Method for suppressing subsynchronous oscillation of permanent magnet direct drive wind turbine generator system

Abstract

The application discloses a permanent magnet direct drive wind turbine subsynchronous oscillation suppression method, and belongs to the technical field of new energy generator set stability control, and comprises the following steps: step 1: constructing an inertia synchronous network type direct drive wind turbine model, obtaining a matching relationship between the established network side DC capacitor voltage and the output voltage angle frequency, and obtaining the DC capacitor voltage variation on the wind power converter DC bus; step 2: judging whether subsynchronous oscillation occurs through the voltage variation, if the judgment is yes, executing step 3, if the judgment is no, repeating step 1; step 3: adding a power system stabilizer to the maximum power reference value of the synchronous generator side converter, calculating the control parameters of oscillation suppression, and suppressing the fluctuation of the DC capacitor voltage. The application realizes the suppression of the subsynchronous oscillation generated by the inertia synchronous network type wind turbine system by using the electrical system stabilizer.

Description

Technical Field

[0001] This invention relates to the field of stable control technology for new energy generator sets, specifically to a method for suppressing subsynchronous oscillations in permanent magnet direct-drive wind turbine generator sets. Background Technology

[0002] Due to the uneven distribution of energy resources in my country, wind farms are mostly concentrated in the northern and central regions, while load centers are mostly located along the southeastern coast. This inverse distribution of wind farms and load centers makes the subsynchronous oscillation of new energy generator units a prominent problem in the power system.

[0003] Subsynchronous oscillations are a type of system instability caused by a unique electromechanical coupling effect in power systems. The greatest danger of subsynchronous oscillations is their potential to severely damage the rotor shaft system of large steam turbine generator units, leading to major accidents and jeopardizing the operational safety of the power system. For example, in July 2015, a subsynchronous oscillation accident spanning five voltage levels occurred at a direct-drive wind turbine wind farm in Hami, Xinjiang, my country, resulting in the complete shutdown of three 660 MW units at the Huayuan Power Plant. The damage to the units and equipment demonstrates the significant challenge that subsynchronous oscillations pose to the safe and stable operation of wind power grid-connected systems.

[0004] Based on this, the present invention designs a method for suppressing subsynchronous oscillations in permanent magnet direct-drive wind turbine generators to solve the above problems. Summary of the Invention

[0005] In view of the above-mentioned shortcomings of the existing technology, the present invention provides a method for suppressing subsynchronous oscillations in permanent magnet direct-drive wind turbine generators.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for suppressing subsynchronous oscillations in permanent magnet direct-drive wind turbine generators includes the following steps:

[0008] Step 1: Construct an inertia-synchronous grid-type direct-drive wind turbine model and obtain the grid-side DC capacitor voltage. With output voltage angular frequency The matching relationship between them is used to obtain the change in DC capacitor voltage on the DC bus of the wind turbine converter;

[0009] Step 2: Determine whether subsynchronous oscillation has occurred by measuring the voltage change. If yes, proceed to step 3; if no, repeat step 1.

[0010] Step 3: Add a power system stabilizer to the maximum power reference value of the synchronous generator-side converter, calculate the control parameters for oscillation suppression, suppress the fluctuation of DC capacitor voltage, and determine whether the harmonics are eliminated. If the determination is yes, the DC capacitor voltage waveform returns to normal; if the determination is no, proceed to step 4.

[0011] Step 4: Reduce the stabilizer gain Repeat step 3.

[0012] Furthermore, the inertia synchronous grid-type direct-drive wind turbine model constructed in step 1 is as follows:

[0013]

[0014] , For stator direct-axis and quadrature-axis voltages, , For stator , shaft current, The synchronization inductor for a permanent magnet synchronous generator. For rotor flux linkage, For stator resistance, The stator side angular frequency;

[0015] The inertia synchronous grid-type permanent magnet direct-drive wind turbine converter consists of a machine-side converter and a grid-side converter. Based on the power balance requirements of the grid-side converter, the dynamic formula for charging and discharging the DC capacitor is as follows:

[0016]

