JP7680976B2 - Power Conversion Equipment - Google Patents

Description

The present invention relates to a power conversion device that converts DC power to AC power by switching switching elements based on pulse width modulation.

Conventionally, the method used to control the power conversion device that drives the induction motor of an electric vehicle is to perform asynchronous control when the vehicle is traveling at low speed and synchronous control when the vehicle is traveling at high speed.

Figure 7 is a diagram showing an example of the configuration of a typical power conversion device. As shown in Figure 7, the power conversion device 90 includes a reactor 3, a capacitor 4, a power converter 5, a current detector 6, a current command generator 11, a voltage command generator 12, a voltage phase and angular frequency calculator 14, and a PWM signal generator 25.

The power converter 5 converts the DC power supplied from the DC power source 2 into AC power in order to drive the AC motor 1 using the DC power source 2. Specifically, the power converter 5 includes a switching element, and by switching the switching element, the power converter 5 converts the DC power from the DC power source 2 into AC power and supplies it to the AC motor 1. The switching of the power converter 5 can be controlled by a PWM method in which a PWM (Pulse Width Modulation) control signal with a different pulse width is used to switch the switching element on and off in response to a comparison between the carrier and the control command.

The current command generator 11 generates a current command using at least a torque command.

The voltage command generating unit 12 generates a voltage command from the current command generated by the current command generating unit 11. Specifically, the voltage command generating unit 12 generates d-axis and q-axis voltage commands using the deviation between the current command from the current command generating unit 11 and the current value from the current detector 6.

The voltage phase/angular frequency calculation unit 14 calculates the voltage phase and angular frequency output from the power converter 5 using the voltage command from the voltage command generation unit 12.

The PWM signal generating unit 25 uses the voltage command generated by the voltage command generating unit 12 and the voltage phase and angular frequency calculated by the voltage phase/angular frequency calculating unit 14 to generate a PWM signal using a carrier such as a triangular wave.

The power converter 5 converts DC power into AC power by switching the switching elements using the PWM signal generated by the PWM signal generating unit 25, and outputs the AC power to the AC motor 1.

Generally, when the frequency of the output voltage of the AC power output from the power converter 5 exceeds a threshold value, the control is switched from asynchronous control to synchronous control of 9 pulses, 15 pulses, etc. Then, for example, when the voltage command increases due to an increase in the speed of an electric vehicle having an AC motor 1 that outputs AC power, an overmodulation state occurs in which the amplitude of the modulated wave (dashed line in FIG. 8) exceeds the amplitude of the carrier (solid line in FIG. 8), as shown in FIG. 8. In the overmodulation state, pulses are lost and the system ultimately enters one-pulse mode. One-pulse mode is a state in which one cycle of output voltage is generated by a single pulse.

The current detector 6 converts the stator current of the AC motor 1 into a d-axis current id and a q-axis current iq, which are the current components on the d-axis and q-axis in a rotating Cartesian coordinate system, and outputs them. When the AC motor 1 is an induction motor, the d-axis is generally defined as the direction of the secondary flux linkage vector of the AC motor 1, and when the AC motor 1 is a permanent magnet synchronous motor, the d-axis is generally defined as the direction of the N pole of the permanent magnet of the rotor of the motor.

However, with the above-mentioned technology, as shown in Figure 8, in the region where the amplitude of the modulated wave exceeds the amplitude of the carrier, an overmodulation state occurs, and losses in the AC motor 1 due to harmonic currents can become large.

Therefore, the invention described in Patent Document 1 creates a table of pulse patterns capable of reducing harmonic currents in advance, and switches the pulse mode between 9-pulse mode, 7-pulse mode (P7 pulse mode), 5-pulse mode (N5 pulse mode), and 3-pulse mode (N3 pulse mode) depending on the modulation rate, thereby reducing harmonic currents in the overmodulation region. Note that the 7-pulse mode, 5-pulse mode, and 3-pulse mode are states in which one cycle of output voltage is generated by 7, 5, and 3 pulses, respectively. In other words, the pulse mode is the number of pulses that make up one cycle of the output voltage.

