JP6960606B2 - Snubber circuit, power conversion system - Google Patents

Description

The present disclosure relates generally to snubber circuits and power conversion systems, and more specifically to snubber circuits for suppressing ringing or surge voltages, and power conversion systems using the snubber circuits.

Conventionally, it is known to use a snubber circuit in a power conversion system such as an AC / DC power converter (see, for example, Patent Document 1).

In Patent Document 1, the AC / AC frequency converter connected to the primary side of the high frequency transformer includes a three-phase rectifier, an inverter, and a snubber circuit. The input side of the three-phase rectifier is connected to the three-phase commercial AC power supply, and the output side is connected to the input side of the inverter. The input side of the inverter is connected to the output side of the three-phase rectifier, and the output side is connected to the primary side of the high-frequency transformer. The snubber circuit is connected between the three-phase rectifier and the inverter.

The snubber circuit described in Patent Document 1 absorbs a spike-shaped overvoltage generated by the leakage inductance of a high-frequency transformer as energy and regenerates the absorbed energy. This snubber circuit is a regenerative RCD unidirectional snubber circuit composed of a resistor, a capacitor, a diode and a semiconductor switch.

Japanese Unexamined Patent Publication No. 2013-158064

It is desired to reduce the inrush current in the snubber circuit.

The present disclosure has been made in view of the above reasons, and an object of the present disclosure is to provide a snubber circuit and a power conversion system capable of reducing inrush current.

The snubber circuit according to one aspect of the present disclosure includes a pair of first voltage points in the main circuit, a pair of second voltage points electrically connected to the pair of first voltage points, a first clamp circuit, and the like. It includes a second clamp circuit, a regeneration circuit, and a start-up circuit. It said first clamp circuit, by absorbing electrical energy of the main circuit from the pair of first voltage point in the main circuit, clamps the voltage between the pair of first voltage point. The second clamp circuit, by injecting electrical energy to the main circuit from the pair of second voltage point in the main circuit, clamps the voltage between the pair of second voltage point. The regeneration circuit is electrically connected to the first clamp circuit and the second clamp circuit, and the electric energy of the first clamp circuit is regenerated to the second clamp circuit. The activation circuit, Ru der enables on / off. The snubber circuit is, by turning on the starter circuit, you charge the second clamp circuit before activation of the regenerative circuit.

The power conversion system according to one aspect of the present disclosure includes the snubber circuit and the main circuit. The main circuit is a power conversion circuit that converts electric power.

The present disclosure has the effect that the inrush current can be reduced.

FIG. 1 is a circuit diagram of a power conversion system including a snubber circuit and a power conversion circuit according to an embodiment of the present disclosure. FIG. 2 is a circuit diagram of the snubber circuit of the same as above. FIG. 3A is a waveform diagram of the bus voltage in the power conversion circuit of the same as above without the snubber circuit. FIG. 3B is a waveform diagram of a bus voltage and a current in the power conversion circuit of the same as above with a snubber circuit. FIG. 4 is a timing chart illustrating the operation at startup in the same power conversion system. FIG. 5 is a timing chart illustrating the operation at the time of stop in the power conversion system described above. FIG. 6 is a circuit diagram of a snubber circuit according to a first modification of the embodiment of the present disclosure.

Each embodiment and modification described below is merely an example of the present disclosure, and the present disclosure is not limited to the embodiment and modification. Even if it is not the embodiment and the modified example, various changes can be made according to the design and the like as long as it does not deviate from the technical idea of the present disclosure.

(1) Outline First, an outline of the snubber circuit 3 according to the present embodiment and the power conversion system 1 using the snubber circuit 3 will be described with reference to FIG.

The power conversion system 1 includes a main circuit 2 and a snubber circuit 3. The main circuit 2 is a power conversion circuit that converts electric power. Hereinafter, the main circuit 2 is also referred to as a power conversion circuit 2. The snubber circuit 3 is a protection circuit for suppressing the ringing or surge voltage generated in the power conversion circuit 2. In the power conversion circuit 2, for example, when converting DC power to AC power or converting AC power to DC power, ringing may occur due to the leakage inductance of the transformer 24 described later. In the power conversion system 1 according to the present embodiment, such ringing can be suppressed by the snubber circuit 3. In the present embodiment, the power conversion circuit 2 electrically insulates one side and the other side of the power conversion, and does not have a capacitance component on the other side. The snubber circuit 3 is connected to the other side of the power conversion circuit 2. In the present embodiment, one side of the power conversion circuit 2 will be described as the primary side, and the other side will be described as the secondary side. The snubber circuit 3 corresponds to a sub circuit with respect to the main circuit 2.

As an example, the power conversion system 1 is used for power conversion between the power system E2 and the storage battery E1 as shown in FIG. The term "electric power system" as used herein means an entire system for an electric power company such as an electric power company to supply electric power to a customer's power receiving equipment. In the example of FIG. 1, the power conversion system 1 has a pair of first terminals H11 and H12 to which the storage battery E1 is electrically connected, and a pair of second terminals H21 and H22 to which the power system E2 is electrically connected. Has. When the storage battery E1 is charged, the power conversion system 1 converts the AC power input from the power system E2 into DC power and supplies the DC power to the storage battery E1. Further, when the storage battery E1 is discharged, the power conversion system 1 converts the DC power input from the storage battery E1 into AC power and outputs the AC power to the power system E2. That is, the power conversion system 1 of the present embodiment is bidirectional between the pair of first terminals H11 and H12 and the pair of second terminals H21 and H22 so as to support both charging and discharging of the storage battery E1. It is configured to perform power conversion. As a result, the power conversion system 1 connects the storage battery E1 to the power system E2 to connect the grids, charges the storage battery E1 with the power supplied from the power system E2, and charges the discharge power of the storage battery E1 to the power system E2. It can supply to the load connected to. In the present embodiment, as an example, a case where such a power storage system including the power conversion system 1 and the storage battery E1 is introduced into a non-residential facility such as an office building, a hospital, a commercial facility, and a school will be described.

(2) Configuration As shown in FIG. 1, the power conversion system 1 of the present embodiment includes an inrush prevention circuit 4, a power conversion circuit 2, a snubber circuit 3, and a control circuit 6. Further, the power conversion system 1 further includes a pair of first terminals H11 and H12, a pair of second terminals H21 and H22, and a pair of third terminals H31 and H32. In the present embodiment, the storage battery E1 is electrically connected between the pair of first terminals H11 and H12 so that the first terminal H11 is on the high potential (positive electrode) side. Further, the power system E2 is electrically connected between the pair of second terminals H21 and H22. The pair of third terminals H31 and H32 are electrically connected to the power conversion circuit 2 and the snubber circuit 3. However, the "terminal" here does not have to be a component for connecting an electric wire or the like, and may be, for example, a lead of an electronic component or a part of a conductor included in a circuit board.

(2.1) Rush Prevention Circuit First, the configuration of the rush prevention circuit 4 will be described.

The inrush prevention circuit 4 is electrically connected between the storage battery E1 and the capacitor C11. The capacitor C11 is, for example, an electrolytic capacitor. The inrush prevention circuit 4 is configured to be able to switch between a high impedance state, a low impedance state, and a cutoff state. The "high impedance state" here means a state in which the impedance is relatively high, and the "low impedance state" means a state in which the impedance is relatively low. The "cut-off state" means a state in which the storage battery E1 and the capacitor C11 are electrically cut off.

The inrush prevention circuit 4 has a first opening / closing portion SW51, a second opening / closing portion SW52, a resistor R51, and a diode D51.

The first opening / closing portion SW51 is electrically connected between the positive electrode of the storage battery E1 and the positive electrode of the capacitor C11 via the first terminal H11. The first opening / closing unit SW51 is opened / closed by the drive signal S51 from the control circuit 6. When the first opening / closing portion SW51 is in the closed state, the positive electrode of the storage battery E1 and the positive electrode of the capacitor C11 are electrically connected, and the inrush prevention circuit 4 is in a low impedance state. In the present embodiment, the first opening / closing unit SW51 is a mechanical switch, but may be a semiconductor switch such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor).

A series circuit of the second opening / closing portion SW52, the resistor R51, and the diode D51 is electrically connected between both ends of the first opening / closing portion SW51. That is, between the positive electrode of the storage battery E1 and the capacitor C11, the series circuit of the second switching portion SW52, the resistor R51, and the diode D51 is electrically connected in parallel with the first switching portion SW51. In the diode D51, the anode terminal is electrically connected to the positive electrode of the storage battery E1 via the resistor R51 and the first terminal H11, and the cathode terminal is electrically connected to the positive electrode of the capacitor C11 via the second opening / closing portion SW52. It is connected to the. The second opening / closing unit SW52 is opened / closed by the drive signal S52 from the control circuit 6. When the second opening / closing portion SW52 is in the closed state, the positive electrode of the storage battery E1 and the positive electrode of the capacitor C11 conduct with each other via the resistor R51 and the diode D51, and the resistance R51 puts the inrush prevention circuit 4 in a high impedance state. The resistor R51 functions as a current limiting resistor that reduces the inrush current to the capacitor C11, the first capacitor C1, and the second capacitor C2, which will be described later. In the present embodiment, the second opening / closing unit SW52 is a semiconductor switch such as a MOSFET, but may be a mechanical switch.

