Power conversion device

Abstract

To provide a power conversion device capable of suppressing a failure due to a temperature rise of a switching element with a simpler configuration even when an output of a low frequency is required.SOLUTION: A converter configured to convert power supplied from a power source into three-phase AC power corresponding to a load by switching of a plurality of switching elements, and a control device configured to control an operation of the converter to convert the power, wherein the control device sets a predetermined frequency range including 0 Hz as a continuous operation prohibition region, and controls an operation of the converter so as to pass through the continuous operation prohibition region when a frequency of the three-phase AC power enters the continuous operation prohibition region, when the control device performs control to pass through the continuous operation prohibition region and then performs control to pass through the next continuous operation prohibition region within a predetermined time, the control device adjusts the current phase and controls the operation of the converter so that a phase different from the previous phase has a strict current pattern.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] 2. Description of the Related Art There is a power conversion device that has a plurality of switching elements, converts power supplied from a power source into three-phase AC power according to a load by switching the plurality of switching elements, and supplies the converted three-phase AC power to the load.

[0003] For example, in a power conversion device that requires a relatively low frequency output, such as a secondary excitation converter that adjusts the rotational speed of a generator-motor by AC exciting the rotor of the generator-motor in an adjustable-speed pumped-storage power generation system, the load on a specific switching element may become excessive depending on the operation of the device, and the element temperature may rise, causing the switching element to break down. For example, in order to cope with the excessive load, it is conceivable to configure the switching elements in parallel, but in this case, there are concerns about the device becoming larger and the manufacturing costs increasing.

[0004] For this reason, it is desirable for a power conversion device to have a simpler configuration that can suppress failures caused by temperature increases in switching elements even when a low frequency output is required. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2023-34924 [Patent Document 2] Patent No. 6900887 [Patent Document 3] Patent No. 4120708 [Patent Document 4] Patent No. 4976144 Summary of the Invention [Problem to be solved by the invention]

[0006] The embodiments of the present invention provide a power conversion device that can suppress failures due to temperature rises in switching elements with a simpler configuration, even when a low frequency output is required. [Means for solving the problem]

[0007] According to an embodiment of the present invention, a converter has a plurality of switching elements, converts power supplied from a power source into three-phase AC power according to a load by switching on and off the plurality of switching elements, and supplies the converted three-phase AC power to the load, and a control device controls the power conversion operation of the converter, wherein the control device arbitrarily changes the frequency of the three-phase AC power supplied from the converter to the load, sets a predetermined frequency range including 0 Hz as a continuous operation prohibited region, and controls the operation of the converter so that the frequency of the three-phase AC power enters the continuous operation prohibited region, and controls the operation of the converter so that the three-phase AC power passes through the continuous operation prohibited region, and Depending on the current phase when passing through the continuous operation prohibited area, one or two phases will have a relatively strict current pattern, and the remaining phases will have a relatively less strict current pattern, and the strict current pattern is a current pattern in which the period of time during which the polarity does not change is longer and the current value is also large for a longer period compared to normal operation which does not pass through the continuous operation prohibited area, and when the control device performs control to pass through the next continuous operation prohibited area within a predetermined time after performing control to pass through the continuous operation prohibited area, it adjusts the current phase and controls the operation of the converter so that a phase different from the previous time will have the strict current pattern, thereby providing a power conversion device which avoids repetition of the strict current pattern in the same phase. [Effects of the Invention]

[0008] A power conversion device is provided that can suppress failures due to temperature rises in switching elements with a simpler configuration even when a low frequency output is required. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a block diagram schematically illustrating an adjustable-speed pumped-storage power generation system and a power conversion device according to an embodiment. [Figure 2] FIG. 2 is a block diagram schematically illustrating an inverter. [Figure 3] 3(a) and 3(b) are graphs that schematically show an example of the operation of the power conversion device according to the embodiment. [Figure 4] 4 is a graph schematically illustrating an example of the operation of the control device according to the embodiment. [Figure 5] 5(a) to 5(d) are graphs that schematically show an example of the operation of the control device according to the embodiment. [Figure 6] 6(a) and 6(b) are graphs that schematically show an example of the operation of the control device according to the embodiment. [Figure 7] 7(a) to 7(d) are graphs that schematically show an example of the operation of the control device according to the embodiment. [Figure 8] 8(a) to 8(d) are graphs that schematically show an example of the operation of the control device according to the embodiment. [Figure 9] 10 is a table schematically illustrating an example of an operation of the control device according to the embodiment. [Figure 10] 4 is a flowchart schematically illustrating an example of an operation of the power conversion device according to the embodiment. [Figure 11] 10 is a flowchart schematically illustrating a modified example of the operation of the power conversion device according to the embodiment. [Figure 12] 10 is a flowchart schematically illustrating a modified example of the operation of the power conversion device according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Each embodiment will be described below with reference to the drawings. The drawings are schematic or conceptual, and the relationship between the thickness and width of each part, the size ratio between parts, etc. are not necessarily the same as those in reality. Furthermore, even when the same part is shown, the dimensions and ratios may be different depending on the drawing. In the present specification and the drawings, elements similar to those described above with reference to the previous drawings are designated by the same reference numerals, and detailed descriptions thereof will be omitted as appropriate.

[0011] FIG. 1 is a block diagram that schematically illustrates an adjustable-speed pumped-storage power generation system and a power conversion device according to an embodiment. 1, a power conversion device 10 is used in an adjustable speed pumped storage power generation system 2. The adjustable speed pumped storage power generation system 2 includes the power conversion device 10, as well as, for example, a main transformer 3, a generator motor 4, a water turbine 5, and an excitation transformer 6.

