CN110729697A - Online identification method for cut-off circuit with optimal enhanced voltage stability - Google Patents
Online identification method for cut-off circuit with optimal enhanced voltage stability Download PDFInfo
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Abstract
The invention discloses an online identification method of an optimal cut-off line with enhanced voltage stability, which is used for searching a group of optimal cut-off line schemes of a mathematical model of the optimal cut-off line with enhanced voltage stability, wherein the mathematical model takes expected accident sets into account, and the method comprises the following steps: establishing a mathematical model of an optimal cut-off line for enhancing voltage stability, which takes the expected accident set into account; identifying the line which is cut off in the most serious accident; performing a voltage stability analysis of the expected ground state power system and the expected incident; the ground state power system is expected to open line identification. The invention realizes the online identification method for enhancing the voltage stability of the expected ground state power system and enabling the voltage stability degrees of all expected accidents to meet the preset requirements of users, solves the safety analysis and control problems of the voltage stability of the expected accident set while considering the expected ground state power system at present, can provide a plurality of open-circuit and cut-off circuit schemes for power system operators, and ensures the voltage stable operation of the power system.
Description
Technical Field
The invention relates to an online identification method for an optimal cut-off line with enhanced voltage stability, and belongs to the technical field of power network structure optimization.
Background
The voltage stabilization of the power system is always an important task for ensuring the safe and stable operation of the power system, and has great economic and social significance. Especially, under the current large-scale access of new energy, the power system faces more challenges threatening the safe and stable operation thereof, so that it is particularly important to develop an online method for voltage stability analysis and control of the power system.
The traditional voltage stabilization control mostly adopts measures of regulating a transformer tap, putting in reactive power compensation devices (such as a series/parallel capacitor, a static reactive power compensator and the like), changing the output of a generator and the like, however, the control measures need to spend a large amount of cost for installation and maintenance, and the control measures are in an idle state for a long time after being installed; the service life of equipment is influenced by frequent operation, the number of control operations is large, and the realization in an actual power system is complex; in general voltage stabilization control, only the voltage stabilization degree of the ground state power system is considered, and the voltage stabilization degree of the expected accident set is not considered, so that voltage stabilization control measures and online calculation methods which are better in economy and take the expected accident set into consideration are required.
Disclosure of Invention
Aiming at the defects of the method, the invention provides the online identification method of the optimal cut-off line for enhancing the voltage stability, which solves the safety analysis and control problems of the voltage stability of the expected ground state power system and the expected accident set, can provide a plurality of cut-off line schemes for power system operators and ensures the stable voltage operation of the power system.
The technical scheme adopted for solving the technical problems is as follows:
the online identification method of the optimal cut-off line with the enhanced voltage stability provided by the embodiment of the invention is characterized in that a group of optimal cut-off line schemes of a mathematical model of the optimal cut-off line with the enhanced voltage stability, which takes expected accident sets into account, are searched, and the optimal cut-off line schemes meet the following conditions: and enabling the load margins of all expected accident sets to meet the threshold requirement, enabling the load margins of the expected ground state power system after disconnection to be maximum, and enabling the expected ground state power system after disconnection to meet the operation safety constraint.
As a possible implementation manner of this embodiment, the method includes the following steps:
establishing a mathematical model of an optimal cut-off line for enhancing voltage stability, which takes the expected accident set into account;
performing a voltage stability analysis of the expected ground state power system and the expected incident;
identifying the line which is cut off in the most serious accident;
the ground state power system is expected to open line identification.
As a possible implementation manner of this embodiment, the process of establishing the mathematical model of the optimal open circuit with enhanced voltage stability considering the expected accident set specifically includes:
giving a current power system state, load prediction data, a power generation plan and a power grid maintenance plan, a group of expected accident sets and a group of alternative breaking lines;
the mathematical model objective function of the optimal open circuit for enhancing voltage stability considering the set of expected accidents is as follows:
the continuous power flow balance equation of the expected ground state power system after the circuit is disconnected is as follows:
the safety operating constraints of the expected ground state power system after a line break are:
the continuous power flow balance equation of all the expected accident power systems is as follows:
the load margin limit requirements of all the power systems after the expected accident are as follows:
wherein, B is a node set; e is a branch set of the power system; c represents a given set of forecasted incidents; c. CiPredicting the accident for the ith, and ci∈C；
And λbRespectively representing expected accidents ciAnd a load margin of the prospective ground state power system; lambda [ alpha ]thA load margin limit for an expected accident; n is a radical ofbA power network after disconnecting the transmission line for the expected ground state power system; vi，Vi，min，Vi，maxRespectively representing the voltage amplitude of the node i, an allowable lower voltage limit and an allowable upper voltage limit; s(i，j)And S(i，j)，maxRepresenting the value of the power connected on the line between nodes i and j and the maximum allowed power, respectively.
