Power conversion system

Abstract

A power conversion system (1) is provided with a transformer (2), a power converter (10), and a filter circuit (5). The transformer has a plurality of primary windings (WPA-WPD) and a secondary winding (WS) that are insulated from each other and magnetically coupled to each other, converts AC power supplied to each of the primary windings into multi-phase AC power, and supplies the multi-phase AC power from the secondary winding to a load. The power converter supplies AC power to each of the primary windings. The filter circuit is connected to the secondary winding and smoothes the multi-phase AC power.

Description

Technical Field

[0001] Embodiments of the present invention relate to a power conversion system. Background Art

[0002] There is a power conversion system that uses a transformer having a primary winding with multiple systems and a secondary winding with one system to centrally output AC power generated by multiple inverters (power converters) to one system. To prevent the quality of the AC power output by such a power conversion system from degrading, a filter is provided between the output of each inverter and the transformer. If the number of inverters is increased to increase the scale of the power conversion system, the scale of this filter section also increases, and it may not be easily configured. Even in such a case, further improvement in the quality of the AC power output by the power conversion system is required.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2015-133856 Summary of the Invention

[0006] Problems to be Solved by the Invention

[0007] An object of the present invention is to provide a power conversion system capable of further improving the quality of the output AC power.

[0008] Means for Solving the Problems

[0009] The power conversion system of the embodiment includes a transformer, a power converter, and a filter circuit. The transformer has a plurality of primary windings and a secondary winding that are mutually insulated and magnetically coupled, converts the AC power supplied to each primary winding into polyphase AC power, and supplies the polyphase AC power from the secondary winding to a load. The power converter supplies AC power to each of the primary windings. The filter circuit is connected to the secondary winding and smoothes the polyphase AC power. Brief Description of the Drawings

[0010] Figure 1A is a schematic configuration diagram of the power conversion system of the embodiment.

[0011] Figure 1B is a schematic configuration diagram of the power conversion system of the comparative example.

[0012] Figure 2A is a diagram for explaining the filter of the embodiment.

[0013] Figure 2B is a diagram for explaining the filter of the embodiment.

[0014] Figure 3 This is a diagram showing an example of the frequency characteristics of the equivalent circuits of the transformer 2 and the filter 5 for explaining the embodiments.

[0015] Figure 4A This is a circuit diagram of the power conversion system 1 to which the filter 5 of the embodiments is applied.

[0016] Figure 4B This is a circuit diagram of the power conversion system 1 to which the filter 5 of the embodiments is applied.

[0017] Figure 5 This is a diagram for explaining the winding structure of the transformer of the embodiments. Detailed Embodiments

[0018] Hereinafter, the power conversion system of the embodiments will be described with reference to the drawings.

[0019] In addition, in the following description, the same reference numerals are given to structures having the same or similar functions. Also, the repeated description of these structures is sometimes omitted. In addition, electrical connection is sometimes simply referred to as "connection".

[0020] (Embodiments)

[0021] Refer to Figure 1A and Figure 1B , and the power conversion system having a four-unit structure will be described.

[0022] Figure 1A This is a schematic structural diagram of the power conversion system 1 of the embodiments.

[0023] Figure 1B This is a schematic structural diagram of the power conversion system 1Z of the comparative example.

[0024] For the power conversion system 1 of the embodiments, the differences from the power conversion system 1Z of the comparative example are shown and described.

[0025] Both the power conversion system 1 and the power conversion system 1Z are configured to output at least one system of AC power from the transformer. The difference between the power conversion system 1 and the power conversion system 1Z lies in the peripheral circuit of the transformer.

[0026] For example, the power conversion system 1 includes a transformer 2, a rectifier 3, an inverter 4, a filter 5, and a control unit 7.

[0027] In contrast, the power conversion system 1Z includes a filter 6 that replaces the filter 5 of the power conversion system 1. Hereinafter, the differences will be described in sequence.

[0028] The transformer 2 is a three-phase AC transformer having multiple primary windings and one secondary winding. The transformer 2 is configured as a multi-winding type transformer, for example.

[0029] Each system on the primary side of the transformer 2 has a winding for three-phase alternating current. The number of systems on the primary side of the transformer 2 is plural. The number of systems on the primary side of the transformer 2 can be expressed as (2N). Further, N is a natural number. In the following description, an example in which (2N) is set to 4 will be described.

