Rapid frequency support control method and system for network-forming converter

Abstract

The invention provides a fast frequency support control method and system for a network construction type converter, and the method comprises the steps: S1, building a heterogeneous power system model which comprises a GFM converter model, a synchronous generator SG model and a GFL wind driven generator model; s2, according to the established GFL wind driven generator model, regarding the output power and the load power of the GFL wind driven generator as disturbance, and designing a preset time nonlinear observer to estimate the disturbance; s3, designing a preset time sliding mode controller, and adjusting the output power increment of the GFM converter so as to adjust the frequency of the heterogeneous power system; and S4, performing simulation on frequency suppression oscillation of the heterogeneous power system through adjustment, and displaying a simulation result. According to the invention, small overshoot and short convergence time can be displayed.

Description

Technical Field

[0001] The present invention relates to the technical field of distributed power generation control of an island AC microgrid, and in particular to a fast frequency support control method and system for a grid-connected converter. Background Art

[0002] With the large-scale access to renewable energy through power electronic devices, the power system is gradually transitioning to a heterogeneous power system. Therefore, a heterogeneous power system consists of different types of power generation sources, including traditional thermal power plants, grid-following (GFL) renewable energy power plants, and grid-forming (GFM) energy storage power plants. However, the integration of renewable energy into the grid through power electronic devices will lead to a decrease in the system's rotational inertia, which in turn causes rapid dynamic changes and large frequency deviations when the heterogeneous power system is subjected to small disturbances. Therefore, it is of great significance to study the frequency stability problem of heterogeneous power systems.

[0003] In heterogeneous power systems, renewable energy is connected to the grid through GFL control, and the main grid maintains synchronous operation. However, the inertia and frequency support capabilities provided by GFL are low. In traditional power grids, frequency support mainly relies on synchronous generator speed regulators. However, as the penetration rate of renewable energy increases, the capacity of synchronous generators decreases, making it difficult to meet the operating requirements of inertia and frequency. In addition, the speed regulator has a slow response and is easily damaged due to frequent use, making it difficult to achieve fast frequency response (FFR).

[0004] In contrast, GFM energy storage technology has been used in power systems to cope with power gaps and provide inertia and frequency support due to its fast response and strong regulation capabilities. The control of GFM converters mainly includes droop control and virtual synchronous generator (VSG) control, the latter of which provides inertia support by simulating the dynamic characteristics of synchronous generators. However, frequency regulation still relies on droop control, which may cause frequency deviation and limit the frequency support capability of GFM converters. In addition, the randomness of user electricity consumption behavior and the intermittent nature of renewable energy in heterogeneous power systems increase the uncertainty of active power supply, making the frequency control problem an anti-disturbance control problem. Observers can be used to estimate active power disturbances, but most existing observers are used for simple systems and are less used for complex heterogeneous power systems with source-load uncertainties.

[0005] Preset time stability can achieve system stability within a specified upper bound time, avoiding the problem that the convergence time of fixed time stability depends on multiple parameters. Based on this, a preset time observer is designed to estimate the disturbance caused by source-load uncertainty in heterogeneous power systems. Sliding Mode Control (SMC) has attracted widespread attention due to its robustness to disturbances and parameter changes, simple implementation, and fast response. Although SMC has been applied to the fields of transient stability and reactive power control of multi-machine power systems, its preset time stability control in complex heterogeneous power systems has not been widely studied. This is mainly due to factors such as system heterogeneity, uncertainty, delay effect, and high computational complexity, which make it difficult for traditional SMC methods to effectively cope with these challenges, thus limiting their application in such systems. Summary of the invention

[0006] In view of the defects in the prior art, the present invention provides a fast frequency support control method and system for a grid-type converter.

[0007] According to a method and system for fast frequency support control of a grid-type converter provided by the present invention, the scheme is as follows:

[0008] In a first aspect, a method for fast frequency support control of a grid-connected converter is provided, the method comprising:

[0009] Step S1: Establish a heterogeneous power system model, including a GFM converter model, a synchronous generator SG model and a GFL wind turbine generator model;

[0010] Step S2: according to the established GFL wind turbine model, the output power and load power of the GFL wind turbine are regarded as disturbances, and a preset time nonlinear observer is designed to estimate the disturbances;

[0011] Step S3: Design a preset time sliding mode controller to adjust the output power increment ΔP of the GFM converter ref , to regulate the frequency of heterogeneous power systems;

[0012] Step S4: Simulate oscillation suppression by adjusting the frequency of the heterogeneous power system and present the simulation results.

[0013] Preferably, in step S1, the heterogeneous power system model is divided into two interconnected areas, namely area A and area B, and power exchange is performed through a tie line;

[0014] The GFM converter includes: a power control loop, a voltage control loop and a current control loop; wherein the power control loop is used to adjust and synchronize the output power with the power grid. In the power control loop, VSG control is used to adjust the active power to achieve inertia and frequency support, and reactive power regulation and output voltage control at the common connection point PCC are completed through droop control in the power control loop; the GFM converter model is as follows:

[0015]

[0016] P1=P AB +P L1 -P ω1 (2)

[0017] Where, ω1 is the frequency of the GFM converter; ω n is the rated frequency; δ1 represents the power angle; t represents the time; ΔP ref Indicates the increment of reference power; P ref and P1 represent the initial active power and output active power respectively; D1 represents the damping coefficient of the GFM converter; H1 represents the inertia time constant of the GFM converter; P AB is the exchange power between area A and area B; P L1 is the load power of load 1, P ω1 is the output power of the GFL wind turbine in area A.

