A transient stability control method and system for a network-forming inverter

Abstract

The present invention discloses a transient stability control method and system for a network-forming inverter. The transient stability control method for the network-forming inverter based on the coordination of current limiting phase angle and reference power adaptively adjusts the current limiting angle and the magnitude of the reference power under different fault drop depths to limit the fault current of the network-forming inverter and ensure the stability of the power angle. This method can solve the risk of transient power angle instability caused by the current limiting link while stably limiting the current. The method of the present invention does not require additional control loops and additional devices, is simple and easy to implement, and is convenient for parameter adjustment. The method of the present invention can also be applied to other network-forming inverters and can be further popularized and applied to fault current limiting and transient power angle control in scenarios such as multi-machine grid connection and islanded microgrids.

Description

Technical Field

[0001] The present invention relates to new energy power generation control technology, in particular to a transient stability control method and system for a grid-forming inverter. Background Art

[0002] With the continuous maturity of new energy power generation technology, the penetration rate of renewable energy based on inverters in the power grid has increased rapidly. This has led to a decrease in the proportion of synchronous generators in the power grid, resulting in a reduction in the inertia of the power grid and endangering the safe and stable operation of the power grid when encountering disturbances. The grid-forming inverter combines the characteristics of synchronous generators and inverters, can simulate the dynamic characteristics of synchronous generators and provide inertia support, and has been widely accepted.

[0003] When the grid-forming inverter is subjected to a fault or disturbance impact, the significantly increased overcurrent is extremely likely to cause damage to the power semiconductor devices in the converter, thereby affecting the safe operation of the converter. In order to prevent the output current of the inverter from being too large and burning out the equipment during a fault, a current limiting link is usually added to the controller of the inverter to restrict the range of the output current. However, existing research has shown that the current limiting link will deteriorate the transient synchronous stability of the grid-forming inverter, bringing new challenges to the transient stable operation of the grid-forming inverter. How to ensure reliable current limiting while taking into account transient synchronous stability is the key to whether the grid-forming inverter grid-connected system can achieve reliable fault ride-through and is a technical problem that urgently needs to be solved. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a transient stability control method and system for a grid-forming inverter, which can reduce the risk of transient power angle instability caused by the current limiting link while stably limiting the current, aiming at the deficiencies of the existing technology.

[0005] To solve the above technical problems, the technical solution adopted by the present invention is: a transient stability control method for a grid-forming inverter, including the following steps:

[0006] S1. Collect the grid-side voltage;

[0007] S2. Judge the grid state according to the grid-side voltage;

[0008] S3. Regulate the reference power and the current limiting phase angle according to the grid state; wherein,

[0009] the regulation formula of the reference power P ref is: U g is the grid-side voltage, I lim is the limit value of the current limiting link, P N is the rated power of the grid-forming inverter, U cris the critical fault grid voltage;

[0010] is the critical current limiting phase angle.

[0011] The present invention first adjusts the reference power to ensure the existence of the equilibrium point (the existence of the equilibrium point is a prerequisite for power angle stability), and secondly uses the current limiting link to limit the current so that it is less than the current limit value I lim of the current limiting link. However, the limiting link may cause power angle instability. Therefore, this application adjusts the reference power and the current limiting phase angle according to the grid state, reducing the risk of transient power angle instability brought by the current limiting link while stably limiting the current.

[0012] The specific implementation process of step S2 includes: If U g = U N , it is judged that the grid is in normal working condition; if U g < U N , it is judged that the grid is in fault condition; where U N is the rated grid voltage.

[0013] When the grid is in fault condition, it further includes:

[0014] Calculating the transmission power of the grid-forming inverter after the fault The maximum transmission power P max = U g * I lim ; δ is the phase angle difference between the grid-forming inverter and the grid output voltage;

[0015] If P max > P ref , that is, U g > P ref / I lim , the grid-forming inverter has an equilibrium point; if P max < P ref , that is, U g < P ref / I lim , the grid-forming inverter has no equilibrium point.

[0016] The above process is simple, convenient and accurate, improving the calculation speed.

