A flexible direct current converter with multi-port direct current power flow control and a control method thereof

Abstract

The application discloses a flexible direct current converter with multi-port direct current flow control and a control method thereof, comprising a modular multi-level flexible direct current converter and a direct current flow controller embedded in the flexible direct current converter. The application has the characteristics of modularity and flexible adjustment of the number of ports, can bidirectionally control the direct current flow of two or more lines, and solves the problem of insufficient direct current flow control freedom in a meshed direct current system. The direct current flow controller is directly embedded in the flexible direct current converter, and energy balance can be achieved without an external power supply and high-voltage isolation device, and the flow regulation range is wide. The control method provided by the application can control the internal and external energy balance of the device. The main circuit parameter design method provided by the application can provide the selection basis of circuit elements for designing the device. The fault protection strategy and the active fault current suppression method can improve the reliability of the device under complex extreme working conditions, and assist the system to realize fault ride-through.

Description

Technical Field

[0001] This invention relates to the fields of flexible DC power transmission and distribution and power electronics, and in particular to a modular multilevel flexible DC converter with multi-port DC power flow control suitable for high voltage DC transmission grids and its control method. Background Technology

[0002] Flexible DC transmission technology features independent controllability of active and reactive power, no need for reactive power compensation, ability to supply power to passive networks, and unchanged voltage polarity during power flow reversal, thus improving the flexibility of system operation and control. Flexible DC transmission and distribution systems have evolved from the initial point-to-point structure with a single sending end and a single receiving end to a multi-terminal flexible DC system structure with multiple power sources and multiple receiving points, and are developing towards a more complex ring network topology in the future. The mesh flexible DC system can further enhance the flexibility of operation and control, system redundancy, and power supply reliability. It is conducive to integrating various types of power sources over a wide area, achieving efficient transmission and optimized distribution of electrical energy over a large area. This meets the current application requirements of large-scale development of new energy sources with multi-terminal grid connection, multi-receiving points, and wide-area interconnection, and can effectively alleviate voltage stability problems caused by new energy power fluctuations and frequency stability problems in large-scale interconnected AC networks.

[0003] In a mesh-structured DC system, multiple loops exist between some controllable nodes, which can improve the flexibility of configuration and control, as well as the reliability of power supply. However, when the number of transmission lines is greater than or equal to the number of controllable nodes, it is not possible to achieve complete control of the DC power flow solely through the control of the converter, which can easily lead to problems such as transmission section blockage, line overload, and excessive line loss. Summary of the Invention

[0004] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0005] In view of the aforementioned existing problems, the present invention is proposed.

[0006] Therefore, the present invention provides a flexible DC converter with multi-port DC power flow control and its control method, which can solve problems such as transmission section blockage, line overload, and excessive line loss.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a flexible DC converter with multi-port DC power flow control, comprising:

[0008] Modular multilevel flexible DC converter; and,

[0009] DC power flow controller embedded in modular multilevel flexible DC converter.

[0010] As a preferred embodiment of the flexible DC converter with multi-port DC power flow control described in this invention, the DC power flow controller includes two or more DC power flow control units, the number of DC power flow control units being equal to the number of ports of the DC power flow controller, and each DC power flow control unit being able to adjust the DC power flow on the connected DC line.

[0011] As a preferred embodiment of the flexible DC converter with multi-port DC power flow control described in this invention, the DC power flow control unit consists of a three-phase star-connected sub-module chain and a bridge arm inductor, and the three-phase neutral point connected by the sub-module chain and the inductor serves as the DC power flow regulation port.

[0012] As a preferred embodiment of the flexible DC converter with multi-port DC power flow control described in this invention, the modular multilevel flexible DC converter is a medium-voltage or high-voltage, three-phase, voltage source converter with a modular multilevel structure, which has the function of AC / DC power conversion and can realize the connection between medium- and high-voltage AC power grids and DC systems.

[0013] As a preferred embodiment of the flexible DC converter with multi-port DC power flow control described in this invention, the embedded DC power flow controller can be symmetrically installed on the upper and lower bridge arms of the modular multilevel flexible DC converter, or it can be installed only on the upper or lower bridge arm of the modular multilevel flexible DC converter.

[0014] As a preferred embodiment of the flexible DC converter with multi-port DC power flow control described in this invention, the submodule chain is a unipolar submodule chain or a bipolar submodule chain, or a hybrid submodule chain composed of unipolar and bipolar submodules.

[0015] The present invention also provides the following technical solution: a control method for an embedded multi-port DC power flow controller includes a DC power flow control loop on multiple lines, an energy balance control loop between a modular multilevel flexible DC converter and the DC power flow controller, and an internal energy balance control loop of the DC power flow controller.

[0016] When the embedded multi-port DC power flow controller is connected to two or more DC lines, the sum of the power on all output lines is equal to the total power of the modular multilevel flexible DC converter. If the embedded multi-port DC power flow controller is connected to N DC output lines, then the power flow on N-1 of these lines can be actively controlled, and the DC power flow on the remaining line is equal to the total power flow minus the sum of the power flows on the other lines.

[0017] As a preferred embodiment of the control method for the embedded multi-port DC power flow controller described in this invention, the DC power flow control loop on the multiple lines includes:

[0018] The line power flow control loop calculates the DC component of the voltage of each DC power flow control unit by subtracting the voltage reference values ​​of other lines from the voltage of the reference circuit. The mathematical equation is as follows:

[0019]

[0020] Among them, U d U is the rated voltage of the DC system, which is also equal to the voltage of the reference line. p1a U pka U pNa These are the DC components of the voltages of the DC power flow control units connected to the 1st, kth, and Nth lines, respectively, U. o1 U ok U oN These are the DC voltage reference values ​​for the 1st line, the kth line, and the Nth line, respectively.

[0021] As a preferred embodiment of the control method for the embedded multi-port DC power flow controller described in this invention, the energy balance control loop between the modular multilevel flexible DC converter and the DC power flow controller includes:

[0022] The energy balance control loop between the modular multilevel flexible DC converter and the DC power flow controller forms AC coupling energy by superimposing energy balance control voltages on the bridge arms of the DC power flow control unit and the modular multilevel converter, respectively, to compensate for the accumulated DC energy. The mathematical equations for the overall input and output energy of the DC power flow controller are as follows:

[0023]

[0024] Among them, P DCPFC_out P represents the total output energy of the multi-port DC power flow controller. DCPFC_in This represents the total input energy of the multi-port DC power flow controller. U pia,dc and U pia,ac These are the DC and AC components of the voltage of the DC power flow control unit connected to the i-th line, respectively. oi I is the DC current of the i-th line. d It is the sum of the DC currents of all lines, i jp,ac It is the AC component of the upper arm current of phase j of the modular multilevel flexible DC converter.

[0025] In a preferred embodiment of the control method for the embedded multi-port DC power flow controller described in this invention, the internal energy balancing control loop of the DC power flow controller includes:

[0026] The mathematical equations for the energy relationships within the DC power flow controller are as follows:

[0027] and

[0028] Among them, Δp1, Δp k and Δp N These are the DC energy values ​​of the 1st, kth, and Nth DC power flow control units, Δp. c1a Δp cka and Δp cNa These are the AC coupling energies of the 1st, kth, and Nth DC power flow control units, respectively.

[0029] The present invention also provides the following technical solution: a main circuit parameter design method for an embedded multi-port DC power flow controller, comprising a method for designing the number of sub-modules of the DC power flow control unit and the modular multilevel flexible DC converter, a method for designing the capacitors of the sub-modules of the DC power flow control unit and the modular multilevel flexible DC converter, a method for designing the bridge arm inductors of the DC power flow controller, and a method for designing the power devices of the sub-modules of the DC power flow control unit and the modular multilevel flexible DC converter.

[0030] As a preferred embodiment of the main circuit parameter design method for the embedded multi-port DC power flow controller described in this invention, wherein:

[0031] The DC power flow controller adopts an asymmetric installation method and is embedded in the end of the upper bridge arm of the modular multilevel flexible DC converter.

[0032] The DC power flow controller adopts a bipolar full-bridge submodule, and the modular multilevel flexible DC converter adopts a unipolar half-bridge submodule.

[0033] As a preferred embodiment of the main circuit parameter design method for the embedded multi-port DC power flow controller described in this invention, wherein:

[0034] The design method for the number of sub-modules in the DC power flow control unit and the modular multilevel flexible DC converter.

