Power conversion equipment and uninterruptible power supply

Abstract

To provide an electric power conversion apparatus capable of improving an electric power conversion efficiency, the electric power conversion apparatus being capable of employing a two-phase modulation method without increasing in size.SOLUTION: An electric power conversion apparatus includes a converter circuit C, a smoothing circuit S, an inverter circuit I, a phase detection unit 11, a converter control unit 12, and an inverter control unit 13. The converter circuit C and the inverter circuit I are configured of three-phase full-bridge circuits. Input filter capacitors 2a to 2c constituting a filter F1 and output filter capacitors 8a to 8c constituting a filter F2 are connected to each other, and a smoothing capacitor 5 of the smoothing circuit S is not connected to the input filter capacitors or the output filter capacitors. When a phase difference θ between a converter AC voltage and an inverter AC voltage detected by the phase detection unit 11 becomes a predetermined value or more, the converter control unit 12 and the inverter control unit 13 switch a modulation method of the converter circuit C and the inverter circuit I from a two-phase modulation method to a three-phase modulation method.SELECTED DRAWING: Figure 1(a)

Description

[Technical Field]

[0001] The present invention relates to a power conversion device and an uninterruptible power supply. [Background technology]

[0002] A power conversion device that functions as a non-isolated uninterruptible power supply is known from Patent Document 1 and the like. In the power conversion device described in Patent Document 1, AC power from a three-phase AC power supply is converted to DC power by a converter circuit, and this DC power is further converted to AC by an inverter circuit, and the AC power obtained by converting this DC power to AC power by an inverter circuit is switched between AC power that is directly supplied from the three-phase AC power supply, thereby causing the power conversion device to function as an uninterruptible power supply.

[0003] Furthermore, a two-phase modulation method is known as a modulation method capable of improving power conversion efficiency, as disclosed in Patent Document 2. In the two-phase modulation method, pulse width modulation signals are supplied to only two of the three phases. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-253262 [Patent Document 2] Japanese Patent Application Publication No. 59-139871 Summary of the Invention [Problem to be solved by the invention]

[0005] In the power conversion device described in Patent Document 1, an AC filter is created using a reactor and a capacitor to smooth the PWM waveform output by the converter and generate a sine wave, and the midpoint of this AC filter is connected to the midpoint of the DC section. Therefore, for example, when two-phase modulation is performed in the power conversion device described in Patent Document 1, the two-phase modulation waveform is applied directly to the AC filter, causing a large current to flow through the reactor and capacitor of the AC filter on both the converter side and the inverter side. This results in larger components and higher costs. Therefore, when using two-phase modulation in a power conversion device, there is room for improvement in these respects.

[0006] Therefore, the present invention provides a power conversion device and an uninterruptible power supply device that can employ a two-phase modulation method without increasing the size of the device and that can improve the efficiency of power conversion. [Means for solving the problem]

[0007] A power conversion device according to one aspect of the present invention comprises: a converter circuit connected to a three-phase AC power source and converting AC from the three-phase AC power source into DC; a smoothing circuit including a smoothing capacitor connected to a DC output terminal of the converter circuit; an inverter circuit connected to a DC terminal of the smoothing circuit, converting the DC from the smoothing circuit into AC and outputting the AC to an AC load; a phase detection unit that detects a phase difference between an AC voltage input to the converter circuit and an AC voltage output from the inverter circuit; a control unit that switches the modulation method of the converter circuit and the inverter circuit between two-phase modulation and three-phase modulation based on the detection result of the phase detection unit; Equipped with the converter circuit and the inverter circuit are configured as three-phase full-bridge circuits made up of switching elements, and an input filter capacitor constituting a filter on the converter circuit side and an output filter capacitor constituting a filter on the inverter circuit side are connected to each other; a smoothing capacitor of the smoothing circuit is not connected to the input filter capacitor and the output filter capacitor; The control unit generates the inverter reference sine wave, and when the phase difference detected by the phase detection unit becomes equal to or greater than a predetermined value, switches the modulation method of the converter circuit and the inverter circuit from the two-phase modulation method to the three-phase modulation method.

[0008] Furthermore, an uninterruptible power supply according to one aspect of the present invention comprises: a converter circuit connected to a three-phase AC power source and converting AC from the three-phase AC power source into DC; a smoothing circuit including a smoothing capacitor connected to a DC output terminal of the converter circuit; an inverter circuit connected to a DC terminal of the smoothing circuit, converting the DC from the smoothing circuit into AC and outputting the AC to an AC load; a bypass circuit connected in parallel to the converter circuit and the inverter circuit; a control unit that controls the converter circuit and the inverter circuit by PWM using a two-phase modulation method; Equipped with the converter circuit and the inverter circuit are configured as three-phase full-bridge circuits made up of switching elements, and an input filter capacitor constituting a filter on the converter circuit side and an output filter capacitor constituting a filter on the inverter circuit side are connected to each other; a smoothing capacitor of the smoothing circuit is not connected to the input filter capacitor and the output filter capacitor; The control unit switches between power supply from the inverter circuit to the AC load and power supply from the bypass circuit to the AC load according to a predetermined condition. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a power conversion device and an uninterruptible power supply device that can employ two-phase modulation without increasing the size of the device and that can improve the efficiency of power conversion. [Brief explanation of the drawings]

