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 secondary winding (WS) and a plurality of primary windings (WPA-WPD) that are insulated from one another and magnetically coupled to one another, converts alternating current power supplied to the primary windings into multi-phase alternating current power, and supplies the multi-phase alternating current power from the secondary windings to a load. The power converter supplies alternating current power to each of the primary windings. The filter circuit is connected to the secondary winding and smooths the multi-phase alternating current power.

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 uses a transformer having multiple primary windings and one secondary winding to combine AC power generated by multiple inverters (power converters) into a single system for output. To prevent a decrease in the quality of the AC power output by such a power conversion system, a filter is provided between the output of each inverter and the transformer. Increasing the number of inverters to increase the scale of the power conversion system increases the size of the filter, which can make it difficult to configure. Even in such cases, there is a demand for a system that can further improve the quality of the AC power output by the power conversion system.

[0003] JP 2015-133856 A

[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] According to an embodiment, a 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 AC power supplied to each primary winding into multi-phase AC power, and supplies the multi-phase AC power from the secondary windings to a load. The power converter supplies AC power to each primary winding. The filter circuit is connected to the secondary windings and smooths the multi-phase AC power.

[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 diagram for explaining a filter according to an embodiment; 4 is a diagram for explaining a filter according to an embodiment; 5 is a diagram for explaining an example of frequency characteristics of an equivalent circuit of a transformer 2 and a filter 5 according to an embodiment; 6 is a circuit diagram of a power conversion system 1 to which a filter 5 according to an embodiment is applied; 7 is a circuit diagram of a power conversion system 1 to which a filter 5 according to an embodiment is applied; 8 is a diagram for explaining a winding configuration of a transformer according to an embodiment;

[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 while 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 peripheral circuitry of the transformer.

[0010] 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. In contrast, the power conversion system 1Z includes a filter 6 instead of the filter 5 of the power conversion system 1. These differences will be explained below in order.

[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. There are multiple primary windings of transformer 2. The number of primary windings of transformer 2 can be expressed as (2N), where N is a natural number. The following description will be given assuming that (2N) is four. The bank configuration shown in FIGS. 1A and 1B is a four-bank configuration, and 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. The same explanation regarding the number of banks will be used hereinafter. The secondary side of the transformer 2 has windings for three-phase AC. 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. Since heat generation due to copper loss in the secondary winding is reduced, the cross-sectional area of ​​the secondary winding may be appropriately reduced accordingly. A more detailed configuration of the transformer 2 will be described later. Note that the following description will be given using an example in which the primary winding of the transformer 2 in the first example above is Y-connected and the secondary winding S is Delta-connected, but the present invention is not limited to this and the connection configuration of the primary winding and secondary winding S may be appropriately changed.

[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 three-phase inverters 4A, 4B, 4C, and 4D shown in FIGS. 1A and 1B are examples of three-phase AC inverters. For example, the three-phase inverters 4A, 4B, 4C, and 4D supply power to each primary winding of the W-phase, as well as each primary winding of the U-phase and each primary winding of the V-phase.

[0014] 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.

[0015] The filter 5 of the embodiment is composed of passive elements and is connected between each line on the secondary side of the transformer 2. The filter 5 acts as a capacitive load on the secondary winding of the transformer 2. The filter 5 includes a circuit that exhibits capacitive load characteristics. The filter 5 is configured to transmit the fundamental wave component of the polyphase AC power output from the secondary side of the transformer 2, and to attenuate normal mode noise components generated by switching of semiconductor elements in the inverter 4 (power converter), using the inductive component of the transformer 2 and the capacitive component of the capacitive load characteristics.

[0016] The filter 5 includes, for example, a capacitor 5 A. In this configuration, the capacitance of the capacitor 5 A may be used as a capacitance component of the circuit that indicates the capacitive load characteristic of the filter 5 .

[0017] The filter 5 includes, for example, a set of a resistor and a capacitor (5A) connected in series.

[0018] The filter 5 is preferably provided between the secondary winding of the transformer 2 and a power distribution switch for disconnecting the load and the connecting cable.

[0019] 4A and 4B show an example of a specific circuit to which the filter 5 of the embodiment is applied. 4A and 4B are circuit diagrams of the power conversion system 1 to which the filter 5 of the embodiment is applied.

[0020] The four primary windings PA, PB, PC, and PD of the transformer 2 shown in Figures 4A and 4B are Y-connected. The primary winding of the transformer 2 shown in Figure 4A is configured as a Y-connection, and the secondary winding is configured as a Δ-connection. The filter 5B (5) is configured as a Δ-connection. By connecting the transformers in this manner, the Δ-connection of the secondary winding of the transformer 2 and the Δ-connection of the filter 5 are connected in parallel. Also, the primary winding of the transformer 2A shown in Figure 4B is configured as a Δ-connection, and the secondary winding is configured as a Y-connection. The filter 5B (5) is similarly configured as a Δ-connection. By connecting the transformers in this manner, the elements of the filter 5 are connected between the wires of the secondary winding of the transformer 2. Note that the winding configurations of the transformers 2 and 2A shown above are merely examples and are not intended to be limiting. For example, the primary winding of the transformer 2 shown in Fig. 4A may be configured as a delta connection, and the primary winding of the transformer 2A shown in Fig. 4B may be configured as a wye connection. For example, a transformer with a winding configuration as shown in Fig. 5 may be used. Fig. 5 is a diagram for explaining an example of the winding configuration of a transformer.

