Coordinated Fault Ride-Through Method for Islanded Wind Farm-Bipolar VSC-HVDC System under Monopole Fault

Abstract

The present invention discloses a cooperative fault ride-through method for an islanded wind farm - bipolar flexible DC system under a monopole fault. Aiming at the deficiency that the existing control method is difficult to maintain the system stability when a monopole fault occurs, when a fault occurs in the voltage control pole of the bipolar flexible DC system, the control strategy of the power control pole is switched to a voltage control strategy. If the maximum power transmitted by the healthy pole in the fault stage is less than the power output of the wind farm, the voltage of the sending-end AC power grid is reduced to make the wind turbine generator set put into its own energy-consuming resistor. Compared with the existing method, this method can make full use of the flexibility of the flexible DC transmission system and the control of the wind turbine generator set, enable the wind turbine generator set and the flexible DC system to cooperate to complete the fault ride-through, and ensure the safe and stable operation of the system.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and particularly relates to a cooperative fault ride-through method for an island wind farm - bipolar flexible DC system under a monopole fault. Background Art

[0002] With the gradual increase in the voltage level and transmission capacity of flexible DC transmission systems, bipolar flexible DC transmission systems have received increasing attention due to their high flexibility and reliability. Flexible DC transmission technology based on the modular multilevel converter (MMC) topology has the advantages of low manufacturing difficulty, low system loss, and high waveform quality, and has very good application prospects in long-distance power collection and transmission.

[0003] When a bipolar MMC converter station is connected to a passive AC grid or an island new energy grid, it is necessary to establish stable frequency and voltage for the AC grid. If both poles adopt constant AC voltage control, the number of system state equations will be less than the number of variables, resulting in inaccurate control of power distribution between the two poles. Therefore, generally, one pole adopts constant AC voltage control and the other pole adopts constant power control. In addition, considering the system construction cost, the single-pole design capacity of a bipolar flexible DC system generally matches half of the rated capacity. If the power distribution between the two poles is unbalanced, it is very likely that the single-pole overload protection will act, affecting the safe and stable operation of the system. Therefore, cooperative control between the two poles also needs to be considered.

[0004] When a monopole fault occurs in a bipolar flexible DC transmission system, if the fault occurs in the voltage control pole and the faulty pole exits operation, then only relying on the power control pole cannot ensure the voltage stability of the island wind farm, and an optimized control strategy needs to be considered; if the fault occurs in the power control pole, although the voltage control pole has the function of controlling the grid voltage, once the output power of the wind farm at this time is greater than the maximum single-pole transmission power, the system still cannot maintain stability. At this time, it is necessary to consider how to quickly reduce the output power of the wind farm. At present, there is little research on the system control strategy of the fan and the MMC converter in the island wind farm - bipolar flexible DC transmission system under a monopole fault. It is urgent to propose a cooperative fault ride-through method for the island wind farm - bipolar flexible DC system under a monopole fault to ensure the safe and stable operation of the system. Summary of the Invention

[0005] The object of the present invention is to overcome the problem that the existing control method for an island wind farm - bipolar flexible DC transmission system is difficult to maintain system stability when a monopole fault occurs, and to provide a cooperative fault ride-through method for an island wind farm - bipolar flexible DC system under a monopole fault. When a fault occurs in the voltage control pole, the control strategy of the power control pole is switched to a voltage control strategy, and the voltage of the sending - end AC grid is reasonably adjusted according to the power balance situation, so that the wind turbines and the flexible DC system cooperate to complete the fault ride - through and ensure the safe and stable operation of the system.

