JP2007097389A - Electric power conversion equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power conversion equipment that permits to fulfill two-arm modulation and the reduction of similar average switching frequency without entailing stepped changes of output potentials, to eliminate the need for large-sized common mode filter and insulating transformer, to achieve reduced size and light weight, and to inhibit output distortions during soft starting and during output transients caused by making and breaking loads. <P>SOLUTION: In the electric power conversion equipment for converting three-phase AC voltage into DC voltage, or DC voltage to three-phase AC voltage, a pulse width modulation is performed using carrier waves with different frequencies depending on phase and instantaneous value of the AC voltage. And, in the vicinity of zero-cross of phase voltage of the AC voltage, the pulse width modulation is performed using a carrier wave with a first frequency; and in the vicinity of peak of phase voltage of the AC voltage, using a carrier wave with a second frequency having a smaller value than that of the first frequency. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to pulse width modulation (PWM) control for reducing loss of a power converter that performs AC-DC conversion or DC-AC conversion.

FIG. 6 shows a configuration example of a three-phase DC-AC conversion circuit as a power conversion device to be controlled, and FIG. 7 shows a sine wave-triangular wave modulation method as an example of the pulse width modulation method. In FIG. 6, 1 and 2 are DC power sources, 3 to 8 are semiconductor switches, 9 to 11 are AC reactors, and 12 to 14 are capacitors. 15 is a common mode reactor, 16 to 18 are grounding capacitors, and 19 is a parasitic capacitance to the ground of the device.
In FIG. 7, the sine wave signal waves and the triangular carrier wave are compared, and the switching elements 3 to 8 in FIG. 6 are turned on / off according to the result.
That is, the semiconductor switch 3 in the U phase signal wave> carrier period, the semiconductor switch 4 in the U phase signal wave <carrier period, the semiconductor switch 5 in the V phase signal wave> carrier period, and the V phase signal wave <carrier. The semiconductor switch 6 is turned on during the period of W, the semiconductor switch 7 is turned on during the period of W-phase signal wave> carrier, and the semiconductor switch 8 is turned on during the period of W-phase signal wave <carrier.
The potentials at the points U1, V1, and W1 are equal to the P point potential when the semiconductor switches 3, 5, and 7 are on, respectively, and are equal to the N point potential when the semiconductor switches 4, 6, and 8 are on.
The voltage between each of the points U1, V1, and W1 is a square wave. By smoothing the voltage with an LC filter including the reactors 9 to 11 and the capacitors 12 to 14, the sine wave voltage is converted into the AC output terminals U, V, and W. Get between each point. This method is widely known as a sinusoidal modulation method.

A semiconductor switch generates a switching loss in accordance with an on / off operation, that is, switching. When the switching loss is large, the conversion efficiency of the device is lowered and the cooling device is enlarged. However, when the switching frequency is lowered to reduce the switching loss, the waveform control performance is lowered.
In FIG. 7, the signal wave and the carrier are drawn at relatively close frequencies to make the drawing easier to see, but the signal wave is actually several tens to several hundreds Hz and the carrier is about 5 kHz to 50 kHz. It is common.
Further, there is a two-arm modulation method shown in FIG. 8 as a method for lowering the switching frequency while minimizing the reduction in control performance. This is to apply an operation so that any one of the signal waves of each phase exceeds the amplitude of the carrier, so that the switching of the phase is stopped for a certain period, and the average switching frequency is lowered. For example, in the modulation system of FIG. 7, the on-duty of the semiconductor switch 3 is close to 100% near the positive peak of the U-phase signal wave, and the on-duty of the semiconductor switch 4 is close to 100% near the negative peak. The effect on the switching waveform is greatest at a duty of 50%, and decreases as the duty is deviated. For this reason, even if switching is stopped, the waveform control performance does not deteriorate so much.

