Abstract:
A device ( 8 ), for adjusting the impedance of a high voltage line ( 9 ), supplying an alternating current, has at least one control coil ( 2 ), which may be inserted in series in the high voltage line ( 9 ) and at least one switching device ( 3,10 ), provided for each control coil ( 2 ). The device is compact and economical. A control unit ( 4 ) for controlling each switching device ( 3,10 ) is also provided, such that the reactance of the control coil ( 2 ), acting in the device may be adjusted by the switching of the switching device ( 3,10 ), whereby each switching device ( 3,10 ) is arranged parallel to the corresponding control coil ( 2 ) in a parallel branch ( 5 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a U.S. national stage application of International Application No. PCT/DE2004/001972 filed Sep. 1, 2004, which designates the United States of America, and claims priority to German application number DE 10345020.3 filed Sep. 23, 2003, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The invention relates to an apparatus for adjustment of the impedance of a high-voltage line which carries an alternating current and comprises a plurality of phases, having at least one control coil which can be inserted into the high-voltage line connected in series, and having at least one switching device which is in each case associated with one of the control coils, with a control unit being provided in order to control each switching device in such a manner that the effective reactance of the control coil in the apparatus can be adjusted by the switching of the switching device.  
         [0003]     The invention also relates to a method and a control unit for adjustment of the impedance of a high-voltage line which carries alternating current.  
       BACKGROUND  
       [0004]     An apparatus such as this, a method such as this and a control unit such as this are known, for example, from DE 37 87 355 T2. The apparatus disclosed there is also illustrated in  FIG. 1  of the attached drawing. The already known apparatus  1  has a control coil  2  and an uncontrolled coil  6  which can be inserted in series into a high-voltage line, which carries alternating current, by means of expedient connections or switches which are not shown in  FIG. 1 . In order to control the effective impedance of the control coil  2  in the apparatus  8 , a switching device  3  is provided, and is formed by two thyristors which are connected in opposite senses. The thyristors are connected to a control unit  4 , whose control signals allow the current flow through the thyristors to be interrupted or permitted. The effective impedance of the control coil  2  in the apparatus, and thus the impedance of the entire apparatus, can be adjusted by expedient choice of the triggering time of the thyristors as a function of the phase of an alternating current in the high-voltage line. This response time of the thyristors, which is dependent on the phase of the alternating current, is referred to in the following text as the trigger angle. The use of the thyristors in the control path  5  for solid-state interruption of the alternating current also results in higher-order harmonic of the fundamental frequency of the alternating current, that is to say integer multiples of the fundamental frequency of the alternating current. For this reason, a filter unit  7  is provided in parallel with the control path  5  and is designed to suppress one or more of these harmonic oscillation components. Furthermore, the filter unit and the filter coil  6  are used to maintain the current path through the apparatus when the switching device  3  is in a position in which the current flow is interrupted. However, the already known apparatus occupies a large amount of space and is costly because of these additional components. Furthermore, the control capabilities of the already known apparatus are restricted, since its impedance cannot be reduced to zero, but only to the impedance of the parallel-connected coils  2  and  6 .  
       SUMMARY  
       [0005]     One object of the present invention is thus to provide an apparatus of the type mentioned initially which is compact and cost-effective, as well as a method as mentioned initially, and a control unit, by means of which the impedance of a high-voltage line can be controlled in a simple manner.  
         [0006]     With regard to the apparatus, this object can be achieved by arranging each switching device in a parallel path in parallel with the control coil associated with it.  
         [0007]     Furthermore, this object can also be achieved by a method in which the control coil is bridged as a function of the phase of the alternating current by triggering a switching device which is arranged in a parallel path in parallel with a control coil which can be inserted in series into the high-voltage line, with the impedance of the high-voltage line being adjusted in this way.  