[0012] The primary side of the main transformer 3 is connected to the power grid PS via, for example, a circuit breaker (not shown). The power of the power grid PS is three-phase AC power. The main transformer 3, for example, steps down the three-phase AC power supplied from the power grid PS and outputs it to the secondary side.

[0013] The generator motor 4 has a stator having a stator winding (primary winding) and a rotor having a rotor winding (secondary winding). The stator is, for example, cylindrical. The rotor is inserted into the stator and rotatably supported. The stator winding is connected to the secondary side of the main transformer 3. The generator motor 4 operates as a motor by supplying three-phase AC current to the stator winding and three-phase AC current to the rotor winding, and setting the rotor rotation speed to be equal to or lower than the rotation speed (synchronous speed) of a rotating magnetic field generated in the stator winding. The generator motor 4 operates as a generator by supplying three-phase AC current to the stator winding and three-phase AC current to the rotor winding, and by rotating the rotor using an external force to make the rotor rotation speed faster than the synchronous speed. The stator winding receives three-phase AC power from the power system PS via the main transformer 3, and also supplies the generated three-phase AC power to the power system PS via the main transformer 3 and the like.

[0014] The water turbine 5 is connected to the rotor of the generator motor 4 via a shaft or the like, and rotates as the rotor rotates. The water turbine 5 rotates due to the water flow, causing the rotor of the generator motor 4 to rotate, thereby operating the generator motor 4 as a generator. When the generator motor 4 operates as an electric motor, the water turbine 5 rotates in conjunction with the rotation of the rotor of the generator motor 4, thereby functioning as a pump for pumping up water.

[0015] The primary side of the excitation transformer 6 is connected to the secondary side of the main transformer 3. The secondary side of the excitation transformer 6 is connected to the power conversion device 10. The excitation transformer 6 further steps down the three-phase AC power stepped down by the main transformer 3, for example, and supplies the power conversion device 10.

[0016] The power conversion device 10 converts three-phase AC power supplied from the excitation transformer 6 (power supply) into three-phase AC power for exciting the rotor winding of the generator motor 4, and supplies the converted three-phase AC power to the rotor winding of the generator motor 4. For example, the power conversion device 10 converts three-phase AC power of a frequency according to the power grid PS supplied from the excitation transformer 6 into three-phase AC power of an arbitrary frequency according to the rotor winding, and supplies the converted three-phase AC power to the rotor winding of the generator motor 4. In other words, the power conversion device 10 is a frequency conversion device. The power conversion device 10 may also be called, for example, a secondary excitation converter.

[0017] The power conversion device 10 includes a converter 12, a control device 14, and a current detector 16. The converter 12 converts the three-phase AC power supplied from the excitation transformer 6 into three-phase AC power for exciting the rotor winding of the generator motor 4, and supplies the converted three-phase AC power to the rotor winding of the generator motor 4.

[0018] Converter 12 includes, for example, a converter 20 that converts the three-phase AC power supplied from excitation transformer 6 into DC power, and an inverter 22 that converts the DC power converted by converter 20 into three-phase AC power for exciting the rotor windings. However, the configuration of converter 12 is not limited to this. Converter 12 may also be, for example, a direct conversion device such as a cycloconverter that converts three-phase AC power into another three-phase AC power without passing through DC. Converter 12 may have any configuration that can appropriately convert the three-phase AC power supplied from excitation transformer 6 into three-phase AC power suitable for the rotor windings.

[0019] The control device 14 controls the power conversion operation of the converter 12. The current detector 16 detects the three-phase AC current supplied from the converter 12 to the rotor windings and inputs the detection result to the control device 14. More specifically, the current detector 16 detects the magnitude of each current of each phase of the three-phase AC current supplied from the converter 12 to the rotor windings and inputs the detection result to the control device 14. The control device 14 controls the power conversion operation of the converter 12 based on the detection result of the current detector 16. It should be noted that the power conversion device 10 does not necessarily have to include the current detector 16. The power conversion device 10 may, for example, obtain information on the detection result of the magnitude of each current of each phase of the three-phase AC current supplied from the converter 12 to the rotor windings from a higher-level controller or the like.

[0020] The adjustable-speed pumped storage power generation system 2 has a power generation operation and a pumping operation. In the power generation operation, water stored in the upper regulating reservoir is dropped into the lower regulating reservoir, and the water flow rotates the water turbine 5, causing the generator motor 4 to operate as a generator. As a result, in the power generation operation, the electric power generated by the generator motor 4 can be supplied to the power grid PS.

[0021] In pumping operation, three-phase AC power is supplied to the stator winding and rotor winding based on the power supplied from the power grid PS side, and the generator motor 4 is operated as an electric motor. As a result, in pumping operation, the water turbine 5 is operated as a pump, and water stored in the lower regulating reservoir is pumped up to the upper regulating reservoir.

[0022] The adjustable speed pumped storage power generation system 2 performs pumping operation to pump water into the upper regulating reservoir during times of low power demand, and performs power generation operation during times of high power demand to supply power to the power grid PS. This allows the adjustable speed pumped storage power generation system 2 to store electricity in the form of water.

[0023] Furthermore, in the adjustable-speed pumped-storage power generation system 2, the rotation speed of the rotor of the generator motor 4 is changed according to the power situation on the power grid PS side, thereby making it possible to adjust the power in both the power generation operation and the pumping operation. The power of the generator motor 4 increases or decreases in proportion to, for example, the cube of the rotation speed of the rotor.

[0024] The rotation speed of the generator motor 4 is adjusted by exciting the rotor with AC and changing the excitation frequency. When excited with DC, the rotation speed of the rotor of the generator motor 4 is the same as the power supply frequency of the power grid PS. On the other hand, when excited with AC, the rotation speed of the rotor of the generator motor 4 is related to the power supply frequency and the excitation frequency. For example, when the power supply frequency is 50 Hz and the rotor winding is excited with AC of 3 Hz, the rotation speed of the rotor of the generator motor 4 corresponds to 53 Hz. When AC is excited with the opposite phase of 3 Hz (corresponding to -3 Hz), the rotation speed of the rotor of the generator motor 4 corresponds to 47 Hz.

