Multi-dc coordinated frequency control method suitable for wind power high penetration rate sending end power system

Abstract

The application discloses a multi-direct-current coordinated frequency control method suitable for a wind power high-penetration sending-end power system, which comprises the following steps: obtaining a system frequency peak value after a fault and a system frequency protection threshold value; establishing a system frequency response model containing a direct-current modulation frequency adjustment behavior; calculating the approximate sensitivity of the system frequency peak value relative to each direct-current power modulation amount; obtaining the bus voltage value of a direct-current transmission system converter station at a fault clearing time, and calculating the real-time lifting space of each direct-current; obtaining the sensitivity value of the power flow increment of each important power flow section relative to each direct-current transmission line based on a perturbation method; establishing an optimization function with the minimum modulation total amount as an optimization target; and solving the optimization function. The application establishes the sensitivity of the frequency peak value relative to the direct-current modulation amount, can effectively evaluate the frequency support effect of different direct currents, selects the direct-current transmission system with high sensitivity to participate in emergency modulation preferentially, and realizes the frequency control effect with less modulation total amount.

Description

Technical Field

[0001] This invention relates to the field of high-penetration power transmission system technology, specifically a multi-DC coordinated frequency control method applicable to high-penetration wind power transmission systems. Background Technology

[0002] The large-scale grid connection of renewable energy generation, especially wind power, via power electronic equipment will also adversely affect the frequency characteristics of AC / DC hybrid systems. First, the high proportion of wind power systems reduces the proportion of traditional turbines, significantly lowering the system's inertia level. In addition, wind turbines do not have primary frequency regulation capabilities, further weakening the system's frequency regulation ability and increasing the risk of severe frequency deviations. This could subsequently lead to automatic disconnection of wind turbines from the grid, further exacerbating the system's frequency stability problem.

[0003] Currently, the main methods for addressing grid frequency regulation include primary frequency regulation, secondary frequency regulation, low-frequency load shedding, and high-frequency generator tripping. Compared to these traditional frequency regulation methods, HVDC transmission systems offer highly controllable power output and rapid adjustment. They can utilize corresponding control mechanisms to regulate line transmission power, providing emergency DC power support to the AC grid after disturbances, thereby effectively reducing generator tripping loads and demonstrating good economic efficiency.

[0004] For existing AC / DC hybrid transmission systems with multiple DC lines, this study investigates how to coordinate DC modulation across these lines to achieve optimal modulation performance. Current research focuses on accurately estimating emergency DC modulation amounts to effectively control steady-state frequency errors. However, for existing power grids with high wind power penetration, many wind turbines are disconnected due to frequency protection to prevent the system frequency from exceeding the threshold. Therefore, wind turbine frequency protection needs to be considered when formulating emergency modulation schemes. Furthermore, to fully utilize the emergency support capabilities of each DC line, the coordination between different DC lines requires further analysis. Based on this, we propose a multi-DC coordinated frequency control method for high-penetration sending-end power systems. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a simple structure and good performance.

[0006] This invention is achieved through the following technical solution: a multi-DC coordinated frequency control method applicable to high-penetration wind power transmission-end power systems, comprising the following steps:

[0007] S100: Obtain the system frequency peak value and system frequency protection threshold after the fault;

[0008] S200: Establish a system frequency response model that includes DC modulation frequency regulation behavior;

[0009] S300: Calculates the approximate sensitivity of the system's peak frequency relative to each DC power modulation.

[0010] S400: Obtain the bus voltage value of the DC transmission system converter station at the time of fault clearance, and calculate the real-time upside potential of each DC transmission line.

[0011] S500: Based on the perturbation method, the sensitivity values ​​of the power flow increment relative to each DC transmission line are obtained for each important power flow section.

[0012] S600: Establish an optimization function with the goal of minimizing the total modulation amount;

[0013] S700: Uses a search method with a limited step size to solve the optimization function and obtain modulation commands for different DC signals.

