Model predictive transient stability control method for multi-machine grid system under saturation constraints

Abstract

The application discloses a kind of model prediction transient stability control methods of multi-organization network system under saturation constraint, belong to power system stability control technical field, including the following steps: considering the influence of current saturation limit, construct the transient analysis mathematical model of grid-forming two-machine parallel system before and after power grid fault;Saturation current phase control strategy is constructed based on transient analysis mathematical model;When saturation current phase control strategy cannot maintain the transient stability of grid-forming two-machine parallel system, the control strategy of active power reference value compensation amount is constructed;A kind of model predictive control strategy of combined saturation current phase control strategy and active power reference value compensation amount control strategy is proposed.The application adopts the model prediction transient stability control method of multi-organization network system under saturation constraint described above, it is favorable to meet the demand of the transient stability of grid-forming multi-machine parallel system.

Description

Technical Field

[0001] This invention relates to the field of power system stability control technology, and in particular to a model predictive transient stability control method for multi-mechanism network systems under saturation constraints. Background Technology

[0002] With the large-scale integration of power electronic devices, the dynamic response characteristics of power systems are gradually shifting from being dominated by synchronous generators to being dominated by power electronic devices. To improve system damping and inertia, researchers have proposed grid-based control strategies represented by virtual synchronous generators (VSGs). However, VSGs may experience transient instability problems similar to those of synchronous generators when subjected to large disturbances. Furthermore, due to the limited overcurrent capacity of power electronic devices, inverters are highly susceptible to overcurrent risks under severe disturbances such as short-circuit faults. Therefore, transient instability and fault overcurrent problems in grid-based parallel systems have become significant challenges for the safe and stable operation of new power systems.

[0003] To address the transient stability problem of a single-unit VSG considering current reference value limitations, some researchers have considered the operating mode switching caused by the current limiter and constructed a Lyapunov energy function to study the impact of different saturation currents on transient stability. Studies have shown that in current source mode, adjusting the phase of the saturation current can significantly improve the transient stability of the grid-connected converter. Therefore, based on the influence mechanism of four saturation currents on transient stability, researchers have analytically calculated the optimal saturation current phase to improve the stability margin. However, due to the complexity of multi-stage systems, this method is difficult to extend to multi-unit parallel systems. Other commonly used transient stability enhancement methods, such as reducing the active power reference value and introducing additional damping control loops, have achieved good results in single-unit systems, but their applicability in multi-unit parallel systems still needs to be verified. Summary of the Invention

[0004] The purpose of this invention is to provide a model predictive transient stability control method for multi-mechanism network systems under saturated constraints, in order to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, this invention provides a model predictive transient stability control method for multi-mechanism network systems under saturated constraints, comprising the following steps: S1. Considering the influence of current saturation limitation, construct a transient analysis mathematical model for a grid-connected two-machine parallel system before and after a grid fault; S2. Construct a saturation current phase control strategy based on a transient analysis mathematical model; S3. When the saturated current phase control strategy cannot maintain the transient stability of the grid-type two-machine parallel system, a control strategy for the active power reference value compensation is constructed. S4. A model predictive control strategy is proposed, which combines a saturated current phase control strategy and an active power reference value compensation strategy.

[0006] Preferably, step S1 includes: S11. Construct the topology of a grid-connected two-machine parallel system, with the two converters connected via a filter inductor. L fi With filter capacitor C fi Access to common coupling point, where Indicates the converter number, and the series impedance at the point of common coupling. Z g Connected to an infinite power grid, the DC bus of each converter is considered a constant DC source. V dc The two converters are VSC1 and VSC2. S12. Each converter uses a virtual synchronous generator (VSG) to generate a reference phase and a reference voltage amplitude. The inner loop of the virtual synchronous generator uses a dual-loop control of voltage and current, including an active control loop and a reactive control loop. The current loop outputs a modulation signal, which is sent to the modulation module to generate the drive signal for the converter. S13. Direct current control is adopted, and a current limiter is placed at the voltage loop output. S14. Derive the active power expressions for the outputs of VSC1 and VSC2, and transform the Y-type topology of the grid-connected two-machine parallel system through Y- Transformation into Type structure.