[0017] This represents the per-unit value of the output power of the machine-side converter; This represents the per-unit value of the grid-side converter's output power; This is the per-unit value of the DC voltage; This represents the per-unit value of the steady-state DC voltage. The inertial time constant of the DC-side capacitor;

[0018] Comparing the rotor motion equations of a synchronous generator, the dynamic formula for the rotational speed of a synchronous generator is as follows:

[0019]

[0020] The per-unit value of the prime mover power input to the synchronous machine; This is the per-unit value of the electromagnetic power output by the synchronous machine; This is the per-unit value of the rotor speed; is the rotor's inertial time constant;

[0021] By comparing the dynamic formulas for charging and discharging a DC capacitor and the dynamic formula for the rotational speed of a synchronous generator, it can be seen that the per-unit value of the DC voltage... It has a per-unit value corresponding to the rotor speed of the synchronous generator. Similar dynamic equations; based on the principle of similarity in dynamic systems, the per-unit value of the DC voltage in the dynamic formula for charging and discharging a DC capacitor. This can be compared to the per-unit value of the rotor speed in the dynamic formula for the speed of a synchronous generator. ;

[0022] Based on the above analogy, the angular frequency of the AC output voltage of the grid-side converter is established. With DC side voltage The analogy is the same, as follows:

[0023]

[0024] The grid-side converter exhibits the external characteristics of a synchronous generator to the power grid. When the power delivered by the generator-side converter to the DC capacitor increases, the per-unit value of the DC voltage... Increase the angular frequency of the output AC voltage. Increase, corresponding to the work angle Increasing the output power of the grid-side converter increases the output power of the grid-side converter, thereby maintaining a constant DC-side voltage; conversely, decreasing the output power increases the output power of the grid-side converter.

[0025] Furthermore, the specific steps for step 2 are as follows:

[0026] S21. Perform Fourier analysis on the DC capacitor voltage waveform to detect harmonic frequencies;

[0027] S22. Determine whether the harmonic frequency is within the set range. If the determination is yes, it indicates that subsynchronous oscillation has occurred, and proceed to step 3. If the determination is no, it indicates that subsynchronous oscillation has not occurred, and repeat step 1.

[0028] Furthermore, the frequency range is set to 40-50Hz.

[0029] Furthermore, the specific steps for step 3 are as follows:

[0030] S31. Add a power system stabilizer to the maximum power reference value of the synchronous generator-side converter, adjust the maximum lead phase of the generator-side transmission loop and the amplitude of the transfer function of the generator-side power control loop, and enhance the damping of the power loop or current loop.

[0031] S32. Calculate the control parameters of the power system stabilizer based on the characteristics of the subsynchronous oscillation frequency band;

[0032] S33. Determine whether the harmonics have been eliminated. If the determination is yes, the DC capacitor voltage waveform returns to normal; if the determination is no, proceed to step 4.

[0033] Furthermore, the control parameters include the proportional coefficient. The time constant of the DC blocking element Leading constant hysteresis constant .

[0034] Furthermore, the proportionality coefficient The calculation is as follows:

[0035]

[0036] This represents the amplitude of the transfer function of the machine-side power control loop. The magnitude of the transfer function of the phase compensation stage in the power system stabilizer. Damping for the power loop or current loop; The generator's moment of inertia and time constant; The angular frequency of the subsynchronous oscillation; This is the inherent gain.

[0037] Furthermore, the transfer function of the stabilizer :

[0038]

[0039] It is a DC blocking element that can filter out signals under steady state, prevent the stabilizer from malfunctioning, and not interfere with the normal operation of the power system.

[0040] It is a leading element used to compensate for phase and provide a leading phase.

[0041] For complex frequency domain variables;

[0042] The output signal phase leads the input signal, and the magnitude of the lead phase is related to the oscillation frequency. , , Directly related, the specific patterns are as follows:

[0043] The maximum lead phase of the leading element occurs at the characteristic angular frequency. At that point, and the oscillation frequency Maximum leading phase .

