Modular multilevel converter, flexible DC power transmission system and control method

Abstract

The invention provides a modular multilevel converter, a flexible direct current power transmission system and a control method, each of six bridge arms of the modular multilevel converter comprises a bridge arm inductor and at least one sub-module which are connected with each other, and each of the at least one sub-module comprises a bridge circuit, a first capacitor, a second capacitor, a third capacitor and a fourth capacitor, each bridge circuit is connected in parallel with a first capacitor; and the DC active power filter is connected in parallel with the two ends of the first capacitor.

Description

Technical Field

[0001] The present application relates to the field of wind power, and more specifically, to a modular multi-level converter, a flexible direct current transmission system and a control method. Background Art

[0002] The modular multilevel converter (MMC) has flexible control, strong scalability, low harmonic order, no need for commutation voltage, no need for filters and transformers, and can independently adjust active power and reactive power at the same time, making it suitable for occasions such as DC transmission.

[0003] However, for the single-phase bridge arm in the modular multilevel converter, the DC current, fundamental current and fundamental voltage produce power fluctuations of the fundamental frequency and the second frequency. The power fluctuation is reflected in the DC capacitor of the modular multilevel converter, which is manifested as the current of the DC capacitor having fundamental frequency and a small amount of second frequency fluctuation. Therefore, when selecting capacitors, it is necessary to select capacitors with larger capacitance to cope with current fluctuations. Therefore, the MMC module is larger in size and higher in cost. Summary of the invention

[0004] In order to solve at least one of the above technical problems, the present disclosure provides a modular multi-level converter.

[0005] One of the objectives of the present disclosure is to provide a modular multi-level converter capable of reducing current fluctuations on a DC capacitor.

[0006] According to a first aspect of the present disclosure, a modular multilevel converter is provided, wherein each of the six bridge arms of the three-phase modular multilevel converter comprises a connected bridge arm inductor and at least one submodule, wherein each of the at least one submodule comprises: a bridge circuit, each of the bridge circuits is connected in parallel with a first capacitor; and a DC active power filter, wherein the DC active power filter is connected in parallel to both ends of the first capacitor.

[0007] According to an embodiment of the present disclosure, the DC active power filter may be a bidirectional buck-boost conversion circuit.

[0008] According to an embodiment of the present disclosure, a bidirectional buck-boost conversion circuit may include: a first inductor, a first end of the first inductor is connected to one end of a first capacitor; a first switching power device, a first end of the first switching power device is connected to the second end of the first inductor, and the second end of the first switching power device is connected to the other end of the first capacitor; a second switching power device, a first end of the second switching power device is connected to the second end of the first inductor, and a second capacitor, a first end of the second capacitor is connected to the second end of the second switching power device, and the second end of the second capacitor is connected to the other end of the first capacitor.

[0009] According to an embodiment of the present disclosure, each of the first switching power device and the second switching power device may be connected in anti-parallel with a diode.

[0010] According to an embodiment of the present disclosure, the DC active power filter may be a dual-inductor bidirectional buck-boost conversion circuit.

[0011] According to an embodiment of the present disclosure, the number of at least one submodule in each bridge arm is N, the number of voltage levels outputted by each phase is M, and N and M may satisfy M=2N+1.

[0012] According to an embodiment of the present disclosure, the bridge circuit may be a half-bridge circuit or a full-bridge circuit, and each switching power device in each of the half-bridge circuit and the full-bridge circuit may be anti-parallel connected with a diode.

[0013] According to a second aspect of the present disclosure, a flexible direct current power transmission system is provided. The flexible direct current power transmission system may include the above-mentioned modular multi-level converter.

[0014] According to an embodiment of the present disclosure, the flexible direct current transmission system may further include: a current detector, which obtains the ripple current in the first capacitor and detects the output current of the direct current active power filter; a PI controller, which performs PI adjustment on the difference between the ripple current and the inverse current of the output current to obtain a control voltage; and a PWM module, which generates a PWM control signal for controlling the switching power device in the direct current active power filter according to the control voltage, so that the direct current active power filter generates a current with the same magnitude and opposite direction as the ripple current.

