CN111431206A

CN111431206A - Cooperative fault ride-through method for large-scale double-fed wind power plant through flexible Direct Current (DC) outgoing

Info

Publication number: CN111431206A
Application number: CN202010271195.9A
Authority: CN
Inventors: 李卫星; 朱蒙; 关万琳; 陈晓光
Original assignee: Dalian University of Technology; Harbin Institute of Technology
Current assignee: Dalian University of Technology; Harbin Institute of Technology
Priority date: 2020-04-08
Filing date: 2020-04-08
Publication date: 2020-07-17
Anticipated expiration: 2040-04-08
Also published as: CN111431206B

Abstract

A cooperative fault ride-through method for a large-scale double-fed wind power plant through flexible direct current outgoing belongs to the technical field of new energy alternating current and direct current grid-connected control. The method solves the problem that the cooperative fault ride-through of the large-scale double-fed wind power plant through flexible direct current delivery cannot be effectively realized by adopting the existing method. The method comprises the following specific processes: during a fault period, the sending end converter station inhibits direct-current voltage fluctuation and realizes self-adaptive balance of direct-current power through two-stage voltage reduction control, and the double-fed wind turbine generator set inhibits rotor overcurrent and direct-current components of stator current through transient current correction control; the two-stage step-down control consists of stepped step-down control and voltage droop control, and the modified transient current control consists of voltage type active current reduction control and feedforward transient stator current control. The method can be applied to cooperative fault ride-through of a large-scale double-fed wind power plant through flexible direct current outgoing.

Description

Cooperative fault ride-through method for large-scale double-fed wind power plant through flexible Direct Current (DC) outgoing

Technical Field

The invention belongs to the technical field of new energy alternating current-direct current grid-connected control, and particularly relates to a cooperative fault ride-through method for a large-scale double-fed wind power plant through flexible direct current delivery.

Background

In recent years, development and utilization of wind power are rapidly developed, and the problem of large-scale wind power transmission and grid connection becomes an important research topic. DFIG has become the mainstream model of the wind power market by virtue of its advantages of high power generation efficiency and low converter capacity. The flexible direct current transmission (MMC-HVDC) technology based on the modular multilevel converter has the advantages of high modularization degree, good waveform quality, small occupied area and the like, and is an effective mode for large-scale wind power integration. However, the fault-ride-through (FRT) capability of doubly-fed wind farms and dc transmission systems has a significant impact on system operational stability and equipment operational safety. Therefore, research needs to be carried out on an FRT control method of a flexible direct current transmission system connected with a double-fed wind power plant.

After a grid fault, the control objective of the system is to prevent dc overvoltages. The main realization mode is as follows: the power transmission capability of a grid side converter station (GSMMC) is improved through alternating current system transient reconstruction, or redundant energy storage of a direct current equivalent capacitor is released through direct current system transient reconstruction, or wind power is rapidly reduced. The alternating current system transient reconstruction is realized by adding a series transformer and a mechanical switch at a receiving end, and the direct current transient reconstruction is realized by adding an unloading circuit. The defects of the scheme are as follows: the response delay of the mechanical switch can reduce the rapidity of the transient reconstruction of the alternating current system, and additional equipment such as a switch, a series transformer and an unloading resistor is introduced, so that the occupied area and the investment cost are increased. The scheme for rapidly reducing the wind power can be divided into the following steps according to different principles: fast load shedding control, frequency up control and voltage down control based on communication. Among them, the communication-based load shedding control causes a response delay, and the system reliability is lowered when the communication fails. The frequency increasing control and the voltage reducing control are both realized by a wind power plant side converter station (WFMMC). The up-conversion control is limited by the tolerance of the wind turbine to the rate of change of frequency, reducing the de-rating efficiency, resulting in significant dc over-voltages. The implementation modes of the voltage reduction control are two types: firstly, short circuit is simulated and demagnetization control is applied to the wind power plant. But the resulting inrush current and rotor overcurrent are a critical issue. In addition, the step-down voltage is too low, which can cause the step-out of the wind turbine; and secondly, droop voltage reduction control is adopted, and additional load reduction control is introduced into the wind turbine generator. However, the scheme cannot realize demagnetization of the wind turbine generator, and the direct-current component injection can cause serious direct-current voltage oscillation generated by MMC-HVDC, which endangers the stable operation of a direct-current system.

