CN104466948A - Multi-microgrid system island partitioning method based on electrical coupling degrees - Google Patents
Multi-microgrid system island partitioning method based on electrical coupling degrees Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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- H—ELECTRICITY
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Abstract
The invention discloses a multi-microgrid system island partitioning method based on electrical coupling degrees, and belongs to the control field of electricity generation or power transformation or power distribution technology. By the application of the method, reasonable island partitioning is carried out on a multi-microgrid system; when preliminary partitioning is carried out on the multi-microgrid system, in order to enable the partitioned islands to meet the certain power balance constraint condition and the upper and lower electrical parameter constraint conditions, load shedding strategy or shedding distributed type power strategy treatment is carried out on the islands which cannot operate stably so that the operating conditions of all the islands can be stabilized, so that it is ensured that the whole microgrid or the multi-microgrid system operates stably, and reasonable island partitioning of the multi-microgrid system is achieved. The island partitioning method is based on the electrical coupling degrees, the method has the definite physical meaning, the higher operation guidance type is achieved, and the required number of original data is small; meanwhile, during isolated power system operation of the microgrid where island operation partitioning is carried out through the multi-microgrid system island partitioning method, stable operation can be achieved.
Description
Technical Field
The invention relates to an island division method for a multi-microgrid system, and belongs to the technical control field of power generation, power transformation or power distribution.
Background
Distributed Generation (DG) refers to a small power Generation system connected near the customer side to meet the special requirements of the end customer. The scale of the energy storage device is not large, the energy storage device is usually dozens of kilowatts to dozens of megawatts, the used energy sources comprise clean energy sources or renewable energy sources such as natural gas (including coal bed gas and methane), solar energy, biological intelligence, hydrogen energy, wind energy, small hydropower and the like, and the energy storage device is mainly a storage battery and the like. In order to improve energy utilization and reduce costs, a system that supplies various energy sources such as cold, heat, and electricity together is often used as a distributed power system, and a power system including distributed energy sources is called a distributed energy power system.
The potential of the distributed power supply technology has not been fully developed so far, and the following points mainly exist for the reason:
(1) the characteristics of the distributed power supply determine that the output of some power supplies changes along with the change of external conditions, and the distributed power supply has the characteristics of intermittence, randomness and the like, so that the power supplies are difficult to meet the power balance of loads only depending on the self regulation capacity, cannot be scheduled, and need the cooperation of other power supplies or energy storage devices to provide support and backup.
(2) The grid-connected operation of the distributed power supply changes the power flow distribution in the system, and for the power distribution network, the system has bidirectional power flow due to the access of the distributed power supply, so that new problems are brought to voltage regulation, protection coordination and energy optimization.
(3) Most distributed power supplies need to be merged into a power grid through a power electronic interface, the introduction of a large number of power electronic devices, capacitors and inductors easily influences the power supply quality of surrounding users, and external interference can cause the asynchronization of frequency and voltage, so that the whole system is dragged down.
(4) The distributed power supplies which are numerous, different in form and not schedulable bring greater difficulty to system operators who depend on the traditional centralized power supply scheduling mode for management, and lack of effective management leads to randomness in running of the distributed power supplies, thereby causing hidden dangers to the safety and stability of the system.
In order to make full use of distributed power generation, some scholars propose a concept of a micro grid (abbreviated as MicroGrid). The microgrid is a small-sized power generation and distribution system formed by collecting a distributed power supply, an energy storage device, an energy conversion device and related load, monitoring and protection devices, is an autonomous system capable of realizing self control, protection and management, and can be connected with a large power grid to run or run in an isolated mode. The existing research and practice shows that the distributed power supply system is connected to a large power grid in a micro-grid mode and operates in a grid-connected mode, and the distributed power supply system and the large power grid are mutually supported, so that the distributed power supply system is the most effective mode for exerting the efficiency of the distributed power generation and supply system. In the microgrid, the distributed power supplies often have different capacity levels, so that a power supply ladder is formed in the microgrid.
