Power converter

Abstract

This addresses the issue of unstable current control in power converters connected to power grids. [Solution] The power converter is connected to the power grid via grid wiring and converts AC power to DC power for output. The power converter includes a current control unit (85) that controls the output current to the current command value based on feedback control that includes a proportional term obtained by multiplying the deviation between the current command value and the output current by a proportionality constant, and a command unit (90) which can change the proportionality constant and commands the current control unit to the current command value. When the degree of oscillation of the output current is greater than a predetermined degree while the command unit has commanded the current control unit to a first current smaller than the rated current of the power converter, the command unit changes the proportionality constant to reduce the degree of oscillation of the output current to less than the predetermined degree, and then commands the current control unit to a second current larger than the first current.

Description

Technical Field

[0005]

[0001] The present invention relates to a power conversion device that converts AC power into DC power and outputs it.

Background Art

[0002] For example, there is a grid-connected inverter connected to a pole-mounted transformer (power grid) via a wire (see Patent Document 1). The grid-connected inverter described in Patent Document 1 estimates the grid impedance, which is the impedance of the wire from the grid-connected inverter to the pole-mounted transformer, and sets control parameters so that the control system becomes stable based on the estimated grid impedance. The control parameter is a set value for controlling the operating characteristics of the grid-connected inverter.

Prior Art Documents

Patent Documents

[0003] <00,00016>

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, in order to charge an in-vehicle battery with the grid voltage of the power grid, there is a charger that converts AC power into DC power with an AC / DC power conversion device and controls the battery charging current with a DC / DC power conversion device. The AC / DC power conversion device generally performs feedback control of the output current to a current command value.

[0005] Depending on the relationship between the inductance of the grid wiring (grid inductance) from the AC / DC power conversion device to the pole-mounted transformer (power grid) and the control constant in the above feedback control, it has been newly found that the current control becomes unstable. If the current control of the AC / DC power conversion device becomes unstable, the power conversion device may be damaged, so the capacity of the AC / DC power conversion device has to be increased, which may hinder miniaturization.

[0006] The present invention was made to solve the above problems, and its main objective is to address the instability of current control in power conversion devices connected to power grids. [Means for solving the problem]

[0007] The first means to solve the above problem is, A power converter (40) connected to a power grid (15) via a grid wiring (13) that converts AC power to DC power and outputs it, A current control unit (85) controls the output current to the current command value based on feedback control that includes a proportional term obtained by multiplying the difference between the current command value and the output current by a proportionality constant, The proportionality constant can be changed, and the system includes a command unit (90) that commands the current command value to the current control unit, When the command unit has commanded the current control unit to output a first current smaller than the rated current of the power converter, and the oscillation degree of the output current is greater than a predetermined degree, the command unit changes the proportionality constant to reduce the oscillation degree of the output current to less than the predetermined degree, and then commands the current control unit to output a second current larger than the first current.

[0008] According to the above configuration, the power converter is connected to the power grid via grid wiring and converts AC power to DC power for output.

[0009] Here, the current control unit controls the output current to the current command value based on feedback control that includes a proportional term obtained by multiplying the deviation between the current command value and the output current by a proportionality constant. When the proportionality constant is large, the responsiveness of the current control is increased, and the total harmonic distortion (THD) of the system current flowing through the system wiring can be reduced. However, it has been newly discovered that when the proportionality constant is large and the inductance of the system wiring (hereinafter referred to as "system inductance") is large, the output current oscillates and the current control becomes unstable. On the other hand, when the proportionality constant is small, the oscillation of the output current can be suppressed. However, when the proportionality constant is small and the system inductance is small, the harmonic distortion of the system current increases, and the current control becomes unstable. The magnitude of the system inductance increases with the length of the system wiring (it changes with length). Neither the manufacturer nor the user can know in advance the magnitude of the system inductance in the operating environment of the power converter.

[0010] In this regard, the command unit can change the proportionality constant of the proportional term in the feedback control. Therefore, even if the magnitude of the system inductance cannot be known in advance, the output current can be stabilized by changing the proportionality constant, that is, the instability of the current control can be addressed. Furthermore, if the degree of oscillation of the output current is greater than a predetermined degree while the command unit has commanded the current control unit to output a first current smaller than the rated current of the power converter, the command unit changes the proportionality constant to reduce the degree of oscillation of the output current to less than the predetermined degree, and then commands the current control unit to output a second current larger than the first current. Therefore, it is possible to suppress the power converter from outputting the rated current when the degree of oscillation of the output current is greater than a predetermined degree. Consequently, damage to the power converter can be suppressed, eliminating the need to increase the capacity of the power converter and allowing the power converter to be miniaturized. [Brief explanation of the drawing]