Therefore, the cooperative fault ride-through of the large-scale double-fed wind power plant through flexible direct current delivery cannot be effectively realized by adopting the existing method.

Disclosure of Invention

The invention aims to solve the problem that the cooperative fault ride-through of a large-scale double-fed wind power plant through flexible direct current delivery cannot be effectively realized by adopting the conventional method, and provides a cooperative fault ride-through method of the large-scale double-fed wind power plant through flexible direct current delivery.

The technical scheme adopted by the invention for solving the technical problems is as follows: a cooperative fault ride-through method for a large-scale double-fed wind power plant through flexible direct current outgoing comprises the following steps:

during a fault period, the sending end converter station inhibits direct-current voltage fluctuation and realizes self-adaptive balance of direct-current power through two-stage voltage reduction control, and the double-fed wind turbine generator set inhibits rotor overcurrent and direct-current components of stator current through transient current correction control;

the two-stage step-down control consists of stepped step-down control and voltage droop control, and the modified transient current control consists of voltage type active current reduction control and feedforward transient stator current control.

The invention has the beneficial effects that: the invention provides a cooperative fault ride-through method for a large-scale double-fed wind power plant through flexible Direct Current (DC) delivery, which can quickly reduce wind power after a power grid fault and ensure that DC voltage is not out of limit; the phenomena of unstable synchronization and transient overcurrent of the wind power plant are avoided, and the running safety of the direct current converter station and the wind power generator set is improved; the cooperative fault ride-through of the large-scale double-fed wind power plant through flexible direct current delivery is effectively realized.

Moreover, the size of the MMC-HVDC sub-module capacitor can be reduced to 80% of the original size, the use of a crowbar circuit in a wind turbine generator is avoided, the equipment investment is further saved, and the economic benefit is improved.

Drawings

FIG. 1 is a schematic diagram of a doubly-fed wind farm access MMC-HVDC system;

wherein: WF stands for equivalent doubly-fed wind turbine generator (DFIG), SEMMC and REMMC stand for sending-end and receiving-end converter stations, respectively, Z_cAnd Z_lRespectively representing equivalent impedances, V, of the current-collecting network and the transmission line_wRepresenting the stator terminal voltage of the wind turbine generator; p_wRepresenting the output active power, V, of an equivalent fan_dchRepresenting the converter station dc voltage; v_gRepresenting GSMMC grid-connected point AC voltage, P_gAnd Q_gThe active and reactive power transmitted by REMMC to the grid are respectively.

FIG. 2 is a schematic diagram of the control method of the present invention;

wherein: RSC represents a rotor side converter of the wind turbine generator, and GSC represents a grid side converter of the wind turbine generator;

I_wrepresenting wind farm output current; i.e. i_sdAnd i_sqRespectively representing d-axis and q-axis components of stator current of the wind turbine generator; v^normalRepresenting the grid-connected voltage of the wind power plant in normal operation; v_dref、V_qrefD-axis and q-axis components of the control voltage reference for the sending end converter station, respectively, L PF stands for low pass filter.

FIG. 3 is a schematic diagram of the MMC-HVDC direct voltage response curve obtained by the TVDC and MTCC methods proposed by the present invention during a fault;

FIG. 4 is a schematic diagram of a wind turbine generator stator terminal voltage response curve obtained by the TVDC and MTCC method according to the present invention during a fault period;

FIG. 5 is a schematic diagram of an active current response curve of a wind turbine generator obtained by the TVDC and MTCC method according to the present invention during a fault period;

the current of a rotor d shaft is normal working;

FIG. 6 is a schematic diagram of a stator terminal voltage d-axis component response curve obtained by the SVDC method according to the present invention during a fault period;

FIG. 7 is a schematic diagram of a stator terminal voltage q-axis component response curve obtained by the SVDC method of the present invention during a fault;

