A control method for seamless switching between grid-following and grid-connecting modes for doubly-fed wind turbines

Abstract

This invention discloses a seamless switching control method for grid-connected and grid-connected modes applicable to doubly-fed induction generator (DFIG) wind turbines, belonging to the field of power electronic control technology. In the grid-connected mode, the rotor-side converter of the DFIG wind turbine employs stator flux linkage phase-locked loop (PLL) control, while in the grid-connected mode, it employs virtual synchronous generator (VRG) control. If the DFIG wind turbine switches from grid-connected to grid-connected mode, the previously measured rotor current value is used as the PLL current reference value, and the real-time physical phase of the PLL is directly used as the reference phase. If switching from grid-connected to grid-connected mode, the real-time physical phase is used as the initial virtual phase, and the previously measured current value is used as the initial integral value of the voltage outer loop PI controller. This method enables the DFIG wind turbine to flexibly and seamlessly switch between grid-connected and grid-connected modes, tapping the wind energy capture and physical inertia support potential of the unit, and significantly improving the grid connection stability and overall adaptability of DFIG wind farms in complex operating environments.

Description

Technical Field

[0001] This invention belongs to the field of power electronic control technology, specifically relating to a seamless switching control method for grid-following and grid-connecting modes applicable to doubly-fed wind turbines. Background Technology

[0002] Wind power generation, as one of the most technologically mature and widely developed new energy power generation methods, plays a crucial role in energy structure adjustment. Among them, doubly-fed wind turbines have become the most widely used wind turbine type due to their significant advantages such as high control flexibility, small converter capacity, and low cost.

[0003] However, with the continuous increase in the proportion of new energy grid connection, the new power system is gradually showing the "dual high" characteristics of high proportion of renewable energy and high proportion of power electronic equipment. Compared with the traditional power system, the new power system puts forward more flexible and diversified requirements for the control performance of grid-connected converters, including providing virtual inertia support under low inertia characteristics, having active voltage support capability when subjected to voltage disturbances, and providing damping to suppress broadband oscillations.

[0004] In current control technologies, grid-connected control and grid-connected control are two mainstream modes, but their adaptability to different grid strengths varies significantly. Grid-connected control is typically equivalent to a controlled current source, capable of quickly tracking dispatch commands and achieving maximum power point tracking (MPPT) for wind energy. However, in weak grid environments, it heavily relies on the stable tracking of phase-locked loops (PLLs), making it prone to instability and unable to provide autonomous inertia support. Grid-connected control, on the other hand, is equivalent to a controlled voltage source, capable of actively constructing and maintaining the voltage and frequency at the grid connection point to adapt to weak grid conditions. However, its stability margin under strong grid conditions is relatively low, and it faces a serious risk of overcurrent damage under large disturbances caused by faults.

[0005] It is obvious that relying on a single control mode is no longer sufficient to cope with the complex and ever-changing operating environment of the new power system. In order to balance efficient energy replenishment and active support, wind turbines must have the ability to flexibly and seamlessly switch between grid-connected and grid-connected modes.

[0006] Although some research has been conducted on grid-connection switching strategies for energy storage converters, targeted research on doubly-fed induction generator (DFIG) wind turbines remains severely lacking. DFIG wind turbines and energy storage converters have fundamental technical differences: the stator side of a DFIG is directly connected to the grid, while the rotor side is controlled by a small-capacity converter. Its electromechanical dynamics are affected by slip frequency, rotor inertia, and the high coupling characteristics of stator and rotor flux linkages. Most existing switching strategies fail to consider the complex nonlinear electromechanical transient processes of DFIG generators and lack a unified control architecture that can accommodate their vector control characteristics and achieve smooth transitions between key state variables such as phase and current. Therefore, it is urgent to conduct in-depth research on control strategies that can achieve smooth grid-connection switching for the dynamic characteristics of DFIG wind turbines to improve the stable operation of wind turbine units in new power systems. Summary of the Invention

[0007] To address the lack of application of existing converter grid-following / grid-connection switching strategies in doubly-fed induction generator (DFIG) wind turbines, this invention provides a seamless grid-following and grid-connection mode switching control method suitable for DFIG wind turbines. The aim is to construct a unified switching framework compatible with the complex electromechanical dynamic characteristics of DFIG wind turbines, achieving seamless and smooth switching across the entire power range. The rotor-side converter of the DFIG wind turbine employs stator flux linkage phase-locked loop control in grid-following mode and virtual synchronous generator control in grid-connection mode. The method includes:

[0008] S1, determine whether the mode of the doubly fed wind turbine has changed. If it has changed from grid-building mode to grid-following mode, proceed to S2; if it has changed from grid-following mode to grid-building mode, proceed to S3.

[0009] S2, when switching to grid-connected mode, the current reference value of the current inner loop in grid-connected mode at the switching moment is set to the actual measured value of the rotor current in grid-connected mode before switching, so as to avoid sudden changes in the current reference value of the current inner loop before and after mode switching.

[0010] At the same time, the reference phase of the rotor-side converter Park transform is directly switched from the virtual phase generated by the network mode to the real-time physical phase output by the stator flux linkage phase-locked loop;

[0011] S3, intercept the real-time physical phase output of the stator flux linkage phase-locked loop at the switching moment, and write it as the initial value into the active-frequency control loop of the virtual synchronous generator in the grid construction mode, so as to ensure that the virtual phase generated by the grid construction mode after the switching starts smoothly from the current real phase point of the grid and eliminates transient active power impact.

