WO2022142812A1

WO2022142812A1 - Multi-end offshore wind power flexible direct current and energy storage cooperative grid-connected system and control method thereof

Info

Publication number: WO2022142812A1
Application number: PCT/CN2021/131065
Authority: WO
Inventors: 翟冬玲; 杨张斌
Original assignee: 中国长江三峡集团有限公司
Priority date: 2020-12-31
Filing date: 2021-11-17
Publication date: 2022-07-07

Abstract

A multi-end offshore wind power flexible direct current and energy storage cooperative grid-connected system and a corresponding control method. The system comprises a plurality of offshore wind power clusters (1), a plurality of offshore converter stations (2) connected in parallel, and an onshore converter station (4) connected to the offshore converter stations (2); each offshore converter station (2) is directly connected to an output end of the corresponding offshore wind power cluster (1); direct current output ends of the offshore converter stations (2) are connected in parallel; the onshore converter station (4) is connected to an alternating current main power grid (5); an energy storage subsystem (6) is connected to the onshore converter station (4), and comprises a plurality of energy storage units, which are used for stabilizing the offshore wind power output fluctuation. A branch-type multi-end offshore wind power flexible direct current power transmission subsystem integrates a plurality of distributed offshore wind power clusters (1), a seabed transmission corridor and an onshore converter station (4) are shared, and an energy storage subsystem (6) is connected to the onshore converter station (4), so that the seabed transmission corridor and landing point resources are saved, the network loss of a power collecting system is reduced, wind power output is smoothed, and the flexibility, stability and reliability of offshore wind power generation are improved.

Description

Multi-terminal offshore wind power flexible DC and energy storage collaborative grid-connected system and its control method

technical field

The invention belongs to the field of offshore wind power control, and in particular relates to a multi-terminal offshore wind power flexible direct current and energy storage coordinated grid-connected system and a control method thereof.

Background technique

With the continuous development of offshore wind power resources in my country, offshore wind power will inevitably go to the deep sea. Flexible DC transmission is one of the most important technical routes for offshore wind power transmission, and has been widely used in Europe. However, the cost of offshore wind power flexible direct current transmission projects is still relatively high. If each offshore wind farm adopts the flexible direct transmission technology, there will be multiple parallel offshore wind power flexible direct transmission projects, and the investment is huge. At the same time, the submarine corridors of multiple flexible and straight transmission lines for offshore wind power take up a lot of space and require multiple landing points, which makes the resources of offshore transmission channels and landing points even more tense, and requires the construction of multiple onshore converter stations, which wastes floor space. Multiple onshore converter stations connected to the grid in a multi-feed manner will also complicate the local grid. In addition, due to the long time period for the implementation of the project and the delivery of the project to start construction, it is difficult to guarantee the construction progress.

In order to reduce engineering cost and save submarine corridors and landing points, multi-terminal flexible DC transmission technology can be used. This technology enables multiple offshore wind power clusters to form a multi-port structure, and the transmission system shares submarine corridors and landing points, and the ports are also very easy to expand. Multi-terminal flexible direct current transmission technology has been applied in many projects on land in my country, but due to the particularity of offshore wind power, there are many differences in the application methods of onshore and offshore multi-terminal flexible direct current systems, such as wiring form, energy consumption device type, whether Adopt DC circuit breaker, black start mode and control mode, etc. The onshore multi-terminal flexible DC transmission system, such as the Zhangbei multi-terminal flexible DC power grid, adopts the form of ring connection, and each DC terminal is equipped with a DC circuit breaker and an AC energy consumption device. However, according to the operating environment and working conditions of offshore wind power, the multi-terminal flexible direct system should not adopt the ring wiring method, and the use of offshore DC circuit breakers will also increase the weight of the offshore platform. In addition, for the offshore wind power system, there is no supporting power supply, and it is necessary to reverse the power to start black. Therefore, it is necessary to fully consider the characteristics of the offshore wind power flexible direct transmission system, and expand the DC terminal, so as to be suitable for the power transmission of multiple offshore wind power clusters.