[0025] The adjustable range of the rotor rotation speed is, for example, about ±5 Hz. Therefore, the power conversion device 10 (converter 12) is required to output a very low frequency. In this specification, the low frequency of the three-phase AC power output by the power conversion device 10 is, for example, a frequency of 10 Hz or less.

[0026] FIG. 2 is a block diagram schematically illustrating an inverter. 2, inverter 22 has a plurality of switching elements Q1 to Q4. Inverter 22 converts DC power into three-phase AC power by switching on and off the plurality of switching elements Q1 to Q4. The plurality of switching elements Q1 to Q4 are, for example, self-excited semiconductor switching elements such as IGBTs. Control device 14 generates a plurality of control signals for controlling the switching of each of the plurality of switching elements Q1 to Q4, and inputs the generated control signals to converter 12 to control the power conversion operation by converter 12 (inverter 22).

[0027] The inverter 22 includes, for example, four switching elements Q1 to Q4, four rectifying elements D11 to D14, two rectifying elements D21 and D22, two charge storage elements C1 and C2, a DC high potential terminal P, a DC low potential terminal N, a DC neutral terminal O, and an AC output terminal AC. For convenience, Fig. 2 shows only the configuration of one phase of the inverter 22 for three-phase AC power. The configuration of the remaining two phases of the inverter 22 is substantially the same as the configuration of one phase shown in Fig. 2, and therefore detailed description thereof will be omitted.

[0028] The inverter 22 is connected to the converter 20 via a DC high potential terminal P and a DC low potential terminal N, and receives DC power output from the converter 20 via the DC high potential terminal P and the DC low potential terminal N. The potential of the DC neutral terminal O is set to an intermediate potential between the potential of the DC high potential terminal P and the potential of the DC low potential terminal N.

[0029] The four switching elements Q1 to Q4 are connected in series between a DC high potential terminal P and a DC low potential terminal N. The four rectifying elements D11 to D14 are connected in parallel to the four switching elements Q1 to Q4, respectively. The rectifying element D21 is provided between the connection point of the rectifying elements D11 and D12 and a DC neutral terminal O. The rectifying element D22 is provided between the connection point of the rectifying elements D13 and D14 and the DC neutral terminal O. The charge storage element C1 is provided between the DC high potential terminal P and the DC neutral terminal O. The charge storage element C2 is provided between the DC low potential terminal N and the DC neutral terminal O. The AC output terminal AC is connected to the connection point of the switching elements Q2 and Q3.

[0030] In this example, the inverter 22 is a so-called three-level inverter. However, the configuration of the inverter 22 is not limited to this, and any configuration that can convert DC power into three-phase AC power by switching a plurality of switching elements may be used.

[0031] 3(a) and 3(b) are graphs that schematically show an example of the operation of the power conversion device according to the embodiment. 3(a) and 3(b) each show an example of an upper carrier signal CRU, a lower carrier signal CRL, a voltage command value VR, an output current Iac output from AC output terminal AC, a control signal SQ1 for controlling the switching of switching element Q1, a control signal SQ2 for controlling the switching of switching element Q2, a control signal SQ3 for controlling the switching of switching element Q3, and a control signal SQ4 for controlling the switching of switching element Q4. Also, FIG. 3(a) shows an example of a case where the frequency of the three-phase AC power supplied from converter 12 to the rotor windings is low, and FIG. 3(b) shows an example of a case where the frequency of the three-phase AC power supplied from converter 12 to the rotor windings is high.

[0032] As shown in Figures 3(a) and 3(b), in the case of low-frequency output, the period during which the output current Iac remains high near its peak value is longer than in the case of high-frequency output. When the output current Iac is high, the losses generated in the switching elements Q1 to Q4 are also large, and the temperature of the switching elements Q1 to Q4 tends to become severe. When the output frequency is high, the period during which the output current Iac remains high near its peak value is short, and the polarity of the output current Iac changes periodically. When the polarity of the output current Iac changes, the conducting element changes. Therefore, in terms of the duties of the switching elements Q1 to Q4, low-frequency output, in which the polarity of the output current Iac does not change for a long time and a high current remains high near its peak value for a long period, poses more severe conditions than high-frequency output.

[0033] FIG. 4 is a graph schematically illustrating an example of the operation of the control device according to the embodiment. The output frequency of the converter 12 (inverter 22) changes depending on the situation. In other words, the rotational speed of the generator motor 4 changes depending on the situation. The control device 14 arbitrarily changes the frequency of the three-phase AC power supplied from the converter 12 to the rotor winding depending on the situation of the power grid PS and the situation of the generator motor 4, which is the load. The control device 14 changes the output frequency depending on the situation of at least one of the power grid PS and the load, for example. The control device 14 changes the output frequency depending on the situation of each part, for example, based on the measurement results of the grid voltage, external commands, etc.

[0034] In some cases, the output frequency of the converter 12 may be close to 0 Hz (DC). In such a case, as described above, the load on the switching elements Q1 to Q4 becomes excessive, which may cause the switching elements Q1 to Q4 to break down. For this reason, the control device 14 sets the region close to 0 Hz as the continuous operation prohibited region, as shown in FIG. 4. The control device 14 sets a predetermined frequency range including 0 Hz as the continuous operation prohibited region. For example, the control device 14 sets a predetermined frequency range centered around 0 Hz as the continuous operation prohibited region.

