AC filter configuration method for offshore wind power medium frequency uncontrolled rectifier DC transmission system

Abstract

The present invention discloses a method for configuring an AC filter for an offshore wind power medium-frequency uncontrolled rectifier DC transmission system, comprising: modeling the main equipment of the offshore wind farm at harmonic frequencies; using the equipment model to construct an offshore wind farm AC network, and establishing a Norton equivalent model of the offshore wind farm suitable for harmonic analysis; establishing differential equations and boundary conditions for the commutation process and non-commutation process of the DRU, respectively, and using an iterative method to calculate the harmonic current injected into the AC system by the uncontrolled rectifier converter station; finally, calculating the harmonic voltage of the AC bus, and determining the number, capacity, and type of AC filter groups according to the power quality standard. The present invention fills the gap in the current research on AC filter configuration methods for DC transmission systems based on medium-frequency uncontrolled rectifier technology, can provide a certain reference for future engineering design, and has strong versatility. In theory, the design method of the AC filter configuration scheme is applicable to offshore wind power grid-connected systems that transmit through diode uncontrolled rectification.

Description

Technical Field

[0001] The present invention belongs to the technical field of power transmission and distribution of electric power systems, and in particular relates to an AC filter configuration method for an offshore wind power medium-frequency uncontrolled rectifier DC transmission system. Background Art

[0002] Offshore wind power plays an increasingly important role in promoting economic growth and enhancing energy security, making its development the most eye-catching and becoming a key technology in the global energy transformation.

[0003] Offshore wind power transmission solutions can be categorized based on three dimensions: the grid-forming capabilities of offshore wind turbines, the frequency characteristics of their output voltage and current, and the transmission methods of offshore high-voltage transmission lines. Grid-forming wind turbines are suitable for grids with a high proportion of renewable energy access. They can improve grid power quality and enhance power system stability, offering broader development prospects within the offshore wind power renewable energy landscape. The future direction of offshore wind power development is deep-sea wind power, so high-voltage transmission must utilize low-frequency AC or DC. Currently, research on AC collection in offshore power collection systems is relatively mature, with medium-frequency collection reducing the weight of offshore equipment and platforms, lowering costs, and improving economic efficiency. Therefore, AC collection and DC transmission will be the preferred offshore wind power transmission solution for the near future.

[0004] In recent years, DRU (Diode Rectifier Unit) has attracted much attention due to its advantages of low cost, small size, light weight and high reliability. The offshore wind power medium frequency convergence DC transmission system based on DRU has stood out from many offshore wind power transmission schemes and has become a research hotspot in this field. Figure 1 As shown in the figure, the system uses DRU as the rectifier device. Since it does not have the active commutation capability, the offshore wind turbine adopts a grid-type configuration. The offshore power collection system and offshore high-voltage transmission line use AC collection and DC transmission respectively. The AC frequency is generally in the medium frequency range of 50Hz~200Hz.

[0005] Because DRUs typically use 12-pulse or 24-pulse rectifier circuits, they introduce significant harmonics on the AC side, impacting the power quality and safe operation of offshore wind power collection systems. Therefore, it's essential to configure appropriate AC filters within offshore wind power transmission systems to reduce harmonic content and ensure proper operation. Proper AC filter configuration is a critical issue facing and urgently needs to be addressed in DRU-based offshore wind power medium-frequency DC transmission systems.

[0006] Reference [Xu Feng, Cao Wenbin, Qian Zhengxu, et al. Harmonic analysis of multiple rectification circuits for offshore wind power diode rectification schemes [J]. Zhejiang Electric Power, 2024, 43(11): 15-25] reveals the AC side current harmonic spectrum characteristics and DC side voltage harmonic distribution law of 12 / 24 pulse rectification type DRU through theoretical derivation and PSCAD (power system computer-aided design) / EMTDC (electromagnetic transient DC program) simulation. However, there are two significant limitations in the research: first, the coupling effect of harmonic injection in offshore wind farms is not included, resulting in limited engineering applicability; second, there is a lack of discussion on the harmonic propagation mechanism under special offshore working conditions. Reference [Zhang Xincheng. Method for harmonic control of AC collection system of offshore wind power transmitted by diode-rectified high-voltage DC [D]. Northeast Electric Power University, 2024] proposed a grid-side converter harmonic suppression method based on voltage control strategy, but there are obvious deficiencies in system-level harmonic control: the overall harmonic impedance model is not established, the AC filter optimization configuration scheme is not given, and the key influence of the distribution parameters of the collection submarine cable on the harmonic amplification effect is ignored. Summary of the Invention

[0007] In view of the above, the present invention provides a method for configuring an AC filter for an offshore wind power medium-frequency uncontrolled rectifier DC transmission system, which can reduce the harmonic content on the DRU AC side and has great practical significance for improving the safety and stability of equipment operation and the normal operation of the offshore wind power transmission system.