[0035] The mathematical equation for selecting the number of DC power flow controller submodules is:

[0036]

[0037] Where, NPFC U represents the number of submodules in a submodule chain within a DC power flow controller. CPFC U is the rated voltage of the submodule within the DC power flow controller. d ΔP is the rated voltage of the DC power grid, ε is the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC power grid. 1,2 P represents the maximum difference in DC power flow between the two DC lines, and P is the total power of the modular multilevel flexible DC converter. Let δ be the phase angle of the AC current in the modular multilevel flexible DC converter, δ be the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E be the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω be the angular frequency, and L be the phase angle of the AC current in the modular multilevel flexible DC converter arm voltage. PFC For the bridge arm inductance of the DC power flow controller;

[0038] The mathematical equation for selecting the number of submodules in a modular multilevel flexible DC converter is:

[0039]

[0040] Where, N MMC,p and N MMC,n These represent the number of submodules in the upper and lower arms of the modular multilevel flexible DC converter, respectively. C The rated voltage of the modular multilevel flexible DC converter submodule, U d ΔP is the rated voltage of the DC power grid, ε is the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC power grid. 1,2 P represents the maximum difference in DC power flow between the two DC lines, and P is the total power of the modular multilevel flexible DC converter. Let δ be the phase angle of the AC current in the modular multilevel flexible DC converter, δ be the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E be the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω be the angular frequency, and L be the phase angle of the AC current in the modular multilevel flexible DC converter arm voltage. PFC This is the bridge arm inductor for the DC power flow controller.

[0041] As a preferred embodiment of the main circuit parameter design method for the embedded multi-port DC power flow controller described in this invention, wherein:

[0042] The design method for the submodule capacitors of the DC power flow control unit and the modular multilevel flexible DC converter.

[0043] The mathematical equation for selecting the capacitance value of the DC power flow controller submodule is as follows:

[0044]

[0045] Among them, C PFCN represents the capacitance of the submodule of the DC power flow controller. PFC U represents the number of submodules in a submodule chain within a DC power flow controller. CPFC R1 represents the rated voltage of the submodule within the DC power flow controller; r1 and r2 represent the fundamental frequency and second harmonic frequency ripple rates of the submodule capacitor voltage, respectively; ε represents the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC grid; and P represents the total power of the modular multilevel flexible DC converter. Let δ be the phase angle of the AC current in the modular multilevel flexible DC converter, δ be the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E be the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω be the angular frequency, and L be the phase angle of the AC current in the modular multilevel flexible DC converter arm voltage. PFC This is the bridge arm inductor for the DC power flow controller.

[0046] The mathematical equation for selecting the capacitance value of the modular multilevel submodule is as follows:

[0047]

[0048] Where C0 is the size of the sub-module capacitor of the modular multilevel flexible DC converter, and N MMC For a modular multilevel flexible DC converter, the number of submodules in one arm, U C ε1 and ε2 represent the rated voltage of the submodule capacitors in the modular multilevel flexible DC converter, respectively, and ε1 and ε2 represent the fundamental frequency and second harmonic frequency ripple rates of the submodule capacitor voltages in the modular multilevel flexible DC converter. P represents the total power of the modular multilevel flexible DC converter. δ represents the phase angle of the AC current in the modular multilevel flexible DC converter, and δ is the phase angle difference between the fundamental AC component of the bridge arm voltage of the modular multilevel flexible DC converter and the grid voltage.

[0049] As a preferred embodiment of the main circuit parameter design method for the embedded multi-port DC power flow controller described in this invention, wherein:

[0050] The bridge arm inductor design method of the DC power flow controller,

[0051] The mathematical equation for selecting the bridge arm inductor of the DC power flow controller is as follows:

[0052]

[0053] Among them, L PFC For the bridge arm inductance of the DC power flow controller, ω0 is the fundamental angular frequency, ω res It is the resonant angular frequency of the sub-module chain and the bridge arm inductor within the phase unit of the DC power flow controller and the modular multilevel flexible DC converter. TL0 is the inductance value of the connecting transformer, and L0 is the inductance value of the modular multilevel flexible DC converter bridge arm. dc N represents the inductance value of the smoothing reactor in the DC system. PFC U represents the number of submodules in a submodule chain within a DC power flow controller. CPFC U is the rated voltage of the submodule within the DC power flow controller. d I is the rated voltage of the DC power grid. d ε is the rated current of the DC grid, λ is the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC grid, λ is the upper limit of the transient current change rate under short-circuit fault, η is the per-unit value of the negative sequence current amplitude, C0 is the size of the submodule capacitor of the modular multilevel flexible DC converter, and N is the rated current of the DC grid. MMC The number of sub-modules in one arm of a modular multilevel flexible DC converter.

[0054] As a preferred embodiment of the main circuit parameter design method for the embedded multi-port DC power flow controller described in this invention, wherein:

[0055] The design method for submodule power devices of the DC power flow control unit and the modular multilevel flexible DC converter.

[0056] The mathematical equations for calculating the voltage and current stresses of power devices in a DC power flow control unit are as follows:

[0057]

[0058]

[0059] Among them, A PFC_U and A PFC_Ures A represents the maximum peak and RMS voltage withstand values ​​for individual power electronic devices in the bridge arm submodules of the DC power flow controller. PFC_I and A PFC_Ires P1 and P2 represent the maximum peak and RMS current withstand values ​​of individual power electronic devices in the bridge arm submodules of the DC power flow controller, respectively. ΔP represents the DC power flow of the two DC lines. 1,2 U is the maximum difference in DC power flow between the two DC lines, ε is the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC grid, and U d The rated voltage of the DC power grid, N PFC P represents the number of submodules in a submodule chain within the DC power flow controller, and P represents the total power of the modular multilevel flexible DC converter. Let δ be the phase angle of the AC current in the modular multilevel flexible DC converter, δ be the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E be the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω be the angular frequency, and L be the phase angle of the AC current in the modular multilevel flexible DC converter arm voltage.PFC This is the bridge arm inductor for the DC power flow controller.

[0060] The mathematical equations for calculating the voltage and current stresses of the power devices in a modular multilevel flexible DC converter are as follows:

[0061]

[0062]

[0063] Among them, A MMC_U and A MMC_Ures For each individual power electronic device in the bridge arm submodule of the modular multilevel flexible DC converter, A represents the maximum peak and effective voltage withstand value. MMC_I and A MMC_Ires The maximum peak and effective current withstand values ​​for individual power electronic devices in the bridge arm submodules of the modular multilevel flexible DC converter are ΔP. 1,2 U is the maximum difference in DC power flow between the two DC lines. d The rated voltage of the DC power grid, N MMC P represents the number of submodules in one arm of the modular multilevel flexible DC converter, and P represents the total power of the modular multilevel flexible DC converter. Let δ be the phase angle of the AC current in the modular multilevel flexible DC converter, δ be the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E be the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω be the angular frequency, and L be the phase angle of the AC current in the modular multilevel flexible DC converter arm voltage. PFC This is the bridge arm inductor for the DC power flow controller.

[0064] The present invention also provides the following technical solution: a protection method for an embedded multi-port DC power flow controller under DC short-circuit fault, the protection method including a protection method for the DC power flow controller under DC short-circuit fault, and a method for actively limiting fault current under DC short-circuit fault.

[0065] As a preferred embodiment of the protection method for the embedded multi-port DC power flow controller under DC short-circuit faults described in this invention, wherein:

[0066] The protection method for the DC power flow controller under DC short-circuit fault includes,

[0067] This can be achieved by controlling the sub-modules within the DC power flow controller to operate in specific switching states, or by adding a protection device to the sub-modules within the DC power flow controller.

[0068] The specific switching state of the submodule refers to the state in which the capacitor of the submodule is bypassed and the current flows only through the switching device. The protection device consists of a load unloading circuit or a thyristor bypass switch.

[0069] The submodule switching device withstands the fault current. By operating the submodule in a specific switching state, the fault current is prevented from flowing into the submodule capacitor, thus achieving fault protection.

[0070] As a preferred embodiment of the protection method for the embedded multi-port DC power flow controller under DC short-circuit faults described in this invention, wherein:

[0071] The protection device is a load-relief circuit connected in parallel with the submodule capacitor.

[0072] The unloading circuit is composed of diodes, resistors, and switching devices connected in series and parallel. It consumes the energy of the fault current, avoids overvoltage of the submodule capacitor, and realizes fault protection.

[0073] As a preferred embodiment of the protection method for the embedded multi-port DC power flow controller under DC short-circuit faults described in this invention, wherein:

[0074] The protection device is a distributed or centralized thyristor bypass switch. Each thyristor bypass switch is composed of anti-parallel thyristors, an RC circuit, and a static resistor connected in parallel. The distributed thyristor bypass switch is installed in each sub-module, and the centralized thyristor bypass switch is chained in parallel with the sub-module. By using the rapid action of bidirectional thyristors to bypass the sub-module, fault current is prevented from flowing through the sub-module, thus achieving fault protection.

[0075] As a preferred embodiment of the protection method for the embedded multi-port DC power flow controller under DC short-circuit faults described in this invention, wherein:

[0076] The method for actively limiting fault current under DC short-circuit fault conditions includes:

[0077] This can be achieved by controlling the sub-modules within the DC power flow controller to operate in specific switching states, or by adding a protection device to the sub-modules within the DC power flow controller.

[0078] The specific switching state of the submodule refers to the state of locking or forward activation of the submodule. The protection device consists of an unloading circuit and a fault current limiter. The unloading circuit is composed of diodes, resistors, and switching devices connected in series and parallel. The fault current limiter is composed of resistors, inductors, and surge arresters connected in series and parallel.

[0079] The submodule capacitor withstands a certain fault voltage, and the submodule switching device withstands the fault current. By operating the submodule in a specific switching state, the submodule is connected in series with the circuit in a negative voltage manner to suppress the fault current.

