A flywheel energy storage assisted wind power system frequency regulation method and system

Abstract

The application discloses a flywheel energy storage assisted wind power based power system frequency modulation method and system, which is equivalent to a first-order inertia link by flywheel energy storage system and dynamically adjusts the inertia time constant, combines a fuzzy logic controller to optimize a virtual damping coefficient in real time, and significantly improves the stability of the power grid frequency. When the power grid frequency fluctuates, the flywheel energy storage system can quickly respond and output close to the rated frequency modulation power in a very short time, so that the steady-state frequency deviation is strictly controlled within a very small range. Meanwhile, the fuzzy logic is used for dynamically correcting the power instruction, so that the frequency modulation instruction tracking error problem caused by the randomness of the wind power output is effectively solved. In addition, after the frequency modulation is completed, the damping coefficient is dynamically adjusted through the fuzzy decision mechanism, so that the flywheel speed recovery time is greatly shortened, the secondary frequency disturbance caused by the speed lag is avoided, and the recycling efficiency of the flywheel energy storage in the continuous frequency modulation scene is ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power system frequency modulation, and in particular to a power system frequency modulation method and system based on flywheel energy storage assisting wind power. BACKGROUND

[0002] With large-scale grid connection of renewable energy such as wind power, the power system faces the severe challenges of inertia loss, frequency fluctuation aggravation and insufficient frequency modulation resources. Wind turbine generators are connected to the grid through power electronic devices, which are decoupled from traditional synchronous generators, resulting in a significant decrease in the equivalent inertia of the system as the wind power penetration rate increases. For every 10% increase in wind power penetration rate, the system inertia decreases by 15-20%. Under load disturbance, the maximum frequency deviation of the high-proportion wind power grid is more than 50% worse than that of the traditional system, and the traditional thermal power and hydroelectric generators are difficult to meet the rapid frequency modulation demand due to their slow response speed and gradual withdrawal.

[0003] In existing frequency modulation schemes, wind turbine generators can provide short-term power support through virtual inertia or variable pitch control, but at the cost of 5-10% of power generation income, which is economically limited. Electrochemical energy storage can respond quickly and achieve millisecond-level response, but it has short service life, safety hazards and recycling problems, and pumped storage is limited by geographical conditions and response speed. Flywheel energy storage has the advantages of high power density, long service life and second-level fast response, but its large-scale application is still in the demonstration stage. Existing control strategies mostly use fixed inertia models and PI control, which do not solve the problems of dynamic damping cooperation and speed recovery lag, resulting in insufficient frequency modulation accuracy and sustainability.

[0004] The existing flywheel frequency modulation method has three defects: first, the flywheel is simplified as a fixed inertia element, ignoring the dynamic coupling of damping and frequency, and the command and actual response have significant deviations; second, it relies on fixed parameter control, which is difficult to adapt to wind power random fluctuations and multi-time scale frequency modulation demand; third, the mechanical damping is fixed after frequency modulation, which slows down the speed recovery and affects the subsequent response capability. SUMMARY

[0005] The embodiments of the present application provide a power system frequency modulation method and system based on flywheel energy storage assisting wind power, which solves the problem of frequency stability after wind power grid connection.

[0006] To achieve the above purpose, the technical scheme of the embodiments of the present application is:

[0007] In a first aspect, the embodiments of the present application provide a power system frequency modulation method based on flywheel energy storage assisting wind power, comprising: establishing a frequency response model of a flywheel energy storage system and a power system, and equivalent the flywheel energy storage system to a first-order inertia element, and its transfer function is represented as:

[0008] ;

[0009] wherein, is a droop control coefficient of the flywheel energy storage system, is an inertia time constant of the flywheel energy storage, the time constant being determined by a ratio of a flywheel moment of inertia to a total damping coefficient of the system;

[0010] The flywheel energy storage system is centrally configured at an outlet bus of a wind farm. The flywheel energy storage system is composed of a plurality of flywheel energy storage units in parallel through a DC bus, and is connected to the power grid through a grid-side converter and a step-up transformer;

[0011] The frequency deviation and its rate of change of the power grid are collected in real time. A fuzzy logic controller is used to dynamically adjust a virtual damping coefficient according to the frequency deviation and its rate of change. The virtual damping coefficient and the mechanical damping together constitute the total damping coefficient.

[0012] By adjusting the moment of inertia and the total damping coefficient, the time constant of the first-order inertia link meets the preset fast response requirement of the power grid frequency regulation.

[0013] The flywheel charging and discharging power instruction is generated based on the frequency deviation, and is dynamically corrected by a fuzzy logic controller to adapt to the trend of the power grid frequency change.

[0014] After the frequency regulation is completed, a fuzzy decision mechanism is used to select the damping coefficient according to the flywheel speed deviation and its rate of change, so that the flywheel speed returns to the initial state.

[0015] In some possible implementations, the input variables of the fuzzy logic controller include the power grid frequency deviation and the rate of change of the frequency deviation, and the output includes the virtual damping coefficient. The fuzzy subsets of the fuzzy logic controller are divided into negative large (NL), negative medium (NM), negative small (NS), zero (Z), positive small (PS), positive medium (PM), and positive large (PL), and inference is performed based on a preset fuzzy rule table. The fuzzy rule table is obtained by combining the fuzzy subsets.

[0016] In some possible implementations, the adjustment rule of the virtual damping coefficient includes:

[0017] When the power grid frequency deviation or the rate of change of the frequency exceeds a preset threshold, the virtual damping coefficient is increased to increase the system damping;

[0018] When the power grid frequency tends to be stable, the virtual damping coefficient is reduced to reduce energy loss.

[0019] In some possible implementations, the flywheel charging and discharging power instruction is obtained by the following formula:

[0020] ;

[0021] wherein, is the flywheel charging and discharging power, is a grid frequency deviation, is a virtual damping coefficient, is a rate of change of the grid frequency deviation.

