A bridge arm current distribution method and system for reducing M3C bridge arm current peak value

Abstract

The application discloses a bridge arm current distribution method and system for reducing M3C bridge arm current peak value, comprising the following steps: constructing a current distribution model based on the relationship between the power frequency side line current, the frequency division side line current and the bridge arm current; optimizing the current distribution model based on the concept of sub-converter upper limit current; obtaining the selection principle of positive sequence current distribution coefficient based on the optimized current distribution model; proposing a solving method of the positive sequence current distribution coefficient with the minimum sub-converter upper limit current as the target based on the selection principle of the positive sequence current distribution coefficient; designing the solving process of the positive sequence current distribution coefficient in actual application based on the solving method of the positive sequence current distribution coefficient with the minimum sub-converter upper limit current as the target, obtaining the positive sequence current distribution coefficient, and determining the distribution scheme of each bridge arm current. The application can effectively reduce the bridge arm current peak value, significantly improve the overload capacity of the M3C converter station, reduce the investment and construction cost of the M3C converter station, and improve the economy of the whole system.

Description

Technical Field

[0001] This invention belongs to the field of electrical control technology and relates to a bridge arm current distribution method and system for reducing the peak value of M3C bridge arm current. Background Technology

[0002] For frequency-division transmission systems, the frequency converter is a key component. M3C is the optimal technical solution for the next-generation flexible frequency-division transmission system's frequency converter. However, the construction cost of M3C converter stations is enormous, and the total capacity of the converter valves is a significant factor influencing the economic cost of the station, as it is directly proportional to the peak current of the bridge arms. Therefore, improving the economic efficiency of converter station construction has always been a crucial issue facing the development of offshore frequency-division transmission systems.

[0003] Deficiencies and shortcomings of existing technologies: In some cases of offshore wind power connected to the grid via flexible DC transmission, when the wind farm output is high, the converter station is forced to operate in unity power factor mode to limit the arm current, thus losing its reactive power regulation capability. Furthermore, when the grid voltage drops due to a fault, the power transmission capacity of the M3C converter station also decreases proportionally. In research on offshore wind power grid connection, no scheme for optimizing the allocation of M3C arm current has been proposed. For frequency-division transmission, the capacity of the M3C converter station is a crucial factor affecting the economics of the entire frequency-division transmission system. Therefore, filling the research gap in M3C current peak reduction is of great significance for improving the economics of the entire offshore frequency-division transmission system. Summary of the Invention

[0004] The purpose of this invention is to solve the problem in the prior art where excessively high peak arm current affects the capacity of the converter valve in the M3C converter station, thus affecting the economic efficiency of the entire converter station. The invention provides a method and system for reducing the peak arm current of the M3C converter station.

[0005] To achieve the above objectives, the present invention employs the following technical solution:

[0006] A method for reducing the peak value of M3C bridge arm currents by allocating bridge arm currents includes:

[0007] A current distribution model is constructed based on the relationship between the power frequency side current, the frequency division side current and the bridge arm current.

[0008] Based on the concept of the upper limit current of the sub-converter, the current allocation model is optimized;

[0009] Based on the optimized current distribution model, the selection principle of the positive sequence current distribution coefficient is obtained;

[0010] Based on the selection principle of the positive sequence current allocation coefficient, a solution method for the positive sequence current allocation coefficient with the objective of minimizing the upper limit current of the sub-converter is proposed.

[0011] Based on the solution method of positive sequence current distribution coefficient with the goal of minimizing the upper limit current of sub-converters, a solution process for positive sequence current distribution coefficient in practical applications is designed to obtain the positive sequence current distribution coefficient and determine the distribution scheme of current for each bridge arm.

[0012] A further improvement of the present invention is that:

[0013] Based on the relationship between the power frequency line current, the frequency division line current, and the bridge arm current, a current distribution model is constructed; specifically:

[0014]

[0015] Among them, i xy i represents the current flowing through any bridge arm xy; x and i y Let x and y represent the single-phase line currents on the frequency division side and the power frequency side, respectively, where x represents one of the three phases u, v, and w on the power frequency side, and y represents one of the three phases a, b, and c on the frequency division side; ω s ω is the angular frequency of the power frequency grid. L This refers to the side angular frequency of the frequency divider network; This is the initial phase angle corresponding to the single-phase line current on the power frequency side; I is the initial phase angle corresponding to the single-phase line current on the frequency division side. mS and I mL These represent the amplitudes of the line currents on the power frequency side and the frequency division side, respectively; each bridge arm current contains two AC components of different frequencies.

