Harmonic suppression method based on offshore wind power ac collection uncontrolled rectification sending-out system

Abstract

The application discloses a harmonic suppression method based on an offshore wind power AC collection non-controlled rectifier sending-out system, and particularly relates to a method for injecting harmonic voltage to suppress a large amount of harmonics generated when a non-controlled rectifier transmits power based on a grid-connected wind generator and a grid-side converter of a back-to-back converter of the wind generator; and solves the problem that a large amount of harmonics are generated when offshore wind power is sent out through a diode DC, and the power quality in the wind power plant is reduced, a transformer and other devices are prone to saturation, and the safe and stable operation of the wind power system is endangered.

Description

Technical Field

[0001] This invention belongs to the technical field of power electronic devices in offshore wind power diode DC transmission systems, and relates to a harmonic suppression method based on an offshore wind power AC collection uncontrolled rectifier transmission system. Background Technology

[0002] Currently, deep-sea wind power generation mainly utilizes high-voltage flexible direct current (HVDC) transmission technology, which offers advantages such as independent control of active and reactive power, current control, and grid formation control. However, its complex control strategies reduce operational reliability and are costly. Some scholars have proposed using uncontrolled rectifiers as rectifier devices in offshore converter stations, and this has been proven to be a viable alternative for improving wind power transmission efficiency and system reliability. While uncontrolled rectifiers can improve the operational reliability of wind power collection converter stations and reduce construction costs, they also generate significant harmonics during power transmission, leading to greater power fluctuations and reduced power quality within the wind farm, jeopardizing system stability. Furthermore, these harmonics can cause transformers to saturate, occupying transformer and submarine cable transmission capacity, resulting in decreased equipment utilization, increased equipment losses, shortened equipment lifespan, and increased system operation and maintenance costs.

[0003] Therefore, further research on current harmonic suppression methods for wind power converter stations based on uncontrolled rectifier DC transmission schemes is of great engineering significance for improving power quality at wind power collection points and enhancing the operational stability of transformers and submarine cables. Summary of the Invention

[0004] The purpose of this invention is to provide a harmonic suppression method based on an uncontrolled rectifier transmission system for offshore wind power AC collection, which solves the problem that offshore wind power generates a large number of harmonics when transmitted via diode DC, thereby reducing the power quality in the wind farm, making transformers and other devices prone to saturation, and endangering the safe and stable operation of the wind power system.

[0005] The technical solution adopted in this invention is a harmonic suppression method based on an offshore wind power AC collection uncontrolled rectifier transmission system. Specifically, based on a grid-connected wind turbine, a method of injecting harmonic voltage is used in the grid-side converter of the wind turbine backing onto the converter to suppress a large number of harmonics generated when the uncontrolled rectifier transmits power.

[0006] The invention is further characterized by:

[0007] The harmonic suppression method based on the offshore wind power AC collection uncontrolled rectifier transmission system is implemented according to the following steps:

[0008] Step 1: Establish the control strategy for the virtual synchronous generator;

[0009] Step 2: Establish the control strategy for the voltage control loop;

[0010] Step 1 specifically involves: The virtual synchronous generator control strategy simulates the working characteristics of a synchronous generator, mainly including active frequency control and reactive voltage control.

[0011] Step 2 specifically involves the voltage control loop, which consists of two parts: first, realizing the sinusoidal fundamental voltage output by the grid-side converter; and second, injecting a certain amount of harmonic voltage into the sinusoidal fundamental voltage to suppress the harmonic components in the three-phase current.

[0012] The power frequency control in step 1 is implemented in the following steps:

[0013] The power frequency controller, as the core control unit of the virtual synchronous generator, adopts the second-order model rotor motion equation of the synchronous generator, while also considering the primary frequency regulation effect of the generator. Its rotor motion equation is as follows:

[0014]

[0015] In the formula, J is the total moment of inertia of the synchronous generator and prime mover, and its unit is kg·m. 2 D is the damping coefficient, representing the effect of the synchronous generator's damping winding; k p This is the first-order frequency regulation coefficient; when the number of pole pairs p = 1, the mechanical angular frequency ω of the synchronous generator is the same as the electrical angular frequency, and the phase θ of the reference voltage can be obtained after a first-order integral transformation; ω N The rated angular frequency; P m P e These are the input mechanical power and the output electromagnetic power, respectively.

