JP7835123B2 - Initial charging device and power conversion system - Google Patents

Description

This invention relates to an initial charging device and a power conversion system.

Conventionally, initial charging devices for initially charging the capacitors of power converters are known (see, for example, Patent Document 1).

Patent Document 1 describes an initial charging circuit (initial charging device) for initially charging the capacitor of a power converter that converts power input from an AC power source and outputs it to a load. The initial charging circuit described in Patent Document 1 is configured to initially charge the capacitor of the power converter while adjusting the frequency of the AC power source to prevent inrush current (excessive current) from flowing through the capacitor of the power converter.

Japanese Patent Publication No. 2009-171800

Here, although not described in Patent Document 1 mentioned above, an initial charging device is known that converts the power input from a power source by switching a switching element and outputs it to the capacitor of a power converter to perform initial charging. In this initial charging device, the switching frequency is kept constant, and the duty cycle is increased at a constant rate from the switching start time when the switching of the switching element begins to the maximum duty cycle arrival time when the duty cycle of the switching element reaches its maximum value. When the rate of increase of the duty cycle is made relatively large, the rate of increase of the capacitor input current to the capacitor of the power converter from the switching start time to the maximum duty cycle arrival time becomes relatively large, and although the charging speed of the capacitor is relatively large, the capacitor input current is likely to exceed a predetermined current value. On the other hand, when the rate of increase of the duty cycle is made relatively small, the rate of increase of the capacitor input current from the switching start time to the maximum duty cycle arrival time becomes relatively small, and although the capacitor input current is less likely to exceed a predetermined current value, the charging speed of the capacitor is relatively small. Furthermore, if the capacitor input current exceeds a predetermined value, the stress on the circuit components of the initial charging device and the capacitors of the power converter will increase, potentially shortening their lifespan. Therefore, a configuration is desired that can shorten the initial charging time of the power converter's capacitors while suppressing the increased stress on the circuit components of the initial charging device and the capacitors of the power converter.

This invention was made to solve the above-mentioned problems, and one of its objectives is to provide an initial charging device and power conversion system that can shorten the initial charging time of the capacitor in the power conversion device while suppressing increased stress on the circuit components of the initial charging device and the capacitor in the power conversion device.

To achieve the above objective, the initial charging device according to the first aspect of this invention is an initial charging device for initial charging a capacitor of a power converter that includes a first power conversion unit that converts a first input power input from an AC power source and outputs it to a load, and a capacitor connected to the input side of the first power conversion unit. The device comprises a second power conversion unit that includes a switching element and converts a second input power by switching the switching element and outputs it to the power converter, and a control unit that controls the switching of the switching element. The control unit is configured to increase the duty cycle at a first rate during a first period, from the switching start time when the switching of the switching element for initial charging the capacitor begins, to the duty cycle switching time when the duty cycle is switched before the maximum duty cycle arrival time when the duty cycle of the switching element reaches its maximum value. During the second period, from the duty cycle switching time to the maximum duty cycle arrival time, the control unit increases the duty cycle at a second rate smaller than the first rate.

In the initial charging device according to the first aspect of this invention, as described above, the control unit is configured to perform duty ratio control such that, during the first period from the switching start time when the switching of the switching element for initial charging the capacitor is started to the duty ratio switching time when the duty ratio is switched before the maximum duty cycle arrival time when the duty ratio of the switching element reaches its maximum value, the duty ratio is increased at a first rate of increase, and during the second period from the duty ratio switching time to the maximum duty cycle arrival time, the duty ratio is increased at a second rate of increase that is smaller than the first rate of increase. As a result, in the first period, the duty ratio is increased at a relatively large first rate of increase, so the rate of increase of the capacitor input current to the capacitor of the power converter can be made relatively large, and the charging speed of the capacitor can be made relatively large. Also, in the second period, the duty ratio is increased at a relatively small second rate of increase, so the rate of increase of the capacitor input current that was increased in the first period can be suppressed, and it is possible to prevent the capacitor input current from exceeding a predetermined current value at the maximum duty cycle arrival time. As a result, the initial charging time of the power converter's capacitor can be shortened while suppressing increased stress on the circuit components of the initial charging device and the capacitors of the power converter.

In the initial charging device according to the first phase described above, preferably, the control unit is configured to perform duty cycle control so that the capacitor input current to the power converter's capacitor does not exceed a predetermined current value at the time of reaching the maximum duty cycle. This configuration prevents the capacitor input current from exceeding a predetermined current value at the time of reaching the maximum duty cycle.

In the initial charging device according to the first phase described above, preferably, the first period is set to be longer than the second period. This configuration allows for a relatively longer first period, during which the duty cycle can be increased at a relatively large first growth rate, thereby increasing the rate of increase in the capacitor input current. This allows for a faster charging speed of the capacitor.

