CN111313354A - Method for determining dynamic equivalent boundary of alternating current-direct current power system - Google Patents
Method for determining dynamic equivalent boundary of alternating current-direct current power system Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H1/00—Details of emergency protective circuit arrangements
- H02H1/0092—Details of emergency protective circuit arrangements concerning the data processing means, e.g. expert systems, neural networks
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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
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
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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
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/28—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for meshed systems
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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
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Abstract
The invention provides a method for determining a dynamic equivalence boundary of an alternating-current and direct-current power system, which is used for solving the problems that safety control measures are not considered in the determination of the dynamic equivalence boundary and the alternating-current and direct-current coupling enhancement trend cannot be comprehensively reflected in the prior art. The dynamic equivalence boundary determining method does not conduct equivalence on all direct current lines and direct current falling points, meanwhile, nodes closely related to the direct current falling points and fault points and nodes provided with safety control are reserved, paths are searched according to fault conditions based on graph theory, iterative equivalence calculation is conducted in an equivalence region, and therefore boundaries are determined. The method determines the equivalence boundary by combining with the new trend of gradual enhancement of the AC-DC coupling in the power system, provides reasonable visual equivalence boundaries for the establishment of different dynamic equivalence models under different expected faults, considers safety control measures, avoids the uncertainty of artificial designated boundaries, improves the accuracy of transient safety assessment of the AC-DC power system, reduces the calculated amount of the transient safety assessment, and ensures the safe operation of the power system.
Description
Technical Field
The invention belongs to the field of power system safety, and particularly relates to a method for determining a dynamic equivalent boundary of an alternating current-direct current power system by considering safety control measures.
Background
The power system is a network system for distributing and transmitting electric energy, and plays an irreplaceable role in production and life. With the development of extra-high voltage alternating current and direct current transmission and networking technologies, the configuration of an electric power system is greatly changed, the structure of the electric power system becomes more complex, higher requirements are also provided for the safe operation of the electric power system, and the traditional safety control measures cannot meet the requirements of a new electric power system. In order to improve the power supply reliability of the power system, transient security evaluation needs to be performed on a multi-resource security control strategy adopted after the power system fails. In order to reduce the amount of calculation in a large number of safety evaluation simulation scenes, it is necessary to perform dynamic equivalence on an ac/dc power system to reduce the scale of the simulation system on the premise of ensuring that the dynamic characteristics and response of the original ac/dc system after a fault occurs can be accurately simulated. In the dynamic equivalence process of an alternating current and direct current power system, due to the fact that the mutual influence degree of alternating current and direct current in the power system is continuously deepened under a new trend, an equivalence boundary is an extremely important parameter.
In the prior art, most researches directly and artificially specify an equivalence boundary on the basis of a certain principle according to a researched system, but for a large power grid, the equivalence precision of the boundary specified according to personal experience cannot be guaranteed in some cases. In addition, a small percentage of research improves the iso-precision by selecting a method of reserving nodes in the iso-region. For example, external network reserved bus method based on node voltage dependency analysis; and the selection principle of the reserved node of the external system and the method for determining the reserved node by using the sensitivity, but the equivalent boundary of the external equivalent area in the method adopted by the prior art is still directly specified, and the division of the internal system and the external system in the alternating current and direct current system is not specifically described. The fault scenes related in most equivalent boundary determination methods do not have the trend of gradual enhancement along with alternating current-direct current coupling, and the conditions of cascading faults and taking safety control measures are considered, so that the effect of equivalent model simulation is influenced, the transient safety assessment is not accurate enough, and the safe operation of a power system cannot be effectively ensured.
Disclosure of Invention
In order to reduce the calculated amount of transient safety evaluation, improve the accuracy of simulation of a dynamic equivalent model of an AC/DC power system, overcome the problems that safety control measures are not taken into consideration and AC/DC coupling trends cannot be comprehensively reflected in dynamic equivalent boundary determination, the invention provides a method for determining the dynamic equivalent boundary of the AC/DC power system taking the safety control measures into consideration, the method combines a new trend of gradual enhancement of AC/DC coupling in the power system, considers corresponding safety control measures required by failure of the AC system to cause DC commutation failure or fault propagation caused by DC blocking, searches equivalent boundaries under different fault conditions by using a shortest path algorithm based on a graph theory idea, adjusts regions needing equivalence in the system according to error requirements, determines the dynamic equivalent boundary, performs dynamic equivalence, and provides reasonable equivalence boundary for establishment of different dynamic equivalent models under different expected faults, the accuracy of the transient safety assessment of the power system is improved, and the safe operation of the power system is guaranteed.
