New energy bearing capacity assessment method considering transient overvoltage constraint

Abstract

The invention discloses a new energy bearing capacity evaluation method considering transient overvoltage constraint, and belongs to the technical field of energy safety. The invention discloses a new energy bearing capacity evaluation method considering transient overvoltage constraint. The method comprises the following steps: S1, calculating a short-circuit ratio of a multi-new energy station sending end system; s2, calculating the transient overvoltage of a new energy station node in the sending end system; and S3, constructing a power grid new energy bearing capacity evaluation model considering the transient overvoltage constraint. According to the new energy bearing capacity evaluation method considering the transient overvoltage constraint, the mathematical relation among the transient overvoltage amplitude, the system short-circuit ratio and the active power of the new energy station is used as a main line, and the system short-circuit ratio is used as an intermediate quantity; a mathematical expression of a new energy station node transient overvoltage amplitude and new energy station active power is established, and a power grid new energy bearing capacity evaluation model considering multiple new energy station transient overvoltage constraints is constructed, so that it is ensured that an optimal new energy station output value can be obtained, and safe operation of a power grid is ensured.

Description

Technical Field

[0001] The present invention relates to the field of energy safety technology, and in particular to a new energy carrying capacity assessment method considering transient overvoltage constraints. Background Art

[0002] Renewable energy represented by wind power and photovoltaics has been developed and utilized on a large scale in my country. my country's heavy-load centers and new energy-rich areas are distributed inversely, and large-scale new energy needs to be centrally transmitted and consumed. Due to its advantages such as long distance and large capacity, high-voltage direct current transmission is widely used in the centralized transmission and cross-regional consumption of new energy bases. However, with the increase in the number of new energy installed in the DC sending-end system, the voltage support strength of the power grid of the high-proportion new energy power system has gradually decreased. When a fault occurs, it is easy to cause serious transient overvoltage problems, causing the new energy station near the sending end to fail to disconnect from the grid, reducing the new energy carrying capacity. Therefore, the overvoltage problem in the sending-end system has become one of the main factors affecting the new energy carrying capacity of the power grid.

[0003] In recent years, domestic and foreign scholars have evaluated the new energy carrying capacity of the system taking into account the safety and stability constraints from the aspects of frequency stability constraints, power angle stability constraints, and small disturbance stability constraints. In terms of the new energy carrying capacity considering frequency stability, Xie Yan et al. analyzed the correlation between wind power penetration rate and the proportion of replacement synchronous machines based on the equivalent inertia of the wind power grid-connected system, and evaluated the new energy carrying capacity of the power grid based on the power system's requirements for the maximum frequency drop value and steady-state frequency deviation index. In terms of the new energy carrying capacity considering transient power angle stability, Lin Haiming et al. transformed the transient stability constraint into the output constraint of the system generator, and then substituted the solution of the wind farm penetration power limit optimization model into the time domain simulation for analysis, and continuously adjusted the generator output until the wind power penetration power limit that meets transient stability is obtained.

[0004] Currently, research on the assessment of renewable energy carrying capacity has not considered the risk constraints of transient overvoltage. Summary of the invention

[0005] The purpose of the present invention is to provide a new energy carrying capacity assessment method taking into account transient overvoltage constraints. The method takes the mathematical relationship between transient overvoltage amplitude, system short-circuit ratio and active power of new energy stations as the main line, takes the system short-circuit ratio as the intermediate quantity, establishes a mathematical expression of the transient overvoltage amplitude of the new energy station node and the active power of the new energy station, and constructs a new energy carrying capacity assessment model for the power grid taking into account the transient overvoltage constraints of multiple new energy stations, so as to ensure that the optimal output value of the new energy station can be obtained and the safe operation of the power grid can be guaranteed.

[0006] To achieve the above object, the present invention provides a method for evaluating the carrying capacity of new energy sources considering transient overvoltage constraints, comprising the following steps:

[0007] S1. Calculate the short-circuit ratio of the sending-end system of multiple renewable energy stations;

[0008] S2. Calculate the transient overvoltage of the nodes of the new energy station in the sending-end system;

[0009] S3. Construct a grid new energy carrying capacity assessment model considering transient overvoltage constraints; the objective function of the grid new energy carrying capacity assessment model is:

[0010]

[0011] Among them, T is the total time period, P w,i,t is the active power flowing into the AC grid from the new energy station at node i at time t, w is the new energy station, Ψ RE It is a collection of new energy station nodes.

[0012] Preferably, in S1, the short-circuit ratio of the sending-end system is defined as

[0013]

[0014] Where: S C is the system short-circuit capacity; P dN Rated transmission power for DC system;

[0015] The short-circuit capacity expression provided by the AC system for the commutation bus is:

[0016] S C =U N |I f |S b (2)

[0017] Where: I f , S b They are the per unit value of busbar three-phase short-circuit current and power reference value respectively;

[0018] The renewable energy grid-connected system is regarded as the superposition of the AC system and renewable energy, and the short-circuit current and short-circuit ratio provided by the AC system are calculated; the relationship between the short-circuit ratio of the sending-end system and the output of the synchronous generator and renewable energy station is obtained.

