Control method for power conversion system and power conversion device

Abstract

This method reduces noise in frequency components related to the carrier frequency of PWM control in a simple way. [Solution] The power conversion system distributes power using an AC bus and a DC bus. The power conversion system comprises a power converter and a controller. The power converter is installed between the AC bus and the DC bus and is capable of converting AC power and DC power to each other. The controller controls the power converter by control modes including a DC-AC conversion mode that can supply AC power to the AC bus and an AC-DC conversion mode that converts the AC power supplied to the AC bus into DC power of a desired DC voltage.

Description

Technical Field

[0001] Embodiments of the present invention relate to a power conversion system and a method for controlling a power conversion device.

Background Art

[0002] Noise of frequency components related to the carrier frequency of PWM control may be included in the AC output of an inverter in a power conversion system.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present invention is to provide a power conversion system and a method for controlling a power conversion device that can reduce noise of frequency components related to the carrier frequency of PWM control in a simple manner.

Means for Solving the Problems

[0005] The power conversion system of the embodiment comprises a transformer, a first inverter, a second inverter, a first controller, and a second controller. The transformer comprises a magnetically coupled first primary winding and a second primary and secondary winding. The first inverter supplies a first three-phase AC power to the first primary winding. The second inverter supplies a second three-phase AC power to the second primary winding. The first controller PWM controls the first inverter at a modulation rate obtained by modulating a command value using a first carrier signal. The second controller PWM controls the second inverter at a modulation rate obtained by modulating the command value using a second carrier signal with a phase inverted to the phase of the first carrier signal. The first controller controls the first inverter to apply a three-phase AC voltage related to the command value to the first primary winding. The second controller controls the second inverter so as to apply a three-phase AC voltage to the primary winding of the second system such that the fundamental wave current flowing through the primary winding of the second system becomes zero. [Brief explanation of the drawing]

[0006] [Figure 1] A schematic diagram of the power conversion system according to the embodiment. [Figure 2] A schematic diagram of the control unit area of ​​the embodiment. [Figure 3] Configuration diagram of the control unit of the embodiment. [Figure 4A] A diagram illustrating the current control of the fundamental wave component of AC in an embodiment. [Figure 4B] A diagram illustrating the current control of the carrier frequency component of PWM control in an embodiment. [Figure 4C] A diagram illustrating the current control of the carrier frequency component in the comparative example of PWM control. [Figure 5] A diagram illustrating the generation of gate pulses for PWM control in an embodiment. [Modes for carrying out the invention]

[0007] The control methods for the power conversion system and power conversion device of the embodiment will be described below with reference to the drawings. In the following explanation, components with the same or similar functions will be denoted by the same reference numeral. Furthermore, redundant explanations of these components may be omitted. Note that the term "connected" may be used to simply refer to an electrical connection.

[0008] (Embodiment) An embodiment of the power conversion system will be described with reference to Figures 1 and 2. Figure 1 is a schematic diagram of the power conversion system 1 of the embodiment. Figure 2 is a schematic diagram of the area around the control unit of the embodiment.

[0009] The power conversion system 1 is configured to output at least one AC power stream from the transformer 2.

[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.

