Power flow control method for improving power stability of key section of AC-DC power grid

Abstract

The invention discloses a power flow control method for improving the power stability of a key section of an AC-DC power grid, and the method comprises the following steps: 1) screening a key power transmission section, analyzing the fluctuation condition of a node, and calculating the multi-node average comprehensive sensitivity of an easy-to-fluctuate output node to the key power transmission section; 2) forming a power flow optimization model by taking improvement of key section power stability as a target; and 3) performing optimization iteration calculation on the change quantity of the direct current active power adjustment to the power flow distribution, and determining the optimal active power reference value of the direct current power transmission system according to the optimization iteration calculation result. Based on the control characteristics of the alternating-current and direct-current hybrid power transmission system, the adverse effect of the new energy output node easy to fluctuate on the transmission power of the key power transmission section can be reduced through section power flow optimization, and safe and stable operation of a power grid is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current transmission, and in particular to a power flow optimization control method for improving power stability of key sections of an AC / DC hybrid power grid. Background Art

[0002] In recent years, with the continuous breakthroughs in key new energy technologies, renewable energy, especially wind power, has developed rapidly.

[0003] Compared with traditional thermal power generation, wind power and other new energy sources have many advantages such as environmental protection, low cost, high flexibility, etc., but there is also the problem of insufficient controllability. Taking wind power as an example, the power generation efficiency of wind turbines is greatly affected by the ambient wind speed, which means that its power generation capacity is restricted by environmental factors. If the wind speed changes greatly, it will cause a large fluctuation in power output, affecting the supply of the entire energy.

[0004] A transmission section refers to a bundle of channels composed of several lines or transformers in a larger power grid under a certain mode. The more standardized definition is as follows: Under a certain base state flow, a group of transmission lines with the same active flow direction and similar electrical distance is called a transmission section. From the definition of a transmission section, it can be seen that since it connects the power center and the load center and undertakes the large-scale transmission of power within the regional power grid, it has very high requirements for the stability of the transmission power, especially for key transmission sections such as cross-river channels. The fluctuation of the transmission power of the section will increase the transmission loss and may even cause safety problems.

[0005] The impact of node output fluctuations on section transmission power can be represented by the index of comprehensive sensitivity, which is based on the sensitivity of the node to each branch in the section, and determines the weight according to the load rate difference of different branches, so as to synthesize a comprehensive sensitivity to reflect the overall effect of the node on the section flow. For new energy output nodes with large power generation output fluctuations, it is considered to reduce their comprehensive sensitivity to key transmission sections by optimizing the flow distribution on the branches, which can greatly reduce the adverse impact of node output fluctuations on key transmission sections, thereby improving the stability of section transmission power.

[0006] As the proportion of new energy sources continues to increase, a large number of fluctuating power sources are connected to the regional power grid, causing large-scale fluctuations in the power flow of the transmission lines. DC transmission provides a possibility for improving the transmission potential of regional power grids based on its high transmission density, rapid power control and flexibility. At present, more and more DC lines are connected to form an AC / DC hybrid power grid, so that the advantages of DC can be considered when optimizing the cross-section power flow.

[0007] Therefore, in the prior art, comprehensive sensitivity and other indicators are used to help screen key power nodes, and the power generation of power nodes is adjusted to alleviate the power congestion of the transmission section. However, the power generation of new energy nodes is not adjustable, and there are always random fluctuations. The impact of the power generation fluctuation of new energy nodes on the stability of the transmission power of key transmission sections is not taken into account, and the power transmission of the power grid has safety and stability issues. Summary of the invention

[0008] In view of the deficiencies in the prior art, the present invention provides a method for improving the power stability of key sections in an AC / DC hybrid power grid by improving the section flow distribution according to the output fluctuation of each renewable energy output node.

[0009] The technical solution of the present invention is: a power flow control method for improving the power stability of key sections of AC and DC power grids, comprising the following steps:

[0010] 1) Screen key transmission sections and analyze node fluctuations, and calculate the average comprehensive sensitivity of multiple nodes of output nodes prone to fluctuations to key transmission sections;

[0011] 2) Taking the minimization of the average comprehensive sensitivity of multiple nodes as the objective function, various equality and inequality constraints are comprehensively considered to form an AC and DC power flow optimization model;

[0012] 3) Perform power flow optimization according to the AC / DC power flow optimization model, perform iterations continuously, and obtain the optimal DC active power reference value based on the optimized distribution.

