EP0026260A1 - Device for controlling the voltage between two conductors of an A.C. supply mains for a rapidly changing load - Google Patents

Abstract

An AC power controller 7 is arranged between the conductors, the valve control of which contains a voltage converter 11, preferably with a downstream function generator 12, an integrator 13 and at least one limit indicator 16, 18. The integrator output voltage and a setpoint M * for the voltage time area of the positive (in relation to the forward direction of the assigned valve) voltage half-wave or the integral of the function generator output variable are connected to the limit indicator. As soon as the voltage time area formed by the integrator exceeds the target value, the voltage is short-circuited by ignition of the valve lying in the forward direction.

Description

The invention relates to a device for keeping the voltage constant between two conductors of an alternating current supply network for a rapidly changing load on a predeterminable half-vibration mean value (target mean value), with an alternating current regulator between the conductors containing two antiparallel controllable valves and a valve control with at least one conductor arranged measuring element.

Such a device is known from "Siemens Research and Development Report", Volume 6 (1977), pages 29 to 38 and is used on a supply network for electric furnaces which are used in steel production for melting scrap. The arc that occurs in such a furnace between the electrodes and the melting material breaks off at irregular intervals when the material melts. The current intensity fluctuates irregularly between zero and short-circuit current. Since the supply network may have a negligible ohmic internal resistance, but it may have a considerable impedance, the reactive current components of the load fluctuations in particular cause significant voltage fluctuations that can disturb other consumers. eg pulse power supplies for synchrotrons or converter drives in rolling mills. Since, for example, with incandescent lamps also connected to the supply network, the human eye in the frequency range from 3 to 10 Hz brightness fluctuations, which are caused by fluctuations in the supply voltage by 0.5%, already perceived as disturbing (so-called "flicker"), it is necessary to suppress the feedback of such consumers on the supply network or to keep them constant.

In the known device, a battery of capacitors is connected in parallel to the consumer connected to a three-phase supply network, which is dimensioned so that it can deliver as much reactive current as the furnace can absorb. When the consumer reactive current is lower, the valves of a three-phase controller with a delta connection, which is also connected to the supply network, are ignited. The three-phase controller consists of a series connection of a choke between two phases and an AC controller formed by two antiparallel controllable valves. To ignite the three-phase control valves, these are controlled by a control system which contains measuring elements for both the current flowing through the furnace and the current flowing through the three-phase control elements and consists of a large number of computing units for coordinate transformation, vector identification and vector rotation. This regulation is complex.

The invention is based on the object of specifying a simpler and very fast-acting control device which keeps the voltage level between the individual conductors of a single-phase, three-phase or multi-phase supply network constant, at least for a short time. The voltage between two conductors (eg one phase and the neutral conductor or between two phases of a multiphase network) should therefore be kept within a preselectable mean value within each half oscillation. This mean doesn't have to be the arithmetic mean of the voltage, rather, it can be advantageous to specify the mean value for a specific function, for example a power, of the voltage in order to influence the effective voltage value, for example. This mean value can be kept constant, or it can fluctuate in the long term within such low frequencies that these fluctuations no longer have a disturbing effect.

According to the invention, the object is achieved by a device of the type mentioned, in which the voltage between the conductors is detected with the measuring element (voltage converter) and the valve control is designed such that an ignition pulse for igniting the valve lying in a voltage half-oscillation in the forward direction is emitted as soon as an integral formed from the actual values of the voltage half-oscillation by means of a first integrator arranged downstream of the measuring element reaches a value corresponding to the target mean value. So should e.g. If a constant arithmetic mean value is maintained for each voltage half-wave, an AC control valve is ignited in each voltage half-wave as soon as the voltage time area of this voltage half-wave reaches the predetermined target mean value.

