Power flow symmetric control method and system for high-voltage flexible direct-current system

Abstract

The invention discloses a power flow symmetrical control method and system for a high-voltage flexible direct-current system, and belongs to the technical field of flexible direct-current transmission, and the method comprises the steps: collecting the direct-current voltage, line power and circuit breaker state signals of each converter station in a direct-current power grid in real time; establishing a mirror symmetry relationship between the available margin of the line power and actual power distribution, and constructing a system symmetric comprehensive index SMPFE for quantitatively evaluating the symmetry of power flow distribution; and on the basis of an SMPFE index, an optimal droop control parameter is solved by adopting an NBI method, and the variable quantity of an output voltage reference value of the converter station is dynamically adjusted, so that autonomous equilibrium distribution of power is realized. According to the method, a mirror symmetry control mechanism of power headroom-actual allocation is provided, power allocation is dynamically matched with the real-time bearing capacity of the current converter and the direct-current line by minimizing the SMPFE index, power out-of-limit is avoided, the method is suitable for application scenes with frequent power fluctuation such as offshore wind power, and safe and stable operation of a multi-terminal direct-current system is effectively guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-voltage flexible direct current (DC) power grid operation control, and in particular relates to a method and system for symmetric power flow control of a high-voltage flexible DC system. Background Art

[0002] The control strategy of high-voltage flexible direct current (HVDC) transmission systems faces the critical challenge of a mismatch between rated parameters and actual dynamic operating conditions. Traditional control methods are primarily based on fixed droop coefficients and rated parameter designs. This static control paradigm struggles to adapt to asymmetric operating conditions commonly found in actual operation, such as line parameter differences, converter station commissioning and decommissioning, and renewable energy fluctuations. Existing evaluation indicators, such as the voltage deviation rate, can only reflect absolute deviations. They are unable to quantify the symmetry characteristics of system operation or characterize the dynamic matching relationship between power distribution and the real-time carrying capacity of the line, making it difficult to accurately assess the system's safety margin. To address this technical bottleneck, this paper innovatively proposes a dynamic symmetric control theory. By establishing a mirror-symmetric relationship between the available power margin and the actual distribution, a comprehensive SMPFE indicator with clear physical meaning is constructed, fundamentally resolving the core contradiction of the traditional method's mismatch between static benchmarks and dynamic demands. This theoretical breakthrough not only enables dynamic adaptation of power distribution and line capacity, but also provides a new solution for adaptive optimization control of multi-terminal direct current systems, particularly suitable for application scenarios with strong fluctuation characteristics, such as offshore wind power grid connection. Summary of the Invention

[0003] In order to solve the problems existing in the prior art, the present invention provides a method and system for symmetrical control of the power flow of a high-voltage flexible DC system, which can realize the distribution of the power flow of the high-voltage flexible DC system according to the symmetrical principle of available margin-actual distribution.

[0004] The technical problem to be solved by the present invention is achieved through the following technical solutions:

[0005] In a first aspect, a method for symmetric power flow control in a high-voltage flexible direct current system is provided, comprising:

[0006] Establish a mirror-symmetric relationship between the available margin of line power and the actual power allocation;

[0007] Construct the comprehensive index of power flow symmetry SMPFE of HVDC flexible system;

[0008] Establish a multi-objective optimization model for power flow symmetry of HVDC flexible system;

[0009] Based on the comprehensive symmetry index SMPFE, the above multi-objective optimization model is transformed into an index minimization problem;

[0010] The SMPFE minimization problem is solved using the NBI method, and the change of the DC voltage reference value is obtained as the control output;

[0011] The power flow of the high-voltage flexible DC system is symmetrically controlled based on the variation of the DC voltage reference value.

[0012] In combination with the first aspect, further, a mirror-symmetric relationship between the line power margin and the actual power allocation is established as follows:

[0013]

[0014] Among them, P L_mirror is the ideal line power value calculated according to the power-margin symmetry principle of the present invention, H Li is the i-th DC line (i=1,2,…,N dcl ) of the available power margin. N dcl is the number of HVDC lines in the HVDC Flexible system. L_mirror is the ideal value of VSC converter power calculated according to the power-margin symmetry principle of the present invention. i is the available power margin of the i-th VSC converter station (i=1,2,...,m). m is the number of converters using droop control in the HVDC flexible system.