[0015] According to an embodiment of the present disclosure, a control method for a modular multilevel converter is provided. The modular multilevel converter is as described above, and the control method includes: obtaining a ripple current in a first capacitor; detecting an output current of a DC active power filter; inputting a difference between the ripple current and an inverted current of the output current into a PI controller to obtain a control voltage; generating a PWM control signal according to the control voltage; and controlling the DC active power filter based on the PWM control signal so that the DC active power filter generates a current having the same magnitude and opposite direction as the ripple current.

[0016] The modular multi-level converter and the control method thereof according to the embodiments of the present disclosure can reduce the volume and cost of the MMC module and improve the power density of the MMC module. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 2 is a diagram showing a topology structure of a modular multilevel converter according to an embodiment of the present disclosure.

[0018] Figure 2 2 is a diagram showing a half-bridge sub-module of a modular multi-level converter according to an embodiment of the present disclosure.

[0019] Figure 3 is a full-bridge sub-module illustrating a modular multi-level converter according to an embodiment of the present disclosure.

[0020] Figure 4 is a block diagram showing a flexible direct current transmission system according to an embodiment of the present disclosure.

[0021] Figure 5 is a flow chart illustrating a control method of a modular multilevel converter according to an embodiment of the present disclosure.

[0022] Hereinafter, the present disclosure will be described in detail with reference to the accompanying drawings, and the same or similar elements will be indicated by the same or similar reference numerals throughout the accompanying drawings. DETAILED DESCRIPTION

[0023] The following detailed description is provided to help gain a comprehensive understanding of the methods, devices and / or systems described herein. However, the order of operations described herein is only an example and is not limited to those orders set forth herein, but may be equivalently replaced or changed except for operations that must occur or be performed in a specific order. In addition, for greater clarity and simplicity, the description of content known in the art will be omitted or simplified.

[0024] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as those generally understood by those skilled in the art to which the present disclosure belongs after understanding the present disclosure. Unless explicitly defined as such herein, terms (such as those defined in general dictionaries) should be interpreted as having a meaning consistent with their meaning in the context of the relevant art and the present disclosure, and should not be interpreted in an idealized or overly formal manner.

[0025] Without special instructions, the same reference numerals generally refer to the same elements (e.g., components, steps, and methods). Reference numerals described in previous embodiments may be omitted if they appear again in later embodiments. In addition, the technical features described in different or the same embodiments may be combined in any manner, as long as the combined embodiments or technical solutions are complete and can solve the technical problems of the present application or achieve the technical effects described or not described in the present disclosure but can be determined based on the above complete technical solutions. The terms used in the present disclosure are explained below.

[0026] Modular multilevel converter (MMC) is an advanced power electronic converter that achieves high voltage and high power power conversion by cascading multiple sub-modules. MMC can overcome the limitations of traditional multilevel converters in high voltage and high power applications and provide higher efficiency, better power quality and higher reliability.

[0027] Each submodule of the modular multilevel converter of the present disclosure is provided with a DC active power filter (DC-APF), thereby reducing the DC fluctuation (for example, the fluctuation of the base frequency and a small amount of double frequency) of the DC capacitor in each submodule. The following is a detailed description in conjunction with the embodiments of the present disclosure.

[0028] Figure 1 is a diagram showing a topology of a modular multilevel converter according to an embodiment of the present disclosure, Figure 2 1 is a diagram showing a half-bridge submodule of a modular multilevel converter according to an embodiment of the present disclosure, Figure 3 is a full-bridge submodule showing a modular multilevel converter according to an embodiment of the present disclosure, Figure 4 is a block diagram showing a flexible direct current transmission system according to an embodiment of the present disclosure, Figure 5 is a flow chart illustrating a control method of a modular multilevel converter according to an embodiment of the present disclosure.

[0029] The modular multilevel converter according to the embodiment of the present disclosure applies a DC active power filter. Each of the three-phase six bridge arms of the modular multilevel converter according to the embodiment of the present disclosure includes a bridge arm inductor connected to each other and at least one sub-module.

[0030] Reference Figure 1 , taking the single-end system of flexible DC transmission as an example, A, B and C are connected to the AC three-phase system. Each phase of phase A, phase B and phase C includes two bridge arms, namely, an upper bridge arm and a lower bridge arm. Each bridge arm contains at least one submodule SM connected in series.