[0012] The initial value of the integral term of the voltage outer loop PI controller in grid mode is reset to the measured value of the rotor current in grid mode before switching, so as to avoid the sudden change in current command when the grid mode single current inner loop structure is changed to the grid mode voltage outer loop and current inner loop structure.

[0013] Furthermore, the doubly-fed wind turbine also includes a grid-side converter, which employs grid voltage-oriented vector control. The control process is as follows:

[0014] The grid phase is obtained in real time based on the grid-side voltage and current during the operation of the doubly fed wind turbine;

[0015] By combining the voltage and current with the grid phase and performing Park transformation, the corresponding measured values ​​of d-axis and q-axis voltage and measured values ​​of d-axis and q-axis current are obtained.

[0016] The PI controller calculates the reference value of the d-axis current of the grid-side converter using the measured values ​​of the d-axis and q-axis voltages, and sets the reference value of the q-axis current of the grid-side converter to 0.

[0017] By combining the reference values ​​of the d-axis and q-axis currents of the grid-side converter with the measured values ​​of the d-axis and q-axis currents, the d-axis and q-axis modulation waves are obtained. The three-phase modulation waves are further obtained by combining the grid phase, and the PWM controls the grid-side converter based on the three-phase modulation waves.

[0018] Furthermore, the process of controlling the rotor-side converter of the doubly-fed wind turbine using a stator magnetic flux phase-locked loop is as follows:

[0019] The stator voltage and stator current of the doubly fed wind turbine in grid-connected mode are obtained, and the three-phase flux linkage is calculated by combining the stator resistance; the stator flux linkage phase-locked loop extracts the real-time physical phase based on the three-phase flux linkage.

[0020] The stator current is transformed using the real-time physical phase to obtain the measured values ​​of the rotor current on the d-axis and q-axis.

[0021] In the inner current loop, by combining the measured values ​​of the d-axis and q-axis rotor currents with the reference values ​​of the d-axis and q-axis rotor currents, the d-axis and q-axis modulation waves of the rotor-side converter are obtained, and the three-phase modulation waves of the rotor-side converter are further obtained. The PWM controls the rotor-side converter in grid-following mode based on the three-phase modulation waves of the rotor-side converter.

[0022] Furthermore, the virtual synchronous generator includes an active-frequency control loop and a reactive-voltage control loop; the process of using the virtual synchronous generator to control the rotor-side converter of the doubly-fed wind turbine is as follows:

[0023] The stator voltage and stator current of the doubly fed wind turbine in grid-connected mode are obtained, and the actual active power and actual reactive power are calculated. At the same time, the stator voltage is converted into measured values ​​of d-axis and q-axis voltages through Park transformation.

[0024] In the active-frequency control loop, based on the active power reference value and the actual active power, the virtual angular velocity is calculated by simulating the swing equation of the synchronous generator rotor, and then further integrated to generate the virtual phase;

[0025] In the reactive-voltage control loop, the stator q-axis voltage reference value is calculated based on the reactive power reference value and the reactive and active power through droop control.

[0026] Set the stator d-axis voltage reference value to 0, and calculate the q-axis and d-axis rotor current reference values ​​by comparing the stator d-axis and q-axis voltage reference values ​​with the measured stator d-axis and q-axis voltage values ​​in the voltage outer loop;

[0027] In the inner current loop, based on the d-axis and q-axis rotor current reference values, the d-axis and q-axis modulation waves of the rotor-side converter are obtained, and the three-phase modulation waves of the rotor-side converter are further obtained. The PWM controls the rotor-side converter in the grid mode based on the three-phase modulation waves of the rotor-side converter.

[0028] Furthermore, the formula for calculating the reference value of the d-axis rotor current is as follows:

[0029] ;

[0030] in, This is the measured value of the stator q-axis voltage. This is the proportionality coefficient. The integral coefficient is... Here is the reference value for the d-axis rotor current, and t is the time interval. This is the reference value for the stator q-axis voltage.

[0031] Furthermore, in S1, the current mode of the doubly fed wind turbine is indicated by an instruction flag bit. A value of 1 indicates that the doubly fed wind turbine is currently in grid-building mode, and a value of 0 indicates that the doubly fed wind turbine is currently in grid-following mode.

[0032] Furthermore, the determination of whether the mode of the doubly fed wind turbine has switched is achieved by detecting the transition edge value of the command flag bit; the transition edge detection value is obtained by subtracting the command flag bit at the current moment from the command flag bit at the previous moment; if the difference between the command flag bits at the current moment and the previous moment is 1, it indicates that the mode has switched from following the grid to building the grid; if the difference between the command flag bits at the current moment and the previous moment is -1, it indicates that the mode has switched from building the grid to following the grid; if the difference between the command flag bits at the current moment and the previous moment is 0, it indicates that no mode switch has occurred.

[0033] Furthermore, in S3, the process of resetting the initial value of the integral term of the voltage outer loop PI controller in grid-connected mode to the measured value of the rotor current in grid-connected mode before switching is as follows:

[0034] Taking the q-axis as an example, let the reference value of the d-axis current output by the PI controller of the outer loop of the q-axis voltage in the virtual synchronous generator reactive-voltage control loop at the switching moment be equal to the d-axis rotor current. By reversing the PI regulation equation, we can obtain the equation for the integral term.

[0035] Since the measured voltage value and the reference voltage value are equal during mode switching, the equation concerning the integral term is transformed into an equal relationship between the initial value of the integrator in the outer loop PI controller of the q-axis voltage and the rotor current of the d-axis at the previous moment.