That is, when the first opening / closing portion SW51 is in the closed state and the second opening / closing portion SW52 is in the open state, the inrush prevention circuit 4 is in the low impedance state. Further, when the first opening / closing portion SW51 is in the open state and the second opening / closing portion SW52 is in the closed state, the inrush prevention circuit 4 is in a high impedance state. Further, when both the first opening / closing portion SW51 and the second opening / closing portion SW52 are in the open state, the inrush prevention circuit 4 is in the cutoff state.

When the inrush prevention circuit 4 is in a high impedance state, the current value of the direct current supplied from the storage battery E1 to the capacitor C11 is smaller than in the case where the inrush prevention circuit 4 is in a low impedance state.

Further, a discharge circuit 5 is electrically connected between both ends of the capacitor C11. The discharge circuit 5 has a resistor R71 and a discharge switch SW71. A series circuit of the resistor R71 and the discharge switch SW71 is electrically connected between both ends of the capacitor C11. The discharge switch SW71 is opened and closed by the drive signal S71 from the control circuit 6. The discharge switch SW71 is normally off, and is turned on when the capacitor C11 is discharged.

(2.2) Power conversion circuit Next, the configuration of the power conversion circuit 2 will be described.

The power conversion circuit 2 includes a first conversion unit 21, a second conversion unit 22, a third conversion unit 23, a transformer 24, and a filter circuit 25.

The first conversion unit 21 is electrically connected between the capacitor C11 and the primary winding L11 of the transformer 24. The first conversion unit 21 is configured to convert DC power into AC power or convert AC power into DC power.

The first conversion unit 21 has four switching elements Q11 to Q14. In the present embodiment, as an example, each of the switching elements Q11 to Q14 is composed of a depletion type n-channel MOSFET. The switching elements Q11 to Q14 are fully bridge-connected. The switching element Q11 is electrically connected in series with the switching element Q12 between both ends of the capacitor C11. The switching element Q13 is electrically connected in series with the switching element Q14 between both ends of the capacitor C11. In other words, the series circuit of the switching elements Q11 and Q12 and the series circuit of the switching elements Q13 and Q14 are electrically connected in parallel between both ends of the capacitor C11. Specifically, the drains of the switching elements Q11 and Q13 are electrically connected to the positive electrode of the capacitor C11. The sources of the switching elements Q12 and Q14 are electrically connected to the negative electrode of the capacitor C11.

The primary winding L11 of the transformer 24 is electrically connected between the connection point of the source of the switching element Q11 and the drain of the switching element Q12 and the connection point of the source of the switching element Q13 and the drain of the switching element Q14. ..

The first conversion unit 21 converts DC power into AC power or AC power into DC power by turning on / off the switching elements Q11 to Q14. The switching elements Q11 to Q14 are individually controlled on / off by the drive signals S11 to S14 from the control circuit 6.

The transformer 24 is a high-frequency isolation transformer having a primary winding L11 and a secondary winding L12 magnetically coupled to each other. The primary winding L11 is electrically connected to the first conversion unit 21, and the secondary winding L12 is electrically connected to the second conversion unit 22. In the present embodiment, the number of turns of the secondary winding L12 is larger than the number of turns of the primary winding L11. Therefore, a voltage obtained by boosting the voltage across the primary winding L11 is generated between both ends of the secondary winding L12. The turns ratio between the primary winding L11 and the secondary winding L12 may be 1: 1.

The second conversion unit 22 is electrically connected between the secondary winding L12 of the transformer 24 and the pair of third terminals H31 and H32.

The second conversion unit 22 has four switching elements Q21 to Q24. In the present embodiment, in the present embodiment, as an example, each of the switching elements Q21 to Q24 is composed of a depletion type n-channel MOSFET. The switching elements Q21 to Q24 are fully bridge-connected. The switching element Q21 is electrically connected in series with the switching element Q22 between the pair of third terminals H31 and H32. The switching element Q23 is electrically connected in series with the switching element Q24 between the pair of third terminals H31 and H32. In other words, the series circuit of the switching elements Q21 and Q22 and the series circuit of the switching elements Q23 and Q24 are electrically connected in parallel between the pair of third terminals H31 and H32. Specifically, the drains of the switching elements Q21 and Q23 are electrically connected to the third terminal H31 on the high potential side, and the sources of the switching elements Q22 and Q24 are electrically connected to the third terminal H32 on the low potential side. It is connected.

The secondary winding L12 of the transformer 24 is electrically connected between the connection point of the source of the switching element Q21 and the drain of the switching element Q22 and the connection point of the source of the switching element Q23 and the drain of the switching element Q24. There is.

The second conversion unit 22 converts AC power into DC power or converts DC power into AC power by turning on / off the switching elements Q21 to Q24. The switching elements Q21 to Q24 are individually controlled on / off by the drive signals S21 to S24 from the control circuit 6.

The third conversion unit 23 is electrically connected to the second conversion unit 22 via a pair of third terminals H31 and H32. Further, a snubber circuit 3 is electrically connected between the second conversion unit 22 and the third conversion unit 23.

The third conversion unit 23 has four switching elements Q31 to Q34. In the present embodiment, as an example, each of the switching elements Q31 to Q34 is composed of a depletion type n-channel MOSFET. The switching elements Q31 to Q34 are fully bridge-connected. The switching element Q31 is electrically connected in series with the switching element Q32 between the pair of third terminals H31 and H32. The switching element Q33 is electrically connected in series with the switching element Q34 between the pair of third terminals H31 and H32. In other words, the series circuit of the switching elements Q31 and Q32 and the series circuit of the switching elements Q33 and Q34 are electrically connected in parallel between the pair of third terminals H31 and H32. Specifically, the drains of the switching elements Q31 and Q33 are electrically connected to the third terminal H31 on the high potential side, and the sources of the switching elements Q32 and Q34 are electrically connected to the third terminal H32 on the low potential side. It is connected.

The filter circuit 25 is electrically connected to the connection point of the source of the switching element Q31 and the drain of the switching element Q32 and the connection point of the source of the switching element Q33 and the drain of the switching element Q34.

The third conversion unit 23 converts DC power into AC power or converts AC power into DC power by turning on / off the switching elements Q31 to Q34. The switching elements Q31 to Q34 are individually controlled on / off by the drive signals S31 to S34 from the control circuit 6.

The filter circuit 25 has two inductors L21 and L22 and a capacitor C21. The filter circuit 25 is electrically connected between the third conversion unit 23 and the pair of second terminals H21 and H22. One end of the inductor L21, in other words, one of the pair of terminals on the third conversion unit 23 side in the filter circuit 25 is electrically connected to the connection point of the source of the switching element Q31 and the drain of the switching element Q32. One end of the inductor L22, in other words, the other of the pair of terminals on the third conversion unit 23 side in the filter circuit 25, is electrically connected to the connection points of the source of the switching element Q33 and the drain of the switching element Q34. The other end of the inductor L21, in other words, the terminal on the second terminal H21 side of the filter circuit 25, is electrically connected to the second terminal H21 via the AC switching portion SW61. The other end of the inductor L22, in other words, the terminal on the second terminal H22 side in the filter circuit 25, is electrically connected to the second terminal H22. In other words, the third conversion unit 23 is electrically connected to the pair of second terminals H21 and H22 via the filter circuit 25. Further, a capacitor C21 is electrically connected between the other end of the inductor L21 and the other end of the inductor L22.

The AC switching unit SW61 is opened and closed by the drive signal S61 from the control circuit 6. The AC switching unit SW61 may be a mechanical switch or a semiconductor switch. The power system E2 is electrically connected between the pair of second terminals H21 and H22.

(2.3) Snubber Circuit Next, the configuration of the snubber circuit 3 will be described. The snubber circuit 3 is electrically connected to the power conversion circuit 2. Specifically, the snubber circuit 3 includes a pair of first voltage points P11 and P12 and a pair of second voltage points P21 and P22 between the second conversion unit 22 and the third conversion unit 23 in the power conversion circuit 2. Is electrically connected to. That is, the power conversion circuit 2 has a pair of first voltage points P11 and P12 and a pair of second voltage points P21 and P22 to which the snubber circuit 3 is electrically connected.

The pair of first voltage points P11 and P12 are electrically connected to the pair of third terminals H31 and H32. Further, the pair of second voltage points P21 and P22 are electrically connected to the pair of third terminals H31 and H32.

The third terminal H31 on the high potential side, the first voltage point P11 on the high potential side, and the second voltage point P21 on the high potential side have the same potential (electrically equivalent). Similarly, the third terminal H32 on the low potential side, the first voltage point P12 on the low potential side, and the second voltage point P22 on the low potential side have the same potential (electrically equivalent).

The snubber circuit 3 includes a first clamp circuit 31, a second clamp circuit 32, a regenerative circuit 33, and a start circuit 34.

The first clamp circuit 31 is electrically connected between the pair of first voltage points P11 and P12. The first clamp circuit 31 clamps the voltage between the pair of first voltage points P11 and P12 by absorbing the electric energy of the power conversion circuit 2 from the pair of first voltage points P11 and P12 in the power conversion circuit 2. .. Here, the voltage between the pair of third terminals H31 and H32 (the pair of first voltage points P11 and P12) in the power conversion circuit 2 is defined as the bus voltage Vbus. That is, the bus voltage Vbus is a DC voltage output by the second conversion unit 22 to the third conversion unit 23, or a DC voltage output by the third conversion unit 23 to the second conversion unit 22. The first clamp circuit 31 is a pair of first voltage points P11 in the power conversion circuit 2 when the magnitude of the voltage (bus voltage Vbus) between the pair of first voltage points P11 and P12 exceeds the first clamp value v1. , P12 is a circuit that absorbs the electric energy of the power conversion circuit 2. As a result, the first clamp circuit 31 clamps the voltage between the pair of first voltage points P11 and P12 to the first clamp value v1. That is, when the bus voltage Vbus in the power conversion circuit 2 exceeds the first clamp value v1, the first clamp circuit 31 extracts the electric energy corresponding to the amount exceeding the first clamp value v1 to obtain the upper limit of the bus voltage Vbus. The value is clamped to the first clamp value v1.

