CN105429183A

CN105429183A - Permanent magnetic direct-drive type offshore wind power plant grid-connected system topology structure and control method thereof

Info

Publication number: CN105429183A
Application number: CN201610006597.XA
Authority: CN
Inventors: 孙黎霞; 陈宇; 王�琦
Original assignee: Hohai University HHU
Current assignee: Hohai University HHU
Priority date: 2016-01-06
Filing date: 2016-01-06
Publication date: 2016-03-23

Abstract

The invention discloses a permanent magnet direct drive type offshore wind farm grid-connected system topology and its control method, wherein the grid-connected system includes a DC bus collector type wind farm connected in sequence, a wind farm grid side converter station, a two-stage Step-up transformers, offshore rectifier stations, submarine DC cables, on-shore inverter stations, grid-connected side step-up transformers and land power grids; DC bus collector wind farms adopt DC bus collector topology, including several groups of wind turbines connected in sequence , permanent magnet synchronous generator and machine-side rectifier, and DC bus for power collection. Its control method includes double closed-loop control of speed outer loop and current inner loop for machine-side rectifiers, double closed-loop control of voltage outer loop and current inner loop for wind farm grid-side converter stations, and constant AC voltage and constant frequency for offshore rectifier stations. Control, the onshore inverter station adopts double closed-loop control with constant DC voltage and constant reactive power. It can realize multi-machine parallel operation, improve power conversion efficiency, and reduce grid-connected AC current harmonics.

Description

Topology structure and control method of grid-connected system of permanent magnet direct drive offshore wind farm

技术领域technical field

本发明涉及一种海上风电场并网系统，特别是涉及一种永磁直驱型海上风电场并网系统拓扑结构及其控制方法，属于风电并网领域。The invention relates to a grid-connected system of an offshore wind farm, in particular to a permanent magnet direct drive type offshore wind farm grid-connected system topology and a control method thereof, belonging to the field of wind power grid-connected.

背景技术Background technique

海上风能具有风速高、风力稳定及干扰少等诸多优点，因此，大力发展海上风电已成为风电发展的新趋势。目前，由于永磁直驱同步风力发电机具有无齿轮箱、可靠性好且效率高等诸多优势，在兆瓦级海上风力发电机组中的应用日益增多。Offshore wind energy has many advantages such as high wind speed, stable wind force and less interference. Therefore, vigorously developing offshore wind power has become a new trend in the development of wind power. At present, because permanent magnet direct drive synchronous wind turbines have many advantages such as no gearbox, good reliability and high efficiency, they are increasingly used in megawatt-scale offshore wind turbines.

对于由永磁直驱同步风力发电机构建的海上风电场，其并网拓扑不仅包括风电场内部集电拓扑结构，还包括并网输电方式。它不仅关系到整个风电场的稳定性、经济性以及使用效率，还关系到风电场接入电网的可靠性。For offshore wind farms built by permanent magnet direct-drive synchronous wind turbines, the grid-connected topology includes not only the internal power collection topology of the wind farm, but also the grid-connected transmission method. It is not only related to the stability, economy and efficiency of the entire wind farm, but also related to the reliability of the wind farm's connection to the grid.

目前，风电场内部一般采用交流母线集电拓扑结构，但其电能转换效率较低，由于每台风力发电机组都有独立的控制系统，整个风电场的控制系统相对更为复杂；海上风电场并网方式一般采用交流输电(HVAC)并网，其中交流输电并网方式电力传输系统结构简单，成本较低，技术成熟，但传输容量和传输距离受到限制。具体可见，如图1和图2所示的风电场并网系统。At present, the AC bus current collection topology is generally used in wind farms, but its power conversion efficiency is low. Since each wind turbine has an independent control system, the control system of the entire wind farm is relatively more complicated; offshore wind farms are not The grid method generally adopts alternating current transmission (HVAC) grid connection, and the AC transmission grid connection mode power transmission system has simple structure, low cost, and mature technology, but the transmission capacity and transmission distance are limited. Specifically, it can be seen that the wind farm grid-connected system shown in Fig. 1 and Fig. 2 .

图1所示为基于HVAC的交流母线集电型风电场并网拓扑结构，其风电场中每台永磁同步电机发出的电能经全功率换流器得到交流电，交流电能在风电场内的交流母线处汇集，再通过35kV:220kV海上升压变电站后经海底交流电缆输送到陆上电网。Figure 1 shows the grid-connected topological structure of the AC bus collector type wind farm based on HVAC. The electric energy generated by each permanent magnet synchronous motor in the wind farm obtains AC power through the full-power converter, and the AC power in the wind farm It is collected at the busbar, and then passed through the 35kV: 220kV offshore step-up substation and then transmitted to the onshore power grid through the submarine AC cable.

图2所示为基于HVAC的直流母线集电型风电场并网拓扑结构，其风电场中每台永磁同步电机发出的电能经整流变换成直流电，直流电能在直流母线处汇集后经换流站集中逆变成交流电，再经过35kV:220kV升压变电站后经海底交流电缆输送到陆上电网。Figure 2 shows the grid-connected topology of the HVAC-based DC bus collector type wind farm. The electric energy generated by each permanent magnet synchronous motor in the wind farm is rectified and converted into DC power, and the DC power is collected at the DC bus and then commutated. The station centrally inverts it into alternating current, and then passes through the 35kV:220kV step-up substation and then transmits it to the land power grid through the submarine AC cable.

所以，风电场的内部集电拓扑结构以及其并网方式将直接关系到风电场远距离输电的稳定性和可靠性。Therefore, the internal power collection topology of the wind farm and its grid connection method will directly affect the stability and reliability of long-distance power transmission of the wind farm.

发明内容Contents of the invention

本发明的主要目的在于，克服现有技术中的不足，提供一种永磁直驱型海上风电场并网系统拓扑结构及其控制方法，不仅可以实现多机并联运行，而且能够提高风电场的电能转换效率，降低并网交流电流谐波量，提高风电场运行的高效性和稳定性，具有产业上的利用价值。The main purpose of the present invention is to overcome the deficiencies in the prior art and provide a permanent magnet direct drive type offshore wind farm grid-connected system topology and its control method, which can not only realize the parallel operation of multiple machines, but also improve the efficiency of the wind farm. Electric energy conversion efficiency, reducing the harmonic content of grid-connected AC current, improving the efficiency and stability of wind farm operation, has industrial application value.

为了达到上述目的，本发明所采用的技术方案是：In order to achieve the above object, the technical scheme adopted in the present invention is:

一种永磁直驱型海上风电场并网系统拓扑结构，包括依次相连的直流母线集电式风电场、风场网侧换流站、两级升压变压器、海上整流站、海底直流电缆、岸上逆变站、并网侧升压变压器和陆地电网。A permanent-magnet direct-drive offshore wind farm grid-connected system topology, including successively connected DC bus collector wind farms, wind farm grid-side converter stations, two-stage step-up transformers, offshore rectifier stations, submarine DC cables, Onshore inverter station, grid-connected side step-up transformer and land power grid.

其中，所述直流母线集电式风电场采用直流母线集电拓扑结构，包括若干组依次相连的风力机、永磁同步发电机和机侧整流器，以及集电用的直流母线；所述风力机将捕获的风能通过永磁同步发电机转换成交流电，交流电经机侧整流器整流变换成直流电在直流母线处汇集后、经风场网侧换流站集中逆变成风场网侧交流电，风场网侧交流电经两级升压变压器升压后输入海上整流站、再经海上整流站变换成升压直流电通过海底直流电缆送至岸上逆变站逆变成并网侧交流电，并网侧交流电最后经并网侧升压变压器升压后并入陆地电网。Wherein, the DC bus collector type wind farm adopts a DC bus collector topology, including several groups of connected wind turbines, permanent magnet synchronous generators, machine-side rectifiers, and DC buses for power collection; the wind turbine The captured wind energy is converted into AC power through the permanent magnet synchronous generator, and the AC power is rectified by the machine-side rectifier and transformed into DC power, which is collected at the DC bus and then centrally inverted by the converter station on the wind farm grid side to become AC power on the wind farm grid side. The side AC power is boosted by a two-stage step-up transformer and then input to the offshore rectification station, then converted into boosted DC power by the offshore rectification station and sent to the onshore inverter station through the submarine DC cable for inversion into grid-connected AC power, and the grid-connected AC power is finally passed through The step-up transformer on the grid-connected side is boosted and connected to the land grid.

本发明的并网系统拓扑结构进一步设置为：所述两级升压变压器包括依次相连的3KV:35KV升压变压器和35KV:115KV升压变压器，所述3KV:35KV升压变压器的输入端与风场网侧换流站的输出端相连，所述35KV:115KV升压变压器的输出端与海上整流站的输入端相连。The topology structure of the grid-connected system of the present invention is further set as follows: the two-stage step-up transformer includes a 3KV:35KV step-up transformer and a 35KV:115KV step-up transformer connected in sequence, and the input end of the 3KV:35KV step-up transformer is connected to the wind The output end of the converter station on the field network side is connected, and the output end of the 35KV:115KV step-up transformer is connected with the input end of the offshore rectification station.

本发明的并网系统拓扑结构进一步设置为：所述海底直流电缆为±100KV直流电缆。The topology structure of the grid-connected system of the present invention is further set as follows: the submarine DC cable is a ±100KV DC cable.

本发明的并网系统拓扑结构进一步设置为：所述海底直流电缆为100km。The topological structure of the grid-connected system of the present invention is further set as follows: the length of the submarine DC cable is 100 km.

本发明的并网系统拓扑结构进一步设置为：所述海上整流站和岸上逆变站为拓扑结构相同的电压源型换流站；所述电压源型换流站为基于全控型开关器件的换流器，具体拓扑结构为三相两电平型换流器、三相三电平型换流器、钳位型多电平电压源换流器、级联型多电平换流器、模块化多电平电压源换流器或多脉波电压源换流器。The topology structure of the grid-connected system of the present invention is further set as follows: the offshore rectification station and the onshore inverter station are voltage source converter stations with the same topology; the voltage source converter station is based on full-control switching devices Converter, the specific topology is three-phase two-level converter, three-phase three-level converter, clamped multi-level voltage source converter, cascaded multi-level converter, Modular multi-level voltage source converter or multi-pulse voltage source converter.