[0022] In some possible implementations, the inertia time constant of the flywheel energy storage is represented as:

[0023]

[0024] wherein, is a flywheel moment of inertia, is a total system damping coefficient.

[0025] In some possible implementations, the fuzzy decision mechanism adopts the following rules in the flywheel speed recovery phase:

[0026] if both the flywheel speed deviation and its rate of change are positive, the total system damping coefficient is reduced to accelerate the speed recovery;

[0027] if the flywheel speed deviation is negative and its rate of change is positive, the total system damping coefficient is kept stable.

[0028] In some possible implementations, the flywheel energy storage unit is connected to the machine-side converter through a bidirectional motor, and the machine-side converter adopts a vector control strategy to realize accurate regulation of the flywheel speed and power.

[0029] In a second aspect, the embodiments of the present application provide a flywheel energy storage assisted wind power frequency modulation system, comprising:

[0030] a wind farm module comprising at least one wind turbine, the wind turbine being connected to the power grid through a power electronic converter;

[0031] a flywheel energy storage module comprising a flywheel body, a bidirectional motor, a machine-side converter and a grid-side converter, the flywheel energy storage module being connected in parallel to the wind farm outlet bus through a DC bus and being connected to the power grid through a step-up transformer;

[0032] a frequency modulation control module configured to detect the grid frequency deviation and the rate of change of the frequency in real time, generate the active power instruction of the flywheel energy storage based on the fuzzy control strategy, and realize the fast charge and discharge response of the flywheel energy storage through virtual droop control;

[0033] The frequency modulation control module further comprises:

[0034] a first-order inertia model for equivalent dynamic response of the flywheel energy storage system;

[0035] a fuzzy logic reasoning unit for dynamically adjusting the output power of the flywheel energy storage according to the fuzzy subset combination of the frequency deviation and the rate of change of the frequency.

[0036] ​The one or more technical solutions provided in the embodiments of the present application have at least the following technical effects or advantages:

[0037] In the embodiments of the present application, the flywheel energy storage system is equivalent to a first-order inertia link and its inertia time constant is dynamically adjusted, and a fuzzy logic controller is combined to optimize the virtual damping coefficient in real time, so that the stability of the power grid frequency is significantly improved. When the power grid frequency fluctuates, the flywheel energy storage system can respond quickly and output close to the rated frequency modulation power in a very short time, so that the steady-state frequency deviation is strictly controlled within a very small range, and the response speed is much faster than that of the traditional thermal power frequency modulation. At the same time, the fuzzy logic is used to dynamically correct the power command, effectively solving the frequency modulation command tracking error problem caused by the randomness of wind power output. In the scene of sudden wind speed change, the frequency minimum deviation is greatly reduced, and the command tracking error is significantly reduced. In addition, after the frequency modulation is completed, the damping coefficient is dynamically adjusted by the fuzzy decision mechanism, so that the flywheel speed recovery time is greatly shortened, the secondary frequency disturbance caused by speed lag is avoided, and the recycling efficiency of the flywheel energy storage in the continuous frequency modulation scene is ensured. The present application takes into account the frequency modulation speed, accuracy and equipment sustainability, and provides an efficient and reliable frequency control solution for new energy high-penetration power grids. BRIEF DESCRIPTION OF DRAWINGS

[0038] In order to more clearly illustrate the embodiments of the present application, the drawings needed to be used in the embodiments of the present application will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0039] Figure 1 An embodiment flowchart of a flywheel energy storage assisted wind power system frequency modulation method provided by the present application is shown in the figure.

[0040] Figure 2 A simplified structure diagram of a regional power grid in the embodiments of the present application is shown in the figure.

[0041] Figure 3 A flywheel energy storage assisted wind power frequency modulation control model in the embodiments of the present application is shown in the figure.

[0042] Figure 4 A fuzzy control principle diagram in the embodiments of the present application is shown in the figure.

[0043] Figure 5 A fuzzy subset relationship in the embodiments of the present application is shown in the figure.

[0044] Figure 6 A thermal power unit and flywheel energy storage replacement ability flowchart in the embodiments of the present application is shown in the figure.

[0045] Figure 7A structure schematic diagram of a power system frequency modulation system based on a flywheel energy storage auxiliary wind power in an embodiment of the present application. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0047] In the related description of the present embodiment, the terms “include, contain, have” and the like are open terms, which are generally preferred to be understood as including but not limited to; the term “at least one” is generally preferred to be understood as one or more, wherein “more” refers to two or more; the term “at least one of” or the like refers to any combination of these terms, including any combination of single or multiple terms, for example, “at least one of a, b or c”, or “at least one of a, b and c”, which can represent a, b, c, a-b (i.e. a and b), a-c, b-c, or a-b-c, wherein a, b, and c can be single or multiple; the symbol “A / B” is used to describe the selection relationship of the associated object, which generally represents the relationship of “or”.

[0048] In the following description of the present embodiment, the terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms “a” and “the” used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0049] Those skilled in the art should understand that in the following description of the present embodiment, the order of the serial numbers does not mean the order of execution, and some or all steps can be executed in parallel or in sequence, and the execution order of each process should be determined according to its function and inherent logic, and should not constitute any limitation on the implementation process of the present embodiment.

[0050] Those skilled in the art should understand that the numerical ranges in the embodiments of the present application should be understood as also specifically disclosing each intermediate value between the upper limit and the lower limit of the range. Each smaller range between any stated value or stated range of values and any other stated value or stated range of values, as well as any other stated value or stated range of values, is also included within the present application. The upper limit and the lower limit of these smaller ranges can be independently included or excluded from the range.

[0051] Technical / scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this application belongs unless otherwise indicated. Although preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application. All documents mentioned herein are incorporated by reference to disclose and describe in full the methods and / or materials which are described herein. In case of conflict between the content of the specification and that of any document incorporated herein by reference, the content of the specification prevails.

[0052] In order to illustrate the technical solutions of the present application, the following will be illustrated by specific examples.

