Optimal proportioning method for network construction type wind turbine generator set and network following type wind turbine generator set in wind power plant station

Abstract

The invention relates to an optimal proportioning method for a grid-constructing type wind turbine generator and a grid-following type wind turbine generator in a wind power plant, solves the problem of subsynchronous oscillation of the grid-following type wind turbine generator in the wind power plant under a weak power grid, and belongs to the technical field of electrical equipment. The method comprises the following steps: respectively establishing state space models of single machines and power grid lines of a network-constructing type wind turbine generator and a network-following type wind turbine generator; according to the established state space model, single-machine port frequency domain impedance in the grid-forming type wind turbine generator set and the grid-following type wind turbine generator set is obtained, black box processing is conducted on all the generator sets, and the equivalent impedance of a whole grid-connected port of the wind power plant is calculated in combination with the connection mode between the wind turbine generator sets and the connection mode between the wind turbine generator sets and a power grid line; positive and negative resistance and capacitive-inductive reactance characteristics of the subsynchronous frequency band are obtained according to the equivalent impedance, the overall interference stability of the wind power plant is judged according to the characteristics, if the stability is not stable, the proportion of the net-forming type wind turbine generator is increased, the equivalent impedance is calculated again till the stability is achieved, and the proportion is output.

Description

Technical Field

[0001] The present invention relates to an optimization ratio method for grid-forming and grid-following wind turbines in a wind farm, belonging to the technical field of electrical equipment. Background Art

[0002] In recent years, the capacity of renewable energy connected to the power grid through power electronic devices has been steadily increasing. Due to the characteristics of new energy such as long transmission distance, low inertia, and uncertainty, the power grid faces challenges such as weakened strength forming a weak power grid and reduced dynamic stability. Various oscillation phenomena characterize these challenges and pose a major threat to the safe and stable operation of modern power systems.

[0003] The grid-following control technology is widely used in the actual operation of existing wind turbines. It uses a phase-locked loop to follow the grid angle to achieve synchronization with the grid, and at the same time uses a vector control strategy to control the output current and power of the unit. It has the advantages of quickly and accurately realizing power decoupling and control, but it has the risk of sub / super-synchronous oscillation in the case of a weak power grid, and at the same time does not have the ability to actively support the power grid. Therefore, it is necessary to study a new control method for wind turbines.

[0004] Drawing on the operation mode of synchronous generators, the application of grid-forming control technology in power systems has received extensive attention in recent years. The existing grid-forming control technology mainly uses virtual synchronous machine control, which uses a power-based or inertia-based synchronization method and has the characteristics of a synchronous voltage source. It can actively show the response and support to the grid frequency and voltage. Research shows that the grid-forming control technology is suitable for application scenarios with weak system strength and low physical inertia. Under weak grid conditions, grid-forming wind turbines have good stability. Therefore, configuring grid-forming wind turbines in a field is beneficial to improving system stability. The connection form of grid-forming and grid-following wind turbines coexisting in a field has become an important research direction for wind power grid connection.

[0005] On the other hand, since most of the existing operating fields use grid-following control, it is costly to transform grid-following control into grid-forming control. At the same time, there are also problems such as high cost and instability in strong grids when applying grid-forming wind turbines on a large scale in additional projects. Therefore, how to adjust the capacity ratio of grid-forming / grid-following wind turbines to make it have better operation stability and stability margin, while reducing the actual cost of the project, is one of the important concerns of existing new energy grid connection projects, and corresponding optimization ratio methods need to be proposed. Summary of the Invention

[0006] Aiming at the problem of sub-synchronous oscillation of grid-following wind turbines in a field under a weak power grid, the present invention provides an optimization ratio method for grid-forming and grid-following wind turbines in a wind farm to improve its small-signal stability.

[0007] An optimization ratio method for grid-forming and grid-following wind turbines in a wind farm, comprising:

[0008] S1. Respectively establish a single-machine state space model of the grid-forming wind turbine, a single-machine state space model of the grid-following wind turbine, and a grid line state space model;

[0009] S2. According to the established single-machine state space model and grid line state space model, obtain the single-machine port frequency-domain impedance in the grid-forming wind turbine and the grid-following wind turbine;

[0010] S3. According to the obtained single-machine port frequency-domain impedance, black-box each unit, and calculate the equivalent impedance of the overall grid connection port of the wind farm in combination with the connection methods between the wind turbines and between the wind turbines and the grid lines;

[0011] S4. Obtain the positive and negative resistance, capacitance and inductance characteristics in the sub-synchronous frequency band according to the equivalent impedance of the overall grid connection port of the wind farm, judge the overall interference stability of the wind farm according to the positive and negative resistance, capacitance and inductance characteristics in the sub-synchronous frequency band. If it is unstable, increase the ratio of the grid-forming wind turbines and transfer to S3. If it is stable, output the ratio of the grid-forming and grid-following wind turbines.