[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 type transformer. Each primary side circuit of transformer 2 has a winding for three-phase AC. There are two primary side circuits of transformer 2. The two windings shown in the figure, WPA and WPB, are examples of primary windings for the W phase of a three-phase AC power supply in transformer 2, which has a Y-connection for its primary winding. A more detailed configuration of transformer 2 will be described later. The secondary side of transformer 2 has windings for three-phase AC power. For example, if the primary winding of the transformer 2 shown in Figures 1 and 2 is connected in a Y configuration and the secondary winding S is connected in a delta configuration, then the primary and secondary sides can be associated, for example, with the winding between the RS 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 Figures 1 and 2 is connected in a delta configuration and the secondary winding S is connected in a wy configuration, then, for example, the T-phase winding corresponding to the primary side U-V line, the R-phase winding corresponding to the primary side VW line, and the S-phase winding corresponding to the primary side WU line can be associated. Furthermore, if both the primary and secondary windings S of the transformer 2 shown in Figures 1 and 2 are connected in a delta configuration, then, for example, the lines on the primary and secondary sides can be associated with each other. Furthermore, if both the primary and secondary windings S of the transformer 2 shown in Figures 1 and 2 are connected in a Y configuration, then, for example, the phases on the primary and secondary sides can be associated with each other. In the following explanation, we will use the example of transformer 2 in the first example above, where the primary winding is Y-connected and the secondary winding S is Δ-connected, as an illustration. However, we are not limited to this and may make appropriate changes based on the above correspondence. Transformer 2 is, for example, a step-up transformer with a transformation ratio a. When transformer 2 is a step-up type, the current flowing through the secondary winding becomes smaller than the current flowing through the primary winding. This reduces the heat generated by copper losses in the secondary winding.

[0012] The rectifier 3, for example, includes a smoothing capacitor and rectifies the AC power supplied from the AC power supply PSA, outputting DC power that has been smoothed by the smoothing capacitor. The rectifier 3 may be configured in multiple separate circuits.

[0013] The inverter 4 is installed on the load side of the rectifier 3. The inverter 4 is equipped with semiconductor switching elements, and generates AC power based on DC power through switching. The inverter 4 may be configured in multiple systems. The inverter 4 systems form a pair. The two inverters 4A and 4B shown in Figures 1 and 2 are examples of inverters for three-phase AC. For example, inverters 4A and 4B supply power to the primary windings of the W phase, as well as the primary windings of the U phase and the V phase.

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

[0015] The control unit 7 causes the inverter 4 to output AC power to the transformer 2 by switching each semiconductor switching element of the inverter 4. The above-described rectifier 3, inverter 4, and 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 a plurality of primary windings of multiple systems and one secondary winding, combines the AC power generated by two inverters into one system, and outputs it from the secondary winding of the transformer 2.

[0017] By operating an inverter by general PWM control, currents of the following frequency components are generated during this operation. a. Current component of the AC fundamental frequency b. Current components of the carrier frequency of PWM control and frequencies related to PWM control

[0018] Note that the carrier frequency of PWM control is the frequency of the carrier signal used in PWM control.

[0019] For example, when supplying a relatively small load capacitance from a converter having a sufficiently large output capacity for that load capacitance, noise due to the current component related to b above may become apparent. Methods for reducing the noise level of such unnecessary frequency components will be described in order.

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

[0021] The power conversion system 1 consists of two inverters, for example, inverter 4A as the first inverter and inverter 4B as the second inverter, configured in a corresponding manner. For the PWM control of the corresponding first and second inverters (inverter 4A and inverter 4B), it is preferable to use out-of-phase carriers as shown in Figure 5. For example, the first connecting conductor BUSA connects inverter 4A, which operates using PWM control of the first carrier (carrier signal CARA) of a pair of carrier signals CARB (a carrier signal CARA and a carrier signal CARB), to the first primary winding of transformer 2. The second connecting conductor BUSB connects the second inverter, which operates using PWM control of the second carrier (carrier signal CARB) that is paired with the first carrier, to the second primary winding of transformer 2. This reduces the effects of voltage imbalances in the windings. The power conversion system 1 includes the first connecting conductor BUSA and the second connecting conductor BUSB described above. The first connecting conductor BUSA and the second connecting conductor BUSB are an example of a busbar for connecting inverter 4A and the second inverter to transformer 2.

[0022] Furthermore, in this embodiment, a second method is applied to reduce the levels of the following unwanted frequency components. This reduces the occurrence of the above-mentioned events.

