Receiving end power grid emergency frequency control method based on optimal frequency track

Abstract

The invention provides a receiving-end power grid emergency frequency control method based on an optimal frequency track, and the method comprises the steps: firstly obtaining a speed regulator-prime mover model and parameters of a synchronous unit, calculating the maximum frequency deviation under the optimal frequency track through an optimal control algorithm, building an equivalent transfer function of a wind power plant participating in frequency response based on the deviation, and carrying out the emergency frequency control of the wind power plant. For an expected accident, an evaluation model is constructed by taking a frequency safety limit value as a constraint, extra demands of system inertia and frequency modulation intensity are solved, a frequency response model containing virtual inertia and droop control is established by combining multi-type frequency modulation resource characteristics and a wind power plant equivalent model, and a frequency response model containing virtual inertia and droop control is established by taking control cost minimization as a target. And the emergency power control quantity of each resource and the corresponding virtual control parameter are optimized and solved in combination with the constraint condition, and finally the virtual inertia coefficient of the network construction type new energy station is adjusted according to the inertia and frequency modulation requirements. Various flexible frequency modulation resources are comprehensively considered, and the solving process of the emergency frequency control optimization model is simplified.

Description

Technical Field

[0001] The present application relates to the technical field of power systems, and in particular to the field of power grid frequency control. Background Art

[0002] As various types of renewable energy power generation devices gradually replace traditional thermal power plants, the power grid in most areas of a certain place gradually shows the characteristics of low inertia and low frequency modulation capability while the penetration rate of renewable energy continues to increase. The resulting system frequency safety and stability issues cannot be ignored. Considering that frequency instability events in the receiving power grid are mainly caused by large-scale active power shortage accidents, when the equivalent inertia and frequency modulation capability are gradually weakened, the frequency of the receiving power grid will be more likely to deviate seriously or even become unstable when accidents such as DC locking occur. In order to effectively deal with low-frequency events after active power shortage in the receiving power grid, it is necessary to fully tap the frequency modulation potential of various types of flexible frequency modulation resources in the power grid and construct an emergency frequency control method that coordinates multiple frequency modulation resources.

[0003] The active power output of wind turbines has highly adjustable flexibility and freedom. The additional active power control link can deeply tap the frequency active support capability of wind farms, effectively improving the frequency safety and stability of the receiving power grid. Traditional wind power additional active power control is comprehensive inertia control, which includes virtual inertia control and virtual droop control links. It can adjust the wind power active power output by tracking system frequency changes to provide power support to the power grid. However, wind farms under this control method need to face the risk of excessive release of rotor kinetic energy leading to disconnection from the grid and exiting frequency modulation causing a secondary drop in frequency. Therefore, it is necessary to improve the additional active power control of wind power to ensure that the rotor speed of wind farms is restored after participating in frequency response and that frequency modulation is exited without loss.

[0004] After determining the frequency regulation control strategy of the wind farm, in order to further reduce the maximum frequency change rate and the maximum frequency deviation after the active power accident occurs in the receiving power grid, it is also necessary to make full use of other types of frequency regulation resources in the power grid for emergency power support. How to reasonably utilize the frequency regulation potential of various flexible frequency regulation resources such as high-voltage direct current, new energy storage power stations, pumped storage power stations and adjustable loads, and coordinate various frequency regulation resources to jointly participate in the emergency power support of the receiving power grid is an urgent problem to be solved. Summary of the invention

[0005] In order to overcome the above technical defects, the present application provides a receiving-end power grid emergency frequency control method based on the optimal frequency trajectory. To achieve the above purpose, the present application is implemented according to the following technical solutions: In a first aspect, the present application provides a receiving-end power grid emergency frequency control method based on an optimal frequency trajectory, comprising: Obtain the specific model and parameter data of the synchronous unit speed regulator-prime mover of the receiving power grid, as well as the control methods of various flexible frequency regulation resources; Based on the specific model and parameter data of the synchronous unit speed governor-prime mover, the maximum frequency deviation value in the frequency response scenario of the wind farm station is obtained; Based on the maximum frequency deviation value, determining the equivalent transfer function of each wind farm station participating in the frequency response; Set the anticipated accidents, take the frequency safety limit as the constraint condition, and build the frequency support strength demand assessment model; Solving the frequency support strength demand assessment model to obtain additional demand for the receiving-end power grid inertia strength and frequency regulation strength; Combining the control methods of various flexible frequency regulation resources and the equivalent transfer functions of each wind farm participating in the frequency response, an optimal frequency trajectory response model including multiple types of frequency regulation resources is established; Construct an emergency frequency control optimization model with the minimization of total control cost as the objective function; Determining the constraint conditions of the emergency frequency control optimization model according to the optimal frequency trajectory response model; Solving the emergency frequency control optimization model to obtain emergency power control amounts for various types of frequency modulation resources; Determining virtual inertia and virtual droop coefficients sent by each type of frequency modulation resource based on the emergency power control amount of each type of frequency modulation resource; Based on the inertia strength of the receiving-end power grid and the additional demand for the frequency regulation strength, the virtual inertia coefficients of each networking-type new energy station in the receiving-end power grid are adjusted.

