A grid-connected method for high-frequency collection and low-frequency transmission of a direct-drive wind farm

Abstract

The application discloses a kind of direct-drive wind farm high-frequency collection low-frequency sending grid-connected methods, comprising: 1, the control structure of wind turbine machine side converter, grid side converter in direct-drive wind farm high-frequency collection system is established;2, the control structure of M3C converter of low-frequency sending system sending end, receiving end is established;3, the optimal frequency of high-frequency collection system and low-frequency sending system is obtained considering resource utilization and stability.The application can establish a new direct-drive wind farm grid-connected mode, wherein high-frequency collection system reduces the size of alternating current transformer, and low-frequency sending system can improve transmission power and reduce voltage loss, thereby improving the resource utilization of direct-drive wind farm grid-connected.

Description

Technical Field

[0001] This invention belongs to the field of wind power grid connection operation and control, specifically relating to a grid connection method for direct-drive wind farms that collects high-frequency data and transmits low-frequency data. Background Technology

[0002] Direct-drive wind turbines, lacking gearboxes and excitation control systems, offer high reliability and efficiency, gradually becoming one of the mainstream models in wind power systems. Currently, grid connection for direct-drive wind farms primarily utilizes power frequency AC and DC transmission methods. However, power frequency AC transmission has a limited power transmission limit and significant line voltage losses over long distances. While DC transmission can effectively increase the power transmission limit, it suffers from issues such as fault current interruption and difficulties in DC transformation. On the other hand, increasing the transmission voltage level can improve the line's power transmission limit, but it places higher demands on equipment manufacturing processes and overvoltage / overcurrent protection. Therefore, proposing a direct-drive wind farm grid connection method with high resource utilization and low technical difficulty is of great significance for the economical and reliable operation of wind power systems. Summary of the Invention

[0003] The present invention addresses the shortcomings of the existing technology by proposing a method for high-frequency collection and low-frequency transmission and grid connection of direct-drive wind farms. This method aims to reduce the size and weight of equipment in the high-frequency collection system and increase the power transmission limit of the low-frequency transmission system, thereby enabling the direct-drive wind farm grid connection system to simultaneously achieve high resource utilization and power transmission capacity.

[0004] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0005] This invention discloses a grid connection method for high-frequency collection and low-frequency transmission in a direct-drive wind farm. It is applied to a grid connection system consisting of a direct-drive wind farm, a high-frequency collection system, a low-frequency transmission system, and an AC power grid. The number of any direct-drive wind turbine in the wind farm is denoted as i, i = 1, ..., n. The high-frequency collection system comprises n high-frequency collection lines. The method is characterized by the following steps:

[0006] Step S1: Establish the control structure for the wind turbine-side converter and grid-side converter in a direct-drive wind farm;

[0007] Step S1.1: Establish the control structure of the wind turbine generator-side converter, including: the constant reactive power control outer loop structure of the wind turbine generator-side converter, the constant reactive power control inner loop structure of the wind turbine generator-side converter, the constant DC voltage control outer loop structure of the wind turbine generator-side converter, and the constant DC voltage control inner loop structure of the wind turbine generator-side converter.

[0008] Step S1.2: Establish the control structure of the grid-side converter of the wind turbine, including: the outer loop structure for the constant AC voltage and frequency control of the grid-side converter of the wind turbine, and the inner loop structure for the constant AC voltage and frequency control of the grid-side converter of the wind turbine.

[0009] Step S2: Establish the control structure for the sending-end M3C converter and the receiving-end M3C converter of the low-frequency transmission system;

[0010] Step S2.1: Establish the control structure of the sending-end M3C converter of the low-frequency transmission system, including: the outer loop structure of the high-frequency side constant DC voltage control and constant reactive power control of the sending-end M3C converter, the inner loop structure of the high-frequency side constant DC voltage control and constant reactive power control of the sending-end M3C converter, the outer loop structure of the low-frequency side constant AC voltage and frequency control of the sending-end M3C converter, the inner loop structure of the low-frequency side constant AC voltage and frequency control of the sending-end M3C converter, and the bridge arm control structure of the sending-end M3C converter;

[0011] Step S2.2: Establish the control structure of the receiving-end M3C converter of the low-frequency transmission system, including: the outer loop structure of the power frequency side constant DC voltage control of the receiving-end M3C converter, the inner loop structure of the power frequency side constant DC voltage control of the receiving-end M3C converter, the outer loop structure of the power frequency side constant AC voltage control of the receiving-end M3C converter, the inner loop structure of the power frequency side constant AC voltage control of the receiving-end M3C converter, the outer loop structure of the low-frequency side constant active power control and constant reactive power control of the receiving-end M3C converter, the inner loop structure of the low-frequency side constant active power control and reactive power control of the receiving-end M3C converter, and the bridge arm control structure of the receiving-end M3C converter;

[0012] Step S3: Obtain the optimal frequency for the high-frequency collection system and the low-frequency transmission system, taking into account resource utilization and stability;

[0013] Step S3.1: Obtain the transmission power limit P of the high-frequency collection system. fi_lmax and line voltage loss ΔU fi %, the transmission power limit P of the low-frequency transmission system low_max and line voltage loss ΔU low %:

[0014] Step S3.2: Obtain the overall loss of the high-frequency collection system, A high The combined loss A of the low-frequency transmission system low for:

[0015] Step S3.3: Obtain the minimum damping ratio ξ of the grid-connected system characteristic values. e :

[0016] Step S3.4: Obtain the optimal frequency of the high-frequency collection system and the optimal frequency of the low-frequency transmission system:

[0017] Step S3.4.1: Using the operating frequency f of the high-frequency collection system high and the operating frequency f of the low-frequency transmission system low To optimize the variables, a frequency optimization model F for the grid-connected system is established using equation (30):

[0018]

[0019] In equation (30), η1 is the minimum damping ratio ξ min The weighting coefficients are: η2 is the weighting coefficient for the transmission power limit of the high-frequency aggregation system and the low-frequency transmission system; η3 is the weighting coefficient for the line voltage loss of the high-frequency aggregation system and the low-frequency transmission system; and η4 is the weighting coefficient for the comprehensive loss of the high-frequency aggregation system and the comprehensive loss of the low-frequency transmission system.