[0053] With large-scale grid connection of renewable energy such as wind power, the power system faces the severe challenges of inertia loss, frequency fluctuation aggravation and insufficient frequency modulation resources. Wind turbine generators are connected to the grid through power electronic devices, which are decoupled from traditional synchronous generators, resulting in a significant decrease in the equivalent inertia of the system as the wind power penetration rate increases. The system inertia decreases by 15-20% for every 10% increase in wind power penetration rate. Under load disturbance, the maximum frequency deviation of the high-proportion wind power grid is more than 50% worse than that of the traditional system, and the traditional thermal power and hydroelectric generators are difficult to meet the demand for rapid frequency modulation due to their slow response speed and gradual withdrawal.

[0054] In the existing frequency modulation scheme, wind turbine generators can provide short-time power support through virtual inertia or variable pitch control, but need to sacrifice 5-10% of the power generation benefit, which is limited in economy; electrochemical energy storage can respond quickly and achieve millisecond-level response, but has the problems of short service life, safety hazards and recycling difficulties, and pumped storage is limited by geographical conditions and response speed. Flywheel energy storage has the advantages of high power density, long service life and second-level fast response, but its large-scale application is still in the demonstration stage. The existing control strategy mainly uses fixed inertia model and PI control, which does not solve the problems of dynamic damping cooperation and speed recovery lag, resulting in insufficient frequency modulation accuracy and sustainability.

[0055] The existing flywheel frequency modulation method has three defects: first, the flywheel is simplified as a fixed inertia element, ignoring the dynamic coupling of damping and frequency, and the deviation between the instruction and the actual response is significant; second, it relies on fixed parameter control, which is difficult to adapt to the random fluctuations of wind power and the demand for multi-time scale frequency modulation; third, the mechanical damping is fixed after frequency modulation, which leads to slow speed recovery and affects the subsequent response capacity.

[0056] Based on this, the embodiments of the present application provide a power system frequency modulation method and system based on flywheel energy storage auxiliary wind power, which solves the problem of frequency stability after wind power grid connection.

[0057] Figure 1 An embodiment flowchart of a power system frequency modulation method based on flywheel energy storage auxiliary wind power provided by the present application is shown in Figure 1 The above method can include:

[0058] S101, a frequency response model of the flywheel energy storage system and the power system is established, the flywheel energy storage system is equivalent to a first-order inertia link, and a transfer function thereof is represented as:

[0059] ;

[0060] wherein, is a droop control coefficient of the flywheel energy storage system, is an inertia time constant of the flywheel energy storage, and the inertia time constant is determined by a ratio of a flywheel moment of inertia to a total damping coefficient of the system;

[0061] In some embodiments, the inertia time constant of the flywheel energy storage is defined as the ratio of the flywheel moment of inertia to the total damping coefficient of the system, and is represented as:

[0062] ;

[0063] wherein, is the flywheel moment of inertia, is the total damping coefficient of the system. The inertia time constant of the flywheel energy storage is defined as a time required for the flywheel energy storage to output a preset target power after receiving a frequency modulation instruction. For example, the inertia time constant can be less than 0.05 s.

[0064] It should be noted that the flywheel moment of inertia is determined by a flywheel rotor mass and geometric size, and can be dynamically adjusted by increasing or decreasing the number of flywheel units or adjusting the material density. The total damping coefficient includes mechanical damping and virtual damping , the mechanical damping includes but is not limited to bearing friction, wind resistance loss, etc., the virtual damping is introduced through a control algorithm, and a relationship between the damping coefficients is: .

[0065] wherein, is a fixed value, and can be dynamically adjusted through a subsequent fuzzy logic controller.

[0066] S102, the flywheel energy storage system is centrally configured at an outlet bus of the wind farm, the flywheel energy storage system is composed of a flywheel array by parallel connection of multiple flywheel energy storage units through a direct-current bus, and is connected to the power grid through a grid-side converter and a step-up transformer;

[0067] For example, assuming that a regional power grid generation unit scenario is mainly composed of a wind farm and a thermal power unit, a flywheel energy storage is used as an auxiliary frequency modulation system for wind power, and the main composition of the flywheel energy storage unit includes a flywheel body, a bidirectional motor, and a machine-side converter. Usually, a plurality of flywheel energy storage units are connected in parallel to a grid-side converter through a DC bus to form a flywheel array system. In order to simplify the control, the flywheel energy storage system is arranged at the outlet bus of the wind farm in a centralized configuration, and is connected to the power grid through a booster transformer and the like. The system topology diagram is shown in FIG. 8. Figure 2

[0068] It should be noted that the wind turbine is a nonlinear system, which can generally have frequency modulation capability through rotor inertia control, overspeed control, and variable pitch control. Among them, the variable pitch control has strong adjustment ability and wide adjustment range, and is suitable for all wind speed modes. When the wind turbine changes the blade angle by using the pitch controller to adjust the unit output to reduce the frequency fluctuation of the power grid, the wind turbine is at a certain operating point below the maximum power point to leave a certain reserve capacity, so the frequency modulation model of the wind turbine is equivalent to a first-order lag transfer function, which is expressed as:

[0069] ;

[0070] Among them, is the frequency modulation coefficient of the wind turbine, is the variable pitch control response time constant of the wind turbine. In order to focus on the effect of flywheel energy storage participating in frequency modulation, a first-order inertia model is usually used as the equivalent model of flywheel energy storage, and a virtual droop control is used as the control method of flywheel energy storage participating in frequency modulation, so the flywheel energy storage control system is composed of a flywheel output control module and a flywheel transfer function module. Therefore, the transfer function of the flywheel energy storage control system can be expressed as:

[0071] .

[0072] With the increasing proportion of wind power, conventional synchronous generators such as thermal power are gradually replaced by wind power, and the adjustment resources providing inertia and frequency modulation response in the power system are gradually reduced, which is manifested as the decrease of the equivalent inertia time constant of the power grid. The coefficient of the conventional synchronous generator is defined as , which is expressed as:

[0073]

[0074] The value range of is 0≤ ≤1, and the coefficient of the synchronous generator is The load-frequency response function of the system is:

[0075] ;

[0076] The response model of flywheel energy storage assisting primary frequency modulation of wind power is shown in FIG. 1, Figure 3 The power provided by the flywheel energy storage for frequency modulation, The power provided by the wind turbine for frequency modulation.

