JP2004260963A - Power converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a power converter wherein magnetic saturation which may occur in a reactor for harmonic noise suppression connected between an alternating-current power supply and the power converter can be prevented. <P>SOLUTION: A PWM circuit 14 compares a command voltage, computed based on a command frequency, with a carrier wave, and outputs a control signal for on/off controlling the power transistor of a switching circuit 13. A comparator 15 compares the magnetic flux flowing through the core of the common mode reactor 6 for noise suppression, detected through a magnetic flux detector 7, with a protection level. If the detected magnetic flux exceeds the protection level, the comparator changes a switching frequency fc, which is the frequency of the carrier wave used in computational comparison with command voltages in the PWM circuit 14. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a power converter, and more particularly to a power converter capable of preventing magnetic saturation of a reactor for suppressing harmonic noise connected between an AC power supply and a power converter.
[0002]
[Prior art]
In an inverter device that drives a motor as a load at a variable speed, when the inverter device is switched at high frequency, the leakage current flows to the ground through the wiring between the inverter device and the motor and the floating capacitor between the conductive part in the motor and the ground. The problem is that it adversely affects the operation of the circuit breaker. When the switching frequency of the inverter device is higher than about 10 kHz, an oscillating current is generated by the floating capacitor and the inductance of the wiring, and even if a zero-phase reactor is inserted, the earth leakage breaker may malfunction. In addition, noise transmitted to the power supply is large, which may adversely affect external devices connected to the power supply.
[0003]
Patent Document 1 (Japanese Patent Laid-Open No. 10-210649) aims at preventing the malfunction of the earth leakage breaker due to the oscillating current generated by the above-mentioned floating capacitor and wiring inductance and the adverse effect on other devices due to noise. ).
[0004]
Patent Document 1 discloses that a DC reactor is provided between each of a DC terminal of a rectifier and a positive terminal and a negative terminal of a smoothing capacitor, and a capacitor is provided between a positive terminal and a ground and between a negative terminal and a ground of the smoothing capacitor. And a resistor connected in series, and the zero-phase current generated by the operation of the inverter flows to a closed circuit including a floating capacitor, ground, a capacitor and a resistor inserted in the inverter DC section, and a rectifier. The reactor inserted in series between the capacitor and the smoothing capacitor has the effect of suppressing the zero-phase current from flowing into the power supply.Also, the voltage noise generated between the inverter and the ground due to the switching of the inverter is absorbed by the capacitor and the resistor. The amount transmitted to the power supply side is reduced.
[0005]
Patent Document 2 (Japanese Patent Application Laid-Open No. 9-93952) discloses a method for protecting a PWM inverter device that can prevent heating and burning of each device due to an increase in harmonic current. Some have been disclosed.
[0006]
Patent Literature 2 discloses that a harmonic earth current via a ground capacitance, which is caused by an output waveform of a PWM inverter device connected to an ungrounded power supply, is detected by a current detector on a DC side of the inverter device, and a switching frequency of the inverter is detected. Only higher components are extracted and a stop command is issued when the level is higher than a certain level. Is detected to protect the PWM inverter device. In setting the protection level, the protection level should be set to the one having the smallest harmonic withstand capability of each device on the path where the harmonic earth current flows. If it exceeds this, the inverter device is stopped due to a failure.
[0007]
Also, the DC input current of the inverter of the PWM power converter includes a ripple current accompanying the PWM operation, which flows into the smoothing capacitor, and its temperature rises. Therefore, a large-capacity smoothing capacitor is provided in the DC circuit according to the ripple current. However, since the magnitude of the ripple current increases in proportion to the magnitude of the output current of the inverter, the size of the smoothing capacitor in the large-capacity inverter becomes large.