[0012] Preferably, the expression of the single-machine state space model of the grid-following wind turbine is:

[0013] Δx GFL =A GFL Δx GFL +B GFL Δu GFL

[0014] Δy GFL =C GFL Δx GFL

[0015] In the formula, Δx GFL is the state variable of the single machine of the grid-following wind turbine, Δu GFL is the input variable of the single machine of the grid-following wind turbine, Δy GFL is the output variable of the single machine of the grid-following wind turbine, A GFL 、B GFL 、C GFL are respectively the state matrix, input matrix, and output matrix of the single-machine state space model of the grid-following wind turbine;

[0016] The expression of the single-machine state space model of the grid-forming wind turbine is:

[0017] Δx GFM =A GFM Δx GFM +BGFM Δu GFM

[0018] Δy GFM = C GFM Δx GFM

[0019] In the formula, Δx GFM is the state variable of a single unit of a grid-forming wind turbine, Δu GFM is the input variable of a single unit of a grid-forming wind turbine, Δy GFM is the output variable of a single unit of a grid-forming wind turbine, A GFM 、B GFM 、C GFM are respectively the state matrix, input matrix, and output matrix of the state space model of a single unit of a grid-forming wind turbine;

[0020] The state space model of a single unit of a grid-following wind turbine includes the state space models of the phase-locked loop, DC voltage loop, current loop, delay link, and LC filter link of the grid-following wind turbine;

[0021] The state space model of a single unit of a grid-forming wind turbine includes the state space models of the power outer loop and voltage-current double closed loop of the grid-forming wind turbine;

[0022] The state variables in each state space model are determined according to the corresponding control equations.

[0023] Preferably, the control equation of the phase-locked loop of the grid-following wind turbine is:

[0024]

[0025] Among them, ΔA 1_GFL represents the state variable obtained after the q-axis component of the grid-following wind turbine access point voltage passes through the phase-locked loop integration link, K pll_P and K pll_I respectively represent the proportional and integral coefficients in the PI link of the phase-locked loop, Δθ GFL is the disturbance amount of the control system angle output by the phase-locked loop of the grid-following wind turbine, Δu pccq_GFL represents the q-axis disturbance amount of the grid-following wind turbine access point voltage in the dq rotating coordinate system, U pccd_GFL represents the steady-state value of the d-axis component of the grid-following wind turbine access point voltage in the dq rotating coordinate system;

[0026] The control equation of the DC voltage loop of the grid-following wind turbine is:

[0027]

[0028] Among them, ΔA 2_GFLrepresents the state variable obtained after the integral link of the DC bus voltage of the grid-connected wind turbine passes through the DC voltage loop, K Udc_P and K Udc_I respectively represent the proportional and integral coefficients in the PI link of the DC voltage loop, Δu dc_GFL is the disturbance amount of the DC bus voltage of the grid-connected wind turbine;

[0029] The current loop control equation of the grid-connected wind turbine is:

[0030]

[0031] Among them, ΔA 3_GFL and ΔA 4_GFL respectively represent the state variables obtained after the integral link of the d-axis and q-axis components of the filter inductor current of the grid-connected wind turbine passes through the current loop, K I_PGFL and K I_IGFL respectively represent the proportional and integral coefficients of the PI link of the current loop, Δi ld_GFL and Δi lq_GFL respectively represent the d-axis and q-axis disturbance amounts of the filter current of the grid-connected wind turbine, Δi lqref_GFL represents the disturbance amount of the given value of the q-axis component of the filter current.

[0032] Preferably, the power outer loop control equation of the grid-forming wind turbine:

[0033]

[0034]

[0035] In the formula, Δω GFM and Δθ GFM are respectively the system frequency disturbance amount and angle disturbance amount actively constructed by the active power loop of the grid-forming wind turbine, ΔP GFM and ΔQ GFM are respectively the active and reactive powers output by the grid-forming unit, J, K, D p and D q are respectively the virtual inertia, reactive power inertia coefficient, and droop coefficients of the active power loop and reactive power loop of the grid-forming unit, Δu pccd_GFM and Δu pccq_GFM respectively represent the d-axis and q-axis disturbance amounts of the grid-forming wind turbine access point voltage in the dq rotating coordinate system, U pccd_GFM and U pccq_GFM respectively represent the d-axis and q-axis steady-state values of the grid-forming wind turbine access point voltage;

[0036] ΔE GFM represents the amplitude of the reference voltage output by the grid-forming unit, U GFM represents the voltage reference value output by the grid-forming unit;

[0037] Voltage and current double closed-loop control equations of grid-forming wind turbines:

[0038]

[0039] In the formula, ΔB 1_GFM and ΔB 2_GFM are state variables corresponding to the integral link of the voltage loop of the grid-forming wind turbine, and ΔB 3_GFM and ΔB 4_GFM are state variables corresponding to the integral link of the current loop of the grid-forming wind turbine. K V_PGFM and K V_IGFM represent the proportional and integral coefficients of the PI link of the voltage loop;

[0040] ω n represents the output frequency reference value.