[0077] According to FIG. 2, when the power generation coefficient of the conventional thermal power unit is Figure 3 , the wind power penetration is 1- , the input reference frequency is set to 0, and the frequency response characteristics when the load power changes can be discussed by taking the following three scenarios as examples.

[0078] Scenario 1: The wind turbine is connected to the power grid, but the frequency modulation capability of the wind turbine is not considered, and only the frequency modulation capability of the thermal power unit is relied on, which is denoted as system S1, and the frequency response function of system S1 is:

[0079] ;

[0080] Scenario 2: The wind turbine participates in frequency modulation by reserving a spare capacity, but the flywheel energy storage system is not connected to the power grid, and the frequency modulation resources of the system are provided by the thermal power unit and the wind turbine, which is denoted as system S2, and the frequency response function of system S2 is:

[0081] ;

[0082] Scenario 3: The flywheel energy storage system is connected to the bus of the wind farm, the wind turbine is connected to the power grid, but the frequency modulation capability of the wind turbine is not considered, and the flywheel energy storage system is used to assist the wind power in frequency modulation, which is denoted as system S3, and the frequency response function of system S3 is:

[0083] ;

[0084] When the power system is subjected to a load disturbance △P L , the steady-state frequency deviation of the above three scenarios is calculated by using the final value theorem, respectively:

[0085] ;

[0086] It can be seen that the steady-state frequency error of the power system with wind power penetration is mainly related to the synchronous generator coefficient ​​​This indicates that if wind turbines do not provide frequency control capabilities, under certain load disturbances, the steady-state error of the system increases with the increase of wind power penetration, and the system's ability to maintain power frequency synchronization and resist disturbances also decreases with the increase of wind power penetration. Compared to wind turbines not participating in frequency regulation, adding flywheel energy storage or having wind turbines participate in frequency regulation can reduce the steady-state error of the power system frequency, thereby enhancing the system's ability to maintain power frequency synchronization and resist disturbances.

[0087] S103 collects the power grid frequency deviation and its rate of change in real time, and dynamically adjusts the virtual damping coefficient according to the frequency deviation and rate of change through a fuzzy logic controller.

[0088] Fuzzy control is an intelligent control method that does not rely on precise data control but uses human-summarized experience as reasoning rules to improve the stability of the control system. To enable the flywheel energy storage system to quickly exchange energy with the grid when the system load is disturbed, such as... Figure 4 As shown, where, The power grid frequency deviation can be expressed as , This represents the rate of change of the grid frequency deviation. Using the system frequency deviation and the rate of change of frequency signals, fuzzy reasoning can be employed to control flywheel energy storage.

[0089] In some embodiments, a wide-area monitoring system based on a synchronous phasor measurement unit (PMU) can be used, for example, to acquire the power grid frequency signal at a sampling rate of 50 frames per second, and to eliminate harmonic interference through fast Fourier transform, so as to ensure that the measurement accuracy of the frequency deviation reaches a preset threshold.

[0090] The rate of change of the power grid frequency deviation can be measured by analyzing multiple consecutive sampling points. The data undergoes differential calculation, and noise is smoothed using a moving average filter to obtain the frequency change rate. The calculation formula can be expressed as:

[0091] ;

[0092] in, The sampling interval can be an empirical value or determined based on the needs of the actual application. For example, The value can be 0.02s.

[0093] according to Figure 4 The fuzzy logic controller shown can dynamically adjust the active power output of the flywheel energy storage in real time by detecting the system frequency and calculating the frequency deviation and frequency change rate. Based on the inertial response and frequency regulation of the synchronous generator, it can dynamically simulate virtual inertia control and droop response control.

[0094] In some embodiments, the input variables of the fuzzy logic controller can include grid frequency deviation and frequency deviation rate of change, and the output includes a virtual damping coefficient; wherein the fuzzy subsets of the fuzzy logic controller can be divided into negative large (NL), negative medium (NM), negative small (NS), zero (Z), positive small (PS), positive medium (PM), and positive large (PL), and inference is performed based on a preset fuzzy rule table; the fuzzy rule table is obtained by combination of the fuzzy subsets.

[0095] Specifically, the relationship between each fuzzy subset can be represented by Figure 5 . The left side of Z represents negative, and the right side represents positive, and the farther away from Z, the larger.

[0096] The frequency deviation and the frequency rate of change of the grid are taken as the input variables of the fuzzy logic controller, and the active power command of the flywheel energy storage output is taken as the output variable, and the relationship between the input and the output is determined by the set inference rule.

[0097] In order to realize the fast energy exchange between the flywheel energy storage system and the grid and make it reach the steady state standard as soon as possible, according to the input variable system frequency deviation and frequency deviation rate of change, 49 kinds of situations can be combined, and the logical inference result of the fuzzy control process is shown in Table 1 below.

[0098] Table 1:

[0099]

[0100] According to the logical inference table of Table 1, the inference rule is that when the frequency deviation is relatively large or the frequency deviation rate of change is large, the energy released by the flywheel energy storage to the grid is as much as possible: when the system frequency deviation is small and the frequency rate of change is close to zero, the flywheel energy storage absorbs the energy of the grid to store it, so that the frequency is restored to the stable state as soon as possible.

[0101] Based on this, in some embodiments, the adjustment rule of the virtual damping coefficient includes:

[0102] When the grid frequency deviation or the frequency rate of change exceeds the preset threshold, the virtual damping coefficient is increased to increase the system damping;

[0103] When the grid frequency tends to be stable, the virtual damping coefficient is reduced to reduce energy loss.