         [0008]     Furthermore, this object can also be achieved by a control unit for adjustment of the impedance of a high-voltage line which carries alternating current, having a phase sensor for production of a zero-crossing signal on verification of a zero crossing of the alternating current, and having at least one trigger unit, which is connected to a phase measurement device and to a trigger angle transmitter for production of a trigger angle for the trigger unit, and which produces a trigger signal after a delay time corresponding to the trigger signal, on receiving a zero-crossing signal, which trigger signal is used to control the impedance of the high-voltage line by using a switching device to bridge a control coil, which is inserted in series into the high-voltage line. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     Further expedient refinements and advantages of the invention are the subject matter of the following description of the exemplary embodiments of the invention with reference to the figures of the drawing, in which components which have the same effect are provided with the same reference symbols, and in which:  
         [0010]      FIG. 1  shows an apparatus of this generic type according to the prior art,  
         [0011]      FIG. 2  shows one exemplary embodiment of the apparatus according to the invention,  
         [0012]      FIG. 3  shows the method of operation of the apparatus shown in  FIG. 2  on the basis of current or voltage curves, resolved on a time basis, and illustrated schematically,  
         [0013]      FIG. 4  shows the reactance of the apparatus shown in  FIG. 2  with respect to the reactance of the control coil, as a function of the trigger angle,  
         [0014]      FIG. 5  shows the fundamental frequency component of the voltage which is dropped across the apparatus at various trigger angles,  
         [0015]      FIG. 6  shows a digital simulation of the change in power flow in the high-voltage line when the trigger angle of the circuit device of the apparatus shown in  FIG. 2  is varied,  
         [0016]      FIG. 7  shows the components of the fundamental frequency and of the higher harmonic oscillations of the voltage which is dropped across the apparatus as shown in  FIG. 2  when the alternating current is constant, as a function of the trigger angle,  
         [0017]      FIG. 8  shows an illustration showing the control unit of the apparatus illustrated in  FIG. 2 ,  
         [0018]      FIG. 9  shows a more detailed illustration of the control unit shown in  FIG. 8 ,  
         [0019]      FIG. 10  and  FIG. 11  show characteristics of the control unit as shown in  FIG. 9 ,  
         [0020]      FIG. 12  shows a further exemplary embodiment of the apparatus according to the invention,  
         [0021]      FIG. 13  shows the control means for an apparatus as shown in  FIG. 12 ,  
         [0022]      FIG. 14  shows calculated voltage profiles of the fundamental and harmonic voltage components as a function of the trigger angle of the apparatus shown in  FIG. 13 ,  
         [0023]      FIG. 15  shows characteristics of the control unit as shown in  FIG. 13 ,  
         [0024]      FIGS. 16 and 17  show characteristics which differ from  FIG. 15  for a control unit for an apparatus as shown in  FIG. 13 ,  
         [0025]      FIG. 18  shows a further exemplary embodiment of the apparatus according to the invention,  
         [0026]      FIG. 19  shows a further exemplary embodiment of the apparatus according to the invention,  
         [0027]      FIG. 20  shows an illustration showing how the apparatus illustrated in  FIG. 19  is controlled,  
         [0028]     FIGS.  21  to  23  show characteristics of the control unit as illustrated in  FIG. 20 , and  
         [0029]      FIG. 24  shows a further exemplary embodiment of the apparatus according to the invention.  
     
    
     DETAILED DESCRIPTION  
       [0030]     According to an embodiment, the control coil is no longer arranged connected in series with the switching device, as in the prior art. In fact, the control coil can be bridged by the switching device, by which means a control method according to the invention, which differs from the prior art, is linked. The switching arrangement according to an embodiment also makes it possible to dispense virtually completely with additional uncontrolled coils or filter units. It should be noted that a control coil need not be in the form of a single component for the purposes of the invention. In fact, according to an embodiment, it is also possible to connect a plurality of coils in series in the respective parallel path. In this case, the expression “control coil” should be understood as meaning the sum of all of the coils in the parallel path.  
         [0031]     The method according to an embodiment is used to control the apparatus according to an embodiment. In this case, the method according to an embodiment is preferably carried out by means of the control unit according to an embodiment.  
         [0032]     The apparatus according to an embodiment advantageously has a parallel path which has no inductive components such as coils or the like even when these are provided only to suppress transient effects, and have no influence on the control characteristic.  
         [0033]     According to an embodiment, the switching unit may be a high-speed mechanical switch. However, at least one switching device expediently comprises thyristors which are connected in opposite senses. The use of thyristors improves the capabilities for controlling the impedance by means of the control unit. According to an embodiment, by way of example, the thyristors are used as high-speed switches. Their switching response can be matched to the respectively required circumstances by means of so-called electrical or optical trigger signals, and thus in a particularly simple manner. All semiconductor components which are already known and are compatible with high voltages, and which are based on silicon or else carbide, are suitable for use as thyristors. Examples of suitable silicon thyristors include electrically triggered thrysitors, optically triggered thyristors, so-called GTOs or else IGBTs.  