本发明的并网系统拓扑结构进一步设置为：所述三相两电平型换流器包括并联于直流侧的滤波电容，与滤波电容并联的电压型三相全桥式逆变电路，以及串联于交流侧、并与电压型三相全桥式逆变电路输出的三相中点分别相连的换流电抗器和滤波器；The grid-connected system topology of the present invention is further set as follows: the three-phase two-level converter includes a filter capacitor connected in parallel to the DC side, a voltage-type three-phase full-bridge inverter circuit connected in parallel with the filter capacitor, and a series A commutation reactor and a filter connected to the AC side and respectively connected to the midpoints of the three phases output by the voltage-type three-phase full-bridge inverter circuit;

其中，所述滤波电容包括串联的第一直流电容器和第二直流电容器，所述第一直流电容器和第二直流电容器的连接节点接地；所述换流电抗器包括依次串联的换流电阻和换流电感，所述换流电阻和换流电感串联后的两端分别与两级升压变压器输出端和电压型三相全桥式逆变电路输出中点相连；所述滤波器为高通滤波器，包括依次串联的滤波电阻、滤波电感和滤波电容器；所述滤波电阻的一端相连于两级升压变压器输出端和换流电阻之间，所述滤波电容器的一端接地。Wherein, the filter capacitor includes a first DC capacitor and a second DC capacitor connected in series, and the connection node of the first DC capacitor and the second DC capacitor is grounded; the commutation reactor includes a commutation resistor and a commutation The two ends of the commutation resistor and the commutation inductance connected in series are respectively connected to the output terminal of the two-stage step-up transformer and the output midpoint of the voltage-type three-phase full-bridge inverter circuit; the filter is a high-pass filter , including a filter resistor, a filter inductor and a filter capacitor connected in series in sequence; one end of the filter resistor is connected between the output terminals of the two-stage step-up transformer and the commutation resistor, and one end of the filter capacitor is grounded.

本发明还提供一种永磁直驱型海上风电场并网系统拓扑结构的控制方法，所述海上整流站的整流控制基于风电场输出功率的间歇性和不可控性，采用定交流电压和定频率控制；具体包括以下步骤，The present invention also provides a method for controlling the topological structure of the permanent magnet direct-drive offshore wind farm grid-connected system. Frequency control; specifically include the following steps,

1)获取风电场输出交流电压有效值U_WF、风电场输出交流电压有效值的参考值U_WFref、风电场输出端的交流电压的基波频率f_WF、风电场输出端的交流电压的基波频率的参考值f_WFref；1) Obtain the effective value U _WF of the output AC voltage of the wind farm, the reference value U _WFref of the effective value of the output AC voltage of the wind farm, the fundamental frequency f _WF of the AC voltage at the output end of the wind farm, and the fundamental frequency of the AC voltage at the output end of the wind farm Reference value f _WFref ;

2)将风电场输出交流电压有效值U_WF和风电场输出交流电压有效值的参考值U_WFref的偏差经PI调节器和[0,1]限幅输出为交流电压幅值M，将风电场输出端的交流电压的基波频率f_WF和风电场输出端的交流电压的基波频率的参考值f_WFref的偏差经PI调节器和[-arcsinX^*,arcsinX^*]限幅输出为交流电压相位δ，通过公式计算换流电抗X的标幺值X^*，其中，S_N为换流站额定容量，U_N为换流站额定电压。2) The deviation between the effective value U _WF of the output AC voltage of the wind farm and the reference value U _WFref of the effective value of the output AC voltage of the wind farm is output as the AC voltage amplitude M through the PI regulator and [0,1] limit, and the wind farm The deviation between the fundamental frequency f _WF of the AC voltage at the output terminal and the reference value f _WFref of the fundamental frequency of the AC voltage at the output terminal of the wind farm is limited by the PI regulator and [-arcsinX ^* , arcsinX ^* ] and output as the AC voltage phase δ, by formula Calculate the per-unit value X ^* of the commutation reactance X, where S _N is the rated capacity of the converter station, and U _N is the rated voltage of the converter station.

本发明的控制方法进一步设置为：所述岸上逆变站的逆变控制采用定直流电压和定无功功率的双闭环控制，维持稳定直流侧电压，均衡系统有功功率；具体包括以下步骤，The control method of the present invention is further set as: the inverter control of the onshore inverter station adopts double closed-loop control of constant DC voltage and constant reactive power, maintains a stable DC side voltage, and balances the active power of the system; specifically includes the following steps,

1)获取VSC-HDC系统逆变侧直流电压V_dc4、VSC-HVDC系统逆变侧直流电压参考值V_dcref4、并入电网的无功功率Q_s4、并网无功功功率参考值Q_sref4；1) Obtain the VSC-HDC system inverter side DC voltage V _dc4 , the VSC-HVDC system inverter side DC voltage reference value V _dcref4 , the reactive power Q _s4 connected to the grid, and the grid-connected reactive power reference value Q _sref4 ;

2)将VSC-HDC系统逆变侧直流电压V_dc4和VSC-HVDC系统逆变侧直流电压参考值V_dcref4的偏差经PI调节器输出d轴参考电流i_sdref4，将并入电网的无功功率Q_s4和并网无功功功率参考值Q_sref4的偏差经PI调节器输出q轴参考电流i_sqref4；2) The deviation _{between the DC voltage V dc4 on the inverter side of the VSC-HDC system and the DC voltage reference value V dcref4} _on the inverter side of the VSC-HVDC system is output through the PI regulator to output the d-axis reference current i _sdref4 , and the reactive power incorporated into the grid The deviation between Q _s4 and grid-connected reactive power reference value Q _sref4 outputs the q-axis reference current i _sqref4 through the PI regulator;

3)将VSC-HDC系统逆变侧交流电流d轴分量i_sd4和VSC-HVDC系统逆变侧交流电流d轴参考电流i_sdref4的偏差经PI调节器和q轴耦合项、d轴交流电压组合输出换流器d轴调制电压，将VSC-HDC系统逆变侧交流电流q轴分量i_sq4和VSC-HVDC系统逆变侧交流电流q轴参考电流i_sqref4的偏差经PI调节器和d轴耦合项、q轴交流电压组合输出换流器q轴调制电压。3) Combine the d-axis component i _sd4 of the AC current on the inverter side of the VSC-HDC system and the d-axis reference current i _sdref4 of the AC current on the inverter side of the VSC-HVDC system through the PI regulator, the q-axis coupling item, and the d-axis AC voltage The d-axis modulation voltage of the output converter is used to couple the deviation between the q-axis component i _sq4 of the AC current on the inverter side of the VSC-HDC system and the q-axis reference current i _sqref4 of the AC current on the inverter side of the VSC-HVDC system through the PI regulator and the d-axis coupling Item, q-axis AC voltage combination output converter q-axis modulation voltage.

本发明的控制方法进一步设置为：所述直流母线集电式风电场的机侧整流器的整流控制策略基于转子磁场定向的零d轴电流控制，采用转速外环和电流内环的双闭环控制，其中转速外环参考值通过最大功率跟踪算法给出，捕获最大风能；具体包括以下步骤，The control method of the present invention is further set as follows: the rectification control strategy of the machine-side rectifier of the DC bus collector type wind farm is based on the rotor field-oriented zero d-axis current control, and adopts the double closed-loop control of the speed outer loop and the current inner loop, The reference value of the outer ring of the speed is given by the maximum power tracking algorithm to capture the maximum wind energy; specifically includes the following steps,

1)以转子永磁体的中心线为d轴，沿转子旋转方向超前d轴90°电角度方向为q轴，在dq0旋转坐标系下，设定永磁同步发电机定子电压方程为式(1)，1) Take the center line of the permanent magnet of the rotor as the d-axis, and the 90° electric angle direction along the rotor rotation direction ahead of the d-axis is the q-axis. In the dq0 rotating coordinate system, set the stator voltage equation of the permanent magnet synchronous generator as formula (1 ),

$\{\begin{matrix} {u u}_{s the s d d 11} = = \frac{{dψ dψ}_{s the s d d}}{d d t t} + + {R R}_{s the s 11} {i i}_{s the s d d 11} - - {ω ω}_{e e} {L L}_{s the s q q 11} {i i}_{s the s q q 11} \\ {u u}_{s the s q q 11} = = \frac{{dψ dψ}_{s the s q q}}{d d t t} + + {R R}_{s the s 11} {i i}_{s the s q q 11} + + {ω ω}_{e e} {L L}_{s the s d d 11} {i i}_{s the s d d 11} + + {ω ω}_{e e} {ψ ψ}_{f f} \end{matrix} - - - - - - ((11))$

式(1)中，u_sd1、u_sq1为发电机定子电压的d、q轴分量，ψ_sd、ψ_sq为定子磁链的d、q轴分量，i_sd1、i_sq1为定子电流的d、q轴分量；ω_e为转子电角频率，ω_e＝pω_g，其中，p为电机极对数，ω_g＝ω_m为发电机的转速；L_sd1、L_sq1为定子d、q轴同步电感；In formula (1), u _sd1 and u _sq1 are the d and q axis components of the stator voltage of the generator, ψ _sd and ψ _sq are the d and q axis components of the stator flux linkage, and i _sd1 and i _sq1 are the d, q axis components of the stator current q-axis component; ω _e is the electrical angular frequency of the rotor, ω _e = pω _g , where p is the number of motor pole pairs, ω _g = ω _m is the rotational speed of the generator; L _sd1 and L _sq1 are the stator d and q-axis synchronous inductance;

2)对式(1)中定子电流的d、q轴分量i_sd1、i_sq1进行解耦控制，根据式(2)引入前馈补偿项u_d'、u_q'，来补偿等效电抗器上的电压降；2) Perform decoupling control on the d and q axis components i _sd1 and i _sq1 of the stator current in formula (1), and introduce feedforward compensation items u _d ' and u _q ' according to formula (2) to compensate the equivalent reactor voltage drop on

$\{\begin{matrix} {u u}_{d d}^{' '} = = \frac{{dψ dψ}_{s the s d d 11}}{d d t t} + + {R R}_{s the s 11} {i i}_{s the s d d 11} \\ {u u}_{q q}^{' '} = = \frac{{dψ dψ}_{s the s q q 11}}{d d t t} + + {R R}_{s the s 11} {i i}_{s the s q q 11} \end{matrix} - - - - - - ((22))$

通过比例积分环节将式(2)变换成式(2-1)，Transform formula (2) into formula (2-1) through proportional integral link,

$\{\begin{matrix} {u u}_{d d}^{' '} = = {K K}_{p p 11} (({i i}_{s the s d d r r e e f f 11} - - {i i}_{s the s d d 11})) + + {K K}_{i i 11} &Integral; &Integral; (({i i}_{s the s d d r r e e f f 11} - - {i i}_{s the s d d 11})) d d t t \\ {u u}_{q q}^{' '} = = {K K}_{p p 11} (({i i}_{s the s q q r r e e f f 11} - - {i i}_{s the s q q 11})) + + {K K}_{i i 11} &Integral; &Integral; (({i i}_{s the s q q r r e e f f 11} - - {i i}_{s the s q q 11})) d d t t \end{matrix} - - - - - - ((22 - - 11))$