[0018] Preferably, in step S1, a synchronous generator SG model is established:

[0019]

[0020] P2=P L2 -P AB -P ω2 (4)

[0021] Among them, δ2, ω2, P2, D2, H2 and P m0 They represent the rotor angle, rotor speed, electromagnetic torque, damping coefficient of the synchronous generator, inertia time constant of the synchronous generator and mechanical torque respectively; P L2 is the load power of load 2, P ω2 is the output power of the GFL wind turbine in area B.

[0022] Preferably, the GFL wind turbine model is established in step S1 as follows:

[0023] The mechanical power generated by the GFL wind turbine is expressed as:

[0024]

[0025] The mechanical torque of the GFL wind turbine is expressed as follows:

[0026]

[0027] Among them, ρ, ω, R and v ω Respectively represent air density, mechanical speed, wind wheel radius and wind speed; C p , λ and θ represent the tip speed ratio, wind energy utilization coefficient and pitch angle respectively; wind energy utilization coefficient C p It is expressed as:

[0028]

[0029] The tip speed ratio λ is expressed as:

[0030]

[0031] By using equations (7) and (8), it can be observed that when the pitch angle θ remains constant, the wind energy utilization coefficient C p Reach the maximum value; implement the maximum power point tracking MPPT strategy to capture the maximum wind energy; the maximum mechanical power of the wind turbine is expressed as follows:

[0032]

[0033] Among them, λ opt and ω opt Respectively represent the best tip speed ratio and the best mechanical speed; C pmax Indicates the maximum value of wind energy utilization coefficient;

[0034] In heterogeneous power systems, wind turbines are connected to the grid in the form of GFLs;

[0035] Based on equations (1)-(4) and (9), the heterogeneous power system model is expressed as:

[0036]

[0037] in, represents the frequency of the center of inertia COI; P ω1 and P ω2 Respectively represent the output power of wind turbine generator set 1 and wind turbine generator set 2; u=ΔP ref , H Σ =H1+H2,D Σ =D1+D2 represents the total damping coefficient of the system.

[0038] Preferably, step S2 comprises:

[0039] Design assistance systems are:

[0040]

[0041] Where, frequency error e = ω COI -η, ρ is a positive constant, η represents the auxiliary state variable of the system;

[0042] The preset time nonlinear observer is designed as:

[0043]

[0044] in, is the estimated value of the disturbance d, is the estimated value of state e, Indicates the rate of change of frequency error; It is expressed as:

[0045]

[0046] in, is the frequency estimation error, the sign function sig(·) γ =sign(·)|·| γ , observer coefficient α1>0,α2>0, and preset time T c1 >0,T c1 Represents the preset time constant of the observer.

[0047] Preferably, step S3 comprises:

[0048] The preset time sliding surface is designed as:

[0049]

[0050] Where σ=ω COI -ω n represents the control error, the controller coefficient T c2 Indicates the preset time constant of the controller;

[0051] When the control error σ reaches the preset time sliding surface, s=0, Therefore, we get:

[0052]

[0053] Based on (10) and (15) and the observation results of the preset time observer Get the equivalent controller u eq for:

[0054]

[0055] Let the approach controller be u ri , which is used to ensure that the control error σ effectively reaches the preset time sliding surface specified in equation (14), and its expression is:

[0056]

[0057] The controller coefficient is T c3 represents the preset time constant of the equivalent controller;

[0058] Based on equations (16) and (17), the preset time sliding mode controller is designed as:

[0059]

[0060] Under the preset time sliding mode controller based on the observer, the control error σ reaches the preset time sliding mode surface within the preset time and converges to 0.

[0061] In a second aspect, a grid-type converter fast frequency support control system is provided, the system comprising:

[0062] Module M1: Establish a heterogeneous power system model, including the GFM converter model, the synchronous generator SG model and the GFL wind turbine model;

[0063] Module M2: Based on the established GFL wind turbine model, the output power and load power of the GFL wind turbine are regarded as disturbances, and a preset time nonlinear observer is designed to estimate the disturbances;

[0064] Module M3: Design a preset time sliding mode controller to adjust the output power increment ΔP of the GFM converter ref , to regulate the frequency of heterogeneous power systems;

[0065] Module M4: Simulate the oscillation suppression by adjusting the frequency of heterogeneous power systems and present the simulation results.

