Hybrid switched-capacitor controlled-inductor static VAR generator and control apparatus

#The subject matter of this invention is related to other inventions taught in concurrently filed copending applicaion Ser. No. 942,837, entitled "Static VAR Generator With Discrete Capacitive Current Levels".

#The subject matter of this invention relates generally to VAR generators and relates more specifically to VAR generators of the type employing banks of switchable fixed capacitors utilized in conjunction with switch controlled inductors for producing positive and negative VARS. It is known to make VAR generators by connecting a fixed capacitor and a switched inductor in parallel across two lines of an electrical system which is to be regulated or controlled by the VAR generator. A suitable control system is provided for sending an output signal to the switch portion of the switched inductor to establish a conduction interval therefor during a predetermined period of time. The conduction interval allows current to flow for a portion of the predetermined period of time thus generating an inductively reactive current which interacts with fixed capacitively reactive current to produce a net reactive current which cooperates with the voltage across the lines to produce reactive power. Th predetermined interval of time is usually one-half cycle of the line voltage. Consequently, on a half cycle by half cycle basis, the switching interval can be changed to provide differing amounts of reactive power as is determined is necessary by the calculating control portion of the system. Systems of the previous type can be found in U.S. Pat. No. 3,936,727, issued Feb. 3, 1976 to F. W. Kelly, Jr. and G. R. E. Laison, and U.S. Pat. 3,999,117, issued Dec. 21, 1976 to L. Gyugyi et al. The latter patent is assigned to the assignee of the present invention. The values of capacitance and inductance are usually chosen in the prior art so that at a moderate conduction interval for the switched capacitor, the produced inductive current is approximately equal to the fixed capacitive current thus producing zero VARS. Consequently, if the conduction interval is increased, the amount of inductive current increases producing a net inductive reactive current. On the other hand, if the conduction interval is decreased, the inductive current is decreased producing a net capacitive reactive current. This capability gives positive and negative VAR capability to the system. A system of this type has a number of problems, however. One problem lies in the fact that at a stand-by condition or of a condition where no VAR generation is required, appreciable power generation may be required in each of the inductive and capacitive components of the system. Said in another way, in a system of the type previously mentioned, significant inductive current is generated at a time when no VAR correction or production is required because the significant inductive current is utilized to cancel the oppositely phased significant capacitive current. This means that there are relatively high stand-by losses. Furthermore, for any given amount of maximum VAR correction either negative or positive, minimum values of capacitance and inductance are required. An improvement of the aforementioned system includes utilization of an inductive branch and a capacitive branch in which the inductive branch operates independently of the capacitive branch and vice verse. In this sytem at standby, neither the inductive portion of the system or the capacitive portion of the system conducts appreciable current and therefore the standby losses are lower than in the aforementioned system. Net inductive current is provided by using the inductive portion of the system exclusively; and net capacitive current is provided by using the capacitive portion of the system exclusively. However, a problem is present with this kind of system in that the use of a capacitive branch is not conducive to continuous switch control over a wide range of capacitive currents as is the use of an inductive branch. In the prior art therefore, the capacitive portion of such a system utilizes a bank of discrete capacitors with each having a separate switch. The net capacitive reactance for capacitive VAR production is provided by judiciously picking cominations of capacitors in the bank of capacitors to provide discrete values of capacitance. As mentioned however, such a system has the inherent disadvantage of only allowing discrete values of capacitive current to be produced. Thus, control over a continuous range is difficult if not impossible. In the range of capacitive VAR demand, only a relatively few values of capacitive current are available. As a consequence, VAR compensation or correction in the capacitive range is at best an approximation. It would be advantageous therefore, if a system could be found which utilized continuous VAR control in both the capacitive and inductive regions, but in which stand-by losses were minimized and in which the relative size of the inductive and capacitive components could be reduced below the previously discussed minimum for a given range of correction. It would be further advantageous if the control arrangement for such a system were such as to provide continuously variable VAR output and fast transient response.