For this purpose, the valve control preferably contains a limit indicator with a pulse generator as a triggering pulse generator downstream of the integrator downstream of the measuring element for each of the anti-parallel valves. The difference between the output variable of the integrator and a variable which corresponds to the target mean value for the voltage half-oscillation positive in the forward direction of the assigned valve is applied to each limit value detector; that is, the limit value detector, the associated valve of which can be conductive in the event of a positive half-oscillation, becomes the target half-value of the voltage half-oscillation The difference between the integrator output variable and a positive setpoint is applied and the other valve the sum of the integrator output variable and the setpoint (difference with the negative setpoint). In order to ignite the valves whenever the integral of the voltage to be kept constant (ie the voltage time area fudf) or a predetermined function of the voltage (eg ∫ | U | ^a dt) reaches the target mean value, the valve control can also be from a another integration and comparator circuit. For example, the output signal of an AC voltage integrator can be rectified and connected to a single limit indicator for comparison with the target mean value, the output signals of the limit indicator being used for alternating firing of the AC control valves.

A choke coil is advantageously connected in series with the AC power controller, which together with the line inductance forms a voltage divider. As a result, the current flowing through the AC controller is limited and the mains voltage is no longer short-circuited when the AC controller valves are ignited.

Furthermore, the integrator is advantageously preceded by a function generator, which generates an output variable ± | U | from the voltage measurement value U supplied ^a (a> 1) forms. As a result, the mean voltage value can be regulated to a value related to the effective value. In addition, the setpoint / actual value comparison takes place earlier, which enables corrective intervention by the AC power controller earlier.

If a choke coil is used, there is still a contribution to the voltage at the output of the integrator, even in some cases, after the valves have been ignited unwanted deviation from the target mean value leads. However, this can be corrected if the current through the AC power controller is detected by a measuring element which is followed by an integrator. The output variable of the integrator is applied to the voltage measurement. By suitably setting the integration time of this further integrator it can be achieved that the ignition point of the corresponding valve is shifted to such an extent that the voltage time area still occurring after the ignition is compensated for and the voltage time area occurring within a half oscillation corresponds to the setpoint.

The device according to the invention regulates short-term voltage changes, e.g. within a second, very quickly. Fluctuations in the range of several seconds are usually not caused by the load, are less disruptive and do not need to be compensated for. Therefore, the target mean value for the voltage can advantageously be tracked such that the device is always in the middle of its control range, seen over several periods. This is achieved by a series connection of a rectifier for the measured value of the current flowing through the AC power controller, a smoothing element and a PI controller. In addition to the output variable of the smoothing element, a setpoint value for the current flowing through the alternating current controller, averaged over several periods, is negatively applied to the input of the PI controller. The output variable of the controller is used as the variable corresponding to the target mean value and is fed to the limit indicator.

It is desirable to control the AC regulator valves so that, regardless of the control device, a certain minimum current in the AC regulator and in the upstream choke coil is not undershot. For this purpose, the valves are fired by an additional ignition signal if their blocking time would exceed a predetermined maximum time. This additional ignition signal can advantageously be formed by a network-synchronized tax rate, wherein the constant maximum blocking time can be predetermined by constant control of the tax rate.

For the device to work precisely, the lowest possible zero point drift of the first integrator connected upstream of the limit value detector is required. Usual measures for suppressing this drift are complex, mean a mostly undesired phase shift for the integrator and can impair the transient behavior of the integrator. When using separate branches, each with its own limit indicator for controlling the anti-parallel valves, there are often certain asymmetries in the two branches, which, like a drift of the integrator, can lead to an undesirable DC component in the network. Also the use of the function generator mentioned for the function t | U | ^a can lead to difficulties if, particularly in the case of weak networks, multiple zero crossings of the network voltage occur.

According to a development of the invention, these difficulties are avoided in that a rectifier with a subsequent potentiator for forming the function y = x ^a with x ≥ 0 and any a is arranged in the valve control between the measuring element and the first integrator. A value corresponding to the target mean value is applied to the integrator output at the input of a limit detector. The limit value detector output signal is input to a pulse shaper, the pulses of which are fed via a pulse distributor to the valve which is currently in current flow. The first The integrator itself can be reset each time an ignition pulse is given and before the next zero crossing of the voltage. In particular, the integrator can be reset by the additional ignition signal.