[0013] Step 1) includes:

[0014] 11) Select key transmission sections according to their importance;

[0015] 12) Divide the new energy output curve in the past day into segments. The number of time periods in a day depends on the frequency of optimization. Observe the node fluctuations in each time period and select the node with the largest fluctuation in each time period as the measurement point.

[0016] 13) The sensitivity standard mathematical model is used to quickly calculate the sensitivity of each measuring point to each branch in the key transmission section. The mathematical model for sensitivity calculation is:

[0017]

[0018] Where B′ is the system susceptance matrix; Δθ is the column vector of the system line voltage phase angle difference; ΔP is the column vector of the system line active power flow change; ΔP L is the change in active power flow of line L; Δθ L is the voltage phase difference between the nodes at the beginning and end of line L; x Lis the reactance of line L; ΔP i is the active power injection variation of node i; S Li is the sensitivity of node i to line L;

[0019] 14) Calculate the load rate of each branch in the key transmission section. The calculation formula is:

[0020]

[0021] Among them, α L is the load rate of branch L, P L and They represent the active power actually delivered by branch L and the rated value of the active power delivered respectively;

[0022] 15) Based on the sensitivity of each measuring point to each branch in the key transmission section, the weight is determined according to the load rate difference of different branches, and the comprehensive sensitivity of each measuring point to the key transmission section is synthesized. The calculation formula of the comprehensive sensitivity is:

[0023]

[0024] Among them, PI(i) is the comprehensive sensitivity of node i to the key transmission section, and T represents all branches on the key transmission section;

[0025] 16) The comprehensive sensitivity of each measuring point to the key transmission section is averaged to synthesize a multi-node average comprehensive sensitivity. The calculation formula is:

[0026]

[0027] Among them, PI avg is the average comprehensive sensitivity of multiple nodes, and N is the number of measurement points.

[0028] In step 2),

[0029] The objective function ob of the AC / DC power flow optimization model is:

[0030] ob=min(PI avg ).

[0031] The constraints of the AC / DC power flow optimization model include equality constraints and inequality constraints.

[0032] Among them, the equality constraints should include active power balance and reactive power balance,

[0033] The inequality constraints should include node voltage constraints, line current constraints, power source active power constraints, and power source reactive power constraints.

[0034] The constraints are specifically:

[0035] Active power balance:

[0036] P g1 +…+P ga +…+P gn =P l1 +…+P lb +…+P lm +ΔP loss

[0037] Among them, P ga represents the active power injected into the system by the ath power source; P lb Indicates the active power absorbed by the bth load from the system; ΔP loss Indicates the active power loss in the system;

[0038] Reactive power balance:

[0039] Q g1 +…+Q ga +…+Q gn =Q l1 +…+Q lb +…+Q lm +ΔQ loss

[0040] Among them, Q ga represents the reactive power injected into the system by the ath reactive source; Q lb Indicates the reactive power absorbed by the bth load from the system; ΔQ loss Indicates the reactive power loss in the system;

[0041] Node voltage constraints:

[0042] U imin ≤U i ≤U imax

[0043] Among them, U i Indicates the actual value of the voltage at the ith node; U imin , U imax They represent the minimum and maximum values ​​allowed for the voltage of the i-th node respectively;

[0044] Line current constraints:

[0045] I Lmin ≤I L ≤I Lmax

[0046] Among them, I L Indicates the actual value of the current in the Lth line; I Lmin ,I Lmax They represent the minimum and maximum values ​​allowed for the current of the Lth line respectively;

[0047] Power supply active power constraint:

[0048] P gamin ≤P ga ≤P gamax

[0049] Among them, P ga Indicates the actual value of the active output of the ath power source; P gamin , P gamax They represent the minimum and maximum values ​​allowed for the active output of the ath power source respectively;

[0050] Power supply reactive power constraint:

[0051] Q gamin ≤Q ga ≤Q gamax

[0052] Among them, Q ga Indicates the actual value of reactive power output of the ath power source; Q gamin , Q gamax They respectively represent the minimum and maximum allowable reactive output of the ath power source.

[0053] The specific process of step 3) is as follows: optimize the power flow according to the AC / DC power flow optimization model, perform continuous iterations, reduce the impact of output fluctuations of new energy nodes that are prone to fluctuations on the power stability of key sections, set a judgment threshold, and end the optimization when the change in the average comprehensive sensitivity of multiple nodes in a certain step is lower than the threshold, and obtain the optimal DC active reference value based on the optimized distribution.

