A method for evaluating load carrying capacity of AC / DC hybrid transmission network

Abstract

The present invention discloses a method for evaluating the load carrying capacity of an AC / DC hybrid transmission network, which relates to the field of electric power technology. The main steps include: establishing a DC power flow equation of an AC / DC hybrid power network; calculating a power transmission allocation factor matrix M of the AC / DC hybrid power network; ptdf ; Calculate the available transmission capacity of each transmission line under the basic load state of the AC / DC hybrid power grid △S line With load factor μ load ; Calculate the maximum value P of the maximum additional amount of each power generation node under the line constraint by increasing the power generation of a single node 1max and the minimum value P 1min ; Calculate the maximum additional generation P of the power generation node under the line constraint according to the proportional increase of the power generation surplus r Considering the two cases of fixed and adjustable transmission power of high-voltage DC lines, the optimal power generation P with fixed DC power and maximum total power generation of power generation nodes is calculated. 0DC , DC power adjustable to maximize the total increase in power generation nodes to achieve the optimal power generation P 1DC ; Then the load carrying capacity of the AC / DC hybrid transmission network under the current base state flow is evaluated.

Description

Technical Field

[0001] The invention relates to the field of electric power technology, and in particular to a method for evaluating the load carrying capacity of an AC / DC hybrid transmission network. Background Art

[0002] As the installed capacity of new energy sources gradually increases, the total load of the power grid is also increasing year by year, and the pressure on the transmission network lines is increasing sharply. At the same time, with the development of the new power system, the backbone transmission network structure has gradually transformed and upgraded from the traditional AC power grid to the new AC / DC hybrid power grid. Therefore, there is an urgent need for an evaluation method for the load carrying capacity of the AC / DC hybrid transmission network, which can effectively reflect the load increment bearing capacity of the power grid under the current trend.

[0003] At present, in the research on improving the load carrying capacity of AC / DC hybrid power grids and the transmission capacity of transmission lines, there is still a lack of effective and accurate evaluation methods for the load carrying capacity of AC / DC hybrid transmission networks. Summary of the invention

[0004] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide an evaluation method for the load carrying capacity of an AC / DC hybrid transmission network. The method can analyze the maximum load carrying capacity under different power generation node generation methods and the optimal power generation method and optimal DC transmission power with the maximum load carrying as the goal, and evaluate the load carrying capacity of the power grid under the current trend.

[0005] The present invention adopts the following technical solutions to solve the above technical problems:

[0006] A method for evaluating the load carrying capacity of an AC / DC hybrid transmission network proposed in the present invention comprises the following steps:

[0007] Step 1: Establishing a DC power flow equation of an AC / DC hybrid power grid based on an AC / DC power grid topology structure;

[0008] Step 2: According to the DC power flow equation of the AC / DC hybrid power grid, the power transmission allocation factor matrix M of the AC / DC hybrid power grid is calculated. ptdf ;

[0009] Step 3: Calculate the available transmission capacity of each transmission line under the basic load state of the AC / DC hybrid power grid △S line With load factor μ load ;

[0010] Step 4: According to M ptdf , △S line With μ load , calculate the maximum value P of the maximum additional amount of each power generation node under the line constraint with single node additional generation 1max and the minimum value P 1min ;

[0011] Step 5: According to M ptdf , △S line With μ load , calculate the maximum additional generation P of the power generation node under the line constraint according to the proportional increase of the power generation surplus r ;

[0012] Step 6: According to M ptdf , △S line With μ load Considering the two cases of fixed and adjustable transmission power of high-voltage DC lines, the optimal power generation P is calculated with the DC power fixed and the total increase of power generation nodes as the goal. 0DC , DC power adjustable to maximize the total increase in power generation nodes to achieve the optimal power generation P 1DC ;

[0013] Step 7: P calculated in step 4 1max and P 1min , P calculated in step 5 r , and P calculated in step 6 0DC and P 1DC , the load carrying capacity of the AC / DC hybrid transmission network under the current base state flow is evaluated.

[0014] As a further optimization scheme of the evaluation method of the load carrying capacity of an AC / DC hybrid transmission network described in the present invention, the DC power flow equation in step 1 is specifically as follows:

[0015] For the AC nodes of the AC / DC power grid, the AC nodes are called nodes, and the node active power balance equation is expressed as:

[0016]

[0017] Among them, P i is the power consumed by the i-th node; P DCik is the power transmitted from the ith node to the kth node through the DC line. If the power flow direction is opposite, the value is negative; U i , U j are the voltage amplitudes of the i-th and j-th nodes respectively, G ij , B ij are the Y in the AC system admittance matrix ij The real and imaginary parts of the elements, Y ij is the admittance matrix of the AC system, N is the total number of AC nodes in the AC / DC power grid system, δ ij is the phase angle difference between the voltages at the i-th node and the j-th node;

[0018] For AC / DC transmission networks, the node power balance equation is:

[0019]

[0020] Among them, δ i ,δ j are the voltage phase angles of the i-th and j-th nodes, respectively, and x ij is the impedance value of the AC line between the i-th node and the j-th node;

[0021] The DC power flow equation further converted into matrix form is:

[0022]

[0023] make

[0024]

[0025] Where B is the system node admittance matrix, is the system node voltage phase angle column vector, is the system node power column vector, is the DC power transmission column vector of the system nodes, and the system refers to the AC / DC power grid system;

[0026] The DC power flow equation is expressed as

[0027]

[0028] As a further optimization scheme of the evaluation method of the load carrying capacity of an AC / DC hybrid transmission network described in the present invention, in step 2, based on the DC power flow equation, the power transmission allocation factor matrix calculation method of the AC / DC hybrid power grid is:

[0029] 1) For the DC power flow equation obtained in step 1, remove the rows and columns where the balance nodes in the line are located, and we get

[0030]

[0031] Where B′ is the system node admittance matrix without the equilibrium node, is the system node voltage phase angle column vector excluding the balance node, is the system node power column vector excluding the balance node, The column vector of DC power delivered to the system nodes excluding the balancing nodes;

[0032] 2) Unit active power is injected into the kth node of the unbalanced node, and the power is balanced through the balanced node. The power change of each line when the kth node injects unit power is calculated. The calculation method is:

[0033] make

[0034]

