A current distribution method to enhance the operation capability of doubly-fed wind turbine generator system under asymmetric fault

Abstract

The present invention discloses a current distribution method for enhancing the operating capability of a doubly-fed wind turbine generator set under asymmetric fault conditions. The method involves providing active current and reactive current instructions to the grid-side converter and rotor-side converter of the doubly-fed wind turbine generator set. The control current instruction obtained by the rotor-side converter is sent to the rotor current loop controller, thereby suppressing the double-frequency fluctuation of the electromagnetic torque output on the stator side of the doubly-fed wind turbine generator set. The control current instruction obtained by the grid-side converter is sent to the grid-side current loop controller, thereby suppressing the double-frequency fluctuation of the total output reactive power of the doubly-fed wind turbine generator set, thereby improving the doubly-fed wind turbine generator set's ability to operate without disconnecting from the grid. The present invention fully utilizes the capacity of the rotor-side and grid-side converters in the doubly-fed wind turbine generator set to suppress the double-frequency fluctuation of the reactive power and electromagnetic torque on the basis of meeting the minimum requirements of the grid-connected guidelines, and maximizes the supply of active power to the grid, thereby improving the operating reliability of the doubly-fed wind turbine generator set.

Description

Technical Field

[0001] The present invention relates to wind power generation technology, and in particular to a current distribution method for enhancing the operating capability of a double-fed induction generator (DFIG) under asymmetric fault conditions, belonging to the field of new energy power generation technology. Background Art

[0002] Compared with symmetrical short-circuit faults in the power grid, asymmetrical short-circuit faults in the power grid occur more frequently, which will have a bad impact on the operation of the doubly fed wind turbine generator set. When the power grid drops asymmetrically, it will cause electromagnetic torque and output power fluctuations in the doubly fed wind turbine generator set. In addition, the double frequency fluctuations of the electromagnetic torque are transmitted to the motor and manifest as mechanical vibrations of the motor, which will cause mechanical wear of the motor and reduce the service life of the motor. At the same time, the "GB / T19963.1-2021 Technical Specifications for Wind Farm Access to the Power System" clearly states that when an asymmetrical fault occurs, the wind farm must inject positive and negative sequence reactive currents into the grid as required. Therefore, in order to enhance the operating capability of the doubly fed wind turbine generator set under asymmetrical faults, it is necessary to further study the total output reactive power of the doubly fed wind turbine generator set and the method of suppressing the double frequency fluctuations of the electromagnetic torque under asymmetrical faults in the power grid. At present, some scholars have conducted research on the control strategy of the DFIG system under asymmetrical faults in the power grid, such as the following published documents:

[0003] (1) Liu Manqin, Zhou Yusheng, Xu Zhenhua, Tang Ci, Xiao Hui, He Yang. Research on stability control of doubly fed wind turbine generator system under asymmetric grid fault[J]. Electrical Measurement and Instrumentation, 2021, 58(01): 131-136.

[0004] (2) Yao Jun, Yu Mengting, Zhao Lei, Li Qing. Method for suppressing total output reactive power fluctuation of doubly-fed wind power system under unbalanced and harmonically distorted grid voltage[P]. Chongqing: CN103997063B, 2016-01-20.

[0005] Reference (1) uses a coordinated control method for machine-side and grid-side converters. During an asymmetric grid fault, the negative sequence component of the rotor current of the machine-side converter is controlled to zero. The grid-side converter uses dual closed-loop positive and negative sequence current control to suppress the negative sequence component on the grid side, thereby effectively suppressing the electromagnetic torque and current double frequency fluctuations of the unit. However, this strategy always keeps the negative sequence reactive component of the rotor current zero, which is contrary to the grid-connected guideline's requirements for the reactive current of the wind turbine during a fault. Reference (2) adds a series grid-side converter to suppress the double and six frequency fluctuations of the total system output reactive power. However, this strategy involves the control of the series grid-side converter, the grid-connected converter, and the machine-side converter, making the control structure too complex and increasing the system hardware cost.

