CN111293896A

CN111293896A - Power conversion device

Info

Publication number: CN111293896A
Application number: CN201910182766.9A
Authority: CN
Inventors: 川端贤彦; 松浦吉高
Original assignee: Toshiba Mitsubishi Electric Industrial Systems Corp
Current assignee: Toshiba Mitsubishi Electric Industrial Systems Corp
Priority date: 2018-12-06
Filing date: 2019-03-12
Publication date: 2020-06-16
Anticipated expiration: 2039-03-12
Also published as: JP2020092540A; JP7020386B2; CN111293896B

Abstract

Provided is a power conversion device having a function as an idle power compensation device, the power conversion device including: a power converter (3) comprising at least a converter (31) and an inverter (32); a transformer (21) having a primary side connected to an AC power supply (1) and a secondary side connected to an input side of the power converter; a switch (4) provided between the output side of the power converter and the AC motor (5); a transformer (22) having a primary side connected to an AC power supply and a secondary side connected to an output side of the power converter; a switch (6) provided between the transformer and the power converter; and a control device (7). The control device is configured to be able to switch the following modes: a motor drive mode for closing and opening the switch and outputting the frequency-converted alternating current from the power converter to the alternating current motor; in the reactive power control mode, the switch is opened and closed, and reactive power is output from the power converter to the ac power supply.

Description

Power conversion device

Technical Field

The present invention relates to a power conversion device, and more particularly to a power conversion device provided with both a speed control function and an reactive power control function of a motor.

Background

There are cases where reactive power compensation is required for stabilization of the power system. Therefore, recently, as disclosed in patent document 1, for example, the case where an reactive power compensation device using a power semiconductor is applied to a power system is increasing. In contrast, for example, as disclosed in patent document 2, a large ac motor is driven at a variable speed by a power conversion device using an inverter.

Documents of the prior art

Patent document 1: japanese patent laid-open publication No. 2018-097410

Patent document 2: japanese patent laid-open publication No. 2017-046402

Disclosure of Invention

Problems to be solved by the invention

Conventionally, a power conversion device for driving a motor and an idle power compensation device are configured as separate devices. Also, the power conversion apparatus does not make any use when the ac motor is stopped. If a power converter when an ac motor can be stopped functions as an reactive power compensation device, it is expected that the number of components of the entire system will be reduced and the cost will be reduced.

The present invention has been made in view of the above problems, and an object thereof is to provide a power conversion device for driving a motor, which has a function as an idle power compensation device in addition.

Means for solving the problems

The power conversion device of the present invention is a power conversion device that frequency-converts alternating-current power received from a three-phase alternating-current power supply and supplies the alternating-current power to an alternating-current motor, and the power conversion device includes at least a power converter, a 1 st transformer, a 1 st switch, a 2 nd transformer, a 2 nd switch, and a control device. The power converter is configured to convert an input ac power into a dc power by a converter, convert the dc power into an ac power by an inverter, and output the ac power. The 1 st transformer connects the primary side to a three-phase ac power supply and the secondary side to the input side of the power converter. The 1 st shutter is provided between the output side of the power converter and the ac motor. The 2 nd transformer connects the primary side to a three-phase ac power supply and the secondary side to the output side of the power converter. The 2 nd switch is provided between the 2 nd transformer and the power converter. The control device is configured to control the power converter and the 1 st and 2 nd switches, and is capable of switching between two control modes. One of the control modes is a motor drive mode in which the 1 st switch is closed and the 2 nd switch is opened, and ac power having a converted frequency is output from the power converter to the ac motor. The other control mode is a reactive power control mode in which the 1 st switch is opened and the 2 nd switch is closed, and reactive power according to a predetermined reactive power reference is output from the power converter to the three-phase ac power supply.

The control device may close the 2 nd shutter after the 1 st shutter is opened at the time of switching from the motor drive mode to the reactive power control mode.

The 1 st transformer and the 2 nd transformer may be provided independently. In this case, the power conversion apparatus may further have a 3 rd shutter. The 3 rd shutter is provided between the three-phase ac power supply and the 2 nd transformer. In this case, the control device may start the inverter from a low voltage in a state where the 2 nd switch is closed and the 3 rd switch is opened at the time of switching from the motor drive mode to the reactive power control mode, and close the 3 rd switch after the voltages of the power source side and the transformer side of the 3 rd switch are equal to each other.

