A control method of network converter for improving transient stability of grid-connected hybrid power system

Abstract

The application discloses a network converter control method for improving transient stability of a grid-following and grid-forming hybrid system, and aims at the problem that the grid-forming converter power angle offset causes the grid-connected point voltage to exceed the limit after fault removal, and causes the phase-locked loop of the grid-following converter to be forced to oscillate and lose stability. The application establishes the upper and lower limits of the safe operation of the grid-forming converter power angle, and further proposes a frequency limiting and power angle integrator freezing cooperative control strategy: when the frequency deviation of the grid-forming converter reaches the limiting threshold during the fault, the power angle integral link is synchronously frozen, and the safe operation trajectory of the grid-forming converter power angle is limited in the upper and lower limits; and after the fault is removed, the normal control logic is restored. The application only needs to implement the control on the grid-forming converter side, does not need to change a large number of dispersed grid-following converters, can effectively avoid the voltage exceeding limit caused by the power angle offset, and significantly improves the transient synchronization stability of the hybrid system.

Description

Technical Field

[0001] This invention belongs to the field of equipment control technology for use with power supplies or similar power supply systems, and specifically relates to a grid-connected converter control method for improving the transient stability of grid-connected hybrid systems. Background Technology

[0002] With the large-scale grid connection of renewable energy, grid-connected converters have become dominant in new energy power generation systems. However, grid-connected converters rely on phase-locked loops (PLLs) for synchronization with the grid, leading to significant transient synchronization stability issues under severe fault disturbances. To enhance the voltage and inertia support capabilities of the system, grid-connected converter technologies such as virtual synchronous generators have been widely adopted. By simulating the voltage source characteristics, inertia, and damping characteristics of synchronous generators, grid-connected converters can effectively improve grid strength, and their introduction into grid-connected converter systems has become the mainstream solution.

[0003] However, the transient synchronization stability of hybrid systems with grid-connected and grid-connected converters has attracted widespread attention. Existing research mainly focuses on the interaction between the two types of converters during faults, including the impact of the grid-connected converter's saturation current angle on the stability of the phase-locked loop, the impact of control parameters of grid-connected and grid-connected converters on overall transient stability, stability criteria based on equivalent damping, and stability boundary analysis under current-limiting modes. While these studies have provided important insights into the dynamic characteristics of hybrid systems, they generally focus on converter interactions during faults, neglecting the dynamic coupling relationship between the converter output power angle and the point of common coupling voltage after fault clearance. This leads to overly optimistic assessments of the system's operating boundaries.

[0004] In AC power systems, voltage dynamics and power angle dynamics are inherently coupled, and power angle fluctuations inevitably lead to voltage fluctuations. This coupling is particularly pronounced in converter-dominated transmission systems with a large number of grid-connected converters. This is because grid-connected converters are subject to strict hardware protection constraints. When a large fluctuation in the power angle of a grid-connected converter causes a voltage drop at the point of common coupling, it directly triggers the low-voltage ride-through control logic of the grid-connected converter, altering its current output characteristics and thus affecting the synchronization stability of the phase-locked loop (PLL).

[0005] After a fault is cleared, a grid-connected converter may operate in two states: If the power angle deviates significantly during the fault, the grid-connected converter will enter a current-limiting control mode, losing its terminal voltage clamping capability. In this mode, the grid-connected voltage becomes highly dependent on the grid-connected converter and fluctuates with the phase angle oscillation of the phase-locked loop (PLL). If the grid-connected converter maintains a constant voltage control mode but the power angle still deviates significantly, the terminal voltage will monotonically decrease as the power angle increases, similarly causing the grid-connected voltage to remain consistently low. In both scenarios, when the grid-connected voltage drops to near the low-voltage ride-through threshold, the phase angle oscillation of the PLL will repeatedly trigger the low-voltage ride-through logic switching. Each switching injects positive transient energy into the PLL, accumulating cycle by cycle, eventually causing forced oscillation instability of the PLL and further exacerbating the risk of voltage exceeding limits, forming a positive feedback instability loop.

[0006] Existing research on the transient stability of hybrid systems has the following shortcomings: First, existing studies do not fully consider the dynamic coupling between the power angle swing of the grid-connected converter and the voltage at the point of common coupling after fault clearance, leading to an overly optimistic assessment of the system stability boundary. Second, the power angle constraints used in existing transient control schemes for grid-connected converters typically only consider the transient stability of the grid-connected converter itself, without incorporating the impact of angle-induced voltage exceeding limits on the stability of the phase-locked loop, resulting in a too-wide constraint range. Third, given the large number and dispersed deployment of grid-connected converters in practical engineering projects, implementing control modifications on each converter individually is extremely costly. There is an urgent need for a method that can ensure the overall stability of the hybrid system simply by controlling the centrally deployed grid-connected converters.

