A flexible straight topology with inertia support performance and a design control method thereof

Abstract

The application provides a flexible direct current topology with inertia support performance, a rectifying side of which comprises an MMC converter station, at least two MMC converter stations in parallel connection at an inverting side, an overcapacity energy cooperative controller and an overcapacity branch controller, the overcapacity energy cooperative controller is connected with the MMC converter station, the overcapacity energy cooperative controller is connected with the overcapacity branch controller, the MMC converter station comprises two cascade MMC structure converters, the MMC converter stations at the rectifying side and the inverting side are connected through a high-voltage direct current bus, and the rectifying side comprises at least two direct-hanging type overcapacity branches. The flexible direct current topology with inertia support performance can effectively solve the problem of insufficient system rotational inertia and reactive power support, the MMC converter adopting a network construction control strategy is used to provide inertia and reactive power support for the system, and a multi-terminal topology is used to reduce the rated current of the converter, so that the network construction MMC converter station has a greater current overload capacity, and the overload demand of power support under transient state is met.

Description

Technical Field

[0001] The present application relates to the field of ultra-high voltage direct current (UHVDC) transmission technology, and in particular to a flexible direct current (FDC) topology with inertia support performance and a design and control method thereof. Background Art

[0002] With the accelerated development of new power systems, the proportion of installed renewable energy capacity has increased significantly, and conventional synchronous generators are being replaced by power electronic devices. Traditional power systems, primarily driven by synchronous motors, are transitioning to low-inertia, weak grids. The resulting problem of insufficient system inertia is becoming increasingly prominent. Flexible DC transmission technology, with its numerous technical advantages, including flexible control, low harmonic content, and strong scalability, has been widely adopted in large-scale clean energy transmission scenarios and has become a key component of new power systems. Previously, flexible DC converters employed a grid-following control strategy, relying on phase-locked loops for grid synchronization. This led to stability issues in weak grids with low physical inertia, making it difficult to meet the critical demands of new power system development.

[0003] Unlike traditional grid-following flexible DC converters, grid-forming flexible DC converters do not rely on external AC system voltage for synchronization. Instead, they control their own output voltage to build the AC output voltage amplitude and phase. Their external characteristics are similar to those of synchronous generators, and they respond quickly to disturbances to provide inertia support for the system. Grid-forming flexible DC converters are more suitable for weak systems. However, to provide sufficient inertia support, the bridge arms of grid-forming converters carry large transient currents during transient periods. Limited by the overcurrent capacity of the converter components, traditional converter topologies struggle to meet the active power overload requirements for inertia support. Furthermore, the transient power required for inertia support can cause a sudden change in the power of the station, which also results in a high overload condition for the station, seriously threatening the safe and stable operation of the flexible DC equipment. Summary of the Invention

[0004] The present application provides a flexible DC topology with inertia support performance and a design and control method thereof to solve the problem that traditional converter topologies are difficult to meet the active overload requirements required for inertia support and the power mutation of the station.

[0005] In order to solve the above technical problems, the present application provides a flexible direct current topology with inertia support performance, wherein the rectifier side includes an MMC converter station, the inverter side includes at least two MMC converter stations in parallel, an ultra-capacity energy collaborative controller and an ultra-capacity branch controller, the ultra-capacity energy collaborative controller is connected to the MMC converter station, the ultra-capacity energy collaborative controller is connected to the ultra-capacity branch controller, the MMC converter station includes two cascaded MMC structure converters, the MMC converter stations on the rectifier side and the inverter side are connected through a high-voltage DC bus, and the rectifier side includes at least two direct-hung ultra-capacity branches.

[0006] Furthermore, the direct-hanging super-capacity branch includes a plurality of super-capacity energy storage modules and a reactor connected in series with the super-capacity energy storage modules.

[0007] Furthermore, one end of the transmitting and receiving converter stations adopts constant DC voltage control, and the other end adopts constant power control, wherein at least one converter station adopts a grid-type control strategy.

