VSG fault recovery capability improvement strategy based on frequency deviation differential feedback

Abstract

This application provides a strategy for improving the fault recovery capability of a VSG based on frequency deviation differential feedback, including: obtaining an unbalanced frequency compensation value; the unbalanced frequency compensation value being determined by a frequency deviation differential feedback control branch; and shortening the time required for the VSG to return to stable operation based on the unbalanced frequency compensation value. The strategy proposed in this application, by introducing frequency deviation differential feedback compensation control during the fault recovery phase, can dynamically adjust the equivalent damping coefficient to quickly consume excess kinetic energy accumulated during the fault, effectively reducing the power angle overshoot during fault recovery and shortening the time required for the VSG to return to stable operation.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of network construction control, and in particular to a VSG fault recovery capability improvement strategy based on frequency deviation differential feedback. BACKGROUND

[0002] Currently, there are two control modes for grid-connected VSC, namely grid-following (GFL) control and grid-forming (GFM) control. In actual working conditions, new energy grid-connected VSC usually adopts GFL control, that is, by measuring the voltage at the grid connection point, the phase locked loop (PLL) is used to track the grid voltage phase, so that the VSC and the grid are kept in synchronous operation. However, under the condition of weak grid subjected to large disturbance fault, the PLL is easy to lose the tracking ability of the grid voltage phase, thereby causing synchronization instability problem. In order to improve the synchronization stability of grid-connected VSC under weak grid, relevant scholars have proposed GFM control strategy. As one of the core controls of grid-forming, the virtual synchronous generator (VSG) simulates the virtual inertia and damping characteristics of the synchronous generator, and provides effective frequency and voltage support for the grid, which has become an important technical means to ensure the stable operation of new energy devices connected to the grid. However, using traditional VSG during voltage sag on the distribution network side will cause transient power angle instability problem, and during the recovery stage after fault removal, if the deceleration area of VSG is not enough to offset the increase of virtual kinetic energy accumulated during the fault, the power angle overshoot or instability phenomenon will occur, which will cause great impact on VSC, and thus threaten the safe operation of VSC connected to the grid. Therefore, it is of great significance to deeply analyze the system fast recovery to steady state of VSG after fault removal during voltage sag on the distribution network side.

[0003] Currently, the research on the transient stability of VSG under the voltage sag scenario on the distribution network side focuses on the transient power angle control during the fault and the expansion of the transient operating boundary. However, the existing research improves the transient power angle stability of VSG during the fault from different angles, but generally ignores the problem of large power angle overshoot and output electrical quantity oscillation of VSG after fault removal. SUMMARY

[0004] The present application provides a VSG fault recovery capability improvement strategy based on frequency deviation differential feedback. In order to solve the above technical problems, the present application adopts the following technical method:

[0005] In a first aspect, the present application provides a VSG fault recovery capability improvement strategy based on frequency deviation differential feedback, comprising:

[0006] obtaining an unbalanced frequency compensation value; the unbalanced frequency compensation value is determined by a frequency deviation differential feedback control branch;

[0007] shortening a time required for the VSG to recover stable operation based on the unbalanced frequency compensation value.

[0008] Optionally, the unbalanced frequency compensation value is determined by the frequency deviation differential feedback control branch, in particular:

[0009] a frequency deviation differential feedback control branch is constructed in an active loop;

[0010] an additional frequency deviation differential compensation coefficient is determined based on the frequency deviation differential feedback control branch;

[0011] an unbalanced frequency compensation value is determined based on the additional frequency deviation differential compensation coefficient.

[0012] Optionally, the shortening of the time required for the VSG to recover stable operation based on the unbalanced frequency compensation value comprises:

[0013] an actual value of a virtual rotor angular velocity of the VSG is obtained;

[0014] the time required for the VSG to recover stable operation is shortened based on the unbalanced frequency compensation value and the actual value of the virtual rotor angular velocity of the VSG.