[0006] During asymmetric grid faults, DFIG wind turbines can experience output power fluctuations, current harmonics, and torque fluctuations, which can jeopardize the operational safety and reliability of the turbine. Therefore, while meeting grid-connection guidelines, this study leverages the coordinated control capabilities of the rotor-side and grid-side converters of DFIG wind turbines to investigate methods for suppressing fluctuations in the total reactive power and electromagnetic torque at the second harmonic frequency of DFIG wind turbines during asymmetric grid faults, thereby enhancing their operational reliability. Summary of the Invention

[0007] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to propose a current distribution method for enhancing the operating capability of a doubly-fed wind turbine generator set under asymmetric faults. This method fully utilizes the capacity of the rotor-side and grid-side converters in the doubly-fed wind turbine generator set to suppress the reactive power and electromagnetic torque double frequency fluctuations on the basis of meeting the minimum requirements of the grid connection guidelines, and maximizes the supply of active power to the grid, thereby improving the operating reliability of the doubly-fed wind turbine generator set.

[0008] The technical solution of the present invention is achieved as follows:

[0009] A current distribution method for enhancing the operating capability of a doubly-fed wind turbine generator system under asymmetric fault conditions, characterized in that: the method involves giving instructions for active current and reactive current to a grid-side converter and a rotor-side converter of the doubly-fed wind turbine generator system;

[0010] (A) Calculation of the positive and negative sequence active current instructions of the rotor-side converter of the doubly fed wind turbine during an asymmetric grid fault and positive and negative sequence reactive current instructions The specific calculation steps are:

[0011] A1) During a fault, the grid connection code requires the injection of positive and negative sequence reactive currents Then the reactive current expression of the rotor side is:

[0012]

[0013] Where, are the positive and negative sequence components of the terminal voltage of the doubly fed wind turbine in the positive and reverse dq synchronous rotating coordinate system; ω1 is the synchronous angular frequency; L s and L m are the equivalent inductance and equivalent mutual inductance of the stator side winding respectively; K + , K - is the dynamic positive and negative sequence reactive current proportional coefficient;

[0014] A2) Suppress the fluctuation of electromagnetic torque, that is, suppress the electromagnetic power sinusoidal quantity P esin2 Sum and cosine quantity P ecos2 , P esin2 and P ecos2The expression is:

[0015]

[0016] Where, ω r is the rotor angular frequency;

[0017] A3) Substitute the rotor side reactive current expression obtained in step A1) into the electromagnetic power sinusoidal value P in step A2) esin2 , the expression for calculating the sinusoidal quantity of electromagnetic power when meeting the grid connection guideline requirements is:

[0018]

[0019] A4) Since the electromagnetic power sinusoidal quantity obtained in step A3) is not zero when meeting the grid connection guideline requirements, in order to minimize the electromagnetic torque fluctuation, let K + =1, K - =1, calculate the positive and negative sequence reactive current on the rotor side The expression is:

[0020]

[0021] A5) In order to make the electromagnetic power cosine value P ecos2 = 0, and meet the maximum wind energy tracking control, calculate the positive and negative sequence active currents on the rotor side The expression is:

[0022]

[0023] Where, P max It is the power setting value of maximum wind energy tracking control;

[0024] A6) Due to the limited capacity of the rotor-side converter, based on the rotor-side positive and negative sequence active current and reactive current expressions obtained in steps A4) and A5), the constraint conditions for determining whether the doubly-fed wind turbine generator system can achieve maximum wind energy tracking control while meeting the grid connection guideline requirements under this operating condition are:

[0025]

[0026] Where, I rmax The maximum current allowed for the rotor-side converter of the doubly-fed wind turbine generator system;

