A method and system for establishing an equivalent model of a doubly-fed wind turbine group

Abstract

The present invention relates to a method and system for establishing an equivalent model of a doubly-fed wind turbine group. Based on the low voltage ride-through characteristics of the doubly-fed wind turbine group, the active power and reactive power of the doubly-fed wind turbine group during the low voltage ride-through period are adjusted, which can not only suppress the doubly-fed wind turbine group from being disconnected from the grid at overspeed during the LVRT period, but also ensure that the doubly-fed wind turbine group provides reactive power support to the power grid during the LVRT period in accordance with the requirements of the National Standard of the People's Republic of China GB / T36995‑2018 "Test Procedure for Fault Voltage Ride-through Capability of Wind Generator Groups". On this basis, a doubly-fed wind turbine group equivalent method is proposed. The method reduces the order of the doubly-fed wind turbine group model, and can improve the simulation speed while ensuring the simulation accuracy.

Description

Technical Field

[0001] The present invention relates to the technical field of wind power networking, and in particular to a method and system for establishing an equivalent model of a double-fed wind turbine group. Background Art

[0002] Large-scale development and utilization of renewable energy is an important way to deal with the depletion of fossil fuels and the greenhouse effect. Wind energy is currently the most commercially viable non-hydro renewable energy source, and wind power grid-connected power is an important way to develop and utilize wind energy on a large scale.

[0003] As the scale of wind power grid connection continues to increase, wind turbines are disconnected from the grid due to grid failures, which brings power shock to the connected grid and, in serious cases, endangers the safe operation of the wind power system. The lack of low voltage ride-through (LVRT) capability of wind turbines is considered to be the main reason for wind turbines disconnecting from the grid. Therefore, mastering the low voltage ride-through characteristics of wind turbines is an important prerequisite for ensuring the safe and stable operation of the power grid.

[0004] Digital simulation is one of the important means to analyze the low voltage ride-through characteristics of wind turbine groups. However, its analysis accuracy depends on the detail of the mathematical model of the wind turbine group. If a detailed model is used for each wind turbine in the wind turbine group, there will be problems of large simulation calculation amount and low simulation analysis efficiency. In serious cases, a suitable solution may not be obtained.

[0005] Therefore, there is an urgent need in this field for a technical solution that can both ensure simulation accuracy and improve simulation speed. Summary of the invention

[0006] The purpose of the present invention is to provide a method and system for establishing an equivalent model of a doubly-fed wind turbine group, which uses the equivalent model to reduce the order of the wind turbine group model and improve the simulation speed while ensuring accuracy, so as to solve the current problems of large simulation calculation amount and low simulation analysis efficiency.

[0007] To achieve the above object, the present invention provides the following solutions:

[0008] A method for establishing an equivalent model of a doubly-fed wind turbine group, the method comprising:

[0009] According to the initial operating speed, the wind turbines are grouped to obtain several groups;

[0010] Calculating the average active power and the average reactive power output by each of the generator groups during the low voltage ride-through period;

[0011] The average value of the active power and the average value of the reactive power output by each of the said groups during the low voltage ride-through period are regarded as the active power and the reactive power output by each equivalent single machine during the low voltage ride-through period;

[0012] The output power of each equivalent single machine during the low voltage ride-through period is calculated according to the active power and reactive power output by each equivalent single machine during the low voltage ride-through period to obtain an equivalent model.

[0013] Optionally, the wind turbines are grouped according to the initial operating speed to obtain a plurality of groups, specifically including:

[0014] According to the initial operating speed, wind turbines in the same speed range are divided into the same group to obtain a plurality of groups.

[0015] Optionally, the calculating the average active power and the average reactive power output by each of the generator sets during the low voltage ride-through period specifically includes:

[0016] Suppose there are n clusters C1, C2, ..., C n ; One of the clusters C i (i=1,2,……,n) contains m wind turbines, and the active power output of each wind turbine during low voltage ride-through is P1,P2,……,P m The output reactive power is Q1, Q2, ..., Q m , then the average active power output of the Ci unit during the low voltage ride-through period is The average reactive power is Wherein, t represents the tth wind turbine.

