A method and system for determining the influence of multi-frequency coupling on power of a doubly-fed wind turbine

Abstract

The application provides a method and system for determining the influence of multi-frequency coupling of a doubly-fed wind turbine on power, comprising: converting a pre-acquired single-phase voltage and current of a power grid into a two-phase orthogonal coordinate system to obtain voltage and current values in the two-phase orthogonal coordinate system; obtaining instantaneous active power and instantaneous reactive power by using instantaneous reactive power theory based on the voltage and current values in the two-phase orthogonal coordinate system; and determining the influence of multi-frequency coupling of a doubly-fed wind turbine on output power under subsynchronous disturbance based on the instantaneous active power and instantaneous reactive power.

Description

Technical Field

[0001] This invention relates to a doubly fed wind turbine, and more specifically to a method and system for determining the power effect of multi-frequency coupling in a doubly fed wind turbine. Background Technology

[0002] Doubly-fed induction generator (DFIG) wind turbines are among the most widely used types of wind turbines, and their grid connection stability is a major concern. Since DFIG wind farms are generally built far from load centers, grid connection oscillations of DFIG wind turbines mostly occur under two grid connection conditions: one is connection to a weak grid with non-negligible grid impedance, where the interaction between the grid impedance and the equivalent impedance of the DFIG wind turbine may lead to grid connection oscillations; the other is connection to a series-compensated grid with series compensation capacitors, in which case the grid-connected system faces the risk of subsynchronous resonance. After subsynchronous oscillations occur, multiple interharmonics of subsynchronous / supersynchronous frequencies will couple into the voltage and current of the DFIG wind turbine, affecting the active and reactive power output of the wind turbine. Current methods for obtaining the oscillation components of the DFIG wind turbine output power are complex to operate and time-consuming, leading to stagnation in DFIG wind turbine technology. Summary of the Invention

[0003] To address the issue that existing methods for obtaining the oscillation component of the output power of doubly-fed induction generator (DFIG) wind turbines are complex and time-consuming, this invention provides a method for determining the impact of multi-frequency coupling on the power of DFIG wind turbines, including:

[0004] The pre-acquired single-phase voltage and current of the power grid are converted into a two-phase orthogonal coordinate system to obtain the voltage and current values ​​in the two-phase orthogonal coordinate system.

[0005] Based on the voltage and current values ​​in the two-phase orthogonal coordinate system, instantaneous active power and instantaneous reactive power are obtained using the instantaneous reactive power theory.

[0006] The impact of multi-frequency coupling on the output power of a doubly-fed wind turbine under subsynchronous disturbance is determined based on the instantaneous active power and instantaneous reactive power.

[0007] Preferably, the acquisition of the single-phase voltage and current of the power grid includes:

[0008] When the system experiences subsynchronous oscillation, the single-phase voltage and current of the power grid are determined based on the amplitude and phase angle of the current and voltage of the power frequency, subsynchronous disturbance, and supersynchronous disturbance.

[0009] Preferably, the step of converting the pre-acquired single-phase voltage and current of the power grid to a two-phase orthogonal coordinate system to obtain the voltage and current values ​​in the two-phase orthogonal coordinate system includes:

[0010] The three-phase voltage is converted to a two-phase orthogonal coordinate system to obtain the voltage value in the two-phase orthogonal coordinate system.

[0011] The three-phase current is converted to a two-phase orthogonal coordinate system to obtain the current value in the two-phase orthogonal coordinate system.

[0012] Preferably, the step of obtaining instantaneous active power and instantaneous reactive power based on the voltage and current values ​​in the two-phase orthogonal coordinate system using instantaneous reactive power theory includes:

[0013] Based on the voltage and current values ​​in the two-phase orthogonal coordinate system, determine the relationship between instantaneous active power, instantaneous reactive power, and the voltage and current values ​​in the two-phase orthogonal coordinate system;

[0014] Based on the relationship between the instantaneous active power, instantaneous reactive power, and the voltage and current values ​​in the two-phase orthogonal coordinate system, the instantaneous active power and instantaneous reactive power are calculated.