In the initial charging device according to the first phase described above, preferably, the first growth rate is set to twice or more the second growth rate. With this configuration, compared to the case where the first growth rate is less than twice the second growth rate, the rate of increase in the capacitor input current during the first period can be made larger, thereby increasing the capacitor charging speed.

In the initial charging device according to the first phase described above, preferably, the control unit is configured to perform frequency control, setting the frequency to a first frequency during the first period, the second period, and the third period from the time to reach the maximum duty cycle until the frequency switching time when the switching frequency of the switching element is switched before the target voltage arrival time when the charging voltage of the capacitor reaches the target voltage, and setting the frequency to a second frequency smaller than the first frequency during the fourth period from the frequency switching time until the target voltage arrival time. Here, the capacitor input current continues to decrease from the time to reach the maximum duty cycle until the target voltage arrival time. Also, the switching frequency has a negative correlation with the current ripple (pulsation) of the capacitor input current. Therefore, with the above configuration, the frequency switching time, when the switching frequency changes from a relatively large first frequency to a relatively small second frequency, is after the time to reach the maximum duty cycle, so it is possible to suppress the capacitor input current from exceeding a predetermined current value due to the current ripple that increases when the frequency is switched from the first frequency to the second frequency. Furthermore, in the fourth period, the switching frequency becomes the relatively low second frequency. Therefore, for example, if a transformer is provided between the voltage conversion section of the initial charging device and the power conversion device, the transformer impedance, which is positively correlated with the frequency, becomes relatively small. In this case, during the fourth period, the capacitor input current, which is negatively correlated with the transformer impedance, can be made relatively large, thereby increasing the capacitor charging speed.

In this case, preferably, the control unit is configured to perform the frequency control such that the capacitor input current to the power converter's capacitor does not exceed a predetermined current value due to the increased current ripple caused by switching the frequency from the first frequency to the second frequency. This configuration prevents the capacitor input current from exceeding a predetermined current value due to the increased current ripple caused by switching the frequency from the first frequency to the second frequency.

In the initial charging device according to the first aspect described above, preferably, the initial charging device has multiple power converters connected in parallel to each other, and is configured to sequentially initial charge the capacitors of each of the multiple power converters. This configuration allows for the initial charging of the capacitors of the multiple power converters to be performed in a relatively short time while suppressing excessive stress on the circuit components of the initial charging device and the capacitors of the power converters.

Furthermore, in order to achieve the above objective, the power conversion system according to the second aspect of this invention comprises a power conversion device including a first power conversion unit that converts a first input power input from an AC power source and outputs it to a load, and a capacitor connected to the input side of the first power conversion unit; and an initial charging device that initially charges the capacitor of the power conversion device. The initial charging device includes a switching element and comprises a second power conversion unit that converts a second input power by switching the switching element and outputs it to the power conversion device; and a control unit that controls the switching of the switching element. The control unit is configured to increase the duty cycle at a first rate of increase during a first period from the switching start time, when the switching of the switching element for initial charging the capacitor begins, to the duty cycle switching time, when the duty cycle is switched before the maximum duty cycle arrival time, when the duty cycle of the switching element reaches its maximum value; and to increase the duty cycle at a second rate of increase, which is smaller than the first rate of increase, during a second period from the duty cycle switching time to the maximum duty cycle arrival time.

In the power conversion system according to the second aspect of this invention, as described above, the control unit is configured to perform duty cycle control such that, during the first period from the switching start time when the switching of the switching element for initial charging the capacitor begins to the duty cycle switching time when the duty cycle is switched before the maximum duty cycle arrival time when the duty cycle of the switching element reaches its maximum value, the duty cycle is increased at a first rate of increase, and during the second period from the duty cycle switching time to the maximum duty cycle arrival time, the duty cycle is increased at a second rate of increase that is smaller than the first rate of increase. This allows for a relatively high charging speed of the capacitor, similar to the initial charging device according to the first aspect, while suppressing the capacitor input current from exceeding a predetermined current value at the maximum duty cycle arrival time. As a result, similar to the initial charging device according to the first aspect, the initial charging time of the capacitor in the power conversion device can be shortened while suppressing increased stress on the circuit components of the initial charging device and the capacitor in the power conversion device.

According to the present invention, as described above, it is possible to provide an initial charging device and a power conversion system that can shorten the initial charging time of the capacitor in the power conversion device while suppressing increased stress on the circuit components of the initial charging device and the capacitor in the power conversion device.

This is a circuit diagram showing the configuration of a power conversion system according to one embodiment of the present invention. This figure illustrates the switching duty cycle control and switching frequency control in an initial charging device according to one embodiment of the present invention. This figure illustrates the relationship between the switching frequency and the current ripple of the capacitor input current in an initial charging device according to one embodiment of the present invention. This figure illustrates the simulation results of initial charging in an initial charging device according to one embodiment of the present invention and in a power conversion device according to a comparative example.

The following describes embodiments of the present invention based on the drawings.