[0031] Reference Figure 1 Each bridge arm may further include a bridge arm inductor L, and the bridge arm inductor L may be connected to at least one submodule SM (e.g., connected in series), and the position of the bridge arm inductor L is not specifically limited. As an example, the bridge arm inductor L may be disposed between a common node of the upper and lower bridge arms and a submodule on the corresponding bridge arm.

[0032] N submodules can be set on each bridge arm, and the number of voltage levels output by each phase can be M, and M=2N+1 is satisfied between N and M. Specifically, as an example, each submodule can be in one of three states: input (capacitor charging), cut-off (capacitor discharging) or intermediate state (capacitor neither charging nor discharging). In the input state: the capacitor of the submodule is charged and the output is positive voltage; in the cut-off state: the capacitor of the submodule is discharged and the output is negative voltage; in the intermediate state, the capacitor of the submodule is neither charged nor discharged, and the output is zero voltage. Taking the upper bridge arm and the lower bridge arm as an example, each of which includes three submodules, the number of voltage levels output by each phase is 7.

[0033] Each submodule in the modular multilevel converter of the present disclosure may include a bridge circuit, which may be a half-bridge circuit or a full-bridge circuit, and each switching power device in each of the half-bridge circuit and the full-bridge circuit may be anti-parallel connected with a diode.

[0034] Reference Figure 2 , each submodule SM1, SM2... or SM N The bridge circuit may be a half-bridge circuit, wherein each of the first switching device T1 and the second switching device T2 is anti-parallel-connected with a diode, and the first switching device T1 and the second switching device T2 form a half-bridge structure, and are parallel-connected with a capacitor C (ie, a DC capacitor C).

[0035] Reference Figure 3 , each submodule SM1, SM2... or SM N The bridge circuit may be a full-bridge circuit, wherein the first switch device T1, the second switch device T2, the third switch device T3 and the fourth switch device T4 are connected in anti-parallel with diodes to form a full-bridge structure, and a capacitor C is connected in parallel.

[0036] The switch device or switch tube in each submodule (i.e., the switch tube in the bridge circuit, etc.) can be realized by IGBT, MOSFET, etc. Taking the modular multi-level converter of the embodiment of the present disclosure applied to the flexible DC transmission system as an example, in normal operation, the capacitor is switched on and off by connecting and disconnecting the upper and lower switch tubes to achieve the control target. In normal operation, DC current and AC current flow through the bridge arm. For a single-phase bridge arm, the DC current, fundamental current and fundamental voltage generate power fluctuations of the fundamental frequency and the double frequency. The power fluctuation is reflected in the DC capacitor of the MMC as the current of the DC capacitor has a fundamental frequency and a small amount of double frequency fluctuations.

[0037] In order to reduce the power fluctuation or DC fluctuation, in addition to the bridge circuit, at least one submodule SM1, SM2, . . . and SM N Each of them includes a DC active power filter (DC-APF).

[0038] The DC active power filter (DC-APF) disclosed in the present invention is connected in parallel to the DC capacitor. In other words, the DC active power filter (DC-APF) disclosed in the present invention is arranged in each submodule, rather than being connected across the upper and lower bridge arms. Compared with connecting the DC active power filter across the upper and lower bridge arms, since each DC-APF only processes local fluctuations, the control is more refined, and the fluctuation of the capacitance can be reduced more effectively. Local control makes the system respond faster to the fluctuation because global coordination is not required. In addition, when the DC active power filter (DC-APF) is set in each submodule, each submodule is independent, which is convenient for modular design and maintenance, and even if the DC-APF of a submodule fails, the other submodules can still work normally, which improves the reliability and redundancy of the system. Furthermore, the fluctuation of the capacitance of each submodule can be effectively controlled, the overall performance optimization is better, and the capacitance value can be reduced to a greater extent, further reducing the module volume and improving the power density.

[0039] DC active power filter (DC-APF) can be used to detect and compensate harmonic current, reactive current or unbalanced current in the system in real time to improve power quality and enhance system stability. DC active power filter offsets these undesirable current components by injecting compensation current that is equal in magnitude and opposite in direction to the harmonic current, reactive current or unbalanced current.

