Adaptive Inertial Damping Cooperative Control Method for Improving the Transient Stability of Grid-Forming Converters

Abstract

An Adaptive Inertia Damping Cooperative Control Method for Improving the Transient Stability of Grid-forming Converters. Among them, the basic control scheme of the power outer loop of the grid-forming converter is Virtual Synchronous Generator (VSG) control. In addition, in order to improve the transient synchronous stability, an Adaptive Inertia Damping Cooperative (AIDC) control method is designed. This method takes into account both frequency stability and transient synchronous stability. The virtual inertia is designed to increase adaptively in the acceleration region and decrease adaptively in the deceleration region. The damping increases adaptively throughout the transient process to enhance the damping effect. The adaptive adjustment of virtual inertia and damping is realized based on the arctan function, effectively ensuring the boundaries of the adaptive virtual inertia and damping, which facilitates the design by engineers in practical applications.

Description

Technical Field

[0001] The present invention relates to the fields of microgrid technology and power control, and specifically to an adaptive inertial damping collaborative control method for improving the transient stability of grid-forming converters. Background Art

[0002] With the increasing popularity of renewable energy generation, synchronous generators are gradually being replaced by inverter-based distributed generation units (IBGs), and the dynamic stability behavior of power systems has gradually changed. On the one hand, IBGs offer more control flexibility. On the other hand, the device vulnerability of converters must be considered under large disturbances. Therefore, how to flexibly configure the available IBG control resources remains an open issue. Traditional IBGs are synchronized with the grid through a phase-locked loop (PLL) and tend to operate in a grid-following mode. This type of grid-following inverter can be equivalent to a controlled current source, and its normal operation depends on the presence of a voltage source in the system to establish a voltage reference. However, the stable operation of grid-following converters will be limited by the gradually decreasing proportion of synchronous generators, resulting in a lack of voltage support. To address this challenge, the concept of grid-forming converters has been widely introduced to provide voltage support and enhance system stability. Droop control and virtual synchronous generator (VSG) control are two typical grid-forming converter control methods. VSG control can be regarded as an improved droop control scheme with more powerful virtual inertia support and optimized dynamic frequency performance.

[0003] One advantage of VSG control is the flexibility of its adjustment control parameters. For example, virtual inertia can be adjusted within a certain range according to the real-time state of system operation, which provides more possibilities for optimizing the dynamic performance of VSG. This flexibility allows for improving frequency stability through technologies such as adaptive inertia. However, current research work only focuses on the impact of variable control parameters on frequency stability, while ignoring the impact of variable virtual inertia and damping on transient synchronization stability. Transient synchronization stability is also crucial for stable operation. To ensure that distributed energy and microgrids can "be connected, generate power, and be consumed", it is urgent to make full use of the control flexibility of converter-interfaced renewable energy to improve the transient crossing and grid-connected synchronous operation capabilities of the system under large disturbances.

[0004] To improve the transient synchronization stability of grid-forming converters under large disturbances, this patent introduces a control idea based on Adaptive Inertia and Damping Coordination (AIDC), which takes into account both frequency stability and transient synchronization stability. The virtual inertia is designed to increase adaptively in the acceleration region and decrease adaptively in the deceleration region. The damping increases adaptively throughout the transient process to enhance the damping effect. The adaptive adjustment of virtual inertia and damping is achieved based on the arctan function, effectively ensuring the boundaries of adaptive virtual inertia and damping. It provides a practical solution for the safe, stable, and reliable operation of current grid-forming converters under large-signal disturbances.

[0005] The current existing technologies are as follows:

[0006] Application No. CN202110784238.8, titled "A VSG Virtual Inertia and Damping Coordination Adaptive Control System and Method". This method establishes a small-signal model containing wind speed variables based on the virtual synchronous control schematic diagram of a doubly-fed wind turbine, uses a fuzzy controller to determine the steady-state value of the virtual inertia coefficient with the operating wind speed and frequency deviation as inputs, and determines the steady-state value of the damping coefficient according to the relationship between the critical oscillation wind speed and the operating wind speed to avoid system oscillation instability; according to the relationship between the frequency deviation, the rate of change of frequency, and the control parameters, as well as the demand changes during the frequency modulation process, the control parameters are adaptively adjusted alternately. The VSG virtual inertia and damping coordination adaptive control system and method provided by the present invention are beneficial for the doubly-fed wind turbine to adapt to different wind speeds and improve the frequency modulation effect on the premise of avoiding system oscillation instability.