[0023] Furthermore, the transformer 2 is configured such that its two primary and secondary windings correspond to, for example, the UVW phases on the primary side. For example, each of the UVW phases has a common structure. The range shown in Figure 2 is for the W phase on the primary side, but the transformer 2 also has U and V phases in addition to this W phase. The rated voltage of transformer 2 used in practice is, for example, 600V on the primary side and 6600V on the secondary side, but it is not limited to this and may be changed as appropriate.

[0024] The control unit 7 of this embodiment comprises a controller 7A (first controller) and a controller 7B (second controller).

[0025] Controller 7A performs PWM control of inverter 4A using a modulation rate obtained by modulating the command value with the first carrier signal. Controller 7A controls inverter 4A so that a three-phase AC voltage relating to the command value is applied to the primary winding of the first system of transformer 2.

[0026] Controller 7B performs PWM control of inverter 4B using a second carrier (carrier signal CARB) with an inverted phase of the first carrier (carrier signal CARA), at a modulation rate modulated by the above command value. Controller 7B controls inverter 4B to apply a three-phase AC voltage to the primary winding of the second system of transformer 2 such that the fundamental wave current flowing through the primary winding of the second system becomes zero.

[0027] Figure 3 is a schematic diagram of the control unit 7 of the embodiment. The controller 7A includes, for example, a power control unit 171 (APR), subtractors 172d and 172q, a current control unit 174 (ACR), a dq inverse conversion unit 175, a PWM control unit 176, a current detection unit 177 (IDET), a dq conversion unit 178, and a reference phase generation unit 179.

[0028] The controller 7B includes, for example, a power control unit 271 (APR), subtractors 272d and 272q, LPF units 273d and 273q, current control unit 274 (ACR), dq inverse conversion unit 275, PWM control unit 276, current detection unit 277 (IDET), dq conversion unit 278, and reference phase generation unit 279.

[0029] Controllers 7A and 7B have similar basic configurations. Controller 7A will be explained first.

[0030] The power control unit 171 adjusts the amount of power output by the inverter 4A based on the command value for the amount of power conversion from the higher-level device. For example, it generates a current command value Id_refA such that the actual amount of power conversion is equal to the command value for the amount of power conversion from the higher-level device. The current command value Id_refA is supplied from the power control unit 171 to the subtractor 172d and to the subtractor 272d of the controller 7B.

[0031] The current control unit 174 generates a voltage command value Vdq_refA such that the calculation result of the subtractor 172d and the calculation result of the subtractor 172q are both 0 (zero). The subtractor 172d subtracts the current detection value Id_fbkA from the current command value Id_refA and outputs the deviation. The subtractor 172q subtracts the current detection value Iq_fbkA from the current reference value Iq_refA and outputs the deviation. The current reference value Iq_refA may be, for example, 0 (zero).

[0032] The dq inverse transform unit 175 performs a coordinate transformation on the two-phase component voltage command value Vdq_refA using the reference phase θ0A to generate the three-phase component voltage command value Vuvw_refA.

[0033] The PWM control unit 176 uses the carrier signal CARA to modulate the voltage command value Vuvw_refA by PWM and generates a gate pulse for inverter 4A.

[0034] The current detection unit 177 (IDET) detects the AC current of the inverter 4A and generates a current detection value Iuvw_fbkA. The dq conversion unit 178 performs a coordinate transformation of the current detection value Iuvw_fbkA using the reference phase θ0A to generate the current detection value Idq_fbkA.

[0035] The reference phase generation unit 179 generates a reference phase θ0A, a carrier signal CARA, and a synchronization signal.

[0036] The above is an example of a controller 7A using a PWM control method.

[0037] The power control unit 271 (APR), subtractors 272d and 272q, current control unit 274 (ACR), dq inverse conversion unit 275, PWM control unit 276, current detection unit 277 (IDET), dq conversion unit 278, and reference phase generation unit 279 of controller 7B correspond to the power control unit 171 (APR), subtractors 172d and 172q, current control unit 174 (ACR), dq inverse conversion unit 175, PWM control unit 176, current detection unit 177 (IDET), dq conversion unit 178, and reference phase generation unit 179 of controller 7A.