式(2-1)中，K_p1、K_i1分别为电流内环比例和积分系数；i_sdref1、i_sqref1分别为电流i_sd1、i_sq1参考值，i_sdref1＝0，i_sqref1由转速参考值ω_mref和转速实际值ω_m的偏差经PI调节器调节得到；In formula (2-1), K _p1 and K _i1 are the current inner loop ratio and integral coefficient respectively; i _sdref1 and i _sqref1 are the reference values of current i _sd1 and i _sq1 respectively, i _sdref1 = 0, and i _sqref1 is determined by the speed reference value The deviation between ω _mref and the actual value of the speed ω _m is adjusted by the PI regulator;

3)根据式(2-1)将式(1)变换成式(3)，3) Transform formula (1) into formula (3) according to formula (2-1),

$\{\begin{matrix} {u u}_{s the s d d 11} = = {K K}_{p p 11} (({i i}_{s the s d d r r e e f f 11} - - {i i}_{s the s d d 11})) + + {K K}_{i i 11} &Integral; &Integral; (({i i}_{s the s d d r r e e f f 11} - - {i i}_{s the s d d 11})) d d t t - - {ω ω}_{e e} {L L}_{s the s q q 11} {i i}_{s the s q q 11} \\ {u u}_{s the s q q 11} = = {K K}_{p p 11} (({i i}_{s the s q q r r e e f f 11} - - {i i}_{s the s q q 11})) + + {K K}_{i i 11} &Integral; &Integral; (({i i}_{s the s q q r r e e f f 11} - - {i i}_{s the s q q 11})) d d t t + + {ω ω}_{e e} {L L}_{s the s d d 11} {i i}_{s the s d d 11} + + {ω ω}_{e e} {ψ ψ}_{f f} \end{matrix} - - - - - - ((33))$

通过调节永磁同步发电机的电流内环比例K_p1、积分系数K_i1和转速实际值ω_m，以及风力机的转速参考值ω_mref，根据式(3)计算永磁同步发电机定子电压的输出。By adjusting the current inner loop ratio K _p1 of the permanent magnet synchronous generator, the integral coefficient K _i1 and the actual speed value ω _m , and the wind turbine speed reference value ω _mref , the stator voltage of the permanent magnet synchronous generator is calculated according to formula (3) output.

本发明的控制方法进一步设置为：所述风场网侧换流站的集中逆变控制基于电网电压定向的矢量控制，采用电压外环和电流内环的双闭环控制，将机侧变流器整流出来的直流电逆变成与电网电压、幅值一样的交流电，同时保持直流侧电压的恒定；具体包括以下步骤，The control method of the present invention is further set as follows: the centralized inverter control of the grid-side converter station of the wind farm is based on the grid voltage-oriented vector control, and the double closed-loop control of the voltage outer loop and the current inner loop is adopted, and the generator-side converter The rectified direct current is inverted into alternating current with the same voltage and amplitude as the grid, while maintaining a constant voltage on the direct current side; specifically, the following steps are included,

1)在d-q旋转坐标系下，确定风场网侧换流站的控制模型为式(4)，1) In the d-q rotating coordinate system, determine the control model of the wind farm grid-side converter station as formula (4),

$\{\begin{matrix} {u u}_{c c d d 22} = = {u u}_{s the s d d 22} + + L L \frac{{di di}_{s the s d d 22}}{d d t t} + + {R R}_{22} {i i}_{s the s d d 22} - - {ω ω}_{s the s} {L L}_{22} {i i}_{s the s q q 22} \\ {u u}_{c c q q 22} = = {u u}_{s the s q q 22} + + L L \frac{{di di}_{s the s q q 22}}{d d t t} + + {R R}_{22} {i i}_{s the s q q 22} + + {ω ω}_{s the s} {L L}_{22} {i i}_{s the s d d 22} \end{matrix} - - - - - - ((44))$

式中，u_cd2、u_cq2为交流侧电压d、q轴分量；R₂、L₂分别为联结变压器和换相电抗器的等效电阻、电抗；i_sd2、i_sq2为逆变器交流侧电流d、q轴分量；ω_s为风场电网电压角频率；u_sd2、u_sq2为风场电网电压d、q轴分量；In the formula, u _cd2 and u _cq2 are the d and q axis components of the AC side voltage; R ₂ and L ₂ are the equivalent resistance and reactance of the connecting transformer and commutation reactor respectively; i _sd2 and i _sq2 are the AC side of the inverter Current d, q-axis components; ω _s is the angular frequency of the wind farm grid voltage; u _sd2 and u _sq2 are the d, q-axis components of the wind farm grid voltage;

并将风场网侧换流站的有功功率和无功功率表示为式(5)，And the active power and reactive power of the wind farm grid side converter station are expressed as formula (5),

$\{\begin{matrix} {P P}_{s the s} = = \frac{33}{22} (({u u}_{s the s d d 22} {i i}_{d d 22} + + {u u}_{s the s q q 22} {i i}_{q q 22})) \\ {Q Q}_{s the s} = = - - \frac{33}{22} (({u u}_{s the s d d 22} {i i}_{q q 22} - - {u u}_{s the s q q 22} {i i}_{d d 22})) \end{matrix} - - - - - - ((55))$

2)采用基于电网电压定向的空间矢量控制方法，在三相电网电压平衡的条件下，取电网电压空间矢量的方向为d轴方向，q轴超前d轴90°电角度，则u_sd2＝U_s，u_sq2＝0，其中的U_s为电网电压空间矢量的模值；2) Using the space vector control method based on grid voltage orientation, under the condition of three-phase grid voltage balance, the grid voltage space vector The direction of the d-axis is the direction of the d-axis, and the q-axis leads the d-axis by an electrical angle of 90°, then u _sd2 = U _s , u _sq2 = 0, where U _s is the modulus of the grid voltage space vector;

将式(5)改写为式(6)，Rewrite formula (5) as formula (6),

$\{\begin{matrix} {P P}_{s the s} = = \frac{33}{22} {U u}_{s the s} {i i}_{d d 22} \\ {Q Q}_{s the s} = = - - \frac{33}{22} {U u}_{s the s} {i i}_{q q 22} \end{matrix} - - - - - - ((66))$

根据式(6)得知，可通过改变d、q轴的电流来控制风场网侧换流站输出的有功功率和无功功率；According to formula (6), it can be known that the active power and reactive power output by the grid-side converter station of the wind farm can be controlled by changing the current of the d and q axes;

3)在风场网侧换流站的内环控制器中引入前馈补偿项，并通过一比例积分环节得以实现，将风场网侧换流站的控制模型变换为式(7)，3) The feed-forward compensation item is introduced into the inner loop controller of the wind farm grid-side converter station, which is realized through a proportional integral link, and the control model of the wind farm grid-side converter station is transformed into formula (7),

$\{\begin{matrix} {u u}_{c c d d 22} = = {u u}_{s the s d d 22} + + {K K}_{p p 22} (({i i}_{s the s d d r r e e f f 22} - - {i i}_{s the s d d 22})) + + {K K}_{i i 22} &Integral; &Integral; (({i i}_{s the s d d r r e e f f 22} - - {i i}_{s the s d d 22})) d d t t - - {ω ω}_{s the s} {L L}_{22} {i i}_{s the s q q 22} \\ {u u}_{c c q q 22} = = {u u}_{s the s q q 22} + + {K K}_{p p 22} (({i i}_{s the s q q r r e e f f 22} - - {i i}_{s the s q q 22})) + + {K K}_{i i 22} &Integral; &Integral; (({i i}_{s the s q q r r e e f f 22} - - {i i}_{s the s q q 22})) d d t t + + {ω ω}_{s the s} {L L}_{22} {i i}_{s the s d d 22} \end{matrix} - - - - - - ((77))$

式(7)中，K_p2、K_i2分别为电流内环比例和积分系数；i_sdref2、i_sqref2分别为有功和无功电流参考值；其中，i_sdref2为网侧变流器直流电压设定值V_dcref2和实际值V_dc2的偏差经PI调节器调节后转化得到；i_sqref2为网侧变流器无功功率参考值Q_ref2和实际值Q₂的偏差经PI调节器调节后转化得到。In formula (7), K _p2 and K _i2 are the proportion and integral coefficient of the current inner loop respectively; i _sdref2 and i _sqref2 are the active and reactive current reference values respectively; among them, i _sdref2 is the grid-side converter DC voltage setting The deviation between the value V _dcref2 and the actual value V _dc2 is adjusted by the PI regulator and converted; i _sqref2 is the deviation between the grid-side converter reactive power reference value Q _ref2 and the actual value Q ₂ is obtained after adjustment by the PI regulator.

与现有技术相比，本发明具有的有益效果是：Compared with prior art, the beneficial effect that the present invention has is:

通过采用直流母线集电拓扑结构的直流母线集电式风电场，将生产的电能在直流母线处汇集后经风场网侧换流站进行集中逆变，以及通过海上整流站、海底直流电缆和岸上逆变站的设置，将交流电经海上整流站变换成直流电通过海底直流电缆送至岸上逆变站逆变回交流电，最后送入并网的陆地电网，整个并网系统拓扑结构不仅可以实现多机并联运行，而且能够提高风电场的电能转换效率，降低并网交流电流谐波量，提高风电场运行的高效性和稳定性。同时，本发明提供的控制方法，能够灵活实现有功功率和无功功率的解耦控制，有效隔离电网故障对风电的影响，具有较强的抗干扰能力。Through the DC bus collector type wind farm adopting the DC bus collector topology, the produced electric energy is collected at the DC bus and then centralized inverter is carried out through the wind farm grid side converter station, and through the offshore rectifier station, submarine DC cable and The setting of the on-shore inverter station converts AC power into DC power through the offshore rectifier station and sends it to the on-shore inverter station to invert back to AC power through the submarine DC cable, and finally sends it to the grid-connected land power grid. The topology of the entire grid-connected system can not only realize multiple The parallel operation of the wind farm can improve the power conversion efficiency of the wind farm, reduce the harmonic content of the grid-connected AC current, and improve the efficiency and stability of the wind farm operation. At the same time, the control method provided by the present invention can flexibly realize the decoupling control of active power and reactive power, effectively isolate the influence of grid failure on wind power, and has strong anti-interference ability.