[0015] Preferably, the single-phase voltage and current of the power grid are calculated using the following formulas:

[0016]

[0017] In the formula: u a The voltage of phase a of the power grid; i a ω is the current in phase a of the power grid; ω1 is the synchronous speed; ω er I1 is the frequency of the three-phase voltage under subsynchronous disturbance; t is time; I1 is the amplitude of the power frequency current; I2 is the amplitude of the subsynchronous disturbance current; I3 is the amplitude of the supersynchronous disturbance current; U1 is the amplitude of the power frequency voltage; U2 is the amplitude of the subsynchronous disturbance voltage; and U3 is the amplitude of the supersynchronous disturbance voltage. The phase angle of the power frequency current; The phase angle of the subsynchronous disturbance current; Phase angle of the supersynchronous disturbance current; The phase angle of the power frequency voltage; Phase angle of the subsynchronous disturbance voltage; The phase angle of the supersynchronous disturbance voltage.

[0018] Preferably, the voltage values ​​in the two-phase orthogonal coordinate system are calculated using the following formula:

[0019]

[0020] In the formula, u α The voltage along the α-axis; u β For β-axis voltage; u a The voltage of phase a of the power grid; u b The voltage of phase b of the power grid; u c This is the voltage of phase c of the power grid.

[0021] Preferably, the current value in the two-phase orthogonal coordinate system is calculated using the following formula:

[0022]

[0023] In the formula, i α i is the α-axis current; β For β-axis current; i a Let i be the current in phase a of the power grid; b For phase b current of the power grid; i c This represents the c-phase current of the power grid.

[0024] Preferably, the relationship between the instantaneous active power, instantaneous reactive power, and the voltage and current values ​​in the two-phase orthogonal coordinate system is as shown in the following formula:

[0025]

[0026] In the formula, p is the instantaneous active power; q is the instantaneous reactive power.

[0027] Preferably, the instantaneous active power is calculated using the following formula:

[0028]

[0029] The instantaneous reactive power is shown in the following formula:

[0030]

[0031] Preferably, determining the impact of multi-frequency coupling of the doubly-fed induction generator on the output power under subsynchronous disturbance based on the instantaneous active power and instantaneous reactive power includes:

[0032] The first three terms of instantaneous active power and instantaneous reactive power are DC components, and the frequencies of the fourth to seventh terms are (ω1-ω). er The subsynchronous power frequency complementary component, the last two terms of which have a frequency of 2(ω1-ω) er The amount of )

[0033] The frequency of the voltage and current of the doubly fed wind turbine is ω er and (2ω1-ω er The disturbance component, with a frequency of (ω1-ω), is coupled into the output power. er ) and 2(ω1-ω er The oscillating component of );

[0034] Consider voltage and current with frequency n(ω1-ω) er If the disturbance is ±ω1, then the wind farm output power will also be coupled with a frequency of n(ω1-ω1). er The oscillation component of ).

[0035] Based on the same inventive concept, this invention provides a system for determining the power impact of multi-frequency coupling in a doubly fed wind turbine, including a module for determining single-phase voltage and current, a module for determining voltage and current in a two-phase orthogonal coordinate system, and an analysis module.

[0036] The module for determining single-phase voltage and current converts the pre-acquired single-phase voltage and current of the power grid into a two-phase orthogonal coordinate system to obtain the voltage and current values ​​in the two-phase orthogonal coordinate system.

[0037] The module for determining the voltage and current in the two-phase orthogonal coordinate system: Based on the voltage and current values ​​in the two-phase orthogonal coordinate system, instantaneous active power and instantaneous reactive power are obtained using instantaneous reactive power theory;

[0038] The analysis module determines the impact of multi-frequency coupling of the doubly-fed wind turbine on the output power under subsynchronous disturbance based on the instantaneous active power and instantaneous reactive power.

[0039] Preferably, the module for determining the voltage and current in the two-phase orthogonal coordinate system includes a voltage conversion submodule and a current conversion submodule.

