Power conversion system

Abstract

This power conversion system includes a transformer, N first inverters, N second inverters, and a control unit. The transformer has 2N-system primary windings for three-phase alternating current and a 1-system secondary winding for three-phase alternating current. The control unit controls each of the inverters by PWM control. A first primary winding corresponding to one of the N first inverters and a second primary winding corresponding to one of the N second inverters are determined on the basis of the magnitude of respective percent impedances from the respective primary windings to the secondary winding. A first carrier phase in PWM control with respect to the first inverter and a second carrier phase in PWM control with respect to the second inverter are inverted. Said N is a natural number of 2 or more.

Description

Power Conversion Systems

[0001] FIELD OF THE INVENTION An embodiment of the present invention relates to a power conversion system.

[0002] There is a power conversion system that combines AC power generated by multiple inverters into a single system and outputs it using a transformer having multiple primary windings and one secondary winding. In a power conversion system with such a configuration, the frequency components of a carrier signal used for PWM control of the multiple inverters can affect the quality of the AC power output from the power conversion system. There has been a demand for further improvement in the quality of the AC power output by the power conversion system.

[0003] Japanese Patent Application Laid-Open No. 2022-030283

[0004] An object of the present invention is to provide a power conversion system that can further improve the quality of AC power that is output.

[0005] A power conversion system according to an embodiment includes a transformer, N first inverters, N second inverters, and a control unit. The transformer has 2N primary windings for three-phase AC and one secondary winding for three-phase AC. The control unit controls each of the first inverter and the second inverter by PWM control. A first primary winding corresponding to one of the N first inverters and a second primary winding corresponding to one of the N second inverters are determined based on the magnitude of each % impedance from each primary winding to the secondary winding. The phase of a first carrier for PWM control of one of the N first inverters is inverted from the phase of a second carrier for PWM control of one of the N second inverters. N is a natural number greater than or equal to 2.

[0006] 1 is a schematic configuration diagram of a power conversion system according to an embodiment. 2 is a schematic configuration diagram of a power conversion system according to a comparative example. 3 is a configuration diagram of a transformer according to an embodiment. 4 is a diagram for explaining an example of a transformer according to an embodiment. 5 is a diagram for explaining variations in impedance of each primary winding in a power conversion system according to a comparative example. 6 is a diagram for explaining variations in impedance of each primary winding in a power conversion system according to a comparative example. 7 is a diagram for explaining measurement results of impedance of each primary winding according to an embodiment. 8 is a diagram for explaining generation of a current waveform according to an embodiment. 9 is a diagram for explaining generation of a current waveform according to a comparative example. 10 is a diagram for explaining generation of a current waveform according to an embodiment. 11 is a diagram for explaining generation of a current waveform according to a comparative example. 12 is a diagram for explaining observation results of current flowing in each primary winding of a W phase. 13 is a diagram for explaining an example of a transformer according to a modified example.

[0007] Hereinafter, a power conversion system according to an embodiment will be described with reference to the drawings. In the following description, components having the same or similar functions will be assigned the same reference numerals. Duplicate descriptions of those components may be omitted. Note that being electrically connected may simply be referred to as being "connected."

[0008] (Embodiment) A four-bank power conversion system will be described with reference to Figures 1A and 1B. Figure 1A is a schematic configuration diagram of a power conversion system 1 of the embodiment. Figure 1B is a schematic configuration diagram of a power conversion system 1Z of a comparative example. The power conversion system 1 of the embodiment will be described by highlighting the differences from the power conversion system 1Z of the comparative example.

[0009] Both the power conversion system 1 and the power conversion system 1Z are configured to output at least one system of AC power from a transformer. The difference between the power conversion system 1 and the power conversion system 1Z lies in the phase relationship of the frequency components contained in the AC power supplied to the primary side of the transformer.

[0010] For example, both the power conversion system 1 and the power conversion system 1Z include a transformer 2, a rectifier 3, an inverter 4, a filter 5, and a control unit 7.