[0008]
A PWM power converter in which the ripple component contained in the DC current of the PWM power converter is effectively absorbed to suppress the ripple current flowing into the smoothing capacitor and the smoothing capacitor is reduced in size, and a capacitor of the series resonant circuit for ripple absorption. The object of the present invention is to provide a control method of a PWM power converter that can effectively absorb a ripple current and reduce the capacity of a smoothing capacitor even if the capacitance value and the inductance value of the reactor fluctuate due to temperature and the like. And Patent Document 3 (JP-A-1-321868).
[0009]
Patent Document 3 discloses a power converter that performs PWM control, includes a series resonance circuit in parallel with a smoothing capacitor, and sets the resonance frequency of the series resonance circuit to the frequency of a ripple component included in the DC current of the power conversion device accompanying the PWM control. The carrier frequency of the PWM control is varied so as to match within a predetermined range.
Since the resonance frequency of the series resonance circuit matches the frequency of the ripple current component, the ripple component is absorbed by the resonance current flowing through the series resonance circuit, and the ripple current hardly flows through the smoothing capacitor.
In addition, the change in resonance frequency is detected from the ripple component flowing through the smoothing capacitor, and the carrier frequency of the PWM control is varied so that the ripple component is minimized. Even if it changes, the ripple component can always be effectively absorbed by the series resonance circuit,
That is, the capacity of the smoothing capacitor of the PWM power converter can be reduced.
[0010]
[Problems to be solved by the invention]
In the above-described conventional example, Patent Document 1 can absorb a leakage current generated when a high-frequency switching element such as an IGBT is subjected to PWM control at a high frequency, not only prevents a malfunction of the leakage breaker but also reduces noise generated to the outside. Although the effect can be reduced, there is a problem in that the DC reactor is provided between each of the DC side terminals of the rectifier and the plus side terminal and the minus side terminal of the smoothing capacitor, so that the device becomes large.
[0011]
Further, in Patent Document 2, the protection level is set in accordance with the device having the smallest harmonic withstand capability, and when the protection level is exceeded, the inverter device is stopped by failure to protect the PWM inverter device. This is to protect the devices on the path where the harmonic earth current flows, but there is a problem that the device stops when the protection circuit operates.
[0012]
Patent Document 3 discloses a technique of controlling a carrier frequency of PWM control so that a resonance frequency fr of a series resonance circuit provided in parallel with a smoothing capacitor and absorbing a ripple current matches a frequency of the ripple current. However, Patent Document 3 focuses on the fact that the frequency of the ripple current is an integral multiple of fc related to the carrier frequency fc of the PWM control, and provides a series resonance circuit for absorbing the ripple current in parallel with the smoothing capacitor. When the resonance frequency fr of the series resonance circuit determined by the inductance L2 and the capacitor size C2 and the frequency of the ripple current are matched, the current component due to the fluctuation of the DC voltage flows through the series resonance circuit and does not flow through the smoothing capacitor. The current flows through the series resonance circuit provided in parallel with this smoothing capacitor to absorb the ripple current. Even if the value of the capacitor and the reactor is changed by with the temperature rise or the like to always be one that controls the carrier frequency so that the resonance condition is satisfied,
There is a problem that it does not prevent magnetic saturation of a reactor for suppressing harmonic noise connected between the AC power supply and the power converter.
[0013]
The present invention has been made in order to solve the above-described problems, and is a power conversion device capable of preventing magnetic saturation of a reactor for suppressing harmonic noise connected between an AC power supply and a power conversion device. The purpose is to obtain.
[0014]
[Means for Solving the Problems]
A power converter according to the present invention includes an AC / DC converter that converts an AC voltage of an AC power supply into a DC voltage, a smoothing capacitor that smoothes the DC voltage, and a power transistor, and controls ON / OFF of the power transistor. A switching circuit that converts a smoothed DC voltage into an AC voltage, a PWM circuit that compares a command voltage with a carrier wave, and outputs a control signal for controlling ON / OFF of a power transistor of the switching circuit; And a comparator that determines whether magnetic flux saturation of the reactor for suppressing harmonic noise connected between the power converter and the power converter and changes the switching frequency of the carrier when it is determined that the magnetic flux is saturated. is there.
