Offshore wind power flexible direct output system based on cascade multi-level energy storage and control method

Abstract

The invention discloses an offshore wind power flexible direct output system based on cascade multi-level energy storage and a control method. The offshore wind power flexible direct delivery system comprises a wind power cluster, an offshore converter station, an energy storage device and an onshore converter station, the offshore converter station is formed by connecting two converters in series on a direct current side, the onshore converter station is formed by connecting four converters in series on the direct current side, and the offshore converter station and the onshore converter station form a flexible direct-current true bipolar transmission system through a direct current cable; the energy storage device is composed of a plurality of low-voltage energy storage units and is connected to the flexible direct output system through two middle converters on the direct current side of the land converter. The charging and discharging state of the energy storage device is controlled according to the output condition of the wind power cluster and the charge state of the energy storage unit, so that the parameters of the four converter sub-module components of the land converter station are consistent with the parameters under the condition that the energy storage device is not connected, and the operation range of the current of the converter bridge arm of the land converter station is not changed when the energy storage device is charged and discharged.

Description

Technical Field

[0001] The present invention relates to the field of offshore wind power transmission and energy storage technology, and in particular to an offshore wind power flexible direct current transmission system based on cascade multi-level energy storage and a control method thereof. Background Art

[0002] Offshore wind power is characterized by strong volatility and randomness. The access of large-scale offshore wind farms will have an adverse impact on the operation of the onshore receiving power grid. Energy storage technology can effectively solve the problem of unstable output of wind farms, balance the contradiction between supply and demand between wind power fluctuations and grid load fluctuations, and improve the controllability of large-scale offshore wind power transmission systems. In addition, the configuration of energy storage devices can improve the reliability of offshore wind power transmission systems. When an overhead line instantaneous grounding fault occurs in the onshore receiving power grid, the energy storage device can quickly absorb the surplus power of the wind farm, overcoming the problems of low utilization rate, large footprint and high cost of the energy-consuming resistor solution. When the DC submarine cable fails, the wind farm does not generate electricity and is in an island operation mode. The energy storage device can be used as an island power supply for auxiliary equipment inside the wind turbine, overcoming the problems of high cost of diesel generator solutions and difficulty in offshore platform construction.

[0003] At present, for the AC access of energy storage devices to offshore wind power transmission systems, the technology of the low-voltage energy storage converter parallel boost access solution is the most mature, but it requires an additional boost transformer, and there are problems such as high loss, high cost, and difficulty in coordinating multiple converters; the AC direct connection solution based on the cascaded H-bridge does not require a transformer, and the energy storage device is directly connected to the medium-voltage power grid, with low loss and low cost, but there are problems such as large circulating current and large secondary current pulsation on the battery side. For the DC access of energy storage devices to offshore wind power transmission systems, since both offshore and onshore converter stations almost all use modular multilevel converter (MMC) topology, the inter-pole DC voltage of MMC is high, while the port DC voltage of the energy storage converter is low, and it still cannot be directly connected to the DC side after series connection; some studies have proposed an energy storage MMC topology, in which a DC / DC converter is connected in parallel at both ends of the DC capacitor of each half-bridge submodule, and then connected to the battery unit, but under this solution, the battery unit is highly coupled with the MMC submodule, and battery failure will affect the operation of the submodule. Summary of the invention

[0004] In view of the problems existing in the above-mentioned prior art, the present invention provides an offshore wind power flexible direct current transmission system based on cascaded multi-level energy storage, which realizes medium-voltage direct current access to the energy storage device, so as to improve the stability, controllability and reliability of offshore wind power transmission; the present invention also provides a control method for the aforementioned offshore wind power flexible direct current transmission system, which controls the charging and discharging state of the energy storage device according to the output of the wind power cluster and the charge state of the energy storage unit, so that the component parameters of the four converter submodules of the onshore converter station are consistent with the parameters when the energy storage device is not connected, and the operating range of the converter bridge arm current of the onshore converter station remains unchanged when the energy storage device is charged and discharged, and the direct current access to the energy storage device can be realized without the need for additional hardware modification costs.