[0030] This Figure 1A and Figure 1B The illustrated unit structure is a 4-unit structure, and the number of systems on the primary side of the transformer 2 is 4. The four windings WPA, WPB, WPC, and WPD shown in the figure are an example of the primary windings for the W phase of the three-phase alternating current in the transformer 2 with the primary winding in Y connection.

[0031] In addition, the following description regarding the number of units is the same.

[0032] Each system on the secondary side of the transformer 2 has a winding for three-phase alternating current.

[0033] The transformer 2 is, for example, a step-up transformer with a turns ratio a. When the transformer 2 is a step-up transformer, the current flowing in the secondary winding is smaller than the current flowing in the primary winding. Since the heat generation due to the copper loss of the secondary winding is small, the cross-sectional area of the secondary winding can be appropriately reduced accordingly. A more detailed structure of the transformer 2 will be described later.

[0034] In addition, in the following description, an example in which the primary winding of the transformer 2 in the above first example is in Y connection and the secondary winding S is in Δ connection will be described, but it is not limited thereto, and the connection structure of the primary winding and the secondary winding S can be appropriately changed.

[0035] The rectifier 3 includes, for example, smoothing capacitors, rectifies the AC power, and outputs DC power smoothed by the smoothing capacitors. The rectifier 3 can also be configured by being divided into multiple systems. For example, the number of systems of the rectifier 3 is set to (2N) in accordance with the above.

[0036] 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 the DC power by switching thereof. The inverter 4 can also be configured by being divided into multiple systems. The number of systems of the inverter 4 is set to (2N) in accordance with the above.

[0037] This Figure 1A and Figure 1B The four three-phase inverters 4A, 4B, 4C, and 4D shown in the figure are an example of three-phase AC inverters. For example, the three-phase inverters 4A, 4B, 4C, and 4D supply power to the primary windings of each U phase and each V phase in addition to the primary windings of each W phase.

[0038] The control unit 7 outputs AC power from the inverter 4 to the transformer 2 by switching each semiconductor switching element of the inverter 4.

[0039] The above-described rectifier 3, inverter 4, and control unit 7 are an example of the power conversion device 10.

[0040] The filter 5 of the embodiment is composed of passive elements and is connected between the lines on the secondary side of the transformer 2. The filter 5 becomes a capacitive load of the secondary winding of the transformer 2. The filter 5 includes a circuit showing capacitive load characteristics. The filter 5 is configured to allow the fundamental wave component of the polyphase AC power output from the secondary side of the transformer 2 to pass through, and to attenuate the normal mode noise components generated by the switching of the semiconductor elements of the inverter 4 (power converter), through the inductive component of the transformer 2 and the capacitive component of the capacitive load characteristics.

[0041] The filter 5 includes, for example, a capacitor 5A. In the case of this structure, the capacitance of the capacitor 5A can be used as the capacitive component of the circuit showing the capacitive load characteristics of the filter 5.

[0042] The filter 5 includes, for example, a group (5A) of a resistor and a capacitor connected in series.

[0043] In addition, the filter 5 can be provided between the secondary winding of the transformer 2 and the distribution switch for disconnecting the load and the connection cable.

[0044] In Figure 4A and Figure 4B An example of a specific circuit applying the filter 5 of the embodiment is shown.

[0045] Figure 4A and Figure 4B are circuit diagrams of the power conversion system 1 to which the filter 5 of the embodiment is applied.

[0046] This Figure 4A and Figure 4B The four primary windings PA, PB, PC, and PD of the transformer 2 shown are Y-connected.

[0047] Figure 4A The primary winding of the transformer 2 shown is configured to be Y-connected, and the secondary winding is configured to be Δ-connected. The filter 5B (5) is configured to be Δ-connected. By wiring as described above, the Δ-connection of the secondary winding of the transformer 2 and the Δ-connection of the filter 5 are connected in parallel.

[0048] In addition, Figure 4BThe primary winding of the transformer 2A shown is configured as a delta connection, and the secondary winding is configured as a wye connection. The filter 5B(5) is also configured as a delta connection. By wiring as described above, the elements of the filter 5 are connected between the lines of the secondary winding of the transformer 2.

