CN113258566B - Hierarchical optimization method for parameters of source network current limiting equipment of complex multi-terminal power grid - Google Patents

Hierarchical optimization method for parameters of source network current limiting equipment of complex multi-terminal power grid Download PDF

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CN113258566B
CN113258566B CN202110524238.4A CN202110524238A CN113258566B CN 113258566 B CN113258566 B CN 113258566B CN 202110524238 A CN202110524238 A CN 202110524238A CN 113258566 B CN113258566 B CN 113258566B
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current
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clr
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CN113258566A (en
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梅军
陈萧宇
朱鹏飞
严凌霄
张丙天
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Southeast University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
    • H02H9/02—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess current
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/10—Power transmission or distribution systems management focussing at grid-level, e.g. load flow analysis, node profile computation, meshed network optimisation, active network management or spinning reserve management
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
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Abstract

The invention discloses a hierarchical optimization method for parameters of a source network current limiting device of a complex multi-terminal power grid, which comprises the following steps: according to the spatial position distribution of a direct current transformer in a power grid, carrying out primary dimension reduction on a complex large power grid topology, dividing the complex large power grid topology into a plurality of sub-power grid topologies with the same voltage level, carrying out secondary dimension reduction on the sub-power grid topologies with the same voltage level, and providing a typical current limiting scheme of each region; judging whether the sub-grid has a low coupling area, if so, limiting the current of the area by virtue of a CLR within milliseconds after the fault occurs, and reducing the CLR of system configuration as much as possible under the condition of meeting the fault ride-through condition of the system; and for the high-coupling area, establishing an optimized mathematical model under the condition that the third harmonic voltage is injected at the source side. According to the optimization method, the maximum current limiting capacity of the source side equipment is excavated through secondary dimensionality reduction and third harmonic voltage injection at the source side, source network side equipment cooperative current limiting is achieved, the current limiting pressure of the network side equipment is reduced, and meanwhile the overall optimization calculation efficiency is improved.

Description

Hierarchical optimization method for parameters of source network current limiting equipment of complex multi-terminal power grid
Technical Field
The invention relates to the field of relay protection of power systems, in particular to a parameter grading optimization method for a current limiting device of a source network of a complex multi-terminal power grid.
Background
The flexible direct current transmission technology has the advantages of flexibly controlling active/reactive power, being beneficial to new energy access, having no commutation failure, providing voltage support for a wind power plant and the like, and becomes an important technical means for solving the problems of long-distance and large-capacity power transmission and interconnection of an ultra-large alternating current and direct current hybrid power grid.
At present, a DCCB (direct current breaker) is mainly adopted for current limiting, but the DCCB is limited in breaking capacity, in practical engineering, a scheme that the DCCB is matched with a CLR (current limiting reactor) is adopted for current limiting, but the CLR inductance value is not too large, otherwise, the dynamic performance of a system is easily reduced, and therefore current limiting needs to be performed in a cooperative fit with other current limiting equipment such as an FCL (fault current limiter) so as to meet the requirement of limiting the breaking capacity of the DCCB. In recent years, with the rapid development of flexible dc power grid fault current limiting technology, an MMC (modular multilevel converter) topology with fault self-clearing capability and a source-side current limiting control strategy are gradually used to limit fault current and reduce the current limiting pressure of a grid-side device.
The current limiting equipment space configuration and parameter collaborative optimization scheme in the current direct-current power grid mainly aims at a three-terminal or four-terminal annular power grid, and the complexity of a future multi-terminal direct-current power grid structure cannot be reflected. In addition, the adopted current limiting means is relatively single, and the method mainly focuses on the cooperative optimization of mutual parameters of the DCCB, the CLR and the FCL at the network side, and does not realize the cooperative optimization current limiting of equipment at two sides of the source network.
Disclosure of Invention
The invention aims to provide a hierarchical optimization method of complex multi-terminal power grid source network current-limiting equipment parameters, which aims at a mixed multi-terminal flexible direct-current large power grid with different voltage grades, considers the same voltage grade and the coupling degree between a converter station and the power grid to perform two-time dimensionality reduction on the power grid, divides the large power grid into a plurality of low-coupling fault areas and high-coupling fault areas in sub-power grid topology with the same voltage grade, provides a basis for subsequent hierarchical optimization equipment parameters, realizes the cooperative current limiting of the source network side equipment and reduces the current-limiting pressure of the network side equipment by injecting third harmonic voltage into the source side to excavate the maximum current-limiting capability of the source side equipment, adopts a multi-objective function to optimize the high-coupling area current-limiting equipment parameters on the basis that the low-coupling area related current-limiting parameters with relatively simple fault current characteristics are firstly determined, and realizes the hierarchical optimization, the overall optimization calculation efficiency is improved.