[0038] Controller 7B includes LPF sections 273d and 273q. The LPF sections 273d and 273q are placed between the output of the dq conversion unit 178 and the inputs of the subtractors 272d and 272q. The LPF sections 273d and 273q are low-pass filters that allow the fundamental frequency component of the AC to pass through and attenuate the harmonic components of the fundamental frequency of the AC. Due to the presence of the LPF sections 273d and 273q, the low-frequency component Idb_lfb of the detected current value Idq_fbkB output from the dq conversion unit 278 is generated.

[0039] Subtractor 272d subtracts the current detection value Id_lpfB from the current command value Id_refA and outputs the deviation. Subtractor 272q subtracts the current detection value Iq_lpfB from the current reference value Iq_refB and outputs the deviation. The current reference value Iq_refB may be, for example, 0 (zero).

[0040] The current control by the power conversion system 1 configured as described above will be explained with reference to Figures 4A to 4C. Figure 4A is a diagram illustrating the current control of the fundamental wave component of AC in the embodiment. Figure 4B is a diagram illustrating the current control of the carrier frequency component of PWM control in the embodiment. Figure 4C is a diagram illustrating the current control of the carrier frequency component of PWM control in the comparative example.

[0041] As shown in Figure 4A, the fundamental wave current component of the AC current passes through transformer 2 and flows from inverter 4A to the AC load. By the way, in the comparative example where inverter 4A and inverter 4B are simply controlled independently, a portion of the AC fundamental wave current component circulates and becomes unusable, resulting in the generation of AC fundamental wave current components. Therefore, in this embodiment, when controlling inverter 4A and inverter 4B independently, the output current of inverter 4B is observed and controlled so that the circulation of its AC fundamental wave current component is eliminated. As a result, the AC fundamental wave current component that cannot be used in the comparative example is canceled out, enabling more efficient power conversion.

[0042] Figure 4C also shows the comparative example power conversion system 1Z. In the case of the comparative example power conversion system 1Z, inverter 4Z is often used alone. In such cases, the carrier frequency component of the PWM control passes through transformer 2 and flows from inverter 4Z to the AC load. When filter 5 is installed in between to attenuate the carrier frequency component, it was sometimes difficult to attenuate it sufficiently, depending on the frequency characteristics of filter 5.

[0043] Therefore, as shown in Figure 4B, in the case of the power conversion system 1 of this embodiment, inverters 4A and 4B are used in combination. As has already been explained, this has the effect of canceling out the fundamental wave current component of the AC. In addition to this, by synchronizing the carriers of the PWM control and making their phases inverse, the frequency component of the current pulse generated by the PWM control becomes twice as high and is output to the AC load side. As a result, if the cutoff frequency of filter 5 is the same as in the example in Figure 4C, the attenuation of the frequency component of the current pulse generated by PWM control will be large.

[0044] Figure 5 is a diagram illustrating the generation of gate pulses for PWM control in an embodiment. Figures 5(a) and (b) show examples of carrier signals for PWM control of inverters 4A and 4B. The phases of the respective carrier signals are inverse. Figures 5(c) and 5(d) show the timing of the gate pulses that occur when the same signal level reference level is set. As described above, because the phases of the respective carrier signals are inverse phase, a phase difference of 180 occurs in the timing of when the gate pulses are generated.

[0045] Figure 5(e) shows a schematic diagram of the pulse current flowing through transformer 2 due to the gate pulses shown in Figures 5(c) and (d) above. Figure 5(ze) shows an example of the pulse current in the case of power conversion system 1Z of the comparative example described above. As can be seen from the example in Figure 5(e), the frequency of the pulse current flowing through transformer 2 can be changed using a simple method.