[0019] Preferably, the specific process of calculating the relationship between the short-circuit ratio of the sending-end system and the output of the synchronous generator and the new energy station is:

[0020] In the AC system, node 1-m1 is a new energy node, node m1+1-m2 is a synchronous generator node, node m2+1 is a DC converter station node, and node m2+2-n is a load node; the node current equation is:

[0021]

[0022] The node admittance matrix after considering the load integration is:

[0023]

[0024] In the formula, P D,j , Q D,j are the voltage, current, active power and reactive power of the load node respectively; the node admittance matrix Y′ is inverted to obtain the node impedance matrix Z′ after the load is incorporated;

[0025] Short-circuit current generated by renewable energy at converter station nodes The calculation formula is:

[0026]

[0027] In the formula, P w,j , Q w,j are the current, active power and reactive power flowing into the AC grid from the new energy station at node j, is the conjugate of the node voltage, is the parallel impedance from node j to the grounded node other than the converter station node;

[0028] Eliminate the load nodes and calculate the short-circuit capacity provided by the synchronous generator nodes;

[0029] The DC converter station nodes, synchronous generator nodes and new energy nodes are classified into one category, and the load nodes are classified into one category, then formula (3) is modified as follows:

[0030]

[0031] Where H is the self-impedance matrix of non-load nodes, which is an m2+1-order square matrix; R is the mutual impedance matrix between non-load nodes and load nodes, which is an (m2+1)*(n-m2-1)-order matrix; G is the mutual impedance matrix between non-load nodes and load nodes, which is an (n-m2-1)*(m2+1)-order matrix; D is the self-impedance matrix of load nodes, which is an n-m2-1-order square matrix; eliminate the load node:

[0032] I ac =(H-RD -1 G)U ac =Y″U ac (9)

[0033] The node admittance matrix after eliminating the load node is inverted to obtain the node impedance matrix Z″ after eliminating the load node; then the no-load voltage generated by the generator at the converter station node is:

[0034]

[0035]

[0036] In the formula, P g,j , Q g,j are the current, active power and reactive power flowing into the AC grid from the synchronous generator node j; the short-circuit current generated by the synchronous generator at the converter station node The calculation formula is:

[0037]

[0038] By superimposing equations (5), (6) and (10), (11) and (12), we can obtain the short-circuit current expression provided by the sending-end system to the converter station node:

[0039]

[0040] Substitute equation (13) into equation (2) to obtain the short-circuit capacity of the commutation busbar. Substitute the obtained short-circuit capacity into equation (1) to obtain the relationship between the short-circuit ratio of the sending-end system and the output of the synchronous generator and the new energy station:

[0041]

[0042] Preferably, S2 is specifically:

[0043] When a fault occurs in the DC transmission system, the transient voltage expression of the steady-state busbar at the sending-end converter station is:

[0044]

[0045] Where: U pr is the bus voltage after the fault; ΔU is the longitudinal component of the voltage drop; δU is the transverse component of the voltage drop; U S is the equivalent potential of the AC system;

[0046] Where ΔU and δU are

[0047]

[0048] Where: ΔP and ΔQ are the deviations of active power and reactive power caused during the fault period respectively;

[0049] Equivalent impedance of the sending end system X s for

[0050]

[0051] Combining equation (15), equation (16) and equation (17) we can get

[0052]

[0053] Simplifying equation (18) into per-unit form is:

[0054]

[0055] Substituting equation (1) into equation (19) and ignoring the influence of the voltage transverse component, the transient overvoltage expression of the commutation bus at the sending end is obtained as follows:

[0056]

[0057] Substituting equation (14) into equation (20), the mathematical expression between the transient overvoltage peak value at the converter station node and the output of the synchronous generator and the new energy station is obtained as follows:

[0058]

[0059] According to the node voltage matrix, the voltage of node i in the new energy station during normal operation is:

[0060]

[0061] Where: Z i,j is the mutual impedance between nodes i and j, Inject current into node j;

[0062] When a lockout fault occurs, it is equivalent to adding a fault branch at the fault point. Then the voltage at the fault node k is the superposition of the steady-state voltage and the transient voltage. The voltage at the fault node k is:

[0063]

[0064] Where: Z k,j is the mutual impedance between nodes k and j, Z k,k is the k-node self-impedance, Inject current into node k;

[0065] The voltage at node i of the new energy station is:

[0066]

[0067] for:

[0068]

[0069] Where: Z k is the ground branch impedance;

[0070] Substituting formula (25) into formula (24), we get:

[0071]

[0072] Where: Z i,k is the mutual impedance between node i and node k;

[0073] The voltage change at node i of the new energy station is:

[0074]

[0075] The voltage change at the converter station node k is:

[0076]

[0077] Combining equations (27) and (28), we can get the ratio of transient voltage rise between the new energy station node i and the converter station bus node k:

[0078]

[0079] According to the relationship between equation (21) and equation (29), the mathematical expression between the transient overvoltage peak value of the new energy station node and the output of the synchronous generator and the new energy station is:

[0080]

[0081] Preferably, the constraint condition of the objective function in S3 is:

[0082] 1) AC system constraints

[0083] The AC / DC hybrid system is coupled through the converter station access point, and the AC system power flow meets the following requirements:

[0084]

[0085] Where P G,i,t , Q G,i,t are the active power and reactive power of the generator node respectively; P w,i,t , Q w,i,t are the active power and reactive power of the new energy node respectively; P D,i,t , Q D,i,t are the active power and reactive power of the load node respectively; P dc,i,t , Q dc,i,t are the active power and reactive power of the DC transmission node respectively;

[0086] The branch flow must meet the following requirements:

[0087] U i,t U j,t [G ij,t cos(θi,t -θ j,t )+B ij,t sin(θ i,t -θ j,t )]-U i,t 2 G ij,t -P ij,t =0 (33)

[0088] U i,t U j,tj [B ij,t cos(θ i,t -θ j,t )-G ij,t sin(θ i,t -θ j,t )]-U i,t 2 B ij,t +Q ij,t =0 (34)

[0089] Where P ij,t , Q ij,t are the active power flow and reactive power flow of branch ij respectively;

[0090] The AC system node voltage must meet the following requirements:

[0091] U i,min ≤U i,t ≤U i,max (35)