[0016] The bridge arm current optimization allocation strategy with the goal of minimizing the peak value of the bridge arm current is described by formulas (2) and (3):

[0017] min{max(|i xy |)} (2)

[0018]

[0019] In the formula, max(|i xy |) represents the peak value of the current in the bridge arm xy, and (2) represents the objective function, that is, the objective function is to minimize the peak value of the bridge arm current; Let y represent the current flowing through the sub-converter y from the power frequency side, where y = u, v, w, representing the three phases on the power frequency side; the current flowing through sub-converter y is equal to the sum of the currents in the bridge arms ay, by, and cy, satisfying Kirchhoff's current law; similarly, Let x represent the current flowing through the sub-converter x divided from the frequency division side, where x = a, b, c, representing the three phases on the frequency division side; the value of the current flowing through the sub-converter x on the frequency division side is equal to the sum of the currents of the bridge arms xu, xv, and xw, satisfying Kirchhoff's current law.

[0020] Based on the concept of the upper limit current of the sub-converter, the current allocation model is optimized as follows:

[0021] For the sub-converter a, the currents in its bridge arms au, av, and aw are expressed as:

[0022]

[0023] Based on the expression for the bridge arm current, it is found that controlling the positive sequence component can avoid the impact on the M3C network-side performance, and adjusting the positive sequence component is determined to be the best optimization scheme.

[0024] The upper limit currents of sub-converters a, b, and c are respectively represented by i SCa i SCb and i SCc Represented as:

[0025]

[0026] Based on formula (5) and the M3C structure diagram, the optimization problem described by formulas (2) and (3) is simplified to the problem of minimizing the upper limit current of the sub-converter.

[0027] The M3C structure diagram includes nine bridge arms. The three-phase output of the M3C converter on the power frequency side is connected to the three-phase input on the frequency division side. Each bridge arm consists of a bridge arm reactor and n full-bridge modules connected in series. Starting from the frequency division side, the M3C converter is divided into three sub-converters a, b, and c; starting from the power frequency side, the M3C converter is divided into three sub-converters u, v, and w.

[0028] Based on the optimized current distribution model, the selection principle for the positive sequence current distribution coefficient is obtained, specifically: k SCx When the value of k satisfies the condition that the instantaneous value of the zero-sequence component of the sub-converter x is maximum, SCx Take the minimum, and when the instantaneous value of the zero-sequence component of the sub-converter x is minimized, k SCx This maximizes the reduction of the upper limit current of the sub-converter x.

[0029] Based on the selection principle of the positive sequence current allocation coefficient, a method for solving the positive sequence current allocation coefficient with the objective of minimizing the upper limit current of the sub-converter is proposed, as follows:

[0030] Based on the current component analysis of the sub-converter, and using the optimized allocation principle of "off-peak distribution" of positive-sequence and zero-sequence currents, the mathematical expression for the positive-sequence current allocation coefficient is constructed as follows:

[0031]

[0032] Where: k SCxM is the positive sequence current distribution coefficient, and β is the normalization coefficient; β is the DC common-mode component in the positive sequence current distribution coefficient. The relationship between M and β is:

[0033]

[0034] And it satisfies:

[0035]

[0036] Based on a method for solving the positive sequence current distribution coefficient with the objective of minimizing the upper limit current of the sub-converter, this paper designs a solution process for the positive sequence current distribution coefficient in practical applications, obtains the positive sequence current distribution coefficient, and determines the current distribution scheme for each bridge arm, specifically as follows:

[0037] Calculate the amplitude of the power frequency line current I mS and frequency divider side line current amplitude I mL ;

[0038] Based on the amplitude of the power frequency side current and the amplitude of the frequency division side line current, the current conversion ratio γ = I is obtained. mS / I mL ;

[0039] Based on the current conversion ratio, obtain the minimum current reduction coefficient η. min Then, the current reduction factor η is determined based on the power on both sides of the converter;

[0040] The current optimization allocation scheme on the power frequency side is determined based on the design margin of the current reduction factor η, the effective value of the bridge arm current, and the voltage ripple of the module capacitor.

[0041] Power frequency side line current amplitude I mS and frequency divider side line current amplitude I mL As shown in formula (9):

[0042]

[0043] In the formula, I mS I mL P represents the amplitude of the line current on the power frequency side and the frequency division side; S Q S P represents the active and reactive power on the power frequency side. L Q L For the active and reactive power on the frequency division side; V mS V mL These represent the phase voltage amplitudes on the power frequency side and the frequency division side.

