Adaptive design method and system for control parameters of network-forming flexible DC converter

Abstract

The invention belongs to the field of active frequency support of power electronic equipment in a network-construction-type flexible direct-current power transmission system, particularly discloses a control parameter adaptive design method and system of a network-construction-type flexible direct-current converter, and aims to solve the problems of insufficient system inertia support and oscillation caused by fixed parameters in a wide-range disturbance scene. The method comprises the steps that a receiving-end converter extracts the deviation value, the change rate and the acceleration of the frequency of a power grid by monitoring the electrical quantity of a grid-connected point in real time, and the system operation state is divided into three modes of steady-state maintenance, fluctuation early warning and emergency response based on threshold judgment; calculating a virtual inertia coefficient by using a saturated nonlinear function, and cooperatively solving a damping coefficient in combination with small signal stability analysis; and establishing a virtual inertia-damping parameter feasible region and outputting the virtual inertia-damping parameter feasible region to a receiving end converter station controller. The limitation of traditional fixed parameter design under the disturbance working condition is broken through, the frequency supporting capacity of the system can be remarkably improved, and the stability problem caused by frequent parameter change is avoided.

Description

Technical Field

[0001] The present application belongs to the field of active frequency support of power electronic equipment in a grid-type flexible direct current transmission system, and more specifically, relates to a control parameter adaptive design method and system for a grid-type flexible direct current converter. Background Art

[0002] With the rapid development of new energy, the power system faces severe challenges of reduced inertia and weakened frequency stability. Grid-forming Modular Multilevel Converter-based High Voltage Direct Current (GFM-MMC-HVDC) has attracted much attention due to its ability to support grid inertia. Currently, the most widely used grid-forming method is the virtual synchronous generator (VSG) control method. However, traditional virtual synchronous generator control methods have significant limitations: virtual inertia ( ) and the damping coefficient ) is difficult to adapt to a wide range of operating conditions and is prone to problems such as frequency overshoot and power oscillation when load changes suddenly or new energy output fluctuates.

[0003] In response to the above problems, there are currently some control parameter adaptive design methods. However, in most control parameter adaptive design methods, the current parameter change is linearly related to the system frequency change rate and frequency deviation, which makes it difficult to adapt to high dynamic disturbance conditions. For example, when the system encounters a large-scale disturbance, the system requires the virtual inertia to remain high to provide inertia support. However, relying solely on linear adjustment may exceed the adjustment range, resulting in an inability to meet the system inertia requirements. Moreover, relying solely on angular frequency deviation and its first-order derivative will have lags, and it is easy to have problems with untimely support. For example, at the beginning of a sudden load increase, the frequency change has not yet reached the threshold, but the system actually needs a higher inertia to suppress the frequency mutation. At this time, linear control cannot provide sufficient support in time due to response delay, resulting in a rapid expansion of the frequency deviation in the initial stage of the transient process. In addition, the traditional adaptive control algorithm will and Independent control can hinder the optimization of system dynamic performance. When virtual inertia increases to suppress overshoot, if the damping coefficient is not adjusted synchronously, the system damping ratio may deviate from the critical value, causing underdamped oscillations or overdamped hysteresis, deteriorating the system's transient performance. Therefore, it is necessary to improve and optimize traditional control parameter adaptive design methods. These shortcomings seriously limit the engineering application value of power electronic equipment controlled by VSG networking technology in flexible DC transmission systems. Summary of the Invention

[0004] In response to the defects of the prior art, the purpose of this application is to provide a control parameter adaptive design method and system for a grid-type flexible DC converter, aiming to solve the problems in the existing control parameter adaptive design method, such as support delay, difficulty in continuously providing high inertia support, and deterioration of system transient performance caused by independent control of virtual inertia and damping coefficient due to the linear correlation between the change amount and the frequency change rate or frequency deviation.

[0005] To achieve the above objectives, in a first aspect, the present application provides a method for adaptively designing control parameters of a grid-type flexible DC converter, comprising the following steps: Real-time extraction of grid frequency deviation, frequency change rate, and frequency change acceleration, combined with preset threshold judgment conditions, to determine the operating status of the grid-connected flexible DC converter; Design the virtual inertia adaptive function and damping coefficient function corresponding to the operating state of the grid-type flexible DC converter. Then, combine the real-time operating parameters of the grid-type flexible DC converter to establish the feasible range of the virtual inertia and damping coefficient. Input the adaptive virtual inertia coefficient and damping coefficient into the controller of the receiving-end converter station. Among them, the construction method of the virtual inertia adaptive function and damping coefficient function corresponding to different operating states in the grid-type flexible DC converter is: Based on the grid frequency deviation, frequency change rate, and frequency change acceleration, combined with preset threshold judgment conditions, the operating status of the grid-connected flexible DC converter is divided into steady-state maintenance, fluctuation warning, and emergency response; Based on the different operating states of the grid-type flexible DC converter, a nonlinear function with saturation characteristics is used to construct the virtual inertia adaptive function, and the small signal analysis method is used to collaboratively construct the damping coefficient function.

[0006] Further preferably, when the absolute value of the grid frequency change rate satisfies < When , it is determined that the grid-type flexible DC converter is in a steady-state maintenance state; is the grid frequency; threshold is the maximum value of the steady-state frequency fluctuation of the grid-type flexible DC converter; when the absolute value of the grid frequency change rate satisfies < < ,and ≥ When , it is determined that the grid-type flexible DC converter is in emergency response state; for 5 times; It represents the worsening trend of the power grid frequency change. According to the principle that twice the order of magnitude is much greater, its value is approximately 100 times; when the absolute value of the grid frequency change rate meets > When , it is determined that the grid-type flexible DC converter is in a fluctuation warning state.