[0011] Transformer 2 is a three-phase AC transformer having multiple primary windings and one secondary winding. Transformer 2 is configured, for example, as a multi-winding transformer. Each primary winding of transformer 2 has a three-phase AC winding. The number of primary windings of transformer 2 is an even number greater than or equal to four. The number of primary windings of transformer 2 can be expressed as (2N). Since N is a natural number greater than or equal to two, (2N) is an even number greater than or equal to four. Since the bank configuration shown in FIGS. 1A and 1B has four banks, a more specific value of N is 2. The number of primary windings of transformer 2 is four. The four windings WPA, WPB, WPC, and WPD shown in the figures are examples of primary windings for the W-phase of three-phase AC in transformer 2, whose primary windings are Y-connected. A more detailed configuration of transformer 2 will be described later. The same applies to the number of banks below. The secondary side of the transformer 2 has windings for three-phase AC. For example, if the primary winding of the transformer 2 shown in FIGS. 1A and 1B is Y-connected and the secondary winding S is Delta-connected, the primary and secondary sides can be associated with each other, such as the winding between the R and S lines corresponding to the primary W phase, the winding between the ST lines corresponding to the primary U phase, and the winding between the TR lines corresponding to the primary V phase. Furthermore, if the primary winding of the transformer 2 shown in FIGS. 1A and 1B is Delta-connected and the secondary winding S is Y-connected, the T-phase winding corresponding to the primary UV lines, the R-phase winding corresponding to the primary VW lines, and the S-phase winding corresponding to the primary WU phase can be associated with each other. Furthermore, if the primary winding and secondary winding S of the transformer 2 shown in FIGS. 1A and 1B are both delta-connected, it is possible to respectively associate the primary and secondary windings. Furthermore, if the primary winding and secondary winding S of the transformer 2 shown in FIGS. 1A and 1B are both wye-connected, it is possible to respectively associate the primary and secondary windings. In the following description, we will use the first example of the transformer 2 in which the primary winding is wye-connected and the secondary winding S is delta-connected. However, this is not limiting and any appropriate modifications may be made with reference to the above correspondences. The transformer 2 is, for example, a step-up transformer with a transformation ratio a. When the transformer 2 is a step-up transformer, the current flowing through the secondary winding is smaller than the current flowing through the primary winding. Heat generation due to copper loss in the secondary winding is reduced.

[0012] The rectifier 3 includes, for example, a smoothing capacitor, rectifies AC power, and outputs DC power smoothed by the smoothing capacitor. The rectifier 3 may be configured with multiple systems. For example, the number of systems of the rectifier 3 is set to (2N) to match the above.

[0013] The inverter 4 is provided on the load side of the rectifier 3. The inverter 4 includes semiconductor switching elements, and generates AC power based on DC power by switching the elements. The inverter 4 may be configured with multiple systems. The number of systems of the inverter 4 is (2N) as described above. The four inverters 4A, 4B, 4C, and 4D shown in FIGS. 1A and 1B are examples of three-phase AC inverters. For example, the inverters 4A, 4B, 4C, and 4D supply power to each primary winding of the W phase, as well as to each primary winding of the U phase and each primary winding of the V phase.

[0014] The filter 5 includes a resistor and a capacitor connected in series and is connected between each line on the secondary side of the transformer 2.

[0015] The control unit 7 switches each semiconductor switching element of the inverter 4 to cause the inverter 4 to output AC power to the transformer 2. The rectifier 3, the inverter 4, and the control unit 7 are an example of a power conversion device 10.

[0016] The power conversion system 1 configured as described above uses, for example, a multi-winding transformer 2 having multiple systems of primary windings and one system of secondary windings to combine AC power generated by multiple inverters into a single system and output it.

[0017] When an inverter is operated using general PWM control, the following frequency components of current are generated during operation: a. Current component of the AC fundamental frequency b. Current component of the harmonic frequency of the AC fundamental frequency c. Current component of the PWM control carrier frequency and frequencies related to PWM control

[0018] The carrier frequency of PWM control refers to the frequency of the carrier signal of PWM control.

[0019] Of the above, the current components b and c contain unnecessary frequency components. Methods for reducing the levels of such unnecessary frequency components will be described below.

[0020] First, a first method for reducing the level of unnecessary frequency components will be described.

[0021] If the switching timing of each semiconductor switching element of multiple inverters is synchronized, the level of unnecessary frequency components will increase in synchronization with this timing. Therefore, the control unit 7 of the power conversion system 1 performs PWM control using carrier phase shift modulation to suppress harmonic currents of the fundamental frequency of AC. When carrier phase shift modulation is applied, the phase of the carrier used by each inverter for PWM control is shifted by 360 / 2N degrees, thereby making it possible to suppress some of the unnecessary current components listed below.