         [0034]     The control unit advantageously has a zero-crossing unit, which is connected to current sensors, in order to verify a zero crossing of the alternating current, and at least one trigger unit, which is connected to a trigger angle transmitter. According to an embodiment, the current sensor can thus be a current sensor which emits digital current values and has a current transformer in order to produce a secondary current that is proportional to the alternating current, a sampling device for sampling the secondary current at a sampling clock rate in order to obtain current values, and an analog/digital converter in order to convert the sampled current values to digital current values. A change in the mathematical sign is identified, for example, by conventional software for this purpose in the zero crossing unit, which then sends a zero crossing signal to the trigger unit which is connected to it. The trigger unit is supplied with a trigger angle via the trigger angle transmitter. This trigger angle corresponds to a time period after which the trigger unit sends a trigger signal to trigger the thyristors, to these thyristors, following a zero crossing of the alternating current. This therefore allows any desired phase shift between the zero crossing of the alternating current and the current bridging by the thyristors by the choice of the trigger angle.  
         [0035]     According to an embodiment which differs from this, the control unit can be in the form of an analog control unit with the normal components relating to this that are known to those skilled in the art.  
         [0036]     According to a further embodiment, the trigger angle transmitter is connected to a current sensor in order to measure the alternating current, and is connected to a voltage sensor in order to measure the voltage on the high-voltage line with respect to the ground potential or with respect to the voltages between the phases, with the control unit having a read only memory element which is provided for storage of control parameters, with at least one matching unit being provided in order to detect discrepancies between the control parameters and the measured values from the current sensor and/or the voltage sensor, or between the control parameters and measurement variables which are calculated from the measured values from the current sensor and/or voltage sensor. In other words, each matching unit is provided with one or more nominal values which, for example, correspond to a nominal current value, to a nominal voltage value or, for example, to a nominal value of a real power or wattless component calculated from these two electrical variables. If the respective matching unit detects a difference which exceeds the tolerance range between the respectively measured actual and nominal values or between the actual values as calculated from the measured variables and the nominal values, it shifts the trigger angle in the direction such that the discrepancy between the actual value and the nominal value is once again shifted into the tolerance range. The tolerance range is dependent on the respective application of the apparatus, and is typically  1 % of the respective nominal parameter.  
         [0037]     According to a further embodiment, the apparatus can be provided with two control coils arranged in series which each have a switching device arranged in a parallel path connected in parallel. According to this further embodiment, it is possible to design the overall apparatus using high-voltage components which occupy less space than apparatuses with a control coil which can be bridged. Furthermore, the harmonic component of the voltage which is dropped across the apparatus is less than that of an apparatus with only one controllable reactance.  
         [0038]     According to a further embodiment, the control unit has two trigger units which each interact with a switching device. The control unit is also connected to two different trigger angle transmitters, so that the timing of the trigger of the switching devices can be controlled independently of one another.  
         [0039]     In the case of this refinement, a further embodiment is possible in which only one of the switching devices has thyristors which are connected in opposite senses, with the other switching device being in the form of a mechanical switch. This admittedly reduces the control capabilities of the apparatus. However, a variant such as this is cost-effective, and its control capability is simplified.  
         [0040]     According to an embodiment, a capacitor can be advantageously provided, which is connected in series with the control coil and can be bridged by means of a capacitor switching unit which is arranged in parallel with the capacitor in a capacitor parallel path. The addition of a capacitor connected in series considerably widens the impedance range which can be controlled by the apparatus. According to this further embodiment, two controllable reactance ranges are provided, although only one common controller is required to control the overall impedance. The series arrangement of controllable reactances can, of course, also be added to by further modules of this type arranged connected in series.  
         [0041]     According to a further embodiment, a filter unit is provided and is arranged in parallel with a series path in which the control coil and the capacitor are connected in series. The filter units can be used only when a capacitor or a coil is also provided in the bridged series circuit, in addition to at least one control coil.  
         [0042]     According to a further embodiment of the control unit, at least one of the switching devices is formed by thyristors which are connected in opposite senses. All of the switching devices may, of course, also comprise thyristors which are connected in opposite senses.  