[0080] As a preferred embodiment of the protection method for the embedded multi-port DC power flow controller under DC short-circuit faults described in this invention, wherein:

[0081] The protection device is a load-relief circuit connected in parallel with the submodule capacitor to achieve active suppression of step-down fault current. The protection device is a distributed or centralized fault current limiting device, which is composed of a thyristor bypass switch, resistor, inductor, surge arrester, etc., connected in series. The distributed fault current limiting device is installed in each submodule, while the centralized fault current limiting device is connected in parallel with the submodule. Through the fault current limiting device, the rise of fault current is limited, assisting the DC power grid in suppressing and clearing faults.

[0082] Compared with existing flexible interconnect devices, the present invention has the following advantages:

[0083] 1. Existing self-balancing series-type DC power flow controllers are relatively complex to control, and their power flow regulation capability and control flexibility are limited. Existing externally balanced DC power flow controllers need to withstand system-level voltage and system-level insulation, increasing device cost and construction difficulty. The multi-port DC power flow controller embedded in the modular multilevel flexible DC converter proposed in this invention achieves energy balance through coordinated control with the main MMC, eliminating the need for an external power supply and isolation transformer. Furthermore, the device's bridge arms are composed of sub-modules, offering advantages such as modularity, multi-port capability, easy expansion, wide adjustment range, strong control capability, applicability to high-voltage systems, and bidirectional power flow controllability.

[0084] 2. The multi-port DC power flow controller in this invention has modular features. By adjusting the number of DC power flow control units, the expansion of output ports can be achieved quickly and economically.

[0085] 3. The DC power flow controller and the modular multilevel flexible DC converter main circuit parameter selection and design method in this invention take into account the impact of embedding the DC power flow controller on the modular multilevel flexible DC converter, and provide a fast, effective and reliable parameter calculation method and selection basis.

[0086] 4. The protection method of the embedded multi-port DC power flow controller under DC short-circuit fault in this invention not only effectively ensures the safe and stable operation of the DC power flow controller under extreme DC fault conditions, but also effectively suppresses fault current and assists the DC power grid in suppressing and clearing faults. Attached Figure Description

[0087] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0088] Figure 1 This is a schematic diagram of the topology of the embedded multi-port DC power flow controller and the embedded modular multilevel flexible DC converter described in this invention.

[0089] Figure 2 This is a schematic diagram of the topology of the embedded multi-port DC power flow controller and the embedded modular multilevel flexible DC converter described in this invention.

[0090] Figure 3 This is a schematic diagram of the topology of the DC power flow control unit in the embedded multi-port DC power flow controller of the present invention, and a schematic diagram of typical topologies of the unipolar submodule and bipolar submodule in the embedded multi-port DC power flow controller.

[0091] Figure 4 This is a typical application scenario diagram of the embedded multi-port DC power flow controller described in this invention.

[0092] Figure 5 This is a schematic diagram of the protection method and protection device of the embedded multi-port DC power flow controller under DC short circuit fault according to the present invention;

[0093] Figure 6 This is a schematic diagram of the topology of the embedded multi-port DC power flow controller and the embedded modular multilevel flexible DC converter in Implementation Plan 1.

[0094] Figure 7 This is a block diagram of the control method for the embedded multi-port DC power flow controller and modular multi-level flexible DC converter in Implementation 1.

[0095] Figure 8 To implement the simulation waveform diagrams of the two operating conditions and four stages in Scheme 1, including the voltage and current waveform diagrams of each port, the voltage difference waveform diagram between each port, and the capacitor voltage waveform diagram of the sub-module in the modular multilevel flexible DC converter and DC power flow controller.

[0096] Figure 9 This is a schematic diagram of a DC short-circuit fault in Implementation Scheme 2;

[0097] Figure 10 The waveforms of the submodule capacitor voltage and fault current of the DC power flow controller after adding a fault protection strategy when a fault occurs in Implementation Scheme 2 are shown.

[0098] Figure 11 The waveforms of fault current and submodule capacitor voltage are shown for the DC power flow controller after adding an active current limiting strategy when a fault occurs in Implementation Scheme 2. Detailed Implementation

[0099] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention.

[0100] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0101] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0102] This invention is described in detail with reference to the schematic diagrams. When detailing the embodiments of this invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged, not adhering to the usual scale. Furthermore, the schematic diagrams are merely examples and should not be construed as limiting the scope of protection of this invention. In actual fabrication, the three-dimensional spatial dimensions of length, width, and depth should be included.

[0103] Furthermore, in the description of this invention, it should be noted that the terms "upper," "lower," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are used solely for the convenience of describing the invention and for 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 limitations on the invention. In addition, the terms "first," "second," or "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0104] Unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" in this invention should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; similarly, they can refer to mechanical connections, electrical connections, or direct connections, or indirect connections through an intermediate medium, or internal connections between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0105] Example 1

[0106] Reference Figures 1-6 This is the first embodiment of the present invention, which provides a flexible DC converter with multi-port DC power flow control and its control method, including:

[0107] Modular multilevel flexible DC converter; and,

[0108] DC power flow controller embedded in modular multilevel flexible DC converter.

[0109] Furthermore, the DC power flow controller includes two or more DC power flow control units, the number of which is equal to the number of ports of the DC power flow controller. Each DC power flow control unit can adjust the DC power flow on the connected DC line;

[0110] The DC power flow control unit consists of a three-phase star-connected sub-module chain and bridge arm inductors. The three-phase neutral point connected by the sub-module chain and inductors serves as the DC power flow regulation port.

[0111] The modular multilevel flexible DC converter is a three-phase, voltage source converter with a modular multilevel structure, which is either medium-voltage or high-voltage. It has the function of AC-DC power conversion and can realize the connection between medium- and high-voltage AC power grids and DC systems.

[0112] Furthermore, the modular multilevel flexible DC converter is a modular multilevel converter with a voltage rating of medium to high voltage.

[0113] Furthermore, such as Figure 1 , Figure 2 The embedded DC power flow controller can be symmetrically installed on the upper and lower arms of the modular multilevel flexible DC converter, or it can be installed only on the upper or lower arm of the modular multilevel flexible DC converter.

[0114] Furthermore, such as Figure 3 The above submodule chain can be a unipolar submodule chain or a bipolar submodule chain, or it can be a hybrid submodule chain composed of unipolar and bipolar submodules.

[0115] The present invention also provides a control method applicable to the embedded multi-port DC power flow controller, the control method comprising a DC power flow control loop on multiple lines, an energy balance control loop between the modular multilevel flexible DC converter and the DC power flow controller, and an internal energy balance control loop of the DC power flow controller;

[0116] When the embedded multi-port DC power flow controller is connected to two or more DC lines, the sum of the power on all output lines equals the total power of the modular multilevel flexible DC converter. If the embedded multi-port DC power flow controller is connected to N DC output lines, then the power flow on N-1 of these lines can be actively controlled, and the DC power flow on the remaining line is equal to the total power flow minus the sum of the power flows on the other lines.

[0117] Furthermore, the control objective of the line power flow control loop is that the DC voltage of the N output lines equals the reference value.

[0118] It should be noted that N is the total number of lines connected to the aforementioned embedded multi-port DC power flow controller; the line power flow control loop first selects the line with the lowest voltage as the reference line based on the output line voltage reference value given by the external system of the device.

[0119] Furthermore, in the DC power flow control unit connected to the reference line, the DC component of the voltage in the internal sub-module chain is set to zero. The line power flow control loop calculates the DC component of the voltage in each DC power flow control unit by subtracting the voltage reference values ​​of other lines from the voltage of the reference circuit. The mathematical equation is as follows:

[0120]

[0121] Among them, U d U is the rated voltage of the DC system, which is also equal to the voltage of the reference line. p1a U pka U pNa These are the DC components of the voltages of the DC power flow control units connected to the 1st, kth, and Nth lines, respectively, U. o1 U ok U oN These are the DC voltage reference values ​​for the 1st line, the kth line, and the Nth line, respectively.

[0122] Furthermore, the energy balance control loop between the modular multilevel flexible DC converter and the DC power flow controller aims to maintain the average capacitor voltage of all sub-modules within the DC power flow controller equal to the reference value.

[0123] It should be noted that the DC power flow controller interacts with the DC system during the control of DC power flow. The accumulation of DC energy causes instability in the capacitor voltage of the sub-modules within the DC power flow controller. The energy balance control loop between the modular multilevel flexible DC converter and the DC power flow controller forms AC-coupled energy by superimposing energy balance control voltages on the bridge arms of the DC power flow control unit and the modular multilevel converter, respectively. The mathematical equations for the overall input and output energy of the DC power flow controller are as follows:

[0124]

[0125] Among them, P DCPFC_out P represents the total output energy of the multi-port DC power flow controller. DCPFC_in This represents the total input energy of the multi-port DC power flow controller. U pia,dc and U pia,ac These are the DC and AC components of the voltage of the DC power flow control unit connected to the i-th line, respectively. oi I is the DC current of the i-th line. d It is the sum of the DC currents of all lines, i jp,ac It is the AC component of the upper arm current of phase j of the modular multilevel flexible DC converter.

