Coordination control method of wind turbine generator and LCC-MMC hybrid direct current system

Abstract

The invention discloses a coordination control method for a wind turbine generator and an LCC-MMC hybrid direct current system. The MMC land converter station adopts network construction type control based on DC voltage synchronization, reflects the frequency change of the AC power grid to the DC voltage change of the MMC land converter station side, and further reflects the DC voltage change of the LCC offshore converter station side through the DC power transmission line. And the wind turbine generator realizes active standby through the overspeed load shedding controller, and reflects the direct-current voltage change of the LCC offshore converter station side to the change of an active power instruction. The method does not depend on communication between the LCC offshore converter station and the MMC land converter station, active frequency support for the land alternating current power grid is achieved by coordinating the wind turbine generator and the LCC-MMC direct current power transmission system, and the method has high reliability and economical efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of wind power generation, and in particular to a coordinated control method for a wind turbine and an LCC-MMC hybrid DC system. This method does not rely on communication between an LCC offshore converter station and an MMC onshore converter station. Instead, it coordinates the wind turbine and the LCC-MMC DC transmission system to achieve active frequency support for an onshore AC power grid, thereby achieving good economic benefits and reliability. Background Art

[0002] With the introduction of relevant environmental protection goals and the gradual saturation of offshore wind power resources, the planning and construction of offshore wind farms in the future tend to extend to deep sea areas. High-voltage AC transmission has problems such as large losses and reduced synchronization stability in long-distance transmission scenarios. LCC-MMC hybrid DC transmission technology is mature, has large capacity, and is low-cost, and is expected to become one of the future offshore wind power transmission solutions. In the current offshore wind power transmission system via DC, the receiving-end converters all adopt grid-following control, which is reflected in the grid as a controlled current source and will not actively respond to grid frequency changes. The isolation effect of the DC system also makes it difficult for wind farms and sending-end converters to perceive grid frequency changes. The inertia-free offshore wind power transmission system via DC will reduce the overall inertia of the grid, posing a huge threat to the frequency stability of the grid. Grid-type control and active reserve of wind turbines can provide inertia and frequency support for the power system, and are important control solutions for offshore wind power DC transmission systems. At present, domestic and foreign scholars have carried out relevant research, such as the following published documents:

[0003] [1]Y.Zhu,Z.Wang and B.Li.Coordinated Grid-Forming Control Strategy for VSC-HVDC Integrating Offshore Wind Farms Based on Hybrid Energy[J].IEEEJournal of Emerging and Selected Topics in Industrial Electronics, 2024,5(4):1350-1361.

[0004] [2] K. Wang, Q. Song, B. Zhao and et al. Grid-Forming Control of Offshore Wind Farms Connected With Diode-Based HVdc Links Based on Remote Active Power Regulation[J]. IEEE Transactions on Sustainable Energy, 2024, 15(2): 1315-1327.

[0005] Document [1] proposes a coordinated control strategy of offshore wind farms through VSC-HVDC system. The strategy enables the VSC converter station to fully utilize the DC capacitor energy to provide inertia support for the onshore AC system through the addition of capacitors, while the wind farm releases the rotor kinetic energy according to the change of the DC side voltage to further provide inertia and frequency modulation capability. The configuration of the DC capacitor in the DC system increases the construction and operation cost, and the wind turbine may cause a secondary frequency drop when it exits the rotor kinetic energy control, so the control scheme has certain limitations. Document [2] proposes a coordinated control strategy of wind turbines and DC systems based on a communication system. The offshore wind turbine measures the DC voltage and frequency of the onshore converter station through the communication system, controls the active power output of the offshore wind turbine, and thus realizes the stability of the DC system voltage and the active frequency support of the onshore AC power grid. This strategy highly depends on the reliability of the communication system. If the communication system fails, the offshore wind turbine will not be able to provide inertia and frequency support for the onshore power grid through the DC system. SUMMARY

[0006] In view of the above deficiencies in the prior art, the purpose of the present application is to provide a coordinated control method of wind turbines and LCC-MMC hybrid DC systems. This method does not require additional devices, only changes the control mode of wind turbines and MMC onshore converter stations, and can realize active frequency support for the onshore AC power grid by coordinating wind turbines and LCC-MMC DC transmission systems. This method does not depend on the communication between the LCC offshore converter station and the MMC onshore converter station, has good economic benefits and reliability.

