A discrete hierarchical optimization method for parameters of multi-terminal power grid current limiting equipment

Abstract

The invention discloses a discrete hierarchical optimization method for parameters of current limiting equipment in a multi-terminal power grid, and relates to the technical field of relay protection of electric power systems. The method comprises: according to the coupling degree between a converter and a power grid in a power grid, a regional division principle and a typical current limiting scheme for a corresponding region are proposed; the influence law of the parameters of the current limiting equipment on the fault current is analyzed, and a discretization design principle for the parameters of the current limiting equipment is proposed in combination with the error range of the inductance parameter; it is judged whether the power grid has a low coupling region, and if so, the parameters of the current limiting equipment in the region are determined according to the analytical formula of the fault current of a single-terminal system; for a high coupling region, an optimization model is established in combination with the discrete parameters of the current limiting equipment, and the parameters of the equipment in the region are discretely optimized, and the optimization space is simultaneously reduced through hierarchical optimization, so that the originally continuous parameters of the current limiting equipment are taken at finite discrete points, thereby improving the optimization calculation efficiency; when the current limiting demand is met, the customization cost of the current limiting equipment is reduced, which is more in line with the standardization and standardization requirements of actual engineering equipment.

Description

Technical Field

[0001] The invention relates to the technical field of relay protection of electric power systems, and in particular to a discrete hierarchical optimization method for parameters of current limiting equipment in a multi-terminal power grid. Background Art

[0002] How to use multiple current limiting devices to effectively limit the fault current on the DC side is an urgent problem to be solved in flexible DC power grids. Currently, DCCB (DC circuit breaker) is mainly used for current limiting. However, due to the limited breaking capacity of DCCB, the actual project adopts the solution of DCCB combined with CLR (current limiting inductor) to limit the current. CLR is always running and in hot standby state in the line, which provides 1-3ms reaction time for protection while suppressing the rapid rise of fault current. However, too large CLR value will increase equipment cost, reduce the dynamic response capability of the system, and even cause system instability. It is difficult to achieve economical, efficient and reliable suppression of fault current by relying on CLR. Therefore, combining the specially designed FCL (fault current limiter) with CLR to coordinate and suppress fault current is a more universal solution.

[0003] At present, the spatial configuration and parameter collaborative optimization schemes of current limiting devices in DC power grids are mainly aimed at simple ring power grids, and cannot reflect the complexity of the future multi-terminal DC power grid structure; in addition, the current collaborative optimization schemes of current limiting devices are all continuous optimization of current limiting device parameters, which requires customization of each current limiting device, greatly increasing the manufacturing cost of the equipment, and there is a lack of research on discrete optimization of current limiting device parameters that is more in line with the standardization and specification of actual equipment; for this reason, a discrete hierarchical optimization method for current limiting device parameters in multi-terminal power grids is proposed. Summary of the invention

[0004] In order to solve the deficiencies mentioned in the above background technology, the purpose of the present invention is to provide a discrete hierarchical optimization method for parameters of multi-terminal power grid current limiting equipment.

[0005] The object of the present invention can be achieved by the following technical solution: a discrete hierarchical optimization method for parameters of a multi-terminal power grid current limiting device, the method comprising the following steps:

[0006] According to the coupling degree between the converter and the power grid, the power grid topology is divided into low-coupling area and high-coupling area, and combined with the current limiting requirements of each area, a typical current limiting scheme for each area is proposed;

[0007] The influence of the selection of CLR and FCL linear discrete parameters on the fault current is analyzed, and the general current limiting sensitivity range is obtained;

[0008] Combined with the error range of inductance parameters, the discretization design principle of current limiting device parameters is proposed;

[0009] Determine whether the power grid has a low coupling area. If so, determine the parameters of the equipment in the area. The low coupling area relies on CLR to limit current within milliseconds after the fault occurs. When the system fault ride-through conditions are met, reduce the system configuration CLR.

[0010] Combining the obtained CLR and FCL linear discrete parameters, an optimization model is established to optimize the discrete parameters of the current limiting device in the high coupling area.