[0011] [Figure 1] A circuit diagram showing the onboard charger and its peripheral components. [Figure 2] Circuit diagram of the PFC converter according to the first embodiment. [Figure 3] Block diagram showing the procedure for determining the d-axis voltage command value of the PFC converter in FIG. 2. [Figure 4] Flowchart showing the procedure for controlling the Id command value. [Figure 5] Graph showing the simulation results of the relationship between the line inductance, phase margin, and proportional constant. [Figure 6] Time chart showing the simulation results when the flowchart of FIG. 4 is executed with the line inductance being 0. [Figure 7] Time chart showing the simulation results when the flowchart of FIG. 4 is executed with the line inductance being the maximum value. [Figure 8] Graph showing the simulation results of the relationship between the inductance of the PFC coil, harmonic distortion rate, line inductance, and proportional constant. [Figure 9] Flowchart showing a modified example of the procedure for controlling the Id command value. [Figure 10] Flowchart showing the procedure for controlling the Id command value in the second embodiment. [Figure 11] Flowchart showing a modified example of the procedure for controlling the Id command value. [Figure 12] Block diagram showing a modified example of the procedure for determining the d-axis voltage command value.

Mode for Carrying Out the Invention

[0012] (First Embodiment) Hereinafter, the first embodiment embodied in an in-vehicle charger for charging an in-vehicle battery will be described with reference to the drawings.

[0013] As shown in FIG. 1, the vehicle 10 includes an in-vehicle battery 11 and an in-vehicle charger 20. The in-vehicle battery 11 is a secondary battery such as a lithium-ion battery, nickel-metal hydride battery, or lead-acid battery.

[0014] The in-vehicle charger 20 includes an isolated DC / DC converter 31, a PFC converter 40, DC terminals DCL1 and DCL2, AC terminals ACL1, ACL2, and ACL3, and a neutral point terminal ACLN, etc.

[0015] The DC terminal DCL1 is connected to the positive electrode of the in-vehicle battery 11. The DC terminal DCL2 is connected to the negative electrode of the in-vehicle battery 11.

[0016] When charging the in-vehicle battery 11, the AC terminals ACL1, ACL2, and ACL3 are connected to the system 15 via the system wiring 13, and the neutral point terminal ACLN is connected to the neutral point N of the system 15. The system 15 (corresponding to the power system) supplies three-phase AC power.

[0017] The PFC converter 40 (corresponding to the power conversion device) is connected to the isolated DC / DC converter 31, the AC terminals ACL1, ACL2, and ACL3, and the neutral point terminal ACLN. That is, when charging the in-vehicle battery 11, the PFC converter 40 is connected to the system 15 via the system wiring 13. The PFC converter 40 converts the AC power input from the AC terminals ACL1, ACL2, and ACL3 into DC power and inputs it to the isolated DC / DC converter 31.

[0018] The isolated DC / DC converter 31 is an isolated type DC / DC converter. When charging the in-vehicle battery 11, the isolated DC / DC converter 31 controls the current flowing through the in-vehicle battery 11 and the voltage applied to the in-vehicle battery 11.

[0019] As shown in FIG. 2, the PFC converter 40 includes an AC filter 50, a PFC coil 41, a current sensor 43, an inverter circuit 60, a smoothing capacitor 69, and a control device 80, etc.

[0020] The AC filter 50 is a circuit that removes AC power (AC current) within a predetermined frequency range from the AC power input from AC terminals ACL1, ACL2, and ACL3. A low-pass filter is used as the AC filter 50. The AC filter 50 has a series connection for each phase, in which two common coils 51 and 52 are connected in series. The AC filter 50 has X capacitors 53, 54, and 55, one end of which is connected to each phase. The other end of each X capacitor 53, 54, and 55 is connected to the neutral terminal ACLN. In other words, the other ends of each X capacitor 53, 54, and 55 are connected to each other.

[0021] The PFC coil 41 is connected between the AC filter 50 and the inverter circuit 60. A PFC coil 41 is provided for each phase. The voltage sensor 42 detects the input voltages Vu, Vv, and Vw that are input to the PFC coil 41 of each phase. A current sensor 43 is provided for each phase and detects the currents Iu, Iv, and Iw that flow through the PFC coil 41 of each phase.

[0022] The inverter circuit 60 is composed of a full-bridge circuit, each having the same number of upper and lower arms as the number of phases in the three-phase system. The PFC coil 41 for each phase is connected to the connection point between the switching element of the upper arm and the switching element of the lower arm in each phase. A smoothing capacitor 69 is connected between the inverter circuit 60 and the isolated DC / DC converter 31. The drive state of the inverter circuit 60 is controlled by the control device 80.