[0006] Optionally, the step of obtaining the maximum frequency deviation value in the frequency response scenario in which the wind farm participates based on the specific model and parameter data of the synchronous unit speed governor-prime mover comprises: Based on the specific model and parameter data of the synchronous unit speed governor-prime mover, a speed regulation system model is established; The speed control system model is reduced and aggregated by using the least square method to obtain the equivalent transfer function of the speed control system of all synchronous units; Obtain the synchronous unit rotation inertia and installed capacity data; According to the synchronous unit rotation inertia data, the installed capacity data and the equivalent transfer function of the speed control system of all synchronous units, a first frequency response equation of the power grid including the active power increment of the wind farm is obtained; Based on the first frequency response equation of the power grid, a frequency response state space equation is established with the active power increment of the wind farm as the control variable; Based on the frequency response state space equation, an optimal control model is established with the maximum frequency deviation value as the objective function; The optimal control model is solved to obtain the maximum frequency deviation value in the frequency response scenario in which the wind farm participates.

[0007] Optionally, the determining, based on the maximum frequency deviation value, an equivalent transfer function of each wind farm participating in the frequency response comprises: Based on the maximum frequency deviation value, determining a second frequency response equation of the power grid; Based on the second frequency response equation of the power grid, determining the total equivalent transfer function of all wind farms; Based on the total equivalent transfer function of all wind farms, the equivalent transfer function of each wind farm participating in the frequency response is determined.

[0008] Optionally, the setting of anticipated accidents, taking frequency safety limits as constraints, and constructing a frequency support strength demand assessment model includes: According to the expected accident screening set of the receiving-end power grid, active power shortage accidents whose impact on frequency safety and stability is greater than a first threshold are screened out; A frequency support strength demand assessment model is established with the minimum sum of the inertia strength of the receiving power grid and the additional demand for frequency regulation strength as the objective function, and the maximum frequency deviation value, quasi-steady-state frequency deviation value, maximum frequency change rate, and average frequency change rate being less than the upper limit value as constraints.

[0009] Optionally, determining the constraint conditions of the emergency frequency control optimization model according to the optimal frequency trajectory response model includes: According to the optimal frequency trajectory response model, the power grid frequency constraint conditions of the emergency frequency control optimization model are determined.

[0010] Optionally, the adjusting the proportion of installed capacity of new energy stations in each network type of the receiving-end power grid based on the inertia strength of the receiving-end power grid and the additional demand for the frequency modulation strength comprises: Determining whether the relationship between the inertia strength of the receiving-end power grid and the additional demand for the frequency modulation strength meets a first preset condition; If it is satisfied, the inertia strength shortfall is determined according to the additional demand for the inertia strength and the frequency modulation strength; Based on the inertia strength shortfall, the virtual inertia coefficients of each networking-type new energy station of the receiving-end power grid are adjusted.

[0011] Optionally, the constraint condition of the emergency frequency control optimization model further includes a resource power adjustment space constraint: , In the formula, are the maximum active powers of the four frequency modulation resources, There are four types of FM resources: Active power at the moment, Respectively represent High-voltage DC converter station, Energy storage power station, Pumped storage power station and The active power increment of an adjustable load, They are the total number of four frequency modulation resources respectively.

[0012] Optionally, the constraint condition of the emergency frequency control optimization model also includes a constraint on the energy that can be released by the energy storage power station: , In the formula, Respectively The rated voltage, rated capacity, charging state, minimum and maximum charging state and transmission line flow constraints of each energy storage power station.

[0013] Optionally, the constraint conditions of the emergency frequency control optimization model also include transmission line power flow constraints: , In the formula, Line The maximum active power and The active power delivery value at the moment, is the total number of transmission lines in the power grid, There are four types of FM resources for lines. The power transfer coefficient.

[0014] This application has the following beneficial effects: The advantages of the method proposed in the above-mentioned embodiment of the present application are: ① It ensures that the wind farm station in the receiving-end power grid can realize autonomous speed recovery during the primary frequency regulation process, thereby reducing the secondary frequency drop accident caused by excessive release of rotor kinetic energy of the wind turbine; ② It proposes how to evaluate the inertia strength and frequency regulation strength requirements of the receiving-end power grid under the optimal frequency trajectory. Compared with the method of evaluating the frequency support strength requirement based on the traditional frequency response model, its principle is simpler and the calculation efficiency is higher; ③ The proposed emergency frequency control method comprehensively considers a variety of flexible frequency regulation resources, and by deriving the numerical relationship between the optimal frequency trajectory and the virtual inertia and virtual droop control of the new energy station, it proves the interoperability of the active power increment in step form with the virtual inertia and virtual droop control, thereby simplifying the solution process of the emergency frequency control optimization model.