In order to achieve the purpose, the invention adopts the following technical scheme.
In a first aspect, an embodiment of the present invention provides a method for determining a dynamic equivalence boundary of an ac/dc power system in consideration of safety control measures, where the method includes:
step S1, converting the alternating current and direct current power system into a topological graph based on graph theory;
step S2, setting the voltage value of the initial boundary division after the fault in the topological graph;
step S3, taking the fault point as a source point, taking the voltage value as a limiting condition, carrying out path search in the topological graph node to obtain an initial boundary, adding a supplementary boundary to form a comprehensive equivalence boundary, and carrying out dynamic equivalence calculation in the equivalence boundary;
step S4, carrying out error evaluation on the result of the equivalence calculation, and when the difference between the maximum error value of the error evaluation and the target error is larger than a preset range, turning to step S5; when the difference is smaller than the predetermined range, the process proceeds to step S6; outputting the isovalent boundary when the predetermined range is met;
step S5, increasing the voltage value divided by the set boundary according to a predetermined rule, and proceeding to step S3;
step S6, the voltage value divided by the set boundary is decreased according to a predetermined rule, and the process proceeds to step S3.
As a preferred embodiment of the present invention, in step S1, the converting the ac/dc power system into a graph based on graph theory further includes:
converting an alternating current-direct current power system into a graph G (V, E);
wherein V ═ { V ═ V1,ν2,ν3,…νnThe vertex points are set of all the vertexes and represent all nodes in the power system network; e ═ E1,e2,e3,…enThe weighting value assigned to each branch circuit represents the absolute value of the voltage difference between nodes at two ends of the branch circuit; in the graph, the length of an edge represents the magnitude of a weight, the direction of a line is determined through the positive and negative of a voltage difference value, a node with low voltage is a predecessor node of a node with high voltage, and the direction is from the end with low voltage to the end with high voltage, so that a weighted directed network is formed.
In a preferred embodiment of the present invention, in step S2, the voltage value is 0.8 pu.
As a preferred embodiment of the present invention, in step S3, a Dijkstra algorithm is used to perform a path search.
As a preferred embodiment of the present invention, the performing a path search in a node of a topological graph with the voltage value as a limiting condition to obtain an equivalence boundary further includes:
s301, selecting a node which is closely related to a direct current drop point and a fault point at the same time and has a node voltage less than or equal to the voltage value as a target end point, and performing path search in a topological graph by adopting a Dijkstra algorithm to obtain a basic boundary;
and S302, outside the basic equivalence boundary, taking the direct current drop point and the node with safety control set in the step S1 as supplementary boundaries, and adding the supplementary boundaries to the basic boundary to obtain a comprehensive equivalence boundary.
As a preferred embodiment of the present invention, the step S3 performs dynamic equivalence calculation within the equivalence boundary, which specifically includes the following steps:
step S311, performing coherent identification on the generator by adopting a time domain method according to the maximum and minimum principle, namely the generator is shown in formula (1);
where ε is the standard of deviation given and τ is the simulation computation time. Preferably, epsilon is 5-10 DEG, and tau is 1-3 s.
Step S312, simplifying the coherent generator bus according to the constant power technology;
step S313, simplifying the network by adopting a current channel CSR method;
and step S314, carrying out parameter aggregation on the coherent generator by adopting a weighting method to obtain an equivalent result.
As a preferred embodiment of the present invention, the step S4 performs error evaluation on the result of the equivalence calculation, specifically:
comparing the dynamic characteristic curves before and after equivalence, selecting node voltage and line active output or a generator power angle as a quantitative index for measuring dynamic errors, and calculating and analyzing errors of each point of the dynamic curves before and after equivalence by adopting a formula (2):
wherein x isiThe value of the ith sampling point of the dynamic characteristic curve is obtained, and N is the number of the sampling points;
the maximum error value is obtained.
As a preferred embodiment of the present invention, the maximum of the error evaluationError value DeltaxRMSThe predetermined range to be satisfied is ± 0.25% from the target error e.
As a preferred embodiment of the present invention, the predetermined rules in step S6 and step S7 are that the initial value of the increase-decrease value is determined according to the degree of deviation of the result of the comparison and is decreased by a dichotomy.