[0023] The control device 80 controls the output current to the isolated DC / DC converter 31 while boosting the system voltage input to the PFC converter 40 through the action of the PFC coil 41, inverter circuit 60, and smoothing capacitor 69. In order to improve the power factor of the PFC converter 40, the control device 80 uses the input voltages Vu, Vv, Vw of each phase PFC coil 41 detected by the voltage sensor 42, and the currents Iu, Iv, Iw flowing through each phase PFC coil 41 detected by the current sensor 43 for control. Generally, the PFC converter 40 uses the input voltage of the PFC coil 41 in a feedforward (FF) term to determine the voltage command value for controlling the output current to the current command value in order to improve control responsiveness.

[0024] Here, the magnitude of the inductance Lg of the system wiring 13 (hereinafter referred to as "system inductance Lg") increases as the length of the system wiring 13 increases (it changes with length). Since the environments in which the on-board charger 20 is used are all different, neither the manufacturer nor the user can know in advance the magnitude of the system inductance Lg in the environment in which the on-board charger 20 is used. It has been newly discovered that when the system inductance Lg is large, the phase margin of the current control system of the PFC converter 40 decreases, and the current control becomes unstable. The voltages input to the above AC terminals ACL1, ACL2, ACL3, and consequently the input voltages Vu, Vv, Vw of the PFC coil 41, are more prone to fluctuation as the system inductance Lg increases. The inventors of this application have focused on the fact that when the system inductance Lg is large, using the input voltages Vu, Vv, Vw of the PFC coil 41 in the FF term exacerbates the oscillation of the currents Iu, Iv, Iw.

[0025] Furthermore, generally, the PFC converter 40 determines a voltage command value to control the output current to the current command value based on feedback control by a PI controller. The PI controller controls the output current to the current command value based on feedback control that includes a proportional term obtained by multiplying the deviation between the current command value and the output current by a proportionality constant Kp, and an integral term obtained by multiplying the integral of the deviation by an integration constant Ki. When the proportionality constant Kp is large, the responsiveness of the current control is increased, and the total harmonic distortion (THD) of the system current flowing through the system wiring 13 can be reduced. However, it has been newly discovered that when the proportionality constant Kp is large and the system inductance Lg is large, the output current oscillates and becomes unstable. On the other hand, when the proportionality constant Kp is small, the oscillation of the output current can be suppressed. However, when the proportionality constant Kp is small and the system inductance Lg is small, the harmonic distortion THD of the system current increases, and the current control becomes unstable.

[0026] The control device 80 performs the following control to solve the above problems. Figure 3 is a block diagram showing the procedure for determining the d-axis voltage command value Vd* of the PFC converter 40 in Figure 2. In this example, the d-axis direction is defined as the active power.

[0027] The control device 80 is mainly composed of a microcomputer equipped with a CPU, memory (ROM, RAM), input / output interface, and drive circuitry. The control device 80 realizes the functions of the dq conversion units 82, 83, subtractor 84, PI controller 85, adder 86, 87, vector control unit 88, and command unit 90 by executing a stored program, for example.

[0028] The dq conversion unit 82 (corresponding to the second current control unit) receives the input voltages Vu, Vv, and Vw of the PFC coils 41 for each phase, detected by the voltage sensors 42 for each phase. The dq conversion unit 82 converts the input voltages Vu, Vv, and Vw of the PFC coils 41 for each phase into the d-axis voltage Vd and the q-axis voltage Vq. The dq conversion unit 82 inputs the d-axis voltage Vd as a feedforward term (corresponding to a predetermined feedforward term) to the adder 86.

[0029] The dq conversion unit 83 converts the currents Iu, Iv, and Iw detected by the current sensors 43 for each phase into the d-axis current Id and the q-axis current Iq. The dq conversion unit 83 inputs the d-axis current Id to the subtractor 84.

[0030] The subtractor 84 receives the Id command value (d-axis current command value Id*) from the command unit 90. The Id command value (corresponding to the current command value) is calculated, for example, based on the Id request value from the isolated DC / DC converter 31. The subtractor 84 inputs the difference ΔId (corresponding to the deviation) between the Id command value and the d-axis current Id (corresponding to the output current) to the PI controller 85. In other words, the command unit 90 commands the PI controller 85 for the Id command value.