[0092] Where U i,min and U i,max are the upper and lower limits of the node voltage respectively;

[0093] 2) Operation constraints of synchronous generator sets

[0094]

[0095] Where P G,i,min and P G,i,max They are the upper and lower limits of the active power of the generator; Q G,i,min and Q G,i,max They are the upper and lower limits of the reactive power of the generator respectively;

[0096] 3) New energy power constraints

[0097]

[0098] In the formula, is the power factor of new energy; the reactive power of new energy is provided by the grid-connected inverter and reactive power compensation device, and the power factor is constant;

[0099] 4) DC system operation constraints

[0100]

[0101] U dc,rec,t =U dc,inv,t +I dc,t R dc (41)

[0102] P dc,i,t =U dc,i,t I dc,i,t (42)

[0103] Q dc,i,t =U dc,i,t I dc,i,t cosα i,t (43)

[0104] Where U dc,i,t and I dc,i,t are DC voltage and DC current respectively; U i,t is the node voltage of the DC converter station; n t is the number of converter bridges; k T,t is the DC conversion ratio; X c k is the commutation reactance of the DC converter; γ is the safety factor; θ dc,i,t and are the trigger angle and commutation angle of the converter respectively; the subscripts rec and inv represent the rectifier side and inverter side of the converter respectively; P dc,i,t and Q dc,i,t are the active power and reactive power of DC transmission respectively;

[0105] The DC system node voltage satisfies:

[0106] U dc,i,min ≤U dc,i,t ≤U dc,i,max (44)

[0107] The DC system power meets the following requirements:

[0108]

[0109] The inverter firing angle and power factor angle satisfy:

[0110]

[0111] 5) Transient overvoltage constraints at each new energy station

[0112]

[0113] The advantages and positive effects of the new energy carrying capacity assessment method considering transient overvoltage constraints described in the present invention are:

[0114] 1. This method can ensure that the voltage is within a safe and reasonable range when a system failure occurs by setting constraints, effectively reducing the risk of triggering the third line of defense of the power system.

[0115] 2. Taking the mathematical connection between transient overvoltage amplitude, system short-circuit ratio and active power of new energy stations as the main line and the system short-circuit ratio as the intermediate quantity, the mathematical expressions of transient overvoltage amplitude of new energy station nodes and active power of new energy stations are established, and a new energy carrying capacity assessment model of the power grid taking into account the transient overvoltage constraints of multiple new energy stations is constructed to ensure that the optimal output value of the new energy station can be obtained and the output of the new energy units can be guaranteed to the greatest extent.

[0116] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0117] Figure 1 is a flow chart of the evaluation method of the present invention;

[0118] Figure 2 An equivalent model for calculating the short-circuit ratio of AC power grids with a high proportion of renewable energy;

[0119] Figure 3 It is the equivalent circuit of the sending end of the high voltage direct current transmission system;

[0120] Figure 4 Simplified model for bus grounding fault in converter station;

[0121] Figure 5 A modified IEEE-39 node test system;

[0122] Figure 6 When only the upper half of the synchronous generator set is replaced by new energy equipment, the optimization results comparison of the active output of the generator with and without constraints when the proportion of new energy is 40%;

[0123] Figure 7 When only the synchronous generator sets in the upper half of the area are replaced by new energy equipment, the optimization results comparison chart of new energy active output with and without constraints when the proportion of new energy is 40%;

[0124] Figure 8 When only the synchronous generator sets in the upper half of the area are replaced by new energy equipment, a comparison chart of transient overvoltage conditions with or without constraints at new energy nodes when the proportion of new energy is 40%;

[0125] Fig. 9 When only the upper half of the synchronous generator set is replaced by new energy equipment, the optimization results comparison of the active output of the generator with and without constraints when the proportion of new energy is 25%;

[0126] Fig.10 When only the synchronous generator sets in the upper half of the area are replaced by new energy equipment, the optimization results comparison of new energy active output with and without constraints when the proportion of new energy is 25%;

[0127] Fig.11 A comparison chart of transient overvoltage conditions with and without constraints at new energy nodes when new energy equipment only replaces the synchronous generator sets in the upper half of the area and the proportion of new energy is 25%;

[0128] Fig.12 The result diagram of the active output of the generator when the proportion of new energy is 20% is the result of replacing only the synchronous generator set in the upper half of the area with new energy equipment;

[0129] Fig.13 The new energy equipment only replaces the synchronous generator sets in the upper half of the area, and the new energy active output result diagram when the proportion of new energy is 20%;

[0130] Fig.14 When only the synchronous generator sets in the upper half of the area are replaced by new energy equipment, the transient overvoltage situation of the new energy node occurs when the proportion of new energy is 20%;

[0131] Fig.15 When only the synchronous generator sets in the lower half of the area are replaced by new energy equipment, the optimization results comparison chart of the active output of the generator with and without constraints when the proportion of new energy is 40%;

[0132] Fig.16 When only the synchronous generator sets in the lower half of the area are replaced by new energy equipment, the optimization results comparison chart of new energy active output with and without constraints when the proportion of new energy is 40%;

[0133] Fig.17 When only the synchronous generator sets in the lower half of the area are replaced by new energy equipment, a comparison chart of transient overvoltage conditions with or without constraints at new energy nodes when the proportion of new energy is 40%;

[0134] Fig.18 When only the synchronous generator sets in the lower half of the area are replaced by new energy equipment, the optimization results comparison chart of the active output of the generator with and without constraints when the proportion of new energy is 25%;

[0135] Fig.19 When only the synchronous generator sets in the lower half of the area are replaced by new energy equipment, the optimization results comparison chart of new energy active output with and without constraints when the proportion of new energy is 25%;