As a possible implementation manner of this embodiment, the performing the voltage stability analysis of the expected ground state power system and the expected accident is specifically:
according to given data, calculating the load margins of the expected ground state power system and analyzing expected accidents, and respectively calculating the load margins of all the expected accidents in the expected ground state power system and the expected accident set; and then selecting the expected accident with the lowest load margin as the most serious accident, and carrying out the identification calculation of the optimal open circuit.
As a possible implementation manner of this embodiment, the most serious broken line identification process specifically includes:
and (3) according to the given alternative broken line, carrying out broken line identification on the most serious expected accident, then selecting the broken line with the load margin meeting the threshold requirement (formula (6)) as a solution of the step, and sending the solution to the next step. As a possible implementation manner of this embodiment, the process of identifying a broken line of the expected ground state power system specifically includes: taking the solution of the identification process of the most serious accident disconnection circuit as an alternative disconnection circuit, identifying the disconnection circuit which can increase the voltage stability margin of the expected ground state power system, and sequencing according to the load margin; meanwhile, carrying out rapid expected accident analysis on the identified solution, checking whether the load margins of all the expected accidents in the expected accident set after the lines are cut off meet the requirements, and if so, retaining the solution; otherwise, the solution is removed from the solution set, and then the next solution is checked continuously until all solutions are checked.
As a possible implementation manner of this embodiment, the most serious accident disconnection line identification process includes the following steps:
1) according to a given network topology structure of the current power system, load prediction data, a maintenance plan of an expected power system, an expected power system power generation plan, an expected accident set, alternative disconnection lines and the like;
2) analyzing the static voltage stability of the current power system and the expected ground state power system by using a continuous power flow method to obtain information such as voltage stabilizing load margins of the current power system and the expected ground state power system, saddle node bifurcation point voltage amplitude values, left eigenvectors corresponding to zero characteristic roots of a Jacobian matrix and the like;
3) performing expected accident analysis, and calculating load margins of all expected accidents;
4) if the load margin of any expected accident does not meet the threshold requirement of the formula (6), solving the identification problem of the line breaking caused by the most serious accident, and continuing to execute downwards; otherwise, switching to the solution of the expected ground state power system broken line identification problem.
As a possible implementation manner of this embodiment, the step 4 specifically includes the following steps:
4.1) selecting the expected accident with the smallest load margin as the most serious expected accident, and extracting the voltage amplitude of the saddle node bifurcation point of the most serious expected accident, the left eigenvector corresponding to the zero characteristic root of the Jacobian matrix and the power value of each branch;
4.2) rapidly scanning all the alternative broken lines by adopting a sensitivity method, wherein the sensitivity calculation method comprises the following steps:
wherein, and
respectively flowing active power and reactive power from the node i to the node j before the lines i-j of the system c with the most serious expected accident are disconnected;
and
respectively the active power and the reactive power flowing from the node j to the node i before the central line i-j of the system c with the most serious expected accident is cut off; omegacA nonzero left eigenvector corresponding to a zero characteristic root of the Jacobian matrix at the saddle node bifurcation point of the system c with the most serious expected accident;
are respectively the feature vector omegacElements corresponding to the positions of the bus i active power balance equation, the bus i reactive power balance equation, the bus j active power balance equation and the bus j reactive power balance equation;
calculating the sensitivity of all the alternative on-off lines one by one, and sending the on-off lines with the sensitivity greater than or equal to zero to the next step for continuous calculation;
4.3) calculating and sequencing the selected alternative cut-off lines screened in the last step by adopting a load margin estimation method;
estimating load margins of all screened off cut-off lines one by one, sorting all alternative cut-off lines from large to small according to the estimated values, and sending the alternative cut-off lines which are sorted in the front to the next step; meanwhile, the broken lines with the load margin estimated value smaller than the expected accident load margin threshold value are removed;
4.4) accurately calculating the load margins of the alternative cut-off power transmission lines sequenced in the last step one by adopting a continuous power flow method until the first expected accident load margin does not meet the cut-off line solution required by the threshold value; after calculation, the load margin of the serious expected accident can be larger than lambdathThe cut-off power transmission line is reserved and sent to the next step for further analysis;
4.5) if the solution of breaking the transmission line exists, the load margin of the most serious expected accident is larger than lambdathSwitching to an expected ground state power system broken line identification process; otherwise, outputting the result.
