Power conversion device and control method

Abstract

To suppress an overvoltage fault and the spread of an accident at the time of the fault while suppressing unnecessary operation of a chopper circuit.SOLUTION: A power conversion device according to an embodiment of the present invention includes two power converters, an overvoltage suppression circuit, and a control circuit. In each of the two power converters, a DC-side terminal is connected to a connection point between the first DC capacitor and the second DC capacitor connected in series, and an AC-side terminal is connected to a common system. The overvoltage suppression circuit includes a first suppression circuit capable of reducing energy stored in the first DC capacitor and a second suppression circuit capable of reducing energy stored in the second DC capacitor. The control circuit is capable of performing, based on a first DC voltage of the first DC capacitor and a second DC voltage of the second DC capacitor, first control of independently operating the first limiting circuit and the second limiting circuit and second control of simultaneously operating the first limiting circuit and the second limiting circuit.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] FIELD An embodiment of the present invention relates to a power conversion device and a control method. [Background technology]

[0002] A power conversion device is known that has two power converters, each with a DC side terminal connected to a junction point of a first DC capacitor and a second DC capacitor connected in series, and each with an AC side terminal connected to a common system. In such a power conversion device, a chopper circuit is connected to the first DC capacitor and the second DC capacitor as an overvoltage suppression circuit to suppress overvoltage. This overvoltage suppression circuit detects the voltage of each capacitor and prevents overvoltage when the voltage exceeds an operating threshold. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 3531506 Summary of the Invention [Problem to be solved by the invention]

[0004] However, power converters require an element protection function to prevent overvoltage failure of switching elements in the power converter. Therefore, to prevent overvoltage failure of switching elements, it is necessary to suppress the DC voltage of the power converter. On the other hand, power converters require a short-circuit protection function to prevent the spread of accidents when switching elements fail due to short-circuit failure.

[0005] However, there is a risk that the operating threshold of the suppression circuit will be different when preventing an overvoltage failure of a switching element and when preventing the spread of an accident when a switching element fails. Therefore, if the operating threshold of the overvoltage suppression circuit is set low so as to be able to prevent both an overvoltage failure and the spread of an accident when a failure occurs, there is a risk that the overvoltage suppression operation of the overvoltage suppression circuit will occur frequently.

[0006] The embodiments of the present invention have been made in consideration of these circumstances, and provide a power conversion device and a control method that can suppress overvoltage faults and the spread of accidents during faults while suppressing unnecessary operation of the overvoltage suppression circuit. [Means for solving the problem]

[0007] A power conversion device according to an embodiment of the present invention includes two power converters, an overvoltage suppression circuit, and a control circuit. The two power converters each have a DC side terminal connected to a junction point between a first DC capacitor and a second DC capacitor connected in series, and an AC side terminal connected to a common system. The overvoltage suppression circuit has a first suppression circuit capable of reducing stored energy in the first DC capacitor and a second suppression circuit capable of reducing stored energy in the second DC capacitor. The control circuit is capable of first control, which operates the first suppression circuit and the second suppression circuit independently, and second control, which operates the first suppression circuit and the second suppression circuit simultaneously, based on a first DC voltage of the first DC capacitor and a second DC voltage of the second DC capacitor. [Effects of the Invention]

[0008] According to the present invention, it is possible to suppress overvoltage failures and the spread of accidents caused by failures while suppressing unnecessary operations of the suppression circuit. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing an example of the configuration of a power generation system. [Figure 2] FIG. 2 is a diagram showing an example of a detailed circuit configuration of a power converter. [Figure 3]FIG. 10 is a diagram showing an example in which one leg is configured with four semiconductor switching elements and two clamping diodes. [Figure 4] FIG. 10 is a diagram showing an example of normal operation. [Figure 5] FIG. 5 is a diagram showing a state in which the switching element is turned off from the state shown in FIG. 4; [Figure 6] FIG. 5 is a diagram showing an example in which a switching element has a short-circuit failure in the state shown in FIG. 4; [Figure 7] FIG. 6 is a diagram showing an example in which a switching element has a short-circuit failure in the state shown in FIG. 5; [Figure 8] FIG. 3 is a diagram showing a control circuit that drives a second chopper circuit and a first chopper circuit. [Figure 9] A timing chart showing an example of operation when an AC fault occurs. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, a power conversion device and a control method according to an embodiment of the present invention will be described in detail with reference to the drawings. Note that the embodiment described below is an example of an embodiment of the present invention, and the present invention should not be interpreted as being limited to these embodiments. Furthermore, in the drawings referred to in this embodiment, identical parts or parts having similar functions are given the same or similar reference numerals, and repeated explanations thereof may be omitted. Furthermore, part of the configuration may be omitted from the drawings.

