A DRU-MMC sending-out system and a steady-state operation method and a black start method thereof

Abstract

This invention belongs to the field of onshore renewable energy cluster transmission technology, and relates to a DRU-MMC transmission system and its steady-state operation method and black-start method. The system includes: an islanded power system and a DRU-HVDC system; the active power of the islanded power system is transmitted to the receiving-end grid through the DRU-HVDC system; the receiving-end MMC converter station adopts constant DC voltage control; all renewable energy power stations in the islanded power system adopt PQ-type grid-f control; all grid-connected energy storage power stations in the islanded power system adopt Q-f reactive power synchronization grid-connected control; this invention effectively solves the problems of high cost and complex control in traditional systems; by adding energy storage on the islanded side, the cost is reduced; during steady-state operation, the control strategy of each power station can accurately adjust power and voltage according to different conditions, ensuring efficient system operation.

Description

Technical Field

[0001] This invention belongs to the field of onshore new energy cluster transmission technology, specifically relating to a DRU-MMC transmission system and its steady-state operation method and black start method. Background Technology

[0002] The steady-state operation control and black-start control of offshore wind power or onshore new energy systems transmitted through diode-type high-voltage direct current transmission (DRU-HVDC) systems include: steady-state operation and black-start strategies with auxiliary converters and steady-state operation and black-start strategies with energy storage devices.

[0003] Steady-state operation and black-start strategies with auxiliary converters include: DC auxiliary converters, dual DC auxiliary converters, and auxiliary AC grids. Although the above topologies are not entirely the same, they all essentially draw power from the grid side / DC side through auxiliary converter equipment to provide black-start or steady-state support for the offshore wind farm's grid.

[0004] The steady-state operation and black-start strategy with energy storage involves configuring a small energy storage device in front of the DRU to achieve steady-state operation and black-start of the islanded grid. New energy sources employ grid-connected control, SVG uses constant voltage control, and energy storage is used for the black-start process.

[0005] However, existing onshore renewable energy transmission via DRU-HVDC has the following drawbacks: Auxiliary AC source: When operating in parallel with three modes, additional power flow distribution control is required, and there are issues such as stability under different operating modes; it increases the size of the converter platform, has poor economic efficiency, complex responsibility division, and there are issues of DC voltage distribution in series and power flow distribution in parallel between DRU and MMC; it has poor versatility for expansion and power distribution issues in parallel operation; it requires a certain amount of offshore platform space and pollutes the environment.

[0006] Furthermore, there are few existing schemes for transmitting onshore renewable energy systems via DRU-HVDC, and grid-type systems use traditional droop (positive droop) control. Traditional onshore MMC-MMC type DC systems are expensive and have complex control.

[0007] Therefore, a lower-cost and simpler-to-control method is needed to solve the above-mentioned technical problems. Summary of the Invention

[0008] This invention provides the following technical solution: a DRU-MMC delivery system, comprising: Islanded power systems are used to achieve local aggregation, stabilization, and coordinated control of large-scale renewable energy sources.

[0009] The DRU-HVDC system is used to efficiently and reliably transmit the active power collected by the islanded power system to the receiving-end grid over long distances.

[0010] An isolated power system includes: Distributed renewable energy power plants serve as the main generators, providing clean energy.

[0011] Distributed grid-based energy storage stations are used to absorb / release power to mitigate the impact of the randomness of new energy sources on isolated power grids, actively provide voltage and frequency regulation, and enhance the stability of isolated grids.

[0012] The grid-type energy storage station at the AC bus of the DRU converter station is used to directly support the AC bus voltage, coordinate the power of the sending end with the DC transmission demand, and prevent the DRU from being blocked due to sudden power changes.

[0013] The DRU-HVDC system includes: Sending-end DRU converter station is used to convert AC power from an islanded power system to DC power.

[0014] DC circuit breakers are used to quickly interrupt fault current on the DC side, prevent fault propagation, and protect converter equipment and lines.

[0015] The receiving-end MMC converter station is used to convert DC power into AC power and then synchronously connect it to the receiving-end power grid.

[0016] The receiving-end power grid is used to receive renewable energy power and balance supply and demand through traditional power grid dispatching.

[0017] The active power of the isolated power system is transmitted to the receiving-end grid via the DRU-HVDC system. The receiving-end MMC converter station adopts constant DC voltage control; all new energy power plants in the isolated power system adopt PQ-type grid-following control; and all grid-connected energy storage power plants in the isolated power system adopt Qf reactive synchronous grid-connected control.