The 1 st transformer and the 2 nd transformer may also share a primary winding. In this case, the capacitance of the primary winding may be matched to the capacitance of the larger of the secondary winding of the 1 st transformer and the secondary winding of the 2 nd transformer.

The converter may be a diode converter and may also be connected to the inverter via a capacitor. In this case, the control device may control the inverter so that the voltage applied to the capacitor becomes equal to or higher than the no-load dc voltage of the converter in the reactive power control mode.

Effects of the invention

According to the power conversion apparatus of the present invention, the power conversion apparatus can be effectively applied to the control of the reactive power of the system when the ac motor is stopped.

Drawings

Fig. 1 is a circuit configuration diagram of a power conversion device according to embodiment 1 of the present invention.

Fig. 2 is a diagram showing states of the respective switches, states of the respective switches in the control device, and flows of electric power in the motor drive mode of the power conversion device shown in fig. 1.

Fig. 3 is a diagram showing states of the respective switches, states of the respective switches in the control device, and flows of electric power in the reactive power control mode of the power conversion device shown in fig. 1.

Fig. 4 is a timing chart showing an operation of the reactive power control mode when the inrush of the excitation current of the transformer is prevented by the power converter shown in fig. 1.

Fig. 5 is a timing chart showing an operation of the reactive power control mode when it is not necessary to prevent the inrush of the excitation current of the transformer by the power converter shown in fig. 1.

Fig. 6 is a circuit configuration diagram of a power conversion device according to embodiment 2 of the present invention.

Fig. 7 is a circuit configuration diagram of a power conversion device according to a modification example of embodiment 2 of the present invention.

Fig. 8 is a timing chart showing an operation of the reactive power control mode when the inrush of the excitation current of the transformer is prevented by the power converter shown in fig. 7.

Fig. 9 is a circuit configuration diagram of a power conversion device according to embodiment 3 of the present invention.

Fig. 10 is a diagram showing states of the respective switches and the respective switches in the control device and flows of electric power in the motor drive mode of the power conversion device shown in fig. 9.

Fig. 11 is a diagram showing states of the respective switches and the respective switches in the control device and flows of electric power in the reactive power control mode of the power conversion device shown in fig. 9.

Fig. 12 is a diagram showing a specific configuration example of a transformer of the power conversion device shown in fig. 9.

Fig. 13 is a circuit configuration diagram of a power conversion device according to embodiment 4 of the present invention.

Detailed Description

Embodiments of the present invention will be described below with reference to the drawings. However, in the embodiments shown below, in the case where numerical values of the number, the amount, the range, and the like of each element are mentioned, the present invention is not limited to the mentioned numerical values unless specifically stated or clearly determined to be the numerical values in principle. Also, the configurations explained in the embodiments shown below are not necessarily required for the present invention unless specifically stated otherwise or clearly determined to be the case in principle.

1. Embodiment 1

Fig. 1 is a circuit configuration diagram of a power conversion device according to embodiment 1 of the present invention. The power converter according to embodiment 1 is a power converter that performs frequency conversion on ac power received from a commercial three-phase ac power supply 1 by a power converter 3 and supplies the ac power to an induction motor 5. The power converter 3 includes a diode converter 31, a dc capacitor 33, and an inverter 32. The alternating current input to the power converter 3 is converted into direct current in the diode converter 31. The dc power output from the diode converter 31 is smoothed in the dc capacitor 33, converted into ac power by the inverter 32, and output from the power converter 3.

The ac power supply 1 and the power converter 3 are connected to a transformer (1 st transformer) 21 via an input switch 2. The power converter 3 and the induction motor 5 are connected by an output switch (1 st switch) 4. The output side of the ac power supply 1 and the output side of the power converter 3 are connected by a power line, and a transformer (2 nd transformer) 22 is provided therein. A switch (2 nd switch) 6 is provided between the transformer 22 and the power converter 3, and an input switch (3 rd switch) 8 is provided between the ac power supply 1 and the transformer 22. Here, for the sake of simplicity, it is assumed that there is no phase difference between the primary side (ac power supply 1 side) and the secondary side (inverter 32 side) of the transformer 22.

The power converter 3 and the

respective switches

2, 4, 6, and 8 are controlled by a control device 7. The inverter 32 of the power converter 3 is a voltage-type PWM converter. The power devices constituting the inverter 32 perform on/off control in accordance with the gate signal supplied from the control device 7. A speed detector 11 is mounted on the induction motor 5, and its output is supplied to the control device 7. A current detector (inverter output current detector) 12 is provided on the output side of the inverter 32, and the output thereof is also supplied to the control device 7. A voltage detector (power supply voltage detector) 14 is provided on the ac power supply side of the input switch 2, and its output is also supplied to the control device 7.

