Transient adaptive control method for grid-connected and off-grid hybrid system considering current-limiting switching

Abstract

The present application relates to a kind of considering the transient adaptive control method of grid-connected and grid-forming hybrid system switching, belong to new energy grid connection and power electronic control technical field.The method is by establishing the mathematical model of hybrid system under the condition of no current limiting and current limiting of grid-forming converter, and the influence law of key control parameters on system transient stability is analyzed by combining phase plane method, the difference of dominant control quantity under different operating conditions can be revealed, and the basis for control strategy design is provided.On this basis, a kind of transient control method is proposed, which adaptively adjusts the active reference value and saturation current phase angle of grid-forming converter according to voltage drop depth and current limiting state, to solve the problem of insufficient adaptability of existing control mode, and improve the transient synchronization stability of hybrid system during fault.

Description

Technical Field

[0001] This invention belongs to the field of new energy grid connection and power electronic control technology, and relates to a transient adaptive control method for grid-connected and grid-connected hybrid systems that considers current limiting switching. Background Technology

[0002] With the large-scale grid connection of new energy sources such as wind power and photovoltaics, the proportion of traditional synchronous generators in the power system is continuously declining, resulting in systems exhibiting characteristics such as low inertia and weak grids. Existing new energy grid-connected devices mostly use grid-following converters, which rely on phase-locked loops to track the grid phase for synchronous operation. Under conditions of weak grids or large disturbances, they are prone to transient synchronization instability. To enhance the voltage and frequency support capabilities of new energy grid-connected systems, grid-connected converters have received widespread attention. Considering the retrofitting costs and technological evolution of new energy power plants, hybrid systems combining grid-following and grid-connected converters will become an important grid connection configuration.

[0003] However, compared to single-type converter systems, the transient synchronization stability problem of grid-connected and hybrid grid-connected systems is more complex. On the one hand, the two types of converters are dynamically coupled through a common grid connection point and network impedance; on the other hand, under large disturbances such as grid voltage sags, the hybrid grid-connected converter often triggers current limiting, causing its external characteristics to shift from an equivalent voltage source to a constrained current source, resulting in a significant switch in the system's dynamic mechanism. This process not only weakens the supporting capacity of the hybrid grid-connected converter itself but also alters the power angle evolution law of the hybrid system, thereby increasing the difficulty of transient stability analysis and control design.

[0004] Existing technologies primarily focus on the transient stability of single-grid or single-connected-grid converters. Control methods for hybrid grid-connected and grid-connected systems, especially those simultaneously considering dynamic coupling and the current-limiting switching characteristics of grid-connected converters, remain relatively lacking. Particularly during faults, when the grid-connected converter operates in both unlimited and current-limited states, the key control variables governing system stability differ. Using fixed parameters or simple switching control logic makes it difficult to meet stability requirements under varying voltage dip depths. Therefore, it is necessary to propose a transient adaptive control method for hybrid grid-connected and grid-connected systems to improve the system's transient synchronous stability during voltage dips. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a transient adaptive control method for a hybrid system of grid connection and network construction that considers current limiting switching, so as to improve the transient synchronization stability capability of the hybrid system during faults.

[0006] To achieve the above objectives, the present invention provides the following technical solution: Option 1: A transient adaptive control method for hybrid grid-connected and grid-connected systems considering current-limiting switching. This method establishes mathematical models of the hybrid system with a grid-connected converter (GFM) under both current-limited and current-limited operating conditions. By combining this with the phase-plane method to analyze the influence of key control parameters on the system's transient stability, the differences in dominant control quantities under different operating states can be revealed, providing a basis for control strategy design. Based on this, a transient control method adaptively adjusts the active power reference value and saturation current phase angle of the grid-connected converter according to the grid voltage sag depth at the point of common coupling (PCC) and the current-limiting state of the grid-connected converter. This addresses the insufficient adaptability of existing control methods and improves the transient synchronous stability capability of the hybrid system during faults.

[0007] Furthermore, a mathematical model of a hybrid system with grid-connected converters under both unlimited and current-limited operating conditions is established. Specifically, the hybrid system is connected to the grid through a point of common coupling, and the grid side is equivalent to an ideal voltage source. U G With equivalent impedance Z G In series; the grid-type converter (GFL) and the grid-type converter (GFM) are connected to the PCC through their respective line impedances.