[0015] Optionally, the actual value of the virtual rotor angular velocity of the VSG is determined by:

[0016] an active power reference value of the VSG, an actual output value of active power of the VSG, a virtual moment of inertia and a damping coefficient are obtained;

[0017] the actual value of the virtual rotor angular velocity of the VSG is determined based on the active power reference value of the VSG, the actual output value of active power of the VSG, the virtual moment of inertia and the damping coefficient.

[0018] Optionally, the time required for the VSG to recover stable operation is shortened based on the unbalanced frequency compensation value and the actual value of the virtual rotor angular velocity of the VSG;

[0019] a dynamic compensation actual value of the virtual rotor angular velocity is calculated by adding the unbalanced frequency compensation value and the actual value of the virtual rotor angular velocity of the VSG;

[0020] a power angle of the VSG is determined based on the dynamic compensation actual value of the virtual rotor angular velocity;

[0021] the time required for the VSG to recover stable operation is shortened based on the power angle.

[0022] Optionally, the step of shortening the time required for the VSG to recover and run stably based on the power angle comprises:

[0023] obtaining a VSG rated voltage instruction value;

[0024] inputting the VSG rated voltage instruction value and the power angle into a VSG system control loop for calculation to obtain a PWM modulation signal;

[0025] shortening the time required for the VSG to recover and run stably based on the PWM modulation signal.

[0026] Optionally, during the fault recovery, the additional frequency deviation differential compensation coefficient is not less than zero.

[0027] In a second aspect, the present application further provides a computer system, comprising:

[0028] a memory for storing instructions executable by the processor;

[0029] a processor for executing the instructions to implement the strategy according to any one of the first aspect.

[0030] In a third aspect, the present application further provides a computer readable medium storing computer program code, which, when executed by a processor, implements the strategy according to any one of the first aspect.

[0031] The present application has the following beneficial effects:

[0032] The strategy provided by the present application can dynamically adjust the equivalent damping coefficient to quickly consume the excess kinetic energy accumulated during the fault by introducing frequency deviation differential feedback compensation control in the fault recovery stage, effectively reduces the power angle overshoot during the fault recovery, and shortens the time required for the VSG to recover and run stably. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 a schematic diagram of a VSG system control loop provided by an embodiment of the present application;

[0034] Figure 2 a VSG system control block diagram provided by an embodiment of the present application;

[0035] Figure 3 a VSG output power angle curve diagram under different voltage drop degrees provided by an embodiment of the present application; Figure 3 (a) is a schematic diagram of the case where there is a power balance point after voltage drop; Figure 3 (b) is a schematic diagram of the case where there is no power balance point after voltage drop;

[0036] Figure 4Waveform diagram of VSG fault ride-through whole process under flexible control of parameters provided by the embodiment of the application;

[0037] Figure 5 Flowchart of VSG fault recovery capability improvement strategy based on frequency deviation differential feedback provided by the embodiment of the application;

[0038] Figure 6 Block diagram of VSG control strategy using differential frequency deviation feedback branch compensation provided by the embodiment of the application;

[0039] Figure 7 VSG phase plane diagram under different control strategies provided by the embodiment of the application;

[0040] Figure 8 Waveform comparison diagram of transient process of different VSG control strategies (grid fault voltage drop to 0.3pu), Figure 8 (a) is a waveform diagram of transient process of traditional VSG control strategy; Figure 8 (b) is a waveform diagram of transient process of the control strategy proposed in the application;

[0041] Figure 9 Comparison diagram of transient process of different VSG control strategies (grid fault voltage drop to 0.5pu); Figure 9 (a) is a waveform diagram of transient process of traditional VSG control strategy; Figure 9 (b) is a waveform diagram of transient process of the control strategy proposed in the application;

[0042] Figure 10 Comparison of transient process of different VSG control strategies (grid fault voltage drop to 0.3pu), Figure 10 (a) is a waveform diagram of transient process of transient parameter flexible adjustment control strategy; Figure 10 (b) is a waveform diagram of transient process of the control strategy proposed in the application. DETAILED DESCRIPTION

[0043] In order to facilitate the understanding of those skilled in the art, the application will be further described below in conjunction with the embodiments and the drawings. The content mentioned in the embodiments is not a limitation of the application.