[0007] Preferably, step S14 specifically includes: Assuming all impedances are inductive, list the node voltage equations for a grid-type two-machine parallel system; Before the fault occurs, the VSG is in voltage source mode. The output voltage amplitude and phase of VSC1 and VSC2 are determined by the active control loop and reactive control loop of the VSG. The output current of VSC1 and VSC2 is obtained by the node voltage equation of the grid-type two-machine parallel system. Calculate the active power of VSC1 and VSC2; When a fault occurs, VSG is in current source mode. The output current amplitude and phase of VSC1 and VSC2 are determined by the current limiter. Since the voltage loop and reactive power loop are bypassed, the output voltage of VSC1 and VSC2 is obtained through the node voltage equation of the grid-type two-machine parallel system. Calculate the active power of VSC1 and VSC2 under current saturation after the fault.

[0008] Preferably, step S2 specifically includes: S21. Measure the three-phase voltage and three-phase current of VSC1 and VSC2 in real time, and calculate the current phase; S22. Based on the operating data of the active power loops of VSC1 and VSC2, measure and collect the power angle and angular frequency deviation of VSC1 and VSC2 in real time. S23. Using the power angle and angular frequency deviation of VSC1 and VSC2 as state variables and the saturation current phase as control variables, respectively, construct a mathematical model for the saturation current phase control strategy based on model prediction.

[0009] Preferably, step S23 specifically includes: In VSC1, the model predictive controller optimizes the objective function at each control time step. Generate the control sequence for the next N steps. , Indicates the first The current saturation angle of VSC1 at time 1, and the first control quantity Apply to the grid-type two-machine parallel system; based on the real-time update of the grid-type two-machine parallel system status, repeat the prediction-optimization-feedback process to form a closed-loop control; objective function The expression is: In the formula, For the prediction interval, At the current moment, the optimization problem of the objective function is constrained by a set of equality and inequality constraints; The equality constraints are discretized by using the forward Euler method on the swing equations of the VSG active control loop; In the case of a minor fault, the swing equation of the VSG active control loop is discretized to obtain: In the formula, The equation constraints represent the conditions for a grid-connected two-machine parallel system under minor faults. express The power angle of VSC1 at time 1. For the first Output angular frequency deviation of the converter express The output angular frequency deviation of VSC1 at any given time express The power angle of VSC2 at time 2. express The current saturation angle of VSC2 at time 2. express The current saturation angle of VSC1 at time 1. The rated angular frequency, This is the reference value for the active power of VSC1; To limit the saturation current within one cycle, it is necessary to... Set upper and lower limits: The above equation forms the inequality constraints for a model-based prediction optimization strategy for saturated current phase.

[0010] Preferably, step S3 specifically involves: using the power angle and angular frequency deviation of VSC1 and VSC2 as state variables and the reference active power compensation amount as control variables, constructing an active power reference value compensation control strategy based on model prediction to achieve dynamic adjustment of reference power and maintenance of stable equilibrium point. In VSC1, the objective function is used as the control objective, and the equality and inequality constraints are modified. Based on the reference power as the control input, the equality constraints of the grid-type two-machine parallel system considering only severe faults are as follows: In the formula, Indicates after the fault occurs The active power of VSC1 at time t. express The active power reference value compensation amount of VSC1 at time _____. The first Damping coefficient of the converter; To prevent excessive reference power reduction from causing reverse power flow, the active power compensation amount of the first converter is adjusted. Set upper and lower limits: The above equation forms the inequality constraints for the compensation design method based on the model-predicted active power reference value.