[0031] The PI controller 85 (corresponding to the current control unit and the first current control unit) calculates a feedback term (FB term) for controlling the d-axis current Id to the Id command value based on the input difference ΔId. Specifically, the FB term is the sum of a proportional term KpΔId, obtained by multiplying the difference ΔId by the proportionality constant Kp, and an integral term Ki∫ΔId·dt, obtained by multiplying the integral of the difference ΔId, ∫ΔId·dt, by the integration constant Ki (FB term = KpΔId + Ki∫ΔId·dt). The PI controller 85 inputs the calculated FB term to the adder 86. Here, the command unit 90 can change the proportionality constant Kp used in the PI controller 85. The procedure for the command unit 90 to change the proportionality constant Kp will be described later.

[0032] Adder 86 inputs the sum of the d-axis voltage Vd (as a feedforward term) and the FB term to adder 87. Adder 87 adds a correction term ωLIq to the sum of the d-axis voltage Vd (as a feedforward term) and the FB term to calculate the d-axis voltage command value Vd*. That is, the d-axis voltage command value Vd* is determined based on the FF term and the FB term. Note that the calculation method of the correction term ωLIq is publicly known, so a detailed explanation is omitted. Adder 87 inputs the d-axis voltage command value Vd* to the vector control unit 88. The voltage command unit is composed of the dq conversion unit 82, adder 86, and adder 87.

[0033] The vector control unit 88 calculates drive signals Duty_u to Duty_w for each switching element of the inverter circuit 60 based on the d-axis voltage command value Vd* using vector control. Various known methods can be applied to the calculation of the drive signals Duty_u to Duty_w, so a detailed explanation is omitted. The inverter circuit 60 is then driven by the drive signals Duty_u to Duty_w.

[0034] Figure 4 is a flowchart showing the procedure for controlling the Id command value. This series of processes is performed by the control device 80.

[0035] First, the proportionality constant Kp is set to Kp1, and the Id command value is set to a current command value equivalent to, for example, 1 / 5 of the rated output (corresponding to the first current) (S10). As shown by the dashed line in Figure 8, the proportionality constant Kp1 is a value that can lower the harmonic distortion rate THD of the system current flowing through AC terminals ACL1, ACL2, and ACL3 to a threshold THD1, even when the system inductance Lg = 0 μH (small). The proportionality constant Kp1 can be set to an appropriate value in advance based on tests and simulations. The Id command value may also be set to a current command value equivalent to 1 / 10 to 1 / 5 of the rated output.

[0036] Next, it is determined whether the difference ΔId between the Id command value and the d-axis current Id is continuously oscillating, or whether the current output has been stopped due to overcurrent (S11). The d-axis current Id is calculated by converting the currents Iu, Iv, and Iw detected by the current sensors 43 for each phase into dq values. It can be determined that the difference ΔId is continuously oscillating, for example, if the peak-to-peak value of the difference ΔId (the difference between the peak maximum value and the peak minimum value) is greater than a threshold, or if the AC RMS value of the difference ΔId is greater than a threshold. It can be determined that the current output has been stopped due to overcurrent, for example, if the RMS value of the d-axis current Id is greater than a threshold. If this determination is negative (S11: NO), the process proceeds to S13. That is, if it is determined that the difference ΔId between the Id command value and the d-axis current Id is not continuously oscillating, and it is determined that the current output has not been stopped due to overcurrent, the control device 80 maintains the proportionality constant Kp=Kp1 and calculates the FB term according to the procedure shown in Figure 3. The control device 80 then drives the inverter circuit 60 based on the d-axis voltage command value Vd* calculated using this FB term. Note that if the difference ΔId between the Id command value and the d-axis current Id continues to oscillate, it corresponds to the degree of oscillation of the current flowing through the PFC coil 41 being greater than a predetermined degree.

[0037] On the other hand, in the determination of S11, if it is determined that the difference ΔId between the Id command value and the d-axis current Id is continuously oscillating, or if it is determined that the current output has stopped due to overcurrent (S11:YES), the value obtained by subtracting the coefficient change amount ΔK from the proportionality constant Kp is set as the new proportionality constant Kp (S12). The coefficient change amount ΔK can be set in advance to an appropriate value by conducting tests or simulations using the magnitude of the system inductance Lg expected in the operating environment of the onboard charger 20. Then, using the new proportionality constant Kp, the FB term is calculated according to the procedure shown in Figure 3. Then, the control device 80 drives the inverter circuit 60 based on the d-axis voltage command value Vd* calculated using this FB term.