[0126] Furthermore, to ensure the stable operation of the DC power flow controller, the output power of the power flow regulation port and the input power of the energy balance port of the DC power flow controller are balanced, as shown in the mathematical equation:

[0127] P DCPFC_out =P DCPFC_in

[0128] It should be noted that a proportional-integral controller is used to control the amplitude and phase angle of the energy balance control voltage of the DC power flow controller and the modular multilevel flexible DC converter. The input is the difference between the average voltage of all sub-modules in the DC power flow controller and the reference value, and the output is the reference value of the amplitude and phase angle of the energy balance control voltage.

[0129] Furthermore, by ensuring that the phase angle of the energy balance control voltage is always equal to the phase angle of the arm current of the modular multilevel flexible DC converter, and only the amplitude of the energy balance control voltage needs to be controlled, and the amplitudes of the three-phase energy balance control voltages are equal, the proportional-integral controller equation becomes:

[0130]

[0131] Where, k p k is the gain coefficient of the proportional element in the proportional-integral controller. i U is the gain coefficient of the integral element of the proportional-integral controller.C* U is the reference value for the capacitor voltage of the submodule inside the DC power flow control unit. C,i U is the average capacitor voltage of the submodule within the k-th DC power flow control unit. PFC* It is the reference value of the energy balance control voltage amplitude between the DC power flow controller and the modular multilevel flexible DC converter, and N is the total number of lines connected to the above-mentioned embedded multi-port DC power flow controller.

[0132] Furthermore, the control objective of the energy balancing control loop inside the DC power flow controller is to maintain consistent capacitor voltages in the sub-modules of each DC power flow control unit.

[0133] It should be noted that the aforementioned line power flow control loop causes the DC voltages of the DC power flow control units to differ, resulting in DC energy deviation, which in turn leads to inconsistent capacitor voltages within the submodules of each DC power flow control unit. Therefore, the energy balancing control loop within the DC power flow controller actively injects AC current circulating between the DC power flow control units, utilizing AC coupling energy to compensate for the DC energy deviation and maintain stable submodule voltages and energy balance. The formula for calculating the DC energy of each DC power flow control unit is as follows:

[0134]

[0135] Where, Δp k It is the DC energy of the k-th DC power flow control unit, U pka,dc and U pia,dc These are the DC components of the voltages of the DC power flow control units connected to the k-th and i-th lines, respectively. ok It is the DC current of the k-th line, and N is the total number of lines connected to the above-mentioned embedded multi-port DC power flow controller.

[0136] Furthermore, under the control of the energy balancing control loop within the DC power flow controller, the mathematical equation for injecting AC voltage in each DC power flow control unit is as follows:

[0137] Δu cka =x k sin(ωt-β k ), k=1,2...N

[0138] Where, Δu cka It is the positive sequence circulating current voltage superimposed on the a-phase modulation voltage of the k-th DC power flow control unit, x i and β i These are the amplitude and phase angle of the fundamental frequency positive sequence circulating current voltage, respectively, and ω is the angular frequency.

[0139] It should be noted that the aforementioned actively injected AC voltage causes AC coupling energy to be generated inside the DC power flow controller. The mathematical equation for the average value of the AC coupling energy during the fundamental frequency period is as follows:

[0140]

[0141] Where, Δp cka L is the AC coupling energy of the k-th DC power flow control unit. PFC It is the value of the bridge arm inductance in the DC power flow control unit, U PFC,ac and δ PFC These are the energy balance control voltage amplitude and phase angle between the aforementioned modular multilevel flexible DC converter and the DC power flow controller, x k and x i These are the circulating current voltage amplitudes of the k-th and i-th DC power flow control units, respectively, β k and β i These are the circulating voltage phase angles of the k-th and i-th DC power flow control units, respectively.

[0142] Furthermore, to achieve energy balance among the DC power flow control units, the internal energy balancing control loop of the DC power flow controller compensates for the aforementioned DC energy deviation by controlling the amplitude and phase angle of the circulating current voltage, i.e., the sum of the AC coupling energy and the DC energy deviation is zero. Simultaneously, the sum of the AC coupling energy of all DC power flow control units is also zero. The mathematical equation for the energy relationship within the DC power flow controller is:

[0143] and

[0144] Among them, Δp1, Δp k and Δp N These are the DC energy values ​​of the 1st, kth, and Nth DC power flow control units, Δp. c1a Δp cka and Δp cNa These are the AC coupling energies of the 1st, kth, and Nth DC power flow control units, respectively.

[0145] It should be noted that a proportional-integral controller is used to control the circulating voltage amplitude and phase angle of the DC power flow control unit. The input is the difference between the voltage of the sub-module within each DC power flow control unit and the average value of the sub-module voltages of all DC power flow control units. The output is the reference value of the circulating voltage amplitude and phase angle of the corresponding DC power flow control unit.

[0146] Furthermore, if we make the circulating voltage phase angle reference values ​​equal for all DC power flow control units, and only control the amplitude of the circulating voltage, then the proportional-integral controller equation is:

[0147]

[0148] Where, k p k is the gain coefficient of the proportional element in the proportional-integral controller. i U is the gain coefficient of the integral element of the proportional-integral controller. C,k and U C,i , respectively, are the average capacitor voltages of the sub-modules inside the k-th and i-th DC power flow control units, xk is the reference value of the circulating current voltage amplitude of the k-th DC power flow control unit, and N is the total number of lines connected to the above-mentioned embedded multi-port DC power flow controller.

[0149] The present invention also provides a main circuit parameter design method for the embedded multi-port DC power flow controller, the main circuit parameter design method including a method for designing the number of sub-modules of the DC power flow control unit and the modular multilevel flexible DC converter, a method for designing the capacitors of the sub-modules of the DC power flow control unit and the modular multilevel flexible DC converter, a method for designing the bridge arm inductors of the DC power flow controller, and a method for designing the power devices of the sub-modules of the DC power flow control unit and the modular multilevel flexible DC converter.

[0150] It should be noted that the aforementioned DC power flow controller adopts an asymmetric installation method, and the DC power flow controller is embedded in the end of the upper bridge arm of the modular multilevel flexible DC converter.

[0151] It should also be noted that the DC power flow controller mentioned above adopts a bipolar full-bridge submodule, while the modular multilevel flexible DC converter adopts a unipolar half-bridge submodule.

[0152] It should also be noted that the multi-port DC power flow controller is connected to two DC lines.

[0153] Furthermore, the design method for the number of submodules in the DC power flow control unit and the modular multilevel flexible DC converter aims to select an appropriate number of submodules for the DC power flow controller and the bridge arm of the modular multilevel flexible DC converter, avoiding problems such as overmodulation and insufficient DC power flow control range. When the system external parameters are determined, the lower limit of the number of submodules in the DC power flow control unit and the lower limit of the number of submodules in the modular multilevel flexible DC converter can be quickly calculated according to the design method for the number of submodules in the DC power flow control unit.

[0154] The mathematical equation for selecting the number of DC power flow controller submodules in this method is:

[0155]

[0156] Where, N PFCU represents the number of submodules in a submodule chain within a DC power flow controller. CPFC U is the rated voltage of the submodule within the DC power flow controller. d ΔP is the rated voltage of the DC power grid, ε is the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC power grid. 1,2 P represents the maximum difference in DC power flow between the two DC lines, and P is the total power of the modular multilevel flexible DC converter. Let δ be the phase angle of the AC current in the modular multilevel flexible DC converter, δ be the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E be the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω be the angular frequency, and L be the phase angle of the AC current in the modular multilevel flexible DC converter arm voltage. PFC This is the bridge arm inductor for the DC power flow controller.

[0157] The mathematical equation for selecting the number of sub-modules in a modular multilevel flexible DC converter is as follows:

[0158]

[0159] Where, N MMC,p and N MMC,n These represent the number of submodules in the upper and lower arms of the modular multilevel flexible DC converter, respectively. C The rated voltage of the modular multilevel flexible DC converter submodule, U d ΔP is the rated voltage of the DC power grid, ε is the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC power grid. 1,2 P represents the maximum difference in DC power flow between the two DC lines, and P is the total power of the modular multilevel flexible DC converter. Let δ be the phase angle of the AC current in the modular multilevel flexible DC converter, δ be the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E be the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω be the angular frequency, and L be the phase angle of the AC current in the modular multilevel flexible DC converter arm voltage. PFC This is the bridge arm inductor for the DC power flow controller.

[0160] Furthermore, the submodule capacitor design method for the DC power flow control unit and the modular multilevel flexible DC converter aims to select appropriate submodule capacitor sizes for the DC power flow controller and the bridge arm of the modular multilevel flexible DC converter, thereby controlling voltage ripple within a reasonable range. Due to the influence of the DC power flow control unit on the modular multilevel flexible DC converter, the capacitor parameter design method differs from the traditional design method for modular multilevel flexible DC converters. When the system external parameters are determined, the lower limit of the submodule capacitor value for the DC power flow control unit, as well as the lower limit of the submodule capacitor value for the modular multilevel flexible DC converter, can be quickly calculated using the submodule capacitor design method for the DC power flow control unit.