         [0043]      FIG. 2  shows an exemplary embodiment of the apparatus  8  which can be inserted into a high-voltage line  9  via switches which cannot be seen in  FIG. 2 , with the high-voltage line  9  being provided in order to transmit power between two nodes of an electrical power distribution network. In this case, the apparatus  8  is arranged in series with the high-voltage line  9  in such a way that all of the alternating current which is carried by the high-voltage line  9  flows via the apparatus  8 . In the normal way, the high-voltage line comprises three phases, that is to say three high-voltage conductors which are routed alongside one another, and of which only one phase is illustrated in the figures of the drawing, for clarity reasons. The other phases, which are not shown, of the high-voltage line  9  are connected to the apparatus  8  in the same way as the phase which is illustrated in the figures of the drawing. The device  8  may, of course, be connected in a corresponding manner to a high-voltage line which has more than three phases or less than three phases.  
         [0044]     The apparatus  8  has a control coil  2  and a switching device  3 , which in this case is formed by two thyristors  10  which are connected in opposite senses. In this case, the switching device  3  is arranged in the parallel path  5 , in parallel with the control coil  2 .  
         [0045]     The thyristors  10  are each connected via expedient control lines  11  to a control unit  4 , which is designed to produce a trigger signal as a function of the phase of the alternating current in the high-voltage line  9 .  
         [0046]      FIG. 3  illustrates the method of operation of the apparatus  8  according to an embodiment as shown in  FIG. 2  on the basis of time-resolved schematic current and voltage curves.  FIG. 3   a  shows two oscillation periods of an idealized sinusoidal alternating current I on  in the high-voltage line  9 , with the alternating current amplitude being 1.5 kA. At the time t 0 , the thyristors  10  in the switching device  3  are in an off position, so that the current passed via the control coil  2  and illustrated in  FIG. 3   b  essentially corresponds to that of the alternating current in the high-voltage line  9 . The current passed via the parallel path  5  is illustrated in  FIG. 3   c.  In consequence, this is equal to zero at the time t 0 .  
         [0047]     When the time indicated by α in  FIG. 3  is reached, the thyristors  10  in the switching device  3  are triggered by the control unit  4 . Because of the self-induction in the coil, the coil current shown in  FIG. 3   b  remains essentially constant. The current which is carried via the parallel path  5  and is shown in  FIG. 3   c  likewise reaches a relative minimum at the vertex point of the alternating current. At the time which is indicated by β, the current carried via the parallel path and thus via the switching device  3  with the thyristors  10  is equal to zero, so that the thyristors  10  are once again changed to the off position. After this time, the current carried via the control coil  2  once again corresponds to the alternating current in the high-voltage line  9 . This process is then repeated, but this time in the positive region of the alternating current, that is to say with the alternating current having the opposite mathematical sign.  
         [0048]      FIG. 3   d  shows the voltage which is dropped across the switching device  3 . Its magnitude is a maximum at the time to, and corresponds to a magnitude of about −40 kV. The thyristors  10  are triggered at the time α, following which the voltage falls to a zero value in order to rise to +28 kV at the time β.  
         [0049]      FIG. 3   e  shows the results of the calculation of the fundamental frequency component of the voltage which is dropped in total across the apparatus  8 . As can be seen, the fundamental frequency component of the voltage is phase-shifted through +90 degrees with respect to the current shown in  FIG. 3   a.  As will be described in the following text, the amplitude of the fundamental frequency component is dependent on the trigger angle α. For this reason, the apparatus  8  acts as a controllable reactance.  