[0005] To this end, the present invention adopts the following technical solution.

[0006] In a first aspect, the present invention provides an offshore wind power flexible direct current transmission system based on cascade multi-level energy storage, which includes a wind power cluster, an offshore converter station, an energy storage device and an onshore converter station; The wind power cluster is composed of n The offshore wind farm wind turbines are assembled together, the offshore converter station is composed of the first converter and the second converter, and the energy storage device is composed of m The onshore converter station is composed of the third to sixth converters; The first converter and the second converter are connected in parallel on the AC side to form a first node, and the wind power cluster is connected to the first node; The third to sixth converters are connected in parallel on the AC side to form a second node, and the second node is connected to the onshore AC power grid; The first converter and the second converter are connected in series on the DC side to form a third node, the fourth converter and the fifth converter are connected in series on the DC side to form a fourth node, and the third node and the fourth node are both grounded; The third converter and the fourth converter are connected in series on the DC side to form a fifth node, the fifth converter and the sixth converter are connected in series on the DC side to form a sixth node, and the energy storage device is connected to the fifth node and the sixth node.

[0007] Furthermore, the first converter and the third converter are connected via a DC cable to form a positive pole of the flexible DC transmission system, and the second converter and the sixth converter are connected via another DC cable to form a negative pole of the flexible DC transmission system.

[0008] Furthermore, the first to sixth converters realize conversion from direct current to alternating current, and the conversion topology is a modular multi-level converter MMC, and the submodule topology is a half-bridge structure.

[0009] Furthermore, the first converter and the second converter both adopt a control mode of fixed AC voltage amplitude and fixed AC voltage frequency to jointly control the stability of the AC voltage and frequency at the outlet of the wind power cluster; the fourth converter and the fifth converter both adopt a control mode of fixed DC voltage and fixed reactive power to jointly control the stability of the DC voltage between the positive port and the negative port of the energy storage device; the third converter and the sixth converter both adopt a control mode of fixed DC voltage and fixed reactive power to jointly control the stability of the DC voltage between the positive pole and the negative pole of the flexible DC transmission system.

[0010] Furthermore, the DC voltage from the first converter to the sixth converter satisfies the following relationship during steady-state operation: , in, U dc1 , U dc2 , U dc3 , U dc4 , U dc5 , U dc6 are the DC voltages of the first converter, the second converter, the third converter, the fourth converter, the fifth converter and the sixth converter, respectively, U dc It is the voltage between the positive pole of the flexible DC transmission system and the negative pole of the flexible DC transmission system.

[0011] Furthermore, the energy storage device is composed of m The energy storage device is composed of a cascade of low-voltage energy storage units, each of which is composed of a power conversion unit (PCS) and a battery unit connected in series; the port DC voltage of the energy storage device is U BAT , the following relationship is satisfied during steady-state operation: .

[0012] Furthermore, the IGBT specifications of each submodule of the third to sixth converters are selected and designed according to the situation where the energy storage device is not connected. When the energy storage device is connected, the magnitude of the DC current of the third to sixth converters is controlled by controlling the charging and discharging conditions of the energy storage device, so that the bridge arm current of the third to sixth converters does not exceed the maximum current capacity allowed by the submodule IGBT.

[0013] Furthermore, assuming that the DC currents of the third to sixth converters are respectively I dc3 , I dc4 , Idc5 , I dc6 , the discharge current of the energy storage device is I BAT , the DC current of the entire flexible DC transmission system is I dc , when the energy storage device is not charging or discharging, the current relationships satisfy: , When the energy storage device is charging and discharging, the current relationships satisfy: .