[0044] Furthermore, the specific operation of S32 is as follows:

[0045] S321. Output power of direct-drive fan without phase compensator Closed-loop transfer function :

[0046] S322, will Substitute into In the process, the lead-lag of the PSS stage is obtained. ;

[0047] S323. Calculate the parameters of the wind turbine PSS;

[0048]

[0049] In the formula, The phase compensation coefficient represents the phase compensation ratio of the PSS lead-lag element. The lag phase at the subsynchronous oscillation frequency, i.e., through S322 The calculated phase value.

[0050] Beneficial effects: This invention detects the change in DC capacitor voltage of the grid-side converter in the wind power system, takes the per-unit value of the DC voltage as the input signal, and superimposes its output into the power control loop to increase the electrical damping of the synchronous generator, thereby suppressing the voltage fluctuations in the power control loop. This achieves the suppression of subsynchronous oscillations generated by the inertial synchronous grid-connected wind turbine system using an electrical system stabilizer. Attached Figure Description

[0051] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0052] Figure 1 This is a topology diagram of the power grid integrated into the present invention;

[0053] Figure 2 This is a diagram of the machine-side converter control structure according to an embodiment of the present invention;

[0054] Figure 3 This is a control structure diagram of the grid-side converter according to an embodiment of the present invention;

[0055] Figure 4 This is a DC capacitor voltage waveform diagram in an embodiment of the present invention without introducing a subsynchronous oscillation anti-oscillation suppression scheme based on an inertial synchronous grid-type wind turbine;

[0056] Figure 5 The above is a spectrum analysis diagram of the DC capacitor voltage in an embodiment of the present invention without the introduction of a subsynchronous oscillation anti-oscillation suppression scheme based on an inertial synchronous grid-type wind turbine.

[0057] Figure 6 The addition of a stabilizer to the rear of the machine in this embodiment of the invention Circuit structure diagram:

[0058] Figure 7 This is a control structure diagram of a power system stabilizer according to an embodiment of the present invention;

[0059] Figure 8 A schematic diagram showing the voltage change on the DC capacitor before and after the implementation of stabilizer suppression measures. Detailed Implementation

[0060] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0061] The present invention will be further described below with reference to embodiments.

[0062] Example 1: Please refer to Figure 1-8 A method for suppressing subsynchronous oscillations in permanent magnet direct-drive wind turbine generators includes the following steps:

[0063] Step 1: Construct an inertia-synchronous grid-type direct-drive wind turbine model and obtain the grid-side DC capacitor voltage. With output voltage angular frequency The matching relationship between them is used to obtain the change in DC capacitor voltage on the DC bus of the wind turbine converter;

[0064] Step 2: Determine whether subsynchronous oscillation has occurred by measuring the voltage change. If yes, proceed to step 3; if no, repeat step 1.

[0065] Step 3: Add a power system stabilizer to the maximum power reference value of the synchronous generator-side converter, calculate the control parameters for oscillation suppression, suppress the fluctuation of DC capacitor voltage, and determine whether the harmonics are eliminated. If the determination is yes, the DC capacitor voltage waveform returns to normal; if the determination is no, proceed to step 4.

[0066] Step 4: Reduce the stabilizer gain Repeat step 3.

[0067] The inertia synchronous grid-type direct-drive wind turbine model constructed in step 1 is as follows:

[0068]

[0069] , For stator direct-axis and quadrature-axis voltages, , For stator , shaft current, The synchronization inductor for a permanent magnet synchronous generator. For rotor flux linkage, For stator resistance, The stator side angular frequency;

[0070] The inertia synchronous grid-type permanent magnet direct-drive wind turbine converter consists of a machine-side converter and a grid-side converter. Based on the power balance requirements of the grid-side converter, the dynamic formula for charging and discharging the DC capacitor is as follows:

[0071]

[0072] This represents the per-unit value of the output power of the machine-side converter; This represents the per-unit value of the grid-side converter's output power; This is the per-unit value of the DC voltage; This is the per-unit value of the steady-state DC voltage; The inertial time constant of the DC-side capacitor;

[0073] Comparing the rotor motion equations of a synchronous generator, the dynamic formula for the rotational speed of a synchronous generator is as follows:

[0074]

