of
the present invention are possible without departing from
the spirit and scope of the present invention. For exam-
pie, only one clamping device may be utilized to discharge
an associated capacitor. Therefore, only one and the same
half of the switch would be kept in conduction for a half
cycle after the firing request has stopped, with the
result that the maximum clamping voltage stress would be
applied to only the same half of the thruster switch and,
therefore only one-half of the thruster switch need be
rated for the clamping voltage level used. Further, three
or more clamping devices across parts of the thruster
switch may be utilized and by selected conduction delays a
variable level control for the residual capacitor voltage
may be realized. Further, a small resistor may be con-
netted between terminals To and To so as to limit the peak
discharge curxe~t available. Additionally, a different
logic scheme for controlling the firing of the thrusters
may also be used.
Therefore, the disclosed invention provides a
relatively simple and inexpensive method of providing VAT
generator control and reliability.
