Power conversion device, control method, and power system using the same

Abstract

To consume a regenerative power early when an abnormality occurs in an AC machine, reduce the stopping time of the AC machine, and achieve energy saving.SOLUTION: A power conversion device comprises: an inverter circuit that can convert a DC power supplied to a DC terminal into an AC power and supply the AC power to an AC machine and can convert a regenerative power generated in the AC machine into a DC power and supply the DC power to the DC terminal; a dynamic brake circuit that can short circuit an AC terminal connected to the AC machine in the inverter circuit with a first resistance to brake the AC machine; a regenerative brake circuit can short circuit the DC terminal in the inverter circuit with a second resistance to brake the AC machine; and a control unit. When applying a brake to the AC machine with the dynamic brake circuit, if a predetermined first condition is satisfied, the control unit further applies a brake to the AC machine with the regenerative brake circuit.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a power conversion device, a control method, and a power system. [Background technology]

[0002] Dynamic braking circuits are used in AC machines such as AC motors to brake the rotational motion of their rotors. It has also been proposed, for example, to monitor the voltage on the DC side of a power conversion device and, when the voltage reaches a preset level, input a fault signal to activate a semiconductor switch and braking resistor to consume regenerative power as heat (see Patent Document 1 below). Such a circuit that consumes regenerative power on the DC side as heat is also called a regenerative braking circuit. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-61376 Summary of the Invention [Problem to be solved by the invention]

[0004] A dynamic braking circuit has a switch that shorts or grounds the AC terminals of the AC machine via a resistor. Dynamic braking circuits often use relays as switches to handle large currents. However, because relays have mechanical contacts, they typically require time to switch on, which often results in a long time required to stop the AC machine.

[0005] On the other hand, regenerative braking circuits use semiconductor switches, which operate at high speeds, but are inferior to relays in terms of capacity.Furthermore, frequent operation of regenerative braking circuits poses problems in terms of energy conservation.

[0006] Therefore, an object of the present invention is to provide a technology that can consume regenerative power early when an abnormality occurs in an AC machine, shorten the downtime of the AC machine, and achieve energy conservation. [Means for solving the problem]

[0007] An embodiment of the present invention is exemplified by the following power conversion device. In a first aspect, the power conversion device includes: an inverter circuit that converts DC power supplied to a DC terminal into AC power and supplies the AC power to an AC machine, and that converts regenerative power generated by the AC machine into DC power and supplies the DC power to the DC terminal; a dynamic braking circuit capable of braking the AC machine by short-circuiting AC terminals of the inverter circuit connected to the AC machine using a first resistor; a regenerative braking circuit capable of braking the AC machine by short-circuiting the DC terminals of the inverter circuit with a second resistor; a control unit, When applying braking to the AC machine by the dynamic braking circuit, the control unit further applies braking to the AC machine by the regenerative braking circuit if a predetermined first condition is satisfied.

[0008] The dynamic braking circuit is operated by, for example, a mechanical relay, while the regenerative braking circuit is operated by, for example, a semiconductor switch. When braking the AC machine by the dynamic braking circuit, the control unit of the power conversion device also controls the regenerative braking circuit. Therefore, this power conversion apparatus can apply braking to the AC machine earlier and stop it in a shorter time than when using only the dynamic braking circuit. Here, when applying braking to the AC machine using the dynamic braking circuit, the control unit applies braking using the regenerative braking circuit if a predetermined first condition is satisfied. Therefore, the control unit can operate the regenerative braking circuit to the extent necessary, thereby saving energy.

[0009] In a second aspect, the first condition is, for example, the occurrence of an abnormality. The control unit can detect the occurrence of the abnormality, for example, by a signal from an abnormality stop button. The control unit can also detect the occurrence of the abnormality, for example, from a sensor that measures a physical quantity of each part, such as temperature, current, voltage, or rotation speed.

[0010] In a third aspect, the control unit, in a state where there is no abnormality in the AC machine, executes a process of applying braking to the AC machine by the regenerative braking circuit when a terminal voltage of the DC terminal of the inverter circuit exceeds a first reference value; The first condition is that an abnormality occurs and the terminal voltage of the DC terminal of the inverter circuit exceeds a second reference value that is smaller than the first reference value. That is, in the event of an abnormality, the control unit activates the regenerative braking circuit when the terminal voltage of the DC terminal reaches a second reference value that is smaller than the normal value. This allows the control unit to activate the regenerative braking circuit earlier in the event of an abnormality. The first reference value is determined based on the rated withstand voltage of capacitor C1 and the voltage from the AC power source input to converter CV1, among other factors. The second reference value must generally be set to a value higher than the instantaneous peak value of the input voltage from the AC power source. For example, the second reference value may be set to a value higher than the input voltage within the expected operating range. Alternatively, the control unit may obtain the peak value of the input voltage of the AC power source detected by a voltage sensor and actively set the second reference value.