[0015] In combination with the first aspect, further, the error comprehensive index SMPFE is expressed as:

[0016]

[0017] Among them, α, β and γ are the weight coefficients of the symmetry factor. mirror It is defined as the original voltage to achieve the goal of reducing the voltage deviation of the high-voltage flexible DC grid. It satisfies α+β+γ=1 and is adjusted in real time by the following formula:

[0018]

[0019] in, Assign the standard deviation to the line power, Represents the average line power. is the VSC converter power distribution standard deviation, Represents the average power of the VSC converter. Assign a standard deviation to the DC voltage, Represents the average DC voltage. This definition can achieve the following effects: For example, when line power distribution is severely unbalanced, the weight of α is automatically increased to strengthen line power symmetry control. The adaptive adjustment process of β and γ is similar.

[0020] Combined with the first aspect, a multi-objective optimization model for the power flow symmetry of the HVDC flexible system is established as follows:

[0021] In order to calculate the accuracy of the symmetrical power distribution of the lines of the present invention, the overall goal of achieving symmetrical power distribution of the DC lines according to the available power margin can be expressed as follows:

[0022]

[0023] Among them, P L,i and P Li_r are the power and rated value of the i-th DC line respectively. is the average rated value of the DC link power.

[0024] In combination with the first aspect, further, in order to calculate the accuracy of the symmetrical power distribution of the converter according to the present invention, the overall goal of achieving symmetrical power distribution of the converter according to the available power margin is expressed as follows:

[0025]

[0026] Among them, P VSC and are the actual power and the average value of the rated power of the converter respectively.

[0027] In combination with the first aspect, further, according to the principle of minimizing the deviation between the actual voltage and the rated voltage, the overall goal of achieving DC voltage control is expressed as follows:

[0028]

[0029] Among them, V dc,i and is the actual voltage and rated voltage of DC node i. N dc Represents the total number of DC nodes.

[0030] In combination with the first aspect, further AC-side constraints mainly include the dynamic balance relationship between key parameters such as the generator's active / reactive output, node voltage amplitude and phase angle, and line transmission power, and are established as follows:

[0031]

[0032] S ac,imin ≤S ac,i ≤S ac,imax ,i=1,2,…,N acl (10)

[0033] Among them, P g,i and P l,i represents the active power output of the generator and load at AC node i; Q g,i and Q l,i Respectively represent the reactive output of the generator and load connected to the AC node i; N g Indicates the total number of generators; Vac,i Represents the voltage amplitude of AC node i; V ac,imin and V ac,imax Represent the minimum and maximum voltage amplitudes of AC node i. ac,i represents the voltage phase angle at node i; δ ac,imin and δ ac,imax Represent the minimum and maximum values ​​of the voltage phase angle at node i. ac Indicates the total number of communication nodes; S ac,i represents the transmission power of AC line i; S ac,imin and S ac,imax Represent the minimum and maximum values ​​of the power transmitted by AC circuit i. acl Indicates the total number of AC lines.

[0034] In conjunction with the first aspect, during the operation of a high-voltage flexible DC system, power flow stability is a key factor affecting the safe operation of the system. Precise control of DC node voltages is particularly important during high-power transmission or when the system is subject to disturbances. A complete DC power flow constraint system is established as follows:

[0035]

[0036] Among them, P dc,i is the active power injected into DC node i; V dc,i is the voltage of DC node i; N dc Represents the total number of DC nodes. The upper and lower limits of voltage and current at each node in the HVDC flexible system can be expressed as:

[0037]

[0038] Among them, V dc,i Represents the voltage amplitude of DC node i; I dc,i Represents the current value of DC node i; N dc represents the total number of DC nodes. The DC line transmission power constraint can be expressed as:

[0039] P l,imin ≤P l,i ≤P l,imax ,i=1,2,…,N dcl (13)

[0040] Among them, P l,i represents the active power transmitted by the i-th DC line; N dcl represents the total number of DC lines in the HVDC flexible system. Arrange (4)-(13) above into a compact format,

[0041]

[0042] Among them, f1(X), f2(X),…, f3(X) are the optimization objective functions corresponding to line power, converter power and DC voltage, g1, g2 and g3 are the equality constraints on the AC side and DC side, respectively, and h is the inequality constraint.