[0014] In a second aspect, the present invention provides a control method for the above-mentioned offshore wind power flexible direct current transmission system based on cascade multi-level energy storage, the contents of which are as follows: The charging and discharging state of the energy storage device is controlled according to the output of the wind power cluster, and the energy storage device is used to smooth the fluctuation of the output of the wind power cluster; assuming that the rated output power of the onshore converter station is P wN The actual output power is P w , when the actual output power P w In 0~ K 1 P wN When the actual power is within the range, the energy storage device is in a discharging state, and the energy storage device injects current into the fifth node; P w exist K 1 P wN ~ K 2 P wN When the actual power is within the range, the energy storage device is in a charging state, and the fifth node injects current into the energy storage device; P w Greater than K 2 P wN When the power conversion unit of the energy storage device is locked, it will no longer charge or discharge. K 1< K 2<1.

[0015] Furthermore, the energy storage device receives charging and discharging instructions according to the output of the wind power cluster. P ref After that, it is necessary to confirm the charge and discharge permission status of the energy storage unit according to the battery's state of charge SOC; when 0< P w < K 1 P wN When, andP ref >0, the energy storage unit needs to be discharged, and the SOC j ≥SOC min , the discharge permission flag is equal to 1; when K 1 P wN < P w < K 2 P wN When, and P ref <0, the energy storage unit needs to be charged, and the SOC j ≤SOC max , the charging permission flag is equal to 1; SOC j Indicates j The state of charge of each energy storage unit, j The value range is 1- m integer; SOC min Indicates the minimum state of charge, SOC max Indicates the maximum value of the state of charge; when one of the discharge permission and charge permission conditions is met, the permission state bit is equal to 1, and the second half-bridge of the power conversion unit in the energy storage unit can be unlocked; if the discharge permission bit is equal to 1, the second half-bridge of the power conversion unit operates in Boost mode; if the charge permission bit is equal to 1, the second half-bridge of the power conversion unit operates in Buck mode.

[0016] The beneficial effects of the present invention are as follows: the present invention reconstructs the sending-end converter station topology of the flexible DC true bipolar system, adopts the form of four converters connected in series on the DC side, constructs a medium-voltage DC access port, so that the voltage level of the access port matches the port voltage level after the energy storage device is cascaded, and realizes the medium-voltage DC access of the energy storage device; and controls the charging and discharging state of the energy storage device according to the output of the wind power cluster and the charge state of the energy storage unit, so that the component parameters of the four converter sub-modules of the onshore converter station are consistent with the parameters when the energy storage device is not connected, and the operating range of the converter bridge arm current of the onshore converter station remains unchanged when the energy storage device is charged and discharged, and the DC access of the energy storage device can be realized without the need for additional hardware modification costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a topological structure diagram of an offshore wind power flexible direct current transmission system based on cascaded multi-level energy storage in an embodiment of the present invention; Figure 2 is a MMC topological structure diagram of the first to sixth converters in an embodiment of the present invention; Figure 3 is a topological structure diagram of an energy storage device in an embodiment of the present invention; Figure 4 It is a logic diagram of the charge and discharge permission state of the energy storage device in an embodiment of the present invention. DETAILED DESCRIPTION

[0018] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0019] Example like Figure 1 As shown, this embodiment provides an offshore wind power flexible direct current transmission system based on cascade multi-level energy storage, including a wind power cluster, an offshore converter station, an energy storage device and an onshore converter station.

[0020] The wind power cluster is composed of n The offshore wind farm wind turbines are assembled together, the offshore converter station is composed of the first converter 1 and the second converter 2, and the energy storage device is composed of m The onshore converter station is composed of a third converter 3 to a sixth converter 6.