[0010] [Figure 1(a)] 1 is a diagram showing a circuit configuration of a power conversion device according to a first embodiment of the present invention. [Figure 1(b)] FIG. 4 is a diagram showing a circuit configuration of a power conversion device according to a second embodiment of the present invention. [Figure 2] FIG. 1 is a diagram showing a circuit configuration of a conventional power conversion device. [Figure 3(a)] These are a control waveform and a carrier triangular wave for generating a drive waveform for a semiconductor switching element. [Figure 3(b)] This is the drive waveform of the u-phase semiconductor switching element. [Figure 4(a)] This is the output voltage of the inverter circuit in Figure 2, and the dashed line is the voltage waveform with the switching frequency included in it removed. [Figure 4(b)] This is the waveform of the current flowing through the output filter reactor of the u-phase of the inverter circuit. [Figure 4(c)] This is the current waveform flowing through the output filter capacitor of the u-phase of the inverter circuit. [Figure 5(a)] This is the output voltage of the u-phase of the inverter circuit in Figure 1(a), and the dashed line is the voltage waveform with the switching frequency included in it removed. [Figure 5(b)] In Figure 1(a), this is the r-phase AC voltage generated in the converter circuit when it is in phase with the inverter circuit, and the dashed line is the voltage waveform with the switching frequency included in it removed. [Figure 6] This is the voltage applied between the output filter reactor and output filter capacitor of the inverter circuit and the input filter capacitor and input filter reactor of the converter circuit in Figure 1(a). The dashed line is the voltage waveform with the switching frequency included in it removed. [Figure 7(a)] This is the waveform of the current flowing through the u-phase output filter reactor of the inverter circuit in FIG. 1(a). [Figure 7(b)] This is the current waveform flowing through the output filter capacitor of the u-phase of the inverter circuit. [Figure 8(a)] This is the output voltage of the u-phase of the inverter circuit in FIG. 1(a). [Figure 8(b)] In Figure 1(a), this is the r-phase AC voltage generated in the converter circuit when the phase with the inverter circuit is shifted. [Figure 9] This is the voltage applied between the u-phase output filter reactor and output filter capacitor of the inverter circuit and the r-phase input filter capacitor and input filter reactor of the converter circuit in FIG. [Figure 10(a)] 9 shows the waveform of the current flowing through the u-phase output filter reactor of the inverter circuit in FIG. 8. [Figure 10(b)] This is the current waveform flowing through the output filter capacitor of the u-phase of the inverter circuit. [Figure 11(a)] 10 is a diagram showing the ripple current flowing through the u-phase filter capacitor with respect to the phase difference between the output voltage of the inverter circuit and the AC voltage input to the converter circuit during two-phase modulation. FIG. [Figure 11(b)] FIG. 11(b) is a partially enlarged view of FIG. [Figure 12(a)] This is the current waveform when the phase difference between the output voltage of the inverter circuit and the AC voltage input to the converter circuit during three-phase modulation is 180 degrees, and is the current waveform flowing through the output filter reactor of the inverter circuit. [Figure 12(b)] This is the current waveform when the phase difference between the output voltage of the inverter circuit and the AC voltage input to the converter circuit during three-phase modulation is 180 degrees, and is also the current waveform flowing through the output filter capacitor. [Figure 13] 10 is a flowchart illustrating an operation of switching a modulation method. [Figure 14] 1 is a diagram illustrating an example of an uninterruptible power supply according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings. For the sake of convenience, the description of components having the same reference numerals as those already described in the description of the embodiments will be omitted. Furthermore, for the sake of convenience, the dimensions of each component shown in the drawings may differ from the actual dimensions of each component.

[0012] Fig. 1(a) is a diagram showing the circuit configuration of a power conversion device 1 according to a first embodiment of the present invention. As shown in Fig. 1(a), the power conversion device 1 includes an input filter F1, a converter circuit C, a smoothing circuit S, an inverter circuit I, an output filter F2, a phase detection unit 11, a converter control unit 12, and an inverter control unit 13.

[0013] The input filter F1 has input filter capacitors 2a to 2c and input filter reactors 3a to 3c. One terminal of each of the input filter reactors 3a to 3c is connected to device input terminals 31a to 31c of the power conversion device 1. The other terminal of each of the input filter reactors 3a to 3c is connected to three-phase (r-phase, s-phase, t-phase) converter input terminals 41a to 41c of the converter circuit C. A three-phase AC power supply 91 is connected to the device input terminals 31a to 31c of the power conversion device 1.

[0014] One terminal of each of the input filter capacitors 2a to 2c is connected to the terminal of each of the input filter reactors 3a to 3c on the device input terminals 31a to 31c side. The other terminal of each of the input filter capacitors 2a to 2c is connected to the neutral conductor n1. The input filter F1 passes three-phase AC power from the three-phase AC power supply 91 to the converter circuit C, while preventing a signal of the carrier frequency (a frequency that determines the pulse width modulation period in the PWM control system) generated by the converter circuit C from passing to the three-phase AC power supply 91.