[0066] Preferably, in the module M1, the heterogeneous power system model is divided into two interconnected areas, namely area A and area B, and power exchange is performed through a tie line;

[0067] The GFM converter includes: a power control loop, a voltage control loop and a current control loop; wherein the power control loop is used to adjust and synchronize the output power with the power grid. In the power control loop, VSG control is used to adjust the active power to achieve inertia and frequency support, and reactive power regulation and output voltage control at the common connection point PCC are completed through droop control in the power control loop; the GFM converter model is as follows:

[0068]

[0069] P1=P AB +P L1 -P ω1 (2)

[0070] Where, ω1 is the frequency of the GFM converter; ω n is the rated frequency; δ1 represents the power angle; t represents the time; ΔP ref Indicates the increment of reference power; P ref and P1 represent the initial active power and output active power respectively; D1 represents the damping coefficient of the GFM converter; H1 represents the inertia time constant of the GFM converter; P AB is the exchange power between area A and area B; P L1 is the load power of load 1, P ω1 is the output power of the GFL wind turbine in area A;

[0071] The synchronous generator SG model is established in the module M1:

[0072]

[0073] P2=P L2 -P AB -P ω2 (4)

[0074] Among them, δ2, ω2, P2, D2, H2 and P m0 They represent the rotor angle, rotor speed, electromagnetic torque, damping coefficient of the synchronous generator, inertia time constant of the synchronous generator and mechanical torque respectively; P L2 is the load power of load 2, P ω2 is the output power of the GFL wind turbine in area B;

[0075] The GFL wind turbine model is established in the module M1 as follows:

[0076] The mechanical power generated by the GFL wind turbine is expressed as:

[0077]

[0078] The mechanical torque of the GFL wind turbine is expressed as follows:

[0079]

[0080] Among them, ρ, ω, R and v ω Respectively represent air density, mechanical speed, wind wheel radius and wind speed; C p , λ and θ represent the tip speed ratio, wind energy utilization coefficient and pitch angle respectively; wind energy utilization coefficient C pIt is expressed as:

[0081]

[0082] The tip speed ratio λ is expressed as:

[0083]

[0084] By using equations (7) and (8), it can be observed that when the pitch angle θ remains constant, the wind energy utilization coefficient C p Reach the maximum value; implement the maximum power point tracking MPPT strategy to capture the maximum wind energy; the maximum mechanical power of the wind turbine is expressed as follows:

[0085]

[0086] Among them, λ opt and ω opt Respectively represent the best tip speed ratio and the best mechanical speed; C pmax Indicates the maximum value of wind energy utilization coefficient;

[0087] In heterogeneous power systems, wind turbines are connected to the grid in the form of GFLs;

[0088] Based on equations (1)-(4) and (9), the heterogeneous power system model is expressed as:

[0089]

[0090] in, represents the frequency of the center of inertia COI; P ω1 and P ω2 Respectively represent the output power of wind turbine generator set 1 and wind turbine generator set 2; H Σ =H1+H2,D Σ =D1+D2 represents the total damping coefficient of the system.

[0091] Preferably, the module M2 comprises:

[0092] Design assistance systems are:

[0093]

[0094] Where, frequency error e = ω COI -η, ρ is a positive constant, η represents the auxiliary state variable of the system;

[0095] The preset time nonlinear observer is designed as:

[0096]

[0097] in, is the estimated value of the disturbance d, is the estimated value of state e, Indicates the rate of change of frequency error; It is expressed as:

[0098]

[0099] in, is the frequency estimation error, the sign function sig(·) γ =sign(·)|·| γ , observer coefficient α1>0,α2>0, and preset time T c1 >0,T c1 Represents the preset time constant of the observer.

[0100] Preferably, the module M3 comprises:

[0101] The preset time sliding surface is designed as:

[0102]

[0103] Where σ=ω COI -ω n represents the control error, the controller coefficient T c2 Indicates the preset time constant of the controller;

[0104] When the control error σ reaches the preset time sliding surface, s=0, Therefore, we get:

[0105]

[0106] Based on (10) and (15) and the observation results of the preset time observer Get the equivalent controller u eq for:

[0107]

[0108] Let the approach controller be u ri , which is used to ensure that the control error σ effectively reaches the preset time sliding surface specified in equation (14), and its expression is:

[0109]

[0110] The controller coefficient is T c3 represents the preset time constant of the equivalent controller;

[0111] Based on equations (16) and (17), the preset time sliding mode controller is designed as:

[0112]

[0113] Under the preset time sliding mode controller based on the observer, the control error σ reaches the preset time sliding mode surface within the preset time and converges to 0.

[0114] Compared with the prior art, the present invention has the following beneficial effects:

[0115] 1. By designing a control strategy based on a preset time sliding mode observer, the present invention realizes the rapid estimation of source-load uncertainty disturbances in heterogeneous power systems, overcoming the limitation of traditional observers that the convergence time depends on parameters;

[0116] 2. The present invention uses a preset time sliding mode controller to effectively suppress system frequency oscillation within a preset time, reduce frequency overshoot and convergence time, and significantly improve the dynamic response performance of the system compared to traditional SMC and PI control methods;

[0117] 3. Combined with the robustness of sliding mode control, the present invention can cope with source-load uncertainty and external disturbances in complex heterogeneous power systems to ensure system frequency stability;

[0118] 4. Through the rapid active power regulation of the grid-type converter, the present invention effectively solves the problem of insufficient frequency regulation capability of heterogeneous power systems and provides efficient inertia and frequency support.