[0022] This carrier phase shift modulation method suppresses the level of the current component b, which occurs in synchronization with the switching timing. However, even when carrier phase shift modulation is used, the current component c cannot be suppressed, and may remain as a harmonic current.

[0023] Therefore, the total (2N) inverters of the power conversion system 1 are divided into a first group consisting of N first inverters and a second group consisting of N second inverters, with the first inverters in the first group corresponding to the second inverters in the second group in a one-to-one correspondence. The first and second inverters may be PWM-controlled using carriers of opposite phases. For example, the first connecting conductor BUSA connects one first inverter in the first group, which operates under the PWM control of the first carrier of a pair of first and second carriers, to the first primary winding of the transformer 2. The second connecting conductor BUSB connects one second inverter in the second group, which operates under the PWM control of the second carrier of the pair of first and second carriers, to the second primary winding of the transformer 2. This reduces the effects of biased voltages in the windings. The power conversion system 1 includes the first connecting conductor BUSA and the second connecting conductor BUSB. The first connecting conductor BUSA and the second connecting conductor BUSB are an example of busbars for connecting the first inverter and the second inverter to the transformer 2.

[0024] However, even if the first method is applied to the power conversion system 1Z of the comparative example, it may be difficult to obtain the effect of reducing the levels of unnecessary frequency components.

[0025] Therefore, in this embodiment, a second method for reducing the level of unnecessary frequency components is applied, thereby reducing the occurrence of the above-mentioned phenomenon. Here, an example will be described in which the current component of a frequency related to the carrier frequency is reduced, focusing on the arrangement of the primary winding of the transformer 2 and the phase of the current flowing through the primary winding.

[0026] More specifically, the symmetry of the electrical characteristics of the winding set is ensured by supplying AC power with a predetermined phase difference to a winding set whose primary winding is arranged symmetrically with respect to the core of the transformer 2. Here, the "symmetry of the electrical characteristics of the winding set" refers to the %Z of the primary winding of the transformer 2.

[0027] First, the configuration of the transformer 2 will be described. FIG. 2 is a configuration diagram of the transformer 2 according to an embodiment. FIG. 3 is a diagram for explaining an example of the transformer 2 according to an embodiment. The transformer 2 is configured such that the primary winding and secondary winding correspond to, for example, the UVW phases on the primary side. For example, the UVW phases have a common structure. The range shown in FIG. 2 is the range related to the W phase on the primary side, but the transformer 2 also has a U phase and a V phase in addition to this W phase. The rated voltage of the transformer 2 actually used is, for example, 600 V on the primary side and 6600 V on the secondary side, but is not limited to this and may be changed as appropriate.

[0028] The photograph shown in Figure 3 was taken from outside the housing of the transformer 2 with part of the housing removed for the purpose of photography. The main body of the transformer 2 is placed in a metal housing made of aluminum or the like. The wiring, sensors, etc. for this transformer 2 are placed for experimental purposes and are not limited to these placements.

[0029] In the image in Figure 3, three columns can be seen in the vertical direction. These columns are assigned to the U, V, and W phases on the primary side from left to right. The U, V, and W phases are configured symmetrically. For example, for each phase, the core extends so that the direction of the main magnetic flux is set in the vertical direction in Figure 3. The core of each phase is made of a magnetic material such as iron.

[0030] Next, an example of the core and winding configuration will be described in more detail using the W-phase as an example. As shown in Figure 2, multiple primary winding coils and secondary winding coils are wound around the W-phase core of the transformer 2. When a voltage is applied to the primary winding coil, a main magnetic flux Φm is generated within the core. This main magnetic flux Φm generates a voltage in the secondary winding (RS) coil.

[0031] This main magnetic flux Φm intertwines both the primary winding and each secondary winding. In addition to the main magnetic flux Φm, leakage magnetic fluxes Φ1σA, Φ1σB, Φ1σC, Φ1σD, and leakage magnetic flux Φ2σ are generated, intertwine with each of the primary and secondary windings. Depending on the situation, leakage magnetic flux Φ1σ related to the primary winding is generated. The leakage magnetic flux Φ1σ intertwines each of the primary windings.

[0032] Even if the primary windings share the same type of wire, number of turns, and winding method, the impedance (%Z) of each primary winding can vary depending on the position of each primary winding on the core. The primary windings of the transformer 2 in this embodiment are arranged at a predetermined interval in the direction of extension of one side of the core.