[0020] Step S3.4.2: Use a multi-objective optimization algorithm to solve the frequency optimization model F shown in equation (30), thereby obtaining the optimal frequency of the high-frequency collection system and the optimal frequency of the low-frequency transmission system.

[0021] The grid connection method for high-frequency collection and low-frequency transmission of a direct-drive wind farm, as described in this invention, is also characterized in that step S3.1 includes:

[0022] Step S3.1.1: Calculate the frequency f of the high-frequency collection system at frequencies greater than 50Hz using equation (23). high The following are the transmission power limits and line voltage losses:

[0023]

[0024] In equation (23), P fi_lmax Let u be the transmission power limit of the i-th direct-drive wind turbine in its high-frequency collection line. wi Let ΔU be the rated voltage of the i-th direct-drive wind turbine on its high-frequency collection line. fi % represents the voltage loss of the i-th direct-drive wind turbine in its high-frequency collection line, Q gi Let be the reactive power of the i-th direct-drive wind turbine in its high-frequency collection line;

[0025] Step S3.1.2: Calculate the frequency f of the low-frequency transmission system at frequencies less than 50Hz using equation (24). low The following are the transmission power limits and line voltage losses:

[0026]

[0027] In equation (24), P low_max For the transmission power limit of the low-frequency transmission system, u low L is the rated voltage of the low-frequency transmission system. lowFor the line inductance of the low-frequency transmission system, ΔU low % represents the voltage loss of the low-frequency transmission system, Q low This refers to the reactive power transmitted by the low-frequency transmission system.

[0028] Step S3.2 includes:

[0029] Step S3.2.1: Calculate the overall loss of the high-frequency collection system using equation (25):

[0030]

[0031] In equation (25), A high For the overall loss of the high-frequency collection system, A wi For the loss of the i-th direct-drive wind turbine, A MSCi For the losses of the i-th direct-drive wind turbine generator side converter, A GSCi Let A be the investment cost of the grid-side converter of the i-th direct-drive wind turbine. gi Let be the loss of the i-th high-frequency collection line;

[0032] Step S3.2.2: Calculate the overall loss of the low-frequency transmission system using equation (26):

[0033] A low =A M3C_s +A M3C_r +A line (26)

[0034] In equation (26), A low For the overall loss of the low-frequency transmission system, A M3C_s For the input loss of the M3C at the transmitting end of the low-frequency transmission system, A M3C_r For the loss of the M3C at the receiving end of the low-frequency transmission system, A line This refers to the losses in low-frequency transmission lines.

[0035] Step S3.3 includes:

[0036] Step S3.3.1: Obtain the initial operating point x0 of the grid-connected system through power flow calculation, and use equation (27) to construct a linearized state-space model of the grid-connected system at the initial operating point x0;

[0037]

[0038] In equation (27), d represents the differential, t is time, and x w_hl A is the state variable of the grid-connected system. w_hl For the state matrix of the grid-connected system, B w_hl For the input matrix of the grid-connected system, u w_hl For the input variables of the grid-connected system; Δxw_hl x represents w_hl The increment, Δu w_hl Indicate u w_hl The increment;

[0039] Step S3.3.2: Solve equation (28) to obtain the state matrix A. w_hl eigenvalues ​​{λ e |e=1,2,…,m};where m is A s The order of λ e A represents w_hl The e-th eigenvalue, and λ e =σ e +jω e ;σ ei For λ e The real part, ω e For λ e The imaginary part, j, denotes a complex number;

[0040] |λ e E w_hl -A w_hl |=0 (28)

[0041] In equation (28), E w_hl To be related to the state matrix A w_hl Identity matrices of the same order;

[0042] Step S3.3.3: Calculate the e-th eigenvalue λ using equation (29) e Damping ratio ξ e :

[0043]

[0044] In equation (29), for all eigenvalues ​​{λ} e The damping ratio {ξ} of |e=1,2,…,m} e After sorting |e=1,2,…,n} in descending order, the minimum value is selected as the minimum damping ratio ξ of the grid-connected system characteristic value. min .

[0045] The present invention provides an electronic device, comprising a memory and a processor, wherein the memory is used to store a program supporting the processor in executing any of the grid-connection methods, and the processor is configured to execute the program stored in the memory.

[0046] The present invention discloses a computer-readable storage medium on which a computer program is stored, wherein the computer program is executed by a processor to perform any of the steps of the grid connection method.

[0047] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0048] 1. By establishing a control structure for the generator-side converter and grid-side converter of the direct-drive wind turbine, this invention enables the direct-drive wind farm collection system to operate at high frequencies, reduces the size and weight of equipment such as transformers in the collection system, and reduces losses during construction and operation, thereby realizing the construction of a green power grid system;

[0049] 2. This invention improves the transmission power limit of the direct-drive wind farm power transmission system by designing the control structure of the modular multilevel matrix converter (M3C) at the sending and receiving ends of the system, which converts the high-frequency electrical energy of the collection system into the low-frequency electrical energy of the transmission system.