[0021] In the case of the filter 5, only a capacitor C or a combination of a resistor R and a capacitor C is provided between the lines on the secondary side. In this case, three capacitors C are required. Three resistors may also be provided as needed.

[0022] 1B 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 and 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. This filter 6 is Y-connected. In the case of the filter 6, it is necessary to provide a set of a reactor L, a resistor R, and a capacitor C for each phase of each bank. In the case of a three-phase circuit with four banks, 12 sets of a reactor L, a resistor R, and a capacitor C are required.

[0023] As described above, the filter 5 of the embodiment differs from the filter 6 of the comparative example in both configuration and number of circuits. The filter 6 of the comparative example processes the AC power generated by each inverter individually, so in a four-bank configuration, four systems would be required. By using the filter 5 of the embodiment in the configuration, the number of component parts can be reduced as described above. Note that if the secondary winding of the transformer 2 is delta-connected as shown in FIG. 1, the elements of the filter 5 and the transformer secondary winding will be connected in parallel with each other.

[0024] Furthermore, the current flowing from inverter 4 to filter 6 contains a current based on the AC power generated by each inverter, as well as a current containing noise components. The presence of filter 6 attenuates these noise components. Without filter 6, the noise components would flow into transformer 2 without being attenuated as described above. In other words, normal mode noise components generated by the switching of semiconductor elements in inverter 4 are supplied to the primary winding of transformer 2. These noise components are generated, for example, in synchronization with the switching timing of each inverter.

[0025] Therefore, in the configuration of the embodiment, the transformer 2 is combined with the filter 5. This allows the frequency characteristics of the magnetic permeability of the core of the transformer 2 to be used to attenuate noise components. In addition, the frequency characteristics of the inductive component of the transformer 2 and the capacitive component of the capacitive load characteristics of the filter 5 can be used to attenuate noise components. These noise components manifest as normal mode noise. Attenuating the noise components can further improve the quality of the output AC power.

[0026] The power conversion system 1 configured as described above uses, for example, a multi-winding transformer 2 having multiple primary windings and one secondary winding to combine AC power generated by multiple inverters into a single system and output the combined power.

[0027] 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).

[0028] The transformer 2 has first to fourth primary windings (for example, W-phase WPA, WPB, WPC, and WPD) that are insulated from one another and magnetically coupled to one another, and a secondary winding S. The filter 5 is connected to the secondary winding S.

[0029] The inverter 4A is connected to a primary winding WPA (first primary winding) and supplies first AC power to the primary winding WPA. The inverter 4B is connected to a primary winding WPB (second primary winding) and supplies second AC power to the primary winding WPB. The inverter 4C is connected to a primary winding WPC (third primary winding) and supplies third AC power to the primary winding WPC. The inverter 4D is connected to a primary winding WPD (fourth primary winding) and supplies fourth AC power to the primary winding WPD.

[0030] The transformer 2 converts 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 a load.

[0031] When the inverter 4 is operated by general PWM control, the following frequency components of current are generated during operation: a. Current component of the fundamental frequency of AC b. Current component of harmonic frequencies of the fundamental frequency of AC c. Current component of the carrier frequency of PWM control and frequencies related to PWM control

[0032] The carrier frequency of PWM control refers to the frequency of the carrier signal of PWM control.

[0033] Of the above, the current components related to b and c contain frequency components that are essentially unnecessary. Methods for reducing the levels of such unnecessary frequency components will be explained below.

[0034] First, a first method for reducing the level of unnecessary frequency components will be described.

[0035] If the switching timing of each semiconductor switching element of the multiple inverters 4 is synchronized, the level of unnecessary frequency components will increase in synchronization with this timing. For example, 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 for PWM control by each inverter is shifted by 360 / N degrees, thereby making it possible to suppress some of the unnecessary current components listed below. For example, N is the number of inverters. In this power conversion system 1, N is 4.

[0036] 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 is not suppressed and may remain as a harmonic current. In this embodiment, the filter 5 is used to reduce noise such as harmonic currents, thereby smoothing the multiphase AC power. As described above, the filter 5 of this embodiment does not individually process the AC power generated by each inverter, but performs filtering on the multiphase AC power that is combined and output into a single system. The filter 5 attenuates noise components superimposed on the current based on the multiphase AC power that the transformer 2 combines and outputs into a single system.

[0037] Next, a specific example of the filter 5 will be described with reference to Figures 2A and 2B. Figures 2A and 2B are diagrams for explaining the filter 5 according to the embodiment. The filter 5A shown in Figure 2A includes a capacitor. The filter 5B shown in Figure 2B includes a capacitor and a resistor. This resistor is a so-called damping resistor.