[0014] Furthermore, in step S200, the system frequency response model is:

[0015]

[0016] Where Δf is the system frequency deviation, and ΔP is the unbalanced power caused by the system disturbance. d τ represents the emergency DC modulation quantity, m represents the number of DC transmission systems participating in the emergency DC modulation, and τ represents the response time.

[0017]

[0018] Among them G g (s) and G L (s) represent the generator model and the dynamic load model, respectively, and H.MVA represents the system's equivalent inertia.

[0019] Define the unit step response as

[0020]

[0021] The system frequency response can then be expressed as At this point, the system frequency peak can be expressed as: Where f0 is the initial frequency before DC is modulated.

[0022] In step S300, the corresponding sensitivity is calculated according to the following formula.

[0023]

[0024] Among them, f max The maximum frequency of the system after DC modulation, ΔP di Let be the power modulation amount of the i-th DC line.

[0025] In step S400, the maximum value of the current command is calculated according to the following formula:

[0026]

[0027] In the formula, N r and N i n represents the number of converter bridges connected in series on the rectifier side and the inverter side, respectively. r and n i These are the transformer turns ratios of the rectifier station and inverter station, respectively, X cr and X ci U is the commutation reactance of the converter transformer referred to the valve side. acr R is the AC bus voltage on the grid side of the converter transformer in the rectifier station. d γ is the resistance of the DC transmission line, γ0 is the inverter-side turn-off advance angle, and α is the inverter-side turn-off angle. min To limit the minimum firing angle.

[0028] The real-time upscaling potential of DC is P dmax =U dr I dmax , among which, U dr This is the actual DC voltage on the rectifier side.

[0029] In step S500, a sensitivity index is established for the cross-sectional power flow relative to each DC emergency modulation quantity:

[0030]

[0031] In the formula, T i,0 and T i,1 Let ΔP be the cross-sectional power flow before and after DC modulation at the i-th power flow section. dk,j This is the power modulation quantity test quantity for the j-th DC transmission system.

[0032] In step S600, the optimization function is:

[0033]

[0034]

[0035] In the formula, F is the total DC modulation amount, and ΔP Mod,j Let f be the modulation amount of the j-th DC transmission system. max,0 and f max,set These are the system frequency peak value before DC modulation and the high-frequency switching action setpoint, T0 and T1, respectively. max These are the initial and limiting values ​​of the tidal current at the cross section, ΔP. Mod and These are the DC modulation matrix and the DC real-time modulated maximum value matrix, respectively, and ξ is the sensitivity matrix of the cross-sectional power flow relative to the DC modulation.

[0036] In step S700, to avoid the obtained modulation quantity always being the transmission power limit of the DC transmission system, the step size of each iteration of the DC modulation quantity solution is limited, as follows:

[0037] S7001: Arrange all non-faulty DC lines in descending order of sensitivity value;

[0038] S7002: Select the DC transmission system with DC line number 1 and calculate the emergency modulation quantity that meets the stability requirements;

[0039] S7003: Set the iteration count to 1 and check if the computational cost is greater than the iteration step size;

[0040] S7004: If so, the emergency modulation quantity of this DC transmission system is set to a step quantity, and then...

[0041] S7005; If not, output the modulation command and the solution ends;

[0042] S7005: Update the modulation amount of the candidate DC, determine whether the constraint conditions are met, if yes, increment the iteration count by 1, and go to S7006; if no, go to S7007.

[0043] S7006: After calculating and outputting the emergency modulation command for this DC line, calculate the peak value of the system frequency and determine whether the frequency stability requirement is met. If yes, output the modulation command for this DC line; otherwise, proceed to step S7007.

[0044] S7007: Select DC serial number plus 1, then switch to S7002.

