HVDC transmission system and control method for offshore wind power based on cascaded multi-level energy storage

Abstract

The present invention discloses a flexible DC transmission system for offshore wind power based on cascaded multi-level energy storage and a control method. The flexible DC transmission system for offshore wind power of the present invention includes a wind power cluster, an offshore converter station, an energy storage device, and an onshore converter station; the offshore converter station is composed of two converters connected in series on the DC side, and the onshore converter station is composed of four converters connected in series on the DC side. The offshore converter station and the onshore converter station form a flexible DC true bipolar transmission system through DC cables; the energy storage device is composed of multiple low-voltage energy storage units and is connected to the flexible DC transmission system through the middle two converters on the DC side of the onshore converter; according to the output of the wind power cluster and the state of charge of the energy storage unit, the charge and discharge state of the energy storage device is controlled, so that the component parameters of the four converter sub-modules of the onshore converter station are the same as those without the energy storage device connected, and the operating range of the converter bridge arm current of the onshore converter station remains unchanged when the energy storage device is charging and discharging.

Description

Technical Field

[0001] The present invention relates to the technical field of offshore wind power transmission and energy storage, and in particular to a flexible DC transmission system for offshore wind power based on cascaded multi-level energy storage and its control method. Background Art

[0002] Offshore wind power is characterized by strong volatility and large 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 supply-demand contradiction between wind power fluctuations and grid load fluctuations, and improve the controllability of large-scale offshore wind power transmission systems. In addition, configuring energy storage devices can improve the reliability of offshore wind power transmission systems. When an instantaneous grounding fault occurs in the overhead line of 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 floor area, and high cost of the energy-consuming resistor scheme. When a DC submarine cable fails, the wind farm stops generating electricity and operates in an island mode. The energy storage device can be used as an island power supply for the internal auxiliary equipment of the wind turbine, overcoming the problems of high cost and difficulty in building an offshore platform of the diesel generator scheme.

[0003] Currently, for the method of AC access of energy storage devices to offshore wind power transmission systems, the technology of the low-voltage energy storage converter parallel boost access scheme is the most mature, but it requires an additional boost transformer and has problems such as large losses, high cost, and difficulty in coordinating multiple converters; the AC direct connection scheme based on cascaded H-bridges does not require a transformer, and the energy storage device is directly connected to the medium-voltage grid, with small losses and low cost, but has problems such as large circulating current and large secondary current pulsation on the battery side. For the method of DC access of energy storage devices to offshore wind power transmission systems, since almost all offshore converter stations and onshore converter stations adopt the topology of modular multilevel converters (MMCs), the DC voltage between the poles of the MMC is relatively high, while the DC voltage at the ports of the energy storage converters is relatively low, and they still cannot be directly connected on the DC side after being connected in series; some research has proposed an energy storage type MMC topology, in which a DC / DC converter is further connected in parallel at both ends of the DC capacitor of each half-bridge sub-module and then connected to the battery unit, but in this scheme, the battery unit is highly coupled with the sub-modules of the MMC, and battery failures will affect the operation of the sub-modules. Summary of the Invention

[0004] In view of the problems existing in the above-mentioned prior art, the present invention provides a flexible HVDC transmission system for offshore wind power based on cascaded multi-level energy storage, which realizes the medium-voltage DC access of the energy storage device to improve the stability, controllability and reliability of offshore wind power transmission. The present invention also provides a control method for the above-mentioned flexible HVDC transmission system for offshore wind power, which controls the charge and discharge state of the energy storage device according to the output of the wind power cluster and the state of charge of the energy storage unit, so that the component parameters of the four converter sub-modules of the onshore converter station are the same as those without the energy storage device connected, and the operating range of the converter bridge arm current of the onshore converter station remains unchanged during the charge and discharge of the energy storage device, and the DC access of the energy storage device can be realized without additional hardware modification costs.

[0005] For this reason, the present invention adopts the following technical solutions.