#In accordance with the invention, a continuous range of VAR correction which covers both negative and positive VAR generation and which has minimum stand-by losses and uses relatively small components for a given range of correction is disclosed. In particular, an inductive apparatus and a capacitive apparatus are utilized in conjuction in such a manner that the inductive apparatus provides essentially the entire VAR generation for net inductive VAR demand. On the other hand discrete capacitors are utilized in conjuction with the same inductive apparatus to provide VAR generation over a continuous range for net capacitive VAR demand. The latter is accomplished by switching in individual capacitors or cominations thereof from a bank of capacitors thus providing a discrete level of capacitive current from which is subtracted any one of a continuum of inductive currents provided by the inductive portion of the circuit to thus produce a continuous range of capacitive VAR generation. A control system is provided for accomplishing the aforementioned purposes. The control system includes an open loop path which senses when VAR correction is necessary and provides an error signal related proportionally to the amount of VAR correction necessary. The error signal is then utilized in appropriate decision making portions of the open loop control system to set the firing angle for the inductive portion of the control system and to select the correct number of appropriate capacitors for the capacitive portion of the control system. Furthermore, the control system has a feed forward portion which is interconnected with the capacitive portion and the inductive portion. The feed forward portion is utilized to determine the conduction angle of the inductive portion of the system when it is utilized as a fine tuning apparatus. That is, when the capacitive portion of the system is used in a discrete manner to produce discrete levels of capacitive VARS.

#For a better understanding of the invention, reference may be had to the preferred embodiments thereof shown in the accompanying drawings in which:

#FIG. 1 shows a plot of VAR demand versus VAR output for the apparatus of the present invention.

#FIG. 2 shows a static VAR generator and control system therefor embodying the teachings of the present invention;

#Referring now to the drawings and FIG. 1 in particular, a plot of VAR output versus VAR demand is shown for an apparatus having the capability for both negative and positive VAR generation. Apparatus whose characteristics may be represented by the plot of FIG. 1 may be of the kind described in copending application Ser. No. 942,837. Apparatus of that type is also shown in FIG. 2 of the present invention. The apparatus in question has the characteristic of reactive current generation over a continuous range in the inductive region and in the capacitive region. However, in the capacitive range, continuous control is provided by a combination of discrete capacitive control and inductive fine tuning control. In the plot of FIG. 1, VAR demand is considered negative to the left while VAR demand is considered positive to the right. In a like manner to the top of FIG. 1, VAR output is considered positive while to the bottom of FIG. 1, VAR output is considered negative. In the ideal situation, VAR output equals VAR demand and the line VART represents such an ideal situation. This means that as circuit conditions change from a disposition in which no VAR correction is required to a disposition in which VAR correction is required or desired, the control system (to be described hereinafter) will operate to calculate the VAR demand for example, to provide appropriate signals for various other portions of the VAR producing circuit, and to produce a VAR output which is ideally equal to VAR demand as is shown in FIG. 1. In actuality, the matching of VAR output with VAR demand in the inductive region shown in the lower left quadrant of FIG. 4 is relatively easily obtainable because of the continuously controllable nature of inductive current as described in the prior art. However, because of the discrete nature of VAR compensation with respect to capacitive elements, VAR generation in the region shown in the upper right portion of FIG. 1 is more difficult. In this case, an appropriate controller senses the time for positive VAR correction and switches in a capacitor to provide a net level of capacitive VAR correction, VAR 1, for example, even though the demand for VAR correction in this region may be significantly less than VAR 1. If the capacitive element were the only element available for VAR correction as is the case in the prior art, the system would have to operate with larger VAR correction than is necessary, thus introducing undesirable error. However, the present invention utilizes the inductive element with its phase back control to compensate for the difference between the actual amount of VAR demanded and that which is produced by the discrete capacitor. Each of the levels VAR 1, VAR 2, etc., shown in FIG. 1, represents a fixed level at which reactive current may be produced in the reactive current region of FIG. 1. Any other reactive current desired must be produced by reducing some of the discrete reactive current by subtracting an appropriate amount of inductive current as determined by a firing angle for a thyristor which is connected in series circuit relationship with the inductive element. The sawtoothed shopped inductive current shown to the bottom right of FIG. 1 represents ranges for continuous control of inductive current. The ranges exactly correspond to the brake points in the discrete levels of capacitive current. Thus, it can be seen that VAR generation in the capacitive region over a continuous range is possible with apparatus that embodies the principles illustrated by FIG. 1.