[0035] 5(a) to 5(d) are graphs that schematically show an example of the operation of the control device according to the embodiment. Figures 5(a) and 5(c) show an example of the output frequency of the converter 12 (inverter 22). Figures 5(b) and 5(d) show an example of the output current of the converter 12 (inverter 22). Figures 5(a) and 5(b) show an example of operation when the output frequency is constant at the upper limit of the continuous operation prohibited region. Figures 5(c) and 5(d) show an example of operation when the output frequency falls within the continuous operation prohibited region.

[0036] 5(c) and 5(d), the control device 14 arbitrarily changes the frequency of the three-phase AC power supplied from the converter 12 to the rotor winding, and if the output frequency enters the continuous operation prohibited region, controls the operation of the converter 12 (inverter 22) so that the converter 12 passes through the continuous operation prohibited region and does not continue operation in the continuous operation prohibited region. When the output frequency enters the continuous operation prohibited region, the control device 14 controls the operation of the converter 12 so that the converter 12 passes through the continuous operation prohibited region, for example, by changing the output frequency at a predetermined rate of change, as shown in FIG. 5(c).

[0037] When passing through the continuous operation prohibited area, depending on the type of control used, there may be a longer period in which the polarity of the output current does not change compared to normal operation, and the current value may remain high for a longer period, as shown in Figure 5(d).

[0038] 6(a) and 6(b) are graphs that schematically show an example of the operation of the control device according to the embodiment. Fig. 6(a) shows an example of the output frequency of the converter 12 (inverter 22), and Fig. 6(b) shows an example of the output current of the converter 12 (inverter 22).

[0039] 6(a) and 6(b), the continuous operation prohibited region may be repeatedly passed through, and depending on the conditions, a current waveform that is severe for a specific switching element among the multiple switching elements Q1 to Q4 may be repeatedly generated. In such a case, the element temperature may become even higher, resulting in a severe condition.

[0040] 7(a) to 7(d) are graphs that schematically show an example of the operation of the control device according to the embodiment. Figures 7(a) and 7(c) schematically show an example of the output frequency of the converter 12 (inverter 22). Figures 7(b) and 7(d) schematically show an example of the output current of the converter 12 (inverter 22). Note that Figure 7(b) shows an example of the output current corresponding to the output frequency of Figure 7(a), and Figure 7(d) shows an example of the output current corresponding to the output frequency of Figure 7(c).

[0041] As shown in Figures 7(a) to 7(d), the current waveform when passing through the continuous operation prohibited area is affected by the current phase when entering the continuous operation prohibited area. Depending on the current phase when entering the continuous operation prohibited area, the current waveform when passing through the continuous operation prohibited area can have strict patterns as shown in Figure 7(b) or less strict patterns as shown in Figure 7(d).

[0042] 8(a) to 8(d) are graphs that schematically show an example of the operation of the control device according to the embodiment. Figures 8(a) and 8(c) schematically show an example of the output frequency of the converter 12 (inverter 22). Figures 8(b) and 8(d) schematically show an example of the output current of the converter 12 (inverter 22). Note that Figure 8(b) shows an example of the output current corresponding to the output frequency of Figure 8(a), and Figure 8(d) shows an example of the output current corresponding to the output frequency of Figure 8(c).

[0043] While Figures 5 to 7 focus on one phase of the three-phase AC power supplied from the converter 12 to the rotor winding, the current patterns of the remaining two phases must also be considered. As shown in Figures 8(a) to 8(d), when considering three phases, one or two phases have a strict current pattern. If the three phases are U, V, and W, for example, in the example shown in Figure 8(b), the U phase has a relatively strict current pattern, while the V and W phases have relatively relaxed current patterns. On the other hand, in the example shown in Figure 8(d), the U phase has a relatively relaxed current pattern, while the V and W phases have relatively strict current patterns. There is no current pattern in which all three phases are strict, or no pattern in which all three phases are relaxed. Therefore, the current pattern of each phase can be adjusted by adjusting the current phase when passing through the continuous operation prohibited area.

[0044] In this way, the current waveform of the three-phase AC power when control is performed to pass through the continuous operation prohibited area has a relatively strict current pattern in one or two phases, depending on the current phase when passing through the continuous operation prohibited area, and a relatively strict current pattern in the remaining phases. More specifically, a strict current pattern is a current pattern in which the period without polarity change is longer and the state with a high current value is longer than in normal operation that does not pass through the continuous operation prohibited area.

[0045] FIG. 9 is a table that schematically illustrates an example of the operation of the control device according to the embodiment. 9, when the continuous operation prohibition region is repeatedly passed through, the control device 14 controls the operation of the converter 12 (inverter 22) to avoid repetition of a severe current pattern in the same phase by adjusting the current phase when passing through the continuous operation prohibition region. In other words, when the control device 14 performs control to pass through the next continuous operation prohibition region within a predetermined time after performing control to pass through the continuous operation prohibition region, the control device 14 adjusts the current phase and controls the operation of the converter 12 so that a phase different from the previous one has a severe current pattern, thereby avoiding repetition of a severe current pattern in the same phase.

[0046] As described above, at least one phase will have a relatively severe current pattern. Therefore, even if the current phase is adjusted, it is difficult to avoid the occurrence of a severe current pattern, but it is possible to prevent a severe current pattern from being repeated in the same phase. This prevents a specific switching element among the multiple switching elements Q1 to Q4 from being overloaded and causing the element temperature of the specific switching element to rise excessively. This prevents failures due to temperature increases in the switching elements.