[0049] In addition, the winding structures of the above-mentioned transformers 2 and 2A represent an example and are not limited thereto. For example, Figure 4A the primary winding of the transformer 2 shown can be configured as a delta connection, or Figure 4B the primary winding of the transformer 2A shown can be configured as a wye connection. For example, it is preferable to use Figure 5 a transformer with the winding structure shown. Figure 5 is a diagram for explaining an example of the winding structure of a transformer.

[0050] In the case of the filter 5, only a capacitor C is provided between the lines on the secondary side, or a set of a resistor R and a capacitor C is provided. In this case, three capacitors C are required. Depending on the need, sometimes three resistors are provided.

[0051] In addition, Figure 1B the filter 6 of the comparative example shown includes a reactor L, a resistor R, and a capacitor C. The reactor L of the filter 6 is provided on a conductor connecting the output of the inverter 4 to each winding on the primary side of the transformer 2. A resistor R and a capacitor C are provided between the transformer 2 side of the reactor L and the neutral point. Such a filter 6 becomes a wye connection. In the case of the filter 6, a set of a reactor L, a resistor R, and a capacitor C needs to be provided for each phase of each unit. In the case of a three-phase circuit of four units, 12 sets of the reactor L, the resistor R, and the capacitor C are required.

[0052] As described above, the structures and the number of circuits of the filter 5 of the embodiment and the filter 6 of the comparative example are different. The filter 6 of the comparative example processes the AC power generated by each inverter separately. Therefore, if it is a structure of four units, four systems of the same number are required. By using the filter 5 of the embodiment, the number of component parts can be reduced as described above.

[0053] In addition, as shown in FIG. 1, when the secondary winding of the transformer 2 is a delta connection, the elements of the filter 5 are connected in parallel with the transformer secondary winding.

[0054] In addition, in the current flowing from the inverter 4 to the filter 6, there are currents based on the AC power generated by each inverter and currents including the noise components therein. By having the filter 6, the noise components are attenuated. Without this filter 6, the noise components flow into the transformer 2 without attenuation as described above. In other words, the normal-mode noise components generated by the switching of the semiconductor elements of the inverter 4 are supplied to the primary winding of the transformer 2. This noise component is generated, for example, in synchronization with the timing of the switching of each inverter.

[0055] Therefore, in the structure of the embodiment, the transformer 2 is combined with the filter 5. Thereby, the frequency characteristics of the magnetic permeability of the iron core of the transformer 2 can be utilized for the attenuation of the noise components. In addition, the frequency characteristics of the capacitive components based on the inductive component of the transformer 2 and the capacitive load characteristics of the filter 5 can be utilized for the attenuation of the noise components. These noise components are manifested as normal-mode noise. By attenuating the noise components, the quality of the output AC power can be further improved.

[0056] The power conversion system 1 configured as described above, for example, uses a multi-winding transformer 2 having multiple systems of primary windings and one system of secondary windings to centrally output the AC power generated by multiple inverters to one system.

[0057] When sorted from another perspective, the power conversion system 1 includes a transformer 2, an inverter 4A (first power converter), an inverter 4B (second power converter), and a filter 5 (filter circuit).

[0058] The transformer 2 has first to fourth primary windings (for example, WPA, WPB, WPC, WPD of the W phase) that are insulated from each other and magnetically coupled, and a secondary winding S. The filter 5 is connected to the secondary winding S.

[0059] The inverter 4A is connected to the primary winding WPA (first primary winding) and supplies first AC power to the primary winding WPA. The inverter 4B is connected to the primary winding WPB (second primary winding) and supplies second AC power to the primary winding WPB. The inverter 4C is connected to the primary winding WPC (third primary winding) and supplies third AC power to the primary winding WPC. The inverter 4D is connected to the primary winding WPD (fourth primary winding) and supplies fourth AC power to the primary winding WPD.

[0060] The transformer 2 converts the AC power supplied to each of its primary windings into polyphase AC power and supplies the polyphase AC power from its secondary winding S to the load.

[0061] However, if the inverter 4 is operated by general PWM control, currents of the following frequency components are generated during this operation.

[0062] a. Current component of the fundamental frequency of the alternating current

[0063] b. Current component of the higher harmonic frequencies of the fundamental frequency of the alternating current

[0064] c. Current component of the carrier frequency of the PWM control and the frequencies associated with the PWM control. Additionally, the carrier frequency of the PWM control refers to the frequency of the carrier signal of the PWM control.