[0161] The mathematical equation for selecting the capacitance value of the DC power flow controller submodule using the above method is as follows:

[0162]

[0163] Among them, C PFC N represents the capacitance of the submodule of the DC power flow controller. PFC U represents the number of submodules in a submodule chain within a DC power flow controller. CPFC R1 represents the rated voltage of the submodule within the DC power flow controller; r1 and r2 represent the fundamental frequency and second harmonic frequency ripple rates of the submodule capacitor voltage, respectively; ε represents the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC grid; and P represents the total power of the modular multilevel flexible DC converter. Let δ be the phase angle of the AC current in the modular multilevel flexible DC converter, δ be the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E be the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω be the angular frequency, and L be the phase angle of the AC current in the modular multilevel flexible DC converter arm voltage. PFC This is the bridge arm inductor for the DC power flow controller.

[0164] The mathematical equation for selecting the capacitance value of the modular multilevel submodule using the above method is as follows:

[0165]

[0166] Where C0 is the size of the sub-module capacitor of the modular multilevel flexible DC converter, and N MMC For a modular multilevel flexible DC converter, the number of submodules in one arm, U C ε1 and ε2 represent the rated voltage of the submodule capacitors in the modular multilevel flexible DC converter, respectively, and ε1 and ε2 represent the fundamental frequency and second harmonic frequency ripple rates of the submodule capacitor voltages in the modular multilevel flexible DC converter. P represents the total power of the modular multilevel flexible DC converter. δ represents the phase angle of the AC current in the modular multilevel flexible DC converter, and δ is the phase angle difference between the fundamental AC component of the bridge arm voltage of the modular multilevel flexible DC converter and the grid voltage.

[0167] Furthermore, the design method for the bridge arm inductor of the DC power flow controller aims to select a suitable value for the bridge arm inductor to meet the power transmission capacity requirements, reduce negative sequence current, suppress short-circuit fault current, and form a resonant angular frequency between the sub-module chain within the phase unit and the bridge arm inductor. When the external parameters of the system are determined, the range of values ​​for the bridge arm inductor of the DC power flow controller can be quickly calculated according to the design method described above.

[0168] The mathematical equation for selecting the bridge arm inductance of the DC power flow controller using the above method is as follows:

[0169]

[0170] Among them, L PFC For the bridge arm inductance of the DC power flow controller, ω0 is the fundamental angular frequency, ω res It is the resonant angular frequency of the sub-module chain and the bridge arm inductor within the phase unit of the DC power flow controller and the modular multilevel flexible DC converter. T L0 is the inductance value of the connecting transformer, and L0 is the inductance value of the modular multilevel flexible DC converter bridge arm. dc N represents the inductance value of the smoothing reactor in the DC system. PFC U represents the number of submodules in a submodule chain within a DC power flow controller. CPFC U is the rated voltage of the submodule within the DC power flow controller. d I is the rated voltage of the DC power grid. d ε is the rated current of the DC grid, λ is the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC grid, λ is the upper limit of the transient current change rate under short-circuit fault, η is the per-unit value of the negative sequence current amplitude, C0 is the size of the submodule capacitor of the modular multilevel flexible DC converter, and N is the rated current of the DC grid. MMC The number of sub-modules in one arm of a modular multilevel flexible DC converter.

[0171] Furthermore, the design method for the submodule power devices of the DC power flow control unit and the modular multilevel flexible DC converter aims to select suitable DC power flow controllers and power electronic devices for the submodules of the modular multilevel flexible DC converter. When the external parameters of the system are determined, the voltage and current stresses of the power electronic devices can be determined according to the design method for the submodule power devices of the DC power flow control unit and the modular multilevel flexible DC converter.

[0172] The mathematical equations for calculating the voltage and current stress of the power devices in the DC power flow control unit using the above method are as follows:

[0173]

[0174] Among them, A PFC_U and A PFC_Ures A represents the maximum peak and RMS voltage withstand values ​​for individual power electronic devices in the bridge arm submodules of the DC power flow controller. PFC_I and A PFC_Ires P1 and P2 represent the maximum peak and RMS current withstand values ​​of individual power electronic devices in the bridge arm submodules of the DC power flow controller, respectively. ΔP represents the DC power flow of the two DC lines. 1,2U is the maximum difference in DC power flow between the two DC lines, ε is the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC grid, and U d The rated voltage of the DC power grid, N PFC P represents the number of submodules in a submodule chain within the DC power flow controller, and P represents the total power of the modular multilevel flexible DC converter. Let δ be the phase angle of the AC current in the modular multilevel flexible DC converter, δ be the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E be the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω be the angular frequency, and L be the phase angle of the AC current in the modular multilevel flexible DC converter arm voltage. PFC This is the bridge arm inductor for the DC power flow controller.

[0175] It should be noted that the upper and lower arm current and voltage stresses of the modular multilevel flexible DC converter with embedded DC power flow controller are different from those of the traditional operating mode.

[0176] Furthermore, the mathematical equations for calculating the voltage and current stress of the power devices in a modular multilevel flexible DC converter using the above method are as follows:

[0177]

[0178]

[0179] Among them, A MMC_U and A MMC_Ures For each individual power electronic device in the bridge arm submodule of the modular multilevel flexible DC converter, A represents the maximum peak and effective voltage withstand value. MMC_I and A MMC_Ires The maximum peak and effective current withstand values ​​for individual power electronic devices in the bridge arm submodules of the modular multilevel flexible DC converter are ΔP. 1,2 U is the maximum difference in DC power flow between the two DC lines. d The rated voltage of the DC power grid, N MMC P represents the number of submodules in one arm of the modular multilevel flexible DC converter, and P represents the total power of the modular multilevel flexible DC converter. Let δ be the phase angle of the AC current in the modular multilevel flexible DC converter, δ be the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E be the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω be the angular frequency, and L be the phase angle of the AC current in the modular multilevel flexible DC converter arm voltage. PFC This is the bridge arm inductor for the DC power flow controller.

[0180] The present invention also provides a protection method for the embedded multi-port DC power flow controller under DC short-circuit fault, the protection method including a protection method for the DC power flow controller under DC short-circuit fault, and a method for actively limiting fault current under DC short-circuit fault.

[0181] Furthermore, the protection method for the DC power flow controller under DC short-circuit fault is achieved by controlling the sub-modules within the DC power flow controller to operate in a specific switching state, or by adding a protection device to the sub-modules within the DC power flow controller.

[0182] Furthermore, the specific switching state of the submodule refers to the state in which the capacitor of the submodule is bypassed and the current flows only through the switching device. The protection device consists of a load unloading circuit or a thyristor bypass switch.

[0183] It should be noted that if the submodule switching device can withstand fault current, this can be achieved by operating the submodule in a specific switching state, such as... Figure 5 (a) This prevents fault current from flowing into the submodule capacitor, thus achieving fault protection.

[0184] It should be noted that the purpose of this method is to prevent damage to the DC power flow controller due to overvoltage of the factor module capacitors or overcurrent of the sub-module switching devices under DC short-circuit faults.

[0185] Furthermore, the protection device is a load-relief circuit connected in parallel with the submodule capacitor, such as... Figure 5 (b) The unloading circuit is composed of diodes, resistors and switching devices connected in series and parallel to consume the energy of the fault current, avoid overvoltage of the submodule capacitor, and realize fault protection.

[0186] It should be noted that the protection device is a distributed or centralized thyristor bypass switch, which is composed of anti-parallel thyristors, an RC circuit, and a static resistor connected in parallel. Figure 5 (c) The distributed thyristor bypass switch is installed in each submodule. For example... Figure 5 (d) The centralized thyristor bypass switch is connected in parallel with the submodule chain. By using the rapid action of the bidirectional thyristor to bypass the submodule, fault current is prevented from flowing through the submodule, thus achieving fault protection.

[0187] Furthermore, the method for the DC power flow controller to actively limit the fault current under DC short-circuit fault is achieved by controlling the sub-modules within the DC power flow controller to operate in a specific switching state, or by adding a protection device to the sub-modules within the DC power flow controller.

[0188] It should be noted that the specific switching state of the submodule refers to the state of locking or forward activation of the submodule. The protection device consists of an unloading circuit and a fault current limiter. The unloading circuit is composed of diodes, resistors, and switching devices connected in series and parallel. The fault current limiter is composed of resistors, inductors, and surge arresters connected in series and parallel.

[0189] Furthermore, if the submodule capacitor can withstand a certain fault voltage and the submodule switching device can withstand a fault current, the submodule can be operated in a specific switching state, such as... Figure 5 (e) The submodule is connected in series with the line with a negative voltage to suppress the fault current.

[0190] It should be noted that the purpose of this method is to use a DC power flow controller to assist the DC power grid in suppressing and clearing faults.

[0191] It should be noted that the protection device is a load-relief circuit connected in parallel with the submodule capacitor, such as... Figure 5 (f) enables active suppression of step-down fault current.

[0192] It should be noted that the protection device is a distributed or centralized fault current limiting device, which is composed of a thyristor bypass switch, a resistor, an inductor, a surge arrester, etc., connected in series. Figure 5 (g) The distributed fault current limiting device is installed in each submodule. For example... Figure 5 (h) The centralized fault current limiting device is connected in parallel with the submodule chain. The fault current limiting device limits the rise of fault current, assisting the DC power grid in suppressing and clearing faults.