The purpose of the invention can be realized by the following technical scheme:
a hierarchical optimization method for parameters of a source network current limiting device of a complex multi-terminal power grid comprises the following steps:
s1: according to the spatial position distribution of a direct current transformer in the current limiting equipment at the power grid source side, the complex large power grid topology is subjected to primary dimensionality reduction, and is divided into a plurality of sub-power grid topologies with the same voltage level.
S2: and performing secondary dimension reduction on the sub-power grid topology with the same voltage level, wherein the secondary dimension reduction divides the sub-power grid topology with the same voltage level into a low coupling area and a high coupling area.
S3: and combining the current limiting requirements of each region to provide a typical current limiting scheme of each region.
S4: and judging whether the sub-grid has a low coupling region, if so, limiting the current of the region by virtue of the CLR within milliseconds after the 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.
S5: and for the high-coupling area, establishing an optimized mathematical model under the condition that the third harmonic voltage is injected at the source side.
Further, the secondary dimension reduction in S2 is to perform spatial division on the sub-grid topology of the same voltage class according to the strength of the coupling relationship between the grid connection lines, whether fault processing has selective characteristics, and the position of a fault point.
Further, the typical current limiting scheme in S3 includes the following two for the low coupling region:
scheme 1: the current limiting is carried out based on half-bridge MMCD, DCCB and CLR, and the scheme adopts network side current limiting.
Scheme 2: the current limiting is carried out based on an actively controlled hybrid MMC, a disconnecting switch and a CLR, and the scheme adopts source side current limiting.
And selecting a specific current limiting configuration scheme of the low coupling area according to whether the AC side has a reactive power support requirement, a requirement of quick restart after a fault and the construction 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.
The CLR in the high coupling area is configured to consider the dynamic performance constraint of the system, the CLR is configured to be 0.1H-0.3H, and the area needs to be limited by cooperating with the FCL.
And third harmonic voltage is injected into the AC side of the converter.
Upper bridge arm voltage u of converterpjLower bridge arm voltage unjDC voltage UdcAnd the voltage amplitude V on the AC valve side1mThe relationship of (a) is as follows:
Figure BDA0003065205040000031
as can be seen from the above formula, the upper bridge arm voltage u can be seen from the above formulapjNot less than 0 and lower bridge arm voltage unjMore than or equal to 0, DC voltage Udc≥2V1m。
After injecting the third harmonic voltage, the ac side voltage becomes:
Figure BDA0003065205040000041
in the formula uvaIs an A-phase voltage of the AC side, uvbIs the B-phase voltage of the AC side, uvcThe ac side C-phase voltages are provided.
The above formula is subjected to derivation to obtain the maximum value V of the voltage on the AC side after the third harmonic wave is added1`mComprises the following steps:
Figure BDA0003065205040000042
from the above formula, after the third harmonic is added, the voltage amplitude of the ac valve side can be reduced to 0.866 times of the original voltage amplitude.
After the third harmonic wave is added, the direct current voltage UdcSatisfies the following conditions:
Figure BDA0003065205040000043
DC voltage UdcEqual to the sub-module input voltage, the rated voltage u of the individual sub-modulecFor constant value, single bridge arm throwIf the number of the sub-modules is N, then:
Udc＝Nuc
Figure BDA0003065205040000044
in the formula, N' is the minimum number of submodules that can be put into the ac side when the third harmonic is injected.
Further, the change law of the fault current of a single converter station is as follows:
Figure BDA0003065205040000045
Figure BDA0003065205040000046
Figure BDA0003065205040000047
Figure BDA0003065205040000051
Figure BDA0003065205040000052
in the formula idcIs a direct line current, I1For the pre-fault DC line current iLIndicating bridge arm current, IaIs the amplitude of the AC side current, UdcFor the converter station outlet DC voltage, ReqIs the sum of the equivalent resistance of the converter station and the equivalent resistance of the direct current line, LeqIs the sum of the equivalent inductance of the converter station, the equivalent inductance of the direct current line and the equivalent inductance of the direct current line.