FIG. 8 is a phasor diagram of the voltage at each time under the SVDC method;

is t₂～t₃The stator terminal voltage magnitude phasors for a time period,

is t₃～t₄The stator terminal voltage magnitude phasors for a time period,

stator terminal voltage phasor in normal operation;

FIG. 9 is a schematic diagram of a VDACC curve proposed by the present invention during a fault;

FIG. 10 is a schematic diagram of the FTSCC method according to the present invention during a failure;

wherein: k_rFor resonant control of gain, z is damping ratio, i_rdFor rotor d-axis current, i_rqFor rotor q-axis current, s is complex frequency, omega_slipAnd the slip is sigma, the magnetic leakage coefficient is sigma, PI + R represents proportional-integral resonance control, dq/abc represents Park inverse transformation, and RPC represents reactive power control.

FIG. 11 is a block diagram of the REMMC output active power P during a fault period using the method of the present invention and the exemplary voltage reduction method_gA simulation comparison graph;

FIG. 12 is a comparison graph of DC voltage simulation during a fault using the method of the present invention and a typical buck method;

FIG. 13 is a wind farm frequency f during a fault using the method of the present invention and a typical voltage reduction method_wA simulation comparison graph;

FIG. 14 is a wind farm grid-connected voltage V during a fault period using the method of the present invention and a typical voltage reduction method_wA simulation comparison graph;

FIG. 15 is a wind farm output current i during a fault using the method of the present invention and an exemplary voltage reduction method_wA simulation comparison graph;

FIG. 16 is a graph of wind farm output power P during a fault period using the method of the present invention and an exemplary voltage reduction method_wA simulation comparison graph;

FIG. 17 is a d-axis component V of grid-connected voltage of a wind farm during a fault period using the method of the present invention and a typical voltage reduction method_wdA simulation comparison graph;

FIG. 18 is a graph of a wind farm grid-connected voltage q-axis component V during a fault period using the method of the present invention and a typical voltage reduction method_wqA simulation comparison graph;

FIG. 19 shows the d-axis component ψ of the wind turbine flux linkage during a fault period using the method of the present invention and the exemplary voltage reduction method_sdA simulation comparison graph;

FIG. 20 is a diagram illustrating the q-axis component ψ of the wind turbine flux linkage during a fault period using the method of the present invention and the exemplary voltage reduction method_sqA simulation comparison graph;

FIG. 21 is a diagram of the d-axis component I of the stator current of a wind turbine during a fault period using the method of the present invention and a typical voltage reduction method_sdA simulation comparison graph;

FIG. 22 is a graph of the wind turbine generator stator current q-axis component I during a fault period using the method of the present invention and a typical voltage reduction method_sqA simulation comparison graph;

FIG. 23 is a wind turbine rotor current I during a fault period using the method of the present invention and a typical voltage reduction method_rA simulation comparison graph;

FIG. 24 is a graph of the rotor voltage V required by a wind turbine during a fault period using the method of the present invention and a typical voltage reduction method_rAnd (5) simulating a comparison graph.

Detailed Description

The first embodiment is as follows: the cooperative fault ride-through method for the large-scale double-fed wind power plant through flexible direct current outgoing comprises the following steps:

Fig. 1 shows a structure of a typical double-fed wind farm access flexible direct current transmission system. In normal operation, SEMMC controls the amplitude and frequency of the wind farm grid-connected voltage, while REMMC controls the dc voltage and the reactive power exchanged with the grid. When the power grid fails, REMMC provides extra reactive current to support the power grid voltage, and the direct current voltage can rise rapidly due to the unbalanced direct current power. Therefore, the SEMMC starts the voltage reduction control to reduce the wind power so as to prevent direct-current overvoltage.

For a flexible direct-current power transmission system connected with a double-fed wind power plant, safe and stable operation of an MMC converter station is ensured in an FRT process, and direct-current voltage ripples are effectively inhibited; meanwhile, Crowbar action of the DFIG rotor is prevented, and the synchronous stability of the wind power system is improved.