[0011] In a fourth aspect, the control unit calculates an operating time corresponding to the amount of energy to be consumed by the regenerative braking circuit based on the kinetic energy of the AC machine when an abnormality occurs in the AC machine, and operates the regenerative braking circuit for the calculated operating time. Therefore, the control unit can operate the regenerative braking circuit to the extent necessary determined from the kinetic energy of the AC machine when an abnormality occurs. As a result, the control unit can achieve energy conservation.

[0012] In a fifth aspect, the dynamic braking circuit comprises: a first dynamic braking circuit capable of braking the AC machine by a resistor having a first resistance value; a second dynamic braking circuit capable of braking the AC machine by a resistor having a second resistance value smaller than the first resistance value, When a predetermined second condition is satisfied while the control unit is executing the process of applying braking to the AC machine using the first dynamic braking circuit, the control unit further applies braking to the AC machine using the second dynamic braking circuit. Here, the first resistance value is determined according to the specifications of the dynamic braking circuit required for normal operation. Furthermore, the second resistance value is determined according to the specifications required for the dynamic braking circuit DB2 when regenerative power from the AC machine is reduced. Examples of specifications include the time constant required for consuming regenerative energy.

[0013] The second dynamic braking circuit uses a resistor with a second resistance value smaller than the first resistance value, and therefore has a small time constant when discharging power. As a result, the control unit can stop the AC machine in a short time by providing additional braking through the second dynamic braking circuit. Furthermore, the control unit activates the second dynamic braking circuit only when the second condition is satisfied. Therefore, the control unit activates the second dynamic braking circuit in more limited situations. It can be activated.

[0014] In a sixth aspect, the second condition is that a predetermined time has elapsed since braking by the first dynamic braking circuit began, or that the rotational speed of the rotor of the AC machine has become equal to or less than a predetermined reference value. That is, the controller can activate the second dynamic braking circuit after waiting for a decrease in regenerative power from the AC machine when the predetermined time has elapsed since braking by the first dynamic braking circuit began. Furthermore, the controller can activate the second dynamic braking circuit after waiting for a decrease in regenerative power from the AC machine when the rotational speed of the AC machine has become equal to or less than a predetermined reference value. Here, the predetermined time may be determined experimentally or empirically based on parameters such as the type of AC machine, the type of abnormality, and the rotational energy accumulated in the rotor of the AC machine when the abnormality occurred. Furthermore, the reference value may also be determined experimentally or empirically based on parameters such as the type of AC machine, the type of abnormality, and the rotational energy accumulated in the rotor of the AC machine when the abnormality occurred.

[0015] In a seventh aspect, the present power conversion device comprises: an inverter circuit that converts DC power supplied to a DC terminal into AC power and supplies the AC power to an AC machine, and that converts regenerative power generated by the AC machine into DC power and supplies the DC power to the DC terminal; a first dynamic braking circuit capable of braking the AC machine by short-circuiting AC terminals of the inverter circuit connected to the AC machine using a resistor having a first resistance value; a second dynamic braking circuit configured to brake the AC machine by connecting an AC terminal of the inverter circuit to the AC machine using a resistor having a second resistance value smaller than the first resistance value; a control unit, The control unit, while executing the process of applying braking to the AC machine by the first dynamic braking circuit, further applies braking to the AC machine by the second dynamic braking circuit if a predetermined second condition is satisfied.

[0016] In the seventh aspect, similar to the fifth aspect, the control unit can stop the AC machine in a short time.

[0017] In an eighth aspect, the present embodiment is exemplified as a control method for a power conversion device. The control method includes applying braking to the AC machine by a dynamic braking circuit; When braking the AC machine, if the above-mentioned predetermined first condition is satisfied, braking is further applied to the AC machine by the regenerative braking circuit.

[0018] In a ninth aspect, the present embodiment is exemplified by a power system including a power converter and an AC machine that receives AC power from the power converter. [Effects of the Invention]

[0019] In at least one aspect of this embodiment, when an abnormality occurs in the AC machine, regenerative power is consumed early, making it possible to shorten the downtime of the AC machine and achieve energy conservation. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram illustrating a power conversion device to which a control circuit according to an embodiment is applied. [Figure 2] FIG. 2 is a diagram illustrating the power conversion device of the first embodiment. [Figure 3] FIG. 3 is a diagram illustrating a time delay caused by a relay. [Figure 4] FIG. 4 is a diagram illustrating the processing of the control circuit of the first embodiment. [Figure 5] FIG. 5 is a flowchart illustrating the processing of the control circuit. [Figure 6] FIG. 6 is a diagram illustrating a power conversion device according to the second embodiment. [Figure 7] FIG. 7 is a diagram illustrating the processing of the control circuit of the second embodiment. [Figure 8] FIG. 8 is a flowchart illustrating the processing of the control circuit. DETAILED DESCRIPTION OF THE INVENTION

[0021] An embodiment according to one aspect of the present invention (hereinafter also referred to as "the present embodiment") will be described below with reference to the drawings. However, the present embodiment described below is merely an example of the present invention in all respects. It goes without saying that various improvements and modifications can be made without departing from the scope of the present invention. In other words, when implementing the present invention, specific configurations according to the present embodiment may be appropriately adopted.

