Method and system for pre-synchronization switching of doubly-fed induction generator to grid under weak grid
By constructing a mode switching criterion based on local operating parameters and a virtual power closed-loop pre-synchronization method, the problem of inaccurate switching of doubly-fed induction generators under weak power grid conditions is solved, smooth switching under weak power grid conditions is achieved, and the system stability and robustness are improved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- INNER MONGOLIA UNIV OF TECH
- Filing Date
- 2026-05-20
- Publication Date
- 2026-06-19
AI Technical Summary
Under weak grid conditions, grid-fed induction generator (GFL) control is prone to increased oscillations, decreased damping, and weakened disturbance suppression capabilities. Existing switching strategies rely on external grid phase references, leading to inaccurate switching and impacting grid operation.
By acquiring local operating parameters of the doubly fed induction generator set, such as stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude, a mode switching criterion is constructed, a forward switching trigger command is generated, and the virtual power closed-loop pre-synchronization process of the standby mode is initiated, thereby achieving smooth switching between GFL and GFM.
It reduces switching complexity, improves switching applicability in weak grids, avoids erroneous switching, achieves bidirectional smooth switching between GFL and GFM, and reduces current transient impact.
Smart Images
Figure CN122246911A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of grid-connected power generation control technology, specifically relating to a method and system for pre-synchronization switching of a doubly fed induction generator with the grid under weak power grid conditions. Background Technology
[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.
[0003] With the continuous large-scale grid connection of new energy power sources via power electronic interfaces, the challenges of long-distance transmission from wind farms, reduced short-circuit capacity at grid connection points, and weakened system inertia are becoming increasingly prominent. Weak grid conditions have gradually become a significant operating scenario for wind turbines. Against this backdrop, grid-following control (GFL) relies on an external grid phase reference and is prone to performance degradation under weak grid conditions, including increased oscillations, decreased damping, and weakened disturbance suppression capabilities, which can potentially lead to instability in severe cases.
[0004] To address the aforementioned issues, existing technologies can be broadly categorized into two types: First, optimizing phase-locked loop parameters or synchronization mechanisms within the GFL control framework to alleviate stability problems in weak power grids. However, this method still relies on the phase-locked loop synchronization mechanism and cannot fundamentally eliminate dependence on external power grid phase references. Second, introducing grid-mode (GFM) control or hybrid control to improve the adaptability of weak power grids by reconstructing the synchronization mechanism. These studies primarily focus on stability analysis under single control modes, but system solutions addressing the switching criteria and organization mechanisms between GFL and GFM remain relatively scarce.
[0005] Regarding switching criteria, existing methods mostly rely on online identification or estimation of external parameters such as short-circuit ratio and grid impedance to construct switching conditions. These methods have significant shortcomings: firstly, external parameters are difficult to obtain accurately under weak grid conditions; secondly, some identification methods require additional perturbations, which can affect grid operation. Regarding smooth switching methods, existing switching strategies require presetting an initial operating point for the standby mode before switching. However, the setting of this operating point usually depends on the current GFL control state, which itself has already degraded under weak grid conditions, leading to inaccurate initial operating point settings and affecting the smoothness of the switching. Summary of the Invention
[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method and system for pre-synchronization switching of a doubly fed induction generator with the grid under weak power grid conditions. This method and system can determine the switching based on the local operating status of the generator unit and achieve pre-synchronization takeover of the target mode during the mode switching process.
[0007] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the technical solution of the present invention provides a method for pre-synchronization switching of a doubly-fed induction generator with the grid under weak power grid conditions, including: Obtain local operating parameters of the doubly-fed induction generator set, including stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude; A mode switching criterion is constructed based on local operating parameters; when the mode switching criterion is met, a forward switching trigger command is generated. Based on the state transition strategy, in response to the forward switching trigger command or the preset reverse switching trigger command, the current active mode is maintained to continue controlling the operation of the doubly fed induction generator set, while the virtual power closed-loop pre-synchronization process of the standby mode is started; when the standby mode converges to the target mode steady state corresponding to the current operating point, the control switch is executed.
[0008] In at least one embodiment, the mode switching criterion is constructed based on local operating parameters, specifically including: Local oscillation components were extracted from the stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude, respectively. Based on the local oscillation components, normalized oscillation intensity indices are constructed corresponding to the stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude, respectively. When the normalized oscillation intensity of the stator active power, the phase-locked loop output frequency, and the grid connection point voltage amplitude all exceed their respective preset thresholds and the duration reaches the preset duration, it is determined that the current GFL control performance has degraded to a performance degradation state, and a forward switching trigger command is generated.
[0009] In at least one embodiment, extracting the local oscillation components from the stator active power, the phase-locked loop output frequency, and the grid connection point voltage amplitude specifically includes: Within the first preset sliding time window, the collected stator active power, phase-locked loop output frequency and grid connection point voltage amplitude are subjected to sliding average filtering to extract their slow-changing steady-state components. Subtracting the original stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude from their slow-varying steady-state components yields the local oscillation components in the stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude.
[0010] In at least one embodiment, a normalized oscillation intensity index is constructed corresponding to the stator active power, the phase-locked loop output frequency, and the grid connection point voltage amplitude, respectively, specifically expressed as: Within the second preset sliding time window, the root mean square values of the local oscillation components in the stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude are calculated, respectively characterizing the oscillation energy intensity of the local oscillation components in the stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude. Within the third preset sliding time window, calculate the background values of the slow-varying steady-state components of the stator active power, the phase-locked loop output frequency, and the grid connection point voltage amplitude. By dividing the oscillation energy intensity by the background value, normalized oscillation intensity indices corresponding to stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude are obtained, respectively.
[0011] In at least one embodiment, the virtual power closed-loop pre-synchronization process in the standby mode specifically includes: Based on the rotor current reference value output by the standby mode itself, calculate the virtual active power and virtual reactive power of the rotor current at the current reference value operating point. The difference between the current actual active power and the virtual active power of the doubly-fed induction generator is used to correct the power closed-loop state of the standby mode. The difference between the current actual reactive power and the virtual reactive power of the doubly-fed induction generator is used to obtain the active power deviation and reactive power deviation, respectively. The active power deviation and reactive power deviation are respectively passed through a proportional-integral regulator until both active power deviation and reactive power deviation converge to within the preset synchronization threshold.
[0012] In at least one embodiment, the forward switching trigger command is used to trigger a switch from GFL mode to GFM mode; the reverse switching trigger command is used to trigger a delayed return switch from GFM mode to GFL mode after the grid conditions are restored.
[0013] In at least one embodiment, when switching control, the target mode current inner loop is reset at the moment of switching to clear the integral residue formed during the inactive period, while the pre-synchronized power outer loop state is retained.