         [0050]     The voltage which is dropped across the triggering apparatus  8  in the time period between the first triggering of the thyristors α and the subsequent interruption of the current by the thyristors π−α, that is to say β, is equal to zero, as can be seen in  FIG. 3d . When the thyristors  10  are switched off, the voltage V(t) which is dropped depends on the change in the current as well as the inductance of the control coil. In this case: 
 
 V ( t )=0 for α&lt;ω t &lt;(π−α) 
 
  
           V   ⁡     (   t   )       =       L   RSP     ⁢       ⅆ     (     I   on     )         ⅆ   t           ,       
 
for (π−α)&lt;ω t &lt;(π+α), 
 
 where I on  corresponds to the alternating current in the high-voltage line  9  and L RSP  corresponds to the inductance of the control coil. The trigger angle is α in radians, and ω—likewise in radians—represents the angular velocity. On the basis that I on =I 0  sin(ωt), 
 
 V ( t )= L   RSP ωI 0  cos(ω t ) for (π−α)&lt;ω t &lt;(π+α) 
 
         [0051]     The fundamental frequency component of V(t) can be calculated using Fourier techniques. This is based on the approximate assumption that the resistance of the apparatus is equal to zero. The only voltage component of the voltage which is dropped across the apparatus  8  that is of interest is thus that which is in phase with cos(ωt). As shown in  FIG. 3 , the positive and negative half cycles of the voltage which is dropped across the apparatus are symmetrical. The magnitude of the component of the fundamental frequency of the voltage V SUM  which is dropped across the apparatus  8  is thus given by:  
               V   SUM     =       2   π     ·       ∫     π   -   α       π   +   α       ⁢       V   ⁡     (   t   )       ·     cos   ⁡     (     ω   ·   t     )       ·     ⅆ     (     ω   ·   t     )                         =       2   π     ·     I   0     ·   ω   ·     L   RSP     ·       ∫     π   -   α       π   +   α       ⁢         cos   2     ⁡     (     ω   ·   t     )       ·     ⅆ     (     ω   ·   t     )                         =       2   π     ·     I   0     ·   ω   ·     L   RSP     ·     (     α   +       sin   ⁡     (     2   ·   α     )       2       )                 
 
 for α between 0 and 90 degrees. 
 
         [0052]     The reactance at the fundamental frequency is thus:  
           X   SUM       X   RSP       =       2   π     ·     (     α   +       sin   ⁡     (     2   ·   α     )       2       )           
 
         [0053]      FIG. 4  shows the reactance of the apparatus  8  X SUM , normalized with respect to the reactance of the control coil  2  X RSP , as a function of various trigger angles α, as is indicated in  FIG. 4 , with the units being radians. As can be seen, the impedance of the apparatus  8  according to an embodiment is negligible at trigger angles in the region of 0 degrees, while it approaches the maximum value asymptotically at trigger angles greater than 70 degrees, with this maximum value being determined by the reactance of the control coil X RSP .  
         [0054]      FIG. 5  shows the total voltage V SUM  which is dropped across the apparatus  8 , with the functional profile being identified by quadrilateral points. Furthermore,  FIG. 5  shows the fundamental frequency component of the voltage, calculated on the basis of a simplified model, in the form of a curve V G , which is identified by round points. The calculation was based on the assumption of an ideal sinusoidal alternating current with an amplitude of 1.5 kA, with the control coil  2  having an inductance of 80 MilliHenrys.  FIG. 5   a  shows the voltage profiles for a trigger angle α of 10°. This means that a control pulse for the thyristors  10  is triggered after just a short time period following a zero crossing of the alternating current in the high-voltage line  9 , as a result of which the control coil  9  is bridged, and the voltage is dissipated.  
         [0055]     The trigger angles a in  FIGS. 5   b,    5   c,    5   d  and  5   e  were 20, 30, 45 and 60°, respectively. As can be seen, the amplitude of the fundamental frequency rises as the trigger angle becomes larger, and, in  FIG. 5   e,  the profile of the fundamental frequency corresponds essentially to the voltage drop across the apparatus  8 .  
         [0056]      FIG. 6  shows a digital simulation based on the same simplified model that was also used as the basis for  FIG. 5 . The illustrated results are based on the assumption of a high-voltage line having a reactance of 45 Ohms at 50 Hz, with the apparatus  8  being fitted with a control coil  2  of 80 MilliHenrys. The respective trigger angle α in radians, chosen for the calculation, is plotted in  FIG. 6   a  as a function of the time in seconds, with α still being zero after 0.2 s, and with α having risen to 90 degrees after 0.8 s.  FIG. 6   b  also shows the voltage of V SUMm  dropped across the apparatus as a function of the time, whose profile approaches a sinusoidal profile to an ever greater extent as the trigger angle increases. The alternating current in the high-voltage line is plotted in  FIG. 6   c,  and its amplitude decreases as the trigger angle increases.  FIG. 6   d  shows the real power P w  which is transported through the high-voltage line and whose magnitude, as expected, decreases as the trigger angles α become greater.  