[0040] The voltage conversion submodule converts the three-phase voltage to a two-phase orthogonal coordinate system to obtain the voltage value in the two-phase orthogonal coordinate system.

[0041] The current conversion submodule converts the three-phase current into a two-phase orthogonal coordinate system to obtain the current value in the two-phase orthogonal coordinate system.

[0042] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0043] 1. This invention provides a method and system for determining the impact of multi-frequency coupling on the power of a doubly-fed induction generator (DFIG) wind turbine, comprising: converting pre-acquired single-phase voltage and current of the power grid into a two-phase orthogonal coordinate system to obtain voltage and current values ​​in the two-phase orthogonal coordinate system; based on the voltage and current values ​​in the two-phase orthogonal coordinate system, using instantaneous reactive power theory to obtain instantaneous active power and instantaneous reactive power; and based on the instantaneous active power and instantaneous reactive power, determining the impact of multi-frequency coupling of the DFIG wind turbine on the output power under subsynchronous disturbance. This invention, based on instantaneous reactive power theory, calculates instantaneous active power and instantaneous reactive power using pre-acquired single-phase voltage and current, effectively determining the impact of multi-frequency coupling of the DFIG wind turbine on the output power under subsynchronous disturbance.

[0044] 2. This invention converts the three-phase voltage to a two-phase orthogonal coordinate system, converts the three-phase current to a two-phase orthogonal coordinate system, and defines the instantaneous active power and instantaneous reactive power. It also determines the expressions for instantaneous active power and instantaneous reactive power. Through these expressions, the influence of multi-frequency coupling of doubly-fed wind turbine generators on output power under subsynchronous disturbances can be analyzed. Attached Figure Description

[0045] Figure 1 This is a schematic diagram illustrating the method for determining the power impact of multi-frequency coupling in a doubly fed wind turbine according to the present invention. Detailed Implementation

[0046] The embodiments of the present invention will be further described with reference to the accompanying drawings.

[0047] Example 1

[0048] Combination Figure 1 This invention provides a method for determining the power impact of multi-frequency coupling in a doubly-fed induction generator (DFIG) wind turbine, comprising:

[0049] Step 1: Convert the pre-acquired single-phase voltage and current of the power grid into a two-phase orthogonal coordinate system to obtain the voltage and current values ​​in the two-phase orthogonal coordinate system;

[0050] Step 2: Based on the voltage and current values ​​in the two-phase orthogonal coordinate system, the instantaneous active power and instantaneous reactive power are obtained using the instantaneous reactive power theory;

[0051] Step 3: Determine the impact of multi-frequency coupling of doubly-fed wind turbine on output power under subsynchronous disturbance based on instantaneous active power and instantaneous reactive power.

[0052] Step one involves converting the pre-acquired single-phase voltage and current of the power grid into a two-phase orthogonal coordinate system to obtain the voltage and current values ​​in the two-phase orthogonal coordinate system. This specifically includes:

[0053] The acquisition of single-phase voltage and current in the power grid includes:

[0054] When the system experiences subsynchronous oscillation, the single-phase voltage and current of the power grid are determined based on the amplitude and phase angle of the current and voltage of the power frequency, subsynchronous disturbance, and supersynchronous disturbance.

[0055] The pre-acquired single-phase voltage and current of the power grid are converted to a two-phase orthogonal coordinate system to obtain the voltage and current values ​​in the two-phase orthogonal coordinate system, including:

[0056] The three-phase voltage is converted to a two-phase orthogonal coordinate system to obtain the voltage value in the two-phase orthogonal coordinate system.

[0057] The three-phase current is converted to a two-phase orthogonal coordinate system to obtain the current value in the two-phase orthogonal coordinate system.

[0058] Based on the voltage and current values ​​in a two-phase orthogonal coordinate system, instantaneous active power and instantaneous reactive power are obtained using instantaneous reactive power theory, including:

[0059] Based on the voltage and current values ​​in the two-phase orthogonal coordinate system, determine the relationship between instantaneous active power, instantaneous reactive power, and the voltage and current values ​​in the two-phase orthogonal coordinate system;

[0060] Based on the relationship between instantaneous active power, instantaneous reactive power, and voltage and current values ​​in a two-phase orthogonal coordinate system, instantaneous active power and instantaneous reactive power are calculated.