JP 2020-137385 A

However, in the invention described in Patent Document 1, the carrier phase differs for each pulse mode, so the carrier frequency needs to be changed when switching between pulse modes. This can cause fluctuations in the torque output from the motor. In addition, the control becomes complicated because the carrier frequency needs to be changed multiple times.

The objective of the present invention, which has been made in consideration of the above problems, is to provide a power conversion device that can reduce harmonic currents and losses generated in AC motors, while also preventing fluctuations in output torque without complicating the control for switching pulse modes.

In order to solve the above problems, the power conversion device of the present invention is a power conversion device that converts DC power to AC power by switching a switching element based on pulse width modulation, and includes a current command generation unit that generates a current command using at least a torque command, a voltage command generation unit that generates a voltage command from the current command, a modulation rate calculation unit that calculates a modulation rate from the voltage command, a voltage phase/angular frequency calculation unit that calculates the voltage phase and angular frequency of the output voltage from the voltage command, and a PWM signal generation unit that generates a PWM signal from the modulation rate, the voltage phase, and the angular frequency, and the PWM signal generation unit uses the modulation rate and the angular frequency to change a pulse mode, which is the number of pulses that constitute one period of the output voltage, and changes a carrier frequency according to the pulse mode, and a modulation wave generation unit that calculates a modulation wave by referring to a modulation wave ratio calculation table using the modulation rate and the voltage phase, and compares the carrier with the modulation wave, and outputs the PWM signal to the power converter according to the comparison result. the carrier generation unit comprises a pulse mode selector that selects the pulse mode in which a harmonic current becomes the lowest according to the modulation factor, and a carrier generator that outputs the carrier and a peak value of the carrier from the pulse mode and the angular frequency, and the pulse mode selector executes only a process of selecting a (9+6(k-1)) pulse mode (k is an integer of 1 or more) and then selecting a 7-pulse mode once while the modulation factor is increasing, and executes only a process of selecting the 7-pulse mode once while the modulation factor is decreasing The process of selecting the (9+6(k-1)) pulse mode is executed once, and the waveform of the 7 pulse mode is characterized in that in the 1 pulse mode, in a waveform that generates an ON signal between the phase of 30 degrees and 210 degrees, an ON signal is generated before the 30 degree phase, an OFF signal is generated after the 30 degree phase, and an OFF signal is generated before and after the 120 degree phase, and in the waveform with a phase of 180 degrees to 360 degrees, the ON signal and the OFF signal are swapped in the waveform with a phase of 0 degrees to 180 degrees.

Furthermore, in the power conversion device control device according to the present invention, the modulated wave generating unit is further characterized in that it is further provided with a modulated wave ratio calculation table that stores in advance the pulse width at which the harmonic current is at its minimum for the modulation factor and the ratio of the modulated wave for outputting the pulse width to the peak value of the carrier in correspondence with each other, converts the modulated wave calculated by the modulation factor calculation unit into a ratio to the peak value of the carrier, and a modulated wave calculator that calculates the modulated wave from the ratio converted by the modulated wave ratio calculation table and the peak value of the carrier generated by the carrier generating unit.

According to the present invention, by reducing harmonic currents in the region where overmodulation occurs, it is possible to reduce losses generated in AC motors and to reduce fluctuations in output torque when switching pulse modes while simplifying control.

1 is a diagram illustrating an example of the configuration of a power conversion device according to an embodiment of the present invention. 2 is a diagram illustrating an outline of a configuration example of a PWM signal generating unit illustrated in FIG. 1 . 3 is a diagram illustrating an example of the configuration of a carrier generating unit illustrated in FIG. 2 ; 3 is a diagram illustrating an example of the configuration of a modulated wave generating unit illustrated in FIG. 2 . FIG. 13 is a diagram showing an example of a phase voltage waveform in the NP7 pulse mode as viewed from the neutral point. 1 is a diagram comparing harmonic currents in the conventional technology with harmonic currents in the present embodiment. FIG. FIG. 1 is a diagram illustrating an example of the configuration of a general power conversion device. FIG. 13 is a diagram showing an overmodulation state in a 9-pulse mode.

