Torque difference feedforward control-based low-frequency oscillation damping method and system for multi-network-construction converter parallel system, and computer readable storage medium

Abstract

The invention discloses a torque difference feedforward control-based low-frequency oscillation damping method for a multi-network-construction converter parallel system, which comprises the following steps of: decomposing a network-construction converter VSG multi-machine parallel system controlled by a virtual synchronous machine into a VSG single machine part and a residual VSG parallel multi-machine part, and transforming an active frequency link of the VSG single machine; adding a torque difference feed-forward loop; the residual VSG parallel multi-machine part is used for comparing with the single-machine part; and based on the improved rotor expression # imgabs0 #, executing a torque difference feed-forward coefficient algorithm based on an equal-area rule and discrete iterative calculation so as to determine a torque difference coefficient range boundary for realizing low-frequency oscillation damping of the system. According to the method, the low-frequency oscillation characteristic of the VSG multi-machine parallel system can be effectively improved, the low-frequency oscillation instability risk after the VSG multi-machine parallel system is disturbed in a weak power grid scene is reduced, and the low-frequency small-disturbance stability of the system is improved.

Description

Technical Field

[0001] The invention belongs to the field of power electronics technology, and in particular relates to a low-frequency oscillation damping method, system and computer-readable storage medium for a multi-grid converter parallel system based on torque difference feedforward control. Background Art

[0002] With the continuous development of renewable energy technologies such as wind power generation and photovoltaic power generation, more and more renewable energy sources are connected to the power grid through grid-connected inverter devices. Due to the special geographical environment in which my country's new energy power generation bases and load centers are distributed inversely, and as the proportion of renewable energy power generation systems connected to the power grid using power electronic devices gradually increases in the power system, the low inertia, low damping, and discretization characteristics of the power electronic devices themselves cannot be ignored. Due to the reduction in damping, the system may experience voltage and frequency instability under external disturbances, which seriously affects the safe and stable operation and power supply reliability of the power system. Virtual Synchronous Generator (VSG) can simulate the output characteristics of synchronous generators such as inertia and damping, and can participate in the frequency and voltage regulation of the power grid like synchronous generators. It is a good solution to improve the stability of new energy grid-connected systems. However, VSG is a power electronic device, and its anti-disturbance ability is still far inferior to that of traditional synchronous generators. Especially when multiple VSGs are operated in parallel, the interaction between VSG monomers after the system is disturbed and the interaction between VSG and weak power grids greatly increase the risk of low-frequency oscillation of the entire system. Therefore, the present invention aims at the VSG multi-machine parallel system, studies the control strategy improvement for the VSG monomer and realizes the effective stabilization strategy design for the multi-machine parallel system, which has very important theoretical and practical engineering significance for the safe and stable operation of the VSG multi-machine parallel system. Summary of the invention

[0003] In view of the above problems, the present invention proposes a low-frequency oscillation damping method for a multi-grid converter parallel system based on torque difference feedforward control. The basic idea of ​​this control method is to introduce the imbalance between the input reference torque and the output electromagnetic torque of the converter system into the acceleration and deceleration process of the system, thereby reducing the acceleration and deceleration intensity of the original system, and at the same time providing a certain initial velocity for the system, thereby ensuring that the system can quickly reach a steady state, and when the system reaches a steady state, the introduced torque difference becomes 0, so the proposed control method will not affect the steady-state characteristics of the system. On this basis, a torque difference coefficient design method based on the equal area rule and discrete iterative calculation is proposed.

[0004] The technical solution of the present invention is: a method for damping low-frequency oscillation of a multi-grid converter parallel system based on torque difference feedforward control, comprising:

[0005] The VSG multi-machine parallel system of the grid-connected converter controlled by the virtual synchronous machine is decomposed into a VSG single machine part and a remaining VSG parallel multi-machine part, the active frequency link of the VSG single machine is transformed, and a torque difference feedforward loop is added; the remaining VSG parallel multi-machine part is used for comparison with the single machine part;

[0006] Based on the improved rotor expression A torque difference feedforward coefficient algorithm based on equal area rule and discrete iterative calculation is executed to determine the torque difference coefficient range boundary for achieving low frequency oscillation damping of the system.