[0045] The present invention has the following advantages: The multi-DC coordinated frequency control method applicable to high-penetration wind power transmission systems of the present invention establishes the sensitivity of frequency peak relative to DC modulation amount, which can effectively evaluate the frequency support effect of different DCs, select high-sensitivity DC transmission systems to participate in emergency modulation first, and achieve better frequency control effect with less total modulation amount. Attached Figure Description

[0046] Figure 1 This is a flowchart illustrating the present invention;

[0047] Figure 2 This is a system dynamic frequency curve diagram under different emergency control methods of the present invention. Detailed Implementation

[0048] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the scope of the present invention.

[0049] like Figure 1 As shown, the multi-DC coordinated frequency control method for high-penetration power transmission systems includes the following steps:

[0050] S100: Obtain the peak system frequency f after the fault max,0 and the system frequency protection threshold f max,set ;

[0051] S200: Establish a system frequency response model that includes DC modulation frequency regulation behavior. The system frequency response model is as follows:

[0052]

[0053] Where Δf is the system frequency deviation, and ΔP is the unbalanced power caused by the system disturbance. d τ represents the DC emergency modulation quantity, m represents the number of DC transmission systems participating in the emergency DC modulation, and τ represents the response time.

[0054]

[0055] Among them G g (s) and G L (s) represent the generator model and the dynamic load model, respectively, and H.MVA is the equivalent inertia of the system;

[0056] Define the unit step response as:

[0057]

[0058] The system frequency response can then be expressed as:

[0059]

[0060] At this point, the system frequency peak can be expressed as:

[0061]

[0062] Where f0 is the initial frequency before DC is modulated;

[0063] S300: Calculate the approximate sensitivity of the system frequency peak relative to each DC power modulation amount: First, define the following sensitivity based on the expression for the system frequency peak after a fault, to describe the relationship between the system frequency peak after a fault and the modulation amount of each DC power participating in DC modulation. After a fault, give the same DC modulation command to each non-faulty DC power transmission line, observe and record the corresponding system frequency peak, and calculate the corresponding sensitivity according to the following formula:

[0064]

[0065] Among them, f maxThe maximum frequency of the system after DC modulation, ΔP di Let be the power modulation amount of the i-th DC line.

[0066] Considering that the analytical solution of this nonlinear function is difficult to obtain, an approximate estimate of the sensitivity is obtained based on the following first-order difference method:

[0067]

[0068] Among them, f max (ΔP di ) represents the i-th DC transmission line participating in emergency modulation with a modulation amount of ΔP. di At that time, the system's peak frequency, ΔP step f is the DC modulation increment. max (ΔP di +ΔP step ) represents the i-th DC transmission line participating in emergency modulation with a modulation amount of ΔP. di +ΔP step Peak system frequency;

[0069] S400: Obtain the bus voltage value of the DC transmission system converter station at the time of fault clearance, and calculate the real-time scalability of each DC line. This method is based on the following assumptions: system disturbances will not cause a switch in the operating mode of the DC transmission system; the receiving-end system is a large-capacity power system, i.e., the AC bus voltage fluctuation at the receiving end of the converter station is small and negligible; for the DC transmission system initially operating in a constant voltage rectifier-side and constant firing angle inverter-side operating mode, the maximum current command is...

[0070]

[0071] In the formula, N r and N i n represents the number of converter bridges connected in series on the rectifier side and the inverter side, respectively. r and n i These are the transformer turns ratios of the rectifier station and inverter station, respectively, X cr and X ci U is the commutation reactance of the converter transformer referred to the valve side. acr R is the AC bus voltage on the grid side of the converter transformer in the rectifier station. d γ is the resistance of the DC transmission line, γ0 is the inverter-side turn-off advance angle, and α is the inverter-side turn-off angle. min To limit the minimum firing angle.

[0072] The real-time upscaling potential of DC is:

[0073] P dmax =U dr I dmax

[0074] Among them, Udr This is the actual DC voltage on the rectifier side;

[0075] S500: Based on the perturbation method, the sensitivity values ​​of the power flow increment relative to each DC transmission line are obtained for each important power flow section. When forming coordinated emergency control measures for multiple DC lines, it is also necessary to consider whether the power flow section will exceed its stability limit and establish the sensitivity index of the power flow of the section relative to the emergency modulation amount of each DC line.