[0041] Preferably, the grid line state space model is:

[0042]

[0043] Among them, Δi gd and Δi gq are the disturbance quantities of the d-axis and q-axis components of the grid line current, and Δu pccd and Δu pccq are the disturbance quantities of the d-axis and q-axis components of the grid voltage. L grid and R grid are the equivalent inductance and equivalent resistance of the grid line respectively. ω n represents the output frequency reference value.

[0044] Preferably, S2 includes:

[0045] Inject a voltage disturbance with an amplitude of ξ and a frequency of ω r at the access points of the grid-following wind turbine and the grid-forming wind turbine, measure the current responses of the grid-following wind turbine and the grid-forming wind turbine at the same frequency ω r , and calculate the single-machine port frequency-domain impedance of the wind turbine at the frequency ω r :

[0046]

[0047] In the formula, γ upccd_GFL / GFM , γ upccq_GFL / GFM , γ ipccd_GFL / GFM , γ ipccq_GFL / GFM respectively represent the components of the eigenvector in the rows where Δu pccd_GFL / GFM , Δu pccq_GFL / GFM , Δi pccd_GFL / GFM , and Δi pccq_GFL / GFM are located; Δu pccd_GFL / GFM , Δupccq_GFL / GFM The disturbances on the d-axis and q-axis of the access point voltage of the distribution-type grid-following wind turbine / grid-forming wind turbine in the dq rotating coordinate system ω n represents the output frequency reference value; Δi pccd_GFL / GFM and Δi pccq_GFL / GFM represent the d-axis and q-axis components of the access point current of the grid-following wind turbine / grid-forming wind turbine in the dq rotating coordinate system; Z GFL / GFM represents the single-machine port frequency-domain impedance of the grid-following wind turbine / grid-forming wind turbine, and R GFL / GFM is the resistance component of the single-machine port impedance of the grid-following wind turbine / grid-forming wind turbine, and X GFL / GFM is the reactance component of the single-machine port impedance of the grid-following wind turbine / grid-forming wind turbine.

[0048] Preferably, in S4, the method for judging the overall interference stability of the wind farm according to the positive and negative resistance, capacitance and inductance characteristics in the sub-synchronous frequency band includes: when the resistance R 总 is negative when the reactance X 总 of the overall grid-connected port impedance of the wind farm passes through the zero point, and the overall grid-connected port impedance of the wind farm exhibits capacitive and negative resistance characteristics, then the overall interference of the wind farm is unstable and there is a sub-synchronous oscillation phenomenon; and when the reactance X 总 of the overall grid-connected port impedance of the wind farm passes through the zero point and the resistance R 总 is positive, and the overall grid-connected port impedance of the wind farm exhibits inductive and positive resistance characteristics, then the overall interference of the wind farm is stable.

[0049] Advantages of the present invention: The present invention provides an optimized ratio method for grid-forming and grid-following wind turbines in a wind farm station, mainly aiming at the small-signal stability problem in the sub-synchronous frequency band of a multi-machine grid-connected system of grid-forming / grid-following wind turbines under a weak grid. By respectively establishing the state-space models of grid-forming and grid-following wind turbines, and then using the idea of frequency-domain impedance to transform them into port impedance models respectively, the multi-machine system can be simply and efficiently transformed into an impedance network for analysis, which is convenient for calculating the port impedance characteristics of the overall wind farm under different capacity ratios, and then using the principle of the second-order RLC oscillation circuit to analyze the stability of the system in the sub-synchronous frequency band under different ratios. It provides a theoretical basis and guiding direction for the reasonable planning of the capacity configuration of grid-forming / grid-following wind turbines in the wind farm station, can adapt to various wind power grid-connected scenarios, and helps to improve the small-signal stability when the wind farm station is grid-connected. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 is a schematic flow chart of the optimized ratio method for grid-forming and grid-following wind turbines in the wind farm station of the present invention.

[0051] Figure 2 is a control block diagram of the grid-following wind turbine of the present invention;

[0052] Figure 3 This is the control block diagram of the network-forming wind turbine generator set of the present invention;

[0053] Figure 4 This is the frequency-domain impedance waveform diagram of the single-machine port of the network-following wind turbine generator set calculated by the present invention, where (a) is the resistance diagram and (b) is the reactance diagram;

[0054] Figure 5 This is the frequency-domain impedance waveform diagram of the single-machine port of the network-forming wind turbine generator set calculated by the present invention, where (a) is the resistance diagram and (b) is the reactance diagram;

[0055] Figure 6 This is the circuit diagram of the multi-machine grid connection of the wind turbine generator set adopted by the present invention. Among them, (a) is the circuit diagram of the multi-machine parallel network, and (b) is the equivalent diagram of the multi-machine parallel impedance;

[0056] Figure 7 This is the overall grid connection port impedance diagram after using the optimization ratio method of the network-forming and network-following wind turbine generator sets in the wind farm of the present invention;

[0057] Figure 8 This is the system grid connection current and its Fourier analysis waveform after using the optimization ratio method of the network-forming and network-following wind turbine generator sets in the wind farm of the present invention. Among them, (a) is the grid connection three-phase current waveform after using the ratio method, and (b) is the Fourier analysis result of the grid connection three-phase current after using the ratio method. Detailed implementation manners

[0058] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts belong to the scope of protection of the present invention.