[0008] GFL adopts grid-following control, and its synchronous operation relies on phase-locked loop to obtain grid phase information: (1) In the formula, δ L The phase angle of the phase-locked loop. U L This refers to the terminal voltage of the GFL. The proportional gain of the GFL phase-locked loop. The integral coefficients of the GFL phase-locked loop; GFM employs network-based control, where the active power loop control is as follows: (2) In the formula, J For rotational inertia, D The damping coefficient is... T m For mechanical torque, T e For electromagnetic torque, T d For damping torque, P m For mechanical power, P e Electromagnetic power, δ M The phase angle is the output phase angle of the active power loop. The instantaneous angular frequency of the voltage source constructed inside the GFM; Reactive power loop control is as follows: (3) In the formula, Q This refers to the reactive power output of the GFM. Q ref Given reactive power; k q This is the reactive power droop coefficient; U This refers to the voltage amplitude output by the reactive power loop control. U n This is the reference value for the reactive power loop control voltage; During a fault, the external characteristics of a GFL are equivalent to a current source, while the external characteristics of a GFM are equivalent to a voltage source when current limiting is not triggered, and equivalent to a current source after current limiting is triggered. When the GFM does not trigger current limiting, the output voltage vector of the GFL can be derived according to the superposition theorem and the principle of coordinate transformation. q Axial components expression: (4) In the formula, Z gm= Z M Z G / ( Z M + Z G ), M g= Z G / ( Z M + Z G ), M m= Z M / ( Z M + Z G ), Z gm This represents the complex impedance coefficient obtained by equating the GFM branch impedance with the mains impedance. M g Indicates the voltage coupling coefficient of GFM. M m Represents the grid voltage coupling coefficient. Z L and Z M These represent the line impedances from GFL and GFM to PCC, respectively. U LG1This indicates the impact of the power grid on the output voltage of the GFL; U LM1 This indicates the coupling effect of GFM on the output voltage of GFL; U LL1 This indicates the interaction between the GFL output current and the line impedance; U M This refers to the terminal voltage of the GFM. I L Inject the grid output current into the GFL; The phase angle of the GFL output current relative to the d-axis of its PLL synchronous rotating coordinate system represents the distribution direction of the GFL's active / reactive current. for Z gm phase angle, for Z L impedance angle, The internal voltage phase angle of the GFM. for M g phase angle, For the PLL synchronization angle of GFL, The phase angle of the grid voltage. for M m The phase angle; For GFM, its output active power can be obtained from complex power calculations. With reactive power : (5) (6) In the formula, P MG1 and Q MG1 These represent the active and reactive power impacts of the power grid on the GFM, respectively. P ML1 and Q ML1 This indicates the coupling effect of GFL on the active and reactive power of GFM; I M The output current injected into the grid by the GFM. for impedance angle, for Z M + Z G , representing the equivalent complex impedance formed by the GFM branch impedance and the grid impedance connected in series; After the GFM triggers current limiting, the reactive power loop becomes imbalanced, and the terminal voltage is no longer controlled by the reactive power loop. Similarly, based on the superposition theorem, the GFL terminal voltage vector can be derived. q Axial components expression: (7) In the formula, Z GL= Z G + Z L , Z GL This represents the equivalent complex impedance formed by the series connection of the mains impedance and the GFL branch impedance; for Z GL The impedance angle; U LL2 Indicates the output current and equivalent impedance of the GFL Z GL The voltage drop term formed by the interaction, U LG2 Indicates the grid voltage relative to the GFL terminal. q The influence of the axis voltage component. U LM2 This indicates the effect of the GFM current-limiting current coupled to the GFL terminal voltage through the grid impedance; Similarly, the active power output of the GFM can be obtained. : (8) In the formula, The saturation current phase angle under GFM current-limited conditions represents the distribution direction of the current-limited current between active and reactive power. P MG2 This term represents the impact of the power grid on the active power output of the GFM. P ML2 This represents the coupling effect of the GFL output current on the GFM output active power.

[0009] Furthermore, the transient control method specifically includes: Obtain the grid voltage at the PCC of the grid-connected and grid-connected hybrid system. U G And calculate the voltage drop depth at the grid connection point. d The calculation formula is: d =max(0,0.9- U G ); Determine whether the GFM in the hybrid system triggers current limiting; If the GFM does not trigger current limiting and maintains voltage source characteristics, then in the unlimited current region, the active power reference value of the GFM is adaptively adjusted according to the grid voltage drop depth using a derating active power control strategy. If the GFM triggers current limiting and enters current source mode, the saturation current phase angle of the GFM is adaptively adjusted according to the grid voltage drop depth using an arctangent saturation phase angle control strategy.