[0119] Other beneficial effects of the present invention will be explained in the specific implementation manner through the introduction of specific technical features and technical solutions. Through the introduction of these technical features and technical solutions, those skilled in the art should be able to understand the beneficial technical effects brought about by the technical features and technical solutions. BRIEF DESCRIPTION OF THE DRAWINGS

[0120] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0121] Figure 1 It is the overall framework diagram of the present invention;

[0122] Figure 2 A heterogeneous power system model of the present invention;

[0123] Figure 3 The control structure of the GFM converter of the present invention;

[0124] FIG. 4 is a diagram showing the simulation results of the present invention. DETAILED DESCRIPTION

[0125] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0126] The embodiment of the present invention provides a fast frequency support control method for a grid-connected converter. By improving the SMC control strategy and customizing the design according to the characteristics of heterogeneous power systems, the above difficulties are successfully overcome, and effective control of the preset time stability is achieved. The stability and dynamic response performance of the system are significantly improved, reflecting the application potential and innovative value of this method in complex power systems. Figure 1 As shown, the invention includes:

[0127] Step S1: Establish a heterogeneous power system model, including a GFM converter model, a synchronous generator SG model and a GFL wind turbine generator model;

[0128] Step S2: according to the established GFL wind turbine model, the output power and load power of the GFL wind turbine are regarded as disturbances, and a preset time nonlinear observer is designed to estimate the disturbances;

[0129] Step S3: Design a preset time sliding mode controller to adjust the output power increment ΔP of the GFM converter ref , to regulate the frequency of heterogeneous power systems;

[0130] Step S4: Simulate oscillation suppression by adjusting the frequency of the heterogeneous power system and present the simulation results.

[0131] 1) Establish a heterogeneous power system model. The details are as follows:

[0132] Figure 2 The heterogeneous power system model of the present invention is a model and structure of a heterogeneous power system consisting of a synchronous generator (SG), a GFL wind turbine generator set and a GFM converter. The heterogeneous power system includes two interconnected areas, namely area A and area B, which exchange power through a tie line. The models of each part are established as follows.

[0133] Figure 3The control structure of the GFM converter is presented, which includes three control loops: power control loop, voltage control loop and current control loop, which together constitute the control system of the GFM converter. The power control loop is mainly used to regulate and synchronize the output power (active power and reactive power) with the grid. In the power control loop, VSG control is used to achieve inertia and frequency support, which is achieved by regulating the active power. Reactive power regulation and output voltage control at the point of common coupling (PCC) are completed by droop control in the power control loop. Therefore, the model of the GFM converter can be expressed as follows:

[0134]

[0135] P1=P AB +P L1 -P ω1 (2)

[0136] Where, ω1 is the frequency of the GFM converter; ω n is the rated frequency; δ1 represents the power angle; t represents the time; ΔP ref Indicates the increment of reference power; P ref and P1 represent the initial active power and output active power respectively; D1 represents the damping coefficient of the GFM converter; H1 represents the inertia time constant of the GFM converter; P AB is the exchange power between area A and area B; P L1 is the load power of load 1, P ω1 is the output power of the GFL wind turbine in area A.

[0137] Establish the dynamic equation model of the synchronous generator:

[0138]

[0139] Among them, δ2, ω2, P2, D2, H2 and P m0 They represent the rotor angle, rotor speed, electromagnetic torque, damping coefficient of the synchronous generator, inertia time constant of the synchronous generator and mechanical torque respectively; P L2 is the load power of load 2, P ω2 is the output power of the GFL wind turbine in area B.

[0140] The GFL wind turbine model is established, and the mechanical power generated by the GFL wind turbine is expressed as:

[0141]

[0142] The mechanical torque of the GFL wind turbine is expressed as follows:

[0143]

[0144] Among them, ρ, ω, R and v ω Respectively represent air density, mechanical speed, wind wheel radius and wind speed; C p , λ and θ represent the tip speed ratio, wind energy utilization coefficient and pitch angle respectively; wind energy utilization coefficient C p It is expressed as:

[0145]

[0146] The tip speed ratio λ is expressed as:

[0147]

[0148] By using equations (7) and (8), it can be observed that when the pitch angle θ remains constant, the wind energy utilization coefficient C p Reach the maximum value. Therefore, the Maximum Power Point Tracking (MPPT) strategy can be implemented to capture the maximum wind energy. The maximum mechanical power of the wind turbine is expressed as follows:

[0149]

[0150] Among them, λ opt and ω opt Respectively represent the best tip speed ratio and the best mechanical speed; C pmax Indicates the maximum value of wind energy utilization coefficient;

[0151] In a heterogeneous power system, wind turbines are connected to the grid in the form of GFLs; therefore, GFL wind turbines cannot provide inertia and operate in MPPT mode. Based on equations (1)-(4) and (9), the heterogeneous power system model is expressed as:

[0152]

[0153] in, Indicates the frequency of the Center of Inertia (COI); P ω1 and P ω2 Respectively represent the output power of wind turbine generator set 1 and wind turbine generator set 2;

[0154]

[0155] H Σ =H1+H2,D Σ =D1+D2 represents the total damping coefficient of the system.

[0156] 2) Design a preset time nonlinear observer, the details are as follows:

[0157] From formula (10), it can be seen that the output power and load power of the GFL wind turbine are regarded as disturbances, considering the randomness of the output power of the GFL wind turbine and the fluctuation of the load. Therefore, it is necessary to design an observer to estimate the disturbance d.