According to the formula, the CLR parameter values under the two schemes in the S3 are determined, the scheme 1 in the S3 can be known according to the requirement of the current limiting function, and the CLR parameter values in the scheme are determined under the condition that the parameters in the CLR meet the maximum on-off current of the DCCB.
According to the requirement of the current limiting function, the scheme 2 in the S3 is known, and under the condition that the CLR parameter is ensured to meet the condition that the CLR parameter is not locked before the converter valve is actively controlled after the fault occurs, the CLR parameter value in the scheme is determined.
Further, the mathematical model in S5 is created based on the system performance of the high coupling region, the cost of the current limiting device, and the current limiting effect of the device, and is targeted to achieve the best current limiting effect in the whole network, the minimum CLR total value, and the minimum FCL total value.
Performing cooperative optimization configuration on the CLR and FCL of the network side current limiting equipment and the voltage reduction coefficient k of the converter of the source side equipment, adopting a multi-objective optimization algorithm, combining constraint conditions, and considering a multi-objective optimization configuration mathematical model with the cooperative cooperation of CLR, FCL and k as follows:
Figure BDA0003065205040000053
wherein x is a decision vector, m (x) is an objective function vector, gi(x) And (3) optimizing to obtain a group of pareto solution sets according to the formula as a constraint condition, and then combining a fuzzy membership function to obtain the following formula:
Figure BDA0003065205040000061
Figure BDA0003065205040000062
in the formula, miFor the ith objective function value, mi,maxIs the upper limit of the objective function, mi,minAnd respectively selecting an optimal compromise solution from the solution set, wherein the lower limit of the objective function is mu, the normalized satisfaction value is mu, and n is the number of the objective functions.
The invention has the beneficial effects that:
1. the optimization method is used for carrying out two-time dimensionality reduction on a power grid by considering the same voltage level and the coupling degree between a converter station and the power grid aiming at a mixed multi-terminal flexible direct-current large power grid with different voltage levels, and dividing the large power grid into a plurality of low-coupling fault areas and high-coupling fault areas in the topology of a sub-power grid with the same voltage level to optimize equipment parameters in a subsequent grading way;
2. according to the optimization method, the third harmonic voltage is injected at the source side, the maximum current limiting capacity of the source side equipment is excavated, the source network side equipment is cooperatively limited in current, and the current limiting pressure of the network side equipment is reduced;
3. the optimization method adopts the multi-objective function to optimize the parameters of the current limiting equipment in the high coupling area on the basis of firstly determining the related current limiting parameters of the low coupling area with relatively simple fault current characteristics, thereby realizing hierarchical optimization and improving the overall optimization calculation efficiency.
Drawings
The invention will be further described with reference to the accompanying drawings.
FIG. 1 is a flow chart of an optimization method of the present invention;
FIG. 2 is a schematic diagram of a 23-terminal multi-voltage-class hybrid DC large power grid according to the present invention;
FIG. 3 is a schematic diagram of a six-terminal power grid according to the present invention;
FIG. 4 is a fault area division diagram for a six-terminal power grid according to the present invention;
FIG. 5 is a graph of the voltage modulation of the source side uninjected third harmonic front arm of the present invention;
FIG. 6 is a graph of bridge arm voltage modulation after the source side injects the third harmonic in accordance with the present invention;
FIG. 7 is a pareto disaggregation effect diagram for a six-terminal power grid in accordance with the present invention;
fig. 8 is a diagram of the current limiting effect of the six-terminal network optimal compromise current limiting scheme 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.
A hierarchical optimization method for parameters of a source network current limiting device of a complex multi-terminal power grid is disclosed, as shown in FIG. 1, and the optimization method comprises the following steps:
s1: according to the spatial position distribution of a direct current transformer in the current limiting equipment at the power grid source side, the complex large power grid topology is subjected to primary dimensionality reduction, and is divided into a plurality of sub-power grid topologies with the same voltage level.
S2: and performing secondary dimension reduction on the sub-grid topology with the same voltage level, wherein the secondary dimension reduction is to perform spatial division on the sub-grid topology with the same voltage level according to the strength of the coupling relation among the grid connecting lines, whether fault processing has the selective characteristic and the position of a fault point, and the secondary dimension reduction divides the sub-grid topology with the same voltage level into a low coupling area and a high coupling area.