[0036] Furthermore, the equation concerning the integral term is:

[0037] ;

[0038] in, for The moment in time is the moment before the mode switch occurred; This refers to the internal state value of the integrator in the outer-loop PI controller for calculating the integral term of the q-axis voltage. for The measured current value of the d-axis rotor-side converter at time 1. This is the proportionality coefficient. for Measured value of stator q-axis voltage at time (i.e.) for Reference value of stator q-axis voltage at any time.

[0039] The beneficial effects of this invention are:

[0040] (1) This invention constructs a unified switching architecture for doubly-fed wind turbines. Considering the high coupling of stator and rotor flux linkages and the dynamic characteristics of slip in doubly-fed wind turbines, this invention fills the technical gap of the lack of application of existing strategies in doubly-fed wind turbines by uniformly reconstructing the mathematical model of the rotor-side control structure while maintaining the voltage regulation of the grid-side converter.

[0041] (2) This invention proposes a control mechanism based on bidirectional instantaneous reset of phase angle and command. It fundamentally eliminates the command step caused by integrator state mismatch and sudden change in control loop structure during switching from a mathematical logic perspective, and completely suppresses phase jump and transient impact during bidirectional switching.

[0042] (3) The method of the present invention enables the doubly fed wind turbine to seamlessly switch between the grid-following mode of efficient energy capture and the grid-building mode of active support. Combined with the unified swing equation configuration, it can effectively capture wind energy for power generation according to demand, and can also make full use of the huge physical rotational inertia contained in the prime mover of the doubly fed wind turbine, deeply explore the flexible support potential of the unit, and provide reliable virtual inertia and transient voltage support for the new power system. Attached Figure Description

[0043] Figure 1 This is the main circuit hardware topology diagram of a doubly fed wind power generation system.

[0044] Figure 2 Diagram of a control architecture for smooth switching between grid-connected and grid-connected dual-mode doubly-fed wind turbines.

[0045] Figure 3 This is a schematic diagram of the control method for smooth switching between dual-mode grid connection and grid construction for doubly-fed wind turbines.

[0046] Figure 4 The system simulation waveforms are shown during the smooth switching process between network following and network construction modes under full load operation.

[0047] Figure 5 The simulation waveforms are for a traditional direct switching system that does not employ the control method of this invention. Detailed Implementation

[0048] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. Technical features in the various embodiments of the present invention can be combined accordingly without mutual conflict.

[0049] Figure 1 This is a hardware topology diagram of the main circuit of the doubly-fed wind power generation system of the present invention. The system mainly consists of wind turbine blades, gearbox, induction motor (i.e., doubly-fed wind turbine), rotor-side converter (RSC), grid-side converter (GSC), three-port transformer, and power grid. (i) sabc i rabc i gabc These are the three-phase currents at the stator outlet, rotor outlet, and grid side, respectively. sabc u gabc These are the three-phase voltages at the stator outlet and the grid side, respectively. dc θ is the DC bus voltage. r The rotor electrical angle.

[0050] Figure 3 The flowchart of a seamless switching control method for grid-following and grid-connecting modes for doubly-fed wind turbines, proposed in this invention, is shown below. The specific steps are as follows:

[0051] Step 1: Set up the basic control architecture and division of labor on the network side and machine side in the network following mode and network construction mode.

[0052] (1) Grid-side converter (GSC)

[0053] Throughout the full-power operation and mode switching process of this invention, the grid-side converter always adopts grid voltage-oriented vector control to maintain a constant DC bus voltage and ensure energy balance on both sides of the converter.

[0054] Specifically, such as Figure 2 As shown, the system collects the three-phase voltage from the grid side. and grid-side three-phase current By using the Park transform and a preset initial grid phase, the stationary three-phase quantities are converted into rotating dq DC quantities, and then the grid phase θ is extracted by the grid voltage-oriented phase-locked loop. g The phase angle is fed back to the Park transform to form a closed loop, thereby continuously completing the cyclic calculation of grid synchronization coordinate transformation and phase-locked loop.

[0055] A PI controller (Proportional-Integral controller) achieves DC bus voltage regulation control through d-axis active current, where the formula for calculating the d-axis active current is:

[0056]

[0057] in, and These are the reference value and the actual sampled value of the DC bus voltage, respectively. K represents the d-axis current command value of the grid-side converter. pdc and K idc These are the proportional and integral coefficients of the DC voltage outer loop PI controller, respectively, and t is time t.

[0058] Simultaneously set the mesh-side q-axis current command. = 0 to achieve unity power factor grid connection.

[0059] Finally, the current control section outputs the modulated wave of the d-axis of the grid-side converter based on the dq-axis current command value and the actual sampled current value after Park conversion. Modulated wave along the q-axis and using grid phase θ g The three-phase modulation wave of the grid-side converter is obtained by coordinate transformation. The grid-side converter is controlled by PWM modulation.

[0060] (2) Rotor-side converter (RSC)

[0061] like Figure 2 As shown, the rotor-side converter (RSC), as the main actuator for mode switching in this invention, is configured with two architectures: a grid-following control architecture based on stator flux orientation and a grid-connected control architecture based on a virtual synchronous generator.

[0062] 1) Follow-up network control architecture (flag = 0): Stator magnetic flux linkage phase-locked loop directional control is adopted.

[0063] The system acquires the stator three-phase voltage u in a three-phase stationary coordinate system. sabc With three-phase current i sabc The three-phase flux linkage of the stator is calculated using an integrator. :

[0064]

[0065] in, This is the stator resistance.