The first clamp circuit 31 has a first diode D1 and a first capacitor C1. The first diode D1 and the first capacitor C1 are electrically connected in series between a pair of first voltage points P11 and P12. When the voltage (Vbus) between the pair of first voltage points P11 and P12 exceeds the first clamp value v1, the first clamp circuit 31 first passes from the first voltage point P11 on the high potential side through the first diode D1. The current Id1 is configured to flow through the capacitor C1. Specifically, a series circuit of the first diode D1 and the first capacitor C1 is electrically connected between the pair of first voltage points P11 and P12. In the first diode D1, the anode terminal is electrically connected to the first voltage point P11 on the high potential side, and the cathode terminal is electrically connected to the first voltage point P12 on the low potential side via the first capacitor C1. ing.

With this configuration, assuming that the magnitude of the voltage across the first capacitor C1 is the first clamp value v1, the first diode when the bus voltage Vbus between the pair of third terminals H31 and H32 exceeds the first clamp value v1. D1 is turned on (conducting state), and the current Id1 flows through the first capacitor C1. Strictly speaking, the voltage obtained by adding the forward voltage drop of the first diode D1 to the voltage across the first capacitor C1 becomes the first clamp value v1. However, since the forward voltage drop of the first diode D1 is sufficiently smaller than the first clamp value v1, the forward voltage drop of the first diode D1 is set to zero, that is, the voltage across the first capacitor C1 is large. Will be described as having a first clamp value v1.

The second clamp circuit 32 is electrically connected between the pair of second voltage points P21 and P22. The second clamp circuit 32 clamps the voltage between the pair of second voltage points P21 and P22 by injecting electric energy from the pair of second voltage points P21 and P22 in the power conversion circuit 2 into the power conversion circuit 2. .. The second clamp circuit 32 converts power from the pair of second voltage points P21 and P22 when the magnitude of the voltage (bus voltage Vbus) between the pair of second voltage points P21 and P22 is less than the second clamp value v2. It is a circuit that injects electric energy into the circuit 2. As a result, the second clamp circuit 32 clamps the voltage between the pair of second voltage points P21 and P22 to the second clamp value v2. That is, when the bus voltage Vbus in the power conversion circuit 2 is lower than the second clamp value v2, the second clamp circuit 32 regenerates the electric energy corresponding to the amount lower than the second clamp value v2 to reduce the bus voltage Vbus. The lower limit is clamped to the second clamp value v2.

The second clamp circuit 32 has a start switching element Q2 and a second capacitor C2.

The start-up switching element Q2 is composed of a semiconductor switch such as a MOSFET, and has a start-up switch unit SW2 and a second diode D2. In the present embodiment, as an example, the activation switching element Q2 is composed of a depletion type n-channel MOSFET. The second diode D2 is a parasitic diode of the start switching element Q2. In the second diode D2, the anode terminal is electrically connected to the source of the start switch portion SW2, and the cathode terminal is electrically connected to the drain of the start switch portion SW2.

The start-up switching element Q2 and the second capacitor C2 are electrically connected in series between the pair of second voltage points P21 and P22. The second clamp circuit 32 connects the second capacitor C2 to the power conversion circuit 2 through the second diode D2 when the voltage (bus voltage Vbus) between the pair of second voltage points P21 and P22 is lower than the second clamp value v2. It is configured so that the current Id2 flows. Specifically, a series circuit of the start switching element Q2 and the second capacitor C2 is electrically connected between the pair of second voltage points P21 and P22. In the start switching element Q2, the drain of the start switch section SW2 is electrically connected to the second voltage point P21 on the high potential side, and the source of the start switch section SW2 is the second voltage on the low potential side via the second capacitor C2. It is electrically connected to point P22. Therefore, in the second diode D2, the cathode terminal is electrically connected to the second voltage point P21 on the high potential side, and the anode terminal is electrically connected to the second voltage point P22 on the low potential side via the second capacitor C2. It is connected. That is, the second clamp circuit 32 has a second diode D2 and a second capacitor C2. The second diode D2 and the second capacitor C2 are electrically connected in series between the pair of second voltage points P21 and P22.

Further, the start switching element Q2 is controlled by the control circuit 6. Specifically, the start switch unit SW2 of the start switching element Q2 is opened and closed by the drive signal S2 from the control circuit 6.

With this configuration, if the magnitude of the voltage across the second capacitor C2 is the second clamp value v2, then when the bus voltage Vbus between the pair of second voltage points P21 and P22 falls below the second clamp value v2, the second capacitor C2 The diode D2 is turned on (conducting state), and the current Id2 flows through the power conversion circuit 2. Strictly speaking, the voltage obtained by adding the forward voltage drop of the second diode D2 to the voltage across the second capacitor C2 becomes the second clamp value v2. However, since the forward voltage drop of the second diode D2 is sufficiently smaller than the second clamp value v2, here, the forward voltage drop of the second diode D2 is set to zero, that is, the voltage across the second capacitor C2 is large. It will be described as having a second clamp value v2.

The start switching element Q2 also functions as a start circuit 34. The start circuit 34 can be turned on and off, and forms a charging path for charging the second clamp circuit 32 before starting the regenerative circuit 33. That is, the starting circuit 34 is a semiconductor switch (for example, MOSFET). The second diode D2 is a parasitic diode of the semiconductor switch. The start circuit 34 includes a start switch unit SW2 electrically connected in parallel with the second diode D2. When the start switch unit SW2 is turned on, a charging path for charging the second capacitor C2 is formed.

The regeneration circuit 33 is electrically connected to the first clamp circuit 31 and the second clamp circuit 32, and regenerates the electric energy of the first clamp circuit 31 to the second clamp circuit 32. Specifically, the regenerative circuit 33 performs voltage conversion (step-down, step-up, or step-up / down pressure) between the first clamp voltage Vc1 and the second clamp voltage Vc2. The "first clamp voltage" referred to here is a voltage that defines the first clamp value v1, and is the voltage across the first capacitor C1 in the present embodiment. The “second clamp voltage” is a voltage that defines the second clamp value v2, and is a voltage across the second capacitor C2 in the present embodiment.

The regenerative circuit 33 includes a first regenerative switch Q41, a second regenerative switch Q42, and an inductor L41. The regeneration circuit 33 is a chopper circuit having a first regeneration switch Q41 and a second regeneration switch Q42 as switching elements, specifically, a chopper type DC / DC converter. In the present embodiment, as an example, the regenerative circuit 33 is a step-down (chopper) circuit, and steps down the first clamp voltage Vc1 to generate a second clamp voltage Vc2. That is, the regenerative circuit 33 steps down the voltage across the first capacitor C1 to generate the voltage across the second capacitor C2. In the present embodiment, as an example, each of the first regenerative switch Q41 and the second regenerative switch Q42 comprises a depletion type n-channel MOSFET.

The first regenerative switch Q41 and the second regenerative switch Q42 are electrically connected in series between both ends of the first capacitor C1. The drain of the first regenerative switch Q41 is electrically connected to the connection point between the cathode terminal of the first diode D1 and the first capacitor C1 in the first clamp circuit 31. The source of the second regenerative switch Q42 is electrically connected to the terminal (first voltage point P12) on the negative electrode side of the first capacitor.

The inductor L41 is electrically connected in series with the second regenerative switch Q42 between both ends of the second capacitor C2. In other words, the inductor L1 is a connection point between the source of the first regenerative switch Q41 and the drain of the second regenerative switch Q42, the source of the start switching element Q2 (the anode of the second diode D2), and the connection point of the second capacitor C2. Is electrically connected between them.

The first-generation switch Q41 and the second-generation switch Q42 are controlled by the control circuit 6. Specifically, the first regenerative switch Q41 and the second regenerative switch Q42 are opened and closed by the drive signals S41 and S42 from the control circuit 6.

The regenerative circuit 33 has a normal mode and a discharge mode as operation modes. The normal mode is an operation mode in which the electric energy of the first clamp circuit 31 is regenerated into the second clamp circuit 32. The discharge mode is an operation mode for discharging the first clamp circuit 31 (first capacitor C1) and the second clamp circuit 32 (second capacitor). In the present embodiment, the regenerative circuit 33 operates in a discharge mode in which the first clamp circuit 31 and the second clamp circuit 32 are discharged after the main circuit 2 (power conversion circuit 2) is stopped. Specifically, the regenerative circuit 33 operates in the discharge mode after the power conversion circuit 2 is stopped by the drive signals S41 and S42 from the control circuit 6. In other words, the control circuit 6 operates the regeneration circuit 33 in a discharge mode in which the first clamp circuit 31 and the second clamp circuit 32 are discharged after the main circuit 2 (power conversion circuit 2) is stopped.

In the present embodiment, the regenerative circuit 33 performs the same operation in the normal mode and the discharge mode. That is, the normal mode is performed during the operation of the power conversion circuit 2, and the discharge mode is performed after the power conversion circuit 2 is stopped. In the regenerative circuit 33, the first regenerative switch Q41 and the second regenerative switch Q42 are alternately turned on in the normal mode and the discharge mode. As a result, in the normal mode, the electrical energy of the first clamp circuit 31 is regenerated (transmitted) to the second clamp circuit 32. Further, in the discharge mode, the first clamp circuit 31 and the second clamp circuit 32 are discharged due to iron loss of the inductor L41 or the like.