[0022] <Application example> First, referring to FIG. 1, a power system will be described as an example of a situation in which the present invention is applied. FIG. 1 illustrates a power system including a power conversion device 1A according to the present embodiment. This power system includes an AC power source, the power conversion device 1A, and a load supplied with power by the power conversion device 1A. The power conversion device 1A includes, for example, a control circuit 10, a converter CV1 that converts AC power from the AC power source into DC power and vice versa, and inverters IV1 to IV6 that convert the DC power output from the converter CV1 into AC power and supply it to an AC machine such as an AC motor, and input regenerative power generated by the AC machine to the converter CV1. The control circuit 10 controls each component of the power conversion device 1A. Note that the AC power source and the AC machine are omitted in FIG. 1. In FIG. 1, multiple inverters IV1 and the like are connected in parallel to DC side terminals (P, N) of the converter CV1. Note that the number of inverters IV1 and the like and the number of AC machines powered by the inverters IV1 and the like are not limited to six. The number of each may be one or seven or more.

[0023] Here, the AC machine refers to an electric motor or the like that inputs and outputs AC power to and from its terminals, and includes, for example, a synchronous motor and an induction motor.

[0024] First Embodiment A power conversion device 1 according to one embodiment (first embodiment) of the present invention will be described below with reference to Fig. 2 to Fig. 5. Fig. 2 is a diagram illustrating the power conversion device 1 according to the first embodiment. The power conversion device 1 includes a converter CV1. The converter CV1 includes a connector CNA having terminals L1, L2, and L3 connected to a three-phase AC power supply. A filter F that removes high-frequency noise may be provided at the terminals L1, L2, and L3.

[0025] The converter CV1 may include, for example, a bridge circuit that performs full-wave rectification on each phase of the three-phase AC circuit and a power factor correction circuit that corrects the power factor of each phase of the three-phase AC circuit. Alternatively, the converter CV1 may be a bridgeless power factor correction circuit.

[0026] A smoothing capacitor C1 is provided between the terminals on the output side of the converter CV1 (the side opposite to the connector CNA). In this embodiment and other embodiments described below, the terminal voltage on the output side of the converter CV1 (the terminal voltage of the capacitor C1) is also referred to as the DC output voltage. The DC output voltage is also referred to as the PN voltage or DC bus voltage. The converter CV1 converts the AC voltage from the AC power supply connected to the connector CNA side into a DC output voltage, and may also convert the DC output voltage into a three-phase AC voltage and send it back to the AC power supply. A fuse is provided between the terminals on the output side of the converter CV1.

[0027] Furthermore, the output side of the converter CV1 is connected to the input terminal of the inverter IV1. A regenerative resistor R1 and a switch SW1 are connected in series between the input terminals of the inverter IV1, and a regenerative braking circuit RB is connected in parallel to the capacitor C1. The converter CV1 is a semiconductor switch, such as a transistor. The regenerative resistor R1 is connected to a second resistor In other words, the regenerative braking circuit RB shorts the DC terminals of the inverter circuit by means of the second resistor.

[0028] A bridge circuit BC is formed between the input terminals of the inverter IV1 by six switch elements Q1 to Q6. Each of the six switch elements Q1, etc. has a diode in parallel with the path between the collector and emitter of the bipolar transistor and in the opposite direction from the collector to the emitter. However, MOSFETs may be used instead of bipolar transistors for the six switch elements Q1, etc. In that case, the parasitic diodes of the MOSFETs may be used as the diodes for the six switch elements Q1, etc.

[0029] The junction between the emitter of switch element Q1 and the collector of switch element Q2 supplies the U-phase voltage of the three-phase AC circuit. That is, switch element Q1 serves as the high-side switch of the upper arm of the inverter circuit that generates the U-phase voltage. Also, switch element Q2 serves as the low-side switch of the lower arm of the inverter circuit that generates the U-phase voltage.

[0030] Similarly, the junction between the emitter of switch element Q3 and the collector of switch element Q4 supplies the V-phase voltage of the three-phase AC circuit. That is, switch element Q3 serves as the high-side switch of the upper arm of the inverter circuit that generates the V-phase voltage. Also, switch element Q4 serves as the low-side switch of the lower arm of the inverter circuit that generates the V-phase voltage.