[0007] Further preferably, the constructed virtual inertia adaptive function is:

[0008]

[0009] in, is the virtual inertia adaptive function; for Initial reference value of and They are Upper and lower limits of change; is the grid frequency deviation; is the grid frequency change rate; is the grid frequency change acceleration.

[0010] Further preferably, the feasible range of the virtual inertia and damping coefficient is:

[0011]

[0012]

[0013]

[0014] in, is the rated capacity of the system, Half of the system's rated capacity; is the VSG output angular frequency; is the initial output angular frequency of VSG; is the maximum value of VSG output angular frequency; is the minimum value of VSG output angular frequency; is the damping coefficient; ; is the frequency-power droop coefficient in active power regulation; is the effective value of the potential in the converter; is the actual converter terminal voltage, is the virtual inertia in VSG; is the virtual reactance; and They are Upper and lower limits of change; and They are Upper and lower limits of change; ; ; is the damping ratio in the second-order system; is the maximum damping ratio in the second-order system.

[0015] In a second aspect, the present application provides a control parameter adaptive design system for a grid-type flexible DC converter, comprising: The operating status determination module is used to extract the grid frequency deviation, frequency change rate and frequency change acceleration in real time, and determine the operating status of the grid-connected flexible DC converter based on the preset threshold judgment conditions; Parameter function extraction module, used to design the virtual inertia adaptive function and damping coefficient function corresponding to the operating state of the grid-type flexible DC converter; The feasible domain range establishment module is used to construct the real-time operating parameters of the grid-type flexible DC converter and establish the feasible domain range of the virtual inertia and damping coefficient; A parameter determination module is used to combine the outputs of the parameter function extraction module and the feasible region range establishment module to determine the virtual inertia coefficient and the damping coefficient, and transmit them to the controller of the receiving end converter station; The operating state classification module is used to classify the operating state of the grid-connected flexible DC converter into steady-state maintenance, fluctuation warning, and emergency response based on the grid frequency deviation, frequency change rate, and frequency change acceleration, combined with preset threshold judgment conditions; The parameter function construction module is used to construct the virtual inertia adaptive function based on the different operating states of the grid-type flexible DC converter using a nonlinear function with saturation characteristics, and to collaboratively construct the damping coefficient function using the small signal analysis method.

[0016] Further preferably, the operating state classification module is used when the absolute value of the grid frequency change rate meets < When , it is determined that the grid-type flexible DC converter is in a steady-state maintenance state; is the grid frequency; threshold is the maximum value of the steady-state frequency fluctuation of the grid-type flexible DC converter; when the absolute value of the grid frequency change rate satisfies < < ,and ≥ When , it is determined that the grid-type flexible DC converter is in emergency response state; for 5 times; It represents the worsening trend of the power grid frequency change. According to the principle that twice the order of magnitude is much greater, its value is approximately 100 times; when the absolute value of the grid frequency change rate meets > When , it is determined that the grid-type flexible DC converter is in a fluctuation warning state.

[0017] Further preferably, the virtual inertia adaptive function constructed in the parameter function construction module is:

[0018]

[0019] in, is the virtual inertia adaptive function; for Initial reference value of and They are Upper and lower limits of change; is the grid frequency deviation; is the grid frequency change rate; is the grid frequency change acceleration.

[0020] The feasible range of the virtual inertia and damping coefficient in the feasible range establishment module is:

[0021]

[0022]

[0023]

[0024] in, is the rated capacity of the system, Half of the system's rated capacity; is the VSG output angular frequency; is the initial output angular frequency of VSG; is the maximum value of VSG output angular frequency; is the minimum value of VSG output angular frequency; is the damping coefficient; ; is the frequency-power droop coefficient in active power regulation; is the effective value of the potential in the converter; is the actual converter terminal voltage, is the virtual inertia in VSG; is the virtual reactance; and They are Upper and lower limits of change; and They are Upper and lower limits of change; ; ; is the damping ratio in the second-order system; is the maximum damping ratio in the second-order system.

[0025] In general, the above technical solutions conceived by this application have the following beneficial effects compared with the existing technologies: This application provides a method for adaptively designing control parameters of a grid-type flexible DC converter by constructing a control parameter adaptive design method including a frequency deviation parameter. , frequency change rate and frequency change acceleration A multidimensional state observer based on the proposed method enables advanced prediction of grid frequency trends and proactive defense mechanisms. Compared to traditional methods that only consider the first-order rate of change of frequency and frequency deviation, the proposed method has a faster dynamic response speed. It can predict the changing trend of system inertia demand through the second-order derivative term at the initial stage of frequency drop, thereby triggering the virtual inertia coefficient increase command in advance, increasing the system inertia, suppressing system frequency deterioration, and significantly improving the system's frequency support capability.

[0026] This application provides a method for adaptively designing control parameters for a grid-type flexible direct current converter, establishing a dynamic regulation model with nonlinear saturation characteristics. This function maintains a high gain characteristic to accelerate parameter convergence when the system fluctuation amplitude is small, and automatically enters the saturation region when the fluctuation amplitude exceeds a preset threshold. Compared with existing linear regulation methods that are prone to parameter overshoot and oscillation instability under strong disturbances, this application effectively avoids the continuous fine-tuning of parameters caused by frequent small disturbances through a flexible constraint mechanism for the parameter change rate, significantly improving the anti-interference robustness of the control system and avoiding stability issues caused by frequent parameter changes.

