CN114884035A - Discrete type grading optimization method for parameters of multi-terminal power grid current limiting equipment - Google Patents

Discrete type grading optimization method for parameters of multi-terminal power grid current limiting equipment Download PDF

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CN114884035A
CN114884035A CN202210625849.2A CN202210625849A CN114884035A CN 114884035 A CN114884035 A CN 114884035A CN 202210625849 A CN202210625849 A CN 202210625849A CN 114884035 A CN114884035 A CN 114884035A
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梅军
陈萧宇
张丙天
张森
雷刘鹏
郭家炜
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
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Abstract

The invention discloses a discrete type grading optimization method for parameters of multi-terminal power grid current limiting equipment, which relates to the technical field of relay protection of a power system and comprises the following steps: providing a region division principle and a corresponding region typical current limiting scheme according to the coupling degree of a current converter and a power grid in the power grid; analyzing the influence rule of the current limiting equipment parameters on the fault current, and providing a discretization design principle of the current limiting equipment parameters by combining the error range of the inductance parameters; judging whether the power grid has a low coupling area, if so, determining the area current limiting equipment parameter according to a single-ended system fault current analytic expression; for a high-coupling area, an optimization model is established by combining discrete parameters of the current-limiting equipment, discrete optimization is carried out on the equipment parameters in the area, and the optimization space is reduced through hierarchical optimization, so that the original continuous current-limiting equipment parameters are valued in limited discrete points, and the optimization calculation efficiency is improved; when the requirement for current limiting is met, the customization cost of the current limiting equipment is reduced, and the requirements for standardization and standardization of actual engineering equipment are met.

Description

Discrete type grading optimization method for parameters of multi-terminal power grid current limiting equipment
Technical Field
The invention relates to the technical field of power system relay protection, in particular to a discrete type grading optimization method for parameters of multi-terminal power grid current limiting equipment.
Background
How to utilize multiple current-limiting devices to cooperate and effectively limit the fault current of the direct current side is a problem to be solved urgently in a flexible direct current power grid; at present, a DCCB (direct current breaker) is mainly adopted for current limiting, but due to the fact that the DCCB is limited in breaking capacity, in practical engineering, a scheme that the DCCB is matched with a CLR (current limiting inductor) is adopted for current limiting, the CLR always runs and is in a hot standby state in a line, and 1-3ms of reaction time is provided for protection while the rapid rise of fault current is restrained; however, an excessively large CLR value increases equipment cost, reduces the dynamic response capability of the system, and even causes system instability. It is difficult to achieve economic, efficient and reliable suppression of fault current in the close proximity of the CLR, so that it is a more universal scheme to combine the design of a dedicated FCL (fault current limiter) and the CLR to cooperatively suppress the fault current.
The current limiting equipment space configuration and parameter collaborative optimization scheme in the direct current power grid mainly aims at a simple ring power grid and cannot reflect the structural complexity of a future multi-terminal direct current power grid; in addition, the current limiting equipment collaborative optimization schemes are all current limiting equipment parameter continuous optimization, so that each current limiting equipment needs to be customized, the manufacturing cost of the equipment is greatly increased, and the current limiting equipment parameter discrete optimization which is more in line with actual equipment standardization and normalization lacks research; therefore, a discrete type grading optimization method for parameters of multi-terminal power grid current limiting equipment is provided.
Disclosure of Invention
In order to solve the above mentioned drawbacks in the background art, the present invention provides a discrete hierarchical optimization method for parameters of a multi-terminal power grid current limiting device.
The purpose of the invention can be realized by the following technical scheme: a discrete type hierarchical optimization method for parameters of multi-terminal power grid current limiting equipment comprises the following steps:
according to the coupling degree of a converter and a power grid, the power grid topology is divided into a low coupling area and a high coupling area, and a typical current limiting scheme of each area is provided by combining the current limiting requirements of each area;
analyzing the rule of influence of CLR and FCL linear discrete parameter selection on fault current to obtain a general current-limiting sensitive range;
the discretization design principle of the parameters of the current limiting equipment is provided by combining the error range of the inductance parameters;
judging whether the power grid has a low coupling area, if so, determining parameters of equipment in the area, limiting the current of the low coupling area by virtue of a CLR within milliseconds after a fault occurs, and reducing the CLR of system configuration under the condition that the fault ride-through of the system is met;
and establishing an optimization model by combining the obtained CLR and FCL linear discrete parameters, and optimizing discrete parameters of the high-coupling area current limiting equipment.