It can be seen from the technical solutions provided by the embodiments of the present invention that, the method for determining a dynamic equivalence boundary of an ac/dc power system considering safety control measures in the embodiments of the present invention combines a new trend of gradual enhancement of ac/dc coupling in the power system, considers the corresponding safety control measures required to be taken for fault propagation caused by failure of the ac system causing dc commutation failure or dc blocking, searches for an equivalence boundary under different fault conditions automatically by using a shortest path algorithm based on the idea of graph theory, adjusts an area in the system requiring equivalence according to an error requirement, determines the dynamic equivalence boundary, provides a reasonable equivalence boundary for establishment of different dynamic equivalence models under different expected faults, fully considers the safety control measures and systematically determines the visualized equivalence boundary, avoids uncertainty of an artificially specified boundary, improves accuracy of transient safety evaluation of the power system, and reduces a calculation amount of simulation evaluation, and the safe operation of the power system is ensured.
Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
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In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.
FIG. 1 is a schematic flow chart of a method for determining a dynamic equivalent boundary of an AC/DC power system according to an embodiment of the present invention;
fig. 2 is a schematic structural diagram of an electric power system when Bus6 in a certain area has a ground fault according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of a conversion diagram of the power system of FIG. 2;
FIG. 4 is a schematic diagram of a two-zone four-DC interconnect system based on an IEEE-39 system in accordance with an embodiment of the present invention;
FIG. 5 is a graph of power angle of the generator before and after a mean value in the power system shown in FIG. 4;
FIG. 6 is a graph of generator node voltage before and after a mean value in the power system of FIG. 4;
FIG. 7 is a graph of frequency before and after a mean value in the power system of FIG. 4;
FIG. 8 is a graph of DC turn-off angle for the power system of FIG. 4;
FIG. 9 is a schematic diagram of the iso-boundary of the power system of FIG. 4 at a voltage setpoint of 0.8 pu;
FIG. 10 is a schematic diagram of the iso-boundary of the power system of FIG. 4 at a voltage setpoint of 0.85 pu.
Detailed Description
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the accompanying drawings are illustrative only for the purpose of explaining the present invention, and are not to be construed as limiting the present invention.
As used herein, the singular forms "a", "an", "the" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may also be present. Further, "connected" or "coupled" as used herein may include wirelessly connected or coupled. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.
It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
Examples
The embodiment provides a method for determining a dynamic equivalent boundary of an alternating current and direct current power system by considering a safety control measure. The dynamic equivalence boundary determining method is characterized by providing an original system needing equivalence aiming at dynamic equivalence of an alternating current and direct current power system considering cascading faults and adopting safety control measures, providing key problems and basic principles which need attention and are different from those of a traditional equivalence system, then designing the dynamic equivalence boundary determining method, converting the alternating current and direct current power system into a graph based on the graph theory idea, searching a path by utilizing a Dijkstra algorithm according to fault conditions, forming an equivalence boundary by combining the provided basic principles, performing equivalence calculation on an equivalence region, finally evaluating an error result, and if the error requirement is not met, adjusting the equivalence boundary through iterative solution until the error meets the requirement, and determining the equivalence boundary. The embodiment of the invention combines the new trend of gradual enhancement of AC-DC coupling in the power system, considers the corresponding safety control measures required by the fault propagation caused by the failure of the AC system to cause the failure of DC commutation or the DC blocking, utilizes the shortest path algorithm to search and automatically form equivalent boundaries under different fault conditions based on the thought of graph theory, adjusts the area needing equivalent in the system according to the error requirement, determines the dynamic equivalent boundary, provides a reasonable equivalent boundary for the establishment of different dynamic equivalent models under different expected faults, fully considers the safety control measures and systematically determines the visual equivalent boundary, avoids the uncertainty of artificially specified boundary, improves the accuracy of transient safety evaluation of the power system, reduces the calculated amount of simulation evaluation, and ensures the safe operation of the power system.
Considering that the mutual coupling degree of alternating current and direct current is enhanced, alternating current faults possibly cause multi-circuit direct current commutation failure and even direct current locking, and further cascading faults are formed through direct current propagation to cause global influence of the faults. In this embodiment, the following principles are adopted to select nodes:
1) because the proportion of the transmission capacity of the direct current system is improved, the response characteristic of the direct current system to the alternating current fault influences the stability of the whole alternating current and direct current system, and all direct current lines and direct current drop points are reserved in order to reserve the mutual influence characteristic between the alternating current and direct current systems and between multiple loops of direct current.