[0011] Furthermore, the typical current limiting scheme includes the following two schemes for the low coupling region:

[0012] Solution 1: Current limiting based on half-bridge MMC, DCCB and CLR;

[0013] Solution 2: Current limiting based on active control hybrid MMC, isolating switch and CLR;

[0014] Select the specific current limiting configuration scheme in the low coupling area based on whether there is reactive power support demand on the AC side, the need for rapid restart after a fault, and the project cost;

[0015] The typical current limiting scheme is for high coupling areas, contains multiple converter stations, and considers equipment cost issues, and adopts half-bridge MMC, CLR, DCCB and FCL current limiting schemes.

[0016] Furthermore, the fault current variation rule of the converter station is as follows:

[0017]

[0018]

[0019]

[0020]

[0021]

[0022] In the formula, i dc is the DC line current, I dc is the DC line current before the fault, i arm Represents the bridge arm current, I a is the AC side current amplitude, U dc is the DC voltage at the converter station outlet, R e is the sum of the equivalent resistance of the converter station and the equivalent resistance of the DC line, L e is the sum of the equivalent inductance of the converter station, the current limiting inductor and the equivalent inductance of the DC line; C e is the equivalent capacitance of the converter station.

[0023] According to the above formula, as the CLR parameter value increases, the fault current gradually decreases. Considering the dynamic performance of the system, CLR provides a 1-3ms reaction time for protection while suppressing the rapid rise of bipolar short-circuit current. Therefore, the minimum CLR value should meet the requirement that the MMC does not lock 3ms after the fault occurs, that is, the maximum bridge arm current of the MMC should be less than 2 times the rated current of the IGBT 3ms after the fault occurs.

[0024] Select the inductive FCL with parallel MOA. MOA can effectively protect the equipment in the power system from overvoltage, failure or even damage. Due to the parallel MOA, the current limiting inductor L F It takes a certain amount of time to put the fault line into operation. When the MOA withstand voltage is lower than the rated voltage U of the MOA, MOAn When the MOA is in operation, the fault current flows through the current limiting inductor L. F and MOA transition stage L F Withstand voltage As follows:

[0025]

[0026] Where i dc is the fault current, i F For the flow through L F The current, i MOA is the current flowing through MOA;

[0027] L F After fully put into operation, MOA will stop running. At this time, the voltage on FCL As shown below:

[0028]

[0029] When L F The voltage analysis after full input is the same as the voltage analysis of the inductive FCL. At the moment the inductive FCL is put into operation, an impulse voltage will be generated. When the fault current flows from the transfer branch to the current limiting inductor L F The time of the branch is t s , L F The voltage across the two ends is shown as follows:

[0030]

[0031] Where t1 is the time when the fault occurs.

[0032] Furthermore, the rated current of the IGBT is 3KA.

[0033] Furthermore, the fault current is transferred from the transfer branch to the current limiting inductor L F The time t of the branch s At the microsecond level.

[0034] Furthermore, the current limiting device parameter discretization design principle is linearly discretized based on the error in inductance selection and the influence of CLR and FCL on faults:

[0035] C min a n =C max

[0036] Where n is the number of discrete parameters; C min is the minimum value that the current limiting device parameter can take, C max is the maximum value that the current limiting device parameters can take, and a is the inductance selection error.

[0037] Furthermore, according to the fault current variation law of the converter station, the CLR parameter values ​​under the two schemes are determined. According to the current limiting function requirement, scheme 1 is obtained while ensuring that the CLR parameter meets the maximum breaking current of the DCCB. The CLR parameter value in scheme 1 is determined;

[0038] According to the current limiting function requirement, scheme 2 determines the CLR parameter value in scheme 2 while ensuring that the CLR parameter satisfies the requirement that the converter valve will not be locked before active control after a fault occurs.

[0039] Furthermore, combined with the discretized parameters of CLR and FCL, according to the system performance of the high-coupling area, the cost of the current limiting device and the current limiting effect of the device, with the best current limiting effect of the whole network, the minimum total CLR value and the minimum total FCL value as the goal, combined with the constraints, the multi-objective optimization configuration mathematical model considering the discrete optimization of CLR and FCL parameters is as follows:

[0040]

[0041] In the formula, x is the decision vector, m(x) is the objective function vector, and g i (x) is the constraint condition.

[0042] The present invention adopts the hierarchical analysis method to determine the weight coefficient of each objective function, overcomes the subjectivity problem of selecting the objective weight coefficient during multi-objective optimization, and based on this, adopts a single-objective optimization algorithm based on discrete optimization problems to perform discrete optimization on the CLR and FCL parameters of the region.