[0104] For example, in the frequency rapid fluctuation scenario: when > 0.2 Hz and > 1.0 Hz / s, the fuzzy logic controller outputs close to the upper limit (for example, the upper limit is 5.0 N·m·s / rad), and the total damping The inertia time constant can be reduced to 5.3 N·m·s / rad, which can reduce the inertia time constant to 60 ms or less, ensuring that the flywheel outputs 90% of the frequency modulation power within 0.5 seconds.

[0105] Take the frequency steady-state scenario as an example: when |<0.03 Hz and lasts for 10 seconds, the fuzzy logic controller will reduce the total damping to 0.5 N·m·s / rad, and the total damping is reduced to 0.8 N·m·s / rad, reducing the flywheel idle loss.

[0106] In the embodiment of the present application, the fuzzy logic dynamically matches the frequency fluctuation state, which can avoid the frequency modulation failure caused by over-damping or under-damping, reduce the virtual damping in the steady-state stage, reduce the flywheel idle loss, and improve the economy of the system. At the same time, the virtual damping mechanism can be used to adapt to different specifications of flywheel energy storage units, support modular expansion, and improve the adaptability of the system.

[0107] S104, by adjusting the moment of inertia and the total damping coefficient, the time constant of the first-order inertia link meets the preset fast response requirement of the power grid frequency modulation;

[0108] The fast response requirement of the power grid frequency modulation is that the flywheel energy storage can have an output that meets the rated frequency modulation power within a predetermined time after the frequency disturbance occurs. For example, the predetermined time is 0.05 s, that is, the time constant needs to meet <0.05 s. By dynamically adjusting the flywheel moment of inertia and the total damping coefficient, the can always be in the optimized interval of Tr<0.05s.

[0109] For dynamic adjustment of the moment of inertia , first, a modular flywheel array design is needed. The flywheel energy storage system is composed of multiple independent units (for example, 10 ), and the moment of inertia of each flywheel is , and the total inertia is ; wherein is the number of flywheel energy storage units in operation.

[0110] When the grid frequency mutation is detected (for example, |>0.2 Hz), all flywheel units are quickly enabled =10), and the total inertia ensures the large power output capability;

[0111] In the frequency recovery stage (| |<0.05 Hz), the number of operating units is gradually reduced to =5, reducing energy loss.

[0112] For dynamic adjustment of virtual damping First, based on the fuzzy logic control of the above step S103, real-time adjustment within its value range, combined with fixed mechanical damping Synthetic total damping . Model predictive control (MPC) optimization combined with The objective function can be expressed as:

[0113] ;

[0114] Where, , , are the weight coefficients of , , The specific weight coefficient value can be determined based on actual application requirements.

[0115] In the embodiment of the application, in terms of dynamic inertia adjustment, the modular flywheel array breaks the limitations of traditional fixed inertia, realizes online optimization of rotational inertia, and can flexibly cope with different working conditions. In terms of damping-inertia coordination, fuzzy logic and MPC algorithm are combined to overcome the disadvantages of single parameter adjustment, and the power grid fast frequency regulation response demand is accurately matched through weight distribution. In terms of energy efficiency and stability balance, the total damping coefficient is maximized in the frequency regulation stage to accelerate the response speed, while the rotational inertia and the total damping coefficient are reduced in the steady state stage to reduce energy loss. Through the above step S104, the application greatly improves the frequency regulation efficiency and economy of flywheel energy storage on the basis of ensuring the stability of the power grid frequency, and provides an extremely scalable solution for power grids with high proportion of renewable energy access.

[0116] S105, based on the frequency deviation, generate flywheel charging and discharging power instructions, and dynamically correct the power instructions through a fuzzy logic controller to adapt to the trend of power grid frequency change;

[0117] In some embodiments, the flywheel charging and discharging power instruction is obtained by the following formula:

[0118] ;

[0119] Where, is the flywheel charging and discharging power.

[0120] After the flywheel charge-discharge power command is obtained by the above formula, the obtained charge-discharge power command can be further corrected by using the fuzzy logic controller in the above step S103. By inputting the tracking error of the actual power and the target value and the frequency deviation change rate , according to the fuzzy rule base as shown in Table 1 above, a correction factor is output by using the weighted average method. The corrected flywheel charge-discharge power command is represented as:

[0121] ;

[0122] For example, when the frequency rapidly decreases (the frequency deviation is large and negative), and the power tracking lags (the tracking error of the actual power and the target value is large and negative), the correction factor > 1, the power shortage is compensated in advance, and the lag time of the power compensation of the flywheel energy storage system is reduced.

[0123] When the frequency rapidly rises (i.e. the frequency deviation is large and positive), and the power overshoots (the tracking error of the actual power and the target value is large and positive), the correction factor < 1, the power overshoot is suppressed.

[0124] S106, after the frequency regulation is ended, according to the flywheel speed deviation and its change rate, a damping coefficient is selected by using a fuzzy decision mechanism, so that the flywheel speed is restored to the initial state.

[0125] In some embodiments, the fuzzy decision mechanism uses the following rules in the flywheel speed recovery stage:

[0126] If the flywheel speed deviation and its change rate are both positive, the total damping coefficient of the system is reduced to accelerate the speed recovery.

[0127] If the flywheel speed deviation is negative and its change rate is positive, the total damping coefficient of the system is kept stable.