[0015] In addition to the above-described purposes, features and advantages, the present application has other purposes, features and advantages. The present application will be further described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings: Figure 1 It is a flow chart of a receiving-end power grid emergency frequency control method based on an optimal frequency trajectory provided in an embodiment of the present application; Figure 2 It is a structural diagram of the IEEE39 node system as a test system in the experimental verification stage of the embodiment of the present application; Figure 3 It is a schematic diagram of an optimal frequency trajectory response model constructed based on an improved IEEE39 system during the experimental verification phase of an embodiment of the present application; Figure 4 It is a control block diagram of a new energy station under the optimal frequency trajectory during the experimental verification phase of the embodiment of the present application; Figure 5 This is a schematic diagram comparing the simulation results of the time domain simulation and the optimal frequency trajectory response model during the experimental verification phase of the embodiment of the present application. Figure 5 (a) is a schematic diagram comparing the time domain simulation and the frequency simulation results of the optimal frequency trajectory response model. Figure 5 (b) Schematic diagram of the comparison between the time domain simulation and the optimal frequency trajectory response model power increase simulation results; Figure 6 This is a schematic diagram showing the frequency, wind farm power and speed curves under the optimal frequency trajectory during the experimental verification phase of the embodiment of the present application. Figure 6 (a) is a schematic diagram of frequency change under the optimal frequency trajectory; Figure 6 (b) is a schematic diagram of the wind farm station changing with the rotation speed; Figure 6 (c) is a schematic diagram of the wind farm station changing over time; Figure 6 (d) is a schematic diagram showing the speed curve; Figure 7 It is a schematic diagram of the change of the power grid frequency curve after each frequency regulation resource participates in the emergency frequency control under the 8% load sudden increase accident in the verification stage of the embodiment of the present application; Figure 8 This is a schematic diagram of the operation curve of each frequency regulation resource under an 8% load sudden increase accident in the verification stage of the embodiment of this application; Figure 8 (a) is a schematic diagram of wind power operation curve; Figure 8 (b) is a schematic diagram of the wind power speed operation curve; Figure 8 (c) is a schematic diagram of the energy storage power operation curve; Figure 8 (d) is a schematic diagram of the pumped storage power operation curve. DETAILED DESCRIPTION

[0017] The embodiments of the present application are described in detail below with reference to the accompanying drawings; however, the present application can be implemented in many different ways as defined and covered by the claims.

[0018] In order to solve the technical problems raised in the above background technology, Figure 1 As shown, the present application proposes a receiving-end power grid emergency frequency control method based on an optimal frequency trajectory, comprising: Step S101: Acquire the specific model and parameter data of the synchronous unit speed regulator-prime mover of the receiving-end power grid, as well as the control methods of multiple flexible frequency regulation resources; The specific model and parameter data of the synchronous unit governor-prime mover generally include the prime mover model and parameters and the governor model and parameters. The prime mover model and parameters include the steam turbine model and the water turbine model. The governor model and parameters include the steam turbine governor and the water turbine governor. The governor-prime mover parameters directly affect the frequency stability and dynamic response of the receiving power grid. The steam turbine parameters focus on the steam volume effect, the water turbine parameters focus on the water hammer effect, and the governor parameters need to balance the regulation speed and stability. In practical applications, it is necessary to combine field tests, multi-machine coordinated control and new energy characteristics to optimize parameters to improve system robustness.

[0019] The control methods of various flexible frequency regulation resources generally include control methods of various flexible frequency regulation resources such as high voltage direct current in the receiving power grid, energy storage power stations, pumped storage power stations and adjustable loads. The control method corresponding to each frequency regulation resource is different. For example, the grid-following energy storage power station generally adopts virtual inertia plus virtual droop control.

[0020] Step S102: based on the specific model and parameter data of the synchronous unit speed governor-prime mover, obtaining the maximum frequency deviation value in the frequency response scenario in which the wind farm participates; After obtaining the above-mentioned specific model and parameter data of the synchronous unit debugger-prime mover, a speed control system model is constructed according to the specific model and parameter data of the synchronous unit debugger-prime mover. The speed control system model is a mathematical model used to describe the dynamic characteristics of the synchronous generator set speed governor and prime mover in the power system. Its core is to simulate the response process of the unit to frequency changes in a parameterized way, and its parameters directly affect the frequency stability of the power grid. The steam turbine model focuses on steam volume delay compensation, the water turbine model focuses on water hammer effect suppression, and the modern electro-hydraulic speed governor improves the regulation accuracy through intelligent algorithms. Against the background of increasing proportion of new energy, the model needs to further integrate virtual synchronous control technology to meet the needs of new power systems.