[0017] Optionally, the control method of the wind turbine set during low voltage ride-through includes:

[0018] When the grid voltage drops and causes the rotor current to exceed the preset current value, the protection circuit is activated for protection;

[0019] Block the rotor-side converter control pulses to put the wind turbine into low voltage ride-through mode;

[0020] When the rotor current decays to the current protection exit value, the protection of the protection circuit is exited and the rotor-side converter is restarted;

[0021] Setting the power output of the wind turbine generator set;

[0022] When the voltage returns to the voltage setting value, the wind turbine generator set is controlled to return to the normal operation mode.

[0023] Optionally, the preset current value is 1.9 times the rated current of the rotor;

[0024] The current protection exit value is 1.7 times the rated current;

[0025] The voltage setting value is 0.9 times the rated value of the stator phase voltage.

[0026] Optionally, the adjusting the power output of the wind turbine generator set specifically includes:

[0027] Calculate the minimum active power to ensure that the wind turbine generator set does not go off-grid due to overspeed;

[0028] The active power reference value and the reactive power reference value of the wind turbine generator set are adjusted according to the minimum active power.

[0029] Optionally, the calculation of the minimum active power for ensuring that the wind turbine generator set does not experience overspeed and grid disconnection specifically includes:

[0030] The minimum active power calculation formula is used to calculate the minimum active power to ensure that the wind turbine generator set does not exceed the speed limit and disconnect from the grid;

[0031] The minimum active power calculation formula is: in The minimum active power per unit value to ensure that the wind turbine does not exceed the grid-connected state, T m is the mechanical torque, the superscript * indicates the per unit value, T j is the inertia time constant, ω1 is the synchronous angular velocity, Ω rlim is the overspeed protection setting value of the wind turbine, Ω r (0) is the rotor mechanical angular velocity at time 0, and the voltage drop time is time 0.

[0032] Optionally, the step of setting the active power reference value and the reactive power reference value of the wind turbine generator set according to the minimum active power specifically includes:

[0033] when hour, is the per-unit value of the minimum active power, and the per-unit value of the active power emitted by the stator is s *=0 to reduce the risk of overspeed disconnection. According to the boundary conditions, the reference power setting result is: in and is the per unit value of the active power reference value and the per unit value of the reactive power reference value emitted by the stator, S n is the rated power of the unit, i cbset is the current preset value, U1 is the effective value of the stator phase voltage, L s1 is the stator leakage inductance, L m is the mutual inductance, k is an intermediate variable for the convenience of discussion,

[0034] when When , the inequality is solved to get: make U 1tip To make and There is a minimum value of U1 for the real number solution, and the reference power setting result is:

[0035] When P smin * >ki cbset / S n When the reactive power compensation device is put into operation, the effective value of the stator phase voltage U1 is increased, so that U1 ≥ U 1tip , the reference power setting result is:

[0036] A system for establishing an equivalent model of a doubly-fed wind turbine group, the system comprising:

[0037] A grouping module is used to group wind turbines according to their initial operating speed to obtain several groups;

[0038] An active and reactive power calculation module, used to calculate the average active power and the average reactive power output by each of the generators during the low voltage ride-through period;

[0039] An equivalent conversion module, used for treating the average value of active power and the average value of reactive power output by each of the machine groups during the low voltage ride-through period as the active power and reactive power output by each equivalent single machine during the low voltage ride-through period;

[0040] The equivalent model establishment module is used to calculate the output power of each equivalent single machine during the low voltage ride-through period according to the active power and reactive power output by each equivalent single machine during the low voltage ride-through period to obtain an equivalent model.

[0041] Optionally, also include:

[0042] The voltage drop judgment module is used to activate the protection circuit for protection when the grid voltage drops and causes the rotor current to exceed the preset current value;

[0043] Converter blocking module, used to block the rotor-side converter control pulses, and the wind turbine enters the low voltage ride-through mode;

[0044] A converter restart module, used to exit the protection of the protection circuit and restart the rotor-side converter when the rotor current decays to the current protection exit value;

[0045] A power setting module, used for setting the power output of the wind turbine;

[0046] The operation determination module is used to return the wind turbine set to a normal operation mode when the voltage recovers to a voltage setting value.