In addition, as of now, the design capacity of the world's largest offshore wind turbine is 14MW, and the maximum single-unit capacity of my country's grid-connected offshore wind turbines has reached 10MW, and the single-unit capacity of offshore wind turbines is still on the rise. With the continuous improvement of the single-unit capacity of offshore wind turbines, the distance between offshore wind turbines is also increasing. If the traditional 35kV collection scheme is adopted, the loss of the power collection system will also increase. Offshore wind farms in Europe have adopted the 66kV power collection system scheme and cancelled the offshore booster station, while my country has also developed a 66kV dynamic submarine cable. Therefore, from reducing the loss of the offshore wind power collection system and canceling the offshore booster From the perspective of meeting the demand for reducing the cost of electricity of offshore wind power in my country, the 66kV power collection system scheme has a very broad application space in my country.

In addition to the above-mentioned problems, the grid-connected consumption of offshore wind power is also the focus of the industry. The grid-connected consumption of offshore wind power is closely related to the power supply scale, load and external transmission capacity of the local power grid. If the load growth rate and the growth rate of new energy installed capacity and the construction of external transmission channels cannot be balanced, and external calls do not participate in peak regulation, it will cause a peak regulation gap in the main AC network; the volatility and randomness of offshore wind power will also increase the problem of wind curtailment. . At present, onshore new energy power stations have begun to configure energy storage systems to balance the randomness of new energy power generation, and participate in grid peak regulation and frequency regulation through the auxiliary service market to improve engineering economy. Although the current cost of energy storage systems is high, with the continuous development of energy storage technology, the configuration of energy storage systems in offshore wind power projects has very good application prospects.

SUMMARY OF THE INVENTION

The purpose of the present invention is to solve the above problems, to provide a multi-terminal offshore wind power flexible DC and energy storage collaborative grid-connected system and its control method, adopting the multi-terminal flexible DC transmission technology, sharing submarine cable corridors and landing points, onshore converter stations Connect the energy storage subsystem, smooth the wind power output, participate in the peak regulation, voltage regulation and frequency regulation of the main power grid, and improve the stability and reliability of offshore wind power generation.

The technical scheme of the present invention is a multi-terminal offshore wind power flexible DC and energy storage collaborative grid-connected system, including multiple offshore wind power clusters, multiple parallel offshore converter stations, and onshore converter stations connected to the offshore converter stations; each The offshore converter station is connected to the output terminal of the corresponding offshore wind power cluster, without the need for an offshore booster station. The DC output terminals of the offshore converter station are connected in parallel to form a branch-type multi-terminal offshore wind power flexible DC transmission and electronic system, sharing the submarine transmission corridor and onshore Converter station; the onshore converter station is connected to the AC mains grid.

Preferably, the generator set of the offshore wind power cluster adopts a semi-direct drive or direct drive type wind generator set, and the AC voltage level of the offshore wind power cluster adopts 66kV.

Further, the energy storage subsystem is connected with the onshore converter station, and includes a plurality of energy storage units, which are used for smoothing the fluctuation of the offshore wind power output.

Further, the offshore converter station includes a first transformer and a first modular multilevel converter connected to the first transformer, and the first modular multilevel converter includes a plurality of half-bridge switch tube modules or full The bridge-type switch tube modules are cascaded to form an upper bridge arm and a lower bridge arm, the lower bridge arm has the same structure as the upper bridge arm, and the connection ends of the upper bridge arm and the lower bridge arm of the offshore converter are connected to the first transformer.

Further, the onshore converter station includes a second modular multi-level converter and a second transformer connected thereto, and the second modular multi-level converter includes a cascade of multiple full-bridge switch tube modules. The upper bridge arm and the lower bridge arm have the same structure as the upper bridge arm. The connecting ends of the upper bridge arm and the lower bridge arm of the second modular multilevel converter lead out wires as the second modular multilevel converter. the output of the streamer.

Preferably, a starting resistor is connected between the second modular multi-level converter and the second transformer, and the connection end between the second transformer and the starting resistor is connected to the grounding device.

