Metal-oxide-semiconductor field effect transistor (MOSFET) module cascade-based network-forming type converter and control method thereof

Abstract

The invention discloses a grid construction type converter based on MOSFET module cascade connection, and discloses a corresponding control method of the grid construction type converter based on MOSFET module cascade connection, the MOSFET has the characteristics of strong overcurrent capability and high switching frequency, the characteristics are very accordant with the current emerging grid construction type control, and a high-frequency cooperative modulation strategy of NLM and PWM is provided. According to the strategy, most sub-modules can adopt an NLM modulation strategy to generate a step wave main body part of output voltage, and the sub-modules mainly work at lower equivalent switching frequency (such as close to fundamental wave frequency) to control switching loss. One or a few sub-modules are assigned to work in a high-frequency PWM mode, and step waves generated by NLM modulation are subjected to fine correction.

Description

Technical Field

[0001] This invention relates to the field of power system safety and stability control, and in particular to a grid-type converter based on cascaded MOSFET modules and its control method. Background Technology

[0002] With the increasing proportion of installed capacity from new energy sources, the energy structure of the power grid will undergo significant changes in the future. The power generation capacity of new energy sources is limited by fluctuations in wind and solar resources, making it difficult to achieve power balance between sources and loads. Problems and contradictions related to wind and solar curtailment, grid stability, and power quality are becoming increasingly prominent. New energy sources are integrated into the grid via power electronic converters. Currently, based on the different synchronization methods with the grid, converters are generally divided into grid-following converters and grid-connecting converters. Grid-following converters often become unstable due to disturbances in systems with low grid strength, even causing new energy installations to disconnect from the grid. Grid-connecting converters, on the other hand, have a synchronization method similar to a synchronous machine. They can adjust the frequency of the reference voltage through power exchange to achieve synchronization with the grid, and their inertia ensures stable operation even under disturbances.

[0003] Currently, there are two common approaches to converter design: Modular Multilevel Converter (MMC) and H-bridge cascade. The MMC approach is frequently used in flexible DC transmission, allowing for different output levels through control. It offers advantages such as low harmonic content, high power quality, and high reliability. The commonly used power electronic device in the MMC approach is the Insulated Gate Bipolar Transistor (IGBT), with some applications using Integrated Gate Commuted Thyristors (IGCT). Both of these power electronic devices have been used in engineering practice, but they have low switching frequencies and high costs. In contrast, MOSFETs, also fully controllable devices, offer advantages such as high switching frequencies and lower costs; therefore, large-capacity converters using MOSFETs are less common.

[0004] The working principle of MOSFETs is highly compatible with grid-based control. Taking the N-channel enhancement-mode MOSFET as an example, it uses a lightly doped P-type silicon wafer as a substrate. Two heavily doped N+ regions are created using a diffusion process, and two electrodes are introduced: the source (S) and the drain (D). A SiO2 insulating layer is fabricated on the semiconductor, followed by an aluminum (Al) layer, which serves as the gate (G). Typically, the substrate and source are connected together. Thus, the gate and substrate each act as an electrode, with the insulating layer in between forming a capacitor. When the gate-source voltage changes, it alters the amount of induced charge near the insulating layer on the substrate, thereby controlling the drain current. In simpler terms, controlling the gate voltage controls the switching of the device, while grid-based control acquires the converter's output power, decouples it to obtain the voltage amplitude and phase, and then precisely controls the converter to adjust the output power. Therefore, grid-based control is a perfect fit for MOSFET devices. The combination of these two technologies offers several advantages: MOSFETs' high-frequency switching capability facilitates the generation of high-quality, controllable three-phase sinusoidal voltage, providing a stable voltage and frequency reference for the power grid; MOSFETs' extremely fast switching speed (microsecond or even sub-microsecond response) allows control algorithms to rapidly adjust power output, achieving millisecond- and sub-microsecond-level support for grid frequency and voltage; MOSFETs' fast control loop effectively suppresses oscillations prone to occur in weak grids, eliminating the need for phase-locked loops to track unstable grid voltages; and advanced control algorithms (such as virtual impedance and current loop optimization) fully utilize the overcurrent capability of MOSFETs, providing several times the rated short-circuit current during faults to aid grid recovery. Specific control logic is detailed in the grid-type control module section of the invention. However, the voltage withstand capability of a converter circuit composed of a single MOSFET device is insufficient for direct grid connection. Modular cascading of the MOSFET-based converter submodules is necessary to improve its voltage withstand level for grid connection at voltage levels of 10kV and above. Summary of the Invention

[0005] This invention aims to address at least one of the technical problems existing in the prior art. To this end, this invention proposes a MOSFET module-level cascaded grid-type inverter architecture, which is lower in cost and more suitable for new power systems than traditional architectures. It can quickly respond to grid-side disturbances and power output fluctuations from renewable energy sources, and is more conducive to smoothing out power output fluctuations from renewable energy sources, achieving system power balance, and improving the safe and stable operation of the power grid.