[0044] In order to facilitate the understanding of the application, the concept background of the application is briefly described as follows:

[0045] The VSG system control loop is as shown in Figure 1 . In the figure, the VSC adopts a constant DC side voltage , and is connected to the grid point PCC through a filter. Among them , and These are the inductance, resistance, and capacitance of the filter, respectively. , and These are the inverter's output current, PCC point voltage, and grid-side voltage under steady-state conditions, respectively. The line reactance is denoted as ; PWM stands for Pulse Width Modulation.

[0046] The VSG system control block diagram is as follows: Figure 2 As shown, its active-frequency and reactive-voltage control equations are respectively given by equation (1) and equation (2):

[0047] (1)

[0048] (2)

[0049] In the formula: and These are the VSG's active power reference value and actual output value, respectively. and These are the actual and reference values ​​of the virtual rotor angular velocity of the VSG, respectively. and These are the reference value and the actual output value of the inverter's reactive power, respectively. This is the droop coefficient; This is a virtual moment of inertia; The damping coefficient; For VSG's offensive role; and These are the VSG rated voltage amplitude and voltage command value, respectively.

[0050] Transient work angle characteristics

[0051] Active power output from VSG to the grid and reactive power As shown in equations (3) and (4) respectively:

[0052] (3)

[0053] (4)

[0054] In the formula: and These are the VSG output voltage and the mains voltage, respectively. For line reactance, The angular frequency of the power grid. It is the inductance of the power grid.

[0055] Substituting equation (3) for the output active power of VSG into equation (1), we get:

[0056] (5)

[0057] Analysis of equation (5) shows that during the period of grid voltage drop, if , Its work angle remains unchanged. Changes and and Related. Ignoring the impact of the reactive power loop on transient stability would lead to a conservative analysis of transient stability characteristics. In practice, the output voltage of the VSG... The value is variable and will be affected by the reactive power loop. The reactive power loop decreases as the reactive power decreases. Therefore, this application explicitly considers the impact of the reactive power loop in the subsequent transient stability analysis.

[0058] Combining equations (2) and (4), the voltage command value of VSG can be obtained as follows:

[0059] (6)

[0060] Substituting equations (6) and (3) into equation (1), we get:

[0061] (7)

[0062] Considering the influence of the reactive power loop, the power angle curves of VSG under different voltage drop conditions can be plotted according to equation (7), as shown below. Figure 3 As shown in the figure. Curves I, II, and III represent the normal operating condition, the grid voltage drop to [value missing], and the [value missing] conditions, respectively. =0.7 and the grid voltage dropped to The VSG output power angle curve with a value of 0.3 and These represent the power balance point and power imbalance point under normal operating conditions. and These are the power balance point and power imbalance point of curve II, respectively.

[0063] Figure 3 (a) In the case of a power balance point after a voltage drop, the initial operating point of the VSG is point A. When a voltage drop occurs on the distribution network side, the operating point abruptly changes to point B. Greater than , The system enters the acceleration region, and as the fault continues, the active power output... Increase until the run point C is reached, satisfying... Then it enters the deceleration zone. Figure 3(b) In the case where there is no power balance point after a voltage drop, when a voltage drop occurs on the distribution network side, due to the deep voltage drop in the grid, the VSG lacks a deceleration area, resulting in a power angle change rate that is always greater than zero, causing the power angle to continuously increase, and ultimately leading to transient instability of the VSG. Therefore, improving the transient power angle stability of the VSG can be achieved by reducing the active power reference value of the VSG to decrease the acceleration area of ​​the system during the fault process and increase the deceleration area.

[0064] In the event of a voltage drop on the distribution network side, although the transient characteristics of the VSG can be altered through parameter control, excessive kinetic energy during the fault may still lead to slippage and resynchronization. Furthermore, the recovery time after fault clearance is relatively long. The waveform of the parameter-controlled VSG during the entire fault ride-through process is as follows: Figure 4 As shown.

