EP0026260B1 - Device for controlling the voltage between two conductors of an a.c. supply mains for a rapidly changing load - Google Patents

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 controller containing two antiparallel controllable valves between the conductors and a valve control, which within a Half voltage oscillation as a function of the voltage detected by means of a measuring element arranged on at least one conductor (voltage converter) emits an ignition pulse for the valve lying in the forward direction.

Such a device is known from US-A-3 435 248 and serves to keep the amplitude of the alternating voltage to be supplied to a consumer by an inverter via a filter. The filter output voltage is rectified and as soon as the rectified voltage reaches the breakdown voltage of a Zener diode, an ignition pulse is emitted with which an inductor arranged between the conductors is switched via an AC power controller. Ultimately, it is a comparison of the actual voltage amplitude with a predetermined target value.

In order to control the effective value of a pulsating controlled variable, according to DE-C-2 334 040 the difference between the square of a setpoint and the square of the actual value is fed to an integral controller. The regulator output voltage, generally a DC voltage, can then be used as a control voltage for an actuator, e.g. the control rate of a DC power supply or converter feeding a load. The subordinate tax rate is necessary because the integral controller output voltage only indicates after a completed half-period due to the simultaneous integration of the actual and target values (which are recorded by their squares as RMS values) whether the RMS actual value averaged over this half-period is equal to the corresponding mean target value, or whether the modulation of the actuator has to be changed for the next half period.

Another 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 occurring 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 considerable voltage fluctuations that can disturb other consumers. Similar irregular or regular drops in the voltage level of a supply network also occur with other consumers, e.g. Pulse power supplies for synchrotrons or converter drives in rolling mills. Because e.g. In the case of incandescent lamps which are also connected to the supply network, the human eye already perceives disturbances in the frequency range from 3 to 10 Hz as brightness fluctuations, which are caused by fluctuations in the supply voltage by 0.5% (so-called "flicker"), it is necessary to react suppress such consumers on the supply network or keep them constant.

In this device, a battery of capacitors is connected in parallel with the consumer connected to a three-phase supply network, which is dimensioned such that it can deliver as much reactive current as the furnace can absorb at maximum. 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 current control 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 (e.g. one phase and the neutral conductor or between two phases of a multiphase network) should therefore be kept within a predeterminable mean value within each half-oscillation. This mean need not be the arithmetic mean of the voltage, rather it may be advantageous to determine the mean for a particular function, e.g. a power to specify the voltage, e.g. to influence the rms voltage value. 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 the introduction, in which an integrator is connected downstream of the measuring element, the output signal of which is compared with a value corresponding to the predetermined target mean value, and the valve control is designed so that it generates the ignition pulse the time when the output signal reaches this value. If, for example, a constant arithmetic mean value is to be maintained for each voltage half-oscillation, an AC control valve is fired in each voltage half-wave as soon as the span voltage time area of this voltage half-wave reaches the predetermined target mean value.

For this purpose, the valve control preferably contains a limit detector 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 that 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, whose assigned valve can be conductive in the event of a positive half-oscillation, uses the difference between the integrator output variable and a positive setpoint as the target mean value of the voltage half-oscillation, and the sum of the integrator output variable and the setpoint (forming the difference with the negative one) for the other valve Setpoint). In order to ignite the valves whenever the integral of the voltage to be kept constant (ie the voltage time area f Udt) or a predetermined function of the voltage (eg ∫ | U | 1 ^a dt) reaches the target mean value, the valve control can also be built from 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.

In addition, 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 in time, 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 after the valves have been ignited, which in some cases leads to an undesired deviation from the target mean value. 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 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 timing of the corresponding valve is shifted so far; 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. However, fluctuations in the range of several seconds are 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 from 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 regulator, 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 a certain minimum current in the AC regulator and in the upstream choke coil is not exceeded, regardless of the control device.