In recent years, certain results have been achieved regarding multi-microgrid islanding research. When the island division of multiple micro-grids mainly considers the maintenance of power balance in the micro-grids and the power supply of important loads, the power distribution grids are divided into optimization problems, and the traditional solution is to establish a power distribution network island division model by taking equivalent effective maximum loads as objective functions. In addition, in consideration of the problems of equipment property right, scheduling range and the like, the multiple micro-grids are often divided based on different types of the area micro-grids or sub-grids thereof, for example, simple classification is performed according to natural topological properties of micro-grids such as unit micro-grids, area micro-grids, system micro-grids and the like in the power distribution network, so that the implementation of scheduling and control protection strategies is facilitated.
Disclosure of Invention
The invention aims to solve the technical problem of providing an island dividing method of a multi-microgrid system based on electrical coupling degree aiming at the defects of the prior art. The method provides a division strategy for the isolated island operation of multiple micro-grids based on the connection characteristic of topology, and is different from the existing simple division. The method has practical physical significance, and has the characteristics of simple calculation, clear steps, high operability, small quantity of required original data and the like.
The method specifically comprises the following steps:
1) calculating to obtain the electrical distance Z between any two nodes i and j in the network topology structure by using a superposition principle according to the node impedance matrix of the multi-microgrid systemij,equ；
2) Calculating the electrical coupling connectivity D of the node i by the following formulae,i；
N is the total number of nodes in the network topology;
3) selecting a node used for expressing a distributed power supply with the highest capacity level in the multi-microgrid system as a first node from the nodes i, and selecting X first nodes from the first nodes as source nodes according to the sequence of the electrical coupling connectivity of the first nodes from small to large;
the number X is the number of islands needing to be divided in the network topology structure;
and the rest nodes which are not selected as source nodes in the node i are used as non-source nodes.
4) Performing island preliminary setting on the multi-microgrid system according to a topological connectivity principle by the selected source node and non-source node, wherein the island takes one source node as a core and contains more than one non-source node;
5) the execution strategy of the source node is selected according to three conditions of a power balance constraint condition, a voltage upper and lower limit constraint condition and a distributed power supply output upper and lower limit constraint condition,
5-1) when the active power output of the distributed power supply in the source node is greater than the rated value of the distributed power supply in the source node, the island executes a load shedding strategy by taking the source node as the center;
5-2) when the active power output of the distributed power supply in the source node is less than 0, the island executes a strategy of cutting off the distributed power supply by taking the source node as a center;
and 5) operating each island according to the step 5) to finish the island division of the multi-microgrid system.
The improvement of the above technical solution is that the power balance constraint condition, the voltage upper and lower limit constraint condition, and the distributed power supply output upper and lower limit constraint condition in step 5) are respectively:
the power balance constraint is as follows,
wherein,
PGias in the node iActive power, Q, of distributed power supplyGiThe reactive power of the distributed power supply of the node i;
PLifor the load active power, Q, in the node iGiLoad reactive power for the node i;
θijis the voltage phase angle difference between the node i and the node j;
m is the number of branches connected to the node i;
the constraint conditions of the upper and lower voltage limits and the constraint conditions of the upper and lower output limits of the distributed power supply are as follows,
wherein,
Uiis the actual voltage amplitude, U, at the node ii,NIs the rated voltage amplitude, P, of the distributed power supply in the node iGi,NIs rated active output, Q, of the distributed power supply in the node iGi,maxAnd the maximum reactive power output of the distributed power supply in the node i.