[0008] The present invention also provides a design method for a flexible DC topology with inertia support performance, comprising:

[0009] Calculate the maximum current I of the direct-mounted overcapacity branch SC_max , Among them, △P fmax is the maximum value of the active power variation of the MMC converter station under the maximum frequency step value of the AC system, △T max is the time required for the active power of the network MMC converter station to recover to the rated power of the network MMC converter station, V SCN It is the rated voltage of the direct-mounted overcapacity branch, which is equal to the high-voltage DC bus voltage;

[0010] If the maximum current of the direct-hanging overcapacity branch is I SC_max If the maximum current of the direct-hanging supercapacitor branch is not more than the maximum current capacity of the supercapacitor module, the number of branches of the direct-hanging supercapacitor branch is n=1; if the maximum current of the direct-hanging supercapacitor branch is I SC_max Exceeding the maximum flow capacity of the super-capacity module, adopting the super-capacity multi-branch parallel installation method, and meeting Among them, I SCN_i (i=1,2…n) is the rated current of each direct-hung overcapacity branch;

[0011] The configuration of the direct-mounted overcapacity branch that meets the device overcurrent constraints, output power constraints, and energy constraints is: Among them, the energy consumed by the internal resistance of the excess capacitance branch is: The active energy required by the excess capacity branch is: E SCN_i =ΔP f max ΔT max , C SC_i is the equivalent capacitance value of the supercapacitors in series when all the supercapacitors are in operation on all branches, R SC_i is the internal resistance value of the overcapacitor when all branches are in operation; determine the open circuit voltage V when all branches are in operation with overcapacitors SC_i , get the equivalent capacitance value C SC_i , according to the equivalent capacitance value C SC_i Determine the number of overcapacity modules for a single branch.

[0012] The present invention also provides a control method for a flexible DC topology with inertia support performance, comprising:

[0013] Step 1: The super-capacity energy cooperative controller receives the active power shortage signal output by the networked MMC converter station;

[0014] Step 2: When the reference value of the output active power of each direct-hanging type over-capacity branch is less than the maximum power support level of each direct-hanging type over-capacity branch, the over-capacity energy collaborative controller adjusts the reference value of the output active power of each direct-hanging type over-capacity branch according to the energy collaborative configuration principle. When the reference value of the output active power of any over-capacity branch of each direct-hanging type over-capacity branch is equal to the maximum power support level of each direct-hanging type over-capacity branch, the direct-hanging type over-capacity branch maintains the maximum power until the energy supply in the transient stage ends;

[0015] Step three: obtain a reference value of the direct-hanging super-capacity branch current based on the reference value of the active power output by the direct-hanging super-capacity branch and the high-voltage DC bus voltage; compare the reference value of the direct-hanging super-capacity branch current with the actual current of the direct-hanging super-capacity branch; add the difference to the high-voltage DC bus voltage after passing through the PI link to obtain the reference voltage of the direct-hanging super-capacity branch; obtain the number of super-capacity clusters required for the super-capacity branch through the reference voltage and the current average voltage of the super-capacity cluster module; and generate a switching pulse after the modulation link to control the on and off of the super-capacity module.

[0016] Furthermore, the overcapacity energy cooperative controller adjusts the reference value of the output active power of each direct-mounted overcapacity branch according to the energy cooperative configuration principle: if ΔP SC ≥βP mmcn , then according to Adjust the reference value of active power output of each direct-mounted over-capacity branch, where ΔP SC =P mmc -P mmcr , ΔP SC Is the active power shortage signal, P mmc is the real-time active power of the MMC converter station, P mmcr is the reference power of the MMC converter station, β is the threshold judgment coefficient, P mmcn is the total rated power of the MMC converter station in the network, μ i (i=1,2…n) is the energy coordination coefficient of the direct-hung type excess capacity branch, n is the total number of direct-hung type excess capacity branches, P SC_i It is the reference value of the output active power of each direct-mounted over-capacity branch.

[0017] Furthermore, the energy synergy coefficient of the direct-hanging type excess capacity branch is related to the electrical distance between the direct-hanging type excess capacity branch and the receiving end station, and the μ of the direct-hanging type excess capacity branch with the shortest electrical distance to the receiving end station is i It is 0.9~1.0.