In this embodiment, therefore, only a single limit indicator is used for both anti-parallel valves. Furthermore, the integrator starts integrating again from zero with each voltage half-wave, so that a zero-point drift always shifts the valve ignitions in the same direction and cannot bring about a constant component.

This embodiment is preferably further developed in that a switch is arranged between the potentiator and the connection of the target mean value, which is always open when the polarity of the voltage half-wave belonging to the last valve ignition matches the instantaneous polarity of the voltage. So if e.g. If the valve located in the positive direction to influence the positive voltage half-wave has been ignited, the switch is opened until the voltage becomes negative. The integrator is now also set to zero. After the zero crossing, negative voltages are input to the integrator by closing the switch; However, if multiple zero crossings occur due to the often unavoidable network fluctuations, all positive voltages are still hidden by opening the switch.

In this embodiment, too, a choke coil is advantageously connected in series with the AC power controller. In this case too, the current through the AC power controller can be detected by means of a measuring element. A second rectifier and a second integrator are then connected downstream of the measuring element. This second integrator is about the same time as the first Resettable integrator. A further switch, which can be opened and closed simultaneously with the first switch, is arranged between the rectifier and the integrator. The output of this second integrator is connected to the input of the first integrator with a negative sign in addition to the output signal of the potentiator. The control of the switches and the distribution of the ignition pulses and the additional ignition signals to the valves can be achieved with a simple logic circuit. The second integrator can advantageously always be set to zero at the same time as the first integrator and can be kept at zero until the next zero crossing of the voltage.

The invention is explained in more detail with reference to 7 exemplary embodiments and 9 figures. FIGS. 1 to 3 show the principle of the invention using the example of a single-phase supply network, FIGS. 4 and 5 using the example of a three-phase supply network, FIGS. 6 and 7 show a further embodiment and its further development, and FIGS. 8 and 9 illustrate the pulse diagram and the construction of a logic circuit for the exemplary embodiments according to FIGS. 6 and 7.

In FIGS. 1 to 3, 1 and 2 denote the conductors of a single-phase AC network, the impedance of this supply network being indicated symbolically by a coil 3, while the conductor 2 is at ground potential. The load 4 is connected to the conductor, the rapidly changing impedance of which causes repercussions from which further consumers 5, for example incandescent lamps, are to be protected. Parallel to the load 4 there is usually a capacitor bank 5 made up of many capacitors with an upstream choke 6. The capacitor bank is dimensioned in such a way that it compensates for the reactive currents that occur at maximum load current can. This capacitor bank makes it possible to maintain a favorable power factor for the system, but it is not absolutely necessary for the operation of the device. Furthermore, an AC power controller 7 is arranged between the conductors 1 and 2, which consists of two anti-parallel thyristor valves 8 and 9. The advantageous coil 10 arranged in series with the alternating current regulator 7 should not be considered for the time being. A measuring element 11 is also provided. In this respect, Figure 1 corresponds to the known device mentioned at the outset for the case of a single-phase network.

In order to control the valves 8 and 9 with ignition pulses, the measuring element 11 detects the voltage U to be kept constant between the conductors and feeds them to the input of an integrator 13 via a function generator 12, which has likewise not been considered for the time being. An integrator is used, the zero point of which does not drift and which automatically adjusts the DC voltage components present at the output (seen over several periods). The output variable of the integrator 13 is fed to the comparison points 14 and 15, which are supplied with a positive variable M ^* , which corresponds to the nominal value of the voltage level, ie the nominal mean value of a voltage half-oscillation.