[0007] The technical scheme of the present application is implemented as follows:

[0008] A coordinated control method for a wind turbine and an LCC-MMC hybrid DC system is disclosed. The LCC is a grid-commutated converter, and the MMC is a modular multilevel converter. The LCC-MMC hybrid DC system includes an offshore converter station composed of the LCC and an onshore converter station composed of the MMC. The LCC offshore converter station is connected to an offshore wind farm, and the MMC onshore converter station is connected to an AC grid. The MMC onshore converter station adopts a grid-connected control based on DC voltage synchronization, reflecting frequency changes of the AC grid to changes in the DC voltage on the MMC onshore converter station side, and further reflecting the DC voltage changes on the LCC offshore converter station side through a DC transmission line. The wind turbine achieves active power standby through an overspeed load reduction controller, and reflects changes in the DC voltage on the LCC offshore converter station side to changes in active power instructions.

[0009] Furthermore, the AC voltage control method based on DC voltage synchronization and grid-type control is:

[0010] The MMC onshore converter station uses the virtual rotor position angle θ MMC The three-phase voltage signal U abc , three-phase current signal I abc Convert to the d and q axis corresponding components u d 、u q 、i d 、i q ; will u d 、u q 、i d 、i q Actual value and corresponding command value u dref 、u qref 、i dref 、i qref The deviation is obtained through the PI link and compensation term of the voltage and current dual inner loop to obtain the d and q axis voltage reference values, and the three-phase voltage reference value is obtained through coordinate inverse transformation. The three-phase voltage reference value is input into the modulation link to realize the control of the MMC onshore converter station; the control equation of the voltage and current dual inner loop is as follows:

[0011]

[0012] Among them, k pd_u 、k pq_u 、k pd_i 、k pq_i are the proportional coefficients of the voltage loop and current loop dq axis, k id_u 、k iq_u 、k id_i 、k iq_i are the integral coefficients of the voltage loop and the current loop dq axis, u d 、u q 、i d, i q is the actual value of dq-axis voltage and current, u dref , u qref , i dref , i qref is the command value of dq-axis voltage and current, ω MMC is the actual value of virtual angular frequency of grid-connected MMC onshore converter station; L eq is the equivalent bridge arm inductance of MMC, u cdref , u cqref is the reference value of modulation d, q-axis voltage of MMC, U MMCref is the effective value of AC voltage of MMC onshore converter station.

[0013] Further, the virtual rotor position angle θ MMC is calculated according to the following formula:

[0014]

[0015] wherein ω MMC_N is the rated value of virtual angular frequency of grid-connected MMC onshore converter station, U dcMMC , U dcMMC_N is the actual value and rated value of DC voltage of grid-connected MMC onshore converter station, and K is the coupling coefficient of DC voltage and frequency.

[0016] Further, the effective value of AC voltage of MMC onshore converter station U MMCref is calculated according to the following formula:

[0017]

[0018] wherein k p_Q and k i_Q are the proportional coefficient and integral coefficient for calculating the reference value of AC voltage of MMC onshore converter station, Q MMC_ref is the reference value of output reactive power of MMC onshore converter station, Q MMC is the output reactive power of MMC onshore converter station.

[0019] Further, the output active power command value P WTref of wind turbine under overspeed tripping is calculated according to the following formula:

[0020] P WTref = P WTref0 + ΔP WTref

[0021] wherein P WTref0 is the active power command value of wind turbine without overspeed tripping; ΔP WTref is the additional active power command of wind turbine; and wind turbine realizes the control of active power command value PWTref tracking, and achieve active frequency support for the onshore AC power grid through the LCC-MMC hybrid DC system.

[0022] Furthermore, the wind turbine additional active power command ΔP WTref It is calculated by the following formula:

[0023] ΔP WTref =K dc (U dcLCC_N -U dcLCC )

[0024] Among them, K dc Refers to the droop coefficient of DC voltage-active power, U dcLCC is the DC voltage of the LCC offshore converter station; U dcLLC_N is the DC voltage rating of the LCC offshore converter station.

[0025] Furthermore, the U dcLCC The DC side voltage of the MMC onshore converter station is obtained by the following equation, which can reflect the DC voltage change on the receiving MMC side to the sending LCC side without relying on the communication line:

[0026] U dcLCC =U dcMMC +I dc R L

[0027] Among them, I dc is the DC current of the hybrid DC system, R L is the DC line resistance; U dcMMC is the DC side voltage of the MMC onshore converter station.