[0038] Next, it is determined whether the difference ΔId between the Id command value and the d-axis current Id is stable (S13). Stability of the difference ΔId between the Id command value and the d-axis current Id can be determined, for example, by whether the peak-to-peak value of the difference ΔId is greater than the threshold, or whether the AC RMS value of the difference ΔId is greater than the threshold, similar to the process in S11. The threshold in the process of S13 may be the same as the threshold in the process of S11, or it may be a different value. If it is determined in this determination that the difference ΔId between the Id command value and the d-axis current Id is not stable (S13:NO), the process is repeated starting from S11.

[0039] On the other hand, in the determination in S13, if it is determined that the difference ΔId between the Id command value and the d-axis current Id is stable (S13: YES), the Id command value is set to, for example, a current command value equivalent to the rated output (corresponding to the second current) (S14). Then, the control device 80 calculates the d-axis voltage command value Vd* using the Id command value equivalent to the rated output and the current proportionality constant Kp in the procedure shown in Figure 3, and drives the inverter circuit 60 based on this d-axis voltage command value Vd*. After that, this series of processes is terminated (END). That is, if the oscillation degree of the current flowing through the PFC coil 41 is greater than a predetermined degree with the Id command value set to the first current, the proportionality constant Kp is made smaller (changed) than the current proportionality constant Kp to reduce the oscillation degree of the current flowing through the PFC coil 41 to less than a predetermined degree, and then the Id command value is set to a second current that is greater than the first current.

[0040] Figure 5 is a graph showing the simulation results of the relationship between system inductance Lg, phase margin, and proportionality constant Kp. Lg_Max is the maximum value of system inductance Lg expected in the operating environment of the onboard charger 20. When the phase margin becomes 0° (phase lag of 180°), the current may oscillate and resonate.

[0041] When the proportionality constant Kp = Kp1, the phase margin is greater than 0° for system inductance Lg between 0 and Lg1, and current control is stable. However, when the system inductance Lg becomes greater than Lg1, the phase margin becomes 0°, and there is a risk of current oscillation and resonance.

[0042] When the proportionality constant Kp = Kp² (for example, Kp² = Kp1 - ΔK), the phase margin is greater than 0° for the system inductance Lg from 0 to Lg_Max, and the current control is stable. However, if the proportionality constant Kp = K² is set when the system inductance Lg is 0 (small), the harmonic distortion (THD) of the system current increases, and the current control becomes unstable.

[0043] Therefore, for example, by setting the proportionality constant Kp = Kp1 when the system inductance Lg is from 0 to Lg1, and the proportionality constant Kp = Kp2 when the system inductance Lg is from Lg1 to Lg_Max, current control can be stabilized when the system inductance Lg is from 0 to Lg_Max.

[0044] Figure 6 is a time chart showing the simulation results when the flowchart in Figure 4 is executed with a system inductance Lg of 0.

[0045] At time t11, the control device 80 is set to a proportionality constant Kp=Kp1 and an Id command value equivalent to 1 / 5 of the rated output. As shown in Figure 5, with system inductance Lg=0 and proportionality constant Kp=Kp1, the phase margin becomes greater than 0°, and current control becomes stable. For this reason, the difference ΔId between the Id command value and the detected Id value (d-axis current Id calculated by dq conversion) is approximately 0, and the difference ΔId is stable.

[0046] At time t12, the control device 80 determines that the difference ΔId is stable and sets the Id command value to the rated output while maintaining the proportionality constant Kp=Kp1. Even after the Id command value becomes equivalent to the rated output, the difference ΔId between the Id command value and the detected Id value is approximately 0, and the difference ΔId is stable.

[0047] Figure 7 is a time chart showing the simulation results when the flowchart in Figure 4 is executed at Lg_Max for the system inductance Lg.

[0048] At time t21, the control device 80 sets the proportionality constant Kp = Kp1 and the Id command value to 1 / 5 of the rated output. As shown in Figure 5, with system inductance Lg = Lg_Max and proportionality constant Kp = Kp1, the phase margin becomes 0 and the current oscillates. Therefore, the difference ΔId between the Id command value and the detected Id value becomes larger than the threshold.

[0049] At time t22, the control device 80 determines that the difference ΔId is oscillating continuously and updates the proportionality constant Kp to the proportionality constant Kp2 (for example, Kp2 = Kp1 - ΔK). As shown in Figure 5, with system inductance Lg = Lg_Max and proportionality constant Kp = Kp2, the phase margin becomes greater than 0°, and current control becomes stable. Therefore, the difference ΔId between the Id command value and the Id detection value is approximately 0, and the difference ΔId is stable. In other words, the control device 80 changes the proportionality constant Kp so that the oscillation degree of the currents Iu, Iv, and Iw flowing through the PFC coil 41 is smaller than the current oscillation degree.