[0059] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0060] Next, the present invention will be further described in conjunction with the accompanying drawings and specific embodiments, but it is not a limitation of the present invention.

[0061] The optimization ratio method of the network-forming and network-following wind turbine generator sets in the wind farm of this embodiment specifically includes the following steps:

[0062] Step 1: Respectively establish the single-machine state space model of the network-forming wind turbine generator set, the single-machine state space model of the network-following wind turbine generator set, and the state space model of the power grid line;

[0063] First, it should be noted that the grid-connected type of the new energy wind turbine adopts a control strategy combining a phase-locked loop with a double closed-loop of voltage and current, and its control block diagram is as shown in Figure 2 shown. The grid-forming type of the wind turbine adopts a virtual synchronous control strategy, which consists of an outer loop of active and reactive power and a double closed-loop of voltage and current, and its control block diagram is as shown in Figure 3 shown; in actual operation, since the interconnected lines and grid lines are mainly inductive, and under the weak grid conditions that this invention focuses on, the grid lines are also mainly inductive, the grid lines and connection impedance are modeled with resistive-inductive as the representative.

[0064] The single-machine state space models of the grid-forming type wind turbine, the grid-connected type wind turbine, and the grid line state space model are established respectively under the above control strategies:

[0065] According to the grid-connected vector control strategy, the phase-locked loop control equation with q-axis voltage orientation can be obtained:

[0066]

[0067] where, ΔA 1_GFL represents the state variable obtained after the q-axis component of the grid-connected wind turbine access point voltage passes through the phase-locked loop integration link, K pll_P and K pll_I represent the proportional and integral coefficients in the PI link of the phase-locked loop, Δθ GFL is the disturbance amount of the control system angle output by the grid-connected wind turbine phase-locked loop, Δu pccq_GFL represents the q-axis disturbance amount of the grid-connected wind turbine access point voltage in the dq rotating coordinate system, and U pccd_GFL represents the steady-state value of the d-axis component of the grid-connected wind turbine access point voltage in the dq rotating coordinate system.

[0068] In the grid-connected control, the DC voltage loop controls the DC bus voltage, and combines with the current loop to form the grid-connected inner loop control strategy. The DC voltage loop control equation of the grid-connected wind turbine is:

[0069]

[0070] The current loop control equation of the grid-connected wind turbine is:

[0071]

[0072] where, ΔA 2_GFL represents the state variable obtained after the grid-connected wind turbine DC bus voltage passes through the DC voltage loop integration link, K Udc_P and K Udc_I represent the proportional and integral coefficients in the PI link of the DC voltage loop, and Δu dc_GFLis the disturbance amount of the DC bus voltage of the grid-following wind turbine, ΔA 3_GFL and ΔA 4_GFL respectively represent the state variables obtained by integrating the dq-axis components of the filter inductor current of the grid-following wind turbine through the current loop. K I_PGFL and K I_IGFL represent the proportional and integral coefficients of the PI link of the current loop. Δi ld_GFL and Δi lq_GFL respectively represent the dq-axis disturbance amounts of the filter current of the grid-following wind turbine, and Δi lqref_GFL represents the disturbance amount of the given value of the q-axis component of the filter current.

[0073] The grid-side output of the wind turbine is connected to the line or the common connection point through an LC filter. Combining the above control equations, the single-machine state space model of the grid-following wind turbine can be derived, including the state space models of the phase-locked loop, DC voltage loop, current loop, delay link, LC filter link, etc. The expression is as follows:

[0074] Δx GFL = A GFL Δx GFL + B GFL Δu GFL

[0075] Δy GFL = C GFL Δx GFL

[0076] In the formula, Δx GFM is the state variable of the single machine of the grid-forming wind turbine, Δu GFM is the input variable of the single machine of the grid-forming wind turbine, Δy GFM is the output variable of the single machine of the grid-forming wind turbine, and A GFM , B GFM , C GFM are the state matrix, input matrix, and output matrix of the single-machine state space model of the grid-forming wind turbine respectively; the state variables in each state space model are determined according to the corresponding control equations;

[0077] Similarly, according to the typical grid-forming control strategy, the state space models satisfied by the power outer loop and the voltage-current double closed loop in the system under virtual synchronous control can be derived. The specific expressions are as follows.