The control device 7 has two operation modes, a motor drive mode and a reactive power control mode. The motor drive mode is an operation mode in which the

switches

2 and 4 are closed and the

switches

6 and 8 are opened, and the induction motor 5 is driven at a variable speed by outputting the ac power of the converted frequency from the power converter 3 to the induction motor 5. The reactive power control mode is an operation mode in which the

switches

2, 6, and 8 are closed and the switch 4 is opened, and reactive power according to a predetermined reactive power reference is output from the power converter 3 to the ac power supply 1. When the induction motor 5 is not operated, the control device 7 switches the operation mode from the motor drive mode to the reactive power control mode, thereby improving the power factor of the system and stabilizing the voltage. Next, an example of the internal configuration of the control device 7 for selectively realizing these two operation modes will be described.

The control device 7 is externally provided with a speed reference. The speed reference supplied from the outside is input as the 1 st input of the subtractor 71. The velocity feedback obtained at the velocity detector 11 is input to the 2 nd input of the subtractor 71. The difference between the 1 st input and the 2 nd input is calculated by a subtractor 71 and supplied to a speed controller 72. The speed controller 72 is, for example, a PI controller, and outputs a torque reference for making the speed feedback follow the speed reference. The torque reference is input to a divider 73, and is divided by a separately set magnetic flux reference to be converted into a torque current reference.

The torque current reference is input as the 1 st input to switch 74A. Also, the 2 nd input of the switch 74A is input 0. The output of the switch 74A is input as the 1 st input of the subtractor 75A. The switching signal S74A of the switch 74A is operated by a switching controller 83 described later.

The flux reference is also provided to flux-to-current converter 73A. A flux current reference, which is the output of the flux current converter 73A, is input as the 1 st input of the switch 74B. The output of the variable limiter 95 described later is input as the 2 nd input of the switch 74B. The output of the switch 74B is input as the 1 st input of the subtractor 75B. The switching signal S74B of the switch 74B is operated by a switching controller 83 described later.

The magnetic flux reference is also supplied to the slip operator 80 together with the torque current reference, which is the output of the divider 73. The slip calculator 80 calculates the slip s of the induction motor based on the magnetic flux reference and the torque current reference. The slip s is added to the speed feedback by an adder 81, and the output frequency of the inverter 32 is calculated. The output frequency is integrated by the integrator 82, thereby obtaining the reference phase θ M of the input terminal voltage of the induction motor 5.

The reference phase (1 st reference phase) θ M as an output of the integrator 82 is input as the 1 st input of the switch 74C. Then, a reference phase (2 nd reference phase) θ S, which is an output of the PLL controller 84 described later, is input as the 2 nd input of the switch 74C. The output of the switch 74C is supplied as a reference phase θ to a three-phase-two-phase converter 79 and a two-phase-three-phase converter 77, which will be described later. The switching signal S74C of the switch 74C is operated by a switching controller 83 described later.

The voltage of the ac power supply 1 detected by the voltage detector 14 is supplied to the PLL controller 84. The PLL controller 84 is a phase synchronization circuit using a phase locked loop, and outputs a reference phase θ S synchronized with the voltage of the ac power supply 1. Here, the reference phase θ S is determined such that a component in phase with the voltage phase of the ac power supply 1 becomes the q-axis and a component orthogonal thereto becomes the d-axis. The reference phase is input as the 2 nd input of switch 74C.

The voltage of the ac power supply 1 detected by the voltage detector 14 is also supplied to the effective value detection circuit 85. The effective value detection circuit 85 detects an effective value of the voltage of the ac power supply 1. The output of the effective value detection circuit 85 is input as the 2 nd input of the divider 86. The externally set inactive power reference is input as the 1 st input to the divider 86. The reactive power reference is divided by the effective value of the voltage of the ac power supply 1 in the divider 86, thereby being converted into a reactive current reference.

The inactive current reference output from divider 86 is an inactive current reference based on the system side voltage. By multiplying the reactive current reference by a gain in which the winding ratio of the transformer 22 is taken into account in the gain circuit 87, the reactive current reference based on the secondary-side voltage of the transformer 22 can be obtained.