[0158] First, the design assistance system is:

[0159]

[0160] Where, frequency error e = ω COI -η, ρ is a positive constant, η represents the auxiliary state variable of the system;

[0161] The preset time nonlinear observer is designed as:

[0162]

[0163] in, is the estimated value of the disturbance d, is the estimated value of state e, Indicates the rate of change of frequency error; It is expressed as:

[0164]

[0165] in, is the frequency estimation error, the sign function sig(·) γ =sign(·)|·| γ , observer coefficient α1>0,α2>0, and preset time T c1 >0,T c1 Represents the preset time constant of the observer.

[0166] The preset time observer proposed in the present invention can estimate the disturbance d, especially the output power and load power of the GFL wind turbine generator, within the preset time.

[0167] 3) Design a preset time sliding mode controller, the details are as follows:

[0168] In order to allow the designed controller to adjust the output power increment ΔP of the GFM converter ref To regulate the frequency of heterogeneous power systems, thereby providing frequency support for heterogeneous power systems.

[0169] First, the preset time sliding surface is designed as:

[0170]

[0171] Where σ=ωCOI -ω n represents the control error, the controller coefficient T c2 Indicates the preset time constant of the controller;

[0172] When the control error σ reaches the preset time sliding surface, s=0, Therefore, we get:

[0173]

[0174] Based on (10) and (15) and the observation results of the preset time observer Get the equivalent controller u eq for:

[0175]

[0176] Let the approach controller be u ri , which is used to ensure that the control error σ effectively reaches the preset time sliding surface specified in equation (14), and its expression is:

[0177]

[0178] The controller coefficient is T c3 represents the preset time constant of the equivalent controller;

[0179] Based on equations (16) and (17), the preset time sliding mode controller is designed as:

[0180]

[0181] Under the proposed observer-based preset time sliding mode controller (PTOBSMC), the control error σ can reach the preset time sliding mode surface and converge to 0 within the preset time.

[0182] The fast frequency support control framework of the grid-type converter based on the preset time sliding mode observer is referenced Figure 1 shown.

[0183] 4) Simulation results:

[0184] Table I Heterogeneous power system parameters

[0185]

[0186] According to the scheduled time theory and the pre-selected parameters, the scheduled time is determined as T c1 =T c2=1, and the relevant parameters are set to ρ=10, γ=0.2, k1=k4=k7=10, k2=k5=k8=5.4, k3=k6=k9=5.2.

[0187] Simulation Example: Load Change

[0188] At t = 2s, a power of 6×10 6 W load, and at t = 7s, a power of 8×10 6 W load. From the simulation results, it can be seen that no matter whether the load is connected or disconnected, the frequency ω COI , ω1 and ω2 will oscillate, and in heterogeneous power systems, by adjusting the active power of the GFM energy storage system under the proposed observer-based preset time sliding mode controller, the oscillation can be effectively suppressed. In addition, compared with the traditional SMC and PI control methods, the system shows smaller overshoot and shorter convergence time under the proposed observer-based preset time sliding mode controller. The simulation results are shown in Figure 4.

[0189] The present invention also provides a grid-type converter rapid frequency support control system, which can be implemented by executing the process steps of the grid-type converter rapid frequency support control method, that is, those skilled in the art can understand the grid-type converter rapid frequency support control method as a preferred implementation of the grid-type converter rapid frequency support control system. The system specifically includes:

[0190] Module M1: Establish a heterogeneous power system model, including the GFM converter model, the synchronous generator SG model and the GFL wind turbine model;

[0191] Module M2: Based on the established GFL wind turbine model, the output power and load power of the GFL wind turbine are regarded as disturbances, and a preset time nonlinear observer is designed to estimate the disturbances;

[0192] Module M3: Design a preset time sliding mode controller to adjust the output power increment ΔP of the GFM converter ref , to regulate the frequency of heterogeneous power systems;

[0193] Module M4: Simulate the oscillation suppression by adjusting the frequency of heterogeneous power systems and present the simulation results.

[0194] 1) Establish a heterogeneous power system model. The details are as follows:

[0195] Figure 2The heterogeneous power system model of the present invention is a model and structure of a heterogeneous power system consisting of a synchronous generator (SG), a GFL wind turbine generator set and a GFM converter. The heterogeneous power system includes two interconnected areas, namely area A and area B, which exchange power through a tie line. The models of each part are established as follows.

[0196] Figure 3 The control structure of the GFM converter is presented, which includes three control loops: power control loop, voltage control loop and current control loop, which together constitute the control system of the GFM converter. The power control loop is mainly used to regulate and synchronize the output power (active power and reactive power) with the grid. In the power control loop, VSG control is used to achieve inertia and frequency support, which is achieved by regulating the active power. Reactive power regulation and output voltage control at the point of common coupling (PCC) are completed by droop control in the power control loop. Therefore, the model of the GFM converter can be expressed as follows:

[0197]

[0198] Where, ω1 is the frequency of the GFM converter; ω n is the rated frequency; δ1 represents the power angle; t represents the time; ΔP ref Indicates the increment of reference power; P ref and P1 represent the initial active power and output active power respectively; D1 represents the damping coefficient of the GFM converter; H1 represents the inertia time constant of the GFM converter; P AB is the exchange power between area A and area B; P L1 is the load power of load 1, P ω1 is the output power of the GFL wind turbine in area A.