As a possible implementation manner of this embodiment, the process of identifying the open circuit of the expected ground state power system includes the following steps:
5) and taking the solution obtained in the step 4 as the alternative cut-off power transmission line in the step, and executing the solution of the expected ground state power system cut-off line identification problem, namely the expected ground state power system cut-off line identification.
As a possible implementation manner of this embodiment, the step 5 specifically includes the following steps:
5.1) the step adopts sensitivity indexes to screen alternative cut-off lines, and the calculation method of the sensitivity indexes is as follows:
wherein,
the variation of the expected load margin of the ground state power system caused by the fact that the power transmission lines i-j are disconnected;ωba non-zero left eigenvector corresponding to a zero characteristic root of the Jacobian matrix at the saddle node bifurcation point of the expected ground state power system;
are respectively the feature vector omegabElements corresponding to the positions of the bus i active power balance equation, the bus i reactive power balance equation, the bus j active power balance equation and the bus j reactive power balance equation;
calculating the sensitivity of the alternative on-off lines one by one, and sending the on-off transmission lines with the sensitivity greater than or equal to zero to the next step for continuous calculation;
5.2) estimating the load margin of the selected alternative on-off lines by adopting a load margin estimation method, sorting the alternative on-off lines from large to small according to the calculation result, and sending the alternative on-off lines sorted in the front to the next step for continuous analysis and calculation;
5.3) accurately calculating the load margin of the alternative cut-off lines sequenced in the last step by adopting a continuous power flow method, sequencing according to the calculation result, and sending the recognition result to the next step for continuous analysis;
5.4) taking the recognition result of the last step as a solution set, extracting a scheme of the cut-off power transmission line from the solution set one by one, taking a system of the cut-off power transmission line as a network structure of an expected ground state power system, performing expected accident analysis and calculation of the power system after cut-off according to input data, if the load margins of all expected accidents meet the requirement of the formula (6), keeping the solution of the cut-off power transmission line, and if not, removing the solution from the solution set;
and continuously extracting the next scheme of the power transmission line on-off, and continuously performing the verification of the expected accident until all solutions are completely verified.
As a possible implementation manner of this embodiment, the method further includes the following steps:
and outputting results, wherein the output results comprise all effective cut-off transmission line solutions, the load margin of the expected ground state power system, the expected accident set analysis result and the expected accident with the minimum load margin.
The technical scheme of the embodiment of the invention has the following beneficial effects:
the invention realizes the online identification method for enhancing the voltage stability of the expected ground state power system and enabling the voltage stability degrees of all expected accidents to meet the preset requirements of users, solves the safety analysis and control problems of the voltage stability of the expected accident set while considering the expected ground state power system at present, can provide a plurality of groups of on-off line schemes for power system operators, and ensures the voltage stable operation of the power system.
According to the method, effective cut-off transmission lines are selected through calculation according to real-time power system data, short-term prediction data (including predicted loads of all nodes of a power system, power generation plans and maintenance plans) and a set of expected accidents, so that a power network structure is optimized, and the static voltage stability of an expected ground state power system is maximized on the premise that all the expected accidents meet the minimum voltage stability margin limit value.
The voltage stability of an expected ground state power system and an expected accident set is considered at the same time, an optimal broken line identification mathematical model meeting the expected accident margin requirement is provided, and the real-time running condition and the safe running requirement of the power system are considered on the model; the calculation method provides a decomposition calculation method based on a decomposition idea, greatly reduces the calculation difficulty of the original problem, avoids repeatedly adopting a calculation mode of a continuous power flow method, has high calculation speed, provides a quick on-off line identification method, and can provide operation guidance for enhancing the voltage stability of a power system.
Description of the drawings:
FIG. 1 is a flow diagram illustrating a method for online identification of an optimal open circuit line for enhanced voltage stability according to an exemplary embodiment;
FIG. 2 is a block diagram illustrating an overall method for online identification of an optimal open circuit line for enhanced voltage stability, according to an exemplary embodiment;
FIG. 3 is a detailed flow diagram illustrating another method for online identification of an optimal open circuit line for enhanced voltage stability according to an exemplary embodiment;
FIG. 4 is a P-V curve comparison of an expected ground state power system before and after a power line is disconnected;
fig. 5 is a comparison graph of load margins of all expected accidents before and after the power transmission line is disconnected.
Detailed Description
The invention is further illustrated by the following examples in conjunction with the accompanying drawings:
in order to clearly explain the technical features of the present invention, the following detailed description of the present invention is provided with reference to the accompanying drawings. The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. To simplify the disclosure of the present invention, the components and arrangements of specific examples are described below. Furthermore, the present invention may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. It should be noted that the components illustrated in the figures are not necessarily drawn to scale. Descriptions of well-known components and processing techniques and procedures are omitted so as to not unnecessarily limit the invention.