[0006] In the first aspect, the present invention provides a flexible HVDC transmission system for offshore wind power based on cascaded multi-level energy storage, which includes a wind power cluster, an offshore converter station, an energy storage device and an onshore converter station;

[0007] The wind power cluster is composed of the wind turbines of n offshore wind farms aggregated together. The offshore converter station is composed of a first converter and a second converter. The energy storage device is composed of m energy storage units. The onshore converter station is composed of a third converter to a sixth converter;

[0008] 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;

[0009] The third converter to the sixth converter are connected in parallel on the AC side to form a second node, and the second node is connected to the onshore AC grid;

[0010] 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. Both the third node and the fourth node are grounded;

[0011] 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. The energy storage device is connected to the fifth node and the sixth node.

[0012] Furthermore, the first converter is connected to the third converter through a DC cable to form the positive pole of the flexible HVDC transmission system, and the second converter is connected to the sixth converter through another DC cable to form the negative pole of the flexible HVDC transmission system.

[0013] Furthermore, the first converter to the sixth converter realize the conversion from direct current to alternating current, and the conversion topology is a modular multi-level converter MMC, and the sub-module topology is a half-bridge structure.

[0014] Further, both the first converter and the second converter adopt a control mode of constant AC voltage amplitude and constant AC voltage frequency to jointly control the stability of the AC voltage and frequency at the outlet of the wind power cluster; both the fourth converter and the fifth converter adopt a control mode of constant DC voltage and constant reactive power to jointly control the stability of the DC voltage between the positive terminal and the negative terminal of the energy storage device; both the third converter and the sixth converter adopt a control mode of constant DC voltage and constant 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.

[0015] Further, the DC voltages of the first converter to the sixth converter satisfy the following relationship during steady-state operation:

[0016] ,

[0017] where, 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 is the voltage between the positive pole and the negative pole of the flexible DC transmission system.

[0018] Further, the energy storage device is composed of m cascaded low-voltage energy storage units, and each low-voltage energy storage unit is formed by connecting a power conversion unit (PCS) and a battery unit in series; the DC voltage at the port of the energy storage device is U BAT , and satisfies the following relationship during steady-state operation:

[0019] .

[0020] Further, the IGBT specifications of each sub-module of the third converter to the sixth converter are selected and designed according to the situation where the energy storage device is not connected. When the energy storage device is connected, the charging and discharging conditions of the energy storage device are controlled to control the magnitude of the DC current of the third converter to the sixth converter, so that the arm current of the third converter to the sixth converter does not exceed the maximum current-carrying capacity allowed by the sub-module IGBT.

[0021] Further, let the DC currents of the third converter to the sixth converter be respectively set as Idc3 , I dc4 , I dc5 , I dc6 , the discharge current of the energy storage device is I BAT , and the DC current of the entire HVDC transmission system is I dc . When the energy storage device does not charge or discharge, the current relationships are satisfied as follows:

[0022] ,

[0023] When the energy storage device charges or discharges, the current relationships are satisfied as follows:

[0024] .

[0025] In a second aspect, the present invention provides a control method for the above-mentioned HVDC transmission system for offshore wind power based on cascaded multi-level energy storage, the content of which is as follows:

[0026] The charge and discharge 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 suppress the fluctuation of the output of the wind power cluster; assume that the rated transmission power of the onshore converter station is P wN , and the actual transmission power is P w . When the actual transmission power P w is in the range of 0 to K 1 P wN , the energy storage device is made to be in the discharge state, and the energy storage device injects current into the fifth node; when the actual transmission power P w is in the range of K 1 P wN ~ K 2 P wN , the energy storage device is made to be in the charge state, and the fifth node injects current into the energy storage device; when the actual transmission power P w is greater than K 2 P wN , the power conversion unit of the energy storage device is locked and no longer charges or discharges, 0 < K 1 < K 2 < 1.