[Configuration of the power conversion system]
Referring to Figures 1 to 3, the configuration of a power conversion system 100 according to one embodiment of the present invention will be described. The power conversion system 100 is, for example, a power conversion system for railway vehicles.

(Overall configuration of the power conversion system)
As shown in Figure 1, the power conversion system 100 comprises a power conversion device 10 and an initial charging device 20.

The power converter 10 is a device that converts the power (first input power) input from the AC power source 200 and outputs it to the load 300. The load 300 is, for example, an electric motor.

The initial charging device 20 adjusts the current input from the DC power supply 400 and outputs it to the power converter 10, thereby initially charging the capacitor 13 of the power converter 10. Initial charging in the power conversion system 100 is performed to prevent excessive current (inrush current) from being input to the capacitor 13 of the power converter 10 from the AC power supply 200. Multiple (two in Figure 1) power converters 10 are connected in parallel to each other in the initial charging device 20.

(Configuration of the power converter)
The power conversion device 10 includes a rectifier circuit 11, a power conversion unit 12, a capacitor 13, and a control unit 14. The power conversion unit 12 is an example of the "first power conversion unit" in the claims.

The rectifier circuit 11 includes switching elements such as IGBTs (Insulated Gate Bipolar Transistors). The rectifier circuit 11 converts the AC power input from the AC power supply 200 into DC power through the switching of these switching elements.

The power conversion unit 12 includes a switching element 12a such as an IGBT. The power conversion unit 12 converts the DC power input from the rectifier circuit 11 into AC power by switching the switching element 12a and outputs it to the load 300. In other words, the power conversion unit 12 converts the power input from the AC power supply 200 (first input power) and outputs it to the load 300.

Capacitor 13 is connected to the output side of the rectifier circuit 11. Capacitor 13 is also connected to the input side of the power conversion unit 12. Capacitor 13 smooths the DC power input from the rectifier circuit 11.

The control unit 14 controls the switching of the switching elements in the rectifier circuit 11 and the switching of the switching elements 12a in the power conversion unit 12 using PWM (Pulse Width Modulation) control. The control unit 14 is a circuit board including a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), etc.

(Configuration of the initial charging device)
The initial charging device 20 includes a power conversion unit 21, a capacitor 22, a reactor 23, a transformer 24, a charging resistor 25, and a control unit 26. The power conversion unit 21 is an example of the "second power conversion unit" in the claims.

The power conversion unit 21 includes two switching elements 21a, such as IGBTs. That is, the initial charging device 20 is configured as a half-bridge converter. The capacitor 22 is connected to the input side of the power conversion unit 21. The reactor 23 is connected to the input side of the capacitor 22. The transformer 24 is connected to the output side of the power conversion unit 21. The transformer 24 is an isolation transformer. That is, the initial charging device 20 is configured as an isolated converter. The charging resistor 25 is connected to the output side of the transformer 24. The control unit 26 controls the switching of the switching elements 21a of the power conversion unit 21 by PWM control. The control unit 26 is a circuit board including a CPU, ROM, RAM, etc.

The power conversion unit 21 converts the input power (second input power) by switching the switching element 21a and outputs it to the power conversion device 10. Specifically, the DC power input from the DC power supply 400 to the initial charging device 20 is input to the power conversion unit 21 via the reactor 23 and capacitor 22. The power conversion unit 21 converts the input DC power (second input power) into AC power by switching the switching element 21a. The AC power output from the power conversion unit 21 is input to the rectifier circuit 11 of the power conversion device 10 via the transformer 24 and charging resistor 25.

(Initial charging of multiple power converters)
The initial charging device 20 is configured to sequentially perform initial charging on each of the capacitors 13 of the multiple power converters 10.

Specifically, a circuit breaker 31 is provided between each of the multiple power conversion devices 10 and the AC power supply 200 to interrupt the current. Furthermore, a circuit breaker 32 is provided between each of the multiple power conversion devices 10 and the initial charging device 20 to interrupt the current. Additionally, a circuit breaker 33 is provided between the initial charging device 20 and the DC power supply 400 to interrupt the current. Of the circuit breakers 31, 32, and 33, circuit breaker 32 is located inside the power conversion system 100, while circuit breakers 31 and 33 are located outside the power conversion system 100.

Then, from the state where all circuit breakers 31, 32, and 33 are open, the circuit breakers 32 and 33 between the first power converter 10 and the initial charging device 20 are closed. The opening and closing of the circuit breakers 32 and 33 between the first power converter 10 and the initial charging device 20 is controlled, for example, by the control unit 14 of the first power converter 10. Then, the control unit 26 of the initial charging device 20 controls the power conversion unit 21 to adjust the current input from the DC power supply 400 and output it to the power converter 10 based on the fact that the circuit breakers 32 and 33 between the first power converter 10 and the initial charging device 20 have been closed, thereby performing the initial charging of the capacitor 13 of the first power converter 10. Then, after the charging voltage Vc (see Figure 2) of the capacitor 13 of the first power converter 10 reaches the target voltage Vct (see Figure 2) and the initial charging is completed, the circuit breaker 32 between the first power converter 10 and the initial charging device 20 is opened.