[0050] 3. This invention simultaneously considers the transmission power limit, line voltage loss, comprehensive loss, and minimum damping ratio of the high-frequency collection system and the low-frequency transmission system of the direct-drive wind farm, and optimizes the frequency of the high-frequency collection system and the low-frequency transmission system, thereby improving the stability and resource utilization of the high-frequency collection and low-frequency transmission grid-connected system of the direct-drive wind farm. Attached Figure Description

[0051] Figure 1 This is a structural diagram of the direct-drive wind farm high-frequency collection and low-frequency transmission grid-connected system of the present invention;

[0052] Figure 2 This is a flowchart of the present invention;

[0053] Figure 3 This is a control block diagram of the direct-drive wind turbine generator-side converter of the present invention;

[0054] Figure 4 This is a control block diagram of the grid-side converter for a direct-drive wind turbine of the present invention;

[0055] Figure 5 This is a control block diagram of the sending-end modular multilevel matrix converter of the present invention;

[0056] Figure 6 This is a control block diagram of the receiving-end modular multilevel matrix converter of the present invention;

[0057] Figure 7 This is a flowchart illustrating the process of obtaining the optimal frequency for the high-frequency collection system and the low-frequency transmission system according to the present invention. Detailed Implementation

[0058] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0059] In this embodiment, as Figure 1As shown, the grid-connected system of direct-drive wind farm high-frequency collection and low-frequency transmission includes: direct-drive wind farm high-frequency collection system, low-frequency transmission system, and AC grid. The number of any direct-drive wind turbine in the direct-drive wind farm is denoted as i, i = 1, ..., n. The high-frequency collection system contains n high-frequency collection lines.

[0060] like Figure 2 As shown, a method for high-frequency collection and low-frequency transmission for grid connection of a direct-drive wind farm is carried out according to the following steps:

[0061] Step S1: Establish the control structure for the wind turbine-side converter and grid-side converter in the high-frequency collection system of the direct-drive wind farm;

[0062] Step S1.1: As Figure 3 As shown, the control structure of the wind turbine generator-side converter is established:

[0063] Step S1.1.1: In the dq rotating coordinate system, establish the constant reactive power control outer loop structure of the wind turbine generator-side converter using equation (1):

[0064]

[0065] In equation (1), i midref Q is the reference value for the d-axis current on the machine side of the i-th direct-drive wind turbine. miref Q is the reference value for the reactive power output on the machine side of the i-th direct-drive wind turbine. mimea Let k be the measured reactive power output value of the i-th direct-drive wind turbine generator. p1 k is the proportional coefficient for the outer loop control of the constant reactive power of the direct-drive wind turbine generator's generator-side converter. i1 The integral coefficient for the constant reactive power outer loop control of the direct-drive wind turbine generator's generator-side converter is s, where s represents the integral.

[0066] In the dq rotating coordinate system, the constant reactive power control inner loop structure of the wind turbine generator-side converter is established using equation (2):

[0067]

[0068] In equation (2), u mid Let i be the d-axis voltage on the machine side of the i-th direct-drive wind turbine. midmea i miqmea Let k be the measured values ​​of the d-axis and q-axis currents on the side of the i-th direct-drive wind turbine. p2 k is the proportional coefficient for the inner loop control of the constant reactive power of the direct-drive wind turbine generator's converter. i2 ω is the integral coefficient for the constant reactive power inner loop control of the direct-drive wind turbine generator's converter. si Let L be the rotor angular velocity of the i-th direct-drive wind turbine. siLet be the stator inductance of the i-th direct-drive wind turbine generator set; by combining the outer and inner loop control structures of the constant reactive power of the generator-side converter of the direct-drive wind turbine generator set, the complete constant reactive power control structure of the generator-side converter of the direct-drive wind turbine generator set can be obtained.

[0069] Step S1.1.2: In the dq rotating coordinate system, establish the constant DC voltage control outer loop structure of the wind turbine generator's converter using equation (3):

[0070]

[0071] In equation (3), i miqref Let u be the reference value for the q-axis current on the machine side of the i-th direct-drive wind turbine. dciref u is the reference value for the DC voltage of the i-th direct-drive wind turbine. dcimea Let k be the measured DC voltage value of the i-th direct-drive wind turbine. p3 k is the proportional coefficient for the outer ring constant DC voltage control of the direct-drive wind turbine generator's generator-side converter. i3 The integral coefficient for the constant DC voltage control of the outer ring of the converter on the generator side of the direct-drive wind turbine unit;

[0072] In the dq rotating coordinate system, the constant DC voltage control inner loop structure of the wind turbine generator-side converter is established using equation (4):

[0073]

[0074] In equation (4), u miq Let k be the q-axis voltage on the machine side of the i-th direct-drive wind turbine. p4 k is the proportional coefficient for the inner loop control of the DC voltage of the direct-drive wind turbine generator's generator-side converter. i4 ψ is the integral coefficient for the inner loop control of the DC voltage setpoint of the direct-drive wind turbine generator. fi Let be the magnetic flux of the i-th direct-drive wind turbine generator set; by combining the outer and inner loop control structures of the constant DC voltage of the direct-drive wind turbine generator set's machine-side converter, the complete constant DC voltage control structure of the direct-drive wind turbine generator set's machine-side converter can be obtained.

