JPH11146657A - Power converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power converter which has little distortion in an output AC voltage waveform and has high efficiency and is economic. SOLUTION: The output of a converter section 11 which is switched at low frequencies and that of a converter section 12 which is switched at high frequencies are combined by a transformer 6 for a converter, and power is transmitted between a DC power supply 7 and an AC power system. In this power converter, the current detected by a current transformer 9 is fed back to an AC current controller 80 through adders 84, 85 to eliminate the distortion in waveform. By this method, most part of power conversion is conducted by the converter section 11 which operates at low frequencies and the converter section 12 which serves as an active filter in high frequency PWM converts only the higher harmonic wave components of the power. Therefore, a small-size and low-cost power converter which has little distortion in output voltage can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power converter for converting DC power into AC power, and more particularly to a power converter suitable for applications requiring a large capacity and a low output AC voltage. Related to the device.

[0002]

2. Description of the Related Art In recent years, a voltage-type self-excited semiconductor inverter device using a self-extinguishing type semiconductor switching element has been widely used. However, this inverter device, that is, a power conversion device for converting DC power into AC power, has been widely used. However, since the DC voltage is converted into an AC voltage by a switching operation, the output voltage is distorted. Therefore, in order to reduce this distortion, a method of increasing the number of PWM (pulse width modulation) control pulses per unit time, that is, a method of increasing the switching frequency and a method of multiplexing the switching converter have been conventionally used. Used.

However, the method of increasing the switching frequency is uneconomical because a semiconductor switching element that can operate even at a high frequency must be used.
Losses also increase. On the other hand, in a method that can cope without increasing the switching frequency of the switching element in each conversion unit by multiplexing, the number of multiplexes must be considerably increased, which complicates the configuration and degrades the control response characteristics. There is a risk of doing so.

Therefore, in order to eliminate these drawbacks, for example, in the National Institute of Electrical and Industrial Engineers of Japan in 1992, No. 4
As seen in "Mid-speed pulse control method for magnetic levitation railway multiplex inverter", a conversion unit with a small number of pulses, that is, a low switching frequency, and a conversion unit with a large number of pulses, that is, a high switching frequency. Have been proposed.

[0005]

The above-mentioned prior art does not take into account both the maintenance of economy by simplification of the configuration and the maintenance of good distortion characteristics. There was a problem that it was difficult to provide the device.

That is, in the prior art, the system is switched according to the operating conditions by combining the same converters, which is not always economical and operates only in accordance with a command value. Therefore, there is a problem of waveform distortion generation.

An object of the present invention is to provide a high-efficiency and economical power conversion device having a small distortion in an output AC voltage waveform.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to convert DC power into AC power using two types of converters, a low-frequency converter having a low switching frequency and a high-frequency converter having a high switching frequency. This is achieved by performing feedback control of the two-system converters based on the AC power.

At this time, according to the present invention, the switching frequency of the low-frequency conversion unit is set to a frequency equal to the fundamental frequency of the AC power, or the combined capacitance of the high-frequency conversion unit is set to 1 to 2 times the capacitance per low-frequency conversion unit. It is possible to make the combined capacity of the high frequency conversion unit substantially equal to the combined capacity of the low frequency conversion unit.

Since the high-frequency conversion section works to cancel the harmonics generated in the low-frequency conversion section, most of the switching frequency in the conversion section can be reduced to a low frequency without causing distortion. Works so as to cancel the remaining distortion, so that the distortion of the AC controlled object caused by various factors can be suppressed.

When the switching frequency of the low-frequency conversion unit is made equal to the fundamental frequency of the AC power, the operation period of the low-frequency conversion unit becomes long, so that the capacity performance can be maximized.

The combined capacitance of the high frequency conversion unit is changed to the low frequency conversion unit 1
When the capacitance per unit is set to 1 to 2 times, the voltage change at the time of the switching operation of the low frequency converter can be absorbed by the high frequency converter.

When there are a plurality of low frequency converters, the switching operation of two or more low frequency converters is prevented from being performed at the same time, so that the voltage change at the time of switching of the low frequency converters can be reduced. Can be absorbed.