[0011] Preferably, in step S4, the model prediction strategy, while keeping the state variables constant, integrates the control concepts of saturated current phase design and active power reference value design. It uses the power angle and angular frequency deviation of VSC1 and VSC2 as state variables, respectively, and jointly employs the saturated current phase... Compensation amount with active power reference value As a control variable; The model prediction strategy improves transient stability by coordinating the adjustment of the saturation current phase and active power reference value compensation: under mild voltage faults, the saturation current phase is adjusted first to enhance transient stability, while the active power reference value compensation is zero to maximize the inverter's power output; however, when the fault is severe and phase adjustment alone is insufficient to maintain stability, the saturation current phase is adjusted in conjunction with the fault. and active power reference value compensation Achieve transient stability of the grid-connected two-machine parallel system and optimize the transient dynamic response trajectory.

[0012] Preferably, in VSC1, the control objective function is designed as follows: In the formula, and These are the weighting factors; Combining the two operating conditions of minor and severe faults, the corresponding equation constraint derivation is as follows: Inequality constraints are and Upper and lower limit constraints.

[0013] Therefore, the present invention adopts the above-mentioned model prediction transient stability control method for multi-mechanism network systems under saturation constraints, which has the following beneficial effects: according to the degree of influence of grid faults on system transient stability, it adaptively selects to adjust only the saturation current phase or to jointly adjust the saturation current phase and the active power reference value compensation amount, so as to avoid unnecessary active power reduction under mild fault conditions and still be able to restore system transient stability under severe fault conditions.

[0014] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0015] Figure 1 This is a flowchart of a model prediction transient stability control method for a multi-mechanism network system under saturated constraints, according to an embodiment of the present invention. Figure 2 This is a structural diagram of a grid-type two-machine parallel system according to an embodiment of the present invention; Figure 3 This is a simplified equivalent circuit diagram of the two-machine parallel circuit according to an embodiment of the present invention. Figure 3 (a) shows the state before the fault occurred; Figure 3 (b) shows the situation after the fault occurred; Figure 4 These are simulation results of the transient stability enhancement control strategy based on MPC when the grid voltage Ug = 0.6pu according to an embodiment of the present invention. Figure 4 (a) and Figure 4 (b) are respectively dq Simulation results of shaft current; Figure 4 (c) and Figure 4 (d) shows the simulation results for active and reactive power, respectively; Figure 4 (e) shows the simulation results of the saturation phase of the MPC output current; Figure 4(f) shows the simulation results of the compensation amount of the MPC output active power reference value; Figure 4 In the middle (g), the simulation results of the angular frequency deviation are shown. Figure 4 The value in (h) represents the simulation result of the power angle; Figure 5 This is the simulation result of the transient stability enhancement control strategy based on MPC when the grid voltage Ug = 0.1pu according to an embodiment of the present invention. Figure 5 (a) and Figure 5 (b) shows the simulation results for the dq-axis currents, respectively. Figure 5 (c) and Figure 5 (d) shows the simulation results for active and reactive power, respectively; Figure 5 (e) shows the simulation results of the saturation phase of the MPC output current; Figure 5 (f) shows the simulation results of the compensation amount of the MPC output active power reference value; Figure 5 In the middle (g), the simulation results of the angular frequency deviation are shown. Figure 5 The value in (h) represents the simulation result of the power angle. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0017] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0018] Example like Figure 1 As shown, this invention provides a model predictive transient stability control method for a multi-mechanism network system under saturated constraints, comprising the following steps: S1. Taking a grid-connected two-machine parallel system as an example, considering the influence of current saturation limitation, a transient analysis mathematical model of the grid-connected two-machine (converter one VSC1 and converter two VSC2) parallel system before and after a grid fault is constructed, as follows: S11. Construct the topology of a grid-connected two-machine parallel system, such as... Figure 2 As shown, the two converters are filtered by an inductor. L fi With filter capacitor C fi Access common coupling point (PCC), where Indicates the converter number, PCC series line impedance Z g It is connected to an infinite power grid. The DC bus of each converter is considered a constant DC source. V dc ; U i , U pcc and U g They represent the first The output voltage of the converter, the voltage at the point of common coupling, and the grid voltage; I i and I g The first The output current of the converter and the current flowing into the power grid; Z i For the first The equivalent line impedance from the filter capacitor of the converter to the PCC.