[0075] Step S1.2: As Figure 4 As shown, the control structure of the grid-side converter of the wind turbine is established:

[0076] Step S1.2.1: In the dq rotating coordinate system, establish the outer loop structure for the constant AC voltage and frequency control of the wind turbine grid-side converter using equation (5):

[0077]

[0078] In equation (5), i hidref i hiqref These are the reference values ​​for the d-axis and q-axis currents on the grid side of the i-th direct-drive wind turbine, respectively.gidref u giqref These are the d-axis and q-axis voltage reference values ​​on the capacitor of the high-frequency collection system of the i-th direct-drive wind turbine, respectively. gidmea u giqmea These are the measured d-axis and q-axis voltage values ​​on the high-frequency collector line capacitor of the i-th direct-drive wind turbine, respectively, and k p5 k i5 These are the proportional and integral coefficients for the fixed d-axis AC voltage outer loop control, respectively, k p7 k i7 These are the proportional and integral coefficients for the fixed q-axis AC voltage outer loop control, i tid i tiq These are the d-axis and q-axis currents at the output of the high-frequency collector transformer of the i-th direct-drive wind turbine, respectively, and ω. h For high-frequency angular velocity, C gi For the capacitor on the high-frequency collection line of the i-th direct-drive wind turbine;

[0079] Step S1.2.2: In the dq rotating coordinate system, establish the inner loop structure for the constant AC voltage and frequency control of the wind turbine grid-side converter using equation (6):

[0080]

[0081] In equation (6), u cd u cq These are the d-axis and q-axis components of the bus voltage in the high-frequency collection system of a direct-drive wind farm, respectively. hidmea i hiqmea These are the measured d-axis and q-axis current values ​​on the grid side of the i-th direct-drive wind turbine, respectively, and k p6 k i6 These are the proportional and integral coefficients for the fixed d-axis AC voltage inner loop control, respectively, k p8 k i8 These are the proportional and integral coefficients for the fixed q-axis AC voltage inner loop control, respectively, L gi The inductance is on the high-frequency collection line of the i-th direct-drive wind turbine unit; combining equations (5) and (6), the complete constant AC voltage and frequency control structure of the grid-side converter of the direct-drive wind turbine unit can be obtained.

[0082] Step S2: Establish the control structure of the modular multilevel matrix converter (M3C) at the sending and receiving ends of the low-frequency transmission system of the direct-drive wind farm;

[0083] Step S2.1: As Figure 5 As shown, the control structure of the M3C at the sending end of the low-frequency transmission system is established:

[0084] Step S2.1.1: In the dq rotating coordinate system, establish the outer loop structure of the constant DC voltage control and constant reactive power control on the high-frequency side of the M3C at the sending end using equation (7):

[0085]

[0086] In equation (7), i hdref i hqref These are the reference values ​​for the d-axis and q-axis currents on the high-frequency side of the M3C at the sending end, respectively. dcsref Q is the reference value for the high-frequency side DC voltage of the M3C at the sending end. href u is the reference value for reactive power on the high-frequency side of the M3C at the sending end. dcsm3c Q represents the measured DC voltage value on the high-frequency side of the M3C transmitter. h k represents the reactive power measurement value on the high-frequency side of the M3C transmitter. p9 k is the proportional coefficient for the outer loop control of the DC voltage on the high-frequency side of the M3C transmitter. i9 k is the integral coefficient of the outer loop control of the high-frequency side fixed DC voltage of the M3C at the sending end. p10 k is the proportional coefficient for the outer loop control of the reactive power on the high-frequency side of the M3C transmitter. i10 The integral coefficient for the outer loop control of reactive power on the high-frequency side of the M3C at the sending end;

[0087] Step S2.1.2: In the dq rotating coordinate system, use equation (8) to establish the inner loop structure of constant DC voltage control and constant reactive power control on the high-frequency side of the M3C at the sending end:

[0088]

[0089] In equation (8), u sumhd u sumhq These are the d-axis and q-axis common-mode voltages on the high-frequency side of the M3C at the sending end, respectively. hd u hq These are the measured d-axis and q-axis voltage values ​​on the high-frequency side of the M3C at the sending end, respectively. hdref i hqref These are the reference values ​​for the d-axis and q-axis currents on the high-frequency side of the M3C at the sending end, respectively. hd i hq These are the measured d-axis and q-axis current values ​​on the high-frequency side of the M3C at the sending end, respectively, k p11 k is the proportional coefficient for the inner loop control of the DC voltage on the high-frequency side of the M3C transmitter. i11 k is the integral coefficient of the inner loop control of the DC voltage on the high-frequency side of the M3C at the sending end. p12 k is the proportional coefficient for the inner loop control of the reactive power on the high-frequency side of the M3C at the sending end. i12 ω is the integral coefficient of the inner loop control of the reactive power on the high-frequency side of the M3C at the sending end. h For the high-frequency side angular velocity of the M3C at the sending end, L M3C_sThe equivalent inductance of the bridge arm of the sending end M3C;

[0090] Step S2.1.3: In the dq rotating coordinate system, use equation (9) to establish the outer loop structure for controlling the AC voltage and frequency on the low-frequency side of the M3C at the sending end:

[0091]

[0092] In equation (9), i sldref i slqref For the reference values ​​of the d-axis and q-axis currents on the low-frequency side of the M3C at the sending end, u sldref u slqref For the low-frequency side d-axis and q-axis voltage reference values ​​of the M3C at the sending end, u sld u slq The measured values ​​of the d-axis and q-axis voltages on the low-frequency side of the M3C at the sending end are given by k. p13 k i13 These are the proportional and integral coefficients for the outer loop control of the fixed d-axis AC voltage on the low-frequency side of the M3C at the sending end, respectively, k. p14 k i14 These are the proportional and integral coefficients for the outer loop control of the fixed q-axis AC voltage on the low-frequency side of the M3C at the sending end;

[0093] Step S2.1.4: In the dq rotating coordinate system, use equation (10) to establish the inner loop structure for controlling the AC voltage and frequency on the low-frequency side of the M3C at the sending end:

[0094]

[0095] In equation (10), u sumld u sumlq These are the d-axis and q-axis common-mode voltages on the low-frequency side of the M3C at the sending end, respectively. sld u slq These are the measured d-axis and q-axis voltage values ​​on the low-frequency side of the M3C at the sending end, respectively. sldref i slqref These are the reference values ​​for the d-axis and q-axis currents on the low-frequency side of the M3C at the sending end, respectively. sld i slq These are the measured d-axis and q-axis current values ​​on the low-frequency side of the M3C at the sending end, respectively, k p15 k is the proportional coefficient for the inner loop control of the DC voltage on the low-frequency side of the M3C transmitter. i15 k is the integral coefficient of the inner loop control of the DC voltage on the low-frequency side of the M3C at the sending end. p16 k is the proportional coefficient for the inner loop control of the reactive power on the low-frequency side of the M3C at the sending end. i16 ω is the integral coefficient of the inner loop control of the fixed reactive power on the low-frequency side of the M3C at the sending end. sl For the low-frequency side angular velocity of the M3C at the sending end;

[0096] Step 2.1.5 Establish the bridge arm control structure of the M3C at the sending end of the low-frequency transmission system:

[0097] Using equations (11) and (12), the common-mode voltages of the high-frequency and low-frequency sides of the sending end M3C are transformed from the dq coordinate system to the abc coordinate system;

[0098]

[0099]

[0100] In equation (11), T dq-abc Let u be the coordinate transformation formula for transforming a variable from the dq coordinate system to the abc coordinate system. sumh_a u sumh_b u sumh_c These are the common-mode voltages of phases a, b, and c on the high-frequency side of the M3C transmitter, respectively. suml_a u suml_b u suml_c These are the common-mode voltages of phases a, b, and c on the low-frequency side of the M3C transmitter, respectively.

[0101] The voltage reference values ​​u of the nine bridge arms ha_la, ha_lb, ha_lc, hb_la, hb_lb, hb_lc, hc_la, hc_lb, and hc_lc of the sending end M3C are obtained by equation (13). hala_cref u halb_cref u halc_cref u hbla_cref u hblb_cref u hblc_cref u hcla_cref u hclb_cref u hclc_cref ;

[0102]

[0103] By combining equations (7) and (13), the complete control structure of the M3C at the sending end of the low-frequency transmission system can be obtained.

[0104] Step S2.2: As Figure 6 As shown, the control structure of the receiving end M3C of the low-frequency transmission system is established:

[0105] Step S2.2.1: In the dq rotating coordinate system, establish the constant DC voltage control outer loop structure of the receiving end M3C power frequency side using equation (14):

[0106]

[0107] In equation (14), i rsdref The reference value for the d-axis current on the power frequency side of the receiving end M3C is u. rdcrefThe reference value for the DC voltage of the receiving end M3C is u. rdcm3c The measured DC voltage value of the receiving end M3C is k. p17 k is the proportional coefficient for the outer loop control of the fixed DC voltage on the M3C power frequency side of the receiving end. i17 The integral coefficient of the outer loop control for the fixed DC voltage on the M3C power frequency side of the receiving end;

[0108] In the dq rotating coordinate system, the constant DC voltage control inner loop structure of the receiving end M3C power frequency side is established using equation (15):

[0109]

[0110] In equation (15), u comrhd For the power frequency side d-axis common-mode voltage of the receiving end M3C, u rsd The measured value of the d-axis voltage on the power frequency side of the receiving end M3C is i. rsd The measured value of the d-axis current on the power frequency side of the receiving end M3C is k. p18 k is the proportional coefficient for the inner loop control of the fixed DC voltage on the M3C power frequency side of the receiving end. i18 ω is the integral coefficient of the inner loop control of the fixed DC voltage on the M3C power frequency side of the receiving end. b For the angular velocity of the M3C power frequency side at the receiving end, L r The equivalent inductance of the bridge arm of the receiving end M3C;

[0111] Step S2.2.2: In the dq rotating coordinate system, establish the constant AC voltage control outer loop structure of the receiving end M3C power frequency side using equation (16):

[0112]

[0113] In equation (16), i rsqref The reference value for the q-axis current on the power frequency side of the receiving end M3C is u. rref u is the reference value for the AC voltage on the power frequency side of the receiving end M3C. rsd The measured value of the d-axis voltage on the power frequency side of the M3C receiver is k. p19 k is the proportional coefficient for the outer loop control of the AC voltage on the M3C power frequency side of the receiving end. i19 The integral coefficient of the outer loop control for the AC voltage on the power frequency side of the receiving end M3C;

[0114] In the dq rotating coordinate system, the constant AC voltage control inner loop structure of the receiving end M3C power frequency side is established using equation (17):

[0115]

[0116] In equation (17), u comrhq For the power frequency side q-axis common-mode voltage of the receiving end M3C, u rsq i represents the measured q-axis voltage on the power frequency side of the M3C receiver. rsqThe measured value of the q-axis current on the power frequency side of the M3C receiver is k. p20 k is the proportional coefficient for the inner loop control of the AC voltage on the M3C power frequency side of the receiving end. i20 The integral coefficient of the AC voltage inner loop control on the power frequency side of the receiving end M3C;

[0117] Step S2.2.3: In the dq rotating coordinate system, establish the outer loop structure of constant active power control and constant reactive power control on the low-frequency side of the receiving end M3C using equation (18):

[0118]

