Direct drive wind turbine online grouping method

Abstract

Embodiments of the application provide a direct drive wind turbine online grouping method, and relate to the technical field of wind power generation. The direct drive wind turbine online grouping method comprises the following steps: step 1: in the case that a fault occurs at the grid side of a wind farm and causes voltage drop at the terminal of a wind turbine generator, the critical power of the active transient response demarcation point of each wind turbine generator is calculated; step 2: simulation test and fitting are performed on the relationship between the input wind speed and the terminal output power of the direct drive wind turbine generator; and step 3: according to the fitting relationship and the critical power, the critical wind speed of each wind turbine generator is inversely calculated, and the category of the active response characteristic of the wind turbine generator is judged according to the actual operating wind speed. The method can quickly group the dynamic response characteristics of each wind turbine generator, is suitable for online analysis, and provides a quick grouping method for future online equivalent modeling of a wind farm.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of wind power generation, in particular to an online grouping method for direct-drive wind turbines. BACKGROUND

[0002] Wind energy has been widely used in power systems due to its safety, cleanliness, high efficiency, low cost and other advantages. However, due to the strong randomness and volatility of wind energy, large-scale wind power integration will affect the safe and stable operation of the power system. In order to maintain the power supply reliability of the power system, simulation analysis needs to be performed on the power system with large-scale wind power integration. However, if each wind turbine is modeled in detail, the computational complexity will be greatly increased, the simulation efficiency will be reduced, and even the "curse of dimensionality" problem will occur. Therefore, dynamic equivalent modeling of wind farms is of great significance for analyzing the stability of power systems with large-scale wind farms.

[0003] At present, equivalent methods use characteristic quantities to group wind turbines, such as calculating the pitch angle, unloading circuit input state, terminal voltage, amplitude limiting element action time and other characteristic quantities, then using clustering algorithms to aggregate wind turbines with similar operating states, and equivalent to a turbine that can represent the operating characteristics of this type of wind turbine, thereby reducing the complexity of wind farm simulation calculation. However, this grouping method is computationally complex and cannot group wind turbines in real time when the operating conditions change. It can only be used for offline analysis. However, the operating state of the wind power system is greatly affected by the working condition, and the offline equivalent model is usually only for a certain specific working condition, with fixed structure and parameters. If the offline equivalent model is used for calculation during online simulation, the calculation result may be affected due to mismatch of working conditions. Therefore, it is of great significance to establish an online equivalent model of the wind farm for online simulation of wind farm integration, and the online grouping method is the basis for online equivalent modeling of the wind farm.

[0004] At present, the methods for online equivalent modeling of wind farms include: using the idea of offline equivalent and online matching to establish an equivalent model library, matching the actual working condition with the typical working condition in the model library, and using the model corresponding to the actual working condition closest to the actual working condition as the online equivalent model, but the method based on matching is always to describe the actual continuous working condition with discrete working condition, and it is difficult to accurately describe the dynamic characteristics of the wind farm under any working condition. Online equivalent of wind farm is realized based on wind speed prediction; for double-fed wind turbines, the wind speed is directly used to group the double-fed wind turbines according to the active response characteristics under fault, and clustering calculation is not needed. However, the above two methods are difficult to online equivalent of direct-drive wind turbines with ramp recovery control strategy. The characteristics of direct-drive wind turbines during fault recovery are related to not only the wind speed but also the voltage drop degree of the machine terminal, and it is difficult to reflect the difference of the operating state of the wind turbine during active ramp recovery by using the wind speed as the grouping index. The equivalent modeling methods are respectively proposed for active instantaneous recovery and ramp recovery of direct-drive wind turbines based on the response characteristics of the direct-drive wind turbines with wind speed as the grouping index, but the ability of the wind turbine to output active power during low voltage ride through is not considered. Under the control of reactive power priority, the grid-side converter should output active power as much as possible under the current limit of the converter to maintain the stability of the DC side capacitor voltage during low voltage ride through. The direct-drive wind turbines are divided into three categories according to the active response, without using clustering algorithm for grouping, but the output of active power of the direct-drive wind turbines to maintain the stability of the DC side capacitor voltage during fault is considered, and the active ramp recovery needs to be further analyzed. SUMMARY

[0005] The purposes of the present application include providing an online grouping method of direct-drive wind turbines, which can quickly group the dynamic response characteristics of each wind turbine and is suitable for online analysis, thereby providing a fast grouping method for future online equivalent modeling of wind farms.