[0040] As an example, the DC active power filter (DC-APF) may be a buck converter, a boost converter, a buck-boost converter (eg, a bidirectional buck-boost converter), or the like.

[0041] As an example, when the DC active power filter (DC-APF) is implemented as a bidirectional buck-boost converter, the bidirectional buck-boost converter may be a single-inductor bidirectional buck-boost converter (which will be described in detail later), a dual-inductor bidirectional buck-boost converter (i.e., a bidirectional buck-boost converter including two inductors), a coupled-inductor bidirectional buck-boost converter, etc. A single-inductor bidirectional buck-boost converter is taken as an example for detailed description.

[0042] Reference Figure 2The DC active power filter DC-APF is a single-inductor bidirectional buck-boost converter, and the DC active power filter DC-APF is connected in parallel to the DC side port of the half-bridge. The capacitor C can be switched on and off by controlling the on and off of the first switch device T1 and the second switch device T2. The switch device in the bidirectional buck-boost converter is used to control the additional DC active power filter DC-APF to absorb the power fluctuations of the base frequency and the double frequency on the DC capacitor C, thereby reducing the power fluctuations and reducing the volume and size of the DC capacitor.

[0043] Specifically, the bidirectional buck-boost conversion circuit may include a first inductor L a , the first switching power device T3, the second switching power device T4, and the second capacitor C a .

[0044] Reference Figure 2 , the first inductor L a The first end of the first switching power device T3 is connected to one end of the first inductor L a The second end of the first switching power device T3 is connected to the other end of the first capacitor C, and the first end of the second switching power device T4 is connected to the first inductor L a The second end of the second capacitor C a The first end of the second capacitor C is connected to the second end of the second switching power device T4. a The second end of the first switching power device T3 can be connected to the other end of the first capacitor C. a The first end and the second end of the switching power device may be the input end and the output end of the switching power device, respectively.

[0045] Reference Figure 3 Taking the full-bridge circuit as an example, the bidirectional buck-boost conversion circuit as a DC active power filter may include a first inductor L a , a first switching power device T5, a second switching power device T6, and a second capacitor C a .

[0046] Reference Figure 3 , the first inductor L a The first end of the first switching power device T5 is connected to one end of the first inductor L a The second end of the first switching power device T5 is connected to the other end of the first capacitor C, and the first end of the second switching power device T6 is connected to the first inductor L a The second end of the second capacitor C a The first end of the second capacitor C is connected to the second end of the second switching power device T6.a The second end of the first switching power device T5 can be connected to the other end of the first capacitor C. a The second end.

[0047] In the submodule including the full-bridge circuit and the bidirectional buck-boost conversion circuit, the DC capacitor C is switched on and off by controlling the switching on and off of the first to fourth switching devices T1, T2, T3 and T4. The first switching power device T5 and the second switching power device T6 are used to control the additional DC active power filter DC-APF to absorb the power fluctuations of the base frequency and the double frequency on the DC capacitor C, thereby reducing the power fluctuations and reducing the capacity and volume of the DC capacitor.

[0048] When the voltage fluctuation on the DC capacitor C causes ripple current, the inductor La absorbs energy from the DC capacitor C (or the first capacitor) by controlling the on and off of the first switching power device. The energy on the inductor La is stored in the second capacitor Ca to smooth the voltage on the inductor La. When it is necessary to compensate for the ripple current on the capacitor C, the inductor La releases the energy back to the capacitor C by controlling the on and off of the second switching device T4. Therefore, the energy on the inductor La can be transferred through the capacitor Ca to compensate for the ripple current on the capacitor C, so that the capacitor C contains only the DC component as much as possible.

[0049] As described above, the DC active power filter of the present disclosure can be implemented in other ways besides the bidirectional buck-boost converter. In addition, the implementation of the bidirectional buck-boost converter is not limited to the single-inductor bidirectional buck-boost converter.

[0050] In addition, each of the first switching power device and the second switching power device (for example, a switching tube implemented by IGBT, MOSFET, etc.) in the bidirectional buck-boost converter can be anti-parallel connected with a diode. The diode here is a freewheeling diode, which has a freewheeling function, can protect the switching device from damage by reverse voltage, and can reduce current pulsation and improve system stability.