[0014] In at least one embodiment, the state transition strategy includes a GFL state, a GFM state, a GFM pre-synchronization state, and a GFL pre-synchronization state, and the state transition process specifically includes: When the system is in GFL state, after the forward switching conditions are met, the state changes from GFL to GFM pre-synchronization state. In this state, GFL continues to maintain unit operation, and GFM performs pre-synchronization. When the synchronization judgment conditions are met, the state changes from GFM pre-synchronization state to GFM state, completing the forward switching of the operating mode. When the system is in GFM state, after the reverse switching trigger command arrives, the state changes from GFM state to GFL pre-synchronization state. In this state, GFM continues to maintain unit operation, and GFL performs pre-synchronization. When the synchronization judgment condition is met, the state changes from GFL pre-synchronization state to GFL state, completing the operation mode switchback.
[0015] Secondly, the technical solution of the present invention also provides a doubly-fed induction generator and grid pre-synchronization switching system under weak grid conditions, including: The parameter acquisition module is configured to acquire the local operating parameters of the doubly-fed induction generator set, including stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude. The criterion generation module is configured to: construct mode switching criteria based on local runtime parameters; and generate a forward switching trigger command when the mode switching criteria are met. The state transition module is configured to: based on the state transition strategy, respond to the forward switching trigger command or the preset reverse switching trigger command, maintain the current active mode to continue controlling the operation of the doubly fed induction generator set, and start the virtual power closed-loop pre-synchronization process of the standby mode; when the standby mode converges to the target mode steady state corresponding to the current operating point, the control switch is executed.
[0016] In at least one embodiment, the state transition module includes a forward switching module and a reverse switching module; wherein, the forward switching module is configured to: when the system is in the GFL state, after the forward switching condition is met, the forward switching module controls the unit state to switch from the GFL state to the GFM pre-synchronization state. In this state, the GFL continues to maintain the unit operation, and the GFM performs pre-synchronization; when the synchronization discrimination condition is met, the forward switching module controls the unit state to switch from the GFM pre-synchronization state to the GFM state, thus completing the forward switching of the operating mode; The reverse switching module is configured such that when the system is in GFM state, after the reverse switching trigger command arrives, the reverse switching module controls the unit state to switch from GFM state to GFL pre-synchronization state. In this state, GFM continues to maintain unit operation, and GFL performs pre-synchronization. When the synchronization judgment condition is met, the reverse switching module controls the unit state to switch from GFL pre-synchronization state to GFL state, completing the operation mode switchback.
[0017] The beneficial effects of the above-described technical solution of the present invention are as follows: 1) The pre-synchronous switching method of the doubly fed induction generator and the grid in weak grid conditions of the present invention utilizes the cooperative oscillation characteristics of the local operating parameters of the DFIG to construct the switching criteria, which does not require complex grid impedance or SCR online identification, reduces the implementation complexity, and improves the applicability of switching determination under weak grid conditions.
[0018] 2) The switching criteria of this invention adopts a three-index joint triggering method and combines it with the duration judgment condition, which effectively avoids false switching caused by transient disturbances; the pre-synchronization method based on virtual power closed loop enables the standby mode to actively approach the current operating point, and combined with the switching strategy of "retaining the outer loop state and resetting the inner loop integral", it realizes bidirectional smooth switching between GFL and GFM, effectively reducing the current transient impact during the switching process. Attached Figure Description
[0019] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.
[0020] Figure 1 This is a schematic diagram of the pre-synchronization switching method between a doubly fed induction generator and the grid structure under a weak power grid as disclosed in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of a typical GFL and GFM control structure of a doubly fed wind turbine disclosed in Embodiment 1 of the present invention; wherein, (a) is a schematic diagram of a typical GFL control structure and (b) is a schematic diagram of a typical GFM control structure. Figure 3 This is a schematic diagram of a typical GFL and GFM control process for a doubly fed induction generator disclosed in Embodiment 1 of the present invention; Figure 4 This is a schematic diagram of the root locus of the closed-loop characteristic root as a function of the short-circuit ratio (SCR) disclosed in Embodiment 1 of the present invention. Figure 5 This is a schematic diagram of the performance characterization based on sensitivity within the stable short-circuit ratio range disclosed in Embodiment 1 of the present invention; wherein, (a) is a frequency response curve of the sensitivity function amplitude under a representative stable SCR value, and (b) is a schematic diagram of the change of the sensitivity peak Ms with SCR. Figure 6 This is a schematic diagram of the bidirectional pre-synchronous GFL / GFM switching control structure based on virtual power closed loop disclosed in Embodiment 1 of the present invention; Figure 7 This is a schematic diagram of the state transition relationship of the GFL / GFM switching system disclosed in Embodiment 1 of the present invention; Figure 8 This is a schematic diagram of the verification results of GFM pre-synchronization switching under rated operating conditions disclosed in Embodiment 1 of the present invention; wherein, (a) is a schematic diagram of the trigger signal; (b) is a schematic diagram of the feedback power curve in GFM mode; (c) is a schematic diagram of the power angle curve in GFM mode; (d) is a schematic diagram of the excitation voltage curve in GFM mode; (e) is a schematic diagram of the rotor current curve; (f) is a schematic diagram of the stator current curve; and (g) is a schematic diagram of the active power and reactive power curves of the generator. Figure 9 This is a schematic diagram of the verification results of GFL pre-synchronization switching under rated operating conditions disclosed in Embodiment 1 of the present invention; wherein, (a) is a schematic diagram of the trigger signal; (b) is a schematic diagram of the feedback power curve in GFL mode; (c) is a schematic diagram of the rotor dq axis current reference value curve in GFL mode; (d) is a schematic diagram of the rotor current curve; (e) is a schematic diagram of the stator current curve; and (f) is a schematic diagram of the active power and reactive power curves of the generator. Figure 10 This is a schematic diagram of the operational adaptability verification results of bidirectional pre-synchronous switching under variable wind speed conditions disclosed in Embodiment 1 of the present invention; wherein, (a) is a schematic diagram of the trigger signal; (b) is a schematic diagram of the wind speed curve; (c) is a schematic diagram of the power angle curve in GFM mode; (d) is a schematic diagram of the excitation voltage curve in GFM mode; (e) is a schematic diagram of the rotor dq shaft current reference value curve in GFL mode; (f) is a schematic diagram of the rotor current curve; (g) is a schematic diagram of the stator current curve; and (h) is a schematic diagram of the active power and reactive power curves of the generator. Figure 11 This is a schematic diagram of the mode switching verification results under the reduced system short-circuit ratio condition disclosed in Embodiment 1 of the present invention; wherein, (a) is a schematic diagram of the system short-circuit ratio curve; (b) is a schematic diagram of the point of common coupling voltage curve; (c) is a schematic diagram of the phase-locked loop (PLL) estimated frequency curve; (d) is a schematic diagram of the generator active power curve; and (e) is the operating performance index. , and (f) is a schematic diagram of the trigger signal curve; (g) is a schematic diagram of the rotor current curve; (h) is a schematic diagram of the stator current curve. Figure 12 Under the condition of reduced system short-circuit ratio disclosed in Embodiment 1 of the present invention Detailed waveform diagrams near the time point; where (a) is a schematic diagram of the voltage curve at the point of common coupling; (b) is a schematic diagram of the rotor current curve; and (c) is a schematic diagram of the stator current curve. Figure 13 This is a schematic diagram of the mode switching verification results under the increased wind speed condition disclosed in Embodiment 1 of the present invention; wherein, (a) is a schematic diagram of the wind speed curve; (b) is a schematic diagram of the system short-circuit ratio curve; (c) is a schematic diagram of the point of common coupling (PCC) voltage curve; (d) is a schematic diagram of the phase-locked loop (PLL) estimated frequency curve; (e) is a schematic diagram of the generator active power curve; and (f) is an operational performance index. , and (g) is a schematic diagram of the trigger signal curve; (h) is a schematic diagram of the rotor current curve; (i) is a schematic diagram of the stator current curve. Detailed Implementation
[0021] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.