上述内容仅是本发明技术方案的概述，为了更清楚的了解本发明的技术手段，下面结合附图对本发明作进一步的描述。The above content is only an overview of the technical solution of the present invention. In order to understand the technical means of the present invention more clearly, the present invention will be further described below in conjunction with the accompanying drawings.

附图说明Description of drawings

图1是现有技术的基于HVAC的交流母线集电型风电场并网拓扑结构；Fig. 1 is the grid-connected topology of the HVAC-based AC bus collector type wind farm in the prior art;

图2是现有技术的基于HVAC的直流母线集电型风电场并网拓扑结构；Fig. 2 is the grid-connected topological structure of the HVAC-based DC bus collector type wind farm in the prior art;

图3是本发明的基于VSC-HVDC的直流母线集电型风电场并网拓扑结构；Fig. 3 is the grid-connected topological structure of the DC bus collector type wind farm based on VSC-HVDC of the present invention;

图4是图3中永磁同步发电机机侧整流器的矢量控制框图；Fig. 4 is the vector control block diagram of the machine-side rectifier of the permanent magnet synchronous generator in Fig. 3;

图5是图3中风场网侧换流站的集中逆变矢量控制框图；Fig. 5 is a block diagram of the centralized inverter vector control of the grid-side converter station of the wind farm in Fig. 3;

图6是图3中海上整流站或岸上逆变站的三相二电平拓扑结构；Fig. 6 is the three-phase two-level topology structure of the offshore rectifier station or onshore inverter station in Fig. 3;

图7是图3中海上整流站的定交流电压控制框图；Fig. 7 is a constant AC voltage control block diagram of the offshore rectifier station in Fig. 3;

图8是图3中岸上逆变站的控制框图；Fig. 8 is a control block diagram of the onshore inverter station in Fig. 3;

图9是风电场风速变化情况；Fig. 9 is the variation of wind speed in the wind farm;

图10是风速变化时图1、图2和图3三种拓扑结构的风电场输出有功功率；Fig. 10 is the wind farm output active power of the three topological structures of Fig. 1, Fig. 2 and Fig. 3 when the wind speed changes;

图11是风速变化时图1、图2和图3三种拓扑结构的风电场输出无功功率；Figure 11 shows the output reactive power of wind farms with three topological structures in Figure 1, Figure 2 and Figure 3 when the wind speed changes;

图12是风速变化时图1、图2和图3三种拓扑结构的并网电网侧有功功率；Figure 12 is the active power of the grid-connected grid side of the three topologies shown in Figure 1, Figure 2 and Figure 3 when the wind speed changes;

图13是风速变化时图1、图2和图3三种拓扑结构的并网电网侧无功功率；Figure 13 is the reactive power of the grid-connected grid side of the three topological structures shown in Figure 1, Figure 2 and Figure 3 when the wind speed changes;

图14是风速变化时图1、图2和图3三种拓扑结构的风电场输出端交流电流；Fig. 14 is the AC current at the output end of the wind farm in Fig. 1, Fig. 2 and Fig. 3 three topological structures when the wind speed changes;

图15是风速变化时图1、图2和图3三种拓扑结构的并网电网侧交流电流；Fig. 15 is the AC current on the grid-connected grid side of the three topological structures in Fig. 1, Fig. 2 and Fig. 3 when the wind speed changes;

图16是风速变化时图3中风电场侧和并网电网侧的直流电压；Figure 16 is the DC voltage on the wind farm side and grid-connected grid side in Figure 3 when the wind speed changes;

图17是电网故障时图1、图2和图3三种拓扑结构的并网电网侧交流电压；Fig. 17 is the grid-connected grid-side AC voltage of the three topological structures shown in Fig. 1, Fig. 2 and Fig. 3 when the power grid is faulty;

图18是电网故障时图1、图2和图3三种拓扑结构的并网电网侧有功功率；Figure 18 is the grid-connected grid-side active power of the three topologies shown in Figure 1, Figure 2 and Figure 3 when the grid is faulty;

图19是电网故障时图1、图2和图3三种拓扑结构的并网电网侧无功功率；Fig. 19 is the reactive power of the grid-connected grid side of the three topological structures shown in Fig. 1, Fig. 2 and Fig. 3 when the power grid is faulty;

图20是电网故障时图1、图2和图3三种拓扑结构的风电场输出有功功率；Fig. 20 is the output active power of the wind farms of the three topological structures of Fig. 1, Fig. 2 and Fig. 3 when the power grid is faulty;

图21是电网故障时图1、图2和图3三种拓扑结构的风电场输出无功功率；Fig. 21 is the output reactive power of wind farms with three topological structures in Fig. 1, Fig. 2 and Fig. 3 when the power grid is faulty;

图22是电网故障时图1、图2和图3三种拓扑结构的风电场输出端交流电压有效值；Figure 22 is the effective value of the AC voltage at the output end of the wind farm in the three topological structures of Figure 1, Figure 2 and Figure 3 when the power grid is faulty;

图23是电网故障时图3中VSC-HVDC系统的直流电压。Figure 23 is the DC voltage of the VSC-HVDC system in Figure 3 when the grid is faulty.

具体实施方式detailed description

下面结合说明书附图，对本发明作进一步的说明。Below in conjunction with accompanying drawing of description, the present invention will be further described.

如图3所示，本发明提供一种永磁直驱型海上风电场并网系统拓扑结构，其是基于VSC-HVDC的直流母线集电型风电场并网拓扑结构，包括依次相连的直流母线集电式风电场1、风场网侧换流站2、两级升压变压器3、海上整流站4、海底直流电缆5、岸上逆变站6、并网侧升压变压器7和陆地电网8；所述直流母线集电式风电场1采用直流母线集电拓扑结构，包括若干组依次相连的风力机11、永磁同步发电机12和机侧整流器13，以及集电用的直流母线14。As shown in Figure 3, the present invention provides a permanent magnet direct drive type offshore wind farm grid-connected system topology, which is a VSC-HVDC-based DC bus collector type wind farm grid-connected topology, including successively connected DC buses Collective wind farm 1, wind farm grid-side converter station 2, two-stage step-up transformer 3, offshore rectifier station 4, submarine DC cable 5, shore inverter station 6, grid-connected side step-up transformer 7 and land power grid 8 The DC bus collector type wind farm 1 adopts a DC bus collector topology, including several groups of connected wind turbines 11, permanent magnet synchronous generators 12 and machine-side rectifiers 13, and a DC bus 14 for power collection.

本发明的海上整流站4、海底直流电缆5和岸上逆变站6构成VSC-HVDC系统10。直流母线集电式风电场1中的风力机11将捕获的风能通过永磁同步发电机12转换成交流电，交流电经机侧整流器13整流变换成直流电在直流母线14处汇集后、经风场网侧换流站2集中逆变成风场网侧交流电，风场网侧交流电经两级升压变压器3升压后输入海上整流站4、再经海上整流站4变换成升压直流电通过海底直流电缆5送至岸上逆变站6逆变成并网侧交流电，并网侧交流电最后经并网侧升压变压器7升压后并入陆地电网8。The offshore rectifying station 4 , the submarine DC cable 5 and the onshore inverter station 6 of the present invention constitute a VSC-HVDC system 10 . The wind turbine 11 in the DC bus collector type wind farm 1 converts the captured wind energy into AC power through the permanent magnet synchronous generator 12, and the AC power is rectified by the machine-side rectifier 13 and converted into DC power, which is collected at the DC bus 14 and passed through the wind farm network. The side converter station 2 centrally inverts the wind farm grid-side AC power, and the wind farm grid-side AC power is boosted by the two-stage step-up transformer 3 and then input to the offshore rectifier station 4, and then transformed into boosted DC by the offshore rectifier station 4 and passed through the submarine DC cable 5 is sent to the onshore inverter station 6 to be inverted into grid-connected AC power, and the grid-connected AC power is finally boosted by the grid-connected step-up transformer 7 and then merged into the land power grid 8 .

其中，两级升压变压器3包括依次相连的3KV:35KV升压变压器21和35KV:115KV升压变压器32；底直流电缆为100km的±100KV直流电缆，并网侧升压变压器7为115kV:220kV升压变压器。Among them, the two-stage step-up transformer 3 includes a 3KV:35KV step-up transformer 21 and a 35KV:115KV step-up transformer 32 connected in sequence; the bottom DC cable is a 100km ±100KV DC cable, and the grid-connected side step-up transformer 7 is 115kV:220kV Step-up transformer.

如图4所示，直流母线集电式风电场中机侧整流器控制的实质是将永磁同步发电机输出的交流电转换成直流电，并保证永磁同步发电机发电机输出功率因数较高的正弦化电流，它与整个风力发电机机组整流部分的运行状态相关。As shown in Figure 4, the essence of machine-side rectifier control in a DC bus collector wind farm is to convert the AC output of the permanent magnet synchronous generator into direct current, and to ensure the sinusoidal output of the permanent magnet synchronous generator with a high power factor. It is related to the operating state of the rectification part of the whole wind turbine unit.

以转子永磁体的中心线为d轴，沿转子旋转方向超前d轴90°电角度方向为q轴。在dq0旋转坐标系下，永磁同步发电机定子电压方程为：Take the center line of the permanent magnet of the rotor as the d-axis, and the 90° electric angle direction ahead of the d-axis along the rotor rotation direction as the q-axis. In the dq0 rotating coordinate system, the permanent magnet synchronous generator stator voltage equation is:

式中，u_sd1、u_sq1为发电机定子电压的d、q轴分量；ψ_sd、ψ_sq为定子磁链的d、q轴分量；i_sd1、i_sq1为定子电流的d、q轴分量；ω_e为转子电角频率，ω_e＝pω_g，其中，p为电机极对数，ω_g＝ω_m为发电机的转速；L_sd1、L_sq1为定子d、q轴同步电感。In the formula, u _sd1 and u _sq1 are the d and q axis components of the generator stator voltage; ψ _sd and ψ _sq are the d and q axis components of the stator flux linkage; i _sd1 and i _sq1 are the d and q axis components of the stator current ; ω _e is the electrical angular frequency of the rotor, ω _e = pω _g , where p is the number of motor pole pairs, ω _g = ω _m is the rotational speed of the generator; L _sd1 and L _sq1 are the synchronous inductances of the stator d and q axes.