[0011] 1 is a diagram showing an example of the configuration of a power generation system 1 to which a power conversion device according to this embodiment is applied. The power generation system 1 is, for example, a variable-speed pumped-storage power generation system, and includes a power conversion device 10, a three-phase induction generator (hereinafter referred to as induction generator) 20, and a transformer 30. To the induction generator 20, for example, a water turbine for pumped-storage power generation is connected.

[0012] The power conversion device 10 includes a power converter 3, a first DC capacitor 4a, a second DC capacitor 4b, an overvoltage suppression circuit 5, a power converter 6, and a control unit 8. The power converters 3 and 6 are doubly excited power converters of an induction generator 20. The power converter 3 is, for example, an NPC inverter, and the power converter 6 is, for example, an NPC converter. The overvoltage suppression circuit 5 includes a first chopper circuit 5a and a second chopper circuit 5b.

[0013] The AC side node of the power converter 3 is connected to the secondary excitation winding of the induction generator 20. The AC side node of the power converter 6 is connected to the AC system via a transformer 30. The DC side nodes of the power converters 3 and 6 are connected to a common series circuit of a first DC capacitor 4a and a second DC capacitor 4b. One end of the first DC capacitor 4a is connected to the P node, and the other end is connected to the O node. One end of the second DC capacitor 4b is connected to the O node, and the other end is connected to the N node. A voltage VPO is applied between the nodes P and O, and a voltage VON is applied between the nodes O and N. The nodes may also be referred to as connection points.

[0014] Power converter 3 supplies power to a secondary excitation winding of induction generator 20 to excite induction generator 20. The stator winding of induction generator 20 is connected to an AC system. Power converter 6 obtains the power required to operate power converter 3 from the AC system via a transformer 30 on the power system side, and supplies the power to a first DC capacitor 4a and a second DC capacitor 4b between power converter 6 and power converter 3.

[0015] One end of the first chopper circuit 5a in the overvoltage suppression circuit 5 is connected to the P node, and the other end is connected to the O node. The first chopper circuit 5a has a switching element 6a and a resistor unit 7a connected in series. The switching element 6a is, for example, a semiconductor switch, and the resistor unit 7a is a resistor. In other words, the first chopper circuit 5a is composed of the switching element 6a and the resistor unit 7a connected in series, and is connected in parallel to the first DC capacitor 4a. As a result, when the switching element 6a is turned on, for example, the stored energy of the first DC capacitor 4a can be reduced by heat generation in the resistor unit 7a.

[0016] One end of the second chopper circuit 5b in the overvoltage suppression circuit 5 is connected to the O node, and the other end is connected to the N node. The second chopper circuit 5b has a switching element 6b and a resistor unit 7b connected in series. The switching element 6b is, for example, a semiconductor switch, and the resistor unit 7b is a resistor. That is, the second chopper circuit 5b is composed of the switching element 6b and the resistor unit 7b connected in series, and is connected in parallel to the second DC capacitor 4b. As a result, when the switching element 6b is in a conductive state, for example, the stored energy of the second DC capacitor 4b can be reduced by heat generation in the resistor unit 7b. Note that the first chopper circuit 5a in this embodiment corresponds to the first suppression circuit, and the second chopper circuit 5b corresponds to the second suppression circuit.