[0007] Therefore, it is necessary to propose a control method that controls only grid-type converters, avoids angle-induced voltage overruns by reasonably constraining their operating power angle, and thus improves the transient synchronization stability of hybrid grid-type and grid-type converter systems. Summary of the Invention

[0008] In grid-connected systems with mixed grid-connected and grid-connected converters, after fault clearance, the grid-connected converter enters current-limiting control mode or constant-voltage control mode with a significant shift in power angle, leading to voltage exceeding the grid connection point limit and subsequently causing forced oscillation instability of the phase-locked loop (PLL) of the grid-connected converter. The purpose of this invention is to propose a grid-connected converter control method to improve the transient stability of mixed grid-connected and grid-connected converter systems. This method derives the voltage exceeding-limit boundary characterized by the power angle of the grid-connected converter, establishes a safe operating boundary for the power angle, and designs a frequency limiting and power angle integrator freezing collaborative control strategy. By controlling only the grid-connected converter, it effectively avoids voltage exceeding the limit induced by angle shift after a fault, significantly improving the transient synchronization stability of the mixed system.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A method for controlling grid-connected converters to improve transient stability in grid-connected hybrid systems includes the following steps: Step 1: Offline calculation of the upper limit of safe operation of the power angle of the grid-type converter and the lower world The upper boundary Angle of unstable equilibrium point of grid converter Upper limit of critical power angle for grid-type converters maintaining constant voltage control mode And the maximum allowable power angle of the grid-connected converter to ensure that all grid-connected converters do not trigger undervoltage ride-through. The minimum of the three, i.e. Lower Boundary Lower critical power angle of grid-type converter in constant voltage control mode. Minimum allowable power angle of grid-type converter to avoid overvoltage limits The maximum of the two, i.e. .

[0010] In step 1, let and These are the branch admittance of the grid-type converter and the equivalent admittance of the power grid, respectively. The network admittance ratio coefficient; This refers to the terminal voltage amplitude of the grid-type converter; The Thevenin equivalent voltage amplitude on the grid side; This refers to the maximum value of the low-voltage ride-through activation threshold for grid-connected converters. Let be the resultant current phasor of the output current of all grid-connected converters at the point of common coupling, where This represents a vector summation of the output current phasors of all grid-connected converters. This represents the vector summation current amplitude at the point of common coupling for the output current of all grid-connected converters. The vector summation current phase angle at the point of common coupling for the output current of all grid-connected converters; For the first The output current phasor of the grid-type converter, among which For the first The output current of the grid-connected converter For the first The phase angle of the output current of the grid-type converter; according to the phasor relationship of the network current, we have: , In the formula, This represents the magnitude of the equivalent impedance of the power grid. The equivalent impedance magnitude of the power grid and the first The sum of the moduli of the branch impedances of the grid-type converter. For the first iTaiwan-type grid converter branch admittance, and They are and The modulus; To match the magnitude of the equivalent voltage phasor of the grid converter, The phase angle of the equivalent voltage phasor; This indicates that the equivalent voltage phasor of the grid converter leads its phase angle. Phasor after 90-degree rotation; This indicates that the resultant current phasor of all grid-connected converter output currents at the point of common coupling leads its phase angle. Phasor after 90-degree rotation; Indicates the first The output current phasor of the grid-connected converter leads its phase angle. Phasors after 90-degree rotation; maximum permissible power angle of grid-connected converters to ensure that all grid-connected converters do not trigger undervoltage ride-through. The determination of is discussed in three scenarios: if satisfies In a network-connected hybrid system operating under unconditional security conditions, There are no valid constraints; if satisfied This indicates that the output of new energy sources is too large, and voltage safety cannot be guaranteed through power angle constraints; when neither of the above two conditions is met, i.e., the following triangle inequality applies... At the time of its establishment, There exists a unique and definite value, calculated as follows: .

[0011] In step 1, let To avoid overvoltage exceeding the high voltage ride-through threshold, the minimum allowable power angle of the grid-type converter is calculated as follows: , In the formula, Minimum allowable power angle for grid-type converters to avoid overvoltage exceeding limits; The network admittance ratio coefficient; This refers to the terminal voltage amplitude of the grid-type converter; The Thevenin equivalent voltage amplitude on the grid side; This is the magnitude of the equivalent voltage phasor of the grid-type converter.

[0012] In step 1, let The maximum current limit for the grid-connected converter, and the upper limit of the critical power angle for the grid-connected converter in constant voltage control mode. Lower critical power angle of grid-type converter maintaining constant voltage control mode The output current of the grid-type converter does not exceed The conditions are determined by the derivation: , In the formula, The phase compensation angle is determined by the relative relationship between the grid-side current of the grid-type converter and the output current of the grid-connected converter at the point of common coupling. d These are intermediate normalized parameters obtained by comprehensively calculating various electrical quantities in a grid-connected system. and d The expression is: , , In the formula, This refers to the maximum current limit value for grid-type converters. The equivalent admittance of the power grid; The Thevenin equivalent voltage amplitude on the grid side; This refers to the terminal voltage amplitude of the grid-type converter; The vector summation current amplitude at the point of common coupling for the output current of all grid-connected converters; The vector summation current phase angle at the point of common coupling for the output current of all grid-connected converters; The network admittance ratio coefficient; yes The modulus.