[0027] A7) If the constraints in step A6) are met, maximum wind energy tracking control is achieved while meeting the grid-connected code requirements and suppressing the double frequency fluctuation of the electromagnetic torque. The rotor-side positive and negative sequence active current and reactive current commands are calculated as:

[0028]

[0029] If the constraints in step A6) are not met, the doubly fed wind turbine generator set cannot achieve maximum wind energy tracking control, but it can use the rotor residual current margin to output a certain amount of active power. The rotor side positive and negative sequence active current and reactive current instructions are calculated as:

[0030]

[0031] Where,

[0032] A8) The control current instruction obtained in step A7) By feeding it into the rotor current loop controller, the double frequency fluctuation of the output electromagnetic torque on the stator side of the doubly fed wind turbine can be suppressed;

[0033] (B) Positive and negative sequence active current instructions of the grid-side converter of the doubly fed wind turbine during asymmetric grid fault and positive and negative sequence reactive current instructions Calculation, the specific calculation steps are:

[0034] B1) In order to suppress the double frequency fluctuation of the total output reactive power, the grid side outputs the sinusoidal value of reactive power Q gsin2 Sum and cosine Q gcos2 The following relationship should be satisfied:

[0035]

[0036] Where, is the positive and negative sequence active current of the grid-side converter; is the positive and negative sequence reactive current of the grid-side converter; are the positive and negative sequence components of the voltage at the grid connection point of the doubly fed wind turbine in the positive and reverse dq synchronous rotating coordinate system;

[0037] B2) Based on the relationship between the positive and cosine quantities of the grid-side output reactive power obtained in step B1), calculate the positive and negative sequence active currents of the grid-side converter and positive and negative sequence reactive current The expression is:

[0038]

[0039] Where, and U dc are the DC bus voltage setting and feedback, k p and k i are the proportional coefficient and integral coefficient of the PI controller of the active current command calculation module of the grid-side converter, and s is the Laplace operator;

[0040] B3) Due to the capacity limitations of the grid-side converter, based on the grid-side positive and negative sequence active current and reactive current expressions obtained in step B2), the constraint conditions for determining whether the grid-side converter can maintain a constant average DC bus voltage while completely suppressing the double frequency fluctuation of the total reactive power under this operating condition are:

[0041]

[0042] Where: I gmax The maximum current allowed for the grid-side converter of the doubly-fed wind turbine;

[0043] B4) If the constraints of step B3) are met, the grid-side converter can ensure a constant average DC bus voltage and completely suppress the double frequency fluctuation of the total reactive power. The positive and negative sequence active current and reactive current commands are calculated as:

[0044]

[0045] If the constraints in step B3) are not met, the grid-side converter prioritizes ensuring a constant average DC bus voltage, and then utilizes the residual current margin on the rotor side to output a certain amount of reactive power to reduce the double frequency fluctuation of the total reactive power. The positive and negative sequence active current and reactive current commands are calculated as:

[0046]

[0047] B5) The control current instruction obtained in step B4) By sending it into the grid-side current loop controller, the double-frequency fluctuation of the total output reactive power of the doubly-fed wind turbine can be suppressed, and the ability of the doubly-fed wind turbine to operate without being disconnected from the grid can be improved.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] The present invention provides positive and negative sequence dq axis current given instructions for the rotor-side and grid-side converters of a doubly-fed wind turbine generator set, taking full account of the converter capacity and the unit operating conditions. This allows the doubly-fed wind turbine generator set to suppress reactive power and electromagnetic torque double frequency fluctuations while meeting the minimum requirements of the grid-connected guidelines, and to provide active power to the grid to the maximum extent, thereby improving the operating reliability of the doubly-fed wind turbine generator set. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 This is a structural diagram of a doubly-fed wind turbine connected to a power system.

[0051] Figure 2 This is a block diagram of the control method for the doubly-fed wind turbine generator system according to the present invention.