[0022] Example 1 Doubly fed induction generators (DFIGs) can be connected to the grid using two typical control modes: grid-following (GFL) and grid-connected (GFM). In GFL, the external grid voltage serves as the synchronization reference, and power injection is achieved through current regulation. In GFM, the grid connection port voltage is actively established through internal control, and power exchange with the grid is performed accordingly. Typical control structures for GFL and GFM are as follows: Figure 2 As shown.
[0023] Typical GFL control structure is as follows Figure 2 As shown in (a), the grid-connected control of the DFIG rotor-side converter typically consists of a phase-locked loop (PLL), an outer loop controller, and an inner loop current controller. The PLL extracts the synchronization angle from the point-of-connection (PCC) voltage, providing a reference for coordinate transformation and controller operation; the outer loop generates the inner loop current reference value based on active and reactive power commands; and the inner loop current controller is used to achieve rapid tracking of the reference value. Therefore, the dynamic response of the GFL directly depends on the state of the PCC voltage, especially its phase information.
[0024] Typical GFM control structure as follows Figure 2 As shown in (b), the rotor-side control of the DFIG no longer relies on the external grid phase as a priori synchronization reference. Instead, it actively generates an output voltage reference through internal control and establishes the amplitude, frequency, and phase of the port voltage. Its control structure typically includes a voltage formation stage and a current inner-loop control stage. Unlike the controlled current source characteristics of the GFL, the GFM is closer to a controlled voltage source at the port side. Its grid-connected power exchange is based on an internal voltage formation mechanism, rather than an external phase-following process.
[0025] In general, GFL and GFM share the commonality of achieving power interaction between the DFIG and the grid through closed-loop control; their main differences lie in the controlled object and synchronization mechanism. GFL uses the external grid phase as a reference, and its control chain focuses on synchronization and current regulation; GFM, on the other hand, relies on an internal voltage formation mechanism to establish port behavior, and its control chain focuses on voltage formation and power regulation. This difference leads to fundamentally different response paths for grid connection point voltage disturbances under weak grid conditions.
[0026] As the equivalent impedance of the external power grid increases, the sensitivity of the grid connection voltage to the injected power of the generator increases. For wind turbine control, this means that the grid connection voltage can no longer be approximated as a rigid external quantity; its phase angle and amplitude will shift with changes in the generator's output power, thus forming a coupling between the generator's power and the grid connection voltage. This coupling means that grid connection voltage disturbances are no longer merely an external given but are further fed back to the generator's active power output, forming an additional coupled closed loop that forms the basis for dynamic problems in weak power grids.
[0027] In engineering, grid strength is typically characterized by the short-circuit ratio (SCR) to represent the support strength of the external grid at the grid connection point, and it can be used to reflect the strength of coupling effects in weak grids. However, under weak grid conditions, the actual coupling strength between the generating unit and the grid connection point voltage depends not only on the external short-circuit capacity but also on the unit's current grid-connected power. Therefore, the static SCR defined based on rated capacity is difficult to accurately characterize the actual coupling strength under different operating conditions, and this deviation is more pronounced when the grid is weak. Therefore, the equivalent short-circuit ratio can be used to reflect the actual grid strength at different operating points, specifically expressed as: (1) In the formula, For the short-circuit capacity of the grid connection point; This represents the unit's current grid-connected power.
[0028] To analyze the dominant coupling relationship between unit power and grid connection voltage under weak grid conditions, the external grid is represented by a simplified model consisting of an ideal voltage source and a series equivalent impedance. Let the ideal grid voltage be... The grid connection point voltage is The two are connected by equivalent impedance Connected. Considering that the transmitting line and the upstream power grid typically meet the characteristics of high reactance and low resistance within the power frequency range, the following analysis adopts... The inductive network approximation is as follows. In this case, the active power transmitted by the generating unit to the grid can be expressed as: (2) At steady-state operating point By linearizing the vicinity, we can obtain the power-phase mapping transfer function, which is specifically expressed as: (3) Near the steady-state operating point, the above equation can be approximated as a static proportional relationship. With... As the voltage increases, the grid strength decreases, and the mapping gain from power disturbances to phase angle disturbances at the grid connection point increases. This relationship forms the entry point for subsequent small-signal coupling analysis.
[0029] As grid strength weakens, the operational issues of a DFIG (Diverterless Power Generation Unit) no longer manifest as static performance problems under a single control mode, but rather transform into an adaptation problem between the operating mode and the external grid conditions. When grid support is strong, the grid connection point voltage is approximately rigid, and the GFL (Government-Fueled Unit) can maintain normal grid connection. However, as grid strength decreases, the additional coupling closed loop between unit power and grid connection point voltage continuously strengthens, and the GFL operating state will gradually approach its stability boundary, potentially exhibiting characteristics such as decreased damping and increased oscillations. At this point, the system needs to switch from GFL to GFM (Government-Fueled Management) mode to maintain stable grid connection operation. Correspondingly, when grid conditions recover, the GFL mismatch caused by weakened grid strength no longer persists, and the system should have the ability to return to GFL operation mode.
[0030] Under weak grid conditions, the DFIG operation requires both forward mode takeover and reverse mode recovery: when the GFL's operating state gradually approaches its stability boundary as the grid strength weakens, it should be smoothly taken over by the GFM mode; when the external grid conditions recover, a stable switchback to the GFL should be achieved.
[0031] The need for mode switching under weak grid conditions essentially stems from the enhanced additional coupling between GFL control and grid connection voltage. The GFL relies on the synchronous phase angle provided by the PLL to achieve power and current control; therefore, the PLL constitutes a key link in the additional coupling between the GFL dynamics and the grid connection voltage. Transient mismatch in grid connection voltage phase angle detection causes a deviation in the projection of the current vector into the actual grid voltage-oriented coordinate system, which is further fed back to the active power output. This is the fundamental reason for the formation of additional coupling. As grid strength decreases, the aforementioned additional coupling effect is continuously amplified, leading to stability issues in the GFL's operating mode under weak grid conditions.
[0032] Define the transient mismatch angle between the actual phase angle at the grid connection point and the PLL output phase angle as: Its small-signal expression near the operating point is: (4) In the formula, This refers to the phase angle disturbance of the PCC voltage; This refers to the phase angle disturbance output by the PLL.