[0035] PD ' Cji =0,j≠i

[0036] Among them, P i 'for The system node power of the i-th node in , P D ' Cji for The DC power transmitted from the jth node to the ith node in the system;

[0037] Depend on get

[0038]

[0039] Then get

[0040]

[0041] in, is the voltage difference between the i-th node and the j-th node, is the current flowing from the ith node to the jth node, is the voltage of the ith node, y ij is the admittance of the AC line between the ith node and the jth node, is the voltage of the jth node, g ij is the conductance of the AC line between the i-th node and the j-th node, δ ij ′The voltage phase difference between the i-th node and the j-th node, b ij is the susceptance of the AC line between the i-th node and the j-th node, δ i ′ is the voltage phase angle of the i-th node, δ j ′ is the voltage phase angle of the jth node;

[0042] Whenever a unit power is injected into the kth node, the active power flowing through the AC line between the i-th node and the j-th node increases by P ij , P ij represents the power transmission distribution coefficient of the kth node on the AC line between the i-th node and the j-th node; when the i-th node absorbs unit power, P ij '=-P ij , P ij ' represents the load power transmission distribution coefficient of the kth node unit load increase on the AC line between the ith node and the jth node;

[0043] 3) For the DC line between the kth node and the tth node, calculate: when the DC line transmission power between the kth node and the tth node increases by unit power, the power change of each line is calculated as follows:

[0044] make

[0045] P i ′=0,P′ DCkt =1,P′ DCtk =-1,k≠t

[0046] Among them, P′ DCkt for The DC power transmitted from the kth node to the tth node in the system, P′ DCtk for The DC power transmitted from the tth node to the kth node in the system;

[0047] Depend on have to

[0048]

[0049] Then get

[0050]

[0051] In summary, the power transmission allocation factor matrix M is obtained ptdf :

[0052]

[0053] Among them, N G 、N L , L, and Z are the number of power generation nodes, load nodes, AC lines, and DC lines, respectively. ln is the power transmission allocation coefficient of the nth generation node to the lth line, b lm is the load power transmission allocation coefficient of the mth load node to the lth line, 1≤n≤N G , 1≤m≤N L , c lz is the power transmission allocation coefficient of the zth DC line at the lth line, 1≤z≤Z, l=1,2,…,L.

[0054] As a further optimization scheme of the evaluation method of load carrying capacity of an AC / DC hybrid transmission network described in the present invention, △S in step 3 line With μ load The calculation expression is:

[0055] ΔS line =S max -S0

[0056]

[0057] Among them, S maxis the maximum power allowed by the line, and S0 is the apparent power of the line under the basic operation state of the power grid.

[0058] As a further optimization scheme of the evaluation method of the load carrying capacity of an AC / DC hybrid transmission network described in the present invention, in step 4, P 1max and P 1min The method to obtain it is:

[0059] In the scenario where the load nodes grow proportionally, the power generation nodes grow proportionally based on the available additional issuance, and always keep the balance between power generation and load growth, the maximum total additional issuance of the power generation nodes when each line reaches congestion is calculated as:

[0060]

[0061] Among them, N G 、N L , L are the number of power generation nodes, load nodes and AC lines respectively, S lmax is the maximum power allowed by the lth line, S l0 is the apparent power of the line under the basic operation state of the power grid, a ln is the power transfer allocation coefficient of the nth generation node at the mth line, ΔP Gln is the additional active power generated by the nth power generation node, b lm is the load power transfer distribution coefficient of the mth load node at the lth line, ΔP Lm is the load added to the mth load node; P Lm is the active load of the mth load node under basic conditions,

[0062] Then

[0063]

[0064] Then, by

[0065] ΔP Gn =min(ΔP Gln )

[0066] Among them, ΔP Gn The minimum value of the additional active power of the nth power generation node;

[0067] The maximum load carrying capacity of each power generation node under the line constraint with single-node additional generation is obtained, and the additional generation amount of each node is checked based on the upper and lower limits of the power generation node. The constraints are as follows:

[0068] P Gnmin -P Gn0 ≤ΔP Gn ≤P Gnmax -P Gn0,1≤n≤N G

[0069] Among them, N G is the number of power generation nodes, ΔP Gn is the additional active power generated by the nth power generation node, P Gnmin is the minimum power generation of the nth power generation node, P Gn0 is the power generation of the nth power generation node under the basic scenario, P Gnmax is the maximum power generation of the nth power generation node;

[0070] If the power generation does not meet the constraint conditions, then the nth power generation node has no line overload problem, and the maximum additional issuance amount at this time takes the maximum value of the constraint; after all power generation nodes are checked, the maximum value P of the maximum additional issuance amount of each power generation node under the line constraint of single node additional issuance is obtained. 1max and the minimum value P 1min .

[0071] As a further optimization scheme of the evaluation method of the load carrying capacity of an AC / DC hybrid transmission network described in the present invention, step 5 is specifically as follows:

[0072] Calculate the maximum total increase in the amount of power generation nodes when each line reaches congestion in the scenario where the load node grows proportionally, the power generation node grows proportionally based on the available increase, and always keeps the balance between power generation and load growth. The calculation expression is:

[0073]

[0074] Among them, N G 、N L , L are the number of power generation nodes, load nodes and AC lines respectively, S lmax is the maximum power allowed by the lth line, S l0 is the apparent power of the line under the basic operation state of the power grid, a ln is the power transmission allocation coefficient of the nth generation node at the lth line, ΔP Gn is the additional active power generated by the nth power generation node, b lm is the load power transfer distribution coefficient of the mth load node at the lth line, ΔP Gl To calculate the maximum total amount of additional issuance for the lth line, ΔP Lm The added load of the mth load node; ΔP Gn is the total additional amount of power generation nodes, P Gn is the remaining additional power generation capacity under the base state power flow of the nth power generation node, P Lm is the active load of the mth load node under the base state power flow;

[0075] Then

[0076]

[0077] At the same time by

[0078] ΔP G =min(ΔP Gl )

[0079] Among them, ΔP G It is the maximum value of the additional active power generated by the power generation node in the scenario where the available additional power increases in proportion;

[0080] The maximum load carrying capacity result is obtained when the power generation increases according to the proportion of the power generation surplus. At the same time, the additional power generation of each node is checked based on the upper and lower limits of the power generation node. The constraints are as follows:

[0081] P Gnmin -P Gn0 ≤ΔP Gn ≤P Gnmax -P Gn0 ,1≤n≤N G

[0082] Among them, N G is the number of power generation nodes, ΔP Gn is the additional active power generated by the nth power generation node, P Gnmin is the minimum power generation of the nth power generation node, P Gn0 is the power generation of the nth power generation node under the basic scenario, P Gnmax is the maximum power generation of the nth power generation node;

[0083] If the power generation of the nth power generation node does not meet the constraint conditions, the power generation of the nth power generation node takes the maximum value and recalculates ΔP G , ΔP G The value of is the maximum additional generation P of the power generation node under the line constraint, which increases in proportion to the additional generation surplus of the power generation node. r .