[0027] The present application provides a method for adaptively designing control parameters of a grid-type flexible DC converter, which coordinates the virtual inertia and the damping coefficient, and ensures that the parameter adjustment process is always within the feasible domain through preset upper and lower limit constraints. Compared with traditional independent control, it can achieve matching changes of the two parameters when the system operating conditions change, so that the system can maintain transient stability and improve system stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a flow chart of a method for adaptively designing control parameters of a grid-type flexible DC converter provided in an embodiment of the present application; Figure 2 This is a structural block diagram of the flexible direct current transmission system provided in an embodiment of the present application; Figure 3 This is a control block diagram of a sending-end converter station of a flexible DC transmission system provided in an embodiment of the present application; Figure 4 This is a control block diagram of a receiving-end converter station of a flexible DC transmission system provided in an embodiment of the present application; Figure 5 This is a schematic diagram of the architecture of the phase-locked loop module and the frequency dynamic monitoring module in the station-level control unit of the flexible DC transmission system provided in an embodiment of the present application; Figure 6 This is a structural diagram of a small signal model of a VSG active loop of a receiving-end converter station provided in an embodiment of the present application; Figure 7 The embodiment of the present application provides a small signal model transfer function of the receiving converter station VSG. When different values ​​are taken, the system Changing root loci; Figure 8 This is an algorithm flow chart of a control parameter adaptive design method for a grid-type flexible DC converter provided in an embodiment of the present application; FIG9( a ) is an image of three saturation functions provided in an embodiment of the present application; FIG9( b ) is an image of a function model actually used in an embodiment of the present application; FIG10( a ) is a comparison diagram of the frequency change rate curve simulation effects when the flexible HVDC transmission system is subjected to a 5% small load disturbance under the three methods of traditional VSG parameter fixed network control, traditional adaptive adjustment parameter control, and the control parameter adaptive adjustment control provided by the present application, provided in an embodiment of the present application; FIG10( b ) is a comparison diagram of the frequency response curve simulation effects of the three control methods described above when the flexible HVDC system is subjected to a 5% small load disturbance, provided by an embodiment of the present application; FIG10( c ) is a comparison diagram of the simulated output power curves of the receiving-end converter station under the above three control methods when the flexible HVDC transmission system is subjected to a 5% small load disturbance, provided in an embodiment of the present application; FIG11( a ) is a comparison diagram of the simulation effects of the frequency change rate curves of the three control methods described above when the flexible HVDC transmission system is subjected to a 25% large load disturbance, provided in an embodiment of the present application; FIG11( b ) is a comparison diagram of the frequency response curve simulation effects of the three control methods described above when the flexible HVDC system is subjected to a 25% large load disturbance, provided by an embodiment of the present application; FIG11( c ) is a comparison diagram of the simulated output power curves of the receiving-end converter station under the above three control methods when the flexible HVDC transmission system is subjected to a 25% large load disturbance, provided in an embodiment of the present application; FIG12( a ) shows the virtual inertia of the flexible DC transmission system under the traditional adaptive adjustment parameter control and the improved adaptive adjustment parameter control of the present application when the system is subjected to a 25% large load disturbance, as provided in an embodiment of the present application. Simulation comparison chart of change curve; FIG12( b ) shows the virtual inertia of the flexible DC transmission system under the adaptive control of the above two control parameters when the system is subjected to a 25% load disturbance, as provided in an embodiment of the present application. Simulation comparison chart of change curves. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solutions and advantages of this application more clear, the following further describes this application in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0030] The term "and / or" as used herein describes an association between related objects, indicating that three possible relationships exist. For example, "A and / or B" can represent: A exists alone, A and B exist simultaneously, or B exists alone. The symbol " / " as used herein indicates that the related objects are in an "or" relationship, for example, A / B means either A or B.

[0031] The terms "first" and "second" and the like in the description and claims herein are used to distinguish different objects rather than to describe a specific order of the objects.

[0032] In the embodiments of this application, words such as "exemplary" or "for example" are used to indicate examples, illustrations, or descriptions. Any embodiment or design described as "exemplary" or "for example" in the embodiments of this application should not be interpreted as being preferred or advantageous over other embodiments or designs. Rather, the use of words such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0033] In the description of the embodiments of the present application, unless otherwise specified, “plurality” means two or more.

[0034] The embodiments of the present application are described below in conjunction with the drawings in the embodiments of the present application.

[0035] like Figure 1 As shown, the present application provides a method for adaptively designing control parameters of a grid-type flexible DC converter, which specifically includes the following steps: Step S1: The receiving-end converter monitors the electrical quantities at the grid connection point in real time, extracts three types of dynamic characteristics: grid frequency deviation, frequency change rate, and frequency change acceleration, and divides the operating state of the grid-connected flexible DC converter into steady-state maintenance, fluctuation warning, and emergency response based on preset threshold judgment conditions. Step S2: Based on the operating state divided in step S1, a nonlinear function with saturation characteristics is used to calculate the virtual inertia coefficient, and a small signal analysis method is used to collaboratively calculate the damping coefficient, and then the generated parameter quantities are sent to the parameter generator; Step S3: The parameter generator calculates the dynamic constraint boundary based on the parameters of the grid-type flexible DC converter, establishes the feasible range of the virtual inertia and damping coefficient, and finally outputs the adaptive parameters to the controller of the receiving converter station; like Figure 2 The figure shows a two-terminal flexible direct current transmission system, which includes a sending-end and a receiving-end power grid, a transformer, a converter valve and a DC line; the sending-end converter valve converts the AC power of the sending-end power grid into DC power, transmits it to the receiving-end converter valve through the DC line, and then the receiving-end converter valve inverts the DC power into AC power and incorporates it into the receiving-end power grid; this application is realized by controlling this system.