Further, the typical current limiting scheme includes the following two schemes for the low coupling region:
scheme 1: performing current limiting based on half-bridge MMC, DCCB and CLR;
scheme 2: carrying out current limiting based on the hybrid MMC, the isolating switch and the CLR which are actively controlled;
selecting a specific current limiting configuration scheme of a low coupling area according to whether the alternating current side has a reactive power support requirement, a requirement of quick restart after a fault and the engineering cost;
the typical current limiting scheme uses half-bridge type MMC, CLR, DCCB and FCL current limiting schemes for high coupling areas containing a plurality of converter stations, considering the equipment cost problem.
Further, the change law of the fault current of the converter station is as follows:
Figure BDA0003677388540000021
Figure BDA0003677388540000031
Figure BDA0003677388540000032
Figure BDA0003677388540000033
Figure BDA0003677388540000034
in the formula i dc Is a direct line current, I dc For the pre-fault DC line current i arm Indicating bridge arm current, I a Is the amplitude of the AC side current, U dc For the converter station outlet DC voltage, R e Is the sum of the equivalent resistance of the converter station and the equivalent resistance of the direct current line, L e The sum of the equivalent inductance of the converter station, the current-limiting inductor and the equivalent inductance of the direct current line; c e Is the converter station equivalent capacitance.
According to the formula, as the CLR parameter value is increased, the fault current is gradually reduced, and the dynamic performance of the system is considered, the CLR provides 1-3ms of reaction time for protection while inhibiting the bipolar short-circuit current from rapidly rising, so that the lowest value of the CLR needs to meet the condition that the MMC is not locked after the fault occurs for 3ms, namely the maximum bridge arm current of the MMC needs to be 2 times less than the rated current of the IGBT after the fault occurs for 3 ms;
the inductance type FCL of the parallel MOA is selected, the MOA can effectively protect equipment in a power system from being subjected to overvoltage to cause failure and even damage, and the current is limited due to the parallel MOAInductor L F The fault line needs a certain time to be put into operation, and the bearing voltage at the MOA is lower than the rated voltage U of the MOA MOAn When the MOA is always running and in the circuit, the fault current flows through the current-limiting inductor L F And MOA transition stage L F Upper withstand voltage
Figure BDA0003677388540000035
The following formula:
Figure BDA0003677388540000041
in the formula i dc For fault current, i F Is flowed through L F Current of (i) MOA Is flowing through MOA current;
L F after the MOA is completely put into operation, the voltage born by the FCL is
Figure BDA0003677388540000044
As shown below:
Figure BDA0003677388540000042
when L is F The voltage analysis after the complete input is the same as that of the inductive FCL, the impact voltage can be generated at the moment of the input of the inductive FCL, and when the fault current is transferred from the transfer branch circuit to the current-limiting inductor L F Time of branch is t s ，L F The voltage across is given by:
Figure BDA0003677388540000043
in the formula t 1 Is the time of occurrence of the failure.
Further, the rated current of the IGBT is 3 KA.
Further, the fault current is transferred from the transfer branch circuit to the current limiting inductor L F Time t of branch s On the order of microseconds.
Further, the discretization design principle of the current limiting equipment parameter is subjected to linear discretization on the basis of considering errors existing in inductance selection and the rule of influence of CLR and FCL on faults:
C min a n ＝C max
wherein n is the number of discrete parameters; c min Minimum value of a desired parameter of the current-limiting device, C max And a is the inductance selection error, which is the maximum value of the current-limiting equipment parameter.
Further, according to a converter station fault current change rule, determining a CLR parameter value under two schemes, and according to the current limiting function requirement, determining the CLR parameter value in the scheme 1 under the condition that the CLR parameter meets the maximum cut-off current of the DCCB according to the scheme 1;
according to the requirement of the current limiting function, under the condition that the CLR parameter meets the requirement of no locking before the converter valve is actively controlled after the fault occurs in the scheme 2, the CLR parameter value in the scheme 2 is determined.
Further, combining discretization parameters of the CLR and the FCL, according to the system performance of a high-coupling area, the cost of current limiting equipment and the current limiting effect of the equipment, aiming at the best current limiting effect of the whole network, the minimum total value of the CLR and the minimum total value of the FCL, combining constraint conditions, and considering a multi-objective optimization configuration mathematical model of CLR and FCL parameter discretization optimization as follows:
Figure BDA0003677388540000051
wherein x is a decision vector, m (x) is an objective function vector, g i (x) Are constraints.