[0050] At time t23, the control device 80 determines that the difference ΔId is stable and sets the Id command value to the rated output while maintaining the proportionality constant Kp=Kp2. After the Id command value becomes equivalent to the rated output, the difference ΔId between the Id command value and the Id detected value temporarily becomes slightly larger before becoming approximately 0, indicating that the difference ΔId is stable. Although the Id detected value fluctuates slightly at time t23, the Id detected value converges to the Id command value due to the action of the FB term calculated by the PI controller 85 in Figure 3.

[0051] Figure 8 is a graph showing the simulation results of the relationship between the inductance Lp of the PFC coil 41, the harmonic distortion THD, the system inductance Lg, and the proportionality constant Kp. If the proportionality constant Kp = Kp1, even when the system inductance Lg = 0 μH (small), the harmonic distortion THD of the system current flowing through AC terminals ACL1, ACL2, and ACL3 can be lower than the threshold THD1. If the proportionality constant Kp = Kp2, although not shown in the figure, when the system inductance Lg = 0 μH (small), the harmonic distortion THD of the system current will be higher than the threshold THD1. However, even with the proportionality constant Kp = Kp2, when the system inductance Lg = Lg_Max (large), the harmonic distortion THD of the system current can be lower than the threshold THD1. Specifically, when the inductance Lp of the PFC coil 41 is in the range of Lp1 to Lp2, the harmonic distortion THD of the system current can be lower than the threshold THD1. In other words, the inductance Lp of the PFC coil 41 can be reduced to Lp1.

[0052] The embodiment described in detail above has the following advantages.

[0053] The command unit 90 can change the proportionality constant Kp of the proportional term KpΔId in the feedback control. Therefore, even if the magnitude of the system inductance Lg cannot be known in advance, the d-axis current Id can be stabilized by changing the proportionality constant Kp, that is, the instability of the current control can be addressed. Furthermore, if the degree of oscillation of the d-axis current Id is greater than a predetermined degree while the command unit 90 has commanded the PI controller 85 to an Id command value (first current) equivalent to 1 / 5 of the rated output of the PFC converter 40, the command unit 90 changes the proportionality constant Kp to reduce the degree of oscillation of the d-axis current Id to less than the predetermined degree, and then commands the PI controller 85 to an Id command value (second current) equivalent to the rated output. Therefore, it is possible to suppress the PFC converter 40 from outputting the rated current when the degree of oscillation of the d-axis current Id is greater than a predetermined degree. Therefore, since damage to the PFC converter 40 can be suppressed, it is not necessary to increase the inductance Lp of the PFC coil 41 (capacitance of the PFC converter 40), and the PFC coil 41 (PFC converter 40) can be made smaller.

[0054] When the degree of oscillation of the d-axis current Id is greater than a predetermined degree due to a large proportionality constant Kp and a large system inductance Lg, the oscillation of the d-axis current Id can be suppressed by reducing the proportionality constant Kp from its current value. Furthermore, when the system inductance Lg is large, even if the proportionality constant Kp is reduced, the harmonic distortion (THD) of the system current can be lowered, thereby stabilizing current control.

[0055] While the control responsiveness can be improved when the dq conversion unit 82 controls the d-axis current Id based on feedforward control (FF control) using the input voltages Vu, Vv, Vw of the PFC coil 41, the following problem arises: When the system inductance Lg is large, the input voltages Vu, Vv, Vw of the PFC coil 41 fluctuate, and using these input voltages Vu, Vv, Vw for FF control exacerbates current oscillations. Even in this case, the d-axis current Id can be stabilized by changing the proportionality constant Kp, thus addressing the instability of the current control.

[0056] The first embodiment can also be implemented with the following modifications. Parts identical to those in the first embodiment are denoted by the same reference numerals, and their descriptions are used accordingly.

[0057] If the oscillation of the d-axis current Id is attenuated when the PI controller 85 is commanded to an Id command value (first current) equivalent to 1 / 5 of the rated output, it is considered that the current oscillation is temporary and not caused by a large system inductance Lg. In this case, changing the proportionality constant Kp may actually make the d-axis current Id unstable. Therefore, the command unit 90 may prohibit changing the proportionality constant Kp when the oscillation of the d-axis current Id is attenuated when the first current is commanded to the PI controller 85. For example, if the peak-to-peak value of the difference ΔId is determined to be greater than the threshold only once, and the peak-to-peak value of the difference ΔId is not determined to be greater than the threshold consecutively, it can be determined that the oscillation of the d-axis current Id is attenuated. Also, if the peak-to-peak value of the difference ΔId is decreasing sequentially, it can be determined that the oscillation of the d-axis current Id is attenuated. With this configuration, unnecessary changes to the proportionality constant Kp can be suppressed when the current oscillation is temporary.