The power conversion device 100 according to one embodiment of the present invention will be described below with reference to the drawings. Note that the same functional units as those of the power conversion device 90 described with reference to FIG. 7 are given the same reference numerals and will not be described. The power conversion device 100 according to this embodiment uses a triangular wave with a constant slope as the carrier, and can change the carrier frequency by changing the peak value of the carrier. The synchronous control pulse mode when the power conversion device 100 according to this embodiment switches from asynchronous control to synchronous control will be described as a 9-pulse mode.

Figure 1 shows an example of the configuration of a power conversion device 100 according to this embodiment.

As shown in FIG. 1, the power conversion device 100 includes a reactor 3, a capacitor 4, a power converter 5, a current detector 6, a current command generator 11, a voltage command generator 12, a modulation factor calculator 13, a voltage phase/angular frequency calculator 14, and a PWM signal generator 15. The power conversion device 100 converts DC power to AC power by switching a switching element based on pulse width modulation. The power conversion device 100 of this embodiment includes a PWM signal generator 15 instead of the PWM signal generator 25 in the conventional power conversion device 90 shown in FIG. 7, and further includes a modulation factor calculator 13 between the voltage command generator 12 and the PWM signal generator 15.

The modulation rate calculation unit 13 calculates the modulation rate α from the voltage command generated by the voltage command generation unit 12. Here, the modulation rate α is the ratio of the output voltage calculated from the voltage command when the possible output voltage in one pulse mode is set to 1.

The voltage phase/angular frequency calculation unit 14 calculates the voltage phase and angular frequency from the voltage command generated by the voltage command generation unit 12.

The PWM signal generating unit 15 generates a PWM signal from the modulation factor α calculated by the modulation factor calculating unit 13 and the voltage phase and angular frequency calculated by the voltage phase and angular frequency calculating unit 14. The PWM signal generating unit 15 outputs the PWM signal to the power converter 5.

Figure 2 shows an outline of an example configuration of the PWM signal generating unit 15.

The PWM signal generating unit 15 includes a carrier generating unit 151, a modulated wave generating unit 152, and a comparator 153.

The carrier generation unit 151 uses the modulation factor α calculated by the modulation factor calculation unit 13 and the angular frequency generated by the voltage phase/angular frequency calculation unit to change the pulse mode, which is the number of pulses that make up one cycle of the output voltage, change the carrier frequency according to the pulse mode, and generate a carrier. The carrier generation unit 151 outputs the carrier to the comparator 153.

The modulated wave generating unit 152 calculates the modulated wave by referring to the modulated wave ratio calculation table using the modulation factor α calculated by the modulation factor calculation unit 13 and the voltage phase calculated by the voltage phase/angular frequency calculation unit 14. The modulated wave generating unit 152 outputs the modulated wave to the comparator 153.

The comparator 153 compares the carrier generated by the carrier generation unit 151 with the modulated wave generated by the modulated wave generation unit 152, and outputs a PWM signal to the power converter 5 according to the comparison result.

Figure 3 shows an example of the carrier generation unit 151.

The carrier generation unit 151 includes a pulse mode selector 154 and a carrier generator 155.

The pulse mode selector 154 selects the pulse mode that minimizes the harmonic current according to the modulation factor α calculated by the modulation factor calculation unit 13. Specifically, the pulse mode selector 154 selects the 9-pulse or 7-pulse pulse mode according to the modulation factor α calculated by the modulation factor calculation unit 13, and outputs the selected pulse mode to the carrier generator 155. More specifically, the pulse mode selector 154 executes only the process of selecting the 9-pulse mode and then the 7-pulse mode once while the modulation factor α is increasing, and executes only the process of selecting the 7-pulse mode and then the 9-pulse mode once while the modulation factor α is decreasing. The specific method by which the pulse mode selector 154 selects the pulse mode will be described in detail later.