[0136] Fig. 20 When only the synchronous generator sets in the lower half of the area are replaced by new energy equipment, a comparison chart of whether there is a transient overvoltage constraint at the new energy node when the proportion of new energy is 25%;

[0137] Fig.21When only the synchronous generator sets in the lower half of the area are replaced by new energy equipment, the optimization results comparison chart of the active output of the generator with and without constraints when the proportion of new energy is 20%;

[0138] Fig. 22 When only the synchronous generator sets in the lower half of the area are replaced by new energy equipment, the optimization results comparison chart of new energy active output with and without constraints when the proportion of new energy is 20%;

[0139] Fig.23 When only the synchronous generator sets in the lower half of the area are replaced by new energy equipment and the proportion of new energy is 20%, a comparison chart of the transient overvoltage conditions with and without constraints at the new energy nodes is shown. DETAILED DESCRIPTION

[0140] The embodiments of the present invention are described in detail below in conjunction with the accompanying drawings.

[0141] like Figure 1 A new energy carrying capacity assessment method considering transient overvoltage constraints includes the following steps:

[0142] S1. Calculate the short-circuit ratio of the sending-end system of multiple renewable energy stations.

[0143] The short-circuit ratio of the sending end system is defined as

[0144]

[0145] Where: S C is the system short-circuit capacity; P dN Rated transmission power of the DC system.

[0146] The short-circuit capacity expression provided by the AC system for the commutation bus is:

[0147] S C =U N |I f |S b (2)

[0148] Where: I f , S b They are the per-unit value of the bus three-phase short-circuit current and the power reference value respectively.

[0149] From equations (1) and (2), we can see that the short-circuit ratio SCR and short-circuit current I f The short-circuit current depends on the network structure and the unit output. The no-load voltage of the bus is different under different unit types and different unit outputs, and the short-circuit current is also different. By calculating the short-circuit current provided by the sending-end system to the commutation bus node at different times, the short-circuit capacity value provided by the sending-end system is calculated, and the display expression of the sending-end short-circuit ratio and the output of different types of units is obtained.

[0150] In the power system, synchronous generators generally present voltage source properties, and renewable energy units generally present current source properties. According to the Thévenin theorem, the equivalent model of renewable energy and generators is as follows: Figure 2 According to the superposition theorem, the renewable energy grid-connected system is regarded as the superposition of the AC system and renewable energy, and then the short-circuit current and short-circuit ratio provided by the AC system are calculated.

[0151] In the AC system, node 1-m1 is a new energy node, node m1+1-m2 is a synchronous generator node, node m2+1 is a DC converter station node, and node m2+2-n is a load node; the node current equation is:

[0152]

[0153] The node admittance matrix after considering the load integration is:

[0154]

[0155] In the formula, P D,j , Q D,j They are the voltage, current, active power and reactive power of the load node respectively; the node admittance matrix Y′ is inverted to obtain the node impedance matrix Z′ after the load is incorporated.

[0156] Short-circuit current generated by renewable energy at converter station nodes The calculation formula is:

[0157]

[0158] In the formula, P w,j , Q w,j are the current, active power and reactive power flowing into the AC grid from the new energy station at node j, is the conjugate of the node voltage, is the parallel impedance from node j to the grounded node other than the converter station node.

[0159] To calculate the short-circuit capacity provided by the generator node, the load node is eliminated. The DC converter station node, synchronous generator node and new energy node are classified into one category, and the load node is classified into one category, then formula (3) is modified as follows:

[0160]

[0161] Where H is the self-impedance matrix of non-load nodes, which is an m2+1-order square matrix; R is the mutual impedance matrix between non-load nodes and load nodes, which is an (m2+1)*(n-m2-1)-order matrix; G is the mutual impedance matrix between non-load nodes and load nodes, which is an (n-m2-1)*(m2+1)-order matrix; D is the self-impedance matrix of load nodes, which is an n-m2-1-order square matrix; eliminate the load node:

[0162] I ac =(H-RD -1 G)U ac =Y″U ac (9)

[0163] The node impedance matrix Z″ after eliminating the load node is obtained by inverting the node admittance matrix after eliminating the load node. Then the no-load voltage generated by the generator at the converter station node is:

[0164]

[0165] In the formula, P g,j , Q g,j are the current, active power and reactive power flowing into the AC grid at the synchronous generator node j respectively.

[0166] Short-circuit current generated by synchronous generator at converter station node The calculation formula is:

[0167]

[0168] By superimposing equations (5), (6) and (10), (11) and (12), we can obtain the short-circuit current expression provided by the sending-end system to the converter station node:

[0169]

[0170] Substitute equation (13) into equation (2) to obtain the short-circuit capacity of the commutation busbar. Substitute the obtained short-circuit capacity into equation (1) to obtain the relationship between the short-circuit ratio of the sending-end system and the output of the synchronous generator and the new energy station:

[0171]

[0172] S2. Calculate the transient overvoltage at the nodes of the new energy station in the sending-end system.

[0173] The equivalent circuit of the sending-end DC transmission system is as follows Figure 3 shown.

[0174] Figure 2 In, P acp and Q acp are respectively the active and reactive power transmitted by a single line; Qcp The reactive power provided by the reactive power compensation device to the system; U s and X s are the equivalent potential and equivalent reactance of the AC system respectively; U p is the commutation bus voltage.

[0175] Depend on Figure 2 From the equivalent circuit, it can be seen that when a fault occurs in the DC transmission system, the steady-state bus transient voltage expression of the sending-end converter station is:

[0176]

[0177] Where: U pr is the bus voltage after the fault; ΔU is the longitudinal component of the voltage drop; δU is the transverse component of the voltage drop, U S is the equivalent potential of the AC system.