[0016] The reactive voltage control in step 1 is implemented according to the following steps:

[0017] Mathematical equations for the excitation controller:

[0018]

[0019] In the formula, Q ref Q e These are the reference value and the actual output value of reactive power, respectively; U N U o These are the effective values ​​of the rated voltage and the output voltage, respectively; k q E is the reactive voltage droop factor. m The effective value of the voltage reference signal; k i The inertia coefficient;

[0020] The above P e Q e It can be calculated using instantaneous power theory, that is:

[0021]

[0022] In the formula, u cd u cq These are the d-axis and q-axis components of the VSG output voltage, respectively. abcd i abcq These are the d-axis and q-axis components of the VSG output current, respectively.

[0023] In summary, the voltage loop reference voltage e can be obtained through the power frequency controller and the excitation controller. mabc The amplitude, frequency, and phase are expressed as follows:

[0024]

[0025] Step 2 shall be implemented as follows:

[0026] To achieve a sinusoidal fundamental voltage output from the wind turbine grid-side converter, a conventional voltage-current dual closed-loop control is employed, using the reference voltage e output from the VSG control. mabc After being transformed to a DC coordinate system via a dq transformation, the fundamental voltage modulation signal is obtained through a voltage-current control loop. This fundamental voltage modulation signal is then fed into the inverter via a control signal generated by a PWM generator, resulting in an output voltage U. c That is, the fundamental sinusoidal voltage;

[0027] Then, based on the harmonic current, a corresponding harmonic voltage is generated and injected into the fundamental voltage.

[0028] The injection of harmonic voltage into the fundamental voltage is implemented according to the following steps:

[0029] The uncontrolled rectifier is theoretically analyzed using a 12-pulse rectifier. In a stationary three-phase coordinate system, the voltage equation is:

[0030]

[0031] In the formula, U c The grid-side converter outputs three-phase voltage; i abc U represents the three-phase alternating current; L and R represent the equivalent inductance and resistance of the line and transformer, respectively; U DR The three-phase terminal voltage on the AC side of the uncontrolled rectifier;

[0032] WhenU c When the voltage is sinusoidal, the uncontrolled rectifier, as a nonlinear element, will generate a large amount of harmonic current. Its harmonic components are (12k±1) order harmonic currents, where k is 1, 2, 3, ...; among them, the 11th, 13th, 23rd, and 25th harmonics have a larger content, so these four harmonics are mainly analyzed.

[0033] In order to suppress the three-phase alternating current i abcTherefore, the harmonic current in the fundamental voltage U is suppressed by using harmonic voltage injection. c When a certain amount of 11th, 13th, 23rd, and 25th harmonic voltages are injected into the circuit, the voltage equation becomes:

[0034]

[0035] In the formula, U h i' is the sum of the 11th, 13th, 23rd, and 25th harmonic voltages injected. abc U' is the three-phase current after the injection of harmonic voltage. DR The AC side voltage of the uncontrolled rectifier after harmonic injection;

[0036] To obtain the voltage equations for each harmonic and to avoid the influence between the fundamental component and the harmonic components, a method is adopted to precisely control each harmonic component in a multi-synchronous rotating coordinate system.

[0037] In the three-phase stationary coordinate system, the rotation direction of the 11th and 23rd harmonic vectors is opposite to that of the fundamental vector, and they are defined as negative sequence harmonics; the rotation direction of the 13th and 25th harmonic vectors is the same as that of the fundamental vector, and they are defined as positive sequence harmonics; the establishment of a multi-synchronous rotating coordinate system is achieved through equations (7) and (8);

[0038]

[0039]

[0040] In the formula, The transformation formula for the nth positive sequence harmonic from the abc stationary coordinate system to the positive sequence harmonic coordinate system is given. The transformation formula for the nth negative harmonic from the abc stationary coordinate system to the negative harmonic coordinate system;

[0041] Analyzing the voltage equation of the 11th harmonic, equation (6) is transformed into the synchronous rotating coordinate system of the 11th harmonic through equation (8), and the voltage equation of the harmonic is:

[0042]

[0043] In the formula, U hd11 and U hq11 These represent the d-axis and q-axis components of the injected 11th harmonic voltage, respectively; to simplify the mathematical model of the injected 11th harmonic voltage, U' DRd11 and U' DRq11 Each is counted in U hd11 and U hq11 The influence of U' hd11 and U' hq11 From equation (9), we obtain equation (10), that is:

[0044]

[0045] Similarly, the voltage equations for the 13th, 23rd, and 25th harmonics are:

[0046]

[0047] From equations (10) and (11), it can be seen that each harmonic voltage is coupled with the corresponding harmonic current. Therefore, when constructing a harmonic voltage controller, the cross-coupling effect between each harmonic voltage and current should be considered, that is, the d-axis harmonic voltage is affected by the q-axis harmonic current, and the q-axis harmonic voltage is affected by the d-axis harmonic current. Under steady-state conditions, combining equations (10) and (11), the control equations for the cross-decoupling of each harmonic current in the grid-side converter under multiple synchronous rotating coordinate systems can be obtained:

[0048]

[0049] In the formula, K pk and K ik These are the proportional and integral coefficients of the PI regulator for the k (k = 11, 13, 23, 25) harmonic currents, respectively; each harmonic current will be converted into a DC component when transformed into the corresponding dq coordinate system, while the fundamental wave and other harmonics will be converted into AC components.

[0050] As can be seen from equation (12), by performing cross-decoupling control on each harmonic current and setting the given value of each harmonic current to 0, the magnitude of each harmonic voltage modulation wave can be obtained. Then, each harmonic voltage modulation wave is transformed from the multi-synchronous rotating coordinate system to the three-phase stationary coordinate system by equations (13) and (14) and added together to obtain the injected harmonic voltage modulation wave. Then, it is injected into the fundamental voltage modulation wave and finally sent to the PWM generator.

[0051]

[0052]

[0053] In the formula, The transformation formula for the nth positive sequence harmonic from the positive sequence harmonic coordinate system to the abc stationary coordinate system is given. The transformation formula for the nth negative harmonic from the negative harmonic coordinate system to the abc stationary coordinate system is given.

[0054] The beneficial effects of this invention are:

[0055] This invention relates to a harmonic suppression method for an offshore wind power AC collection uncontrolled rectifier transmission system. It employs a permanent magnet synchronous wind turbine generator, with back-to-back turbine-side converters controlling the back-to-back DC terminal voltage. The grid-side converter utilizes a virtual synchronous generator control strategy to provide a stable support voltage to the 12-pulse uncontrolled rectifier, while also providing frequency support. The uncontrolled rectifier is connected to the onshore converter station via a high-voltage DC line and then to the power grid. During power transmission, the uncontrolled rectifier generates a large amount of harmonic current. To suppress harmonics, this invention proposes injecting a certain amount of corresponding harmonic voltage into the output voltage of the grid-side converter to suppress the generation of harmonic current. This method addresses the problem of offshore wind power generating a large amount of harmonics through diode-based DC transmission, which reduces the power quality within the wind farm, easily saturates transformers and other equipment, and jeopardizes the safe and stable operation of the wind power system. Attached Figure Description

[0056] Figure 1 This is a schematic diagram of the topology of the offshore wind power back-to-back grid-side converter of the present invention;

[0057] Figure 2 This is a schematic diagram of the virtual synchronous generator control strategy for the back-to-back grid-side converter of the present invention.

[0058] Figure 3 This is a schematic diagram of the harmonic current extraction method of the present invention;

[0059] Figure 4 This is a schematic diagram of the harmonic voltage control strategy of the present invention;

[0060] Figure 5 This is a schematic diagram of the overall control strategy for the back-to-back grid-side inverter of the present invention;

[0061] Figure 6 This is a schematic diagram of the AC voltage and current of the grid-side converter of the conventional virtual synchronous generator control system according to the present invention.

[0062] Figure 7 This is a schematic diagram of the 11th harmonic content of the AC current in the dq coordinate system of the conventional virtual synchronous generator control network-side converter of the present invention.