[0008] A method for configuring an AC filter for an offshore wind power medium-frequency uncontrolled rectifier DC transmission system comprises the following steps:

[0009] (1) Modeling of offshore wind farms at various harmonic frequencies;

[0010] (2) A DRU equivalent model suitable for harmonic analysis is established for the AC side of the DRU in the offshore converter station. The model consists of a second equivalent current source I2, and the DC side of the DRU is equivalent to the DC loop impedance.

[0011] (3) The iterative method is used to calculate the harmonic currents injected by the DRU into the offshore AC system. The harmonic currents correspond to the current phasors of the second equivalent current source I2 at each harmonic frequency.

[0012] (4) Calculate the harmonic voltage of the PCC node at each harmonic frequency based on the equivalent model parameters of the offshore wind farm and DRU;

[0013] (5) Calculate the distortion rate of a single harmonic, the total effective harmonic distortion rate, and the telephone harmonic waveform factor based on the harmonic voltage at the PCC node, and compare them with the power quality standards. If they meet the standards, there is no need to install an AC filter; if they do not meet the standards, install an AC filter of appropriate capacity and type.

[0014] The offshore wind power medium-frequency uncontrolled rectifier DC transmission system includes an offshore wind farm, an offshore converter station based on DRU, and an onshore converter station based on MMC, wherein the offshore wind farm contains multiple wind turbines, which are connected to the AC busbar of the offshore converter station, namely the PCC (common connection point) node, through an AC submarine cable. The AC side of the DRU in the offshore converter station is connected to the AC busbar through a converter transformer, and the DC side of the DRU is connected to the DC side of the MMC in the onshore converter station through a DC submarine cable. The AC side of the MMC is connected to the onshore AC system through a converter transformer; the offshore wind farm, the AC busbar and the converter transformer of the DRU constitute the offshore AC system; the DC side of the DRU, the DC submarine cable and the DC side of the MMC constitute the DC transmission system.

[0015] Furthermore, the specific implementation method of step (1) is as follows: a Norton circuit and a π-shaped circuit are used as equivalent circuits of wind turbines and AC submarine cables in an offshore wind farm, respectively, and an offshore wind farm equivalent model suitable for harmonic analysis is established based on these equivalent circuits. The equivalent model is composed of a first equivalent current source I1 and an equivalent impedance Z in parallel, one end of which is grounded and the other end is connected to the AC bus of the offshore converter station.

[0016] Furthermore, the numerical expressions of the first equivalent current source I1 and the equivalent impedance Z are as follows:

[0017]

[0018]

[0019] in: Z owf,h Indicates harmonic frequency h · f The impedance value of the equivalent impedance Z under 0, Indicates harmonic frequency h · f The current phasor of the first equivalent current source I1 under 0, Y owf,h Indicates harmonic frequency h · f 0 node admittance matrix of the offshore AC system, V owf is a column vector, the first element of which is 1 and the rest are 0; I owf,h is a column vector, in which the second n wt +1 element corresponds to the number of wind turbines in the offshore wind farm h Subharmonic current, the rest of the elements are 0; superscript T represents transpose, h is a natural number greater than 0, f0 is the fundamental frequency, n wt is the number of wind turbines in the offshore wind farm. Assume that there are N nodes, the first node is the AC busbar of the offshore converter station, the second to n wt Node +1 is the corresponding wind turbine node (the high-voltage side of the wind turbine box step-up transformer).

[0020] Furthermore, the DC loop impedance is composed of the DRU DC side impedance and other DC impedances in the DC transmission system in series, wherein the numerical expression of the DRU DC side impedance is as follows:

[0021]

[0022] in: Z DRU,h,k Indicates harmonic frequency h · f 0Next k The impedance value of the DRU DC side impedance is iterated step by step. k b Indicates the number of six-pulsation valves in series in the DRU, μ k Indicates the k The commutation angle of the DRU is iterated step by step, k DRU Indicates the transformation ratio of the DRU converter transformer, Z owf,h Indicates harmonic frequency h · f The impedance value of the equivalent impedance Z under 0, Z filt,h Indicates the AC filter installed on the AC bus at the harmonic frequency h · f 0 (if no AC filter is installed, this value is infinite), j is the imaginary unit, h is a natural number greater than 0, X tD Indicates fundamental frequency f 0 DRU converter transformer leakage reactance, k is a natural number.