[0033] Inside the PLL, the corresponding grid connection point voltage q-axis detection error can be expressed as: (5) Take it near the work site Linearizing equation (5) yields: (6) PLL for error input A PI closed-loop regulation is performed, and an estimated grid-connected voltage phase angle is generated through the integral stage, specifically as follows: (7) In the formula, and These are the proportional and integral coefficients of the PI controller in the PLL, respectively.
[0034] From equations (4), (6), and (7), the PLL phase tracking transfer function can be obtained, specifically expressed as: (8) Furthermore, the transient mismatch angle preservation transfer function can be obtained, specifically expressed as: (9) It should be pointed out that, Defined by the difference between the actual phase angle at the grid connection point and the PLL output phase angle, therefore, all... All control loops that control the synchronization angle include this mismatch angle. In addition to participating in the PLL phase tracking closed loop, this transient mismatch angle also causes a deviation in the projection of the current vector in the actual grid voltage orientation coordinate system, and further affects the unit power control process.
[0035] Under grid voltage orientation conditions, based on steady-state power relations, the active power of the unit near the operating point can be linearized into a function of the rotor d-axis current disturbance in the actual grid voltage orientation coordinate system, specifically expressed as: (10) In the formula, and These are the magnetizing inductance and the stator inductance, respectively. The rotor d-axis current is given in the actual grid voltage orientation coordinate system.
[0036] On the other hand, the rotor d-axis current command output by the active outer loop PI controller in the controller coordinate system is specifically expressed as follows: (11) In the formula, and These are the proportional and integral parameters of the power outer loop PI controller, respectively; The disturbance to the unit power reference value is taken during disturbance analysis. .
[0037] After closed-loop tracking of the rotor current inner loop, the actual rotor d-axis current in the controller coordinate system can be expressed as: (12) In the formula, It is the equivalent inertial time constant of the current inner loop control.
[0038] Furthermore, the projection relationship of the rotor current in the controller coordinate system and the actual grid voltage orientation coordinate system satisfies: (13) Linearization near the steady-state operating point yields: (14) In the formula, This represents the steady-state value of the q-axis current in the controller coordinate system at the operating point.
[0039] Substituting equation (12) into equation (14) and combining it with equation (10), we get: (15) Further simplification yields the following feedback transfer function from transient mismatch angle to active power output: (16) Equation (16) shows that the transient mismatch angle enters the active power control channel through the projection deviation of the current vector and forms dynamic feedback on the active power output.
[0040] Equations (3), (9), and (16) correspond to the power-phase angle mapping transfer function, the transient mismatch angle retention transfer function, and the transient mismatch angle-active power feedback transfer function, respectively. When connected in series, they form the open-loop path of the dominant additional coupling loop under weak grid conditions. Therefore, the equivalent open-loop transfer function of the dominant additional coupling loop in a weak grid can be written as: (17) Substituting equations (3), (9), and (16) into (17), we get: (18) Equation (18) shows that the external power grid parameters pass through the coefficient. Entering this coupling loop, where The increase of will enhance the additional coupling effect.
[0041] Based on the open-loop transfer function (18), the closed-loop characteristic equation of the GFL with an added coupling loop under weak grid conditions can be expressed as: (19) Therefore, the stability characteristics of the system under different power grid intensities can be analyzed from two perspectives: closed-loop characteristic roots and sensitivity functions.
[0042] Figure 4A schematic diagram of the root locus of the closed-loop eigenvalues as a function of SCR is presented under different horizontal axis display units and scaling ratios. It can be seen that as SCR decreases, a pair of conjugate poles gradually moves closer to the real axis and eventually transforms into real poles; simultaneously, the dominant real pole on the right continuously moves towards the imaginary axis, and under weaker grid conditions, crosses the imaginary axis into the right half-plane, leading to system instability. This result indicates that the influence of a weak grid not only changes the position of the dominant closed-loop poles but also alters the form of the dominant mode. From the perspective of the root locus, weakened grid strength will continuously amplify the additional coupling effect and push the GFL operating mode closer to the stability boundary.
[0043] However, for weak network operation modes, the system's closed-loop performance begins to degrade before the closed-loop poles enter the right half-plane. Therefore, a sensitivity function is introduced to characterize the evolution of system performance as SCR decreases. The sensitivity function is defined as: (20) And define the peak sensitivity as: (twenty one) Among them, sensitivity Used to characterize the sensitivity of a closed-loop system to disturbances and model uncertainties, the peak sensitivity value. This reflects the level of disturbance amplification of the system in the most unfavorable frequency band. Generally speaking, The larger the value, the weaker the system's ability to suppress disturbances and the worse its closed-loop robustness.
[0044] Figure 5 (a) gives the amplitude of the sensitivity function under representative SCR conditions. The frequency response curves were obtained. The results show that as the SCR decreases, the peak value of the curve continues to rise, indicating that the system's sensitivity to disturbances and parameter deviations increases continuously within a specific frequency band. Figure 5 (b) further provides the sensitivity peak value. The trend of SCR change. It can be seen that as SCR continues to decrease, The increase shows a nonlinear accelerating trend, especially with a significant increase in the critical range. This indicates that the robustness of the closed-loop system is continuously weakening, the system's tolerance to disturbances and uncertainties is continuously decreasing, and the closed-loop performance is deteriorating at an accelerated pace.
[0045] In summary, the above analysis shows that the stability evolution of the closed-loop system is accompanied by a synchronous weakening of its closed-loop performance. Under weak grid conditions, the GFL operating mode exhibits two stability characteristics: firstly, the decrease in SCR (Specialized Reduction Capacity) drives the dominant closed-loop pole to continuously move towards the stability boundary, eventually leading to instability; secondly, before the pole enters the right half-plane, the peak sensitivity has already increased significantly, indicating that the system's closed-loop performance deteriorates continuously prior to instability in the strict sense. Therefore, the impact of a weak grid on the GFL is not only reflected in the eventual loss of stability, but more importantly, in the continuous weakening of its closed-loop robustness and disturbance suppression capability.
[0046] Based on the above mechanism analysis of GFL control performance degradation under weak power grid conditions, in a typical embodiment of the present invention, such as Figures 1 to 13 As shown, this embodiment discloses a method for pre-synchronization switching of a doubly-fed induction generator with the grid under weak power grid conditions, specifically including the following steps: S1. Obtain the local operating parameters of the doubly-fed induction generator set, including stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude; S2. Construct mode switching criteria based on local operating parameters; when the mode switching criteria are met, generate a forward switching trigger command; S3. Based on the state transition strategy, in response to the forward switching trigger command or the preset reverse switching trigger command, maintain the current active mode to continue controlling the operation of the doubly fed induction generator set, and start the virtual power closed-loop pre-synchronization process of the standby mode; when the standby mode converges to the target mode steady state corresponding to the current operating point, the control switch is executed.
[0047] The following detailed description of the above-mentioned method for switching doubly fed induction generators and grid pre-synchronization in weak power grids, with reference to specific implementation methods, is provided below.
[0048] S1. Obtain the local operating parameters of the doubly-fed induction generator set, including stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude.