The carrier generator 155 outputs a carrier and a peak value of the carrier from the pulse mode selected by the pulse mode selector 154 and the angular frequency calculated by the voltage phase/angular frequency calculation unit 14. Specifically, the carrier generator 155 generates a carrier from the pulse mode and angular frequency and outputs it to the comparator 153, while also calculating the peak value of the carrier and outputting it to the modulated wave generation unit 152.

Figure 4 shows an example of the modulated wave generating unit 152. The modulated wave generating unit 152 includes a modulated wave ratio calculation table 156 and a comparison value calculator 157.

The modulation wave ratio calculation table 156 stores the ratio of the modulation wave to the peak value of the carrier for outputting the pulse width at which the harmonic current is at its minimum (comparison value in FIG. 4) in advance in correspondence with the modulation rate α, and converts the modulation rate calculated by the modulation rate calculation unit 13 to the ratio to the peak value of the carrier. Specifically, the modulation wave ratio calculation table 156 stores the ratio to the peak value of the carrier in advance in correspondence with the modulation rate α calculated by the modulation rate calculation unit 13 and the voltage phase calculated by the voltage phase/angular frequency calculation unit 14. The modulation wave ratio calculation table 156 then calculates the ratio of the peak value of the modulation wave to the peak value of the carrier using the modulation rate α calculated by the modulation rate calculation unit 13 and the voltage phase calculated by the voltage phase/angular frequency calculation unit 14, and outputs it to the modulation wave calculator 157.

The modulated wave calculator 157 calculates the modulated wave from the ratio of the peak value of the modulated wave to the peak value of the carrier, which is converted by the modulated wave ratio calculation table 156, and the peak value of the carrier generated by the carrier generation unit 151. The modulated wave calculator 157 outputs the modulated wave to the comparator 153.

Next, we will explain the pulse mode switching point and the method for calculating the modulated wave ratio used by the pulse mode selector 154 and the modulated wave ratio calculation table 156.

When the power conversion device 100 of this embodiment switches from asynchronous control to synchronous control, the synchronous control uses a 9-pulse mode in which control is performed with 9 pulses in one period. Therefore, in order to reduce harmonic currents, the power conversion device 100 of this embodiment reduces harmonic currents by introducing multiple pulses between the 9-pulse mode and the 1-pulse mode.

As pulses that can be introduced between 9 pulses and 1 pulse, 7 pulses, 5 pulses, and 3 pulses are considered. Here, we consider inserting two 0 vectors into a 3-pulse mode (N3 pulse mode) in which a 0 vector is not inserted. That is, in the 7-pulse mode, when an ON signal is generated between a phase of 30 degrees and 210 degrees in a 1-pulse mode, an ON signal is generated before a phase of 30 degrees, an OFF signal is generated after a phase of 30 degrees, and an OFF signal is generated before and after a phase of 120 degrees, respectively, and a waveform with a phase of 180 degrees to 360 degrees is a waveform in which an ON signal and an OFF signal are swapped in a waveform with a phase of 0 degrees to 180 degrees. The phase that generates an OFF signal before and after a phase of 120 degrees is a phase with a phase width of θ ₀₇₁ centered on a phase of 120 degrees, and in the following description, the phase width of the OFF signal is θ _072. In the following description, the 7-pulse mode generated in this way will be described as an NP7-pulse mode.

Fig. 5 is a diagram showing an example of a phase voltage waveform at the neutral point in the NP7 pulse mode. In the NP7 pulse mode, the combination of pulse widths that minimizes the harmonic current is obtained by changing three phases (pulse widths) of θ ₀₇₁ , θ ₀₇₂ , and θ ₀₇₀ , which is the width of the phase of the ON signal generated before the 30-degree phase, as shown in Fig. 5. Note that the waveform when θ ₀₇₂ in the NP7 pulse mode becomes 0 becomes the N3 pulse mode.