Preferably, the DC side of the second modular multilevel converter is provided with an energy consumption device connected in parallel with it, and the energy consumption device is used for unloading to prevent the DC voltage from being too high.

Preferably, both the upper bridge arm and the lower bridge arm of the first modular multilevel converter are provided with first bridge arm reactors connected in series.

Preferably, both ends of the DC side of the second modular multilevel converter are provided with smoothing reactors connected in series.

Preferably, both the upper bridge arm and the lower bridge arm of the second modular multilevel converter are provided with second bridge arm reactors connected in series.

Further, the energy storage subsystem includes a plurality of energy storage units, a first-stage step-up transformer, and a plurality of second-stage step-up transformers connected to the first-stage step-up transformer. Transformer connection.

Preferably, the first multi-level converter half-bridge switch tube module includes two series-connected power switch tubes, each power switch tube is anti-parallel freewheeling diode, and the capacitor C1 is connected in parallel with the series-connected power switch tubes.

Preferably, the second full-bridge switch tube module of the multilevel converter includes four power switch tubes and a capacitor C2, and the power switch tubes are connected in parallel with the capacitor C2 after being connected in series.

Preferably, the energy-consuming device includes a plurality of cascaded energy-consuming sub-modules, the energy-consuming sub-modules include a resistor R1, a resistor R2, diodes VD1 to VD3, a DC capacitor C _dc , a switch Q and a switch S3, and a diode VD2 The anode is connected to the collector of the switch Q, the anode of the DC capacitor C _dc is connected to the cathode of the diode VD2, the cathode of the DC capacitor C _dc is connected to the emitter of the switch Q and the anode of the diode VD3 respectively, and one end of the resistor R1 is connected to The anode of the DC capacitor Cdc is connected, the other end of the resistor R1 is connected to the cathode of the diode VD3, one end of the resistor R2 is connected to the anode of the diode VD2, the other end of the resistor R2 is connected to the cathode of the diode VD3, and the switch S3 is connected in parallel with the diode VD3. The tube Q is connected in anti-parallel with the freewheeling diode VD1; the two ends of the energy consumption device are respectively provided with switches S1 and S2.

The control method for the above-mentioned multi-terminal offshore wind power flexible DC and energy storage coordinated grid-connected system includes the following steps:

Step 1: Compare the output power P _wind of the offshore wind power cluster according to the dispatch command power P _ord of the main power grid, and control the charging or discharging of the energy storage system and the output of the offshore converter station according to the comparison result;

Step 2: According to the voltage change ΔU of the grid connection point with the main grid, carry out constant reactive power control based on Q-U droop for onshore converter stations, and carry out constant AC voltage control for energy storage inverters or constant reactive power based on Q-U droop. Power Control;

Step 3: Detect the main grid frequency deviation Δf, and use P-f droop control to control the power output of the multi-terminal offshore wind power flexible DC and energy storage coordinated grid-connected system to adjust the main grid frequency.

The multi-terminal offshore wind power flexible DC and energy storage collaborative grid-connected system also includes an energy storage control unit, a flexible DC control unit, and an energy storage and flexible direct coordination control unit. The power reference value of the energy subsystem, detects the frequency deviation of the power grid, and calculates the power adjustment amount; the energy storage control unit controls the energy storage unit with constant active power, and controls the inverter with constant DC voltage and reactive power; the flexible DC control unit Constant AC voltage control and active power control are performed for offshore converter stations, and constant DC voltage control and constant reactive power control based on Q-U droop are performed for onshore converter stations.