[0075] The per-unit value of the prime mover power input to the synchronous machine; This is the per-unit value of the electromagnetic power output by the synchronous machine; This is the per-unit value of the rotor speed; is the rotor's inertial time constant;

[0076] By comparing the dynamic formulas for charging and discharging a DC capacitor and the dynamic formula for the rotational speed of a synchronous generator, it can be seen that the per-unit value of the DC voltage... It has a per-unit value corresponding to the rotor speed of the synchronous generator. Similar dynamic equations; based on the principle of similarity in dynamic systems, the per-unit value of the DC voltage in the dynamic formula for charging and discharging a DC capacitor. This can be compared to the per-unit value of the rotor speed in the dynamic formula for the speed of a synchronous generator. ;

[0077] Based on the above analogy, the angular frequency of the AC output voltage of the grid-side converter is established. With DC side voltage The analogy is the same, as follows:

[0078]

[0079] The grid-side converter exhibits the external characteristics of a synchronous generator to the power grid. When the power delivered by the generator-side converter to the DC capacitor increases, the per-unit value of the DC voltage... Increase the angular frequency of the output AC voltage. Increase, corresponding to the work angle Increasing the output power of the grid-side converter increases the output power of the grid-side converter, thereby maintaining a constant DC-side voltage; conversely, decreasing the output power increases the output power of the grid-side converter.

[0080] This method establishes a real-time linkage mechanism between the DC voltage of the grid-side converter and the angular frequency of the output voltage. It achieves autonomous synchronization with the power grid based on the inertia of the DC-side capacitor, and is therefore called "inertial synchronization" control.

[0081] The specific steps for step 2 are as follows:

[0082] S21. Perform Fourier analysis on the DC capacitor voltage waveform to detect harmonic frequencies;

[0083] S22. Determine whether the harmonic frequency is within the set range. If the determination is yes, it indicates that subsynchronous oscillation has occurred, and proceed to step 3. If the determination is no, it indicates that subsynchronous oscillation has not occurred, and repeat step 1.

[0084] The set range is 40-50Hz.

[0085] The specific steps for step 3 are as follows:

[0086] S31. Add a power system stabilizer to the maximum power reference value of the synchronous generator-side converter, adjust the maximum lead phase of the generator-side transmission loop and the amplitude of the transfer function of the generator-side power control loop, and enhance the damping of the power loop or current loop.

[0087] S32. Calculate the control parameters of the power system stabilizer based on the characteristics of the subsynchronous oscillation frequency band;

[0088] S33. Determine whether the harmonics have been eliminated. If the determination is yes, the DC capacitor voltage waveform returns to normal; if the determination is no, proceed to step 4.

[0089] Control parameters include proportional coefficient The time constant of the DC blocking element Leading constant hysteresis constant ;

[0090] proportionality coefficient Adjusting the amplitude of the transfer function of the phase compensation stage in the power system stabilizer directly affects the strength of the damping after compensation; increasing... It can enhance the positive damping amplitude and improve the oscillation suppression speed; if it is too small, the damping enhancement will be insufficient and it will not be able to effectively suppress oscillation.

[0091] DC blocking element time constant : Filters out DC signals under steady-state conditions to prevent stabilizer malfunctions; in steady state, No fluctuations It can block steady-state signal input without interfering with the normal operation of the terminal voltage control loop; it only occurs when oscillation occurs. c changes dynamically, allowing AC signals to pass through and triggering the stabilizer to work;

[0092] Lead time constant To compensate for phase lag in the damping transmission loop, the oscillation frequency must be matched to ensure that the leading phase can be accurately compensated. Closed-loop transfer function The lag phase makes the total phase fall within the 90°-270° positive damping range;

[0093] Lag time constant :and In conjunction with optimizing the bandwidth of the lead phase, a standalone lead element can cause high-frequency signal interference. It can limit the effective frequency range of the leading phase, and only has an adverse effect on subsynchronous oscillations.