[0031] Similarly, the connection point between the emitter of switch element Q5 and the collector of switch element Q6 supplies the W-phase voltage of the three-phase AC circuit. That is, switch element Q5 serves as a high-side switch of the upper arm of the inverter circuit that generates the W-phase voltage. Also, switch element Q6 serves as a low-side switch of the lower arm of the inverter circuit that generates the W-phase voltage.

[0032] The U, V, and W phases on the output side of the inverter IV1 supply three-phase AC power to an AC machine serving as a load via the U, V, and W phase terminals of the connector CNC.

[0033] A dynamic braking circuit DB is provided in each power path from each phase on the output side of the inverter IV1 to the U-phase terminal, V-phase terminal, and W-phase terminal of the connector CNC. That is, resistors RU, RV, and RW and relays SU, SV, and SW are connected in series in parallel to the load of each phase. This forms the U-phase, V-phase, and W-phase circuits of the dynamic braking circuit DB, respectively. That is, the circuits of each phase of the dynamic braking circuit DB are connected in parallel to the load connected to the connector CNC. The resistors RU, RV, and RW are an example of a first resistor. That is, the dynamic braking circuit DB can be said to short-circuit the AC terminals by the first resistors (resistors RU, RV, and RW).

[0034] The control circuit 10 controls the converter CV1 and the inverter IV1. That is, during power running, the control circuit 10 converts three-phase AC power into DC power using the converter CV1, and converts the DC power into three-phase AC power using the inverter IV1, and supplies the power to the AC machine connected to the connector CNC.

[0035] Here, at least a part of the control circuit 10 is a central processing unit (CPU) and The control circuit 10 may be provided by a main storage device such as a memory and a CPU, etc. That is, the processing of the control circuit 10 may be performed by a computer program that is deployed in an executable manner on the memory by one or more CPUs.

[0036] A CPU is also called a processor. A CPU is not limited to a single processor. The main memory may be a multi-processor configuration. Also, a single CPU may have a multi-core configuration. The main memory stores computer programs executed by the CPU, data processed by the CPU, etc. The control circuit 10 may also include a digital signal processor (DSP), a graphics processing unit (GPU), etc., and the CPU may cooperate with these processors. The main memory may be a dynamic random access memory (DRAM), a static random access memory (SRAM), a read only memory (ROM), etc.

[0037] The control circuit 10 also includes an analog-to-digital (AD) converter, a digital-to-analog It may also include a (DA) converter, other digital circuits, and analog circuits.

[0038] (Specific operation of the control circuit) If an abnormality occurs in the AC machine, the control circuit 10 cuts off the power supply from the converter CV1 and the inverter IV1. However, at this time, the rotor of the AC machine continues to rotate, and the energy of this rotation generates regenerative power. The control circuit 10 drives the relays SU, SV, and SW of each phase of the dynamic braking circuit DB to turn on in order to consume the regenerative power in a short period of time and stop the rotation. As a result, the U-phase terminal, V-phase terminal, and W-phase terminal of the connector CNC are short-circuited by the resistors RU, RV, and RW of the dynamic braking circuit DB. As a result, the regenerative power from the AC machine that is supplied to the U-phase terminal, V-phase terminal, and W-phase terminal is dissipated as heat.

[0039] At this time, at least a part of the regenerative power charges the capacitor C1 through the full-wave rectifier circuit of the inverter IV1. This is because the relays SU, SV, and SW are mechanically operated and require a certain time until they are turned on after receiving a command from the control circuit 10.

[0040] FIG. 3 illustrates an example of a time delay caused by a relay. For example, in the event of an abnormality, the control circuit 10 shuts off the motor drive signal and activates the dynamic braking circuit DB to quickly stop the rotation of an AC machine such as an AC motor. In this way, when the motor suddenly stops, the control circuit 10 attempts to stop the motor as quickly as possible by absorbing regenerative power into the resistors RU, RV, and RW of the dynamic braking circuit DB via the relays SU, SV, and SW. However, in the relays SU, SV, and SW, a delay occurs between receiving the drive signal and the switch contacts turning on. For this reason, the time required for an AC motor or other device to stop may not be sufficiently short. For example, empirically, the opening and closing time required for the open contacts of a typical relay to close is several tens to hundreds of milliseconds.

[0041] Therefore, if the load-side AC machine must be shut down in a short period of time due to an abnormality, the response time delay of the relays SU, SV, and SW may not be acceptable. Therefore, the control circuit 10 shorts the output DC voltage via the regenerative resistor R1 by turning on the switch SW1 of the regenerative braking circuit RB. The control unit 10 can detect the occurrence of an abnormality, for example, by a signal from an abnormality stop button provided on the power conversion device 1 or the AC machine. The control unit 10 can also detect the occurrence of an abnormality, for example, from sensors that measure physical quantities of each component, such as temperature, current, voltage, and rotation speed.