Therefore, the FRT control target of the DFIG wind power plant accessing the MMC-HVDC system is as follows:

t1) the direct voltage of MMC-HVDC is in a safe range;

t2) the injection current of MMC-HVDC is in a safe range, and the direct-current component of the stator current of the DFIG is effectively inhibited;

t3) DFIG rotor current is in the allowable range;

t4) power self-adaptive balancing of two ends of direct current after fault;

in order to realize the control target 1, effective voltage reduction control can be implemented on the wind power system through the SEMMC so as to rapidly reduce the wind power. In order to achieve the goal 2, effective demagnetization control needs to be applied to the DFIG, and the transient component of the output current of the wind turbine generator is reduced. To achieve goal 3, the rotor current of the wind turbine needs to be reduced during the step-down. To achieve goal 4, adaptive voltage control needs to be introduced in the buck control loop.

Fig. 2 is a proposed FRT control system architecture. The proposed solution consists of two parts: two-stage buck control (TVDC) for SEMMC and Modified Transient Current Control (MTCC) for DFIG. Wherein the two-stage buck control consists of a stepped buck control (SVDC) and a Voltage Droop Control (VDC). The SVDC aims to realize the reduction of wind power and restrain the stator free flux linkage of the DFIG; the function of the Voltage Droop Control (VDC) is to realize the rapid balance of the active power of the direct current system after the SVDC process is finished. The Modified Transient Current Control (MTCC) of the RSC of the wind turbine generator consists of a voltage type active current reduction control (VDACC) and a feedforward transient current control (FTSCC). The VDACC is used for adjusting the rotor current and preventing MMC-HVDC overvoltage caused by step loss of a wind power system; the purpose of the FTSCC is to further suppress the dc component of the stator current.

FIGS. 3, 4, and 5 show the proposed FRT control strategy applied at t₁MMC-HVDC direct-current voltage V after three-phase short-circuit fault of PCC at moment_dchTerminal voltage V of DFIG stator_sAnd an active current i_rdrefTime domain response curve of (1). t is t₂Time direct voltage V_dchReaches its protective action threshold

Then, ISMMC initiates a two-stage buck control, where t₂～t₄Stage is SVDC, at t₄The time instant switches to VDC. Meanwhile, VDACC of DFIG is at t₂And the SVDC is activated after the moment, and the reduction of the active power output by the fan is realized in cooperation with the SVDC. t is t₄After the moment, VDACC acts on V according to VDC_sRapidly adjust its output power P_wRealization of P_wAnd P_gAdaptive balancing of (2). At this time V_dchRise to the maximum value

And V_sAnd i_rdRespectively reduced to the minimum

And

the second embodiment is as follows: the first difference between the present embodiment and the specific embodiment is: the specific process of the step-type pressure reduction control is as follows:

the stepped buck control strategy is shown in fig. 6 and 7. FIG. 6 and FIG. 7 show stator terminal voltage d-axis component V_sdAnd q-axis component V_sqTime domain response curve of (1).

At t₁At the moment, the PCC has a three-phase short-circuit fault, at t₂Time of day, DC voltage V_dchReaching a protection action threshold

And the moment when the first step reduction occurs is defined as t₃The second step-down occurs at a time t₄As can be seen from fig. 6 and 7, the stator terminal voltage V at each period_sAre respectively expressed as d-axis and q-axis components

In the formula, t represents time, V_sdRepresenting stator terminal voltage V_sThe d-axis component of (a) is,

representing the stator terminal voltage during normal operation,

is t₂～t₃The d-axis component of the stator terminal voltage magnitude of the time period,

is t₂～t₃The magnitude of the stator terminal voltage over a period of time,

is t₃～t₄The d-axis component of the stator terminal voltage magnitude of the time period,

is t₃～t₄Stator terminal voltage amplitude of time period, theta₁Represents t₂～t₃Phase angle of stator terminal voltage in time interval;

V_sqrepresenting stator terminal voltage V_sThe q-axis component of (a) is,

is t₂～t₃A q-axis component of stator terminal voltage amplitude of the time period;

the formulas (1) and (2) show that the stator voltage amplitude and the phase of the wind turbine generator are t₂～t₄The duration is continuously varied.