[0094] proportionality coefficient The calculation is as follows:

[0095]

[0096] This represents the amplitude of the transfer function of the machine-side power control loop. Let the amplitude of the transfer function of the phase compensation stage of the power system stabilizer be the value. Damping for the power loop or current loop; The generator's moment of inertia and time constant; The angular frequency of the subsynchronous oscillation; This is the inherent gain.

[0097] stabilizer transfer function :

[0098]

[0099] It is a DC blocking element that can filter out signals under steady state, prevent the stabilizer from malfunctioning, and not interfere with the normal operation of the power system.

[0100] It is a leading element used to compensate for phase and provide a leading phase.

[0101] For complex frequency domain variables;

[0102] The output signal phase leads the input signal, and the magnitude of the lead phase is related to the oscillation frequency. , , Directly related, the specific patterns are as follows:

[0103] The maximum lead phase of the leading element occurs at the characteristic angular frequency. At that point, and the oscillation frequency Maximum leading phase ;

[0104] Fourier analysis of the DC capacitor voltage shows that the oscillation frequency of the inertial synchronous grid-type permanent magnet direct-drive wind turbine model is 44Hz. This can be corrected by adjusting... and make Matching the oscillation frequency, when the oscillation frequency decreases, increase It will make This reduces phase lag, making it suitable for compensating for low-frequency oscillations.

[0105] The specific operation of S32 is as follows:

[0106] S321. Output power of direct-drive fan without phase compensator Closed-loop transfer function :

[0107]

[0108] , These are the transfer functions for the power loop and current loop regulators, respectively. This is the base value of the stator side angular frequency; This refers to the quadrature-axis component of the stator voltage; Synchronization inductance for permanent magnet synchronous generator; The quadrature-axis component of the output current of the machine-side converter. For complex frequency domain variables;

[0109] S322, will Substitute into In the process, the lead-lag of the PSS stage is obtained. ;

[0110] The specific operators are as follows:

[0111] A. Transform the complex frequency domain variables Replace with frequency domain variables (j is the imaginary unit, satisfying) ), thus obtaining the frequency domain expression of the transfer function. ,at this time Both the numerator and denominator are converted to complex form:

[0112] molecular:

[0113] Denominator:

[0114] The frequency domain form of the power loop and current loop PI controller conversion. , The expression is:

[0115] , real part imaginary part ;

[0116] , real part imaginary part ;

[0117] B. Further splitting the molecules The real and imaginary parts;

[0118] Expand and rearrange the molecular expression, separating the real part. and the virtual part :

[0119]

[0120] make The real part of the product, For the imaginary part of the product:

[0121] Then: Real part:

[0122] Virtual part:

[0123] C. Splitting the molecule real part and imaginary part

[0124] Expand and rearrange the denominator expression, separating the real part. and the virtual part :

[0125]

[0126] Substitution ,but:

[0127] Real part:

[0128] Virtual part:

[0129] D. Simplifying complex number division

[0130] According to the rules of complex number division Multiply by the conjugate of the denominator. Simplify to obtain the standard complex form. ,in:

[0131] General Department:

[0132] Total imaginary part:

[0133] E. Solving for the lag phase

[0134] According to the definition of complex phase angle, the phase angle is the angle between a vector in the complex plane and the positive direction of the real axis, and is calculated from the real part A and the imaginary part B using the arctangent function:

[0135]

[0136] Phase result determination: If This indicates that the output of the machine-side control loop lags behind the input in phase, and its absolute value is... This refers to the leading phase angle that needs to be compensated in the PSS stage; This indicates that the loop has no lag or has a lead, and no additional phase compensation is required.

[0137] S323. Calculate the parameters of the wind turbine PSS;

[0138]

[0139] In the formula, The phase compensation coefficient represents the phase compensation ratio of the PSS lead-lag element.

[0140] The lag phase at the subsynchronous oscillation frequency, i.e., through S322 The calculated phase value is determined Key parameters;

[0141] Determines the lead time constant With lag time constant The proportional relationship affects the accuracy of PSS compensation for the lagging phase of the power system; It is the oscillation frequency, ensuring that the compensation frequency band of the PSS matches the actual dominant oscillation frequency of the power system, so as to avoid compensation failure or interference to other frequency bands; The magnitude of the leading phase that the PSS can provide must be determined in relation to the lagging phase of the power system. Precise matching is used to effectively dampen subsynchronous oscillations; Determine the dynamic characteristics of the lag element, and By forming a lead-lag network, the PSS can be ensured to provide stable phase compensation at the target oscillation frequency.