[0015]
The power converter according to the present invention includes a magnetic flux detector that detects a magnetic flux flowing through an iron core of a reactor for suppressing harmonic noise connected between the AC power supply and the power converter. The magnetic flux detected by the detector is compared with a preset protection level, and the switching frequency of the carrier is changed when the detected magnetic flux exceeds the protection level.
[0016]
Further, the power converter according to the present invention sets the reference frequency of the protection level in advance based on the relationship between the frequency of the harmonic current and the magnetic flux flowing through the core of the reactor for suppressing harmonic noise, and sets the comparator to the carrier wave. This switching frequency is compared with the reference frequency of this protection level to determine whether or not the magnetic flux saturation of the reactor for suppressing harmonic noise is appropriate.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiment 1 FIG.
FIG. 1 is a diagram showing a configuration of a power conversion device according to Embodiment 1 of the present invention. In the figure, a power converter 1 converts an AC power supply 2 into AC power of a predetermined frequency and voltage, and drives an induction motor 3 as a load. The AC power supply 2 is grounded 4, a grounding capacitor 5 is grounded between the AC power supply 2 and the ground, and a noise suppression common mode reactor 6 is connected between the power converter 1 and the AC power supply 2. . The magnetic flux detector 7 detects a magnetic flux flowing through the core of the noise suppression common mode reactor 6. In addition, a stray capacitance 8 is generated between the wiring between the power converter 1 and the induction motor 3 as a load and the ground.
[0018]
In the power converter 1, the AC / DC converter 11 converts the AC voltage of the AC power supply 2 into a DC voltage, and smoothes the DC voltage with the smoothing capacitor 12. The switching circuit 13 is composed of a power transistor, and by turning on / off the power transistor according to a control signal from the PWM circuit 14, converts the smoothed DC voltage into an AC voltage, and switches the induction motor 3 as a load. Drive.
In addition, the PWM circuit 14 compares the command voltage calculated based on the command frequency with the carrier, and outputs a control signal for controlling ON / OFF of the power transistor.
[0019]
The comparator 15 compares the magnetic flux flowing through the iron core of the noise suppression common mode reactor 6 detected by the magnetic flux detector 7 with the protection level, and when the detected magnetic flux exceeds the protection level, the PWM circuit At 14, the switching frequency fc, which is the frequency of the carrier used for the comparison operation with the command voltage, is changed.
[0020]
FIG. 2 is a diagram showing a propagation path of a harmonic current generated when the power transistor of switching circuit 13 is turned on / off in the power converter according to Embodiment 1 of the present invention. In the figure, reference numerals 2 to 5, 8, and 11 to 13 are the same as those in FIG. When the power transistor of the switching circuit 13 is turned on / off by a control signal from the PWM circuit 14, a harmonic current Ih oscillating at the frequency fh is generated, and wiring between the power converter 1 and the induction motor 3 as a load. The current flows to the switching circuit 13 via the ground 4 and the ground capacitor 5 via the stray capacitance 8 between the ground and the ground.
[0021]
FIG. 3 is a diagram showing an equivalent circuit of a propagation path of the harmonic current Ih generated when the power transistor of the switching circuit 13 is turned on / off in the power converter according to Embodiment 1 of the present invention. In the figure, 5, 6, and 8 are the same as those in FIG. 1, and the description thereof is omitted. C1 is the capacitance value of the grounding capacitor 5 grounded between the AC power supply 2 and the ground; C2 is the capacitance value of the stray capacitance 8 between the wiring between the power converter 1 and the induction motor 3 as a load; L is the inductance value of the common mode reactor 6 for noise suppression connected between the power converter 1 and the AC power supply 2, and fh is the frequency of the harmonic current Ih. Here, the frequency fh of the harmonic current Ih generated when the power transistor of the switching circuit 13 is turned on / off at the switching frequency fc is substantially equal to the switching frequency fc.