[0043] In combination with the first aspect, further, the NBI method is used to solve the multi-objective optimization model in claim 4, as shown below:

[0044] If only the minimum value of the objective function f1(X) is considered, the optimal solution can be obtained The corresponding points are as follows:

[0045] f 1* (f1(x 1* ),f2(x 1* ),f3(x 1* ))(14)

[0046] Similarly, by only considering the minimization of f2(X) and f3(X), we can get the optimal solutions respectively. and Corresponding to f 2* (f1(x 2* ),f2(x 2* ),f3(x 2* )) and f 3* (f1(x 3* ),f2(x 3* ),f3(x 3* )). In the coordinate space formed by each objective function, point f 1* ,f 2* and f 3* The endpoints of the Pareto front, and the plane defined by them is called the Utopian plane.

[0047] In addition, when the objective function is scaled by different coefficients, the optimal point on the Pareto front can be determined by the following formula:

[0048]

[0049] Where d is the distance parameter; β is the vector of equidistant points; e = [1,1...1] T is a unit vector. As d increases, The objective function corresponding to the determined feasible solution is gradually improved. When d increases to the maximum value d max When , each objective function reaches Pareto optimality. Therefore, given the equidistant point vector β, the multi-objective optimization problem in (14) can be transformed into a single-objective optimization problem, that is, the goal is to maximize the distance between the points on the Utopia surface and the corresponding points on the Pareto front, as shown below:

[0050]

[0051] By varying the value of β, the multi-objective optimization problem is transformed into multiple single-objective optimization problems. Solving these problems using the original pairwise interior point method yields a set of uniformly distributed Pareto optimal solutions.

[0052] In combination with the first aspect, further, based on a set of uniformly distributed Pareto optimal solutions obtained by the NBI method, the process of obtaining the converter voltage reference value adjustment amount is as follows:

[0053] Substitute each set of solutions into the expression of the symmetry index SMPFE and take a set of solutions that satisfies the minimization of the index, as follows:

[0054] X_opt={X|X∈X NBI ,min(SMPFE)} (17)

[0055] Among them, X NBI is a set of uniformly distributed Pareto optima obtained by solving the optimization model using the NBI method; X_opt is the optimal solution that satisfies the minimization of the indicator. NBI is the adjustment value of the converter voltage reference value for droop control, which is expressed as follows:

[0056]

[0057] In a second aspect, the present invention further provides a control system for converter power and DC voltage, comprising:

[0058] A data acquisition module is used to obtain DC line power, converter power and DC voltage in the power grid;

[0059] Indicator construction module, used to construct the symmetric composite indicator SMPFE;

[0060] A solution module, used to solve the problem using the NBI method based on the symmetry comprehensive index SMPFE;

[0061] A control module, configured to obtain a DC voltage reference value adjustment amount of the converter based on a minimized SMPFE value;

[0062] The power flow of the high-voltage flexible DC system is symmetrically controlled based on the DC voltage reference value adjustment.

[0063] Beneficial technical effect: Beneficial technical effect: The present invention innovatively constructs a comprehensive index of power flow symmetry SMPFE by establishing a mirror-symmetric relationship between the power available margin and actual distribution of the high-voltage flexible DC system, and adopts the NBI method to solve the multi-objective optimization problem, thereby realizing accurate symmetrical control of the power flow of the high-voltage flexible DC system. This method dynamically optimizes the coordinated control of line power, converter power and DC voltage through an adaptive weight adjustment mechanism, significantly improving the balance of system power flow distribution and operational stability. The proposed NBI solution method effectively solves the trade-off problem in multi-objective optimization, and the obtained Pareto optimal solution set ensures the optimality of the control effect. The complete AC / DC constraint system and compact format optimization model greatly improve the computational efficiency and are suitable for real-time control of large-scale systems. The present invention realizes the coordinated optimization of AC / DC systems by comprehensively considering the dynamic balance of the AC side and the power flow control of the DC side. It can effectively cope with complex working conditions such as high-power transmission and system disturbances, and provides reliable guarantee for the safe and stable operation of the high-voltage flexible DC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 This is a single-line diagram of a multi-terminal flexible DC test system with five nodes;

[0065] Figure 2 This is a block diagram of the method for symmetrical power flow control of a high-voltage flexible DC system proposed in the present invention;