[0084] As a further optimization scheme of the evaluation method of the load carrying capacity of an AC / DC hybrid transmission network according to the present invention, in step 6, P 0DC The calculation method is:

[0085] Considering that the transmission power of the DC line is fixed, the maximum total increase in the generation node and the power generation of each generation node in the scenario where the load of each line has not reached the blocking state, the load increases proportionally, the generation node meets the upper and lower limits of generation, and the load growth and generation growth are always balanced are calculated; the calculation optimization model is:

[0086] The optimization variables are

[0087]

[0088] Among them, ΔP Gn is the additional issuance amount of the nth power generation node, 1≤n≤N G , λ is the ratio of the load node load increase to the basic load, X1 is the optimization model solution column vector of the first optimization model, and the superscript T is the transpose;

[0089] The objective function is

[0090]

[0091] Constraints

[0092]

[0093] Among them, f1 is the objective function solved by the first optimization model, N G 、N L , L are the number of power generation nodes, load nodes and AC lines respectively, S lmax is the maximum power allowed by the lth line, S l0 is the apparent power of the line under the basic operation state of the power grid, a ln is the power transmission allocation coefficient of the nth generation node at the lth line, ΔP Gn is the additional active power generated by the nth power generation node, b lm is the load power transfer distribution coefficient of the mth load node at the lth line, P Lm is the active load of the mth load node under the base state power flow, P Gnmin is the minimum power generation of the nth power generation node, P Gn0 is the power generation of the nth power generation node under the base state power flow, P Gnmax is the maximum power generation of the nth power generation node;

[0094] Based on the first optimization model, the optimal power generation P is calculated with the DC power fixed and the maximum total increase of the power generation node as the goal. 0DC And the power generation of each power generation node under the optimal condition.

[0095] As a further optimization scheme of the evaluation method of the load carrying capacity of an AC / DC hybrid transmission network according to the present invention, in step 6, P 1DC The calculation method is:

[0096] Considering that the transmission power of DC lines is adjustable, the maximum total increase in generation nodes and the generation capacity of each generation node in this case are calculated under the scenario that the load of each line has not reached the blocking state, the load increases proportionally, the generation nodes meet the upper and lower generation constraints, and the load growth and generation growth are always balanced. The calculation optimization model is:

[0097] The optimization variables are

[0098]

[0099] Among them, X2 is the optimization model solution column vector of the second optimization model, ΔP Gn is the additional issuance amount of the nth power generation node, 1≤n≤N G , λ is the ratio of the load increase at the load node to the base load, ΔP DCz is the increase in the transmission power of the zth DC line, 1≤z≤Z;

[0100] The objective function is

[0101]

[0102] Constraints

[0103]

[0104] Among them, f2 is the objective function solved by the second optimization model, N G 、N L , L, and Z are the number of power generation nodes, load nodes, AC lines, and DC lines, respectively. lmax is the maximum power allowed by the lth line, S l0 is the apparent power of the line under the basic operation state of the power grid, a ln is the power transmission allocation coefficient of the nth generation node at the lth line, ΔP Gn is the additional active power generated by the nth power generation node, b lm is the load power transfer distribution coefficient of the mth load node at the lth line, P Lm is the active load of the mth load node under the base state power flow, c lz is the power transfer distribution coefficient of the zth DC line at the lth line, ΔP DCz is the increase in power transmitted by the zth DC line, P Gnmin is the minimum power generation of the nth power generation node, P Gn0 is the power generation of the nth power generation node under the base state power flow, P Gnmax is the maximum power generation of the nth power generation node, P DCzmin is the minimum transmission capacity of the zth DC line, P DCz0 is the power transmission capacity of the zth DC line under the base state power flow scenario, P DCzmax is the maximum transmission capacity of the zth DC line;

[0105] Based on this optimization model, the optimal power generation P of the DC power can be calculated with the maximum total increase in power generation nodes as the goal. 1DCAnd the power generation of each power generation node and the increase in DC line power in this case.

[0106] As a further optimization scheme of the evaluation method of the load carrying capacity of the AC / DC hybrid transmission network described in the present invention, in step 7, based on P 1max , P 1min , P r , P 0DC and P 1DC The calculation results in steps 4 to 6 are used to comprehensively evaluate the load carrying capacity of the AC / DC hybrid transmission network under the current base state flow.

[0107] Compared with the prior art, the present invention adopts the above technical solution and has the following technical effects:

[0108] This method can evaluate the maximum load carrying capacity of the AC / DC hybrid power grid, analyze the maximum load carrying capacity under different power generation node increase methods, and the optimal power generation method and optimal DC transmission power with the maximum load carrying as the goal, and evaluate the load carrying capacity of the power grid under the current trend. BRIEF DESCRIPTION OF THE DRAWINGS

[0109] Figure 1 The present invention is a flow chart of a method for evaluating the load carrying capacity of an AC / DC hybrid transmission network;

[0110] Figure 2 This is the topological diagram of the transmission network connection in Jiangsu Province in the example;

[0111] Figure 3 It is the maximum additional issuance amount of each node subject to line constraints in the single-node additional issuance scenario in the example;

[0112] Figure 4 It is the additional generation of each node under the optimal power generation scenario under the HVDC fixed scenario in the example;

[0113] Figure 5 It shows the increase in power of each node and the increase in power of the DC line under the optimal power generation scenario in the high-voltage DC adjustable scenario in the example. DETAILED DESCRIPTION

[0114] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0115] The present invention conducts research on the load carrying capacity evaluation method of the AC / DC hybrid transmission network, focusing on analyzing the maximum load carrying capacity under different power generation node increase modes and the optimal power generation mode and optimal DC transmission power with the maximum load carrying as the goal, so as to evaluate the load carrying capacity of the power grid under the current trend.