[0021] The first converter 1 and the second converter 2 are connected in parallel on the AC side to form a first node, and the wind power cluster is connected to the first node; the third converter 3 to the sixth converter 6 are connected in parallel on the AC side to form a second node, and the second node is connected to the onshore AC power grid; the first converter 1 and the second converter 2 are connected in series on the DC side to form a third node, the fourth converter 4 and the fifth converter 5 are connected in series on the DC side to form a fourth node, and the third node and the fourth node are both grounded; the third converter 3 and the fourth converter 4 are connected in series on the DC side to form a fifth node, the fifth converter 5 and the sixth converter 6 are connected in series on the DC side to form a sixth node, and the energy storage device is connected to the fifth node and the sixth node.

[0022] like Figure 2 As shown, the first converter 1 to the sixth converter 6 realize the conversion of DC to AC, and the conversion topology is a modular multilevel converter MMC, and the submodule topology is a half-bridge structure. The first converter 1 and the second converter 2 both adopt the control mode of fixed AC voltage amplitude and fixed AC voltage frequency to jointly control the stability of the AC voltage and frequency at the outlet of the wind power cluster; the fourth converter 4 and the fifth converter 5 both adopt the control mode of fixed DC voltage and fixed reactive power to jointly control the stability of the DC voltage between the positive and negative ports of the energy storage device; the third converter 3 and the sixth converter 6 both adopt the control mode of fixed DC voltage and fixed reactive power to jointly control the stability of the DC voltage between the positive and negative ports of the flexible DC transmission system.

[0023] The DC voltages of the first to sixth converters 1 to 6 satisfy the following relationship during steady-state operation: , in, U dc1 , U dc2 , U dc3 , U dc4 , U dc5 , U dc6 are the DC voltages of the first converter 1, the second converter 2, the third converter 3, the fourth converter 4, the fifth converter 5 and the sixth converter 6, U dc It is the voltage between the positive pole of the flexible DC transmission system and the negative pole of the flexible DC transmission system.

[0024] like Figure 3 As shown, the energy storage device consists of m The power supply of the power converter is composed of a series of low-voltage energy storage units, each of which is composed of a power conversion unit (PCS) and a battery unit in series. The PCS consists of two half-bridges. The input sides of the first half-bridges of each PCS are cascaded with each other, and the output side of the first half-bridge is connected in parallel with a DC capacitor. The function of the first half-bridge is to balance the DC voltage of each energy storage unit; the input side of the second half-bridge is connected in parallel with both ends of the DC capacitor, and the output side is connected to the battery unit through an inductor. The function of the second half-bridge is to achieve bidirectional DC / DC power transmission. When the energy storage unit works in the discharge mode, the second half-bridge of the power conversion unit operates in the Boost mode, the upper tube is locked, and the lower tube is turned on or off according to the modulation signal; when the energy storage unit works in the charging mode, the second half-bridge of the power conversion unit operates in the Buck mode, the lower tube is locked, and the upper tube is turned on or off according to the modulation signal.

[0025] The DC voltage at the port of the energy storage device is U BAT , the following relationship is satisfied during steady-state operation: , The IGBT specifications of each submodule of the third converter 3 to the sixth converter 6 are selected and designed according to the situation where the energy storage device is not connected. When the energy storage device is connected, the magnitude of the DC current of the third converter 3 to the sixth converter 6 is controlled by controlling the charging and discharging conditions of the energy storage device, so that the bridge arm current of the third converter 3 to the sixth converter 6 does not exceed the maximum current capacity allowed by the submodule IGBT.

[0026] Assume that the DC currents of the third converter 3 to the sixth converter 6 are respectively I dc3 ,I dc4 , I dc5 , I dc6 , the discharge current of the energy storage device is I BAT , the DC current of the entire flexible DC transmission system is I dc , when the energy storage device is not charging or discharging, the current relationships satisfy: , When the energy storage device is charging and discharging, the current relationships satisfy: , The charging and discharging state of the energy storage device is controlled according to the output of the wind power cluster, and the energy storage device can smooth the fluctuation of the wind power cluster output. Assume that the rated output power of the onshore converter station is P wN The actual output power is P w ,when P w In 0- K 1 P wN When the energy storage device is in a discharge state, the energy storage device injects current into the fifth node; P w exist K 1 P wN - K 2 P wN When the energy storage device is in the charging state, the fifth node injects current into the energy storage device; P w Greater than K 2 P wN When the power conversion unit of the energy storage device is locked, it will no longer charge or discharge, where 0< K 1< K 2<1.