[0066] Figure 3a To verify the key DC line power diagram in Example 1;

[0067] Figure 3b To verify the DC voltage schematic diagram in Example 1;

[0068] Figure 3c This is a schematic diagram to verify the DC average voltage in Example 1;

[0069] Figure 3d To verify the converter power diagram in Example 1;

[0070] Figure 3e This is a schematic diagram of the symmetric comprehensive index SMPFE in Example 1;

[0071] Figure 4a To verify the DC line power schematic diagram in Example 2;

[0072] Figure 4b To verify the DC voltage schematic diagram in Example 2;

[0073] Figure 4c This is a schematic diagram to verify the DC average voltage in Example 2;

[0074] Figure 4d Schematic diagram of the converter power in verification example 2

[0075] Figure 4e Schematic diagram of the symmetric comprehensive index SMPFE in verification example 2. DETAILED DESCRIPTION

[0076] In order to make the objectives, technical solutions and advantages of the present application more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0077] Example 1

[0078] The present invention proposes a method for symmetrical power flow control of a high-voltage flexible DC system, establishes a mirror-symmetrical relationship between the available power margin and the actual power distribution, takes minimizing the corresponding symmetry index SMPFE as the optimization control goal, and uses the voltage reference point of the VSC converter based on droop control as the control variable to achieve symmetrical control of the power flow of the high-voltage flexible DC system.

[0079] The process of the HVDC flexible system power flow symmetry control method provided by the present invention is mainly as follows:

[0080] The mirror-symmetric relationship between the available power margin and the actual power allocation is established as follows:

[0081]

[0082] Among them, P L_mirror is the ideal line power value calculated according to the power-margin symmetry principle of the present invention, H Li is the i-th DC line (i=1,2,…,N dcl ) of the available power margin. N dcl is the number of HVDC lines in the HVDC Flexible system. L_mirror is the ideal value of VSC converter power calculated according to the power-margin symmetry principle of the present invention. i is the available power margin of the i-th VSC converter station (i=1,2,...,m). m is the number of converters using droop control in the HVDC system. Based on the above symmetry relationship, the comprehensive index of power flow symmetry of the HVDC system SMPFE is constructed as follows:

[0083]

[0084] Among them, α, β and γ are the weight coefficients of the symmetry factor. mirror It is defined as the original voltage to achieve the goal of reducing the voltage deviation of the high-voltage flexible DC grid. It satisfies α+β+γ=1 and is adjusted in real time by the following formula:

[0085]

[0086] in, Assign the standard deviation to the line power, Represents the average line power. is the VSC converter power distribution standard deviation, Represents the average power of the VSC converter. Assign a standard deviation to the DC voltage, Represents the average DC voltage. This definition can achieve the following effects: For example, when line power distribution is severely unbalanced, the weight of α is automatically increased to strengthen line power symmetry control. The adaptive adjustment process of β and γ is similar.

[0087] The multi-objective optimization model for HVDC system power flow symmetry is established as follows:

[0088] In order to calculate the accuracy of the symmetrical power distribution of the lines of the present invention, the overall goal of achieving symmetrical power distribution of the DC lines according to the available power margin can be expressed as follows:

[0089]

[0090] Among them, P L,i and P Li_r are the power and rated value of the i-th DC line respectively. is the average rated value of the DC link power.

[0091] In combination with the first aspect, further, in order to calculate the accuracy of the symmetrical power distribution of the converter according to the present invention, the overall goal of achieving symmetrical power distribution of the converter according to the available power margin is expressed as follows:

[0092]

[0093] Among them, P VSC and are the actual power and the average value of the rated power of the converter respectively.

[0094] In combination with the first aspect, further, according to the principle of minimizing the deviation between the actual voltage and the rated voltage, the overall goal of achieving DC voltage control is expressed as follows:

[0095]

[0096] Among them, V dc,i and is the actual voltage and rated voltage of DC node i. N dc Represents the total number of DC nodes.

