A power flow control method for improving the power stability of key sections in AC-DC power grids

Abstract

The present invention discloses a power flow control method for improving the power stability of key sections of AC-DC power grids, which comprises the following steps: 1) screening key transmission sections and analyzing the node fluctuation conditions, and calculating the multi-node average comprehensive sensitivity of the output nodes prone to fluctuation to the key transmission sections; 2) forming a power flow optimization model with the goal of improving the power stability of key sections; 3) optimizing and iteratively calculating the change in power flow distribution caused by DC active power adjustment, and determining the optimal active power reference value of the DC transmission system according to the results of the optimized iterative calculation. Based on the control characteristics of the AC-DC hybrid transmission system, the present invention can achieve reducing the adverse impact of the new energy output nodes prone to fluctuation 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.

Description

Technical Field

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

[0002] In recent years, with the continuous breakthrough of key new energy technologies, the development of renewable energy, especially wind power, has been rapid.

[0003] New energy power generation such as wind power has many advantages over traditional thermal power generation, such as environmental protection, renewable, 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 environmental wind speed, which means that its power generation capacity is restricted by environmental factors. If the wind speed changes greatly, it will lead to a large fluctuation in power generation 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 large power grid under a certain mode. The relatively standard definition is as follows: Under a certain base-state power flow, a set of transmission lines with the same active power flow direction and close electrical distances is called a transmission section. From the definition of the transmission section, it can be seen that since it connects the power source center and the load center and undertakes the large-scale transmission of power within the regional power grid, its requirement for the stability of the transmitted power is very high. Especially for key transmission sections such as cross-river channels, the fluctuation of the section transmission power will increase the transmission loss and may even cause safety problems.

[0005] The influence of node output fluctuation on the section transmission power can be represented by the index of comprehensive sensitivity. This index 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, thereby reflecting the overall effect of the node on the section power flow. For new energy output nodes with large fluctuations in power generation output, considering optimizing the power flow distribution on the branches to reduce their comprehensive sensitivity to key transmission sections can greatly reduce the adverse impact of node output fluctuations on key transmission sections, thereby improving the stability of section transmission power.

[0006] In the trend of continuously increasing the proportion of new energy, a large number of fluctuating power sources are connected to the regional power grid, causing large-scale fluctuations in the power flow of transmission lines. Based on its large transmission density, fast and flexible power control, HVDC transmission provides a possibility for improving the transmission potential of the regional power grid. Currently, more and more DC lines are connected, forming an AC-DC hybrid power grid. In this way, the advantages of DC can be considered when optimizing the section power flow.

[0007] Therefore, in the prior art, key power nodes are screened by integrating indicators such as sensitivity, and the power generation power of the power nodes is adjusted to relieve the power flow congestion of the transmission section. However, the power generation power of new energy nodes is not adjustable and there are always random fluctuation characteristics, and the impact of the power generation power fluctuation of new energy nodes on the transmission power stability of the key transmission section is not considered, and there are safety and stability problems in the power transmission of the power grid. Summary of the Invention

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

[0009] The technical solution of the present invention is: a power flow control method for improving the power stability of a key section of an AC / DC power grid, including the following steps:

[0010] 1) Screen the key transmission section and analyze the node fluctuation conditions, and calculate the multi-node average comprehensive sensitivity of the easily fluctuating output nodes to the key transmission section;

[0011] 2) Taking the minimization of the multi-node average comprehensive sensitivity as the objective function, comprehensively considering each equality and inequality constraint, and forming an AC / DC power flow optimization model;

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

[0013] Step 1) includes:

[0014] 11) Select the key transmission section according to the importance of the transmission section;

[0015] 12) Segment the new energy output curve in the past day. The number of time periods in a day depends on the frequency at which optimization is desired to be performed. Observe the node fluctuation conditions in each time period, and select the node with the largest fluctuation amount in each time period as the measurement point;

[0016] 13) Adopt the standard mathematical model of sensitivity to quickly calculate the sensitivity of each measurement 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 active power flow change of line L; Δθ L is the voltage phase difference between the head and end nodes of line L; x LReactance of line L; ΔP i Change in active power injection at node i; S Li Sensitivity of node i to line L;

[0019] 14) Calculate the load factor of each branch in the key transmission section, and the calculation formula is:

[0020]

[0021] where, α L is the load factor of branch L, P L and respectively represent the active power actually transmitted by branch L and the rated value of the transmitted active power;