[0006] In order to achieve the above - mentioned object of the invention, the following technical solutions are adopted:

[0007] A cooperative fault ride - through method for an island wind farm - bipolar flexible DC system under a monopole fault. The control system adopted to implement the method includes: a grid voltage reference value calculation module, a negative - pole power calculation module, a positive - pole sampling module and a negative - pole sampling module, a positive - pole Park transformation module and a negative - pole Park transformation module, a positive - pole voltage control module, a negative - pole power control module, a negative - pole voltage control module, a positive - pole modulation module and a negative - pole modulation module;

[0008] The grid voltage reference value calculation module calculates the grid voltage reference value U wind according to the active power P gmax output by the wind farm and the maximum monopole output power P gdref of the flexible DC system;

[0009] The negative - pole power calculation module calculates the negative - pole active and reactive powers P gabc and Q gabc2 according to the voltage vector U g2 and the negative - pole current vector I g2 ;

[0010] In the positive - pole sampling module, it includes:

[0011] A positive - pole voltage sampling module samples the three - phase voltage U gabc of the MMC AC grid;

[0012] A positive - pole current sampling module samples the three - phase current I gabc1 of the positive - pole MMC AC grid and the internal circulating current I cabc1 of the positive - pole MMC;

[0013] In the negative - pole sampling module, it includes:

[0014] A negative - pole voltage sampling module samples the three - phase voltage U gabc of the MMC AC grid;

[0015] A negative - pole current sampling module samples the three - phase current I gabc2, the internal circulating current I of the negative - pole MMC cabc2 is sampled;

[0016] The positive - pole Park transformation module performs Park transformation on the three - phase voltage U of the MMC AC grid gabc and the three - phase current I of the positive - pole MMC AC grid gabc1 to obtain the corresponding voltage vector U gdq and current vector I gdq1 in the synchronous rotating d - q coordinate system;

[0017] The negative - pole Park transformation module performs Park transformation on the three - phase voltage U of the MMC AC grid gabc and the three - phase current I of the negative - pole MMC AC grid gabc2 to obtain the corresponding voltage vector U gdq and current vector I gdq2 in the synchronous rotating d - q coordinate system;

[0018] The angles used for Park transformation by the positive - pole Park transformation module and the negative - pole Park transformation module are the reference phase θ r ;

[0019] The positive - pole voltage control module uses the d - axis and q - axis voltages U gdq as the outer control loop, and the positive - pole d - axis and q - axis currents I gdq1 as the inner control loop. Then, combined with the internal circulating - current control link and the coordinate - transformation link, the reference voltages U prefabc1 and U nrefabc1 of the upper and lower arms of the positive - pole MMC are obtained;

[0020] The negative - pole power control module uses the negative - pole active and reactive powers P g2 and Q g2 as the outer control loop, and the negative - pole d - axis and q - axis currents I gdq2 as the inner control loop. Then, combined with the internal circulating - current control link and the coordinate - transformation link, the reference voltages U prefabc2 and U nrefabc2 of the upper and lower arms of the negative - pole MMC output by the negative - pole power control module are obtained;

[0021] The negative - pole voltage control module uses the d - axis and q - axis voltages U gdq as the outer control loop, and the negative - pole d - axis and q - axis currents I gdq2 as the inner control loop. Then, combined with the internal circulating - current control link and the coordinate - transformation link, the reference voltages U prefabc3 and U nrefabc3 of the upper and lower arms of the negative - pole MMC output by the negative - pole voltage control module are obtained;

[0022] The positive - pole modulation module, according to the reference voltages U of the upper and lower arms of the positive - pole MMCprefabc1 With U nrefabc1 , the control of the positive MMC is realized;

[0023] For the negative modulation module, when the system is operating normally, according to the reference voltages U of the upper and lower arms of the negative MMC output by the negative power control module prefabc2 and U nrefabc2 , the control of the negative MMC is realized; when a single-pole fault occurs in the positive pole and it exits the operation, according to the reference voltages U of the upper and lower arms of the negative MMC output by the negative voltage control module prefabc3 and U nrefabc3 , the control of the negative MMC is realized.

[0024] Furthermore: In the grid voltage reference value calculation module, when the system is operating normally, the grid voltage reference value is given as 1 p.u., and the base value is the grid rated voltage; when a single-pole grounding fault occurs in the system, the grid voltage reference value U is calculated according to the following method gdref :

[0025]

[0026] where, P wind is the active power output of the wind farm, P gmax is the maximum single-pole output power of the flexible DC system, and the voltage base value is the grid rated voltage.

