The "N-1" safety standard is a crucial criterion in the planning of distribution networks. N-1 safety refers to ensuring power supply security for unloading loads after an N-1 fault occurs in the main transformer or feeder of the distribution network, by transferring the load appropriately.
Research indicates that the integration of distributed energy resources (DERs) significantly enhances the power supply reliability and security of distribution networks. When the capacity and location of DERs are reasonably planned, the overall reliability and safety of the distribution system can be improved. In future distribution network planning, the impact of DERs must be carefully considered. N-1 safety verification remains a critical step in this process, yet the challenge of implementing N-1 security in distribution networks with integrated DERs remains unresolved. Addressing this issue is essential for the safe and efficient planning of future distribution systems.
To tackle this, this paper proposes a new method for N-1 security verification in distribution networks incorporating distributed generation. This approach provides foundational tools and strategies for planning and managing distribution networks with distributed power sources.
**1. Processing Method of DG After Distribution Network N-1**
**1.1 DG Classification Related to Distribution Network N-1**
Distributed generation (DG) can act as a backup power source during N-1 faults in the distribution network. Based on this function, DGs can be classified into two categories: standby DGs and non-standby DGs. Standby DGs typically have controllable output power and include devices such as generator sets, micro-turbines, fuel cells, wind turbines with energy storage, and photovoltaic systems with storage capabilities. Non-standby DGs, on the other hand, often exhibit intermittent and variable output, heavily influenced by weather conditions. Examples include wind turbines and photovoltaics without energy storage.
Additionally, DGs can be categorized based on whether they remain connected to the main grid after an N-1 fault. Grid-connected DGs continue operating in parallel with the main grid, while off-grid DGs fall into three types: those used as backup during normal operation but not connected to the grid, non-standby DGs that disconnect immediately after a fault, and islanded DGs that continue operating independently after a fault.
Furthermore, DGs can be divided into fault area DGs and non-fault area DGs, depending on whether they are located within the power-off zone caused by the N-1 fault. These classifications are not absolute, as DGs may transition between states during the fault recovery process.
**1.2 DG Processing After Distribution Network N-1**
When an N-1 fault occurs, different DGs require distinct handling. It should be noted that when a standby DG forms an island, it should expand its power supply as much as possible—beyond local users—to support multiple users, thereby enhancing the overall reliability of the distribution network. These DGs can be treated as PQ nodes in power flow calculations.
**2. N-1 Safety Verification Process in DG-Integrated Distribution Networks**
Based on the fault recovery process in distribution networks with DGs, the N-1 safety verification procedure is as follows:
1) Assume an N-1 fault occurs at a main transformer or feeder outlet. Identify the affected area. If non-standby DGs exist in the fault zone, they are disconnected. Standby DGs (including both grid-connected and off-grid DGs) should form multi-user islands if possible; otherwise, they operate as single-user islands to serve local loads.
2) Locate the tie lines between the faulty and non-faulty areas. Perform power flow analysis on each non-faulty area connected to the fault zone to determine the available capacity margin and node voltage distribution.
3) Use the feeder with the largest capacity margin and acceptable voltage levels to restore power to the fault area. Attempt to restore power to a certain number of loads and perform power flow calculations each time. If voltages and branch flows remain within limits, continue restoring power; otherwise, stop the process.
During recovery, if the island formed by the fault area can reconnect to the grid, it should do so. Otherwise, it continues operating in island mode.
Throughout the restoration process, the distribution network must comply with traditional constraints:
(1) Node voltage must stay within allowable limits.
(2) Branch flows must not exceed their rated capacities.
(3) The total load on the outgoing feeders of the main transformer must not exceed its rated capacity.
(4) The network must maintain radial configuration during power restoration, regardless of DG influence.
4) Once power is restored to the fault area using a non-faulty feeder, a new fault zone may form. Repeat steps 2 and 3 until the entire fault area is fully restored or no further non-faulty feeders are available.
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