(2.4) Control circuit The control circuit 6 is composed of a microcomputer having a processor and a memory. That is, the control circuit 6 is realized in a computer system having a processor and a memory. Then, when the processor executes an appropriate program, the computer system functions as the control circuit 6. The program may be pre-recorded in a memory, may be recorded through a telecommunication line such as the Internet, or may be recorded and provided on a non-temporary recording medium such as a memory card.

The control circuit 6 is configured to control the power conversion circuit 2, the snubber circuit 3, the inrush prevention circuit 4, the AC switching unit SW61, and the discharge switch SW71.

Specifically, the control circuit 6 outputs drive signals S51 and S52 for driving the first opening / closing section SW51 and the second opening / closing section SW52, respectively, to the inrush prevention circuit 4.

Further, the control circuit 6 outputs a drive signal S61 for driving the AC opening / closing section SW61 to the AC opening / closing section SW61.

Further, the control circuit 6 outputs a drive signal S71 for driving the discharge switch SW71 to the discharge switch SW71.

Further, the control circuit 6 outputs drive signals S11 to S14 (hereinafter, also referred to as drive signal Sigma 10) for driving the switching elements Q11 to Q14 to the first conversion unit 21 of the power conversion circuit 2. The control circuit 6 outputs drive signals S21 to S24 (hereinafter, also referred to as drive signals Sigma 20) for driving the switching elements Q21 to Q24, respectively, to the second conversion unit 22 of the power conversion circuit 2. The control circuit 6 outputs drive signals S31 to S34 (hereinafter, also referred to as drive signals Sigma 30) for driving the switching elements Q31 to Q34, respectively, to the third conversion unit 23 of the power conversion circuit 2.

Further, the control circuit 6 outputs a drive signal S2 for driving the start switching element Q2 (start switch unit SW2) to the second clamp circuit 32 of the snubber circuit 3. The control circuit 6 outputs drive signals S41 and S42 (hereinafter, also referred to as drive signals Sig40) for driving the first regenerative switch Q41 and the second regenerative switch Q42, respectively, to the regenerative circuit 33 of the snubber circuit 3.

Further, the drive signal S2 and the regenerative enable signal Sen1 for determining whether or not to output the drive signals S41 and S42 are input to the control circuit 6. The regenerative enable signal Sen1 is a digital signal having a signal level of High / Low. The control circuit 6 outputs the drive signal S2 and the drive signal Sigma 40 based on the signal level of the regenerative enable signal Sen1.

When the signal level of the regenerative enable signal Sen1 is Low, the control circuit 6 outputs the drive signal S2 and stops the drive signal Sig40. Specifically, when the signal level of the regenerative enable signal Sen1 is Low, the control circuit 6 sets the signal level of the drive signal S2 to High and turns on the start switching element Q2 (start switch unit SW2). Further, when the signal level of the regenerative enable signal Sen1 is Low, the control circuit 6 stops the drive signal Sigma 40 and turns off both the first regenerative switch Q41 and the second regenerative switch Q42.

Further, when the signal level of the regenerative enable signal Sen1 is High, the control circuit 6 stops the drive signal S2 and outputs the drive signal Sig40. Specifically, when the signal level of the regenerative enable signal Sen1 is High, the signal level of the drive signal S2 is set to Low to turn off the start switching element Q2 (start switch unit SW2). Further, when the signal level of the regenerative enable signal Sen1 is High, the control circuit 6 outputs the drive signal Sig40 and alternately turns on the first regenerative switch Q41 and the second regenerative switch Q42.

(3) Operation Example (3.1) Operation of Power Conversion Circuit The operation of the power conversion circuit 2 will be described below with reference to FIG.

The power conversion circuit 2 of the present embodiment is configured to perform bidirectional power conversion between the pair of first terminals H11 and H12 and the pair of second terminals H21 and H22 via a transformer 24. .. Therefore, the power conversion circuit 2 has two operation modes, an "inverter mode" and a "converter mode". The inverter mode is an operation mode in which the DC power input to the pair of first terminals H11 and H12 is converted into AC power and output from the pair of second terminals H21 and H22. The converter mode is an operation mode in which the AC power input to the pair of second terminals H21 and H22 is converted into DC power and output from the pair of first terminals H11 and H12.

First, the operation of the power conversion circuit 2 in the inverter mode will be described.

The control circuit 6 outputs drive signals S11 to S14, S21 to S24 so that the combination of the switching elements Q11, Q14, Q21, and Q24 and the combination of the switching elements Q12, Q13, Q22, and Q23 are turned on alternately. do. Here, the duty ratio of the switching elements Q11, Q14, Q21, Q24 (or the switching elements Q12, Q13, Q22, Q23) is 50%.

As a result, a voltage (bus voltage Vbus) that boosts the voltage Vc11 across the capacitor C11 is generated between the pair of third terminals H31 and H32.

During the period when the voltage between the pair of third terminals H31 and H32 is fixed, the control circuit 6 controls the switching elements Q31 to Q34 of the third conversion unit 23 by PWM control (PWM: Pulse Width Modulation). 3 Controls the output voltage of the conversion unit 23. Specifically, during the supply period when the switching elements Q31 and Q34 (or the switching elements Q32 and Q33) are turned on, a pair of second winding elements L12 of the transformer 24 is passed through the second conversion unit 22 and the third conversion unit 23. Current is supplied to the two terminals H21 and H22.

On the other hand, during the circulation period in which the switching elements Q31 and Q33 (or the switching elements Q32 and Q34) are turned on, a current flows from the inductors L21 and L22 through the third conversion unit 23 as a reflux path.

The control circuit 6 controls the output voltage of the third conversion unit 23 by changing the ratio between the supply period and the circulation period. The reversing operation in the secondary winding L12 of the transformer 24 is performed during the circulation period.

By repeating the operation as described above, the power conversion circuit 2 converts the DC power from the storage battery E1 into AC power and outputs the DC power from the pair of second terminals H21 and H22 to the power system E2.

Further, the power conversion circuit 2 operates the first conversion unit 21, the second conversion unit 22, and the third conversion unit 23 in the same sequence as in the inverter mode, even in the converter mode. That is, in the power conversion circuit 2, if the output voltage of the third conversion unit 23 is lower than the voltage of the power system E2, the AC power from the power system E2 is converted into DC power, and the pair of first terminals H11 and H12 Is output to the storage battery E1.

(3.2) Operation of Snubber Circuit Next, the operation of the snubber circuit 3 will be described with reference to FIGS. 1 to 3B.

With the operation of the power conversion circuit 2 described above, ringing may occur in the DC bus voltage Vbus generated between the pair of third terminals H31 and H32. That is, since the third conversion unit 23 is connected to the storage battery E1 which is a DC power supply via the transformer 24, the third conversion unit 23 supplies electricity to the DC power supply (storage battery E1) via the leakage inductance of the transformer 24. Can be considered to be connected. Therefore, ringing may occur in the bus voltage Vbus during the switching operation of the switching elements Q31 to Q34 of the third conversion unit 23.

FIG. 2 is a description in which the arrangement of the snubber circuit 3 shown in FIG. 1 is changed on the circuit diagram, and the snubber circuit 3 shown in FIG. 2 and the snubber circuit 3 shown in FIG. 1 are equivalent.

In FIG. 3A, the bus voltage Vbus is shown with the horizontal axis as the time axis. In FIG. 3B, the horizontal axis is the time axis, the upper graph shows the bus voltage Vbus, and the lower graph shows the currents Id1 and Id2 (see FIG. 2). The current Id1 represents the current flowing through the first diode D1 of the snubber circuit 3, and the current Id2 represents the current flowing through the second diode D2 which is the parasitic diode of the start switching element Q2 in the snubber circuit 3. Further, in FIG. 3B, the bus voltage Vbus before being clamped is shown by a broken line.

First, in the absence of the snubber circuit 3, as shown in FIG. 3A, positive ringing and negative ringing may occur in the DC bus voltage Vbus generated at the pair of bus voltage points P1 and P2. The term "positive ringing" as used herein means ringing in the direction in which the voltage increases (positive direction) with respect to the average value v0 of the bus voltage Vbus (in FIG. 3A, it increases to "vr1"). .. The “negative ringing” means ringing in the direction in which the voltage drops (negative direction) with respect to the average value v0 of the bus voltage Vbus (the ringing drops to “vr2” in FIG. 3A).

When positive ringing occurs in the bus voltage Vbus, the snubber circuit 3 absorbs the electric energy of the power conversion circuit 2 in the first clamp circuit 31, so that the bus voltage Vbus is changed as shown in FIG. 3B. 1 Clamp to the clamp value v1. That is, if the magnitude of the bus voltage Vbus exceeds the first clamp value v1 as a result of positive ringing in the bus voltage Vbus, the first diode D1 is turned on and the first clamp circuit 31 is operated. At this time, as shown in FIG. 3B, a pulsed current Id1 flows through the first diode D1 as the electric energy is absorbed by the first clamp circuit 31. Therefore, when the magnitude of the bus voltage Vbus exceeds the first clamp value v1, the snubber circuit 3 extracts the electric energy exceeding the first clamp value v1 from the power conversion circuit 2 and uses this electric energy as the first capacitor. It can be accumulated in C1. Therefore, even if positive ringing occurs in the bus voltage Vbus, the maximum value of the bus voltage Vbus is suppressed to the first clamp value v1.