[0010] Furthermore, the aforementioned derating active power control strategy specifically employs a quadratic power curve derating strategy, and its control law is as follows:

[0011] In the formula, This is the adjusted GFM active power reference value. This represents the initial active power before the fault. This is the derating ratio coefficient; clip() is the clipping function, ensuring that the active power command is within [0, ... P It is valid within the range of 0].

[0012] Furthermore, the control law of the arctangent saturation phase angle control strategy is:

[0013] In the formula, The phase angle of the adaptively adjusted saturation current; As the steady-state reference phase angle, take φ 0 = -π / 2 (corresponding to) q Axis-priority current limiting helps maximize reactive power support. The gain is adjusted by the phase angle; For control coefficients; For the steady-state operating point d The baseline value; and These are the lower and upper limits for phase angle adjustment, set to [-π, 0]; clip() is the clipping function.

[0014] Option 2: A transient adaptive control system for a hybrid grid-connected and network-connected system considering current limiting switching, comprising: The voltage sag calculation module is used to obtain the grid voltage at the PCC of the grid-connected and grid-connected hybrid system, and to calculate the grid voltage sag depth. The operating condition determination module is used to determine whether the GFM in the hybrid system is triggered by a voltage dip in the grid. The unlimited current zone control module is used to adaptively adjust the active power reference value of the GFM based on the voltage drop depth output by the voltage drop calculation module and the power-square curve derating active power control strategy when the grid-type converter does not trigger current limiting. The current limiting zone control module is used to adaptively adjust the saturation current phase angle of the GFM based on the drop depth when the grid-type converter triggers current limiting.

[0015] The beneficial effects of this invention are as follows: (1) The transient analysis model of the hybrid system established in this scheme shows that there is deep power and voltage bidirectional coupling between the converters. In particular, after the GFM trigger current is limited, its external characteristics will change abruptly, fundamentally changing the coupling path and exacerbating the transient instability risk of the hybrid system; (2) This scheme reveals through phase plane analysis that, under non-current-limited conditions, the deceleration area of ​​the system can be reshaped by reasonably adjusting the active power reference value of GFM; under current-limited conditions, the saturated current phase angle is a completely new control degree of freedom, and its optimized configuration can significantly reduce the power angle deviation before and after the fault. (3) The adaptive control strategy proposed in this scheme can adaptively control the voltage drop depth at the PCC point. In the unlimited current region, an active power derating strategy is adopted, and in the current-limited region, the saturation current phase angle is adaptively adjusted. Simulation results show that the strategy can effectively suppress the power angle divergence and maintain a constant power angle under voltage sag conditions at different depths, significantly improving the transient stability level of the hybrid system during fault ride-through.

[0016] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0017] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein: Figure 1 This is a schematic diagram of the topology and control structure of a GFL / GFM hybrid system; Figure 2 Equivalent circuit diagram of GFL / GFM hybrid system; Figure 3 A transient analysis model for a hybrid system when current limiting is triggered and not triggered; Figure 4 The effect of the GFM power reference value on the phase trajectory of the hybrid system when current limiting is not triggered; Figure 5 The effect of the phase angle of the GFM saturation current on the phase trajectory of the hybrid system when current limiting is triggered; Figure 6 The virtual power angle response curves of the hybrid system under different control strategies when current limiting is not triggered; Figure 7 The influence of the phase angle value of the GFM saturation current on the phase trajectory of the hybrid system when the current limiting is triggered. Specific implementation manners

[0018] The following uses specific specific examples to illustrate the implementation manners of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through different specific implementation manners. Each detail in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the diagrams provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0019] Please refer to Figure 1 , the research object of this embodiment includes a power grid, a grid-following converter, and a grid-forming converter. The hybrid system is connected to the power grid through a point of common coupling, and the grid side is equivalent to an ideal voltage source U G in series with an equivalent impedance Z G . The GFL (grid-following converter) and GFM (grid-forming converter) are respectively connected to the PCC (point of common connection) through their respective line impedances, where Z L , Z M represent the line impedances of the GFL and GFM to the PCC, R fL , L fL , C fL and so on represent filtering parameters. U L and U M are respectively the terminal voltages of the GFL and GFM; I L and I M are respectively the output currents injected into the power grid by the GFL and GFM.