First, from the phase voltage waveform as shown in FIG. 5, the pulse widths θ ₇₀₀ , θ ₀₇₁ , and θ ₀₇₂ that are variables are changed to calculate the harmonic currents.

Specifically, the phase voltage waveforms shown in FIG. 5 are defined as the U-phase voltage waveform _vu , a waveform delayed by 120 degrees from the U-phase voltage waveform _vu is defined as the V-phase voltage waveform _vv , and a waveform delayed by 240 degrees from the _U -phase voltage waveform vu is defined as the W-phase voltage waveform _vw , and a three-phase composite wave v is generated using equation (1).

The voltage waveform of the three-phase composite wave v is subjected to FFT (Fast Fourier Transform) analysis to calculate the peak value of the voltage of each order. The magnitude of the harmonic current is calculated by calculating the square root of the sum of squares of the values obtained by dividing the peak value of the voltage of each order by the respective orders. The magnitude of the harmonic current calculated at this time is calculated as a ratio of the magnitude of the harmonic current in the one-pulse mode to 1. At this time, the modulation factor α is also derived. Then, by changing the above-mentioned pulse widths θ ₇₀₀ , θ ₀₇₁ , and θ ₀₇₂ , the pulse widths θ ₇₀₀ , θ ₀₇₁ , and θ ₀₇₂ at which the harmonic current has the minimum value at the calculated modulation factor α are derived.

The solid line in FIG. 6 indicates the harmonic current value according to the modulation factor α in the power conversion device 100 of this embodiment. The dashed line in FIG. 6 indicates the change in harmonic current value when the power conversion device 90 in the conventional technology (specifically, the technology described in Patent Document 1) changes the pulse mode. In FIG. 6, when the modulation factor α changes from less than 0.7 to 0.7 or more, the 9-pulse mode is switched to the NP7-pulse mode. When the modulation factor α is 0.87 or more, the harmonic current becomes larger than in the conventional technology, and at 0.98, the harmonic current is equivalent to that in the conventional technology.

The power conversion device 100 of this embodiment can reduce harmonic current more than the conventional power conversion device 90 by switching from the 9-pulse mode to the NP7 pulse mode at a modulation rate α that is about 8% lower than when the power conversion device 90 of the conventional technology switches from the 9-pulse mode to the P7 pulse mode. Note that, during the process of accelerating an electric vehicle driven by AC power converted by the power conversion device 100, the time during which the modulation rate α is 0.87 or more and less than 0.98, and the harmonic current is higher than in the conventional technology, is short. Therefore, in this embodiment, even during the time during which the harmonic current in the acceleration process is higher than in the conventional technology, the power conversion device 100 continues to use the NP7 pulse mode, thereby reducing the number of switching controls and avoiding the execution of complex control.

As described above, the NP7 pulse mode becomes the N3 pulse mode by setting θ ₀₇₂ to 0. Therefore, the power conversion device 100 of the present embodiment can switch the pulse mode by changing only the modulated wave without changing the carrier, and can switch continuously without requiring complicated control.

Considering the above, the power conversion device 100 of this embodiment switches from the 9-pulse mode to the NP7-pulse mode, and then the output changes continuously according to the modulation rate α. When the 0 vector disappears, the power conversion device 100 switches to the N3-pulse mode. This simple control makes it possible to reduce harmonic currents in the region where overmodulation occurs while avoiding or reducing fluctuations in the output torque. As shown in FIG. 3, the pulse mode selector 154 is set with the switching points of the pulse modes derived as described above (from 9-pulse to NP7-pulse mode, from NP7-pulse mode to 9-pulse mode). As shown in FIG. 4, the modulation wave ratio calculation table 156 in FIG. 4 is set with information indicating the pulse width at which the harmonic current is at its lowest.