[0007] Furthermore, the multi-machine parallel system of the Virtual Synchronous Generator (VSG) controlled by the virtual synchronous machine is decomposed into a VSG single machine part and a remaining VSG parallel multi-machine part; the remaining VSG parallel multi-machine part is used for comparison with the single machine part; by adding a torque difference feedforward loop to the single machine, the low-frequency oscillation suppression of the whole system of the single machine + the remaining multi-machines is effectively achieved; the active frequency link of the VSG single machine is transformed, and a torque difference feedforward loop is added to establish a virtual synchronous machine system model based on torque difference feedforward. Through this model, the specific topological structure of the torque difference feedforward is explained. Because the position of the feedforward is different, its topology is also quite different;

[0008] It includes the following sub-steps:

[0009] S11. Decompose the VSG multi-machine parallel system into two parts: a single VSG machine and the remaining VSG parallel multi-machine system. The two parts are decentralized and the coordinated control of the entire system does not rely on any communication facilities, such as Figure 1 As shown;

[0010] S12, transform the active frequency link of the VSG unit and add a torque difference feedforward control loop, such as Figure 2 As shown, the VSG output frequency can be expressed as:

[0011]

[0012] In the above formula: ω N represents the angular frequency generated by the traditional VSG; ω VSG0 represents the angular frequency generated by the traditional VSG; ω VSG It represents the actual output angular frequency of VSG after synthesis; P ref Indicates the VSG output active power given value; P e Indicates the actual value of VSG output active power; K TEFF Represents the feedforward coefficient of the torque difference feedforward loop.

[0013] S13. Combining the second-order rotor equation of the traditional VSG and the torque difference control model, the updated rotor equation expression of the virtual synchronous machine system based on torque difference feedforward is obtained:

[0014]

[0015] In the above formula: J represents the moment of inertia coefficient of the VSG active frequency link; D represents the virtual damping coefficient of the VSG active frequency link; U represents the VSG output voltage; U g represents the grid voltage; δ represents the power angle difference between the potential inside the VSG and the grid voltage. It can be seen from the formula that the torque difference feedforward coefficient K TEFF The introduction of adds a time-varying damping term to the system; the larger its value, the greater the damping and the better the anti-disturbance performance of the system; but at the same time, the frequency mutation of the system also increases accordingly, which is not conducive to the rapid stabilization of the system.

[0016] The source of the algorithm is: The equal area law is that in the transient process of the power system, the changes of various electrical quantities show the law of time-amplitude equal area changes. In the same time, the change areas of various electrical quantities are equal. In the ferromagnetic resonance problem, the equal area law is used to solve the relationship between the acceleration area and the deceleration area to determine whether the system will enter the resonance area.

[0017] The discrete iterative calculation is to approach the optimal result step by step, so that the selected torque difference feedforward coefficient can damp the low-frequency oscillation of the system.

[0018] The relationship between the results obtained after executing the algorithm and the low-frequency oscillation damping:

[0019] The use of torque difference feedforward can introduce time-varying damping to ensure that the system does not become unstable during transient periods, and the torque difference feedforward coefficient has an important influence on the magnitude of the introduced damping and the magnitude of the frequency mutation. If the torque difference coefficient is too large, the larger the frequency mutation introduced, the greater the kinetic energy of the system will be, which will lead to system instability. If the torque difference coefficient is too small, it may lead to insufficient damping in the system, causing the inverter to continue to oscillate and become unstable for a long time during fault recovery. This algorithm is used to determine the torque difference coefficient range boundary that effectively realizes the system's low-frequency oscillation damping.

[0020] The comparison between the multi-machine part and the single-machine part shows that the single-machine introduces torque difference and achieves effective stabilization of the entire system without the help of communication facilities. Compared with most existing control strategies that require the modification of each single-machine control of the system and the use of communication equipment to achieve stable control of the entire system, this method is more practical and economical.

[0021] The result of the transformation is:

[0022] The rotor motion equation before transformation:

[0023]

[0024] The rotor motion equation after transformation:

[0025]

[0026] After the transformation, the original constant damping term in the virtual synchronous machine rotor equation based on torque difference feedforward is changed into a time-varying damping term related to the power angle change rate. By introducing the imbalance degree between the input torque and the output electromagnetic torque into the damping term, it can be found that the greater the imbalance degree, the greater the damping of the system, and vice versa. Therefore, it is beneficial for the system to achieve rapid stabilization.