[0178] Where ΔU and δU are

[0179]

[0180] Where: ΔP and ΔQ are the deviations of active power and reactive power caused during the fault period.

[0181] Equivalent impedance of the sending end system X s for

[0182]

[0183] Combining equation (15), equation (16) and equation (17) we can get

[0184]

[0185] Simplifying equation (18) into per-unit form is:

[0186]

[0187] Substituting equation (1) into equation (19) and ignoring the influence of the voltage transverse component, the transient overvoltage expression of the commutation bus at the sending end is obtained as follows:

[0188]

[0189] Substituting equation (14) into equation (20), the mathematical expression between the transient overvoltage peak value at the converter station node and the output of the synchronous generator and the new energy station is obtained as follows:

[0190]

[0191] When a lockout fault occurs, the voltage at the converter station node will rise rapidly. The new energy stations at different locations in the AC system will be affected by the difference in distance from the converter station node, causing different degrees of voltage increase at the nodes of each new energy station. As a result, the new energy station may experience overvoltage and disconnection from the grid, which seriously restricts the power grid's new energy carrying capacity.

[0192] In order to ensure that the new energy station does not disconnect from the grid, it is necessary to further derive the calculation method of the transient voltage rise of the new energy grid connection point near the converter station. The simplified model of the converter station busbar grounding fault is as follows: Figure 4 shown.

[0193] According to the node voltage matrix, the voltage of node i in the new energy station during normal operation is:

[0194]

[0195] Where: Z i,j is the mutual impedance between nodes i and j, Inject current into node j;

[0196] When a lockout fault occurs, it is equivalent to adding a fault branch at the fault point. Then the voltage at the fault node k is the superposition of the steady-state voltage and the transient voltage. The voltage at the fault node k is:

[0197]

[0198] Where: Z k,j is the mutual impedance between nodes k and j, Z k,k is the k-node self-impedance, Inject current into node k;

[0199] The voltage at node i of the new energy station is:

[0200]

[0201] Using Thevenin's theorem to analyze the fault point k, It is expressed as:

[0202]

[0203] Where: Z k is the ground branch impedance.

[0204] Substituting formula (25) into formula (24), we get:

[0205]

[0206] Where: Z i,k is the mutual impedance between node i and node k;

[0207] The voltage change at node i of the new energy station is:

[0208]

[0209] The voltage change at the converter station node k is:

[0210]

[0211] Combining equations (27) and (28), we can get the ratio of transient voltage rise between the new energy station node i and the converter station bus node k:

[0212]

[0213] According to the relationship between equation (21) and equation (29), the mathematical expression between the transient overvoltage peak value of the new energy station node and the output of the synchronous generator and the new energy station is:

[0214]

[0215] S3. Construct a grid new energy carrying capacity assessment model considering transient overvoltage constraints; the objective function of the grid new energy carrying capacity assessment model is:

[0216]

[0217] Where T is the total time period, which is 24 hours. w,i,t is the active power flowing into the AC grid from the new energy station at node i at time t, w is the new energy station, Ψ RE It is a collection of new energy station nodes.

[0218] The constraints of the objective function are:

[0219] 1) AC system constraints

[0220] The AC / DC hybrid system is coupled through the converter station access point, and the AC system power flow meets the following requirements:

[0221]

[0222] Where P G,i,t , Q G,i,t are the active power and reactive power of the generator node respectively; P w,i,t , Q w,i,t are the active power and reactive power of the new energy node respectively; P D,i,t , Q D,i,t are the active power and reactive power of the load node respectively; P dc,i,t , Q dc,i,t are the active power and reactive power of the DC transmission node respectively.

[0223] The branch flow must meet the following requirements:

[0224] U i,t U j,t [G ij,t cos(θ i,t -θ j,t )+B ij,t sin(θ i,t -θ j,t )]-U i,t 2 G ij,t -P ij,t =0 (33)

[0225] U i,t U j,tj [B ij,t cos(θ i,t -θ j,t )-G ij,t sin(θ i,t -θ j,t )]-U i,t 2 B ij,t +Q ij,t =0 (34)

[0226] Where P ij,t , Q ij,t They are the active power flow and reactive power flow of branch ij respectively.

[0227] The AC system node voltage must meet the following requirements:

[0228] U i,min ≤U i,t ≤U i,max (35)

[0229] Where U i,min and U i,max are the upper and lower limits of the node voltage respectively.

[0230] 2) Operation constraints of synchronous generator sets

[0231]

[0232] Where P G,i,min and P G,i,max They are the upper and lower limits of the active power of the generator; Q G,i,min and Q G,i,max They are the upper and lower limits of the generator reactive power respectively.

[0233] 3) New energy power constraints

[0234]

[0235] In the formula, is the power factor of new energy; the reactive power of new energy is provided by the grid-connected inverter and reactive power compensation device, and the power factor is constant.

[0236] 4) DC system operation constraints

[0237]

[0238] U dc,rec,t =U dc,inv,t +I dc,t R dc (41)

[0239] P dc,i,t =U dc,i,t I dc,i,t (42)

[0240] Q dc,i,t =U dc,i,t I dc,i,t cosα i,t (43)

[0241] Where U dc,i,t and I dc,i,t are DC voltage and DC current respectively; U i,t is the node voltage of the DC converter station; n t is the number of converter bridges; k T,t is the DC conversion ratio; X c k is the commutation reactance of the DC converter; γ is the safety factor; θ dc,i,t and are the trigger angle and commutation angle of the converter respectively; the subscripts rec and inv represent the rectifier side and inverter side of the converter respectively; P dc,i,t and Q dc,i,t They are the active power and reactive power of DC transmission respectively.