[0142] Specific cases:

[0143] The following will demonstrate the simulation experiment of the inertia synchronous grid-type permanent magnet direct-drive wind turbine model in PSCAD, as described in this embodiment. Figure 4 As shown, the system parameters are as follows: power base value: 2MW; AC phase voltage base value: 0.563kV; DC voltage base value: 1.1kV; stator side angular frequency base value: 84.6rad / s; grid side angular frequency base value: 314rad / s; DC capacitor inertia constant: 3.025ms; steady-state DC voltage: 1.0pu. Four seconds after the experiment began, the controller parameters were changed to cause the DC capacitor voltage to oscillate.

[0144] System power control loop proportional coefficient When 10 is equal to 100, Fourier analysis of the DC capacitor voltage shows that the inertia synchronous grid-type permanent magnet direct-drive wind turbine model oscillates at 44Hz. Figure 5 As shown.

[0145] Depend on Figure 5 It is evident that after the fault occurred, the DC capacitor voltage oscillation of the unit gradually increased, resulting in a subsynchronous oscillation phenomenon.

[0146] After detecting an oscillation in the DC capacitor voltage, the oscillation suppression system and control method described in the above embodiment are used to suppress the synchronous oscillation after 0.01 seconds of oscillation. Figure 8 As can be seen, after the power system stabilizer with the parameters set was added, the DC capacitor voltage oscillation of the system decayed rapidly. 0.7 seconds after the fault occurred, the DC capacitor voltage fluctuation dropped from the initial 0.01 pu to below 0.001 pu.

[0147] Through the description and verification of this embodiment, it can be seen that after introducing the subsynchronous oscillation anti-oscillation suppression scheme of the power system stabilization device based on phase compensation, the subsynchronous oscillation of the system is effectively suppressed and the DC capacitor fluctuation is greatly reduced.