S3: and (3) combining the current limiting requirements of each region, and providing a typical current limiting scheme of each region.
For a low-coupling region, because the low-coupling region belongs to a single-end converter node, after a fault occurs, the low-coupling region is decoupled with a power grid, the fault current characteristic of the low-coupling region is clear, and two typical current limiting configuration schemes are provided for the region as follows:
scheme 1: the current limiting is carried out based on half-bridge type MMC, DCCB and CLR, and the scheme adopts network side current limiting.
Scheme 2: the current limiting is carried out based on an actively controlled hybrid MMC, a disconnecting switch and a CLR, and the scheme adopts source side current limiting.
And selecting a specific current limiting configuration scheme according to whether the alternating current side has a reactive power support requirement, a requirement of quick restart after a fault and the construction 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.
The CLR in the high coupling region needs to be configured by considering the dynamic performance constraint of a system, the CLR is configured to be 0.1H-0.3H, the CLR needs to be matched with the FCL in a coordinated mode to limit the current of the region, and because the cost of the FCL is high, the CLR is not suitable to be configured too much, and third harmonic voltage is injected into the alternating current side of the converter.
The third harmonic voltage is injected into the AC side of the converter, the current-limiting capacity of high-coupling area network side equipment is reduced, the current-limiting capacity of a half-bridge type MMC of typical source side equipment of a DC power grid is fully excavated, the utilization rate of the DC voltage is improved, and further when the half-bridge type MMC adopts an emergency current-limiting control strategy for reducing the input number of sub-modules, the input number of the sub-modules and a voltage-reducing coefficient k can be further reduced on the premise that the work of the AC power grid is not influenced, the source network equipment is cooperatively limited, and the current-limiting pressure of the network side equipment is reduced.
The injected third harmonic voltage appears in the output voltage of the converter at the alternating current side, a star-shaped transformer and a delta-shaped transformer are actually used at the alternating current side, a zero sequence loop does not exist, the normal operation of the converter cannot be influenced by the injected third harmonic voltage, and the input quantity of the submodules which can be reduced after the third harmonic voltage is injected is analyzed as follows:
upper bridge arm voltage u of converterpjLower bridge arm voltage unjDC voltage UdcAnd the voltage amplitude V on the AC valve side1mThe relationship of (a) is as follows:
Figure BDA0003065205040000081
as can be seen from the above formula, the upper bridge arm voltage upjNot less than 0 and lower bridge arm voltage unjUnder the condition of more than or equal to 0, the direct current voltage Udc≥2V1m。
After the third harmonic voltage is added, the ac side voltage becomes:
Figure BDA0003065205040000091
in the formula uvaIs an A-phase voltage of the AC side, uvbIs the B-phase voltage of the AC side, uvcIs the AC side C phase voltage.
The above formula is subjected to derivation to obtain the maximum value V of the voltage on the AC side after the third harmonic wave is added1`mComprises the following steps:
Figure BDA0003065205040000092
according to the formula, after the third harmonic wave is added, the voltage amplitude of the alternating current valve side can be reduced to 0.866 times of the original voltage amplitude, and further the direct current voltage U is improveddcThe utilization ratio of (2).
In summary, after the third harmonic wave is added, the DC voltage U is obtaineddcIt should satisfy:
Figure BDA0003065205040000093
DC voltage UdcEqual to the sub-module input voltage, the rated voltage u of the individual sub-modulecIf the number of the submodules thrown into a single bridge arm is N, the following steps are performed:
Udc＝Nuc
in summary, the DC voltage UdcThe number N of the submodules input by a single bridge arm is reduced, compared with the number N of the submodules input by a non-injected harmonic wave, the number N of the submodules input by 13.4% is reduced after the third harmonic voltage is injected into the alternating current side, and a direct current voltage reduction strategy is utilized to the maximum extent under the condition that the normal operation of a power grid is not influenced.
Figure BDA0003065205040000094
In the formula, N' is the minimum number of submodules that can be put into the ac side when the third harmonic is injected.
S4: and judging whether the sub-grid has a low coupling region, if so, because the sub-grid belongs to a single-end converter node, the sub-grid is decoupled with the grid after the fault occurs, and the sub-grid belongs to a single-end converter, the fault current characteristic of the sub-grid is clear, and no matter which current limiting scheme in S3 is adopted in the region, the region is limited by the CLR within milliseconds after the fault occurs.
