The "N-1" safety standard is a crucial criterion in the planning of distribution networks. N-1 safety ensures that, after an N-1 fault occurs in the main transformer or feeder of the distribution network, the power supply to the affected loads remains secure through load transfer. This principle is fundamental in maintaining the reliability and stability of the grid, especially as modern distribution systems become more complex with the integration of new technologies.
Research indicates that the integration of distributed energy resources (DERs) has a significant positive impact on enhancing the security and resilience of distribution networks. When the capacity and location of these distributed power sources are appropriately planned, they can improve the overall reliability and safety of the power supply. As the future distribution network evolves, it becomes essential to consider the influence of DERs during the planning phase. The N-1 safety verification process plays a key role in this, but challenges remain in effectively implementing N-1 security checks once DERs are connected to the network. Addressing these issues is vital for the safe and efficient development of next-generation distribution systems.
To tackle this challenge, this paper proposes a novel N-1 security verification method tailored for distribution networks with distributed power sources. It provides foundational tools and methodologies to support the planning and operation of distribution networks that incorporate renewable and decentralized energy generation.
**1. Processing Method of DG After Distribution Network N-1**
**1.1 Classification of DG Related to Distribution Network N-1**
Distributed Generation (DG) can be classified based on its behavior during an N-1 fault. If the DG can serve as a backup power source, it is categorized as a standby DG. These types of DGs typically have controllable output and include technologies such as generator sets, microturbines, fuel cells, wind turbines with energy storage, and photovoltaic systems equipped with storage. On the other hand, non-standby DGs, such as solar panels or wind turbines without storage, have intermittent and variable outputs, often influenced by weather conditions.
Additionally, DGs can be divided into grid-connected and off-grid types. Grid-connected DGs continue to operate in synchronization with the main grid, while off-grid DGs may either be isolated during normal operations or form islanded systems after a fault occurs. Off-grid DGs can also be further categorized based on whether they are used as backup power or automatically disconnect upon fault detection.
Another classification is based on the area affected by the N-1 fault. Fault zone DGs are those located within the power-off area caused by the fault, while non-fault zone DGs are situated in areas still receiving power. It's important to note that 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, the handling of DGs depends on their type. For standby DGs, forming an islanded operation is recommended to maximize the power supply to local users and potentially expand to multiple users if feasible. This approach enhances the reliability of the distribution network. In power flow calculations, these DGs are typically modeled as PQ nodes.
**2. N-1 Safety Verification Process for DG-Integrated Distribution Networks**
Based on the fault recovery process in a distribution network with DGs, the N-1 safety verification process includes the following steps:
1. Assume an N-1 fault occurs at a main transformer or feeder outlet. Identify the affected area. If non-standby DGs are present, they are disconnected. Standby DGs (including both grid-connected and off-grid types) should be utilized to form multi-user islands if possible, otherwise, they will be limited to powering only local loads.
2. Locate the tie lines connecting the faulty power-off area to the non-faulty power supply area. Perform power flow analysis on each non-faulty area that is in contact with the faulty region, calculating the feeder 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, performing power flow calculations each time. If the voltage and load limits are not exceeded, continue the restoration; otherwise, stop the process.
4. During the recovery, if the island formed by the fault area can reconnect to the grid, it should do so. Otherwise, continue operating in island mode.
Throughout the restoration, the distribution network must adhere to traditional constraints, including:
- Node voltage limits: Ensure all node voltages remain within acceptable ranges.
- Branch flow limits: Prevent overloading of feeders.
- Main transformer capacity limits: The total load on the outgoing feeders should not exceed the transformer’s rated capacity.
- Radial configuration: Maintain a radial structure during power restoration, regardless of DG presence.
5. After restoring power to the faulty area using a non-faulty feeder, identify any new faulted zones and repeat steps 2 and 3 until full restoration is achieved or no further restoration is possible due to lack of available feeders.
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