[0027] Furthermore, the energy storage device receives charge and discharge commands according to the output of the wind power cluster P refAfter that, it is also necessary to confirm the charge and discharge permission status bit of the energy storage unit according to the state of charge (SOC) of the battery; when 0 < P w < K 1 P wN When, and P ref > 0, the energy storage unit needs to discharge. At this time, it is necessary to satisfy SOC j ≥SOC min , and the discharge permission flag bit 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 charge. At this time, it is necessary to satisfy SOC j ≤SOC max , and the charge permission flag bit is equal to 1; SOC j represents the state of charge of the j th energy storage unit, and the value range of j is an integer from 1 to m ; SOC min represents the minimum state of charge, and SOC max represents the maximum state of charge; when one of the discharge permission and charge permission conditions is met, the permission status bit is equal to 1. At this time, 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.

[0028] The beneficial effects of the present invention are as follows: The present invention reconstructs the topology of the sending converter station of the flexible DC true bipolar system, adopts the form of four converters connected in series on the DC side, constructs the access port for medium-voltage DC, makes the voltage level of the access port match the port voltage level after cascading with the energy storage device, and realizes the medium-voltage DC access of the energy storage device; and controls the charge and discharge state of the energy storage device according to the output of the wind power cluster and the state of charge of the energy storage unit, so that the component parameters of the four converter sub-modules of the onshore converter station are the same as those without the energy storage device connected, and the operating range of the converter bridge arm current of the onshore converter station remains unchanged when the energy storage device charges and discharges, and the DC access of the energy storage device can be realized without additional hardware modification costs. Description of the Drawings

[0029] Figure 1 is a topology structure diagram of an offshore wind power flexible DC transmission system based on cascaded multilevel energy storage in an embodiment of the present invention;

[0030] Figure 2 is the MMC topology diagram of the first to sixth converters in the embodiments of the present invention;

[0031] Figure 3 is the topology diagram of the energy storage device in the embodiments of the present invention;

[0032] Figure 4 is the logic schematic diagram of the charge and discharge allowable state of the energy storage device in the embodiments of the present invention. Detailed Embodiments

[0033] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and detailed implementation manners and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments.

[0034] Embodiment

[0035] As Figure 1 shown, this embodiment provides a flexible DC transmission system for offshore wind power based on cascaded multilevel energy storage, including a wind power cluster, an offshore converter station, an energy storage device, and an onshore converter station.

[0036] The said wind power cluster is composed of the wind turbines of n offshore wind farms aggregated together. The offshore converter station is composed of a first converter 1 and a second converter 2. The energy storage device is composed of m energy storage units. The onshore converter station is composed of a third converter 3 to a sixth converter 6.

[0037] The first converter 1 and the second converter 2 are connected in parallel on the AC side to form a first node, and the said 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 both the third node and the fourth node are 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.

[0038] As Figure 2As shown, the first to sixth converters 1 to 6 achieve the conversion from direct current to alternating current. The conversion topologies are all modular multilevel converters (MMCs), and the sub-module topology is a half-bridge structure. The first converter 1 and the second converter 2 both adopt the control modes of constant AC voltage amplitude and constant 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 modes of constant DC voltage and constant 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 modes of constant DC voltage and constant reactive power to jointly control the stability of the DC voltage between the positive and negative poles of the flexible DC transmission system.

[0039] The DC voltages of the first to sixth converters 1 to 6 satisfy the following relationship during steady-state operation:

[0040] ,

[0041] where, 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 respectively, U dc is the voltage between the positive and negative poles of the flexible DC transmission system.