[0006] The embodiments of the present application can be implemented as follows:

[0007] The present application provides an online grouping method of direct-drive wind turbines, which comprises the following steps:

[0008] Step 1: in the case that a fault occurs at the grid side of a wind farm and causes voltage drop at the machine terminal of the wind turbine, the critical power of the active transient response dividing point of each wind turbine is calculated.

[0009] Step 2: the relationship between the input wind speed and the output power of the direct-drive wind turbine is simulated and tested and fitted.

[0010] Step 3: according to the fitted relationship and the critical power, the critical wind speed of each wind turbine is inversely calculated, and the category of the active response characteristics of the wind turbine is judged according to the actual operating wind speed.

[0011] In an optional embodiment, in step 1, the critical power includes a lower critical power P cri1 and an upper critical power P cri2 , and the calculation formula is as follows:

[0012]

[0013] P cri2 = αP cri1 (4)

[0014] In the formula, e g is a grid-side voltage comprehensive vector; I max is a converter current upper limit value; ɑ is a machine-side voltage drop degree value; I N is a wind turbine rated current.

[0015] In an optional embodiment, step 2 includes:

[0016] The input wind speed of the wind turbine is set from the cut-in wind speed 6 m / s to the cut-out wind speed 20 m / s at an interval of 0.005 m / s, and the relationship between the input wind speed of the wind turbine and the electromagnetic power and the machine-side output active power is simulated and tested.

[0017] In an optional embodiment, in step 3, the categories of the active response characteristics include:

[0018] Category 1: When the voltage drops to the first range, the wind turbine needs to provide reactive power support to the grid, and since the grid-side converter capacity is all used for output reactive power, the active power output by the wind turbine during the fault is 0;

[0019] Category 2: The wind turbine has the ability to output active power during the fault, but the active power does not return to the normal value before the fault after the fault is recovered, and needs to go through a ramp-up process to return to the normal value;

[0020] Category 3: When the voltage drops to the second range, which is higher than the first range, the active transient response curve restores the grid-side output active power to the normal value during the fault process, maintains the stability of the DC-side capacitor voltage, and after the fault disappears, it can be restored to the normal value through oscillation.

[0021] In an optional embodiment, step 3 includes:

[0022] In the case of P0>P cri1 , the active power transient response characteristic of the wind turbine belongs to Category 1;

[0023] In the case of P cri2 <P0≤P cri1 , the active power transient response characteristic of the wind turbine belongs to Category 2;

[0024] In the case of P0 < P cri2 The active power transient response characteristic of the wind turbine belongs to the third type.

[0025] Wherein, P0 is the active power output by the wind turbine before the fault.

[0026] In the optional implementation, before step 1, the online grouping method of the direct-drive wind turbine further comprises:

[0027] The mathematical models of the parts of the direct-drive wind turbine are established, including the wind turbine model, the transmission system model, the direct-drive wind turbine model and the frequency converter model.

[0028] In the optional implementation, the frequency converter model comprises the machine-side frequency converter control and the grid-side frequency converter control.

[0029] In the optional implementation, the grid-side frequency converter control comprises the current control strategy during normal operation, the current control strategy and the voltage inner loop control during the fault and recovery.

[0030] In the optional implementation, after the step of establishing the mathematical models of the parts of the direct-drive wind turbine, the online grouping method of the direct-drive wind turbine further comprises:

[0031] The mechanism of the active transient response is analyzed, and the type of the active response characteristic of the wind turbine is determined.

[0032] In the optional implementation, the step of analyzing the mechanism of the active transient response and determining the type of the active response characteristic of the wind turbine comprises:

[0033] The active transient response characteristic during the fault is analyzed.