[0021] After the speed regulation system model is constructed, the least squares method is used to reduce the order and aggregate the speed regulation system model, so that the equivalent transfer function (third-order form) of the speed regulation system of all synchronous units is obtained: (1) In the formula, is the third-order equivalent transfer function, are the numerator and denominator coefficients of the transfer function. Decomposed into three first-order inertial links, is the equivalent gain of the first-order inertia link, is the time constant of the first-order inertia link; At this time, the synchronous unit rotation inertia and installed capacity data are obtained. After obtaining the synchronous unit rotation inertia and installed capacity data, the equivalent transfer function of the entire synchronous unit speed control system is combined with formula (1) to establish the first frequency response equation containing the active power increment of the wind farm station as follows: (2) In the formula, is the equivalent inertia coefficient of the power grid, For the The rotational inertia of the synchronous unit, For the The installed capacity of synchronous machines is is the total number of synchronous units in the power grid, is the equivalent damping coefficient, For the Damping coefficient of synchronous units, For the Wind farm installed capacity, is the total number of wind farms in the power grid, is the total capacity, is the frequency deviation, is the active power increment of the wind farm, It is the unbalanced active power of the power grid.

[0022] At this time, when the active power imbalance occurs in the power grid, if the wind farm has not yet participated in the frequency response, the frequency will drop from the rated value until it recovers to the quasi-steady-state frequency. : (3) After obtaining the first frequency response equation containing the active power increment of the wind farm, the active power increment of the wind farm is used as a control variable to establish a frequency response state space equation, as shown below: (4) In the formula, They are the three components of the active power increment of the synchronous unit, for The derivative of Similarly, The energy released during the frequency regulation of the wind farm station. and They are the starting time and the ending time of a frequency modulation respectively.

[0023] by As the control variable, the objective function is the frequency deviation The optimal control model is established at the minimum, and the endpoint constraints of each state variable are as follows: (5) Solve the optimal control model to obtain the maximum frequency deviation Under the optimal frequency trajectory, the maximum frequency deviation will be greater than or equal to the quasi-steady-state frequency when the wind farm station does not participate in the response. ,Right now: (6) In the formula, is the optimal tracking coefficient of the wind farm. and equal to When the wind farm station releases a total energy of 0 during the end of a frequency regulation, the speed can be restored autonomously during the frequency regulation period.

[0024] Step S103: determining the equivalent transfer function of each wind farm participating in the frequency response based on the maximum frequency deviation value; By solving the optimal control model and After that, the second frequency response equation of the power grid can be obtained as follows: (7) In the formula, and is the optimal frequency response coefficient, is the frequency modulation intensity coefficient of the power grid.

[0025] According to the above second frequency response equation of the power grid, the total equivalent transfer function of all wind farms can be determined: : (8) After obtaining the total equivalent transfer function of all wind farms, the weight of each wind farm participating in the frequency response is determined by comprehensively considering the real-time wind speed, rotor kinetic energy and power adjustment space of different wind farms. By allocating them to each wind farm, we can get the equivalent transfer function of each wind farm participating in the frequency response. At this time, the optimal frequency trajectory response model of the power grid can be expressed as: (9) In the formula, Respectively The weight of each wind farm participating in the frequency response and its active power increment, For the The real-time or predicted value of the active power output of each wind farm. is the total number of wind farms in the power grid.

[0026] Active power increment of wind farm When it intersects with its maximum power point tracking (MPPT) curve, it will automatically exit the frequency modulation, and the frequency will return to the quasi-steady-state value. . No. The small signal model of the MPPT link of a wind farm station can be expressed as: (10) In the formula, Add active signal to MPPT, is the MPPT coefficient, are the initial value and speed change of the wind turbine respectively. ,when The wind farm will exit frequency regulation at this time.

[0027] Step S104: setting the anticipated accident, taking the frequency safety limit as a constraint condition, and constructing a frequency support strength demand assessment model; Set the expected accident, that is, according to the expected accident set of the receiving power grid, select the active power shortage accident whose impact on frequency safety and stability is greater than the first threshold. The first threshold can be set by yourself. If it exceeds the threshold, it can be understood that the predetermined accident has a greater impact on frequency safety and stability. Take the frequency safety limit as a constraint, that is, the maximum frequency deviation value, quasi-steady-state frequency deviation value, maximum frequency change rate, and average frequency change rate are less than the upper limit value. The frequency support strength demand assessment model is established, and its specific process is as follows: Determine the active power unbalance caused by anticipated accidents (such as large-scale active power shortage accidents such as DC bipolar blocking) And set the maximum frequency change rate limit at the moment of accident After the accident Average frequency change rate limit within one second , Maximum frequency deviation limit And the frequency quasi-steady-state deviation limit ; make and Represent the additional demand for the inertia strength and frequency regulation strength of the receiving grid respectively. Then the time domain expression of the grid frequency deviation and the corresponding frequency safety limit (including the maximum frequency change rate) are , frequency average change rate , Maximum frequency deviation and the frequency quasi-steady-state deviation The fourth) are: (11) In the formula, is the average time.