[0033] The following describes the variation in impedance (%Z) of each primary winding. Figures 4A and 4B are diagrams for explaining the variation in impedance of each primary winding in a power conversion system 1Z of a comparative example. Figure 5 is a diagram for explaining the measurement results of the impedance of each primary winding. The short-circuit current is measured with the secondary winding short-circuited, and the %Z of each primary winding is derived. The results are shown in the table of Figure 5. Note that the primary windings other than the primary winding being measured are left open.

[0034] The %Z of the first primary winding (Primary 1) is 12.99%. The %Z of the second primary winding (Primary 2) is 11.37%. The %Z of the third primary winding (Primary 3) is also 11.37%. The %Z of the fourth primary winding (Primary 4) is 13.01%.

[0035] These results show that the %Z values ​​of the first primary winding (Primary 1) and the fourth primary winding (Primary 4) are relatively close. The %Z values ​​of the second primary winding (Primary 2) and the third primary winding (Primary 3) are the same. The difference between the %Z of the first primary winding (Primary 1) and the fourth primary winding (Primary 4), and the difference between the %Z of the second primary winding (Primary 2) and the third primary winding (Primary 3) are relatively small. This is just one example, but even in measurement results of similar cases, the %Z values ​​of the second primary winding (Primary 2) and the third primary winding (Primary 3) tend to be relatively close. Among these four %Z, the %Z values ​​of the first primary winding (Primary 1) and the fourth primary winding (Primary 4) are relatively large. Comparing the %Z values ​​of the first primary winding (Primary 1) and the second primary winding (Primary 2), the %Z of the first primary winding (Primary 1) is larger.

[0036] The current of each phase was examined for the transformer 2 shown in Figure 5. The waveform diagrams shown in Figures 4A and 4B show the observation results of the current flowing in each W-phase primary winding when a resistive load of a predetermined impedance is connected to the secondary winding of the transformer 2 under the connection configuration of the comparative example shown in Figure 1B. The waveform diagram shown in Figure 7 shows the observation results of the current flowing in each W-phase primary winding when a resistive load of a predetermined impedance is connected to the secondary winding of the transformer 2 under the connection configuration of the embodiment shown in Figure 1A. Figure 7 is a diagram for explaining the observation results of the current flowing in each W-phase primary winding.

[0037] First, we will explain the measurement results using the observation configuration of the comparative example. The waveforms shown in Figure 4A are waveforms representing the observed results (instantaneous values) of the currents flowing through the first to fourth primary windings. The waveforms shown in Figure 4B are enlarged versions of the waveforms shown in Figure 4A.

[0038] The original graph of the measurement results shows the changes in the current flowing through each secondary winding using four colored lines: blue (B), red (R), green (G), and pink (P). These have been converted to grayscale and are shown here. The peak and bottom values ​​of each waveform, which are difficult to distinguish due to the grayscale conversion, are shown with auxiliary lines.

[0039] For example, the peak and trough values ​​of each waveform in Figure 4B were extracted by taking the moving average of the current and selecting the peak values ​​during a period when the moving average value had relatively little change. The results shown in Figure 4B show that the ranges of change in current for red (R) and green (G) are consistent, and the ranges of change in current for blue (B) and pink (P) are consistent.

[0040] In this example, the primary windings are arranged in the order of blue (B), red (R), green (G), and pink (P), with blue (B) and pink (P) at the ends of the order and red (R) and green (G) in the middle. For example, the primary windings corresponding to blue (B), red (R), green (G), and pink (P) above correspond to the first to fourth primary windings. It can be seen that the tendency that can be read from the amplitude of this waveform is consistent with the tendency of %Z shown in Figure 5 above.

[0041] Next, with reference to FIG. 7 , observation results using the configuration of the embodiment will be described. This will be compared with the corresponding FIG. 4A . Although the time scales of the comparative example in FIG. 4A and FIG. 7 are different, the current values ​​in the observation results output roughly the same amplitude. For example, spikes exceeding the steady-state amplitude range (the length of the arrows in the figure) are detected. However, the observation results in FIG. 7 show that the spikes are smaller in magnitude and occur less frequently. While this result is based on observation of the current in the primary winding, similar disturbances occur on the secondary side at the same times as the spikes are observed. It was confirmed that the method of the embodiment can reduce the degree and frequency of disturbances, thereby improving the quality of the AC power output by the transformer 2.