[0018] The flexible DC topology with inertia support performance provided by the present invention can effectively solve the problems of insufficient rotational inertia and reactive power support in the current system. By adopting the MMC converter with a network control strategy to provide inertia and reactive power support for the system, and at the same time adopting a multi-terminal topology to reduce the rated current of the converter, the networked MMC converter station has a larger current overload capacity margin, meeting the overload demand of power support under transient conditions. In addition, a direct-hung super-capacity branch is installed on the DC side to maintain transient power stability, providing energy support for the MMC converter station, which can effectively reduce the overload level of the opposite converter station. While meeting the inertia support energy demand on the DC side, the output circuit of a single energy storage branch is reduced, ensuring the safe and stable operation of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the specific implementation methods or the description of the prior art. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0020] Figure 1 A schematic diagram of a flexible DC topology structure with inertia support performance provided by an embodiment of the present invention;

[0021] Figure 2 Schematic diagram of the topological structure of the MMC converter provided by an embodiment of the present invention;

[0022] Figure 3 Schematic diagram of the topological structure of the series-connected submodule (SM) provided in an embodiment of the present invention;

[0023] Figure 4 This is a simulation waveform of the active power of the MMC converter station in the receiving-end network under frequency step provided by an embodiment of the present invention;

[0024] Figure 5 This is a simulated waveform diagram of the DC side voltage at the transmitting and receiving ends under frequency steps provided by an embodiment of the present invention;

[0025] Figure 6 This is a simulation waveform diagram of the active power of the sending-end network MMC converter station, the receiving-end network MMC converter station and the over-capacity branch under the frequency step provided by the embodiment of the present invention;

[0026] Figure 7 A schematic diagram of the overcapacity energy coordinated control structure of any overcapacity branch provided in an embodiment of the present invention;

[0027] Figure 8 It is a schematic diagram of the super-capacity energy collaborative control structure provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0028] The following is a clear and complete description of the technical solutions in this application in conjunction with the accompanying drawings. Obviously, the embodiments described are part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of this application.

[0029] In the description of this application, it should be noted that the terms "center", "up", "down", "left", "right", "vertical", "horizontal", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, they cannot be understood as limitations on this application.

[0030] In the description of this application, it should be noted that, unless otherwise expressly specified or limited, the terms "installed," "connected," and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; they can refer to internal connections between two components; they can refer to wireless connections or wired connections. Those skilled in the art will understand the specific meanings of the above terms in this application based on the specific circumstances.

[0031] In addition, the technical features involved in the different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0032] Figure 1 Schematic diagram of a flexible DC topology structure with inertia support performance provided by an embodiment of the present invention. Figure 1 The present application provides a flexible direct current topology with inertia support performance, wherein the rectifier side includes an MMC converter station 101, the inverter side includes at least two MMC converter stations 101 in parallel, an overcapacity energy collaborative controller, and an overcapacity branch controller, the overcapacity energy collaborative controller being connected to the MMC converter station 101, the overcapacity energy collaborative controller being connected to the overcapacity branch controller, the MMC converter station 101 including two cascaded MMC structure converters, the MMC converter stations on the rectifier side and the inverter side being connected via a high-voltage DC bus, and the rectifier side including at least two direct-hung overcapacity branches 102. The at least two MMC converter stations connected in parallel at the receiving end adopt a multi-terminal structure to effectively reduce the rated current of the converter.

[0033] The direct-mounted super-capacity branch includes multiple super-capacity energy storage modules (ES1, ES2, ..., ESn) connected in series with reactors. The transmitting and receiving converter stations utilize constant DC voltage control at one end and constant power control at the other. At least one of these converter stations employs a grid-based control strategy, which provides active and reactive power support for the system during transients by structuring the internal potential amplitude and phase.