[0027] Furthermore: In the positive voltage control module, the reference voltages U of the upper and lower arms of the positive MMC are obtained according to the following method prefabc1 and U nrefabc1 ;

[0028] First, the d-axis and q-axis voltages U gdq are controlled by a PI controller to make them follow the given reference values U gdref and U gqref , U gqref is set to 0. After passing through the limiter, the output of the PI controller is used as the reference values I gdref1 and I gqref1 of the positive d-axis and q-axis currents; then, the positive d-axis and q-axis currents I gdq1 are controlled by a PI controller to make them follow the reference values I gdref1 and I gqref1 . The output of the PI controller is used as the reference differential-mode voltage U difdq1 of the positive MMC, and then through the Park inverse transformation, the reference differential-mode voltage U difabc1 in the stationary three-phase coordinate system is obtained; next, the internal circulating current I cabc1 inside the positive MMC is controlled by a resonant controller, and the output of the internal circulating current controller of the positive pole is used as the reference common-mode voltage U of the positive MMCcomabc1 ; Finally, using the positive reference differential mode voltage U difabc1 and the reference common mode voltage U comabc1 , the reference voltages U prefabc1 and U nrefabc1 of the upper and lower arms of the positive MMC are obtained through calculation.

[0029] Furthermore: In the negative power control module, the reference voltages U prefabc2 and U nrefabc2 of the upper and lower arms of the negative MMC are obtained according to the following method;

[0030] First, the negative active and reactive powers P g2 and Q g2 are controlled by a PI controller to make them follow the given reference values P g2ref and Q g2ref , and Q g2ref is set to 0. After passing through a limiting link, the output of the PI controller is used as the reference values I gdref2 and I gqref2 of the negative d and q axis currents; Then, the negative d and q axis currents I gdq2 are controlled by a PI controller to make them follow the reference values I gdref2 and I gqref2 . The output of the PI controller is used as the negative MMC reference differential mode voltage U difdq2 , and after Park inverse transformation, the reference differential mode voltage U difabc2 in the stationary three-phase coordinate system is obtained; Next, the negative MMC internal circulating current I cabc2 is controlled by a resonant controller, and the output of the negative internal circulating current controller is used as the negative MMC reference common mode voltage U comabc2 ; Finally, using the negative reference differential mode voltage U difabc2 and the reference common mode voltage U comabc2 , the reference voltages U prefabc2 and U nrefabc2 of the upper and lower arms of the negative MMC are obtained through calculation.

[0031] Furthermore: In the negative voltage control module, the reference voltages U prefabc3 and U nrefabc3 of the upper and lower arms of the negative MMC are obtained according to the following method;

[0032] First, the d and q axis voltages U gdq are controlled by a PI controller to make them follow the given reference values U gdref and U gqref , and U gqref is set to 0. After passing through a limiting link, the output of the PI controller is used as the reference values I gdref3and I gqref3 ; Then, for the negative - terminal d - axis and q - axis currents I gdq2 a PI controller is used for control to make it follow the reference values I gdref3 and I gqref3 . The output of the PI controller is used as the reference differential - mode voltage U difdq3 of the negative - terminal MMC. After Park inverse transformation, the reference differential - mode voltage U difabc3 in the stationary three - phase coordinate system is obtained; Next, for the internal circulating current I cabc2 inside the negative - terminal MMC, a resonant controller is used for control. The output of the internal circulating - current controller of the negative - terminal is used as the reference common - mode voltage U comabc3 of the negative - terminal MMC; Finally, by using the reference differential - mode voltage U difabc3 and the reference common - mode voltage U comabc3 , the reference voltages U prefabc3 and U nrefabc3 of the upper and lower arms of the negative - terminal MMC are obtained through calculation.