[0047] For example, when control is performed to pass through the continuous operation prohibited region, the control device 14 stores information regarding the current pattern of each phase of the three-phase AC power supplied from the converter 12 to the rotor winding. For example, as shown in FIG. 9, the control device 14 stores information indicating a relatively strict current pattern or information indicating a relatively loose (less strict) current pattern in association with each phase. For example, the control device 14 may store information regarding the current phase when control is performed to pass through the continuous operation prohibited region. The information stored by the control device 14 is not limited to the above, and may be any information that allows the control device 14 to appropriately recognize the current pattern of each phase when control is performed to pass through the continuous operation prohibited region.

[0048] The control device 14 determines the current pattern (current phase) of each phase when control is performed to pass through the continuous operation prohibited region, for example, based on the control timing of the multiple switching elements Q1 to Q4 of the converter 12 (inverter 22). The control device 14 may also determine the current pattern (current phase) of each phase when control is performed to pass through the continuous operation prohibited region, for example, based on the detection result of the current detector 16. The method for determining the current pattern (current phase) of each phase when control is performed to pass through the continuous operation prohibited region is not limited to the above, and any method that can be appropriately determined by the control device 14 may be used.

[0049] The control device 14 starts counting the predetermined time from the timing of the end of the control to pass through the continuous operation prohibition region (see FIG. 6(a)). The predetermined time is, for example, 10 seconds. For example, if the interval between the control to pass through the continuous operation prohibition region is 10 seconds or more, even if a severe current pattern occurs consecutively in the same phase, it is considered that the influence of the previous control to pass through the continuous operation prohibition region is reduced, and it is possible to prevent the element temperature of a specific switching element from becoming excessively high. However, the predetermined time is not limited to 10 seconds and may be any time that can suppress the influence of the previous control to pass through the continuous operation prohibition region. The predetermined time may be set appropriately depending on, for example, the characteristics of the switching elements and the capacity of the cooling mechanism of the switching elements.

[0050] For example, when the control device 14 starts timing and then performs control to pass through the next continuous operation prohibited region within a predetermined time, the control device 14 adjusts the current phase to avoid repetition of a severe current pattern in the same phase. For example, the control device 14 adjusts the current phase by delaying the timing of starting control to pass through the next continuous operation prohibited region as necessary, thereby avoiding repetition of a severe current pattern in the same phase.

[0051] Furthermore, when storing information related to the current pattern, the control device 14 may further store information on the sign of the current pattern (information on the direction of the flowing current). For example, as shown in Fig. 6(b), when severe current patterns occur in the same phase in the same current direction (both positive in Fig. 6(b)), current flows through the same switching element in the converter 12, which may cause an excessive load on a particular switching element.

[0052] Therefore, when the control device 14 starts timing and controls the passage through the next continuous operation prohibited area within a predetermined time, if the current direction of the severe current pattern is the same, it adjusts the current phase to avoid repetition of the severe current pattern in the same phase.

[0053] For example, depending on the configuration of converter 12, if the current direction of the strict current pattern is different, the switching element through which current flows in converter 12 may be different. Therefore, in this case, when control is performed to pass through the next continuous operation prohibited region within a predetermined time after starting timing, control device 14 does not adjust the current phase if the current direction of the strict current pattern is different. In other words, when control is performed to pass through the next continuous operation prohibited region within a predetermined time after starting timing, control device 14 controls the operation of converter 12 so that the strict current pattern is repeated in the same phase if the current direction of the strict current pattern is different. In this case, for example, delays in the timing of changing the frequency can be suppressed, and frequency control responsiveness can be further improved.

[0054] However, depending on the configuration of the converter 12, current may flow through the same switching element even when the current direction of the strict current pattern is different. Therefore, in such a case, the control device 14 adjusts the current phase when controlling the passage of the next continuous operation prohibited region within a predetermined time after starting clocking, regardless of the current direction of the strict current pattern. This prevents the repetition of the strict current pattern in the same phase. In this case, it is possible to more appropriately prevent the load of a specific switching element from becoming excessive and the element temperature of the specific switching element from becoming excessive. This makes it possible to more appropriately prevent failures due to temperature increases in the switching elements.

[0055] In this way, when control that passes through the continuous operation prohibited area is continuous, it is preferable that the control device 14 appropriately changes whether or not to adjust the current phase depending on the current direction of the strict current pattern, depending on the configuration of the converter 12.

[0056] FIG. 10 is a flowchart schematically illustrating an example of the operation of the power conversion device according to the embodiment. As shown in FIG. 10, the control device 14 of the power conversion device 10 determines whether or not to execute control to pass through the continuous operation prohibited region (step S101 in FIG. 10).

[0057] When control to pass through the continuous operation prohibited region is executed, control device 14 starts measuring a predetermined time from the timing of ending control to pass through the continuous operation prohibited region, and determines whether or not the predetermined time has elapsed (step S102 in FIG. 10). Furthermore, when control device 14 executes control to pass through the continuous operation prohibited region, for example, it stores information regarding the current pattern of each phase of the three-phase AC power supplied from converter 12 to the rotor winding.

[0058] After starting to measure the predetermined time, the control device 14 determines whether or not to execute control to pass through the next continuous operation prohibited region (step S103 in FIG. 10). If the predetermined time has elapsed before the control device 14 determines to execute control to pass through the next continuous operation prohibited region, the control device 14 returns to the processing of step S101.

[0059] When the control device 14 determines that control for passing through the next continuous operation prohibited region will be performed within a predetermined time, it adjusts the current phase to avoid repetition of strict current patterns in the same phase, and then executes control for passing through the continuous operation prohibited region (step S104 in FIG. 10). At this time, if the current direction of the strict current pattern is different as described above, adjustment of the current phase may not be performed.

[0060] After executing the control to pass through the continuous operation prohibited region, the control device 14 determines, for example, based on the detection result of the current detector 16 and the stored information on the current pattern of each phase, whether or not the sign of the current of the phase that had a strict current pattern in the previous control to pass through the continuous operation prohibited region has changed for a certain period of time among the three-phase AC current (output current) supplied from the converter 12 to the rotor winding (load) (step S105 in FIG. 10).