[0018] Preferably, the steady-state operation method of the DRU-MMC output system includes: Control methods for distributed grid-type energy storage stations: During steady-state operation, the m-th distributed grid-type energy storage station adjusts the d-axis voltage at its port according to equation (1). and q-axis voltage Adjust the port frequency according to formula (2) and phase angle : (1) (2) In equations (1) and (2), These are the active reference power, actual active power, and theoretical port voltage amplitude used for feedforward of the m-th distributed grid-type energy storage station, respectively. These are the equivalent port active PI controller parameters of the m-th distributed grid-type energy storage station, and S is the Laplace differential operator; These are the active reference power and actual active power of the m-th distributed grid-type energy storage station, respectively. It is the equivalent reactive power droop parameter of the m-th distributed grid-type energy storage station. This represents the system's global theoretical frequency.

[0019] More preferably, the steady-state operation method of the DRU-MMC transmission system further includes: a grid-type energy storage site control method at the AC bus of the DRU converter station; the steady-state operation process of the grid-type energy storage site at the AC bus of the DRU converter station is consistent with the distributed grid-type energy storage control method.

[0020] More preferably, in the control method for the grid-type energy storage site at the AC bus of the DRU converter station, the reactive power reference is provided by the upper-level control, and the active power reference value is provided in the following ways: Case R1: Active power reference value Calculated using equation (3): (3) Case R2: Active power reference value Calculated by the following formula (4): (4) In equations (3) and (4), Provide a reference for upper-level control. These are the equivalent port voltage PI controller parameters for the grid-type energy storage station at the AC bus of the DRU converter station. These are the reference and actual values ​​of the AC bus voltage at the DRU converter station, respectively.

[0021] When the system is running normally, the active power reference value is provided by condition R2; when the AC bus voltage control is unlocked, and the system detects that the AC bus voltage value satisfies the constraint of equation (5): (5) In equation (5), Let be a proportionality constant, satisfying .

[0022] The active reference value is provided by situation R1. Once situation R1 is activated, it will not switch to R2 unless actively switched.

[0023] More preferably, the black boot method of the DRU-MMC output system includes the following steps: Step S1: After the receiving-end grid completes the charging of the MMC, bypass the receiving-end start-up resistor, unlock the MMC DC voltage control, and provide the DC voltage reference value as a ramp input. Once the DC voltage stabilizes, the receiving-end system is ready to start.

[0024] Step S2: Start the energy storage station used for system black start, denoted as the power balance node. During the startup process, each energy storage converter in the energy storage station adopts a Pu droop control mode with AC voltage amplitude feedforward. The AC voltage amplitude feedforward is given in a ramp manner to gradually restore the grid voltage and communication power supply. The reactive power control of the power balance node is consistent with its steady-state operation control.

[0025] Step S3: Each distributed renewable energy power station starts charging from the islanded grid, unlocks the grid-side inverter to a grid-following PQ control based on phase-locked loop synchronization, and sets both active and reactive power reference commands to 0; each distributed energy storage power station directly adopts its own grid-building control strategy during steady-state operation, and sets both active and reactive power reference commands to 0; the grid-building energy storage power station at the AC bus of the DRU converter station adopts the same control structure as the distributed energy storage power station, and controls the AC bus voltage when the power is off.

[0026] Step S4: Once the islanded power system is stabilized, the energy storage at the power balance node smoothly switches to the grid control strategy for steady-state operation, and the reference commands for both active and reactive power are set to 0; the grid-type energy storage station at the AC bus of the DRU converter station activates AC bus voltage control.

[0027] Step S5: The DRU converter station determines the difference between the actual DC voltage value and the expected stable value. If the difference is within a reasonable range, the DC circuit breaker is closed to connect the islanded power system to the receiving-end grid.

[0028] Step S6: The active and reactive power reference values ​​of each station in the isolated power system are obtained by ramping up to the reference command provided by the upper control. Step S7: The system enters steady-state operation mode, and the entire system startup is complete.

[0029] More preferably, in S1, the formula for calculating the DC voltage reference value is: (6) In equation (6), This is the DC voltage control reference value. The slope of the DC voltage rise. This is the expected stable value of DC voltage. It's time.

[0030] More preferably, in step S2, the formula for calculating the d-axis voltage control reference value of the j-th energy storage converter is: (7) In equation (7), These are the d-axis voltage control reference value, Pu droop coefficient, active power reference value, and actual port active power value of the j-th energy storage converter. k is the slope of the AC voltage amplitude rise due to feedforward. It is the expected amplitude of AC voltage.