[0066] Subsequently, the three-phase magnetic flux Ψ sabc The phase is fed into the stator magnetic flux phase-locked loop to extract the directional phase. And use it as the reference angle for coordinate transformation (i.e. ).

[0067] When switching to this mode, the control loop retains only the inner loop of the rotor current, and its d-axis and q-axis rotor current reference values ​​( , The current is directly given by an external scheduling system or upper-level controller, and finally controlled by the current control section using directional phase. The stator three-phase current is subjected to Park transformation to obtain the rotor current feedback value in the rotating dq coordinate system. This value is then combined with the rotor current reference value to output the modulation wave of the rotor-side converter's d-axis. Modulated wave along the q-axis The three-phase modulation wave of the rotor-side converter was obtained through coordinate transformation. By controlling the rotor-side converter through PWM modulation, the unit exhibits highly efficient and controlled current source characteristics.

[0068] 2) Network-type control architecture (flag=1): adopts virtual synchronous generator control strategy.

[0069] The rotor-side converter operating in Virtual Synchronous Generator (VSG) control mode uses an active-frequency control loop that simulates the swing equation of a synchronous generator rotor, autonomously integrating to generate a virtual angular velocity ω and synchronous phase θ. vsg :

[0070]

[0071] In the formula, J is the virtual inertia. Let P0 and P be the active damping coefficients. e These represent the commanded value and the actual output value of the active power, respectively. This is the rated angular frequency of the power grid. At this time, the control switch is oriented towards this virtual phase (i.e., θ).s =θ vsg ).

[0072] Its reactive power-voltage control loop generates a stator q-axis voltage reference value u through droop control. sqref :

[0073]

[0074] In the formula, Let be the integral time constant, s be the complex frequency variable, and D be the integral time constant. q The reactive power damping coefficient is given by U0 and U, respectively, where U0 and U are the commanded and actual amplitudes of the generator terminal voltage, and Q0 and Q are the reactive power damping coefficients. e This refers to the reactive power command and the actual value.

[0075] The control system generates a rotor current reference command through the stator voltage outer loop. The q-axis voltage deviation is adjusted via a PI controller to generate a d-axis rotor current reference value. :

[0076]

[0077] in, This is the measured value of the stator q-axis voltage. The proportional gain of the outer loop PI controller. The integral coefficient of the voltage outer loop PI controller. This is the reference value for the stator q-axis voltage.

[0078] Simultaneously set the d-axis voltage reference value u. sdref = 0, the q-axis rotor current reference value is generated through PI regulation. :

[0079]

[0080] in, This is the measured value of the stator d-axis voltage.

[0081] The above-generated reference instructions ( , The given value in the network configuration mode is input to the inner loop of the rotor current, and finally, the three-phase modulation wave of the rotor-side converter is obtained. Furthermore, by controlling the rotor-side converter through PWM modulation, the unit exhibits the characteristics of a controlled voltage source with inertial response and active support capabilities.

[0082] Step 2: Monitor the system's operating status and mode command flag changes in real time.

[0083] This invention first defines a flag to indicate whether the current doubly-fed induction generator (DFIG) is operating in grid-following mode or grid-connecting mode according to instructions from the upper level. The control system reads the flag in real time to switch between grid-following and grid-connecting modes. When the system receives flag = 0, it is set to grid-following mode; conversely, when the system receives flag = 1, it is set to grid-connecting mode.

[0084] In addition, in order to accurately capture the transients of mode switching and trigger a smooth transition mechanism, the system also needs to detect and capture the transition edge of the command flag bit, and perform corresponding judgments based on the transition edge detection value Δflag:

[0085] If Δflag = 0: it means that the instructions have not changed, the system is in a steady state, and the currently active network following or network construction control mode will continue to be maintained;

[0086] If Δflag = 1 (rising edge detection): it indicates that the system has received a command to switch from follow-mesh to network configuration mode.

[0087] If Δflag = -1 (falling edge detection): it indicates that the system has received a command to switch from network construction mode to network following mode.

[0088] The edge detection value Δflag is obtained by subtracting the flag at the current sampling time from the flag at the previous sampling time.

[0089] Step 3: Execute the corresponding seamless switching control strategy.

[0090] Figure 2 This diagram illustrates the dual-mode smooth switching control architecture for a doubly-fed induction generator (DFIG) wind turbine, encompassing both grid-connected and grid-connected modes. It includes the grid-side converter control architecture and the turbine-side (rotor-side) control architecture. (Figure i) sdq i rdq i dq These are the dq-axis currents at the stator outlet (i.e., the d-axis currents at the stator outlet, i). sd Stator outlet q-axis current i sq ), the dq-axis current at the rotor outlet (i.e., the d-axis current i at the rotor outlet) rd Rotor outlet q-axis current i rq ) and the dq-axis current on the grid side (i.e., the d-axis current i on the grid side) d , grid-side q-axis current i q ), u sdq u dq These are the dq-axis voltages at the stator outlet (i.e., the d-axis voltage u at the stator outlet). sd Stator output q-axis voltage u sq ) and the dq-axis voltage on the grid side (i.e., the d-axis voltage u on the grid side) d , grid-side q-axis voltage u q).

[0091] In this invention, the grid-side converter employs grid voltage-oriented vector control, which maintains the DC bus voltage u by controlling the AC / DC switching power. dc The constancy of.