[0065] Based on the above analysis, such as Figure 5 As shown, this application proposes a strategy to improve VSG fault recovery capability based on frequency deviation differential feedback, including:

[0066] Step S101: Obtain the unbalanced frequency compensation value; the unbalanced frequency compensation value is determined by the frequency deviation differential feedback control branch;

[0067] like Figure 6 As shown, a frequency deviation differential feedback control branch (blue part) is constructed in the active power converter of the VSG. During the fault recovery phase, this branch can dynamically adjust the VSG frequency to balance the frequency difference between the VSG and the grid, effectively reduce the power angle overshoot after the fault is cleared, and significantly shorten the time required for the VSG to restore stable operation.

[0068] from Figure 6 It can be seen that the frequency deviation differential feedback control branch can calculate the unbalanced frequency deviation compensation value. The unbalanced frequency deviation compensation value can be obtained from the additional frequency deviation differential compensation coefficient. The calculation yields the following formula:

[0069] (8)

[0070] Step S102: Based on the unbalanced frequency compensation value, shorten the time required for the VSG to restore stable operation.

[0071] After determining the unbalanced frequency compensation value, such as Figure 6 As shown, the actual value of the virtual rotor angular velocity of the VSG can then be obtained. This will further transform the actual value of the virtual rotor angular velocity of the VSG. And unbalanced frequency deviation compensation value Adding the actual values ​​of the dynamically compensated virtual rotor angular velocity after frequency deviation differential feedback compensation, As shown in the following formula:

[0072] (9)

[0073] Actual value of virtual rotor angular velocity of VSG The calculation method is as follows: Obtain the active power reference value of VSG. Actual active power output value of VSG Virtual moment of inertia and damping coefficient Then, based on the VSG's active power reference value, the VSG's actual active power output value, the virtual moment of inertia, and the damping coefficient, it is calculated as shown in the following formula:

[0074] (10)

[0075] After obtaining the actual value of the dynamically compensated virtual rotor angular velocity, the power angle of the VSG can be calculated based on this actual value. .

[0076] This allows the angle of attack to be adjusted. and VSG rated voltage command value enter Figure 1 In the control loop of the VSG system shown, a PWM modulation signal is calculated, and based on this PWM modulation signal, the time required for the VSG to recover stable operation is shortened.

[0077] Combining and rearranging formulas (8), (9), and (10), we get:

[0078] (11)

[0079] because It remains constant, that is, it exists. From equation (11), we can obtain:

[0080] (12)

[0081] Substituting equations (3) and (6) into equation (11), we get:

[0082] (13)

[0083] (14)

[0084] (15)

[0085] From equations (13), (14), and (15), it can be seen that the equivalent damping coefficient is quite complex after differentiation. If the VSG reactive power loop contains... If the value is large enough, the VSG output voltage can remain approximately constant during fault recovery as the power angle oscillates. Therefore, equations (13), (14), and (15) can be simplified to:

[0086] (16)

[0087] By comparing equation (1) and equation (16), it can be seen that the equivalent damping coefficient after frequency deviation differential feedback compensation is:

[0088] (17)

[0089] From the analysis of the above equation (15), it can be seen that, due to and Its function will dynamically adjust the damping coefficient during the fault recovery phase. .

[0090] To ensure that VSG power angle overshoot can be reduced during the fault recovery phase, this application addresses the compensation coefficient. The rationale for determining the value is as follows: during fault recovery, It should be no less than zero, so that the equivalent damping coefficient exist When greater than the damping coefficient .

[0091] At the same time, considering The characteristics of the equivalent damping coefficient It will dynamically adjust according to changes in the angle of work. For ease of analysis, this paper will use the angle of work... Divided into two intervals:

[0092] (1) When the fault is cleared, the VSG power angle hour, , The equivalent damping is enhanced, which helps to consume the kinetic energy accumulated during the fault after the fault is cleared, and accelerates the system recovery.

[0093] (2) When the VSG power angle hour, , The equivalent damping is reduced.

[0094] After the fault is cleared, if the power angle Greater than 90°, i.e., at When the interval is, This reduces the equivalent damping coefficient. However, as the power angle continues to operate, it re-enters... When the interval is, The change from negative to positive results in an increase in the equivalent damping coefficient, which in turn can absorb the kinetic energy accumulated during the fault duration to the greatest extent and accelerate the VSG's rapid recovery to stable operation.