[0116] like Figure 2The figure shows the wiring diagram of part of the power transmission network in Jiangsu. The network includes 500kV AC power transmission network lines and 3 500kV flexible DC lines. The numbers 1 to 38 in the figure represent nodes 1 to 38. Among them, nodes 1 to 6 are power generation nodes, nodes 7 to 11 are load nodes, and the lines between nodes 25 and 38, nodes 36 and 37, and nodes 32 and 33 are 500kV high-voltage DC lines. The remaining lines are 500kV cross lines. The base state flow data is taken from the Jiangsu power transmission network. Some nodes and 220kV lines of the power transmission network are not drawn in the figure.

[0117] For the above system, the evaluation method of the load carrying capacity of the AC / DC hybrid transmission network proposed in the present invention is as follows: Figure 1 As shown in the process, the following steps are included:

[0118] Step 1: Establish the DC power flow equation of the AC / DC hybrid power grid based on the AC / DC power grid topology;

[0119] The specific steps of step 1 are:

[0120] For the AC nodes of the AC / DC power grid, the AC nodes are called nodes, and the node active power balance equation is expressed as:

[0121]

[0122] Among them, P i is the power consumed by the i-th node; P DCik is the power transmitted from the ith node to the kth node through the DC line. If the power flow direction is opposite, the value is negative; U i , U j are the voltage amplitudes of the i-th and j-th nodes respectively, G ij , B ij are the Y in the AC system admittance matrix ij The real and imaginary parts of the elements, Y ij is the admittance matrix of the AC system, N is the total number of AC nodes in the AC / DC power grid system, δ ij is the phase angle difference between the voltages at the i-th node and the j-th node;

[0123] For AC / DC transmission networks, the node power balance equation is:

[0124]

[0125] Among them, δ i ,δ j are the voltage phase angles of the i-th and j-th nodes, respectively, and x ij is the impedance value of the AC line between the i-th node and the j-th node;

[0126] Then the DC power flow equation in matrix form for all AC nodes in the power grid is:

[0127]

[0128] Step 2: According to the DC power flow equation of the AC / DC hybrid power grid, the power transmission allocation factor matrix M of the AC / DC hybrid power grid is calculated. ptdf ;

[0129] The specific steps of step 2 are:

[0130] 1) For the equation obtained in step 1, remove the row and column where the balanced node in the circuit is located, and we can get

[0131]

[0132] 2) Unit active power is injected into the kth node of the unbalanced node, and the power is balanced through the balanced node. The power change of each line when the kth node injects unit power is calculated. The calculation method is:

[0133] make

[0134]

[0135] P′ DCji =0,j≠i

[0136] Among them, P i 'for The system node power of the i-th node in , P′ DCji for The DC power transmitted from the jth node to the ith node in the system;

[0137] Depend on get

[0138]

[0139] Then get

[0140]

[0141] in, is the voltage difference between the i-th node and the j-th node, is the current flowing from the ith node to the jth node, is the voltage of the ith node, y ij is the admittance of the AC line between the ith node and the jth node, is the voltage of the jth node, g ij is the conductance of the AC line between the i-th node and the j-th node, δ ij ′The voltage phase difference between the i-th node and the j-th node, bij is the susceptance of the AC line between the i-th node and the j-th node, δ i ′ is the voltage phase angle of the i-th node, δ j ′ is the voltage phase angle of the jth node;

[0142] Whenever a unit power is injected into the kth node, the active power flowing through the AC line between the i-th node and the j-th node increases by P ij , P ij represents the power transmission distribution coefficient of the kth node on the AC line between the i-th node and the j-th node; when the i-th node absorbs unit power, P ij '=-P ij , P ij ' represents the load power transmission distribution coefficient of the kth node unit load increase on the AC line between the ith node and the jth node;

[0143] 3) For the DC line between the kth node and the tth node, calculate: when the DC line transmission power between the kth node and the tth node increases by unit power, the power change of each line is calculated as follows:

[0144] make

[0145] P i ′=0,P′ DCkt =1,P′ DCtk =-1,k≠t

[0146] Among them, P′ DCkt for The DC power transmitted from the kth node to the tth node in the system, P′ DCtk for The DC power transmitted from the tth node to the kth node in the system;

[0147] Depend on have to

[0148]

[0149] Then get

[0150]

[0151] In summary, the power transmission allocation factor matrix M is obtained ptdf :

[0152]

[0153] Among them, N G 、N L , L, and Z are the number of power generation nodes, load nodes, AC lines, and DC lines, respectively.ln is the power transmission allocation coefficient of the nth generation node to the lth line, b lm is the load power transmission allocation coefficient of the mth load node to the lth line, 1≤n≤N G , 1≤m≤N L , c lz is the power transmission allocation coefficient of the zth DC line at the lth line, 1≤z≤Z, l=1,2,…,L.

[0154] Since there are too many nodes and lines in the power grid in this example, only the power generation and load nodes with high power generation and load and greater impact on the network, as well as the AC lines and DC lines with high load rates and greater impact on the load and power generation in the power grid are considered here for power transmission allocation factor matrix M ptdf The calculation, node and line selection are shown in Table 1 below:

[0155] Table 1 Selection of power generation nodes, load nodes and important lines

[0156]

[0157]

[0158] The AC lines are marked as lines 1 to 13 and the DC lines as lines DC1 to DC3 in order, thus M can be obtained. ptdf matrix:

[0159]

[0160] Among them, the first 6 columns of the matrix are the power transmission allocation coefficients of the power generation node to the line l, columns 7 to 11 are the power transmission allocation coefficients of the load node to the line l, and columns 11 to 14 are the power transmission allocation coefficients of the DC line to the line l.

[0161] Step 3: Calculate the available transmission capacity △S of each transmission line under the basic load state of the AC / DC hybrid power grid line With load factor μ load ;

[0162] The specific steps of step 3 are:

[0163] The calculation expressions of the remaining capacity and load rate of each transmission line under the basic operation state are as follows:

[0164] ΔS line =S max -S0

[0165]

[0166] Among them, S maxis the maximum power allowed by the line, and S0 is the apparent power of the line under the basic operation state of the power grid.