         [0057]      FIG. 7  shows the amplitudes of the fundamental frequency and of the higher harmonic oscillation components of the voltage which is dropped across the apparatus  8  up to the 13th harmonic V x  with respect to the amplitude of the total voltage V SUM , as a function of the trigger angle α. As can be seen, the amplitude Y of the higher harmonics decreases as the trigger angles become greater, and the fundamental frequency component of the total voltage is virtually 100% at trigger angles of 90 degrees.  
         [0058]      FIG. 8  shows the apparatus  8  as shown in  FIG. 2 , but with the control unit  4  being illustrated in more detail. As can be seen in  FIG. 8 , the control unit  4  comprises a zero crossing sensor  12  as well as a trigger unit  13 , with the zero crossing sensor  12  being connected to a current sensor  14 . For its part, the current sensor  14  comprises, for example, a current transformer, which is not shown but produces a secondary current which is proportional to the alternating current in the high-voltage line  9 , is sampled by a sampling unit (which is likewise not shown) in the current sensor  14  in order to obtain sample values, with the sample values then being converted by an analog/digital converter (which is not shown) to digital current values I on , and being supplied to the zero crossing sensor  12  via a connecting line  15 . When the zero crossing sensor  12  detects a change in the mathematical sign of the digital current values I on , it sends zero crossing pulses  16  to the trigger unit  13 . Receiving the zero crossing pulses  16 , the trigger unit  13  sends trigger pulses  17 , after a delay time α, to the thyristors  10  in the switching device  3 , which are then switched from an off position, in which the current carried via the parallel path  10  is interrupted, to an on position, in which current can flow via the parallel path  5 . The delay time by which the trigger unit  13  delays the emission of the trigger pulses  17  after receiving the zero crossing pulses  16  corresponds to the trigger angle α, which is supplied to the trigger unit  13  via a signal line  18 . The impedance can thus be controlled and thus the real power transmitted via the high-voltage line  9  can be regulated, by varying the trigger angle parameter α via the signal line  18 .  
         [0059]      FIG. 9  shows a more detailed illustration of the control unit  4  of the apparatus  8  as shown in  FIG. 8  and, in particular, the components of a trigger angle transmitter  19  for production of a trigger angle α which is suitable for controlling the apparatus  8 . The trigger angle transmitter  19  has a real-current sensor  20  and a real-power sensor  21 . In this case, the real-current sensor  20  is connected to the current sensor  14  and is designed to receive the digital current values  19  which are produced by the current sensor, as described above.  
         [0060]     The real-power sensor  21  is connected both to the current sensor  14  and to the voltage divider  22 , whose output signal V on  is proportional to the voltage on the high-voltage line  9  with respect to ground potential. The analog signals which are emitted from the voltage divider  22  are sampled and digitized by the real-power sensor  21 , and are converted with the digital current values of the voltage standard  14  to digital power values, which correspond to the power transmitted through the high-voltage line  9 .  
         [0061]     The trigger angle transmitter  19  can also be supplied via a nominal current line  23  and via a nominal power line  24  with control parameters, with the control parameters in each case being supplied to a matching unit  25  and  26 . The nominal current line  23  or the nominal power lines  24  is or are connected, for example, to a computer (which is not shown) or to a control console (which is not shown), so that a user is able to supply expedient control parameters to the control unit  4 . The matching units  25  and  26  are each connected to a proportional/integral regulator  27  or  28 , respectively, which is followed by a selection unit  29 . The selection unit  29  is used to select the measurement variable which will be monitored by comparison with a control parameter and will be used to control the apparatus  8 . In order to select the respective control parameter, the selection unit  29  is connected via a selection line  30  to, for example, the computer or the control console.  
         [0062]     The selection unit  29  is also connected to a nominal trigger angle line  31 . The trigger angle transmitter can thus be supplied directly with a nominal trigger angle, which can then be used to control the apparatus  8 , by expedient adjustment of the selection unit  29  via the selection line  30 .  
         [0063]     Furthermore, additional control signals can be introduced via further control lines  32  into the trigger angle transmitter  19  from the exterior, for example via the control computer. The additional control signals  32  can also be supplied to the open-loop and closed-loop control measures described above, and can be used for open-loop control purposes. By way of example a matching unit  33  which is connected downstream from the selection unit  29  is used for closed-loop control purposes. Further control signals in this sense are, for example, known control variables to increase the transient stability of the power supply network. Furthermore, it is possible to use the apparatus  8  to damp sub-synchronous resonances.  