[0031] More preferably, in step S4, the formula for calculating the voltage reference value is: (8) In equation (8), Here, T represents the leakage inductance of the DRU port transformer, and T represents the turns ratio of the DRU port transformer. This provides a reference for higher-level control; it can be set to 0 or a small value during the startup process. For frequency.

[0032] More preferably, in step S6, the power calculation formula for any power station is: (9) In equation (9), These are the active power reference values ​​and reactive power reference values ​​for the nth power station. These are the rising slopes of the active and reactive power of the nth power station, respectively. These are the active and reactive power references for the nth power station.

[0033] More preferably, in the black start method, the control method for the energy storage station as a power balancing node during system black start is as follows: When energy storage stations, initially used as power balance nodes during system black start, are smoothly switched to their steady-state operation control process, the initial power reference value after the switch needs to inherit the actual power value before the switch and decrease linearly to 0, satisfying the following equation (10): (10) In equation (10), This is the power reference command for the station after the switch. It is the actual value of active power at the moment of switching. It is the slope of the decrease in active power.

[0034] The beneficial effects of this invention are: 1. Compared with the traditional onshore MMC-MMC type flexible DC system, the present invention has lower cost and simpler control.

[0035] 2. This invention adds energy storage to the island side of a traditional deep-sea wind power type DRU-HVDC system and transforms it for use in onshore systems. The energy storage can replace the reactive power compensator and additional black start equipment of the traditional system, and can smooth out fluctuations in new energy sources.

[0036] 3. This invention employs a grid-type energy storage system with reactive power drooping, and achieves black start and steady-state operation of the system through control, thus making it more flexible and reliable, and also having good scalability. Attached Figure Description

[0037] Figure 1 This is a simplified topology diagram of a DRU-MMC output system and its steady-state operation method and black start method according to the present invention. Figure 2 This is a block diagram of the energy storage steady-state control of the present invention; Figure 3 This is a black start flowchart of the present invention; Figure 4 This is a control switching block diagram of the present invention; Figure 5 This is a simulation result diagram of the islanded system according to an embodiment of the present invention; Figure 6 This is a power simulation result diagram of the MMC-side system according to an embodiment of the present invention; Figure 7 The figure shows the simulation results of the MMC-side system voltage and current in an embodiment of the present invention. Detailed Implementation

[0038] The relevant technologies of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0039] like Figures 1-7 As shown, the DRU-MMC transmission system of this embodiment is mainly used for onshore transmission systems (offshore systems cannot deploy large amounts of energy storage due to size limitations). The system includes an islanded power system and a DRU-HVDC system (DRU: Diode Rectifier Unit; HVDC: High Voltage Direct Current; DRU-HVDC system: diode-type high-voltage direct current transmission system). The islanded power system includes distributed renewable energy power plants, distributed grid-type energy storage power plants, and grid-type energy storage power plants at the AC bus of the DRU converter station, with different power plants converging via AC lines. The DRU-HVDC system includes a sending-end DRU converter station, DC circuit breakers, DC transmission lines, a receiving-end MMC converter station, and a receiving-end power grid. The active power collected by the islanded power system is transmitted to the receiving-end power grid through the DRU-HVDC system. The simplified system topology of this embodiment is as follows: Figure 1 As shown.

[0040] The receiving-end MMC adopts constant DC voltage control; all new energy power stations in the islanded power system adopt PQ type grid-following control. All grid-connected energy storage power stations adopt Qf reactive synchronous grid-connected control.

[0041] System steady-state operation strategy: Control of distributed grid-type energy storage sites: The m-th distributed grid-type energy storage station adjusts its port d-axis voltage according to the following formula (1) during steady-state operation. and q-axis voltage ; (1) in These are the active reference power, actual active power, and theoretical port voltage amplitude used for feedforward of the m-th distributed grid-type energy storage station, respectively. These are the equivalent port active PI controller parameters of the m-th distributed grid-type energy storage station, and S is the Laplace differential operator.

[0042] The m-th distributed grid-type energy storage station adjusts its port frequency according to the following formula (2) during steady-state operation. and phase angle ; (2) in These are the active reference power and actual active power of the m-th distributed grid-type energy storage station, respectively. It is the equivalent reactive power droop parameter of the m-th distributed grid-type energy storage station. For the system's global theoretical frequency The active and reactive power reference values ​​for distributed grid-type energy storage stations are provided by the upper-level control. The energy storage steady-state control framework of this embodiment is as follows: Figure 2 As shown.