Thus, stator terminal voltage phasor

Is shown as

In the formula (I), the compound is shown in the specification,

e is the natural logarithm, j is the unit of the imaginary number, omega_sIs the synchronous angular frequency;

stator flux linkage vector psi corresponding to different voltage drop degrees_sExpressed as:

in the formula I_rRepresenting rotor current, R_sAnd L_sStator resistance and inductance, L respectively_mFor generator excitation inductance, C₁And C₂Respectively represents t₂And t₃Time stator free flux linkage psi_snAn initial value of (1);

from the formula (4), at t₂～t₃And t₃～t₄Time period, psi_sAre all free components psi_sn(first term) and the mandatory component ψ_sf(item two) composition;

for a MW class wind turbine, R_sAre small. To simplify the analysis, assume the rotor current I_rAt t₂To t₄The time interval is not changed, and C is obtained according to the flux linkage conservation principle₁And C₂Expression (c):

in formula (5), (t) is₃-t₂) Defined as the duration of SVDC.

As can be seen from the formulas (4) and (5), if at t₃At a moment of time satisfies psi_snIf 0, then C must be present₂0, therefore, must satisfy

For visual explanation, the phasor expression corresponding to the formula (13) obtained by the phasor method is shown as

FIG. 8 shows θ₁And the phasor diagram corresponding to the SVDC is larger than or equal to 0. As can be seen from FIG. 8, if t is the same₃At the moment of time, the time of day,

and

are of the same amplitude and in opposite directions, then C₂And 0 holds.

Therefore, when

And theta₁When the value of (b) satisfies the phasor relationship corresponding to equation (13) or fig. 8, the embodiment can simultaneously realize the reduction of the wind power and the demagnetization of the doubly-fed wind turbine generator.

The third concrete implementation mode: the second embodiment is different from the first embodiment in that: the specific process of the voltage droop control stage is as follows:

t₃after a short interval of time at t₄And at the moment, the SEMMC is switched to a voltage droop control mode to realize the rapid balance of the power at two ends of the direct current system. At this stage, V_sq＝0，V_sdAnd V_dchDesigned to be linear, i.e.

In the formula (I), the compound is shown in the specification,

representing the voltage reduction control exiting the corresponding DC voltage threshold, K_FRTIs the sag factor, K_FRTIs expressed as

In the formula (I), the compound is shown in the specification,

the upper limit of the MMC-HVDC direct-current voltage is mainly determined by the overvoltage tolerance level of the sub-module capacitor,

represents the lower limit of the depressurization depth.

The fourth concrete implementation mode: the third difference between the present embodiment and the specific embodiment is that: the specific process of the voltage type active current reduction control is as follows:

FIG. 9 shows a VDACC curve suitable for use in a DFIG. When the d-axis voltage of the power grid is oriented, the d-axis component of the rotor current is controlledTo control the active power emitted by the DFIG, so in fig. 9, the rotor d-axis current reference i_rdrefRepresenting a reference value of active current of the wind turbine generator during voltage reduction;

rotor d-axis current reference value i_rdrefIs expressed as

In the formula (I), the compound is shown in the specification,

the voltage threshold for switching from constant power control mode to VDACC mode,

representing the d-axis current of the rotor in normal operation;

to prevent conflict with the buck control, the rotor q-axis current reference i_rqrefIs expressed as

i_rqref＝-V_s/L_m(9)。

The fifth concrete implementation mode: the fourth difference between this embodiment and the specific embodiment is that: the specific process of the feedforward transient stator current control is as follows:

in the design process of SVDC, the wind farm frequency and DFIG rotor current are assumed to be unchanged during the buck period. But in fact, V_sqThe change in (c) may cause the frequency of the islanded grid to drift. Moreover, since the DFIG is switched to the VDACC mode, the rotor current value also changes. Thus, ψ_snCannot be completely eliminated, which implies that the stator current still contains a certain dc component. To further suppress the influence of the dc current component on the MMC-HVDC dc voltage fluctuations, the stator current may generally be proportional to the free component of the rotor current.