[0199] Establish the dynamic equation model of the synchronous generator:

[0200]

[0201] P2=P L2 -P AB -P ω2 (4)

[0202] Among them, δ2, ω2, P2, D2, H2 and P m0 They represent the rotor angle, rotor speed, electromagnetic torque, damping coefficient of the synchronous generator, inertia time constant of the synchronous generator and mechanical torque respectively; P L2 is the load power of load 2, P ω2 is the output power of the GFL wind turbine in area B.

[0203] The GFL wind turbine model is established, and the mechanical power generated by the GFL wind turbine is expressed as:

[0204]

[0205] The mechanical torque of the GFL wind turbine is expressed as follows:

[0206]

[0207] Among them, ρ, ω, R and v ω Respectively represent air density, mechanical speed, wind wheel radius and wind speed; C p , λ and θ represent the tip speed ratio, wind energy utilization coefficient and pitch angle respectively; wind energy utilization coefficient C p It is expressed as:

[0208]

[0209] The tip speed ratio λ is expressed as:

[0210]

[0211] By using equations (7) and (8), it can be observed that when the pitch angle θ remains constant, the wind energy utilization coefficient C p Reach the maximum value. Therefore, the Maximum Power Point Tracking (MPPT) strategy can be implemented to capture the maximum wind energy. The maximum mechanical power of the wind turbine is expressed as follows:

[0212]

[0213] Among them, λ opt and ω opt Respectively represent the best tip speed ratio and the best mechanical speed; C pmax Indicates the maximum value of wind energy utilization coefficient;

[0214] In a heterogeneous power system, wind turbines are connected to the grid in the form of GFLs; therefore, GFL wind turbines cannot provide inertia and operate in MPPT mode. Based on equations (1)-(4) and (9), the heterogeneous power system model is expressed as:

[0215]

[0216] in, Indicates the frequency of the Center of Inertia (COI); P ω1 and P ω2 Respectively represent the output power of wind turbine generator set 1 and wind turbine generator set 2; u=ΔPref , H Σ =H1+H2,D Σ =D1+D2 represents the total damping coefficient of the system.

[0217] 2) Design a preset time nonlinear observer, the details are as follows:

[0218] From formula (10), it can be seen that the output power and load power of the GFL wind turbine are regarded as disturbances, considering the randomness of the output power of the GFL wind turbine and the fluctuation of the load. Therefore, it is necessary to design an observer to estimate the disturbance d.

[0219] First, the design assistance system is:

[0220]

[0221] Where, frequency error e = ω COI -η, ρ is a positive constant, η represents the auxiliary state variable of the system;

[0222] The preset time nonlinear observer is designed as:

[0223]

[0224] in, is the estimated value of the disturbance d, is the estimated value of state e, Indicates the rate of change of frequency error; It is expressed as:

[0225]

[0226] in, is the frequency estimation error, the sign function sig(·) γ =sign(|·|) γ , observer coefficient α1>0,α2>0, and preset time T c1 >0,T c1 Represents the preset time constant of the observer.

[0227] The preset time observer proposed in the present invention can estimate the disturbance d, especially the output power and load power of the GFL wind turbine generator, within the preset time.

[0228] 3) Design a preset time sliding mode controller, the details are as follows:

[0229] In order to allow the designed controller to adjust the output power increment ΔP of the GFM converter refTo regulate the frequency of heterogeneous power systems, thereby providing frequency support for heterogeneous power systems.

[0230] First, the preset time sliding surface is designed as:

[0231]

[0232] Where σ=ω COI -ω n represents the control error, the controller coefficient T c2 Indicates the preset time constant of the controller;

[0233] When the control error σ reaches the preset time sliding surface, s=0, Therefore, we get:

[0234]

[0235] Based on (10) and (15) and the observation results of the preset time observer Get the equivalent controller u eq for:

[0236]

[0237] Let the approach controller be u ri , which is used to ensure that the control error σ effectively reaches the preset time sliding surface specified in equation (14), and its expression is:

[0238]

[0239] The controller coefficient is T c3 represents the preset time constant of the equivalent controller;

[0240] Based on equations (16) and (17), the preset time sliding mode controller is designed as:

[0241]

[0242] Under the proposed observer-based preset time sliding mode controller (PTOBSMC), the control error σ can reach the preset time sliding mode surface and converge to 0 within the preset time.

[0243] The fast frequency support control framework of the grid-type converter based on the preset time sliding mode observer is referenced Figure 1 shown.

[0244] 4) Simulation results:

[0245] Table I Heterogeneous power system parameters

[0246]

[0247] According to the scheduled time theory and the pre-selected parameters, the scheduled time is determined as T c1 =T c2 =1, and the relevant parameters are set to ρ=10, γ=0.2, k1=k4=k7=10, k2=k5=k8=5.4, k3=k6=k9=5.2.

[0248] Simulation Example: Load Change

[0249] At t = 2s, a power of 6×10 6 W load, and at t = 7s, a power of 8×10 6 W load. From the simulation results, it can be seen that no matter whether the load is connected or disconnected, the frequency ω COI , ω1 and ω2 will oscillate, and in heterogeneous power systems, by adjusting the active power of the GFM energy storage system under the proposed observer-based preset time sliding mode controller, the oscillation can be effectively suppressed. In addition, compared with the traditional SMC and PI control methods, the system shows smaller overshoot and shorter convergence time under the proposed observer-based preset time sliding mode controller. The simulation results refer to Figures 4a to 4d shown.