[0128] It should be noted that after the frequency regulation of the power system is ended, the flywheel energy storage system needs to quickly and smoothly restore the speed to the initial state to prepare for the next frequency fluctuation. The traditional method uses a fixed or segmented damping strategy, which has problems such as long reset time, large overshoot, and high mechanical loss. The present application dynamically adjusts the damping coefficient by using a fuzzy decision mechanism, which significantly optimizes the dynamic performance and safety of the speed recovery process. In some embodiments, in order to intuitively show the replacement ability of the flywheel energy storage for the frequency regulation of the thermal power unit, the present application quantitatively analyzes the replacement indexes of the two by designing different wind power proportion scenarios, and the calculation process of the rating index of the replacement capacity of the flywheel energy storage and the thermal power unit is specifically shown in Figure 6

[0129] ​​​In some embodiments, the application can also evaluate the replacement ability of flywheel energy storage for thermal power units, specifically including:

[0130] Definition of power replacement ability index and frequency index replacement ability index , respectively:

[0131] ;

[0132] ;

[0133] wherein φ(t) is the replacement ability evaluation index of flywheel energy storage power and thermal power unit at t, is the maximum output power of the newly added thermal power, is the maximum output power of the newly added flywheel energy storage, ΔP F (t) is the output power of flywheel energy storage at t, G (t) is the output power of thermal power unit at t;

[0134] The replacement effect of flywheel energy storage at the initial stage of step disturbance and the steady-state replacement index are verified through simulation or actual measurement data.

[0135] It can be understood that when studying the participation of flywheel energy storage in power system frequency modulation, the flywheel energy storage system is usually equivalent to a first-order inertia link, and the time constant of the first-order inertia link depends on the characteristics of the flywheel itself and the power conversion system, which is represented as:

[0136] ;

[0137] In the formula, is the inertia time constant of flywheel energy storage.

[0138] Assuming that only thermal power units participate in frequency modulation in the system, the transfer function is as follows:

[0139] ;

[0140] In the formula, M represents the inertia time constant, D represents the system damping, R G represents the modulation difference coefficient of thermal power unit, T G represents the speed regulator time constant, T CH , T RH , F HP are all time constants of steam turbine generators.

[0141] Assuming that only flywheel energy storage participates in frequency modulation in the system, the frequency modulation transfer function of the system is as follows:

[0142] ;

[0143] In order to compare the frequency modulation capabilities of the thermal power unit and the flywheel energy storage in the same scenario, the performance indicators of the active power input into the system when the flywheel energy storage and the thermal power unit participate in frequency modulation respectively are set to evaluate the performance indicators of the active power input into the system when the same load disturbance occurs, and the evaluation formula is shown in the above formula.

[0144] The method of the application is described below with a specific embodiment:

[0145] The application takes a certain regional power grid as the analysis basis, and the power system parameters are set as shown in Table 2. The flywheel energy storage replacement thermal power unit capability of the following two scenarios is analyzed: in scenario 1, the newly added frequency modulation resources are all borne by the flywheel energy storage, and in scenario 2, the newly added frequency modulation resources are all borne by the thermal power unit, and the remaining system load and frequency modulation installed capacity remain the same.

[0146] Table 2:

[0147]

[0148] The calculated replacement capability indicators are shown in Table 3 and Table 4:

[0149] Table 3:

[0150]

[0151] Table 4:

[0152]

[0153] According to the above analysis, it can be found that in the initial stage of the change of the grid frequency, the flywheel energy storage responds immediately, and at this time, the output is much larger than that of the thermal power unit, the performance indicators of the flywheel energy storage replacing the thermal power unit for frequency modulation are higher, and with the gradual recovery of the grid frequency to a stable state, the power response of the flywheel energy storage frequency modulation and the power response of the thermal power unit frequency modulation are both sent to a stable value, at this time, the replacement capability of the flywheel energy storage for the thermal power unit will also reach a stable value.

[0154] Considering that in the future, a higher proportion of wind power will be connected to the grid, but the scenario of compensating for the frequency fluctuation caused by the connection of wind power by increasing the number of thermal power units is difficult to appear, the advantages of the flywheel energy storage participating in frequency modulation will gradually appear, and therefore the flywheel energy storage will become one of the most valuable frequency modulation resources in the grid.

[0155] For example, based on the above-mentioned flywheel energy storage auxiliary wind power frequency control model, the simulation model of the flywheel energy storage system auxiliary wind power unit frequency modulation system shown in Table 5 is established in Matlab / imulink. Figure 3 The parameters are selected as shown in Table 5, and the parameters are standardized based on the rated load 1000MW and the rated frequency 50Hz.

[0156] Table 5:

[0157]

[0158] The dynamic performance index under different frequency modulation modes is shown in Table 6 as follows:

[0159] Table 6:

[0160]

[0161] As can be seen from Table 6, when the system load power occurs a step disturbance, in the system S1, the wind turbine does not participate in frequency modulation, the frequency of the power system drops greatly, the minimum value of the frequency is 49.66 Hz, and the steady-state frequency is 49.82 Hz; in the system S2, the wind turbine participates in frequency modulation, the wind turbine releases standby power to adjust the frequency, the minimum value of the frequency is 49.78 Hz, and the steady-state frequency is 49.88 Hz; in the system S3, the flywheel energy storage alone assists the wind power plant to participate in frequency modulation, the output power of the flywheel energy storage is basically equal to the release power of the wind turbine frequency modulation, the power is obviously reduced compared with the power of the system S1 that relies on the thermal power unit for frequency modulation, and due to the faster response speed of the flywheel energy storage and the larger output energy per unit time, the minimum value of the frequency of the flywheel energy storage frequency modulation is 49.83 Hz, the frequency change rate is reduced compared with the systems S1 and S2, the minimum value of the frequency deviation and the steady-state frequency deviation are increased by 50% and 33% respectively compared with the case without frequency modulation, and the minimum value of the frequency deviation is increased by 29% compared with the case of wind turbine frequency modulation. The frequency modulation response effect of the flywheel energy storage system is obviously better than that of the wind turbine frequency modulation response effect through variable pitch control, and the decline of the system frequency characteristics caused by the wind power grid connection is effectively suppressed.

[0162] When the load power occurs a step disturbance, the addition of the flywheel energy storage can improve the frequency modulation effect of the wind turbine, but the fluctuation of the load power is usually irregular and continuous, and therefore the frequency modulation effect under continuous load power disturbance needs to be analyzed.

[0163] The frequency deviation data is shown in Table 7.

[0164] Table 7:

[0165]

[0166] When the load occurs a continuous disturbance, the flywheel energy storage device can quickly follow the power change of the system load, reduce the frequency deviation of the system, and has a better performance effect than the wind turbine participating in frequency modulation.