[0033] Due to the adoption of the technical solution of the present invention, when a fault occurs in the voltage - control pole of the island - type wind farm - bipolar flexible DC transmission system, the control strategy of the power - control pole can be switched to the voltage - control strategy; if the maximum power transmitted by the healthy pole in the fault stage is less than the power output of the wind farm, the voltage of the sending - end AC power grid can also be reduced to make the wind turbines turn on their own energy - consuming resistors, and cooperate with the flexible DC system to complete the fault - ride - through, ensuring the safe and stable operation of the system. Description of the Drawings

[0034] Figure 1 is a typical topological diagram of the island - type wind farm - bipolar flexible DC transmission system.

[0035] Figure 2 is a specific example structure diagram of a single - pole MMC in the bipolar flexible DC transmission system, where u ga , u gb , u gc are the grid voltages; i ga , i gb , i gc are the grid currents; u pa , u pb , u pc are the upper - arm voltages of the MMC; u na , u nb , u nc are the lower - arm voltages of the MMC; i pa , i pb , i pc are the upper - arm currents of the MMC; i na , i nb , i nc are the lower - arm currents of the MMC; U dcis the DC bus voltage, and i dc is the DC bus current, L0 is the arm inductor, and SM (N) is the sub-module in the MMC; N is the sub-module serial number.

[0036] Figure 3 is a specific example system schematic diagram of the control method of the present invention.

[0037] Figure 4 is the specific control schematic diagram of the positive voltage control module.

[0038] Figure 5 is the specific control schematic diagram of the negative power control module.

[0039] Figure 6 is the specific control schematic diagram of the negative voltage control module. Specific implementation mode

[0040] To describe the present invention more specifically, the technical solution of the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0041] The system implementation of the collaborative fault ride-through method for the island wind farm - bipolar VSC-HVDC system under single-pole faults in the present invention is as Figure 3 shown, and includes a grid voltage reference value calculation module 1, a negative power calculation module 2, a positive voltage sampling module 3, a positive current sampling module 4, a positive Park transformation module 5, a positive voltage control module 6, a positive modulation module 7, a negative voltage sampling module 8, a negative current sampling module 9, a negative Park transformation module 10, a negative power control module 11, a negative voltage control module 12, and a negative modulation module 13.

[0042] As Figure 3 shown, the collaborative fault ride-through method for the island wind farm - bipolar VSC-HVDC system in the present invention includes the following steps:

[0043] Collect the three-phase voltage U gabc of the MMC AC grid side through the positive voltage sampling module 3 and the negative voltage sampling module 8, and respectively collect the three-phase current I gabc1 of the positive MMC AC grid side and the three-phase current I gabc2 of the negative MMC AC grid side through the positive current sampling module 4 and the negative current sampling module 9, as well as the internal circulating current I cabc1 of the positive MMC and the internal I cabc2 of the negative MMC.

[0044] Using the negative power calculation module 2, according to the voltage vector U gabc and the negative current vector I gabc2 , calculate the negative active and reactive powers P g2 and Qg2 。

[0045] Using the grid voltage reference value calculation module 1, when the system is operating normally, the grid voltage reference value is given as 1 p.u. (the base value is the grid rated voltage); when a single-pole grounding fault occurs in the system, the grid voltage reference value U is calculated according to the following method gdref :

[0046]

[0047] where P wind is the active power output of the wind farm, P gmax is the maximum output power of a single pole of the VSC-HVDC system, and the voltage base value is the grid rated voltage.