[0054] During operation, the present invention can improve the section active power distribution by modifying the DC active power control parameters in the section without adjusting the power generation power of the power generation node, so as to reduce the influence of certain nodes prone to fluctuations on the power stability of key sections, thereby improving the stability of power transmission and facilitating the safe and stable operation of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 It is a flow chart of the present invention. DETAILED DESCRIPTION

[0056] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0057] like Figure 1 As shown, the present invention provides a power flow control method for improving the power stability of key sections of AC and DC power grids, comprising the following steps:

[0058] 1) According to the importance of the transmission section, a key transmission section that urgently needs to improve power stability is selected. According to the new energy output curve, the node fluctuation is analyzed in sections, and the comprehensive sensitivity of a series of nodes with the largest fluctuation to the key transmission section is calculated. A multi-node average comprehensive sensitivity is synthesized, which will serve as the basis for subsequent power flow optimization;

[0059] 2) Form a power flow optimization model with the goal of improving the power stability of key sections: take the DC active reference value in the key section as the optimization variable, take minimizing the average comprehensive sensitivity of the power nodes that are prone to fluctuations to the multi-nodes of the key section as the objective function, and comprehensively consider various equations and inequality constraints to form an AC / DC power flow optimization model;

[0060] 3) Optimize and iterate the power flow distribution of each branch of the transmission section: Optimize the power flow according to the AC / DC power flow optimization model, and iterate continuously to reduce the impact of the output fluctuations of the new energy nodes that are prone to fluctuations on the power stability of the key sections, and obtain the optimal DC active power reference value based on the optimized distribution.

[0061] The present invention takes into account that the power generation power of new energy nodes is not adjustable and always has random fluctuation characteristics. Without changing the power generation power of the nodes, the active power reference value of the DC line in the key transmission section is adjusted to change the power flow distribution of the corresponding AC / DC power grid, thereby reducing the impact of the fluctuating new energy nodes on the power stability of the key section, thereby improving the safety and stability of power transmission in the power grid.

[0062] Specifically,

[0063] In step 1) of the present invention, the specific process of screening key transmission sections and analyzing node fluctuations, and calculating the multi-node average comprehensive sensitivity of the output nodes prone to fluctuations to the key transmission sections is as follows:

[0064] 11) Select a key transmission section where power stability needs to be improved urgently according to its importance;

[0065] The specific screening of the key transmission section includes: disconnecting each branch one by one, and detecting the load rate of the undisconnected branches one by one, judging whether there is a branch whose load rate among the undisconnected branches is greater than a preset load rate threshold value, and if so, selecting the corresponding disconnected branch as the key branch; using the DC power flow method to calculate the power flow change of each branch when the unit injected active power changes at each selected node, and the branch whose power flow change is greater than the preset threshold value is a strongly correlated branch, and the strongly correlated branch and the key branch constitute the key transmission section;

[0066] 12) Divide the new energy output curve in the past day into segments. The number of time periods in a day depends on the frequency of optimization. Observe the node fluctuations in each time period and select the node with the largest fluctuation in each time period as the measurement point.

[0067] 13) The sensitivity standard mathematical model is used to quickly calculate the sensitivity of each measuring point to each branch in the key transmission section. The mathematical model for sensitivity calculation is:

[0068]

[0069] Where B′ is the system susceptance matrix; Δθ is the column vector of the system line voltage phase angle difference; ΔP is the column vector of the system line active power flow change; ΔP L is the change in active power flow of line L; Δθ L is the voltage phase difference between the nodes at the beginning and end of line L; x L is the reactance of line L; ΔP i is the active power injection variation of node i; S Li is the sensitivity of node i to line L. Its physical meaning is that when the active power injection of node i increases by 1 unit, the corresponding balancing machine decreases by 1 unit. At this time, the active power change of branch L is

[0070] 14) Calculate the load rate of each branch in the key transmission section. The calculation formula is:

[0071]

[0072] Among them, α L is the load rate of branch L, P L and They represent the active power actually delivered by branch L and the rated value of the active power delivered respectively;

[0073] 15) Based on the sensitivity of each measuring point to each branch in the key transmission section, the weight is determined according to the load rate difference of different branches, and the comprehensive sensitivity of each measuring point to the key transmission section is synthesized to reflect the overall impact of the fluctuation of each node on the flow change of the key transmission section. The calculation formula of the comprehensive sensitivity is:

[0074]

[0075] Among them, PI(i) is the comprehensive sensitivity of node i to the key transmission section, and T represents all branches on the key transmission section;

[0076] 16) The comprehensive sensitivity of each measuring point to the key transmission section is averaged to synthesize a multi-node average comprehensive sensitivity. The calculation formula is:

[0077]

[0078] Among them, PI avg is the average comprehensive sensitivity of multiple nodes, and N is both the number of time periods divided in a day and the number of measurement points.