[0167] It can be known from the above analysis that the method has the following advantages:

[0168] (1) By comparing the flywheel energy storage system with electrochemical energy storage under the same power, and comparing them under step disturbance and continuous disturbance, it is shown that the flywheel energy storage system responds faster and has a stronger load following ability than electrochemical energy storage. Then, the frequency regulation capability of flywheel energy storage and thermal power unit is compared and analyzed. Finally, the substitution capability of flywheel energy storage and thermal power unit under the same power and the capacity substitution capability under the same frequency index are defined respectively. The results show that when the substitution capability is the same power, the substitution effect is tens of times in the early stage of frequency change. As the output gradually stabilizes, the substitution index remains at about 5.76. Under the same frequency index, the capacity substitution effect gradually increases with the continuous increase of wind power proportion.

[0169] (2) A flywheel energy storage-assisted wind power frequency regulation control model was established using a linear frequency model. The frequency characteristics of flywheel energy storage participating in frequency regulation were analyzed based on the transfer function. Then, under conditions of step and continuous load power disturbances, the frequency characteristics of the regional power grid were simulated and verified through time-domain simulation. The results showed that configuring a certain proportion of flywheel energy storage systems can quickly respond to frequency deviation signals. During the frequency regulation simulation, both the maximum frequency deviation and steady-state deviation of the system were reduced, meeting the performance requirements of the power system and effectively improving the system's frequency quality. Furthermore, by improving droop inertial control and adopting a fuzzy control method, the frequency regulation process was improved. The results showed that the frequency results obtained using the fuzzy control method were significantly better than those obtained using traditional control methods.

[0170] Based on the same inventive concept, this application also provides a flywheel energy storage-assisted wind power frequency regulation system for implementing the above-mentioned power system frequency regulation method based on flywheel energy storage-assisted wind power. Figure 7 This is a schematic diagram of a power system frequency regulation system based on flywheel energy storage assisted by wind power, as described in an embodiment of the present invention. (See attached diagram.) Figure 7 As shown, the power system frequency regulation system 700 based on flywheel energy storage assisted wind power may include:

[0171] Wind farm module 701 includes at least one wind turbine generator, which is connected to the power grid via a power electronic converter;

[0172] The flywheel energy storage module 702 includes a flywheel body, a bidirectional motor, a machine-side converter, and a grid-side converter. The flywheel energy storage module is connected in parallel to the wind farm outlet bus via a DC bus and connected to the power grid via a step-up transformer.

[0173] The frequency control module 703 is configured to detect the grid frequency deviation and frequency change rate in real time, generate active power commands for flywheel energy storage based on fuzzy control strategy, and realize the rapid charging and discharging response of flywheel energy storage through virtual droop control.

[0174] The frequency modulation control module 703 further includes:

[0175] a first order inertia model 7031 for equivalent dynamic response of the flywheel energy storage system;

[0176] a fuzzy logic inference unit 7032 for dynamically adjusting the output power of the flywheel energy storage according to the combination of the fuzzy subsets of the frequency deviation and the frequency change rate.

[0177] The various embodiments in the specification are described in a progressive manner, and the same or similar parts between the various embodiments can be referred to each other, and each embodiment focuses on the difference from other embodiments.

[0178] The above examples are only used to illustrate the technical solutions of the present application, and are not limited to the present application; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that the technical solutions recorded in the foregoing examples can still be modified, or some or all of the technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the present application.

Claims

1. A flywheel energy storage assisted wind power system frequency regulation method, characterized in that, The application relates to a frequency response model of a flywheel energy storage system and a power system, wherein the flywheel energy storage system is equivalent to a first-order inertia link, and a transfer function of the flywheel energy storage system is represented as: The flywheel energy storage system is centrally arranged at an outlet bus of a wind power plant, the flywheel energy storage system is composed of a plurality of flywheel energy storage units in parallel through a direct-current bus, and the flywheel energy storage system is connected to a power grid through a grid-side converter and a step-up transformer; ； wherein, a droop control coefficient for a flywheel energy storage system, an inertia time constant for the flywheel energy storage, the time constant being determined by a ratio of a flywheel moment of inertia to a total damping coefficient of the system; Real-time acquisition of a power grid frequency deviation and a change rate thereof, dynamic adjustment of a virtual damping coefficient according to the frequency deviation and the change rate through a fuzzy logic controller, wherein the virtual damping coefficient and a mechanical damping jointly constitute the total damping coefficient, wherein the power grid frequency deviation is obtained based on a wide-area monitoring system of a synchronous phasor measurement unit at a sampling rate of 50 frames per second, and a fast Fourier transform is used to eliminate harmonic interference, so that the measurement accuracy of the frequency deviation reaches a preset threshold value, the change rate of the power grid frequency deviation is obtained by performing difference operation on the power grid frequency deviation data of a plurality of continuous sampling points and by performing sliding average filtering on the noise, and a calculation formula is represented as: Generation of a flywheel charging and discharging power instruction based on the frequency deviation, and dynamic correction of the power instruction through the fuzzy logic controller to adapt to the frequency change trend of the power grid; ； wherein, is the grid frequency deviation, is the rate of change of the grid frequency deviation, is the sampling interval; By adjusting the moment of inertia and the total damping coefficient, the time constant of the first-order inertia link meets the preset fast response requirement of grid frequency modulation; wherein the fast response requirement of grid frequency modulation is that the grid requires the flywheel energy storage to have an output meeting the rated frequency modulation power within a predetermined time after the frequency disturbance occurs, and the corresponding inertia time constant needs to meet <0.05s; For dynamic regulation of moment of inertia Firstly, modular flywheel array design, flywheel energy storage system consists of independent units, each flywheel moment of inertia , the total inertia ; wherein, is the number of flywheel energy storage units in operation; when the grid frequency mutation is detected, i.e. |>0.2Hz, all flywheel units are quickly enabled, and the total inertia ensures the large power output capacity; in the frequency recovery stage, i.e. |<0.05Hz, the number of operating units is gradually reduced to =5, and the energy loss is reduced; For virtual damping dynamic adjustment, first, based on the fuzzy logic controller, in its range of values, real-time adjustment, with fixed mechanical damping synthetic total damping , using model predictive control optimization and combination, the objective function is expressed as: ； wherein, , , are weight coefficients, respectively, , , ​ After frequency modulation, a damping coefficient is selected by using a fuzzy decision mechanism according to a flywheel speed deviation and a change rate thereof, so that the flywheel speed returns to an initial state. Input variables of the fuzzy logic controller include the power grid frequency deviation and the change rate of the frequency deviation, and output includes the virtual damping coefficient; wherein fuzzy subsets of the fuzzy logic controller are divided into negative large (NL), negative medium (NM), negative small (NS), zero (Z), positive small (PS), positive medium (PM) and positive large (PL), and reasoning is carried out based on a preset fuzzy rule table; the fuzzy rule table is obtained by combination of the fuzzy subsets.