[0034] The active transient response characteristic during the fault recovery is analyzed.

[0035] The active transient response characteristic during the complete fault process is classified.

[0036] The online grouping method of the direct-drive wind turbine provided by the embodiment of the application has the following beneficial effects:

[0037] Based on the analysis of control strategy, all possible dynamic response characteristics of wind turbine and corresponding boundary conditions are derived, and a grouping method for wind turbines based on wind speed and terminal voltage is proposed, which is suitable for online analysis. Since the active response characteristics are related to the actual output power of the wind turbine, and the wind speed can only reflect the wind power received by the wind turbine, in order to more accurately judge the dynamic process of the wind turbine during the fault period, the relationship between the wind speed and the terminal output power under normal operating conditions is fitted, and the grouping method is verified based on the active transient response of the direct-drive wind turbine under different fault and wind speed conditions. Based on the method, if the wind speed and voltage distribution of the wind farm are known, the dynamic response characteristics of each wind turbine can be quickly grouped, which is suitable for online analysis and provides a fast grouping method for future online equivalent modeling of wind farms. BRIEF DESCRIPTION OF DRAWINGS

[0038] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced as follows. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0039] Figure 1 The flow chart of the online grouping method of direct-drive wind turbine provided by the embodiments of the present application;

[0040] Figure 2 The schematic diagram of three types of active response characteristics;

[0041] Figure 3 The active transient response curve of the wind turbine under different voltage drop and operating wind speed;

[0042] Figure 4 The measured power curve of the wind turbine under different wind speed;

[0043] Figure 5 The segmented fitting curve of the output power on the grid side;

[0044] Figure 6 The active response curve of each type when the voltage drop is 0.5 p.u.;

[0045] Figure 7 The d-axis current curve under the critical wind speed. DETAILED DESCRIPTION

[0046] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the following will be combined with the accompanying drawings for the embodiments of the present application to make a clear and complete description of the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the accompanying drawings can be arranged and designed in various different configurations.

[0047] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0048] It should be noted that: similar reference numerals and letters represent similar items in the following drawings, therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0049] It should be noted that: the features in the embodiments of the present application can be combined with each other without conflict.

[0050] Please refer to Figure 1 The embodiment provides an online grouping method of direct-drive wind turbine, according to the control strategy of direct-drive wind turbine, the possible dynamic process in the fault process is analyzed from the mechanism, and the boundary condition is deduced, and the method for grouping the active response characteristics of the direct-drive wind turbine by using the wind speed and the machine terminal voltage drop is proposed. The online grouping method of direct-drive wind turbine includes the following steps:

[0051] S1: establishing mathematical models of each part of the direct-drive wind turbine.

[0052] 1.1 wind turbine model

[0053] The power captured by the wind wheel from the wind can be expressed as:

[0054]

[0055] In the formula, P w is the mechanical power obtained by the wind turbine blade from the wind energy; ρ is the air density; R is the radius of the wind turbine rotor; V w is the wind speed; β is the pitch angle; λ is the tip speed ratio; C p is the wind energy utilization coefficient of the wind turbine, which is a function of β and λ. The expressions of the tip speed ratio λ and the wind energy utilization coefficient are as follows:

[0056] λ = ω w R / V w (6)

[0057] Cp = 0.22(116 / a - 0.4 β - 5)exp(-12.5 / a) (7)

[0058] 1 / a = 1 / l + 0.08 β - 0.035 / β 3 + 1 (8)

[0059] where: ω w is the mechanical angular speed of the wind turbine rotor.

[0060] 1.2 Drive train model

[0061] Since the direct drive wind turbine has no gearbox between the wind turbine and the direct drive generator, the drive train part can be modeled as a lumped mass, the mathematical expression is as follows:

[0062]

[0063] where: ω r is the mechanical speed of the generator rotor; T w is the mechanical torque of the wind turbine; T e is the electromagnetic torque of the generator; B m is the rotational viscous coefficient; J eq is the equivalent moment of inertia of the unit.