[0058] The procedure for controlling the Id command value shown in Figure 4 can also be changed as shown in Figure 9. That is, the control device 80 is set to proportionality constant Kp = Kp1 and the Id command value is set to a current command value equivalent to the rated output, for example (S10A). The processes in S11 to S13 are the same as the processes in S11 to S13 in Figure 4. The process in S14 in Figure 4 is omitted. Even with this configuration, it is possible to stabilize the d-axis current Id by changing the proportionality constant Kp, that is, to address the instability of current control.

[0059] (Second Embodiment) The second embodiment will now be described, focusing on the differences from the first embodiment. In this embodiment, the method for setting the proportionality constant Kp differs from that of the first embodiment. Parts identical to those in the first embodiment will be denoted by the same reference numerals for further explanation.

[0060] The procedure for controlling the Id command value shown in Figure 4 is changed as shown in Figure 10. Specifically, the control device 80 is set to a proportionality constant Kp = Kp3, and the Id command value is set to a current command value equivalent to, for example, 1 / 5 of the rated output (corresponding to the first current) (S10B). Alternatively, the Id command value may be set to a current command value equivalent to 1 / 10 to 1 / 5 of the rated output.

[0061] Next, it is determined whether the harmonic distortion rate (THD) of the system current is high, or whether the current output has been stopped due to an overcurrent (S11B). The harmonic distortion rate (THD) can be calculated, for example, by identifying the fundamental wave of system 15, measuring the harmonic components other than the fundamental wave, and dividing the sum of the RMS values ​​of the harmonic components by the RMS value of the fundamental wave. Various known methods can be applied to the calculation of the harmonic distortion rate (THD), so a detailed explanation will be omitted. It can be determined, for example, that the harmonic distortion rate (THD) of the system current is high if the calculated harmonic distortion rate (THD) is higher than the threshold THD1 (corresponding to a predetermined distortion rate). If this determination is negative (S11B: NO), the process proceeds to S13B. That is, if it is determined that the harmonic distortion rate (THD) of the system current is not high, and it is determined that the current output has not been stopped due to an overcurrent, the control device 80 maintains the proportionality constant Kp=Kp3 and calculates the FB term according to the procedure shown in Figure 3. Then, the control device 80 drives the inverter circuit 60 based on the d-axis voltage command value Vd* calculated using this FB term.

[0062] On the other hand, in the determination of S11B, if it is determined that the harmonic distortion rate (THD) of the system current is high, or if it is determined that the current output has been stopped due to overcurrent (S11B: YES), the value obtained by adding the coefficient change amount ΔK to the proportionality constant Kp is set as the new proportionality constant Kp (S12B). The coefficient change amount ΔK can be set in advance to an appropriate value by conducting tests or simulations using the magnitude of the system inductance Lg expected in the operating environment of the onboard charger 20. Then, using the new proportionality constant Kp, the FB term is calculated according to the procedure shown in Figure 3. Then, the control device 80 drives the inverter circuit 60 based on the d-axis voltage command value Vd* calculated using this FB term.

[0063] Next, it is determined whether the harmonic distortion (THD) of the system current is low or not (S13B). Low harmonic distortion of the system current can be determined, for example, by checking if the calculated harmonic distortion (THD) is lower than the threshold THD2. Note that the threshold THD2 in S13B may be the same as the threshold THD1 in S11B, or it may be a different value from the threshold THD1 in S11B. If it is determined in this check that the harmonic distortion of the system current is not low (S13B: NO), the process is repeated starting from S11B.

[0064] On the other hand, in the determination in S13B, if it is determined that the harmonic distortion rate (THD) of the system current is low (S13B: YES), the Id command value is set to, for example, a current command value equivalent to the rated output (corresponding to the second current) (S14). Then, the control device 80 calculates the d-axis voltage command value Vd* using the Id command value equivalent to the rated output and the current proportionality constant Kp in the procedure shown in Figure 3, and drives the inverter circuit 60 based on this d-axis voltage command value Vd*. After that, this series of processes is terminated (END).

[0065] The embodiment described in detail above has the following advantages.

[0066] - It is possible to suppress the PFC converter 40 from outputting the rated current when the harmonic distortion rate (THD) of the system current is higher than the threshold THD1. Therefore, damage to the PFC converter 40 can be suppressed, eliminating the need to increase the inductance Lp of the PFC coil 41 (capacitance of the PFC converter 40), and allowing the PFC coil 41 to be miniaturized.