[0048] Using the positive Park transformation module 5 and the negative Park transformation module 10, the three-phase voltages U gabc of the MMC AC grid, the three-phase currents I gabc1 of the positive MMC, and the three-phase currents I gabc2 of the negative MMC are respectively subjected to Park transformation to obtain the corresponding voltage vectors U gdq and the positive MMC current vector I gdq1 and the negative MMC current vector I gdq2 in the synchronous rotating d-q coordinate system. The angle used for Park transformation is the reference phase θ r 。

[0049] Now refer to Figure 4 to describe the control process of the positive voltage control module 6. Among them, sub-module 14 is the positive voltage outer-loop controller, sub-module 15 is the positive current inner-loop controller, sub-module 16 is the positive Park inverse transformation module, sub-module 17 is the positive internal circulating current controller, and sub-module 18 is the positive arm voltage calculation module. First, the d-axis and q-axis voltages U gdq are controlled by a PI controller so that they respectively follow the given reference values U gdref and U gqref (U gqref is set to 0). After passing through the limiter, the output of the PI controller is used as the reference values I gdref1 and I gqref1 of the positive d-axis and q-axis currents; then, the positive d-axis and q-axis currents I gdq1 are controlled by a PI controller so that they follow the reference values I gdref1 and I gqref1 . The output of the PI controller is used as the reference differential-mode voltage U difdq1 of the positive MMC, and then through Park inverse transformation, the reference differential-mode voltage U difabc1 in the stationary three-phase coordinate system is obtained; next, the internal circulating current I cabc1 of the positive MMCControl is carried out using a resonant controller, and the output of the positive internal circulating current controller is used as the reference common-mode voltage U of the positive MMC comabc1 ; Finally, using the reference differential-mode voltage U of the positive electrode difabc1 and the reference common-mode voltage U comabc1 , the reference voltages U of the upper and lower arms of the positive MMC are obtained through calculation prefabc1 and U nrefabc1 . Among them, the implementation methods of the voltage control outer loop and the current control inner loop are as follows:

[0050]

[0051] Among them: F PI1 (s) is the transfer function of the positive PI controller, k p1 is the proportional coefficient, k i1 is the integral coefficient, I gdref1 , I gqref1 correspond to the d-axis and q-axis components of the current vector I gdqref1 .

[0052]

[0053] Among them: L is the equivalent inductance including the commutation transformer and the arm reactor, U difd1 , U difq1 correspond to the d-axis and q-axis components of the voltage vector U difdq1 .

[0054] Using the positive modulation module 7, according to the reference voltages U of the upper and lower arms of the positive MMC prefabc1 and U nrefabc1 , the control of the positive MMC is realized.

[0055] Now refer to Figure 5 Describe the control process of the negative power control module 11, where the sub-module 19 is the negative power outer loop controller, the sub-module 20 is the negative current inner loop controller, the sub-module 21 is the negative Park inverse transformation module, the sub-module 22 is the negative internal circulating current controller, and the sub-module 23 is the negative arm voltage calculation module. First, the active and reactive powers P of the negative electrode g2 and Q g2 are controlled by a PI controller so that they respectively follow the given reference values P g2ref and Q g2ref (Q g2ref is set to 0). After passing through the limiter, the output of the PI controller is used as the reference values I of the negative d-axis and q-axis currents gdref2 and I gqref2 ; Then, the negative d-axis and q-axis currents I gdq2 are controlled by a PI controller so that they follow the reference values I gdref2 and Igqref2 , the output of the PI controller serves as the negative - terminal MMC reference differential - mode voltage \(U\) difdq2 , and after Park inverse transformation, the reference differential - mode voltage \(U\) in the stationary three - phase coordinate system is obtained difabc2 ; Next, the internal circulating current \(I\) of the negative - terminal MMC cabc2 is controlled by a resonant controller, and the output of the internal circulating - current controller of the negative - terminal serves as the negative - terminal MMC reference common - mode voltage \(U\) comabc2 ; Finally, using the negative - terminal reference differential - mode voltage \(U\) difabc2 and the reference common - mode voltage \(U\) comabc2 , through calculation, the reference voltages \(U\) prefabc2 and \(U\) nrefabc2 of the upper and lower arms of the negative - terminal MMC are obtained. Among them, the implementation methods of the outer power - control loop and the inner current - control loop are as follows:

[0056]

[0057] Where: \(F\) PI2 (s) is the transfer function of the negative - terminal power PI controller, \(k\) p2 is the proportional coefficient, \(k\) i2 is the integral coefficient, \(I\) gdref2 , \(I\) gqref2 respectively correspond to the d - axis and q - axis components of the current vector \(I\) gdqref2 .