[0015] The converter circuit C has six semiconductor switching elements 4a to 4f connected in a three-phase bridge. The semiconductor switching elements 4a to 4f are configured, for example, with IGBTs (insulated gate bipolar transistors) and anti-parallel connected freewheeling diodes. The converter circuit C is configured as a three-phase (r-phase, s-phase, t-phase) full bridge circuit. The converter circuit C converts AC voltage from a three-phase AC power supply 91 into DC voltage.

[0016] The smoothing circuit S has a smoothing capacitor 5. In the example shown in the figure, the smoothing capacitor 5 is configured as a single capacitor. The smoothing capacitor 5 is a capacitor that smoothes the output of the converter circuit C, and is, for example, an electrolytic capacitor. The smoothing capacitor 5 is connected to the converter output terminals 42a and 42b of the converter circuit C.

[0017] The inverter circuit I has six semiconductor switching elements 6a to 6f connected in a three-phase bridge. The semiconductor switching elements 6a to 6f are configured, for example, with IGBTs and anti-parallel connected free wheel diodes. The inverter circuit I is configured as a three-phase (u-phase, v-phase, w-phase) full bridge circuit. In the inverter circuit I, inverter input terminals 61a and 61b are connected to both polarity terminals of the smoothing capacitor 5, respectively. The inverter circuit I converts the DC voltage from the smoothing circuit S (the DC voltage output by the converter circuit C) into an AC voltage through the switching operation of the semiconductor switching elements 6a to 6f.

[0018] The output filter F2 has output filter reactors 7a-7c and output filter capacitors 8a-8c. One terminal of each of the output filter reactors 7a-7c is connected to inverter output terminals 62a-62c of a three-phase (u-phase, v-phase, w-phase) inverter circuit I. The other terminal of each of the output filter reactors 7a-7c is connected to device output terminals 71a-71c of the power conversion device 1. An AC load 92 to which the three-phase AC power output from the inverter circuit I is supplied is connected to the device output terminals 71a-71c of the power conversion device 1.

[0019] One terminal of each of output filter capacitors 8a to 8c is connected to the terminal of output filter reactors 7a to 7c on the device output terminals 71a to 71c side, respectively. The other terminal of each of output filter capacitors 8a to 8c is connected to neutral conductor n1. Output filter F2 passes AC power output from inverter circuit I to AC load 92, while preventing a carrier frequency signal generated by inverter circuit I from passing to AC load 92.

[0020] The phase detector 11 is connected to the device input terminals 31a to 31c of the power conversion device 1 and the device output terminals 71a to 71c of the power conversion device 1. The phase detector 11 detects a phase shift of the converter side relative to the inverter side. The phase detector 11 detects the phase difference between the input AC voltage input to the three-phase (r-phase, s-phase, t-phase) device input terminals 31a to 31c and the output AC voltage output to the three-phase (u-phase, v-phase, w-phase) device output terminals 71a to 71c. The phase detector 11 outputs the detected phase differences between the input AC voltage and the output AC voltage to the converter control unit 12 and the inverter control unit 13. In a second embodiment of the present invention, as shown in FIG. 1(b), the phase detector 11 may detect a phase difference between the input AC voltage input to the device input terminals 31a to 31c and an inverter reference sine wave used in the inverter control unit 13 when detecting a phase shift of the converter side relative to the inverter side. The inverter reference sine wave is an ideal sine wave generated by a program in the inverter control unit 13.

[0021] The converter control unit 12 controls the switching of the semiconductor switching elements 4a to 4f of the converter circuit C to a desired state. The converter control unit 12 receives the phase difference between the input AC voltage and the output AC voltage from the phase detection unit 11. The converter control unit 12 switches the modulation method of the converter circuit C between two-phase modulation and three-phase modulation according to the phase difference between the input AC voltage and the output AC voltage.

[0022] The inverter control unit 13 controls the switching of the semiconductor switching elements 6a to 6f of the inverter circuit I to a desired state. The inverter control unit 13 receives the phase difference between the input AC voltage and the output AC voltage from the phase detection unit 11. The inverter control unit 13 switches the modulation method of the inverter circuit I between two-phase modulation and three-phase modulation according to the phase difference between the input AC voltage and the output AC voltage.

[0023] FIG. 2 is a diagram illustrating an example of a circuit configuration of a conventional power conversion device 200. As shown in FIG. 2, the power conversion device 200 differs from the power conversion device 1 of the present invention shown in FIGS. 1(a) and 1(b) in that the smoothing circuit S is configured with two smoothing capacitors 5a and 5b connected in series, which is a single capacitor. The neutral point of the DC voltage, between the smoothing capacitors 5a and 5b, is connected to the neutral line n1 to which the input filter capacitors 2a to 2c and the output filter capacitors 8a to 8c are connected. The power conversion device 200 also differs from the power conversion device 1 of the present invention in that it does not include a phase detector 11 and does not switch the switching method of the converter circuit C by the converter control unit 12 and the inverter circuit I by the inverter control unit 13 between two-phase modulation and three-phase modulation. The power conversion device 200 is similar in configuration to the power conversion device 1 of the present invention, including the input filter F1, converter circuit C, smoothing circuit S, inverter circuit I, output filter F2, converter control unit 12, and inverter control unit 13.