[0047] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0048] The method and system for establishing an equivalent model of a doubly-fed wind turbine group provided in an embodiment of the present invention reduce the model order of the doubly-fed wind turbine group and can improve the simulation speed while ensuring the simulation accuracy. The existing doubly-fed wind turbine group equivalent method does not consider the LVRT characteristics of the doubly-fed wind turbine set. The method provided by the present invention considers the LVRT characteristics of the doubly-fed wind turbine set. At the same time, the amount of simulation calculation is reduced, and the simulation speed is accelerated while ensuring the accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0050] Figure 1 A flow chart of a method for establishing an equivalent model of a doubly-fed wind turbine group provided in Embodiment 1 of the present invention.

[0051] FIG. 2( a ) is a schematic diagram of the equivalent model of a wind turbine group provided in the first embodiment of the present invention before being equivalent.

[0052] FIG. 2( b ) is a schematic diagram of the equivalent model of the wind turbine group provided in the first embodiment of the present invention.

[0053] Figure 3 This is a flow chart for setting the power output of a wind turbine generator set provided in Embodiment 1 of the present invention.

[0054] Figure 4 This is a control block diagram of a rotor-side converter provided in Embodiment 1 of the present invention.

[0055] Figure 5 A system block diagram of a system for establishing an equivalent model of a doubly-fed wind turbine group provided in Embodiment 1 of the present invention. DETAILED DESCRIPTION

[0056] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0057] The purpose of the present invention is to provide a method and system for establishing an equivalent model of a doubly-fed wind turbine group, which uses the equivalent model to reduce the order of the wind turbine group model and improve the simulation speed while ensuring accuracy, so as to solve the current problems of large simulation calculation amount and low simulation analysis efficiency.

[0058] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0059] like Figure 1 As shown, an embodiment of the present invention provides a method for establishing an equivalent model of a doubly-fed wind turbine group, the method comprising:

[0060] S1. Grouping wind turbines according to their initial operating speed to obtain several wind turbine groups;

[0061] Specifically, according to the initial operating rotation speed, wind turbine sets in the same rotation speed range are divided into the same group to obtain a plurality of groups.

[0062] S2. Calculate the average active power and the average reactive power output by each of the generator sets during the low voltage ride-through period;

[0063] Specifically, suppose there are n clusters C1, C2, ..., C n ; One of the clusters C i (i=1,2,……,n) contains m wind turbines, and the active power output of each wind turbine during low voltage ride-through is P1,P2,……,P m The output reactive power is Q1, Q2, ..., Q m , then the average active power output of the Ci unit during the low voltage ride-through period is The average reactive power is Wherein, t represents the tth wind turbine.

[0064] S3, considering the average value of the active power and the average value of the reactive power output by each of the groups during the low voltage ride-through period as the active power and reactive power output by each equivalent single machine during the low voltage ride-through period;

[0065] S4. Calculate the output power of each equivalent single machine during the low voltage ride-through period according to the active power and reactive power output by each equivalent single machine during the low voltage ride-through period to obtain an equivalent model.

[0066] Specifically, the output power of a certain cluster Ci during low voltage ride-through = the output power of the equivalent single machine in the Ci cluster × m; thus, C i Single machine equivalent model of a cluster.

[0067] Then we get C1, C2, ..., C n The single-machine equivalent model of the wind turbine group is obtained, so that the wind turbine group is equivalent to n single machines, and the wind turbine group equivalent model considering the low voltage ride-through characteristics can be obtained. The schematic diagram of the wind turbine group equivalent model is shown in Figure 2(a) and Figure 2(b), where Figure 2(a) is the schematic diagram before equivalent, and Figure 2(b) is the schematic diagram after equivalent.