[0023] Furthermore, the iterative method in step (3) is to calculate the DRU of each iteration a Phase AC current time domain expression, obtained by phase shifting bc The time domain expression of the two-phase AC current is then transformed into the time domain expression of the three-phase AC current of the DRU when the iteration converges, and the harmonic currents injected by the DRU into the offshore AC system are obtained through Fourier transformation;

[0024] For a 12-pulse DRU, a The time domain expression of phase AC current is as follows:

[0025]

[0026] For a 24-pulse DRU, a The time domain expression of phase AC current is as follows:

[0027]

[0028] in: i a,k ( t ) indicates the k Step Iteration t Moment DRU a Phase AC current, μ k-1 Indicates the k -1 step iterative DRU commutation angle, k DRU Indicates the transformation ratio of the DRU converter transformer, ω represents the fundamental angular frequency, n represents the order of characteristic harmonics, q is a natural number, i dc,k-1 ( t ) indicates the k -1 iteration t The DC current of DRU at the moment, t Indicates time, k is a natural number.

[0029] Furthermore, the harmonic frequencies h · f 0Next k Step-by-step iteration of DRU DC current harmonic phasor The expression of is as follows, and the Fourier transform relationship can be used to obtain the first k Step Iteration t DC current of DRU at time i dc,k ( t );

[0030]

[0031] Where: Z dc,h,k Indicates harmonic frequency h · f 0Next k The impedance value of the DC loop impedance is iterated step by step, Indicates harmonic frequency h · f 0Nextk The harmonic phasors of the potential inside the DC side of the DRU are iterated step by step.

[0032] Further, the k Step Iteration t The internal potential of the DRU DC side at time u dc,k ( t ) is as follows, and the harmonic frequency can be obtained by Fourier transform relationship h · f 0Next k Step-by-step iteration of the harmonic phasor of the potential inside the DC side of the DRU ;

[0033]

[0034]

[0035] in: p Indicates the number of the six-pulsation converter valve in the DRU. u dc,k,1 ( t )and Respectively represent k Step Iteration t Moment and The DC voltage of the six-pulsation converter valve No. 1 at this moment, φ p Indicates the p The DC voltage phase difference between the No. 6 pulsating converter valve and the No. 1 six-pulsating converter valve is u r ( t ), u s ( t ), u x ( t ) represent AC busbars r Mutually, s Harmony x The instantaneous value of the phase voltage, r Phase indicates the phase entering commutation, s Phase indicates the phase to exit commutation. x Phases that do not participate in commutation, m Indicates the number of the commutation interval, μ k Indicates the k Iterate the commutation angle of the DRU in steps.

[0036] Furthermore, the step (4) calculates the harmonic voltage of the PCC node at each harmonic frequency by the following expression;

[0037]

[0038]

[0039]

[0040] in: U PCC,h Indicates harmonic frequency h · f 0, the harmonic voltage of the PCC node, U PCC,h,1 Indicates the AC bus voltage when only the first equivalent current source I1 is considered h Subharmonic voltage effective value, U PCC,h,2 Indicates the AC bus voltage when only the second equivalent current source I2 is considered h Subharmonic voltage effective value, h is a natural number greater than 0, k pcc,h is a given coefficient, which is taken as 1.62, 1.28 and 0.72 at the 3rd, 5th and 7th harmonics respectively, and is taken as 0 at other harmonics; k b Indicates the number of six-pulsation valves in series in the DRU, k DRU Indicates the transformation ratio of the DRU converter transformer, X tD Indicates fundamental frequency f 0 DRU converter transformer leakage reactance, U dc0 Indicates the rated DC voltage of the DRU. I dc,0 Indicates the initial value of the DRU's DC current iteration, Indicates the AC bus voltage when only the second equivalent current source I2 is considered h Subharmonic voltage phasors, Indicates harmonic frequency h · f The current phasor of the first equivalent current source I1 under 0, Indicates harmonic frequency h · f The current phasor of the second equivalent current source I2 under 0, Z owf,h Indicates harmonic frequency h · f The impedance value of the equivalent impedance Z under 0, Z filt,h Indicates the AC filter installed on the AC bus at the harmonic frequency h · f Harmonic impedance at 0.

[0041] Furthermore, the initial value of the DC current iteration of the DRU I dc,0 And the initial value of the commutation angle iteration μ The calculation expression of 0 is as follows:

[0042]

[0043]

[0044] in: Represents the sum of the active power of all wind turbines in the offshore wind farm.

[0045] Furthermore, in step (5), an AC filter of appropriate capacity and type is installed. The specific implementation method is as follows: first, consider using multiple groups of AC filters, each group of filters has equal fundamental wave capacity, and the upper limit of the total capacity is Q max and lower limit Q min 40% and 10% of the DRU rated DC power respectively. Q min Start to gradually increase the capacity, the capacity step size Δ Q The value is 2% of the DRU rated DC power. The AC filter types include double-tuned filters, single-tuned filters, and shunt capacitors. After the capacity is fixed each time, each group of AC filters is traversed through all type combinations. The distortion rate of the single harmonic, the total effective harmonic distortion rate, and the telephone harmonic waveform coefficient of each combination configuration are calculated and compared with the power quality standard. If only one combination configuration meets the standard, the current capacity and this type of combination are used as the AC filter configuration scheme. If multiple combination configurations meet the standard, the type combination with the simplest structure is selected, and the current capacity and this type of combination are used as the AC filter configuration scheme. If no combination configuration meets the standard, the capacity is increased and the traversal judgment is repeated according to the above method.