[0043] Control of grid-type energy storage sites at the AC bus of DRU converter stations: The grid-type energy storage facility at the AC bus of the DRU converter station operates under the same control strategy as the distributed grid-type energy storage facility during steady-state operation. Reactive power reference is provided by the upper-level control. Active power reference values ​​are provided in two ways: Case R1: Active power reference value Calculated by the following formula (3): (3) in, Provide a reference for upper-level control. These are the equivalent port voltage PI controller parameters for the grid-type energy storage station at the AC bus of the DRU converter station. These are the reference and actual values ​​of the AC bus voltage at the DRU converter station, respectively.

[0044] Case R2: Active power reference value Calculated by the following formula (4): (4) During normal system operation, the active power reference value is provided by R2. When the AC bus voltage control is unlocked, and the system detects that the AC bus voltage value meets the following constraints, (5) The active reference value will be provided by R1. Once R1 is activated, it will not switch to R2 unless actively switched.

[0045] in Let be a proportionality constant, satisfying .

[0046] Black boot method: When an islanded power system is isolated from the receiving-end system by a DC circuit breaker, the following steps need to be completed: Step S1: After the receiving-end grid completes the charging of the MMC, bypass the receiving-end start-up resistor, unlock the MMC DC voltage control, and provide the DC voltage reference value as a ramp input. Once the DC voltage stabilizes, the receiving-end system is ready to start.

[0047] The formula for calculating the DC voltage reference value is: (6) in This is the DC voltage control reference value. The slope of the DC voltage rise. This is the expected stable value of DC voltage. It's time.

[0048] Step S2: Start the energy storage station used for system black start, denoted as the power balance node. During startup, each energy storage converter in this station employs a Pu-droop control method with AC voltage amplitude feedforward. The AC voltage amplitude feedforward is given in a ramp manner to gradually restore the grid voltage and communication power supply. The reactive power control of the power balance node is consistent with its steady-state operation control.

[0049] The formula for calculating the d-axis voltage control reference value of the j-th energy storage converter is as follows: (7) in These are the d-axis voltage control reference value, Pu droop coefficient, active power reference value, and actual port active power value of the j-th energy storage converter. k is the slope of the AC voltage amplitude rise due to feedforward. It is the expected amplitude of AC voltage.

[0050] Step S3: Each distributed renewable energy power station starts charging from the isolated grid. The grid-side inverter is unlocked to use grid-following PQ control based on phase-locked loop synchronization, and the reference commands for both active and reactive power are set to 0. Each distributed energy storage power station directly adopts its own grid-connected control strategy during steady-state operation, and the reference commands for both active and reactive power are set to 0. The grid-connected energy storage power station at the AC bus of the DRU converter station adopts the same control structure as the distributed energy storage power station, with voltage control of the AC bus in the event of power failure.

[0051] Step S4: Once the islanded power system is stabilized, the energy storage at the power balance node smoothly switches to its steady-state operation grid control strategy, and both active and reactive power reference commands are set to 0. The grid-type energy storage facility at the DRU converter station's AC bus activates AC bus voltage control; the voltage reference value is calculated using the following formula: (8) in, Here, T represents the leakage inductance of the DRU port transformer, and T represents the turns ratio of the DRU port transformer. This provides a reference for higher-level control; it can be set to 0 or a small value during the startup process. For frequency.

[0052] Step S5: The DRU converter station determines the difference between the actual DC voltage value and the expected stable value. If the difference is within a reasonable range, the DC circuit breaker is closed to connect the islanded power system to the receiving-end grid.

[0053] Step S6: The active and reactive power reference values ​​for each station in the isolated power system are obtained by ramping up to the reference command provided by the upper-level control. For any station (denoted as n), the calculation formula is as follows: (9) in These are the active power reference values ​​and reactive power reference values ​​for the nth power station. These are the rising slopes of the active and reactive power of the nth power station, respectively. These are the active and reactive power references for the nth power station.

[0054] The system then enters a steady-state operating mode, and the entire system startup is complete. The startup process is as follows: Figure 3 As shown.

[0055] System black start as a power balancing node for energy storage site control: When energy storage stations, initially used as power balance nodes during system black start, are smoothly switched to their steady-state operation control process, the initial power reference value after the switch needs to inherit the actual power value before the switch and decrease linearly to 0, satisfying the following equation (10): (10) in, This is the power reference command for the station after the switch. It is the actual value of active power at the moment of switching. It is the slope of active power decrease. Control switching is as follows: Figure 4 As shown.