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

[0023]

[0024] where, 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) Average the comprehensive sensitivity of each measurement point to the key transmission section, and synthesize a multi-node average comprehensive sensitivity. The calculation formula is:

[0026]

[0027] where, PI avg is the multi-node average comprehensive sensitivity, 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 constraint conditions of the AC-DC power flow optimization model include equality constraints and inequality constraints,

[0032] where, 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 specific constraint conditions are:

[0035] Active power balance:

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

[0037] Wherein, P ga represents the active power injected by the a-th power source into the system; P lb represents the active power absorbed by the b-th load from the system; ΔP loss represents 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] Wherein, Q ga represents the reactive power injected by the a-th reactive power source into the system; Q lb represents the reactive power absorbed by the b-th load from the system; ΔQ loss represents the reactive power loss in the system;

[0041] Node voltage constraint:

[0042] U imin ≤ U i ≤ U imax

[0043] Wherein, U i represents the actual value of the voltage of the i-th node; U imin , U imax respectively represent the minimum and maximum allowable values of the voltage of the i-th node;[[ID=7�]]

[0044] Line current constraint:

[0045] I Lmin ≤ I L ≤ I Lmax

[0046] Wherein, I L represents the actual value of the current of the L-th line; I Lmin , I Lmax respectively represent the minimum and maximum allowable values of the current of the L-th line;

[0047] Active power constraint of power source:

[0048] P gamin ≤P ga ≤P gamax

[0049] Wherein, P ga represents the actual value of the active power output of the a-th power source; P gamin , P gamax respectively represent the minimum and maximum allowable values of the active power output of the a-th power source;

[0050] Reactive power constraint of power source:

[0051] Q gamin ≤Q ga ≤Q gamax

[0052] Wherein, Q ga represents the actual value of the reactive power output of the a-th power source; Q gamin , Q gamax respectively represent the minimum and maximum allowable values of the reactive power output of the a-th power source.

[0053] The specific process of step 3) is as follows: perform power flow optimization according to the AC-DC power flow optimization model, continuously iterate, reduce the influence of the output fluctuation of the new energy nodes with easy fluctuations on the power stability of the key section, set a discrimination threshold, and end the optimization when the change amount of the average comprehensive sensitivity of multiple nodes in a certain step is lower than the threshold, and obtain the optimal DC active power reference value according to the optimized distribution.

[0054] In the operation of the present invention, it is possible to improve the active power distribution of the section by modifying the DC active power control parameters in the section without adjusting the power generation power of the power generation nodes, so as to reduce the influence of some nodes with easy fluctuations on the power stability of the key section, thereby improving the stability of power transmission and being beneficial to the safe and stable operation of the power grid. Description of the Drawings

[0055] Figure 1 is the flow chart of the present invention. Detailed Embodiment

[0056] The following further describes the technical solution of the present invention with reference to the drawings.

[0057] As Figure 1 shown, the present invention provides a power flow control method for improving the power stability of the key section of the AC-DC power grid, including the following steps:

[0058] 1) Select a key transmission section that urgently needs to improve power stability according to the importance of the transmission section. According to the new energy output curve, analyze the node fluctuation situation in segments, calculate the comprehensive sensitivity of a series of nodes with the largest fluctuation amount to the key transmission section, and synthesize a multi-node average comprehensive sensitivity. This index will be used 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 the key section: Take the DC active power reference value in the key section as the optimization variable, and take the minimization of the multi-node average comprehensive sensitivity of the volatile power supply nodes to the key section as the objective function. Considering various equality and inequality constraints comprehensively, form an AC-DC power flow optimization model;

[0060] 3) Optimize and iterate the power flow distribution of each branch of the transmission section: Perform power flow optimization according to the AC-DC power flow optimization model, continuously iterate, reduce the impact of the output fluctuation of the volatile new energy nodes on the power stability of the key section, and obtain the optimal DC active power reference value according to the optimized distribution.