[0242] The DC system node voltage satisfies:

[0243] U dc,i,min ≤U dc,i,t ≤U dc,i,max (44)

[0244] The DC system power meets the following requirements:

[0245]

[0246] The inverter firing angle and power factor angle satisfy:

[0247]

[0248] 5) Transient overvoltage constraints at each new energy station

[0249]

[0250] Example

[0251] The following is a specific example. Figure 5 The feasibility of the solution of the embodiment is verified.

[0252] The example is based on a 10-machine 39-node system to illustrate the effectiveness of the proposed new energy carrying capacity assessment method. Figure 5 The block diagram of the 10-machine 39-node system is given in the figure, where nodes 30-39 are connected to synchronous generator nodes, and a traditional DC line is connected at node 6. The IEEE-39 node test system is regarded as the sending end system. The overall grid structure refers to the standard IEEE 39 node system model.

[0253] Two situations are considered in the present invention: (1) the new energy equipment only replaces the synchronous generator in the upper half area, and the impact on the overvoltage peak of the nodes of the new energy station under different proportions of new energy is analyzed; (2) the new energy equipment only replaces the synchronous generator in the lower half area, and the impact on the overvoltage peak of the nodes of the new energy station under different proportions of new energy is analyzed.

[0254] (1) New energy equipment only replaces the synchronous generator sets in the upper half of the area

[0255] 1) New energy accounts for 40%

[0256] First, the capacity of nodes 36, 37, 38 and 39 synchronous generator sets is replaced with new energy. At this time, the proportion of new energy in the entire system is about 40%. The optimization operation results of new energy stations and synchronous generator sets with and without transient overvoltage constraints are compared. Figure 6 , Figure 7 As shown in the figure, the comparison of overvoltage at each new energy station node is as follows: Figure 8 .

[0257] from Figure 8 It can be seen that when there is no transient overvoltage constraint, each new energy station is in full power state, and the grid's new energy carrying capacity is 31.09pu, but its corresponding transient overvoltage value is close to 1.3pu. If the corresponding control measures are not taken in time, it is easy to cause the new energy station to be disconnected from the grid, which seriously threatens the safe operation of the grid. After adding the transient overvoltage constraint, the grid's new energy carrying capacity is reduced to 27.23pu, and the corresponding transient overvoltage values ​​of each new energy station node are controlled within a safe range.

[0258] 2) New energy accounts for 25%

[0259] Secondly, the capacity of the synchronous generator sets at nodes 38 and 39 is replaced with new energy. At this time, the proportion of new energy in the entire system is about 25%. The optimized operation results of the new energy station and the synchronous generator sets with and without transient overvoltage constraints are compared. Fig. 9 , Fig.10 As shown in the figure, the comparison of overvoltage at each new energy station node is as follows: Fig.11 .

[0260] from Fig.11 It can be seen that when there is no transient overvoltage constraint, each new energy station is in full power state, and the grid's new energy carrying capacity is 19.65pu, but its corresponding transient overvoltage value is slightly more than 1.25pu, which is smaller than when the proportion of new energy is 40%, and it is at the boundary of safe and stable operation of new energy. The output of new energy stations should be reasonably reduced. After adding transient overvoltage constraints, the grid's new energy carrying capacity is 16.11pu, and the corresponding transient overvoltage values ​​of each new energy station are controlled within a safe range.

[0261] 3) New energy accounts for 20%

[0262] Finally, the capacity of the synchronous generator sets at nodes 37 and 38 is replaced with new energy. At this time, the proportion of new energy in the entire system is about 20%, and the grid's new energy carrying capacity is 14.29pu. When no constraints are added, the new energy station is in full power and the transient overvoltage values ​​of each new energy station node are within a reasonable range. The optimized operation results of the new energy station and synchronous generator sets when no transient overvoltage constraints are added are shown in Figure 2. Fig.12 , Fig.13 As shown in the figure, the comparison of overvoltage at each new energy station node is as follows: Fig.14 .

[0263] (2) New energy equipment only replaces the synchronous generator sets in the lower half of the area

[0264] 1) New energy accounts for 40%

[0265] First, the capacity of nodes 31, 32, 33, 34 and 35 synchronous generator sets is replaced with new energy. At this time, the proportion of new energy in the entire system is about 40%. The optimization operation results of new energy stations and synchronous generator sets with and without transient overvoltage constraints are compared. Fig.15 , Fig.16 As shown in the figure, the comparison of overvoltage at each new energy station node is as follows: Fig.17 .

[0266] from Fig.17It can be seen that when there is no transient overvoltage constraint, each new energy station is in full power state, and the grid's new energy carrying capacity is 32.18pu, but its corresponding transient overvoltage value has exceeded 1.3pu, and the transient overvoltage values ​​of other new energy nodes have also exceeded 1.25pu. If the corresponding control measures are not taken in time, it is easy to cause the new energy station to be disconnected from the grid, which seriously threatens the safe operation of the grid. After adding the transient overvoltage constraint, the grid's new energy carrying capacity is reduced to 23.24pu, and the corresponding transient overvoltage values ​​of each new energy station node are controlled within the safe range.

[0267] Depend on Fig.17 It can be seen that under the condition of approximately the same new energy carrying capacity of the power grid, compared with the case where the new energy equipment only replaces the synchronous generator in the upper half area, when no constraints are added, since the nodes of the new energy stations in the lower half area are closer to the fault location, when a fault occurs, the transient overvoltage value of each new energy station is slightly higher than the transient overvoltage value of each new energy station when the new energy equipment only replaces the synchronous generator in the upper half area; after adding constraints, since the transient overvoltage value of each station is higher, the new energy carrying capacity of the power grid is also reduced by 4pu compared with the case where the new energy equipment only replaces the synchronous generator in the upper half area.