The improvement of the technical scheme is that in the step 5):
the load shedding strategy comprises the following steps:
5-1-1) in the island of the step 4), calculating the electrical distance from a boundary point in the island to the source node by taking the source node as a center;
the boundary point refers to the non-source node with m being 1 in the island;
5-1-2) selecting a point with the largest electrical distance with the source node from the boundary points as a first load point, and cutting off the minimum load in the first load point;
5-1-3) after the load of the island is removed,
whether a distributed power supply in the source node meets a power balance constraint condition or not is judged, and if yes, the load in the first load point is not cut off;
if the distributed power source in the source node does not meet the power balance constraint condition, continuing to perform minimum load removal processing on the first load point with the overload removed until the load in the first load point is completely removed;
in step 5-1-3), if the load in the first load point is completely removed by the island, and the distributed power supply in the source node still cannot meet the power balance constraint condition, performing a load shedding strategy again on the island in the current state until the distributed power supply in the source node can meet the power balance constraint condition, and completing the multi-microgrid system island division;
the method for cutting off the distributed power supply strategy comprises the following steps:
5-2-1) selecting a secondary active node in the island in the step 4), wherein the secondary active node is a node which contains the distributed power supply except a source node in the island;
5-2-2) sorting the secondary active nodes in descending order according to the variable cost of the distributed power supply in the secondary active nodes;
in the sorting process in the step 5-2-2), if secondary active nodes with the same variable cost exist, comparing the electrical distance between each secondary active node and the source node, and arranging the secondary active nodes with small electrical distance in front of the secondary active nodes with large electrical distance according to the comparison result;
5-2-3) cutting off the distributed power supplies in the secondary active nodes one by one from head to tail according to the sequence of the secondary active nodes obtained in the step 5-2-2) until the source nodes do not absorb energy from the multi-microgrid system any more, namely when the active output of the distributed power supplies in the active nodes is greater than or equal to 0, adjusting the output of the distributed power supplies of the last cut-off secondary active nodes at the moment, so that the active output and the reactive output of the distributed power supplies in the active nodes are both 0, and completing the island division of the multi-microgrid system.
The invention adopts the technical scheme that the method has the beneficial effects that: according to the method, from the aspect of topological connectivity, isolated network operation of a multi-microgrid system is subjected to isolated island division, a node corresponding to a distributed power supply at the highest level is used as a center in the divided isolated island, and the center does not refer to the center of an actual distance but refers to an electrical center taking the electrical distances of all nodes in the isolated island as a standard. All nodes in the island have a line to the electrical center, and the line is divided according to topological connectivity, so that in each island, there is only one electrical center (or called a source node).
The method comprises the steps of calculating the electrical distance between nodes, calculating the electrical coupling degree of the point through the electrical distance, comparing the electrical coupling degrees which can be used as source nodes, using the points with smaller electrical coupling degrees as the source nodes (or the electrical centers) and dividing an island by using the source nodes as the centers, and achieving a preliminary island division by using a topological connectivity principle to obtain the island.
The adjusting method adopts two strategies, determines which strategy should be adopted for adjusting the island according to the limitation of the constraint conditions, and operates according to the corresponding strategy, so that the island finally reaches a stable working state.
As can be seen from the above, no matter what kind of operation is carried out in the method according to a practical possible situation, the operation of each step has more physical significance, and the method can play a good role in learning and guiding the practice.
Drawings
The invention will be further explained with reference to the drawings.
FIG. 1 is a schematic flow diagram of the process of the present invention.
FIG. 2 is a diagram illustrating an example partition result according to an embodiment of the present invention.
Fig. 3 is a graph of the example island voltage stability margin of fig. 2.
Fig. 4 is a graph of the calculated active load shedding amount of fig. 2.
Fig. 5 is a graph of the loss of the example island of fig. 2.
Detailed Description
Examples
Fig. 1 is an operation flow of the method, in which the method includes the following specific steps:
1) calculating to obtain the electrical distance Z between any two nodes i and j in the network topology structure by using a superposition principle according to the node impedance matrix of the multi-microgrid systemij,equ；
Zij,epu＝(Zii-Zij)-(Zij-Zjj)
Wherein Z isijRepresenting the ith row and the jth column of elements of the node impedance matrix;
2) calculating the electrical coupling connectivity D of the node i by the following formulae,i；
Wherein i is not equal to j, and N is the total number of nodes in the network topology;
3) selecting a node used for expressing a distributed power supply with the highest capacity level in the multi-microgrid system as a first node from the nodes i, and selecting X first nodes from the first nodes as source nodes according to the sequence of the electrical coupling connectivity of the first nodes from small to large;
the number X is the number of islands needing to be divided in the network topology structure;
and the rest nodes which are not selected as source nodes in the node i are used as non-source nodes.