[0022]
The noise suppression common mode reactor 6 and the ground capacitor 5 constitute an LC circuit, and when the resonance frequency f1 of the LC circuit matches the frequency fh of the harmonic current Ih, the harmonic current Ih is amplified by LC resonance. . The resonance frequency f1 at which resonance occurs can be obtained by equation (1).
f1 = 1 / 2π · (L · C1) ^1/2 ···· (1)
Further, the magnetic flux Φ generated in the iron core of the noise suppression common mode reactor 6 by the harmonic current Ih flowing through the noise suppression common mode reactor 6 can be obtained by Expression (2).
Φ = L · Ih = B · S (2)
Here, B is the magnetic flux density, and S is the cross-sectional area of the iron core. From the equation (2), the harmonic current Ih is obtained by the equation (3).
Ih = BS / L (3)
[0023]
FIG. 4 is a diagram showing the relationship between the magnetic flux generated in the core of the noise suppression common mode reactor 6 and the frequency fh of the harmonic current Ih. In the figure, Φ is a magnetic flux generated in the core of the noise suppression common mode reactor 6, Φmax is the maximum value of the magnetic flux, f1 is the resonance frequency, fa is the first reference frequency as the protection level, and fb is the second reference frequency as the protection level. Reference frequency. As shown in the figure, the magnetic flux Φ generated in the iron core of the noise suppression common mode reactor 6 has a maximum Φmax when the frequency fh of the harmonic current Ih is the resonance frequency f1. The magnetic flux density B is determined by the material of the iron core. The saturation magnetic flux density Bm is determined by the material of the iron core. When the saturation magnetic flux density Bm is reached, the noise suppression common mode reactor 6 does not function as the inductance value L, and abnormal heat generation, vibration and noise are reduced. appear.
The first reference frequency fa and the second reference frequency fb are reference values as protection levels for preventing magnetic saturation, and the figure shows an example in which the frequency is Φmax × 0.8.
[0024]
FIG. 5 is a flowchart for automatically changing the switching frequency fc when the harmonic current exceeds the protection level in the power converter according to Embodiment 1 of the present invention.
[0025]
In the power converter according to Embodiment 1 of the present invention, when the magnetic flux Φ generated in the core of noise-reducing common mode reactor 6 exceeds the protection level, the switching frequency is automatically changed to prevent magnetic saturation. As shown in FIG. 4, the frequency fh of the harmonic current Ih is in a range where the iron core of the noise suppression common mode reactor 6 is magnetically saturated (first reference frequency fa ≦ frequency fh of harmonic current Ih ≦ fh ≦ In order not to be within the second reference frequency fb),
The frequency fh of the harmonic current Ih <the first reference frequency fa,
Or the frequency fh of the harmonic current Ih> the second reference frequency fb,
The control method will be described with reference to FIGS.
[0026]
In step S1, the magnetic flux Φn flowing through the iron core of the noise suppression common mode reactor 6 is detected by the magnetic flux detector 7.
In step S2, the comparator 15 compares the magnetic flux Φn detected by the magnetic flux detector 7 with the protection level, and determines that the magnetic flux Φn detected is smaller than the protection level as a normal state (in FIG. The frequency fh of the wave current Ih <the area of the first reference frequency fa or the frequency fh of the harmonic current Ih> the area of the second reference frequency fb), and returns to step S1 (in this case, the switching frequency fc is not changed).
[0027]
If the detected magnetic flux Φn is equal to or higher than the protection level in step S2, it is determined that there is a possibility of causing magnetic saturation, and the switching frequency fc is reduced in step S3.
In step S4, the magnetic flux detector 7 detects the magnetic flux Φn + 1 flowing through the iron core of the noise suppression common mode reactor 6 after reducing the switching frequency fc.