[0193] In the first embodiment, the installation scenario and the specific topology of the DC power flow controller are as follows: Figure 4 and Figure 6 This invention utilizes a multi-port DC power flow controller embedded in a modular multilevel flexible DC converter to achieve power flow control of three DC lines. The modular multilevel flexible DC converter is a modular multilevel flexible DC converter. The multi-port DC power flow controller is asymmetrically installed and embedded at the end of the upper arm of the modular multilevel flexible DC converter. The multi-port DC power flow controller includes three DC power flow control modules, each containing a three-phase star-connected arm. Each arm consists of a bipolar full-bridge sub-module chain and an arm inductor. By adjusting the amplitude and phase of the DC and AC voltages on the DC power flow control modules, energy balance is achieved between the DC power flow controller and the modular multilevel flexible DC converter, as well as internal energy balance within the multi-port DC power flow controller. Furthermore, active control of the DC power flow on the three output lines is achieved.

[0194] for Figure 6 The system shown is based on a multi-port DC power flow controller. Through a DC power flow control loop, the DC voltage of the multi-port DC power flow controller is adjusted, thereby changing the DC voltage of the output line and achieving DC power flow control. The energy balance between the DC power flow controller and the modular multilevel flexible DC converter, as well as the energy balance within the multi-port flexible AC interconnect device, are controlled by the energy balance control loop between the modular multilevel flexible DC converter and the DC power flow controller, and the energy equalization control loop within the DC power flow controller, respectively, to achieve stable capacitor voltages of all sub-modules of the multi-port DC power flow controller.

[0195] Example 2

[0196] Reference Figures 7-11 The multi-port DC power flow controller submodule topology is the same as in the first embodiment, but the number of DC power flow controller connection lines is changed to two. In this embodiment, the method of actively limiting fault current under DC short-circuit faults is used to control the multi-port DC power flow controller. When a DC short-circuit fault occurs, the fault protection method and the method of actively limiting fault current under DC short-circuit faults of the DC power flow controller are applied respectively to suppress the rise of the submodule capacitor voltage and the rise of the fault current, ensuring stable operation of the device under complex and extreme conditions and assisting the DC grid in suppressing and clearing faults.

[0197] In this embodiment, the principles for achieving DC power flow control for multiple lines, energy balance between the multi-port DC power flow controller and the modular multilevel flexible DC converter, and internal energy balance of the multi-port DC power flow controller are the same as those in the above embodiments, and will not be repeated here.

[0198] The following will use specific simulation examples to further illustrate the application of the structures and methods in the two embodiments described above.

[0199] Based on the above embodiments, the system is simulated and verified using MATLAB / Simulink software. The simulation parameters are shown in Table 1.

[0200]

[0201] Simulation Example 1:

[0202] The multi-port DC power flow controller based on the modular multilevel flexible DC converter in Implementation Scheme 1 is installed in the DC system at the following location: Figure 4 For a detailed topology diagram of the DC power flow controller and modular multilevel flexible DC converter, please refer to... Figure 7 The control method of implementation plan one is as follows: Figure 8As shown, the multi-port DC power flow controller is connected to three DC lines and controls the DC power flow by adjusting the DC voltage at the output port; the corresponding control loop is the DC power flow control loop. The DC power flow controller embeds a modular multilevel flexible DC converter, achieving overall energy balance through joint control; the corresponding control loop is the energy balance control loop between the modular multilevel flexible DC converter and the DC power flow controller. Internally, the DC power flow controller achieves sub-module energy balance by injecting a base frequency circulating current; the corresponding control loop is the internal energy equalization control loop of the DC power flow controller.

[0203] To verify the power flow control capability, energy balance control stability, and main circuit parameter design rationality of the multi-port DC power flow controller, the simulation was conducted based on two operating conditions set for the power direction of the modular multilevel flexible DC converter. Each operating condition was divided into two stages: power flow distribution under uncontrolled and controlled power flow conditions. The specific operating condition design is as follows:

[0204] Operating Condition 1 (Rectification Mode):

[0205] Phase 1 (Natural Power Flow): The modular multilevel flexible DC converter has a rectified power of 1500MW. At this stage, the DC power flow controller is not activated, and the DC power flow is naturally distributed according to the line impedance. Since the resistances of the three DC lines are different, the currents in the three DC lines are inconsistent. Figure 8 (c) The interval [0~3s] is shown;

[0206] Phase Two (Controlled Power Flow): The DC voltage difference at each port of the DC power flow controller, the port voltage, and the port voltage difference are respectively as follows... Figure 8 (a) and Figure 8 As shown in (e), the current in the DC lines connected to the three DC ports is kept consistent, as follows: Figure 8 (c) The interval [3~6s] is shown. The capacitor voltage waveforms of the internal sub-modules of the modular multilevel flexible DC converter and DC power flow controller in operating condition one are shown below. Figure 8 As shown in (g).

[0207] Operating Condition 2 (Inverter Mode):

[0208] Phase 1 (Natural Power Flow): The modular multilevel flexible DC converter has an inverter power of 1500MW. At this stage, the DC power flow controller is not activated, and the DC power flow is naturally distributed according to the line impedance. Since the resistances of the three DC lines are different, the currents in the connected DC lines are inconsistent. Figure 8 (d) The interval [0~3s] is shown;

[0209] Phase Two (Controlled Power Flow): The DC voltage difference at each port of the DC power flow controller, the port voltage, and the port voltage difference are respectively as follows... Figure 8 (b) and Figure 8 As shown in (f), the current in the DC lines connected to the three DC ports is kept consistent, as follows: Figure 8 (d) The interval [3~6s] is shown. The capacitor voltage waveforms of the internal sub-modules of the modular multilevel flexible DC converter and DC power flow controller in operating condition two are shown below. Figure 8 As shown in (h).

[0210] Figure 8 To illustrate the simulation results for operating conditions 1 and 2 in implementation scheme 1, eight waveform diagrams are included. From left to right and from top to bottom, they represent the DC voltage waveforms at each line port of operating conditions 1 and 2, the DC current waveforms at each line port of operating conditions 1 and 2, the DC voltage difference waveforms at each line port of operating conditions 1 and 2, and the capacitor voltage waveforms of the internal sub-modules of the modular multilevel flexible DC converter and DC power flow controller in operating conditions 1 and 2.

[0211] Simulation waveform results show that, under different DC current flow conditions, the embedded multi-port DC power flow controller can effectively regulate the power flow of multiple DC lines and has bidirectional power flow control capability. The energy balance control loop between the modular multilevel flexible DC converter and the DC power flow controller, as well as the internal energy balance control loop of the DC power flow controller, can effectively maintain the energy balance of the device's sub-modules. The main circuit parameter design method can also effectively design and select the main circuit electrical parameters.

[0212] Simulation Example 2:

[0213] In implementation scheme two, the multi-port DC power flow controller is connected to two lines, such as... Figure 9 As shown. The control strategy during steady-state operation is also consistent with that in Implementation Scheme 1, such as... Figure 7 As shown. The difference is that implementation scheme two adds a current suppression strategy under DC short-circuit faults.

[0214] The simulation assumes a bipolar short circuit occurs at DC port 1 of the multi-port DC power flow controller, such as... Figure 9 As shown.

[0215] Operating Condition 1 (DC Power Controller Fault Protection):

[0216] Fault location is completed 1.5ms after the fault occurs, and the DC power flow controller submodule is bypassed to prevent fault current from entering the submodule capacitor, thus achieving fault protection.

[0217] Operating Condition 2 (Active Fault Current Suppression):

[0218] 1.5ms after the fault occurs, an active current limiting strategy is executed, which puts the bipolar submodule in the forward direction to form a reverse voltage, slows down the rise of the fault current, and assists the system in fault ride-through.

[0219] Figure 10and Figure 11 The simulation results are for working condition one and working condition two, respectively. Figure 10 The diagram contains a total of 4 waveforms, which, from left to right and from top to bottom, represent: the capacitor voltage of the submodules in the two DC power flow control units with and without fault protection strategies, and the switching device current of the submodules in the two DC power flow control units with and without fault protection strategies. Figure 11 The chart contains a total of four waveforms, which are arranged from left to right and from top to bottom as follows: a comparison waveform of fault current before and after the addition of fault current limiting strategy to the modular multilevel flexible DC converter; a comparison waveform of fault current at two ports before and after the addition of fault current limiting strategy to the multi-port DC power flow controller; a comparison waveform of sub-module capacitor voltage before and after the addition of fault current limiting strategy to the modular multilevel flexible DC converter; and a comparison waveform of sub-module capacitor voltage before and after the addition of fault current limiting strategy to the multi-port DC power flow controller.

[0220] Simulation waveform results show that the fault protection strategy of the embedded multi-port DC power flow controller can effectively prevent fault current from entering the sub-module capacitor and avoid overvoltage of the sub-module capacitor; the protection method of the embedded multi-port DC power flow controller under DC short circuit fault can effectively suppress the fault current of the DC power flow controller, and also has a certain suppressive effect on the fault current of the modular multilevel flexible DC converter.

