## 4 Case Studies
### Test System Specification
#### 4.1.1 IEEE-8 Bus System  
The standard IEEE-8 bus system is a meshed distribution system with multiple sources [54]. It consists of eight buses and seven line segments. Three-phase faults were simulated at the midpoint of each line segment. There were a total of 14 directional overcurrent relays (DOCR). These relays comprised 20 primary-backup relay pairs, as given in Table 3. To study the effect of wind speed on FCL variation, two wind farms were integrated at bus-3 and bus-6. The details of WTGs in the wind farms are given in Appendix A Table 11. A one-line diagram of the IEEE-8 bus system is given in Figure 10. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Fault** & **Pair** & **PR** & **BR** & **Fault** & **Pair** & **PR** & **BR** & **Fault** & **Pair** & **PR** & **BR** \\ \hline \multirow{4}{*}{F1} & 1 & 1 & 6 & F3 & 7 & 3 & 2 & 14 & 6 & 5 \\  & 2 & 8 & 7 & 8 & 10 & 11 & F6 & 15 & 6 & 14 \\  & 3 & 8 & 9 & F4 & 9 & 4 & 3 & 16 & 13 & 8 \\  & 4 & 2 & 1 & 10 & 11 & 12 & & 17 & 7 & 5 \\  & 5 & 2 & 7 & 11 & 5 & 4 & & 18 & 7 & 13 \\ \multirow{4}{*}{F2} & 6 & 9 & 10 & F5 & 12 & 12 & 13 & & 19 & 14 & 1 \\  & & & & & 13 & 12 & 14 & & 20 & 14 & 9 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Primary (PR) and backup (BR) relay pairs for the IEEE-8 bus system. Figure 10: Standard IEEE-8 bus system modified by integrating wind turbine generators. GE Multilin750/760 numerical relays were employed here. The CT ratio for all relays was 400:1. The minimum and maximum limits of TMS were 0.05 and 1.1, respectively, whereas the limits for I\({}_{\text{p}}\) were \(1.1\times\text{I}_{\text{Load}}\) and \(1.5\times\text{I}_{\text{Load}}\). The CTI can take a value between 0.2 and 0.5 (REFF). However, for analysis of the IEEE-8 bus system, it was taken as 0.3. The minimum and maximum operation times of the relay were kept as 0.1 and 2.5, respectively, to assure the reliability of the proposed protection scheme. Two case studies were carried out. In the first case, the wind speed and wind direction were taken as 10 m/s and 5\({}^{\circ}\), respectively, whereas, in the second case, the wind speed was 20 m/s and the wind direction was 10\({}^{\circ}\). Figure 11 depicts the variation in FCL due to wind speed for all 20 relay pairs for both cases. It is reflected that the fault current increased as the wind speed increased, as described in Equation (16). If the conventional relay settings are not updated according to wind-speed variation after WTG integration, then the variation in FCL due to WS variation can cause miscoordination in primary-backup relay pairs. Thus, using the predicted FCL by ANFIS-SARIMA, the relay settings were optimally updated by hybrid HHO-LP. The relay settings for a wind speed of 10 m/s obtained using conventional, PSO-LP [55], and HHO-LP methods are given in Table 4. Table 5 provides the time of operation of primary-backup relays for the first case. It can be seen that, in some cases, relay pairs violated the CTI limit. For example, in relay pair 6 in the first case, the backup relay R10 and primary relay R9 could not maintain a CTI of 0.3, thereby disturbing the protection coordination. On the other hand, in relay pair 10, both primary and backup relays operated at the same time. Figure 12a shows the characteristic curve of relay pair 6 taken during the simulation in ETAP, with conventional relay settings during the first case. The relay settings were updated optimally by HHO-LP on the basis of the predicted fault current, and the relay characteristics for the same relay pair 6 with the updated relay settings are shown in Figure 12b. It can be seen that the CTI of 0.3 was maintained between primary and backup relays for this pair.