[0052] Figure 3This is a schematic diagram of a current instruction calculation module for a rotor-side converter of a doubly-fed wind turbine generator system according to the present invention.

[0053] Figure 4 This is a schematic diagram of the current instruction calculation module of the grid-side converter of the doubly-fed wind turbine of the present invention.

[0054] Figure 5 The following is a simulation waveform diagram of the operating characteristics of the doubly fed wind turbine when the voltage imbalance at the grid connection point is 23% and the voltage positive sequence component drops to 0.71pu.

[0055] Figure 6 The following is a simulation waveform diagram of a doubly-fed wind turbine generator system using the control method of the present invention when the voltage imbalance at the grid connection point is 23% and the voltage positive sequence component drops to 0.71pu.

[0056] Figure 7 The figure is a simulation waveform diagram of a doubly-fed wind turbine generator system using the control method of the present invention when the voltage imbalance at the grid connection point is 46% and the voltage positive sequence component drops to 0.52 pu. DETAILED DESCRIPTION

[0057] The specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0058] The present invention provides a current distribution method for enhancing the operating capability of a doubly-fed wind turbine generator set under an asymmetric fault, which is used to improve the non-offline operating capability of the doubly-fed wind turbine generator set under an asymmetric fault in the power grid and suppress the total output reactive power and electromagnetic torque double frequency fluctuations of the doubly-fed wind turbine generator set during the fault process.

[0059] Figure 1 This is a structural diagram of a 50MW doubly-fed wind turbine connected to the power system. The doubly-fed wind turbine is connected to the large power grid through a public connection point.

[0060] Figure 2 The control block diagram of the doubly-fed wind turbine generator system according to the present invention under a grid asymmetric fault condition includes the following control objects: a rotor-side converter 1, a grid-side converter 2, a rotor-side converter current command calculation module 3, a grid-side converter current command calculation module 4, a rotor current loop control module 5, and a grid-side current loop control module 6.

[0061] This method involves giving instructions for active current and reactive current of a grid-side converter and a rotor-side converter of a doubly-fed wind turbine. The specific implementation steps of the present invention are as follows:

[0062] (A) Calculation of the positive and negative sequence active current instructions of the rotor-side converter of the doubly fed wind turbine during an asymmetric grid fault and positive and negative sequence reactive current instructions The specific calculation steps are:

[0063] A1) During a fault, the grid connection code requires the injection of positive and negative sequence reactive currents Then the reactive current expression of the rotor side is:

[0064]

[0065] Where, are the positive and negative sequence components of the terminal voltage of the doubly fed wind turbine in the positive and reverse dq synchronous rotating coordinate system; ω1 is the synchronous angular frequency; L s and L m are the equivalent inductance and equivalent mutual inductance of the stator side winding respectively; K + , K - is the dynamic positive and negative sequence reactive current proportional coefficient;

[0066] A2) Suppress the fluctuation of electromagnetic torque, that is, suppress the electromagnetic power sinusoidal quantity P esin2 Sum and cosine quantity P ecos2 , P esin2 and P ecos2 The expression is:

[0067]

[0068] Where, ω r is the rotor angular frequency;

[0069] A3) Substitute the rotor side reactive current expression obtained in step A1) into the electromagnetic power sinusoidal value P in step A2) esin2 , the expression for calculating the sinusoidal quantity of electromagnetic power when meeting the grid connection guideline requirements is:

[0070]

[0071] A4) Since the electromagnetic power sinusoidal quantity obtained in step A3) is not zero when meeting the grid connection guideline requirements, in order to minimize the electromagnetic torque fluctuation, let K + =1, K - =1, calculate the positive and negative sequence reactive current on the rotor side The expression is:

[0072]

[0073] A5) In order to make the electromagnetic power cosine value P ecos2 = 0, and meet the maximum wind energy tracking control, calculate the positive and negative sequence active currents on the rotor side The expression is:

[0074]

[0075] Where, P max It is the power setting value of maximum wind energy tracking control;