The control process of the storage control unit, flexible DC control unit, energy storage and flexible DC coordination control unit includes:

1) Calculate the active power control reference value P _ref =P _ord -P _wind , when -P _{lim2 ≤P} _ref ≤P _lim1 , the fixed active power control reference value of the energy storage unit P _batt =P _ref , P _lim1 represents the energy storage unit The critical value of charging power, P _lim2 represents the critical value of the discharge power of the energy storage unit. According to the size of P _ref , the discharge or charging of the energy storage unit is _controlled , and the offshore converter station adopts constant frequency control _; When the critical value of the charging power of the energy unit, P _batt =P _lim1 , the offshore converter station adopts constant active power control, and the active power reference value of the i-th offshore converter station is P _refi =(P _ord -P _batt )/P _t * P _i , P _t is the total rated capacity of the system, and P _i is the rated capacity of the i-th offshore converter station;

2) When detecting the voltage change ΔU at the grid connection point, -ΔU ₁ ≤ ΔU ≤ ΔU ₂ , the inverter of the energy storage control unit adopts constant AC voltage control; when ΔU <-ΔU ₁ or ΔU > ΔU ₂ , the energy storage control unit inverter The constant reactive power control based on QU droop is adopted in the converter and onshore converter station;

3) Detect the frequency deviation Δf of the main grid, adopt Pf droop control, use the simulated rotor operation equation (Hω ₀ s+D)Δf=ΔP to calculate the active power change ΔP, and calculate the power output adjustment value of the energy storage unit P _s =ΔP+ P _batt , if -P _{lim2 ≤P} _s ≤P _lim1 , at this time, the variation of the active power reference value of the energy storage unit ΔP _batt =ΔP, and the new active power reference value is P _s ; if P _s <-P _lim2 , it means that If the active power regulation exceeds the critical value of the discharge power of the energy storage unit, let P _batt =-P _lim2 ; if P _s >P _lim1 , it means that the active power regulation exceeds the critical value of the charging power of the energy storage unit, let P _batt =P _lim1 , at this time , the offshore converter station adopts constant active power control, the active power reference value of the i-th offshore converter station is P _refi =(P _ord -P _batt )/P _t *P _i , P _t is the total rated capacity of the system, P _i is the rated capacity of the i-th offshore converter station.

Compared with the prior art, the beneficial effects of the present invention:

1) The present invention adopts a branch-type multi-terminal offshore wind power flexible DC power transmission system, and a plurality of independent offshore converter stations integrate the scattered offshore wind power clusters, share the submarine transmission corridor and the onshore converter station, and save the submarine transmission corridor and landing point. Resources, the offshore wind power flexible DC transmission system has strong scalability, and it is easy for new offshore wind power clusters to be connected through DC terminals; the 66kV power collection system is adopted, the offshore booster station is cancelled, and the network loss and engineering cost of the power collection system are reduced; The station is connected to the energy storage subsystem to smooth the wind power output and improve the controllability, stability and reliability of offshore wind power generation;

2) The present invention responds to the power dispatch command of the main grid and the voltage regulation and frequency regulation of the main grid through the joint control of the energy storage unit and the energy storage inverter, the offshore converter station and the onshore converter station, so that the main grid is more stable. , efficient;

3) The onshore converter station of the present invention adopts a full-bridge power switch tube module. When a short-circuit fault occurs in the DC system, the short-circuit current injected by the grid into the DC short-circuit point can be suppressed, and the system stability margin can be improved.

Description of drawings

The present invention will be further described below with reference to the accompanying drawings and embodiments.

FIG. 1 is a topology circuit wiring diagram of a multi-terminal offshore wind power flexible DC and energy storage coordinated grid-connected system according to an embodiment of the present invention.

FIG. 2 is a wiring diagram of a topology circuit of an offshore converter station according to an embodiment of the present invention.

FIG. 3 is a wiring diagram of a topology circuit of an onshore converter station according to an embodiment of the present invention.

FIG. 4 is a wiring diagram of a topology circuit of an energy storage subsystem according to an embodiment of the present invention.

FIG. 5 is a schematic diagram of a topology circuit of an energy consuming device according to an embodiment of the present invention.

FIG. 6 is a schematic diagram of a control method according to an embodiment of the present invention.