Under the condition of meeting the system fault ride-through, the system configuration CLR is reduced as much as possible, the system performance can be improved, the equipment cost investment can be reduced, and the fault current change rule of a single converter station is as follows:
Figure BDA0003065205040000101
Figure BDA0003065205040000102
Figure BDA0003065205040000103
Figure BDA0003065205040000104
Figure BDA0003065205040000105
in the formula idcIs a direct line current, I1For the pre-fault DC line current iLIndicating bridge arm current, IaIs the amplitude of the AC side current, UdcFor the converter station outlet DC voltage, ReqIs the sum of the equivalent resistance of the converter station and the equivalent resistance of the direct current line, LeqIs the sum of the equivalent inductance of the converter station, the equivalent inductance of the direct current line and the equivalent inductance of the direct current line.
According to the formula, determining the CLR parameter value under two schemes in S3, knowing scheme 1 in S3 according to the requirement of the current limiting function, and determining the CLR parameter value in the scheme under the condition of ensuring that the parameter in the CLR meets the maximum on-off current of the DCCB;
according to the requirement of the current limiting function, scheme 2 in S3 is known, and the CLR parameter value in the scheme is determined under the condition that the CLR parameter is ensured to meet the condition that the CLR parameter is not locked before the converter valve is actively controlled after the fault occurs.
S5: and for the high coupling area, under the condition that third harmonic voltage is injected at the source side, establishing an optimized mathematical model, wherein the mathematical model is established according to the fixed parameters of the current limiting equipment in the low coupling area, the system performance of the high coupling area, the cost of the current limiting equipment, the current limiting effect of the equipment, the minimum CLR total value and the minimum FCL total value as targets.
Performing cooperative optimization configuration on the CLR and FCL of the network side current-limiting equipment and the voltage reduction coefficient k of the current converter of the source side equipment to realize cooperative current limiting of the source network equipment, adopting a multi-objective optimization algorithm and combining constraint conditions, and considering a multi-objective optimization configuration mathematical model with the cooperative cooperation of the CLR, FCL and k as follows:
Figure BDA0003065205040000111
wherein x is a decision vector, m (x) is an objective function vector, gi(x) For constraint conditions, a group of pareto solution sets are obtained according to the optimization of the formula, and then the fuzzy membership function is combined as the following formula:
Figure BDA0003065205040000112
Figure BDA0003065205040000113
in the formula, miFor the ith objective function value, mi,maxIs the upper limit of the objective function, mi,minThe lower limit of the objective function, mu the normalized satisfaction value and n the number of objective functions, respectively, is selected from the solution set.
The effectiveness of the current limiting effect of the parameter grading optimization of the source network current limiting equipment in the complex power grid is verified through simulation software PSCAD/EMTDC, as shown in FIG. 2, the example is a 23-terminal multi-voltage grade mixed direct-current large power grid.
According to the step S1, the 23-end large power grid topology is subjected to primary dimensionality reduction, the example is divided into 5 sub-power grid topologies with the same voltage level, and a six-end power grid is selected as an example to verify the effectiveness of a typical configuration scheme of each regional source network device and the effectiveness of a parameter grading optimization method of a current limiting device on the basis of the primary dimensionality reduction, as shown in FIG. 3, in the example, the voltage of the six-end power grid is +/-500 kV.
In this example, 12 dc bipolar short-circuit fault points are set in the six-terminal power grid, and according to S2, space division results of "secondary dimension reduction" and "secondary dimension reduction" are performed on the six-terminal power grid, as shown in fig. 4, and according to S3, validity of various typical schemes of current limiting devices of the source grid provided for different areas is verified.
In this example, the low coupling zone contains two converter stations, respectively denoted C4And C6The high coupling area contains a plurality of converter stations, respectively denoted C1、C2、C3And C5，f1-f12For a fault point, the current limiting scheme of each region of the six-terminal power grid is selected as follows:
converter station C in low coupling area4Selecting half-bridge MMC, CLR and DCCB current limiting scheme, C6Selecting MMC, CLR and isolating switch current limiting scheme, converter station C in high coupling area1、C2、C3And C5MMC, CLR, FCL and DCCB current limiting schemes are adopted.
After each zone current limiting scheme is determined, according to S4, this example is a low coupling zone fault point f8And f10When a fault occurs, two fault points f are calculated according to the formula S48And f10The CLR parameters were 0.24H and 0.0183H, respectively.