Then, the control unit 14 of the second power converter 10 closes the circuit breaker 32 between the second power converter 10 and the initial charging device 20. The opening and closing of the circuit breaker 32 between the second power converter 10 and the initial charging device 20 is controlled by the control unit 14 of the second power converter 10. Then, based on the closing of the circuit breaker 32 between the second power converter 10 and the initial charging device 20, the control unit 26 of the initial charging device 20 controls the power conversion unit 21 to adjust the current input from the DC power supply 400 and output it to the power converter 10, thereby performing the initial charging of the capacitor 13 of the second power converter 10. After the charging voltage Vc (see Figure 2) of the capacitor 13 of the second power converter 10 reaches the target voltage Vct (see Figure 2) and the initial charging is completed, the circuit breaker 32 between the second power converter 10 and the initial charging device 20 is opened.

Furthermore, if the power conversion system 100 is equipped with three or more power conversion devices 10, the initial charging of the capacitor 13 of the third power conversion device 10 is performed in the same manner as for the second power conversion device 10.

(Duty ratio control during initial charging)
As shown in Figure 2, the control unit 26 (see Figure 1) controls the switching duty cycle D of the switching element 21a (see Figure 1) in order to adjust the capacitor input current Ic to the capacitor 13 of the power converter 10 (see Figure 1). The control unit 26 controls the duty cycle D so that when the capacitor input current Ic during initial charging increases the duty cycle D from 0 and reaches its maximum value (a predetermined duty cycle Dp), the duty cycle D becomes the maximum value Icm. Since the initial charging device 20 (see Figure 1) is a half-bridge type converter, the predetermined duty cycle Dp is 0.5.

The control unit 26 (see Figure 1) controls the duty cycle D from the switching start time t0, when the switching element 21a for initial charging of the capacitor 13 begins switching, until the maximum duty cycle arrival time t2, when the duty cycle D reaches its maximum value (a predetermined duty cycle Dp). From the maximum duty cycle arrival time t2 until the target voltage arrival time t4, when the charging voltage Vc of the capacitor 13 reaches the target voltage Vct, the control unit maintains the duty cycle D at its maximum value (a predetermined duty cycle Dp). In other words, the capacitor input current Ic is gradually increased (soft-start) from the switching start time t0 to the maximum duty cycle arrival time t2. After the duty cycle D reaches its maximum value (a predetermined duty cycle Dp), a general CR charging circuit is formed between the power conversion unit 21 (see Figure 1) of the initial charging device 20 (see Figure 1) and the capacitor 13 of the power conversion device 10 (see Figure 1).

The control unit 26 (see Figure 1) is configured to perform duty cycle control by increasing the duty cycle D at a first increase rate R1 during the first period P1, from the switching start time t0 to the duty cycle switching time t1, which is before the maximum duty cycle arrival time t2; and increasing the duty cycle D at a second increase rate R2, which is smaller than the first increase rate R1, during the second period P2, from the duty cycle switching time t1 to the maximum duty cycle arrival time t2. In other words, during the first period P1, increasing the duty cycle D at a relatively large first increase rate R1 increases the rate of increase of the capacitor input current Ic, while during the second period P2, increasing the duty cycle D at a relatively small second increase rate R2 suppresses the rate of increase of the capacitor input current Ic that was increased during the first period P1.

The control unit 26 (see Figure 1) is configured to perform duty cycle control so that the capacitor input current Ic does not exceed a predetermined current value Icp at the time t2 to reach the maximum duty cycle. Specifically, the first increase rate R1, the second increase rate R2, the first period P1, and the second period P2 are set so that the capacitor input current Ic is less than the predetermined current value Icp at the time t2 to reach the maximum duty cycle. The predetermined current value Icp is set based on the allowable current of the circuit components of the initial charging device 20 (transformer 24, charging resistor 25, etc.) and the capacitor 13 of the power converter 10.

The first period P1 is set to be longer than the second period P2. That is, the length of the first period P1 is made relatively long to allow for a larger increase in the capacitor input current Ic by increasing the duty cycle D with a relatively large first growth rate R1. The first growth rate R1 is set to more than twice the second growth rate R2.

(Frequency ratio control during initial charging)
The control unit 26 (see Figure 1) is configured to perform frequency control in the first period P1, the second period P2, and the third period P3 from the time to reach the maximum duty cycle t2 to the frequency switching time t3, which is before the time to reach the target voltage t4, setting the frequency f to a first frequency f1. In the fourth period P4 from the frequency switching time t3 to the time to reach the target voltage t4, it sets the frequency f to a second frequency f2, which is smaller than the first frequency f1. In other words, at the frequency switching time t3, which is after the time to reach the maximum duty cycle t2, the switching frequency f is switched from a relatively large first frequency f1 to a relatively small second frequency f2. The first frequency f1 is set to, for example, twice the second frequency f2.