[0027] like Figure 4 As shown, the energy storage device receives charging and discharging instructions based on the output of the wind power cluster. P ref After that, it is necessary to confirm the charge and discharge permission status of the energy storage unit according to the battery's state of charge SOC; when 0< P w < K 1 P wN When, and P ref>0, the energy storage unit needs to be discharged, and the SOC j ≥SOC min , the discharge permission flag is equal to 1; when K 1 P wN < P w < K 2 P wN When, and P ref <0, the energy storage unit needs to be charged, and the SOC j ≤SOC max , the charging permission flag is equal to 1; SOC j Indicates j The state of charge of each energy storage unit, j The value range is 1- m integer; SOC min Indicates the minimum state of charge, SOC max Indicates the maximum value of the state of charge; when one of the discharge permission and charge permission conditions is met, the permission state bit is equal to 1, and the second half-bridge of the power conversion unit PCS can be unlocked; if the discharge permission bit is equal to 1, the second half-bridge of the power conversion unit PCS operates in Boost mode; if the charge permission bit is equal to 1, the second half-bridge of the power conversion unit PCS operates in Buck mode.

[0028] Application Examples An offshore wind power flexible direct current transmission system based on cascaded multi-level energy storage and a control method thereof described in the embodiment are used as follows.

[0029] Rated output power of onshore converter station P wN =300MW, DC cable pole voltage U dc =800kV, the DC voltage of the first converter 1 and the second converter 2 is U dc1 = U dc2 =400kV, DC voltage at the port of the energy storage device U BAT =160kV, the DC voltage of the fourth converter 4 and the fifth converter 5 U dc4 = U dc5 =80kV, the DC voltage of the third converter 3 and the sixth converter 6 U dc3 = U dc6=320kV. The AC side connection transformer ratios of the fourth converter 4 and the fifth converter 5 are both 84kV / 220kV, and the AC side connection transformer ratios of the third converter 3 and the sixth converter 6 are both 336kV / 220kV. Then the rated voltage of the AC side of the onshore converter station is U wN =420kV.

[0030] Ignoring converter losses, when the wind power cluster actually transmits power through the onshore converter station P w Equal to rated power P wN When the energy storage device is not charged or discharged, the rated current of the submodule switch device in the converter I mN for:

[0031] Number of energy storage units m =50, the charge and discharge threshold coefficients of the energy storage device are respectively K 1=0.5, K 2=0.8. When the actual output power of the onshore converter station is P w In the range 0< P w <150MW, the energy storage device discharges to the onshore converter, and the maximum value of the submodule switch device current of the third converter 3 and the sixth converter 6 is K 1 I mN =0.5 I mN At this time, the maximum value of the submodule switch device current of the fourth converter 4 and the fifth converter 5 needs to be controlled not to be greater than I mN , then the energy storage device discharge current I BAT satisfy:

[0032] The discharge current of the energy storage device can be solved as I BAT The range is:

[0033] Energy storage device discharge current I BAT The maximum value of I BAT_max =0.625kA, then the energy storage device discharge instruction P ref Maximum possible value Pref_max for:

[0034] When the actual output power of the onshore converter station P w In the range 150MW< P w <240MW, the energy storage device is charged from the onshore converter, and the maximum value of the submodule switch device current of the fourth converter 4 and the fifth converter 5 is K 2 I mN =0.8 I mN At this time, the maximum value of the submodule switch device current of the third converter 3 and the sixth converter 6 needs to be controlled not to be greater than I mN , then the energy storage device charging current I BAT satisfy:

[0035] The discharge current of the energy storage device can be solved as I BAT The range is:

[0036] Energy storage device discharge current I BAT Minimum value of I BAT_min =-0.25kA, then the energy storage device charging instruction P ref Possible minimum value P ref_min for:

[0037] Let the minimum state of charge SOC allowed for discharge be min =0.1, the maximum value of the state of charge SOC allowed for charging max =0.9.

