A wind turbine reactive power control method and system based on virtual synchronization technology

Abstract

This invention discloses a reactive power control method and system for wind turbine generators based on virtual synchronization technology, belonging to the field of wind power generation technology. First, the control equations of the virtual synchronizer are established based on the synchronous shaft and power angle system. Then, by changing the amplitude and phase angle of the voltage at the grid connection point of the virtual synchronizer, the active and reactive power transmitted by the inverter to the grid are controlled. Finally, the reactive power-voltage droop coefficient of the reactive power control link of the virtual synchronizer is adjusted to achieve the maximum reactive power output of the wind turbine generator. This invention can effectively improve the grid connection characteristics of wind turbine generators, leverage the inertia and damping of the wind turbine generators, and provide strong support for the grid. It can also improve the grid connection performance of wind turbine generators and maximize their reactive power characteristics, showing promising application prospects.

Description

Technical Field

[0001] This invention belongs to the field of wind power generation technology, specifically relating to a reactive power control method and system for wind turbine generators based on virtual synchronization technology. Background Technology

[0002] A wind turbine is an energy conversion device. Energy transmission mainly consists of a wind turbine and a generator. The wind turbine is responsible for capturing wind energy, and the generator is responsible for converting mechanical energy into electrical energy. The generator's electrical energy control is achieved by a frequency converter. Currently, the mainstream generator types for wind turbines include permanent magnet synchronous generators and doubly-fed induction generators (DFIG) generators.

[0003] Doubly fed asynchronous wind turbines are stable and low-cost wind turbines, belonging to the variable-speed constant-frequency type. Control of the wind turbine system relies on PWM back-to-back converters within the doubly fed asynchronous wind turbine unit. To ensure stable power conversion, the rotating magnetic fields constructed by the stator and rotor of the doubly fed asynchronous turbine need to meet the conditions for synchronous operation. Because the unit uses an AC-excited wound-rotor motor, the relative magnetic field speed of the rotor can be changed by altering the frequency of the rotor voltage, thus freeing the wind turbine rotor speed from the limitation of the grid frequency. When the rotor's electrical angular velocity is greater than the power frequency electrical angular velocity (i.e., slip is greater than 0), the wind turbine operates in a supersynchronous state, and the rotor transmits power to the grid via the converter. When the rotor's electrical angular velocity is less than the power frequency electrical angular velocity (i.e., slip is less than 0), the wind turbine operates in a subsynchronous state, and the grid, in turn, transmits power to the rotor via the converter. Meanwhile, since the power flowing through the converter is proportional to the slip, and the slip typically ranges from -0.3 pu to 0.3 pu, the converter capacity can be significantly reduced, greatly saving equipment costs. Currently, the commonly used control technology for doubly-fed asynchronous wind turbines in actual production is vector control. Its basic principle is to make the vector of the stator voltage or stator flux of the doubly-fed asynchronous wind turbine coincide with the synchronously rotating shaft, thereby achieving the purpose of decoupling reactive power control. While conventional control of doubly-fed asynchronous wind turbines effectively meets the requirements of variable speed and constant frequency operation, it also decouples the turbine rotor speed from the grid frequency, failing to provide inertia to the system. The inertia and damping of the wind turbine are thus hidden.

[0004] Wind turbines, as a renewable energy source, are poised for greater development opportunities. With the large-scale grid connection of wind turbines, the proportion of wind power in the power system is gradually increasing, leading to a significant reduction in power system inertia. Simultaneously, the uncertainty of wind energy exacerbates the burden on the power system. While the existing vector control of doubly-fed induction generator (DFIG) wind turbines possesses strong stability and can meet the normal grid connection requirements, its initial design focused on fulfilling grid connection demands does not provide inertial support to the grid. This results in poor frequency stability and may even trigger large-scale faults, threatening the safe and stable operation of the power grid. Furthermore, the reactive power generation of DFIG units is decoupled through frequency converters. Limited by this control algorithm, the reactive power generation capacity of wind turbines is restricted, preventing them from playing a significant role in the power system through substantial reactive power. Summary of the Invention

[0005] To address the aforementioned issues, this invention provides a method and system for controlling the reactive power of wind turbine generators based on virtual synchronization technology. This method can improve the grid connection performance of wind turbine generators and maximize the reactive power characteristics of wind turbine generators.

[0006] This invention is achieved through the following technical solution:

[0007] This invention discloses a reactive power control method for wind turbine generators based on virtual synchronization technology, comprising:

[0008] Establish the control equations of the virtual synchronizer based on the synchronization axis and power angle of the virtual synchronizer;

[0009] By changing the amplitude and phase angle of the voltage at the grid connection point of the virtual synchronous machine through the control equations of the virtual synchronous machine, the active and reactive power transmitted by the inverter to the grid can be controlled.