[0036] Figure 2 In the control structure diagram of the grid-type flexible DC converter with adaptive control parameters shown in FIG, the sending-end converter valve generates a first control pulse signal PWM through a first controller, and the receiving-end converter valve generates a second control pulse signal PWM through a second controller; specifically, the structural block diagram of the first controller is as follows: Figure 3 As shown in the figure, the control method adopted is vector control, which includes phase-locked loop, DC voltage and reactive power outer loop control, current inner loop control and circulating current suppression; the structural block diagram of the second controller is shown in the figure. Figure 4 As shown in the figure, it adopts the network control of virtual synchronous machine method, including VSG control and current inner loop and circulating current suppression link; among them, the parameters involved in VSG control are and Adopting the adaptive parameters calculated by this application; The present application provides a method for adaptively designing control parameters of a grid-type flexible DC converter, which specifically includes the following steps: Step S1: The receiving-end converter extracts three types of dynamic characteristic quantities, namely, grid frequency deviation, frequency change rate, and frequency change acceleration, by monitoring the electrical quantity at the grid connection point in real time. Figure 5 The schematic diagram of the architecture of the phase-locked loop module and frequency dynamic monitoring module in the receiving-end converter of the flexible DC transmission system is presented; the phase-locked loop module is used to extract the grid fundamental frequency information and phase parameters, and then derive the grid frequency change rate and grid frequency second-order derivative parameters; the specific implementation process includes: collecting the output voltage of the synchronous machine of the receiving-end grid ; Map it from the stationary three-phase coordinate system to the rotating two-phase coordinate system through Park coordinate transformation , where the phase reference required for the rotation transformation is Provided by the synchronization unit; the q-axis component After being processed by the proportional integral regulator, the real-time angular frequency of the power grid is generated By integrating the angular frequency, the grid-side voltage phase ; At the same time, differentiate the angular frequency signal and divide it by , and finally output the grid frequency change rate parameter RoCoF (Right now ); then you will get RoCoF Perform a differential process again and filter it through a low-pass filter to remove high-frequency interference components and obtain the second-order derivative of the grid frequency ( ); Step S2: Select the saturation function model and the variable segmentation principle For the selection of nonlinear functions of saturation characteristics, the commonly used nonlinear functions of saturation characteristics are sigmoid function, hyperbolic tangent function and Function; The function graphs of the three are shown in Figure 9(a); Since the parameters need to be able to change in both directions during the adaptive design of control parameters, it is not appropriate to use the sigmoid function with a value range of (0,1). The tanh(x) tends to saturation too quickly, which will cause the system inertia to be excessive or too little. In addition, the tanh(x) function involves exponential operations and has high computational complexity. The function calculation is more efficient and smoother, so it is suitable to be used as the basis for the design of virtual inertia adaptive parameter adjustment; therefore, the virtual inertia adaptive function of this application is:

[0037] in, is the changed moment of inertia, for The initial reference value of and They are The upper and lower limits of the change to ensure the dynamic stability requirements of the system; To decide The function graph is shown in Figure 9(b). Depend on The function graph shows that the function shown can be used in the variable =0 to realize the initial reference value output. >0, the function value gradually moves toward Approaching, when <0, the function value gradually moves toward approach; In traditional adaptive control That is, the rate of change of angular frequency. In order to avoid the hysteresis problem of traditional adaptive control, this application The selection follows the following model:

[0038] The above formula defines Adaptive adjustment process variables Since the stability of the power system is more intuitively expressed as frequency-related quantities, here The value and segmentation principles are also related to the frequency Related; the specific value rules are as follows: Steady-state condition judgment: When the absolute value of the frequency change rate meets < When the system is in steady state, there is no need to adjust the virtual inertia and control the output = ;Threshold The setting is based on the steady-state frequency fluctuation range of the system operation. Under normal circumstances, the frequency change rate limit for the power system to maintain stable operation is 0.5Hz / s, leaving a certain margin. Select 0.1; this ensures that virtual inertia regulation is not triggered under normal load fluctuations.

[0039] Disturbance condition determination: When The value is greater than the threshold When the system is significantly disturbed, the control module starts virtual inertia adjustment. This stage is divided into two stages according to the frequency change rate and its derivative: When the system meets < < When the absolute value of the second derivative of the frequency is further detected , if it exceeds the threshold This indicates that although the frequency change rate has not reached an emergency state, its change trend shows a rapid deterioration. For example, the frequency drops twice due to insufficient spinning reserve capacity. At this time, the control algorithm introduces the second-order derivative term, which can be adjusted in advance. Achieve rapid support for emergency response to the system; Consider the dynamic response of the system and select 5 times , that is, 0.5; and The principle of selecting the value is that if the system frequency change rate reaches a peak value of 0.5Hz / s within 1 / 4 cycle (0.005s), it means that the system change trend is worsening. At this time, the secondary change rate of the system frequency is 100Hz / s 2 ,therefore Take it as 60; when > When , it means that the system has entered a state of severe disturbance. Although the quadratic rate of change may be large at this time, in order to avoid the risk of oscillation caused by noise amplified by the second-order derivative, the control variable still regresses to the first-order derivative. To establish the virtual inertia in VSG and The dynamic constraint relationship of the system requires small signal modeling analysis. The physical quantity in the system can be regarded as the sum of the steady-state component and the disturbance component. Therefore, the power angle , VSG output angular frequency , grid angular frequency , and the active power output of the converter It can be expressed as:

[0040] Among them, the subscript 0 represents the reference value when the system is running in steady state. Then it represents the disturbance component of the corresponding physical quantity pair; Reference internal potential amplitude in VSG The expression is:

[0041] in, is the internal potential reference amplitude of the converter, is the reference value of the converter output reactive power, is the reactive power output by the converter, is the reference value of the converter terminal voltage, is the actual converter terminal voltage, is the voltage regulation coefficient, is the adjustment coefficient, which is used to adjust the response speed of the entire adjustment process; the integral link 1 / s The purpose is to achieve zero-static error regulation, that is, to integrate and accumulate the reactive power deviation and voltage deviation, so that the internal potential is continuously adjusted until the system reactive power and voltage reach the reference value, thus eliminating the steady-state error; The VSG model is based on the second-order electromechanical transient mathematical model of the synchronous generator, so its equation is as follows:

[0042] in, is the virtual moment of inertia in VSG, is the virtual damping coefficient, is the virtual mechanical power of the input converter; According to the approximate formula for power system calculation, the active power and reactive power output by the receiving-end converter can be expressed as:

[0043] in, is the effective value of the potential in the converter, is the virtual reactance; Combining the internal potential reference formula, the VSG second-order electromechanical transient mathematical model, and the power expression with the above disturbance expression, and ignoring the secondary disturbance, the small signal model expression of the system can be obtained:

[0044] Where, is the equivalent damping coefficient, Figure 4 It can be seen that the input variable of power droop control is , damping coefficient The input variables are ,according to The equivalent damping coefficient of the receiving-end converter is The expression is as follows:

[0045] in, for Figure 4 Frequency-power droop coefficient in active power regulation shown; Based on the above analysis, the system small signal model expression is Laplace transformed to obtain the small signal model of the VSG active loop as shown in Figure 6. Therefore, the transfer function expression of the dynamic response characteristics of the VSG active power can be expressed as:

[0046] Further analysis It can be seen that it can be transformed into a standard second-order system:

[0047] Therefore, the natural angle oscillation frequency corresponding to this second-order system is and damping ratio They are:

[0048] To ensure the rationality of the inertia and damping coefficient designs, it is necessary to clarify the relationship between the two parameters and the system stability. The following is based on the root locus method to study the open-loop transfer function corresponding to different parameters, thereby analyzing the performance of different parameters. The characteristic equation is:

[0049] In order to study the influence of parameters on the system, this characteristic equation is transformed into the form of root locus:

[0050] Its equivalent open-loop transfer function is:

[0051] when When different values ​​are taken, the system The root locus of the change can be represented by Figure 7; Figure 7 It can be seen that when the system's moment of inertia As the value gets larger, the root locus of the system gets closer to the origin, and the intercept with the real axis also decreases, which reflects the decrease in the attenuation rate of the system response and the degradation of the dynamic characteristics. Do not take too large a value.

[0052] When the moment of inertia When it is a fixed value, As the value of becomes larger, the root locus of the system moves from two conjugate imaginary roots to the real axis and intersects with the real axis. Then one branch converges to the origin and the other moves to the infinity of the real axis. When is small, the system behaves as an underdamped mode corresponding to the complex dominant pole, accompanied by significant overshoot. As the two roots intersect on the real axis, the system is in a critical damping state. When the two roots separate, the system is in an overdamped state. As the value gets larger, one of the roots gets closer to the origin, and the system decays slower and slower; therefore Too small will cause the system to oscillate violently, while too large will result in slow dynamic response; In order to improve the dynamic comprehensive performance of the power system, the damping coefficient The selection, through and The mathematical relationship between the two is used to achieve the matching in the adaptive process; the specific implementation is to first calculate the damping ratio of the obtained second-order system. The formula is transformed and deduced The expression:

[0053] To highlight and The relationship, , ;therefore It can be expressed as:

[0054] Among them, A integrates the inherent parameters of the grid voltage, impedance, etc., and B represents the influence of the active control link; according to the classical control theory Taking 0.707 can suppress overshoot and ensure response speed; Step S3: Setting the virtual inertia and damping coefficient parameter range First determine Initial value of ; Due to the nature of VSG, its virtual inertia The physical meaning is the same as the synchronous machine inertia, the synchronous generator inertia coefficient The calculation formula is as follows:

[0055] Therefore, the virtual inertia of VSG can be obtained by using the inertia time constant Quantitative characterization:

[0056] in, is the rated capacity of the system; due to the physical constraints of traditional synchronous units, Remain constant; in contrast, VSG has parameter flexibility, so in this application The selection is combined with the new energy grid connection technical standards and system capacity constraints to adapt the parameters; Then determine and Because the structure of VSG simulates the inertia characteristics of synchronous generator and the frequency regulation characteristics of speed regulator, the active power instruction output by VSG The virtual inertia power and speed regulator to adjust power It consists of two parts:

[0057] When the grid frequency starts to change, the power supported by the system almost comes from the virtual inertia power due to the slow response speed of the primary frequency regulation. , in order to fully utilize the capacity of the converter, the inertia The following relationship should be satisfied:

[0058] because In fact, it can be equivalent to the total droop coefficient of the system. When the angular frequency range is Sometimes:

[0059] in, The value of is set to 314rad / s±3.14rad / s according to the frequency deviation of 50Hz±0.5Hz specified in GB / T15945 "Permissible deviation of frequency of power system for power quality". is the rated capacity of the system, Set to half the system's rated capacity; Therefore, the damping coefficient The minimum value of is:

[0060] Combine and The relational expression can be deduced The minimum value of is:

[0061] Finally, regarding the damping coefficient The maximum value of the system root locus image shows that although Increasing can increase the damping of the system, but if the system damping ratio When it increases to greater than 1, one branch of the system root locus will gradually move toward the origin along the real axis, slowing down the dynamic response speed; therefore The value of should also satisfy the following relationship:

[0062] This relationship shows that During the system adaptive adjustment process, Dynamic adjustment to meet the system transient response requirements; In summary, the control flow chart of the adaptive design method for control parameters of grid-type flexible DC converter is shown in Figure 8. This algorithm monitors the frequency variation characteristics of the system in the transient process in real time and ensures the dual constraints of dynamic response and static stability. and Implement adaptive regulation; when the power grid experiences disturbances, the control module constructs adjustment criteria based on the frequency fluctuation and its related parameters to improve the system's transient support capacity and steady-state performance.

[0063] On the other hand, the present application provides a control parameter adaptive design system for a grid-type flexible DC converter, comprising: The operating status determination module is used to extract the grid frequency deviation, frequency change rate and frequency change acceleration in real time, and determine the operating status of the grid-connected flexible DC converter based on the preset threshold judgment conditions; Parameter function extraction module, used to design the virtual inertia adaptive function and damping coefficient function corresponding to the operating state of the grid-type flexible DC converter; The feasible domain range establishment module is used to construct the real-time operating parameters of the grid-type flexible DC converter and establish the feasible domain range of the virtual inertia and damping coefficient; A parameter determination module is used to combine the outputs of the parameter function extraction module and the feasible region range establishment module to determine the virtual inertia coefficient and the damping coefficient, and transmit them to the controller of the receiving end converter station; The operating state classification module is used to classify the operating state of the grid-connected flexible DC converter into steady-state maintenance, fluctuation warning, and emergency response based on the grid frequency deviation, frequency change rate, and frequency change acceleration, combined with preset threshold judgment conditions; The parameter function construction module is used to construct the virtual inertia adaptive function based on the different operating states of the grid-type flexible DC converter using a nonlinear function with saturation characteristics, and to collaboratively construct the damping coefficient function using the small signal analysis method.

[0064] Further preferably, the operating state classification module is used when the absolute value of the grid frequency change rate meets < When , it is determined that the grid-type flexible DC converter is in a steady-state maintenance state; is the grid frequency; threshold is the maximum value of the steady-state frequency fluctuation of the grid-type flexible DC converter; when the absolute value of the grid frequency change rate satisfies < < ,and ≥ When , it is determined that the grid-type flexible DC converter is in emergency response state; for 5 times; It represents the worsening trend of the power grid frequency change. According to the principle that twice the order of magnitude is much greater, its value is approximately 100 times; when the absolute value of the grid frequency change rate meets > When , it is determined that the grid-type flexible DC converter is in a fluctuation warning state.

[0065] Further preferably, the constructed virtual inertia adaptive function is:

[0066]

[0067] in, is the virtual inertia adaptive function; for Initial reference value of and They are Upper and lower limits of change; is the grid frequency deviation; is the grid frequency change rate; is the grid frequency change acceleration.

[0068] The feasible range of the virtual inertia and damping coefficient in the feasible range establishment module is:

[0069]

[0070]

[0071]

[0072] in, is the rated capacity of the system, Half of the system's rated capacity; is the VSG output angular frequency; is the initial output angular frequency of VSG; is the maximum value of VSG output angular frequency; is the minimum value of VSG output angular frequency; is the damping coefficient; ; is the frequency-power droop coefficient in active power regulation; is the effective value of the potential in the converter; is the actual converter terminal voltage, is the virtual inertia in VSG; is the virtual reactance; and They are Upper and lower limits of change; and They are Upper and lower limits of change; ; ; is the damping ratio in the second-order system; is the maximum damping ratio in the second-order system.

[0073] To further illustrate the control effect of the control parameter adaptive design method for the grid-type flexible DC converter provided by this application, the following is an explanation in conjunction with specific embodiments: To meet the engineering application requirements of HVDC Flexible technology for long-distance, high-capacity power transmission, a two-terminal HVDC Flexible transmission system (Figure 2) was constructed on the MATLAB / Simulink platform. Its DC voltage level is 400 kV, its rated transmission power is 1100 MW, and its initial load is 550 MW. The receiving-end converter station and the receiving-end power grid each bear half of the load, i.e., 275 MW. The main system parameters are shown in Table 1. Table 1