[0038] When 0≤ P w <150MW, charge and discharge instructions P ref according to P w Change, the range is 0< P ref ≤100MW. j Energy storage unit, its state of charge is SOC j , if SOC j≥0.1, the discharge permission flag is equal to 1, the first half-bridge and the second half-bridge of the power conversion unit PCS are unlocked, the first half-bridge controls the port voltage of the energy storage unit to be constant at 160 / 50=3.2kV, and the second half-bridge operates in Buck mode.

[0039] When 150MW≤ P w <240MW, charge and discharge instructions P ref according to P w Changes, the range is -40MW≤ P ref ≤0. j Energy storage unit, its state of charge is SOC j , if SOC j ≤0.9, the charging permission flag is equal to 1, the first half-bridge and the second half-bridge of the power conversion unit PCS are unlocked, the first half-bridge controls the port voltage of the energy storage unit to be constant at 160 / 50=3.2kV, and the second half-bridge operates in Boost mode.

[0040] When 240MW≤ P w ≤300MW, charge and discharge instructions P ref =0, the discharge permission flag is equal to 0, the charge permission flag is equal to 0, and the first half bridge and the second half bridge of the power conversion unit PCS are locked.

[0041] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention.

Claims

1. An offshore wind power flexible direct current transmission system based on cascaded multi-level energy storage, characterized in that: Including wind power clusters, offshore converter stations, energy storage devices and onshore converter stations; The wind power cluster is composed of n The offshore wind farm wind turbines are assembled together, the offshore converter station is composed of the first converter and the second converter, and the energy storage device is composed of m The onshore converter station is composed of the third to sixth converters; The first converter and the second converter are connected in parallel on the AC side to form a first node, and the wind power cluster is connected to the first node; The third to sixth converters are connected in parallel on the AC side to form a second node, and the second node is connected to the onshore AC power grid; The first converter and the second converter are connected in series on the DC side to form a third node, the fourth converter and the fifth converter are connected in series on the DC side to form a fourth node, and the third node and the fourth node are both grounded; The third converter and the fourth converter are connected in series on the DC side to form a fifth node, the fifth converter and the sixth converter are connected in series on the DC side to form a sixth node, and the energy storage device is connected to the fifth node and the sixth node.

2. The offshore wind power flexible direct current transmission system based on cascade multi-level energy storage according to claim 1 is characterized in that: The first converter and the third converter are connected via a DC cable to form the positive pole of the flexible DC transmission system, and the second converter and the sixth converter are connected via another DC cable to form the negative pole of the flexible DC transmission system.

3. The offshore wind power flexible direct current transmission system based on cascade multi-level energy storage according to claim 1 is characterized in that: The first to sixth converters realize conversion from direct current to alternating current, and the conversion topology is a modular multi-level converter MMC, and the submodule topology is a half-bridge structure.

4. The offshore wind power flexible direct current transmission system based on cascade multi-level energy storage according to claim 1 is characterized in that: The first converter and the second converter both adopt a control mode of fixed AC voltage amplitude and fixed AC voltage frequency to jointly control the stability of the AC voltage and frequency at the outlet of the wind power cluster; the fourth converter and the fifth converter both adopt a control mode of fixed DC voltage and fixed reactive power to jointly control the stability of the DC voltage between the positive port and the negative port of the energy storage device; the third converter and the sixth converter both adopt a control mode of fixed DC voltage and fixed reactive power to jointly control the stability of the DC voltage between the positive pole and the negative pole of the flexible DC transmission system.