[0067] When the harmonic distortion (THD) of the system current is higher than the threshold THD1 due to a small proportionality constant Kp and a small system inductance Lg, increasing the proportionality constant Kp from its current value can reduce the harmonic distortion (THD) of the system current to below the threshold THD1. Furthermore, if the system inductance Lg is small, even if the proportionality constant Kp is increased, the oscillation of the d-axis current Id can be reduced, thereby stabilizing the current control.

[0068] Furthermore, the procedure for controlling the Id command value shown in Figure 10 can be changed as shown in Figure 11. Specifically, the control device 80 is set to a proportionality constant Kp = Kp3, and the Id command value is set to a current command value equivalent to the rated output, for example (S10C). The processing in S11B to S13B is the same as the processing in S11B to S13B in Figure 10. The processing in S14 in Figure 10 is omitted. Even with this configuration, it is possible to stabilize the d-axis current Id by changing the proportionality constant Kp, that is, to address the instability of current control.

[0069] Furthermore, the first and second embodiments can also be implemented with the following modifications. Parts identical to those in the first and second embodiments are denoted by the same reference numerals, and their descriptions are used accordingly.

[0070] As a feedback control that includes a proportional term KpΔId, which is obtained by multiplying the difference ΔId (deviation) between the Id command value (current command value) and the d-axis current Id (output current) by a proportionality constant Kp, not only PI control but also PID control, PD control, and P control can be employed.

[0071] As shown in Figure 12, the FF term can also be omitted when determining the d-axis voltage command value Vd*. This configuration also allows for stabilizing the d-axis current Id by changing the proportionality constant Kp, thus addressing the instability of current control. [Explanation of Symbols]

[0072] 13...System wiring, 15...System, 40...PFC converter, 85...PI controller, 90...Command unit.

Claims

1. A power converter (40) connected to a power grid (15) via a grid wiring (13) that converts AC power to DC power and outputs it, A current control unit (85) controls the output current to the current command value based on feedback control that includes a proportional term obtained by multiplying the difference between the current command value and the output current by a proportionality constant, The proportionality constant can be changed, and the system includes a command unit (90) that commands the current command value to the current control unit, The power converter is configured such that, when the command unit commands the current control unit to output a first current smaller than the rated current of the power converter and the oscillation degree of the output current is greater than a predetermined degree, the command unit changes the proportionality constant to reduce the oscillation degree of the output current to less than the predetermined degree, and then commands the current control unit to output a second current larger than the first current.

2. The power conversion device according to claim 1, wherein, when the oscillation degree of the output current is greater than a predetermined degree while the command unit has commanded the current control unit to the first current, the command unit reduces the proportionality constant to less than the current proportionality constant to reduce the oscillation degree of the output current to less than the predetermined degree, and then commands the current control unit to the second current.

3. The power conversion device according to claim 1 or 2, wherein the command unit prohibits changing the proportionality constant when the oscillation of the output current is attenuated while the command unit has commanded the current control unit to supply the first current.

4. The power converter according to claim 1, wherein the command unit commands the current control unit to supply a first current smaller than the rated current of the power converter, and if the harmonic distortion rate of the system current flowing through the system wiring is greater than a predetermined distortion rate, the command unit changes the proportionality constant to reduce the harmonic distortion rate of the system current to less than the predetermined distortion rate, and then commands the current control unit to supply a second current larger than the first current.

5. A power converter (40) connected to a power grid (15) via a grid wiring (13) that converts AC power to DC power and outputs it, A current control unit (85) controls the output current to the current command value based on feedback control that includes a proportional term obtained by multiplying the difference between the current command value and the output current by a proportionality constant, The proportionality constant can be changed, and the system includes a command unit (90) that commands the current command value to the current control unit, The power converter is configured such that, when the command unit commands the current control unit to supply a first current smaller than the rated current of the power converter, and the harmonic distortion rate of the system current flowing through the system wiring is greater than a predetermined distortion rate, the command unit changes the proportionality constant to reduce the harmonic distortion rate of the system current to less than the predetermined distortion rate, and then commands the current control unit to supply a second current larger than the first current.

6. The power conversion device according to claim 4 or 5, wherein, when the harmonic distortion rate of the system current is greater than a predetermined distortion rate while the command unit has commanded the current control unit to the first current, the command unit increases the proportionality constant to less than the current proportionality constant to reduce the harmonic distortion rate of the system current to less than the predetermined distortion rate, and then commands the current control unit to the second current.

7. The current control unit is a first current control unit (85), PFC coil (41) and A second current control unit (82) controls the output current based on feedforward control using the input voltage of the PFC coil, A power conversion device according to any one of claims 1, 2, 4, or 5, comprising:

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