[0095] According to equations (1) and (16), the VSG phase plane diagrams under different control strategies with a fault duration of 1s are as follows: Figure 7 As shown. By Figure 7 It is known that traditional VSGs become completely unstable during a fault. However, VSGs using frequency deviation differential feedback compensation control during the fault recovery phase can regain stability even if transient instability occurs during the fault. This is because after the fault is cleared, the frequency deviation differential feedback compensation control can dynamically adjust the equivalent damping coefficient, further consuming the acceleration kinetic energy accumulated during the fault, thus enabling the VSG, which was originally in a transiently unstable state, to return to stable operation after the fault is cleared.

[0096] Simulation Experiment

[0097] To verify the correctness of the theoretical analysis and the effectiveness of the proposed control strategy, a system was built in Matlab / Simulink. Figure 1 The VSG grid-connected simulation model is shown below. The simulation parameters are shown in Table 1.

[0098] The grid-connected inverter initially operates in a stable state. At 1 second, a voltage drop occurs in the grid, lasting for 1 second. The following simulations compare the traditional VSG control and the VSG inverter with the proposed control strategy under two operating conditions: voltage drops to 0.5 pu and 0.3 pu.

[0099] Table 1 Simulation Parameters

[0100] ;

[0101] Figure 8 A comparison of transient waveforms for different VSG control strategies when the grid voltage drops to 0.5 pu. Figure 8 (a) is a waveform diagram under traditional VSG control. It can be seen from the figure that the traditional VSG experiences transient instability during the fault process. Figure 8 (b) The VSG using frequency deviation differential compensation control experiences power angle oscillation during a fault, but can still resume stable operation after the fault is cleared.

[0102] Figure 9 A comparison of transient waveforms for different VSG control strategies when the grid voltage drops to 0.3 pu. Figure 9(a) is a waveform diagram under traditional VSG control. It can be seen from the figure that the traditional VSG experiences transient instability during the fault process. Figure 9 (b) The VSG using frequency deviation differential compensation control exhibits power angle oscillation during a fault, but is able to return to stable operation after the fault is cleared. Ultimately, simulation results demonstrate that the proposed frequency deviation differential feedback compensation control strategy can effectively improve transient power angle stability.

[0103] To further demonstrate the superiority of the proposed control strategy in the fault recovery phase, the parameter adjustment control strategy proposed in the prior art is compared with the output power angle, output active power and current of VSG under frequency deviation differential feedback compensation control, as shown in Figure 10.

[0104] Figure 10 (a) A flexible control strategy for adjusting the transient parameters of the VSG is adopted. This strategy can effectively extend the fault clearance time and optimize the transient stability performance of the VSG by flexibly adjusting the VSG's moment of inertia and damping coefficient. However, after the fault is cleared, the power angle exhibits a slippage resynchronization phenomenon with an overshoot of 4 pu, and it takes 0.47 s to recover stable operation. Figure 10 (b) This paper presents the frequency deviation differential feedback compensation control strategy. During the fault persistence phase, this strategy employs flexible adjustment of VSG transient parameters to improve the transient stability of the VSG. After fault clearance, the frequency deviation differential feedback compensation control strategy reduces the power angle overshoot to 2.4 pu and shortens the fault recovery time to 0.35 s. Finally, simulation results demonstrate that the proposed frequency deviation differential feedback compensation control strategy significantly reduces the power angle overshoot during fault recovery and shortens the time required for the VSG to return to stable operation.

[0105] In summary, the strategy proposed in this application, by introducing frequency deviation differential feedback compensation control during the fault recovery phase, can dynamically adjust the equivalent damping coefficient to quickly consume the excess kinetic energy accumulated during the fault, effectively reduce the power angle overshoot during the fault recovery phase, and shorten the time required for the VSG to return to stable operation.

[0106] In some embodiments, this application also provides a computer system including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps in the above-described method embodiments.

[0107] This application also provides a computer-readable storage medium for storing a computer program. This computer-readable storage medium can be applied to a computer device, and the computer program causes the computer device to execute the corresponding processes in the methods described above in the embodiments of this application; for brevity, further details are omitted here.

