JPS634430B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention converts a DC input current supplied via a DC reactor from a controllable DC power supply device such as a thyristor rectifier connected to a grid power supply into an AC output current and supplies the AC output current to a load. The present invention relates to a current control device for an inverter (so-called current source inverter), and particularly to improvement of an inverter output current waveform.

For example, various types of alternating current are used as a power source for electrochemical surfaces of metals, and one of them is a method using square wave alternating current. This rectangular wave current requires a strictly rectangular waveform. The electrolytic cell for surface treatment of metal serving as a load can be considered as a parallel circuit of a resistor R and a capacitor C as an equivalent circuit. This CR load generally has a relatively large time constant. A current source inverter as shown in FIG. 1 can be considered as a device for supplying a rectangular wave current to such a load. In addition to the main thyristors 1 to 4, the main circuit of the inverter 10 is provided with a commutation circuit consisting of an arc-extinguishing thyristor and a commutating capacitor (not shown), and the inverter 10 is configured as a self-excited inverter. For example, a thyristor converter 20 connected to a 50 Hz grid power supply is provided as a controllable DC power supply device that supplies a smoothed DC input current via a DC reactor 5. A differential voltage between the output voltage Vdc of the thyristor converter 20 and the DC terminal voltage Vdi of the inverter 10 is applied to the DC reactor 5. When the differential voltage applied to the DC reactor 5 is large, the current Id changes greatly. For electrochemical surface treatment of metals, it is required that fluctuations in current Id be kept below a specified slight value. However, for example, main thyristors 1 and 4
When commutation is performed from a state in which thyristors are conducting to a state in which main thyristors 2 and 3 are conducting, the equivalent circuit immediately before the commutation becomes as shown in Fig. 2A, whereas,
The equivalent circuit immediately after commutation is as shown in FIG. 2B. That is, immediately before commutation, the load voltage V _L had a polarity opposite to the thyristor converter output voltage Vdc, whereas immediately after commutation, the load voltage V L had a polarity opposite to the thyristor converter output voltage Vdc.
The polarity changes in the same direction as Vdc. Therefore, the voltage applied to the reactor 5 changes from Vdc−V _L to Vd+V _L. As a result, the DC input current Id rises as shown in Figure 3A immediately after commutation, and then attenuates due to the time constant of the circuit passing through the load and reactor 5, but it takes several tens of milliseconds for the current to settle. vinegar. Therefore, the AC output current of the inverter deviates from a rectangular wave and has a rounded waveform as shown in FIG. 3B. The effect of suppressing the change in the DC current Id depends on the inductance of the DC reactor 5 and the response of the current regulator acting on the thyristor converter 20, but since there is a limit to the response of the current regulator, the DC current Id An extremely large DC reactor is required to suppress the variation in the required tolerance.

The purpose of the present invention is to improve the output current waveform of a current source inverter that supplies power to a load containing an electromotive force or a capacitive load having a relatively large time constant, that is, to improve commutation operation even when using a relatively small DC reactor. An object of the present invention is to provide a current control device for an inverter that can sufficiently suppress changes in direct current caused by.

The inverter current control device according to the present invention that achieves this object is characterized by: current detection means for detecting the inverter DC input current; means for setting a target value for the inverter DC input current; current adjustment that compares the detected inverter DC input current with a target value set by the setting means and controls the output voltage of the controllable DC power supply so that the inverter DC input current matches the target value; and voltage compensation means for forming a signal containing an amount equivalent to a change in the DC input voltage of the inverter, superimposing this signal on the output signal of the current control means, and applying the signal to the controllable DC power supply device. There are some places.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 4 is a block diagram showing an embodiment of the present invention, in which a self-excited inverter 10, a thyristor converter 2
0, DC reactor, load equivalent circuit C and R are the first
It may have the same configuration as the circuit shown in the figure. The frequency of the inverter 10 is set by a setter 11. Firing pulses are supplied to the thyristors in the self-exciting inverter 10 by a pulse distributor 13 which receives the output pulses of a voltage/frequency converter 12 which generates pulses with a frequency proportional to the input voltage from the setter 11. The actual value of the inverter DC input current Id is detected by a current detector 21 provided on the AC side of the thyristor rectifier. The actual current value detected by the current detector 21 is compared with the current setpoint value from the current setting device 23 in the input circuit of the current regulator 22 . The current regulator 22 serves to control the output voltage of the thyristor converter 20 via the starting angle regulator 24 so that the actual current value corresponds to the current setpoint value. However, it has already been shown in Figures 2 and 3.
As explained in the figure, it is impossible to make the response of the current control system including the current regulator 22 fast enough to compensate for the change in the inverter DC terminal voltage Vd _I due to commutation operation, and the DC reactor 5 Unless the current waveform is made extremely large, it is unavoidable that the current waveforms will become as shown in FIGS. 3A and 3B. In order to improve this, according to the invention a voltage compensation circuit 25 is inserted between the current regulator 22 and the firing angle regulator 24. This voltage compensation circuit 2
5, the voltage detector 1 is connected to the output signal of the current regulator 22.
Inverter DC terminal voltage detected by 4
A signal including a change in Vd _I is superimposed.