[0068] The control method of the wind turbine set during low voltage ride-through includes:

[0069] A1. When the grid voltage drops and causes the rotor current to exceed the preset current value, the protection circuit is activated for protection; the preset current value is 1.9 times the rated rotor current;

[0070] A2. Block the rotor-side converter control pulse to put the wind turbine into low voltage ride-through mode;

[0071] Specifically, when the grid voltage drops and the rotor current reaches i cbset When the current exceeds the preset value, the crowbar protection circuit is activated (when the current exceeds the preset value, the rotor side converter is shut down and the crowbar circuit is activated to prevent the converter from being damaged by excessive current), and the rotor side converter control pulse is blocked at the same time, and the wind turbine enters the LVRT mode. cbset Take the rotor rated current I rN 1.9 times of

[0072] A3. When the rotor current decays to the current protection exit value, the protection of the protection circuit is exited and the rotor-side converter is restarted; the current protection exit value is 1.7 times the rated current;

[0073] A4. Setting the power output of the wind turbine generator set;

[0074] Specifically, during the period of grid voltage drop, the wind turbine adopts the control strategy in LVRT (low voltage ride through) mode, that is, wait for the rotor current to decay to a certain extent before exiting the crowbar protection, restart the rotor side converter, and then the DFIG (doubly fed induction generator) adopts the power control strategy of LVRT mode, that is, adjust the power output of the wind turbine. The crowbar protection exit conditions are as follows: when the instantaneous values ​​of the three-phase current of the rotor are not greater than the crowbar protection exit value i cbback (i cbback Take 1.7I rN ) and lasts for 20ms, the crowbar protection is exited. The wind turbine generator set in this embodiment is DFIG.

[0075] As a specific implementation method, Figure 3 As shown, in this embodiment, the power output of the wind turbine generator set is adjusted, specifically including:

[0076] A41 calculates the minimum active power to ensure that the wind turbine generator set does not experience overspeed and disconnection from the grid;

[0077] The minimum active power calculation formula is used to calculate the minimum active power to ensure that the wind turbine generator set does not overspeed and disconnect from the grid;

[0078] The minimum active power calculation formula is: in The minimum active power per unit value to ensure that the wind turbine does not overspeed and disconnect from the grid, T m is the mechanical torque, the superscript * indicates the per unit value, T j is the inertia time constant, ω1 is the synchronous angular velocity, Ω rlim is the overspeed protection setting value of the wind turbine, Ω r (0) is the rotor mechanical angular velocity at time 0, and the voltage drop time is time 0.

[0079] Specifically, when the wind turbine is operating normally, the output active power reference value is set to the maximum wind energy capture power. To reduce the stator winding copper loss, the output reactive power reference value is set to 0.

[0080] During low voltage ride-through, according to the generator rotor motion equation, the minimum active power value P of the wind turbine to ensure that the wind turbine does not overspeed and disconnect from the grid can be derived. smin , P smin As the active power reference value of the unit during low voltage ride-through. smin The calculation process is as follows:

[0081] The generator rotor motion equation is as follows:

[0082]

[0083] Where: Ω r is the rotor speed; t is the time; ω1 is the synchronous angular velocity; T j is the inertia time constant; T m is the mechanical torque; T e is the electromagnetic torque; the superscript * indicates the per-unit value.

[0084] The voltage drop time is taken as t = 0. When t = 0, the crowbar protection is activated. b When t=625ms, the voltage returns to normal. bThe instantaneous values ​​of the rotor three-phase currents during the voltage drop are not greater than the crowbar protection exit value i for the first time cbback (i cbback Take 1.7I rN ) and lasts for 20ms. According to the requirements of the National Standard of the People's Republic of China GB / T 36995-2018 "Test Procedure for Fault Voltage Ride-Through Capability of Wind Turbine Generators" (hereinafter referred to as the "Procedure"), the doubly fed wind turbine generator set must have the ability to operate continuously for 625ms without being disconnected from the grid during the LVRT period and provide reactive power support to the grid. The actual duration of the voltage drop is unknown, but 625ms is the maximum duration of the voltage drop specified in the "Procedure". Therefore, it is assumed that 625ms is the fault clearing and the voltage returns to normal. If the grid voltage does not return to normal, according to the "Procedure", the wind turbine can be disconnected from the grid, which has no research significance.