In the above embodiment, an example of introducing the NP7 pulse mode between the 9-pulse mode and the 1-pulse mode has been described. However, by using a semiconductor element with a wide band gap such as silicon carbide (SiC) to reduce switching losses, the power conversion device 100 is not limited to 9 pulses, and can also reduce harmonic currents in the region where the modulation rate α is lower than 0.7 by switching from a (9+6(k-1)) pulse mode (k is an integer equal to or greater than 1) (e.g., 15-pulse mode, 21-pulse mode) to the NP7 pulse mode.

As described above, by introducing a pulse mode in the region where overmodulation occurs after switching to synchronous control mode, it is possible to reduce harmonic currents. Reducing harmonic currents makes it possible to reduce losses generated in AC motors.

Although the invention has been described, these embodiments are presented as examples and are not intended to limit the scope of the invention.

The present invention is useful for electric vehicles that are powered by AC power converted by a power conversion device.

REFERENCE SIGNS LIST 1 AC motor 2 DC power supply 3 Reactor 4 Capacitor 5 Power converter 6 Current detector 11 Current command generator 12 Voltage command generator 13 Modulation factor calculator 14 Voltage phase/angular frequency calculator 15 PWM signal generator 100 Power conversion device 151 Carrier generator 152 Modulated wave generator 153 Comparator 154 Pulse mode selector 155 Carrier generator 156 Modulated wave ratio calculation table 157 Modulated wave calculator

Claims (2)

A power conversion device that converts DC power into AC power by switching a switching element based on pulse width modulation,
a current command generating unit that generates a current command using at least a torque command;
a voltage command generating unit that generates a voltage command from the current command;
a modulation factor calculation unit that calculates a modulation factor from the voltage command;
a voltage phase and angular frequency calculation unit that calculates a voltage phase and an angular frequency of an output voltage from the voltage command;
a PWM signal generating unit that generates a PWM signal from the modulation factor, the voltage phase, and the angular frequency,
The PWM signal generating unit
a carrier generating unit that uses the modulation rate and the angular frequency to change a pulse mode, which is the number of pulses that constitute one cycle of the output voltage, changes a carrier frequency in accordance with the pulse mode, and generates a carrier;
a modulated wave generating unit that calculates a modulated wave by referring to a modulated wave ratio calculation table using the modulation factor and the voltage phase;
a comparator that compares the carrier with the modulated wave and outputs the PWM signal to a power converter according to a comparison result;
The carrier generating unit includes:
a pulse mode selector that selects the pulse mode that minimizes the harmonic current in response to the modulation factor;
a carrier generator that outputs the carrier and a peak value of the carrier based on the pulse mode and the angular frequency,
the pulse mode selector executes only a process of selecting a (9+6(k-1)) pulse mode (k is an integer equal to or greater than 1) and then a 7-pulse mode once while the modulation rate is increasing, and executes only a process of selecting the (9+6(k-1)) pulse mode and then a 7-pulse mode once while the modulation rate is decreasing;
A power conversion device characterized in that the waveform in the 7-pulse mode is a waveform in which an ON signal is generated between a phase of 30 degrees and 210 degrees in a 1-pulse mode, an ON signal is generated before a phase of 30 degrees, an OFF signal is generated after a phase of 30 degrees, and an OFF signal is generated both before and after a phase of 120 degrees, and a waveform from a phase of 180 degrees to 360 degrees is a waveform in which the ON signal and the OFF signal are swapped in a waveform from a phase of 0 degrees to 180 degrees. The modulated wave generating unit includes:
a modulation wave ratio calculation table that stores in advance a pulse width at which a harmonic current is at a minimum for the modulation factor and a ratio of a modulation wave for outputting the pulse width to a peak value of the carrier in correspondence with each other, and converts the modulation factor calculated by the modulation factor calculation unit into a ratio to the peak value of the carrier;
a modulated wave calculator that calculates the modulated wave from the ratio converted by the modulated wave ratio calculation table and a peak value of the carrier generated by the carrier generation unit;
The power conversion device according to claim 1 , further comprising:

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