At U> 0, the valve 8 of the AC power controller 7 is polarized in the forward direction. The variable M ^* assigned to this valve is therefore negatively applied to the variable M ^* . The difference signal is fed to a limit value detector 16, from whose output signal an ignition pulse for the valve 8 is formed in the pulse shaper 17. For the ignition pulses of the valve 9, which can only be live when U <0, the positive variable M ^{* is added to} the corresponding reference junction 15. The comparison point 15 thus delivers a target / actual value ver equal between the voltage time area of voltage U and the (negative) setpoint for the negative voltage half-oscillation. The sum size obtained is in turn used via a limit detector 18 with a pulse shaper 19 connected downstream to ignite the valve 9.

To explain the mode of operation, a positive semi-oscillation is considered first. The integrator 13, which is initially at a negative initial value at the beginning of this half-wave, integrates the measured values (actual voltage values) U until the value M ^{* is} reached. Then the valve 8 is ignited via the limit indicator. As a result, the voltage is short-circuited as soon as the voltage time area has reached a value determined by M ^* within a half oscillation. In the negative half-oscillation, the valve 8 goes out and the negative half-oscillation is regulated to the negative mean value by corresponding control of the valve 9.

With this control, M ^{* is} specified as the setpoint for the voltage time area JU dt of a half oscillation. However, it is also possible to choose M ^* not directly as the half-vibration mean value, but rather as a mean value for another, predetermined function of the voltage U. In particular, M ^* can be used as a setpoint for + ∫U ^a dt for the positive half-wave, -M ^* as a setpoint for - ∫ | U | ^a dt for the negative half wave. By suitable selection of the parameter a> 1 1, the voltage can be regulated to a value related to the effective value. The potentiating function generator 12, which is connected upstream of the AC voltage integrator 13, is used for this purpose. This function generator 12 can be switched so that its output signal! | U | ^{a is} positive if its input signal is positive, or negative if the input signal is negative.

In order to reduce the distortion of the mains voltage and the high short-circuit currents, it is advantageous to insert the choke coil 10. This prevents short circuits in the mains voltage when the AC power controller 7 is ignited.

In the embodiment according to FIG. 2, the current Ib flowing through the alternating current controller is measured by means of a current transformer 20, the measured values of which are integrated in a second integrator 21. The output voltage proportional to the value ∫Ib dt is applied negatively to the measured values of the mains voltage measured via the measuring element 11 on a summing element 22. By using the choke coil 10, even after the voltage has been short-circuited, a mains voltage still occurs, which is detected by the first integrator 13 and would lead to the integral exceeding the predetermined value M ^* . By a suitable choice of the time constant T _{1 of} the integrator it can now be achieved that, at least in the steady state, the input variable for the integrator 13 is adjusted such that the condition M * = ∫U ^a dt is actually maintained in each half-oscillation. For comparison with the setpoint M ^* , the integral of the voltage U ^a- lIb dt is now used, whereby the ignition timing is brought forward to such an extent that the voltage still present after the ignition timing and integrated in the first integrator 13 is approximately compensated.

According to FIG. 3, this device can be expanded by a series circuit branching off at the output of the measuring element 20, comprising a rectifier 30, a smoothing element 31 (time constant T _{2 for} several seconds) and a PI regulator 32. The rectified and smoothed output voltage of the current measuring element 20 becomes compared at the input of the controller 32 with a target value for the corresponding longer-term mean current value. The controller The output signal is fed to the comparison points 15 and 16 instead of the input variable which corresponds directly to the short-term nominal voltage value M ^* . Seen over several periods, the reactive current fluctuations of the rapidly changing load 4 average to an approximately constant reactive current, so that there is a fixed relationship between the setpoint value M ^* and the average current Ib flowing through the alternating current controller. By specifying the setpoint Ib * For the long-term average of this current it is thus achieved that the target value for the half-vibration mean value, for example the effective voltage, follows longer-term fluctuations in the amplitude of the mains voltage. The effects of short-term load fluctuations on the mains voltage, which lead to the annoying flicker, are still compensated for by the rapid regulation via the integrator 13 and the limit value detectors 16 and 18.

Furthermore, as shown in FIG. 3, the AC power controller 7 can be connected to the supply network via a transformer 33. If its leakage inductance is higher than normal, it may a separate choke coil 10 can be dispensed with.