Establishing a proportional relation of free components of the stator current and the rotor current:

I_rn＝rI_sn(10)

in which r is a scaling factor, I_rnRepresenting rotor currentsFree component, I_snRepresenting the free component of the stator current, the stator free flux linkage psi_snIs shown as

ψ_sn＝L_mI_sn+L_rI_rn(11)

In the formula, L_rIs a generator stator inductance;

by substituting formula (11) for formula (10) to obtain

As shown in formula (12), r>When 0, through reasonably selecting r, I can be obviously reduced_snThe value of (c). However, in the process of separating the free component of the stator current, delay is introduced in the filtering link, and amplitude and phase errors are increased in the transient state, so that the dynamic control effect is poor. Moreover, the active power regulation performance of the DFIG is inevitably influenced by the indirect control of the free component of the stator current through a rotor current control loop.

To overcome this problem, a feed Forward Transient Stator Current Control (FTSCC) method as shown in fig. 10 is proposed. The implementation method of the FTSCC may be described as follows:

the method comprises the following steps: d-axis component i of stator current_sdQ-axis component i_sqGenerating transient compensation voltage through a second-order band-pass filter;

step two: introducing the voltage transient compensation term in the step one as additional feedforward compensation at the output end of the RSC current regulator, so that a control closed loop is formed to ensure the correct orientation of the output voltage of the rotor converter and the transient induction voltage; when the voltage at the stator terminal is reduced, more accurate transient voltage compensation is provided on the rotor circuit, so that the direct current component of the stator current is restrained;

step three: the rotor current control loop also adopts a resonance control link, the resonance frequency is 50Hz, and the resonance control link is used for improving the d-axis component i of the rotor current_rdAnd rotor current q-axis component i_rqThe tracking accuracy of the command value reduces the ripple component in the rotor current.

In order to verify the effectiveness of the coordinated control strategy of the invention, a simulation model shown in FIG. 1 is built on a PSCAD/EMTDC platform.

Considering the most severe conditions: the wind power plant operates according to the maximum power, and a three-phase short-circuit fault is simulated at a grid-connected point of the REMMC. Lower limit of decompression depth is taken during simulation

And the equivalent capacity inertia time constant H of the MMC-HVDC is 36 ms. Taking in simulation

θ₁0.3218 (corresponding to the depressurization duration Δ t)_2-3＝T_s/4). The scheme of the present invention is compared to two typical buck control strategies. A typical buck control scheme can be described as: the first scheme is as follows: applying demagnetization control on the wind power plant only through the SEMMC without changing the control mode of the DFIG, and reducing the voltage of the island power grid to 0; scheme II: the voltage droop control is employed while reducing the torque command value of the DFIG to 0. The simulation results obtained are shown in fig. 11-16.

Fig. 11-16 show the dynamic variation of the electrical quantities of the dc system during a fault. As shown in fig. 11, the real power output by REMMC after grid fault is reduced to 0. As can be seen from fig. 12, in case of the scheme one, the dc voltage continues to increase because the islanded grid loses step (fig. 13), resulting in the voltage V of the islanded grid_w(FIG. 14) and active Power P_w(FIG. 16) cannot be lowered to 0. And the wind power plant transient output current i_w(fig. 15) a very high value is reached. When the second scheme is adopted, because the DFIG is not demagnetized in the voltage reduction process, the direct current voltage (figure 12) and the active power (figure 16) output by the wind farm fluctuate dramatically. Compared with the scheme, the scheme of the invention realizes the demagnetization control of the DFIG through the SVDC and the FTSCC, and prevents the direct-current voltage fluctuation; the synchronization stability of the islanded grid is guaranteed by introducing a VDACC to the DFIG (FIG. 13), and wind farm output overcurrent is prevented (FIG. 15).

FIGS. 17-24 illustrate dynamic responses within a DFIG unit under different buck control schemes. As can be seen from fig. 17 and 18, both the first and the second embodiments of the present invention achieve demagnetization of the DFIG by simultaneously changing d-axis and q-axis components of the islanded grid voltage, so that the free component of the stator flux linkage can be significantly suppressed (fig. 19 and 20). And in the second scheme, only the d-axis component of the island grid voltage is reduced, and the demagnetization function is not realized, so that the stator flux linkage fluctuation is obvious. It should be noted that in the solution of the present invention, the stator flux linkage still has some fluctuation, which is caused by SEMMC switching to droop control. As can be seen from fig. 21 and 22, the fluctuation of the stator current can be sufficiently suppressed by the proposed scheme, which is a result of the combined action of SVDC and FTSCC. As can be seen from fig. 23 and 24, the rotor transient current is minimized and the required rotor voltage is also minimized by using the proposed solution.