[0065] Among the above, the current components related to b and c contain frequency components that are not originally required. A method for reducing the level of such unnecessary frequency components will be described in sequence.

[0066] First, a first method for reducing the level of unnecessary frequency components will be described.

[0067] If the switching timings of the semiconductor switching elements of each of the multiple inverters 4 are the same, the level of the unnecessary frequency components becomes high in synchronization with that timing. For example, the control unit 7 of the power conversion system 1 implements PWM control in the carrier phase shift modulation method to suppress the higher harmonic current of the fundamental frequency of the alternating current. In the case of applying the carrier phase shift modulation method, by staggering the phase of the carrier used in the PWM control of each inverter by 360 / N degrees each time, a part of the unnecessary current components in the subsequently supplied current components can be suppressed. For example, N is the number of inverters. In this power conversion system 1, N is 4.

[0068] Through this carrier phase shift modulation method, the level of the current component of b generated in synchronization with the switching timing is suppressed. However, even when using the carrier phase shift modulation method, the current component of c described above is not suppressed, so it sometimes remains as a higher harmonic current. In the present embodiment, by using the filter 5 to reduce such noises such as higher harmonic current, the above-mentioned polyphase alternating current power is smoothed.

[0069] In this way, the filter 5 of the embodiment does not process the alternating current power generated by each inverter individually, but performs a filtering process on the polyphase alternating current power centrally output to one system. The filter 5 attenuates the noise components overlapping in the current of the polyphase alternating current power centrally output to one system based on the transformer 2.

[0070] Next, with reference to Figure 2A and Figure 2B , a specific example of the filter 5 will be described.

[0071] Figure 2A and Figure 2B are diagrams for explaining the filter 5 of the embodiment.

[0072] Figure 2A The filter 5A shown includes a capacitor.

[0073] Figure 2B The filter 5B shown includes a capacitor and a resistor. The resistor is a so-called damping resistor.

[0074] The transformer 2 is an insulated type transformer with a primary winding and a secondary winding separated. Modeling this transformer 2 as a single-phase circuit, it is equivalent to an inductor L including the windings and magnetic circuit of the transformer 2.

[0075] Through this modeling, the equivalent circuits of the transformer 2 and the filter 5 can be converted into Figure 2A or Figure 2B like that.

[0076] Figure 3 is a diagram showing an example of the frequency characteristics of the equivalent circuits of the transformer 2 and the filter 5 for explaining the embodiments.

[0077] The resonance frequency of the inductor L and the capacitor C is obtained by the following formula (1).

[0078]

[0079] The equivalent circuits of the transformer 2 and the filter 5 function as a second-order low-pass filter. For example, assume that the resonance frequency of the inductor L and the capacitor C in the equivalent circuits of the transformer 2 and the filter 5 is 100 kHz.

[0080] According to this equivalent circuit, when exceeding this frequency, an attenuation characteristic with a gradient of 40 dB / dec is shown, and the higher the frequency, the greater the attenuation amount.

[0081] For example, if the carrier frequency fcar of the inverter 4 is 1 MHz, the attenuation amount of this carrier frequency fcar is -40 dB with respect to the 0 dB attenuation at low frequency, so it can be known that the components of the carrier frequency fcar can be sufficiently attenuated.

[0082] In addition, it is preferable to use a circuit provided with a damping resistor R as needed ( Figure 2B ). Figure 2A The frequency characteristics of the filter 5A shown have a resonance point, so sometimes the components of the frequency near this resonance point are amplified. In order not to amplify such components, it is preferable to set a damping resistor to adjust the frequency characteristics to desired frequency characteristics.

[0083] According to at least one embodiment described above, the power conversion system includes a transformer, a power converter, and a filter circuit. The transformer has a plurality of primary windings and secondary windings that are insulated from each other and magnetically coupled, converts the AC power supplied to each primary winding into polyphase AC power, and supplies the polyphase AC power from the secondary winding to a load. The power converter supplies AC power to each of the primary windings. The filter circuit is connected to the secondary winding, and by smoothing the polyphase AC power, the quality of the output AC power can be further improved.