[0063] Figure 8 This is a schematic diagram of the 13th harmonic content of the AC current in the dq coordinate system of the conventional virtual synchronous generator control network-side converter of the present invention.

[0064] Figure 9 This is a schematic diagram of the 23rd harmonic content of the AC current in the dq coordinate system of the conventional virtual synchronous generator control network-side converter of the present invention.

[0065] Figure 10 This is a schematic diagram of the 25th harmonic content of the AC current in the dq coordinate system of the conventional virtual synchronous generator control network-side converter of the present invention.

[0066] Figure 11 A schematic diagram of the AC voltage and current of the grid-side converter of the virtual synchronous generator with injected harmonic voltage for this invention;

[0067] Figure 12 A schematic diagram of the 11th harmonic content of the AC current dq coordinate system of the virtual synchronous generator control grid-side converter for injecting harmonic voltage in this invention.

[0068] Figure 13 A schematic diagram of the 13th harmonic content of the AC current in the dq coordinate system of the virtual synchronous generator control grid-side converter for injecting harmonic voltage in this invention.

[0069] Figure 14 A schematic diagram of the 23rd harmonic content of the AC current in the dq coordinate system of the virtual synchronous generator control grid-side converter for injecting harmonic voltage in this invention.

[0070] Figure 15 A schematic diagram of the 25th harmonic content of the AC current in the dq coordinate system of the virtual synchronous generator control grid-side converter for injecting harmonic voltage in this invention.

[0071] Figure 16 This is a schematic diagram of the harmonic current spectrum of the conventional virtual synchronous generator control strategy of the present invention;

[0072] Figure 17 A schematic diagram of the harmonic current spectrum of the virtual synchronous generator control strategy for injecting harmonic voltage in this invention. Detailed Implementation

[0073] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0074] Example 1

[0075] This invention provides a harmonic suppression method for an offshore wind power AC collection uncontrolled rectifier power transmission system. Based on a grid-connected wind turbine, a harmonic voltage injection method is used in the grid-side converter of the back-to-back converter of the wind turbine to suppress a large number of harmonics generated when the uncontrolled rectifier transmits power. This method is applicable to low-frequency, power frequency and medium-frequency operating conditions of wind power systems. In this case, a medium-frequency system was selected for verification.

[0076] Since this scheme is based on the DC transmission of offshore wind power through an uncontrolled rectifier, the wind turbines need to be grid-type to meet the system operation requirements; therefore, this scheme belongs to an active harmonic suppression method.

[0077] Because transformers are large, heavy, expensive, and difficult to transport and install, increasing the operating frequency within the wind farm can reduce their size and weight, thereby lowering costs and resolving transportation and installation issues. While increasing the system operating frequency will have some impact on the transmission capacity of long-distance submarine cables, the transmission capacity is largely unaffected considering the relatively short cable distances within wind farms.

[0078] Example 2

[0079] This invention employs a virtual synchronous generator control strategy to output the fundamental voltage in a back-to-back grid-side converter of a wind turbine, setting the system frequency to 100Hz. Then, based on the virtual synchronous generator control strategy, a harmonic voltage injection method is used to suppress harmonics generated by a 12-pulse uncontrolled rectifier. The specific implementation method is as follows:

[0080] Step 1: First, establish the control strategy for the virtual synchronous generator. The virtual synchronous generator control strategy simulates the operating characteristics of a synchronous generator and mainly includes active frequency control and reactive voltage control.

[0081] As the core control unit of the virtual synchronous generator, the power frequency controller adopts the second-order model rotor motion equation of the synchronous generator to simplify the controller structure. Considering the primary frequency regulation function of the generator, its rotor motion equation is as follows:

[0082]

[0083] In the formula, J represents the total moment of inertia of the synchronous generator and prime mover, and its unit is kg·m. 2 D is the damping coefficient, representing the effect of the synchronous generator's damping winding; k p This is the primary frequency regulation coefficient; when the number of pole pairs p = 1, the mechanical angular frequency ω of the synchronous generator is the same as the electrical angular frequency, and the phase θ of the reference voltage can be obtained after a first integral transformation; ω N The rated angular frequency; P m P e These represent the input mechanical power and the output electromagnetic power, respectively.