由式(1)可知，定子d、q轴电流分量i_sd1、i_sq1除受控制电压u_sd1、u_sq1的影响外，还受耦合补偿项ω_eL_sq1i_sq1和ω_eL_sd1i_sd1的影响。因此，为了实现对电流i_sd1、i_sq1的解耦控制，引入前馈补偿项u_d'、u_q'，从而补偿等效电抗器上的电压降。It can be seen from formula (1) that the stator d and q axis current components i _sd1 and i _sq1 are not only affected by the control voltage u _sd1 and u _sq1 , but also affected by the coupling compensation items ω _e L _sq1 i _sq1 and ω _e L _sd1 i _sd1 Impact. Therefore, in order to realize the decoupling control of the current i _sd1 and i _sq1 , the feed-forward compensation items u _d ', u _q ' are introduced to compensate the voltage drop on the equivalent reactor.

由式(2)可以看出，u_d'、u_q'分别为与i_sd1、i_sq1具有一阶微分关系的电压分量，It can be seen from formula (2) that u _d ', u _q ' are voltage components that have a first-order differential relationship with i _sd1 and i _sq1 respectively,

其可以通过比例积分环节将式(2)变换成式(2-1)。It can transform formula (2) into formula (2-1) through proportional integral link.

式(2-1)中，K_p1、K_i1分别为电流内环比例和积分系数；i_sdref1、i_sqref1分别为电流i_sd1、i_sq1参考值，i_sdref1＝0，i_sqref1由转速参考值ω_mref和转速实际值ω_m的偏差经PI调节器调节得到。In formula (2-1), K _p1 and K _i1 are the current inner loop ratio and integral coefficient respectively; i _sdref1 and i _sqref1 are the reference values of current i _sd1 and i _sq1 respectively, i _sdref1 = 0, and i _sqref1 is determined by the speed reference value The deviation between ω _mref and the actual speed value ω _m is adjusted by the PI regulator.

则根据式(2-1)将式(1)变换成式(3)，Then transform formula (1) into formula (3) according to formula (2-1),

可通过调节永磁同步发电机的电流内环比例K_p1、积分系数K_i1和转速实际值ω_m，以及风力机的转速参考值ω_mref，根据式(3)计算永磁同步发电机定子电压的输出。The permanent magnet synchronous generator stator voltage can be calculated according to formula (3) by adjusting the current inner loop ratio K _p1 of the permanent magnet synchronous generator, the integral coefficient K _i1 and the actual speed value ω _m , and the wind turbine speed reference value ω _mref Output.

如图5所示，风场网侧换流站的集中逆变控制的主要作用是维持直流侧电压的恒定，并根据电网的需要进行无功功率的调节。As shown in Figure 5, the main function of the centralized inverter control of the grid-side converter station of the wind farm is to maintain a constant voltage on the DC side and adjust the reactive power according to the needs of the grid.

在d-q旋转坐标系下网侧变流器的数学模型为：The mathematical model of the grid-side converter in the d-q rotating coordinate system is:

式中，u_cd2、u_cq2为变流器交流侧电压d、q轴分量；R₂、L₂分别为联结变压器和换相电抗器的等效电阻、电抗；i_sd2、i_sq2为逆变器交流侧电流d、q轴分量；ω_s为电网电压角频率；u_sd2、u_sq2为电网电压d、q轴分量。In the formula, u _cd2 and u _cq2 are the d and q axis components of the AC side voltage of the converter; R ₂ and L ₂ are the equivalent resistance and reactance of the connection transformer and commutation reactor respectively; i _sd2 and i _sq2 are the inverter AC side current d, q axis components; ω _s is grid voltage angular frequency; u _sd2 , u _sq2 are grid voltage d, q axis components.

有功功率和无功功率可以表示为：Active power and reactive power can be expressed as:

采用基于电网电压定向的空间矢量控制方法时，在三相电网电压平衡的条件下，取电网电压空间矢量的方向为d轴方向，q轴超前d轴90°电角度，则u_sd2＝U_s(电网电压空间矢量的模值)，u_sq2＝0。When using the space vector control method based on grid voltage orientation, under the condition of three-phase grid voltage balance, the grid voltage space vector The direction of is the d-axis direction, and the q-axis leads the d-axis by 90° electrical angle, then u _sd2 =U _s (the modulus value of the grid voltage space vector), u _sq2 =0.

则(5)可改写为：Then (5) can be rewritten as:

由式(6)可以看出，交流侧的有功功率和d轴的电流成正比，交流侧无功功率只与q轴电流成正比。因此，可通过改变d、q轴的电流来控制换流站输出的有功功率和无功功率。It can be seen from formula (6) that the active power on the AC side is proportional to the d-axis current, and the reactive power on the AC side is only proportional to the q-axis current. Therefore, the active power and reactive power output by the converter station can be controlled by changing the current of the d and q axes.

与直流母线集电式风电场中机侧整流器的内环控制器设计类似，在风场网侧换流站内环控制器的设计过程中也引入了前馈补偿项，并通过一比例积分环节得以实现，其最终的系统控制方程为：Similar to the design of the inner-loop controller of the machine-side rectifier in the DC bus collector wind farm, the feed-forward compensation item is also introduced in the design process of the inner-loop controller of the wind farm grid-side converter station, and obtained through a proportional-integral link Realized, the final system governing equation is:

式中，K_p2、K_i2分别为电流内环比例和积分系数；i_sdref2、i_sqref2分别为有功和无功电流参考值。其中i_sdref2为网侧变流器直流电压设定值V_dcref2和实际值V_dc2的偏差经PI调节后转化得到；i_sqref2为网侧变流器无功功率参考值Q_ref2和实际值Q₂的偏差经PI调节器调节后转化得到。In the formula, K _p2 , K _i2 are current inner loop ratio and integral coefficient respectively; i _sdref2 , i _sqref2 are active and reactive current reference values respectively. Among them, i _sdref2 is the deviation between the grid-side converter DC voltage set value V _dcref2 and the actual value V _dc2 after PI adjustment; i _sqref2 is the grid-side converter reactive power reference value Q _ref2 and the actual value Q ₂ The deviation is obtained after being adjusted by the PI regulator.

由于VSC-HVDC系统位于两端的海上整流站WFVSC和岸上逆变站GSVSC结构相同，现以一端换流器VSC为例。换流器VSC一般有两电平、三电平和多电平等多种拓扑结构，本文主要研究典型的三相两电平VSC拓扑结构，如图6所示，以岸上逆变站为三相两电平型换流器进行详细说明。Since the structure of the offshore rectifier station WFVSC located at both ends of the VSC-HVDC system is the same as that of the onshore inverter station GSVSC, the one-end converter VSC is taken as an example. Converter VSC generally has two-level, three-level and multi-level topological structures. This paper mainly studies the typical three-phase two-level VSC topology, as shown in Figure 6. The onshore inverter station is three-phase two-level The level type converter will be described in detail.

所述三相两电平型换流器包括并联于直流侧的滤波电容61，与滤波电容61并联的电压型三相全桥式逆变电路62，以及串联于交流侧、并与电压型三相全桥式逆变电路62输出的三相中点分别相连的换流电抗器63和滤波器64(如图3所示)；其中，所述滤波电容61包括串联的第一直流电容器2C1和第二直流电容器2C2，所述第一直流电容器2C1和第二直流电容器2C2的连接节点接地；所述换流电抗器63包括依次串联的换流电阻R和换流电感L，所述换流电阻R和换流电感L串联后的两端分别与两级升压变压器3输出端和电压型三相全桥式逆变电路62输出中点相连；所述滤波器64为高通滤波器，包括依次串联的滤波电阻、滤波电感和滤波电容器(图中未示出)；所述滤波电阻的一端相连于两级升压变压器输出端和换流电阻之间，所述滤波电容器的一端接地。The three-phase two-level converter includes a filter capacitor 61 connected in parallel to the DC side, a voltage-type three-phase full-bridge inverter circuit 62 connected in parallel with the filter capacitor 61, and a voltage-type three-phase full-bridge inverter circuit 62 connected in series with the AC side and connected to the voltage-type three-phase converter. A commutation reactor 63 and a filter 64 (as shown in FIG. 3 ) are respectively connected to the three-phase midpoints output by the phase full-bridge inverter circuit 62; wherein, the filter capacitor 61 includes a series connected first DC capacitor 2C1 and The second DC capacitor 2C2, the connection node of the first DC capacitor 2C1 and the second DC capacitor 2C2 is grounded; the commutation reactor 63 includes a commutation resistance R and a commutation inductance L connected in series in sequence, and the commutation resistance The two ends of R and the commutation inductance L connected in series are respectively connected to the output terminal of the two-stage step-up transformer 3 and the output midpoint of the voltage-type three-phase full-bridge inverter circuit 62; the filter 64 is a high-pass filter, including A filter resistor, a filter inductor and a filter capacitor (not shown in the figure) are connected in series; one end of the filter resistor is connected between the output terminals of the two-stage step-up transformer and the commutation resistor, and one end of the filter capacitor is grounded.

对于三相两电平型换流器，根据基尔霍夫电压定律可建立交流侧的三相VSC电压回路方程为：For a three-phase two-level converter, according to Kirchhoff's voltage law, the three-phase VSC voltage loop equation on the AC side can be established as:

$\{\begin{matrix} {u u}_{c c A A} = = L L \frac{{di di}_{A A}}{d d t t} + + {Ri Ri}_{A A} + + {u u}_{s the s A A} \\ {u u}_{c c B B} = = L L \frac{{di di}_{B B}}{d d t t} + + {Ri Ri}_{B B} + + {u u}_{s the s B B} \\ {u u}_{c c C C} = = L L \frac{{di di}_{C C}}{d d t t} + + {Ri Ri}_{C C} + + {u u}_{s the s C C} \end{matrix} - - - - - - ((88))$

式中，u_sA、u_sB、u_sC是电网侧或风电场侧三相交流电压，u_cA、u_cB、u_cC为换流器输出的PWM电压，L为换流电抗器的电感，R为换流电抗器的电阻。In the formula, u _sA , u _sB , u _sC are the three-phase AC voltages on the grid side or wind farm side, u _cA , u _cB , u _cC are the PWM voltage output by the converter, L is the inductance of the commutation reactor, R is the resistance of the commutation reactor.

由基尔霍夫电流定律可得到直流侧的动态方程为：According to Kirchhoff's current law, the dynamic equation of the DC side can be obtained as:

${I I}_{d d l l} - - {I I}_{d d c c} = = C C \frac{{dV dV}_{d d c c}}{d d t t} - - - - - - ((99))$

式中，I_dl为直流电流，I_dc为注入到换流器的直流电流，C为直流侧电容，V_dc为VSC直流侧电压。In the formula, I _dl is the DC current, I _dc is the DC current injected into the converter, C is the DC side capacitance, and V _dc is the VSC DC side voltage.