4) Performing island preliminary setting on the multi-microgrid system according to a topological connectivity principle by the selected source node and non-source node, wherein the island takes one source node as a core and contains more than one non-source node;
5) the execution strategy of the source node is selected according to three conditions of a power balance constraint condition, a voltage upper and lower limit constraint condition and a distributed power supply output upper and lower limit constraint condition as follows,
5-1) when the active power output of the distributed power supply in the source node is greater than the rated value of the distributed power supply in the source node, the island executes a load shedding strategy by taking the source node as the center;
the load shedding strategy execution steps are as follows:
5-1-1) in the island of the step 4), calculating the electrical distance from a boundary point in the island to the source node by taking the source node as a center;
the boundary point refers to the non-source node with m being 1 in the island;
5-1-2) selecting a point with the largest electrical distance with the source node from the boundary points as a first load point, and cutting off the minimum load in the first load point;
5-1-3) after the load of the island is removed,
whether a distributed power supply in the source node meets a power balance constraint condition or not is judged, and if yes, the load in the first load point is not cut off;
if the distributed power source in the source node does not meet the power balance constraint condition, continuing to perform minimum load removal processing on the first load point with the overload removed until the load in the first load point is completely removed;
in step 5-1-3), if the load in the first load point is completely removed by the island, and the distributed power supply in the source node still cannot meet the power balance constraint condition, performing a load shedding strategy again on the island in the current state until the distributed power supply in the source node can meet the power balance constraint condition, and completing island division of the multi-microgrid system;
5-2) when the active power output of the distributed power supply in the source node is less than 0, the island executes a strategy of cutting off the distributed power supply by taking the source node as a center;
the execution steps of the strategy for cutting off the distributed power supply are as follows:
5-2-1) selecting a secondary active node in the island in the step 4), wherein the secondary active node is a node which contains the distributed power supply except a source node in the island;
5-2-2) sorting the secondary active nodes in descending order according to the variable cost of the distributed power supply in the secondary active nodes;
in the sorting process in the step 5-2-2), if secondary active nodes with the same variable cost exist, comparing the electrical distance between each secondary active node and the source node, and arranging the secondary active nodes with small electrical distance in front of the secondary active nodes with large electrical distance according to the comparison result;
5-2-3) cutting off the distributed power supplies in the secondary active nodes one by one from head to tail according to the sequence of the secondary active nodes obtained in the step 5-2-2) until the source nodes do not absorb energy from the multi-microgrid system any more, namely when the active output of the distributed power supplies in the active nodes is greater than or equal to 0, adjusting the output of the distributed power supply with the most cut-off secondary active node at the moment, so that the active output and the reactive output of the distributed power supplies in the active nodes are both 0, and completing the island division of the multi-microgrid system.
The power balance constraint conditions, the voltage upper and lower limit constraint conditions and the distributed power supply output upper and lower limit constraint conditions in the step 5) are respectively as follows:
the power balance constraint is as follows,
wherein,
PGiis the active output, Q, of the distributed power supply in the node iGiThe reactive power of the distributed power supply of the node i;
PLifor the load active power, Q, in the node iGiLoad reactive power for the node i; thetaijIs the voltage phase angle difference between the node i and the node j;
m is the number of branches connected to the node i;
the constraint conditions of the upper and lower voltage limits and the constraint conditions of the upper and lower output limits of the distributed power supply are as follows,
wherein,
Uiis the actual voltage amplitude, U, at the node ii,NFor distributed power supply in said node iRated voltage amplitude, PGi,NIs rated active output, Q, of the distributed power supply in the node iGi,maxAnd the maximum reactive power output of the distributed power supply in the node i.
Through the adjustment, islands which cannot stably work in the divided islands in the multi-microgrid system are processed one by one, and the island division of the multi-microgrid system is completed.
The connectivity in step 4) above is a basic concept in point set topology, which is defined as follows: if there is no other subset in X that is both open and closed except for empty set and X itself, then the topology space X is said to be connected.
If E is connected as a subspace of X under the induced topology, then the subset E of the topology space X is said to be connected.
Equivalents are described as:
1. the topology space X is said to be connected if X cannot be represented as a union of two non-null disjoint sets.