[0042] As Figure 3 shown, the energy storage device consists of m cascaded low-voltage energy storage units. Each low-voltage energy storage unit is composed of a power conversion unit (PCS) and a battery unit connected 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 sides of the first half-bridges are connected in parallel with DC capacitors. The function of the first half-bridge is to balance the DC voltages of each energy storage unit; the input side of the second half-bridge is connected in parallel across 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 operates in the discharge mode, the second half-bridge of the power conversion unit operates in the Boost mode, the upper switch is blocked, and the lower switch conducts or turns off according to the modulation signal; when the energy storage unit operates in the charging mode, the second half-bridge of the power conversion unit operates in the Buck mode, the lower switch is blocked, and the upper switch conducts or turns off according to the modulation signal.

[0043] The DC voltage of the port of the energy storage device is U BAT , and the following relationship is satisfied during steady-state operation:

[0044] ,

[0045] The IGBT specifications of each sub-module 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 charging and discharging conditions of the energy storage device are controlled to control the magnitude of the DC current of the third converter 3 to the sixth converter 6, so that the arm current of the third converter 3 to the sixth converter 6 does not exceed the maximum current-carrying capacity allowed by the sub-module IGBT.

[0046] Let the DC currents of the third converter 3 to the sixth converter 6 be respectively set as I dc3 , I dc4 , I dc5 , I dc6 , the discharge current of the energy storage device is I BAT , and the DC current of the entire flexible DC transmission system is I dc . When the energy storage device does not charge or discharge, the current relationships are satisfied as follows:

[0047] ,

[0048] When the energy storage device charges and discharges, the current relationships are satisfied as follows:

[0049] ,

[0050] The charging and discharging states of the energy storage device are controlled according to the output of the wind power cluster, and the energy storage device can suppress the fluctuations of the output of the wind power cluster. Let the rated transmission power of the onshore converter station be P wN , and the actual transmission power be P w . When P w is in the range of 0 - K 1 P wN , the energy storage device is made to be in the discharge state, and the energy storage device injects current into the fifth node; when P w is in K 1 P wN - K 2 P wNWhen it is within the range, the energy storage device is in the charging state, and the fifth node injects current into the energy storage device; when P w is greater than K 2 P wN , the power conversion unit of the energy storage device is locked and no longer charges or discharges, where 0 < K 1 < K 2 < 1.

[0051] As Figure 4 shown, after the energy storage device receives the charge and discharge command according to the output of the wind power cluster P ref , it is also necessary to confirm the charge and discharge permission status bit of the energy storage unit according to the state of charge SOC of the battery; when 0 < P w < K 1 P wN and P ref > 0, the energy storage unit needs to discharge. At this time, it is necessary to satisfy SOC j ≥SOC min , and the discharge permission flag bit is equal to 1; when K 1 P wN < P w < K 2 P wN and P ref < 0, the energy storage unit needs to charge. At this time, it is necessary to satisfy SOC j ≤SOC max , and the charge permission flag bit is equal to 1; SOC j represents the state of charge of the j th energy storage unit, j the value range of which is an integer from 1 to m ; SOC min represents the minimum state of charge, and SOC max represents the maximum state of charge; when one of the discharge permission and charge permission conditions is satisfied, the permission status bit is equal to 1. At this time, 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.

[0052] Application Example

[0053] The one kind of flexible DC transmission system for offshore wind power based on cascaded multi-level energy storage and its control method described in the embodiment are adopted for the following application.

[0054] Rated output power of onshore converter station P wN = 300 MW, the voltage between poles of DC cable U dc = 800 kV, the DC voltages of the first converter 1 and the second converter 2 are U dc1 = U dc2 = 400 kV, the DC voltage at the port of energy storage device U BAT = 160 kV, the DC voltages of the fourth converter 4 and the fifth converter 5 U dc4 = U dc5 = 80 kV, the DC voltages of the third converter 3 and the sixth converter 6 U dc3 = U dc6 = 320 kV. The turns ratios of the transformers connected to the AC sides of the fourth converter 4 and the fifth converter 5 are both 84 kV / 220 kV, and the turns ratios of the transformers connected to the AC sides of the third converter 3 and the sixth converter 6 are both 336 kV / 220 kV. Then the rated voltage of the AC side of the onshore converter station U wN = 420 kV.