[0119] In equation (18), i rldref i rlqref P is the reference value for the d-axis and q-axis currents on the low-frequency side of the receiving end M3C. rlref Q rlref These are the reference values ​​for active and reactive power on the low-frequency side of the receiving end M3C, respectively. rl Q rl These are the measured values ​​of active power and reactive power on the low-frequency side of the receiving end M3C, respectively, k p21 k i21 These are the proportional and integral coefficients for the constant active power outer loop control on the low-frequency side of the M3C receiver, respectively, k p22 k i22 These are the proportional and integral coefficients of the outer loop control of the constant reactive power on the low-frequency side of the M3C receiver;

[0120] In the dq rotating coordinate system, the inner loop structure of constant active power control and reactive power control on the low-frequency side of the receiving end M3C is established using equation (19):

[0121]

[0122] In equation (19), u comrld u comrlq These are the d-axis and q-axis common-mode voltages on the low-frequency side of the receiving end M3C, respectively. rld u rlq These are the measured d-axis and q-axis voltage values ​​on the low-frequency side of the M3C receiver, respectively. rld i rlq These are the measured d-axis and q-axis current values ​​on the low-frequency side of the receiving end M3C, respectively, k p23 k i23 These are the proportional and integral coefficients for the constant active power inner loop control on the low-frequency side of the M3C receiver, respectively, k p24 k i24 These are the proportional and integral coefficients for the low-frequency reactive power inner loop control of the receiving-end M3C, ω. rl The low-frequency side angular velocity of the receiving end M3C;

[0123] Step 2.2.4 Establish the bridge arm control structure of the receiving end M3C of the low-frequency transmission system:

[0124] Using equations (20) and (21), the common-mode voltages of the high-frequency and low-frequency sides of the receiving end M3C are transformed from the dq coordinate system to the abc coordinate system;

[0125]

[0126]

[0127] In equation (20), u comrh_a u comrh_b u comrh_c These are the common-mode voltages of phases a, b, and c on the power frequency side of the receiving end M3C, respectively; in equation (21), u comrl_a u comrl_b u comrl_c These are the common-mode voltages of phases a, b, and c on the low-frequency side of the receiving end M3C, respectively.

[0128] The voltage reference values ​​u of the nine bridge arms sa_la, sa_lb, sa_lc, sb_la, sb_lb, sb_lc, sc_la, sc_lb, and sc_lc of the receiving end M3C are obtained using equation (22). sala_cref u salb_cref u salc_cref u sbla_cref u sblb_cref u sblc_cref u scla_cref u sclb_cref u sclc_cref ;

[0129]

[0130] By combining equations (14) and (22), the complete control structure of the receiving end M3C of the low-frequency transmission system can be obtained;

[0131] Step S3: As Figure 7 As shown, the optimal frequencies for the high-frequency collection system and the low-frequency transmission system, taking into account resource utilization and stability, are obtained.

[0132] Step S3.1: Obtain the transmission power limit and line voltage loss of the high-frequency collection system and the low-frequency transmission system:

[0133] Step S3.1.1: Calculate the frequency f of the high-frequency collection system at frequencies greater than 50Hz using equation (23). high The following are the transmission power limits and line voltage losses:

[0134]

[0135] In equation (23), Pfi_lmax For the i-th direct-drive wind turbine, the maximum power transmission capacity of the collection line is u. wi Let ΔU be the rated voltage on the collection line of the i-th direct-drive wind turbine. fi % represents the voltage loss of the collection line of the i-th direct-drive wind turbine, Q gi P represents the reactive power of the collection line of the i-th direct-drive wind turbine; in equation (23) P fi_max The solution method has certain limitations and is not applicable to high-frequency cable lines whose transmission capacity is limited by charging power.

[0136] Step S3.1.2: Calculate the frequency f of the low-frequency transmission system at frequencies less than 50Hz using equation (24). low The following are the transmission power limits and line voltage losses:

[0137]

[0138] In equation (24), P low_max For the transmission power limit of the low-frequency transmission system, u low L is the rated voltage of the low-frequency transmission system. low For the line inductance of the low-frequency transmission system, ΔU low % represents the voltage loss of the low-frequency transmission system, Q low P represents the reactive power transmitted by the low-frequency transmission system; in equation (24) P low_max The solution method has certain limitations and is not applicable to low-frequency cable lines where the transmission capacity is limited by the charging power.

[0139] Step S3.2: Obtain the combined losses of the high-frequency collection system and the low-frequency transmission system:

[0140] Step S3.2.1: Calculate the overall loss of the high-frequency collection system using equation (25):

[0141]

[0142] In equation (25), A high For the overall loss of the high-frequency collection system, A wi For the loss of the i-th direct-drive wind turbine, A MSCi For the losses of the i-th direct-drive wind turbine generator side converter, A GSCi For the losses of the grid-side converter of the i-th direct-drive wind turbine, A gi Let be the loss of the i-th high-frequency aggregation line; the loss includes construction loss and subsequent operation and maintenance loss.

[0143] Step S3.2.2: Calculate the overall loss of the low-frequency transmission system using equation (26):

[0144] A low =AM3C_s +A M3C_r +A line (26)

[0145] In equation (26), A low For the overall loss of the low-frequency transmission system, A M3C_s For the loss of M3C at the transmitting end of the low-frequency transmission system, A M3C_r For the loss of the M3C at the receiving end of the low-frequency transmission system, A line This refers to the losses in low-frequency transmission lines;

[0146] Step S3.3: Obtain the minimum damping ratio of the characteristic values ​​of the direct-drive wind farm high-frequency collection and low-frequency transmission grid-connected system:

[0147] Step S3.3.1: Obtain the initial operating point x0 of the direct-drive wind farm high-frequency collection and low-frequency transmission grid-connected system through power flow calculation, and use Equation (27) to obtain the linearized state space model of the direct-drive wind farm high-frequency collection and low-frequency transmission grid-connected system at the initial operating point x0.