[0006] The present invention also proposes a control method for the above-mentioned grid-type converter system based on cascaded MOSFET modules.

[0007] A grid-type converter based on cascaded MOSFET modules according to a first aspect embodiment of the present invention is characterized in that it comprises:

[0008] The overall architecture topology of the modular multilevel converter includes three phases and six arms, each arm consisting of an arm inductor and multiple sub-modules connected in series.

[0009] The submodule is a MOSFET full-bridge submodule, whose core components include MOSFET switching transistors, high-voltage DC capacitors, and diodes;

[0010] The grid-type control module is used to control the converter. It adopts a virtual synchronous control method based on the generator rotor motion equation and transient voltage equation, and outputs internal potential voltage reference value and phase angle reference value.

[0011] The modulation module employs a high-frequency collaborative modulation strategy to generate trigger pulses for the sub-modules. The high-frequency collaborative modulation strategy includes: controlling most of the sub-modules to use nearest-level approximation modulation to generate the main body of the stepped wave of the output voltage; and simultaneously controlling one or a few designated sub-modules to operate in a high-frequency pulse width modulation mode to finely correct the main body of the stepped wave.

[0012] The grid-type converter based on MOSFET module cascading according to embodiments of the present invention has at least the following beneficial effects:

[0013] The grid-type converter based on cascaded MOSFET modules provided in this invention has lower cost and is more suitable for new power systems compared to traditional structures. It can quickly respond to grid-side disturbances and power output fluctuations on the renewable energy side, which is more conducive to smoothing out renewable energy output fluctuations, achieving system power balance, and improving the safe and stable operation of the power grid.

[0014] According to some embodiments of the present invention, the MOSFET full-bridge submodule includes four MOSFET switches and a high-voltage DC capacitor, forming a full-bridge topology; by controlling the on and off combinations of different switches, the submodule can output a positive capacitor voltage, a negative capacitor voltage, or a zero voltage.

[0015] According to some embodiments of the present invention, the total number N of submodules in each bridge arm total Determined in the following ways:

[0016] Basic quantity N min =V dc / U n V dc U is the DC side voltage. n The rated voltage of the submodule;

[0017] Total quantity N total =N min+n, where n is an integer from 1 to 3, representing the number of spare submodules.

[0018] According to some embodiments of the present invention, the parameters of the capacitor C0 and the bridge arm inductor L0 of the submodule must be selected to avoid resonance, and the resonant frequency ω res satisfy:

[0019]

[0020] Where N is the number of sub-modules actually put into operation.

[0021] According to some embodiments of the present invention, the high-frequency cooperative modulation strategy further includes: the control unit monitors the capacitor voltage of each submodule in real time, and, in combination with the bridge arm current direction, periodically rotates the designated submodules undertaking the high-frequency pulse width modulation mode task through a sorting algorithm, so as to achieve uniform distribution of loss among the submodules.

[0022] According to some embodiments of the present invention, in the virtual synchronization control method executed by the network-type control module, the typical model of the network-type outer loop control can be expressed as:

[0023]

[0024] Among them, T J For virtual inertial time constant P ref Number; ω is the virtual angular velocity of the converter output internal electromotive force; is the power reference value, corresponding to the input mechanical power of a traditional synchronous machine; P e V is the actual output active power of the converter; D is the virtual damping coefficient; ω0 is the rated angular velocity of the system; θ is the phase angle of the virtual internal potential of the energy storage converter output; E is the amplitude of the virtual internal potential of the converter output; G(s) is the virtual excitation control transfer function of the converter; V * V is the reference voltage; V is the calculated actual voltage signal at the control point.

[0025] According to some embodiments of the present invention, the network control module further includes an overcurrent limiting model, which is used to limit the internal potential voltage reference value and the phase angle reference value before outputting them to the modulation module.