[0076] A6) Due to the limited capacity of the rotor-side converter, based on the rotor-side positive and negative sequence active current and reactive current expressions obtained in steps A4) and A5), the constraint conditions for determining whether the doubly-fed wind turbine generator system can achieve maximum wind energy tracking control while meeting the grid connection guideline requirements under this operating condition are:

[0077]

[0078] Where, I rmax The maximum current allowed for the rotor-side converter of the doubly-fed wind turbine generator system;

[0079] A7) If the constraints in step A6) are met, maximum wind energy tracking control is achieved while meeting the grid-connected code requirements and suppressing the double frequency fluctuation of the electromagnetic torque. The rotor-side positive and negative sequence active current and reactive current commands are calculated as:

[0080]

[0081] If the constraints in step A6) are not met, the doubly fed wind turbine generator set cannot achieve maximum wind energy tracking control, but it can use the rotor residual current margin to output a certain amount of active power. The rotor side positive and negative sequence active current and reactive current instructions are calculated as:

[0082]

[0083] Where,

[0084] A7) If the constraints in step A6) are met, maximum wind energy tracking control is achieved while meeting the grid-connected code requirements and suppressing the double frequency fluctuation of the electromagnetic torque. The rotor-side converter current command calculation module 3 calculates the rotor-side positive and negative sequence active current and reactive current commands as follows:

[0085]

[0086] If the constraints in step A6) are not met, the doubly fed wind turbine generator set cannot achieve maximum wind energy tracking control, but can output a certain amount of active power by utilizing the rotor residual current margin. The rotor-side converter current command calculation module 3 calculates the rotor-side positive and negative sequence active current and reactive current commands as follows:

[0087]

[0088] Where,

[0089] A8) The control current instruction obtained in step A7) The current is sent to the rotor current loop control module 5 to suppress the double frequency fluctuation of the output electromagnetic torque on the stator side of the doubly fed wind turbine.

[0090] (B) Calculation of the positive and negative sequence active current commands of the grid-side converter (GSC) of the doubly-fed wind turbine during a grid asymmetric fault and positive and negative sequence reactive current instructions The specific calculation steps are:

[0091] B1) In order to suppress the double frequency fluctuation of the total output reactive power, the grid side outputs the sinusoidal value of reactive power Q gsin2 Sum and cosine Q gcos2 The following relationship should be satisfied:

[0092]

[0093] Where, is the positive and negative sequence active current of the grid-side converter; is the positive and negative sequence reactive current of the grid-side converter; are the positive and negative sequence components of the voltage at the grid connection point of the doubly fed wind turbine in the positive and reverse dq synchronous rotating coordinate system;

[0094] B2) Based on the relationship between the positive and cosine quantities of the grid-side output reactive power obtained in step B1), calculate the positive and negative sequence active currents of the grid-side converter and positive and negative sequence reactive current The expression is:

[0095]

[0096] Where, and U dc are the DC bus voltage setting and feedback, k p and k i are the proportional coefficient and integral coefficient of the PI controller of the active current command calculation module of the grid-side converter, and s is the Laplace operator;

[0097] B3) Due to the capacity limitations of the grid-side converter, based on the grid-side positive and negative sequence active current and reactive current expressions obtained in step B2), the constraint conditions for determining whether the grid-side converter can maintain a constant average DC bus voltage while completely suppressing the double frequency fluctuation of the total reactive power under this operating condition are:

[0098]

[0099] Where: I gmax The maximum current allowed for the grid-side converter of the doubly-fed wind turbine;

[0100] B4) If the constraints of step B3) are met, the grid-side converter can ensure a constant average DC bus voltage and completely suppress the double frequency fluctuation of the total reactive power. The grid-side converter current command calculation module 4 calculates the positive and negative sequence active current and reactive current commands as:

[0101]