[0046] According to the above embodiment, the power conversion system 1 comprises a transformer 2, an inverter 4A, an inverter 4B, a controller 7A, and another controller 7B. The transformer 2 comprises a magnetically coupled first primary winding and a second primary and secondary winding. Controller 7A controls inverter 4A to apply a three-phase AC voltage corresponding to the command value to the first primary winding of the transformer 2. As a result, inverter 4A supplies first three-phase AC power to the first primary winding of the transformer 2. Controller 7B controls inverter 4B to apply a three-phase AC voltage to the second primary winding of the transformer 2 such that the fundamental wave current flowing through the second primary winding becomes zero. As a result, inverter 4B supplies second three-phase AC power to the second primary winding of the transformer 2. Controller 7A also performs PWM control of inverter 4A with a modulation rate obtained by modulating the command value using a first carrier signal. Controller 7B uses a second carrier signal (CARB) with a phase inverted to that of the first carrier signal (CARA) to control inverter 4B with PWM at a modulation rate modulated from the command value. This allows for a simple reduction of noise in frequency components related to the carrier frequency of the PWM control.

[0047] For example, it is advisable to configure the power conversion system 1 by matching the rated capacities of inverters 4A and 4B. In this case, a redundant configuration can also be achieved by combining inverters 4A and 4B. For example, if the power conversion system 1 is in a normal state, the control unit 7 controls the system to reduce noise in frequency components related to the carrier frequency of the PWM control. If an abnormality occurs in the power conversion system 1, the control unit 7 should suspend the control to reduce noise in the frequency components related to the carrier frequency of the PWM control, and, taking advantage of the above redundant configuration, use the normal units to control the power conversion system 1 in a way that increases its operating rate.

[0048] (modified version) Let me explain one variation. The case in which the rated capacities of inverter 4A and inverter 4B of the power conversion system 1 of the above embodiment are matched has been described. This section describes the case where the rated capacities of inverter 4A and inverter 4B in this modified example are not matched. The degree of mismatch in rated capacities is assumed to be sufficiently large compared to the range of variation caused by individual differences among units with a common rated capacity.

[0049] For example, suppose the rated capacity of inverter 4A is greater than the rated capacity of inverter 4B. In this case, by supplying more load current from inverter 4A, the current component flowing through inverter 4B can be reduced.

[0050] Furthermore, for example, when selecting between inverter 4A and inverter 4B from a particular manufacturer's product line, the rated capacity of inverter 4B may be the smaller product within that line. Needless to say, the actual selection should be made from products capable of supplying the required current capacity.

[0051] According to at least one embodiment described above, the power conversion system comprises a transformer, a first inverter, a second inverter, a first controller, and a second controller. The transformer comprises a magnetically coupled first primary winding and a second primary and secondary winding. The first inverter supplies a first three-phase AC power to the first primary winding. The second inverter supplies a second three-phase AC power to the second primary winding. The first controller PWM controls the first inverter at a modulation rate obtained by modulating a command value using a first carrier signal. The second controller PWM controls the second inverter at a modulation rate obtained by modulating the command value using a second carrier signal with a phase inverted to the phase of the first carrier signal. The first controller controls the first inverter to apply a three-phase AC voltage related to the command value to the first primary winding. The second controller controls the second inverter to apply a three-phase AC voltage to the primary winding of the second system such that the fundamental wave current flowing through the primary winding of the second system becomes zero. This makes it possible to reduce noise of frequency components related to the carrier frequency of PWM control in a simple manner.

[0052] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel 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. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents. Furthermore, the embodiments described above can be implemented in combination with each other.