In step S5, the comparator 15 compares the magnetic flux Φn detected last time with the magnetic flux Φn + 1 detected after reducing the switching frequency fc, and the magnetic flux Φn + 1 detected after reducing the switching frequency fc becomes the magnetic flux Φn detected last time. In the following cases, it is determined in step S3 that the switching frequency fc can be protected from magnetic saturation by decreasing the switching frequency fc, and the process returns to step S1 (in FIG. 4, the frequency fh of the harmonic current Ih ≦ the area of the resonance frequency f1). .
[0028]
In step S5, if the magnetic flux Φn + 1 detected after reducing the switching frequency fc is larger than the magnetic flux Φn detected last time, it is determined that reducing the switching frequency fc cannot be protected from magnetic saturation (in FIG. 4, (Resonance frequency f1 ≦ frequency fh area of harmonic current Ih), and switching frequency fc is increased in step S6.
In step S7, the magnetic flux detector 7 detects the magnetic flux Φn + 2 flowing through the iron core of the noise suppression common mode reactor 6 after increasing the switching frequency fc.
In step S8, the comparator 15 compares the magnetic flux Φn + 2 detected after increasing the switching frequency fc with the protection level, and determines that the detected magnetic flux Φn + 2 is normal if it is smaller than the protection level (FIG. 4). , The second reference frequency fb <the area of the frequency fh of the harmonic current Ih), and the process returns to step S1.
[0029]
In step S8, if the detected magnetic flux Φn + 2 is equal to or higher than the protection level, it is determined that the magnetic flux cannot be protected from magnetic saturation (in FIG. 4, the first reference frequency fa ≦ the frequency fh of the harmonic current Ih ≦ the second reference frequency fb). Area), In step S6, the switching frequency fc is increased.
[0030]
As described above, in the power conversion device according to the first embodiment, the power transistor of the switching circuit 13 is turned on and off at the switching frequency fc by the influence of the harmonic current Ih generated by the power conversion device 1. Since the switching frequency fc is changed so that the noise suppressing common mode reactor 6 connected to the power supply 2 does not saturate,
It is possible to suppress abnormal heat generation, noise, and vibration of the noise suppression common mode reactor 6 without stopping the power converter.
[0031]
By the way, in the above description, the magnetic flux of the noise suppression common mode reactor 6 which is a target of suppressing abnormal heat generation, noise, and vibration is detected, and the switching frequency fc is changed so that the noise suppression common mode reactor 6 is not saturated. Although the example described above has been described, the same applies when the switching frequency fc is changed by detecting the zero-phase current flowing in the AC power supply 2 or the core temperature of the noise suppression common mode reactor 6 to determine magnetic saturation. The effect can be obtained.
[0032]
【The invention's effect】
Since the present invention is configured as described above, it has the following effects.
[0033]
A power converter according to the present invention includes an AC / DC converter that converts an AC voltage of an AC power supply into a DC voltage, a smoothing capacitor that smoothes the DC voltage, and a power transistor, and controls ON / OFF of the power transistor. A switching circuit that converts the smoothed DC voltage into an AC voltage, a PWM circuit that compares a command voltage with a carrier wave, and outputs a control signal for controlling ON / OFF of a power transistor of the switching circuit, and an AC power supply. And a comparator that changes the switching frequency of the carrier when judging the magnetic flux saturation of the reactor for harmonic noise suppression connected between the power conversion device and the magnetic saturation, Abnormal heat generation, noise, etc. of the noise suppression common mode reactor without stopping the power converter It is possible to suppress the movement.
[0034]
Also, the power converter according to the present invention includes a magnetic flux detector that detects a magnetic flux flowing through an iron core of a reactor for suppressing harmonic noise connected between the AC power supply and the power converter, and the comparator uses the magnetic flux detector. The magnetic flux detected by the detector is compared with a preset protection level, and when the detected magnetic flux exceeds the protection level, the switching frequency of the carrier is changed, so that the magnetic saturation can be accurately determined.