[0064] 1.3 Direct drive wind turbine model

[0065] The direct drive wind turbine uses permanent magnets to replace the field winding in the synchronous motor, which can be regarded as a synchronous motor with constant field current. Its stator voltage equation in the d-q synchronous rotating coordinate is:

[0066]

[0067] where: u sd , u sq are the d-axis and q-axis stator voltages respectively; i sd , i sq are the d-axis and q-axis stator currents respectively; R s is the stator resistance; ω e is the electrical angular speed of the generator; ψ sd and ψ sq are the d-axis and q-axis stator flux linkages respectively.

[0068] Using the rotor flux oriented vector control technology, the d-axis of the rotating coordinate system is located at the direction of the rotor permanent magnet flux linkage, then the flux linkage equations of the d-axis and q-axis on the stator side are:

[0069]

[0070] where: L d and Lq respectively are the generator stator d-axis and q-axis inductances; ψ0is the permanent magnet flux linkage.

[0071] The output electromagnetic torque equation of the direct-drive wind turbine is:

[0072] T e = 1.5n p (ψ sd i sq - ψ sq i sd ) (12)

[0073] wherein n p is the pole pair number of the direct-drive wind turbine.

[0074] Substituting equation (7) into equation (8) can obtain:

[0075] T e = 1.5n p [(L d - L q )i sq i sd + i sq ψ0] (13)

[0076] Assuming L d = L q , then:

[0077] T e = 1.5n p i sq ψ0 (14)

[0078] As can be seen from equation (10), the electromagnetic torque of the generator can be controlled by controlling the stator side q-axis current, and further control the generator rotor speed.

[0079] 1.4 Converter model

[0080] 1.4.1 Machine-side converter control

[0081] The machine-side converter is composed of a diode uncontrolled rectifier bridge and a Boost voltage-boosting chopper circuit, and the main function of this part is to control the generator rotor speed to achieve maximum wind power tracking.

[0082] 1.4.2 Grid-side converter control

[0083] The grid-side converter is composed of a controllable inverter bridge of insulated gate bipolar transistors. The main function of the grid-side converter is to maintain the stability of the DC side capacitor voltage, to transmit the active power outputted by the machine-side converter to the grid, and to output given reactive power according to the reactive power reference value. For the grid-side converter, the voltage-oriented vector control technology is generally adopted, the d-axis of the synchronous rotating coordinate system is coincided with the grid voltage vector, and the projection of the grid voltage on the q-axis is 0, i.e.

[0084]

[0085] In the formula, u gd and u gq are the d-axis and q-axis components of the grid voltage; e g is the grid voltage vector.

[0086] At this time, the active power and the reactive power exchanged between the grid-side converter and the grid can be expressed as:

[0087]

[0088] In the formula, P g and Q g are the active power and the reactive power exchanged between the grid-side converter and the grid; i gd and i gq are the d-axis and q-axis currents of the grid side.

[0089] As can be seen from the formula (12), the output of the active power and the reactive power can be independently controlled by controlling the projections of the grid-side current vector on the d-axis and the q-axis, and thus the d-axis current can also be called the active current and the q-axis current can also be called the reactive current.

[0090] 1) Current control strategy during normal operation

[0091] During normal operation, the grid-side converter maintains the stability of the DC side capacitor voltage by controlling the DC side voltage to determine the reference value i gdref of the d-axis current. In addition, to realize the unit power factor operation of the wind turbine, the reactive power reference value is usually set to 0, and thus the reference value i gqref of the q-axis current is also 0.

[0092] 2) Current control strategy during fault and recovery

[0093] The control strategy of active power during fault is still to keep the DC voltage constant. The sudden drop of grid voltage leads to the fact that the grid-side converter can not send out active power normally, and the active power accumulates on the DC capacitor, which makes the voltage of the DC capacitor rise. When the voltage reaches the set threshold, the unloading circuit connected in parallel with the DC capacitor operates to consume the excess active power, so that the DC voltage no longer continues to rise. However, since the voltage of the DC capacitor is always higher than the reference value after the fault occurs, the grid-side converter will continuously increase the active current.