[0049] Under weak grid conditions, the deterioration of GFL (Power Flow Facility) performance typically does not initially manifest as abrupt changes in steady-state quantities, but rather as a sustained increase in local oscillations in the local response. As grid strength decreases, the oscillating components in the power response, synchronization chain dynamics, and PCC (Power Control Chain) voltage coupling response gradually rise, exhibiting a clear synergistic enhancement characteristic. Therefore, the forward switch to GFM (Power Flow Facility) requires identifying not just a single instantaneous anomaly, but rather the synergistic representation of the overall mismatch state near the weak grid boundary in local operating quantities.
[0050] Therefore, the forward switching criterion in this embodiment no longer relies on online grid strength identification, but rather on identifying whether the current GFL is approaching the mismatch operating boundary under weak grid conditions through local operating status. Near this boundary, although the system may not immediately become unstable, the oscillation modes in the power chain, synchronization chain, and PCC voltage coupling response have significantly increased, indicating a continuous decline in closed-loop robustness and disturbance suppression capability. Continuing to maintain GFL operation will bring further deterioration risks. Therefore, the switching criterion should be directly based on the characterization of the mismatch state by the local operating response.
[0051] To quantify the aforementioned mismatch characteristics, this step selects the stator active power when obtaining the local operating parameters of the doubly-fed induction generator. Phase-locked loop (PLL) output frequency and grid connection point (PCC) voltage amplitude As observables, they correspond to the power chain response, synchronization chain response, and PCC voltage coupling response, respectively.
[0052] S2. Construct mode switching criteria based on local operating parameters; when the mode switching criteria are met, generate a forward switching trigger command.
[0053] In this step, for any observation First, within the first preset sliding time window, the collected local operating parameters are processed. By performing a moving average filter, its slowly varying steady-state component is extracted, as specifically expressed as: (twenty two) In the formula, Index for the current time; Indicates the sampling period; For summation index; The first preset sliding time is defined as follows. As a further implementation, the first preset sliding time... Preferred .
[0054] Then, subtracting the original observation from its slowly varying steady-state component yields the local oscillation component, specifically expressed as: (twenty three) In the formula, This represents a local oscillation component.
[0055] Through the above processing, the local oscillation mode can be separated from the original response, thus providing a data basis for weak network mismatch identification based on local operating status.
[0056] Subsequently, based on the local oscillation components, normalized oscillation intensity indices corresponding to stator active power, phase-locked loop output frequency, and PCC voltage amplitude are constructed respectively.
[0057] Specifically, within the second preset sliding time window, the root mean square value of the local oscillation component is calculated to characterize its oscillation energy intensity, as specifically expressed as: (twenty four) In the formula, This is the second preset sliding time. In this embodiment, It is not used to extract frequency components at a fixed frequency point, but rather to characterize the energy intensity of local oscillations. Since the oscillation frequency near the boundary of a weak network changes with operating conditions, RMS characterization can more stably reflect the enhancement degree of the mismatch mode than fixed-frequency detection.
[0058] To eliminate the impact of oscillation baseline differences under different operating conditions, the background component of the slowly varying steady-state component is calculated within the third preset sliding time window, specifically expressed as: (25) In the formula, This is the third preset sliding time.
[0059] Dividing the oscillation energy intensity by the background component yields the normalized oscillation intensity index, which is specifically expressed as: (26) This yields normalized oscillation intensity indices corresponding to stator active power, phase-locked loop output frequency, and PCC voltage amplitude, respectively. , representing the enhancement of active power oscillation, PLL synchronization link oscillation, and PCC voltage coupling oscillation relative to their respective background levels. Thus, the weak network mismatch state can be transformed into a comparable index of local oscillation energy enhancement.
[0060] Since the mismatch near the weak network boundary manifests as an overall oscillation enhancement involving the power chain, synchronization chain, and PCC voltage coupling response, this step is based on a normalized oscillation intensity index corresponding to stator active power, PLL output frequency, and PCC voltage amplitude. The constructed mode switching criterion can only reliably indicate that the current GFL has entered an overall mismatch state near the weak network boundary when all three types of responses simultaneously show a continuous enhancement. This can effectively avoid misjudgment of switching caused by transient fluctuations of a single index due to individual active power regulation, reactive power regulation, or local voltage disturbances.
[0061] Based on this, the mode switching criterion constructed in this step is: when All exceeded their respective preset thresholds, and the duration was... When the preset time is reached, it is determined that the current GFL control performance has degraded to a weak network mismatch state, and a forward handover trigger command is generated to trigger a forward handover from GFL to GFM.
[0062] As an example, if the preset threshold is set to 2 and the preset duration is 500ms, then only if the following conditions are met: (27) and The duration satisfies When it is determined that the current GFL has entered a mismatch operation state near the weak grid boundary, a forward switching trigger command needs to be generated to trigger the forward switching from GFL to GFM.
[0063] The three-indicator combined triggering method in this step can not only suppress false triggering caused by single-channel transient disturbances, but also more accurately reflect the continuous enhancement of the overall mismatch mode.
[0064] S3. Based on the state transition strategy, in response to the forward switching trigger command or the preset reverse switching trigger command, maintain the current active mode to continue controlling the operation of the doubly fed induction generator set, and start the virtual power closed-loop pre-synchronization process of the standby mode; when the standby mode converges to the target mode steady state corresponding to the current operating point, the control switch is executed.
[0065] In this step, the state transition strategy includes a forward switching strategy and a reverse switching strategy. Forward switching refers to the state transitioning from GFL to GFM pre-synchronization state, which is conditionally triggered, i.e., triggered by the forward switching trigger command generated in step S2. Reverse switching refers to the state transitioning from GFM to GFL pre-synchronization state after grid conditions are restored, which is triggered by a preset reverse switching trigger command. As a further implementation, the preset reverse switching trigger command is a preset delayed return command or a mode return command automatically generated based on grid condition restoration.
[0066] like Figure 6 As shown, the control structure for pre-synchronization switching in this embodiment is configured with two controllers, GFL and GFM, where the current active mode directly controls the DFIG rotor-side converter. When the forward switching command generated in step S2 arrives, in response to the forward switching trigger command, the current active mode is maintained to continue controlling the doubly-fed induction generator set, while the standby mode is started. A power outer loop state is established through a virtual power closed loop to perform the pre-synchronization process; when the pre-synchronization conditions are met, the control switch is then executed.
[0067] In this step, the pre-synchronization structure is constructed for the power outer loop in standby mode. Specifically, during standby mode pre-synchronization, the virtual active power and virtual reactive power of the rotor current at the current reference value operating point are first calculated based on the rotor current reference value output by the standby mode itself, as expressed as follows: (28) In the formula, and These represent the virtual active power and virtual reactive power during the standby mode pre-synchronization period, respectively. This indicates the d-axis reference value of the rotor current; This indicates the q-axis reference value for the rotor current; This represents the d-axis value of the PCC voltage; Indicates the magnetizing inductance; Indicates stator inductance; This represents the angular frequency of the grid voltage.