FIG. 5 shows an example of the voltage compensation circuit 25. The filter 251 to which the inverter DC terminal voltage Vd _I detected by the voltage detector 14 is guided is a first-order lag element having a time constant larger than the inverter period T _I , and is a first-order lag element that takes the inverter DC terminal voltage Vd _I to a steady value. Outputs the corresponding output voltage. Additional points 252
Now, the output voltage of the voltage detector 14 and the filter 251
A difference between the output voltage and the output voltage is formed. This difference corresponds to a change in the inverter DC terminal voltage Vd _I , and is added to the output voltage of the current regulator 22 at the addition point 253. The added voltage is applied to the function generator 254.
is input. The output voltage of function generator 254 becomes the control input voltage for firing angle regulator 24 . The function generator 254 serves to compensate for non-linearities between the control input voltage Vd of the firing angle regulator 24 and the output voltage Vdc of the thyristor converter and may be optional.

In the voltage compensation circuit 26 shown in FIG. 6 as a modification of the voltage compensation circuit 25 shown in FIG. 5, the filter 251 in FIG. It is being Switches S ₁ and S 2 serving as samplers for _{each sample and hold circuit 251A and 251B, and switches S 3 and} S ₄ that alternately connect the output terminals of both sample and hold circuits to the summing point 252 are electronic switches such as FETs.
Switches S ₁ and S ₄ are turned on by turning on the main thyristors 1 and 4, and switches S ₂ and S ₃ are turned on by turning on the main thyristors 1 and 4.
It is turned on by the ON command of 3. Other configurations are 5th
This is the same as the embodiment shown in the figure.

FIG. 7 shows an example of the time course of the on/off command signals for the main thyristors 1, 4 and 2, 3 and the signal voltages at each point in the circuit in FIG. 6. When the DC input current Id, which was flowing in the positive direction to the load circuit via the main thyristors 1 and 4, is commutated so that it flows in the negative direction to the load via the main thyristors 2 and 3, or vice versa. As can be seen from the point signal voltage waveform in FIG. 7, the DC terminal voltage Vd _I of the inverter 10 opposes the output voltage Vdc of the thyristor converter 20 in the opposite direction with a value almost equal to it immediately before commutation. However, immediately after commutation, the thyristor converter output voltage Vdc assumes a forward polarity with a value almost equal to that, and then slowly reverses again according to the load time constant. Back to polarity. Such fluctuations in the DC terminal voltage Vd _I of the inverter 10 due to commutation cause the DC input current Id to fluctuate as described above. In the voltage compensation circuit 25 provided to compensate for this, sample and hold circuits 251A and 251B alternately track the inverter output voltage each time, hold the value immediately before commutation, and alternately transmit it to the addition point. As a result, the steady value (≒Vdc) of the inverter DC voltage Vd _I as shown as the point signal voltage waveform shown in Fig. 7 is detected,
At the summing point 252, a difference between the point signal voltage and the point signal voltage is formed, resulting in a signal voltage corresponding to the change in the inverter DC voltage Vd _I as shown as the point signal voltage. A signal voltage corresponding to this change is applied to the current regulator 22 at the addition point 253.
The output signal voltage (point signal voltage) is superimposed. The output signal voltage (point signal voltage) of the summing point 253 has a waveform corresponding to the inverter DC voltage Vd _I , and thanks to the nonlinear compensation of the function generator 254, the thyristor converter 10 corresponds to the inverter DC voltage Vd _I. Can generate output voltage Vdc. In other words, without waiting for the response of the current regulator 22, the thyristor converter output voltage Vdc is controlled in a feed-forward manner according to fluctuations in the inverter DC voltage Vd _I , and the voltage between the terminals of the DC reactor 5 Vdc - Vd _I This is done to prevent changes in the DC current Id by keeping it almost zero. Therefore, the DC reactor 5 can be made smaller.