[0148]

[0149] In equation (27), d represents the differential, t is time, and x w_hl For the state variables of the direct-drive wind farm's high-frequency collection and low-frequency transmission grid-connected system, A w_hl For the state matrix of the direct-drive wind farm high-frequency collection and low-frequency transmission grid-connected system, B w_hl For the input matrix of the direct-drive wind farm high-frequency collection and low-frequency transmission grid-connected system, u w_hl For the input variable of the direct-drive wind farm high-frequency collection and low-frequency transmission grid-connected system; Δx w_hl x represents w_hl The increment, Δu w_hl Indicate u w_hl The increment;

[0150] Step S3.3.2: Solve equation (28) to obtain the state matrix A. w_hl eigenvalues ​​{λ e |e=1,2,…,m};where m is A s The order of λ e A represents w_hl The e-th eigenvalue, λ e =σ e +jω e ;σ ei For λ e The real part, ω e For λ e The imaginary part, j, denotes a complex number;

[0151] |λ e E w_hl-A w_hl |=0 (28)

[0152] In equation (28), E w_hl To be related to the state matrix A w_hl Identity matrices of the same order;

[0153] Step S3.3.3: Calculate the eigenvalue {λ} using equation (29). e Damping ratio of |e=1,2,…,m}:

[0154]

[0155] In equation (29), ξ e For the eigenvalue λ e The damping ratio for {ξ} e After sorting |e=1,2,…,n} in descending order, select the smallest number ξ. min The minimum damping ratio for the characteristic values ​​of the direct-drive wind farm high-frequency collection and low-frequency transmission grid-connected system;

[0156] Step S3.4: Obtain the optimal frequency of the high-frequency collection system and the optimal frequency of the low-frequency transmission system of the direct-drive wind farm:

[0157] Step S3.4.1: Operate the high-frequency collection system at frequency f high and the operating frequency f of the low-frequency transmission system low To optimize the variables, a frequency optimization model F for the direct-drive wind farm high-frequency collection and low-frequency transmission grid-connected system is established using equation (30):

[0158]

[0159] In equation (30), η1 is the minimum damping ratio ξ min The weighting coefficients are: η2 is the weighting coefficient for the transmission power limit of the high-frequency collection system and the low-frequency transmission system; η3 is the weighting coefficient for the line voltage loss of the high-frequency collection system and the low-frequency transmission system; and η4 is the weighting coefficient for the comprehensive loss of the high-frequency collection system and the comprehensive loss of the low-frequency transmission system.

[0160] Step S3.4.2: Use a multi-objective optimization algorithm to solve the frequency optimization model F shown in equation (30), thereby obtaining the optimal frequency of the high-frequency collection system and the optimal frequency of the low-frequency transmission system.

[0161] In this embodiment, an electronic device includes a memory and a processor. The memory stores a program that supports the processor in executing the above-described method, and the processor is configured to execute the program stored in the memory.

[0162] In this embodiment, a computer-readable storage medium stores a computer program, which is executed by a processor to perform the steps of the above method.

Claims

1. A grid-connected method for high-frequency collection and low-frequency transmission in a direct-drive wind farm, applied to a grid-connected system consisting of a direct-drive wind farm, a high-frequency collection system, a low-frequency transmission system, and an AC power grid, wherein any direct-drive wind turbine in the direct-drive wind farm is denoted as i, i=1,…,n, and the high-frequency collection system comprises n high-frequency collection lines; characterized in that… The grid connection method is carried out according to the following steps: Step S1: Establish the control structure for the wind turbine-side converter and grid-side converter in a direct-drive wind farm; Step S1.1: Establish the control structure of the wind turbine generator-side converter, including: the constant reactive power control outer loop structure of the wind turbine generator-side converter, the constant reactive power control inner loop structure of the wind turbine generator-side converter, the constant DC voltage control outer loop structure of the wind turbine generator-side converter, and the constant DC voltage control inner loop structure of the wind turbine generator-side converter. Step S1.2: Establish the control structure of the grid-side converter of the wind turbine, including: the outer loop structure for the constant AC voltage and frequency control of the grid-side converter of the wind turbine, and the inner loop structure for the constant AC voltage and frequency control of the grid-side converter of the wind turbine. Step S2: Establish the control structure for the sending-end M3C converter and the receiving-end M3C converter of the low-frequency transmission system; Step S2.1: Establish the control structure of the sending-end M3C converter of the low-frequency transmission system, including: the outer loop structure of the high-frequency side constant DC voltage control and constant reactive power control of the sending-end M3C converter, the inner loop structure of the high-frequency side constant DC voltage control and constant reactive power control of the sending-end M3C converter, the outer loop structure of the low-frequency side constant AC voltage and frequency control of the sending-end M3C converter, the inner loop structure of the low-frequency side constant AC voltage and frequency control of the sending-end M3C converter, and the bridge arm control structure of the sending-end M3C converter; Step S2.2: Establish the control structure of the receiving-end M3C converter of the low-frequency transmission system, including: the outer loop structure of the power frequency side constant DC voltage control of the receiving-end M3C converter, the inner loop structure of the power frequency side constant DC voltage control of the receiving-end M3C converter, the outer loop structure of the power frequency side constant AC voltage control of the receiving-end M3C converter, the inner loop structure of the power frequency side constant AC voltage control of the receiving-end M3C converter, the outer loop structure of the low-frequency side constant active power control and constant reactive power control of the receiving-end M3C converter, the inner loop structure of the low-frequency side constant active power control and reactive power control of the receiving-end M3C converter, and the bridge arm control structure of the receiving-end M3C converter; Step S3: Obtain the optimal frequency for the high-frequency collection system and the low-frequency transmission system, taking into account resource utilization and stability; Step S3.1: Obtain the transmission power limit P of the high-frequency collection system. fi_max and line voltage loss ΔU fi %, the transmission power limit P of the low-frequency transmission system low_max and line voltage loss ΔU low %: Step S3.2: Obtain the overall loss of the high-frequency collection system, A high The combined loss A of the low-frequency transmission system low for: Step S3.3: Obtain the minimum damping ratio ξ of the grid-connected system characteristic values. e : Step S3.4: Obtain the optimal frequency of the high-frequency collection system and the optimal frequency of the low-frequency transmission system: Step S3.4.1: Using the operating frequency f of the high-frequency collection system high and the operating frequency f of the low-frequency transmission system low To optimize the variables, a frequency optimization model F for the grid-connected system is established using equation (30): (30) In equation (30), η1 is the minimum damping ratio ξ min The weighting coefficients are: η2 is the weighting coefficient for the transmission power limit of the high-frequency aggregation system and the low-frequency transmission system; η3 is the weighting coefficient for the line voltage loss of the high-frequency aggregation system and the low-frequency transmission system; and η4 is the weighting coefficient for the comprehensive loss of the high-frequency aggregation system and the comprehensive loss of the low-frequency transmission system. Step S3.4.2: Use a multi-objective optimization algorithm to solve the frequency optimization model F shown in equation (30), thereby obtaining the optimal frequency of the high-frequency collection system and the optimal frequency of the low-frequency transmission system.