Further, the snubber circuit 3 is a regenerative circuit 33 electrically connected between the first clamp circuit 31 and the second clamp circuit 32, and is a voltage between the first clamp voltage Vc1 and the second clamp voltage Vc2. Perform the conversion. In the regeneration circuit 33, the first regeneration switch Q41 and the second regeneration switch Q42 are alternately turned on by the drive signals S41 and S42 from the control circuit 6, and the first clamp voltage Vc1 is stepped down to generate the second clamp voltage Vc2. do. The drive frequency of the first regenerative switch Q41 and the second regenerative switch Q42 is, for example, 100 kHz. Therefore, the value of the voltage across the second capacitor C2 as the second clamp voltage Vc2 (second clamp value v2) is the value of the voltage across the first capacitor C1 as the first clamp voltage Vc1 (first clamp value v1). Will be lower than. In short, when the first clamp circuit 31 operates and electric energy is accumulated in the first capacitor C1, at least a part of the electric energy is transferred to the second capacitor C2 of the second capacitor circuit 32 via the regenerative circuit 33. Is sent and stored in the second capacitor C2.

Further, in the snubber circuit 3, when negative ringing occurs in the bus voltage Vbus, the bus voltage Vbus is as shown in FIG. 3B by injecting electric energy into the power conversion circuit 2 in the second clamp circuit 32. To the second clamp value v2. That is, if the magnitude of the bus voltage Vbus falls below the second clamp value v2 as a result of negative ringing in the bus voltage Vbus, the second diode D2, which is a parasitic diode of the start switching element Q2, is turned on. The second clamp circuit 32 operates. At this time, as shown in FIG. 3B, a pulsed current Id2 flows through the second diode D2 as the electric energy is injected in the second clamp circuit 32. Therefore, when the magnitude of the bus voltage Vbus is less than the second clamp value v2, the snubber circuit 3 can regenerate the electric energy less than the second clamp value v2 from the second capacitor C2 to the power conversion circuit 2. can. Therefore, even if negative ringing occurs in the bus voltage Vbus, the minimum value of the bus voltage Vbus is suppressed to the second clamp value v2.

Here, the electric energy stored in the second capacitor C2 is the electric energy sent from the first capacitor C1 via the regenerative circuit 33 as described above. That is, the snubber circuit 3 receives the electric energy absorbed by the first clamp circuit 31 from the power conversion circuit 2 when the bus voltage Vbus has a positive ringing, and the snubber circuit 3 has a second when a negative ringing occurs in the bus voltage Vbus. It is regenerated from the clamp circuit 32 to the power conversion circuit 2. In other words, in the snubber circuit 3, the electric energy absorbed when positive ringing occurs is temporarily stored and regenerated when negative ringing occurs. In this way, the positive ringing electrical energy generated in the bus voltage Vbus and the negative ringing electrical energy cancel each other out, so that both positive and negative ringing of the bus voltage Vbus are suppressed.

Further, in the snubber circuit 3, the first clamp value v1 and the second clamp value v2 can be adjusted by adjusting the duty ratio of the drive signal S41 for controlling the first regenerative switch Q41. That is, if the duty ratio of the drive signal S41 changes, the step-down ratio in the regenerative circuit 33 changes, so that the first clamp value v1 and the second clamp value v2 also change accordingly.

Specifically, the larger the duty ratio of the drive signal S41, that is, the larger the proportion of the period in which the first regenerative switch Q41 is on in the switching cycle of the first regenerative switch Q41, the smaller the first clamp value v1. The second clamp value v2 becomes large. Then, as the duty ratio of the drive signal S41 approaches the maximum value "1", the first clamp value v1 and the second clamp value v2 become closer to the average value v0 of the bus voltage Vbus. However, the first clamp value v1 does not fall below the average value v0 of the bus voltage Vbus, and the second clamp value v2 does not exceed the average value v0 of the bus voltage Vbus. Further, as the duty ratio of the drive signal S41 increases, the power (regenerated power) regenerated by the snubber circuit 3 in the power conversion circuit 2 increases. In other words, the smaller the duty ratio of the drive signal S41, that is, the smaller the proportion of the period in which the first regenerative switch Q41 is on in the switching cycle of the first regenerative switch Q41, the larger the first clamp value v1 and the second. The clamp value v2 becomes smaller. Further, the smaller the duty ratio of the drive signal S41, the smaller the regenerative power.

(3.3) Startup Sequence Next, the startup sequence of the power conversion system 1 will be described with reference to FIG.

In FIG. 4, the voltage Vc11, the voltage Vc1, Vc2, the drive signal S52, the drive signal S51, the drive signals Sigma10, Sigma20, the regenerative enable signal Sen1, and the drive signal Sigma30 are shown in order from the top with the horizontal axis as the time axis. .. In FIG. 4, the voltage Vc11 is the voltage across the capacitor C11 in the power conversion circuit 2. The voltage Vc1 is the voltage across the first capacitor C1 of the first capacitor circuit 31 in the snubber circuit 3, and the voltage Vc2 is the voltage across the second capacitor C2 of the second capacitor circuit 32 in the snubber circuit 3. The drive signal S52 is a drive signal of the second opening / closing portion SW52 in the inrush prevention circuit 4. The drive signal S51 is a drive signal of the first opening / closing portion SW51 in the inrush prevention circuit 4. The drive signal Sigma 10 is a drive signal for the switching elements Q11 to Q14 of the first conversion unit 21 in the power conversion circuit 2, and the drive signal Sigma 20 is a drive for the switching elements Q21 to Q24 of the second conversion unit 22 in the power conversion circuit 2. It is a signal. The regeneration enable signal Sen1 is a signal for determining whether or not to output the drive signal S2 of the start switching element Q2 in the snubber circuit 3 and the drive signals S41 and S42 of the first regeneration switch Q41 and the second regeneration switch Q42. The drive signal Sigma 30 is a drive signal of the switching elements Q31 to Q34 of the third conversion unit 23 in the power conversion circuit 2.

In the initial state, the switching elements Q11 to Q14, Q21 to Q24, Q31 to Q34, the start switching element Q2 (start circuit 34), the first regenerative switch Q41, the second regenerative switch Q42, the first open / close section SW51, and the second open / close section. All units SW52 are in the off state.

At time t0, a start command signal is input to the control circuit 6.

When the start command signal is input, the control circuit 6 first starts outputting the drive signals Sig10 and Sig20, and starts the operations of the first conversion unit 21 and the second conversion unit 22 (time t1). Specifically, in the control circuit 6, the drive signals S11 to S14, so that the combination of the switching elements Q11, Q14, Q21, and Q24 and the combination of the switching elements Q12, Q13, Q22, and Q23 are turned on alternately. Outputs S21 to S24. At this time, the drive frequency f1 (hereinafter, also referred to as the first frequency f1) of the switching elements Q11 to Q14 and Q21 to Q24 is, for example, 40 kHz.

Further, at time t1, the signal level of the regenerative enable signal Sen1 is Low. Therefore, the control circuit 6 turns on the start switching element Q2 (start switch unit SW2). That is, the start circuit 34 is turned on. At this time, the regenerative circuit 33 (1st regenerative switch Q41 and 2nd regenerative switch Q42) remains in the off state, that is, before activation.

At time t2, the control circuit 6 outputs a drive signal S52 to the second opening / closing section SW52 to turn on the second opening / closing section SW52. When the second opening / closing portion SW52 is turned on, the capacitor C11 starts to be charged via the second opening / closing portion SW52. Here, the resistor R51 is connected in series to the second opening / closing portion SW52. Therefore, the capacitor C11 is charged by the time constant of the resistor R51 and the capacitor C11.

Further, at time t2, the switching elements Q11 to Q14 and Q21 to Q24 are driven. Therefore, a DC voltage obtained by boosting the voltage Vc11 across the capacitor C11 is generated between the pair of third terminals H31 and H32. The DC voltage between the pair of third terminals H31 and H32 starts charging the first capacitor C1 via the first diode D1. Further, the start switching element Q2 (start circuit 34) is in the ON state. That is, a charging path for charging the second clamp circuit 32 (second capacitor C2) is formed. Therefore, the DC voltage between the pair of third terminals H31 and H32 starts charging the second capacitor C2 via the start switching element Q2 (start circuit 34). Strictly speaking, the voltage Vc1 across the first capacitor C1 is lower than the voltage Vc2 across the second capacitor C2 by the forward voltage drop of the first diode D1. However, the forward voltage drop of the first diode D1 is sufficiently smaller than the voltages Vc1 and Vc2 across the first capacitor C1 and the second capacitor C2. Therefore, in FIG. 4, the forward voltage drop of the first diode D1 is regarded as zero, and the voltage Vc1 across the first capacitor C1 and the voltage Vc2 across the second capacitor C2 are described as being the same.

The timing at which the second opening / closing unit SW52 is turned on may be the same as the output of the drive signals Sig10 and Sig20.

At time t3 after the time t2 and the period T1 (for example, 310 ms), the voltage Vc11 across the capacitor C11 is compared with the predetermined threshold value Vth. When the voltage Vc11 across the capacitor C11 is equal to or higher than the threshold value Vth, the control circuit 6 continues to output the drive signals Sigma10 and Sigma20. When the voltage Vc11 across the capacitor C11 is less than the threshold value Vth, the control circuit 6 turns off the second opening / closing unit SW52 and stops the start of the power conversion system 1. Here, it is assumed that the voltage Vc11 across the capacitor C11 is equal to or higher than the threshold value Vth.