[0042] FIG. 4 illustrates the processing of the control circuit 10 in this embodiment. In this embodiment, when an abnormality occurs, the control circuit 10 cuts off the drive signal for the AC machine, issues an operation command for the dynamic braking circuit DB, and then operates the regenerative braking circuit RB. More specifically, when the terminal voltage of the capacitor C1, i.e., the DC output voltage, reaches the operation threshold (regeneration ON threshold), the control circuit 10 turns on the switch SW1 of the regenerative braking circuit RB to protect the system. Then, due to the time delay described above, the regenerative braking circuit RB operates first before the relays SU, SV, and SW actually turn on. Furthermore, in this embodiment, the operation threshold (regeneration ON threshold) of the regenerative braking circuit RB is set lower than normal. This setting allows the control circuit 10 to start the regenerative braking circuit RB earlier. Activate the brake circuit RB. Note that, in order to reduce the size of the regenerative resistor R1 and to achieve energy savings, the operating threshold of the regenerative brake circuit RB is usually set as high as possible so as to be as close as possible to the rated voltage of the smoothing capacitor C1.

[0043] By setting an operation threshold (regeneration ON threshold) that corresponds to such an abnormality, in this power conversion device 1, the regenerative braking circuit RB is turned ON before the contacts of the relay in the dynamic braking circuit DB are turned ON. This is because the switch SW1 of the regenerative braking circuit RB is a semiconductor switch such as a bipolar transistor or a unipolar transistor (MOSFET), and has a shorter operation delay time than the relays SU, SV, and SW. This process allows the regenerative energy of the AC machine to be absorbed early, shortening the stop time of the AC motor, etc.

[0044] The control circuit 10 may calculate the motor power immediately before the motor stops and determine the maximum on-time of the regenerative braking circuit RB, in order to shorten the operating time of the regenerative braking circuit RB as much as possible.

[0045] 4, the regenerative power converted into DC power by the inverter IV1 is consumed as heat by the regenerative resistor R1. Note that the control circuit 10 may also send a portion of the power charged in the capacitor C1 back to the AC power supply side by the converter CV1.

[0046] FIG. 5 is a flowchart illustrating the processing of the control circuit 10. This processing is executed when an abnormality occurs in either the load AC machine or the power conversion apparatus 1 and the power supply to the AC machine is interrupted. This processing is referred to as processing after abnormality detection. In the processing after abnormality detection, the control circuit 10 first transmits a control signal to the relays SU of the dynamic braking circuit DB to turn on the dynamic braking circuit DB (S1).

[0047] Next, the control circuit 10 obtains the operation threshold E0 of the regenerative braking circuit RB when an abnormality is detected from a predetermined address in the main memory (S2). As described above, the operation threshold E0 is a value lower than the operation threshold of the regenerative braking circuit RB under normal conditions. The operation threshold E0 is determined from the perspective of quickly shutting down the load AC converter and is stored at a predetermined address in the main memory. The operation threshold E0 is set higher than the input voltage within the expected operating range and is stored at a predetermined address in the main memory. The control circuit 10 can also detect the peak value of the input voltage of the AC power supply detected by a voltage sensor, actively set the operation threshold E0, and store it at a predetermined address in the main memory. In this embodiment and other embodiments described below, the term "experimental" naturally includes computer simulation. The operation threshold E0 is an example of a second reference value lower than the first reference value. The operation threshold of the regenerative braking circuit RB under normal conditions is also an example of the first reference value.

[0048] Next, the control circuit 10 calculates the on-time DT of the regenerative braking circuit RB (S3). The on-time DT of the regenerative braking circuit RB is determined based on the relationship between the regenerative power predicted to be generated from the kinetic energy calculated from the current rotational speed of an AC machine, such as an AC motor, the operation delay time TX of the relays SU, and the regenerative power to be consumed during that operation delay time. For example, if the regenerative braking circuit RB only needs to operate for a period shorter than the operation delay time TX of the relays SU, the on-time DT of the regenerative braking circuit RB is set shorter than the operation delay time TX. Note that if the regenerative braking circuit RB needs to operate until the full operation delay time TX of the relays SU, the on-time DT of the regenerative braking circuit RB can be set to match the operation delay time TX of the relays SU. In other words, the regenerative braking circuit RB may be turned off when the dynamic braking circuit DB is turned on. The on-time DT can be considered an example of an operation time corresponding to the amount of energy to be consumed by the regenerative braking circuit. The kinetic energy calculated from the current rotational speed of an AC motor can be considered an example of the kinetic energy possessed by an AC machine.