[0019] S12. Each converter uses a virtual synchronous generator (VSG) to generate a reference phase and reference voltage amplitude. To quickly track the target voltage and current values, the inner loop of the virtual synchronous generator adopts conventional dual-loop control of voltage and current, including an active control loop and a reactive control loop. The current loop outputs a modulation signal, which is sent to the modulation module to generate the converter's drive signal.

[0020] Reference Figure 3 The swing equation for the active power control loop of the virtual synchronous generator is: In the formula, Indicates the first Taiwan converter d axis and grid voltage vector V g The angle difference between them, i.e., the power angle, and These are the rated angular frequency and the first The output angular frequency of the converter For the first Output angular frequency deviation of the converter and The first The virtual inertia and damping coefficient of the converter. and The first The active power reference value and output active power of the converter.

[0021] The control law of the reactive power control loop is as follows: In the formula, For the first Reference value for the output voltage amplitude of the VSG converter. Rated voltage, For the first The integral coefficient of the reactive power loop of the converter. Represents the Laplace operator. and The first The reactive power reference value and output reactive power of the converter.

[0022] S13. To reduce fault current, direct current control is used, and a current limiter is placed at the voltage loop output. The output of the current limiter can be expressed as: In the formula, and The first Taiwan converter dq Shaft current reference value, For the first The voltage loop output current amplitude of the converter. and The first The voltage loop output current amplitude of the converter dq Axial components, For the first The maximum saturation current amplitude of the converter. and The first Maximum saturation current amplitude of the converter dq Axial components, For the first The current saturation angle of the converter.

[0023] S14. Derive the active power expressions for the outputs of VSC1 and VSC2. For ease of modeling, the Y-type topology of the grid-connected two-machine parallel system is transformed through Y-... Transformation into Type structure. Assuming all impedances are inductive, by... Figure 3 The node voltage equations for a grid-type two-machine parallel system can be listed as follows: In the formula, , and These are the output currents of VSC1, VSC2, and the mains power, respectively. , and These are the terminal voltages of VSC1, VSC2, and the mains grid, respectively. For self-guided admittance, ; For mutual admittance, ; For the self-admittance of the power grid, The mutual admittance between VSC1 and the power grid. The mutual admittance between the power grid and VSC1; The mutual admittance between VSC2 and the power grid. This is the mutual admittance between the power grid and VSC2.

[0024] Before the fault occurs, the VSG is in voltage source mode, such as Figure 3 As shown in (a), the output voltage amplitude and phase of VSC1 and VSC2 are determined by the active and reactive power control loops of the VSG, while the output current is uncontrollable and unknown. From the expression of the node voltage equations of the grid-connected two-machine parallel system, the expressions for the output currents of VSC1 and VSC2 can be obtained as follows: The active power of VSC1 and VSC2 is further obtained as follows: In the formula, and These represent the active power of VSC1 and VSC2 before the fault occurred, respectively. , , and They are , , and The modulus, , and They are respectively , and The modulus.

[0025] When a fault occurs, the VSG enters current source mode, such as Figure 3As shown in (b), the output current amplitude and phase of VSC1 and VSC2 are determined by the current limiter, as shown in the output expression of the current limiter. Since the voltage loop and reactive power loop are bypassed, the output voltage is uncontrollable and unknown. From the node voltage equations of the grid-connected two-machine parallel system, the output voltage expressions for VSC1 and VSC2 can be obtained as follows: Further derivation of the active power of VSC1 and VSC2 under current saturation after the fault is as follows: In the formula, and This indicates the active power of VSC1 and VSC2 after the fault occurred. and They are respectively and The modulus; and The first term represents the power coupling between VSC1 and VSC2, and its algebraic sum is zero, indicating that power is transferred between the two converters; the second term corresponds to the power interaction components between VSC1 and VSC2 and the grid, respectively.