[0056] Example This embodiment uses a 35kV AC distributed wind-solar-storage collection system, with the DRU-MMC transmission system as an example, to simulate and verify the scheme. The main electrical parameters are shown in Table 1 below:

[0057] The simulation results of this embodiment are as follows: Figure 5-7 As shown.

[0058] In summary, this invention effectively solves the problems of high cost and complex control in traditional systems by employing innovative control strategies and black-start methods in the DRU-MMC transmission system. Adding energy storage on the islanded side not only reduces costs but also improves system stability and flexibility. During steady-state operation, the control strategies of each station can precisely adjust power and voltage according to different conditions, ensuring efficient system operation. The black-start method provides a reliable startup solution for fault recovery and other situations, enabling the system to quickly and stably restore power supply.

[0059] In practical applications, the system of this invention can be widely used in onshore power transmission systems, providing strong support for the large-scale integration and stable transmission of new energy sources. With the continuous development of new energy sources, the requirements for system stability and reliability are also increasing, and the technical solution of this invention can well meet these needs. At the same time, the system's scalability also provides possibilities for future upgrades and optimizations, enabling it to adapt to the ever-changing electricity market and technological developments.

[0060] Furthermore, the technical solution of this invention also has good economic and social benefits. By reducing costs and improving energy efficiency, it can bring more profits to enterprises. At the same time, a stable power supply also helps to ensure the normal operation of society and promote economic development.

[0061] It should be emphasized that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A DRU-MMC delivery system, characterized in that, include: Islanded power systems are used to achieve local aggregation, stabilization, and coordinated control of large-scale renewable energy sources; The DRU-HVDC system is used to efficiently and reliably transmit the active power collected by the islanded power system to the receiving-end power grid over long distances. The isolated power system includes: Distributed renewable energy power plants serve as the main power generation entities, providing clean energy. Distributed grid-based energy storage stations are used to absorb / release power to mitigate the impact of the randomness of new energy sources on isolated power grids, actively provide voltage and frequency regulation, and enhance the stability of isolated grids; The grid-type energy storage station at the AC bus of the DRU converter station is used to directly support the AC bus voltage, coordinate the power at the sending end with the DC transmission demand, and prevent the DRU from being blocked due to power fluctuations. The DRU-HVDC system includes: Sending-end DRU converter station is used to convert AC power in an islanded power system into DC power. DC circuit breakers are used to quickly interrupt fault current on the DC side, prevent fault propagation, and protect converter equipment and lines. The receiving-end MMC converter station is used to convert DC power into AC power and then synchronously connect it to the receiving-end power grid. The receiving-end power grid is used to receive new energy power and balance supply and demand through traditional power grid dispatch. The active power of the isolated power system is transmitted to the receiving-end power grid through the DRU-HVDC system. The receiving-end MMC converter station adopts constant DC voltage control; all new energy power stations in the isolated power system adopt PQ type grid-following control; all grid-type energy storage power stations in the isolated power system adopt Qf reactive synchronous grid-following control.

2. The DRU-MMC delivery system according to claim 1, characterized in that, The steady-state operation method of the DRU-MMC output system includes: Control methods for distributed grid-type energy storage stations: During steady-state operation, the m-th distributed grid-type energy storage station adjusts the port d-axis voltage according to equation (1). and q-axis voltage Adjust the port frequency according to formula (2) and phase angle : （1） （2） In equations (1) and (2), These are the active reference power, actual active power, and theoretical port voltage amplitude used for feedforward of the m-th distributed grid-type energy storage station, respectively. These are the equivalent port active PI controller parameters of the m-th distributed grid-type energy storage station, and S is the Laplace differential operator; These are the active reference power and actual active power of the m-th distributed grid-type energy storage station, respectively. It is the equivalent reactive power droop parameter of the m-th distributed grid-type energy storage station. This represents the system's global theoretical frequency.

3. The DRU-MMC delivery system according to claim 2, characterized in that, The steady-state operation method of the DRU-MMC transmission system also includes: a control method for a grid-type energy storage site at the AC bus of the DRU converter station; the steady-state operation process of the grid-type energy storage site at the AC bus of the DRU converter station is consistent with the control method for distributed grid-type energy storage.