[0076]

[0077] In the formula, T i,0 and T i,1 Let ΔP be the cross-sectional power flow before and after DC modulation at the i-th power flow section. dk,j For the power modulation quantity test of the j-th DC transmission system;

[0078] S600: An optimization function is established with the goal of minimizing the total modulation amount, while considering constraints such as frequency threshold limitations, DC real-time improvement potential, and power flow section stability requirements. The optimization function is as follows:

[0079]

[0080]

[0081] In the formula, F is the total DC modulation amount, and ΔP Mod,j Let f be the modulation amount of the j-th DC transmission system. max,0 and f max,set These are the system frequency peak value before DC modulation and the high-frequency switching action setpoint, T0 and T1, respectively. max These are the initial and limiting values ​​of the tidal current at the cross section, ΔP. Mod and These are the DC modulation matrix and the DC real-time moduliable maximum value matrix, respectively, and ξ is the sensitivity matrix of the cross-sectional power flow relative to the DC modulation.

[0082] S700: A search method with a limited step size is used to solve the optimization function and obtain the modulation commands for different DC signals. During the optimization function solution process, to avoid the obtained modulation quantity always being the transmission power limit of the DC transmission system, the step size of each iteration of the DC modulation quantity solution is limited, as follows:

[0083] S7001: Arrange all non-faulty DC lines in descending order of sensitivity value;

[0084] S7002: Select the DC transmission system with DC line number 1, and calculate the emergency modulation quantity that meets the stability requirements:

[0085]

[0086] S7003: Set the iteration count to 1, and check if the computational cost is greater than the iteration step size, i.e., ΔP. Mod,j ≥ΔP step_up ;

[0087] S7004: If so, the emergency modulation amount of this DC transmission system is set to a step size: ΔP Mod,j =ΔP step_up If not, proceed to S7005; otherwise, output the modulation command and end the solution.

[0088] S7005: Update the modulation of the candidate DC: Determine whether the constraints are met, i.e. If yes, increment the iteration count by 1 and go to S7006; otherwise, go to S7007.

[0089] S7006: After calculating and outputting the emergency modulation command for this DC signal, the system frequency peak value... And determine whether the frequency stability requirement is met, i.e., |f max -f max,set If |≤ε, output the DC modulation command; otherwise, proceed to step S7007.

[0090] S7007: Select DC serial number plus 1, then switch to S7002.

[0091] Once the modulation commands under different emergency control methods are calculated according to the process shown in the invention, the system dynamic frequency curves under different emergency control methods are as follows: Figure 2 As shown in Table 1, the comparative data under different emergency control methods are as follows:

[0092] Table 1 Comparison of Schemes under Different Emergency Control Methods

[0093]

[0094] Combination Figure 2 It can be seen that the multi-DC coordinated frequency control method proposed in this invention, by giving full play to the effectiveness of high-sensitivity DC in frequency regulation, still has a good frequency control effect while greatly reducing the total modulation amount.