[0017] The control unit 8 drives the switching elements 6a and 6b in accordance with a VPO voltage signal having information about the voltage value of the voltage VPO measured by a voltage meter (not shown) and a VON voltage signal having information about the voltage value of the voltage VON. The control unit 8 is capable of performing a first control, in which the first chopper circuit 5a and the second chopper circuit 5b are operated independently based on the voltages VPO and VON, and a second control, in which the first chopper circuit 5a and the second chopper circuit 5b are operated simultaneously. In the first control, the control unit 8 controls the first chopper circuit 5a based on the voltage VPO and the second chopper circuit 5b based on the voltage VON. In the second control, the control unit 8 simultaneously controls the first chopper circuit 5a and the second chopper circuit 5b based on the sum of the voltages VPO and VON. The control unit 8 also drives the switching elements of the power converters 3 and 6 as inverters and converters using a general control method.

[0018] FIG. 2 is a diagram showing an example of the detailed circuit configuration of the power converters 3 and 6. As shown in FIG. 2, the power converter 3 is an example of the circuit configuration of a neutral-point-clamped (NPC) inverter, and the power converter 6 is an example of the circuit configuration of a neutral-point-clamped inverter. Note that in this embodiment, an NPC inverter is used as an example of a neutral-point-clamped power converter, but this is not limiting. For example, the neutral point clamp may be a T-type neutral point clamp, or may be another type.

[0019] As shown in Figure 2, one leg is configured with four semiconductor switching elements (hereinafter sometimes referred to as switching elements) and two clamp semiconductor switching elements (which may be diodes). In the DC link section, a first DC capacitor 4a and a second DC capacitor 4b are connected in series. The AC power supply is connected to the AC side terminals of the UVW phase converter including the U phase power converter 3, and the load is connected to the AC side terminals of the UVW phase inverter including the u phase power converter 6.

[0020] The power converter 3 includes four switching elements SW_C1, SW_C2, SW_C3, and SW_C4 connected in series from the high-potential side to the low-potential side to form legs; four freewheeling diodes D_C1, D_C2, D_C3, and D_C4 connected in anti-parallel with each switching element in a one-to-one relationship; two switching elements SW_C5 and SW_C6 connected in series from the interconnection point of the switching elements SW_C1 and SW_C2 to the interconnection point of the switching elements SW_C3 and SW_C4; and two diodes D_C5 and D_C6 connected in anti-parallel with the switching elements SW_C5 and SW_C6 in a one-to-one relationship. The first DC capacitor 4a holds a DC voltage VPO, and the second DC capacitor 4b holds a DC voltage VON. The collector of each switching element is the high-potential side, and the emitter is the low-potential side. The cathode of each diode is the high-potential side, and the anode is the low-potential side.

[0021] A current iuc is output from the interconnection point of switching elements SW_C2 and SW_C3 to the outside of the power converter 3. The interconnection point of switching elements SW_C5 and SW_C6 is connected to the neutral point O (potential is VON), which is the interconnection point of the first DC capacitor 4a and the second DC capacitor 4b, and a current inc flows through it. The configuration of the V-phase and W-phase converters is the same as that of the U-phase power converter 3.

[0022] The u-phase power converter 6 also has four switching elements SW_I1, SW_I2, SW_I3, and SW_I4 connected in series from the high potential side to the low potential side to form legs, four freewheeling diodes D_I1, D_I2, D_I3, and D_I4 connected in anti-parallel to each switching element in a one-to-one relationship, two switching elements SW_I5 and SW_I6 connected in series from the interconnection point of the switching elements SW_I1 and SW_I2 to the interconnection point of the switching elements SW_I3 and SW_I4, and two diodes D_I5 and D_I6 connected in anti-parallel to the switching elements SW_I5 and SW_I6 in a one-to-one relationship.

[0023] The current iui is output from the interconnection point of the switching elements SW_I2 and SW_I3 to the outside of the power converter 6. The interconnection point of the switching elements SW_I5 and SW_I6 is connected to the neutral point O, and a current ini flows through it. The configuration of the v-phase and w-phase inverters is the same as that of the u-phase power converter 6.

[0024] Here, an example of the operation of the switching elements will be described. Fig. 3 is a diagram showing an example in which four semiconductor switching elements and two clamp diodes are configured in one leg. For simplicity of explanation, an example will be described in which the two clamp semiconductor switching elements (see Fig. 2) are not used.