Then, at time t4, the voltage Vc11 across the capacitor C11 is charged to the voltage Va1, and the voltages Vc1 and Vc2 across the first capacitor C1 and the second capacitor C2 are charged to the voltage Va2. Here, the voltage Vc11 across the capacitor C11 is charged only to a voltage Va1 lower than the output voltage Vbat of the storage battery E1 due to the voltage drop of the resistor R51 and the iron loss of the transformer 24. Further, the voltages Vc1 and Vc2 across the first capacitor C1 and the second capacitor C2 are charged only to a voltage Va2 lower than the rated voltage Vx1 of the bus voltage Vbus due to the iron loss of the transformer 24.

At time t5 after the time t2 and the period T2 (for example, 3 ms), the drive signals Sig10 and Sig20 are stopped so that the voltage Vc11 across the capacitor C11 becomes the voltage Vbat. As a result, the switching elements Q11 to Q14 and Q21 to Q24 are stopped, and the voltage Vc11 across the capacitor C11 is gradually increased so as to approach the voltage Vbat. On the other hand, the voltages Vc1 and Vc2 across the first capacitor C1 and the second capacitor C2 gradually decrease due to natural discharge.

At time t6 after the time t2 and the period T3 (for example, 5 ms), the control circuit 6 outputs a drive signal S51 in order to turn on the first opening / closing unit SW51. Since the first opening / closing unit SW51 is a mechanical switch, it is turned on at time t7 after a period T4 (for example, 27 ms to 50 ms) from time t6. When the first switching portion SW51 is turned on, the capacitor C11 is charged without passing through the resistor R51, so that the voltage Vc11 across the capacitor C11 increases to the voltage Vbat. On the other hand, the voltages Vc1 and Vc2 across the first capacitor C1 and the second capacitor C2 are further reduced by natural discharge because the switching elements Q11 to Q14 and Q21 to Q24 are stopped. In the period T3, the switching elements Q11 to Q14 and Q21 to Q24 are stopped, and the first capacitor C1 and the second capacitor C2 spontaneously discharge. Therefore, the period T3 is preferably a short period.

At time t8 after the time t6 and the period T5 (for example, 60 ms), the control circuit 6 resumes the output of the drive signals Sigma10 and Sigma20. As a result, the driving of the switching elements Q11 to Q14 and Q21 to Q24 is restarted. At this time, the drive frequency f2 (hereinafter, also referred to as the second frequency f2) of the switching elements Q11 to Q14 and Q21 to Q24 is, for example, 20 kHz.

By restarting the driving of the switching elements Q11 to Q14 and Q21 to Q24, the voltages Vc1 and Vc2 across the first capacitor C1 and the second capacitor C2 increase to the rated voltage Vx1 of the bus voltage Vbus.

Further, at time t8, the control circuit 6 stops the output of the drive signal S52 to the second opening / closing unit SW52. As a result, the second opening / closing portion SW52 is turned off.

At time t9 after the time t8 and the period T6 (for example, 50 μs), the signal level of the regenerative enable signal Sen1 becomes High, and the control circuit 6 turns off the start switching element Q2 (start circuit 34). Further, the control circuit 6 activates the regenerative circuit 33. Specifically, the control circuit 6 alternately turns on the first regenerative switch Q41 and the second regenerative switch Q42. That is, the start circuit 34 is turned off, the regenerative circuit 33 is started, and the ringing of the bus voltage Vbus can be suppressed.

At time t10 after the time t9 and the period T7 (for example, 14.95 ms), the control circuit 6 starts the output of the drive signal Sigma 30 and starts the operation of the third conversion unit 23. At this time, the drive frequency f3 of the switching elements Q31 to Q34 is, for example, 20 kHz.

In the present embodiment, as described above, when the power conversion system 1 is started, the inrush prevention circuit 4 is set to a high impedance state, and the first conversion unit 21 and the second conversion unit 22 are operated. At this time, the start switching element Q2 (start circuit 34) of the second clamp circuit 32 in the snubber circuit 3 is in the ON state. As a result, the second capacitor C2 can be charged via the start switching element Q2. Therefore, each of the capacitor C11, the first capacitor C1, and the second capacitor C2 can be charged. Further, the second capacitor C2 is charged via the start switching element Q2. For example, as a snubber circuit of the comparative example, there is a configuration in which a resistor is connected in parallel with the series circuit of the first regenerative switch and the inductor, and the second capacitor is charged through the resistor. In the snubber circuit 3 of the present embodiment, the power loss is suppressed as compared with the snubber circuit of the comparative example.

Then, after starting the operation of the first conversion unit 21 and the second conversion unit 22, the inrush prevention circuit 4 is switched from the high impedance state to the low impedance state. Therefore, as compared with the case where the operation of the first conversion unit 21 and the second conversion unit 22 is started after the inrush prevention circuit 4 is switched from the high impedance state to the low impedance state, the capacitors C11, the first capacitor C1, and The inrush current to the second capacitor C2 is reduced. As a result, it is suppressed that an overcurrent flows through the elements included in the charging paths of the capacitors C11, the first capacitor C1, and the second capacitor C2.

Further, the start switching element Q2 (start circuit 34) is turned on only when the power conversion system 1 is started, that is, only before the regenerative circuit 33 is started, and forms a charging path for the second clamp circuit 32 (second capacitor C2). do. Then, the start switching element Q2 (start circuit 34) is turned off during normal operation of the power conversion system 1, and the second diode, which is a parasitic diode, receives the current regenerated from the second capacitor C2 into the power conversion circuit 2. Form a pathway. Further, when the regenerative circuit 33 is driven when the power conversion system 1 is started, the first capacitor C1 and the second capacitor C2 are not sufficiently charged due to iron loss of the regenerative circuit 33 and the like. Therefore, when the power conversion system 1 is started, it is preferable that the first regenerative switch Q41 and the second regenerative switch Q42 of the regenerative circuit 33 are in the off state. During normal operation of the power conversion system 1, the first regenerative switch Q41 and the second regenerative switch Q42 need to operate in order to suppress ringing of the bus voltage Vbus.

That is, the start-up circuit 34 (start-up switching element Q2) and the regenerative circuit 33 have an exclusive relationship, and when the start-up circuit 34 (start-up switching element Q2) is in the ON state, the rejuvenation circuit 33 stops. When the start circuit 34 (start switching element Q2) is in the off state, the regenerative circuit 33 operates. Therefore, it is possible to control on / off of the operation of both the start circuit 34 (start switching element Q2) and the regenerative circuit 33 with only one control signal (regeneration enable signal Sen1), and the control is simplified. be able to. In the present embodiment, the start-up circuit 34 (start-up switching element Q2) is turned on for a predetermined time before the start-up of the regenerative circuit 33. The predetermined time includes at least the period T1 of FIG. As a result, the second clamp circuit 32 (second capacitor C2) can be sufficiently charged.

Further, in the present embodiment, the drive frequencies of the switching elements Q11 to Q14 and Q21 to Q24 are such that the inrush prevention circuit 4 has a low first frequency f1 in a high impedance state (period T2 in FIG. 3) and the intrusion prevention circuit 4 has a low drive frequency. It is higher than the second frequency f2 in the impedance state (during normal operation). Therefore, the iron loss of the transformer 24 can be reduced as compared with the case where the first frequency f1 is the second frequency f2 or less.

Further, in the present embodiment, the first conversion unit 21 and the second conversion unit 22 are stopped before the drive frequencies of the switching elements Q11 to Q14 and Q21 to Q24 are switched from the first frequency f1 to the second frequency f2. .. Therefore, the influence of iron loss can be reduced as compared with the case where the first conversion unit 21 and the second conversion unit 22 are not stopped.

Further, in the present embodiment, when a predetermined period (a period from time t1 to time t6) elapses after the operations of the first conversion unit 21 and the second conversion unit 22 are started, the inrush prevention circuit 4 is low-impedance. Switching to the state. Therefore, the inrush current to the capacitor C11, the first capacitor C1 and the second capacitor C2 can be reduced as compared with the case where the inrush prevention circuit 4 is switched to the low impedance state before the predetermined period elapses.

Further, in the present embodiment, the operations of the first conversion unit 21 and the second conversion unit 22 are started at the switching timing for switching the inrush prevention circuit 4 from the cutoff state to the high impedance state. Therefore, the inrush current to the first capacitor C1 and the second capacitor C2 can be reduced as compared with the case where the operations of the first conversion unit 21 and the second conversion unit 22 are started after the switching timing.

(3.4) Stop Sequence Next, the stop sequence of the power conversion system 1 will be described with reference to FIG.