The online identification method of the optimal cut-off line with the enhanced voltage stability provided by the embodiment of the invention is characterized in that a group of optimal cut-off line schemes of a mathematical model of the optimal cut-off line with the enhanced voltage stability, which takes expected accident sets into account, are searched, and the optimal cut-off line schemes meet the following conditions: and enabling the load margins of all expected accident sets to meet the threshold requirement, enabling the load margin of the expected ground state power system after disconnection to be the largest, and enabling the expected power system after disconnection to meet the operation safety constraint.
FIG. 1 is a flow diagram illustrating a method for online identification of an optimal open circuit line for enhanced voltage stability according to an exemplary embodiment. As shown in fig. 1, an online identification method for an optimal open circuit line with enhanced voltage stability according to an embodiment of the present invention includes the following steps:
establishing a mathematical model of an optimal cut-off line for enhancing voltage stability, which takes the expected accident set into account;
performing a voltage stability analysis of the expected ground state power system and the expected incident: according to given data, calculating the load margins of the expected ground state power system and analyzing expected accidents, and respectively calculating the load margins of all the expected accidents in the expected ground state power system and the expected accident set; then selecting the expected accident with the lowest load margin as the most serious accident, and carrying out the identification calculation of the optimal cut-off line;
identifying the line broken by the most serious accident: according to a given alternative cut-off line, identifying the cut-off line of the most serious expected accident, selecting the cut-off line with the load margin meeting the threshold requirement (formula (6)) as a solution of the step, and sending the solution to the next step;
expected ground state power system open line identification: identifying a broken line which can enable the voltage stability margin of the expected ground state power system to be maximum by taking the solution of the identification process of the most serious expected accident broken line as an alternative broken line; meanwhile, carrying out rapid expected accident analysis on the identified solution, checking whether the load margins of all the expected accidents in the expected accident set after the lines are cut off meet the threshold requirement, and if so, retaining the solution; otherwise, the solution is removed from the solution set, and then the next solution is checked continuously until all solutions are checked.
As a possible implementation manner of this embodiment, the method further includes the following steps:
and outputting results, wherein the output results comprise all effective cut-off transmission line solutions, the load margin of the expected ground state power system, the expected accident set analysis result and the expected accident with the minimum load margin.
As a possible implementation manner of this embodiment, the process of establishing the mathematical model of the optimal open circuit with enhanced voltage stability considering the expected accident set specifically includes:
giving a current power system state, load prediction data, a power generation plan and a power grid maintenance plan, a group of expected accident sets and a group of alternative breaking lines; the mathematical model objective function of the optimal open circuit for enhancing voltage stability considering the set of expected accidents is as follows:
the continuous power flow balance equation of the expected ground state power system after the circuit is disconnected is as follows:
the safety operating constraints of the expected ground state power system after a line break are:
the continuous power flow balance equation of all the expected accident power systems is as follows:
the load margin limit requirements of all the power systems after the expected accident are as follows:
wherein, B is a node set; e is a branch set of the power system; c represents a given set of forecasted incidents; c. CiPredicting the accident for the ith, and ci∈C；
And λbRespectively representing expected accidents ciAnd a load margin of the prospective ground state power system; lambda [ alpha ]thA load margin limit for an expected accident; n is a radical ofbA power network after disconnecting the transmission line for the expected ground state power system; vi，Vi，min，Vi，maxRespectively representing the voltage amplitude of the node i and the allowable voltageA lower limit and an upper limit; s(i，j)And S(i，j)，maxRepresenting the value of the power connected on the line between nodes i and j and the maximum allowed power, respectively.
Equations (1) - (6) are mathematical models of voltage stabilization enhancing the optimal open circuit to account for the set of expected incidents. The model describes that a group of optimal broken lines is found, so that the load margins of all expected accident sets meet the threshold requirement, the load margin of the expected ground state power system after the line breakage is the largest, and the expected power system after the line breakage meets the operation safety constraint.
The method for identifying the optimal cut-off line with the enhanced voltage stability considering the expected accident set is divided into two problems to be solved, as shown in fig. 2, the two problems are respectively:
the sub-problem is the identification of the most severe accident disconnect line. And selecting the expected accident with the lowest load margin as the most serious accident according to the analysis result of the expected accidents, and carrying out the identification calculation of the optimal cut-off line.