[0268] 2) New energy accounts for 25%

[0269] Secondly, the capacity of the synchronous generator sets at nodes 31, 32 and 34 is replaced with new energy. At this time, the proportion of new energy in the entire system is about 25%, and the optimization operation results of the new energy station and the synchronous generator sets with and without transient overvoltage constraints are compared. Fig.18 , Fig.19 As shown in the figure, the comparison of overvoltage at each new energy station node is as follows: Fig. 20 .

[0270] from Fig. 20 It can be seen that when there is no transient overvoltage constraint, each new energy station is in full power state, and the grid's new energy carrying capacity is 18.79pu. However, the transient overvoltage values ​​of new energy nodes 31 and 32, which are closer to the fault, exceed 1.25pu. Compared with when the proportion of new energy is 40%, its transient overvoltage value is reduced, which is at the boundary of safe and stable operation of new energy. The output of new energy stations should be reasonably reduced. After adding transient overvoltage constraints, the grid's new energy carrying capacity is reduced to 10.36pu, and the corresponding transient overvoltage values ​​of each new energy station are controlled within a safe range.

[0271] 3) New energy accounts for 20%

[0272] Finally, the capacity of the synchronous generator sets at nodes 34 and 35 is replaced with new energy. At this time, the proportion of new energy in the entire system is about 20%, and the optimization operation results of the new energy station and the synchronous generator sets with and without transient overvoltage constraints are compared. Fig.21 , Fig. 22 As shown in the figure, the comparison of overvoltage at each new energy station node is as follows: Fig.23 .

[0273] from Fig.23 It can be seen that when there is no transient overvoltage constraint, each new energy station is in full power state, and the grid's new energy carrying capacity is 11.95pu, but the transient overvoltage peak of the new energy station node is slightly more than 1.25pu, and the output of the new energy station should be appropriately reduced. After adding the transient overvoltage constraint, the grid's new energy carrying capacity is reduced to 9.87pu, and the corresponding transient overvoltage values ​​of each new energy station are controlled within a safe range.

[0274] Depend on Fig.23 It can be seen that when the new energy carrying capacity of the power grid is roughly the same, compared with the replacement of only the synchronous generator in the upper half of the area by new energy equipment, since the nodes of the new energy stations in the lower half are closer to the fault location, when a fault occurs, the transient overvoltage value of the new energy station is slightly higher than 1.25pu. The output of the new energy station should be reasonably controlled to keep its transient overvoltage value within the safe operating range.

[0275] Therefore, the new energy carrying capacity assessment method considering transient overvoltage constraints described in the present invention is adopted, with the mathematical relationship between transient overvoltage amplitude, system short-circuit ratio and active power of new energy stations as the main line, and the system short-circuit ratio as the intermediate quantity. A mathematical expression of the transient overvoltage amplitude of the new energy station node and the active power of the new energy station is established, and a new energy carrying capacity assessment model of the power grid considering transient overvoltage constraints of multiple new energy stations is constructed to ensure that the optimal new energy station output value can be obtained and the safe operation of the power grid can be guaranteed.

[0276] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A new energy carrying capacity assessment method considering transient overvoltage constraints, characterized in that: The following steps are involved: S1. Calculate the short-circuit ratio of the sending-end system of multiple renewable energy stations; S2. Calculate the transient overvoltage of the nodes of the new energy station in the sending-end system; S3. Construct a grid new energy carrying capacity assessment model considering transient overvoltage constraints; the objective function of the grid new energy carrying capacity assessment model is: Among them, T is the total time period, P w,i,t is the active power flowing into the AC grid from the new energy station at node i at time t, w is the new energy station, Ψ RE It is a collection of new energy station nodes.

2. A new energy carrying capacity assessment method considering transient overvoltage constraints according to claim 1, characterized in that: In S1, the short-circuit ratio of the sending end system is defined as Where: S C is the system short-circuit capacity; P dN Rated transmission power for DC system; The short-circuit capacity expression provided by the AC system for the commutation bus is: S C =U N |I f |S b (2) Where: I f , S b They are the per unit value of busbar three-phase short-circuit current and power reference value respectively; The renewable energy grid-connected system is regarded as the superposition of the AC system and renewable energy, and the short-circuit current and short-circuit ratio provided by the AC system are calculated; the relationship between the short-circuit ratio of the sending-end system and the output of the synchronous generator and renewable energy station is obtained.

3. A new energy carrying capacity assessment method considering transient overvoltage constraints according to claim 2, characterized in that: The specific calculation process of the relationship between the short-circuit ratio of the sending-end system and the output of the synchronous generator and the new energy station is as follows: In the AC system, node 1-m1 is a new energy node, node m1+1-m2 is a synchronous generator node, node m2+1 is a DC converter station node, and node m2+2-n is a load node; the node current equation is: The node admittance matrix after considering the load integration is: In the formula, P D,j , Q D,j are the voltage, current, active power and reactive power of the load node respectively; the node admittance matrix Y′ is inverted to obtain the node impedance matrix Z′ after the load is incorporated; Short-circuit current generated by renewable energy at converter station nodes The calculation formula is: In the formula, P w,j , Q w,j are the current, active power and reactive power flowing into the AC grid from the new energy station at node j, is the conjugate of the node voltage, is the parallel impedance from node j to the grounded node other than the converter station node; Eliminate the load nodes and calculate the short-circuit capacity provided by the synchronous generator nodes; The DC converter station nodes, synchronous generator nodes and new energy nodes are classified into one category, and the load nodes are classified into one category, then formula (3) is modified as follows: Where H is the self-impedance matrix of non-load nodes, which is an m2+1-order square matrix; R is the mutual impedance matrix between non-load nodes and load nodes, which is an (m2+1)*(n-m2-1)-order matrix; G is the mutual impedance matrix between non-load nodes and load nodes, which is an (n-m2-1)*(m2+1)-order matrix; D is the self-impedance matrix of load nodes, which is an n-m2-1-order square matrix; eliminate the load node: I ac =(H-RD -1 G)U ac =Y″U ac (9) The node admittance matrix after eliminating the load node is inverted to obtain the node impedance matrix Z″ after eliminating the load node; then the no-load voltage generated by the generator at the converter station node is: In the formula, P g,j , Q g,j are the current, active power and reactive power flowing into the AC grid from the synchronous generator node j; the short-circuit current generated by the synchronous generator at the converter station node The calculation formula is: By superimposing equations (5), (6) and (10), (11) and (12), we can obtain the short-circuit current expression provided by the sending-end system to the converter station node: Substitute equation (13) into equation (2) to obtain the short-circuit capacity of the commutation busbar. Substitute the obtained short-circuit capacity into equation (1) to obtain the relationship between the short-circuit ratio of the sending-end system and the output of the synchronous generator and the new energy station:

4. The method for evaluating the new energy carrying capacity considering transient overvoltage constraints according to claim 3 is characterized in that: The S2 is specifically: When a fault occurs in the DC transmission system, the transient voltage expression of the steady-state busbar at the sending-end converter station is: Where: U pr is the bus voltage after the fault; ΔU is the longitudinal component of the voltage drop; δU is the transverse component of the voltage drop; U S is the equivalent potential of the AC system; Where ΔU and δU are Where: ΔP and ΔQ are the deviations of active power and reactive power caused during the fault period respectively; Equivalent impedance of the sending end system X s for Combining equation (15), equation (16) and equation (17) we can get Simplifying equation (18) into per-unit form is: Substituting equation (1) into equation (19) and ignoring the influence of the voltage transverse component, the transient overvoltage expression of the commutation bus at the sending end is obtained as follows: Substituting equation (14) into equation (20), the mathematical expression between the transient overvoltage peak value of the converter station node and the output of the synchronous generator and the new energy station is obtained as follows: According to the node voltage matrix, the voltage of node i in the new energy station during normal operation is: Where: Z i,j is the mutual impedance between nodes i and j, Inject current into node j; When a lockout fault occurs, it is equivalent to adding a fault branch at the fault point. Then the voltage at the fault node k is the superposition of the steady-state voltage and the transient voltage. The voltage at the fault node k is: Where: Z k,j is the mutual impedance between nodes k and j, Z k,k is the k-node self-impedance, Inject current into node k; The voltage at node i of the new energy station is: for: Where: Z k is the ground branch impedance; Substituting formula (25) into formula (24), we get: Where: Z i,k is the mutual impedance between node i and node k; The voltage change at node i of the new energy station is: The voltage change at the converter station node k is: Combining equations (27) and (28), we can get the ratio of transient voltage rise between the new energy station node i and the converter station bus node k: According to the relationship between equation (21) and equation (29), the mathematical expression between the transient overvoltage peak value of the new energy station node and the output of the synchronous generator and the new energy station is:

5. The method for evaluating the new energy carrying capacity considering transient overvoltage constraints according to claim 4 is characterized in that: The constraints of the objective function in S3 are: 1) AC system constraints The AC / DC hybrid system is coupled through the converter station access point, and the AC system power flow meets the following requirements: Where P G,i,t , Q G,i,t are the active power and reactive power of the generator node respectively; P w,i,t , Q w,i,t are the active power and reactive power of the new energy node respectively; P D,i,t , Q D,i,t are the active power and reactive power of the load node respectively; P dc,i,t , Q dc,i,t are the active power and reactive power of the DC transmission node respectively; The branch flow must meet the following requirements: U i,t U j,t [G ij,t cos(θ i,t -θ j,t )+B ij,t sin(θ i,t -θ j,t )]-U i,t 2 G ij,t -P ij,t =0 (33) U i,t U j,tj [B ij,t cos(θ i,t -θ j,t )-G ij,t sin(θ i,t -θ j,t )]-U i,t 2 B ij,t +Q ij,t =0 (34) Where P ij,t , Q ij,t are the active power flow and reactive power flow of branch ij respectively; The AC system node voltage must meet the following requirements: IN i,min ≤U i,t ≤U i,max (35) Where U i,min and U i,max are the upper and lower limits of the node voltage respectively; 2) Operation constraints of synchronous generator sets Where P G,i,min and P G,i,max They are the upper and lower limits of the active power of the generator; Q G,i,min and Q G,i,max They are the upper and lower limits of the reactive power of the generator respectively; 3) New energy power constraints In the formula, is the power factor of new energy; the reactive power of new energy is provided by the grid-connected inverter and reactive power compensation device, and the power factor is constant; 4) DC system operation constraints U dc,rec,t =U dc,inv,t +I dc,t R dc (41) P dc,i,t =U dc,i,t AND dc,i,t (42) Q dc,i,t =U dc,i,t I dc,i,t cosα i,t (43) Where U dc,i,t and I dc,i,t are DC voltage and DC current respectively; U i,t is the node voltage of the DC converter station; n t is the number of converter bridges; k T,t is the DC conversion ratio; X c k is the commutation reactance of the DC converter; γ is the safety factor; θ dc,i,t and are the trigger angle and commutation angle of the converter respectively; the subscripts rec and inv represent the rectifier side and inverter side of the converter respectively; P dc,i,t and Q dc,i,t are the active power and reactive power of DC transmission respectively; The DC system node voltage satisfies: IN dc,i,min ≤U dc,i,t ≤U dc,i,max (44) The DC system power meets the following requirements: The inverter firing angle and power factor angle satisfy: 5) Transient overvoltage constraints at each new energy station

Images