[0061] The present invention considers that the power generation of new energy nodes is not adjustable and there is always a random fluctuation characteristic. Without changing the node power generation, by adjusting the active power reference value of the DC line in the key transmission section, the power flow distribution of the corresponding AC-DC power grid is changed, and then the impact of the volatile new energy nodes on the power stability of the key section is reduced, 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 the key transmission section, analyzing the node fluctuation situation, and calculating the multi-node average comprehensive sensitivity of the volatile output nodes to the key transmission section is as follows:

[0064] 11) Select a key transmission section that urgently needs to improve power stability according to the importance of the transmission section;

[0065] [[ID=2i]]Specifically, the screening of the key transmission section includes: disconnect each branch one by one, and successively detect the load rate of the branches that are not disconnected, and judge whether there is a branch with a load rate greater than the preset load rate threshold among the branches that are not disconnected. If so, select the corresponding disconnected branch as the key branch; Use the DC power flow method to calculate the power flow change amount of each branch when a unit injection active power change occurs at each of the selected nodes. The branch with a power flow change amount greater than the preset threshold is a strongly correlated branch, and the strongly correlated branch and the key branch constitute the key transmission section;

[0066] 12) Segment the new energy output curve in the past day. The number of time periods in a day depends on the frequency at which optimization is desired to be performed. Observe the node fluctuation situation in each time period, and select the node with the largest fluctuation amount in each time period as the measurement point;

[0067] 13) Adopt the sensitivity standard mathematical model to quickly calculate the sensitivities of each measurement point to each branch in the key transmission section. The mathematical model for sensitivity calculation is as follows:

[0068]

[0069] Among them, B′ is the system susceptance matrix; Δθ is the column vector of the phase angle difference of the system line voltage; ΔP is the column vector of the change in the active power flow of the system line; ΔP L is the change in the active power flow of line L; Δθ L is the voltage phase difference between the head and tail nodes of line L; x L is the reactance of line L; ΔP i is the change in the active power injection 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, and at this time, the change in the active power of branch L;

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

[0071]

[0072] Among them, α L is the load factor of branch L, P L and respectively represent the actual transmitted active power and the rated value of the transmitted active power of branch L;

[0073] 15) Based on the sensitivities of each measurement point to each branch in the key transmission section, determine the weights according to the load factor differences of different branches, and synthesize the comprehensive sensitivity of each measurement point to the key transmission section to reflect the overall impact of the fluctuations of each node on the change in the power flow of the key transmission section. The calculation formula for 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) Average the comprehensive sensitivities of each measurement point to the key transmission section and synthesize a multi-node average comprehensive sensitivity. The calculation formula is:

[0077]

[0078] Among them, PI avg is the multi-node average comprehensive sensitivity, and N is both the number of time periods divided within 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 the key section:

[0080] 21) To reduce the adverse effects of new energy output nodes with volatile power on the transmission power of the key transmission section and improve the power stability of the key section, minimizing the multi-node average comprehensive sensitivity of each measurement to the key transmission section is used as the optimization objective function ob, and 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, and the inequality constraints should include node voltage constraints, line current constraints, power source active power constraints, and power source reactive power constraints;

[0083] The specific optimization constraints are:

[0084] (1) Active power balance:

[0085] P[[ID=2)) g1 +…+P ga +…+P gn =P l1 +…+P lb +…+P lm +ΔP loss (6)

[0086] Among them, P ga represents the active power injected by the a-th power source into the system; P lb represents the active power absorbed by the b-th load from the system; ΔP loss represents 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 by the a-th reactive power source into the system; Q lb represents the reactive power absorbed by the b-th load from the system; ΔQ loss represents the reactive power loss in the system;

[0090] (3) Node voltage constraint:

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

[0092] Among them, U i represents the actual value of the voltage of the i-th node; U imin , U imax respectively represent the minimum and maximum allowable values of the voltage of the i-th node;

[0093] (4) Line current constraint:

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

[0095] Among them, I L represents the actual value of the current of the L-th line; I Lmin , I Lmax respectively represent the minimum and maximum allowable values of the current of the L-th line;

[0096] (5) Active power constraint of power source:

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

[0098] Among them, P ga represents the actual value of the active power output of the a-th power source; P gamin , P gamax respectively represent the minimum and maximum allowable values of the active power output of the a-th power source;

[0099] (6) Reactive power constraint of power source:

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

[0101] Among them, Q ga represents the actual value of the reactive power output of the a-th power source; Q gamin , Q gamax respectively represent the minimum and maximum allowable values of the reactive power output of the a-th power source;

[0102] (23) Determine that the optimized variable is the DC active power reference value of each DC branch in the key transmission section, and form an optimization model 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 iteratively:

[0104] Power flow optimization is carried out according to the AC-DC power flow optimization model, and iterative optimization is continuously performed to reduce the multi-node average comprehensive sensitivity of each measurement point to the key section, thereby reducing the impact of the output fluctuation of the volatile nodes on the power stability of the key section. A discrimination threshold ΔPI is set. min , when the change amount ΔPI avg of the multi-node average comprehensive sensitivity PI avg is lower than the threshold ΔPI min , the optimization ends, 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 transmission system, the present invention can reduce the adverse impact of the volatile new energy output nodes on the transmission power of the key transmission section through section power flow optimization, which is beneficial to the safe and stable operation of the power grid.