[0007] It uses a complex fuzzy controller algorithm for adaptive control of virtual inertia and damping values, determines the steady-state value of the virtual inertia coefficient with the operating wind speed and frequency deviation as inputs, and determines the steady-state value of the damping coefficient according to the relationship between the critical oscillation wind speed and the operating wind speed to avoid system oscillation instability;

[0008] There are three major differences in this application:

[0009] 1) The core objective of the above document is to improve the active frequency modulation and power oscillation capabilities, while the core objective of this application is to improve the fault ride-through and transient synchronization stability capabilities of the converter under faults, with different design objectives;

[0010] 2) The above document uses a fuzzy control algorithm with complex and unclear design mechanisms, while this application uses a direct construction method with mathematical expressions to quantitatively describe the relationship between virtual inertia and damping and the system operating state, with different implementation control algorithms;

[0011] 3) The above document uses the operating wind speed and frequency deviation as input quantities. In this application, the frequency deviation and the rate of change of frequency are used as input variables. Moreover, the algorithm constructed in this application uses the actan function to ensure the upper and lower boundedness of inertia and frequency. Both the control means variable and the constructed function are different. Summary of the Invention

[0012] To solve the above problems, the present invention proposes an adaptive inertia damping cooperative control method for improving the transient stability of a network-forming converter, which takes into account both frequency stability and transient synchronization stability. The virtual inertia is designed to increase adaptively in the acceleration region and decrease adaptively in the deceleration region. The damping increases adaptively throughout the transient process to enhance the damping effect. The adaptive adjustment of the virtual inertia and damping is achieved based on the arctan function, effectively ensuring the boundaries of the adaptive virtual inertia and damping, which facilitates the design by engineers in practical applications.

[0013] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0014] An adaptive inertia damping cooperative control method for improving the transient stability of a network-forming converter, based on virtual synchronous generator control, is as follows:

[0015] The control expression of the adaptive inertia damping cooperative control method is:

[0016]

[0017] Wherein, P * represents the active power reference, P is the output active power of the network-forming converter, J and D m respectively represent the virtual inertia constant and the damping coefficient, ω * is the voltage angular velocity reference, and ω is the angular velocity of the output voltage of the network-forming converter;

[0018] The design criteria for the virtual inertia damping of the control method are as follows:

[0019] Time period Δω dω / dt ω state Region ΔJ ΔD <![CDATA[t1-t2]]> >0 >0 Deviation Acceleration >0 >0 <![CDATA[t2 - t3]]> >0 <0 Return Deceleration <0 >0 <![CDATA[t3-t4]]> <0 <0 Deviation Acceleration >0 >0 <![CDATA[t4-t5]]> <0 >0 Return Deceleration <0 >0

[0020] Wherein, ΔJ and ΔD m represent the dynamic control variables of the designed virtual inertia constant and damping coefficient;

[0021] The virtual inertia is designed to increase adaptively in the acceleration region and decrease adaptively in the deceleration region, and the damping increases adaptively throughout the transient process to enhance the damping effect;

[0022] Specifically, it is expressed as follows:

[0023] A power grid fault occurs at point B and time t1. During the first stage from B to C, from t1 to point t2, the frequency will accelerate, and the transient power angle increases. This region is called the acceleration region. At this time, a large virtual inertia is needed to slow down the deviation trend and reduce the maximum frequency deviation. At point C, it reaches the steady state point. In the second stage from C to D, from t2 to t3, the frequency will decelerate, but Δω is greater than zero, and the power angle still increases. This region is called the deceleration region;

[0024] During this process, a small inertia is needed to enable the grid-forming converter to quickly return to the frequency reference;

[0025] The adaptive adjustment of the virtual inertia and damping is realized based on the arctan function, and the specific expression is as follows

[0026]

[0027]

[0028] where J0 and D m0 represent the static control variables of the virtual inertia constant and damping coefficient, while ΔJ and ΔD m represent the dynamic control variables of the designed virtual inertia constant and damping coefficient, and k J and k D represent the adaptive coefficients of virtual inertia and damping respectively.