[0097] In combination with the first aspect, further AC-side constraints mainly include the dynamic balance relationship between key parameters such as the generator's active / reactive output, node voltage amplitude and phase angle, and line transmission power, and are established as follows:

[0098]

[0099]

[0100] S ac,imin ≤S ac,i ≤S ac,imax ,i=1,2,…,N acl (10)

[0101] Among them, P g,i and P l,i represents the active power output of the generator and load at AC node i; Q g,i and Q l,i Respectively represent the reactive output of the generator and load connected to the AC node i; N g Indicates the total number of generators; V ac,i Represents the voltage amplitude of AC node i; V ac,imin and V ac,imax Represent the minimum and maximum voltage amplitudes of AC node i. ac,i represents the voltage phase angle at node i; δ ac,imin and δ ac,imax Represent the minimum and maximum values ​​of the voltage phase angle at node i. ac Indicates the total number of communication nodes; S ac,i represents the transmission power of AC line i; S ac,imin and S ac,imax Represent the minimum and maximum values ​​of the power transmitted by AC circuit i. acl Indicates the total number of AC lines.

[0102] In conjunction with the first aspect, during the operation of a high-voltage flexible DC system, power flow stability is a key factor affecting the safe operation of the system. Precise control of DC node voltages is particularly important during high-power transmission or when the system is subject to disturbances. A complete DC power flow constraint system is established as follows:

[0103]

[0104] Among them, P dc,i is the active power injected into DC node i; V dc,i is the voltage of DC node i; N dc Represents the total number of DC nodes. The upper and lower limits of voltage and current at each node in the HVDC flexible system can be expressed as:

[0105]

[0106] Among them, V dc,i Represents the voltage amplitude of DC node i; I dc,i Represents the current value of DC node i; N dcrepresents the total number of DC nodes. The DC line transmission power constraint can be expressed as:

[0107] P l,imin ≤P l,i ≤P l,imax ,i=1,2,…,N dcl (13)

[0108] Among them, P l,i represents the active power transmitted by the i-th DC line; N dcl represents the total number of DC lines in the HVDC flexible system. Arrange (4)-(13) above into a compact format,

[0109]

[0110] Among them, f1(X), f2(X),…, f3(X) are the optimization objective functions corresponding to line power, converter power and DC voltage, g1, g2 and g3 are the equality constraints on the AC side and DC side, respectively, and h is the inequality constraint.

[0111] The process of solving the SMPFE minimization problem using the NBI method and obtaining the DC voltage reference value change as the control output is as follows:

[0112] If only the minimum value of the objective function f1(X) is considered, the optimal solution can be obtained The corresponding points are as follows:

[0113] f 1* (f1(x 1* ),f2(x 1* ),f3(x 1* ))(14)

[0114] Similarly, by only considering the minimization of f2(X) and f3(X), we can get the optimal solutions respectively. and Corresponding to f 2* (f1(x 2* ),f2(x 2* ),f3(x 2* )) and f 3* (f1(x 3* ),f2(x 3* ),f3(x 3* )). In the coordinate space formed by each objective function, point f 1* ,f 2* and f 3* The endpoints of the Pareto front, and the plane defined by them is called the Utopian plane.

[0115] In addition, when the objective function is scaled by different coefficients, the optimal point on the Pareto front can be determined by the following formula:

[0116]

[0117] Where d is the distance parameter; β is the vector of equidistant points; e = [1,1...1] T is a unit vector. As d increases,

[0118] The objective function corresponding to the determined feasible solution is gradually improved. When d increases to the maximum value d max When , each objective function reaches Pareto optimality. Therefore, given the equidistant point vector β, the multi-objective optimization problem in (14) can be transformed into a single-objective optimization problem, that is, the goal is to maximize the distance between the points on the Utopia surface and the corresponding points on the Pareto front, as shown below:

[0119]

[0120] By varying the value of β, the multi-objective optimization problem is transformed into multiple single-objective optimization problems. Solving these problems using the original pairwise interior point method yields a set of uniformly distributed Pareto optimal solutions.

[0121] In combination with the first aspect, further, based on a set of uniformly distributed Pareto optimal solutions obtained by the NBI method, the process of obtaining the converter voltage reference value adjustment amount is as follows:

[0122] Substitute each set of solutions into the expression of the symmetry index SMPFE and take a set of solutions that satisfies the minimization of the index, as follows:

[0123] X_opt={X|X∈X NBI ,min(SMPFE)} (17)

[0124] Among them, X NBI is a set of uniformly distributed Pareto optima obtained by solving the optimization model using the NBI method; X_opt is the optimal solution that satisfies the minimization of the indicator. NBI is the adjustment value of the converter voltage reference value for droop control, which is expressed as follows:

[0125]

[0126] Finally, the HV flexible DC system power flow is symmetrically controlled based on the DC voltage reference value variation in (18).