Detailed ways

As shown in Figure 1-4, the multi-terminal offshore wind power flexible DC and energy storage collaborative grid-connected system includes multiple offshore wind power clusters 1, multiple parallel offshore converter stations 2, and onshore converters connected to the offshore converter stations. Station 4; each offshore converter station is connected to the output end of the corresponding offshore wind power cluster, without the need for an offshore booster station, and the DC output ends of the offshore converter station are connected in parallel to form a branch-type multi-terminal offshore wind power flexible DC transmission electronic system. The flow station 2 is connected to the onshore converter station 4 via the DC transmission cable 3 , and shares the submarine transmission corridor and the onshore converter station; the onshore converter station is connected to the AC main grid 5 .

As shown in FIG. 2 , the offshore converter station 2 includes a first transformer 201 and a first modular multilevel converter 202 connected to the first transformer. The first modular multilevel converter 202 includes a plurality of half-level converters. The upper bridge arm and the lower bridge arm formed by the cascade connection of the bridge switch tube module HSM, the lower bridge arm has the same structure as the upper bridge arm, and the connection ends of the upper bridge arm and the lower bridge arm of the offshore converter are connected to the first transformer 201; The upper bridge arm and the lower bridge arm of the first modular multilevel converter are both provided with first bridge arm reactors 203 connected in series.

The half-bridge switch tube module HSM of the first multilevel converter includes two series-connected power switch tubes, each power switch tube is anti-parallel freewheeling diode, and the voltage stabilizing capacitor C1 is connected in parallel with the series-connected power switch tubes.

As shown in FIG. 3 , the onshore converter station 4 includes a second modular multilevel converter 402 and a second transformer 404 connected in series therewith, and the second modular multilevel converter 402 includes a plurality of full bridges The upper bridge arm and the lower bridge arm are cascaded into the FSM type switch tube module. The lower bridge arm has the same structure as the upper bridge arm. as the output of the second modular multilevel converter. A start-up resistor 407 is connected between the second modular multi-level converter 402 and the second transformer 404 , and the connection end of the second transformer 404 and the start-up resistance 407 is connected to the grounding device 405 . Both the upper bridge arm and the lower bridge arm of the second modular multilevel converter are provided with second bridge arm reactors 403 connected in series. Both ends of the DC side of the second modular multilevel converter 402 are provided with smoothing reactors 401 connected in series. The full-bridge switch tube module FSM of the second multi-level converter includes four power switch tubes and a capacitor C2, and the power switch tubes are connected in parallel with the capacitor C2 after being connected in series.

The DC side of the second modular multilevel converter 402 is provided with an energy dissipation device 406 connected in parallel therewith. When an AC fault occurs on the AC grid side, the energy consuming device 406 is used for unloading to prevent the DC voltage from being too high.

As shown in FIG. 4 , the energy storage subsystem 6 includes a plurality of energy storage units, a first-stage step-up transformer, and a plurality of second-stage step-up transformers connected to the first-stage step-up transformer. The energy storage unit is connected to the corresponding inverter via the corresponding inverter. Secondary step-up transformer connection. The energy storage subsystem 6 is connected to the onshore converter station.

As shown in FIG. 5 , the energy-consuming device 406 includes a plurality of cascaded energy-consuming sub-modules SM, and the energy-consuming sub-module SM includes a resistor R1, a resistor R2, diodes VD1-VD3, a DC capacitor C _dc , a switch tube Q and a switch S3, the anode of the diode VD2 is connected to the collector of the switch Q, the anode of the DC capacitor C _dc is connected to the cathode of the diode VD2, the cathode of the DC capacitor C _dc is connected to the emitter of the switch Q and the anode of the diode VD3 respectively, the resistance One end of R1 is connected to the anode of the DC capacitor Cdc, the other end of the resistor R1 is connected to the cathode of the diode VD3, one end of the resistor R2 is connected to the anode of the diode VD2, the other end of the resistor R2 is connected to the cathode of the diode VD3, and the switch S3 is connected to the diode VD3 is connected in parallel, and the switch tube Q is inversely connected in parallel with the freewheeling diode VD1; the two ends of the energy consuming device 406 are respectively provided with switches S1 and S2.