#Referring now to FIG. 2, a VAR generator system 10 is shown. The VAR generator system 10 is connectable at terminals X and Y to two lines of an electrical system across which the voltage v is present. In this embodiment of the invention, the only means of communication with the electrical system is provided at the terminals X and Y although that is not limiting. The production of VARS or volt amps reactive, is useful for many purposes in an electrical system. It tends to be very good for voltage compensation among other things. In another instance, the network voltage v may drop or rise from a nominal ideal value as load conditions change in the electrical system. Change in the voltage v beyond a certain predetermined tolerance range is undersirable in an electrical system. The VAR generator may produce sufficient reactive current or reactive power to compensate for the change in the voltage thus boosting it when it has fallen or reducing it when it has risen. The production of the reactive curent is dependent upon VAR generating apparatus which usually includes capacitive elements and inductive elements and a control system for firing the various elements or introducing them into the circuit on half cycle by half cycle intervals in accordance with a recently calculated VAR demand. In this particular embodiment of the invention, the control system 11 includes a voltage measuring circuit 14, a synchronous timing circuit 18, an error generator 20, an error amplifier 22, a VAR demand comparator 24, an error adjusting circuit 26, an error to firing angle converter 28, and firing circuits 29. In addition a decision logic for capacitor switching circuit 32 and a capacitor switching time selector 34 are also provided. The elements are interconnected with each other and with the various capacitors, inductors and switches of the VAR generator circuit 10 in the following manner. The voltage measuring circuit 14 is interconnected with the terminals X and Y to measure phase voltages v1, v2 and v3, for example, in the case of a three phase electrical system and to provide an output VAVE which is somehow related to them. The voltage measuring circuit 14 therefor comprises the input threshold logic control device for the circuit. It may in other embodiments of the invention include a current measuring circuit, a power measuring circuit, or the like. Its principal function is to provide an output which is related to AC network variables and from which an error signal can be derived. In conjunction with the latter point, the VAVE signal from the voltage measuring circuit 14 is provided to the negative input of an error generator 20. To the positive input thereof is provided a predetermined reference signal VREF. These signals are added algebraically to provide an error signal Ve. The error signal is provided to an error amplifier 22 from which a enlarged error signal VARD is produced. The enlarged error signal VARD is proportional to the error signal Ve. The output of the error amplifier 22 is provided simultaneously to the three places. The output is provided by way of a droop resistor DR to another negative input terminal of the error generator 20 for closed loop feedback. The droop resistor DR may be adjusted to provide an appropriate droop range for the voltage. The output of the error amplifier 22 is also provided to a VAR demand comparator 24 and to an error adjusting circuit 26. The VAR demand comparator compares the VAR demand signal VARD with a reference signal VART which may be derived from a fixed voltage or from a controlled voltage which is dependent upon network variables. Apparatus 24 provides outputs BK1 through BK4, each of which is related to firing signals for fixed capacitors. These signals are in turn provided in tandem to the decision logic for capacitor switching circuit 32. This circuit compares the descrete levels of VAR demand signals BK1 through BK4 with signals SP1 through SP4 from the capacitor switching time selector 34 and with signals VPS and VNS from the synchronous timing circuit 18. Two sets of output signals are provided from the decision logic for capacitor switching 32. One set of outputs, represented by the lines 36, may be considered the closed loop signals DP1, DN1, etc. These signals are provided to the firing circuits 29 for actuation of the switches THC for appropriate capacitors C1, C2, C3 or C4. The other set of output signals from the decision logic capacitor switching 32 is represented by the lines 38 upon which the signals G01 through G04 are present. This may be called the feed forward portion of the control circuit. These latter signals are provided as one set of inputs to an error adjusting circuit 26. The other input of error adjusting circuit 26 is the VAR demand signal VARD. The output from the error adjusting circuit 26 is the adjusted error signal VARLD. This signal is provided to the error to firing angle converter 28 which cooperates with the synchronous timing circuit 18 to provide an output signal .alpha. on the line 30 which in turn is provided to the firing circuits 29 for controlling the phase back or conduction angles as the case may be of the thyristor switches THL for the inductor L. The capacitor switching time selector 34 is interconnected to measure the voltages across the switches THC so as to provide the output signals SP1 through SP4 in conjuction with the output signal TS1 from the synchronous timing circuit 18. Essentially, the capacitor switching time circuit 34 picks the appropriate instant of time based on the voltage conditions across the capacitors C1 through C4, for example, for firing the capacitors in the most efficient transient free way in accordance with the status of the signals BK1 through BK4. The synchronous timing circuit 18 may be similar to the synchronous timing circuit taught in the previously mentioned U.S. Pat. No. 3,999,117. The synchronous timing circuit 18 provides the time reference base for the operation of the entire static VAR generator 10. It has an input which is interconnected with the terminals X and Y for utilizing the AC network terminal voltage v in the preferred embodiment of the invention as the basic timing reference for all of the operations of the controller of the static VAR generator 10.