If the number of low-frequency converters is, for example, four, the combined capacity of the high-frequency converters is less than half of the total capacity of the low-frequency converters. It is possible to reduce the size of the power conversion device and thereby obtain an economical and low-loss power conversion device.

When the combined capacitance of the high-frequency converter and the combined capacitance of the low-frequency converter are substantially equal, the combined output voltage of the low-frequency converter is canceled by the combined output voltage of the high-frequency converter to obtain an AC output. Since the voltage can be instantaneously reduced to zero, it is possible to effectively suppress overcurrent and the like at the time of an accident.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a power converter according to the present invention will be described in detail with reference to the illustrated embodiments. FIG.
Is an embodiment of the present invention, and in the figure, the power conversion device according to this embodiment includes two types of conversion units, a low-frequency conversion unit 11 and a high-frequency conversion unit 12.
1 AC side A and the AC side B of the high frequency converter 12 are coupled to two sets of DC side windings of the transformer 6 for the converter, and the AC side winding of the transformer 6 for the converter is used to connect an AC power system or load
(Not shown).

The low-frequency converter 11 and the high-frequency converter 12 are controlled by a switching signal from the control device 8 to perform DC and AC power conversion between the DC power supply 7 and the AC power system. I have.

Here, the DC side winding is the one connected to the converter of the converter transformer 6, and the AC side winding is also connected to the AC power system. Is the winding that is used.

The low-frequency converter 11 and the high-frequency converter 12 are composed of semiconductor elements connected in a single-phase bridge. First, the low-frequency converter 11 is a GTO (gate turn-off thyristor) serving as an upper arm. 31, free wheeling diode 51, and GTO to be the lower arm
32 and two arm pairs 21 and 22 each including a free wheeling diode 52.

Next, the high-frequency conversion section 12 includes an IGBT (insulated gate bipolar transistor) 41 serving as an upper arm.
And the free wheeling diode 53, and the IGBT 42 and the free wheeling diode 5 serving as the lower arm.
4 comprises two pairs of arms.

Here, as is well known to those skilled in the art, GTO has a large-capacity element, but at present the switching frequency is low. On the other hand, the IGBT can perform high-speed switching, but at present, only an element having a smaller capacity than the GTO can be obtained. Thus, in the embodiment of FIG. 1, the arm pair of the low-frequency conversion unit 11 uses a GTO, and the arm pair of the high-frequency conversion unit 12 uses an IGBT.

In the present invention, these semiconductor switching elements are not necessarily limited to the GTO and the IGBT, but all the arms may be constituted by the GTO or the STO.
Another semiconductor switching device such as an I-thyristor may be used.

Next, the power conversion operation by the low frequency converter 11 and the high frequency converter 12 will be described. Now
Assuming that the required voltage waveform on the AC side is a required AC voltage having a sine wave shape shown in FIG. 2A, at this time, first, the low-frequency converter 11 is switched at the same frequency as this voltage waveform. The voltage at the point A on the AC side in FIG.
It becomes a rectangular wave shape as shown in (b).

Then, as shown in FIG. 2C, the difference between the required AC voltage shown in FIG. 2A and the voltage at point A shown in FIG. A subtraction waveform is obtained. by the way,
As shown in FIG. 1, the converter transformer 6 is configured such that the voltage at the point A and the voltage at the point B are added by two sets of DC side windings. Therefore, if the high-frequency converter 12 is controlled so that the voltage at the point B on the AC side has the waveform shown in FIG. 2D, the waveform shown in FIG.
The waveform shown in (d) is added by the transformer for converter 6 and FIG.
The required AC voltage having a sine waveform shown in (a) is obtained from the AC side winding.

For this purpose, the voltage of the waveform shown in FIG. 2D may be used as a modulation signal, and the arm pair of the high-frequency conversion unit 12 may be PWM-controlled by a high-frequency switching signal.

This control is executed by the control device 8 shown in FIG. First, the current control device 80 generates the required AC voltage a shown in FIG. 2A based on the current set value inputted through the adder 85, and outputs the required AC voltage a to the comparator 82 of the pulse control device 81. To the container 84. Therefore, first, the comparator 82 determines whether the required AC voltage a is positive or negative, and generates a rectangular wave b shown in FIG. 2B having pulse widths of a period in which the required AC voltage a is positive and a period in which the required AC voltage a is negative. I do.

