# Development of A Loss Minimization Based Operation Strategy for Embedded BTB VSC HVDC
## 4 Case Study
### STLM Strategy in Normal Condition  
Before determining the operating point in normal operation, Equations (7)-(9) were applied to reduce the number of observed branches to be considered in the overload constraint. The active power capacity \(P_{\alpha p}\) is 400 MW, weighting factor \(W\) is 0.3, and other necessary values were applied using Korean power system data. The monitoring branches determined using Equations (7)-(9) are in Appendix A.  
To apply the STLM strategy, the initial conditions must be found. The initial conditions of this case study are as follows.  
* Initial operating point \(S_{0}^{(1)}\): (0, 0, 0),
* Independent directions used for initial searching \(\epsilon=\{\overrightarrow{P},\overrightarrow{Q_{rec}},\overrightarrow{Q_{inv}}\}\),
* Step size \(\delta=1\).  
The loss reduction from the operating point found by using the STLM can be seen in Tables 5 and 6. For the case when the HVDC is in operation, we compared the losses for (1) the case where the STLM strategy is applied and (2) the case where the operating condition is applied to the amount of power flow in the existing AC line without applying the STLM. The HVDC operating point in the case of \"with HVDC (AC flow)\" does not have a special strategy, and the operating point is determined based on the power flow through the AC line when there is no HVDC (when connected to the AC line instead of the separation between buses). More than 400 MW of power flowed to the AC line before the bus separation, and 200 MW \(\times\) 2 was selected as the operation point according to the HVDC rating limit. When the STLM is applied, the system losses are the least and the technology of the conversion loss of the converter further develops, and the economy caused by the reduction of the loss can be improved. Additionally, at the operating point obtained by applying the STLM strategy, there are no overload lines, and the maximum/minimum voltage of the system is 1.060/0.984 at the 154 kV level, and 1.039/0.995 at the 345 kV level. These values meet the system overload and voltage criteria described in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Bus** & **Bus** & **With** & **Without** & **Bus** & **Bus** & **With** & **Without** \\
**Number** & **HVDC** & **HVDC** & **Number** & **Base kV** & **HVDC** & **HVDC** \\ \hline
1200 & 345 & 54.8899 & 55.0129 & 1810 & 154 & 48.2441 & 53.2228 \\
1400 & 345 & 48.2340 & 50.1957 & 1811 & 154 & 47.7596 & 52.6257 \\
1410 & 154 & 49.7672 & 65.5842 & 1865 & 154 & 46.2224 & 51.5447 \\
1411 & 154 & 25.7956 & 65.5842 & 1870 & 154 & 47.0455 & 52.7775 \\
1475 & 154 & 46.1315 & 51.8724 & 1895 & 154 & 47.1782 & 53.1212 \\
1490 & 154 & 46.2564 & 52.0078 & 1955 & 154 & 44.5498 & 50.1641 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Fault current comparison with/without HVDC [kA].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**P (1410 \(\rightarrow\) 1411)** & _AC Voltage (Rectifier)_ & _AC Voltage (Inverter)_ \\ \hline
**Setpoint** & \(131\times 2\) MW & \
\begin{tabular}{c} 1.048 kV \\ (Supply 95.31 MVAR) \\ \end{tabular} &
\begin{tabular}{c} 1.039 kV \\ (Supply 3.80 MVAR) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 5: HVDC setpoint using the STLM control.