Considering the high capacity of the offshore wind farm, it needs to be sent to the ultra-high voltage grid for consumption. In the embodiment, the energy storage subsystem 6 is incorporated into the low-voltage side of the transformer of the onshore converter station 4, but the AC on the low-voltage side of the transformer The voltage level is still very high. Therefore, in the embodiment, the AC power output by the battery block through the inverter is boosted in two stages.

In the embodiment, the role of the energy storage subsystem is mainly to stabilize wind power fluctuations and participate in peak regulation, voltage regulation, and frequency regulation of the system. The battery of the energy storage unit adopts electrochemical energy storage. Load and delivery channel construction, energy storage charging and discharging characteristics, and return on investment, etc.

Step 2: According to the voltage change ΔU of the grid connection point with the main grid, carry out constant DC voltage control and constant reactive power control based on Q-U droop for the onshore converter station, and carry out constant DC voltage control and AC voltage control for the energy storage inverter. Control or constant reactive power control based on Q-U droop;

Step 3: Detect the frequency deviation Δf of the main grid, use P-f droop control, use the simulated rotor equation of motion to calculate the active power regulation amount ΔP, and control the power output of the multi-terminal offshore wind power flexible DC and energy storage collaborative grid-connected system to adjust the main grid frequency. The equation of motion of the rotor is as follows:

(Hω ₀ s+D)Δf=ΔP

where H is the inertia time constant, ω ₀ is the rated speed, D is the damping parameter, and s is the complex frequency in the complex frequency domain.

The P-f droop control in step 3 refers to the P-f droop control method disclosed in the paper "Adaptive Droop Control of VSC-MTDC Connected to Low Inertia Systems" published in the journal "Electric Power Automation Equipment", No. 39, 2019.

The Q-U droop control of the onshore converter station and the Q-U droop control of the energy storage inverter refer to the Q-U slope control method disclosed in "High Voltage Direct Current Transmission Technology Based on Voltage Source Converters" published by China Electric Power Press in 2010.

As shown in Figure 6, the control process of the energy storage control unit, the flexible DC control unit, the energy storage and the flexible DC coordination control unit includes:

(1) When the energy storage subsystem is in the state of charge and discharge, the logical value L ₁ takes the value of 0, the offshore converter station adopts constant frequency control, and the active power control reference values of the energy storage unit P _batt =P _ref , P _ref = P _ord -P _wind , where P _ord is the power grid dispatch command, and P _wind is the power generated by the wind farm;

(2) When the dispatch instruction P _ord is less than the offshore wind power output P _wind , and the energy storage subsystem is fully charged to the limit, that is, the magnitude of P _ref exceeds the critical value P _lim1 of the charging power of the energy storage unit, the logic value L ₁ The value is -1, the active power control reference value of the energy storage subsystem P _batt =P _lim1 , and at the same time, the active power control mode of offshore wind power will be switched from constant frequency control to constant active power control, and the active power of the i-th offshore converter station will be Power reference value P _refi =(P _ord -P _batt )/P _t *P _i , P _t is the total rated capacity of the system, P _i is the rated capacity of the ith offshore converter station, where i∈{1,2, 3}, representing offshore converter stations #1, #2, and #3 respectively;

(3) When the energy storage subsystem needs to perform active power adjustment ΔP in response to the system frequency change Δf, calculate the power output adjustment value of the energy storage unit P _s =ΔP+P _batt , if P _s >P _lim1 , the logical value L ₂ is taken as If the value is -1, the active power control reference value of the energy storage subsystem is set to P _lim1 , if P _s <-P _lim2 , the logical value L ₂ takes the value of 1, and the active power control reference value of the energy storage subsystem is set to P lim1 . _lim2 and P _lim2 are the critical values of the discharge power of the energy storage unit. At the same time, when the charging limit of the energy storage subsystem is P _lim1 , that is, the value of L ₂ is -1, it is necessary to reduce the output power of the offshore wind farm and the active power of the offshore wind power. The control mode will be switched from constant frequency control to constant active power control, the active power reference value of the i-th offshore converter station P _refi =(P _ord -P _batt )/P _t *P _i , P _t is the total rated capacity of the system, P _i is the rated capacity of the ith offshore converter station, where i∈{1, 2, 3}, representing offshore converter stations #1, #2, and #3 respectively;