[0078] The power outer loop control includes an active power loop and a reactive power loop:

[0079]

[0080] In the formula, Δω GFM and Δθ GFMThe system frequency disturbance quantity and angle disturbance quantity actively constructed for the active power loop of the network-forming wind turbine, ΔP GFM 、ΔQ GFM are the active and reactive power outputs of the network-forming unit. J, K, Dp, and Dq are the virtual inertia, reactive inertia coefficient, droop coefficients of the active power loop and reactive power loop of the network-forming unit respectively. Δu pccd_GFM 、Δu pccq_GFM represent the dq-axis disturbance quantities of the voltage at the connection point of the network-forming wind turbine in the dq rotating coordinate system. U pccd_GFM 、U pccq_GFM represent the steady-state values of the dq axes of the voltage at the connection point of the network-forming wind turbine. ΔE GFM represents the amplitude of the reference voltage output by the network-forming unit. U GFM represents the voltage reference value output by the network-forming unit.

[0081] The voltage-current double closed-loop control equation of the network-forming wind turbine:

[0082]

[0083] In the formula, ΔB 1_GFM 、ΔB 2_GFM are the state variables corresponding to the integral link of the voltage loop of the network-forming wind turbine. ΔB 3_GFM 、ΔB 4_GFM are the state variables corresponding to the integral link of the current loop of the network-forming wind turbine. K V_PGFM and K V_IGFM represent the proportional and integral coefficients of the PI link of the voltage loop.

[0084] After organizing the above equations, the expression of the single-machine state space model of the network-forming wind turbine can be obtained as:

[0085] Δx GFM =A GFM Δx GFM +B GFM Δu GFM

[0086] Δy GFM =C GFM Δx GFM

[0087] In the formula, Δx GFM is the state variable of the single machine of the network-forming wind turbine, Δu GFM is the input variable of the single machine of the network-forming wind turbine, Δy GFM is the output variable of the single machine of the network-forming wind turbine, A GFM 、B GFM 、C GFM are the state matrix, input matrix, and output matrix of the single-machine state space model of the network-forming wind turbine respectively.

[0088] In addition, the power grid line satisfies the following state space equation:

[0089]

[0090] where Δi gd and Δi gq are the disturbance quantities of the dq-axis components of the power grid line current, and Δu pccd and Δu pccq are the disturbance quantities of the dq-axis components of the power grid voltage. L grid and R grid are the equivalent inductance and equivalent resistance of the power grid line respectively.

[0091] Thus, the state space models of the grid-forming and grid-following wind turbine units, the single unit, and the power grid line are obtained.

[0092] Step 2: According to the established single unit state space model and power grid line state space model, obtain the single unit port frequency domain impedance in the grid-forming and grid-following wind turbine units;

[0093] Based on the established models, the relationship between the state space model of the grid-forming / grid-following wind turbine unit and its port frequency domain impedance is deduced, the equivalent port impedance of the grid-following and grid-forming wind turbine units is obtained, and its impedance characteristics in the sub-synchronous frequency band are clarified, providing a theoretical basis for the subsequent analysis of the second-order RLC oscillation circuit.

[0094] The state space method can accurately and detailedly analyze the oscillation modes and damping ratios of a single wind turbine unit. Extended to the multi-unit dimension, the state space models of each unit can be transformed into port impedances using the idea of frequency domain impedance. The main steps are as follows:

[0095] Inject a voltage disturbance with an amplitude of ξ and a frequency of ω r at the connection point of the grid-following / grid-forming wind turbine unit, and then measure the current response of the unit at the same frequency to calculate the unit frequency domain impedance at this frequency. The voltage disturbance expression is as follows:

[0096]

[0097] Transform the above formula into the dq coordinate system and perform differential processing simultaneously to obtain the voltage differential equation of the voltage at the connection point:

[0098]

[0099] Combine the above formula with the single unit state space models of the grid-following and grid-forming wind turbine units and the power grid line state space model to obtain a new state space model for calculating the unit port frequency domain impedance as follows:

[0100]

[0101] In the formula, B 1_GFL / GFM and B 2_GFL / GFM are the first column and the second column of matrix B GFL / GFM respectively.

[0102] After arranging the above formula, it is simplified as follows:

[0103]

[0104] Then, from the structure of , it can be known that this matrix has eigenvalues ±j(ω n -ω r ), which corresponds to the oscillation mode of the unit when the voltage disturbance frequency is ω r . Therefore, its eigenvector γ s and are introduced, and then the equivalent port impedance of the access point of the wind turbine generator set is obtained as:

[0105]

[0106] In the formula, γ upccd_GFL / GFM , γ upccq_GFL / GFM , γ ipccd_GFL / GFM , γ ipccq_GFL / GFM represent the components of the eigenvector γ s in the rows where Δu pccd_GFL / GFM , Δu pccq_GFL / GFM , Δi pccd_GFL / GFM , Δi pccq_GFL / GFM are located; Z GFL / GFM represents the single-machine port frequency-domain impedance of the grid-connected wind turbine generator set / grid-forming wind turbine generator set, R GFL / GFM is the resistance component of the single-machine port impedance of the grid-connected wind turbine generator set / grid-forming wind turbine generator set, and X GFL / GFM is the reactance component of the single-machine port impedance of the grid-connected wind turbine generator set / grid-forming wind turbine generator set.