4. The DRU-MMC delivery system according to claim 3, characterized in that, In the control method for the grid-type energy storage site at the AC bus of the DRU converter station, the reactive power reference is provided by the upper-level control, and the active power reference value is provided in the following ways: Case R1: Active power reference value Calculated using equation (3): （3） Case R2: Active power reference value Calculated by the following formula (4): （4） In equations (3) and (4), Provide a reference for upper-level control. These are the equivalent port voltage PI controller parameters for the grid-type energy storage station at the AC bus of the DRU converter station. These are the reference and actual values ​​of the AC bus voltage at the DRU converter station, respectively. When the system is running normally, the active power reference value is provided by condition R2; when the AC bus voltage control is unlocked, and the system detects that the AC bus voltage value satisfies the constraint of equation (5): （5） In equation (5), Let be a proportionality constant, satisfying ; The active reference value is provided by situation R1. Once situation R1 is activated, it will not switch to R2 unless actively switched.

5. A DRU-MMC delivery system according to claim 2, characterized in that, The black boot method for the DRU-MMC delivery system includes the following steps: Step S1: After the receiving-end grid completes charging of the MMC, bypass the receiving-end start-up resistor, unlock the MMC DC voltage control, and provide the DC voltage reference value as a ramp input. Once the DC voltage stabilizes, the receiving-end system is ready to start. Step S2: Start the energy storage station used for system black start, denoted as the power balance node. During the startup process, each energy storage converter in the energy storage station adopts a Pu droop control mode with AC voltage amplitude feedforward. The AC voltage amplitude feedforward is given in a ramp manner to gradually restore the grid voltage and communication power supply. The reactive power control of the power balance node is consistent with its steady-state operation control. Step S3: Each distributed renewable energy power station starts charging from the isolated grid, unlocks the grid-side inverter to a grid-following PQ control based on phase-locked loop synchronization, and sets both active and reactive power reference commands to 0; each distributed energy storage power station directly adopts its own grid-building control strategy during steady-state operation, and sets both active and reactive power reference commands to 0; the grid-building energy storage power station at the AC bus of the DRU converter station adopts the same control structure as the distributed energy storage power station, with power failure AC bus voltage control; Step S4: Once the islanded power system is stabilized, the energy storage at the power balance node smoothly switches to the grid control strategy for steady-state operation, and the reference commands for both active and reactive power are set to 0; the grid-type energy storage station at the AC bus of the DRU converter station activates AC bus voltage control. Step S5: The DRU converter station determines the difference between the actual DC voltage value and the expected stable value. If the difference is within a reasonable range, the DC circuit breaker is closed to connect the isolated power system to the receiving-end grid. Step S6: The active and reactive power reference values ​​of each station in the isolated power system are obtained by ramping up to the reference command provided by the upper control layer; Step S7: The system enters steady-state operation mode, and the entire system startup is complete.

6. A DRU-MMC delivery system according to claim 5, characterized in that, In S1, the formula for calculating the DC voltage reference value is as follows: （6） In equation (6), This is the DC voltage control reference value. The slope of the DC voltage rise. This is the expected stable value of DC voltage. It's time.

7. A DRU-MMC delivery system according to claim 5, characterized in that, In step S2, the formula for calculating the d-axis voltage control reference value of the j-th energy storage converter is as follows: （7） In equation (7), These are the d-axis voltage control reference value, Pu droop coefficient, active power reference value, and actual port active power value of the j-th energy storage converter. k is the slope of the AC voltage amplitude rise due to feedforward. It is the expected amplitude of AC voltage.

8. A DRU-MMC delivery system according to claim 5, characterized in that, In step S4, the formula for calculating the voltage reference value is: （8） In equation (8), Here, T represents the leakage inductance of the DRU port transformer, and T represents the turns ratio of the DRU port transformer. Provide a reference for upper-level control. For frequency.

9. A DRU-MMC delivery system according to claim 5, characterized in that, In step S6, the power calculation formula for any power station is as follows: （9） In equation (9), These are the active power reference values ​​and reactive power reference values ​​for the nth power station. These are the rising slopes of the active and reactive power of the nth power station, respectively. These are the active and reactive power references for the nth power station.

10. A DRU-MMC delivery system according to claim 5, characterized in that, In the aforementioned black-start method, the control method for energy storage stations, where the system black-start is used as a power balance node, is as follows: When energy storage stations, initially used as power balance nodes during system black start, are smoothly switched to their steady-state operation control process, the initial power reference value after the switch needs to inherit the actual power value before the switch and decrease linearly to 0, satisfying the following equation (10): （10） In equation (10), This is the power reference command for the station after the switch. It is the actual value of active power at the moment of switching. It is the slope of the decrease in active power.

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