[0094] When the grid voltage drops due to external fault, the wind turbine needs to send out reactive power to support the grid voltage. According to the grid connection standard, the calculation formula of the reactive current output by the wind turbine during low voltage ride through is:

[0095] I gqref ≥1.5×(0.9-U T )I N ,(0.2≤U T ≤0.9) (17)

[0096] In the formula, I gqref is the reference value of the reactive current output by the wind turbine; U T is the fault voltage of the grid; and I N is the rated current.

[0097] In the example in this paper, the reference value of the reactive current is taken as:

[0098] I gqref =2×(0.9-U T )I N ,(0.2≤U T ≤0.9) (18)

[0099] During the fault, the ability of the wind turbine to provide reactive power to the grid needs to be ensured first, so the strategy of reactive power priority control is adopted, that is, under the premise of the capacity constraint of the converter, the reactive current control is satisfied first, and then the reference value of the active current is determined. Therefore, the upper limit value of the active current is:

[0100]

[0101] In the formula, I dmax is the upper limit value of the active current; and I max is the upper limit value of the current of the converter.

[0102] Under the above limitation, the reference value of the active current can be expressed as:

[0103] I gdref =min{I gd1 ,I dmax} (20)

[0104] In the formula, I gdrefis the active current reference value; I gd1 is the active current reference value obtained by the DC voltage control.

[0105] When the fault is cleared, if the active power does not recover to the level before the low voltage ride through, the active current should be controlled to recover to the normal value with a specified slope. This control method of recovering the active current with a slope can reduce the impact of active power surge on the unit during voltage recovery.

[0106] 3) Inner loop control of voltage

[0107] The mathematical model of the grid-side converter in the d-q rotating coordinate system is:

[0108]

[0109] In the formula, u d , u q are the d-axis and q-axis components of the output of the grid-side converter; R g , L g are the grid-side resistance and reactance, respectively; ω g is the synchronous angular velocity of the grid.

[0110] As can be seen from equation (17), after the closed-loop PI control of the d-axis and q-axis currents, the cross-coupling terms Δu d and Δu q need to be added to obtain the final voltage control quantity, Δu d and Δu q can be expressed as:

[0111]

[0112] S2: Analyze the mechanism of the active transient response and determine the category of the active response characteristics of the wind turbine generator.

[0113] 2.1 Analysis of the active transient response characteristics during the fault

[0114] As can be seen from equation (16), the active current reference value during the fault process is the smaller value of I gd1 and I dmax . Assuming that the grid voltage drops to α p.u., when the grid-side voltage drops to α p.u., the output power of the grid-side converter instantaneously decreases to αP g , in order to maintain the stability of the DC voltage, the fault steady-state value of I gd should satisfy:

[0115]

[0116] In the formula, I d0 is the d-axis current of the grid-side before the fault.

[0117] Therefore, the steady-state d-axis current reference value during the fault process can be expressed as:

[0118]

[0119] When , it is illustrated that the grid-side converter can stabilize the DC-side voltage by increasing the d-axis current during the fault steady state; when , gdref I dmax , the grid-side d-axis current is limited by the capacity of the converter, and the output power of the grid-side during the fault steady state is lower than the power input to the DC-side by the machine-side converter, so the DC-side voltage cannot be stabilized at the reference value.

[0120] 2.2 Analysis of the active transient response characteristics during the fault recovery

[0121] The active transient response characteristics during the fault recovery mainly differ in whether there is a part of active ramp recovery. Since the direct-drive wind turbine limits the recovery rate of the d-axis current during the fault recovery process, whether there is a dynamic characteristic of active ramp recovery depends on whether the active power is restored to the normal value before the fault at the fault recovery moment, that is, whether the d-axis current during the fault steady state is restored to the normal value before the fault.