[0025] First, an example of operation during normal operation will be described using Figures 4 and 5. Figure 4 is a diagram showing an example of operation during normal operation. In this example, switching elements SW_C1 and SW_C2 are in the ON state, and switching elements SW_C3 and SW_C4 are in the OFF state. The output current passes through switching elements SW_C1 and SW_C2, and the power converter 3 outputs the potential of the first DC node.

[0026] Figure 5 shows the state after switching element SW_C1 is turned off from the state shown in Figure 4. When switching element SW_C1 is turned off, current flows in the direction of the arrow along the commutation path shown by the dashed line. As a result, the output current is commutated to a path passing through diode D_C5 and switching element SW_C2, and power converter 3 outputs the potential of the O-side DC node.

[0027] While the switching element SW_C1 is turned off, the sum of voltage VPO and surge voltage Vsurge1, which is generated by the inductance of the commutation path indicated by the dashed line, is applied to the switching element SW_C1. Therefore, the first chopper circuit 5a must suppress voltage VPO so that the sum of voltage VPO and surge voltage Vsurge1 is equal to or less than the device breakdown voltage Vces. Thus, during normal operation, the second chopper circuit 5b controls voltage VPO to be equal to or less than the device breakdown voltage Vces minus surge voltage Vsurge1. In other words, if the first operating threshold Vth1, which is the upper limit voltage for operating the first chopper circuit 5a, is set to Vces-Vsurge1, voltage VPO will be equal to or less than the device breakdown voltage Vces minus surge voltage Vsurge1. Note that surge voltage Vsurge1 is determined by the turn-off time and inductance of the switching element SW_C1.

[0028] Similarly, when the switching elements SW_C1 and SW_C2 are in the OFF state and the switching elements SW_C3 and SW_C4 are in the ON state, and then SW_C4 is turned OFF, the voltage VON must be suppressed by the second chopper circuit 5b. Thus, during normal operation, if the first operating threshold Vth1, which is the upper limit voltage for operating the second chopper circuit 5b based on the voltage VON, is set to Vces-Vsurge1, the voltage VPO will be equal to or less than the value obtained by subtracting the surge voltage Vsurge1 from the element withstand voltage Vces.

[0029] Next, an example of operation when an element fails will be described using Figures 6 and 7. Figure 6 is a diagram showing an example when switching element SW_C1 has a short-circuit failure in the state of Figure 4. As in Figure 4, the output current passes through switching elements SW_C1 and SW_C2, and the power converter 3 outputs the potential of the first DC node.

[0030] FIG. 7 illustrates an example in which the switching element SW_C1 in the state illustrated in FIG. 5 experiences a short-circuit fault. Because the switching element SW_C1 experiences a short-circuit fault, turning off the switching element SW_C2 causes current to flow along the commutation path indicated by the dashed line in the direction indicated by the arrow. This causes the output current to be commutated to a path passing through diodes D_C3 and D_C4, and the power converter outputs the voltage of the O-side DC node. While the switching element SW_C2 is turned off, the sum of the voltage VPO across the first DC capacitor 4a, the voltage VON across the second DC capacitor 4b, and a surge voltage Vsurge2 generated by the inductance of the commutation path indicated by the dashed line is applied to the switching element SW_C2. Therefore, it is necessary to control the voltage VPO + VON + Vsurge2 so that it is equal to or less than the breakdown voltage Vces. In this case, suppressing the voltage using only the first chopper circuit 5a or the second chopper circuit 5b will result in the voltage exceeding the breakdown voltage Vces. Therefore, in the event of an element failure, the chopper circuits 5a and 5b are used to simultaneously suppress the voltages VPO and VON.

[0031] In other words, if the second operating threshold Vth2, which is the upper limit voltage for simultaneously operating the first chopper circuit 5a and the second chopper circuit 5b, is set to a value obtained by subtracting the surge voltage Vsurge2 from the voltages VPO and VON, the sum of the voltages VPO and VON will be equal to or less than the value obtained by subtracting the surge voltage Vsurge2 from the element breakdown voltage Vces. In other words, the chopper circuits 5a and 5b operate so that (VPO+VON+Vsurge2) / 2 is equal to or less than the element breakdown voltage Vces / 2.