Next, the adder 84 receives the required AC voltage a supplied from the current controller 80 and the rectangular wave b supplied from the comparator 82, and subtracts the rectangular wave b from the required AC voltage a. And outputs a subtraction waveform d.
The PWM pulse generator 83 receives the subtracted waveform d from the adder 84, and uses this as a modulation signal to PWM modulate a carrier signal of a predetermined frequency to generate a PWM modulated pulse signal p.

The rectangular wave b generated by the comparator 82 is supplied to the arm pairs 21 and 22 of the low-frequency conversion unit 11, and the upper arm and the lower arm of each arm pair are supplied according to the polarity of the rectangular wave b. They are switched alternately. As a result,
On the AC side A of the low-frequency converter 11, an AC voltage having a rectangular waveform shown in FIG. 2B is generated with a voltage value determined by the voltage of the DC power supply 7.

Each pair of arms of the high-frequency converter 12 is supplied with a PWM modulated pulse signal p generated by the PWM pulse generator 83, and the upper and lower arms of each pair of arms are alternately switched by the pulse signal p. Is switched to As a result, an AC voltage having a predetermined voltage value and the same waveform as the subtraction waveform d shown in FIG. Therefore, according to this embodiment, a voltage having a sine waveform corresponding to the required AC voltage can be easily obtained from the transformer for converter 6.

In this embodiment, the low-frequency converter 1
The comparator 82 generates the square wave b used for the control of
However, instead of this, a more accurate pulse generation method of detecting the phase of the signal wave and generating a pulse may be used.

Here, the required AC voltage a input to the pulse control device 81 is supplied from the current control device 80. The current control device 80 is connected to an external higher-order A required AC voltage a is generated by a current or voltage set value cs given from the control device of the above. At this time, an AC current or voltage corresponding to the current or voltage set value cs is obtained from the converter transformer 4. A required AC voltage a of a value necessary for the above is generated.

However, in this state, since the feedforward control is performed, the AC output is distorted due to various factors. In view of this, the present invention is characterized in that an AC current or an AC voltage to be controlled is detected and feedback control is performed. The form will be described.

In FIG. 1, reference numeral 9 denotes a current transformer, which detects a current flowing in the AC side winding of the transformer 6 for the converter, supplies the detected current value ct to the adder 85, and sets a current set value. A deviation ε is obtained by matching with cs, and the deviation ε is input to the current control device 80 to generate a required AC voltage a.

If there is a distortion in the current flowing from the AC side winding of the converter transformer 6 to the AC power system, the current detection value ct
Does not match the current set value cs, and a deviation ε appears in the output of the adder 85. The waveform of the required AC voltage a is corrected by the current controller 80 in accordance with the deviation ε. The feedback control in the direction of making ε zero works, so that the alternating current flowing through the transformer for converter 6 has a good waveform with sufficiently suppressed distortion.

It is to be noted that the control of the alternating current may be performed not only by the feedback control as in the above-described embodiment but also by adding another control method so as to obtain better control characteristics. Further, in this embodiment, the present invention is implemented as a single-phase power converter, but when implemented as a three-phase power converter, a method such as two-axis conversion and control may be used. It is possible to control the AC voltage in the same manner.

Here, the switching frequency of the low-frequency converter 11 does not necessarily have to be the same as the frequency of the AC voltage. However, by setting the same frequency, the utilization rate of the converter can be maximized, and There is an advantage that loss can be minimized.

In the above embodiment, at the time C or D in the waveform of FIG. 2D, it is necessary to cause the high-frequency converter 12 to output a reverse voltage that only cancels the voltage of the low-frequency converter 11. However, in the case of the PWM control as in the above embodiment, there is a limitation of the modulation degree,
When the two sets of DC-side windings of the transformer 6 for the conversion device are the same number of turns, it is practically almost impossible to output such a steep voltage from the high-frequency converter 12. However, in practice, there is not much problem.