After parameters of the current limiting device in the low coupling area are determined, the current limiting devices CLR and FCL in the high coupling area and the voltage reduction degree k are optimally selected according to S5, and an MCFCL (inductive magnetic coupling fault current limiter) is selected according to the invention aiming at the high coupling area in the six-terminal power grid, and has the advantages of low conduction loss and fast impedance variation.
Aiming at a high coupling area of a six-terminal power grid, substituting three objective functions m into a multi-objective optimization configuration mathematical model in S5 by taking three objective functions m as examples, wherein the three objective functions m are respectively1、m2And m3The expression is as follows:
Figure BDA0003065205040000121
Figure BDA0003065205040000122
wherein m is1Indicating the best current limiting effect of the whole network, Ii(t4) For fault point near-end line current during DCCB action, IDmaxIs the maximum off current of the DCCB.
m2A parameter value L representing CLRRAnd MCFCL leakage reactance LF1Total inductance value L ofcMinimum, MCFCL leakage reactance LF1The equivalent inductance of MCFCL during normal operation is the same as CLR operation mode, so CLR and LF1Considered as a whole.
m3Represents the total excitation reactance value L of the whole network MCFCLF2And minimum.
According to the four constraint conditions of the formula, the fault current I is shown when the DCCB acts according to the current breaking capacity of the DCCBi(t4) Is less than IDmaxBridge arm current I of current converterarmLess than twice the rated current of the IGBT (insulated gate bipolar transistor), 3kA in this example, and in order to maintain the dynamic performance of the dc network, the inductance parameter is limited, LCi(i-1-12) is LRAnd LF1And (4) summing.
In order to solve the current limiting capability of the half-bridge type MMC, on the basis of injecting third harmonic voltage into the source side, the input quantity of sub-modules can be further reduced after a fault occurs, as shown in fig. 5 and 6, after the third harmonic voltage is added into the source side, the utilization rate of direct current voltage is improved, and the voltage reduction degree is too low, so that the current on the alternating current side is increased sharply, the normal operation of a system is influenced, and therefore the voltage reduction coefficient k needs to be restrained.
When the fault occurs, on the premise of not influencing the normal work of the alternating current power grid, the direct current voltage is further reduced, the input number of sub-modules is reduced, the current limiting capacity of the source side equipment is fully developed, the current limiting pressure of the network side equipment is reduced, the source network cooperative current limiting is realized, and k is not less than 0.65 and not more than 1 in the example.
Based on the multi-objective function and the constraint condition, a group of pareto solution sets is obtained by combining a multi-objective mixed frog leaping algorithm, as shown in fig. 7.
In the actual operation of the power grid, a dispatcher needs to select an optimal compromise solution from a group of pareto solution sets, substitute the optimal compromise solution into a fuzzy membership function to calculate the standardized satisfaction degree, and find the optimal compromise solution from the group of pareto solution sets as follows: k is 0.885, LC1＝0.280H，LC2＝0.268H，LC3＝0.115H，LC4＝0.248H，LC5＝0.254H，LC6＝0.341H，LC7＝0.352H，LC8＝0.24H，LC9＝0.203H，LC10＝0.147H，LC11＝0.218H，LC12＝0.083H。
In this example there are 12 FCLs, denoted as FCLs respectively1、FCL2、...、FCL12The MCFCL is configured in the FCL2，FCL3，FCL4，FCL10Where the excitation reactance values are LF22＝0.148H，LF23＝0.123H，LF24＝0.139H，LF210＝0.189H。
In order to prove the influence of the voltage reduction coefficient k of the source side device on the configuration of the network side device, when the current limitation of the source side device is not considered and only the current limitation of the network side device CLR and FCL is relied on, the optimal compromise solution is obtained as follows: l is a radical of an alcoholC1＝0.137H，LC2＝0.283H，LC3＝0.329H，LC4＝0.121H，LC5＝0.359H，LC6＝0.320H，LC7＝0.248H，LC8＝0.24H，LC9＝0.276H，LC10＝0.169H，LC11＝0.235H，LC12＝0.083H。
The MCFCL is configured in the FCL1，FCL4，FCL7，FCL10，FCL11Where the excitation reactance values are LF21＝0.152H，LF24＝0.108H，LF27＝0.167H，LF210＝0.182，LF211＝0.146H。
The optimization results under the two schemes are shown in Table 1
TABLE 1 optimization results under different current limiting schemes
Figure BDA0003065205040000141
As can be seen from table 1, the source network cooperative current limiting scheme provided in the present invention can reduce the current limiting pressure of the network side device, reduce the number of FCLs configured, and reduce the device cost on the basis of improving the current limiting effect of the entire network.
