The “N-1†safety standard is a crucial criterion in the planning of distribution networks. N-1 safety ensures that, even after a fault occurs in the main transformer or feeder of the distribution network, the power supply to the unloaded load remains secure through load redistribution. This principle plays a vital role in maintaining grid reliability and minimizing disruptions.
Research has shown that the integration of distributed energy resources (DERs) significantly enhances the security and resilience of distribution networks. When the capacity and location of these distributed power sources are appropriately planned, they can greatly improve the reliability and safety of power delivery. As the future of distribution networks evolves, it becomes essential to incorporate the impact of DERs into the planning process. However, the challenge of conducting N-1 safety verification in the presence of distributed generation remains unresolved, making this an important area for further study and development.
Therefore, this paper introduces a new method for N-1 safety verification in distribution networks with distributed power sources. This approach provides foundational tools and techniques for planning and operating distribution systems that integrate renewable and decentralized energy resources.
### 1. Processing of DG After Distribution Network N-1 Fault
#### 1.1 Classification of DG Related to N-1 Fault
Distributed Generators (DGs) can be categorized based on their behavior during an N-1 fault. If a DG can act as a backup power source, it is considered a standby DG. These types of DGs, such as generator sets, microturbines, fuel cells, wind turbines with storage, and photovoltaic systems with energy storage, usually have controllable output. On the other hand, non-standby DGs, like solar panels or wind turbines without storage, often exhibit intermittent and variable output, heavily influenced by weather conditions.
Additionally, DGs can be classified as either grid-connected or off-grid. Grid-connected DGs continue to operate in synchronization with the main grid, while off-grid DGs may disconnect from the grid during a fault. Off-grid DGs include those that function as backup during normal operation but are not connected, those that automatically disconnect after a fault, and those that form islanded microgrids.
Furthermore, DGs can also be divided into those located within the fault area and those outside of it. Fault area DGs are found in regions that lose power due to the N-1 fault, while non-fault area DGs remain operational in unaffected zones.
It's important to note that these classifications are not absolute, as all DGs will eventually transition back to grid-connected operation once the fault is resolved.
#### 1.2 Handling of DG After N-1 Fault
When an N-1 fault occurs, the handling of DGs varies depending on their type. Standby DGs that can form an island should aim to expand their power supply to serve more users, enhancing the overall reliability of the network. These DGs are typically modeled as PQ nodes in power flow calculations.
### 2. N-1 Safety Verification Process in DG-Integrated Networks
Based on the recovery process of distribution networks with DGs, the N-1 safety verification procedure involves the following steps:
1. Assume an N-1 fault occurs at a main transformer or feeder. Identify the affected area. If non-standby DGs are present, they are disconnected. If standby DGs (including both grid-connected and off-grid types) are available, they should be used to form multi-user islands to maximize power supply.
2. Locate the tie lines between the faulty and non-faulty areas. Perform power flow analysis on each non-faulty section connected to the faulty zone, determining the capacity margin and voltage distribution of each feeder.
3. Use the feeder with the highest capacity margin and stable voltage to restore power to the faulted area. Attempt to restore power to a certain number of loads and perform power flow calculations. If no overloads or voltage violations occur, continue the restoration; otherwise, stop the process.
4. During the restoration, if the isolated island can reconnect to the grid, it should do so. Otherwise, continue operating in island mode.
Throughout the process, the network must comply with traditional constraints, including:
- Node voltage limits: All node voltages must stay within acceptable ranges.
- Branch flow limits: No branch should exceed its rated capacity.
- Transformer capacity limits: The total load on each outgoing feeder must not exceed the main transformer’s rating.
- Radial structure: The network should maintain a radial configuration during restoration, regardless of DG presence.
After restoring power to the faulted area using a healthy feeder, a new faulted zone may emerge. Repeat steps 2 and 3 until the entire area is restored or no further feasible restoration options exist.
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