[0079] In step 2) of the present invention, a power flow optimization model is formed with the goal of improving the power stability of key sections:

[0080] 21) In order to reduce the adverse impact of the fluctuating new energy output nodes on the transmission power of key transmission sections and improve the power stability of key sections, the calculated multi-node average comprehensive sensitivity of each measurement to the key transmission section is minimized as the optimization objective function ob. The objective function formula is:

[0081] ob=min(PI avg )(5)

[0082] 22) Determine the optimization constraints. The equality constraints should include active power balance and reactive power balance. The inequality constraints should include node voltage constraints, line current constraints, power source active power constraints, and power source reactive power constraints.

[0083] The optimization constraints are as follows:

[0084] (1) Active power balance:

[0085] P g1 +…+P ga +…+P gn =P l1 +…+P lb +…+P lm +ΔP loss (6)

[0086] Among them, P ga represents the active power injected into the system by the ath power source; P lb Indicates the active power absorbed by the bth load from the system; ΔP loss Indicates the active power loss in the system;

[0087] (2) Reactive power balance:

[0088] Q g1 +…+Q ga +…+Q gn =Q l1 +…+Q lb +…+Q lm +ΔQ loss (7)

[0089] Among them, Q ga represents the reactive power injected into the system by the ath reactive source; Q lb Indicates the reactive power absorbed by the bth load from the system; ΔQ loss Indicates reactive power loss in the system;

[0090] (3) Node voltage constraints:

[0091] U imin ≤U i ≤U imax (i=1,2,3,……) (8)

[0092] Among them, U i represents the actual value of the voltage at the ith node; U imin , U imax They represent the minimum and maximum values ​​allowed for the voltage of the i-th node respectively;

[0093] (4) Line current constraints:

[0094] I Lmin ≤I L ≤I Lmax (L=1,2,3,……) (9)

[0095] Among them, I L Indicates the actual value of the current in the Lth line; I Lmin ,I Lmax They represent the minimum and maximum values ​​allowed for the current of the Lth line respectively;

[0096] (5) Power supply active power constraint:

[0097] P gamin ≤P ga ≤P gamax (a=1,2,3,……) (10)

[0098] Among them, P ga Indicates the actual value of the active output of the ath power source; P gamin , P gamax They represent the minimum and maximum values ​​allowed for the active output of the ath power source respectively;

[0099] (6) Power supply reactive power constraint:

[0100] Q gamin ≤Q ga ≤Q gamax (a=1,2,3,……) (11)

[0101] Among them, Q ga Indicates the actual value of reactive power output of the ath power source; Q gamin , Q gamax They represent the minimum and maximum values ​​allowed for reactive output of the ath power source respectively;

[0102] 23) The optimized variable is determined as the DC active power reference value of each DC branch in the key transmission section, and the optimization model is formed by integrating the objective function, constraint conditions and optimization variables.

[0103] In step 3) of the present invention, the power flow distribution of each branch of the transmission section is optimized and iterated:

[0104] According to the AC / DC power flow optimization model, power flow optimization is performed, and iterations are continuously performed to reduce the average comprehensive sensitivity of each measurement point to multiple nodes of the key section, thereby reducing the impact of the output fluctuation of the easily fluctuating node on the power stability of the key section, and setting the judgment threshold ΔPI min , when the average comprehensive sensitivity of multiple nodes at a certain step PI avg The change in ΔPI avg Below the threshold ΔPI min The optimization is completed when , and the optimal DC active power reference value of the key section is obtained according to the optimized distribution.

[0105] Based on the control characteristics of the AC / DC hybrid power transmission system, the present invention can reduce the adverse effects of volatile new energy output nodes on the transmission power of key transmission sections through section power flow optimization, which is beneficial to the safe and stable operation of the power grid.

[0106] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A power flow control method for improving power stability of key sections of AC and DC power grids, characterized in that: The following steps are involved: 1) Screen key transmission sections and analyze node fluctuations, and calculate the average comprehensive sensitivity of multiple nodes of output nodes prone to fluctuations to key transmission sections; 2) Taking the minimization of the average comprehensive sensitivity of multiple nodes as the objective function, various equality and inequality constraints are comprehensively considered to form an AC and DC power flow optimization model; 3) Perform power flow optimization according to the AC / DC power flow optimization model, perform iterations continuously, and obtain the optimal DC active power reference value based on the optimized distribution.