[0020] The GFL adopts grid-following control, and its synchronous operation depends on the phase-locked loop to obtain the power grid phase information: (1) In the formula, δ L is the phase angle of the phase-locked loop, U L is the terminal voltage of the GFL, is the proportional coefficient of the GFL phase-locked loop, The integral coefficients of the GFL phase-locked loop; GFM employs network-based control, where the active power loop control is as follows: (2) In the formula, J For rotational inertia, D The damping coefficient is... T m For mechanical torque, T e For electromagnetic torque, T d For damping torque, P m For mechanical power, P e Electromagnetic power, δ M The phase angle is the output phase angle of the active power loop. The instantaneous angular frequency of the voltage source constructed inside the GFM; Reactive power loop control is as follows: (3) In the formula, Q This refers to the reactive power output of the GFM. Q ref Given reactive power; k q This is the reactive power droop coefficient; U This refers to the voltage amplitude output by the reactive power loop control. U n This is the reference value for the reactive power loop control voltage.

[0021] During a fault, the external characteristics of a GFL are equivalent to a current source, while the external characteristics of a GFM are equivalent to a voltage source when current limiting is not triggered, and equivalent to a current source after current limiting is triggered. The equivalent circuit of the hybrid system is as follows: Figure 2 As shown, we will now analyze two cases.

[0022] When the GFM does not trigger current limiting, the output voltage vector of the GFL can be derived according to the superposition theorem and the principle of coordinate transformation. q Axial components expression: (4) In the formula, Z gm= Z M Z G / ( Z M + Z G ), M g=Z G / ( Z M + Z G ), M m= Z M / ( Z M + Z G ), Z gm This represents the complex impedance coefficient obtained by equating the GFM branch impedance with the mains impedance. M g Indicates the voltage coupling coefficient of GFM. M m Represents the grid voltage coupling coefficient. U LG1 This indicates the impact of the power grid on the output voltage of the GFL; U LM1 This indicates the coupling effect of GFM on the output voltage of GFL; U LL1 This indicates the interaction between the GFL output current and the line impedance; The phase angle of the GFL output current relative to the d-axis of its PLL synchronous rotating coordinate system represents the distribution direction of the GFL's active / reactive current. for Z gm phase angle, for Z L impedance angle, The internal voltage phase angle of the GFM. for M g phase angle, For the PLL synchronization angle of GFL, The phase angle of the grid voltage. for M m The phase angle.

[0023] For GFM, its output active power can be obtained from complex power calculations. With reactive power : (5) (6) In the formula, P MG1 and Q MG1 These represent the active and reactive power impacts of the power grid on the GFM, respectively.P ML1 and Q ML1 This indicates the coupling effect of GFL on the active and reactive power of GFM; for impedance angle, for Z M + Z G , which represents the equivalent complex impedance formed by the GFM branch impedance and the grid impedance connected in series.

[0024] After the GFM triggers current limiting, the reactive power loop becomes imbalanced, and the terminal voltage is no longer controlled by the reactive power loop. Similarly, based on the superposition theorem, the GFL terminal voltage vector can be derived. q Axial components expression: (7) In the formula, Z GL= Z G + Z L , Z GL This represents the equivalent complex impedance formed by the series connection of the mains impedance and the GFL branch impedance; for Z GL The impedance angle; U LL2 Indicates the output current and equivalent impedance of the GFL Z GL The voltage drop term formed by the interaction, U LG2 Indicates the grid voltage relative to the GFL terminal. q The influence of the axis voltage component. U LM2 This indicates the effect of the GFM current-limiting current coupled to the GFL terminal voltage through the grid impedance; Similarly, the active power output of the GFM can be obtained. : (8) In the formula, The saturation current phase angle under GFM current-limited conditions represents the distribution direction of the current-limited current between active and reactive power. P MG2 This term represents the impact of the power grid on the active power output of the GFM. P ML2 This represents the coupling effect of the GFL output current on the GFM output active power.

[0025] Combining the previously derived single-machine and coupled equivalent equations, we can construct, as follows: Figure 3 The transient analysis model of the hybrid system is shown. This model intuitively reflects the dynamic coupling mechanism between the two types of converters under two operating conditions: untriggered and triggered current limiting by the GFM.

[0026] like Figure 3 As shown in Figures 3(a) and 3(b), they represent the active loop transient model of GFM and the phase-locked loop transient model of GFL, respectively. Figure 3 The logic switches S1 and S2 correspond to two different operating conditions: GFM non-triggered current limiting and triggered current limiting, respectively.

[0027] When the GFM does not trigger current limiting, the coupling strength of the hybrid system mainly depends on the electrical state of the GFL side and the relative phase relationship between them. At this time, the magnitude of the coupling term is mainly related to... I L , δ M , δ L as well as φ L related.