The invention adopts an analytic hierarchy process to determine the weight coefficient of each objective function, overcomes the subjectivity problem of objective weight coefficient selection in multi-objective optimization, and adopts a single-objective optimization algorithm based on a discrete optimization problem to perform discrete optimization on the CLR and FCL parameters of the region.
The invention has the beneficial effects that:
the optimization method of the invention provides a region division principle for a complex multi-terminal power grid and provides a typical current limiting scheme of each region by combining the current limiting requirements of each region; on the basis, the hierarchical optimization of the current limiting equipment parameters in different areas is realized, and the calculation efficiency of the overall optimization of the current limiting equipment parameters of the complex multi-terminal direct-current power grid is improved; the original continuous current limiting equipment parameters are valued in limited discrete points, and the optimization space is greatly reduced; and when meeting the current-limiting requirement, the customization cost of the current-limiting equipment can be greatly reduced, the requirements of normalization and standardization of actual engineering equipment are met, and the method has an actual engineering application value.
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In order to more clearly illustrate the embodiments or technical solutions in the prior art of the present invention, the drawings used in the description of the embodiments or prior art will be briefly described below, and it is obvious for those skilled in the art to obtain other drawings without creative efforts;
FIG. 1 is a flow chart of an optimization method of the present invention;
FIG. 2 is a schematic diagram of a six-terminal power grid according to the present invention;
FIG. 3 is a fault area division diagram for a six-terminal power grid according to the present invention;
FIG. 4 is the effect of the CLR of the present invention on bridge arm current;
FIG. 5 is the effect of the CLR of the present invention on fault current;
FIG. 6 is the effect of the FCL of the present invention on fault current;
FIG. 7 is a CLR and FCL parameter linearization design of the present invention;
fig. 8 is a diagram of discrete type grading optimization current limiting effect of the current limiting device parameter of the invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
As shown in fig. 1, a discrete hierarchical optimization method for parameters of a multi-terminal power grid current limiting device includes the following steps:
s1: according to the coupling degree of the converter and the power grid, the power grid topology is divided into a low coupling area and a high coupling area, and a typical current limiting scheme of each area is provided by combining the current limiting requirements of each area;
the low coupling region belongs to single-end MMC, and fault current characteristic is simple, CLR current limiting is adopted, and DCCB opens and closes fault current to the effect, therefore, two current limiting schemes are provided for the region: scheme 1: performing current limiting based on half-bridge MMC, DCCB and CLR; scheme 2: carrying out current limiting based on the hybrid MMC, the isolating switch and the CLR which are actively controlled;
selecting a specific current limiting configuration scheme of a low coupling area according to whether the alternating current side has a reactive power support requirement, a requirement of quick restart after a fault and the engineering cost;
for a high-coupling area, a plurality of converter stations are contained, and a half-bridge type MMC, CLR, DCCB and FCL current limiting scheme is adopted in consideration of the equipment cost;
s2: and analyzing the rule of influence of CLR and FCL parameter selection on fault current, and revealing the general current limiting sensitivity range. The single converter station fault current change rule is as follows:
Figure BDA0003677388540000071
Figure BDA0003677388540000072
Figure BDA0003677388540000073
Figure BDA0003677388540000074
Figure BDA0003677388540000075
in the formula i dc Is a direct line current, I dc For the pre-fault DC line current i arm Indicating bridge arm current, I a Is the amplitude of the AC side current, U dc For the converter station outlet DC voltage, R e Is the sum of the equivalent resistance of the converter station and the equivalent resistance of the direct current line, L e The sum of the equivalent inductance of the converter station, the current-limiting inductor and the equivalent inductance of the direct current line; c e Is the converter station equivalent capacitance.
According to the above formula, it can be obtained that the fault current gradually decreases as the value of the CLR parameter increases in S2. The CLR cannot be taken too small considering the dynamic performance of the system, and provides a 1-3ms reaction time for protection while suppressing the rapid rise of the bipolar short-circuit current. Therefore, the lowest value of the CLR is required to meet the condition that the MMC is not locked after the fault occurs for 3ms, namely the maximum bridge arm current of the MMC is required to be 2 times less than the rated current (3kA) of the IGBT after the fault occurs for 3 ms.