[0049] Next, the control circuit 10 determines whether the DC voltage (terminal voltage of the capacitor C1) charged by the regenerative power exceeds the operation threshold E0 (S4). If the DC voltage exceeds the operation threshold E0 (YES in S4), the control circuit 10 turns on the regenerative brake circuit RB for the ON time DT (S5). This turns on the switch SW1, and DC power is consumed by the regenerative resistor R1 for the time DT. The condition under which S4 is determined to be YES can be said to be the first condition. Furthermore, the processing of S5 is executed when an abnormality occurs in either the load AC machine or the power conversion device 1. Therefore, the first condition can be said to be when an abnormality occurs and the terminal voltage of the DC terminal of the inverter circuit exceeds a second reference value (operation threshold E0) that is smaller than the first reference value.

[0050] Next, the control circuit 10 determines whether the system has been shut down (S6). The system shutdown refers to, for example, the cessation of rotation of the AC load. Once the system shutdown is complete (YES in S6), the control circuit 10 transmits a control signal to the relay SU of the dynamic braking circuit DB to turn off the dynamic braking circuit DB, and then terminates processing.

[0051] (Effects of the first embodiment) As described above, in this embodiment, the control circuit 10 turns on the regenerative braking circuit RB in addition to issuing a command to turn on the dynamic braking circuit DB. Therefore, even if there is an operational delay time in the relay SU or the like of the dynamic braking circuit DB, the power conversion device 1 can quickly consume the regenerative energy of the load AC machine, for example, an AC motor, and stop the AC machine in a short time.

[0052] In this case, the control circuit 10 monitors the DC output voltage (PN voltage) across the capacitor C1 of the DC circuit, which is charged by regenerative power, and turns on the regenerative braking circuit RB when the DC output voltage exceeds the operating threshold E0. Therefore, the regenerative braking circuit RB does not operate while the DC output voltage does not exceed the operating threshold E0, allowing the power conversion device 1 to avoid unnecessary power consumption. Furthermore, the operating threshold E0 is set to a value lower than the operating threshold of the regenerative braking circuit RB under normal conditions. Therefore, the control circuit 10 can control the regenerative braking circuit RB so that its operation is suppressed under normal conditions and so that it operates when an abnormality occurs. As a result, the control circuit 10 can suppress unnecessary power consumption when no abnormality occurs and effectively operate the regenerative braking circuit RB only when an abnormality occurs.

[0053] Furthermore, the control circuit 10 determines the on-time DT of the regenerative braking circuit RB based on the kinetic energy of the AC machine when the power is cut off. Therefore, the control circuit 10 can operate the regenerative braking circuit RB to the extent necessary to stop the AC machine load as soon as possible.

[0054] <Modification> In the above embodiment, in the process of Fig. 5, when an abnormality occurs, if the DC voltage charged by regenerative power (terminal voltage of capacitor C1) exceeds the operation threshold value E0, the control circuit 10 turns on the regenerative braking circuit RB. However, instead of this process, when an abnormality occurs, the control circuit 10 may immediately turn on the regenerative braking circuit RB, for example, depending on the level of the abnormality. In this case, it can be said that the first condition is that an abnormality occurs.

[0055] <Second embodiment> Hereinafter, a power conversion device 1B of the second embodiment will be described with reference to FIGS.

[0056] (composition) 6 is a diagram illustrating a power conversion device 1B of this embodiment. As shown in FIG. 6, the power conversion device 1B has the same configuration as that of FIG. 2 except that it has two dynamic braking circuits DB1 and DB2. The power conversion apparatus 1B is similar to the power conversion apparatus 1. The power conversion apparatus 1B also includes a control circuit 10B. The control circuit 10B is similar to the control circuit 10 of the first embodiment except that it controls two dynamic braking circuits DB1 and DB2. The dynamic braking circuit DB1 is an example of a first dynamic braking circuit, and the dynamic braking circuit DB2 is an example of a second dynamic braking circuit.

[0057] The dynamic braking circuit DB1 has a configuration similar to that of the dynamic braking circuit DB in Figure 2. That is, the dynamic braking circuit DB1 comprises resistors RU, RV, and RW connected in series with relays SU, SV, and SW, forming U-phase, V-phase, and W-phase circuits, respectively. Each phase circuit of the dynamic braking circuit DB is connected in parallel to the load connected to the connector CNC.

[0058] The dynamic braking circuit DB2 has a configuration similar to that of the dynamic braking circuit DB1. That is, the dynamic braking circuit DB2 has relays SU2, SV2, and SW2 for the U, V, and W phases, respectively. However, the dynamic braking circuit DB2 has a resistance for each phase that is smaller than the resistances RU, RV, and RW for each phase of the dynamic braking circuit DB1. Therefore, the resistances of the dynamic braking circuit DB2 are omitted from Figure 6. This allows the dynamic braking circuit DB2 to consume regenerative power in a short time with a small time constant. Note that if the resistances RU, RV, and RW of the dynamic braking circuit DB1 are taken as an example of a first resistance value, the resistance of the dynamic braking circuit DB2 can be considered to have a second resistance value that is smaller than the first resistance value. The first resistance value is determined according to the specifications required for the dynamic braking circuit DB1 during normal operation. The first resistance value may be determined experimentally or empirically. The second resistance value is determined according to the specifications required for the dynamic braking circuit DB2 when the regenerative power from the AC machine is reduced. The specification may be, for example, a time constant when the regenerative energy is consumed, etc. The second resistance value may be determined experimentally or empirically.