对式(8)进行Park变换，得到VSC在dq0坐标系下的数学模型：Carry out Park transformation on formula (8) to obtain the mathematical model of VSC in the dq0 coordinate system:

$\{\begin{matrix} L L \frac{{di di}_{d d}}{d d t t} = = {u u}_{c c d d} - - {Ri Ri}_{d d} - - {u u}_{s the s d d} + + {ωLi ωLi}_{q q} \\ L L \frac{{di di}_{q q}}{d d t t} = = {u u}_{c c q q} - - {Ri Ri}_{q q} - - {u u}_{s the s q q} - - {ωLi ωLi}_{d d} \end{matrix} - - - - - - ((1010))$

根据瞬时无功功率理论，dq0坐标系下VSC与交流系统交换的有功功率P_s1和Q_s1可表示为：According to the instantaneous reactive power theory, the active power P _s1 and Q _s1 exchanged between the VSC and the AC system in the dq0 coordinate system can be expressed as:

$\{\begin{matrix} {P P}_{s the s 11} = = \frac{33}{22} (({u u}_{s the s d d} {i i}_{d d} + + {u u}_{s the s q q} {i i}_{q q})) \\ {Q Q}_{s the s 11} = = - - \frac{33}{22} (({u u}_{s the s d d} {i i}_{d d} + + {u u}_{s the s q q} {i i}_{d d})) \end{matrix} - - - - - - ((1111))$

稳态情况下，假设系统三相对称运行，因此，没有零序分量。当d轴与交流母线基波电压同相位时，u_sq＝0，则式(11)更改为式(12)：In the steady state, it is assumed that the three-phase system operates symmetrically, so there is no zero-sequence component. When the d-axis is in the same phase as the AC bus fundamental voltage, u _sq =0, then formula (11) is changed to formula (12):

$\{\begin{matrix} {P P}_{s the s 11} = = \frac{33}{22} {u u}_{s the s d d} {i i}_{d d} \\ {Q Q}_{s the s 11} = = - - \frac{33}{22} {u u}_{s the s d d} {i i}_{q q} \end{matrix} - - - - - - ((1212))$

对于VSC稳定运行时，假设交流系统足够强大，因此，u_sd为恒定值，由式(12)可知，交流侧的有功和d轴的电流成正比，交流侧无功功率只与q轴电流成正比。因此，交流电流可分解为两个独立的分量i_d和i_q。由式(12)可知，通过改变d、q轴的电流，可以改变换流站输出的有功功率和无功功率，该模型实现了有功功率和无功功率的解耦，根据三相VSC的dq0坐标系数学模型设计稳态运行条件下的VSC-HVDC控制器。For the stable operation of VSC, assuming that the AC system is strong enough, therefore, u _sd is a constant value. From formula (12), it can be seen that the active power on the AC side is proportional to the d-axis current, and the reactive power on the AC side is only proportional to the q-axis current Proportional. Therefore, the alternating current can be decomposed into two independent components i _d and i _q . From equation (12), it can be seen that by changing the d and q axis currents, the active power and reactive power output by the converter station can be changed. This model realizes the decoupling of active power and reactive power. According to the dq0 of the three-phase VSC The coordinate coefficient mathematical model is used to design the VSC-HVDC controller under steady-state operating conditions.

图7为VSC-HVDC系统中风电场侧换流站，即海上整流站的定交流电压控制框图。由于风电场具有间歇性和不可控性，风电场发出的有功功率也是随着风速不断变化的。因此，风电场采用VSC-HVDC系统并网时，海上整流站很难实现定有功功率的控制。为了尽可能使风电场发出的功率输送到VSC-HVDC系统并送入电网，本发明在海上整流站的风电场侧采用定交流电压控制和定频率控制。图7中，U_WF为风电场输出交流电压有效值，U_WFref为风电场输出交流电压有效值的参考值；f_WF为风电场输出端的交流电压的基波频率，f_WFref为风电场输出端的交流电压的基波频率的参考值；将风电场输出交流电压有效值U_WF和风电场输出交流电压有效值的参考值U_WFref的偏差经PI调节器和[0,1]限幅输出交流电压幅值M，将风电场输出端的交流电压的基波频率f_WF和风电场输出端的交流电压的基波频率的参考值f_WFref的偏差经PI调节器和[-arcsinX^*,arcsinX^*]限幅输出交流电压相位δ，通过为计算换流电抗X的标幺值X^*，其中，S_N为换流站额定容量，U_N为换流站额定电压。Fig. 7 is a block diagram of the constant AC voltage control of the wind farm side converter station in the VSC-HVDC system, that is, the offshore rectifier station. Due to the intermittent nature and uncontrollability of wind farms, the active power generated by wind farms also changes with the wind speed. Therefore, when the wind farm is connected to the grid using the VSC-HVDC system, it is difficult for the offshore rectifier station to achieve constant active power control. In order to transmit the power from the wind farm to the VSC-HVDC system as much as possible and send it to the power grid, the present invention adopts constant AC voltage control and constant frequency control on the wind farm side of the offshore rectifier station. In Fig. 7, U _WF is the effective value of the output AC voltage of the wind farm, U _WFref is the reference value of the effective value of the output AC voltage of the wind farm; f _WF is the fundamental frequency of the AC voltage at the output end of the wind farm, and f _WFref is the The reference value of the fundamental wave frequency of the AC voltage; the deviation between the effective value U _WF of the output AC voltage of the wind farm and the reference value U _WFref of the effective value of the output AC voltage of the wind farm is output through the PI regulator and [0,1] limiting the output AC voltage Amplitude M, the deviation between the fundamental frequency f _WF of the AC voltage at the output of the wind farm and the reference value f _WFref of the fundamental frequency of the AC voltage at the output of the wind farm is limited by the PI regulator and [-arcsinX ^* , arcsinX ^* ] Output AC voltage phase δ, through To calculate the per-unit value X ^* of the commutation reactance X, wherein, _SN is the rated capacity of the converter station, and U _N is the rated voltage of the converter station.

VSC-HVDC系统中网侧换流站，即岸上逆变器的控制主要是维持直流侧电压的稳定，以维持有功功率的平衡。其控制原理与风场网侧换流站的集中逆变控制基本一致，其控制框图如图8所示。图8中，V_dc4为VSC-HDC系统逆变侧直流电压，V_dcref4为VSC-HVDC系统逆变侧直流电压参考值，Q_s4为并入电网的无功功率，Q_sref4为并网无功功功率参考值。将VSC-HDC系统逆变侧直流电压V_dc4和VSC-HVDC系统逆变侧直流电压参考值V_dcref4的偏差经PI调节器输出d轴参考电流i_sdref4，将并入电网的无功功率Q_s4和并网无功功功率参考值Q_sref4的偏差经PI调节器输出q轴参考电流i_sqref4；将VSC-HDC系统逆变侧交流电流d轴分量i_sd4和VSC-HVDC系统逆变侧交流电流d轴参考电流i_sdref4的偏差经PI调节器和q轴耦合项ω_sL₄i_sq4、d轴交流电压u_sd4组合输出换流器d轴调制电压u_cd4，将VSC-HDC系统逆变侧交流电流q轴分量i_sq4和VSC-HVDC系统逆变侧交流电流q轴参考电流i_sqref4的偏差经PI调节器和d轴耦合项ω_sL₄i_sd4、q轴交流电压u_sq4组合输出换流器q轴调制电压u_cq4。In the VSC-HVDC system, the grid-side converter station, that is, the control of the on-shore inverter is mainly to maintain the stability of the DC side voltage to maintain the balance of active power. Its control principle is basically the same as the centralized inverter control of the wind farm grid-side converter station, and its control block diagram is shown in Figure 8. In Fig. 8, V _dc4 is the DC voltage on the inverter side of the VSC-HDC system, V _dcref4 is the reference value of the DC voltage on the inverter side of the VSC-HVDC system, Q _s4 is the reactive power connected to the grid, and Q _sref4 is the grid-connected reactive power Power reference value. The deviation _{between the DC voltage V dc4 on the inverter side of the VSC-HDC system and the DC voltage reference value V dcref4} _on the inverter side of the VSC-HVDC system is output through the PI regulator to output the d-axis reference current i _sdref4 , and the reactive power Q _s4 incorporated into the grid The deviation from the grid-connected reactive power reference value Q _sref4 is output through the PI regulator to output the q-axis reference current i _sqref4 _; The deviation of the d-axis reference current i _sdref4 is combined with the PI regulator and the q-axis coupling item ω _s L ₄ i _sq4 , and the d-axis AC voltage u _sd4 to output the d-axis modulation voltage u _cd4 of the converter, and the VSC-HDC system inverter side The deviation between the q-axis component i _sq4 of the AC current and the reference current i _sqref4 of the AC current q-axis on the inverter side of the VSC-HVDC system is output through the combination of the PI regulator and the d-axis coupling item ω _s L ₄ i _sd4 , and the q-axis AC voltage u _sq4 The inverter q-axis modulation voltage u _cq4 .

本发明利用PSCAD/EMTDC软件对所设计的永磁直驱型海上风电场并网系统拓扑结构进行了仿真分析，通过对风速变化和电网侧单相接地故障两种情况进行仿真，可验证本发明的有效性和优越性。The present invention uses PSCAD/EMTDC software to simulate and analyze the topology of the designed permanent magnet direct drive type offshore wind farm grid-connected system, and can verify the present invention by simulating the two situations of wind speed change and single-phase ground fault on the grid side effectiveness and superiority.

图9为风速变化曲线。当风电场的风速发生变化时，图10-11为风电场的有功功率和无功功率输出曲线，从图中可以看到，风电场输出的有功功率均随着风速的增大而增加，而无功功率不随风速的变化而变化。Figure 9 is the wind speed change curve. When the wind speed of the wind farm changes, Figure 10-11 shows the active power and reactive power output curves of the wind farm. It can be seen from the figure that the active power output by the wind farm increases with the increase of the wind speed, while Reactive power does not change with wind speed.