The main problem is the expected ground state power system open line identification. In the stage, the solution of the sub-problem is used as an alternative open circuit, and the open circuit which can enable the voltage stability margin of the expected ground state power system to be maximum is identified; and meanwhile, carrying out rapid expected accident analysis on the identified solution, and checking whether the load margins of all the expected accidents in the expected accident set after the line is cut meet the threshold requirement (namely, the formula (6)). If the requirement is met, the solution is reserved; otherwise, the solution is removed from the solution set, and then the next solution is checked continuously until all solutions are checked.
As a possible implementation manner of this embodiment, as shown in fig. 3, the most serious accident disconnection line identification process includes the following steps:
1. according to a given network topology structure of the current power system, load prediction data, a maintenance plan of an expected power system, an expected power system power generation plan, an expected accident set, alternative broken lines and the like;
2. calculating the static voltage stability of the current power system and the expected ground state power system by using a continuous power flow method to obtain information such as voltage stability load margins of the current power system and the expected ground state power system, saddle node bifurcation point voltage amplitude values, left eigenvectors corresponding to zero characteristic roots of a Jacobian matrix and the like;
3. performing expected accident analysis, and calculating load margins of all expected accidents;
4. if the load margin of any expected accident does not meet the requirement of the formula (6), solving the identification problem of the line breaking caused by the most serious accident, and continuing to execute downwards; otherwise, switching to the 5 th step to solve the problem of line identification of the expected ground state power system disconnection.
4.1, selecting the expected accident with the smallest load margin as the most serious expected accident, and extracting the voltage amplitude of the saddle node bifurcation point of the most serious expected accident, the left eigenvector corresponding to the zero characteristic root of the Jacobian matrix and the power value of each branch;
4.2 the step adopts a sensitivity method to rapidly scan all the alternative broken lines, and the sensitivity calculation method comprises the following steps:
wherein,
and
respectively flowing active power and reactive power from the node i to the node j before the lines i-j of the system c with the most serious expected accident are disconnected;
and
respectively the active power and the reactive power flowing from the node j to the node i before the central line i-j of the system c with the most serious expected accident is cut off; omegacTo the most severe pre-stageA non-zero left eigenvector corresponding to a zero characteristic root of the Jacobian matrix at the saddle node bifurcation point of the accident thinking system c;are respectively the feature vector omegacElements corresponding to the positions of the bus i active power balance equation, the bus i reactive power balance equation, the bus j active power balance equation and the bus j reactive power balance equation;
calculating the sensitivity of all the alternative on-off lines one by one, and sending the on-off lines with the sensitivity greater than or equal to zero to the next step for continuous calculation;
4.3, calculating and sequencing the selected alternative cut-off lines screened in the last step by adopting a load margin estimation method;
estimating load margins of all screened off cut-off lines one by one, sorting all alternative cut-off lines from large to small according to the estimated values, and sending the alternative cut-off lines which are sorted in the front to the next step; meanwhile, the broken lines with the load margin estimated value smaller than the expected accident load margin threshold value are removed;
4.4, accurately calculating the load margins of the alternative cut-off power transmission lines sequenced in the last step one by adopting a continuous power flow method until the first expected accident load margin does not meet the cut-off line solution required by the threshold value; after calculation, the load margin of the most serious expected accident can be larger than lambdathThe cut-off power transmission line is reserved and sent to the next step for further analysis;
4.5 if there is a disconnected transmission line solution, the load margin of the most serious expected accident can be made larger than lambdathSwitching to an expected ground state power system broken line identification process; otherwise, outputting the result.
As a possible implementation manner of this embodiment, as shown in fig. 3, the process of identifying the open circuit of the expected ground state power system includes the following steps:
5. and taking the solution obtained in the step 4 as the alternative cut-off power transmission line in the step, and executing the solution of the expected ground state power system cut-off line identification problem, namely the expected ground state power system cut-off line identification.
5.1 the step adopts sensitivity indexes to screen the alternative cut-off lines, and the calculation method of the sensitivity indexes is as follows:
wherein,
the variation of the expected load margin of the ground state power system caused by the fact that the power transmission lines i-j are disconnected;
ωba non-zero left eigenvector corresponding to a zero characteristic root of the Jacobian matrix at the saddle node bifurcation point of the expected ground state power system;
are respectively the feature vector omegabElements corresponding to the positions of the bus i active power balance equation, the bus i reactive power balance equation, the bus j active power balance equation and the bus j reactive power balance equation;
calculating the sensitivity of the alternative on-off lines one by one, and sending the on-off transmission lines with the sensitivity greater than or equal to zero to the next step for continuous calculation;
5.2, estimating the load margin of the selected alternative on-off lines by adopting a load margin estimation method, sorting the alternative on-off lines from large to small according to the calculation result, and sending the alternative on-off lines which are sorted in the front to the next step for continuous analysis and calculation;
5.3, accurately calculating the load margin of the alternative cut-off lines sequenced in the previous step by adopting a continuous power flow method, sequencing according to the calculation result, and sending the recognition result to the next step for continuous analysis;
5.4 taking the recognition result of the last step as a solution set, extracting a scheme of the cut-off power transmission line from the solution set one by one, taking a system of the cut-off power transmission line as a network structure of an expected ground state power system, performing expected accident analysis and calculation of the power system after cut-off according to input data, if the load margins of all expected accidents meet the requirement of the formula (6), keeping the solution of the cut-off power transmission line, and if not, removing the solution from the solution set;
and continuously extracting the next scheme of the power transmission line on-off, and repeatedly executing the verification of the expected accident until all solutions are completely verified.