[0028] Combination , , as well as , build a frequency support intensity demand assessment model: (12) Step S105: solving the frequency support strength demand assessment model to obtain additional demand for the receiving-end power grid inertia strength and frequency modulation strength; Solving the above formula (10), that is, solving the frequency support strength demand assessment model, can determine the current receiving-end power grid inertia strength: and the additional demand for FM intensity .

[0029] Step S106: combining the control methods of various flexible frequency regulation resources and the equivalent transfer functions of each wind farm station participating in the frequency response, establishing an optimal frequency trajectory response model including multiple types of frequency regulation resources; Combining the control methods of various flexible frequency regulation resources such as voltage DC, energy storage water station, pumping station and adjustable load in the receiving-end power grid and the equivalent transfer function of each wind farm participating in the frequency response, an optimal frequency trajectory response model containing multiple types of frequency regulation resources under the optimal frequency trajectory is established, specifically: (13) In the formula, Respectively represent High-voltage DC converter station, Energy storage power station, Pumped storage power station and The active power increment of an adjustable load, are the total number of four types of FM resources; Under the optimal frequency trajectory, the virtual inertia coefficient and virtual droop coefficient of a new energy station (such as high-voltage direct current, energy storage power station) are and , then the station active power increment It can be expressed as: (14) From the above formula, we can see that when When the station output , that is, reasonable setting and The value of can make the active power increment of the new energy station in the frequency response process become a step signal form to offset the unbalanced active power of the receiving end power grid. , and will not damage the power absorption and speed recovery effect of the wind farm station under the optimal frequency trajectory; Furthermore, by observing equations (11) and (14), we can see that under the optimal frequency trajectory, the receiving power grid Mainly and have an impact, but will not and Therefore, the emergency frequency control optimization model can only consider and Constraints are used to preliminarily evaluate the emergency power control amount of each type of frequency modulation resources, and then and The numerical relationship between the two can be used to determine whether the grid-forming new energy station in the receiving power grid needs to provide additional virtual inertia. How to make the judgment will be described in detail later.

[0030] Step S107: constructing an emergency frequency control optimization model with minimization of total control cost as the objective function; According to the above conditions, corresponding deductions can be made, that is, an emergency frequency control optimization model with the minimization of total control cost as the objective function and the active power increment of each type of frequency regulation resource as the decision variable can be constructed to minimize the total control cost: (15) In the formula, are the weight coefficients of the four frequency modulation resources, They are the control costs per kW of active power of the four frequency regulation resources (in pu / kW); Step S108: determining the constraint conditions of the emergency frequency control optimization model according to the optimal frequency trajectory response model; After constructing the emergency frequency control optimization model with the goal of minimizing the total control cost, it is necessary to determine the constraints. The model constraints include grid frequency constraints, frequency regulation resource power adjustment space constraints, energy storage power station releasable energy constraints, and transmission line flow constraints.

[0031] 1) Grid frequency constraint: According to formula (13), it can be directly derived that the grid frequency constraint condition of the emergency frequency control optimization model is determined according to the optimal frequency trajectory response model: (16) 2) Frequency modulation resource power adjustment space constraints: (17) In the formula, are the maximum active powers of the four frequency modulation resources, There are four types of FM resources: Active power at the moment; 3) Energy storage power stations can release energy constraints: (18) In the formula, Respectively The rated voltage, rated capacity, charging state, minimum and maximum charging state of each energy storage power station; 4) Transmission line flow constraints: (19) In the formula, Line The maximum active power and The active power delivery value at the moment, is the total number of transmission lines in the power grid, There are four types of FM resources for lines. The power transfer coefficient.

[0032] Step S109: solving the emergency frequency control optimization model to obtain emergency power control amounts for various types of frequency modulation resources; The above emergency frequency control model is solved. According to the solution results and the above derivation, the emergency power control amount of each type of resource can be determined, and the active power increment signal is sent to each pumped storage power station operating under electric conditions and adjustable loads, so that they can slow down the frequency drop by cutting off the water pump or load after the expected accident occurs; the virtual inertia and virtual droop parameters are sent to each high-voltage DC converter station, energy storage power station and pumped storage power station operating under power generation conditions, so that they can change their power reference value according to the frequency change after the expected accident occurs, thereby achieving power increase.