[0043] Beneficial effects of the present invention:

[0044] The optimization method of the present invention proposes a regional division principle for complex multi-terminal power grids and proposes typical current limiting schemes for each region in combination with the current limiting requirements of each region; on this basis, hierarchical optimization of current limiting device parameters in different regions is realized, and the calculation efficiency of the overall optimization of current limiting device parameters in complex multi-terminal DC power grids is improved; the originally continuous current limiting device parameters are taken at limited discrete points, greatly reducing the optimization space; and when meeting the current limiting requirements, the customization cost of the current limiting equipment can be greatly reduced, which is more in line with the requirements of standardization and standardization of actual engineering equipment and has practical engineering application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0046] Figure 1 is a flow chart of the optimization method of the present invention;

[0047] Figure 2 is a schematic diagram of a six-terminal power grid of the present invention;

[0048] Figure 3 It is a six-terminal power grid fault area division diagram of the present invention;

[0049] Figure 4 It is the influence of CLR of the present invention on the bridge arm current;

[0050] Figure 5 It is the influence of CLR of the present invention on the fault current;

[0051] Figure 6 is the effect of the FCL of the present invention on the fault current;

[0052] Figure 7 It is the linearization design of CLR and FCL parameters of the present invention;

[0053] Figure 8 It is a diagram of the current limiting effect of the discrete graded optimization of the current limiting device parameters of the present invention. DETAILED DESCRIPTION

[0054] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0055] like Figure 1As shown, a discrete hierarchical optimization method for parameters of a multi-terminal power grid current limiting device comprises the following steps:

[0056] S1: According to the coupling degree between the converter and the grid, the grid topology is divided into low-coupling area and high-coupling area, and typical current limiting schemes are proposed for each area in combination with the current limiting requirements of each area;

[0057] The low coupling area belongs to the single-end MMC, and the fault current characteristics are simple. CLR is used for current limiting, and DCCB can break the fault current. Therefore, two current limiting schemes are proposed for this area: Scheme 1: Current limiting based on half-bridge MMC, DCCB and CLR; Scheme 2: Current limiting based on active control hybrid MMC, disconnector and CLR;

[0058] Select the specific current limiting configuration scheme in the low coupling area based on whether there is reactive power support demand on the AC side, the need for rapid restart after a fault, and the project cost;

[0059] For high coupling areas, which contain multiple converter stations, half-bridge MMC, CLR, DCCB and FCL current limiting solutions are adopted considering equipment cost.

[0060] S2: Analyze the influence of CLR and FCL parameter selection on fault current and reveal its general current limiting sensitivity range. The fault current variation formula of a single converter station is as follows:

[0061]

[0062]

[0063]

[0064]

[0065]

[0066] In the formula, i dc is the DC line current, I dc is the DC line current before the fault, i arm Represents the bridge arm current, I a is the AC side current amplitude, U dc is the DC voltage at the converter station outlet, R e is the sum of the equivalent resistance of the converter station and the equivalent resistance of the DC line, L e is the sum of the equivalent inductance of the converter station, the current limiting inductor and the equivalent inductance of the DC line; C e is the equivalent capacitance of the converter station.

[0067] According to the above formula, it can be obtained that as the CLR parameter value increases in S2, the fault current gradually decreases. Considering the dynamic performance of the system, CLR cannot be too small. CLR provides a 1-3ms reaction time for protection while suppressing the rapid rise of bipolar short-circuit current. Therefore, the minimum CLR value should meet the requirement that the MMC does not lock after 3ms of the fault, that is, the maximum bridge arm current of the MMC should be less than 2 times the rated current of the IGBT (3kA) 3ms after the fault occurs.