[0059] Figure 7 illustrates the processing of the control circuit 10B in this embodiment. When the load AC machine abnormally stops, the control circuit 10B first controls the dynamic braking circuit DB1 to on, and then controls the dynamic braking circuit DB2 to on. In this case, as a first control method, the control circuit 10B controls the dynamic braking circuit DB1 to on, and then controls the dynamic braking circuit DB2 to on after a predetermined time has elapsed. As a second control method, the control circuit 10B controls the dynamic braking circuit DB2 to on when the rotational speed of the AC machine falls below a predetermined rotational speed.

[0060] 8 is a flowchart illustrating the processing of the control circuit 10B. As in the first embodiment, this processing (processing after abnormality detection) is also executed when an abnormality occurs in either the load AC machine or the power conversion apparatus 1 and the power supply to the AC machine is interrupted. In this processing after abnormality detection, the control circuit 10B first transmits a control signal to the relay SU of the dynamic braking circuit DB1 to turn on the dynamic braking circuit DB1 (S11).

[0061] Next, the control circuit 10B determines whether a predetermined condition is satisfied (S12). Here, the predetermined condition is an example of the second condition, and is the elapse of a predetermined time after the dynamic braking circuit DB1 is controlled to be on. In other words, the predetermined condition can be said to be the elapse of a predetermined time after the first dynamic braking circuit is instructed to start braking. Here, the predetermined time may be determined experimentally or empirically depending on parameters such as the type of AC machine, the type of abnormality, and the rotational kinetic energy accumulated in the rotor of the AC machine when the abnormality occurred. Alternatively, the predetermined condition can be the rotational speed of the AC machine falling below a predetermined rotational speed. In other words, the predetermined condition can be said to be the rotational speed of the AC machine rotor falling below a predetermined reference value. This reference value can also be determined depending on the type of AC machine, the type of abnormality, and the rotational kinetic energy accumulated in the rotor of the AC machine when the abnormality occurred. It may be determined experimentally or empirically depending on parameters such as the temperature.

[0062] If the predetermined condition is satisfied (YES in S12), the control circuit 10B transmits a control signal to the relay SU2 of the dynamic braking circuit DB2 to turn on the dynamic braking circuit DB2 (S13). The control circuit 10B then determines whether the system has been shut down (S14). If the system has been shut down (YES in S14), the control circuit 10B transmits control signals to the dynamic braking circuits DB1 and DB2 to turn off the dynamic braking circuits DB1 and DB2, and then ends the processing (S15).

[0063] (Effects of the second embodiment) According to this embodiment, in a region where regenerative energy is large, the control circuit 10B uses resistors with high resistance values to suppress the current value and consume the regenerative power. In a region where regenerative energy is small, the control circuit 10B uses resistors with low resistance values to reduce the time constant and consume the regenerative power in a short time. In other words, the power conversion device 1B can consume the regenerative power in a short time by using two dynamic braking circuits DB1 and DB2, each having two types of resistance, large and small.

[0064] <Other variations> It goes without saying that a regenerative braking circuit RB may be used in conjunction with the dynamic braking circuits DB1, DB2 of the second embodiment. Furthermore, in both the first and second embodiments, the power conversion devices 1, 1A, 1B, etc. are illustrated as being in a three-phase AC circuit. However, the present invention is not limited to being implemented in a three-phase AC circuit. In other words, the control of the regenerative braking circuit RB and the dynamic braking circuits DB, DB1, DB2, etc. by the control circuits 10, 10B, etc. can also be implemented in a single-phase AC circuit.

[0065] <Additional Notes> 1. An inverter circuit (IV1) that converts DC power supplied to a DC terminal into AC power and supplies it to an AC machine, and that converts regenerative power generated by the AC machine into DC power and supplies it to the DC terminal; a dynamic braking circuit (DB, DB1, DB2) capable of braking the AC machine by short-circuiting AC terminals (U, V, W) of the inverter circuit (IV1) connected to the AC machine using first resistors (RU, RV, RW); a regenerative braking circuit (RB) capable of braking the AC machine by short-circuiting the DC terminals of the inverter circuit (IV1) with a second resistor (R1); a control unit (10, 10B), The power conversion device (1), wherein the control unit (10, 10B), when applying braking to the AC machine by the dynamic braking circuit (DB, DB1), further applies braking to the AC machine by the regenerative braking circuit (RB) if a predetermined first condition is satisfied. [Explanation of symbols]