[0026] According to some embodiments of the present invention, the converter is characterized in that it is applied to new energy power generation grid connection, grid-type energy storage system or grid-type static var generator (SVG).

[0027] A control method for a grid-type converter architecture according to a second aspect embodiment of the present invention is characterized in that it includes:

[0028] A network-based control strategy is adopted, and the virtual internal potential voltage reference value and phase angle reference value of the converter output are calculated based on the principle of virtual synchronous machine;

[0029] A high-frequency collaborative modulation strategy is employed to generate trigger pulses for each submodule in the modular multilevel converter architecture, including:

[0030] Most of the control submodules use the nearest-level approximation modulation to generate the main part of the stepped waveform of the output voltage;

[0031] Control one or a few designated submodules to operate in high-frequency pulse width modulation mode for fine correction of the main body of the stepped wave.

[0032] According to some embodiments of the present invention, the high-frequency collaborative modulation strategy includes: real-time monitoring of the capacitor voltage of each submodule, and periodically rotating the designated submodule that undertakes the task of the high-frequency pulse width modulation mode in combination with the direction of the bridge arm current.

[0033] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0034] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0035] Figure 1 This is a schematic diagram of the MOSFET full-bridge module provided in an embodiment of the present invention;

[0036] Figure 2 This is a schematic diagram of the overall architecture of MMC provided in an embodiment of the present invention;

[0037] Figure 3 The structured control framework diagram of the grid-connected converter in MMC provided in the embodiments of the present invention is shown. Detailed Implementation

[0038] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0039] In the description of this invention, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., are based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0040] In the description of this invention, "several" means one or more, "more than" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0041] In the description of this invention, unless otherwise explicitly defined, terms such as "set up," "install," and "connect" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this invention in conjunction with the specific content of the technical solution.

[0042] Example 1

[0043] To address the limitations of existing technologies, this invention proposes a MOSFET-based MMC network-type converter architecture. This converter can be divided into an overall MMC architecture, sub-modules, and a network-type control framework.

[0044] The MMC architecture consists of six bridge arms, each containing numerous sub-modules connected in series with an arm inductor. The main components of each sub-module are high-voltage DC capacitors, semiconductor switching devices, and diodes. There are typically hundreds of sub-modules in an MMC bridge arm. Therefore, under the control of the controller, these sub-modules can cause the bridge arm to output a stepped voltage (i.e., a multi-level voltage), thus allowing the bridge arm voltage to approximate its reference value in a near-continuous manner. The converter's control section employs a network-based control system.

[0045] The overall architecture topology of MMC is as follows: Figure 1 As shown, the three-phase MMC overall architecture consists of six bridge arms, each with numerous sub-modules connected in series, along with bridge arm inductors. The sub-modules are responsible for the output level, and each sub-module can be independently switched. Through unified and coordinated control, multi-level stepped wave synthesis is achieved, reducing the harmonic content of the output voltage. Bridge arm inductors are used to suppress circulating current and buffer the rate of change of current, improving the system's dynamic response characteristics. Each bridge arm maintains energy balance during inter-phase power exchange, ensuring symmetrical and stable three-phase output.

[0046] The submodule adopts a MOSFET full-bridge topology, such as Figure 2 As shown, the core components of each submodule are MOSFETs, high-voltage DC capacitors, and diodes. In addition to these core components, it also includes a semiconductor device driver board, heat sink, discharge resistor, bypass switch, housing, etc. (not shown in the diagram). Figure 2(Drawn in the middle). The main reason for using MOSFETs is that they have a higher switching frequency than thyristors such as IGBTs and IGCTs. They have advantages in responding to the rapid changes in new energy sources or supporting equipment (grid-based energy storage, grid-type SVG), which can improve control accuracy, reduce harmonic content and improve power quality.

[0047] Example 2

[0048] Another embodiment of the present invention provides a submodule calculation method considering redundant configuration. The submodule calculation method is divided into basic quantity calculation and spare submodule quantity calculation.