[0167] The maximum power allowed by each line selected in this example and the apparent power under the basic operation state of the power grid are as follows:

[0168] Table 2 Maximum allowable power of the line and apparent power under base state power flow

[0169]

[0170] The remaining capacity and load rate of each selected line can be calculated from the data in the table as shown in Table 3:

[0171] Table 3 Line base state remaining capacity and load rate

[0172]

[0173]

[0174] Step 4: According to M ptdf , △S line With μ load , calculate the maximum additional generation of each generation node under the line congestion constraint in the scenario where the load increases proportionally and the system power balance is achieved by increasing the generation of a single generation node;

[0175] The specific steps of step 4 are:

[0176] In the scenario where the load nodes grow proportionally, the power generation nodes grow proportionally based on the available additional issuance, and always keep the balance between power generation and load growth, the maximum total additional issuance of the power generation nodes when each line reaches congestion is calculated as:

[0177]

[0178] Among them, N G 、N L , L are the number of power generation nodes, load nodes and AC lines respectively, S lmax is the maximum power allowed by the lth line, S l0 is the apparent power of the line under the basic operation state of the power grid, a ln is the power transfer allocation coefficient of the nth generation node at the mth line, ΔP Gln is the additional active power generated by the nth power generation node, b lm is the load power transfer distribution coefficient of the mth load node at the lth line, ΔP Lm is the load added to the mth load node; P Lm is the active load of the mth load node in the basic case, then

[0179]

[0180] Then, by

[0181] ΔP Gn =min(ΔP Gln )

[0182] Among them, ΔP Gn The minimum value of the additional active power of the nth power generation node;

[0183] The maximum load carrying capacity of each power generation node under the line constraint with single-node additional generation is obtained, and the additional generation amount of each node is checked based on the upper and lower limits of the power generation node. The constraints are as follows:

[0184] P Gnmin -P Gn0 ≤ΔP Gn ≤P Gnmax -P Gn0 ,1≤n≤N G

[0185] Among them, N G is the number of power generation nodes, ΔP Gn is the additional active power generated by the nth power generation node, P Gnmin is the minimum power generation of the nth power generation node, P Gn0 is the power generation of the nth power generation node under the basic scenario, P Gnmax is the maximum power generation of the nth power generation node;

[0186] If the power generation does not meet the constraints, then there is no line overload problem at the nth power generation node, and the maximum additional generation amount at this time takes the maximum value of the constraint.

[0187] The maximum additional issuance amount for each single node additional issuance scenario can be calculated from the above formula as shown in Table 4 and Figure 3 As shown,

[0188] Table 4: Additional issuance amount and blocking information in a single node additional issuance scenario

[0189]

[0190]

[0191] It is verified that the additional generation of each node meets the constraint conditions. The maximum value of the additional generation of a single power generation node under line constraints can be obtained from the table: 1max The maximum value P is 2544MW. 1min 388MW;

[0192] Step 5: According to M ptdf , △S line With μ load, calculate the maximum additional generation capacity of the generation node under the line congestion constraint in the scenario where the load increases proportionally and each generation node increases proportionally based on the available additional generation capacity;

[0193] The specific steps of step 5 are:

[0194] Calculate the maximum total increase in the amount of power generation nodes when each line reaches congestion in the scenario where the load node grows proportionally, the power generation node grows proportionally based on the available increase, and always keeps the balance between power generation and load growth. The calculation expression is:

[0195]

[0196] Among them, N G 、N L , L are the number of power generation nodes, load nodes and AC lines respectively, S lmax is the maximum power allowed by the lth line, S l0 is the apparent power of the line under the basic operation state of the power grid, a ln is the power transmission allocation coefficient of the nth generation node at the lth line, ΔP Gn is the additional active power generated by the nth power generation node, b lm is the load power transfer distribution coefficient of the mth load node at the lth line, ΔP Gl To calculate the maximum total amount of additional issuance for the lth line, ΔP Lm The added load of the mth load node; ΔP Gn is the total additional amount of power generation nodes, P Gn is the remaining additional power generation capacity under the base state power flow of the nth power generation node, P Lm is the active load of the mth load node under the base state power flow;

[0197] Then

[0198]

[0199] At the same time by

[0200] ΔP G =min(ΔP Gl )

[0201] Among them, ΔP G It is the maximum value of the additional active power generated by the power generation node in the scenario where the available additional power increases in proportion;

[0202] The maximum load carrying capacity result is obtained when the power generation increases according to the proportion of the power generation surplus. At the same time, the additional power generation of each node is checked based on the upper and lower limits of the power generation node. The constraints are as follows:

[0203] P Gnmin -PGn0 ≤ΔP Gn ≤P Gnmax -P Gn0 ,1≤n≤N G

[0204] Among them, N G is the number of power generation nodes, ΔP Gn is the additional active power generated by the nth power generation node, P Gnmin is the minimum power generation of the nth power generation node, P Gn0 is the power generation of the nth power generation node under the basic scenario, P Gnmax is the maximum power generation of the nth power generation node;

[0205] If the power generation of the nth power generation node does not meet the constraint conditions, the power generation of the nth power generation node takes the maximum value and recalculates ΔP G , ΔP G The value of is the maximum additional generation P of the power generation node under the line constraint, which increases in proportion to the additional generation surplus of the power generation node. r .

[0206] The maximum additional issuance amount and line congestion information in this scenario can be calculated from the above formula, as shown in Table 5:

[0207] Table 5: The additional generation and congestion information in the scenario where the generation is increased in proportion to the generation surplus

[0208] Calculated variables Calculation results <![CDATA[Maximum total additional power generation P r / MW]]> 1057 Blocked line number Line 7

[0209] Verify that the additional issuance of each node meets the constraints.