[0042] As described above, the power conversion system 1 includes a transformer 2, N first inverters, N second inverters, and a control unit 7. The transformer 2 has 2N primary windings for three-phase AC and one secondary winding for three-phase AC. The control unit 7 controls each of the first inverter and the second inverter by PWM control. A first primary winding corresponding to one of the N first inverters and a second primary winding corresponding to one of the N second inverters are determined based on the magnitude of each % impedance from each primary winding to the secondary winding. The phase of a first carrier for PWM control of one of the N first inverters is inverted from the phase of a second carrier for PWM control of one of the N second inverters. By making the value of N a natural number greater than or equal to 2, the quality of the output AC power can be further improved.

[0043] The primary windings of the 2N systems described above are insulated for each system. As described above, the combination of the connection configurations of the primary windings and secondary windings of the 2N systems is arbitrary. If a delta connection is included among the primary windings and secondary windings of the 2N systems, third harmonics can be reduced without providing a delta-connected tertiary winding. If a delta connection is not included among the primary windings and secondary windings of the 2N systems, a delta-connected tertiary winding may be provided.

[0044] The 2N primary windings may be arranged for each phase of the three-phase AC so that the direction of the leakage magnetic flux generated by each of them during the common control cycle of the PWM control is oriented in a first direction. The 2N primary windings may be arranged side by side so that the direction of the leakage magnetic flux is oriented in a first direction along the extension direction of the core. The transformer 2 may be configured such that each core for each phase has N primary windings on one side and a secondary winding on the opposite side. Furthermore, the arrangement of the windings relative to the core makes it easy to ensure structural symmetry. This also makes it easy to ensure symmetry in the electrical characteristics.

[0045] Of the 2N primary windings arranged side by side in the direction of the leakage magnetic flux, a pair of primary windings arranged at both ends may have a first end primary winding connected to one of the N first inverters by a first conductor, and a second end primary winding connected to one of the N second inverters by a second conductor. In the PWM control of one of the N first inverters and one of the N second inverters, carrier phases of the PWM control may be inverted from each other. This allows the pair of primary windings to be supplied with drive pulses for semiconductor switching elements, which are generated with carrier phases inverted from each other.

[0046] The difference in %Z between a pair of primary windings arranged at both ends of the 2N primary windings arranged side by side in the direction of the leakage magnetic flux is smaller than the difference in %Z between one of the pair of primary windings and the primary winding arranged adjacent to that one. As a result, the level of carrier components can be reduced more when drive pulses for semiconductor switching elements generated with carrier phases inverted from each other are supplied to a pair of primary windings than when drive pulses for semiconductor switching elements generated with carrier phases inverted from each other are supplied to one of the pair of primary windings and the primary winding arranged adjacent to that one.

[0047] By pairing windings with approximately the same %Z, the carrier frequency components generated by those windings can be canceled out, thereby suppressing harmonic currents. Note that the above phrase "approximately the same %Z" may also include cases where the error is between 0 and a predetermined ratio (for example, about 0.02%).

[0048] The magnitude of the %Z of a pair of primary windings arranged at both ends of the 2N systems of primary windings arranged side by side in the direction of the leakage magnetic flux is larger than the magnitude of the %Z of the other primary windings excluding the pair of primary windings.

[0049] (Analysis of current waveform generated by transformer) Generation of a current waveform in the embodiment will be described with reference to Fig. 6A to Fig. 6D. Fig. 6A and Fig. 6C are diagrams for explaining generation of a current waveform in the embodiment. Fig. 6B and Fig. 6D are diagrams for explaining generation of a current waveform in a comparative example.

[0050] 6A to 6D , the upper part of each figure shows the current flowing from the inverters of banks A to D within one PWM control cycle. When the voltage (current) output from transformer 2 is a positive AC voltage (current), each bank is assigned a period (phase angle) during which current flows within one carrier cycle from phase 0 to 180 degrees. Of these four figures, 6A to 6D , figures 6A and 6B show cases where the output level is relatively small, while figures 6C and 6D show cases where the output level is relatively large. Depending on the modulation factor of PWM control, no current is output in the darkly shaded area within one half cycle of the PWM control carrier signal, and current is output in the lightly shaded area.

[0051] There is a 90-degree phase difference between banks A to D. Therefore, the timing at which current is output from each bank is shifted in the time axis direction (phase axis direction) due to the phase difference.