[0024] Next, we will explain the currents that flow through the output filter reactors 7a to 7c and the output filter capacitors 8a to 8c when the inverter circuit I is PWM (Pulse Width Modulation) controlled using a two-phase modulation method in the power conversion device 1 of the present invention shown in Figure 1(a) and the conventional power conversion device 200 shown in Figure 2.

[0025] First, PWM control using two-phase modulation will be described with reference to Fig. 3. Fig. 3(a) is a waveform diagram showing control waveforms 101u, 101v, and 101w for three phases (u-phase, v-phase, and w-phase) in two-phase modulation, and a carrier triangular wave 102. The control waveforms 101u, 101v, and 101w are waveforms generated based on an inverter reference sine wave. Fig. 3(b) shows a drive waveform 103 for the u-phase semiconductor switching element 6a, generated by the control waveform 101u and the carrier triangular wave 102.

[0026] As shown in Figures 3(a) and 3(b), the inverter control unit 13 generates a drive waveform 103 by comparing the control waveform 101u with the carrier triangular wave 102. The inverter control unit 13 compares the control waveform 101u with the carrier triangular wave 102, and generates a drive waveform 103 in which the upper semiconductor switching element 6a is turned on and the lower semiconductor switching element 6b is turned off when the control waveform 101u is larger. Furthermore, the inverter control unit 13 generates a drive waveform 103 in which the lower semiconductor switching element 6b is turned on and the upper semiconductor switching element 6a is turned off when the control waveform 101u is smaller. Note that Figure 3(b) only shows the drive waveform 103 for the upper semiconductor switching element 6a.

[0027] In two-phase modulation, the phase of one of the three-phase control waveforms 101u, 101v, and 101w is fixed at a maximum or minimum value during a period in which the phase of the control waveform changes by 60 degrees, and the remaining two phases are modulated. In this way, in two-phase modulation, the control waveform of one of the three phases is always fixed at a maximum or minimum value, stopping the switching of the semiconductor switching elements. This reduces the number of switching operations compared to three-phase modulation, thereby suppressing switching losses. While Figure 3 illustrates the u-phase, control waveforms for semiconductor switching elements 6c, 6d, 6e, and 6f are generated in a similar manner for the v-phase and w-phase.

[0028] 4A and 4B are diagrams illustrating the currents flowing through the output filter reactor 7a and the output filter capacitor 8a when the converter circuit C and the inverter circuit I in the conventional power conversion device 200 shown in FIG. 2 are PWM-controlled using two-phase modulation. FIG. 4A shows the output voltage Eu2 from the u-phase inverter output terminal 62a in the inverter circuit I, and the dashed line shows the voltage waveform with the switching frequency included therein removed. FIG. 4B shows a current waveform 221 flowing through the u-phase output filter reactor 7a in the inverter circuit I. FIG. 4C shows a current waveform 222 flowing through the u-phase output filter capacitor 8a in the inverter circuit I.

[0029] As shown in FIG. 4(a), the output voltage Eu2 is output as a waveform that varies between the power supply voltage ±Ed through PWM control. In the circuit configuration of the power conversion device 200, a closed loop is formed by the smoothing capacitor 5, the semiconductor switching elements 6a and 6b, the output filter reactor 7a, the output filter capacitor 8a, and the smoothing capacitor 5. Therefore, a low-frequency voltage 211, indicated by a dashed line, is applied between the output filter reactor 7a and the output filter capacitor 8a in this loop. The voltage 211 exhibits large voltage fluctuations at its transition points, as shown in the circular regions 212a-212e. This causes the output filter reactor 7a and the output filter capacitor 8a to oscillate, resulting in current waveforms 221 and 222 of approximately 1.2 Ap-p, as shown in FIGS. 4(b) and 4(c), flowing through the output filter reactor 7a and the output filter capacitor 8a. This requires larger components for the output filter reactor 7a and the output filter capacitor 8a.

[0030] In the example shown in the figure, the current flowing through u-phase output filter reactor 7a and output filter capacitor 8a in inverter circuit I is shown, but the same currents flow through v-phase and w-phase output filter reactors 7b and 7c and output filter capacitors 8b and 8c. The same currents flow through r-phase, s-phase, and t-phase input filter capacitors 2a-2c and input filter reactors 3a-3c in converter circuit C (the same applies in the following explanation).

[0031] FIG. 5 is a diagram illustrating the currents flowing through the output filter reactor 7a and the output filter capacitor 8a when the converter circuit C and the inverter circuit I in the power conversion device 1 of the present invention shown in FIG. 1(a) are PWM-controlled using two-phase modulation. FIG. 5(a) shows the output voltage Eu at the u-phase inverter output terminal 62a of the inverter circuit I, with the dashed line indicating the voltage waveform with the switching frequency removed. FIG. 5(b) shows the voltage output by the converter circuit C to the u-phase input side when the converter circuit C is in phase with the inverter circuit in FIG. 1(a), i.e., the input voltage Er generated at the u-phase converter input terminal 41a of the converter circuit C, with the dashed line indicating the voltage waveform with the switching frequency removed. Because the converter circuit is in phase with the inverter circuit, the waveform of the input voltage Er is substantially the same as the waveform of the output voltage Eu.