[0085] From the above formula, we can get from time t1 to time t2 (t2>t1), Ω r The changes over time are:

[0086]

[0087] In the formula, Ω r (t1) and Ω r (t2) are the mechanical angular velocities of the rotor at time t1 and time t2 respectively.

[0088] When t = t b According to the above formula, Ω r (t b )for:

[0089]

[0090] At t = t b During t = 625ms, Te* = Ps*. Among them, Ps* is the per unit value of the active power emitted by the stator. When t = 625ms, according to the above formula, Ωr (0.625) is:

[0091]

[0092] If the double-fed wind turbine generator set does not disconnect from the grid due to overspeed during the low voltage ride-through period, that is, Ωr(0.625)≤Ω rlim ,Ω rlim is the overspeed protection setting value of the doubly-fed wind turbine. It can be obtained:

[0093]

[0094] in, The per unit value of the minimum stator active power to ensure that the wind turbine does not overspeed and disconnect from the grid. bGenerally, it is tens of milliseconds. j It is usually a number s. It can be seen from the formula that Ωr is between 0 and t b The time period changes slightly, ignoring tb and Ωr in the range of 0 to t b The change of time period, P is obtained from the above formula smin * The approximate calculation expression is

[0095]

[0096] A42 adjusts the active power reference value and the reactive power reference value of the wind turbine generator set according to the minimum active power;

[0097] Specifically, when crowbar protection is not activated, P s * and Q s *satisfy:

[0098]

[0099] Where (i cbset ) 2 =(i rd ) 2 +(i rq ) 2 ; S n is the rated power of the unit; P s * and Q s * is the per-unit value of active power and reactive power emitted by the stator; L is the inductance; subscripts s and r represent the stator quantity and rotor quantity respectively; subscripts d and q represent the d-axis component and q-axis component respectively; subscripts l and m represent leakage inductance and mutual inductance respectively. i cbset Current limit value to avoid overcurrent damage to the rotor-side converter. rd is the d-axis component of the rotor current, i rq is the q-axis component of the rotor current, k has no meaning and is an intermediate variable for the convenience of discussion. s1 is the stator leakage inductance, L m For mutual induction.

[0100] From step A41, we can see that P smin * Determined by the initial working condition. Under different initial working conditions, it can be obtained from step A41 that P smin * There are 3 possible sizes:

[0101] (1) When hour, is the per-unit value of the minimum active power, and the per-unit value of the active power emitted by the stator is s*=0 to reduce the risk of overspeed disconnection. According to the boundary conditions, the reference power setting result is: in and is the per unit value of the active power reference value and the per unit value of the reactive power reference value emitted by the stator, S n is the rated power of the unit, i cbset is the current preset value, U1 is the effective value of the stator phase voltage, L s1 is the stator leakage inductance, L m is the mutual inductance, k is an intermediate variable for the convenience of discussion,

[0102] (2) When When , the inequality is solved to get:

[0103]

[0104] make U 1tip To make and There is a minimum value of U1 for the real number solution. In this case, according to the boundary conditions, the reference power setting result is:

[0105] (3) When P smin * >ki cbset / S n When U1 1tip In this case, the boundary conditions show that: P sref * and Q sref * There is no real number solution. If U1 remains unchanged, there is no P that can avoid overspeed disconnection. sref * In this case, reactive power compensation equipment needs to be put into use to quickly increase U1 so that U1 ≥ U 1tip Then, the reference power is adjusted according to the possible situation (2). The reference power adjustment result is:

[0106] A5. When the voltage returns to the voltage setting value, the wind turbine is controlled to return to the normal operation mode. The voltage setting value is 0.9 times the rated value of the stator phase voltage.