[0061] If the control device 14 determines that the sign of the current of the phase that had a strict current pattern in the previous control passing through the continuous operation prohibited area has not changed for a certain period of time, it performs a protection operation for the converter 12 (step S106 in Figure 10).

[0062] Adjusting the current phase can suppress the temperature rise of the switching elements. However, there is a possibility that the current phase cannot be adjusted correctly due to the influence of external disturbances, resulting in an unintentional repetition of a severe current pattern. In such cases, the temperature of the switching elements may become excessive, which may cause the switching elements to fail.

[0063] Therefore, after adjusting the current phase and executing control to pass through the continuous operation prohibited region, the control device 14 checks whether or not the repetition of a severe current pattern in the same phase has been appropriately avoided based on the detection result of the current detector 16 and the stored information about the current pattern of each phase. For example, as described above, the control device 14 checks whether or not the repetition of a severe current pattern in the same phase has been appropriately avoided by determining whether or not the sign of the current in the phase that became the severe current pattern in the previous control to pass through the continuous operation prohibited region has changed for a certain period of time.

[0064] If the sign of the current of the phase that became a severe current pattern in the previous control passing through the continuous operation prohibited area changes within a certain time, i.e., if the control device 14 has been able to appropriately avoid repetition of the severe current pattern in the same phase, it returns to processing of step S101.

[0065] On the other hand, if the sign of the current in a phase that resulted in a severe current pattern in the previous control to pass through the continuous operation prohibited region does not change for a certain period of time, i.e., if the repetition of the severe current pattern in the same phase cannot be appropriately avoided and the severe current pattern is unintentionally repeated, the control device 14 executes a protection operation for the converter 12. The certain period of time is set according to, for example, the current frequency, the set range of the continuous operation prohibited region, the rate of change of the output frequency in the continuous operation prohibited region, etc.

[0066] The protection operation of the converter 12 is, for example, at least one of stopping the power conversion operation by the converter 12 (stopping the switching of the multiple switching elements), reducing the switching frequency of the multiple switching elements of the converter 12, and reducing the magnitude (amplitude) of the three-phase AC current supplied from the converter 12 to the rotor winding (load). This makes it possible to protect the converter 12 even when a severe current pattern is unintentionally repeated. Note that the protection operation of the converter 12 is not limited to the above, and may be any operation that can appropriately protect the converter 12 and prevent breakdowns or the like of the converter 12.

[0067] As described above, in power conversion device 10 according to this embodiment, when the continuous operation prohibited region is repeatedly passed through, control device 14 controls the operation of converter 12 to avoid repetition of severe current patterns in the same phase by adjusting the current phase when passing through the continuous operation prohibited region. This prevents the load of a particular switching element among multiple switching elements Q1 to Q4 from becoming excessive, which in turn prevents the element temperature of the particular switching element from becoming excessive, thereby making it possible to prevent failures due to temperature rises in the particular switching element.

[0068] In the power conversion device 10 according to this embodiment, temperature rise of the switching elements can be suppressed, thereby reducing the risk of the converter 12 shutting down due to a failure of the switching elements and improving the utilization rate of the power conversion device 10. Furthermore, the need for a parallel configuration of switching elements to handle excessive loads can be reduced, and the number of parallel-connected switching elements can be reduced, enabling the converter 12 to be made smaller and less expensive. Furthermore, in addition to reducing the number of parallel-connected switching elements, it is also possible to use, for example, smaller and less expensive switching elements. This allows for a reduction in the number of parallel-connected switching elements and the use of smaller switching elements, simplifying the configuration of the converter 12.

[0069] Furthermore, for example, it is possible to increase the output current of the converter 12 while suppressing an increase in the number of parallel connections. In this case, for example, it is possible to increase the capacity of the converter 12 while suppressing an increase in the complexity of the configuration of the converter 12.

[0070] In this way, the power conversion device 10 according to this embodiment can suppress failures due to temperature rises in the switching elements with a simpler configuration, even when a low frequency output is required.

[0071] FIG. 11 is a flowchart schematically illustrating a modified example of the operation of the power conversion device according to the embodiment. In FIG. 11, the processing from steps S201 to S204 is substantially the same as the processing from steps S101 to S104 described with reference to FIG. 10, and therefore detailed description thereof will be omitted.

[0072] As shown in FIG. 11, in this example, the control device 14 adjusts the current phase and executes control to pass through the continuous operation prohibited region, and then, for example, based on the detection result of the current detector 16 and the stored information on the current pattern of each phase, calculates a moving average value of the current of the phase that resulted in a strict current pattern in the previous control to pass through the continuous operation prohibited region, out of the three-phase AC current (output current) supplied from the converter 12 to the rotor winding (load), and determines whether the calculated moving average value is equal to or greater than a predetermined value (step S205 in FIG. 11).

[0073] In this example, the control device 14 determines whether the moving average value of the current of the phase that resulted in a severe current pattern in the previous control passing through the continuous operation prohibited area is equal to or greater than a predetermined value, thereby confirming whether the repetition of a severe current pattern in the same phase has been appropriately avoided.

[0074] For example, the control device 14 starts calculating the moving average value of the current from the timing when the control to pass through the continuous operation prohibited area by adjusting the current phase starts, and determines whether the moving average value is equal to or greater than a predetermined value after a certain time has elapsed since the start of the calculation of the moving average value.

[0075] If the moving average value after a certain time of the current of the phase that became a severe current pattern in the previous control passing through the continuous operation prohibited area is less than a predetermined value, that is, if the control device 14 has been able to appropriately avoid repetition of the severe current pattern in the same phase, it returns to processing of step S201.