[0084] The excitation control system is a voltage closed-loop control system. It measures the generator's terminal voltage and controls the excitation voltage based on its changes to maintain a constant generator terminal voltage. The mathematical equation of the excitation controller is as follows:

[0085]

[0086] In the formula, Q ref Q e These are the reference value and the actual output value of reactive power, respectively; U N U oThese are the effective values ​​of the rated voltage and the output voltage, respectively; k q E is the reactive voltage droop factor. m The effective value of the voltage reference signal; k i The inertia coefficient;

[0087] The above P e Q e It can be calculated using instantaneous power theory, that is:

[0088]

[0089] In the formula, u cd u cq These are the d-axis and q-axis components of the VSG output voltage, respectively. abcd i abcq These are the d-axis and q-axis components of the VSG output current, respectively.

[0090] In summary, the voltage loop reference voltage e can be obtained through the power frequency controller and the excitation controller. mabc The amplitude, frequency, and phase are expressed as follows:

[0091]

[0092] The virtual synchronous generator controller is composed of a power frequency controller and an excitation controller, such as... Figure 2 As shown;

[0093] Step 2: Establish the control strategy for the voltage control loop. The voltage control loop consists of two parts: first, to achieve a sinusoidal fundamental voltage output from the grid-side converter; and second, to inject a certain amount of harmonic voltage into the sinusoidal fundamental voltage to suppress harmonic components in the three-phase current.

[0094] To achieve a sinusoidal fundamental voltage output from the wind turbine grid-side converter, a conventional voltage-current dual closed-loop control is employed, using the reference voltage e output from the VSG control. mabc After being transformed to a DC coordinate system via a dq transformation, the fundamental voltage modulation signal is obtained through a voltage-current control loop. This fundamental voltage modulation signal is then fed into the inverter via a control signal generated by a PWM generator, resulting in an output voltage U. c That is, the fundamental sinusoidal voltage;

[0095] Secondly, based on the harmonic current, a corresponding harmonic voltage is generated and injected into the fundamental voltage. The specific analysis method is as follows:

[0096] The topology of the wind turbine grid-side converter is as follows: Figure 1 As shown, a 12-pulse rectifier is selected for theoretical analysis of the uncontrolled rectifier; in the stationary three-phase coordinate system, the voltage equation is:

[0097]

[0098] In the formula, U c The grid-side converter outputs three-phase voltage; i abc U represents the three-phase alternating current; L and R represent the equivalent inductance and resistance of the line and transformer, respectively; U DR The three-phase terminal voltage on the AC side of the uncontrolled rectifier bridge;

[0099] When U c When the voltage is sinusoidal, the uncontrolled rectifier, as a nonlinear element, will generate a large amount of harmonic current. Its harmonic components are (12k±1) order harmonic currents, where k is 1, 2, 3, ...; among them, the 11th, 13th, 23rd, and 25th harmonics have a larger content, so the following analysis mainly focuses on these four types of harmonics.

[0100] In order to suppress the three-phase alternating current i abc Therefore, the harmonic current in the fundamental voltage U is suppressed by using harmonic voltage injection. c When a certain amount of 11th, 13th, 23rd, and 25th harmonic voltages are injected into the circuit, the voltage equation becomes:

[0101]

[0102] In the formula, U h i' is the sum of the 11th, 13th, 23rd, and 25th harmonic voltages injected. abc U' is the three-phase current after the injection of harmonic voltage. DR The AC side voltage of the uncontrolled rectifier after harmonic injection;

[0103] In order to obtain the voltage equations for each harmonic and to avoid the influence between the fundamental component and each harmonic component, a method of precisely controlling each harmonic component in a multi-synchronous rotating coordinate system can be adopted.