图12-13为风电场并网的有功功率和无功功率输出，可以看出，风电场输出的有功功率经过100km的交流传输线路或直流传输线路之后有一定幅度的降低；对于直流母线集电型风电场，经过VSC-HVDC传输系统送到电网的有功功率略少。这是因为虽然直流电缆损耗比交流电缆要小，但是VSC-HVDC系统中两端换流站存在开关损耗，但是随着输电距离的增加，直流电缆节省的损耗将逐渐明显；VSC-HVDC输电系统并网电网侧无功功率一直稳定在设定值0Mvar附近，而HVAC输电系统并网电网侧无功功率会随着输送的有功功率的变化而变化，这将不利于风电场并网的稳定运行。Figure 12-13 shows the active power and reactive power output of the wind farm connected to the grid. It can be seen that the active power output by the wind farm decreases to a certain extent after passing through the 100km AC transmission line or DC transmission line; Type wind farm, the active power sent to the grid through the VSC-HVDC transmission system is slightly less. This is because although the loss of the DC cable is smaller than that of the AC cable, there are switching losses in the converter stations at both ends of the VSC-HVDC system, but as the transmission distance increases, the loss saved by the DC cable will gradually become obvious; the VSC-HVDC transmission system The reactive power on the grid-connected grid side has been stable around the set value of 0Mvar, while the reactive power on the grid-connected grid side of the HVAC transmission system will change with the change of the transmitted active power, which will not be conducive to the stable operation of the wind farm grid-connected .

图14和15分别为风电场输出的交流电流和风电场并网的交流电流，从图中可以看到，风电场内部采用交流母线集电时，风电场输出的交流电流谐波量最多，并且风电场并网的交流电流谐波量也是最多。Figures 14 and 15 show the AC current output by the wind farm and the grid-connected AC current of the wind farm respectively. It can be seen from the figure that when the AC busbar is used to collect power inside the wind farm, the AC current output by the wind farm has the most harmonics, and The AC current harmonics of wind farms connected to the grid are also the largest.

基于VSC-HVDC系统并网，逆变侧直流电压一直稳定在设定值200kV，而整流侧直流电压随着输送的有功功率的增大而有所抬升，如图16所示。Based on the grid connection of the VSC-HVDC system, the DC voltage on the inverter side has been stable at the set value of 200kV, while the DC voltage on the rectifier side has increased with the increase of the transmitted active power, as shown in Figure 16.

电网侧发生单相接地故障，引起了系统交流电压的跌落，电压跌落情况如图17所示。并网电网侧的有功功率和无功功率如图18-19所示。风电场输出的有攻功率和无功功率如图20-21所示。风电场输出的交流电压如图22所示。从图中可以看到，风电场采用HVAC并网时，无法有效隔离故障，电网侧有功功率和无功功率均受到了明显的影响，并且风电场输出的有功功率、无功功率和交流电压也都受到了明显的影响，出现了一定的波动和下降。A single-phase ground fault occurred on the power grid side, causing the AC voltage of the system to drop. The voltage drop is shown in Figure 17. The active power and reactive power on the grid-connected grid side are shown in Figure 18-19. The active power and reactive power output by the wind farm are shown in Figure 20-21. The AC voltage output by the wind farm is shown in Figure 22. It can be seen from the figure that when the wind farm adopts HVAC to connect to the grid, the fault cannot be effectively isolated, the active power and reactive power of the grid side are obviously affected, and the active power, reactive power and AC voltage output by the wind farm are also affected. All have been significantly affected, and there have been certain fluctuations and declines.

图23为风电场采用VSC-HVDC系统并网时风电场侧和并网电网侧的直流电压，从图中可以看到，VSC-HVDC系统逆变侧的直流电压出现了轻微波动，但是VSC-HVDC系统整流侧直流电压基本没有变化。Figure 23 shows the DC voltage on the wind farm side and the grid-connected grid side when the wind farm is connected to the grid using the VSC-HVDC system. It can be seen from the figure that the DC voltage on the inverter side of the VSC-HVDC system fluctuates slightly, but the VSC- The DC voltage on the rectification side of the HVDC system basically does not change.

所以可以看到，风电场采用直流母线集电拓扑结构有助于提高电能转换效率，减少风电场输出交流电流的谐波量；以及能够顺利实现电网故障穿越；而采用VSC-HVDC系统输电方式则能够有效隔离电网故障对风电场的影响，具有良好的运行性能。Therefore, it can be seen that the DC bus collector topology used in wind farms can help improve power conversion efficiency and reduce the harmonics of the wind farm output AC current; and can smoothly achieve grid fault ride-through; It can effectively isolate the impact of grid faults on wind farms, and has good operating performance.

综上分析，对本发明提供的基于VSC-HVDC的直流母线集电型风电场并网拓扑结构，通过风速变化和电网侧单相接地故障两种情况进行仿真分析，可充分表明本发明具备可行性和优越性。In summary, the simulation analysis of the VSC-HVDC-based DC bus collector type wind farm grid-connected topology provided by the present invention, through the simulation analysis of wind speed change and single-phase ground fault on the grid side, can fully demonstrate the feasibility of the present invention and superiority.

以上显示和描述了本发明的基本原理、主要特征及优点。本行业的技术人员应该了解，本发明不受上述实施例的限制，上述实施例和说明书中描述的只是说明本发明的原理，在不脱离本发明精神和范围的前提下，本发明还会有各种变化和改进，这些变化和改进都落入要求保护的本发明范围内。本发明要求保护范围由所附的权利要求书及其等效物界定。The basic principles, main features and advantages of the present invention have been shown and described above. Those skilled in the industry should understand that the present invention is not limited by the above-mentioned embodiments. What are described in the above-mentioned embodiments and the description only illustrate the principle of the present invention. Without departing from the spirit and scope of the present invention, the present invention will also have Variations and improvements are possible, which fall within the scope of the claimed invention. The protection scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A permanent-magnet direct-drive offshore wind farm grid-connected system topology, characterized in that it includes successively connected DC bus collector wind farms, wind farm grid-side converter stations, two-stage step-up transformers, and offshore rectifiers substation, submarine DC cable, onshore inverter station, grid-connected side step-up transformer and land power grid;

The DC bus collector type wind farm adopts a DC bus collector topology, including several groups of connected wind turbines, permanent magnet synchronous generators and machine-side rectifiers, and a DC bus for power collection;

The wind turbine converts the captured wind energy into AC power through the permanent magnet synchronous generator, and the AC power is rectified and converted into DC power by the machine-side rectifier, collected at the DC bus, and centrally inverted by the wind farm grid-side converter station into wind farm grid-side AC power. The AC power on the grid side of the wind farm is boosted by a two-stage step-up transformer and then input to the offshore rectification station, and then converted into boosted DC power by the offshore rectifier station and sent to the onshore inverter station through the submarine DC cable to be converted into grid-connected AC power and grid-connected The alternating current on the grid-connected side is finally boosted by the step-up transformer on the grid-connected side and then merged into the land grid.

2. The topological structure of the permanent magnet direct drive offshore wind farm grid-connected system according to claim 1, wherein the two-stage step-up transformer includes a 3KV:35KV step-up transformer and a 35KV:115KV step-up transformer connected in sequence Transformer, the input end of the 3KV:35KV step-up transformer is connected to the output end of the wind farm grid side converter station, and the output end of the 35KV:115KV step-up transformer is connected to the input end of the offshore rectification station.

3. The topological structure of the grid-connected system of the permanent magnet direct-drive offshore wind farm according to claim 1, wherein the submarine DC cable is a ±100KV DC cable.

4. The topology structure of the grid-connected system of the permanent magnet direct drive type offshore wind farm according to claim 1 or 3, characterized in that: the submarine DC cable is 100km long.

5. The topology structure of permanent magnet direct drive offshore wind farm grid-connected system according to claim 1, characterized in that: the offshore rectifier station and the onshore inverter station are voltage source converter stations with the same topology; The above-mentioned voltage source converter station is a converter based on full-control switching devices, and the specific topological structures are three-phase two-level converter, three-phase three-level converter, and clamped multi-level voltage converter. source converter, cascaded multilevel converter, modular multilevel voltage source converter or multi-pulse voltage source converter.

6. The topology structure of permanent magnet direct drive offshore wind farm grid-connected system according to claim 5, characterized in that: the three-phase two-level converter includes a filter capacitor connected in parallel to the DC side, and the filter capacitor A voltage-type three-phase full-bridge inverter circuit connected in parallel, and a commutation reactor and filter connected in series on the AC side and connected to the three-phase midpoints output by the voltage-type three-phase full-bridge inverter circuit;

The filter capacitor includes a first DC capacitor and a second DC capacitor connected in series, and a connection node of the first DC capacitor and the second DC capacitor is grounded;

The commutation reactor includes a commutation resistance and a commutation inductance connected in series in sequence, and the two ends of the commutation resistance and the commutation inductance connected in series are respectively connected to the output terminal of the two-stage step-up transformer and the voltage-type three-phase full-bridge inverter. The output midpoint of the variable circuit is connected;

The filter is a high-pass filter, including filter resistors, filter inductors and filter capacitors connected in series in sequence; one end of the filter resistor is connected between the output terminals of the two-stage step-up transformer and the commutation resistor, and one end of the filter capacitor grounded.

7. The method for controlling the topology of the permanent magnet direct drive offshore wind farm grid-connected system according to any one of claims 1 to 6, characterized in that: the rectification control of the offshore rectifier station is based on the intermittent output power of the wind farm and uncontrollability, using constant AC voltage and constant frequency control; specifically include the following steps,

1) Obtain the effective value U _WF of the output AC voltage of the wind farm, the reference value U _WFref of the effective value of the output AC voltage of the wind farm, the fundamental frequency f _WF of the AC voltage at the output end of the wind farm, and the fundamental frequency of the AC voltage at the output end of the wind farm Reference value f _WFref ;

2) The deviation between the effective value U _WF of the output AC voltage of the wind farm and the reference value U _WFref of the effective value of the output AC voltage of the wind farm is output as the AC voltage amplitude M through the PI regulator and [0,1] limit, and the wind farm The deviation between the fundamental frequency f _WF of the AC voltage at the output terminal and the reference value f _WFref of the fundamental frequency of the AC voltage at the output terminal of the wind farm is limited by the PI regulator and [-arcsinX ^* , arcsinX ^* ] and output as the AC voltage phase δ, by formula Calculate the per-unit value X ^* of the commutation reactance X, where S _N is the rated capacity of the converter station, and U _N is the rated voltage of the converter station.