[0068] FCL has almost no negative impact on the normal operation of the system, and actively places it in the short-circuit path to suppress the fault current after a short circuit. At present, the impact of FCL parameter values ​​on fault current is mostly considered, and the relationship between FCL's own voltage stress and fault current and current limiting inductance parameters is not considered. Whether FCL's own voltage stress margin affects parameter selection and current limiting effect needs further research to provide support for the subsequent selection of FCL parameter optimization range. Select an inductive FCL with parallel MOA. MOA can effectively protect equipment in the power system from overvoltage and failure or even damage. Due to the parallel MOA, the current limiting inductor L F It takes a certain amount of time to put the fault line into operation. When the MOA withstand voltage is lower than the rated voltage U MOAn When the MOA is in operation, the fault current flows through the current limiting inductor L. F and MOA transition stage L F Withstand voltage As follows:

[0069]

[0070] Where i dc is the fault current, i F For the flow through L F The current, i MOA is the current flowing through MOA.

[0071] L F After fully put into operation, MOA will stop running. At this time, the voltage on FCL As shown below:

[0072]

[0073] When L F The voltage analysis after full input is the same as the voltage analysis of the inductive FCL. At the moment the inductive FCL is put into operation, an impulse voltage will be generated. When the fault current flows from the transfer branch to the current limiting inductor L F The time of the branch is t s (at the microsecond level), L F The voltage across the two ends is shown as follows:

[0074]

[0075] Where t1 is the time when the fault occurs.

[0076] S3: Combined with the error range of inductance parameters, the discrete design principle of current limiting device parameters is proposed.

[0077] Linear discretization is performed based on the certain errors in inductance selection and the influence of CLR and FCL on faults:

[0078] C min a n =C max

[0079] Where n is the number of discrete parameters; C min is the minimum value that the current limiting device parameter can take, C max is the maximum value that the current limiting device parameters can take, and a is the inductance selection error.

[0080] S4: Determine whether the power grid has a low coupling area. If so, determine the parameters of the equipment in the area. Within a few milliseconds after the fault occurs, the area relies on CLR to limit the current. When the system fault ride-through conditions are met, the system configuration CLR is reduced as much as possible, and there is no need to optimize the equipment parameters in the area.

[0081] S5: Combine the CLR and FCL linear discrete parameters obtained in S3, and perform discrete optimization of CLR and FCL parameters based on the system performance in the high coupling area, the cost of the current limiting device, and the current limiting effect of the device, with the goal of achieving the best current limiting effect in the entire network, the minimum total CLR value, and the minimum total FCL value.

[0082] Combined with the constraints, the multi-objective optimization mathematical model considering the discrete optimization of CLR and FCL parameters is as follows:

[0083]

[0084] In the formula, x is the decision vector, m(x) is the objective function vector, and g i (x) is the constraint condition.

[0085] Because multi-objective optimization obtains a set of solutions and cannot directly obtain the optimal solution, the present invention adopts the hierarchical analysis method to determine the weight coefficient of each objective function, overcomes the subjective problem of selecting the objective weight coefficient during multi-objective optimization, and based on this, adopts a single-objective optimization algorithm based on discrete optimization problems to perform discrete optimization on the CLR and FCL parameters of the region.

[0086] The simulation software PSCAD / EMTDC is used to verify the effectiveness of the current limiting effect of discrete hierarchical optimization of current limiting device parameters in complex power grids, such as Figure 2As shown, this example is a six-terminal flexible DC grid with a voltage of ±500kV.

[0087] According to S1, the six-terminal power grid topology is divided into regions. The division results are as follows: Figure 3 The effectiveness of the typical solutions of current limiting equipment proposed for different regions is verified by combining the six-terminal power grid.

[0088] In this example, the low coupling region contains two converter stations, denoted as C4 and C6, and the high coupling region contains multiple converter stations, denoted as C1, C2, C3, and C5. 12 As the fault point, the current limiting schemes for each area of ​​the six-terminal power grid are selected as follows:

[0089] In the low-coupling area, converter station C4 selects the half-bridge MMC, CLR and DCCB current limiting scheme, and C6 selects the hybrid MMC, CLR and disconnector current limiting scheme. Considering the cost of equipment, converter stations C1, C2, C3 and C5 in the high-coupling area all adopt the half-bridge MMC, CLR, FCL and DCCB current limiting scheme.