[0061] The single-phase voltage and current of the power grid are calculated using the following formulas:

[0062]

[0063] In the formula: u a The voltage of phase a of the power grid; i a ω is the current in phase a of the power grid; ω1 is the synchronous speed; ω er I1 is the frequency of the three-phase voltage under subsynchronous disturbance; t is time; I1 is the amplitude of the power frequency current; I2 is the amplitude of the subsynchronous disturbance current; I3 is the amplitude of the supersynchronous disturbance current; U1 is the amplitude of the power frequency voltage; U2 is the amplitude of the subsynchronous disturbance voltage; and U3 is the amplitude of the supersynchronous disturbance voltage. The phase angle of the power frequency current; The phase angle of the subsynchronous disturbance current; Phase angle of the supersynchronous disturbance current; The phase angle of the power frequency voltage; Phase angle of the subsynchronous disturbance voltage; The phase angle of the supersynchronous disturbance voltage.

[0064] The voltage values ​​in a two-phase orthogonal coordinate system are calculated using the following formula:

[0065]

[0066] In the formula, u α The voltage along the α-axis; u β For β-axis voltage; u a The voltage of phase a of the power grid; u b The voltage of phase b of the power grid; u c This is the voltage of phase c of the power grid.

[0067] The current value in a two-phase orthogonal coordinate system is calculated using the following formula:

[0068]

[0069] In the formula, i α i is the α-axis current; β For β-axis current; i a Let i be the current in phase a of the power grid; b For phase b current of the power grid; i c This represents the c-phase current of the power grid.

[0070] Step two involves obtaining instantaneous active power and instantaneous reactive power based on the voltage and current values ​​in a two-phase orthogonal coordinate system using instantaneous reactive power theory. This includes:

[0071] The relationship between instantaneous active power, instantaneous reactive power, and the voltage and current values ​​in the two-phase orthogonal coordinate system is shown in the following equation:

[0072]

[0073] In the formula, p is the instantaneous active power; q is the instantaneous reactive power.

[0074] Instantaneous active power is calculated using the following formula:

[0075]

[0076] Instantaneous reactive power is shown in the following formula:

[0077]

[0078] Step 3 involves determining the impact of multi-frequency coupling of the doubly-fed induction generator (DFIG) on the output power under subsynchronous disturbances based on instantaneous active and reactive power, including:

[0079] The first three terms of instantaneous active power and instantaneous reactive power are DC components, and the frequencies of the fourth to seventh terms are (ω1-ω). er The subsynchronous power frequency complementary component, the last two terms of which have a frequency of 2(ω1-ω) er The amount of )

[0080] The frequency of the voltage and current of the doubly fed wind turbine is ω er and (2ω1-ω er The disturbance component, with a frequency of (ω1-ω), is coupled into the output power. er ) and 2(ω1-ω er The oscillating component of );

[0081] Consider voltage and current with frequency n(ω1-ω) er If the disturbance is ±ω1, then the wind farm output power will also be coupled with a frequency of n(ω1-ω1). er The oscillation component of ).

[0082] Example 2

[0083] This invention proposes a method and system for determining the impact of multi-frequency coupling on the power output of a doubly-fed induction generator (DFIG) wind turbine. The method includes: after subsynchronous oscillation occurs in the system, multiple interharmonics of subsynchronous / supersynchronous frequencies will couple into the voltage and current of the DFIG wind turbine, affecting the active and reactive power output of the wind turbine. Only frequencies with larger amplitudes, such as ω, are considered. er and (2ω1-ωer When the components of ) are present, the single-phase voltage and current of the power grid can be expressed as:

[0084]

[0085] In the formula: ω1 is the synchronous speed; ω er t represents the frequency of the three-phase voltage under subsynchronous disturbance; I1, I2, and I3 represent the amplitudes of the power frequency, subsynchronous disturbance, and supersynchronous disturbance currents, respectively; U1, U2, and U3 represent the amplitudes of the power frequency, subsynchronous disturbance, and supersynchronous disturbance voltages, respectively. These are the phase angles of the power frequency, subsynchronous disturbance, and supersynchronous disturbance currents, respectively. These are the phase angles of the power frequency, subsynchronous disturbance, and supersynchronous disturbance voltages, respectively.