[0029] As a further improvement of the present invention, the adaptive inertia-damping collaborative control method further includes two aspects:

[0030] 1) Adaptive virtual inertia based on Δω and dω / dt;

[0031] The virtual inertia J0 + ΔJ needs to increase in the acceleration region and decrease in the deceleration region, and is adaptively changed according to the combination of Δω and dω / dt. In addition, the maximum virtual inertia J max is related to the instantaneous power capacity limit of the inverter, and the minimum virtual inertia J min is required to meet the frequency stability requirements. Therefore, the adaptive virtual inertia needs to have an upper and lower limit, and the mathematical function ΔJ = arctan(Δω·dω / dt) ensures the boundary of the adaptive virtual inertia;

[0032] 2) Adaptive damping based on dω / dt;

[0033] The damping coefficient D0 + ΔD m is designed to increase adaptively according to dω / dt and expand the system damping effect under disturbances. ΔD m is the transient damping term, which needs to be positive during the transient stability problem and return to zero at steady state. The mathematical function ΔDm = arctan 2 (dω / dt) also provides an upper limit and ensures that ΔD m remains positive throughout the transient process of acceleration / deceleration.

[0034] Beneficial effects:

[0035] The present invention discloses an Adaptive Inertia and Damping Coordination (AIDC) control method for improving the transient synchronization stability of a network-forming converter. Among them, the basic control scheme of the power outer loop of the network-forming converter is Virtual Synchronous Generator (VSG) control. In addition, in order to improve the transient synchronization stability, an Adaptive Inertia and Damping Coordination (AIDC) control method is designed. This method takes into account both frequency stability and transient synchronization stability. The virtual inertia is designed to adaptively increase in the acceleration region and adaptively decrease in the deceleration region. The damping increases adaptively throughout the transient process to enhance the damping effect. The adaptive adjustment of virtual inertia and damping is realized based on the arctan function, effectively ensuring the boundaries of adaptive virtual inertia and damping, which facilitates the design of engineers in practical applications. Description of the drawings

[0036] Figure 1 Shows the schematic diagram of the structure of a network-forming converter system based on VSG;

[0037] Figure 2 Shows the schematic diagram of the time-domain response of the power angle curve and power angle / frequency;

[0038] Figure 3 Shows the schematic diagram of the AIDC control method and system structure of a network-forming converter based on VSG;

[0039] Figure 4 Shows the result diagram of the hardware-in-the-loop experiment. Detailed implementation manners

[0040] The present invention will be further described in detail below in conjunction with the drawings and specific implementation manners:

[0041] The adaptive inertial damping cooperative control method for enhancing the transient stability of the network-forming converter in the present invention takes into account both frequency stability and transient synchronization stability. The virtual inertia is designed to adaptively increase in the acceleration region and decrease in the deceleration region. The damping adaptively increases throughout the transient process to enhance the damping effect. The adaptive adjustment of the virtual inertia and damping is achieved based on the arctan function, effectively ensuring the boundaries of the adaptive virtual inertia and damping, which facilitates the design by engineers in practical applications.

[0042] The basic control scheme of the power outer loop of the network-forming converter in the proposed method is the virtual synchronous generator (VSG) control, as Figure 1 shown, and its control expression is

[0043]

[0044] where, P * represents the active power reference, and P is the output active power of the network-forming converter. J and D m represent the virtual inertia constant and the damping coefficient respectively. ω * is the voltage angular velocity reference, and ω is the angular velocity of the output voltage of the network-forming converter.