[0026] Therefore, the output active power of the converter is simultaneously affected by the coupling between converters and the interaction with the grid, and there is transient power exchange among the three. With fixed branch parameters, the magnitude of the interaction depends mainly on the grid voltage amplitude, the current limiting amplitude of each VSG, and the phase of its saturation current.

[0027] S2. Construct a saturation current phase control strategy based on model prediction, specifically as follows: S21. Measure the three-phase voltage and three-phase current of VSC1 and VSC2 in real time, and calculate the current phase.

[0028] S22. Based on the operating data of the active power loops of VSC1 and VSC2, measure and collect the power angle and angular frequency deviation of VSC1 and VSC2 in real time. S23. Using the power angle and angular frequency deviation of VSC1 and VSC2 as state variables and the saturation current phase as control variables, respectively, construct a mathematical model for a saturation current phase control strategy based on model prediction (MPC).

[0029] Under mild voltage faults, improper saturation current phase design may result in an unfavorable initial operating point, preventing the operating trajectory from entering the stable attraction region and leading to transient instability. Therefore, properly adjusting the saturation current phase can effectively enhance the transient stability of the system. To this end, the power angle and angular frequency deviation of VSC1 and VSC2 are used as indicators. , , , As a state variable, the phase of the saturated current ( , Using ) as the control variable, a saturated current phase control strategy based on MPC is constructed.

[0030] Taking VSC1 as an example, the MPC controller optimizes the objective function at each control time step. Generate the control sequence for the next N steps. , Indicates the first The current saturation angle of VSC1 at time t, and the first control variable is selected. The control is applied to a grid-connected two-machine parallel system; based on the real-time updates of the grid-connected two-machine parallel system's state, the "prediction-optimization-feedback" process is repeated to form a closed-loop control. Considering the computational delay, a three-step prediction method is adopted.

[0031] objective function The expression is: In the formula, For the prediction interval, At the current moment, the optimization problem of the objective function is constrained by a set of equality and inequality constraints. The equality constraints are obtained by discretizing the dynamic equations (i.e., the swing equations of the VSG active control loop) using the forward Euler method, namely: In the formula, For discrete time, General The state variable at any given time.

[0032] In the case of a minor fault, the swing equation of the VSG active control loop is discretized to obtain: In the formula, The equation constraints represent the conditions for a grid-connected two-machine parallel system under minor faults. express The power angle of VSC1 at time 1. express The output angular frequency deviation of VSC1 at any given time express The power angle of VSC2 at time 2. express The current saturation angle of VSC2 at time 2. express The current saturation angle of VSC1 at time VSC1.

[0033] In addition, in order to limit the saturation current within one cycle, it is necessary to... Set upper and lower limits: The above equation forms the inequality constraints for the MPC-based optimization strategy for saturated current phase.

[0034] S3. Construct a control strategy for active power reference value compensation based on model prediction.

[0035] As the voltage drop deepens, the active power output amplitudes of VSC1 and VSC2 will fall below their respective reference values. At this point, regardless of the design of the saturation current phase, the grid-connected two-machine parallel system will experience transient instability. Therefore, in this situation, the equilibrium point must be restored by lowering the active power reference value. Based on this, the power angle and angular frequency deviation of VSC1 and VSC2 ( , , , As a state variable, the reference active power compensation amount (active power compensation amount of the first converter) is used. The active power compensation of the second converter Using MPC as the control variable, an active power reference value compensation control strategy is constructed to achieve dynamic adjustment of the reference power and maintenance of a stable equilibrium point.

[0036] Taking VSC1 as an example, using the objective function as the control objective, the MPC-based active power reference value compensation design method modifies the equality and inequality constraints compared to the MPC-based saturated current phase design strategy. Based on the reference power as the control input, the equality constraints of the grid-type two-machine parallel system considering only severe faults are: In the formula, Indicates after the fault occurs The active power of VSC1 at time t. express The active power reference value compensation amount of VSC1 at time t.