[0034] Figure 2 Schematic diagram of the topology of the MMC converter provided by the embodiment of the present invention. Figure 2 The MMC converter has the same three-phase six-bridge structure, and each bridge arm has N series submodules (SM) (SM1, SM2, ... SM N ), L arm is the bridge arm inductance, R arm The bridge arm resistor is composed of multiple full-bridge and half-bridge sub-modules. Each sub-module contains a switching device and a parallel diode. By controlling the number of series sub-modules on and off, the bridge arm output voltage is adjusted, thereby adjusting the AC voltage output value. Figure 3 Schematic diagram of the topology of the series submodule (SM) provided by the embodiment of the present invention. Figure 3 The series submodule is composed of a full-bridge submodule, which includes switching devices T1, T2, T3, and T4 and a parallel diode C0.

[0035] The DC side includes at least one direct-mounted energy storage branch. The super-capacity energy storage device branch is composed of multiple energy storage sub-modules and export reactors in series. The energy storage device actively identifies the transmission power changes of the sending and receiving converter stations during the system steady transient period. According to the identified power changes, it quickly responds to the output power for power compensation, effectively reducing the overload level of the sending and receiving converter stations and maintaining the stability of the DC bus power.

[0036] The present invention also provides a design method for a flexible DC topology with inertia support performance, comprising:

[0037] Calculate the maximum current I of the direct-mounted overcapacity branch SC_max , Among them, △P fmax is the maximum value of the active power variation of the MMC converter station under the maximum frequency step value of the AC system, △T max is the time required for the active power of the network MMC converter station to recover to the rated power of the network MMC converter station, V SCN is the rated voltage of the direct-hanging super-capacity branch, which is equal to the high-voltage DC bus voltage; first determine the maximum frequency step value f of the AC system connected to the network MMC converter station maxThrough the established system simulation model, the rated power output of the network MMC converter station is tested. Under the condition of no additional overcapacity, the maximum frequency step value f of the AC system of the network MMC converter station is max The maximum value of the active power change △P fmax。 Figure 4 This is a simulation waveform of the active power of the MMC converter station in the receiving network under the frequency step provided by the embodiment of the present invention. Figure 4 At time t=8.0s, the system experiences a frequency step, with the AC system frequency jumping from 50Hz to 49.5Hz. Active power is injected into the AC system to provide inertia support for the system. The energy storage device recognizes the power change at the receiving end and then outputs power, maintaining the DC side voltage and power at the sending and receiving ends stable.

[0038] If the maximum current of the direct-hanging overcapacity branch is I SC_max If the maximum current of the direct-hanging supercapacitor branch is not more than the maximum current capacity of the supercapacitor module, the number of branches of the direct-hanging supercapacitor branch is n=1; if the maximum current of the direct-hanging supercapacitor branch is I SC_max Exceeding the maximum flow capacity of the super-capacity module, adopting the super-capacity multi-branch parallel installation method, and meeting Among them, I SCN_i (i=1,2…n) is the rated current of each direct-hung overcapacity branch; Figure 5 This is a simulation waveform of the DC voltage at the transmitting and receiving ends under the frequency step provided by the embodiment of the present invention. Figure 5 , where simulation waveform 1 represents the change of receiving end voltage over time, and simulation waveform 2 represents the change of sending end voltage over time. When inertia support is provided, the voltages at both ends of the sending and receiving ends remain basically unchanged. After the direct-hanging super-capacity branch is added to the system, the voltage changes at both ends are relatively small.

[0039] The configuration of the direct-mounted overcapacity branch that meets the device overcurrent constraints, output power constraints, and energy constraints is: Among them, the energy consumed by the internal resistance of the excess capacitance branch is: The active energy required by the excess capacity branch is: E SCN_i =ΔP f max ΔT max , C SC_i is the equivalent capacitance value of the supercapacitors in series when all the supercapacitors are in operation on all branches, R SC_i is the internal resistance value of the overcapacitor when all branches are in operation; determine the open circuit voltage V when all branches are in operation with overcapacitors SC_i , get the equivalent capacitance value C SC_i , according to the equivalent capacitance value C SC_i Determine the number of overcapacity modules for a single branch.

[0040] In the embodiment of the present invention, the rated voltage V of the excess capacity branch is determined. SCN : Rated voltage of the overcapacity branch and MMC DC bus voltage Vdc Consistent, that is, V SCN =400kV.

