In recent years, distributed power (DG) penetration rates in power distribution systems have continued to increase. More and more attention has been paid to the wide-ranging access to DG to meet increasing load demands, optimize energy structure, and improve energy comprehensive utilization. The relevant standard IEEE Std1547-2003, which was established in 2003, supports DG to operate on islands through coordinated control and other technical means. IEEE Std1547.4-2011 revised in 2011 extended the definition of microgrid to the distribution system with DG, and regarded the distribution system with higher DG permeability as composed of multiple microgrids based on DG coordinated control system; It has multiple operating modes such as grid-connected mode and island mode. When the power distribution area is disconnected from the upper-level power grid due to faults, etc., reasonable load islanding can be used to restore the load in the island area, improve power supply reliability, and reserve sufficient time for troubleshooting.
The existing islanding strategy is mainly aimed at maximizing the total amount of load recovery, taking into account factors such as the amount of important load recovery. The division models used are mainly based on the description of the network structure in the island's power balance and island operation. Most of the existing literature does not consider the operability of the tie switch, and fails to give a strict method to ensure the radial operation of the island region from the distribution network structure model. The algorithm is based on heuristic search algorithm and intelligent algorithm.
In this context, this paper proposes a distribution network structure model with variable branch direction based on directed graphs by introducing the concepts of “virtual node†and “virtual demandâ€; The new strategy of islanding, the importance of load, the importance of load, the controllability of load and the influence of DG operation characteristics on island operation, and transform it into mixed integer linear programming (MILP) problem.
Second, the structural modeling of the DG distribution networkDistribution networks often use closed-loop design for open-loop operation. Therefore, it is necessary to ensure the radial operation structure of the island area when performing the island division of the distribution network. This paper draws on the concept of directed graph in graph theory, and proposes a distribution network structure model based on directed graph, which strictly guarantees the connectivity and radial structure of the island running area.
In this paper, the tree-shaped structure is used to describe the radial operation structure under normal conditions of the distribution network; the distribution network is described as a tree with the busbar as the node and the power supply line as the edge. First, the two concepts of "virtual node" and node "virtual demand" are defined.
A "virtual node" has the following properties:
1 The node is a virtual node independent of the distribution network node, and is connected to each DG node in the distribution network only through a breakable "virtual branch";
2 "Virtual node" does not consume, send or transfer power, that is, the "virtual branch" connection rate connected to the DG node.
The node "virtual demand" is similar to the node load and its properties are as follows:
1 When the distribution network is operating, each live node in the isolated island, including the DG node, has a unit “virtual demandâ€;
2 "virtual supply" is provided only by "virtual node";
The transmission path of 3 "virtual demand" consists of a power supply branch and a "virtual branch".
The islanding scheme obtained after the island division of the distribution network is a forest composed of one or more island regions. After the introduction of "virtual demand" and "virtual node" and the "virtual branch" connected to each DG node, since the power supply branch also bears the function of transmitting "virtual demand", in order to ensure the transmission path of "virtual demand", a single The live nodes in the island area will be connected to each other; the "virtual node" as the only "virtual supply" point will also be in the same connection diagram as several island areas, as shown in Figure 1. Through this method, the connectivity of the live nodes in the island operating area is guaranteed.
After the introduction of the "virtual node" and "virtual demand", the "virtual node" and "virtual branch" will be in a connected graph with the island running area; on this basis, the concept of power distribution is introduced by introducing the concept of the root tree in the directed graph. The radial operating structure of the net. In order to describe the changes in the direction of the branch and the state of the branch when the island is divided, each branch (including the tie switch and the "virtual branch") consists of two positive and negative directed edges, which can be selected when the network structure changes. In order to ensure the radial structure of the connected graph, any branch can only select one directed edge or choose to open. At the same time, the virtual node is used as the only root node of the connected graph. In the case that the connected graph is a root tree, each island running region is a plurality of subtrees.
Third, the island division strategyIn this paper, the power supply is prioritized to restore the important load, and the total amount of load recovery is maximized as the objective function. Constraints include:
1 Current constraints, ensuring active power balance in the isolated island area;
2DG output constraints, it should be noted that similar wind turbines, photovoltaics, etc. can not provide continuous stable output DG, can not be used alone for the island area, need to work with the DG with frequency modulation capabilities, the processing method is to remove the "virtual node" and not equipped a "virtual branch" connected between the DGs of the FM capability;
3 Load controllability constraints, using demand side management technology, the load is roughly divided into controllable load and uncontrollable load, to play the controllable load adjustment ability, in the island mode to make the important load priority supply;
4 line transmission limit constraints;
5 connectivity constraints: describe the supply balance of “virtual demandâ€;
6 Radial structural constraints: a mathematical description of the number of roots;
7-way-node state consistency constraint: describes the relationship between the branch opening and closing state and the charged state of the two connected nodes;
8 active backup constraints: ensure that each island area reserves a certain proportion of spare capacity.
The island partitioning model proposed in this paper contains integer variables, quadratic objective functions and quadratic constraints. It is a mixed integer nonlinear programming (MINLP) problem with certain difficulty in solving. By reducing the order of the model, the problem is transformed into a MILP problem, and the speed and reliability of the numerical solution are improved. In turn, a practical islanding scheme can be obtained.
Fourth, research conclusionsThis paper uses the US PG&E69 node power distribution system to verify the effectiveness of the proposed partition model. Table 1 compares the islanding schemes when considering different constraints.
By comparing and analyzing the above-mentioned island division scheme, the following conclusions can be drawn: Through the operation of the tie switch, the node that is expected to be restored can be flexibly selected, and the recovery rate of the important load is improved; when the DG type is considered, in order to prevent the frequency modulation capability The DG does not operate in an isolated island area alone. The division scheme adjusts the position of the load shedding and the load shedding. By sacrificing a part of the second type of load, the DG without the frequency modulation capability is connected to the DG with the frequency modulation capability. Each island area reserves a certain proportion of spare capacity, which can reduce the impact of load fluctuations and the uncertainty of DG output such as wind turbines and photovoltaics, and ensure that the isolated islands can continue to operate safely and stably under certain extreme conditions.
It should be noted that the islanding scheme proposed in this paper mainly focuses on the active power flow distribution of the power grid, but does not consider the reactive power flow distribution and voltage constraints of the power grid. In order to consider the reactive power flow distribution and voltage constraints, one idea is to change the DC power flow model used in this paper into an AC power flow model, and consider the optimal distribution of active and reactive power flows while the island is divided, but the model thus established is For the MINLP problem, the convergence reliability of the numerical solution will be reduced, and the calculation amount will be significantly increased. Another way is to perform reactive power on each island based on the island division scheme and the active power flow distribution given in this paper. Voltage optimization, regulation of reactive voltage by adjusting DG voltage, switching reactive power compensation device, etc., if necessary, cutting off some non-critical loads to ensure a proper voltage level. The reactive power distribution of the actual distribution network generally needs to meet the needs of the hierarchical localization of the hierarchical partition. The second approach is generally feasible in practical applications.
LED Underwater light,Full colour underwater luminaires,Waterproof lighting underwater lamp
Kindwin Technology (H.K.) Limited , https://www.ktlleds.com