[0037] To prevent excessive reference power reduction from causing reverse power flow, the active power compensation of the first converter needs to be adjusted. Set upper and lower limits: The above equation forms the inequality constraints for the MPC active power reference value compensation design method.

[0038] S4. A unified MPC control strategy is proposed. This strategy, while keeping the state variables constant, integrates the control concepts of saturated current phase design and active power reference value design. It uses the power angle and angular frequency deviation of VSC1 and VSC2 as state variables, respectively, and jointly employs the saturated current phase... Compensation amount with active power reference value As a control variable, this strategy improves transient stability by coordinating the adjustment of the saturation current phase and the active power reference value compensation: under mild voltage faults, the saturation current phase is adjusted first to enhance transient stability, while the active power reference value compensation is zero to maximize the inverter's power output; however, when the fault is severe and stability cannot be maintained by phase adjustment alone, the saturation current phase is adjusted in conjunction with the voltage adjustment. and active power reference value compensation Achieve transient stability of the grid-connected two-machine parallel system and optimize the transient dynamic response trajectory.

[0039] Taking VSC1 as an example, the control objective function is designed as follows: In the formula, and These are the weighting factors. Since the angular frequency and power differ significantly in magnitude, they are typically taken as... This is to avoid power compensation having a dominant influence on the optimization objective.

[0040] Combining the two operating conditions of minor and severe faults, the corresponding equality constraints can be derived as follows: Inequality constraints are and The upper and lower bound constraints are defined. Similarly, the MPC control strategy of VSC2 can be derived, which will not be elaborated here. Thus, a unified MPC-based transient stability enhancement control strategy is constructed, which greatly improves the transient stability of networked multi-machine parallel systems.

[0041] To verify the proposed MPC-based transient stability enhancement control strategy, a control system was constructed on MATLAB / Simulink software as follows: Figure 2 The VSG two-machine parallel system model is shown, and two fault conditions are set for time-domain simulation analysis. The saturation current amplitudes of VSC1 and VSC2 are set to... , The simulation was set to cause a power grid fault at 3 seconds and clear the fault at 7 seconds. The main simulation parameters are shown in Table 1, and the simulation results are as follows: Figure 4 and Figure 5 As shown.

[0042] Table 1 Simulation parameters of the grid-type two-machine parallel system

[0043] When the grid voltage temporarily drops to 0.6 pu, the simulation results of the grid-connected two-machine parallel system are as follows: Figure 4 As shown. After the fault occurs, with the optimization objective of minimizing the angular frequency deviation and the active power reference value compensation, the optimal current saturation phase angles of VSC1 and VSC2 are obtained as follows: and ,like Figure 4 As shown in (e). From Figure 4 As can be seen in (f), the active power reference value compensation is almost zero. This indicates that under slight voltage dips, transient stability can be improved simply by adjusting the current saturation phase angle, thus avoiding capacity waste caused by reducing the active power reference value. Figure 4 (a)- Figure 4 As shown in Figure (d), under the proposed control strategy, both VSC1 and VSC2 can maintain stable current and power output after a fault occurs and is cleared. Furthermore, by... Figure 4 As can be observed in (g), after adopting the MPC control strategy, the angular frequency deviation during the fault period is effectively suppressed to within 10. -5 Orders of magnitude, the power angle remains almost unchanged after a fault, such as Figure 4 As shown in Figure (h), the simulation verifies the effectiveness of the MPC-based transient stability enhancement control strategy.