[0076] On the other hand, if the moving average value after a certain time of the current of a phase that became a strict current pattern in the previous control passing through the continuous operation prohibited area is equal to or greater than a predetermined value, that is, if the control device 14 is unable to properly avoid the repetition of the strict current pattern in the same phase and unintentionally repeats the strict current pattern, it performs a protective operation on the converter 12 (step S206 in Figure 11).

[0077] In this way, a severe current pattern may be determined not only from the sign of the current but also from the moving average value of the current. When a severe current pattern is determined from the moving average value of the current, the magnitude of the detected current is taken into account, and the occurrence of a severe current pattern can be more appropriately determined than when the sign of the current is used for determination. On the other hand, when a severe current pattern is determined from the sign of the current, the severe current pattern can be determined with a simpler configuration than when the moving average value of the current is used for determination.

[0078] FIG. 12 is a flowchart schematically illustrating a modified example of the operation of the power conversion device according to the embodiment. In FIG. 12, the processing from steps S301 to S304 is substantially the same as the processing from steps S101 to S104 described with reference to FIG. 10, and therefore detailed description thereof will be omitted.

[0079] As shown in FIG. 12, in this example, the control device 14 adjusts the current phase and executes control to pass through the continuous operation prohibited region, and then, for example, based on the detection result of the current detector 16 and the stored information on the current pattern of each phase, calculates an integral value of the current of the phase that had a strict current pattern in the previous control to pass through the continuous operation prohibited region, out of the three-phase AC current (output current) supplied from the converter 12 to the rotor winding (load), and determines whether the calculated integral value is equal to or greater than a predetermined value (step S305 in FIG. 12).

[0080] In this example, the control device 14 determines whether the integrated value of the current of the phase that resulted in a severe current pattern in the previous control passing through the continuous operation prohibited area is equal to or greater than a predetermined value, thereby confirming whether the repetition of a severe current pattern in the same phase has been appropriately avoided.

[0081] For example, the control device 14 starts calculating the integral value of the current from the timing when the control to pass through the continuous operation prohibited area by adjusting the current phase is started, and determines whether the integral value is equal to or greater than a predetermined value after a certain time has elapsed since the start of the calculation of the integral value.

[0082] If the integral value after a certain time of the current of the phase that became a severe current pattern in the previous control of passing through the continuous operation prohibited area is less than a predetermined value, that is, if the control device 14 has been able to appropriately avoid repetition of the severe current pattern in the same phase, it returns to processing of step S301.

[0083] On the other hand, if the integral value of the current of a phase that became a strict current pattern in the previous control of passing through the continuous operation prohibited area after a certain time is equal to or greater than a predetermined value, that is, if the control device 14 is unable to properly avoid repetition of the strict current pattern in the same phase and unintentionally repeats the strict current pattern, the control device 14 performs a protection operation for the converter 12 (step S306 in Figure 12).

[0084] In this way, a severe current pattern may be determined from the integrated value of the current. When a severe current pattern is determined from the integrated value of the current, the magnitude of the detected current is taken into consideration, and the occurrence of a severe current pattern can be determined more appropriately than when a severe current pattern is determined from the sign of the current, just as in the case of determining a severe current pattern from the moving average value of the current.

[0085] In the above embodiment, an example is shown in which the power conversion device 10 is applied to the adjustable-speed pumped-storage power generation system 2. The power conversion device 10 is not limited to being used in the adjustable-speed pumped-storage power generation system 2, and may be used for other purposes. For example, the power conversion device 10 may be applied to an adjustable-speed wind power generation system in which the water turbine 5 is replaced with a wind turbine, or a flywheel power storage system in which the water turbine 5 is replaced with a flywheel. The power conversion device 10 is applicable to, for example, a secondary excitation device of any wound-rotor induction machine. Furthermore, the load of the power conversion device 10 is not limited to the rotor winding of a wound-rotor induction machine, but may be any load that requires the supply of low-frequency three-phase AC power.

[0086] Furthermore, the power supply that supplies power to the power conversion device 10 is not limited to the excitation transformer 6, but may be any power supply that can appropriately supply power to the power conversion device 10. The power supplied to the power conversion device 10 is not limited to three-phase AC power, but may be DC power or the like. The power supplied to the power conversion device 10 may be any power. The configuration of the converter 12 is not limited to the above, but may be any configuration that has a plurality of switching elements, converts power supplied from a power supply into three-phase AC power according to a load by switching the plurality of switching elements, and can supply the converted three-phase AC power to the load.

[0087] The present embodiment includes the following aspects. (Appendix 1) a converter having a plurality of switching elements, converting power supplied from a power source into three-phase AC power according to a load by switching the plurality of switching elements, and supplying the converted three-phase AC power to the load; a control device for controlling the power conversion operation of the converter; Equipped with the control device arbitrarily changes the frequency of the three-phase AC power supplied from the converter to the load, sets a predetermined frequency range including 0 Hz as a continuous operation prohibited region, and when the frequency of the three-phase AC power falls within the continuous operation prohibited region, controls the operation of the converter so that the frequency passes through the continuous operation prohibited region; a current waveform of the three-phase AC power when control is performed to pass through the continuous operation prohibition region has a relatively strict current pattern in one or two phases and a relatively lenient current pattern in the remaining phases, depending on the current phase when passing through the continuous operation prohibition region; The severe current pattern is a current pattern in which the period in which the polarity does not change is long and the state in which the current value is large is long, compared to normal operation that does not pass through the continuous operation prohibited area, When the control device performs control to pass through the continuous operation prohibited region again within a predetermined time after performing control to pass through the continuous operation prohibited region, the control device adjusts the current phase and controls the operation of the converter so that a phase different from the previous time has the severe current pattern, thereby avoiding repetition of the severe current pattern in the same phase.