[0148] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions will not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for suppressing subsynchronous oscillations in permanent magnet direct-drive wind turbine generators, characterized in that: Includes the following steps: Step 1: Construct an inertia-synchronous grid-type direct-drive wind turbine model and obtain the grid-side DC capacitor voltage. With output voltage angular frequency The matching relationship between them is used to obtain the change in DC capacitor voltage on the DC bus of the wind turbine converter; Step 2: Determine whether subsynchronous oscillation has occurred by measuring the voltage change. If yes, proceed to step 3; if no, repeat step 1. Step 3: Add a power system stabilizer to the maximum power reference value of the synchronous generator-side converter, calculate the control parameters for oscillation suppression, suppress the fluctuation of DC capacitor voltage, and determine whether the harmonics are eliminated. If the determination is yes, the DC capacitor voltage waveform returns to normal; if the determination is no, proceed to step 4. Step 4: Reduce the stabilizer gain Repeat step 3; The inertia synchronous grid-type direct-drive wind turbine model constructed in step 1 is as follows: ； , For stator direct-axis and quadrature-axis voltages, , For stator , shaft current, The synchronization inductor for a permanent magnet synchronous generator. For rotor flux linkage, For stator resistance, The stator side angular frequency; The inertia synchronous grid-type permanent magnet direct-drive wind turbine converter consists of a machine-side converter and a grid-side converter. Based on the power balance requirements of the grid-side converter, the dynamic formula for charging and discharging the DC capacitor is as follows: ； This represents the per-unit value of the output power of the machine-side converter; This represents the per-unit value of the grid-side converter's output power; This is the per-unit value of the DC voltage; This represents the per-unit value of the steady-state DC voltage. The inertial time constant of the DC-side capacitor; Comparing the rotor motion equations of a synchronous generator, the dynamic formula for the rotational speed of a synchronous generator is as follows: ； The per-unit value of the prime mover power input to the synchronous machine; This is the per-unit value of the electromagnetic power output by the synchronous machine; This is the per-unit value of the rotor speed; is the rotor's inertial time constant; By comparing the dynamic formulas for charging and discharging a DC capacitor and the dynamic formula for the rotational speed of a synchronous generator, it can be seen that the per-unit value of the DC voltage... It has a per-unit value corresponding to the rotor speed of the synchronous generator. Similar dynamic equations; based on the principle of similarity in dynamic systems, the per-unit value of the DC voltage in the dynamic formula for charging and discharging a DC capacitor. This can be compared to the per-unit value of the rotor speed in the dynamic formula for the speed of a synchronous generator. ; Based on the above analogy, the angular frequency of the AC output voltage of the grid-side converter is established. With DC side voltage The analogy is the same, as follows: ； The grid-side converter exhibits the external characteristics of a synchronous generator to the power grid. When the power delivered by the generator-side converter to the DC capacitor increases, the per-unit value of the DC voltage... Increase the angular frequency of the output AC voltage. Increase, corresponding to the work angle Increasing the output power of the grid-side converter increases the output power of the grid-side converter, thereby maintaining a constant DC-side voltage; conversely, decreasing the output power increases the output power of the grid-side converter. The specific steps for step 2 are as follows: S21. Perform Fourier analysis on the DC capacitor voltage waveform to detect harmonic frequencies; S22. Determine whether the harmonic frequency is within the set range. If the determination is yes, it indicates that subsynchronous oscillation has occurred, and proceed to step 3. If the determination is no, it indicates that subsynchronous oscillation has not occurred, and repeat step 1. The set range is 40-50Hz. The specific steps for step 3 are as follows: S31. Add a power system stabilizer to the maximum power reference value of the synchronous generator machine-side converter, adjust the maximum lead phase of the machine-side transmission loop and the amplitude of the transfer function of the machine-side power control loop, and enhance the damping of the power loop or current loop. S32. Calculate the control parameters of the power system stabilizer based on the characteristics of the subsynchronous oscillation frequency band; S33. Determine whether the harmonics have been eliminated. If the determination is yes, the DC capacitor voltage waveform returns to normal; if the determination is no, proceed to step 4.

2. The method for suppressing subsynchronous oscillations in a permanent magnet direct-drive wind turbine generator according to claim 1, characterized in that, Control parameters include proportional coefficient The time constant of the DC blocking element Leading constant hysteresis constant .

3. The method for suppressing subsynchronous oscillations in a permanent magnet direct-drive wind turbine generator according to claim 2, characterized in that, proportionality coefficient The calculation is as follows: ； This represents the amplitude of the transfer function of the machine-side power control loop. Let the amplitude of the transfer function of the phase compensation stage of the power system stabilizer be the value. Damping for the power loop or current loop; The generator's moment of inertia and time constant; The angular frequency of the subsynchronous oscillation; This is the inherent gain.

4. The method for suppressing subsynchronous oscillations in a permanent magnet direct-drive wind turbine generator according to claim 3, characterized in that, stabilizer transfer function : ； It is a DC blocking element that can filter out signals under steady state, prevent the stabilizer from malfunctioning, and not interfere with the normal operation of the power system. It is a leading element used to compensate for phase and provide a leading phase. For complex frequency domain variables; The output signal phase leads the input signal, and the magnitude of the lead phase is related to the oscillation frequency. , , Directly related, the specific patterns are as follows: The maximum lead phase of the lead element occurs at the characteristic angular frequency. At that point, and the oscillation frequency Maximum leading phase .

5. The method for suppressing subsynchronous oscillations in a permanent magnet direct-drive wind turbine generator according to claim 4, characterized in that, The specific operation of S32 is as follows: S321. Output power of direct-drive fan without phase compensator Closed-loop transfer function : S322, will Substitute into In the process, the lead-lag of the PSS stage is obtained. ; S323. Calculate the parameters of the wind turbine PSS; ； In the formula, The phase compensation coefficient represents the phase compensation ratio of the PSS lead-lag element. The lag phase at the subsynchronous oscillation frequency, i.e., through S322 The calculated phase value.

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