2. The topological space X is said to be connected if, when it is divided into two non-empty subsets A, B and a ∪ B, there is a closure of a intersection B that is not empty, or B that intersects a that is not empty.
3. The topology space X is said to be connected, if the subset of X that is both open and closed has only X and an empty set.
Therefore, the connectivity principle mentioned in step 4) of this embodiment mainly uses a principle to achieve the purpose of providing basis for island division so that there is only one source node in each island.
The application of the connectivity principle in this embodiment can be understood by the following columns, and the relationship between the source node and the non-source node in the island is set according to the connectivity principle in the following manner (where the source node A, B, C, D is any source node, it has no special meaning):
a. if the source node B, C, D is not touched in the line from source node a to the end, then all non-source nodes in the line will default to source node a.
b. If the line between two source nodes is crossed when the nodes are selected, the line cross section compares and selects the node number of the two source nodes, and all the nodes of the cross section B-C-D select the source nodes with more node numbers; if the number of the nodes of the two parties is the same, comparing Zij,equThe sum of the two is selected to be small.
The Variable Costs (Variable Costs), also called Variable Costs, referred to in step 5-2-2) of this embodiment refer to cost items that vary with the variation of the production amount in the total cost, mainly the value of production elements such as raw materials, fuel, power, etc., when the production amount increases for a certain period of time, the consumption of raw materials, fuel, power will increase proportionally, and the generated Costs will also increase proportionally, so called Variable Costs. The variable cost equals the total cost minus the fixed cost.
Example analysis based on the method:
the method selects the CIGRE medium-voltage radial network as an example analysis, and selects different load levels to apply the division strategy to analyze the island division result. FIG. 2 shows the results of the division at the light load and heavy load levels, respectively; FIG. 3 shows a voltage index value S for two load levelsVC，SVCIf the voltage is more than 0, the voltage is stable (see fig. 1), and it can be seen that all non-source nodes in the islands have good voltage stability in both cases; fig. 4 and 5 respectively show the number of active loads removed by the system and the whole isolated network loss value under the gradually increasing load level, and it can be seen that the load removed by the system is also steadily increased along with the increase of the load, and the network loss shows N-shaped distribution due to the influence of the removed load
The present invention is not limited to the above-described embodiments. All technical solutions formed by equivalent substitutions fall within the protection scope of the claims of the present invention.
Claims (3)
1. A multi-microgrid system island division method based on electrical coupling degree is characterized by comprising the following steps:
1) calculating to obtain the electrical distance Z between any two nodes i and j in the network topology structure by using a superposition principle according to the node impedance matrix of the multi-microgrid systemij,equ；
2) Calculating the electrical coupling connectivity D of the node i by the following formulae,i；
i is not equal to j, and N is the total number of nodes in the network topology;
3) selecting a node used for expressing a distributed power supply with the highest capacity level in the multi-microgrid system as a first node from the nodes i, and selecting X first nodes from the first nodes as source nodes according to the sequence of the electrical coupling connectivity of the first nodes from small to large;
the number X is the number of islands needing to be divided in the network topology structure;
taking the rest nodes which are not selected as source nodes in the node i as non-source nodes;
4) performing island preliminary setting on the multi-microgrid system according to a topological connectivity principle by the selected source node and non-source node, wherein the island has only one source node;
5) the execution strategy of the source node is selected according to three conditions of a power balance constraint condition, a voltage upper and lower limit constraint condition and a distributed power supply output upper and lower limit constraint condition as follows,
5-1) when the active power output of the distributed power supply in the source node is greater than the rated value of the distributed power supply in the source node, the island executes a load shedding strategy by taking the source node as the center;
5-2) when the active power output of the distributed power supply in the source node is less than 0, the island executes a strategy of cutting off the distributed power supply by taking the source node as a center;
and 5) operating each island according to the step 5) to finish the island division of the multi-microgrid system.