[0028] When a large disturbance causes the GFM to trigger current limiting, its control model switches. At this point, the magnitude of the coupling terms depends not only on the original state variables but also on... φ M Closely related. At this time. φ M It becomes a new degree of control freedom under current-limited operating conditions, through reasonable selection φ M The value of can effectively influence the amplitude and phase of the coupling term, thereby improving the transient stability of the system.

[0029] Based on the transient analysis model described above, it is evident that regardless of whether the GFM triggers current limiting, the connection of the GFL introduces a significant power coupling term on the GFM side, causing dynamic changes in the GFM's equivalent mechanical power and directly affecting its power balance characteristics. Simultaneously, the connection of the GFM also introduces a corresponding voltage coupling term on the GFL side. This coupling term acts on the front end of the phase-locked loop (PLL), causing a bias in the equivalent input of the GFL PLL, thereby affecting its phase tracking and voltage balance characteristics.

[0030] Due to this bidirectional coupling effect, the transient instability risk of hybrid systems increases significantly. Therefore, it is necessary to further explore the mechanism by which key converter control parameters affect the transient stability of hybrid systems, thereby providing a solid theoretical basis for optimizing control parameter configuration schemes to improve the stability of hybrid systems.

[0031] When a slight voltage dip occurs in the power grid and the GFM does not trigger current limiting, the GFM retains its voltage source characteristics. At this time, the presence of coupling terms alters the equivalent mechanical power of the GFL and GFM, thus disrupting the original power balance. The power reference value of the GFM is a key factor determining the system's initial equilibrium point and the evolution trend of the phase trajectory during a fault.

[0032] Figure 4 The phase plane diagram and transient phase trajectory of the hybrid system are shown when the active power command value of the GFM is changed under the condition that the current limiting is not triggered by the GFM.

[0033] Analysis combining mathematical models and phase trajectory diagrams reveals that when the active power reference value of the GFM is increased during a fault, the equivalent mechanical power of the GFM and GFL increases accordingly, leading to an increase in the system's acceleration area and a decrease in the maximum deceleration area. In the phase plane diagram, this manifests as a rightward shift of the equilibrium point during the fault, moving it further away from the initial equilibrium point, and a gradual decrease in the system's transient stability margin. When the active power reference value increases beyond a critical value, the phase trajectory will fail to converge to a new equilibrium point, resulting in transient synchronous instability. Conversely, moderately decreasing the active power reference value can shift the equilibrium point to the left, increasing the deceleration area and facilitating rapid phase trajectory convergence.

[0034] Therefore, it can be seen that when current limiting is not triggered, the virtual power angle curve during a fault can be reshaped by reasonably adjusting the power reference value of the GFM. Theoretically, there exists an optimal set of active power reference values ​​that can keep the virtual power angle balance point of the GFM constant before and after the fault. At this point, the theoretical acceleration area is minimized, and the transient synchronization stability of the hybrid system reaches its optimal level.

[0035] When a severe voltage dip occurs in the power grid, the GFM will trigger current saturation limiting, and its external characteristics will change from a voltage source to a constrained current source. At this time, the original reactive voltage control loop fails, and the saturation current phase angle... φ M It has become a new and crucial degree of freedom for control under current-limited operating conditions.

[0036] Figure 5 The phase plane diagram and transient response trajectory of the hybrid system are shown under the GFM trigger current limiting condition, with different saturation current phase angles.

[0037] Under the parameters and fault settings of this embodiment, φ M The impact on the fault equilibrium point location of the GFM is relatively limited, but it can significantly alter the grid connection point voltage / phase coupling term, thereby affecting the equivalent input and power angle deviation of the GFL. A suitable... φ M It can effectively reduce the virtual power angle offset of GFL before and after a fault and accelerate system convergence.

[0038] If the saturation current phase angle is set improperly (e.g., using extreme values), q Axis priority current limiting or d If the axis-priority current limiting is implemented without considering network coupling, the fault equilibrium point of the GFL may deviate significantly from the initial equilibrium point, leading to system instability.

[0039] In summary, while adjusting the saturated current phase angle does not directly alter the internal control structure of the GFM, it can significantly improve the power angle response characteristics of the GFL through the network coupling path. Similar to the unlimited current condition, there is also an optimal saturated current phase angle when current limiting is triggered, which allows the power angle balance point during the fault period to approach the initial power angle balance point as closely as possible, thereby achieving transient stability of the hybrid system.