[0027] Furthermore, based on the opposite effects of the time-varying damping introduced by the feedforward loop and the VSG frequency mutation on the transient stability of the system, a design idea of ​​the torque difference feedforward coefficient based on the equal area rule and discrete iterative calculation is proposed, which specifically includes the following steps:

[0028] S21. At the moment of disturbance recovery, the system input reference power is instantaneously adjusted from the fault reference active power to the rated reference active power, causing the introduced frequency term related to the unbalanced torque to undergo a sudden change, injecting a larger initial forward motion velocity into the system. The frequency mutation introduced by the system can be specifically calculated as follows:

[0029] The dq-axis steady-state component of the inverter LC filter port voltage can be expressed as:

[0030]

[0031] At the same time, U gd with U gq are the dq axis components of the grid voltage, respectively, which can be expressed as:

[0032]

[0033] According to the instantaneous active power calculation formula:

[0034]

[0035] Among them, the dq axis current can be calculated as:

[0036]

[0037] The angle of the dq axis current components can be calculated using the ratio of active to reactive power when current limiting is not used:

[0038]

[0039] Where δ represents the steady-state power angle.

[0040] Therefore, the output active power mutation of the VSG inverter can be calculated as shown in the formula:

[0041]

[0042] Then the frequency mutation introduced by the torque difference feedforward term can be calculated as:

[0043] Δω VSG =ΔP ref -ΔP e (9)

[0044] S22. For the time-varying damping and frequency mutation expressions introduced by the torque feedforward term, combined with the equal-area rule and using the discrete iterative calculation idea, the feedforward coefficient design idea can be characterized as follows, and its flow chart is as follows: Figure 3 :

[0045] The first step is parameter initialization; considering that the fault depth is greater than 80%, the inverter can be directly cut off from the grid according to the grid guideline standard. Therefore, the deepest voltage drop of 80% during the system oscillation can be selected to calculate the torque difference feedforward coefficient, and the relevant parameters at this time can be calculated respectively;

[0046] The second step is to perform parameter forward iteration calculation, and use the rectangular approximation curve to discretely calculate the frequency of a power angle at each power angle, where the acceleration and deceleration area demarcation point δ is set. a , it is in an accelerated state before the dividing point, then the frequency calculation formula is as follows:

[0047]

[0048] If the power angle of the discrete iteration is greater than the cutoff point, the frequency distribution is calculated as follows:

[0049]

[0050] The third step is to determine whether it is possible to decelerate to 0 before the overcurrent protection triggers the power angle. If it is not possible to decelerate to 0, it means that the system does not provide enough damping at this time and needs to continue to increase the torque difference feedforward coefficient; otherwise, reduce the torque difference feedforward coefficient; and finally obtain the optimal torque difference feedforward coefficient value for the system.

[0051] Among them, the equal area law is that in the transient process of the power system, the changes of various electrical quantities show the law of time-amplitude equal area changes; in the same time, the change areas of various electrical quantities are equal; in the ferromagnetic resonance problem, the equal area law is used to solve the relationship between the acceleration area and the deceleration area to determine whether the system will enter the resonance area;

[0052] The discrete iterative calculation is to approach the optimal result step by step, so that the selected torque difference feedforward coefficient can damp the low-frequency oscillation of the system.

[0053] Furthermore, a real-time simulation model of a multi-grid converter parallel system is established to verify the effectiveness of the torque difference feedforward control and the correctness of the torque difference feedforward coefficient design method, including:

[0054] S31. Establish a real-time simulation model based on the system circuit model to verify the effectiveness of the torque difference feedforward control strategy. The simulation results are as follows: Figure 4 , 5 As shown;

[0055] S32, set different torque difference feedforward coefficients to verify the correctness of the torque difference feedforward coefficient design analysis ideas. The simulation results are as follows: Figure 6 , 7 shown.

[0056] A low-frequency oscillation damping system of a multi-grid converter parallel system based on torque difference feedforward control is also proposed, which is characterized by comprising:

[0057] The virtual synchronous machine system model building module is used to decompose the VSG multi-machine parallel system of the grid-connected converter controlled by the virtual synchronous machine into a VSG single machine part and a remaining VSG parallel multi-machine part, transform the active frequency link of the VSG single machine, and add a torque difference feedforward loop; the remaining VSG parallel multi-machine part is used for comparison with the single machine part;

[0058] Torque difference coefficient range boundary determination module for the improved rotor expression A torque difference feedforward coefficient algorithm based on equal area rule and discrete iterative calculation is executed to determine the torque difference coefficient range boundary for achieving low frequency oscillation damping of the system.

[0059] A computer-readable storage medium is also proposed, wherein a computer program is stored in the computer-readable storage medium, and when the computer program is executed by a processor, the steps of any one of the methods of claims 1 to 4 are implemented.