[0068] Then, the virtual power is directly incorporated into the closed-loop feedback of the standby mode to establish its power outer loop. The difference between the current actual active power and the virtual active power of the doubly-fed induction generator (DFIG) is calculated, and the difference between the current actual reactive power and the virtual reactive power is also calculated, yielding the active power deviation and reactive power deviation, respectively. These deviations are then passed through a proportional-integral (PI) regulator to correct the power outer loop command of the standby mode, gradually aligning the standby mode's operating point with the unit's current operating point. This process is specifically represented as follows: (29) In the formula, and These represent the current active power and reactive power of the doubly-fed induction generator set, respectively.
[0069] When both active power deviation and reactive power deviation converge to within the preset synchronization threshold, the synchronization judgment condition is met. (30) When the current operating point of the standby mode is matched with the current operating point of the doubly fed induction generator set, a switching operation can be performed.
[0070] The pre-synchronization structure of the virtual power closed-loop-based pre-synchronization switching method proposed in this step only applies to the power outer loop; the current inner loop is not included in the pre-synchronization. At the moment of switching, the target mode's current inner loop is reset to clear any integral residues formed during the inactive period, while the pre-synchronized power outer loop state is retained. Thus, after the target mode takes over the unit, its power outer loop state matches the unit's current operating state. The above structure applies uniformly to both switching directions: when the GFL is in active mode, the GFM acts as the standby mode, completing pre-synchronization through the virtual power closed loop after the forward switching command arrives; when the GFM is in active mode, the GFL uses the same structure to complete pre-synchronization. When the switching direction changes, only the active and standby modes exchange roles; the pre-synchronization structure itself remains unchanged.
[0071] In this embodiment, the state transition strategy includes two main operating states and two transition states, namely, GFL state, GFM state, GFM pre-synchronization state, and GFL pre-synchronization state. Based on the above-described pre-synchronization switching method based on virtual power closed loop, the state transition strategy in this step is as follows: Figure 7 As shown, it can be specifically described as follows: When the system is in GFL state, after the forward switching conditions are met, the state transitions from GFL to GFM pre-synchronization state. In this state, GFL continues to maintain unit operation, and GFM performs pre-synchronization. When the synchronization judgment condition is met, the state transitions from GFM pre-synchronization state to GFM state, completing the forward switching of operating modes. When the system is in GFM state, upon the arrival of the reverse switching trigger command, the state transitions from GFM state to GFL pre-synchronization state. In this state, GFM continues to maintain unit operation, and GFL performs pre-synchronization. When the synchronization judgment condition is met, the state transitions from GFL pre-synchronization state to GFL state, completing the reverse switching of operating modes.
[0072] Based on this state transition strategy, the power grid triggers the generation of a forward switching trigger command based on a mode switching criterion constructed from local operating parameters. The power grid then performs a forward switching of its operating mode, changing its state from GFL to GFM. After the power grid state transitions to GFM, a reverse switching trigger command is generated periodically, and the power grid performs a reverse switching of its operating mode, changing its state back from GFM to GFL. After the power grid state returns to GFL, it then determines in real time, based on local operating parameters, whether the GFL performance is still in a mismatched operating state near the weak grid boundary. If so, it quickly performs a forward switch to the synchronized GFM. If the performance does not deteriorate after switching back to GFL, it does not switch back.
[0073] To verify the effectiveness of the pre-synchronization switching method for doubly-fed induction generators (DFIGs) in weak power grids proposed in this embodiment, this embodiment conducts HIL verification focusing on the bidirectional switching of DFIG GFL / GFM, verifying the smoothness and adaptability of pre-synchronization switching under different operating conditions. The basic parameter settings for the example are shown in Table 1.
[0074] Table 1 HIL Simulation System Parameters
[0075] (1) Verification of GFM pre-synchronization switching under rated operating conditions The system is set to operate under rated conditions, using GFL control mode. The test results of triggering the switch to GFM mode are as follows: Figure 8 As shown. By Figure 8 (a) It can be seen that the pre-synchronization trigger signal is at The time has arrived. Previously, GFM was in standby mode; since From that moment on, GFM enters the pre-synchronization state. At this time, Figure 8 The feedback power shown in (b) is given by the virtual power defined by equation (29). Figure 8 (c) and Figure 8 (d) shows the GFM power angle The excitation potential E gradually converges towards the current steady-state operating point of the unit.
[0076] When the deviation between the virtual power feedback and the current power of the unit satisfies the threshold condition defined by equation (30), The mode switching signal is triggered at all times, such as Figure 8 As shown in (a). At this time, the GFM feedback power switches from virtual power feedback to the actual measured power of the unit, the GFM mode takes over system control, and the GFL mode switches to standby, completing the mode switch. The pre-synchronization duration is 0.94s.
[0077] Depend on Figure 8 (e)– Figure 8 (g) It can be seen that the rotor current during the switching process Stator current and the active power of the unit and reactive power All transitions were smooth, with no significant disturbances observed.
[0078] (2) Verification of GFL pre-synchronization switching under rated operating conditions The system is set to operate under rated conditions, using GFM control mode. The test results of triggering the switch to GFL mode are as follows: Figure 9 As shown. By Figure 9 (a) It can be seen that the pre-synchronization trigger signal is at The time has arrived. Previously, GFL was in standby mode; since From that moment on, GFL enters the pre-synchronization state. At this time, Figure 9 The feedback power shown in (b) is given by the virtual power defined by equation (29). Figure 9 (c) shows the rotor dq axis current command in GFL mode. and It gradually approaches the steady-state value corresponding to the current operating conditions of the DFIG unit.
[0079] When the deviation between the virtual power feedback and the current power of the unit satisfies the threshold condition defined by equation (30), The mode switching signal is triggered at all times, such as Figure 9 As shown in (a). At this time, the GFL feedback power switches from virtual power feedback to the actual measured power of the unit, the GFL mode takes over the system control, and the GFM mode switches to standby, completing the mode switch.
[0080] Depend on Figure 9 (d)– Figure 9 (f) It can be seen that the rotor current during the switching process... Stator current and the active power of the unit and reactive power All transitions were smooth, with no significant disturbances observed.
[0081] (3) Verification of the operational adaptability of bidirectional pre-synchronization switching under variable wind speed conditions The system was set to operate under variable wind speed conditions, initially using GFM control mode. The test results for bidirectional mode switching are as follows: Figure 10 As shown. By Figure 10 (a) It can be seen that the system is respectively at , Switch to GFL mode at any time. , Switching to GFM mode at all times, each switch is in Figure 10 (b) shows the completion of the process during the continuous change of wind speed. - and - During this period, the system operates in GFL mode. Figure 10 (e) shows that the rotor dq shaft current reference value is the actual control value, while Figure 10 (c) and Figure 10 Power angle shown in (d) The magnetomotive force E corresponds to the standby or pre-synchronization state. Before, - During the period and Afterwards, the system runs in GFM mode, at which point... Figure 10 (c) and Figure 10 Power angle shown in (d) The magnetomotive force E is the actual control quantity, while Figure 10 (e) shows the rotor dq shaft current reference value corresponding to the standby or pre-synchronization state.