If necessary, the number of turns of the DC side winding of the converter transformer 6 is changed, and the current / voltage specifications of the low-frequency converter 11 and the high-frequency converter 12 are adjusted. If the voltage equal to or larger than the AC voltage change width is output from the high-frequency converter 12, this problem can be solved and sufficient cancellation can be obtained.

If necessary, the DC power supply 7 may be divided into two systems, and the high-frequency converter 12 may be operated with a DC voltage higher than the voltage of the DC power supply of the low-frequency converter 11. However, in this case, it is necessary to adjust the power capacity of each converter. Therefore, it is appropriate that the combined capacity of the high-frequency conversion unit 12 is set to be 1 to 2 times the capacity of one low-frequency conversion unit 11.

The reason why the combined capacity is used here is that the total capacity may not always be the capacity during operation when the operation phases of a plurality of converters are shifted due to the configuration of the conversion unit. The details will be described in another embodiment described later.

On the other hand, when the power capacity of the low-frequency converter 11 and the power capacity of the high-frequency converter 12 are substantially the same, the output voltage of the low-frequency converter 11 is canceled by the output voltage of the high-frequency converter 12. The output AC voltage can be reduced to zero. For this reason, there is an effect that overcurrent can be suppressed at the time of an accident in the AC / AC circuit. As described above, for this purpose, it is necessary to make the combined capacitance of the low-frequency converter and the combined capacitance of the high-frequency converter substantially equal.

FIG. 5 shows an embodiment of the present invention in which the pulse control device 81 in the embodiment of FIG. 1 is modified. In the embodiment of FIG. 1, the subtraction waveform d is obtained by subtracting the rectangular wave b supplied from the comparator 82 from the required AC voltage a. In the embodiment of FIG. 5, an instrument transformer 10 for detecting the voltage of the low-frequency converter is added, and the subtracted waveform d is converted from the required AC voltage a to the actual voltage of the low-frequency converter detected by the instrument transformer 10. v is calculated by subtracting it.

Thus, it is clear that better control can be achieved. This is directly used as a PWM pulse generator 83
However, in the present embodiment, in order to obtain a better waveform, a new AC voltage controller 86 is added, and the AC voltage controller 86 is input to the PWM pulse generator 83 via this. The AC voltage controller 83 sets the subtracted voltage d to 0.
As a result, the second feedback control is performed at a high speed, and an even better output can be obtained.

Next, another embodiment of the present invention will be described. In the embodiment described here, in the configuration of FIG. 1, the switching phases of the two pairs of arms 21 and 22 of the low-frequency conversion unit 11 are changed as shown in FIGS. 3 (b-1) and 3 (b-2). , 180 ° -φ before and after. For this purpose, a phase control circuit may be provided at the output of the comparator 82 in FIG. 1 to control the phases of the rectangular waves b supplied to the arm pair 21 and the arm pair 22, respectively.

As a result, the output voltage generated on the AC side A of the low-frequency converter 11 is as shown in FIG. 3 (b-3), and the fundamental wave voltage is as shown in FIG. Co compared to
The power capacity is reduced by a factor of sφ, and the power capacity is also reduced by a factor of cosφ. On the other hand, the required AC voltage shown in FIG.
The subtraction waveform which is the difference between the voltages at the point A shown in FIG. 3 (b-3) is as shown in FIG. 3 (d), and the change in amplitude is considerably reduced as compared with FIG. 2 (d). .

Therefore, according to this embodiment, without changing the number of turns of the DC side winding of the transformer 6 for the converter,
The subtraction waveform shown in FIG. 3D can be easily output from the high-frequency conversion unit 12, and as a result, waveform distortion can be sufficiently suppressed.

In this embodiment, as is apparent from the comparison between the waveform chart of FIG. 2 (b) and the waveform chart of FIG. 3 (b-3), the occurrence of harmonics in the low-frequency converter 11 is small. So you can
The output AC waveform can be more easily made into a sine wave.