[0032] Furthermore, since switching occurs only once in the event of an element failure, a clamp circuit that suppresses surge voltage Vsurge2 may also be provided. If a clamp circuit is provided, it is sufficient to suppress Vpo+Von to be equal to or less than the element's breakdown voltage. In other words, when configuring a clamp circuit in the event of an element failure, the threshold Vth1 can be set so that the sum of voltages Vpo and Von becomes the element's breakdown voltage Vces. For example, an active gate circuit that detects the element voltage and feeds it back to the element gate can be used as the clamp circuit.

[0033] 2 again, an active gate circuit is configured for each of the switching elements SW_C1 to C6 and SW_I1 to I6. This active gate circuit detects the element voltage between the collector and emitter of each of the switching elements SW_C1 to C6 and SW_I1 to I6, and when the element voltage reaches a clamp voltage, controls the voltage and current at the gate terminal of each of the switching elements SW_C1 to C6 and SW_I1 to I6 to prevent the surge voltage Vsurge2 from rising above the element breakdown voltage Vces. Note that a general circuit configuration can be used for the active gate circuit.

[0034] Thus, the required chopper operation threshold differs between normal operation and when an element fails. However, to ensure safe switching in either state, the DC voltage must be suppressed below both thresholds. Furthermore, when a power grid fails, control is also required to keep VPO+VON below the element breakdown voltage Vces. For this reason, in this embodiment, the operation threshold when a power grid fails is also set to the second operation threshold Vth2.

[0035] 8 is a diagram showing an example of the configuration of a control circuit 8a that drives the first chopper circuit 5a and the second chopper circuit 5b. The control circuit 8a has an adder circuit 12, a divider circuit 14, comparators 16, 18, and 20, and OR circuits 22 and 24.

[0036] The input terminal of the adder circuit 12 is connected to a voltage meter, and receives the VPO voltage signal and the VON voltage signal. The output terminal of the adder circuit 12 is connected to one input terminal of a divider circuit 14. The other input terminal of the divider circuit 14 receives a division coefficient of 2. The output terminal of the divider circuit 14 is input to one input terminal of a comparator circuit 16. The other input terminal of the comparator circuit 16 receives a second operating threshold Vth2. The output terminal of the comparator circuit 16 is connected to one input terminal of an OR circuit 22 and one input terminal of an OR circuit 24. The comparator circuit 16 according to this embodiment corresponds to the third circuit unit.

[0037] One input terminal of the comparator circuit 18 is connected to a voltage meter and receives a VPO voltage signal. The other input terminal of the comparator circuit 18 receives a first operating threshold Vth1. The output terminal of the comparator circuit 18 is connected to the other input terminal of the OR circuit 22. The output terminal of the OR circuit 22 is connected to the switching element 6a of the first chopper circuit 5a. The comparator circuit 18 according to this embodiment corresponds to the first circuit unit.

[0038] One input terminal of the comparator circuit 20 is connected to a voltage meter and receives a VON voltage signal. The other input terminal of the comparator circuit 20 receives a first operating threshold Vth1. The output terminal of the comparator circuit 20 is connected to the other input terminal of the OR circuit 24. The output terminal of the OR circuit 24 is connected to the switching element 6b of the second chopper circuit 5b. The comparator circuit 20 according to this embodiment corresponds to the second circuit unit.

[0039] Here, an example of operation of the control circuit 8a will be described. First, an example of operation when the voltage VP0 of the first DC capacitor 4a becomes greater than the first operating threshold Vth1 will be described. The value of the voltage VP0 is input to the comparator circuit 18, which compares it with the first operating threshold Vth1. When the value of the voltage VP0 is greater than the first operating threshold Vth1, the comparator circuit 18 outputs a true value to the OR circuit 22. As a result, the OR circuit 22 outputs a true value (e.g., a high-level signal) that turns on the switching element 6a of the first chopper circuit 5a. Subsequently, the first chopper circuit 5a operates, and energy from the first DC capacitor 4a is consumed. This prevents breakdown of the switching elements of the power converters 3 and 6.