[0107] Through the above steps, the equivalent port impedance of the grid-connected / grid-forming wind turbine generator set can be obtained. Through the numerical results of R GFL / GFM and X GFL / GFM , the positive and negative resistance, capacitive and inductive reactance characteristics of the unit in each frequency band, especially in the sub-synchronous frequency band, can be judged. As Figure 4 shows the theoretical analysis diagram of the port impedance of the grid-connected wind turbine generator set as an example, Figure 5 is the theoretical analysis diagram of the port impedance of the grid-forming wind turbine generator set as an example. Based on this, the second-order system analysis after the unit is connected to the power grid line can be carried out.

[0108] Step 3: According to the obtained single-machine port frequency-domain impedance, black-box each unit, and calculate the equivalent impedance of the overall grid-connected port of the wind farm in combination with the connection modes between wind turbine generator sets and between wind turbine generator sets and the power grid line:

[0109] Using the equivalent port impedance of each grid-connected / network-forming wind turbine unit calculated in Step 2, each unit is black-boxed and connected in series or parallel to the line in the form of impedance. The line impedance of each unit connected to the main line is also considered to be resistive-inductive. From the above analysis, it can be seen that the grid-connected unit has negative resistance and capacitive port characteristics in the sub-synchronous frequency band, while the network-forming unit has positive resistance and inductive characteristics. Therefore, a certain proportion of network-forming units needs to be configured at the substation. The multi-machine parallel network circuit diagram and its impedance network diagram are as Figure 6 shown. Finally, the equivalent impedance Z of the overall grid-connected port of the wind turbine units can be calculated through the impedance network diagram composed of each unit and the line 总 = R 总 + jX 总 .

[0110] Step 4: Obtain the positive and negative resistance, capacitance and inductance characteristics in the sub-synchronous frequency band according to the equivalent impedance of the overall grid-connected port of the wind farm. Judge the overall interference stability of the wind farm according to the positive and negative resistance, capacitance and inductance characteristics in the sub-synchronous frequency band. If it is unstable, increase the proportion of network-forming wind turbine units and transfer to Step 3. If it is stable, output the proportion of network-forming and grid-connected wind turbine units;

[0111] In Step 4, according to the above calculation steps, the equivalent port impedance of the multi-machine parallel system of grid-connected / network-forming wind turbine units with different proportions, different control structures and different connection methods in the wind farm can be obtained. The overall small-signal stability of the system can be judged according to the second-order system composed of the overall impedance of the wind farm and the grid line impedance. The stability here mainly focuses on the stability in the sub-synchronous frequency band under a weak grid. Here, it is considered that the grid impedance X grid is inductive, and there is a potential second-order RLC oscillation risk between the impedance of the wind farm and the grid impedance. When the reactance X 总 in the system port impedance passes through zero, the resistance R 总 in the port impedance is negative, and the port impedance exhibits capacitive and negative resistance characteristics, then the system is unstable at this proportion. The port impedance Z 总 = R 总 + jX 总 and X grid constitute a second-order RLC oscillation circuit, and there is a sub-synchronous oscillation phenomenon; when the reactance X 总 in the system port impedance passes through zero, the resistance R 总 in the port impedance is positive, and the port impedance exhibits inductive and positive resistance characteristics, then the system is stable at this proportion and there is no sub-synchronous oscillation risk.

[0112] Furthermore, when the system is unstable in the sub-synchronous frequency band, the capacity ratio of grid-following / grid-forming wind turbines in the wind farm can be changed, and the ratio of grid-forming units can be appropriately increased. Then, the port impedance of the overall grid connection point of the wind farm is calculated again using the above calculation steps, and stability analysis is carried out according to the method described in step 4 to obtain a method for the capacity ratio of the multi-machine grid-connected system of wind turbines that meets system stability. Finally, the above analysis steps are also verified through simulation and a semi-physical platform to check the system stability before and after adjusting the system ratio using this method.

[0113] As Figure 7 shown, it is the overall grid connection port impedance characteristic diagram after adjusting the capacity ratio of wind turbines using the method of the present invention. It can be seen that there are no negative resistance and capacitive impedance regions in the sub-synchronous frequency band of the system port impedance under this capacity ratio, and the system stability is good. Furthermore, in the simulation, the multi-machine grid-connected system of wind turbines under this ratio in the embodiment is simulated. As Figure 8 shown, it is the grid connection current and Fourier analysis waveform diagram of the system after using the optimized ratio method. It can be seen that the three-phase grid connection current of the system is stable and there is no oscillation phenomenon. The current harmonic distortion rate in the Fourier analysis is 1.88%, which meets the grid connection requirements, achieving the optimization effect of improving the system stability by optimizing the capacity ratio of the wind farm station.

[0114] Although the present invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed, as long as they do not depart from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the different dependent claims and the features described herein can be combined in a manner different from that described in the original claims. It should also be understood that the features described in connection with a single embodiment can be used in other described embodiments.