[0044] Furthermore, when the grid voltage drops to 0.1 pu, the time-domain simulation results of the grid-connected two-machine parallel system are as follows: Figure 5 As shown. After the fault occurs, the optimal saturation current phase angles obtained by the MPC algorithm are respectively and ,like Figure 5 As shown in (e). Under this severe fault condition, adjusting the saturation current phase angle alone is insufficient to maintain system transient stability. Therefore, the MPC further outputs an active power reference value compensation to enhance system stability. Figure 5 As can be seen in (f), the active power compensation amounts of VSC1 and VSC2 are respectively and .Depend on Figure 5 (a)- Figure 5 (d) and Figure 5 (g)- Figure 5As can be observed in (h), both VSC1 and VSC2 can achieve stable output of current, power, and power angle after the fault occurs and is cleared, verifying that the proposed MPC control strategy still has effective transient stability enhancement capability under severe voltage drop conditions. Thus, the goal of improving transient stability is achieved by adaptively compensating for the saturated current phase and active power reference value compensation through model predictive control method.

[0045] Therefore, the present invention adopts the above-mentioned model prediction transient stability control method for multi-machine network systems under saturation constraints, which can coordinate the control behavior of each converter under the condition of multi-machine parallel operation, effectively suppress power angle divergence and frequency oscillation, and improve the overall transient stability capability of multi-machine network systems under large disturbance conditions.

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. 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 still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A model predictive transient stability control method for a multi-mechanism network system under saturated constraints, characterized in that, Includes the following steps: S1. Considering the influence of current saturation limitation, construct a transient analysis mathematical model for a grid-connected two-machine parallel system before and after a grid fault; S2. Construct a saturation current phase control strategy based on a transient analysis mathematical model; S3. When the saturated current phase control strategy cannot maintain the transient stability of the grid-type two-machine parallel system, a control strategy for the active power reference value compensation is constructed. S4. A model predictive control strategy is proposed, which combines a saturated current phase control strategy and an active power reference value compensation strategy.

2. The model predictive transient stability control method for multi-mechanism network systems under saturated constraints according to claim 1, characterized in that, Step S1 includes: S11. Construct the topology of a grid-connected two-machine parallel system, with the two converters connected via a filter inductor. L fi With filter capacitor C fi Access to common coupling point, where Indicates the converter number, and the series impedance at the point of common coupling. Z g Connected to an infinite power grid, the DC bus of each converter is considered a constant DC source. V dc The two converters are VSC1 and VSC2. S12. Each converter uses a virtual synchronous generator (VSG) to generate a reference phase and a reference voltage amplitude. The inner loop of the virtual synchronous generator uses a dual-loop control of voltage and current, including an active control loop and a reactive control loop. The current loop outputs a modulation signal, which is sent to the modulation module to generate the drive signal for the converter. S13. Direct current control is adopted, and a current limiter is placed at the voltage loop output. S14. Derive the active power expressions for the outputs of VSC1 and VSC2, and transform the Y-type topology of the grid-connected two-machine parallel system through Y- Transformation into Type structure.

3. The model predictive transient stability control method for multi-mechanism network systems under saturated constraints according to claim 2, characterized in that, Step S14 is as follows: Assuming all impedances are inductive, list the node voltage equations for a grid-type two-machine parallel system; Before the fault occurs, the VSG is in voltage source mode. The output voltage amplitude and phase of VSC1 and VSC2 are determined by the active control loop and reactive control loop of the VSG. The output current of VSC1 and VSC2 is obtained by the node voltage equation of the grid-type two-machine parallel system. Calculate the active power of VSC1 and VSC2; When a fault occurs, VSG is in current source mode. The output current amplitude and phase of VSC1 and VSC2 are determined by the current limiter. Since the voltage loop and reactive power loop are bypassed, the output voltage of VSC1 and VSC2 is obtained through the node voltage equation of the grid-type two-machine parallel system. Calculate the active power of VSC1 and VSC2 under current saturation after the fault.