[0033] Step S110: determining the virtual inertia and virtual droop coefficient sent by each type of frequency modulation resource based on the emergency power control amount of each type of frequency modulation resource; After obtaining the emergency power control amount of each type of frequency modulation resource, it is necessary to determine the virtual inertia and virtual droop coefficient sent by each type of frequency modulation resource. The specific process of sending is as follows: The first Pumped storage power station (electric operation) and Active power increment of adjustable load and The unbalanced power of the power grid is reduced by cutting off the pumps of the pumped storage power station and the adjustable load. High-voltage DC converter station, Energy storage power station and Active power increment of pumped storage power station (generating condition) and It is sent in the form of virtual inertia and virtual droop coefficient: (20) In the formula, Respectively The virtual inertia coefficient, virtual droop coefficient, and The virtual inertia coefficient, virtual droop coefficient and the Virtual inertia coefficient and virtual droop coefficient of pumped storage power station; Step S111: Based on the grid inertia strength and the additional demand for the frequency regulation strength, the virtual inertia coefficients of each grid-forming new energy station of the receiving-end grid are adjusted.

[0034] According to formula (14) and the corresponding derivation, combined with formula (20), it can be seen that the equivalent inertia strength provided by all frequency regulation resources of the power grid (except wind farms) is , which is related to the additional requirement of inertia strength They are not strictly equal. Therefore, after issuing the increment of active power of frequency regulation resources according to the above steps, it is necessary to determine the inertia strength of the receiving power grid and the additional demand for frequency regulation strength. and The numerical relationship between , that is, when the first preset condition is met, the inertia strength shortage still existing in the receiving-end power grid is determined according to the additional demand for inertia strength and frequency modulation strength. : (twenty one) In order to meet the inertia strength requirements of the receiving power grid, it is necessary to adjust the virtual inertia coefficients of each grid-type new energy station of the receiving power grid, that is, the shortfall The virtual inertia coefficient is secondary distributed according to the power adjustment space of each grid-forming type (GFM) new energy station in the receiving power grid, as shown below: (twenty two) In the formula, Respectively The secondary distribution results of the virtual inertia coefficient of the network-type new energy station and its installed capacity, is the total number of network-building new energy stations, For the The power adjustment space of the grid-type new energy station is set. The secondary distribution result After being sent to each networking station, the frequency response of the receiving power grid will meet the constraints of the frequency safety limit.

[0035] Experimental verification In order to verify the feasibility and accuracy of the above-mentioned receiving-end power grid emergency frequency control method based on the optimal frequency trajectory provided by the embodiment of the present invention, an IEEE 39-node system was built on the DIgSILENT / PowerFactory2022 software platform and a specific implementation example analysis was carried out. Among them, the calculation programs were compiled on the computer using MATLAB.

[0036] Application example: Take the IEEE39 node system as the test system, such as Figure 2 As shown. The conventional synchronous units include 9 thermal power units and 1 hydropower unit. The thermal power units adopt the IEEE-G1 speed governor-prime mover model, and the hydropower units adopt the IEEE-G3 speed governor-prime mover model. In terms of new energy, 30 double-fed wind turbines with a rated output of 6MW are connected to nodes 3, 12, 15, 16, 23, and 29 to form 6 wind farms, of which W1-W4 are grid-following stations, and W5 and W6 are grid-forming stations. The power factor is constant at 0.9; 4 energy storage power stations with a rated power of 60MW are connected to nodes 2, 8, 19, and 24, of which B1 and B4 are grid-following stations, and B2 and B3 are grid-forming stations; 3 45MW pumped storage power stations are connected to nodes 4, 23, and 29 respectively; 1 high-voltage DC line with a rated power of 400MW is connected to node 26; the loads at nodes 8, 18, and 21 are adjustable loads. The total installed capacity of the system's synchronous units is 6500MVA, the inertia time constant is 4.153s, the total system load demand is 7500MW, and the load damping coefficient is 1.

[0037] Based on the above improved IEEE39 system, the optimal frequency trajectory response model is constructed in MATLAB / Simulink, as shown in Figure 3 The control block diagram of the new energy station under the optimal frequency trajectory is shown in Figure 4 The expected accident is set as an 8% sudden increase in power load in the power grid. The PowerFactory time domain simulation results are compared with the constructed optimal frequency trajectory response model (ωop=1.0724). The results are shown in Figure 5 As shown, Figure 5 (a) Comparison of the frequency curves before and after wind power participates in the frequency response. It can be seen that the wind farm effectively reduces the frequency deviation after participating in the frequency response through the equivalent transfer function; Figure 5(b) shows the additional power of all synchronous units and all wind turbines in the power grid and the sum of the two. It can be seen that the additional power of the wind farm station shows the characteristics of increasing first and then decreasing. The additional power in the first half is greater than 0, which is in the stage of rotor kinetic energy release. The additional power in the second half is less than 0, which makes it enter the speed recovery stage and recover to the initial speed at the end of a frequency modulation (the duration of a frequency modulation in this embodiment is set to 20s). The comparison of the calculation results of the time domain simulation and the optimal frequency trajectory response model under the expected accident is shown in Table 1, including the average frequency change rate RoCoFav within 200ms and the maximum frequency deviation ∆fmax. By comparison, it can be seen that the proposed optimal frequency trajectory response model has high accuracy.