[0060] Technical effects and advantages of the present invention:

[0061] The present invention effectively improves the low-frequency oscillation characteristics of the VSG multi-machine parallel system, reduces the risk of low-frequency oscillation instability after the VSG multi-machine parallel system is disturbed in the weak power grid scenario, and improves the low-frequency small disturbance stability of the system. At the same time, the control method proposed in the present invention completely gets rid of the dependence on communication, and is a typical decentralized form, which overcomes the problem that the existing centralized and decentralized low-frequency oscillation suppression methods need to rely on communication, and improves the flexibility and scalability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 The topological structure of the VSG multi-machine parallel system of the present invention;

[0063] Figure 2 This is a control block diagram of a virtual synchronous machine based on torque difference feedforward according to the present invention;

[0064] Figure 3 The invention provides a torque difference coefficient design method based on the equal area rule and discrete iterative calculation;

[0065] Figure 4 This is a simulation result diagram of the oscillation divergence of the torque difference feedforward system of the present invention;

[0066] Figure 5 This is a simulation result diagram of oscillation convergence of a system with a small feedforward coefficient according to the present invention;

[0067] Figure 6 This is a simulation result diagram of the feedforward coefficient being moderate and effective in stabilizing the present invention;

[0068] Figure 7 This is a simulation result diagram of bounded oscillation caused by a large feedforward coefficient of the present invention. DETAILED DESCRIPTION

[0069] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0070] Embodiment 1

[0071] During specific implementation: The present invention takes the VSG dual-machine system as the research object, in which VSG1 introduces torque difference feedforward control, and VSG2 does not perform any processing. Table 1 gives the relevant parameters of VSG1, Table 2 shows the relevant parameters of VSG2, and Table 3 shows the relevant parameters of the infinite power grid. Its operating conditions are characterized as follows: first, VSG1 and VSG2 operate normally with isolated loads; then VSG1 is connected to the grid and operates normally; then VSG2 is connected to the grid, and the system oscillates and becomes unstable. On this basis, torque difference feedforward control is introduced to VSG1, and the stability of the system under different torque difference feedforward coefficients is compared to verify the effectiveness of the torque difference feedforward control method and the correctness of the torque difference feedforward coefficient design method.

[0072] Table 1 VSG1 parameters

[0073]

[0074] Table 2 VSG2 parameters

[0075]

[0076]

[0077] Table 3 Infinite power grid parameters

[0078]

[0079] The results are as follows: Figure 4 , 5 , 6, and 7 respectively give the torque difference feedforward coefficient K when the system does not use torque difference feedforward TEFF =0.05, K TEFF =0.2, K TEFF =3.22, the active power response curve of each VSG in the system. From the simulation results, it can be concluded that: when K TEFF =0.05, the output power of VSG changes from oscillation divergence to oscillation convergence. However, due to the small torque difference feedforward coefficient, the damping provided by the torque difference feedforward term is insufficient, so the system still cannot quickly enter a stable state during the disturbance recovery process. However, compared with the case without torque difference feedforward, the oscillation form of the system gradually converges, which proves the rationality of introducing the torque difference feedforward term to a certain extent. When K TEFF =0.2, the output power of the VSG decreases rapidly and continuously and converges to the rated value. The system can recover and maintain stable operation after being disturbed and oscillating. When the torque difference feedforward coefficient continues to increase to 3.22, although the system does not oscillate and diverge and cause instability, bounded oscillations of the power angle and frequency occur, which in turn causes bounded oscillations in the output power of the VSG, proving that the system cannot remain stable at the operating point, and proving the effectiveness of the torque difference feedforward upper bound design based on small signal analysis.

[0080] Embodiment 2

[0081] The present invention proposes a method for damping low-frequency oscillation of a multi-grid converter parallel system based on torque difference feedforward control. The topology of the VSG multi-machine parallel system is as follows: Figure 1 As shown; wherein the torque difference feedforward control is introduced in the active synchronization link of VSG1, and the remaining VSG multi-machine system does not perform any processing; the virtual synchronous machine control block diagram based on torque difference feedforward proposed in the present invention is as follows Figure 2 The present invention effectively improves the low-frequency oscillation characteristics of the VSG multi-machine parallel system, reduces the risk of low-frequency oscillation instability after the VSG multi-machine parallel system is disturbed in a weak power grid scenario, and improves the low-frequency small disturbance stability of the system.

[0082] The source of the algorithm is: The equal area law is that in the transient process of the power system, the changes of various electrical quantities show the law of time-amplitude equal area changes. In the same time, the change areas of various electrical quantities are equal. In the ferromagnetic resonance problem, the equal area law is used to solve the relationship between the acceleration area and the deceleration area to determine whether the system will enter the resonance area.