[0058]

[0059] Where: \(L\) is the equivalent inductance including the commutation transformer and the arm reactor, \(U\) difd2 , \(U\) difq2 respectively correspond to the d - axis and q - axis components of the voltage vector \(U\) difdq2 .

[0060] Now refer to Figure 6 to describe the control process of the negative - terminal voltage control module 12. Among them, sub - module 24 is the negative - terminal voltage outer - loop controller, sub - module 25 is the negative - terminal current inner - loop controller, sub - module 26 is the negative - terminal Park inverse - transformation module, sub - module 27 is the negative - terminal internal circulating - current controller, and sub - module 28 is the negative - terminal arm - voltage calculation module. First, the d - axis and q - axis voltages \(U\) gdq are controlled by a PI controller to make them respectively follow the given reference values \(U\) gdref and \(U\) gqref (\(U\) gqref is set to 0). After passing through the limiting link, the output of the PI controller serves as the reference values \(I\) gdref3 and \(I\) gqref3 of the negative - terminal d - axis and q - axis currents; Then, the negative - terminal d - axis and q - axis currents \(I\) gdq2 are controlled by a PI controller to make them follow the reference values \(I\)gdref3 and I gqref3 , the output of the PI controller serves as the negative - pole MMC reference differential - mode voltage U difdq3 , and through the Park inverse transformation, the reference differential - mode voltage U in the stationary three - phase coordinate system is obtained difabc3 ; Next, the internal circulating current I of the negative - pole MMC cabc2 is controlled by a resonant controller, and the output of the negative - pole internal circulating - current controller serves as the negative - pole MMC reference common - mode voltage U comabc3 ; Finally, using the negative - pole reference differential - mode voltage U difabc3 and the reference common - mode voltage U comabc3 , through calculation, the reference voltages U prefabc3 and U nrefabc3 of the upper and lower arms of the negative - pole MMC are obtained. Among them, the implementation methods of the outer voltage - control loop and the inner current - control loop are as follows:

[0061]

[0062] Among them: F PI3 (s) is the transfer function of the negative - pole voltage PI controller, k p3 is the proportional coefficient, k i3 is the integral coefficient, I gdref3 , I gqref3 correspond to the d - axis and q - axis components of the current vector I gdqref3 .

[0063]

[0064] Among them: L is the equivalent inductance including the commutation transformer and the arm reactor, U difd3 , U difq3 correspond to the d - axis and q - axis components of the voltage vector U difdq3 .

[0065] Using the negative - pole modulation module 13, when the system is operating normally, according to the reference voltages U prefabc2 and U nrefabc2 of the upper and lower arms of the negative - pole MMC output by the negative - pole power - control module, the control of the negative - pole MMC is realized; when the positive - pole has a single - pole fault and exits operation, according to the reference voltages U prefabc3 and U nrefabc3 of the upper and lower arms of the negative - pole MMC output by the negative - pole voltage - control module, the control of the negative - pole MMC is realized.

[0066] The above description of the embodiments is to enable those of ordinary skill in the art to understand and apply the present invention. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and all improvements and modifications made by those skilled in the art based on the disclosure of the present invention should fall within the protection scope of the present invention.