The control unit 26 (see Figure 1) is configured to perform the frequency control described above so that the capacitor input current Ic does not exceed a predetermined current value Icp due to the current ripple that increases when switching the frequency f from the first frequency f1 to the second frequency f2. Here, the capacitor input current Ic continues to decrease from the time t2 to reach the maximum duty cycle until the time t4 to reach the target voltage. Also, as shown in Figure 3, the switching frequency f has a negative correlation with the current ripple (pulsation) of the capacitor input current Ic. That is, the frequency switching time t3 is set so that the capacitor input current Ic does not exceed a predetermined current value Icp due to the current ripple that increases when switching the frequency f from the first frequency f1 to the second frequency f2.

[Initial charging simulation]
Referring to Figure 4, the power conversion system 100 according to this embodiment and the simulation results of initial charging in the power conversion system 100 according to Comparative Examples 1 to 3 will be explained.

(Simulation of initial charging in the power conversion system according to this embodiment)
As shown in Figure 4, in the simulation of initial charging in the power conversion system 100 according to this embodiment, the length of the first period P1 from the switching start time t0 to the duty cycle switching time t1 was set to 1.5 times the length of the second period P2 from the duty cycle switching time t1 to the time to reach the maximum duty cycle t2. Also, the first increase rate R1 of the duty cycle D in the first period P1 was set to approximately 2.67 times the second increase rate R2 of the duty cycle D in the second period P2. Furthermore, the first frequency f1 was set to twice the second frequency f2. Also, the frequency switching time t3 was set to 61 times the time to reach the maximum duty cycle t2.

In the simulation of initial charging in the power conversion system 100 according to this embodiment, the charging voltage Vc of the capacitor 13 reached the target voltage Vct within a predetermined target time tp from the switching start time t0. This is thought to be because the charging speed of the capacitor 13 was relatively large for the following two reasons. The first reason is that in the first period P1, by increasing the duty cycle D at a relatively large first increase rate R1, the rate of increase of the capacitor input current Ic in the first period P1 was relatively large. The second reason is that in the fourth period P4, from the frequency switching time t3 to the target voltage arrival time t4 (see Figure 2), by setting the switching frequency f to a relatively small second frequency f2, the impedance of the transformer 24 (see Figure 1) of the initial charging device 20 (see Figure 1) was relatively small, resulting in a relatively large capacitor input current Ic.

Furthermore, in the simulation of initial charging in the power conversion system 100 according to this embodiment, the capacitor input current Ic did not exceed a predetermined current value Icp. This is thought to be due to the following two reasons. The first reason is that, in the second period P2, by increasing the duty cycle D at a second increase rate R2 that is sufficiently smaller than the first increase rate R1, the rate of increase of the capacitor input current Ic, which was increased in the first period P1, was sufficiently suppressed, preventing the capacitor input current Ic from exceeding a predetermined current value Icp at the time t2 when the maximum duty cycle is reached. The second reason is that, by setting the frequency switching time t3, when the relatively small second frequency f2 occurs, to a sufficiently later time than the time t2 when the maximum duty cycle is reached, the capacitor input current Ic from exceeding a predetermined current value Icp due to the current ripple that increases when switching the frequency f from the first frequency f1 to the second frequency f2 was prevented.

Based on the above, it has been confirmed that the power conversion system 100 according to this embodiment has a favorable configuration in terms of both the charging speed of the capacitor 13 and the capacitor input current Ic.

(Simulation of initial charging in a power conversion system according to Comparative Example 1)
In the simulation of initial charging in the power conversion system according to Comparative Example 1, the duty cycle D was increased by an increase rate R10 from the switching start time t0 to the time to reach the maximum duty cycle t12. The increase rate R10 was set to approximately 0.25 times the first increase rate R1 and approximately 0.67 times the second increase rate R2 in the simulation of initial charging in the power conversion system 100 according to this embodiment. Consequently, the time to reach the maximum duty cycle t12 became three times the time to reach the maximum duty cycle t2 in the simulation of initial charging in the power conversion system 100 according to this embodiment. In addition, the frequency f from the switching start time t0 to the time to reach the target voltage t4 (see Figure 2) was set to the second frequency f2.

In the simulation of initial charging in the power conversion system according to Comparative Example 1, the charging voltage Vc of capacitor 13 reached the target voltage Vct within a predetermined target time tp from the switching start time t0. This is thought to be because setting the switching frequency f to a relatively small second frequency f2 resulted in a relatively small impedance of the transformer 24 (see Figure 1) of the initial charging device 20 (see Figure 1), and a relatively large capacitor input current Ic.

Furthermore, in the simulation of initial charging in the power conversion system according to Comparative Example 1, the capacitor input current Ic exceeded a predetermined current value Icp around the time t12 when the maximum duty cycle was reached. This is thought to be because setting the switching frequency f to a relatively small second frequency f2 resulted in a relatively large current ripple in the capacitor input current Ic.