[0107] Specifically, after the voltage returns to normal, the DFIG returns to normal operation. 1N (U 1N When U1 is the rated value of the stator phase voltage, the voltage is considered to have returned to normal. Wherein, U1 is the effective value of the stator phase voltage.

[0108] ​During the low voltage ride-through process, the rotor side converter control block diagram is as follows: Figure 4 As shown, where σ=1-(L m ) 2 / [(L s1 +L m )(L r1 +L m )]; s is the slip, P opt is the optimal power for capturing the maximum wind energy of the unit, and σ has no meaning but is an intermediate variable for the convenience of discussion.

[0109] According to step A41, the active power and reactive power output by the DFIG during low voltage ride-through are determined by the initial operating conditions of the DFIG before the grid voltage drops. Therefore, the units in the wind turbine group can be grouped according to the initial operating speed, and the total output power of the units in the same group is equivalent to the sum of the output power of a single equivalent power unit and a controlled current source with the same output current as the equivalent power unit. In this way, the wind turbine group is equivalent to several single units, and then the equivalent model of the wind turbine group considering the low voltage ride-through characteristics is obtained.

[0110] The wind turbine group has n equivalent single units through an equivalent impedance Z eq Connect to the power grid. eq It can be calculated as follows:

[0111]

[0112] Where S is the total output power of the cluster. h is the actual output power of each wind turbine generator (h=1, 2, ..., g), and I is the total output current of the group.

[0113] like Figure 5 As shown, an embodiment of the present invention further provides a system for establishing an equivalent model of a doubly-fed wind turbine group, the system comprising:

[0114] The grouping module M1 is used to group the wind turbines according to the initial operating speed to obtain a number of groups;

[0115] The active and reactive power calculation module M2 is used to calculate the average active power and the average reactive power output by each of the generators during the low voltage ride-through period;

[0116] The equivalent conversion module M3 is used to regard the average value of active power and the average value of reactive power output by each of the groups during the low voltage ride-through period as the active power and reactive power output by each equivalent single machine during the low voltage ride-through period;

[0117] The equivalent model building module M4 is used to calculate the output power of each equivalent single machine during the low voltage ride-through period according to the active power and reactive power output by each equivalent single machine during the low voltage ride-through period to obtain an equivalent model.

[0118] As an optional implementation, the system further includes:

[0119] The voltage drop judgment module is used to activate the protection circuit for protection when the grid voltage drops and causes the rotor current to exceed the preset current value;

[0120] Converter blocking module, used to block the rotor-side converter control pulses, and the wind turbine enters the low voltage ride-through mode;

[0121] A converter restart module, used to exit the protection of the protection circuit and restart the rotor-side converter when the rotor current decays to the current protection exit value;

[0122] A power setting module, used for setting the power output of the wind turbine;

[0123] The operation determination module is used to return the wind turbine set to a normal operation mode when the voltage recovers to a voltage setting value.

[0124] The method and system for establishing an equivalent model of a doubly-fed wind turbine group provided in an embodiment of the present invention adjust the active power and reactive power of the doubly-fed wind turbine group during the low voltage ride-through period based on the low voltage ride-through characteristics of the doubly-fed wind turbine group, which can not only suppress the doubly-fed wind turbine group from being disconnected from the grid at overspeed during the LVRT period, but also ensure that the doubly-fed wind turbine group provides reactive power support to the power grid during the LVRT period in accordance with the requirements of the National Standard of the People's Republic of China GB / T 36995-2018 "Test Procedure for Fault Voltage Ride-through Capability of Wind Generator Groups". On this basis, a doubly-fed wind turbine group equivalent method is proposed. The method reduces the order of the doubly-fed wind turbine group model, and can improve the simulation speed while ensuring the simulation accuracy. The existing doubly-fed wind turbine group equivalent method does not consider the LVRT characteristics of the doubly-fed wind turbine group. The method proposed in the present invention takes into account the LVRT characteristics of the doubly-fed wind turbine group. At the same time, the simulation calculation amount is reduced, and the simulation speed is accelerated while ensuring the accuracy.

[0125] As for the system disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the description of the method part.