The above-described current compensation effect can be completed by providing the function generator 254 for nonlinear compensation, but this can be omitted, and in this case, the voltage compensation effect will be somewhat incomplete.

The signal voltage waveforms at points ~ in the embodiment of FIG. 5 shown above also correspond to the waveform diagram of FIG. 7. However, since the filter 251 outputs an average value, the point signal voltage will be smaller than in the case of FIG. The output voltage level of the regulator decreases and eventually the function generator 254
The input voltages of will be equal. This means that the point signal voltage superimposed on the output voltage of the current regulator 22 may include a DC component. Therefore, the fifth
It is also possible to omit the filter section and addition point 252 in the figures and FIG. 6, and to directly lead the detected value of the inverter DC voltage Vd _I to the adder 253. The important point is that the point signal voltage includes an amount equivalent to the change in voltage Vd _I.

8 and 9 show other possible variations of the voltage compensation circuit 25. In each case, a function generator 254 similar to before is provided, at the summing point 25 on the input side of this function generator 254,
A voltage corresponding to a change in the inverter output voltage having a point signal waveform shown in FIG. 7 is superimposed on the output voltage of the current regulator 22. In the embodiment shown in FIG. 8, the current regulator 22 is turned on by the switch _S1 or _S2 , which is turned on by the ON command of the main thyristors 1, 4 or 2, 3, to the incomplete differentiation circuit 256A or 256B. An output voltage is input, and the output of each differentiating circuit is led to an adder 253 via coefficient multipliers 257A and 257B. Current regulator 2
Since the output voltage of No. 2 corresponds to the thyristor converter output voltage Vdc, by making the time constants of both differentiating circuits match the time constants of changes in the load voltage and giving an appropriate gain K by both coefficient multipliers, A signal corresponding to the change in the inverter DC voltage Vd _I can be generated. Although the compensation accuracy will be slightly degraded, if the variation range of the load circuit resistance R is narrow and therefore the variation range of the load voltage is narrow, the differential circuit 256A is connected to the differential circuit 256A via the switches S ₁ and S ₂ as shown in FIG. ,2
The voltage input to 56B can also be appropriately set in advance by setting device 257.

In addition to the above-mentioned modifications, various modifications can be made without departing from the technical idea of the present invention.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an example of the main circuit configuration of an inverter to which the control device according to the present invention is applied, and Figs. 2 and 3 are equivalent diagrams immediately before and after commutation to explain problems in conventional control devices. A circuit diagram and a current waveform diagram; FIG. 4 is a block diagram showing a schematic configuration of a device for implementing the control device according to the present invention; FIGS. 5 and 6 show mutually different implementations of main parts of the device in FIG. FIG. 7 is an operational waveform diagram for the embodiment shown in FIG. 6, and FIGS. 8 and 9 are block diagrams showing different modifications of the above-mentioned main parts. 1 to 4... Main thyristor, 10... Inverter, 20... Thyristor converter, 22... Current regulator, 24... Firing angle regulator, 25... Voltage compensation circuit, 14... Inverter DC terminal voltage Detector,
251... Filter, 252, 253... Addition point, 254... Function generator, 251A, 251B
...Sample hold circuit, 256A, 256B
...differentiation circuit, 257A, 257B...coefficient unit.

Claims (1)

[Claims] 1. Converting a DC input current supplied from a controllable DC power supply via a DC reactor into a rectangular wave AC output current to generate a load including an electromotive force or a capacitive type having a relatively large time constant. In an inverter that supplies power to a load, a current detection means 21 for detecting an inverter DC input current, a means 23 for setting a target value for the inverter DC input current, and an inverter DC input current detected by the current detection means. current adjusting means 22 for controlling the controllable DC power supply device so that the inverter DC input current matches the target value by comparing the current to the target value set by the setting means; An inverter comprising: voltage compensation means 25 for forming a signal including the amount of change, superimposing this signal on the output signal of the current control means and applying it to the controllable DC power supply device. Current control device.