[0083] The discrete iterative calculation is to approach the optimal result step by step, so that the selected torque difference feedforward coefficient can damp the low-frequency oscillation of the system.

[0084] The relationship between the results obtained after executing the algorithm and the low-frequency oscillation damping:

[0085] The use of torque difference feedforward can introduce time-varying damping to ensure that the system does not become unstable during transient periods, and the torque difference feedforward coefficient has an important influence on the magnitude of the introduced damping and the magnitude of the frequency mutation. If the torque difference coefficient is too large, the larger the frequency mutation introduced, the greater the kinetic energy of the system will be, which will lead to system instability. If the torque difference coefficient is too small, it may lead to insufficient damping in the system, causing the inverter to continue to oscillate and become unstable for a long time during fault recovery. This algorithm is used to determine the torque difference coefficient range boundary that effectively realizes the system's low-frequency oscillation damping.

[0086] Embodiment 3

[0087] A low-frequency oscillation damping system for a multi-grid converter parallel system based on torque difference feedforward control, characterized in that it comprises:

[0088] The virtual synchronous machine system model building module is used to decompose the virtual synchronous machine control virtual synchronous generator (VSG) multi-machine parallel system into a VSG single machine part and the remaining VSG parallel multi-machine part, transform the active frequency link of the VSG single machine, add a torque difference feedforward loop, and establish a virtual synchronous machine system model based on torque difference feedforward;

[0089] Torque difference coefficient range boundary determination module for the improved rotor expression A torque difference feedforward coefficient algorithm based on equal area rule and discrete iterative calculation is executed to determine the torque difference coefficient range boundary for achieving low frequency oscillation damping of the system.

[0090] Furthermore, a virtual synchronous machine system model building module is used to decompose a virtual synchronous machine control grid-connected converter (Virtual Synchronous Generator, VSG) multi-machine parallel system into a VSG single machine part and a remaining VSG parallel multi-machine part, transform the active frequency link of the VSG single machine, add a torque difference feedforward loop, and establish a virtual synchronous machine system model based on torque difference feedforward; specifically, it includes the following sub-steps:

[0091] 1.1. Decompose the VSG multi-machine parallel system into two parts: VSG single machine and remaining VSG parallel multi-machine system. The two parts are decentralized controlled, and the coordinated control of the entire system does not rely on any communication facilities;

[0092] 1.2. The active frequency link of the VSG unit is modified and a torque difference feedforward control loop is added. Then the VSG output frequency can be expressed as:

[0093]

[0094] In the above formula: ω N represents the angular frequency generated by the traditional VSG; ω VSG0 represents the angular frequency generated by the traditional VSG; ω VSG It represents the actual output angular frequency of VSG after synthesis; P ref Indicates the VSG output active power given value; P e Indicates the actual value of VSG output active power; K TEFF Represents the feedforward coefficient of the torque difference feedforward loop.

[0095] 1.3. Combining the second-order rotor equation of the traditional VSG and the torque difference control model, the updated rotor equation expression of the virtual synchronous machine system based on torque difference feedforward is obtained:

[0096]

[0097] In the above formula: J represents the moment of inertia coefficient of the VSG active frequency link; D represents the virtual damping coefficient of the VSG active frequency link; U represents the VSG output voltage; U g represents the grid voltage; δ represents the power angle difference between the potential inside the VSG and the grid voltage. It can be seen from the formula that the torque difference feedforward coefficient K TEFF The introduction of adds a time-varying damping term to the system; the larger its value, the greater the damping and the better the anti-disturbance performance of the system; but at the same time, the frequency mutation of the system also increases accordingly, which is not conducive to the rapid stabilization of the system.

[0098] Furthermore, a torque difference coefficient range boundary determination module is used to propose a torque difference feedforward coefficient design method based on the equal area rule and discrete iterative calculation based on the opposite effects of the time-varying damping introduced by the feedforward loop and the VSG frequency mutation on the transient stability of the system; specifically, the following steps are included:

[0099] 2.1. At the moment of disturbance recovery, the system input reference power is instantaneously adjusted from the fault reference active power to the rated reference active power, causing the introduced frequency term related to the unbalanced torque to undergo a sudden change, injecting a larger initial forward motion velocity into the system. The frequency mutation introduced by the system can be specifically calculated as follows:

[0100] The dq-axis steady-state component of the inverter LC filter port voltage can be expressed as:

[0101]

[0102] At the same time, U gd with U gq are the dq axis components of the grid voltage, respectively, which can be expressed as:

[0103]

[0104] According to the instantaneous active power calculation formula:

[0105]

[0106] Among them, the dq axis current can be calculated as:

[0107]

[0108] The angle of the dq axis current components can be calculated using the ratio of active to reactive power when current limiting is not used:

[0109]

[0110] where δ F Characterizes the steady-state power angle.