[0046] A computer device includes a memory and a processor, wherein the memory stores a computer program, and the processor is used to execute the computer program to implement the above-mentioned method for configuring an AC filter in an offshore wind power medium-frequency uncontrolled rectifier DC transmission system.

[0047] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the above-mentioned method for configuring an AC filter in an offshore wind power medium-frequency uncontrolled rectifier DC transmission system.

[0048] Based on the above technical solution, the present invention has the following beneficial technical effects:

[0049] 1. For 12-pulse and 24-pulse DRU rectifiers, this paper proposes a harmonic analysis method based on iterative solution. Based on this method, a method for configuring AC filters for offshore converter stations is proposed. This method fills the gap in DC power management in offshore AC systems for medium-frequency uncontrolled rectifier DC transmission systems of offshore wind power and can provide a reference for future engineering design.

[0050] 2. The present invention has strong versatility and relatively simple parameter selection. In theory, it is applicable to all offshore wind power transmission systems based on uncontrolled rectification, and meets the requirements of complying with power quality standards, small reactive power compensation, and simple overall AC filter structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 This is a structural diagram of an offshore wind power medium-frequency uncontrolled rectifier DC transmission system according to an embodiment of the present invention.

[0052] Figure 2 Schematic diagram of the structure of a neutron wind farm in an embodiment of the present invention.

[0053] Figure 3 Schematic diagram of the resistance change per unit length of submarine cables of five line types under different harmonic orders in an embodiment of the present invention.

[0054] Figure 4 Schematic diagram of the change in inductance per unit length of submarine cables of five line types under different harmonic orders in an embodiment of the present invention.

[0055] Figure 5 Schematic diagram of the capacitance change per unit length of submarine cables of five line types under different harmonic orders in an embodiment of the present invention.

[0056] Figure 6 Schematic diagram of the change of equivalent harmonic impedance of a wind turbine under different harmonic orders in an embodiment of the present invention.

[0057] Figure 7 Schematic diagram of the change of equivalent harmonic current of a wind turbine at different power levels in an embodiment of the present invention.

[0058] Figure 8 Schematic diagram of the Norton equivalent circuit structure of an offshore wind farm in an embodiment of the present invention.

[0059] Figure 9 Schematic diagram of the change of equivalent harmonic current of an offshore wind farm at different power levels in an embodiment of the present invention.

[0060] Figure 10 Schematic diagram of the change of equivalent harmonic impedance of an offshore wind farm under different harmonic orders in an embodiment of the present invention.

[0061] Figure 11Schematic diagram of the Thevenin equivalent circuit structure of the DRU DC side in an embodiment of the present invention.

[0062] Figure 12 Schematic diagram of the change in the effective value of the harmonic voltage (AC component) on the DC side of the DRU under different harmonic orders of rated power in an embodiment of the present invention.

[0063] Figure 13 Schematic diagram of the change of equivalent harmonic impedance on the DC side of a DRU under different harmonic orders of rated power in an embodiment of the present invention.

[0064] Figure 14 Schematic diagram of the change of equivalent harmonic current on the DC side of the DRU at different power levels in an embodiment of the present invention.

[0065] Figure 15 Schematic diagram of the structures of three AC filters in the embodiments of the present invention.

[0066] Figure 16 The figure is a flow chart of determining the AC filter configuration scheme in an embodiment of the present invention.

[0067] Figure 17 This is a line graph of the harmonic voltage of the AC bus before and after the AC filter is installed in an embodiment of the present invention. DETAILED DESCRIPTION

[0068] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0069] The structure of the offshore wind power medium frequency uncontrolled rectifier DC transmission system in this embodiment is as follows: Figure 1 As shown in Figure 1, the offshore wind farm consists of 16 sub-wind farms. The AC filter is connected to the AC busbar. Each sub-wind farm is identical and uses five full-power converter-type wind turbines with a rated capacity of 12.5MW. They are connected to the AC busbar of the offshore converter station via five sections of 66kV AC submarine cables. Figure 2 The DRU of this embodiment adopts 24 pulsations, the frequency of the offshore wind farm is 100 Hz, and the main parameters of the system are shown in Table 1:

[0070] Table 1

[0071]

[0072] First, the harmonic impedance modeling of wind turbine and AC submarine cable is carried out respectively. The AC submarine cable adopts π circuit equivalent. The resistance per unit length of the submarine cable model in Table 1 is R 0. Inductor L 0 and capacitance parameters C The calculation results of 0 at the 2nd to 50th fundamental frequency points are as follows Figures 3 to 5As shown, then calculate the equivalent impedance of the line at both ends of the π circuit to the ground branch at the harmonic frequency according to the following formula Z 0,h The equivalent impedance of the series circuit between the two ends of the line Z 12,h .