[0074] Verification Aspect 1: Frequency Support Capability of the Technical Solution of This Application Scenario I: Traditional VSG parameter fixed network control; Scenario II: Traditional adaptive parameter control; Scenario III: Improved adaptive parameter control proposed in this invention; Table 2 and Table 3 compare the three scenarios when the system is subjected to 5%~25% load disturbance in 3s~6s. RoCoF and maximum frequency drop Figure 10 (a) and (b) show the data under 5% load disturbance. RoCoF and the receiving-end grid frequency change diagram, characterizing the frequency response of the system under small disturbance conditions; Figures 11 (a) and 11 (b) present the corresponding curves when the load disturbance is 25%, reflecting the frequency response of the system under large disturbance conditions; From the data in Table 2 combined with Figures 10 (a) and 11 (a), it can be seen that the control parameter adaptive design method provided by this application is better than the two traditional control methods. RoCoF The maximum value is reduced by 39.2% and 20.1% on average. The frequency change rate of the control parameter adaptive design method provided by this application is smaller than that of the previous two methods. In terms of frequency stability, the data in Table 3 combined with the curves in Figure 10 (b) and Figure 11 (b) prove that the improved adaptive control Compared with the two control strategies, the average reduction is 44.2% and 34.4%, respectively, which verifies the effectiveness of the proposed strategy in suppressing frequency drops.

[0075] Table 2

[0076] Table 3

[0077] Verification Aspect 2: Power Support Capacity of the Technical Solution of This Application Scenario I: Traditional VSG parameter fixed network control; Scenario II: Traditional adaptive parameter control; Scenario III: Improved adaptive parameter control proposed in this invention; Table 4 quantitatively shows the maximum power support of the receiving-end converter in three scenarios when the system is subjected to a 5% to 25% load disturbance for 3s to 6s. P maxComparison results. Figure 10(c) is the output power curve of the receiving converter station after a 5% load disturbance, which reflects the power response of the system under small disturbance conditions. Figure 11(c) is the output power curve of the receiving converter station after a 25% load disturbance, which reflects the power response of the system under large disturbance conditions. It can be seen from Table 4, Figure 10(c) and Figure 11(c) that after a load disturbance occurs, the power support capacity of the method proposed in the present invention is the largest, which is an average increase of 7.9% and 4.4% respectively compared with the other two control strategies, verifying that the control parameter adaptive design method of the grid-type flexible DC converter proposed in the present invention has a stronger power support capability.

[0078] Table 4

[0079] Verification Aspect 3: Parameter Adaptive Response Comparison For the 25% load disturbance condition, Figures 12(a) and (b) respectively compare the dynamic parameters of the traditional adaptive control and the improved adaptive control proposed in this invention. and Figure 12 (a) shows the dynamic adjustment curve of virtual inertia parameters: in the key adjustment period of 0-20ms, the improved scheme improves the parameter response speed by 2ms-5ms compared with the traditional method, and in the transient process The instantaneous value is improved by 83.3% compared with the traditional method; the damping coefficient adjustment curve in Figure 12(b) further shows that the improved The increase in parameters during the system disturbance phase is significantly greater than that of traditional methods, which increases the system damping during disturbance and correspondingly reduces the system frequency drop.

[0080] Experimental data demonstrates that the improved adaptive algorithm proposed in this paper, through an optimized parameter adjustment mechanism, creates a synergistic enhancement effect between virtual inertia and the damping coefficient. This characteristic not only improves dynamic response speed but also enhances the system's instantaneous support capacity during disturbances, validating the proposed method's combined advantages in dynamic adjustment capability and transient support strength. In summary, compared with the prior art, this application has the following advantages: This application provides a method for adaptively designing control parameters of a grid-type flexible DC converter by constructing a control parameter adaptive design method including a frequency deviation parameter. , frequency change rate and frequency change acceleration A multidimensional state observer based on the proposed method enables advanced prediction of grid frequency trends and proactive defense mechanisms. Compared to traditional methods that only consider the first-order rate of change of frequency and frequency deviation, the proposed method has a faster dynamic response speed. It can predict the changing trend of system inertia demand through the second-order derivative term at the initial stage of frequency drop, thereby triggering the virtual inertia coefficient increase command in advance, increasing the system inertia, suppressing system frequency deterioration, and significantly improving the system's frequency support capability.

[0081] The present application provides a method for adaptively designing control parameters of a grid-type flexible direct current converter. The proposed method establishes a dynamic regulation model with nonlinear saturation characteristics. The function maintains a high gain characteristic to accelerate parameter convergence when the system fluctuation amplitude is small, and automatically enters the saturation zone when the fluctuation amplitude exceeds the preset threshold. Compared with the existing linear regulation methods that are prone to parameter overshoot and oscillation instability under strong disturbances, this solution effectively avoids the continuous fine-tuning of parameters caused by frequent small disturbances through a flexible constraint mechanism of the parameter change rate, significantly improving the anti-interference robustness of the control system. Avoid stability problems caused by frequent parameter changes.

[0082] The present application provides a method for adaptively designing control parameters of a grid-type flexible DC converter. The proposed method coordinates the virtual inertia and the damping coefficient, and ensures that the parameter adjustment process is always within the feasible domain through preset upper and lower limit constraints. Compared with traditional independent control, it can achieve matching changes of the two parameters when the system operating conditions change, so that the system can maintain transient stability and improve system stability.