[0041] Determine the maximum active support power △P of the overcapacity branch fmax and maximum support time △T max :First, determine the maximum frequency step value f of the AC system connected to the network control MMC converter station max , f max 0.5Hz is selected. Through the established system simulation model, the maximum frequency step value f of the AC system of the MMC converter station is tested under the condition of rated power output of the MMC system and no additional overcapacity. max The maximum active power change is the maximum active support power △P of the overcapacity branch. fmax , and the time required for active power to recover to rated power is the maximum support time △T max According to the simulation test results, △P fmax 1200MW, maximum support time △T max 0.5s.

[0042] Determine the number of overcapacity branches n: Based on the system active power and duration requirements, determine the number and maximum capacity of directly connected overcapacity energy storage branches that meet the overcurrent constraints of the energy storage branch devices. In this embodiment, the number of branches is 1, and the total energy released is 600MJ.

[0043] Select an overcapacity branch configuration that meets the device overcurrent constraints, output power constraints, and energy constraints. Reserve an overcapacity redundant cluster to improve the reliability of the energy storage equipment. Considering that the DC current of a single branch is no more than 3kA, select a 144V / 62.5F module with a single module internal resistance of 96mΩ. The total number of overcapacity valve sub-modules is 325.

[0044] Figure 6 The following is a simulation waveform of the active power of the sending-end network MMC converter station, the receiving-end network MMC converter station and the over-capacity branch under the frequency step provided by the embodiment of the present invention. Figure 6 Curve a represents the change of active power of the MMC converter station at the sending end, curve b represents the change of active power of the MMC converter station at the receiving end, and curve c represents the change of active power of the overcapacity branch. When the overcapacity branch is used to provide inertia support for active power changes, the power change amplitude at the sending end is relatively small, and the output power is basically output by the overcapacity branch of the energy storage, ensuring that the change amplitude of the energy transmission power is reduced and the protection system is not overloaded or damaged.

[0045] An embodiment of the present invention also provides a control method for a flexible DC topology with inertia support performance, comprising: step 1, a super-capacitor energy cooperative controller receives an active power shortage signal output by a networked MMC converter station;

[0046] Step 2: When the reference value of the output active power of each direct-hanging type over-capacity branch is less than the maximum power support level of each direct-hanging type over-capacity branch, the over-capacity energy cooperative controller adjusts the reference value of the output active power of each direct-hanging type over-capacity branch according to the energy cooperative configuration principle. When the reference value of the output active power of any over-capacity branch in each direct-hanging type over-capacity branch is equal to the maximum power support level of each direct-hanging type over-capacity branch, the direct-hanging type over-capacity branch maintains the maximum power until the energy supply in the transient stage ends; under the same voltage level, the operating performance of different power electronic devices and over-capacity equipment is different. Therefore, when determining the maximum power support level of each direct-hanging type over-capacity branch, it is necessary to consider the overload capacity of the power electronic device, the capacity of the over-capacity equipment, and the overload capacity of the over-capacity equipment under the selected voltage level, and make a comprehensive evaluation and selection based on the above factors.

[0047] Figure 7 Schematic diagram of the overcapacity energy cooperative control structure of any overcapacity branch provided by the embodiment of the present invention. Figure 7 , the power input P of any overcapacitance branch i sc_i As the starting input quantity of the entire control process, it is compared with the maximum power support level of each direct-hanging type super-capacity branch. When the reference value of the output active power of each direct-hanging type super-capacity branch is less than the maximum power support level of each direct-hanging type super-capacity branch, the super-capacity energy cooperative controller adjusts the reference value of the output active power of each direct-hanging type super-capacity branch according to the energy cooperative configuration principle. When the reference value of the output active power of any super-capacity branch of each direct-hanging type super-capacity branch is equal to the maximum power support level of each direct-hanging type super-capacity branch, the direct-hanging type super-capacity branch maintains the maximum power until the energy supply in the transient stage ends; then, according to the reference value of the output active power of the direct-hanging type super-capacity branch and the high-voltage DC bus voltage V dc Calculate the reference value of the direct-mounted overcapacity branch current I dc_refi , the direct-hung type over-capacity branch current reference value I dc_refi The actual current of the direct-hung overcapacity branch I sc_i The difference is compared with the high-voltage DC bus voltage V after passing through the PI link. dc Add together to obtain the reference voltage V of the direct-hung over-capacitance branch scsm The number of super-capacity clusters required for the super-capacity branch is obtained by combining the reference voltage and the current average voltage of the super-capacity cluster module. After the modulation link, a switching pulse is generated to control the on and off of the super-capacity module.