Based on the above, it was confirmed that the power conversion system according to Comparative Example 1 has a favorable configuration with respect to the charging speed of capacitor 13, but an unfavorable configuration with respect to the capacitor input current Ic.

(Simulation of initial charging in a power conversion system according to Comparative Example 2)
In the simulation of initial charging in the power conversion system according to Comparative Example 2, the duty cycle D was increased by an increase rate R20 from the switching start time t0 to the time to reach the maximum duty cycle t22. The increase rate R10 was set to approximately 0.05 times the first increase rate R1 and 0.125 times the second increase rate R2 in the simulation of initial charging in the power conversion system 100 according to this embodiment. Consequently, the time to reach the maximum duty cycle t12 became 16 times the time to reach the maximum duty cycle t2 in the simulation of initial charging in the power conversion system 100 according to this embodiment. In addition, the frequency f from the switching start time t0 to the time to reach the target voltage t4 (see Figure 2) was set to the second frequency f2.

In the simulation of initial charging in the power conversion system according to Comparative Example 2, the charging voltage Vc of capacitor 13 did not reach the target voltage Vct within the predetermined target time tp from the switching start time t0. Furthermore, in the simulation of initial charging in the power conversion system according to Comparative Example 2, the capacitor input current Ic did not exceed the predetermined current value Icp. These results are thought to be due to the relatively small increase rate R10 of the duty cycle D from the switching start time t0 to the maximum duty cycle arrival time t22, thereby relatively suppressing the increase rate of the capacitor input current Ic during this period.

Based on the above, it was confirmed that while the power conversion system according to Comparative Example 2 has a favorable configuration with respect to the capacitor input current Ic, it has an unfavorable configuration with respect to the charging speed of the capacitor 13.

(Simulation of initial charging in a power conversion system according to Comparative Example 3)
In the simulation of initial charging in the power conversion system according to Comparative Example 3, the voltage was increased at the same rate of increase R10 as in the power conversion system according to Comparative Example 1 from the switching start time t0 to the time to reach the maximum duty cycle t12. Furthermore, the frequency f from the switching start time t0 to the time to reach the target voltage t4 (see Figure 2) was defined as the first frequency f1.

In the simulation of initial charging in the power conversion system according to Comparative Example 3, the charging voltage Vc of capacitor 13 did not reach the target voltage Vct within the predetermined target time tp from the switching start time t0. This is thought to be because setting the switching frequency f to a relatively large first frequency f1 resulted in a relatively large impedance of the transformer 24 (see Figure 1) of the initial charging device 20 (see Figure 1), and consequently, a relatively small capacitor input current Ic.

Furthermore, in the simulation of initial charging in the power conversion system according to Comparative Example 3, the capacitor input current Ic did not exceed the predetermined current value Icp. This is thought to be because setting the switching frequency f to a relatively large first frequency f1 resulted in a relatively small current ripple in the capacitor input current Ic.

Based on the above, it was confirmed that the power conversion system according to Comparative Example 3 has a favorable configuration with respect to the capacitor input current Ic, but an unfavorable configuration with respect to the charging speed of the capacitor 13.

[Effects of the Embodiment]
In this embodiment, the following effects can be obtained.

In this embodiment, as described above, the control unit 26 is configured to perform duty ratio control such that, during the first period P1 from the switching start time t0, when the switching of the switching element 21a for initial charging of the capacitor 13 is started, to the duty ratio switching time t1, when the duty ratio D is switched before the maximum duty cycle arrival time t2, when the duty ratio D of the switching of the switching element 21a reaches its maximum value, the duty ratio D is increased by a first increase rate R1, and during the second period P2, from the duty ratio switching time t1 to the maximum duty cycle arrival time t2, the duty ratio D is increased by a second increase rate R2, which is smaller than the first increase rate R1. As a result, during the first period P1, the duty ratio D is increased by a relatively large first increase rate R1, so that the rate of increase of the capacitor input current Ic to the capacitor 13 of the power converter 10 can be made relatively large, and the charging speed of the capacitor 13 can be made relatively large. Furthermore, in the second period P2, the duty cycle D is increased at a relatively small second increase rate R2. This suppresses the rate of increase in the capacitor input current Ic, which was increased in the first period P1, preventing the capacitor input current Ic from exceeding a predetermined current value Icp at the time t2 when the maximum duty cycle is reached. As a result, the initial charging time of the capacitor 13 in the power converter 10 can be shortened while suppressing increased stress on the circuit components of the initial charging device 20 and the capacitor 13 of the power converter 10.

Furthermore, in this embodiment, as described above, the control unit 26 is configured to perform duty cycle control so that the capacitor input current Ic to the capacitor 13 of the power converter 10 does not exceed a predetermined current value Icp at the time t2 to reach the maximum duty cycle. This prevents the capacitor input current Ic from exceeding the predetermined current value Icp at the time t2 to reach the maximum duty cycle.