[0055] Ignoring the losses of the converters, when the actual output power of the wind power cluster through the onshore converter station P w is equal to the rated power P wN and the energy storage device does not charge or discharge, the rated current of the sub-module switching devices in the converter I mN is:

[0056]

[0057] The number of energy storage units m = 50, and the charge and discharge threshold coefficients of the energy storage device are taken as K 1 = 0.5, K 2 = 0.8. When the actual output power of the onshore converter station P w is in the range 0 < P w < 150 MW, the energy storage device discharges to the onshore converter, and the maximum value of the current of the sub-module switching devices of the third converter 3 and the sixth converter 6 is K 1 I mN = 0.5 I mN, at this time, it is necessary to control the maximum value of the current of the sub-module switching devices of the fourth converter 4 and the fifth converter 5 not to be greater than I mN , then the discharge current of the energy storage device I BAT satisfies:

[0058]

[0059] It can be solved that the discharge current of the energy storage device at this time I BAT The range is:

[0060]

[0061] The discharge current of the energy storage device I BAT The maximum value I BAT_max = 0.625 kA, then the maximum possible value of the discharge command of the energy storage device P ref is: P ref_max is:

[0062]

[0063] When the actual power output of the onshore converter station P w is in the range of 150 MW < P w < 240 MW, the energy storage device charges from the onshore converter, and the maximum value of the current of the sub-module switching devices of the fourth converter 4 and the fifth converter 5 is K 2 I mN = 0.8 I mN , at this time, it is necessary to control the maximum value of the current of the sub-module switching devices of the third converter 3 and the sixth converter 6 not to be greater than I mN , then the charging current of the energy storage device I BAT satisfies:

[0064]

[0065] It can be solved that the discharge current of the energy storage device at this time I BAT The range is:

[0066]

[0067] The discharge current of the energy storage device I BAT The minimum valueI BAT_min =-0.25 kA, then the energy storage device charging instruction P ref The possible minimum value P ref_min is:

[0068]

[0069] Let the minimum state of charge SOC for allowing discharge min = 0.1, and the maximum state of charge SOC for allowing charging max = 0.9.

[0070] When 0 ≤ P w < 150 MW, the charge and discharge instruction P ref According to P w varies, and its range is 0 < P ref ≤ 100 MW. For the j th energy storage unit, its state of charge is SOC j , if SOC j ≥ 0.1, the discharge permission flag bit 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 = 3.2 kV, and the second half-bridge operates in the Buck mode.

[0071] When 150 MW ≤ P w < 240 MW, the charge and discharge instruction P ref According to P w varies, and its range is -40 MW ≤ P ref ≤ 0. For the j th energy storage unit, its state of charge is SOC j , if SOC j ≤ 0.9, the charge permission flag bit 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 = 3.2 kV, and the second half-bridge operates in the Boost mode.

[0072] When 240 MW ≤ P w ≤ 300 MW, the charge and discharge instruction P ref= 0, the discharge permission flag bit is equal to 0, the charge permission flag bit is equal to 0, and the first half-bridge and the second half-bridge of the power conversion unit PCS are locked.

[0073] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included in the protection scope of the present invention.