[0122] Therefore, the size of I dmax and I d0 can be compared to determine whether there is a dynamic characteristic of active ramp recovery. When I dmax ≥ I d0 , the d-axis current during the fault steady state is not less than the d-axis current before the fault occurs, the active power rises above the normal value after the fault recovery, and there is no ramp recovery process, but a short oscillation process to restore to the normal value; when I dmax < I d0 , the d-axis current during the fault steady state is less than the d-axis current before the fault occurs, the active power is still below the normal value at the fault recovery moment, and due to the limitation of the recovery rate of the d-axis current, the active power will have a ramp recovery process.

[0123] 2.3 Determine the category of the active transient response characteristics during the complete fault process

[0124] Considering the active transient response characteristics of the direct-drive wind turbine during the fault and recovery, it can be divided into three categories, and the active transient response curve of each category is shown in Figure 2 .

[0125] 1) When I dmax < I d0 , the output active power of the wind turbine during the fault steady state is lower than the normal value before the fault, and there is a ramp recovery process after the fault recovery.

[0126] 2) When , the wind turbine output active power during the fault steady state is still lower than the normal value before the fault, but the active power output also recovers to above the normal value at the fault recovery moment, and then recovers to the normal value after a short oscillation.

[0127] 3) When , the wind turbine output active power during the fault steady state has recovered to the normal value, and the DC side capacitor voltage can be maintained stable, and after the fault recovery, the active power also recovers to the normal value after a short oscillation.

[0128] The direct-drive wind turbine is connected to an infinite grid, and the grid voltage drops at 1.5 s and recovers at 2.1 s. The operating wind speed is set from 6 m / s, and is increased to the rated wind speed 11 m / s at intervals of 0.2 m / s. When the wind speed is above the rated wind speed, the active power is maintained at the rated value in the steady state through the pitch angle control, and the active power curve is consistent with the response under the rated wind speed. The terminal voltage drop is increased from 0.1 p.u. to 0.9 p.u. at intervals of 0.1 p.u. The active power response characteristics of the wind turbine are simulated and analyzed under different operating wind speeds and different grid voltage drops, and the active power response curve is shown in Figure 3 , wherein each curve in the figure corresponds to a simulation result under a wind speed and a voltage drop.

[0129] As can be seen from Figure 3 , the active power response curves of the wind turbine operating at different wind speeds and voltage drops are obviously different. When the voltage drop is in a lower range, for example, a), b) and c) in Figure 3 , the wind turbine needs to provide a large amount of reactive power support to the grid, and since the capacity of the grid-side converter is used for outputting reactive power, the active power output by the wind turbine during the fault is 0; while in d) of Figure 3 , the wind turbine has a certain ability to output active power during the fault, but the active power does not recover to the normal value before the fault after the fault recovery, and needs to experience a ramp-up process for a period of time to recover to the normal value; when the voltage drop is not deep, for example, g), h) and i) in Figure 3 , most of the curves can recover the grid-side output active power to the normal value during the fault process, and can maintain the stability of the DC side capacitor voltage, and after the fault disappears, the active power can recover to the normal value after a short oscillation, which is basically consistent with the theoretical derivation result.

[0130] S3: online classification of active power transient response.

[0131] When the wind turbine operates normally, since the grid-side voltage oriented control is adopted, the relationship between the grid-side d-axis current and the output active power and the grid-side voltage is:

[0132]

[0133] The upper limit of d-axis current due to the converter capacity limitation during fault can be obtained by combining equations (14) and (15) as:

[0134]

[0135] According to the classification method in section 2.1, when I dmax = I d0 , the pre-fault active power satisfies:

[0136]

[0137] When , the pre-fault active power is:

[0138] P cri2 = αP cri1 (28)

[0139] Therefore, when the grid-side voltage drop depth is known, P cri1 and P cri2 can be calculated, and by comparing them with the pre-fault active power P0 of the wind turbine, the active power transient response characteristic of the wind turbine can be determined.

[0140] 1) When P0 > P cri1 , the active power transient response characteristic of the wind turbine belongs to the first type in Figure 3 (a), b), c) in Figure 3 );

[0141] 2) When P cri2 < P0≤ P cri1 , the active power transient response characteristic of the wind turbine belongs to the second type in Figure 3 (part of the curves in d), e), f) in Figure 3 );

[0142] 3) When P0 < P cri2 , the active power transient response characteristic of the wind turbine belongs to the third type in Figure 3 (g), h), i) in Figure 3 ).