[0040] Based on the preceding transient stability analysis, it is evident that during grid voltage dips, appropriately adjusting the active power reference value and saturation current phase angle of the fault current generator (GFM) is crucial for improving the transient stability of the hybrid system. However, the voltage dip depth caused by actual grid faults is random, and a single fixed parameter is insufficient to adapt to complex and variable transient conditions. Therefore, this embodiment combines the fault voltage dip depth with the current limiting state of the GFM to design an adaptive fault ride-through control strategy covering both unlimited and current-limited operating conditions.

[0041] First, define the voltage sag depth at the grid connection point. d =max(0,0.9- U G ).

[0042] When the GFM does not trigger current limiting, the GFM maintains its voltage source characteristics. At this time, to effectively suppress the accelerated divergence of the power angle caused by power imbalance during a fault, this invention proposes a derating active power control strategy in the unlimited current region. Its control law design is as follows: (9) In the formula, P m * This is the adjusted GFM active power reference value; P 0 represents the initial active power before the fault; k p This is the derating ratio coefficient; clip() is the clipping function, ensuring that the active power command is within [0, ... P It is valid within the range of 0].

[0043] In the unrestricted current zone, a quadratic power curve is used for derating. Compared to traditional linear derating, its advantage lies in: [the following applies to drop depth]. d When the current drop is small, the derating is gradual, avoiding over-response to small disturbances; while when the drop depth is large and approaches the current limiting critical point, the active power reference value can be reduced rapidly and significantly.

[0044] As the drop depth increases further, the GFM triggers current limiting and enters current source mode. At this point, simple active power derating is insufficient to dominate system dynamics, and the converter must prioritize injecting reactive current into the grid to support voltage. To meet reactive power support requirements while also ensuring transient synchronization stability, this invention proposes an arctangent saturation phase angle control strategy in the current-limiting region. Its control law design is as follows: (10) In the formula, The phase angle of the adaptively adjusted saturation current; As the steady-state reference phase angle, take φ 0 = -π / 2 (corresponding to) q Axis-priority current limiting helps maximize reactive power support. The gain is adjusted by the phase angle; For control coefficients; and These are the lower and upper limits for phase angle adjustment, respectively, set to [-π, 0].

[0045] Introducing the arctangent function in the current-limiting region has inherent advantages. On the one hand, the arctangent function possesses smooth, continuous, and bounded mathematical properties, enabling the saturation current phase angle to revolve around a reference value. The flexible offset avoids the control command from jumping abruptly and triggering a new oscillation mode when the fault worsens; on the other hand, the adaptive phase angle command effectively compensates for the power offset caused by trigger limiting and deep coupling of GFL, and achieves system resynchronization with minimal transient overshoot.

[0046] Combining the two control strategies described above, the hybrid system can automatically determine its current fault location based on the grid connection point voltage during a fault. The two strategies are interconnected, achieving a smooth transition and stable ride-through throughout the entire grid voltage dip process.

[0047] Verification experiment: To verify the effectiveness of the proposed transient adaptive control strategy for hybrid systems, a time-domain simulation model of a grid-connected / interconnected hybrid system was built. For the grid voltage sag condition, the control effects were compared and verified under two scenarios: no current limiting and current limiting triggered. The grid was set to... t A voltage dip occurs at 0.4s, and... t The fault was cleared at 1.4 seconds, and the normal grid voltage was restored.

[0048] When the grid voltage drops to 0.6 pu, the output current of the GFM converter does not reach the limiting threshold, thus maintaining the voltage source characteristics. Figure 6 The simulation results show the comparison between the traditional control strategy and the adaptive active power control strategy proposed in this invention under this operating condition.

[0049] Under the traditional control strategy (dashed line), due to the limited network transmission power and unchanged mechanical command power during a fault, the system generates excess acceleration kinetic energy. As a result, the virtual power angles of the GFM and GFL increase significantly after the fault occurs, and after transient oscillations, stabilize at a new equilibrium point much higher than the initial value. This leads to a significant compression of the system's transient stability margin during the fault. In contrast, by adopting the adaptive active power control strategy proposed in this invention (solid line), the system can adaptively reduce the active power reference value according to the voltage drop depth. Figure 6 As shown, during the entire fault period (0.4s~1.4s), the virtual power angles of the GFM and GFL were locked near the initial equilibrium point. This indicates that the strategy effectively offsets the acceleration area caused by power imbalance, achieves constant control of the power angle of the hybrid system, and greatly improves the transient synchronization stability under unlimited current conditions.