2) Because of close coupling with the direct current drop point, the alternating current node which is closer in electrical distance and closer to the fault point can aggravate the sudden phase change voltage drop of the inversion side of the direct current line when the alternating current system fails, and the direct current phase change failure is easily caused. In order to more accurately simulate a cascading failure scenario in which a failure propagates due to a failure of a direct current commutation caused by an alternating current failure, nodes closely related to a direct current drop point and a failure point are reserved.
3) Considering the construction of the current system protection technology, after a system has a cascading failure, the coordination of direct-current emergency control and safety control measures such as a generator tripping, load shedding, a pump storage and pump cutting and the like needs to be adopted, and in order to check the effectiveness of a system protection strategy under the failure and ensure the accuracy degree of the equivalent model for simulating the dynamic characteristics under the strategy, nodes set for safety control in the dynamic equivalence process are reserved.
Fig. 1 is a schematic flow chart of the method for determining the dynamic equivalent boundary of the ac/dc power system. As shown in FIG. 1, the dynamic iso-boundary determination method includes the following steps:
step S1, converting the alternating current and direct current power system into a topological graph based on graph theory;
step S2, setting the voltage value of the initial boundary division after the fault in the topological graph;
step S3, taking the fault point as a source point, taking the voltage value as a limiting condition, carrying out path search in the topological graph node to obtain an initial boundary, adding a supplementary boundary to form a comprehensive equivalence boundary, and carrying out dynamic equivalence calculation in the equivalence boundary;
step S4, carrying out error evaluation on the result of the equivalence calculation, and when the difference between the maximum error value of the error evaluation and the target error is larger than a preset range, turning to step S5; when the difference is smaller than the predetermined range, the process proceeds to step S6; outputting the isovalent boundary when the predetermined range is met;
step S5, increasing the voltage value divided by the set boundary according to a predetermined rule, and proceeding to step S3;
step S6, the voltage value divided by the set boundary is decreased according to a predetermined rule, and the process proceeds to step S3.
Wherein, in the step S1, the converting the ac/dc power system into a topological graph based on graph theory further includes:
in (V, E), V ═ ν1,ν2,ν3,…νnThe vertex is a set of all vertexes and is used for representing each node in the power system network; e ═ E1,e2,e3,…enThe vertex is a set of edges between all the vertexes, and is used for representing each branch in the system; the method comprises the steps of applying a ground fault to a certain node in an alternating current-direct current system, obtaining the voltage of each node after the fault through simulation, and representing the absolute value of the voltage difference between the nodes at two ends of each branch by using the assigned weight value of each branch. In the graph, the length of an edge represents the magnitude of a weight, the direction of a line is determined by the positive and negative of a voltage difference value, a node with low voltage is a predecessor node of a node with high voltage, namely, the direction is from the end with low voltage to the end with high voltage, and a weighted directed network is formed. In addition, the direct current drop points and the nodes for setting safety control in the system are marked in the figure.
Since ac systems in which ac/dc systems are a plurality of zones are interconnected by dc lines, this embodiment describes the conversion of an electric power system into a diagram by taking one zone as an example. The proposed method for determining the equivalence boundary is directed to a specific fault, in this embodiment, taking a Bus6 in an electric power system as an example, fig. 2 is a schematic diagram of an electric power system structure when a Bus6 in a certain area has a ground fault, and fig. 3 is a schematic diagram of a topology of the electric power system when a Bus6 in a certain area has a ground fault. As shown in fig. 2 and 3, dots are dc drop points and nodes for setting safety control in the power system, and are reserved in the equivalence process.
In step S2, the voltage value is obtained by balancing the degree of simplification and the equivalent accuracy, and is preferably 0.8 pu.