[0088] (Appendix 2) a current detector that detects a three-phase AC current supplied from the converter to the load and inputs the detection result to the control device; The power conversion device according to claim 1, wherein the control device adjusts the current phase to perform control to pass through the continuous operation prohibited area, and then checks based on the detection result of the current detector whether the repetition of the severe current pattern in the same phase has been avoided, and if the repetition of the severe current pattern in the same phase has not been avoided, performs a protective operation for the converter.

[0089] (Appendix 3) The power conversion device according to claim 2, wherein the control device checks whether the repetition of the severe current pattern in the same phase has been avoided by determining whether the sign of the current in the phase that resulted in the severe current pattern in the previous control of passing through the continuous operation prohibited region has not changed for a certain period of time.

[0090] (Appendix 4) The power conversion device according to claim 2, wherein the control device checks whether the repetition of the severe current pattern in the same phase has been avoided by determining whether the moving average value of the current of the phase that became the severe current pattern in the previous control of passing through the continuous operation prohibited region is equal to or greater than a predetermined value.

[0091] (Appendix 5) The power conversion device according to claim 2, wherein the control device checks whether the repetition of the severe current pattern in the same phase has been avoided by determining whether the integral value of the current of the phase that resulted in the severe current pattern in the previous control of passing through the continuous operation prohibited region is equal to or greater than a predetermined value.

[0092] (Appendix 6) The power conversion device according to any one of appendices 1 to 5, wherein when the control device performs control to pass through the next continuous operation prohibited region within the specified time, if the current direction of the strict current pattern is the same, the control device adjusts the current phase to avoid repetition of the strict current pattern in the same phase, and if the current direction of the strict current pattern is different, the control device does not adjust the current phase.

[0093] (Appendix 7) A power conversion device as described in any one of appendices 1 to 5, wherein the control device, when performing control to pass through the next continuous operation prohibited region within the specified time, adjusts the current phase regardless of the current direction of the strict current pattern, thereby avoiding repetition of the strict current pattern in the same phase.

[0094] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims. [Explanation of symbols]

[0095] 2...Adjustable speed pumped storage power generation system, 3...Main transformer, 4...Generator motor, 5...Water turbine, 6...Excitation transformer, 10...Power conversion device, 12...Converter, 14...Control device, 16...Current detector, 20...Converter, 22...Inverter, AC...AC output terminal, C1, C2...Charge storage element, D11 to D14...Rectifier element, D21, D22...Rectifier element, N...DC low potential terminal, O...DC neutral terminal, P...DC high potential terminal, PS...Power system, Q1 to Q4...Switching element

Claims

1. a converter having a plurality of switching elements, converting power supplied from a power source into three-phase AC power according to a load by switching the plurality of switching elements, and supplying the converted three-phase AC power to the load; a control device for controlling the power conversion operation of the converter; Equipped with the control device arbitrarily changes the frequency of the three-phase AC power supplied from the converter to the load, sets a predetermined frequency range including 0 Hz as a continuous operation prohibited region, and when the frequency of the three-phase AC power falls within the continuous operation prohibited region, controls the operation of the converter so that the frequency passes through the continuous operation prohibited region; a current waveform of the three-phase AC power when control is performed to pass through the continuous operation prohibition region has a relatively strict current pattern in one or two phases and a relatively lenient current pattern in the remaining phases, depending on the current phase when passing through the continuous operation prohibition region; The severe current pattern is a current pattern in which the period in which the polarity does not change is long and the state in which the current value is large is long, compared to normal operation that does not pass through the continuous operation prohibited area, When the control device performs control to pass through the continuous operation prohibited region again within a predetermined time after performing control to pass through the continuous operation prohibited region, the control device adjusts the current phase and controls the operation of the converter so that a phase different from the previous time has the severe current pattern, thereby avoiding repetition of the severe current pattern in the same phase.

2. a current detector that detects a three-phase AC current supplied from the converter to the load and inputs the detection result to the control device; The power conversion device according to claim 1, wherein the control device adjusts the current phase to perform control to pass through the continuous operation prohibited area, and then checks based on the detection result of the current detector whether or not the repetition of the severe current pattern in the same phase has been avoided, and if the repetition of the severe current pattern in the same phase has not been avoided, performs a protective operation of the converter.

3. The power conversion device according to claim 2, wherein the control device checks whether the repetition of the severe current pattern in the same phase has been avoided by determining whether the sign of the current in the phase that resulted in the severe current pattern in the previous control of passing through the continuous operation prohibited region has not changed for a certain period of time.

4. 3. The power conversion device according to claim 2, wherein the control device checks whether or not the repetition of the severe current pattern in the same phase has been avoided by determining whether or not a moving average value of the current of the phase that became the severe current pattern in the previous control of passing through the continuous operation prohibited region is equal to or greater than a predetermined value.

5. The power conversion device according to claim 2, wherein the control device checks whether or not the repetition of the severe current pattern in the same phase has been avoided by determining whether or not an integral value of the current of the phase that resulted in the severe current pattern in the previous control of passing through the continuous operation prohibited region is equal to or greater than a predetermined value.

6. The power conversion device according to claim 1, wherein when the control device performs control to pass through the next continuous operation prohibited region within the specified time, if the current direction of the strict current pattern is the same, the control device adjusts the current phase to avoid repetition of the strict current pattern in the same phase, and does not adjust the current phase if the current direction of the strict current pattern is different.

7. The power conversion device according to claim 1, wherein when the control device performs control to pass through the next continuous operation prohibited region within the specified time, the control device adjusts the current phase regardless of the current direction of the strict current pattern, thereby avoiding repetition of the strict current pattern in the same phase.

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