For this purpose, the valves are fired by an additional ignition signal each time 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 to suppress 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 ± | 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 further development of the invention, these difficulties are avoided by arranging a rectifier in the valve control between the measuring element and the first integrator with a subsequent potentiator to form the function y = x ^a with x ≥ 0 and any a. A value corresponding to the target mean value is connected to the integrator output at the input of a limit value 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 integrator itself is after the delivery of an ignition pulse and before resettable at 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 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 entered into 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 can be reset approximately at the same time as the first 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 development, and FIGS. 8 and 9 illustrate the pulse diagrams 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 in front of which further consumers 5, e.g. Incandescent lamps 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 such that it can compensate for the reactive currents that occur at maximum load current. 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 it 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 have a positive variable M ^* applied to them, which corresponds to the nominal value of the voltage level, ie the nominal mean value of a half voltage oscillation.

At U> 0, the valve 8 of the AC power controller 7 is polarized in the forward direction. The size M is therefore applied negatively to the comparison point 14 associated with this valve. The difference signal is fed to a limit value detector 16, from the output signal of which 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 quantity M ^{* is added to} the corresponding comparison point 15. The comparison point 15 thus provides a target / actual value comparison between the voltage time area of the voltage U and the (negative) target value for the negative voltage half-oscillation. The size obtained is again used to ignite the valve 9 via a limit indicator 18 with a pulse shaper 19 connected downstream.

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 indicated by de control of the valve 9 regulated to the negative mean.

With this control, M ^{* is} specified as the target value for the voltage time area fU 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 specified as a setpoint for + ∫U ^a dt for the positive half-wave, -M ^* as a setpoint for - f 1 U 1 "dt for the negative half-wave. The voltage can be increased by suitable selection of the parameter a> 1 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 such 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 exemplary 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 ∫lb 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 *. Through 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 in such a way that the condition M ^* = dU ^a dt is actually maintained in each half-oscillation. For comparison with the setpoint M, the integral of the voltage U ^a - ∫ lb 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 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 are averaged to an approximately constant reactive current, so that a fixed relationship between the setpoint M ^* and the average current flowing through the AC power controller lb results. By specifying the setpoint lb * For the long-term average of this current, it is achieved that the target value for the half-vibration mean value, for example the effective voltage, continues 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 indicators 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 1 R, 1 S and 1 T, the three-phase load with 40, each between one phase and the star point or in pairs between two phases capacitor banks and upstream chokes with 50 and 60 and each also between one Phase and the neutral conductor or between two phases arranged AC power controller and the upstream chokes designated 70 and 30 respectively. To control the alternating current thyristors, the phase voltages are measured via measuring elements 11 R, 11 S, 11 T 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 indicators, pulse shapes, function transmitters, rectifiers, smoothing elements and controllers. These control units also receive the setpoint for the long-term average lb * The currents flowing through the AC power controller are specified via a common line 90.

The arrangement according to Figure 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 controller 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 based on the voltage U tapped by the voltage converter 11 is synchronized and its modulation is set by specifying a constant control vector such that a fixed period of time before the end of each half oscillation, an additional ignition signal L or M is applied to the ignition pulse F or G for the valve located in the respective half oscillation in the forward direction.

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, 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. By choosing a you can influence the quality of the flicker control.

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, together with the negative target mean value M ^{*, is} fed 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 when the integrator output = size 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 Figure 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 reset of 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 the positive edge of the additional ignition signal L if this edge lies before 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 Figure 8, a weak network usually follows several zero crossings in quick succession, difficulties can arise. Therefore, in this exemplary 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 the integrator z. B. to determine the negative voltage time areas, only the sections of the voltage curve are supplied that have a negative polarity. 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 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 set via a delay element 90, e.g. a second-order delay element, fed to a limit indicator 91. 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, 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 receive the pulse K for closing the switches 103 and 115, the additional ignition signals L, M taking into account the sign of U derived from a limit value indicator 97. For this purpose, the additional ignition signal L or M at the R to 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 Q 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 1), 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. A device for an a.c. supply network feeding a rapidly changing load regulating the voltage between two conductors (1, 2) to a predeterminable half- wave mean value (theoretical mean value), with an a.c. adjusting device (7) which contains two controllable rectifiers (8, 9) arranged between the conductors in anti-parallel fashion and with a rectifier control device (12 to 19) which, within a voltage half-cycle, in dependence upon the voltage (U) detected by means of a measuring element (11) arranged on at least one conductor, emits an ignition pulse for the rectifier located in the forwards direction, characterised in that the measuring element (11) is followed by an integrator (13) whose output signal is compared with a value which corresponds to the predetermined theoretical mean value (M^*), and that the rectifier control unit emits the ignition pulse at the time at which the output signal reaches this value (M^*).