[0010] The reactive power-voltage droop coefficient of the virtual synchronous machine reactive power control link is adjusted according to the active and reactive power transmitted by the inverter to the grid, so as to achieve the maximum reactive power output of the wind turbine.

[0011] Preferably, the control equations of the virtual synchronizer are:

[0012]

[0013] Where J is the virtual inertia constant of VSG control, θ is the virtual rotor angle, K is the proportional coefficient, and T... e For electromagnetic torque, T ref For electromagnetic reference torque, D p and D q The damping coefficients are the frequency and voltage, respectively, ω is the VSG virtual electric angular velocity, and M is the voltage damping coefficient. f i f For virtual DC excitation, ω refU is the synchronous electric angular velocity of the power grid. ref U is the AC reference voltage, and U is the grid connection point voltage.

[0014] Preferably, when both the active and reactive power transmitted between the virtual synchronous machine and the power grid are 0, the grid connection condition that the voltage at the grid connection point matches the voltage of the power grid is met.

[0015] More preferably, active power and reactive power are calculated using the following formula:

[0016]

[0017] Preferably, the virtual synchronous machine has a speed system built inside the inverter, through which the power angle relationship of the synchronous motor can be established.

[0018] Preferably, whether the active and reactive power delivered by the VSG to the grid is zero is used as a criterion for determining whether the virtual synchronous machine is connected to the grid or disconnected from the grid.

[0019] Preferably, the reactive power-voltage droop coefficient D q Determined by the following formula:

[0020]

[0021] Where ΔU is the change in the magnitude of the grid voltage, and the negative sign indicates that the change in the reactive power reference value is in the opposite direction to the change in the grid voltage.

[0022] This invention discloses a reactive power control system for wind turbines based on virtual synchronization technology, including a virtual synchronizing machine control unit, a rotor-side converter, a grid-side converter, and a DC filter capacitor;

[0023] The virtual synchronous machine control unit is used to establish the control equations of the virtual synchronous machine. By changing the amplitude and phase angle of the voltage at the grid connection point of the virtual synchronous machine, it controls the active and reactive power transmitted by the inverter to the grid and adjusts the reactive power-voltage droop coefficient of the reactive power control link of the virtual synchronous machine.

[0024] The stator of the doubly fed wind turbine is connected to the grid, and the rotor is connected to the grid through a rotor-side converter and a grid-side converter. The rotor-side converter and the grid-side converter form a DC control loop through a DC filter capacitor. The inner loop of the DC control loop is a frequency downslope control loop, and the outer loop includes a closed loop for feedback from external circuits.

[0025] Preferably, the grid connection point is equipped with a grid connection point voltage sensor and a grid connection point current sensor, which are respectively connected to the grid-side converter.

[0026] Preferably, the rotor-side converter outlet is equipped with a rotor-side converter outlet voltage sensor and a rotor-side converter outlet current sensor.

[0027] Compared with the prior art, the present invention has the following beneficial technical effects:

[0028] This invention discloses a reactive power control method for wind turbines based on virtual synchronous technology. It combines virtual synchronous machine control with stator voltage-oriented control of doubly-fed induction generator (DFIG) wind turbines. This allows the wind turbine to meet grid connection performance requirements while reconstructing the coupling relationship between the DFIG wind turbine speed and the grid frequency. Furthermore, when the grid experiences voltage and frequency drops, the wind turbine can provide reactive power support, improving its grid connection performance and reactive power capacity. The core idea of ​​virtual synchronous machine control is to improve the control strategy of the power electronic converter to achieve operation similar to a synchronous generator, even though the converter itself does not contain inertial components. By improving the inverter control strategy, the power angle relationship from synchronous generators is reintroduced into current PLL-based inverters, enabling the DC source to achieve operating characteristics similar to a synchronous generator when connected to the grid.

[0029] While conventional control of doubly-fed asynchronous wind turbines effectively meets the requirements of variable-speed, constant-frequency operation, it also decouples the turbine rotor speed from the grid frequency, failing to provide inertia to the system. The inertia and damping of the wind turbine are hidden, unable to provide inertial support to the grid. This invention can effectively improve the grid connection characteristics of wind turbine generators, leveraging the turbine's own inertia and damping to provide robust support to the grid. Simultaneously, current wind turbine generators possess a certain reactive power capacity, which is decoupled from reactive power control via frequency converters. A control strategy based on a virtual synchronous machine can maximize the reactive power characteristics of wind turbine generators, providing reactive power support to the grid when voltage and frequency drops occur. This invention improves the grid connection performance of wind turbine generators and maximizes their reactive power characteristics, demonstrating promising application prospects.