[0013] The reason for using the above method to determine the power angle safety boundary in step 1 is that the power angle deviation of the grid-connected converter poses a threat to the stability of the hybrid system from three aspects: First, the power angle exceeds the unstable equilibrium point. This will cause the grid-type converter to lose its synchronization stability; secondly, the power angle will exceed the critical value. This will cause the grid-connected converter to enter current-limiting control mode, losing its terminal voltage clamping capability; thirdly, an excessively large power angle will cause the grid connection point voltage to drop to the low voltage crossing threshold, triggering forced oscillation instability of the grid-connected converter's phase-locked loop. All three types of constraints are indispensable. The most stringent constraint (i.e., the minimum value) should be taken as the upper bound to simultaneously ensure the stability of the grid-connected converter itself and the safety of the grid connection point voltage. Lower bound Similarly, both constraints of not entering reverse current-limiting mode and not triggering overvoltage limits must be satisfied simultaneously. The aforementioned power angle safety boundaries all have analytical calculation expressions, and offline calculations do not require real-time identification of system parameters, making engineering implementation convenient.

[0014] Step 2: Based on the energy function of the grid-type converter, design the frequency limiting threshold offline using quantization. The upper bound ensures that the operating trajectory of the grid-type converter can be constrained within a safe power angle range after frequency limiting triggering. Inside.

[0015] In step 2, let The virtual inertia coefficient for a grid-type converter. The damping coefficient is... and These are the angular frequencies of the grid-type converter and the grid angular frequency, respectively. For the frequency deviation of the grid-type converter, δ For grid-type converters, The power angle at the stable equilibrium point of the grid-type converter under constant voltage control mode. The actual output active power of the grid-type converter. As the active power reference value, the energy function of a grid-type converter is defined as follows:

[0016] In the formula, For grid-type converters with power angle Frequency deviation Lyapunov energy functions for state variables characterize the grid converter at the operating point. Transient energy at the point; frequency limiting threshold The quantitative design is based on the following stability criteria: , The stability criterion represents the transient energy of the grid converter at the frequency limiting trigger moment. Subtract the lower bound of damped dissipation energy Afterwards, it should be less than the upper limit of the safe operation of the power angle of the grid-type converter. Lower limit of safe operation of grid-type converter power angle Minimum energy at the location This ensures that the operating trajectory of the grid-type converter is constrained within the safe operating range of the grid-type converter's power angle after frequency limiting triggering. Inside; among them, The power angle of the grid-type converter at the frequency limiting trigger moment. For frequency limiting threshold, This represents the transient energy of the grid-type converter at the moment of frequency limiting triggering. and The power angle of the grid-type converter reaches the upper limit of safe operation. and the lower world And the transient energy corresponding to zero frequency deviation, This is the lower bound of the damping dissipation energy from the frequency limiting trigger moment until the grid-type converter's power angle reaches the safe operating boundary; Based on the worst-case failure condition, the estimated value is: , In the formula, Duration of the fault; This refers to the terminal voltage amplitude of the grid-type converter; The vector summation current amplitude at the point of common coupling for the output current of all grid-connected converters; The network admittance ratio; satisfying the above criteria The upper bound is obtained through numerical solution.

[0017] Step 2 uses the energy function method of a grid-type converter to determine The reason for the upper bound is: frequency limiting threshold. The timing of intervention is determined, if If the value is too large, the power angle will have already deviated too far when the control is triggered, and even freezing the power angle integral stage will not be able to constrain the running trajectory within a safe range; if If the value is too small, it will prematurely intervene in normal operating conditions, reducing the system's normal regulation capability. Therefore, the Lyapunov energy function is introduced. The stability criterion is given by using the condition that "the transient energy at the frequency limiting trigger moment minus the damping dissipation energy does not exceed the critical energy at the safety boundary". The quantitative design upper bound avoids the uncertainty of trial and error based on experience in engineering parameter tuning. This step also uses offline calculation, so no additional computational burden is required in the real-time control stage.

[0018] Step 3: During normal operation of the hybrid system, the grid-connected converter maintains its original constant voltage control logic; when a fault is detected and the frequency deviation of the grid-connected converter is found... Reaching the frequency limiting threshold At that time, frequency limiting control is triggered and the power angle integral stage is frozen simultaneously, thus constraining the safe operating trajectory of the grid-type converter's power angle within a specified range. Inside.