[0086] For sinusoidal steady-state circuits, traditional power theory uses average power values ​​to define active power and instantaneous maximum reactive power values ​​to define reactive power. However, for non-sinusoidal cases where voltage and current contain interharmonics, the power definitions in traditional power theory cannot accurately describe the power fluctuation characteristics caused by interharmonics. In such cases, instantaneous reactive power theory can be used for analysis.

[0087] The theory of instantaneous reactive power requires converting the three-phase voltage and current into a two-phase orthogonal coordinate system:

[0088]

[0089]

[0090] Instantaneous active power p and instantaneous reactive power q are defined as follows:

[0091]

[0092] The expressions for p and q are calculated as follows:

[0093]

[0094]

[0095] The first three terms of p and q are DC components, and the fourth to seventh terms are frequencies of (ω1-ω). er The subsynchronous power frequency complementary components, the last two terms are at a frequency of 2(ω1-ω). er The components of ) are analyzed using instantaneous reactive power theory. The frequency ω in the DFIG voltage and current is... er and (2ω1-ω er The disturbance component, with a frequency of (ω1-ω), is coupled into the output power. er ) and 2(ω1-ω erThe oscillating component of the voltage and current with a frequency of n(ω1-ω) er If the disturbance is ±ω1, then the wind farm output power will also be coupled with a frequency of n(ω1-ω1). er The oscillation component of the electromagnetic torque will also produce similar oscillations, leading to mechanical stress fatigue in the fan and hindering the unit's operation.

[0096] Example 3

[0097] Based on the same inventive concept, this invention provides a system for determining the power impact of multi-frequency coupling in a doubly fed wind turbine, including a module for determining single-phase voltage and current, a module for determining voltage and current in a two-phase orthogonal coordinate system, and an analysis module.

[0098] Determine the single-phase voltage and current module: Convert the pre-acquired single-phase voltage and current of the power grid to a two-phase orthogonal coordinate system to obtain the voltage and current values ​​in the two-phase orthogonal coordinate system;

[0099] Determine the voltage and current modules in the two-phase orthogonal coordinate system: Based on the voltage and current values ​​in the two-phase orthogonal coordinate system, the instantaneous active power and instantaneous reactive power are obtained using the instantaneous reactive power theory;

[0100] The instantaneous active power is calculated using the following formula:

[0101]

[0102] The instantaneous reactive power is calculated using the following formula:

[0103]

[0104] The module for determining voltage and current in a two-phase orthogonal coordinate system includes determining the voltage conversion submodule and the current conversion submodule.

[0105] Voltage conversion submodule: Converts three-phase voltage to a two-phase orthogonal coordinate system to obtain the voltage value in the two-phase orthogonal coordinate system;

[0106] The voltage values ​​in a two-phase orthogonal coordinate system are calculated using the following formula;

[0107]

[0108] In the formula, u α The voltage along the α-axis; u β For β-axis voltage; u a The voltage of phase a of the power grid; u b The voltage of phase b of the power grid; u c This is the voltage of phase c of the power grid.

[0109] Current conversion submodule: Converts three-phase current to a two-phase orthogonal coordinate system to obtain the current value in the two-phase orthogonal coordinate system;

[0110] The current value in a two-phase orthogonal coordinate system is calculated using the following formula:

[0111]

[0112] In the formula, i α i is the α-axis current; β For β-axis current; i a Let i be the current in phase a of the power grid; b For phase b current of the power grid; i c This represents the c-phase current of the power grid.

[0113] Analysis module: Determine the impact of multi-frequency coupling on output power of doubly-fed wind turbines under subsynchronous disturbances based on instantaneous active power and instantaneous reactive power.