[0044] Based on the amplitude of the power frequency side current and the amplitude of the frequency division side line current, the current conversion ratio γ = I is obtained. mS / I mL Based on the current conversion ratio, obtain the minimum current reduction coefficient η.min Then, the current reduction factor η is determined based on the power on both sides of the converter; taking sub-converter a as an example, the specific details are as follows:

[0045] For sub-converter a, the current distribution described by equation (6) is i SCa Represented as:

[0046]

[0047] i SCa The derivative with respect to time is:

[0048]

[0049] Setting its derivative to 0, we obtain i SCa The extreme points satisfy:

[0050]

[0051] According to formula (12), when M≥6γ, i SCa There is only one maximum point, that is At this point, the extreme point is called Peak I; at Peak I, i SCa The maximum value is:

[0052]

[0053] When M < 6γ, i SCa The maximum point becomes At this point, the extreme point is called Peak II, and at this point, i SCa The maximum value is:

[0054]

[0055] In summary, i SCa The maximum value is expressed as:

[0056]

[0057] Define the current reduction factor η as:

[0058]

[0059] According to equation (15), for Find the derivative of M:

[0060]

[0061] Taking the value of equation (17) as 0, after monotonicity analysis, we obtain... The expression for the minimum value is in the form of a piecewise function:

[0062]

[0063] From equation (16), when When the minimum value is taken, η is at its minimum; substituting (18) into equation (16) yields η. min The specific process for determining the current reduction factor η based on the power on both sides of the converter is as follows:

[0064]

[0065] in:

[0066]

[0067] In the formula, η temp I is the current reduction factor obtained based on the active and reactive power on the grid side; rated This refers to the rated value of the peak current of the sub-converter under defined active and reactive power control commands.

[0068] The current optimization allocation scheme on the power frequency side is determined based on the design margin of the current reduction factor η, the effective value of the bridge arm current, and the module capacitor voltage ripple. Specifically, the effective value of the bridge arm current is the amplitude of the bridge arm current. The design margin for module capacitor voltage ripple is taken as 10% of the capacitor's rated voltage; current optimization distribution is performed from the power frequency side, letting η = η S And according to η S The normalization coefficient M is derived in reverse; after obtaining M, β can be obtained from the relationship between M and β; once M and β are determined, the positive sequence current distribution coefficient of each sub-converter, that is, the distribution scheme of the bridge arm current, is also determined.

[0069]

[0070] Where: B=12[γ-(1+γ)η S ];η S The current reduction factor is denoted by S, and the subscript S indicates that the current optimization allocation scheme is performed from the power frequency side.

[0071] A bridge arm current distribution system for reducing the peak current of an M3C bridge arm includes:

[0072] The first construction module constructs a current distribution model based on the relationship between the power frequency side current, the frequency division side current and the bridge arm current.

[0073] The optimization module optimizes the current distribution model based on the concept of the upper limit current of the sub-converter.

[0074] The first acquisition module obtains the selection principle of the positive sequence current allocation coefficient based on the optimized current allocation model.

[0075] The second construction module is based on the selection principle of the positive sequence current allocation coefficient and constructs a method for solving the positive sequence current allocation coefficient with the goal of minimizing the upper limit current of the sub-converter.

[0076] The second acquisition module is based on the solution method of the positive sequence current allocation coefficient with the goal of minimizing the upper limit current of the sub-converter. It designs the solution process of the positive sequence current allocation coefficient in practical applications, obtains the positive sequence current allocation coefficient, and determines the allocation scheme of the current of each bridge arm.

[0077] Compared with the prior art, the present invention has the following beneficial effects:

[0078] This invention optimizes the constructed current distribution model and obtains the selection principle of the positive sequence current distribution coefficient. With the goal of minimizing the upper limit current of the sub-converter, the positive sequence current distribution coefficient is obtained, and the bridge arm current optimization distribution scheme from the power frequency side is determined. This invention can effectively reduce the peak value of the bridge arm current. At the same time, the bridge arm current optimization distribution scheme significantly improves the overload capacity of the M3C converter station and reduces the investment and construction cost of the M3C converter station, thereby improving the economy of the entire system. Attached Figure Description

[0079] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0080] Figure 1 This is a diagram showing the grid connection structure of an offshore wind farm containing an M3C converter station via a frequency-division transmission system in this invention.

[0081] Figure 2 This is a circuit structure diagram of the M3C;

[0082] Figure 3 Taking sub-converter a as an example, the relationship between the upper limit current of the sub-converter and the instantaneous arm current is shown.