Claims

1. Optimization ratio method for grid-forming and grid-following wind turbines in a wind farm, characterized in that, The method includes: S1. Respectively establish the single - unit state - space model of the grid - forming wind turbine, the single - unit state - space model of the grid - following wind turbine, and the state - space model of the power grid line; S2. According to the established single - unit state - space model and the state - space model of the power grid line, obtain the single - unit port frequency - domain impedance of the grid - forming wind turbine and the grid - following wind turbine; S3. According to the obtained single - unit port frequency - domain impedance, black - box each unit, and calculate the equivalent impedance of the overall grid - connection port of the wind farm in combination with the connection modes between wind turbines and between wind turbines and the power grid line; S4. Obtain the positive and negative resistance, capacitance - inductance reactance characteristics in the sub - synchronous frequency band according to the equivalent impedance of the overall grid - connection port of the wind farm, and judge the overall interference stability of the wind farm according to the positive and negative resistance, capacitance - inductance reactance characteristics in the sub - synchronous frequency band. If it is unstable, increase the ratio of the grid - forming wind turbines, and transfer to S3. If it is stable, output the ratio of the grid - forming and grid - following wind turbines.

2. The optimization ratio method of grid-forming and grid-following wind turbines in a wind farm according to claim 1, wherein The expression of the single - unit state - space model of the grid - following wind turbine is: Δx GFL = A GFL Δx GFL + B GFL Δu GFL Δy GFL = C GFL Δx GFL where, Δx GFL is the state variable of a single unit of the network-connected wind turbine, Δu GFL is the input variable of a single unit of the network-connected wind turbine, Δy GFL is the output variable of a single unit of the network-connected wind turbine, A GFL , B GFL , C GFL are respectively the state matrix, input matrix, and output matrix of the state space model of a single unit of the network-connected wind turbine; The expression of the single - unit state - space model of the grid - forming wind turbine is: Δx GFM = A GFM Δx GFM + B GFM Δu GFM Δy GFM = C GFM Δx GFM where, Δx GFM is the state variable of a single unit of a grid-forming wind turbine, Δu GFM is the input variable of a single unit of a grid-forming wind turbine, Δy GFM is the output variable of a single unit of a grid-forming wind turbine, A GFM , B GFM , C GFM are respectively the state matrix, input matrix, and output matrix of the state space model of a single unit of a grid-forming wind turbine; The single - unit state - space model of the grid - following wind turbine includes the state - space models of the phase - locked loop, DC voltage loop, current loop, delay link, and LC filter link of the grid - following wind turbine; The single - unit state - space model of the grid - forming wind turbine includes the state - space models of the power outer loop and the double - closed loop of voltage and current of the grid - forming wind turbine; The state variables in each state - space model are determined according to the corresponding control equations.

3. The optimization ratio method of grid-forming and grid-following wind turbines in a wind farm according to claim 2, characterized in that The control equation of the phase - locked loop of the grid - following wind turbine is: Among them, ΔA 1_GFL represents the state variable obtained after the q-axis component of the grid-connected wind turbine access point voltage in the dq rotating coordinate system passes through the phase-locked loop integration link, K pll_P and K pll_I respectively represent the proportional and integral coefficients in the PI link of the phase-locked loop, Δθ GFL is the disturbance amount of the control system angle output by the phase-locked loop of the grid-connected wind turbine, Δu pccq_GFL represents the q-axis disturbance amount of the grid-connected wind turbine access point voltage in the dq rotating coordinate system, U pccd_GFL represents the steady-state value of the d-axis component of the grid-connected wind turbine access point voltage in the dq rotating coordinate system; The control equation of the DC voltage loop of the grid - following wind turbine is: Among them, ΔA 2_GFL represents the state variable obtained after the DC bus voltage of the grid-connected wind turbine passes through the integral link of the DC voltage loop. K Udc_P and K Udc_I respectively represent the proportional and integral coefficients in the PI link of the DC voltage loop. Δu dc_GFL is the disturbance amount of the DC bus voltage of the grid-connected wind turbine; The control equation of the current loop of the grid - following wind turbine is: Among them, ΔA 3_GFL and ΔA 4_GFL respectively represent the state variables obtained by integrating the d-axis and q-axis components of the filter inductor current of the grid-connected wind turbine through the current loop. K I_PGFL and K I_IGFL respectively represent the proportional and integral coefficients of the PI link of the current loop. Δi ld_GFL and Δi lq_GFL respectively represent the d-axis and q-axis disturbance quantities of the filter current of the grid-connected wind turbine. Δi lqref_GFL represents the disturbance quantity of the given value of the q-axis component of the filter current.