[0019] In step 3, the coordinated triggering logic for triggering frequency limiting control and simultaneously freezing the power angle integral stage is as follows: when a fault occurs and When the enable signal is set to 1, the frequency limiting will be... Pincer At the same time, the input of the power angle integral stage is forced to zero, so that the power angle of the grid-type converter is reduced. The value is maintained at the time of the limiting trigger; when the above conditions are not met, the enable signal is set to 0, neither of the two controls intervenes, and the grid-type converter maintains the original constant voltage control logic.

[0020] Step 3 employs a coordinated triggering of frequency limiting and power angle integrator freezing because: simple frequency limiting can only prevent the frequency deviation from increasing further, but the power angle integrator will continue to integrate, causing the power angle to continue to shift; while simple power angle integrator freezing cannot limit the frequency deviation, and the system may still become unstable due to excessive frequency deviation. Only by triggering both in tandem can further deviations in frequency deviation and power angle be constrained simultaneously, locking the grid-type converter's operating trajectory within a safe power angle range. Furthermore, this coordinated control only intervenes when a fault occurs and the frequency deviation reaches a threshold. Under normal operating conditions, the grid-type converter maintains its original constant voltage control logic, without affecting the system's normal operating performance.

[0021] Step 4: After clearing the fault, unfreeze the power angle integrator and restore the original constant voltage control logic of the grid-type converter.

[0022] Compared with the prior art, the present invention has the following advantages: I. Existing transient control schemes for grid-connected converters only consider the stability of the grid-connected converter itself, without considering the impact of its power angle oscillation on the grid connection point voltage and the stability of the phase-locked loop (PLL) of the grid-connected converter. This invention uses the condition of avoiding voltage over-limit and the stability constraint of the grid-connected converter itself as the design basis for the safe operation boundary of the power angle, ensuring that the power angle of the grid-connected converter is always constrained within a range that does not trigger voltage over-limit in the grid-connected converter, thus fundamentally blocking the positive feedback loop of power angle oscillation—voltage over-limit—forced oscillation instability of the PLL.

[0023] II. This invention provides and The analytical calculation expression covers three types of constraints: transient stability constraints of the grid-type converter itself (from the perspective of unstable equilibrium point). constraint ), maintaining the current constraint in constant voltage control mode (ensuring the output current of the grid-connected converter does not exceed the maximum limit value). The upper bound of the critical work angle is derived from the conditions. and the lower world Constraints and ), and grid-connected converter voltage safety constraints (defined by the maximum permissible power angle that ensures all grid-connected converters do not trigger undervoltage ride-through). constraint The minimum permissible power angle to avoid overvoltage limits constraint Furthermore, the frequency limiting threshold was derived based on the energy function of the grid-type converter. The quantitative design upper bound is defined. Both designs have clear analytical basis, avoiding the uncertainty of trial and error based on experience in engineering parameter tuning.

[0024] Third, control is only applied to grid-connected converters, resulting in low engineering implementation costs. This invention utilizes the centralized deployment of grid-connected converters, implementing frequency limiting and power angle integrator freezing coordinated control only on the grid-connected converter side. No modifications are required to the numerous distributed grid-connected converters in the grid-connected hybrid system, resulting in a small control modification scope suitable for large-scale renewable energy power plant engineering applications. Attached Figure Description

[0025] Figure 1 The present invention provides a flowchart of a grid-connected converter control method for improving the transient stability of a grid-connected hybrid system.

[0026] Figure 2 This is a schematic diagram of the topology and control framework of a grid-connected renewable energy power plant system that includes grid-connected converters and centralized grid-connected converters.

[0027] Figures 3(a) and 3(b) are the control block diagram and phase plane trajectory schematic diagram of the frequency limiting and power angle integrator freezing coordinated control strategy of the present invention, respectively.

[0028] Figure 4(a) shows the dynamic response of the power angle of the grid-connected converter and the output phase angle of the phase-locked loop of a single grid-connected converter; Figure 4(b) shows the dynamic response of the grid-connected voltage of a single grid-connected converter; Figure 4(c) shows the dynamic response of the active and reactive current commands of a single grid-connected converter.

[0029] Figure 5(a) shows the dynamic response of the power angle of the grid-connected converter and the output phase angle of the phase-locked loop of multiple grid-connected converters; Figure 5(b) shows the dynamic response of the grid-connected voltage of multiple grid-connected converters.

[0030] Figure 6(a) shows the dynamic response of the power angle of the grid-connected converter and the output phase angle of the phase-locked loop of the grid-connected converter; Figure 6(b) shows the dynamic response of the grid-connected voltage of the grid-connected converter as the power angle of the grid-connected converter continuously increases, which leads to the forced oscillation and instability of the phase-locked loop.