[0052] The bottom row of each diagram shows the result of adding up the amplitudes of the current pulses output from the inverters 4 of Banks A to D. The timing shown by the result of adding up the amplitudes of the current pulses corresponds to the timing at which current flows through the secondary winding of the transformer 2. The magnitude of the amplitudes shown by the results also corresponds. In this diagram, the amplitudes are shown as an example for a transformer with a turns ratio of 1, but it is advisable to convert them to a scale according to the transformation ratio. As mentioned above, the waveforms at the bottom row of each diagram will be explained as the output of the secondary winding.

[0053] The amplitude of the current pulse in the primary winding and the phase of each bank are explained below. The amplitude of the current pulse in the primary winding in each diagram is the same for the pair of banks A and D, and the same for the pair of banks B and C.

[0054] 6A and 6C, the C bank and the D bank are shifted 270 degrees and 180 degrees in phase with the A bank. In the cases of FIGS. 6A and 6C, the periods during which the A bank and the D bank, which have relatively small amplitudes, output current pulses do not overlap. Similarly, the periods during which the B bank and the C bank, which have relatively large amplitudes, output current pulses do not overlap. This causes the output of the secondary winding to have one of four amplitude levels: 0, one small-amplitude pulse corresponding to the output of the A bank (or the D bank), one large-amplitude pulse corresponding to the output of the B bank (or the C bank), or the sum of one small-amplitude pulse corresponding to the output of the A bank (or the D bank) and one large-amplitude pulse corresponding to the output of the B bank (or the C bank).

[0055] In contrast, the C and D banks in Figures 6B and 6D have phase differences of 180 degrees and 270 degrees with respect to the A bank. In the cases of Figures 6B and 6D, there are times when the periods in which the A and D banks, which have relatively small amplitudes, output current pulses overlap. There are also times when the periods in which the B and C banks, which have relatively large amplitudes, output current pulses overlap. Compared to the cases of Figures 6A and 6C described above, the output of the secondary winding has an amplitude equal to the sum of two large-amplitude pulses corresponding to the output of the B bank (or the C bank), and the output is one of five levels.

[0056] In this way, it was verified through modeling analysis that the example of the embodiment has less fluctuation in the output of the secondary winding than the example of the comparative example.

[0057] (First Modification) In the above embodiment, a four-bank configuration was exemplified. Each bank in the configuration had a phase difference of 90 degrees from the others. Therefore, the timing at which current was output from each bank was set with a phase difference in 90-degree increments. Instead, in this modification, a case where the number of banks is set to a number other than four will be described. For example, the description will be generalized to an even number of banks equal to or greater than four, i.e., (2N) banks. In this case, the phase angle of each bank may be calculated using the following equations (1) and (2).

[0058] 180x(n-1) / N (1) 180x(n+1) / N (2) where n is a natural number from 1 to N.

[0059] The pair of banks set to the phase angles calculated using equations (1) and (2) can be controlled using the results of PWM control with carrier signals of opposite phases. As shown in Figures 2 and 3, leakage flux from the windings of transformer 2 is generated along the extension direction of the core edges (the normal direction of the surface formed by the windings). Therefore, excluding the windings at both ends, two windings selected in the order of winding arrangement along the direction of leakage flux are paired. Current resulting from PWM control using carrier signals with a 180-degree phase difference is supplied to the pair of windings. This makes it possible to reduce the carrier component even in cases with six or more banks.

[0060] (Second Modification) In the above embodiment, a transformer with a multi-winding configuration was exemplified. Instead, in this modification, a configuration using a transformer with a combination of single-winding cores will be described. FIG. 8 is a diagram illustrating an example of a modified transformer 2A. The transformer 2A has the same number of cores per phase as the number of banks. For example, a configuration in which the number of cores in the W phase on the primary side is set to four is exemplified. The secondary winding of the transformer 2A is divided in accordance with the cores. These are connected in series to form a secondary winding.

[0061] When the core is divided in this manner, it is preferable to arrange the cores so that leakage flux generated in each core interlinks with the winding of the adjacent core.