8. The method for controlling the topological structure of the permanent magnet direct drive type offshore wind farm grid-connected system according to claim 7, characterized in that: the inverter control of the onshore inverter station adopts a method of constant DC voltage and constant reactive power Double closed-loop control to maintain a stable DC side voltage and balance the active power of the system; specifically includes the following steps,

1) Obtain the DC voltage V dc4 on the inverter side of the VSC-HDC system, the reference value V _dcref4 of the DC voltage on the inverter side of the VSC-HVDC system, the reactive power Q _s4 _connected to the grid, and the reference value Q _sref4 of the grid-connected reactive power;

2) The deviation _{between the DC voltage V dc4 on the inverter side of the VSC-HDC system and the DC voltage reference value V dcref4} _on the inverter side of the VSC-HVDC system is output through the PI regulator to output the d-axis reference current i _sdref4 , and the reactive power incorporated into the grid The deviation between Q _s4 and grid-connected reactive power reference value Q _sref4 outputs the q-axis reference current i _sqref4 through the PI regulator;

3) Combine the d-axis component i _sd4 of the AC current on the inverter side of the VSC-HDC system and the d-axis reference current i _sdref4 of the AC current on the inverter side of the VSC-HVDC system through the PI regulator, the q-axis coupling item, and the d-axis AC voltage The d-axis modulation voltage of the output converter is used to couple the deviation between the q-axis component i _sq4 of the AC current on the inverter side of the VSC-HDC system and the q-axis reference current i _sqref4 of the AC current on the inverter side of the VSC-HVDC system through the PI regulator and the d-axis coupling Item, q-axis AC voltage combination output converter q-axis modulation voltage.

9. The method for controlling the topological structure of the grid-connected system of permanent magnet direct-drive offshore wind farms according to claim 7, characterized in that: the rectification control strategy of the machine-side rectifier of the DC bus collector wind farm is based on the rotor magnetic field The directional zero d-axis current control adopts the double closed-loop control of the speed outer loop and the current inner loop, wherein the reference value of the speed outer loop is given by the maximum power tracking algorithm to capture the maximum wind energy; the specific steps are as follows,

1) Take the center line of the permanent magnet of the rotor as the d-axis, and the 90° electric angle direction along the rotor rotation direction ahead of the d-axis is the q-axis. In the dq0 rotating coordinate system, set the stator voltage equation of the permanent magnet synchronous generator as formula (1 ),

\{\begin{matrix} {u u}_{s the s d d 11} = = \frac{{dψ dψ}_{s the s d d}}{d d t t} + + {R R}_{s the s 11} {i i}_{s the s d d 11} - - {ω ω}_{e e} {L L}_{s the s q q 11} {i i}_{s the s q q 11} \\ {u u}_{s the s q q 11} = = \frac{{dψ dψ}_{s the s q q}}{d d t t} + + {R R}_{s the s 11} {i i}_{s the s q q 11} + + {ω ω}_{e e} {L L}_{s the s d d 11} {i i}_{s the s d d 11} + + {ω ω}_{e e} {ψ ψ}_{f f} \end{matrix} - - - - - - ((11))

In formula (1), u _sd1 and u _sq1 are the d and q axis components of the stator voltage of the generator, ψ _sd and ψ _sq are the d and q axis components of the stator flux linkage, and i _sd1 and i _sq1 are the d, q axis components of the stator current q-axis component; ω _e is the electrical angular frequency of the rotor, ω _e = pω _g , where p is the number of motor pole pairs, ω _g = ω _m is the rotational speed of the generator; L _sd1 and L _sq1 are the stator d and q-axis synchronous inductance;

2) Perform decoupling control on the d and q axis components i _sd1 and i _sq1 of the stator current in formula (1), and introduce feedforward compensation items u _d ' and u _q ' according to formula (2) to compensate the equivalent reactor voltage drop on

\{\begin{matrix} {u u}_{d d}^{' '} = = \frac{{dψ dψ}_{s the s d d 11}}{d d t t} + + {R R}_{s the s 11} {i i}_{s the s d d 11} \\ {u u}_{q q}^{' '} = = \frac{{dψ dψ}_{s the s q q 11}}{d d t t} + + {R R}_{s the s 11} {i i}_{s the s q q 11} \end{matrix} - - - - - - ((22))

Transform formula (2) into formula (2-1) through proportional integral link,

\{\begin{matrix} {u u}_{d d}^{' '} = = {K K}_{p p 11} (({i i}_{s the s d d r r e e f f 11} - - {i i}_{s the s d d 11})) + + {K K}_{i i 11} &Integral; &Integral; (({i i}_{s the s d d r r e e f f 11} - - {i i}_{s the s d d 11})) d d t t \\ {u u}_{q q}^{' '} = = {K K}_{p p 11} (({i i}_{s the s q q r r e e f f 11} - - {i i}_{s the s q q 11})) + + {K K}_{i i 11} &Integral; &Integral; (({i i}_{s the s q q r r e e f f 11} - - {i i}_{s the s q q 11})) d d t t \end{matrix} - - - - - - ((22 - - 11))

In formula (2-1), K _p1 and K _i1 are the current inner loop ratio and integral coefficient respectively; i _sdref1 and i _sqref1 are the reference values of current i _sd1 and i _sq1 respectively, i _sdref1 = 0, and i _sqref1 is determined by the speed reference value The deviation between ω _mref and the actual value of the speed ω _m is adjusted by the PI regulator;

3) Transform formula (1) into formula (3) according to formula (2-1),

\{\begin{matrix} {u u}_{s the s d d 11} = = {K K}_{p p 11} (({i i}_{s the s d d r r e e f f 11} - - {i i}_{s the s d d 11})) + + {K K}_{i i 11} &Integral; &Integral; (({i i}_{s the s d d r r e e f f 11} - - {i i}_{s the s d d 11})) d d t t - - {ω ω}_{e e} {L L}_{s the s q q 11} {i i}_{s the s q q 11} \\ {u u}_{s the s q q 11} = = {K K}_{p p 11} (({i i}_{s the s q q r r e e f f 11} - - {i i}_{s the s q q 11})) + + {K K}_{i i 11} &Integral; &Integral; (({i i}_{s the s q q r r e e f f 11} - - {i i}_{s the s q q 11})) d d t t + + {ω ω}_{e e} {L L}_{s the s d d 11} {i i}_{s the s d d 11} + + {ω ω}_{e e} {ψ ψ}_{f f} \end{matrix} - - - - - - ((33))

By adjusting the current inner loop ratio K _p1 of the permanent magnet synchronous generator, the integral coefficient K _i1 and the actual speed value ω _m , and the wind turbine speed reference value ω _mref , the stator voltage of the permanent magnet synchronous generator is calculated according to formula (3) output.

10. The method for controlling the topological structure of the permanent magnet direct-drive offshore wind farm grid-connected system according to claim 9, characterized in that: the centralized inverter control of the grid-side converter station of the wind farm is based on the grid voltage-oriented vector Control, using the double closed-loop control of the voltage outer loop and the current inner loop, inverting the DC rectified by the machine-side converter into an AC with the same voltage and amplitude as the grid, while keeping the DC side voltage constant; specifically includes the following steps ,

1) In the d-q rotating coordinate system, determine the control model of the wind farm grid side converter station as formula (4),

\{\begin{matrix} {u u}_{c c d d 22} = = {u u}_{s the s d d 22} + + L L \frac{{di di}_{s the s d d 22}}{d d t t} + + {R R}_{22} {i i}_{s the s d d 22} - - {ω ω}_{s the s} {L L}_{22} {i i}_{s the s q q 22} \\ {u u}_{c c q q 22} = = {u u}_{s the s q q 22} + + L L \frac{{di di}_{s the s q q 22}}{d d t t} + + {R R}_{22} {i i}_{s the s q q 22} + + {ω ω}_{s the s} {L L}_{22} {i i}_{s the s d d 22} \end{matrix} - - - - - - ((44))

In the formula, u _cd2 and u _cq2 are the d and q axis components of the AC side voltage; R ₂ and L ₂ are the equivalent resistance and reactance of the connecting transformer and commutation reactor respectively; i _sd2 and i _sq2 are the AC side of the inverter Current d, q-axis components; ω _s is the angular frequency of the wind farm grid voltage; u _sd2 and u _sq2 are the d, q-axis components of the wind farm grid voltage;

And the active power and reactive power of the wind farm grid side converter station are expressed as formula (5),

\{\begin{matrix} {P P}_{s the s} = = \frac{33}{22} (({u u}_{s the s d d 22} {i i}_{d d 22} + + {u u}_{s the s q q 22} {i i}_{q q 22})) \\ {Q Q}_{s the s} = = - - \frac{33}{22} (({u u}_{s the s d d 22} {i i}_{q q 22} - - {u u}_{s the s q q 22} {i i}_{d d 22})) \end{matrix} - - - - - - ((55))

2) Using the space vector control method based on grid voltage orientation, under the condition of three-phase grid voltage balance, the grid voltage space vector The direction of the d-axis is the direction of the d-axis, and the q-axis leads the d-axis by an electrical angle of 90°, then u _sd2 = U _s , u _sq2 = 0, where U _s is the modulus of the grid voltage space vector;

Rewrite formula (5) as formula (6),

\{\begin{matrix} {P P}_{s the s} = = \frac{33}{22} {U u}_{s the s} {i i}_{d d 22} \\ {Q Q}_{s the s} = = - - \frac{33}{22} {U u}_{s the s} {i i}_{q q 22} \end{matrix} - - - - - - ((66))

According to formula (6), the active power and reactive power output by the grid-side converter station of the wind farm can be controlled by changing the current of the d and q axes;

3) The feed-forward compensation item is introduced into the inner loop controller of the wind farm grid-side converter station, which is realized through a proportional integral link, and the control model of the wind farm grid-side converter station is transformed into formula (7),

\{\begin{matrix} {u u}_{c c d d 22} = = {u u}_{s the s d d 22} + + {K K}_{p p 22} (({i i}_{s the s d d r r e e f f 22} - - {i i}_{s the s d d 22})) + + {K K}_{i i 22} &Integral; &Integral; (({i i}_{s the s d d r r e e f f 22} - - {i i}_{s the s d d 22})) d d t t - - {ω ω}_{s the s} {L L}_{22} {i i}_{s the s q q 22} \\ {u u}_{c c q q 22} = = {u u}_{s the s q q 22} + + {K K}_{p p 22} (({i i}_{s the s q q r r e e f f 22} - - {i i}_{s the s q q 22})) + + {K K}_{i i 22} &Integral; &Integral; (({i i}_{s the s q q r r e e f f 22} - - {i i}_{s the s q q 22})) d d t t + + {ω ω}_{s the s} {L L}_{22} {i i}_{s the s d d 22} \end{matrix} - - - - - - ((77))

In formula (7), K _p2 and K _i2 are the proportion and integral coefficient of the current inner loop respectively; i _sdref2 and i _sqref2 are the active and reactive current reference values respectively; among them, i _sdref2 is the grid-side converter DC voltage setting The deviation between the value V _dcref2 and the actual value V _dc2 is adjusted by the PI regulator and converted; i _sqref2 is the deviation between the grid-side converter reactive power reference value Q _ref2 and the actual value Q ₂ is obtained after adjustment by the PI regulator.