The reactive current reference as the output of the gain circuit 87 is input to the variable limiter 95. The variable limiter 95 specifies a limit value of the reactive current reference. When the input reactive current reference is within the range of the limit value, the variable limiter 95 outputs a value equal to the input reactive current reference, and when the input reactive current reference exceeds the range of the limit value, the variable limiter 95 outputs the limit value. The output of the variable limiter 95 is input as the 2 nd input of the switch 74B. The limit change signal S95 for changing the limit value of the variable limiter 95 is operated by the switching controller 83 described later.

The three-phase output current detected by current detector 12 is supplied to three-phase to two-phase converter 79. The three-phase two-phase converter 79 converts the three-phase current into 2-axis components orthogonal to each other in accordance with a reference phase θ, which is an output of the switch 74C described later. By appropriately selecting the reference phase θ, the current component of the 2-axis can be made into q-axis current feedback, which is torque current feedback, and d-axis current feedback, which is flux current feedback orthogonal to the q-axis current feedback.

An output of three-phase-to-two-phase converter 79, q-axis current feedback, is provided as the 2 nd input of subtractor 75A, and the difference between the 1 st input and the 2 nd input is provided by subtractor 75A to q-axis current controller 76A. The q-axis current controller 76A is, for example, a PI controller, and outputs a q-axis voltage command so that the q-axis current feedback follows the output of the switch 74A.

The other output of three-phase-to-two-phase converter 79, i.e., d-axis current feedback, is given as the 2 nd input of subtractor 75B, and the difference between the 1 st input and the 2 nd input is provided by subtractor 75B to d-axis current controller 76B. The d-axis current controller 76B is, for example, a PI controller, and outputs a d-axis voltage command so that the d-axis current feedback follows the output of the switch 74B.

Both the q-axis voltage command and the d-axis voltage command are provided to a two-phase to three-phase converter 77. The two-phase-three-phase converter 77 converts the q-axis voltage command and the d-axis voltage command into three-phase voltage commands using the reference phase θ as the output of the switch 74C, and supplies the three-phase voltage commands to the PWM controller 78. The PWM controller 78 supplies PWM-modulated gate signals to the respective high power devices of the inverter 32 so that the output voltages of the respective phases of the inverter 32 reach the supplied voltage commands of the three phases.

The voltage of the ac power supply 1 detected by the voltage detector 14, the reference phase θ S as the output of the PLL controller 84, the reference phase θ M as the output of the integrator 82, and the three-phase voltage command as the output of the two-phase-three-phase converter 77 are input to the switching controller 83 as monitoring inputs. The three-phase voltage command is a signal corresponding to the fundamental wave voltage output of the inverter 32.

A signal from an upper layer not shown in the figure or a signal from a device not shown in the figure included in the control device 7 or the like is input to the switching controller 83. The switching controller 83 switches the opening/closing operation signal S4 supplied to the shutter 4, the opening/closing operation signal S6 for the shutter 6, and the opening/closing operation signal S8 for the shutter 8, respectively, in accordance with an operation mode described later and indicated by these signals. The switching controller 83 switches the switching signal S74A supplied to the switch 74, the switching signal S74B supplied to the switch 74B, the switching signal S74C supplied to the switch 74C, and the limit change signal S95 supplied to the variable limiter 95 in the control device 7 according to the operation mode.

Next, a method of realizing each operation mode will be described. Fig. 2 is a diagram showing states and flows of electric power of the

respective switches

2, 4, 6, 8 and the

respective switches

74A, 74B, 74C in the control device 7 in the motor drive mode. During the motor drive mode, the

switches

2, 4 are closed and the

switches

6, 8 are opened. When switching from the reactive power control mode to the motor drive mode, the

switches

6 and 8 are opened, and then the switch 4 is closed. In addition, the input shutter 2 is substantially always closed. In the motor drive mode, the induction motor 5 is connected to the ac power supply 1 via the transformer 21 and the power converter 3. Although the switch 8 may be closed, the switch 8 is preferably opened to reduce the no-load loss of the transformer 22.