[0104] In a three-phase stationary coordinate system, the 11th and 23rd harmonic vectors rotate in the opposite direction to the fundamental vector, and are defined as negative-sequence harmonics; the 13th and 25th harmonic vectors rotate in the same direction as the fundamental vector, and are defined as positive-sequence harmonics; the establishment of a multi-synchronous rotating coordinate system is achieved through equations (7) and (8):

[0105]

[0106]

[0107] In the formula, The transformation formula for the nth positive sequence harmonic from the abc stationary coordinate system to the positive sequence harmonic coordinate system is given. The transformation formula for the nth negative harmonic from the abc stationary coordinate system to the negative harmonic coordinate system;

[0108] The following analysis uses the 11th harmonic voltage equation; transforming equation (6) into the 11th harmonic synchronous rotating coordinate system using equation (8), the harmonic voltage equation is:

[0109]

[0110] In the formula, U hd11 and U hq11 These represent the d-axis and q-axis components of the injected 11th harmonic voltage, respectively; to simplify the mathematical model of the injected 11th harmonic voltage, U' DRd11 and U' DRq11 Each is counted in U hd11 and U hq11 The influence of U' hd11 and U' hq11 From equation (9), we obtain equation (10), that is:

[0111]

[0112] Similarly, the voltage equations for the 13th, 23rd, and 25th harmonics are:

[0113]

[0114] From equations (10) and (11), it can be seen that each harmonic voltage is coupled with the corresponding harmonic current. Therefore, when constructing a harmonic voltage controller, the cross-coupling effect between each harmonic voltage and current should be considered, that is, the d-axis harmonic voltage is affected by the q-axis harmonic current, and the q-axis harmonic voltage is affected by the d-axis harmonic current. The traditional PI regulator can theoretically achieve zero steady-state error tracking of a DC constant signal, with high steady-state accuracy, good dynamic response, and simple and practical engineering applications. Based on this idea, under the steady state of the system, by combining equations (10) and (11), the control equations for the cross-decoupling of each harmonic current of the grid-side converter under multiple synchronous rotating coordinate systems can be further obtained:

[0115]

[0116] In the formula, K pk and K ik These are the proportional and integral coefficients of the PI regulator for the k (k = 11, 13, 23, 25) harmonic currents, respectively. Each harmonic current, when converted to the corresponding dq coordinate system, becomes a DC component, while the fundamental and other harmonics become AC components. To achieve precise control of each harmonic current and reduce the influence of the fundamental and other harmonic currents on this harmonic current, a low-pass filter can be used to filter out the AC components, as shown in Figure 3.

[0117] As can be seen from equation (12), the harmonic current can be cross-decoupled and controlled. The given value of each harmonic current is set to 0 to obtain the magnitude of each harmonic voltage modulation wave. Then, the multi-synchronous rotating coordinate system is transformed to the three-phase stationary coordinate system by equations (13) and (14) to obtain the injected harmonic voltage modulation wave. Then, it is injected into the fundamental voltage modulation wave and finally sent to the PWM generator. Figure 4 This is a control block diagram for a harmonic voltage controller. Figure 5 This is a schematic diagram of the overall control strategy for back-to-back grid-side inverters.

[0118]

[0119]

[0120] In the formula, The transformation formula for the nth positive sequence harmonic from the positive sequence harmonic coordinate system to the abc stationary coordinate system is given. The transformation formula for the nth negative harmonic from the negative harmonic coordinate system to the abc stationary coordinate system is given.

[0121] Example 3

[0122] In the traditional VSG control network-side converter, by Figures 6-10 It can be seen that the AC voltage of the wind turbine grid-side converter has good sinusoidal characteristics, but the AC current contains a certain amount of 11th, 13th, 23rd, and 25th harmonic currents, with a current distortion rate of 8.74%, indicating poor sinusoidal characteristics. In the synchronous rotating coordinate system of the 11th, 13th, 23rd, and 25th harmonic currents, the dq-axis components are all relatively large, demonstrating that the traditional VSG strategy lacks the ability to effectively suppress harmonic currents. Figure 16 Spectral analysis of the alternating current reveals a high content of 11th, 13th, 23rd, and 25th harmonics, representing 7.36%, 4.28%, 1.32%, and 1.20% of the fundamental component, respectively. This can easily saturate the transformer and severely degrade the power quality at the wind power collection point, resulting in large fluctuations in transmitted power, which is detrimental to system energy transmission and jeopardizes system stability. However, by adopting a virtual synchronous generator control strategy that injects harmonic voltage, such as… Figures 11-15 As shown, the AC voltage of the wind turbine grid-side inverter is injected with a certain amount of harmonic voltage, which worsens the sinusoidal characteristics but does not affect the normal operation of the system. However, the distortion of the AC current waveform is significantly reduced, the sinusoidal characteristic is greatly improved, the current harmonic distortion rate is 1.08%, and the 11th, 13th, 23rd, and 25th harmonic current components are effectively suppressed and controlled to around 0. The harmonic current suppression and control effect is good, the power quality at the wind power collection point is improved, and the range of transmission power fluctuations is reduced, which is conducive to the stable operation of the system. Figure 17Spectral analysis of the alternating current shows that the proportions of the 11th, 13th, 23rd, and 25th harmonic current components relative to the fundamental current component have all decreased significantly, further demonstrating that the virtual synchronous generator control strategy with injected harmonic voltage has good performance in suppressing harmonic currents.