2. The grid connection method for high-frequency collection and low-frequency transmission in a direct-drive wind farm according to claim 1, characterized in that, Step S3.1 includes: Step S3.1.1: Calculate the frequency f of the high-frequency collection system at frequencies greater than 50Hz using equation (23). high The following are the transmission power limits and line voltage losses: (23) In equation (23), P fi_lmax Let u be the transmission power limit of the i-th direct-drive wind turbine in its high-frequency collection line. wi Let ΔU be the rated voltage of the i-th direct-drive wind turbine on its high-frequency collection line. fi % represents the voltage loss of the i-th direct-drive wind turbine in its high-frequency collection line, Q gi Let be the reactive power of the i-th direct-drive wind turbine in its high-frequency collection line; Step S3.1.2: Calculate the frequency f of the low-frequency transmission system at frequencies less than 50Hz using equation (24). low The following are the transmission power limits and line voltage losses: (24) In equation (24), P low_max For the transmission power limit of the low-frequency transmission system, u low L is the rated voltage of the low-frequency transmission system. low For the line inductance of the low-frequency transmission system, ΔU low % represents the voltage loss of the low-frequency transmission system, Q low This refers to the reactive power transmitted by the low-frequency transmission system.

3. The grid connection method for high-frequency collection and low-frequency transmission in a direct-drive wind farm according to claim 2, characterized in that, Step S3.2 includes: Step S3.2.1: Calculate the overall loss of the high-frequency collection system using equation (25): (25) In equation (25), A high For the overall loss of the high-frequency collection system, A wi For the loss of the i-th direct-drive wind turbine, A MSCi For the losses of the i-th direct-drive wind turbine generator side converter, A GSCi Let A be the investment cost of the grid-side converter of the i-th direct-drive wind turbine. gi Let be the loss of the i-th high-frequency collection line; Step S3.2.2: Calculate the overall loss of the low-frequency transmission system using equation (26): (26) In equation (26), A low For the overall loss of the low-frequency transmission system, A M3C_s For the input loss of the M3C at the transmitting end of the low-frequency transmission system, A M3C_r For the loss of the M3C at the receiving end of the low-frequency transmission system, A line This refers to the losses in low-frequency transmission lines.

4. The grid connection method for high-frequency collection and low-frequency transmission in a direct-drive wind farm according to claim 3, characterized in that, Step S3.3 includes: Step S3.3.1: Obtain the initial operating point x0 of the grid-connected system through power flow calculation, and use equation (27) to construct a linearized state-space model of the grid-connected system at the initial operating point x0; (27) In equation (27), d represents the differential, t is time, and x w_hl A is the state variable of the grid-connected system. w_hl For the state matrix of the grid-connected system, B w_hl For the input matrix of the grid-connected system, u w_hl For the input variables of the grid-connected system; Δx w_hl x represents w_hl The increment, Δu w_hl Indicate u w_hl The increment; Step S3.3.2: Solve equation (28) to obtain the state matrix A. w_hl eigenvalues ​​{λ e |e=1,2,…,m};where m is A s The order of λ e A represents w_hl The e-th eigenvalue, and λ e =σ e +jω e ;σ ei For λ e The real part, ω e For λ e The imaginary part, j, denotes a complex number; (28) In equation (28), E w_hl To be related to the state matrix A w_hl Identity matrices of the same order; Step S3.3.3: Calculate the e-th eigenvalue λ using equation (29) e Damping ratio ξ e : (29) In equation (29), for all eigenvalues ​​{λ} e The damping ratio {ξ} of |e=1,2,…,m} e After sorting |e=1,2,…,n} in descending order, the minimum value is selected as the minimum damping ratio ξ of the grid-connected system characteristic value. min .

5. An electronic device, comprising a memory and a processor, characterized in that, The memory is used to store a program that supports the processor in executing any of the grid connection methods described in claims 1-4, and the processor is configured to execute the program stored in the memory.

6. A computer-readable storage medium storing a computer program thereon, characterized in that, The computer program, when run by a processor, performs the steps of any of the grid connection methods described in claims 1-4.

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