The device according to the invention, which has so far been described for the case of an AC network with two conductors, can also be applied analogously to an N-phase AC network. For this, e.g. one of the devices shown in FIGS. 1 to 3 can each be arranged between one of the conductors and the neutral conductor. For circuits without a neutral conductor, it is also possible to arrange such a device between two conductors which follow one another in the direction of rotation of the AC voltage. Such possibilities for a three-phase three-phase network are shown in FIGS. 4 (star connection) and 5 (delta connection).

The phases of this network are with 1R, 1S and 1T, the three-phase load with 40, each between a phase and the star point or in pairs between two phases, capacitor banks and upstream chokes with 50 and 60, and each also between a phase and the Neutral conductor or alternating current controller arranged in pairs between two phases and the chokes connected upstream of them are designated 70 and 30 respectively. To control the alternating current thyristors, the phase voltages are measured via measuring elements 11R, 11S, 11T and the currents flowing through the alternating current regulators 70 via measuring elements 20 ', 20 ", 20"'. The measured values are fed to the control units 80 ', 80 ", 80"', which are constructed in accordance with FIGS. 1 to 3 from alternating current integrators, limit value detectors, pulse formers, function transmitters, rectifiers, smoothing elements and controllers. These control units are also given the setpoint for the long-term mean value Ib ^* of the currents flowing through the AC controllers via a common line 90.

The arrangement according to FIG. 6, in which the elements 1 to 11 correspond to the elements in FIGS. 1 to 5, differs from the previous exemplary embodiments primarily in the use of a central logic circuit 100, which generates the ignition pulses F and G for the ignition devices 111 and 112 of the valves 8 and 9 of the AC controller 7 delivers. In order to maintain a minimum current in the alternating current regulator 7 and in the choke coil 10, a commercially available control set, in this case a two-pulse control set 110, is provided, which is synchronized with the voltage U tapped by the voltage converter 11 and whose modulation is set by presetting a constant control vector is set so that a fixed period of time before the end of each half oscillation an additional ignition signal L or M the ignition pulse F or G for the valve lying in the respective half oscillation in the forward direction is switched on.

In this preferred embodiment, the function generator 12 in FIGS. 1 to 3 corresponds to the rectifier 101 and the downstream potentiator 102, which supplies the function y = x ^a (x 0 0, a as desired, preferably a> 1). Such a circuit is known, for example, from Tietze-Schenk "semiconductor circuit technology", Berlin, Heidelberg, New York, 4th ed. 1978, page 212. Multipliers can also be used for integer values of a. The quality of the flicker control can be influenced by choosing a.

The output variable of the potentiator 102 is passed via a switch 103 to the first integrator 105, the reset input of which is indicated by the switch 104. The switches 103 and 104 can be kept closed by a "high" pulse of the control signals K and H. The integrator output variable is fed together with the negative target mean value M ^{* to} a summation point 106 at the input of a first limit value detector 107. An ignition pulse is input to the logic circuit 100 by means of a pulse shaper 108 each time the integrator output value exceeds the value M ^* . The logic circuit 100 distributes the ignition pulses A together with the additional ignition signals L, M to the ignition devices 111, 112.

The diagrams of the previously mentioned pulses A, F, G, H, K, L, M are shown in FIG. 8 together with the course of the voltage U. The control angle of the network-synchronized headset 110, which supplies the additional ignition signals L, M to limit the maximum blocking time of the AC control valves, is designated by α _o . Arrows 70 indicate the times at which the integrator output signal reaches the target mean value. The Resetting the switch 104 by means of the signal H and thus the preparation of the integrator for the formation of the voltage time area of a half oscillation (e.g. the negative half oscillation) takes place at the earliest with the first ignition belonging to the previous (in the example of the positive) voltage half wave, i.e. with the signal A or positive edge of the additional ignition signal L if this edge is earlier than the pulse A. The reset should ideally be completed when the new (in the example the negative) voltage half-wave begins after the voltage U has passed zero. However, since, as shown in FIG. 8, a weak network usually follows several zero crossings in quick succession, difficulties can arise. Therefore, in this embodiment, the reset pulses occur simultaneously with the positive edges of the additional ignition signals L and M.