[0017] The calculation formula for the phase angle difference δ between the grid-forming inverter and the grid output voltage is: where D p is the damping coefficient, J is the virtual inertia, V g is the grid voltage amplitude, is the second derivative of the phase angle difference between the grid-forming inverter and the grid output voltage with respect to time, is the first derivative of the phase angle difference between the grid-forming inverter and the grid output voltage with respect to time.

[0018] Adjust the current limit phase angle The specific implementation process includes: 1) Initialize the current limit phase angle

[0019] 2) Calculate the phase angle difference δ between the grid-forming inverter and the grid output voltage, and determine whether δ converges to the steady-state value. If so, go to step 3); otherwise, update the value and repeat step 2);

[0020] 3) Determine the critical current limit phase angle Then In the present invention, the time derivative of the power angle is taken. If this value is equal to 0, it converges to the steady-state value, and the grid-forming inverter is determined to be stable; if it is not equal to 0, it is unstable.

[0021] Determine the critical current limit phase angle The implementation process includes:

[0022] a) Initialize the current limit phase angle to 0;

[0023] b) When δ converges to the steady-state value, the saturation angle at this time is the critical current limit phase angle End; when δ does not converge to the steady-state value, add the step size to the value and repeat step b).

[0024] Determine the critical current limit phase angle The implementation process includes:

[0025] A) Initialize the current limit phase angle to π;

[0026] B) When δ converges to the steady-state value, the saturation angle at this time is the critical current limit phase angle End; when δ does not converge to the steady-state value, subtract the step size from the value and repeat step B).

[0027] Saturation angle = -arctan(Iq / Id), where Id is the d-axis component of the current vector after saturation and Iq is the q-axis component of the current vector after saturation.

[0028] The process of adjusting the current limit phase angle in the present invention is accurately calculated, ensuring reliable current limiting and stable power angle.

[0029] As an inventive concept, the present invention also provides a transient stability control system for a grid-forming inverter, which includes a memory, a processor, and a computer program stored on the memory; the processor executes the computer program to implement the steps of the above method.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows: Based on the transient stability control method of the grid-forming inverter coordinated by current limiting phase angle and reference power, the current limiting angle and the magnitude of the reference power are adaptively adjusted under different fault drop depths to limit the fault current of the grid-forming inverter and ensure the stability of the power angle. While stably limiting the current, this method can solve the risk of transient power angle instability caused by the current limiting link. The method of the present invention does not require additional control loops and additional devices, is simple and easy to implement, and is convenient for parameter adjustment. The method of the present invention can also be applied to other grid-forming inverters and can be further popularized and applied to fault current limiting and transient power angle control in scenarios such as multi-machine grid connection and islanded microgrids. Description of the Drawings

[0031] Figure 1 It is the structure of the grid-forming inverter control system according to the embodiment of the present invention;

[0032] Figure 2 It is a schematic diagram of the current saturation angle;

[0033] Figure 3 It is the reference power regulation rule according to the embodiment of the present invention;

[0034] Figure 4 It is the method flow of the feasible region of the current limiting phase angle according to the embodiment of the present invention;

[0035] Figure 5 It is the feasible region of the current limiting phase angle under different fault depths according to the embodiment of the present invention;

[0036] Figure 6 It is the transient control block diagram of the grid-forming inverter according to the embodiment of the present invention;

[0037] Figure 7 It is the simulation waveform of the transient control method of the grid-forming inverter under different fault drop depths according to the embodiment of the present invention;

[0038] Figure 8 It is the simulation waveform of controlling the grid-forming inverter by using the traditional method and the method according to the embodiment of the present invention under severe fault conditions. Detailed Embodiments

[0039] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0040] Figure 1 Shown is the structure diagram of the grid-forming inverter control system. In the main circuit part, the inverter, the filter circuit, and the transformer are connected in sequence, and the transformer is connected to the power grid Grid. Z L and Z T are the transmission line impedance and the transformer impedance respectively. L f and C f are the output LC filter of the inverter. Generally, an energy storage unit device is connected to the DC side of the inverter or DC bus control is adopted, so the DC side voltage U dc can be regarded as constant. The amplitude of the grid voltage is V g , and the phase angle is θ g . In the control part, it mainly includes an active power control link (APC), a reactive power control link (RPC), a current limiting link (CSU), a reference voltage generation link (RV), and a voltage and current control link (IC). APC and RPC are mainly used for the active and reactive power control of the virtual synchronous generator. The mathematical expression of APC can be expressed by (2):

[0041]

[0042] In the formula, Δω = ω - ω N , ω N and ω are the rated angular frequency and the output angular frequency of the virtual synchronous generator respectively; P ref and P are the reference active power and the actual output active power respectively; J is the virtual inertia, and D p is the damping coefficient; θ ref is the reference phase angle.