In FIG. 5, the horizontal axis is the time axis, and the voltage Vac, the drive signal Sigma 30, the drive signal S51, S61, the drive signal Sigma 10, Sigma 20, the drive signal S71, the drive signal S41, S42, the voltage Vc11, and the voltage Vc1 are in order from the top. , Vc2, current It is shown. In FIG. 5, the voltage Vac is an AC voltage input from the power system E2 or output to the power system E2. The drive signal Sigma 30 is a drive signal of the switching elements Q31 to Q34 of the third conversion unit 23 in the power conversion circuit 2. The drive signal S51 is a drive signal of the first opening / closing section SW51 in the inrush prevention circuit 4, and the drive signal S61 is a drive signal of the AC opening / closing section SW61. The drive signal Sigma 10 is a drive signal for the switching elements Q11 to Q14 of the first conversion unit 21 in the power conversion circuit 2, and the drive signal Sigma 20 is a drive for the switching elements Q21 to Q24 of the second conversion unit 22 in the power conversion circuit 2. It is a signal. The drive signal S71 is a drive signal of the discharge switch SW71 in the discharge circuit 5. The drive signal S41 is a drive signal of the first regenerative switch Q41 of the regenerative circuit 33 in the snubber circuit 3, and the drive signal S42 is a drive signal of the second regenerative switch Q42 of the regenerative circuit 33 in the snubber circuit 3. The voltage Vc11 is the voltage across the capacitor C11 in the power conversion circuit 2. The voltage Vc1 is the voltage across the first capacitor C1 of the first capacitor circuit 31 in the snubber circuit 3, and the voltage Vc2 is the voltage across the second capacitor C2 of the second capacitor circuit 32 in the snubber circuit 3. The current It is the current flowing through the primary winding L11 (or the secondary winding L12) of the transformer 24.

In the state immediately before the start of the stop sequence, the first open / close section SW51 and the AC open / close section SW61 are in the ON state, and the power conversion circuit 2 and the snubber circuit 3 are operating. Further, the second opening / closing unit SW52 and the discharge switch SW71 are in the off state.

At time t20, a stop command signal is input to the control circuit 6.

When the stop command signal is input, the control circuit 6 stops the drive signals Sigma30, S51, and S61 (time t21). As a result, the third conversion unit 23 is stopped and the switching elements Q31 to Q34 are turned off. Further, the first switching unit SW51 and the AC switching unit SW61 are turned off, and the power supply path between the power conversion system 1 and the storage battery E1 and the power system E2 is cut off.

At the time t22 after the time t21 and the period T21 (for example, 40 ms), the control circuit 6 stops the drive signals Sigma10 and Sigma20 and outputs the drive signal S71. As a result, the first conversion unit 21 and the second conversion unit 22 are stopped, and the switching elements Q11 to Q14 and Q21 to Q24 are turned off. That is, the power conversion circuit 2 is stopped. Further, the discharge switch SW71 is turned on, and both ends of the capacitor C11 conduct with each other via the resistor R71. As a result, the capacitor C11 is discharged at the time constant of the resistor R71 and the capacitor C11, and the voltage Vc11 across the capacitor C11 gradually decreases.

Here, the regenerative circuit 33 continues to operate after the power conversion circuit 2 (first conversion unit 21, second conversion unit 22, third conversion unit 23) is stopped. That is, in the regenerative circuit 33, after the power conversion circuit 2 is stopped, the first regenerative switch Q41 and the second regenerative switch Q42 are driven in the discharge mode. As a result, the first capacitor C1 and the second capacitor C2 are discharged, and the voltages Vc1 and Vc2 decrease.

In the present embodiment, the discharge of the first capacitor C1 and the second capacitor C2 is completed at the time t23 after the period T22 (for example, 350 ms) from the time t22. The term "discharge completed" here is not limited to the case where the voltage values of the voltages Vc1 and Vc2 across the first capacitor C1 and the second capacitor C2 are zero, and is not limited to a predetermined voltage value or less, or a predetermined% or less of the rated value. Including the case where.

At time t24 after the time t22 and the period T23 (for example, 500 ms), the control circuit 6 stops the drive signals S41 and S42 and turns off the first-generation switch Q41 and the second-generation switch Q42. That is, the snubber circuit 3 (regenerative circuit 33) is stopped. The period T23 is set to a time longer than the time estimated that the discharge of the first capacitor C1 and the second capacitor C2 is completed after the first conversion unit 21 and the second conversion unit 22 are stopped (from time t22). Has been done.

In the present embodiment, the discharge of the capacitor C11 is completed at the time t25 after the period T24 (for example, 5 s) from the time t21. The term "discharge completed" as used herein is not limited to the case where the voltage value of the voltage Vc11 across the capacitor C11 is zero, but also includes the case where the voltage value is equal to or less than a predetermined voltage value or not more than a predetermined% of the rated value.

In the present embodiment, as described above, when the power conversion system 1 is stopped, the power conversion circuit 2 (first conversion unit 21, second conversion unit 22, and third conversion unit 23) is regenerated even after the power conversion circuit 2 is stopped. The circuit 33 continues to operate in the discharge mode. That is, after the power conversion system 1 is stopped, the first regenerative switch Q41 and the second regenerative switch Q42 of the regenerative circuit 33 are alternately turned on. As a result, the first capacitor C1 and the second capacitor C2 are discharged, and the voltages Vc1 and Vc2 decrease. At this time, the discharge path of the first capacitor C1 and the second capacitor C2 includes the inductor L41 of the regenerative circuit 33. Therefore, the discharge time of the first capacitor C1 and the second capacitor C2 can be shortened due to the iron loss of the inductor L41. Compared with the case where the regenerative circuit 33 is stopped at the same timing (time t22) as the first conversion unit 21 and the second conversion unit 22, the time until the discharge of the first capacitor C1 and the second capacitor C2 is completed is shorter. Become. In FIG. 5, the drive signals S41 and S42 and the first capacitor C1 and the second capacitor C2 when the regeneration circuit 33 is stopped at the same timing as the power conversion circuit 2 (first conversion unit 21 and second conversion unit 22). The voltages Vc1 and Vc2 across the above are indicated by broken lines. When the regenerative circuit 33 is stopped at the same timing as the power conversion circuit 2 (first conversion unit 21 and second conversion unit 22), the power conversion circuit 2 (first conversion unit 21 and second conversion unit 22) is stopped. It takes 16 seconds to complete the discharge of the 1st capacitor C1 and the 2nd capacitor C2. On the other hand, in the present embodiment, the operation of the regenerative circuit 33 continues even after the power conversion circuit 2 is stopped, and the time from the stop of the power conversion circuit 2 to the completion of the discharge of the first capacitor C1 and the second capacitor C2. (Period T22) is 350 ms.

(4) Modified Example The above embodiment is only an example of the present invention, and the present invention is not limited to the above embodiment, and even if it is other than the above embodiment, it deviates from the technical idea of the present invention. As long as it does not, various changes can be made depending on the design and the like. Hereinafter, modifications of the above embodiment will be listed.

(4.1) First Modified Example A first modified example of the snubber circuit 3 will be described with reference to FIG.

In the above-described embodiment, the start-up switching element Q2 also serves as the first clamp circuit 31 and the start-up circuit 34. In the snubber circuit 3 of this modification, the first regenerative switch Q41 also serves as the start circuit 34A and the regenerative circuit 33. Further, in the snubber circuit 3 of this modification, a second diode D2A is provided instead of the start switching element Q2. In the second diode D2A, the cathode terminal is electrically connected to the second voltage point P21 on the high potential side, and the anode terminal is electrically connected to the second voltage point P22 on the low potential side via the second capacitor C2. ing. If the magnitude of the bus voltage Vbus falls below the second clamp value v2 as a result of negative ringing in the bus voltage Vbus, the second diode D2A is turned on and the second clamp circuit 32 is operated. At this time, a pulsed current Id2 flows through the second diode D2A as the electric energy is injected in the second clamp circuit 32 (see FIG. 3). Therefore, when the magnitude of the bus voltage Vbus is less than the second clamp value v2, the snubber circuit 3 can regenerate the electric energy less than the second clamp value v2 from the second capacitor C2 to the power conversion circuit 2. can. Therefore, even if negative ringing occurs in the bus voltage Vbus, the minimum value of the bus voltage Vbus is suppressed to the second clamp value v2.

In this modification, the control circuit 6 can individually control the first regenerative switch Q41 and the second regenerative switch Q42. When the power conversion system 1 is started, the control circuit 6 puts the inrush prevention circuit 4 into a high impedance state and operates the first conversion unit 21 and the second conversion unit 22 (time t1 to time t5 in FIG. 3). .. At this time, the control circuit 6 turns on the start circuit 34A (first regenerative switch Q41). As a result, the second capacitor C2 can be charged from the first voltage point P11 on the high potential side via the first diode D1, the first regenerative switch Q41, and the inductor L41. Therefore, when starting the power conversion system 1, the capacitor C11, the first capacitor C1, and the second capacitor C2 can be charged, respectively. After the power conversion system 1 is started, the control circuit 6 alternately turns on the first regenerative switch Q41 and the second regenerative switch Q42.

When the power conversion system 1 is started, the control circuit 6 puts the inrush prevention circuit 4 into a high impedance state and operates the first conversion unit 21 and the second conversion unit 22 (time t1 to time t5 in FIG. 4). .. As a result, a DC voltage is generated between the pair of third terminals H31 and H32 by boosting the voltage Vc11 across the capacitor C11. The DC voltage between the pair of third terminals H31 and H32 charges the first capacitor C1 via the first diode D1. Further, the DC voltage between the pair of third terminals H31 and H32 charges the second capacitor C2 via the first diode D1 and the resistor R41.

(Other variants)
Other variants are listed below.

In the above-described embodiment, when the power conversion system 1 is started, the first conversion unit 21 and the first conversion unit 21 and the first conversion unit 21 and after a predetermined time (period T2) elapses after the second opening / closing unit SW52 is turned on (time t2 in FIG. 4). 2 The conversion unit 22 has been stopped, but the present invention is not limited to this. The predetermined time (period T2) is a time longer than the time estimated that each of the capacitor C11, the first capacitor C1, and the second capacitor C2 is sufficiently charged. The control circuit 6 monitors the voltages Vc11, Vc1 and Vc2 across the capacitor C11, the first capacitor C1 and the second capacitor C2, and when charged to a predetermined value or more, the first conversion unit 21 and the second conversion unit 22 May be stopped.