[0114] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0115] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0116] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0117] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0118] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of the claims of the present invention pending approval.

Claims

1. A method for determining the power impact of multi-frequency coupling in a doubly-fed induction generator (DFIG), characterized in that, include: The pre-acquired single-phase voltage and current of the power grid are converted into a two-phase orthogonal coordinate system to obtain the voltage and current values ​​in the two-phase orthogonal coordinate system. Based on the voltage and current values ​​in the two-phase orthogonal coordinate system, instantaneous active power and instantaneous reactive power are obtained using the instantaneous reactive power theory. The impact of multi-frequency coupling of the doubly-fed induction generator on the output power under subsynchronous disturbance is determined based on the instantaneous active power and instantaneous reactive power. The acquisition of single-phase voltage and current of the power grid includes: When the system experiences subsynchronous oscillation, the single-phase voltage and current of the power grid are determined based on the amplitude and phase angle of the current and voltage of the power frequency, subsynchronous disturbance, and supersynchronous disturbance. The instantaneous active power is calculated using the following formula: The instantaneous reactive power is shown in the following formula: In the formula: p Instantaneous active power; q Instantaneous reactive power; Synchronous speed; The frequency of occurrence in the three-phase voltage under subsynchronous disturbance; For time; This represents the amplitude of the power frequency current. The magnitude of the subsynchronous disturbance current; The magnitude of the supersynchronous disturbance current; U 1 represents the amplitude of the power frequency voltage; U 2 represents the amplitude of the subsynchronous disturbance voltage. U 3 represents the amplitude of the supersynchronous disturbance voltage; i1 The phase angle of the power frequency current; i2 The phase angle of the subsynchronous disturbance current; i3 The phase angle of the supersynchronous disturbance current; u1 The phase angle of the power frequency voltage; u2 The phase angle of the subsynchronous disturbance voltage; u3 The phase angle of the supersynchronous disturbance voltage; The impact of multi-frequency coupling of a doubly-fed induction generator (DFIG) on its output power under subsynchronous disturbance is determined based on the instantaneous active power and instantaneous reactive power, including: The first three terms of instantaneous active power and instantaneous reactive power are DC components, and the frequencies of the fourth to seventh terms are (…). ω 1- ω er The subsynchronous power frequency complementary components, the last two terms have a frequency of 2 ( ω 1- ω er The amount of ) The frequency of voltage and current in a doubly fed wind turbine is er and (2) 1- er A disturbance component of frequency () is coupled into the output power. ω 1- ω er ) and 2 ( ω 1- ω er The oscillating component of ); Considering the frequency of voltage and current is n ( ω 1- ω er )± ω A disturbance of 1 will correspondingly couple a frequency of into the wind farm's output power. n ( ω 1- ω er The oscillation component of ).

2. The method as described in claim 1, characterized in that, The step of converting the pre-acquired single-phase voltage and current of the power grid to a two-phase orthogonal coordinate system to obtain the voltage and current values ​​in the two-phase orthogonal coordinate system includes: The three-phase voltage is converted to a two-phase orthogonal coordinate system to obtain the voltage value in the two-phase orthogonal coordinate system. The three-phase current is converted to a two-phase orthogonal coordinate system to obtain the current value in the two-phase orthogonal coordinate system.

3. The method as described in claim 2, characterized in that, The instantaneous active power and instantaneous reactive power are obtained based on the voltage and current values ​​in the two-phase orthogonal coordinate system using instantaneous reactive power theory, including: Based on the voltage and current values ​​in the two-phase orthogonal coordinate system, determine the relationship between instantaneous active power, instantaneous reactive power, and the voltage and current values ​​in the two-phase orthogonal coordinate system; Based on the relationship between the instantaneous active power, instantaneous reactive power, and the voltage and current values ​​in the two-phase orthogonal coordinate system, the instantaneous active power and instantaneous reactive power are calculated.

4. The method as described in claim 3, characterized in that, The single-phase voltage and current of the power grid are calculated using the following formulas: In the formula: The voltage of phase a of the power grid; Let be the current in phase a of the power grid.