[0143] On this basis, the relationship between wind speed and grid-side converter output power of the wind turbine can be obtained through experiments or historical data, and after fitting, the corresponding critical wind speed can be obtained through P cri1 and P cri2 .

[0144] S4: Perform fitting of wind speed and generator-side output power.

[0145] Since the dynamic process of the wind turbine during the fault is related to the power output at the generator terminal before the fault, errors may occur if the wind power received by the wind turbine is directly used for calculation. In order to more accurately determine the dynamic process of the wind turbine during the fault, the relationship between the wind speed and the output power at the generator terminal in the normal operating state needs to be fitted. The input wind speed of the wind turbine is set from the cut-in wind speed of 6 m / s, with an interval of 0.005 m / s, to the cut-out wind speed of 20 m / s. The relationship between the input wind speed of the wind turbine and the electromagnetic power and the active power output at the generator terminal is simulated and tested, and the simulation results are shown in Figure 4 , and the fitting of each operating interval is shown in Figure 5 .

[0146] In actual use, the detailed steps of the online grouping method are as follows:

[0147] Step 1: In the case that a fault occurs at the grid side of the wind farm, causing voltage drop at the generator terminal of the wind turbine, the critical power of the active transient response dividing point of each wind turbine is calculated according to formulas (23) and (24).

[0148] Step 2: The relationship between the input wind speed and the output power at the generator terminal of the direct-drive wind turbine is simulated and tested according to the method of S4 and fitted.

[0149] Step 3: The critical wind speed of each wind turbine is inversely calculated according to the fitted relationship and the critical power, and the active response characteristics of the wind turbine are determined according to the actual operating wind speed.

[0150] The wind turbines with the same active transient response characteristics are classified into a group, and there is no complex clustering algorithm in the calculation process, so the method can be used as an online grouping method.

[0151] The method provided in this embodiment uses the built model of the direct-drive wind turbine to verify the above grouping method. The critical output active power of the three types of active characteristics is calculated, and the critical wind speed is calculated based on the fitted V w -P g curve, and the active power and d-axis current of the wind turbine at the critical operating condition are output to verify the accuracy of the calculated critical operating condition.

[0152] When the voltage drop is 0.5 p.u., the calculated P cri1 = 1.075 MW, P cri2 = 0.5375 MW, and the corresponding critical wind speed V cri1 = 10.02 m / s, V cri2 = 7.93 m / s can be obtained according to the functions fitted in section 5.1. The active power and d-axis current of the wind turbine at the two types of critical wind speed are shown in Figure 6 , 7 .

[0153] The I can be calculated by using formula (14) and (15) gqref = 2234A, I dmax = 2002A. From Figure 7 It can be found that when V cri1 = 10.02 m / s, the active power of the wind turbine rises to the normal value before the fault at the moment of fault recovery, but the DC side capacitor voltage has not recovered to the reference value, and the active power still has a small slope rising process, but the time duration is short, and the lower the wind speed, the shorter the duration, which can be ignored when grouping, and the d-axis current recovers to the same value as before the fault during the fault. When V cri2 = 7.93 m / s, the active power recovers to the normal value before the fault during the fault steady state, and the two critical curves are consistent with the theoretical derivation.

[0154] The online grouping method of the direct drive wind turbine provided by the embodiment of the application has the following beneficial effects:

[0155] Firstly, the direct drive wind turbine with reactive power priority control and active slope recovery characteristics during the fault process is modeled; then, based on the analysis of the control strategy, the dynamic response characteristics of the wind turbine and the corresponding boundary conditions are derived, and a wind turbine grouping method based on wind speed and terminal voltage suitable for online analysis is proposed; finally, since the active response characteristics are related to the actual output power of the wind turbine, and the wind speed can only reflect the wind power received by the wind turbine, in order to more accurately judge the dynamic process of the wind turbine during the fault, the relationship between the wind speed and the terminal output power under normal operating conditions is fitted, and the grouping method is verified based on the active transient response of the direct drive wind turbine under different fault and wind speed conditions.