[0210] Step 6: According to M ptdf , △S line With μ load , considering the two cases of fixed and adjustable transmission power of high-voltage DC lines, taking the maximum total increase in power generation at the power generation node as the goal, the optimal power generation of each power generation node and the optimal transmission power of the DC transmission line constrained by the transmission capacity of the transmission line are obtained;

[0211] The specific steps of step 6 are:

[0212] 1) Considering that the transmission power of the DC line is fixed, the maximum total increase in the generation node and the power generation of each generation node in the scenario where the load of each line has not reached the blocking state, the load increases proportionally, the generation node meets the upper and lower generation constraints, and the load growth and generation growth are always balanced are calculated; the calculation optimization model is:

[0213] The optimization variables are

[0214]

[0215] Among them, ΔP Gn is the additional issuance amount of the nth power generation node, 1≤n≤N G , λ is the ratio of the load node load increase to the basic load, X1 is the optimization model solution column vector of the first optimization model, and the superscript T is the transpose;

[0216] The objective function is

[0217]

[0218] Constraints

[0219]

[0220] Among them, f1 is the objective function solved by the first optimization model, N G 、N L , L are the number of power generation nodes, load nodes and AC lines respectively, S lmax is the maximum power allowed by the lth line, S l0 is the apparent power of the line under the basic operation state of the power grid, a ln is the power transmission allocation coefficient of the nth generation node at the lth line, ΔP Gn is the additional active power generated by the nth power generation node, b lm is the load power transfer distribution coefficient of the mth load node at the lth line, P Lm is the active load of the mth load node under the base state power flow, P Gnmin is the minimum power generation of the nth power generation node, P Gn0 is the power generation of the nth power generation node under the base state power flow, P Gnmax is the maximum power generation of the nth power generation node;

[0221] Based on the optimization of the first optimization model, the maximum total increase in the generation node in this scenario and the generation capacity of each generation node and the increase in load of each load node in this case can be obtained as shown in Table 6 and Figure 4 As shown:

[0222] Table 6 Node information under DC fixed optimization

[0223]

[0224] 2) Considering that the transmission power of DC lines is adjustable, the maximum total increase in generation capacity of the generation nodes and the generation capacity of each generation node in this case are calculated under the scenario that the load of each line has not reached the blocking state, the load increases proportionally, the generation nodes meet the generation upper and lower limit constraints, and the load growth and generation growth are always balanced. The calculation optimization model is:

[0225] The optimization variables are

[0226]

[0227] Among them, X2 is the optimization model solution column vector of the second optimization model, ΔP Gn is the additional issuance amount of the nth power generation node, 1≤n≤N G , λ is the ratio of the load increase at the load node to the base load, ΔP DCz is the increase in the transmission power of the zth DC line, 1≤z≤Z;

[0228] The objective function is

[0229]

[0230] Constraints

[0231]

[0232] Among them, f2 is the objective function solved by the second optimization model, N G 、N L , L, and Z are the number of power generation nodes, load nodes, AC lines, and DC lines, respectively. lmax is the maximum power allowed by the lth line, S l0 is the apparent power of the line under the basic operation state of the power grid, a ln is the power transmission allocation coefficient of the nth generation node at the lth line, ΔP Gn is the additional active power generated by the nth power generation node, b lm is the load power transfer distribution coefficient of the mth load node at the lth line, P Lm is the active load of the mth load node under the base state power flow, c lz is the power transfer distribution coefficient of the zth DC line at the lth line, ΔP DCz is the increase in power transmitted by the zth DC line, P Gnmin is the minimum power generation of the nth power generation node, P Gn0 is the power generation of the nth power generation node under the base state power flow, P Gnmax is the maximum power generation of the nth power generation node, P DCzmin is the minimum transmission capacity of the zth DC line, P DCz0 is the power transmission capacity of the zth DC line under the base state power flow scenario, P DCzmax is the maximum transmission capacity of the zth DC line;

[0233] Based on this optimization model, the maximum total increase in the generation capacity of the power generation nodes in this scenario and the power generation capacity of each power generation node and the increase in DC line power in this case can be obtained as shown in Table 7 and Figure 5 As shown:

[0234] Table 7 Information of each node under DC adjustable optimization

[0235]

[0236]

[0237] Step 7: Based on the maximum load growth calculated under the above multiple scenarios, evaluate the load carrying capacity of the AC / DC hybrid transmission network under the current base state flow.

[0238] The specific method is to establish indicators as follows:

[0239] 1) The maximum value P of the maximum additional amount of each power generation node under the line constraint is a single node additional amount 1max ;

[0240] 2) The minimum value P of the maximum additional amount of each power generation node under the line constraint based on the single node additional amount 1min ;

[0241] 3) The generation node increases in proportion to the surplus of power generation that can be increased. The maximum increase in power generation under line constraints is P r ;

[0242] 4) The optimal power generation P with the DC power fixed to maximize the total increase in power generation nodes 0DC ;

[0243] 5) The DC power can be adjusted to maximize the total increase in the power generation node. 1DC ;

[0244] The above five indicators and the calculation results in steps 4 to 6 are shown in Table 8:

[0245] Table 8 Load bearing capacity index and calculated value

[0246] index Index value Calculated Value <![CDATA[P Smax ]]> 2000 2544 <![CDATA[P Smin ]]> 200 388 <![CDATA[P R ]]> 1000 1057 <![CDATA[P 0DC ]]> 4000 4998 <![CDATA[P 1DC ]]> 6000 9100

[0247] From the comparison in the above table, it can be seen that the calculated values ​​of each indicator in this example have reached and exceeded the indicator value. In particular, the optimal load under DC control far exceeds the indicator value, reaching 1.5 times the indicator value. Therefore, a comprehensive analysis shows that the load carrying capacity of the AC / DC hybrid transmission network under the base state flow selected in this example is strong.

[0248] In summary, through the above calculations and index analysis, a relatively comprehensive and rapid evaluation and analysis of the load carrying capacity of the AC / DC hybrid transmission network under base state power flow can be carried out.