[0126] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for establishing an equivalent model of a doubly-fed wind turbine group, characterized in that: The method comprises: According to the initial operating speed, the wind turbines are grouped to obtain several groups; Calculating the average active power and the average reactive power output by each of the generator groups during the low voltage ride-through period; The average value of the active power and the average value of the reactive power output by each of the said groups during the low voltage ride-through period are regarded as the active power and the reactive power output by each equivalent single machine during the low voltage ride-through period; Calculating the output power of each equivalent single machine during the low voltage ride through period according to the active power and reactive power output by each equivalent single machine during the low voltage ride through period to obtain an equivalent model; The wind turbine group is equivalent to several single units, and then the equivalent model of the wind turbine group considering the low voltage ride-through characteristics is obtained; The wind turbine group has n equivalent single units through an equivalent impedance Z eq Access to the power grid; Z eq Calculate as follows: Among them, S is the total output power of the cluster; S h is the actual output power of each wind turbine generator (h = 1, 2, ..., g), and I is the total output current of the group; The control method of the wind turbine set during low voltage ride-through includes: When the grid voltage drops and causes the rotor current to exceed the preset current value, the protection circuit is activated for protection; Block the rotor-side converter control pulses to put the wind turbine into low voltage ride-through mode; When the rotor current decays to the current protection exit value, the protection of the protection circuit is exited and the rotor-side converter is restarted; Setting the power output of the wind turbine generator set; When the voltage returns to the voltage setting value, controlling the wind turbine set to return to the normal operating mode; The step of adjusting the power output of the wind turbine generator set specifically includes: Calculate the minimum active power to ensure that the wind turbine generator set does not go off-grid due to overspeed; According to the minimum active power, an active power reference value and a reactive power reference value of the wind turbine generator set are set; The calculation of the minimum active power to ensure that the wind turbine generator set does not experience overspeed and disconnection from the grid specifically includes: The minimum active power calculation formula is used to calculate the minimum active power to ensure that the wind turbine generator set does not overspeed and disconnect from the grid; The minimum active power calculation formula is: in The minimum active power per unit value to ensure that the wind turbine does not overspeed and disconnect from the grid, T m is the mechanical torque, the superscript * indicates the per unit value, T j is the inertia time constant, ω1 is the synchronous angular velocity, Ω rlim is the overspeed protection setting value of the wind turbine, Ω r (0) is the rotor mechanical angular velocity at time 0, and the voltage drop time is time 0; The step of setting the active power reference value and reactive power reference value of the wind turbine generator set according to the minimum active power specifically includes: when hour, is the per-unit value of the minimum active power, and the per-unit value of the active power emitted by the stator is s *=0 to reduce the risk of overspeed disconnection. According to the boundary conditions, the reference power setting result is: in and is the per unit value of the active power reference value and the per unit value of the reactive power reference value emitted by the stator, S n is the rated power of the unit, i cbset is the current preset value, U1 is the effective value of the stator phase voltage, L s1 is the stator leakage inductance, L m is the mutual inductance, k is an intermediate variable for the convenience of discussion, when When , the inequality is solved to get: make U 1tip To make and There is a minimum value of U1 for the real number solution, and the reference power setting result is: When P smin * >ki cbset / S n When the reactive power compensation device is put into operation, the effective value of the stator phase voltage U1 is increased, so that U1 ≥ U 1tip , the reference power setting result is:

2. The method for establishing an equivalent model of a doubly-fed wind turbine group according to claim 1, characterized in that: The wind turbines are grouped according to the initial operating speed to obtain a plurality of groups, specifically including: According to the initial operating speed, wind turbines in the same speed range are divided into the same group to obtain a plurality of groups.

3. The method for establishing an equivalent model of a doubly-fed wind turbine group according to claim 1, characterized in that: The calculating of the average active power and the average reactive power output by each of the generator groups during the low voltage ride-through period specifically includes: Suppose there are n clusters C1, C2, ..., C n ; One of the clusters C i (i=1,2,……,n) contains m wind turbines, and the active power output of each wind turbine during low voltage ride-through is P1,P2,……,P m The output reactive power is Q1, Q2, ..., Q m , then the average active power output of the Ci unit during the low voltage ride-through period is The average reactive power is Wherein, t represents the tth wind turbine.