[0038] Transformer 2 is an isolated transformer with a separated primary and secondary winding. This transformer 2 is modeled as a single-phase circuit, and the windings and magnetic circuit of transformer 2 are equivalent to inductance L. This modeling allows the equivalent circuit of transformer 2 and filter 5 to be converted as shown in Figure 2A or 2B.

[0039] 3 is a diagram illustrating an example of frequency characteristics of an equivalent circuit of the transformer 2 and the filter 5 according to the embodiment. The resonance frequency of the inductance L and the capacitor C is calculated by the following equation (1).

[0040] fc=1 / (2π√(LC)) (1)

[0041] The equivalent circuit of the transformer 2 and the filter 5 functions as a second-order low-pass filter. For example, assume that the resonant frequency of the inductance L and the capacitor C in the equivalent circuit of the transformer 2 and the filter 5 is 100 kHz. According to this equivalent circuit, above this frequency, the attenuation characteristic exhibits a gradient of 40 dB / dec, and the higher the frequency, the greater the attenuation amount.

[0042] For example, if the carrier frequency fcar of the inverter 4 is 1 MHz, the attenuation of the carrier frequency fcar is −40 dB compared to 0 dB in the low range, so it can be seen that the components of the carrier frequency fcar can be sufficiently attenuated.

[0043] If necessary, a circuit (FIG. 2B) equipped with a damping resistor R may be used. The frequency characteristics of the filter 5A shown in FIG. 2A have a resonance point, which may amplify frequency components near the resonance point. To prevent such components from being amplified, a damping resistor may be provided to adjust the frequency characteristics to the desired one.

[0044] According to at least one embodiment described above, a 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 AC power supplied to each primary winding into multi-phase AC power, and supplies the multi-phase AC power from the secondary windings to a load. The power converter supplies AC power to each primary winding. The filter circuit is connected to the secondary winding and smooths the multi-phase AC power, thereby further improving the quality of the output AC power.

[0045] 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.

[0046] 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.

[0047] 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 group of inverters 4, thereby halving the processing load of the PWM control. For example, the illustrated inverter 4 configuration is not limited, and may be either a half-bridge type or a full-bridge type. Each inverter (4A, 4B, 4C, 4D) may be a 180-degree conduction type or a 120-degree conduction type. The voltage waveform (current waveform) formed by combining the outputs of each inverter is not limited, and any desired waveform can be selected. For example, while the example shows a case in which each inverter outputs one voltage pulse (current pulse) per PWM control cycle, this is not limited, and multiple pulses may be output per cycle. Furthermore, for example, the secondary voltage waveform (current waveform) synthesized from the outputs of each inverter may be multilevel.

[0048] REFERENCE SIGNS LIST 1... Power conversion system 2... Transformer 3... Rectifier 4... Inverter 5... Filter 7... Control unit 10... Power conversion device

Claims

1. a transformer having a plurality of primary windings and secondary windings that are insulated from each other and magnetically coupled to each other, converting AC power supplied to each primary winding into polyphase AC power and supplying the polyphase AC power from the secondary windings to a load; a power converter for supplying AC power to each of the primary windings; a filter circuit including a Δ-connection capacitor connected to the secondary winding or a set of a resistor and a capacitor connected in series in a Δ-connection, and smoothing the polyphase AC power; Equipped with a power conversion system.

2. The filter circuit is composed of a capacitor in the delta connection or a passive element consisting of a pair of the resistor and capacitor in the delta connection, and acts as a capacitive load on the secondary winding. The power conversion system of claim 1 .

3. The filter circuit comprises: The power converter includes a capacitor in the delta connection that exhibits capacitive load characteristics or a combination of the resistor and capacitor in the delta connection, and transmits the fundamental wave component of the polyphase AC power by the inductive component of the transformer and the capacitive component of the capacitive load characteristics, and attenuates normal mode noise components generated by switching of semiconductor elements of the power converter. The power conversion system of claim 1 .

4. The filter circuit comprises: The capacitance of the capacitor is used as a capacitance component of a circuit that includes a capacitor in the delta connection and exhibits the capacitive load characteristics. The power conversion system according to claim 3 .

5. (delete)

6. Each of the power converters is supplying normal mode noise components generated by switching of semiconductor elements of each of the power converters to a primary winding of the transformer; By combining the transformer and the filter circuit, The frequency characteristics of the magnetic permeability of the core of the transformer and the frequency characteristics due to the inductive component of the transformer and the capacitive component of the capacitive load characteristic are utilized to attenuate the normal mode noise component. The power conversion system according to claim 3 .

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

8. a transformer having first and second primary windings and secondary windings that are insulated from each other and magnetically coupled to each other, converting AC power supplied to the first and second primary windings into polyphase AC power and supplying the polyphase AC power from the secondary windings to a load; a first power converter connected to the first primary winding to supply first AC power to the first primary winding; a second power converter connected to the second primary winding to supply second AC power to the second primary winding; a filter circuit including a Δ-connection capacitor connected to the secondary winding or a set of a resistor and a capacitor connected in series in a Δ-connection, and smoothing the polyphase AC power; Equipped with a power conversion system.