[0083] The above description is merely a specific embodiment of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

Claims

1. A control parameter adaptive design method for a grid-type flexible DC converter, characterized in that: The following steps are involved: Real-time extraction of grid frequency deviation, frequency change rate, and frequency change acceleration, combined with preset threshold judgment conditions, to determine the operating status of the grid-connected flexible DC converter; Design a virtual inertia system function and damping coefficient function corresponding to the operating state of the grid-type flexible DC converter. Combined with the real-time operating parameters of the grid-type flexible DC converter, establish the feasible range of the virtual inertia and damping coefficients. Input the adaptive virtual inertia coefficient and damping coefficient into the controller of the receiving-end converter station. Among them, the construction method of the virtual inertia coefficient function and damping coefficient function corresponding to different operating states in the grid-type flexible DC converter is: Based on the grid frequency deviation, frequency change rate, and frequency change acceleration, combined with preset threshold judgment conditions, the operating status of the grid-connected flexible DC converter is divided into steady-state maintenance, fluctuation warning, and emergency response; Based on the different operating states of the grid-type flexible DC converter, a nonlinear function with saturation characteristics is used to construct the virtual inertia coefficient function, and the small signal analysis method is used to collaboratively construct the damping coefficient function.

2. The control parameter adaptive design method according to claim 1, characterized in that: When the absolute value of the grid frequency change rate satisfies < When , it is determined that the grid-type flexible DC converter is in a steady-state maintenance state; is the grid frequency; threshold is the maximum value of the steady-state frequency fluctuation of the grid-type flexible DC converter; when the absolute value of the grid frequency change rate satisfies < < ,and ≥ When , it is determined that the grid-type flexible DC converter is in emergency response state; for 5 times; Indicates that the frequency trend of the power grid is deteriorating, and its value is 100 times; when the absolute value of the grid frequency change rate meets > When , it is determined that the grid-type flexible DC converter is in a fluctuation warning state.

3. The control parameter adaptive design method according to claim 2, characterized in that: The constructed virtual inertia adaptive function is: in, is the virtual inertia adaptive function; for Initial reference value of and They are Upper and lower limits of change; is the grid frequency deviation; is the grid frequency change rate; is the grid frequency change acceleration.

4. The control parameter adaptive design method according to any one of claims 1 to 3, characterized in that: The feasible range of virtual inertia and damping coefficient is: in, is the rated capacity of the system, Half of the system's rated capacity; is the VSG output angular frequency; is the initial output angular frequency of VSG; is the maximum value of VSG output angular frequency; is the minimum value of VSG output angular frequency; is the damping coefficient; ; is the frequency-power droop coefficient in active power regulation; is the effective value of the potential in the converter; is the actual converter terminal voltage, is the virtual inertia in VSG; is the virtual reactance; and They are Upper and lower limits of change; and They are Upper and lower limits of change; ; ; is the damping ratio in the second-order system; is the maximum damping ratio in the second-order system.

5. The control parameter adaptive design system of the grid-type flexible DC converter is characterized by: include: The operating status determination module is used to extract the grid frequency deviation, frequency change rate and frequency change acceleration in real time, and determine the operating status of the grid-connected flexible DC converter based on the preset threshold judgment conditions; Parameter function extraction module, used to design the virtual inertia adaptive function and damping coefficient function corresponding to the operating state of the grid-type flexible DC converter; The feasible domain range establishment module is used to construct the real-time operating parameters of the grid-type flexible DC converter and establish the feasible domain range of the virtual inertia and damping coefficient; A parameter determination module is used to combine the outputs of the parameter function extraction module and the feasible region range establishment module to determine the virtual inertia coefficient and the damping coefficient, and transmit them to the controller of the receiving end converter station; The operating state classification module is used to classify the operating state of the grid-connected flexible DC converter into steady-state maintenance, fluctuation warning, and emergency response based on the grid frequency deviation, frequency change rate, and frequency change acceleration, combined with preset threshold judgment conditions; The parameter function construction module is used to construct the virtual inertia adaptive function based on the different operating states of the grid-type flexible DC converter using a nonlinear function with saturation characteristics, and to collaboratively construct the damping coefficient function using the small signal analysis method.

6. The control parameter adaptive design system according to claim 5, characterized in that: The operation state classification module is used when the absolute value of the grid frequency change rate meets < When , it is determined that the grid-type flexible DC converter is in a steady-state maintenance state; is the grid frequency; threshold is the maximum value of the steady-state frequency fluctuation of the grid-type flexible DC converter; when the absolute value of the grid frequency change rate satisfies < < ,and ≥ When , it is determined that the grid-type flexible DC converter is in emergency response state; for 5 times; Indicates that the frequency trend of the power grid is deteriorating, and its value is 100 times; when the absolute value of the grid frequency change rate meets > When , it is determined that the grid-type flexible DC converter is in a fluctuation warning state.

7. The control parameter adaptive design system according to claim 6, characterized in that: The constructed virtual inertia adaptive function is: in, is the virtual inertia adaptive function; for Initial reference value of and They are Upper and lower limits of change; is the grid frequency deviation; is the grid frequency change rate; is the grid frequency change acceleration.

8. The control parameter adaptive design system according to any one of claims 5 to 7, characterized in that: The feasible range of virtual inertia and damping coefficient is: in, is the rated capacity of the system, Half of the system's rated capacity; is the VSG output angular frequency; is the initial output angular frequency of VSG; is the maximum value of VSG output angular frequency; is the minimum value of VSG output angular frequency; is the damping coefficient; ; is the frequency-power droop coefficient in active power regulation; is the effective value of the potential in the converter; is the actual converter terminal voltage, is the virtual inertia in VSG; is the virtual reactance; and They are Upper and lower limits of change; and They are Upper and lower limits of change; ; ; is the damping ratio in the second-order system; is the maximum damping ratio in the second-order system.

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