The invention adopts the calculation of an IEEE118 node power system example, and data is derived from an IEEE standard example. In simulation verification, node 69 is a balance node, the thermal limit of each line is 500MVA, and 175 power transmission lines (excluding 9 radiation type lines) in the example are all used as alternative on-off power transmission lines. Prediction data: the 39 loads in the region 2 are increased by 10% on the basis of the current ground state power system, and the load increment is met by adjusting the output of the generator of the node 1, the node 4 and the node 31, wherein the output is 75.71, 54.08 and 86.52MW respectively. Expected ground state power system load margin is lambdab11.7039, the load margin requirement for the set of expected accidents is λthA total of 175 failures were envisioned for the set of accidents, 8.0. Firstly, the most serious expected accident is the disconnection of the lines 37-39 and the load margin is lambdac＝7.5666。
When the subproblem is solved, the weighting sensitivity of 61 lines in the 175 alternative open-circuit transmission lines is positive, so the 61 alternative open-circuit lines are sent to the 4.3 th step line for sorting. Finally, 6 alternative disconnected transmission lines can enable the load margin of the serious expected accident to meet the requirement, as shown in the 2 nd column of the table 1.
Table 1 example simulation results:
when the main problem is solved, firstly, sensitivity calculation is executed, the alternative open-close lines 30-17 are removed because the sensitivity is negative, the remaining 5 alternative open-close transmission lines are sent to the 5.2 th step and the 5.3 th step, and calculation results are listed in the 4 th, 5 th and 6 th columns of the table 1. And finally, only one alternative cut-off power transmission line 15-17 is verified to meet the expected accident margin requirement. Therefore, disconnecting the transmission lines 15-17 is the solution of this example. The PV curves of the expected ground state power system before and after the power transmission line 15-17 is disconnected are shown in fig. 4, and the load margins of all the expected accidents before and after the disconnection are shown in fig. 5.
As can be seen from fig. 4 and 5, after the transmission lines 15 to 17 are disconnected, the load margin of the ground state power system is expected to be increased from 11.7039 to 11.8797, which is increased by 1.5%; the load margin of the most serious expected accident is improved from 7.5666 to 9.0483, which is improved by 19.58%; and the load margins of all the expected accidents meet the predetermined requirements.
The online identification method based on the decomposition idea solves the sub-problem (identification of the broken line of the most serious expected accident) and the main problem (identification of the broken line of the expected ground state power system) respectively. According to the real-time data and the prediction data of a given power system, firstly, calculating the load margins of an expected ground state power system and analyzing expected accidents, respectively estimating the load margins of all the expected accidents in the ground state power system and the expected accidents, selecting the expected accident with the lowest load margin as the most serious expected accident, and then, carrying out identification calculation on the optimal cut-off line; then, the solution of the sub-problem is used as an alternative cut-off power transmission line, a cut-off line which can enable the voltage stability margin of the expected ground state power system to be maximum is identified, finally, all identified cut-off schemes are subjected to expected accident analysis, and whether the load margins of all accidents in the expected accident set after the cut-off power transmission lines meet the load margin requirement or not is verified (namely, an equation (6)). If the requirement is met, the solution is retained; otherwise, deleting the solution, and then continuously checking the next solution until all solutions are checked. The voltage stability control measure is to cut off the current running power transmission line, so that the voltage stability of the expected ground state power system can be enhanced, the voltage stability degree of all expected accidents in the expected accident set meets the preset requirements of users, the solving difficulty of the original problem is reduced, and the calculation speed is increased.