[0090] After the current limiting schemes for each area are determined, according to S2, this example takes the inductive FCL (MOA can effectively protect the equipment in the power system from overvoltage and failure or even damage) of the parallel MOA and the single-ended system with a rated voltage of 400kV as an example to analyze the impact of CLR and FCL on the fault current. As shown in Figure 4, when the CLR value is lower than 50mH, the system will not meet the requirement that the MMC will not be locked within the protection response time. Therefore, this example takes the minimum CLR value of 50mH as an example for research. Figure 5 It is shown that when CLR increases to the boundary value: 200mH, the suppression effect of fault current is significantly reduced if the CLR value continues to increase. Considering the influence of CLR value on current limiting capability and current recovery time, the value range of CLR is generally [0.05, 0.2]H. s For 1 microsecond, the MOA rated voltage U of the inductive FCL connected in parallel with MOA MOAn The influence of current limiting inductance on fault current is as follows Figure 6 Show: With U MOAn Increase, the current limiting inductor effectively suppresses the fault current sensitivity range gradually increases, but because FCL is used for fault current limiting, its parallel U MOAn It cannot be higher than the system DC rated voltage. MOAn Take it as 75% of the rated DC voltage, then L F Taking 200mH as an example, the boundary value with obvious suppression effect on fault current provides a basis for the discrete design of equipment parameters.

[0091] Based on the analysis of the current limiting sensitivity range of CLR and FCL, combined with the inductance parameter selection error and the influence of CLR and FCL on fault current, the equipment parameters are discretized and designed.

[0092] C min a n =C max

[0093] Where n is the number of discrete parameters; C min is the minimum value that the current limiting device parameter can take, C max is the maximum value that the current limiting device parameters can take. Based on the above analysis of the current limiting sensitivity range of CLR and FCL, in this example, C min and C max They are 50mH and 200mH respectively. In addition, due to the certain error in the selection of inductor parameters, this example takes a 15% inductor parameter selection error as an example, that is, a = 1.15, and then we can get n = 10. The discrete parameters of CLR and FCL are: 50mH, 58mH, 68mH, 78mH, 91mH, 105mH, 122mH, 142mH, 164mH and 190mH. The fitting curve of the discrete parameter points of the current limiting device selected by S3 is as follows: Figure 7 It is shown that the selected discrete parameters of the equipment make the fault current change approximately according to a linear law, thus realizing the linear discretization of the current limiting device parameters.

[0094] According to S2, this example is the low coupling area fault points f8 and f 10 When a fault occurs, no optimization is required. According to formula S2, two fault points f8 and f 10 The CLR parameters are 0.2H and 0.083H respectively.

[0095] Three objective functions are proposed for parameter optimization of current limiting devices in high coupling areas: m1, m2, m3:

[0096]

[0097] Among them, m1 indicates the best current limiting effect in the whole network, I q (t2) is the line current near the fault point when the DCCB operates, I Dmax is the maximum breaking current of DCCB. m2 represents the parameter value of CLR of the whole network. Cq The total inductance value is the smallest. m3 represents the total current limiting inductance value L of the entire network FCL Fp Minimum.

[0098] The following uses the analytic hierarchy process to obtain the weight coefficients of each objective: First, the importance of the objective functions is sorted. Since the primary goal of optimizing the configuration of the current limiting device parameters is to meet the current limiting requirements of the system, the objective function m1 that reflects the current limiting effect is the most important. Secondly, since CLR is a current limiting device that has been running in the power grid, it not only has a suppressive effect on fault current, but also plays an important role in the normal operation of the system in a steady state, so the objective function m2 is more important. Then, the objective functions are sorted from high to low in importance as m1, m2, and m3. This example selects the importance scale value 1, 2, and 3 according to the analytic hierarchy process, and calculates the multi-objective weight coefficients.

[0099] The judgment matrix C established according to the scale value is as follows.

[0100]

[0101]

[0102] Where h is the number of objective functions, and w is the weight coefficient corresponding to each objective function.

[0103] Combining the above formula, the weight coefficient of each target can be obtained as: w = [0.539, 0.297, 0.164]. In order to verify the correctness of the calculated weight coefficient, the consistency check of the weight coefficient is carried out according to the following formula.

[0104]

[0105]

[0106] CI=(λ max -h) / (h-1)=0.004

[0107] CR=CI / RI=0.004 / 0.52=0.007<ε=0.1

[0108] The value of RI in the formula can be directly obtained by looking up the table in combination with the number of objective functions.