[0084] A part or all of each functional part of the control unit 7 in the power conversion system 1 of the embodiment described above may, for example, also have a software functional part implemented by a program (computer program, software component) stored in a storage part (memory, etc.) of a computer being executed by a processor (hardware processor) of the computer. In addition, a part or all of each functional part of the control unit 7 can be implemented by hardware such as LSI (Large Scale Integration), ASIC (Application Specific Integrated Circuit), or FPGA (Field-Programmable Gate Array), or can also be implemented by a combination of a software functional part and hardware.

[0085] Several embodiments of the present invention have been described, but these embodiments are presented as examples and are not intended to limit the scope of the invention. These new embodiments can be implemented in various other ways, and various omissions, substitutions, and changes can be made without departing from the gist of the invention. For example, the structures of the respective embodiments can also be combined and implemented, and can be applied to the structural parts that have been omitted from the description. These embodiments and their modifications are included in the scope and gist of the invention, and are included in the invention described in the claims and its equivalent scope.

[0086] In the above embodiment, the above PWM control can be implemented separately for each unit of the inverter 4, or alternatively, by implementing the above PWM control for each group of the inverter 4, the processing load of the PWM control can be halved.

[0087] For example, there is no limitation on the structure of the exemplary inverter 4, and it can be in any form such as a half-bridge type and a full-bridge type. Each inverter (4A, 4B, 4C, 4D) can be of the 180-degree conduction type or the 120-degree conduction type. There is no limitation on the voltage waveform (current waveform) formed by the combination of the outputs of the respective inverters, and a desired waveform can be selected. For example, a case where the number of voltage pulses (current pulses) output by each inverter in one cycle of PWM control is one is illustrated, but it is not limited thereto, and it can also be configured to output multiple pulses in one cycle. Further, for example, the voltage waveform (current waveform) on the secondary side synthesized from the outputs of the respective inverters can be set to a multi-level.

[0088] Description of Reference Numerals

[0089] 1... Power conversion system;

[0090] 2... Transformer;

[0091] 3... Rectifier;

[0092] 4... Inverter;

[0093] 5... Filter;

[0094] 7... Control unit;

[0095] 10... Power conversion device.

Claims

1. A power conversion system comprising: A transformer having a plurality of primary windings and secondary windings that are mutually insulated and magnetically coupled, converting the AC power supplied to each primary winding into polyphase AC power, and supplying the polyphase AC power from the secondary winding to a load; A power converter for supplying AC power to each of the primary windings; and A filter circuit connected to the secondary winding to smooth the polyphase AC power.

2. The power conversion system according to claim 1, wherein the filter circuit is composed of passive elements and forms a capacitive load of the secondary winding.

3. The power conversion system according to claim 1, wherein the filter circuit includes a circuit showing capacitive load characteristics, and allows the fundamental component of the polyphase AC power to pass through by the inductive component of the transformer and the capacitive component of the capacitive load characteristics, and attenuates the normal mode noise components generated by the switching of the semiconductor elements of the power converter.

4. The power conversion system according to claim 3, wherein the filter circuit includes a capacitor, and uses the capacitance of the capacitor as the capacitive component of the circuit showing the capacitive load characteristics.

5. The power conversion system according to claim 3, wherein the filter circuit is configured in a Δ connection.

6. The power conversion system according to claim 3, wherein each of the power converters supplies the normal mode noise components generated by the switching of the semiconductor elements of each of the power converters to the primary winding of the transformer, and by combining the transformer and the filter circuit, utilizes the frequency characteristics of the magnetic permeability of the core of the transformer and the frequency characteristics of the capacitive component based on the inductive component of the transformer and the capacitive load characteristics for the attenuation of the normal mode noise components.

7. The power conversion system according to claim 1, wherein the filter circuit is provided between the secondary winding of the transformer and the distribution switch.

8. A power conversion system comprising: A transformer having a first and a second primary winding and a secondary winding that are mutually insulated and magnetically coupled, converting the AC power supplied to the first and second primary windings into polyphase AC power, and supplying the polyphase AC power from the secondary winding to a load; A first power converter connected to the first primary winding for supplying first AC power to the first primary winding; A second power converter connected to the second primary winding for supplying second AC power to the second primary winding; and A filter circuit connected to the secondary winding to smooth the polyphase AC power.

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