Since the change in potential due to this operation does not appear in the voltage between the phases, that is, the value of the signal wave in the other phase is also operated so that the difference between the signal waves becomes a sine wave, the output line voltage is the sine wave. To be kept. The two-arm modulation method is disclosed in Patent Document 1, for example.
As a method for switching the switching frequency, there is a method of changing the carrier. For example, Patent Document 2 and Patent Document 3 show the technique for changing the carrier. However, in Patent Document 2, the carrier frequency is continuously changed by feedback control. This is not appropriate because it causes inconsistencies in the mountains and valleys. In Patent Document 3, the carrier is switched according to the output current, and no consideration is given to reducing the carrier frequency at a timing with a small influence on the voltage waveform, which is a feature of the present invention. Is inevitable.
JP-A-1-274668 JP-A-3-52565 JP 2002-314345 A

In the sine wave modulation method shown in FIG. 7 and the two-arm modulation method shown in FIG. 8, the line voltage is equal, but the behavior of the potential of each phase (for example, the potential at point M in FIG. 6) is different.
The potentials at points U1, V1, and W1 with respect to point M in FIG. 6 fluctuate at a high frequency with switching. However, in the two-arm modulation method, step-like potential fluctuations are further superimposed due to step-like changes in the signal wave. This includes lower frequency components than those due to switching. Although the potential fluctuation of the switching frequency component can be removed by a common mode filter including the common mode reactor 15 and the ground capacitors 16 to 18, in order to remove the step-like potential fluctuation, the common corresponding to the low frequency component is used. A mode filter or an isolation transformer is required, and in either case, a significant increase in external shape and mass is involved. For example, in a load such as an electronic device, if there is a large potential fluctuation, there is a high risk of malfunction due to a leakage current flowing between the ground and the ground. For this reason, the two-arm modulation method is difficult to apply to a device that uses an electronic device or the like as a load, and is used exclusively for a device that targets a load such as an electric motor that is less likely to cause a potential fluctuation.

According to a first aspect of the present invention, there is provided a power converter that includes a semiconductor switch and converts a three-phase AC voltage into a DC voltage or a DC voltage into a three-phase AC voltage by signal wave-carrier wave comparison pulse width modulation control. Depending on the phase or instantaneous value of the AC voltage, pulse width modulation is performed using a carrier wave of a different frequency, and the carrier wave of the first frequency is set near the zero cross of the phase voltage of the AC voltage, and the peak of the phase voltage of the AC voltage. In the vicinity, pulse width modulation is performed using a carrier wave having a second frequency lower than the first frequency.
In the second invention, in the first invention, the first frequency is an integer and an odd multiple of the second frequency.
According to a third aspect of the present invention, there is provided a power converter configured by a semiconductor switch, which converts a three-phase AC voltage into a DC voltage or converts a DC voltage into a three-phase AC voltage by signal wave-carrier wave comparison pulse width modulation control. By correcting the signal wave in synchronism with the above, a pulse width modulation operation similar to that of the first and second inventions is performed using a carrier wave having a single frequency.

  In the fourth invention, the first to third inventions described above are implemented only during a period when the conversion device output command exceeds a predetermined value.

  In the present invention, since the average switching frequency can be reduced similarly to the two-arm modulation without stepwise change in output potential, a large common mode filter and an insulation transformer are not required, and the apparatus can be downsized. Weight reduction can be achieved. In addition, it is possible to suppress output distortion at the time of soft start or load transient fluctuation due to load interruption.

  The gist of the present invention is that pulse width modulation control is performed using two carriers having different frequencies and the frequency ratio of which is an odd multiple, so that the magnitude relationship between the phase potentials is not reversed temporarily, and W1 and U1 The ripples of the AC reactors 9 to 11 are minimized by distributing the pulse width modulation results to the semiconductor switches of the respective phases so that the voltage between them or between W1 and V1 is unipolar.

FIG. 1 shows a first embodiment of the present invention. This is a timing portion where the U and W phases are near the positive and negative peaks and the V phase is near the zero cross from one period of the signal wave.
Pulse width modulation is performed with carrier 1 for the V phase and carrier 2 with a period three times that of carrier 1 for the U and W phases. In other words, the V phase, which has a large influence on the waveform when the duty is near 50%, secures the waveform control performance by switching at a high frequency, and the switching frequency is lower than the V phase in the U phase and W phase, which deviate from 50%. Thus, the average switching frequency is lowered within a range that does not impair the control performance as much as possible.
The reason why the period of carrier 2 is three times the period of carrier 1 is that the two carriers are synchronized and the peak of the carrier (positive peak) and the valley of the carrier (negative peak) match. Because. This is due to the following reason.