[0031] Figures 7(a), 7(b), and 7(c) show the simulation results of the phase plane trajectory, phase-locked loop output phase angle, and grid-connected voltage of the grid-connected converter after applying the control method proposed in this invention to the scenarios shown in Figures 4(a), 4(b), and 4(c), respectively. Figures 7(d), 7(e), and 7(f) show the simulation results of the phase plane trajectory, phase-locked loop output phase angle, and grid-connected voltage of the grid-connected converter after applying the control method proposed in this invention to the scenarios shown in Figures 5(a) and 5(b), respectively. Figures 7(g), 7(h), and 7(i) show the simulation results of the phase plane trajectory, phase-locked loop output phase angle, and grid-connected voltage of the grid-connected converter after applying the control method proposed in this invention to the scenarios shown in Figures 6(a) and 6(b), respectively. Detailed Implementation

[0032] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0033] This invention relates to a control method for grid-connected converters that improves the transient stability of hybrid grid-connected and grid-connected converter systems. For example... Figure 2 As shown, the system targeted by this invention is a grid-connected renewable energy power plant system comprising multiple grid-connected converters and centralized grid-connected converters. Each grid-connected converter and centralized grid-connected converter converges to a common coupling point via its respective filter, and then is connected to the grid via transmission lines and the Thevenin equivalent circuit. The overall execution flow of the method described in this invention is as follows: Figure 1 As shown in Figures 3(a) and 3(b), the specific structural block diagram and phase plane trajectory of the frequency limiting and power angle integrator freezing coordinated control are as follows: Step 1: Offline calculation of the upper limit of safe operation of the power angle of the grid-type converter and the lower world The upper boundary Angle of unstable equilibrium point of grid converter Upper limit of critical power angle for grid-type converters maintaining constant voltage control mode And the maximum allowable power angle of the grid-connected converter to ensure that all grid-connected converters do not trigger undervoltage ride-through. The minimum of the three, i.e. Lower Boundary Lower critical power angle of grid-type converter in constant voltage control mode. Minimum allowable power angle of grid-type converter to avoid overvoltage limits The maximum of the two, i.e. .

[0034] In step 1, let and These are the branch admittance of the grid-type converter and the equivalent admittance of the power grid, respectively. The network admittance ratio coefficient; This refers to the terminal voltage amplitude of the grid-type converter; The Thevenin equivalent voltage amplitude on the grid side; This refers to the maximum value of the low-voltage ride-through activation threshold for grid-connected converters. Let be the resultant current phasor of the output current of all grid-connected converters at the point of common coupling, where This represents a vector summation of the output current phasors of all grid-connected converters. This represents the vector summation current amplitude at the point of common coupling for the output current of all grid-connected converters. The vector summation current phase angle at the point of common coupling for the output current of all grid-connected converters; For the first The output current phasor of the grid-type converter, among which For the first The output current of the grid-connected converter For the first The phase angle of the output current of the grid-type converter; according to the phasor relationship of the network current, we have: , In the formula, This represents the magnitude of the equivalent impedance of the power grid. The equivalent impedance magnitude of the power grid and the first The sum of the moduli of the branch impedances of the grid-type converter. For the first i Taiwan-type grid converter branch admittance, and They are and The modulus; To match the magnitude of the equivalent voltage phasor of the grid converter, The phase angle of the equivalent voltage phasor; This indicates that the equivalent voltage phasor of the grid converter leads its phase angle. Phasor after 90-degree rotation; This indicates that the resultant current phasor of all grid-connected converter output currents at the point of common coupling leads its phase angle. Phasor after 90-degree rotation; Indicates the first The output current phasor of the grid-connected converter leads its phase angle. Phasors after 90-degree rotation; maximum permissible power angle of grid-connected converters to ensure that all grid-connected converters do not trigger undervoltage ride-through. The determination of is discussed in three scenarios: if satisfies In a network-connected hybrid system operating under unconditional security conditions, There are no valid constraints; if satisfied This indicates that the output of new energy sources is too large, and voltage safety cannot be guaranteed through power angle constraints; when neither of the above two conditions is met, i.e., the following triangle inequality applies... At the time of its establishment, There exists a unique and definite value, calculated as follows: .

[0035] In step 1, let To avoid overvoltage exceeding the high voltage ride-through threshold, the minimum allowable power angle of the grid-type converter is calculated as follows: , In the formula, Minimum allowable power angle for grid-type converters to avoid overvoltage exceeding limits; The network admittance ratio coefficient; This refers to the terminal voltage amplitude of the grid-type converter; The Thevenin equivalent voltage amplitude on the grid side; This is the magnitude of the equivalent voltage phasor of the grid-type converter.

[0036] In step 1, let The maximum current limit for the grid-connected converter, and the upper limit of the critical power angle for the grid-connected converter in constant voltage control mode. Lower critical power angle of grid-type converter maintaining constant voltage control mode The output current of the grid-type converter does not exceed The conditions are determined by the derivation: , In the formula, The phase compensation angle is determined by the relative relationship between the grid-side current of the grid-type converter and the output current of the grid-connected converter at the point of common coupling. d These are intermediate normalized parameters obtained by comprehensively calculating various electrical quantities in a grid-connected system. and d The expression is: , , In the formula, This refers to the maximum current limit value for grid-type converters. The equivalent admittance of the power grid; The Thevenin equivalent voltage amplitude on the grid side; This refers to the terminal voltage amplitude of the grid-type converter; The vector summation current amplitude at the point of common coupling for the output current of all grid-connected converters; The vector summation current phase angle at the point of common coupling for the output current of all grid-connected converters; The network admittance ratio coefficient; yes The modulus.