[0030] This invention discloses a wind turbine control system. The system is simple to construct, can improve the grid connection performance of wind turbines, and can maximize the reactive power characteristics of wind turbines. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the overall structure of the wind turbine reactive power control system based on virtual synchronization technology of the present invention;

[0032] Figure 2 This is a block diagram of the virtual synchronous machine control principle of the present invention. Detailed Implementation

[0033] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. These descriptions are intended to explain the invention and not to limit it.

[0034] like Figure 1 This is a schematic diagram of the overall structure of the wind turbine reactive power control system based on virtual synchronization technology of the present invention, including a virtual synchronizing machine control unit, a rotor-side converter, a grid-side converter, and a DC filter capacitor.

[0035] The virtual synchronous machine control unit is used to establish the control equations of the virtual synchronous machine. By changing the amplitude and phase angle of the voltage at the grid connection point of the virtual synchronous machine, it controls the active and reactive power transmitted by the inverter to the grid and adjusts the reactive power-voltage droop coefficient of the reactive power control link of the virtual synchronous machine.

[0036] The stator of the doubly fed wind turbine is connected to the grid, and the rotor is connected to the grid through a rotor-side converter and a grid-side converter. The rotor-side converter and the grid-side converter form a DC control loop through a DC filter capacitor. The inner loop of the DC control loop is a frequency downslope control loop, and the outer loop includes a closed loop for feedback from external circuits.

[0037] The grid connection point is equipped with a grid connection point voltage sensor and a grid connection point current sensor, which are respectively connected to the grid-side converter.

[0038] The rotor-side converter outlet is equipped with a rotor-side converter outlet voltage sensor and a rotor-side converter outlet current sensor.

[0039] like Figure 2 This is a block diagram of the control principle of the Virtual Synchronizer (VSG), and its control strategy specifically includes:

[0040] 1. Establish a virtual synchronizer, synchronous axis, and angle of attack system.

[0041] The active power loop in VSG control forms a mesh structure, with the inner loop being a frequency downslope control loop and the outer loop being a complex closed loop incorporating external circuit feedback. The active power loop implicitly enables the inverter to automatically track the grid frequency. This differs from the phase-locked loop (PLL) used in traditional inverter control, which completely tracks the grid phase angle. In contrast, the virtual synchronous machine (VSM) constructs a relatively independent speed system within the inverter, similar to the rotating elements of a synchronous motor, establishing the power angle relationship of a synchronous motor.

[0042] Because the controlled device is a fully controlled power electronic device, the control has two degrees of freedom, allowing for separate control of the output active and reactive power. To make the VSG control more similar to the synchronous motor in actual operation, the active power frequency regulation and reactive power voltage regulation characteristics of the synchronous generator are also reproduced, resulting in the final control equations for the VSG control as follows:

[0043]

[0044]

[0045] In the above formula, J is the virtual inertia constant of VSG control, θ is the virtual rotor angle, K is the proportional coefficient, and T... e For electromagnetic torque, T ref For electromagnetic reference torque, D p and D q The damping coefficients are the frequency and voltage, respectively, ω is the VSG virtual electric angular velocity, and M is the voltage damping coefficient. f i f For virtual DC excitation, ω ref U is the synchronous electric angular velocity of the power grid. ref U is the AC reference voltage, and U is the grid connection point voltage.

[0046] Active power and reactive power can be calculated using the following formula:

[0047]

[0048] The active power loop in VSG control forms a mesh structure, with the inner loop being a frequency downslope control loop and the outer loop being a complex closed loop incorporating external circuit feedback. The active power loop implicitly enables the inverter to automatically track the grid frequency. This differs from the phase-locked loop (PLL) used in traditional inverter control, which completely tracks the grid phase angle. In contrast, the virtual synchronous machine (VSM) constructs a relatively independent speed system within the inverter, similar to the rotating elements of a synchronous motor, establishing the power angle relationship of a synchronous motor.

[0049] 2. Virtual Synchronizer Self-Synchronization

[0050] According to the control strategy proposed above, although its own power angle relationship has been established inside the inverter, the initial phase angle of the grid still needs to be input by the phase-locked loop during the startup process, and it cannot achieve autonomous grid connection like a synchronous motor.

[0051] By changing the amplitude and phase angle of the VSG grid connection point voltage, the active and reactive power transmitted by the inverter to the grid can be controlled. Conversely, when both the active and reactive power transmitted between the VSG and the grid are 0, the grid connection condition of matching the grid connection point voltage with the grid voltage can be met.

[0052] Whether the active and reactive power delivered by the VSG to the grid is zero is used as a criterion for determining whether the virtual synchronous machine is connected to or disconnected from the grid.