Further, in the above-described embodiment, when the power conversion system 1 is stopped, the power conversion circuit 2 (first conversion unit 21, second conversion unit 22) is stopped (time t22 in FIG. 5), and then a predetermined time is reached. After the elapse of (period T23), the regeneration circuit 33 was stopped (time t24), but the present invention is not limited to this. The predetermined time (period T23) is longer than the time estimated that the discharge of the first capacitor C1 and the second capacitor C2 is completed. The control circuit 6 monitors the voltages Vc1 and Vc2 across the first capacitor C1 and the second capacitor C2, and when the voltage is discharged to a predetermined value or less, the regenerative circuit 33 (first regenerative switch Q41, second regenerative switch Q42) is turned on. You may stop. For example, the regenerative circuit 33 may be configured to operate in the discharge mode until the voltages Vc1 and Vc2 of the first clamp circuit 31 and the second clamp circuit 32 are at least 50% or less.

Further, in the above-described embodiment, when the power conversion system 1 is stopped, after the power conversion circuit 2 (first conversion unit 21 and second conversion unit 22) is stopped (time t22 in FIG. 5), the power conversion circuit 2 is continuously regenerated. The circuit 33 is operated, but the present invention is not limited to this. The control circuit 6 may also temporarily stop the regenerative circuit 33 after the power conversion circuit 2 is stopped. In this case, when the control circuit 6 receives the start command signal while the regenerative circuit 33 is temporarily stopped and restarts the power conversion system 1, the control circuit 6 stops the operation of the regenerative circuit 33 in the discharge mode. As a result, when the power conversion system 1 is restarted in a short period of time, the amount of discharge of the first capacitor C1 and the second capacitor C2 is small, and the inrush current is suppressed from flowing through the first capacitor C1 and the second capacitor C2. NS. On the other hand, the control circuit 6 operates the regenerative circuit 33 in the discharge mode when the start command signal is not received within a predetermined period after the regenerative circuit 33 is temporarily stopped. As a result, the first capacitor C1 and the second capacitor C2 are discharged.

In the above-described embodiment, the first conversion unit 21 in the power conversion circuit 2 is a full-bridge type DC / AC conversion circuit, but may be configured by a center tap type DC / AC conversion circuit. Further, although the second conversion unit 22 is a full bridge type DC / AC conversion circuit, it may be composed of a center tap type DC / AC conversion circuit. Further, both the first conversion unit 21 and the second conversion unit 22 may be configured by a center tap type DC / AC conversion circuit.

In the above-described embodiment, the third conversion unit 23 in the power conversion circuit 2 is a single-phase DC / AC conversion circuit, but may be composed of a three-phase DC / AC conversion circuit.

Further, the power conversion system 1 is not limited to a configuration in which power is converted in both directions. The power conversion system 1 may be configured to convert power in only one direction from the pair of first terminals H11 and H12 to the pair of second terminals H21 and H22. Further, the power conversion system 1 may be configured to convert power only in one direction from the pair of second terminals H21 and H22 to the pair of first terminals H11 and H12.

The power storage system including the power conversion system 1 and the storage battery E1 is not limited to non-residential facilities, and may be introduced into a house, for example, or may be applied to other than facilities such as an electric vehicle. Further, the power conversion system 1 is not limited to power conversion between the power system E2 and the storage battery E1, and is not limited to power conversion between, for example, a power generation facility such as a solar power generation device or a fuel cell, and the power system E2 or a load. It may be used for conversion.

(summary)
The snubber circuit (3) according to the first aspect includes a first clamp circuit (31), a second clamp circuit (32), a regenerative circuit (33), and a start-up circuit (34, 34A). The first clamp circuit (31) absorbs the electrical energy of the main circuit (2) from the pair of first voltage points (P11, P12) in the main circuit (2), thereby absorbing the electric energy of the main circuit (2), thereby causing the pair of first voltage points (P11, P12). Clamp the voltage (Vbus) between P12). The second clamp circuit (32) is a pair of second voltage points (P21, P22) by injecting electrical energy from the pair of second voltage points (P21, P22) in the main circuit (2) into the main circuit (2). The voltage (Vbus) between P22) is clamped. The regeneration circuit (33) is electrically connected to the first clamp circuit (31) and the second clamp circuit (32), and the electrical energy of the first clamp circuit (31) is regenerated to the second clamp circuit (32). .. The start-up circuit (34, 34A) can be turned on / off and forms a charging path for charging the second clamp circuit (32) before starting the regenerative circuit (33).

According to this aspect, the second clamp circuit (32) for injecting electric energy into the main circuit (2) can be charged before the regenerative circuit (33) is started, so that the inrush current can be suppressed. ..

In the snubber circuit (3) according to the second aspect, in the first aspect, the regenerative circuit (33) is a chopper circuit having a switching element (first regenerative switch Q41, second regenerative switch Q42).

According to this aspect, the electric energy of the first clamp circuit (31) can be efficiently regenerated to the second clamp circuit (32).

In the snubber circuit (3) according to the third aspect, in the first or second aspect, the first clamp circuit (31) is electrically connected in series between a pair of first voltage points (P11, P12). It has one diode (D1) and a first capacitor (C1). The second clamp circuit (32) has a second diode (D2, D2A) and a second capacitor (C2) electrically connected in series between a pair of second voltage points (P21, P22).

According to this aspect, the first clamp circuit (31) and the second clamp circuit (32) can be realized by a relatively simple circuit configuration using a diode and a capacitor.

In the snubber circuit (3) according to the fourth aspect, in the third aspect, the start circuit (34) includes a start switch unit (SW2) electrically connected in parallel with the second diode (D2).

According to this aspect, the starting circuit (34) can be realized by a relatively simple circuit configuration.

In the snubber circuit (3) according to the fifth aspect, in the fourth aspect, the start circuit (34) is a semiconductor switch (start switching element Q2). The second diode (D2) is a parasitic diode of the semiconductor switch (Q2).

According to this aspect, the semiconductor switch (Q2) can be used as both the start circuit (34) and the second clamp circuit (32), and the circuit configuration of the snubber circuit (3) can be simplified.

In the snubber circuit (3) according to the sixth aspect, in any one of the first to fifth aspects, the activation circuit (34, 34A) is turned on for a predetermined time before the activation of the regenerative circuit (33).

According to this aspect, the second clamp circuit (32) can be fully charged.

The power conversion system (1) according to the seventh aspect includes a snubber circuit (3) according to any one of the first to sixth aspects and a main circuit (2). The main circuit (2) is a power conversion circuit (2) that converts electric power.

According to this aspect, in the power conversion system (1), the second clamp circuit (32) for injecting electric energy into the main circuit (2) is provided before the power conversion circuit (2) and the regeneration circuit (33) are started. Since it can be charged, the inrush current can be suppressed.

In the power conversion system (1) according to the eighth aspect, in the seventh aspect, the power conversion circuit (2) electrically insulates one side and the other side of the power conversion, and has a capacitance component on the other side. I don't have it. The snubber circuit (3) is connected to the other side of the power conversion circuit (2).

According to this aspect, the second clamp circuit (32) for injecting electric energy into the main circuit (2) can be charged before the power conversion circuit (2) and the regenerative circuit (33) are started. The current can be suppressed.

1 Power conversion system 2 Main circuit (power conversion circuit)
3 Snubber circuit 31 1st clamp circuit 32 2nd clamp circuit 33 Regeneration circuit 34, 34A Start circuit P11, P12 1st voltage point P21, P22 2nd voltage point D1 1st diode D2, D2A 2nd diode C1 1st capacitor C2 2nd capacitor

Claims (8)

A pair of first voltage points in the main circuit and
A pair of second voltage points electrically connected to the pair of first voltage points,
By absorbing electrical energy of the main circuit from the pair of first voltage point in the main circuit, a first clamp circuit for clamping a voltage between the pair of first voltage point,
By injecting electrical energy to the main circuit from the pair of second voltage point in the main circuit, and a second clamp circuit for clamping a voltage between the pair of second voltage point,
A regenerative circuit that is electrically connected to the first clamp circuit and the second clamp circuit and regenerates the electrical energy of the first clamp circuit into the second clamp circuit.
And the start-up circuit on / off is possible, the Bei example,
By turning on the start circuit, the second clamp circuit is charged before the start of the regenerative circuit.
Snubber circuit. The regenerative circuit is a chopper circuit having a switching element.
The snubber circuit according to claim 1. The first clamp circuit has a first diode and a first capacitor electrically connected in series between the pair of first voltage points.
The second clamp circuit has a second diode and a second capacitor electrically connected in series between the pair of second voltage points.
The snubber circuit according to claim 1 or 2. The start circuit includes a start switch unit electrically connected in parallel with the second diode.
The snubber circuit according to claim 3. The starting circuit is a semiconductor switch and
The second diode is a parasitic diode of the semiconductor switch.
The snubber circuit according to claim 4. The activation circuit is turned on for a predetermined time before the activation of the regenerative circuit.
The snubber circuit according to any one of claims 1 to 5. The snubber circuit according to any one of claims 1 to 6.
With the main circuit
The main circuit is a power conversion circuit that converts electric power.
Power conversion system. The power conversion circuit electrically insulates one side and the other side of power conversion, and has no capacitance component on the other side.
The snubber circuit is connected to the other side of the power conversion circuit.
The power conversion system according to claim 7.

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