2. A power flow control method for improving power stability of key sections of AC and DC power grids according to claim 1, characterized in that: Step 1) includes: 11) Select key transmission sections according to their importance; 12) Divide the new energy output curve in the past day into segments. The number of time periods in a day depends on the frequency of optimization. Observe the node fluctuations in each time period and select the node with the largest fluctuation in each time period as the measurement point. 13) The sensitivity standard mathematical model is used to quickly calculate the sensitivity of each measuring point to each branch in the key transmission section. The mathematical model for sensitivity calculation is: Where B′ is the system susceptance matrix; Δθ is the column vector of the system line voltage phase angle difference; ΔP is the column vector of the system line active power flow change; ΔP L is the change in active power flow of line L; Δθ L is the voltage phase difference between the nodes at the beginning and end of line L; x L is the reactance of line L; ΔP i is the active power injection variation of node i; S Li is the sensitivity of node i to line L; 14) Calculate the load rate of each branch in the key transmission section. The calculation formula is: Among them, α L is the load rate of branch L, P L and They represent the active power actually delivered by branch L and the rated value of the active power delivered respectively; 15) Based on the sensitivity of each measuring point to each branch in the key transmission section, the weight is determined according to the load rate difference of different branches, and the comprehensive sensitivity of each measuring point to the key transmission section is synthesized. The calculation formula of the comprehensive sensitivity is: Among them, PI(i) is the comprehensive sensitivity of node i to the key transmission section, and T represents all branches on the key transmission section; 16) The comprehensive sensitivity of each measuring point to the key transmission section is averaged to synthesize a multi-node average comprehensive sensitivity. The calculation formula is: Among them, PI avg is the average comprehensive sensitivity of multiple nodes, and N is the number of measurement points.

3. A power flow control method for improving power stability of key sections of AC and DC power grids according to claim 1, characterized in that: In step 2), The objective function ob of the AC / DC power flow optimization model is: ob=min(PI avg )。 4. A power flow control method for improving power stability of key sections of AC and DC power grids according to claim 3, characterized in that: The constraints of the AC / DC power flow optimization model include equality constraints and inequality constraints. Among them, the equality constraints should include active power balance and reactive power balance, The inequality constraints should include node voltage constraints, line current constraints, power source active power constraints, and power source reactive power constraints.

5. A power flow control method for improving power stability of key sections of AC and DC power grids according to claim 4, characterized in that: The constraints are specifically: Active power balance: P g1 +…+P ga +…+P gn =P l1 +…+P lb +…+P lm +ΔP loss Among them, P ga represents the active power injected into the system by the ath power source; P lb Indicates the active power absorbed by the bth load from the system; ΔP loss Indicates the active power loss in the system; Reactive power balance: Q g1 +…+Q ga +…+Q gn =Q l1 +…+Q lb +…+Q lm +ΔQ loss Among them, Q ga represents the reactive power injected into the system by the ath reactive source; Qlb represents the reactive power absorbed by the bth load from the system; ΔQ loss Indicates the reactive power loss in the system; Node voltage constraints: IN imin ≤U i ≤U imax Among them, U i Indicates the actual value of the voltage at the ith node; U imin , U imax They represent the minimum and maximum values ​​allowed for the voltage of the i-th node respectively; Line current constraints: I Lmin ≤I L ≤I Lmax Among them, I L Indicates the actual value of the current in the Lth line; I Lmin ,I Lmax They represent the minimum and maximum values ​​allowed for the current of the Lth line respectively; Power supply active power constraint: P gain ≤P ga ≤P gamax Among them, P ga Indicates the actual value of the active output of the ath power source; P gamin , P gamax They represent the minimum and maximum values ​​allowed for the active output of the ath power source respectively; Power supply reactive power constraint: Q gamin ≤Q ga ≤Q gamax Among them, Q ga Indicates the actual value of reactive power output of the ath power source; Q gamin , Q gamax They respectively represent the minimum and maximum allowable reactive output of the ath power source.

6. A power flow control method for improving power stability of key sections of AC and DC power grids according to claim 1, characterized in that: The specific process of step 3) is as follows: The power flow optimization is performed according to the AC / DC power flow optimization model, and it is continuously iterated to reduce the impact of the output fluctuation of the new energy nodes that are prone to fluctuations on the power stability of the key sections. The judgment threshold is set. When the change in the average comprehensive sensitivity of multiple nodes in a certain step is lower than the threshold, the optimization ends, and the optimal DC active power reference value is obtained according to the optimized distribution.

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