Claims

1. A coordinated fault ride-through method for an islanded wind farm - bipolar flexible DC system under single-pole faults, characterized in that, The control system adopted to implement the method includes: a grid voltage reference value calculation module, a negative power calculation module, a positive electrode sampling module and a negative electrode sampling module, a positive electrode Park transformation module and a negative electrode Park transformation module, a positive electrode voltage control module, a negative electrode power control module, a negative electrode voltage control module, a positive electrode modulation module and a negative electrode modulation module; The grid voltage reference value calculation module calculates the grid voltage reference value U wind according to the active power P output by the wind farm gmax and the maximum monopole output power P of the flexible DC system gdref . The negative electrode power calculation module calculates the active and reactive powers P gabc and the negative electrode current vector I gabc2 based on the voltage vector U g2 and Q g2 ; In the positive electrode sampling module, it includes: Positive electrode voltage sampling module, sampling the three-phase voltage U of the MMC AC power grid gabc for sampling; Positive current sampling module, sampling the three-phase current I of the positive MMC AC power grid gabc1 and the internal circulating current I of the positive MMC cabc1 for sampling; In the negative electrode sampling module, it includes: The negative electrode voltage sampling module samples the three-phase voltages U of the MMC AC power grid gabc for sampling; The negative electrode current sampling module samples the three-phase current I of the negative electrode MMC AC power grid gabc2 and the internal circulating current I of the negative electrode MMC cabc2 for sampling; The positive - pole Park transformation module performs Park transformation on the three - phase voltage U of the MMC AC power grid gabc and the three - phase current I of the positive - pole MMC AC power grid gabc1 to obtain the corresponding voltage vector U gdq and current vector I gdq1 ; The negative - terminal Park transformation module performs Park transformation on the three - phase voltages U gabc of the MMC AC grid and the three - phase currents I gabc2 of the negative - terminal MMC AC grid to obtain the corresponding voltage vector U gdq and current vector I gdq2 in the synchronous rotating d - q coordinate system; The angles used for Park transformation in the positive and negative Park transformation modules are the reference phase θ r ; The positive electrode voltage control module uses the d-axis and q-axis voltages U gdq As the outer control loop, the positive electrode d-axis and q-axis currents I gdq1 As the inner control loop, combined with the internal circulating current control link and the coordinate transformation link, the reference voltages U of the upper and lower arms of the positive electrode MMC are obtained prefabc1 And U nrefabc1 ; The negative - terminal power control module takes the negative - terminal active and reactive powers P g2 and Q g2 as the outer control loop, and the negative - terminal d - axis and q - axis currents I gdq2 as the inner control loop. Then, combined with the internal circulating - current control link and the coordinate - transformation link, the reference voltages U prefabc2 and U nrefabc2 of the upper and lower arms of the negative - terminal MMC output by the negative - terminal power control module are obtained; The negative - terminal voltage control module uses the d - axis and q - axis voltages U gdq As the outer control loop, the negative - terminal d - axis and q - axis currents I gdq2 As the inner control loop, combined with the internal circulating - current control link and the coordinate - transformation link, the reference voltages U of the upper and lower arms of the negative - terminal MMC output by the negative - terminal voltage control module are obtained prefabc3 And U nrefabc3 ; The positive electrode modulation module controls the positive electrode MMC according to the reference voltages U prefabc1 and U nrefabc1 of the upper and lower arms of the positive electrode MMC; The negative electrode modulation module, when the system is operating normally, controls the upper and lower arms of the negative MMC according to the reference voltages U prefabc2 and U nrefabc2 output by the negative power control module, so as to control the negative MMC; when the positive electrode fails in a single-pole mode and exits the operation, it controls the upper and lower arms of the negative MMC according to the reference voltages U prefabc3 and U nrefabc3 output by the negative voltage control module, so as to control the negative MMC.

2. The coordinated fault ride-through method for an islanded wind farm - bipolar flexible DC system under single-pole faults according to claim 1, characterized in that: In the grid voltage reference value calculation module, when the system operates normally, the grid voltage reference value is given as 1 p.u., and the base value is the grid rated voltage; When a single-pole ground fault occurs in the system, calculate the grid voltage reference value U according to the following method gdref : Among them, P wind is the active power output of the wind farm, and P gmax is the maximum output power of a single pole of the VSC-HVDC system, and the voltage base value is the rated voltage of the power grid.