[0038] Table 1 Comparison of frequency response indicators between time domain simulation and optimal frequency trajectory response model in the embodiment of the present invention .

[0039] After the wind farm enters the speed recovery stage, when its active power increment is equal to the MPPT increment, it will switch to the MPPT operation model and return to the initial operating point. Figure 6 (a) shows the frequency changes during the process of wind farm station participating in frequency regulation and exiting frequency regulation. It can be seen that the lowest frequency point has been significantly improved. Figure 6 (b) shows the changing relationship between the speed and power of the wind farm during the frequency regulation process. It can be seen that under the optimal frequency trajectory, the wind farm goes through three stages and finally returns to the initial operating point, effectively avoiding the wind farm disconnection accident caused by excessive release of rotor kinetic energy. Figure 6 (c) Figure 6 (d) shows the changes in the active power increment and speed change of the four wind farms during a frequency modulation period. It can be seen that during a frequency modulation period, the active power of the wind farms first increased, then decreased and returned to the initial level, and all completed autonomous speed recovery.

[0040] For the three anticipated accidents of 8% power load surge, synchronous unit G4 tripping, and G7 tripping + 5% load surge, the grid frequency security constraint limits are set as shown in Table 2. By constructing a frequency support strength demand evaluation model and evaluating the additional demands Hex and Rex of inertia strength and frequency regulation strength under the three accidents, the results are shown in Table 2. Figure 7 The frequency curves of some nodes before and after each frequency regulation resource participated in the emergency frequency control under the 8% load surge accident are shown. It can be seen that the frequency drop of the power grid has been significantly improved after the emergency frequency control. Figure 8The output active power curves of each frequency regulation resource under an 8% load surge accident are shown. It can be seen that under the optimal frequency trajectory, setting the virtual inertia and virtual droop coefficients of energy storage, DC and pumped storage power stations according to formula (20) can make their output power approximate to a step form, verifying the feasibility and effectiveness of using the active power increment of each frequency regulation resource as a decision variable in the emergency frequency control optimization model.

[0041] Table 2 Frequency safety constraint limits in the embodiment of the present invention .

[0042] Table 3 lists the active power increments of various types of frequency regulation resources under three types of anticipated accidents. The compound anticipated accident of G7 tripping + 5% load surge has the largest active power shortage, so the sum of emergency control power is the largest. Since the control cost of adjustable load is often higher than other frequency regulation costs, the adjustable load under the three accidents is 0.

[0043] Table 3 Emergency control power of various types of frequency modulation resources under different accidents in the embodiment of the present invention .

[0044] Table 4 lists the frequency response indicators of the power grid after emergency frequency control under three anticipated accidents. By comparing the frequency safety constraint limits given in Table 2, it can be seen that the emergency frequency control method proposed in the present invention effectively improves the frequency stability of the power grid under large-scale active power accidents, so that various frequency response indicators are within the safety constraint range.

[0045] Table 4 Frequency response indicators under different accidents in the embodiment of the present invention (after emergency frequency control) .

[0046] In summary, the advantages of the method proposed in the above embodiments of the present application are: ① It ensures that the wind farm station in the receiving power grid can achieve autonomous speed recovery during the primary frequency regulation process, thereby reducing the secondary frequency drop accident caused by excessive release of rotor kinetic energy of the wind turbine; ② It proposes how to evaluate the inertia strength and frequency regulation strength requirements of the receiving power grid under the optimal frequency trajectory. Compared with the method of evaluating the frequency support strength requirement based on the traditional frequency response model, its principle is simpler and the calculation efficiency is higher; ③ The proposed emergency frequency control method comprehensively considers a variety of flexible frequency regulation resources, and by deriving the numerical relationship between the optimal frequency trajectory and the virtual inertia and virtual droop control of the new energy station, it proves the interoperability of the active power increment in step form with the virtual inertia and virtual droop control, and simplifies the solution process of the emergency frequency control optimization model.