[0073]

[0074] in: l 0 is the line length.

[0075] At harmonic frequencies (2nd to 50th fundamental frequency points), the wind turbine adopts the Norton circuit equivalent, and its equivalent impedance is independent of the power level. It is obtained by the test signal method (for details, see the reference [Xue Yinglin, Xu Zheng, Zhang Zheren, et al. Analysis of impedance frequency characteristics of MMC-HVDC converter [J]. Proceedings of the CSEE, 2014, 34(24): 4040-4048]). The calculation results are as follows: Figure 6 As shown, its equivalent current source is related to the power level, such as Figure 7 shown.

[0076] Then an equivalent model of offshore wind farm suitable for harmonic analysis is established, such as Figure 8 As shown, the equivalent model of the offshore wind farm consists of an equivalent current source I h(n) and equivalent impedance Z w(n) The DRU equivalent model consists of an equivalent current source The nodes of each wind turbine and each line are numbered in sequence, the offshore wind farm is converted into an AC power network, and the node admittance matrix is ​​solved. h · f 0 equivalent impedance of offshore wind farm Z owf,h and equivalent current phasors During the calculation process, the AC busbar of the offshore converter station is marked as node 1, and the high-voltage side of all wind turbine boxes is defined as wind turbine nodes (numbered 2 to n wt +1), using the short-circuit current method and external power supply method of AC bus grounding, the equivalent harmonic current source and equivalent harmonic impedance in the Norton equivalent model of the offshore wind farm are solved respectively, and the Norton equivalent model of the offshore wind farm under different harmonics is established. Assuming that there are N nodes, we can calculate Z owf,h and They are shown in the following formulas:

[0077]

[0078]

[0079] in: Z owf,h Indicates harmonic frequency h · f 0 equivalent impedance Z w(n) The impedance value, Indicates harmonic frequency h · f 0 equivalent current source I h(n) The current phasor, Y owf,h Indicates harmonic frequency h · f 0 node admittance matrix of the offshore AC system, V owf is a column vector, the first element of which is 1 and the rest are 0; I owf,h is a column vector, in which the second n wt +1 element corresponds to the number of wind turbines in the offshore wind farm h Subharmonic current, the rest of the elements are 0; superscript T represents transpose, h is a natural number greater than 0, f 0 is the fundamental frequency, n wt is the number of wind turbines in the offshore wind farm.

[0080] Considering the impact of different power levels on the Norton equivalent model of offshore wind farms, the rated power is 100%, the step size is 10%, and there are 10 power levels from 10% to 100%. The effective values ​​of harmonic currents at different power levels are calculated as follows: Figure 9 As shown, the harmonic impedance is Figure 10 shown.

[0081] The iterative method is used to calculate the harmonic current injected into the AC system by the uncontrolled rectifier converter station. k =1, assuming the DC current is constant DC, the DC current I dc,0 and commutation angle μ 0 is calculated according to the following formula, and the DC component of the DC current is kept constant during the subsequent iterations. I dc,0 constant.

[0082]

[0083]

[0084] in: Represents the sum of active power of all fans, U dc0 Indicates the rated DC voltage, X tD Indicates fundamental frequency f 0 DRU converter transformer leakage reactance, k b Indicates the number of 6-pulse converters connected in series.

[0085] Further obtain 12 pulse (or 24 pulse) DRU a The time domain expression of the phase AC current is shown in the following formula:

[0086]

[0087]

[0088] in: μ k-1 Indicates the k -1 step iterative calculation of the commutation angle, k DRU represents the commutation ratio, ω represents the fundamental angular frequency, q is a natural number, i dc,k-1 ( t ) indicates the k -1 iteration t The DC current of DRU at the moment, n represents the order of characteristic harmonics, t represents time. Assume that the harmonic frequency corresponding to the above formula is h · f The output current phasor of DRU under 0 is , the harmonic frequencies can be calculated h · f 0 AC bus node voltage phasor of the converter station as follows:

[0089]

[0090] in: Indicates harmonic frequency h · f 0 equivalent current source The current phasor, Z filt,h Indicates the AC filter installed on the AC bus at the harmonic frequency h · f 0 (if no AC filter is installed, this value is infinite).