[0083] Figure 4 A block diagram of the control system for optimizing current distribution from the power frequency side;

[0084] Figure 5 A flowchart of a bridge arm current distribution method to reduce the peak value of M3C bridge arm current;

[0085] Figure 6 A structural diagram of a bridge arm current distribution system for reducing the peak value of M3C bridge arm current;

[0086] Figure 7 The system architecture diagram for the simulation example;

[0087] Figure 8 The simulation waveforms are shown when the current optimization allocation strategy is not applied; among them... Figure 8 (a) is a schematic diagram of the three-phase line current on the frequency division side; Figure 8 (b) is a schematic diagram of the three-phase line current on the power frequency side; Figure 8 (c) is a schematic diagram of active and reactive power on the power frequency side and the frequency division side; Figure 8 (d) is a schematic diagram of the bridge arm current of sub-converter a; Figure 8 (e) is a schematic diagram of the power frequency circulating current component; Figure 8 (f) is a schematic diagram of the bridge arm current of sub-converter b; Figure 8 (g) is a schematic diagram of the average module capacitor voltage of the bridge arm of sub-converter a; Figure 8 (h) is a schematic diagram of the bridge arm current of sub-converter c;

[0088] Figure 9 The simulation waveforms are shown for optimizing current distribution in sub-converters a, b, and c on the power frequency side; among them, Figure 9 (a) is a schematic diagram of the three-phase line current on the frequency division side; Figure 9 (b) is a schematic diagram of the three-phase line current on the power frequency side; Figure 9 (c) is a schematic diagram of active and reactive power on the power frequency side and the frequency division side; Figure 9 (d) is a schematic diagram of the bridge arm current of sub-converter a; Figure 9 (e) is a schematic diagram of the power frequency circulating current component; Figure 9 (f) is a schematic diagram of the bridge arm current of sub-converter b; Figure 9 (g) is a schematic diagram of the average module capacitor voltage of the bridge arm of sub-converter a; Figure 9 (h) is a schematic diagram of the bridge arm current of sub-converter c. Detailed Implementation

[0089] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0090] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0091] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0092] In the description of the embodiments of the present invention, it should be noted that if terms such as "upper," "lower," "horizontal," or "inner" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of the invention is in use, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. Furthermore, terms such as "first" and "second" are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0093] Furthermore, the use of the term "horizontal" does not imply that the component must be absolutely horizontal, but rather that it can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal than "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0094] In the description of the embodiments of the present invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in the present invention according to the specific circumstances.

[0095] The present invention will now be described in further detail with reference to the accompanying drawings:

[0096] refer to Figure 1 A multi-frequency AC hybrid system comprises a power frequency system, a frequency division system, and an M3C converter. The frequency division system can be a frequency division power generation system and a frequency division power network. The power frequency system is located on land. The electricity generated by the frequency division power generation system is transmitted to land via a step-up substation and transmission lines. The frequency converter substation changes the frequency of the electricity generated by the frequency division power generation system from 50 / 3 Hz to 50 Hz, and finally integrates it into the land-based power frequency grid.

[0097] refer to Figure 2 The M3C structure diagram includes nine bridge arms. The three-phase output of the M3C converter on the power frequency side is connected to the three-phase input on the frequency division side. Each bridge arm consists of a bridge arm reactor and n full-bridge modules connected in series. Starting from the frequency division side, the M3C converter is divided into three sub-converters a, b, and c; starting from the power frequency side, the M3C converter is divided into three sub-converters u, v, and w.

[0098] refer to Figure 3 Taking sub-converter a as an example, the relationship between the upper limit current of the sub-converter and the instantaneous arm current is explained. Specifically, the mathematical expression for the upper limit current of the sub-converter is the sum of the magnitude of the positive-sequence component and the instantaneous value of the zero-sequence component. Figure 3 It can be seen that, taking sub-converter a as an example, the upper limit current i of sub-converter a is... SCa The physical meaning is the bridge arm current i au i av and i aw The upper envelope of .

[0099] refer to Figure 4 and Figure 5 This invention discloses a bridge arm current distribution method for reducing the peak value of M3C bridge arm current, comprising:

[0100] S101. Based on the relationship between the power frequency side current, the frequency division side current and the bridge arm current, a current distribution model is constructed.

[0101]

[0102] Among them, i xy i represents the current flowing through any bridge arm xy; x and i y Let x and y represent the single-phase line currents on the frequency division side and the power frequency side, respectively, where x represents one of the three phases u, v, and w on the power frequency side, and y represents one of the three phases a, b, and c on the frequency division side; ω s ω is the angular frequency of the power frequency grid. L This refers to the side angular frequency of the frequency divider network; This is the initial phase angle corresponding to the single-phase line current on the power frequency side; I is the initial phase angle corresponding to the single-phase line current on the frequency division side. mS and I mL These represent the amplitudes of the line currents on the power frequency side and the frequency division side, respectively; each bridge arm current contains two AC components of different frequencies;

[0103] The bridge arm current optimization allocation strategy with the goal of minimizing the peak value of the bridge arm current is described by formulas (2) and (3):

[0104] min{max(|i xy |)} (2)

[0105]