[0109] It can be seen from the formula that the calculated weight coefficient meets the consistency requirement. Therefore, based on the calculation of each objective weight coefficient, the multi-objective optimization is converted into the following single-objective optimization problem:

[0110]

[0111] In addition, according to the overcurrent protection constraint of the converter valve and the converter outlet voltage constraint, the bridge arm current of the converter should be less than twice the rated current of the IGBT (3kA) and the DC voltage should be greater than 0.7 times the rated value of the grid voltage (U dc ). In summary, the constraints of the objective function m are as follows.

[0112]

[0113] Based on the above objective function and constraints, the particle swarm optimization algorithm based on discrete optimization is used to perform discrete optimization on the parameters of the current limiting device, and the optimization result is: L C1 =0.122H,L C2 =0.190H,L C3 =0.190H,L C4 =0.164H,L C5 =0.190H,L C6 =0.190H,L C7 =0.190H,L C8 =0.200H, L C9 =0.190H,L C10 =0.190H,L C11 =0.190H,L C12 =0.083H.

[0114] In this example, there are 12 FCLs, which are represented as FCL1, FCL2, ..., FCL 12 , FCL is configured in FCL1, FCL4, FCL5, FCL9, FCL 11 At, the current limiting inductance values ​​are L F1 =0.164H,L F4 =0.122H,L F5 =0.164H,L F9 =0.164H,L F11 =0.142H.

[0115] In order to compare and analyze the discrete and continuous optimization schemes of equipment parameters, taking the CLR optimization interval of [50,200]mH and the FCL optimization interval of [100,200]mH as an example, the CLR and FCL parameters are continuously optimized, and the optimization results are called optimization scheme 1: The specific optimization results are: L C1 =0.164H,L C2 =0.188H,L C3 =0.175H,L C4 =0.178H,L C5 =0.185H,L C6 =0.186H,L C7 =0.187H,L C8 =0.200H, L C9 =0.178H,L C10 =0.189H,L C11 =0.145H,L C12=0.083H. FCL is configured in FCL1, FCL4, FCL5, FCL9, FCL 11 At, the current limiting inductance values ​​are L F1 =0.152H,L F4 =0.108H,L F5 =0.167H,L F9 =0.182H,L F11 =0.146H; the continuous optimization result of the current limiting device parameters is directly rounded according to the optional discrete parameters. This is a pseudo-discrete optimization method, which is called optimization scheme 2. The specific optimization result is: L C1 =0.164H,L C2 =0.190H,L C3 =0.190H,L C4 =0.190H,L C5 =0.190H,L C6 =0.190H,L C7 =0.190H,L C8 =0.200H, L C9 =0.190H,L C10 =0.190H,L C11 =0.164H,L C12 =0.083H. FCL is configured in FCL1, FCL4, FCL5, FCL9, FCL 11 At, the current limiting inductance values ​​are L F1 =0.164H,L F4 =0.122H,L F5 =0.190H,L F9 =0.190H,L F11 =0.164H; the discrete optimization result of the current limiting device parameters is called optimization scheme 3.

[0116] The comparison of CLR and FCL optimization results under the three optimization schemes is shown in Table 1. It can be seen that the number of FCLs configured in the entire network is the same under the three schemes. Based on optimization scheme 1, the configuration parameters of the current limiting devices in the entire network are different, the device parameters are non-standardized, and each current limiting device needs to be customized. Compared with optimization scheme 1, optimization schemes 2 and 3 will increase the total CLR and FCL inductance values ​​of the entire network, but will greatly reduce the number of non-standardized devices.

[0117] Table 1 Optimization results under different optimization schemes

[0118]

[0119] The CLR cost is linearly positively correlated with its own inductance. For every 1mH increase in CLR cost, the cost increases by about 520,000 yuan. F Its price is the same as CLR and its own L F The optimization scheme 1 will lead to different equipment parameter specifications, which will greatly increase the equipment design and customization costs. Therefore, this paper considers 10% of the equipment customization cost for the optimization scheme 1, that is, for every 1mH increase in the total inductance of the whole network, the equipment cost increases by 572,000 yuan. The equipment cost comparison under the three optimization schemes is shown in Table 2.