Next, still another embodiment of the present invention is shown in FIG. In the embodiment shown in FIG. 4, the present invention is embodied as a three-phase AC power converter, in which three-phase converters 14, 15, 16, and 17 operating at low frequency are multiplexed in four, and high-frequency converters are used. The three-phase converters 18 and 19 to be operated are 2
It is multiplexed, and is composed of 6 multiplexes as a whole.

In response to this, a phase winding is added to the winding on the low frequency side of the transformer 61 for the conversion device, and the operation phase of each conversion unit is shifted by 15 degrees to form a so-called 24-phase configuration. Things.

In this embodiment, the basic control method is the same as that of FIG. However, since the transformer 61 for the converter is provided with the phase winding, the four low-frequency converters 14 to 16 are provided.
Are converted in consideration of the phase winding of the transformer 61 for the conversion device.
16 need to be controlled individually. At this time,
A control method using two-axis conversion may be applied.

Next, in this embodiment, a phase winding is not provided in the winding of the transformer for the converter on the low frequency side, and a winding of 12 phases and two sets is formed only by the combination of the delta winding and the star winding. A configuration may be adopted, in which only the operation phase of the conversion unit is shifted by about 15 degrees, so that an approximate 24-phase operation may be obtained.

In this case, since a complicated phase winding is not required, the configuration of the transformer for the converter is simplified, and high reliability can be achieved at low cost. Further, the windings may be all the same, and a voltage approximate to a predetermined voltage may be generated only by control of the low-frequency conversion unit. In addition,
The conversion section on the high frequency side does not always need to be multiplexed.

Here, the characteristics in the case of multiplexing are as follows. (1) If the equivalent switching frequency is doubled, it can be doubled, so that better control becomes possible. (2) The capacity per conversion unit of the low frequency can be reduced. Although the DC voltage is the same, the current capacity can be reduced, so that a semiconductor switching element which can operate at a high frequency with a small capacity can be used.

[0054]

According to the present invention, a highly efficient and economical power converter can be obtained with a small distortion in the output AC voltage waveform, which may affect the voltage waveform and the current waveform of the AC power system. Therefore, it is possible to easily exchange AC power.

[Brief description of the drawings]

FIG. 1 is a block circuit diagram showing an embodiment of a power converter according to the present invention.

FIG. 2 is a waveform chart for explaining the operation of one embodiment of the present invention.

FIG. 3 is a waveform diagram for explaining the operation of another embodiment of the present invention.

FIG. 4 is a block circuit diagram showing still another embodiment of the present invention.

FIG. 5 is a block circuit diagram showing a modification of the embodiment of FIG. 1 according to still another embodiment of the present invention.

[Explanation of symbols]

Reference Signs List 6 transformer for converter 7 DC power supply 8 controller 9 current transformer 11 low-frequency converter 12 high-frequency converter 13-16 three-phase converter operating at low frequency 17, 18 three-phase converter operating at high frequency Unit 21, 22 Arm pair 31, 32 GTO (Gate Turn-Off Thyristor) 41, 42 IGBT (Insulated Gate Bipolar Transistor) 51, 52, 53, 54 Free Wheeling Diode 61 Transformer for Converter 80 AC Current Controller 81 Pulse controller 82 Comparator 83 PWM (pulse width modulation) pulse generator 84, 85 Adder

Claims (4)

[Claims] 1. A converter comprising at least two systems of converters having different switching frequencies, wherein a harmonic generated in a converter operating at a low frequency is suppressed by an output of the converter operating at a high frequency. In the voltage-type multiplex power conversion device, one of the current and the voltage of the AC side winding of the transformer for the conversion device is detected, and based on the detection result of the current or the current, the feedback control of the at least two systems of the conversion units is performed. A power converter characterized by the above-mentioned. 2. The switching device according to claim 1, wherein the switching frequency of the converter operating at the low frequency is:
A power converter having a frequency equal to a frequency of an output AC voltage. 3. The invention according to claim 1, wherein a combined capacitance of the converter operating at the high frequency is set to be 1 to 2 times a capacitance per one converter operating at the low frequency. Power converter. 4. The power converter according to claim 1, wherein a combined capacitance of the converter operating at the high frequency is set substantially equal to a combined capacitance of the converter operating at the low frequency. .