[0106] In the formula, max(|i xy |) represents the peak value of the current in the bridge arm xy, and (2) represents the objective function, that is, the objective function is to minimize the peak value of the bridge arm current; Let y represent the current flowing through the sub-converter y from the power frequency side, where y = u, v, w, representing the three phases on the power frequency side; the current flowing through sub-converter y is equal to the sum of the currents in the bridge arms ay, by, and cy, satisfying Kirchhoff's current law; similarly, Let x represent the current flowing through the sub-converter x divided from the frequency division side, where x = a, b, c, representing the three phases on the frequency division side; the value of the current flowing through the sub-converter x on the frequency division side is equal to the sum of the currents of the bridge arms xu, xv, and xw, satisfying Kirchhoff's current law.

[0107] S102 optimizes the current distribution model based on the concept of the upper limit current of the sub-converter.

[0108] For the sub-converter a, the currents in its bridge arms au, av, and aw are expressed as:

[0109]

[0110] Based on the expression for the bridge arm current, it is found that controlling the positive sequence component can avoid the impact on the M3C network-side performance, and adjusting the positive sequence component is determined to be the best optimization scheme.

[0111] The upper limit currents of sub-converters a, b, and c are respectively represented by i SCa i SCb and i SCc Represented as:

[0112]

[0113] Based on formula (5) and the M3C structure diagram, the optimization problem described by formulas (2) and (3) is simplified to the problem of minimizing the upper limit current of the sub-converter.

[0114] S103, based on the optimized current distribution model, obtain the selection principle of the positive sequence current distribution coefficient.

[0115] k SCx When the value of k satisfies the condition that the instantaneous value of the zero-sequence component of the sub-converter x is maximum, SCx Take the minimum, and when the instantaneous value of the zero-sequence component of the sub-converter x is minimized, k SCx This maximizes the reduction of the upper limit current of the sub-converter x.

[0116] S104. Based on the selection principle of the positive sequence current allocation coefficient, a method for solving the positive sequence current allocation coefficient with the objective of minimizing the upper limit current of the sub-converter is constructed.

[0117] Based on the current component analysis of the sub-converter, and using the optimized allocation principle of "off-peak distribution" of positive-sequence and zero-sequence currents, the mathematical expression for the positive-sequence current allocation coefficient is constructed as follows:

[0118]

[0119] Where: k SCx Here, M is the positive sequence current distribution coefficient, M is the normalization coefficient, and β is the DC common-mode component in the positive sequence current distribution coefficient. The relationship between M and β is:

[0120]

[0121] And it satisfies:

[0122]

[0123] S105, based on the solution method of positive sequence current allocation coefficient with the goal of minimizing the upper limit current of sub-converters, designs the solution process of positive sequence current allocation coefficient in practical applications, obtains the positive sequence current allocation coefficient, and determines the allocation scheme of current for each bridge arm.

[0124] S105.1: Calculate the amplitude of the power frequency line current I mS and frequency divider side line current amplitude I mL ;

[0125]

[0126] In the formula, I mS I mL P represents the amplitude of the line current on the power frequency side and the frequency division side; S Q S P represents the active and reactive power on the power frequency side. L Q L For the active and reactive power on the frequency division side; V mS V mL These represent the phase voltage amplitudes on the power frequency side and the frequency division side.

[0127] S105.2: Obtain the current conversion ratio γ = I based on the amplitude of the power frequency side current and the amplitude of the frequency division side line current. mS / I mL ;

[0128] S105.3: Obtain the minimum current reduction coefficient η based on the current conversion ratio. min Then, the current reduction factor η is determined based on the power on both sides of the converter;

[0129] For sub-converter a, the current distribution described by equation (6) is i SCa Represented as:

[0130]

[0131] i SCa The derivative with respect to time is:

[0132]

[0133] Setting its derivative to 0, we obtain i SCa The extreme points satisfy:

[0134]

[0135] According to formula (12), when M≥6γ, i SCa There is only one maximum point, that is At this point, the extreme point is called Peak I; at Peak I, i SCa The maximum value is:

[0136]

[0137] When M < 6γ, i SCa The maximum point becomes At this point, the extreme point is called Peak II, and at this point, i SCa The maximum value is:

[0138]

[0139] In summary, i SCa The maximum value is represented as:

[0140]

[0141] Define the current reduction factor η as:

[0142]

[0143] According to equation (15), for Find the derivative of M:

[0144]

[0145] Taking the value of equation (17) as 0, after monotonicity analysis, we obtain... The expression for the minimum value is in the form of a piecewise function:

[0146]

[0147] From equation (16), when When the minimum value is taken, η is at its minimum; substituting (18) into equation (16) yields η. min The specific process for determining the current reduction factor η based on the power on both sides of the converter is as follows:

[0148]

[0149] in:

[0150]

[0151] In the formula, η temp I is the current reduction factor obtained based on the active and reactive power on the grid side; rated This refers to the rated value of the peak current of the sub-converter under defined active and reactive power control commands.