During the motor drive mode, the switching signals S74A, S74B, S74C supplied to the

switches

74A, 74B, 74C are set so that the 1 st input is selected by the outputs of the

switches

74A, 74B, 74C. Thus, during this period, the output of the divider 73, i.e., the torque current reference, is supplied to the 1 st input of the subtractor 75A through the switch 74A. During this period, the flux current reference, which is the output of the flux current converter 73A, is supplied to the 1 st input of the subtractor 75B through the switch 74B. During this period, reference phase θ M of the input terminal voltage of induction motor 5 becomes reference phase θ, and reference phase θ M is supplied to two-phase-three-phase converter 77 and three-phase-two-phase converter 79 via switch 74C. As described above, in the motor drive mode, the power converter 3 can perform variable speed drive so that the induction motor 5 follows the supplied speed reference.

Fig. 3 is a diagram showing states and flows of electric power of the

switches

2, 4, 6, and 8 and the

switches

74A, 74B, and 74C in the control device 7 in the reactive power control mode. During the reactive power control mode, the

switches

2, 6, 8 are closed and the switch 4 is opened. When switching from the motor drive mode to the reactive power control mode, the inverter 32 is gate-locked, the switch 4 is opened, and then the

switches

2, 6, and 8 are closed.

During the inactive power control mode, the switching signals S74A, S74B, S74C supplied to the

respective switches

74A, 74B, 74C are set so that the 2 nd input is selected by the output of the

respective switches

74A, 74B, 74C. Thus, the output of the switch 74A becomes 0 in this period. During this period, the output of the variable limiter 95 is supplied to the 1 st input of the subtractor 75B through the switch 74B. During this period, a reference phase θ S synchronized with the voltage of ac power supply 1 becomes a reference phase θ, and the reference phase θ S is supplied to two-phase-three-phase converter 77 and three-phase-two-phase converter 79 through switch 74C.

The output of the switch 74A corresponds to the torque current reference, and is used as the q-axis current reference in the q-axis current control loop of the inverter 32. Since the output voltage phase of inverter 32 and the voltage phase of ac power supply 1 are synchronized, the q-axis current component output from inverter 32, that is, the effective current component becomes 0 during the reactive power control mode. During the reactive power control mode, the output of the variable limiter 95, that is, the reactive current reference subjected to the limiting process becomes the d-axis current reference in the d-axis current control loop of the inverter 32. Since the output voltage phase of the inverter 32 is synchronized with the voltage phase of the ac power supply 1, the d-axis current component output from the inverter 32 becomes an ineffective current component. Therefore, in the reactive power control mode, the q-axis current, which is the active current output from the inverter 32, becomes 0, and the d-axis current, which is the reactive current, becomes the reactive current reference to which the limiting process is applied.

Here, the operation of variable limiter 95 in the reactive power control mode will be described with reference to a timing chart. Fig. 4 is a timing chart showing an operation of the reactive power control mode when preventing the inrush of the excitation current. Fig. 5 is a timing chart showing an operation of the reactive power control mode when it is not necessary to prevent the inrush of the excitation current.

In the reactive power control mode, the switch 2(CB2) is always closed, the switch 4(CB4) is always open, and the switch 6(CB6) is always closed. In order to prevent the inrush of the excitation current of the transformer 22, as shown in fig. 4, the switch 8(CB8) is first opened. Then, at time t1, inverter 32 is unlocked (DEB), and up to time t3, the limit value of variable limiter 95 is gradually increased from 0 to the 1 st limit value. The 1 st limit value is set to a value slightly larger than the exciting current level of the transformer 22. Along with this operation, the output voltage of the inverter 32 increases.

At time t2 between time t1 and time t3, when the output voltage of inverter 32 and the winding voltage on the secondary side (inverter side) of transformer 22 are equal to each other, switch 8 is closed at that timing. In this way, the transformer 22 can be gradually excited by the inverter 32, and thus the inrush of the excitation current can be suppressed.

From time t4 onward until time t6, the limit value of variable limiter 95 is gradually increased from the 1 st limit value to the 2 nd limit value. The 2 nd limit value is set to the rated current level of the inverter 32. In addition, as for the rate of change in the limit value of the variable limiter 95, it is preferable that the rate of change from the time t1 to the time t2 is smaller than the rate of change from the time t4 to the time t 6. The interval between time t3 and time t4 may be zero.

When the limit value of the variable limiter 95 exceeds the output value of the gain circuit 87 from time t5 between time t4 and time t6, the output value of the variable limiter 95 becomes equal to the input value. Therefore, the reactive current, which is the output current of the inverter 32, follows the reactive current reference, which is the output of the gain circuit 87.