         [0064]     At this point, it should be noted that  FIGS. 8 and 9  illustrate the control process only in a schematic form, and any limiters, signal filters and the like which are used however, covered by the scope of the invention, although they are not expressively mentioned.  
         [0065]     The output signal SIN from the matching unit  33  is supplied to a linearization unit  34 , which is provided in order to compensate for any non-linear behavior of the impedance of the apparatus  8  with respect to said output signal S IN .  
         [0066]      FIGS. 10 and 11  show the method of operation of the linearization unit  34 . In  FIG. 10 , the output signal α from the linearization unit  34  is plotted on the ordinate as a function of the output signal from the selection unit  29  or from the matching unit  33 , that is to say the input signal SIN to the linearization unit  34 . As can be seen, there is a non-linear relationship between these signals. In  FIG. 11 , the reactance of the apparatus  8  X SUM , normalized with respect to the maximum value X MAX , is plotted on the ordinate between 0 and 1, as a function of the input signal SIN to the linearization unit  34 , that is to say of the output signal from the selection unit  29 . As can be seen, the linearization unit  34  produces the desired proportionality between these two variables.  
         [0067]      FIG. 12  shows a further exemplary embodiment of an apparatus which has two control coils  2 , which are arranged connected in series and can each be bridged by a separate parallel path  5 . In this case, each parallel path is provided with a switching device  3 , whose thyristors  10 , which are connected in opposite senses, are controlled by a common control unit  4 . The common control of two switching devices  3  is illustrated in  FIG. 13 . As can be seen, the control unit  4  now has two trigger units  13   a  and  13   b,  which are each associated with a switching device  3   a  or  3   b,  respectively. In this case, the trigger units  13   a  and  13   b  are supplied with different trigger angles α a  and α b . For this purpose, the output signal from the selection unit  29  or from the matching unit  33  is split into two signals, and is respectively supplied to a linearization unit  34   a,  which is connected to the trigger unit  13   a,  and to a linearization unit  34   b,  which is provided in order to cause the trigger unit  13   b  to respond.  
         [0068]      FIG. 15   a  shows the characteristic of the linearization unit  34   a,  with the characteristic of the linearization unit  34   b  being plotted in  FIG. 15   b.  In this case, the trigger angles α a  and α b  which are emitted from the respective linearization unit  34   a  or  34   b  are plotted as a function of the output signal S IN  from the matching unit  33 .  
         [0069]      FIG. 14  shows an illustration, corresponding to  FIG. 7 , of the calculated amplitudes of the fundamental frequency and of the higher harmonic frequencies of the voltage V x  which is dropped across the apparatus  8 , normalized with respect to the total voltage V SUM  dropped in total across the apparatus  8 , as a function of the output signal S IN  from the matching unit  33  of a control unit  4  as shown in  FIG. 9 . As can be seen, there is a particularly advantageous linear relationship between the output signal S IN  from the matching unit  33  and the amplitude of the fundamental frequency.  
         [0070]      FIGS. 16 and 17  show a configuration, which is different to this, of the linearization units  34   a  and  34   b  as shown in  FIG. 13 . As can be seen in this case, the first switching device  3   a,  whose switching behavior is controlled by the linearization unit  34   a,  is operated as a high-speed switch. In the case of normalized output signals S IN  from the matching unit  33  below 0.5, the switching device  3   a  remains in an on position, in which the thyristors  10  which are connected in opposite senses in the switching device  3  allow current to flow through the parallel path  10   a.  In the case of output signals S IN  from the matching unit  33  of more than 0.5, the switching device  3   a  is switched to an off position, so that no current can flow via the parallel path  5   a.  This control process makes it possible to achieve a profile of the voltage amplitudes of the fundamental frequency components corresponding essentially to  FIG. 14 .  
         [0071]     According to one different exemplary embodiment, which is not illustrated, the switching device annotated  3   a  in  FIG. 12  is not formed by two thyristors which are connected in opposite senses but by a mechanical switch which interacts with the linearization unit  34   a,  whose characteristic corresponds to the characteristic shown in  FIG. 16 . The use of a mechanical switch instead of thyristors  10  results in cost advantages.  