[0049] The calculation of the basic quantity is related to the rated voltage and DC side voltage of the submodule, as follows:

[0050]

[0051] The backup submodule configuration is to ensure stable operation of the converter in the event of a partial submodule failure. Typically, 1-3 backup submodules are used. The total number of backup submodules considered is:

[0052] N total =N min +n (2)

[0053] In addition, the number of submodules should also take into account the resonance problem between the submodule capacitors and the bridge arm inductors. The capacitors are calculated as follows:

[0054]

[0055] The bridge arm inductor needs to consider suppressing bridge arm current harmonics and avoiding resonance with the submodule capacitor. The resonant frequency is:

[0056]

[0057] Therefore, the selection of the number of sub-modules should also take into account the capacitance and inductance of the entire structure.

[0058] Example 3

[0059] Based on the MOSFET module cascaded grid converter provided in Embodiment 1 above, this embodiment of the invention provides a high-frequency cooperative modulation strategy + grid converter control strategy. For example... Figure 3As shown, the modulation strategy employs a high-frequency collaborative modulation strategy: most submodules in the bridge arm use improved nearest-level approximation modulation (NLM) to generate the main body of the stepped waveform of the output voltage. These submodules primarily operate at lower equivalent switching frequencies (e.g., close to the fundamental frequency) to control switching losses. One or a few submodules are designated to operate in high-frequency PWM mode to finely correct ("shaping") the stepped waveform generated by NLM modulation, eliminating low-order harmonics and improving waveform quality. These high-frequency switching tasks are undertaken by specific submodules. The control unit monitors the capacitor voltage of each submodule in real time. Combining the bridge arm current direction, a specific sorting algorithm periodically rotates the submodules undertaking high-frequency PWM (pulse width modulation) tasks. For example, submodules with higher capacitor voltages can be given priority in switching tasks, utilizing their charging and discharging effects during switching to help balance the capacitor voltage. This dynamic role allocation avoids overheating or excessive losses in a few submodules due to prolonged high-frequency switching, achieving a uniform distribution of losses.

[0060] The PCS control strategy employs a virtual synchronous control method based on the generator rotor motion equation and transient voltage equation. It can also incorporate optimized control strategies such as power command limiting control, primary frequency regulation control, primary voltage regulation control, and additional damping control (virtual power system stabilizer). This treats new energy sources or supporting equipment as equivalent controllable voltage sources with synchronous generator output characteristics. The structured control framework for grid-connected converters includes... Figure 3 As shown. A typical model for network-type outer-loop control can be represented as:

[0061]

[0062] Among them, T J P is the virtual inertial time constant; ω is the virtual angular velocity of the converter output internal potential; ref This is a power reference value, corresponding to the input mechanical power of a traditional synchronous machine; P e V is the actual output active power of the converter; D is the virtual damping coefficient; ω0 is the rated angular velocity of the system; θ is the phase angle of the virtual internal potential of the energy storage converter output; E is the amplitude of the virtual internal potential of the converter output; G(s) is the virtual excitation control transfer function of the converter; V * V is the reference voltage; V is the calculated actual voltage signal at the control point.

[0063] Through virtual synchronization control, a virtual internal potential voltage reference value E is output. ref and phase angle reference value θ ref After passing through the overcurrent limiting model, current limiting can be achieved through virtual impedance or vector reshaping. Finally, the output control voltage is converted to a three-phase stationary coordinate system and sent to the PWM generator to generate trigger pulses that meet the control requirements, thus completing the overall output control.

[0064] The device embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs.

[0065] It will be understood by those skilled in the art that all or some of the steps and systems in the methods disclosed above can be implemented as software, firmware, hardware, and suitable combinations thereof. Some or all of the physical components can be implemented as software executed by a processor, such as a central processing unit, digital signal processor, or microprocessor, or as hardware, or as an integrated circuit, such as an application-specific integrated circuit. Such software can be distributed on a computer-readable medium, which can include computer storage media (or non-transitory media) and communication media (or transient media). As is known to those skilled in the art, the term computer storage media includes volatile and non-volatile, removable and non-removable media implemented in any method or technology for storing information (such as computer-readable instructions, data structures, program modules, or other data). Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technologies, CD-ROM, digital versatile disc (DVD) or other optical disc storage, magnetic cartridges, magnetic tape, disk storage or other magnetic storage devices, or any other medium that can be used to store desired information and is accessible to a computer. Furthermore, as is known to those skilled in the art, communication media typically contain computer-readable instructions, data structures, program modules, or other data in modulated data signals such as carrier waves or other transmission mechanisms, and may include any information delivery medium.