FCLs have little negative impact on the system's normal operation and are actively placed in the short circuit path after a short circuit to suppress fault current. At present, the influence of FCL parameter value on fault current is considered, the relation between FCL self voltage stress and fault current and current-limiting inductance parameters is not considered, whether FCL self voltage stress margin influences parameter selection and current-limiting effect needs further research, and support is provided for selection of subsequent FCL parameter optimization range. And selecting an inductance type FCL of the parallel MOA. The MOA can effectively protect equipment in the power system from being subjected to overvoltage, thereby avoiding failure and even damage. Current limiting inductance L due to parallel MOA F A certain time is required to put in a faulty line. The bearing voltage at the MOA is lower than the rated voltage U of the MOA MOAn When the MOA is always running and in the circuit, the fault current flows through the current-limiting inductor L F And MOA transition stage L F Upper withstand voltage
Figure BDA0003677388540000081
The following formula:
Figure BDA0003677388540000082
in the formula i dc For fault current, i F Is flowed through L F Current of (i) MOA Is flowing through the MOA current.
L F After the MOA is completely put into operation, the voltage born by the FCL is
Figure BDA0003677388540000083
As shown below:
Figure BDA0003677388540000084
when L is F The voltage analysis after the complete input is the same as that of the inductive FCL, the impact voltage can be generated at the moment of the input of the inductive FCL, and when the fault current is transferred from the transfer branch circuit to the current-limiting inductor L F Time of branch is t s (in the order of microseconds), L F The voltage across is given by:
Figure BDA0003677388540000091
in the formula t 1 Is the time of occurrence of the failure.
And S3, providing a discretization design principle of the parameters of the current limiting equipment by combining the error range of the inductance parameters.
The method comprises the following steps of performing linear discretization on the basis of considering certain errors existing in inductance selection and the rule of influence of CLR and FCL on faults:
C min a n ＝C max
wherein n is the number of discrete parameters; c min Minimum value of a desired parameter of the current-limiting device, C max And a is the inductance selection error, which is the maximum value of the current-limiting equipment parameter.
S4: judging whether the power grid has a low coupling area, if so, determining parameters of equipment in the area, limiting the current of the area by virtue of a CLR within milliseconds after a fault occurs, and reducing the CLR configured by the system as much as possible under the condition of meeting the fault ride-through condition of the system without optimizing the parameters of the equipment in the area;
s5: and combining the CLR and FCL linear discrete parameters obtained in the step S3, and according to the system performance of the high coupling area, the cost of the current limiting equipment and the current limiting effect of the equipment, aiming at the best current limiting effect of the whole network, the minimum total CLR value and the minimum total FCL value. The CLR and FCL parameters are discretely optimized.
Combining constraint conditions, and considering a multi-objective optimization mathematical model of CLR and FCL parameter discrete optimization as follows:
Figure BDA0003677388540000092
wherein x is a decision vector, m (x) is an objective function vector, g i (x) Are constraints.
Because the multi-objective optimization obtains a group of solutions, the optimal solution cannot be directly obtained, therefore, the invention adopts an analytic hierarchy process to determine the weight coefficient of each objective function, overcomes the subjectivity problem of target weight coefficient selection in the multi-objective optimization, and adopts a single-objective optimization algorithm based on the discrete optimization problem to carry out the discrete optimization on the CLR and FCL parameters of the region.
The effectiveness of the current limiting effect of discrete grading optimization of parameters of the current limiting equipment in the complex power grid is verified through simulation software PSCAD/EMTDC, as shown in FIG. 2, the example is a six-terminal flexible direct-current power grid with the voltage of +/-500 kV.
According to the step S1, area division is carried out on the topology of the six-end power grid, the division result is shown in fig. 3, and the effectiveness of the typical scheme of the current limiting device provided for different areas is verified by combining the six-end power grid.
In this example, the low coupling zone contains two converter stations, denoted C4 and C6, respectively, and the high coupling zone contains a number of converter stations, denoted C1, C2, C3 and C5, respectively, f 1 -f 12 For a fault point, the current limiting scheme of each region of the six-terminal power grid is selected as follows:
the converter station C4 in the low coupling area selects a half-bridge type MMC, CLR and DCCB current limiting scheme, the converter station C6 selects a mixed type MMC, CLR and isolating switch current limiting scheme, and considering the cost of equipment, the converter stations C1, C2, C3 and C5 in the high coupling area all adopt the half-bridge type MMC, CLR, FCL and DCCB current limiting schemes.
