# Faulty Links' Fast Recovery Method Based on Deep Reinforcement Learning
## 3 Faulty-Path Recovery Scheme Based on DDPG-LBBP
### DDPG-LBBP Agent Interacts with the Environment
It can be seen from the figures that the faulty-link recovery delays of the four faulty-link recovery algorithms in the IEEE 57 communication network were higher than those in the IEEE 30 communication network. The reason for this phenomenon is that some links in the IEEE 57 communication network were longer, resulting in larger link delays, which led to high faulty-link recovery delays in the IEEE 57 communication network. Since the DDPG-LBBP algorithm comprehensively considers the network topology status information to calculate disjoint backup paths, it can effectively reduce the fault recovery delay compared with the other three fault recovery algorithms. ### Packet Loss Rate After a Faulty-Link Is Restored

Considering the fact that the phasor data flow in the communication network of the wide-area measurement system changes dynamically in real time, this paper designed an experimental environment with different traffic loads for the link design in the network topology. The Wireshark packet capture tool was used to analyze the phasor data packet loss in the network topology. The phasor packet loss rate is calculated as the ratio of phasor packets that have not reached the destination node \(d\) to the total number of phasor packets sent. To ensure the reliability of the experimental results, the packet loss rate was determined by averaging the results from several experiments in the IEEE 30 and IEEE 57 benchmark test power system communication networks. The packet loss rates of the DDPG-LBBP, FFRLI, (1+2\(\varepsilon\))-BPCA, and LIR faulty-link recovery algorithms were then compared. The experimental results are shown in Figure 9.

Figure 9: Packet loss rate of different fault recovery algorithms. As shown in Figure 9, the packet loss rate of the DDPG-LBBP algorithm fluctuated around 3.8% with the continuous increase in phasor data traffic in the IEEE 30 and IEEE 57 benchmark test power system communication networks, which was lower than that of other faulty-link recovery algorithms. The packet loss rates of the FFRLI (1+2\(\varepsilon\))-BPCA, and LIR faulty-link recovery algorithms showed an overall upward trend. The FFRLI fault recovery method formulates a backup path based on the importance of the link in the network topology, which can ensure the phasor data transmission of the main path, and thus had a lower packet loss rate than (1+2\(\varepsilon\))-BPCA and LIR. The (1+2\(\varepsilon\))-BPCA backup path recovery algorithm achieves faulty-link recovery using the faulty-link approximation backup path algorithm, without assessing the criticality of the link. This led to a higher packet loss rate in the network topology after fault recovery compared to the FFRLI fault recovery method. The LIR faulty-link recovery method was susceptible to excessive network link load during the experiment, resulting in the maximum phasor packet loss rate of the algorithm. In terms of the packet loss rate, the DDPG-LBBP algorithm demonstrated remarkable adaptive capabilities. Confronted with varying traffic intensities, the algorithm intelligently optimized link utilization and data transmission strategies. In high-traffic-intensity scenarios, it balanced the traffic loads among multiple links to avert congestion. In low-traffic-intensity scenarios, it enhanced data transmission efficiency, thereby reducing the packet loss rate. The DDPG-LBBP algorithm proposed in this paper uses the improved DDPG method to optimize low link utilization, low transmission delay and low packet loss rate as its objectives. It ensures the load balancing of the backup path while achieving fast fault recovery and reducing the packet loss rate of the network topology. Compared with the better FFRLI fault recovery method, the packet loss rate in the network topology was reduced by about 15.31% after the faulty link was restored. ### Faulty-Link Recovery Success Rate

To verify the faulty-link recovery performance of the DDPG-LBBP algorithm, the experiment used the faulty-link recovery success rate as the evaluation standard.