[0053] 3. Optimize the control parameters of the virtual synchronous machine to tap the reactive power potential of the wind turbine generator.

[0054] The active power control stage of a VSG has two degrees of freedom: the virtual inertia J and the active power-frequency droop coefficient Dp. The reactive power control stage also has two degrees of freedom: the reactive power-voltage droop coefficient Dq and the virtual excitation coefficient K. Although the virtual synchronous machine has a similar control relationship to an actual synchronous generator set, its hardware, being essentially power electronic devices, offers faster response and shorter delays compared to actual synchronous motors. Therefore, by optimizing control parameters, the reactive power potential of the wind turbine can be maximized, achieving optimal control of the wind turbine's reactive power.

[0055] The reactive power droop factor Dq is mainly used to realize the reactive power-voltage droop characteristic of VSG control. The value of Dq determines the primary voltage regulation capability of the final control. Dq can be determined by the following formula:

[0056]

[0057] In the formula, ΔU represents the change in the grid voltage amplitude, and the negative sign indicates that the change in the reactive power reference value is opposite to the change in the grid voltage. Adjusting this value yields the maximum value of Dq, which, combined with other control parameters, enables the wind turbine to achieve its maximum reactive power output.

[0058] The above description is merely a specific embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A reactive power control method for wind turbine generators based on virtual synchronization technology, characterized in that, include: The control equations of the virtual synchronizer are established based on the synchronous shaft and power angle of the virtual synchronizer. The control equations of the virtual synchronizer are as follows: Where J is the virtual inertia constant of the VSG control. The virtual rotor angle is K, the proportional coefficient is T. e For electromagnetic torque, T ref For electromagnetic reference torque, D p and D q The damping coefficients are the frequency and voltage damping coefficients, respectively. M is the virtual electric angular velocity of the VSG. f i f For virtual DC excitation, ref U is the synchronous electric angular velocity of the power grid. ref U is the AC reference voltage, and U is the grid connection point voltage. The virtual synchronous machine has a speed system built inside the inverter, through which the power angle relationship of the synchronous motor can be established; By changing the amplitude and phase angle of the voltage at the grid connection point of the virtual synchronous machine through the control equations of the virtual synchronous machine, the active and reactive power transmitted by the inverter to the grid can be controlled. The reactive power-voltage droop coefficient of the virtual synchronous machine reactive power control link is adjusted according to the active power and reactive power transmitted by the inverter to the grid, so as to achieve the maximum reactive power output of the wind turbine when the grid voltage frequency drops. The reactive power-voltage droop coefficient D q Determined by the following formula: in, This represents the change in the magnitude of the grid voltage; the negative sign indicates that the change in the reactive power reference value is in the opposite direction to the change in the grid voltage. When both the active and reactive power transmitted between the virtual synchronous machine and the grid are zero, the grid connection condition that the grid connection point voltage matches the grid voltage is met. Furthermore, whether the active and reactive power transmitted by the VSG to the grid is zero is used as the judgment condition before the virtual synchronous machine is connected to the grid and disconnected from the grid.

2. The reactive power control method for wind turbine generators based on virtual synchronization technology according to claim 1, characterized in that, Active power and reactive power are calculated using the following formula: 。 3. A reactive power control system for wind turbines based on virtual synchronization technology, used to implement the reactive power control method for wind turbines based on virtual synchronization technology as described in claim 1, characterized in that, This includes a virtual synchronous machine control unit, a rotor-side converter, a grid-side converter, and DC filter capacitors; The virtual synchronous machine control unit is used to establish the control equations of the virtual synchronous machine. By changing the amplitude and phase angle of the voltage at the grid connection point of the virtual synchronous machine, it controls the active and reactive power transmitted by the inverter to the grid. When the grid experiences a voltage frequency drop, it adjusts the reactive power-voltage droop coefficient of the reactive power control link of the virtual synchronous machine to achieve the maximum reactive power output of the wind turbine. The stator of the doubly fed wind turbine is connected to the grid, and the rotor is connected to the grid through a rotor-side converter and a grid-side converter. The rotor-side converter and the grid-side converter form a DC control loop through a DC filter capacitor. The inner loop of the DC control loop is a frequency downslope control loop, and the outer loop includes a closed loop for feedback from external circuits.

4. The wind turbine reactive power control system based on virtual synchronization technology according to claim 3, characterized in that, The grid connection point is equipped with a grid connection point voltage sensor and a grid connection point current sensor, which are respectively connected to the grid-side converter.

5. The wind turbine reactive power control system based on virtual synchronization technology according to claim 3, characterized in that, The rotor-side converter outlet is equipped with a rotor-side converter outlet voltage sensor and a rotor-side converter outlet current sensor.

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