Claims

1. A control method for a flexible DC transmission system of offshore wind power based on cascaded multilevel energy storage, characterized in that, The described flexible HVDC transmission system for offshore wind power 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 the wind turbines of n offshore wind farms, the offshore converter station is composed of a first converter and a second converter, the energy storage device is composed of m energy storage units, and the onshore converter station is composed of a third converter to a sixth converter; 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 converter to the sixth converter are connected in parallel on the AC side to form a second node, and the second node is connected to the onshore AC 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 both the third node and the fourth node are 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; in the form of series connection of the DC sides of the third to sixth converters, an access port for medium-voltage DC is constructed so that the voltage level of the access port matches the port voltage level after cascading of the energy storage device, realizing the medium-voltage DC access of the energy storage device; The control method of the offshore HVDC flexible power transmission system is as follows: the charge and discharge 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 suppress the fluctuation of the output of the wind power cluster; assume that the rated transmission power of the onshore converter station is P wN , and the actual transmission power is P w . When the actual transmission power P w is in the range of 0 to K 1 P wN , the energy storage device is in the discharge state, and the energy storage device injects current into the fifth node; when the actual transmission power P w is in the range of K 1 P wN to K 2 P wN , the energy storage device is in the charge state, and the fifth node injects current into the energy storage device; when the actual transmission power P w is greater than K 2 P wN , the power conversion unit of the energy storage device is blocked and no longer charges or discharges, 0 < K 1 < K 2 < 1.

2. The control method according to claim 1, characterized in that, The first converter and the third converter are connected by a DC cable to form the positive pole of the flexible DC transmission system. The second converter and the sixth converter are connected by another DC cable to form the negative pole of the flexible DC transmission system.

3. The control method according to claim 1, characterized in that The first converter to the sixth converter realize the conversion from direct current to alternating current. The conversion topology is a modular multilevel converter (MMC), and the sub-module topology is a half-bridge structure.

4. The control method according to claim 1, wherein Both the first converter and the second converter adopt the control modes of constant AC voltage amplitude and constant AC voltage frequency to jointly control the stability of the AC voltage and frequency at the outlet of the wind power cluster. Both the fourth converter and the fifth converter adopt the control modes of constant DC voltage and constant reactive power to jointly control the stability of the DC voltage between the positive and negative ports of the energy storage device. Both the third converter and the sixth converter adopt the control modes of constant DC voltage and constant 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 control method according to claim 1, characterized in that, The DC voltages of the first converter to the sixth converter satisfy the following relationship during steady-state operation: ， Among them, 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 is the voltage between the positive pole and the negative pole of the HVDC transmission system.

6. The control method according to claim 1, wherein The energy storage device consists of m cascaded low-voltage energy storage units, and each low-voltage energy storage unit is formed by connecting a power conversion unit and a battery unit in series; the port DC voltage of the energy storage device is U BAT , and 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 control method according to claim 1, wherein The IGBT specifications of each sub-module of the third converter to the sixth converter are selected and designed according to the situation where the energy storage device is not connected. After the energy storage device is connected, the magnitude of the DC current of the third converter to the sixth converter is controlled by controlling the charge and discharge of the energy storage device, so that the arm current of the third converter to the sixth converter does not exceed the maximum current-carrying capacity allowed by the sub-module IGBT.

8. The control method according to claim 1, wherein Let the DC currents of the third converter to the sixth converter be set as I dc3 , I dc4 , I dc5 , I dc6 , the discharge current of the energy storage device is I BAT , and the DC current of the entire HVDC transmission system is I dc . When the energy storage device is not charging or discharging, the current relationships are satisfied as follows: ， When the energy storage device is charging and discharging, the following current relationships are satisfied: 。 9. The control method according to claim 1, characterized in that The energy storage device receives charge and discharge commands according to the output of the wind power cluster P ref After that, it is also necessary to confirm the charge and discharge permission status bit of the energy storage unit according to the state of charge (SOC) of the battery; when 0 < P w < K 1 P wN When, and P ref > 0, the energy storage unit needs to discharge. At this time, it is necessary to satisfy SOC j ≥ SOC min , and the discharge permission flag bit 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 charge. At this time, it is necessary to satisfy SOC j ≤ SOC max , and the charge permission flag bit is equal to 1; SOC j Indicates the state of charge of the j th energy storage unit, j The value range of is 1 - m An integer; SOC min Indicates the minimum state of charge, SOC max Indicates the maximum state of charge; when one of the discharge permission and charge permission conditions is met, the permission status bit is equal to 1. At this time, 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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