[0082] As wind speed increases Figure 10 The active power of the unit shown in (h) increased from 3MW to 5.12MW, while the reactive power remained near 0; correspondingly, Figure 10 (f) and Figure 10 As shown in (g), both the rotor current and stator current increase accordingly. Simultaneously, the rotor current phase sequence changes, indicating that the unit's operating state transitions from subsynchronous to supersynchronous. Overall, under conditions of wind speed variation and operating point migration, the unit's power and stator and rotor currents maintain a smooth transition during multiple bidirectional switching processes, without significant disturbances, demonstrating that the proposed pre-synchronous switching method has good adaptability under different operating conditions.
[0083] (4) Verification of mode switching under the condition of reduced system short-circuit ratio like Figure 11 As shown in (a), the system short-circuit ratio is... As the short-circuit ratio decreases, the system control performance gradually deteriorates; until... time, Figure 11 (b)- Figure 11 (d) shows the grid connection point voltage. Phase-locked loop detection frequency and the active power of the unit The oscillations were all significantly enhanced. Figure 11 The power index shown in (e) Frequency indicators and voltage index The aforementioned performance degradation was quantitatively characterized. Subsequently, all three metrics simultaneously and continuously increased from around 1, reaching a maximum of 4.3. Once the performance metrics exceeded the set threshold, Figure 11 (f) shows the GFM pre-synchronization signal at... Triggered at any time; after pre-synchronization is complete, at A mode switching signal is generated continuously, and the system switches to GFM operating mode. After the switch is completed, voltage, frequency, and power oscillations are significantly reduced, and the operating performance indicators return to the pre-deterioration level.
[0084] To verify the bidirectional switching capability, the system was... The system switches back to GFL mode according to a preset timing sequence. However, because the short-circuit ratio condition is not improved, the system control performance deteriorates again after switching back. Once the forward switching criterion is met again, the system... Restart GFM pre-synchronization at all times, and The system will switch back to GFM mode at any time. It should be noted that during the reverse switch from GFM to GFL, the GFM channel uses a delayed freeze mechanism to allow the system to quickly return to GFM should performance deteriorate after switching back to GFL. Figure 11 (f) It can be seen that the pre-synchronization duration of switching back to GFM is significantly shorter than that of the first switch, and the corresponding duration of GFL performance degradation is also shorter.
[0085] Detailed waveforms of the nearby grid connection point voltage and stator and rotor currents are as follows: Figure 11 As shown. By Figure 12 (a)- Figure 12 (c) It can be seen that, Prior to this time, the grid connection point voltage was affected by the deterioration of GFL control performance. Rotor current and stator current Significant distortion was observed in all samples; however, the distortion was rapidly suppressed after switching to GFM mode.
[0086] The above results show that when the system short-circuit ratio decreases and causes GFL control performance degradation, the proposed switching strategy can trigger GFM takeover in a timely manner based on the operating performance indicators. After switching back to GFL, if performance degradation occurs again, the system can still quickly return to GFM operation, thus verifying the effectiveness and repeated response capability of the bidirectional switching strategy in weak network scenarios.
[0087] (5) Verification of mode switching under increased wind speed conditions The system was initially set to operate under rated conditions, using GFL control mode. The HIL test results for mode switching under increased wind speed conditions are as follows: Figure 13 As shown.
[0088] like Figure 13 As shown in (a), the wind speed gradually increases from the initial moment, and the active power of the unit increases accordingly. As the operating point moves, the system's equivalent short-circuit ratio gradually decreases, as... Figure 13 As shown in (b). During this process, the system control performance gradually deteriorates; until... time, Figure 13 (c)- Figure 13 (e) shows the grid connection point voltage. Phase-locked loop detection frequency and the active power of the unit The oscillations were all significantly enhanced.
[0089] Figure 13 The power index shown in (f) Frequency indicators and voltage index The aforementioned performance degradation was quantitatively characterized. It should be noted that... Before, It will fluctuate to some extent during the power adjustment process, and and Basically remained stable; only Subsequently, the three indicators showed a synchronized upward trend. This phenomenon indicates that the constructed indicator system can distinguish between fluctuations in a single indicator caused by normal operation and regulation and coordinated anomalies in multiple indicators corresponding to control performance degradation, thereby achieving reliable identification of system deterioration states. When the performance indicators exceed the set threshold, Figure 13 (g) The GFM pre-synchronization signal shown is at Triggered at any time, and A mode switching signal is generated at all times, and the system switches to GFM operating mode.
[0090] Depend on Figure 13 (c)- Figure 13(f) As can be seen, after switching to GFM mode, the voltage, frequency, and power oscillations at the grid connection point were significantly reduced, and the operating performance indicators returned to the pre-deterioration level. Meanwhile, Figure 13 (h) and Figure 13 The rotor current shown in (i) With stator current It adjusts accordingly to power changes and maintains a smooth transition during switching without any obvious disturbances.
[0091] The above results indicate that the operating point migration caused by increased wind speed leads to a decrease in the system's equivalent short-circuit ratio and further induces GFL control performance degradation. In response, the switching strategy proposed in this embodiment can promptly trigger GFM takeover based on operating performance indicators, thus verifying the effectiveness of this strategy in weak network scenarios caused by changes in operating conditions.
[0092] In summary, under both rated and variable wind speed operating conditions, the method proposed in this embodiment can achieve smooth bidirectional switching between GFL and GFM. In scenarios where the system short-circuit ratio decreases and the equivalent weak grid condition deepens due to increased wind speed, the constructed index can distinguish between local fluctuations caused by normal operation regulation and coordinated anomalies corresponding to control performance degradation, and trigger GFM takeover in a timely manner accordingly. After switching back to GFL, if performance deterioration occurs again, the system can still quickly return to GFM. The results from various operating conditions show that the proposed method can not only effectively suppress oscillations near the weak grid boundary and reduce switching transients, but also improve the adaptability of DFIG units to changes in grid strength and operating point migration.
[0093] Example 2 In a typical embodiment of the present invention, this embodiment discloses a doubly-fed induction generator and grid pre-synchronization switching system under weak power grid conditions, comprising: The parameter acquisition module is configured to acquire the local operating parameters of the doubly-fed induction generator set, including stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude. The criterion generation module is configured to: construct mode switching criteria based on local runtime parameters; and generate a forward switching trigger command when the mode switching criteria are met. The state transition module is configured to: based on the state transition strategy, respond to the forward switching trigger command or the preset reverse switching trigger command, maintain the current active mode to continue controlling the operation of the doubly fed induction generator set, and start the virtual power closed-loop pre-synchronization process of the standby mode; and perform the control switch after it converges to the target mode steady state corresponding to the current operating point.