[0062] According to at least one embodiment described above, the power conversion system includes a transformer, N first inverters, N second inverters, and a control unit. The transformer has 2N primary windings for three-phase AC and one secondary winding for three-phase AC. The control unit controls each of the first inverter and the second inverter by PWM control. A first primary winding corresponding to one of the N first inverters and a second primary winding corresponding to one of the N second inverters are determined based on the magnitude of each % impedance from each primary winding to the secondary winding. The phase of a first carrier for PWM control of one of the N first inverters is inverted from the phase of a second carrier for PWM control of one of the N second inverters. The value of N is a natural number greater than or equal to 2, thereby further improving the quality of the output AC power.

[0063] Some or all of the functional units of the control unit 7 in the power conversion system 1 of the embodiment described above may have software functional units that are realized, for example, by a computer processor (hardware processor) executing a program (computer program, software component) stored in a storage unit (memory, etc.) of the computer. Note that some or all of the functional units of the control unit 7 may be realized by hardware such as an LSI (Large Scale Integration), an ASIC (Application Specific Integrated Circuit), or an FPGA (Field-Programmable Gate Array), or may be realized by a combination of software functional units and hardware.

[0064] 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 embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. For example, the configurations of each embodiment may be implemented in combination with each other, and can be applied to components omitted from the description. These embodiments and their modifications are intended to be included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims.

[0065] In the above embodiment, the PWM control may be performed individually for each bank of inverters 4. Alternatively, the PWM control may be performed for each set of inverters 4, thereby halving the processing load of the PWM control. For example, instead of performing PWM control using a carrier with a phase of 0 degrees on one inverter in a set and then performing PWM control using a carrier with a phase of 180 degrees, the result of the PWM control using the carrier with a phase of 0 degrees can be used to control the other inverter in the set.

[0066] Furthermore, the illustrated inverter 4 is not limited to any particular configuration, and may be either a half-bridge or full-bridge type. Each inverter (4A, 4B, 4C, 4D) may be either a 180-degree conduction type or a 120-degree conduction type. The voltage waveform (current waveform) formed by combining the outputs of the inverters is not limited to any particular configuration, and any desired waveform can be selected. For example, while the inverters each output one voltage pulse (current pulse) per PWM control cycle, this is not a limitation, and multiple pulses may be output per cycle. Furthermore, the secondary voltage waveform (current waveform) synthesized from the outputs of the inverters may be multilevel.

[0067] REFERENCE SIGNS LIST 1... Power conversion system 2... Transformer 3... Rectifier 4... Inverter 7... Control unit 10... Power conversion device

Claims

1. A power conversion system comprising: a transformer having 2N systems of primary windings for three-phase AC and one system of secondary windings for three-phase AC; N first inverters; N second inverters; and a control unit that controls each of the N first inverters and the N second inverters by PWM control, wherein a first primary winding corresponding to one of the N first inverters and a second primary winding corresponding to one of the N second inverters are determined based on the magnitude of each % impedance from each primary winding to the secondary winding, a phase of a first carrier of PWM control for one of the N first inverters and a phase of a second carrier of PWM control for one of the N second inverters are inverted, and the value of N is a natural number greater than or equal to 2.

2. The power conversion system according to claim 1, wherein the 2N primary windings are arranged for each phase of the three-phase AC so that the direction of leakage magnetic flux generated by each of the primary windings during a common control cycle of the PWM control is oriented in a first direction.

3. The power conversion system according to claim 2, wherein the 2N primary windings are arranged side by side such that the direction of the leakage magnetic flux is oriented in the first direction along the extension direction of the core.

4. The power conversion system according to claim 3, wherein a pair of primary windings arranged at both ends of the 2N systems of primary windings arranged side by side in the direction of the leakage magnetic flux is connected to one of the N first inverters and one of the N second inverters.

5. The power conversion system according to claim 4, wherein the difference in %Z between a pair of primary windings arranged at both ends of the 2N systems of primary windings arranged side by side in the direction of the leakage magnetic flux is smaller than the difference in %Z between one of the pair of primary windings and the primary winding arranged adjacent to that one.

6. The power conversion system described in claim 4, wherein the magnitude of %Z of a pair of primary windings arranged at both ends of the 2N systems of primary windings arranged side by side in the direction of the leakage magnetic flux is larger than the magnitude of %Z of the other primary windings excluding the pair of primary windings.

7. A power conversion system as described in claim 1, comprising: a first connecting conductor connecting the one first inverter operated under PWM control of the first carrier and a first primary winding of the 2N systems; and a second connecting conductor connecting the one second inverter operated under PWM control of the second carrier and a second primary winding of the 2N systems.

Images