[0050] To verify the effectiveness of the strategy under deep voltage drop conditions, a severe voltage drop to 0.3 pu was simulated in the power grid. At this point, the GFM output current exceeded the limit, triggering the current saturation limiting module, and the external characteristic switched to current source mode. Figure 7 The simulation results show the comparison between traditional fixed-phase angle control and the adaptive saturated phase angle control strategy proposed in this invention under this extreme condition.

[0051] Under traditional fixed-phase-angle current limiting control (dashed line), the equivalent mechanical power of the GFM is severely unbalanced due to the failure to consider the nonlinear abrupt changes caused by the limiting and the deterioration of the dynamic coupling effect between the two machines. This leads to a continuous drift of its virtual power angle during the fault period, significantly deviating from the initial equilibrium point, resulting in poor convergence and an unstable trend. Simultaneously, this divergence trend is transmitted to the GFL through the voltage coupling term, causing the virtual power angle of the GFL to also drift. By adopting the adaptive saturation phase angle control strategy proposed in this invention (solid line), the control system can adaptively adjust the saturation current phase angle according to the drop depth. Figure 7 As shown in (a), this strategy successfully suppressed the divergence trend of the GFM power angle and drove it to converge rapidly to the initial equilibrium point; Figure 7 In (b), the virtual power angle of the GFL also remains highly stable. Simulation results fully demonstrate that even under severe operating conditions with trigger current limiting, the proposed strategy can still ensure the safe and stable fault passage of the hybrid system by optimizing the saturation phase angle.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A transient adaptive control method for a hybrid system of grid connection and network construction considering current limiting switching, characterized in that, This method includes: establishing a mathematical model of a hybrid system with grid-connected converters under both current-limited and current-limited operating conditions; A transient control method that adaptively adjusts the active power reference value and saturation current phase angle of the grid converter based on the grid voltage drop depth at the point of common coupling and the current limiting state of the grid converter.

2. The transient adaptive control method for a hybrid system of grid connection and network construction considering current limiting switching as described in claim 1, characterized in that, A mathematical model of a hybrid system with a grid-connected converter under both current-limited and current-limited operating conditions is established. Specifically, the hybrid system is connected to the grid through a point of common coupling, and the grid side is equivalent to an ideal voltage source. U G With equivalent impedance Z G In series; GFL and GFM are connected to PCC through their respective line impedances, where GFL is a grid-connected converter, GFM is a network-connected converter, and PCC is a common connection point; GFL adopts grid-following control, and its synchronous operation relies on phase-locked loop to obtain grid phase information: （1） In the formula, δ L The phase angle of the phase-locked loop. U L This refers to the terminal voltage of the GFL. The proportional gain of the GFL phase-locked loop. The integral coefficients of the GFL phase-locked loop; GFM employs network-based control, where the active power loop control is as follows: （2） In the formula, J For rotational inertia, D The damping coefficient is... T m For mechanical torque, T e For electromagnetic torque, T d For damping torque, P m For mechanical power, P e Electromagnetic power, δ M The phase angle is the output phase angle of the active power loop. The instantaneous angular frequency of the voltage source constructed internally by the GFM; Reactive power loop control is as follows: （3） In the formula, Q This refers to the reactive power output of the GFM. Q ref Given reactive power; k q This is the reactive power droop coefficient; U This refers to the voltage amplitude output by the reactive power loop control. U n This is the reference value for the reactive power loop control voltage; During a fault, the external characteristics of a GFL are equivalent to a current source, while the external characteristics of a GFM are equivalent to a voltage source when current limiting is not triggered, and equivalent to a current source after current limiting is triggered. When the GFM does not trigger current limiting, the output voltage vector of the GFL is derived based on the superposition theorem and the principle of coordinate transformation. q Axial components expression: （4） In the formula, Z gm= Z M Z G / ( Z M + Z G ), M g= Z G / ( Z M + Z G ), M m= Z M / ( Z M + Z G ), Z gm This represents the complex impedance coefficient obtained by equating the GFM branch impedance with the mains impedance. M g Indicates the voltage coupling coefficient of GFM. M m Represents the grid voltage coupling coefficient. Z L and Z M These represent the line impedances from GFL and GFM to PCC, respectively. U LG1 This indicates the impact of the power grid on the output voltage of the GFL; U LM1 This indicates the coupling effect of GFM on the output voltage of GFL; U LL1 This indicates the interaction between the GFL output current and the line impedance; U M This refers to the terminal voltage of the GFM. I L Inject the grid output current into the GFL; The phase angle of the GFL output current relative to the d-axis of its PLL synchronous rotating coordinate system represents the distribution direction of the GFL's active / reactive current. for Z gm phase angle, for Z L impedance angle, The phase angle of the internal voltage of the GFM. for M g phase angle, For the PLL synchronization angle of GFL, The phase angle of the grid voltage. for M m The phase angle; For GFM, its output active power can be calculated using complex power. With reactive power : （5） （6） In the formula, P MG1 and Q MG1 These represent the active and reactive power impacts of the power grid on the GFM, respectively. P ML1 and Q ML1 This indicates the coupling effect of GFL on the active and reactive power of GFM; I M The output current injected into the grid by the GFM. for impedance angle, for Z M + Z G , representing the equivalent complex impedance formed by the GFM branch impedance and the grid impedance connected in series; After the GFM triggers current limiting, the reactive power loop becomes imbalanced, and the terminal voltage is no longer controlled by the reactive power loop. Similarly, based on the superposition theorem, the GFL terminal voltage vector can be derived. q Axial components expression: （7） In the formula, Z GL= Z G + Z L , Z GL This represents the equivalent complex impedance formed by the series connection of the mains impedance and the GFL branch impedance; for Z GL The impedance angle; U LL2 Indicates the output current and equivalent impedance of the GFL Z GL The voltage drop term formed by the interaction, U LG2 Indicates the grid voltage relative to the GFL generator terminal. q The influence of the axis voltage component. U LM2 This indicates the effect of the GFM current-limiting current coupled to the GFL terminal voltage through the grid impedance; Similarly, the active power output of the GFM is obtained. : （8） In the formula, The phase angle of the saturated current under the GFM current-limiting state characterizes the distribution direction of the current-limiting current between active and reactive power. P MG2 This term represents the impact of the power grid on the active power output of the GFM. P ML2 This represents the coupling effect of the GFL output current on the GFM output active power.