[0094] As a further implementation, the state transition module includes a forward switching module and a reverse switching module; wherein, the forward switching module is configured to: when the system is in the GFL state, after the forward switching condition is met, the forward switching module controls the unit state to switch from the GFL state to the GFM pre-synchronization state. In this state, the GFL continues to maintain the unit operation, and the GFM performs pre-synchronization; when the synchronization discrimination condition is met, the forward switching module controls the unit state to switch from the GFM pre-synchronization state to the GFM state, completing the forward switching of the operating mode; The reverse switching module is configured such that when the system is in GFM state, after the reverse switching trigger command arrives, the reverse switching module controls the unit state to switch from GFM state to GFL pre-synchronization state. In this state, GFM continues to maintain unit operation, and GFL performs pre-synchronization. When the synchronization judgment condition is met, the reverse switching module controls the unit state to switch from GFL pre-synchronization state to GFL state, completing the operation mode switchback.
[0095] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.
Claims
1. A method for pre-synchronization switching of a doubly-fed induction generator to grid under weak grid, characterized in that, include: Obtain local operating parameters of the doubly-fed induction generator set, including stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude; Construct mode switching criteria based on local runtime parameters; When the mode switching criterion is met, a forward switching trigger command is generated; Based on the state transition strategy, in response to the forward switching trigger command or the preset reverse switching trigger command, the current active mode is maintained to continue controlling the operation of the doubly fed induction generator set, while the virtual power closed-loop pre-synchronization process of the standby mode is started; when the standby mode converges to the target mode steady state corresponding to the current operating point, the control switch is executed.
2. The method for pre-synchronization switching of a doubly-fed induction generator to a grid under a weak grid of claim 1, wherein, The mode switching criteria are constructed based on local runtime parameters, specifically including: Local oscillation components were extracted from the stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude, respectively. Based on the local oscillation components, normalized oscillation intensity indices are constructed corresponding to the stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude, respectively. When the normalized oscillation intensity of the stator active power, the phase-locked loop output frequency, and the grid connection point voltage amplitude all exceed their respective preset thresholds and the duration reaches the preset duration, it is determined that the current GFL control performance has degraded to a performance degradation state, and a forward switching trigger command is generated.
3. The method for pre-synchronization switching of a doubly-fed induction generator to a grid under a weak grid of claim 2, wherein, Extracting local oscillation components from stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude, specifically including: Within the first preset sliding time window, the collected stator active power, phase-locked loop output frequency and grid connection point voltage amplitude are subjected to sliding average filtering to extract their slow-changing steady-state components. Subtracting the original stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude from their slow-varying steady-state components yields the local oscillation components in the stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude.
4. The method for pre-synchronization switching of a doubly-fed induction generator to a grid under a weak grid of claim 3, wherein, The normalized oscillation intensity indices corresponding to the stator active power, phase-locked loop output frequency, and grid connection point voltage are constructed respectively, and are specifically expressed as follows: Within the second preset sliding time window, the root mean square values of the local oscillation components in the stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude are calculated, respectively characterizing the oscillation energy intensity of the local oscillation components in the stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude. Within the third preset sliding time window, calculate the background values of the slow-varying steady-state components of the stator active power, the phase-locked loop output frequency, and the grid connection point voltage amplitude. By dividing the oscillation energy intensity by the background value, normalized oscillation intensity indices corresponding to stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude are obtained, respectively.
5. The method for pre-synchronization switching of DFIG grid synchronization under weak grid of claim 1, wherein, The virtual power closed-loop pre-synchronization process in standby mode specifically includes: Based on the rotor current reference value output by the standby mode itself, calculate the virtual active power and virtual reactive power of the rotor current at the current reference value operating point. The difference between the current actual active power and the virtual active power of the doubly-fed induction generator is used to correct the power closed-loop state of the standby mode. The difference between the current actual reactive power and the virtual reactive power of the doubly-fed induction generator is used to obtain the active power deviation and reactive power deviation, respectively. The active power deviation and reactive power deviation are respectively passed through a proportional-integral regulator until both active power deviation and reactive power deviation converge to within the preset synchronization threshold.
6. The method for pre-synchronization switching of a doubly-fed induction generator to grid under weak grid of claim 1, wherein, The forward switching trigger command is used to trigger the switch from GFL mode to GFM mode; the reverse switching trigger command is used to trigger a delayed return switch from GFM mode to GFL mode after the grid conditions are restored.
7. The method for pre-synchronization switching of a doubly-fed induction generator to grid under weak grid of claim 1, wherein, When switching control, the target mode current inner loop is reset at the moment of switching to clear the integral residue formed during the inactive period, while the power outer loop state that has completed pre-synchronization is retained.
8. The method for pre-synchronization switching of a doubly-fed induction generator to grid under weak grid of claim 1, wherein, The state transition strategies include GFL state, GFM state, GFM pre-synchronization state, and GFL pre-synchronization state. The state transition process specifically includes: When the system is in GFL state, after the forward switching conditions are met, the state changes from GFL to GFM pre-synchronization state. In this state, GFL continues to maintain unit operation, and GFM performs pre-synchronization. When the synchronization judgment conditions are met, the state changes from GFM pre-synchronization state to GFM state, completing the forward switching of the operating mode. When the system is in GFM state, after the reverse switching trigger command arrives, the state changes from GFM state to GFL pre-synchronization state. In this state, GFM continues to maintain unit operation, and GFL performs pre-synchronization. When the synchronization judgment condition is met, the state changes from GFL pre-synchronization state to GFL state, completing the operation mode switchback.
9. A pre-synchronization switching system for grid connection of a doubly-fed induction generator under a weak grid, characterized in that, include: The parameter acquisition module is configured to acquire the local operating parameters of the doubly-fed induction generator set, including stator active power, phase-locked loop output frequency, and grid connection point voltage amplitude. The criterion generation module is configured to: construct mode switching criteria based on local runtime parameters; and generate a forward switching trigger command when the mode switching criteria are met. The state transition module is configured to: based on the state transition strategy, respond to the forward switching trigger command or the preset reverse switching trigger command, maintain the current active mode to continue controlling the operation of the doubly fed induction generator set, and start the virtual power closed-loop pre-synchronization process of the standby mode; when the standby mode converges to the target mode steady state corresponding to the current operating point, the control switch is then executed.
10. The pre-synchronization switching system for a doubly-fed induction generator to grid synchronization under weak grid of claim 9, wherein, The state transition module includes a forward switching module and a reverse switching module. The forward switching module is configured as follows: when the system is in the GFL state, after the forward switching conditions are met, the forward switching module controls the unit state to switch from GFL to GFM pre-synchronization state. In this state, GFL continues to maintain unit operation, and GFM performs pre-synchronization. When the synchronization discrimination conditions are met, the forward switching module controls the unit state to switch from GFM pre-synchronization state to GFM state, completing the forward switching of the operating mode. The reverse switching module is configured such that when the system is in GFM state, after the reverse switching trigger command arrives, the reverse switching module controls the unit state to switch from GFM state to GFL pre-synchronization state. In this state, GFM continues to maintain unit operation, and GFL performs pre-synchronization. When the synchronization judgment condition is met, the reverse switching module controls the unit state to switch from GFL pre-synchronization state to GFL state, completing the operation mode switchback.