[0249] The above description is only a specific implementation of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a technician familiar with the technical field within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for evaluating the load carrying capacity of an AC / DC hybrid transmission network, characterized in that: The following steps are involved: Step 1: Establishing a DC power flow equation of an AC / DC hybrid power grid based on an AC / DC power grid topology structure; Step 2: According to the DC power flow equation of the AC / DC hybrid power grid, the power transmission allocation factor matrix M of the AC / DC hybrid power grid is calculated. ptdf ; Step 3: Calculate the available transmission capacity of each transmission line under the basic load state of the AC / DC hybrid power grid △S line With load factor μ load ; Step 4: According to M ptdf , △S line With μ load , calculate the maximum value P of the maximum additional amount of each power generation node under the line constraint with single node additional generation 1max and the minimum value P 1min ; Step 5: According to M ptdf , △S line With μ load , calculate the maximum additional generation P of the power generation node under the line constraint according to the proportional increase of the power generation surplus r ; Step 6: According to M ptdf , △S line With μ load Considering the two cases of fixed and adjustable transmission power of high-voltage DC lines, the optimal power generation P is calculated with the DC power fixed and the total increase of power generation nodes as the goal. 0DC , DC power adjustable to maximize the total increase in power generation nodes to achieve the optimal power generation P 1DC ; Step 7: P calculated in step 4 1max and P 1min , P calculated in step 5 r , and P calculated in step 6 0DC and P 1DC , evaluate and obtain the load carrying capacity of the AC / DC hybrid transmission network under the current base state power flow; The DC power flow equation in step 1 is as follows: For the AC nodes of the AC / DC power grid, the AC nodes are called nodes, and the node active power balance equation is expressed as: Among them, P i is the power consumed by the i-th node; P DCik is the power transmitted from the ith node to the kth node through the DC line. If the power flow direction is opposite, the value is negative; U i , U j are the voltage amplitudes of the i-th and j-th nodes respectively, G ij , B ij are the Y in the AC system admittance matrix ij The real and imaginary parts of the elements, Y ij is the admittance matrix of the AC system, N is the total number of AC nodes in the AC / DC power grid system, δ ij is the phase angle difference between the voltages at the i-th node and the j-th node; For AC / DC transmission networks, the node power balance equation is: Among them, δ i , δ j are the voltage phase angles of the i-th and j-th nodes, respectively, and x ij is the impedance value of the AC line between the i-th node and the j-th node; The DC power flow equation further converted into matrix form is: make Where B is the system node admittance matrix, is the system node voltage phase angle column vector, is the system node power column vector, is the DC power transmission column vector of the system nodes, and the system refers to the AC / DC power grid system; The DC power flow equation is expressed as In step 2, based on the DC power flow equation, the power transmission allocation factor matrix calculation method of the AC / DC hybrid power grid is: 1) For the DC power flow equation obtained in step 1, remove the rows and columns where the balance nodes in the line are located, and we get Where B′ is the system node admittance matrix without the equilibrium node, is the system node voltage phase angle column vector excluding the balance node, is the system node power column vector excluding the balance node, The column vector of DC power delivered to the system nodes excluding the balancing nodes; 2) Unit active power is injected into the kth node of the unbalanced node, and the power is balanced through the balanced node. The power change of each line when the kth node injects unit power is calculated. The calculation method is: make P′ DCji =0,j≠i Among them, P i 'for The system node power of the i-th node in , P′ DCji for The DC power transmitted from the jth node to the ith node in the system; Depend on get Then get in, is the voltage difference between the i-th node and the j-th node, is the current flowing from the i-th node to the j-th node, is the voltage of the ith node, y ij is the admittance of the AC line between the ith node and the jth node, is the voltage of the jth node, g ij is the conductance of the AC line between the i-th node and the j-th node, δ ij ′The voltage phase difference between the i-th node and the j-th node, b ij is the susceptance of the AC line between the i-th node and the j-th node, δ i ′ is the voltage phase angle of the i-th node, δ j ′ is the voltage phase angle of the jth node; Whenever a unit power is injected into the kth node, the active power flowing through the AC line between the i-th node and the j-th node increases by P ij , P ij represents the power transmission distribution coefficient of the kth node on the AC line between the i-th node and the j-th node; when the i-th node absorbs unit power, P ij '=-P ij , P ij ' represents the load power transmission distribution coefficient of the kth node unit load increase on the AC line between the ith node and the jth node; 3) For the DC line between the kth node and the tth node, calculate: when the DC line transmission power between the kth node and the tth node increases by unit power, the power change of each line is calculated as follows: make P i ′=0,P′ DCkt =1,P′ DCtk =-1,k≠t Among them, P′ DCkt for The DC power transmitted from the kth node to the tth node in the system, P′ DCtk for The DC power transmitted from the tth node to the kth node in the system; Depend on have to Then get In summary, the power transmission allocation factor matrix M is obtained ptdf : Among them, N G 、N L , L, and Z are the number of power generation nodes, load nodes, AC lines, and DC lines, respectively. ln is the power transmission allocation coefficient of the nth generation node to the lth line, b lm is the load power transmission allocation coefficient of the mth load node to the lth line, 1≤n≤N G , 1≤m≤N L , c lz is the power transfer distribution coefficient of the zth DC line at the lth line, 1≤z≤Z, l=1,2,…,L; △S in step 3 line With μ load The calculation expression is: ΔS line =S max -S0 Among them, S max is the maximum power allowed by the line, S0 is the apparent power of the line under the basic operation state of the power grid; Step 4 P 1max and P 1min The method to obtain it is: In the scenario where the load nodes grow proportionally, the power generation nodes grow proportionally based on the available additional issuance, and always keep the balance between power generation and load growth, the maximum total additional issuance of the power generation nodes when each line reaches congestion is calculated as: Among them, N G 、N L , L are the number of power generation nodes, load nodes and AC lines respectively, S lmax is the maximum power allowed by the lth line, S l0 is the apparent power of the line under the basic operation state of the power grid, a ln is the power transfer allocation coefficient of the nth generation node at the mth line, ΔP Gln is the additional active power generated by the nth power generation node, b lm is the load power transfer distribution coefficient of the mth load node at the lth line, ΔP Lm is the load added to the mth load node; P Lm is the active load of the mth load node under basic conditions, Then Then, by ΔP Gn =min(ΔP Gln ) Among them, ΔP Gn The minimum value of the additional active power of the nth power generation node; The maximum load carrying capacity of each power generation node under the line constraint with single-node additional generation is obtained, and the additional generation amount of each node is checked based on the upper and lower limits of the power generation node. The constraints are as follows: P Gnmin -P Gn0 ≤ΔP Gn ≤P Gnmax -P Gn0 ,1≤n≤N G Among them, N G is the number of power generation nodes, ΔP Gn is the additional active power generated by the nth power generation node, P Gnmin is the minimum power generation of the nth power generation node, P Gn0 is the power generation of the nth power generation node under the basic scenario, P Gnmax is the maximum power generation of the nth power generation node; If the power generation does not meet the constraint conditions, then the nth power generation node has no line overload problem, and the maximum additional issuance amount at this time takes the maximum value of the constraint; after all power generation nodes are checked, the maximum value P of the maximum additional issuance amount of each power generation node under the line constraint of single node additional issuance is obtained. 