[0048] Figure 8 This is a schematic diagram of the super-capacity energy collaborative control structure provided by an embodiment of the present invention. Figure 8The super-capacity energy cooperative controller receives the active power shortage signal output by the network control converter station, and dynamically adjusts the reference value of the active power output by each super-capacity branch according to the energy cooperative configuration principle. Each direct-connected super-capacity branch adjusts the active power output by the super-capacity branch according to the obtained active power reference. In an embodiment of the present invention, the super-capacity energy cooperative controller adjusts the reference value of the active power output by each direct-connected super-capacity branch according to the energy cooperative configuration principle: if ΔP SC ≥βP mmcn , then according to Adjust the reference value of active power output of each direct-mounted over-capacity branch, where ΔP SC =P mmc -P mmcr , ΔP SC Is the active power shortage signal, P mmc is the real-time active power of the MMC converter station, P mmcr is the reference power of the MMC converter station, β is the threshold judgment coefficient, P mmcn is the total rated power of the MMC converter station in the network, μ i (i=1,2…n) is the energy coordination coefficient of the direct-hung type excess capacity branch, n is the total number of direct-hung type excess capacity branches, P SC_i The threshold determination coefficient β is a reference value for the output active power of each direct-mounted overcapacity branch. The selection of the threshold determination coefficient requires a preliminary assessment of the AC system inertia level connected to the networked MMC converter station. In this embodiment of the present invention, if the assessment result indicates that the AC system inertia level is low or unstable, the threshold determination coefficient β is set to 0.5%-1%. If the assessment result indicates that the AC system inertia level is sufficient or relatively stable, the threshold determination coefficient β is set to 1%-3%.

[0049] In the embodiment of the present invention, the energy coordination coefficient of the direct-hanging type excess capacity branch is related to the electrical distance between the direct-hanging type excess capacity branch and the receiving end station. The energy coordination coefficient μ of the direct-hanging type excess capacity branch with the shortest electrical distance to the receiving end station is i The value assigned is relatively large, ranging from 0.9 to 1.0. The maximum power support level of each overcapacity branch under the rated DC voltage of the converter station is calculated based on the capacity of the overcapacity equipment and the overload capacity constraint of the equipment. The support capacity of each overcapacity branch is comprehensively evaluated in combination with the electrical distance of each overcapacity branch. Based on the comprehensive evaluation of the support capacity of each overcapacity branch, the energy coordination coefficient μ of each overcapacity branch is finally selected. i .

[0050] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of this application.

Claims

1. A design method for a flexible DC topology with inertia support performance, characterized in that: The rectifier side of the flexible direct current topology with inertia support performance includes an MMC converter station, at least two MMC converter stations in parallel on its inverter side, an overcapacity energy collaborative controller, and an overcapacity branch controller, the overcapacity energy collaborative controller is connected to the MMC converter station, the overcapacity energy collaborative controller is connected to the overcapacity branch controller, the MMC converter station includes two cascaded MMC structure converters, the MMC converter stations on the rectifier side and the inverter side are connected via a high-voltage DC bus, and the rectifier side includes at least two direct-hung overcapacity branches; Calculate the maximum current of a direct-mounted overcapacity branch Among them, △P fmax is the maximum value of the active power variation of the MMC converter station under the maximum frequency step value of the AC system, △T max is the time required for the active power of the network MMC converter station to recover to the rated power of the network MMC converter station, V SCN It is the rated voltage of the direct-mounted overcapacity branch, which is equal to the high-voltage DC bus voltage; If the maximum current of the direct-hanging overcapacity branch is I SC_max If the maximum current of the direct-hanging supercapacitor branch is not more than the maximum current capacity of the supercapacitor module, the number of branches of the direct-hanging supercapacitor branch is n=1; if the maximum current of the direct-hanging supercapacitor branch is I SC_max Exceeding the maximum flow capacity of the super-capacity module, adopting the super-capacity multi-branch parallel installation method, and meeting Among them, I SCN_i , i=1,2…n is the rated current of each direct-hung over-capacity branch; The configuration of the direct-mounted overcapacity branch that meets the device overcurrent constraints, output power constraints, and energy constraints is: Among them, the energy consumed by the internal resistance of the excess capacitance branch is: The active energy required by the excess capacity branch is: E SCN_i =ΔP fmax ΔT max , C SC_i is the equivalent capacitance value of the supercapacitors in series when all the supercapacitors are in operation on all branches, R SC_i is the internal resistance value of the overcapacitor when all branches are in operation; determine the open circuit voltage V when all branches are in operation with overcapacitors SC_i , get the equivalent capacitance value C SC_i , according to the equivalent capacitance value C SC_i Determine the number of overcapacity modules for a single branch.