The effectiveness of the optimal compromise configuration scheme of the source network equipment in the six-terminal power grid obtained through hierarchical optimization is simulated and verified in a PSACD/EMTDC simulation platform, and the simulation result is shown as table 2:
TABLE 2 simulation results of fault current and DC voltage under optimal compromise
Figure BDA0003065205040000142
Figure BDA0003065205040000151
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 0.65 time of the rated direct current voltage of the system, the constraint condition is met, and the effectiveness of the compromise scheme is shown.
Furthermore, a fault point f in the six-terminal network1In the case of a fault, when the head and the tail of a direct-current line in a six-terminal power grid are configured to be 100mHCLR, the fault current and the fault current under the optimal compromise scheme of source network equipment obtained through hierarchical optimization are compared, for example, as shown in fig. 8, in the present example, after the fault occurs for 6ms, when a DCCB acts, the fault current is reduced from 25kA to 10.9kA, the current limiting effect is significant, the open-close capacity of the DCCB is reduced, which indicates that the present embodiment is suitable for a power grid system with a fault current limiting function of 6msThe current limiting effect effectiveness of the proposed method.
Taking a six-terminal power grid as an example, the optimization method can determine the device parameter values in two low-coupling areas of the direct-current line, optimize the device parameter values in ten high-coupling areas, and further improve the overall optimization calculation efficiency by 16.7%, so that for the x-terminal power grid, if the y-position low-coupling area is included, the overall optimization calculation efficiency of y/2 x% can be improved.
The simulation result proves the feasibility of the optimization method, and provides a theoretical basis for reducing the pressure of the network side equipment for the complex multi-terminal direct-current power grid, realizing the source network current-limiting equipment collaborative optimization current limiting and improving the overall optimization calculation efficiency.
In the description herein, references to the description of "one embodiment," "an example," "a specific example" or the like are intended to mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
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 (4)

1. A hierarchical optimization method for parameters of a source network current limiting device of a complex multi-terminal power grid is characterized by comprising the following steps:
s1: according to the spatial position distribution of the direct-current transformers of the current-limiting equipment at the power grid source side, performing primary dimensionality reduction on a complex large power grid topology, and dividing the complex large power grid topology into a plurality of sub power grid topologies with the same voltage level;
s2: performing secondary dimensionality reduction on the sub-grid topology with the same voltage level, wherein the secondary dimensionality reduction divides the sub-grid topology with the same voltage level into a low coupling area and a high coupling area;
s3: providing a typical current limiting scheme of each region by combining the current limiting requirements of each region;
s4: judging whether the sub-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;
s5: for a high-coupling area, under the condition that third harmonic voltage is injected at the source side, an optimized mathematical model is established, and source network coordinated current limiting is realized;
the typical current limiting scheme in S3 includes the following two for the low coupling region:
scheme 1: carrying out current limiting based on half-bridge MMC, DCCB and CLR, wherein the scheme adopts network side current limiting;
scheme 2: performing current limiting based on an actively controlled hybrid MMC, a disconnecting switch and a CLR, wherein the scheme adopts source side current limiting;
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;
the CLR configuration in the high coupling area needs to consider the dynamic performance constraint of the system, the CLR configuration is 0.1H-0.3H, and the CLR configuration needs to cooperate with the FCL to limit the current of the area;
injecting third harmonic voltage at the AC side of the converter;
upper bridge arm voltage u of converterpjLower bridge arm voltage unjDC voltage UdcAnd the voltage amplitude V on the AC valve side1mThe relationship of (a) is as follows:
Figure FDA0003571304400000021
as can be seen from the above formula, the upper bridge arm voltage upjNot less than 0 and lower bridge arm voltage unjMore than or equal to 0, DC voltage Udc≥2V1m；
After injecting the third harmonic voltage, the ac side voltage becomes:
Figure FDA0003571304400000022
in the formula uvaIs an A-phase voltage on the AC side, uvbIs the B-phase voltage of the AC side, uvcIs the AC side C phase voltage;
the derivation is carried out on the formula to obtain the maximum value V' of the AC side voltage after the third harmonic wave is added1mComprises the following steps:
Figure FDA0003571304400000023
according to the formula, after the third harmonic is added, the voltage amplitude of the alternating current valve side can be reduced to 0.866 times of the original voltage amplitude;
after the third harmonic wave is added, the direct current voltage UdcSatisfies the following conditions:
Figure FDA0003571304400000024
DC voltage UdcEqual to the sub-module input voltage, the rated voltage u of the individual sub-modulecIf the number of the submodules thrown into a single bridge arm is N, the following steps are performed:
Udc＝Nuc
Figure FDA0003571304400000025
in the formula, N' is the minimum number of submodules that can be put into the ac side when the third harmonic is injected.