In the voltage curve in FIG. 8, the input signal of the first integrator 105 for a = 1 is drawn in with the dashed line 71. Since, for example, to influence the negative voltage half-waves, the voltage time areas are not formed at the ideal time, which would be given by the zero crossing of the fundamental voltage, but rather the integrator begins to integrate from zero, an error could arise. However, this error is kept small in that, by closing and opening the switch 103, only those sections of the voltage profile which have a negative polarity are fed to the integrator, for example to determine the negative voltage time areas. In FIG. 8, the voltage time area determined by the integrator 105 is shown hatched, which is monitored at the limit detector 107 for exceeding the target mean value M ^* .

The further pulse diagrams shown in FIG. 8 relate to the example of a logic circuit 100 shown in FIG. 9 and the exemplary embodiment according to FIG. 7. For the reasons already explained in connection with FIG. 2, there is also a current measuring element 20 in the AC power supply line according to FIG and a second integrator 116 is provided. A second rectifier 114 is connected upstream of the integrator 116 via a switch 115, which can be opened by the pulses K like the switch 103. This second integrator 117, like the first integrator 105, should ideally be reset with the zero crossing of the fundamental voltage oscillation. However, it is simpler in terms of circuitry and results in practically no error if the reset switch 117 is closed with the start of the additional ignition signal and remains closed until the first zero crossing of the actual voltage curve, as represented by the corresponding closing pulse I.

The elements 30, 31 and 32 are structurally identical to the elements already described in FIG. 3 and fulfill the same task.

In the logic circuit according to FIG. 9, the voltage curve U is fed to a limit value detector 91 via a delay element 90, for example a delay element of the second order. The signal C at the output of this limit value detector provides information about the polarity of the last voltage half-wave for the time period in which both the ignition pulses A and the zero crossings of the actual voltage curve are to be expected. With this information, the ignition pulses A and the additional ignition signals L and M can be distributed to the lines to the ignition devices 111 and 112 of the corresponding AC control valves. For this purpose, the pulse C is fed to an AND gate 92 and negated to an AND gate 93. The Additional ignition signals L and N in turn are combined on an OR gate 94. The signal H is formed from the positive edges of the combined signal by means of a pulse shaper 95, which signal is combined with the ignition pulses A at an OR gate 96 to form the ignition pulse sequence E, which is applied to the other inputs of the AND gates 92 and 93 . The signal H is also led out of the logic circuit for actuating the reset switch 104 of the first integrator.

In order to obtain the pulse K for closing the switches 103 and 115, the additional ignition signals L, M are formed taking into account the sign of U derived from a limit indicator 97. For this purpose, the additional ignition signal L or M to the R -respectively. S - Input of an RS flip-flop 98 given, the output Q (pulse B) together with the negated limit value output signal to an AND gate 99 and the 5 output together with the limit signal output to the input of an AND gate 99 '. The negated outputs of these two AND gates become the signal of the K output via an AND gate 89.

In order to close the reset switch 117 of the second integrator with the positive edge of the additional ignition signal and to keep it closed until the first zero crossing (pulse I), the pulses E and K are fed to the corresponding I output via a memory 88.

Analogously to FIGS. 4 and 5, the circuit according to FIGS. 6 and 7 can also be applied to a multi-phase network.