2. The design method of flexible DC topology with inertia support performance according to claim 1, characterized in that: The direct-hanging super-capacity branch includes a plurality of super-capacity energy storage modules and a reactor connected in series with the super-capacity energy storage modules.

3. The design method of flexible DC topology with inertia support performance according to claim 1, characterized in that: One end of the sending and receiving converter stations adopts constant DC voltage control, and the other end adopts constant power control, and at least one converter station adopts a grid-type control strategy.

4. A control method for a flexible DC topology with inertia support performance, comprising the flexible DC topology with inertia support performance according to claim 1, characterized in that: include: Step 1: The super-capacity energy cooperative controller receives the active power shortage signal output by the networked MMC converter station; Step 2: When the reference value of the output active power of each direct-hanging type over-capacity branch is less than the maximum power support level of each direct-hanging type over-capacity branch, the over-capacity energy collaborative controller adjusts the reference value of the output active power of each direct-hanging type over-capacity branch according to the energy collaborative configuration principle. When the reference value of the output active power of any over-capacity branch of each direct-hanging type over-capacity branch is equal to the maximum power support level of each direct-hanging type over-capacity branch, the direct-hanging type over-capacity branch maintains the maximum power until the energy supply in the transient stage ends; Step three: obtain a reference value of the direct-hanging super-capacity branch current based on the reference value of the active power output by the direct-hanging super-capacity branch and the high-voltage DC bus voltage; compare the reference value of the direct-hanging super-capacity branch current with the actual current of the direct-hanging super-capacity branch; add the difference to the high-voltage DC bus voltage after passing through the PI link to obtain the reference voltage of the direct-hanging super-capacity branch; obtain the number of super-capacity clusters required for the super-capacity branch through the reference voltage and the current average voltage of the super-capacity cluster module; and generate a switching pulse after the modulation link to control the on and off of the super-capacity module.

5. The control method of flexible DC topology with inertia support performance according to claim 4, characterized in that: The overcapacity energy cooperative controller adjusts the reference value of the output active power of each direct-mounted overcapacity branch according to the energy cooperative configuration principle: if ΔP SC ≥βP mmcn , then according to Adjust the reference value of active power output of each direct-mounted over-capacity branch, where ΔP SC =P mmc -P mmcr , ΔP SC Is the active power shortage signal, P mmc is the real-time active power of the MMC converter station, P mmcr is the reference power of the MMC converter station, β is the threshold judgment coefficient, P mmcn is the total rated power of the MMC converter station in the network, μ i , i=1,2…n is the energy coordination coefficient of the direct-hanging type over-capacity branch, n is the number of branches of the direct-hanging type over-capacity branch, P SC_i It is the reference value of the output active power of each direct-mounted over-capacity branch.

6. The control method of flexible DC topology with inertia support performance according to claim 5, characterized in that: The energy synergy coefficient of the direct-hanging type excess capacity branch is related to the electrical distance between the direct-hanging type excess capacity branch and the receiving end station, and the μ of the direct-hanging type excess capacity branch with the shortest electrical distance to the receiving end station is i It is 0.9~1.0.

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