[0108] The above embodiments are preferred implementations of this application. In addition, this application can be implemented in other ways. Any obvious substitutions without departing from the concept of this technical solution are within the protection scope of this application.

[0109] To facilitate understanding by those skilled in the art of the improvements made by this application compared to the prior art, some of the accompanying drawings and descriptions have been simplified, and for clarity, some other elements have been omitted from this application. Those skilled in the art should realize that these omitted elements may also constitute the content of this application.

Claims

1. A strategy for improving VSG fault recovery capability based on frequency deviation differential feedback, characterized in that, include: Obtain the unbalanced frequency compensation value; the unbalanced frequency compensation value is determined by the frequency deviation differential feedback control branch; Based on the aforementioned unbalanced frequency compensation value, the time required for the VSG to restore stable operation is shortened; The equivalent damping coefficient after frequency deviation differential feedback compensation is: ； In the formula, The damping coefficient; For VSG's offensive role, These are the grid-side voltages of the inverter under steady-state conditions; For line reactance, These are the VSG voltage command values, The differential compensation coefficient for frequency deviation; After the fault is cleared, the equivalent damping coefficient is dynamically adjusted through frequency deviation differential feedback compensation to consume the acceleration kinetic energy accumulated during the fault duration, shorten the time required for the VSG to return to stable operation, and enable the VSG, which was originally in a transient unstable state, to return to stable operation after the fault is cleared.

2. The strategy according to claim 1, characterized in that, The unbalanced frequency compensation value is determined by the frequency deviation differential feedback control branch, specifically: Construct a frequency deviation differential feedback control branch in the active power loop; Based on the frequency deviation differential feedback control branch, the additional frequency deviation differential compensation coefficient is determined. The unbalanced frequency compensation value is determined based on the additional frequency deviation differential compensation coefficient.

3. The strategy according to claim 2, characterized in that, The reduction of the time required for the VSG to restore stable operation based on the unbalanced frequency compensation value includes: Obtain the actual value of the virtual rotor angular velocity of the VSG; Based on the unbalanced frequency compensation value and the actual value of the virtual rotor angular velocity of the VSG, the time required for the VSG to restore stable operation is shortened.

4. The strategy according to claim 3, characterized in that, The actual value of the virtual rotor angular velocity of the VSG is determined in the following way: Obtain the active power reference value of the VSG, the actual active power output value of the VSG, the virtual moment of inertia and the damping coefficient; Based on the active power reference value of the VSG, the actual active power output value of the VSG, the virtual moment of inertia, and the damping coefficient, the actual value of the virtual rotor angular velocity of the VSG is determined.

5. The strategy according to claim 3, characterized in that, Based on the unbalanced frequency compensation value and the actual value of the virtual rotor angular velocity of the VSG, the time required for the VSG to restore stable operation is shortened. The unbalanced frequency compensation value and the actual value of the virtual rotor angular velocity of the VSG are added together to calculate the actual value of the dynamically compensated virtual rotor angular velocity after frequency deviation differential feedback compensation. Based on the actual value of the dynamically compensated virtual rotor angular velocity, the power angle of the VSG is determined; Based on the aforementioned power angle, the time required for the VSG to restore stable operation is shortened.

6. The strategy according to claim 5, characterized in that, The reduction of the time required for VSG to restore stable operation based on the power angle includes: Obtain the VSG rated voltage command value; The VSG rated voltage command value and the power angle are input into the VSG system control loop for calculation to obtain the PWM modulation signal; Based on the PWM modulation signal, the time required for the VSG to resume stable operation is shortened.

7. The strategy according to claim 2, characterized in that, During fault recovery, the additional frequency deviation differential compensation coefficient is not less than zero.

8. A computer system, characterized in that, include: Memory is used to store instructions that can be executed by the processor; A processor for executing the instructions to implement the strategy as described in any one of claims 1 to 7.

9. A computer-readable medium, characterized in that, The system stores computer program code that, when executed by a processor, implements the strategy as described in any one of claims 1 to 7.

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