In the step S3, the path search preferably adopts Dijkstra algorithm. The major idea of Dijkstra's algorithm is to traverse all nodes with a source point as the center, update the point set after each iteration, and reserve the shortest path from the source point to the target point found so far for each node. The performing path search in the nodes of the topological graph with the voltage value as a limiting condition to obtain an equivalent boundary, further comprising:
and S301, selecting a node which is closely related to the direct current drop point and the fault point at the same time and has the node voltage less than or equal to the voltage value as a target end point, and performing path search in the topological graph by adopting a Dijkstra algorithm to obtain a basic boundary. In the step, the fault point is used as a source point of algorithm search, the fault point is expanded to an outer layer until the fault point is expanded to a target node, and the topological graph is extended according to the sequence of voltage from low to high. For each node, the algorithm outputs the calculated shortest distance (namely the minimum weight sum) from the fault point to the target node and the predecessor node reaching the target point, so that the connection trend of the shortest path node in the system network required by the text can be obtained, and the boundary of the target node naturally forms a basic boundary.
And S302, outside the basic equivalence boundary, taking the direct current drop point and the node with safety control set in the step S1 as supplementary boundaries, and adding the supplementary boundaries to the basic boundary to obtain a comprehensive equivalence boundary. In this step, both the direct current drop point and the node adopting the safety control measure need to be reserved, so if the node exists outside the equivalent boundary, the node is taken as a supplementary boundary of the initial boundary in the previous step, and the equivalent area outside the comprehensive boundary is determined. In addition, nodes which are closely related to a direct current drop point and a fault point at the same time are selected according to the position of the specific fault point in the system, and direct current commutation failure is closely related to commutation voltage of an inverter side.
The step S3 of performing dynamic equivalence calculation within the equivalence boundary specifically includes the following steps:
step S311, performing coherent identification on the generator by adopting a time domain method according to the maximum and minimum principle, namely the generator is shown in formula (1);
where ε is the standard of deviation given and τ is the simulation computation time. Preferably, epsilon is 5-10 DEG, and tau is 1-3 s.
Step S312, simplifying the coherent generator bus according to the constant power technology;
step S313, simplifying the network by adopting a current Channel (CSR) method;
and step S314, carrying out parameter aggregation on the coherent generator by adopting a weighting method to obtain an equivalent result.
In the step S4, the result of the equivalence calculation is subjected to error evaluation, specifically, the dynamic characteristic curves before and after the equivalence are compared, since the voltage is not only closely related to the dynamic characteristic of the system, but also has a large influence on the dc commutation process, and at the same time, in order to make the index not single, the node voltage and the line active output or the generator power angle are simultaneously selected as quantization indexes for measuring the dynamic error, and the errors of each point of the dynamic curve before and after the equivalence are calculated and analyzed by using the relative root mean square in the formula (2), so as to obtain the maximum error value, thereby reflecting the overall average deviation.
Wherein x isiThe value of the ith sampling point of the dynamic characteristic curve is shown, and N is the number of the sampling points.
The difference between the maximum error value and the target error satisfies a predetermined range. Preferably, the pre-range is ± 0.25%.
In the steps S4 to S6, the maximum error value Deltax of the error estimation result is calculatedRMSComparing with the target error epsilon, if the difference value is larger than the preset range, taking a voltage value of 0.8pu as an example, increasing the voltage set value on the basis of 0.8pu to reduce the equivalent area, thereby reducing the error; otherwise, the voltage set value is reduced. And after the voltage value is adjusted, returning to the step S3, and performing iterative search to obtain a new equivalence boundary. In the iteration process, the preset rule changes after each iteration so as to continuously narrow the boundary adjustment range and gradually reduce the increasing and decreasing values of the voltage set value. Preferably, the predetermined rule is a dichotomy decreasing, e.g., Δ V is 0.5pu for the first iteration and Δ V is 0.25pu for the second iteration. And the initial increment and decrement value is determined according to the difference degree between the result error and the target error in the actual situation. The number of iterations varies with the degree of deviation of the initial error and the node voltage after the fault. After several iterations, the equivalent boundary can be obtained until the error meets the requirement.
It can be seen from the above technical solutions that, the method for determining a dynamic equivalence boundary of an ac/dc power system considering safety control measures of this embodiment combines a new trend of gradual enhancement of ac/dc coupling in the power system, considers the corresponding safety control measures required to be taken for propagation of a fault caused by a failure of the ac system causing a failure of dc commutation or dc blocking, searches for an equivalence boundary automatically formed under different fault conditions by using a shortest path algorithm based on the idea of graph theory, adjusts an area requiring equivalence in the system according to an error requirement, determines a dynamic equivalence boundary, provides a reasonable equivalence boundary for establishment of different dynamic equivalence models under different expected faults, fully considers safety control measures and systematically determines a visual equivalence boundary, avoids uncertainty of an artificially specified boundary, improves accuracy of transient safety evaluation of the power system, and reduces a calculation amount of simulation evaluation, and the safe operation of the power system is ensured.