[0045] Table 1 Design principles of adaptive virtual inertia damping

[0046] Time period Δω dω / dt ω state Region ΔJ ΔD <![CDATA[t1 - t2]]> >0 >0 Deviation Acceleration >0 >0 <![CDATA[t2-t3]]> >0 <0 Return Deceleration <0 >0 <![CDATA[t3 - t4]]> <0 <0 Deviation Acceleration >0 >0 <![CDATA[t4 - t5]]> <0 >0 Return Deceleration <0 >0

[0047] On this basis, the control method will adaptively adjust the virtual inertia and damping, and its adaptive adjustment criteria are shown in Table 1, where, ΔJ and ΔD m represent the dynamic control variables of the designed virtual inertia constant and damping coefficient. The virtual inertia is designed to adaptively increase in the acceleration region and decrease in the deceleration region. The damping adaptively increases throughout the transient process to enhance the damping effect. The specific description is as Figure 2 shown. The grid fault occurs at point B and time t1. During the first period from point B to point C (from t1 to point t2), the frequency will accelerate, and the transient power angle increases. This region is called the acceleration region. At this time, a large virtual inertia is required to slow down the deviation trend and reduce the maximum frequency deviation. At point C, it reaches the steady state point. During the second period from point C to point D (from t2 to t3), the frequency will decelerate, but Δω is greater than zero, and the power angle still increases. This region is called the deceleration region. During this process, a small inertia is required to enable the network-forming converter to quickly return to the frequency reference. For example, if the virtual inertia becomes 0 at point C, the frequency can instantly return to the frequency reference. Then, the network-forming converter will remain stable at point C, and no deceleration region is required anymore, which is completely different from the traditional synchronous generator.

[0048] Based on this adaptive regulation criterion, the adaptive regulation of virtual inertia and damping in the adaptive inertia damping coordination (AIDC) control method can be achieved through the arctan function, and its control expression is

[0049]

[0050]

[0051] where, J0 and D m0 represent static control variables of the virtual inertia constant and damping coefficient, while ΔJ and ΔD m represent dynamic control variables of the designed virtual inertia constant and damping coefficient. k J and k D represent the adaptive coefficients of virtual inertia and damping respectively.

[0052] The proposed AIDC control scheme is as Figure 3 shown, and the proposed method includes two aspects:

[0053] 1) Adaptive virtual inertia based on Δω and dω / dt

[0054] The virtual inertia (J0 + ΔJ) needs to increase in the acceleration region and decrease in the deceleration region. It can be adaptively changed according to the combination of Δω and dω / dt. In addition, the maximum virtual inertia J max is related to the instantaneous power capacity limit of the inverter, and the minimum virtual inertia J min is required to meet the frequency stability requirement. Therefore, there need to be upper and lower limits for the adaptive virtual inertia. The mathematical function ΔJ = arctan(Δω·dω / dt) guarantees the boundary of the adaptive virtual inertia, which facilitates the engineering practice design.

[0055] 2) Adaptive damping based on dω / dt

[0056] The damping coefficient (D0 + ΔD m ) is designed to increase adaptively according to (dω / dt) and expand the system damping effect under disturbances. ΔD m can be understood as a transient damping term, which needs to be positive during transient stability problems and return to zero at steady state. The mathematical function ΔD m = arctan 2 (dω / dt) also provides an upper limit and ensures that ΔD m remains positive throughout the transient process of acceleration / deceleration.

[0057] Figure 4 The hardware-in-the-loop experimental results of the proposed adaptive inertia damping coordination (AIDC) control method are given.