[0127] Example 2

[0128] A high-voltage flexible direct current system power flow symmetry control system, characterized by comprising:

[0129] A data acquisition module is used to obtain DC line power, converter power and DC voltage in the power grid;

[0130] Indicator construction module, used to construct the symmetric composite indicator SMPFE;

[0131] A solution module, used to solve the problem using the NBI method based on the symmetry comprehensive index SMPFE;

[0132] A control module, configured to obtain a DC voltage reference value adjustment amount of the converter based on a minimized SMPFE value;

[0133] The power flow of the high-voltage flexible DC system is symmetrically controlled based on the DC voltage reference value adjustment.

[0134] Below we verify the effect of the present invention through two specific cases.

[0135] like Figure 1 As shown in the figure, a five-node multi-terminal flexible HVDC transmission system connected to an offshore wind farm (OWF) is used as a test model. In this system, offshore wind farms OWF-3 and OWF-5 are connected to the DC network via voltage source converters VSC-3 and VSC-5, respectively. The three AC grids AC-1, AC-2, and AC-4 use ideal voltage source models and are connected to VSC-1, VSC-2, and VSC-4, respectively, via impedances. The system adopts a hybrid control strategy: the converters connected to the wind farm (VSC-3 and VSC-5) operate in active power control mode. The converters connected to the AC grid (VSC-1, VSC-2, and VSC-4) use adaptive voltage droop control.

[0136] Verification Example 1

[0137] In Example 1, the symmetrical control method proposed in this invention is used to achieve symmetrical power flow control for all high-voltage flexible DC systems after a large power fluctuation occurs in the offshore wind turbine converter. Specifically, at t = 0.5s, the VSC-5 experiences a significant increase in active power from 500MW to 750MW due to a significant fluctuation in offshore wind power. The control method proposed in this invention is initiated at t = 1s. This includes key DC line power, DC voltage, DC average voltage, converter power, and the symmetry index SMPFE. Figure 3a The study showed that after a large power fluctuation in voltage source converter VSC-5, the power distribution on the two key DC lines connected to VSC-5 was severely asymmetric. Specifically, line 5-1 exceeded its power limit, while line 5-4 still had a large power margin. After the control method proposed in this invention was activated at t = 1 second, the power on the lines was symmetrically distributed according to their available margins. Figure 3b and Figure 3c It shows that after a large power fluctuation occurs in the voltage source converter VSC-5, the risk of voltage overlimit in the DC voltage curve increases sharply. The control method proposed in the present invention can make the system voltage close to the steady-state set value after startup, thereby avoiding the risk of voltage overlimit. Figure 4d It shows that after a large power fluctuation occurs in the voltage source converter VSC-5, the power distribution of VSC-1, VSC-2 and VSC-4 has serious asymmetry, which is specifically manifested in that: VSC-4 exceeds the power limit, while VSC-1 and VSC-2 still have a large power margin. After the control method proposed in the present invention is started at t=1s, the three realize symmetrical power distribution based on the available power margin, and all are stable below the limit value, without the risk of exceeding the limit. Figure 4e It shows that after the control method proposed in the present invention is started at t=1s, the established symmetry comprehensive index SMPFE is significantly reduced by 19 percentage points, verifying the effectiveness of the present invention in Example 1 for symmetric control of the power flow of the high-voltage flexible DC system.