[0221] It is important to note that the constructions and arrangements of this application shown in several different exemplary embodiments are merely illustrative. Although only a few embodiments are described in detail in this disclosure, those who consult this disclosure will readily understand that many modifications are possible (e.g., changes in the size, dimensions, structure, shape, and proportions of various elements, as well as parameter values ​​(e.g., temperature, pressure, etc.), mounting arrangements, use of materials, color, orientation, etc.) without substantially departing from the novel teachings and advantages of the subject matter described in this application). For example, an element shown as integrally formed may be composed of multiple parts or elements, the position of elements may be inverted or otherwise altered, and the nature or number or position of discrete elements may be changed or altered. Therefore, all such modifications are intended to be included within the scope of the invention. The order or sequence of any process or method steps may be changed or rearranged according to alternative embodiments. In the claims, any "device plus function" clause is intended to cover the structure described herein that performs the function, and not only structurally equivalent but also equivalent in structure. Other substitutions, modifications, alterations, and omissions may be made in the design, operation, and arrangement of the exemplary embodiments without departing from the scope of the invention. Therefore, the present invention is not limited to the specific embodiments, but extends to various modifications that still fall within the scope of the appended claims.

[0222] Furthermore, in order to provide a concise description of exemplary embodiments, not all features of actual embodiments (i.e., those features that are not relevant to the currently considered best mode for carrying out the invention, or those features that are not relevant to implementing the invention) may be omitted.

[0223] It should be understood that numerous specific implementation decisions can be made during the development of any practical implementation, such as in any engineering or design project. Such development efforts may be complex and time-consuming, but for those skilled in the art who benefit from this disclosure, the development effort will be a routine work of design, manufacturing, and production without requiring much experimentation.

[0224] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

[0225] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

[0226] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. The solutions in the embodiments of this application can be implemented in various computer languages, such as the object-oriented programming language Java and the interpreted scripting language JavaScript.

[0227] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0228] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0229] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0230] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0231] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A flexible DC converter with multi-port DC power flow control, characterized in that: include, Modular multilevel flexible DC converter; and, DC power flow controller embedded in modular multilevel flexible DC converter; The energy balance control loop between the modular multilevel flexible DC converter and the DC power flow controller includes... The energy balance control loop between the modular multilevel flexible DC converter and the DC power flow controller forms AC coupling energy by superimposing energy balance control voltages on the bridge arms of the DC power flow control unit and the modular multilevel converter, respectively, to compensate for the accumulated DC energy. The mathematical equations for the overall input and output energy of the DC power flow controller are as follows: Among them, P DCPFC_out P represents the total output energy of the multi-port DC power flow controller. DCPFC_in U represents the total input energy of the multi-port DC power flow controller. pia,dc and U pia,ac These are the DC and AC components of the voltage of the DC power flow control unit connected to the i-th line, respectively. oi I is the DC current of the i-th line. d It is the sum of the DC currents of all lines, i jp,ac It is the AC component of the upper arm current of phase j of the modular multilevel flexible DC converter.

2. A flexible DC converter with multi-port DC power flow control as described in claim 1, characterized in that: The DC power flow controller includes two or more DC power flow control units, the number of which is equal to the number of ports of the DC power flow controller. Each DC power flow control unit can adjust the DC power flow on the connected DC line.

3. A flexible DC converter with multi-port DC power flow control as described in claim 2, characterized in that: The DC power flow control unit consists of a three-phase star-connected sub-module chain and a bridge arm inductor. The three-phase neutral point connected by the sub-module chain and the inductor serves as the DC power flow regulation port.

4. A flexible DC converter with multi-port DC power flow control as described in claim 3, characterized in that: The modular multilevel flexible DC converter is a three-phase, voltage source converter with a modular multilevel structure, which is either medium-voltage or high-voltage. It has the function of AC / DC power conversion to realize the connection between medium- and high-voltage AC power grids and DC systems.

5. A flexible DC converter with multi-port DC power flow control as described in claim 4, characterized in that: The DC power flow controller is symmetrically installed on the upper and lower bridge arms of the modular multilevel flexible DC converter, or only on the upper or lower bridge arm of the modular multilevel flexible DC converter.

6. A flexible DC converter with multi-port DC power flow control as described in claim 5, characterized in that: The submodule chain is a unipolar submodule chain, a bipolar submodule chain, or a hybrid submodule chain composed of unipolar and bipolar submodules.

7. A control method applied to a flexible DC converter with multi-port DC power flow control as described in claim 1, characterized in that, The control method includes a DC power flow control loop on multiple lines, an energy balance control loop between the modular multilevel flexible DC converter and the DC power flow controller, and an internal energy balance control loop of the DC power flow controller. When the flexible DC converter with multi-port DC power flow control is connected to two or more DC lines, the sum of the power on all output lines is equal to the total power of the modular multilevel flexible DC converter. If the flexible DC converter with multi-port DC power flow control is connected to N DC output lines, then the power flow on N-1 of these lines is actively controlled, and the DC power flow on the remaining line is equal to the total power flow minus the sum of the power flows on the other lines.

8. The control method for a flexible DC converter with multi-port DC power flow control as described in claim 7, characterized in that: The DC power flow control loops on the multiple lines include, The line power flow control loop calculates the DC component of the voltage of each DC power flow control unit by subtracting the voltage reference values ​​of other lines from the voltage of the reference circuit. The mathematical equation is as follows: Among them, U d U is the rated voltage of the DC system, which is also equal to the voltage of the reference line. p1a U pka U pNa These are the DC components of the voltages of the DC power flow control units connected to the 1st, kth, and Nth lines, respectively, U. o1 U ok U oN These are the DC voltage reference values ​​for the 1st line, the kth line, and the Nth line, respectively.

9. The control method for a flexible DC converter with multi-port DC power flow control as described in claim 8, characterized in that: The internal energy balancing control loop of the DC power flow controller includes... The mathematical equations for the energy relationships within the DC power flow controller are as follows: Among them, Δp1, Δp k and Δp N These are the DC energy values ​​of the 1st, kth, and Nth DC power flow control units, Δp. c1a Δp cka and Δp cNa These are the AC coupling energies of the 1st, kth, and Nth DC power flow control units, respectively.

10. A method for designing main circuit parameters for a flexible DC converter with multi-port DC power flow control as described in claim 1, characterized in that, The main circuit parameter design method includes a method for designing the number of sub-modules of the DC power flow control unit and the modular multilevel flexible DC converter, a method for designing the capacitors of the sub-modules of the DC power flow control unit and the modular multilevel flexible DC converter, a method for designing the bridge arm inductors of the DC power flow controller, and a method for designing the power devices of the sub-modules of the DC power flow control unit and the modular multilevel flexible DC converter.

11. The main circuit parameter design method for a flexible DC converter with multi-port DC power flow control as described in claim 10, characterized in that: The DC power flow controller adopts an asymmetric installation method and is embedded in the end of the upper arm of the modular multilevel flexible DC converter. The DC power flow controller adopts a bipolar full-bridge submodule, and the modular multilevel flexible DC converter adopts a unipolar half-bridge submodule.

12. The main circuit parameter design method for a flexible DC converter with multi-port DC power flow control as described in claim 11, characterized in that: The design method for the number of sub-modules in the DC power flow control unit and the modular multilevel flexible DC converter. The mathematical equation for selecting the number of DC power flow controller submodules is: Where, N PFC U represents the number of submodules in a submodule chain within a DC power flow controller. CPFC U is the rated voltage of the submodule within the DC power flow controller. d ΔP is the rated voltage of the DC power grid, ε is the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC power grid. 1,2 φ is the maximum difference in DC power flow between the two DC lines, P is the total power of the modular multilevel flexible DC converter, φ is the phase angle of the AC current of the modular multilevel flexible DC converter, δ is the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E is the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω is the angular frequency, and L is the maximum difference in DC power flow between the two DC lines, P is the total power of the modular multilevel flexible DC converter, φ is the phase angle of the AC current of the modular multilevel flexible DC converter arm voltage, δ is the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E is the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω is the angular frequency, and L is the phase angle of the AC current of the modular multilevel flexible DC converter arm voltage. PFC For the bridge arm inductance of the DC power flow controller; The mathematical equation for selecting the number of submodules in a modular multilevel flexible DC converter is: Where, N MMC,p and N MMC,n These represent the number of submodules in the upper and lower arms of the modular multilevel flexible DC converter, respectively. C The rated voltage of the modular multilevel flexible DC converter submodule, U d ΔP is the rated voltage of the DC power grid, ε is the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC power grid. 1,2 φ is the maximum difference in DC power flow between the two DC lines, P is the total power of the modular multilevel flexible DC converter, φ is the phase angle of the AC current of the modular multilevel flexible DC converter, δ is the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E is the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω is the angular frequency, and L is the maximum difference in DC power flow between the two DC lines, P is the total power of the modular multilevel flexible DC converter, φ is the phase angle of the AC current of the modular multilevel flexible DC converter arm voltage, δ is the phase angle difference between the fundamental AC component of the modular multilevel flexible DC converter arm voltage and the grid voltage, E is the amplitude of the fundamental AC component of the modular multilevel flexible DC converter arm voltage, ω is the angular frequency, and L is the phase angle of the AC current of the modular multilevel flexible DC converter arm voltage. PFC This is the bridge arm inductor for the DC power flow controller.