After each zone current limiting scheme is determined, the present example analyzes the effect of CLR and FCL on fault current by taking as an example the inductive FCL of a parallel MOA (the MOA can effectively protect equipment in a power system from overvoltage and even damage) and a single-ended system rated at 400kV, according to S2. As shown in fig. 4, when the CLR value is lower than 50mH, the requirement that the MMC of the system does not lock in the protection response time will not be met, so this example takes the minimum CLR value of 50mH as an example to perform research. As shown in fig. 5, when CLR increases to the boundary value: 200mH, the CLR value is continuously increased, the suppression effect on the fault current is obviously reduced, the influence of the CLR value on the current limiting capability and the current recovery time is considered, and the CLR value range is generally [0.05,0.2 ]]H, get t s MOA rated voltage U of inductance type FCL (current collector inductor) connected with MOA in parallel for 1 microsecond MOAn And the effect of the current limiting inductance on the fault current is shown in fig. 6: following U MOAn The sensitivity range of the current-limiting inductor for effectively inhibiting the fault current is increased gradually, but the FCL is used for limiting the fault current and is connected with the U in parallel MOAn And can not be higher than the rated direct-current voltage of the system. This example uses U MOAn At 75% of the rated DC voltage, L F The boundary value with obvious suppression effect on the fault current is 200mH as an example, and a basis is provided for the discretization design of equipment parameters.
On the basis of analyzing the current limiting sensitive ranges of the CLR and the FCL, the discretization design of the equipment parameters is carried out by combining inductance parameter selection errors and the influence rule of the CLR and the FCL on fault current.
C min a n ＝C max
Wherein n is the number of discrete parameters; c min Minimum value of a desired parameter of the current-limiting device, C max Based on the above analysis of CLR and FCL current limiting sensitivity ranges for the maximum value of a current limiting device parameter that is desirable, this example C min And C max 50mH and 200mH, respectively. In addition, since there is some error in the inductance parameter selection, this example considers 15%Taking the inductance parameter selection error of (a) as an example, that is, a equals to 1.15, then n equals to 10, and the discrete parameters of CLR and FCL are selected as follows: 50mH, 58mH, 68mH, 78mH, 91mH, 105mH, 122mH, 142mH, 164mH and 190 mH. As shown in fig. 7, the fitting curve of the discrete parameter points of the current limiting device selected in S3 is used to select discrete parameters of the device, so that the fault current changes approximately according to a linear rule, thereby realizing linear discretization of the parameters of the current limiting device.
According to S2, the low coupling region fault point f is shown in this example 8 And f 10 When the fault occurs, two fault points f are calculated according to the formula S2 without optimization 8 And f 10 The CLR parameters were 0.2H and 0.083H, respectively.
Three objective functions are proposed for parameter optimization of high coupling area current limiting equipment: m is 1 ，m 2 ，m 3 ：
Figure BDA0003677388540000111
Wherein m is 1 Indicating the best current limiting effect of the whole network, I q (t 2 ) For fault point near-end line current during DCCB action, I Dmax Is the maximum off current of the DCCB. m is 2 Parameter value L representing a full network CLR Cq The total inductance value is minimal. m is 3 Representing the total current-limiting inductance L of the whole-network FCL Fp And minimum.
Each target weight coefficient is obtained by combining the analytic hierarchy process as follows: firstly, the importance of the objective function is sequenced, and the objective function m reflecting the current limiting effect is used as the primary objective of the optimal configuration of the parameters of the current limiting equipment is to meet the current limiting requirement of the system 1 Most importantly, the CLR is used as a current limiting device which is always operated in a power grid, so that the CLR not only has a function of inhibiting fault current, but also has an important function on the steady normal operation of a system, and therefore, the objective function m 2 It is important. Further, the objective function is ordered m from high to low in importance 1 ，m 2 ，m 3 . This example selects the importance scale values according to an analytic hierarchy process: 1, 2, and 3, calculating multi-target weight coefficients.
The decision matrix C established from the scale values is as follows.
Figure BDA0003677388540000121
Figure BDA0003677388540000122
In the formula, h is the number of objective functions, and w is the weight coefficient corresponding to each objective function.
The weight coefficient of each target obtained by combining the above formula is: w ═ 0.539, 0.297, 0.164 ]. In order to verify the correctness of the weight coefficient obtained by calculation, the consistency check is carried out on the weight coefficient according to the following formula.
Figure BDA0003677388540000123
Figure BDA0003677388540000124
CI＝(λ max -h)/(h-1)＝0.004
CR＝CI/RI＝0.004/0.52＝0.007＜ε＝0.1
The RI value in the formula can be directly obtained by looking up a table by combining the number of objective functions.