[0138] Verification Example 2

[0139] In Example 2, the symmetrical control method proposed in this invention is used to achieve symmetrical power flow control for all HVDC systems after the grid-side converter is shut down. Specifically, VSC-2 is shut down at t = 0.5s, and the control method proposed in this invention is initiated at t = 1s. Figures 4a to 4e Results are displayed, including DC link power, DC voltage, DC average voltage, converter power, and the symmetry index SMPFE. Figure 4a It shows that after the voltage source converter VSC-2 is completely shut down, the line power distribution is seriously asymmetric. After the control method proposed in the present invention is started at t=1s, the line power is symmetrically distributed according to the respective available margins. Figure 4b and Figure 4c It shows that after the voltage source converter VSC-2 is completely shut down, the DC voltage curve increases significantly and each converter is at risk of voltage exceeding the limit. After the control method proposed in the present invention is started at t=1s, the system voltage is close to the steady-state set value, avoiding the risk of exceeding the limit. Figure 4d It shows that after the voltage source converter VSC-2 is completely shut down, the power distribution of the two voltage source converters VSC-1 and VSC-4 that are not shut down also has serious asymmetry. Specifically, VSC-4 has a power limit exceeding, while VSC-1 still has a large power margin. After the control method proposed in the present invention is started at t=1s, the two achieve symmetrical power distribution based on the available power margin. Figure 4eIt shows that after the control method proposed in the present invention is started at t=1s, the established symmetry comprehensive index SMPFE is significantly reduced by 21 percentage points, verifying the effectiveness of the present invention in Example 2 for symmetric control of the power flow of the high-voltage flexible DC system.

[0140] The present invention is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of the processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0141] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 a process or multiple processes and / or boxes Figure 1 The function specified in one or more boxes.

[0142] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0143] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A method for symmetrical power flow control of a high-voltage flexible DC system, characterized in that: include: Establish a mirror-symmetric relationship between the available margin of line power and the actual power allocation; Construct the comprehensive index of power flow symmetry SMPFE of HVDC flexible system; Establish a multi-objective optimization model for power flow symmetry of HVDC flexible system; Based on the symmetry comprehensive index SMPFE, the above multi-objective optimization model is transformed into an index minimization problem; The NBI method is used to solve the symmetry comprehensive index SMPFE minimization problem, and the DC voltage reference value change is obtained as the control output; The power flow of the high-voltage flexible DC system is symmetrically controlled based on the variation of the DC voltage reference value.

2. A method for symmetrical power flow control of a high-voltage flexible DC system according to claim 1, characterized in that: The mirror-symmetric relationship between the available margin of line power and the actual power allocation is established as follows: Among them, P L_mirror is the ideal line power value calculated according to the power-margin symmetry principle of the present invention, H Li is the i-th DC line (i=1,2,…,N dcl ) of the available power margin, N dcl is the number of HVDC lines in the HVDC Flexible system, P L_mirror is the ideal value of VSC converter power calculated based on the power-margin symmetry principle, H i is the available power margin of the i-th VSC converter station (i=1,2,...,m), and m is the number of converters using droop control in the HVDC flexible system.

3. The method for symmetrical power flow control of a high-voltage flexible DC system according to claim 1, characterized in that: The error comprehensive index SMPFE expression is: SMPFE=α(P L -P L_mirror ) / P L r +β(P VSC -P VSC_mirror ) / P r +γ(VV mirror ) / V r (2) Among them, α, β and γ are the symmetry factor weight coefficients, V mirror It is defined as the original voltage, α+β+γ=1 and is adjusted in real time by the following formula: in, Assign the standard deviation to the line power, represents the average line power, is the VSC converter power distribution standard deviation, represents the average power of the VSC converter, Assign a standard deviation to the DC voltage, Represents the average DC voltage.