[0092] The rotor-side converter performs dual-mode switching control based on the flag bit, such as... Figure 2 The red arrow in the image indicates: θ s θ is the reference phase for the stator flux linkage. s1 This refers to the slip phase. When flag=0, the system is in grid-following mode, and the system outputs phase θ via a stator flux linkage phase-locked loop. Ψ Orientation (θ) s =θ Ψ The control loop retains only the inner loop of rotor current, directly following the current reference value given by the front end (i.e., the rotor d-axis current reference value in grid-following mode). Rotor q-axis current reference value When flag=1, the system is in network construction mode, switches to virtual synchronous generator control, and autonomously generates the synchronous phase θ using the swing equation. vsg Orientation (θ) s =θ vsg And the current reference value is generated by adjusting the stator voltage outer loop (i.e., the rotor d-axis current reference value in grid mode). Rotor q-axis current reference value At the instant of switching between the two modes, the control system assigns the real-time phase or current physical quantity to the integrator state terminal of the target loop, thereby resetting the initial value of the reference phase and control command, and thus completing the smooth and seamless switching of the doubly fed wind turbine between grid-connected and grid-connected control modes.

[0093] Specifically:

[0094] Based on the transition direction of the mode command flag detected in step two, the doubly-fed wind turbine performs a mode switch according to the received command. The moment the switch is triggered (recorded as ) The rotor-side converter is subjected to a seamless switching control strategy to eliminate stator phase angle jumps and rotor reference current jumps caused by abrupt changes in the control structure.

[0095] (1) When the system determines that the switching direction is from network following mode to network building mode (Δflag = 1), perform the following operations in sequence:

[0096] 1) Reset the initial value of the active loop phase integrator:

[0097] In the vector control system of a doubly-fed generator, the dq-axis decomposition of all stator or rotor electrical quantities (voltage, current, flux linkage) depends on a rotating coordinate transformation matrix (i.e., the Park transformation matrix), and the input angle of this matrix is ​​the orientation reference angle θ. s In grid-connected mode, this angle is output as phase by the stator flux linkage phase-locked loop (PLL). Provided; while in the network configuration mode, this angle is generated by the active-frequency loop in the Virtual Synchronous Generator (VSG) control algorithm through time integration of the frequency deviation.

[0098] When a mode switch occurs, since the rotating reference frame in the network mode is generated by the active power loop integral of the virtual synchronous generator, if the active power loop of the virtual synchronous generator is used to generate... The initial value of the phase accumulator (i.e., the register that implements the integration function in a digital control system, usually a 32-bit floating-point variable) is compared with the PLL output at the moment before the switch. There is a discrepancy: This will cause a step jump in the orientation reference angle used for coordinate transformation after the switch. Since the transformation relationship between the dq axis and the abc axis is a nonlinear trigonometric function, this angular step will directly cause a drastic redistribution of the active / reactive components of the stator current at the moment of switching. Macroscopically, this manifests as a surge in transient active power, which can, in severe cases, trigger the converter overcurrent protection.

[0099] To ensure that the coordinate system does not rotate or shift during the transition, a truncation is taken. The real-time physical phase (i.e., orientation phase) output by the stator flux linkage phase-locked loop at all times. This initial value is forcibly written into the status register of the phase integrator in the active-frequency control loop via direct memory assignment, and this value serves as the starting point for the discrete integral accumulation of the virtual synchronous generator's active-frequency loop. Mathematically, this operation is equivalent to assigning a non-zero initial condition to the swing equation of the virtual synchronous generator. Its mathematical reset expression is:

[0100]

[0101] After writing the initial value, from From that moment on, the phase integrator of the active power loop will be based on this initial value. Based on this, the speed deviation is calculated in real time. Perform incremental accumulation.

[0102] This operation ensures that the virtual phase generated by the grid configuration mode after the switch starts smoothly from the current real phase point of the power grid, achieving seamless synchronization of the stator side phase and eliminating the transient active power surge caused by the sudden change in phase angle.

[0103] 2) Reset the initial value of the voltage outer loop PI controller:

[0104] In grid-connected mode, the rotor-side converter control structure is a single current loop, meaning only the inner current loop PI controller operates, and its reference value is directly given externally. In grid-connected mode, the control structure becomes a dual closed-loop series structure of outer voltage loop and inner current loop, where the output of the outer voltage loop PI controller serves as the reference value for the inner current loop. When switching from grid-connected mode to grid-connected mode, if the outer voltage loop is directly put into operation, because it was in an open-loop, uncontrolled state during the previous grid-connected mode operation, the initial value of the integrator inside its PI controller will continuously integrate the voltage error until integration saturation. This will result in the initial output of the outer voltage loop (i.e., the reference value of the inner current loop) being an integral process value or the upper / lower limit of integrator saturation at the time of mode switching, which deviates significantly from the actual current reference value of the inner current loop just before the switch, causing transient active power surges.

[0105] To avoid abrupt changes in the current reference value when the control architecture changes from a single current loop to a voltage outer loop and current inner loop, sampling is used. The actual components i of the measured values ​​of rotor current d-axis and q-axis currents at any given time rd i rq The initial value of the integral term of the voltage outer loop PI controller in the network configuration mode is reset. It should be noted that since the voltage outer loop in the network configuration mode contains two independent control channels, the d-axis and the q-axis, and their PI controllers are independent digital arithmetic units, the integrators of the two channels must be reset independently.