The method is based on the decomposition idea, the original problem is decomposed into two problems to be solved and calculated, a large amount of calculation of one-by-one analysis of the disconnected circuit and the expected accident is eliminated, the problem that the detailed calculation speed is low by singly adopting a continuous power flow method is solved, various disconnected circuit schemes can be provided, and the expected ground state power system after the disconnected circuit meets the static safe operation requirement, so that the method is particularly suitable for the online calculation and analysis of the power grid topology optimization of a large-scale power system considering the expected accident set.
The foregoing is only a preferred embodiment of the present invention, and it will be apparent to those skilled in the art that various modifications and improvements can be made without departing from the principle of the present invention, and these modifications and improvements are also considered to be within the scope of the present invention.
Claims (10)
1. An online identification method of an optimal cut-off line with enhanced voltage stability is characterized in that a group of optimal cut-off line schemes of a mathematical model of the optimal cut-off line with enhanced voltage stability, which takes a forecast accident set into account, are searched, and the optimal cut-off line schemes meet the following conditions: the load margins of all expected accident sets meet the threshold requirement, the load margin of the ground state power system after disconnection is the largest, and the power system after disconnection meets the operation safety constraint.
2. The method of claim 1 for on-line identification of an optimal open circuit line with enhanced voltage stability, comprising the steps of:
establishing a mathematical model of an optimal cut-off line for enhancing voltage stability, which takes the expected accident set into account;
performing a voltage stability analysis of the expected ground state power system and the expected incident;
identifying the line which is cut off in the most serious accident;
the ground state power system is expected to open line identification.
3. The method for identifying an optimal open circuit with enhanced voltage stability as claimed in claim 2, wherein the step of establishing the mathematical model of the optimal open circuit with enhanced voltage stability, which takes into account the expected accident set, comprises:
giving a current power system state, load prediction data, a power generation plan and a power grid maintenance plan, a group of expected accident sets and a group of alternative breaking lines;
the mathematical model objective function of the optimal open circuit for enhancing voltage stability considering the set of expected accidents is as follows:
the continuous power flow balance equation of the expected ground state power system after the circuit is disconnected is as follows:
the safety operating constraints of the expected ground state power system after a line break are:
the continuous power flow balance equation of all the expected accident power systems is as follows:
the load margin limit requirements of all the power systems after the expected accident are as follows:
wherein, B is a node set; e is a branch set of the power system; c represents a given set of forecasted incidents; c. CiPredicting the accident for the ith, and ci∈C；
And λbRespectively representing expected accidents ciAnd a load margin of the prospective ground state power system; lambda [ alpha ]thA load margin limit for an expected accident; n is a radical ofbA power network after disconnecting the transmission line for the expected ground state power system; vi，Vi,min，Vi,maxRespectively representing the voltage amplitude of the node i, an allowable lower voltage limit and an allowable upper voltage limit; s(i,j)And S(i,j),maxRepresenting the value of the power connected on the line between nodes i and j and the maximum allowed power, respectively.
4. The method of claim 2, wherein the performing of the voltage stability analysis of the expected ground state power system and the expected fault is specifically:
according to given data, calculating the load margins of the expected ground state power system and analyzing expected accidents, and respectively calculating the load margins of all the expected accidents in the expected ground state power system and the expected accident set; and then selecting the expected accident with the lowest load margin as the most serious accident, and carrying out the identification calculation of the optimal open circuit.
5. The method for identifying the most disconnected line with the enhanced voltage stability as claimed in claim 2, wherein the identification process of the most serious disconnected line is as follows:
and according to the given alternative cut-off line, identifying the cut-off line of the most serious expected accident, selecting the cut-off line with the load margin meeting the threshold requirement as the solution of the step, and sending the solution to the next step.
6. The method as claimed in claim 2, wherein the identification process of the expected ground state power system open circuit is as follows: taking the solution of the identification process of the most serious accident disconnection circuit as an alternative disconnection circuit, identifying the disconnection circuit which can increase the voltage stability margin of the expected ground state power system, and sequencing according to the load margin; meanwhile, carrying out rapid expected accident analysis on the identified solution, checking whether the load margins of all the expected accidents in the expected accident set after the lines are cut off meet the requirements, and if so, retaining the solution; otherwise, the solution is removed from the solution set, and then the next solution is checked continuously until all solutions are checked.
7. The method of any one of claims 2 to 6, wherein the analysis of the expected ground state power system and the voltage stability of the expected fault comprises the steps of:
1) according to a given network topology structure of the current power system, load prediction data, a maintenance plan of an expected power system, an expected power system power generation plan, an expected accident set and the like;
2) calculating the static voltage stability of the current power system and the expected ground state power system by using a continuous power flow method, and obtaining information such as voltage stability load margins of the current power system and the expected ground state power system, saddle node bifurcation point voltage amplitude values, left eigenvectors corresponding to zero characteristic roots of a Jacobian matrix and the like;
3) performing expected accident analysis, and calculating load margins of all expected accidents;
4) if the load margin of any expected accident does not meet the requirement of the formula (6), solving the identification problem of the line breaking caused by the most serious accident, and continuing to execute downwards; otherwise, switching to the solution of the expected ground state power system broken line identification problem.