2. The method for optimizing the grading of the parameters of the current-limiting equipment of the source network of the complex multi-terminal power grid according to claim 1, wherein the secondary dimension reduction in the step S2 is to perform spatial division on the sub-power grid topology of the same voltage class according to the strength of the coupling relationship between the grid connecting lines, whether fault processing has selective characteristics, and the position of a fault point.
3. The method for optimizing the grading of the parameters of the current limiting equipment of the source network of the complex multi-terminal power grid according to claim 1, wherein the change law of the fault current of a single converter station is as follows:
Figure FDA0003571304400000031
Figure FDA0003571304400000032
Figure FDA0003571304400000033
Figure FDA0003571304400000034
Figure FDA0003571304400000035
in the formula idcIs a direct line current, I1For the pre-fault DC line current iLIndicating bridge arm current, IaIs the amplitude of the AC side current, UdcFor the converter station outlet DC voltage, ReqFor the equivalent of a converter stationSum of resistance and equivalent resistance of DC line, LeqIs the sum of the equivalent inductance of the converter station, the equivalent inductance of the direct current line and the equivalent inductance of the direct current line, CeqThe sum of the equivalent inductance of the converter station, the equivalent reactance of the direct current line and the equivalent capacitance of the direct current line;
according to the formula, the CLR parameter values under the two schemes in the S3 are determined, the scheme 1 in the S3 can be known according to the requirement of the current limiting function, and the CLR parameter values in the scheme are determined under the condition that the parameters in the CLR meet the maximum on-off current of the DCCB;
according to the requirement of the current limiting function, the scheme 2 in the S3 is known, and under the condition that the CLR parameter is ensured to meet the condition that the CLR parameter is not locked before the converter valve is actively controlled after the fault occurs, the CLR parameter value in the scheme is determined.
4. The method for optimizing the parameter classification of the current-limiting equipment of the source network of the complex multi-terminal power grid according to claim 3, wherein the mathematical model in the S5 is established 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, and aiming at the best current-limiting effect of the whole power grid, the minimum total CLR value and the minimum total FCL value;
performing cooperative optimization configuration on the CLR and FCL of the network side current limiting equipment and the voltage reduction coefficient k of the converter of the source side equipment, adopting a multi-objective optimization algorithm, combining constraint conditions, and considering a multi-objective optimization configuration mathematical model with the cooperative cooperation of CLR, FCL and k as follows:
Figure FDA0003571304400000041
wherein x is a decision vector, m (x) is an objective function vector, gi(x) And (3) optimizing to obtain a group of pareto solution sets according to the formula as a constraint condition, and then combining a fuzzy membership function to obtain the following formula:
Figure FDA0003571304400000042
Figure FDA0003571304400000043
in the formula, miFor the ith objective function value, mi,maxIs the upper limit of the objective function, mi,minThe lower limit of the objective function, mu the normalized satisfaction value and n the number of objective functions, respectively, is selected from the solution set.
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CN110416983A (en) * 2019-07-03 2019-11-05 东南大学 Current-limiting reactor and fault current limiter cooperates with Optimal Configuration Method in soft straight power grid
CN111769530A (en) * 2020-05-21 2020-10-13 东北电力大学 Flexible direct-current transmission fault current cooperative inhibition method for large-scale wind power access

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN110416983A (en) * 2019-07-03 2019-11-05 东南大学 Current-limiting reactor and fault current limiter cooperates with Optimal Configuration Method in soft straight power grid
CN111769530A (en) * 2020-05-21 2020-10-13 东北电力大学 Flexible direct-current transmission fault current cooperative inhibition method for large-scale wind power access

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