[0040] Similarly, an example of operation when the voltage VON of the second DC capacitor 4b becomes greater than the first operating threshold Vth1 will be described. The value of the voltage VON is input to the comparator circuit 20, which compares it with the first operating threshold Vth1. When the value of the voltage VON is greater than the first operating threshold Vth1, the comparator circuit 20 outputs a true value to the OR circuit 24. This causes the OR circuit 24 to output a true value (e.g., a high-level signal) that turns on the switching element 6b of the second chopper circuit 5b. This activates the second chopper circuit 5b, consuming energy from the second DC capacitor 4b. This prevents breakdown of the switching elements of the power converters 3 and 6.

[0041] An example of operation when half the sum of voltages VP0 and VON is greater than second operating threshold Vth2 will be described. The values ​​of voltages VP0 and VON are input to adder circuit 12. Adder circuit 12 outputs the sum of voltages VP0 and VON to divider circuit 14. Divider circuit 14 divides the sum by 2 and outputs the result to comparator circuit 16.

[0042] When half of the sum of voltages VP0 and VON is greater than the second operation threshold Vth2, the comparator circuit 16 outputs a true value to the OR circuits 22 and 24. As a result, the OR circuit 22 outputs a true value (e.g., a high-level signal) that turns on the switching element 6a of the first chopper circuit 5a. Subsequently, the first chopper circuit 5a operates, consuming energy from the first DC capacitor 4a. At the same time, the OR circuit 24 outputs a true value (e.g., a high-level signal) that turns on the switching element 6b of the second chopper circuit 5b. As a result, the second chopper circuit 5b operates simultaneously with the first chopper circuit 5a, consuming energy from the second DC capacitor 4b. This makes it possible to prevent the spread of accidents, such as short-circuit failures of switching elements, in the power converters 3 and 6.

[0043] In this way, the first operating threshold Vth1 used during normal operation and the second operating threshold Vth2 used in the event of an element failure can be set to different values. This allows for separate control of the switching element protection operation during normal operation and the operation to prevent the spread of an accident such as a short-circuit failure of the switching element. This suppresses unnecessary operation of the first chopper circuit 5a and the second chopper circuit 5b. Generally, the surge voltage Vsurge1 associated with normal switching is designed to be smaller than Vces / 2. Therefore, setting Vth1 > Vth2 can further suppress unnecessary chopper operation. Furthermore, the first operating threshold Vth1 and the second operating threshold Vth2 can be set to different operation start and stop thresholds to provide a hysteresis width. This enables control operation that is more in line with safety objectives.

[0044] Figure 9 is a time chart showing an example of operation when a fault occurs in the AC system. The horizontal axis represents time. From the top, the horizontal axis represents the operating waveforms of the switching element 6a of the first chopper circuit 5a, the switching element 6b of the second chopper circuit 5b, the first operating threshold Vth1, the second operating threshold Vth2, and the midpoint target potential 1pu.

[0045] At time t0, an AC system fault occurs, causing both voltages VPO and VON to start rising, and at time t1, voltage VPN (VPO+VON) exceeds second operating threshold Vth2. As a result, both switching elements 6a and 6b enter a conducting state (ON state) at time t1. This consumes energy from the first DC capacitor 4a and the second DC capacitor 4b, and the rise in voltage VPN is maintained at the second operating threshold Vth2.

[0046] Next, at time t2, the AC system fault is removed. As a result, voltage VPN becomes smaller than second operating threshold Vth2, and both switching elements 6a and 6b enter a non-conducting state (OFF state). When normal element driving returns at time t2, voltage VPO begins to rise, while voltage VON begins to fall.

[0047] Next, at time t3, the voltage VPO exceeds the first operating threshold Vth1. This causes the switching element 6a to enter a conductive state (ON state) at time t3. This consumes energy from the first DC capacitor 4a, maintaining the rise in the voltage VPO at the first operating threshold Vth1. Next, at time t4, the voltage VPO becomes smaller than the first operating threshold Vth1, causing the switching element 6a to enter a non-conductive state (OFF state). In this way, the first operating threshold Vth1 can also respond to an AC system fault, suppressing the operation of the first chopper circuit 5a and the second chopper circuit 5b while preventing breakdown of the switching elements in the power converters 3 and 6. Furthermore, from time t3 to t4, only the energy of the first DC capacitor 4a is consumed, suppressing the rise in the voltage VPO, while the voltage VON continues to rise. In this way, being able to independently suppress the voltages VPO and VON also contributes to suppressing voltage imbalance. [Explanation of symbols]