(4) When the multi-terminal offshore wind power flexible DC and energy storage collaborative grid-connected system responds to the voltage change ΔU at the grid connection point, when the value of ΔU is within the allowable range of the system -ΔU ₁ ~ ΔU ₂ , the logical value L ₃ takes the value of 0, and the storage The reactive power control mode of the inverter that can control the unit is constant AC voltage control;

(5) When the multi-terminal offshore wind power flexible DC and energy storage coordinated grid-connected system responds to the voltage change ΔU at the grid connection point, when ΔU <-ΔU ₁ , the logical value L ₃ is -1, and when ΔU > ΔU ₂ , the logical value The value of L3 is ₁ , and the reactive power control mode of the inverter is constant reactive power control based on QU droop, and provides reactive power support to the grid together with the onshore converter station.

Claims

A multi-terminal offshore wind power flexible direct current and energy storage coordinated grid-connected system, characterized in that it includes a plurality of offshore wind power clusters (1), a plurality of parallel offshore converter stations (2), and an onshore converter connected to the offshore converter stations Station (4); each offshore converter station is connected to the output terminal of the corresponding offshore wind power cluster, and the DC output terminals of the offshore converter station are connected in parallel to form a branch-type multi-terminal offshore wind power flexible DC transmission electronic system, sharing the submarine transmission corridor and land The onshore converter station; the onshore converter station is connected to the AC mains grid (5);

The energy storage subsystem (6) is connected with the onshore converter station, and includes a plurality of energy storage units, which are used for smoothing output fluctuations of offshore wind power;

The offshore converter station (2) includes a first transformer (201) and a first modular multilevel converter (202) connected to the first transformer, the first modular multilevel converter (202) including multiple The upper bridge arm and the lower bridge arm are cascaded into two half-bridge type switch tube modules or full bridge type switch tube modules. The lower bridge arm has the same structure as the upper bridge arm, and the connection between the upper and lower bridge arms of the offshore converter The terminal is connected to the first transformer;

The onshore converter station (4) comprises a second modular multilevel converter (402) and a second transformer (404) connected thereto, the second modular multilevel converter (402) comprising a plurality of full The upper bridge arm and the lower bridge arm are cascaded into bridge-type switch tube modules. The lower bridge arm has the same structure as the upper bridge arm. The connecting ends of the upper bridge arm and the lower bridge arm of the second modular multi-level converter lead out wires. as the output of the second modular multilevel converter.
The multi-terminal offshore wind power flexible DC and energy storage coordinated grid-connected system according to claim 1, wherein a start-up resistor (407) is connected between the second modular multi-level converter and the second transformer.
The multi-terminal offshore wind power flexible DC and energy storage coordinated grid-connected system according to claim 1, wherein the DC side of the second modular multi-level converter is provided with an energy consumption device (406) connected in parallel with it, The energy consumption device (406) is used for unloading to prevent the DC voltage from being too high.
The multi-terminal offshore wind power flexible DC and energy storage coordinated grid-connected system according to claim 1, wherein the upper arm and the lower arm of the first modular multi-level converter are both provided with a series-connected first Bridge arm reactor (203).
The multi-terminal offshore wind power flexible DC and energy storage collaborative grid-connected system according to claim 1, characterized in that, both ends of the DC side of the second modular multi-level converter are provided with smoothing reactors ( 401).
The multi-terminal offshore wind power flexible DC and energy storage coordinated grid-connected system according to claim 1, wherein the energy storage subsystem comprises a plurality of energy storage units (604), a first-stage step-up transformer (601) and a plurality of A second-level booster transformer (602) connected to the first-level booster transformer, and the energy storage unit is connected to the second-level booster transformer via a corresponding inverter (603).
The multi-terminal offshore wind power flexible DC and energy storage coordinated grid-connected system according to claim 1, wherein the first multi-level converter comprises two power switch tubes connected in series, and each power switch tube is anti-parallel freewheeling The diode and the capacitor C1 are connected in parallel with the power switch tube connected in series.
The multi-terminal offshore wind power flexible DC and energy storage collaborative grid-connected system according to claim 1, wherein the second multi-level converter includes four power switch tubes and a capacitor C2, and the power switch tubes are connected in series with each other. Capacitor C2 is connected in parallel.
The control method for a multi-terminal offshore wind power flexible DC and energy storage coordinated grid-connected system according to any one of claims 1-8, characterized in that, it comprises the following steps:

Step 1: Compare the output power P wind of the offshore wind power cluster according to the dispatch command power P ord of the main power grid, and control the charging or discharging of the energy storage system and the output of the offshore converter station according to the comparison result;

Step 2: According to the voltage change ΔU of the grid connection point with the main grid, carry out constant reactive power control based on Q-U droop for onshore converter stations, and carry out constant AC voltage control for energy storage inverters or constant reactive power based on Q-U droop. Power Control;

Step 3: Detect the main grid frequency deviation Δf, and use P-f droop control to control the power output of the multi-terminal offshore wind power flexible DC and energy storage coordinated grid-connected system to adjust the main grid frequency.
The control method according to claim 9, wherein the multi-terminal offshore wind power flexible DC and energy storage coordinated grid-connected system further comprises an energy storage control unit, a flexible DC control unit, and an energy storage and flexible DC coordinated control unit, the energy storage and The flexible direct coordination control unit calculates the power reference value of the energy storage subsystem according to the dispatch command power of the main grid, detects the frequency deviation of the grid, and calculates the power adjustment amount; the energy storage control unit performs constant active power control on the energy storage unit, and controls the inverter. Carry out constant DC voltage control and reactive power control; the flexible DC control unit performs constant AC voltage control and active power control for offshore converter stations, and constant DC voltage control and constant reactive power control based on Q-U droop for onshore converter stations;

The control process of the energy storage control unit, flexible DC control unit, energy storage and flexible DC coordination control unit includes:

1) Calculate the active power control reference value P ref =P ord -P wind , when -P lim2 ≤P ref ≤P lim1 , the fixed active power control reference value of the energy storage unit P batt =P ref , P lim1 represents the energy storage unit The critical value of charging power, P lim2 represents the critical value of the discharge power of the energy storage unit, according to the size of P ref , the discharge or charging of the energy storage unit is controlled, and the offshore converter station adopts constant frequency control ; When the critical value of the charging power of the energy unit, P batt =P lim1 , the offshore converter station adopts constant active power control, and the active power reference value of the i-th offshore converter station is P refi =(P ord -P batt )/P t * P i , P t is the total rated capacity of the system, and P i is the rated capacity of the i-th offshore converter station;

2) When detecting the voltage change ΔU at the grid connection point, -ΔU 1 ≤ ΔU ≤ ΔU 2 , the inverter of the energy storage control unit adopts constant AC voltage control; when ΔU <-ΔU 1 or ΔU > ΔU 2 , the energy storage control unit inverter The constant reactive power control based on QU droop is adopted in the converter and onshore converter station;

3) Detect the frequency deviation Δf of the main grid, adopt Pf droop control, use the simulated rotor operation equation (Hω 0 s+D)Δf=ΔP to calculate the active power change ΔP, and calculate the power output adjustment value of the energy storage unit P s =ΔP+ P batt , if -P lim2 ≤P s ≤P lim1 , at this time, the variation of the active power reference value of the energy storage unit ΔP batt =ΔP, and the new active power reference value is P s ; if P s <-P lim2 , it means that If the active power regulation exceeds the critical value of the discharge power of the energy storage unit, let P batt =-P lim2 ; if P s >P lim1 , it means that when the active power regulation exceeds the critical value of the charging power of the energy storage unit, let P batt =P lim1 , at this time , the offshore converter station adopts constant active power control, the active power reference value of the i-th offshore converter station is P refi =(P ord -P batt )/P t *P i , P t is the total rated capacity of the system, P i is the rated capacity of the i-th offshore converter station.