Claims (14)

1.Device for regulating the voltage between two conductors (1, 2) of an alternating current supply network for a rapidly changing load to a predeterminable half-vibration mean value (target mean value), with an alternating current controller (7) containing two antiparallel controllable valves (8, 9) the conductors and a valve control with a measuring element (11) arranged on at least one conductor, characterized in that the voltage (U) is detected with the measuring element (11) and in that the valve control is designed such that in order to ignite the voltage oscillation in An ignition pulse is emitted in the direction of passage of the valve as soon as an integral formed from the actual values (U), the voltage half-oscillation by means of a first integrator (13) downstream of the measuring element (11), reaches a value (M ^* ) corresponding to the target mean value (Fig. 1). 2. Device according to claim 1, characterized in that the valves are each fired by an additional ignition signal (L, M) when their blocking time exceeds a predetermined maximum time (α _o ) (Fig. 8). 3. Apparatus according to claim 2, characterized in that the additional ignition signal (L, M) formed by a network-synchronized tax rate (110) and the constant maximum blocking time is predetermined by a constant modulation (α _o ) (Fig. 6). 4. Device according to one of claims 1 to 3, characterized in that for each of the anti-parallel valves (9, 10) a limit detector (16, 18) with a downstream pulse shaper (17, 19) before is seen and that each limit detector (16 or 18) the difference between the output variable of the first integrator (13) and a value (M ^* ) corresponding to the target mean value for the positive half-wave in the forward direction of the associated valve (9, 10) is (Fig. 2). 5. Device according to one of claims 1 to 3, characterized in that a rectifier downstream of the first integrator and a limit detector with a downstream pulse shaper is provided that the limit detector the difference between the output variable of the rectifier and a value corresponding to the target mean value of the voltage half-wave (M ^* ) is switched on and that the anti-parallel valves are alternately fired with the output signals of the limit detector. 6. Device according to one of claims 1 to 5, characterized by a function generator (12) connected upstream of the first integrator (13), the output variable (± y = ± | x | ^a ) of which is a power greater than 1 of the voltage measurement value (FIG. 1 ). 7. Device according to one of claims 1 to 6, characterized by a choke coil (10) connected in series with the AC power controller (7) (Fig. 1). 8. The device according to claim 7, characterized by a measuring element (20) for the current through the AC power controller (7) and a downstream second integrator (21), the output variable of which the voltage measurement value is negative (Figure 2). 9. Device according to one of claims 1 to 3, characterized in that in the valve control between the measuring element (11) and the first integrator (105) a rectifier (101) with subsequent potentiator (102) is arranged that the integrator output at the input a limit value detector (107) is connected to a value (M ^* ) corresponding to the target mean value, so that the output signal of the limit value detector (107) is input to a pulse shaper (108), the pulses of which are fed in a pulse distributor to the valve currently lying in the direction of current flow, and that the integrator (105) can be reset (switch 104) after a firing pulse has been given and before the next zero crossing of the voltage (FIG. 6). 10. The device according to claim 9, characterized in that the integrator (105) by an additional signal (H), in particular the additional ignition signal (L, M) according to claim 2 or 3, is reset (Fig. 6). 11. The device according to claim 9 or 10, characterized in that the potentiator is followed by a switch (103) which is kept open when the polarity of the voltage half-wave associated with the last valve ignition corresponds to the polarity of the current actual values (U) of the voltage ( Fig. 6). 12. Device according to one of claims 9 to 11, characterized by a choke coil (10) connected in series with the alternating current regulator (7), a measuring element (20) for the current through the alternating current regulator (7), a downstream second rectifier (114) and a second integrator (117) which can be reset approximately simultaneously with the first integrator, the input of which via a further one for opening and closing Simultaneously with the first switch (103) operated switch (115) is connected to the output of the second rectifier and the output of which is additionally connected negatively to the input of the first integrator (FIG. 7). 13. The apparatus according to claim 12, characterized in that the second integrator simultaneously with. the first integrator is set to zero and remains at zero until the next zero crossing of the voltage. 14. The device according to one of claims 1 to 13, characterized by a series connection of a rectifier (30) for the measured value of the current flowing through the AC current controller (Ib), a smoothing element (31) and a PI controller (32), the Input of the controller additionally a setpoint (Ib ^* ) for the current averaged over several periods and flowing through the alternating current controller (7), and the controller output variable is fed to the limit value detector as the variable corresponding to the setpoint average value (Fig. 3, Fig. 7).

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