[0028] Furthermore, the active power command value P of the wind turbine generator set WTref0 The calculation is based on the following formula according to different wind speed ranges under overspeed load reduction:

[0029]

[0030] Where ρ is the density of air, del is the load reduction coefficient, C p 、C pmax is the wind energy utilization coefficient and the maximum wind energy utilization coefficient, ω max is the maximum speed of the wind turbine, r is the radius of the wind turbine; v is the wind speed; β is the pitch angle of the wind turbine; S is the area swept by the wind turbine, P WTmax Output the maximum active power for the wind turbine.

[0031] Furthermore, the low wind speed interval is v in ≤v <v nom0 , the medium wind speed range is vnom0 ≤v <v nom , the high wind speed range is v nom ≤v <v out , where v in 、v nom 、v out are the cut-in wind speed, rated wind speed and cut-out wind speed of the wind turbine respectively, v nom0 The wind speed required for the wind turbine to reach the rated speed under overspeed load reduction.

[0032] Furthermore, the wind speed v required for the wind turbine to reach the rated speed under the overspeed load reduction is nom0 The calculation method is:

[0033]

[0034] Wherein, λ is the tip speed ratio of wind turbine; λ del is the tip speed ratio of the wind turbine under overspeed load reduction, λ i is an intermediate variable related to the tip speed ratio and pitch angle.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] This invention, through grid-based control of the MMC onshore converter station, reflects frequency changes in the onshore power grid as changes in the DC voltage at the MMC onshore converter station. This is then reflected via DC lines as changes in the DC voltage at the LCC offshore converter station, achieving line-free transmission of onshore power grid frequency information. The wind turbines, through an overspeed load shedding controller, release active reserve power when the DC voltage at the LCC offshore converter station changes, thereby providing active frequency support for the onshore power grid through the DC system. This achieves excellent economic efficiency and reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is the topology diagram of the wind turbine generator system through the LCC-MMC hybrid DC transmission system.

[0038] Figure 2 This is a schematic diagram of the control structure of the grid-type MMC converter station proposed in the present invention.

[0039] Figure 3 This is a block diagram of the active power instruction calculation of the wind turbine generator set proposed in the present invention.

[0040] Figure 4 The figure shows the simulation waveforms of the wind turbine generator system using the traditional method and the method proposed in the present invention when the onshore power grid frequency changes through the LCC-MMC hybrid DC transmission system. DETAILED DESCRIPTION

[0041] The application is used for coordinating wind turbine generators and LCC-MMC DC transmission system to realize active frequency support for onshore AC power grid. Figure 1 It is a topology diagram of wind turbine generator through LCC-MMC hybrid DC transmission system. Figure 2 It is a control structure diagram of network type MMC onshore converter station. Figure 3 It is an active power instruction calculation block diagram of wind turbine generator. The application realizes communication line-free transmission of onshore power grid frequency information through network type control of MMC onshore converter station, reflects frequency change of onshore power grid to DC voltage change at the side of MMC onshore converter station, and transmits DC voltage change at the side of LCC offshore converter station through DC line, so that the wind turbine generator releases active reserve when DC voltage changes at the side of LCC offshore converter station through overspeed load shedding controller, and then provides active frequency support for onshore power grid through DC system, which has good economic benefit and reliability.

[0042] The application discloses a kind of wind turbine generator and the coordination control method of LCC-MMC hybrid DC system, LCC is grid commutated converter, MMC is modular multilevel converter, LCC-MMC hybrid DC system includes offshore converter station of LCC composition and onshore converter station of MMC composition, LCC offshore converter station is connected with offshore wind farm, and MMC onshore converter station is connected with AC power grid;MMC onshore converter station uses network type control based on DC voltage synchronization, reflects the frequency change of AC power grid to the change of DC voltage at the side of MMC onshore converter station, and further reflects the change of DC voltage at the side of LCC offshore converter station through DC transmission line;Wind turbine generator realizes active reserve through overspeed load shedding controller, and reflects the change of DC voltage at the side of LCC offshore converter station to the change of active power instruction.This method does not depend on communication between LCC offshore converter station and MMC onshore converter station, realizes active frequency support for onshore AC power grid by coordinating wind turbine generator and LCC-MMC DC transmission system, and has high reliability and economy.