[0156] Based on the method, if the wind speed and voltage distribution of the wind farm are known, the dynamic response characteristics of each wind turbine can be quickly grouped, which is suitable for online analysis and provides a fast grouping method for future online equivalent modeling of wind farms.

[0157] The above is only a specific embodiment of the application, but the protection scope of the application is not limited thereto, and any changes or replacements within the technical range disclosed by the application can be easily thought of by those skilled in the art, which should be covered within the protection scope of the application. Therefore, the protection scope of the application should be subject to the protection scope of the claims.

Claims

1. A method for online grouping of direct-drive fans, characterized in that, The online grouping method for direct-drive fans includes: Step 1: In the event of a grid-side fault causing a voltage drop at the wind turbine terminals, calculate the critical power at the active transient response boundary of each wind turbine. The critical power includes the lower critical power. and upper critical power The calculation formula is as follows: (23) (24) In the formula, For grid-side voltage composite vector; I max α is the upper limit of the converter current; I is the voltage drop at the generator terminals; N This refers to the rated current of the wind turbine generator. Step 2: Simulate and fit the relationship between the input wind speed and the output power at the turbine end of the direct-drive wind turbine. Step 3: Based on the fitting relationship and the critical power, calculate the critical wind speed for each wind turbine, and determine the category of the active power response characteristics of the wind turbine based on the actual operating wind speed. The categories of active power response characteristics include: Category 1: When the voltage drops to the first range, the wind turbine needs to provide reactive power support to the grid. Since the grid-side converter capacity is used entirely for reactive power output, its active power output during the fault period is 0. Category 2: The wind turbine has the ability to output active power during the fault, but after the fault is restored, the active power does not return to the normal value before the fault and needs to go through a ramp-up process to return to the normal value. Category 3: When the voltage drops to the second range, which is higher than the first range, the active transient response curve restores the active power output from the grid side to the normal value during the fault process, maintains the DC side capacitor voltage stability, and can recover to the normal value after the fault disappears through oscillation. exist In this case, the active power transient response characteristics of the wind turbine belong to the first category. exist In this case, the active power transient response characteristics of the wind turbine belong to the second category. exist In this case, the active power transient response characteristics of the wind turbine belong to the third category. in, This represents the active power output of the wind turbine before the fault.

2. The online grouping method for direct-drive wind turbines according to claim 1, characterized in that, Step 2 includes: The input wind speed of the wind turbine was set from a cut-in wind speed of 6 m / s, and increased in increments of 0.005 m / s to a cut-out wind speed of 20 m / s. The relationship between the input wind speed, electromagnetic power, and active power output of the wind turbine was simulated and tested.

3. The online grouping method for direct-drive wind turbines according to claim 1, characterized in that, Prior to step 1, the online grouping method for direct-drive wind turbines further includes: Establish mathematical models for each part of the direct-drive wind turbine, including the wind turbine model, transmission system model, direct-drive wind turbine model, and frequency converter model.

4. The online grouping method for direct-drive wind turbines according to claim 3, characterized in that, The inverter model includes machine-side inverter control and grid-side inverter control.

5. The online grouping method for direct-drive fans according to claim 4, characterized in that, The grid-side inverter control includes current control strategies during normal operation, current control strategies during faults and recovery, and voltage inner loop control.

6. The online grouping method for direct-drive wind turbines according to claim 3, characterized in that, Following the step of establishing mathematical models for each part of the direct-drive wind turbine, the online clustering method for direct-drive wind turbines further includes: Analyze the mechanism of active transient response and determine the category of active response characteristics of wind turbine units.

7. The online grouping method for direct-drive wind turbines according to claim 6, characterized in that, The steps for analyzing the mechanism of active transient response and determining the category of active response characteristics of wind turbine units include: Analyze the active transient response characteristics during the fault; Analyze the active transient response characteristics during fault recovery; Classify the active transient response characteristics of the complete fault process.

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