[0095] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A multi-DC coordinated frequency control method applicable to high-penetration wind power transmission-end power systems, characterized in that: Includes the following steps: S100: Obtain the system frequency peak value and system frequency protection threshold after the fault; S200: Establish a system frequency response model that includes DC modulation frequency regulation behavior; S300: Calculates the approximate sensitivity of the system's peak frequency relative to each DC power modulation. S400: Obtain the bus voltage value of the DC transmission system converter station at the time of fault clearance, and calculate the real-time upside potential of each DC transmission line. S500: Based on the perturbation method, the sensitivity values ​​of the power flow increment relative to each DC transmission line are obtained for each important power flow section. S600: Establish an optimization function with the goal of minimizing the total modulation amount; S700: Uses a search method with a limited step size to solve the optimization function and obtain the modulation commands for different DC signals; In step S300, the corresponding sensitivity is calculated according to the following formula: ； Among them, f max The maximum frequency of the system after DC modulation, ΔP di Let be the power modulation amount of the i-th DC line. For the first Frequency sensitivity of a DC line participating in power modulation; In step S500, a sensitivity index is established for the cross-sectional power flow relative to each DC emergency modulation quantity: ； In the formula, and Let i represent the cross-sectional power flow before and after DC modulation at the i-th power flow cross-section. This is the power modulation quantity test quantity for the j-th DC transmission system. Representing the The tidal current at the first cross section is relative to the first Sensitivity when a DC component participates in power modulation; In step S600, the optimization function is: In the formula, This is the total amount of DC modulation. Let j be the modulation quantity of the j-th DC transmission system. and These are the system frequency peak value before DC modulation and the high-frequency switching action setpoint, respectively. and These are the initial value and the limit of the tidal current at the cross section, respectively. and These are the DC modulation matrix and the DC real-time modulated maximum value matrix, respectively. This is the sensitivity matrix of the cross-sectional power flow relative to the DC modulation. For the first Frequency sensitivity when a DC line participates in power modulation.

2. The multi-DC coordinated frequency control method for high-penetration wind power transmission systems as described in claim 1, characterized in that: In step S200, the system frequency response model is: in For system frequency deviation, The unbalanced power caused by system disturbances For DC emergency modulation, ΔP di Let m be the power modulation amount of the i-th DC line, and m be the number of DC transmission systems participating in emergency DC modulation. For response time, in and These are the generator model and the dynamic load model, respectively. This is the equivalent inertia of the system.

3. The multi-DC coordinated frequency control method applicable to high-penetration wind power transmission systems as described in claim 2, characterized in that: Define the unit step response as The system frequency response can then be expressed as At this point, the system frequency peak can be expressed as Where f0 is the initial frequency before DC is modulated. For Laplace transform, This is the inverse Laplace transform operation.

4. The multi-DC coordinated frequency control method for high-penetration wind power transmission systems as described in claim 1, characterized in that: In step S400, the maximum value of the current command is calculated according to the following formula: In the formula, and These represent the number of converter bridges connected in series on the rectifier side and the inverter side, respectively. and These refer to the transformer turns ratios of the converter transformers in the rectifier station and inverter station, respectively. and The commutation reactance of the converter transformer referred to the valve side. This refers to the AC bus voltage on the grid side of the converter transformer in the rectifier station. For the resistance of a DC transmission line, For the inverter side to turn off the leading angle, To limit the minimum firing angle.

5. The multi-DC coordinated frequency control method for high-penetration wind power transmission systems as described in claim 4, characterized in that: DC real-time upscaling potential is ,in, This is the actual DC voltage on the rectifier side.

6. The multi-DC coordinated frequency control method for high-penetration wind power transmission systems as described in claim 1, characterized in that: In step S700, to avoid the obtained modulation quantity always being the transmission power limit of the DC transmission system, the step size of each iteration of the DC modulation quantity solution is limited, as follows: S7001: Arrange all non-faulty DC lines in descending order of sensitivity value; S7002: Select the DC transmission system with DC line number 1 and calculate the emergency modulation quantity that meets the stability requirements; S7003: Set the iteration count to 1 and check if the computational cost is greater than the iteration step size; S7004: If yes, set the emergency modulation amount of the DC transmission system to a step value and proceed to S7005; if no, output the modulation command and end the solution. S7005: Update the modulation amount of the candidate DC, determine whether the constraint conditions are met, if yes, increment the iteration count by 1, and go to S7006; if no, go to S7007. S7006: After calculating and outputting the emergency modulation command for this DC line, calculate the peak value of the system frequency and determine whether the frequency stability requirement is met. If yes, output the modulation command for this DC line; otherwise, proceed to step S7007. S7007: Select DC serial number plus 1, then switch to S7002.

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