4. The method for establishing an equivalent model of a doubly-fed wind turbine group according to claim 1, characterized in that: The preset current value is 1.9 times the rated current of the rotor; The current protection exit value is 1.7 times the rated current; The voltage setting value is 0.9 times the rated value of the stator phase voltage.

5. A system for establishing an equivalent model of a doubly-fed wind turbine group, characterized in that: The system comprises: A grouping module is used to group wind turbines according to their initial operating speed to obtain several groups; An active and reactive power calculation module, used to calculate the average active power and the average reactive power output by each of the generators during the low voltage ride-through period; An equivalent conversion module, used for treating the average value of active power and the average value of reactive power output by each of the machine groups during the low voltage ride-through period as the active power and reactive power output by each equivalent single machine during the low voltage ride-through period; An equivalent model building module is used to calculate the output power of each equivalent single machine during the low voltage ride-through period according to the active power and reactive power output by each equivalent single machine during the low voltage ride-through period to obtain an equivalent model; The wind turbine group is equivalent to several single units, and then the equivalent model of the wind turbine group considering the low voltage ride-through characteristics is obtained; The wind turbine group has n equivalent single units through an equivalent impedance Z eq Access to the power grid; Z eq Calculate as follows: Among them, S is the total output power of the cluster; S h is the actual output power of each wind turbine generator (h = 1, 2, ..., g), and I is the total output current of the group; Also includes: The voltage drop judgment module is used to activate the protection circuit for protection when the grid voltage drops and causes the rotor current to exceed the preset current value; Converter blocking module, used to block the rotor-side converter control pulses, and the wind turbine enters the low voltage ride-through mode; A converter restart module, used to exit the protection of the protection circuit and restart the rotor-side converter when the rotor current decays to the current protection exit value; A power setting module, used for setting the power output of the wind turbine; The step of adjusting the power output of the wind turbine generator set specifically includes: Calculate the minimum active power to ensure that the wind turbine generator set does not go off-grid due to overspeed; According to the minimum active power, an active power reference value and a reactive power reference value of the wind turbine generator set are set; The calculation of the minimum active power to ensure that the wind turbine generator set does not experience overspeed and disconnection from the grid specifically includes: The minimum active power calculation formula is used to calculate the minimum active power to ensure that the wind turbine generator set does not overspeed and disconnect from the grid; The minimum active power calculation formula is: in The minimum active power per unit value to ensure that the wind turbine does not overspeed and disconnect from the grid, T m is the mechanical torque, the superscript * indicates the per unit value, T j is the inertia time constant, ω1 is the synchronous angular velocity, Ω rlim is the overspeed protection setting value of the wind turbine, Ω r (0) is the rotor mechanical angular velocity at time 0, and the voltage drop time is time 0; The step of setting the active power reference value and reactive power reference value of the wind turbine generator set according to the minimum active power specifically includes: when hour, is the per-unit value of the minimum active power, and the per-unit value of the active power emitted by the stator is s *=0 to reduce the risk of overspeed disconnection. According to the boundary conditions, the reference power setting result is: in and is the per unit value of the active power reference value and the per unit value of the reactive power reference value emitted by the stator, S n is the rated power of the unit, i cbset is the current preset value, U1 is the effective value of the stator phase voltage, L s1 is the stator leakage inductance, L m is the mutual inductance, k is an intermediate variable for the convenience of discussion, when When , the inequality is solved to get: make U 1tip To make and There is a minimum value of U1 for the real number solution, and the reference power setting result is: When P smin * >ki cbset / S n When the reactive power compensation device is put into operation, the effective value of the stator phase voltage U1 is increased, so that U1 ≥ U 1tip , the reference power setting result is: The operation determination module is used to return the wind turbine set to a normal operation mode when the voltage recovers to a voltage setting value.

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