3. The coordinated fault ride-through method for an islanded wind farm - bipolar flexible DC system under single-pole faults according to claim 1, characterized in that: In the positive electrode voltage control module, the reference voltages U of the upper and lower arms of the positive electrode MMC are obtained according to the following method prefabc1 and U nrefabc1 ; First, the d-axis and q-axis voltages U gdq are controlled by PI controllers to respectively follow the given reference values U gdref and U gqref , and U gqref is set to 0. After passing through the limiter, the outputs of the PI controllers serve as the reference values I gdref1 and I gqref1 of the positive d-axis and q-axis currents; then, the positive d-axis and q-axis currents I gdq1 are controlled by PI controllers to follow the reference values I gdref1 and I gqref1 . The outputs of the PI controllers serve as the reference differential-mode voltage U difdq1 of the positive MMC. After Park inverse transformation, the reference differential-mode voltage U difabc1 in the stationary three-phase coordinate system is obtained; next, the internal circulating current I cabc1 of the positive MMC is controlled by a resonant controller, and the output of the internal circulating current controller of the positive serves as the reference common-mode voltage U comabc1 of the positive MMC; finally, using the reference differential-mode voltage U difabc1 and the reference common-mode voltage U comabc1 , the reference voltages U prefabc1 and U nrefabc1 of the upper and lower arms of the positive MMC are obtained through calculation.

4. The coordinated fault ride-through method for an islanded wind farm - bipolar flexible DC system under single-pole faults according to claim 1, characterized in that: In the negative electrode power control module, the reference voltages U of the upper and lower arms of the negative electrode MMC are obtained according to the following method prefabc2 and U nrefabc2 ; First, the active and reactive power of the negative electrode, P g2 and Q g2 are controlled by a PI controller to make them follow the given reference values P g2ref and Q g2ref , and Q g2ref is set to 0. After passing through a limiting link, the output of the PI controller serves as the reference values of the d-axis and q-axis currents of the negative electrode, I gdref2 and I gqref2 ; then, the d-axis and q-axis currents of the negative electrode, I gdq2 are controlled by a PI controller to make them follow the reference values I gdref2 and I gqref2 . The output of the PI controller serves as the reference differential-mode voltage U difdq2 of the negative electrode MMC. After Park inverse transformation, the reference differential-mode voltage U difabc2 in the stationary three-phase coordinate system is obtained; next, the internal circulating current I cabc2 inside the negative electrode MMC is controlled by a resonant controller. The output of the internal circulating current controller of the negative electrode serves as the reference common-mode voltage U comabc2 of the negative electrode MMC; finally, using the reference differential-mode voltage U difabc2 and the reference common-mode voltage U comabc2 , the reference voltages U prefabc2 and U nrefabc2 of the upper and lower arms of the negative electrode MMC are obtained through calculation.

5. The coordinated fault ride-through method for an islanded wind farm - bipolar flexible DC system under single-pole faults according to claim 1, characterized in that: In the negative electrode voltage control module, the reference voltages U of the upper and lower arms of the negative electrode MMC are obtained according to the following method prefabc3 and U nrefabc3 ; First, the d-axis and q-axis voltages U gdq are controlled by PI controllers to respectively follow the given reference values U gdref and U gqref , and U gqref is set to 0. After passing through the limiting link, the outputs of the PI controllers serve as the reference values I gdref3 and I gqref3 of the negative d-axis and q-axis currents; then, the negative d-axis and q-axis currents I gdq2 are controlled by PI controllers to follow the reference values I gdref3 and I gqref3 . The outputs of the PI controllers serve as the reference differential-mode voltage U difdq3 of the negative MMC. After Park inverse transformation, the reference differential-mode voltage U difabc3 in the stationary three-phase coordinate system is obtained; next, the internal circulating current I cabc2 inside the negative MMC is controlled by a resonant controller, and the output of the internal circulating current controller of the negative serves as the reference common-mode voltage U comabc3 of the negative MMC; finally, using the reference differential-mode voltage U difabc3 and the reference common-mode voltage U comabc3 , the reference voltages U prefabc3 and U nrefabc3 of the upper and lower arms of the negative MMC are obtained through calculation.

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