[0037] Step 2: Based on the energy function of the grid-type converter, design the frequency limiting threshold offline using quantization. The upper bound ensures that the operating trajectory of the grid-type converter can be constrained within a safe power angle range after frequency limiting triggering. Inside.

[0038] In step 2, let The virtual inertia coefficient for a grid-type converter. The damping coefficient is... and These are the angular frequencies of the grid-type converter and the grid angular frequency, respectively. For the frequency deviation of the grid-type converter, δ For grid-type converters, The power angle at the stable equilibrium point of the grid-type converter under constant voltage control mode. The actual output active power of the grid-type converter. As the active power reference value, the energy function of a grid-type converter is defined as follows:

[0039] In the formula, For grid-type converters with power angle Frequency deviation Lyapunov energy functions for state variables characterize the grid converter at the operating point. Transient energy at the point; frequency limiting threshold The quantitative design is based on the following stability criteria: , The stability criterion represents the transient energy of the grid converter at the frequency limiting trigger moment. Subtract the lower bound of damped dissipation energy Afterwards, it should be less than the upper limit of the safe operation of the power angle of the grid-type converter. Lower limit of safe operation of grid-type converter power angle Minimum energy at the location This ensures that the operating trajectory of the grid-type converter is constrained within the safe operating range of the grid-type converter's power angle after frequency limiting triggering. Inside; among them, The power angle of the grid-type converter at the frequency limiting trigger moment. For frequency limiting threshold, This represents the transient energy of the grid-type converter at the moment of frequency limiting triggering. and The power angle of the grid-type converter reaches the upper limit of safe operation. and the lower world And the transient energy corresponding to zero frequency deviation, This is the lower bound of the damping dissipation energy from the frequency limiting trigger moment until the grid-type converter's power angle reaches the safe operating boundary; Based on the worst-case failure condition, the estimated value is: , In the formula, Duration of the fault; This refers to the terminal voltage amplitude of the grid-type converter; The vector summation current amplitude at the point of common coupling for the output current of all grid-connected converters; The network admittance ratio; satisfying the above criteria The upper bound is obtained through numerical solution.

[0040] Step 3: During normal operation of the hybrid system, the grid-connected converter maintains its original constant voltage control logic; when a fault is detected and the frequency deviation of the grid-connected converter is found... Reaching the frequency limiting threshold At that time, frequency limiting control is triggered and the power angle integral stage is frozen simultaneously, constraining the operating trajectory of the grid-type converter within a certain range. Inside.

[0041] In step 3, as shown in Figure 3(a), the coordinated triggering logic for triggering frequency limiting control and simultaneously freezing the power angle integral stage is as follows: when a fault occurs and When the enable signal is set to 1, the frequency limiting will be... Pincer At the same time, the input of the power angle integral stage is forced to zero, so that the power angle of the grid-type converter is reduced. The value remains constant at the time of the limiting trigger. When the above conditions are not met, the enable signal is set to 0, neither control intervenes, and the grid-type converter maintains its original constant voltage control logic. As shown in Figure 3(b), in the phase plane, after the fault occurs, the operating trajectory of the grid-type converter changes from the stable equilibrium point. The trajectory deviates to the right upon departure; when the trajectory reaches the frequency limiting trigger point... And frequency deviation achieve Time (in Figure 3(b)) (Point), triggering frequency limiting and power angle freezing coordinated control, the power angle is locked at After the fault is cleared, the trajectory converges back to the vicinity of the stable equilibrium point (point O in Figure 3(b)), and is constrained within the safe power angle range throughout the entire process. Inside, among which It is an unstable equilibrium point.

[0042] Step 4: After clearing the fault, unfreeze the power angle integrator and restore the original constant voltage control logic of the grid-type converter.

[0043] Example: To verify the effectiveness of the present invention, a test system consisting of three grid-connected converters and one grid-connected converter was built on an electromagnetic transient simulation platform. A fault scenario in which the grid voltage drops to 0.2 per unit and lasts for 200 milliseconds was set for simulation verification.

[0044] Figure 4(a) shows the dynamic response of the power angle of the grid-connected converter and the output phase angle of the phase-locked loop of the grid-connected converter; Figure 4(b) shows the dynamic response of the grid-connected voltage of the grid-connected converter; Figure 4(c) shows the dynamic response of the active and reactive current commands of the grid-connected converter. Under the condition of a single grid-connected converter, after the grid-connected converter fails, it enters the current-limiting control mode. Its power angle swing causes the grid-connected voltage of the grid-connected converter to repeatedly reach the low voltage ride-through threshold of 0.8 per unit. The repeated switching of the low voltage ride-through control causes the phase angle oscillation of the phase-locked loop output to continuously intensify, eventually losing synchronization stability, and further triggering the grid-connected voltage amplification oscillation.