[0048] 1: power generation system, 3, 6: power converter, 4a: first DC capacitor, 4b: second DC capacitor, 5a: first chopper circuit, 5a, 5b: second chopper circuit, 8: control unit, 8a: control circuit, 10: power conversion device, 16, 18, 20: comparison circuit, 20: three-phase induction generator

Claims

1. two power converters each having a DC side terminal connected to a junction point of a first DC capacitor and a second DC capacitor connected in series and having an AC side terminal connected to a common system; an overvoltage suppression circuit including a first suppression circuit capable of reducing stored energy in the first DC capacitor and a second suppression circuit capable of reducing stored energy in the second DC capacitor; a control circuit capable of performing a first control for independently operating the first suppression circuit and the second suppression circuit and a second control for simultaneously operating the first suppression circuit and the second suppression circuit, based on a first DC voltage of the first DC capacitor and a second DC voltage of the second DC capacitor; A power conversion device comprising:

2. In the first control, the first suppression circuit is controlled based on the first DC voltage, and the second suppression circuit is controlled based on the second DC voltage. The power conversion device according to claim 1 , wherein the second control simultaneously controls the first suppression circuit and the second suppression circuit based on a sum voltage of the first DC voltage and the second DC voltage.

3. the first suppression circuit is composed of a first semiconductor switch and a first resistor connected in series, and is connected in parallel to the first DC capacitor, and the first semiconductor switch is in a conductive state when either the first control or the second control is being executed; 3. The power conversion device according to claim 1, wherein the second suppression circuit is composed of a second semiconductor switch and a second resistor connected in series, is connected in parallel to the second DC capacitor, and the second semiconductor switch is in a conductive state when either the first control or the second control is executed.

4. The control circuit a first circuit unit that operates the first suppression circuit based on the first DC voltage; a second circuit unit that operates the second suppression circuit based on the second DC voltage; a third circuit unit that simultaneously operates the first suppression circuit and the second suppression circuit based on the sum of the first DC voltage and the second DC voltage; The power converter of claim 1 , comprising:

5. the first circuit unit operates the first suppression circuit when the first DC voltage is greater than a first operation threshold; The power conversion device according to claim 4 , wherein the second circuit unit operates the second suppression circuit when the second DC voltage is greater than the first operating threshold value.

6. 6. The power conversion device according to claim 5, wherein the third circuit unit operates the first suppression circuit and the second suppression circuit when half the sum of the first DC voltage and the second DC voltage is greater than a second operation threshold.

7. The power conversion device according to claim 6 , wherein the second operating threshold is set to be smaller than the first operating threshold.

8. 2. The power conversion device according to claim 1, wherein the two power converters are a neutral point clamped inverter and a neutral point clamped converter, each of which is configured with four semiconductor switching elements and two clamping semiconductor switching elements or diodes.

9. 9. The power conversion device according to claim 8, wherein the neutral point clamped inverter supplies power to a secondary excitation winding of a generator having a stator winding connected to a power grid.

10. The power conversion device according to claim 7 , wherein the first operation threshold and the second operation threshold are capable of providing a hysteresis width by changing an operation start threshold and an operation stop threshold.

11. 9. The power conversion device according to claim 8, further comprising a clamp circuit configured to adjust a gate voltage when a voltage between connection terminals of the semiconductor switching element reaches a predetermined voltage so that a surge voltage does not reach or exceed a withstand voltage of the semiconductor switching element.

12. two power converters each having a DC side terminal connected to a junction point of the first DC capacitor and the second DC capacitor connected in series, and each having an AC side terminal connected to a common system; A control method for a power conversion device having an overvoltage suppression circuit including a first suppression circuit capable of reducing stored energy in the first DC capacitor and a second suppression circuit capable of reducing stored energy in the second DC capacitor, A control method for a power conversion device, which executes a first control for independently operating the first suppression circuit and the second suppression circuit based on a first DC voltage of the first DC capacitor and a second DC voltage of the second DC capacitor, and a second control for simultaneously operating the first suppression circuit and the second suppression circuit.

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