The “N-1†safety standard is an important criterion for distribution network planning. N-1 safety means that after the N-1 fault occurs in the main transformer and feeder of the distribution network, the power supply safety of the unloaded load is ensured through the load transfer.
Research shows that the access of distributed power sources plays an important role and positive impact on improving the power supply security of distribution networks. When the capacity and location of the distributed power supply distribution network are reasonable and appropriate, the reliability and safety of the power supply of the distribution network can be improved. In the future distribution network planning, the influence of distributed power supply should be considered. N-1 safety verification is an important step in distribution network planning, and how to implement N-1 security for distribution network after distributed power supply is connected to the distribution network. The problem of sexual verification has not been resolved, which has important significance and role for the planning and safety evaluation of the future distribution network.
Therefore, this paper proposes a N-1 security verification method for distribution network with distributed power supply. Provides basic tools and methods for distribution network planning with distributed power.
1 Processing method of DG after distribution network N-1
1.1 DG classification related to distribution network N-1
According to the distributed power supply (DG, Distributed GeneraTIon), after the fault of the distribution network N-1 can be used as the backup power supply of the distribution network, the distributed power supply can be divided into the standby power source DG and the non-standby power source DG. The output power of the backup power DG can usually be controlled, including generator sets, micro gas turbines, fuel cells, wind turbines with energy storage devices and photovoltaics; non-backup power DG output power is often intermittent and volatility, subject to weather conditions The influence of other factors is relatively large, such as fans and photovoltaics that are not equipped with energy storage equipment.
According to whether the distributed power supply is connected to the main power grid after the fault of the distribution network N-1, it can be divided into the grid connection DG and the off-network DG. The grid-connected DG maintains grid-connected operation with the main grid; the off-grid DG includes three types: one is the DG that is used as the backup power source during the normal operation of the grid but is not connected to the grid; the other is the non-standby power source DG that directly exits the operation after the fault occurs; After the failure, an island-running DG is formed.
According to the distributed power supply, the area where the distribution network N-1 fault occurs can be further divided into a fault area DG and a non-fault area DG. The fault zone DG refers to the DG in the power grid in the power-off area formed after the N-1 fault occurs; the non-fault zone DG is the DG in the non-power-off zone after the N-1 fault occurs.
It is not difficult to see that the concept of grid-connected DG and off-network DG and fault zone DG and non-fault zone DG are not absolute. Any type of distributed power supply will follow the fault recovery process after the fault occurs. change.
1.2 DG processing after distribution network N-1
When an N-1 fault occurs in the distribution network, the processing method is different for different distributed power sources. It should be pointed out that when the standby DG forms an island operation, on the basis of ensuring the power supply to the local users, according to its own capacity margin, the power supply range is expanded as much as possible to form a multi-user island operation mode to improve the power supply of the distribution network. reliability. The above types of DG can be regarded as PQ nodes in the power flow calculation.
2 DG distribution network N-1 safety verification process
According to the distribution network fault recovery process with distributed power supply, the N-1 safety verification flow of the distribution network with distributed power supply is as follows.
1) Assume that an N-1 fault occurs in a main transformer (at the outlet of a feeder) in the distribution network, and the fault area is found down from the fault point. If there is a non-standby DG in the fault area, it will be taken out of operation; if there is a standby DG (including grid-connected DG and off-network DG), the multi-user island operation should be formed as close as possible, otherwise it will be decomposed into unit islands only for local load. powered by.
2) Find the tie line between the faulty power-off area and the non-faulty normal power supply area. The power flow calculation is performed on each non-faulty normal power supply area that is in contact with the faulty power-off area, and the respective feeder capacity margin and node voltage distribution are obtained.
3) Using the feeder with the largest capacity margin and the terminal node voltage is still not limited, the power supply of the fault area is restored. Every time an attempt is made to restore the power supply to a certain number of loads, a power flow calculation is performed, such as the feeder and the main variable capacity of the feeder. If the voltage is not overloaded and the voltage of each node on the feeder is not exceeded, the power supply attempt is resumed, otherwise the recovery is terminated.
During the recovery process, if the isolated island formed by the fault area can be connected to the grid, it will be connected to the grid. Otherwise continue the island operation.
In the process of power restoration, the distribution network needs to meet the constraints of the traditional power restoration process:
(1) The node voltage is constrained, and the voltage of each node should be kept within the voltage constraint range;
(2) The branch flow constraint, the branch flow cannot exceed the upper limit of the capacity of each feeder, that is, it cannot be overloaded;
(3) The main variable capacity constraint, the sum of all loads on the outgoing feeder of the main transformer shall not exceed the rated capacity of the main transformer;
(4) Radial power supply constraints, regardless of the influence of DG, the distribution network should maintain radial power supply during power restoration.
4) After restoring the power supply to the faulty area with the non-faulty normal power supply feeder, re-form a new fault power-off area and repeat steps 2) and 3). Until the faulty power-off area is fully restored or there is still a faulty power-off area but there is no non-faulty normal power supply feeder, the power supply can be restored.
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