[0045] Figure 5(a) shows the dynamic response of the power angle of the grid-connected converter and the output phase angle of the phase-locked loop of each grid-connected converter; Figure 5(b) shows the dynamic response of the grid-connected converter's grid voltage. Under the condition of multiple grid-connected converters, the grid-connected converter is also in the current-limiting control mode after the fault is cleared. Among them, the phase-locked loops of two grid-connected converters successively lost synchronization stability, and the grid voltage continued to oscillate and could not be recovered, verifying that this instability phenomenon also exists under the condition of multiple machines.

[0046] Figure 6(a) shows the dynamic response of the power angle of the grid-connected converter and the output phase angle of the phase-locked loop (PLL) of the grid-connected converter; Figure 6(b) shows the dynamic response of the grid-connected converter's grid voltage monotonically decreasing as the power angle of the grid-connected converter continuously increases, leading to forced oscillation and instability of the PLL. Under the condition that the grid-connected converter is in constant voltage control mode and the power angle continues to increase after a fault, the grid voltage of the grid-connected converter continuously decreases. When the voltage drops to near the low voltage crossover threshold of 0.8 per unit, the PLL also loses synchronization stability and the voltage increases oscillates, verifying that the grid-connected converter has the same instability risk when the power angle is too large under constant voltage control mode.

[0047] As can be seen from Figures 7(a) to 7(i), the phase plane trajectory of the grid-type converter of the present invention is applied ( Figures 7(a) to 7(g) The phase angle of the phase-locked loop output is effectively constrained within the safe power angle range. Figures 7(b) to 7(h) It quickly recovered to stable condition after the fault, and the grid-connected voltage with the grid converter ( Figures 7(c) to 7(i) The system returned to normal, and the transient synchronization stability of the hybrid system was significantly improved.

Claims

1. A method for controlling a grid-connected converter to improve the transient stability of a grid-connected hybrid system, characterized in that, Includes the following steps: Step 1: Offline calculation of the upper limit of safe operation of the power angle of the grid-type converter and the lower world The upper boundary Angle of unstable equilibrium point of grid converter Upper limit of critical power angle for grid-type converters maintaining constant voltage control mode And the maximum allowable power angle of the grid-connected converter to ensure that all grid-connected converters do not trigger undervoltage ride-through. The minimum of the three, i.e. Lower Boundary Lower critical power angle of grid-type converter in constant voltage control mode. Minimum allowable power angle of grid-type converter to avoid overvoltage limits The maximum of the two, i.e. ; Step 2: Based on the energy function of the grid-type converter, design the frequency limiting threshold offline using quantization. The upper bound ensures that the operating trajectory of the grid-type converter can be constrained within a safe power angle range after frequency limiting triggering. Inside; Step 3: During normal operation of the hybrid system, the grid-connected converter maintains its original constant voltage control logic; when a fault is detected and the frequency deviation of the grid-connected converter is found... Reaching the frequency limiting threshold At that time, frequency limiting control is triggered and the power angle integral stage is frozen simultaneously, thus constraining the safe operating trajectory of the grid-type converter's power angle within a specified range. Inside; Step 4: After clearing the fault, unfreeze the power angle integrator and restore the original constant voltage control logic of the grid-type converter.