[0250] Those skilled in the art know that, in addition to realizing the system and its various devices, modules, and units provided by the present invention in a purely computer-readable program code, it is entirely possible to realize the same functions in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system and its various devices, modules, and units provided by the present invention can be considered as a hardware component, and the devices, modules, and units included therein for realizing various functions can also be regarded as structures within the hardware component; the devices, modules, and units for realizing various functions can also be regarded as both software modules for realizing the method and structures within the hardware component.

[0251] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A fast frequency support control method for a grid-type converter, characterized in that: include: Step S1: Establish a heterogeneous power system model, including a GFM converter model, a synchronous generator SG model and a GFL wind turbine generator model; Step S2: according to the established GFL wind turbine model, the output power and load power of the GFL wind turbine are regarded as disturbances, and a preset time nonlinear observer is designed to estimate the disturbances; Step S3: designing a preset time sliding mode controller to adjust the output power increment of the GFM converter to adjust the frequency of the heterogeneous power system; Step S4: Simulate oscillation suppression by adjusting the frequency of the heterogeneous power system and present the simulation results.

2. The method for fast frequency support control of a grid-connected converter according to claim 1, characterized in that: In the step S1, the heterogeneous power system model is divided into two interconnected areas, namely area A and area B, and power exchange is performed through a tie line; The GFM converter includes: a power control loop, a voltage control loop and a current control loop; wherein the power control loop is used to adjust and synchronize the output power with the power grid. In the power control loop, VSG control is used to adjust the active power to achieve inertia and frequency support, and reactive power regulation and output voltage control at the common connection point PCC are completed through droop control in the power control loop; the GFM converter model is as follows: P1=P AB +P L1 -P ω1 (2) Where, ω1 is the frequency of the GFM converter; ω n is the rated frequency; δ1 represents the power angle; t represents the time; ΔP ref Indicates the increment of reference power; P ref and P1 represent the initial active power and output active power respectively; D1 represents the damping coefficient of the GFM converter; H1 represents the inertia time constant of the GFM converter; P AB is the exchange power between area A and area B; P L1 is the load power of load 1, P ω1 is the output power of the GFL wind turbine in area A.

3. The fast frequency support control method for a grid-type converter according to claim 2, characterized in that: In step S1, a synchronous generator SG model is established: P2=P L2 -P AB -P ω2 (4) Among them, δ2, ω2, P2, D2, H2 and P m0 They represent the rotor angle, rotor speed, electromagnetic torque, damping coefficient of the synchronous generator, inertia time constant of the synchronous generator and mechanical torque respectively; P L2 is the load power of load 2, P ω2 is the output power of the GFL wind turbine in area B.

4. The method for fast frequency support control of a grid-type converter according to claim 1, characterized in that: The GFL wind turbine model is established in step S1 as follows: The mechanical power generated by the GFL wind turbine is expressed as: The mechanical torque of the GFL wind turbine is expressed as follows: Among them, ρ, ω, R and v ω Respectively represent air density, mechanical speed, wind wheel radius and wind speed; C p , λ and θ represent the tip speed ratio, wind energy utilization coefficient and pitch angle respectively; wind energy utilization coefficient C p It is expressed as: The tip speed ratio λ is expressed as: By using equations (7) and (8), it can be observed that when the pitch angle θ remains constant, the wind energy utilization coefficient C p Reach the maximum value; implement the maximum power point tracking MPPT strategy to capture the maximum wind energy; the maximum mechanical power of the wind turbine is expressed as follows: Among them, λ opt and ω opt Respectively represent the best tip speed ratio and the best mechanical speed; C pmax Indicates the maximum value of wind energy utilization coefficient; In heterogeneous power systems, wind turbines are connected to the grid in the form of GFLs; Based on equations (1)-(4) and (9), the heterogeneous power system model is expressed as: in, represents the frequency of the center of inertia COI; P ω1 and P ω2 Respectively represent the output power of wind turbine generator set 1 and wind turbine generator set 2; H Σ =H1+H2,D Σ =D1+D2 represents the total damping coefficient of the system.

5. The method for fast frequency support control of a grid-connected converter according to claim 4, characterized in that: The step S2 comprises: Design assistance systems are: Where, frequency error e = ω COI -η, ρ is a positive constant, η represents the auxiliary state variable of the system; The preset time nonlinear observer is designed as: in, is the estimated value of the disturbance d, is the estimated value of state e, Indicates the rate of change of frequency error; It is expressed as: in, is the frequency estimation error, the sign function sig(·) γ =sign(·)|·| γ , observer coefficient α1>0,α2>0, and preset time T c1 >0,T c1 Represents the preset time constant of the observer.

6. The method for fast frequency support control of a grid-connected converter according to claim 1, characterized in that: The step S3 comprises: The preset time sliding surface is designed as: Where σ=ω COI -ω n represents the control error, the controller coefficient T c2 Indicates the preset time constant of the controller; When the control error σ reaches the preset time sliding surface, s=0, Therefore, we get: Based on (10) and (15) and the observation results of the preset time observer Get the equivalent controller u eq for: Let the approach controller be u ri , which is used to ensure that the control error σ effectively reaches the preset time sliding surface specified in equation (14), and its expression is: The controller coefficient is T c3 represents the preset time constant of the equivalent controller; Based on equations (16) and (17), the preset time sliding mode controller is designed as: Under the preset time sliding mode controller based on the observer, the control error σ reaches the preset time sliding mode surface within the preset time and converges to 0.