In this example, a two-zone four-dc interconnection system constructed based on an IEEE-39 system is taken as an example, and fig. 4 is a schematic structural diagram of the two-zone four-dc interconnection system. As shown in fig. 4, the left side and the right side are respectively an ac sending end system and an ac receiving end system, which are connected by four dc lines. Wherein, the four direct current transmission capacities account for 39.68% of the total load of the receiving end, and account for a large proportion, and the receiving end is provided with 1 pumping storage power station. The actual dynamic equivalence process is partition equivalence, namely the sending end system and the receiving end system respectively conduct equivalence.
The method for determining the dynamic equivalent boundary of the alternating current-direct current power system comprises the following steps:
step S1001, applying a ground fault to the node 25 in the receiving end system, extracting the voltage of each node after the fault, and forming an initial isoboundary according to the method for determining the isoboundary. As shown in fig. 4, the upper dashed boundary is the initial iso-contour boundary.
And step S1002, carrying out a coherence identification test on the generator. After the ac node 25 fails, the DC2 fails commutation and latches at 0.95 s. And taking safety control measures under the cascading failure, and adopting a system protection strategy. And reserving the direct current drop point and the nodes with the safety control as supplementary boundaries, and performing dynamic equivalence calculation. As shown in fig. 4, the triangulated region is the supplemental boundary.
Step S1003, after cascading failure occurs and a system protection strategy is adopted, relation graphs of a power angle curve, a node voltage curve, a frequency curve and a direct current turn-off angle curve of the generator before and after equivalence are obtained, and further quantitative evaluation is conducted on dynamic characteristic results. Fig. 5 is a graph of the power angle of the generator before and after the equivalence in this example, fig. 6 is a graph of the node voltage of the generator before and after the equivalence in this example, fig. 7 is a graph of the frequency before and after the equivalence in this example, and fig. 8 is a graph of the dc off angle in this example. As shown in fig. 5 to 8, the node voltage and the line active output index are selected, and the dynamic characteristic effect error before and after the equivalence is calculated. The calculation results are shown in table 1:
TABLE 1 equal front and back relative root mean square error (0.8pu)
As shown in table 1, the maximum error of the active output curve of the line before and after the equivalent value exceeds 10%.
Step S1004, adjust the boundary to meet the equivalent error requirement. The voltage set point is increased to 0.85pu and the isoboundary is re-delineated. Searching by utilizing Dijkstra algorithm. Fig. 9 is a schematic diagram illustrating a dynamic adjustment process of the power system equivalent boundary (voltage set value is from 0.8pu to 0.85 pu).
As shown in FIG. 3, the lower dotted line is the boundary and equivalent region after the adjustment of the original system after the iteration, and the amplified reserved region is the region between the two dotted boundary lines.
Step S1005, performing dynamic equivalence calculation and dynamic characteristic simulation test on the system again. After the boundary adjustment, as shown in table 2, the relative root mean square error before and after the active output curve of the receiving end line is equivalent is obviously reduced, and the maximum error meets the range of the error requirement. The effectiveness of the method for determining the AC/DC dynamic equivalent boundary by considering the AC/DC cascading failure and adopting safety control measures is verified.
TABLE 2 equal front and back relative root mean square error (0.85pu)
Those of ordinary skill in the art will understand that: the figures are merely schematic representations of one embodiment, and the blocks or flow diagrams in the figures are not necessarily required to practice the present invention.
The embodiments in the present specification are described in a progressive manner, and the same and similar parts among the embodiments are referred to each other, and each embodiment focuses on the differences from the other embodiments. In particular, for apparatus or system embodiments, since they are substantially similar to method embodiments, they are described in relative terms, as long as they are described in partial descriptions of method embodiments. The above-described embodiments of the apparatus and system are merely illustrative, and the units described as separate parts may or may not be physically separate, and the parts displayed as units may or may not be physical units, may be located in one place, or may be distributed on a plurality of network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the solution of the present embodiment. One of ordinary skill in the art can understand and implement it without inventive effort.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (9)
1. A method for determining a dynamic equivalent boundary of an AC/DC power system is characterized by comprising the following steps:
step S1, converting the alternating current and direct current power system into a topological graph based on graph theory;
step S2, setting the voltage value of the initial boundary division after the fault in the topological graph;
step S3, taking the fault point as a source point, taking the voltage value as a limiting condition, carrying out path search in the topological graph node to obtain a basic boundary, adding a supplementary boundary to form a comprehensive equivalence boundary, and carrying out dynamic equivalence calculation in the equivalence boundary;
step S4, carrying out error evaluation on the result of the equivalence calculation, and when the difference between the maximum error value of the error evaluation and the target error is larger than a preset range, turning to step S5; when the difference is smaller than the predetermined range, the process proceeds to step S6; outputting the isovalent boundary when the predetermined range is met;
step S5, increasing the voltage value divided by the set boundary according to a predetermined rule, and proceeding to step S3;
step S6, the voltage value divided by the set boundary is decreased according to a predetermined rule, and the process proceeds to step S3.