[0152] S105.4: Determine the current optimization allocation scheme on the power frequency side based on the design margin of the current reduction factor η, the effective value of the bridge arm current and the voltage ripple of the module capacitor.

[0153] The effective value of the bridge arm current is the amplitude of the bridge arm current. The design margin for module capacitor voltage ripple is taken as 10% of the capacitor's rated voltage; current optimization distribution is performed from the power frequency side, letting η = η S And according to η S The normalization coefficient M is derived in reverse.

[0154]

[0155] Where: B=12[γ-(1+γ)η S ];η S The current reduction factor is denoted by S, and the subscript S indicates that the current optimization allocation scheme is performed from the power frequency side.

[0156] After obtaining M, β can be calculated from equation (7); substituting M and β into equation (6) yields the positive sequence current distribution coefficient k. SCa k SCb and k SCc Once the positive sequence current distribution coefficient is determined, the current distribution scheme for each bridge arm is also determined.

[0157] See Figure 6 This invention discloses a bridge arm current distribution system for reducing the peak current of an M3C bridge arm, comprising:

[0158] The first construction module constructs a current distribution model based on the relationship between the power frequency side current, the frequency division side current and the bridge arm current.

[0159] The optimization module optimizes the current distribution model based on the concept of the upper limit current of the sub-converter.

[0160] The first acquisition module obtains the selection principle of the positive sequence current allocation coefficient based on the optimized current allocation model.

[0161] The second construction module is based on the selection principle of the positive sequence current allocation coefficient and constructs a method for solving the positive sequence current allocation coefficient with the goal of minimizing the upper limit current of the sub-converter.

[0162] The second acquisition module is based on the solution method of the positive sequence current allocation coefficient with the goal of minimizing the upper limit current of the sub-converter. It designs the solution process of the positive sequence current allocation coefficient in practical applications, obtains the positive sequence current allocation coefficient, and determines the allocation scheme of the current of each bridge arm.

[0163] Example 1

[0164] See Figure 7 Assuming the total installed capacity of the offshore wind farm is 400MW, consisting of 100 4MW permanent magnet synchronous wind turbines, but for the purpose of simplifying the model structure and reducing the computational burden, a model of "single wind turbine + single full-power converter" is adopted as the equivalent model of the wind farm, and the line parameters of the power collection system are ignored. The head converter is equivalent to a two-level voltage source converter.

[0165] Table 1: Specific parameters of the simulation examples:

[0166]

[0167] The active power transmitted through the M3C is 270MW, while both sides operate at unity power factor, meaning the reactive power is 0. At this point, calculations show that the line current amplitudes on the power frequency side and the sub-frequency side are 1kA and 2kA, respectively. Figure 8 and Figure 9 Simulation waveforms of current allocation without optimization and current allocation optimized from the power frequency side are shown respectively. The objective of the current allocation optimization strategy is set to minimize the peak current of the bridge arm.

[0168] Comparison Figure 8 Subgraphs (a), (b), and (c) and Figure 9 As shown in sub-figures (a), (b), and (c), the active power through the M3C is 270MW in both simulation tests, while the line current amplitudes on the power frequency side and the frequency division side are 1kA and 2kA, respectively, which are not affected by the current optimization allocation strategy. Figure 8 Subgraphs (d), (f), and (h) and Figure 9 Subgraphs (d), (f), and (h) illustrate the currents of each sub-converter before and after implementing the current optimization allocation strategy from the power frequency side. Under the power frequency side current optimization allocation strategy, the currents of each sub-converter maintain good symmetry. Figure 8 In the middle, the peak current of the bridge arm is 1.0kA, while... Figure 9 The value is 0.805kA, a reduction of 19.5%. This means that the total capacity of the converter valves can also be reduced proportionally, thus significantly reducing the construction cost of the converter station. Due to the high voltage level and small line current amplitude on the power frequency side, optimizing the current distribution from the sub-converters a, b, and c on the power frequency side can reduce even more of the bridge arm current peaks.

[0169] Comparison Figure 8 subgraph (e) and Figure 9 Subgraph (e) illustrates the circulating current component that needs to be injected into the M3C to achieve optimized current distribution. Figure 8 In (e), since no current optimization allocation strategy was adopted, the circulating current component is 0. However, Figure 9 In (e), the circulating current component appears in the power frequency synchronous coordinate system, and its frequency is twice the frequency of the grid on the frequency division side.