4. The optimization ratio method of grid-forming and grid-following wind turbines in a wind farm according to claim 2, characterized in that The control equation of the power outer loop of the grid - forming wind turbine: where, Δω GFM , Δθ GFM are respectively the system frequency disturbance amount and the angle disturbance amount actively constructed by the active power loop of the network-forming wind turbine, ΔP GFM , ΔQ GFM are respectively the active power and reactive power output by the network-forming unit, J, K, D p , D q are respectively the virtual inertia, reactive inertia coefficient, and droop coefficients of the active power loop and reactive power loop of the network-forming unit, Δu pccd_GFM , Δu pccq_GFM respectively represent the d-axis and q-axis disturbance amounts of the voltage at the connection point of the network-forming wind turbine in the dq rotating coordinate system, U pccd_GFM , U pccq_GFM respectively represent the steady-state values of the d-axis and q-axis of the voltage at the connection point of the network-forming wind turbine; ΔE GFM represents the amplitude of the output reference voltage of the network-forming unit, U GFM represents the voltage reference value of the output of the network-forming unit; The control equation of the double - closed loop of voltage and current of the grid - forming wind turbine: where, ΔB 1_GFM and ΔB 2_GFM are the state variables corresponding to the integral link of the voltage loop of the grid-forming wind turbine, and ΔB 3_GFM and ΔB 4_GFM are the state variables corresponding to the integral link of the current loop of the grid-forming wind turbine. K V_PGFM and K V_IGFM represent the proportional and integral coefficients of the PI link of the voltage loop; ω n represents the output frequency reference value.

5. The optimization ratio method of grid-forming and grid-following wind turbines in a wind farm according to claim 1, characterized in that The state - space model of the power grid line is: where, Δi gd , Δi gq are the disturbance quantities of the d-axis and q-axis components of the grid line current, and Δu pccd , Δu pccq are the disturbance quantities of the d-axis and q-axis components of the grid voltage. L grid , R grid are the equivalent inductance and equivalent resistance of the grid line respectively, and ω n represents the output frequency reference value.

6. The optimization ratio method of grid-forming and grid-following wind turbines in a wind farm according to claim 1, wherein The said S2 includes: Inject a voltage disturbance with an amplitude of ξ and a frequency of ω at the access points of both the grid-following wind turbines and the grid-forming wind turbines. r Measure the current responses of the grid-following wind turbines and the grid-forming wind turbines at the same frequency ω. r Calculate the single-machine port frequency-domain impedance of the wind turbines at the frequency ω. r : where γ upccd_GFL / GFM , γ upccq_GFL / GFM , γ ipccd_GFL / GFM , γ ipccq_GFL / GFM respectively represent the components of the eigenvector in the rows where Δu pccd_GFL / GFM , Δu pccq_GFL / GFM , Δi pccd_GFL / GFM , Δi pccq_GFL / GFM are located; Δu pccd_GFL / GFM , Δu pccq_GFL / GFM are the d-axis and q-axis disturbance quantities of the access point voltage of the grid-following wind turbine / grid-forming wind turbine in the dq rotating coordinate system, ω n represents the output frequency reference value; Δi pccd_GFL / GFM and Δi pccq_GFL / GFM represent the d- and q-axis components of the current at the connection point of the grid-connected wind turbine / grid-forming wind turbine in the dq rotating coordinate system; Z GFL / GFM represents the single-machine port frequency-domain impedance of the grid-connected wind turbine / grid-forming wind turbine, and R GFL / GFM is the resistance component of the single-machine port impedance of the grid-connected wind turbine / grid-forming wind turbine, and X GFL / GFM is the reactance component of the single-machine port of the grid-connected wind turbine / grid-forming wind turbine.

7. The optimization ratio method of grid-forming and grid-following wind turbines in a wind farm according to claim 1, wherein In S4, the method for judging the overall interference stability of the wind farm according to the positive and negative resistance and capacitive-inductive reactance characteristics in the sub-synchronous frequency band includes: when the resistance R 总 at the zero crossing of the reactance X 总 in the overall grid-connected port impedance of the wind farm is negative, and the overall grid-connected port impedance of the wind farm exhibits capacitive and negative resistance characteristics, then the overall interference of the wind farm is unstable and there is a sub-synchronous oscillation phenomenon; while when the resistance R 总 at the zero crossing of the reactance X 总 in the overall grid-connected port impedance of the wind farm is positive, and the overall grid-connected port impedance of the wind farm exhibits inductive and positive resistance characteristics, then the overall interference of the wind farm is stable.

8. A computer-readable storage device storing a computer program, characterized in that, When the computer program is executed by the processor, it realizes the steps of the method for optimizing the ratio of grid - forming and grid - following wind turbines in the wind farm as described in any one of claims 1 to 7.

9. An optimization ratio device for grid-forming and grid-following wind turbines in a wind farm, comprising a storage device, a processor, and a computer program stored in the storage device and executable on the processor, characterized in that, The processor executes the computer program to realize the steps of the method for optimizing the ratio of grid - forming and grid - following wind turbines in the wind farm as described in any one of claims 1 to 7.

10. A computer program product, comprising a computer program, characterized in that, When the computer program is executed by the processor, it realizes the steps of the method for optimizing the ratio of grid - forming and grid - following wind turbines in the wind farm as described in any one of claims 1 to 7.

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