3. The transient adaptive control method for a hybrid system of following and constructing networks considering current limiting switching as described in claim 2, characterized in that, The transient control method specifically includes: Obtain the grid voltage at the PCC of the grid-connected and grid-connected hybrid system. U G And calculate the voltage drop depth at the grid connection point; Determine whether the GFM in the hybrid system triggers current limiting; If the GFM does not trigger current limiting and maintains voltage source characteristics, then in the unlimited current region, the active power reference value of the GFM is adaptively adjusted according to the grid voltage drop depth using a derating active power control strategy. If the GFM triggers current limiting and enters current source mode, the saturation current phase angle of the GFM is adaptively adjusted according to the grid voltage drop depth using an arctangent saturation phase angle control strategy.

4. The transient adaptive control method for a hybrid system of grid connection and network construction considering current limiting switching as described in claim 3, characterized in that, The voltage sag depth of the power grid d The calculation formula is: d =max(0,0.9- U G ).

5. The transient adaptive control method for a hybrid system of grid connection and network construction considering current limiting switching as described in claim 3, characterized in that, The aforementioned active power derating control strategy specifically employs a quadratic power curve derating strategy, and its control law is as follows: In the formula, This is the adjusted GFM active power reference value. This represents the initial active power before the fault. `clip()` is the reduction ratio coefficient; `clip()` is the clipping function.

6. The transient adaptive control method for a hybrid system of grid connection and network construction considering current limiting switching as described in claim 3, characterized in that, The control law for the arctangent saturation phase angle control strategy is: In the formula, The phase angle of the adaptively adjusted saturation current; The steady-state reference phase angle; The gain is adjusted by the phase angle; For control coefficients; For the steady-state operating point d The baseline value; and These are the lower and upper limits for phase angle adjustment, respectively; clip() is a clipping function.

7. The transient adaptive control method for a hybrid system of following and constructing networks considering current limiting switching as described in claim 6, characterized in that, The steady-state reference phase angle The value is ,correspond q Axis-priority current limiting; the lower limit of the phase angle adjustment. -π, upper limit It is 0.

8. A transient adaptive control system for a hybrid network-connected and grid-connected system considering current limiting switching, characterized in that, include: The voltage sag calculation module is used to obtain the grid voltage at the PCC of the grid-connected and grid-connected hybrid system, and to calculate the grid voltage sag depth. The operating condition determination module is used to determine whether the GFM in the hybrid system is triggered by a voltage dip in the grid. The unlimited current zone control module is used to adaptively adjust the active power reference value of the GFM based on the voltage drop depth output by the voltage drop calculation module and the power-square curve derating active power control strategy when the grid-type converter does not trigger current limiting. The current limiting zone control module is used to adaptively adjust the saturation current phase angle of the GFM based on the drop depth when the grid-type converter triggers current limiting.

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