2. The method for determining the dynamic isovolumetric boundary of the ac/dc power system of claim 1, wherein the step S1 is implemented by converting the ac/dc power system into a graph based on graph theory, and further comprising:
converting an alternating current-direct current power system into a graph G (V, E);
wherein V ═ { V ═ V1,ν2,ν3,…νmThe vertex points are set of all the vertexes and represent all nodes in the power system network; e ═ E1,e2,e3,…enThe weighting value assigned to each branch circuit represents the absolute value of the voltage difference between nodes at two ends of the branch circuit; in the graph, the length of an edge represents the magnitude of a weight, the direction of a line is determined through the positive and negative of a voltage difference value, a node with low voltage is a predecessor node of a node with high voltage, and the direction is from the end with low voltage to the end with high voltage, so that a weighted directed network is formed.
3. The method according to claim 1, wherein in step S2, the voltage value set for the initial boundary is 0.8 pu.
4. The method for determining the dynamic isovolumetric boundary of the ac/dc power system of claim 1, wherein in step S3, a Dijkstra algorithm is used for the path search.
5. The method for determining the dynamic isovolumetric boundary of the ac-dc power system according to claim 4, wherein the step of performing a path search in the topological graph node under the condition that the voltage value is used as a constraint to obtain the isovolumetric boundary further comprises the steps of:
s301, selecting a node which is closely related to a direct current drop point and a fault point at the same time and has a node voltage less than or equal to the voltage value as a target end point, and performing path search in a topological graph by adopting a Dijkstra algorithm to obtain a basic boundary;
and S302, outside the basic equivalence boundary, taking the direct current drop point and the node with safety control set in the step S1 as supplementary boundaries, and adding the supplementary boundaries to the basic boundary to obtain a comprehensive equivalence boundary.
6. The method for determining the dynamic equivalence boundary of the alternating current-direct current power system according to claim 5, wherein the step S3 is performed for dynamic equivalence calculation, and is performed within the equivalence boundary, and the method specifically comprises the following steps:
step S311, performing coherent identification on the generator by adopting a time domain method according to the maximum and minimum principle, namely the generator is shown in formula (1);
where ε is the standard of deviation given and τ is the simulation computation time. Preferably, epsilon is 5-10 DEG, and tau is 1-3 s.
Step S312, simplifying the coherent generator bus according to the constant power technology;
step S313, simplifying the network by adopting a current channel CSR method;
and step S314, carrying out parameter aggregation on the coherent generator by adopting a weighting method to obtain an equivalent result.
7. The method for determining the dynamic equivalence boundary of the ac-dc power system according to claim 1, wherein in step S4, the error evaluation is performed on the result of the equivalence calculation, specifically:
comparing the dynamic characteristic curves before and after equivalence, selecting node voltage and line active output or a generator power angle as a quantitative index for measuring dynamic errors, and calculating and analyzing errors of each point of the dynamic curves before and after equivalence by adopting a formula (2):
wherein x isiFor the ith sample of the dynamic characteristic curveThe value of the point, N is the number of sampling points;
the maximum error value is obtained.
8. The method of claim 7, wherein the maximum error value Δ x of the error estimation is determined by a dynamic equivalence boundary determination method of the AC/DC power systemRMSThe predetermined range to be satisfied is ± 0.25% from the target error e.
9. The method for determining the dynamic equivalence boundary of an AC/DC power system according to any one of claims 1-8, wherein the predetermined rules in steps S5 and S6 are that the initial value of the increase/decrease value is determined according to the deviation degree of the comparison result and decreased by dichotomy.
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