[0170] Figure 8 (g) and Figure 9 (g) The amplitudes of the average module capacitor voltages in each arm of sub-converter a were compared before and after adopting the current optimization distribution strategy. It can be seen that the amplitudes of the capacitor voltage ripple are all between 2 and 3 kV. In the scenario where the current optimization distribution strategy is adopted, the amplitude of the capacitor voltage ripple is relatively large.

[0171] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for reducing the peak value of M3C bridge arm current by allocating bridge arm currents, characterized in that, include: A current distribution model is constructed based on the relationship between the power frequency side current, the frequency division side current and the bridge arm current. Based on the concept of the upper limit current of the sub-converter, the current allocation model is optimized; Based on the optimized current distribution model, the selection principle of the positive sequence current distribution coefficient is obtained; Based on the selection principle of the positive sequence current allocation coefficient, a solution method for the positive sequence current allocation coefficient with the objective of minimizing the upper limit current of the sub-converter is proposed. Based on the method for solving the positive sequence current distribution coefficient with the objective of minimizing the upper limit current of the sub-converter, this paper designs a solution process for the positive sequence current distribution coefficient in practical applications, obtains the positive sequence current distribution coefficient, and determines the current distribution scheme for each bridge arm, specifically as follows: Calculate the amplitude of the power frequency line current. I mS and frequency divider side line current amplitude I mL ; The current conversion ratio is obtained based on the amplitude of the power frequency side current and the amplitude of the frequency division side line current. γ = I mS / I mL ; Obtain the minimum current reduction factor based on the current conversion ratio. η min Then, determine the current reduction factor based on the power on both sides of the converter. η ; Based on current reduction factor η The design margins of the effective value of the bridge arm current and the voltage ripple of the module capacitor determine the current optimization allocation scheme on the power frequency side. The amplitude of the power frequency side current I mS and frequency divider side line current amplitude I mL As shown in the following formula: In the formula, , These represent the amplitudes of the line currents on the power frequency side and the frequency division side. , These represent the active and reactive power on the power frequency side. , The active and reactive power on the frequency division side; , These represent the phase voltage amplitudes on the power frequency side and the frequency division side.

2. The bridge arm current distribution method for reducing the peak value of M3C bridge arm current according to claim 1, characterized in that, The current distribution model is constructed based on the relationship between the power frequency side current, the frequency division side current, and the bridge arm current; specifically: in, i xy This represents the current flowing through any bridge arm xy; i x and i y These represent the single-phase line currents on the frequency division side and the power frequency side, respectively, where x represents one of the three phases u, v, w on the power frequency side, and y represents one of the three phases a, b, c on the frequency division side; This refers to the side angular frequency of the power frequency grid; This refers to the side angular frequency of the frequency divider network; This is the initial phase angle corresponding to the single-phase line current on the power frequency side; This is the initial phase angle of the single-phase line current corresponding to the frequency division side; and These represent the amplitudes of the line currents on the power frequency side and the frequency division side, respectively; each bridge arm current contains two AC components of different frequencies; The bridge arm current optimization allocation strategy with the goal of minimizing the peak value of the bridge arm current is described by formulas (2) and (3): In the formula, (2) represents the peak value of the current in the bridge arm xy, and (3) represents the objective function, that is, the objective function is to minimize the peak value of the bridge arm current. Let y represent the current flowing through the sub-converter y from the power frequency side, where y = u, v, w, representing the three phases on the power frequency side; the current flowing through sub-converter y is equal to the sum of the currents in the bridge arms ay, by, and cy, satisfying Kirchhoff's current law; similarly, Let x represent the current flowing through the sub-converter x divided from the frequency division side, where x = a, b, c, representing the three phases on the frequency division side; the value of the current flowing through the sub-converter x on the frequency division side is equal to the sum of the currents of the bridge arms xu, xv, and xw, satisfying Kirchhoff's current law.

3. The bridge arm current distribution method for reducing the peak value of M3C bridge arm current according to claim 2, characterized in that, The current allocation model optimized based on the concept of the upper limit current of the sub-converter is as follows: For the sub-converter a, the currents in its bridge arms au, av, and aw are expressed as: Based on the expression for the bridge arm current, it is found that controlling the positive sequence component can avoid the impact on the M3C network-side performance, and adjusting the positive sequence component is determined to be the best optimization scheme. The upper limit currents of sub-converters a, b, and c are respectively represented by i SCa i SCb and i SCc Represented as: Based on formula (5) and the M3C structure diagram, the optimization problem described by formulas (2) and (3) is simplified to the problem of minimizing the upper limit current of the sub-converter; The M3C structure diagram includes nine bridge arms. The three-phase output of the M3C converter on the power frequency side is connected to the three-phase input on the frequency division side. Each bridge arm consists of a bridge arm reactor and n full-bridge modules connected in series. Starting from the frequency division side, the M3C converter is divided into three sub-converters a, b, and c; starting from the power frequency side, the M3C converter is divided into three sub-converters u, v, and w.