8. The method as claimed in claim 7, wherein the step 4) comprises the following steps:
4.1) selecting the expected accident with the smallest load margin as the most serious expected accident, and extracting the voltage amplitude of the saddle node bifurcation point of the most serious expected accident, the left eigenvector corresponding to the zero characteristic root of the Jacobian matrix and the power value of each branch;
4.2) rapidly scanning all the alternative broken lines by adopting a sensitivity method, wherein the sensitivity calculation method comprises the following steps:
wherein, 
and
respectively flowing active power and reactive power from the node i to the node j before the lines i-j of the system c with the most serious expected accident are disconnected;
and
respectively the active power and the reactive power flowing from the node j to the node i before the central line i-j of the system c with the most serious expected accident is cut off; omegacA nonzero left eigenvector corresponding to a zero characteristic root of the Jacobian matrix at the saddle node bifurcation point of the system c with the most serious expected accident;
are respectively the feature vector omegacElements corresponding to the positions of the bus i active power balance equation, the bus i reactive power balance equation, the bus j active power balance equation and the bus j reactive power balance equation;
calculating the sensitivity of all the alternative on-off lines one by one, and sending the on-off lines with the sensitivity greater than or equal to zero to the next step for continuous calculation;
4.3) calculating and sequencing the selected alternative cut-off lines screened in the last step by adopting a load margin estimation method;
estimating load margins of all screened off cut-off lines one by one, sorting all alternative cut-off lines from large to small according to the estimated values, and sending the alternative cut-off lines which are sorted in the front to the next step; meanwhile, the broken lines with the load margin estimated value smaller than the expected accident load margin threshold value are removed;
4.4) accurately calculating the load margins of the alternative cut-off power transmission lines sequenced in the last step one by adopting a continuous power flow method until the first expected accident load margin does not meet the cut-off line solution required by the threshold value; after calculation, the load margin of the serious expected accident can be larger than lambdathThe cut-off power transmission line is reserved and sent to the next step for further analysis;
4.5) if there is a disconnected transmission line solution, the load margin of the most serious expected accident can be larger than lambdathSwitching to an expected ground state power system broken line identification process; otherwise, outputting the result.
9. The method of claim 8 wherein said expected ground state power system open circuit identification process comprises the steps of:
5) and taking the solution obtained in the step 4 as the alternative cut-off power transmission line in the step, and executing the solution of the expected ground state power system cut-off line identification problem, namely the expected ground state power system cut-off line identification.
10. The method as claimed in claim 9, wherein the step 5 comprises the steps of:
5.1) the step adopts sensitivity indexes to screen alternative cut-off lines, and the calculation method of the sensitivity indexes is as follows:
wherein,
to openThe variation of the expected load margin of the ground state power system caused by the disconnection of the power transmission lines i-j;
ωba non-zero left eigenvector corresponding to a zero characteristic root of the Jacobian matrix at the saddle node bifurcation point of the expected ground state power system;
are respectively the feature vector omegabElements corresponding to the positions of the bus i active power balance equation, the bus i reactive power balance equation, the bus j active power balance equation and the bus j reactive power balance equation;
calculating the sensitivity of the alternative on-off lines one by one, and sending the on-off transmission lines with the sensitivity greater than or equal to zero to the next step for continuous calculation;
5.2) estimating the load margin of the selected alternative on-off lines by adopting a load margin estimation method, sorting the alternative on-off lines from large to small according to the calculation result, and sending the alternative on-off lines sorted in the front to the next step for continuous analysis and calculation;
5.3) accurately calculating the load margin of the alternative cut-off lines sequenced in the last step by adopting a continuous power flow method, sequencing according to the calculation result, and sending the recognition result to the next step for continuous analysis;
5.4) taking the recognition result of the last step as a solution set, extracting a scheme of the cut-off power transmission line from the solution set one by one, taking a system of the cut-off power transmission line as a network structure of an expected ground state power system, performing expected accident analysis and calculation of the power system after cut-off according to input data, if the load margins of all expected accidents meet the requirement of the formula (6), keeping the solution of the cut-off power transmission line, and if not, removing the solution from the solution set;
and continuously extracting the next scheme of the power transmission line on-off, and repeatedly executing the verification of the expected accident until all solutions are completely verified.
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