2. The islanding method for multiple microgrid systems based on electrical coupling degree of claim 1, wherein the power balance constraint condition, the upper and lower voltage limit constraint conditions and the upper and lower distributed power output limit constraint conditions in step 5) are respectively:
the power balance constraint is as follows,
wherein,
PGiis the active output, Q, of the distributed power supply in the node iGiThe reactive power of the distributed power supply of the node i;
PLifor the load active power, Q, in the node iGiLoad reactive power for the node i;
θijis the voltage phase angle difference between the node i and the node j;
m is the number of branches connected to the node i;
the constraint conditions of the upper and lower voltage limits and the constraint conditions of the upper and lower output limits of the distributed power supply are as follows,
wherein,
Uiis the actual voltage amplitude, U, at the node ii，NIs the rated voltage amplitude, P, of the distributed power supply in the node iGi，NIs rated active output, Q, of the distributed power supply in the node iGi，maxAnd the maximum reactive power output of the distributed power supply in the node i.
3. The electrical coupling degree-based multi-microgrid system islanding method of claim 1, characterized in that in step 5):
the load shedding strategy comprises the following steps:
5-1-1) in the island of the step 4), calculating the electrical distance from a boundary point in the island to the source node by taking the source node as a center;
the boundary point refers to the non-source node with m being 1 in the island;
5-1-2) selecting a point with the largest electrical distance with the source node from the boundary points as a first load point, and cutting off the minimum load in the first load point;
5-1-3) after the load of the island is removed,
whether a distributed power supply in the source node meets a power balance constraint condition or not is judged, and if yes, the load in the first load point is not cut off;
if the distributed power source in the source node does not meet the power balance constraint condition, continuing to perform minimum load removal processing on the first load point with the overload removed until the load in the first load point is completely removed;
in step 5-1-3), if the load in the first load point is completely removed by the island, and the distributed power supply in the source node still cannot meet the power balance constraint condition, performing a load shedding strategy again on the island in the current state until the distributed power supply in the source node can meet the power balance constraint condition, and completing the multi-microgrid system island division;
the method for cutting off the distributed power supply strategy comprises the following steps:
5-2-1) selecting a secondary active node in the island in the step 4), wherein the secondary active node is a node which contains the distributed power supply except a source node in the island;
5-2-2) sorting the secondary active nodes in descending order according to the variable cost of the distributed power supply in the secondary active nodes;
in the sorting process in the step 5-2-2), if secondary active nodes with the same variable cost exist, comparing the electrical distance between each secondary active node and the source node, and arranging the secondary active nodes with small electrical distance in front of the secondary active nodes with large electrical distance according to the comparison result;
5-2-3) cutting off the distributed power supplies in the secondary active nodes one by one from head to tail according to the sequence of the secondary active nodes obtained in the step 5-2-2) until the source nodes do not absorb energy from the multi-microgrid system any more, namely when the active output of the distributed power supplies in the active nodes is greater than or equal to 0, adjusting the output of the distributed power supplies of the last cut-off secondary active nodes at the moment, so that the active output and the reactive output of the distributed power supplies in the active nodes are both 0, and completing the island division of the multi-microgrid system.
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| CN107508315A (en) * | 2017-08-24 | 2017-12-22 | 南京南瑞继保电气有限公司 | The power distribution network isolated island division methods of meter and the equivalent electrical distance in power supply group inside and outside |
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| CN110690719A (en) * | 2019-09-18 | 2020-01-14 | 国网重庆市电力公司电力科学研究院 | Micro-grid battery energy storage configuration method and readable storage medium |
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| CN110752597B (en) * | 2019-10-22 | 2022-04-12 | 太原理工大学 | Minimum load shedding strategy of electricity-gas coupling comprehensive energy system |
| CN111049185B (en) * | 2019-12-29 | 2023-02-14 | 国网辽宁省电力有限公司电力科学研究院 | Island division method for power distribution network comprising distributed power supply |
| CN111049185A (en) * | 2019-12-29 | 2020-04-21 | 国网辽宁省电力有限公司电力科学研究院 | Island division method for power distribution network comprising distributed power supply |
| CN116646978A (en) * | 2023-07-26 | 2023-08-25 | 国网上海市电力公司 | Self-healing device based on diamond type power distribution network |
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