As can be seen from the equation, the calculated weight coefficients satisfy the consistency requirement. Therefore, on the basis of calculating the weight coefficient of each target, multi-target optimization is converted into a single-target optimization problem of the following formula:
Figure BDA0003677388540000125
in addition, according to the overcurrent protection constraint of the converter valve and the outlet voltage constraint of the converter, the bridge arm current of the converter is less than two times of the rated current (3kA) of the IGBT and the direct-current voltage is more than 0.7 times of the rated value (U) of the grid voltage dc ). To sum upThe constraints of the objective function m are as follows.
Figure BDA0003677388540000131
Based on the target function and the constraint condition, a particle swarm optimization algorithm based on discrete optimization improvement is adopted to perform discrete optimization on the parameters of the current limiting equipment, and the obtained optimization result is as follows: l is 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。
The FCLs in this example are 12 in number, denoted as FCLs respectively 1 、FCL 2 、...、FCL 12 The FCL is configured in the FCL 1 ，FCL 4 ，FCL 5 ，FCL 9 ，FCL 11 Where, the current-limiting inductance value is L F1 ＝0.164H，L F4 ＝0.122H，L F5 ＝0.164H，L F9 ＝0.164H，L F11 ＝0.142H。
For comparing the discrete and continuous optimization schemes of the analysis equipment parameters, the CLR optimization interval is [50,200 ]]mH, FCL optimization interval is [100,200]mH is taken as an example, the CLR and FCL parameters are continuously optimized, and the optimization result is called an optimization scheme 1: the specific optimization result is as follows: l is 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. The FCL is configured in the FCL 1 ，FCL 4 ，FCL 5 ，FCL 9 ，FCL 11 Where, the current-limiting inductance value is L F1 ＝0.152H，L F4 ＝0.108H，L F5 ＝0.167H，L F9 ＝0.182H，L F11 0.146H; the result of continuous parameter optimization of the parameters of the current limiting equipment,rounding the continuous optimization result directly according to the selectable discrete parameters, which is a pseudo-discrete optimization mode and is called as an optimization scheme 2, wherein the specific optimization result is as follows: l is 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. The FCL is configured in the FCL 1 ，FCL 4 ，FCL 5 ，FCL 9 ，FCL 11 Where, the current-limiting inductance value is L F1 ＝0.164H，L F4 ＝0.122H，L F5 ＝0.190H，L F9 ＝0.190H，L F11 0.164H; the result of the discrete optimization of the parameters of the current limiting device is referred to as an optimization scheme 3.
The optimization results of the CLR and the FCL under the 3 optimization schemes are shown in table 1, for example, it can be seen that the number of the FCLs configured in the whole network under the 3 schemes is the same, based on the optimization scheme 1, the configuration parameters of the current limiting devices in the whole network are different, the device parameters are non-normalized, and each current limiting device needs to be customized. Optimization scheme 2 and optimization scheme 3 will increase the total net CLR and FCL inductance values, but will greatly reduce the number of device denormals, compared to optimization scheme 1.
TABLE 1 optimization results under different optimization schemes
Figure BDA0003677388540000141
The cost of the CLR is in linear positive correlation with the inductance of the CLR, and the cost of the CLR is increased by about 52 ten thousand yuan for every 1mH increase. Major source of equipment cost for FCL and L F Its price is as same as CLR as its own L F And is linearly related. Because the optimization scheme 1 results in different device parameter specifications and greatly increases the device design and customization cost, 10% of the device customization cost is considered in the text for the optimization scheme 1, namely, the device cost is increased by 57.2 ten thousand yuan for every 1mH increase of the total inductance value of the whole network. The equipment cost pair ratios under the 3 optimization schemes are shown in table 2.
TABLE 2 different optimization scheme Equipment costs
Figure BDA0003677388540000142
As can be seen from table 2, the optimization schemes 2 and 3 increase the configuration of the inductance of the full-network current-limiting device compared to the optimization scheme 1, but greatly reduce the customization cost of the device; the optimization scheme 2 and the optimization scheme 3 belong to discrete parameter optimization, extra cost brought by equipment customization is not required to be considered, but the configuration of inductance values of the whole-network current-limiting equipment is greatly increased by comparing the optimization scheme 2 with the optimization scheme 3; in general, the optimization scheme 3 is most economical while meeting the requirements of actual engineering equipment normalization and standardization.