Claims

1. A harmonic suppression method for an offshore wind power AC collection uncontrolled rectifier transmission system, characterized in that, Specifically, based on grid-connected wind turbines, a method of injecting harmonic voltage is used in the grid-side converter of the back-to-back converter of the wind turbine to suppress a large number of harmonics generated when the uncontrolled rectifier transmits power. The uncontrolled rectifier is a 12-pulse rectifier. The specific steps are as follows: Step 1: Establish the control strategy for the virtual synchronous generator; Step 2: Establish the control strategy for the voltage control loop; Step 2 is implemented as follows: To achieve a sinusoidal fundamental voltage output from the wind turbine grid-side converter, a conventional voltage-current dual closed-loop control is employed, using the reference voltage output from the VSG control. The fundamental voltage modulation signal is obtained by transforming the coordinate system to DC through a dq transformation and then passing it through a voltage-current control loop. This fundamental voltage modulation signal is then fed into the inverter via a control signal generated by a PWM generator, resulting in the output voltage. That is, the fundamental sinusoidal voltage; Then, based on the harmonic current, a corresponding harmonic voltage is generated and injected into the fundamental voltage. In order to suppress three-phase alternating current The harmonic current in the fundamental voltage is used to suppress it, so the harmonic voltage injection method is adopted. When a certain amount of 11th, 13th, 23rd, and 25th harmonic voltages are injected into the circuit, the voltage equation becomes: （6） In the formula, The sum of the injected 11th, 13th, 23rd, and 25th harmonic voltages. The three-phase current after the injection of harmonic voltage, The AC side voltage of the uncontrolled rectifier after harmonic injection; Cross-decoupling control is performed on each harmonic current. The setpoint of each harmonic current is set to 0 to obtain the magnitude of each harmonic voltage modulation wave. Then, the multi-synchronous rotating coordinate system is inversely transformed to the three-phase stationary coordinate system and the harmonic voltage modulation waves are added to obtain the injected harmonic voltage modulation wave. This wave is then injected into the fundamental voltage modulation wave and finally sent to the PWM generator.

2. The harmonic suppression method for an offshore wind power AC collection uncontrolled rectifier transmission system according to claim 1, characterized in that, Step 1 specifically involves: The virtual synchronous generator control strategy simulates the working characteristics of a synchronous generator, mainly including active frequency control and reactive voltage control. Step 2 specifically involves the voltage control loop, which consists of two parts: first, realizing the output sinusoidal fundamental voltage of the grid-side converter; and second, injecting a certain amount of harmonic voltage into the sinusoidal fundamental voltage to suppress the harmonic components in the three-phase current.

3. The harmonic suppression method for an offshore wind power AC collection uncontrolled rectifier transmission system according to claim 2, characterized in that, The power frequency control in step 1 is implemented according to the following steps: The power frequency controller, as the core control unit of the virtual synchronous generator, adopts the second-order model rotor motion equation of the synchronous generator, while also considering the primary frequency regulation effect of the generator. Its rotor motion equation is as follows: （1） In the formula, The moment of inertia of the synchronous generator and prime mover is given by the unit _____. ; is the damping coefficient, representing the effect of the damping winding of the synchronous generator; This is the first-order frequency modulation coefficient; when the number of pole pairs At that time, the mechanical angular frequency of the synchronous generator This is the electrical angular frequency; the phase of the reference voltage can be obtained through a single integral transformation. ; The rated angular frequency; , These are the input mechanical power and the output electromagnetic power, respectively.