#Referring concurrently to FIGS. 1 and 2, it can be seen that the static VAR generator 10 has three modes of operation. The first mode of operation may be called the exclusive inductive current production mode. This mode of operation corresponds to the VAR generator activity depicted in the lower left quadrant of FIG. 1. In this case, net inductive current is required and the inductor L of FIG. 2 is utilized exclusively for producing the inductive current. The voltage measuring circuit 14 produces an output signal VAVE which is compared with the VREF signal by the error generator 20. The output signal Ve therefrom is provided to the error amplifier 22 from where it is passed through the error adjusting circuit 26 in a generally unchanged state to produce the signal VARLD. This latter signal is fed to the error to firing angle converter 28 where it produces an appropriate firing angle .alpha. for the firing circuits 29. Since no capacitive action is required at this time, the firing circuit 29 fires the thyristor gates THL during appropriate half cycles to allow current iL to flow through the inductor L during appropriate half cycles of voltage v to thus provide net inductively reactive current for compensating the voltage between the terminals X and Y. In the next mode of operation, the error amplifier provides the signal VARD which is oppositely signed from the signal VARD produced during the situation where inductive reactive current was desired. In this case, the signal VARD is operated upon by the VAR demand comparator 24 and provides appropriate signals BK1 through BK4 to the decision logic for capacitor switching block 32. At an appropriate time, when the voltages across the capacitive elements C1 through C4 are in a proper dispositon for firing, the capacitor switching time monitor 34 will alert the decision logic for capacitor switching module 32 of this fact by way of the signals SP1 through SP4. When the line voltage v is in an appropriate disposition as determined by the synchronous timing circuit 18, the decision logic for capacitor switching module 32 will provide appropriate signals DP1, DN1, etc. to the firing circuits 29 for actuation of the appropriate thyristor switches THC to switch the properly determined number of capacitors C1 through C4 into parallel circuit relationship with the terminals X and Y to thus produce a discrete level of reactive current IC. In the event that the discrete level of reactive current IC chosen is exactly equal to the current sufficient to satisfy the VAR demanded as determined by the VAR demand comparator 24, no further action will be required in the static VAR generator 10. However, the latter event is statistically unlikely. What is more likely is that the third mode of operation will be utilized in which the appropriate VAR demanded will require a capacitive current which is somewhere between the various discrete levels available through all of the combinations of capacitors C1 through C4. If this is the case, the decision logic for capacitor switching circuit will supply a signal by way of line 38 to the error adjusting circuit 26 which in turn will produce the signal VARLD which is fed to the error to firing angle converter 28 which in turn provides an appropriate signal for angle .alpha. on the line 30 to thus fire the thyristors THL to cause the inductor L to conduct. This is sufficient to cause a component of inductive current to flow which will cancel a portion of the fixed capacitive current thus producing the exact amount of capactive current, within limits, for producing the amount of capacitive VARS which are equal to the VARS demanded as called for by the error amplifier 22. In summary therefore, it can be seen that in the negative, that is the inductive range of VAR demand, all capacitor banks are switched off and the inductive VAR is varied by the usual method of phase control of the reactor or inductor L. As the VAR demand becomes positive, that is capacitive, the first capacitor bank is switched in and the reactor is phase controlled to absorb surplus capacitive VARS and thus to provide the VAR output required. When the VAR demand exceeds the rating of the first, second, third, etc. capacitor banks, the second, third, fourth, etc. capacitor bands respectively are switched in and the inductor bank is controlled each time so as to make the VAR output generally equal to the VAR demand. To ensure that the capacitor banks are not switched in and out in an oscillatory manner above the scheduled switching points, an appropriate hysteresis is provided between the VAR demand values and the places where switching in and switching out actually takes place as illustrated in FIG. 1. This requires that the rating of the inductor reactor bank be higher than that of the single capacitor bank but is deemed necessary in one embodiment of the invention to provide the hysteresis function. Examination of FIG. 2 will show that the control arrangement depicted therein provides the following main functions. It determines how many capacitor banks are to be switched in or out to approximate the compensation VAR demand with the positive (Capacitive) VAR surplus. It determines which capacitor banks can be switched at the switching instance defined for minimum transient disturbance. It determines the inductive VARS necessary to absorb the surplus capacitive VARS provided by the capacitor banks in order to make the VAR generator and the VAR demand equal and it provides an overall closed loop control for the VAR output so as to match the compensating requirement of the AC network. Consequently, it can be seen that the above circuit provides both closed loop and feed forward control utilizing the same apparatus where necessary in each case.