1max and the minimum value P 1min ; Step 5 is as follows: Calculate the maximum total increase in the amount of power generation nodes when each line reaches congestion in the scenario where the load node grows proportionally, the power generation node grows proportionally based on the available increase, and always keeps the balance between power generation and load growth. The calculation expression is: Among them, N G 、N L , L are the number of power generation nodes, load nodes and AC lines respectively, S lmax is the maximum power allowed by the lth line, S l0 is the apparent power of the line under the basic operation state of the power grid, a ln is the power transmission allocation coefficient of the nth generation node at the lth line, ΔP Gn is the additional active power generated by the nth power generation node, b lm is the load power transfer distribution coefficient of the mth load node at the lth line, ΔP Gl To calculate the maximum total amount of additional issuance for the lth line, ΔP Lm The added load of the mth load node; ΔP Gn is the total additional amount of power generation nodes, P Gn is the remaining additional power generation capacity under the base state power flow of the nth power generation node, P Lm is the active load of the mth load node under the base state power flow; Then At the same time by ΔP G =min(ΔP Gl ) Among them, ΔP G It is the maximum value of the additional active power generated by the power generation node in the scenario where the available additional power increases in proportion; The maximum load carrying capacity result is obtained when the power generation increases according to the proportion of the power generation surplus. At the same time, the additional power generation of each node is checked based on the upper and lower limits of the power generation node. The constraints are as follows: P Gnmin -P Gn0 ≤ΔP Gn ≤P Gnmax -P Gn0 ,1≤n≤N G Among them, N G is the number of power generation nodes, ΔP Gn is the additional active power generated by the nth power generation node, P Gnmin is the minimum power generation of the nth power generation node, P Gn0 is the power generation of the nth power generation node under the basic scenario, P Gnmax is the maximum power generation of the nth power generation node; If the power generation of the nth power generation node does not meet the constraint conditions, the power generation of the nth power generation node takes the maximum value and recalculates ΔP G , ΔP G The value of is the maximum additional generation P of the power generation node under the line constraint, which increases in proportion to the additional generation surplus of the power generation node. r ; Step 6 0DC The calculation method is: Considering that the transmission power of the DC line is fixed, the maximum total increase in the generation node and the power generation of each generation node in the scenario where the load of each line has not reached the blocking state, the load increases proportionally, the generation node meets the upper and lower limits of generation, and the load growth and generation growth are always balanced are calculated; the calculation optimization model is: The optimization variables are Among them, ΔP Gn is the additional issuance amount of the nth power generation node, 1≤n≤N G , λ is the ratio of the load node load increase to the basic load, X1 is the optimization model solution column vector of the first optimization model, and the superscript T is the transpose; The objective function is Constraints Among them, f1 is the objective function solved by the first optimization model, N G 、N L , L are the number of power generation nodes, load nodes and AC lines respectively, S lmax is the maximum power allowed by the lth line, S l0 is the apparent power of the line under the basic operation state of the power grid, a ln is the power transmission allocation coefficient of the nth generation node at the lth line, ΔP Gn is the additional active power generated by the nth power generation node, b lm is the load power transfer distribution coefficient of the mth load node at the lth line, P Lm is the active load of the mth load node under the base state power flow, P Gnmin is the minimum power generation of the nth power generation node, P Gn0 is the power generation of the nth power generation node under the base state power flow, P Gnmax is the maximum power generation of the nth power generation node; Based on the first optimization model, the optimal power generation P is calculated with the DC power fixed and the maximum total increase of the power generation node as the goal. 0DC and the power generation of each power generation node under the optimal condition; Step 6 1DC The calculation method is: Considering that the transmission power of DC lines is adjustable, the maximum total increase in generation nodes and the generation capacity of each generation node in this case are calculated under the scenario that the load of each line has not reached the blocking state, the load increases proportionally, the generation nodes meet the upper and lower generation constraints, and the load growth and generation growth are always balanced. The calculation optimization model is: The optimization variables are Among them, X2 is the optimization model solution column vector of the second optimization model, ΔP Gn is the additional issuance amount of the nth power generation node, 1≤n≤N G , λ is the ratio of the load increase at the load node to the base load, ΔP DCz is the increase in the transmission power of the zth DC line, 1≤z≤Z; The objective function is Constraints Among them, f2 is the objective function solved by the second optimization model, N G 、N L , L, and Z are the number of power generation nodes, load nodes, AC lines, and DC lines, respectively. lmax is the maximum power allowed by the lth line, S l0 is the apparent power of the line under the basic operation state of the power grid, a ln is the power transmission allocation coefficient of the nth generation node at the lth line, ΔP Gn is the additional active power generated by the nth power generation node, b lm is the load power transfer distribution coefficient of the mth load node at the lth line, P Lm is the active load of the mth load node under the base state power flow, c lz is the power transfer distribution coefficient of the zth DC line at the lth line, ΔP DCz is the increase in power transmitted by the zth DC line, P Gnmin is the minimum power generation of the nth power generation node, P Gn0 is the power generation of the nth power generation node under the base state power flow, P Gnmax is the maximum power generation of the nth power generation node, P DCzmin is the minimum transmission capacity of the zth DC line, P DCz0 is the power transmission capacity of the zth DC line under the base state power flow scenario, P DCzmax is the maximum transmission capacity of the zth DC line; Based on this optimization model, the optimal power generation P of the DC power can be calculated with the maximum total increase in power generation nodes as the goal. 1DC and the power generation of each power generation node and the increase in DC line power in this case; In step 7, based on P 1max , P 1min , P r , P 0DC and P 1DC The calculation results in steps 4 to 6 are used to comprehensively evaluate the load carrying capacity of the AC / DC hybrid transmission network under the current base state flow.

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