[0043] Further, the AC voltage control method of network type control based on DC voltage synchronization is as follows:

[0044] MMC onshore converter station converts three-phase voltage signal U MMC of AC side of MMC onshore converter station abc , three-phase current signal I abc to corresponding component u d , u q , i d , i q of d, q axis. d , u q , i d , i qActual value and corresponding command value u dref 、u qref 、i dref 、i qref The deviation is obtained through the PI link and compensation term of the voltage and current dual inner loop to obtain the d and q axis voltage reference values, and the three-phase voltage reference value is obtained through coordinate inverse transformation. The three-phase voltage reference value is input into the modulation link to realize the control of the MMC onshore converter station; the control equation of the voltage and current dual inner loop is as follows:

[0045]

[0046] Among them, k pd_u 、k pq_u 、k pd_i 、k pq_i are the proportional coefficients of the voltage loop and current loop dq axis, k id_u 、k iq_u 、k id_i 、k iq_i are the integral coefficients of the voltage loop and the current loop dq axis, u d 、u q 、i d 、i q is the actual value of dq axis voltage and current, u dref 、u qref 、i dref 、i qref is the dq axis voltage and current command value, ω MMC is the actual value of the virtual angular frequency of the network-type MMC onshore converter station; L eq is the equivalent bridge arm inductance of MMC, u cdref 、u cqref is the MMC modulation d and q axis voltage reference value, U MMCref is the effective value of the AC voltage at the MMC onshore converter station.

[0047] Furthermore, the virtual rotor position angle θ MMC Calculated according to the following formula:

[0048]

[0049] Among them, ω MMC_N is the virtual angular frequency rating of the grid-type MMC onshore converter station, U dcMMC 、U dcMMC_N are the actual and rated values ​​of the DC voltage of the grid-type MMC onshore converter station, and K is the coupling coefficient between the DC voltage and frequency.

[0050] Furthermore, the effective value of the AC voltage of the MMC onshore converter station U MMCref Calculated according to the following formula:

[0051]

[0052] wherein k p_Q and k i_Q are proportional and integral coefficients for calculating the reference value of the AC voltage of the MMC onshore converter station, Q MMC_ref is the reference value of the output reactive power of the MMC onshore converter station, Q MMC is the output reactive power of the MMC onshore converter station.

[0053] Further, the output active power instruction value P WTref of the wind turbine under overspeed tripping is calculated according to the following formula:

[0054] P WTref = P WTref0 + ΔP WTref

[0055] wherein P WTref0 is the active power instruction value of the wind turbine without overspeed tripping; ΔP WTref is the additional active power instruction of the wind turbine; the wind turbine tracks the active power instruction value P WTref through the controller and realizes the active frequency support for the onshore AC power grid through the LCC-MMC hybrid DC system, which has high reliability and economy.

[0056] Further, the wind turbine additional active power instruction ΔP WTref is calculated according to the following formula,

[0057] ΔP WTref = K dc (U dcLCC_N - U dcLCC )

[0058] wherein K dc is the droop coefficient of DC voltage-active power, U dcLCC is the DC voltage of the LCC offshore converter station; and U dcLLC_N is the rated DC voltage of the LCC offshore converter station.

[0059] Further, the U dcLCC is obtained from the DC side voltage of the MMC onshore converter station through the following equation to reflect the DC voltage change of the receiving end MMC side to the sending end LCC side without relying on the communication line:

[0060] U dcLCC = U dcMMC + IR dc L

[0061] wherein I​dc is the DC current of the hybrid DC system, R L is the DC line resistance; U dcMMC is the DC side voltage of the MMC onshore converter station.

[0062] Furthermore, the active power command value P of the wind turbine generator set WTref0 The calculation is based on the following formula according to different wind speed ranges under overspeed load reduction:

[0063]

[0064] Where ρ is the density of air, del is the load reduction coefficient, C p 、C pmax is the wind energy utilization coefficient and the maximum wind energy utilization coefficient, ω max is the maximum speed of the wind turbine, r is the radius of the wind turbine; v is the wind speed; β is the pitch angle of the wind turbine; S is the area swept by the wind turbine, P WTmax Output the maximum active power for the wind turbine.

[0065] Furthermore, the low wind speed interval is v in ≤v <v nom0 , the medium wind speed range is v nom0 ≤v <v nom , the high wind speed range is v nom ≤v <v out , where v in 、v nom 、v out are the cut-in wind speed, rated wind speed and cut-out wind speed of the wind turbine respectively, v nom0 The wind speed required for the wind turbine to reach the rated speed under overspeed load reduction.