5. The method as described in claim 4, characterized in that, The voltage values ​​in the two-phase orthogonal coordinate system are calculated using the following formula: In the formula, The voltage along the α-axis; This is the voltage along the β axis; The voltage of phase a of the power grid; This refers to the voltage of phase b of the power grid. This is the voltage of phase c of the power grid.

6. The method as described in claim 5, characterized in that, The current values ​​in the two-phase orthogonal coordinate system are calculated using the following formula: In the formula, For α-axis current; For β-axis current; The current in phase a of the power grid; This refers to the current in phase b of the power grid. This represents the c-phase current of the power grid.

7. The method as described in claim 6, characterized in that, The relationship between the instantaneous active power, instantaneous reactive power, and the voltage and current values ​​in the two-phase orthogonal coordinate system is shown in the following formula: 。 8. A system for determining the power influence of multi-frequency coupling in a doubly-fed wind turbine, characterized in that, This includes modules for determining single-phase voltage and current, determining voltage and current in a two-phase orthogonal coordinate system, and analysis modules. The module for determining single-phase voltage and current converts the pre-acquired single-phase voltage and current of the power grid into a two-phase orthogonal coordinate system to obtain the voltage and current values ​​in the two-phase orthogonal coordinate system. The module for determining the voltage and current in the two-phase orthogonal coordinate system: Based on the voltage and current values ​​in the two-phase orthogonal coordinate system, instantaneous active power and instantaneous reactive power are obtained using instantaneous reactive power theory; The analysis module determines the impact of multi-frequency coupling of the doubly-fed induction generator on the output power under subsynchronous disturbance based on the instantaneous active power and instantaneous reactive power. The module for determining single-phase voltage and current: when the system experiences subsynchronous oscillation, it determines the single-phase voltage and current of the power grid based on the amplitude and phase angle of the current and voltage of the power frequency, subsynchronous disturbance, and supersynchronous disturbance. The instantaneous active power is calculated using the following formula: The instantaneous reactive power is shown in the following formula: In the formula: p Instantaneous active power; q Instantaneous reactive power; Synchronous speed; The frequency of occurrence in the three-phase voltage under subsynchronous disturbance; For time; This represents the amplitude of the power frequency current. The amplitude of the subsynchronous disturbance current; The magnitude of the supersynchronous disturbance current; U 1 represents the amplitude of the power frequency voltage; U 2 represents the amplitude of the subsynchronous disturbance voltage. U 3 represents the amplitude of the supersynchronous disturbance voltage; i1 The phase angle of the power frequency current; i2 The phase angle of the subsynchronous disturbance current; i3 The phase angle of the supersynchronous disturbance current; u1 The phase angle of the power frequency voltage; u2 The phase angle of the subsynchronous disturbance voltage; u3 The phase angle of the supersynchronous disturbance voltage; The analysis module: The first three terms of instantaneous active power and instantaneous reactive power are DC components, and the frequencies of the fourth to seventh terms are (…). ω 1- ω er The subsynchronous power frequency complementary components, the last two terms have a frequency of 2 ( ω 1- ω er The amount of ) The frequency of voltage and current in a doubly fed wind turbine is er and (2) 1- er A disturbance component of frequency () is coupled into the output power. ω 1- ω er ) and 2 ( ω 1- ω er The oscillating component of ); Considering the frequency of voltage and current is n ( ω 1- ω er )± ω A disturbance of 1 will correspondingly couple a frequency of into the wind farm's output power. n ( ω 1- ω er The oscillation component of ).

9. The system as described in claim 8, characterized in that, The module for determining the voltage and current in a two-phase orthogonal coordinate system includes a voltage conversion submodule and a current conversion submodule. The voltage conversion submodule converts the three-phase voltage to a two-phase orthogonal coordinate system to obtain the voltage value in the two-phase orthogonal coordinate system. The current conversion submodule converts the three-phase current into a two-phase orthogonal coordinate system to obtain the current value in the two-phase orthogonal coordinate system.

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