2. The method according to claim 1, characterized in that: In step 1, let and These are the branch admittance of the grid-type converter and the equivalent admittance of the power grid, respectively. The network admittance ratio coefficient; This refers to the terminal voltage amplitude of the grid-type converter; The Thevenin equivalent voltage amplitude on the grid side; This refers to the maximum value of the low-voltage ride-through activation threshold for grid-connected converters. Let be the resultant current phasor of the output current of all grid-connected converters at the point of common coupling, where This represents a vector summation of the output current phasors of all grid-connected converters. This represents the vector summation current amplitude at the point of common coupling for the output current of all grid-connected converters. The vector summation current phase angle at the point of common coupling for the output current of all grid-connected converters; For the first The output current phasor of the grid-type converter, among which For the first The output current of the grid-connected converter For the first Phase angle of output current of grid-type converters; Based on the network current phasor relationship, we have: ， In the formula, This represents the magnitude of the equivalent impedance of the power grid. The equivalent impedance magnitude of the power grid and the first The sum of the moduli of the branch impedances of the grid-type converter. For the first i Taiwan-type grid converter branch admittance, and They are and The modulus; To match the magnitude of the equivalent voltage phasor of the grid converter, The phase angle of the equivalent voltage phasor; This indicates that the equivalent voltage phasor of the grid converter leads its phase angle. Phasor after 90-degree rotation; This indicates that the resultant current phasor of all grid-connected converter output currents at the point of common coupling leads its phase angle. Phasor after 90-degree rotation; Indicates the first The output current phasor of the grid-connected converter leads its phase angle. Phasors after 90-degree rotation; maximum permissible power angle of grid-connected converters to ensure that all grid-connected converters do not trigger undervoltage ride-through. The determination of is discussed in three scenarios: if satisfies In a network-connected hybrid system operating under unconditional security conditions, There are no valid constraints; if satisfied This indicates that the output of new energy sources is too large, and voltage safety cannot be guaranteed through power angle constraints; when neither of these conditions is met, i.e., the following triangle inequality applies. At the time of its establishment, There exists a unique and definite value, calculated as follows: 。 3. The method according to claim 1, characterized in that: In step 1, let To avoid overvoltage exceeding the high voltage ride-through threshold, the minimum allowable power angle of the grid-type converter is calculated as follows: ， In the formula, Minimum allowable power angle for grid-type converters to avoid overvoltage exceeding limits; The network admittance ratio is the coefficient. Y M and Y g These are the branch admittance of the grid-type converter and the equivalent admittance of the power grid, respectively. This refers to the terminal voltage amplitude of the grid-type converter; The Thevenin equivalent voltage amplitude on the grid side; This is the magnitude of the equivalent voltage phasor of the grid-type converter.

4. The method according to claim 1, characterized in that: In step 1, let The maximum current limit for the grid-connected converter, and the upper limit of the critical power angle for the grid-connected converter in constant voltage control mode. Lower critical power angle of grid-type converter maintaining constant voltage control mode The output current of the grid-type converter does not exceed The conditions are determined by the derivation: ， In the formula, The phase compensation angle is determined by the relative relationship between the grid-side current of the grid-type converter and the output current of the grid-connected converter at the point of common coupling. d These are intermediate normalized parameters obtained by comprehensively calculating various electrical quantities in a grid-connected system. and d The expression is: ， ， In the formula, This refers to the maximum current limit value for grid-type converters. The equivalent admittance of the power grid; The Thevenin equivalent voltage amplitude on the grid side; This refers to the terminal voltage amplitude of the grid-type converter; The vector summation current amplitude at the point of common coupling for the output current of all grid-connected converters; The vector summation current phase angle at the point of common coupling for the output current of all grid-connected converters; The network admittance ratio coefficient; yes The modulus.

5. The method according to claim 1, characterized in that: In step 2, let The virtual inertia coefficient for a grid-type converter. The damping coefficient is... and These are the angular frequencies of the grid-type converter and the grid angular frequency, respectively. For the frequency deviation of the grid-type converter, δ For grid-type converters, The power angle at the stable equilibrium point of the grid-type converter under constant voltage control mode. The actual output active power of the grid-type converter. As the active power reference value, the energy function of a grid-type converter is defined as follows: In the formula, For grid-type converters with power angle Frequency deviation Lyapunov energy functions for state variables characterize the grid converter at the operating point. Transient energy at the point; frequency limiting threshold The quantitative design is based on the following stability criteria: ， The stability criterion represents the transient energy of the grid converter at the frequency limiting trigger moment. Subtract the lower bound of damped dissipation energy Afterwards, it should be less than the upper limit of the safe operation of the power angle of the grid-type converter. Lower limit of safe operation of grid-type converter power angle Minimum energy at the location This ensures that the operating trajectory of the grid-type converter is constrained within the safe operating range of the grid-type converter's power angle after frequency limiting triggering. Inside; among them, The power angle of the grid-type converter at the frequency limiting trigger moment. For frequency limiting threshold, This represents the transient energy of the grid-type converter at the moment of frequency limiting triggering. and The power angle of the grid-type converter reaches the upper limit of safe operation. and the lower world And the transient energy corresponding to zero frequency deviation, This is the lower bound of the damping dissipation energy from the frequency limiting trigger moment until the grid-type converter's power angle reaches the safe operating boundary; Based on the worst-case failure condition, the estimated value is: ， In the formula, Duration of the fault; This refers to the terminal voltage amplitude of the grid-type converter; The vector summation current amplitude at the point of common coupling for the output current of all grid-connected converters; The network admittance ratio; satisfying the above criteria The upper bound is obtained through numerical solution.

6. The method according to claim 1, characterized in that: In step 3, the coordinated triggering logic for triggering frequency limiting control and simultaneously freezing the power angle integral stage is as follows: when a fault occurs and When the enable signal is set to 1, the frequency limiting will be... Pincer At the same time, the input of the power angle integral stage is forced to zero, so that the power angle of the grid-type converter is reduced. The value is maintained at the time of the limiting trigger; when the condition is not met, the enable signal is set to 0, neither of the two controls intervenes, and the grid-type converter maintains the original constant voltage control logic.

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