[0066] Furthermore, the wind speed v required for the wind turbine to reach the rated speed under the overspeed load reduction is nom0 The calculation method is:

[0067]

[0068] Wherein, λ is the tip speed ratio of wind turbine; λ del is the tip speed ratio of the wind turbine under overspeed load reduction, λ i is an intermediate variable related to the tip speed ratio and pitch angle.

[0069] Effect description of the present invention:

[0070] Figure 4 The time domain simulation results of wind turbines passing through the LCC-MMC hybrid DC system are compared under the control strategy proposed in this invention and the traditional strategy when the onshore grid frequency changes. Figure 4(a) can be seen, the receiving end DC voltage in the traditional strategy is dropped about 0.019 p.u. and then rises to 1 p.u. and remains stable, the receiving end DC voltage in the strategy of the application is stable to about 0.991 p.u. during the disturbance. It can be seen that the DC voltage of the receiving end in the traditional strategy is about 0.019 p.u. higher than that in the strategy of the application. Figure 4 (b) (c) at this time, located in the middle wind speed interval, the disturbance before the pitch angle is about 2°. During the disturbance, the strategy of the application releases the rotor kinetic energy and active reserve of the wind turbine by reducing the pitch angle. As shown in (d), the output power of the wind turbine in the traditional strategy remains unchanged during the disturbance, and the active power after the strategy of the application rises from about 0.68 p.u. to about 0.75 p.u. As shown in (e), the grid frequency after the strategy of the application drops from 50 Hz to about 49.93 Hz during the disturbance, higher than 49.91 Hz in the traditional strategy. Figure 4 Figure 4

[0071] It can be seen that the coordination control method proposed in the application can transmit the information of the onshore grid frequency to the LCC offshore converter station through the DC voltage, so that the wind turbine can release the active reserve when the DC system voltage drops, thereby supporting the active frequency of the onshore grid, and the effectiveness of the proposed scheme is verified. The scheme does not depend on the communication between the DC converter stations, has good economic benefit and reliability.

[0072] Finally, it should be pointed out that the specific examples mentioned above are only used to explain the application and do not constitute a limitation on the embodiments of the application. Although the preferred examples are selected and described, other forms of adjustment and improvement can be made by those skilled in the art based on the above explanation. It is impossible to list all possible embodiments here. Any direct or indirect adjustment and improvement within the scope of the technical scheme of the application can be considered as the protection scope of the application.​​

Claims

1. A coordinated control method for a wind turbine generator set and an LCC-MMC hybrid DC system, characterized by: LCC is a grid-commutated converter, and MMC is a modular multilevel converter. The LCC-MMC hybrid DC system includes an offshore converter station composed of LCC and an onshore converter station composed of MMC. The LCC offshore converter station is connected to the offshore wind farm, and the MMC onshore converter station is connected to the AC grid. The MMC onshore converter station adopts a grid-connected control based on DC voltage synchronization, reflecting the frequency changes of the AC grid to the changes in the DC voltage on the MMC onshore converter station side, and further reflecting the DC voltage changes on the LCC offshore converter station side through the DC transmission line. The wind turbines achieve active power standby through the overspeed load reduction controller, and reflect the DC voltage changes on the LCC offshore converter station side to the changes in the active power command.

2. The coordinated control method of a wind turbine and an LCC-MMC hybrid DC system according to claim 1, characterized in that: The AC voltage control method based on DC voltage synchronization and grid-type control is as follows: The MMC onshore converter station uses the virtual rotor position angle θ MMC The three-phase voltage signal U abc , three-phase current signal I abc Convert to the d and q axis corresponding components u d 、u q 、i d 、i q ; will u d 、u q 、i d 、i q Actual value and corresponding command value u dref 、u qref 、i dref 、i qref The deviation is obtained through the PI link and compensation term of the voltage and current dual inner loop to obtain the d and q axis voltage reference values, and the three-phase voltage reference value is obtained through coordinate inverse transformation. The three-phase voltage reference value is input into the modulation link to realize the control of the MMC onshore converter station; the control equation of the voltage and current dual inner loop is as follows: Among them, k pd_u 、k pq_u 、k pd_i 、k pq_i are the proportional coefficients of the voltage loop and current loop dq axis, k id_u 、k iq_u 、k id_i 、k iq_i are the integral coefficients of the voltage loop and the current loop dq axis, u d 、u q 、i d 、i q is the actual value of dq axis voltage and current, u dref 、u qref 、i dref 、i qref is the dq axis voltage and current command value, ω MMC is the actual value of the virtual angular frequency of the network-type MMC onshore converter station; L eq is the equivalent bridge arm inductance of MMC, u cdref 、u cqref is the MMC modulation d and q axis voltage reference value, U MMCref is the effective value of the AC voltage at the MMC onshore converter station.