[0106] Taking the q-axis as an example, in the network configuration mode, the outer loop of the q-axis voltage controls the stator q-axis voltage u. sq Follow its reference value According to the network control strategy adopted in this invention, Generated by the reactive power-voltage control loop, the output of this channel is the reference value for the d-axis rotor current. .

[0107] At the switching time To ensure that the output of the outer voltage loop is operational, To prevent abrupt changes, the ideal goal is to make this output value equal to the actual d-axis rotor current measurement at the moment before the switch (i.e., in grid mode). .

[0108] Therefore, the output reference value of the outer loop PI controller for the q-axis voltage is set at the moment of switching. Exactly equal to the current d-axis rotor current measurement value The initial internal state value of the voltage outer loop integrator can be derived by reversing the discretized regulation equation of the voltage outer loop PI controller.

[0109] In this invention, the q-axis voltage outer loop PI controller adopts a position-based digital PI algorithm, and its discretized mathematical model is as follows:

[0110]

[0111] in, These are the internal state values ​​of the q-axis voltage outer loop integrator. This is the voltage outer loop proportionality coefficient (dimensionless).

[0112] The logic for establishing the equivalent relationship between the discretized mathematical model and the current d-axis rotor current measurement value, and then reversing and executing the integrator initial value reset based on this relationship, is as follows:

[0113]

[0114] in, The controller sampling time preceding the mode switch is defined as follows: , This refers to the control cycle of the inner loop of the rotor-side converter current.

[0115] Considering that there is no steady-state error between the actual voltage and the reference value when the system is running stably (i.e., u... sq = u sqref Therefore, the integrator reset value can be directly changed to:

[0116]

[0117] Through this assignment operation, when the outer voltage loop is... When the closed-loop control is officially engaged, the reference value of the first output current is... This is completely consistent with the actual value that the inner current loop was tracking before the switch. Afterward, the integrator will start from this initial value and calculate based on subsequent accumulated values. The voltage deviation is adjusted incrementally to achieve a smooth transition without impact.

[0118] Similarly, the state of the outer loop integrator of the d-axis voltage. The initial reset value is set to:

[0119] .

[0120] (2) When the system determines that the switching direction is from network construction mode to network following mode (i.e. Δflag = -1), a smooth transition is achieved through the current command preset strategy.

[0121] In grid-connected mode, the reference value of the rotor current inner loop is calculated and generated in real time by the voltage outer loop (virtual synchronous generator control). In grid-following mode, the reference value of the rotor current inner loop is directly given by the external dispatch system or the upper-level energy management system. When switching from grid-connected mode to grid-following mode, if the reference value of the current inner loop is switched directly from the voltage outer loop output to the external dispatch value at the moment of switching, and the external dispatch value may deviate significantly from the actual current reference value output by the voltage outer loop just before the switch, it will cause a step change in the reference value of the current inner loop, thereby causing a stator current surge.

[0122] For example, at the instant of mode switching, the current reference value calculated by the virtual synchronous generator is 800A, while the external dispatch value is 700A. When mode switching occurs, the reference value of the inner loop of rotor current jumps from 800A to 700A instantly, resulting in a step jump.

[0123] To solve this problem, the present invention performs a current reference value preset operation before switching triggering, and the specific implementation method is as follows:

[0124] At the controller sampling moment before officially switching to network mode The three-phase instantaneous current at the rotor-side converter outlet is read, and then subjected to Clark and Park transformations (the virtual phase of the network configuration is still used at this time). (Perform coordinate transformation) to obtain the actual measured value of the rotor current in the rotated dq coordinate system, where the d-axis and q-axis are... and .

[0125] When switching to grid-following mode, the reference value of the inner current loop in grid-following mode will be... and Forced to be set to the above sample value:

[0126]

[0127] By aligning the command reference value with the actual physical value, the reference value of the current inner loop is ensured to be equal before and after the switching, and there is no step jump. The modulation signal output by the current inner loop PI controller remains unchanged, thereby eliminating the command error caused by the change in the control loop structure and ensuring the continuity of the stator current of the doubly fed wind turbine during the switching process of different modes.

[0128] Meanwhile, since the stator flux linkage phase-locked loop operates continuously in both grid-connected and grid-following modes, at the switching moment, the reference phase for the stator flux linkage coordinate transformation is directly obtained from the virtual phase θ given by the virtual synchronous generator. vsg Switch to phase-locked loop output phase θ Ψ This allows coordinate transformations in the grid-following mode to be re-based on the stator flux linkage orientation.

[0129] Step 4: After the integrator initial value reset or the current command preset alignment operation is completed, the control system seamlessly switches to the corresponding target control mode to eliminate the transient impact caused by the sudden change in the control structure and realize the stable operation of the doubly fed wind turbine.

[0130] It should be noted that in grid-connected mode, the current reference value gradually changes from... and Transition to the given current reference value from an external dispatcher.

[0131] To verify the practicality of this invention, this embodiment conducts a grid-connected mode / grid-connected mode switching simulation experiment on a doubly-fed induction generator (DFIG) using the following parameter settings: the base value of the grid-side AC voltage is 35kV, the fundamental frequency of the AC grid is 50Hz, the rated capacity of the DFIG is 6.5MVA, the rated voltage of the stator side is 1.14kV (connected to the 35kV grid via a step-up transformer), and the rated voltage of the DC bus is 2.4kV; under normal conditions, the switching frequency of the rotor-side converter is 2kHz, and the switching frequency of the grid-side converter is 3kHz.