[0047] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A receiving-end power grid emergency frequency control method based on optimal frequency trajectory, characterized in that: include: Obtain the specific model and parameter data of the synchronous unit speed governor-prime mover of the receiving power grid and the control methods of various flexible frequency regulation resources; Based on the specific model and parameter data of the synchronous unit speed governor-prime mover, the maximum frequency deviation value in the frequency response scenario of the wind farm station is obtained; Based on the maximum frequency deviation value, determining the equivalent transfer function of each wind farm station participating in the frequency response; Set the anticipated accidents, take the frequency safety limit as the constraint condition, and build the frequency support strength demand assessment model; Solving the frequency support strength demand assessment model to obtain additional demand for the receiving-end power grid inertia strength and frequency regulation strength; Combining the control methods of the various flexible frequency regulation resources and the equivalent transfer functions of each wind farm station participating in the frequency response, an optimal frequency trajectory response model including multiple types of frequency regulation resources is established; Construct an emergency frequency control optimization model with the minimization of total control cost as the objective function; Determining the constraint conditions of the emergency frequency control optimization model according to the optimal frequency trajectory response model; Solving the emergency frequency control optimization model to obtain emergency power control amounts for various types of frequency modulation resources; Determining virtual inertia and virtual droop coefficients sent by each type of frequency modulation resource based on the emergency power control amount of each type of frequency modulation resource; Based on the inertia strength of the receiving-end power grid and the additional demand for the frequency regulation strength, the virtual inertia coefficients of each networking-type new energy station in the receiving-end power grid are adjusted.

2. The method according to claim 1, characterized in that The step of obtaining the maximum frequency deviation value in the frequency response scenario of the wind farm station based on the specific model and parameter data of the synchronous unit speed governor-prime mover comprises: Based on the specific model of the synchronous unit speed governor-prime mover, a speed control system model is established; The speed control system model is reduced and aggregated by using the least square method to obtain the equivalent transfer function of the speed control system of all synchronous units; Obtain the synchronous unit rotation inertia and installed capacity data; According to the synchronous unit rotation inertia data, the installed capacity data and the equivalent transfer function of the speed control system of all synchronous units, a first frequency response equation of the power grid including the active power increment of the wind farm is obtained; Based on the first frequency response equation of the power grid, a frequency response state space equation is established with the active power increment of the wind farm as the control variable; Based on the frequency response state space equation, an optimal control model is established with the maximum frequency deviation value as the objective function; The optimal control model is solved to obtain the maximum frequency deviation value in the frequency response scenario in which the wind farm participates.

3. The method according to claim 2, characterized in that The step of determining the equivalent transfer function of each wind farm station participating in the frequency response based on the maximum frequency deviation value comprises: Based on the maximum frequency deviation value, determining a second frequency response equation of the power grid; Based on the second frequency response equation of the power grid, determining the total equivalent transfer function of all wind farms; Based on the total equivalent transfer function of all wind farms, the equivalent transfer function of each wind farm participating in the frequency response is determined.

4. The method according to claim 1, characterized in that: The aforementioned setting of anticipated accidents and taking the frequency safety limit as a constraint condition to construct a frequency support intensity demand assessment model includes: According to the expected accident screening set of the receiving-end power grid, active power shortage accidents whose impact on frequency safety and stability is greater than a first threshold are screened out; A frequency support strength demand assessment model is established with the minimum sum of the inertia strength of the receiving power grid and the additional demand for frequency regulation strength as the objective function, and the maximum frequency deviation value, quasi-steady-state frequency deviation value, maximum frequency change rate, and average frequency change rate being less than the upper limit value as constraints.

5. The method according to claim 1, characterized in that Determining the constraint conditions of the emergency frequency control optimization model according to the optimal frequency trajectory response model includes: According to the optimal frequency trajectory response model, the power grid frequency constraint conditions of the emergency frequency control optimization model are determined.

6. The method according to claim 1, characterized in that The additional demand for the inertia strength of the receiving-end power grid and the frequency modulation strength is used to adjust the installed capacity ratio of the new energy stations in each grid type of the receiving-end power grid; including: Determining whether the relationship between the inertia strength of the receiving-end power grid and the additional demand for the frequency modulation strength meets a first preset condition; If it is satisfied, the inertia strength shortfall is determined according to the additional demand for the inertia strength and the frequency modulation strength; Based on the inertia strength shortfall, the virtual inertia coefficients of each networking-type new energy station of the receiving-end power grid are adjusted.

7. The method according to claim 5, characterized in that The constraint conditions of the emergency frequency control optimization model also include adjustment resource power adjustment space constraints: ， In the formula, are the maximum active powers of the four frequency modulation resources, There are four types of FM resources. Active power at the moment, Respectively represent High-voltage DC converter station, Energy storage power station, Pumped storage power station and The active power increment of an adjustable load, They are the total number of four frequency modulation resources respectively.

8. The method according to claim 7, characterized in that The constraint conditions of the emergency frequency control optimization model also include the energy release constraint of the energy storage power station: ， In the formula, Respectively The rated voltage, rated capacity, charging state, minimum and maximum charging state and transmission line flow constraints of each energy storage power station.

9. The method according to claim 7, characterized in that: The constraints of the emergency frequency control optimization model also include transmission line power flow constraints: ， In the formula, Line The maximum active power and The active power delivery value at the moment, is the total number of transmission lines in the power grid, There are four types of FM resources for lines. The power transfer coefficient.

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