[0051] The modular multi-level converter disclosed in the present invention can be applied to a flexible direct current power transmission system, which can flexibly control active power and reactive power.

[0052] Reference Figure 4 In addition to the modular multilevel converter 410 , the flexible direct current transmission system 400 of the present disclosure may further include a current detector 420 , a PI controller 430 and a PWM modulation module 440 .

[0053] The current detector 420 may obtain the ripple current in the first capacitor and detect the output current of the DC active power filter (eg, the current flowing through the inductor in the DC active power filter, ie, the inductor current).

[0054] The current detector 420 may include a current sensor, a filter circuit, and a differential circuit. The current sensor may sample the current in the DC capacitor C in real time, the filter circuit may be a low-pass filter, the low-pass filter may extract the DC component from the sampled current, and the differential circuit may perform a difference between the sampled current in the DC capacitor C and the DC component to obtain the ripple current. The current detector 420 here may be implemented by a controller or processor such as an MCU or a DSP.

[0055] The PI controller 430 may perform PI regulation on the difference between the ripple current and the negative-phase current of the output current to obtain a control voltage.

[0056] That is to say, the ripple current can be used as the target value to control the DC active power filter DC-APF to generate a current with the same magnitude as the current ripple but opposite direction. The PI controller can be a proportional integral (PI) controller, which can adjust its output (control voltage) according to the error between the target value and the inductor current, thereby continuously optimizing the PWM signal so that the inductor current's reverse current tracks the ripple current.

[0057] The PWM modulation module 440 can generate a PWM control signal for controlling each switching power device (i.e., the first switching power device and the second switching power device) in the DC active power filter according to the control voltage output by the PI controller 430, so that the DC active power filter generates a current with the same magnitude and opposite direction as the ripple current. As an example, the duty cycle of the PWM control signal can be adjusted to adjust the current amplitude of the ripple current. The PWM modulation module 440 can also be implemented by an MCU, a DSP, etc.

[0058] Reference Figure 5 , the control method of the modular multilevel converter according to the embodiment of the present disclosure may include step S510, step S520, step S530, step S540 and step S550.

[0059] In step S510, a ripple current in the first capacitor is obtained. The ripple current can be implemented by hardware or software.

[0060] In step S520, the output current of the DC active power filter is detected. The inductor current of the DC active power filter can be fed back through a current sensor, and then fed back to the PI controller after being inverted.

[0061] In step S530 , the difference between the ripple current and the negative-phase current of the output current is input into a PI controller to obtain a control voltage.

[0062] As an example, a PI controller can continuously adjust the output according to the error between the target value and the feedback value to obtain different control voltages.

[0063] In step S540, a PWM control signal is generated according to the control voltage. The control voltage generated by the PI controller can be loaded into the PWM modulation module, and the PWM modulation module can generate a PWM control signal using the control voltage.

[0064] In step S550, the DC active power filter is controlled based on the PWM control signal so that the DC active power filter generates a current of the same magnitude and opposite direction as the ripple current. The current of the same magnitude as the ripple current here means that the generated current is the same as the ripple current within a certain error range.

[0065] Each step of the above control method can be implemented by a corresponding software or hardware module or unit.

[0066] The above has been referred to Figures 1 to 5 A modular multi-level converter, a flexible direct current transmission system, and a control method according to exemplary embodiments of the present disclosure are described. However, it should be understood that the units, modules, and systems shown in the accompanying drawings may be configured as software, hardware, firmware, or any combination of the above items to perform specific functions. For example, these systems and devices may correspond to dedicated integrated circuits, pure software codes, or modules that combine software and hardware. In addition, one or more functions implemented by these systems or devices may also be uniformly performed by components in physical entity devices (e.g., processors, clients, or servers, etc.).

[0067] For example, according to an exemplary embodiment of the present disclosure, a computer-readable storage medium storing instructions may be provided, wherein when the instructions are executed by at least one computing device, the at least one computing device is prompted to perform at least one of the above steps.

[0068] It should be noted that the control method according to the exemplary embodiment of the present disclosure may rely on the execution of a computer program or instructions to implement the corresponding functions, that is, each device corresponds to each step in the functional architecture of the computer program, so that the entire system is called through a special software package (e.g., lib library) to implement the corresponding functions.