[0132] Figure 4 This diagram illustrates the system simulation waveforms of a doubly-fed induction generator (DFIG) switching bidirectionally between grid-connected and grid-fed modes under full-load conditions, according to an embodiment of the present invention. From top to bottom, the waveforms show the changes in active power / reactive power, DC bus voltage, grid connection point voltage, and grid connection point current over time. At 0.5 seconds, the system issues a mode-switching command, and the DFIG switches from grid-connected mode to grid-fed mode. Figure 3 The magnified waveforms of the grid-connected voltage and current show that during the switching process, the voltage and current waveforms basically did not show amplitude jumps or phase shifts; at the same time, the output active power and reactive power transitioned smoothly without low-frequency oscillations.

[0133] At 1.0s, the system issues another command, and the doubly-fed induction generator switches from grid-fed mode to grid-connected mode. As can be seen from the waveform diagram, at the moment of switching triggering at 1.0s, the three-phase grid-connected current still maintains a highly continuous sinusoidal current, completely eliminating the transient impact common in traditional hard switching strategies.

[0134] Furthermore, throughout the two mode transitions at 0.5s and 1.0s, the grid-side converter consistently performs high-frequency voltage regulation control on the DC bus, precisely clamping the DC bus voltage to near the rated value of 2.4kV. This absorbs the minor transient imbalance energy between the stator and rotor during the mode switching, ensuring the safety of the converter hardware.

[0135] To further highlight the superiority and necessity of the bidirectional smooth switching strategy proposed in this invention, Figure 5The simulation waveforms are shown for a scenario where the state reset and command alignment mechanism of this invention is not used (i.e., the traditional hard switching method is employed). From top to bottom, the waveforms represent the changes in active power / reactive power, DC bus voltage, grid connection point voltage, and grid connection point current over time. Simulation conditions and... Figure 4 Maintain complete consistency (6.5MW full-load grid-connected operation).

[0136] like Figure 5 As shown, at 0.5s, when the doubly-fed induction generator (DFIG) switches directly from grid-connected mode back to grid-following mode, the lack of a pre-defined alignment mechanism for the current command causes a sudden dimensionality reduction in the control structure, resulting in a sudden change in the reference setpoint of the inner current loop. As can be seen from the magnified view, this command abrupt change triggers a significant drop in the three-phase current, accompanied by a sudden surge in active power.

[0137] Secondly, at time 1.0s, when the unit directly switches from grid-connected mode to grid-connected mode, the lack of phase angle pre-synchronization and voltage outer loop integrator initial value reset leads to a more drastic change in rotor reference current due to the deviation between the outer loop integrator state and phase angle. As shown in the figure, this results in severe transient overcurrent in the three-phase grid-connected current, and the power exhibits low-frequency, violent oscillations, easily triggering the overcurrent protection of the generator-side converter and causing the wind turbine to disconnect from the grid.

[0138] The simulation results above fully demonstrate that the grid mode switching method proposed in this invention is perfectly compatible with the complex stator-rotor magnetic flux coupling dynamic characteristics of the doubly fed generator, and realizes seamless and smooth switching between current source mode and voltage source mode across the entire power range, significantly improving the flexibility and active support capability of wind turbine generator sets in new power systems.

[0139] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention. Therefore, all technical solutions obtained through equivalent substitution or transformation fall within the protection scope of the present invention.

Claims

1. A seamless switching control method for grid-following and grid-connecting modes applicable to doubly-fed wind turbines, characterized in that, The rotor-side converter of the doubly-fed wind turbine adopts stator flux linkage phase-locked loop control in grid-following mode and virtual synchronous generator control in grid-connection mode; the method includes: S1, determine whether the mode of the doubly fed wind turbine has changed. If it has changed from grid-building mode to grid-following mode, proceed to S2; if it has changed from grid-following mode to grid-building mode, proceed to S3. S2, when switching to grid-connected mode, the current reference value of the current inner loop in grid-connected mode at the switching moment is set to the actual measured value of the rotor current in grid-connected mode before switching, so as to avoid sudden changes in the current reference value of the current inner loop before and after mode switching. At the same time, the reference phase of the rotor-side converter Park transform is directly switched from the virtual phase generated by the network mode to the real-time physical phase output by the stator flux linkage phase-locked loop; S3, intercept the real-time physical phase output of the stator flux linkage phase-locked loop at the switching moment, and write it as the initial value into the active-frequency control loop of the virtual synchronous generator in the grid construction mode, so as to ensure that the virtual phase generated by the grid construction mode after the switching starts smoothly from the current real phase point of the grid and eliminates transient active power impact. The initial value of the integral term of the voltage outer loop PI controller in grid mode is reset to the measured value of the rotor current in grid mode before switching, so as to avoid the sudden change in current command when the grid mode single current inner loop structure is changed to the grid mode voltage outer loop and current inner loop structure.

2. The seamless switching control method for grid-following and grid-connection modes applicable to doubly-fed wind turbines according to claim 1, characterized in that, The doubly fed wind turbine also includes a grid-side converter, which employs grid voltage-oriented vector control. The control process is as follows: The grid phase is obtained in real time based on the grid-side voltage and current during the operation of the doubly fed wind turbine; By combining the voltage and current with the grid phase and performing Park transformation, the corresponding measured values ​​of d-axis and q-axis voltage and measured values ​​of d-axis and q-axis current are obtained. The PI controller calculates the reference value of the d-axis current of the grid-side converter using the measured values ​​of the d-axis and q-axis voltages, and sets the reference value of the q-axis current of the grid-side converter to 0. By combining the reference values ​​of the d-axis and q-axis currents of the grid-side converter with the measured values ​​of the d-axis and q-axis currents, the d-axis and q-axis modulation waves are obtained. The three-phase modulation waves are further obtained by combining the grid phase, and the PWM controls the grid-side converter based on the three-phase modulation waves.