4. The method for reducing the peak value of M3C bridge arm current by allocating bridge arm current according to claim 3, characterized in that, The selection principle for the positive sequence current allocation coefficient based on the optimized current allocation model is as follows: k SCx When the value satisfies the condition that the instantaneous value of the zero-sequence component of the sub-converter x is at its maximum, k SCx Take the minimum, and when the instantaneous value of the zero-sequence component of sub-converter x is minimum, k SCx This maximizes the reduction of the upper limit current of the sub-converter x.

5. The method for reducing the peak value of M3C bridge arm current according to claim 4, characterized in that, Based on the selection principle of the positive sequence current allocation coefficient, a method for solving the positive sequence current allocation coefficient with the objective of minimizing the upper limit current of the sub-converter is proposed, specifically as follows: Based on the current component analysis of the sub-converter, and using the optimized allocation principle of "staggered distribution" of positive-sequence and zero-sequence currents, the mathematical expression for the positive-sequence current allocation coefficient is constructed as follows: in: k SCx M is the positive sequence current distribution coefficient, and M is the normalization coefficient; β For the DC common-mode component in the positive-sequence current distribution coefficient, the relationship between M and β is: And it satisfies:

6. The method for reducing the peak value of M3C bridge arm current distribution according to claim 1, characterized in that, The current conversion ratio is obtained based on the amplitude of the power frequency side current and the amplitude of the frequency division side line current. γ = I mS / I mL ; Obtain the minimum current reduction factor based on the current conversion ratio. η min Then, determine the current reduction factor based on the power on both sides of the converter. η Taking sub-converter a as an example, specifically: For sub-converter a, the current distribution described by equation (6) is as follows: Represented as: The derivative with respect to time is: Setting its derivative to 0, we obtain The extreme points satisfy: According to formula (12), when hour There is only one maximum point, that is This extreme point is called PeakI; At Peak I, The maximum value is: when hour, The maximum point becomes At this point, the extreme point is called Peak II. The maximum value is: In conclusion, The maximum value is represented as: Define current reduction factor η for: According to equation (15), beg The derivative: Taking the value of equation (17) as 0, after monotonicity analysis, we obtain... The expression for the minimum value is in the form of a piecewise function: From equation (16), when When taking the minimum value, η Minimum; Substituting (18) into equation (16) yields the result. η min Determine the current reduction factor based on the power on both sides of the converter. η The specific process is as follows: in: In the formula, The current reduction factor is obtained based on the active and reactive power on the grid side. This refers to the rated value of the peak current of the sub-converter under defined active and reactive power control commands.

7. The bridge arm current distribution method for reducing the peak value of M3C bridge arm current according to claim 6, characterized in that, The current reduction factor η The design margins for the effective value of the bridge arm current and the voltage ripple of the module capacitor determine the current optimization allocation scheme on the power frequency side. Specifically, the effective value of the bridge arm current is the amplitude of the bridge arm current. The design margin for module capacitor voltage ripple is 10% of the capacitor's rated voltage; current optimization distribution is performed from the power frequency side, making... η = η S And according to η S Reverse derivation of normalization coefficients M ; obtain M Later by M and β The relationship between them can be determined β ; when M and β Once determined, the positive sequence current distribution coefficient of each sub-converter, i.e. the distribution scheme of the arm current, is also determined accordingly. in: ; The current reduction factor is denoted by S, and the subscript S indicates that the current optimization allocation scheme is performed from the power frequency side.

8. A bridge arm current distribution system for reducing the peak value of M3C bridge arm current, based on the bridge arm current distribution method for reducing the peak value of M3C bridge arm current as described in claim 1, characterized in that, include: The first construction module constructs a current distribution model based on the relationship between the power frequency side current, the frequency division side current and the bridge arm current. The optimization module optimizes the current distribution model based on the concept of the upper limit current of the sub-converter. The first acquisition module obtains the selection principle of the positive sequence current allocation coefficient based on the optimized current allocation model. The second construction module is based on the selection principle of the positive sequence current allocation coefficient and constructs a method for solving the positive sequence current allocation coefficient with the goal of minimizing the upper limit current of the sub-converter. The second acquisition module is based on the solution method of the positive sequence current allocation coefficient with the goal of minimizing the upper limit current of the sub-converter. It designs the solution process of the positive sequence current allocation coefficient in practical applications, obtains the positive sequence current allocation coefficient, and determines the allocation scheme of the current of each bridge arm.

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