[0102] If the constraints of step B3) are not met, the grid-side converter prioritizes ensuring a constant average DC bus voltage, and then utilizes the residual current margin on the rotor side to output a certain amount of reactive power to reduce the double frequency fluctuation of the total reactive power. The grid-side converter current command calculation module 4 calculates the positive and negative sequence active current and reactive current commands as follows:

[0103]

[0104] B5) The control current instruction obtained in step B4) By sending it to the grid-side current loop control module 6, the double-frequency fluctuation of the total output reactive power of the doubly-fed wind turbine can be suppressed, and the ability of the doubly-fed wind turbine to operate without being disconnected from the grid can be improved.

[0105] Effect description of the present invention:

[0106] Figure 5 The following waveforms illustrate the simulated operating characteristics of a DFIG unit when the voltage positive sequence component drops to 0.71 pu and the imbalance reaches 23%. Under normal voltage conditions, the DFIG unit outputs 0.5 pu of active power. An asymmetric fault occurs in the grid within 1.5 seconds. If the RSC does not employ appropriate control strategies during the duration of the fault, the electromagnetic torque and total reactive power of the DFIG system will experience large double-frequency fluctuations, significantly degrading the power quality of the grid-connected system and potentially leading to wind turbine tripping. Figure 6 The following waveforms show the simulation results of the proposed suppression method when the voltage positive sequence component drops to 0.71 pu and the imbalance reaches 23%. As can be seen from the figure, this method can reduce electromagnetic torque and the double frequency fluctuations of total reactive power while meeting the minimum reactive current requirements of grid-connection guidelines, while also achieving maximum wind energy tracking control. Figure 7 The following waveforms are simulated when the voltage positive sequence component drops to 0.52 pu and the imbalance reaches 46%, using the proposed suppression method. This figure demonstrates that this method can reduce the double-frequency fluctuations of the total reactive power while meeting the minimum reactive current requirements of grid-connection guidelines. While maximum wind energy tracking control is not possible due to converter capacity limitations, a certain amount of active power can still be output using the current headroom.

[0107] Finally, it should be noted that the above examples of the present invention are merely illustrative of the present invention and are not intended to limit the embodiments of the present invention. Although the applicant has described the present invention in detail with reference to preferred embodiments, those skilled in the art will appreciate that other variations and modifications can be made based on the above description. It is not possible to enumerate all embodiments here. Any obvious variations or modifications derived from the technical solution of the present invention remain within the scope of protection of the present invention.