[0032] 5(a), the output voltage Eu is output as a waveform that varies between a power supply voltage ±Ed by PWM control, similar to the output voltage Eu2 in the conventional power conversion device 200. However, in the circuit configuration of the power conversion device 1, a closed loop is formed by the smoothing capacitor 5-semiconductor switching elements 6a and 6b-output filter reactor 7a-output filter capacitor 8a-input filter capacitor 2a-input filter reactor 3a-semiconductor switching elements 4a and 4b-smoothing capacitor 5. As a result, a voltage obtained by subtracting the input voltage Er from the output voltage Eu is applied between the output filter reactor 7a, the output filter capacitor 8a, the input filter capacitor 2a, and the input filter reactor 3a in the loop of the smoothing capacitor 5-semiconductor switching elements 6a and 6b-output filter reactor 7a-output filter capacitor 8a-input filter capacitor 2a-input filter reactor 3a-semiconductor switching elements 4a and 4b-smoothing capacitor 5.

[0033] 6 shows the applied voltage Eu-Er obtained by subtracting the input voltage Er shown in Fig. 5(b) from the output voltage Eu shown in Fig. 5(a), i.e., the voltage applied to output filter reactor 7a, output filter capacitor 8a, input filter capacitor 2a, and input filter reactor 3a, with dashed voltage 111 indicating a voltage waveform with the switching frequency included therein removed. Because input voltage Er and output voltage Eu have substantially the same voltage waveform, voltage 111 exhibits significantly smaller voltage fluctuations at transition points, as shown in circled regions 112a-112e, for example, than the voltage fluctuations at transition points shown in circled regions 212a-212e in Fig. 4(a).

[0034] Figure 7(a) shows a waveform 121 of a current flowing through output filter reactor 7a when the applied voltage Eu-Er shown in Figure 6 is applied to output filter reactor 7a, output filter capacitor 8a, input filter capacitor 2a, and input filter reactor 3a. Figure 7(b) shows a waveform 122 of a current flowing through output filter capacitor 8a when the same applied voltage Eu-Er is applied.

[0035] As described above, the voltage fluctuation at the change point of voltage 111 included in the voltage applied to output filter reactor 7a, output filter capacitor 8a, input filter capacitor 2a, and input filter reactor 3a is small, so current oscillation is unlikely to occur in output filter reactor 7a and output filter capacitor 8a. As a result, the current flowing through output filter reactor 7a and output filter capacitor 8a is suppressed to current waveforms 121 and 122 of about 100 mAp-p, as shown in Figures 7(a) and 7(b).

[0036] In a power conversion device, for example, a converter circuit on the input side may become out of synchronization with an inverter circuit on the output side. When the converter circuit and the inverter circuit are out of synchronization, a phase difference occurs between the input voltage of the converter circuit and the output voltage of the inverter circuit.

[0037] Therefore, the inventors have studied the currents that flow through the output filter reactor 7a and the output filter capacitor 8a when the converter circuit C side and the inverter circuit I side are not synchronized in the power conversion device 1 of the present invention shown in Figure 1(a), i.e., when a phase difference occurs between the input voltage Er of the converter circuit C and the output voltage Eu of the inverter circuit I. Note that the current waveforms 121 and 122 that flow through the output filter reactor 7a and the output filter capacitor 8a in the power conversion device 1 described above in Figures 5 to 7 are currents that flow when the converter circuit C and the inverter circuit I are synchronized, i.e., when the phase difference between the input voltage Er of the converter circuit C and the output voltage Eu of the inverter circuit I is 0 degrees.

[0038] 8 to 10, the currents flowing through the output filter reactor 7a and the output filter capacitor 8a when the phase difference between the input voltage Er of the converter circuit C and the output voltage Eu of the inverter circuit I in the power conversion device 1 is 180 degrees will be described below.

[0039] Figure 8(a) shows the u-phase output voltage Eu in inverter circuit I, with the dashed line representing the voltage waveform with the switching frequency included therein removed. Figure 8(b) shows the u-phase input voltage Er in converter circuit C, with the dashed line representing the voltage waveform with the switching frequency included therein removed. As shown in Figure 8(a), the output voltage Eu is output as a waveform that varies between the power supply voltage ±Ed through PWM control, similar to the output voltage Eu when the phase difference is 0 degrees in Figure 5(a). On the other hand, as shown in Figure 8(b), the input voltage Er has a voltage waveform whose plus and minus are approximately inverted relative to the output voltage Eu.