[0111] Therefore, the output active power mutation of the VSG inverter can be calculated as shown in the formula:

[0112]

[0113] Then the frequency mutation introduced by the torque difference feedforward term can be calculated as:

[0114] Δω VSG =ΔP ref -ΔP e

[0115] 2.2. For the expressions of time-varying damping and frequency mutation introduced by the torque feedforward term, combined with the equal-area rule and using the discrete iterative calculation idea, the design idea of ​​the feedforward coefficient can be characterized as follows:

[0116] The first is parameter initialization; considering that the fault depth is greater than 80%, the inverter can be directly cut off from the grid according to the grid guideline standard. Therefore, the deepest voltage drop of 80% during the system oscillation can be selected to calculate the torque difference feedforward coefficient, and the relevant parameters at this time can be calculated respectively;

[0117] The second is to perform parameter forward iteration calculation, and use the rectangular approximation curve to discretely calculate the frequency of a power angle at each power angle, where the acceleration and deceleration area demarcation point δ is set. a , it is in an accelerated state before the dividing point, then the frequency calculation formula is as follows:

[0118]

[0119] If the power angle of the discrete iteration is greater than the cutoff point, the frequency distribution is calculated as follows:

[0120]

[0121] The third is to determine whether it is possible to decelerate to 0 before the overcurrent protection triggers the power angle. If it cannot decelerate to 0, it means that the system does not provide enough damping at this time and needs to continue to increase the torque difference feedforward coefficient; otherwise, reduce the torque difference feedforward coefficient; and finally obtain the optimal torque difference feedforward coefficient value for the system.

[0122] Furthermore, it also includes a simulation model establishment and verification module, which is used to establish a real-time simulation model of a multi-grid converter parallel system to verify the effectiveness of the torque difference feedforward control and the correctness of the torque difference feedforward coefficient design method; specifically, it includes the following steps:

[0123] 3.1. Establish a real-time simulation model based on the system circuit model to verify the effectiveness of the torque difference feedforward control strategy;

[0124] 3.2. Set different torque difference feedforward coefficients to verify the correctness of the design and analysis ideas of the torque difference feedforward coefficient.

[0125] Embodiment 4

[0126] A computer-readable storage medium, characterized in that a computer program is stored in the computer-readable storage medium, and when the computer program is executed by a processor, the steps of any one of the methods of claims 1-4 are implemented.

[0127] Although the specific embodiments of the present invention are described above in conjunction with the accompanying drawings, it should be understood by those skilled in the art that these are only examples, and that various variations or modifications may be made to these embodiments without departing from the principles and essence of the present invention. The scope of the present invention is limited only by the appended claims.

Claims

1. A method for damping low-frequency oscillation of a multi-grid converter parallel system based on torque difference feedforward control, characterized in that: The method comprises the following steps: The VSG multi-machine parallel system of the grid-connected converter controlled by the virtual synchronous machine is decomposed into a VSG single machine part and a remaining VSG parallel multi-machine part, the active frequency link of the VSG single machine is transformed, and a torque difference feedforward loop is added; the remaining VSG parallel multi-machine part is used for comparison with the single machine part; Based on the improved rotor expression A torque difference feedforward coefficient algorithm based on equal area rule and discrete iterative calculation is executed to determine the torque difference coefficient range boundary for achieving low frequency oscillation damping of the system.

2. According to claim 1, a method for damping low-frequency oscillation of a multi-grid converter parallel system based on torque difference feedforward control is characterized in that: The active frequency link of the VSG single machine is modified, a torque difference feedforward loop is added, and a virtual synchronous machine system model based on torque difference feedforward is established; It includes the following sub-steps: The VSG multi-machine parallel system is decomposed into two parts: a single VSG machine and the remaining VSG parallel multi-machine system. The two parts are controlled in a decentralized manner, and the coordinated control of the entire system does not rely on any communication facilities. The active frequency link of the VSG unit is modified and a torque difference feedforward control loop is added. Then the VSG output frequency can be expressed as: In the above formula: ω N represents the angular frequency generated by the traditional VSG; ω VSG0 represents the angular frequency generated by the traditional VSG; ω VSG It represents the actual output angular frequency of VSG after synthesis; P ref Indicates the VSG output active power given value; P e Indicates the actual value of VSG output active power; K TEFF represents the feedforward coefficient of the torque difference feedforward loop; Combining the second-order rotor equation of the traditional VSG and the torque difference control model, the updated rotor equation expression of the virtual synchronous machine system based on torque difference feedforward is obtained: In the above formula: J represents the moment of inertia coefficient of the VSG active frequency link; D represents the virtual damping coefficient of the VSG active frequency link; U represents the VSG output voltage; U g represents the grid voltage; δ represents the power angle difference between the potential inside the VSG and the grid voltage; It can be seen from the formula that the torque difference feedforward coefficient K TEFF The introduction of adds a time-varying damping term to the system; The larger the value, the greater the damping and the better the system's anti-disturbance performance; but at the same time, the frequency mutation of the system also increases accordingly, which is not conducive to the rapid stabilization of the system.