[0043] The mathematical expression of RPC can be expressed by (2):

[0044] V ref = V N + D q (Q ref - Q) (3)

[0045] Q ref and Q are the reference reactive power and the actual output reactive power respectively; V ref and V N are the reference voltage and the rated voltage respectively; D qis the sag coefficient.

[0046] When the system does not fail, the current limiting link is not activated; when the system fails and the fault current surges, the reference current I ref exceeds the current limit value I lim of the current limiting link, and the current limiting link is activated. Its function is shown in Equation (4):

[0047]

[0048] In the formula, is the current limiting phase angle, and I dlim , I qlim are the d-axis component and q-axis component of the current after current limiting by the current limiting link respectively.

[0049] When the current limiting link is not activated, the output power of the network-forming inverter is:

[0050]

[0051] In the formula, δ is the phase angle difference between the output voltage of the inverter and the power grid, named the power angle. When the current limiting link is activated, the output power of the network-forming inverter is:

[0052]

[0053] Substituting Equation (6) into Equation (2) can obtain the second-order swing equation of the network-forming inverter, as shown in Equation (7).

[0054]

[0055] After the fault, if P max >P ref , that is, U g >P ref / I lim , the system has an equilibrium point; if P max <P ref , that is, U g <P ref / I lim , the system has no equilibrium point. Define the critical fault grid voltage U cr =P ref / I lim . When U g >U cr , the system always has an equilibrium point, and P ref =P N . U g <U cr , the system has no equilibrium point, so it is necessary to reduce P ref to search for a new equilibrium point again. The reference power P refThe control rule is expressed by Equation (8), and the reference power adaptive curve is as Figure 3 shown.

[0056]

[0057] The reference power P ref adapts to decrease, which ensures the existence of an equilibrium point after a fault. However, inappropriate current saturation phase angle values will still cause the system to experience transient synchronous instability. To obtain the appropriate current limiting phase angle, i.e., the feasible region of the current limiting phase angle, under different fault depths, it is mainly implemented based on an iterative algorithm, and its algorithm block diagram is as Figure 4 shown.

[0058] First, initialize the current limiting phase angle to 0 and the grid voltage Ug. Then, use the second-order swing equation (7) of the V-configuration network inverter to solve for the power angle δ through the MATLAB command "ode45", and determine whether the system can be stable under the action of the current limiting phase angle at this time based on whether the power angle δ converges to the steady-state value. Further, increase it by 0.1 after each judgment of stability, so as to find the critical current limiting phase angle that makes the system stable. At this time, under this grid voltage, the variable range of the current limiting phase angle is: Next, change the grid voltage and repeat the above steps to obtain the variable range of the current limiting phase angle, i.e., the feasible region of the current limiting phase angle, under different grid voltages, as Figure 5 shown.

[0059] Figure 6 This is the transient control block diagram of the network-forming inverter of the present invention. The grid voltage is detected in real time. If the grid voltage U g is lower than the set rated value U N , it is determined that the system enters the fault mode; further, adjust the reference power value according to Equation (8), and set the current limiting phase angle according to the Figure 4 feasible region, so as to achieve fault current limiting and transient synchronous stability.

[0060] Figure 7 In (1) of Figure 7 and sag <V sag,cr ), the simulation waveforms of the network-forming inverter grid-connected system adopting traditional control and the control strategy of the embodiment of the present invention are given respectively under light fault conditions (V Figure 7 In (1) of Figure 7 ), when the traditional control strategy is adopted, the system remains synchronous but the output current exceeds the set threshold. In (2) of

[0061] Figure 8 in (1) and Figure 8 in (2) gives the simulation waveforms of the grid-forming inverter grid-connected system when a severe fault condition (V sag > V sag,cr ) occurs, respectively using the traditional control and the control strategy of the embodiment of the present invention. A grid fault occurs at the 4s moment. From Figure 8 in (1), it can be seen that when the traditional control strategy is adopted, the output current exceeds the set threshold and transient synchronization instability occurs. From Figure 8 in (2), it can be seen that when the control strategy of the embodiment of the present invention is adopted, the system realizes reliable current limiting and maintains synchronous and stable operation with the grid.