On the other hand, when it is not necessary to prevent the inrush of the excitation current of the transformer 22, the shutter 8 is closed before the inverter 32 is operated as shown in fig. 5. Then, the inverter 32 is unlocked (DEB) at time t7, and up to time t9, the limit value of the variable limiter 95 is gradually increased from 0 to the 2 nd limit value. Along with this operation, the output voltage of the inverter 32 increases. In this case, immediately after the inverter 32 is unlocked, the inverter 32 outputs a voltage equivalent to the ac power supply 1 through 2-time voltage conversion by the transformer 22.

When the limit value of the variable limiter 95 exceeds the output value of the gain circuit 87 from time t8 between time t7 and time t9, the output value of the variable limiter 95 becomes equal to the input value. Therefore, the reactive current, which is the output current of the inverter 32, follows the reactive current reference, which is the output of the gain circuit 87.

As described above, in the reactive power control mode, the power converter 3 can output the reactive power in accordance with the reactive power reference and compensate the reactive power. The amount of loss consumed by the power converter 3 is supplied from the diode converter 31 through the transformer 21.

2. Embodiment 2

Fig. 6 is a circuit configuration diagram of a power conversion device according to embodiment 2 of the present invention. In the figure, the same elements as those of embodiment 1 are assigned the same reference numerals. In the following description, the configuration already described in embodiment 1 is omitted, and a configuration unique to embodiment 2 is described.

In embodiment 2, the power converter 3 is provided with a dc voltage detector 34 for detecting a dc voltage that is an output of the diode converter 31. The control device 7 is provided with a subtractor 88 and a voltage controller 89, the subtractor 88 calculates a difference between the voltage detected by the dc voltage detector 34 and a predetermined voltage reference, and the voltage controller 89 outputs a q-axis current reference for minimizing the difference. The output of the voltage controller 89 is provided to the 2 nd input of the switch 74A.

In embodiment 2, during the reactive power control mode, the switch 74A selects the q-axis current reference, which is the output of the voltage controller 89, as the 2 nd input instead of 0. Here, the voltage reference supplied to the subtractor 88 is set to a voltage equal to or higher than the no-load dc voltage of the diode converter 31, for example. In this way, inverter 32 performs a regenerative operation so that the voltage applied to dc capacitor 33 follows the set voltage reference, and an effective current that is equivalent to the amount of power loss in power converter 3 flows from ac power supply 1 into inverter 32. The dc voltage is maintained at or above the no-load dc voltage of the diode converter 31, and thus no current is supplied to the power converter 3 via the diode converter 31. According to embodiment 2, the consumption of reactive power of the diode converter 31 and the transformer 21 can be suppressed.

Further, since the dc voltage of the power converter 3 is maintained at a fixed value irrespective of the power supply variation of the ac power supply 1, reactive power compensation can be performed more stably. In the configuration of embodiment 2, since the switch 2 can be released in the reactive power control mode, the amount of no-load loss of the transformer 21 can be reduced.

< modification of embodiment 2 >

As a modification of embodiment 2, a switch 94D may be provided between the subtractor 88 and the voltage controller 89 as shown in fig. 7. The 1 st input of the switch 94D is connected to 0 and the 2 nd input of the switch 94D is connected to the output of the subtractor 88. The output of the switch 94D is connected to the input of the voltage controller 89. The signal output from the switch 94D is selected in accordance with the switching signal S94D from the switching controller 83.

In the modification example of embodiment 2 shown in fig. 7, the field current of the transformer 22 can be suppressed by starting the reactive power compensation mode in accordance with the following procedure. The operation in the motor drive mode is the same as that in embodiment 1 or embodiment 2.

When switching from the motor drive mode to the reactive power compensation mode, the inverter 32 is gate-locked, the shutter 4 is opened, the shutter 6 is closed, and the shutter 8 is closed. During the reactive power compensation mode, the switching signals S74A, S74B, S74C provided to the

respective switches

74A, 74B, 74C are set so that the output of the

respective switches

74A, 74B, 74C selects the 2 nd input. Thus, in the initial stage, the switching signal S94D provided to the switch 94D is set so that the output of the switch 94D selects the 1 st input, i.e., 0. Therefore, during the initial stage of switching of the reactive power compensation mode, the output of the switch 74A becomes 0.