[0069] On the other hand, when the system, unit, or module is implemented in software, firmware, middleware, or microcode, the program code or code segment for performing the corresponding operation may be stored in a computer-readable medium such as a storage medium, so that at least one processor or at least one computing device may perform the corresponding operation by reading and running the corresponding program code or code segment. In addition, the computer-readable medium or storage medium may cause the processor to execute the above-mentioned control method when the computer program is executed by the processor.

[0070] For example, according to an exemplary embodiment of the present disclosure, a computer device including a readable medium storing computer program instructions may be provided, wherein the instructions, when executed by at least one computing device, prompt the at least one computing device to perform at least one of the above steps.

[0071] According to an exemplary embodiment of the present disclosure, an MMC controller may be provided. The MMC controller may include a memory and a processor. The memory stores instructions or programs. When the instructions or programs are executed by the processor, the processor is prompted to execute the above-mentioned control method, for example, at least one of the various steps of the above-mentioned control method may be executed.

[0072] The modular multi-level converter and the control method thereof according to the embodiments of the present disclosure can reduce the current ripple on the capacitor.

[0073] The modular multi-level converter and the control method thereof according to the embodiments of the present disclosure can reduce the volume and cost of the module and improve the power density of the MMC module.

Claims

1. A modular multilevel converter, characterized in that: Each of the three-phase six bridge arms of the modular multilevel converter comprises a bridge arm inductor and at least one submodule connected to each other, wherein each of the at least one submodule comprises: A bridge circuit, each of the bridge circuits being connected in parallel with a first capacitor; A direct current active power filter is connected in parallel at both ends of the first capacitor.

2. The modular multilevel converter according to claim 1, characterized in that: The DC active power filter is a bidirectional buck-boost conversion circuit.

3. The modular multilevel converter according to claim 2, characterized in that: The bidirectional buck-boost conversion circuit comprises: a first inductor, wherein a first end of the first inductor is connected to one end of a first capacitor; A first switching power device, wherein a first end of the first switching power device is connected to a second end of the first inductor, and a second end of the first switching power device is connected to the other end of the first capacitor; a second switching power device, wherein a first terminal of the second switching power device is connected to a second terminal of the first inductor, A second capacitor, wherein a first end of the second capacitor is connected to a second end of the second switching power device, and a second end of the second capacitor is connected to the other end of the first capacitor.

4. The modular multilevel converter according to claim 3, characterized in that: Each of the first switching power device and the second switching power device is connected in anti-parallel with a diode.

5. The modular multilevel converter according to claim 2, characterized in that: The DC active power filter is a dual-inductor bidirectional buck-boost conversion circuit.

6. The modular multilevel converter according to any one of claims 1 to 5, characterized in that: The number of at least one submodule in each bridge arm is N, the number of voltage levels output by each phase is M, and N and M satisfy M=2N+1.

7. The modular multilevel converter according to claim 1, characterized in that: The bridge circuit is a half-bridge circuit or a full-bridge circuit, and each switching power device in each of the half-bridge circuit and the full-bridge circuit is anti-parallel connected with a diode.

8. A flexible direct current transmission system, characterized in that: Comprising a modular multilevel converter according to any one of claims 1 to 7.

9. The flexible direct current transmission system according to claim 8, characterized in that: The flexible direct current transmission system further comprises: A current detector, which obtains a ripple current in the first capacitor and detects an output current of the DC active power filter; A PI controller performs PI regulation on the difference between the ripple current and the reverse current of the output current to obtain a control voltage; The PWM modulation module generates a PWM control signal for controlling a switching power device in the DC active power filter according to the control voltage, so that the DC active power filter generates a current with the same magnitude and opposite direction as the ripple current.

10. A control method for a modular multilevel converter, characterized in that: The modular multilevel converter is as claimed in any one of claims 1 to 7, wherein the control method comprises: Obtaining a ripple current in the first capacitor; Detecting the output current of the DC active power filter; Inputting the difference between the ripple current and the anti-phase current of the output current into a PI controller to obtain a control voltage; generating a PWM control signal according to the control voltage; The DC active power filter is controlled based on the PWM control signal so that the DC active power filter generates a current having the same magnitude as the ripple current and opposite direction.

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