[0120] Table 2 Equipment costs of different optimization solutions

[0121]

[0122] As can be seen from Table 2, compared with Optimization Scheme 1, Optimization Scheme 2 and Optimization Scheme 3 will increase the configuration of the inductance value of the current limiting equipment in the entire network, but will greatly reduce the customization cost of the equipment; Optimization Scheme 2 and Scheme 3 are both discrete parameter optimizations, and there is no need to consider the additional cost brought by equipment customization, but Optimization Scheme 2 will greatly increase the configuration of the inductance value of the current limiting equipment in the entire network compared with Optimization Scheme 3; In general, Optimization Scheme 3 has the highest economy and meets the requirements of standardization and standardization of actual engineering equipment.

[0123] The discrete optimization results of the current limiting device parameters in the six-terminal power grid obtained through hierarchical optimization are simulated and verified in the PSACD / EMTDC simulation platform. The simulation results are shown in Table 3:

[0124] Table 3 Simulation results of discrete hierarchical optimization scheme for current limiting device parameters

[0125]

[0126] It can be seen that within 6ms of the DC bipolar short-circuit fault occurring at the 12 fault points in the table, when the DCCB is actuated, the fault current of the fault line is less than the breaking current capacity of the DCCBs, which is 15kA in this example. The current of the converter arm connected to the fault point does not trigger the overcurrent protection threshold, which is 6kA in this example. The converter outlet voltage connected to each fault point is maintained at more than 0.7 times the rated DC voltage of the system, which meets the constraint conditions, indicating the effectiveness of the optimization scheme.

[0127] In addition, when the fault point f1 in the six-terminal power grid fails, the fault current comparison under the condition that the DC line head and end in the six-terminal power grid are configured as 150mHCLR and under the discrete hierarchical optimization scheme of the current limiting device parameters is as follows: Figure 8In this example, 6ms after the fault occurs, when the DCCB operates, the fault current drops from 21.61kA to 11.61kA, and the current limiting effect is significant, which reduces the breaking capacity of the DCCB, indicating the effectiveness of the current limiting effect of the method proposed in the present invention.

[0128] Taking the six-terminal power grid as an example, this optimization method can first determine the equipment parameter values ​​in two low-coupling areas, and then optimize the equipment parameter values ​​in ten high-coupling areas, thereby improving the overall optimization calculation efficiency by 16.7%. It can be seen that for the x-terminal power grid, if it contains y low-coupling areas, the "first low and then high" hierarchical optimization method can improve the overall optimization calculation efficiency by y / 2x%.

[0129] The above simulation results prove the feasibility of the optimization method: it realizes the hierarchical optimization of the current limiting equipment of the whole network of complex multi-terminal DC power grid and improves the overall optimization calculation efficiency; it adopts the discrete optimization scheme of the current limiting equipment parameters, which can greatly reduce the customization cost of the current limiting equipment when meeting the current limiting requirements, and is more in line with the requirements of standardization and standardization of actual engineering equipment, and has practical engineering application value. The method proposed in the present invention can provide a theoretical basis for the discrete hierarchical optimization of the current limiting equipment parameters of complex multi-terminal DC power grid.

[0130] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of the present invention to be protected.

Claims

1. A discrete hierarchical optimization method for parameters of multi-terminal power grid current limiting equipment, characterized in that: The method comprises the following steps: According to the coupling degree between the converter and the power grid, the power grid topology is divided into low coupling area and high coupling area, and combined with the current limiting requirements of each area, a typical current limiting scheme for each area is proposed; Typical current limiting solutions for low coupling areas include the following two solutions: Solution 1: Current limiting based on half-bridge MMC, DCCB and CLR; Solution 2: Current limiting based on active control hybrid MMC, isolating switch and CLR; Select the specific current limiting configuration scheme in the low coupling area based on whether there is reactive power support demand on the AC side, the need for rapid restart after a fault, and the project cost; The typical current limiting scheme is for high coupling areas, contains multiple converter stations, and considers equipment cost issues, using half-bridge MMC, CLR, DCCB and FCL current limiting schemes; The influence of the selection of CLR and FCL linear discrete parameters on the fault current is analyzed, and the general current limiting sensitivity range is obtained; Combined with the error range of inductance parameters, the discretization design principle of current limiting device parameters is proposed; Determine whether the power grid has a low coupling area. If so, determine the parameters of the equipment in the low coupling area. The low coupling area relies on CLR to limit current within milliseconds after the fault occurs. When the system fault ride-through conditions are met, reduce the system configuration CLR. Combining the obtained CLR and FCL linear discrete parameters, an optimization model is established to optimize the discrete parameters of the current limiting device in the high coupling area; Combining the discretized parameters of CLR and FCL, according to the system performance of the high-coupling area, the cost of the current limiting device and the current limiting effect of the device, with the best current limiting effect of the whole network, the minimum total CLR value and the minimum total FCL value as the goal, combined with the constraints, the multi-objective optimization configuration mathematical model considering the discrete optimization of CLR and FCL parameters is as follows: In the formula, x is the decision vector, m(x) is the objective function vector, and g i (x) is a constraint condition; The analytic hierarchy process is used to determine the weight coefficient of each objective function to overcome the subjectivity of objective weight coefficient selection in multi-objective optimization. Based on this, a single-objective optimization algorithm based on discrete optimization problems is used to perform discrete optimization of regional CLR and FCL parameters.