The operation of the modification of embodiment 2 will be described in detail with reference to fig. 8. Fig. 8 is a timing chart showing the operation of the reactive power control mode when preventing the inrush of the excitation current, as in fig. 4. However, fig. 8 adds a curve of the switching signal S94D of the switch 94D and a curve of the dc voltage of the power converter 3. The operation until the shutter 8 is closed at time t2 is the same as the operation shown in fig. 4. As a result, it is understood that the inrush of the excitation current of the transformer 22 can be suppressed even in the modification example according to embodiment 2.

The dc voltage of the power converter 3 becomes the output voltage of the diode converter 31. After the shutter 8 is closed at the time t2, the switching controller 83 switches the output of the switch 94 from the 1 st input to the 2 nd input at the time t3a in accordance with the switching signal S94D. Thus, the q-axis current command of the inverter 32 is output so that the dc voltage follows the voltage reference, and the dc voltage gradually approaches the voltage corresponding to the dc voltage reference from the time t3 a. At time t3b, the dc voltage almost reaches a voltage corresponding to the dc voltage reference.

At time t3c, the switch controller 83 switches the opening/closing operation signal S2 for the shutter 2 to release, whereby the shutter 2 is opened. The operation from time t3c is the same as the operation shown in fig. 4. Further, from time t3a, after the dc voltage exceeds the output voltage of diode converter 31, the amount of loss of power converter 3 is compensated by the regenerative operation of inverter 32 (based on the q-axis current command). According to the modification of embodiment 2, in the reactive current compensation mode, since the switch 2 can be released while suppressing the inrush of the excitation current of the transformer 22, the amount of no-load loss of the transformer 21 can be reduced.

3. Embodiment 3

Fig. 9 is a circuit configuration diagram of a power conversion device according to embodiment 3 of the present invention. In the figure, the same elements as those of embodiment 1 are assigned the same reference numerals. In the following description, the configuration already described in embodiment 1 is omitted, and a configuration unique to embodiment 3 is described.

In embodiment 3, the two

transformers

21 and 22 used in embodiment 1 are integrated into one transformer 23. The shutter 8 used in embodiment 1 is omitted accordingly.

The transformer 23 includes a motor driving transformer (1 st transformer) 23a and an reactive power control transformer (2 nd transformer) 23b that share a primary winding. The motor driving transformer 23a corresponds to the transformer 21 used in embodiment 1, and the capacitances of the secondary windings of both are the same. The reactive power control transformer 23b corresponds to the transformer 22 used in embodiment 1, and the capacitances of the secondary windings of both are the same. On the other hand, since the speed control and the reactive power control of the induction motor 5 are not performed simultaneously, the capacitance of the shared primary winding can be made equal to the capacitance of the larger one of the secondary winding of the motor driving transformer 23a and the secondary winding of the reactive power control transformer 23 b. Therefore, embodiment 3 has an advantage that the transformer can be downsized and can be realized at low cost as compared with embodiment 1.

The method of implementing each operation mode of embodiment 3 is the same as that of embodiment 1, and therefore, the description thereof is omitted. Fig. 10 is a diagram showing states and flows of electric power of the

respective switches

2, 4, and 6 and the

respective switches

74A, 74B, and 74C in the control device 7 in the motor drive mode of embodiment 3. During the motor drive mode, the

shutters

2 and 4 are closed and the shutter 6 is opened. When switching from the reactive power control mode to the motor drive mode, the gate of the inverter 32 is closed, and then the shutter 6 is opened, and then the shutter 4 is closed.

Fig. 11 is a diagram showing states and flows of electric power of the

switches

2, 4, and 6 and the

switches

74A, 74B, and 74C in the control device 7 in the reactive power control mode according to embodiment 3. During the reactive power control mode, the

switches

2 and 6 are closed and the switch 4 is opened. When switching from the motor drive mode to the reactive power control mode, the gate of the inverter 32 is closed, and then the shutter 4 is opened, and then the shutter 6 is closed.

Fig. 12 shows a specific configuration example of the transformer 23. In this example, the transformer 23 is configured as a 12-pulse transformer. However, the structure of the transformer 23 is not limited to this, and can be applied to 24 pulses, 36 pulses, and more multiple pulses.

4. Embodiment 4

Fig. 13 is a circuit configuration diagram of a power conversion device according to embodiment 4 of the present invention. In the figure, the same elements as those of embodiment 3 are assigned the same reference numerals. Embodiment 4 corresponds to a combination of features of embodiment 3 and embodiment 2. The operation and effects of embodiment 4 are the same as those of the combination of embodiment 3 and embodiment 2, and therefore the description thereof is omitted.