[0040] The waveform shift between the input voltage Er and the output voltage Eu varies depending on the phase difference between the input voltage Er and the output voltage Eu, and is maximum when the phase difference is 180 degrees. For this reason, the voltage applied to the output filter reactor 7a, the output filter capacitor 8a, the input filter capacitor 2a, and the input filter reactor 3a in the loop of the power conversion device 1, smoothing capacitor 5, semiconductor switching elements 6a, 6b, output filter reactor 7a, output filter capacitor 8a, input filter capacitor 2a, input filter reactor 3a, semiconductor switching elements 4a, 4b, and smoothing capacitor 5, i.e., the voltage obtained by subtracting the input voltage Er from the output voltage Eu, becomes larger as the phase difference between the two voltages becomes larger.

[0041] 9 shows the applied voltage Eu-Er obtained by subtracting the input voltage Er shown in Fig. 8(b) from the output voltage Eu shown in Fig. 8(a), i.e., the voltage applied to output filter reactor 7a, output filter capacitor 8a, input filter capacitor 2a, and input filter reactor 3a, with dashed voltage 131 indicating a voltage waveform with the switching frequency included therein removed. Because the output voltage Eu and the input voltage Er have voltage waveforms that are approximately inverted, as shown in Fig. 9, the voltage fluctuations at the change points of voltage 131 are larger than the voltage fluctuations at the change points of voltage 111 when the phase difference in Fig. 6 is 0 degrees, as indicated by circled regions 132a-132e.

[0042] Figure 10(a) shows a waveform 141 of a current flowing through output filter reactor 7a when the applied voltage Eu-Er shown in Figure 9 is applied to output filter reactor 7a, output filter capacitor 8a, input filter capacitor 2a, and input filter reactor 3a. Figure 10(b) shows a waveform 142 of a current flowing through output filter capacitor 8a when the same applied voltage Eu-Er is applied.

[0043] As described above, when the phase difference between input voltage Er and output voltage Eu is 180 degrees, the voltage fluctuations at the change points of applied voltage Eu-Er applied to output filter reactor 7a, output filter capacitor 8a, input filter capacitor 2a, and input filter reactor 3a become large, making current oscillations more likely to occur in output filter reactor 7a and output filter capacitor 8a. As a result, the currents flowing through output filter reactor 7a and output filter capacitor 8a may have current waveforms 141 and 142 of approximately 400 mAp-p, as shown in Figures 10(a) and 10(b).

[0044] In response to this, the inventors have studied the relationship between the phase difference between the input voltage Er of the converter circuit C and the output voltage Eu of the inverter circuit I, and the current flowing through the output filter capacitor 8a (or the output filter reactor 7a).

[0045] Fig. 11(a) is a diagram showing the relationship between the phase difference between the input voltage Er and the output voltage Eu and the ripple current flowing through the output filter capacitor 8a, and Fig. 11(b) is a partially enlarged diagram showing the ripple current around a phase difference of 0 degrees.

[0046] The inventors considered the shape and cost of the components used as the output filter capacitor 8a and studied the phase difference with the goal of keeping the ripple current at or below twice the ripple current when the phase difference is 0 degrees. As a result, they determined that the phase difference necessary to keep the ripple current at or below twice the ripple current was ±0.5 degrees. This revealed that when the phase difference between the input voltage Er and the output voltage Eu is less than ±0.5 degrees, even if the converter circuit C and the inverter circuit I are PWM-controlled using two-phase modulation, the capacitor components do not become larger and the cost does not increase.

[0047] Next, the inventors investigated the current flowing through the output filter reactor 7a and the output filter capacitor 8a when the converter circuit C and the inverter circuit I are PWM controlled using a three-phase modulation method in a case where the converter circuit C side and the inverter circuit I side in the power conversion device 1 are not synchronized.

[0048] FIG. 12(a) shows the current flowing through the output filter reactor 7a when the phase difference between the input voltage Er of the converter circuit C and the output voltage Eu of the inverter circuit I is 180 degrees under three-phase modulation PWM control. FIG. 12(b) shows the current flowing through the output filter capacitor 8a when the phase difference is 180 degrees. As shown in FIGS. 12(a) and 12(b), when PWM control is performed using three-phase modulation, the current flowing through the output filter reactor 7a and the output filter capacitor 8a is suppressed to current waveforms 151 and 152 of approximately 50 mA p-p. This is because the input voltage Er and the output voltage Eu do not have any transition points due to two-phase modulation, resulting in ideal sine waves (not shown). Note that while the example shown in FIG. 12 describes the case where the phase difference between the input voltage Er and the output voltage Eu is 180 degrees, the current flowing through the output filter reactor 7a and the output filter capacitor 8a is similarly suppressed even when the phase difference is less than 180 degrees.

[0049] Next, the modulation method switching operation in the power conversion device 1 will be described with reference to FIG. The phase detection unit 11 receives the input AC voltage input to the converter circuit C and the output AC voltage output from the inverter circuit I. The phase detection unit 11 calculates a phase difference θ between the received input AC voltage and the output AC voltage (step S10). The phase detection unit 11 transmits the calculated phase difference θ to the converter control unit 12 and the inverter control unit 13.

[0050] The converter control unit 12 and the inverter control unit 13 determine whether the received phase difference θ is less than a predetermined value (for example, -0.5 degrees < θ < 0.5 degrees), i.e., whether the converter circuit C and the inverter circuit I are in a synchronous or asynchronous state (step S11).