3. According to claim 1, a method for damping low-frequency oscillation of a multi-grid converter parallel system based on torque difference feedforward control is characterized in that: The active frequency link of the VSG stand-alone machine is transformed, and the rotor motion equation after the transformation is:

4. The method for damping low-frequency oscillation of a multi-grid converter parallel system based on torque difference feedforward control according to claim 1 is characterized in that: Based on the opposite effects of the time-varying damping introduced by the feedforward loop and the VSG frequency mutation on the transient stability of the system, that is, based on the improved rotor expression: The torque difference feedforward coefficient algorithm based on the equal area rule and discrete iterative calculation is executed to determine the torque difference coefficient range boundary for achieving low-frequency oscillation damping of the system, specifically including the following steps: At the moment of disturbance recovery, the system input reference power is instantaneously adjusted from the fault reference active power to the rated reference active power. The frequency mutation introduced by the system can be specifically calculated as follows: The dq-axis steady-state component of the inverter LC filter port voltage can be expressed as: At the same time, U gd with U gq are the dq axis components of the grid voltage, respectively, which can be expressed as: According to the instantaneous active power calculation formula: Among them, the dq axis current can be calculated as: The angle of the dq axis current components can be calculated using the ratio of active to reactive power when current limiting is not used: where δ F Characterize the steady-state power angle; Therefore, the output active power mutation of the VSG inverter can be calculated as shown in the formula: Then the frequency mutation introduced by the torque difference feedforward term can be calculated as: Give VSG =ΔP ref -ΔP e According to the expressions of time-varying damping and frequency mutation introduced by the torque feedforward term, combined with the equal-area rule and using the discrete iterative calculation method, the feedforward coefficient calculation method is characterized as follows: Take 80% of the deepest voltage drop during system oscillation to calculate the torque difference feedforward coefficient, and calculate the relevant parameters at this time; At each power angle, the rectangular approximation curve is used to discretely calculate the frequency of a power angle under the frequency, where the delimiting point of the acceleration and deceleration area δ is set a , it is in an accelerated state before the dividing point, and the frequency calculation formula is as follows: If the power angle of the discrete iteration is greater than the cutoff point, the frequency distribution is calculated as follows: Determine whether it is possible to decelerate to 0 before the overcurrent protection triggers the power angle. If it is not possible to decelerate to 0, continue to increase the torque difference feedforward coefficient; otherwise, reduce the torque difference feedforward coefficient; and finally obtain the optimal torque difference feedforward coefficient value for the system.

5. The method for damping low-frequency oscillation of a multi-grid converter parallel system based on torque difference feedforward control according to claim 1, characterized in that: It also includes establishing a real-time simulation model of a multi-grid converter parallel system to verify the effectiveness of the torque difference feedforward control and the correctness of the torque difference feedforward coefficient design method, which specifically includes the following steps: A real-time simulation model is established based on the system circuit model to verify the effectiveness of the torque difference feedforward control strategy; Set different torque difference feedforward coefficients to verify the correctness of the design and analysis ideas of the torque difference feedforward coefficient.

6. A low-frequency oscillation damping system for a multi-grid converter parallel system based on torque difference feedforward control, characterized in that: include: The virtual synchronous machine system model building module is used to decompose the VSG multi-machine parallel system of the grid-connected converter controlled by the virtual synchronous machine into a VSG single machine part and a remaining VSG parallel multi-machine part, transform the active frequency link of the VSG single machine, and add a torque difference feedforward loop; the remaining VSG parallel multi-machine part is used for comparison with the single machine part; Torque difference coefficient range boundary determination module for the improved rotor expression A torque difference feedforward coefficient algorithm based on equal area rule and discrete iterative calculation is executed to determine the torque difference coefficient range boundary for achieving low frequency oscillation damping of the system.