[0091] For the k Step iteration, solve the commutation angle according to the differential equation of the commutation process and boundary conditions, u dc,k ( t ) is the sum of the DC output voltages of the six-pulsation converter valves, as shown in the following formula:

[0092]

[0093] in: p Indicates that each 6-pulse converter is numbered, and it is assumed that the 6-pulse converter valve with YY connection is numbered 1. u dc,k,p ( t )express p The time domain expression of the DC voltage of the six-pulsation converter valve is: φ p express p The phase difference between the DC voltage of the six-pulsation converter valve No. 1 and No. 2 is Indicates the k Step Iteration DC voltage of the 1st six-pulse converter valve at this moment, 12-pulse converter , 24 pulsation converter .

[0094] When calculating the DC voltage of the No. 1 six-pulse converter valve, first divide one cycle into 12 intervals according to the conduction state time of each bridge arm, including 6 non-commutation intervals and 6 commutation intervals. Take the moment when the No. 6 bridge arm is turned on and the No. 1 bridge arm just enters the commutation process of the No. 5 bridge arm as the starting point (this interval is marked as No. 1 interval) and number the 6 commutation intervals in sequence. m The voltage in the commutation interval and the subsequent non-commutation interval can be described by the following formula:

[0095]

[0096] Among them: subscript r Indicates the phase entering commutation, subscript s Indicates the phase to exit commutation, subscript x Indicates the non-commutating phase; u r ( t ) represents the AC busbar of the offshore converter station r Instantaneous value of phase voltage, u s ( t ) represents the AC busbar of the offshore converter station s Instantaneous value of phase voltage, u x ( t ) represents the AC busbar of the offshore converter stationx The instantaneous value of the phase voltage can be obtained by the AC bus harmonic voltage component And the AC bus fundamental voltage component shown in the following formula Calculate; the commutation angle can be calculated by solving the following equation :

[0097]

[0098]

[0099] in: μ k Indicates the k The commutation angle of the DRU is iterated step by step, I dc,k-1 ( t a+μ ) indicates the k -1 iteration t a+μ The DC current of DRU at the moment, i r Indicates the AC busbar of the offshore converter station r Phase current, i s Indicates the AC busbar of the offshore converter station s Phase current, t a Indicates the a The start time of a commutation process, t a+μ Indicates the a The end time of a commutation process, a =1,2,3,4,5,6.

[0100] On this basis, the harmonic frequencies are calculated h · f 0 DC circuit impedance Z dc,h,k Including the DC side impedance of the sending end DRU Z DRU,h,k and the DC impedance of the rest of the DC system Z res,h,k ,like Figure 11 As shown, it can be calculated by the following formula:

[0101]

[0102] Among them: calculation Z res,h,k The equipment that needs to be considered includes the smoothing reactor of the sending-end converter station, the DC line and the receiving-end converter station. In actual calculation, only the smoothing reactor of the sending-end converter station can be considered to improve the calculation efficiency.

[0103] for u dc,k ( t )Medium harmonic frequency h · f 0 harmonic phasor , calculate the corresponding DC side harmonic current phasor according to the following formula :

[0104]

[0105] For the frequency range considered, if the harmonic phasors and the previous step result If the difference is less than the preset threshold, the calculation is considered convergent; otherwise, the DC side harmonic current result needs to be calculated. The time domain expression of the restored DC side current is I dc,k ( t ), repeat the above steps until the calculation converges. Taking the rated power as an example, the DC component of the DC side voltage is calculated to be 639.92kV, and the harmonic phasor is The valid values ​​are as follows Figure 12 As shown, the DC circuit impedance Z dc,h,k like Figure 13 As shown; at different power levels, the DRU equivalent current source The valid values ​​are as follows Figure 14 shown.

[0106] In the process of calculating the above parameters, the harmonics of the AC bus voltage of the offshore converter station are calculated using the following formula:

[0107]

[0108] in: U PCC,h,1 To consider only the equivalent current source in the offshore AC system AC bus voltage of offshore converter station h Subharmonic voltage effective value, U PCC,h,2 Considering only the harmonic current injected by DRU into the AC system AC bus voltage of offshore converter station h Subharmonic voltage effective value, k pcc In the case of the 3rd, 5th and 7th harmonics, the values ​​are 1.62, 1.28 and 0.72 respectively, and in the case of other harmonic orders, the values ​​are all 0.

[0109] Three indicators are considered for power quality: single harmonic distortion, total effective harmonic distortion, and telephone harmonic waveform factor, as shown in Table 2 (for details, see GB / T 26870-2011 Application of Filters and Shunt Capacitors in Industrial AC Power Grids Affected by Harmonics and GB / T 14549-1993 Power Quality - Public Power Grid Harmonics):

[0110] Table 2

[0111]

[0112] For this embodiment, the upper and lower limits of reactive power capacity are Q max and Q min 40% rated DC power and 10% rated DC power are selected, which are 400Mvar and 100Mvar respectively. Therefore, 4 groups of AC filters with the same fundamental wave capacity are selected. There are three types of AC filters: single tuned filter (ST), double tuned filter (DT), and shunt capacitor (SC). Figure 15 As shown in the figure; considering 15 combinations of 4 groups of filters, the structures of the AC filter configurations from complex to simple are: 4×DT, 3×DT+1×ST, 3×DT+1×SC, 2×DT+2×ST, 2×DT+1×ST+1×SC, 2×DT+2×SC, 1×DT+3×ST, 1×DT+2×ST+1×SC, 1×DT+1×ST+2×SC, 1×DT+3×SC, 4×ST, 3×ST+1×SC, 2×ST+2×SC, 1×ST+3×SC, 4×SC.