2. A device as claimed in claim 1, characterised in that the rectifiers are each ignited by an additional ignition signal (L, M) when their blockage time exceeds a predetermined maximum time (a_o) (Fig. 8).

3. A device as claimed in claim 2, characterised in that the additional ignition signal (L, M) is formed by a network-synchronised control set (110) and the constant maximum blockage time is governed by a constant level control (a_o).

4. A device as claimed in one of the claims 1 to 3, characterised in that for each of the anti-parallel rectifiers (9, 10) there is a limit value indicator (16, 18) followed by a pulse shaper (17, 19), and that the difference between the output value of the first integrator (13) and a value (M^*) corresponding to the theoretical mean value for the positive voltage half- wave in the forwards direction of the assigned rectifier (9, 10) is superimposed upon each limit value indicator (16, 18), (Fig. 2).

5. A device as claimed in one of the claims 1 to 3, characterised in that there are provided a rectifier arranged following the first integrator and a limit value indicator followed by a pulse shaper, that the difference between the output value of the rectifier and a value (M^*) corresponding to the theoretical mean value of the voltage half-cycle is superimposed onto the limit value indicator, and that the anti-parallel rectifiers are alternately ignited by the output signals of the limit value indicator.

6. A device as claimed in one of the claims 1 to 5, characterised by a function forming device (12) which is connected preceding the first integrator (13) and whose output value (± y = ± |x|^a) is the voltage measured value raised to a power of more than 1 (Fig. 1).

7. A device as claimed in one of the claims 1 to 6, characterised by a choke coil (10) connected in series with the a.c. adjusting device (7) (Fig. 1).

8. A device as claimed in claim 7, characterised by a measuring element (20) for the current flowing through the a.c. adjusting device (7) and a subsequently connected second integrator (21) whose output value is negatively superimposed upon the voltage measured value (Fig. 2).

9. A device as claimed in one of the claims 1 to 3, characterised in that in the rectifier control unit between the measuring element (11) and the first integrator (105) there is arranged a rectifier (101) with an adjoining power-forming device (102), that a value (M^*) corresponding to the theoretical mean value is superimposed upon the integrator output at the input of a limit value indicator (107), that the output signal of the limit value indicator (107) is input into a pulse shaper (108) whose pulses are fed in a pulse distributor to that rectifier which is presently located in the current-conducting direction, and that the integrator (105) can be reset following the emission of an ignition pulse and preceding the next zero transition of the voltage (switch 104) (Fig. 6).

10. A device as claimed in claim 9, characterised in that the integrator (105) can be reset by an additional signal (H), in particular the additional ignition signal (L, M) as claimed in claim 2 or 3 (Fig. 6).

11. A device as claimed in claim 9 or 10, characterised in that the power-forming device is followed by a switch (103) which is held open when the polarity of the voltage half-cycle belonging to the last rectifier ignition is identical to the polarity of the instantaneous actual values (U) of the voltage (Fig. 6).

12. A device as claimed in one of the claims 9 to 11, characterised by a choke coil (10) which is connected in series with the a.c. adjusting device (7), by a measuring element (20) for the current flowing through the a.c. adjusting device (7), by a following second rectifier (114), and by a second integrator (117) which can be reset approximately simultaneously to the first integrator and whose input is connected to the output of the second rectifier via a further switch (115) which is operated in order to open and close simultaneously with the first switch (103) and whose output is additionally negatively superimposed upon the input of the first integrator (Fig. 7).

13. A device as claimed in claim 12, characterised in that the second integrator is set at zero simultaneously with the first integrator and remains at zero until the next zero transition of the voltage.

14. A device as claimed in one of the claims 1 to 13, characterised by a series arrangement composed of a rectifier (30) for the measured value of the current (Ib) flowing through the a.c. adjusting device, of a smoothing device (31) and a PI-regulator (32), where a theoretical value (lb*) of the current flowing through the a.c. adjusting device (7) averaged over a plurality of periods is additionally negatively superimposed upon the input of the regulator, and the regulator output values are fed to the limit value indicator as values which correspond to the theoretical mean value (Fig. 3, Fig. 7).

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