5. Other embodiments

For example, the speed detector 11 according to each embodiment may be a position detector. In this case, the position may be differentiated to obtain the velocity. Further, the speed may be indirectly obtained by calculation without providing a speed detector.

In each of the embodiments, only the switching from the motor drive mode to the reactive power control mode is described, but if the control procedure is reversed, it is obvious that the switching from the reactive power control mode to the motor drive mode is possible.

In the embodiments, although the explanation is made assuming that there is no phase difference between the primary side and the secondary side of the transformer 22, when there is a phase difference, the same effect can be obtained by correcting the phase difference amount by the PLL controller 84 or the voltage detector 14.

Further, the control of the inverter 32 in the motor drive mode according to each embodiment is described as vector control, but V/f fixed control may be used.

Further, although the induction motor is described in each of the embodiments, the same effects can be obtained also in the power converter for driving the synchronous motor if the control circuit for the motor drive mode is replaced with the control circuit for the synchronous motor.

Description of the reference symbols

1, an alternating current power supply; 2 input shutter; 3a power converter; 4 output shutter (1 st shutter); 5 an induction motor; 6 shutter (2 nd shutter); 7 a control device; 8 input shutter (3 rd shutter); 11 a speed detector; 12 a current detector; 14 a voltage detector; 21 transformer (1 st transformer); 22 transformer (2 nd transformer); a 23 three-winding transformer; 23a motor driving transformer (1 st transformer); 23b a transformer for reactive current control (2 nd transformer); 31 a diode converter; 32 an inverter; 33 a direct current capacitor; 34 a DC voltage detector; 71. 75A, 75B, 88 subtracter; 72 a speed controller; 73. 86 a divider; 73A flux-to-current converter; 74A, 74B, 74C, 94D switches; 76A, 76B current controllers; 77 two-phase to three-phase converter; 78 a PWM controller; a 79 three-phase to two-phase converter; 80 a sliding operator; an adder 81; 82 an integrator; 83 switching the controller; a PLL controller 84; 85 effective value detection circuit; 87 a gain circuit; 89 voltage controller.

Claims

1. A power conversion device for frequency-converting AC power received from a three-phase AC power supply and supplying the converted AC power to an AC motor,

the power conversion device includes:

a power converter for converting an input ac power into a dc power by a converter, and converting the dc power into an ac power by an inverter and outputting the ac power;

a 1 st transformer having a primary side connected to the ac power supply and a secondary side connected to an input side of the power converter;

a 1 st shutter provided between an output side of the power converter and the ac motor;

a 2 nd transformer having a primary side connected to the ac power supply and a secondary side connected to an output side of the power converter;

a 2 nd switch provided between the 2 nd transformer and the power converter; and

a control device that controls the power converter and the 1 st and 2 nd shutters,

the control device is configured to be capable of switching the following modes:

a motor drive mode in which the 1 st switch is closed and the 2 nd switch is opened, and the ac power having the converted frequency is output from the power converter to the ac motor; and

and a reactive power control mode for opening the 1 st switch and closing the 2 nd switch, and outputting a reactive power based on a predetermined reactive power reference from the power converter to the ac power supply.

2. The power conversion apparatus according to claim 1,

the 1 st transformer and the 2 nd transformer are independently provided.

3. The power conversion apparatus according to claim 1,

the 1 st transformer and the 2 nd transformer share a primary winding.

4. The power conversion apparatus according to claim 3,

the capacitance of the primary winding is equal to the capacitance of the larger of the secondary winding of the 1 st transformer and the secondary winding of the 2 nd transformer.

5. The power conversion apparatus according to any one of claims 1 to 4,

the converter is a diode converter and the converter is,

the converter and the inverter are connected by a capacitor,

the control device controls the inverter so that the voltage applied to the capacitor reaches a voltage above a no-load direct current voltage of the converter in the reactive power control mode.

6. The power conversion apparatus according to any one of claims 1 to 5,

the control device opens the 1 st switch and closes the 2 nd switch when switching from the motor drive mode to the reactive power control mode.

7. The power conversion apparatus according to claim 2,

the power conversion apparatus further has a 3 rd switch provided between the ac power source and the 2 nd transformer,

the control device starts the inverter in a state where the 2 nd switch is closed and the 3 rd switch is opened at the time of switching from the motor drive mode to the reactive power control mode, and closes the 3 rd switch after voltages on a power supply side and a transformer side of the 3 rd switch are equal to each other.