CA1229380A - Residual charge controller for switched capacitor installation - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A device is taught for controlling the residual charge on a thyristor switched capacitor. A capacitor is in series circuit relationship with at least two pairs of back-to-back thyristor switches which is then connected across an AC network. Each back-to-back thyristor combi-nation is shunted by a non-linear clamping device which exhibits a very high resistance below a specified voltage level and a very low resistance above that voltage level.
A pulse stretcher and a sequential firing circuit ar-rangement is utilized so as to prolong the conduction of one of the back to-back pair of thyristor switches at a time. During an overvoltage condition in the AC network the non-linear clamping device will limit the residual charge on the capacitor. A plurality of capacitor, thy-ristor switch, non-linear clamping device combinations may be utilized in parallel with the AC network.

Description

I

1 50,489 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

2 I 50~4 9 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 owe

3 50,489 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 I

4 50,4~9 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, I
50,4~9 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 7J~
6 50,48~
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.

~%~

7 50,489 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.

3~3~3 8 50,48~
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-9 I 50, 489 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.

50,489 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 P~3~3~ 50,4~9 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 I 50,~89 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.

Claims (9)

What I claim is:

1. A VAR generator of the type which supplies reactive power to an electrical system for regulation thereof, comprising:
a capacitive reactive means interconnectable with said electrical system for supplying said reactive power thereto during a predetermined interval of time;
at least two controllable switch means inter-connected in series circuit relationship with said capaci-tive reactive means for connecting said capacitive reac-tive means in reactive circuit relationship with said electrical system during said interval of time; and at least two non-linear clamping means each of which is connected in parallel circuit relationship with each of said switch means for conducting therethrough capacitive reactive means discharge current while said switch means is in an off state but only when the voltage across said switch means is above a maximum predetermined allowable value and during a period of time which is subsequent to said interval to thus limit the voltage across said capacitive reactive means and said switching means to a predetermined safe level.

2. The combination as claimed in claim 1 where-in said clamping means is comprised of a zinc oxide de-vice.

3. The combination as claimed in claim 1 where-in said clamping means comprises a voltage surge-arrestor means.

4. The combination as claimed in claim 1 wherein said switch means comprises thyristor means.

5. The combination as claimed in claim wherein said clamping means comprises a zinc-oxide device.

6. The combination as claimed in claim 4 wherein said clamping means comprises a voltage surge-arrestor means.

7. A device according to claim 1 wherein each of said controllable switch means is interconnected with said reactive circuit and said electrical system so as to permit only one of each of said pair of controllable switch means to conduct during said interval of time.

8. A device according to claim 1 wherein each of said controllable switch means is interconnected so that during said interval of time at least one of said controllable switch means is in an off state.

9. Device according to claim 1 wherein each of said controllable switch means is comprised of at least one pair of back-to-back thruster switches in parallel circuit relationship so as to allow each of said pair of thruster switches to conduct current in a direction opposite to that of the other thyristor switch.