Furthermore, in this embodiment, as described above, the first period P1 is set to be longer than the second period P2. This allows for a relatively longer first period P1, which enables a larger increase in the capacitor input current Ic by increasing the duty cycle D at a relatively large first increase rate R1. Therefore, the charging speed of the capacitor 13 can be increased.

Furthermore, in this embodiment, as described above, the first growth rate R1 is set to more than twice the second growth rate R2. This allows for a greater increase in the capacitor input current Ic during the first period P1, compared to the case where the first growth rate R1 is less than twice the second growth rate R2, thereby increasing the charging speed of the capacitor 13.

Furthermore, in this embodiment, as described above, the control unit 26 is configured to perform frequency control such that, in the first period P1, the second period P2, and the third period P3 from the maximum duty cycle arrival time t2 to the frequency switching time t3, which is before the target voltage arrival time t4 when the charging voltage Vc of the capacitor 13 reaches the target voltage Vct, the frequency f is set to the first frequency f1, and in the fourth period P4 from the frequency switching time t3 to the target voltage arrival time t4, the frequency f is set to the second frequency f2, which is smaller than the first frequency f1. As a result, the frequency switching time t3, when the switching frequency f changes from the relatively large first frequency f1 to the relatively small second frequency f2, is after the maximum duty cycle arrival time t2, when the capacitor input current Ic reaches its maximum value Icm. Therefore, it is possible to suppress the capacitor input current Ic from exceeding a predetermined current value Icp due to the current ripple that increases when switching the frequency f from the first frequency f1 to the second frequency f2. Furthermore, during the fourth period P4, the switching frequency f becomes a relatively small second frequency f2. Therefore, if a transformer 24 is provided between the power conversion unit 21 of the initial charging device 20 and the power conversion device 10, the impedance of the transformer 24, which has a positive correlation with frequency f, becomes relatively small. In this case, during the fourth period P4, the capacitor input current Ic, which has a negative correlation with the impedance of the transformer 24, can be made relatively large, thereby increasing the charging speed of the capacitor 13.

Furthermore, in this embodiment, as described above, the control unit 26 is configured to perform the frequency control so that the capacitor input current Ic to the capacitor 13 of the power converter 10 does not exceed a predetermined current value Icp due to the current ripple that increases when switching the frequency f from the first frequency f1 to the second frequency f2. This prevents the capacitor input current Ic from exceeding a predetermined current value Icp due to the current ripple that increases when switching the frequency f from the first frequency f1 to the second frequency f2.

Furthermore, in this embodiment, as described above, the initial charging device 20 has multiple power converters 10 connected in parallel to each other, and the initial charging device 20 is configured to sequentially perform initial charging on each of the capacitors 13 of the multiple power converters 10. This allows for the initial charging of the capacitors 13 of the multiple power converters 10 to be performed in a relatively short time while suppressing excessive stress on the circuit components of the initial charging device 20 and the capacitors 13 of the power converters 10.

[Variations]
The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than the description of the embodiments above, and further includes all modifications (modifications) within the meaning and scope equivalent to the claims.

For example, in the above embodiment, the initial charging device 20 is configured to have multiple power converters 10 connected in parallel, and to sequentially initial charge each capacitor 13 of the multiple power converters 10. However, the present invention is not limited to this. In the present invention, the initial charging device may be connected to one power converter, and the initial charging device may be configured to initial charge the capacitor of the one power converter.

Furthermore, in the above embodiment, the control unit 26 is configured to perform frequency control, setting the frequency f to a first frequency f1 during the first period P1, the second period P2, and the third period P3 from the maximum duty cycle arrival time t2 to the frequency switching time t3, which is before the target voltage arrival time t4 when the charging voltage Vc of the capacitor 13 reaches the target voltage Vct. It also sets the frequency f to a second frequency f2, which is smaller than the first frequency f1, during the fourth period P4 from the frequency switching time t3 to the target voltage arrival time t4. However, the present invention is not limited to this configuration. In the present invention, the control unit may be configured not to perform frequency switching control.

Furthermore, while the above embodiment shows an example where the first growth rate R1 is set to twice or more the second growth rate R2, the present invention is not limited to this. In the present invention, the first growth rate may be set to less than twice the second growth rate.

Furthermore, while the above embodiment shows an example where the first period P1 is set to be longer than the second period P2, the present invention is not limited to this. In the present invention, the first period may be set to be equal to the second period, or it may be set to be shorter than the second period.

Furthermore, in the above embodiment, the control unit 26 was shown to control the capacitor input current Ic during initial charging so that it reaches its maximum value Icm at the timing when the duty cycle D increases from 0 and reaches its maximum value (a predetermined duty cycle Dp) (maximum duty cycle arrival time t2). However, the present invention is not limited to this. In the present invention, the control unit may control the capacitor input current during initial charging so that it reaches its maximum value before the maximum duty cycle arrival time, and so that the capacitor input current remains below its maximum value during the period between the time the capacitor input current reaches its maximum value and the time the maximum duty cycle arrival time.