The above-described calculation examples of the present invention are merely to explain the calculation model and the calculation flow of the present invention in detail, and are not intended to limit the embodiments of the present invention. It will be apparent to those skilled in the art that other variations and modifications of the present invention can be made based on the above description, and it is not intended to be exhaustive or to limit the invention to the precise form disclosed, and all such modifications and variations are possible and contemplated as falling within the scope of the invention.

Claims

1. The cooperative fault ride-through method for the large-scale double-fed wind power plant through flexible direct current outgoing is characterized by comprising the following steps:

2. The cooperative fault ride-through method for the large-scale doubly-fed wind farm through the flexible direct current outgoing according to claim 1, characterized in that the specific process of the stepped voltage reduction control is as follows:

And the moment when the first step reduction occurs is defined as t₃The second step-down occurs at a time t₄Stator terminal voltage V at each period_sAre respectively expressed as d-axis and q-axis components

In the formula, t represents time, V_sdRepresenting stator terminal voltage V_sD-axis component of (V)_s ^normalRepresenting the stator terminal voltage during normal operation,

is t₂～t₃D-axis component of stator terminal voltage amplitude, V, of time period_s ^drop1Is t₂～t₃The magnitude of the stator terminal voltage over a period of time,

is t₃～t₄D-axis component of stator terminal voltage amplitude, V, of time period_s ^drop2Is t₃～t₄Stator terminal voltage amplitude of time period, theta₁Represents t₂～t₃Phase angle of stator terminal voltage in time interval;

V_sqrepresenting stator terminal voltage V_sThe q-axis component of (a) is,

stator terminal voltage phasor

Is shown as

In the formula (I), the compound is shown in the specification,

at t₂～t₃And t₃～t₄Time period, psi_sAre all free components psi_snAnd the mandatory component psi_sfComposition is carried out;

rotor current I_rAt t₂To t₄The time interval is not changed, and C is obtained according to the flux linkage conservation principle₁And C₂Expression (c):

3. the cooperative fault ride-through method for the large-scale doubly-fed wind farm through the flexible direct current outgoing according to claim 2, wherein the specific process of the voltage droop control stage is as follows:

at t₄Moment, SEMMC switches to voltage droop control mode, V_sq＝0，V_sdAnd V_dchDesigned to be linear, i.e.

In the formula (I), the compound is shown in the specification,

In the formula (I), the compound is shown in the specification,

is the upper limit of the MMC-HVDC direct voltage, V_s ^minlimRepresents the lower limit of the depressurization depth.

4. The cooperative fault ride-through method for the large-scale doubly-fed wind farm through the flexible direct current outgoing according to claim 3, characterized in that the specific process of the voltage type active current reduction control is as follows:

rotor d-axis current reference value i_rdrefIs expressed as

In the formula, V_s ^thresThe voltage threshold for switching from constant power control mode to VDACC mode,

representing d-axis electricity of rotor in normal operationA stream;

rotor q-axis current reference value i_rqrefIs expressed as

i_rqref＝-V_s/L_m(9)。

5. The cooperative fault ride-through method for the large-scale doubly-fed wind farm through the flexible direct current outgoing according to claim 4, characterized in that the specific process of the feedforward transient stator current control is as follows:

step two: introducing the voltage transient compensation term in the step one as additional feedforward compensation at the output end of the RSC current regulator, so that a control closed loop is formed to ensure the correct orientation of the output voltage of the rotor converter and the transient induction voltage; when the voltage of the stator terminal is reduced, transient voltage compensation is provided on the rotor circuit, and the direct current component of the stator current is restrained;

step three: the rotor current control loop adopts a resonance control link with a resonance frequency of 50Hz and is used for improving a d-axis component i of the rotor current_rdAnd rotor current q-axis component i_rqThe tracking accuracy of the command value reduces the ripple component in the rotor current.