4. The harmonic suppression method for an offshore wind power AC collection uncontrolled rectifier transmission system according to claim 2, characterized in that, The reactive voltage control in step 1 is implemented according to the following steps: Mathematical equations for the excitation controller: （2） In the formula, , These are the reference value and the actual output value of reactive power, respectively. , These are the effective values ​​of the rated voltage and the output voltage, respectively. This is the reactive voltage droop factor; This is the effective value of the voltage reference signal; The inertia coefficient; The above , It can be calculated using instantaneous power theory, that is: （3） In the formula, , These are the d-axis and q-axis components of the VSG output voltage, respectively. , These are the d-axis and q-axis components of the VSG output current, respectively. In summary, the voltage loop reference voltage can be obtained through the power frequency controller and the excitation controller. The amplitude, frequency, and phase are expressed as follows: （4）。 5. The harmonic suppression method for an offshore wind power AC collection uncontrolled rectifier transmission system according to claim 1, characterized in that, The injection of harmonic voltage into the fundamental voltage is specifically implemented according to the following steps: In a stationary three-phase coordinate system, the voltage equation is: （5） In the formula, The grid-side converter outputs three-phase voltage; It is a three-phase alternating current; and For the equivalent inductance and resistance of the line and transformer; The three-phase terminal voltage on the AC side of the uncontrolled rectifier bridge; when When the voltage is sinusoidal, the uncontrolled rectifier, as a nonlinear element, will generate a large amount of harmonic current. Its harmonic components are (12k±1) order harmonic currents, where k is 1, 2, 3, ...; among them, the 11th, 13th, 23rd, and 25th harmonics have a larger content, so these four harmonics are mainly analyzed. To obtain the voltage equations for each harmonic and to avoid the influence between the fundamental component and the harmonic components, a method is adopted to precisely control each harmonic component in a multi-synchronous rotating coordinate system. In the three-phase stationary coordinate system, the rotation direction of the 11th and 23rd harmonic vectors is opposite to that of the fundamental vector, and they are defined as negative sequence harmonics; the rotation direction of the 13th and 25th harmonic vectors is the same as that of the fundamental vector, and they are defined as positive sequence harmonics; the establishment of a multi-synchronous rotating coordinate system is achieved through equations (7) and (8); （7） （8） In the formula, The transformation formula for the nth positive sequence harmonic from the abc stationary coordinate system to the positive sequence harmonic coordinate system is given. The transformation formula for the nth negative harmonic from the abc stationary coordinate system to the negative harmonic coordinate system; Analyzing the voltage equation of the 11th harmonic, equation (6) is transformed into the synchronous rotating coordinate system of the 11th harmonic through equation (8), and the voltage equation of the harmonic is: （9） In the formula, and These represent the d-axis and q-axis components of the injected 11th harmonic voltage, respectively; to simplify the mathematical model of the injected 11th harmonic voltage, and separately included and The impact, received and From equation (9), we obtain equation (10), that is: （10） Similarly, the voltage equations for the 13th, 23rd, and 25th harmonics are: （11） From equations (10) and (11), it can be seen that each harmonic voltage is coupled with the corresponding harmonic current. Therefore, when constructing a harmonic voltage controller, the cross-coupling effect between each harmonic voltage and current should be considered, that is, the d-axis harmonic voltage is affected by the q-axis harmonic current, and the q-axis harmonic voltage is affected by the d-axis harmonic current. Under steady-state conditions, combining equations (10) and (11), the control equations for the cross-decoupling of each harmonic current in the grid-side converter under multiple synchronous rotating coordinate systems can be obtained: （12） In the formula, and These are the proportional and integral coefficients of the PI regulator for the k-th harmonic current, where k = 11, 13, 23, and 25. Each harmonic current, when converted to the corresponding dq coordinate system, becomes a DC component, while the fundamental and other harmonics become AC components. （13） （14） In the formula, The transformation formula for the nth positive sequence harmonic from the positive sequence harmonic coordinate system to the abc stationary coordinate system is given. The transformation formula for the nth negative harmonic from the negative harmonic coordinate system to the abc stationary coordinate system is given.

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