Furthermore, while the above embodiment shows an example where the initial charging device 20 is configured as an isolated converter, the present invention is not limited to this. In the present invention, the initial charging device may be configured as a non-isolated converter or as a boost chopper.

Furthermore, in the above embodiment, the circuit breaker 32 is provided inside the power conversion system 100, while the circuit breakers 31 and 33 are provided outside the power conversion system 100. The circuit breaker 31 is provided between the power conversion device 10 and the AC power supply 200, the circuit breaker 32 is provided between the power conversion device 10 and the initial charging device 20, and the circuit breaker 33 is provided between the initial charging device 20 and the DC power supply 400. However, the present invention is not limited to this. In the present invention, the circuit breaker provided between the power conversion device and the AC power supply may be provided inside the power conversion system, the circuit breaker provided between the power conversion device and the initial charging device may be provided outside the power conversion system, and the circuit breaker provided between the initial charging device and the DC power supply may be provided inside the power conversion system.

10 Power conversion device 12 Power conversion unit (first power conversion unit)
13 Capacitor 20 Initial charging device 21 Power conversion unit (second power conversion unit)
21a Switching element 26 Control unit 100 Power conversion system 200 AC power supply 300 Load D Duty cycle (of the switching element) f Frequency (of the switching element) f1 First frequency f2 Second frequency Ic Capacitor input current Icm Maximum value (of the capacitor input current) Icp Predetermined current value P1 First period P2 Second period P3 Third period P4 Fourth period R1 First increase rate R2 Second increase rate t0 Switching start time t1 Duty cycle switching time t2 Time to reach maximum duty cycle t3 Frequency switching time t4 Time to reach target voltage Vc Charging voltage (of the capacitor) Vct Target voltage (of the capacitor charging voltage)

Claims (8)

An initial charging device for a capacitor in a power conversion device, which includes a first power conversion unit that converts a first input power input from an AC power source and outputs it to a load, and a capacitor connected to the input side of the first power conversion unit,
A second power conversion unit includes a switching element, which converts the second input power by switching the switching element and outputs it to the power conversion device,
The system comprises a control unit that controls the switching of the switching element,
Initial charging device, wherein the control unit is configured to perform duty ratio control such that, during a first period from a switching start time when switching of the switching element for initial charging of the capacitor is started to a duty ratio switching time when the duty ratio is switched before the maximum duty ratio arrival time when the duty ratio of the switching element reaches its maximum value, the duty ratio is increased by a first rate of increase, and during a second period from the duty ratio switching time to the maximum duty ratio arrival time, the duty ratio is increased by a second rate of increase which is smaller than the first rate of increase. The initial charging device according to claim 1, wherein the control unit is configured to perform duty cycle control so that the capacitor input current to the capacitor of the power converter does not exceed a predetermined current value during the time to reach the maximum duty cycle. The initial charging device according to claim 1, wherein the first period is set to be longer than the second period. The initial charging device according to claim 1, wherein the first increase rate is set to twice or more the second increase rate. The control unit is configured to perform frequency control in the first period, the second period, and the third period from the maximum duty cycle arrival time to the frequency switching time for switching the switching frequency of the switching element before the target voltage arrival time when the charging voltage of the capacitor reaches the target voltage, setting the frequency to a first frequency, and in the fourth period from the frequency switching time to the target voltage arrival time, setting the frequency to a second frequency smaller than the first frequency, as described in claim 1. The initial charging device according to claim 5, wherein the control unit is configured to perform frequency control so that the capacitor input current to the capacitor of the power converter does not exceed a predetermined current value due to the current ripple that increases when the frequency is switched from the first frequency to the second frequency. The initial charging device has a plurality of power conversion devices connected in parallel to each other.
The initial charging device according to claim 1, configured to sequentially initial charge the capacitors of each of the plurality of power converters. A power conversion device comprising: a first power conversion unit that converts a first input power received from an AC power source and outputs it to a load; and a capacitor connected to the input side of the first power conversion unit;
The power converter comprises an initial charging device for initial charging the capacitor,
The initial charging device,
A second power conversion unit includes a switching element, which converts the second input power by switching the switching element and outputs it to the power conversion device,
The system comprises a control unit that controls the switching of the switching element,
A power conversion system wherein the control unit is configured to perform duty ratio control such that, during a first period from a switching start time when the switching of the switching element for initial charging the capacitor is started, to a duty ratio switching time when the duty ratio is switched before the maximum duty cycle arrival time when the duty ratio of the switching element reaches its maximum value, the duty ratio is increased by a first rate of increase, and during a second period from the duty ratio switching time to the maximum duty cycle arrival time, the duty ratio is increased by a second rate of increase that is smaller than the first rate of increase.