[0058] To verify the enhanced transient stability of the proposed AIDC control, a grid voltage sag is selected as a large disturbance. In this case, when a fault occurs, the grid voltage drops to 0.55 p.u., and after the fault is cleared, the grid voltage recovers to 0.85 p.u. under normal conditions. Figure 4 (a1 - a2) shows the waveforms of the traditional VSG control at different fault clearing times ΔT. In this case, the critical clearing time CCT1 is 1.23 s. Figure 4 (a3) shows the virtual inertia J and damping coefficient D of the traditional VSG control m values. J and D m are kept fixed. Figure 4 (b1 - b2) shows the waveforms of the adaptive virtual inertia control at different fault clearing times ΔT. In this case, the critical clearing time CCT2 is 1.55 s. When ΔT = 1.56 s, the system converges to another equilibrium point, which is considered unstable in engineering. Figure 4 (b3) shows the virtual inertia J and damping coefficient D of the adaptive virtual inertia control m values. The method of only adaptively changing the virtual inertia during the transient process can adaptively change J to expand the CCT and enhance the transient synchronous stability of the grid-forming converter. Figure 4 (c1 - c2) shows the waveforms of the AIDC control at different fault clearing times ΔT. In this case, the critical clearing time CCT3 is 2.00 s, which is much longer than the previous two methods. When ΔT = 2.01 s, the system converges to another equilibrium point, i.e., it is unstable. Figure 4 (c3) shows the value curves of the virtual inertia J and damping coefficient D of the AIDC control m values. Both J and D m are adaptively changed to expand the CCT and enhance the transient stability during the transient process. The proposed AIDC control method has more advantages in improving transient stability.

[0059] As described above, it is only the preferred embodiment of the present invention, and it is not a limitation of the present invention in any other form. Any modification or equivalent change made according to the technical essence of the present invention still belongs to the scope protected by the present invention.

Claims

1. An adaptive inertia damping cooperative control method for improving the transient stability of a grid-forming converter, based on virtual synchronous generator control, characterized in that, The details are as follows: The control expression of the adaptive inertial damping cooperative control method is as follows: Among them, P * represents the active power parameter. P is the output active power of the grid-forming converter. J and D m respectively represent the virtual inertia constant and the damping coefficient. ω * is the voltage angular velocity reference, and ω is the angular velocity of the output voltage of the grid-forming converter; The design criterion of the virtual inertial damping of the control method is as follows: where, ΔJ and ΔD m represent the dynamic control variables of the virtual inertia constant and damping coefficient of the design; The virtual inertia is designed to increase adaptively in the acceleration region and decrease adaptively in the deceleration region, while the damping increases adaptively during the entire transient process to enhance the damping effect; The specific description is as follows: The power grid fault occurs at point B and time t1. During the first stage from B to C, from t1 to point t2, the frequency will accelerate and the transient power angle will increase. This region is called the acceleration region. At this time, a large virtual inertia is required to slow down the deviation trend and reduce the maximum frequency deviation. At point C, it reaches the steady state point. During the second stage from C to D, from t2 to t3, the frequency will decelerate, but Δω is greater than zero and the power angle still increases. This region is called the deceleration region; During this process, a small inertia is required to enable the grid-forming converter to quickly recover to the frequency reference; The adaptive adjustment of the virtual inertia and damping is realized based on the arctan function. The specific description is as follows Among them, J0 and D m0 represent the static control variables of the virtual inertia constant and damping coefficient, while ΔJ and ΔD m represent the dynamic control variables of the designed virtual inertia constant and damping coefficient, k J and k D represent the adaptive coefficients of virtual inertia and damping respectively.

2. The adaptive inertia damping collaborative control method for improving the transient stability of the grid-forming converter according to claim 1, characterized in that, The adaptive inertial damping cooperative control method also includes two aspects: 1) Adaptive virtual inertia based on Δω and dω / dt; The virtual inertia J0+ΔJ needs to increase in the acceleration region and decrease in the deceleration region, and is adaptively changed according to the combination of Δω and dω / dt. In addition, the maximum virtual inertia J max is related to the instantaneous power capacity limit of the inverter and requires a minimum virtual inertia J min to meet the frequency stability requirements. Therefore, there needs to be an upper and lower limit for the adaptive virtual inertia. The mathematical function ΔJ = arctan(Δω·dω / dt) ensures the boundary of the adaptive virtual inertia; 2) Adaptive damping based on dω / dt; Damping coefficient D0 + ΔD m is designed to increase adaptively according to dω / dt and expand the system damping effect under disturbances, ΔD m is the transient damping term, which needs to be positive during the transient stability problem and return to zero at steady state. The mathematical function ΔD m = arctan 2 (dω / dt) also provides an upper limit and ensures that ΔD m remains positive throughout the transient process of acceleration / deceleration.

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