[0051] In step S11, if it is determined that the phase difference θ is less than a predetermined value (synchronized state) (YES in step S11), the converter control unit 12 and the inverter control unit 13 PWM control the semiconductor switching elements 4a to 4f of the converter circuit C and the semiconductor switching elements 6a to 6f of the inverter circuit I using the two-phase modulation method (step S12).

[0052] On the other hand, if it is determined in step S11 that the phase difference θ is not less than the predetermined value (asynchronous state) (NO in step S11), the converter control unit 12 and the inverter control unit 13 PWM control the semiconductor switching elements 4a to 4f of the converter circuit C and the semiconductor switching elements 6a to 6f of the inverter circuit I using the three-phase modulation method (step S13).

[0053] The series of processes from step S10 to step S13 is continuously repeated. Then, when the phase difference θ changes from less than a predetermined value to greater than or equal to the predetermined value, the converter control unit 12 and the inverter control unit 13 switch the PWM control of the converter circuit C and the inverter circuit I from two-phase modulation to three-phase modulation. Also, when the phase difference θ changes from greater than or equal to the predetermined value to less than the predetermined value, the converter control unit 12 and the inverter control unit 13 switch the PWM control of the converter circuit C and the inverter circuit I from three-phase modulation to two-phase modulation.

[0054] As shown in FIG. 14 , the power conversion device 1 may be provided with a bypass circuit 301 configured to connect the r-phase device input terminal 31a to the u-phase device output terminal 71a, the s-phase device input terminal 31b to the v-phase device output terminal 71b, and the t-phase device input terminal 31c to the w-phase device output terminal 71c, respectively. A storage battery 9 may also be provided, which is connected in parallel with the smoothing capacitor 5 to the converter output terminals 42a and 42b of the converter circuit C. In the configuration provided with the bypass circuit 301, a power supply route can be formed that supplies AC power directly from the three-phase AC power source 91 to the AC load 92 via the bypass circuit 301. This allows power to be continuously supplied to the AC load 92 even when maintenance of the power conversion device 1 is performed. In the configuration provided with the storage battery 9, a power supply route can be formed that supplies power stored in the storage battery 9 to the AC load 92 via the inverter circuit I. As a result, even when the three-phase AC power supply 91 experiences a power outage, the power stored in the storage battery 9 can be continuously supplied to the AC load 92 via the inverter circuit I, and the power conversion device 1 can be used as an uninterruptible power supply 300.

[0055] As described above, the power conversion device 1 according to this embodiment includes a converter circuit C connected to a three-phase AC power supply 91 and converting AC from the three-phase AC power supply 91 into DC, a smoothing circuit S including a smoothing capacitor 5 connected to the DC output terminal of the converter circuit C, an inverter circuit I connected to the DC terminal of the smoothing circuit S and converting the DC from the smoothing circuit S into AC and outputting the AC to an AC load 92, a phase detection unit 11 detecting a phase difference between the AC voltage input to the converter circuit C and the AC voltage output from the inverter circuit I, and a converter control unit 12 and an inverter control unit 13 switching the modulation method of the converter circuit C and the inverter circuit I between two-phase modulation and three-phase modulation based on the detection result of the phase detection unit 11. The converter circuit C and the inverter circuit I are each configured as a three-phase full-bridge circuit including semiconductor switching elements 4a to 4f and semiconductor switching elements 6a to 6f. Input filter capacitors 2a-2c that constitute input filter F1 on the converter circuit side and output filter capacitors 8a-8c that constitute output filter F2 on the inverter circuit side are connected, while smoothing capacitor 5 of smoothing circuit S is not connected to input filter capacitors 2a-2c and output filter capacitors 8a-8c. Converter control unit 12 and inverter control unit 13 switch the modulation method of converter circuit C and inverter circuit I from two-phase modulation to three-phase modulation when phase difference θ detected by phase detection unit 11 becomes equal to or greater than a predetermined value. With this configuration, by not connecting the midpoint of input filter F1 on the converter circuit side and the midpoint of output filter F2 on the inverter circuit side to smoothing capacitor 5 of smoothing circuit S, and by switching control of converter circuit C and inverter circuit I from two-phase modulation to three-phase modulation when phase difference θ becomes equal to or greater than the predetermined value, it is possible to suppress the current flowing through input filter F1 of converter circuit C and output filter F2 of inverter circuit I.As a result, the components that make up the input filter F1 and the output filter F2 (input filter capacitors 2a to 2c, output filter capacitors 8a to 8c, input filter reactors 3a to 3c, output filter reactors 7a to 7c, etc.) can be made smaller, so that the two-phase modulation method can be adopted without increasing the size of the entire power conversion device 1, and by adopting the two-phase modulation method, the efficiency of power conversion can be improved.

[0056] Furthermore, according to the power conversion device 1, the converter control unit 12 and the inverter control unit 13 switch the modulation mode of the inverter circuit I from three-phase modulation to two-phase modulation when the phase difference θ detected by the phase detection unit 11 becomes less than a predetermined value. Therefore, when the phase difference θ becomes less than the predetermined value, the control of the inverter circuit I can be returned from three-phase modulation to two-phase modulation, thereby appropriately improving the efficiency of power conversion.