[0066] 1, 1A, 1B Power Converter 10, 10A, 10B control circuit DB, DB1, DB2 dynamic brake circuit RB Regenerative brake circuit R1 Regenerative braking resistor RU, RV, RW resistance SU, SV, SW relays

Claims

1. An inverter circuit that converts DC power supplied to a DC terminal from a converter that receives first AC power from an AC power source into second AC power and supplies the second AC power to an AC machine, and that converts regenerative power generated by the AC machine into DC power and supplies the DC power to the DC terminal; a dynamic braking circuit capable of braking the AC machine by short-circuiting AC terminals of the inverter circuit connected to the AC machine using a first resistor; a regenerative braking circuit capable of braking the AC machine by short-circuiting the DC terminals of the inverter circuit by a second resistor; a control unit, the control unit, when applying braking to the AC machine by the dynamic braking circuit, further applies braking to the AC machine by the regenerative braking circuit if a predetermined first condition is satisfied, the control unit, when there is no abnormality in the AC machine, executes a process of applying braking to the AC machine by the regenerative braking circuit when a terminal voltage of the DC terminal of the inverter circuit exceeds a first reference value; The predetermined first condition includes a state in which an abnormality occurs in the AC machine and a terminal voltage of the DC terminal of the inverter circuit exceeds a second reference value that is smaller than the first reference value and higher than an instantaneous peak value of the first AC power.

2. 2. The power conversion device according to claim 1, wherein the control unit calculates an operating time corresponding to an amount of energy to be consumed by the regenerative braking circuit based on the kinetic energy of the AC machine when an abnormality occurs in the AC machine, and operates the regenerative braking circuit for the calculated operating time.

3. The dynamic braking circuit comprises: a first dynamic braking circuit capable of braking the AC machine with a resistor having a first resistance value; a second dynamic braking circuit capable of braking the AC machine by a resistor having a second resistance value smaller than the first resistance value, The control unit is configured to further apply braking to the AC machine by the first dynamic braking circuit when a predetermined second condition is satisfied while the control unit is executing the process of applying braking to the AC machine by the first dynamic braking circuit.

3. The power conversion device according to claim 1, wherein braking is added to the AC machine by a brake circuit.

4. 4. The power conversion apparatus according to claim 3, wherein the second condition is that a predetermined time has elapsed since braking by the first dynamic braking circuit started, or that the rotation speed of the rotor of the AC machine is equal to or less than a predetermined reference value.

5. An inverter circuit that converts DC power supplied to a DC terminal from a converter that receives first AC power from an AC power source into second AC power and supplies it to an AC machine, and that can convert regenerative power generated by the AC machine into DC power and supply it to the DC terminal; a dynamic braking circuit capable of braking the AC machine by short-circuiting AC terminals of the inverter circuit connected to the AC machine using a first resistor; a regenerative braking circuit capable of braking the AC machine by short-circuiting the DC terminals of the inverter circuit using a second resistor, applying braking to the AC machine with the dynamic braking circuit; when applying braking to the AC machine, if a predetermined first condition is satisfied, further applying braking to the AC machine by the regenerative brake circuit; the process of applying braking to the AC machine by the regenerative braking circuit further includes a process of applying braking to the AC machine by the regenerative braking circuit when a terminal voltage of the DC terminal of the inverter circuit exceeds a first reference value in a state in which there is no abnormality in the AC machine; The control method for a power conversion device, wherein the predetermined first condition includes a condition in which an abnormality occurs and the terminal voltage of the DC terminal of the inverter circuit exceeds a second reference value that is smaller than the first reference value and higher than an instantaneous peak value of the first AC power.

6. A power system having a power conversion device and an AC machine supplied with AC power from the power conversion device, The power conversion device is an inverter circuit that converts DC power supplied to a DC terminal from a converter that receives first AC power from an AC power supply into second AC power and supplies the second AC power to an AC machine, and that converts regenerative power generated by the AC machine into DC power and supplies the DC power to the DC terminal; a dynamic braking circuit capable of braking the AC machine by short-circuiting AC terminals of the inverter circuit connected to the AC machine using a first resistor; a regenerative braking circuit capable of braking the AC machine by short-circuiting the DC terminals of the inverter circuit by a second resistor; a control unit, the control unit, when applying braking to the AC machine by the dynamic braking circuit, further applies braking to the AC machine by the regenerative braking circuit if a predetermined first condition is satisfied, the control unit, when there is no abnormality in the AC machine, executes a process of applying braking to the AC machine by the regenerative braking circuit when a terminal voltage of the DC terminal of the inverter circuit exceeds a first reference value; The predetermined first condition includes a condition in which an abnormality occurs and the terminal voltage of the DC terminal of the inverter circuit exceeds a second reference value that is smaller than the first reference value and higher than an instantaneous peak value of the first AC power.

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