7. A low-frequency oscillation damping system for a multi-grid converter parallel system based on torque difference feedforward control according to claim 6, characterized in that: The virtual synchronous machine system model building module is used to decompose the VSG multi-machine parallel system of the grid-connected converter controlled by the virtual synchronous machine into a VSG single machine part and a remaining VSG parallel multi-machine part, transform the active frequency link of the VSG single machine, and add a torque difference feedforward loop; the remaining VSG parallel multi-machine part is used for comparison with the single machine part; It includes the following sub-steps: The VSG multi-machine parallel system is decomposed into two parts: a single VSG machine and the remaining VSG parallel multi-machine system. The two parts are controlled in a decentralized manner, and the coordinated control of the entire system does not rely on any communication facilities. The active frequency link of the VSG unit is modified and a torque difference feedforward control loop is added. Then the VSG output frequency can be expressed as: In the above formula: ω N represents the angular frequency generated by the traditional VSG; ω VSG0 represents the angular frequency generated by the traditional VSG; ω VSG It represents the actual output angular frequency of VSG after synthesis; P ref Indicates the VSG output active power given value; P e Indicates the actual value of VSG output active power; K TEFF represents the feedforward coefficient of the torque difference feedforward loop; Combining the second-order rotor equation of the traditional VSG and the torque difference control model, the updated rotor equation expression of the virtual synchronous machine system based on torque difference feedforward is obtained: In the above formula: J represents the moment of inertia coefficient of the VSG active frequency link; D represents the virtual damping coefficient of the VSG active frequency link; U represents the VSG output voltage; U g represents the grid voltage; δ represents the power angle difference between the potential inside the VSG and the grid voltage; It can be seen from the formula that the torque difference feedforward coefficient K TEFF The introduction of adds a time-varying damping term to the system; The larger the value, the greater the damping and the better the system's anti-disturbance performance; but at the same time, the frequency mutation of the system also increases accordingly, which is not conducive to the rapid stabilization of the system.

8. The low-frequency oscillation damping system of a multi-grid converter parallel system based on torque difference feedforward control according to claim 6 is characterized in that: Torque difference coefficient range boundary determination module for the improved rotor expression Executing a torque difference feedforward coefficient algorithm based on the equal area rule and discrete iterative calculation to determine the torque difference coefficient range boundary for achieving low-frequency oscillation damping of the system; specifically comprising the following steps: At the moment of disturbance recovery, the system input reference power is instantaneously adjusted from the fault reference active power to the rated reference active power. The frequency mutation introduced by the system can be specifically calculated as follows: The dq-axis steady-state component of the inverter LC filter port voltage can be expressed as: At the same time, U gd with U gq are the dq axis components of the grid voltage, respectively, which can be expressed as: According to the instantaneous active power calculation formula: Among them, the dq axis current can be calculated as: The angle of the dq axis current components can be calculated using the ratio of active to reactive power when current limiting is not used: where δ F Characterize the steady-state power angle; Therefore, the output active power mutation of the VSG inverter can be calculated as shown in the formula: Then the frequency mutation introduced by the torque difference feedforward term can be calculated as: Give VSG =ΔP ref -ΔP e According to the expressions of time-varying damping and frequency mutation introduced by the torque feedforward term, combined with the equal-area rule and using the discrete iterative calculation idea, the design idea of ​​the feedforward coefficient can be characterized as follows: Parameter initialization; Considering that the fault depth is greater than 80%, according to the grid guideline standard, the inverter can be directly cut off from the grid, so the deepest voltage drop of 80% during the system oscillation can be selected to calculate the torque difference feedforward coefficient, and the relevant parameters at this time can be calculated respectively; Perform parameter forward iteration calculation, and use the rectangular approximation curve to discretely calculate the frequency of a power angle at each power angle, where the acceleration and deceleration area demarcation point δ is set a , it is in an accelerated state before the dividing point, then the frequency calculation formula is as follows: If the power angle of the discrete iteration is greater than the cutoff point, the frequency distribution is calculated as follows: Determine whether it is possible to decelerate to 0 before the overcurrent protection triggers the power angle. If it is not possible to decelerate to 0, it means that the system does not provide enough damping at this time and needs to continue to increase the torque difference feedforward coefficient; otherwise, reduce the torque difference feedforward coefficient; and finally obtain the optimal torque difference feedforward coefficient value for the system.

9. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of any one of the methods of claims 1 to 4 are implemented.

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