3. The seamless switching control method for grid-following and grid-connection modes of doubly-fed wind turbines according to claim 1, characterized in that, The process of controlling the rotor-side converter of a doubly-fed wind turbine using a stator flux linkage phase-locked loop is as follows: The stator voltage and stator current of the doubly fed wind turbine in grid-connected mode are obtained, and the three-phase flux linkage is calculated by combining the stator resistance; the stator flux linkage phase-locked loop extracts the real-time physical phase based on the three-phase flux linkage. The stator current is transformed using the real-time physical phase to obtain the measured values ​​of the rotor current on the d-axis and q-axis. In the inner current loop, by combining the measured values ​​of the d-axis and q-axis rotor currents with the reference values ​​of the d-axis and q-axis rotor currents, the d-axis and q-axis modulation waves of the rotor-side converter are obtained, and the three-phase modulation waves of the rotor-side converter are further obtained. The PWM controls the rotor-side converter in grid-following mode based on the three-phase modulation waves of the rotor-side converter.

4. The seamless switching control method for grid-following and grid-connection modes applicable to doubly-fed wind turbines according to claim 1, characterized in that, The virtual synchronous generator includes an active-frequency control loop and a reactive-voltage control loop; the process of using the virtual synchronous generator to control the rotor-side converter of the doubly-fed wind turbine is as follows: The stator voltage and stator current of the doubly fed wind turbine in grid-connected mode are obtained, and the actual active power and actual reactive power are calculated. At the same time, the stator voltage is converted into measured values ​​of d-axis and q-axis voltages through Park transformation. In the active-frequency control loop, based on the active power reference value and the actual active power, the virtual angular velocity is calculated by simulating the swing equation of the synchronous generator rotor, and then further integrated to generate the virtual phase; In the reactive-voltage control loop, the stator q-axis voltage reference value is calculated based on the reactive power reference value and the reactive and active power through droop control. Set the stator d-axis voltage reference value to 0, and calculate the q-axis and d-axis rotor current reference values ​​by comparing the stator d-axis and q-axis voltage reference values ​​with the measured stator d-axis and q-axis voltage values ​​in the voltage outer loop; In the inner current loop, based on the d-axis and q-axis rotor current reference values, the d-axis and q-axis modulation waves of the rotor-side converter are obtained, and the three-phase modulation waves of the rotor-side converter are further obtained. The PWM controls the rotor-side converter in the grid mode based on the three-phase modulation waves of the rotor-side converter.

5. The seamless switching control method for grid-following and grid-connection modes of doubly-fed wind turbines according to claim 4, characterized in that, The formula for calculating the reference value of the d-axis rotor current is: ； in, This is the measured value of the stator q-axis voltage. This is the proportionality coefficient. The integral coefficient is... Here is the reference value for the d-axis rotor current, and t is the time interval. This is the reference value for the stator q-axis voltage.

6. The seamless switching control method for grid-following and grid-connection modes of doubly-fed wind turbines according to claim 1, characterized in that, In S1, the current mode of the doubly fed wind turbine is indicated by the command flag bit. A value of 1 indicates that the doubly fed wind turbine is currently in the grid construction mode, and a value of 0 indicates that the doubly fed wind turbine is currently in the grid following mode.

7. The seamless switching control method for grid-following and grid-connection modes of doubly-fed wind turbines according to claim 6, characterized in that, The determination of whether the mode of the doubly fed wind turbine has switched is achieved by detecting the tipping edge of the command flag; the tipping edge detection value is obtained by subtracting the command flag at the current moment from the command flag at the previous moment. If the difference between the current time and the previous time's instruction flag is 1, it indicates that the system is switching from network following mode to network building mode; if the difference between the current time and the previous time's instruction flag is -1, it indicates that the system is switching from network building mode to network following mode. If the difference between the instruction flags at the current time and the previous time is 0, it means that no mode switching has occurred at this time.

8. The seamless switching control method for grid-following and grid-connection modes of doubly-fed wind turbines according to claim 1, characterized in that, In S3, the process of resetting the initial value of the integral term of the voltage outer loop PI controller in grid mode to the measured value of the rotor current in grid mode before switching is as follows: Taking the q-axis as an example, let the reference value of the d-axis current output by the PI controller of the outer loop of the q-axis voltage in the virtual synchronous generator reactive-voltage control loop at the switching moment be equal to the d-axis rotor current. By reversing the PI regulation equation, we can obtain the equation for the integral term. Since the measured voltage value and the reference voltage value are equal during mode switching, the equation concerning the integral term is transformed into an equal relationship between the initial value of the integrator in the outer loop PI controller of the q-axis voltage and the rotor current of the d-axis at the previous moment.

9. A seamless switching control method for grid-following and grid-connection modes applicable to doubly-fed wind turbines according to claim 8, characterized in that, The equation concerning the integral term is: ； in, for The moment in time refers to the moment before the mode switch occurred; This refers to the internal state value of the integrator in the outer-loop PI controller for calculating the integral term of the q-axis voltage. for The measured current value of the d-axis rotor-side converter at time 1. This is the proportionality coefficient. for Measured value of stator q-axis voltage at time (i.e.) for Reference value of stator q-axis voltage at any time.

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