         [0072]      FIG. 18  shows a further exemplary embodiment of the apparatus  8 . As can be seen, the apparatus  8  according to an embodiment, which is surrounded by a dashed line, comprises an inducted unit  35 , which is surrounded by a further dashed line, and a capacitive unit  36 , which is surrounded by a dashed line, which units  35  and  36  can be connected in series with one another by means of mechanical switches  37 . The inductive unit  35  corresponds very largely to the exemplary embodiment of the apparatus as shown in  FIG. 2 , but with a bridging path  38  being provided in order to bridge the inductive unit  35 . The capacitive unit  36  can likewise be bridged by means of a bridging path  38 .  
         [0073]     The capacitive unit  36  also has a capacitor  40  and a parallel path  41  in which a mechanical switch  42  is arranged. In addition, a varistor  43  and a spark discharge gap  44  are connected in parallel with the capacitor  40  and with the switch  42 . The capacitive unit  36  thus corresponds to a solution which is already known per se in order to compensate for the impedance of a high-voltage line  9 , with the exception that the switch  42  is opened and closed in order to bridge the capacitor  40  by means of the same control unit  4  which is also used to control the switching device  3  of the inductive unit  35 . Connection of the inductive unit  35  and capacitive unit  36  in series in these ways widens the control range of the apparatus  8 .  
         [0074]      FIG. 19  shows an exemplary embodiment of the apparatus  8  which largely corresponds to the exemplary embodiment shown in  FIG. 18 . In the exemplary embodiment shown in  FIG. 19 , the control unit  4  in the inductive unit  36  causes a switching device  3  which is arranged in series with the control coil  2  to respond, however. In this case, the switching device  3  is once again formed by two thyristors  10  which are connected in opposite senses. The reactance of the control coil  2  of the apparatus is connected by triggering of the thyristors  10 .  
         [0075]     The capacitive unit  36  as shown in  FIG. 19  correspond to an apparatus which is already known per se but which is used in this case, by means of the illustrated combination, to extend the application range of the apparatus  8  according to an embodiment, and at the same time to control the impedance of the high-voltage line  9 .  
         [0076]      FIG. 20  shows, schematically, the control unit  4  of the apparatus  8  shown in  FIG. 19 . As can be seen, in this case as well, two trigger units  13   a  and  13   b  are provided, and are connected to a respective switching device  3   a  and  3   b.  The trigger units  13   a  and  13   b  are once again connected to respectively associated linearization units  34   a  and  34   b,  which each have a different but expedient characteristic.  
         [0077]     One example of this circuitry for production of an apparatus  8  which operates continuously and linearly with respect to S IN  is shown in  FIGS. 21, 22  and  23 . This is based on the assumption that the reactance of the apparatus  8  is symmetrical, with positive values plotted on the abscissa representing inductive reactances, while negative abscissa values, in contrast, represent capacitive reactances.  FIG. 21  shows the reactance of the capacitive unit  36  X ASC  as a function of the output signal S IN  from the matching unit  33 .  FIG. 22  shows the reactance of the inductive unit  35  X TRIC  as a function of the output signal S IN  of the matching unit  33 . The resultant reactance X SUM  of the apparatus  8 , which comprises the total reactance of the series-connected inductive unit  35  and capacitive unit  36 , is illustrated in  FIG. 23  as a function of the output signal S IN  from the matching unit  33 . As can be seen, the relationship is linear.  
         [0078]      FIG. 24  shows a further exemplary embodiment of an apparatus  8  with an inductive unit  35  and a capacitive unit  36 , which are connected in series, with a filter unit  45  being connected in parallel with this series circuit. The filter unit  45  represents a so-called single-tuned filter which is designed, for example, to trigger a specific harmonic frequency component of the voltage which is dropped across the apparatus  8 .  
         [0079]     Finally, it should be noted once again that a filter unit  45  can be connected in parallel with the apparatus  8  according to an embodiment only when a capacitive unit  36  is also provided, in addition to an inductive unit  35 . In the case of one exemplary embodiment, which differs from this, an uncontrollable inductive unit, that is to say a coil, is provided instead of the capacitive unit  36 , connected in series with the control coil  2 , for this purpose, that is to say for connection of the filter unit  45 .