[0106] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution of the present invention and its inventive concept, makes equivalent substitutions or changes, and all should be covered within the protection scope of the present invention.

Claims

1. A power flow control method for improving the power stability of key sections of AC-DC power grids, characterized in that, It includes the following steps: 1) Screen the key transmission sections and analyze the node fluctuation conditions, and calculate the multi-node average comprehensive sensitivity of the output nodes prone to fluctuation to the key transmission sections; 2) Take the minimization of the multi-node average comprehensive sensitivity as the objective function, and comprehensively consider various equality and inequality constraints to form an AC-DC power flow optimization model; 3) Conduct power flow optimization according to the AC-DC power flow optimization model, continuously iterate, and obtain the optimal DC active power reference value according to the optimization distribution; Step 1) includes: 11) Select the key transmission sections according to the importance of the transmission sections; 12) Segment the new energy output curve within the past day. The number of time periods within a day depends on the frequency of optimization to be performed. Observe the node fluctuation conditions in each time period, and select the node with the largest fluctuation amount in each time period as the measurement point; 13) Adopt the standard mathematical model of sensitivity to quickly calculate the sensitivity of each measurement 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 phase angle differences of the system line voltages; ΔP is the column vector of the active power flow changes of the system lines; ΔP L is the active power flow change of line L; Δθ L is the voltage phase difference between the two ends of line L; x L is the reactance of line L; ΔP i is the change in the active power injection at 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 respectively represent the active power actually transmitted by branch L and the rated value of the transmitted active power; 15) Based on the sensitivity of each measurement point to each branch in the key transmission section, determine the weight according to the load rate difference of different branches, and synthesize the comprehensive sensitivity of each measurement point to the key transmission section. The calculation formula for 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) Average the comprehensive sensitivity of each measurement point to the key transmission section, and synthesize a multi-node average comprehensive sensitivity. The calculation formula is: Among them, PI avg is the multi-node average comprehensive sensitivity, and N is the number of measurement points; In step 2), The objective function ob of the AC-DC power flow optimization model is: ob = min(PI avg ); The specific process of step 3) is: Conduct power flow optimization according to the AC-DC power flow optimization model, continuously iterate, reduce the influence of the output fluctuation of the new energy nodes prone to fluctuation on the power stability of the key section, set a discrimination threshold, and end the optimization when the change amount of the multi-node average comprehensive sensitivity in a certain step is lower than the threshold. Obtain the optimal DC active power reference value according to the optimization distribution.

2. A power flow control method for improving the power stability of the key section of an AC-DC power grid according to claim 1, characterized in that The constraint conditions 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.

3. A power flow control method for improving the power stability of the key section of an AC-DC power grid according to claim 2, characterized in that The constraint conditions 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 a-th power source; P lb represents the active power absorbed from the system by the b-th load; ΔP loss represents 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 a-th reactive power source; Q lb represents the reactive power absorbed from the system by the b-th load; ΔQ loss represents the reactive power loss in the system; Node voltage constraint: U imin ≤U i ≤U imax Among them, U i represents the actual value of the voltage of the i-th node; U imin , U imax respectively represent the minimum allowable value and the maximum allowable value of the voltage of the i-th node; Line current constraint: I Lmin ≤I L ≤I Lmax Among them, I L represents the actual value of the current of the L-th line; I Lmin and I Lmax respectively represent the minimum allowable value and the maximum allowable value of the current of the L-th line; Power source active power constraint: P gamin ≤P ga ≤P gamax Among them, P ga represents the actual value of the active power output of the a-th power source; P gamin , P gamax respectively represent the minimum and maximum allowable values of the active power output of the a-th power source; Power source reactive power constraint: Q gamin ≤Q ga ≤Q gamax Among them, Q ga represents the actual value of the reactive power output of the a-th power source; Q gamin , Q gamax respectively represent the minimum allowable value and the maximum allowable value of the reactive power output of the a-th power source.

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