[0053] (Note) The embodiment can be configured as follows. (1) A transformer comprising a magnetically coupled primary winding of a first system and a primary and secondary winding of a second system, A first inverter that supplies first three-phase AC power to the primary winding of the first system, A second inverter supplies a second three-phase AC power to the primary winding of the second system, A first controller that PWM controls the first inverter with a modulation rate obtained by modulating the command value using the first carrier signal, A second controller that PWM controls the second inverter with a modulation rate obtained by modulating the above command value using a second carrier signal having a phase inverted to the phase of the first carrier signal, Equipped with, The first controller is, The first inverter is controlled to apply a three-phase AC voltage corresponding to the command value to the primary winding of the first system. The second controller is, The second inverter is controlled to apply a three-phase AC voltage to the primary winding of the second system such that the fundamental wave current flowing through the primary winding of the second system becomes zero. Power conversion system. (2) In the power conversion system relating to (1) above, The first controller and the second controller are: The PWM control is performed using the respective carrier signals, with the frequencies of the carrier signals being equal and the phases of the carrier signals being inverse. (3) In the power conversion system relating to (1) or (2) above, The rated output capacity of the first inverter is greater than the rated output capacity of the second inverter. (4) A transformer having magnetically coupled primary and secondary windings for a first and second three-phase AC system, A first inverter that supplies first three-phase AC power to the primary winding of the first system, A second inverter supplies a second three-phase AC power to the primary winding of the second system, A first controller that PWM controls the first inverter with a modulation rate obtained by modulating the command value using the first carrier signal, A second controller that PWM controls the second inverter with a modulation rate obtained by modulating the above command value using a second carrier signal having a phase inverted to the phase of the first carrier signal, A control method for a power converter equipped with, The second controller, The first inverter is controlled to apply a three-phase AC voltage corresponding to the command value to the primary winding of the first system. The second controller, The second inverter is controlled to apply a three-phase AC voltage to the primary winding of the second system such that the fundamental wave current flowing through the primary winding of the second system becomes zero. A method for controlling a power converter. [Explanation of Symbols]

[0054] 1. Power Conversion System 2 Transformers 3 Rectifier 4, 4A, 4B inverter 5 Filters 7 Control Unit 7A, 7B Controllers 10 Power converter

Claims

1. A transformer comprising a magnetically coupled primary winding of a first system and a primary and secondary winding of a second system, A first inverter that supplies first three-phase AC power to the primary winding of the first system, A second inverter supplies a second three-phase AC power to the primary winding of the second system, A first controller that PWM controls the first inverter with a modulation rate obtained by modulating the command value using the first carrier signal, A second controller that PWM controls the second inverter with a modulation rate obtained by modulating the command value using a second carrier signal having a phase inverted to the phase of the first carrier signal, Equipped with, The first controller is, The first inverter is controlled to apply a three-phase AC voltage corresponding to the command value to the primary winding of the first system. The second controller is, The second inverter is controlled to apply a three-phase AC voltage to the primary winding of the second system such that the fundamental wave current flowing through the primary winding of the second system becomes zero. Power conversion system.

2. The first controller and the second controller are, The PWM control is performed using the respective carrier signals, with the frequencies of the carrier signals being equal and the phases of the carrier signals being inverse. The power conversion system according to claim 1.

3. The rated output capacity of the first inverter is greater than the rated output capacity of the second inverter. The power conversion system according to claim 1.

4. A transformer comprising magnetically coupled primary and secondary windings for a first and second three-phase AC system, A first inverter that supplies first three-phase AC power to the primary winding of the first system, A second inverter supplies a second three-phase AC power to the primary winding of the second system, A first controller that PWM controls the first inverter with a modulation rate obtained by modulating the command value using the first carrier signal, A second controller that PWM controls the second inverter with a modulation rate obtained by modulating the command value using a second carrier signal having a phase inverted to the phase of the first carrier signal, A control method for a power converter equipped with, The first controller, The first inverter is controlled to apply a three-phase AC voltage corresponding to the command value to the primary winding of the first system. The second controller, The second inverter is controlled to apply a three-phase AC voltage to the primary winding of the second system such that the fundamental wave current flowing through the primary winding of the second system becomes zero. A method for controlling a power converter.

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