[0113] The process of determining the AC filter configuration solution is as follows: Figure 16 As shown, the standard for AC filter configuration is to minimize reactive power compensation and keep the structure of the AC filter configuration as simple as possible, while ensuring that harmonics at all power levels meet power quality requirements. Therefore, the AC filter configurations for a given reactive power compensation level are connected to the AC busbar of the offshore wind farm, and the harmonic voltages at different power levels are calculated to determine whether a suitable AC filter configuration exists for that reactive power compensation level. If so, the simplest solution is determined. By varying the reactive power compensation level, all suitable AC filter configurations are compared, and the solution with the smallest reactive power compensation level is selected as the AC filter configuration for the offshore wind power transmission system.

[0114] For this implementation, the AC filter configuration for the 24-pulse DRU offshore wind power transmission system at 100Hz is 4 SCs with a total fundamental wave capacity of 240Mvar. Calculate the AC bus harmonic voltage before and after installing the AC filter and compare them. Figure 17 As shown in the figure, after installing the AC filter, all odd and even harmonics are reduced to within the specified limits, with THD (total harmonic distortion) dropping from 8.16% to 2.01%, and THFF (telephone harmonic form factor) dropping from 0.63% to 0.17, meeting power quality standards. Calculation results show that the AC filter configuration scheme proposed in this paper can control the harmonics of the DRU-based offshore wind power transmission system within the power quality requirements, ensuring the normal operation of the power system.

[0115] The above description of the embodiments is intended to facilitate understanding and application of the present invention by those skilled in the art. It is apparent that those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without requiring creative effort. Therefore, the present invention is not limited to the above embodiments. Any improvements or modifications made by those skilled in the art based on the disclosure of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for configuring an AC filter for an offshore wind power medium frequency uncontrolled rectifier DC transmission system, characterized in that: The steps include: (1) Modeling of offshore wind farms at various harmonic frequencies is achieved by using the Norton circuit as the equivalent circuit of the wind turbines in the offshore wind farm and the π-shaped circuit as the equivalent circuit of the AC submarine cable in the offshore wind farm. Based on these equivalent circuits, an equivalent model of the offshore wind farm suitable for harmonic analysis is established. The equivalent model consists of a first equivalent current source I1 and an equivalent impedance Z in parallel, one end of which is grounded and the other end is connected to the AC busbar of the offshore converter station. (2) A DRU equivalent model suitable for harmonic analysis is established for the AC side of the DRU in the offshore converter station. The model consists of a second equivalent current source I2, and the DC side of the DRU is equivalent to the DC loop impedance. (3) The iterative method is used to calculate the harmonic currents injected by the DRU into the offshore AC system. The harmonic currents correspond to the current phasors of the second equivalent current source I2 at each harmonic frequency. (4) According to the equivalent model parameters of the offshore wind farm and DRU, the harmonic voltage of the PCC node at each harmonic frequency is calculated by the following expression; Among them: U PCC,h represents the harmonic voltage of the PCC node at the harmonic frequency h·f0, U PCC,h,1 It represents the effective value of the hth harmonic voltage of the AC bus voltage when only the first equivalent current source I1 is considered, U PCC,h,2 It represents the effective value of the hth harmonic voltage of the AC bus voltage when only the second equivalent current source I2 is considered, where h is a natural number greater than 0, and k pcc,h is a given coefficient; k b k represents the number of six-pulsation valves in series in the DRU, DRU Indicates the transformation ratio of the DRU converter transformer, X tD It represents the leakage reactance of the DRU converter transformer at the fundamental frequency f0, U dc0 Indicates the rated DC voltage of the DRU, I dc,0 Indicates the initial value of the DRU's DC current iteration, It represents the hth harmonic voltage phasor of the AC bus voltage when only the second equivalent current source I2 is considered. represents the current phasor of the first equivalent current source I1 at the harmonic frequency h·f0, represents the current phasor of the second equivalent current source I2 at the harmonic frequency h·f0, Z owf,h Indicates the impedance value of the equivalent impedance Z at the harmonic frequency h·f0, Z filt,h Indicates the harmonic impedance of the AC filter installed and connected to the AC bus at the harmonic frequency h·f0; (5) Calculate the distortion rate of a single harmonic, the total effective harmonic distortion rate, and the telephone harmonic waveform factor based on the harmonic voltage at the PCC node, and compare them with the power quality standards. If they meet the standards, there is no need to install an AC filter; if they do not meet the standards, install an AC filter of appropriate capacity and type.