4. The method for symmetrical power flow control of a high-voltage flexible DC system according to claim 1, characterized in that: A multi-objective optimization model for the power flow symmetry of the HVDC system is established, as follows: The overall goal of achieving symmetrical power distribution in DC lines based on the available power margin is as follows: Among them, P L,i and P Li_r are the power and rated value of the i-th DC line respectively, is the average rated value of the DC line power; The overall goal of achieving symmetrical converter power distribution based on the available power margin is as follows: Among them, P VSC and are the actual power and the average value of the rated power of the converter respectively; According to the principle of minimizing the deviation between the actual voltage and the rated voltage, the overall goal of achieving DC voltage control is expressed as follows: Where V dc,i and is the actual voltage and rated voltage of DC node i, N dc represents the total number of DC nodes; Under fault conditions, the generator sets switch to power flow control mode. The dynamic balance relationship between the AC side constraints, including the active / reactive output of the generators, the node voltage amplitude and phase angle, and the key parameters of the line transmission power, is as follows: Among them, P g,i and P l,i represents the active power output of the generator and load at AC node i; Q g,i and Q l,i Respectively represent the reactive output of the generator and load connected to the AC node i; N g Indicates the total number of generators; V ac,i Represents the voltage amplitude of AC node i; V ac,imin and V ac,imax Represent the minimum and maximum voltage amplitudes of AC node i, δ ac,i represents the voltage phase angle at node i; δ ac,imin and δ ac,imax Represent the minimum and maximum values ​​of the voltage phase angle at node i, respectively, N ac Indicates the total number of communication nodes; S ac,i represents the transmission power of AC line i; S ac,imin and S ac,imax Represent the minimum and maximum values ​​of the AC circuit i transmission power, N acl Indicates the total number of AC lines; A complete DC power flow constraint system is established, as shown in formula (11): Among them, P dc,i is the active power injected into DC node i; V dc,i is the voltage of DC node i; N dc represents the total number of DC nodes. The upper and lower limits of voltage and current at each node in the HVDC flexible system are expressed as: Among them, V dc,i Represents the voltage amplitude of DC node i; I dc,i Represents the current value of DC node i; N dc represents the total number of DC nodes, and the DC line transmission power constraint is expressed as: P l,imin ≤P l,i ≤P l,imax ,i=1,2,…,N dcl (13) Among them, P l,i represents the active power transmitted by the i-th DC line; N dcl Indicates the total number of DC lines in the HVDC-flexible system; Arrange equations (4)-(13) into a compact format: Among them, f1(X), f2(X),…, f3(X) are the optimization objective functions corresponding to line power, converter power and DC voltage, g1, g2 and g3 are the equality constraints on the AC side and DC side, respectively, and h is the inequality constraint.

5. The method for symmetrical power flow control of a high-voltage flexible DC system according to claim 3 is characterized in that: The NBI method is used to solve the multi-objective optimization model of the HVDC system power flow symmetry, as shown below: Considering the minimum value of the objective function f1(X), we can get the optimal solution. The corresponding points are as follows: f 1* (f1(x 1* ),f2(x 1* ),f3(x 1* ))(14) Consider minimizing f2(X) and f3(X), and get the optimal solutions respectively and Corresponding to f 2* (f1(x 2* ),f2(x 2* ),f3(x 2* )) and f 3* (f1(x 3* ),f2(x 3* ),f3(x 3* )), in the coordinate space formed by each objective function, point f 1* ,f 2* and f 3* The endpoints of the Pareto front are called the Utopian plane. When the objective function is scaled by different coefficients, the optimal point on the Pareto front is determined by the following formula: Where d is the distance parameter; β is a vector of equidistant points; e = [1,1...1] T is a unit vector. As d increases, The objective function corresponding to the determined feasible solution is gradually improved. When d increases to the maximum value d max When , each objective function reaches Pareto optimality. Given the equidistant point vector β, the multi-objective optimization problem in Equation (14) is transformed into a single-objective optimization problem, that is, the goal is to maximize the distance between the points on the Utopia surface and the corresponding points on the Pareto front, as shown below: By changing the value of β, the multi-objective optimization problem is transformed into multiple single-objective optimization problems, and a set of evenly distributed Pareto optimal solutions are obtained using the original pairwise interior point method.

6. A method for symmetrical control of power flow in a high-voltage flexible DC system according to claim 5, characterized in that: The process of obtaining the converter voltage reference value adjustment amount based on a set of uniformly distributed Pareto optimal solutions obtained by the NBI method is as follows: Substitute each set of solutions into the expression of the symmetry index SMPFE and take a set of solutions that satisfies the minimization of the index, as follows: X_opt={X|X∈X NBI ,min(SMPFE)} (17) Among them, X NBI is a set of uniformly distributed Pareto optima obtained by solving the optimization model using the NBI method; X_opt is the optimal solution that satisfies the minimization of the index, corresponding to X NBI is the adjustment value of the converter voltage reference value for droop control, which is expressed as follows:

7. A system using the method according to any one of claims 1 to 6, characterized in that: include: A data acquisition module is used to obtain DC line power, converter power and DC voltage in the power grid; Indicator construction module, used to construct the symmetric composite indicator SMPFE; A solution module, used to solve the problem using the NBI method based on the symmetry comprehensive index SMPFE; A control module is used to obtain a DC voltage reference value adjustment amount of the converter based on a minimized symmetry comprehensive index SMPFE value; The power flow of the high-voltage flexible DC system is symmetrically controlled based on the DC voltage reference value adjustment.

Images