4. The model predictive transient stability control method for multi-mechanism network systems under saturated constraints according to claim 3, characterized in that, Step S2 is as follows: S21. Measure the three-phase voltage and three-phase current of VSC1 and VSC2 in real time, and calculate the current phase; S22. Based on the operating data of the active power loops of VSC1 and VSC2, measure and collect the power angle and angular frequency deviation of VSC1 and VSC2 in real time. S23. Using the power angle and angular frequency deviation of VSC1 and VSC2 as state variables and the saturation current phase as control variables, respectively, construct a mathematical model for the saturation current phase control strategy based on model prediction.

5. The model predictive transient stability control method for multi-mechanism network systems under saturated constraints according to claim 4, characterized in that, Step S23 is as follows: In VSC1, the model predictive controller optimizes the objective function at each control time step. Generate the control sequence for the next N steps. , Indicates the first The current saturation angle of VSC1 at time 1, and the first control quantity Apply to the grid-type two-machine parallel system; based on the real-time update of the grid-type two-machine parallel system status, repeat the prediction-optimization-feedback process to form a closed-loop control; objective function The expression is: In the formula, For the prediction interval, At the current moment, the optimization problem of the objective function is constrained by a set of equality and inequality constraints; The equality constraints are discretized by using the forward Euler method on the swing equations of the VSG active control loop; In the case of a minor fault, the swing equation of the VSG active control loop is discretized to obtain: In the formula, The equation constraints represent the conditions for a grid-connected two-machine parallel system under minor faults. express The power angle of VSC1 at time 1. For the first Output angular frequency deviation of the converter express The output angular frequency deviation of VSC1 at any given time express The power angle of VSC2 at time 2. express The current saturation angle of VSC2 at time 2. express The current saturation angle of VSC1 at time 1. The rated angular frequency, This is the reference value for the active power of VSC1; To limit the saturation current within one cycle, it is necessary to... Set upper and lower limits: The above equation forms the inequality constraints for a model-based prediction optimization strategy for saturated current phase.

6. The model predictive transient stability control method for multi-mechanism network systems under saturated constraints according to claim 5, characterized in that, Step S3 specifically involves: using the power angle and angular frequency deviation of VSC1 and VSC2 as state variables and the reference active power compensation amount as control variables, constructing an active power reference value compensation control strategy based on model prediction to achieve dynamic adjustment of reference power and maintenance of stable equilibrium point. In VSC1, the objective function is used as the control objective, and the equality and inequality constraints are modified. Based on the reference power as the control input, the equality constraints of the grid-type two-machine parallel system considering only severe faults are as follows: In the formula, Indicates after the fault occurs The active power of VSC1 at time t. express The active power reference value compensation amount of VSC1 at time _____. The first Damping coefficient of the converter; To prevent excessive reference power reduction from causing reverse power flow, the active power compensation amount of the first converter is adjusted. Set upper and lower limits: The above equation forms the inequality constraints for the compensation design method based on the model-predicted active power reference value.

7. The model predictive transient stability control method for multi-mechanism network systems under saturated constraints according to claim 6, characterized in that: In step S4, the model prediction strategy, while keeping the state variables constant, integrates the control concepts of saturated current phase design and active power reference value design. It uses the power angle and angular frequency deviation of VSC1 and VSC2 as state variables, respectively, and jointly employs the saturated current phase... Compensation amount with active power reference value As a control variable; The model prediction strategy improves transient stability by coordinating the adjustment of the saturation current phase and active power reference value compensation: under mild voltage faults, the saturation current phase is adjusted first to enhance transient stability, while the active power reference value compensation is zero to maximize the inverter's power output; however, when the fault is severe and phase adjustment alone is insufficient to maintain stability, the saturation current phase is adjusted in conjunction with the fault. and active power reference value compensation Achieve transient stability of the grid-connected two-machine parallel system and optimize the transient dynamic response trajectory.

8. The model predictive transient stability control method for multi-mechanism network systems under saturated constraints according to claim 7, characterized in that: In VSC1, the control objective function is designed as follows: In the formula, and These are the weighting factors; Combining the two operating conditions of minor and severe faults, the corresponding equation constraint derivation is as follows: Inequality constraints are and Upper and lower limit constraints.

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