[0062] The simulation results show that: under different fault ride-through depths, the proposed control strategy can effectively limit the fault current of the grid-forming inverter and ensure the stability of the power angle. While stably limiting the current, it solves the risk of transient power angle instability brought by the current limiting link and realizes reliable fault ride-through. Therefore, the method of the embodiment of the present invention does not require additional control loops and additional devices, is simple and easy to implement, and is convenient for parameter adjustment. The method of the embodiment of the present invention can also be applied to other grid-forming inverters and can be further popularized and applied to fault current limiting and transient power angle control in scenarios such as multi-machine grid connection and island microgrid.

[0063] Although the preferred embodiments of the present application have been described, those skilled in the art can make additional changes and modifications once they know the basic creative concept. Therefore, the appended claims are intended to be construed as including the preferred embodiments and all changes and modifications falling within the scope of the present application.

[0064] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalent technologies, the present application also intends to include these changes and modifications.

Claims

1. A transient stability control method for a network-forming inverter, characterized in that, Including the following steps: S1. Collect the grid-side voltage; S2. Judge the grid state according to the grid-side voltage; If U g = U N , it is determined that the power grid is in a normal operating condition; if U g < U N , it is determined that the power grid is in a fault condition; where U N is the rated voltage of the power grid. When the grid is in a fault condition, it further includes: Calculate the transmission power of the network-forming inverter after a fault, and the maximum transmission power P max = U g * I lim ; is the phase angle difference between the network-forming inverter and the grid output voltage; If P max > P ref , that is, U g > P ref / I lim , the grid-forming inverter has an equilibrium point; If P max < P ref , that is, U g <P ref / I lim , the grid-forming inverter has no equilibrium point; Phase angle difference between the network-forming inverter and the grid output voltage The calculation formula is as follows: ; where D p is the damping coefficient, J is the virtual inertia, is the grid voltage amplitude, is the second derivative of the phase angle difference between the network-forming inverter and the grid output voltage with respect to time, and is the first derivative of the phase angle difference between the network-forming inverter and the grid output voltage with respect to time; S3. Adjust the reference power and current limiting phase angle according to the grid state wherein The reference power P ref The regulation formula is: ; U g is the grid-side voltage, I lim is the limit value of the current limiting link, P N is the rated power of the grid-forming inverter, U cr is the critical fault grid voltage; is the critical current limiting phase angle; Adjust the current limiting phase angle The specific implementation process includes: 1) Initialize the current limiting phase angle ; 2) Calculate the phase angle difference between the grid-forming inverter and the grid output voltage , and judge whether it converges to the steady-state value. If so, proceed to step 3); otherwise, update the value and repeat step 2); 3) Determine the critical current limiting phase angle , then .

2. The transient stability control method of the network-forming inverter according to claim 1, wherein, Determine the critical current limiting phase angle The implementation process includes: Initialize the current limiting phase angle to 0; When converges to the steady-state value, the saturation angle at this time is the critical current limiting phase angle , end; When does not converge to the steady-state value, add the step size to the value of, and repeat step b); saturation angle = -arctan(Iq / Id), where Id is the d-axis component of the current vector after saturation and Iq is the q-axis component of the current vector after saturation.

3. The transient stability control method of the network-forming inverter according to claim 1, characterized in that Determine the critical current limiting phase angle The implementation process includes: A) Initialize the current limiting phase angle to π; B) When converges to the steady-state value, the saturation angle at this time is the critical current limiting phase angle , end; when does not converge to the steady-state value, subtract the step size from the value of, and repeat step B); saturation angle = -arctan(Iq / Id), where Id is the d-axis component of the current vector after saturation and Iq is the q-axis component of the current vector after saturation.

4. A transient stability control system for a network-forming inverter, characterized in that, It includes a memory, a processor, and a computer program stored on the memory; characterized in that, the processor executes the computer program to implement the steps of the method described in any one of claims 1 to 3 above.

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