2. The method for configuring an AC filter for an offshore wind power medium frequency uncontrolled rectifier DC transmission system according to claim 1, characterized in that: The numerical expressions of the first equivalent current source I1 and the equivalent impedance Z are as follows: Where: Z owf,h The impedance value of the equivalent impedance Z at the harmonic frequency h·f0 is expressed as follows: represents the current phasor of the first equivalent current source I1 at the harmonic frequency h·f0, Y owf,h represents the node admittance matrix of the offshore AC system at the harmonic frequency h·f0, V owf Is a column vector, the first element of which is 1 and the rest are 0; I owf,h is a column vector, in which the 2nd to nth wt +1 element corresponds to the hth harmonic current of each wind turbine in the offshore wind farm, and the rest of the elements are 0; T Indicates transposition, h is a natural number greater than 0, f0 is the fundamental frequency, n wt is the number of wind turbines in the offshore wind farm.

3. The method for configuring an AC filter for an offshore wind power medium frequency uncontrolled rectifier DC transmission system according to claim 1, characterized in that: The DC loop impedance is composed of the DRU DC side impedance and other DC impedances in the DC transmission system in series, where the numerical expression of the DRU DC side impedance is as follows: Where: Z DRU,h,k k represents the impedance value of the DRU DC side impedance at the kth iteration under the harmonic frequency h·f0, b Indicates the number of six-pulsation valves in series in the DRU, μ k represents the commutation angle of the k-th iteration DRU, k DRU Indicates the transformation ratio of the DRU converter transformer, Z owf,h Indicates the impedance value of the equivalent impedance Z at the harmonic frequency h·f0, Z filt,h It represents the harmonic impedance of the AC filter installed on the AC bus at the harmonic frequency h·f0, j is an imaginary unit, h is a natural number greater than 0, X tD represents the leakage reactance of the DRU converter transformer at the fundamental frequency f0, and k is a natural number.

4. The method for configuring an AC filter for an offshore wind power medium frequency uncontrolled rectifier DC transmission system according to claim 1, characterized in that: The iterative method in step (3) is to calculate the time domain expression of the a-phase AC current of the DRU in each iteration, obtain the time domain expression of the bc two-phase AC current by phase shifting, and then obtain the harmonic currents injected by the DRU into the offshore AC system by Fourier transforming the time domain expression of the three-phase AC current of the DRU when the iteration converges.

5. The method for configuring an AC filter for an offshore wind power medium frequency uncontrolled rectifier DC transmission system according to claim 4, characterized in that: The DRU's DC current iteration initial value I dc,0 And the calculation expression of the initial value μ0 of the commutation angle iteration is as follows: Where: ∑P wt represents the sum of the active power of all wind turbines in the offshore wind farm, U dc0 Indicates the rated DC voltage of the DRU, k b Indicates the number of six-pulsation valves in series in the DRU, X tD It represents the leakage reactance of the DRU converter transformer at the fundamental frequency f0.

6. The method for configuring an AC filter for an offshore wind power medium frequency uncontrolled rectifier DC transmission system according to claim 1, characterized in that: In step (5), an AC filter of appropriate capacity and type is installed. The specific implementation method is as follows: first, multiple groups of AC filters are considered. The fundamental wave capacity of each group of filters is equal, and the upper limit of the total capacity is Q max and the lower limit Q min 40% and 10% of the DRU rated DC power, respectively, from Q min Start by gradually increasing the capacity, with the capacity increase step size ΔQ being 2% of the DRU rated DC power. The AC filter types include double-tuned filters, single-tuned filters, and shunt capacitors. After each fixed capacity, each group of AC filters traverses all type combinations, calculates the distortion rate of the single harmonic, the total effective harmonic distortion rate, and the telephone harmonic waveform coefficient under each combination configuration, and compares them with the power quality standard: If only one combination configuration meets the standard, the current capacity and type combination are used as the AC filter configuration scheme; if multiple combination configurations meet the standard, the type combination with the simplest structure is selected, and the current capacity and type combination are used as the AC filter configuration scheme; if no combination configuration meets the standard, increase the capacity and repeat the traversal judgment according to the above method.

7. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: The processor is used to execute the computer program to implement the AC filter configuration method for the offshore wind power medium-frequency uncontrolled rectifier DC transmission system according to any one of claims 1 to 6.

8. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, it implements the method for configuring an AC filter in an offshore wind power medium-frequency uncontrolled rectifier DC transmission system according to any one of claims 1 to 6.

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