2. The method of claim 1, wherein, The adjustment rule of the virtual damping coefficient includes:

3. The method of claim 2, wherein, When the power grid frequency deviation or the frequency change rate exceeds a preset threshold value, the virtual damping coefficient is increased to increase the system damping; When the power grid frequency tends to be stable, the virtual damping coefficient is reduced to reduce energy loss. The flywheel charging and discharging power instruction is obtained through the following formula:

4. The method of claim 3, wherein, An inertia time constant of the flywheel energy storage is represented as: ； wherein, is the flywheel charge-discharge power, is the grid frequency deviation, is the virtual damping coefficient, is the rate of change of the grid frequency deviation.

5. The method of claim 4, wherein, The fuzzy decision mechanism adopts the following rules in the flywheel speed recovery stage: ； wherein, is the flywheel moment of inertia, is the total system damping coefficient.

6. The method of claim 5, wherein, If the flywheel speed deviation and the change rate thereof are both positive, the total damping coefficient of the system is reduced to accelerate the speed recovery; If the flywheel speed deviation is negative and the change rate thereof is positive, the total damping coefficient of the system is kept stable. The flywheel energy storage unit is connected with a machine-side converter through a bidirectional motor, and the machine-side converter adopts a vector control strategy to realize accurate regulation of the flywheel speed and power.

7. The method of claim 6, wherein, The application relates to a frequency response model of a flywheel energy storage system and a power system, wherein the flywheel energy storage system is equivalent to a first-order inertia link, and a transfer function of the flywheel energy storage system is represented as:

8. A flywheel energy storage assisted wind power system frequency regulation system for implementing the method of any one of claims 1 to 7, characterized in that, The flywheel energy storage system is centrally arranged at an outlet bus of a wind power plant, the flywheel energy storage system is composed of a plurality of flywheel energy storage units in parallel through a direct-current bus, and the flywheel energy storage system is connected to a power grid through a grid-side converter and a step-up transformer; Real-time acquisition of a power grid frequency deviation and a change rate thereof, dynamic adjustment of a virtual damping coefficient according to the frequency deviation and the change rate through a fuzzy logic controller, wherein the virtual damping coefficient and a mechanical damping jointly constitute the total damping coefficient, wherein the power grid frequency deviation is obtained based on a wide-area monitoring system of a synchronous phasor measurement unit at a sampling rate of 50 frames per second, and a fast Fourier transform is used to eliminate harmonic interference, so that the measurement accuracy of the frequency deviation reaches a preset threshold value, the change rate of the power grid frequency deviation is obtained by performing difference operation on the power grid frequency deviation data of a plurality of continuous sampling points and by performing sliding average filtering on the noise, and a calculation formula is represented as: Generation of a flywheel charging and discharging power instruction based on the frequency deviation, and dynamic correction of the power instruction through the fuzzy logic controller to adapt to the frequency change trend of the power grid; After frequency modulation, a damping coefficient is selected by using a fuzzy decision mechanism according to a flywheel speed deviation and a change rate thereof, so that the flywheel speed returns to an initial state. Input variables of the fuzzy logic controller include the power grid frequency deviation and the change rate of the frequency deviation, and output includes the virtual damping coefficient; wherein fuzzy subsets of the fuzzy logic controller are divided into negative large (NL), negative medium (NM), negative small (NS), zero (Z), positive small (PS), positive medium (PM) and positive large (PL), and reasoning is carried out based on a preset fuzzy rule table; the fuzzy rule table is obtained by combination of the fuzzy subsets. The adjustment rule of the virtual damping coefficient includes: When the power grid frequency deviation or the frequency change rate exceeds a preset threshold value, the virtual damping coefficient is increased to increase the system damping; When the power grid frequency tends to be stable, the virtual damping coefficient is reduced to reduce energy loss. The flywheel charging and discharging power instruction is obtained through the following formula: An inertia time constant of the flywheel energy storage is represented as: The fuzzy decision mechanism adopts the following rules in the flywheel speed recovery stage: If the flywheel speed deviation and the change rate thereof are both positive, the total damping coefficient of the system is reduced to accelerate the speed recovery; If the flywheel speed deviation is negative and the change rate thereof is positive, the total damping coefficient of the system is kept stable. The flywheel energy storage unit is connected with a machine-side converter through a bidirectional motor, and the machine-side converter adopts a vector control strategy to realize accurate regulation of the flywheel speed and power. The frequency modulation control module is configured to detect grid frequency deviation and frequency change rate in real time, generate flywheel energy storage active power instruction based on fuzzy control strategy, and realize fast charge and discharge response of the flywheel energy storage through virtual droop control. The frequency modulation control module further comprises: A first-order inertia model for equivalent dynamic response of the flywheel energy storage system; A fuzzy logic reasoning unit for dynamically adjusting the output power of the flywheel energy storage according to the fuzzy subset combination of the frequency deviation and the frequency change rate.

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