Simulation verification is carried out on the discrete optimization result of the parameters of the current limiting equipment in the six-terminal power grid obtained through hierarchical optimization in a PSACD/EMTDC simulation platform, and the result obtained through simulation is shown as table 3:
TABLE 3 simulation results under the discrete type hierarchical optimization scheme of current limiting device parameters
Figure BDA0003677388540000151
It can be seen that, within 6ms of the occurrence of the dc bipolar short-circuit fault at 12 fault points in the table, when the DCCB operates, the fault current of the fault line is smaller than the open current capacity of the DCCBs, which in this example is 15 kA. The current of a bridge arm of the current converter connected with the fault point does not trigger an overcurrent protection threshold, the overcurrent protection threshold in the example is 6kA, the voltage of the outlet of the current converter connected with each fault point is kept to be more than 0.7 time of the rated direct current voltage of the system, the constraint condition is met, and the effectiveness of the optimization scheme is shown.
Furthermore, a fault point f in the six-terminal network 1 When a fault occurs, the fault current pair is shown in fig. 8 under the condition that the head end and the tail end of a direct current line in a six-terminal power grid are configured to be 150mHCLR and under the discrete type grading optimization scheme of parameters of a current limiting device. In this example, after a fault occurs for 6ms, when the DCCB is operated, a fault current flowsThe current limiting effect is reduced from 21.61kA to 11.61kA, the current limiting effect is obvious, the DCCB breaking capacity is reduced, and the effectiveness of the current limiting effect of the method is shown.
Taking a six-terminal power grid as an example, the optimization method can determine the equipment parameter values in two low-coupling areas, and then optimize to obtain the equipment parameter values in ten high-coupling areas, so that the overall optimization calculation efficiency can be improved by 16.7%. Therefore, for the x-end power grid, if the y-position low coupling area is included, the overall optimization calculation efficiency of y/2 x% can be improved by adopting a hierarchical optimization method of 'low first and high last'.
The simulation result proves the feasibility of the optimization method: the grading optimization of the complex multi-terminal direct-current power grid whole-network current limiting equipment is realized, and the overall optimization calculation efficiency is improved; by adopting the discrete optimization scheme of the parameters of the current limiting equipment, the customization cost of the current limiting equipment can be greatly reduced when the current limiting requirement is met, the requirements of normalization and standardization of actual engineering equipment are met, and the method has an actual engineering application value. The method provided by the invention can provide a theoretical basis for discrete type grading optimization of the parameters of the current limiting equipment of the complex multi-terminal direct current power grid.
The foregoing shows and describes the general principles, essential features, and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed.

Claims (8)

1. A discrete type grading optimization method for parameters of multi-terminal power grid current limiting equipment is characterized by comprising the following steps:
according to the coupling degree of the converter and the power grid, the power grid topology is divided into a low coupling area and a high coupling area, and a typical current limiting scheme of each area is provided by combining the current limiting requirements of each area;
analyzing the rule of influence of CLR and FCL linear discrete parameter selection on fault current to obtain a general current-limiting sensitive range;
the discretization design principle of the parameters of the current limiting equipment is provided by combining the error range of the inductance parameters;
judging whether the power grid has a low coupling area, if so, determining parameters of equipment in the low coupling area, limiting the current of the low coupling area by virtue of a CLR within milliseconds after a fault occurs, and reducing the CLR of system configuration under the condition that the fault ride-through of the system is met;
and establishing an optimization model by combining the obtained CLR and FCL linear discrete parameters, and optimizing discrete parameters of the high-coupling area current limiting equipment.
2. The method for optimizing the parameter discrete type grading of the multi-terminal grid current limiting device according to claim 1, wherein the typical current limiting scheme comprises the following two schemes for a low coupling region:
scheme 1: performing current limiting based on half-bridge MMC, DCCB and CLR;
scheme 2: carrying out current limiting based on the hybrid MMC, the isolating switch and the CLR which are actively controlled;
selecting a specific current limiting configuration scheme of a low coupling area according to whether the alternating current side has a reactive power support requirement, a requirement of quick restart after a fault and the engineering cost;
the typical current limiting scheme uses half-bridge type MMC, CLR, DCCB and FCL current limiting schemes for high coupling areas containing a plurality of converter stations, considering the equipment cost problem.