[0057] Furthermore, according to the power conversion device 1, the phase detection unit 11 uses the inverter reference sine wave used by the inverter control unit 13 when detecting the phase difference θ. In this way, it is possible to appropriately switch the control modulation method by detecting the phase difference between the AC voltage input to the converter circuit C and the ideal inverter reference sine wave used by the inverter control unit 13.

[0058] Moreover, the power conversion device 1 further includes a bypass circuit 301 in parallel with the converter circuit C and the inverter circuit I, and the converter control unit 12 and the inverter control unit 13 can switch, depending on a predetermined condition (for example, during a power outage or maintenance), between a route for supplying the power stored in the storage battery 9 from the inverter circuit I to the AC load 92 and a route for supplying the power from the bypass circuit 301 to the AC load 92. In this way, the power conversion device 1 can also function as a non-insulated uninterruptible power supply.

[0059] Although the embodiments of the present invention have been described above, it goes without saying that the technical scope of the present invention should not be construed as being limited by the description of the present embodiments. The present embodiments are merely examples, and it will be understood by those skilled in the art that various modifications of the embodiments are possible within the scope of the invention described in the claims. The technical scope of the present invention should be determined based on the scope of the invention described in the claims and its equivalents. [Explanation of symbols]

[0060] 1,200 Power Converters 2a~2c Input filter capacitors 3a~3c Input filter reactor 4a~4f Semiconductor switching elements 5, 5a, 5b smoothing capacitors 6a to 6f Semiconductor switching elements 7a~7c Output filter reactor 8a~8c Output filter capacitors 9. Storage battery 11 Phase detection section 12 Converter control section 13 Inverter control unit 31a~31c Device input terminals 41a~41c Converter input terminals 91 Three-phase AC power supply 92 AC load 101u, 101v, 101w control waveform 102 Carrier Triangular Wave 103 Drive waveform 300 Uninterruptible Power Supply 301 Bypass circuit C converter circuit Eu,Eu2 output voltage Er Input voltage F1 Input Filter F2 Output Filter I Inverter circuit nl neutral line S smoothing circuit

Claims

1. a converter circuit connected to a three-phase AC power source and converting AC from the three-phase AC power source into DC; a smoothing circuit including a smoothing capacitor connected to a DC output terminal of the converter circuit; an inverter circuit connected to a DC terminal of the smoothing circuit, converting a DC current from the smoothing circuit into an AC current and outputting the AC current to an AC load; a phase detection unit that detects a phase difference between an AC voltage input to the converter circuit and an AC voltage output from the inverter circuit; a control unit that switches a modulation method of the converter circuit and the inverter circuit between a two-phase modulation method and a three-phase modulation method based on a detection result of the phase detection unit; Equipped with the converter circuit and the inverter circuit are each formed of a three-phase full-bridge circuit made of switching elements, and an input filter capacitor constituting a filter on the converter circuit side and an output filter capacitor constituting a filter on the inverter circuit side are connected to each other; a smoothing capacitor of the smoothing circuit is not connected to the input filter capacitor and the output filter capacitor; the control unit switches the modulation method of the converter circuit and the inverter circuit from the two-phase modulation method to the three-phase modulation method when the phase difference detected by the phase detection unit becomes equal to or greater than a predetermined value. Power conversion equipment.

2. the control unit switches the modulation method of the converter circuit and the inverter circuit from the three-phase modulation method to the two-phase modulation method when the phase difference detected by the phase detection unit becomes less than a predetermined value. The power conversion device according to claim 1 .

3. When detecting a phase difference between the converter circuit and the inverter circuit, the phase detection unit detects a phase difference between an AC voltage input to the converter circuit and an inverter reference sine wave used in the control unit. The power conversion device according to claim 1 or 2.

4. a bypass circuit in parallel with the converter circuit and the inverter circuit; the control unit switches between power supply from the inverter circuit to the AC load and power supply from the bypass circuit to the AC load according to a predetermined condition. The power conversion device according to any one of claims 1 to 3.

5. a converter circuit connected to a three-phase AC power source and converting AC from the three-phase AC power source into DC; a smoothing circuit including a smoothing capacitor connected to a DC output terminal of the converter circuit; an inverter circuit connected to a DC terminal of the smoothing circuit, converting a DC current from the smoothing circuit into an AC current and outputting the AC current to an AC load; a bypass circuit connected in parallel to the converter circuit and the inverter circuit; A control unit that PWM controls the converter circuit and the inverter circuit using a two-phase modulation method; Equipped with the converter circuit and the inverter circuit are each formed of a three-phase full-bridge circuit made of switching elements, and an input filter capacitor constituting a filter on the converter circuit side and an output filter capacitor constituting a filter on the inverter circuit side are connected to each other; a smoothing capacitor of the smoothing circuit is not connected to the input filter capacitor and the output filter capacitor; the control unit switches between power supply from the inverter circuit to the AC load and power supply from the bypass circuit to the AC load according to a predetermined condition. Uninterruptible power supply.

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