## 5 General Comparison of the Topologies  
General comparative case study for several MT-HVdc grid topologies is presented in Figure 8, considering the techno-economic factors such as length, number, and ratings of dc cables, number and  
Figure 7: 2 WF-SSRT topology operation in the event of a fault on the line L\({}_{12}\): (**a**) at \(t_{0}\), 2 WF-ISST topology during normal operation, (**b**) at \(t_{1}\), fault appears on the line L\({}_{12}\), (**c**) at \(t_{2}\), CB\({}_{12}\) and CB\({}_{34}\) operates, (**d**) at \(t_{3}\), IS\({}_{122}\), UFMA\({}_{12}\), and UFMA\({}_{21}\) open (**e**) at \(t_{4}\), first UFMA\({}_{15}\) and UFMA\({}_{23}\) are closed and then CB\({}_{12}\) and CB\({}_{34}\) re-closed as well. ratings of VSCs and DCCBs, redundancy, and loss of VSC station in case of a fault. The geographical positions of VSCs are presented in Table 2. PPT is not a multi-terminal topology while SGRT is a combination of GRT and ST. Therefore, these are not considered for comparative analysis. ST, GRT, SSRT GRT, and ST require the number of DCCBs equal to the number of WF-VSCs plus GS-VSCs while WFRT and SSRT require DCCBs equal to the WF-VSCs. The 2 WF-ISST and the proposed 2 WF-SSRT need DCCBs equal to half the number of WF-VSCs. The dc grid carries six GS-VSC and offshore WF-VSC stations, each one 400 MW for PPT. The star topology requires six GS-VSCs of 480 MW and six offshore VSC stations of 400 MW, respectively. Twelve VSCs of 2400 MW are required by the GRT. SSRT and WFRT need six offshore VSCs each of them 400 MW and 800 MW, respectively, with 800 MW and 480 MW mainland substations, respectively [30]. Six WF-VSC of 400 MW and three onshore VSCs of 1200 MW are designed for the 2 WF-ISST and proposed 2 WF-SSRT topology, respectively. An extra offshore platform is required for only ST and SGRT to install circuit breakers while dc circuits have the same power ratings as an associated VSC station. The dc circuits in WF/SS rings \begin{table}
\begin{tabular}{c c c c c c} \hline
**Wind Farm** & **Horizontal (km)** & **Vertical (km)** & **Sub Station** & **Horizontal (km)** & **Vertical (km)** \\ \hline WF\({}_{1}\) & 50 & 10 & SS\({}_{1}\) & 101 & 10 \\ WF\({}_{2}\) & 20 & 20 & SS\({}_{2}\) & 128 & 45 \\ WF\({}_{3}\) & 10 & 35 & SS\({}_{3}\) & 125 & 70 \\ WF\({}_{4}\) & 11 & 60 & SS\({}_{4}\) & 123 & 80 \\ WF\({}_{5}\) & 30 & 82 & SS\({}_{5}\) & 110 & 91 \\ WF\({}_{6}\) & 50 & 92 & SS\({}_{6}\) & 102 & 95 \\  & & Centre point for ST & & 75 & 45 \\ \hline \end{tabular}
\end{table} 
Table 2: Geographical positions of WF-VSCs and GS-VSCs for topologies. Figure 8: General case study established for various topologies (**a**) GRT, (**b**) ST, (**c**) WFRT, (**d**) SSRT, and (**e**) 2 WF-ISST, (**f**) 2 WF-SSRT. need a rating equal to the power of two adjacent VSCs while the rating of the dc circuit connecting GS/WF-VSCs from WF/SS ring depends upon the rating of the respective GS/WF-VSC. For 2 WF-1 SST and proposed 2 WF-SSRT, dc-links connecting WFs to the common point need to be rated at double capacity than the WF-VSCs while the dc circuits connecting neighboring-WF's are rated equal to the WF-VSC. Rating of the lines to the mainland grid from common point depends upon the rating of the GS-VSC for 2 WF-1 SST, while, for 2 WF-SSRT, it is equal to the rating of the respective unit. The rating of the cables within the SS ring of 2 WF-SSRT would be equal to the rating of two adjacent GS-VSCs. In the event of a dc-link fault, connected WF will be lost for ST and SSRT. However, in general, it will not affect the operation of GRT, WFRT, 2 WF-1 SST, and 2 WF-SSRT topologies. However, the operation of 2 WF-1 SST will be greatly affected if the fault persists on end sectioned lines (i.e., L\({}_{12}\) and L\({}_{56}\)). Thus, 2 WF-SSRT topology shall support in this regard.