[0035]
Further, the power converter according to the present invention sets the reference frequency of the protection level in advance based on the relationship between the frequency of the harmonic current and the magnetic flux flowing through the core of the reactor for suppressing harmonic noise, and sets the comparator to the carrier wave. Is compared with the reference frequency of this protection level to judge whether magnetic flux saturation of the reactor for suppressing harmonic noise is appropriate or not, so that magnetic saturation can be easily determined.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a power conversion device according to Embodiment 1 of the present invention.
FIG. 2 is a diagram showing a propagation path of a harmonic current Ih generated when the power transistor of the switching circuit 13 is turned on / off in the power converter according to Embodiment 1 of the present invention.
FIG. 3 is a diagram showing an equivalent circuit of a propagation path of a harmonic current Ih generated when the power transistor of the switching circuit 13 is turned on / off in the power conversion device according to the first embodiment of the present invention.
FIG. 4 is a diagram illustrating a relationship between a magnetic flux generated in an iron core of a noise suppression common mode reactor 6 and a frequency fh of a harmonic current Ih.
FIG. 5 is a flowchart for automatically changing the switching frequency fc when the harmonic current exceeds the protection level in the power converter according to Embodiment 1 of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Power converter, 2 AC power supply, 3 Induction motor, 4 Grounding, 5 Grounding capacitor, 6 Common mode reactor for noise suppression, 7 Magnetic flux detector, 8 Floating capacitance, 11 AC / DC converter, 12 Smoothing capacitor, 13 Switching circuit , 14 PWM circuit, 15 comparator, C1 capacitance value of ground capacitor 5 grounded between AC power supply 2 and ground, C2 stray capacitance between wiring between power converter 1 and induction motor 3 as a load and ground 8, the inductance value of the common mode reactor 6 for noise suppression connected between the power converter 1 and the AC power supply 2, fc switching frequency, and Ih the power transistor of the switching circuit 13 is turned on at the switching frequency fc. Harmonic current generated when / OFF control is performed, fh harmonic Frequency of current Ih, f1 resonance frequency, first reference frequency as fa protection level, second reference frequency as fb protection level, Φ magnetic flux generated in the core of noise suppressing common mode reactor 6, Φmax maximum value of magnetic flux, The present invention relates to protection of a reactor connected to a power converter. Reactor resonance avoidance function,

Claims (3)

An AC / DC converter for converting an AC voltage of an AC power supply into a DC voltage;
A smoothing capacitor for smoothing this DC voltage,
The power transistor is composed of a power transistor. By controlling ON / OFF of the power transistor, a switching circuit for converting a smoothed DC voltage into an AC voltage is compared with a command voltage and a carrier wave, and the power transistor of the switching circuit is turned ON. A PWM circuit that outputs a control signal for performing the / OFF control;
A comparator that determines whether magnetic flux saturation of the reactor for harmonic noise suppression connected between the AC power supply and the power conversion device, and changes the switching frequency of the carrier when it is determined that the magnetic flux is saturated,
Power conversion device provided with. A magnetic flux detector for detecting a magnetic flux flowing through an iron core of a reactor for harmonic noise suppression connected between the AC power supply and the power converter,
The comparator compares the magnetic flux detected by the magnetic flux detector with a preset protection level, and changes the switching frequency of the carrier when the detected magnetic flux exceeds the protection level. The power converter according to claim 1. The reference frequency of the protection level is set in advance based on the relationship between the magnetic flux flowing through the iron core of the reactor for harmonic noise and the frequency of the harmonic current, and the comparator sets the switching frequency of the carrier wave to the reference of this protection level. 3. The power converter according to claim 2, wherein it is determined whether magnetic flux saturation of the reactor for suppressing higher harmonic noise is compared with the frequency.

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