5. The offshore wind power flexible direct current transmission system based on cascade multi-level energy storage according to claim 1 is characterized in that: The DC voltage from the first converter to the sixth converter satisfies the following relationship during steady-state operation: ， in, U dc1 , U dc2 , U dc3 , U dc4 , U dc5 , U dc6 are the DC voltages of the first converter, the second converter, the third converter, the fourth converter, the fifth converter and the sixth converter, respectively, U dc It is the voltage between the positive pole of the flexible DC transmission system and the negative pole of the flexible DC transmission system.

6. The offshore wind power flexible direct current transmission system based on cascade multi-level energy storage according to claim 1 is characterized in that: The energy storage device comprises m The energy storage device is composed of a cascade of low-voltage energy storage units, each of which is composed of a power conversion unit and a battery unit in series; the port DC voltage of the energy storage device is U BAT , the following relationship is satisfied during steady-state operation: ， In the formula, U dc4 , U dc5 are the DC voltages of the fourth converter and the fifth converter respectively.

7. The offshore wind power flexible direct current transmission system based on cascade multi-level energy storage according to claim 1 is characterized in that: The IGBT specifications of each submodule of the third to sixth converters are selected and designed according to the situation where the energy storage device is not connected. When the energy storage device is connected, the size of the DC current of the third to sixth converters is controlled by controlling the charging and discharging conditions of the energy storage device, so that the bridge arm current of the third to sixth converters does not exceed the maximum current capacity allowed by the submodule IGBT.

8. The offshore wind power flexible direct current transmission system based on cascade multi-level energy storage according to claim 1 is characterized in that: Assume that the DC currents of the third to sixth converters are respectively I dc3 , I dc4 , I dc5 , I dc6 , the discharge current of the energy storage device is I BAT , the DC current of the entire flexible DC transmission system is I dc , when the energy storage device is not charging or discharging, the current relationships satisfy: ， When the energy storage device is charging and discharging, the current relationships satisfy: 。 9. A control method for an offshore wind power flexible direct current transmission system based on cascaded multi-level energy storage as described in any one of claims 1 to 8, characterized in that: The charging and discharging state of the energy storage device is controlled according to the output of the wind power cluster, and the energy storage device is used to smooth the fluctuation of the output of the wind power cluster; assuming that the rated output power of the onshore converter station is P wN The actual output power is P w , when the actual output power P w In 0~ K 1 P wN When the actual power is within the range, the energy storage device is in a discharging state, and the energy storage device injects current into the fifth node; P w exist K 1 P wN ~ K 2 P wN When the actual power is within the range, the energy storage device is in a charging state, and the fifth node injects current into the energy storage device; P w Greater than K 2 P wN When the power conversion unit of the energy storage device is locked, it will no longer charge or discharge. K 1< K 2<1.

10. The control method according to claim 9, characterized in that: The energy storage device receives charging and discharging instructions based on the output of the wind power cluster. P ref After that, it is necessary to confirm the charge and discharge permission status of the energy storage unit according to the battery's state of charge SOC; when 0< P w < K 1 P wN When, and P ref >0, the energy storage unit needs to be discharged, and the SOC j ≥SOC min , the discharge permission flag is equal to 1; when K 1 P wN < P w < K 2 P wN When, and P ref <0, the energy storage unit needs to be charged, and the SOC j ≤SOC max , the charging permission flag is equal to 1; SOC j Indicates j The state of charge of each energy storage unit, j The value range is 1- m integer; SOC min Indicates the minimum state of charge, SOC max Indicates the maximum value of the state of charge; when one of the discharge permission and charge permission conditions is met, the permission state bit is equal to 1, and the second half-bridge of the power conversion unit in the energy storage unit can be unlocked; if the discharge permission bit is equal to 1, the second half-bridge of the power conversion unit operates in Boost mode; if the charge permission bit is equal to 1, the second half-bridge of the power conversion unit operates in Buck mode.

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