At the timing of FIG. 1, the potential of each phase has a relationship of U phase> V phase> W phase. Here, in the period (T1) in which the W1 potential is equal to P, the potentials of U1 and V1 are also equal to P. In the other periods, the W1 potential is N, so there is no period in which the W1 potential exceeds the other phases. Similarly, in the period (T2) in which the U1 potential is N, the potentials of V1 and W1 are also equal to N, and there is no period in which the U1 potential is lower than the other phases. Thus, the magnitude relationship between the phase potentials is not reversed temporarily, and the voltage between W1 and U1 or between W1 and V1 is unipolar, so that ripples in the AC reactors 9 to 11 can be minimized. .
If the peaks or valleys of the carriers do not match, for example, if carrier 1 is not a valley but a peak in T1, V1 becomes the N-point potential, and a potential difference of opposite polarity to the original line voltage occurs between V1 and W1. . As a result, the voltage between W1 and V1 becomes bipolar, and the ripples of the AC reactors 9 to 11 increase.

If the period of the carrier 2 is an even multiple of the period of the carrier 1, for example, twice, it is inevitable that either the peak or valley of the carrier will not match. Needless to say, when the periods of the carriers 1 and 2 are not in an integral multiple relationship, a mismatch between the peaks and valleys of the carriers occurs.
This control is possible by providing a carrier for each signal wave and switching its frequency according to the magnitude or phase of the signal wave, or by always comparing either of the two types of carriers and selecting one of the comparison results. .
When the AC voltage is three-phase, the range controlled based on the carrier 2 is at least one when the phase of the signal wave is within ± 60 ° of the positive / negative peak, or the instantaneous value of the signal wave is 0.5 times or more of the amplitude. Since the phase is controlled based on the carrier 1, the deterioration of the waveform control performance can be minimized.

  The frequency range of the potential fluctuation generated by this control is equal to or higher than the frequency of the carrier 2 and is sufficiently higher than the frequency component included in the step-like potential fluctuation of the two-arm modulation.

FIG. 2 shows a second embodiment of the present invention. FIG. 2A shows the same modulation method as FIG. 1, but the same comparison result can be obtained by manipulating the signal wave without changing the carrier frequency, as shown in FIG. 2B. For example, in the U phase, to triple the carrier period means (1) the number of times that the signal wave <carrier becomes one out of the three peaks of the original carrier.
(2) The period in which the signal wave per carrier <carrier is tripled.
So that means
(A) New signal wave is set to carrier amplitude or more in 2 of 3 periods of the carrier, regardless of the original signal wave.
(B) In one of the three carrier cycles, a value obtained by subtracting a value obtained by multiplying the difference ΔA between the original signal wave and the carrier amplitude by three from the carrier amplitude is a new signal wave.
By the above operation, a result equivalent to changing the carrier cycle can be obtained.

  When a one-chip microcomputer or the like is used for a control device, pulse width modulation control is often performed using a built-in timer. However, it may be difficult to provide two carriers due to restrictions on the functions of the microcomputer. According to this embodiment, a desired operation is possible even in such a case.

In general, when starting up the power converter, a so-called soft start is performed in which the output is output from zero to the rated value with a slope. If soft start is performed in a state where the pulse width modulation method of the first and second embodiments is applied, output distortion may occur in a low output region, which may adversely affect the load. The reason will be described below.
In a three-phase full-bridge circuit, the V phase is pulse width modulated (PWM) by the carrier wave of the first frequency (carrier 1), and the U phase and the W phase are the carrier waves of the second frequency (carrier 2 and carrier 3 times). FIG. 4 and FIG. 5 show gate signals of respective phases when pulse width modulation (PWM) is performed according to (period).
FIG. 4 shows the gate signal of each phase when the output control command is in the vicinity of the rating. The number of times of switching that occurs within the peak-peak period of the first carrier wave is shown in the lower part of the figure. The portion where the number of switching is “2” is a period in which two phases are performing waveform control, and the portion where the number of switching is “1” is a period in which only one phase is performing waveform control. FIG. 5 shows a gate signal for each phase when the output control command is in the low output region. As is clear from comparison between FIG. 4 and FIG. 5, in the low output region, the period during which only one phase performs waveform control increases. Normally, since the waveform control gain and the circuit constant of the converter are adjusted in the rated output state, the control performance is reduced and the output distortion is increased in the low output region.
In particular, at the time of soft start, there is a possibility that the output distortion is further amplified because there is a period in which the switching mode is mixed in the transition period of the output control command.