3. The coordinated control method of a wind turbine and an LCC-MMC hybrid DC system according to claim 2, characterized in that: The virtual rotor position angle θ MMC Calculated according to the following formula: Among them, ω MMC_N is the virtual angular frequency rating of the grid-type MMC onshore converter station, U dcMMC 、U dcMMC_N are the actual and rated values ​​of the DC voltage of the grid-type MMC onshore converter station, and K is the coupling coefficient between the DC voltage and frequency.

4. The coordinated control method of a wind turbine generator set and an LCC-MMC hybrid DC system according to claim 2, characterized in that: The effective value of the AC voltage U of the MMC onshore converter station MMCref Calculated according to the following formula: Among them, k p_Q and k i_Q are the proportional coefficient and integral coefficient used to calculate the AC voltage reference value of the MMC onshore converter station, Q MMC_ref is the reference value of reactive power output by the MMC onshore converter station, Q MMC Output reactive power to the MMC onshore converter station.

5. The coordinated control method of a wind turbine and an LCC-MMC hybrid DC system according to claim 1, characterized in that: The wind turbine output active power command value P under overspeed load reduction WTref Calculated as follows: P WTref =P WTref0 +ΔP WTref Among them, P WTref0 is the active power command value of the wind turbine when overspeed load reduction is not performed; ΔP WTref Add active power instruction to the wind turbine; the wind turbine realizes the active power instruction value P through the controller WTref tracking, and achieve active frequency support for the onshore AC power grid through the LCC-MMC hybrid DC system.

6. The coordinated control method of a wind turbine generator set and an LCC-MMC hybrid DC system according to claim 5, characterized in that: Additional active power command of wind turbine ΔP WTref Calculated by the following formula, ΔP WTref =K dc (U dcLCC_N -U dcLCC ) Among them, K dc Refers to the droop coefficient of DC voltage-active power, U dcLCC is the DC voltage of the LCC offshore converter station; U dcLLC_N is the DC voltage rating of the LCC offshore converter station.

7. The coordinated control method of a wind turbine generator set and an LCC-MMC hybrid DC system according to claim 6, characterized in that: The U dcLCC The DC side voltage of the MMC onshore converter station is obtained by the following equation, which reflects the DC voltage change on the receiving MMC side to the sending LCC side without relying on the communication line: U dcLCC =U dcMMC +I dc R L Among them, I dc is the DC current of the hybrid DC system, R L is the DC line resistance; U dcMMC is the DC side voltage of the MMC onshore converter station.

8. The coordinated control method of a wind turbine generator set and an LCC-MMC hybrid DC system according to claim 5, characterized in that: The active power command value P of the wind turbine generator set WTref0 The calculation is based on the following formula according to different wind speed ranges under overspeed load reduction: Where ρ is the density of air, del is the load reduction coefficient, C p 、C pmax is the wind energy utilization coefficient and the maximum wind energy utilization coefficient, ω max is the maximum speed of the wind turbine, r is the radius of the wind turbine; v is the wind speed; β is the pitch angle of the wind turbine; S is the area swept by the wind turbine, P WTmax Output the maximum active power for the wind turbine.

9. The coordinated control method of a wind turbine generator set and an LCC-MMC hybrid DC system according to claim 8, characterized in that: The low wind speed interval is v in ≤v <v nom0 , the medium wind speed range is v nom0 ≤v <v nom , the high wind speed range is v nom ≤v <v out , where v in 、v nom 、v out are the cut-in wind speed, rated wind speed and cut-out wind speed of the wind turbine respectively, v nom0 The wind speed required for the wind turbine to reach the rated speed under overspeed load reduction.

10. The coordinated control method of a wind turbine generator set and an LCC-MMC hybrid DC system according to claim 9, characterized in that: The wind speed v required for the wind turbine to reach the rated speed under the overspeed load reduction nom0 The calculation method is: Wherein, λ is the tip speed ratio of wind turbine; λ del is the tip speed ratio of the wind turbine under overspeed load reduction, λ i is an intermediate variable related to the tip speed ratio and pitch angle.

Images