[0066] The above is a detailed description of the preferred embodiments of this application. However, this application is not limited to the above embodiments. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of this application. All such equivalent modifications or substitutions are included within the scope defined by the claims of this application.

Claims

1. A grid-type converter based on cascaded MOSFET modules, characterized in that, include: The overall architecture topology of the modular multilevel converter includes three phases and six arms, each arm consisting of an arm inductor and multiple sub-modules connected in series. The submodule is a MOSFET full-bridge submodule, whose core components include MOSFET switching transistors, high-voltage DC capacitors, and diodes; The grid-type control module is used to control the converter. It adopts a virtual synchronous control method based on the generator rotor motion equation and transient voltage equation, and outputs internal potential voltage reference value and phase angle reference value. The modulation module employs a high-frequency collaborative modulation strategy to generate trigger pulses for the submodule; The high-frequency collaborative modulation strategy includes: controlling most of the sub-modules to use nearest-level approximation modulation to generate the main body of the stepped wave of the output voltage; and simultaneously controlling one or a few designated sub-modules to operate in high-frequency pulse width modulation mode to finely correct the main body of the stepped wave.

2. The converter according to claim 1, characterized in that, The MOSFET full-bridge submodule includes four MOSFET switches and a high-voltage DC capacitor, forming a full-bridge topology. By controlling the on and off combinations of different switches, the submodule can output a positive capacitor voltage, a negative capacitor voltage, or zero voltage.

3. The grid-type converter architecture according to claim 1, characterized in that, The total number of submodules in each bridge arm is N total Determined in the following ways: Basic quantity N min =V dc / U n V dc U is the DC side voltage. n The rated voltage of the submodule; Total quantity N total =N min +n, where n is an integer from 1 to 3, representing the number of spare submodules.

4. The grid-type converter architecture according to claim 3, characterized in that, The parameters of the capacitor C0 and bridge arm inductor L0 in the submodule must be selected to avoid resonance, with a resonant frequency ω. res satisfy: Where N is the number of sub-modules actually put into operation.

5. The grid-type converter architecture according to claim 1, characterized in that, The high-frequency collaborative modulation strategy further includes: the control unit monitors the capacitor voltage of each submodule in real time, and, in conjunction with the direction of the bridge arm current, periodically rotates the designated submodules undertaking the high-frequency pulse width modulation mode task through a sorting algorithm, so as to achieve a uniform distribution of losses among the submodules.

6. The grid-type converter architecture according to claim 1, characterized in that, In the virtual synchronization control method executed by the network-type control module, the typical model of the network-type outer loop control can be expressed as: Among them, T J For virtual inertial time constant P ref Number; ω is the virtual angular velocity of the converter output internal electromotive force; is the power reference value, corresponding to the input mechanical power of a traditional synchronous machine; P e V is the actual output active power of the converter; D is the virtual damping coefficient; ω0 is the rated angular velocity of the system; θ is the phase angle of the virtual internal potential of the energy storage converter output; E is the amplitude of the virtual internal potential of the converter output; G(s) is the virtual excitation control transfer function of the converter; V * V is the reference voltage; V is the calculated actual voltage signal at the control point.

7. The grid-type converter architecture according to claim 1, characterized in that, The network-type control module also includes an overcurrent limiting model, which is used to limit the internal potential voltage reference value and phase angle reference value before outputting them to the modulation module.

8. The grid-type converter architecture according to any one of claims 1 to 7, characterized in that, The converter is used in grid-connected new energy power generation, grid-type energy storage systems, or grid-type static var generators (SVG).

9. A control method for a grid-type converter architecture as described in claim 1, characterized in that, include: A network-based control strategy is adopted, and the virtual internal potential voltage reference value and phase angle reference value of the converter output are calculated based on the principle of virtual synchronous machine; A high-frequency collaborative modulation strategy is employed to generate trigger pulses for each submodule in the modular multilevel converter architecture, including: Most of the control submodules use the nearest-level approximation modulation to generate the main part of the stepped waveform of the output voltage; Control one or a few designated submodules to operate in high-frequency pulse width modulation mode for fine correction of the main body of the stepped wave.

10. The control method according to claim 9, characterized in that, The high-frequency collaborative modulation strategy includes: real-time monitoring of the capacitor voltage of each submodule, and periodically rotating the designated submodule that undertakes the high-frequency pulse width modulation mode task in combination with the direction of the bridge arm current.

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