Claims

1. A current distribution method for enhancing the operating capability of a doubly-fed wind turbine generator system under asymmetric fault conditions, characterized by: The method involves giving instructions for active current and reactive current of a grid-side converter and a rotor-side converter of a doubly-fed wind turbine; (A) Calculation of the positive and negative sequence active current instructions of the rotor-side converter of the doubly fed wind turbine during an asymmetric grid fault and positive and negative sequence reactive current instructions The specific calculation steps are: A1) During a fault, the grid connection code requires the injection of positive and negative sequence reactive currents Then the reactive current expression of the rotor side is: Where, are the positive and negative sequence components of the terminal voltage of the doubly fed wind turbine in the positive and reverse dq synchronous rotating coordinate system; ω1 is the synchronous angular frequency; L s and L m are the equivalent inductance and equivalent mutual inductance of the stator side winding respectively; K + , K - is the dynamic positive and negative sequence reactive current proportional coefficient; A2) Suppress the fluctuation of electromagnetic torque, that is, suppress the electromagnetic power sinusoidal quantity P esin2 Sum and cosine quantity P ecos2 , P esin2 and P ecos2 The expression is: Where, ω r is the rotor angular frequency; A3) Substitute the rotor side reactive current expression obtained in step A1) into the electromagnetic power sinusoidal value P in step A2) esin2 , the expression for calculating the sinusoidal quantity of electromagnetic power when meeting the grid connection guideline requirements is: A4) Since the electromagnetic power sinusoidal quantity obtained in step A3) is not zero when meeting the grid connection guideline requirements, in order to minimize the electromagnetic torque fluctuation, let K + =1, K - =1, calculate the positive and negative sequence reactive current on the rotor side The expression is: A5) In order to make the electromagnetic power cosine value P ecos2 = 0, and meet the maximum wind energy tracking control, calculate the positive and negative sequence active currents on the rotor side The expression is: Where, P max It is the power setting value of maximum wind energy tracking control; A6) Due to the limited capacity of the rotor-side converter, based on the rotor-side positive and negative sequence active current and reactive current expressions obtained in steps A4) and A5), the constraint conditions for determining whether the doubly-fed wind turbine generator system can achieve maximum wind energy tracking control while meeting the grid connection guideline requirements under this operating condition are: Where, I rmax The maximum current allowed for the rotor-side converter of the doubly-fed wind turbine generator system; A7) If the constraints in step A6) are met, maximum wind energy tracking control is achieved while meeting the grid-connected code requirements and suppressing the double frequency fluctuation of the electromagnetic torque. The rotor-side positive and negative sequence active current and reactive current commands are calculated as: If the constraints in step A6) are not met, the doubly fed wind turbine generator set cannot achieve maximum wind energy tracking control, but it can use the rotor residual current margin to output a certain amount of active power. The rotor side positive and negative sequence active current and reactive current instructions are calculated as: Where, A8) The control current instruction obtained in step A7) By feeding it into the rotor current loop controller, the double frequency fluctuation of the output electromagnetic torque on the stator side of the doubly fed wind turbine can be suppressed; (B) Positive and negative sequence active current instructions of the grid-side converter of the doubly fed wind turbine during a grid asymmetric fault and positive and negative sequence reactive current instructions Calculation, the specific calculation steps are: B1) In order to suppress the double frequency fluctuation of the total output reactive power, the grid side outputs the sinusoidal value of reactive power Q gsin2 Sum and cosine Q gcos2 The following relationship should be satisfied: Where, is the positive and negative sequence active current of the grid-side converter; is the positive and negative sequence reactive current of the grid-side converter; are the positive and negative sequence components of the voltage at the grid connection point of the doubly fed wind turbine in the positive and reverse dq synchronous rotating coordinate system; B2) Based on the relationship between the positive and cosine quantities of the grid-side output reactive power obtained in step B1), calculate the positive and negative sequence active currents of the grid-side converter and positive and negative sequence reactive current The expression is: Where, and U dc are the DC bus voltage setting and feedback, k p and k i are the proportional coefficient and integral coefficient of the PI controller of the active current command calculation module of the grid-side converter, and s is the Laplace operator; B3) Due to the capacity limitations of the grid-side converter, based on the grid-side positive and negative sequence active current and reactive current expressions obtained in step B2), the constraint conditions for determining whether the grid-side converter can maintain a constant average DC bus voltage while completely suppressing the double frequency fluctuation of the total reactive power under this operating condition are: Where: I gmax The maximum current allowed for the grid-side converter of the doubly-fed wind turbine; B4) If the constraints of step B3) are met, the grid-side converter can ensure a constant average DC bus voltage and completely suppress the double frequency fluctuation of the total reactive power. The positive and negative sequence active current and reactive current commands are calculated as: If the constraints in step B3) are not met, the grid-side converter prioritizes ensuring a constant average DC bus voltage, and then utilizes the residual current margin on the rotor side to output a certain amount of reactive power to reduce the double frequency fluctuation of the total reactive power. The positive and negative sequence active current and reactive current commands are calculated as: B5) The control current instruction obtained in step B4) By sending it into the grid-side current loop controller, the double-frequency fluctuation of the total output reactive power of the doubly-fed wind turbine can be suppressed, and the ability of the doubly-fed wind turbine to operate without being disconnected from the grid can be improved.

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