2. A discrete hierarchical optimization method for parameters of a multi-terminal power grid current limiting device according to claim 1, characterized in that: The converter station fault current variation rule is as follows: In the formula, i dc is the DC line current, I dc is the DC line current before the fault, i arm Represents the bridge arm current, I a is the AC side current amplitude, U dc is the DC voltage at the converter station outlet, R e is the sum of the equivalent resistance of the converter station and the equivalent resistance of the DC line, L e is the sum of the equivalent inductance of the converter station, the current limiting inductor and the equivalent inductance of the DC line; C e is the equivalent capacitance of the converter station; According to the above formula, as the CLR parameter value increases, the fault current gradually decreases. Considering the dynamic performance of the system, CLR provides a 1-3ms reaction time for protection while suppressing the rapid rise of bipolar short-circuit current. Therefore, the minimum CLR value should meet the requirement that the MMC does not lock 3ms after the fault occurs, that is, the maximum bridge arm current of the MMC should be less than 2 times the rated current of the IGBT 3ms after the fault occurs. Select the inductive FCL with parallel MOA. MOA effectively protects the equipment in the power system from overvoltage, failure or even damage. Due to the parallel MOA, the current limiting inductor L F It takes a certain amount of time to put the fault line into operation. When the MOA withstand voltage is lower than the rated voltage U of the MOA, MOAn When the MOA is in operation, the fault current flows through the current limiting inductor L. F and MOA transition stage L F Withstand voltage U FCL_LF / / MOA As follows: Where i dc is the fault current, i F For the flow through L F The current, i MOA is the current flowing through MOA; L F After fully put into operation, MOA will stop running. At this time, the voltage on FCL As shown below: When L F The voltage analysis after full input is the same as the voltage analysis of the inductive FCL. At the moment the inductive FCL is put into operation, an impulse voltage will be generated. When the fault current flows from the transfer branch to the current limiting inductor L F The time of the branch is t s , L F The voltage across the two ends is shown as follows: Where t1 is the time when the fault occurs.

3. A discrete hierarchical optimization method for parameters of a multi-terminal power grid current limiting device according to claim 2, characterized in that: The rated current of the IGBT is 3KA.

4. A discrete hierarchical optimization method for parameters of a multi-terminal power grid current limiting device according to claim 2, characterized in that: The fault current is transferred from the branch to the current limiting inductor L F The time t of the branch s At the microsecond level.

5. A discrete hierarchical optimization method for parameters of a multi-terminal power grid current limiting device according to claim 1, characterized in that: The current limiting device parameter discretization design principle is linearly discretized based on the error in inductance selection and the influence of CLR and FCL on faults: C min a n =C max Where n is the number of discrete parameters; C min is the minimum value that the current limiting device parameter can take, C max is the maximum value that the current limiting device parameters can take, and a is the inductance selection error.

6. A discrete hierarchical optimization method for parameters of a multi-terminal power grid current limiting device according to claim 1, characterized in that: According to the fault current variation law of the converter station, the CLR parameter values ​​under the two schemes are determined. According to the current limiting function requirements, the CLR parameter value in scheme 1 is determined while ensuring that the CLR parameter meets the maximum breaking current of the DCCB. According to the current limiting function requirement, scheme 2 determines the CLR parameter value in scheme 2 while ensuring that the CLR parameter satisfies the requirement that the converter valve will not be locked before active control after a fault occurs.

Images