A control block diagram for solving this problem is shown in FIG. The operation will be described below.
The difference between the output reference V * and the output detection V is obtained by the subtracter 21 and is input to the voltage regulator (AVR) 22 at the subsequent stage to obtain the output control signal λ. The output control signal λ is generated by the PWM circuit 23 according to the first carrier wave (carrier 1) which is the output of the carrier generator 31 and the second carrier wave (carrier 2 and lower frequency than the carrier 1) which is the output of the carrier generator 32. Pulse width modulated (PWM). The output control signal λ of the voltage regulator (AVR) 22 is further compared with the output λ1 of the comparison signal setter 33 and the output λ2 of the comparison signal setter 34, respectively. λ1 and λ2 are comparison levels for determining whether the gate signal is a pulse width modulated (PWM) signal using the first carrier wave or a pulse width modulated (PWM) signal using the second carrier wave, When λ> λ1 or λ <λ2, the changeover switch 29 selects the PWM signal based on the second carrier wave. By setting the comparison signals (λ1, λ2) to appropriate values, PWM signals are determined by the first carrier wave (carrier 1) for all phases at the time of low output, and output distortion is suppressed. In the vicinity of the rating, the PWM signal is determined by the second carrier wave (carrier 2), and the switching loss of the converter can be reduced.

  The present invention can be applied to an uninterruptible power supply apparatus, a DC power supply apparatus, an AC stabilized power supply apparatus, and the like using an AC-DC conversion circuit or a DC-AC conversion circuit.

2 shows an operation time chart of the first embodiment of the present invention. The operation | movement time chart of 2nd Example of this invention is shown. FIG. 5 is a control circuit diagram showing a third embodiment of the present invention. FIG. 4 shows a first operation explanatory diagram of FIG. FIG. 4 shows a second operation explanatory diagram of FIG. 3; The structural example of the three-phase DC-AC conversion circuit used as control object is shown. The principle diagram of sine wave-triangular wave modulation is shown. The principle figure of 2 arm modulation is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2 ... DC power supply 3-8 ... Semiconductor switch 9-11 ... AC reactor 12-14 ... Capacitor 15 ... Common mode reactor 16-18 ... Grounding capacitor 19 ... Parasitic capacitance to ground 21 ... Subtractor 22 ... Voltage regulator (AVR)
DESCRIPTION OF SYMBOLS 23 ... PWM circuit 24 ... Determination circuit 25, 26, 27, 28 ... Comparator 29 ... Changeover switch 30 ... AND circuit 31, 32 ... Carrier generator 33, 34. ..Comparison signal setting device

Claims (4)

In a power converter configured by a semiconductor switch and converting a three-phase AC voltage to a DC voltage or converting a DC voltage to a three-phase AC voltage by signal wave-carrier wave comparison pulse width modulation control,
Depending on the phase or instantaneous value of the AC voltage, pulse width modulation is performed using a carrier wave of a different frequency, and the carrier wave of the first frequency is set near the zero cross of the phase voltage of the AC voltage. A power converter that performs pulse width modulation using a carrier having a second frequency lower than the first frequency near a peak.   The power converter according to claim 1, wherein the first frequency is an integer and an odd multiple of the second frequency. In a power converter configured by a semiconductor switch and converting a three-phase AC voltage to a DC voltage or converting a DC voltage to a three-phase AC voltage by signal wave-carrier wave comparison pulse width modulation control,
3. A power conversion device that performs a pulse width modulation operation similar to that of claim 1 or 2 using a carrier wave having a single frequency by correcting a signal wave in synchronization with the carrier wave. 4. The power converter according to claim 1, wherein the pulse width modulation according to claim 1 is performed only during a period when an output command of the converter exceeds a predetermined value.

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