7. A grid-type converter fast frequency support control system, characterized in that: include: Module M1: Establish a heterogeneous power system model, including the GFM converter model, the synchronous generator SG model and the GFL wind turbine model; Module M2: Based on the established GFL wind turbine model, the output power and load power of the GFL wind turbine are regarded as disturbances, and a preset time nonlinear observer is designed to estimate the disturbances; Module M3: Design a preset time sliding mode controller to adjust the output power increment of the GFM converter to adjust the frequency of the heterogeneous power system; Module M4: Simulate the oscillation suppression by adjusting the frequency of heterogeneous power systems and present the simulation results.

8. The grid-type converter fast frequency support control system according to claim 7, characterized in that: In the module M1, the heterogeneous power system model is divided into two interconnected areas, namely area A and area B, and power exchange is performed through the tie line; The GFM converter includes: a power control loop, a voltage control loop and a current control loop; wherein the power control loop is used to adjust and synchronize the output power with the power grid. In the power control loop, VSG control is used to adjust the active power to achieve inertia and frequency support, and reactive power regulation and output voltage control at the common connection point PCC are completed through droop control in the power control loop; the GFM converter model is as follows: P1=P AB +P L1 -P ω1 (2) Where, ω1 is the frequency of the GFM converter; ω n is the rated frequency; δ1 represents the power angle; t represents the time; ΔP ref Indicates the increment of reference power; P ref and P1 represent the initial active power and output active power respectively; D1 represents the damping coefficient of the GFM converter; H1 represents the inertia time constant of the GFM converter; P AB is the exchange power between area A and area B; P L1 is the load power of load 1, P ω1 is the output power of the GFL wind turbine in area A; The synchronous generator SG model is established in the module M1: P2=P L2 -P AB -P ω2 (4) Among them, δ2, ω2, P2, D2, H2 and P m0 They represent the rotor angle, rotor speed, electromagnetic torque, damping coefficient of the synchronous generator, inertia time constant of the synchronous generator and mechanical torque respectively; P L2 is the load power of load 2, P ω2 is the output power of the GFL wind turbine in area B; The GFL wind turbine model is established in the module M1 as follows: The mechanical power generated by the GFL wind turbine is expressed as: The mechanical torque of the GFL wind turbine is expressed as follows: Among them, ρ, ω, R and v ω Respectively represent air density, mechanical speed, wind wheel radius and wind speed; C p , λ and θ represent the tip speed ratio, wind energy utilization coefficient and pitch angle respectively; wind energy utilization coefficient C p It is expressed as: The tip speed ratio λ is expressed as: By using equations (7) and (8), it can be observed that when the pitch angle θ remains constant, the wind energy utilization coefficient C p Reach the maximum value; implement the maximum power point tracking MPPT strategy to capture the maximum wind energy; the maximum mechanical power of the wind turbine is expressed as follows: Among them, λ opt and ω opt Respectively represent the best tip speed ratio and the best mechanical speed; C pmax Indicates the maximum value of wind energy utilization coefficient; In heterogeneous power systems, wind turbines are connected to the grid in the form of GFLs; Based on equations (1)-(4) and (9), the heterogeneous power system model is expressed as: in, represents the frequency of the center of inertia COI; P ω1 and P ω2 Respectively represent the output power of wind turbine generator set 1 and wind turbine generator set 2; u=ΔP ref , H Σ =H1+H2,D Σ =D1+D2 represents the total damping coefficient of the system.

9. The grid-type converter fast frequency support control system according to claim 7, characterized in that: The module M2 comprises: Design assistance systems are: Where, frequency error e = ω COI -η, ρ is a positive constant, η represents the auxiliary state variable of the system; The preset time nonlinear observer is designed as: in, is the estimated value of the disturbance d, is the estimated value of state e, Indicates the rate of change of frequency error; It is expressed as: in, is the frequency estimation error, the sign function sig(·) γ =sign(·)|·| γ , observer coefficient α1>0,α2>0, and preset time T c1 >0,T c1 Represents the preset time constant of the observer.

10. The grid-type converter fast frequency support control system according to claim 7, characterized in that: The module M3 comprises: The preset time sliding surface is designed as: Where σ=ω COI -ω n represents the control error, the controller coefficient T c2 Indicates the preset time constant of the controller; When the control error σ reaches the preset time sliding surface, s=0, Therefore, we get: Based on (10) and (15) and the observation results of the preset time observer Get the equivalent controller u eq for: Let the approach controller be u ri , which is used to ensure that the control error σ effectively reaches the preset time sliding surface specified in equation (14), and its expression is: The controller coefficient is T c3 represents the preset time constant of the equivalent controller; Based on equations (16) and (17), the preset time sliding mode controller is designed as: Under the preset time sliding mode controller based on the observer, the control error σ reaches the preset time sliding mode surface within the preset time and converges to 0.

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