13. The main circuit parameter design method for a flexible DC converter with multi-port DC power flow control as described in claim 12, characterized in that: The design method for the submodule capacitors of the DC power flow control unit and the modular multilevel flexible DC converter. The mathematical equation for selecting the capacitance value of the DC power flow controller submodule is as follows: Among them, C PFC N represents the capacitance of the submodule of the DC power flow controller. PFC U represents the number of submodules in a submodule chain within a DC power flow controller. CPFC R1 represents the rated voltage of the submodule within the DC power flow controller; r1 and r2 represent the fundamental frequency and second harmonic frequency ripple rates of the submodule capacitor voltage, respectively; ε is the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC grid; P is the total power of the modular multilevel flexible DC converter; φ is the phase angle of the AC current of the modular multilevel flexible DC converter; δ is the phase angle difference between the fundamental AC component of the arm voltage of the modular multilevel flexible DC converter and the grid voltage; E is the amplitude of the fundamental AC component of the arm voltage of the modular multilevel flexible DC converter; ω is the angular frequency; L PFC For the bridge arm inductance of the DC power flow controller; The mathematical equation for selecting the capacitance value of the modular multilevel submodule is as follows: Where C0 is the size of the sub-module capacitor of the modular multilevel flexible DC converter, and N MMC For a modular multilevel flexible DC converter, the number of submodules in one arm, U C ε1 and ε2 are the fundamental frequency and second harmonic frequency fluctuation rates of the submodule capacitors of the modular multilevel flexible DC converter, respectively. P is the total power of the modular multilevel flexible DC converter, φ is the phase angle of the AC current of the modular multilevel flexible DC converter, and δ is the phase angle difference between the fundamental AC component of the bridge arm voltage of the modular multilevel flexible DC converter and the grid voltage.

14. The main circuit parameter design method for a flexible DC converter with multi-port DC power flow control as described in claim 13, characterized in that: The bridge arm inductor design method of the DC power flow controller, The mathematical equation for selecting the bridge arm inductor of the DC power flow controller is as follows: Among them, L PFC For the bridge arm inductance of the DC power flow controller, ω0 is the fundamental angular frequency, ω res It is the resonant angular frequency of the sub-module chain and the bridge arm inductor within the phase unit of the DC power flow controller and the modular multilevel flexible DC converter. T L0 is the inductance value of the connecting transformer, and L0 is the inductance value of the modular multilevel flexible DC converter bridge arm. dc N represents the inductance value of the smoothing reactor in the DC system. PFC U represents the number of submodules in a submodule chain within a DC power flow controller. CPFC U is the rated voltage of the submodule within the DC power flow controller. d I is the rated voltage of the DC power grid. d ε is the rated current of the DC grid, λ is the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC grid, λ is the upper limit of the transient current change rate under short-circuit fault, η is the per-unit value of the negative sequence current amplitude, C0 is the size of the submodule capacitor of the modular multilevel flexible DC converter, and N is the rated current of the DC grid. MMC The number of sub-modules in one arm of a modular multilevel flexible DC converter.

15. The main circuit parameter design method for a flexible DC converter with multi-port DC power flow control as described in claim 14, characterized in that: The design method for submodule power devices of the DC power flow control unit and the modular multilevel flexible DC converter. The mathematical equations for calculating the voltage and current stresses of power devices in a DC power flow control unit are as follows: Among them, A PFC_U and A PFC_Ures A represents the maximum peak and RMS voltage withstand values ​​for individual power electronic devices in the bridge arm submodules of the DC power flow controller. PFC_I and A PFC_Ires P1 and P2 represent the maximum peak and RMS current withstand values ​​of individual power electronic devices in the bridge arm submodules of the DC power flow controller, respectively. P1 and P2 represent the DC power flow of the two DC lines, ΔP. 1,2 U represents the maximum difference in DC power flow between the two DC lines, ε is the ratio of the maximum regulating voltage of the DC power flow controller to the rated voltage of the DC grid, and U d The rated voltage of the DC power grid, N PFC Here, P represents the number of submodules in a submodule chain within the DC power flow controller, φ represents the total power of the modular multilevel flexible DC converter, δ represents the phase angle of the AC current in the modular multilevel flexible DC converter, E represents the phase angle difference between the fundamental AC component of the arm voltage of the modular multilevel flexible DC converter and the grid voltage, ω represents the amplitude of the fundamental AC component of the arm voltage of the modular multilevel flexible DC converter, and L represents the angular frequency. PFC For the bridge arm inductance of the DC power flow controller; The mathematical equations for calculating the voltage and current stresses of the power devices in a modular multilevel flexible DC converter are as follows: Among them, A MMC_U and A MMC_Ures For each individual power electronic device in the bridge arm submodule of the modular multilevel flexible DC converter, A represents the maximum peak and effective voltage withstand value. MMC_I and A MMC_Ires The maximum peak and effective current withstand values ​​for individual power electronic devices in the bridge arm submodules of the modular multilevel flexible DC converter are ΔP. 1,2 U is the maximum difference in DC power flow between the two DC lines. d The rated voltage of the DC power grid, N MMC Let P be the number of submodules in one arm of the modular multilevel flexible DC converter, φ be the phase angle of the AC current in the modular multilevel flexible DC converter, δ be the phase angle difference between the fundamental AC component of the arm voltage and the grid voltage, E be the amplitude of the fundamental AC component of the arm voltage, ω be the angular frequency, and L be the total power of the modular multilevel flexible DC converter. PFC This is the bridge arm inductor for the DC power flow controller.

16. A protection method for a flexible DC converter with multi-port DC power flow control as described in claim 1 under a DC short-circuit fault, characterized in that, The protection methods include a protection method for a DC power flow controller under a DC short-circuit fault, and a method for actively limiting the fault current under a DC short-circuit fault.

17. The protection method for a flexible DC converter with multi-port DC power flow control under DC short-circuit fault as described in claim 16, characterized in that: The protection method for the DC power flow controller under DC short-circuit fault includes, This can be achieved by controlling the sub-modules within the DC power flow controller to operate in specific switching states, or by adding a protection device to the sub-modules within the DC power flow controller. The specific switching state of the submodule refers to the state in which the capacitor of the submodule is bypassed and the current flows only through the switching device. The protection device consists of a load unloading circuit or a thyristor bypass switch. The submodule switching device withstands the fault current. By operating the submodule in a specific switching state, the fault current is prevented from flowing into the submodule capacitor, thus achieving fault protection.

18. The protection method for a flexible DC converter with multi-port DC power flow control under DC short-circuit fault as described in claim 17, characterized in that: The protection device is a load-relief circuit connected in parallel with the submodule capacitor. The unloading circuit is composed of diodes, resistors, and switching devices connected in series and parallel. It consumes the energy of the fault current, avoids overvoltage of the submodule capacitor, and realizes fault protection.

19. The protection method for a flexible DC converter with multi-port DC power flow control under DC short-circuit fault as described in claim 18, characterized in that: The protection device is a distributed or centralized thyristor bypass switch. Each thyristor bypass switch is composed of anti-parallel thyristors, an RC circuit, and a static resistor connected in parallel. The distributed thyristor bypass switch is installed in each sub-module, and the centralized thyristor bypass switch is chained in parallel with the sub-module. By using the rapid action of bidirectional thyristors to bypass the sub-module, fault current is prevented from flowing through the sub-module, thus achieving fault protection.

20. The protection method for a flexible DC converter with multi-port DC power flow control under DC short-circuit fault as described in claim 19, characterized in that: The method for actively limiting fault current under DC short-circuit fault conditions includes: This can be achieved by controlling the sub-modules within the DC power flow controller to operate in specific switching states, or by adding a protection device to the sub-modules within the DC power flow controller. The specific switching state of the submodule refers to the state of locking or forward activation of the submodule. The protection device consists of an unloading circuit and a fault current limiter. The unloading circuit is composed of diodes, resistors, and switching devices connected in series and parallel. The fault current limiter is composed of resistors, inductors, and surge arresters connected in series and parallel. The submodule capacitor withstands a certain fault voltage, and the submodule switching device withstands the fault current. By operating the submodule in a specific switching state, the submodule is connected in series with the circuit in a negative voltage manner to suppress the fault current.

21. The protection method for a flexible DC converter with multi-port DC power flow control under DC short-circuit fault as described in claim 20, characterized in that: The protection device is a load-relief circuit connected in parallel with the submodule capacitor to achieve active suppression of step-down fault current. The protection device is a distributed or centralized fault current limiting device, which is composed of a thyristor bypass switch, resistor, inductor, and surge arrester connected in series. The distributed fault current limiting device is installed in each submodule, and the centralized fault current limiting device is connected in parallel with the submodule. Through the fault current limiting device, the rise of fault current is limited, assisting the DC grid in suppressing and clearing faults.

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