3. The discrete type hierarchical optimization method for the parameters of the multi-terminal power grid current limiting equipment according to claim 2, wherein the change law of the fault current of the converter station is as follows:
Figure FDA0003677388530000021
Figure FDA0003677388530000022
Figure FDA0003677388530000023
Figure FDA0003677388530000024
Figure FDA0003677388530000025
in the formula i dc Is a direct line current, I dc For the pre-fault DC line current i arm Indicating bridge arm current, I a Is the amplitude of the AC side current, U dc For the converter station outlet DC voltage, R e Is the sum of the equivalent resistance of the converter station and the equivalent resistance of the direct current line, L e The sum of the equivalent inductance of the converter station, the current-limiting inductor and the equivalent inductance of the direct current line; c e Is the equivalent capacitance of the converter station;
according to the formula, as the value of the CLR parameter is increased, the fault current is gradually reduced, and considering the dynamic performance of the system, the CLR provides 1-3ms of reaction time for protection while inhibiting the rapid rise of the bipolar short-circuit current, so that the MMC is not locked after the lowest value of the CLR meets the condition that the fault occurs for 3ms, namely the maximum bridge arm current of the MMC is less than 2 times of the rated current of the IGBT after the fault occurs for 3 ms;
the inductance type FCL of the parallel MOA is selected, the MOA effectively protects equipment in a power system from being subjected to overvoltage to cause failure and even damage, and the current-limiting inductance L is caused by the parallel MOA F The fault line needs a certain time to be put into operation, and the bearing voltage at the MOA is lower than the rated voltage U of the MOA MOAn When the MOA is always running and in the circuit, the fault current flows through the current-limiting inductor L F And MOA transition stage L F Upper withstand voltage
Figure FDA0003677388530000026
The following formula:
Figure FDA0003677388530000031
in the formula i dc For fault current, i F Is flowed through L F Current of (i) MOA Is flowing through MOA current;
L F after the MOA is completely put into operation, the voltage born by the FCL is
Figure FDA0003677388530000032
As shown below:
Figure FDA0003677388530000033
when L is F The voltage analysis after the complete input is the same as that of the inductive FCL, the impact voltage can be generated at the moment of the input of the inductive FCL, and when the fault current is transferred from the transfer branch circuit to the current-limiting inductor L F Time of branch is t s ，L F The voltage across is given by:
Figure FDA0003677388530000034
in the formula t 1 Is the time of occurrence of the failure.
4. The method for optimizing parameters of the multi-terminal power grid current-limiting device in a discrete type step according to claim 3, wherein the rated current of the IGBT is 3 KA.
5. The method as claimed in claim 3, wherein the fault current is diverted from the branch to the limiting inductor L F Time t of branch s On the order of microseconds.
6. The discrete hierarchical optimization method for the parameters of the multi-terminal power grid current-limiting equipment according to claim 1, wherein the discretization design principle of the parameters of the current-limiting equipment is that linear discretization is performed on the basis of considering errors existing in inductance selection and the rule that CLR and FCL influence the fault:
C min a n ＝C max
wherein n is the number of discrete parameters; c min Minimum value of a desired parameter of the current-limiting device, C max And a is the inductance selection error, which is the maximum value of the current-limiting equipment parameter.
7. The discrete hierarchical optimization method for the parameters of the multi-terminal power grid current-limiting equipment according to claim 2 is characterized in that the CLR parameter values under two schemes are determined according to a converter station fault current change rule, and the CLR parameter value in the scheme 1 is determined under the condition that the CLR parameter meets the maximum cut-off current of the DCCB according to the current-limiting function requirement in the scheme 1;
according to the requirement of the current limiting function, under the condition that the CLR parameter meets the requirement of no locking before the converter valve is actively controlled after the fault occurs in the scheme 2, the CLR parameter value in the scheme 2 is determined.
8. The discrete hierarchical optimization method for the parameters of the multi-terminal power grid current-limiting equipment according to claim 6, wherein the objective of the best overall network current-limiting effect, the minimum total CLR value and the minimum total FCL value is taken as the target according to the system performance of the high-coupling region, the cost of the current-limiting equipment and the current-limiting effect of the equipment by combining the discrete parameters of the CLR and the FCL, and the objective optimization configuration mathematical model considering the discrete optimization of the CLR and the FCL parameters by combining the constraint conditions is as follows:
Figure FDA0003677388530000041
wherein x is a decision vector, m (x) is an objective function vector, g i (x) Is a constraint condition;
and determining the weight coefficient of each objective function by adopting an analytic hierarchy process, overcoming the subjectivity problem of target weight coefficient selection in multi-objective optimization, and performing discrete optimization on the parameters of the CLR and the FCL by adopting a single-objective optimization algorithm based on a discrete optimization problem.
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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN116565848A (en) * 2023-05-12 2023-08-08 华北电力大学（保定） Global optimization method for current limiting parameters
CN116565848B (en) * 2023-05-12 2023-10-13 华北电力大学（保定） Global optimization method for current limiting parameters

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